Spray dryers sizing example

The main variables in the operation of atomizers are feed pressure, orifice diameter, flow rate and motive pressure for nozzles and geometry and rotation speed of wheels. Enough is known about these factors to enable prediction of size distribution and throw of droplets in specific equipment. Effects of some atomizer characteristics and other operating variables on spray dryer performance are summarized in Table 9.18. A detailed survey of theory, design and performance of atomizers is made by Masters (1976), but the conclusion is that experience and pilot plant work still are essential guides to selection of atomizers. A clear choice between nozzles and spray wheels is rarely possible and may be arbitrary. Milk dryers in the United States, for example, are equipped with nozzles, but those in Europe usually with spray wheels. Pneumatic nozzles may be favored for polymeric solutions, although data for PVC emulsions in Table 9.16(a) show that spray wheels and pressure nozzles also are used. Both pressure nozzles and spray wheels are shown to be in use for several of the applications of Table 9.16(a). 

The design of spray dryers is based on experience and pilot plant determinations of residence time, air conditions, and air flow rate. Example 9.10 utilizes such data for the sizing of a commercial scale spray dryer. 

Example The PCD method is used for monitoring a laboratory-scale spray dryer operation where fine aluminum oxide powder is produced by dr3dng dilute solutions of water and aluminum oxide. On-line particle size and velocity, inlet hot air and exhaust air temperatures were measured. The SPM scheme based on on-line temperature measurements checks if the process is operating under the selected settings, and producing the desired particle size distribution [213]. j4f (3) models are used for both temperatures. The exhaust air temperature is modeled by... 

Sizing a continuous spray dryer begins with defining the hourly quantity which is broken down into solids and liquids. The quantity of the liquid present is used to calculate the energy input necessary to evaporate that amount of liquid. For example ... 

Most food-processing companies use spray dryers to produce powdered products. Spray drying has the ability to handle heat-sensitive foods with maximum retention of their nutritive content. The flexibility of spray-dryer design enables powders to be produced in the various forms required by consumer and industry. This includes agglomerated and nonagglomerated powders having precise particles size distribution, residual moisture content, and bulk density. As examples, spray drying of milk, tomato juice, tea extracts, and coffee is discussed. 

Another example of dryer selection is related to the choice of a suitable atomizer for a spray dryer. A spray dryer is indicated when a pumpable slurry, solution, or suspension is to be reduced to a free-flowing powder. With proper choice of atomizer, spray chamber design, gas temperature, and flow rate it is possible to engineer powders of desired particle size and size distribution. Table 47.2 shows how the choice of the atomizer affects chamber design, size, as well as energy consumption of atomization and particle size distribution. The newly developed two-fluid sonic nozzles appear to be especially attractive choices when nearly monodisperse powders need to be produced from relatively moderate viscosity feeds (e.g., under 250 cP) at capacities up to 80 t/h by using multiple nozzles. More examples may be found in Kudra and Mujumdar [20]. 

Table 10.2 presents as an example the basic characteristics of zinc phosphate dried in a pilot spray dryer with reference to industrial standards (Lyulin, 1998). It is evident that pigments dried in a spray dryer with the shock-wave atomizer satisfy all requirements for the commercial product. In addition, pigments obtained with the shock-wave atomizer have a narrower size distribution so the throughput of the ball mills used in a downstream process of standardizing is increased by about 20% to 30%. 

With knowledge of the parameters influencing the particle size distribution of spray dried powders, and process cperience with different products, new spray dryers with, for example, integrated fluidized beds are developed. Production of almost dust-free powders is now possible [5.110]. 

Once the scale of spray dryer is established, it is important to select an atomization nozzle appropriate to the scale. The purpose of the atomization stage is to produce a fine mist (spray) from a liquid feed to substantially increase the liquid surface area and improve the efficiency of heat and mass transfer. For example, 50 ml of a solvent atomized in 800 million droplets of 50 xm creates a surface area about 6 m. By the generation of such high surface area, droplets dry fast, in the order of seconds or fraction of a second depending on the drying conditions. Moreover, the control of the atomization process dictates droplet size and consequently the particle size. 

---