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What is the difference between evaporator and dryer?

March 26, 2014

Typically an evaporator is used to remove water from a mostly liquid stream. Typically, heat is added in a pump-around loop and a large vapor/liquid chamber is used for disengaging the two fluids. Vacuum can be used to assist this flashing, but is not essential. Improved efficiency can be achieved by stringing together units so that the steam from one stage can be used in a subsequent (lower pressure) stage (multi-effect) or sent through a compressor (mechanical recompression) or a steam jet (thermal recompression) to raise the steam pressure and allow it to be re-used on the same stage. Typically an evaporator is behaving as a flash system and is controlled by the VLE.

Dryers are typically used for removing water from a mostly solid material, with the goal of reducing its moisture content. Typically, this is done by providing heat, and is often enhanced by blowing a large volume of heated air through the bed and providing mechanical agitation. With the large volume of air involved, dryers are typically less thermally efficient and its harder to re-use the evaporated water. VLE certainly influences their performance, but often liquid absorption isotherms and mass transfer are the dominant mechanisms.

Evaporators produce a concentrated liquid product (sometimes extremely viscous) whereas dryers produce a solid product. The phase of the feed is not a full distinction, because there are some dryers that operate with liquid feeds such as spray dryers, film-drum dryers and thin-film dryers. The last type can be used either as dryers or as evaporators (also known as wiped-film or scraped-surface).

Liquid feeds to dryers are usually of much higher solids content than those to evaporators, to minimise the evaporative load and energy consumption. You can’t normally use multi-effect systems with dryers. It’s fairly common to see a multi-effect evaporator used to concentrate a dilute solution as efficiently as possible, followed by a dryer to produce the solid product.

In saying “complete removal of moisture”, we should remember that there will always be a small amount of residual moisture left in a solid even after drying, whether it is 10%, 1%, 0.1%, ppm or ppb. This is regulated by the pore structure of the solid and its level of affinity for the liquid (surface tension, physisorption, chemisorption etc). Understanding the “equilibrium moisture content” and “moisture sorption isotherm”, which varies with relative humidity, is often key to specifying and troubleshooting drying equipment and operational parameters to meet a specification for final moisture content.

Nevertheless, this residual moisture from drying is indeed far less than in evaporation, where the final product is still a solution or suspension, and therefore typically >>50% liquid.

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