Indian Journal of Chemical Technology Vol. 7, September 2000, pp. 242-249

Low pressure equilibrium between H2S and C02 over aqueous alkanolamine solution

!VI V Jagushte & V V Mahajani*

Department of Chemical Technology, University of Mumbai, Matunga, Mumbai 400 019, Indi a

Received 25 October / 999; accepted 15 May 2000

The simultaneous absorption of H2S and C02 in aqueous alkanolamine sol uti on is of considerable commerci al impor­tance. The reversible nature of reactions always results in equilibrium parti al pressure o f C02 and H2S 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 H2S and C02 with the select- ion electrode. The entire equilibrium is divided into two parts, namely first equilibrium and the final equilibrium. A (H2S + C02)- aqueous alkanol amine system is highly non-ideal. Therefore, an attempt has been made to correlate data by using ' final equilibrium' expression based on idea l behaviour and then introducing a lumped correction fac to r (p) to predict observed dat a between H2S-C02 and diethanolamine soluti on . This methodology being process engineering friend ly can be extended to other systems also .

The simultaneous absorption of H2S 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 some extent removal of C02, to overcome corrosion related prob­lems in pipelines . Bio-gas from fermentation units, anaerobic digester is often used to generate power by firing the same as fuel in gas engines. Though C02

alongwith CH4 is acceptable, H2S levels are desired to be as low as possible to avo id corrosion related problems.

By and large aqueous solutions of alkanolamines are used for the s imultaneous absorpti on of H2S and C02

1. The react ion between H2S, C02 and alkanol­

amine is reversible in nature. Therefore , in the top section of the absorber, the knowledge of equilibrium partial pressure of H2S and C02 over partiall y regen­erated amine solution is desirab le for the process de­sign of the absorber. The extent of sweetening of gas stream (removal of H2S and C02) depends upon the equilibrium consideration in the top section of the countercurrentl y operated absorption column .

The equilibrium in C02-H2S-alkanolamine system has been well analysed by Asterita et al.2

, Kent and Eisenberg 3 and Deshmukh and Mather4

. The models of Kent-Ei senberg and Deshmukh-mather 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 the various equilibrium con­stants at infinite dilution and correlated them with temperature in the polynomial exponential form. In order to fit the experimental data they forced values of Ka and K,q to obtain the best fit. Thus the Ka and Keq were derived. No wonder their model could we ll -predict behaviour of single solute system C02 and H2S. However, when the mixture is present, their model cou ld not predict the observed values well.

Deshmukh and Mather4 proposed the thermody­namic model for the solubility of C02 and H2S in al­kanolamine solution. They accounted for variation in activity coefficients through modified form of Debye­Huckel expression . The model by Deshmukh and Mather is rather complex to use for it requires the knowledge of various interaction parameters . The ir model fails to predict acceptable partial pressures of H2S and C02 at higher load ing of solutions. The ne(;d for reliable information on carbamate hydro lysi s equilibrium constants was highlighted by these re­searchers.

Very scanty info rmati on is available in the pub­li shed lite rature on very low-pressure equilibrium data. Lal et ae have published some data on low pressure equilibrium of H2S and C02 over aqueous diethanolamine solution . Here, a nove l and simpler method of measuring equ ilibrium pressure of H2S and C02 with the he lp of ion-se lecti ve e lectrodes is pre-

JAGUSHTE & MAHAJANI: LOW PRESSURE EQUILIBRIUM BETWEEN H2S AND C0 2 243

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 highly non-ideal aqueous system containing various ionic spec ies such as amines, protonated amine, carbamate, bicarbonate, hydroxyl ions etc.

Experimental Procedure Materials-Diethanolamine (DEA), sodium car­

bonate (Na2C03) 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. C02 gas is supplied in cylinder obtained from Maharashtra gas Company which is of 99% purity .. Sulphuric acid was used to generate H2S from iron sulphide so that con­tamination due to HCl and water is avoided. Also suf­fi cient quan tity of gas is generated and purged out so that air contamination in H2S is eliminated.

Experimental set-up-The experimental 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 reservoir. The gas-c irculating blower takes gas from reservoir and passes into the equilibrium cell. The pressure maintained in this system is practically near atmosphere. The entire as­sembly, except gas ci rcu lating blower, is placed into a constant temperature bath maintained at 30°C. The heat losses to the 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 quantity of al­kanolamine soluti on was taken in an equilibrium cell. H2S and C02 gases were inj ected into reservoir to get desired partial pressure. The gas-circulating blower was then started . Some H2S and C02 would get ad­sorbed into alkanol amine solution . To compensate this, additional quantity of H2S and C02 gases were inj ected, so that the system is near atmospheric pres­sure. The approach to equilibrium is monitored with the help of a conductivity probe. Since the reaction of H2S and C02 with aqueous alkanolamine solution is ionic in nature, the concentration of ionic species re­mains constant after reaching equilibrium. The con­stant reading of conductivity probe over a two-three days means the equilibrium was achieved. At thi s stage, the gas compos ition was identical in the ce ll as well as in the reservoir. The reservoir was then iso-

TO CONDUCTIVITY METER

J ~TIC ]

I ' II

Fig. !-Experimental set-up for vapour liquid equilibrium meas­urement. I . Conductivity probe, 2. Equilibrium cell , 3. Saturator, 4. Gas reservoir, 5. Gas circul ating blower, 6. Magnetic sti rrer, 7. Constant temperatu re bath , and 8. Temperature Indi cator 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 reservoir with the he lp of liquid syringe . It was then well mixed by shaking and left for about 48 h so that the entire amount of H2S and C02 gases are absorbed into aqueous NaOH solution . A sample was taken from amine solution with the help of a gas-tight syringe and introduced into caustic soluti on to convert it into corresponding Na2S and Na2C03. With the he lp of sulphide and C02 ion-selective e lectrodes, both sam­ples were analyzed for sulphide and carbonate content and hence corresponding H2S and C02 content was back calculated both in the gas phase and in the liquid phase .

The sulphide and C02 ion selecti ve e lectrodes ORION (USA) make, were calibrated before the reading with the help of standard aqueous solutions of Na2S and Na2C03 respectively . It was ensured that all readings were obtained in linear behaviour of the electrodes.

Results and Discussion During the si multaneous removal of H2S and C02

the following equilibri a are established between H2S and C02 and a lkanol amine.

Reaction of amine with H2S

H2S+AmH HS- + Am H2+ ( Ia)

Ks = (AmH; ).(HS - ) ( lb) (H 2S). (AmH)

244 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2000

Reaction of amine with C02

C02+2AmH ... AmH2+ + Amcoo-

K _ (C0 2) (AmH) 2

c- (AmH ; )(AmCOO- )

Dissociation of H2S

H2S HS- + H+

(H +) (HS- ) K - --'-------'-------'----

Hs - (H 2S)

Profanation of amine

(AmH)+H+

K = (H+) (AmH)

" (AmH ; )

Hydrolysis of carbamate

(2a)

(2b)

(3a)

(3b)

. .. (4a)

. . . (4b)

Amcoo- + H20 +:::=:::~., (AmH)+HC03- .. . (Sa)

K = (AmH) (HCO~ ) eq (AmCOO-)

Hydration of C02

C02+ H20

K _ (H+) (HCO~ ) I - (C02)

Amine dissociation in water

(AmH)+H20 .,. ., (AmH/)+OH-

K _ (AmH ; ) (OH - ) b- (AmH)

Dissociation of water

H20 H+ + OH-

Kw = (H+) (OH - )

Dissociation of bicarbonate HC03- C03

2- + H+

(C0 2- ) (H +) K - 3 2

- (HCO~ )

... (Sb)

... (6a)

... (6b)

... (7a)

... (7b)

. .. (8a)

... (8b)

. . . (9a)

. . . (9b)

Equilibrium pressure of H2S

P*H2S= (H 2S)

HH 2S

Equilibrium pressure of C02

p * CO2 = (CO 2) Hco 2

... (I 0)

. . . (II)

The absorption of H2S and C02 in aqueous solution at the microscopic level consists of the reaction of dis­solved H2S and C02 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 H2S.

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 bulk. 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 . Maha­jani 6 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 Equilibrium' .

First Equilibrium The chemical reaction between H2S-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 H2S and C02 with OH- can be ne­glected.

JAGUSHTE & MAHAJANI: LOW PRESSURE EQUILIBRIUM BETWEEN H2S AND C02 245

Table l-H2S-C02 aqueous diethanolamine (DEA) equilibrium data at 313 K by ac tual experimental technique

System Partial pressure of Carbonation ratio Partial pressure of Sulfidation ratio C02 (kPa)

H2S- C02.-DEA (2M) 0.143 0.070 0.064 0.78

Let the degree of carbonation a.1 and degree of sul­

phidation a.2, may be defined as

a.1= kmols C02 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

Eqs (I b), (3b) and (4b), give,

Ks = KHs = (HS-) (AmH;)

Ka (H 2S) (AmH)

... ( 12)

... (13)

(14)

.. . ( 15)

. .. ( 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 H2S over amine solution is given by

P * H ?S=~ I a.2(a.l +a.2) (AmH)o ... ( 19) - K HS HH 2S (1- 2a.,- a. 2)

Similarly, substituting for (AmCOO-), (AmH) and (AmH/ ) in Eq. (2b) from Eqs (12), ( 13) and (14), it was found that

(20)

.. . (2 1)

(a I) H2S (kPa) (az)

0.182 0.058 0.020 0.150 0.074 0.038 0.101 0.130 0.062 0.131 0.170 0.075

Eqs (2b), (4b), (Sb) and (6b) give,

... (22)

Thus, from Eq. (9), the equ ilibrium partial pressure of C02 over amine solution is g iven by

P * C02=Keq Ka a.l(a.l+a.2) ... (23) Kl H co2 (1- 2a.l - a. 2)2

The Eqs ( 19) and (23) could be used to correlate 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 established. 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 H2S 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 lution which is in equi librium, one has :

Amine balance equation

(AmH)o=(AmH)+(AmH2 +)+(Amcoo-) ... (24)

Carbon dioxide balance yields

... (25)

and H2S balance gives

.. . (26)

246 INDIAN 1. CHEM. TECHNOL., SEPTEMBER 2000

Table 2-Various equilibria as a function of temperature

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 Perrin7)

2 First dissoci ation constant for H2S in water

Reference

pKHs = -106.67 + 6045.2 IT+ 37.744 log T , KHs= (kmol.m-3)

3 Solubility of H2S in water

log HH2s = 651.2 I T- 8.206 , H H

2s = (kmol. m·3 Pa. 1

)

4 Solubility of C02 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, a= 3.12055 x 10·2R

298 < T < 323 Kc4 = (kmol. m·3 )

b =-I 0. 887 I 8

[BlauwhoffiC1]

(From the data given by Blauwhoff10, we have derived the above correlation)

6 Equilibrium constant fo r C02 hydration log 10 K1 = -3404.7 I T + I 4.843 - 0.03279 T (kmol m·3)

The electroneutrality equation results in

... (27)

from Eqs (25) and (26), gives

. . . (28)

substituting above values of (AmH2 +) and (AmCOO-) in Eq. (24), one gets

(AmH)0=(AmH)+(AmH2 +)+(AmCOO-) . . . (29)

=(AmH)+( a, +a2) (AmH)o+a, (AmH)o-(HC03 -)

. . . (30)

From Eq . (5b)

_ Keq (Amcoo-) (HC0

1 ) = _..:..___ __ _

. (AmH) . . . (3 1)

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)

[Danckwerts and Sharma 11]

Eq. (32) is quadratic equation in (AmH), therefore ,

2(AmH)= {-Keq+(2a,+a2-l ) (AmH)0 }

[K,q + (2a 1 + a 2 -1) (AmH) 0 f} +4Keq [l- (a 1 + a 2 )](AmH)

0

(33)

Substituting the values of (HS- ) and (AmH ; ) from

Eqs (26) and (27) in Eq. ( 16), one gets

Similarly, the partial pressure of C02 could be ob­tained from Eq. (2b) as

.. . (35)

Now,

.. . (36)

JAGUSHTE & MAHAJANI : LOW PRESSURE EQUILIBRIUM BETWEEN H2S AND C02

Table 3a-Comparison of computed equilibrium partial pressures of C02 from the model with the actual experimental value

Experimental equilibrium partial

pressure of C02 Carbonation P*co2 (kPa)

ratio

( al)

0.182 0.143 0.150 0.070 0.101 0.064 0.131 0.078

For C02

P1 = ~xp(-a 1~ -a2~)} a1 = 3.520

Predicted equilibrium parti al pressure of

C02 P * C0 2 (kPa)

0.708 0.495 0.250 0.478

a2 = 1.727

CPI)

0.174 0.183 0.212 0.174

Computed equilibrium partial pressure of C02

p* co2 =P*C02 x

PI (kPa)

0.123 0.090 0.053 0.083

Table 3b-Comparison of computed equilibrium partial pressures of H2S from the model with the actual experimental value

Sulfidation Experimental equilibrium Predicted equilibrium Computed equilibrium ratio partial pressure of partial pressure of H2S partial pressure of H2S ( a 2) H2S (kPa) P * H2S (kPa) P * H2S = P H2Sxp2

<P2) (kPa)

0.020 0.058 0.337 0.157 0.053 0.038 0.074 0.566 0.154 0.087 0.062 0.130 0.736 0.168 0.124 0.075 0.170 1.230 0.135 0.166

For H2S

P2 =~xp(-b1~ - b2~)} bl = 3.408 b2=2.815

247

Therefore substitution of (AmH ; ), (Amcoo- ),

Pc02 and K c in Eq. (35) gives,

~ Keq K.,

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

P *C0 2 = ' Kl H co2

[{(l-a 1 -a2 ) (AmH)0

}-(AmH)](a1 +a2 )(AmH)0

(AmH)2

. . . (38)

Eqs (34) and (38) can now be used to obtain partial pressures of H2S and C02 over aqueous 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

a 2.

Thus,

P *C0 2 = P *C0 2 ~ 1

P * C0 2 = P *C0 2 ~xp(-a 1 .Ja: -a2 ~)}

(39) where •

. . . (40)

248 INDIAN J. CHEM . TECHNOL., SEPTEMBER 2000

... (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).

Table I presents data obtained by actual experi­mental technique. This agrees reasonably well with that reported by Lal et a/.5

Various equi libria used to compute equilibrium pressure have been presented in Table 2. The com­puted values of equilibrium partial pressure of C02

and H2S are compared with the actual experimental values in Table 3a and 3b respectively, and constants a,, a2, b1 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 would help design engi­neers .

Conclusion A simplified approach to correlate equilibrium

partial pressures of H2S 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 namely ~ 1 and

~2 were introduced by using 'final equilibrium' ex­

pression to predict the observed data.

Nomenclature (Ami-1) =concentration of amine, (kmol.m.3)

(Am H) ., = original concentration of amine, (kmol.m.3)

(AmH 2 ) = concentration of piOtonated amine (kmol m·3)

(Am COO-) = concentration of carbamate(kmol nf·')

(C0 2) = concentrati on of C02 (kmol m·')

(I-1 2S) =concentration of H2S, (kmol m··')

I-1 1-12s =solubility of H2S, (kmol.m·3.Pa. 1

)

K a, (pKa) =equilibrium constant in Eq. (4b), (kmol.nf·')

K b , (pK b) =equilibrium constant in Eq. (7b), (kmol.m··')

Kc =equilibrium constant for COramine reaction in Eq. (2b),

(kmol m·3)

K =carbamate hydrolysi s equilibrium constant in Eq. (5b), eq

(kmol m-3)

KHs =equilibrium constant for reaction of amine with H2S in

Eq. (3b), (kmol.m.3)

K1 =equilibrium constant for C02 hydration in Eq. (6b),

(kmol m·3)

K2 =equilibrium constant for dissociation of bicarbonate in Eq.

(9b), (kmol m·3)

P * co2

=experimental equilibrium partial pressure of C02, (kPa)

P *H2s =experimental equilibrium partial pressure of H2S, (kPa)

P * C02 =predicted equilibrium partial pressure of C02 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 C02 in the

bulk of gas phase (kPa)

P * H2S =computed equilibrium partial pressure of H2S in the

bulk of gas phase, (k Pa)

a 1 =,carbonation ratio, kmols C02 absorbed I kmols of amine

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

Kohl A L & Riesenfeld F C, Gas Purification, Third Ed . (Gulf Pub. Co., Houston), 1974.

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.

4 Deshmukh R D & Mather A E, Chem Eng Sci, 36 ( 1981) 355.

5 Lal D, Otto F D & Mather A E, Can 1 Chem Eng, 63 (1985) 681.

6 Mahajani V V, Gas Sep Purif, 7 ( 1993) 53 .

7 Perrin D D, Dissociation Constants of Organic Bases in Aqueous Solutions (Butterwort, London), 1965.

JAGUSHTE & MAHAJANI: LOW PRESSURE EQUILIBRIUM BETWEEN H2S AND C02 249

8 Bosch H, Solvents and reactors for acid gas treating, Ph D Thesis, Twente University of Technology, The Netherlands , 1989.

9 Versteeg G F, & van Swaaij WPM , J Chem Eng Data , 33 (1988)29.

I 0 Blauwhoff P M M, Solvents and reactors for acid gas treat­ing, Ph D Thesis, Twente University of Technology, The Netherlands, 1982.

II Danckwerts P V & Sharma M M, The Chern Eng (I Chem E Rev Ser ). 1966, CE244.