ABSTRACT
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Thermal drying has long been recognized as one of the most energy-intensive operations in solids-liquid separation. The share of drying energy in industrial energy use ranges from 10 to 25%, although in some industrial sectors it is much higher, reaching 35% in paper production and even 50% in fi nishing of textile fabrics (Kudra, 2004; Mujumdar, 2007). It is diffi cult to estimate drying energy use in the agrifood sector because of the diversity of food products, multiplicity of food producers, and frequent use of process coproducts as supplementary fuel. Nevertheless, 12% was quoted for England and 27% for France over two decades ago (Strumillo et al., 2007). At present, higher numbers can be anticipated because the percentage of drying energy is progressively increasing (Baker, 2005). For example, Kemp
(1998) indicates that the drying proportion of the U.K. industrial energy consumption has risen by 4% over 12 years. Because almost 99% of applications involve removal of water (Mujumdar, 2007), such high energy consumption is primarily due to the latent heat of water vaporization, which is 2500 kJ/kg at 0°C. The fraction of thermal energy can be favorably reduced by upstream mechanical separation or nonthermal dewatering such as membrane processing and osmotic dehydration. An interesting option for drying certain fruits such as cranberries is osmotic dehydration prior to thermal drying because such a hybrid technology not only reduces energy consumption but also offers a product of enhanced quality (Grabowski et al., 2002). Basically, ineffi cient convective dryers, which account for about 85% of all industrial dryers (Mujumdar, 2007), are the second contributor to the high energy demands for drying. Thus, concentrating the liquid feed by evaporation prior to spray drying can notably reduce the heat load to the dryer because the energy demand for evaporation ranges from 360 to 3600 kJ/kg versus 2900 to 5400 kJ/kg for a single spray dryer, even though higher values were also reported, such as 9040 kJ/kg for spray drying of sweet whey (Trägårdh, 1986).
