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    University of JordanFaculty of Science & Technology

    Department of Chemical Engineering

    Chemical Engineering Laboratory III

    "Distillation Column"

    Date of

    Performing : 4/4/2005

    Date of

    submission : 19/4/2005

    "This report is made due to the request of chemical Engineering Department to study the

    effects of varying the reflux ratio on the Distillation Column efficiency"

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    Table of Contents :

    1- Summary

    3

    2- Introduction.. 4

    3- Theory..

    6

    4- Equipment11

    5- Procedure.13

    6- Results.. 15

    7- Discussion of Results 20

    8- Conclusion 22

    9- Nomenclature 23

    10- References 24

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    11- Appendices 25

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    Summary:

    The Distillation Column is an apparatus which separates two

    substances with difference in their relative volatilities. This separation

    process is studied by varying the reflux ratio between the reflux stream

    and the product stream, and so the composition of the distillate coming

    out also varies. Also the number of stages loaded inside the column

    affects the performance of the separation process, and so the efficiencyof each plate or stage loaded, which is called "murphee plate

    efficiency".

    Equilibrium relations besides two methods: McCabe & Thiele andPanchon & Savarit methods; were used to obtain the Theoretical

    Number of stages required.

    As a main results, The Distillate composition (with highervolatility material) rises with the reflux ratio. Theoretical number of

    stages was unable to be calculated with neither of the two methods

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    Introduction:

    Distillation is an operation whereby the vaporization of a liquidmixture yields a vapor phase containing more than one component, and

    it is desired to recover one or more of these components in a pure state.Or in another way, it's 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 columns are classified on the basis of how they'reoperated, so they classify into:Batch columns & Continuous columns.

    Our experiment is carried under Continuous column section,

    which 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

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

    extractive distillation - where the extra feed appears in the bottomproduct stream

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

    stream

    the type of column internals

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

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    liquid to provide better contact between vapor and liquid, hence better

    separation

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

    contact between vapor and liquid

    This experiment is done to:

    1. Demonstrate the effect of variation of reflux ratio upon

    distillate composition, which is the composition of thedesired product.

    2. Determine the number of theoretical plates within thecolumn using the methods of McCabe-Thiele & Panchon-

    Savarit.

    3. Find the murphee plate efficiency of one plate,

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    Theory:

    Separation of components from a liquid mixture via distillation

    depends on the differences in boiling points of the individualcomponents. Also, depending on the concentrations of the components

    present, the liquid mixture will have different boiling point

    characteristics.

    Mathematical-graphical methods for determining the number of

    theoretical trays or stages needed for a given separation of a binary

    mixture of A and B has been developed by:

    1) McCabe-Thiele method:

    The main assumption made in this method is that there must

    be equimolar overflow through the tower between the feed inletand the top tray and the feed inlet and bottom tray. A total

    material balance gives:

    Vn+1 + Ln-1 = Vn + Ln

    A component balance on one material (A) gives:

    Vn+1 yn+1 + Ln-1 xn+1 = Vnyn + Lnxn

    As the feed is from the bottom of the tower, then the whole

    sections is considered to be enriching section, and followingequation are derived:

    An overall material balance around the entire column statesthat the entering feed of (F) must equal the distillate (D) plus the

    bottoms (W) in:

    F = D + W

    A total material balance on component (A) gives:

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    F xF = D xD + W xW

    The vapor form the top tray having a composition y1 passes

    to the condenser, where it's condensed so that the resulting liquidis at the boiling point. The reflux stream L and distillate D have

    the same composition, so y1 = xD. Since equimolal overflow is

    assumed, L1 = L2 = Ln and V1 = V2 = Vn = Vn+1.

    Making a total material balance around the upper three stages:

    Vn+1 = Ln + D

    Making a balance on component (A):

    Vn+1 yn+1 = Ln xn + D xD

    Solving for yn+1, the enriching section operating line is:

    11

    1

    ++

    +

    +=

    n

    D

    n

    n

    n

    nV

    xDx

    V

    Ly

    Since DLV nn +=+1 ,1

    1+

    =

    +RR

    VL

    n

    n

    with the upper equation gives:

    111

    +

    +

    +

    =+

    R

    xx

    R

    Ry Dnn

    2) Ponchon-Savarit Method:

    This method, it obviates the need for the constant phase-ratio

    flow assumptions.

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    Start numbering the stages from the top as n, stage n-1 of the

    column shown in fig (a) is a mixing device where streams Ln,Vn-2enter and the equilibrated streams Vn-1, Ln-1 leave.

    The vaporVn-2 and liquid Ln are mixed to give overall

    composition z, which then separates into two equilibrium vapor

    and liquid phases Vn-1, Ln-1 that are connected by a tie line

    through z, as shown in fig (b).

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    The ponchon diagram embodies both enthalpy and material

    balance relationships as well as phase equilibrium conditions.

    Since it is unnecessary to assume constant molal overflow, the

    calculations can be done on per mole or per pound basis.

    By making material and enthalpy balances about the

    portion of the enriching section of the column in fig (a) enclosedwith the dotted line:

    For the more volatile component:

    Yn-2 Vn-2 = xn-1 Ln-1 + DxD

    Total material balance:Vn-2 = Ln-1 +D

    And the enthalpy balance:

    qD D + Hn-2 Vn-2 = hn-1 Ln-1 + hDD

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    By solving the material balance equations for Ln-1/D:

    12

    21

    =

    nn

    nDn

    xy

    yx

    D

    L

    And by simultaneous solution of the upper two equations:

    ( )

    12

    21

    =

    nn

    nDDn

    hH

    Hqh

    D

    L

    Which these two equations represent the operating line for two passingstreams Vn-2 and Ln-1 by the equation:

    ( )

    12

    12

    2

    2

    =

    nn

    nn

    nD

    nDD

    xy

    hH

    yx

    Hqh

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    Equipments:

    As shown on fig(C) :

    Bubble cap column: 1500mm x 80mm dia fitted with 8 type 316SS bubble cap trays

    Condenser: 100mm dia x 0.5ft.2

    Product cooler: 40mm dia x 0.2ft.2

    Boiler: 150mm dia x 0.5ft.2 (steam)

    Reflux control: Variable area flow meters infinitely variable

    Safety: Graphite rupture discs, element temperature sensor, zener

    barriers (intrinsically safe electricals)

    Water: Control valve, flowmeter, pressure gauge, twotemperature indicators

    Vacuum: Control valve, pressure gauge

    Steam: Reducer, control valve, pressure gauge

    Process: Reflux controller, fifteen temperature indicators

    The major components with their duties are:

    1. A verticalshellwhere the separation of liquid components is

    carried out.

    2. Column internals trays/plates which are used to enhance

    component separations.

    3. a re-boilerto provide the necessary vaporization for the

    distillation process

    4. a condenserto cool and condense the vapor leaving the top of the

    column

    5. 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

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    Fig(C)

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    Procedure:

    Starting the cooling water pump and slowly adjusting the

    flowrate of cooling water to about 4L/min on flow meter.

    The pressure should not exceed 2 bars

    Open slowly the steam inlet valve and also the (steam trap by-

    pass).

    The pressure of steam should not exceed 1.5 bar.

    Vapor generated in the re-boiler rises through the column and

    is

    condensed in a Vertical water-cooled condenser.

    When distillate liquid is seen on top of the column, open the

    valve that leads to the flowmeters.

    The condensed product leaves the column and passes into an

    infinitely variable reflux ratio controller incorporating variable

    area flowmeters.

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    Calibration of the flow rate should be done by the flowmeter

    controllers.

    When done, adjusting the flowrate of the Product stream.

    Taking the temperature readings across the column, and the

    flowrate of the reflux stream.

    Reflux is returned to the column and product passes through a

    cooler and graduated pipe section and can be passed either to a

    receiving vessel, which allows product removal while operating

    under vacuum, or back to the boiler.

    Vapor and liquid compositions throughout are determined by

    temperature measurement.

    Repeating the experiment for different reflux ratios.

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    Results:

    Table (4): Rotameter reading and the actual reading.

    Reflux Product

    Rotameter readingActual reading

    rotameter readingActual reading

    ml/min ml/min

    3.75 65.35 1.00 27.403

    3.20 57.76 1.20 30.1628

    2.20 43.96 1.60 35.6824

    2.00 41.20 2.00 41.202

    1.70 37.06 2.40 46.7216

    Table(5): reflux ratio vs. composition of distillate

    reflux ratio xD

    2.385 0.980

    1.915 0.960

    1.232 0.915

    1.000 0.835

    0.793 0.790

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    7017

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    Discussion of Results:

    It's apparent that the relation between the rotameter scale

    readings and the actual flowrate is a direct linear relation, andof course it should be that, because the utility of the rotameter

    will be much better and easier for the user to calculate byinterpolation the flowrate of the streams; than if it was non

    linear.

    In fig (5), the relation of the Reflux Ratio and the distillate

    composition is direct. And that is logically correct, because, when

    the reflux ratio is high; it means that the reflux flowrate that goes

    back to the column will go for further separation through the

    stages, and so for further concentrating for the distillate and thebottom streams, which is desirable and more profitable with the

    product distillate stream to be high concentrated. And that is the

    target of the experiment.

    when using the McCabe-Thiele method to get the number of

    theoretical stages, the result was infinity; which isunbelievable, but the only convincing reason for that is: when

    first the steam pressure if so high that cause a large amount of

    vapor to pass through the stages, and the following effectsmay occur:

    o Foaming

    Foaming refers to the expansion of liquid due to

    passage of vapor or gas. Although it provides high

    interfacial liquid-vapor contact, excessive foaming often

    leads to liquid buildup on trays. In some cases, foaming

    may be so bad that the foam mixes with liquid on the tray

    above. Whether foaming will occur depends primarily on

    physical properties of the liquid mixtures, but is sometimesdue to tray designs and condition. Whatever the cause,

    separation efficiency is always reduced.

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    o Entrainment

    Entrainment refers to the liquid carried by vapor up tothe tray above and is again caused by high vapor flow rates.

    It is detrimental because tray efficiency is reduced: lowervolatile material is carried to a plate holding liquid of

    higher volatility. It could also contaminate high purity

    distillate. Excessive entrainment can lead to flooding.

    o Flooding

    Flooding is brought about by excessive vapor flow,

    causing liquid to be entrained in the vapor up the column.

    The increased pressure from excessive vapor also backs upthe liquid in the downcomer, causing an increase in liquid

    holdup on the plate above. Depending on the degree of

    flooding, the maximum capacity of the column may be

    severely reduced. Flooding is detected by sharp increases in

    column differential pressure and significant decrease in

    separation efficiency.

    Also the effects will hit the Panchon-Savarit method whencalculating the theoretical number of stages in the same way.

    Due to these effect the efficiency of the plate will by

    undetermined, because the temperature of the liquid and the

    vapor above the stage will be the same, and the liquid of stage

    n-1 due to flooding or any other effect will approach the stage

    n and so differs it's really efficiency.

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    Recommendation:

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    Nomenclature:

    Em Murphee plate efficiency in vapor terms

    R Reflux Ratio

    T Temperature Co

    xD composition of distillate @ top of column %

    y mole fraction of methanol in vapor phase %

    x mole fraction of methanol in liquid phase %

    x rota rotameter reading

    y Act Actual flow rate ml/min

    Xn composition of vapor at stage n %

    Yn composition of liquid at stage n %

    XB the composition of the bottom vapor in the tower %

    y*n Composition in the hypothetical vapor phase that

    would be in equilibrium with liquid composition

    leaving the actual stage %

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    References:

    1. " Equilibrium-Stage Separation Operations in Chemical Engineering"_

    Ernest J.Henley & J.D.seader ,unknown edition, John Wiley

    2. " Transport Processes and Unit Operations" _ Christie J.Geankoplis,Third Edition, 1993, PTR PH

    3. " Unit Operations of Chemical Engineering" _ McCabe & Smith &

    Harriott, sixth edition, 2001, McGraw-Hill

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    Appendices: _

    Sample of Calculations:

    Trial no. 3 will be chosen as a sample:

    From the given data, table no.(1) of rotameter, reading its actual

    flow rate, fig(1) is drawn; with an equation:

    ( ) 604.13799.13 += xy

    By interpolation, the actual flow rates of obtained rotameter

    readings are gathered.

    Rotameter reading for Reflux = 2.2 = x rota (1)Rotameter reading for product = 1.6 = x rota (2)

    Actual flow rate for Reflux = y Act (1) = 13.799(2.2) + 13.604

    = 43.96 minml

    Actual flow rate for Product = y Act (2) = 13.799(1.6) + 13.604

    = 35.68 minml

    And so fig (1') is generated

    The reflux ratio:

    rateflowproduct

    rateflowrefluxR =

    232.1

    35.68

    43.96

    =

    =

    R

    R

    from the T-x-y diagram , fig(2), arbitrarily X (mole fraction of

    methanol in liquid) values are chosen from the x-axis, going

    vertically from there, till hitting the liquid saturation curve, then

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    Starting drawing horizontally from 45o line @ XD till the

    equilibrium line @ (Vn, Ln) ; then going vertically till the

    operating line @ (Ln, Vn-1), and so on in the same way till

    reaching XB or step off it. The number of triangles madebetween the equilibrium line & the operating line are the

    Theoretical no. of stages required.

    As shown in fig (3); the theoretical no. of stages is infinity

    .

    o Using Panchon - Savarit method:

    From the given data in table (4); the enthalpy

    diagram for methanol-water system is drawn; fig (4).

    To get the Theoretical no. of stages ; locate the

    composition of the distillate & bottom @ the x-axis, going

    upward vertically from XD; the distance between the

    saturated liquid & vapor curves is [a]; the distance from the

    saturated vapor till an unknown point [ D] is [b]. From

    lever Rule; the distance ratio a/b = R = L/D, R is 1.232, [a]

    is measured by a ruler to be 4.4cm, so b = a*R =

    4.4*1.232= 4.928cm. Locating [ D] from the saturated

    vapor curve by a distance 4.928cm.

    [a]: represents the distillate flow rate at the top of the

    column.

    [b]: represents the liquid flow rate.

    Starting at point (Vn) on fig(4) and using theequilibrium curve fig(2), we get Ln @ Xn=0.72 on the

    saturated liquid curve; from this pint going on a straight

    line toward [ D]; when crossing the saturated vapor

    curve; Vn-1 is located @ Yn-1.

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    Completing in the same way to get a number of tie

    lines (lines that represents the equilibrium between the two

    outgoing phases from a stage); these tie lines indicate the

    Theoretical no. of stages required.

    It's obviously seen that also the Theoretical number

    of stages is .

    o The Murphee plate efficiency:

    %100*

    1

    *

    1

    =

    nn

    nnv

    yy

    yyE

    = %100*217.0217.0

    217.0195.0

    =

    Given Data:

    Table(1): Calibration curve data:

    RI/(B+1A)flow rate

    mL/min

    0.8 25.7

    1.1 28

    1.2 30.5

    1.5 35.1

    1.9 37.2

    2.6 50.1

    3 55.6

    Table(2): Equilibrium data

    xM yM0 0

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    0.1 0.26

    0.2 0.45

    0.3 0.62

    0.4 0.72

    0.5 0.803

    0.6 0.8710.7 0.916

    0.8 0.95

    0.9 0.984

    1 1

    Table(3): Enthalpy data for methanol/water mixture

    HL x y HVKJ/g.mole KJ/g.mole

    6.061 0.00 0.000 47.625

    5.267 0.05 0.273 45.144

    4.723 0.10 0.418 43.681

    4.389 0.15 0.517 42.678

    4.138 0.20 0.579 42.009

    3.929 0.30 0.665 41.089

    3.846 0.40 0.729 40.379

    3.804 0.50 0.779 39.835

    3.804 0.60 0.825 39.3343.804 0.70 0.870 38.832

    3.804 0.80 0.915 38.331

    3.816 0.90 0.958 37.871

    3.887 0.95 0.979 37.620

    3.954 1.00 1.000 37.453

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