Mass transfer operations

(V semester Sec A/B)

By

Dr.M.SUBAS CHANDRA BOSE(Assistant Prof)(BT34),&

Mrs.SABARUNISHA BEGUM(Assistant Prof)(BT29) Bio Technology, REC

Dr.M.SUBAS.C.BOSE

MASS TRANSFER 0PERATIONS

MASS TRANSFER ● Transfer of material from one homogeneous phase to another. With or without phase change.

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UNIT - I DIFFUSION & MASS TRANSFER

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DIFFUSIONThe term mass transfer Diffusion is used to denote the transference of a component in a mixturefrom a region where its concentration is high to a region where the concentration is lower.Mass transfer process can take place in a gas or vapour or in a liquid, and it can resultfrom the random velocities of the molecules (molecular diffusion) or from the circulatingor eddy currents present in a turbulent fluid (eddy diffusion).

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A simple example of a mass transfer process is that occurring in a box consisting oftwo compartments, each containing a different gas, initially separated by an impermeablepartition. When the partition is removed the gases start to mix and the mixing processcontinues at a constantly decreasing rate until eventually (theoretically after the elapse ofan infinite time) the whole system acquires a uniform composition. The process is oneof molecular diffusion in which the mixing is attributable solely to the random motionof the molecules. The rate of diffusion is governed by Pick's Law, first proposed byFlCK(I) in 1855 which expresses the mass transfer rate as a linear function of the molarconcentration gradient. In a mixture of two gases A and B, assumed ideal, Pick's Lawfor steady state diffusion may be written as:

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DIFFUSION IN BINARY GAS MIXTURES10.2.1, Properties of binary mixturesIf A and B are ideal gases in a mixture, the ideal gas law, equation 2.15, may be appliedto each gas separately and to the mixture:PAV = nART PBV = nRRT PV = nRT where nA and HB are the number of moles of A and B and n is the total number ofmoles in a volume V, and PA , PB and P are the respective partial pressures and the totalpressure.

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Equimolecular counterdiffusionWhen the mass transfer rates of the two components are equal and opposite the process issaid to be one of equimolecular counterdiffusion. Such a process occurs in the case of thebox with a movable partition, referred to in Section 10.1. It occurs also in a distillationcolumn when the molar latent heats of the two components are the same. At any pointin the column a falling stream of liquid is brought into contact with a rising stream ofvapour with which it is not in equilibrium. The less volatile component is transferred fromthe vapour to the liquid and the more volatile component is transferred in the oppositedirection. If the molar latent heats of the components are equal, the condensation ofa given amount of less volatile component releases exactly the amount of latent heatrequired to volatilise the same molar quantity of the more volatile component. Thus atthe interface, and consequently throughout the liquid and vapour phases, equimolecularcounterdiffusion is taking place.

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Mass transfer through a stationary second componentIn several important processes, one component in a gaseous mixture will be transportedrelative to a fixed plane, such as a liquid interface, for example, and the other will undergono net movement. In gas absorption a soluble gas A is transferred to the liquid surfacewhere it dissolves, whereas the insoluble gas B undergoes no net movement with respectto the interface. Similarly, in evaporation from a free surface, the vapour moves awayfrom the surface but the air has no net movement.

The concept of a stationary component may be envisaged by considering the effect ofmoving the box, discussed in Section 10.1, in the opposite direction to that in which B isdiffusing, at a velocity equal to its diffusion velocity, so that to the external observer Bappears to be stationary. The total velocity at which A is transferred will then be increasedto its diffusion velocity plus the velocity of the box.For the absorption of a soluble gas A from a mixture with an insoluble gas B, therespective diffusion rates are given by:NA = - D AB

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Mass transfer velocities

It is convenient to express mass transfer rates in terms of velocities for the species underconsideration where:As a result of the diffusional process, there is no net overall molecular flux arising fromdiffusion in a binary mixture, the two components being transferred at equal and oppositerates. In the process of equimolecular counterdiffusion which occurs, for example, in adistillation column when the two components have equal molar latent heats, the diffusionalvelocities are the same as the velocities of the molecular species relative to the walls ofthe equipment or the phase boundary.

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Thermal diffusion

If a temperature gradient is maintained in a binary gaseous mixture, a concentrationgradient is established with the light component collecting preferentially at the hot endand the heavier one at the cold end. This phenomenon, known as the Soret effect, may beused as the basis of a separation technique of commercial significance in the separationof isotopes.Conversely, when mass transfer is occurring as a result of a constant concentrationgradient, a temperature gradient may be generated; this is known as the Dufour effect.In a binary mixture consisting of two gaseous components A and B subject to a temperaturegradient, the flux due to thermal diffusion is given by GREW and IBBS

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UNIT - II GAS LIQUID OPERATIONS

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Gas absorption principles

Liquid/Gas Absorption opsincludes absorption, stripping and desorption

soluble vapour absorbed from mixture with a liquid (solute), solute is then regenerated can also have gas absorption with reaction.

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The removal of one or more selected components from a mixture of gases by absorption into a suitable liquid is the second major operation of chemical engineering that is based on interphase mass transfer controlled largely by rates of diffusion. Thus, acetone can be recovered from an acetone–air mixture by passing the gas stream into water in which the acetone dissolves while the air passes out. Similarly, ammonia may be removed from an ammonia–air mixture by absorption in water. In each of these examples the process of absorption of the gas in the liquid may be treated as a physical process, the chemical reaction having no appreciable effect

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The transfer unitThe group (dy/ye − y), which is used in Chapter 11, has been defined by CHILTON andCOLBURN(52) as the number of overall gas transfer units NOG. The concept of the transferunit is also introduced in Volume 1, Chapter 10. The application of this group to thecountercurrent conditions in the absorption tower is now considered.Over a small height dZ, the partial pressure of the diffusing component A will changeby an amount dPAG.

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UNIT - III VAPOUR LIQUID OPERATIONS

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Total condenser

Feed

Overhead vapor

BoilupN

2

1

Distillation

f

Reflux drum

Rectifying section stages

Stripping section stages

Feed Stage

Bottoms

Partial reboiler

Reflux

Distillate


Distillation and V-L Equilibria Types of DistillationAction on an Ideal PlateMass Balance in a Distillation ColumnDetermination of Ideal Number of Plates – McCabe –Thiele Analysis

Differential or batch distillationFlash or equilibrium distillationContinuous Rectification – Binary systems

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Differential distillation:

The simplest examples of batch distillation at a single stage.

Starting with a still pot, initially full, heated at a constant rate. In this process the vapour formed on boiling the liquid is removed at once from the system.

Since this vapour is richer in the more volatile component, with this result the composition of the product progressively alters.

Thus, whilst the vapour formed over a short period is in equilibrium with the liquid

At the end of the process the liquid, which has been vaporized, is removed as the bottom product.

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Let S be the number of mols of material in the still and x be the mol fraction of component A.

Suppose an amount dS, containing a mol fraction y of A, be vaporised.

Then a material balance on component A gives:

ydS = d (Sx)

= S dx + x DsThe integral on then right-hand side can be solved graphically if the equilibrium relationship between y and x is available.

Thus, if over the range concerned the equilibrium relationship is a straight line of the form y= m x + c

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This process consists of only a single stage, a complete

separation is impossible unless the relatively volatility is

finite. Application is restricted to conditions where a

preliminary separation is to be followed by a more

rigorous distillation, where high purifies is not required, or

where the mixture is very easily separated

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Flash & continuous distillation This method is frequently carried out as a continuous process.

Consist of vaporizing a definite fraction of liquid feed in such a way that the vapour evolved is in equilibrium with the residual liquid.

The feed is usually pumped through a fired heater and enters the still through a valve where the pressure is reduced.

The still is essentially a separator in which the liquid and vapour produced is reduced by reduction in pressure with sufficient time to reach equilibrium. The vapour is removed from the top of the separator and is then usually condensed, while the liquid leaves from the bottom.

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Flash & continuous distillation It is used in petroleum refining, in which petroleum fractions are heated in pipe stills and the heated fluid flashed in to vapour and residual streams, each containing many components.

Flash distillation is used most for separating components that boil at widely different temperatures.

It is not effective separating components of comparable volatility, which requires the use of distillation with reflux

By definition a vapour leaving a plate are brought into equilibrium.Assume that the plates are numbered serially from top down and that the plate under consideration is the nth plate from the bottom.

Then the immediately above plate n is plate n-1, and the immediately below is n+1.

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Material Balance diagram for plate n:

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Plate nVn,yn

Ln-1,Xn-1

Vn-1,yn-1

Ln-2,Xn-2

Plate n+1

Plate n -1

Vn+1,yn+1

Ln ,xn


For example if two fluid enter plate n and two leave it, the liquid, Ln-1 mol/h, from plate n-1 and the stream of vapour, Vn+1 mol/h, from plate n+1 are brought into intimate contact.

A stream of vapour, Vn mol/h, rises to plate n-1 and a stream of liquid, Ln mol/h, descends to plate n+1.

Since the vapour streams are the V phase, their considerations are denoted by y; the liquid streams are the L phase and their concentrations are denoted by x. Then the concentrations of the streams entering and leaving the nth plate are as follows:

Vapour leaving plate, ynLiquid leaving plate, xnVapour entering plate, yn+1Liquid entering plate, xn-1

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Boiling-Point Diagram showing rectification on ideal plate:

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Number of Plates Required in a Distillation Column:

To develop a method for the design of distillation units to give the desired fractionation, it is necessary to determine the numbers of trays.

Before that the heat and material flows over the trays, the condenser and the reboiler must be established

Thermodynamic data is required to establish how much mass transfer is needed to establish equilibrium between the stream leaving each tray.

The diameter of the column will be dictated by the necessity to accommodate the desired flow rates, to operate within the available drop in pressure, while at the same time affecting the desired degree of mixing

of the stream on each tray.

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Summary of the material balances for two components systems:

Let the process be analysed simply for a binary mixture of A and B as follows:

Let F be the number of mols per unit of feed of mol fraction xf of A.

D be the number of mols per unit time of vapour formed with y the mol fraction of A and

B be the number of mols per unit time of liquid with x the mol fraction of A. Then an overall mass balance gives:

F = D + BComponent A balance

FxF = D xD + B xB

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Eliminating B and D from these equations give the follow:

Net flow rates. Quantity D is the difference between the flow rates of the streams entering and leaving the top of the column. A material balance around the condenser and accumulator in the gives:

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BD

FD

BD

BF

xx

xx

F

B

xx

xx

F

D

D V La a


Eliminating B and D from these equations gives the following:

The difference between the flow rates of vapour and liquid anywhere in the upper section of the column is also equal to D. This surface includes the condenser and all plates above n+1. A total material balance around this control surface gives:

Similar material balances for A give the following equations:

Quantity D xD is the net flows rate of the component A upward in the upper section of the column. It too is constant throughout this part of the equipment. In the lower section of the column the net flow rates are also constant but, are in a downward direction.

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D V Ln n 1

D x V Y L x V y L xD a a a a n n n n 1 1


The net flow of total material equals B; that of the component A is BxB. The following equations apply:

Operating lines:There are two sections in the column; there are also two operating lines, one for the rectifying section and other for the stripping section.

For the first section (rectifying section) the operation line is represented by:

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B L L L V

Bx L x V y L x V yb b m m

B b b b b m m m m

1

1 1

111

n

aaaan

n

nn V

xLyVx

V

Ly

DL

Dxx

DL

Ly

n

Dn

n

nn

1


Substituting for DxD in the equation above and eliminating Vn+1

For the section below the feed plate, a material balance over control surface II gives

Rearranging this equation and taking into account that the slope is the ratio of liquid flow to the vapour flow, and also eliminating Vm+1

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Bmmmm BxxLyV 11

DL

Dxx

DL

Ly

n

Dn

n

nn

1

BL

Bxx

BL

Ly

m

Bm

m

mm

1


Feed Line:

The conditions of the vapour rate or the liquid rate may change depending of the thermal condition of the feed.

It is related to the heat to vaporize one mole of feed divided by molar latentheat (q)

Various type of feed conditions:

Cold feed, q>1

Feed at bubble point (saturated liquid), q=1

Feed partially vapour, 0<q<1

Feed at dew point (saturated vapour), q=0

Feed superheated vapour q<0

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Feed Line - Diagram:

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Feed Line:Cold feed : It is assumed that the entire feed stream adds to the liquid flowing down the column.Feed at bubble point: no condensation is required to heat the feed.Feed partial vapour: the liquid portion of the feed becomes part of the L and the vapour portion becomes part of VFeed saturated vapour the entire feed becomes part of the VFeed superheated: part of the liquid from the rectifying column is vaporized to cool the feed to a state of saturated vapour.

Feed Line Equation:If xq = xF, and yq =xF then;

The point of intersection of the two operating lines lies on the straight line of slope (q/q -1) and intercept (xF, yF)

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11

q

xx

q

qy F

qq


Reflux Ratio:The analysis of fractionating columns is facilitated by the use of a quantity called reflux ratio.Two ratios are used, one is the ratio of the reflux to the overhead product and the other is the ratio of the reflux to the vapour. Both ratios refer to quantities in the rectifying section. The equations for those ratios are

If the operation lines equations are divided D, the result is, for constant molar overflow,

This equation is an operation line of the rectifying section

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DL

L

V

LRand

D

DV

D

LR VD

111

D

Dn

D

Dn R

xx

R

Ry


Reflux Ratio:The y intercept of this line is xD/(RD+1).

The concentration xD is set by the conditions of the design.

RD, the reflux ratio, is an operating variable that can be controlled at will by adjusting the split between reflux and overhead product or by changing the amount of vapour formed in the reboiler for a given flow rate of the overhead product.A point at the upper end of the operating line can be obtained by setting xn equal to xD in the equation above.A point at the upper end of the operating line can be obtained by setting xn equal to xD in the previous equation.

The operating line for the rectifying section then intersects the diagonal at point (xD, xD).

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Dn

D

DD

D

DD

D

Dn xyor

R

Rx

R

xx

R

Ry

11 1

1

11


The operation lines represented by those two equations are plotted with the equilibrium curve on the x-y diagram.Those equations also show that unless Ln and Lm are constant, the operating lines are curved.The lines can be plotted only if the change in these internal streams with concentration is known.

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ae

b

xF

Minimum Reflux

Total (Maximum) Reflux


Influence of the Number of Reflux Ratio:Any change in R will therefore modify the slope of the operation line as can be seen from

the Figure

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ae

b

xF

Minimum Reflux


If no product is withdrawn from the still (D=0), the column is said to operate under conditions of total reflux and, as seen from equation , the top operating line has its maximum slope of unity, and coincides with the line x=y.

The step become very close to the plate above, these conditions are known as minimum reflux and Rm denotes the reflux ratio.

Any small increase in R beyond Rm will give a workable system, though a large numbers of plate will be required.

The minimum number of plates is required for a given separation at conditions of total reflux

There is a minimum reflux ratio below which it is impossible to obtain the desired enrichment, however many plates are used.

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ae

b

xF

Minimum Reflux


Calculation of Minimum Reflux Ratio Rm

Based on the previous figure, the slope of the line ad is given by

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ae

b

xF

Minimum Reflux

1m D F D F

mm D F F F

R x x x yor R

R x x y x


McCabe -Thiele principles Construction the operation lines:Locate the feed line Calculate the y-axis intercept xD/(RD + 1) of the rectifying line and plot that line through the intercept and the point (xD, xD)

Draw the stripping line through point (xB,xB) and the intersection of the rectifying line with the feed line.

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Effect the feed condition on feed line:

If the feed is a cold liquid, the feed line slopes will be upward and to the right;

if the feed is a saturated liquid, the line is vertical;if the feed is a mixture of liquid and vapour, the lines

slopes upward and to the left and the slope is the negative of the ratio of the liquid to the vapour;

if the feed is saturated vapour the line is horizontal andif the feed is superheated vapour. The lines slope

downward and to the left.

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Feed Plate location

After the location of the feed plate the construction of the number of ideal trays is found by the usual step-by step construction. The process can begin at the top and also a total condenser is used.

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UNIT - IV EXTRACTION OPERATIONS

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Liquid–Liquid Extraction13.1. INTRODUCTIONThe separation of the components of a liquid mixture by treatment with a solvent in which one or more of the desired components is preferentially soluble is known as liquid–liquid extraction—an operation which is used, for example, in the processing of coal tar liquids and in the production of fuels in the nuclear industry, and which has been applied extensively to the separation of hydrocarbons in the petroleum industry. In this operation, it is essential that the liquid-mixture feed and solvent are at least partially if not completely immiscible and, in essence, three stages are involved:

(a) Bringing the feed mixture and the solvent into intimate contact,(b) Separation of the resulting two phases, and(c) Removal and recovery of the solvent from each phase.

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Stage wise Extraction

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Continuous Extraction

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General principles of LeachingLeaching is concerned with the extraction of a soluble constituent from a solid by meansof a solvent. The process may be used either for the production of a concentrated solutionof a valuable solid material, or in order to remove an insoluble solid, such as a pigment,from a soluble material with which it is contaminated. The method used for the extractionis determined by the proportion of soluble constituent present, its distribution throughoutthe solid, the nature of the solid and the particle size.

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Factors influencing the rate of extractionThe selection of the equipment for an extraction process is influenced by the factors whichare responsible for limiting the extraction rate. Thus, if the diffusion of the solute throughthe porous structure of the residual solids is the controlling factor, the material should beof small size so that the distance the solute has to travel is small. On the other hand, ifdiffusion of the solute from the surface of the particles to the bulk of the solution is thecontrolling factor, a high degree of agitation of the fluid is required.

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EQUIPMENT FOR LEACHING Processes involvedThree distinct processes are usually involved in leaching operations:(a) Dissolving the soluble constituent.(b) Separating the solution, so formed, from the insoluble solid residue.(c) Washing the solid residue in order to free it of unwanted soluble matter or to obtainas much of the soluble material as possible as the product.Leaching has in the past been carried out mainly as a batch process although manycontinuous plants have also been developed. The type of equipment employed depends onthe nature of the solid—whether it is granular or cellular and whether it is coarse or fine.The normal distinction between coarse and fine solids is that the former have sufficientlylarge settling velocities for them to be readily separable from the liquid, whereas thelatter can be maintained in suspension with the aid of only a small amount of agitation.

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UNIT - V SOLID FLUID OPERATIONS

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INTRODUCTION - ADSORPTION

Although adsorption has been used as a physical-chemical process for many years, it isonly over the last four decades that the process has developed to a stage where it is nowa major industrial separation technique. In adsorption, molecules distribute themselvesbetween two phases, one of which is a solid whilst the other may be a liquid or a gas.The only exception is in adsorption on to foams, a topic which is not considered in thischapter.Unlike absorption, in which solute molecules diffuse from the bulk of a gas phase tothe bulk of a liquid phase, in adsorption, molecules diffuse from the bulk of the fluid tothe surface of the solid adsorbent forming a distinct adsorbed phase.

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The adsorption which results from the influence of van der Waals forces is essentiallyphysical in nature. Because the forces are not strong, the adsorption may be easily reversed.In some systems, additional forces bind absorbed molecules to the solid surface. These are chemical in nature involving the exchange or sharing of electrons, or possibly moleculesforming atoms or radicals. In such cases the term chemisorption is used to describe the phenomenon. This is less easily reversed than physical adsorption, and regeneration may be a problem. Chemisorption is restricted to just one layer of molecules on the surface, although it may be followed by additional layers of physically adsorbed molecules.

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MOISTURE CONTENT RELATIONSHIPS:MOISTURE/SOLID EQUILIBRIUM RELATIONSHIPS DEFINED ON THE BASIS OF RELATIVE HUMIDITY AT A SPECIFIC TEMPERATURE EQUILIBRIUM AMOUNT OF MOISTURE TENDS TO DECREASE WITH INCREASING TEMPERATURE.

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MOISTURE CONTENT VARIABLES BASED ON THE MASS OF MOISTURE RELATIVE TO THE MASS BONE DRY SOLID

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DRYING RATE CURVES:DEPEND ON WHETHER HEAT ORMASS TRANSFER CONTROLS FREE MOISTURE VS. TIME DRYING RATE VS. MOISTURE CONTENT

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DRYING REGIMES CONSTANT RATE - NO LIMIT TO MASS TRANSFER IN SOLID PHASE SURFACE MOISTURE TRANSFER NEAR SURFACE FALLING RATE – MOISTURE FLUX THROUGH THE SOLID IS HINDERED CRITICAL POINTS OCCUR BETWEEN CONSTANT RATE AND FALLING RATE WITH A CHANGE IN THE FALLING RATE DRYING MECHANISM.

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FALLING RATE EXAMPLE:

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Mass transfer operations

Technology

Transcript of Mass transfer operations