Constrained thin film desorption through membrane separation

Graduation date: 2005Best scan available for tables and computer code in the appendices. The original is faded.A constrained thin film desorption scheme has been experimentally tested to\ud determine the desorption rates for water from an aqueous lithium bromide mixture\ud through a confining membrane. Variable conditions include the inlet\ud concentration, pressure differential across the membrane, and channel height.\ud Desorption takes place in a channel created between two parallel plates with one of\ud the walls being both heated and porous. A hydrophobic porous membrane creates\ud a liquid-vapor interface and allows for vapor removal from the channel. Inlet\ud concentrations of 32 wt%, 40 wt%, and 50 wt% lithium bromide were tested at an\ud inlet sub-atmospheric pressure of 33.5 kPa. Pressure differentials across the\ud membrane of 6 kPa and 12 kPa were imposed along with two channel heights of\ud 170 μm and 745 μm. All cases were run at an inlet mass flow rate of 3.2 g/min,\ud corresponding to Reynolds numbers of approximately 2.5 to 4.5. The membrane\ud surface area for desorption was 16.8 cm². A maximum desorption rate (vapor\ud mass flow rate) of 0.51 g/min was achieved, for the 32 wt%, 12 kPa pressure\ud differential, and 170 μm channel. Increasing the pressure differential across the\ud channel allowed for higher desorption rates at a fixed wall superheat, and delayed\ud the transition to boiling. As the inlet concentration increased the desorber's\ud performance decreased as more energy was required to produce a fixed desorption\ud rate. Results are also presented for the variation in the heat transfer coefficient\ud with the wall superheat temperature. The increase in the channel height had a\ud negative influence on the heat transfer coefficient, requiring larger superheat\ud values to produce a fixed desorption rate