Optimization of Membrane Properties for CO2 Removal from Biogas

Biomethane from biogas is a valuable renewable energy source.  Biogas contains approximately 60% CH₄ and 40% CO₂ and other contaminants, such as water.  The CO₂ must be removed below 4% before biomethane can be used as fuel for transportation or delivered to pipelines.  Membrane technology is an attractive approach for CO₂ removal, due to its inherent advantages, such as compactness, small footprint and simple operation.  The objective of this work is to evaluate a two‑step membrane process for CO₂ removal and identify the optimal membrane properties for low capital cost and energy consumption.  A biogas separation and optimization model for a binary CO₂/CH₄ mixture is created, which is incorporated with the calculation for power, capital and operating cost.  Membranes with separation properties on the Robeson’s upper bound were evaluated.  In disagreement with the conventional approach utilizing higher membrane selectivity to lower costs, it was found that the membrane system achieves best economics when the membrane has high permeance and moderate selectivity.  For example, membranes with a CO₂/CH₄ selectivity of 10-20 provided the lowest operating cost.  This presentation will also describe the effect of the biomethane purity and methane recovery on the economics of the CO₂ removal.