Absorption - Dehumidification A Unit Operation for Heat Recovery from Dryers

Abstract

Drying operations account for 10% of primary energy consumption of the main industrial sectors in Europe. Energy recovery is uncommon, and most drying processes discharge hot, humid air to atmosphere. The purpose of this work was to develop an absorption heat transformer (A.H.T.) to simultaneously dehumidify and reheat dryer exhaust streams for recycle to the drying process.

Experiments have been carried out on a single-stage, direct-contact A.H.T. utilising aqueous Lithium Bromide as absorbent. It was concluded that, in order to maximise the drying capacity of the recycled airstream, the dehumidification and reheating processes must be carried out as separate operations. A two-stage variant has been constructed and operated with a wide range of inlet temperatures and humidities. Using a ‘typical’ spray dryer exhaust stream at 100°C and humidity 0.2 kg/kg, outlet air temperatures of 170°C and humidities of 0.04 kg/kg may be obtained: the recycling of such a stream to the spray dryer could give rise to energy savings of 20%.

Specification of A.H.Ts. for a variety of dryer types indicates that absorption dehumidification may be accorded the status of a unit operation applicable to any hot, humid exhaust gas stream. The design of an A.H.T. for use with a large industrial spray dryer is developed as a case study.