Design and Control of the Cryogenic Distillation Process for Purification of Synthetic Natural Gas from Methanation of Coke Oven Gas

A cryogenic distillation process for the upgrade of synthetic natural gas (SNG) from methanation of coke oven gas (COG) is designed and controlled using a method of gradually reducing independent variables. Freedom analysis is performed to decide independent variables of the cryogenic distillation column. Based on the equilibrium stage model and Peng-Robinson-Boston-Mathias (PR-BM) thermodynamics method, parameters sensitivity analysis is implemented to obtain the optimal operation conditions using Aspen Plus software. After supplying the physical dimensions and control variables (reflux flow rate and reflux ratio) of the distillation column, three control structures that involve fixed reflux flow rate, fixed reflux ratio and dual-composition controllers are developed to control the cryogenic distillation process. Under the significant disturbances of feed flow rate and feed composition, evaluation results show that the dual-composition control system displays the best effect for maintaining the mole percent of CH4 in column bottoms and gas distillates, which are 99.87% and 0.43%, respectively.