Introduction to Distillation

Distillation is defined as: – a process in which a liquid or

vapor mixture of two or more substances is separated into its component fractions of desired purity, by the application and removal of heat.

DISTILLATION PRINCIPLES

• The vapor of a boiling mixture will be richer in the components that have lower boiling points.

• When this vapor is condensed, the condensate will contain more volatile components.

• At the same time, the original mixture will contain more of the less volatile material.

DISTILLATION PRINCIPLES

DISTILLATION PRINCIPLES• As we add more “stages”

the vapor becomes richer in the more volatile component

• Conversely, the liquid becomes richer in the less volatile component

• We can achieve the purity we require by adding more “stages”

• Since the vapor is in “equilibrium” with the liguid, each stage is an “equilibrium stage”.

VAPOR-LIQUID EQUILIBRIUMVapor Pressure and Boiling

The vapor 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 vapor pressure:

energy input raises vapor pressure

vapor pressure is related to boiling

a liquid is said to ‘boil’ when its vapor pressure equals the surrounding pressure

the ease with which a liquid boils depends on its volatility

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

the vapor 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

VAPOR-LIQUID EQUILIBRIUMRaoult’s and Dalton’s Law

• For the design of distillation the relationship yi(xi) at thermodynamic equilibrium between the mole fraction xi of component i in the liquid phase and mole fraction yi of component i in the vapor phase must be known.

• For ideal mixtures this relationship is given by Raoult's law:

• p0,i is the vapor pressure of pure component i and p is the total pressure.

• The vapor pressure p0,i depends on the temperature but not on the pressure and the other components in the mixture.

• Pure component vapor pressures for many substances can be calculated with the Antoine Equation.

VAPOR-LIQUID EQUILIBRIUM• The term on the left-hand side of Raoult's law is also known as the

partial pressure pi of component i in the mixture:

• The sum of all partial pressures in a mixture equals the total pressure. This is known as Dalton’s Law.

p = pi

• For binary mixtures (two components) vapor-liquid equilibria can be graphically illustrated on:

• Boiling point diagrams • Equilibrium diagrams

• On boiling point diagrams the bubble-point and dew-point curves are plotted at constant pressure.

VAPOR-LIQUID EQUILIBRIUMThe Boiling Point Diagram

• The bubble-point curve t(x) connects the temperatures at which the liquid starts to boil.

• The dew-point curve t(y) shows the temperatures at which the saturated vapor starts to condense.

• The equation describing the bubble-point curve of an ideal binary mixture is

• The equation describing the dew-point curve is

VAPOR-LIQUID EQUILIBRIUMThe Boiling Point Diagram

• The region above the dew-point curve shows the composition of the superheated vapor while the region below the bubble-point curve shows the composition of the subcooled liquid. In these regions, a single phase exists.

• 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 vapors 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.

VAPOR-LIQUID EQUILIBRIUMVapour-Liquid-Equilibrium (VLE) Curves

• On equilibrium diagrams the vapor phase composition y is plotted against the liquid phase composition x at fixed pressure.

• The equilibrium curve of an ideal binary mixture may be derived from Raoult's and Dalton's laws:

VAPOR-LIQUID EQUILIBRIUMEquilibrium K

• The K-value is defined as

• An ideal system is one where the vapor obeys the ideal gas law and the liquid obeys Raoult’s Law. In this case

• Unfortunately, there are very few ideal systems in real life. K-values are obtained from Thermodynamic Equations of State (EOS) for non-polar or slightly polar systems or Activity Coefficient Models for polar systems.

• Non-ideality is due to the interaction between molecules due to intermolecular forces. The following groups all contribute to non-ideality hydroxyl groups (-OH), ketone groups (-C=O), aldehyde groups (-CHO), halogens (-Cl, -Br), and carboxylic acid groups (-COOH).

K = yi/xi

K = yi/xi = p0ixi/p

VAPOR-LIQUID EQUILIBRIUMEquilibrium K Models

Activity Coefficient ConditionsModelNRTL Strongly non-ideal including electrolytes

Wilson Slightly non-ideal

UNIQUAC Highly non-ideal systems

UNIFAC Non-ideal systems below 10 bars

Equations of State

Chao-Seader Light hydrocarbons,<1500 psia, 0-500 F

Grayson-Streed Improved CS, <3000 psia, 0-800 F

Soave-Redlich Kwong Hydrocarbon systems, supercritical P,T

Peng-Robinson Improved SRK, more stable near critical regions

VAPOR-LIQUID EQUILIBRIUMEquilibrium K Graphs

Source:

Perry’s Chemical Engineer’s Handbook

VAPOR-LIQUID EQUILIBRIUMRelative Volatility

Relative volatility is a measure of the differences in volatility between 2 components, and hence their boiling points. It indicates how easy or difficult a particular separation will be. The relative volatility of component ‘i’ with respect to component ‘j’ is defined as

yi = mole fraction of component ‘i’ in the vaporxi = mole fraction of component ‘i’ in the liquidKi = Equlibrium K of component ‘ i ’Kj = Equlibrium K of component ‘ j ’

Thus if the relative volatility between 2 components is very close to one, it is an indication that they have very similar vapor pressure characteristics. This means that they have very similar boiling points and therefore, it will be difficult to separate the two components via distillation.

ij = Ki / Kj

or

McCABE-THIELE METHOD

McCABE-THIELE METHOD

The McCabe-Thiele approach is a graphical one, and uses the VLE plot to determine the theoretical number of stages required to effect the separation of a binary mixture. It assumes constant molar overflow and this implies that:

– molal heats of vaporization of the components are roughly the same

– heat effects (heats of solution, heat losses to and from column, etc.) are negligible

– for every mole of vapor condensed, 1 mole of liquid is vaporized

• Given the VLE diagram of the binary mixture, operating lines are drawn first.

• Operating lines define the mass balance relationships between the liquid and vapor phases in the column.

• There is one operating line for the bottom (stripping) section of the column, and one for the top (rectification or enriching) section of the column.

• Use of the constant molar overflow assumption also ensures the the operating lines are straight lines.

McCABE-THIELE METHOD

Operating Line for the Rectification Section

• The desired top product composition is located on the VLE diagram, and a vertical line produced until it intersects the diagona.

• A line with slope R/(R+1) is then drawn from this intersection point as shown in the diagram below.

McCABE-THIELE METHOD

• R is the ratio of reflux flow (L) to distillate flow (D) and is called the reflux ratio

• It is a measure of how much of the material going up the top of the column is returned back to the column as

reflux.

Operating Line for the Stripping Section

• The desired bottom product composition is located on the VLE diagram.

• A vertical line is drawn from this point to the diagonal line, and a line of slope Ls/Vs is drawn as illustrated in the diagram below.

McCABE-THIELE METHOD

• Ls is the liquid rate down the stripping section of the column, while

• Vs is the vapor rate up the stripping section of the column.

• The slope of the operating line for the stripping section is a ratio between the liquid and vapor flows in that part of the column.

Number of Stages and Trays

• Doing the graphical construction repeatedly will give rise to a number of 'corner' sections, and each section will be equivalent to a stage of the distillation.

• Given the operating lines for both stripping and rectification sections, the graphical construction described above was applied.

McCABE-THIELE METHOD

• This particular example shows that 7 theoretical stages are required to achieve the desired separation.

• The required number of trays (as opposed to stages) is one less than the number of stages since the graphical construction includes the contribution of the reboiler in carrying out the separation.

Equilibrium and Operating Lines

• The McCabe-Thiele method assumes that the liquid on a tray and the vapor above it are in equilibrium.

• The liquid in stage 'n' and the vapor above it are in equilibrium, therefore, xn and yn lie on the equilibrium line.

McCABE-THIELE METHOD

• Since the vapor is carried to the tray above without changing composition, this is depicted as a horizontal line on the VLE plot.

• Its intersection with the operating line will give the composition of the liquid on tray 'n+1' as the operating line defines the material balance on the trays.

• The composition of the vapor above the 'n+1' tray is obtained from the intersection of the vertical line from this point to the equilibrium line.

The Feed Line (q-line)

• The condition of the feed determines the slope of the feed line or q-line. The q-line is that drawn between the intersection of the operating lines, and where the feed composition lies on the diagonal line.

McCABE-THIELE METHOD

• Depending on the state of the feed, the feed lines will have different slopes. For example,

q = 0 (saturated vapor) q = 1 (saturated liquid) 0 < q < 1 (mix of liquid and vapor) q > 1 (subcooled liquid) q < 0 (superheated vapor)

• The q-lines for the various feed conditions are shown in the diagram on the left.

The Feed Line (q-line)

• q is defined as the molar fraction of feed that is liquid

• The slope of the q-line is q/(q-1), which intersects the diagonal at the feed composition Xf.

• Other relationships from Kister’s book “Distillation Design” below:

McCABE-THIELE METHOD

Using Operating Lines and the Feed Line in McCabe-Thiele Design

• If we have information about the condition of the feed mixture, then we can construct the q-line and use it in the McCabe-Thiele design.

• However, excluding the equilibrium line, only two other pairs of lines can be used in the McCabe-Thiele procedure. These are:

feed-line and rectification section operating line

feed-line and stripping section operating line

stripping and rectification operating lines

• This is because these pairs of lines determine the third.

McCABE-THIELE METHOD

Multicomponent Distillation

• The McCabe-Thiele method can be used for analysis of multicomponent distillation columns by the use of a pseudo-binary method.

• In this method, we assume that separation occurs only between Key components. Other non-Key components are neglected.

• Key components are the two components in the feed mixture whose separation is specified.

• The more volatile component is called the light key and is recovered in the distillate.

• The less volatile component is called the heavy key and is recovered in the bottoms.

McCABE-THIELE METHOD

Multicomponent Distillation

• In the pseudo-binary method, xF, xD, and xB are computed using only the Light and Heavy Keys.

• The VLE curve is computed using the following equation, where is the relative volatility of the two key components.

McCABE-THIELE METHOD

y = x/(1+(-1)x)

Using the McCabe-Thiele Method

• In modern column design, using a process simulator is a must, however using shortcut methods such as the McCabe-Thiele can save time and money in setting up the problem.

• Other uses of the method according to Kister:– Detecting pinched regions

– Identifying mislocated feed points

– Identifying excessive reflux and reboil

– Guiding column optimization

McCABE-THIELE DESIGN METHOD

Main Components of Distillation Columns

• Distillation columns are made up of several components, each of which is used either to transfer heat energy or enhance material transfer.

DISTILLATION COLUMN

A typical distillation contains several major components:

a vertical shell where the separation of liquid components is carried out

column internals such as trays/plates and/or packing which are used to enhance component separations

a reboiler to provide the necessary vaporization for the distillation process

a condenser to cool and condense the vapor leaving the top of the column

a reflux drum to hold the condensed vapor from the top of the column so that liquid (reflux) can be recycled back to the column

Basic Operation and Terminology

• The liquid mixture that is to be processed is known as the feed and this is introduced usually somewhere near the middle of the column to a tray known as the feed tray.

• The feed tray divides the column into a top (enriching or rectification) section and a bottom (stripping) section.

• The feed flows down the column where it is collected at the bottom in the reboiler.

DISTILLATION COLUMN

Heat is supplied to the reboiler to generate vapor.

The vapor raised in the reboiler is re-introduced into the unit at the bottom of the column.

The liquid removed from the reboiler is known as the bottoms product or simply, bottoms.

• The vapor moves up the column, and as it exits the top of the unit, it is cooled by a condenser.

• The condensed liquid is stored in a holding vessel known as the reflux drum.

• Some of this liquid is recycled back to the top of the column and this is called the reflux.

DISTILLATION COLUMN

The condensed liquid that is removed from the system is known as the distillate or top product.

Thus, there are internal flows of vapor and liquid within the column as well as external flows of feeds and product streams, into and out of the column.

ANTOINE EQUATIONParameters of Antoine equation for selected substances

Substance Temp. range

/ °C A B C

Acetic acid 17 - 118 7.68454 1644.048 233.524 Acetone 57 - 205 7.75624 1566.690 273.419 Benzene 8 - 80 7.00481 1196.760 219.161 n-Butyl alcohol 89 - 126 7.48860 1305.198 173.427 i-Butyl alcohol 72 - 107 7.32625 1157.000 168.270 Carbon tetrachloride -20 - 77 6.96577 1177.910 220.576 Ethyl alcohol 20 - 93 8.23714 1592.864 226.184 n-Heptane -3 - 127 7.01880 1264.370 216.640 n-Hexane -25 - 92 7.00270 1171.530 224.366 Methyl alcohol 15 - 84 8.20591 1582.271 239.726 i-Octane 24 - 100 6.92798 1252.590 220.119 n-Pentane -50 - 58 7.00126 1075.780 233.205 Phenol 63 - 182 7.05545 1382.650 159.493 Isopropyl alcohol -26 - 83 9.00323 2010.330 252.636 Toluene -27 - 111 7.07581 1342.310 219.187 Water 1 - 100 8.19625 1730.630 233.426

Where:

• Temperature T in °C

• Vapor pressure p in mbar

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