Drying cocurrent spray dryer

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. 

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

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. 

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. 

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). 

A cylinder-on-cone short-form pilot-scale spray dryer with cocurrent flow of drying air and spray of droplets (Figure 10.3) is adopted from the literature [30]. The diameter and position of air outlet pipe are assumed to be equal to the corresponding data published by Huang et al. [31]. 

As the flow velocity of the desiccant is increased, the drying good may be transported pneumatically. This kind of flow is found in a continuous pneumatic conveyor dryer as shown in Fig. 10.1-9. Here, the drying good is trarrsported by the heated air. The solid as well as the gaseous phase may also be directed vertically downward in cocurrent flow. Figure 10.1-10 shows a spray dryer. The suspension to be dried is dispersed by nozzles at the top and moves downward with the heated air in cocurrent flow. 

Another slurry process reported to have been developed in Europe is similar to the Jacobs-Dorrco process [7]. However, granulation and drying are combined by spraying the slurry onto a cascading curtain of granules at the feed end of a cocurrent rotary dryer. 

The common types of dryers are rotary, hearth, flash (spray), and fluidized beds (10). Hot gases are used invariably to remove moisture. The gas flow can be either cocurrent or countercurrent to the flow of soHds, the former tends to be more efficient. In the hearths, the gas flow is countercurrent as the soHds are raked down from one hearth to the next below. Flash dryers are very rapid because the soHds are exposed only briefly to the hot gases. Fluidized-bed dryers, which use hot gases to suspend the soHds, are rapid and efficient, but require elaborate dust coHection systems. These are preferred when fine soHds are involved, and are used commonly for drying fine coal. Indirect-fired dryers are used when the soHds are heat sensitive or combustible. 

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. 

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