Simulation of binary gas separation with asymmetric hollow fibre membranes and case studies of air separation

A mathematical model for high‐flux asymmetric hollow fibre membrane was developed considering the effect of permeate pressure build‐up inside the fibre bore. A new solution technique was developed to solve the model equations, which constitute a boundary value problem. The ordinary differential equations were solved as an initial value problem in two successive steps using the Gear's BDF method. The technique is advantageous since it requires minimum computational time and effort with improved solution stability, and the computational complexity does not multiply as the number of components increases. The model predictions and the robustness of the numerical technique were validated with experimental data for several membrane systems with different flow configurations. The model and the solution technique were applied to evaluate the separation characteristics of air using representative membranes with different configurations, including single‐stage, single‐stage with permeate recycle, single‐stage with retentate recycle, air blending, and two stages in series. The study demonstrates that the new solution technique can conveniently handle the high‐flux hollow fibre membrane problems with different module configurations. © 2011 Canadian Society for Chemical Engineering     

生物甲烷膜分离提纯系统的设计与优化

以厌氧发酵生物气为原料生产压缩天然气是大规模利用生物质资源的重要途径。首先,在过程模拟软件UniSim Design中基于有限元方法建立了中空纤维膜的离散数值计算模型,适合于模拟渗透切割比非常高的生物甲烷膜分离过程。以单级聚酰亚胺膜分离系统为例研究了关键操作条件--膜的进料压力对处理能力、甲烷收率及压缩天然气生产单耗的影响。目前的评估体系下,提高进料压力有利于提高处理能力和甲烷回收率,而压缩天然气生产单耗在2.70 MPa时最低,为0.46 kW·h·m^-3压缩天然气。通过分析渗透气的甲烷浓度变化趋势,开发了一级二段气体膜分离系统,兼具流程简单、设备投资低、甲烷收率高、产值高的优点。以处理1000 m³·h^-1生物气为例,甲烷收率达95.0%,压缩天然气产量500 m³·h^-1。对应地,装置总投资为3.8×10⁶ CNY,年运行费用及设备折旧为1.5×10⁶ CNY,年经济效益（毛利）超过2.50×10⁶ CNY。     

Pure hydrogen generation in a fluidized-bed membrane reactor: Experimental findings

A pilot-scale fluidized-bed membrane reactor was tested for the production of hydrogen. The prototype reactor operated under steam methane reforming (SMR) and autothermal reforming (ATR) conditions, without membranes and with membranes of different total areas. Heat was added either externally or via direct air addition. Hydrogen permeate purity of up to 99.995+% as well as a pure-H₂-to-natural-gas yield of 2.07 were achieved with only half of the full complement of membrane panels active under SMR conditions. A permeate-H₂-to reactor natural gas feed molar ratio >3 was achieved when all of the membrane panels were installed under SMR conditions. Experimental tests investigated the influence of such parameters as reactor pressure, hydrogen permeate pressure (vacuum vs atmospheric pressure), air top/bottom split, feed flowrate and membrane area. Reactor performance was strongly dependent on the active membrane surface area.     

Gas permeation — module design and arrangement

Gas permeation by means of membranes is a very simple operation in chemical engineering, and its technical use is already possible in some cases. The dThis paper points out the criteria which enable us to estimate whether the flow pattern in the module is of importance under the given conditions of opFurthermore, suitable module arrangements for enrichment of air with oxygen and of biogases or landfill gases with methane are presented and compared rThis paper also deals with the method of operating with a low pressure on the permeate side, which is frequently mentioned in connection with gas permeFinally, the influence of the Joule-Thomson effect on the separation characteristic is discussed. An example demonstrates that if this effect is not taThe paper analyses flow pattern in gas permeation modules and module arrangements. The analysis leads to the conclusions that in most technically relevThe optimal module arrangements for two important cases, enrichment of oxygen from air and production of methane from biogas, are one- or two-stage casThe question whether the application of vacuum at the permeate side is economical or not has been discussed for the case of methane production from bio     

Single and Binary CO2/CH4 Separation of a Zeolitic Imidazolate Framework‐8 Membrane

The performance of a zeolitic imidazolate framework‐8 (ZIF‐8) membrane in single and binary CO₂/CH₄ gas separation was investigated by means of a gas transport model that included generalized Maxwell‐Stefan and binary friction models. The model concerns gas diffusion through the membrane layer, gas flow through membrane intercrystalline pores, and resistance of the support layer. The effective membrane area considering the actual area for the gas permeated through the membrane was also introduced in this model. The selective ZIF‐8 membrane was successfully synthesized using a microwave‐assisted solvothermal method on an α‐alumina support pre‐attached with ZIF‐8 seeds by solvent evaporation. The simulated data agreed well with the experimental data. The model revealed that the membrane intercrystalline pores and its effective area significantly affected the CO₂/CH₄ gas permeation and separation performance.     

A New Numerical Approach for a Detailed Multicomponent Gas Separation Membrane Model and AspenPlus Simulation

A new numerical solution approach for a widely accepted model developed earlier by Pan [1] for multicomponent gas separation by high‐flux asymmetric membranes is presented. The advantage of the new technique is that it can easily be incorporated into commercial process simulators such as AspenPlus^TM [2] as a user‐model for an overall membrane process study and for the design and simulation of hybrid processes (i.e., membrane plus chemical absorption or membrane plus physical absorption). The proposed technique does not require initial estimates of the pressure, flow and concentration profiles inside the fiber as does in Pan's original approach, thus allowing faster execution of the model equations. The numerical solution was formulated as an initial value problem (IVP). Either Adams‐Moulton's or Gear's backward differentiation formulas (BDF) method was used for solving the non‐linear differential equations, and a modified Powell hybrid algorithm with a finite‐difference approximation of the Jacobian was used to solve the non‐linear algebraic equations. The model predictions were validated with experimental data reported in the literature for different types of membrane gas separation systems with or without purge streams. The robustness of the new numerical technique was also tested by simulating the stiff type of problems such as air dehydration. This demonstrates the potential of the new solution technique to handle different membrane systems conveniently. As an illustration, a multi‐stage membrane plant with recycle and purge streams has been designed and simulated for CO₂ capture from a 500 MW power plant flue gas as a first step to build hybrid processes and also to make an economic comparison among different existing separation technologies available for CO₂ separation from flue gas.     

Technico‐economical assessment of MFI‐type zeolite membranes for CO2 capture from postcombustion flue gases

A detailed survey of the effect of moisture on the CO₂/N₂ permeation and separation performance of Mobile Five (MFI) zeolite membranes in view of downstream postcombustion CO₂ capture applications in power plants and incinerators is presented. The membranes, displaying a nanocomposite architecture, have been prepared on α‐alumina tubes by pore‐plugging hydrothermal synthesis at 443 K for 89 h using a precursor clear solution with molar composition 1 SiO₂:0.45 tetrapropylammonium hydroxide:27.8 H₂O. The synthesized membranes present reasonable permeation and CO₂/N₂ separation properties even in the presence of high water concentrations in the gas stream. A critical discussion is also provided on the technico‐economical feasibility (i.e., CO₂ recovery, CO₂ purity in the permeate, module volume, and energy consumption) of a membrane cascade unit for CO₂ capture and liquefaction/supercritical storage from standard flue gases emitted from an incinerator. Our results suggest that the permeate pressure should be kept under primary vacuum to promote the CO₂ driving force within the membrane. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3183–3194, 2012     

Designing an atmosphere controlling hollow fiber membrane system for mango preservation

We devised an atmosphere controlling facility to gain a longer life span for mango. A membrane module made of polyethersulfone/polydimethylsiloxane (PES/PDMS) composite membrane was applied to selectively permeate CO₂ from the gas mixture of the fruit container. To design the membrane separation system, two models were introduced into our mathematical simulations: (1) an equilibrium model giving the optimal membrane area, the compositions of CO₂ and O₂ in the fruit container, feed flow rate and pressures on both the feed and permeate sides of the module, and (2) a dynamic model simulating the change in the gas composition of the fruit container with time. The pressure build-up in the bore side of the hollow fiber was also discussed using the Hagen-Poiseuille equation. The best membrane module configuration was obtained based on the pressure build-up analysis. That was (1) the vacuum pressure should be set at 0.1 bar, (2) the hollow fiber inner diameter should be 0.45 mm, and (3) the vacuum should be applied at both ends of the hollow fiber bore sides.     

Development of ethanolamine‐based ionic liquid membranes for efficient CO2/CH4 separation

This study is focused on the development of ionic liquids (ILs) based polymeric membranes for the separation of carbon dioxide (CO₂) from methane (CH₄). The advantage of ILs in selective CO₂ absorption is that it enhances the CO₂ selective separation for the ionic liquid membranes (ILMs). ILMs are developed and characterized with two different ILs using the solution‐casting method. Three different blend compositions of ILs and polysulfone (PSF) are selected for each ILMs 10, 20, and 30 wt %. Effect of the different types of ILs such as triethanolamine formate (TEAF) and triethanolamine acetate (TEAA) are investigated on PSF‐based ILMs. Field emission scanning electron microscopy analysis of the membranes showed reasonable homogeneity between the ILs and PSF. Thermogravimetric analysis showed that by increasing the ILs loading thermal stability of the membranes improved. Mechanical analysis on developed membranes showed that ILs phase reduced the amount of plastic flow of the PSF phase and therefore, fracture takes place at gradually lower strains with increasing ILs content. Gas permeation evaluation was carried out on the developed membranes for CO₂/CH₄ separation between 2 bar to 10 bar feed pressure. Results showed that CO₂ permeance increases with the addition of ILs 10–30 wt % in ILMs. With 20–30 wt % TEAF‐ILMs and TEAA‐ILMs, the highest selectivity of a CO₂/CH₄ 53.96 ± 0.3, 37.64 ± 0.2 and CO₂ permeance 69.5 ± 0.6, 55.21 ± 0.3 is observed for treated membrane at 2–10 bar. The selectivity using mixed gas test at various CO₂/CH₄ compositions shows consistent results with the ideal gas selectivity. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45395.     

Design of metallic nickel hollow fiber membrane modules for pure hydrogen separation

Cost‐effective and robust nickel (Ni) membrane for H₂ separation is a promising technology to upgrade the conventional H₂ industries with improved economics and environmental benignity. In this work, Ni hollow fibers (HFs) with one closed end were fabricated and assembled into a membrane module for pure H₂ separation by applying vacuum to the permeate side. The separation behavior of the HF module was investigated both experimentally and theoretically. Results indicate that H₂ recovery can be improved significantly by changing the operation conditions (temperature or feed pressure). Ni HF is a promising membrane geometry, but the negative effect of pressure drop when H₂ passes through the lumen cannot be ignored. Under the vacuum operation mode, there is little difference in term of H₂ recovery efficiency whether the feed gas flow is controlled in countercurrent or recurrent operation. This work provides important insight to the development of superior membrane H₂ separation system. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3662–3670, 2018     

Separation of Diacteone Alcohol‐Water Mixtures by Membrane Pervaporation

Abstract

A study was conducted to evaluate membrane pervaporation for the separation of diacetone alcohol‐water mixtures using commercially available membranes for organic enrichment and dehydration. Empirical correlations for the effect of the process parameters of feed concentration, feed temperature, permeate‐side pressure, and scale‐up were developed. The solvent‐water mixture was successfully separated with a poly(vinyl alcohol) based Sulzer PERVAP 2210 dehydration membrane. Various dehydration membranes were evaluated and a comparison of the flux and separation factor was made. The membrane performance in separating acetone‐water mixtures was also studied. An overall model to predict the membrane area needed for a scale‐up was developed based on the results.     

Separation of C2–C4 hydrocarbons from methane by zeolite MFI hollow fiber membranes fabricated from 2D nanosheets

Separation of higher hydrocarbons from methane is an important and energy-intensive operation in natural gas processing. We present a detailed investigation of thin and oriented MFI zeolite membranes fabricated from 2D MFI nanosheets on inexpensive α-alumina hollow fiber supports, particularly for separation of n-butane, propane, and ethane (“natural gas liquids”) from methane. These membranes display high permeances and selectivities for C₂–C₄ hydrocarbons over methane, driven primarily by stronger adsorption of C₂–C₄ hydrocarbons. We study the separation characteristics under unary, binary, ternary, and quaternary mixture conditions at 298 K and 100–900 kPa feed pressures. The membranes are highly effective in quaternary mixture separation at elevated feed pressures, for example allowing n-butane/methane separation factors of 170–280 and n-butane permeances of 710–2,700 GPU over the feed pressure range. We parametrize and apply multicomponent Maxwell–Stefan transport equations to predict the main trends in separation behavior over a range of operating conditions.     

Modeling and simulation of gas separations with spiral-wound membranes

Models for gas separations with spiral-wound membranes are developed and found to exhibit good agreement with experiments performed on N₂/O₂ mixtures. The two-dimensional (2D) model can be accurately approximated by a one-dimensional (1D) surrogate model when the spacer widths are chosen to make the channel pressure drops small. Subsequently, the separation of propane/propylene mixtures from the recycle purge stream of a polypropylene reactor is investigated. Assuming ideal gas is found to lead to significant overestimations in membrane stage cuts (sometimes more than 10%), an extent comparable to that associated with extrapolating constant olefin permeance from a low-pressure condition. While olefin permeance can change significantly with pressure, using a constant-permeance formulation can result in a small (< 2.5%) underprediction in stage cut if the value for the permeance is taken from the feed condition. Finally, membrane properties and costs necessary for a viable separation process are discussed.     

Oxygen and methane enrichment — a comprison of module arrangements in gas permeation

In the separation of gaseous mixtures by gas permeation, it is in some cases impossible to achieve the desired product quality in a single stage and, therefore, several stages may be necessary. Multistage processes can be implemented by membrane modules arranged in the form of a cascade or by a membrane column design. On the basis of an economic analysis, this paper discusses different possible module arrangements for 2 cases, i.e. the enrichment of oxygen from air and the separation of methane from biogas. Present calculations indicate that, in the first case, two-stage cascades with or without recycle and, in the second case, one-stage cascade without recycle constitute the optimum module arrangements. However, depending on the selling price of the methane enriched gas, one- or two-stage cascades with recycle have to be considered. Finally, It was shown that, in the permeation of non-ideal gases, the Joule-Thomson effect has to be taken into account.     

Micellar enhanced ultrafiltration: A comparative study

A pilot plant study was carried out to evaluate the performance of continuous cross flow micellar enhanced ultrafiltration (MEUF) method for the simultaneous separation of phenol and ortho‐cresol from the aqueous phase using a cationic surfactant, cetyltrimethyl ammonium bromide. The membranes used in this study are polysulfone tubular hollow fibre membrane with 6 kDa and a ceramic membrane having pore diameter 50 nm with an active layer of microporous zirconia (ZrO₂) supported on α‐alumina, respectively. The work includes comparing the effectiveness of membranes based on pollutants structural parameters, membrane properties and membrane modules. The characteristic parameters such as the distribution coefficient (D), micelle binding constant (K_p) and micelle loading (L_m) were obtained at optimal experimental conditions. High initial rejections were observed (above 95%) for both the membranes and the permeate flux remained almost constant at 17 LMH. It was found that the polysulfone material membrane suited better for MEUF with higher and steady rejections over a period of time. High micelle leakage was observed in ceramic membrane. The energy consumption for hollow fibre polysulfone membrane was less than the ceramic membrane. © 2011 Canadian Society for Chemical Engineering     

Hydrogen separation at elevated temperatures using metallic nickel hollow fiber membranes

Nickel is a cheaper metallic material compared to palladium membranes for H₂ separation. In this work, metallic Ni hollow fiber membranes were fabricated by a combined phase inversion and atmospheric sintering method. The morphology and membrane thickness of the hollow fibers was tuned by varying the spinning parameters like bore liquid flow rate and air gap distance. H₂ permeation through the Ni hollow fibers with N₂ as the sweep gas was measured under various operating conditions. A rigorous model considering temperature profiles was developed to fit the experimental data. The results show that the hydrogen permeation flux can be well described by using the Sieverts’ equation, implying that the membrane bulk diffusion is still the rate‐limiting step. The hydrogen separation rate in the Ni hollow fiber module can be improved by 4–8% when switching the co‐current flow to the countercurrent flow operation. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3026–3034, 2017     

Hydrogen separation from the H2/N2 mixture by using a single and multi-stage inorganic membrane

The separation characteristics of hydrogen from a gas mixture were investigated by using a single and two-stage inorganic membrane. Three palladium impregnated membranes were prepared by using the sol-gel, hydrolysis, and soaking-and-vapor deposition (SVD) techniques. A two-stage gas separation system without a recycling stream was constructed to see how much the hydrogen separation factor would be increased. Numerical simulation for the separation system was conducted to predict the separation behavior for the multi-stage separation system and to determine the optimal operating conditions at which the highest separation factor is obtained. Gas separation through each prepared membrane was achieved mainly by Knudsen diffusion. The real separation factor for the H₂/ N₂ mixture was increased with the pressure difference and temperature for a single stage, respectively. For the twostage separation system, there was a maximum point at which the highest separation factor was obtained and the real hydrogen separation factor for H₂/N₂ mixture was increased about 40 % compared with a single stage separation. The numerical simulation for the single and two-stage separation system was in a good agreement with the experimental results. By numerical simulation for the three-stage separation system, which has a recycle stream and three membranes that have the same permeability and hydrogen selectivity near to the Knudsen value, it is clear that the hydrogen separation factors for H₂/N₂ mixture are increased from 1.8 to 3.65 and hydrogen can be concentrated up to about 80 %. The separation factors increased with increasing recycle ratio. Optimal operating conditions exist at which the maximum real separation factor for the gas mixture can be obtained for three-stage gas separation and they can be predicted successfully by numerical simulation.     

Effect of titanium dioxide nanoparticles on polydimethylsiloxane/polyethersulfone composite membranes for gas separation

Introducing inorganic nanoparticles into the structure of polymeric membranes is an interesting approach for the enhancement of physical, chemical, and separation properties of the membranes. In this article, the performance of a two‐layer nanocomposite membrane for gas separation was studied. Three different methods for embedding titanium dioxide (TiO₂) nanoparticle were employed for the membrane preparation. The techniques include blending TiO₂ in the polydimethylsiloxane (PDMS) coating layer, blending TiO₂ in the polyethersulfone (PES) support and dip coating of PES support with TiO₂ accompanied by PDMS coating. The aim of the current research was finding the optimum technique for introducing TiO₂ into the membrane to obtain superior performance for gas separation. The results indicated that PES support containing TiO₂ nanoparticles possessed favorable effect on gas separation capability. The optimum performance was obtained by PDMS‐coated membranes prepared with 7 wt% TiO₂‐embedded PES support. Carbon dioxide (CO₂) permeance, CO₂/nitrogen, and CO₂/methane selectivity were obtained as 188.7 GPU, 8.6, and 3.4, respectively. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Pure H2 production through hollow fiber hydrogen‐selective MFI zeolite membranes using steam as sweep gas

Hollow fiber MFI zeolite membranes were modified by catalytic cracking deposition of methyldiethoxysilane to enhance their H₂/CO₂ separation performance and further used in high temperature water gas shift membrane reactor. Steam was used as the sweep gas in the MR for the production of pure H₂. Extensive investigations were conducted on MR performance by variations of temperature, feed pressure, sweep steam flow rate, and steam‐to‐CO ratio. CO conversion was obviously enhanced in the MR as compared with conventional packed‐bed reactor (PBR) due to the coupled effects of H₂ removal as well as counter‐diffusion of sweep steam. Significant increment in CO conversion for MR vs. PBR was obtained at relatively low temperature and steam‐to‐CO ratio. A high H₂ permeate purity of 98.2% could be achieved in the MR swept by steam. Moreover, the MR exhibited an excellent long‐term operating stability for 100 h in despite of the membrane quality. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3459–3469, 2015     

Abreicherung von Flüssiggas aus Erdgas mittels Zeolithmembranen

The depletion of higher alkanes from methane is a key aspect during the conditioning of natural gases or accompanying gases. Membrane technologies could be used as alternative to energy and cost intensive purifications. Against this background the influence of membrane geometry, composition of the gas mixture as well as temperature and pressure was investigated in separation experiments for methane/n‐butane mixtures using MFI membranes.