drying

DRYING

Dr. Ibtihal O. Alkarim


Definition Vaporization and removal of water

and/or other liquid from a solution, suspension or other solid-liquid mixture to form a dry solid.


DON’T CONFUSED DRYING WITH EVAPORIZATION

EVAPORIZATION is more often applied to the concentration of solution.

But drying associated with the removal of relatively small quantities of liquid to give a dry product.


IMPORTANCE1. Preservation2. Preliminary to size reduction3. Improve Properties of final product 4. eliminate moisture which may lead to corrosion and decrease

the product or drug stability.5. good properties of a material, e.g. flow-ability, compressibility.6. reduce the cost of transportation of large volume materials

(liquids)7. material easy or more suitable for handling.


Theory of Drying

HEAT TRANSFE

R

MASS TRANSFE

R DRYING


Rate of evaporization dw/dϕ = q/λ Rate of diffusion dw/dϕ = KA(Hs-Hg) dw/dϕ = q/λ= KA(Hs-Hg) dw/dϕ = (qc+qr+qk)/λ= KA(Hs-Hg)


GENERAL PRINCIPLES

General

terms

Saturated humidity

Absolute humidity

Relative humidity

Dew point

Moisture

content


GENERAL PRINCIPLES ABSOLUTE HUMIDITY: weight of water vapor per

unit weight of dry air. SATURATION HUMIDITY: is the absolute humidity

at which the partial pressure of water vapor in the air is equal to the vapor pressure of free water at the same temperature.

RELATIVE HUMIDITY: expresses the degree of saturation


PRINCIPLES… MOISTURE CONTENT: usually determined

by measuring the los of weight of a sample when heated in an oven

DEW POINT: is temperature to which a given mixture of air and water vapor must be cooled to became saturated.


HUMIDITY MEASUREMENT

Gravimetric method

Sling pyschromet

erDew point Electric

hygrometerMechanical hygrometer

GRAVIMETRIC METHOD Moisture

absorbing material such as Phosphorus Pentoxide

Known amount of air



SLING PYSCHROMETER


DEW POINT Temperature at

which moisture begins to form on a polished surface in contact with air.


MECHANICAL HYGROMETER Moisture-sensitive

element


ELECTRICAL HYGROMETER measurement

resistance in moisture-absorbing material.

Equilibrium moisture content (E.M.C)

Exposure to air at a definite temperature and humidity will cause a material to lose or gain moisture until an E.M.C is attained.

Starch /Talc OVERDRYING!!!!!



RATE OF DRYING


A-B initial adjustment B-C constant rate period C-D first falling rate period D-E second falling rate period


DISRIBUTION OF MOISTURE


DISRIBUTION OF MOISTURE1- Capillary state Removal of water at one surface causes a general flow through the bed.

DISRIBUTION OF MOISTURE….


2- Funicular stateMovement of water still possible BUT restricted by friction Film around the particle is lost. In LENS-LIKE RING


DISRIBUTION OF MOISTURE…3- Pendular stateAir movement is possible.Migration of water only take place by evaporation


CLASSIFICATION OF DRYERS


Nature of materialAbsence or presence of agitation

BASED ON THE METHOD OF SOLID HANDELING


STATIC MOVING FLUID PNEUMATIC


Static bed dryer No movement Only fraction exposed directly thichness


Moving bed dryer Gravity or mechanical agitation More rapid mass and heat transfer


Fluid bed dryer Suspended in upward moving gas

stream


Pneumatic bed dryer Entrain and conveyed in high velocity

gas stream Very rapid mass and heat transfer Shortening drying time


Demonstrate major differences in dryer design, operation, and energy requirement

BASED ON METHOD OF HEAT TRANSFER


CONVENCTION

COMPARTMENTAL

TUNNELROTARY

FLUIDIZED

CONDUCTION

VACUUM

FREEZ


Shelf dryer Arrays Air gaps Heater and fan Very common versatility


Tunnel dryer Same as shelf

dryer but in a tunnel shape


Tunnel dryer


Rotary dryer Individual drying Large scale Continuous drying Air rate,

temperature, mean time


Fluid-bed dryer


Fluid-bed dryer Advantages:1. Efficient heat and

mass transfer2. Individual drying3. Uniform drying4. Produce free flowing

product5. Short drying time

Disadvantages:1. May cause

attrition2. Loss of fine 3. Generation of

charges


Vacuum dryer Stout

construction Work under

pressure 0.3-0.06 bar at which water boils at 25-35C.

Unstable material.


Freeze dryer

drying

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