DistillationDistillation

By : Govind N ManglaniBy : Govind N Manglani

TopicsTopics

Types of DistillationTypes of Distillation Action on an Ideal PlateAction on an Ideal Plate Mass Balance in a Distillation Mass Balance in a Distillation

ColumnColumn Determination of Ideal Number of Determination of Ideal Number of

Plates – McCabe –Thiele AnalysisPlates – McCabe –Thiele Analysis

The methods of The methods of distillationdistillation Differential or batch distillationDifferential or batch distillation Flash or equilibrium distillationFlash or equilibrium distillation Continuous Rectification – Binary Continuous Rectification – Binary

systemssystems

Differential distillationDifferential distillation The simplest examples of batch distillation The simplest examples of batch distillation

at a single stage.at a single stage. Starting with a still pot, initially full, heated Starting with a still pot, initially full, heated

at a constant rate. In this process the vapour at a constant rate. In this process the vapour formed on boiling the liquid is removed at formed on boiling the liquid is removed at once from the system.once from the system.

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

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

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

Differential DistillationDifferential Distillation

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

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

Then a material balance on component Then a material balance on component A gives:A gives:

ydS = d (Sx)ydS = d (Sx) = S dx + x dS= S dx + x dS

Differential DistillationDifferential Distillation

The integral on then right-hand The integral on then right-hand side can be solved graphically if side can be solved graphically if the equilibrium relationship the equilibrium relationship between y and x is available. between y and x is available.

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

Differential distillationDifferential distillation

This process consists of only a single This process consists of only a single stage, a complete separation is stage, a complete separation is impossible unless the relatively impossible unless the relatively volatility is finite. Application is volatility is finite. Application is restricted to conditions where a restricted to conditions where a preliminary separation is to be followed preliminary separation is to be followed by a more rigorous distillation, where by a more rigorous distillation, where high purifies is not required, or where high purifies is not required, or where the mixture is very easily separated the mixture is very easily separated

Flash vaporisationFlash vaporisationor Equilibrium or Equilibrium DistillationDistillation This method is frequently carried out as a This method is frequently carried out as a

continuos process.continuos process. Consist of vaporizing a definite fraction of Consist of vaporizing a definite fraction of

liquid feed in such a way that the vapour liquid feed in such a way that the vapour evolved is in equilibrium with the residual evolved is in equilibrium with the residual liquid.liquid.

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

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

Flash or Equilibrium Flash or Equilibrium Distillation Distillation It is used in petroleum refining, It is used in petroleum refining,

in which petroleum fractions are in which petroleum fractions are heated in pipe stills and the heated in pipe stills and the heated fluid flashed in to vapour heated fluid flashed in to vapour and residual streams, each and residual streams, each containing many components.containing many components.

Continuous Distillation Continuous Distillation with Refluxwith Reflux Flash distillation is used most for Flash distillation is used most for

separating components that boil separating components that boil at widely different temperatures.at widely different temperatures.

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

Action on an Ideal Action on an Ideal PlatePlate By definition a vapour leaving a plate By definition a vapour leaving a plate

are brought into equilibrium.are brought into equilibrium. \Assume that the plates are numbered \Assume that the plates are numbered

serially from top down and that the serially from top down and that the plate under consideration is the nth plate under consideration is the nth plate from the bottom.plate from the bottom.

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

Material Balance diagram for plate nMaterial Balance diagram for plate n

Plate n

Plate n+1

Plate n-1

Vn,yn

Ln-1,Xn-1

Vn-1,yn-1

Ln-2,Xn-2

Vn+1,yn+1

Ln,xn

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

A stream of vapour, VA stream of vapour, Vnn mol/h, rises to plate mol/h, rises to plate n-1n-1 and and a stream of liquid, La stream of liquid, Lnn mol/h, descends to plate mol/h, descends to plate n+1n+1..

Since the vapour streams are the V phase, their Since the vapour streams are the V phase, their considerations are denoted by considerations are denoted by yy; the liquid streams ; the liquid streams are the L phase and their concentrations are are the L phase and their concentrations are denoted by denoted by xx. Then the concentrations of the . Then the concentrations of the streams entering and leaving the streams entering and leaving the nnth plate are as th plate are as follows:follows:

Vapour leaving plate, ynVapour leaving plate, yn Liquid leaving plate, xnLiquid leaving plate, xn Vapour entering plate, yn+1Vapour entering plate, yn+1 Liquid entering plate, xn-1Liquid entering plate, xn-1

Boiling-Point Diagram showing rectification on ideal plate

xn xn-1 yn+1 yn

Number of Plates Number of Plates Required in a Distillation Required in a Distillation ColumnColumn

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

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

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

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

Summary of the material Summary of the material balances for two balances for two components systems components systems Let the process be analysed Let the process be analysed

simply for a binary mixture of A simply for a binary mixture of A and B as follows:and B as follows:

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

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

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

F = D + BF = D + B Component Component AA balance balance

FxFxFF = D x = D xDD + B x + B xBB

Eliminating B and D from these equations Eliminating B and D from these equations give the follow:give the follow:

BD

FD

BD

BF

xx

xx

F

B

xx

xx

F

D

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

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

D V La a

D V Ln n 1

Similar material balances for A give the Similar material balances for A give the following equations:following equations:

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

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

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

Operating linesOperating lines There are two sections in the column; There are two sections in the column;

there are also two operating lines, one there are also two operating lines, one for the rectifying section and other for for the rectifying section and other for the stripping section.the stripping section.

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

111

n

aaaan

n

nn V

xLyVx

V

Ly

DL

Dxx

DL

Ly

n

Dn

n

nn

1

Substituting for DxSubstituting for DxDD in the equation above and in the equation above and eliminating Veliminating Vn+1n+1

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

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

Bmmmm BxxLyV 11

BL

Bxx

BL

Ly

m

Bm

m

mm

1

DL

Dxx

DL

Ly

n

Dn

n

nn

1

Feed LineFeed Line

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

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

Various type of feed Various type of feed conditionsconditions Cold feed, q>1Cold feed, q>1 Feed at bubble point (saturated Feed at bubble point (saturated

liquid), q=1liquid), q=1 Feed partially vapour, 0<q<1Feed partially vapour, 0<q<1 Feed at dew point (saturated Feed at dew point (saturated

vapour), q=0vapour), q=0 Feed superheated vapour q<0Feed superheated vapour q<0

Feed LineFeed Line

Feed LineFeed Line

Cold feed : It is assumed that the Cold feed : It is assumed that the entire feed stream adds to the liquid entire feed stream adds to the liquid flowing down the column.flowing down the column.

Feed at bubble point: no condensation Feed at bubble point: no condensation is required to heat the feed.is required to heat the feed.

Feed partial vapour: the liquid portion Feed partial vapour: the liquid portion of the feed becomes part of the L and of the feed becomes part of the L and the vapour portion becomes part of Vthe vapour portion becomes part of V

Feed LineFeed Line

Feed saturated vapour the entire Feed saturated vapour the entire feed becomes part of the Vfeed becomes part of the V

Feed superheated: part of the Feed superheated: part of the liquid from the rectifying column liquid from the rectifying column is vaporized to cool the feed to a is vaporized to cool the feed to a state of saturated vapour.state of saturated vapour.

Feed Line EquationFeed Line Equation

If xIf xqq = x = xFF, and y, and yqq =x =xFF then; then;– The point of intersection of the two The point of intersection of the two

operating lines lies on the straight operating lines lies on the straight line of slope (q/q -1) and intercept line of slope (q/q -1) and intercept (x(xFF, y, yFF))

11

q

xx

q

qy F

qq

Reflux RatioReflux Ratio The analysis of fractionating columns is The analysis of fractionating columns is

facilitated by the use of a quantity called facilitated by the use of a quantity called reflux ratio.reflux ratio.

Two ratios are used, one is the ratio of the Two ratios are used, one is the ratio of the reflux to the overhead product and the reflux to the overhead product and the other is the ratio of the reflux to the other is the ratio of the reflux to the vapour. vapour.

Both ratios refer to quantities in the Both ratios refer to quantities in the rectifying section. The equations for those rectifying section. The equations for those ratios areratios are

DL

L

V

LRand

D

DV

D

LR VD

Reflux RatioReflux Ratio If the operation lines equations are divided If the operation lines equations are divided

D, the result is, for constant molar D, the result is, for constant molar overflow,overflow,

This equation is an operation line of the This equation is an operation line of the rectifying sectionrectifying section

111

D

Dn

D

Dn R

xx

R

Ry

Reflux RatioReflux Ratio

The y intercept of this line is xThe y intercept of this line is xDD/(R/(RD+1D+1).). The concentration xThe concentration xDD is set by the is set by the

conditions of the design.conditions of the design. RRDD, the reflux ratio, is an operating , the reflux ratio, is an operating

variable that can be controlled at will variable that can be controlled at will by adjusting the split between reflux by adjusting the split between reflux and overhead product or by changing and overhead product or by changing the amount of vapour formed in the the amount of vapour formed in the reboiler for a given flow rate of the reboiler for a given flow rate of the overhead product.overhead product.

Reflux RatioReflux Ratio

A point at the upper end of A point at the upper end of the operating line can be the operating line can be obtained by setting xobtained by setting xnn equal to xequal to xDD in the equation in the equation above.above.

Reflux RatioReflux Ratio A point at the upper end of the operating A point at the upper end of the operating

line can be obtained by setting xline can be obtained by setting xnn equal to equal to xxDD in the previous equation. in the previous equation.

The operating line for the rectifying section The operating line for the rectifying section then intersects the diagonal at point (xthen intersects the diagonal at point (xDD, x, xDD))

Dn

D

DD

D

DD

D

Dn xyor

R

Rx

R

xx

R

Ry

11 1

1

11

Reflux RatioReflux Ratio

The operation lines represented by those The operation lines represented by those two equations are plotted with the two equations are plotted with the equilibrium curve on the x-y diagram.equilibrium curve on the x-y diagram.

Those equations also show that unless Those equations also show that unless Ln and Lm are constant, the operating Ln and Lm are constant, the operating lines are curved.lines are curved.

The lines can be plotted only if the The lines can be plotted only if the change in these internal streams with change in these internal streams with concentration is known. concentration is known.

Reflux RatioReflux Ratio

a

e

b

xF

Minimum Reflux

Total (Maximum) Reflux

Influence of the Number Influence of the Number of Reflux Ratioof Reflux Ratio Any change Any change

in R will in R will therefore therefore modify the modify the slope of the slope of the operation operation line as can line as can be seen from be seen from the Figure the Figure b

a

f

e

d

g

Maximum or Total Maximum or Total RefluxReflux If no product is withdrawn from If no product is withdrawn from

the still (D=0), the column is said the still (D=0), the column is said to operate under conditions of to operate under conditions of total reflux and, as seen from total reflux and, as seen from equation , the top operating line equation , the top operating line has its maximum slope of unity, has its maximum slope of unity, and coincides with the line x=y.and coincides with the line x=y.

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

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

Important deductionsImportant deductions

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

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

Calculation of Calculation of Minimum Reflux Ratio Minimum Reflux Ratio RRmm Based on the previous figure, the Based on the previous figure, the

slope of the line ad is given byslope of the line ad is given by

1m D F D F

mm D F F F

R x x x yor R

R x x y x

McCabe - ThieleMcCabe - Thiele

Construction the operation lines:Construction the operation lines: Locate the feed lineLocate the feed line Calculate the y-axis intercept xCalculate the y-axis intercept xDD/(R/(RDD + +

1) of the rectifying line and plot that 1) of the rectifying line and plot that line through the intercept and the line through the intercept and the point point

(x(xDD, x, xDD)) Draw the stripping line through point Draw the stripping line through point

(x(xBB,x,xBB) and the intersection of the ) and the intersection of the rectifying line with the feed line.rectifying line with the feed line.

Effect the feed condition Effect the feed condition on feed lineon feed line

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

– if the feed is a saturated liquid, the line if the feed is a saturated liquid, the line is vertical;is vertical;

– if the feed is a mixture of liquid and if the feed is a mixture of liquid and vapour, the lines slopes upward and to vapour, the lines slopes upward and to the left and the slope is the negative of the left and the slope is the negative of the ratio of the liquid to the vapour;the ratio of the liquid to the vapour;

– if the feed is saturated vapour the line is if the feed is saturated vapour the line is horizontal andhorizontal and

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

McCabe - ThieleMcCabe - Thiele

Feed Plate locationFeed Plate location After the location of the feed plate After the location of the feed plate

the construction of the number of the construction of the number of ideal trays is found by the usual ideal trays is found by the usual step-by-step construction.step-by-step construction.

The process can begin at the top The process can begin at the top and also a total condenser is used. and also a total condenser is used.

McCabe - ThieleMcCabe - Thiele

ENDEND

Summary Summary