In the evaporation process, concentration of a product is accomplished by boiling out a solvent, generally water, so that the end product may be recovered at the optimum solids content consistent with desired product quality and operating economics. It is a unit operation that is used extensively in processing foods, chemicals, pharmaceuticals, fruit juices, dairy products, paper and pulp, and both malt and grain beverages. It also is a unit operation, which, with the possible exception of distillation, is the most energy-intensive.
Although the design criteria for evaporators are the same regardless of the industry involved, the question always arises as to whether evaporation is being carried out in the equipment best suited to the duty and whether the equipment is arranged for the most efficient and economical use. As a result, many types of evaporators and many variations of processing techniques have been developed to take into account different product characteristics and operating parameters.
The more common types of evaporators include the following:
1. Batch pan
2. Natural circulation
3. Rising film tubular
4. Falling film tubular
5. Rising-falling film tubular
6. Forced circulation
7. Wiped film
8. Plate equivalents of tubular evaporators Batch Pan
Next to natural solar evaporation, the batch pan as shown in Figure 1 is one of the oldest methods of concentration. It is somewhat outdated in today's technology but still is used in a few limited applications such as the concentration of jams and jellies where whole fruit is present and in processing some pharmaceutical products. Up until the early 1960s, it also enjoyed wide use in the concentration of corn syrups.
With a batch pan evaporator, product residence time normally is many hours. Therefore, it is essential to boil at low temperatures and high vacuum when a heat-sensitive or thermodegradable product is involved. The batch pan is either jacketed or has internal coils or heaters. Heat-transfer areas normally are quite small as a result of vessel shapes, and heat-transfer coefficients tend to be low under natural convection conditions. Heat transfer is improved by agitation within the vessel. Low surface areas together with low HTCs (heat-transfer coefficients) generally limit the evaporation capacity of such a system. In many cases, large temperature differences cannot be used for fear of rapid fouling of the heat-transfer surface. Relatively low evaporation capacities, therefore, limit its use.
Natural Circulation. Evaporation by natural circulation is achieved through the use of a short tube bundle within the batch pan or by having an external shell and tube heater outside the main vessel as illustrated by Figure 2.
To condenser or vacuum
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