principle and mechanism of drying

7/29/2019 principle and mechanism of drying

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Drying is the thermal process through whichliquid is separated from a solid-liquid

mixture by the application ofheat and alsoby accelerating evaporation throughvacuum.

It is accomplished by transfer of the liquidfrom a surface into an unsaturated vapourphase


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Drying & evaporation are distinguishable bythe relative quantities of liquid removed from

the solid.

In evaporation, theproductobtained is eitherconcentrated solution or suspension or wetslurry.

In drying, dry solidis the product.


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The rate of evaporation depends upon:

o Nature of the liquid, which is to evaporateo Amount of heatappliedo Nature of air saturationo Surface area of the solid or solid mixture


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1. Stability

Dried products are often more stable thanmoist ones because here, deterioration canbe avoided e.g. effervescent salts, penicillinetc.

Actually drying inhibits microbial growth &chemical decomposition.


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2. Preparation of bulk drugs:Drying is also used in the preparation of bulk drugs

where this is the final stage of processing. e.gPreparation of dried aluminium hydroxide.

3. Preparation of granules:

Drying is most commonly used in pharmaceuticalmanufacturing as a unit process in thepreparation of granules, which can be dispersed

in bulk or converted into the tablets or capsules.


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4. Flow property of powder: Flow property of a powder or granules is greatly

dependent on the moisture content.

Viscous & sticky materials are not free flowing.

In pharmaceutical manufacturing of tablets andcapsules, drying is essential to maintain the flowproperty of the granules to improve the fluidity& compression characteristics.


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5. Improved handling:Drying can also be used to reduce bulk &

weight, thereby lowering the cost oftransportation & facilitating storage.

6. Sterilization:Drying is also important for sterilization of

many pharmaceutical equipments or glasswares.


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The amount of water vapor in the air at anygiven time is usually less than that required to

saturate the air. The relative humidity is the percent of

saturation humidity, generally calculated inrelation to saturated vapor density.

Relative Humidity(RH)=Actual vapor density

Saturation vapor density

X 100
http://hyperphysics.phy-astr.gsu.edu/HBASE/Kinetic/vappre.htmlhttp://hyperphysics.phy-astr.gsu.edu/HBASE/Kinetic/vappre.html


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Its approximately equal to the percentsaturation which is =mass of vapor present

per kg of dry air/mass of vapor required tosaturate 1 kg of air at the same temperaturex100

So, RH of air is dependent both on theamount of moisture in the air & on itstemperature.


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This is the water content of a material that iscalculated as the percentage of the weight of

the wet solid.

LOD= Weight of water in sample (kg)/Totalweight of wet sample (kg) x 100


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BOUND MOISTURE: This is the minimumwater held by the material that exerts an

equilibrium vapor pressure less than the purewater at the same temperature.

UNBOUND MOISTURE: This is the amount ofwater held by the material that exerts anequilibrium vapor pressure equal to that ofpure water at the same temperature.


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This is the amount of water present in the solidthat exerts a vapor pressure equal to the vaporpressure of the atmosphere surrounding it.

At EMC, the materials neither loose moisturenor gain moisture


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It helps in selecting proper drying conditions

It helps in selecting proper storage conditions


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Temperature of surrounding air 1/EMC

Relative humidity EMC

Nature of solid: For non-porous, non-hygroscopic materials, EMC is essentiallyzero at all temperature & humidity. Forcolloidal or fibrous materials, EMC is higher


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Basically drying process involves twophenomena---

a) Heat transfer &

b) Mass transfer


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Heat must be transferred to the material to be

dried in order to supply the latent heat requiredfor the vaporization of moisture.

Heating source may be flame, steam, hot gas,

hot air etc.


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Heat may betransferred in the

following ways:1. Conduction

2. Convection

3. Radiation


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Transfer of heatbetween

substances thatare in directcontact witheach other. It isan indirectprocess.


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It is a very simple method

Cheap and rapid drying process

The wet material does not come in directcontact with heat


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Heat distribution is uneven

Stirring of material is required during drying

Compact materials require preliminary

granulation


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A heating process bywhich heat from thesource is directlyapplied to the solid-liquid mixturethorough a media.

The heating mediamay be hot air orhot gas.


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It is a simple method

More uniform distribution of heat occurs

Prompt drying is obtained

Control of heat is easier


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Fixing of temperature is essential

Product loss may occur if a stirring device ismaintained.


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The process ofheating where

radiant energy isused as heatingsource.

Usually infraredradiation is used.


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It is a unique process where drying period is

short

Heating source can easily be stopped


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It is very expensive

Stirring is required for even drying

Worker safety problems


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When heat is applied to the wet material,water is removed from the material as vapor.

This is called mass transfer.

Mass transfer involves the diffusion of waterfrom the interior to the evaporating surfaceand subsequent evaporation of water fromthe surface to the passing air stream.


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Mass transfer occurs by two mechanisms:

1. Internal mechanisms

2. External mechanisms


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1. DiffusionIt is an important mechanism by which water is

transferred through the material toevaporating surface & the resulting vapor istransferred to the passing air stream.


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2. Flow caused by shrinkage & pressuregradients

On heating when water is removed, the poresbecome empty and solid particles take theempty place and the solid shrinks. Whenparticles shrink, a tremendous pressure acts onthe squeezed surface and the surface becomescracked. So hot air or heat reaches quickly inthe inner area and the drying rate is enhanced.


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3. GravityThe moisture content of a wet solid is

gravitationally pulled downward and tends toaccumulate beneath the lower surface of thesolid particles. Water comes downwards

through the inner capillaries.


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4. Vaporization condensation sequence: When heat is applied, the water from the surfaces

of the solid granules becomes vaporized and it

tries to expel out but fails to do so because thesurface is rigid.

This trapped moisture is condensed when comes

in contact with relatively less heated area. Again heat is applied to vaporize the moisture

and a series of vaporization-condensation

sequence occurs until finally the vapor comes out.


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5. Capillary flow: When capillary tube is immersed into water,

water enters into it and climbs up to a height,which is due to surface tension and capillaryaction.

In this case, inner moisture expels out throughthe continuous intermolecular space of thegranular particles due to surface tensioncapillary action.


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1. Temperature:The rate of drying is directly proportional to the

temperature.In higher temperature rapid drying occurs. Butexcess heat can hamper the stability of theproduct, especially for the thermo labileones.


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2. Humidity:Rate of drying is inversely proportional to

humidity of surrounding area.If the surrounding area is saturated withmoisture, application of even highertemperature dose not influences drying. i.e.

less humid or less saturated area is neededfor better drying.


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3. Agitation:Agitation of material from time to time enhances

evaporating by bringing the unexposed area ofthe particle to heat contact.Otherwise some particles will remain wet & some

will be burnt.

Adequate mechanical agitation enhances dryingthrough a uniform heat distribution.


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4. Method of supporting the solid:The different types of dryer support the materials in

different ways and mechanism of applying heat is

different.Metallic tray is preferable to others because it can be

sufficiently hot which will facilitate drying process.

The relative drying capacity of different types ofbeds is:

Dilute bed > Fluidized bed > Moving bed > Staticbed.


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5. State of subdivision:If the particles are more subdivided, the surface

areas are increased & more surfaces areexposed to heat, thus drying occurs rapidly.

6. Nature of the particle: Porous particles are

easier to dry but fibrous materials take longertime to dry.


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7. Vacuum:Thermo labile substances are efficiently dried

by creating vacuum with low temperature.

8. Others: Contact between hot surface and wet solid

Bulk of the material Period of drying

Heating time


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Beds are the supporting devices on which thewet materials (to be dried) are placed. There

are four types of drying beds:

1. Static bed2. Moving bed3. Fluidized bed4. Pneumatic bed or dilute bed


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It is the system in which there is no relativemovement among the solid particles being

dried.Only a fraction of the total number of particles

is directly exposed to heat sources.

The exposed surface can be increased bydecreasing the thickness of the bed andallowing drying air to flow through it.


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The bed is perforated and mostly made ofstainless steel. Perforation of the bed allows

hot air to come in contact with the drugmaterials from all directions.

Example- beds present in tray dryer.


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Advantages: Very economic

Simple

Disadvantages: Non uniform distribution of heat Drying time is high Chance of burning Chance of formation of cluster


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It is the system in which the drying particles arepartially separated so that they flow over

each other.Motion may be induced by either gravity(downward flow) or the mechanical agitationor lifting (upward flow).


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The resultant separation of the particles and

continuous exposure of new surfaces allowmore rapid heat and mass transfer than canoccur in static bed.

E.g. Beds in agitated pan dryer.


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Advantages: Slightly expanded bed

Drying time is low More uniform distribution of heat Less chance of burning & cluster formulation

Disadvantages: Not simple Mechanical device is required for agitation


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Here hot air or gas at high

pressure is applied under theparticles so that they are

expanded, can move upward& then suspended in air.

The particles are lifted and

then fall back in a randommanner so that the resultantmixture of solid and gas acts

like a boiling liquid.


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The gas-solid contact is excellent and resultsin better heat and mass transfer than instatic and moving beds.

Advantages: Expanded bed

Drying time is less

Disadvantages: 1) Expensive


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Here air is applied under various pressures, soparticles are in flying condition.

This is a fully expanded condition in which theparticles are so widely separated that iscompletely surrounded by gas of highvelocity.

Thus, the resultant heat and mass transfer areextremely rapid & the drying time is veryshort.


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Advantages: More expanded bed

Least drying time

Disadvantage: Very costly


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The pattern of passing air over the material iscalled airflow and the method by which gas maycome in contact with a bed is the gas-solid

contact.The gas used for drying may be directed by the

following flow:

1. Parallel 2. Perpendicular3. Through circulation 4. Co-current5. Counter-current 6. Cross flow


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The direction of gas flow is parallel to the solidphase.

Contact occurs primarily at the interfacebetween phases, with possibly somepenetration of gas into the voids among thesolids near the surface.

The bed is usually in a static condition, i.e.parallel flow in tray dryer.


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The direction of gas flow is perpendicular to the

phase interface.

The gas impinges on the solid bed and the bed

is usually kept in a static condition.


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The gas penetrates and flows throughcirculating pores or less freely around the

individual particles.

This may occur when solids are in static,fluidized or dilute condition.


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The gas phase and solid particles both flow in

the same direction i.e. flow pattern ofpneumatic dryer.


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The direction of gas flow is exactly opposite to

the direction of solid movement, e.g. the flowof spray dryer.

Collision between air & particles are the highest

here.


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The direction of gas flow is at a right angle to

that of solid movement across the solid bed.


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Thank You

principle and mechanism of drying

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