Indian Journal of Chemical Technology
Vol. 7, September 2000, pp. 242-249
Low pressure equilibrium between H2S and C0 2 over aqueous alkanolamine
solution
!VI V Jagushte & V V Mahajani*
Department of Chemical Technology, University of Mumbai, Matunga, Mumb ai 400 019, Indi a
Received 25 October / 999; accepted 15 May 2000
The simultaneous absorpti o n of H 2S and C0 2 in aqueous alkanolamine sol uti on is of considerable commerci al importance. The reversible nature of reactions always results in eq uilibrium parti al pressure o f C0 2 and H 2S over amine sol ut ion.
A si mple method is presented to determine the equilibrium parti al pressure havi ng very low loadi ng of H 2S and C0 2 with
th e select-ion electrode. The entire equilibrium is divided into two parts, namely first equ ilibrium and the final equilibrium.
A (H2S + C0 2)- aqueous alkanol amin e system is hi ghl y no n-idea l. Therefore, an attempt has been made to correlate data by
usin g ' final equilib rium ' expression based o n idea l behaviour and th en introducing a lumped correction fac to r (p) to predict
observed dat a between H2S-C0 2 and diethano lamine soluti on . This methodology being process engineering friend ly can be
extended to other systems also .
The simultaneous absorption of H 2S and C02 is of
considerable importance in sweetening of natural gas,
assoc iated gas and biogas . For instance, transportation of natural gas or associated gas needs almost total removal of H2S (say <I ppm) and to so me extent
removal of C02, to overcome corrosion related problems in pipelines . Bi o-gas from fermentation units,
anaerobi c digester is often used to generate power by
firing the same as fuel in gas engines. Though C02
alongw ith CH 4 is acceptabl e, H2 S levels are desired to
be as low as possible to avo id corrosion related
problems.
By and large aq ueous solu tion s of alkanolamines
are used for th e simultaneous absorpti on of H 2S and
1
C02 . The reaction between H 2S, C02 and alkanolamine is reversible in nature. Therefore , in the top
section of th e absorber, the knowl edge of equilibrium
partial pressure of H 2S and C0 2 over partiall y regenerated amine so lution is des irab le for the process design of the absorber. The ex tent of sweetening of gas
stream (removal of H 2S and C02) depends upon the
equilibrium consideration in the top sec tion of the
countercurrentl y operated absorpti on co lumn .
Th e equilibrium in C0 2-H 2S-alkanolamine system
2
has been well analysed by Asterita et al. , Kent and
4
Eisenberg 3 and Deshmukh and Mather . The mode ls
of Kent-Ei senberg and Deshmukh-math er are bri efl y
reviewed in the following:
*For correspondence.
Kent and Eisenberg assumed activ ity coefficient of
all spec ies to be unity . Further they used all the published information on th e various equilibrium constants at infinite dilution and correlated them with
temperature in the polynomi al exponential form. In
order to fit the experimental data they forced values
of Ka and K,q to obtain the best fit. T hu s the Ka and
Keq were derived. No wonder their mode l could we ll
-predict behaviour of single solute system C0 2 and
H 2S. However, when the m ixtu re is present, their
model cou ld not predict the observed values we ll.
4
Deshmukh and M ather proposed th e thermodynamic model for the solubility of C0 2 and H 2S in alkanolamine solution. They accounted for variati on in
activity coefficients through modifi ed form of DebyeHuckel expression . The model by Deshmukh and
Mather is rather complex to use fo r it requires the
knowledge of various interaction parameters . The ir
model fails to predict acceptable partial pressures of
H 2S and C02 at hi gher load ing of solution s. The ne(;d
for reliable information on carbamate hydro lysi s
equilibrium constants was highli ghted by these researchers.
Very scanty info rmati on is available in the publi shed lite rature on very low-pressure equilibriu m
data. Lal et
have published some data on low
pressure equilibrium of H 2S and C0 2 over aqueous
di ethanolamine sol ution . Here, a nove l and simpler
method of measuring equ ilibrium pressure of H 2 S and
C0 2 with the he lp of ion-se lecti ve e lectrodes is pre-
ae
JAGUSHTE & MAHAJANI: LOW PRESSURE EQUILIBRIUM BETWEEN H2S AND C0
243
2
TO CONDUCTIVITY METER
sented here. In addition, a unified approach is also
presented to analyze data based on the various equilibrium constants at infinitely dilute solutions alongwith a lumped correction factor taking into consideration highl y non-ideal aqueous system containing
various ionic spec ies such as amines, protonated
amine, carbamate, bicarbonate, hydroxyl ions etc.
J ~TIC
]
Experimental Procedure
Materials-Diethanolamine (DEA), sodium carbon ate (Na 2C0 3) and sodium sulphide (Na2S) used
were of analytical grade with 99.5 % purity and obtained from S.D. Fine Chern. Ind ., Mumbai (India).
H2S gas was gene rated in Kipp' s apparatus. C0 2 gas
is supplied in cylinder obtained from Maharashtra gas
Company which is of 99% purity .. Sulphuric acid was
used to generate H 2S from iron sulphide so that contamination due to HCl and water is avoided. Also suffi cient qu an tity of gas is generated and purged out so
that air contamination in H 2S is eliminated.
Experimental set-up-The ex perimental set-up
consisted of a gas bubbler with magnetic stirrer to
enhance equilibrium process. The equilibrium cell is
fitted with a conductivity probe. The ex it of the ce ll is
connected to a g lass reservo ir. The gas-c irculating
blower takes gas from reservo ir and passes into the
equilibrium cell. The pressure maintained in this
system is practi ca ll y near atmo sphere. The entire assembl y, except gas ci rcu lating blower, is placed into a
constant temperature bath maintained at 30°C. The
heat losses to th e surrounding can safely be neglec ted
at the ambient temperature close to 30°C. Fig.l shows
the entire set-up schematicall y.
Experimental procedure-A known qu antity of alkanolamine so luti on was taken in an equilibrium ce ll.
H 2S and C0 2 gases were inj ected into reservo ir to get
desired partial pressure. The gas-circulating blower
was then started . Some H 2S and C02 would get adsorbed into alkanol amine sol ution . T o compensate
thi s, additional quantity of H 2S and C02 gases were
inj ected , so that the system is near atmospheric pressure. The approac h to equilibrium is monitored with
the help of a conductivity probe . Since the reaction of
H2S and C0 2 with aqueous alkanol amine solution is
ionic in nature, the concentration of ioni c spec ies remains constant after reaching equilibrium. The constant readin g of co nductivity probe over a two-three
days means the equilibrium was ac hieved. At thi s
stage, the gas compos ition was identical in the ce ll as
well as in th e reservo ir. The reservoir was then iso-
I
' II
Fi g. !-Experimental set-up for vapour liquid equilibrium measurement. I . Conductivity probe, 2. Equilibrium cell , 3. Saturator,
4. Gas reservo ir, 5. Gas circul atin g blower, 6. Magneti c sti rrer, 7.
Constant temperatu re bath , and 8. Temperature Indi cato r and
Controll er (TIC)
lated from the system with the help of valves. A
known quantity of caustic , which was in far excess
than required, was added to the reservo ir with the
he lp of liquid syringe . It was then well mixed by
shaking and left for about 4 8 h so that the entire
amount of H 2S and C0 2 gases are absorbed into
aqueous NaOH so lution . A sampl e was taken from
amine solution with the help of a gas-tight syringe
and introduced into caustic so luti on to convert it into
correspondin g Na2 S and Na 2C0 3 . With the he lp of
sulphide and C0 2 ion-selective e lectrodes, both samples were analyzed for sulphide and carbonate content
and hence corresponding H 2S and C0 2 content was
back calculated both in the gas phase and in the liqui d
phase .
The sulphide and C02 ion se lecti ve e lectrodes
ORION (USA) make, were calibrated before the
reading with the help of standard aqueous so lutions of
Na 2S and Na 2C03 respectively . It was ensured that all
read ings were obtained in linear behav iour of the
electrodes.
Results and Discussion
During the si multaneou s removal of H 2S and C02
the following equilibri a are establi shed between H 2S
and C0 2 and a lkanol amine.
Reaction of amine with H2 S
H2S+AmH
Ks
= (AmH ; ).(HS - )
(H 2S). (AmH)
HS - + Am H 2+
( Ia)
( lb)
INDIAN J. CHEM. TECHNOL., SEPTEMBER 2000
244
Reaction of amine with C0 2
Equilibrium pressure of H2S
... AmH2+ + AmcooK _ (C0 2) (AmH) 2
c- (AmH ; )(AmCOO- )
C02+2AmH
(2a)
Equilibrium pressure of C0 2
p * CO2 = (CO 2)
Hco 2
HS- + H+
(3a)
(H +) (HS - )
K - --'-------'-------'---Hs (H 2S)
(3b)
Profanation of amine
(AmH)+H+
. .. (4a)
K = (H+) (AmH)
"
. . . (4b)
(AmH ; )
Hydrolysis of carbamate
+:::=:::~., (AmH)+HC0 3-
Amcoo- + H20
K
eq
= (AmH) (HCO ~ )
(AmCOO-)
.. .
(Sa)
... (Sb)
Hydration of C02
... (6a)
C02+ H20
K _ (H +) (HCO ~ )
I -
... (6b)
(C0 2)
Amine dissociation in water
(AmH)+H 20
.,.
., (AmH/)+OH-
K _ (AmH ; ) (OH - )
b-
(AmH)
... (7a)
... (7b)
Dissociation of water
H20
H+ + OH-
Kw = (H+) (OH - )
Dissociation of bicarbonate
HC0 3C0 32- + H+
2
(C0 - ) (H +)
3
K2 (HCO ~ )
... (I 0)
HH 2S
(2b)
Dissociation of H2S
H2S
P*H2S= (H 2S)
. .. (8a)
... (8b)
. . . (9a)
. . . (9b)
.. . (II)
The absorption of H 2S and C0 2 in aqueous solution at
the microscopic level consists of the reaction of dissolved H 2S and C0 2 with amine and OH- present in
the diffusion films [Eqs (I a) and (2a)]. The carbamate
formed during the chemical react ion between C02
and amine undergoes hydrolysis (partial hydrolysis
may occur in the gas liquid diffusion film in the liquid phase also) into the bulk of the liquid to regenerate amine [Eq .(Sa)]. The regenerated amine is then
available for absorption of C02 and H 2S.
If K eq (Eq . Sb) is very high, there is a distinct possibility of hydrolysis of carbamate, (forward reaction
in Eq. Sa) in the film. However the hydrolysis of carbamate in the bulk of liquid would be completed over
a larger contact time. The contact time in diffusion
film being very low, it is generally considered that the
hydrolysis of carbamate takes place entirely in the
bu lk. When equilibrium experiments are performed as
in the case explained below, there is enough contact
time for hydrolysis of carbamate to take place. Therefore, for equilibrium data correlation, carbamate hydrolysis to amine (Eqs Sa & b) be considered . Maha6
jani has analysed such a situation and shown that
during C02 absorption, carbamate hydrolysis is not
predominant at the top of the absorption column .
Thus there are two equilibria in the diffusion film,
namely, one in the diffusion film known as the 'First
Equilibrium' and the other due to the hydrolysis of
carbamate known as the ' Final Equ ilibrium' .
First Equilibrium
The chemical reaction between H 2S-C02 and amine
is reversible in nature, an equilibrium exists between
all reacting species, as represented by Eqs (l b) and
(2b). Simultaneously all the other equilibria, except
for carbamate hydrolysis (Eq . Sb) are established. As
the basicity of diethanolamine is such that concentration of OH- can safely be neglected, hence the reaction between H 2S and C02 with OH- can be neglected.
JAGUSHTE & MAHAJANI: LOW PRESSURE EQUILIBRIUM BETWEEN H2S AND C0 2
245
Table l-H 2S-C0 2 aqueous diethanolamine (DEA) equilibrium data at 313 K by ac tual experimental technique
System
Partial pressure of
C0 2 (kPa)
Carbonation ratio
Partial pressure of
H2S (kPa)
Sulfidation ratio
(a I)
0.143
0.070
0.064
0.78
0.182
0.150
0.101
0.131
0.058
0.074
0.130
0.170
0.020
0.038
0.062
0.075
H2S- C0 2.-DEA (2M)
Let the degree of carbonation a. 1 and degree of sulphidation a.2, may be defined as
a. 1= kmols C0 2 absorbed I kmols of amine
<X2= kmols of H2S absorbed I kmols of amine
The amine balance equation is
(AmH)=( l-2a.l-a.2) (AmH)o
(AmH2+)=(a.1 +a.2) (AmH)o
(AmCOO-) =a. l (AmH)o
(HS_)=a.2 (AmH)o
... ( 12)
... (13)
(14)
.. . ( 15)
Eqs (I b), (3b) and (4b), give,
Ks = KHs = (HS-) (AmH;)
Ka
(H 2S) (AmH)
. .. ( 16)
Substituting the relevant values in Eq. ( 15) one gets,
... (17)
(H2S) = ~ a. 2 (a. I + a. 2) (AmH)o
K HS
( l - 2a.l - a. 2 )
... ( 18)
From Eq. (I 0), the equi librium partial pressure of H 2S
over amine solution is given by
P * H ? S=~
-
I
K HS HH 2S
a.2(a.l +a. 2) (AmH)o ... ( 19)
(1- 2a.,- a. 2)
Similarly, substituting for (A mCOO-), (AmH) and
(AmH/ ) in Eq. (2b) from Eqs (12), ( 13) and (14), it
was found that
(az)
Eqs (2b), (4b), (Sb) and (6b) give,
... (22)
Thus, from Eq. (9), the equ ilibrium partial pressure of
C02 over ami ne so luti on is g iven by
P * C02=Keq Ka
Kl
a.l(a.l+a. 2)
H co2 (1- 2a.l - a. 2)2
... (23)
The Eqs ( 19) and (23) could be used to corre late the
data when the First Equi librium exists. When vapourliquid-equilibrium data are generated, there is enough
contact time so that carbamate gets hydrolyzed and
fina l or total equilibrium is estab lished. Therefore, it
is suggested that the equi librium pressure measurement data shou ld be analyzed considering on ly Final
Equi librium (carbamate hydrolysis).
Here a methodology is presented to compute the
partial pressures of H 2S and C02 considering Final
Equi librium.
Final Equilibrium
Final equi librium is said to be establi shed when
carbamate hydro lysis takes place (Eq. Sa). The procedure followed to obtai n fi nal equilibrium is :
In the whole so luti on which is in equi librium, one
has :
Amine balance equation
(AmH)o=(AmH)+(AmH2+)+(Amcoo-)
... (24)
Carbon dioxide balance yields
... (25)
(20)
and H2 S balance gives
.. . (2 1)
.. . (26)
INDIAN 1. CHEM. TECHNOL., SEPTEMBER 2000
246
Table 2- Various equilibria as a fun ction of temperature
Re ference
DEA (diethanolamine)
pK,
= a.(T) ~ ,
K, = (kmol. m·3 )
a= I I I 1.406 and ~ =- 0.8477
298 < T < 323 (based on the data by Perrin 7 )
2
First dissoci ation constant for H2 S in water
pKHs = -106.67 + 6045.2 IT+ 37.744 log T , KHs= (kmol.m-3 )
3
Solubility of H 2S in water
log HH 2s = 651.2 I T- 8.206 ,
H H 2s
=
3
(k mol. m· Pa.
4
Solubility of C0 2 in water
Hc 0 2 =3.54 x 10·4 exp (2044 I 1), (kmol m·3 Pa. 1)
5
Carbamate hydrolysis Equilibrium Constant
Kc4
=
a{X.J
where,
298 < T < 323 Kc4
a= 3.12055
=(kmo l. m·
3
b =-I 0. 887 I 8
[BlauwhoffiC 1]
)
(From the data given by Bl auwhoff
6
x 10· 2R
1
)
10
,
we have derived the above correlation)
Equilibrium constant fo r C0 2 hydration
log 10 K 1 =-3404.7 I T + I 4.843 - 0.03279 T (kmol m·3 )
The electroneutrality equation results in
[Danc kwerts and Sharma 11 ]
Eq. (32) is quadratic equation in (AmH), therefore ,
... (27)
2(AmH)= {-Keq+(2a,+a2-l ) (AmH)0 }
from Eqs (25) and (26), gives
[K ,q + (2a 1 + a 2 -1) (AmH) 0
f}
+4Keq [l- (a 1 + a 2 )](AmH) 0
. . . (28)
(33)
substituting above values of (AmH 2+) and (AmCOO-)
in Eq. (24), one gets
Substituting the values of (HS - ) and (AmH ; ) from
(AmH) 0=(AmH)+(AmH 2+)+(AmCOO-)
. . . (29)
Eqs (26) and (27) in Eq. ( 16), one gets
=(AmH)+(a, +a2) (AmH)o+a, (AmH)o-(HC0 3-)
. . . (30)
From Eq . (5b)
_
Keq (Amcoo-)
(HC0 1 ) = _..:..____ __
.
(AmH)
Similarly, the partial pressure of C0 2 could be obtained from Eq. (2b) as
. . . (3 1)
.. . (35)
Therefore from, Eqs. (30) and (31)
(AmH)
2
+
{K,,1 + (2a 1 + a 2 -I ) (AmH) o J(AmH)
-K,q [l- (a 1 +a 2 )](AmH) 0 = 0
. .. (32)
Now,
.. . (3 6)
JAGUSHTE & MAHAJANI : LOW PRESSURE EQUILIBRIUM BETWEEN H2S AND C0 2
247
Table 3a-Comparison of computed equilibrium partial pressures of C0 2 from the model with the actual experimental value
Experimental
equilibrium partial
pressure of C0 2
Carbonation
ratio
( al)
P *co 2 (kPa)
0.182
0.150
0. 10 1
0.131
0.143
0.070
0.064
0.078
Predicted equilibrium
parti al pressure of
Computed equilibrium
partial pressure of C0 2
P* C0 2
p* co 2 =P*C0 2 x
C0 2
(kPa)
0.708
0.495
0.250
0.478
CPI)
PI
(kPa)
0.174
0.183
0.212
0. 174
0. 123
0.090
0.053
0.083
For C0 2
P1 =
~xp(-a 1 ~ -a 2 ~)}
a 1= 3.520
a 2 = 1.727
Table 3b-Comparison of computed equilibrium partial pressures of H2S from the model with the actual experimental value
Sulfidation
ratio
( a 2)
Experimental equilibrium
partial pressure of
H2S (kPa)
0.020
0.038
0.062
0.075
Computed equilibrium
partial pressure of H2S
Predicted equilibrium
partial pressure of H2S
P * H 2S (kPa)
0.058
0.074
0.130
0. 170
0.337
0.566
0.736
1.230
P* H 2S = PH 2Sxp 2
<P2)
(kPa)
0.157
0.154
0.168
0.135
0.05 3
0.087
0. 124
0. 166
For H2S
P2 =~xp(-b1~ - b2 ~)}
bl = 3.408
Therefore substitution of (AmH ; ),
b2 =2.815
(Amcoo - ),
Pc02 and K c in Eq. (35) gives,
~
Keq K.,
P *C0 2 =
'
Kl
H co2
quantifying effect of various ionic species on individual equilibrium constant, a process engineer friendly
approach is being recommended wherein this effect is
lumped together in a single parameter say ~ , which
would then be correlated to the system characteristic
such as carbonation ratio a 1 and sulphidation rati o
a 2.
[{(l-a 1 -a 2 ) (AmH)0 }-(AmH)](a 1 +a 2 )(AmH)0
(AmH) 2
. . . (38)
Eqs (34) and (38) can now be used to obtain partial
pressures of H 2S and C0 2 over aqueou s alkanolamine
solution.
The aqueous solution of alkanolamine is a nonideal solution. At any given time, it contains various
ionic species. These species do exert their influence
on various equilibria explained above. Instead of
Thus,
P *C0 2
= P *C0 2
P * C0 2
= P *C0 2 ~xp(-a 1 .Ja: -a 2 ~)}
~1
(39)
where •
. . . (40)
INDIAN J. CHEM . TECHNOL., SEPTEMBER 2000
248
I-1 1-1
s =solubility of
2
H2S, (kmol.m· 3.Pa. 1)
K a, (pKa) =equilibrium constant in Eq. (4b), (kmol.nf·')
... (42)
In the ~'s, the effect of ionic strength on all equilibrium constants and solubi lity parameters lumped together, has been taken into consideration.
(The particular form of the Eqs. (39) and (41) are
chosen as the activity coefficients because they are
related to ionic strength in a similar manner, i.e.
yoc..{i).
K b , (pK b) =equilibrium constant in Eq . (7b), (kmol.m··')
Kc =equilibrium constant for COramine reaction in Eq . (2b),
(kmol m·3)
Keq =carbamate hydrolysi s equilibrium constant in Eq. (5b),
(kmol m-3)
KH s =equilibrium
constant for reactio n of amine with H2S in
Eq. (3b), (kmol.m.3)
K1 =equilibrium constant for C0 2 hydration in Eq. (6b),
(kmol m·3)
Table I presents data obtained by actual experimental technique. This agrees reasonably well with
5
that reported by Lal et a/.
Various equi libria used to compute equilibrium
pressure have been presented in Table 2. The computed values of equilibrium partial pressure of C02
and H 2S are compared with the actual ex perimental
values in Table 3a and 3b respectively, and constants
a,, a 2, b 1 and b2 of Eqs (39) and (41) are also presented. More data points are required to test this
model universally. However, in present work, the aim
is to demonstrate the approach. The above process
engineer friendly approach woul d help design engineers .
K2 =equilibrium const ant for dissoci ation of bicarbonate in Eq.
(9b), (kmol m·3)
P *co 2 =experimental equilibrium partial pressure of C0 2, (kPa)
P * H2s =experimental equilibrium partial pressure of H2 S, (kPa)
P* C0 2
=predicted equilibrium partial pressure of C0 2 in the
bulk of gas phase, (kPa)
P* H2S =predicted equilibrium parti al
pressure of H2S in the
bulk of gas phase, (kPa)
P * C0 2 = computed equilibrium partial pressure of C0 2 in the
bulk of gas phase (kPa)
P * H2S =computed equilibrium partial pressure of H2 S in the
bulk of gas phase, (k Pa)
a 1 =,carbonation ratio, kmols C0 2 absorbed I kmols of amine
Conclusion
A simplified approach to correlate equilibrium
partial pressures of H 2S and C02 over aqueous amine
solution is demonstrated based on the equilibrium
aspect. Based on the carbonation ratio, sulphidation
ratio and nature of amine, two factors name ly ~ 1 and
~ 2 were
a 2 = sulphidation ratio , kmols of H2S absorbed I kmols of amine
p, =correction factor defined by Eq . (39)
P2 =correction factor dell ned by Eq. (41)
References
introduced by using 'final equilibrium' ex-
Kohl A L & Riesenfeld F C, Gas Purification, Third Ed .
(Gulf Pub. Co., Houston), 1974.
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2
Asterita G, Savage D W & Bisio A, Gas Treatment with
Chemical Solvents (John Wiley & Sons, New York) , 1982.
3
Kent R L & Eisenberg B, Hydrocarbon Processing, 55
(1976) 87.
(Am H) ., = original concentration of amine, (kmol.m.3)
4
Deshmukh R D & Mather A E, Chem Eng Sci, 36 ( 198 1)
355.
(AmH 2 ) = concentration of piOtonated amine (kmol m·3 )
5
Lal D, Otto F D & Mather A E, Can 1 Chem Eng, 63 ( 1985)
681.
6
Mahaj ani V V, Gas Sep Purif, 7 ( 1993) 53 .
7
Perrin D D, Dissociation Constants of Organic Bases in
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Nomenclature
(Ami-1) =concentration of amine, (kmol.m.3 )
(Am COO-) = concentration of carbamate(kmol nf·')
(C0 2) = concentrati on of C0 2 (kmol m·')
(I-1 2 S) =concentration of H2S, (kmol m··')
JAGUSHTE & MAHAJANI: LOW PRESSURE EQUILIBRIUM BETWEEN H2S AND C0 2
249
8
Bosch H, Solvents and reactors for acid gas treating, Ph D
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I0
Blauwhoff P M M, Solvents and reactors for acid gas treating, Ph D Thesis, Twente University of Technology, The
9
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II
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