Numerical investigation of water and heat recovery from flue gas by using nanoporous ceramic membrane tubes

Direct emission of high humidity flue gas results in great wastage of water and thermal energy. Ceramic membrane technology has been widely regarded as a potential technology for water and heat recovery from flue gas. This study presents numerical investigations of water and heat recovery processes from flue gas by using nanoporous ceramic membrane tubes (CMTs). A comprehensive mathematical model is developed based on the Kelvin equation, and the Hagen-Poiseuille equation is applied to calculate the bilateral pressure difference needed for continuous recovery operation. Decreasing the pore size of the separation layer of the ceramic membrane improves the capillary condensation flux but also drastically increases the required bilateral pressure difference. CMT with a pore size of 20 nm is appreciated for the studied conditions. The performances of the CMTs located in the last rows along the flow direction are limited by local low gas temperature and humidity. Twelve rows of tubes are suggested to be arranged along the gas flow direction for ceramic membrane devices with pore sizes of 20 nm and higher. A low transverse spacing obviously improves the overall water and heat recovery efficiency of the device, decreases the performance of each tube, and enhances the gas flow resistance. A transverse spacing ratio of less than 1.4 is suitable for the studied conditions. A combined arrangement by substituting the tubes located in the last rows with those of smaller pore size is a better method than using uniform tubes. The device with combined 20 and 10 nm CMTs is suggested for the studied conditions.