Question

(7} Explain in detail the theory of drying. What are its applications in pharmacy
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Answer 1

Answer:

Drying is defined as the final removal of liquid from solids by vaporization with the aid of heat.” The equipment used for drying is called dryer.

Theory of drying: Drying involves both heat & mass transfer operations. To dry material, two steps are needed which are:

• Heat must be supplied to provide latent heat of vaporization.
• The liberated vapour must be removed by moving an air steam.

Consider that, a film of liquid is formed at the surface of the material being dried. The rate of evaporation of this film is related to the rate of heat transfer, as per equation..  dw/dØ = q / λ ……. eq.1 Where, dw/dØ = the rate of evaporation pounds of water per hour q = overall rate of heat transfer (BTU per hr) λ = latent heat of vaporisation of water (BTU Per

The rate of diffusion of moisture into the air stream is expressed by the rate equations, similar to those for heat transfer. The driving force is a humidity differential, whereas, heat transfer is a temperature differential. Thus eq.1 can be written as.. dw/dØ = K’ A (Hs – Hg)……..eq.2 Where, dw/dØ = the rate of diffusion as pounds of water per hr. K’ = the coefficient of mass transfer. A = area of the evaporating surface in square feet. Hs = the absolute humidity at the evaporating surface. Hg = the absolute humidity in the passing air stream

K’ is not a constant but it varies with the velocity of the air stream passing over the evaporating surface thus.. K’ = cGn where, c = a proportionality constant G = the rate of flow of air n = a fractional exponent usually about 0.82. After certain period, the rate of evaporation is equal to the rate of diffusion of vapour. The rate of heat transfer can be equated with the rate of mass transfer i.e eq.1 can be equated with eq.2

There are four principles to properly drying a structure:

• removal of standing water
• evaporation through air movement
• dehumidification
• temperature control

Applications of drying in pharmacy are as follows:

• Used in manufacturing of granules.
• Used to reduce the bulk & weight of material.
• It helps in the preservation of crude drugs or plant from mould growth.
• It helps in the size reduction process.