Physics of Membrane-Based Desorption Process From LiBr Solution Flow in Microchannels

Abstract

This study investigates the physics of water desorption from a lithium bromide (LiBr) solution film. The study was conducted on a membrane-based desorber in which the solution flows through an array of microchannels capped by a porous membrane. The membrane allows the vapor to exit the flow and retains the liquid. The solution film velocity and thickness as well as the solution and vapor pressures are independently controlled. Effects of different parameters such as wall temperature, solution and vapor pressures, solution flow velocity, and the solution inlet temperature on desorption rate were studied. Two different mechanisms of desorption are observed and analyzed. These mechanisms consisted of: (1) direct diffusion of water molecules out of the solution and their subsequent flow through the membrane and (2) formation of water vapor bubbles within the solution and their exit through the membrane. Direct diffusion was the dominant desorption mode at low surface temperatures and its magnitude was directly related to the vapor pressure, the solution concentration, and the heated wall temperature. Desorption at the boiling regime was predominantly controlled by the solution flow pressure. Overall, an order of magnitude higher desorption rate compare to a previous study on a membrane-based desorber was achieved.