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Natural gas sweetening using a cellulose triacetate hollow fiber membrane illustrating controlled plasticization benefits
Journal Pre-proof Natural gas sweetening using a cellulose triacetate hollow fiber membrane illustrating controlled plasticization benefits Yang Liu, Zhongyun Liu, Atsushi Morisato, Nitesh Bhuwania, Daniel Chinn, William J. Koros PII:
S0376-7388(19)33827-X
DOI:
https://doi.org/10.1016/j.memsci.2020.117910
Reference:
MEMSCI 117910
To appear in:
Journal of Membrane Science
Received Date: 17 December 2019 Revised Date:
30 January 2020
Accepted Date: 31 January 2020
Please cite this article as: Y. Liu, Z. Liu, A. Morisato, N. Bhuwania, D. Chinn, W.J. Koros, Natural gas sweetening using a cellulose triacetate hollow fiber membrane illustrating controlled plasticization benefits, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2020.117910. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Author Statements Yang Liu: Visualization, Methodology, Investigation, Writing-Original draft preparation; Zhongyun
Liu:
Methodology;
Atsushi
Morisato:
Resources;
Nitesh
Bhuwania:
Conceptualization; Daniel Chinn: Conceptualization; William J. Koros: Supervision, Funding Acquisition, Project Administration, Writing-Reviewing and Editing
Natural Gas Sweetening Using a Cellulose Triacetate Hollow Fiber Membrane Illustrating Controlled Plasticization Benefits Yang Liua, Zhongyun Liua, Atsushi Morisatob, Nitesh Bhuwaniac, Daniel Chinnc, William J. Korosa,* a
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst
Drive, Atlanta, GA 30332, USA b
Cameron, A Schlumberger Company, 2504 Verne Roberts Circle, Suite 102, Antioch, CA
94509 c
Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94802-0627,
USA *
Corresponding author: [email protected] (W.J.K).
ABSTRACT: Plasticization is a well-understood drawback of polymer membranes in many applications; however, recent studies have demonstrated surprising advantages of this phenomenon for demanding natural gas sweetening for some glassy polymer dense film membranes. Moving beyond dense film membranes, the current study focuses on cellulose triacetate (CTA) hollow fiber membranes to use the benefits of controlled plasticization for realistic raw natural gas sweetening. Natural gas sweetening can be complicated by co-existence of condensable hydrocarbons, e.g. C2H6, C3H8 and toluene with the main H2S/CO2/CH4 ternary mixture; moreover, the operating temperature and pressure adds another dimension to this important separation. In this study, we consider an aggressive gas composition of high H2S (20 mol.%), low CO2 (5 mol.%), and significant amounts of C2H6 (3 mol.%) and C3H8 (3 mol.%) as well as trace amount of toluene (100 - 300 ppm) with CH4 comprising the rest of the feed. Various 1
temperatures (35 oC and 50 oC) and pressures (6.9 - 31.3 bar) are also considered. We show a controlled plasticization benefit for the CTA hollow fiber membrane, with attractive CO2 and H2S permeance (> 110 GPU) and selectivity (22 - 28) for CO2 and H2S over CH4 at 35 oC and 31.3 bar. The current study represents a major step forward in processes for membrane-based natural gas sweetening using practical asymmetric membranes. Keywords: membrane, CTA, hydrogen sulfide, carbon dioxide.
Introduction Natural gas is an attractive energy source compared to coal, due to its reduced carbon footprint [1-3]. Methane (CH4) typically comprises 50% - 90 % of natural gas; however, undesirable impurities, such as H2O, CO2, H2S, N2, C2H6, C3H8, and toluene, are often present in the raw natural gas [1]. The acid gases, CO2 and H2S, with coexistence of H2O, can corrode processing and transporting equipment; moreover, H2S is toxic, so raw natural gas must be sweetened prior to use [4, 5]. Roughly 40% of global natural gas resources contain high concentrations of H2S and CO2, and H2S concentration in the gas reservoirs in the Middle East can reach 30 mol.% along with significant CO2 [6]. Advanced membranes for combined CO2 and H2S separation from CH4 are high priorities for the membrane community and the natural gas processing industry [6-9]. Previous studies have shown that both rubbery [10, 11] and glassy [12-17] polymer membranes exhibit promising H2S capture performance. Glassy polymer membranes in dense film forms have been shown to also benefit from controlled plasticization for H2S/CH4 separation as compared to rubbery polymer membranes, as demonstrated recently [18, 19]. Swelling of the glassy polymer offers 2
increased sorption for the more highly sorbing H2S vs. CH4, promoting a simultaneous increase of H2S permeability and H2S/CH4 selectivity. Surprisingly, swelling-induced controlled plasticization of some glassy polymer membranes even offers much higher CO2 removal efficiency than rubbery polymer membranes due to retained molecular discrimination features under conditions of controlled plasticization. Moreover, glassy polymers offer straightforward spinning into self-supporting asymmetric hollow fiber membranes with high productivity and selectivity using thin selective layers [20-22]. In this regard, asymmetric cellulose acetate hollow fiber membranes represents the current industry standard for CO2 removal from natural gas [2327]; however, its H2S removal capacity has not been systematically studied in detail, primarily due to the difficulty in executing experiments, even in a lab-scale, due to the toxic nature of H2S. Herein, we report application of an asymmetric cellulose triacetate (CTA) hollow fiber membrane for natural gas sweetening under realistic application conditions. The CTA hollow fiber membrane was tested under gas compositions of high H2S (20 mol.%), low CO2 (5 mol.%), and significant amounts of C2H6 (3 mol.%) and C3H8 (3% mol.%) as well as trace amounts of toluene (100 ppm or 300 ppm) with CH4 comprising the rest of the feed (balance gas). Herein, to avoid cumbersome notations we use the terminology 20/5/3/3/X ppm, where X refers to either 100 ppm or 300 ppm as discussed later. The effects of temperature (35 oC and 50 oC) and pressure (6.9 bar to 31.3 bar) on the membrane performance are also considered. In this work, plasticization benefits were observed for the CTA hollow fiber membrane under the aggressive conditions, providing attractive productivity and selectivity for both CO2 and H2S removal. Our work demonstrates the high separation potential of CTA hollow fiber membranes under realistic natural gas sweetening process conditions. 3
Experimental details The CTA hollow fiber membrane samples were specially prepared and provided by Cameron, A Schlumberger Company, for this study, and used without further pretreatment before measurements. The outer diameters of the membranes were measured by an optical microscope and were averaged by multiple measurements to provide accurate values. The membrane area was calculated using the effective membrane length and the outer diameter. After each measurement, the plasticized membrane sample was replaced by a new membrane sample to provide a valid comparison of membrane performance under different test conditions. Deviations of the separation performance under the same condition for different membrane samples are estimated to be less than 3%. Figure 1 shows the schematic of natural gas sweetening using the CTA hollow fiber membranes with shell side feed.
Cellulose Triacetate
Figure 1. Schematic of natural gas sweetening using a cellulose triacetate (CTA) hollow fiber membrane.
All gas permeation measurements were conducted in a variable pressure, constant-volume apparatus under mixed gas conditions [13]. Specifically, two kinds of mixed gas with gas compositions of 20/5/3/3/100_ppm and 20/5/3/3/300_ppm for H2S/CO2/C2H6/C3H8/toluene 4
balanced with CH4 were used in the measurements. Additionally, two temperatures, i.e. 35 oC and 50 oC, and four pressures, ranging from 6.9 bar to 31.3 bar, were considered in the study. During the measurements, the downstream composition was determined using a gas chromatograph (Varian 450-GC). The stage cut representing the flow rate ratio of permeate to feed was maintained below 0.06 to minimize concentration polarization on the upstream side of the permeation cell despite the high flux of the hollow fiber membranes. To ensure that all mixed gas data were collected at steady state, the membranes were pre-saturated under the target pressure for 3 to 6 hours, depending on the temperature and pressure used. The overall downstream pressure change with time (dp/dt) and the product gas composition were monitored continuously in the process, after which the final data were collected by averaging the stabilized data points, usually 2-3 points with negligible variation from each other. The vented exhaust H2S containing gas mixture was saturated with NaOH solution to avoid potential environmental and health issues. Permeance and permselectivity were used to characterize membrane separation performance. The gas permeance coefficient of component i (Pi/l) in mixed gas was calculated using its mole fraction in the permeate (xi) and the transmembrane fugacity difference (∆fi): / =
∙ ∙ ∙ ∙ ∙∆
(Eq. 1)
where, dp/dt is the slopes of permeate pressure vs. time, A is the membrane area, R is the gas constant, T is the operating temperature. The fugacity (NIST software standard reference database) was used instead of partial pressure to account for non-idealities of gases. The perm-
5
selectivity, αij, is determined by the ratio of the component i permeance to the component j permeance: =
/ /
(Eq. 2)
Results and Discussions Figure 2 shows the H2S/CH4 and CO2/CH4 separation efficiency of the CTA hollow fiber membranes under a H2S/CO2/CH4/C2H6/C3H8/toluene composition of 20/5/bal./3/3/100_ppm at 35 oC and 50 oC. As expected, plasticization occurs for the CTA hollow fiber membrane samples with increased pressure at both temperatures as evidenced by the gradual increase of permeance of H2S, CO2, and CH4 (Figure 2a and 2c). This response is expected due to the high condensability of H2S, CO2, C2H6, C3H8, and toluene, which increased mobility of the polymer chain segments, thereby increasing gas transport rates through the membrane. Also, as expected, CO2/CH4 selectivity decreases gradually as plasticization occurs due to reduction of molecular discrimination effect (Figure 2b and 2d). In addition, however, the plasticized CTA hollow fiber membrane shows an unexpected increase in H2S/CH4 selectivity, which is contrary to traditional understanding of the plasticization effect. Remarkably, at 35 oC and 31.3 bar, the CTA hollow fiber membrane exhibits a H2S permeance of ~140 GPU with a H2S/CH4 selectivity of ~28; moreover, the CO2 permeance is ~115 GPU with a CO2/CH4 selectivity of ~22. The higher H2S separation efficiency allows the membrane to remove more H2S than CO2 to assist in reducing final polishing to meet pipeline specifications 4 ppm H2S and 2% of CO2. These promising results clearly demonstrate potential for economical CTA hollow fiber membranes under realistic natural gas sweetening applications: simultaneous removal of H2S and CO2. 6
(b) 180 o H 2S CO2 CH4 160 35 C 140 120 100 80 60 10 8 6 4 2 H S/CO /CH /C2/C3/toluene (20/5/bal./3/3/100ppm) 2 2 4 0 5 10 15 20 25 30 35
35
35 oC
H2S/CH4
25 20 15 H2S/CO2/CH4/C2/C3/toluene (20/5/bal./3/3/100ppm)
10 5
10
20
25
30
35
(d)
180 o H2S CO2 CH4 160 50 C 140 120 100 80 60 10 8 6 4 2 H S/CO2/CH4/C2/C3/toluene (20/5/bal./3/3/100ppm) 0 2 5 10 15 20 25 30 35
35 50 oC
H2S/CH4
CO2/CH4
30
Selectivity
Gas permeance (GPU)
15
Pressure (bar)
Pressure (bar)
(c)
CO2/CH4
30
Selectivity
Gas permeance (GPU)
(a)
25 20 15 H2S/CO2/CH4/C2/C3/toluene (20/5/bal./3/3/100ppm)
10 5
10
Pressure (bar)
15
20
25
30
35
Pressure (bar)
Figure 2. H2S/CH4 and CO2/CH4 separation performance of the cellulose triacetate hollow fiber membrane under a gas composition of 20/5/3/3/100 ppm for H2S/CO2/C2H6/C3H8/toluene balanced with CH4. (a) gas permeance and (b) gas selectivity at 35 oC; (c) gas permeance and (d) gas selectivity at 50 oC.
Notably, the plasticization-promoted H2S/CH4 separation performance agrees with our previous studies on glassy polyimide-based dense film membranes for H2S/CO2/CH4 ternary mixture separation [18]. In both cases, plasticization provides a dramatic benefit for the sorptiondominated H2S/CH4 separation by providing more sorption capacity in the newly created free volume in the glassy polymer regardless the type of polymers. This fact notwithstanding, this 7
study provides the first report of the plasticization benefit in an actual CTA asymmetric hollow fiber membrane at high pressure with a significant amount of heavy hydrocarbons and H2S. Indeed, the responses to plasticization between hollow fiber membranes and dense film membranes can be different due to the morphology difference [28], asymmetric vs. symmetric, as well as the effective membrane thickness difference [29], submicron vs. tens of microns, respectively. Nevertheless, the reserved plasticization benefit in the CTA hollow fiber membranes with ~200 nm effective membrane thickness demonstrate the feasibility of the plasticization benefit for realistic natural gas sweetening applications using membranes. Another noteworthy phenomenon we observed involves the significant temperature effect on membrane performance as illustrated in Figure 3. Typically, for simple gas mixture, permeance/permeability increases and selectivity decreases with increasing gas permeation temperature measurements, e.g. CO2/CH4 [30], C2H4/C2H6 [31], and C3H6/C3H8 [32,33]. Indeed, we observed lower H2S/CH4 and CO2/CH4 selectivities for the CTA hollow fiber membrane under higher temperatures, i.e. 50 oC vs. 35 oC; however, at the lower temperature 35 oC, both H2S and CO2 permeance at 31.3 bar are higher than those at 50 oC. This unexpected phenomenon reflects the plasticization benefit. Plasticization is a sorption-induced phenomenon, and increasing the operating temperature decreases gas sorption capacities, thereby suppressing plasticization. Specifically, less plasticization benefit occurs at a higher operating temperature for the sorption-dominated gas separation process, resulting in lower H2S and CO2 permeance at 50 oC vs. 35 oC at equivalent feed composition. In real applications, the operation temperature of the feed streams in natural gas separation process varies from 65 oC (to prevent condensation in membrane modules) to sub-ambient temperature (to capture and remove C2+ hydrocarbon 8
components) [22]. Our work suggests opportunities in system design using glassy polymer membranes to optimize the plasticization benefit for the natural gas sweetening process using temperature to optimize membrane performance for different feed composition. This work is planned for future studies. (b) 180 o H2S CO2 CH4 160 35 C 140 50 oC H2S CO2 CH4 120 100 80 60 10 8 6 4 2 H S/CO2/CH4/C2/C3/toluene (20/5/bal./3/3/100ppm) 0 2 5 10 15 20 25 30 35
Pressure (bar)
35 35 oC o
50 C
30
Selectivity
Gas permeance (GPU)
(a)
H2S/CH4
CO2/CH4
H2S/CH4
CO2/CH4
25 20 15 10
H2S/CO2/CH4/C2/C3/toluene (20/5/bal./3/3/100ppm)
5
10
15
20
25
30
35
Pressure (bar)
Figure 3. Comparison of gas separation performance of the CTA hollow fiber membrane at 35 oC and 50 oC under a gas composition of 20/5/3/3/100 ppm for H2S/CO2/C2H6/C3H8/toluene balanced with CH4. (a) gas permeance and (b) gas selectivity.
We also considered the effect of hydrocarbon contaminant concentration on the CTA hollow fiber membrane performance. Specifically, the concentration of toluene was increased from 100 ppm to 300 ppm while holding the H2S, CO2, C2H6, and C3H8 compositions the same and using CH4 as the balancing gas. In this case, the plasticization benefit is again observed for H2S/CH4 separation at 35 oC (Figure 4), whereas the H2S/CH4 selectivity drops at 50 oC and 31.3 bar (Figure 4b). Comparing with the results in Figure 2c-d with a toluene concentration of 100 ppm at same temperature, the unexpected H2S/CH4 selectivity drop at 50 oC reflects the increased concentration of toluene (300 ppm). As the absolute amount of the large toluene 9
molecules (kinetic diameter = 5.8 Å) is further increased in the feed stream at high pressure (31.3 bar), larger transient gaps enable diffusion of the smaller H2S, CO2 and CH4 molecules, which subsequently reduces the H2S/CH4 selectivity at high feed pressure. (b)
180 o H2S CO2 CH4 160 35 C o 50 C H S CO CH4 140 2 2 120 100 80 60 10 8 6 4 2 H S/CO2/CH4/C2/C3/toluene (20/5/bal./3/3/300ppm) 0 2 5 10 15 20 25 30 35
35 30
Selectivity
Gas permeance (GPU)
(a)
35 oC
H2S/CH4
CO2/CH4
50 oC
H2S/CH4
CO2/CH4
25 20 15 H2S/CO2/CH4/C2/C3/toluene (20/5/bal./3/3/300ppm)
10 5
10
Pressure (bar)
15
20
25
30
35
Pressure (bar)
Figure 4. H2S/CH4 and CO2/CH4 separation performance of the cellulose triacetate hollow fiber membrane under a gas composition of 20/5/3/3/300 ppm for H2S/CO2/C2H6/C3H8/toluene balanced with CH4. (a) gas permeance and (b) gas selectivity at 35 oC; (c) gas permeance and (d) gas selectivity at 50 oC.
Increased toluene concentration also affects gas separation performance at 35 oC and low pressure; however, this effect is somewhat offset by the plasticization benefit at high pressure, as shown in Figure 5. At low pressure, the highly condensable toluene with large kinetic size can compete with H2S and CO2 by sorption and can also hinder their diffusion; therefore a higher toluene concentration results in decreased H2S/CH4 and CO2/CH4 selectivities (300 ppm vs. 100 ppm). Nevertheless, plasticization of the polymer at high pressure creates new free volume (sorption-favored) and passage (diffusion-favored) for all gas molecules and thus competition among H2S, CO2 and toluene is offset by the new “opportunities” for permeation. Such an 10
interesting phenomenon again demonstrates tuning performance for CTA hollow fiber membranes using temperature and feed pressure to rationally optimize membrane performance for this important application. A wide ranges of options exist, including optimizing the downstream as well as upstream operating conditions to tune the transmembrane plasticization profile; however, this topic is beyond the scope of this study and will be considered in detail in a subsequent paper. Clearly, a limit will eventually be encountered at extremely high pressure; however, the CTA hollow fiber membranes will be extremely well-suited for many applications. (b) 180 100 ppm 160 140 300 ppm 120 100 80 60 10 8 6 4 2 0 5 10
H 2S
CO2
CH4
H 2S
CO2
CH4
35 30
Selectivity
Gas permeance (GPU)
(a)
100 ppm
H2S/CH4
CO2/CH4
300 ppm
H2S/CH4
CO2/CH4
25 20 15 10
15
20
25
Pressure (bar)
30
35
5
10
15
20
25
30
35
Pressure (bar)
Figure 5. Comparison of gas separation performance of the CTA hollow fiber membrane at 35 oC under different gas compositions of 20/5/3/3/100 ppm and 20/5/3/3/300 ppm for H2S/CO2/C2H6/C3H8/toluene balanced with CH4. (a) gas permeance and (b) gas selectivity.
Conclusions In conclusion, we have demonstrated the plasticization benefit of a commercial asymmetric CTA hollow fiber membrane on natural gas sweetening under aggressive mixed gas conditions: high H2S, low CO2 and significant amounts of heavy hydrocarbons (C2, C3 and Toluene). The H2S and CO2 separation efficiency of the membrane is promising to achieve the simultaneous 11
H2S+CO2 removal target with attractive productivity. In applications, the temperature effect can be further considered as an economical tuning tool to engineer the plasticization benefit and optimize separation performance. The CTA hollow fiber membrane shows excellent tolerance to the toluene concentration in the mixed gas, demonstrating the feasibility of the membrane in real applications with significant aromatic contents.
Acknowledgments The authors acknowledge the support by Chevron Energy Technology Company (1906BXE)
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Highlights •
Simultaneous removal of H2S and CO2 from natural gas using a CTA hollow fiber membrane
•
The CTA hollow fiber membrane illustrates the plasticization benefits
•
The CTA hollow fiber membrane shows excellent tolerance to various hydrocarbon concentrations in realistic natural gas sweetening conditions
1
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0376-7388(19)33827-X
DOI:
https://doi.org/10.1016/j.memsci.2020.117910
Reference:
MEMSCI 117910
To appear in:
Journal of Membrane Science
Received Date: 17 December 2019 Revised Date:
30 January 2020
Accepted Date: 31 January 2020
Please cite this article as: Y. Liu, Z. Liu, A. Morisato, N. Bhuwania, D. Chinn, W.J. Koros, Natural gas sweetening using a cellulose triacetate hollow fiber membrane illustrating controlled plasticization benefits, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2020.117910. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Author Statements Yang Liu: Visualization, Methodology, Investigation, Writing-Original draft preparation; Zhongyun
Liu:
Methodology;
Atsushi
Morisato:
Resources;
Nitesh
Bhuwania:
Conceptualization; Daniel Chinn: Conceptualization; William J. Koros: Supervision, Funding Acquisition, Project Administration, Writing-Reviewing and Editing
Natural Gas Sweetening Using a Cellulose Triacetate Hollow Fiber Membrane Illustrating Controlled Plasticization Benefits Yang Liua, Zhongyun Liua, Atsushi Morisatob, Nitesh Bhuwaniac, Daniel Chinnc, William J. Korosa,* a
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst
Drive, Atlanta, GA 30332, USA b
Cameron, A Schlumberger Company, 2504 Verne Roberts Circle, Suite 102, Antioch, CA
94509 c
Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94802-0627,
USA *
Corresponding author: [email protected] (W.J.K).
ABSTRACT: Plasticization is a well-understood drawback of polymer membranes in many applications; however, recent studies have demonstrated surprising advantages of this phenomenon for demanding natural gas sweetening for some glassy polymer dense film membranes. Moving beyond dense film membranes, the current study focuses on cellulose triacetate (CTA) hollow fiber membranes to use the benefits of controlled plasticization for realistic raw natural gas sweetening. Natural gas sweetening can be complicated by co-existence of condensable hydrocarbons, e.g. C2H6, C3H8 and toluene with the main H2S/CO2/CH4 ternary mixture; moreover, the operating temperature and pressure adds another dimension to this important separation. In this study, we consider an aggressive gas composition of high H2S (20 mol.%), low CO2 (5 mol.%), and significant amounts of C2H6 (3 mol.%) and C3H8 (3 mol.%) as well as trace amount of toluene (100 - 300 ppm) with CH4 comprising the rest of the feed. Various 1
temperatures (35 oC and 50 oC) and pressures (6.9 - 31.3 bar) are also considered. We show a controlled plasticization benefit for the CTA hollow fiber membrane, with attractive CO2 and H2S permeance (> 110 GPU) and selectivity (22 - 28) for CO2 and H2S over CH4 at 35 oC and 31.3 bar. The current study represents a major step forward in processes for membrane-based natural gas sweetening using practical asymmetric membranes. Keywords: membrane, CTA, hydrogen sulfide, carbon dioxide.
Introduction Natural gas is an attractive energy source compared to coal, due to its reduced carbon footprint [1-3]. Methane (CH4) typically comprises 50% - 90 % of natural gas; however, undesirable impurities, such as H2O, CO2, H2S, N2, C2H6, C3H8, and toluene, are often present in the raw natural gas [1]. The acid gases, CO2 and H2S, with coexistence of H2O, can corrode processing and transporting equipment; moreover, H2S is toxic, so raw natural gas must be sweetened prior to use [4, 5]. Roughly 40% of global natural gas resources contain high concentrations of H2S and CO2, and H2S concentration in the gas reservoirs in the Middle East can reach 30 mol.% along with significant CO2 [6]. Advanced membranes for combined CO2 and H2S separation from CH4 are high priorities for the membrane community and the natural gas processing industry [6-9]. Previous studies have shown that both rubbery [10, 11] and glassy [12-17] polymer membranes exhibit promising H2S capture performance. Glassy polymer membranes in dense film forms have been shown to also benefit from controlled plasticization for H2S/CH4 separation as compared to rubbery polymer membranes, as demonstrated recently [18, 19]. Swelling of the glassy polymer offers 2
increased sorption for the more highly sorbing H2S vs. CH4, promoting a simultaneous increase of H2S permeability and H2S/CH4 selectivity. Surprisingly, swelling-induced controlled plasticization of some glassy polymer membranes even offers much higher CO2 removal efficiency than rubbery polymer membranes due to retained molecular discrimination features under conditions of controlled plasticization. Moreover, glassy polymers offer straightforward spinning into self-supporting asymmetric hollow fiber membranes with high productivity and selectivity using thin selective layers [20-22]. In this regard, asymmetric cellulose acetate hollow fiber membranes represents the current industry standard for CO2 removal from natural gas [2327]; however, its H2S removal capacity has not been systematically studied in detail, primarily due to the difficulty in executing experiments, even in a lab-scale, due to the toxic nature of H2S. Herein, we report application of an asymmetric cellulose triacetate (CTA) hollow fiber membrane for natural gas sweetening under realistic application conditions. The CTA hollow fiber membrane was tested under gas compositions of high H2S (20 mol.%), low CO2 (5 mol.%), and significant amounts of C2H6 (3 mol.%) and C3H8 (3% mol.%) as well as trace amounts of toluene (100 ppm or 300 ppm) with CH4 comprising the rest of the feed (balance gas). Herein, to avoid cumbersome notations we use the terminology 20/5/3/3/X ppm, where X refers to either 100 ppm or 300 ppm as discussed later. The effects of temperature (35 oC and 50 oC) and pressure (6.9 bar to 31.3 bar) on the membrane performance are also considered. In this work, plasticization benefits were observed for the CTA hollow fiber membrane under the aggressive conditions, providing attractive productivity and selectivity for both CO2 and H2S removal. Our work demonstrates the high separation potential of CTA hollow fiber membranes under realistic natural gas sweetening process conditions. 3
Experimental details The CTA hollow fiber membrane samples were specially prepared and provided by Cameron, A Schlumberger Company, for this study, and used without further pretreatment before measurements. The outer diameters of the membranes were measured by an optical microscope and were averaged by multiple measurements to provide accurate values. The membrane area was calculated using the effective membrane length and the outer diameter. After each measurement, the plasticized membrane sample was replaced by a new membrane sample to provide a valid comparison of membrane performance under different test conditions. Deviations of the separation performance under the same condition for different membrane samples are estimated to be less than 3%. Figure 1 shows the schematic of natural gas sweetening using the CTA hollow fiber membranes with shell side feed.
Cellulose Triacetate
Figure 1. Schematic of natural gas sweetening using a cellulose triacetate (CTA) hollow fiber membrane.
All gas permeation measurements were conducted in a variable pressure, constant-volume apparatus under mixed gas conditions [13]. Specifically, two kinds of mixed gas with gas compositions of 20/5/3/3/100_ppm and 20/5/3/3/300_ppm for H2S/CO2/C2H6/C3H8/toluene 4
balanced with CH4 were used in the measurements. Additionally, two temperatures, i.e. 35 oC and 50 oC, and four pressures, ranging from 6.9 bar to 31.3 bar, were considered in the study. During the measurements, the downstream composition was determined using a gas chromatograph (Varian 450-GC). The stage cut representing the flow rate ratio of permeate to feed was maintained below 0.06 to minimize concentration polarization on the upstream side of the permeation cell despite the high flux of the hollow fiber membranes. To ensure that all mixed gas data were collected at steady state, the membranes were pre-saturated under the target pressure for 3 to 6 hours, depending on the temperature and pressure used. The overall downstream pressure change with time (dp/dt) and the product gas composition were monitored continuously in the process, after which the final data were collected by averaging the stabilized data points, usually 2-3 points with negligible variation from each other. The vented exhaust H2S containing gas mixture was saturated with NaOH solution to avoid potential environmental and health issues. Permeance and permselectivity were used to characterize membrane separation performance. The gas permeance coefficient of component i (Pi/l) in mixed gas was calculated using its mole fraction in the permeate (xi) and the transmembrane fugacity difference (∆fi): / =
∙ ∙ ∙ ∙ ∙∆
(Eq. 1)
where, dp/dt is the slopes of permeate pressure vs. time, A is the membrane area, R is the gas constant, T is the operating temperature. The fugacity (NIST software standard reference database) was used instead of partial pressure to account for non-idealities of gases. The perm-
5
selectivity, αij, is determined by the ratio of the component i permeance to the component j permeance: =
/ /
(Eq. 2)
Results and Discussions Figure 2 shows the H2S/CH4 and CO2/CH4 separation efficiency of the CTA hollow fiber membranes under a H2S/CO2/CH4/C2H6/C3H8/toluene composition of 20/5/bal./3/3/100_ppm at 35 oC and 50 oC. As expected, plasticization occurs for the CTA hollow fiber membrane samples with increased pressure at both temperatures as evidenced by the gradual increase of permeance of H2S, CO2, and CH4 (Figure 2a and 2c). This response is expected due to the high condensability of H2S, CO2, C2H6, C3H8, and toluene, which increased mobility of the polymer chain segments, thereby increasing gas transport rates through the membrane. Also, as expected, CO2/CH4 selectivity decreases gradually as plasticization occurs due to reduction of molecular discrimination effect (Figure 2b and 2d). In addition, however, the plasticized CTA hollow fiber membrane shows an unexpected increase in H2S/CH4 selectivity, which is contrary to traditional understanding of the plasticization effect. Remarkably, at 35 oC and 31.3 bar, the CTA hollow fiber membrane exhibits a H2S permeance of ~140 GPU with a H2S/CH4 selectivity of ~28; moreover, the CO2 permeance is ~115 GPU with a CO2/CH4 selectivity of ~22. The higher H2S separation efficiency allows the membrane to remove more H2S than CO2 to assist in reducing final polishing to meet pipeline specifications 4 ppm H2S and 2% of CO2. These promising results clearly demonstrate potential for economical CTA hollow fiber membranes under realistic natural gas sweetening applications: simultaneous removal of H2S and CO2. 6
(b) 180 o H 2S CO2 CH4 160 35 C 140 120 100 80 60 10 8 6 4 2 H S/CO /CH /C2/C3/toluene (20/5/bal./3/3/100ppm) 2 2 4 0 5 10 15 20 25 30 35
35
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25 20 15 H2S/CO2/CH4/C2/C3/toluene (20/5/bal./3/3/100ppm)
10 5
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(d)
180 o H2S CO2 CH4 160 50 C 140 120 100 80 60 10 8 6 4 2 H S/CO2/CH4/C2/C3/toluene (20/5/bal./3/3/100ppm) 0 2 5 10 15 20 25 30 35
35 50 oC
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25 20 15 H2S/CO2/CH4/C2/C3/toluene (20/5/bal./3/3/100ppm)
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Figure 2. H2S/CH4 and CO2/CH4 separation performance of the cellulose triacetate hollow fiber membrane under a gas composition of 20/5/3/3/100 ppm for H2S/CO2/C2H6/C3H8/toluene balanced with CH4. (a) gas permeance and (b) gas selectivity at 35 oC; (c) gas permeance and (d) gas selectivity at 50 oC.
Notably, the plasticization-promoted H2S/CH4 separation performance agrees with our previous studies on glassy polyimide-based dense film membranes for H2S/CO2/CH4 ternary mixture separation [18]. In both cases, plasticization provides a dramatic benefit for the sorptiondominated H2S/CH4 separation by providing more sorption capacity in the newly created free volume in the glassy polymer regardless the type of polymers. This fact notwithstanding, this 7
study provides the first report of the plasticization benefit in an actual CTA asymmetric hollow fiber membrane at high pressure with a significant amount of heavy hydrocarbons and H2S. Indeed, the responses to plasticization between hollow fiber membranes and dense film membranes can be different due to the morphology difference [28], asymmetric vs. symmetric, as well as the effective membrane thickness difference [29], submicron vs. tens of microns, respectively. Nevertheless, the reserved plasticization benefit in the CTA hollow fiber membranes with ~200 nm effective membrane thickness demonstrate the feasibility of the plasticization benefit for realistic natural gas sweetening applications using membranes. Another noteworthy phenomenon we observed involves the significant temperature effect on membrane performance as illustrated in Figure 3. Typically, for simple gas mixture, permeance/permeability increases and selectivity decreases with increasing gas permeation temperature measurements, e.g. CO2/CH4 [30], C2H4/C2H6 [31], and C3H6/C3H8 [32,33]. Indeed, we observed lower H2S/CH4 and CO2/CH4 selectivities for the CTA hollow fiber membrane under higher temperatures, i.e. 50 oC vs. 35 oC; however, at the lower temperature 35 oC, both H2S and CO2 permeance at 31.3 bar are higher than those at 50 oC. This unexpected phenomenon reflects the plasticization benefit. Plasticization is a sorption-induced phenomenon, and increasing the operating temperature decreases gas sorption capacities, thereby suppressing plasticization. Specifically, less plasticization benefit occurs at a higher operating temperature for the sorption-dominated gas separation process, resulting in lower H2S and CO2 permeance at 50 oC vs. 35 oC at equivalent feed composition. In real applications, the operation temperature of the feed streams in natural gas separation process varies from 65 oC (to prevent condensation in membrane modules) to sub-ambient temperature (to capture and remove C2+ hydrocarbon 8
components) [22]. Our work suggests opportunities in system design using glassy polymer membranes to optimize the plasticization benefit for the natural gas sweetening process using temperature to optimize membrane performance for different feed composition. This work is planned for future studies. (b) 180 o H2S CO2 CH4 160 35 C 140 50 oC H2S CO2 CH4 120 100 80 60 10 8 6 4 2 H S/CO2/CH4/C2/C3/toluene (20/5/bal./3/3/100ppm) 0 2 5 10 15 20 25 30 35
Pressure (bar)
35 35 oC o
50 C
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(a)
H2S/CH4
CO2/CH4
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H2S/CO2/CH4/C2/C3/toluene (20/5/bal./3/3/100ppm)
5
10
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Figure 3. Comparison of gas separation performance of the CTA hollow fiber membrane at 35 oC and 50 oC under a gas composition of 20/5/3/3/100 ppm for H2S/CO2/C2H6/C3H8/toluene balanced with CH4. (a) gas permeance and (b) gas selectivity.
We also considered the effect of hydrocarbon contaminant concentration on the CTA hollow fiber membrane performance. Specifically, the concentration of toluene was increased from 100 ppm to 300 ppm while holding the H2S, CO2, C2H6, and C3H8 compositions the same and using CH4 as the balancing gas. In this case, the plasticization benefit is again observed for H2S/CH4 separation at 35 oC (Figure 4), whereas the H2S/CH4 selectivity drops at 50 oC and 31.3 bar (Figure 4b). Comparing with the results in Figure 2c-d with a toluene concentration of 100 ppm at same temperature, the unexpected H2S/CH4 selectivity drop at 50 oC reflects the increased concentration of toluene (300 ppm). As the absolute amount of the large toluene 9
molecules (kinetic diameter = 5.8 Å) is further increased in the feed stream at high pressure (31.3 bar), larger transient gaps enable diffusion of the smaller H2S, CO2 and CH4 molecules, which subsequently reduces the H2S/CH4 selectivity at high feed pressure. (b)
180 o H2S CO2 CH4 160 35 C o 50 C H S CO CH4 140 2 2 120 100 80 60 10 8 6 4 2 H S/CO2/CH4/C2/C3/toluene (20/5/bal./3/3/300ppm) 0 2 5 10 15 20 25 30 35
35 30
Selectivity
Gas permeance (GPU)
(a)
35 oC
H2S/CH4
CO2/CH4
50 oC
H2S/CH4
CO2/CH4
25 20 15 H2S/CO2/CH4/C2/C3/toluene (20/5/bal./3/3/300ppm)
10 5
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15
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Figure 4. H2S/CH4 and CO2/CH4 separation performance of the cellulose triacetate hollow fiber membrane under a gas composition of 20/5/3/3/300 ppm for H2S/CO2/C2H6/C3H8/toluene balanced with CH4. (a) gas permeance and (b) gas selectivity at 35 oC; (c) gas permeance and (d) gas selectivity at 50 oC.
Increased toluene concentration also affects gas separation performance at 35 oC and low pressure; however, this effect is somewhat offset by the plasticization benefit at high pressure, as shown in Figure 5. At low pressure, the highly condensable toluene with large kinetic size can compete with H2S and CO2 by sorption and can also hinder their diffusion; therefore a higher toluene concentration results in decreased H2S/CH4 and CO2/CH4 selectivities (300 ppm vs. 100 ppm). Nevertheless, plasticization of the polymer at high pressure creates new free volume (sorption-favored) and passage (diffusion-favored) for all gas molecules and thus competition among H2S, CO2 and toluene is offset by the new “opportunities” for permeation. Such an 10
interesting phenomenon again demonstrates tuning performance for CTA hollow fiber membranes using temperature and feed pressure to rationally optimize membrane performance for this important application. A wide ranges of options exist, including optimizing the downstream as well as upstream operating conditions to tune the transmembrane plasticization profile; however, this topic is beyond the scope of this study and will be considered in detail in a subsequent paper. Clearly, a limit will eventually be encountered at extremely high pressure; however, the CTA hollow fiber membranes will be extremely well-suited for many applications. (b) 180 100 ppm 160 140 300 ppm 120 100 80 60 10 8 6 4 2 0 5 10
H 2S
CO2
CH4
H 2S
CO2
CH4
35 30
Selectivity
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100 ppm
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35
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Figure 5. Comparison of gas separation performance of the CTA hollow fiber membrane at 35 oC under different gas compositions of 20/5/3/3/100 ppm and 20/5/3/3/300 ppm for H2S/CO2/C2H6/C3H8/toluene balanced with CH4. (a) gas permeance and (b) gas selectivity.
Conclusions In conclusion, we have demonstrated the plasticization benefit of a commercial asymmetric CTA hollow fiber membrane on natural gas sweetening under aggressive mixed gas conditions: high H2S, low CO2 and significant amounts of heavy hydrocarbons (C2, C3 and Toluene). The H2S and CO2 separation efficiency of the membrane is promising to achieve the simultaneous 11
H2S+CO2 removal target with attractive productivity. In applications, the temperature effect can be further considered as an economical tuning tool to engineer the plasticization benefit and optimize separation performance. The CTA hollow fiber membrane shows excellent tolerance to the toluene concentration in the mixed gas, demonstrating the feasibility of the membrane in real applications with significant aromatic contents.
Acknowledgments The authors acknowledge the support by Chevron Energy Technology Company (1906BXE)
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Highlights •
Simultaneous removal of H2S and CO2 from natural gas using a CTA hollow fiber membrane
•
The CTA hollow fiber membrane illustrates the plasticization benefits
•
The CTA hollow fiber membrane shows excellent tolerance to various hydrocarbon concentrations in realistic natural gas sweetening conditions
1
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: