sp lecture 1

7/31/2019 SP Lecture 1

1/70

1/28/2010 1engineering-resource.com


2/70

Separation processes

Anam Saeed



3/70

1/28/2010 3

Synopsis of this lecture: Separation processes

Importance of mass transfer in Chemical Engineering

What is Mass Transfer?

What are mass transfer operations?

Different mass transfer operations

Course outline

engineering-resource.com


4/70

Separation process:

Is used to transform a mixture of substances into two or

more distinct products. The separated products could

differ in chemical properties or some physical property,

such as size, or crystal modification or other separation

into different components.

Separation processes can essentially be termed as mass

transfer processes.

The mixture at hand could exist as a combination of anytwo or more states: solid-solid, solid-liquid, solid-gas,

liquid-liquid, liquid-gas, gas-gas, solid-liquid-gas mixture,

etc.



5/70

1/28/2010 5

Mass Transfer:Mass Transfer is the transport of material from one

point to another in a single phase or from one phase to

another generally in the presence of difference of

conc., vapour pressure, in activity that provides thedriving force.

The mass transfer operations include such techniques

as Distillation, gas absorption, adsorption, liquid

extraction, leaching etc.



6/70



7/701/28/2010 7

Importance of Mass Transfer in Chemical

Engineering: Key part of unit operations

Unit Operations; is the systematic study of the operations that

are helpful in the manufacturing of final product.

Depending on the raw mixture, various processes can be

employed to separate the mixtures. Many times two or more of

these processes have to be used in combination to obtain the

desired separation.

Mass transfer operations are widely used in refinery, fertilizer,

food industries etc.



8/70

Different Mass Transfer Operations:1. Distillation:

Is a unit operation or a physical separation process

whereby two or more miscible liquids are separatedusing their volatility differences( differences in

boiling points).

Application:

Water is distilled to remove impurities, such as salt

from seawater.



9/701/28/2010 Distillation of water 9engineering-resource.com


10/701/28/2010 10

Unit operation and is mass transfer phenomena where

the solute of a gas are removed by being placed in

contact with a nonvolatile liquid solvent that removes the

components from the gas.

Application:

The washing of ammonia from the mixture of ammonia

and air by means of liquid water.

Absorption:



11/701/28/2010 Removal of SO2 gas 11



12/70

Liquid Liquid Extraction:This is the separation of a substance from a mixture

by preferentially dissolving that substance in a

suitable solvent. By this process a soluble compound

is usually separated from an insoluble compound.

Application:

Acetic acid can be separated from water by using

MIBK solvent.



13/70



14/70

Leaching:Leaching is concerned with the extraction of a soluble

constituent from a solid by means of a solvent.

Application:

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 iscontaminated.



15/70

1/28/2010

Soil leaching

15engineering-resource.com


16/70

1/28/2010 16

Different Mass Transfer Operations

Separation

process

Feed Separating

Agent

Product Principle of

separation

Practical

example

Distillation Liquid Heat Liquid +

vapor

Difference in

relative

volatility

Fractionation

of crude oil

Liquid

liquid

extraction

Liquid Immiscible

liquid

Two liquids Different

solubilities

of different

species in thetwo liquid

phases

Recovery of

acetic acid



17/70

1/28/2010 17

ContSeparation

process

Feed Separating

Agent

Product Principle of

separation

Practical

example

Absorption Mixture

of gases

Liquid

absorbent

Rich

liquidand pure

gas

Difference in

concentration

Good for

recovery ofsoluble

gases

Leaching solid Liquid

solvent

Concent-

rated

solid

Concentratio

n difference

Extraction

of metals

from ores



18/70

Course outline:

Introduction to mass transfer operations; selection

criteria among different separation processes;

complete design calculations for equilibrium stage

processes; distillation; Absorption; Leaching; liquidliquid extraction: design calculations for differential

contractors; applications of mass transfer

fundamentals, calculations of rate of mass transfer:

limiting processes; case study taking environmental,

manufacturing and economic effects



19/70

Books:Text books:

1. Coulson and Richardsons CHEMICAL ENGINEERING

Volume 2 Fifth Edition2. Unit operations of chemical Engineering by julian C. Smith.

Warren L. McCabe Peter Harriott Fifth Edition

3. Mass transfer operations by Robert E. Treybal; 3rd edition



20/70

Distillation



21/70

Introduction:The separation of liquid mixtures into their various

components is one of the major operations in the

process industries, and distillation, the most widely used

method of achieving this end, is the key operation inany oil refinery.

In this chapter consideration is given to the theory of

the process, methods of distillation and calculation of

the number of stages required for binary system.



22/70

Distillation: Is a unit operation or a physical separation process

whereby two or more miscible liquids are separated

using their volatility differences( differences inboiling points).

Is simply defined as a process in which a liquid or

vapour mixture of two or more substances are

separated into its component fractions of desiredpurity by the application and removal of heat.



23/70

Vapourliquid equilibrium:

The composition of the vapour in equilibrium with a

liquid of given composition is determined

experimentally using an equilibrium still. The results are

conveniently shown on a temperaturecompositiondiagram.



24/70



25/70

1/28/2010 25

For distillation purposes it is more convenient to ploty against x at a constant pressure,

since the majority of industrial distillations take place

at substantially constant pressure.

This is shown in Figure 11.4



26/70



27/70

Partial vaporization and partial

condensation:If a mixture of benzene and toluene is heated in a vessel,

closed in such a way that the pressure remains atmospheric

and no material can escape and the mole fraction of the

more volatile component in the liquid, that is benzene, is

plotted as abscissa, and the temperature at which the mixture

boils as ordinate.



28/70



29/70

Continue..Partial vaporisation of the liquid gives a vapour richer in

the more volatile component than the liquid If the

vapour initially formed, as for instance at point E, is at

once removed by condensation, then a liquid ofcomposition x3 is obtained, represented by point C. The

step BEC may be regarded as representing an ideal

stage, since the liquid passes from composition x2 to a

liquid of composition x3. Thus, partial condensationbrings about enrichment of the vapour in the more

volatile component in the same manner as partial

vaporisation.



30/70

Daltons law:

By Daltons law of partial pressures, P = PA, that is thetotal pressure is equal to the summation of the partial

pressures.

Since in an ideal gas or vapour the partial pressure is

proportional to the mole fraction of the constituent, then:

PA = yAP

Raoults law:

For an ideal mixture, the partial pressure is related to theconcentration in the liquid phase by Raoults law which may be

written as:

PA = PAxA



31/70

Example 11.1The vapour pressures of n-heptane and toluene at 373 K

are 106 and 73.7 kN/m2 respectively.

What are the mole fractions of n-heptane in the vapour

and in the liquid phase at 373 K if the total

pressure is 101.3 kN/m2?



32/70



33/70



34/70



35/70



36/70



37/70



38/70



39/70



40/70



41/70



42/70

Methods of distillation:There are three main methods used in distillation practice.

(a) Differential distillation.

(b) Flash or equilibrium distillation, and

(c) Rectification.



43/70

Differential distillation:

The simplest example of batch distillation is a single stage,

differential distillation, 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 than the

liquid. it follows that the liquid remaining becomes

steadily weaker in this component, with the result that the

composition of the product progressively alters. Thus, whilst

the vapour formed over a short period is in equilibrium with the

liquid, the total vapour formed is not in equilibrium with the

residual liquid.



44/70

Cont.At the end of the process the liquid which has not been

vaporised is removed as the bottom product. As this

process consists of only a single stage, a complete

separation is impossible unless the relative volatility isinfinite.



45/70

Flash or equilibrium distillation:

Flash or equilibrium distillation, frequently carried out as

a continuous process, consists of vaporising a definite

fraction of the liquid feed in such a way that the vapour

evolved is in equilibrium with the residual liquid. The feedis 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 by the reduction in pressure have

sufficient time to reach equilibrium. The vapour is

removed from the top of the separatorand is then usually

condensed, while the liquid leaves from the bottom.



46/70



47/70

Example 11.6:An equimolar mixture of benzene and toluene is subjected

to flash distillation at 100 kN/m2 in the separator. Using

the equilibrium data given in Figure 11.9, determine the

composition of the liquid and vapour leaving the separatorwhen the feed is 25 per cent vaporised. For this condition,

the boiling point diagram in Figure 11.10 may be used to

determine the temperature of the exit liquid stream.



48/70



49/70

Rectification:In the two processes considered, the vapour leaving the

still at any time is in equilibrium with the liquid

remaining, and normally there will be only a small

increase in concentration of the more volatile component.

The essential merit of rectification is that it enables a

vapour to be obtained that is substantially richer in the

more volatile component than is the liquid left in the still.

This is achieved by an arrangement known as a

fractionating column which enables successive

vaporisation and condensation.



50/70

Fractionating Column:Is a series of stills placed one on top of the other. As

vaporization occurs, the lighter components of the mixture

moves up to the tower and are distributed on the various

trays. Heavier components goes down lighter componentsgoes up.

The feed is introduced at the middle section of the

column at intermediate tray where the liquid has the same

composition as that of feed.



51/70



52/70

Types of section:Rectifying section:

The part of column above the feed point is known as

rectifying section.

Stripping section:

The part of the column below the feed point is known

as stripping section.



53/70

Distillation Equipments

Major components of a typical

distillation are:

Vertical Shell

Column internals

Reboiler

Condenser

Reflux Drum



54/70

Distillation Equipmentsengineering-resource.com


55/70

Continuous distillation: liquid mixture is continuously (without interruption) fed

into the process and separated fractions are removed

continuously as output stream.

The liquid in the base of column heated, and the vapour

rises to the bottom try. where it is partially condensed and

partially vaporized.

The operation of partial condensation of raising vapour

and partial vaporization of the reflux liquid is repeated on

each try.



56/70

Vapours are

moving upward

while the liquid is

flowing downward

in the distillation

column and

mixing of vapour

and liquid take

place on each tray.



57/70

Continue. Vapour of composition yt (MVC) from the top try is

condensed to give top product D and the reflux R, both of

same composition yt.

The vapour rising from an ideal tray will be in equilibrium

with the liquid leaving, although in practice smaller

degree of enrichment occur.



58/70

The system tends to reach equilibrium on each tray: Some of the less volatile component condenses from the

rising vapour into the liquid thus increasing the

concentration of the more volatile component in the

vapour.

Some of the MVC is vaporized from the liquid on the tray

thus decreasing the concentration of the MVC in the

liquid.



59/70

Equimolar counter diffusion: The number of molecules passing in each direction

from vapour to liquid and in reverse is approximately

the same since the heat given out by one mole of the

vapour on condensing is approximately equal to the

heat required to vapourize one mole of the liquid.this

is called Equimolar counter diffusion.



60/70

Constant molar overflow: If the molar heat of vaporisation are approximately

constant, the flow of liquid and vapour in each part of

the column will not vary from tray to tray. This

concept is called constant molar overflow.



61/70

Number of plates required in a distillation

column:In order to develop a method for the design of distillation units

to give the desired fractionation, it is necessary, in the first

instance, to develop an analytical approach which enables

the necessary number of trays to be calculated. First the heatand material flows over the trays, the condenser, and the

reboiler must be established. Thermodynamic data are required

to establish how much mass transfer is needed to establish

equilibrium between the streams leaving each tray. The requireddiameter of the column will be dictated by the necessity to

accommodate the desired flowrates, to operate within the

available drop in pressure, while at the same time effecting the

desired degree of mixing of the streams on each tray.1/28/2010 61engineering-resource.com


62/70



63/70

Heat balance over a plate:



64/70

Assumptions: The heat losses will be small and may be neglected, and

for an ideal system the heat of mixing is zero.

For such mixtures, the molar heat of vaporization

may be taken as constant and independent of thecomposition.

VN-1 =VN

so that the molar vapour flow is constant up the column

unless material enters or is withdrawn from the section.

The temperature change from one plate to the next will

be small, and HL

n may be taken as equal to H n+1.



65/70

Cont. Ln = Ln+1,so that the moles of liquid reflux are also constant

in this section of the column. Thus

Vn and Ln are constant over the rectifying section, and Vm and

Lm are constant over the stripping section.

on the basis of these assumptions there are two methods to find

out the number of plates

1. LewisSorel method

2. The method of McCabe and Thiele



66/70

Calculation of number of plates using the

LewisSorel methodA binary feed F is distilled to give

a top product D and a bottom product

W, with xf , xd, and xw as the

corresponding mole fractions of the

more volatile component, and the

vapour Vt rising from the top plate is

condensed, and part is run back as

liquid at its boiling point to the column

as reflux, the remainder being

withdrawn as product, then a materialbalance above plate n.



67/70



68/70



69/70

The method of McCabe and Thiele



70/70

Thank you

sp lecture 1

Documents