Separation of greenhouse gases from gas mixtures using nanoporous polymeric membranes

Removal of greenhouse gases from gas streams using porous membranes was carried out in this work. Theoretical studies were performed in terms of mathematical modeling and numerical simulation of CO₂ capture in a flat‐sheet membrane contactor. Numerical simulation was performed using computational fluid dynamics (CFD) of mass and momentum transfer in the membrane module for laminar flow conditions. Physical absorption was considered in the simulations for absorption of CO₂ in pure water. CO₂ concentration distribution in the membrane module was determined through numerical solution of continuity equation coupled with the Navier‐Stokes equations. The modeling predictions indicated that the CO₂ concentration difference is not appreciable in the membrane direction. Moreover, velocity distribution was determined in the liquid side of membrane contactor. CFD also represents a design and optimization tool for membrane gas separation processes. POLYM. ENG. SCI., 55:975–980, 2015. © 2014 Society of Plastics Engineers     

Modelling of multicomponent gas separation with asymmetric hollow fibre membranes—methane enrichment from biogas

A mathematical model for the dynamic performance of gas separation with high flux, asymmetric hollow fibre membranes was developed considering the permeate pressure build‐up inside the fibre bore and cross flow pattern with respect to the membrane skin. The solution technique provides reliable examination of pressure and concentration profiles along the permeator length (both residue/permeate streams) with minimal effort. The proposed simulation model and scheme were validated with experimental data of gas separation from literature. The model and solution technique were applied to investigate dynamic performance of several membrane module configurations for methane recovery from biogas (landfill gas or digester gas), considering biogas as a mixture of CO₂, N₂ and CH₄. Recycle ratio plays a crucial role, and optimum recycle ratio vital for the retentate recycle to permeate and permeate recycle to feed operation was found. From the concept of two recycle operations, complexities involved in the design and operation of continuous membrane column were simplified. Membrane permselectivity required for a targeted separation to produce pipeline quality natural gas by methane‐selective or nitrogen‐selective membranes was calculated. © 2012 Canadian Society for Chemical Engineering     

A New Chemical System Solution for Acid Gas Removal

An energy‐efficient absorbent formulation for separating acid gases (e.g. CO₂, H₂S) from gas streams such as natural gas, syngas or flue gas is important for a number of industrial applications. In many cases, a substantial share of their costs is driven by the operational expenditure (OPEX) of the CO₂ separation unit. One possible strategy for reducing OPEX is the improvement of the absorbent performance. Although a number of absorbents for the separation of CO₂ from gas streams exist, there is still a need to develop CO₂ absorbents with an improved absorption performance, less corrosion and foaming, no nitrosamine formation, lower energy requirement and therefore less OPEX. Selected performance results of a new family of amine‐based CO₂ absorbents are summarized. High cyclic capacities in the range of 2.9 to 3.2 mol CO₂ kg^–1 absorbent and low absorption enthalpies of about –30 kJ mol^–1 allow for significant savings in the regeneration energy of the absorbent. Calculations with the modified Kremser model indicated a reduction in the specific reboiler heat duty of 45 %. Furthermore, the absorbents developed show much lower corrosion rates than state‐of‐the‐art solutions that are currently employed.     

Carbon Dioxide Mass Transfer into Solvents in a Zeolite Hybrid Device

Carbon dioxide (CO₂) mass transfer processes are analyzed in hybrid equipment which involves a zeolitic membrane and a physical or chemical solvent. This separation device was chosen because the membrane can be used to produce a stream of higher CO₂ concentration to be treated by gas‐liquid absorption. The analysis of the mass transfer behavior of this gas through the solid phase is an important step before more complicated gas streams are applied. The combined use of both techniques can improve the global separation process because they allow performing a previous separation with a positive effect on the cost of the later separation operations. The influence of the liquid phase nature used in one chamber of the membrane contactor upon CO₂ global mass transfer is analyzed. Also the effect caused by the absorption regime, liquid and gas flow rates, and the pressure corresponding to the gas chamber on CO₂ mass transfer is studied to evaluate the importance of each variable.     

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     

Numerical Simulation of Absorbing CO2 with Ionic Liquids

Although separating CO₂ from flue gas with ionic liquids has been regarded as a new and effective method, the mass transfer properties of CO₂ absorption in these solvents have not been researched. In this paper, a coupled computational fluid dynamic (CFD) model and population balance model (PBM) was applied to study the mass transfer properties for capturing CO₂ with ionic liquids solvents. The numerical simulation was performed using the Fluent code. Considering the unique properties of ionic liquids, the Eulerian‐Eulerian two‐flow model with a new drag coefficient correlation was employed for the gas‐liquid fluid dynamic simulation. The gas holdup, interfacial area, and bubble size distribution in the bubble column reactor were predicted. The mass transfer coefficients were estimated with Higbie's penetration model. Furthermore, the velocity field and pressure field in the reactor were also predicted in this paper.     

Modeling and control of CO2 separation process with hollow fiber membrane modules

A multi-stage model is developed for CO₂ separation by hollow-fiber membrane. The model permits rapid solution of the governing differential mass and pressure distribution in hollow-fiber gas separation modules using a computational scheme that does not rely on commercial software and conventional numerical methods such as shooting techniques. For 1-stage, 2-stage and 3-stage configurations the changes of required separation areas according to stage cuts are computed. A simple model predictive control technique is employed to provide optimal operation conditions based on the proposed model. Values of stage cuts can easily be identified for various desired mole fractions and recovery rates. From the results of numerical simulations, we can see that the proposed model can be effectively used in the control of gas separation process by hollow-fiber membrane modules.     

A new recovery process of carbon dioxide from alkaline carbonate solution via electrodialysis

Bipolar membrane electrodialysis is applied to CO₂ recovery from alkaline carbonate solution. CO₂ in flue gas is captured by an alkaline hydroxide absorbing solution to form an alkaline carbonate solution. The captured CO₂ is recovered from the alkaline carbonate solution via bipolar membrane electrodialysis, and the alkaline solution is regenerated simultaneously. To reduce the power requirement for CO₂ recovery, this study considers optimal design and operation. Three membrane arrangements were compared, and the results indicate the membrane arrangement comprising a bipolar membrane and cation exchange membrane is the most energy saving. With further optimization of operation conditions, the minimum power requirement for CO₂ recovery was reduced to 2.1 MJ/kg‐CO₂ (or 2.1 GJ/t‐CO₂). © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Simulations of Membrane Gas Separation: Chemical Solvent Absorption Hybrid Plants for Pre- and Post-Combustion Carbon Capture

Solvent absorption and membrane gas separation are two carbon capture technologies that show great potential for reducing emissions from stationary sources such as power plants. Here, plants combining chemical solvent absorption and membrane gas separation are considered for post-combustion capture as well as pre-combustion capture. In all ASPEN HYSYS simulations the membrane stage initially concentrates CO₂ into either the permeate or the retentate stream, which is then passed to a monoethanolamine (MEA) based solvent absorption process. In particular, post-combustion capture scenarios examined a membrane that is selective for CO₂ against N₂, while for the pre-combustion scenario a H₂-selective membrane was studied. It was found the energy demand of the combined hybrid plant was always more than that of a stand alone MEA solvent process. This was mainly due to the need to generate a pressure driving force upstream of the membrane in the post-combustion scenario or to recompress downstream gas streams in the pre-combustion scenarios. For both scenarios concentrating the CO₂ in the feed to the solvent system reduced the absorber column height and diameter, which could represent a CAPEX saving for the hybrid plant, dependent upon the membrane price. The use of a hydrogen selective membrane downstream of an oxygen fired gasifier was identified as the most prospective scenario, as it led to significant reductions in absorber size, for a relatively small membrane area and energy penalty.     

Membranes for CO2 /CH4 and CO2/N2 Gas Separation

Membrane technology has emerged as a leading tool worldwide for effective CO₂ separation because of its well-known advantages, including high surface area, compact design, ease of maintenance, environmentally friendly nature, and cost-effectiveness. Polymeric and inorganic membranes are generally utilized for the separation of gas mixtures. The mixed-matrix membrane (MMM) utilizes the advantages of both polymeric and inorganic membranes to surpass the trade-off limits. The high permeability and selectivity of MMMs by incorporating different types of fillers exhibit the best performance for CO₂ separation from natural gas and other flue gases. The recent progress made in the field of MMMs having different types of fillers is emphasized. Specifically, CO₂/CH₄ and CO₂/N₂ separation from various types of MMMs are comprehensively reviewed that are closely relevant to natural gas purification and compositional flue gas treatment     

Polyurethane/polyethersulfone dual-layer anisotropic membranes for CO2 removal from flue gas

Removing CO₂ from flue gas streams has been a permanent challenge regarding environmental issues. Membrane technology is a solution for this problem but more efficient membranes are required. The fabrication of dual-layer polyurethane/polyethersulfone membrane by the co-casting technique is undertaken and the effects of previous evaporation time and coagulation water bath temperature on membrane morphology are explored. Uniform layers with excellent adhesion are obtained. The effect of feed pressure and temperature on membrane permeability and selectivity for CO₂, N₂, and O₂ are studied. Increasing the pressure from 1 to 8 bar results in a reduction of CO₂ permeability and CO₂/N₂ ideal selectivity from 19.6 to 13.0 barrer, and from 66 to 60, respectively. Temperature in the range of 25–45°C enhances CO₂ permeability from 19.6 to 28.9 barrer, although CO₂/N₂ selectivity decreases from 66 to 43, yet showing good potential for applications.     

Entwicklung und Erprobung eines Wäschersystems zur CO2‐Abreinigung aus Rauchgasen mithilfe alkalischer Reststoffe

An innovative, technical approach for the reduction of CO₂ emissions is presented that utilizes alkaline wastes to capture CO₂ from flue gases in stable mineral form. Comprehensive pilot‐scale experiments were conducted with the developed flue gas scrubbing system at a power plant site. By optimizing the process parameters gas flux, CO₂ partial pressure, circulation flux and suspension liquid‐to‐solid ratio, a CO₂ binding of 40 – 90 g kg^–1 waste could be reached and up to 25 % of the CO₂ could be captured. The new technique is economically advantageous especially when both alkaline waste and CO₂ are produced on site and when the carbonated products can be used as secondary resources.     

Modeling and parametric analysis of hollow fiber membrane system for carbon capture from multicomponent flue gas

The modeling and optimal design/operation of gas membranes for postcombustion carbon capture (PCC) is presented. A systematic methodology is presented for analysis of membrane systems considering multicomponent flue gas with CO₂ as target component. Simplifying assumptions is avoided by namely multicomponent flue gas represented by CO₂/N₂ binary mixture or considering the co/countercurrent flow pattern of hollow‐fiber membrane system as mixed flow. Optimal regions of flue gas pressures and membrane area were found within which a technoeconomical process system design could be carried out. High selectivity was found to not necessarily have notable impact on PCC membrane performance, rather, a medium selectivity combined with medium or high permeance could be more advantageous. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Process parameter optimization for ionic liquids as gas separation membranes based on a GWO–BPNN model

Gas separation membranes offer a cost-effective solution for capturing greenhouse gases, mitigating the global greenhouse effect. Ionic liquids (ILs) have emerged as one of the promising materials for greenhouse gas separation due to their strong affinity for CO₂. In this study, we propose a laboratory-scale method for preparing IL–PVDF blend membranes with high CO₂/N₂ selectivity. The separation performance of the membranes was evaluated using a custom gas permeation measurement system. The effects of casting solution composition, solidification method, and film-forming processes on separation performance were experimental investigated, and the obtained experimental data were used to train a back propagation neural network (BPNN) optimized by the Gray Wolf Optimizer (GWO) algorithm. This hybrid GWO–BPNN model was utilized to predict separation membrane efficiency, optimize the film-forming process, and identify the optimal range of process parameters. Notably, the GWO–BPNN model demonstrated a 2.76% higher prediction accuracy compared to a standalone BPNN. The results indicated that the GWO–BPNN algorithm has a great potential to accurately predict membrane separation efficiency and apply in optimal membrane process design (OMPD), and this method can significantly reduce the number of experimental trials required to achieve OMPD.     

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.     

CO2 separation performance by chitosan/tetraethylenepentamine/poly(ether sulfone) composite membrane

CO₂ separation from CO₂/N₂ (20:80) gas mixture has been demonstrated by tetraethylenepentamine blended with chitosan (CS‐TEPA) membrane. Optimization of CS and TEPA weight ratio were carried out based on characterization details involving thermogravimetric analysis, Fourier transform infrared spectroscopy, X‐ray diffraction, atomic force microscope, and field emission scanning electron microscope. Effects of water flow rate, pressure, and temperature were concurrently studied on CS‐TEPA membranes through gas permeation. Almost twofold increase in CO₂ permeance (24.7 GPU) was detected in CS blend with 30% (w/w) of TEPA (CS70) as compared to pure CS membrane (12.5 GPU). CS70 yielded CO₂/N₂ selectivity of 80 whereas CS demonstrated a maximum of 54 at 90 °C. The membrane also exhibited improved stability at temperatures less than 120 °C which was evident from TGA isotherm trace. The proposed composite membrane can be a promising candidate for flue gas separation. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45206.     

Amine-functionalized polycarbosilane hybrids for CO2-selective membranes

Commercially available polycarbosilane has been chemically modified with primary and secondary amino silane derivatives in order to provide amine-functionalized polycarbosilane as hydrophobic solid sorbent capable of reversibly capturing CO₂ from flue gas streams. CO₂ uptake by the samples was investigated at the molecular level using thermogravimetric analysis under CO₂ atmosphere, in situ DRIFTS analysis, and CO₂ sorption isotherm. The reaction paths and sorption mechanisms were examined by comparing with the CO₂ adsorbing behaviors previously studied for amine-functionalized SiO₂, and proven to be related to the presence of adsorbed water, as well as the nature of the grafted amino silanes. With effective CO₂ adsorption rate, regeneration capacity at 40–50 °C, and lesser sensitivity to moist due to its hydrophobic Si-C backbone, secondary amine-functionalized polycarbosilane hybrids have potential applications in membrane gas separation through facilitated transport of CO₂.     

Quest for high performance carbon capture membranes: Fabrication of SAPO-34 and CNTs-doped polyethersulfone-based mixed-matrix membranes

Gas separation process is an effective method for capturing and removing CO₂ from post-combustion flue gases. Due to their various essential properties such as ability to improve process efficiency, polymeric membranes are known to dominate the market. Trade-off between gas permeability and selectivity through membranes limits their separation performance. In this study, solution casting cum phase separation method was utilized to create polyethersulfone-based composite membranes doped with carbon nanotubes (CNTs) and silico aluminophosphate (SAPO-34) as nanofiller materials. Membrane properties were then examined by performing gas permeation test, chemical structural analysis and optical microscopy. While enhancing membranes CO₂ permeance, SAPO-34 and CNTs mixture improved their CO₂/N₂ selectivity. By carefully adjusting membrane casting factors such as filler loadings. Using Taguchi statistical analysis, their carbon capture efficiency was improved. The improved mixed-matrix membrane with loading of 5 wt% CNTs and 10 wt% SAPO-34 in PES showed highly promising separation performance with a CO₂ permeability of 319 Barrer and an ideal CO₂/N₂ selectivity of 12, both of which are within the 2008 Robeson upper bound. A better mixed-matrix membrane with outstanding CO₂/N₂ selectivity and CO₂ permeability was produced as a result of the synergistic effect of adding two types of fillers in optimized loading.     

Application of Water‐Swollen Thin‐Film Composite Membrane in Flue Gas Purification

A water‐swollen thin‐film composite membrane, which was a reverse osmosis membrane with a thin polyamide layer, was used to separate a model mixture of N₂, CO₂, and SO₂. The polyamide swells with water, and thus, becomes more permeable to polar gases. The flue gas contains water vapor, which must be removed before it is subjected to SO₂ removal. Here moisture is employed to keep the membrane swollen. Using the model mixture, the humidified feed stream is brought to the membrane, where it is cooled below the dew point, so that water condenses on the membrane to keep the polyamide swollen. The membrane showed high CO₂ and SO₂ permeance, but low selectivity, so it could be applied to separate these two gases from N₂, and thus, is suitable for flue gas purification.     

Mathematical Modeling,Simulation, and Experimental Verification of CO2 Removal in a Turbulent Contact Absorber

A highly efficient technique of contaminant gas reduction, Turbulent Contact Absorber (TCA), is applied to CO₂ removal from a typical flue gas. Aqueous K₂CO₃ sorbent was evaluated as a regenerable sorbent for CO₂ from the flue gas. In order to identify the system, momentum and mass balance equations were written for the TCA tower. A flat plate falling film model was employed to simulate the TCA tower and the effect of turbulence was included in mass and momentum transfer coefficients. To check the accuracy of the model, a pilot scale TCA was built and operated. A Testo type gas analyzer was used to detect gas concentrations at the inlet and outlet of the rig. The model was validated successfully with pilot plant data. The effect of velocity and K₂CO₃ concentration on the TCA performance has also been carried out. It was found that the bed pressure drop increases linearly with gas velocity and then remains constant. An increase in the liquid flow rate increases liquid holdup, which leads to a rise in bed pressure drop. Higher turbulence within the TCA causes a velocity peak to shift from hypothetical gas‐liquid interface towards the falling film plate. An increase of the K₂CO₃ concentration from 1.0 g mol/L to 2.0 g mol/L was found to give an increase in CO₂ removal by about 4 %.