Simultaneous Separation of H2S and CO2 from Biogas by GasâLiquid

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Simultaneous separation of H2S and CO2 from biogas by gasliquid membrane contactor using single and mixed absorbents Pengrui Jin, Chuan Huang, Yadong Shen, Xinyuan Zhan, Xinyue Hu, Lei Wang, and Liao Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02114 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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Simultaneous separation of H2S and CO2 from biogas by gas-liquid membrane contactor using single and mixed absorbents Pengrui Jin,ab Chuan Huang,*ab Yadong Shen,ab Xinyuan Zhan,*ab Xinyue Hu,ab Lei Wangab and Liao Wangab a

State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University,

Chongqing 400044, People’s Republic of China. E-mail: [email protected]. b

College of Resources and Environmental Science, Chongqing University, Chongqing 400044,

People’s Republic of China. *Corresponding authors: [email protected]

ABSTRACT The present work studied the simultaneous separation of H2S and CO2 from biogas by gas-liquid membrane contactor (GLMC) using single and mixed absorbents. The synthetic biogas contained 300 to 900 ppm H2S, 30% to 50% CO2 and CH4. To better understand the effects of different absorbents on simultaneous separation of H2S and CO2 from biogas, water, monoethanolamine (MEA, primary amine), potassium carbonate (K2CO3, inorganic salt), potassium hydroxide (KOH, inorganic salt), and potassium sarcosine (PS, organic salt) were applied as absorbent solutions. Poly(vinylidene fluoride) (PVDF) hollow fiber membrane was

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used in the membrane contactor modules. The simultaneous absorption performance of CO2 and H2S into single and mixed absorbents were investigated. In addition, the effects of liquid and gas velocities, absorbent concentration, acid gas content of the feed gas, and gas pressure, on the absorption performance and the analysis of mass transfer coefficients were investigated. The results indicated that the highest H2S absorption flux was obtained when KOH and K2CO3 were used as single absorbents, and the highest CO2 flux was obtained using PS as the single absorbent. The use of promoted K2CO3 with PS solutions could simultaneously improve the absorption flux of H2S and CO2. Increasing the liquid flow rate and absorbent concentration led to an increase in the CO2 absorption flux, while increasing the gas flow rate led to a significant increase in H2S absorption; The change of liquid flow rate has little effect on H2S absorption flux. A long-term stability test revealed that partial wetting of membrane could reduce the CO2 absorption flux; but has little effect on H2S absorption flux. The detailed analysis of the mass transfer coefficients showed that liquid side resistance was negligible in comparison with membrane and gas side resistances for H2S absorption. On the contrary, the mass transfer process of CO2 was controlled by liquid mass transfer resistance.

KEYWORDS: Membrane contactor; Simultaneous absorption; Potassium carbonate; Potassium sarcosine; Carbon dioxide; Hydrogen sulfide 1 Introduction It is believed that in the next few decades, biogas will play an important role in the world’s industrial and domestic energy supply. The removal of acid gases such as CO2 and H2S from biogas is a necessary step in the biogas purification process.1, 2 A large amount of acid CO2 in


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biogas will reduce the heating value of the biogas and corrode equipment. H2S is a very toxic and corrosive impurity which also must be removed before application of biogas.3, 4 Technologies for biogas treatment include absorption (physical or chemical solvent), solid adsorption and membrane separation.5-7 Implementation of these traditional absorption technologies has revealed some unavoidable shortcomings such as foaming, flooding, entraining, channelling, and high capital costs.8, 9 As a result, many researchers have been looking for new gas purification processes that can eliminate these shortcomings while maintaining lower operation costs. The gas-liquid membrane contactor (GLMC) is an efficient technology with the potential for large-scale application. It integrates membrane separation (low cost, high selectivity) and liquid absorption (high surface-area-to-volume ratio, modularity, compact equipment). In a GLMC device, as the gas and the absorbent flow along different sides of the microporous membrane, the impurities in the gas dissolve in reaction with the absorbent and are removed. Compared with traditional absorption processes, the GLMC permits independent control of gas and liquid flow rates, thereby preventing the aforementioned operational problems.10 Qi and Cussler pioneered use of the GLMC for the absorption of acid gases.11, 12 Since their pioneering work, more studies have been done on CO2 removal than on H2S removal using GLMC.9, 13, 14 This imbalanced focus may be due to the need for additional protective measures to study H2S absorption, since H2S is highly toxic. There are also few literatures on the simultaneous absorption of CO2 and H2S. Wang et al.15 investigated the selective removal of H2S from gas streams containing CO2 by GLMC. They found that GLMC was very effective in the selective removal of trace H2S. Faiz and Al-Marzouqi continued their work by performing a modelling with an aqueous potassium carbonate solution as the absorbent.16


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In recent years, the performance of GLMC to absorb CO2 and H2S at the same time has been predicted by mathematical models. Keshavarz et al.17 designed a mathematical model with diethanolamine (DEA) as the absorbent for simultaneous absorption of CO2 and H2S from CO2H2S-N2 mixture in GLMC. The modelling results showed that membrane wetting has a greater impact on the absorption of CO2 than H2S. A mathematical model with MEA as the absorbent was designed by Faiz and Al-Marzouqi.18 It was concluded that low concentration of MEA can effectively remove H2S, and increasing the concentration of MEA can increase removal efficiency of CO2. Recently, Hedayat et al.19 investigated simultaneous removal of CO2 and hydrogen sulfide using a PVDF and polysulfone GLMC and amine solutions. They found that in the H2S absorption process, compared with the membrane and gas side resistance, the liquid side resistance can be ignored. Marzouk et al. reported an experimental investigation of the high pressure simultaneous removal of CO2 and H2S from CO2-H2S-CH4 mixture in GLMC.20 The authors concluded that gas phase mass transfer resistance has a significant effect on the overall resistance at high pressure, although the effect of gas phase mass transfer resistance on the overall mass transfer resistance is negligible at low pressure. These studies15-20 show that the simultaneous absorption of H2S and CO2 using GLMC has been studied, both experimentally and theoretically, but analysis of acid gas simultaneous absorption performance of various types of absorbents is still lacking. Although aqueous alkanolamine solutions are widely used in the chemical industry for the absorption of acid gas, amino acid salts have some more advantageous characteristics.54,

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For example, in contrast to alkanolamine solutions, amino acid salts are

thermally stable and easy to be regenerated.9 However, thus far no experimental investigation has been conducted on the simultaneous removal of CO2 and H2S into potassium sarcosine (PS) solution using a GLMC. The use of mixed absorbents is an efficient method because it can


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simultaneously enhance the absorption of CO2 and H2S. However, to the best of our knowledge, no experimental investigation has studied on the simultaneous removal of CO2 and H2S using amino acid salt promoted carbonate solution in a GLMC. In this study, simultaneous removal of CO2 and H2S from feed gas similar to crude biogas by a GLMC using single (i.e. water, MEA, K2CO3, KOH, PS) and mixed absorbents (i.e. PS promoted K2CO3) has been studied. The effects of liquid and gas velocities, absorbent concentration and acid gas content of the feed gas, and pressure, on the absorption performance and the analysis of mass transfer coefficients were investigated. 2 Experimental 2.1 Materials Commercial-grade PVDF hollow fibers were purchased from Tianjin Haizhihuang Technology Co., Ltd., China. The characteristics of the membrane module are listed in Table 1. KOH, K2CO3, and MEA were procured from Chongqing Chuandong Chemical Industry Co., Ltd., China. Sarcosine was obtained from Aladdin. All the experimental chemicals were analytically pure reagents. Feed gas A (CO2-H2S-CH4 system: Cin,co2: 40%, Cin,H2S: 300 ppm, CH4 balanced), feed gas B (CO2-H2S-CH4 system: Cin,co2: 40%, Cin,H2S: 600 ppm, CH4 balanced), feed gas C (CO2-H2S-CH4 system: Cin,co2: 40%, Cin,H2S: 900 ppm, CH4 balanced), feed gas D (CO2-H2S-CH4 system: Cin,co2: 40%, Cin,H2S: 0 ppm, CH4 balanced), feed gas E (CO2-H2S-CH4 system: Cin,co2: 30%, Cin,H2S: 300 ppm, CH4 balanced), and feed gas F (CO2-H2S-CH4 system: Cin,co2: 50%, Cin,H2S: 300 ppm, CH4 balanced) were obtained from Zhongli Industrial Gases Co., Ltd., China. Potassium sarcosine was prepared by the reaction of sarcosine and potassium hydroxide.21


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2.2 Experiment The pH value, surface tension and viscosity were measured with a pH meter (IP67, Mettler), surface tension meter (K11, Kruss) and rotary rheometer (Anton Paar MCR302), respectively. These measurements are shown in Table 2. All experiments were carried out at room temperature (25±2 °C) using a GLMC system as shown in Fig 1. Feed gasses (A-F) were fed into the tube side of the GLMC module, while the various absorbents were circulated through the shell side. The gas and liquid flow rates were adjusted and controlled by the mass flow controllers and liquid flow meter, respectively. In the experiment, to avoid the formation of bubbles in the liquid phase, the liquid phase operation pressure was controlled at approximately 0.11 bar higher than the gas phase pressure.22 The gas volumetric flow rates were measured with a soap film flowmeter. A peristaltic pump (JSX25/5 LIGAO) was used to deliver the liquid from the rich storage tank through a rotameter to the membrane contactor. A gas chromatograph (9790 Fu Li, TCD) was used to analyze the concentrations of CH4 and CO2. In addition, a gas chromatograph (9790 Fu Li, FPD) was used to analyze the concentration of H2S. All experimental results were collected after the system reached the steady state (less than 10 min). Because H2S is highly toxic, purified biogas was cleaned with KOH solution to absorb the remaining H2S. To ensure the safe operation, an H2S detector with a response range of 0-500 ppm was installed in the laboratory to detect any leaks. 2.3 Estimation of overall mass transfer coefficient in GLMC Film theory has been used to describe the resistance-in-series model in the gas absorption process. Fig. 2 shows the mass-transfer mechanism of the hollow fiber membrane with nonwetted pores. It can be seen that the interested gas diffuses from the gas phase boundary layer,


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penetrates the membrane, and comes into contact with the liquid phase boundary layer. If the liquid phase is on the shell side (i.e. if the gas phase is on the tube side), the overall mass transfer coefficient based gas phase (KOG) can be written as follows:23

= +

+

(1a)

= + +

(1b)

where KOG is the overall mass transfer coefficient on the basis of gas phase (m/s), and kG, kM, and kL are the gas, non-wetted membrane, and liquid phase mass transfer coefficients (m/s), respectively. di, do, dln are the inner, outer, and logarithmic mean diameters of the hollow fiber membrane (m), respectively. H represents the Henry’s Law constant. E is the enhancement factor. R is the mass transfer resistance. The tube side mass-transfer coefficient is described by the well-known Graetz-lévêque mass transfer correlation:24, 25

ℎ =

= 1.62

⁄!

(2)

where Sh is the Sherwood number, µG is the fluid velocity (m/s), L is the tube length (m), and DG is the diffusivity in gas phase (m2/s). Many correlations have been proposed to calculate the mass transfer coefficient on the shell side,25-27 but each one is applicable only under specific operating conditions. In general, the mass transfer coefficient on the shell side can be calculated by the following equation: ℎ = #$ % & '

(3)


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where Re and Sc are the Reynolds and Schmidt numbers, respectively. For the liquid flow in the shell side, the outside fiber diameters can be used as the characteristic length.24 Considering the physical properties of the liquid phase and the outside fiber diameters, the dimensionless numbers can be expressed as follows: ℎ =

$ =

(

(4)

(5)

)

and )

Sc =

(6)

where µL is the liquid velocity (m/s), νL is the viscosity of liquid phase (m2/s), and DL is the diffusivity in the liquid phase (m2/s). For entirely gas-filled membrane pores, i.e. non-wetted mode, the membrane mass transfer coefficient can be calculated using the following equation:28 , =

,.// Ɛ

(7)

1 2

where ƐM and lM are the porosity thickness of membrane, respectively. τM is the tortuosity which can be calculated from the following equation: 3 =

456Ɛ 7

(8)

Ɛ

DG,eff is the effective diffusion coefficient of gas in the gas-filled membrane pores (m2/s), which is a combination of molecular and Knudsen diffusivity. It is given as:


8

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,.//

8

= +

(9)

where DM and DKn are the molecular and Knudsen diffusion coefficients, respectively. 2.4 Estimation of the absorption performance The separation performance of the hollow fiber membrane module was determined by CO2 and H2S absorption flux, which can be calculated by the following equation:29 9: =

;< ×> , 6 ,?@ A×5B!.C×DDD 55.E×F×G

(10)

where Ji is the molar flux (mol/m2 s). Ci,in and Ci,out represent the inlet and outlet gas-phase concentrations of CO2 or H2S, respectively (%v/v). Qin and Qout are the gas volumetric flow rate at the inlet and outlet, respectively (m3/s). T is gas temperature (K), and S is the gas-liquid interfacial area (m2). 2.5 Breakthrough pressure Compared with wetted mode (liquid-filled pores), non-wetted mode is more conducive to the absorption of acid gas in GLMC, because the gas diffusion coefficient is higher in non-wetted mode. When the penetration pressure is larger than the pressure difference between the gas and liquid stream in the membrane pores, the porous membrane will not permit absorbents to enter the pores. This phenomenon can be estimated using the Laplace-Young equation: △I =

65JKLMN OP

(11)


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where △P is the penetration pressure or wetting pressure, KPa. σL is the surface tension of the liquid, mN/m. θ is the contact angle between the liquid phase and the membrane, °. rP is the membrane pore size, m. 3 Results and discussion 3.1 CO2 and H2S absorption performance with various absorbents 3.1.1 Single absorbents and effect of liquid flow rates. Fig.3 and Fig. 4 show the effect of liquid flow rate and types of solutions on CO2 and H2S flux for the five absorbents tested: water, 0.05M MEA, 0.05M KOH, 0.05M K2CO3, and 0.05M PG. The inlet gas contained CO2 40%v/v and H2S 300 ppm in the balance of CH4. Gas flow rate was fixed at 250 ml/min. Liquid flow rate was varied from 50 to 140 ml/min. The increase of the liquid flow rate had a good effect on the increase of CO2 absorption flux. The liquid phase mass transfer coefficient increased with increase of the Reynolds number or liquid flow rate (see Eq. (3)). A number of previous studies have reported a similar conclusion, namely that the mass transfer process of CO2 absorption is controlled by the liquid phase in GLMC.30, 31 In contrast, the liquid flow rate had little effect on the H2S absorption flux. Section 3.2 shows the mass transfer resistance calculation results (Table 4) that illustrate the effect of the liquid flow rate. The above results order the CO2 absorption flux of the absorbents as follows: PS＞MEA＞ KOH＞K2CO3＞water. This phenomenon may be due to the difference in reaction rate constants between CO2 and different absorbents. For example, the reaction rate constant at 30℃ between CO2 and PS was about 22368.49 m3kmol-1s-1,32 while the reaction rate constant between CO2 and hydroxide ions (OH-) was above 11,0000 m3kmol-1s-1 as reported by Kucka et al..33 In addition,


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the reaction rate constant between CO2 and MEA was about 7740 m3kmol-1s-1 at 30℃ according to Seda et al.’s calculation.34 It is clear that the separation efficiency of water is lower than that of chemical absorption (PS, MEA, KOH and K2CO3 as chemical absorbents). This is because the effect of physical absorption is determined by the solubility of acid gas in physical absorbents. Kentish et al. have reported that the reaction rate constant between K2CO3 and CO2 was lower than the reaction rate constant between MEA and CO2 at 35℃, and hence the absorption of CO2 by MEA under the same operating conditions is better than the removal of CO2 from K2CO3.35 Similar results have been found for single absorbents. For example, the CO2 absorption performance sequence is NaOH＞MEA＞K2CO3 as reported by Zhang et al..36 As can be seen from Fig. 4, the removal efficiency of H2S by different absorbents is KOH=K2CO3＞PS＞MEA＞water. This is because the higher the PH value of absorbent liquid, the greater the concentration of OH- produced by the absorbent liquid. Therefore, the chemical reaction rate of OH- and H2S in the high PH value of absorbent solution is higher, resulting in higher H2S flux.37 It can be seen from Table 2 that the PH values of the five solutions of KOH, K2CO3, PS, MEA, and water are 12.30, 11.07, 10.83, 10.20, and 7.13, respectively. 3.1.2 Single absorbents and effect of gas flow rates. The effect of gas flow velocity in the range of 150 to 300 ml/min on the CO2 and H2S absorption flux was studied using 0.05M PS and 0.05M K2CO3 as absorbents. The inlet gas contained CO2 40%v/v and H2S 300 ppm in the balance of CH4. Liquid flow rate was fixed at 110ml/min. The results are shown in Fig. 5. The results revealed that both CO2 and H2S absorption flux increases with an increase in gas velocity. This increase is due to improvement of the Reynolds number or gas velocity, as that improvement enhances the gas phase mass transfer coefficient (see Eq. (2)). Thus, the mass


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transfer resistance in gas decreases with the increase of the gas flow rate. Similar observations were reported in the literature.19, 38 At the same time, it can be found that the H2S absorption flux increases with the increase of gas flow rate more obviously than does the CO2. This finding indicates that gas side resistance is more significant in this process for H2S mass transfer than for CO2 mass transfer. More details of the mass transfer resistance are presented in Section 3.2. 3.1.3 Mixed absorbents. The comparison of CO2 and H2S absorption flux by using water, 0.05 M PS, 0.05 M K2CO3 and mixed absorbent (0.025 M PS+0.025 M K2CO3) is shown in Fig. 6 and Fig. 7. The inlet gas contained CO2 40%v/v and H2S 300 ppm in the balance of CH4. Liquid flow rate was varied from 50 to 140 ml/min. Gas flow rate was fixed at 250 ml/min. The purpose of these two figures is to show the effect of the mixed absorbents on the absorption properties. It can be seen from Fig. 6 that the promoted K2CO3 with PS enhances the CO2 flux significantly compared to not-promoted K2CO3. This result occurs because PS is an amino acid salt with a higher reaction rate constant with CO2 (see also Section 3.1.1). Fig. 6 also reveals that promoted K2CO3 with PS has even better performance than single PS. Surprisingly, this finding means that 0.05 M PS can have a lower CO2 absorption flux than mixed absorbent (0.025 M PS+0.025 M K2CO3). To explain this result, it should be noted that the surface tension of PS/K2CO3 of 1:1 mole ratio at a total concentration of 0.05 M (71.29 mN/m) is higher than at a total concentration of 0.05 M PS solution (71.07 mN/m). The higher surface tension of the promoted K2CO3 with PS may result in lower probability of membrane wetting, leading to higher CO2 absorption flux than that in 0.05M PS solution (see Eq. (11)). Zhao et al. have reported that a high surface tension in the absorbent will improve the pore-wetting resistance of the membrane.9 In addition, the viscosity of promoted K2CO3 (1.055 mPa·s) was higher than that of


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0.05 M PS (0.984 mPa· s). Therefore, the promoted K2CO3 struggled to penetrate into the membrane pores, resulting in less membrane wetting. Lin et al. have reported that the wetting ability is related to the viscosity of the liquid absorbent.37, 39 Fig. 7 represents the effect of promoted K2CO3 on the H2S absorption flux. The H2S absorption performance sequence is promoted K2CO3=0.05M K2CO3＞0.05M PS＞water. The figure shows that using promoted K2CO3 with PS yields the highest H2S absorption flux. When the price and the performance of simultaneous separation of H2S and CO2 are taken into account, the promoted K2CO3 solution with PS exhibits better application potential as the absorbent in a GLMC. Moreover, Zhao et al.9 and Mehdipour et al.40 have reported that aqueous amino acid salts and K2CO3 solution are easy to regenerate. Fig. 8 compares the CO2 and H2S absorption flux per mixed absorbent (0.025 M PS+0.025 M K2CO3, 0.025 M PS+0.05 M K2CO3, 0.025 M PS+0.1 M K2CO3). The inlet gas contained CO2 40%v/v and H2S 600 ppm in the balance of CH4. Liquid flow rate was fixed at 110 ml/min. Gas flow rate was fixed at 250 ml/min. It is evident that increasing the amount of K2CO3 in the mixture enhances the CO2 absorption flux. This is because increasing the concentration of the absorbent liquid enhances the chemical reaction rate between CO2 and K2CO3. Similar experimental results were reported in other literatures.38, 40 According to Fig. 8, the value of H2S absorption flux does not vary with the blended absorption solutions concentration. In the case of promoted K2CO3 solution with PS, the H2S removal percentage is about 100%. Therefore, the increase of K2CO3 concentration in the blended absorption solution did not affect the H2S absorption flux.


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3.1.4 Effect of initial CO2 and H2S concentration. Fig. 9 shows the effect of CO2 concentration on CO2 and H2S flux when using mixed absorbent (0.025 M PS+0.025 M K2CO3) as the absorbent solution. Liquid flow rate was fixed at 110ml/min. Gas flow rate was fixed at 250 ml/min. The inlet gas contained CO2 30% to 50%v/v (for biogas) and H2S 600 ppm in the balance of CH4. Increasing the inlet CO2 concentration in the synthetic gas, thereby enhancing the driving force, improved CO2 absorption flux. Similar results have been reported by some literatures.16, 41 With the introduction of more CO2 in the synthetic gas, more CO2 reacted with absorbent. Thus, lower H2S absorption is achieved. The influence of H2S concentration on CO2 and H2S flux when using mixed absorbent (0.025M PS+0.025M K2CO3) as the absorbent solution is shown in Fig. 10. Liquid flow rate was fixed at 110ml/min. Gas flow rate was fixed at 250ml/min. The inlet gas contained CO2 40%, and the H2S concentrations varied between 0 and 900 ppm (for biogas) in the balance of CH4. The H2S absorption flux in this system increases with the increase of H2S concentration. In addition, the change of H2S concentration in the gas phase did not significantly influence CO2 absorption flux. In the current work, the concentration of H2S is much lower than CO2 (CO2/H2S about 450 times). Hence, the mass transfer process of CO2 was not influenced by change of H2S concentration. 3.1.5 Effect of feed gas pressure. Fig. 11 shows the effect of feed gas pressure on CO2 and H2S flux when using mixed absorbent (0.025M PS+0.025M K2CO3) as the absorbent solution. Liquid flow rate was fixed at 110ml/min. Gas flow rate was fixed at 250ml/min. The inlet gas contained CO2 40%v/v and H2S 600 ppm in the balance of CH4. It was evident that enhancing the feed gas pressure led to growth in CO2 absorption flux, because the solubility of CO2 under high pressure was improved. A similar result has been reported by Marzouk et al..42 In contrast,


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the increase in the feed gas pressure from 0.1 to 0.4 Mpa led to a reduction in the H2S absorption flux from 1.95×10-6 to 1.75×10-6 mol/m2s. Since this process is a simultaneous removal of CO2 and H2S, the increase in feed gas pressure resulted in more CO2 reacting with the absorbent and, thus, lower H2S absorption. Moreover, the gas diffusion coefficient is inversely proportional to the gas pressure.15 Therefore, the increasing feed gas pressure can also enhance the gas phase mass transfer resistance of H2S. The gas phase resistance controlled the mass transfer of H2S.38 Therefore the H2S absorption flux decreased with increasing the gas pressure. 3.1.6 Effect of operation time. To study the effect of operation time on membrane flux, experiments were conducted over a 5-hour period. Fig. 12 shows the effect of operation time on CO2 and H2S flux for using mixed absorbent (0.025 M PS+0.025 M K2CO3) as the absorbent solution. The inlet gas contained CO2 40%v/v and H2S 600 ppm in the balance of CH4. Gas flow rate was fixed at 250 ml/min. Liquid flow rate was fixed at 110 ml/min. The CO2 absorption flux decreased by approximately 30% after 90 min of operation. The decrease in the CO2 flux came from the partial wetting of the PVDF membrane. The surface tension of 0.025M K2CO3 + 0.025M PS was 71.29 mN/m, lower than the surface tension of water (see Table 2). To prevent wetting, the penetration pressure is larger than the pressure difference between the gas and liquid stream in the membrane pores. The Kelvin equation predicts that unsaturated water vapor is more prone to capillary condensation in small-sized channels.43 A similar result has been reported by Sadoogh et al.44 As can be seen from Fig. 12, with the increase in operation time, the H2S absorption flux did not change significantly. It can be inferred that while partial wetting of the membrane can reduce the CO2 absorption flux, it has little effect on H2S absorption flux. Because the value of E for


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H2S is high, the instantaneous reaction of H2S-K2CO3/PS occurs in the membrane pores that are wetted. Keshavarz et al.17 reported that a wetted membrane did not affect H2S absorption in low wettings from their mathematical model results. 3.2 Analysis of the mass transfer coefficients The individual mass transfer resistances, RG, RL and RM, were calculated by Eqs. (2), (3) and (7), respectively, using the properties listed in Table 3. The calculated values of KOG were obtained using Eq. (1). These values are shown in Table 4 and Fig. 13 and 14 for CO2 and H2S absorption based on the gas mixture with 40% CO2, 600 ppm H2S, and with 0.05 M MEA as the absorbent. Table 4, Fig. 15 and Fig. 16 show the percentages of CO2 and H2S mass transfer resistance in gas, membrane and liquid. As can be seen from Fig. 13, the overall mass transfer coefficient of H2S increases with increasing gas flow rate, while the overall mass transfer coefficient of CO2 is not significantly influenced by gas flow rate. The gas phase resistance has a great influence on the H2S mass transfer process, and the value of gas phase mass transfer resistance for H2S absorption is 68.36-73.13% of the total mass transfer resistance. Similar single gas H2S absorption studies have been reported in other literature.19, 45 For the mass transfer of CO2, the result was the opposite, i.e., the CO2 absorption was controlled by the resistance of liquid phase, because the value of liquid phase mass transfer resistance is very high. A similar result has been reported by Wu et al.46 and Rezaei et al.47. Taking into account the low reaction rate constant between CO2 and MEA, the amount of CO2 at the gas-liquid interface is much larger than that of H2S. The driving force (concentration difference between gas and liquid phase) of H2S is much lower than that of CO2. Therefore, the gas-phase is the main factor affecting the mass transfer of H2S.


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The results presented in Fig. 14 show that the overall mass transfer coefficient of CO2 increases as the liquid flow rate, but increasing the liquid flow rate has little influence on the overall mass transfer coefficient of H2S. This is because the enhancement factor (E) is much higher than the enhancement factor of CO2.48 In addition, it can be observed that increasing the liquid flow rate slightly increases the H2S absorption flux (see Fig. 4). This phenomenon may be caused by an increase in the gradient of the H2S concentration between gas and liquid. The reaction mechanism between MEA and H2S is as follows:49 R47 R457 NH + H5 S ↔ R47 R457 NH5 V + HS 6

(12)

For MEA, R(1) =(CH2)2OH, and R(2) =H. Since the reaction between H2S and MEA is reversible, increasing the liquid flow rate provided fresher MEA for the mass transfer zone of the absorbent, enhancing the positive reaction of Eq. (12). Thus, the reversible reaction of H2S-MEA is shifted to the right, and the concentration of H2S in the absorbent liquid is reduced. Unfortunately, the mass transfer coefficients of K2CO3 and promoted K2CO3 with PS solution cannot be calculated due to the lack of relevant parameters for the calculation of individual mass transfer coefficients. 4 Conclusions A comprehensive study on the simultaneous absorption of H2S and CO2 from synthetic biogas using a GLMC and different solvents (i.e. water, MEA, K2CO3, KOH, PS and PS promoted K2CO3) has been conducted. It was found that the highest H2S absorption flux was obtained when KOH and K2CO3 were used as single absorbents, and the highest CO2 flux was obtained using PS as the single absorbent. Both H2S and CO2 absorption fluxes were higher when using


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promoted K2CO3 with PS than when using single absorbents. The CO2 absorption flux significantly increased as the liquid flow rate increased, while the H2S absorption flux highly enhanced as the gas flow rate increased. Since the conference of CO2 in the synthetic biogas was much higher than that of H2S, the H2S absorption was strongly influenced by CO2 absorption. Increasing the concentration of CO2 and gas phase pressure will increase the CO2 absorption flux and decrease the H2S absorption flux. With the increase in operation time, the CO2 absorption flux declined while the H2S absorption flux did not change significantly. The mass transfer in the simultaneous absorption of H2S and CO2 was analyzed. It was found that the gas phase mass transfer resistance dominates in the mass transfer process of H2S, whereas the liquid phase mass transfer resistance contributes considerably in the mass transfer process of CO2.

Table 1. Specifications of the membranes used in this study solution Parameter (unit)

Values

Fiber o.d. (mm)

1.1

Fiber i.d. (mm)

0.8

Module i.d. (mm)

15

Number of fibers

37

Membrane pores size (µm)

0.2

Membrane porosity

0.7

Table 2. Surface tensions, viscosity, PH of the various absorbents


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Energy & Fuels

Absorbents Water MEA 0.05M K2CO3 0.05M KOH 0.05M PS 0.05M K2CO3 0.025M+PS 0.025M K2CO3 0.05M+PS 0.025M K2CO3 0.1M+PS 0.025M

Surface tension (mN/m) 72.12 71.84 71.61 70.83 71.07 71.29 71.51 71.57

Viscosity (mPa·s) 0.932 1.149 1.535 1.482 0.984 1.055 1.256 1.393

PH 7.13 10.20 11.07 12.30 10.83 10.99 11.10 11.36

Table 3. Values for the calculation of individual mass transfer coefficients of simultaneous absorption of CO2 and H2S in 0.05M MEA solution (298K and 1bar) Parameter Henry’s constant in liquid phase HCO2(-) HH2S(-) Diffusion coefficient in gas phase DCO2,G (m2/s) DH2S,G (m2/s) Diffusion coefficient in liquid phase DCO2,G (m2/s) DH2S,G (m2/s) Diffusion coefficient in membrane DM,H2S DKn,H2S DM,CO2 DKN,CO2 Enhancement factor ECO2 EH2S Others Density of feed gas (g/cm3) Density of liquid (g/cm3) Viscosity of feed gas (g/cm s) Viscosity of liquid (mPa s)

Value

Reference

0.835 2.3

50

1.64×10-5 1.70×10-5

51

1.92×10-9 1.52×10-9

50

1.07×10-5 2.87×10-5 1.07×10-5 2.52×10-5

52

1 1000

48

1.1×10-3 1.149 1.424×10-4 1.149

Ideal gas law Table 2

18

51

18

52 52 52

48

53

Table 2

Table 4. Gas, membrane and liquid phase mass transfer resistances of simultaneous absorption of CO2 and H2S in 0.05M MEA solution (non-wetted mode)


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Parameters CO2

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H2S

%RG %RM %RL

%RG

%RM

%RL

Gas flow rate (ml/min) 150

0.45

0.15

99.40

73.13 23.62 3.25

200

0.41

0.15

99.44

71.20 25.31 3.49

250

0.38

0.15

99.47

69.66 26.67 3.68

300

0.35

0.15

99.50

68.36 27.81 3.83

Liquid flow rate (ml/min) 50

0.22

0.09

99.69

67.92 26.00 6.08

80

0.30

0.12

99.58

69.05 26.44 4.51

110

0.38

0.15

99.47

69.66 26.67 3.67

140

0.44

0.17

99.39

70.04 26.82 3.14

Figure 1 Flow diagram of experimental setup for simultaneous absorption of CO2 and H2S.


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Energy & Fuels

Figure 2 A schematic drawing of the resistance-in-series based on film theory in non-wetted membrane contactor.

Figure 3 Effect of liquid flow rate and types of solutions on CO2 absorption flux (gas flow rate: 250ml/min, feed gas contained CO2 40%v/v and H2S 300 ppm in the balance of CH4).


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Figure 4 Effect of liquid flow rate and types of solutions on H2S absorption flux (gas flow rate: 250ml/min, feed gas contained CO2 40%v/v and H2S 300 ppm in the balance of CH4).

Figure 5 Effect of gas flow rate on CO2 and H2S absorption flux (liquid flow rate: 110ml/min, feed gas contained CO2 40%v/v and H2S 300 ppm in the balance of CH4).


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Energy & Fuels

Figure 6 Effect of single and mixed solutions on CO2 absorption flux (gas flow rate: 250ml/min, feed gas contained CO2 40%v/v and H2S 300 ppm in the balance of CH4).

Figure 7 Effect of single and mixed solutions on H2S absorption flux (gas flow rate: 250ml/min, feed gas contained CO2 40%v/v and H2S 300 ppm in the balance of CH4).


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Figure 8 Effect of K2CO3 concentration on CO2 and H2S absorption flux (gas flow rate: 250 ml/min, liquid flow rate: 110 ml/min, feed gas contained CO2 40%v/v and H2S 600ppm in the balance of CH4).

Figure 9 Effect of CO2 concentration on CO2 and H2S absorption flux (gas flow rate: 250 ml/min, liquid flow rate: 110 ml/min, feed gas contained CO2 40% to 50%v/v and H2S 600 ppm in the balance of CH4).


24

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Energy & Fuels

Figure 10 Effect of H2S concentration on CO2 and H2S absorption flux (gas flow rate was fixed at 250 ml/min, liquid flow rate: 110 ml/min, feed gas contained CO2 40%v/v and H2S 0 to 900 ppm in the balance of CH4).

Figure 11 Effect of pressure on CO2 and H2S absorption flux (gas flow rate: 250 ml/min, liquid flow rate: 110 ml/min, feed gas contained CO2 40%v/v and H2S 600 ppm in the balance of CH4).


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Energy & Fuels

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Figure 12 Effect of operation time on CO2 and H2S absorption flux (gas flow rate: 250 ml/min, liquid flow rate: 110 ml/min, feed gas contained CO2 40%v/v and H2S 600 ppm in the balance of CH4).

Figure 13 Effect of gas flow rate on the calculated overall mass transfer coefficients for CO2 and H2S absorption based on the gas mixture with 40% CO2, 600 ppm H2S in the balance of CH4, and 0.05 M MEA as the absorbent.


26

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Energy & Fuels

Figure 14 Effect of liquid flow rate on the calculated overall mass transfer coefficients for CO2 and H2S absorption based on the gas mixture with 40% CO2, 600 ppm H2S in the balance of CH4, and 0.05 M MEA as the absorbent.

(a)

(b)

Figure 15 Effect of gas flow rate on the calculated percentages of gas, membrane and liquid phase mass transfer resistances for (a) CO2 and (b) H2S absorption based on the gas mixture with 40% CO2, 600 ppm H2S in the balance of CH4, and 0.05 M MEA as the absorbent.


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Energy & Fuels

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(a)

Page 28 of 36

(b)

Figure 16 Effect of liquid flow rate on the calculated percentages of gas, membrane and liquid phase mass transfer resistances for (a) CO2 and (b) H2S absorption based on the gas mixture with 40% CO2, 600 ppm H2S in the balance of CH4, and 0.05 M MEA as the absorbent.

Acknowledgements This work was supported by the Chinese National Science and Technology Support Program (Contract No. 2014BAC29B01) for which due acknowledgement is given.


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Simultaneous Separation of H2S and CO2 from Biogas by GasâLiquid

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