Page 27 - Chemical and process design handbook

Speight_Part 1_C&D 11/7/01 3:03 PM Page 1.13

DEHYDRATION

Dehydration is the removal of water or the elements of water, in the cor-
rect proportion, from a substance or system or chemical compound. The
elements of water may be removed from a single molecule or from more
than one molecule, as in the dehydration of alcohol, which may yield eth-
ylene by loss of the elements of water from one molecule or ethyl ether by
loss of the elements of water from two molecules:
CH CH OH → CH =CH + H O
3 2 2 2 2
2CH CH OH → CH CH OCH CH + H O
3 2 3 2 2 3 2
The latter reaction is commonly used in the production of ethers by the
dehydration of alcohols.
Vapor-phase dehydration over catalysts such as alumina is also prac-
ticed. Hydration of olefins to produce alcohols, usually over an acidic
catalyst, produces substantial quantities of ethers as by-products. The
reverse reaction, ethers to alcohols, can be accomplished by recycling
the ethers over a catalyst.
In food processing, dehydration is the removal of more than 95% of the
water by use of thermal energy. However, there is no clearly defined line
of demarcation between drying and dehydrating, the latter sometimes
being considered as a supplement of drying.
The term dehydration is not generally applied to situations where there
is a loss of water as the result of evaporation. The distinction between the
terms drying and dehydrating may be somewhat clarified by the fact that
most substances can be dried beyond their capability of restoration.
Rehydration or reconstitution is the restoration of a dehydrated food
product to its original edible condition by the simple addition of water,
usually just prior to consumption or further processing.

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