Commercially available polymeric membrane materials may also show their potential for CO₂ capture by the association of the membrane process with other separation techniques in a hybrid system. In the current study, PRISM PA1020/Air Products and UBE UMS-A5 modules with membrane formed of modified polysulfone and polyimide, respectively, were assessed as a second stage in the hybrid vacuum swing adsorption (VSA)–membrane process developed in our laboratory. For this purpose, the module permeances of CO₂, N₂, and O₂ at different temperatures were determined, and the separation of CO₂/N₂ and CO₂/N₂/O₂ mixtures was investigated in an experimental setup. An appropriate mathematical model was also developed and validated based on experimental data. It was found that both modules can provide CO₂-rich gas of the purity of > 95% with virtually the same recovery (40.7−63.6% for maximum carbon dioxide content in permeate) when fed with pre-enriched effluent from the VSA unit. It was also found that this level of purity and recovery was reached at a low feed to permeate the pressure ratio (2−2.5) in both modules. In addition, both modules reveal stable separation performance, and thus, their applicability in a hybrid system depends on investment outlays and will be the subject of optimization investigations, which will be supported by the model presented and validated in this study.

Due to the low CO2 concentrations in flue gases (below 20vol.%), the adequate recovery and purity of carbon dioxide in the product can only be achieved by using two-stage adsorptive or membrane systems. In these systems, the high recovery is commonly obtained by minimising CO2 content in the gas leaving stage 1 and recycling the CO2 that remains after stage 2 to the inlet of the installation. The hybrid technique is an obvious extension of the two -stage adsorptive or membrane process. However, there are a number of problems that have to be tackled before such a system becomes practically attractive. One of these problems is the sequence in which the two sections (adsorption and membrane separation) are incorporated into the complete installation. Preliminary studies show that, in the case of flue gas purification, the adsorptive section based on pressure swing adsorption (PSA) should come first. In the present paper results of detailed numerical simulations are presented for the arrangement proposed. A demonstration installation processing around 10 m3(STP)/h of the flue gas is briefly described

The agriculture sector in Poland could provide 7.8 billion m³ of biogas per year, but this potential would be from dispersed plants of a low capacity. In the current study, a membrane process was investigated for the upgrading biogas to biomethane that conforms to the requirements for grid gas in Poland. It was assumed that such a process is based on membranes made from modified polysulfone or polyimide, available in the market in Air Products PRISM PA1020 and UBE UMS-A5 modules, respectively. The case study has served an agricultural biogas plant in southern Poland, which provides the stream of 5 m³ (STP) h⁻¹ of biogas with a composition of CH₄ (52 vol.%), CO₂ (46.3 vol.%), N₂ (1.6 vol.%) and O₂ (0.1 vol.%), after a pretreatment. It was theoretically shown that this is possible to obtain the biomethane stream of at least 96 vol.% of CH₄ purity, with the concentration of the other biogas components below their respective thresholds, as required in Poland for gas fuel “E”, with methane recovery of up to 87.5% and 71.6% for polyimide and polysulfone membranes, respectively. The energetic efficiency of the separation process is comparable for both membrane materials, as expressed by power excess index, which reaches up to 51.3 kW_th kW_el⁻¹ (polyimide) and 40.7 kW_th kW_el⁻¹ (polysulfone). In turn, the membrane productivity was significantly higher in the case of the polyimide membrane (up to 38.3 kW_th m⁻²) than those based on the polysulfone one (up to 3.13 kW_th m⁻²).

Biogas, one of the important controllable renewable energy sources, may be split into two streams: bio-CH₄ and bio-CO₂ using, among others, membrane processes. The proper optimization of such processes requires the knowledge of phenomena accompanying each specific CH₄–CO₂–membrane system (e.g., competitive sorption or swelling). The phenomena were analyzed for the polysulfone-based membrane used in a developed adsorptive–membrane system for biogas separation. The Dual Mode Sorption and partial immobilization models were used to describe the solubility and diffusion of CO₂, CH₄ and their mixtures in this material. The parameters of the models were determined based on pure-gas sorption isotherms measured gravimetrically and permeances of CO₂/CH₄ mixture components from our previous studies. It was found, among other things, that the membrane swelling caused by CO₂ was observed for pressures higher than 5 bar. The real selectivity (permselectivity) of CO₂ vs. CH₄ is significantly lower than the selectivity of pure gases (ideal selectivity), while the solubility selectivity of CO₂ vs. CH₄ in the mixture is higher than that of pure gases. This is due to the better affinity of CO₂ towards the tested polysulfone membrane, making CO₂ the dominant component in competitive sorption. The reduction in the permselectivity is mainly due to an approximately two-fold decrease in the CO₂ diffusion rate in the presence of CH₄. It was also found that the fraction of solubility in the fractional free volume (FFV) is dominant for both gases, pure and mixed, reaching 65–73% of the total solubility. Moreover, in CO₂/CH₄ mixtures, the mobility of methane in FFV disappears, which additionally confirms the displacement of methane by CO₂ from FFV.