Ind. Eng. Chem. Res. 2002, 41, 2953-2956
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Selective H2S Removal by Nonaqueous Methyldiethanolamine Solutions in an Experimental Apparatus Hong-Jian Xu,* Cheng-Fang Zhang, and Zhi-Sheng Zheng Research Institute of Chemical Technology, East China University of Science and Technology, P.O. Box 274, 130 Meilong Road, Shanghai 200237, People’s Republic of China
An experimental apparatus consisting of adiabatic packed absorption and regeneration columns was established to simulate the H2S removal process. With this apparatus, nine nonaqueous methyldiethanolamine (MDEA) solutions were evaluated under fixed operating conditions such as circulating solution flow rate and heating power of regeneration. The results indicated that the selectivity of H2S removal increased by 3.31 and 2.74 times with N-methylpyrrolidone and ethylene glycol as the nonaqueous solvents, respectively, but that the efficiency of desulfurization was somewhat lower compared with aqueous MDEA solutions. However, it is necessary to keep a limited amount of water in nonaqueous solutions that can cause stripping steam in the reboiler to prevent the solutions from overheating. For ethylene glycol and diethylene glycol as examples, the effect of the water content on the selectivity, efficiency of desulfurization, and stripping temperature were studied, and the results indicated that the efficiency of desulfurization would decrease significantly as a result of insufficient regeneration if the water content were lower than 0.5 mol/mol of amine. 1. Introduction Selective H2S removal from gases with high CO2/H2S ratios is increasingly valued in the field of gas purification. In view of some advantages such as energy savings, reduced corrosion, chemical stability, and lower solvent losses for lower vapor pressures, methyldiethanolamine (MDEA) solutions have successfully been applied in the natural gas industry, in oil refineries, and inClaus units.1-3 However, it is often difficult to obtain good selectivity for H2S removal with aqueous MDEA solutions, especially in the presence of a high content of CO2, and to achieve a qualified purification of the treated gas at the same time.4 To further increase the selectivity of desulfurization, much effort has been focused on the development of new gas-sweetening solvents for the removal of all kinds of sulfurous gases since the 1980s. Tertiary amines cannot react with CO2 in the absence of water, but they can instantaneously react with H2S through proton transfer with a fairly high reaction rate.5 The reaction is as follows
R3N + H2S f R3NH+ + HS-
(1)
R3N + CO2 + H2O f R3NH+ + HCO3-
(2)
The mechanism of reaction 2 requires that CO2 first be hydrated to carbonic acid, which is a relatively slow reaction.5 If the amine is anhydrous, CO2 hydration is almost completely avoided, and bicarbonate formation is almost negligible, whereas the H2S neutralization reaction still occurs because it is only proton transfer. According to this principle, nonaqueous solvents based on tertiary amines have been of great interest to some * To whom correspondence should be addressed. Telephone: 0086-21-64252386. Fax: 0086-21-64250884. E-mail:
[email protected] researchers.5-7 A suitable chosen nonaqueous mixed solvent might be superior to a pure solvent in terms of both selectivity and capacity, especially at high pressure.6 Gazzi et al.7 published the results of a pilot test of H2S removal in nonaqueous tertiary amine solutions in which very high selectivities were obtained. However, the type of nonaqueous solvent and the type of tertiary amine have not yet been reported in detail for proprietary reasons. In this article, an experimental apparatus consisting of adiabatic packed absorption and regeneration columns was established to simulate the selective H2S removal process. With this apparatus, nine nonaqueous MDEA solutions were evaluated under fixed operating conditions such as circulating solution flow rate and heating power of regeneration. 2. Experimental Section 2.1. Experimental Simulation Apparatus Diagram. The experimental simulation in this study was performed under atmospheric pressure. To simulate the industrial H2S removal process well, both the absorption and regeneration columns were adiabatic and made of stainless steel tubing with an inside diameter of 30 mm and a height of 900 mm, in which a 500-mm height of regular packing was packed. The regular packing was made of corrugated stainless steel sheet with a peak height of 2.2 mm and an angle of inclination of 45°. The structure of the adiabatic packed column is described elsewhere.8 A diagram of the experimental simulation apparatus for the selective removal of H2S is shown in Figure 1. Carbon dioxide and nitrogen gases having purities of 99.95% and 99.99, respectively, were blended with hydrogen sulfide made from a gas generator. The mixed gas was then continuously passed through the absorption column from its bottom, and the tail gas emitted at its top was measured with a flowmeter. The rich
10.1021/ie0109253 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/11/2002
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Figure 1. Diagram of experimental simulation desulfurization apparatus: 1, cylinder of carbon dioxide; 2, cylinder of nitrogen; 3, pressure buffer tube; 4, gas mixer; 5, absorption column; 6, regeneration column; 7, pump; 8, filter; 9, water cooler; 10, rich solution preheater; 11, acid gaseous condenser; 12, reboiler; 13, reflux gutter; 14, flowmeter.
solution drawn from the bottom of the absorption column was filtered and preheated; afterward, it flowed into the regeneration column, where it was stripped by a constant-heating power reboiler. The acid gases H2S and CO2 were stripped and separated from the regeneration steam at the acid gaseous condenser. The condensate flowed back into the top of the regeneration column to maintain a constant concentration in the solutions. The lean solution drawn from the regeneration column, after being cooled and filtered, was sent back to the absorption column at a constant flow rate. After the recycling process maintained stability for a certain length of time time (at least 6 h), so that the lean and rich solution loadings did not change with time, samples of the solutions and gases were analyzed. The gas composition at the beginning and end of absorption was analyzed using gas chromatography. The CO2 loading in the liquid phase was determined by volumetric analysis. The H2S content in the liquid was determined by using iodine-thiosulfate titration with starch as the indicator. The water content in the solutions was measured by distillation methods every other hour. During the experiments, the loss of water in the regeneration column was made up according to the results of the analysis. It should be noted that the whole experimental system must be extensively purged with pure nitrogen prior during the experiment to prevent oxidative degradation of the solution and to avoid the formation of sulfur. 2.2. Determination of Experimental Operating Conditions. (1) Composition and Flow Rate of Feed Gas. Acid gases with high CO2/H2S ratios are often found in some natural gases, synthetic gases from coal gasification, and tail gases from sulfur plants.10 In this paper, the content of H2S in the feed gas was 1.964.82%, and that of CO2 was 40.1-70% (mole ratio). The ratio of CO2/H2S was 11.2-32.2. The flow rate of feed gas was limited by the efficiency of desulfurization at a constant flow rate of circulating solution. In this experiments, the flow rate of feed gas ranged from 60 × 10-3 to 140 × 10-3 m3/h (at standard state). (2) Flow Rate of Circulating Solution. When the flow rate of the circulating solution was too great, flooding would occur in the regeneration column. When it was too low, the absorption solution could not be distributed evenly in the packing, and a state of gasliquid equilibrium was reached in the bottom of absorp-
tion column, thus causing abnormal results. In this study, the flow rate of the circulating solution was 1.7 × 10-3 m3/h. (3) Entry Temperature of Regeneration and Absorption Columns. The rich solution prior to regeneration is always preheated by a heat exchanger in the industrial process. In this study, the entry temperature of the regeneration column was adjusted to about 90 °C by setting the heating power of the solution preheater to 35 W, and the entry temperature of the absorption column was fixed at 22 °C. (4) Heating Power of Regeneration. It is evident that MDEA degradation is a strong function of temperature.9 When the heating power of regeneration is too high, the temperature of the solution in the reboiler is also excessively high, which might accelerate the tendency of thermal degradation of the solvent. In contrast, when the heating power is too low, the regeneration is incomplete. Through experimentation, a fixed reboiler heating power of 60 W was found to be appropriate. 3. Solvents Selection and Method of Its Evaluation The absorption and regeneration characteristics of nonaqueous MDEA solutions for the selective removal of H2S were evaluated simultaneously in this experimental simulation apparatus under constant operating conditions such as circulating solution flow rate and heating power of regeneration. The principles of selection of the nonaqueous solvents were as follows: the solvents must be nontoxic, noncorrosive, chemically and thermally stable, and regenerative; they must have a high absorptivity for H2S and a low absorptivity for hydrocarbons; they must have a high boiling point (i.e., low vapor pressure), low freezing point, low specific heat, and low viscosity; and they must be available and inexpensive. Nine nonaqueous solvents were selected, and the evaluation results are shown in Table 1. Because of the high boiling points of the solvents, however, some water had to be present in the circulating solution to produce stripping steam in the regenerator and to avoid excessively high temperatures, which might cause the sovents to become degraded. The method of evaluation of desulfurization in this simulation apparatus included a consideration of three respects: the selectivity of desulfurization, the efficiency of desulfurization, and the characteristic of regeneration.
Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002 2955 Table 1. Test Data for Nonaqueous Solutions in Experimental Simulation Apparatusa organic solvent
xH2Ob yC′ RS RC′ yS yC yS′ RC RS′ (wt %) (mol %) (mol %) (mol %) (mol %) (mol/mol) (mol/mol) (mol/mol) (mol/mol)
PEG200 EG DEG TEG benzyl alcohol 1,2-propanediol DGME TBP NMP
70.0 3.35 4.22 5.15 4.11 4.91
3.16 2.15 3.18 2.36 4.82 3.31
67.8 69.1 70.0 50.0 53.9 58.0
0.09 1.20 0.53 0.65 1.76 1.26
64.6 68.2 70.5 48.5 53.2 56.5
0.0252 0.0107 0.0187 0.0165 0.0270 0.0244
0.129 0.0507 0.0350 0.0435 0.0485 0.0467
0.0069 0.0025 0.0047 0.0053 0.0049 0.0021
5.93
1.96
57.2
0.38
50.6
0.0232
0.171
4.48 4.61 4.46
2.43 2.42 2.62
47.5 50.0 40.1
0.87 0.92 0.51
43.5 49.0 39.5
0.0143 0.0147 0.0207
0.0609 0.0283 0.0248
ηCc (%)
R
Ta (°C)
Tb (°C)
0.0388 0.0093 0.0108 0.0154 0.0106 0.0113
38.8 41.1 34.2 41.2 42.5 38.5
27.8 25.7 24.9 25.1 26.2 25.8
108.1 134.9 124.1 130.8 130.9 123.7
0.0082
0.0496
42.7 29.5 114.9 83.8 26.3
0.0053 0.0046 0.0056
0.0174 0.0072 0.0095
39.5 27.2 140.4 67.8 17.6 4.04 0.88 41.3 25.3 126.9 64.0 6.89 9.89 2.17 44.1 24.7 117.4 81.4 5.94 15.1 3.31
ηS (%)
S
97.6 21.9 4.56 47.6 7.27 6.37 84.5 6.76 12.5 74.2 9.12 8.45 66.5 9.54 6.52 65.1 10.7 6.08
βd 1.00 1.39 2.74 1.85 1.43 1.33
3.61 0.79
a C b c MDEA ) 2.55 mol/L. Value of xH2O corresponding to a mole ratio of water to MDEA equal to 1. ηC ) 1 - yC′/yC × [(1 - yS - yC)/(1 - yS′ - yC′)]. d β ) Ratio of selectivity with nonaqueous MDEA solutions to selectivity with aqueous MDEA solutions.
Table 2. Effect of Water Content on the Performance for H2S Removala organic γb yS yC yS′ yC′ RS RC RS′ RC′ solvent (mol/mol) (mol %) (mol %) (mol %) (mol %) (mol/mol) (mol/mol) (mol/mol) (mol/mol) EG DEG
a
2 1 0.5 2 1 0.5
3.32 3.18 3.27 3.05 2.36 2.64
70.0 70.0 68.3 50.0 50.0 49.2
0.49 0.53 1.19 0.81 0.65 1.06
69.5 70.5 68.5 47.7 48.5 48.0
0.0184 0.0187 0.0211 0.0173 0.0165 0.0191
0.0498 0.0350 0.0336 0.0484 0.0435 0.0351
0.0030 0.0047 0.0103 0.0039 0.0053 0.0094
0.0082 0.0108 0.0127 0.0134 0.0154 0.0171
R
Ta (°C)
Tb (°C)
32.8 34.2 35.5 37.4 41.9 38.0
25.8 24.9 24.1 26.2 25.1 23.9
119.3 124.1 134.7 125.2 130.8 143.1
ηS (%)
ηCc (%)
S
86.9 11.7 7.81 84.5 6.76 12.5 65.9 5.92 10.8 75.8 13.0 6.27 74.2 9.12 8.45 62.0 7.76 7.98
CMDEA ) 2.55 mol/L. b γ ) Mole ratio of water to MDEA. c ηC ) 1 - yC′/yC × [(1 - yS - yC)/(1 - yS′ - yC′)].
The selectivity of desulfurization can be defined as the ratio of the H2S/CO2 content in the absorbent to that in the feed gases. It can be expressed as
S)
RS - RS′ yC RC - RC′ yS
The efficiency of desulfurization is given by the material balance as
ηS ) 1 -
yS′ 1 - yS - yC yS 1 - yS′ - y′C
A characteristic of regeneration can be judged by RS′ and RC′, the contents of H2S and CO2 in the lean solution. The smaller the value of RS′ or RC′, the easier the regeneration of the solvent, which would be more advantageous for the absorption of H2S into nonaqueous MDEA solutions. However, if the value of RS′ or RC′ were too high, the regeneration of solutions would be incomplete, which would cause difficulties in the selective H2S removal process. 4. Results and Discussion 4.1. Evaluation of Nonaqueous MDEA Solutions for the Selective Removal of H2S. First, an aqueous MDEA solution with a concentration of 2.55 mol/L was tested in this experimental simulation apparatus for the selective removal of H2S. Then, nine nonaqueous MDEA solutions with different kinds of organic solvents were compared with aqueous MDEA solutions in this experimental simulation apparatus under the fixed operating conditions mentioned previously. The experimental results are listed in Table 1. To guarantee qualified regeneration of the rich solution and to avoid degradation of the solution, the mole ratio of water to MDEA was adjusted to 1:1, while the content of water was changed from 3.35 to 5.15% (weight ratio) according to the different molecular weights of the organic solvents.
From Table 1, the nonaqueous solvents can be divided into two groups according to their effects on the selectivity of H2S removal compared with that of aqueous MDEA solutions. One group includes solvents that can increase the selectivity, such as NMP, EG, TBP, DEG, TEG, PEG200, and benzyl alcohol. In this group, the selectivity of H2S removal is increased by 3.31 and 2.74 times with N-methylpyrrolidone and ethylene glycol, respectively, but the efficiencies of desulfurization for these two solvents are only 81.4 and 84.5% (mole ratio). These values are somewhat lower than can be obtained with aqueous MDEA solutions, for which the efficiency of desulfurization can reach 97.6%. A reason for the lower efficiency of desulfurization could be that the solubility of H2S might be lower in nonaqueous MDEA solvents than in aqueous MDEA solutions. On the other hand, the rate of mass transfer would be suffer with increasing viscosity in the nonaqueous solutions. The second group includes solvents that decrease, rather than increase, the selectivity, such as 1,2propanediol and diethylene glycol monomethyl ether (DGME), which can be explained by the fact that the efficiency of CO2 removal for these solvents is abnormally high. The formation of complexes or the occurrence of a chemical reaction between CO2 and the solvent would explain this phenomenon.11 For the regeneration process, the loadings of H2S and CO2 in the nonaqueous MDEA solutions, RS′ and RC′, were much lower than the loadings in the aqueous MDEA solutions, because of the higher temperatures of regeneration caused by the high boiling points of the nonaqueous solvents. Therefore, the nonaqueous MDEA solutions can be regenerated more easily. The physical properties of the desulfurization solutions, including viscosity and solubility, are very important in affecting the selective H2S removal process. The solubility of H2S in nonaqueous solutions will be further studied in our future work. 4.2. Effect of Water Content on Performance for H2S Removal. Some water must be present in the circulating solution to produce stripping steam in the
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regenerator and to avoid excessively high temperatures of regeneration in the reboiler. In this paper, different contents of water, mole ratios of water to MDEA of 2/1, 1/1, and 0.5/1 and a constant MDEA concentration of 2.55 mol/L, were studied also in this experimental simulation apparatus. In Table 2 are reported the results for ethylene glycol and diethylene glycol as examples. It can be seen that the suitable mole ratio of water to MDEA is 1/1, for which the greatest selectivity was found. The reason for this finding is that the regeneration of the solutions is insufficient when the ratio of water to MDEA is less than 0.5 because of the difficulty in producing regeneration steam. This also leads to a dramatic drop in the efficiency of desulfurization. At the same time, the temperature of reboiling is excessively high, which might accelerate the thermal degradation of the solvents. In contrast, when the mole ratio of water to MDEA is increased, the selectivity of the nonaqueous solutions decreases because higher water-to- MDEA ratios favor of the co-absorption of CO2. These results indicate that the ratio of water to MDEA plays a very important role in nonaqueous solutions for selective H2S removal. 5. Conclusions An experimental simulation apparatus consisting of adiabatic packed absorption and regeneration columns was established to simulate the H2S removal process. With this apparatus, nine nonaqueous MDEA solutions were evaluated under fixed operating conditions, and the results indicated that the selectivity for H2S removal can be increased by 3.31 and 2.74 times with Nmethylpyrrolidone and ethylene glycol as the nonaqueous solvent, respectively. However, the efficiency of desulfurization was somewhat lower than can be obtained with aqueous MDEA solutions. For ethylene glycol and diethylene glycol as examples, the effect of different water contents on the selectivity and efficiency of desulfurization were also studied, and the results indicated that the efficiency and selectivity of desulfurization would decrease as a result of the insufficient regeneration of the lean solution if the water content were lower than 0.5 mol/mol of amine. Nomenclature CMDEA ) concentration of MDEA solution, mol/L R ) volume ratio of feed gas flow rate (at standard state) to circulating solution flow rate S ) selectivity of solution for H2S removal Ta ) exit liquid temperature of absorption column, °C Tb ) temperature of regeneration in the reboiler, °C xH2O ) weight percentage of water content, % y ) mole fraction of feed gas, %
y′ ) mole fraction of off gas, % η ) efficiency of acid gas removal, % R ) loading of acid gases in rich solution, mol/mol of amine R′ ) loading of acid gases in lean solution, mol/mol of amine β ) ratio of selectivity with nonaqueous MDEA solutions to selectivity with aqueous MDEA solutions γ ) mole ratio of water to MDEA Subscripts S ) H2S C ) CO2 Amine Abbreviations MDEA ) methylenediethanolamine EG ) ethylene glycol DEG ) diethylene glycol DGME ) diethylene glycol monomethyl ether NMP ) N-methyl-2-pyrrolidone PEG ) triethylene glycol TBP ) tributyl phosphate
Literature Cited (1) Sigmud, P. W.; Butwell, K. F.; Wussler, A. J. HS Process removes H2S selectively. Hydrocarbon Process. 1981, 5, 118. (2) Blanc, C.; Elgue, J. MDEA process selects H2S. Hydrocarbon Process. 1991, 8, 111. (3) Mak, H. Y. Gas plant converts amine unit to MDEA-based solvent. Hydrocarbon Process. 1992, 10, 91. (4) Gazzi L.; Rescalli, C. Solvent-based process has very high H2S/CO2 selectivity. Oil Gas J. 1984, 16, 76. (5) Robert, N. M.; Gilbert, T. M.; Mahmud, A. R. Reactions of carbon dioxide and hydrogen sulfide with some alkanolamines. Ind. Eng. Chem. Res. 1987, 26, 27. (6) Rivas, O. R.; Prausnitz, J. M. Sweetening of sour natural gases by mixed-solvent absorption: Solubility of ethane, carbon dioxide, and hydrogen sulfide in mixtures of physical and chemical solvents. AIChE J. 1979, 25, 975. (7) Gazzi, L.; Rescalli, C.; Sguerea, O. Selefining Process: A New Route for Selective H2S Removal. Chem. Eng. Prog. 1986, 5, 47. (8) Zhang, X.; Zhang, C.-F.; Xu, G.-W.; Gao, W.-H.; Wu, Y.-Q. An Experimental Apparatus to Mimic CO2 Removal and Optimum Concentration of MDEA Aqueous Solution. Ind. Eng. Chem. Res. 2001, 40, 898. (9) Chakma, A.; Meisen, A. Methyldiethanolamine degradations Mechanism and kinetics. Can. J. Chem. Eng. 1997, 75, 861. (10) Savage, D. W.; Funk, E. W.; Yu, W. C.; Astarita, G. Selective absorption of H2S and CO2 into aqueous solutions of methyldiethanolamine. Ind. Eng. Chem. Fundam. 1986, 25, 326. (11) Henni, A.; Maham, Y.; Paitoon, T.; Chakma, A.; Mather, A. E. Densities and Viscosities for Binary Mixtures of N-Methyldiethanolamine + Triethylene Glycol Monomethyl Ether from 25 °C to 70 °C and N-Methyldiethanolamine + Ethanol Mixtures at 40 °C. J. Chem. Eng. Data 2000, 45, 247.
Resubmitted for review February 12, 2002 Revised manuscript received April 17, 2002 Accepted April 18, 2002 IE0109253
