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DRYING Drying denotes the separation of volatile liquids from solid materials by vaporizing the liquid and removing the vapour Hot air is blown over wet solids. The water evaporates, leaving a dry solid product. Over 85 percent of industrial dryers are of the convective type with hot air or direct combustion gases as the drying medium. Over 99 percent of the applications involve removal of water. Drying - final removal of water, or another solute often follows evaporation, filtration, crystallisation. Carried out for the following reasons: (a) To reduce the cost of transport. (b) To make a material more suitable for handling e.g. soap powders, dyestuffs and fertilisers. (c) To provide definite properties, e.g., maintaining the free-flowing nature of salt. (d) Drying or dehydration of biological materials, especially foods, is used for preservation Sometimes drying is essential part of the process, e.g. in paper making or in the seasoning of timber

almost all drying processes involve the removal of water by vaporisation -requires addition of heat. The effective utilisation of the heat supplied is the major consideration.

DRYER TYPES Drying processes can be classified as batch material is inserted into the dryer and drying proceeds for a period of time, or as continuous material continuously added to the dryer and dried material continuously removed. solids exposed to heated gas by various methods: Blown across the surface - cross circulation Blown through a bed of solids supported on a screen - through-circulation Dropped slowly through a slow moving gas stream - rotary dryer Blown through a bed of solids that fluidize the particles - fluidized bed dryer Solids enter a high velocity hot gas stream and conveyed pneumatically to a collector - Flash Dryer

Continuous Tunnel Dryers (Belt Dryer) Cross flow of hot gas

rotary dryer consists of a hollow cylinder which is rotated and slightly inclined toward the outlet. Wet granular solids fed at the high end moves through the shell as it rotates. Heating by direct contact with hot gases in counter-current flow or by indirect contact through the heated wall The granular particles move forward slowly a short distance before they are showered downward through the hot gases as shown.

Drum Dryers A drum dryer consists of a heated metal roll shown in Fig. on the outside o[which a thin layer of liquid or slurry is evaporated to dryness. The final dry solid is scraped off the roll, which is revolving slowly.

Spraying (atomization) of product increases surface area of contact with hot air for drying spray dryer - liquid or slurry solution sprayed in to a hot gas stream in the form of a mist of fine droplets - water rapidly vaporized from droplets,

leaving particles of dry solid which are separated from the gas stream. Three basic unit processes are involved in spray drying: liquid atomization, gasdroplet mixing, and drying from liquid droplets..

MOISTURE IN SOLID MATERIALS Water contained within solids is in two forms, unbound or free water and bound water. Unbound water is free to be in equilibrium with water in the vapour phase; thus, unbound water exerts vapour pressure equal to that of pure liquid at the same temperature. Unbound water is mainly on the surface or held in the voids of the solid. Bound moisture - Liquid physically and/or chemically bound to solid matrix so as to exert a vapour pressure lower than that of pure liquid at the same temperature can exist in several conditions: (1) water in fine capillaries that exerts an abnormally low vapour pressure because of the highly concave curvature of the surface, (2) water containing a high level of dissolved solids, and (3) water in physical or chemical combination with the solids. Solids containing bound water are called hygroscopic;

Driving force of drying phenomena - Equilibrium moisture of drying gas is less than in the material to be dried \

Equilibrium Moisture Content Of Materials When a solid is exposed to a continual supply of air at constant temperature and humidity, having a fixed partial pressure of the vapour, p, the solid will either lose moisture by evaporation or gain moisture from the air until the vapour pressure of the moisture of solid equals p. The solid and the gas are then in equilibrium, and the moisture content of the solid in equilibrium with the surrounding conditions is known as equilibrium moisture content E.M.C.

Equilibrium data for moist solids in contact with humid air give valuable information about the water capacity of the solids. The water content of the solid is plotted as a function of the relative humidity of air Curves of the type shown are nearly independent of temperature

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The moisture content of a solid cannot be lower than the equilibrium moisture content corresponding the humidity of the incoming air. e.g. for 50% RH air equilibrium moisture for Wool =12.5 % and Newspaper = 5.5% A non-porous insoluble solid, such as sand or china clay, has an equilibrium moisture content approaching zero Bound Water. Vapour pressure of moist component (water) is less than free water Moisture is bound to the material in such a way that lowers the vapour pressure Water is in capillaries, cells and/or fibres, ”keeping hold” of the water

Unbound Water Vapour pressure of moist component (water) is constant Equal to vapour pressure of water at temperature of solid material Water is on the surface of solid material, material does not affect

e.g. a wood sample contains 34 kg H20/100 kg dry solid, Determine the amount of bound and unbound water Free moisture, Xf Xf = X - X* Moisture content in excess of the equilibrium moisture content (hence free to be removed) at given air humidity and temperature e.g. a sample of silk contains 10 kg H20/100 kg dry material, in contact with air of 50% relative humidity and 25°C. Determine the amount of water removable by drying i.e. determine the free moisture content

EXAMPLE 11.1 Air at 1 atm and 25°C with a RH of 50% is to be heated to 50°C and then to be used in drying wet crystals of the antibiotic cefazolin sodium. The wet crystals contain 30 g of water per 100 g of dry antibiotic. In the drying process, the air at 50°C and the crystals reach equilibrium with respect to the moisture. Determine the following: the % of bound and unbound water in the wet crystals before drying, the moisture content of the crystals after drying, and the water partial pressure at the drying temperature

The equilibrium moisture curve for cefazolin sodium in the Figure can be extrapolated to 100% relative humidity to give a water content of 23 g/100 g dry weight. The water corresponding to concentrations lower than 23 g/100 g dry weight is bound water, because it exerts a vapour pressure less than that of liquid water at the same temperature. The balance of water contained in the crystals is therefore unbound water. Thus,

Using the psychrometric chart at a temperature of 50°C, relative humidity of the air is 13%. At this relative humidity, from the EMC for cefazolin sodium Moisture content of antibiotic after drying = 8 g water / 100 g dry weight assumed that the equilibrium curve is independent of temperature for the temperature range in this example

Now

From psychrometric chart at 13% relative humidity and 50°C,

so that the water partial pressure at 50°C is

DRYING RATES Drying Rate Curves for Convection Drying drying rate curves are experimentally determined Data obtained from a batch-drying experiment – W total weight of the wet solid (dry solid plus moisture) at different times t hours in the drying period. converted to rate-of- drying. If W is the weight of the wet solid in kg total water plus dry solid and Ws is the weight of the dry solid in kg, then the moisture content at time t, Xt

Then the free moisture content Xf in kg free water/kg dry solid is calculated for each value of Xt . Xf = Xt - X* a plot of free moisture content Xf versus time t in h is made

To obtain the rate-of-drying curve from this plot, the slopes of the tangents drawn to the curve in Fig. can be measured, which give values of dX/dt at given values of t. The rate R is calculated for each point by