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DRYING Drying is defined as removal of the liquid from a material by application of heat, & is accomplished by transfer of a liquid from the surface into an unsaturated vapor phase.

Drying and Evaporation are distinguishable by relative quantities of liquid removed from the solid. : 

Drying and Evaporation are distinguishable by relative quantities of liquid removed from the solid . In evaporation water boils until it reaches it boiling point while drying just passage of heat until the substance get dried. EVAPORATION DRYING In evaporation, the product obtained is either concentrated solution or suspension or wet slurry. In drying, dry solid is the product.

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Non-thermal methods of drying:- Expression Extraction Adsorption Absorption, e.t.c .


Expression Squeezing of a wetted sponge


Adsorption… Of water from solvent by use of desiccants, such as anhydrous calcium chloride.

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Extraction… of liquid from solid by use of solvent Absorption … of moisture from gases by passage through sulfuric acid column


Applications…. Preservation of drug products Preparation of bulk drugs Improved handling Improved characteristics

Preservation of drug products….: 

Preservation of drug products…. Drying is necessary in order to avoid deterioration. A few examples are… --blood products, tissues… undergo microbial growth --effervescent tablets, synthetic & semi synthetic drugs undergo chemical decomposition.

Preparation of bulk drugs….: 

Preparation of bulk drugs…. Drying is the final stage of processing . Eg : *dried aluminium hydroxide *spray dried lactose *powdered extracts (in fig..chamomile powder), e.t.c

Improved characteristics… drying produces materials of spherical shape, uniform size, free flowing & enhanced solubility. Some specific areas of imp are…: 

Improved characteristics… drying produces materials of spherical shape, uniform size, free flowing & enhanced solubility. Some specific areas of imp are… 1. Granules are dried to improve the fluidity & compression characteristics. These are essential for production of tablets and capsules. 2. Viscous & sticky materials are not free flowing. Drying modifies these characteristics

Improved handling….: 

Improved handling…. Removal of moisture makes the material light in weight and reduces bulk. Thus cost of transportation will be less & storage will be efficient. If moisture is present, size reduction of drugs is difficult. Drying reduces the moisture content.

Theories of Drying….: 

Theories of Drying…. Drying involves both heat transfer as well as mass transfer

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The rate of evaporation is related to rate of heat transfer by the equation: (temperature differential) dW / dø = q/ λ -------❶ Where, dW / dø = rate of evaporation q = rate of heat transfer λ = latent heat of vapourization of water

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The rate of diffusion of moisture into air is expressed by rate equations as : (humidity differential) dW / dø = K´A(Hs-Hg) ---------- ❷ Where, dW / dø = rate of diffusion k´= coefficient of mass transfer (it is not constant,but varies with velocity of passing air stream) A = area of evaporating surface Hs = abs.humidity at evaporating surface Hg = abs.humidity at passing air stream

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After initial period of adjustment, the rate of evaporation is equal to rate of diffusion of vapor and rate of heat transfer… dW / dø = q/ λ = K´A(Hs-Hg) ------❸ if overall heat transfer, q , is expressed as sum of rates of heat transfer by conduction, convection and radiation, eq.3 is expanded to form : dW / dø = (qc + qk + qr )/ λ = K´A(Hs-Hg) ------ ❹ rate of drying can be accelerated by increasing any of the individual terms in eq.4.

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Drying of solids: The moisture in a solid can be expressed on a wet-weight or dry-weight basis. Wet-weight basis : loss on drying %LOD = wt. of water in sample × 100 tot. wt.of wet sample Dry-weight basis: moisture content %MC = wt.of water in sample × 100 wt.of dry sample

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Drying rate = wt of water in sample__ time × wt of dry solid a graph was drawn b/w Drying rate vs MC gives information about periods of drying. FMC: amt of water i.e free to evaporate from solid surface. FMC = tot. water content - EMC EMC: amt of water present in solid which exerts V.P equal to that of V.P of atmosphere surrounding it. Moisture in solid > EMC = desorption (lose water) Moisture in solid < EMC = sorption (gain water)

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Emc curve can be drawn b/w %RH on x-axis & % MC on y-axis. This EMC curve permits us the selection of exp conditions to be used for drying of a product. Such that over drying can be avoided by stopping when MC reaches level of EMC.

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1. Material not agitated : static bed dryer : mechanism: no relative movement b/w solid particles eg : tray, freeze dryers Adv: attrition is not observed Dis : only a fraction of solid is directly exposed. 2. Material agitated : a) moving bed dryers b)fluidized bed dryers c)pneumatic dryers

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A)Moving bed dryers : Mechanism: particles are partially separated so that particles flow over each other. Eg : drum dryer Adv: entire mass is continuosly exposed to heat surface. Dis : attrition is possible

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B) Fluidised bed dryer : Mechanism: particles partially suspended in an upward moving heated gas system Eg : fluidised bed dryer Adv: uniform drying Dis : attrition C) Pneumatic dryer : Mechanism: drying particles are entrained & conveyed at high velocity gas stream. Eg : spray dryer Adv: efficient & rapid drying.

Freeze dryer:: 

Freeze dryer:

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Principle: water removed from frozen state by sublimation( direct change of water from solid to vapor phase without forming liq phase ). Drying is achieved by subjecting material below triple point (4.579mm hg & 0.0099°C)

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Working of freeze dryer consists of following steps: 1. prepn & pre-treatment 2. pre-freezing to solidify water 3. primary drying 4. secondary drying 5. packing.

-Naveen chand.V: 

- Naveen chand.V