Separation and Purification Technology 222 (2019) 152–161
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Preparation and characterization of multilayer thin-film composite hollow fiber membranes for helium extraction form its mixtures
T
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Seung-Hak Choi , Melhan M. Ben Sultan, Abdulrahman A. Alsuwailem, Sattam M. Zuabi Research & Development Center, P.O. Box 62, Saudi Aramco, Dhahran 31311, Saudi Arabia
A R T I C LE I N FO
A B S T R A C T
Keywords: Composite hollow fiber Helium enrichment He/CO2 separation Helium separation
The unique and distinctive physico-chemical properties of helium make it an irreplaceable resource in a wide range of applications, including cryogenics. Commercially-produced helium is extracted from natural gas. For the use of a membrane to be viable for the helium extraction and enrichment process, the membrane-based process must be economically competitive against other conventional technologies, and therefore, developments in membrane and optimized process design are crucial. This work demonstrates the successful fabrication of a highly helium-selective multilayer thin-film composite hollow fiber membrane for the enrichment of helium from natural gas. An aromatic polyamide selective layer was formed on top of a mesoporous polyacrylonitrile support by interfacial polymerization, followed by a caulking process to plug defects on the polyamide layer. The permeation and separation properties of the multilayer thin-film composite hollow fiber membrane were characterized using pure and quaternary mixed gases. The effects of feed pressure (up to 300 psig) and stage-cut (1.5–20%) on helium purity and CH4 loss were systematically investigated.
1. Introduction The unique characteristics of helium make it a valuable commodity with numerous applications. Currently, the extraction of helium from natural gas during its processing is the only cost-effective method of helium production. The composition of natural gas, and more specifically, the presence or concentration of helium in natural gas, varies depending on the type, depth, and location of the gas well and the geology of the area. Feed streams containing more than 0.3 percent helium are economically feasible for helium extraction in the United States, although the economics of helium extraction often depend on the other products in a natural gas stream [1]. The conventional process used to produce liquid helium from natural gas has been well explained elsewhere [2–6]. In brief, this process consists of four steps. The first step is the extraction (or recovery) of crude helium from natural gas (up to 80 vol% helium with impurities). The second step is upgrading the crude helium to 90 vol% helium. The third step is purification of the helium to 99.99 vol% or higher purity. Then, the final step is the liquefaction of the helium. Most helium production processes are integrated into liquid natural gas (LNG) processing, and helium extraction starts after LNG production. In order to produce LNG, which is the final product of natural gas processing, that can be easily and safely stored and transported, helium-containing natural gas is sequentially
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processed to remove impurities such as CO2, H2S, heavy hydrocarbons, water, and mercury before liquefying the natural gas. Therefore, the major components of the feed gas in the helium extraction step are CH4 and N2, with 1–3 vol% of helium. After cryogenic distillation, in which most of CH4 is removed from the stream, the helium concentration is increased to 80 vol%, with the balance being N2 and other minor components, e.g., Ar, Ne, and H2. The subsequent upgrading and purification steps are a multi-stage cryogenic process, which is expensive and energy-intensive [2,6]. Then, pressure swing adsorption (PSA) is used to purify the upgraded helium to the level required for liquefaction and/or sale. The economics and technical feasibility of membrane processes as a potential alternative to energy-intensive cryogenic distillation for helium extraction and upgrading have been widely studied [2–5,7]. For instance, Schole et al. simulated a membrane process for the recovery and upgrading of helium from natural gas and set the helium selectivity target in various operating conditions [5]. The same author also reported the economics of helium recovery and upgrading using a multistage membrane process [3]. However, it should be noted that these previous studies focused on the separation of helium from N2 and CH4, and that the influence of other components such as CO2 on the helium extraction rate and purity was not investigated, since the major components after the N2 rejection unit (NRU) are CH4 and N2.
Corresponding author. E-mail address:
[email protected] (S.-H. Choi).
https://doi.org/10.1016/j.seppur.2019.04.036 Received 7 March 2019; Received in revised form 11 April 2019; Accepted 11 April 2019 Available online 12 April 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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must be well-integrated with each other to maximize the permeation and separation performances. The fabrication of multilayer TFC hollow fiber membranes begins with the development of the mesoporous support, which provides mechanical stability to the resultant membrane. Pinnau et al. reported that a highly permeable porous support is needed for the composite membrane in order to obtain high-flux composite membranes [9]. Furthermore, a uniform pore size and a narrow pore size distribution are preferred for the support in order to form a uniform coating layer. The chemical compatibility between the support material and the solvent used for dissolving the coating material is also a critical factor to be considered when selecting the support material. The formation of the aromatic polyamide thin layer on top of the porous support via interfacial polymerization is the next step in the production of multilayer TFC hollow fiber membranes. In our previous work, commercially available seawater desalination RO membranes were used to fabricate composite membranes, and therefore, the formation of a similar aromatic polyamide layer on the porous support is essential to reproduce the results we reported using the flat sheet membranes. Finally, the polyamide-coated TFC hollow fiber membrane undergoes a dip coating process to plug the defects with caulking material. Depending on the coating material, the resulting gas transport properties can vary significantly [8,10]. TFC membranes for desalination, which were a precursor of multilayer TFC membrane, are produced using interfacial polymerization to form a thin selective layer on a porous support, and were first introduced by Scala et al. and later optimized by Cadotte [11,12]. Interfacial polymerization occurs at the interface between two immiscible solutions, each of which contains different reactants. For instance, the two immiscible solutions can be water containing 1,3-phenylenediamine (m-PDA) and n-hexane containing trimesoyl chloride (TMC). Upon contact of the two immiscible solutions on the outer surface of the porous support, m-PDA and TMC polymerize only at the interface. As the polymerization reaction proceeds, the interfacial film becomes a barrier that slows further reaction; hence, interfacial polymer films are generally ultrathin (< 500 nm in thickness). During the interfacial polymerization, various parameters such as the concentration of the reactants, the presence and types of additives, the reaction temperature and time, and the curing temperature and time can significantly
When a natural gas process plant does not have a helium recovery process, or natural gas is processed for power generation rather than LNG production, after minimal treatment of the raw natural gas (e.g., to remove H2S, some CO2, and water), the helium-containing natural gas is fed into a combustion chamber to generate power. In this case, the unrecovered helium will be vented to the atmosphere. Depending on the composition of raw natural gas, especially the H2S and CO2 content, and the design factor of the sulfur recovery unit (SRU), the degree of CO2 removal in amine-based acid gas removal processes operated in various manners is primarily focused on the removal of H2S. Furthermore, low British Thermal Unit (BTU) natural gas has become a more attractive source of power generation due to the increasing population and continuous economic growth, which have led to greatly increased energy demand. To extract and upgrade helium from natural gas in the presence of CO2, the development of a new membrane with higher selectivity for helium over not only N2 and CH4, but also CO2, and the evaluation of its performance are essential to explore their feasibility in helium production. Recently, our research group demonstrated the technical feasibility of a novel approach involving the fabrication of a multilayer thin-film composite (TFC) using commercially available polyamide reverse osmosis (RO) membranes for recovering and enriching helium from low BTU natural gas [8]. In brief, RO membranes, which are defective in the dry state, were coated with different polymeric materials; the resulting membranes showed superior selectivity for helium over other gases, including CO2. The gas permeance was found to be relatively low; thus, in order to achieve a more economically feasible separation process, the development of multilayer TFC membranes with a hollow fiber geometry to maximize the packing density was proposed. In this study, the fabrication of such multilayer TFC hollow fiber membranes and their characterization using pure and simulated natural gas streams containing helium were demonstrated.
2. Methodologies and strategies As shown in Fig. 1, in the preparation of a multilayer TFC membrane, each individual layer (the support layer, polyamide-coated selective layer, and caulking layer) must be optimized, and the layers
Fig. 1. Factors to be considered during the multilayer TFC hollow fiber membrane development process. 153
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Subsequently, the fiber was collected on a take-up bobbin, which was partially immersed in a water bath. The water in the take-up bath was continuously circulated and replaced with fresh water to allow complete removal of the residual solvent.
influence the physical and chemical properties of the polyamide layer. These factors can be optimized to obtain membranes with improved performance [13–19]. As discussed above, the first step in fabricating multilayer TFC hollow fiber membranes involves a dry-wet diffusion (or non-solvent) induced phase separation (DIPS or NIPS) process to produce the mesoporous support. Then, interfacial polymerization is performed to form a polyamide layer on the porous support. Finally, the caulking process is conducted by dip coating the polyamide-coated hollow fiber membrane to plug defects in the polyamide layer. In this report, the influences of the operating parameters on the permeation and separation properties of the resultant membrane were systematically investigated using a quaternary gas mixture.
3.2.2. Continuous fabrication of the thin-film composite hollow fiber membrane and caulking The aromatic polyamide layer was formed on top of the mesoporous PAN hollow fiber support via interfacial polymerization between an aqueous amine (i.e., m-PDA) solution and TMC dissolved in an n-hexane solution. A schematic of the continuous coating apparatus and a photo of the system are presented in Fig. 3. The wetted PAN hollow fiber membrane was first immersed in a known concentration of the aqueous amine solution (Solution bath 1 in Fig. 3) for a certain time to saturate the support with the amine monomer on the outer surface of the hollow fiber membrane. The excess aqueous amine solution was removed by using an air nozzle with adjustable pressure. Then, the PAN hollow fiber membrane was guided into the organic solution bath (Solution bath 2 in Fig. 3) containing TMC for a given time to carry out the interfacial polymerization. Finally, the resultant polyamide-coated TFC hollow fiber membrane was washed in methanol and then dried at room temperature. The final multilayer TFC hollow fiber membrane was formed by dip coating the polyamide-coated TFC with a PTMSP solution using the same system used for interfacial polymerization (Fig. 3). During the caulking process, the concentration of the PTMSP solution, drying temperature, and coating speed were carefully controlled to achieve complete plugging of the defects with minimum thickness (approximately 1 μm).
3. Experimental 3.1. Materials and reagents Polyacrylonitrile (PAN, Mw = 230,000 g/mol) was purchased as a fine powder from GoodFellow, UK and used for the fabrication of the mesoporous hollow fiber substrate. Reagent grade dimethyl sulfoxide (DMSO, ≥99.9%, Fisher Scientific) was selected as the solvent, and poly(ethylene glycol) (PEG600, ACROS Organics) and propylene carbonate (anhydrous PPC, 99.7%, Sigma-Aldrich) were used as additives to prepare the polymer spinning solution. Tap water was used as the external coagulant, and a mixture of tetraethylene glycol (TEG, 99%, Alfa Aesar) and DMSO was used as the bore fluid during the spinning. 1,3,5-Benzenetricarbonyl trichloride (TMC, > 98%, TCI Co.) and mphenylenediamine (m-PDA, 99%, Sigma-Aldrich) were dissolved in nhexane and distilled water, respectively, and used to form the aromatic polyamide layer on top of the PAN support by interfacial polymerization. A 0.5–2.0 wt% solution of poly[1-(trimethylsilyl)-1-propyne] (PTMSP, Gelest Inc.) in n-hexane or cyclohexane was used to seal the defects on the polyamide TFC hollow fibers. A two-component epoxy resin (Stycast 1266, Emerson & Cuming, Belgium) was used for potting during the membrane module preparation. The permeation properties of the multilayer TFC hollow fiber membranes were characterized using a constant pressure system with pure He, N2, CH4, and CO2. The sample with the best performance was then selected for further investigation using a membrane module with a larger membrane area. The gas separation properties were analyzed using a constant pressure system with a mixed gas containing 0.13 vol% He, 4.4 vol% CO2, 28.1 vol% N2, and 67.34 vol% CH4.
3.3. Membrane characterization The morphologies of the mesoporous PAN support, polyamidecoated thin-film composite, and PTMSP-coated multilayer hollow fiber membrane were characterized by scanning electron microscopy (SEM, FEI QANTA 400F E-SEM). The membranes were freeze-fractured in liquid nitrogen and observed at 20 kV after gold sputtering. The permeation of the hollow fiber membrane in each stage (i.e., as the support and polyamide TFC before and after caulking) was characterized after making test modules. As shown in Fig. 4, to select the best fiber samples, the hollow fiber modules were prepared by potting 30 randomly selected fibers in a bundle with an effective fiber length of 25–30 cm at one side in a short stainless-steel tube. At the other side, the loose open end of the fibers was plugged with a drop of epoxy resin. The prepared module was then fitted into a stainless-steel tube connected bore through a reducing union tee (Swagelok, SS-810-3-4-4BT, photo in Fig. 4(b)). After the screening and selection of the best fibers, a relatively large test module with an effective membrane area of 90.5 cm2 was fabricated for further investigation in the mixed gas test. Prior to mixed gas testing, the pure gas permeation properties of each module were evaluated and screened via ideal selectivity and permeance tests using pure He, N2, CH4, and CO2 gases at different feed pressures using a constant pressure system (shown in Fig. 5). Only some of the prepared modules were selected for characterization of their permeation and separation properties using the mixed gas. The effects of the pressure ratio and stage-cut on the helium purity and recovery in the permeate, methane loss, and the permeance of each gas were systematically evaluated. The flow rate of the permeate was measured using mass flow meters with different ranges (Alicat Scientific, USA). For the pure gas permeation experiments, at least three modules were tested. Unless stated otherwise, average results are reported with their standard deviation. In general, the permeability can be calculated using Eq. (1) by measuring the volumetric permeate flow rate (Qi , cm3/s) of gas component i, effective membrane thickness (l , cm), effective membrane area (A, cm2), and trans-membrane partial pressure difference (Δpi ,
3.2. Membrane preparation 3.2.1. Preparation of the mesoporous PAN hollow fiber support A PAN polymer solution was prepared by mixing the polymer powder with the mixture of the solvent and the additives in a glass flask under mechanical stirring for 1 day at 65 °C to obtain a homogeneous solution. The dope was then transferred into the dope tank after filtration and kept at a constant temperature of 65 °C for 24 h to allow air bubbles to escape from the solution. Hollow fibers were spun using the dry-wet phase inversion process with the spinning apparatus illustrated in Fig. 2. The dope solution and the bore fluid were co-extruded to the spinneret by the precision gear pumps. The dope solution and the spinneret were kept at 65 °C. The nascent hollow fiber was passed through the desired air-gap distance, in which the fiber was exposed to the ambient temperature (27 ± 1 °C) and relative humidity (23 ± 2%) before entering the coagulation bath at room temperature. The extrusion rate of the dope solution and bore fluid were carefully controlled to obtain the desired fiber dimensions. The take-up speed of the fibers was 25 m/min. The fibers were coagulated in a bath with continuous circulation of water to avoid local buildup of the solvent concentration. The nascent fiber was pulled out of the coagulation bath by take-up rolls rotating at an adjustable speed. 154
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Fig. 2. Schematic of the hollow fiber spinning apparatus. (1: N2 gas cylinder, 2: dope tank, 3: inner coagulant reservoir, 4: liquid pump, 5: precision gear pump, 6: spinneret, 7: coagulation bath, 8: washing bath, 9: tension controller, 10: take-up bath, 11: heater, 12: circulation pump).
Fig. 3. Photo and schematic of the hollow fiber coating apparatus.
permeability ratio of certain gas pairs, while the separation factor αi∗/ j is often used as a measure of efficiency for mixed gas permeation. This is conventionally given by Eq. (5).
cmHg). In the permeability calculation, the effective membrane thickness is critical. For asymmetric porous support and composite membranes, the effective membrane thickness cannot be directly measured. Therefore, the permeance (P', in GPU, defined in Eq. (3)) was used to evaluate the permeation property instead of the permeability.
Pi =
l Q × i Δpi A
1 Barrer =
P '=
(1)
10−10
× cm cm2 × s × cmHg
10−6
(2) (3)
cm3 (STP )
cm2 × sec × cmHg
pi / pj fi / f j
(5)
where pi and pj are the mole fractions of components i and j , respectively, on the permeate side, while fi and f j are the mole fractions of components i and j , respectively, on the feed side of the membrane. A quaternary gas mixture containing He, CO2, N2, and CH4 was used to characterize the separation properties of the multilayer TFC hollow fiber membranes. The permeance of each gas was calculated using the trans-membrane partial pressure difference and the composition of the permeate and retentate streams. Detailed calculation methods for the permeance and separation factor can be found elsewhere [8]. The effects of the feed pressure and stage-cut on the permeate helium purity, recovery, and separation factor were systematically analyzed. All the mixed gas experiments were carried out at room temperature (25 ± 1 °C) using different feed pressures of up to 300 psig and maintaining the permeate pressure at 1/3 psig. The retentate flow rate was carefully controlled to vary the stage-cut within the range of 1% to
cm3 (STP )
Q Δp × A
1 GPU =
αi∗/ j =
(4)
The performance of membranes can be characterized in terms of productivity and efficiency. The productivity can be expressed by the permeability (in Barrer; defined in Eq. (2)) or permeance (in GPU; defined in Eq. (4)). In addition to the productivity, the efficiency of the process can be evaluated by the ideal selectivity or separation factor. For the pure gas permeation test, the ideal selectivity is defined as the 155
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Fig. 4. Prepared (a) multilayer TFC hollow fiber membrane bundles and modules with (b) a module test tube.
20%. The gas composition of the permeate stream was continuously analyzed by micro GC (INFICON 3000 with a thermal conductivity detector) until a steady state was reached. Based on GC analysis and monitoring of changes in the flow rate of the permeate, 18–20 h were required to reach steady state at each operating condition. The retentate and feed gas composition were analyzed at the end of each run. The stage-cut and helium recovery were calculated using Eqs. (6) and (7).
Stage − cut(%) =
QPerm × 100 QFeed
Recovery(%) =
QPerm × [He]Perm × 100 QFeed × [He]Feed
(7)
where QPerm and QFeed are the permeate flow rate and feed flow rate, respectively, and [He]Feed and [He]Perm are the helium concentration at the feed and permeate side of the membrane. The permeation and separation properties using the mixed gas were measured using a single module.
(6)
Fig. 5. Process flow diagram for the pure and mixed gas permeation test set-up. 156
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Fig. 6. SEM images of the (a) cross-section, (b) inner surface, (c) outer surface of the prepared PAN support and (d) cross-section of the PTMSP coated multilayer TFC hollow fiber membranes.
coating). As can be seen in Fig. 6(c), the PAN hollow fiber prepared in this study had 10–30 nm pores on its surface, and its porosity was 8 to 10%. The porosity was estimated using a software routine (ImageJ ver. 1.51j8, Wayne Rasband, National Institute of Health, USA) based on the SEM image. In addition, the cross-sectional image of PTMSP caulked multilayer thin-film composite hollow fiber membrane was shown in Fig. 6(d). The overall coating thickness including polyamide and PTMSP caulking layer was approximately 0.5–0.6 μm. The surface morphologies of the polyamide thin-film composite RO membranes produced in this study are shown in Fig. 7(a) and (b). In general, the polyamide TFC membrane showed a characteristic ridge-and-valley structure. It is well known that depending on the interfacial polymerization conditions, the surface roughness can vary. In this study, the monomer concentration in each phase was controlled to tune the surface properties (i.e., roughness). Finally, the caulked multilayer thin-film composite hollow fiber membrane was produced by dip coating; SEM images are shown in Fig. 7(c) and (d). The rough polyamide layer was covered with the caulking material. By controlling the concentration of the caulking material, the caulking thickness could be adjusted. For instance, the rough surface could be totally covered, making the surface smooth (Fig. 7(c)), by coating the surface using a higher concentration of caulking solution (1.5 wt%), while peaks of the polyamide layer were still seen when a thinner caulking layer was formed using a diluted polymer coating solution (0.7 wt%).
4. Results and discussion 4.1. Structural analysis using SEM The SEM images of the produced hollow fiber support and thin-film composite membranes before and after caulking are shown in Figs. 6 and 7, respectively. Several spinning parameters, such as the composition of the polymer solution, including the polymer concentration, type of solvent, and the presence of additive(s) in the dope, and the composition of bore fluid and external coagulant, all strongly affect the membrane morphology and permeation properties [20–22]. Among these parameters, the bore fluid composition, air-gap length, and takeup speed can be easily changed during the spinning process, while the composition and the temperature of the dope and external coagulant are predetermined before the experiment. The extruded dope is exposed to ambient conditions when it passes through the air-gap; during this time the solvent(s) in the dope solution evaporates into the air, and the water vapor (humidity) in the air diffuses into the nascent hollow fiber membranes. Finally, complete precipitation occurs when the nascent fiber is immersed in the coagulation bath. Therefore, the temperature and humidity of the spinning room need to be controlled precisely. In a prior study, our research group evaluated the influence of individual spinning parameters on the membrane structure and permeation property to produce a hollow fiber support capable of withstanding the pressure normally used in natural gas processing, and established optimum spinning conditions, which are beyond the scope of this work. Fig. 6 shows the SEM images of the hollow fibers prepared using typical spinning conditions. As reported by Tham et al., a hollow fiber membrane with a large outer diameter (OD) to inner diameter (ID) ratio (OD/ID = 2.5–3.0) is also required to ensure mechanical stability [23]. As shown in Fig. 6(a), a hollow fiber membrane with a macrovoidfree sponge-like structure, an OD/ID ratio of approximately 2.5, and an outer diameter of 320 μm was successfully fabricated. The inner surface, Fig. 6(b), of the obtained fiber presented a highly porous open structure, which will minimize the permeation resistance of gases. The outer surface pore characteristics are another critically important issue, especially for further processing of the composite membrane (i.e.,
4.2. Permeation properties of the composite hollow fibers As discussed earlier, it is widely known that the properties of the support, including the pore size, pore size distribution, and permeation property have a significant influence on the permeation and separation properties of the composite membrane [9,10,24]. In addition, the support layer must provide mechanical stability while minimizing mass transfer resistance. The pure nitrogen permeance of the mesoporous PAN hollow fiber support prepared in this study was 6000–7000 GPU. The pure gas permeance of the fabricated polyamide TFC hollow fiber membrane was compared to that of a commercial RO membrane, 157
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(a) after IP: rough polyamide surface
(b) after IP: relatively smooth polyamide surface
(c) Caulked multilayer TFC: 1.5 wt%
(d) Caulked multilayer TFC: 0.7 wt%
Fig. 7. SEM images of prepared polyamide TFC and multilayer TFC hollow fiber membranes.
polyamide TFC hollow fiber (symbol: empty circle) membrane showed approximately 40% lower permeance. The ideal selectivity for helium over CO2 through the TFC hollow fiber was in the range of 4.2 and 4.3, which was slightly higher than the Knudsen diffusion selectivity (He/ CO2 = 3.32). This indicated the effect of the solution-diffusion based gas transport through the membrane. The pure gas permeation property of the PTMSP caulked multilayer TFC hollow fiber was investigated as a function of feed pressure; the results are summarized in Table 1. The helium permeance increased slightly from 5.96 to 7.79 GPU when the feed pressure was varied from 50 psig to 300 psig, while the CO2 permeance increased from 0.16 to 0.26 GPU. The ideal selectivities of He/CH4 and He/CO2 were in the range of 36–40 and 29–38, respectively. As reported in our previous work, the selectivity for helium over CO2 is much higher than any reported values [8].
Table 1 Pure gas permeation properties of multilayer thin-film composite hollow fiber membranes. Pressure (psig)
Fig. 8. Pure gas permeation properties of the commercial polyamide flat sheet membranes and fabricated TFC hollow fiber membranes.
50 100 150 200 250 300
and is summarized in Fig. 8. As shown in the figure, the pure helium permeance (symbol: solid circle) of the commercially available flat sheet RO membrane was in the range of 150–200 GPU, while the 158
Permeance, GPU
Ideal selectivity
He
N2
CH4
CO2
He/CH4
He/CO2
5.96 6.95 7.37 7.65 7.77 7.79
0.06 0.06 0.07 0.09 0.11 0.11
0.16 0.17 0.16 0.17 0.19 0.19
0.16 0.18 0.22 0.23 0.27 0.28
36.39 41.53 46.33 43.81 40.72 40.13
38.05 38.27 34.11 32.77 29.33 28.31
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the pressure increases, the driving force for mass transfer increases due to the higher trans-membrane pressure difference. Thus, the permeation of less permeable gases, such as N2, CH4, and CO2 through the membrane also increases up to a certain point. At a fixed stage-cut of 10%, the partial pressure difference of helium can be maximized and the residence time effectively used for selective helium permeation. Further increasing the pressure increases the partial pressure of the other gases, and longer residence time allows the less-permeable gases to pass through the membrane. Similarly, due to the clear correlation between the pressure ratio and membrane selectivity, one of these parameters will limit the membrane performance; these behaviors have been well established [26,27]. Due to the higher selectivity for helium over other gases, the helium concentration dropped quickly after the feed gas came into contact with the membrane, and then the helium concentration in the retentate became too low. These trends became more severe at the higher stage-cut range; more details will be discussed in a later section. The permeation and separation behavior of a mixed gas are known to be much different than those of pure gases [28–31]. Depending on the membrane material (i.e., whether it is glassy or rubbery) and the gas system (i.e., the gas pair), different transportation behaviors are observed. For instance, Haraya et al. studied the pure and mixed gas permeation and separation properties through an asymmetric polyimide hollow fiber membrane using the gas pairs CO2-CH4, O2-N2, and H2-CO. They found that the permeation of mixtures of CO2-CH4 and O2N2 appears to follow the constant permeability model, while that of H2CO mixtures does not. The permeability of H2 was depressed considerably due to the presence of CO; a prediction using the dual-mode transport model showed good agreement. Other researchers have also reported the effects of a second component in the feed stream on the permeation and separation properties using different materials. It should be noted that most of the reported studies have focused on binary mixtures. Therefore, the gas transport phenomena in quaternary mixtures may be different from those of pure and binary mixture systems. For better understanding of the current results, further investigation is needed, such as a sorption experiment, which is currently in progress. The effect of the feed pressure on the helium recovery and CH4 loss is summarized in Fig. 10. Since CH4 is the valuable main product, while helium is a by-product of natural gas processing, CH4 loss must be monitored and minimized. At the same time, the helium recovery should be high enough to make the process economical. At a constant stage-cut of 10% and 50 psig, less than 25% of the He was recovered,
(a) permeance
(b) helium concentration and separation factor Fig. 9. Permeation and separation properties of the multilayer TFC hollow fibers.
4.3. Separation properties of the multilayer TFC hollow fibers 4.3.1. Effect of the feed pressure Fig. 9 depicts the effect of the feed pressure on the gas permeance, helium concentration on the permeate side, and separation factor. When the feed pressure of the gas mixture was increased, the permeance of all gases increased. When the feed pressure was increased from 50 psig to 300 psig, the calculated helium permeance increased from 3.4 GPU to 7.8 GPU, while the CO2 permeance increased from 0.42 GPU to 0.58 GPU. Additionally, the helium purity continuously increased to reach 0.93 vol% at 300 psig of feed pressure. It should be noted that the stage-cut was maintained at 10% while the feed pressure was increased. The He/CO2 separation factor increased from 2.27 to 7.1 when the feed pressure was increased from 50 psig to 300 psig. As Hosseini et al. discussed, at low pressures, the competitive sorption of components is dominant, which results in reduced permeance [25]. As Fig. 10. Effect of the feed pressure on the helium recovery and methane loss. 159
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Fig. 12. Effect of the stage-cut on the helium recovery and methane loss.
(a) permeance
a constant pressure of 300 psig, and are summarized in Figs. 11 and 12. To design and evaluate the actual helium extraction and enrichment process, required membrane area, the number of membrane stages and target purity and recovery rate of helium must be optimized. In general, it is well known that the membrane-based separation process is suitable for bulk removal/recovery and to produce liquid helium, eventually, further purification process (i.e. pressure swing adsorption; PSA) is needed. As discussed above, the stage-cut can be controlled by changing either the (a) membrane area, (b) the flowrate of the feed or retentate, or (c) the pressure ratio. A higher stage-cut results in the passage of a larger amount of gas through the membrane along the extended membrane area. When the membrane area is increased, the retentate stream is gradually depleted of the more permeable component (i.e., helium) and enriched with the less permeable components, such as N2, CH4 and CO2. In other words, a higher stage-cut provides a longer residence time for all the gases in the feed stream, especially for the lesspermeable gases. As a result, the permeate side becomes diluted with the less permeable component, and the purity of the more permeable component at the permeate side decreases. Clearly, there is a trade-off between the product recovery and purity [25–27]. In the current study, at a fixed pressure, the stage-cut decreased as the retentate flow rate was increased due to the increasing feed flow rate on the fixed membrane area. Fig. 11(a) shows the gas permeance measured at various stage-cuts (i.e., 1.5–20%) and a constant feed pressure of 300 psig. The helium permeance dropped from 46 to 4.2 GPU when the stage-cut was increased 1.5–20%, while the permeance of all the other gases remained constant. The significant drop in helium permeance was due to the significant depletion of the helium concentration in the retentate. For instance, increasing the stage-cut from 1.5 to 20% caused the retentate helium concentration drop to 0.03 vol% from 0.12 vol% (Fig. 11(b)). The logarithmic mean value of the feed concentration was used to calculate the gas permeance, and due to this significant drop in the helium concentration, a decreased helium permeance was found. As a result of the low driving force, as shown in Fig. 11(b), the separation factor also decreased from 11.9 to 5.62. It is well known that when the stage-cut is decreased at a fixed pressure, the recovery (refer to Fig. 12) and the purity of the most permeable component show a trade-off: at high recovery, the purity is low, and vice-versa. As the stage-cut was increased from 1.5 to 20%, the retentate helium concentration dropped from 2.2 vol% to 0.53 vol%, while the helium recovery increased from 25.7% to 79.4%. Due to the high stage-cut, the less permeable gases, including CH4, began to
(b) helium concentration and separation factor Fig. 11. Mixed gas permeation and separation properties of the multilayer TFC hollow fiber membrane.
but this value increased to more than 70% at 300 psig. Similar results for CO2 removal from flue gas using a membrane process were reported and discussed by Brunetti et al. [27]. 4.3.2. Effect of stage-cut During the evaluation of the membrane performances, several parameters could be varied, such as the pressure ratio (i.e., the feed and/or permeate pressure), temperature, composition, and stage-cut (i.e. feed and/or retentate flow rate) while maintaining same membrane area. These factors directly influence the purity and recovery of the desired product. However, in practical cases, the feed composition, temperature, and feed flow rate are kept constant, while the pressure ratio and stage-cut can be easily controlled to meet target values. The pressure ratio, of course, can be controlled by the changing permeate pressure. To change the stage-cut, rather than the changing retentate or feed flow rate as is typically done in lab-scale experiments, the membrane area can be modulated during practical industrial operation. The effect of the stage-cut on the gas permeance, separation factor, helium purity, and helium recovery were systematically investigated at 160
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penetrate the membrane, and the CH4 loss increased from 1.5% to 20.2% as the stage-cut was increased from 1.5% to 20%.
for helium enrichment, J. Membr. Sci. 553 (2018) 180–188. [9] I. Pinnau, J.G. Wijmans, I. Blume, T. Kuroda, K.V. Peinemann, Gas permeation through composite membranes, J. Membr. Sci. 37 (1988) 81–88. [10] J.M.S. Henis, M.K. Tripodi, Composite hollow fiber membranes for gas separation: the resistance model approach, J. Membr. Sci. 8 (1981) 233–246. [11] L.C. Scala, D.F. Ciliberti, D. Berg, Interface Condensation Desalination Membranes, Westinghouse Electric Corporation, United States, 1973. [12] J.E. Cadotte, Reverse Osmosis Membrane, Midwest Research Institute, United States, 1981. [13] G.S. Lai, W.J. Lau, P.S. Goh, Y.H. Tan, B.C. Ng, A.F. Ismail, A novel interfacial polymerization approach towards synthesis of graphene oxide-incorporated thin film nanocomposite membrane with improved surface properties, Arab. J. Chem. (2017). [14] J. Jegal, S.G. Min, K.H. Lee, Factors affecting the interfacial polymerization of polyamide active layers for the formation of polyamide composite membranes, J. Appl. Polym. Sci. 86 (2002) 2781–2787. [15] B.-H. Jeong, E.M.V. Hoek, Y. Yan, A. Subramani, X. Huang, G. Hurwitz, A.K. Ghosh, A. Jawor, Interfacial polymerization of thin film nanocomposites: a new concept for reverse osmosis membranes, J. Membr. Sci. 294 (2007) 1–7. [16] K.P. Lee, T.C. Arnot, D. Mattia, A review of reverse osmosis membrane materials for desalination—development to date and future potential, J. Membr. Sci. 370 (2011) 1–22. [17] A. Prakash Rao, N.V. Desai, R. Rangarajan, Interfacially synthesized thin film composite RO membranes for seawater desalination, J. Membr. Sci. 124 (1997) 263–272. [18] B. Khorshidi, T. Thundat, B.A. Fleck, M. Sadrzadeh, A novel approach toward fabrication of high performance thin film composite polyamide membranes, Sci. Rep. – UK 6 (2016) 22069. [19] W.J. Lau, S. Gray, T. Matsuura, D. Emadzadeh, J. Paul Chen, A.F. Ismail, A review on polyamide thin film nanocomposite (TFN) membranes: history, applications, challenges and approaches, Water Res. 80 (2015) 306–324. [20] R.M. Boom, I.M. Wienk, T. van den Boomgaard, C.A. Smolders, Microstructures in phase inversion membranes. Part 2. The role of a polymeric additive, J. Membr. Sci. 73 (1992) 277–292. [21] N. Peng, T.-S. Chung, K.Y. Wang, Macrovoid evolution and critical factors to form macrovoid-free hollow fiber membranes, J. Membr. Sci. 318 (2008) 363–372. [22] N. Vogrin, Č. Stropnik, V. Musil, M. Brumen, The wet phase separation: the effect of cast solution thickness on the appearance of macrovoids in the membrane forming ternary cellulose acetate/acetone/water system, J. Membr. Sci. 207 (2002) 139–141. [23] H.M. Tham, K.Y. Wang, D. Hua, S. Japip, T.-S. Chung, From ultrafiltration to nanofiltration: hydrazine cross-linked polyacrylonitrile hollow fiber membranes for organic solvent nanofiltration, J. Membr. Sci. 542 (2017) 289–299. [24] A.K. Ghosh, E.M.V. Hoek, Impacts of support membrane structure and chemistry on polyamide–polysulfone interfacial composite membranes, J. Membr. Sci. 336 (2009) 140–148. [25] S.S. Hosseini, S. Najari, P.K. Kundu, N.R. Tan, S.M. Roodashti, Simulation and sensitivity analysis of transport in asymmetric hollow fiber membrane permeators for air separation, RSC Adv. 5 (2015) 86359–86370. [26] Y. Huang, T.C. Merkel, R.W. Baker, Pressure ratio and its impact on membrane gas separation processes, J. Membr. Sci. 463 (2014) 33–40. [27] A. Brunetti, E. Drioli, Y.M. Lee, G. Barbieri, Engineering evaluation of CO2 separation by membrane gas separation systems, J. Membr. Sci. 454 (2014) 305–315. [28] R.T. Chern, W.J. Koros, E.S. Sanders, R. Yui, “Second component” effects in sorption and permeation of gases in glassy polymers, J. Membr. Sci. 15 (1983) 157–169. [29] M. Sadrzadeh, M. Amirilargani, K. Shahidi, T. Mohammadi, Pure and mixed gas permeation through a composite polydimethylsiloxane membrane, Polym. Adv. Technol. 22 (2011) 586–597. [30] T.C. Merkel, R.P. Gupta, B.S. Turk, B.D. Freeman, Mixed-gas permeation of syngas components in poly(dimethylsiloxane) and poly(1-trimethylsilyl-1-propyne) at elevated temperatures, J. Membr. Sci. 191 (2001) 85–94. [31] K. Haraya, K. Obata, N. Itoh, Y. Shndo, T. Hakuta, H. Yoshitome, Gas permeation and separation by an asymmetric polyimide hollow fiber membrane, J. Membr. Sci. 41 (1989) 23–35.
5. Conclusions Multilayer thin-film composite hollow fiber membranes have been developed, and their permeation and separation properties were analyzed using pure and quaternary mixed gases. The fabricated multilayer TFC hollow fiber membranes showed a high ideal He/CO2 selectivity in the range of 30–38 in the tested pressure range. The separation factor varied from 2.3 to 11.9, while the helium permeance varied from 3.4 to 46.2 GPU depending on operating parameters (i.e., the stage-cut and feed pressure) in a mixed gas system. A higher feed pressure at a constant stage-cut of 10% leads to higher purity and higher recovery of helium, with values reaching 0.93 vol% and 69%, respectively. The effect of the stage-cut was investigated at a constant feed pressure of 300 psig, and the stage-cut was varied from 1.5% to 20% by increasing or decreasing the retentate flow rate. Under these experimental conditions, the helium purity varied between 2.2 and 0.93 vol%, while increasing the stage-cut improved the helium recovery from 25.7% to 79.4%. It should be noted that increasing the stage-cut also increased the CH4 loss. Therefore, to minimize the loss of CH4 while maximizing helium recovery and purity, operating parameters such as the pressure ratio and stage-cut must be optimized. Acknowledgments The authors would like to thank the management of Saudi Aramco for their support and permission to publish this article. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.04.036. References [1] N.R. Council, The Impact of Selling the Federal Helium Reserve, The National Academies Press, Washington, DC, 2000. [2] T.E. Rufford, K.I. Chan, S.H. Huang, E.F. May, A review of conventional and emerging process technologies for the recovery of helium from natural gas, Adsorpt. Sci. Technol. 32 (2014) 49–72. [3] C.A. Scholes, U.K. Gosh, M.T. Ho, The economics of helium separation and purification by gas separation membranes, Ind. Eng. Chem. Res. 56 (2017) 5014–5020. [4] J. Sunarso, S.S. Hashim, Y.S. Lin, S.M. Liu, Membranes for helium recovery: an overview on the context, materials and future directions, Sep. Purif. Technol. 176 (2017) 335–383. [5] C.A. Scholes, U. Ghosh, Helium separation through polymeric membranes: selectivity targets, J. Membr. Sci. 520 (2016) 221–230. [6] N.R. Council, Selling the Nation's Helium Reserve, The National Academies Press, Washington, DC, 2010. [7] M. Alders, D. Winterhalder, M. Wessling, Helium recovery using membrane processes, Sep. Purif. Technol. 189 (2017) 433–440. [8] S.-H. Choi, M.S. Qahtani, E.A. Qasem, Multilayer thin-film composite membranes
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