Simple Pharmanotes: Definition of Evaporation and Factors affecting it.

Theoretically, evaporation means simply vaporization from the surface of the liquid.
Evaporation is an unit operation by which a solvent is evaporated from a solution by boiling the liquor in a suitable vessel and withdrawing the vapor, leaving a concentrated liquid residue.

Factors affecting evaporation:

Heat is necessary to provide the latent heat of vaporization, and in general, the rate of evaporation is controlled by the rate of heat transfer.
Rate of heat transfer depends on the temperature gradient.
Many pharmaceutical agents are thermolabile. So the temperature that will cause the least possible decomposition should be used.
e.g. Many glycosides and alkaloids are decomposed at temperatures below 1000C.
e.g. Hormones, enzymes and antibiotics are extremely heat sensitive substances. e.g. Malt extract (containing enzyme) is prepared by evaporation under reduced pressure to avoid loss of enzymes.
Some antibiotics are concentrated by freeze-drying.

Exposure to a relatively high temperature for a short period of time may be less destructive of active principles than a lower temperature with exposure for a longer period.
Film evaporators use a fairly high temperature but the time of exposure is very short.
An evaporating pan involves prolonged heating.

Some drug constituents decompose more rapidly in the presence of moisture, especially at a raised temperature (by hydrolysis). Hence, evaporation should be carried out at a low controlled temperature, although the final drying can be performed at higher temperature when little moisture remains.
e.g. Belladonna Dry Extract is an example of this type.

Evaporating pans or stills will produce liquid or dry products, but film evaporators will yield only liquid products. So a dilute extract can be first concentrated in a film evaporator and then the concentrated extract may be dried in an evaporating pan.

As the liquor becomes concentrated, the increasing proportion of solids results in elevation of the boiling point of the solution. This leads to a greater risk of damage to thermolabile constituents and reduction of the temperature gradient.
In general concentrated solutions will have increased viscosity, causing thicker boundary layers, and may deposit solids that may build up on the heating surface that reduce heat transfer.
All these problems may be minimized by turbulent flow conditions.