X= lb moisture / lb dry solid Figure 9. Typical rate-of-drying curve, constant drying conditions.

X= lb moisture / lb dry solid Figure 9. Typical rate-of-drying curve, constant drying conditions.

When manufacturing large quantities of a product which does not require tight batch controls, a more efficient operation (usually less expensive) results by drying the product in a continuous or semi-continuous fashion. The product, in one case, can be batch-stored in large vessels and fed at a continuous rate to the dryer. The product is usually dried in small quantities thus requiring a long time to process the entire amount in a smaller, more efficient piece of equipment. In another situation, ideal for continuous operation, the product would be manufactured upstream of the dryer in a true plug flow manner and transferred to the dryer at a constant rate. In other words, the dryer's capacity matches that of the upstream equipment. This is the most efficient manner.

5.1 Batch Direct Dryers

Most direct batch dryers are fluid bed types such as those which retain the batch on a screen while pneumatically fluidizing the product. Mechanically agitated or tumble rotary dryers also exist. If the product is temperature sensitive, the user should consider a vacuum dryer as an alternative. Vacuum or lower pressure can be utilized to assist in drying the product. However, since most of the mass transfer occurs as a result of the heat input transferred via conduction through the walls of the dryer's jacket, that is considered to be an indirect dryer. For more information on indirect dryers please refer to the first section of this chapter.

5.2 Batch Fluid Bed Dryers

In the category of fluid bed dryers, there are two types of processes commonly used to suspend the material—pneumatic and mechanical fluidi-zation.

1. Pneumatic Fluid Bed Dryers. In the pneumatic fluidiza-tion process, the wet cake is placed in the dryer and dry heated gas is introduced at a very high velocity (under the bed of product) through a fine screen or a porous plate in order to fluidize the product. There is a visible layer of material which is sustained as the gas passes through the bed. The wet gas leaves the chamber through a sock or bag type dust collector which removes the fines and returns them to the batch. [More recently, stainless steel cartridge filters are becoming very popular because they can be cleaned-in-place (CIP). This has been developed by the Aeromatic-Fielder Division of Niro.] If the carrier medium is air containing only clean water vapor, the gas can then be exhausted to the atmosphere if it contains clean water vapor. If the medium is an inert gas, it can be recycled back to the dryer while removing contaminants or solvents via a condenser and filter. However, this inert gas must then be reheated to the proper inlet temperature.

2. Mechanically Agitated Fluid Bed Dryers. In the fluidi-zation process, the wet cake is gently lifted by rotating paddle type agitators thus blending the product into the gas stream creating an intimate mixing of the wet solids with the dry gas stream. This results in a very efficient exposure of the wet product's surface area. The advantage of such a dryer is a faster drying time and a lower total energy input due to lower overall energy requirements (see Fig. 10).

Exhaust air

Exhaust duct A

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