Experimental Study and Numerical Optimization of a Thermal Swing Adsorber for Biogas Upgrading

Abstract

Biogas is a renewable energy source that can be adopted as a reliable and sustainable alternative when upgraded. The composition of carbon dioxide in biogas of up to 45% reduces its energy density. Thermal swing adsorption has proven to be a promising technology in the biogas upgrading process, due to its ease of integration with renewable electricity sources and its suitability for water-deficient areas. The experimental study of the biogas upgrading process has been complicated by the dynamic nature of the process and the high sensitivity to operating conditions, such as pressure, temperature, and gas flow rate. To understand the complex interaction between the process parameters, numerical simulation was utilised. There are, however, limited numerical studies evaluating multi-objective optimization of these process parameters. This study assessed the performance of thermal swing adsorption technology, utilising resistive heating, in upgrading biogas produced from the anaerobic digestion of organic waste. Commercial coconut shell-based activated carbon was used as an adsorbent in the experimental cyclic process to capture carbon dioxide. Aspen Adsorption software was used to develop a thermal swing adsorption numerical model. The simulation model was validated using experimental data obtained from a laboratory-scale setup. A good agreement was observed between the simulation and experimental carbon dioxide breakthrough times, with a mean absolute percentage error of 2%. Dynamic adsorption tests were conducted to evaluate the system performance in carbon dioxide capture. The maximum resistive heating regeneration temperature of 60℃ resulted in a peak carbon dioxide concentration of 39% in the waste gas, an energy requirement of 0.1538 kWh per cycle, and an energy efficiency of 87%. This was a good trade-off between adsorbent recovery for subsequent biogas upgrading cycles and system energy efficiency. In the second phase of the study, the adsorbent particle radius, regeneration temperature, and purge-to-feed flow rate ratio were investigated to determine the system's sensitivity. The adsorption and desorption processes were based on methane and carbon dioxide adsorption isotherms, which were fitted to the Langmuir-Freundlich model. The model described the adsorption behaviour on a heterogeneous adsorbent surface, where adsorption sites had different affinities and capacities. Adsorbent particle radius, steam regeneration temperature, and purge-to-feed flow rate ratio range of 1 to 9 mm, 77 to 227℃, and 0.1 to 0.7, respectively, were adopted. Multi-objective numerical optimization of the selected variables was carried out using the Box-Behnken design response surface methodology. The target output responses maximized were the methane purity and recovery. From the analysis of variance, the purge-to-feed flow rate ratio made the highest contribution to both methane purity and recovery, of 92.37% and 99.90%, respectively. While the particle radius had a negligible influence on the methane recovery model, its contribution to the methane purity was significant. The optimal values for maximum methane purity and recovery obtained were 82.12% and 37.21%, respectively, achieved at a particle radius of 9 mm, steam regenerating temperature of 227℃, and a purge-to-feed flow rate ratio of 0.4152. This study offers valuable insights into the design of a thermal swing adsorption biogas upgrading model, as well as the impact of various variables and configurations on the process. The developed model provides practical guidelines for selecting optimal biogas upgrading process parameters to maximize both methane purity and recovery.