In the evaporation process, concentration of a product is accomplished by boiling out a solvent, generally water. 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. Also it is a unit operation which, with the possible exception of distillation, is the most energy intensive.
While the design criteria for evaporators are the same regardless of the industry involved, two questions always exist: is this equipment best suited for the duty, and is the equipment arranged for the most efficient and economical use? As a result, many types of evaporators and many variations in processing techniques have been developed to take into account different product characteristics and operating parameters.
5.5.1:EVAPORATOR TYPE SELECTION
The choice of an evaporator best suited to the duty on hand requires a number of steps. Typical rules of thumb for the initial selection are detailed below. A selection guide (Figure ), based on viscosity and the fouling tendency of the product is shown below on next page.
MODE OF EVAPORATION
The user needs to select one or more of the various types of evaporator modes
that were described in the previous section. To perform this selection, there are a
number of ‘rules of thumb’ which can be applied.
1) FALLING FILM EVAPORATION:
Ø either plate or tubular, provides the highest heat transfer coefficients.
Ø is usually the mode chosen if the product permits.
Ø will usually be the most economic.
Ø is not suitable for the evaporation of products with viscosities over 300cp.
Ø is not suitable for products that foul heavily on heat transfer surfaces
2) FORCED CIRCULATION EVAPORATORS:
Ø can be operated up to viscosities of over 5,000cp.
Ø will significantly reduce fouling.
Ø are expensive; both capital and operating costs are high.
3) PARAVAP EVAPORATORS:
Ø are suitable for viscosities up to 10,000cp for low fouling duties.
Ø are suitable for very high viscosities, i.e., over 20,000cp, usually the
only suitable evaporation modes are the wiped film and thin film systems.
FILM EVAPORATORS–PLATE OR TUBULAR
Ø Provide a gentle type of evaporation with low residence times and are
often the choice for duties where thermal degradation of product can occur.
Ø Often provide enhanced quality of food products.
Ø Require low headroom and less expensive building and installation costs.
Ø Are usually the choice for very large evaporators.
Ø Are usually the choice for evaporators operating above 25 psia (1.7 bar).
Ø Are better at handling large suspended solids.
Ø Require less floor space than plate evaporators.
Have fewer gasket limitations.
FALLING FILM EVAPORATORS
Vertical shell-and-tube heat exchanger, with laterally or concentrically arranged separator.
The liquid to be concentrated is supplied to the top of the heating tubes and distributed in such a way as to flow down the inside of the tube walls as a thin film. The liquid film starts to boil due to the external heating of the tubes and is partially evaporated as a result. The downward flow, caused initially by gravity, is enhanced by the parallel, downward flow of the vapour formed. Residual film liquid and vapour is separated in the lower part of the calandria and in the down stream centrifugal droplet separator. It is essential that the entire film heating surface, especially in the lower regions, be evenly and sufficiently wetted with liquid. Where this is not the case, dry spots will result that will lead to incrustation and the build-up of deposits.
For complete wetting it is important that a suitable distribution system is selected for the head of the evaporator. Wetting rates are increased by using longer heating tubes, dividing the evaporator into several compartments or by recirculating the product.
Ø PARTICULAR FEATURES
Best product quality – due to gentle evaporation, mostly under vacuum, and extremely short residence times in the evaporator.
High energy efficiency – due to multiple-effect arrangement or heating by thermal or mechanical vapour recompressor, based upon the lowest theoretical temperature difference.
Simple process control and automation – due to their small liquid content falling film evaporators react quickly to changes in energy supply, vacuum, feed quantities, concentrations, etc. This is an important prerequisite for a uniform final concentrate.
Flexible operation – quick start-up and easy switchover from operation to cleaning, uncomplicated changes of product.
Ø Fields of application
Capacity ranges of up to 150 t/hr, relatively small floor space requirement. Particularly suited for temperature-sensitive products.
For liquids which contain small quantities of solids and have a low to moderate tendency to form incrustations.