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SELECTIVE ABSORPTION OF HYDROGEN
SULFIDE IN AQUEOUS SODIUM CARBONATE
SOLUTIONS
by
Mohammad Al- Wohoush
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A Thesis submitted to the Faculty of Graduate Studies
and Research in partial fulfillment of the
requirements for the Degree of
Master of Engineering
McGILL
Department of Chemical Engineering
McGiIl University
Montréal. Canada
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Copyright© 1994
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ABSTRACT
The absorption of hydrogcn sullide and carbon dlo,ide in aqul'ollS sodium
carbonate solution was studied on a pilot scale pad. ed column opelall:d cOllnter-cllllently
under atmospheric pressure. J'he ahsOIptlon COIUIllIl \\as 7.5
('fil ~n
diamder and ISO
('/11
in height and packcd randomly wlth 6 mm Intalox Saddlcs Il \\'ll!-. deslgncd to lemo\ c
hydrogen sultide from a gus mixture of 1.5 % hydrogen slIltide and 15 (} (} cm hon dio~ilk
at an cfficiency of 95 %.
Rcliahiltty of the cxpcrimental setllp ha<; heen pelll<;ed hy
investigating the rcsidence time distribution of th...: gas phase in the colllllln and hy
studying thc absorption or carbon dioxidc
111
\\all'r.
In the first pDrt of the cxpcrimental \York. the lepcatahlhty or e~penl1lCllts and
•
material balances \Vere studicd and round to be within the acceptahle range
I\lso. thl'
interactions betwecn tempcrature, carbonate concentration. initIai concentration or
hydrogen sulfide and carbon dioxide. and the gas/liqUld ratio wele Itllll1d to he negllgiblc.
Furthcrmore, it has been investigated how ail these parameter:-. eontnhllte to hydJ(\gen
sulfide and carbon dioxide absorption. Il was 1{)Und that carbonate conccntmtlOll has a
major cffect on the absorption of hydrogen sullide, whilc the ah!-.(lI pt ion or carboll
dioxide is atTectcd signilicantly by temperatllle. 1Il the second part or the expenlllclltal
work, the mflucncc of ail parameters on the ab~orptlOn or hydlOgen <;ullide and t:arboll
dioxide has been invcstigatcd. Results ""cre in good agreement with tho~c rcported hy
other researchers; thcse rcsulls were analyzcd in terms
or rCl1loyal cf'liciclleies, the (lyerall
mass transfer coefficients and the sclcctivity of the proccs<.; l'or hydrogell
~lIlfide.
Il was
observed that hydrogen sullide ean he absorhed sclec\lvcly ill the presence 01 carboll
dioxide at low operating tcmperaturcs. high carbonatc conccntratloll and high gas to
liquid ratios. A rcmoval cfficicncy
or hydrogen su 1fide of ahout 92'X, at:compallled with
about 17% of initial amount of carbon dioxlde has been achicyed .
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III
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Résumé
L'ahsorption d\: sulfure d'hydrogène et de dioxyde de carbone dans une solution
aqu\:use de carhonate de sodium a été étudiée sur une colonne garnie, à échelle pilote,
utilisée dans le ~cns opposé au courant sous pression atmosphérique.
d'ahsorptioll
me~urait
La colonne
7.5 cm de diamètre et ISO cm de haut ct était garnie de façon
arbitrair\: ù l'aide de "lntalox Saddles" de 6 I1Im. Elle était conçue pour extraire le sulfure
d'hydrogi:nc d'un mélange gazeux
de 1.5 % d.! sulfure d'hydrogène ct de 15 % de
dioxyde de carhone avec un rendement de 95 %. La fiabilité du montage expérimental a
été sOigneusement vérifiée cn recherchant la répartition du temps de résidence de la phase
gazcuse dans la colonne ct en étudiant l'ahsorption du dioxyde de carhone dans l'eau.
Dans la première partie du travail expérimentaL la répétition des expériences et la
balance des maténaux fur\:nt étudiécs \:t se placèrent dans un intervalle acceptable. De
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même
Ics interactions entre la température. la concentration dc carbonate, la
concentration mitialc de sulfure d'hydrogène et de dioxyde de carbone et le ratio
gas/liqUIde s'avérèrent négligeables.
De plus.
la manière dont ces paramètres
contribuèrent il l'absorption du sulfure d'hydrogène ct du dioxyde de carbone fut étudiée.
II en ressortit que la concentration de carbonate a le plus d'influence sur l'absorption du
sulfure d'hydrogène tandis que la tempérrtarc a un effet maJeur sur l'absorption du
dioxyde de
Cal bone.
Dans la Jeuxième partie du travail expérimental. l'influence de tous
ces paramètres sur l'ahsorption du sulfure d'hydrogène et du dioxyde de carbone fut
étudiée. Les résultats correspondaient d'une manière satisfaisante à ceux mentionnés par
d'autres chercheurs: ces résultats furent analysés en fonction de l'efficacité d'extraction,
des coeflicients de translCrt de la masse totale ct de la selectivité du processus pour je
sulfure d'hydrogène.
lia été observé quc le sulfure d'hydrogènc peut être absorbé
sélectivement en présence de dio:-.ydc de carbone à de basses températures, à haute
concentratIon de em bonatc ct à ratios gas/liquide élevés. Un rendement d'(:xtraction de
sulfure d'hydlOgène d'environ <)2 '% accompagné d'environ 17 % de la quantité initiale de
dioxyde de carbone fut atteint.
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ACKNOWLEDGMENTS
1 would like to e:~press Illy gratitude 10 Dr. G. Kuhcs IlH' hls \'aluahk guidance.
encouragemcnt and tinancial support throughout the period it took 10 eompktc this tlll~sis.
Special thanks go as weil to my co-supcrvisor. Dr. Adlian vall 1il.'inillgcll (lINB)
for his valuablc comments.
AIso, J would like to sinccrely th<lI1k the InternatlOllal Energy Agelle)' and its
supporting members for providing mos! of the funding l'or this research.
1 am
also
grateful to the Pulp and Paper Research Institute of Canada for the scholarships i1wardcd
and for building the equipment.
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Many thanks go to the Chemical Engineering Ikpartl11ent. especially J Dumont
for his very kind hclp in the selcction and orderillg of malerials and suppllcs.
1 wish to thank ail members of thc CRIS» group. particularly i\kilcndra, Sabina.
Yujing, Stuart, Aliyc, Biao and Amir for thcir fricndship. A spccial word or thanks 10
François for his help with the expclÏmcntal design and Emmanuelle lor translating the
abstract to French.
Last, but not least, many thanks go tn my loving wi rc, Mariam. I(lr her sacri Iiccs,
much encouragement and mor:11 ~upport; a very special thanks go to our seven wceks old
son Anass .
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TABLE OF CONTENTS
ABSTRACT
111
Résumé
IV
ACKNOWIJmGMENTS
LIST OF FIGURES
LIST OF TABLES
V
IX
Xl
CHAPTERONE
INTRODUCTION
1.1 IMPORTANCE OF THIS RESEARCH
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1.2 BASIC OF RESEARCH
1.3 OUTLINE OF TIIESIS
2
3
4
CHAPTERTWO
LITERATURE REVIEW
2.1 ABSORPTION IN ALKANOLAMINE SOLUTIONS
2.2 ABSORPTION IN AÇùEOUS ALKALI SOLUTIONS
2.2.1 SIMULTANEOUS ABSORPTION OF H2S AND CO 2
2.2.2 SELECTIVE ABSORPTION OF ll2S
2.3 CIIEMISTRY OF TI lE PROCESS
2.4 EQlIILlBRIU1\I1 DATA
2.4.1 VAPOR LlQUID EQUILII3RIA
2.4.2 CARBONATl:-l3ICARBONA TE EQUILII3RIUM
2.5 CONCU ISIONS
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7
8
9
10
13
16
16
17
18
vi
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CHAPTER THREE
EXPERIMENTAL SET-lJP
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DESCRIPTION OF EXPFRIMEN rAI SE r-llp
3.2 CHARACTERIZA l'ION OF EXPFRIl\tFN l'AI. SI-' r-llP
3.2.1 GAS RESIDENCE T1MF DIS 1RlBllllON
3.2.2 CARBON DIOXIDE ABSORP lION IN WA rFR
3.3 OPERATING PARAME rERS
3.4 CHEMICAL ANAL YSIS OF SAMPLES
3.4.1 GAS SAMPLES ANAL YSES
3.4.2 LIQUID SAMPLES ANALYSES
.., ,
--'
l'
--'
"l.7
JO
.1 1
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CHAPTER FOlJR
PRELIMINARY EXPERIMENTAL WORK
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4.1 PRELIMINARY EXPERIMENTAL DESICiN
4.2 PRELIMINARY EXPERIMENTAL RESl ILTS
4.2.1 REPEATABILITY OF EXPERIMI:N rs
4.2.2 INTERACTIONS & EFFECT OF PARAMETI':RS
4.3 MATERIAL BALANCE
:'7
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4.4 EFFECT OF BED IIEICiHT ON 11 2 S & CO 2 ABSORPTION
47
CHAPTER FIVE
RESULTS AND DISCUSSION
5.1 EXPERIMENTAL DESIGN
5.2 DISCUSSION OF RESUL 1S
5.2.1 EFFECT ON THE RbMOVAL EFFICIENC'Y
5.2.2 OVERALL MASS TRANSFER COI:FFICIEN 1S
5.2.3 SELECTIVITY OF TI lE PROCESS
50
51
51
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VII
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CHAPTER SIX
CONCLUSIONS & RECOMMENDATIONS
() 1 ('ONCLlJSIONS
69
() 2 RECOMMEND;\TIONS FOR FtrJ URL WORK
70
IU;:FERENCES
71
APPENDIX A : DESIGN OF TI lE ABSORPTION COLUMN
73
APPENDIX B: AXIAL DISPERSION PLUG FLOW MODEL
79
APPENDIX C: DATA AND CALCULATIONS
82
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viii
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LIST OF FIGlIRES
Figure 1./
Altclnatlve Inw tempel atule J..r,lft Ililld-bed chenllc,1I1 eUl\ el\ PIlILL''''
Figure 2./
Concenlratloll profile of rem:teu
Figure 3.1
Schematlc dhlgJ am of the e\peflmcnl.ll ... cl-llp
Figure 3.2
The 1l0n-d"nenslOnal COnCel1tl.1t Ion 01 Ile al Ihe
or Ume (L 1/'11/111" '1' -2:i"(')
Figllre 3.3
Residence "1 II1H!
Figllre 3.4
E\.pcml1ental and Theoretical Re.,idellLe Illne nl,lllblillOll 01 thc !!.l' ph,\.,c
Figllre 3.5
Ab~orptioll
C\ Il
,,
01 Ihe COIUlIlll ,l, IUIlL 111111
.' 1
the pacJ..cd lllllllllil (1. 1 1111/1/. 1 .2 ,,"( ')
ne
Figllre 4.1
InteractIOn bctween temperalure and c.." bondte LOncenllallllll ,!Ild Ih cl kL\ 01
the absorptIon 01' 1Ils and COl
Figure 4.2
Interactioll betwcen temperature and hydlOgcn
Its erfect of the absorptIon of II~S and CO 2
Figllre 4.3
Interaction between tempcrature ,lIld cmboll dlo\,lde II1lel LOnL.entl,\llIlIl .llllilh
effcct of the ab~orptlon of 11 2S and cO 2
flgllre 4.4
Interaction between tempclature ilnd
absorptIon of 1Ils and co}
Figllre 4.5
Interaction bet\Vecn cmbonate conCCllIl,\IIOn and hydlOgell
concentration und Ils cffect of the ab~orptHlll of Il,S and co,
Figure 4.6
InteractIon bctween carbonate concclltl,ltlOIl and c,uholl
concentration and It~ clTect 01 the ab~orrtlOn ofll,S and co,
~lIllide
ga~/liqllld
11)
mlet LOIlLCIlII.lllon ,lIld
lalm and
clkLI III Ihe
11\
10
~lIllidc
IIIleI
·11
dlo\llie
111 IcI
·11
Figure 4.7
1nteractlon between carbonate COllœntratlon and ga.,/lIqllld 1at III ,lIltl Ih ellecl
of the ab~orptlon of II~S and co}
,12
Figure 4.8
InteractIOn between mlet conœntratlon 01 hydlOgen .,lIllide .lIld
concentratIOn and Il<, clTeet of the ab~orptlon 01 Il)S and ('(),
miel
,12
Figure 4.9
Intel action between mlet conccntratlon or hydlOgen
ratIo and It<, efrect of the ab~orptlon 01 II~S and co!
!!a~/llqulll
·Il
Figllre 4.1 () InteractIOn between mlet conccntratlon of Cdl hon d IO\( Ide and g;I~!llqllld 1atlO
'Il
and It., elTect orthe ab,>orption 01 Il,'') and
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1~
,1I1d lonlL "penc,
of carbon dlo\lde III \V.ltel a, a 11IIlLlI01l 01 g.t., ,lIld liqllld
velocilie~ at 25
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[)I~tllbulion 111
ga.,c~
~lIllidc
,lIId
co,
Fig.lre 4.// Relative elrect of parametcr., on the ah,orptlOl1 01 11 2 ~ and ((),
Itl
Figure 4./2 Effect bed helght on the relnoval cflillency 01 Il, ~
tl()
Figure 4. /3 CITect bed helght on the relnoval ellk lency 01 CO,
tl')
Figure 5./
Elrect oftemperaturc on the removal ellillency olll,~ and ( 0,
'if)
FigllTe 5.2
Effect of carbonate concentration on the relllovai cl fillcncy of Il, ~ and ( (),
'il)
Figure 5.3
Effeet or hydrogcn ,>ullidc miel C\lnecntratloll on thl! rCl110val dlitll!IICy 01
')7
H}S and
co}
IX
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l';~,,re
5.4
Llleet of l.arhon dloxlde
ln let
concentration on the removal efliclency of H2S
57
and co)
N~lIre 5.5
U lect 01 ga,> ',uperficlal veloclty on tlle rCllloval efliciency of H2S and CO 2
58
N~,,,e 5.6
1.1 fCl.t 01 hqu Id <,uperfic lai veloclty on the removal efficlcncy of 11 2Sand CO 2
58
Lffect 01 temper,lture on the overall l1las~ tran'ilèr coefficients of H2S and
61
N~"re
.;.7
('° 2
N~"re
5.11
I:flel.l 01 carbonate concentration on the over-all
11 2S and CO 2
transfer coefficients of
61
1';~lIre
5.9
Fffect 01 hydrogen ,>ulfide mlet concentration on the over-all mass transfer
coefficient,> or /IlS and cO 2
62
N~lIre
5.m
Effect 01 carbon dloxlde mlet concentration on the over-all mass transfer
coefficient,> of /IlS alld cO 2
62
l-ï~lIre
5./1 1:lIèct ga~ ~uperlicJaI veloelty on the ovel-all mass transfer coefficients of
11 2S and CO 2
63
N1:lIre 5.12 I:ffeet liquid
~tlperlieJaI
ll1a,>~
veloclty on the over-allmass
tran~fer
roefficlents of
63
11 2S and CO 2
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N1:lIre 5. /3 I:flèct of tcmperature on the <,electlvlty factor
66
N~lIre
66
5.14 Effect 01 carbonate concentl atlon on the seleetlvlty factor
l-ï1:lIre 5./5 LI l'cL! of hydrogen <,ulfidc mlct concentratIon on the 'iclcctivity tàctor
67
l-ï~lIre
67
5.16 Lllcet 01 carbon dlo7\lde in let concentration on the selectlvlty factor
l-ï1:lIre 5./7 Elfect gas '>tlrerticral veloclty on the selcctivlty làctor
68
I-ï~IIre
68
5.111 ,. flect hqUld superlicial velocity on the selectivlty factor
I-ï1:IIre ,.1.1
Floodmg and pressure drop
III
random packed columns[J 9]
78
x
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LIST OF TABLES
Table 2.1 Dissocmtlon constant of carbonate m water Iio.,t .
17
Table 3.1 Charactenstics of the packed bed ....... .
25
Table 3.2 Ranges of operating
parameter~
JI
Table 4.1 Prehminary Expenmental design
Table 4.2 Repeatability of expenments ..
J7
T(Jble 4.3 Effect parameters and mteractions on the removal cl1icicncic . . of Il,S and {'()
l
lK
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TClble 4.4 Material balance ....
·17
Table 5.1 Final experimental de'ilgn
51
Table 5.2 Effect of gas flow rate. ........ ..
Table 5.3 Effect of hquid flow rate ..................... .
Table A.I Composition and properties of the gas ..,tream at the in lei and at the out let 01
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the column.. ...... ........ ........ .... ........ .. ...... . ...
Table CI Operating condition and removal efficlencle.., of hydrogen !'luI tide and
dlOxide m the tinal expenmcntal work
T(Ible C2 Concentration of
10nlC
C,II
bon
....... . ..
. K3
!'pecies m the eXit hqulù ... tream and the CqlllllhllUIlI
partial pressures of hydrogen sulfide and carbon dloxlde .. .
T(Ible C3 Absorption rates and the overall
mas~
tram fer coefficlen .... and the ...clcctlvlty
. .. XH
factor...... .. .............. ..
Table C4 Effeet of temperature on the rcmoval efficicncy and the ovcr
coeffiCient of 11 2S and CO 2 and on the <,electlvlty factor
allllla~ ...
trano.,fer
91
..
Table C5 Effect of carbonate concentratIOn on the rell10val efficlenl.y and the over ail
mass transfer coefticlCnt of 11 2 S and CO 2 ,md on the o.,elcctivlty factOl
l)
1
Table C6 Effect of 11 2S mlet concentration on th.: rell10val efficlenl.y and the ovcr .dl
mass tran!.fer coefticlCnt of 11 2 S and CO 2 and on the ... clel\lvlty lal\or .
92
Table C7 Effect of CO 2 inlet concentJatlOn on the rell10val cl1iclelKy and the ovcr ail
mass tran~fer coefficient of 11 2 S and CO] and on the '>Clel\lvlty fm.\or
Table
. 92
ca Effect of the gas superficlal veloclty on the rell10val efficlcncy and the ovcr ail
mass transfer coefficient of H2S and CO 2 and on the ,>clettlvlty factor (data
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obtumed wllh the expermlel1tul de\l~f1).
.. ....... 93
XI
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1t,hle C9 !:Ilect 01 the ga., ,>uperficml vcloclty on thc rcmoval cfficlcncy and the over ail
ma.,., tran.,fcr coefficlcnt or "2S and CO 2 and on thc selcctivity factor (duta
ohill/lled wllh Ihe conventlOnal expel/II/cntal wrJrk)..
. ................................... 93
lt/hle CH Enect of the ga., ~lIpcrficJaI veloclty on the rcmoval efficlcncy and the over ail
IlHl.,.,
lran.,fcr coerticlCnt or I/ 2S and COl and on the sdectivity factor .................. 94
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xii
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CHAPTERONE
INTRODlJCTION
Hydrogen sulfide is one of the undesircd gascs which IS prodm:cd hy Ihe
combustion or gasification of fuels that contain
especially in petroleum rdincries and
111
~lIlfur
compollnds. Il i. . I<HlIld gas 'ilrcall1s
Ihe klaft (1l1lping 'iceluI'
III
the pulp and papci
industry.
For vanous reasons, such as the ollcn'ilve odm 01 hydrogcn ,,\lllidc, It:-.
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corrosiveness and its toxicity, the presence of hydrogen sullidc has heen consldell:d to be
environmentally objectionable l'rom practically the beginning 01 the coal indllstry
Hydrogen sulfide is cspccially
CorroSI vc
if the Il lie gases arc cuolcd
10 li
tcmperature
below their dew pomt. Its toxlcity has been 'itlldled ... crioll~ly and Il \Va ... round that the
maximum permlssible concentratIOn fi)r exposure not cxceedmg elghl hOlll 'i a day i'i
20 ppm. according to The .lmel'1Clll1 ,\"lIIulard
.1.\.\(JcuIlI0I1
1he IIllplcaSllnl Odor 01
hydrogen sultide can be detected al concentration., a . . Iowa., live pal h pel hllllOIl
For those rcasons, there has hcen a great cOl1ccrn over thc CIllI., ... IOI1 01 hydrogen
sulfide. Very shortly after coul
ga~
was rntroulIccd a'i
il
that its sulfur content was a declded dbadvalltage. In
dome'itlc t'ueL Il hecame apparent
1~l1glalld, III
1X()(), mrlk Illlle wa . .
utilized for the removal ofslIlfur compounds lrom fuel ga'i Iwo year:-. alter the lir:-.t use 01
gas for street lighting, a tcw years Iater scrubbers u . . ing .,olld lime \Vere elllployed.
Nowadays, many mdustnal proce:-.se'i, IIldudmg pctrolcu/ll Ic/inerre:-.. coal ga:-.
manutàcture and kraft pulping. generatc hydrogen :-.ullidc
operations.
•
III
large q\lantitics
The gas is usually unwantcd. J\ ... a component 01 prOl:e ......
ga~
III
thcir
. . trcam'i,
hydrogen sultidc intcrfcres with important chcmical rcaetionc.;, while a., a compo/lcnt 01
wastc gases. It usually imparts an
ohnoxi{)u~
odor to the nearhy atmo.,phcre. Ilowcvcr,
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Iho,>c indu~lne~ genemte carhon dioxide togcthcr \Vith hydrogen sulfidc
quuntltlc .... lor Ihis rcu ... on.
eXI~tll1g procc,>~es
rcmoval. hecau ... c Ihe ,>elcct!vlty f()r hydrogcn
cllough
111
much larger
arc inadequatc for hydrogen sulfide
~ultide
ovcr carbon dim.ide is not high
achlevc a ... ati~lilctory hydrogen sul/ide removal efficiency without the
10
cO/lsumptÎon of Ul1eCOIlOmlC quantllle,> of chemlcal through carbon dloxlde remm al.
1.1 IMPORTANCE OF THIS RESEARCH
In the pulp and parer II1dustry, \\ood libers arc scparated from cach other by
mcchanical or chcmical mean~ or a combinatlon of the two.
In the conventional kraft
pulping. the maJor chel11lcal pulpll1g proccss. \"ond libers arc liberated hy cooking the
wood chlP'i
whlle Ilqllor. a !--l)llltiol1 of
III
~otiIum ~ullidc
and
~odium
hydroxide. at
devated temperatures and pressures As a rcsult. the so-called. \\"eak h/ack !tC/IlO,. is
Il contains about 50 % of the dissolvcd wood and mo~t of the pulping
produccd.
chemicals. In thc collventIOllal k.ralt chemical recovery process. this liquor is evaporated
•
tn a sol id content of ahout 70
0/r)
to form a combustible liquor. which is then is burned in
the recovery furnacc. wherc several "unit operations" are performed
compartl11ent
In
a single
1n the recovery fumace. a rcducmg atmosphere is maintmned in the lower
/one \\lhcre carbol1
IS
gasIficd and the inorganic chel111cals are convcrtcd to a liquid smelt
of pnll1anly "'OdllIll1 ~ultidc and sodium carbonate \\ hich is then rcmoved and dlssoived
111
water
prnducc green liquor. (irccn 1iquor i" processcd in a caustlclzing stcp to
10
producc licsh white IIquor Furthermore. the substantIaI heat of combustion IS rccovered
III
Ihe uppcr part
f)c~rlte
01
the l'limace and high pressure stream is gem:rated.
the facl that the objectives of the present chemical recovery proeess are
surticlcntly achic\'ed. therc are several drawback.s associated with the conventional kraft
chcl11lcal reco\'cry proccss. such as its hlgh li:-..ed capital cos1. the hazard of smclt-water
CXplO~IOI1. wrrosi\ enes~ of Ihe ..,mclt and the encrgy consumption charaeter of the
caustici/ing
~tcp.
For these
leaSOIl~.
therc
\\:1S
a need for an alternatl\,c chemical recovery proeess.
An altcrnatiw process for black. IIquor reeo\'cry based on low temperature proccssing in
•
tluidi/cd bed'i \\as proposcd tn the InternatiOnal Encrgy i\gency (lEA) [II. The proposed
prm:css
j"
the synergl"tlc integration of se\'eral black. Iiquor trcatments: superheated
... tcam dr~ 1I1g. tluid hed pyrnl~ SIS. leduction. gasitic:.lllOn and combustion. and gas liquid
•
absorption. A now diagrmn of this proccss is sho\\ n 111 Figllle 1.1
1hl'
i'tllCC~,S IS
IlHlll'
energy efficient and has a rclativdy small eapllal el)st compared tn the l'om l'ntInllal
recovery proccss and Jt docs Ilot sulTer from the smdt-\\ater e'plo<;\()11 h,I/:1I Li
this process is expectcd to 1clease hydrogCll sullide
111
lm ger quantlt ICS
recovered in the form green/ù/zlOr by thc absorptlnn in cal bOllatc
\\
\10\\ CH'I.
hlch
I~
tl) hl'
~llllltlon".
1.2 BASIS FOR RESEARCH
There have bccn several
stlldie~
on the absorptlOll of h) dw/:!,cn sullidc
presence of carbon dioxide I\1to sodium carbonate
~olutions
111
the
,lI1d -;lldllll11 cal honate-
bicarbonate bulTer solution reported in the literatureI5-141. Ilowcvcl. thcle
ha~
heen no
systematic study to absorb hydrogen sullide selectively l'rom gas stre<lI1lS that contam
carbon dioxide and to. simuItaneously, recovcr hydrogcn sullide in the lorlll of green
•
liquor .
The presence
or
carbon dioxide in the gas -;tream
conversion of sodium carbonate mto "lldium hicarbonate.
minimize the absorption of carbon dioxidc which,
selective absorption of hydrogen
green Iiquor
15
~ullide.
III
kad~ ft)
tlle 1IlH.ksircd
l'herc!'(lIc It was dc","ed to
rcturn, rc ... ult ... in optlllllling the
hlrthermorc, the Slll1ultancoll-; ploductlon 01
an important operalion in the alternallve procc,>" I(lr the n:covcry 01
pulping liquor. Thus, the basls of tlm
hydrogen sulfide in carbonate solutions
re~earch
111
wa,> to IllvcstIgak tlte ahsorptlon or
a packed colllllln reactor
a~
11I1lction of the
operating parameters .
•
1
•
Heat Exchanger
Flue Gas
CHAR
Fluid Bed .......--,
Combustor
700-750°C
!-Ieat Exchanger
•
...
QJ
.D
Fluid Bed
Gasifier
650-700 Oc
Air
Stearn
Figllre /./
~
2
u
en
Green Liquor
to causticizing
Black Liquor
Alternative low tcmpcrature "'raft fluid-bed chemical recovery process
•
4
•
1.3 OUTLINE OF THESIS
This thesis
IS
concerned with thc l'undamcntal
111\ cstigation
simultaneous absorption of hydrogen sultide and carbon dio\.H.lc
sodium carbonate.
111
nt the seb:tl\'l' and
aqll\:olls solutions ot
In Chapter Two. the literaturc of hydrngcn sullidc and carhon dill\.it\e ahsOlptlOJ1
as weIl as the chemistry of absorptIOn arc rc\'ie\\'t'tl.
In Chapter Threc. the cxpcrimcntal
~et-lip
t.. descnhed. and the absorption colulllt1
is charactenzed by investigatmg the resldcnce time distribution and the
ab~OIptlon
of
carbon dioxidc in water. The analytical procedures for the gas and liquld phases arc abo
described.
•
In Chapter Four. results of the prclimmary expertmcntal \\'01 ~ alc presentcd 111
terms of interactions between the paramcters and their relattve eltect on the absorption of
both gases. Aiso. material balances for the
~pccics
IOvolved arc
estahlt~hed
the repeatability of experiments and the ctTccl of the hed helghl on the
ln addition.
ah~orpllOlI
of holh
gases are discussed.
ln Chapter Five. the final results of this \York are presented and
dlsc.:lI~sed.
'1 hls
includes the int1uence of cach paramcter on the removal erticiencie ... the overall mass
transfer coefficients of both gases. and the proccss sclcctlvity fiJr hydrogcn sul tide.
Chapter Six comprises the gencral conclUSIOns and rccommcndattons li)r future
work .
•
5
•
CHAPTERTWO
LITERA TURE REVIEW
The prohlem of the ab3orption of hydrogen sulfide and carbon dioxide was often
encollntereu in natllral gas processing plants and in synthetic ammonia plants.
•
In
synthetic ammoma industry, wh en the synthesls gas is made by the partial oxidation of
petrolcul11 Ihu.:tlOl1S contammg signiticant quantitles of sulfur, the gas stream may contain
hydlOgen sultidc, carhon)'1 sultide and carbon dioxide.
Several processes have hecn proposcd and stlldicd for the removal of hydrogen
sullide and carhon dlOxide l'rom gasification gases.
One of the most important gas
purilication techmques, is absorptIon. It involves the transfer of hydrogen sultide from
the gaseous to the liqUlu phase through the phase boundary. Hydrogen sulfide absorption
may he physical \"hen mercI)' dissolvcd in the absorbing solvent such as water, or it may
be chcmical \\hen hydrogen sultide reacts with the absorbing solution sueh as
al"'anola11lll1es and al"'ali solutions.
l'he ahsorhent solutions generally cmployed for treating a gaseous containing
hydrogcn sultiuc and carhon dioxide arc:
1.
Sodium hydroxide solutions.
2. Solutions of 1110no- or di-ethanolamines or solutions of di-isopropanolamine
•
3. Aqucous ammonia solutions.
4. Potassium carhonate or sodium carbonate solutions.
6
•
5. Potassium phosphate solutions
6. Tri-ethanolamine solutions.
The last threc solutions have a relatÏ\ e1y high selectivity \\ IIh
1e~peet
III hydrogcn
sulfide.
2.1 ABSORPTION IN ALKANOLAMINE SOLlJTIONS
Aqueous alkanolamincs solutions havc hecn utili/cd
III
ll1dU~lIy
Ihe
to ab",ol h
hydrogen sulfide and carbon dioxide from gas ... treums elther ... illlllllaneously
selectively cspecially
1I1
petroleum proccssmg plants and
111
Ihe
'i)
nthetlc
01
:t1ll11l01lla
industry. The alkanolamines have at Icast one hydroxyl group that scrvcs to rcduce Ihclr
vapor pressure and increase their solubility in water.
•
rhey also have one aml11e group
which provides the necessary alkalinity for the ahsorptlOn of aCldic
hydrogen sulfide and carbon dioxldc
ga~es
... lIeh as
PI.
Hydrogen sultide reacts with alkanolamincs instantancously in accordancc to the
following reaction:
---(2.1 )
There are
several
types of alkanolamines.
methyldiethanolamme (MDEA). which can absorb
the
most
Important
hcing
hydrogen :-.ullidc rea~onably
selectively under proper operating conditions involving short contact
tlllle~.
A study by 01lwerkerk 13] in which 'ideetlve ab~or;1tJ()n wlth c.,cvcral
•
ethanolamines was investigated on a pilot ~cale and a commercial plant dClllol1stratcd lhal
purified gas containing as )ittle as 5 ppm of hydrogen ~ulfidc could he obtained at a
7
•
rcmoval cfticicncy of 91 % while only about 30 % of carbon dioxide that existed in the
Iccd gas was removcd.
A study hy
Verstee~
et al
r-II
was carried out ta investigate the selective
absorptIon of hydrogcn sulfidc in the presence of carbon dioxide in several aqueous
solutions of albnnlamines.
'r heir
study was donc using a co-cmrent down flow fixed
bcd reactor operated in the pulse now rcgime. They lound that this type of reactor is
more suitable for the selective absorption of hydrogen sultide l'rom aCld gases.
Ilowevcr, thcsc proccsses are not suitable when hydrogen sulfide
IS
to be
rccovcrcd as sodium sul/ide.
•
2.2 ABSORPTION IN AQUEOUS ALKALI SOLUTIONS
Sodium and potassium carbonate solutions have been utilized to absorb hydrogen
sulfidc and carbon dioxide because of their low cost and their ready availability.
Most nI' the proccsses fOlmd in the literature describe the phenomena of the
simultancous rcmoval of hydrogen sulfidc and carbon dioxide from gasification gases.
Howcvcr, thcrc arc only a tew processes in which hydrogen sultide is selectively
absorbcd in the presence of carbon dioxide.
One of t'le 1110st important processes, that have bcen used widely in the industry
I()r the rcmoval of hydrogen sultide, is The Seaboard Process [2]. ft is based on the
Icactlon hctween hydrogen sultidc and the carbonate ions. The etliciency of hydrogen
sulti~ic
•
rcmoval in a s111glc stage packed column i'\ between 85 and 95 % for a solution
!low rate het\\ccn 60 to 150 gallIOOO
hydro~:cn
L'li.
/1 of gases depending on the concentration of
sultidc and carhon dioxldc in the l'eed gas.
8
~----
•
------------------------.
2.2.1 SIMULTANEOUS ABSORPTION OF H2S AND ('° 2
As/araa and GlOra [5.15]. studied the Chcllllstr) nI' thc Sll11ultallcnu" ahsOIptlllll nI'
hydrogen sul1ide in aqueous hydroxidc and C,lI honate-blcarblln·lh.' bul tl-r "nlut 1l1l1S ln thc
tirst paper, they derived an equatlOn for t:\C absorptIOn ratc ni h) d\(l!!.cn sullidc and
carbon dioxide. In addItion. they abo mcaslII'cd thc ahsorptlon 1ate 01
water and sodium hydroxide solutions \Vith a wctted-\\all collll11n
stainless steel tube which was I.u cm
111
diumctcr and ::!O
('11/
III
h~
drogen
ah~or her
~lIllide 111
Illalk (lI' a
helght at 1X "('
1hcy
determined the rates of absorptIon \OlllIllctfll:all) hy Il1cans or" ,,('ap lillll tL'l'hlllqllc
th()~c
experimental results showed good agrccmcnt \\ 1th
theory. They also studied the absorptIon of hydrogcn
potassium carbonatc-bicarbonate butTer solution
•
111
1hc
ca\culatcd USIIll,!. thL' pendl at Ion
~ultidc
the saille
and laI bon dioxlde
ab~orhcl.
III
1he)' also 100ll1d
that the absorption rate of carbon dloxide decreases !l' the concentratIOn ni the carbon.lte
bn is increased, while the absorptIOn rate of hydrogen sullide
IIlcrca~c'i
\VIth the
carbonate ion concentration.
In the second paper, they have glven a thcOlctical solution hascd on a lilm mode!
assuming that ail the ditfusivities of thc rcacting spccles arc l'quaI. and that two rl'actlon
planes exist in the liquid phase. the tirst one \Vilae hydrogen <,ulfidc l'i cOlllplctcly
consumed and the second plane whcre carbon dloxldc
studied the slmultaneous absorpt\()n of hydmgen
~lIl1ide
1'1
partially IcacteJ. Abo they
and carbon dioxlde
hydroxide solutions experimentally using the ..,ume wetted-wall COlllll111.
was 50% H;!S and 50% CO;!.
III
aqucolls
1he gas lIsed
rhc Ilow rate of ga<; was hlgh 'iO thal the exit ga..,
composition was practically the same as the IIllet
compo~ltion.
1hey Illlllld that the rate
of absorption of hydrogen sulfide incrcases hy lI1crea~ing the concentratloll (lI hydroxlde
ions and that the ratio of the chemlcal ah<;orptlon ,.lI1d
rhy~lcal
ah..,orptlOll rate was
independent of the liquid tlow : .\tc. Ilowl'vcr. there wa~ no comparl'>ol) made
111
their
work, between thc experimcntal rcsults and their theoretical ..,olutlO!I'i l'urthermore, thelr
•
experimental date were not discusscd quantltativc\y
1)
•
A simdar study was pcrformcd by Onda
of hydrogcn 'iultidc ab~orption
In
el
al. r7]. They investlgated the kinetics
a 'iodium carbonate-bIcarbonate butTer solutions. and
~ullide
'ihowed that the rcactlOn hctwccn hydrogen
and carbonate Ion IS instantaneous.
Also, they studied the '>Imultancou~ absorptIon of h~ drogc'l sulfidc and carbon dloxlde at
25 0(. In an aqueou, ~od\Um hydroxlde ,>olutlon u~lI1g a \\ettcd-\\all column which agam
was 1 cm
In
dmmelcr and 20 cm
hydrogen ,>ulfidc
ah~orptlon
11lcrea,>c~
wlth
In
hcight.
increa~mg
1hey round that the late of absorptIOn of
Ils partial pressure while it causes the rate of
of carbon dloxlde to occrcasc. On the other hand. the y found that the rate of
ahsorptlon of carhon dloxloe IrIcreases slightly by II1crcasmg Its partIal pressure. In other
words. lhclr result'i 'iho\Vcd that
~c1ectlvc
absorptIon of hydrogen
~olutions
~ulfide
of sodium hydroxlde could be employed for
m the presence of carbon dioxidc The reliability
of the expcnmcntal apparatu~ \Vas charactcriled hy
carhon dlOXldc III water.
•
~tudymg
the physical absorption of
2.2.2 SELECTIVE ABSORPTION OF H 2 S
Oloman
al. [8]. studicd the
L'I
~cJecti\ c
presence of carnon dIO:-"lde to reducc the odor
absorption of hydrogcn sulfidc in the
a~socIated
stnek gases. in \\'hich the concentratIon of hydrogen
\Vith kraft pulp mill recovery
~ul1idc
is about 0.1 %. A buffer
'iolution of ~odlllln carbonate-hicarhonate havmg a pli or 9.5 and WlllCh is 2 molell in
sodIUm \Vas
u~ed
SIIlCC
thls solution \Vas
III
cquilibnum \Vith the carbon dioxide
tced gas al an optImum temperaturc. the ahsorptlon
Ieadmg to sclectl\'c absorptIon of hydrogen
~ulfidc
or carbon dloxldc
III
the
\\as minimized.
The study ",as carried out using a
laboratory :-.calc packcd column opcrated countcr-currently uSlIlg a gas mixture of 0 1%
11 2S and 15% ('0 2
\Vas trcatcd
lhiosulf;ltc
111
111
nitrogcn. 1he
~lIltidc
an o'\ldatlon lcactor in order
(S20j
) '\0
loadcd liquor leaving the absorption column
10
that the butTer solutIon could be recycled. ft was found that an
dlicient absorptIOn :-.ystell1 for a rate of 500
counler
•
currcntl~
convcrt the absorbed hydrogcn sulfide to
IOl1s/day
kraft pulp rnill \\,;ould reqUlre a
operatcd packed column rcactor of 20 Il diameter and 10ft height of
packing 10 .1cIllCVC a hydrogen sultide rcmo\'al cfticiency of 95 % with sodium carbonate-
10
•
bicarbonate solution reclrculated at a r.lte nI' JO()l) gal da\' opel ated .It :1 templ.'latllll' nI'
160 oF.
(Jarner et al. [91. studied the ~eb:ti\'C absorptInn III h~ dmgl'n
carbonate solutions in the presence nf carbon din\.ide \\ Ith
li \\ dll'lI
"li 1li dl.'
diS,," and
li \\
\\ IIh
elled-
wall column. The feed gas stream conlained carbon dio:-"Ide and h~ dmgell "ultide ,It ,1
ratio of 30: 1. They II1vestigated the dTeet of temperatllle. concentlatHln or the ~nl\ltlnn.
gas and liquid tlow ratcs and hydrogen sullide IIlttlal eonceJ1t1 atlon
the g,as pha<;e
111
lln
the rates of absorptIon of hydrogcn slIlfiùc and carbon dio'lde alld (ln tlll.' ')l.'b:tt\'ity
factor. They claimed that about 85°";) or 11 2S l11itlal .11110unt l'an be ahsorbed accol11pal\\I:d
\\ith about 10-15 % of thc IIlltial amount of CO,.
Aiso. they 100llld that the 1ate 01
hydrogen sulfide absorption can be increased hy II1creasing the "olutHln CIlIlCl.'ntmtloll
while thr rate of carbon dlOxide absorption
\Va~
found to <.teneuse 1he dkcl (lI' gas Ilow
rate was more signiticant on 11 2S absorption. because it is gas
~Ide
èOnttolkd whllc Ihe
flow rate of liquid was !(mnd to IX' mOle sigmticanl in Ihe ca<;c of
•
C()2
ahsorptlon.
because it is liqUld sldc controlled ln tcrms or sc1ectlvity. they lillllld thallhe "idCclivlly
for hydrogen sulfidc absorptIOn decreascs \\ Ith lIlcrca-ang thc IlqUlt! tlow 1ate and
temperature. and it lI1creases \VIth 1I1creaslIlg the ga:-, Ilmv 1ale and the coneentl alion 01
the carbonate solution.
They also
~howcd
that the <;elecllvity
I~
II1depclldent 01 the
hydrogen suifide content in gas ahove 1 3 (%. hut It declca"e" whcn the concentration 01
hydrogen sulfidc is decreased below this value
Bent/al!
el
al [1 (JI studled the selcctlve absorption or hydlOgell ,>ullidc 1rom lai ger
quantities of carbon dicxide by ahsorption and reaction in lin\.! spray'" U<;lI1g hullcred
solutions of carbonate-bicarbonate. rhey I()tllld Ihat the ahsorptron 01 hydrogcn sullidc
1...
virtually independent of the presence of l'arhlln dloxldc. On the other hand. they provcd
that hydrogen su\fidc reacts at the gas-liqllld Il1tcrluœ whilc carhon dloXldc
the bulk of liquid.
rcadcd in
i\lso. thcy c1almed tliat thelr procc...... wa ... elliclcnt enough
selectively reduce the concentration 01 hydrogcn ... ullide in the nch gas 1rom
\ow as 0.0004 % (4 l'pm) l'ven
If
is about 10 %. Their rcsults \Vere
•
1...
the concentration 01 carhon dloxlde
dlscll~sed
in terms of the
"'I/C
III
the
(J.!) 1
ga~
IYt, lo
II)
(1:-'
mixture
01 the dropleh and the
distance from the spray mg nozzle .
Il
•
ffohlfeld [11[. studicd thc selective absorption of hydrogen sulflde from gas
,>trcam~
contallling hydrogen sultide and carbon dioxide into a minimum amount of
,>odiulll hytlroxiuc <;olution. In ortler to reduce the total co st of the process they used a
hOrll'ontal ,>tatlc
III 1xer
ga~/lIqUltl ~cparat()r.
contactor operated co-currently. and It emptIed duectly into a
Part of the liquid was recycled to maintain an optimum 0a!1/Liquid
rallo \VIth thc LlIIll to utilllc. more completely. the unreacted sodium hydrmode. and to
gcnerate morc concentratetl eftluent streams. l'he expenmcnts \\ere carried out at room
temperatllre. Reslilts \\ere analY7ed t y means of the contact. detined as the ratio of void
volume
III
the contactor to the total gas llow rate. He found that if the contact lime is
decreaseu the consumption of sodium hydroxlde is deereased.
Also the absorption of
carnon dioxluc uccreases ~lgJ1i ficantly \Vith decreasmg the contact time. beeause it reacts
\Vith hydroxldc Ions
111
a vcry
~Iow
reactlon compared to the instantancous reaction of
hydrogen ~ullide wlth hydroxlde. rhis results in the preferential absorptIon of hydrogen
~ullide
ovcr carnon dlOxlue absorption. The pressure drop in the contactor became severe
if the contact time \\as reuuced to 0.01 .,el'. He also tound that the ahsorption rate of
•
hydrogen sultiue lllcrcascs if its concentration at the inlet is increased. He showed that a
removal efliclelKy of hyurogen ~ultide of the order of 90 % was easy to be achieved by
l11eans of this t) pe of gas-liquid contactor.
NlIl1IlIclllll1drllll and .\ïlllrma [1':1 studied the absorption phenomena of hydrogen
... ullidc and cal hon dlo\lde in carbonate ~olutions.
and li"c
I~cl
long \\ctlcd \\all column.
rhey employed a one-lOch diameter
rhc mam lindings of thcir cxperiments was that
the rate of carhon dioxide absorption was not affected slgniticantly hy the gas tlow rate
",hile IIqllld Ihm rate had a larger erfeet. because the transtèr of carbon dioxide was
\:ontrolh:d hy the hqUld slde mass transter reslstance. On the other hand. they found that
the gas Ilo\\' rate had a marked effect on the rate of absorptton of hydrogen sulfide since it
\\a!'. largdy controlled hy the gas side mass transfer reslstance.
They also found that
1lH:lea!'>ll1g the concentratIOn of carbonate n:sulted in the 1l1crcase in the ahsorption rate of
h~ dlOgen !'>llitide and it decreased absorption rate of carbon dioxide.
The ciTeet of
IllcrcaslOg tempcrature \\as found to redllcc the absorption rate of hydrogen sulfide and to
incrcasc that of carhon dioxlde.
•
12
•
A similar study \Vas performcd hy
(]IOW
1/31
to imestigate th~ ~c\ectiH~
absorption of hydrogen sultide (from gas strcams that contalllcd h~ drngen sultide and
carbon dioxide at a ratio nI' 5: 10) in potassium carhonate-hicarbllnatc hulkr solutinns.
The solution \\-as fOlmd to have a Il1gh sclecti\'it~ \\ tth respect to h~ dlOgen ~llllïde.
selectivity \Vas round to increase If either the concentratIon
or carhonate
l'he
or the partial
pressure ofhydrogen sultide \Vas increased.
Cir/io and Melchor
rJ-II
investigated the selective ahsorption
or hydrogen sultide
and carbon dioxide in potassium carbonate solutIOns in a wcttcd \\'all clllul11n at 70 "('
under atmospheric pressure.
The found that hydrogcn
~llltidc
coult! he ah-;orbcd
selectively at high gas t10w rates and ln\\' liquiJ no\\' lates. i\lsll. thcy IIm: . . . tlgaled the
effect of promoting the potassIUm solutions hy addlllg ammc. and Ihey Clllll'ludL'd Ihat Ihe
absorption of hydrogen suItidc \Vas Ilnproved slgl1lticantly \\hich rcsllited in 1I1lproVIIlg
the seleetivity of the proeess .
•
2.3 CHEMISTRY OF THE PROCESS
Hydrogen sulfidc and carbon dioxidc arc wcak aClds: llll:rc/ore. Ihey have Ihe
tendency to reaet \Vith basic solutions ~uch as aqucolls carhonate ,,()llIlJ()n~ whlch the
conjugate base of earbonie acid.
When hydrogen sulfidc and carbon dioxidc arc allowcd ln contact with aquC()ll~
carbonate solutions. the following cquilibrium reactlons take place:
•
co.,
+ OH •
.. He0 3
H 2S
+ OH •
.. 112°
H.,S
+ C0 3
..
4
(2.2)
+ liS
HC0 3 + liS
-
(2.3)
(2.4 )
Il
•
IICO; + 011 • ~ C0 3
liS
C0 3
+ OH
j-
1fS
.
.
~
~
+ 1-1,0
(2.5)
112°
+ S
(Z.6)
HeO;
+
(2.7)
S
Experimental evidences lound in the literature [5.6.15]. suggests that reactions 2.6
and 2.7 can bc ncglectcd. bccausc at cquilibrium. those rcactions are shifted to the left. In
othcr words. It may hc assumcd Ihat no sulfide ions (S-2) are l'ormed.
/\Iso. as far as the kllletics
IS
concerned. reactions 2.3. 2.4 and 2.5 may be
considered as lI1~tantaneOllS rcactlons. regardless of the concentration of the reactant.
when they are compared to the rate of the diffusion process [6,15]. Thus. the reaction of
hydrogen slIlfide with carbonate ions and its reaction with the hydroxide ions take place
•
at or near the gas liqllid interface. This makes the concentration of hydrogen sulfide in
the liquid hulk l'cm. In other words. the absorption of hydrogen sulfide in the carbonate
solution may he said to be physically controlled.
On the other hand. the reaction of
carbon dioxidc and hydroxide ions (reaction 2.2)
IS
fast and it might be considered
IIlstantaneous only If the concentratIOn of ol-f is substantial ( i.e. [OH-l > 10- 2 mole//). At
slIch a Icvcl of 01 r concentratIOn. the equilibrium of reaction 2.5 is completely displaced
to the nght. sn that reaction 2.2 is followed by reaction 2.5.
Th, refore. at low
concentrations of 01 r. that may be encountercd when CO; and HCOj ions coexist.
feaction 2.2 may be considered as a slow Iirst order reaction with respect to carbon
dioxide and Il has a ratc constant of 0.1 .\ec-Jat 25"C [I.J 1. Thereafter. the absorption of
carhon dioxide
111
carhonate solutions is said to be !iquid side controlled as it reaets in the
liquid huI\.... thus. the absorption of carbon dioxide is chemically controlled [6,12]. Figure
2.\ illustrates the concentration protiles of the gases being absorbed and the various ionie
specics i n\'ol\'ed in the reactions .
•
14
•
UAS FIL\(
-~
.
•
,,
,,
,,
U(}I In "'1.11
,,
()
Figure 2./
{l,
Concentration profile of reactcd ga,>c'i and IOnlC "pCCIC~
•
15
•
2.4 EQUILIBRIUM DATA
2.4.1 VAPOR LIQUID EQUILIBRIA
As <.,œn. ,>everal proccs<; for the absorption of hydrogen sultide and carbon
dloxltle
III
very lew
alkall ,>olution have hcen stutlicd in the literaturc. However.
111
the literature.
phy~lcal-chemical
design data \Vere round concermng systems of H2S-C0 2NaIICOrNa2COl-NaIIS-Na:zS-II;!0.
An experirnental study \Vas pertormed by J'v/m et al. [/6] in order to develop a
mathematlcal expressIOn lor the cquilibrium constants which govern the above mentioned
system.
I"hey dcvelopcd a method for the contmuos analysis of the vapor and liquid
phases.
rhey il1vc~tlgated the clJuilibrium conditions at temperaturcs hetwecn 20 and
whlle the pressure \Vas betwecn 50 to 3500 mmHg. The ICSUItS of their
(l5 <1(.,
cxpenmental work were litted to a mathematical correlation for equilibrium partial
pressures of Il;!S and CO;! as a function of the concentration of the differcnt species in the
•
liquid phase and the opcrating temperature .
l'he se cOiTclations arc given in Equation 2.8 and Equation 2.9. and they were used
in this study to evaluatc the equilibrium partial pressure of H2 S and CO 2 above the
absorhing solution.
(2.8)
(2.9)
whcn::
l'.: the equilihriul11 partial pressure abovc the solution, atm.
MN,,:
t:
•
the concentration of sodium ions in the solution, mo/ell.
the fraction of sodium ions as sodium bicarbonate in the sodium carbonatehicarhonatc solution.
r:
the operating tcmperature, K .
16
•
2.4.2 CARBONATE-BICARBONATE EQUILIBRIlJM
Data for the cquilibnum bct\\ccn carhonate and bicarbonalc ions arc IcqUll'cd in
ordcr to estimate the concentration ofthesc t\\O specics in the solution.
Carbonate is the conJugate base of cathOl1lc aCld
Il dlssoclalcs ln \\,IICI
ln
produce bicarbonate which is furthcr dissocwlcs \VIth \,\atcr ln plOduc': carbnnH: acid in
accordance to reaction 1.10 and Icaction 1.11 l'cspccti\'cly.
l'hcoretlcal cvidcnee
suggests that. rcaction 2.11 has a very small equilihrium constant. 1 e
Therefore, it could be ncglectcd. llowcvcl. rcaction 2.10. \\ hil:h is the
2.5. is important determining the cquilihnum concentratIOn nI
III
Ihc ortler of 10
1C\
\1('()1
1/-1.
~.
cr'\e of 1cactu 1I1
Valllcs of the
equilibrium constant of this rcaction at diflàcnt tempcratures aIl' Itstcd ln l'ahle 2.1 as
given by Lalifner el al. [f ï].
•
C0 3
+ H20
Il
HCO; + H.,O·
BCO; + 011
(2.10)
• 11 2C0 1 + OH
(2 Il)
•
,
Table:2.1 DiSSOCiatIOn constant 01 carbonate
III wa'cr
-
Temperature, "e
KI fI
'fcmpcnlturc, "( .
K "1
5.0
4.463x Hl"
35.0
"\ 4))110
10.0
6.455x 10'
40 ()
4721/10\
15.0
9 172-< 1(j"'
450
() 4(1)". 10 '
20.0
1 2(2)( \(f'
50.0
X(,') 1,.. 10 '
15.0
1. 797x Hf'
55.0
1 14(,/ 10 \
30.0
1.506x 10-'
60 ()
l ')01/10 \
1
•
17
•
2.5 CONCLUSIONS
As far as the sclectivity of the absorption process is concerned. most of the
previous studies
aqueous alkali
1.
a~rcc
on the fàct that hydrogen sultidc could be absorbed selectively in
~olutlons
under the followmg conditions:
lligh gas and liquid Ilow rates because
the absorption of hydrogen sulfide is
considered to he gas side controlled and the absorption of carbon dioxidc is liquid
side controlled /5.9.//.12./-1).
2. Iligh concentratlon of carbonate in the liquid phase. because the absorption rate of
hydrogen sulfide is the highest in pure carbonate solutions [5.6.9.12.13].
•
3. Low opcratll1g temperatures. hecause the absorption rate of carbon dioxide
incrcases with temperature whereas the absorption rate of hydrogen sulfide is not
aflccted apprcciably
19.1.n
4. Ilighcr operating pressures are rccommended. because the absorption rate of
hydrogcn
~1I1fidc
incrcascs significantly with ils increased partial pressure while
the absorptIon rate of carbon dioxide incrcases slightly if its partial pressure is
incrcascd 1~)
On the othcr hand. the effect of sodium sulfide (Na2S) is not wel1 documented.
For examplc. Olmall el al. 18] c1aimed that sodium sulfide formation hinders the
absorption of hydrogcn sulfide in carbonate-bicarbonate butTer solutions.
This
contradicts the bdief that the reactions which produce sultide ions in the alkali solutions
arc ncgligible .
•
18
•
CHAPTER THREE
EXPERIMENTAL SET-UP
A packed column has bcen selecteu as lhe equipment
•
10
absorb hydrogell slillide
in an aqueous sodium carbonate solution because of the fiJllowing reasons.
J. The specifie surface area of the packing material compared
10
\Vetted wall colllmns
is very high thcreby reducing the required Sl/.C of the absorptioll wlllllln sillCl! the
over ail mass transfer coefficient heeomes 11Igher
2. In terms of the complexily 01 cOl1slmclum paeked beu reactors ,Ile \'l:I)' '>lIl1ple since
there are no complicated internais IIke
III
tlay columns
1his means Ihat
maintenance ofpacked columns IS easy.
3. In packed columns, the /tlfwd Iwld-up
IS ~lIb~talltially ~l11allcr
(han that ln other
types of contactors. This is an advantage sinec it reuuces the ah<;orptlol1
or ('C)2
which occurs in the bulk of the liquiu.
4. Packed columns can be operated in a "Indic hel/" \VIth ks'i huhhlrng (JI gm Ihrough
the liquid. thereby further rcdueing the Irc/uul lili/li-III' a", weil a", the chance 01
foaming.
5. Packed beds are casier to seule up than wettcd wall columns or lray colllmns.
6. In packed columns high valucs of liquuJ/gos IloH! raie.' l'olim are ca'iily achicvable.
•
1C)
•
3.1 DESCRIPTION OF EXPERIMENTAL SET-UP
A schemallc diagram of the expenmcntal set-up is shown in Figure 3.1. An
aqueous carhonate
~olution
IS prepared hatch-wisc hy dissolving a suitable amount of
sodium carbonate mono-hydrate (Na2C01.lIi)) in 501 of de-ionized \Vater to obtain the
desired concentratIOn
ln
111
the carhonate solution tced tank. An electric heater is installed
the tank ln pre-heat the solution to the desircd operating temperature. The solution is
pumped to the Ilquid thstributor at the top of the absorption column.
The Iiquid
distrihuting system is deslgncd in such a way that the liquid is uniformly distributed over
the packing matenal and liqUid droplcts are prevented From being carried out hy the gas.
1he ~()llItlOn kaves the column at the hottom through a siphon type tube. The tube
mamtams a certain hcad of liquid to prevent the gas l'rom leavmg at the bottom. A
'iample of this IIqUid is collccted for later analysis. while the rest of the solution is treated
with an ex cess 10% CuS0.t solutIOn to precipitate the absorbed H2S as CuS .
•
The ahsorption gas mixture is generated by mlxing two streams of pure hydrogen
sullide and pure carbon dioxide \VIth a nitrogen stream. Before mixing the nitrogen with
the two gascs. I1Itrogen
IS
bubbled through the saturator where it gets saturated with water
vapor. and al"o. prchcatcd to thc deSlrcd opcrating tcmperature. The Ilow rates of the
tlnce componcnts are ad,usted to obtain the rcquircd compOSition and total gas flow rate.
1he gas mixturc cnters at the bottom of the absorption column where Il is uniformly
distriouted o\'cr the column cross section by passing through agas distributor. The gas
Icaving at the top of the column is further trcated
ln
a caustic scrubber where the
unreaetcd Il,,S is remm'ed completely l'rom the gas before heing released to the
atl1111sphere.
Th~~
absorption column \Vas designed to absorb hydrogen sulfide into carbonate
solutions at an dticiency of 95% l'rom a gas mixture of 1.5% H2S and 15% CO 2 in
nitrogen (see
•
appcndi~
:\ lor details). The three sections of the absorption column are
made of t\Hl concentric P/exlglll.\ tubes.
The outer tube has an
0
cl. of 15 cm and a
thid.ncss nI' 6 mm "hile the inner tube is 7 5 ('/11 in 1 d. and has a thickness of 3 mm. The
clearance oet,,"een the tubes is uscd as a heatmg jacket in which hot water tlows upward
10
•
to maintain an isothermal operation hy suhstitlltmg thc heat cnnsumcd by the
endothermic reactions.
The column sections arc cqual
separated from each other
b~
III
height (50
('11/).
rhc~
arc
a thick plate contammg a thcrmocoupk and sampling tube
as weil as a stainless steel scrccn ,\- hich supports the 6 mm llIfa/O\ .....·(/Cid/es pad,ing
material. and also it redistributes the liquid across the \m\'cr colul11l1 tll .I\'nid channding
in the liquid phase. This design \Vas adopted since it is rccommended to ledlstnhutc the
liquid across the height of packed co\umns evely 6-7 timcs Hs dWl11ctcl 1/8. jl)1.
For safety reasons, a hydrogcn sullide almm and control de\ l'>e \\as installed. The
device measures the concentration of 11 2S outslde the absorption clliumn hut IIlslde the
fume hood. The alarm gocs off when the 11 2S cllnœntration rcatls 1() {1{1/I1 or 11Ighcr ami
automatically shuts off an electrical valve instalh.:d at the outlet ni the hydrogl'n "lIllide
cylinder. Since the expenmental set-up cou Id not lit
\Il
a stalldatd
11IIllC
hom\. it was
surrounded by a heavy nylon plastic supported by a metallic structure. 1he who le set-up
was connected to an induced draft fan placcd on the roof of the hllliding \'la li 110"e that
•
IS
20 cm in diameter.
•
21
•
lJquld DI'inbnlor
Alorm
'"
"'.Ier J)ryer
\\'ater
('au.hc
~IS
Jacket
~crubh.r
Intol", ~oddlc
,mil)!... ro nie AUlh/L.'f
C arbORai., ~o'ulion
Feed Tank
Pac~mg
.-------~==:::;----I~I.
~~~-,
•
WaterBath
11,<,
co
N
l "IUld Qut
Va,lIIur !.aluralor
®
.Jackel Feed Tank
Ab\orphlln C. olumn
f'igllre 3./ Schematic diagram of the cxperimental set-up
•
22
•
3.2 CHARACTERIZATION OF EXPERIMENTAL SET-tIP
The experimental set-up "as charactcrt/cd
b~
mcasunng the Icsldencc time
ab~ol pt III Il
distnbutlon of the gas phase in the packed column. and the
carbon dioxide in \Vater \Vas im cstigated and comparL'd to that gl\'L'1l
III
phenomcnon nI'
the Ittcratule.
3.2.1 GAS RESIDENCE TIME DISTRIBLJTION
The residence time distnbutlon (RTD) or the gas
the racked wlumn \Vas
111
studied experimentally using stm1Ulus rcsponse e:-..periments
111
onk!' to ddel mine the
degree ofback mixing cffects m the column. l'he ga!:> phase R rD \\as Il1L'.I'atn:d
reportcd
f/81 that for H,S ahsorption the ga,>
pha~e 1'>
SIllCC
it is
controlling thl.: ,Ihsorptlon
efficiency.
The RTD experiments wert' carried nut hy creatlllg a "tep up (,;hange in
•
concentration of helium at the inlet of the column. cu' (t'rom 0
10
50/'1'111
111
IlItrogcn )
The concentration ofhelium at the exit of the eolumn, \Vas mcasured as a fundlon 01' lime
(every 0.1 sec) using a Mass Spectrometcr
The exit concentration. ('. which IS
nondimensionalized by the !ced concentration (' - LIe,,)
I~
plotted agal\1"t tlllle, 1.
Figure 3.1. The time has been corrcctcd by "uhtrm:ting the dcaJ timc
III
III
thc tubes. The
experiments were repeated at threc dIffèrent gas no\\' rates (20. j5 and 50//11/111 ) whde
water t10ws at a constant rate
or
II1111m
under
atl11o~plll:ric
ple~"lIrC
and loom
temperature.
From a mass balance, it is obvious that the area bctwecn the IlIle { .- 1 and Ihe
response curve in Figure 3.1 must be equal tn the me an residencc time as givcn hy
Equation 3.1
1=00
e= f (I - C)dl
n.l )
1=0
The mean residence time. 0, \Vas cvaluutcd for the thrcc gus Ilow rates. The time,
•
1.
was nondimensionalized as
o.?)
21
•
12
10
""'"".J
OX
"';:::
è
~
(,-"'O/mm
OC!
:!!
~
•
Q
04
~
02
(lO
180
00
t,
Figllre 3.2
•
SèC
1he l1on-dll11cn<;lonal concentration 01 Ile al thc exit of thc column as functlOn of
III11e (1. -1 "'/lm. r -25"( Î
•
The high frcquency noise \\as ICl11o\'l'd h;. liltl'ling thl' data uSlllg ~1.\ 1'1 :\H .
Subsequently the RTD l'un cs \\cre ohtaincd \1:- plottlllg the dll1lCnslOl1te"s hehlll1l
concentration \s. the dimenslOnlcss time as ~ho\\'n in Figure 3.3. rhese rL'slIlts ittustrates
that the degree of back 111lxing eftccts in the gas phase decrcasL's \\ tth IIH:reasmg Ihl\\'
rate.
The degree of bad.. mixing \\as charactcri/cd by litting thesc R rI)
mathematical solution of thc axial dispcrsion modcl obtallled h;. /1,.('l/l/a
l'lII\
1': /1
L'S to the
dl'''Cllhcd
in Appendix (8). Figure 3 4. shows that thc c'Xperimcntal RTl) nhtalllcd at (i 20 1 1111/1
1:-'
weil represented by the RTO ohtall1cd \\Ith the malhcmulical model ,lIld a l'eckt
or
(l't')
32. The Pec1et numhers obtatned by liUing the I11casured R rD ClIl\l'-; 101 tlll' thll'C gas
tlow rates are tabulated
111
rable 3.1.
l'he high \ ailles or the-;e Pœlct 1l1l1111K'i!-. IIldlcate
that RTO in thc cotumn is close tll plug Ihm
1hl' colull1l1 \ old Il:\CIIOI1.
1..
ha-; beell
cvaluated at each gas tlo\v rate us mg l:quattol1 -' 1.•lI1d the le!-.lIlt-; aIl' al..,ll ...,howll 111
Table 3.1. The theoretical \Old rraction l'or the
(l 111111
manufacturer. NORTON chemlcallJJ'oCI!.ls l'J'Oc/I/CII)
•
Int.llo'\ SaddIL-:-- (as l!-1\CIl hy Ihe
HuI. .. hout 1 11 (llll'a..,lIIcd
1..., () (l7.
experimentally) of the total void volume II., equlpJ1ed by the IlqUIt! \\ hCIl \V.Ilcr Ilmv:-. al a
rate of 1.0 llmm. and at stcady state conditions. Iherdùre. the 1rcc
about 0.48. Since this value is closc
10
1(lI
lhat orthe three \'oid Iluctlonl., Il,,lcd
and the se values are almost the salllc. 1 c. not u runetlon
or the
concluded that the present expcrimcntal and analy-;i-; :--ystcl11 1..,
f.
I.,pac~
ga..,
t1()\V
()rcrallll~
l!-"" hCCllIlICI.,
III
1ahlc 1 1.
ratc. It
Gill
he
propclly
Ge
=-
V
where:
G: gas flow rate.
e:
rnean residcnce time.
V: total volume
TlIhle J.I
•
or packing
Charactemllc~
01 the paLl... cd heu
No.
G.I/II1111
1
..,
20
~'1
\-
() 42X
~5
(lX
() 414
")
50
X4
() 41()
l'e :...Jo
!:,
\'()1l1 /1 {/CIlU/I
2'\
•
12
-".
r---------------------------------------------------------~
oK
~
,
-
06
~
~
\"j
O·,
(j=20 l'mm
(J=50 IImm
G=35/1mm
02
(1
0
o
NJ:llrf! 1.3 Residence Time Distribution
III
25
15
0)
,D//1/t!Il.\·/(mlt!.\.\
the packed column (L= II/mm. T=25°C)
•
12
~--------------------------------------------------------~
.., os
~
.::
-
1hcorctlcal RrD
"~'
PI! =32
Oh
~
::::
r ,pcnmcntal RTD
',ÎI
0=20 /, mm
èS o ,
~
02
Il
0
05
15
D/IIlenslOnle~s
:2
:2 5
o.
•
1ïJ:llrl! 1.4
,
.'
I:\penOlental and Theorellcal Residence Time Distribution of the gas phase
26
•
3.2.2 CARBON DIOXIDE ABSORPTION IN WATER
Another \Vay to test the expenmental set-ur,
\\.IS
tn study the ahsllrptinn of carbon
dioxide in water bccause this problem has hœn !-ooh ed theoretically, and cxpcnmental
results are available in the literature.
The expcriments \Vere performed lIsing
li
gas stream of IOt!/" ('():! and 90°1c1 N:!
saturated \Vith \Vater. The gas stream tlows cOllntcr-currcntly to asti eam 01 distillcd watcr
running down l'rom the top of the column. l'he ~:xit concentratton 01
gas phase was measured using a mass spcctrophotomctcr and a
C~II hon
('()2
dwxldc in the
IIlfra Icd analy:!cl
operated simultaneously, and an acid-base tltration procedure \\a'i Il'icd tll dC!':rmlllc Ihe
steady state concentratIOn of CO 2
111
the liqllld phase at the hottom of thc colullln.
check the material balance for CO", the total amount
-
or co,-
10
Ihat \Vas ahsol hcd \Vas
detennined from the analysis of both phases, and it \Vas Itllllld that the dc\'wtion bclwcen
•
them was less than 5 % in most cases.
The absorption of carbon dioxide in \Vater has heen sludlcd a'! a lunclion of gas
and liquid flow rates at 25
"e.
The rcsulls arc prescnted in the form of the (lverall mass
transfer coefficient. The overall mass transfcr cocrticlents have heen cvaluated hy llIeans
of the known carbon dioxide concentration in the inlel and outlet ga'- stream'! lI'!ing
Equation 3.4.
K(j
J
= - - - -9t= , - -
(3.4 )
{l.A.h.IlP,.1I
where:
KG: overall mass transfer coefficient, molelmm II11 210lm
•
a:
specifie area of the packing material. 981 m'lm 1.
A:
cross sectional arca of the bed, 4.56xH)"' m 2 •
Il:
height of the packed bed, 1.50 m
27
•
The rate of absorption of carbon dioxide. ~H mole/min).
IS
calculated using
Equation 3.5
G·!'
~n=--'x(y-y)
RT
(3.5)
"'
where:
G: gas flow rate. I/min.
l',: total operating pressure. 1 atm
R: gas constant. 0.082057 (! atm)j(mole.K).
T: operating temperature. 297 K
Y and
II
)', arc the measured inlet
111
outlet CO:! mole fraction in the gas
respectivcly.
The logarithmic mean pressure differencc. L\ P 'If
•
was calculated from Equation
3.6 assuming that the equilibrium partial pressure of carbon dioxide above water, P'. is
•
negligiblc comparcd to its partial pressure in the gas phase .
(3.6)
whcre:
!'" and
P, are the inlet and out/et partial pressures respective/y.
It was found that the absorption rate of CO:! increases with increasing liquid or
gas /low rate. Morcover. the ctrect of the liquid now rate on the absorption is much
larger since the Iiquid film resistance is high. The results are summarized in Figure 3.5 as
a fllllctioll of the sllperticial liqUld veIocity. L' = LIA. which also. illustrates that the
present rcsults \Vere tound to be in a good agreement with those reported by Onda [7] .
•
28
•
0010
-
(;=35/1",1/1
(/=201/1/11/1
~
-...
:.:;-:-:
~
() 001!
Rc~ults
"-1
•
~
0004
0"
~
. .',.' . --------------.-,'
",
,...:.,
U
..
~.:.
~
0
111/1/11
Ilotamcd 0\ 0111'"
0006
:::::
-
(,~~()
. ' .•. j-'.
,,,
0002
o,ooe
(/0
() 1
Il·'
0.1
02
L·. /1/1111111
Figure 3.5
Absorption of carbon dioxidc
25 Oc
III
waler
a~
a tUl1ctlOI1 ot
ga~
.lIld hqllld
VciOCIIIC~
.11
•
29
•
3.3 OPERATING PARAMETERS
r he paramctcr~ that have a malor effect on the absorption of hydrogen sulfide and
carhon Jioxidc arc tcmperature. concentration of carbonate ions in the solution. partial
pressures of l '2S and ("02' and gas and ItqUlJ flow rates.
The range of the parameters listed in Table 3.2 were determined based on the
prcliminary e'_penments. calculations and practlcal considerations. For example, the
maximum temperature was chosen to be 60 oC to avoid material problems since most
parts of the experimental apparatus are made of PleXiglas which does not stand
tempe ratures ahove 70 O( •
l'he operatmg lirlllts of the gas and liquid tlow rates were determined visually
during the study of the hydrodynamics of the packed column using water and air. The
minimum liquid flow rate. bclow which channcling of the liquid phase became apparent
•
was lound to he about 0.5 l/mm It was also observed that liquid started ta build up at the
hottol11
of each section of the column when the liqurd f10w rate was increased above 1.5
I/mm causing the gas to be hubbled through liquid
The minimum gas tlow rate was
found to he 20 1/111/11 at the maximum liquid flow rate. rhe maximum gas flow rate was
50 1/111111 otherwise the pressure drop across the column became relatively high and
tlooding started to occur.
ln order 10 determine the working range of hydrogen sultide and carbon dioxide
wnccntrations. chemical equrlibrium calculations concerning black liquor gasification
\Vere made \VIth a module of the FACT computer pro gram by Thompson el a/.[22] which
is availahle on the McUlll mamframe. fhe results of this program show that about 50%
of the total sul fur
111
black liquor is converted to hydrogen sultide in the gaseous phase
\Vhile the lest remarns
II~S
a~
Na,S
111
the smclt. It \Vas also found that the concentration of
in the gas vunes approximate1y hetween 0.1 and 1.0 % depending on the conditions
nI' hlach. liqum gasllication. Similarly. the concentration of CO 2 in the gas phase was
round to he hct\\'een 1.0 and 10.0 % .
•
30
•
Since the concentration of carhonate in gn:en liqum
carbonate concentration in the solution \\as sekctcd 10
\ ar~
1"
ahout 2 II/ole 1. Ihe
het\\ccn n.5 and 5.0 mole 1.
Ail the expcnments \\cre carru:d out \\ 1Ih thc cql11pmcnt c\haust opcncd to the
atmosphere so that the pressure inslde the co\umn \\as atmo~phcnc.
Table 3.2 Ranges of opcratmg paramctcr'i
Operating
P~lrnmeter
Minimum
Temperature ("(1
25 ()
hO ()
Carbonate concentration (molell)
()j
')()
n. \
JO
\0
10 ()
Gas tlow rate (//min.)
20.0
50 ()
Liquid flow rate (//111111.)
0.5
1.5
Hydrogen sultide concentration (mole
Carbon dioxide concentration (mol!!
•
1\ 1~"iltlu III
0'1l)
(~'o)
3.4 CHEMICAL ANALYSIS OF SAMPLES
Chemlcal analysis was performed in order to cvaluatc thc ahsorptlon rate,;
hydrogen sulfidc and carbon dioxlde
111
or
the carhonate ~oll1liolls Il \Va" carrll:d out lin
samplcs from both the liquld and gas phases
111
ortlcr to check Ihe lotal and
CO III pO Ill: n1
material balances.
3.4.1 GAS SAMPLES ANALYSES
A Mass Spectrometer was employed fi.>r the analysis of the ga'i ~trcalll~ ft was
calibrated for H2S and CO 2 .
rhe calihrallon
fiH
hydrogen ,>ulfidl! wa,> carned out by
measuring the response of the MS tn diffèrent '>t"lI1dard ga,> mixturl!'> of 11 2 S
111
hehurn
(0.010.0.100 and 1.000 mole %). Slmtlarly. the calibration of the MS for carbon dloxldc
•
was done using threc diftercnt standard ga~ mixturcs of CO 2
III
nllrogcn (0500. 5.000
11
•
and 10.000 mole !Yr,) simuitancously with an Infra Red (IR)
(,02
analyzer to double check
the accuracy.
'1 he MS was used to dctermine the composition of the gaseous streams in terrns of
hydrogcn 'illitide and carbon dioxldc.
A continuous gas strea,TI \Vas analyzed at a
rrequency or 10 /1=. The flow rate of this stream
Ilow rate.
IS
negligible comparcd to the total gas
Watcr \Vas rcmovcd l'rom thls ~ample stream by hllbbling the gas through a
rclativcly large quantlty of conccntrated sulfuric acid and then passing it through a bed of
Si/ka (jel hcforc entering the MS.
At the hegmnll1g of cach cxpcnmcnl. the inlet composition of the gas stream
cntcring at the holtom
or column was analyzed us mg the MS in order to adjust the tlow
rate of cach specics Lo obtain the dcsired gas composition and tlow late. Also. it \Vas used
to analyzc the cxhallst gas stream at the top of the colllmn after the steady state conditions
•
\Vere achicved. The amount ahsorbed of each gas \Vas calculated from the difference
hctwcen Ils concentration at thc inlet and thc outlet streams.
3.4.2 LIQUID SAMPLES ANALYSES
Alter steady state conditions v.ere reached. a sample of the liquid at the bottom of
thc ahsorption columl1 \Vas takcn to determine its composition in terms of HS". C0 1 and
. liCOl"' The procedurc or Uarnerl91 was rollowed and is described below.
1. Absorbe,/ "\',lrl1l:eII su/Ode: A sample or the liquld was neutralized by addir'g an
cxccss HI110unt of 1() % cadmium chloride.
hydrochlonc acid
\HlS
Then a sufticicnt amount of
addcd to hring the normality of the acid to 0.2 N.
An
cxcess amount of standard iodine solution \Vas added. and the unconsumed iodine
\Vas detcrmined by titration \Vith a standard solution of sodium thiosulfate using
•
starch as an IIldicator. From thesc titration rcsults. the concentration of HS" in the
liquid phase \\as calculatcd.
•
2. Absorbed carboll (lioxide: a) The total alkalil1lty of the solution \\as deternllned
by titration a sam pie of the liquid agamst a \ nlul11ctnc solution nI' sul furie ,Il'id
using methyl orange as an indicator. h) :\n c:-..ccss amount
l)1
n.1 .\'
~tandard
sodium hydroxidc solution \\ as added to anothcr sampk tn com ert hlcarhonate
into carbonate, which \Vas then prccipitated as barium carbonate hy adding a
sufficient amount of 10°;;) banul11 ehloride solution.
bicarbonate
In
the initial sample \Vas then determincd hy haek ttlwtlOIl of the
unreacted NaOH with a 0.1 N IICI standard solution
Ih:n
balance was applied to detcrminc the concentration
or caeh
amount of carbon dioxidc that was absorhcd.
•
•
l'he CllnœntlatlOll of
li
"impie matenal
~pceic~
and the
•
CHAPTER FOUR
PRELIMINARY EXPERIMENTAL WORK
The experimcntal work has bcen carried out in two main parts: The preliminary
cxpcrimcntal part, in which an cxpcrimental design proccdure was followed to investigate
the repeatability of the experiments, the interaction between parameters. the material
•
halancc of species bctween liqmd and gas phase and the effect of bed height: The final
cxperimental part l'rom which the tinal results were obtained and they are discussed in
Chapter Five.
4.1 11RELIMINARY EXPERIMENTAL DESIGN
ln order to minimiLe the total number of experiments. the interaction between
paramclcrs has hcen studicd by pcrforming an cxperimental design of 16 experiments at
lwn Icvcls as 11Illstrated hy Tagllclll el al. f23]. For each experiment the outlet
concentratIOn of hydrogen sultide and carbon dioxide at steady state conditions were
Illeasured. thcn the response of each cxpcrimcnt \Vas determined in terms of the removal
cfticiencles of Il:!S and CO:! . ll( Il;!S) and 1l( CO::!). as shown in Table 4.1.
ln Table 4.1. the values 1 and 2 correspond to the lowest and highest level of each
paral11ctcr .previollsly shown in Tahle 3.2. respectively.
•
To simplify the expcrimental
design. gas and liquid !low ratcs \\cre combmed in a single parameter (GIL), where the
valuc 1 corresponds to the lo\Vest Icvel of G divided by the highest level of L while the
34
•
value :2 is the highest lcvel of G dividcd by the hmest k\ d of L
experimental design. the follO\\ing
t\\O
Also.
\Il
titis
important comhtions \\cre applted in 4.lnicr to
obtain a balanced expenmental design:
1. Any parameter appears eight tllnes at k"d
and elght times at kwl 2. fllr the
whole set of experiments.
2. Any two parameters meet togctlter at the samc combination lIf le, ds (1-1. 1-2. 2-
1 or 2-2) only four times in the whole set or e:\periments.
The removal cfticiencies of hydrogcn sultidc and carbon dlOxide \Vere calculatcd
on the basis of the analyses of gas streams. The rel110val dliciency lIf the ahsOI'hed gas
(l')) is defined as the amount of gas being ahsorbed hy the carbonate solution dl"idcd hy
the total amount oflhat gas entcring thc cnlumn.
Tl = Pli - PI
PlI
•
X
100%
I.C.
(4.\ )
where:
Po and
PI
are the inlct and thc oullet partial pressures or the ahsorhed gus and
the y are obtained by means of Equation 4.2
(4.2 )
where:
•
y:
is the mole fraction in the gas phase.
PI:
is the total operating pressure. alm .
1)
•
Tt,ble 4.1 Prchmmary L,,,pcrlmcntal
Run No.
r
ICO; 1
A-I
1
1
y (II,S) YjCO:!)
-
"
10.01
78.90
9.50
1
,.,
86.13
7.25
,.,
,.,
1
80.23
11.13
1
1
2
86.80
5.10
,.,
1
81.09
9.30
1
1
88.10
7.17
90.91
6.29
1
A-3
1
1
,.,
A-4
1
1
A-5
1
2
A-6
1
,.,
A-7
1
,.,
A-X
1
,.,
A-II
")
11 (CO:!),%
74.50
1
,.,
l1(H:!S).%
1
1
Â-IO
G/L
,.,
1
A-2
i\-9
•
dC~lgn
1
,.,
-
1
-'1
,.,
-
,
,.,
1
1
1
'1
77.13
12.36
1
1
'1
1
72.20
16.81
1
2
1
1
78.41
14.50
81.90
13.34
,.,
-
A-12
2
1
")
'1
,.,
A-13
'1
")
1
1
1
79.29
12.45
A-14
")
,.,
1
")
")
82.80
11.39
1
")
89.88
9.39
'1
1
83.86
13.27
A-15
,
-
")
,.,
A-16
'1
'1
'1
4.2 PRELIMINARY EXPERIMENTAL RESULTS
"'.2.1 REPEATABILITY OF EXPERIMENTS
'\'0 test the expenmcntal results some experiments listed in fable 4.1 were
repeated.
rhe ùifferenccs in the r('moval efticiencies of hydrogen sultide and carbon
dioxiùe of the duplicate experiments. respectlvely Tl' and
Tl". are very small
as shown in
rahle 4.2. suggcsting that the rcpeatability of the experiments is very good.
•
36
•
Table 4.2 Repeatabthty of e-..pertments
Run No.
11'(CO ~)
T] '( Il,S)
1l"(H 2S)
Dc,'.
0/
0
O{,
III
A-5
86.80
86.21
o6X
5.10
5 .\()
5 10
A-8
90.91
91.70
(U~6
() 29
(, 53
\ X2
A-15
89.88
89.10
I).X7
l)
01
40)
'1"(('0,)
II
II li
li
39
1)
DC\'.
II
n
n
4.2.2 INTERACTIONS & EFFECT OF PARAMETERS
The removal efticicncy of Il:!S and CO;! have also bcen ploltcd as functioll of the
operating parameters in Figure 4 1 through Figurc 4 10
l',ach pomt on tho<;e ligures is
calculated by taking the average valuc of thc four rcmoval cf liclcllcles where the two
•
parameters are set together at the samc comhlllation of Icvcb as glycn
figure shows two straight lines onc
IS
III
1ahle ·1.1. I:adl
hctwcen the avcraged n.:moval d liclcncles wherc
the two parameters are at the Icvel eOlllhinations ( 1-1 ) and (2-1 ). and the :-,ccolld IlIlc is
between (1-2) and (2-2),
The linding in Figures 4.8b and 4.11 a that the IlI1es arc alll1oo.;t hori/Olltal shows
that the efficiencies of H::!S and CO:! arc hardly intluenced hy the IIllet COI1(.:cntratloll 01
respectively CO:! and H::!S. Moreover, l'rom thosc li gure 0.;. olle can "ce tllat tlle :-.traight
lines are almost parallel to caeh other, indieating that there i.., no IIltcracliOIl
The procedure of TlIglichl et al
1231 was followcd to calculate the IIlteraclions
and the eftèct of parameters on the absorptIOn of hydrogcn ... ullidc and carhon dloxJ<.lc
using the distances between the lines on those ligures and thcir
~Iopc<;.
The resuIts are listed in l'able 4.3 III terms of pcrcentagc valucs 01 how the
•
parameters and the interactions contributc in the removal eflicicm:y 01 1/ 2S and
(,02
ft
or
the
shows that the ealculated IIlteractions are very ..,mall compared lo the cf leet
"
•
paramctcr~,
thu,;, they mlght he ncglected. Figure 4 II also shows the percentage of the
an"olute relative cf tcct of each paramctcr on the remo\'al efficienc) of H:!S and CO:!. It
';lIggc~t';
that carhonate conccntration has the larger effcct on the of absorptIOn H2S. and
that tcmperature
lilhle 431 ffcet
1';
the mu,;t Important parameter ln the ease CO:! absorption
p,lramctcr~
,lIld
II1tcr<lctlon~
flanameterll and Interactions
'J emperature
Effèet on T](I1 2S). %
Erfeet on T](C0 2), %
6.50
57.61
1
41.09
17.32
Y,,( 112~)
31.51
0.96
Y)C()~)
1.01
6.92
19.51
16.42
* ICO, 1
* Y.,(11 2S)
* Y.,(C0 2 )
* G/L
* J'J 11 2S)
* Y.,(C0 2 )
001
0.01
0.03
0.06
0.00
0.05
0.01
0.03
0.02
0.02
0.09
0.04
IC'O~
G/L
Temp
•
on the rcmoval cfticlcnclcs of H 2S and CO 2
Temp.
Temp.
Temp.
ICO~ -1
ICO,
1
!COl 1
*
G/L
001
007
Y.,(11 2S)
*
Y,,(<-'°2)
0.12
0.28
.v,,( 11 2S)
*
G/L
0.00
0.09
Y)('()2)
*
(l'lL
0.09
0.03
100.00
100.00
Tohll
•
38
•
lU)
IU\I-I
!CO, I-~
-
'(\
1\ Il,11
1" 1,1 è
X~
-
l'
;t:.
0
0'
,-:...
....:.
rJl
:c"
'"'
r=-
III
~
~
'-"
1="
75
"1)
~_
_ _ _ _ _ _ _ _ _ _ _.-I
Temperature
il)
h)
Figllre 4. / Interaction bctween tcmpcraturc anù cMbonalc lOl1ccnll <111011
Il) Effect on H2 S absorption. h) EtTcct on C()~ ab\OlpllOIl
•
'JO
\' III ~ 1 - 1
\' III ~I - ~
-
'0
1 fil""
1
"0,
,
1 dl
85
...
l'
;t:.
:>
,-:...
rJl
~'
....:.
so
:r:"
S
'-,
1="
1="
"-"
7~
70
'+____________
Temperature
(/)
•
~
"'-!-------------""":""
rcmpcraturc
h)
Figllre 4.2 Interaction bctween tcmperaturc and hydrogcn ,>ulfidc mlct lom.cntratlOI1
a) Effect on H2S absorptIOn. h) !.IfCll on CO~ Jb<,orptlon
•
:1) ,..._____________
"1,..._ _ _ _ _ _ _ _ _ _ _ _.....
IllO )-1
l'Il ()
...
)-:!
15
~<
,0
0
'"....:
0'
cr;
....:
~II
,-."
=
-
III
'-'
-
V
.-
~
,<
'11 ....._ _ _ _ _ _ _ _ _ _ _ _......
,,~------------~
Temperature
1crnpcralurc
h)
Il)
/'ï1:lIre 4.3
Intcr.JeliOn octwccn tcmperaturc and carbon dlO'lde mlct concentration
Il) Urccl on Il:S absorptIOn. h) Efreet on CO:! absorption
•
')fI
(,'11. - 1
:0
...
(,'iL - 1
(i/I. - 2
(,'lI. -:!
~<
15
,0
0'
?J(
....:..
::r:'.
'Jl
...
,-..
~II
0
"
III
-
u
~
~
'<
'11
~_ _ _ _ _ _ _ _ _ _ _ _~
•
Temperature
Icmpcraturc
,,)
/'ï1:lIrt! 4.4
Il~_ _ _ _ _ _ _ _ _ _ _ _ _~
h)
InteractIOn oct\\cen tClllpcrature and gas'hqUld rallo
Il)
UrCel on Il:S "bsorpllOn. h) Efreet on CO: absorption
40
•
..
'JO
y,(l15)-1
1',111 SI-
~ll
1 III \1
2
1
1 III \i
liS
--
l'
~
'<f?,
.-,
r/l
"-'
RO
0
"
J:"
U
1='
1='
'-'
III
75
I\~_ _ _ _ _ _ _ _ _ _ _ _.u
70~______________________-U
[CO~l
h)
lI)
Figu,e 4,5 Interaction betwecn carbonate concentrallon and hydrogcll ~lIl1idc
III IcI
COl1lcnt raI 1011
a) Effect on H~S absorptIOn. h) [ffccl 011 CO 2 ,lb~O!pllOl1
•
90
J',llO,)-1
Y.110 1-2
..
20
1'1111 1 1
11111 1
"
l,
!!5
,0
0'
'<f?,
--:...
r/l
--
,-:
80
J:"
0
,."'
'-'
"
III
U
.....
1='
1='
75
70~_____________________~
"'-!--____
a)
Figu,e4.6
•
[COi1
h)
-------~
ICO Il
Interaction betweel1 carbonate concentratIOn and carhon dlOxldc IIIlel LOIILcntratJOII
a) Effect on H 2S absorptIOn. h) Effeet on c..0 2 Jh~orpllOn
41
•
f}f,
(,'lI - 1
(,'lI. - 2
-
20
(i/I. - 1
(in. - 2
X~
15
0
0
0'
'"
.--!
If)
--.-
.,
.--!
XI)
0"
:c
10
u
~
::-
~
::7~
71)
L.!-------------~
2
1)
[COlI
1.!-------------~2
[C0 1
h)
l')
3
l-ï1:ure 4.7 Interaction between carbonate concentration and ga~/hqllld ratio
Il)
Efreet 011 11 2 S ab~orptlon. h) Erfeet on CO~ absorption
•
tm
l',tl Il 1 - 1
l'III) 1
-
~()
l'ot< 0,)-1
, I( 0 )-2
10;
X~
~
~
,...:.
V1
--
-:
0"
Kti
:r:"
'" "
. ..
10
u
~
~
s::-
'~
"II L.J._ _ _ _ _ _ _ _ _ _ _ _....
Il)
o ~------------~2
h)
Fi1:IIre 4.8 Interaction bet\\'ccn mlet concentratIOn 01 hydrogen sullide and mlet concentration
•
of carbon dio'ode, Il) [ffect on H~S absorption, h) Erfect on CO~ absorption
42
•
'10
Gfl.-I
G/L - ~
-
~o
(. 1 - 1
(;/1 -
85
...
..
l'
";f..
--'
rJl
~
""
RI!
"
J:
--"
..
•
,0
0'
1(\
'-'
U
~
~
75
70~______________________~
IIL!---_____________!_'
.l'JII,S)
(1)
h)
Figure 4.9
Interaction between mlct concentratIOn of hydrogcn ~ullidc .lI\d gol~/ilqllld
1.1\10
(/) Effect on H2S absorption, h) Effel:! on CO~ oIbsorplllln
•
'JO
(,ïf. - 1
...
20
!oÏl • 1
aiL - 2
H5
o.
l'
";f..
--'
rJl
ro
(;// - 2
0
0'
,...:.
0"
U
HO 1_
J:
'-"
~
III
...
..
---
~
7,
70~______________________~
a)
•
Figure 4.10
hl
.1',,(( 0,)
Interaction between mlct concentration of carbon dloxldc and ga.,/llquld ratio
a) Effeet on H2S absorption, h) Effecl on CO; ao,>orpllon
tll
•
G/L
a) H2S absorption
•
T
[CO)]
h) CO2 absorption
fïg"re 4.11
Relative cffect of parameters on the absorption of H2S and CO 2
•
44
•
4.3 MATERIAL BALANCE
Material balance of the SpCCICS invol"cd in this system has bccn lI1\'Cstigatcd
order to contirm the rcliability of the experimental
re~ults,
the
anal~
111
tkal I1Icthmls and the
calibration of the measuring instruments. RcslIlts of thls study 'lIe SlIll1m.lll1cd in l'ahk
4.4 based on the concentration of the ionic spccics (I1~'f, IICO,- and COl )
liquid stream.
III
thc outlet
The concentrations of thosc -;pccics wcre dctcrlllmcd analytlcally as
described earlier in Chapter Threc and then comparcd to thosc calclliated liulll the total
and component material balances bascd on the "-nO\\'11 composition of thc mlct and the
outlet gas streams and the inlet liqUld stream as described hellow, t,,"-ing mto account thal
the following two assumptions have bcen made tn calclliate the com:enll atlOn or IICO,-'
J. Hydrogen sultide is absorbed by l11eans ofrcaclion 2.3 onl)'.
•
2. The amount of HC0 3- produced by lhe ahsorptlOl1 of 11 2S and
('()2
is negliglhle
compared to that produced by the dissociation of the carbonate IOns according tn
Reaction (2.5) so that the mathematical expression of the eqllllihl illm constant I(lr
this reaction is given as:
(4.3 )
Where:
Ke" is the equilibrium constant
[HCO~
1.'1
is the concentration of bicarbonate ions at equilihriull1, mo/ell.
[CO~]" is the inlet concentration of carbonate. lIlole/l.
The corresponding values of the cquilibrium constant K"II' lor rcaction 4.3 arc
given in Table 2.1 as a function of tempcraturc .
•
45
•
The concentrations of the spccies (ilS". HC0 3" and CO, = ) in the outlet liquid
stream were calculated using the following equatlons'
. l' Tolal moles 01 ahsorhed
1Ils =
I!..,S
-
(4.5)
V()lumelrtc Ilqu III flow raIe
fllco-]'
:1
= Tolal moles of lIhsorhed
co:! + [I-IC0 ],.,/ +[I-I..,sr
('(Jlumelrie liquid flow raIe
3
-
[CO~ ( =[c071., -[IICO~L,
-
(4.6)
(4.7)
The deviation is givcn as the ratio of the absolute deference between the measured
and the calclllatcd concentrations to the mcasured concentration .
•
%Dev.=
I[ Measllred] -l ('alL'ulated]1 x J()()%
[ l'vIel/sured]
Results of this ~tudy arc tabulated in Table 4.4.
(4.8)
The deviation between the
l11easllred and calculated concentration for the three ionic species (HS". I-IC03" and
CO, ) was I()lmd to he Icss than 5 % in ail cases. Therefore these results shows that the
experimental set-up \Vas
111
good operation. the tlow meters were \Vell calibrated. and it
also suggcsts that the analytical instruments and procedures llsed to analyze gas and
liquid sumples \Vere accurate .
•
46
•
Table 4.4 Malenal balance
Run.
No.
(ns-)"
molell
A-\
A-2
0.00341
Del'.
IYt.
IHCO~ )"
l''c()~ l'
De\'.
I('u ,1"
1CO, l'
1
mole 1
I>t"'.
Iy'.
~ ()l)
Il 4X(,7
Il 4 l )4X
1 (,)
0.0520
128
o 43llS
Il 4529
; ()l)
0.0520
() 0501
.; (lh
() ·n·H
() 454X
474
0.00468 0.00455 2.63
() 020X
0.0200
'UN
o 4hh 1
Il 4X49
402
A-5
0.00384 0.00370 3.80
0.0355
O.OJ49
174
478.17
4
I)(,(ln
1 (ll)
A-6
0.00051 0.00049 3.86
O.O}53
O.OJ49
106
47715
4
,>(,(ln
1.'>5
A-7
0.00523 000501 409
0.0347
0.0344
() 74
48347
4
1)hO",
~
A-8
0.03917 0.03805 2.87
0.0957
0.0933
245
48471\
4 l)O J(,
III
A-9
0.00364 0.00359 1.44
0.0369
() 035()
.\,(,1
0442",
o ·.~X \
1.59
A-IO 0.00043 0.00041 3.77
0.0372
O.OJ66
179
(4)02
(1 4~7\
I:W
0.00447 0.00438 1.96
0.0329
003}7
1.66
04494
04h22
2 Xl)
A-12 0.03606 0.03467 3.86
0.1178
0.1172
055
() 1714
o 17()7 n. Kt)
0.00049 0.00049 0.69
0.0913
O.mnl
4.57
4
~N11
4 lJl XX
Il 56
A-14 0.00362 0.00344 4.77
0.1441
0.1184
3.95
475S')
.. 8() 75
2.35
A-15
0.04028 0.03860 4.15
0.1334
0.1286
3.57
470()S
.. 8771
3.63
A-16 0.00498 0.00489 1.90
n.IOIS
0.0985
2.()]
4 XX94
4. 1)!)74
0.17
molell
1
/IIoll.!d
11/01 ...
0.00046 0.00045 }.II
() () 104
00101
1.~2
().05~7
A-3
0.038\ 0 0.03776 089
A-4
A-Il
•
IHS-)'
A-13
Nole 1he
sllper~CrlplS
0.00337
'\1 and c
corre~p(llld, ln
IVo.
1110'"
(,0
Ihe mc.t,urcd.md Ihc L.IILIII.IICd Llll1lClllr.llloll' rc,plLlIVd\
4.4 EFFECT OF BED HEIGHT ON H2S & C02 ABSOIU'TION
The cffcct of thc bcd hcight on thc rcmoval ctfieJ(.:ncy of' hythogcn <;ullidc and
carbon dioxide was invcsttgatcd by measunng the gas compo<;ttion alter caeh of the three
50 cm high sections orthe co\umn bec Figure 3 1). Bcfitrc !'>witching l'rom one "iumphng
point 10 another steady state condition,> wcrc al\owcd tn he rccovcred agum. 1he removal
•
efficiencies H2S. CO:! at the diffcrent !-Jcctions of the column werc determmco. RC!-Jults
terms of the cffect of bcd height on the n:movul cfliciency of IlzS and
('():!
lU
arc .,hown in
Figure 4.12 and Figure 4.13 respcctivcly.
-17
•
Figure 4.12 shows that at the conditions of the present set of experiments which
gave the luwest hydrogen sultide removal efliciency (Run no. A-ID. where 11=72.2 %),
cf/cet of the hed height on the absorption of hydrogen sultide seems to be almost linear.
Ifowever, at conditions which gave hlgher rcmoval efficiencies (Run no. A-8. where
.,=909 %), this rclationship starts to take a power shape.
The results
FIgure 4.13 c1early show that the re1ationship bctween the removal
cfliciency of carbon dioxide and the hcight of the bed is 1inear at the conditions where the
rcmoval efticiency is lowcst (Run no. 5. .,=5.1 %). But at the conditions where the
III
highcr rcmoval cfticiency was obtaincd (Run no. 10. 11= 16.81 %). the relationship takes a
power shape strongcr than that of hydrogen sultide.
Thesc
•
IWO
ligures suggest that the packed bed was not high enough for hydrogen
sullide in the gas phase to reach the equilibrium conditions with the absorbed hydrogen
sullide in the liqllid phase taking into account its inlet concentration in the gas phase. On
the othcr hand. this indicates that carbon dioxide in the gas phase is not tàr from the
ct\uilibrillm conditions \Vith that in the liqllid phase.
Finally, an important conclusion of this study IS that the absorption of hydrogen
sullidc could approach 100 % if the height of the packed bed is increased ta
approximately 2.0 m. and it does not affect the absorption of carbon dioxide very much.
•
48
•
100
80
~
--..
60
,-::;
rJ:J
~
40
20
20
.10
60
XO
100
120
l,Ill
l3ed Ilclght. ,'/II
Figure 4.12
EtTect bed height on the rcmoval clliclcncy of Il!S
•
20
Run no A-5
16
::R
0
12
r-.
O~·
U
r::
!!
4
.
()
"
,
()
20
'
40
W
XO
100
110
140
IW
Bcd Ilcight. cm
Figure 4,/3
•
Erreel bed height on the removal cfficlcncy of CO,
49
•
CHAPTER FIVE
RESUL TS & DISCUSSION
The absorption or hydrogen sultide and carbon dioxide have been investigated as
a limction of the opcratlllg parameters, I.e. temperaturc, carbonate concentration, gas and
1iquid supcrlicial vclocltlcs, and thc inlet concentration of both gases.
•
RcsuIts are presentcd in tcnns of the influence of these parameters on the removal
cfficienclcs and the ovcrall mass transter coefficients of hydrogen sulfide and carbon
dioxidc.
Furthermore, the process selcctivity for hydrogen sulfide removal has been
discussed.
5.1 EXPERIMENTAL DESIGN
For this cxpcrimcntal investigation, the same experimental design procedure
dcscribcd by Ta~lidl1
1231
was follo'Wed.
Another set of sixteen cxperiments was
pcrli.mncd. hascd on thc prcliminury cxperimental resuIts (Chapter Four), where the
interactions between parametcrs arc negliglblc. In this sct of experiments, the liquid flow
rate was maintaincd constant at the avcrage valuc between its lowest and highest levels
(at 111cdllllll Ic\'cl.
m).
For thc olhcr paramelers, i.e. lcmperature, carbonatc concentration,
Il;!S and '-'()2 inlct concentratIOn, and the now rate of the gas, four equally spaced levels
•
\\erc in\'cstigatcd. rhus, thc value 1 corresponds to the lowest level of each parameter, 4
is the highest kwl. 2 is at the level one third of the ditference between the lowest and the
50
- -
•
highest levels above the lowest kvel and 3
IS
-----------------.
at the le\ cl t\\O thmls of the same
difference ubove the lowest levcl. The conditions ot"these e:-.pertlllellls arc
gl\l.'1l
in l'abk
5.1.
This expenmental design IS considered 10 be balalH.:ed "Illec Il "atl~,ilies lhl.'
follo\\-ing two conditions:
1. Each parametcr
IS
repcated at the saille levd only tour tl\l1eS
III
thl.' \\holt: set of
experirncnts.
2. Any cornbination oflevcls for any two parametcrs appems only ollce 1I11he who le
set of cxperiments.
Table 5./ Fmal cxpcnmental deSign
•
Run No.
Temp.
ICO;I
B- 1
1
1
B- 1
1
2
B-3
1
B-4
Y,,(1I 2S)
1
Y.,((·02 )
1
lllt'O,), n'i,
GIL
'1(I1.,"'i). no
1
7495
XhX
ln 15
7 9(1
.,
.,
.,
-'
3
.)
...
...,
X935
761
1
4
4
4
() 2
().~
775
B-5
2
1
.,
-
4
...)
4
XI (14
12 26
B-6
")
'1
1
4
3
7() XI
1167
")
.,
X712
X61
X4 9()
1)
B-7
8-8
")
..,
.,
.)
1
-
1
4
...
.'
.,
1
3
4
4
.'
8-9
-'
B-I0
-'
Î
B-ll
3
-'
B-12
3
4
..,
4
.,
.,
..,
.,
-
07
7X X4
14.02
1
XII 'i
1220
1
.,
4
X1. 50
II.()()
1
3
Xô 1X
1() 41
...)
XI
1"\ (n
.,
B-13
4
1
4
.,-
B-14
4
'1
3
1
4
X6 X5
B-15
4
3
")
4
1
XO 5(1
1222
B-16
4
4
1
3
7') 7()
12 10
.,
(J'i
Il ()5
.
•
'il
•
The removal efticiencics of H2S and CO 2 have been determined based on the
analy'ils of the gas streams at thc inlct and the outlct of the column by means of Equation
4 1 and EquatIon 4.2.
"he Influence of gas Ilow rate on the absorption of hydrogen sultide and carbon
dioxldc has been ~tudled by running a set of expenments at tive levels (Table 5.2) where
aIl other parameters werc kept constant at thctr average level between the lowest and the
highest leveb (m,.
rhis was done in order to compare the results obtained from the
TaguL'hl cxpenmental design wlth those obtained l'rom the ordinary cxpenmental design.
i\ simiJar
~ct
of cxpeflments \Vas carned out to study the intluence of liqUld flow rate on
the absorplJon of 112S and CO 2 ('rab le 5.3). The designation of those levels. 1 to 4, in
thcse two tables IS the ~amt.' as for that of the final cxperimental design given in
1able 5.1.
Ttlhle 5.2 Erfeet 01 gas tlow rate
•
No.
% 1l(CÛ2),0/0
G
ll(l-l:!S),
80.41
10.54
('-2
1
...,
82.25
10.72
C-3
3
84.08
10.91
('-4
4
85.92
1109
C-5
'"
83.06
10.71
({un
C- 1
Ttlhle J.3 Erfeet 01 IiqUid tlow rate
({un
No.
D- 1
•
L
1
...,
1l(H 2S), % r1(,02)' %
82.71
9.35
83.01
10.33
D-2
-
))-3
-'
83.32
11.3/
D-4
4
83.61
12.28
1)-5
ln
83
n
10.93
...
52
•
5.2 DISCUSSION OF RESUL TS
The final rcsults \vcre evalualed \\'Ilh lespecl
10
the el'll'cl of the -;tudlcd
parameters on the rcmoval effieiencies of h)drogen sullide and carbon dloxlde, Oll the
overall mass transtèr coefticients and also on the selcctivity 1:lctor
5.2.1 EFFECT ON THE REMOV AL EFFICIENCY
The eITeet of tempcrature, concentration of carhonate, concentratlOll 01 hnlrogen
sulfide and concentration of carbon dioXH.ic on the rel110val dliciellcy 01 Il ~S ,lIId CO:,
has been estimated trom thc data shown in rahle 5,1. 1he cllcet
level has been cstimated by takll1g the average value or Ihe
•
or cach
J'Olll
Il'1I10\'al cl licIl'l1CICS
obtained wlth paramctcr at that Icvel. An ad<.htional data pOlIl1. al Ic\'el
taking the average of thc last two expenmental
details see Appendix C),
re~uIts
pawlllctcr al ally
III.
I~
ohlalllcd hy
in 1ahlcs 5 2 .lIld 5 J (Ior
rhis gl ves the removal eflicH:ncy
a~
a 11IIlCIlOll 01 each
parameter while thc rcst orthe parameters \Verc kepl constant at thell IIlcthUI11 Icvcl
the form of five data points. Thc calculated data have hcen plotted
IIIOIC
1\1
(III) 111
thl' 1;lgures 5 1 ln
5.6. Thus, each ligure shows the effect of a slllgle paramctcr Oll the lellloval eflielcllcy (lI
H;!S and CO:! at live levels while the five othcr parumeters are
cOII~lallt
al thclr medium
level.
Figure 5.1 shows that incrcasing the opcrating temperaturc dcercascs the
absorption rate of hydrogen sullidc and incrcases the absorption rate of carhon dlOxide.
According to theory, the solubility,
or buth
gases, ln water decrea<;c.., wlth increasing
temperature [18,19]. Although at non-equtlibrium
c()ndition~,
whcn chcllllcal
rcael\()n~
between the gus phase and the liqll1d phase are 1I1volvcd. thl.., doc" Il()t alway.., apply
However. increasing the tcmpcraturc
re~ulb
ln a
~lgnJlicant
of CO:! (reaction 2.2), Also, It decrcme,; the IIquld lilm
•
I\1Cre,N! ln the retlctJon rate
re"l~tancc
hccLllI"e of the lowcr
liquid vlscosity. Since thc absorption rate of ( ():! i.., controlled hy the IllJuld "Ide
ma~s
transfer reslstance as was rcportcd by Garner el al, 1 f)1 thi,; might explmn the l\1f1ucnce of
temperaturc.
)1
•
()n the other hand. tempcraturc atfccts the absorption of H 2S is an opposite way.
The ab'iorptlon "2S i. . control/cd by the gas side mass transter resistance: Therefore, the
c.lccreasc in the "quld film rcsistancc does Ilot increase the absorption of H 2S in a
<;iglllficant all10unt
Ilowever. an incrcasc in temperature shifts the cquilibrium
lh~""oclatlon
reactlOn (rcaction 5
IICO,' ionc;; arc produced
1)
of COl in \\ater to the right side 118./6]; thus. more
Thcrcf(.)re. the cquilibrium reaction bet\\een H 2S and C0 3
(reactlon 2 4) is <;hi flet! to the Idi slde. resulting in a dccrease in the absorption rate of
Il..,S. 1hese reslIlts arc
ln
agreement \Vith those rcported by Ramachandran el al. [12].
CO 3
(5.1 )
rhe intlllencc of the mlet concentratIOn of carbonate Ions on the removal
cfficicncy of hydrogen ~ultide and carbon dioxide IS in good agreement \Vith those
rcportet! in literature 15.6,9,12,131 As illustrated in Figure 5.2. the removal etliciency of
"2S increases with II1creasing concentration of carbonate IOns. because the equilibrium
•
reaction hctwccn ('0 1-- and 11 2S (reaction 2.4) is shifted to the right side. However, an
increase in the carbonate ions concentratIon shifts reaclion 5.1 to the right side, thus,
incrcasing the concentration of 1ICO) '.
Therefore. reaction 2.2 becomes doser to
equilihnull1. \\hiLh resll/ts in redllcmg the absorption rate of CO 2• as reported by AslarÎla
e( al
1() 1·
From Ficure 5 3. it is dear that H,S
- removal efficiencv. increases signiticantly
~
when its concentration at the inlet of the column is increased. The concentration gradient
of Il,:!S het\\een the liquid and the gas phase becomes higher if its partial pressure is
incrcased.
rherct~.)fe.
a higher drivmg force is crcated. which reslllts in an increase in
ahsorptlon rate hy allo\\ ing more H 2S to he absorbcd in accordance to reaction 2.3 and
n:actllln :2.4. (his linding is similar to those rcported in the literature [-.9, Il,13].
ln the saille \\av.
- increasinc the inlet concentration of CO, increases its removal
~
efticicnc~
•
-
(Figure 5.4).
Hnwever. carbon dioxide is absorbed by means of a slow
reacttllll (reaction 2.2).
rhus, the ciTect of CO, inlet concentration on its removal
efticicncy appcars to he sm aller than the cHect of H,S inlet concentration on the
54
•
absorption efficiency of H~S (Figure 5.3). Furtht:rmore. rmlll th~se 1"'0 ligures. lht:
removal efficiency of carbon dioxide and that of h~ drogl.'n ~ullidl.' arl' not alfl.'l.'ted
significantly by the inlet concentration of rt:spt:cti\ I.'ly. Il.!S and CO.!. l'lm linding.
corroborates the preliminary experimentallt:sults gi\1.'1l t:atlil.'r 111 l'ahll.' 4.3 and Figurl.'
4.11. and also agrees \Vith that rcportcd by Belldal! e{ al Il () 1.
The influence of gas and liquid supcrlicial velocitics is Illustratcd in Figure 5.5
and Figure 5.6. The results arc in excellent agrct:l1lent \\ith thost: round in lJterature
[-1,5,10,111. The removal efticiency of Il:!S incrcases
signilicantl~
\\Ith incrcasing the
gas velocity. white the influencc of the gas vclocity on the rClllo\'al dlicil.'lll'Y or co:! is
very small as can be seen in Figure 5.5. On the otht:r hand. l'rom Figlln: .5 () Il is ohviolls
that increasing the liquid \'clocity. results in a signilicant IIlcrca'il.' 111 the Il.'llloval
efficieney of CO;!, while 1t has a sl1lall cffect on the rt:moval efliclcncy (lI' 11 2 S 1he main
reason for this phenomenon is that the absorption of Il;!S is agas "i<.k controllcd proccss
as il reaets at the gas-liquid intertace, while the absorption of ('{)2 is a litJuid 'iidc
•
eontrolled proeess; therefore. it reacts in the Itquid bulk.
In thesc Iwo tigurl.'S, thc
relationship between the rcmoval elliciencics and the \'clocitlcs of ga'i and liquid appcars
to be linear. The only cxplanalion for thb hchavior is that thc hClght of the packed
eolumn was not high enough to reach the eqUllibriulll hetwccn the
IWO pha~es
as was
previously shown in Figure 4.12 and Figure 4.13
•
55
•
~____________________________________________~
16
XX
Q
Q'
X"
14
X4
12~
X2
10~
,-:.
'fi
-r=
0"
"
-'
~
XO
8
.'
~____________~____________________________~____~6
7&
:W
JO
40
50
60
70
Temperature. oC
fïg'ITe .'U
•
H8
Effeet oftemperature on the removal eftieiency of H2S and CO~
-
II zS
!Ill
,0
Q'
18
CO2
16
14~
N.t
....:.
"""
0"
[/J
:r:"
12~
r= X2
~
sn
10
7!!
Il
lco~ 1. II/ole / 1
Fig"re .f.l
[nect of carbonate concentration on the removal efficiency of H~S and CO 2
•
56
•
R8
r-------------------------------------------~18
ILS
co,
-
86
~
16
R"
I.I~
R2
12
RO
10
.-:.,
rJ)
rt
J:
'-'
ç:-
7R
0~------~------------------------------------~8
0002
000"
() O(}6
Il 008
() (JI
0012
Figure 5.3
•
~
Effect of hydrogcn sullidc mlet conccntratlon on Ihc 1cl1\oval d
IiCICIlCY
IH
R6
I(l
H4
110°
-:-
rJ)
0"
rt
~
lit Il!S and ('(),
R8
.J:
'-'
0"
~
ç:-
H2
12~
~
RO
10
.'
7R
000
Figure 5.4
•
002
o 114
() Of!
(J
OH
() If)
{J
H
12
Eftèct ot carbon dloxlde mlet concentration on the rcmoval efliucncy (lt " 1S and CO ..
'i7
•
XX
0
0
r---------------------------------------------~
JI,S
('0,
IX
X"
16
H4
14~
H2
12~
HO
10
-.:.
..
'fJ
7X
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
~
~
~
0"
-
~
____________________________ 8
,1
X
10
12
G'. /Il/mm
"Igllre 5.5
•
Hol
-----------~
-.:.
r./l
"
:I:
?:=
velllclt)' on the removal efticiency of H2S and CO2
-
X6
,0
~uJlerfïclal
r---------------------------------------------,IX
II,S
CO,
X8
0'
EfTect of ga~
16
1ol~
.....-
HZ
10
HO
Il ~---------------11~2-------(-)2-5-------(-).-1-------()~3~s------~()1
1
Il 15
7H
L·. III/mm
"Ig"r#! J.6
l'nect of hqUld sUJll!rtictal veloclty on the removal efficiency of H2S and CO 2
•
58
•
5.2.2 OVERALL MASS TRANSFER COEFFICIENTS
The overall mass translcr cocflicicnl of hydrogcn sullidc and cal bon dlll)o.ldc ha"c
been evaluated at the operating conditions of all cxpcflmcnls shO\\ Il in l'ahlcs ='.1. 5.2
and 5.3. i.e.
9l
KG =----=-(l' A· h· ô.P 1.\/
(5.2)
where:
KG: overall mass transtcr cocfticicnt. mole/lI/lIl /111~/atll/
•
a:
specifie area of the packing matcriaI. 98 1 II/~/m\.
A:
eross seetional area orthe bed. m~.
Il:
height of the packed bed.
ni
The absorption rates of hydrogcn slilIidc ami carhon dioXllh:. ~H (II/oldmm ). arc
given by Equation 5.3:
G·/', (
)
9l=--lxy_y
RT
n
(5.1 )
1
where:
G: gas flow rate. 'Imm.
PI: total operating pressure. atm
R : gas constant. 0.082057 (/ atm)/(mole K).
T: operatmg tcmperaturc. K
Yo and
are the inlct in nul let 11 2S and ('° 2 mole fractIOn
respecti vcl y.
•
YI
The logarithmic mean pressure diffcrencc. Ill' 1\/
•
III
the
gU!l
~tream
was cakulated l'rom I·.quation
5.4 where the equilibrillm partial pressures of hydrogen 'iullide and carholJ dioxide anove
•
the carbonate solution. p' (/lJS) and p' (CO~), were estimated by means of Equations 2.8
and 2.9 respcctivcly.
(5.4)
whcrc:
Ji" and Ji
1
are the inlct and outlet partial pressures respectively and the y are given
as P = y.p,.
p' is thc cquilibrium partial pressure above the carbonate solution .
•
The cftect of the operating parameters on the overall mas transter coefficient, KG,
of both Il:!S and CO:! have been cvaluated using the same averaging technique followed
in the cvaluation ofthcir effect on the removal efficiencies.
The results of this study arc shown on Figure 5.7 through Figure 5.12. Those
ligures show that the influcnce of the operating parameters on the absorption of H2 S and
CO:! \Vere in good agrcemcnt with the results reported by Garner el al. [9].
Ilowcvcr, sincc both the overall mass transtèr coefficient. and the removal
cfficiency. 'l, arc functions of the absorption rate. the effect of ail parameters on the
oyerall mass transfer coefficients nf hydrogen sulfide and carbon dioxide has the same
trend as the crleet on their removal efticicncles. Moreover. it was found that KdH2S) is
one order of magnitude higher than KdCO~).
•
In terms of magnitudes. KG(H~S) values
l,btained l'rom this study \Vere much higher than those reported by (Jarner [9] for wetted
\\ ail columns .
60
•
o22 1""- - - - - -_ _ _ _ _ _ _ _ _
-
1LS
(120
1
~
_ _ _ _ _ _., 0016
CO,
(l011 :::
"5
...
Oll!~
tlOI:!~
-:",
016
".:::
~-----------1 () tl \tl ~:::;:,
d'
,,'
.... .'
0,12
() OON '"
~
. . .----------------...I
,-=----~_._JO
20
Figure 5.7
•
u
,,'
40
50
rcmpcraturc.
0('
(,0
7()
00116
Effeet of temperaturc on the ovcrall ma.,., tramler coefliclCnh of Il!S ,lI1d CO!
0,22 , . . . . . - - - - - -_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _-,Olllll
S
020
.
~
-::::
....
rI-
-...
OIS
-----!!----I Il Il 12
"
=,'"
.'9
016
••
P'"
o010
A
00N
:I:
'--'
~o
::
~
....::
::
...
~
0'
U
014
OOON .~
0,12 ~-----------....- - - - - - - -......- - -.....
2
.•
(,
0
1)
<
1COlI.
Figure 5.8
00(,
mflle /1
Effeet of carbonate concentratIOn on the nver-all ma.,., tran.,lcr cocflicienh 01 IllS and
CO 2
•
61
•
() 22 , . . . . - - - - - - - - - - - - - - - - - - - - - - - , 0 0 1 6
-::::
() 20
0014.§
~
...
::::
'!...
,l::!
. __----.....---1 0 OIL~-
-
() II!
"'::-
::::
~::::
-.""
o010 ~
0.16
,-.,
.-!
0'
U
rfl
:é'
-'
OOO!!~
'<..:- () 14
o 12
0006
() 012
L..._ _ _......_ _ _......_ _ _....._ _ _ _....._ _......_ _ _....
()
1/ 002
() 004
0 006
() OO!!
() 0 1
Y,,(H~S)
/<ïgllre 5.9
EITect of hydrogen
~ullide
inlet concentration on the over-allmass transfer coefficients
of H,S and CO,
-
•
0.22
:;::
-
~--------------------------------~OOI6
02
0.014 :;::
§
~
:::s
-o
::::
"!...
::::
....::
II!
::
() 16
0
•
.' .....
...
,...:;,
r;:;
:é'
'-"
'i<.:J () 1·\
0'
U
0008~
() 12 - - -___~------__- - -___- - - - - -.... 0006
0
002
0 04
() 06
() 08
() 1
() 12
Figllre 5. /(J EITcct of carbon dioxide mlet concentration on the over-all mass transfer coefficients of
Il,S
. and CO,-
•
62
--
•
,
-....
E:...
-
----------------------------.
030
li 010
025
Il OO! ~ :::
...
~
~
~
o~,'"
020
\:)
o 15
~
~
--
Il o.'!o
::::
~
::.::'"'
d'
u
0,10
0010'0
:.::
.'
"
4
6
(i'.
Figure 5. / /
X
III/mm
Effect gas superticml veloe Ity on the
() 22
-
"
021
10
ovcr-alll11a,,>~
12
110115
Inlll,,>lcr CllcllÏl:ICIII"> 01 Il ,S .m<l l '( ),
001"
1I 1S
~
--...
:::
...:.
005
•
~
Il Il l 'Ï
r/J
r'
::r:
'-'
~
r'....
CO,
0011
§
':l
0,20
0012
§
019
0011
-"
,'"
N
r.:
,~
.....
~
~
::::
,-..
r/J
:t'
'-'
::.::~
-
l""
018
",
017
",
0,01 (),.-;,
0
",
....
-
•
-
016
01
,..
() 15
u- '
o ()()9:C:
';J
OOOK
02
Il 25
Il 1
Il 1-;
(J,I
1. '. mlf/un
Figure 5./2 Effect hquld .,upcrlieml vcloclty on thc ovcr-all ma.,., lIan.,lcr LOclfkicnl., 01 1I 1 S and
CO 2
•
63
•
5.2.3 SELECTIVITY OF THE PROCESS
ln ordcr to reduce the total cost of the process, and for the purpose of producing
green liquor, it is desircd to maximize the removal of hydrogen sulfide and to minimize
that of carbon dioxide, thus improving the efticiency of the absorption process which
il1volvcs the translcr of both gases simultaneously to the liquid phase.
Selectivity of
carbonate lèJr hydrogen sultide absorption may be defined as the tendency for the ratio of
hydrogcn sullide to carbon dioxidc contents in the Iiquid phase to be larger than it is in
the gas phase. As a measure of this. the so-called sclectivity factor. S. \Vas introduced by
(il1rner el
al
191.
The seIectivity factor for hydrogen sultide is defined as the ratio of the
overall mass transler coefficient of hydrogen sultide to that of carbon dioxide. i.e.
•
S= K(j(H 2S)
(5.2)
Kc;{CO)
The higher the selectivity tactor the more efficient is the absorption process. In
other words. to absorb hydrogen sulfide selectively in the presence of carbon dioxide a
high value of the sclectivity lactor
calculatcd, and it was found
by Garner el a/
191
10
IS
needed.
For each run, the selectivity factor was
be bctwcen 13 and 20 which
IS
higher than those reported
or those reported by Ramac/umdran et al. [12l The intluence of
cach paramcter on the selectivity factor was evaluated by the same averaging technique
describcd earlicr. and they ure shown in Figure 5 13 through Figure 5.18.
Figure 5.13 shows that if the tempe rature
IS
increased. the selectivity factor
dccreases because the rate of carbon dioxide absorption increases while the absorption
•
rate of hydrogen sultide decrcascs. This indicates that in order to improve the selectivity
of hydrogcn sultide absorption lower operating temperatures are required.
64
•
From Figure 5.14. it is clear that the s~kctJ, it~ t'actor IIlCleaSl'S \\ ith inca.'asing
the concentration of carbonate in the absorptton solution. 1his nccurs hecause the ratl' ot'
absorption of H:!S II1creases hy increasing the concentratIOn nt' Ct)l
\\'hich can he
attributed to the increases in the reaction rate b~t\\een cal ho na te ions ,md II,S and dll~ to
the decrease in the absorption rate of carbon dioxide as discusscd belole
Increasing the concentration of Il:!S at the ml et or the colull1n causes Ils rate or
absorption to increase, while it does not affect consldcrahly the absol ptlon late of ('( ):!.
Thus. the selectivity tàctor increases signilicuntly as Illustrated in Figure 5.15. Similarly,
the rate of absorption of CO ~ ~ltghtly increascs hy II1crcasing Its concentratloll at tl1l' miel.
Thus. it results in a
decrcast~
this behavior suggests that
in the sdcctivity Illctor as shown
111
111
l'Îgule .;; 16
1 hClcfore,
ordcr to improvc the sclcctlvity of the plOCl.'SS. high partial
pressure of H 2S is preferable which can be obtained by operatl11g the coluI1111 al highcl
•
total pressures .
Because the absorption of hydrogen sultide is controlled hy gas side mass translcr
resistance. and that of carbon dioxide is controlled by the Irquid "Ide mass transfcr
resistance. the selectivity factor appears to increase If the gas vcloclly
nlso it decreases \Vith increasing the liquit: vclocity, as ~h()wn
1/1
1"
increased, and
l'Îgure 'i 17 and l'Îgulc
5.18. Thus. it is recommcndcd to operatc the ubsorption column at high gus/liquld latlos
in order to improve the selectivrty for hydrogen sul/ide absorptIOn .
•
•
2R~
______--__________________________
x~
20
~
________________
~
____________________________________________~
:;0
.tu
50
Temperature. 0('
hO
70
I-ïgure J./3 Effect of temperature on the selecuvity factor
•
2Xr---------------------------------______________________
20
--•
16
-
~
•
L!
N~______________________________________________~
o
2:;
4
:;
(,
[C03], mu/ell
Figure J./4 Effec! of carbonate concentration on the selectivity factor
•
66
•
28r-----------------------------------__________________
~
20
16
•
•
12
1101
() 01.'
000 ...
()OOl!
8~--------------------------------------------~
o
On02
IIOOH
J',,\II~S)
Fig"re 5./5 Erreet of hydrogcn !>ullidc J11let concentr,ltloll
•
011
the \CleLlIVIt) 1,ILlm
28~----------------------------------------------------~
24
20
.
16
12
8L-__________________________________________~
o
(J
02
(J
04
0 06
Il Ol!
Fig"re 5./6 Effeet of carbon dloxidc mlet tollCentratlOn on the
•
Il 12
Il 1
~electlvlty
f,lLtor
•
22
20
IX
~
16
14
12
.t
G'.
12
10
Il
h
111/111/11
/''i1:IIre 5./7 EfTect gas superticml velocity on the sclectlvtty factor
•
2Xr------------------------------------------------------,
20
16
12
() 15
02
025
03
() 35
04
L'. III/mm
H1:lIre J.18 Hfect hquid superticial veloclt)' on the selectivity làctor
•
68
•
CHAPTER SIX
CONCLUSIONS & RECOMMENDATI()NS
6.1 CONCLUSIONS
Based on the present expenmental work. it can he concluded Ihat a 11Igh rcrnoval
efficiency of hydrogen sulfide accompanicd \Vith a rclativcly low lel11o\'al efliciellcy of
•
carbon dioxide can be achieved by the absorption or gases contallling the"e Iwo species in
an aqueous sodium carbonate solution lIsing a packed collimn operatcd
current mode.
The selectivity for hydrogen slilIidc absorption
IS
III
Ihe cOllntcr-
IlIcreased IInder the
following operating conditions:
J. Low operatmg temperatures, to minimize the absorption rate 01 carbon dioxide, by
reducing its reaction rate \Vith carbonate. and to maximi/.e Ihe ah"orption rate of
hydrogen sulfidc.
2. High concentration of sodium carbonate in the lilJuid stream to reôtlce the
absorption rate of carbon dioxide and to increase the ahsorption rate of hydrogell
sulfide by crcating a higher driving force hetween the two pha~e"
3. High total pressure to increasc the partial pre~sure of hydrogen ... ullide hecall~e ils
absorption rate increases by increasing il<.; partial pres ... ure. whrk the ah"orptroll
rate of carbon dioxide incrcases slightly hy illcrcasrng it~ parual prc~sure.
•
4. l-ligh gas flow rate to incrca~,
he ab~orptlon mte of hydrogcn ... ullide hecause its
mass transter rate is gas side controllcd.
•
5. Low liquid Ilow rate to reduce the absorption rate of carbon dioxidc since it is
controlled by the liqUid side rcsistance.
Green liquor b a solution in which the concentration of sodium carbonate is about
2 mo/el/ and the concentration of sodium sultide is about 1 mo/ell. According to the
conditions of the expenrnents in this work. even if the removal cfficiency of hydrogen
')ul/ide is about 100%. the maximum concentration of sodium bisulfide that can be
produccd (atthe highe~t G/L ratio) is about 4.6xlO· 2 M. Which I11cans that green liquor
can not be produced
lI~ing
the present experimental set-up.
6.2 RECOMMENDA TIONS FOR FUTURE WORK
•
Since we are concerned \Vith the removal of hydrogen sulfide. it is recommended
that the column hcight should be JI1creased.
A theoretical extrapolation of the results
ohtailled l'rom this study indicates that tf'the column hcight is increased by 50 cm (total of
2.0
m)
the rcmoval ellieiency could he increased up to about 98.5 % at G=50 /Imm., L=
0.5 //ml11 • J' (l1~S )=2.0 (Yc, and a carbonate concentration of 2.0 M. under these conditions
•
Il
_
thc concentratIon of bisultide ion~ (HS') \Vould he around 0.1 mo/el 1.
Moreover, to
rllrther II1Clca!:JC the concentration of IlS' il is neccssary to mcrease the GIL ratio. This
ean he donc hy gOIl1g to a triekle hed mode of operation by further decreasing the Iiquid
Ilow rate and/or hy II1crcasmg the SILe of packing which nllows highcr gas tlow rates.
Furthcnnore. another Iden to I\1crcase the concentration of bislIltidc ions in the
effluent. is to recycle some or the liquor back to the absorption column in order to utilize
mon: completcly the unrcaeted carbonates. This wIll improve the rcmoval efficiency of
hydrogcn sultide. pro\'lding that the equilibrium conditions between hydrogen sulfide in
the gas phase and blsultidc Ions in liquid phase are not achieved. On the other hand, the
rell1o\'al of carhon dio'\\(.lc will not he signiticantly affccted since its equilibrium
•
~nI1ccntration is rclati\ ely \ cr~ 11Igh comparcd to that of hydrogen sulfide .
70
•
REFERENCES
1. Avedesian. M. M .. Kubes. G. J . and \'an 1Icmingen. A. R. Il. "!>e\'c/opf11t'11I 0/1111
Alternalive Kraft Black Liquor ReL'O\'e,:v l'ro(.'e.\·\ l/a.\('eI 011 1 ou'- l',,ml,el'alw'('
Processlll~ in FllIldi=ed-Beds. .1 l'I'OIJO.\1I1 10 1he 1-.'\'('('III1\'C ('ommlI1L'e loI' lire
InternatIOnal Ene,.~y AgenC.'y's Impleme11l111g 011 Fllcrgy ('011.\'-" mliOl11l11!rc /'111" l1/1d
Paper InduSI1:l'''. Dept. of Chem. Eng .. Ivtcl Jilll 1ni\'erslty. 1\ 1ontrcal. ('anada. 1()lN.
2. Kohl, A. and Reinscnfeld. F.. "Uas PunjiCal/ml". 2nd cd .. (Iull' Puhlishing. Ilouston.
1974.
3. Ouwerkerk. C., " De,\'lg1l jol' Se/eell\'e "'2.\' ,/hM"."IWII". Ilydrncarhnn PlOœssin~.
89-94, April. 1978.
4. Versteeg, G. F. and van Swaaij. W. P. M .. " ,1h.\orIJlltIIl 0/ ('()~ and Il'2.S /lllllflU'O/l.\'
a/kano/amine sol III/ons lIsmg a fixe" hed relll.:lol' Il'lI1t (O-CI/n ('1/1 dI/lin/loI\' 0IJ('rlll/lJll
in the pu/se /low f'e~ime". Chemical Engl\1ccring and Proccssing. 240). }(ll-17(,.
1988.
•
5. Astarita. G. and Gioia, F ..
19.963-971, 1964 .
1t
1~vdrogen ,""lIlfide ( 'hemICll/ .. 1h,\'()/'l1 I/IJ/I " • ('he. Eng. Sc ..
6. Astarita. G. and Gioia. F., " Simliitaneous Ah.\Orpl/OI/ of III'drogeli Sill/icie mlll
Carbon Dioxide in Aqueolis I~vdrox/(Ie Solu/ions". I&E(' Fundamcntals. 4( 1). j 17320. 1965.
7. Onda. K .. Takenchi. IL Kuboyasi. T. and Yokola. K.. Il Simlllllll11!OIlS Ahsorp(lIm 01
Hydro~en Sul/ide and Carh(}n Dioxule ill //(/I/eOIlS Sod/llm f fI,d/'(/x/({c So/IIlW1/S".
Jour. ofChem. Eng. of Japan. 5( 1),27-33. 1972
8. Oloman, c.. Murray. F. E. and Risk. J. IL Tite Se/eclll'e Ah.\'ol'/J/101l 01 IIvdrogl'11
Su/fide From StackGases". Pulp & Paper Mag .. 5. 69-74. Dcc. 11)(,9
Il
9. Garner. F. 1-1.. Long, R. and Pcnnc11. A .... '/11e Se/ectl1'e Ah.\'(JIIJ/1011 0/ 1/\'(lrogl'l1
Sulfide 111 Carhonale S'OIWIOI1S ", Jour. Appl. Chl:m .. 325-)3(), May. 1l)5X.
10. 8endall. E.. Aikcn, R.
c.. ami Manda~. l,
Quantilies of C02 and react/On 111
. " Selec/lve /lh,WJrpllOl1 01 "].\' lrom I.urger
FmI.' ....iJf'(l.v\". AIChF Journal. 29( 1), M)-n. 1()'n.
II. Hohlfeld. R. W .. Selective /lh\'Orpl/011 of I/~,r..,' fmn7 Sour (;m" r()ur. Pet. 1 œh ..
32(6), 1083-1089. 1980 .
Il
•
12. Ramachandran. P. A. and Sharma. M. M.. " SlIlluitaneOIl,\' Ah.\·of'/JI/OI1 of 'f'wo (ia.\e.l'".
Trans. Insl. Chcm. Eng., 49.253-280. 1971
71
•
13. (iioia, F., " .<"'e/ecllve Ahsorption of H2'<'" in PotassIUm Carbonate-bicarbonate Bliffèr
,\'olutu)11s ".Chimica Ind. Milano. 49. 1287-1298. 1967.
14. Cirilo, R. and Mclchor. !\., "Carhon Dio:ade and I~vdmgen Sullide Absorption hy
Prolnoted Potas.Hum Carhonate SolutIOns". Revista Technica INTEVEP, 8(2),
129-133, 19RH.
15. Doraiswamy, L. K. and Sharma M. M ..
Wiley & Sons. New York. 1984.
"Hetero~eneous
ReactIOns", vol. 2. John
16. Mai, K. L. and Babb !\. L.. "Vapor-Liqllld Eqllilibria hy Radioactive Tracer
Techniques .\:vstem Carhon Dioxide-Hydrogen Sulfide-Sodium Carbonate-Sodium
lJiclirhonate-,\'odi /lm ,c;,'ulfide- Water". Ind. Ellg. Chcm .. 47(1749-1 757),1955.
17. Latimcr. W. M. and lIildebrand. 1. IL "Reference Book of Or~a11lC Chelnl.\·I1/" 3rd
ed.,'lhe Macmillan Company. NcwYork. 1965.
1H. Perry. ''l'ef'l)<'' 01(.'l11lcal Engll1eering Handbook". 5th ed., McGraw Hill Book Co.,
New York, 1985.
•
19. Treybal. R.. E.. "Alass Tran.\/er Operations", 3rd ed .. McGraw Hill Book Co., New
York, ,1981.
20. Shah. Y. T .. Sticgcl. G. .1. and Sharrna, M. M.,
Rel/ctors", AIChE Journal. 24(3),369-400.1978.
Il
Back mlxing in Gas-Liquid
2\. Brenner. II.. "7ïle dlflusion mm/el of longitudinal mixmg m hells of fimte length
Numem:al l'ailles". L'hem. Eng. Sc .. 17. 229-243.1987.
r.. Pc Iton. A. D.. and BaIes. C. W.. " Factlily loI' the Analysis of
Chemu:al ThernwJynamics - GlIIde to Opera/IOn". McGill University, MTL., May,
22. Thompson. W.
1985.
23. Taguchi. G. and Konishi. S.. " TA GUCHI METI-IODS: Orthogonal Arrays and Linear
Uraphs ". Arncrican Supplier Institute. Michigan. 1987.
•
T2
APPENDIXA
DESIGN OF THE ABSORPTION COLUMN
A preliminary desIgn procedure was followed in order tn estimate the sllitahle
height of packing that should be used to removc 95 °11) of hydrogcn sullidc h(lm
li
gas
mixture of 1.5 % H 2 S and 15 % CO:! in nitrogen. The optimum gas and liquid sllPcrlicial
velocities were estimated.
The design was carried out under the following assumptions:
1. The gas flow rate does not change significantly betwecn the intet
and the oudet since the amount \lI' gas absorbcd is ncgligihtc
compared to the amount of nitrogcn.
2. Isothermal operation.
3. The density of gas and tiquid strcams is constant.
4. Constant pressure ( 1 alm.).
5. Only 75 % of the initial amount of carbonate in the liqllid stream is
consumed during the absorption of hydrogen sullide.
A.t THEORY OF H2S ABSORPTION:
The absorption of hydrogcn sultide occurs according ln the lollowmg two
reactÏons:
(A.I)
73
(A.2)
Reaction A.I IS considered to b\! the controlling reaction in this process since
hydrogcn sul/ide is ahsorhed mostly by the reaction with carbonate ions in the Iiquid
phase. Since this reaction is considered to be instantaneous and it is gas side controlled,
the ahsorption of hydrogen sulfide takes place at the gas-liquid interface which means
that the equilibrium partial pressure of H2S is zero. Thus. the rate expression for the
absorption can be wriUen as f()lIows:
(A.3)
and the height of the packed bed may bl.! writtcn as:
G'~!11=-
M
K.C, ·u· P,1
l',
ft/y
\'"
(A.4)
y
where:
li:
packing average specitie area for mass transfer, m2/m\
G':
Il:
superticial gas velocity,
height of packing. m
K(J:
overallmass transt'er coetlicient in the
l'II~~,
l',:
partial pressure of 11 2S in the gas bulk. atm,
total pressure in the column, alm
~H
rate of dhsorption of H2S. mole/min.
:
1l1OIe/m~/min.
ga~
phase. mole/m2flmin.
mole fraction of Il,S in the gas bulk.
Y, andY~:ll1ole fraction of 11 2S in the gas phase at the inlet and at the outlet of the
YII ..,:
column respccti"c\y.
74
A.2 COMPOSITION AND PROPERTIES OF STREAMS:
A.2.1 LIQUID STREAM:
The liquid stream cnters the colllmn at atmosphcric pressure and
~5 O( '.
and il has
a concentration of 10 % 11'/" as Na::!C0 1 which corresponds to a Illolarily (Ill) of
0.095 r,wlell. The solution dcnslty (P,). and \ iscosity Ul,) as gi"en h)' Ila""I.:' 1 aIe
1007 kg/m'and 1.15x 10- 1 kg/m/sec. respcctivcly.
A.2.2 GAS STREAM:
The propertics and tl1I' composition of the inlct und the oullet gas streams are gi ven in
Table A.I.
Table A.I Composition and propl'rtleS orthe
ga~
COMPONENT M wt.. ~/m()le
stream at the II1lct and at the out Id 01 the
Uensity.
k~/m'
•
.'
LOhllllll
• 1
\1
"
H2S
34
1.518
0.0150
O.()()O77
CO 2
44
1.964
0.1500
O.1J910
N2
28
1 250
0.8350
0.86020
The average density (Pu) and molccular welght
'"
(fl.-/)
allhe
II1lcl
of Ihe
COIUIlIl1
arc
30.49 g/mole and 1.361 kwm'. respectivcly.
A.3 CALCULA TIONS
A.3.t LIQUID/GAS RATIO
The ratio of Iiquid slIpcrficial vclocity to that 01 the gas (L'/(i').
15
calculated hy
means of Equation A.5 as:
L' L __ Yn-Y
_=_=
G'
G
075
M
X
p,
M
(A.5)
7')
!:.:.. = L = __1 _
G'
G
x 0.01500-1) 00077 x J.,907
0.75
0.095
30 49
= 6 596
A.3.2 (;AS AND LIQUID FLOW RATES:
lo determine the supcrficial gas vc10city (G 1 ), figure A.l is used. First, the term
[( I ..
r
/G)(p",/Cr, - pg) 5] whieh corresponds to the x-axis was calculated. and under the
abovc conditions it has a value of (0243) , and then the corresponding y-axis value
[G,2 C, ~l/J/(rH/(p,- rJ)~.]
was determined at 50 % of the tlooding line. which was
'()Und to be equivalcnt to (0.02). Then, the optimum superficial gas velocity, G', is
calculated, i.e.
(A.6)
G=
Taking into aeeount that J=I. ~(=l and the packing factor (Cf) for 1/4" Inta/ox
.')'udlile.\' is 725, G' was found to he 0.2745 kg/m 2/.\'ec. which is equivalent to a total
volumetrie flow rate of 55.18 //m;l1. irthe diameler of the column is assumed to he 3".
At a ratio of 6.596 the superticial liquid velocity is ea1culated l'rom Equation A.5
and \Vas found to he 1.838 kg/m 21.\'t!c. which is equivalent to a totalliquid volumetrie flow
rate
l)f 0.50
//mi".
76
A.3.3 ESTIMATION OFTHE BED HEIGHT:
To evaluatc the requircd bed hcight, Equation 1\.4 was solved ta~ing
\1110
account
nw/eIl1l 2/mi1l lalm.
as gÎ\'cll
that a=981, PI = 1 atm., Pg= 1.36\ kg/m'. M anJ K(j=7 .26x Hr'
by Garner[-I] and then, integrate fromY,,=O.O\50 to)\==0.00077. The value ol'heighl 01'
the packed bed, Il, was found to be 1,48 fil.
77
o.•
~~J
r.o;r-.
.J-
010
0.01
006
.
~
l'-- ro.... ~
~
001
0001
0.006
-
~o
-
~
"
p..,. t"-
r-...
1"'-
-....
~
~
.'"," l'-L'- t\
r"'!o.
"
"""",
--
NI.'
III
...
~
OO!
004
01
0 :!
o.
10
~
~
~
'" l'
000 2
000 1
001
•
fi
GII ,rnau~ 1110'
000t
III
.......
1 .......
~
ri
i"HzG • 1 22 •• 10" NI.'
........
,......
""'"
Z
-"
~
002
i,)-
.!! '~/rt'. 6 J 7 • ur' N/ra'
'"
l'-,..
,~
~
:
../
"_"'1
......
~OO
00.
• l,:
~... t
' ,
'Ap,,",I,",II.
,..,..~
'"
~ l~
~
'"
2
"
1\ 1"
r\. ~~
"",1'1'
•
10
!:. (2L)"1
Ci'
"-"
fïgllre .'l.I Flnodmg and pressure drop in random packcd columns[l9]
78
APPENDIX B
AXIAL DISPERSION PLUG FLOW MODEL
This model describes the tlow of a non reactlve component thmug,h a hcd or incll
packing material wherc the effccts of aB mcchamsms th ut contrihutc!'> tn the axial mi.xl\1g
ure lumped together into a !>ingle clTcctive dispersion coefficient (0,,). Il
\\'a~
tkvolped by
Brenner[J 2] in 1962. And it \Vas uscd in order to obtaill the l'eclct Ilumbcr values that lit
the experimental residence time distributions
111
the ga!> phase in the uhsorplion colull1ll.
The transport equation which is governing this modd ts:
oc
at
oc
ax
El c
-+U·-=Dd - ,
8x'
( B.2)
where c: is the concentration of the non-reactive component. , is the tin1l': ..\
along the direction of the f10w and U is the supcrtieial vclocity
or the Iluid
IS
distance.:
throllgh the
bed. In a dimensionless form, Equation B.2 may be written as:
ac ac
1
a2 C
- + - = - - -2
Be ax Pe a.x
(B.2)
Here. distances arc rendered dimentionless with respcct tn the hcght of the packed
bed (h), the concentration by the Înlet concentration (c,,) and the tl/ne hy the average
residcnce time of the tluid in the bed (0,,) while Pc is the P(,!Clet nllm ber whu,;h l~ defincd
by means of Equation B.3:
79
hU
Pe=-
(8.3)
0"
The initial and boundary conditions which reprcsent a step change in the feed
concentration arc:
at 0=0
('=0
for 0::; X sI
(B.4)
at .,Y = 0
('=1+_1 Be
Pe BX
lor
e> 0
(B.5)
at .\'=1
-=0
ae
ax
for
e> 0
(B.6)
Equations B.5 and B.6 give the Dankwerts boundery conditions that arc written
gcnerally for packcd beds.
The analytical solution for this model was obtained by B/'enner[l2]. The
symptotic solution at large values of Pc was expressed in terms of the dimensionless
lime and Pcclct numbcr as shown below.
0
P) 1(1-0)1+-1 (e
- - )5exp [P(1- )2] X\I/('t) (B.7)
1
[( C,,=I--erjc
2
0
0+ 1
~1tP
0
whcrc 1" and 'l' arc dctincd by the following two equations
()
"C=--
(8.8)
0+1
80
'It(-t ) = ( 1-6t+4t"') - t(l-t)( \-lth+24t-') t ... +
2P
(B.9)
... + ( -1 )J.[t(l-t)]J.
P
x (1)( t )
where k is a summntion index and P is equivalenl to (Pel 4 }and the rUl1cliol1 (Il( t )) IS
given by Equation B. 10
<D(-r) == 1.3 ... (2 k+ \)(_1- - 6t: + 4(k+ 1)-r2 )
2k+1
(1l.IO)
The values of Peclct numbcr for an experimel1lal breakthrollgh 01 LI 1l0n-IC'H:tIVC
component can be obtained by comparing the cxpcrimcntal RTf) with li 1111111crical
solution of Equation B. 7
XI
APPENDIX C
DATA AND CALCULATIONS
The following table shows the opcratmg conditions
or
each
l'lill
in the tinal
experimental part ofthis study. And it also shows the conccntrations of hydrogcll slIltilk
and carbon dioxide mcasured at the outlet of the columll.
Ih: Icmoval crticlcncics \lf
both gases were calculated in aecordance to the !()llowlIlg equations.
11 = P
Il -
[> 1 X
100 %
Pli
(<.'.1 )
where:
Po and PI are the inlet and the outlet partial pressures of the absOl hcd gas and
they are obtained by means of Equation 4.2
P=y'[>1
({' 2)
where:
PI is the total pressure, 1.05 alm.
H2
Table ( . J ()pcratllll.! Londltlon and rcmoval ct ficicnCiC'i of hydrogen sulfide and carbon dJoxlde
c,<pcrlllIcnt.II work
NO.
r
"(
.
IC0 3 1 Yjll~~)
JI (CO.,)
"
-
G
L
24
o 4X7
(l
B.2 24
2.019
B.3 24
(J009~
the tinal
YI(II 2S) )'I(C0 2 ) 11(H 2S) 11 (C0 2)
//mm 1/111/11
mo/eli
111
/!/
//)
%
() 0101
20.19
1.02 0.00025
() 0092
74.95
8.68
() 00400
0.0399
29.84
1.03 0.00067
0.0366
83.15
7.96
1 479
000692
(J.()702
39.82
098 0.00074
0.0648
89.35
7.61
BA 24
5098
O.OIO()X
01008
50.46
0.99 0.00080
0.0929
92.04
7.75
B.5 36
o 4X7
() 004(15
() 0699
50.03
1.02 0.00074
0.0612
81.64
12.26
136 16
2.019
() 00103
(J.IOI3
4026
1 03 0.00021
00883
79.81
11.67
)C)
3479
001021
() 0099
30.09
0.98 0.00129
0.0090
87.32
8.61
B.X 36
5.098
o (JO'/OO
(J 0199
19.87
() 99 0.00105
00362
84.9Q
907
B.9 48
04";7
() 00709
() 0990
30.36
1.02 0.00150
0.0851
78.84
14.02
B.IO 48
2.019
0.00992
0.0697
20.33
1.03 000187
0.0612
81.15
12.20
B Il 48
3.479
o (J0102
() 0403
49.76
0.98 0.00017
00354
83.50
II.90
B.12 48
5.098
() 00392
0.0101
40.19
0.99 0.00054
0.0090
86.18
10.43
B.13 60
0.487
0.01032
0.0405
39.78
1.02 0.00196
0.0348
81.05
13.93
B.14 (,0
2.019
000703
0.0\02
50.29
1.03 0.00092
00089
86.85
Il.95
B 15 60
3.479
o O()W6
() 1009
20.33
0.98 0.00077
0.0885
80.56
12.22
B 16 60
.5 098
(),()O097
0.0703
30.11
099 0.00020
00616
79.70
12.30
42
2.765
(J.00559
0.0554
2019
1.04 0.00109
00495
80.41
10.54
(',2 42
2765
() 00541
00552
29.92
1.04 0.00096
00493
82.25
10.72
CJ
42
2765
0.00553
0.0547
39.89
104 0.00088
00487
84.08
10.91
C.4 42
2.765
0.00561
00549
5016
1.04 0.00079
0.0487
85.92
Il 09
(',5 42
2.7(15
() 0052-+
() 0549
'14.89
0.99 0.00089
0.0490
83.06
10.71
D.I 42
.2 759
0.00556
0.0549
3489
0.52 0.00096
0.0497
82.71
9.35
)).2 42
2.759
0.00556
0.0549
34.89
084 0.00094
0.0492
83.01
10.33
lL\ 42
.2 759
0.00556
o0549
34.89
1.\8 0.00093
0.0487
83.32
Il.31
D,4 42
2.759
000556
0.0549
34.89
1.61
0.00091
0.0481
83.61
12.28
·~2
2.75C)
000556
0.0555
35.53
1.09 0.00093
0.0494
83.26
10.93
B.I
B7
CI
1).5
83
The concentration of HS-. IICO, - and COl
each mn are given in Table C 2.
fhe
t Iatio
I11casurcd 111 outle! liquid sllcam lilr
and thc eqllllihriul11 partial pn.'ssurcs
llf
hydrogen sultide and carbon dioxide above the out let liqUld stream \\cre cakulatcd lIsing
Equations C.3. C.4 and C.5 respccti"cly. Data arc listel! in the samc table.
[HCO~]
l=
[ HCO~] + 2 x
(t '.3)
[C0 3]
where:
[ HCO;] and [CO;] arc the I11casured conccntrations or bicarbonate ilnd
carbonate at the bottom of the column.
Z
!
l,62
p·(CO..,)=1.654xIO 1 MNol exp(-2729/Tl
(I-{)
.( s) =9746x10 -4
PH.,
-
125 [I1S'-]
p·(c 0') )MN-[
p ("""75/7')
., lIeo 1_]ex __ _
(('.5 )
where:
p.: the equilibrium partial prcssure ahovc the solution. atm.
MNa:the concentration ofsodium ions ln the solution and il is given as twice the
inlet concentration of carbonate "sted in Table C. L mo/elf.
j:
the fraction of sodium ions as sodium bicarbonate in thc 'iOdlUlll carhonatebicarbonate solution,
T:
Pu' and
the operatmg tcmpcraturc. "K.
~.
114
li/hIe C2 Col1Lcntrallol1 of IOOle
partial
NO.
prc<;\lIn!~ of
(IIS-,
hvdro!!t!n
-
~
\pecle~ ln
~ulfide
(lIeO;,
the eXit IIqUid stream and the cqUlllbrium
and t.arbon dlox Ide
(CO;I
m()lell
f
P'(II,S)
P,,·(CO:.J
atm,
alm,
()
4
/IIo/ell
molell
B,I
000063
(J,O 103
(J
477
(J,OIOR
5 17x 10")
1 91x 1a'('
13,2
0.00415
0.0265
1992
0,0066
2.55x 10'x
4.97x 1a'('
lU
OOlOX2
0,0443
3.435
0.0064
8.1 6x lO'x
9.82x 10'1'
BA
002037
0.0668
~
031
() 0066
I.96x 10,7
1.75xlO,5
13.5
O'()O671
00373
0.450
0,0399
8.79xlO,7
3.85x 10,5
B,6
000133
0.0473
1.91'2
0.0119
2.84x 10'x
2.30x 10"
B.7
001133
Il 0481
] 431
0,0070
1.07x](('
1.65x1O"
B,X
000494
(J,0512
5047
00051
2.94xlO'x
IA6x 10"
B,l)
0.00663
0,0421
0.445
00452
1.20xlO'h
6.93x 10"
B.IO
() 00633
00522
1.967
0.0131
I.74xl0,7
3.90x 10,5
B.II
0.00172
00629
3.416
0.0091
2.97xlO'x
3.96x 10. 5
B.12
0.00547
0,069(1
5,028
0.0069
6.39x1O's
3.77x 10'5
B.I]
0.01253
00"173
0.440
0.051 1
3.IOxlO'll
1.21 x 1O'~
B.14
001145
0.0680
1.951
0,01 ï2
5.76x1O,7
9.12x 10,5
B.15
() 00254
00839
3.395
0,0122
8.32xlO,l!
9.65x 10"
III Cl
O,OOOl)O
00978
5000
0,0097
2.22xHr!!
I.02x 1O'~
CI
O.(lO]54
0,0464
2.716
0,0084
458xlO'x
2.12xlfr '
C2
0.00520
0,0504
2712
0,0092
2.5lxl0"
CJ
0.00724
0,0548
2.707
0.0100
7.94xlO'!!
7
1.32xlO·
2.97x 10,5
CA
(),OO944
0,0596
2.702
0.0109
2.04x1O,7
3.53 xl 0,5
C5
000623
0.0524
2706
0,0097
I.05xIO,7
2.77x 10,5
D,I
o (l1253
(),0648
2,694
0011 R
3.21xlO'7
4, 18x 10'5
D.2
0,00779
00556
2703
0,0101
1.46x 10,7
3.06x 10"
DJ
000556
0.0513
2.708
0,0093
8.82xlO'X
2.60x 10"
DA
0,00409
0,0483
2.711
0,0088
5.74xlO'K
2.30x 10'5
D.5
000613
00525
2.712
0,0097
1.02xlO,7
2.72x 10.5
,-
85
From the data given in Table c.l. the absorption rates nI' hydrngen sultide and
carbon dioxidc \Vere calculated for each run
G·p
9t = _
_
1 x (v -.'
RT
. n .1
b~
l11eans
or Fquatilll1 C.6
)
j
C.6)
where:
G: gas now rate. IImil1.
PT: total opcrating pressure. 1 OS atm.
R :gas constant. 0082057 (! atlll)/(mo/e "K).
T: operating temperature. "K
Yo and YI are the inlet in nutlet 'I:;S and
respectively.
CO~
mole tiactwll in the gas
sllCHm
Then, using the data of Table C .2. the logarithmic mean prcssure dllïèl"l:ncc t\ I~,\I
for hydrogen sulfide and carbon dioxide \Vere calculatcd assuming that the equilihlllllll
partial pressures
If, of bath gases ubove the inlct liquld stream 15 Icro. i.e.
(<.'.7)
The overall mass tnmstè:r cocrtlcicnts K c,' lor hydrogen sul/iùe and carhon dio.xide
were caleulate using their absorption rates and thcir logarithmic Illcun rn:ssurc
differences as follows:
~H
KG =----=A. h· I!. P lAI
J
(/.
where:
KG: overall mass transfer coefticicnt. mo/e/min.lm%l1m
l
a: specifie area of the packmg matcnaI. 9RI m'/m •
A: cross scctional arca orthe bcd, 4.56x\{f\ m2•
h: hcight of the packed bcd. 1.50 m
Then, the sclccflvity factor S, \Vas cvaluatcd as the ration of the overall mass
trans/cr coefficients, i .c.
/}'=
K (Il,S)
(,
.
K (, (CO,)
(C.9)
Data ohtaincd from lhcse calculations are tabulated in Table C.3
87
Table CoI Absorption rates and the ovemll ma~s translel coetliclt:llh and tht: .,elecll\ 11\ t.lctllr
No.
~H(H2S)
~
P'1/(I1,S)
K(, (II~S)
~H(C02 )
:\1'/1 ' ( CO: )
K(I(CO~ )
S
!nole/mm
(//11/
II/oi/mm '(//m 'II/~
11/ ni<' 1//1/1
,am
8.1
r--8.2
0.00064
0.0012
0.0756
000076
o 022X
() (}O4 l )
15 Ihh
0.00428
0.0044
O.14J5
n.0040S
0.0901
() OOh 7
~1
B.3
0.01060
0.0065
0.2406
000917
0.1593
() OOX 5
2X Oh!)
B.4
0.02020
0.0087
03441
n.o 1700
() 2284
(l(l110
31 ()X9
8.5
0.00684
0.0046
0.2201
0.01770
() 15-l5
() () \70
12 X77
B.6
0.00137
0.0012
() 1673
001950
o 2220
Il 0130
12 XIX
B.7
0.01110
00102
0.1611
0.00106
() 0222
() 0070
22
8.8
0.004R9
0.0074
(J.097R
0.00298
() 0897
() O()4l)
1() X'I')
8.9
0.00676
0.00R5
o 1179
0.01(;80
() 2 {(lX
(lO115
1() 223
8.10 0.00652
0.0114
0.0848
0.00689
0.1543
(). ()()( )()
12 7·H)
0.00169
0.001 1
0.2241
0.00951
() 0892
o Ol5X
14.125
B.t2 0.00541
0.0040
0.1986
000169
0.0225
() 0 1 11
17
8.13 0.01280
0.0119
0.1595
000863
O.08X7
o () 144
1 1 () JO
B.14 0.01180
00071
02456
000236
o 0225
()O155
157'i7
B.15 0.00249
0.0046
00802
O.Ü0963
0.2232
() ()OM
12A87
B.16 0.00089
0.0011
0.1157
0.01000
0.1555
o 0(1)')
12071
C.l
0.00369
0.0065
o 083R
0.00479
o 1237
o O()57
14 548
C.2
0.00541
0.0061
0.1317
0.00719
() 1212
() OOS7
15.145
C.3
0.00753
0.0060
o 1867
0.00967
O.121!)
(JOlIS
J ') 71)!)
C.4
0.00982
0.0058
0.2503
0.01240
() 1221
()O151
\() 555
C.5
0.00617
0.0058
() 1578
0.00833
o 1225
() 0101
15 'iM
D.l
0.00652
0.0062
() 1560
O.OO72S
0.1234
() 0087
17 7(,0
D.2
0.00654
00061
o 1575
O.OOS04
o 1227
O.O()<J7
1h 150
D.3
0.00656
00061
0.1591
O.OOSSO
o 1221
(J.O 107
14 S20
D.4
0.00659
0.0061
0.1607
0.00956
0.1215
o () 117
11 711
D.5
0.00669
0.0062
0.1616
0.00&77
0.1236
() 0105
15.344
. 8.11
!IIO/IIIIII
a/III 11/
32X
--
74~
-
-~()6
The cf fect of the individual pararneters on the removal efficiency and the overall
I11USS
trans/cr coefficient of hydrogen ~ulfide and carbon dIOXidc. and on the scleCl!vity
fuctor were evaluated l'rom the prcvious tahles. An averaging technique \Vas followed to
ohtain the corrc~ponding values 01 II ' K(, and S at euch level for ail the parameters,
exccpt 111 the cu,>c of the hqUld flow rate. where the results were obtained directly through
the sel of expenrnents. D.
ln the experimental design, cach parameter was repeated at the sarne level only
four tlllles in the wholc sel of cxpenrnents (set B). Theretor. the corresponding value of
II ,K(, and S (at level, 1 and for pararnctcr. A) arc given by the average of the tour values
where
(1\)
is at Icvel (i).
EXilmplc:
The rernoval efficicncy of hydrogen sulfide as a function of its inlet concentration,
may bc obtained l'rom Table CI at tour levels.
at levcl 1 JI
. "'
= (Yom. 1)+Y,,(B.6)+Yo(B.ll )+Y()(B.16))/4
= (0.00098+0.00103+0.00102+0.00097)4
at level 2 Y",2 =(Y,,(B.2)+Y,,(B.5)+YJB.12)+Y,,(B.15))/4
=(0.00400+0,00405+0.00392+0.00396)/4
atlevel3 -J'Il,' ~-::(Y,,(8.3 )+J',,( B,8)+)',,(R.9)+Y,,( 8.14»/4
at levcl4
.)'Il''
= 0.00100
==
0.00398
=(0.00692+0,00700+0,00709+0,00703)/4 = 0.00701
=(Yn~B.4)+J',,(B. 7)+YJR.I O)+Y.,(B, 13»/4
=(0.01008+0,001021 +0.00992+0,0 i OJ2)/4 = 0.01 013
similarly. at Icvcl 1 11,
=(11(13.1 )+1l(B,6)+11(B.l1 )+11(B.16»/4
=( 74.95+ 79.81 +83.50 t 79.70)/4
at Icvel 2 ll"
at Icycl .3
al
=79.49
=-( 11(B.2)+11(R.5)+11(B.12) tl1( 8.15))/4
=( 83 .15+81.64+86.18+80.56)/4
=82.88
'11 =( 11(B.3)+11(B.8)+11(B.9)+1l( 8.14 ))/4
=(89.35+84.99+78.84+86.85)/4
=85.01
Icvel 3 111 =(11(B.4)+'1(B. 7)+11(B.1O)+11( 8.13))/4
=(92.04+87.32+81.15+81.05)/4
=85.39
89
In addition to these four data points. the removal efticiency llfhydwgcn sultidc al
level
III
(medium level)of is obtaincd by taking the average value nt" 1) nhla1l1cd fwm run
no. C.5 and 0.5, i.e.
at levelm 11
=(1)(C.5)+(11 0.5))/2
=(83.06+83.26)/2
Note that if the rest of pararnctcrs (T, COl ~,Y.,(CO:!'., (i and L) \Vcrc al ~.vcragcd
the different tive Icvels of Y,,(H:!S). cach parameter will show a constant valuc,
approximately at its medium levcl.
The above calculations gave the rcmoval cfticiency of hydrogcn sullidc as
LI
function of the inlet concentration of 11 2S at live levcls. Similurly, ail thc data gl\'cn III
Table C.I and those in Table C.3 have avcraged using the same pl Ol:ctlure. And the
rernoval efficiencies and the mass transfcr coefficients as \vell as the seb:tlvlly ladol ail
function of the different operating parameters \Vere obtained. Urcel 01 liquld and gas
flow rates were expressed by means of the superticial velocities
or the gas and the hquid
strcarns, L' and 6. i.e.
L'= LI A
(C.IO)
G'=GI A
where:
Land G are the liquid and gas l10w rates respectively
A is the cross sectional area of the absorption column, 4.56x 10'\
/Il! .
Results of these calculatlOns arc listcd in TabIc C.4 through Tahlc
c.) O. Tahlc C.X
shows the effect of the superficial gas velocity ohtaincd l'rom this cxperimcntal design,
while Table C.9 shows the same crfeet obtatncd l'rom the conventlonal cxpcnlncntal
work. It is obvious that the data
ln
both tablc~ arc almost Idcntical. which mcans tl1at the
experimental design procedure followed in this ')tudy gave accuratc re~ult~
90
0\
Table C4 Errec! of temperature on the remO\ al efficlenc) and the over ail mass transfer coefficient of 11 2S and COl and on the
selectivltv factor
NO.
T
Oc'
ICO~I Y (H,S)
- Y.,(C0 2 )
llOI~S) 11(C02 )
G'
L'
111/111 III
111 1111111
%
%
()
l1Iole 1
K(,(C0 2 )
S
mol, 1111/1 , atm /m~ mol/mm /atm ,'m 2
1
24
2771
0.0055
0.0552
7.69
0.22
84.87
8.00
0.2010
0.0078
23.91
2
36
2.771
00056
0.0549
7.69
0.22
83.4-1
10.40
0.16/6
0.0/05
17.06
.)
"
42
2.762
0.0054
00552
7.72
0.23
83.16
10.82
0.1598
0.0103
15.45
4
-18
2771
0.0055
0.0548
7.71
0.22
82.42
12.14
0.1564
0.0113
13.73
5
60
2.771
0.0056
0.0555
7.70
l 0.22
82.04
12.6C
0.1503
0.0115
12.83
---_.-
- - - - - - - -----
------
-
Table C" Efkct l,f ';.ubon:lle concl'ntrallon on the rell1O\al effïcil'nc) .1I1d the o\cr
and on the selectl\ Il:- factor
~o.
T
C
ICO;I
II/oh'
)' (11,,\) )'(C(),)
"
"
-
i
G'
L'
11/ 11/ /11
1/1 11/ 11/
Tl(H,S) ll( CO,)
n
'u
allll1d~s
transrer coeffïclent of !lèS and CO 2
K(,O I::!S)
K(,(C0 2 )
S
"
12.22
1110 1 111"1 (/11/'
0.1433
0.0120
12.32
111'
11101 111111 alm f;j'
1
-L2
O.4S7
o (lO56
00549
769
U 22
'"
79./2
.,
....f,-
2.lH9
00055
00550
771
023
82.74
10.95
0.1604
0.0105
/5.66
-'
...
42
~.76~
00054
0055:
772
023
83./6
10.82
0./598
0.0/03
/5.45
4
42
3.479
00055
00553
7.67
021
85.18
10.09
0./766
0.0095
19.36
5
42
5.098
0.0055
0.0553
7.71
0.22
85.73
9.89
0.1891
0.0092
20.19
-
e
K(,(1I 2S)
e
!
N
=-
Tahle (,6 f.îf':Cl "f fI,"
- ml.::t .:onc.:ntrJllc'll l'n rh" rè010\31 ètficlèllC; JnJ rh,' c'h'r ,III m,l"
an,j on the
~o.
~elecu\
T
(
i
tr,1n~tè"r
cl'dliclcm
,)fH,~ ,li1J
'
CC),
,
It\ factùr
{CO)
1 y
/111)": 1
11(1--!:"»[
T1(Co:)
G'
L'
III m 1/1
mil/III
" '1
"
(Il, <..,) Y(C(),)'
1\l. (l1,S)
_
mol 1/1111
0)
Li/ll/ 111=
S
1\)1...'0: )
II/(l! IIlIII ,lflll
/1/=
1
-t.2
:2 f7i
0.0010
00552
769
022
i9.49
Il. 14
O.145i
O.OI09
/335
2
42
2 771
0.0040
U 0552
769
022
82.88
10.72
0.160-;
0.0104
16.12
.)
....
42
'2762
0.0054
00552
772
0.23
83.16
/0.82
0.1598
0.0103
/5.45
4
41
2771
0.0070
0.0548
7.69
0.11
85.01
10.66
0.1755
0.0102
18.46
5
42
2771
0.0101
0.0552
7.71
0.22
85.39
10.62
0.1874
0.0098
19.40
1
7able C 7 Effett of CO, mlet conccntratlon on the remo\ul effierenc\ and the
and on the selccliv It\ fac-tor
NO.
T
0\
er ,III
tJl,I~S
Iransfer eoeflierent
01
!l,S and CO,
J
G'
ICO;I Y.,( Il, ~) )'jC():)
"c
II/v/e /
1
42
2.771
00055
0.0101
7.72
Î
42
2.771
00056
0.0401
.)
....
42
2.762
0.0054
4
42
2.771
5
42
2.771
L'
-
l1(H,,,) ll(COJ
K(,{
Il:S)
K (CO,)
(,
-
S
?~
IJ'
/0
lIIo/mlll :allll 1Il~
mo/m/11 atm m~
0.22
83.82
9.92
0.1703
(}.0097
17.87
7.63
0.22
83.17
10.72
0.1563
0.0105
16.57
0.0552
7.72
0.23
83.16
10.82
0.1598
0.0103
15.45
0.0055
0.0700
7.69
0.22
82.96
11.09
0.1653
0.0105
16.44
0.0055
0.1002
7.75
0.22
82.81
11.42
0.1775
0.0105
16.65
m
II/Ill
m
1111/1
i
!
i
e
e
....,
0'
Table CI~ Erfcct of the gas superficwl \cloclt)
011
the rem 0\ al eftïclenc) and the
0\
er ail mass tlansfer coeftïclent of H2S and
CO, and on the selectlvlty factor (duta abtllllled \1'/(11 the etpCI Imentul design)
-
NO.
T
ICO;I
Y (1),';;) Y,,(0)2)
" -
"( ,
mole /
1
42
2771
00055
'1
42
2771
3
42
4
42
5
42
G'
L'
l1(1I~S)
11«('°2)
Kd H 2 S )
K(,(C0 2 )
0/
,ri
mO/'IIll11 a[1II '/11'
mO"'I1//1 atm Ill'
S
1/1 111 11/
/11, 1/1//1
()/
0.0552
4.43
0.22
80.41
10.54
0.0846
0.0058
15.05
00056
00548
6.60
022
82.25
/0.72
0./346
0.0087
/6.59
2762
0.0054
0.0552
7.72
0.23
83.16
10.82
0.1598
0.0103
15.45
2 771
00055
00552
8.77
0.22
84.10
10.91
0.1916
0.OIl8
17.43
2.771
0.0055
0.0553
10.99 0.22
86.01
10.97
0.2585
0.0149
18.46
/u
------
Ji/hie C 9 l:tkct of the gJS ~llperlicJaI \c1uclt;. (Ill the rellll)\al efticllml;' JilL! the mer allma<;<; translà c(ldticlent uf H2~ and
CO, .md l'Il the ~eh:ctl\ Il;' factor (d.i/Li ONd"I"'; 111111 tlll! COIll Cnl/o/1<..,/ l:\r-'rJ/III!I1f<l/ 11 ()/,,)
~o.
e
r
ICO~I
3
C
fIIO/,' /
Y,(II:S)
.r (( (), ) G'
L'
11 (II: ~;
11(CO:)
K(,I Il: S)
S
K(,(l():.)
l
1
0,
,
mol
mol mm
023
"
80.41
10.54
0.0839
0.0058
14.55
!II Ill/II
III 1/1111
~"
III/Il
a(1II
111=
1.1(11/
111=
1
42
2 Î62
o OÜ56
-"
4:2
:2 76:2
() U054
0055:2
6.56
023
82.25
/O.i2
0.JJ/8
0.0087
/5.15
~
-'
42
2
ï«~
00054
Ü
0552
7.ï2
U 23
83.16
10.82
0.1598
0.0103
15.45
4
4~
~
762
00055
o0547
8.75
023
84.08
/0.91
0./867
0.01l8
/5.80
5
42
2.762
0.0056
00549
11.00 023
85.92
11.09
0.2503
0.0151
16.56
l)
0554
4.43
e
e
~
=-
Table C8 EffLCt of the gas '-.Upèl fÏclal \ èloCIl~ (ln the remo\ a!
èrticlenc~
and the
L'\
er .111 111.155 transfer coeftïClènt
l)f
Il,~
CO, and on the 'ielcctl\ It~ factor
-
~O.
T
ICO;I
.r(ll/; ) J' ,( ( ():)
nc
mole 1
1
42
:. 759
() ()O56
2
..,
42
-'
-+
5
G'
L'
Tl(H:~)
n,
11(COJ
0
mn/III/Il
,11111
s
K t ,( C( 2 )
K[,(II:S)
m'
mO/1II111 ,UII1
1
/r/ mlll
00549
765-
0./1
82.71
9.35
0.1560
0.0088
1 Î. -r6
2,759
o 0056 o 0549
7,65
0.18
83.01
10.33
0.1576
0.0098
/6./5
42
2 762
00054
o0552
7.72
(J.23
83.16
10.82
0.1598
0.0/03
/i45
42
2759
<L0056
0.0549
7.65
0.26
83.32
Il.31
0.1591
0.0107
14.82
42
2.759
0.0056 0.0549
7.65
0.35
83.61
12.28
0.1607
0.0117
13.71
"
e
<)
1
1
11/ III 1Il
....
e
.111..1
-
/1/'
,