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High pressure removal of acid gases using hollow fiber membrane contactors: Further characterization and long-term operational stability
Journal of Natural Gas Science and Engineering 37 (2017) 192e198
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High pressure removal of acid gases using hollow fiber membrane contactors: Further characterization and long-term operational stability Mohamed H. Al-Marzouqi a, *, Sayed A.M. Marzouk b, c, Nadia Abdullatif a a b c
Department of Chemical and Petroleum Engineering, College of Engineering, United Arab Emirates University, Al Ain, 15551, United Arab Emirates Department of Chemistry, College of Science, United Arab Emirates University, Al Ain, 15551, United Arab Emirates Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, 11566, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 August 2016 Received in revised form 19 September 2016 Accepted 17 November 2016 Available online 19 November 2016
Our previously reported high pressure hollow fibers membrane (HFM) modules were specifically developed with aim to meet the operation requirements for the industrial sweetening of natural gas. The present work presents further characterization of CO2 and H2S high pressure removal under different experimental conditions, which are in particular relevant to the ultimate intended industrial application. These include module packing density, feed gas/absorption liquid flow rate ratio, absorption liquid temperature as well as solvent loading. The operational stability of the HFM module, equipped with the PFA hollow fibers, was also investigated and reported. The presented results on the long-term operational stability of membrane modules based on hollow fibers, under industrially relevant conditions are believed to be the first report of its kind. The PFA-HFM modules showed excellent operational stability during the test period, i.e., ~200 operation hours over a 7-week period using pressurized feed gas pressure (50 bar) and hot absorption solvent (100 C). © 2016 Elsevier B.V. All rights reserved.
Keywords: Natural gas treatment Acid gases Membrane contactors Operational stability
1. Introduction The world's energy needs are projected to rise by 48% from 2012 to 2040 (International Energy Outl, 2016). Among different energy sources, natural gas represents the largest increase in world's energy utilization from 120 trillion cubic feet (Tcf) in 2012 to 203 Tcf in 2040 (International Energy Outl, 2016), which places natural gas in the spotlight. Despite this promising trend, almost all natural gas requires pretreatment before commercial utilization. Approximately 30% of the world's gas fields are significantly contaminated with CO2, H2S, in addition to some other impurities such as mercaptans and organic sulfides. Industrial natural gas sweetening aims to meet strict product specifications (Kohl and Nielsen, 1997; Alcheikhhamdon and Hoorfar, 2016; Mansourizadeh and Ismail, 2009) for H2S levels, for safety and environmental reasons, and CO2 for the required range of gross calorific value (GCV) (Lambert et al., 2006; Selene and Chou, 2003). Membrane system processes are attractive candidates for several industrial applications (Khulbe
* Corresponding author. E-mail address: [email protected] (M.H. Al-Marzouqi). http://dx.doi.org/10.1016/j.jngse.2016.11.039 1875-5100/© 2016 Elsevier B.V. All rights reserved.
and Matsuura, 2016). Applications of HFM module in various gas separation processes have attracted considerable interest of several research groups (Marzouk et al., 2012, 2010a, 2010b; Al-Marzouqi et al., 2009; Simons et al., 2009; Khaisri et al., 2009; Rajabzadeh et al., 2009; Fosi-Kofal et al., 2016; Park et al., 2008; Atchariyawut et al., 2007; Kosaraju et al., 2005; Dindore and Brilman, 2004; Yeon et al., 2003; Hedayat et al., 2011; Li et al., 1998; Zhang, 2016). The main advantages of membrane separation over other technologies include its compact size, convenience of installation, lower energy consumption and lower cost. Different solvents have been used with HFM modules for the capture of CO2 from different gas streams (Karror and Sirkar, 1993; Rangwala, 1996; Kumar et al., 2002; Wang et al., 2005; Li and Chen, 2005). The capture of H2S using HFM modules and different solvents was also reported, albeit at lower rate, due to the extra safety precautions requirements for experimenting with H2S (Marzouk et al., 2010b; Li et al., 1998; Wang et al., 2002, 2004; Boucif et al., 2007). Over the past few years, the authors reported stepwise progress towards the ultimate goal of utilizing HFM modules in industrial natural gas sweetening. In summary, our previous efforts can be divided into five phases: (i) comparing the commercially available HFM modules at low feed gas pressure (Al-Marzouqi et al., 2009), (ii) designing and custom
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constructing of HFM modules suitable for high pressure treatment of gas stream up to 50 bar (Marzouk et al., 2010a, 2010b) as the natural gas is typically explored as a pressured stream of 50 bar or more (Dindore and Brilman, 2004) and the treatment process is highly preferred to be conducted at the natural pressure to avoid the cost of re-pressurizing the gas, which is normally needed in the subsequent processing (Marzouk and Al-Marzouqi, 2010), (iii) assessing two potential microporous hollow fibers, i.e., expanded polytetrafluoroethylene (ePTFE) fibers and poly(tetrafluoroethylene-co-perfluorinated alkyl vinyl ether (PFA) fibers which were selected because of their known superior chemical compatibility of with the aggressive solvents such as amine solutions and strong alkalis, (iv) evaluating of the simultaneous CO2 and H2S removal from synthetic feed gas stream mimicking natural gas composition under similar conditions are used in industry (Marzouk et al., 2012), and (v) developing of a comprehensive mathematical model (Al-Marzouqi et al., 2008a, 2008b; Faiz and AlMarzouqi, 2009; Faiz and Al-Marzouqi, 2011). The primary objective of the present work is to investigate the capture of CO2 and H2S from gas stream under different experimental conditions; such as loading, temperature, gas/liquid velocity ratio and long term stability; to gain more understanding of the parameters which may affect the optimum design of HFM modules for industrial applications. The parameters considered in this study were module's surface area, feed gas/absorption solvent flow ratio, gas and solvent temperatures and solvent loading. The assessment of the operational stability of a PFA membrane module at high pressure and temperature will also be reported. 2. Experimental 2.1. Materials Nitrogen (99.99%) and gas mixtures cylinders (5% CO2e 2.0% H2Sein CH4 balance) for the simultaneous H2S and CO2 removal and (5% CO2 and 95% CH4) for CO2 removal were acquired from Air Products (UAE). PFA fibers (0.25 mm ID, 0.65 mm OD) were purchased from Entegris (Germany). A low viscosity epoxy was acquired from Buehler (Resin No. 20-8140-128, Hardener No. 208142-064). Sodium Hydroxide and Diethanolamine (99%) were received from Sigma-Aldrich. The sulfide abatement solution was a sodium hypochlorite stock solution (12% w/v) and was purchased from the local stores. Deionized water was used throughout in preparing all solutions. 2.2. Experimental setup The experimental setup used in the present work is shown in Fig. 1. A pneumatic high-pressure pump (Knauer) was used to deliver the absorption solvent, in a counter flow arrangement, to the tube side of the module. Mass flow controller (Parker, Porter model 201) was used to set the required flow rate of the feed gas mixture, which was let to pass through the shell side of the module. The upstream gas and solvent pressures were manually controlled using two back-pressure regulators (Tescom), respectively, in such a way that the solvent pressure was kept at slightly higher pressure (0.5 bar) than that of the feed gas stream. The initial concentration readings of CO2 and H2S which corresponds to the inlet gas composition were established at the beginning of each experiment by sending the gas stream directly to the gas analyzers. Steady state concentration levels for CO2 and H2S in the exit gas stream were used to calculate the flux and percent removal of these gases, respectively, for various sets of experimental conditions. The temperature of both the feed gas and absorption solvents were controlled using heating tapes. For ambient temperature
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experiments (i.e., 22 ± 1 C), the control units of the heating tapes were turned off.
2.3. Modules with different membrane surface areas Two different modules were fabricated using 300 and 500 PFA fibers in stainless steel shell, respectively, using our previous procedure (Marzouk et al., 2010b, 2012). These two modules are labelled M300 and M500, respectively, and used throughout the present work. The specifications of the modules are presented in Table 1.
3. Results and discussions 3.1. Effect of the module packing density and membrane surface area The simultaneous H2S and CO2 removal using the M300 and M500 modules was studied at ambient temperature and at different feed gas pressures. The obtained results were presented in Fig. 2a and 2b, respectively. In this investigation, the feed gas (2% H2S, 5% CO2 and 93% CH4) was fed at 1500 mL min1 thorough the shell side of the module. Whereas the absorption sodium hydroxide solution (0.5 mol/L) was pumped, counter flow, through the tube side of the module at 10 mL min1. The results showed that the percent removal of both H2S and CO2 were enhanced by increasing the feed gas pressure as a result of the enhanced solubility at higher pressures for both modules. However, M500 showed better H2S and CO2 removal which was attributed to the higher surface area as compared with M300. This can be further explained by the more favorable lower gas/liquid velocity ratio obtained with the M500 compared to that obtained with M300. The calculated gas/liquid velocity ratios were 24 and 10.6 for M300 and M500, respectively. For each stream, the velocity (m/s) was calculated by dividing the stream flow rate (m3/s) by the corresponding cross sectional area (m2) available for flow. Lower gas/liquid velocity ratio implies higher contact time and enhanced absorption. The theoretical calculation of overall gas mass transfer coefficient, KOGj,cal, was obtained by using resistances in series approach, as described in our previous report (Marzouk et al., 2012). Equation (1) is used to calculate overall mass transfer coefficient from the experimental data on the basis of the overall gas phase molar concentration difference, KOG:
KOGj;exp
CGj;in Q ¼ G ln Ao CGj;out
! (1)
where QG is the volumetric feed gas flow rate at the operation pressure and temperature, CG,in and CG,out are the concentrations in the gas phase at the inlet and outlet of the module, respectively, and A is the total outer membrane area. The overall mass transfer coefficient based on the theoretical predictions and experimental data were shown in Fig. 3a and 3b, respectively. As expected, the mass transfer coefficient decreases with increasing pressure due to the decreased gas phase diffusivities (Marzouk et al., 2012). The agreement between the model predictions and the experimental results was better for M300 module than that of M500 module. The appreciable difference between the experimental results and model predictions for M500 at pressures less than 30 Bar is not fully understood at this stage and requires further investigation.
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Fig. 1. The experimental setup utilized in the study of H2S and CO2 absorption from pressurized feed gas at pressures up to 50bar using HFM modules under different experimental conditions.
Table 1 Comparison of the Specifications of the PFA-HFM Modules Equipped with 300 and 500 PFA fibers.
No. of fibers Fiber ID, OD (mm) Active fiber length (m) Module shell ID (mm) Total Lumen cross section (m2) Total inner membrane area (m2) Fiber packing density
M300
M500
300 0.25, 0.65 0.14 20.1 1.47 105 0.034 0.308
500 0.25, 0.65 0.14 20.1 2.45 105 0.055 0.523
3.2. The effect of gas and liquid rates at fixed flow ratio The feed gas mixture of composition (5% CO2 and 95% CH4) was used in the shell side. Whereas the NaOH solution (0.5 M) was used in the tube side of the M500 module at ambient temperature, for the study of Gas/Liquid (G/L) ratio. G/L ratio was arbitrarily kept constant at 200 for 3 different gas and liquid flow rates. The obtained results for the percent CO2 removal at different feed gas pressure for the same G/L ratio were shown in Fig. 4. Increasing both gas flow and liquid flow rates, while keeping the same G/L ratio, decreased the percent CO2 removal. Simultaneous and proportional increase of both G and L flow rates leads to two opposing effects, i.e., (i) shorter gas residence time which should considerably decrease the percent removal and (ii) an enhanced absorption efficiency at higher solvent flow rate. The observed net decline of the percent removal by proportional increase of both G and L flow rates is reasonably explained by the larger dependence of the overall gas absorption process on the gas flow rate than on the absorption liquid flow rate (Marzouk et al., 2010a).
3.3. Temperature effect Experimental evaluation of the physical and chemical absorption of H2S and CO2 at different temperatures was carried out using the M300 module.
3.3.1. Physical absorption Distilled water, as a physical solvent, was used to evaluate the physical absorption of H2S and CO2 at different temperatures for different feed gas pressures up to 50 bar. The effect of feed gas pressure on the simultaneous physical flux data for H2S and CO2 using a fixed water flow rate (10 mL min1) and a fixed gas flow rate (1000 mL min1) at three different temperatures were investigated and the obtained results were shown in Fig. 5a and 5b, respectively. Both H2S and CO2 fluxes were decreased with increasing temperature. This trend was explained by the decreased gas physical solubility with increase in temperature (Alcheikhhamdon and Hoorfar, 2016; Handbook of Chemistry (2004)). The observed decrease in the flux values was slightly lower than that expected on the basis of the lowered solubility on increasing the temperature from ~22 C to 100 C (http://www.engineeringtoo, 1148). This could be possibly explained assuming that the actual water temperature inside the module was lower than the recorded water temperature measured immediately after the heating tapes in a prior calibration experiment and reported in Fig. 5a and b. Such lower temperature was reasonably attributed to the heat loss and the feed gas lower temperature (i.e., 50 C) which was adjusted as such to simulate the crude natural gas temperature during the industrial sweetening process.
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Fig. 2. Comparison between the effect of feed gas pressure on the simultaneous percent removal of H2S (A) and CO2 (B) using M300 and M500 modules and 0.5 M NaOH absorption liquid at ambient temperature. Feed gas and absorption liquid flow rates were 1500 and 10 mL min1, respectively.
3.3.2. Chemical absorption Aqueous NaOH, DEA and K2CO3 solutions were used to evaluate the combined chemical and physical absorption of H2S and CO2 at different temperatures. The effect of feed gas pressure on H2S and CO2 flux using 0.5 M NaOH, 0.5 M DEA and 0.5 M K2CO3 solutions of different temperatures at fixed gas and liquid flow rates (i.e., 1000 and 10 mL min1) were studied. In case of NaOH and DEA there was a slight enhancement (~2e4% at 50 bar) in both H2S and CO2 fluxes with temperature (data not shown). These solvents absorb CO2 and H2S both physically and chemically (Marzouk et al., 2010a, 2010b). However, the mass transfer of such acid gases from the gaseous phase to the liquid phase is increased by the chemical reaction. The fast kinetics of H2S as well as CO2 with the amine and NaOH solutions (Yildirim et al., 2012; Anderson et al., 2011) have two effects, i.e., (i) lower the positive dependence of the rate of the chemical reaction on temperature and (ii) minimizes the importance of the physical absorption step, which has a negative dependence on temperature, to the overall absorption process. These two effects are not only opposing but also have small temperature dependence which explains well the observed slight dependence of H2S and CO2 absorption into NaOH and DEA in the present investigation. On the other hand, the flux of both H2S and CO2 showed significantly different behavior when 0.5 M K2CO3 was used as a stripping solvent as shown in Fig. 6. The small dependence of H2S flux on temperature was similarly explained by its fast kinetics (Yildirim et al., 2012). However, the significant positive dependence
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Fig. 3. Comparison between mass transfer coefficients for the absorption of H2S (A) and CO2 (B) using M300 and M500 modules and 0.5 M NaOH absorption solvent at ambient temperature. Feed gas and absorption liquid flow rates were 1500 and 10 mL min1, respectively.
Fig. 4. Effect of feed gas pressure on CO2 removal using the M500 module and 0.5 M NaOH absorption liquid at three different feed gas and liquid flow rates. The feed gas flow rates were 1000. 2000 and 4000 mL min1 and the absorption liquid flow rates were 5, 10 and 20 mL min1, respectively to keep a constant gas/liquid flow ratio.
of CO2 flux into K2CO3 solution was reasonably explained by its slow kinetics (Abdeen et al., 2016). The positive dependence of the reaction rate on temperature (because of the high activation energy) outweighs the already small negative dependence of the physical absorption step in the overall chemical absorption process.
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Fig. 5. Effect of feed gas pressure on H2S (A) and CO2 (B) respective flux using the M300 module and water as a physical solvent at different temperatures. The percentage values correspond to the H2S removal for different liquid temperatures at 50 bar.
Fig. 6. Effect of feed gas pressure on H2S and CO2 respective flux using the M300 module and 0.5 M K2CO3 solution as a chemical solvent at different temperatures. The percentage values correspond to the H2S removal for different liquid temperatures at 50 bar.
3.4. Effect of solvent loading
shell side with a flow rate of 2000 mL min1 at 50 bar. The solvent which composed of 30 wt% K2CO3 þ 1 wt% DEA solution was heated to 100 C and pumped through the tube side at a flow rate of 7 mL min1 at 50.5 bar to maintain a pressure difference across the membrane of ~0.5 bar. The module was operated continuously under the above conditions for 6e8 h each day for a period of 36 working days. The obtained results (Fig. 8) showed impressive stable flux values (with 100% removal) for CO2 and H2S with no sign of membrane wetting. Up to the authors’ best knowledge, this is the first report on the long term stability of modules based on polymeric hollow fibers under industrial natural gas sweetening conditions. Such results should pave the road for future industrial utilization of the hollow fiber membranes modules equipped with PFA fibers.
Simultaneous H2S and CO2 removal was also investigated using different loaded carbonate solutions as the absorbing solvents. The effect of feed gas pressure on H2S and CO2 fluxes was investigated using the M300 module and fixed gas and liquid flow rates (i.e., 1000 and 10 mL min1, respectively). The obtained results were shown in Fig. 7. The results showed that using loaded carbonate solution decreased H2S removal percentage by 3e4% but CO2 removal percentage was remarkably decreased by 20e25%. The more significant effect of bicarbonate loading on CO2 absorption could be attributed to (i) the lower pH value of the absorption solvent which significantly affects the rate of reaction of CO2 in particular (Abdeen et al., 2016; Astarita et al., 1981) and (ii) the bicarbonate common ion effect which inhibits specifically the absorption of CO2. However, the relatively smaller inhibition effect of bicarbonate loading on H2S absorption could be attributed to the lower pH value of the loaded absorption solvent only.
4. Conclusions 3.5. Assessment of the operational stability of a PFA-module at high pressure and temperature conditions The long term stability of the fibers was also investigated using the M500 module to seriously assess the performance of the fibers under similar industrial conditions of natural gas sweetening. The feed gas of composition mimicking the natural gas (i.e., Methane 81.64%m, Ethane 6.9%m, Propane 3.6%m, I-butane 0.3%m and NButane 0.56%m, Carbon dioxide 4.2%m, Hydrogen Sulfide 1.4%m, and Nitrogen 1.4%m) was heated to 50 C and passed through the
The high pressure hollow fibers membrane (HFM) modules were critically evaluated for the intended ultimate application in natural gas sweetening process. The developed modules based on PFA hollow fibers showed excellent operational stability under the conditions of high feed gas pressure and absorption liquid temperature over extended period. The presented results suggest the suitability and readiness of our presented/characterized HFM modules for the next phase of pilot plant testing for natural gas sweetening.
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References
Fig. 7. Effect of feed gas pressure on H2S and CO2 flux using the M300 module and different loaded carbonate solutions as a chemical solvent. Feed gas and liquid flow rates were 1000 and 10 mL. min1, respectively. The percentage values correspond to the H2S removal at 50 bar.
Fig. 8. The effect of long term operation on the flux stability of H2S and CO2 from pressurized feed gas (50 bar) using the M500 module operated at 100 C. Feed gas and liquid flow rates were 1000 and 7 mL. min1, respectively.
Acknowledgement The authors would like to acknowledge UAE University (21N004) for the financial support.
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High pressure removal of acid gases using hollow fiber membrane contactors: Further characterization and long-term operational stability Mohamed H. Al-Marzouqi a, *, Sayed A.M. Marzouk b, c, Nadia Abdullatif a a b c
Department of Chemical and Petroleum Engineering, College of Engineering, United Arab Emirates University, Al Ain, 15551, United Arab Emirates Department of Chemistry, College of Science, United Arab Emirates University, Al Ain, 15551, United Arab Emirates Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, 11566, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 August 2016 Received in revised form 19 September 2016 Accepted 17 November 2016 Available online 19 November 2016
Our previously reported high pressure hollow fibers membrane (HFM) modules were specifically developed with aim to meet the operation requirements for the industrial sweetening of natural gas. The present work presents further characterization of CO2 and H2S high pressure removal under different experimental conditions, which are in particular relevant to the ultimate intended industrial application. These include module packing density, feed gas/absorption liquid flow rate ratio, absorption liquid temperature as well as solvent loading. The operational stability of the HFM module, equipped with the PFA hollow fibers, was also investigated and reported. The presented results on the long-term operational stability of membrane modules based on hollow fibers, under industrially relevant conditions are believed to be the first report of its kind. The PFA-HFM modules showed excellent operational stability during the test period, i.e., ~200 operation hours over a 7-week period using pressurized feed gas pressure (50 bar) and hot absorption solvent (100 C). © 2016 Elsevier B.V. All rights reserved.
Keywords: Natural gas treatment Acid gases Membrane contactors Operational stability
1. Introduction The world's energy needs are projected to rise by 48% from 2012 to 2040 (International Energy Outl, 2016). Among different energy sources, natural gas represents the largest increase in world's energy utilization from 120 trillion cubic feet (Tcf) in 2012 to 203 Tcf in 2040 (International Energy Outl, 2016), which places natural gas in the spotlight. Despite this promising trend, almost all natural gas requires pretreatment before commercial utilization. Approximately 30% of the world's gas fields are significantly contaminated with CO2, H2S, in addition to some other impurities such as mercaptans and organic sulfides. Industrial natural gas sweetening aims to meet strict product specifications (Kohl and Nielsen, 1997; Alcheikhhamdon and Hoorfar, 2016; Mansourizadeh and Ismail, 2009) for H2S levels, for safety and environmental reasons, and CO2 for the required range of gross calorific value (GCV) (Lambert et al., 2006; Selene and Chou, 2003). Membrane system processes are attractive candidates for several industrial applications (Khulbe
* Corresponding author. E-mail address: [email protected] (M.H. Al-Marzouqi). http://dx.doi.org/10.1016/j.jngse.2016.11.039 1875-5100/© 2016 Elsevier B.V. All rights reserved.
and Matsuura, 2016). Applications of HFM module in various gas separation processes have attracted considerable interest of several research groups (Marzouk et al., 2012, 2010a, 2010b; Al-Marzouqi et al., 2009; Simons et al., 2009; Khaisri et al., 2009; Rajabzadeh et al., 2009; Fosi-Kofal et al., 2016; Park et al., 2008; Atchariyawut et al., 2007; Kosaraju et al., 2005; Dindore and Brilman, 2004; Yeon et al., 2003; Hedayat et al., 2011; Li et al., 1998; Zhang, 2016). The main advantages of membrane separation over other technologies include its compact size, convenience of installation, lower energy consumption and lower cost. Different solvents have been used with HFM modules for the capture of CO2 from different gas streams (Karror and Sirkar, 1993; Rangwala, 1996; Kumar et al., 2002; Wang et al., 2005; Li and Chen, 2005). The capture of H2S using HFM modules and different solvents was also reported, albeit at lower rate, due to the extra safety precautions requirements for experimenting with H2S (Marzouk et al., 2010b; Li et al., 1998; Wang et al., 2002, 2004; Boucif et al., 2007). Over the past few years, the authors reported stepwise progress towards the ultimate goal of utilizing HFM modules in industrial natural gas sweetening. In summary, our previous efforts can be divided into five phases: (i) comparing the commercially available HFM modules at low feed gas pressure (Al-Marzouqi et al., 2009), (ii) designing and custom
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constructing of HFM modules suitable for high pressure treatment of gas stream up to 50 bar (Marzouk et al., 2010a, 2010b) as the natural gas is typically explored as a pressured stream of 50 bar or more (Dindore and Brilman, 2004) and the treatment process is highly preferred to be conducted at the natural pressure to avoid the cost of re-pressurizing the gas, which is normally needed in the subsequent processing (Marzouk and Al-Marzouqi, 2010), (iii) assessing two potential microporous hollow fibers, i.e., expanded polytetrafluoroethylene (ePTFE) fibers and poly(tetrafluoroethylene-co-perfluorinated alkyl vinyl ether (PFA) fibers which were selected because of their known superior chemical compatibility of with the aggressive solvents such as amine solutions and strong alkalis, (iv) evaluating of the simultaneous CO2 and H2S removal from synthetic feed gas stream mimicking natural gas composition under similar conditions are used in industry (Marzouk et al., 2012), and (v) developing of a comprehensive mathematical model (Al-Marzouqi et al., 2008a, 2008b; Faiz and AlMarzouqi, 2009; Faiz and Al-Marzouqi, 2011). The primary objective of the present work is to investigate the capture of CO2 and H2S from gas stream under different experimental conditions; such as loading, temperature, gas/liquid velocity ratio and long term stability; to gain more understanding of the parameters which may affect the optimum design of HFM modules for industrial applications. The parameters considered in this study were module's surface area, feed gas/absorption solvent flow ratio, gas and solvent temperatures and solvent loading. The assessment of the operational stability of a PFA membrane module at high pressure and temperature will also be reported. 2. Experimental 2.1. Materials Nitrogen (99.99%) and gas mixtures cylinders (5% CO2e 2.0% H2Sein CH4 balance) for the simultaneous H2S and CO2 removal and (5% CO2 and 95% CH4) for CO2 removal were acquired from Air Products (UAE). PFA fibers (0.25 mm ID, 0.65 mm OD) were purchased from Entegris (Germany). A low viscosity epoxy was acquired from Buehler (Resin No. 20-8140-128, Hardener No. 208142-064). Sodium Hydroxide and Diethanolamine (99%) were received from Sigma-Aldrich. The sulfide abatement solution was a sodium hypochlorite stock solution (12% w/v) and was purchased from the local stores. Deionized water was used throughout in preparing all solutions. 2.2. Experimental setup The experimental setup used in the present work is shown in Fig. 1. A pneumatic high-pressure pump (Knauer) was used to deliver the absorption solvent, in a counter flow arrangement, to the tube side of the module. Mass flow controller (Parker, Porter model 201) was used to set the required flow rate of the feed gas mixture, which was let to pass through the shell side of the module. The upstream gas and solvent pressures were manually controlled using two back-pressure regulators (Tescom), respectively, in such a way that the solvent pressure was kept at slightly higher pressure (0.5 bar) than that of the feed gas stream. The initial concentration readings of CO2 and H2S which corresponds to the inlet gas composition were established at the beginning of each experiment by sending the gas stream directly to the gas analyzers. Steady state concentration levels for CO2 and H2S in the exit gas stream were used to calculate the flux and percent removal of these gases, respectively, for various sets of experimental conditions. The temperature of both the feed gas and absorption solvents were controlled using heating tapes. For ambient temperature
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experiments (i.e., 22 ± 1 C), the control units of the heating tapes were turned off.
2.3. Modules with different membrane surface areas Two different modules were fabricated using 300 and 500 PFA fibers in stainless steel shell, respectively, using our previous procedure (Marzouk et al., 2010b, 2012). These two modules are labelled M300 and M500, respectively, and used throughout the present work. The specifications of the modules are presented in Table 1.
3. Results and discussions 3.1. Effect of the module packing density and membrane surface area The simultaneous H2S and CO2 removal using the M300 and M500 modules was studied at ambient temperature and at different feed gas pressures. The obtained results were presented in Fig. 2a and 2b, respectively. In this investigation, the feed gas (2% H2S, 5% CO2 and 93% CH4) was fed at 1500 mL min1 thorough the shell side of the module. Whereas the absorption sodium hydroxide solution (0.5 mol/L) was pumped, counter flow, through the tube side of the module at 10 mL min1. The results showed that the percent removal of both H2S and CO2 were enhanced by increasing the feed gas pressure as a result of the enhanced solubility at higher pressures for both modules. However, M500 showed better H2S and CO2 removal which was attributed to the higher surface area as compared with M300. This can be further explained by the more favorable lower gas/liquid velocity ratio obtained with the M500 compared to that obtained with M300. The calculated gas/liquid velocity ratios were 24 and 10.6 for M300 and M500, respectively. For each stream, the velocity (m/s) was calculated by dividing the stream flow rate (m3/s) by the corresponding cross sectional area (m2) available for flow. Lower gas/liquid velocity ratio implies higher contact time and enhanced absorption. The theoretical calculation of overall gas mass transfer coefficient, KOGj,cal, was obtained by using resistances in series approach, as described in our previous report (Marzouk et al., 2012). Equation (1) is used to calculate overall mass transfer coefficient from the experimental data on the basis of the overall gas phase molar concentration difference, KOG:
KOGj;exp
CGj;in Q ¼ G ln Ao CGj;out
! (1)
where QG is the volumetric feed gas flow rate at the operation pressure and temperature, CG,in and CG,out are the concentrations in the gas phase at the inlet and outlet of the module, respectively, and A is the total outer membrane area. The overall mass transfer coefficient based on the theoretical predictions and experimental data were shown in Fig. 3a and 3b, respectively. As expected, the mass transfer coefficient decreases with increasing pressure due to the decreased gas phase diffusivities (Marzouk et al., 2012). The agreement between the model predictions and the experimental results was better for M300 module than that of M500 module. The appreciable difference between the experimental results and model predictions for M500 at pressures less than 30 Bar is not fully understood at this stage and requires further investigation.
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Fig. 1. The experimental setup utilized in the study of H2S and CO2 absorption from pressurized feed gas at pressures up to 50bar using HFM modules under different experimental conditions.
Table 1 Comparison of the Specifications of the PFA-HFM Modules Equipped with 300 and 500 PFA fibers.
No. of fibers Fiber ID, OD (mm) Active fiber length (m) Module shell ID (mm) Total Lumen cross section (m2) Total inner membrane area (m2) Fiber packing density
M300
M500
300 0.25, 0.65 0.14 20.1 1.47 105 0.034 0.308
500 0.25, 0.65 0.14 20.1 2.45 105 0.055 0.523
3.2. The effect of gas and liquid rates at fixed flow ratio The feed gas mixture of composition (5% CO2 and 95% CH4) was used in the shell side. Whereas the NaOH solution (0.5 M) was used in the tube side of the M500 module at ambient temperature, for the study of Gas/Liquid (G/L) ratio. G/L ratio was arbitrarily kept constant at 200 for 3 different gas and liquid flow rates. The obtained results for the percent CO2 removal at different feed gas pressure for the same G/L ratio were shown in Fig. 4. Increasing both gas flow and liquid flow rates, while keeping the same G/L ratio, decreased the percent CO2 removal. Simultaneous and proportional increase of both G and L flow rates leads to two opposing effects, i.e., (i) shorter gas residence time which should considerably decrease the percent removal and (ii) an enhanced absorption efficiency at higher solvent flow rate. The observed net decline of the percent removal by proportional increase of both G and L flow rates is reasonably explained by the larger dependence of the overall gas absorption process on the gas flow rate than on the absorption liquid flow rate (Marzouk et al., 2010a).
3.3. Temperature effect Experimental evaluation of the physical and chemical absorption of H2S and CO2 at different temperatures was carried out using the M300 module.
3.3.1. Physical absorption Distilled water, as a physical solvent, was used to evaluate the physical absorption of H2S and CO2 at different temperatures for different feed gas pressures up to 50 bar. The effect of feed gas pressure on the simultaneous physical flux data for H2S and CO2 using a fixed water flow rate (10 mL min1) and a fixed gas flow rate (1000 mL min1) at three different temperatures were investigated and the obtained results were shown in Fig. 5a and 5b, respectively. Both H2S and CO2 fluxes were decreased with increasing temperature. This trend was explained by the decreased gas physical solubility with increase in temperature (Alcheikhhamdon and Hoorfar, 2016; Handbook of Chemistry (2004)). The observed decrease in the flux values was slightly lower than that expected on the basis of the lowered solubility on increasing the temperature from ~22 C to 100 C (http://www.engineeringtoo, 1148). This could be possibly explained assuming that the actual water temperature inside the module was lower than the recorded water temperature measured immediately after the heating tapes in a prior calibration experiment and reported in Fig. 5a and b. Such lower temperature was reasonably attributed to the heat loss and the feed gas lower temperature (i.e., 50 C) which was adjusted as such to simulate the crude natural gas temperature during the industrial sweetening process.
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Fig. 2. Comparison between the effect of feed gas pressure on the simultaneous percent removal of H2S (A) and CO2 (B) using M300 and M500 modules and 0.5 M NaOH absorption liquid at ambient temperature. Feed gas and absorption liquid flow rates were 1500 and 10 mL min1, respectively.
3.3.2. Chemical absorption Aqueous NaOH, DEA and K2CO3 solutions were used to evaluate the combined chemical and physical absorption of H2S and CO2 at different temperatures. The effect of feed gas pressure on H2S and CO2 flux using 0.5 M NaOH, 0.5 M DEA and 0.5 M K2CO3 solutions of different temperatures at fixed gas and liquid flow rates (i.e., 1000 and 10 mL min1) were studied. In case of NaOH and DEA there was a slight enhancement (~2e4% at 50 bar) in both H2S and CO2 fluxes with temperature (data not shown). These solvents absorb CO2 and H2S both physically and chemically (Marzouk et al., 2010a, 2010b). However, the mass transfer of such acid gases from the gaseous phase to the liquid phase is increased by the chemical reaction. The fast kinetics of H2S as well as CO2 with the amine and NaOH solutions (Yildirim et al., 2012; Anderson et al., 2011) have two effects, i.e., (i) lower the positive dependence of the rate of the chemical reaction on temperature and (ii) minimizes the importance of the physical absorption step, which has a negative dependence on temperature, to the overall absorption process. These two effects are not only opposing but also have small temperature dependence which explains well the observed slight dependence of H2S and CO2 absorption into NaOH and DEA in the present investigation. On the other hand, the flux of both H2S and CO2 showed significantly different behavior when 0.5 M K2CO3 was used as a stripping solvent as shown in Fig. 6. The small dependence of H2S flux on temperature was similarly explained by its fast kinetics (Yildirim et al., 2012). However, the significant positive dependence
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Fig. 3. Comparison between mass transfer coefficients for the absorption of H2S (A) and CO2 (B) using M300 and M500 modules and 0.5 M NaOH absorption solvent at ambient temperature. Feed gas and absorption liquid flow rates were 1500 and 10 mL min1, respectively.
Fig. 4. Effect of feed gas pressure on CO2 removal using the M500 module and 0.5 M NaOH absorption liquid at three different feed gas and liquid flow rates. The feed gas flow rates were 1000. 2000 and 4000 mL min1 and the absorption liquid flow rates were 5, 10 and 20 mL min1, respectively to keep a constant gas/liquid flow ratio.
of CO2 flux into K2CO3 solution was reasonably explained by its slow kinetics (Abdeen et al., 2016). The positive dependence of the reaction rate on temperature (because of the high activation energy) outweighs the already small negative dependence of the physical absorption step in the overall chemical absorption process.
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Fig. 5. Effect of feed gas pressure on H2S (A) and CO2 (B) respective flux using the M300 module and water as a physical solvent at different temperatures. The percentage values correspond to the H2S removal for different liquid temperatures at 50 bar.
Fig. 6. Effect of feed gas pressure on H2S and CO2 respective flux using the M300 module and 0.5 M K2CO3 solution as a chemical solvent at different temperatures. The percentage values correspond to the H2S removal for different liquid temperatures at 50 bar.
3.4. Effect of solvent loading
shell side with a flow rate of 2000 mL min1 at 50 bar. The solvent which composed of 30 wt% K2CO3 þ 1 wt% DEA solution was heated to 100 C and pumped through the tube side at a flow rate of 7 mL min1 at 50.5 bar to maintain a pressure difference across the membrane of ~0.5 bar. The module was operated continuously under the above conditions for 6e8 h each day for a period of 36 working days. The obtained results (Fig. 8) showed impressive stable flux values (with 100% removal) for CO2 and H2S with no sign of membrane wetting. Up to the authors’ best knowledge, this is the first report on the long term stability of modules based on polymeric hollow fibers under industrial natural gas sweetening conditions. Such results should pave the road for future industrial utilization of the hollow fiber membranes modules equipped with PFA fibers.
Simultaneous H2S and CO2 removal was also investigated using different loaded carbonate solutions as the absorbing solvents. The effect of feed gas pressure on H2S and CO2 fluxes was investigated using the M300 module and fixed gas and liquid flow rates (i.e., 1000 and 10 mL min1, respectively). The obtained results were shown in Fig. 7. The results showed that using loaded carbonate solution decreased H2S removal percentage by 3e4% but CO2 removal percentage was remarkably decreased by 20e25%. The more significant effect of bicarbonate loading on CO2 absorption could be attributed to (i) the lower pH value of the absorption solvent which significantly affects the rate of reaction of CO2 in particular (Abdeen et al., 2016; Astarita et al., 1981) and (ii) the bicarbonate common ion effect which inhibits specifically the absorption of CO2. However, the relatively smaller inhibition effect of bicarbonate loading on H2S absorption could be attributed to the lower pH value of the loaded absorption solvent only.
4. Conclusions 3.5. Assessment of the operational stability of a PFA-module at high pressure and temperature conditions The long term stability of the fibers was also investigated using the M500 module to seriously assess the performance of the fibers under similar industrial conditions of natural gas sweetening. The feed gas of composition mimicking the natural gas (i.e., Methane 81.64%m, Ethane 6.9%m, Propane 3.6%m, I-butane 0.3%m and NButane 0.56%m, Carbon dioxide 4.2%m, Hydrogen Sulfide 1.4%m, and Nitrogen 1.4%m) was heated to 50 C and passed through the
The high pressure hollow fibers membrane (HFM) modules were critically evaluated for the intended ultimate application in natural gas sweetening process. The developed modules based on PFA hollow fibers showed excellent operational stability under the conditions of high feed gas pressure and absorption liquid temperature over extended period. The presented results suggest the suitability and readiness of our presented/characterized HFM modules for the next phase of pilot plant testing for natural gas sweetening.
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References
Fig. 7. Effect of feed gas pressure on H2S and CO2 flux using the M300 module and different loaded carbonate solutions as a chemical solvent. Feed gas and liquid flow rates were 1000 and 10 mL. min1, respectively. The percentage values correspond to the H2S removal at 50 bar.
Fig. 8. The effect of long term operation on the flux stability of H2S and CO2 from pressurized feed gas (50 bar) using the M500 module operated at 100 C. Feed gas and liquid flow rates were 1000 and 7 mL. min1, respectively.
Acknowledgement The authors would like to acknowledge UAE University (21N004) for the financial support.
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