Introduction

General Principles

Rate of Drying

Time of Drying

Drying Periods

Time of Drying

Constants Rate of Drying Period

First Falling Rate Period

Second Falling Rate Period

Total Time of Drying

Course Contents of Prof. Dr. Aamir Ijazs Portion Course Contents of Drying

Capillary Theory of Drying

Principal of the Theory

Drying of a Granular Material according to the capillary theory

Freeze Drying

Drying Equipment

Classification and selection of dryers

Tray or shelf dryers

Tunnel dryers

Rotary dryers

Design considerations

Drum dryers

Spray dryers

Course Contents of Prof. Dr. Aamir Ijaz Portion Course Contents of Drying

Course Contents of Prof. Dr. Aamir Ijaz Portion

The Drying of Gases

Drying

Drying

Crystallization

Importance of Crystal SizeCrystal Geometry Crystallographic systems Invariant crystals Crystal size and shape factors.Principals of crystallizations

Purity of ProductSolubility Vs. Temperature Equilibria yields

Phase Equilibrium DiagramPhase Equilibrium Diagram for MgSO4 - H2OYields

Enthalpy BalancesSuper-saturation Units of Super-saturation Temperature difference as a potential

Course Contents of Crystallization

Nucleation Origins of Crystals in Crystallizers Primary NucleationHomogenousEquilibrium Heterogeneous Nucleation Secondary Heterogeneous Nucleation Crystal Growth

Individual and overall growth coefficientsMass transfer coefficientsSurface-growth coefficients

Course Contents of Crystallization

The L Law of Crystal Growth Crystallization EquipmentsVariations in Crystallizers Vacuum crystallizersDraft-Tube-Baffle CrystallizersYield of Vacuum Crystallizers

Application of Principles to design Increase the crystal sizeContact Nucleation in Crystallizers Crystallization of ORGANIC Chemicals Crystallizations from Melts

Course Contents of Crystallization

Chemical Engineering, Vol. 2, by Coulson &Richardsons

Unit Operations of Chemical Engineering byWarrant L. McCabe d, Julian C. Smith, PeterHarriott

Text Books:

Reference Books:

Introduction to Chemical Engineering, by Walter L.Badger & Julius T. Banchero

Difference between Drying and Evaporation

The term drying usually infers the removal of relatively smallamounts of water from solid or nearly solid material, and the termevaporation is usually limited to the removal of relatively largeamount of water from solutions.

In drying processes the major emphasis is usually on the solidproduct. In most cases drying involves the removal of water attemperatures below its boiling point, whereas evaporation meansthe removal of water by boiling a solution.

Another distinction is that in evaporation the water is removedfrom the material as practically pure water vapor, mixed with othergases only because of unavoidable leaks. In drying, on the otherhand, water is usually removed by circulating air or some other gasover the material in order to carry away water vapor; but in somedrying processes no carrier gas is used.

Difference between Drying and Evaporation

Strictly speaking drying processes all cases of the removal of smallamounts of water from gases or liquids. For example, air issometimes dried for iron blast furnaces by passing it overrefrigerating coils which removes water as either liquid water or ice.

Organic liquids or gases are dried by passing them through a bed ofsolid adsorbent, such as silica gel or activated alumina. Occasionallythey may also be dried by countercurrent contact with strongcalcium chloride brine or caustic soda. In the past batches of organicliquids have been dried by adding calcium carbide.

Introduction to Drying TheoryThe process of drying must be approached from twopoints of view: first, the equilibrium relationships and,second, the rate relationships. Thus, there is always heattransfer to the materialin variety of ways. In addition,there are the mechanisms by which moisture (either asliquid or vapor) travels from the interior of the solid to thesurface, Since a wide variety of materials are encounteredin drying operations, and many of these may be complexsystems, such as soap, wood, textiles, etc., it is notsurprising that the equilibrium relationships may be morecomplex than those encountered in previous operations.And just as these equilibrium relationships arecomplicated, it is also to be expected that, as themechanism by which water travels through the solidvaries, the form of rate equations may also vary.

It may be noted that J=kt/l2, where k is a constant, t the time in ks and 2l is the thickness of the sheet of

material in millimeters.

They are used for lumpy or pasty in small quantity. The material is spread

uniformly over the trays and the hot air is passed parallel and over the trays. The

velocity of air varies from 1 to 10m/s and 80 to 90% of air passed is re circulated.

When drying is complete, the cabinet is opened and a new set of trays is

introduced.

In case when the material is granular, it is placed at the bottom of each tray, which

takes the form of screen. Hot air drying is passed through the permeable bed and

drying time is short due to large surface area. The drying rate is 0.2 2 kg

water/h.m2 surface and Thermal efficiency: is 20-25%.

Advantages:-

No loss of product

Low space requirement

Ease of cleaningDisadvantages:

Long drying time

High lab our cost

Tray Dryer

They are mainly used for granular solids and may be heated directly or

indirectly. It consists of hollow inclined rotating cylinder. Feed is introduced at

the upper end and moves through the shell due to rotation, head effect and

slope of cylinder. Dried product is discharged at the lower end.

In case of direct heating hot gas is passed counter currently over the material

and in case of indirect heating, heat is transferred through the wall of the

cylinder. The drying rate is 10-50kg water/h m3 shell volume and thermal

efficiency is 50 80 %.

Advantages:-

Good gas contacting

Moderate drying time

High thermal efficiency

Low capital cost

Disadvantages:

Difficult scaling

Product builds up on interior walls

High structural load

Non-uniform residence time

They are used for slurries, fine suspensions and for solutions. They consist of one or more

metal rolls heated internally by steam. Material is dried outside the roll and is scrapped by

knife scrapper from slowly revolving roll. Drum pertly acts as an evaporator and partly as

dryer. They can no be used for salt solutions with limited solubility and for abrasive

material that have tendency to settle out and create high pressure blew the drum.

It is used for the material which is in the state of fine division to increase surfaceper unit volume. A pneumatic dryer is shown in figure. Wet material is introducedinto dryer by means of some from of mechanical feeder to give short length ofmaterial, such as 5-10 mm. Hot gases from furnace are introduced from the bottomwhich picks up the particles and carry them to column. Evaporation causes dryingof material. Hot gases are discharged out from the top and the dry product iscollected at the bottom. The contact time b/w gases and particles is small, and theparticles temperature do not exceed from the temperature of the hot gases.Thermal and power requirements are 4.5 MJ/kg and 0.2 MJ/kg respectively

Pneumatic Dryers

Applications of Fluidized Bed Dryers: At large scale they are used for drying of

Fertilizers

Plastic materials

Foundry sand

Inorganic salts

At small scale they are used for drying of tablet granulations are pharmaceuticals.

Dryer evaporative capacity varies from 0.2 0.3 kg/s m2 grid area. Specific air rates are

0.5 -2 kg/s m2 grid are and take total energy demand is 2.5-7.5 MJ/kg moisture

evaporated.

Freeze Dryer:-

In this process the material is first frozen and then dried by

sublimation in a very high vacuum, 10-40 N/m2 at a

temperature for 240 260 K. During the sublimation of ice a

dry surface layer is left. During the sublimation the

temperature is so maintained that highest vapor pressure of

water vapors is obtained without melting the material.

During this stage 95% Drying is completed and then

remaining water is removed by increasing the temperature to

ambient temperature. A typical freeze dryer is shown in fig

16.35. Heat is supplied to plates which interleave with trays

containing the product either by conduction or radiation.

Sublimed moisture condenses on refrigeration coil, located at

the far end of chamber.

Advantages:

(i) Process is carried out at low temperature

(ii) It avoids surface hardening

(iii) Useful for heat sensitive materials

Application: Used for drying of

(i) Penicillin and other biological materials

(ii) Foodstuffs

(iii) Meat and vegetables.

Introduction A rigid definition of drying that shall sharply differentiate it fromevaporation is difficult to formulate. The term drying usually infers the removal ofrelatively small amounts of water from solid or nearly solid material, and the termevaporation is usually limited to the removal of relatively large amount of waterfrom so1utions. In drying processes the major emphasis is usually on the solidproduct. In most cases drying involves the removal of water at temperaturesbelow its boiling point, whereas evaporation means the removal of water byboiling a solution. Another distinction is that in evaporation the water is removedfrom the material as practical1y pure water vapor, mixed with other gases onlybecause of unavoidable leaks. In drying, on the other hand, water is usuallyremoved by circulating air or some other gas over the material in order to carryaway water vapor; but in some drying processes no carrier gas is used. The abovedefinitions hold in many cases, but there are also notable exceptions to every oneof them. In the last analysis, the question of whether a given operation is calledevaporation or drying is largely a question of common usage. Thus the removal ofwater from a solution by spraying it into a current of superheated steam fulfillsmost of the definitions of evaporation; but, because this is done in an apparatusexactly like the apparatus in which true drying operations are carried out, it iscustomarily considered a drying operation..

Drying

In drying, on the other hand, water is usually removed by circulating airor some other gas over the material in order to carry away water vapor;but in some drying processes no carrier gas is used. The abovedefinitions hold in many cases, but there are also notable exceptions toevery one of them. In the last analysis, the question of whether a givenoperation is called evaporation or drying is largely a question ofcommon usage. Thus the removal of water from a solution by spraying itinto a current of superheated steam fulfills most of the definitions ofevaporation; but, because this is done in an apparatus exactly like theapparatus in which true drying operations are carried out, it iscustomarily considered a drying operation..

Drying

A strict interpretation of the first three sentences of this chapter willeliminate from consideration as drying processes all cases of the removal ofsmall amounts of water from gases or liquids. For example, air is sometimesdried for iron blast furnaces by passing it over refrigerating coils whichremove water as either liquid water or ice. Organic liquids or gases are driedby passing them through a bed of solid adsorbent, such as silica gel oractivated alumina. Occasionally they may also be dried by countercurrentcontact with strong calcium chloride brine or caustic soda. In the past batchesof organic liquids have been dried by adding calcium carbide. These are calleddrying processes in ordinary usage but are somewhat too specialized to treatin this book.

This chapter will first classify dryers and describe typical forms. Then thebasic course of the drying process will be discussed. The application of suchtheory to the actual calculation of dryer size can be attempted only the caseof one type. Reference to any discussion of dryer design in the literature isalmost monotonous in the repeated statements that This class of dryers canbe designed only by actual tests of the material in question in a dryer of thetype involved. Finally a few general principles applicable to most dryers willbe discussed.

This process is suitable only for the drying of thin films on the surface of thematerial to be dried and never for cases where the water (or solvent) to beremoved penetrates the solid. It is a very expensive dryer.Dielectric heating is accomplished by passing the object to be dried through avery-high-frequency (2 to 100 X 106 cycles) electrostatic field.This generates heat uniformly throughout the object. Its only important 4 fieldis in polymerizing the resin that forms the bond between layers ofplywood, which is scarcely a drying operation. It has been suggested for dryingbut is far too expensive for any important applications.Vaporization from ice has been applied in special cases. The vapor pressure ofwater from pure ice is 4.6 mm. Consequently, if a substance containing water isexposed to a vacuum of less than this amount, it will freeze and water willsublime from solid ice. If substances are in solution,- the pressure at which vaporization takes place will be lower. The method isslow and expensive and calls for very large equipment. Its usefulness ispractically confined to the drying of biological products that must not beexposed to elevated temperatures or oxidation. It has been suggested for fruitjuices.

10-19. Introduction to drying theory. The process of drying must beapproached (as in other operations discussed in this book) from two pointsof view: first, the equilibrium relationships and, second, the raterelationships. Operations already discussed are of some help. Thus, there isalways heat transfer to the materialin variety of ways, but allunderstandable from previous sections. The vaporization of water from asurface into a stream of air was discussed in Chap. 8. In addition to thesethere are the mechanisms by which moisture (either as liquid or vapor)travels from the interior of the solid to the surface, Since a wide variety ofmaterials are encountered in drying operations, and many of these may becomplex systems, such as soap, wood, textiles, etc., it is not surprising thatthe equilibrium relationships may be more complex than those encounteredin previous operations. And just as these equilibrium relationships arecomplicated, it is also to be expected that, as the mechanism by which watertravels through the solid varies, the form of rate equations may also vary.A review of drying apparatus in the previous pages shows that, while somedryers (vacuum drum dryers, for instance) dry with the material in contactwith water vapor alone, the majority of methods use air as a carrier of thewater vapor. The following discussion will be confined to these last methods.

10-20. Equilibrium moisture content. Suppose that a wet solid is broughtinto contact with a stream of air, of constant temperature and humidity, insuch amounts that the properties of the air stream remain constant, andthat the exposure is sufficiently long for equilibrium to be reached. In such acase the solid will reach definite moisture contentMoisture content may be expressed either on the wet basis, i.e., pounds ofmoisture per pound of solid plus moisture, or on the dry basis, i.e., poundsof moisture per pound of moisture-free solid. The dry basis is moreconvenient from the standpoint of ricii1ation (see a similar choice forhumidity, Sec. 8-10) and will be used throughout the remainder of thischapter that will be unchanged by further exposure to this same air. This isknown as the equilibrium moisture content of the material under thespecified conditions. For many materials the equilibrium moisture contentdepends on the direction in which equilibrium is approached. A differentvalue is obtained according to whether a wet sample is allowed to dry(desorption) or whether a dry sample is allowed to adsorb moisture(sorption). For drying calculations only the desorption value should be used.

If the material contains more moisture than the equilibrium value, it will dryuntil its moisture content reaches the equilibrium value on the desorption curve.On the other hand, if the material is dryer than the equilibrium value and isbrought into contact with air of the stated temperature and humidity, it willadsorb water until it reaches the equilibrium point on the sorption curve. For airof zero humidity, the equilibrium moisture content of all materials is zero.For any given percentage humidity, the equilibrium moisture content variesgreatly with the type of material. For example, a nonporous insoluble solid willhave an equilibrium moisture content of practically zero, as far as the buld of thesolid is concerned, for any humidity and temperature. On the other hand,certain organic materials of fibrous or colloidal.Some typical equilibrium moisture curvesare given in Fig. 10-15. These aremerely sample curves and must not be considered to hold for all varieties of thesubstance described. So, for instance, curve 7 is not general for all samples ofleaf tobacco, but holds only for the particular sample tested. Relative humidity isused as the abscissa for Fig. 10-15, since this is the customary form in whichequilibrium-moisture-content curves are given. The relative humidity (seefootnote, Sec. 8-10) defined as the ratio of the partial pressure of water vapor inthe gas phase to the vapor pressure of liquid water at the same temperature,and is usually expressed as a percentage.

The equilibrium moisture content of a solid decreases with an increase inthe air temperature. Figure 10-16 shows the effect of temperature on theequilibrium moisture content of raw cotton,2 Temperatures are in degreesFahrenheit.1. International Critical Tables, vol. 2, pp. 322-325.2. The curves presented for raw cotton are based on theexperimental data reported by J. G. Wiegerink, J. Research Nat. Bur.Standards, 24: 645664 (1940), as recalculatedby R.K Toner, C. F. Bowen, and J. C. Whitwell, Textile Research J., 17:7 7-18(1947).will have an equilibrium moisture content of practically zero, as far as the

bulk of the solid is concerned, for any humidity and temperature. On theother hand, certain organic materials of fibrous or colloidal structure suchas wood, paper, textiles, soap, and leather have equilibrium moisturecontents that vary regularly and through wide ranges as the humidity andtemperature of the air with which

10-21. Bound, unbound, and frees water. If the equilibrium curves of Fig. 10-15are continued to their intersection with the axis for 100 per cent humidity, themoisture content so defined is the least moisture that this material can containand still exert a vapor pressure as high as that exerted by ordinary liquid waterat the same temperature.2 If such a material contains more water than thatindicated by this intersection, it can still exert only the vapor pressure of waterat the given temperature. This makes possible a distinction between two typesof water held in a given substance. The water up to the lowest concentrationthat is in equilibrium with saturated air (given by the intersection of the curvesof Fig. 10-15 with the line for 100 per cent humidity) is called bound water,because it exerts vapor pressure les than that of liquid water at the sametemperature. Substances containing bound water are called hygroscopicsubstances.Bound water may exist under several conditions. Liquid water in very finecapillaries will exert an abnormally low vapor pressure because of high concavecurvature of the surface; moisture in cell or fiber walls may suffer a vaporpressure lowering because of solids dissolved in it; water in natural organicstructures is in physical and chemical combination, the nature and strength ofwhich vary greatly with the nature and moisture content of the solid. Unboundwater, on the other hand, exerts its full vapor pressure.

Free moisture content is the moisture in a sample above the equilibriummoisture content. Since the equilibrium moisture is the limit to which thematerial can he dried under a specific set of conditions, it is the moisture abovethis point that can be removed by the drying processnot the Eotal moisturecontent. So, for instance, a sample of wool for which curve 2 of Fig. 10-15 is validhas an equilibrium moisture content of 12.5 per cent in contact with air of 50 percent relative humidity and 25C. if a given sample of wool contains 20 per centmoisture, all this 20 per cent is not removable by drying in a current of air at 25Cand 50 per cent humidity. Only 20 12.5 or 7.5 per cent is so removable, andthis is the free moisture of this sample for these conditions.

10-22. Rate-of-drying curves. The experimental data obtained in aninvestigation of the effect of external conditions on the drying of a solid by airare usually the moisture content of the solid as a function of time under constantdrying conditions. The term constant drying conditions means that the airvelocity, temperature, humidity, and pressure are maintained constant and thatthe outlet air conditions are substantially the same as those at the inlet.Differentiation of the data either graphically or numerically gives the drying rate,which may be plotted vs. either free moisture content or Lime. A plot of thedrying rate per unit area of drying surface vs. free moisture content is the formmost often used. Figure 10-17 illustrates such a curve for the drying of sand. r1hesand was held McCrvacly and McCabe, Trun.s. A in. 1nt, Citem. Enjrs., 29: 131160 (1933).

In a tray whose bottom and sides were insulated, and heated air at constanthumidity was blown over the surface of the tray. The time required for apredetermined loss in weight was read, and this was repeated for successivechanges in weight. The temperature near the surface of the solid, as measuredby a thermocouple, is also shown.The drying-rate curve (Fig. 10-17) may be divided into a constant-rate period,such as the portion AB, and the falling-rate period BD.* The free moisturecontent 1 at point B is called the critical moisture content. The moisturecontent plotted here is the average moisture content of the solid, since at anytime during the drying operation the actual local moisture content is notuniform throughout the solid but varies with position. The drying periodsdescribed do not occur in all cases. If the desired moisture content is largerthan the critical moisture content, only he constant-rate period will occur. Inother cases, for example the drying of soap, the initial moisture content islower than the equilibrium moisture content and the entire drying operationtakes place in the falling period.

Figure 10-17 is only one of the types of drying-rate curves that may beobtained and represents the case of a granular solid composed of nonporousparticles. Figure 10-18 shows other typical drying-rate curves that may beobtained. These curves are for the air drying of slabs, with the air flowingpast both surfaces of the slab. The form of the drying-rate curve depends onthe structure and composition of the solid and on the mechanism by whichmoisture moves within the solid.

* In most cases there is an unsteady-state period that precedes the constant-rate pe nod. During this period conditions in the solid are changing from thevalues at which the solid was introduced into the dryer to thosecorresponding to the constant-rate period. This unsteady-state period hasnot been shown in Fig. 10-17. Usually, the unsteady-state period is only asmall fraction of the constant-rate period.