Name of presentation

Marmara University Chemical Engineering

Drying of Ceramic Ware

By Asila Osamn 524513921

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Outline IntroductionFactor Affecting Drying Of Ceramic Drying period Rate of dryingequilibrium moisture content of the ceramic waresCombined Convection Radiation and Conduction Heat Transfer in Const Rate PeriodLiquid Diffusion of moisture in Drying

Introduction drying generally refer to the removal of liquid from a solid by evaporation commonly it involves transfer heat from the surrounding environment such as flowing warm air to the most article on the simultaneous transfer of vapor in the reverse direction . The process is accompanied by the movement of moisture either liquid or vapor within the pores of the body and often by shrinking of article

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Factor Affecting Drying Of Ceramic Permeability of the clay body is laying largely affected by the amount of non plastics the clay.Mineralogy of the clay also affects the rate of drying in the same clay minerals such as the smectite group for example absorb a great deal of water and takes long to dry than clay minerals (like halloysite) that does not absorb so much water.

Factor Affecting Drying Of Ceramic Size and shape of body affects the drying processing that larger pieces require more time to dry than smaller pieces for example W. Arthur White (personal communication) reports that some pieces of China Candry in a couple of hours where as large as sewer pipes may require a week to dry under controlled temperature and relative humidity.

Drying Period The process of ceramic drying is perform in three period :

Rate of Drying of ceramic

Constant Rate Period In the const rate drying period the surface of solid is initially very wet and continuous film of water exist on the drying surface. The heat transfer from surrounding air by convection and the rate of vaporization during CRP is independent of material being dry , the evaporation is close to that from open dish of liquid .

First Falling Rate The rate of vaporization decrease and temperature of surface is rise above the wet bulb temperature The rate of vaporization control by diffusion of vapor through the solid and also by moisture move through the capillaries and voids of the solid by capillary action .

Second falling RateAs the dry front recedes further into the body if the body is thick enough flow of liquid becomes so slow that liquid near the surface of body becomes isolated in pocket the liquid in this region is said to enter the pendulum condition flow of liquid to the surface stop and liquid is removed from the body only by diffusion of it vapor.

Rate of drying A. Constant rate period 1. Method using drying curve This method based on actual experimental data obtained under conditions where the feed material relative exposed surface area gas velocity temperature and humidity are the same as in the final drier then the time required for cost rate period can determined directly

A. Constant rate period2. Using the rate of drying curve for constant rate period:

Where, R is the drying-rate flux, Ls is the mass of bone dry solid A is the interfacial area of contact

A. Constant rate periodPredicting constant rate Rcheat transfer by convection neglect heat transfer by radiation to the solid surface. Assume no heat transfer by conduction from dryer surfaceq = h(T-Tw)A

NA=ky(yw-y)

where h is heat transfer coefficient (W/m2K) , T gas temperature Tw the solid yw=mole is mole fraction of water vapor in the gas at the surface and y is mol fraction in gas ky is mass transfer coefficient in kg mol/s MB is the molecular weight of air MA is the molecular weight of water H is humidity of gas in kg water/ kg dry air Hw humidity of gas at the surface in kg water/ kg dry air

Heat Transfer Coefficient The value of heat transfer coefficient in (W/m2.k) can be calculate according to air mass velocity GFor G (2450-29300 kg/h.m2):

For G (3900-19500 kg/h.m2

Effect of process variable on constant Rate Period Effect of air velocity: when conduction and radiation heat are not present the rate Rc is proportional to h and hence to G0.8 . The effective of velocity is less important when radiation and conduction are present. Effect of gas humidity : if the gas humidity H is decreased the wet bulb temperature Tw will decrease then Rc will increase Effect of gas temperature: if the gas temperature T is increased Rc increasing

B. Falling Rate drying period In falling rate drying period the rate is not constant but decreasing we can find the time of drying is the integration of rate relation

We can use numerical integration or we can integrated analytically

1.Rate is a linear function of X R= aX+b Where a is the slop of line and b is a constant the drying time is

2. Rate is a liner function through originR=a XThe drying time is

Equilibrium Moisture Content of the Ceramic Wares if you ceramic wares containing moisture is brought into contact with a stream of air after exposure of the solid sufficiently long for equilibrium to be reached the solid will attain a definite moisture content .This is known as the equilibrium moisture content

Equilibrium moisture content of the ceramic wares

Combined Convection Radiation and Conduction Heat Transfer in Const Rate Period Often the drying is done in an enclosure where the enclosure surface radiates heat to the drying solid . Also in some case the solid may be resting on metal tray and heat transfer by conduction through the metal to the bottom of solid The total rate of heat transfer to the drying surface :

Where qc is the convection heat transfer from the gas at T to the solid surface at Ts in W qR is the radiation heat transfer from the surface and qK is rate of heat transfer by conduction from the bottom .

The rate of convection heat transfer

The heat of radiation is

Where hR is the radiant heat transfer coefficient

The heat by conduction is

Where zM the metal thickness in m , kM the metal thermal conductivity in W/m.K , zS the solid thickness in m , kS the solid thermal conductivity in W/m.KThe rate of drying is

Liquid Diffusion of moisture in Drying When liquid diffusion of moisture controls the rate of dryng in falling rate period Fick's second low can be used

Where DL is liquid diffusion coefficient in m2/h the n the equation may be integrated by the

Where X=average free moisture at time t X1= initial free moisture X* = equilibrium free moisture x1 is the thickness of slab If long drying time only the first term is significant

Then the rate of drying when diffusion control

Effect of porosity on drying rate

It was confirmed that the heat transfer through the ceramic layer was more intense in the case of the dense droplet than the loose one due to the enhanced thermal conductionThe mass transfer was facilitated in the crust layer of loose droplet owing to the large pore space available for diffusion of water vapor from the evaporation interface to the outer droplet surface.

References Christie . J. Geankoplis , Transport processes and unit operations , third edition, 1993, Prentice Hall interation Inc, New Jersey Deane Aranold , Ceramic theory and cultural process, 1985, Press Syndicate of the university of Cambridge, New York .Mohamed N Rahaman , Ceramic Processing ,2007, CRC Taylor & Francis Group , United Stated of America.

Thank You

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