Drying paddle dryer

In the vacuum batch dryer, approximately 60% total volume is required for optimum drying result. A very simple and approximate equation can be used for the scaleup of vertical pan and horizontal paddle dryers ... 

FIGURE 18.5 Influence of mechanical stress on drying (A) API after filtration (B) API after drying in a paddle dryer. 

A layout of a multipurpose plant is shown in Fig. 25.7. The wet section comprises three production trains, a distillation unit for solvent recovery or liquid product purification, and a scrubbing system for effluent gas. The dry section consists of a paddle dryer, a silo, a sieve, a mill, a mixer-homogenizer, and the filling and packaging station. 

Conduction or indirect dryers are more appropriate for thin products or for very wet solids. Heat for evaporation is supplied through heated surfaces (stationary or moving) placed within the dryer to support, convey, or confine the solids. The evaporated moisture is carried away by vacuum operation or by a stream of gas that is mainly a carrier of moisture. Vacuum operation is recommended for heat-sensitive solids. Because the enthalpy lost with the drying air in convective dryers is large, their thermal efficiency tends to be low. For conduction dryers the thermal efficiency is higher. Paddle dryers for drying of pastes, rotary dryers with internal steam tubes, and drum dryers for drying thin slurries are examples of indirect dryers. 

For the first group the following types of dryers are generally used convective (tray, band, fluid bed, flash dryers, and their modifications) or contact (vacuum dryers such as double-cone dryer-blender, conical dryer with rotating helical mixer, paddle dryer). Pastelike materials are dried in tray dryers, band dryers equipped with extruding devices, and spin-flash dryers. Finally, thin pastes can be dried in spray dryers or on fluid beds or spouted beds of inert particles. Small amounts of solutions and suspensions are generally freeze-dried, especially if the product is thermolabile. 

Paddle dryers have been successfully used in drying such polymers as VC resin, nylon pellets, and polypropylene (PP), as well as polyethylene. Operated in a closed-cycle mode they can recover organics from such solvent-laden products as polyethylene or PP and can reduce the air volume requirement to only 5%-10% of that used in direct dryers. 

Energy requirements for such dryers are also lower. It is seen that 1300-1500 Btu is required to dry 1 lb of moisture with the paddle dryer compared with 3000 Btu/lb for a suspended air unit. Because of the smaller air volume needed, the sizes of downstream condensers and refrigeration system units are reduced. 

Polystyrene (PS) and acrylonitrile-styrene (AS) are two other polymers produced in bulk quantities. Previously, these polymers were dried with FD. Later, C-FBD replaced all previous FD dryers because of their energy savings advantage. In recent years, paddle dryers have made rapid gains. The heat-resisting power of these materials is comparatively low. Melted material will adhere to the walls of the equipment if the processing temperature is not properly regulated. 

Polypropylene oxide (PPO) is a recently developed resin with an application that is rapidly expanding. It requires a comparatively long drying time since it contains superfine particles and has high affinity for water. Of various kinds of polymers, this is the one that requires the most difficult processing techniques. The paddle dryer is found to process this material economically. 

In other cases not the vessel is rotated, but revolving rotors are placed inside it. An example is the paddle dryer shown in Fig. 10.1-3. For sticky drying goods, a selfcleaning twin screw such as in Fig. 10. 1-4 can be employed, where the heat is supplied via the hollow screws as well as via the outer jacket. 

Viscous solutions, polymers, and pastes are preferably dried by contaet diying on the surface of hot drams and belts. Also these processes offer a certain variety in possible product forms like flakes, chips, and pastilles. Also paddle dryers and kneaders are used in this case. 

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