Efficient Facilitated Transport Polymer Membrane for CO2/CH4 Separation from Oilfield Associated Gas

CO2 enhanced oil recovery (CO2-EOR) technology is a competitive strategy to improve oil field economic returns and reduce greenhouse gas emissions. However, the arbitrary emissions or combustion of the associated gas, which mainly consists of CO2 and CH4, will cause the aggravation of the greenhouse effect and a huge waste of resources. In this paper, the high-performance facilitated transport multilayer composite membrane for CO2/CH4 separation was prepared by individually adjusting the membrane structure of each layer. The effect of test conditions on the CO2/CH4 separation performance was systematically investigated. The membrane exhibits high CO2 permeance of 3.451 × 10−7 mol·m−2·s−1·Pa−1 and CO2/CH4 selectivity of 62 at 298 K and 0.15 MPa feed gas pressure. The cost analysis was investigated by simulating the two-stage system. When the recovery rate and purity of CH4 are 98%, the minimum specific cost of separating CO2/CH4 (45/55 vol%) can be reduced to 0.046 $·Nm−3 CH4. The excellent short-to-mid-term stability indicates the great potential of large industrial application in the CH4 recovery and CO2 reinjection from oilfield associated gas.


Introduction
The energy and environmental crisis are the key issues for sustainable development in the world today. With the continuous progress of the economy and society, energy demand is increasing year by year. Compared with traditional fossil energy, the large-scale promotion of renewable energy such as wind energy, solar energy, and biomass energy is still facing greater difficulties. The contradiction between energy supply and demand has become more severe. Therefore, the production capacity of fossil energy needs to be expanded urgently [1].
Abundant low-permeability oil and gas resources are buried in petroliferous basins dominated by continental deposits [2]. According to incomplete statistics, Yanchang Oilfield is the second-largest low-permeability oilfield in China [3], with proven reserves of 1.12 × 10 8 t and an oil-bearing area of 215.5 km 2 [4]. However, most oil fields in this area are currently in the late stage of water-enhanced oil recovery. The high-water content and low pressure of the oil reservoirs lead to reduced oil production and poor production efficiency.
In recent years, CO 2 enhanced oil recovery (CO 2 -EOR) technology has faced rapid development and has been widely used in many oil and gas fields in the world [5][6][7][8]. According to statistics, compared with other types of oil displacement technology, CO 2 -EOR could increase oil and gas recovery by 8-25% [9]. CO 2 miscible flooding is a very surface but abundant water in the internal pore channels, so that the PDMS solution could not infiltrate into the pore of PSf ultrafiltration membrane, avoiding the excessive drop in gas permeance.
The main component of the solution utilized for coating the selective layer is PVAm, while the remaining ingredients include 20 mol% PVA, 1 mol% PAAS, and 10 mol% SDS, which was shortened to PPPS in this paper. The additives in the PPPS solution were mainly used to enhance the CO 2 separation ability and interlayer compatibility [22]. The PPPS/PDMS/PSf composite membranes were manufactured by coating PPPS solution directly on the dried PDMS gutter layer with a wet coating thickness of 150 µm. The wet membranes were placed and dried in a constant environment with the temperature and humidity of 303 K and 40 %RH respectively for subsequent tests.

Characterization
The cross-section topographies of the PDMS/PSf and PPPS/PDMS/PSf composite membranes were screened by scanning electron microscopy (SEM, Nova NanoSEM 430, FEI, Hillsboro, OR, USA). The functional groups of the membranes were evaluated by Fourier Transform Infrared (FTIR) spectroscopy (FTS-6000, Bio-Rad, Hercules, CA, USA). The CO 2 /CH 4 mixed gas separation performance was estimated by the laboratory-made gas permeance analysis platform. As shown in Figure 1, the prepared CO 2 /CH 4 mixed gases with different CO 2 concentrations served as the feed gas flow into membrane cell at a set pressure. The CO 2 -rich penetrate gas driven by He was analyzed in the gas chromatograph (7890B, Agilent, Palo Alto, CA, USA) with a certain flow rate. The ratio of humidified gas (saturated) and dry gas in the feed gas can be controlled by adjusting the precision needle valve, thereby controlling the relative humidity of the feed gas. All error bars represented the standard errors of the performance of three membranes prepared under the same conditions. thickness of 300 μm on the PSf ultrafiltration membrane that possesses no obvious w on the surface but abundant water in the internal pore channels, so that the PDMS solu could not infiltrate into the pore of PSf ultrafiltration membrane, avoiding the exces drop in gas permeance.
The main component of the solution utilized for coating the selective layer is PVA while the remaining ingredients include 20 mol% PVA, 1 mol% PAAS, and 10 mol% S which was shortened to PPPS in this paper. The additives in the PPPS solution w mainly used to enhance the CO2 separation ability and interlayer compatibility [22]. PPPS/PDMS/PSf composite membranes were manufactured by coating PPPS solution rectly on the dried PDMS gutter layer with a wet coating thickness of 150 μm. The membranes were placed and dried in a constant environment with the temperature humidity of 303 K and 40 %RH respectively for subsequent tests.

Characterization
The cross-section topographies of the PDMS/PSf and PPPS/PDMS/PSf compo membranes were screened by scanning electron microscopy (SEM, Nova NanoSEM FEI, Hillsboro, OR, USA). The functional groups of the membranes were evaluated Fourier Transform Infrared (FTIR) spectroscopy (FTS-6000, Bio-Rad, Hercules, CA, US The CO2/CH4 mixed gas separation performance was estimated by the laboratory-m gas permeance analysis platform. As shown in Figure 1, the prepared CO2/CH4 mi gases with different CO2 concentrations served as the feed gas flow into membrane ce a set pressure. The CO2-rich penetrate gas driven by He was analyzed in the gas ch matograph (7890B, Agilent, Palo Alto, CA, USA) with a certain flow rate. The ratio of midified gas (saturated) and dry gas in the feed gas can be controlled by adjusting precision needle valve, thereby controlling the relative humidity of the feed gas. All e bars represented the standard errors of the performance of three membranes prepa under the same conditions.

Performance Assessment
The CO 2 /CH 4 separation performance of the composite membranes was evaluated by two main parameters, permeance, and selectivity. Permeance indicates how fast the gas passes through the membrane, which can be calculated by the following formula: where, Q i is defined as the flow rate of gas composition (CO 2 or CH 4 ) penetrating through the membrane at the steady-state, the unit of which used in this paper is mol·s −1 ·Pa −1 ; ∆p i stands for the partial pressure difference of the gas composition between the retentate and the permeate side of the membrane; A is the effective separation area of the membrane. Under ideal circumstances, the CO 2 /CH 4 selectivity of the membrane is approximately equal to:

PDMS Gutter Layer
PDMS is the most widely used gutter layer material, the thickness of which is directly related to the gas permeability of the composite membrane [23,24]. The concentration of PDMS solution determines the thickness of the gutter layer. Therefore, in this section, the adjustment of the gutter layer thickness was achieved by fixing the wet coating thickness (300 µm) and changing the concentration of the cross-linked PDMS solution. To select the optimal PDMS concentration, the CO 2 /CH 4 mixed gas (45/55 vol%) separation performance was examined at 298 K, saturated humidity and 0.15 MPa feed gas pressure after coating 0.15 wt% PPPS solution with a wet coating thickness of 150 µm on the prepared PDMS/PSf composite membranes.
The infrared spectrum of PDMS/PSf and PPPS/PDMS/PSf composite membranes were implemented to characterize the functional groups of both membranes. As presented in Figure 2, the absorption peaks at 690 cm −1 and 1100 cm −1 are the characteristic bands of Si-C and Si-O respectively, indicating the successful coating of PDMS on PSf substrate. However, due to the thin PPPS separation layer, the infrared spectrum of the PPPS/PDMS/PSf composite membrane contains all the absorption bands of PDMS. The absorption bands around 3300 cm −1 -3000 cm −1 and 1670 cm −1 confirm the presence of -OH and -NH 2 , which proves the PPPS/PDMS/PSf composite membranes were successfully prepared. Figure 3 shows the cross-section topographies of the PPPS/PDMS/PSf composite membranes prepared based on the gutter layers with the PDMS concentration of 0.5-2.5 wt%. The continuous interfaces are clearly presented between the gutter layers and PSf substrate, indicating that the wet-dry combined coating method can effectively inhibit the pore penetration of PDMS in the polysulfone. The thickness of the composite membranes prepared by different PDMS concentrations were investigated further. As shown in Figure 3, with the increase of PDMS concentration, the thickness of PPPS/PDMS/PSf composite membranes increased from 161 nm to 364 nm.
Si-C and Si-O respectively, indicating the successful coating of PDMS on PSf su However, due to the thin PPPS separation layer, the infrared spectrum PPPS/PDMS/PSf composite membrane contains all the absorption bands of PDM absorption bands around 3300 cm −1 -3000 cm −1 and 1670 cm −1 confirm the presence and -NH2, which proves the PPPS/PDMS/PSf composite membranes were succ prepared.
.   The CO2/CH4 (45/55 vol%) mixed gas separation performance of PPPS/PDM composite membranes prepared from different PDMS concentrations is shown in F 4. The separation performance of the composite membranes increases first and the creases with the increasing PDMS concentration. When the PDMS concentration re 1.5 wt%, the composite membrane possesses the best performance. The reasons are a lows. When the PDMS concentration is too low (≤1.0 wt%), the PDMS cannot comp cover the surface of the PSf substrate, which causes the PPPS solution to partially trating into the pores of the PSf substrate. According to the resistance model of the posite membrane, the total resistance of the composite membrane increases linearly the increasing thickness of the polymer penetration area [25]. The incomplete mem structure results in the fluctuating CO2 permeance and low CO2/CH4 selectivity. How when the PDMS concentration increases to more than 2 wt%, the increasing thickn the gutter layer increases the gas resistance, which reduces the gas permeance. Be the strong hydrophobicity of the prepared PDMS/PSf membranes affects the mallea of the PPPS solution, leading to the unevenness of the separation layer, and event increases the CH4 permeance. Thus, the CO2/CH4 selectivity was reduced. The CO 2 /CH 4 (45/55 vol%) mixed gas separation performance of PPPS/PDMS/PSf composite membranes prepared from different PDMS concentrations is shown in Figure 4. The separation performance of the composite membranes increases first and then decreases with the increasing PDMS concentration. When the PDMS concentration reaches 1.5 wt%, the composite membrane possesses the best performance. The reasons are as follows. When the PDMS concentration is too low (≤1.0 wt%), the PDMS cannot completely cover the surface of the PSf substrate, which causes the PPPS solution to partially penetrating into the pores of the PSf substrate. According to the resistance model of the composite membrane, the total resistance of the composite membrane increases linearly with the increasing thickness of the polymer penetration area [25]. The incomplete membrane structure results in the fluctuating CO 2 permeance and low CO 2 /CH 4 selectivity. However, when the PDMS concentration increases to more than 2 wt%, the increasing thickness of the gutter layer increases the gas resistance, which reduces the gas permeance. Besides, the strong hydrophobicity of the prepared PDMS/PSf membranes affects the malleability of the PPPS solution, leading to the unevenness of the separation layer, and eventually increases the CH 4 permeance. Thus, the CO 2 /CH 4 selectivity was reduced.

PPPS Separation Layer
With hydrophilic PVAm as the main ingredient, the PPPS solution achieves excellen compatibility between the hydrophilic separation layer and the hydrophobic PDMS sur face by the synergistic effect of the surfactant and thickener [22]. Based on the optima PDMS concentration determined in Section 3.1.1, the effect of the PPPS concentration on the CO2/CH4 separation performance of the PPPS/PDMS/PSf composite membrane wa further explored fixing the wet coating thickness (150 μm) and changing the concentration of the PPPS solution.
The cross-section topographies of the PPPS/PDMS/PSf composite membranes with the PPPS concentration between 0.05 wt% and 0.20 wt% were investigated by SEM. A shown in Figure 5, the thickness of the PPPS/PDMS/PSf composite membranes increase from 250 nm to 333 nm with the increasing PPPS concentration, which indicates that th separation layer is complete and thin.

PPPS Separation Layer
With hydrophilic PVAm as the main ingredient, the PPPS solution achieves excellent compatibility between the hydrophilic separation layer and the hydrophobic PDMS surface by the synergistic effect of the surfactant and thickener [22]. Based on the optimal PDMS concentration determined in Section 3.1.1, the effect of the PPPS concentration on the CO 2 /CH 4 separation performance of the PPPS/PDMS/PSf composite membrane was further explored fixing the wet coating thickness (150 µm) and changing the concentration of the PPPS solution.
The cross-section topographies of the PPPS/PDMS/PSf composite membranes with the PPPS concentration between 0.05 wt% and 0.20 wt% were investigated by SEM. As shown in Figure 5, the thickness of the PPPS/PDMS/PSf composite membranes increases from 250 nm to 333 nm with the increasing PPPS concentration, which indicates that the separation layer is complete and thin.

PPPS Separation Layer
With hydrophilic PVAm as the main ingredient, the PPPS solution achieves excellent compatibility between the hydrophilic separation layer and the hydrophobic PDMS surface by the synergistic effect of the surfactant and thickener [22]. Based on the optimal PDMS concentration determined in Section 3.1.1, the effect of the PPPS concentration on the CO2/CH4 separation performance of the PPPS/PDMS/PSf composite membrane was further explored fixing the wet coating thickness (150 μm) and changing the concentration of the PPPS solution.
The cross-section topographies of the PPPS/PDMS/PSf composite membranes with the PPPS concentration between 0.05 wt% and 0.20 wt% were investigated by SEM. As shown in Figure 5, the thickness of the PPPS/PDMS/PSf composite membranes increases from 250 nm to 333 nm with the increasing PPPS concentration, which indicates that the separation layer is complete and thin.    As the PPPS concentration increases, the CO 2 permeance gradually decreases, while the CO 2 /CH 4 selectivity gradually increases. When the PPPS concentration is at 0.15 wt%, the PPPS/PDMS/PSf membrane exhibits the best separation performance that the CO 2 permeance of 3.451 × 10 −7 mol·m −2 ·s −1 ·Pa −1 and CO 2 /CH 4 selectivity of 62. This is mainly due to the following reasons. When the PPPS concentration is less than 0.10 wt%), due to the low viscosity, the PPPS solution can hardly form an intact separation layer, resulting in the formation of defects without gas selectivity. While the increasing PPPS solution (>0.15 wt%) thickens the separation layer, which increases the gas resistance through the membrane. embranes 2021, 11, x FOR PEER REVIEW PPPS/PDMS/PSf membrane exhibits the best separation performance that th ance of 3.451 × 10 −7 mol·m −2 ·s −1 ·Pa −1 and CO2/CH4 selectivity of 62. This is mai following reasons. When the PPPS concentration is less than 0.10 wt%), d viscosity, the PPPS solution can hardly form an intact separation layer, re formation of defects without gas selectivity. While the increasing PPPS s wt%) thickens the separation layer, which increases the gas resistance thro brane.

Relative Humidity of the Feed Gas
The effect of the relative humidity of the feed gas on CO2/CH4 (45/55 vol performance of the PPPS/PDMS/PSf composite membrane was explored un 0.15 MPa feed pressure. The relative humidity of the feed gas was adjusted the opening of the two high-precision needle valves.
As shown in Figure 7, with the increasing relative humidity of the feed permeance and CO2/CH4 selectivity of the PPPS/PDMS/PSf composite mem crease. This is due to the following two aspects. Firstly, PVAm with abun amine groups, which is the dominance in the separation layer, can react sp reversibly with CO2 and promote the transfer of CO2 across the membran nism of primary amine groups promoting CO2 transfer can be expressed as formula [26]:

RNH +H O+CO HCO +RNH
Due to a large number of active hydrogens, the primary amine grou react with CO2 without water. However, water can not only be used as rea

Relative Humidity of the Feed Gas
The effect of the relative humidity of the feed gas on CO 2 /CH 4 (45/55 vol%) separation performance of the PPPS/PDMS/PSf composite membrane was explored under 298 K and 0.15 MPa feed pressure. The relative humidity of the feed gas was adjusted by controlling the opening of the two high-precision needle valves.
As shown in Figure 7, with the increasing relative humidity of the feed gas, the CO 2 permeance and CO 2 /CH 4 selectivity of the PPPS/PDMS/PSf composite membrane also increase. This is due to the following two aspects. Firstly, PVAm with abundant primary amine groups, which is the dominance in the separation layer, can react specifically and reversibly with CO 2 and promote the transfer of CO 2 across the membrane. The mechanism of primary amine groups promoting CO 2 transfer can be expressed as the following formula [26]: Due to a large number of active hydrogens, the primary amine groups can slowly react with CO 2 without water. However, water can not only be used as reactants to participate in the CO 2 facilitated transport, but also promote the ionization of primary amine groups and CO 2 , thereby accelerate the reversible reaction. Therefore, with the increasing relative humidity of feed gas, the positive effect of water on the reversible reaction improves, leading to an increase in CO 2 permeance. Secondly, water also promotes the CO2 adsorption on the membrane surface, w the adsorption capacity of CH4 on the membrane surface is reduced due to the compet adsorption. Since CH4 penetrates through the solution-diffusion mechanism in the c posite membrane, the permeation rate of CH4 will also decrease with the increase of tive humidity. Thirdly, as the relative humidity increases, PPPS with strong hydrop groups (amine and hydroxyl groups) adsorb and solve a lot of water, resulting in swe and increase of the free volume of the composite membrane, and thus accelerate the meance of CO2 and CH4 in the PPPS/PDMS/PSf composite membrane. Under the c bined effect of the above, with the increasing relative humidity of the feed gas, CO2 meance greatly increases while CH4 permeance slowly increases, which ultimately l to an increase in the CO2/CH4 selectivity. Therefore, the PPPS/PDMS/PSf composite m branes are more suitable for use under high humidity conditions.

Temperature of the Feed Gas
The effect of the feed gas temperature on CO2/CH4 (45/55 vol%) separation pe mance of the PPPS/PDMS/PSf composite membrane was explored under saturated midity and 0.15 MPa feed pressure. As shown in Figure 8, with the increasing feed temperature, the CO2 permeance of PPPS/PDMS/mPSf composite membrane gradu increases, and the CO2/CH4 selectivity reduces rapidly, which can be attributed to the lowing reasons. Firstly, the saturated vapor pressure increased with the feed gas temperature, w Secondly, water also promotes the CO 2 adsorption on the membrane surface, while the adsorption capacity of CH 4 on the membrane surface is reduced due to the competitive adsorption. Since CH 4 penetrates through the solution-diffusion mechanism in the composite membrane, the permeation rate of CH4 will also decrease with the increase of relative humidity. Thirdly, as the relative humidity increases, PPPS with strong hydrophilic groups (amine and hydroxyl groups) adsorb and solve a lot of water, resulting in swelling and increase of the free volume of the composite membrane, and thus accelerate the permeance of CO 2 and CH 4 in the PPPS/PDMS/PSf composite membrane. Under the combined effect of the above, with the increasing relative humidity of the feed gas, CO 2 permeance greatly increases while CH 4 permeance slowly increases, which ultimately leads to an increase in the CO 2 /CH 4 selectivity. Therefore, the PPPS/PDMS/PSf composite membranes are more suitable for use under high humidity conditions.

Temperature of the Feed Gas
The effect of the feed gas temperature on CO 2 /CH 4 (45/55 vol%) separation performance of the PPPS/PDMS/PSf composite membrane was explored under saturated humidity and 0.15 MPa feed pressure. As shown in Figure 8, with the increasing feed gas temperature, the CO 2 permeance of PPPS/PDMS/mPSf composite membrane gradually increases, and the CO 2 /CH 4 selectivity reduces rapidly, which can be attributed to the following reasons. Secondly, water also promotes the CO2 adsorption on the membrane surface, w the adsorption capacity of CH4 on the membrane surface is reduced due to the compet adsorption. Since CH4 penetrates through the solution-diffusion mechanism in the posite membrane, the permeation rate of CH4 will also decrease with the increase of tive humidity. Thirdly, as the relative humidity increases, PPPS with strong hydrop groups (amine and hydroxyl groups) adsorb and solve a lot of water, resulting in swe and increase of the free volume of the composite membrane, and thus accelerate the meance of CO2 and CH4 in the PPPS/PDMS/PSf composite membrane. Under the bined effect of the above, with the increasing relative humidity of the feed gas, CO2 meance greatly increases while CH4 permeance slowly increases, which ultimately l to an increase in the CO2/CH4 selectivity. Therefore, the PPPS/PDMS/PSf composite m branes are more suitable for use under high humidity conditions.

Temperature of the Feed Gas
The effect of the feed gas temperature on CO2/CH4 (45/55 vol%) separation pe mance of the PPPS/PDMS/PSf composite membrane was explored under saturated midity and 0.15 MPa feed pressure. As shown in Figure 8, with the increasing feed temperature, the CO2 permeance of PPPS/PDMS/mPSf composite membrane gradu increases, and the CO2/CH4 selectivity reduces rapidly, which can be attributed to the lowing reasons. Firstly, the saturated vapor pressure increased with the feed gas temperature, w causes the increase of moisture content in the feed gas. As mentioned in Section 3.2.1 increase of the water content in the feed gas strengthens the swelling degree and free Firstly, the saturated vapor pressure increased with the feed gas temperature, which causes the increase of moisture content in the feed gas. As mentioned in Section 3.2.1, the increase of the water content in the feed gas strengthens the swelling degree and free volume of the composite membrane, resulting in an increase in the gas permeance. In addition, the increase of water content in the composite membrane also enhances the CO 2 facilitated transport. Secondly, as the feed gas temperature increases, due to the exothermically reversible reaction between CO 2 and primary amine groups, the reaction rate increases but the forward reaction equilibrium constant decreases, which is not conducive to the CO 2 facilitated transport. Thirdly, the solution and diffusion process of CH 4 adheres to Arrhenius' law [27,28], thus the CH 4 permeance increases significantly with the increasing feed gas temperature.
In summary, with the increscent feed gas temperature, the CO 2 and CH 4 permeance in the PPPS/PDMS/mPSf composite membrane are both confronted with an upward trend. However, due to the multiple factors analyzed above, the CO 2 permeance could increases more slowly, which ultimately leads to a decline in the CO 2 /CH 4 selectivity. Thus, the ideal operating temperature of the PPPS/PDMS/PSf composite membrane is at 298 K.

CO 2 Concentration of the Feed Gas
As the CO 2 concentration of the oilfield associated gas changes greatly during the entire life of the oilfield, thus, it is especially significant to make a comprehensive study of the effect of the feed gas CO 2 concentration on the CO 2 /CH 4 separation performance of the PPPS/PDMS/PSf composite membranes. As presented in Figure 9, with the increasing CO 2 concentration, the CO 2 permeance keeps dropping, while the CO 2 /CH 4 selectivity constantly grows. The main reasons are as follows.
As the CO2 concentration of the oilfield associated gas changes greatly du entire life of the oilfield, thus, it is especially significant to make a comprehensiv of the effect of the feed gas CO2 concentration on the CO2/CH4 separation perform the PPPS/PDMS/PSf composite membranes. As presented in Figure 9, with the in CO2 concentration, the CO2 permeance keeps dropping, while the CO2/CH4 se constantly grows. The main reasons are as follows.
According to the formula obtained by Zhang et al. [29], the CO2 permeanc facilitated transport membranes can be defined by Formula (5).

( )
where RA is the CO2 permeance, pA,0 is the CO2 partial pressure of the feed gas, DA are the diffusion coefficients of CO2 and CO2-carrier complexes respectively, l is t ness of the membrane, HA is the Henry coefficient of CO2, K is the equilibrium con the reversible reaction between CO2 and the carrier, CT is the total concentratio carrier. Therefore, the growing CO2 partial pressure in the feed gas leading to th bution of facilitated transport to the total CO2 permeance decline. As for CH4, on hand, the solubility coefficient and diffusion coefficient of CH4 in the membrane with the falling CH4 partial pressure [30]. On the other hand, due to the compet sorption, the amount of CH4 adsorbed on the membrane surface tends to downw the increase of the CO2 partial pressure. To sum up, as the CO2 partial pressure in gas increases, the CO2 permeance of the PPPS/PDMS/PSf facilitated transport me keeps decreasing, while the CH4 permeance drops faster, leading to the CO2/CH4 ity showing an upward trend. According to the formula obtained by Zhang et al. [29], the CO 2 permeance of the facilitated transport membranes can be defined by Formula (5).
where R A is the CO 2 permeance, p A,0 is the CO 2 partial pressure of the feed gas, D A and D AX are the diffusion coefficients of CO 2 and CO 2 -carrier complexes respectively, l is the thickness of the membrane, H A is the Henry coefficient of CO 2 , K is the equilibrium constant of the reversible reaction between CO 2 and the carrier, C T is the total concentration of the carrier. Therefore, the growing CO 2 partial pressure in the feed gas leading to the contribution of facilitated transport to the total CO 2 permeance decline. As for CH 4 , on the one hand, the solubility coefficient and diffusion coefficient of CH 4 in the membrane decrease with the falling CH 4 partial pressure [30]. On the other hand, due to the competitive adsorption, the amount of CH 4 adsorbed on the membrane surface tends to downward with the increase of the CO 2 partial pressure. To sum up, as the CO 2 partial pressure in the feed gas increases, the CO 2 permeance of the PPPS/PDMS/PSf facilitated transport membrane keeps decreasing, while the CH 4 permeance drops faster, leading to the CO 2 /CH 4 selectivity showing an upward trend.

Pressure of the Feed Gas
To further examine the effect of feed gas pressure on CO 2 /CH 4 (45/55 vol%) separation performance, the PPPS/PDMS/PSf composite membrane was tested at a feed pressure of 0.15~1.8 MPa. As shown in Figure 10, the CO 2 permeance and CO 2 /CH 4 selectivity decrease rapidly with increasing feed gas pressure, which is mainly due to the following reasons. First of all, PVAm is a typical material mainly utilized in facilitated transport membrane, thus the PPPS/PDMS/PSf composite membrane displays the representative characteristics of facilitated transport, that is, CO 2 permeance decreases with increasing CO 2 partial pressure [29,31]. In addition, CH 4 penetrates through the composite membrane by solution-diffusion mechanism, causing the CH 4 permeance to be almost unchanged. Therefore, the CO 2 /CH 4 selectivity decreases as the feed gas pressure increases.

Pressure of the Feed Gas
To further examine the effect of feed gas pressure on CO2/CH4 (45/55 vol%) separation performance, the PPPS/PDMS/PSf composite membrane was tested at a feed pressure of 0.15~1.8 MPa. As shown in Figure 10, the CO2 permeance and CO2/CH4 selectivity decrease rapidly with increasing feed gas pressure, which is mainly due to the following reasons. First of all, PVAm is a typical material mainly utilized in facilitated transport membrane, thus the PPPS/PDMS/PSf composite membrane displays the representative characteristics of facilitated transport, that is, CO2 permeance decreases with increasing CO2 partial pressure [29,31]. In addition, CH4 penetrates through the composite membrane by solution-diffusion mechanism, causing the CH4 permeance to be almost unchanged. Therefore, the CO2/CH4 selectivity decreases as the feed gas pressure increases. Figure 10. Effect of the feed gas pressure on the CO2/CH4 mixed gas separation performance of PPPS/PDMS/PSf composite membrane. Test conditions: 298 K and saturated humidity.

System and Simulation Condition Setting
To investigate the industrial potential of the PPPS/PDMS/PSf composite membranes, a two-stage membrane process was designed for the CO2 separation and CH4 recovery from the oilfield associated gas, which is the most economical separation process in the case of high product purity and recovery [32][33][34]. In this simulation, the CH4 purity (98 vol%) and CH4 recovery rate (98%) were chosen as the separation requirements [35,36]. The feed pressure of the two-stage membrane process was optimized when the gas composition of raw gas was 45 vol% CO2 and 55 vol% CH4.
The two-stage membrane separation process is shown in Figure 11. The flow rate of raw gas is set as 1000 Nm 3 ·h −1 , and the pressure is 0.15 MPa (absolute pressure, the same below). After being compressed by the compressor, the raw gas encountered with the recycle gas was separated by the 1st-stage membrane. The retentate gas (CH4 concentration at 98 vol%) from the 1st-stage was stored for further purification, while the permeate gas was recompressed and divided by the 2nd-stage membrane into the recycle gas and the penetrate gas rich in CO2. The feed pressures of the 1st-and 2nd-stages are in the range of 0.3~1.5 MPa, and the pressures on the permeate side are set as 0.1 MPa.
The gas separation process was described by the cross-flow model [37,38] and computed by MATLAB software, according to previous work [39,40]. Detailed information on the mathematical model is provided in S2 (Supplementary Information).

System and Simulation Condition Setting
To investigate the industrial potential of the PPPS/PDMS/PSf composite membranes, a two-stage membrane process was designed for the CO 2 separation and CH 4 recovery from the oilfield associated gas, which is the most economical separation process in the case of high product purity and recovery [32][33][34]. In this simulation, the CH 4 purity (98 vol%) and CH 4 recovery rate (98%) were chosen as the separation requirements [35,36]. The feed pressure of the two-stage membrane process was optimized when the gas composition of raw gas was 45 vol% CO 2 and 55 vol% CH 4 .
The two-stage membrane separation process is shown in Figure 11. The flow rate of raw gas is set as 1000 Nm 3 ·h −1 , and the pressure is 0.15 MPa (absolute pressure, the same below). After being compressed by the compressor, the raw gas encountered with the recycle gas was separated by the 1st-stage membrane. The retentate gas (CH 4 concentration at 98 vol%) from the 1st-stage was stored for further purification, while the permeate gas was recompressed and divided by the 2nd-stage membrane into the recycle gas and the penetrate gas rich in CO 2 . The feed pressures of the 1st-and 2nd-stages are in the range of 0.3~1.5 MPa, and the pressures on the permeate side are set as 0.1 MPa.  Figure 11. Schematic diagram of a two-stage membrane process.

Simulation of the Total Membrane Area and Specific Electricity Consumption
The total membrane area required to complete the set conditions was simulated by adjusting the feed pressure of the 1st-stage membrane and the 2nd-stage membrane separately. As shown in Figure 12a, with the increase of the 1st-stage feed pressure, the total membrane area decreases significantly, while the 2nd-stage feeding pressure has little influence on the total membrane area. The reason is that the gas processing capacity of the 1st-stage membrane process is much higher than that of the 2nd-stage. With the increasing feed gas in the 1st-stage, the gas flux per membrane area also increases significantly, thus the membrane area decreases dramatically.
However, although the increase in the feed pressure can reduce the membrane area, the energy consumption to compress the gas is relatively increased. As shown in Figure  12b, when the 1st-stage feed pressure is fixed, with the increasing 2nd-stage feed pressure, the specific electricity consumption of the compressor increases. When fixing the 2ndstage feed pressure, the increasing 1st-stage feed pressure leads to the specific electricity consumption first decreasing and then increasing. Therefore, when the 1st-and 2nd-stage feed pressures are 0.7 MPa and 0.3 MPa respectively, the minimum specific electricity consumption can be reduced to 0.13 kWh/Nm 3 raw gas.

Simulation of the Effect of CO2 Concentration on Specific Cost
The specific cost of the two-stage process using PPPS/PDMS/PSf composite membranes was calculated mainly referring to the method mentioned in the previous papers [39,41]. The system operation time was set as 8000 h per year. The membrane module cost was considered as 50 $·m −2 [36], the referenced fixed investment cost was selected as 394,000$ per 2000 m 2 membranes [42] and the electricity cost was consulted as 0.1 $·kWh −1 [32,43]. In brief, the total annual cost is the sum of the investment cost, the cost for annual operating and maintenance, and energy cost. Detailed information on the economic evaluation is provided in S3 (Supplementary Information). The gas separation process was described by the cross-flow model [37,38] and computed by MATLAB software, according to previous work [39,40]. Detailed information on the mathematical model is provided in S2 (Supplementary Information).

Simulation of the Total Membrane Area and Specific Electricity Consumption
The total membrane area required to complete the set conditions was simulated by adjusting the feed pressure of the 1st-stage membrane and the 2nd-stage membrane separately. As shown in Figure 12a, with the increase of the 1st-stage feed pressure, the total membrane area decreases significantly, while the 2nd-stage feeding pressure has little influence on the total membrane area. The reason is that the gas processing capacity of the 1st-stage membrane process is much higher than that of the 2nd-stage. With the increasing feed gas in the 1st-stage, the gas flux per membrane area also increases significantly, thus the membrane area decreases dramatically. As presented in Figure 12c, when the raw gas was fixed as 45 vol% CO2 and 55 vol% CH4, the minimum specific cost can be reduced to 0.046 $·Nm −3 CH4 with the 1st-and 2ndstage pressures of 1.1 MPa and 0.3 MPa, respectively. Compared with the 2nd-stage feed pressure, the 1st-stage feed pressure exerts a greater influence on the specific cost, for the 1st-stage feed pressure affects the total membrane area more obviously. However, although the increase in the feed pressure can reduce the membrane area, the energy consumption to compress the gas is relatively increased. As shown in Figure 12b, when the 1st-stage feed pressure is fixed, with the increasing 2nd-stage feed pressure, the specific electricity consumption of the compressor increases. When fixing the 2ndstage feed pressure, the increasing 1st-stage feed pressure leads to the specific electricity consumption first decreasing and then increasing. Therefore, when the 1st-and 2nd-stage feed pressures are 0.7 MPa and 0.3 MPa respectively, the minimum specific electricity consumption can be reduced to 0.13 kWh/Nm 3 raw gas.

Simulation of the Effect of CO 2 Concentration on Specific Cost
The specific cost of the two-stage process using PPPS/PDMS/PSf composite membranes was calculated mainly referring to the method mentioned in the previous papers [39,41]. The system operation time was set as 8000 h per year. The membrane module cost was considered as 50 $·m −2 [36], the referenced fixed investment cost was selected as 394,000$ per 2000 m 2 membranes [42] and the electricity cost was consulted as 0.1 $·kWh −1 [32,43]. In brief, the total annual cost is the sum of the investment cost, the cost for annual operating and maintenance, and energy cost. Detailed information on the economic evaluation is provided in S3 (Supplementary Information).
As presented in Figure 12c, when the raw gas was fixed as 45 vol% CO 2 and 55 vol% CH 4 , the minimum specific cost can be reduced to 0.046 $·Nm −3 CH 4 with the 1st-and 2nd-stage pressures of 1.1 MPa and 0.3 MPa, respectively. Compared with the 2nd-stage feed pressure, the 1st-stage feed pressure exerts a greater influence on the specific cost, for the 1st-stage feed pressure affects the total membrane area more obviously.
However, CO 2 concentration varies greatly in the life of oilfield associated gas. Based on the above pressures, the specific cost was further simulated by changing the CO 2 concentration. As shown in Figure 12d, with the increase of CO 2 concentration in the raw gas, the required membrane area gradually decreases. Simultaneously, the product flow rate (98 vol% CH 4 ) declines, but the flow rate of the gas that needs to be compressed increasingly augment. Thus, when CO 2 concentration is at 20 vol%, the specific cost is 0.030 $·Nm −3 CH 4 , while when CO 2 concentration reaches up to 75 vol%, the specific cost gradually rises to 0.097 $·Nm −3 CH 4 .

Short-to-Mid-Term Stability of PPPS/PDMS/PSf Composite Membrane
The PPPS/PDMS/PSf composite was continuously tested at 298 K, saturated humidity, and 0.15 MPa of the feed gas pressure for 70 h to verify the short-to-mid-term CO 2 /CH 4 separation performance. As shown in Figure 13, the short-to-mid-term stability test was divided into three stages. The feed gas in the first 30 h consisted of 45 vol% CO 2 and 55 vol% CH 4 (stage A). At the 30th hour, the feed gas was replaced with the mixed gas containing 79 vol% CO 2 and 21 vol% CH 4 (stage B). Then at the 50th hour, it was restored to the mixed gas same with stage A (stage C). In the three stages of A, B, and C, the PPPS/PDMS/PSf composite membrane maintained stable CO 2 /CH 4 separation performance, indicating that the composite membrane exhibited excellent short-to-mid-term stability.
Membranes 2021, 11, x FOR PEER REVIEW Figure 13. Short-to-mid-term stability of PPPS/PDMS/PSf composite membrane. Test co 298 K, saturated humidity, and 0.15 MPa. Table 1 exhibited some commercial and reported membranes for CO2/CH4 tion. Compared with other membranes, the PPPS/PDMS/PSf membrane displays ble CO2 permeance and excellent CO2/CH4 selectivity, which has great potential fo scale application in CO2 reuse and CH4 recovery in oilfield associated gas.

Conclusions
Compared with traditional separation methods such as cryogenic rectificat adsorption, the membrane separation process features operation flexibility, ease up, no phase change, and no need for regeneration. The membrane separation s feasible to be designed as an integrated skid-mounted structure, which is conven transportation and movement in the remote wilderness. Therefore, membrane sep possesses obvious advantages in CO2 reuse and CH4 recovery in oilfield associate In this paper, the PPPS/PDMS/PSf composite membrane with proper thickn defect-free composite layers was successfully prepared by 1.5 wt% PDMS and 0 PPPS, which exhibited high CO2 permeance of 3.451 × 10 −7 mol·m −2 ·s −1 ·Pa −1 and C selectivity of 62 at 298 K, saturated humidity and 0.15 MPa. The two-stage me separation process using PPPS/PDMS/PSf composite membranes was simulated justing the feed gas CO2 concentration and pressure. When the 1st-and 2nd-sta sures are 1.1 MPa and 0.3 MPa respectively, the minimum specific cost of the tw membrane separation process using the PPPS/PDMS/PSf composite membranes rate CO2/CH4 (45/55 vol%) mixed gas can be controlled within 0.046 $·Nm −3 CH4.  Table 1 exhibited some commercial and reported membranes for CO 2 /CH 4 separation. Compared with other membranes, the PPPS/PDMS/PSf membrane displays acceptable CO 2 permeance and excellent CO 2 /CH 4 selectivity, which has great potential for large-scale application in CO 2 reuse and CH 4 recovery in oilfield associated gas.

Conclusions
Compared with traditional separation methods such as cryogenic rectification and adsorption, the membrane separation process features operation flexibility, ease of scale-up, no phase change, and no need for regeneration. The membrane separation system is feasible to be designed as an integrated skid-mounted structure, which is convenient for transportation and movement in the remote wilderness. Therefore, membrane separation possesses obvious advantages in CO 2 reuse and CH 4 recovery in oilfield associated gas.
In this paper, the PPPS/PDMS/PSf composite membrane with proper thickness and defect-free composite layers was successfully prepared by 1.5 wt% PDMS and 0.15 wt% PPPS, which exhibited high CO 2 permeance of 3.451 × 10 −7 mol·m −2 ·s −1 ·Pa −1 and CO 2 /CH 4 selectivity of 62 at 298 K, saturated humidity and 0.15 MPa. The two-stage membrane separation process using PPPS/PDMS/PSf composite membranes was simulated by adjusting the feed gas CO 2 concentration and pressure. When the 1st-and 2nd-stage pressures are 1.1 MPa and 0.3 MPa respectively, the minimum specific cost of the two-stage membrane separation process using the PPPS/PDMS/PSf composite membranes to separate CO 2 /CH 4 (45/55 vol%) mixed gas can be controlled within 0.046 $·Nm −3 CH 4 . In addition, the PPPS/PDMS/PSf composite membrane shown outstanding short-to-mid-term stability, indicating the potential of industrial application in CO 2 reuse and CH 4 recovery in oilfield associated gas.

Data Availability Statement:
The data presented in this study are available in the article and the supplementary material.

Conflicts of Interest:
The authors declare no conflict of interest.