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    Distillation is defined as:

    a process in which a liquid or vapour mixture of two or more

    substances is separated into its component fractions of desiredpurity, 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 moreof the less volatile material.

    Distillation columns are designed to achieve this separation efficiently.

    Although many people have a fair idea what distillation means, the importantaspects that seem to be missed from the manufacturing point of view are that:

    distillation is the most common separation technique

    it consumes enormous amounts of energy, both in terms of cooling and

    heating requirements

    it can contribute to more than 50% of plant operating costs

    The best way to reduce operating costs of existing units, is to improve their efficiencyand operation via process optimisation and control. To achieve this improvement, a

    thorough understanding of distillation principles and how distillation systems aredesigned is essential.

    The purpose of this set of notes is to expose you to the terminology used in

    distillation practice and to give a very basic introduction to:

    types of columns

    basic distillation equipment and operation

    column internals


    distillation principles

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    vapour liquid equilibria

    distillation column design and

    the factors that affect distillation column operation


    There are many types of distillation columns, each designed to perform specific typesof separations, and each design differs in terms of complexity.

    Batch and Continuous Columns

    One way of classifying distillation column type is to look at how they are operated.Thus we have:

    batch and

    continuous columns.

    Batch Columns

    In batch operation, the feed to the column is introducedbatch-wise. That is, the column is charged with a 'batch'and then the distillation process is carried out. When thedesired task is achieved, a next batch of feed isintroduced.

    Continuous Columns

    In contrast, continuous columns process a continuous feedstream. No interruptions occur unless there is a problemwith the column or surrounding process units. They arecapable of handling high throughputs and are the mostcommon of the two types. We shall concentrate only onthis class of columns.

    Types of Continuous Columns

    Continuous columns can be further classified according to:

    the nature of the feed that they are processing,binary column - feed contains only two components

    multi-component column - feed contains more than two components

    the number of product streams they have

    multi-product column - column has more than two product streams

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    where the extra feed exits when it is used to help with the separation,

    extractive distillation - where the extra feed appears in the bottom productstream

    azeotropic distillation - where the extra feed appears at the top productstream

    the type of column internals

    tray column - where trays of various designs are used to hold up the liquid to

    provide better contact between vapour and liquid, hence better separation

    packed column - where instead of trays, 'packings' are used to enhance

    contact between vapour and liquid


    Main Components of Distillation Columns

    Distillation columns are made up of several components, each of which is used either

    to tranfer heat energy or enhance materail transfer. 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 packings which are used toenhance component separations

    a reboiler to provide the necessary vaporisation for the distillationprocess

    a condenser to cool and condense the vapour leaving the top of the


    a reflux drum to hold the condensed vapour from the top of the column

    so that liquid (reflux) can be recycled back to the columnThe vertical shell houses the column internals and together with the condenser andreboiler, constitute a distillation column. A schematic of a typical distillation unit with

    a single feed and two product streams is shown below:

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    Basic Operation and TerminologyThe liquid mixture that is to be processedis known as the feed and this is

    introduced usually somewhere near themiddle of the column to a tray known as

    the feed tray. The feed tray divides thecolumn 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 thereboiler.

    Heat is supplied to the reboiler to generate vapour. The source of heatinput can be any suitable fluid, although in most chemical plants this isnormally steam. In refineries, the heating source may be the outputstreams of other columns. The vapour raised in the reboiler is re-introduced into the unit at the bottom of the column. The liquidremoved from the reboiler is known as the bottoms product or simply,bottoms.

    The vapour moves up the column, and as it exits the top of the unit, itis cooled by a condenser. The condensed liquid is stored in a holdingvessel known as the reflux drum. Some of this liquid is recycled back to

    the top of the column and this is called the reflux. The condensedliquid that is removed from the system is known as the distillate or topproduct.

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

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    Trays and Plates

    The terms "trays" and "plates" are usedinterchangeably. There are many types of tray designs,

    but the most common ones are :

    Bubble cap traysA bubble cap tray has riser or chimney fitted

    over each hole, and a cap that covers the riser.The cap is mounted so that there is a space

    between riser and cap to allow the passage ofvapour. Vapour rises through the chimney and is directed downward by the

    cap, finally discharging through slots in the cap, and finally bubbling throughthe liquid on the tray.

    Valve trays

    In valve trays, perforations are covered byliftable caps. Vapour flows lifts the caps, thus selfcreating a flow area for the passage of vapour.

    The lifting cap directs the vapour to flowhorizontally into the liquid, thus providing better

    mixing than is possible in sieve trays.

    Sieve traysSieve trays are simply metal plates with holes in

    them. Vapour passes straight upward throughthe liquid on the plate. The arrangement,

    number and size of the holes are designparameters.

    Because of their efficiency, wide operating range, ease of maintenance and costfactors, sieve and valve trays have replaced the once highly thought of bubble cap

    trays in many applications.

    Liquid and Vapour Flows in a Tray Column

    The next few figures show the direction of vapour and liquid flow across a tray, and

    across a column.

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    Each tray has 2 conduits, one on each side, called downcomers.

    Liquid falls through the downcomers by gravity from one tray to theone below it. The flow across each plate is shown in the above diagramon the right.A weir on the tray ensures that there is always some liquid (holdup) onthe tray and is designed such that the the holdup is at a suitable

    height, e.g. such that the bubble caps arecovered by liquid.Being lighter, vapour flows up the column andis forced to pass through the liquid, via theopenings on each tray. The area allowed forthe passage of vapour on each tray is called

    the active tray area.The picture on the left is a photograph of asection of a pilot scale column equiped withbubble capped trays. The tops of the 4bubble caps on the tray can just be seen. Thedown- comer in this case is a pipe, and isshown on the right. The frothing of the liquid

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    on the active tray area is due to both passage of vapour from the traybelow as well as boiling.As the hotter vapour passes through the liquid on the tray above, ittransfers heat to the liquid. In doing so, some of the vapour condensesadding to the liquid on the tray. The condensate, however, is richer in

    the less volatile components than is in the vapour. Additionally,because of the heat input from the vapour, the liquid on the tray boils,generating more vapour. This vapour, which moves up to the next trayin the column, is richer in the more volatile components. Thiscontinuous contacting between vapour and liquid occurs on each trayin the column and brings about the separation between low boilingpoint components and those with higher boiling points.

    Tray Designs

    A tray essentially acts as a mini-column, each accomplishing a fraction of the

    separation task. From this we can deduce that the more trays there are, the better

    the degree of separation and that overall separation efficiency will dependsignificantly on the design of the tray. Trays are designed to maximise vapour-liquidcontact by considering the

    liquid distribution and

    vapour distribution

    on the tray. This is because better vapour-liquid contact means better separation ateach tray, translating to better column performance. Less trays will be required to

    achieve the same degree of separation. Attendant benefits include less energy usageand lower construction costs.

    There is a clear trend to improve separations by supplementing the

    use of trays by additions of packings.Packings

    Packings are passive devices that are designed to increase the interfacial area for

    vapour-liquid contact. The following pictures show 3 different types of packings.

    These strangely shaped pieces are supposed to impart good vapour-liquid contact when a particular type is placed together in numbers,without causing excessive pressure-drop across a packed section. Thisis important because a high pressure drop would mean that moreenergy is required to drive the vapour up the distillation column.

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    Packings versus Trays

    A tray column that is facing throughput problems may be de-bottlenecked by

    replacing a section of trays with packings. This is because:

    packings provide extra inter-facial area for liquid-vapour

    contactefficiency of separation is increased for the same column


    packed columns are shorter than trayed columns

    Packed columns are called continuous-contact columns while trayed columns are

    called staged-contact columns because of the manner in which vapour and liquid arecontacted.

    COLUMN REBOILERSThere are a number of designs of reboilers. It is beyond the scope of this set of

    introductory notes to delve into their design principles. However, they can beregarded as heat-exchangers that are required to transfer enough energy to bring the

    liquid at the bottom of the column to boiling boint. The following are examples oftypical reboiler types.

    Photo courtesy of Brian Kennedy

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    A novel development in reboiler designis the self-cleaning shell-and-tube heatexchangers by Klarex Technology forapplications where heat exchangesurfaces are prone to fouling by theprocess fluid. Particles are introducedinto the process stream and theseproduce a scouring action on the heatexchange surfaces. An example is

    shown in the diagram on the left. [Click on it to find out more]


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

    differences in boiling points of the individual components. Also, depending on theconcentrations 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 surroundingpressure

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    the ease with which a liquid boils depends on its volatility

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


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

    on the relative amounts of the components in the mixturedistillation occurs because of the differences in the volatility of the

    components in the liquid mixture

    The Boiling Point Diagram

    The boiling point diagram shows how the

    equilibrium compositions of thecomponents in a liquid mixture vary with

    temperature at a fixed pressure.

    Consider an example of a liquid mixturecontaining 2 components (A and B) - a

    binary mixture. This has the followingboiling point diagram.

    The boiling point of A is that atwhich the mole fraction of A is 1.The boiling point of B is that atwhich the mole fraction of A is 0.In this example, A is the morevolatile component and thereforehas a lower boiling point than B.

    The upper curve in the diagram is called the dew-point curve while the

    lower one is called the bubble-point curve.

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

    The bubble-point is the temperature at which the liquid startsto boil.

    The region above the dew-point curve shows the equilibriumcomposition of the superheated vapour while the region below thebubble-point curve shows the equilibrium composition of the subcooled


    For example, when a subcooled liquid with mole fraction of A=0.4(point A) is heated, its concentration remains constant until it reachesthe bubble-point (point B), when it starts to boil. The vapours evolvedduring the boiling has the equilibrium composition given by point C,approximately 0.8 mole fraction A. This is approximately 50% richer in

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    A than the original liquid.

    This difference between liquid and vapour compositions is thebasis for distillation operations.

    Relative Volatility

    Relative volatility is a measure of the differences in volatility

    between 2 components, and hence their boiling points. It indicateshow easy or difficult a particular separation will be. The relative

    volatility of component i with respect to component j is definedas

    yi = mole fraction of component i in thevapour

    xi = mole fraction of component i in the liquid

    Thus if the relative volatility between 2 components is very close toone, it is an indication that they have very similar vapour pressurecharacteristics. This means that they have very similar boiling pointsand therefore, it will be difficult to separate the two components viadistillation.


    Distillation columns are designed based on the boiling point properties of thecomponents in the mixtures being separated. Thus the sizes, particularly the height,

    of distillation columns are determined by the vapour liquid equilibrium (VLE) data forthe mixtures.

    Vapour-Liquid-Equilibrium (VLE) Curves

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    Constant pressure VLE data is obtained from

    boiling point diagrams. VLE data of binary

    mixtures is often presented as a plot, as shownin the figure on the right. The VLE plot expresses

    the bubble-point and the dew-point of a binarymixture at constant pressure. The curved line is

    called the equilibrium line and describes thecompositions of the liquid and vapour inequilibrium at some fixed pressure.

    This particular VLE plot shows a binarymixture that has a uniform vapour-liquidequilibrium that is relatively easy to separate.The next two VLE plots below on the otherhand, shows non-ideal systems which will

    present more difficult separations. We can tell from the shapes of the curves andthis will be explained further later on.

    The most intriguing VLE curves are generated by azeotropic systems. An azeotrope isa liquid mixture which when vaporised, produces the same composition as the liquid.

    The two VLE plots below, show two different azeotropic systems, one with aminimum boiling point and one with a maximum boiling point. In both plots, the

    equilibrium curves cross the diagonal lines, and this are azeotropic points where theazeotropes occur. In other words azeotropic systems give rise to VLE plots where the

    equilibrium curves crosses the diagonals.

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    Note the shapes of the respective equilibrium lines in relation to thediagonal lines that bisect the VLE plots.

    Both plots are however, obtained from homogenous azeotropicsystems. An azeotrope that contains one liquid phase in contact withvapour is called a homogenous azeotrope. A homogenous azeotropecannot be separated by conventional distillation. However, vacumndistillation may be used as the lower pressures can shift the azeotropicpoint.Alternatively, an additional substance may added to shift theazeotropic point to a more favourable position.

    When this additional component appears in appreciable amounts at the topof the column, the operation is called azeotropic distillation.

    When the additional component appears mostly at the bottom of the column,

    the operation is called extractive distillation

    The VLE curve on the left is also generated by an

    azeotropic system, in this case a heterogenous

    azeotrope. Heterogenous azeotropes can beidentified by the flat portion on the equilibrium


    They may be separated in 2 distillation

    columns since these substances usuallyform two liquid phases with widelydiffering compositions. The phases maybe separated using settling tanks underappropriate conditions.

    Next, we will look at how VLE plots/data are used to design distillation columns.

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    As mentioned, distillation columns are designed using VLE data for the mixtures to

    be separated. The vapour-liquid equilibrium characteristics (indicated by the shape ofthe equilibrium curve) of the mixture will determine the number of stages, and hence

    the number of trays, required for the separation. This is illustrated clearly byapplying the McCabe-Thiele method to design a binary column.


    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 binarymixture. It assumes constant molar overflow and this implies that:

    molal heats of vaporisation of the components are roughly the same

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


    for every mole of vapour condensed, 1 mole of liquid is vaporised

    The design procedure is simple. Given the VLE diagram of the binary mixture,

    operating lines are drawn first.

    Operating lines define the mass balance relationships between the liquid and

    vapour phases in the column.

    There is one operating line for the bottom (stripping) section of the column, and

    on 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.

    Operating Line for the Rectification Section

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    The operating line for the rectification section is constructed asfollows. First the desired top product composition is located on theVLE diagram, and a vertical line produced until it intersects thediagonal line that splits the VLE plot in half. A line with slope R/(R+1) is then drawn from this instersection point as shown in the

    diagram below.

    R is the ratio of reflux flow (L) to distillate flow (D) and is called thereflux ratio and is a measure of how much of the material going upthe top of the column is returned back to the column as reflux.

    Operating Line for the Stripping Section

    The operating line for the stripping section is constructed in a similar manner.However, the starting point is the desired bottom product composition. A

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

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    Ls is the liquid rate down the stripping section of the column, whileVs is the vapour rate up the stripping section of the column. Thusthe slope of the operating line for the stripping section is a ratiobetween the liquid and vapour flows in that part of the column.

    Equilibrium and Operating Lines

    The McCabe-Thiele method assumes that the liquid on a tray and the vapourabove it are in equilibrium. How this is related to the VLE plot and the operating

    lines is depicted graphically in the diagram on the right.

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    A magnified section of the operating line for the stripping sectionis shown in relation to the corresponding n'th stage in the column.L's are the liquid flows while V's are the vapour flows. x and ydenote liquid and vapour compositions and the subscripts denote

    the origin of the flows or compositions. That is 'n-1' will meanfrom the stage below stage 'n' while 'n+1' will mean from thestage above stage 'n'. The liquid in stage 'n' and the vapourabove it are in equilibrium, therefore, xn and yn lie on theequilibrium line. Since the vapour is carried to the tray abovewithout changing composition, this is depicted as a horizontal lineon the VLE plot. Its intersection with the operating line will givethe composition of the liquid on tray 'n+1' as the operating linedefines the material balance on the trays. The composition of thevapour above the 'n+1' tray is obtained from the intersection ofthe vertical line from this point to the equilibrium line.

    Number of Stages and Trays

    Doing the graphical constructionrepeatedly will give rise to a number

    of 'corner' sections, and each sectionwill be equivalent to a stage of the

    distillation. This is the basis of sizingdistillation columns using the

    McCabe-Thiele graphical designmethodology as shown in the

    following example.

    Given the operating lines forboth stripping and rectificationsections, the graphicalconstruction described abovewas applied. This particularexample shows that 7

    theoretical stages are required to achieve the desired separation.The required number of trays (as opposed to stages) is one lessthan the number of stages since the graphical constructionincludes the contribution of the reboiler in carrying out theseparation.

    The actual number of trays required is given by the formula:

    (number of theoretical trays)/(tray efficiency)

    Typical values for tray efficiency ranges from 0.5 to 0.7 anddepends on a number of factors, such as the type of trays beingused, and internal liquid and vapour flow conditions. Sometimes,additional trays are added (up to 10%) to accomodate the

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    possibility that the column may be under-designed.

    The Feed Line (q-line)

    The diagram above also shows thatthe binary feed should be introduced

    at the 4'th stage. However, if thefeed composition is such that it does

    not coincide with the intersection ofthe operating lines, this means that

    the feed is not a saturated liquid. Thecondition of the feed can be deduced

    by the slope of the feed line or q-line.

    The q-line is that drawn between theintersection of the operating lines,

    and where the feed composition lieson the diagonal line.

    Depending on the state of thefeed, the feed lines will have

    different slopes. For example,q = 0 (saturated vapour)q = 1 (saturated liquid)

    0 < q < 1 (mix of liquid and vapour)q > 1 (subcooled liquid)

    q < 0 (superheated vapour)

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

    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, excludingthe 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.[see Flash tutorial on Distillation Basics written by Jon Lee]


    Determining the number of stages required for the desired degree of separation and

    the location of the feed tray is merely the first steps in producing an overall

    distillation column design. Other things that need to be considered are tray spacings;column diameter; internal configurations; heating and cooling duties. All of these can

    lead to conflicting design parameters. Thus, distillation column design is often aniterative procedure. If the conflicts are not resolved at the design stage, then the

    column will not perform well in practice. The next set of notes will discuss the factorsthat can affect distillation column performance.

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    Here we will expand on the design of columns by looking briefly at the effects of

    the number of trays, and

    the position of the feed tray, and

    on the performances of distillation columns.

    Effects of the Number of Trays

    It can be deduced from the previous section on distillation columndesign that the number of trays will influence the degree ofseparation. This is illustrated by the following example.

    Consider as a base case, a 10 stage column. The feed is a binarymixture that has a composition of 0.5 mole fraction in terms of themore volatile component, and introduced at stage 5. The steady-state terminal compositions of about 0.65 at the top (stage 1) and0.1 at the bottom (stage 10) are shown below:

    Composition Profile: 10 stages, feed at stage 5

    Suppose we decrease the number of stages to 8, and keep thefeed at the middle stage, i.e. stage 4. The resulting compositionprofile is:

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    Composition Profile: 8 stages, feed at stage 4

    We can see that the top composition has decreased while the

    bottom composition has increased. That is, the separation ispoorer.

    Now, if we increase the number of stages to 12, and againintroduce the feed at mid-column, i.e. stage 6, the compositionprofile we get is:

    Composition Profile: 12 stages, feed at stage 6

    Again, the composition has changed. This time the distillate ismuch richer in the more volatile component, while the bottomshas less, indicating better separation.

    Thus, increasing the number of stages will improveseparation.

    Effect of Feed Tray Position

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    Here we look at how the position of the feed tray affectsseparation efficiency. Suppose we have a 20 stage column, againseparating a binary mixture that has a composition of 0.5 molefraction in terms of the more volatile component. The terminalcompositions obtained when the feed is introduced at stages 5, 10

    and 15 (at fixed reflux and reboil rates) are shown in the followingplots.

    Composition profile: 20 stages, feed at stage 5

    Composition profile: 20 stages, feed at stage 10

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    Composition profile: 20 stages, feed at stage 15

    [Click on green button to see animated display of how the compositionprofiles change with feed stage position]

    As the feed stage is moved lower down the column, the topcomposition becomes less rich in the more volatile componentwhile the bottoms contains more of the more volatile component.However, the changes in top composition is not as marked as thebottoms composition.

    The preceding examples illustrate what can happen if the positionof the feed tray is shifted for this particular system. They shouldnot be used to generalise to other distillation systems, as theeffects are not straightforward.

    In Depth Look at Extractive Distillation

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    1. Distillation for azeotropic mixture

    Distillation is the most widely used separation technique in thechemical and petroleum industry. However, not all liquid mixture areamenable to ordinary fractional distillation. When the components of

    the system have low relative volatilities (1.00 < < 1.05), separationbecomes difficult and expansive because a large number of trays arerequired and, usually, a high reflux ratio as well. Both equipment andutilities costs increase markedly and the operation can becomeuneconomical. If the system forms azeotropes, as in a benzene andcyclohexane system, a different problem arises, - the azeotropiccomposition limit the separation, and for a better separation thisazeotrope must be bypassed in some way. At low to moderatepressure, with the assumption of ideal-gas model for the vapor phase,the vapor-liquid phase equilibrium (VLE) of many mixture can beadequately describe by the following Modified Raoults Law:

    When i = 1, the mixture is said to be ideal Equation 1 simplifies to

    Raoults Law. Nonideal mixtures (i 1) can exhibit either positive (i >1) or negative deviations (i < 1) from Raoults Law. In many highly

    nonideal mixtures these deviations become so large that the pressure-composition (P-x, y) and temperature-composition (T-x, y) phasediagrams exhibit a minimum or maximum azeotrope point. In thecontext of the T-x, yphase diagram, these points are called theminimum boiling azeotrope (where the boiling temperature of theazeotrope is less than that of the pure component) or maximumboiling azeotrope (the boiling temperature of the azeotrope is higherthan that of the pure components). About 90% of the knownazeotropes are of the minimum variety. At these minimum and

    maximum boiling azeotrope, the liquid phase and its equilibrium vaporphase have the same composition, i.e.,xi = yifor i = 1, , c(2)Two main types of azeotropes exist, i.e. the homogeneous azeotrope,where a single liquid phase is in the equilibrium with a vapor phase;and the heterogeneous azeotropes, where the overall liquidcomposition which form two liquid phases, is identical to the vaporcomposition.

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    Most methods of distilling azeotropes and low relative volatilitymixtures rely on the addition of specially chosen chemicals to facilitatethe separation. The selection of the separating agent will be discussedlater.The five methods for separating azeotropic mixtures are:

    i. Extractive distillation and homogeneous azeotropic distillation where theliquid separating agent is completely miscible.

    ii. Heterogeneous azeotropic distillation, or more commonly, azeotropic

    distillation where the liquid separating agent, called the entrainer, forms

    one or more azeotropes with the other components in the mixture and

    causes two liquid phases to exist over a wide range of compositions. Thisimmiscibility is the key to making the distillation sequence work.

    iii. Distillation using ionic salts. The salts dissociates in the liquid mixture

    and alters the relative volatilities sufficiently that the separation becomepossible.

    iv. Pressure-swing distillation where a series of column operating at different

    pressures are used to separate binary azeotropes which change appreciablyin composition over a moderate pressure range or where a separating agent

    which forms a pressure-sensitive azeotrope is added to separate a pressure-

    insensitive azeotrope.

    v. Reactive distillation where the separating agent reacts preferentially andreversibly with one of the azeotropic constitutes. The reaction product is

    then distilled from the nonreacting components and the reaction is

    reversed to recover the initial component.

    2. Residue Curve Maps

    The most simple form of distillation, called simple distillation, is aprocess in which a muticomponent liquid mixture is slowly boiled in anopen pot and the vapors are continuously removed as they form. Atany instant in time the vapor is in equilibrium with the liquid remainingon the still. Because the vapor is always richer in the more volatilecomponents than the liquid, the liquid composition changescontinuously with time, becoming more and more concentrated in theleast volatile species. A simple distillation residue curve is a graphshowing how the composition of the liquid residue curves on the potchanges over time. A residue curve map is a collection of the liquidresidue curves originating from different initial compositions. Residue

    curve maps contain the same information as phase diagrams, butrepresent this information in a way that is more useful forunderstanding how to synthesize a distillation sequence to separate amixture.All of the residue curves originate at the light (lowest boiling) purecomponent in a region, move towards the intermediate boilingcomponent, and end at the heavy (highest boiling) pure component inthe same region. The lowest temperature nodes are termed as

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    unstable nodes (UN), as all trajectories leave from them; while thehighest temperature points in the region are termed stable nodes (SN),as all trajectories ultimately reach them. The point that the trajectoriesapproach from one direction and end in a different direction (as alwaysis the point of intermediate boiling component) are termed saddle

    point(S). Residue curve that divide the composition space intodifferent distillation regions are called distillation boundaries. A betterunderstanding of the residue curve map may be view in Fig. 1. Noticethat the trajectories move from the lowest temperature componenttowards the highest.

    Fig. 1 Residue curve mapfor a ternary mixture with a

    distillation boundaryrunning from pure

    component D to the binaryazeotrope C.

    Residue curve maps would be of limited usefulness if they could onlybe generated experimentally. Fortunately that is not the case. Usingvarious references, the simple distillation process can be described bythe set of equations:

    Research studies have also been done to determine the relationshipbetween the number of nodes (stable and unstable) and saddle points

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    one can have in a legitimately drawn ternary residue plot. Theequation is based on topological arguments. One form for this equationis:

    4(N3 - S3) + 2(N2 - S2) + (N1 - S1) = 1Where:

    Ni = number of nodes (stable and unstable) involving i speciesSi = number of saddles involving i speciesMany different residue curve maps are possible when azeotropes arepresent. Ternary mixtures containing only one azeotrope may exhibitsix possible residue curve maps that differ by the binary pair formingthe azeotrope and by whether the azeotrope is minimum or maximumboiling.Even though the simple distillation process has no practical use as amethod for separating mixtures, simple distillation residue curve mapshave extremely useful applications, such as:

    i. Testing of the consistency of experimental azeotropic data;

    ii. Predict the order and content of the cuts in batch distillation;iii. In distillation, to check whether the given mixture is separable by distillation,

    identification of the entrainers / solvents, prediction of attainable product

    compositions, qualitative prediction of composition profile shape, and synthesisof the corresponding distillation column.

    By identifying the limiting separation achievable by distillation, residuecurve maps are also useful in synthesizing separation sequencescombining distillation with other methods.

    3. Homogeneous Azeotropic Distillation

    The most general definition of homogeneous azeotropic distillation isthe separation of any single liquid-phase mixture containing one ormore azeotropes into the desired pure component or azeotropicproducts by continuous distillation. Thus, in addition to azeotropicmixtures which require the addition of a miscible separating agent inorder to be separated, homogeneous azeotropic distillation alsoincludes self-entrained mixtures that can be separated without theaddition of a separating agent.The first step in the synthesis of a homogeneous azeotropic distillationsequence is to determine the separation objective. Sometimes it is

    desirable to recover all of the constituents in the mixture as purecomponents other times it is sufficient to recover only some of thepure components as product. In our example, we would like to recoverthe cyclohexane product at 90% purity and recycle the separatingagent back to the initial separating column for further use.The second step is to sketch the residue curve map for the mixture tobe separated. The residue curve map allows one to determine whether

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    the goal can be reached and if so how to reach it, or the goal needs tobe redefined.Distillation boundaries for continuous distillation are approximated bysimple distillation boundaries. It is a good approximation for mixtureswith nearly simple distillation boundaries. For a separation to be

    feasible by distillation, the simple distillation boundary should not becrossed, i.e. the distillate and bottom composition should lie in thesame distillation region. A more detail calculation method involving thecomposition will be discuss in the later section.In the most common situation, a separating agent is added to separatea minimum boiling binary azeotrope into its two constituent purecomponents by homogeneous azeotropic distillation. Michael F. D. andJeffrey P. K. presented seven of the most favorable residue curve mapsfor this task. Of the seven, the map representing extractive distillationis by far the most common and the most important. Its correspondingresidue curve and column sequences are shown in Fig. 2 below.

    4. Extractive distillation

    Extractive distillation is defined as distillation in the present of amiscible, high boiling, relatively nonvolatile component, the solvent,that forms no azeotropes with the other components in the mixture. Itis widely used in the chemical and petrochemical industries forseparating azeotropic, close-boiling, and others low relative volatilitymixture.Extractive distillation works because the solvent is specially chosen tointeract differently with the components of the original mixture,

    thereby altering their relative volatilities. Because these interactionsoccur predominantly in the liquid phase , the solvent is continuouslyadded near the top of the extractive distillation column so that anappreciable amount is present in the liquid phase on all of the traysbelow. The mixture to be separated is added through second feedpoint further down the column. In the extractive column, thecomponent having the greater volatility, not necessarily thecomponent having the lowest boiling point, is taken overhead as arelatively pure distillate. The other component leaves with the solventvia the column bottoms. The solvent is separated from the remainingcomponents in a second distillation column and then recycled back to

    the first column.

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    Fig. 2 Extractive distillation with a heavy solventwhich introduce no new azeotrope for a minimum boiling

    azeotrope. In some case, B can come off the top of the first column.

    There are several industrial application for homogeneous azeotropicdistillation listed in the Encyclopedia of Separation TechnologybyMichael F. D., Jeffrey P. K.Extractive distillations can be divided into three categories, eachcorrespond to the different residue curve maps, the minimum boilingazeotropes, maximum boiling azeotropes and the nonazeotropemixtures. Since our benzene-cyclohexane mixture to be separated is ofthe second type of mixture, i.e. the minimum boiling azeotrope, we willfocus our attention on column sequencing this type of azeotropicseparation method in the following section.As in azeotropic distillation, design of extractive distillation system willalso requires significant preliminary work including:

    Choosing the solvent

    Developing or finding necessary data, such as azeotropic condition or residue


    Preliminary screening

    Computer simulation

    Small scale testing

    For our example, we will consider the first four steps.

    5. Solvent screening and selection

    5.1 Solvent criteriaOne of the most important steps in developing a successful

    (economical) extractive distillation sequence is selecting a goodsolvent. Approaches to the selection of an extractive distillation solventare discussed by Berg, Ewell et al. , and Tassions. In general, selectioncriteria include the following :

    i. Should enhance significantly the natural relative volatility of the key component.

    ii. Should not require an excessive ratio of solvent to nonsolvent (because of cost ofhandling in the column and auxiliary equipment.

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    iii. Should remain soluble in the feed components and should not lead to the

    formation of two phase.

    iv. Should be easily separable from the bottom product.v. Should be inexpensive and readily available.

    vi. Should be stable at the temperature of the distillation and solvent separation.

    vii. Should be nonreactive with the components in the feed mixture.viii. Should have a low latent heat.

    ix. Should be noncorrosive and nontoxic.

    Naturally no single solvent or solvent mixture satisfy all the criteria,and compromises must be reached.

    5.2 Solvent screeningPerry's handbook serve as a good reference for the solvent selectionprocedure, which can be thought of as a two-step process, i.e.:

    5.2.1 First level: Broad screening by functional group or chemical

    familyi. Homologous series: Select candidate solvent from the high boiling homologousseries of both light and heavy key components.

    ii. Robins Chart: Select candidate solvents from groups in the Robbins Chart (part ofthe chart is shown in Table 3) that tend to give positive (or no) deviations from

    Raoult's law for the key component desire in the distillate and negative (or no)

    deviations for the other key.iii. Hydrogen-bonding characteristic: are likely to cause the formation of hydrogen

    bonds with the key component to be removed in the bottoms, or disruption of

    hydrogen bonds with the key to be removed in the distillate. Formation and

    disruption of hydrogen bonds are often associated with strong negative and

    positive deviations, respectively from Raoult's Law.iv. Polarity characteristic: Select candidate solvents from chemical groups that tend

    to show higher polarity than one key component or lower polarity than the otherkey.

    5.2.2 Identification of individual candidate solventsi. Boiling point characteristic: Select only candidate solvents that boil at least 30-

    40oC above the key components to ensure that the solvent is relatively nonvolatile

    and remains largely in the liquid phase. With this boiling point difference, thesolvent should also not form azeotropes with the other components.

    ii. Selectivity at the infinite dilution: Rank the candidate solvents according to their

    selectivity at infinite dilution.iii. Experimental measurement of relative volatility: Rank the candidate solvents bythe increase in relative volatility caused by the addition of the solvent.

    Residue curve maps are of limited usefulness at the preliminaryscreening stage because there is usually insufficient informationavailable to sketch the them, but they are valuable and should be

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    sketched or calculated as part of the second stage of the solventselection.

    6 The scenario of our distillation process6.1 The azeotropic condition

    For our example which deals with the azeotropic mixture formedbetween benzene and cyclohexane, we have chosen extractivedistillation (one of the homogeneous azeotropic distillation methods).The reason of choosing this method is due to the availability ofinformation regarding this separation technique and its tendency tooperate more efficiently, i.e. in separating and recycling the separatingagent. A brief discussion of the process is given below.After the mixture exited as the bottom product of the flash unit, itcontains mostly our desire product of cyclohexane and also asignificant amount of unreacted benzene, which is to be recycled backto the reactor for further conversion. Our main goal now is to further

    separate the remaining components in the mixture. As cyclohexaneand benzene have been encounter most of the remaining compositionwith the mole % of 44.86 and 54.848 respectively (Table 1), we willconsider this to be a binary mixture in our further discussion.From the process flowsheeting, we would like to operate the distillationcolumn at the pressure of 150 kPa. At this condition, cyclohexane andbenzene will have boiling points of 94.34oC and 93.49oC respectively(Fig. 3). This is a typical case where conventional distillation wouldstruggle to perform the separation of this type of close boiling mixture.Thus, a special type of distillation technique, i.e. extractive distillationhas been chosen in order to purify the desire product, i.e. cyclohexane

    to our desired purity of 99.3%.As can be shown from Fig. 3, this binary composition will form aminimum boiling, homogeneous azeotrope at the temperature of 91oCand the corresponding composition at this point will be 45.5 mole % forcyclohexane and 55.5 mole % for benzene (Fig. 4).

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    Table 1 Properties of Stream 26 (bottom product of Flash Drum (F13)

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    6.2 Solvent selection for benzene-cyclohexane binary

    mixtureIn order to perform a successive extractive distillation, a solvent needsto be chosen to "break" the azeotrope that forms at the operatingpressure of the distillation column. Recommended solvent for thebenzene-cyclohaxane mixture from the literature,,, is aniline, with a

    solvent to feed ratio (S/F) of 4, which will shift the azeotropic pointtoward the corner of the high-boiling component cyclohexane, and theequilibrium curve of the original components fall below the diagonal(Fig. 5).

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    As was stated in the above section, the primary goal of solventselection is to identify a group of feasible solvents to perform a goodseparation. The desired product, i.e. cyclohexane should have a purityof above 99% to meet the market standard. Aniline was the firstsolvent that had been put to the simulator to be tried out, as it is of thesame homologous group as benzene. As can be shown from the resultin Table 2, this solvent will produce the desire production rate of 150

    with the solvent flow rate of 3500, i.e. a S/F ratio of 9.85. However, theproduct purity can only reach 70.08% and this does not meet ourproduct specification. As a result, other solvent may have to beresearched to perform the desire separation.We will have to perform the solvent selection criteria as stated in thepreceding section. At the column pressure of 150 kPa, cyclohexaneand benzene boil at 94.34oC and 93.49oC respectively and form aminimum-boiling azeotrope at 91oC. The natural volatility of the systemis benzene > cyclohexane, so the favored solvents most likely will bethose that cause the benzene to be recovered in the distillate.However, in order to get a better quality of product, we would like to

    recover cyclohexane as the distillate rather than from the bottomstream. Thus, solvent to be chosen should give positive deviationsfrom Raoult's law for cyclohexane and negative (or zero) deviation forbenzene.

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    Table 2 Result from computer simulation using aniline as solvent.

    Table 3 Part ofRobbins Charttaken from Perry's Chemical Handbook.

    Turning to the Robbins Chart(Table 3), we note that solvents thatmay cause the positive deviation for cyclohexane (Class 12) andnegative (or zero) to benzene (Class 11) came from the groups of 4, 7,

    8 and 9, which consist of polyol, amine and ether. We further considerthe solubility, the hydrogen bonding effect, and also the homologouscharacteristic of the solvent with the corresponding components in thefeed mixture. As few candidate solvents that had been put to thecomputer simulation, includedphenol (homologous to benzene), 1,2-benzenediol (homologous to benzene, with -OH group that will producehydrogen bonding), 1,3-butanediol (with -OH group that will producehydrogen bonding), and also 1,2-propanediol (same characteristic as

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    with 1,3-butanediol). 1,2-propanediol (often known aspropyleneglycol), seem to give the most promising results compared to the othersolvents. This result may be caused from the high solubility of benzenein this solvent and the hydrogen bonding that were formed betweenthe two constituents. Simulation result of this solvent can be view in

    Table 4.6.3 Construction of the residue curve

    Equation 3 and 4 were used to sketch the corresponding residue curvefor the three species. From the above information, we know that thesespecies have boiling points at 94.34 (cyclohexane), 93.49 (benzene)and 200.35oC (propylene glycol) at the pressure of 150 kPa, and anazeotrope that boils at 91oC between the two more volatile species. Aswere shown from Fig. 6 and Fig. 7 there were no new azeotropesformed between the solvent 1,2-propanediol respectively with theanother two component in the feed.

    Table 3 Result from computer simulation using 1,2-propanediol as solvent.

    We then start to sketch our residue curve map by sketching thetriangular diagram in Fig. 8, and placing the arrows pointing from thelower to higher temperatures around the edge. The corner points forbenzene and cyclohexane are single species point, and both areunstable nodes - all residue curves leave. The corner point forpropylene glycol is a single species point which is a stable node - allresidue curve enter. All three are nodes; none are saddles, thus;

    N1 = 3 and S1 = 0

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    We then further assume that there will be no ternary azeotrope beenform among the three constituents, i.e.,

    N3 = S3 = 0

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    The remaining steps here require the identification of the only binaryazeotrope that form between benzene and cyclohexane, to be either anode or a saddle point. From equation 4:

    4(0-0) + 2(N2 - S2) + (3-0) = 12(N2 - S2) = -2

    N2 - S2 = -1Thus, the only way we can satisfy the above equation is letting N2 = 0and S2 = 1, i.e. the binary azeotrope is a saddle point, which directsthe trajectories in another direction.

    6.4 Column operationThe extractive distillation unit of this cyclohexane production plantconsists of two distillation columns (Fig. 10), which we can easilyclassify as direct sequence columns. The first column acts as anextractive column where the solvent is introduced at the second stageof the column, so that it will be present throughout the column andexits with the bottoms. As were stated above, the solvent alters thenatural volatility of the binary mixture by forming hydrogen bonds withbenzene and allowing it to be recovered as the bottom product.The bottom product of the first column will then fed to the secondcolumn, i.e. the solvent recovery column, to undergo the normaldistillation to separate both the components for further usage, i.e.benzene being recycled to the reactor for further conversion whilesolvent to the first column for reuse. The main operation parameter ofthe distillation unit is shown in Table 4.

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    Table 4 Distillation unit summary

    Fig. 10 Extractive distillation unit for cyclohexane production plant

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