Helium Recovery through Inorganic Membranes Incorporated with a

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Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Helium Recovery through Inorganic Membranes Incorporated with a Nitrogen Rejection Unit Colin A. Scholes* Department of Chemical Engineering, The University of Melbourne, Parkville 3010 VIC, Australia ABSTRACT: Helium recovery and purification from a natural gas process is increasingly being investigated globally to address rising market demand, as traditional helium sources become depleted. Here, process simulations of two types of inorganic membranes were undertaken in Aspen HYSYS to investigate the possibility of recovering and purifying helium from the Nitrogen Rejection Unit (NRU) offgas close to the NRU’s operating temperature. The two membranes were a cobalt-silica membrane that has He/N2 selectivity through molecular sieving and a zeolite membrane that has N2/He selectivity at low temperatures, because of surface diffusion. Both membranes were able to achieve the desired He recovery and purification through a three-membrane-stage process, and for a feed of 4% He, the cobalt-silica membrane could achieve the same separation performance through a two-membrane-stage process above 340 K, because of increasing selectivity with temperature. In contrast, the zeolite membrane could not operate above 220 K, because of the loss of the surface diffusion mechanism. The difference in permeance of the two membranes significantly affected the membrane area, with the cobalt-silica membrane requiring three orders of magnitude more area than the zeolite membrane to recover and purify the same amount of helium. However, the zeolite membrane’s selectivity for N2 meant that the vast majority of the NRU offgas passed through the membrane into the permeate streams. Hence, to ensure a high helium recovery, the permeate streams from the second and third membrane stages must be recycled, resulting in permeate gas throughputs that are orders of magnitude higher than the cobalt-silica membrane process. This placed significant recompression duty on the zeolite membrane process, compared to the cobalt-silica process, and, as such, the zeolite membrane’s power duty for helium separation was at least five times greater than that of the cobalt-silica membrane. Hence, there is a tradeoff between the two inorganic membranes for helium recovery and purification, based on required membrane area and power demand. separation applications.4 Only the final helium purification stage to achieve ultrahigh purity is poorly suited for membrane separation. An issue of this process design is the difference in operating temperature of the NRU and polymeric membranes. NRU are cryogenic distillation processes operating below the boiling temperature of methane (95%. The NRU exit gas is assumed to be 2.5 MPa and at 120 K (−153 °C), comparable to the operating conditions of standard NRU columns in natural gas processing.2 The membrane

Several inorganic membranes have reported helium permeances, generally as part of a range of gases to characterize the performance of the membrane.5−9 These inorganic membranes are porous and include microporous silica and metal organic frameworks (MOFs),10−13 which can achieve high selectivity, because of the strong pore diameter control leading to excellent molecular sieving. In this investigation, process simulations are undertaken to investigate the potential of inorganic membranes to achieve helium recovery and purification over a range of temperatures, based on the NRU offgas. In particular, two inorganic membranes are studied: a cobalt-silica membrane that has He selectivity over N2 for a wide range of temperatures14,15 and a zeolite-silica membrane that has selectivity for N2 over He at low temperatures, because of surface diffusion16,17 (Figure 1). The selection of these two different selective membranes enables the process options of generating the purified He product through the permeate stream (cobaltsilica) or retentate stream (zeolite-silica) to be investigated.

2. METHODOLOGY All membrane simulations were undertaken in Aspen HYSYS (version 8.8), using the Peng−Robinson fluid package. The membrane simulations used an in-house module specifically designed for gas separation processes, based on mass-transfer equations for cross-flow and counter-flow configurations.18,19 The membrane separation process is broken down into 100 discrete stages and the mass balance is determined iteratively for each stage. For the membrane modules, a cross-flow configuration without sweep gas was used, and a pressure loss of 0.01 kPa was assumed across the retentate stream. Applying the total pressure drop through each membrane stage corresponds to a permeate pressure of 0.12 MPa. All compressors were centrifugal with an adiabatic efficiency of 75%, based on default Aspen parameters and settings. Heat exchanges have an assumed pressure drop of 0.05 kPa. Gas flow through piping was assumed to have negligible temperature and pressure losses. The inorganic membrane was based on the performance of two inorganic materials: a cobalt-silica membrane that had He/ N2 selectivity,14,15 and a zeolite-silica membrane that had N2/ B

DOI: 10.1021/acs.iecr.8b00314 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

sieving of the smaller gas molecule and the lack of defects in the fabricated membrane.14 The membrane displays Arrhenius behavior with temperature, because of the activation energy associated with the molecular sieving mechanism.20 This enables He permeance to be extrapolated to low temperatures, and the cobalt-silica membrane displays selectivity down to the temperature of the NRU offgas. Hence, the molecular sieving process remains active at low temperature, although with a reduced permeance, because of the required activation energy. At the lowest temperature, the He/N2 selectivity remains above that of the Knudsen diffusion selectivity of 1.9. Therefore, the cobalt-silica membrane can be used to separate He from the NRU offgas, with increased efficiency at high temperatures associated with the higher selectivity. 3.1.1. Total Pressure Drop Per Membrane Stage. For the cobalt-silica membrane, separation performance is maximized when each membrane stage experiences the total pressure drop. This is achieved when the permeate stream is repressurized back to the pressure of 2.5 MPa and feeds the next membrane stage. The ability of each membrane stage to concentrate He is demonstrated by the composition of the permeate stream, which is provided in Figure 4, as a function of membrane operating temperature. Figure 2. Schematic of three-membrane-stage processes for the recovery and purification of helium from the NRU exit gas: (a) for a He/N2 selective membrane and (b) for a N2/He selective membrane.

process is incorporated directly downstream of the NRU. The helium composition of the exit gas is variable between 1 mol % and 4 mol %, to simulate different helium compositions in the natural gas.

3. RESULTS AND DISCUSSION 3.1. Cobalt-Silica Membrane (He/N2 Selectivity). The helium permeance of the cobalt-silica membrane is provided in Figure 3, as a function of temperature (K). The membrane has selectivity for He over N2, which increases with temperature. This is attributed to the precise pore size enabling molecular

Figure 4. Helium mole fraction in the permeate stream from the first (solid line) and second (dash line) membrane stages, as a function of membrane operating temperature (K) and the feed gas helium composition.

The permeate from the second-membrane-stage He concentration is higher than the first stage, irrespective of the feed composition, because of the progressive increase in purity through the membrane cascade. The He concentration in the permeate streams increases with temperature, because of the improved selectivity of the cobalt-silica membrane with temperature (Figure 3). For a feed composition of 4% helium, above 340 K, the cobalt-silica membrane selectivity is high enough that a two-membrane-stage process can achieve the necessary recovery and purity. This temperature is above ambient; hence, to take advantage of the cryogenic temperatures associated with the NRU, the cobalt-silica membrane

Figure 3. Helium permeance (× 1010 mol/(m2 s Pa)) and He/N2 selectivity of the cobalt-silica inorganic membrane, as a function of temperature (K);14 those points below 400 K are extrapolated based on an Arrhenius relationship. C

DOI: 10.1021/acs.iecr.8b00314 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

stage, which was focused on He recovery, with the second and third membrane stages used to increase the concentration of helium to the desired purity. The increase in membrane area with decreasing He concentration in the NRU offgas is due to the diluted feed requiring a larger area in the first membrane stage to achieve the same amount of He recovery, because of the reduction in driving force. As such, the operating temperature will have a strong impact on the viability of the cobalt-silica membrane, from an economic perspective. The perspective cost of an inorganic membrane is considerably higher than that of polymeric membranes, ∼$3000 US per m2, relative to $50 US per m2 for a commercial polymeric membrane.21 This will lead to a strong economic incentive to minimize the inorganic membrane’s area. Hence, an economic tradeoff between membrane area cost and energy duty, based on operating temperature, will exist; a study on this tradeoff for polymeric membranes has already been presented in the literature.22 The required power per thousand cubic feet of He produced for the cobalt-silica membrane is provided in Figure 6, based on

requires a three-stage process. Below 140 K, there is a slight increase in the helium concentration in the first permeate stream, irrespective of feed composition. This is due to the membrane process operating at marginal stagecuts around the second and third membrane stages to ensure He recovery, because of the low selectivity of the membrane at these temperatures. The feed helium composition has a large impact on the composition of each permeate stream. The cobalt-silica membrane can only achieve the necessary recovery and purity at the NRU temperature of 120 K when the offgas has 4% helium; for a 2% helium feed, the cobalt-silica membrane can only undertake the separation in a three-stage process above 140 K, because of the poor selectivity at lower temperatures, and, for the 1% helium feed, the offgas needs to be above 170 K. For a feed composition of 2%, the transition from a threemembrane-stage process to a two membrane stage process is at 360 K, because the He/N2 selectivity is high enough to process the diluted feed. For a 1% helium feed, the transition is observed at 460 K. Hence, operating at higher temperatures simplifies the membrane processes by reducing the number of stages, but the tradeoff is the increased heating duty necessary to increase the temperature of the membranes and gas streams. Importantly, the cobalt-silica membrane can achieve helium recovery and purification at cryogenic temperatures and therefore there exists the possibility of operating an inorganic membrane helium recovery process, in combination with the NRU at these low temperatures. Hence, the cobalt-silica membrane has an advantage over polymeric membranes and traditional adsorption processes. The total membrane area required to perform the necessary helium recovery and purification is provided in Figure 5, with

Figure 6. Power demand (MW per thousand cubic feet He produced), as a function of temperature (K) and feed He composition for the cobalt-silica membrane.

He concentration in the NRU offgas; this power does not include the final compressor duty to pressurize He for transport. Including this duty increases the power demand by 0.3 MW per thousand cubic feet of He. A power duty comparison with a conventional cryogenic distillation process is not possible, because such information is not readily available in the literature. At low temperatures, the power demand is significant, because of the poor selectivity of the membrane leading to significant amounts of N2 in the recycle streams, especially the retentate return from the second stage, leading to high compression duties. The power demand decreases as the operating temperature of the membrane stages increases, because of improved selectivity in the cobalt-silica membrane. Hence, more N2 is excluded from the second and third membranes permeate streams, reducing the compression duties. For the cobalt-silica membrane there is a minimum in

Figure 5. Cobalt-silica membrane total area (m2) required to recover and purify helium from the NRU offgas at different feed compositions.

the area decreasing as the temperature increased, because of improved selectivity of the membrane. The majority of the membrane area was associated with the first membrane stage (>95%), with the second membrane stage representing, at most, 3%, and the third membrane representing 61, which is achieved at temperatures of >340 K; for the 2% He feed composite, the selectivity needs to be

Figure 8. Helium permeance (× 107 mol/(m2 s Pa)) and N2/He selectivity of zeolite-silica inorganic membrane, as a function of temperature (K).17

selectivity.16,17 The helium permeance increases with temperature, following an Arrhenius relationship, similar to the cobaltsilica membrane (Figure 3). The nature of the zeolite membrane results in permeance three orders of magnitude greater than the cobalt-silica membrane. Importantly, below ambient temperature, the membrane has selectivity for N2, with a maximum N2/He selectivity at 125 K due to surface diffusion. At higher temperatures, the N2/He selectivity decreases, because of the poor condensability of N2, while at lower temperatures, selectivity is lost, because of the lack of energy for N2 to diffuse along the pore walls. This zeolite-silica membrane

Figure 7. Power demand (MW per thousand cubic feet He produced) when staging the pressure drop through the membrane series, as a function of temperature (K) and feed He composition (mol %). E

DOI: 10.1021/acs.iecr.8b00314 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

third stages. Importantly, the process design of producing a high-purity pressurized He and cryogenic operating temperatures is beneficial for processing the NRU offgas directly; hence, this zeolite membrane has the potential to be incorporated into a NRU process. The total membrane area to achieve the recovery and purity of He is provided in Figure 10, as a function of temperature and

selectivity enables N2 to be removed from the NRU off gas keeping the helium at high pressure through a three-membranestage process (recall Figure 2). 3.2.1. Total Pressure Drop through Each Membrane Stage. The zeolite membrane achieves the maximum separation performance when the total pressure drop is experienced in each stage and the He product is maintained at pressure through the retentate stream. The process is only viable up to a temperature of 220 K, after which the N2/He selectivity is too low to achieve the desired separation. The most important feature of this process is the removal of the major component nitrogen into the permeate stream, which is counter to most gas separation configurations.23 As such, there are significant throughputs associated with the recycle streams from the second and third membrane stages, to ensure high He recovery. The He composition in the retentate stream from the first and second membrane stages is provided in Figure 9. The

Figure 10. Zeolite membrane total area (m2) required to recover and purify helium from the NRU offgas at different feed compositions.

feed He composition. The area reduces with temperature, because of the increased permeance of both gases in the zeolite membrane (Figure 8). Diluting the He composition in the feed gives rise to increased membrane area to accommodate the larger amount of N2 that must be removed from the process. The higher permeance of the zeolite membrane, relative to the cobalt-silica, means that this process configuration has a total area that is three orders of magnitude less than that of the aforementioned cobalt-silica membrane (recall Figure 5). The second membrane stage accounts for 80%−90% of the total membrane area, depending on N2/He selectivity, while the third membrane stage accounts for 6%−11% of the total area. The third membrane stage has a large stage cut to remove the majority of the nitrogen and achieve the high-purity He in the retentate stream. This removal of nitrogen resulted in large amounts of nitrogen being recycled to the second membrane hence, this membrane’s larger area. The area of the first membrane stage is low, because its primary purpose is to remove the majority of the nitrogen from the process while minimizing He loss. Critically, the zeolite membrane requires significantly less area than the cobalt-silica membrane (Figure 5), which is an advantage for the zeolite membrane, given that the high cost of inorganic membrane fabrication favors smaller area requirements. The power demand of the zeolite membrane process is provided in Figure 11, as a function of the He produced. This process requires significantly more power than the cobalt-silica membrane (Figure 6), by a factor of 5 for the temperature range studied and feed He composition. This is due to the major component of N2 being removed through the membrane into the permeate stream and the large gas throughputs

Figure 9. He mole fraction in the retentate stream from the first (solid line) and second (dash line) stages of the zeolite membrane, as a function of membrane operating temperature (K) and the feed gas helium composition.

He composition in the retentate stream from the first membrane stage is only slightly higher than the feed composition for most of the temperature range studied. This is due to N2 in the large recycle stream from the second membrane stage diluting the He in the feed to the first membrane stage; hence, the first membrane role is bulk N2 removal. The second membrane stage retentate stream clearly concentrates the He product, with some dilution of the feed to the second stage caused by the recycle from the third membrane stage. The increase in He concentration observed at temperatures of >200 K is due to the low N2/He selectivity of the membrane forcing the process to operate at marginal membrane stage-cuts to ensure that recovery and purity targets are met. This has the effect of concentrating the He in the retentate streams, but the marginal operating conditions mean that the process is not likely to be feasible. Diluting the He composition in the feed gas has the effect of reducing the He concentration in the respective rententate streams, because of greater N2 carry over in the recycle stream of the second and F

DOI: 10.1021/acs.iecr.8b00314 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

and gas. This leads to an optimal operating temperature of 240 K, when the feed gas is 4% He. For the zeolite membrane, the opposite design of removing the main gas component (N2) through the membrane stages generates significant recycle streams and compression duties. This results in a power demand that is five times greater than the cobalt-silica membrane, but the higher permeance of the zeolite membrane results in significantly reduced membrane area. Hence, inorganic membranes have potential in He processing, but must be optimized based on membrane selectivity and operating temperature.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Colin A. Scholes: 0000-0002-3810-2251 Notes

The author declares no competing financial interest. Biography Figure 11. Power demand (MW per thousand cubic feet He produced) as a function of temperature (K) and feed He composition for the zeolite membrane.

associated with the recycle streams from the second and third membrane stages. Hence, the large recompression duty on the recycle streams accounts for the higher power demand. The increase in power with temperature is due to the decrease in N2/He selectivity of the membrane, resulting in more He being present in the permeate streams; hence, greater recycles are required to ensure that He is not lost from the process and recovery levels are achieved. Diluting the He in the feed stream results in a more significant increase in the power demand, because of the greater amount of N2 being processed through the membrane stages’ permeate streams. Importantly, this process design produced a He product at high pressure; therefore, the compression needed for transportation is minor. Staging the pressure drop through the respective membrane processes provides no advantage in this configuration. A reduced pressure drop decreases the driving force in each membrane stage; this results in a larger membrane area to remove the required N2, which correspondingly increases the amount of He loss through the membrane. This produces larger recycle streams around the second and third membrane stages and, hence, large compression duties. Hence, the power loss because of reduced compression ratios is offset by the increased gas throughput of the recycle streams.

Dr. Colin Scholes is a Senior Lecturer in the Department of Chemical Engineering at the University of Melbourne. His research is focused on achieving a low-emissions future through the development and implementation of membrane separation technology across the energy sector. His research covers all aspects of membrane technology, from novel polymer synthesis, membrane morphology and fabrication modifications, developments to membrane transport theory, as well as modeling membrane processes, including economics, and pilot-plant studies. A career highlight has been successful demonstration trials of his membrane contactor technology for low carbon emissions in Australian industry.

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ACKNOWLEDGMENTS This invited contribution is part of the I&EC Research special issue for the 2018 Class of Influential Researchers.

4. CONCLUSION Inorganic membranes are able to recover and purify He from the offgas out of a nitrogen rejection unit (NRU) through twoor three-stage membrane processes. This is achievable through a He-selective cobalt-silica membrane, as well as a N2-selective zeolite membrane, over a range of temperatures. Critically, both membrane designs are able to achieve the separation at temperatures below ambient that are associated with a NRU; hence, there is the potential to incorporate the membrane process within the NRU. For the cobalt-silica membrane, there is a tradeoff in improved selectivity with temperature, which reduces the compressor duty on the recycle streams against the heating duty required to raise the temperature of the membrane

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DOI: 10.1021/acs.iecr.8b00314 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX