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Renin Angiotensin Mechanism.

Definition of Evaporation and Factors affecting it.

 

Definition:

  • 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.

Objective of evaporation:

  • To make a solution more concentrated. 

  • Generally extracts are concentrated in this way.

Factors affecting evaporation:

  1. Temperature:

  • 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.

  1. Temperature and time of evaporation:

  • 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.

  1. Temperature and moisture content

  • 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.

  1. Type of product required

  • 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.

  1. Effect of concentration

  • 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.


Commonly Asked Questions.

  1. Define evaporation and discuss various factors affecting evaporation.
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