Evaporation refers to the change in the phase of a component from liquid to gas. In the pharmaceutical industry, evaporation is chiefly associated with removing water and other solvents in batch operations. The suspended components do not appear in the vapour phase, if they do appear then the same operation is referred to as distillation. However, the same principles apply to both operations and are derived from studying heat transfer to the boiling liquid, the pertinent physical properties of the liquid, and the thermal constancy of its components.
Principle Of Evaporation
The heat required to boil a liquid in any vessel is usually transferred from a heating fluid, such as steam or hot water, across the wall of a jacket or tube around or inside which the liquid boils. Rate of heat flow is governed by the following equation:
$$ Q=IAΔT $$
where, Q is the rate of heat flow, U is the overall heat transfer coefficient, A is the area over which heat is transferred, and ΔT is the difference in temperature between the fluids. The overall heat transfer coefficient is derived from a series of individual coefficients that characterize the thermal barriers. It includes the resistance given by metal wall and liquid films (both condensed steam film and liquid side film) present on the either side of the wall.
If the solid barrier consists of a thin metal wall, the resistance to heat flow is small, whereas a glass wall may provide the largest thermal resistance of the system. As we know that, for the heating fluid, the film coefficient for a condensing vapour, such as steam is high, therefore, the condensed liquid on the steam side should be removed immediately, as soon as it is formed using a suitable steam trap. With liquid heating media, the velocity of fluid inside should be as high as possible as this will maintain a low boundary film thickness and thus resistance on the liquid side can be minimized. High boiling temperature and rapid circulation of the liquid promote high film coefficients.
Other factors affecting the heat transfer includes area across which the heat transfer has to take place and the difference in temperature of the boiling liquid and the heating surface. Surface area can be kept high by making use of long and coiled tubes. It is important to keep the boiling point of the liquid low, which rises due to material in the solution and hydrostatic head. For dilute solutions, the rise in the boiling point with solute concentration can be calculated from Raoult’s law. However, this procedure is not applicable to concentrated solutions or to solutions of uncertain composition. For aqueous concentrated solutions, Duhring’s rule may be used to obtain the boiling point rise of a solution at any pressure. This rule states that the boiling point of a given solution is a linear function of the boiling point of water at the same pressure. Therefore, a plot of the temperature of the constant concentration solution versus the temperature of a reference substance where the reference substance and solution exert the same pressure, results in a straight line. Reference substance is generally pure water. Duhring line can be drawn by knowing two vapour pressures and temperature points required for the solution to boil. A family of lines is required to cover a range of concentration.
The liquid which is at the bottom is subjected to the pressure of the liquid column above it. Therefore, hydrostatic boiling occurs at the bottom at higher temperature than the surface. The difference between boiling point of the solution under pressure and solution at evaporating surface is boiling point rise due to hydrostatic head. Thus, hydrostatic head should be kept minimum, so as to keep high evaporation rates.
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.