Distillation Principle

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INTRODUCTION

Distillation is a process in which a liquid or vapour mixture of two or more substances

is separated into its component fractions of desired purity, by the application and removal of

heat. Distillation is based on the fact that the vapour of a boiling mixture will be richer in the

components that have lower boiling points. Therefore when this vapour is cooled and

condensed, the condensate will contain more volatile components. At the same time, the

original mixture will contain more of the less volatile material.

Distillation columns are designed to achieve this separation efficiently. Although many

people have a fair idea what distillation means, the important aspects that seem to be missed

from the manufacturing point of view are that:

Distillation is the most common separation technique.

Distillation consumes enormous amounts of energy, both in terms of cooling and heatingrequirements.

It can attribute to more than 50% of plant operating costs.

The best way to reduce operating costs of existing units, is to improve their efficiency via

process optimization and control. To achieve this improvement, a thorough understanding

of distillation principles and how distillation systems are designed is essential.

DISTILLATION PRINCIPLES

Separation of components from a liquid mixture via distillation depends on the differences in

boiling points of the individual components. Also, depending on the concentrations of the

components present, the liquid mixture will have different boiling point characteristics.

Therefore, distillation processes depends on the vapour pressure characteristics of liquid

mixtures.

Vapour Pressure and Boiling

The vapour pressure of a liquid at a particular temperature is the equilibrium pressure exerted

by molecules leaving and entering the liquid surface. Here are some important points

regarding vapour pressure:

energy input raises vapour pressure

vapour pressure is related to boiling

a liquid is said to boil when its vapour pressure equals the surrounding pressure

the ease with which a liquid boils depends on its volatility

liquids with high vapour pressures (volatile liquids) will boil at lower temperatures

the vapour pressure and hence the boiling point of a liquid mixture depends on the

relative amounts of the components in the mixture

distillation occurs because of the differences in the volatility of the components in the

liquid mixture


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The Boiling Point Diagram

The boiling point diagram shows how the equilibrium compositions of the

components in a liquid mixture vary with temperature at a fixed pressure. Consider an

example of a liquid mixture containing 2 components (A and B) - a binary mixture. This has

the following boiling point diagram.

The boiling point of A is that at which the B is that at which the mole fraction of A is

0. In this example, A is the more volatile component and therefore has a lower boiling point

than B. The upper curve in the diagram is called the dew-point curve while mole fraction of

A is 1. The boiling point of the lower one is called the bubble-point curve.

The dew-point is the temperature at which the saturated vapour starts to condense.The

bubble-point is the temperature at which the liquid starts to boil. The region above the dew-

point curve shows the equilibrium composition of the superheated vapour while the region

below the bubble-point curve shows the equilibrium composition of the subcooled liquid. For

example, when a subcooled liquid with mole fraction of A=0.4 (point A) is heated, its

concentration remains constant until it reaches the bubble-point (point B), when it starts to

boil. The vapours evolved during the boiling has the equilibrium composition given by point

C, approximately 0.8 mole fraction A. This is approximately 50% richer in A than the

original liquid. This difference between liquid and vapour compositions is the basis for

distillation operations.


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Specify Separation:

A material balance around the column is the first step in fractionation calculations. In order to

perform this balance, assumption of the product stream compositions must be made. There

are three ways to specifying desired product from the fractionators:

A percentage recovery of a component in the overhead or bottom stream.

A composition of one component in either product.

A specific physical property, such as vapor pressure, for either product.

In a multicomponent mixture, there are typically two components, which are keys to the

separation.

Light Key (LK) Component: Defined as lightest component in the bottom product in a

significant amount. Heavy Key (HK) Component: Defined as heaviest component in the overhead product

in a significant amount.

Normally, these two components are adjacent to each other in the volatility list. For hand

calculations, it is normally assumed for material balance purpose that all components lighter

than the light key are produced overhead and all components heavier than the heavy product

are produced with the bottom product

Set Column Pressure:

Before any design calculations can be made on a fractionation problem, a tower operating

pressure must be determined. One of the primary considerations for operating pressure is

cooling medium available for reflux condenser. The cooling media typically used are:

Air: The least expensive cooling method. Design limits the process to an 11C

approach to the ambient summer temperature.

Cooling Water: The satisfactory temperature approach is 5 to 10C.

Mechanical Refrigeration: For process temperature below 35C. This is the most

expensive cooling method from both a capital and operating cost.

Example: If cooling water supply is at 32C and return temperature of 40C using above

guideline the condensing temperature is 45C. For total condenser with a distillate

composition of C1, C2, C3 as 5/5/90%, the vapor pressure of the distillate @ 45C is 23.3

kg/cm2a. Set the pressure of the reflux drum equal to or slightly above the vapor pressure of

the distillate at 45C.

Generally, it is desirable to operate at as low a pressure as possible to maximize the relative

volatility between the key components of the separation. For a quick estimate of a gas

oil/resid operation a pressure of 50 mm Hg at the overhead line and a flash zone temp of


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370C may be used. However, if reducing the pressure requires a change to a more expensive

cooling method, this is the usually not a desirable choice.

Whereas, in case of lighter hydrocarbon separation, say C2 from C3, use of a suitable

refrigerant is required when the distillate liquid temperature is lower than the cooling water

temperature. Designing such a column at the higher pressure will make:

The refrigerant system smaller, but

Reboiler duty and the column diameter will be larger.

In deciding the operating pressure of a column, a number of factors have to be considered in

relation to the total system. A table is summarized below to indicate the advantages and

disadvantages of operating a column at higher pressure.

Advantages

It increases the boiling points of the distillate and enables a cheaper cooling medium

to be used. If the cooling medium remains unchanged, it reduces the condenser area

requirements.

It reduces energy costs in vacuum operation.

It increases vapor density and vapor handling capacity. Leads to smaller column

diameter for pressure upto 3.5 to 10.5 Kg/cm2.

It increases boiling points of liquids in the column for distillation of liquefied gases

and allows the use of cheaper construction materials.

It reduces the size of pipes and valves for the vapor line.

Below 1 atm, reduces air leakage into the system.

Disadvantages

It decreases relative volatility and makes separation more difficult. It will therefore

increase reflux and stage requirements, and requires higher reboiler and condenser

duties.

It increases bottom product temperature and increases the risk of chemical

degradation, polymerization and fouling. Above 7 kg/cm2g, column shell thickness will probably increase, whereas below 7

kg/cm2g, the effect is not as significant.

Increases reboiler temperature, needs a more expensive heating medium or increase in

reboiler area requirement.

It increases hazard potential if process materials are inflammable or toxic.

For mature technologies the operating pressures of the column have been standardized except

for some variation due to site condition, reference can be taken from similar plant/equipment.


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Determine minimum reflux and minimum number of stage

Relation between Reflux Ratio and Number of stages:

The design of a fractionation column is a capital cost versus energy cost trade-off problem.

The primary parameters are the number of stages and the reflux ratio. For most calculation,

reflux ratio is defined as the ratio of the molar rate of reflux liquid divided by the molar rate

of net overhead product. The desired separation can be achieved between the limits of

minimum reflux and minimum stages. The relationship between reflux ratio and number of

stages for a given separation is shown on figure 1 below. At minimum reflux an infinite

number of stages are required. At total reflux a minimum number of stages are required

Determination of Minimum Number of Stages

Fenskes

equation can be used to calculate the minimum number of stages


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If volatility varies widely, the approach of Winn is suggested, in which a modified volatility

is used


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Determination of Minimum Reflux Ratio

The Underwood method can be used for calculating minimum reflux ratio. The Underwood

method assumes:

Constant molar overflow, and

Constant relative volatility at the average column temperature.

The first step is to evaluate? (Which must lie between the relative volatilities of the keys) by

trail and error from

Once s is determined, the minimum reflux ratio is


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Where,

Lo = Liquid Reflux Rate, kgmole/hr.

D = Distillate Product Rate, kgmole/hr

T = Correlating Parameter

a = Relative Volatility

i = any component

X = Liquid Rate, kgmole/h

Set reflux ratio and calculate number of theoretical stages

It is customary to always add a certain safety margin above the minimum reflux level to

avoid potential errors from VLE estimation or from misdistribution at the desired reflux

ratio. The rule of thumb for R/Rminwhich is regarded satisfactory as a general guideline is

listed below

There are several considerations in deciding the reflux ratio; the following comments indicate

the effect of a change in the reflux ratio in relation to the number of stages.

The higher the reflux rate, the higher the cost of condensing and reboiling. Thesemake up the bulk of the column operating costs.

As reflux ratio increases, the number of stages decreases, but column diameter

increases.

Close to minimum reflux rate, small increases in reflux ratio will reduce the number

of stages considerably and therefore column height, but produce only a small increase

in column diameter.

Further increases in reflux will have a smaller effect in the height reduction but

greater increase in column diameter.

The closest possible approach of design to minimum reflux requires accurate methods such as

vigorous stage wise calculations, developed by multiple cases on computers.


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Determination of Number of theoretical stages:

The number of theoretical stages required for a given separation at a reflux ratio between

minimum and total reflux can be determined from Erbar and Maddox Correlation. This

correlation relates the ratio of minimum stages to theoretical stages to the minimum reflux

ratio and the operating reflux ratio as shown below:

We have calculated

Minimum Reflux Ratio = Rm = (Lo/V) m

Minimum number of stages = Sm

R = (Lo/V) = Reflux Ratio.

Knowing Sm/S from the graph below we can calculate S = Number of stage


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Adjust actual reflux for feed vaporization if necessary

The Erbar and Maddox correlation is based on bubble point feed. For a feed stream between

the bubble point and dew point the following relationship adjusts the vapor rate from the top

tray for non bubble point feed.

V = Vapor rate, kg mol/hr

D = Overhead product rate, kg mol/hr

F = Feed rate, kg mol/hr

HVF = Vaporized feed stream enthalpy, Kcal/Kg mol

HBP = Bubble point feed stream enthalpy, Kcal/Kg mol

QC = Condenser duty Kcal/hr

LO = Liquid condensed in a partial condenser, Kg mol/h

Determination of feed location

Once a base case tower has been simulated, optimization studies on trays vs. reflux, feed tray

location, etc. can be carried out. One criterion often used for optimum location of the feed

tray is to try and match the feed composition as closely as possible to a corresponding tray

composition. The approximate feed location can be determined by the ratio of the total

number of theoretical stages above and below the feed plate from following equation:

Fenskes Equation Kirkbridges Equation

Akashahetals Correction

Fenskes Equation


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CONCLUSION

Generally, the efficiency of any distillation column is its ability to produce maximum outputwith minimum cost as much as possible. Thus the design considerations discussed in the

preceding section tend to achieve this goal; perhaps the is no other consideration that can be

place above this in terms of importance.


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REFERENCE

1.

Coulson and Richardsons chemical engineering series volume 2 and 6 by J. M.

COULSON & J. F. RICHARDSON

2. Perrys Chemical Engineers Handbook; 7thEdition

3.

http://www.artisan-distiller.net/phpbb3/viewypic.php?f=2326&start=15

4. http://www.lorien.ncl.ac.uk/ming/distil/distil0.htm

5. http://www.seperatontechnology.com/distillation-design/
http://www.artisan-distiller.net/phpbb3/viewypic.php?f=2326&start=15http://www.artisan-distiller.net/phpbb3/viewypic.php?f=2326&start=15http://www.lorien.ncl.ac.uk/ming/distil/distil0.htmhttp://www.lorien.ncl.ac.uk/ming/distil/distil0.htmhttp://www.seperatontechnology.com/distillation-design/http://www.seperatontechnology.com/distillation-design/http://www.seperatontechnology.com/distillation-design/http://www.lorien.ncl.ac.uk/ming/distil/distil0.htmhttp://www.artisan-distiller.net/phpbb3/viewypic.php?f=2326&start=15

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Distillation Principle