Indi an Journal or Chemi cal Technology
Vol. 7, Nove mber 2000, pp. 326-33 1
A study of the design of solar evaporation pans
C Srinivasa Kannan * & P G Rao
Chemi cal Engineeri ng Division, Central Leather Research Institute, Chennai 600 020, Indi a
Received I April / 999; accepted 9 August 2000
In the present work an ex perimen tal and theoretica l stud y is attempted , on the usc of so lar evaporation pans in tanneries
for evaporat ion or water from the soak li quo r. A design methodo logy is identilicd and checked with the ex periment al resu lts
or the present stud y. The model is fo und to match the ex perimental data sati sfactoril y. Increase in the air te mpe ratu re, win d
veloci ty and sa lt bath temperatu re arc rou nd to increase the evaporation rate whereas increase in concent rat ion or the sa lt
bath soluti on and air humidit y arc round to dec rease evapo rati on rate. Salt bath temperature is sound to be major influencing
parameter to increase the evaporation rate.
The proper des ig n of so lar evapo rati o n pan s in tanneri es is ve ry impo rtant for no n-sto p ope ration of th e
tanneri es. In tro pi ca l countries like Indi a, solar e ne rgy
is utili zed to e vapo rate water assoc iated with the soak
I iqu or to recover sa lt. Isol ati o n of soa k I iqu o r fro m
ot he r streams in leathe r process ing is ve ry important,
as th e soak I iqu o r affects the bi o logica l treatme nt
sys tem. Furthe r, the segregati on and di sposal of soak
liqu or will brin g about substanti a l reduction in tota l
di sso lved so lids load by abo ut 30% ( i.e reduction to
14000 to 20000 mg/L). Thi s is very important in o rder to meet the e mi ss io ns stand ard s of 2100 mg/L
tota l di ssol ved so lid s in treated e fflu ent. All the process stream s o f the leath e r process in g except th e soak
liqu o r, pi ckl e liqu o r and the c hrome li q uo r of le ath e r
process in g are mix ed togeth e r a nd se nt to the treatme nt pl ant, to make treatment of th e tannery e ffluent
e ffec ti ve in meetin g the stand ard s co mfo rtably .
The salt assoc iated with raw skin/hide fo r prese rvati on is no rma ll y re mo ved by soakin g the sk in s in
300 to 600 % of water based o n th e we ig ht of the
sa lted skin . The spe nt liquor from soa kin g has a sa lt
concentrati o n of 4 to 6% w/v. The salt bath mainl y
consists o f salt , ha ir, mud , dung etc . Pre-treatme nt of
soa k liqu o r would be necessary fo r reusing th e recove red sa lt. Ho wever, reuse of sa lt recove red fro m soa k
liqu o r is not do ne, as the salt is associated with dirt ,
bacte ria and th e use of th e fres h sa lt is eco nomi cal.
At present , the so lar e vaporati on pan s a re des ig ned
ass umin g an e va po rati o n rate of 5 to 7 mm pe r clay
without ta kin g into co ns ide rati o n th e vari ous para-
*For correspondence
mete rs controlling e vaporati o n rate suc h as,
•
•
•
•
•
Atmo spheri c te mpe rature
Wind ve loc it y
Atmos phe ri c humidity
Salt bath te mpe rature
Conce ntrati on of the salt bath
It is inte res tin g to kno w the effects o f th ese paramete rs o n th e rate of evapo rati on . Th e mode l of
1
Sherwood and Pi g ford deve lo ped fo r evapo rati on
fro m surfaces in wind tunn e ls is used to predi ct the
dime nsion s of the pan fo r a spec ifi ed evaporati o n
load . With the he lp of thi s mode l, g iven the me teoro log ica l data of the parti c ul ar site, it is poss ibl e to design the so lar evapo ration pan rea li sticall y to meet th e
lad of the tann e ry.
Evapo rati o n o f wate r fro m free surfaces has been
of inte rest to many sci e nti sts fo r a lo ng time and a
s izeable a mount of work has been ca rri ed out o n thi s
subj ec t. Earli e r wo rk on free surface e vapo rat io n has
2
bee n don e by Mai se l and She rwood , Luri e and
Mich a il o rff3, Hinchil ey a nd Himu s'\ Powe l a nd G riffith s5, Powe l6 , e tc . The above auth o rs have carri ed out
wind tunne l expe rime nt s unde r diffe re nt ex pe rime nta l
condition s and gene rated a vast literatu re on eva poration from free surfaces . Th e work o f vari o us auth ors
such as Wade 7, Pasquiii R, Po we l and Griffith s 5,
Po we l6 , Luri e a nd Mich a il o rff3 , were co mpil ed by
1
S he rwood and Pi gfo rd , and a generali zed corre lati on
has been presented coverin g the above wo rk s.
The present work cove rs and ex pe rime ntal stud y
on e vapo rati o n rate in so lar evapo rati o n pans under
vari ed atmosphe ri c conditi o ns. T he mode l of
1
S he rwood and Pi g fo rd , is simul ated to qu anti fy the
KA NNAN & RAO : A STUDY OF THE DESIGN OF SOLAR EV APORAT!ON PA NS
effects of the contro lling parameters . The model is
verified with th e ex perimental data due to the present
study and th e data were found to match sati sfactorily .
Experimental Procedure
Experiments were carried out 1n a solar evaporation pan of O.S xO.S xO. l meters in dimension. Atmospheric humidity and temperature are determined using
dry and wet bulb thermometers . A psychometric chart
is used to ca lculate the humidity. Wind velocity is
measured using an anemometer. Change in pan
weight is used to assess the eva poration rate and co ncentration l>uild-up in the pan. Bath temperature is
registered usin g a th ermometer. The pan is weighed at
an interva l of one hour. Atmospheric temperature,
humidity and wind velocity are monitored for th e
who le duration of the ex periments.
Mode/for prediction of evaporation rate
Evaporation rate is expressed by the eq uation ,
( I)
where th e mass transfer coefficient 1s predicted by
usi ng th e generali zed correlations given below,
Sh = 0.664 Re05 Sco_n for laminar flow
(2)
Sh=0.036 Re o_s Sc03 ·1 for turbul ent flow
(3)
Laminar flow ex ists up to a Reyn olds number of
3,00,000 and above 3,00,000 turbulent fl ow ex ists.
The model is simulated for various parameters
such as,
• Pan area
• Atmospheric temperature
• Salt bath temperature
• Salt bath concentration
• Wind velocity
• Atmospheric humidity
Results and Discussion
An increase in pan area increases th e amount of
water evaporated (Fig. I), but the rate of evaporati on
per square meter of the pool remains co nstant or in
other words regardl ess of the pan area the reducti on
in height due to evaporation remains constant. An
increase in air temperature, keeping the other parameters constant, on one hand increases th e diffusion
coefficient and on th e other hand decreases the mass
transfer coefficient (because of reduction in th e Reynolds number), res ulting in a marginal decrease in the
evaporation rate (Fig. 2). However, in a practical
situation an increase in air temperature causes an increase in the salt bath temperature.
During the process of evaporation , if the temperature of the salt bath is the same as th at of the air and
the air is not saturated, th e liquid evaporates and th e
latent heat of evaporation is supplied by cooling the
salt bath. As the temperature of the salt bath becomes
lower than th e temperature of the air, sensible heat is
transferred to the liquid from th e air, until th e heat
needed to eva porate the liquid is exactl y matched by
the sensible heat fl ow from th e air phase to the liqui d
phase. The temperature of the salt bath will reduce
until it attains a steady state. Assuming the salt bath is
at wet bulb temperature (the lowest poss ible temperature) corresponding to th e increase in air temperature at a fixed atmospheric humidity , the increase
in air flow rate increases the eva poration rate (Fig. 3).
Fig. 4 shows an increase in eva poration rate with increase in temperature of th e salt bath solution. The
~
I
i
!
,,
i
100
327
'"
Pool &re8, m
300
2
Fig. 1-Effcct of p ~ n area o n eva poratio n rate. T.,-25 °C, T,- 18.3 °C , Ha-O.OOH. C,-6 %, 11-0.5 m/s_
328
INDI AN J. CHEM. TECHNOL. , NOVEMBER 2000
10
--·----------------..--------------------·--------·-- ----------------------- ------------ -----,
50
-10
Air tempera~ure, ·c
Fig. 2-EfTcct of ai r temperature on evapo rati on rate at constant bath temperature. A- 10 m2, T,-20°C, H"-0, 0.008, C,-6%, V-0.5 m/s.
"
---------------------
- ----- --------- ------ -- ------:
"
Fig. 3-Eflect of air temperature on evaporation rate at wet bulb bath temperature. A-100m 2, H"-0.008, C,-6%, V- 0.5 m/s.
"' ------------ --------------------------------------------·--:
i
I
I
100
!
I
I!
I
!
20
3S
"
Salt bath te mperatu~"e , "c
2
""
Fig. 4-Eflect of salt bath temperature on evapo rati on rate. A- l 00 m , T"-40°C, H"-0.0 19, C-6%, V-0.5 m/s.
KAN NAN & RAO: A STUDY OF THE DESIGN OF SOLAR EV APORATION PANS
"" - -
~---
--
---
- ~-v~
~
I
I
10
.
50
"
25
Sell bath concentration, %
Fig. 5-Eilect o r salt bath concent rati on on evapo ration rate V:
0.5 m/s, 6.- I m/s. A- 100 m2 , T.. -40°C, T,-40°C, H .. -0.019.
120
*
- · - - - - - ---------~
""
g "'
l"' "'
20
15
25
Wind velocity, mls
Fig. 6-Ellect or win d ve loc it y on evaporation rate. A- 100 m2, T.. 400C. T,-°C, H-0 .0 19, C,-6%.
50
increase in temperature of the salt bath increases the
vapour pressure of the salt bath soluti on, whi ch results in hi gher evaporati on rate.
Fi g. 5 show s a dec rease in evap orati on rate
with increase in sa lt bath co nce ntrati on. T hi s is
du e to dec rease in th e vapour press ure o f th e sa lt
bath with increase in sa lt bath co nce ntrat ion.
Th e increase in eva porati on rate with win d veloc ity is du e to in c rease in mass trans fe r coeffi c ie nt. As th e wind ve loc it y is in c reased , th e film
res istance betw ee n th e gas and liquid ph ases is
reduc ed, whi c h res ult s in hi ghe r mass tr ansfer
coe ffi c ie nt (Fi g. 6). Fi g. 7 shows a dec rease in
eva porati on rate at hi ghe r atm os ph e ri c hu mid ity.
As th e humidit y is in c reased , th e res ultin g dr iv in g fo rce fr om th e surfa ce of th e bat h is red uced ,
whi c h res ult s in dec reased eva porati on rat e.
Howeve r, Yos hid o and Hyo do 9 ex pe rim e nt all y
noted an in version te mp e rature for pure water
eva porati on above whi c h an in c rease in a ir humidity in creases th e ev ap orati on rat e. Thi s Inve rs ion po int is approx im ate ly 170°C. T he in ve rsion te mpe rature was furth e r st udi ed by
C how and Chun g 10 and is exp la in ed by th e combin ed effec ts o f hi ghe r hea t tr ansfe r coefficient
fr om strea m flows and int e rfac ia l te mperature
depress ion by th e prese nce of a ir .
Th e above simul ati on in dicates a marked
va ri ati on in eva porati on rate w ith inc rease in air
te mpe rature, wind ve loc ity, humidit y and salt
bath te mperature. Howeve r, th ese paramete rs
depe nd on atm os ph e ri c co nditi ons and a re beyo nd co ntro l. Th e eva porati on rate in c reases six-
---~-------~--------. - - - - - - - - - - - - - - - - - - - - - -
45
40
35
10
0 01
329
002
003
Air humidity, kg/kg
Fi g. 7- EITec t or humidity on evaporation rate. A- 100m 2, T.. -40°C, T,-35°C, C,-6%, V- 1 m/s.
INDIAN J. CHEM . TECHNOL., NOVEMBER 2000
330
..c: 25.00
~20DO
j
Conclusion
..... .
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~
i""1
i
":..
IC.OO
<:
_g
~
5 .08
Experiments are carried out for pred icting the
evaporation rat e of water from a sal t bath in a
laboratory scale pan. The experimental evaporation rate is found to mate h th e model of
1
Sherwood and Pi gfo rd sa ti sfac torily . The mod e l
is simulated to assess the influ e nc e of variou s
parameters affecting the evaporation rate. Increase in air temperature, wind ve locity a nd sa lt
b a th temperature increase the evaporation rate ,
whereas increases in humidity and salt bath co ncentration decrease th e evaporation rate . An increase in salt bath temperature of 25 °C is found
to increase the evapo ration rate b y s ix-fo ld .
~
c
~J
~
c
.
u
c
/"
"'
w
mrrrorrrrrp·,,~~~,..,...,..,~-rl
0.00
0
5
10
15
20
25
Pr·ed icl ed evap orat ion r ole XlOOO, kg/ h
Fi g. 8-Comparison of experimenta l eva po ration rate with
the predi cted eva poration rate usin g th e model.
Nomenclature
A
sa lt bath co nce ntrati on , kg of sa lt / kg of water
DAR
fo ld , if th e temperature of th e sa lt bath is increased by 25 °C. If the temp e ra ture of salt bath
is above atmospheric temp era ture , as evaporation proceeds the temperature of t he bath will
decrease as latent heat of eva poration is ta ken
from the bath. To sustain th e same evaporation
rate , he at energy equivalent to the latent hea t
must be su pplied. This ca n be effec ted by external hea tin g medium s uch as s te am, hot o il
( which requires high operating cos t) or by the
use of solar evaporation cell s (w hich neces si tate
a higher initial investment ).
Comparison of the model equation with the
experimental data
F ig. 8 compares th e mod e l prediction with the
ex perimental data of th e prese nt s tudy. It is see n
from the f ig ure that the mod e l matches sa ti s factorily with th e expe rimenta l data . Th e exper ime ntal evapora tion rat e indi cated in th e figure is
compared with model takin g int o co ns ideration
t he changes in the air temperatures, humidity 's
a nd wi nd velocities th at prevailed over th e tim e
inte r val. The model prediction is uniformly
hig her than the experi me ntal data is due to th e
diffe rence in vapo ur press ure be tween th e pure
s alt sol ution and th e soak liquor. The vapour
p ress u re o f the soak l iquor is lower than th e
ure salt s olution becau se of ot her co ntamin a tio n.
area of th e evapora ti on pan , m2
diffu s ivity of water in ai r, m2 /s
air humidity. kg of moisture/kg of air
K
mass trans fer coefficien t, m/s
L
le ngth of th e po nd in th e di rec ti o n of wind fl ow,
p
atmospheric pre ss ure, atm
Ill
( P-P v i )+(P -P v2)/2
vapour pre ss ure o f· sa lt bath at sa lt bath tempe rature, atm
parti al press ure of wa te r vapou r in a ir , atm .
R
gas constant
Re
Reynolds numb e r,
Sh
Sherwood numbe r, KL/D AB
Sc
Schmidt number,
VpL/~
~/pD A B
atmospheric te mp eratu re,
sa lt bath tempe rature,
V
velocit y of air , m/s
~
viscos ity of air , ka/ ms
p
den sit y o f a ir, ka!Jn 3
oc
oc
Heferences
Sherwood T K & Pi gford R L, Abso rption a nd Extra ction (McG raw Hill , NY) , 1952 .
2 Maisel D S & Sherwo od T K, ChCJn Eng Prog, 46
( 1950) 13 1.
3 Luri e M & Mich a ilorff M, lnd L'nr; Ch em , 28 ( 1936 )
34 5.
4 Hin chiley J W & Himu s G W, Tran s In s/ Che m Eng, 2
( 1924) 57 .
5 Powell R W & Griffiths E, Tra ns Ins/ Chem f:..'n g , 13
( 19 35) 17 5.
KANNAN & RAO: A STUDY OF THE DESIGN OF SOLAR EVAPORATION PA S
6 Powci R W, Trans Ins/ Chem Eng, I 8 ( I 940) 36.
7 W adeS H , Tans In s/ Chem Eng , 20 ( 1942) I .
8 Pasquil F, Pro c Roy So c , Al82 (1943) 175 .
33 1
9 Yoshida T & H yo do T , ln d Eng Ch em Pro Des Dn·, 9
( 1970) 207 .
10 Chow L & Chung J N, ln t J Hea t & Mass Transfer,
26(3) (I 983) 373.