Process Design

The multiple effect evaporation is used in cases where a large amount of evaporation is required, and steam (or hot water, in rare cases) is an inexpensive energy source for the process. By utilizing multiple effects for the evaporation, the specific energy consumption in reduced significantly.

While the first effect is heated directly with steam or hot water, the vapours that are created through boiling in that evaporator are withdrawn, cleaned, and used to heat the following (second) effect. The vapours from the second effect are used to heat the third effect, and so on.

The number of effects is limited by the temperature difference between the heating medium and the vacuum that is applied, and ultimately by the temperature of the condensing medium for the last effect. The maximum number of effects is typically limited to 6 or 7.

Thermal Vapour Recompression (TVR) utilises a steam booster as central part of the technology. The vapours that are generated in the evaporator are combined with high-pressure steam in the booster, and the resulting vapour stream discharged by the booster is the energy driving the evaporation process.

This technology can easily be combined with a multiple effect, increasing the overall energy efficiency of the operation.

Theoretically, TVR counts as an additional effect in terms of energy efficiency: a TVR single effect unit has, approximately, the energy efficiency of a double effect, at a much lower cost (the cost of the booster, compared with that of an additional evaporator unit).

TVR operations have the disadvantage of contaminating the boiler steam they use, so the condensate from such an operation is usually not returned directly to the boiler.

The mechanical vapour recompression (MVR) utilises compression of the vapours, which are generated in the evaporator, to a higher pressure/temperature, and their subsequent reuse for heating of the process.

In this case of MVR, high-speed centrifugal compressors achieve the compression; in recent years, the high-speed compressors have been successfully replaced by two or three fans (operating at significantly lower speed) arranged in series.  

While MVR installations have higher initial capital cost, as compared to other concentration options (such as TVR or multiple effect), their energy cost can be substantially lower, due to a high cost differential between electrical energy bought from a power company, as compared to steam generated in-house.

Recrystallization is a purification process, in which impure crystals are dissolved in a clean solvent, and then recrystallized, to produce crystals that are significantly (10X or better) purer than those used for the feedstock.

The actual method of crystallization can be that of evaporation or cooling, depending on the solubility of the product; usually the recrystallization operation is a repetition of the first crystallization unit.   Recrystallization is used for products that require high levels of purity (usually >99.9%), such as pharmaceutical grade or reagent grade components.

Flash evaporation or crystallization is used as a means to cool a brine (and precipitate a compound) without the use of heat exchange on the process liquid or slurry.

This method provides for longer operation between cleanings of the evaporator or crystallizer, since there is far less scale formation on the process equipment surfaces.   The hot feedstock is introduced into a vessel that operates at low pressure (and temperature), where it is flashed under controlled conditions.

Usually, forced circulation or Draft Tube Crystallizers are used for this operation. The flash generates vapours, which are conducted to a condenser. The evaporation concentrates and cools the process feed, and, combined with the lower end temperature, causes the precipitation of the product crystals.

The process equipment designs and arrangements discussed above can be combined as necessary to provide a path to the desired outcome.

The expertise of SEP allows this combination to be seamless and efficient, so that the resulting Plant is a model of efficient and robust operation, utilizing modern technology on a solid foundation of decades of experience in all sectors of evaporation and crystallization operations.