Abstract
M. Tech. (Chemical Engineering)
The usual target of an upgrading process using membrane is to produce a retentate stream, the product, with high CH4 concentration. This work presents a simulation of two possible membrane configurations, single stage without recycle (SSWR) and double stage with permeate recycle (DSPR), of an existing operational biogas upgrading plant. The simulation was conducted using ChemCAD and AlmeeSoft gas permeation software to investigate the performance of the configurations on product purity, recovery and required compressor power with a view to determine the optimal operational conditions for maximising the concentration of CH4 and its recovery. Thereafter, an economic assessment on the optimal configuration was conducted to determine the gas processing cost (GPC), the profitability of producing biomethane and cost-benefit of utilising biomethane as a vehicular fuel.
The simulation was validated against plant data with a maximum percentage error of 2.64%. Increasing CO2 in feed reduced product recovery and purity. Increasing feed pressure and selectivity increased product recovery and purity up to the pressure limit of the membrane module. Increasing feed flow rate increased product recovery but reduces purity. In both configurations, increasing CO2 in the feed and increasing feed pressure increased the GPC. However, increasing feed flow rate reduced the GPC. The overall performance of DSPR configuration was much higher due to increased trans-membrane area available for separation.
At optimal conditions, a product purity of 91% and 96% CH4 recovery was achieved from the initial plant result of 87.2% product purity and 91.16% CH4 recovery. The total compression duty was 141 kW. The GPC was $0.46/m3 of biomethane. The cumulative discounted NPV, IRR and BCR for producing biomethane was R15,240,343, 22.41% and 2.05 respectively, with a break-even in the 5th year after plant start-up considering a prime lending rate at 9%. Using CBG instead of gasoline saves 34% of annual fuel cost with a payback period of one year and three months for the cost of retrofitting the vehicle.