(V semester Sec A/B)

1
(V semester Sec A/B)
AN INSIGHT INTO MASS TRANSFER OPERATIONS (Notes
on Lesson)

• By
• Dr.M.SUBAS CHANDRA BOSE(Assistant Prof)(BT34),
• Mrs.SABARUNISHA BEGUM(Assistant Prof)(BT29)
• Bio Technology, REC

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MASS TRANSFER 0PERATIONS

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

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MASS TRANSFER 0PERATIONS

• UNIT - I DIFFUSION MASS TRANSFER

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MASS TRANSFER 0PERATIONS

• DIFFUSION
• The term mass transfer Diffusion is used to
  denote the transference of a component in a
  mixture
• from 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 result
• from the random velocities of the molecules
  (molecular diffusion) or from the circulating
• or eddy currents present in a turbulent fluid
  (eddy diffusion).

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MASS TRANSFER 0PERATIONS

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

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MASS TRANSFER 0PERATIONS

• DIFFUSION IN BINARY GAS MIXTURES
• 10.2.1, Properties of binary mixtures
• If A and B are ideal gases in a mixture, the
  ideal gas law, equation 2.15, may be applied
• to 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 of
• moles in a volume V, and PA , PB and P are the
  respective partial pressures and the total
• pressure.

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MASS TRANSFER 0PERATIONS

• Equimolecular counterdiffusion
• When the mass transfer rates of the two
  components are equal and opposite the process is
• said to be one of equimolecular counterdiffusion.
  Such a process occurs in the case of the
• box with a movable partition, referred to in
  Section 10.1. It occurs also in a distillation
• column when the molar latent heats of the two
  components are the same. At any point
• in the column a falling stream of liquid is
  brought into contact with a rising stream of
• vapour with which it is not in equilibrium. The
  less volatile component is transferred from
• the vapour to the liquid and the more volatile
  component is transferred in the opposite
• direction. If the molar latent heats of the
  components are equal, the condensation of
• a given amount of less volatile component
  releases exactly the amount of latent heat
• required to volatilise the same molar quantity of
  the more volatile component. Thus at
• the interface, and consequently throughout the
  liquid and vapour phases, equimolecular
• counterdiffusion is taking place.

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MASS TRANSFER 0PERATIONS

• Mass transfer through a stationary second
  component
• In several important processes, one component in
  a gaseous mixture will be transported
• relative to a fixed plane, such as a liquid
  interface, for example, and the other will
  undergo
• no net movement. In gas absorption a soluble gas
  A is transferred to the liquid surface
• where it dissolves, whereas the insoluble gas B
  undergoes no net movement with respect
• to the interface. Similarly, in evaporation from
  a free surface, the vapour moves away
• from the surface but the air has no net movement.
  
• The concept of a stationary component may be
  envisaged by considering the effect of
• moving the box, discussed in Section 10.1, in the
  opposite direction to that in which B is
• diffusing, at a velocity equal to its diffusion
  velocity, so that to the external observer B
• appears to be stationary. The total velocity at
  which A is transferred will then be increased
• to 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, the
• respective diffusion rates are given by
• NA - D AB

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MASS TRANSFER 0PERATIONS

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

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MASS TRANSFER 0PERATIONS

• Thermal diffusion
• If a temperature gradient is maintained in a
  binary gaseous mixture, a concentration
• gradient is established with the light component
  collecting preferentially at the hot end
• and the heavier one at the cold end. This
  phenomenon, known as the Soret effect, may be
• used as the basis of a separation technique of
  commercial significance in the separation
• of isotopes.
• Conversely, when mass transfer is occurring as a
  result of a constant concentration
• gradient, 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 temperature
• gradient, the flux due to thermal diffusion is
  given by GREW and IBBS

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MASS TRANSFER 0PERATIONS

• UNIT - II GAS LIQUID OPERATIONS

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MASS TRANSFER 0PERATIONS

• Gas absorption principles
• Liquid/Gas Absorption ops
• includes 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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MASS TRANSFER 0PERATIONS

• 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
  acetoneair 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 ammoniaair 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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MASS TRANSFER 0PERATIONS

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

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MASS TRANSFER 0PERATIONS

• UNIT - III VAPOUR LIQUID OPERATIONS

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MASS TRANSFER 0PERATIONS

• Distillation and V-L Equilibria
• Types of Distillation
• Action on an Ideal Plate
• Mass Balance in a Distillation Column
• Determination of Ideal Number of Plates McCabe
  Thiele Analysis
• Differential or batch distillation
• Flash or equilibrium distillation
• Continuous Rectification Binary systems

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MASS TRANSFER 0PERATIONS

• 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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MASS TRANSFER 0PERATIONS

• 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 Ds
• The 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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MASS TRANSFER 0PERATIONS

• 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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MASS TRANSFER 0PERATIONS

• 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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MASS TRANSFER 0PERATIONS

• 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 n1.

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MASS TRANSFER 0PERATIONS

• Material Balance diagram for plate n

Vn-1,yn-1 Ln-2,Xn-2
Plate n -1
Vn,yn Ln-1,Xn-1
Plate n

Vn1,yn1 Ln ,xn
Plate n1
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MASS TRANSFER 0PERATIONS

• 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, Vn1 mol/h, from plate
  n1 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 n1.
• 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, yn
• Liquid leaving plate, xn
• Vapour entering plate, yn1
• Liquid entering plate, xn-1

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MASS TRANSFER 0PERATIONS

• Boiling-Point Diagram showing rectification on
  ideal plate

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MASS TRANSFER 0PERATIONS

• 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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MASS TRANSFER 0PERATIONS

• 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 B
• Component A balance
• FxF D xD B xB

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MASS TRANSFER 0PERATIONS

• 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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MASS TRANSFER 0PERATIONS

• 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 n1. 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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MASS TRANSFER 0PERATIONS

• 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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MASS TRANSFER 0PERATIONS

• Substituting for DxD in the equation above and
  eliminating Vn1
• 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 Vm1

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MASS TRANSFER 0PERATIONS

• 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 latent
• heat (q)
• Various type of feed conditions
• Cold feed, qgt1
• Feed at bubble point (saturated liquid), q1
• Feed partially vapour, 0ltqlt1
• Feed at dew point (saturated vapour), q0
• Feed superheated vapour qlt0

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MASS TRANSFER 0PERATIONS

• Feed Line - Diagram

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MASS TRANSFER 0PERATIONS

• 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 V
• Feed saturated vapour the entire feed becomes
  part of the V
• Feed 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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MASS TRANSFER 0PERATIONS

• 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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MASS TRANSFER 0PERATIONS

• Reflux Ratio
• The y intercept of this line is xD/(RD1).
• 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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MASS TRANSFER 0PERATIONS

• 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.

Minimum Reflux
a
e

b
Total (Maximum) Reflux
xF
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MASS TRANSFER 0PERATIONS

• 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

Minimum Reflux
a
e
b
xF
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MASS TRANSFER 0PERATIONS

• If no product is withdrawn from the still (D0),
  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 xy.
• 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.

Minimum Reflux
a
e
b
xF
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MASS TRANSFER 0PERATIONS

• Calculation of Minimum Reflux Ratio Rm
• Based on the previous figure, the slope of the
  line ad is given by

Minimum Reflux
a
e
b
xF
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MASS TRANSFER 0PERATIONS

• 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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MASS TRANSFER 0PERATIONS

• 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 and
• if the feed is superheated vapour. The lines
  slope downward and to the left.

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MASS TRANSFER 0PERATIONS

• 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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MASS TRANSFER 0PERATIONS
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MASS TRANSFER 0PERATIONS

• UNIT - IV EXTRACTION OPERATIONS

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MASS TRANSFER 0PERATIONS

• LiquidLiquid Extraction
• 13.1. INTRODUCTION
• The 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 liquidliquid
  extractionan 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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MASS TRANSFER 0PERATIONS

• Stage wise Extraction

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MASS TRANSFER 0PERATIONS

• Continuous Extraction

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MASS TRANSFER 0PERATIONS
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MASS TRANSFER 0PERATIONS

• General principles of Leaching
• Leaching is concerned with the extraction of a
  soluble constituent from a solid by means
• of a solvent. The process may be used either for
  the production of a concentrated solution
• of 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 extraction
• is determined by the proportion of soluble
  constituent present, its distribution throughout
• the solid, the nature of the solid and the
  particle size.

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MASS TRANSFER 0PERATIONS

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

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MASS TRANSFER 0PERATIONS

• EQUIPMENT FOR LEACHING
• Processes involved
• Three 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 obtain
• as much of the soluble material as possible as
  the product.
• Leaching has in the past been carried out mainly
  as a batch process although many
• continuous plants have also been developed. The
  type of equipment employed depends on
• the nature of the solidwhether 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 sufficiently
• large settling velocities for them to be readily
  separable from the liquid, whereas the
• latter can be maintained in suspension with the
  aid of only a small amount of agitation.

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MASS TRANSFER 0PERATIONS

• UNIT - V SOLID FLUID OPERATIONS

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MASS TRANSFER 0PERATIONS

• INTRODUCTION - ADSORPTION
• Although adsorption has been used as a
  physical-chemical process for many years, it is
• only over the last four decades that the process
  has developed to a stage where it is now
• a major industrial separation technique. In
  adsorption, molecules distribute themselves
• between 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 this
• chapter.
• Unlike absorption, in which solute molecules
  diffuse from the bulk of a gas phase to
• the bulk of a liquid phase, in adsorption,
  molecules diffuse from the bulk of the fluid to
• the surface of the solid adsorbent forming a
  distinct adsorbed phase.

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MASS TRANSFER 0PERATIONS

• The adsorption which results from the influence
  of van der Waals forces is essentially
• physical 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 molecules
• forming 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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MASS TRANSFER 0PERATIONS

• 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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MASS TRANSFER 0PERATIONS

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

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MASS TRANSFER 0PERATIONS

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

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MASS TRANSFER 0PERATIONS

• 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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MASS TRANSFER 0PERATIONS

• FALLING RATE EXAMPLE

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MASS TRANSFER 0PERATIONS

• FALLING RATE EXAMPLE

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MASS TRANSFER 0PERATIONS