Drying of suspensions is a very important process in the chemical, pharmaceutical and food processing industries. This unit operation is a most energy-consuming operation. Although there are hundreds of variants actually used in drying, the research efforts throughout the world are associated with the development of more sophisticated systems. In general, the trends in drying technology are associated with higher energy efficiency, enhanced drying rates and the development of more compact dryers, better control for enhanced quality and optimal capacity, developments of multi-processing units, etc. Mujumdar (1990Mujumdar ( , 1995 pointed out that numerous new or improved drying technologies are currently at various stages of development. Each type of dryer has specific characteristics, which make it suitable or unsuitable for specific applications.An efficient drying system should include good quality of the final product as well as meet several conditions: high values of heat and mass transfer coefficients, high contact area, high input of heat carrier gas, uniform temperature distribution over the drying chamber, the use of concentrated suspensions (as high as possible) in order to minimize the water amount to be evaporated and the use of a high inlet air temperature as much as possible. However, many of these conditions are in conflicts.Drying of suspensions of inert particles is a relatively novel technology to produce powdery materials. It was originally developed for drying pigments, chemicals and some biomaterials to eliminate the constrains of spray, drum and paddle dryers. Independent of the hydrodynamic configuration of the dryer, the principle behind this technology is based on the drying of a thin slurry layer that coats the surface of inert particles. Depending on the dryer type, these particles can be vibrated, fluidized or spouted either by hot air only, or in combination with a mechanical device installed within the dryer, such as an agitator or conveyor screw (Kudra and Mujumdar, 2002).In our prior investigations, a fluid bed dryer with inert particles was used for drying solutions, suspensions and pastes, as described in detail in Grbavcic et al. (1998). With respect to the main efficiency criteria, i.e. specific water evaporation rate, specific heat consumption and specific air consumption, a fluidized bed dryer with inert particles represents a very attractive alternative to other drying technologies. A high drying efficiency results from the large contact area and from the large temperature difference between the inlet and outlet air.The most important disadvantage of a fluid bed dryer with inert particles lies in the possibility of bed sintering in certain applications. For example, our experimental trials of drying several organic and biologi-*Author to whom correspondence may be addressed. E-mail address: grbavcic@elab.tmf.bg.ac.yu A draft tube spouted bed dryer with inert particles was used for drying suspensions. The effects of the operating conditions on dryer throughput and product ...
