Energy input into recovery of renewable gaseous fuels using permeator systems equipped with polyimide membranes

The scarcity of fossil energy resources together with intensifying anthropic emissions that influence earth’s climate and human health drive the contemporary research on the energy production in the direction of sustainable systems. A considerable part of this research focuses on the production of renewable gaseous fuels, mainly natural gas substitute and bio-hydrogen. Both can be obtained in a large variety of biomass and biowaste conversion processes. The production of the former includes principally processes like anaerobic fermentation, gasification and methanation. The latter can be obtained for example through photofermentation and also gasification. Since the gases obtained in the exemplified processes usually do not meet the desired product purities, the employment of gas upgrading methods is necessary. This opens wide spectrum of applications for membrane gas permeation processes.
It is likely that membrane gas permeation will provide a series of advantages over other competing separation processes in the field of the production of renewable gases. The reasons for it are process simplicity, high safety, high reliability and easy scale-down which hardly influences process economics.
The contemporary engineering of polyimide materials is already developed enough to allow the production of robust membranes with high permeances and selectivities in large scale. Glassy membranes possess advantageous selectivity order for the upgrading of renewable gaseous fuels obtained in biomass decomposition processes. The selectivity order is particularly important in the upgrading of gas mixtures obtained in biomass gasifications; here the methane with longer aliphatics are at least permeating and remain in the high pressure retentate; hydrogen is one of the fastest permeating components.
This work presents several membrane gas permeations processes for the upgrading of both methane and hydrogen from gas mixtures obtained in typical biomass conversion processes. The provided examples are based upon already realised processes and piloting experiments as well as rigorously modelled renewable gas production and recovery methods that are a matter of the near future. The evaluation of the separation processes focuses principally on the energy input that is required for the upgrading of the gases. The energy requirement is an important factor in the production of renewable gases because it affects directly the energy conversion efficiency which is decisive for the process sustainability and economics. In the work it is investigated how further developments in the membrane technology concerning increase of membrane selectivites may influence the energy conversion efficiency of the upgrading processes. It is shown that the energy requirement can be minimised not only by the augmentation of membrane selectivity but also through the correct choice and optimisation of permeator configurations.