Foam-mat drying: Energy and cost analyses

Kudra, T. and Ratti, C. 2006. Foam-mat drying: Energy and cost analyses. Canadian Biosystems Engineering/Le genie des biosystemes au Canada 48: 3.27 3.32. Foam-mat drying allows processing of hardto-dry materials such as tomato paste and a variety of fruit pulps and juices. Preferential product quality stems from accelerated drying at generally lower drying temperatures. Reduced density of foamed materials leads, however, to a decreased dryer load, which has to be compensated for by shorter drying time to maintain the dryer throughput. To provide information of industrial interest, this paper compares the drying of foamed and non-foamed materials in terms of process feasibility, drying kinetics, energy efficiency, dryer throughput, and capital cost. Convective drying of both foamed and non-foamed apple juice dried in a 19-mm layer at 55°C has indicated higher drying rates for foamed juice which resulted in reduced drying time from 500 to 200 min. Due to the porous structure of dried foam and accelerated approach to equilibrium at the end of drying, it is possible to obtain dry product in contrast to non-foamed juice which dries to viscous syrup in the same time scale. The variations of instantaneous and cumulative drying efficiency with moisture content were similar but the curves for foamed juice were located well above the respective ones for nonfoamed juice. Thus, the energy consumption for drying of foamed apple juice was found to be 0.2 of that for drying of non-foamed juice. The dryer throughput was calculated as 0.83 and 0.68 kg mh, respectively. Because of higher throughput and shorter drying time, the foam-mat dryer can be smaller which would reduce capital costs by about 11% for a belt conveyor dryer and by 10% for a drum dryer.