The Economics of Helium Separation and Purification by Gas

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The economics of helium separation and purification by gas separation membranes. Colin A Scholes, Ujjal Kumar Ghosh, and Minh T. Ho Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00976 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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Industrial & Engineering Chemistry Research

The economics of helium separation and purification by gas separation membranes.

Colin A. Scholesa*, Ujjal Kumar Goshb, Minh T. Hoc. a

Department of Chemical & Biomolecular Engineering, The University of Melbourne, VIC 3010, Australia b

Department of Chemical Engineering, College of Engineering, Qatar University, Doha 2713, Qatar c

School of Chemical and Biomedical Engineering, The University of Sydney, NSW 2006, Australia

Abstract Membrane gas separation has potential to recover and purify helium, either directly from natural gas or as part of the off gas produced from the nitrogen rejection unit in conventional natural gas processing. Process designs to achieve high helium recovery and purity have previously been published by the authors, based on permeability and selectivity performances already present in current polymeric membranes. Here, the application of polymeric gas separation membranes for helium recovery is extended further with the economic viability of process designs investigated and evaluated against the commercial helium price. For direct recovery and purification of helium from natural gas, membrane gas separation is economically competitive when the natural gas field has a helium concentration of 0.3 mol% or greater. The final compression of the purified helium dominates the economics, accounting for 45% of the CAPEX of the process as well as 80% of the OPEX. This is because the membrane area is relatively small and the staggering of the high pressure of the feed through the membrane stages reduces the need for significant compression on the membranes’ recycle streams. For the recovery and purification of helium from the NRU off gas, the economics of a combined membrane – PSA process is more economical against conventional technologies. Again, the final compression of the purified helium dominates the process, accounting for almost 90% of the production price. Sensitivity analysis demonstrates that the compressors CAPEX and cost of electricity significantly affect the production price. Hence, to increase the economic viability of membranes for helium recovery and purification, reducing the costs of the compressors and improving their efficiency will have a much greater impact than reducing the costs associated with the membrane itself.

Keywords: Membranes, Helium, economics, natural gas * Corresponding author ([email protected])

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1. Introduction Membrane gas separation has been commercially successful for air enrichment and natural gas sweetening 1; as demonstrated by the technology’s increasing market share in these applications. A range of other potential applications have been suggested for membrane gas technology, of which helium recovery and purification has been discussed since membrane technology was first proven in the 1960s 2. Helium is a high valued gas that has a range of end usages, primarily in cryogenic applications such as MRI scanners and in generating inert atmospheres for the electronics industry 3. The market for helium has expanded over the past few decades because of economic development in Asia, which has been represented in the increasing price of helium (Figure 1). The majority of helium is currently sourced from natural gas fields, primarily in the USA, where helium concentrations can be as high as 4 mol% 4. The increased demand for helium has been met in part by the USA federal government sell off of its helium reserves, with a set price for USA crude well established (Figure 1) 5. This has suppressed the need for new helium production plants in the preceding decades, but recently the reserve sell off has tapered and market forces have returned to dominate the commercial helium price. As such, natural gas fields with low helium concentrations are becoming increasingly viable and new helium processing facilities are being constructed and planned. For example, in 2010, BOC launched Australia’s first helium processing facility in Darwin, while RasGas has recently opened the largest helium processing facility in Qatar, directly aimed at the Asian market. Conventional helium recovery and purification is achieved through cryogenic distillation and pressure swing adsorption (PSA), processing either the natural gas directly or the off gas from the nitrogen rejection unit (NRU) as part of a traditional natural gas processing plant 6. The final product is helium of purity 99.995%, at pressures above 50 MPa for transportation 7. This is required because the consumers of helium are geographically dispersed relative to the small number of helium production plants. Recent research by the authors has demonstrated that membrane gas separation technology is feasible for 99% recovery of helium from the source gas and purification of helium to over 99.995% 8. Processing natural gas directly is achievable with three membranes in cascade with recycle streams (Figure 2); where the membrane has a He/CH4 selectivity of 25 or greater (dependent on the operating pressure of each membrane stages). More feasible is the recovery and upgrading of helium from the NRU off gas (Figure 3); through two separate membrane processes in series with PSA to achieve the final purity product. This requires membranes to have He/N2 selectivity of 7.5 or greater, which is currently achievable through polymeric materials that have been commercialized for natural gas sweetening 9. In this investigation, the membrane gas separation process simulations the authors’ have previously undertaken are extended further through economic analysis to determine the feasibility of gas separation membranes in helium recovery and purification. This techno-economic study is undertaken for membranes recovering helium directly from natural gas; as well as membranes recovering helium from the NRU off gas (Figures 2 and 3). The economic comparison of helium produced from both membrane based processes is benchmarked against the commercial helium price (Figure 1); as it enables the viability of both membrane process designs to be determined. This techno-economic study


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also provides insight into those process units and costs that need to be improved to further increase the economic competitiveness of the membrane processes. Figure 1 Figure 2 Figure 3

2. Economic simulation All membrane simulations were undertaken in Aspen HYSYS (version 8.8), the details of the process simulations and outcomes can be found in the authors’ previous publication 8. The process is designed to produce 300 kg/h of Grade A helium based on 99% recovery of helium from the feed gas, which is of comparable output to the RasGas plant in Qatar. All membranes were operated in a cross-flow configuration without the use of a sweep gas, and assumed to operate at 35 oC. The membranes had a He permeance of 1000 GPU, with varying selectivity dependent on application. For comparison, cellulose acetate membranes have reported He permeability of 1990 barrer and a He/CH4 selectivity of 11.8 10, polypyrrolone has a He permeability of 166 barrer and a He/CH4 selectivity of 184 11, while Teflon AF-2400 has a He permeability of 3600 barrer and a He/CH4 selectivity of 6 12. A pressure drop of 0.01 MPa was assumed for the retentate stream across the membrane. For direct recovery and purification from natural gas, the helium composition was modelled between 0.05 and 4 mol%, with a feed pressure between 2.5 and 10 MPa, to simulate various natural gas fields. For processing the off gas from the NRU, the helium composition was varied between 1 and 3 mol% for a feed pressure of 0.12 MPa. The final product had a purity of 99.995% and undergoes compression to 50 MPa, to meet transport specifications. This product specification also enables comparison with conventional technology, such as cryogenic distillation and pressure swing adsorption, which can produce a pure helium product at pressure. All compressors were centrifugal with an adiabatic efficiency of 75%. The pressure swing adsorption process was based on Molecular Sieve 4X 13, modelled through the four steps of the Skarstrom cycle 14, 15. All process simulations were operated at steady-state conditions. The capital cost (CAPEX) estimates of individual pieces of equipment were determined using the Aspen Economic Evaluation package, 2015 1st Quarter, with default parameters. This includes equipment costs for the heat exchangers and compressors. The cost of the gas separation membranes was estimated at US$100 /m2, with a membrane installation factor of 1.6 16. CAPEX of the pressure swing adsorption process was based on pressure vessels costings 14, while PSA cycle’s compressors were costed through Aspen, adsorbent Molecular Sieve 4X was costed at $4/kg, and an assumption of 10% was made for the valves required for the process cycle. Other capital costs assumptions cover instrumentation, piping and electrical costs (25% of equipment costs), engineering and start-up costs (13% of equipment costs) and contingency (10% of equipment costs) 17. The operating costs (OPEX) are based on electrical energy duty, at a cost of US$ 40/MWh, along with a cost associated with a 5 year replacement period for all membranes and adsorbent in the PSA unit. 3 ACS Paragon Plus Environment


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These replacement periods are based on literature estimates 18. A fixed cost of 6% was added onto this for maintenance and labor 17. Economic evaluation was undertaken using a NPV method based on the amount of helium produced (thousand cubic feet). The project life was 25 years, with a 2 year construction period and an 85% load factor. The discount rate was 7% (real). The economics take into account final compression of the product, but no pretreatment of the natural gas or NRU off gas. Furthermore, the cost of purchasing the natural gas is not included in this economic analysis, as it can vary considerably dependent on location and market forces. Inclusion of the purchase of natural gas is expected to increase the price of helium production between US$ 2 and 10 per thousand cubic feet.

3. Results and Discussion 3.1 Direct Recovery from Natural Gas The cost of producing helium directly from natural gas, as part of a three membrane cascade process with recycle, is provided in Figure 4, as a function of natural gas feed helium composition and pressure. The helium production price is at ~US$ 90 per thousand cubic feet for natural gas fields with a helium composition of 2 to 4%, while lower grade fields result in a production price rise to between US$ 300 and 415 per thousand cubic feet, for the lowest concentration studied of 0.05 mol% dependent on feed pressure. The increase in production price with decreasing feed composition is indicative of the increased CAPEX and OPEX of the process because of higher methane concentrations in the membrane stages’ permeate, requiring larger membrane area and additional throughput on the compressors to process the diluted gas. The variation in feed pressure is a result of the change in pressure driving force across each individual membrane stage. For lower feed pressures, the low driving force ensures increased methane in the permeate streams of the first and second membrane stages, resulting in higher compressor duty to recycle the gas to achieve target helium recovery. However, operating polymeric membranes at very high pressure driving force conditions can increase the probability of membrane rupturing and/or gas plasticization of the polymer; both of which will dramatically reduce the performance of the membrane module to undertake the desired separation. This difference in production price is more notable at the lower feed composition because of the stronger dependence of those processes on membrane area and membrane permeate compressors. Importantly, the economic study strongly suggests that direct recovery of helium from natural gas through membranes is economically competitive when the feed gas compositions is 0.3 mol% or higher, dependent on feed pressure.

Figure 4


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The breakdown of the CAPEX and OPEX of the direct recovery from natural gas process is provided in Figure 5, as a function of class of equipment (excluding CAPEX contingency). In both the CAPEX and OPEX the membrane’s contribution is negligible, in terms of initial installation and ongoing replacement. This is because the total membrane area for the process is between 46,000 and 930,000 m2, dependent on feed helium composition and pressure. These are relatively small areas for a membrane process when compared to natural gas sweetening or reverse osmosis desalination 9. In addition, in membrane processes for natural gas sweetening, the membrane unit can be less than 5% of the total CAPEX of the process because of the pressures involved and gas handling equipment 18. Hence, the total CAPEX of the membranes is between US$ 4.6 and 93 million, dependent on feed composition and pressure. The largest contributor to both the CAPEX and OPEX of the process are the final compressors stages to pressurize the helium product to 50 MPa for transport. This accounts for 45% of the CAPEX and 80% of the OPEX. This sizeable contribution is because the helium product leaving the third membrane stage is at low pressure and significant compression is required to reach product specification pressure. Reducing this compression duty will have significant impact on the economics of the process, from both a CAPEX and OPEX perspective. However, the need to stagger the feed pressure through the membrane stages to achieve the separation driving force means that there will always be considerable final compression duty for a membrane process. Importantly, producing a final helium product at a much lower pressure will result in significant lower production price. For example lowering transportation pressure to 10 MPa will reduce the cost of production by 63%. The current commercial approach of cryogenic distillation and pressure swing adsorption can mitigate this compression duty somewhat by operating at a higher pressure than a gas separation membrane process, and produce a purity product that does not require significant compression for transportation specifications 6. The membranes’ permeate compressors account for less than 10% of the CAPEX and less than 15% of the OPEX of the process. This is because the staggering of feed gas pressure drop through the membrane stages enables the permeate compressors duty to be relatively minor, pressurizing between 1 and 5 MPa compared to the final compressors. This duty can be reduced further if the membranes are more permeable for helium. For the CAPEX, the piping and instrumentation as well as engineering and start-up costs account for almost 40%, while for OPEX maintenance is 6%, due to the load factors applied in the economic analysis. A gas separation membrane demonstration plant for helium recovery and purification was operated in Alberta, Canada 18. That plant processed 3 MMscfd of natural gas containing 0.05% helium, and produced a helium product of 72 to 90% purity. The CAPEX of the plant was $50 million and OPEX was $2.8 million, in 1980, not including the final product gas compression. Adjusted for inflation, this represents a CAPEX of $140 million and an OPEX of $8 million, which are comparable to the costs determined for the process configurations investigated here. Similarly, the demonstration plant estimated a production price of US$ 288 per thousand cubic feet of helium (accounting for inflation), which is comparable to the production price determined here for the same feed composition (Figure 4).

Figure 5



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For most techno-economic studies of membrane processes, one of the largest uncertainties is the membrane price; because commercial gas separation membranes are sold as complete processes, not as individual modules 16, 18. Here, the membrane contribution to the CAPEX and OPEX of the direct recovery from natural gas is negligible and hence variability in the membrane price has only a minor impact on the production price. As such, economic sensitivity analysis is better focused on the compressors, both between membrane stages and the final compressors, as these dominate the CAPEX and OPEX of the process. This sensitivity analysis is provided in Figure 6 as a relative change to both the economic variable and the production price. The CAPEX of the compressors has the largest impact on the production price, with a doubling of the compressors capital resulting in the production price increasing by 62%, while a halving of the compressors capital results in the production price decreasing by 32%. As such, achieving real reductions in production price will be possible through cheaper compressor technology. The cost of electricity (CoE) has a direct impact on the production price because of the OPEX strong dependency on the compressors’ duty. The CoE can vary considerable between locations, with energy rich regions, such as the Middle East, having a lower CoE compared to OECD countries. A CoE increase to US$60 /MWh increases the production price of helium by 18%, irrespective of membrane separation performance. While a doubling of the CoE to US$80 /MWh results in the production price increasing by 36%; alternatively halving the CoE in the simulation to US$20 /MWh leads to a corresponding decrease in production price by 19%. Hence, the electrical demand and therefore pricing are significant factors and applying more efficient compressor technology will have a positive impact on the economics. The sensitivity of the economics to CoE is part of the reason for the recent strong interest in the Middle East to undertake helium recovery and purification. As with all economic studies, the discount rate influences the production price. However, in this study the compressor CAPEX and CoE have a more significant impact on price change than comparable changes to the discount rate. This highlights the dominance the compressors have on economics of the overall process. Hence, for direct recovery and purification of helium from natural gas through membrane technology it is the compressors and their efficient operation that dictate the helium production price rather than the membrane modules.

Figure 6

3.2 Helium recovery from NRU process In a traditional natural gas treatment process, helium can also be recovered from the nitrogen rejection unit (NRU) which removes nitrogen from the natural gas. This off gas has helium concentrations of between 1 to 3 mol%, dependent on the helium in the raw natural gas and is at near atmospheric pressure. The conventional approach is to separate and purify this helium from the off gas through cryogenic distillation and PSA, and a similar approach can be undertaken with membrane technology, as 6 ACS Paragon Plus Environment

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demonstrate in Figure 3. Here, two distinct membrane process stages are applied; the first is the recovery of helium from the off gas via a two stage membrane cascade, with recycle, that concentrates the helium to between 50 and 70 mol%. The second membrane process is upgrading the helium to >90%, again through a two membrane cascade process, with recycle. For both membrane processes, a He/N2 selectivity of 7.5 or greater can achieve the required separation 8. The final stage is undertaken through PSA, to achieve specification purity, making the overall simulation a combined membrane – PSA process. The economic assumptions for applying membranes for helium recovery from the NRU assumes that the LNG process train is already present, and the helium process is a separate entity in terms of the CAPEX, OPEX and NPV calculation. As such, the economics of the LNG process, i.e. gas sweetening and liquefaction, have no impact on the economics of the helium process. The production price of helium from the NRU off gas through the combined membrane – PSA process is provided in Figure 7, as a function of membrane He/N2 selectivity. The helium production price decreases with increasing membrane selectivity because at higher selectivities more nitrogen is excluded from the permeate streams reducing both the required membrane area and permeate compressor throughput. A sharp decrease in production price occurs between He/N2 selectivities of 7.5 and 15 because this marks the transition in the process simulations going from marginal operating conditions, in terms of stage-cut and recycle amount, to more tolerable membrane operating conditions 8. This impacts both CAPEX and OPEX. Irrespective of membrane selectivity, the helium price of production is competitive against the current market price (Figure 1). Hence, the incorporating of membranes into natural gas processing for the distinct purpose of helium recovery and upgrading is competitive with conventional technology. The viable production price can be achieved with polyimide and perfluoropolymer membranes, that have He/N2 selectivities greater than 25 19, 20; which are classes of polymers that have been commercialized as membranes in other gas separation applications. Hence, the adoption of gas separation membranes for helium recovery from the NRU is a feasible economic proposal that utilizes existing membrane modules.

Figure 7

The breakdown in the helium production price as a function of the individual stages of NRU recovery is provided in Figure 8. The final compressors dominate the production price, accounting for up to 90%, dependent on membrane selectivity. This is for the same reason as observed in direct recovery from natural gas, the high final compression of pure helium to meet specifications. This final compression puts a floor on the production price of US$ 75 per thousand cubic feet, and to reduce the production costs requires improvements in both the efficiency of the compressors as well as reduced CAPEX. Similar to direct recovery from natural gas study, reducing the pressure of the final helium product will significantly reduce the production price. This can be achieved by supplying a lower pressure product to market as well as directly connecting the helium processing plant to end users to reduce transportation requirements. If the transportation pressure is lowered to 10 MPa, then the cost of production from the 7 ACS Paragon Plus Environment


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NRU is reduced by ~72%, dependent on membrane selectivity. The membrane process for recovery accounts for 2.5% while the membrane process for upgrading the helium only accounts for 1%. This is because both processes require small membrane area and low compressor sizes and duties, especially when compared to direct recovery from natural gas because of the higher helium concentrations involved. The PSA stage contributes 7% to the overall production price, in part because the process requires four pressure tanks and additional gas handling equipment compared to the simpler membrane process.

Figure 8

For this membrane – PSA combined process; the uncertainty in the membrane price is not a significant factor in the production price compared to the compressors. A doubling in the membrane price will only increase the production price by less than 7%. So similar to the direct recovery from natural gas, sensitivity analysis of the economics is focused on the compressors CAPEX and CoE and the impact they have on the production price, which is provided in Figure 9. The sensitivity analysis trend is very similar to that observed for the direct recovery from natural gas, in part because compressors dominate both process simulations. The doubling of the compressors CAPEX results in the production price increasing by 68%, while halving the compressors CAPEX results in a price reduction of 35%. Similarly, the CoE has a direct impact on the OPEX of the process, because it is dominated by the electrical duty needed to power the compressors. An increase in the CoE to US$60 /MWh results in a 20% rise in the helium production price, irrespective of the membrane stages’ performance; while doubling the CoE to US$80 /MWh results in an increase in the production price by 38% for the process conditions studied here. Alternatively, halving the CoE to US$20 /MWh decreases the production price by 20%. For the CoE this variation has a greater effect on the process recovering helium from the NRU off gas, compared to direct recovery from natural gas, because of the additional compressors and vacuum pumps involved in the PSA process. Hence, the viability of helium recovery from the NRU off gas is strongly dependent on the CoE, and more favourable for regions where electricity pricing is low. Changing the discount rate applied to the economic simulation alters the production price, but has a smaller influence on price compared to the compressors CAPEX. Again, this indicates that the final product compressors dominate both process designs for recovering and purifying helium.

Figure 9

4. Conclusion The economic study of gas separation membranes for helium separation and purification has demonstrated that the technology is cost competitive, based on current helium price, for both direct 8 ACS Paragon Plus Environment

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separation from natural gas as well as incorporation into a standard natural gas processing facility. The study revealed that for both process options the overall dominating unit stage was the final compression needed to bring the pressure up to 50 MPa for transport. For direct recovery from natural gas, it was found that fields with helium concentration of 0.3 mol% or greater were economic with a three membrane stage cascade process, and the economic feasibility of the process dependent on minimizing compressors CAPEX and OPEX. For recovery of helium from the NRU off gas through a combined membrane – PSA process, it was found that the two membrane process stages contributed

The Economics of Helium Separation and Purification by Gas

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