ENERGY RECOVERY AND PLUME REDUCTION FROM AN INDUSTRIAL SPRAY-DRYING UNIT USING AN ABSORPTION HEAT TRANSFORMER

Spray drying is a widely used unit operation in the chemical industry. It is highly energy intensive, requiring hot, dry airstreams, at temp eratures up to 550-degrees-C, to dry a wide range of products. Exhaust air from a dryer is usually vented to the atmosphere with little or n o heat recovery. At best only the sensible heat associated with the ai r stream is recovered, although the majority of the stream's energy is in the form of latent heat associated with the evaporated water. Exha ust airstreams usually contain moisture to cause visible plumes upon l eaving the dryer stack. A two-stage absorption heat transformer (A.H.T .) has been designed and constructed to investigate the potential for dehumidifying and reheating a simulated dryer exhaust stream to make i t suitable for recycling to the dryer inlet. The amount of air vented to atmosphere and also the amount of wasted heat would be reduced by i ncorporating an A.H.T. into the drying operation. Performance data for the A.H.T. indicates that an airstream can be reheated to a temperatu re of 160-degrees-C, using a lithium bromide solution of 68% w/w, with a circulation ratio (LiBr: steam flow) of 14.8. Temperature lifts bet ween 50 and 70-degrees-C are possible in the reheat column when using a low circulation ratio and a high LiBr concentration. Experiments sho w that a humid air stream can be dehumidified to a level suitable for recycling by direct contact with a concentrated lithium bromide stream .