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
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.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.