Cocurrent spray dryer

Fig. 18. Product removal arrangements for cocurrent spray dryers (a) simple outlet (b) product separation ia an agglomeration chamber and (c) classifyiag...

The flow pattern is such that a cocurrent spray dryer must be relatively long and small in diameter (Fig. 2), whereas a countercurrent dryer is shorter and larger in diameter. A third type, sometimes referred to as a mixed-flow dryer (Fig. 3), uses an air pattern similar to a cyclone collector, i.e., the spray is introduced at the upcoming air stream (countercurrent) and the particles transfer to the air sweeping the wall (cocurrent). 

Crowe et al. (1977) proposed an axi-symmetric spray drying model called Particle-Sonrce-In-Cell model (PSI-Cell model). This model includes two-way mass, momentum, and thermal conpling. In this model, the gas phase is regarded as a continuum (Eulerian approach) and is described by pressure, velocity, temperature, and humidity fields. The droplets or particles are treated as discrete phases which are characterized by velocity, temperature, composition, and the size along trajectories (Lagrangian approach). The model incorporates a finite difference scheme for both the continuum and discrete phases. The authors used this PSI-Cell model to simulate a cocurrent spray dryer. But no experimental data were compared with it. More details can be found in the woik by Crowe et al. (1977). 

Papadakis and King (1988a,b) used this PSI-Cell model to simulate a spray dryer and compare their predicted results with limited experimental results associated with a lab-scale spray dryer. They have shown that the measured air temperatures at various levels below the roof of the spray drying chamber were well predicted by the computational fluid dynamics (CFD) model. Negiz et al. (1995) developed a program to simulate a cocurrent spray dryer based on the PSI-Cell model. Straatsma et al. (1999) developed a drying model, named NIZO-DrySim, to simulate aspects of... 

Huang, L.X., Kumar, K., and Mujumdar, A.S., A parametric study of the gas flow patterns and drying performance of cocurrent spray dryer Results of a computational fluid dynamics study, Drying TechnoL, 2003, 21(6), 957-978. 

Fig. 17. Open-cycle, cocurrent, disk atomizer spray dryer.

The majority of spray dryers in commercial use employ cocurrent flow of gas and solids. Countercurrent-flow diyers are used primarily for diying soaps and detergents. Their classif ng ability is useful in these applications. Air flow is upward, cariying entrained fines from the top of the chamber. The coarse product settles and is removed... 

Gas-droplet contact can be either cocurrent or countercvirrent (and sometimes crosscurrent). Up imtil now we have considered that the gas has a large voliune with constant temperatvire and partial pressure. In a spray dryer the gas in contact with a particle constantly changes due to the gas-droplet contact pattern. These different dryer configu-... 

Gas flow and solids flow are usually cocurrent, one exception being a countercurrent-flow spray dryer. The method of gas-solids contact-... 

A spray dryer consists of a feed tank, a rotary or nozzle atomizer, an air heater, a drying chamber, and a cyclone to separate the powder from the air. A rotary atomizer uses centrifugal energy to form the droplet. Pressure nozzle atomizers feed solution to a nozzle under pressure, which forms the droplet. Two fluid nozzles feed solution separately into a nozzle head, which produces high-speed atomizing air that breaks the solution into tiny droplets. Both the feed solution and the drying air are fed into the drying chamber in a standard cocurrent flow. ... 

Use cocurrent spray drying for heat-sensitive products of fine as well as coarse particle size, where the final product temperature must be kept lower than the dryer outlet temperature. 

Gas flow and solids flow are usually cocurrent, one exception being a countercurrent flow spray dryer. The method of gas-solids contacting is best described as through-circulation however, in the dilute condition, solids particles are so widely dispersed in the gas that they exhibit apparently no effect upon one another, and they offer essentially no resistance to the passage of gas among them. 

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. 

SPRAY DRYERS. In a spray dryer a slurry or liquid solution is dispersed into a stream of hot gas in the form of a mist of fine droplets. Moisture is rapidly vaporized from the droplets, leaving residual particles of dry solid, which are then separated from the gas stream. The flow of liquid and gas may be cocurrent, countercurrent, or a combination of both in the same unit. 

Average drop diameters in a spray dryer range from 20 /rm when a disk atomizer is used to 180 /im with a coarse spray nozzle. Residence times vary from 3 to 6 s in cocurrent dryers to as much as 25 to 30 s in countercurrent dryers. 

Particularly if dry powder is produced in a spray dryer plant from solutions, suspensions, or slurries, agglomeration can be accomplished if the partially solidified but still moist particles are tumbled in an associated fluidized bed where, in most cases, final drying also takes place. Fig. 6.2-17 is the schematic flow diagram of a continuous fluidized spray dryer (FSD). As compared with the conventional spray dryer [B.48, B.49, B.71, B.93], a somewhat modified gas handling system is the most obvious new feature of the FSD. Drying gas (9) not only enters the top of the tower for cocurrent drying but also a so-called plenum , a specially designed chamber at the bot-... 

In the cocurrent flow configuration, the liquid spray and air pass through the drying chamber in the same direction, although spray-air movement in reality is far from cocurrent in initial contact. This type of contact is commonly used in a centrifugal atomization spray dryer. It can lead to product temperatures lower than those obtained by the other two flow patterns (Masters 1991). 

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