Energy recovery in drying by adsorption heat pumping
Drying is one of the most energy intensive operations of the chemical industry and accounts for about 6% of the total energy used by U. K. industry and about 2.5 of the overall energy demand of the U.K. Measurements taken on a typical industrial tumble dryer confirmed that generally convective dryers operate at thermal efficiencies less than 4 and that over 5Y of the energy input is lost as sensible heat content of the moist exhaust air. Any significant improvement in the thermal performance of dryers would therefore require a means of gainfully recovering the heat lost in the exhaust air. Some conventional heat recovery methods have been considered. Recirculation of exhaust air was shown to improve thermal efficiency but at the cost of reduced drying rates. Heat recovery by heat exchange was found unattractive because very large heat exchange surface areas would be required. A new type of adsorption heat pump (AHP) which may be operated either as a temperature swing cycle (TSC) or a pressure swing cycle (PSC), has therefore been proposed here for heat recovery from dryers. An experimental rig was built and used to investigate the adsorption of moisture on silica-gel in a 1.5 m. high, 0.25m. diameter column under conditions that simulate an industrial dryer, including high temperatures and humidities. Correlation of the experimental data led to the derivation of a polynomial function, similar to the system equilibrium equation, which relates the breakpoint capacity of the adsorbent to the breakpoint bed relative humidity. This function was used to develop a new theoretical model for predicting the performance of the proposed heat pump dryer. The predictions of this model enabled similar predictions obtained from two other models synthesized from various proposals put forward by other investigators previously to be tested against experimental results. This new model was found to be the most appropriate for the conditions encountered and was therefore considered to be the most suitable for predicting the performance of the adsorption heat pump drying system. Theoretical predictions based on this model indicate that the heat pump drL-er may be attractive for low temperature ( <1000C) drying. Under high temperature drying conditions low thermal efficiencies and unfavourably large adsorption bed size requirements make the heat pump unattractive.