Evaporation is basically a separation step which uses heat transfer to separate products presenting differences at boiling point. This technology results in several different downstream processes such as concentration, crystallization and drying.
|Industrial falling film evaporator||Falling film evaporator basic diagram|
In chromatographic processes, fractions of the desired compound(s) diluted in solvent (pure or mixed) are produced. This creates a need for concentration, evaporation and crystallization operations.
Evaporation and crystallization equipment can be configured as multiple effect, thermo-compression and mechanical recompression. Among many others, the following examples of applications can be given:
The thermal separation technology offers effective solutions to many customer concerns such as:
Nowadays, numerous designs are operating industrially.
Stirrer evaporator: This type of evaporation is rarely used today; it applies mostly for highly viscous feed and for very specific products.
Rising film evaporator: The operation principle is quite straightforward: a powerful pump moves the boiling feed up along the tubes, and the lower portion can eventually be used to preheat the solution.
Circulation evaporator: The operations are similar to the rising film principle, but the liquid recovered in the separator is sent back to the bottom of the evaporator creating a closed loop.
Forced circulation evaporator: This system is used when the product has a strong tendency to foul the heating surfaces; therefore, it is recirculated at a rather high rate through the tubes.
Falling film evaporator: In this case, both liquid and vapors move downwards, the key point being a uniform wetting of the tubes in order to avoid any incrustation.
Fluidized bed evaporator: This system operates under the same principle as forced circulation evaporators and is used when the feed solution contains particles.
Plate evaporator: As the name indicates, this type of evaporator is composed of a series of thin, corrugated plates compressed together in a rigid frame evaporator instead of using a shell and tube design. The feed solution and steam both flow in alternate channels.
Spiral tube evaporator: This is a very compact system where the long heating tubes are in spiral form to minimize space.
The characteristics of the feed solution such as heat sensitivity, viscosity, turbidity, boiling points, tendency to precipitation or foaming will determine the best selection.
Typically, an evaporation unit consists of three parts: preheaters in order to bring the feed solution up to the required temperature to start evaporation on the first unit; separators for the separation of the vapor and liquid phases; and lastly, condensers for residual vapors and deaeration units.
The energy consumption to evaporate an aqueous solution is fairly significant; therefore, in order to reduce the energy cost, systems such as multiple effect evaporation and thermal vapor recompression are often used. The steam consumption of the evaporator unit can be reduced by using the vapor from the first chamber to heat the second one.
The principle consists in recompressing low pressure/temperature vapors at a higher pressure so that they can be condensed in the evaporator heat exchanger. This can be achieved either through a thermal vapor recompression device (typically a venture-shaped steam ejector) or mechanical vapor recompression (turbo compressor).
Multiple effect evaporation with thermocompression
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