Drying unit operation

1. By: Mujeeb Qureshi
DRYING….

2. Content:
Introduction to Drying.
Principles of Drying..
Types of Dryer.
Classification of Dryers.
.Temperature Patterns in Dryer
Heat Transfer in dryer
Phase Equilibria
•
•
•
.The Drying Curve.

3. Drying a Solid:
Removal of relatively small amount of water or other liquid
from solid material.
Reduce the content of residual liquid to an acceptable low
value.

4. What is Drying?
Drying is a unit operation in which a
liquid is separated from a solid.
This generally requires supplying heat
and resulting in evaporation of liquid.

5. Drying Principle:
Migration of moisture
from the interior of a
material to its surface
Evaporation of moisture
from surface to the
surrounding air.

6. • In pharmaceutical technology, drying is
carried out for one or more of the following
reasons:
1. To avoid or eliminate moisture which may
lead to
corrosion and decrease the product or drug
stability.
2. To improve or keep the good properties of
a material, e.g. Flowability, compressibility.
3. To reduce the cost of transportation of
large volume materials ( liquids).
4. To make the material easy or more
suitable for handling.
5. Preservative.
6. The final step in Evaporation, Filtration,
Crystallization

7. Difference between drying
and evaporation:

8. Feed Stock for
Drying Process:
Wet Solids
Slurry
Solution
Suspension

9. OPTIONS AVAILABLE
TO REDUCE / REMOVE LIQUID FROM SOLIDS
Mechanically by PRESS
Centrifuges
Drying (thermally by Vaporization)

10. Location of Liquid to be Vaporized.
May be on the surface of the solid (e.g. Drying
salts, crystals)
May be inside the solid (e.g. Solvent removal from
sheet of polymer)
May be partly outside and partly inside
Feed fordryers may in the form of
Liquid in which the solid is suspended as particles.
Solution.

11. Which type of
Industry
uses
Dryers
?

12. Textiles Industries
Paper and Allied Product Industries
Chemical Industries
Food Industries
Herbal Industries
Pharmaceutical Industries
Dairy Industries
Tea Industries

13. Different FORMS of Solids.
1. Flakes
2. Granules
3. Crystals
4. Powders
5. Slabs
6. Continuous sheets
All
have
widely
different
properties

18. Types of Dryers on the basis of feed
DRYERS FOR SOLIDS:
TRAY DRYER
SCREEN-CONVEYOR DRYER
FLUID BED DRYER
ROTARY DRYER
TOWER DRYER
SCREW-CONVEYOR DRYER
FLASH DRYER

19. Dryers for slurries:
 Spray Dryer
 Thin Film Dryer
 Drum Dryer

20. Classification of DRYERS
1. Adiabatic orDirect Dryers
Dryersthat exposethe solids to ahot gas (usuallyair)arecalled
AdiabaticorDirect Dryers.
2. Non-Adiabatic orIndirect Dryers
Dryersinwhichheatistransferredfromanexternal
mediumareknown asnon-adiabaticorIndirectDryers.

21. Solids Handling in Dryers
Heat is important only to describe the different patterns of motion of solid particles through
dryers
In Adiabatic Dryers solidsareexposed
to thegasinthefollowingways.
1.Cross-Circulation Drying:
Gas isblown acrossthesurface of abedor slabof solids.
2.Through Circulation Drying:
Gas isblown through abedof coarsegranularsolidsthatare
supportedon ascreen
3.Solidsareshowereddownwardthrough aslowly movinggas streaminarotary dryers
4.Gas passes through thesolidsat a velocitysufficientto fluidize
the bed
5.Solidsareallentrainedinahivelocity gasstreamandare pneumaticallyconveyedfrom amixingdevice
to amechanical separator.

23. In Non-Adiabatic Dryers solids are exposed to the hot surface or
other source of heatinthefollowing ways.
1.Solidsarespreadoverastationary orslowly moving
horizontalsurfaceand
cookeduntildry.
Surfacemaybeheated
Electrically
Heattransferfluidstreamorhot water
Radiant heater.
2.Solidsaremovedoveraheatedsurface,cylindrical by an
agitator ora screw orpaddle conveyer.
3.Solidsslidebygravity overaninclinedheatedsurface

24. Commercially DRYERS are chiefly differ in
The way of solid are moved through the drying zone
The way in which heat is transferred
Some dryers are Continuous or other are Batch wise
Some dryers Agitate the Solids and other are Essentially
un Agitated
Some dryers can handle almost any kind of material
while others are severely limited to
specified type of feed

25. Major Division of Dryers on the basis
of mode of heat transfer
1. Dryersinwhichthesolidisdirectlyexposedtoahotgas(usually air)
2. Dryers in which heat is transfer to the solid from anexternal medium such as condensing
steam
3. Dryersthatareheatedbydielectric,radiantormicrowaveenergy
4. Dryerswhichusesmorethenonemodeof H/T
e.g.Hotgaswithheatedsurface
HotgaswithRadiation
Temperaturevarydependsupon
Natureandliquidcontentof thefeedstock
Temperatureof heatingmedium
Drying Time
Allowablefinaltemperatureof thedry solids

26. Typical Temperature Pattern
Batch Dryers
Withheatingmediumatconstanttemperature.
Tsa= feedtemp./Initialsolidtemp.
Tsb= FinalSolidstemp.
Tv= Vaporization temp.
Th= constantheatingmediumtemp.
Inanon-adiabaticdryerswithnosweepgas
Tv isessentiallytheB.Pof theliquidatthe
pressureprevailinginthe dryers.
Inaadiabaticdryerswithsweepgas
Tv isneartheWBT of thegas.

27. Typical Temperature Pattern
cont.
Continuous Dryers
In steady-state operation, the temp at any given
point in a continuous dryer is constant, but it
variesalongthelengthof the dryers.
Tsa= feed temp./Initial solid temp.
Tsb= FinalSolidstemp.
Tv= Vaporization temp.
Thb= gas temp. at inlet
Tha= gastemp.atoutlet Temperature pattern for an adiabaticcountercurrent dryer

28. Heat Transfer in Dryers
By definition it is athermal process
It is often complicated by diffusion in the
solid or through agas.
Solids can be dried by exposure to highly
superheated steam, there are no diffusion
limitation.
Surface temp. depends on the rates of
mass transfer and heat transfer and
this concept is used in dryer calculation.

29. Calculation of Heat Duty
Heatmustbeappliedto adryerto accomplishthe following.
1. Heatthefeed(solids& liquids)to thevaporizationtemp.
2. Vaporizetheliquid.
3. Heatthesolidto theirfinal temp.
4. Heat the vapor to its final temp.
5. Heat the air or other added gas to its final temp.
All the other items are often negligible compared to the vaporization of
liquid.

30. Total rate of heat transfer
If msisthemassofbone-drysolidsto bedriedperunittime.
Xa andXb aretheinitialandfinalliquidcontentinmassof liquidperunitmassof bone-drysolid.
HeatqT transferredperunitmassofbone-drysolidperunit time
qT = cps (Tsb-Tsa) + Xa cpl (Tv - Tsa) + (Xa-Xb) λ
ms + Xb cpl (Tsb - Tv) + (Xa –Xb) cpv (Tva - Tv)
Where,
Tsa–feedtemperature
Xa–initialliquidcontents(massof liquidperunitmassof bone-dry solid)
Xb –finalliquidcontents
λ–heatof vaporization
cps,cpL, cpv–specific heats

31. Inadiabaticdryerheattransferredto thesolids,liquidsandvapor,comesfromthe
coolingof gas.
Forcontinuousadiabaticdryerstheheatbalancegives.
qT = mg Csb (Thb - Tha)
Where,
mg=massrateof dry gass
Csb = humidheatof gasatinlethumidity

32. Basic heat transfer
equation
q= UA∆T
q= rateof heattransferinasectionof thedryer
U= Overall co-efficient
A= Heattransferarea
∆T = Averagetemp.difference
Value of U can be predicted
Empirical correlation
Experimental data
Theory and correlation

33. Basic heat transfer
equation (cont)
Value of A
Fortray Dryers and moving Belt Dryers.
Aistheareaof horizontalsurfacecarryingthewet solids
ForDrum Dryers.
Ais the active surface area of the drum
ForThrough-Circulation Dryers.
Ais the total surface area of theparticles
ForScrew Conveyor Dryers orRotary Dryers
The effective area for heat and mass transferis hard to determine. Such dryers are design on the basis of a
volumetric heat transfercoefficient (Ua)

34. Mechanism of drying depends on the
• natureof solids
• methods of contracting the solids andgas.
Solids are of three kinds.
1. Crystalline:No Interior liquid, drying occur at the. surface
2. Porous : such as pallets, contain liquid in Interior channel.
3. Non- Porous : such as soap, glue and plastic clay, dense cellular
solids such as wood and leather and many polymeric materials.

35. For each and every product, there is
a representative curve that describes
the drying characteristics for that
product at specific temperature,
velocity and pressure conditions.
This curve is referred to as the
drying curve for a specific product.
Variations in the curve will occur
principally in rate relative to carrier
velocity and temperature.