Evaluation of a membrane permeation system for biogas upgrading using model and real gaseous mixtures: The effect of operating conditions on separation behaviour, methane recovery and process stability

In this paper, the enrichment of methane by membrane technology was studied by employing (i) a model as well as (ii) a real biogas mixture produced on a laboratory-scale. Thereafter, the endurance of the process was tested at an existing biogas plant. The commercial gas separation module under investigation contained hollow fiber membranes with a polyimide selective layer. During the measurements, the effect of critical factors (including the permeate-to-feed pressure ratio and the splitting factor) was sought in terms of the (i) CH₄ content on the retentate-side and (ii) CH₄ recovery, which are important measures of biogas upgrading efficiency. The results indicated that a retentate with 93.8 vol% of CH₄ – almost biomethane (>95 vol% of CH₄) quality – could be obtained using the model gas (consisting of 80 vol% of CH₄ and 20 vol% of CO₂) along with 77.4% CH₄ recovery in the single-stage permeation system. However, in the case of the real biogas mixture, ascribed primarily to inappropriate N₂/CH₄ separation, the peak methane concentration noted was only 80.7 vol% with a corresponding 76% CH₄ recovery. Besides, longer-term experiments revealed the adequate time-stability of membrane purification, suggesting such a process is feasible under industrial conditions for the improvement of biogas quality.