Contact-mixing dryer

With contact-mixing dryers [5.1, 5.2] the product is mechanically mixed, redistributed and intensively dried. Milling and scraping devices may be installed for lumpy products. Drying is usually carried out at atmospheric pressure or under vacuum conditions. 

In a plate dryer [5.29, 5.30, 5.46] (Fig. 5-29), without hot gas peripherals, free-flowing product is moved across heated discs using wiper arms, turnover blades, side scrapers, rakes, etc. The product is constantly overturned and alternately transferred from disc to disc through central openings or openings in the dryer walls and dried in this way. 

With a paddle dryer (Fig. 5-40) the liquid, pasty, or free-flowing wet product is evenly distributed by rotating paddles over the heated areas (jacket, hollow shaft, and paddles) and discharged at atmospheric pressure or under vacuum. 

Representation according to Luwa — SMS Verfahrenstechnik, Ziirich/Butzbach. 

RO Rotor with paddles DR Drive FR Frame VF Vapor filter VC Vapor condenser DD Discharge device VP Vacuum pump 

---

Between 1 s and 1 min specific contact time, conduction heat-transfer performance decreases theoretically as the 0.29 power of contact time. This is consistent with empirical data from several forms of indirect-heat dryers which show performance variation as the 0.4 power of rotational speed (21). In agitator-stirred and rotating indirect-heat dryers, specific contact time can be related to rotational speed provided that speed does not affect the physical properties of the material. To describe the mixing efficiency of various devices, the concept of a mixing parameter is employed. An ideal mixer has a parameter of 1. 

All parts of the freeze-dryer that come in contact with solvent vapour must be solvent-resistant. The solvent blend must dissolve all components of the formula and should have its freezing point well above the shelf temperature of the freeze dryer. Blends of tert-butanol and chloroform (from 2 1 to 1 4 mixing ratio) are suited for most lipid blends. 

All raw materials are procured either in liquid or powder form. While ensuring a certain quality, the selection is mostly based on price. The physical characteristics of the components is not important since they will be dispersed or dissolved and mixed with other liquids to form a solution or slurry. This slurry is pumped by low- or medium-pressure pumps to the spray dryer (Fig. 6.3-2). An air vessel (pressure accumulator) is used to even-out pressure peaks. The spray towers can work in con- or counter-current fashion [B.97]. Concurrent contact yields light powders (mostly made-up of hollow particles) with a bulk density of about 100-150 g/L and a moisture content of 3-10%. Also, the hollow beads tend to break-up and form dust. Counter-current drying (Fig. 6.3-2) produces powder with a bulk density of 300-500 g/L and a moisture content of 7-15% (commonly 10%). Most plants use this method because the higher bulk density is almost always desired. 

The mixed-flow spray drying system is a combination of the previous two systems. The droplets may contact the drying medium in the same and opposite direction in one drying chamber. It is usually found in spray dryers fitted with pressure nozzle or pneumatic nozzle. It is sometimes used to obtain agglomerated powders in a small drying chamber. It should be noted that the product must be nonheat sensitive. 

---