Chemical potential analysis for directing the optimal design of gas membrane separation frameworks

• A thermodynamic model is proposed for mass transfer in gas membrane separation.
• Chemical potential change is used to discern work loss in RMC and CMC frameworks.
• The mix of local permeate differing in composition reduces efficiency seriously.
• Membrane stages with lower efficiency are ascertained in retrofitting frameworks.
• Efficiency is enhanced by partially retrofitting RMC with proper capital cost.

Thermodynamic analysis can discern those energy requirements which might be decreased in the separation system. In this work a theoretical model based on nonequilibrium thermodynamics was established to analyze the free energy change and the key points governing separation efficiency in gas membrane separation. Mass transfer in membranes was treated to be a two-step process in the model: species selectively permeate across membrane to form a series of local permeate gases, then the local permeate gases converge into a bulk permeate stream. Thus the total work required by the membrane process was identified as (i) the work to drive mass permeation, (ii) the work wasted by permeate mixing, and (iii) the minimum separation work. The analysis of two typical systems – the recycle membrane cascade (RMC) and the continuous membrane column (CMC) – revealed that the work wasted by permeate mixing seriously affects the free energy efficiency. This work wastage can be reduced by retrofitting process frameworks. Subsequently, the analysis was used to identify the serious work-losing stages in the RMC. Finally, a partially retrofitted RMC was proposed with tangible reductions in both equipment investment and energy consumption.