Spray dryer, atomizing devices

As in spray dryers, a variety of devices have been used or suggested for producing droplets from the melt. Centrifugal devices, such as spinning discs and rotating perforated baskets impart an initial radial velocity to the droplets. Such devices require larger tower cross-sections and may lead to inefficient air/droplet contact due to non-uniform prill distribution across the tower [6]. These devices are best-suited to prill tubes of circular cross-section. Atomizing nozzles produce small droplets which are only suitable when fine prills are required. 

Many different typas of spray device have been developed and no generalized correlations are available for predicting performance. They can be categorized into two basic types (I) preformed spray, which includes countercurrent, cocurrent, and crosscurrent spray lowers, spray dryers, cyclonic spray devices, and injector venturis and (2) gas atomized spray, which consists primarily of venturi scrubbers and related designs. 

FIGURE 9.33 Spray dryer for paste-like materials (1) feed tank (2) atomizing device (3) compressed air inlet (4) drying gas (5) heater (6) drying chamber (7) cyclone (8) bag filter (9) exhaust fan (10) product outlet. (From Strumillo, C., Podstawy Teorii i Techniki Suszenia, 2nd edn., Wydawnic-two Naukowo-Techniczne, Warsaw, Poland, 1983.)... 

When the spray-drying operation is cocurrent, i.e., hot air introduced into the dryer close to the atomizing device, there is less danger of overheating as the evaporation rates are high (34-160 kg/h/m of particle area) [3], Thus, cocurrent drying chambers are preferred to minimize heat deactivation of enzymes during the process. 

The figure 12 shows droplet size distribution for water and hydrogel suspension. The atomizing speed is 80 m.s and the feed flowrate 3 kg.hr (curves 1 and 2). Curve 3 represents the particle size distribution of a xerogcl, which is produced from the hydrogel of curve 1 dried in the labmatory spray dryer using the atomizing device described here. 

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