The process of evaporation is based on several factors and their relationship may be expressed mathematically as:
$$ M=\frac{KS(b-b’)}p $$
where, M is mass of vapour formed in unit time (m3/s), K is a constant (m/s), S is surface area of the liquid exposed (m3), b stands for the maximum vapour pressure at the temperature of air, b’ stands for pressure due to vapour of the liquid actually present in the air and p stands for atmospheric pressure.
Therefore, by the formula we can observe that evaporation is directly proportional to the surface of the liquid exposed, temperature used during the evaporation, dryness of the air and indirectly proportional to the atmospheric pressure.
Factors Affecting Evaporation
Surface Area of the Liquid
Equation depicts that with increase in surface area of the liquid exposed the rate of evaporation increases. This justifies the efficiency of hemispherical pans providing maximum surface area.
Pressure
The relation between the vapour pressure of the liquid and its evaporation rate is directly proportional. Liquids with low boiling point have high vapour pressure and such liquids evaporate quickly. If the outer atmosphere is dry the value of b’ will be low and will be greater. If the vapours of the liquid are removed as soon as they are formed (applying vacuum) the space above the liquid does not become saturated with the vapours and hence evaporation proceeds faster. Furthermore, applying vacuum to the system also reduces the atmospheric pressure and facilitates evaporation.
Temperature
For the substances that decompose at temperatures below 100°C, the evaporation temperatures have to be carefully selected so, these operations are generally carried at reduced pressures. Residence time also plays a significant role. Therefore, evaporation carried out at relatively high temperatures but for a very short period of time is less destructive as compared to lower temperatures for a longer period of time. Selection of evaporating temperature also depends upon the moisture because for some cases the moisture is destructive as it can cause hydrolysis whereas, for others water may be required as a reaction medium.
Type of the Product
The method of evaporation used also depends upon the type of the product desired. Pans or stills can be used to obtain liquid or dry products, respectively, whereas film evaporators will yield only liquid products.
Concentration
With evaporation liquid becomes concentrated, and its viscosity increases. The increasing solid content elevates the boiling point of the solution. Moreover, increased viscosity will adversely affect heat transfer and also there will be greater risk of decomposition of the thermolabile substances.
Economic Considerations
The method of evaporation should provide economy of labour, fuel and materials. The recovery of solvents and heat also contributes significantly to cost reduction. A multiple-effect evaporator is operated primarily with the aim to achieve increased steam economies.
Reference:
- Khar, R.,Vyas, S., Ahmad, F., & Jain, G. (2016). Lachman/Lieberman’s The Theory and Practice of Industrial Industrial Pharmacy. New Delhi, ND: CBS Publishers & Distributors Pvt Ltd.