Experimental Study of UDS Solvents for Purifying Highly Sour Natural Gas at Industrial Side-stream Plant

Process Research China Petroleum Processing and Petrochemical Technology 2016, Vol. 18, No. 1, pp 15-21 March 31, 2016 Experimental Study of UDS Solvents for Purifying Highly Sour Natural Gas at Industrial Side-stream Plant Ke Yuan; Shen Benxian; Sun Hui; Liu Jichang; Liu Lu; Xu Shenyan (State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237) Abstract: The desulfurization performance of the UDS solvents was investigated at an industrial side-stream plant and was compared with that of MDEA solvent. A mass transfer performance model was employed for explaining the COS absorption into different solvents. Meanwhile, the regeneration performance of the UDS solvents was evaluated in side-stream tests. Results indicate that under the conditions covering an absorption temperature of 40, a pressure of 8.0 MPa, and a gas to liquid volume ratio (V/L) of around 230, the H 2 S content in purified gas can be reduced to 4.2 mg/m 3 and 0 by using solvents UDS-II and UDS-III, respectively. Moreover, the total sulfur content in both purified gases is less than 80 mg/m 3. As a result, the UDS-III solvent shows by 30 percentage points higher in COS removal efficiency than MDEA. In addition, the total volume mass transfer coefficient of UDS solvent is found to be twice higher than that of MDEA. Furthermore, the UDS solvents exhibit satisfactory thermal stability and regeneration performance. Key words: highly sour natural gas; UDS solvent; COS; desulfurization 1 Introduction As a clean and efficient energy, the rapid development of natural gas has become a trend and requisite nowadays. With its increasing domestic demand, the proven reserve of Puguang gas field in Northeastern Sichuan of China is up to 381 billion m 3. The Puguang natural gas has a high content of sour components, especially a high carbonyl sulfide (COS) content [1-3]. As the requirements for natural gas purification are increasingly stringent, the academic and industrial investigation on efficient purification technology and high-performance desulfurization solvents have been attracting increasingly wide interests [4-6]. Thanks to a lot of advantages including the large processing capacity, a wide range of adaptability to sulfide concentration and the cheap equipment requirement, the absorption processes involving various aqueous alkanolamine solutions are considered as the most suitable technologies for removal of impurities from acid natural gas. However, the commonly used alkanolamines including monoethanolamine (MEA), diethanolamine (DEA), di-2-propanolamine (DIPA), and methyldiethanolamine (MDEA) feature a low efficiency for removal of organosulfurs, such as COS and mercaptans [7-9]. Therefore, it is really a challenge to reducing the total sulfur content in purified gas to a required low level, since the sour natural gas has a high organosulfur concentration. Based on the organosulfur removal mechanism in relation to different solvent components, the efficient formulated solvents called UDS (unitedly developed desulfurization solvent) have been developed in this lab and proven to have excellent performance for both H 2 S and organosulfurs removal [10]. The UDS solvents are composed of UDS formula component (UDS-F) and MDEA solvent. Furthermore, their formulas can be varied according to the compositions of raw natural gas in compliance with the purification requirements. The UDS formula component is mainly composed of alkanolamine compound, heterocyclic amine compound, and sulfur-containing heterocyclic compound. Previous investigations have indicated that the UDS solvents show excellent performance for removing sour components including organosulfur compounds from simulated highly sour natural gas [11-13]. In present work, we studied the desulfurization perfor- Received date: 2015-10-21; Accepted date: 2015-12-25. Corresponding Author: Prof. Shen Benxian, Telephone: +86-21-64252851; E-mail: sbx@ecust.edu.cn. 15

mance of UDS solvents in an industrial side-stream unit by using the actual natural gas feed under the operation conditions that were similar to typical industrial process. The results can provide some guidance for industrial application of the UDS solvents. 2 Experimental 2.1 Feed and reagents The actual natural gas from an industrial plant was used as the feed gas. Its average composition is given in Table 1. It had a high content of acid components, and the concentration of H 2 S and organosulfur compounds was 15.9% and 390.9 mg/m 3, respectively. The UDS solvent, which was prepared in this laboratory, contained more than 99% of active components, with its properties (on an anhydrous basis) shown in Table 2. UDS-I, UDS-II and UDS- III solvents contained 5%, 10%, and 15% of UDS formula component (UDS-F), respectively. The mass fractions of the UDS aqueous solutions were fixed at 50%. Deionized water was used in all cases. Table 1 Composition of sour natural gas feed Components Content CH 4 volume fraction, % 76.0 CO 2 volume fraction, % 8.5 H 2 S volume fraction, % 15.9 COS concentration, mg/m 3 368.8 MeSH concentration, mg/m 3 15.8 EtSH concentration, mg/m 3 6.3 Table 2 Properties of anhydrous UDS solvent Properties Value Content of active components, % 99.0 Density (20 ), g/cm 3 1.035.065 Viscosity (20 ), mpa s 90.0 2.2 Experimental apparatus and procedure The industrial side-stream experimental unit with a processing capacity of 80 Nm 3 of feed natural gas per h is shown in Figure 1. The highly sour natural gas feedstock enters the absorption tower at the bottom and contacts with the lean solution introduced from the top of the tower on the packing. The purified gas was sampled from the top and the content of sulfur compounds was analyzed by a gas chromatograph. The rich solution is heated at the regeneration tower for stripping the absorbed sour components. The regenerated lean solution is then discharged from the bottom of the regeneration tower followed by cooling prior to being recycled to the absorption tower for absorption of sulfur compounds. According to a typical industrial process, the absorption temperature is specified at 40 for these side-stream experiments. Figure 1 Flowchart of industrial side-stream unit for highly sour natural gas purification 1 Absorber; 2 Cooler of lean solution; 3 Lean solution circulation pump; 4 Rich solution heater; 5 Stripper; 6 Condenser; 7 Reboiler 2.3 Analytical method A Clarus 500 gas chromatograph equipped with a flame photometric detector (FPD) (Perkin Elmer Chromatograph Instrument Co., Ltd., USA) was used to analyze the sulfur content in the feed gas and the purified gas. An SE- 30 capillary column (Lanzhou Institute of Chemical Physics, China) was used to separate the sulfur compounds. 2.4 Efficiency for COS removal The efficiency E(%) for COS removal is calculated using the following expression: c E = 2 1 ϕ( H2S+CO2) 1 1 c1 1 ϕ(h2s+ CO 2) 2 (1) where c 1 and c 2 are the concentration of COS in the feed gas and the purified gas, respectively, mg/m 3 ; φ(h 2 S+CO 2 ) 1 and φ(h 2 S+CO 2 ) 2 are the total volume fraction of H 2 S and CO 2 in the feed gas and in the purified gas, respectively, %. 16

2.5 Total volume mass transfer coefficient The total volume mass transfer coefficient K g a (kmol/ (m 3 s kpa)) for COS is calculated using the following expression: j K a = c g PH ln 1 c 2 (2) where j is the total gas phase velocity, kmol/(m 2 s); P is the gas phase pressure, kpa; H is the packing height, m. 3 Results and Discussion 3.1 Mechanism for COS removal According to the composition analysis results presented in Table 1, COS accounts for the main part of organosulfurs in natural gas feed, suggesting that the overall efficiency of organosulfurs removal can be largely determined by the COS removal rate. Optimization in the formula of solvent should be focused on promoting its efficiency for COS removal. COS can be absorbed into tertiary amine like MDEA through hydrolysis reaction based on the alkali catalytic reaction (see Equation (3)). + - R 3N + COS + H2O R 3NH +HCO2S (3) In addition to the catalytic hydrolysis reaction of COS, primary amine and secondary amine can also react with COS through forming the zwitterions (RNH + 2 COS - and R 2 NH + COS - ) [14-16]. On the other hand, the primary amine and secondary amine components can catalyze the hydrolysis reaction of COS more effectively, and therefore, improve the chemical absorptive efficiency for COS removal [17-20]. The reaction can be written as Equations (4) and (5). + - RNH 2+COS RNH2COS + - H +RNHCOS (thiocarbamate) + - R 2NH+COS R 2NH COS + - H +R NCOS (thiocarbamate) 2 Furthermore, heterocyclic amine (R NH 2 ) is the major component of UDS solvent affecting the removal of COS. Compared with primary amines and tertiary amines, MOR has a faster reaction rate with COS [14, 20]. The reaction is shown in Equation (6). R 2 + NH 2 +COS R NH2COS + H R (6) + NHCOS ( thiocarbamate) (4) (5) Meanwhile, the UDS solvents have good physical solubility of COS. Components of molecules with the S=O groups in UDS-F solvent can enhance the combination of organosulfurs and solvent molecules, increase the solubility of COS in the solvent, and largely improve the selective absorption of organosulfurs. Therefore, the UDS solvents have better organosulfurs removal performance than MDEA solvent because of their high physical absorption and chemical absorption nature. Furthermore, according to the compositions of raw natural gas as well as the purification requirements, the removal performance of UDS solvents can be updated through adjusting the content of UDS-F solvent. 3.2 Purification performance of UDS solvent 3.2.1 H 2 S removal under different gas to solvent ratios Figure 2 shows the H 2 S removal performance of UDS-I, UDS-II and UDS-III at different gas to solvent volume ratios (V/L). The H 2 S removal performance of MDEA solvent under the similar operating conditions is also given for comparison. It can be seen from Figure 2 that three kinds of UDS solvents and MDEA solvent all exhibit excellent H 2 S removal performance. The H 2 S content in the purified gas can be reduced up to less than the detection limit at a V/L ratio of less than 200. As the V/L ratio rises to 230, different solvents show a clear distinction in H 2 S removal performance. As for MDEA solvent, the H 2 S content in purified gas is a highest value, 34.7 mg/m 3. As regards the UDS solvents, the H 2 S content in purified gas is 6.5 mg/m 3, 4.2 mg/m 3 and 0 for UDS-I, UDS-II and UDS-III solvents, respectively. Figure 2 Effect of gas to solvent ratio on the performance of different solvents for H 2 S removal at 40 MDEA; UDS-I; UDS-II; UDS-III 17

3.2.2 Total sulfur removal performance under different gas to solvent ratios At different V/L ratios, the performance of solvents for removal of total sulfur is shown in Figure 3. With the V/L ratio increasing from 140 to 230, the total sulfur content in the purified gas treated by MDEA is in the range of 150 to 290 mg/m 3. The total sulfur content of the purified gas is below 80 mg/m 3 at the same V/L ratio range upon using the UDS solvents. Three kinds of UDS solvents show significantly higher total sulfur removal performance as compared with MDEA solvent. It is mainly because the UDS solvents are designed according to the mechanism for removal of different types of sulfur compounds. Therefore, the UDS solvents not only can effectively remove H 2 S, but also reduce the total sulfur content in the purified gas to a lower level and improve the quality of purified gas. Figure 3 Effect of gas to solvent ratio on the performance for removal of total sulfur at 40 MDEA; UDS-I; UDS-II; UDS-III 3.2.3 CO 2 removal performance at different gas to solvent ratios Figure 4 shows the relationship between the CO 2 content in the purified gas and V/L of different solvents. With the increase of V/L ratio, the CO 2 content in purified gas follows a different rising speed. Because of the similar molecular structure of CO 2 and COS, the mechanism for removal of CO 2 is similar to that of COS in the absorption process. The composition of UDS solvents that can improve the physical and chemical dissolution performance of COS could also increase the CO 2 removal rate to a certain extent. Therefore, in comparison with MDEA solvent, the UDS solvents possess higher CO 2 absorption rate. As a result, varying performance for removal of CO 2 can also be achieved by adjusting the composition of UDS solvents based on different process requirements. Figure 4 Effect of gas to solvent ratio on the performance of solvent for CO 2 removal at 40 MDEA; UDS-I; UDS-II; UDS-III 3.2.4 Comparison of COS removal performance between UDS and MDEA solvents Judging from the analysis of sulfur compounds in feed gas shown in Table 1, the organosulfurs in raw materials contain mainly COS, so the organosulfurs removal performance of the solvent is mainly determined by the COS removal rate. The COS removal efficiency and total volume mass transfer coefficient of UDS solvents are compared with those of MDEA at a V/L ratio of 230, with the results presented in Figure 5. It can be seen that the COS removal efficiency and total volume mass transfer coefficient increase with an increasing content of UDS-F solvent. The COS removal efficiency of UDS solvents is by more than 30 percentage points higher than that of MDEA solvent. At a V/L ratio of 230, the total volume mass transfer Figure 5 Comparison on removal of COS between UDS and MDEA solvents at 40 (at V/L=230) E; K g a 18

coefficient of UDS-I is 8.55 10-7 kmol/(m 3 s kpa), which is twice higher than that of MDEA solvent, viz. 3.02 10-7 kmol/(m 3 s kpa). 3.3 Mass transfer performance model for COS absorption The UDS solvents have definite chemical and physical solubility of COS. But in the presence of H 2 S and CO 2, the chemical solubility of COS will be affected by different degree of inhibition. For the case of highly sour gas absorption process, there is a high concentration of H 2 S and CO 2. The acidic component load in UDS solution is higher. Therefore, we set up an absorption model for removing organosulfurs from highly sour natural gas by the UDS solvents [21-22] (see Equation (7)). mc 1 L bah mg exp y 0. 3 1 = G L (7) y mg 2 1 L where b is the mass transfer performance factor of COS, mol 0.3 m -0.6 s -0.3 kpa -1 ; a is the packing specific area, m 2 /m 3 ; H is the height of absorption tower, m; G is the gas phase flow rate, mol/s; L is the liquid flow rate, mol/s; m is the Henry constant of COS; y 1 and y 2 are the organosulfurs concentration in the gas feed and in the purified gas, respectively. The Henry constant m and the mass transfer performance factor b of COS in UDS and MDEA solvents are calculated using the above model, with the results listed in Table 3. The Henry constant m is used to characterize the capacity of COS dissolved, and the smaller m indicates the higher solubility of COS in the solvent. The mass transfer performance factor b characterizes the mass transfer performance of COS in the absorption process. Under the same condition, the greater mass transfer performance factor b of COS is more beneficial to the absorption process. The model parameters in Table 3 show that the Henry constant of COS for the UDS solvents is less than that of MDEA solvent, which indicates that the solubility of COS in the UDS solvent is higher than that in MDEA under the same absorption condition. At the same time, the mass transfer performance factor of COS in UDS solvent is higher than that in MDEA. The UDS-F component, which can improve the chemical absorption rate and the physical solubility of COS, provides UDS solvent with higher COS solubility and mass transfer performance. The industrial side-stream test results confirm our conclusion that the UDS solvents can improve the COS removal efficiency by more than 30 percentage points as compared with MDEA solvent. Table 3 Henry's constant and mass transfer factors of organosulfur for UDS solvents at 40 Solvent m b 10 3-1, mol 0.3 m -0.6 s -0.3 kpa MDEA 4.15 1.93 UDS-I 1.34 2.94 UDS-II 1.02 3.37 UDS-III 0.91 3.52 The model analysis is carried out to study the effect of operating conditions on the purification performance. For the same set of devices, the main adjustable parameters are the gas liquid ratio, namely the gas phase flow and the liquid phase flow. The effect of gas phase flow and liquid phase flow on the purification performance using the UDS-III solvent is shown in Figure 6. With the decrease of the gas phase flow or the increase of liquid phase flow, the purification performance becomes better. At a low gas velocity, a higher solvent circulation volume can improve the purification performance. But at a high gas velocity, an increasing solvent flow, which means reduction of the gas liquid ratio, does not have obvious effect on improvement of the purification performance. Therefore, an appropriate gas liquid ratio not only can improve the effect of purification, but also reduce the energy consumption. Figure 6 Model analysis on effect of operating conditions on purification performance 3.4 Regeneration performance of UDS solvents The regeneration performance of the solvent in the re- 19

cycling process is an important factor that can affect the absorption performance of the solvent, and the content of acid components in the lean solution will directly influence the quality of the purified gas. Industrial side-stream experiment examines the regeneration performance of the UDS and MDEA solvents. The results are listed in Table 4. Regenerative steam consumption is specified at 0.35 kg per kg of solution circulated. Under the same regeneration conditions, the contents of H 2 S and CO 2 in the UDS and MDEA lean solutions are maintained at under 0.04 mol/l, indicating to the good regeneration performance. Table 4 Content of H 2 S and CO 2 in the UDS and MDEA lean solutions Solvent H 2 S, mol/l CO 2, mol/l MDEA 0.025 0.028 UDS I 0.032 0.017 UDS II 0.023 0.014 UDS III 0.035 0.023 4 Conclusions The desulfurization performance of MDEA solvent and three kinds of UDS solvents was studied in an industrial side-stream unit. At an absorption temperature of 40, an absorption pressure of 8.0 MPa, a V/L ratio of about 230, the H 2 S content in purified gas was reduced to 34.7 mg/m 3, 6.5 mg/m 3, 4.2 mg/m 3, and 0 by using MDEA, UDS-I, UDS-II and UDS-III, respectively. In comparison with MDEA solvent, three kinds of the UDS solvents not only showed good effect of H 2 S removal, but also achieved better COS removal performance. When the V/L ratio changed within the range from 140 to 230, the total sulfur content in the purified gas ranged from 150 mg/m 3 to 290 mg/m 3 for MDEA solvent, while the total sulfur content in the purified gas was below 80 mg/m 3 for the UDS solvents. The UDS solvents achieved by more than 30 percentage points higher in COS removal efficiency as compared to that of MDEA. In addition, the total volume mass transfer coefficient of UDS solvent was found to be twice higher than that of MDEA. Furthermore, the UDS solvents exhibited satisfactory regeneration performance. Acknowledgements: The authors are grateful for the financial support from the National Key Science and Technology Project of China (2011ZX05017-005) and the Fundamental Research Funds for the Central Universities (No.22A201514010). References [1] Long S X, Zhu H, Zhu T, et al. Prospect of Sinopec's exploration for natural gas [J]. Natural Gas Industry, 2008, 28(1): 17-21 (in Chinese) [2] Li L, Chen J F, Xu L H. Component and carbon isotope characteristics of natural gas in the Puguang gas field, Sichuan [J]. Inner Mongolia Petrochem Indus, 2008(4): 106-108 (in Chinese) [3] Ma Y S. Geochemical characteristics and origin of natural gases from Puguang gas field in Eastern Sichuan Basin [J]. Natural Gas Geoscience, 2008, 19(1): 1-7 (in Chinese) [4] Ghanbarabadi H, Khoshandam B. Simulation and comparison of Sulfinol solvent performance with amine solvents in removing sulfur compounds and acid gases from natural sour gas [J]. Journal of Natural Gas Science and Engineering, 2015, 22: 415-420 [5] Luo X W. Development and application of natural gas purification techniques [J]. Natural Gas and Oil, 2006, 24(2): 30-34 (in Chinese) [6] Angaji M T, Ghanbarabadi H, Gohari F K Z. Optimizations of sulfolane concentration in propose sulfinol-m solvent instead of MDEA solvent in the refineries of Sarakhs [J]. Journal of Natural Gas Science and Engineering, 2013, 15: 22-26 [7] Zong L, Chen C C. Thermodynamic modeling of CO2 and H 2 S solubility in aqueous DIPA solution, aqueous sulfolane- DIPA solution, and aqueous sulfolane-mdea solution with electrolyte NRTL model [J]. Fluid Phase Equilibria, 2011, 306(2): 190-203 [8] Rivera-Tinoco R, Bouallou C. Reaction kinetics of carbonyl sulfide (COS) with diethanolamine in methanolic solutions[j]. Industrial & Engineering Chemistry Research, 2008, 47(19): 7375-7380 [9] Yu M, Zhou L. Review and forecast of purification of H2 S in natural gas[j]. Tianjin Chemical Industry, 2002(5): 18-20 (in Chinese) [10] Shen B X, Zhang J H, Chu Z, et al. High-efficiency purification desulfurizer for high-acid oil and gas: China Patent, ZL200910233505.1[P], 2014-04-02 20

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