Ind. Eng. Chem. Res. 1993,32, 3051-3055
306 1
SEPARATIONS Selective Removal of H2S from Gases Containing H2S and C02 by Absorption into Aqueous Solutions of 2-Amino-2-methyl-1-propanol Asit K. Saha,t Symalendu S. Bandyopadhyay,’J P. Sajuf and Asok K. Biswast Cryogenic Engineering Centre and Department of Chemical Engineering, Indian Institute of Technology, Kharagpur 721302, India
Selective absorption of H2S from N2 streams containing H2S and C02 into aqueous solutions of 2-amino-2-methyl-1-propanol was investigated in a 2.55 X le2m 0.d. stainless steel wetted-wall column a t atmospheric pressure and constant feed gas ratio. In the range of gas flow rates studied (90 X 10-6 to 180 X 104 m3/s), the effect of gas-phase resistance t o mass transfer on the absorption of H2Swas significant. The rates of absorption of H2S and the selectivity factor decreased with the increase in the contact time. For an increase in the concentration of the amine from 0.5 to 2.0 kmol/m3, the rates of absorption of both CO2 and H2S increased, but to a larger extent for C02 resulting in a consequent decrease in the selectivity factor. In the temperature range of 290.5-301.5 K the rates of absorption of C02 increased marginally with the increase in temperature while the rates of absorption of H2Sand the selectivity factor decreased. The maximum value of the selectivity factor observed in this work was 22.5.
Introduction Removal of acid gas components, H2S and C02, from sour natural gas, refinery gases, and synthesis gas by absorption into aqueous or mixed solutions of alkanolamines has become a well-established process. Alkanolamines widely used for this purpose are monoethanolamine (MEA), diethanolamine (DEA),N-methyldiethanolamine (MDEA), and di-2-propanolamine (DIPA). In industrial gas sweetening recently there has been an increasing interest in gas absorption processes for the selective removal of H2S from gas streams containing both C02 and H2S with a high C02/H2S ratio. The primary reason for this processing route is to increase the H2S/C02 ratio in the regenerator acid gas going to the sulfur recovery unit. In the case of low heat content fuel gas, the H2S must be removed for environmental reasons but C02 removal is not essential. Although the use of aqueous solutions of MDEA was proposed by Frazier and Kohl in 1950, the selectivity of MDEA for H2S was mainly of scientific interest at that time. However, a number of processing applications for the selective removal of H2S have arisen in the past few years, and MDEA technology has been reviewed by Goar (1980). Besides MDEA, DIPA has also been reported to have a greater selectivity for H2S over C02 than either MEA or DEA. DIPA has been used in the commercial ADIP process and as a constituent in the Sulfinol process (Klein, 1970). A number of studies on the simultaneous absorption of C02 and H2S in aqueous solutions of MDEA and DIPA have been reported (Vidaurri and Kahre, 1977;Savage et al., 1986;Blauwhoff and van Swaaij, 1985; Haimour et al., 1987;Yih and Sun, 1987). Recently a new class of amines, sterically hindered amines, has been introduced as important absorbents for the gas-sweetening processes (Sartori and Savage, 1983). These amines are capable of high loading of C02 because
* To whom correspondence should be addressed. t t
Cryogenic Engineering Centre. Department of Chemical Engineering.
0888-5885/93/2632-3051$04.00/0
their carbamates are very unstable due to the steric hindrance of the amine molecule. For severely hindered amines carbamate formation is not at all possible and these amines form bicarbonate directly (Sartori et al., 1987). 2-Amino-2-methyl-1-propanol (AMP) is a hindered primary alkanolamine that forms a carbamate with a much lower stability than the carbamates of MEA and DEA (Sartori et al., 1987). In these amines the formation of bicarbonate becomes the only important reaction, and loadings approaching 1 mol of COz/mol of AMP with appreciable rates are possible at moderate partial pressures of carbon dioxide. It is also indicated that hindered amines demonstrate very good selectivity toward H2S in the presence of C02 in the sour gas streams. Recently, hindered amine based H2S-selectivegas treating processes have been commercialized by Exxon Research and Engineering Company (Goldstein et al., 1986). It is claimed that the new hindered amine based processes are potentially attractive replacements for the existing selective HzS removal processes including MDEA-based and direct conversion processes. However, in spite of the importance of the sterically hindered amines as selective H2S removal agents, published information on simultaneous absorption of H2S and C02 in solutions of sterically hindered amines is extremely limited. Say et al. (1984)presented a process developmenmt work using hindered amine as the promoter of hot carbonate solutions for simultaneous absorption of HzS and C02. According to them the hindered amine promoted hot carbonate process offers a clear advantage over conventional promoters, e.g., DEA. Goldsteien et al. (1986) reported pilot plant results and results of a commercial test of a new hindered amine based selective H2S removal process, FLEXSORB SE. I t is claimed that FLEXSORB SE has a distinct advantage over MDEA for selective HzS removal with respect to circulation rate, which is much lower for the FLEXSORB SE process, that translates to substantial energy-saving. However, no detailed results about the selectivity and the rates for simultaneous absorption were presented. Ziodas and Dadach (1986) 0 1993 American Chemical Society
3052 Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993
reported coabsorption rates of COz and H2S mixtures into quiescent solutions of AMP. However, the influences of different parameters on the rates of absorption and selectivity were not investigated in detail. In this paper new results on the selective absorption of HzS in the presence of COz into aqueous solutions of AMP, a sterically hindered primary amine, are reported. The effects of amine concentration, contact time, and temperature on the rates of coabsorption of H2S and COZand the selectivity factor are presented.
Basic Chemistry Following the mechanism proposed by Danckwerts (1979)and subsequent experimental evidence (Blauwhoff et al., 1983),the general consensus for the reaction of C02 with primary and secondary alkanolamines is the formation of a zwitterion intermediate, rather than one-step carbamate formation. For an amine with a stable carbamate (e.g., for MEA), the following reactions take place: RR’NH + CO, + RR’NH’COO[R’ = H for primary amines] (1) RR’NH’COO-
+ B ==RR’NCOO- + BH’
(2)
Here B is a base which could be amine, OH-, or H20 (Blauwhoff et al., 1983). If the carbamate ion is unstable, as in the case of a sterically hindered amine, e.g., AMP, the following reaction proceeds subsequently (Sartori and Savage, 1983; Alper, 1990). RR’NCOO- + H,O
RR’NH + HC0,-
(3)
The reaction between H2S and aqueous amines involves a proton transfer as shown in eq 4, and can be regarded RR’NH
+ H,S * RR’NH; + HS-
(4)
as reversible and instantaneously fast with respect to mass transfer (Cornelissen, 1980). Everywhere in the liquid phase including the interfacial liquid film, HzS-amine equilibrium exists always. Since bicarbonate is the ultimate product of the COzhindered amine reaction (eq 3), it is reasonable to expect good HzS selectivity with the hindered amines as suppression of carbamate formation and, consequently, the rate of COz absorption, without affecting the rate of H2S absorption, should result in a better selectivity compared to that exhibited by the normal primary and secondary alkanolamines (Kohl and Riesenfeld, 1985). In fact, it is claimed that better selectivity can be obtained with the hindered amines than with the presently used tertiary or secondary alkanolamines (Kohl and Riesenfeld, 1985).
Selectivity Selectivity of a solvent for H2S may be defined as the tendency for the ratio of HzS to COZcontents to be larger in the liquid phase than it is in the gas phase. For a simultaneous absorption process involving absorption of HzS and COz, the selectivity factor S is used as a yardstick for the H2S selectivity. The selectivity factor is expressed as
concn of H,S s = (molar molar concn of CO, molar concn of H,S molar concn of CO, in this work.
Experimental Section Simultaneous absorption of COZand HzS was studied in a 2.55 X lo-, m 0.d. stainless steel wetted-wall column which was used earlier by Saha et al. (1993). Thermostated water was circulated through the double wall of the column and the jacketed glass shroud to control the temperature of the liquid film and the gas phase, respectively. The liquid was fed from an overhead vessel to provide aconstant flow rate and was thermostated before entering the wettedwall column. The liquid flow rate was measured with a rotameter. HzS, COz, and N2 at a volumetric ratio of 1:lO: 89 were metered with calibrated rotameters, mixed, and saturated with water at the temperature of absorption before entering the absorption chamber. The composition of the feed gas was determined from the respective flow rates. After allowing for steady state to be reached, sample solutions were collected from the outlet sampling points and immediately analyzed for HzS and C02 contents. HzS contents in the liquid samples were determined by titration with standard AgN03 solution (Savage et al., 1986). An autotitrator (Mettler DL-25) with a silver electrode (Mettler DM-141) was used for this titration. For the determination of COz content of the liquid, a known volume of the liquid sample was acidified with concentrated HzS04 and the volume of evolved gas measured with a gas buret. The evolved gas was the total H2S and COZcontent of the liquid sample. After temperature and pressure correction the HzS content of the liquid as found earlier by titration was subtracted from the total gas volume to get the COz content of the liquid sample. The temperature of absorption was controlled within about f0.2 K of the desired level. AMP was supplied by Loba Chemie Indoaustranal Company, Bombay, and was 97 % pure. To make amine solutions, freshly distilled water degassed by prolonged boiling was used. For COz and Nz commercialgas cylinders were used. The purities of both gases were better than 99%. HzS was supplied by Speciality Gas Company, Bombay, as 49% HzS and 51 5% N2 and was of calibration standard quality. The liquid flow rate for all runs was fixed at 2 X 10-6 m3/s. It was verified that the liquid flow rate was always in the laminar region. The gas flow rate was changed from 90 X lo4 to 180 X 10-6 m3/s to study the effect of gas flow rate on the rates of absorption and selectivity. Experiments were performed at 290.5,296.0, and 301.5 K. The total pressure in the absorption chamber was about 100 kPa. The amine concentrations were 0.5,1.0, and 2.0 kmol/ m3. The contact time was varied in the range 0.26-0.65 s. For a particular amine concentration and temperature the contact time was varied by changing the absorption to 10 X 10-, m. length from 5 X Results and Discussion Experimental data of simultaneous absorption are summarized in Table I, and the results are presented in Figures 1-7. Effect of Gas Flow Rate. Figure 1 shows the effect of gas flow rate on specific rates of absorption of HzS and COPand the selectivity factor at a particular temperature,
Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993 3063 Table I. Experimental Data for Simultaneous Absorption of HzS and COZin Aqueous AMP Solutions (Feed Gas Ratio H&COa:Nz = 1:1089;Pressure = 100 kPa)
l@V,,
run no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
m3/s 90 130 180 180 180 180 180 180 180 180 180 180 180 180 180
T,K
102h,m 7 7 7 5 10 10 7 5 7 10 7 5 10 7 5
301.5 301.5 301.5 301.5 301.5 296.0 296.0 296.0 290.5 301.5 301.5 301.5 301.5 301.5 301.5 a kL values in this table are a function of COz.
[AMPI, km0i/m3 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.5 0.5 0.5 2.0 2.0 2.0
RH&,
104kL,'
m/s 0.730 0.730 0.730 0.864 0.613 0.567 0.678 0.802 0.626 0.690 0.822 0.974 0.480 0.571 0.679
tc, s
0.394 0.394 0.394 0.284 0.562 0.589 0.412 0.294 0.432 0.526 0.368 0.263 0.648 0.454 0.324
kmol/(m2 s) 0.907 2.030 2.413 2.703 1.964 2.119 2.554 2.686 2.557 1.774 2.068 2.402 2.239 2.580 2.940
l@Rco kmol/(mfs) 0.880 1.275 1.318 1.381 1.299 1.173 1.278 1.193 1.217 1.005 1.062 1.101 1.934 1.858 1.952
S 10.3 15.9 18.3 19.6 15.1 18.1 20.0 22.5 21.0 17.6 19.5 21.8 11.6 13.9 15.1
3 . 5 -16
t
3 2.5"E 1 '.
E
Y v
1
-
-12
v)
a
51
1.5-
X
L Y
-8
80
120
V,XIO'
160
(rn'/s)
-
10' 0.2
20;
Figure 1. Effect of gas flow rate on specific rates of absorption and selectivity factor at 301.5K. [AMP], 1.0kmol/m3; t,, 0.394 s.
amine concentration, and contact time. It can be seen that in the range of 90 X lo4 to 180 X 10-6 m3/s gas flow rate the specific rates of absorption of both C02 and H2S increase with the gas flow rate. However, the effect of gas flow rate on H2S absorption is more significant, indicating a predominant role of gas-phase resistance to mass transfer for H2S absorption, while at a gas flow rate above 150 X 10-6 m3/s there is negligible effect of gas film resistance on the absorption of C02. As expected from the trends of absorption of C02 and H2S, the selectivity factor increases with the gas flow rate. Because of limitation of the experimental setup, the gas flow rate could not be increased beyond 180 X lo4 m3/s for this work, and all other runs for the present work were performed at a gas flow rate of 180 X lo4 m3/s. Effect of Contact Time. For a particular amine concentration and temperature the contact time was varied by changing the absorption length of the wetted-wall column from 5 X 1 0 - 2 to 10 X 1 0 - 2 m. Figure 2 shows the effect of contact time on the specific rates of absorption of C02 and HzS, and Figure 3 shows the effect of contact time on the selectivity factor. While the specific rate of absorption of C02 does not change with contact time, that of H2S decreases with increasing contact time, resulting in a decrease in the selectivity factor. This suggests that the absorption of C02 for simultaneous absorption of C02 and H2S in aqueous AMP with negligible interaction in the absorbed phase remains in the fast pseudo-mth-order
I
I
0.4
0.6
tc (4
-
I 0.8
Figure 2. Effect of contact time on specific rates of absorption in aqueous AMP at 301.5 K. *, 0.5kmol/m3 AMP; 0,1.0 kmol/ms AMP; 0,2.0 kmol/m3 AMP.
3.0
-
2.5
-
t h
m E
1 '.
E
2.0 -
Y
v
a
51
1.5
-
L1L
regime. This is in agreement with the observation of Yih and Shen (1988) for absorption of C02 alone in aqueous solutions of AMP. H2S reaction in amine is instantaneously fast. Hence, a decrease in the specific absorption rate of H2S resulting in a consequent decrease in the
3054 Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993 !8
4.0 3.0
I
1 3.0
h
Y
E
' I 2.0
E 5 2.0 E Y
e
\
t m
Y
V
V
D
2 B
D
E
li2
!2
Y
h
1.0
0.0 0.0
16
1.0
IO
0.0
0.5
1.0
1.5
[AMP] (krnol/rns)
2.0
-
i
288
292
296 T (K)
Figure 4. Effect of amine concentration on specific rates of absorption at 301.5 K for different absorption lengths. *, 5 X 10-* m; 0,7 x 10-2 m; 0,10 x 10-2 m.
300
304
-
Figure 6. Effect of temperature on specific rates of absorption and selectivity factor in aqueous AMP. [AMP], 1.0 kmol/ma.
3.0
t
20.0
h
Y
E 2.0
1
1
E
16.0
Y v
m
D
E 2
12.0
1.0
,
8.0 0.0
0.0
0.5
1.0
1.5
[AMP] (krnol/rn')
2.0
2.5
-
Figure 5. Effect of amine concentration on selectivity factor at 301.5' K for different absorption lengths. *, 5 X 10-2 m; 0,7 X 10-* m; 0, 10 x 10-2 m.
selectivity factor with increasing contact time, i.e., decreasing kL, is expected. For selective H2S absorption, therefore, a relatively lower contact time is preferred. Effect of Amine Concentration. The effect of concentration of AMP on the specific rates of absorption of C02 and H2S and the selectivity factor is shown in Figures 4 and 5 , respectively. Within the amine concentration range of 0.5-2.0 kmol/m3, the rate of absorption of COz is seen to increase rapidly with the amine concentration while that of H2S increases to a much lesser extent with the amine concentration. Thus, there is a consequent decrease in the selectivity factor with increasing amine concentration. It appears that the influence of increased amine concentration on the rate of absorption of H2S is adversely affected by the lower values of k L associated with the increase in viscosity with increasing amine concentration. In order to show clearly the change in kL associated with the change in the amine concentration, absorption length, and temperature, k L values corresponding to all runs have been estimated as a function of C02 using the N2O-analogy method (Saha et al., 1993) and presented in Table I. Effect of Temperature. The rates of absorption of C02 and H2S and the selectivity factor are plotted against temperature in Figure 6. It is observed from Figure 6 that
2813
292
296
T (K)
300
304"
-
Figure 7. Effect of temperature on specific ratee of absorption and selectivity factor at constant k ~ .[AMP], 1.0 kmol/mg; kL, 6.26 X 1W m/s.
with the rise in temperature from 290.5 to 301 K the rate of absorption of H2S decreases slowly while the rate of absorption of COz increases, although marginally, resulting in a decrease in the selectivity factor. In order to find out the effect of temperature on the selectivity factor free from the influence of the change in k L with temperature, the values of the selectivity factor for a fixed amine concentration of 1kmol/m3 are plotted against temperature for a fixed value of k L in Figure 7. This shows that even at constant k~ the H2S selectivity decreases with the rise in temperature. This may be due to the higher rate of increase of the CO2-amine reaction with the rise in temperature compared to that of the H2Samine reaction and possible desorption of H2S at higher temperatures. Similar findings have been reported by Savage et al. (1986) and Yih and Sun (1987) for simultaneous absorption of H2S and C02 in MDEA and DIPA, respectively. For absorption of H2S in aqueous MDEA Ouwerkerk (1978) observed that the rate of absorption was lower at higher temperature for low H2S pressures (up to about 40 mmHg). However, there was an inversion of the temperature effect on the rate of absorption at higher pressure. Ouwerkerk (1978) concluded that, at low H2S pressures, the effect of temperature on the equilibrium loading of the amine at the interface dominated the
absorption of HzS in amine and, at high HzS pressures, the temperature effect on viscosity and k~ dominated.
Conclusions For simultaneous absorption of HzS and COP from a feed gas with a constant H2S:COz molar ratio of 1 : l O at the contactor inlet, a significant effect of the gas-phase resistance on the rate of absorption of H2S and selectivity has been observed within the range of gas flow rate studied in this work. For COZabsorption the gas-phase resistance was negligible above a gas flow rate of 150 X 10-6 m3/s. H2S partial pressure in the feed gas was low as the emphasis of this simultaneous absorption studies was more on the selective absorption of HzS in the presence of COz. Under the conditions of this work absorption of C02 conformed to the fast pseudo-mth-order regime while HzS-amine reaction was instantaneously fast. HzS absorption rate and the selectivity factor were observed to be higher at shorter contact times, i.e., for higher values of k ~ .HzS selectivity decreased with increasing concentration of AMP. With the rise in temperature the selectivity was observed to decrease. In this work the highest value of the selectivity factor was found to be 22.5, which is comparable to the values of selectivity factor reported by Savage et al. (1986) for simultaneous absorption of H2S and COz in aqueous solutions of MDEA. Acknowledgment The authors gratefully acknowledge the assistance of Mr. R. N. Tarafdar of the Department of Chemical Engineering, IIT, Kharagpur, in the experimental part of the work. Nomenclature [AMP] = concentration of AMP in the bulk of the liquid, kmol/m3 h = length of absorption section, m k~ = liquid-film mass-transfer coefficient, m/s R = specific rate of absorption over contact time t,, kmol/ (m2.s) S = selectivity factor, defined by eq 5 t , = contact time, s T = temperature, K V, = volumetric gas flow rate, m3/s Literature Cited Alper, E. Reaction mechanism and kinetics of aqueous solutions of 2-amino-2-methyl-1-propanoland carbondioxide. Znd. Eng. Chem. Res. 1990,29,1725-1728. Astarita, G.; Savage, D. W. Gas absorption and desorption with reversible instantaneous chemicalreaction. Chem.Eng. Sci. 1980, 35,1755.
Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993 3055 Blauwhoff, P. M. M.; van Swaaij, W. P. P. Simultaneous Mass Transfer of HzS and COz with complex chemical reactions in an aqueous di-isopropanolaminesolution. Chem.Eng. Process. 1986, 19,67-83. Blauwhoff, P. M. M.; Versteeg, C. G.; van Swaaijk, W. P. M. A study on the reaction between COz and alkanolamines in aqueous solution. Chem. Eng. Sci. 1983, 38, 1411-1429. Cornelissen, A. E. Absorption of HzS and COz into aqueous alkanolamines. Trans. Znst. Chem. Eng. 1980,58, 242-250. Danckwerts, P. V. The reactions of COz with ethanolamines. Chem. Eng. Sci. 1979, 34, 443-446. Frazier, F. D.; Kohl, A. L. Selective absorption of hydrogen sulfide from gas streams. Znd. Eng. Chem. 1960,42, 2282. Goar, B. G. Selective gas treating produces better claus feed. Oil Gas J. 1980, 78, 239-242. Goldstein, A. M.; Brown, E. C.; Heinzelmann, F. J.; Say, G. R. New FLEXSORB gas treating technologyfor acid gas removal. Energy Prog. 1986,6,67-70. Haimour, N. K.; Bidarian, A.; Sandall, 0.C. Simultaneous absorption of HzS and COz into aqueous methyldiethanolamine. Sep. Sci. Technol. 1987,22,921-947. Johnson, R. R.; Say, G. R. Presented at the 3rd International Conference on control of sulfur and other gaseous emissions, University of Salford, England, April 1979. Klein, J. P. Developments in Sulfinol and Adip processes increase uses. Oil Gas Int. 1970,10, 109-112. Kohl, A. L.; Riesenfeld, F. C. Gas Purification; Gulf Publishing Co.: Houston, 1985. Ouwerkerk, C. Design for selective HzS absorption. Hydrocarbon Process. 1978, 4, 89-94. Saha, A. K.: Bandyopadhyay, S. S.; Biswas, A. K. Solubility and diffusivity of NzO and COz in aqueous solutions of 2-amino-2methyl-1-propanol. J. Chem. Eng. Data 1993, 38, 78-82. Sartori, G.; Savage,D. W. Sterically hindered amines for COz removal from gases. Ind. Eng. Chem. Fundam. 1983,22,239-249. Sartori, G.; Ho, W. S.; Savage, D. W.; Chludrinski, G. R.; Wiechert, S.Sterically hindered amines for acid-gas absorption. Sep. Purif. Methods 1987,16, 171-200. Savage, D. W.; Funk, E. W.; Yu, W. C.; Astarita, G. Selective absorption of HzS and COz into aqueous solutions of methyldiethanolamine. Znd. Eng. Chem. Fundam. 1986,25,326-330. Say, G. R.; Heinzelmann, F. J.; Iyengar, J. N.; Savage, D. W.; Bisio, A,; Sartori, G. Treating acid and sour gas: A new hindered amine concept for simultaneous removal of COz and HzS from gases. Chem. Eng. Process. 1984,10, 72-77. Vidaurri, F. C.; Kahre, L. C. Recover HzS selectivity from sour gas streams. Hydrocarbon Process. 1977,56 (Nov), 333-337. Yih, S. M.; Sun, C. C. Simultaneous absorption of HzS and COz into DIPA solution. Can. J. Chem. Eng. 1987,65, 581-585. Yih, S. M.; Shen, K. P. Kinetics of carbon dioxide reaction with sterically hindered 2-amino-2-methyl-1-propanol aqueous solutions. Znd. Eng. Chem. Res. 1988,27,2237-2241. Ziodas,A. P.; Dadach, Z.Absorption rates of COz and HzS in sterically hindered amines. Chem. Eng. Sci. 1986,41,405-408. Received for review August 6 , 1993 Accepted August 24, 1993. Abstract published in Advance ACS Abstracts, October 15, 1993. @
