The biogas treatment does not involve only the separation and recovery of CO2, but also the recovery of a concentrated CH4 added value stream, which could be directly supplied - under the strict purity standards - to feed pipelines for domestic or small plants. Hence, the evaluation of an integrated membrane-based process in terms of process efficiency, cost and footprint occupied by the installation could give a preliminary evaluation about benefits and drawbacks for the design of future integrated membrane plants. For this purpose, Brunetti et al. [66] proposed different operation schemes for an integrated polymeric membrane-based plant useful for biogas treatment. Two different kinds of polymeric membranes were chosen (Hyflon AD60 and Matrimid 5218) to carry out the aforementioned separation process in a multistage configuration system. On one hand, the first polymeric membrane possesses higher CO2 permeability with respect to other membrane materials, but a quite low CO2/CH4 selectivity.
On the other hand, the second polymeric membrane exhibits higher selectivity but lower permeability. The operation schemes were studied depending on the main design parameters such as pressure ratio and permeation number, comparing the results with the current targets related to the biogas streams treatment. In a two-stages configuration, the aforementioned authors demonstrated that a further treatment is needed for the permeate streams coming from the two stages in order to recover as much as possible CH4 in the outlet streams, simultaneously increasing CO2 concentration. The third stage is exercised at 40 as a pressure ratio, as done for the other stages, with the purpose of supplying the CH4 recovered directly into the grid, which operates normally under 30–50 bar. It stated that their analyses confirmed that it was not possible to achieve contemporary the purity and recovery targets for CO2 and CH4. Fig. reports the process scheme in which the CO2 stream targets are well reached, providing high recovery (97.5%) and a purity close to target.