Distillation: It is a very old separation technology for separating liquid mixtures mixtures that can be traced back to the chemists in Alexandria. Today distillation is the most important industrial separation technology well suited for high purity separation and any degree of separation can be achieved by fixed energy consumption by increasing the equilibrium stages.
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Distillation can be carried out by two principal methods: –
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First method is based on the production of vapor by boiling the liquid mixture to be separated and condensing vapor with no liquid return i.e. with no Reflux .
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Second method method is returning returning a part of liquid so it can come in intimate contact with the vapor on their way to condenser i.e. with Reflux
Distillation •
Distillation: It is a very old separation technology for separating liquid mixtures mixtures that can be traced back to the chemists in Alexandria. Today distillation is the most important industrial separation technology well suited for high purity separation and any degree of separation can be achieved by fixed energy consumption by increasing the equilibrium stages.
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Distillation can be carried out by two principal methods: –
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First method is based on the production of vapor by boiling the liquid mixture to be separated and condensing vapor with no liquid return i.e. with no Reflux .
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Second method method is returning returning a part of liquid so it can come in intimate contact with the vapor on their way to condenser i.e. with Reflux
Flash Distillation •
This method consist of vaporizing a definite fraction of liquid in such a way that evolved vapor is in equilibrium with the residual liquid separating the vapor from the liquid and condensing vapor.
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The diagram shows the flash distillation configuration the feed is pumped by a through heater b, and pressure is reduced through valve c the intimate mixture of vapor and liquid enters the vapor separator d and sufficient time is allowed for separating vapor and liquid.
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Vapor leaves through e and liquid through g.
Flash Distillation •
Flash distillation is used extensively in petroleum refining in which petroleum fractions are heated in pipe stills and the heated fluid is flashed into vapor and liquid streams.
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We will consider 1 mole of binary mixture and the let the mole fraction of volatile component be xf and f be the molal fraction of the feed that is vaporized. Also let the mole fraction yD and xB be the concentration of the vapor and liquid respectively.
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Applying material balance we have.
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Using equilibrium relationship the points xB & yD are 2 replaced with x and y.
Flash Distillation •
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The fraction f is not fixed directly depend on the enthalpy of the hot in coming liquid and can be increased by lowering the pressure.
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The equation 2 is a straight line equation with slope of –(1f)/f and can be plotted on the equilibrium diagram.
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The coordinates of the intersection of this material balance line and the diagonal x=y can be used easily as a point on the line.
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The material balance line crosses the diagonal at x=xf for all values of f .
Flash Distillation
Ex-18.1 A mixture of 50 mole % Benzene and 50 mole % Toluene is subjected to flash distillation at a separator pres of 1 atm. Using the Vap-Liq equilibrium curves and boiling-point diagram plot the following as function of f. a) The temp in separator b) Composition of liquid leaving separator c) Composition of vapor leaving separator
Flash Distillation •
Solution: Several values of f corresponding quantities – [(1/f)-1] are calculated using these as slopes a series of straight lines each passing through point(x f , xf ) is drawn on the equilibrium curves these lines cuts the equilibrium curves at corresponding values of x B and yD the temp is then found corresponding to the vapor composition and be plotted as function of f .
Continuous Distillation •
Flash distillation is used for separating components that boils at widely different temperature for separating close boiling component we require distillation with reflux and for large scale production we call it continuous distillation.
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Ideal Plate: On an ideal plate vapor liquid come in contact and leave the plate in the state of being equilibrium with each other.
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We consider a single plate in an ideal cascade as shown in the fig.
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We can see two streams enters and two stream leaves each plate and the concentration of streams entering and leaving are in equilibrium.
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Yn & Xn are in Equilibrium also Y n+1 & Xn+1 are in Equilibrium.
Continuous Distillation •
By the definition of an ideal plate the vapor and liquid leaving plate n are in equilibrium concentrations. The vapor is enriched in more volatile component A as it travels up the column and liquid is depleted of A as it flows downwards.
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The heat requires for vaporizing more volatile component must be supplied by the heat released in the condensation of less volatile component.
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Each plate act as an interchange apparatus.
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The concentration of more volatile component increases in both liquid and vapor phase as we go up in the column but the temp and pressure is decreased from top to bottom.
Continuous Fractionating Column •
For getting pure products from top and the bottom of the column the feed is admitted in the central portion and the plate where feed enters is called Feed Plate.
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All plates above feed are called Rectifying section and all plates below the feed including feed tray called as Stripping section.
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Feed flows down the stripping column by gravity to re boiler and the volatile components are being stripped off from feed in the stripping section.
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Vapor rising the column are condensed in the condenser and the condensate is collected from which a definite part is sent back to the column called as Reflux.
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Reflux provide the down flowing liquid in the rectifying section which helps to rectify the more volatile component in this section.
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With no reflux no rectification would occur in
Material balance for Continuous Fractionating Column •
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Total material balance B Component A balance x D + B x B.
F=D+ Fx F = D
Material balance Around the condenser and total balance around the control surface respectively D = V a – La D = V n+1 - Ln
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Operating Lines Applying the component balance and overall balance we can get the operating line for rectifying section as:
Substituting
For rectifying section
with Dx D
Reflux Ratio •
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The analysis of fractionating column is facilitated by the use of a quality called reflux ratio. Two types of reflux ratio are often used
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Reflux to the overhead product
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Reflux to the vapor
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Using reflux ratio the operating line for enriching section reduced to A
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The concentration xD is set by the condition of design and Reflux ratio is an operating variable and can be controlled by split between reflux and overhead product.
Constant Molal overflow •
In designing the column the concept of constant molal overflow is generally used which means the subscripts n,n+1, m, m+1 on L and V are dropped .
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Using this modification the operating lines are almost straight.
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This simplification means that each mole of high boiler that condenses provide energy to vaporize about the 1 mole of low boiler.
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Although there are some changes between the enthalpy of low and high boiler but because of heat loses from the column often require slightly more vapor to be formed at the bottom.
Condenser & Top Plate •
The condenser is used to get reflux to the column and the overhead product.
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Types of condenser
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Total condenser : It condenses all the vapor coming to it from the top tray by removing latent heat and the liquid inside is at its bubble point then the concentration XC =XD=Y1 also
V1= V & L = LC.
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Partial condenser: It condenses some vapor into liquid and the remaining liquid are condenses in the final condenser so the concentration XC ≠ XD but XD=Y1 .
Condenser & Top Plate •
If the condenser removes the latent heat only the liquid is at its bubble point and then the reflux L= Lc & V=V1.
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If the reflux is cooled below the bubble point then portion of vapor coming to the plate 1 must condense to heat the reflux so V1< V & L> Lc so the additional amount of liquid condenses found by the equation.
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The actual reflux ration in the column become
Bottom Plate & Reboiler •
Material Balance for bottom plate and reboiler.
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In common reboiler the vapor leaving the reboiler is in equilibrium with the liquid leaving as bottom product.
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Action of bottom plate is analogous to the top plate for constant molal overflow the operating line for stripping section is written as
Feed Plate •
At the plate where feed enters the liquid and vapor rate both changes according to the condition of the feed.
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If the feed is cold the entire feed stream adds to the liquid flowing down the column and in addition some vapor condenses to heat the feed to the bubble point this makes the liquid flow greater in the stripping section and decrease the flow of vapor in the rectifying section.
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If the feed is at its bubble point then no condensation is require to heat the feed so
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If the feed is partly vapor the liquid portion of feed become a part of and the vapor becomes a part of V.
Feed Plate •
If the feed is saturated vapor the entire feed become part of
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If the feed is superheated vapor part of the liquid from rectifying section is vaporized to cool the feed to a state of saturated vapor then vapor in the rectifying section consist of the vapor from stripping section, the feed and the extra mole vaporized in cooling the feed.
Feed Plate •
All the five types of feed can be characterized by using a single factor ‘q’ .
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‘q’ is defined as the moles of liquid flowing in the stripping section results from the introduction of each mole of feed.
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The value of q for cold-liquid feed is found from the equation
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The value of q calculated for super heated vapor is
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q has following numerical limits for the condition . –
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Cold feed ,q>1 Feed at bubble point (saturated liquid), q = 1 Feed partially vapor , 0 < q < 1 Feed at dew point (saturated vapor), q = 0 Feed superheated vapor q < 0 .
Feed Line •
By applying the value of q we can get the feed line equation on which all intersection of operating line must fall.
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Material balance equation for the contribution of feed stream to the total flow rate of reflux in the stripping section is
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Similarly the contribution of feed to the internal flow of vapor is F(1-q) so total flow rate of vapor in the rectifying section is
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Constant molal overflow material balance equation for two sections are
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Feed line Equation –
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position of feed line depends on xF and q
Construction of Operating lines •
Steps for plotting operating line are –
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Locate feed line Calculate the y-axis intercept of the rectifying line and plot that line through the intercept and the point . Draw the stripping line through point and the intersection of the rectifying line with the feed line.
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Line Line Line Line Line
a corresponds to the b corresponds to the c corresponds to the d corresponds to the e corresponds to the
q >1 q =1 0
Feed Plate location •
After the construction of operating lines and the feed lines the no of trays required for the given purity is found by the step-by-step construction as shown in the figure.
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Construction can begin at either the bottom of the stripping line or at the top of the rectifying line with the assumption of total condenser at the top.
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The change of steps from rectifying line to the stripping line is made in such a manner that maximum enrichment per plate is obtain so that minimum no of plates are required.
Heating & Cooling Requirements
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Heat losses from a large column are generally small and the column is itself essentially adiabatic. Heat effects of entire unit are confined to the condenser and reboiler. When the feed is liquid at the bubble point then heat supplied in the reboiler is approximately equal to that removed in the condenser. If the average latent heat is and the total sensible heat change in the liquid stream is small then the heat added to the reboiler is . If saturated heat is used as heating medium the steam requirement at the reboiler is
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If water is used as cooling medium then its requirement is
Minimum No of Plates •
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The slope of rectifying line from equation “A” is RD/(RD+1) and it increases as the reflux increases and when reflux is infinite also V=L then slope is 1.
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When RD is infinite then operating lines both coincide with the diagonal and this condition is called Total Reflux and at total reflux the no of plates for given separation is minimum.
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At total reflux the no of plates is minimum but the rate of feed and of both overhead and bottom product are zero.
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Since column is operating under total reflux there is no discontinuity between upper and lower section and the operating line is the 45 line for both section of the
Fenske Equation •
The relative volatility of two components terms of equilibrium concentration as
defined in
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Form Raoult’s law the relative volatility can be written as
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Relative volatility does not changes much over the range of temp in column so it is taken as constant.
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For binary system and can be replaced by and so the can be written in the below form for n+1 plate
Fenske Equation •
At total reflux D=0 and L/V=1 then we can write so 45 line becomes
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We multiply the succession of equations we get for the plates from 1 to n and we will get
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To reach the bottom recharge from the column Nmin plates and a reboiler are needed then the equation is modified to
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The equation for Nmin by algorithm gives
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This is Fenske equation applies when
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Minimum Reflux Ratio •
At any Reflux than the total reflux the no of plates needed is large for a given separation and increases as reflux is decreased.
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As the reflux ratio becomes smaller the no of plates becomes very large and at a definite minimum called the minimum reflux ratio the no of plates become infinite.
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All actual column must operate in between minimum reflux and the total reflux.
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At total reflux both operating lines coincide with the diagonal and as reflux is reduced moved towards equilibrium line along the feed line so the are on diagonal for steps shrinks and steps increases.
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Further decreases of reflux brings the intersection of operating lines outside the equilibrium curve then even an infinite plates cannot
Minimum Reflux Ratio •
The slope of line ad which passes through point x’ & y’ and xD, xD is
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Invariant Zone is the place where the intersection of operating line along with equilibrium curve take place forming an acute angle and an infinite no of plates are called for representing an infinite no of plates where there is no change in either liquid or vapor composition from
Optimum Reflux Ratio •
As the Reflux ratio increases from minimum the no of plates decreases rapidly and then more and more slowly until at total reflux the number of plates is a minimum.
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As reflux increases both V and L increase for given production and a point reached where the increase in column diameter is more rapid than the decrease in the no of plates.
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The cost of column unit is proportional to total plate area so the fixed charges first decreases then increases with the reflux ratio.
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The cost of heating equipments the reboiler and condenser also increases steadily with the Reflux Ratio this cost is small at low reflux but significantly increases with the reflux ratio and causes the curve upturn.
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Optimum Reflux Ratio is usually in the range of 1.1 to 1.5 times the minimum Reflux ratio.
Enthalpy Balance for fractionating Column •
The actual variation imposed by constant molal overflow can be removed by enthalpy balance along with material balance and phase equilibria.
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The overall enthalpy balance for the system when HF , HD , HB are specific enthalpies of the feed is
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When feed is liquid at the boiling point then HF is in between HD and HB then the term cancels each other making the heat supplies at the reboiler qr equals to that removed in the condenser.
Enthalpy Balance for Rectifying and stripping column •
Assuming specific enthalpy of vapor rising form plate n+1 is H y , n+1 and the enthalpy of liquid leaving tray n as H x ,n.
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Applying enthalpy balance on the control surface we get
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Eliminating qc using the below relationship
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Equation for enthalpy balance for Rectifying section
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Applying enthalpy balance for the stripping section we have
Design of Sieve-Plate Columns •
There are enormous variety of rectifying columns and their applications are enormous petroleum industry is the single largest user of these columns.
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Column diameter may vary form 1 ft to more than 30 ft and the plates form few to hundreds.
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Formerly bubble cap trays were used in the column manufacture but today most common contain sieve trays or lift valve trays.
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The column can operate from very low pressure to high one and temp can reach upto 900 oC.
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The sound design of fractionating column relies on a few principles and on a no of empirical correlations and also need much experience and is best done by experts
Operation of Sieve Plate •
A sieve tray is designed to bring a rising stream of vapor into intimate contact with a descending stream of liquid.
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The flow pattern on each plate is cross flow rather than counter flow but column as a whole considered to have counter flow of liquid and vapor.
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Down comer are used to bring the liquid form one plate to other plate it occupies about 10 to 15% area of the plate and leaving 70 to 80 % area for the bubbling of liquid.
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Weir is used to improve the liquid distribution and to prevent vapor bubbles from entering the down comer.
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The vapor passes through the perforated region of the plate between down comer and vapor velocity is maintain high enough to make froth on the plate for having high mass transfer area.
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apor ressure rop n eve trays The flow of vapor through the holes and the liquid on the plate require a difference in pressure. The require pressure is automatically developed by the reboiler which generate vapor at sufficient pressure to overcome the pressure drop in the column and condenser. The pressure drop across plate is divided into two parts the frictional pressure loss and also due to liquid holdup.
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Pressure drop from holes can be predicted by the definition of flow through orific as
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Vapor Pressure Drop in Sieve trays The amount of liquid on the plate increases with the weir height and with the flow rate of liquid but decreases with increasing vapor flow rate.
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The liquid hold up also depends on the physical property of liquid and vapor.
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Empirical formula for calculating liquid hold up is
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Where hi is liquid holdup hwis weir height how liquid over weir Correlation factor is β.
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Weir height can be calculated form Francis Equation for straight segmental weir is
Operating limit for Sieve Trays •
At low vapor velocities the pressure drop is not great enough to prevent liquid from flowing down through some of the holes this condition is called weeping.
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Weeping decreases the plate efficiency since some liquid passes to the next plate without contacting the vapor.
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The lower limit of operation is fixed by weeping and it could be extended by using smaller holes.
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Sieve tray operate over a three to fourfold range of floe rates between the weeping and flooding.
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When the liquid in the downcomer backs up to the next plate and the plate spacing or the entrainment become excessive then flooding in column occurs.
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Maximum permissible velocity in the column is given by
Plate Efficiency •
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We need to define plate efficiency to use ideal plate into actual plate. Types of plate efficiencies –
Overall efficiency which concerns the entire column is the least fundamental and is defined as the ratio of the no of ideal plates needed in an entire column to the no of actual plates.
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Murphree efficiency which concerns single plate defined as
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Murphree efficiency is therefore the change in vapor composition from one plate to the next divided by the change in vapor composition form one plate to the next divided by the change that would have occurred if the vapor leaving were in equilibrium with the liquid leaving.
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local efficiency which pertains to a specific location on a single plate is defined as
Relation Between Murphree and Local Efficiencies •
In the small columns the concentrations of liquid is same every where because the liquid is sufficiently agitated by the vapor flow so no gradients exists in the vapor stream then
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Local and Murphree Efficiencies are equal.
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In large columns liquid mixing in the direction of flow is not complete and a concentration gradient does exist in the liquid on the plate so the local efficiency is therefore considerably lower than Murphree efficiency.
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The relation between depends on the degree of liquid mixing and whether or not the vapor is mixed before going to the next plate calculations shows only a small difference in efficiency for completely mixed vapor or unmixed vapor but the effect of no liquid mixing can be quiet large.
