Mdelling and Simu

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    ABSTRACT

    Modelling and Simulation of Reactive Distillation Columns

    Reactive distillation (RD), simultaneous reaction and separation within a single unit,

    representing an existing alternative to conventional reaction followed by separation

    processes, leading to significantly reduction in initial investment and operating cost. The

    modeling and simulation is done for both batch and continuous reactive distillation in the

    production of ethyl acetate from ethyl alcohol and acetic acid. For the batch reactive

    distillation done in 11 stages, the optimum reboiler heat duty is determined. For that

    optimum reboiler heat duty, the dynamics of composition in reflux drum and reboiler is

    studied until it reaches a steady state. In the continuous reactive distillation column the

    reactants are fed at different trays, the optimum reboiler heat duty, feed plate location,

    distillate flow rate, feed flow temperature are determined.

    Keywords: Reactive distillation, modeling, simulation, ethyl acetate column.

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    Chapter-1

    1.1 Introduction and Literature survey

    The combination of chemical reaction with distillation in only one unit is called reactive

    distillation (RD). Reactive distillation has received increasing attention over the past

    several years as a promising alternative to conventional processes. Although invented in

    1921, the industrial application of reactive distillation did not take place before 1980s.

    Especially, interesting equilibrium reactions suitable for reactive distillation are

    esterification, ester hydrolysis reactions, etherification and transesterification. In recent

    years, attention has been paid to ethyl acetate synthesis and hydrolysis, which serves as a

    model for reactive distillation processes.

    The performance of reaction with separation in one piece of equipment offers distinct

    advantage over the conventional sequential approach. As a advantage of this integration,

    chemical equilibrium limitations can be overcome, higher selectivities can be achieved,

    the heat of reaction can be used for distillation, auxiliary solvents can be avoided and

    azeotropic mixtures can be more easily separated than in conventional distillation. This

    May lead to enormous reduction of capital and investments costs and may be important

    for sustainable development due to lower consumption of resources. Some industrial

    processes where reactive distillation is used are the esterification processes.

    However reactive distillation is not suitable for every process where reaction and

    separation steps occur. Operating conditions, such as pressure and temperature of the

    reactive and separation processes and perhaps other requirements, must overlap in order

    to assure the feasibility of the combined process. This limitation can be overcome by

    fixing adequate operating conditions in the cases where this is possible.

    In addition to the low capital investment, the RD column requires low operating costs

    (energy, water, solvents, etc). however, the mathematical model for this piece of

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    Chapter 2

    Modelling and Simulation of Ethyl Acetate Reactive Distillation

    Columns

    2.1.Process Description:

    2.1.1. For Batch Reactive Distillation Column:

    The reboiler is fully charged, all the trays and the condensers are specified with

    initial holdups. The reboiler is then heated. Vapor flows upwards in the

    rectifying column and condenses at the top. The entire

    condensate is returned to the column as reflux. This contacting

    of vapor and liquid considerably improves the reaction. After

    some time, a part of the overhead condensate is withdrawn

    continuously as distillate and it is accumulated in the receivers,

    and the other part is recycled into the column as reflux. Owing to

    the differing vapour pressures of the distillate, there will be a

    change in the overhead distillation with time.

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    In the column the lighter (lower-boiling) component tend to concentrate in the vapor

    phase, while the heavier (higher-boiling) components tend towards the liquid phase. The

    result is the top product becomes richer in light components as the feed and the bottom

    product becomes richer in heavy components as the feed.

    Figure 2.2: Schematic ofa Continuous Reactive Distillation Column

    The following is a reversible liquid phase reaction:

    Acetic acid + Ethanol Ethyl acetate + Water

    (1) (2) (3) (4)

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    This is a two reactant- two product system. Among the two product water is the heaviest

    one and it comes out as the bottom product. On the other hand, the main product, ethyl

    acetate, comes out at the top section as the distillate product

    2.2.Assumptions

    Ethyl acetate RD column with trays numbered from bottom to top.

    Liquid on the tray is perfectly mixed and incompressible.

    Tray vapor holdups are negligible.

    Variable liquid holdup in each tray.

    No heat accumulation on each tray.

    Vapor phase Murphree efficiency of 75% is considered.

    Nonlinear Francis weir formula for tray hydraulics calculation.

    Raoults law for vapor-liquid equilibrium.

    2.2.1 Bubble point calculation

    For solving the vapor rates i.e. the energy balance equation, one requires the enthalpy

    data; and to calculate the enthalpy, the temperature should be known. Therefore, it is

    necessary to have the temperature-composition correlation. The vapor phase composition

    in equilibrium with the liquid phase is given by,

    i i iy x =

    where k is equilibrium ratio, for this study k is calculated as follows,

    s

    ii

    t

    Pk

    P=

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    where is Pis the vapor pressure was calculated by using Antoine equation, and Pt is the

    total pressure.

    The Antoine equation is given as,

    expiB

    P AT C

    = +

    Where A, B, C are the constants and T is the temperature. Here the pressure is in

    mmHgand the temperature in C.

    Step-wise bubble point calculation

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    2.2.2. Enthalphy Calculations

    Assume Temperature

    Calculate equilibrium ratio

    Calculate yi

    using xi

    Check(T) = = y

    i-1

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    The liquid and vapor enthalpies were calculated based on the following equations (Yaws,

    1996). Here the Temperature is in (K) and the enthalphy is in (J/mol.K).

    *vH Cp T = (2.2.2 1)

    2 3 4

    Cp A BT CT DT ET = + + + + (2.2.2.2)l vH H= (2.2.2.3)v v

    i iH y H = (2.2.2.4)

    ( )

    2

    2

    i

    i

    aRT

    b T

    = +

    (2.2.2.5)

    where viH = Enthalpy of Vapor

    l

    iH = Enthalpy of liquid

    = Latent heat.

    R=gas constant

    A,B,C,D,E, =constants

    a , b =B,C of Antonie constants.

    2.2.3. Liquid flow rate Calculations

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    For the calculation of liquid flow rates the Francis weir formula (L.Wang.et,al.

    Computers and Chemical engineering 27 (2003) 1485 - 1497). Liquid flow rate is a

    function of holdup, weir height, area of the tray and the volume held.

    liq tray

    j jif h h> (2.2.3.1)

    1.5liq tray

    j j

    j weir liq

    j

    h h

    L lvol

    =

    (2.2.3.2)

    0jelse L = (2.2.3.3)

    liq

    j jliq

    j

    tray

    M vol where h

    A= (2.2.3.4)

    Where jL = Liquid flow rate (mole/s)

    weirl = length of the weir (m).

    , = constants.

    trayjh =Height of the weir (m)

    j = mass holdup (mole)

    liq

    jvol = Volume of mass held in the tray (m3).

    trayA = Area of the tray (m2).

    2.2.4. Rate of reaction

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    2.4.1 : Batch Reactive Distillation

    0 2 4 6 8 10 120.87

    0.88

    0.89

    0.90

    0.91

    0.92

    0.93

    0.94

    0.95

    0.96

    After 500 min

    Heat duty QR=10 5 J/min

    PurityofethylacetateinRefluxdrum(

    mole

    Figure:2.3 Effect of Ethyl acetate purity in Reflux Drum due to Reboiler heat duty

    The effect of ethyl acetate purity in reflux drum due to reboiler heat duty is shown above.

    With less than 5*105 J/min, the vapor produced is very less and hence there is less

    interaction between the upcoming vapor and down coming condensed liquid and the

    purity is low. With heat duty more than 5*105 J/min the vapor produced is very high,

    there is less xi in the reaction, hence the purity is decreasing. The optimum high purity is

    produced when the reboiler heat duty is 5*105 J/min.

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    0 100 200 300 400 500 600335

    340

    345

    350

    355

    360

    365

    Time(min)

    Temperature(K)

    ReboilerReflux drum

    Figure:2.4 Dynamics of Temperature Profile in Reboiler and Reflux drum

    The dynamics of temperature profile in the reboiler and distillate shows that, initially

    both the temperatures are at the same point, as all trays includin