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  • Wednesday, 6 April 2016

    [HOW TO] Calculate required Vapour column Size

    Hello readers.... Good Day!!!
    Today i gonna give you a small brief Introduction about Vapour Column and its necessity and also what effect it can have over regular pharma operations.

    Usually Vapour Column is a part of a reactor , a part of distillation setup whether it is a product concentration distillation or Solvent Recovery Distillation, its secondary, but vapour column always plays a major role, selection of a wrong Vapour Column can lead to unimaginable cases which will have adverse effects over the .....  Hard to Say, but i have to say even it can have effect over distillation rate, recovery rate, and rate of drying also.

    Because if i ask somebody what is the use of Vapour Column / what you know about Vapour Column,
    Mostly the simply reply comes as a Distance piece connected from a Equipment to a condenser or a receiver or a blower/vacuum pump to transfer the vapours that were generated in an operation.

    But this is time to grow up from that kiddo stage and have to know its Importance, what ever Vacuum Pump Capacity or whatever Condenser or whatever Utility or whatever heat transfer area of the equipment you are using doesn't matter, the thing that only matters is whether you selected a right Dia Vapour Column or not, this is the base of Distillation or Drying operations that were done in pharma field. So, right now i think you you gotta brief about importance of vapour column in pharma engineering. So, now i think you are clear, and i'll start my show...

    Do Read:-

    What is a Vapour Column??

    ** Vapour column is a simple line with a 90deg or 45deg elbow bend towards a condenser,

    What is the need of Vapour Column??

    ** A vapour Column used to transfer vapour out of a reactor,

    How Does Vapour Column has an effect over an Distillation or Drying Operation??

    **  Everyone knows how to calculate the flowrate inside a pipeline, so similar calculation need to be applied here, but in a pipe flow we will be taking velocity of fluid, but here our hero is vapour so we need to consider vapour velocity in place of fluid velocity,

    What is the Calculation involved in calculating the Flowrate inside Vapour Column??

    ** Back to Basics again, Flowrate = Fluid Velocity x Cross section area.

    So, i think you are Well Prepared and Can get to point easily,

    For Calculation of Vapour Column Size we need to know, what amount of boilup need to be carried to condenser [ if this is a normal distillation], how much amount of Suction velocity need to be generated towards reactor [ in case of vacuum distillation].

    So, i think now i need to give you some info regarding batch boilup, batch boilup is nothing but the amount of volume that has been generated from the heat transfer equipment where we are adding heat to the batch volume, so this boilup majorly depends upon two operating parameters, 1) Pressure, 2) Temperature.

    * So usually whenever we need to calculate the boilup based on these parameters, we have to stick to basics and start using ideal gas law, right now i dont want to mention the law, but have a look at the derived equation,

    P xV = n x R x T
    so here, P, T are operating parameters, n is number of moles, V is Boilup Volume, R is universal gas constant,

    So in order to calculate the Boilup volume, i'll transform the equation as V = (n x R x T)/P.

    and some more transformation i'll prefer V = ((W/M) x R x T)/P,

    So here right now everything is clear , based on the composition of batch volume you consider, you can generate the data for W, M.
    W- mass of the solvent (volume of solvent x Density of solvent),
    M- molecular weight of solvent.

    In case of binary mixture, you can split that W/M to (W1/M1)+(W2/M2).

    So, Now you got a Clear vision over Boilup,
    Its time to jump to main concept Vapour Column size,
    So first of all we need to know what amount of boilup we need to generate, for that we use the formula,

    V = W x (359/M) x (1/60), by this we will be getting in Cu.ft/sec,
     So, now as usually i'll prefer customizations to make it work for different operating parameters, so i'll be adding some extra functions like, (760/P) for pressure and (T/489.67) for temperature..,

    Finally it will be V = W x (359/M) x (760/P) x (T/489.67) x (1/60),
     here you need to convert T to rankine, for that you need to know,

    T, R= (9/5) x T, K

    So, now integrating all the above,

    V = W x (359/M) x (760/P) x (T/489.67) x (1/60),

    Input W in Kg/hr, we will be getting V in Cu.ft/sec and for Airload we can add 2-5% overall of volumetric flowrate of boilup,

    And after this we need to decide whether its a Normal distillation or Vacuum Distillation,
    if its Normal Distillation we Can have Vapour Velocity, = 100ft/sec, and
    for Vacuum Distillation we can have vapour velocity, v = 170-180ft/sec,

    So, finally for calculating Dia we use optimum pipe diameter,

    D, ft = 0.147 x SQRT( V / v)
    , convert this to inches, 1ft = 12 inch

    For Those who need, we are providing the Excel sheet, Download Here

    About the Author

    Hi! I am Ajay Kumar Kalva, Currently serving as the CEO of this site, a techoholic geek by passion, and a chemical process engineer by profession, i'm interested in writing articles regarding technology, hacking and pharma technology.
    Follow Me on Twitter AjaySpectator

    Pharma Engineering


    1. M value to be taken is g/mol or Kg/mol

      1. There is not that much to think over the units for molecular weight, anyway for your knowledge the equation above mentioned is in FPS units, so it need to be mentioned in Lb / Mole.

    2. Hello ajay.your blogs r very knowledgeable and interesting.I want your help.I want to find out u loop calculation which we found in reflux line of api reactors for traping the vapour from going in condenser . specifically the hight of loop.waiting for your response.

      1. Hey Rutika, thanks for your comment,

        Basically there wont be any straight way calculation for that U loop located reflux condensate line, but based on just your approximation you can derive the height, and you may use P = rho x g x H, H is height, P is pressure, based on the temperature of the distillate using antonie equation you can get the vapour pressure, that's it,

        Still any queries feel free to comment


    3. Hai ajay i'm venkat,

      i didn't got this step. Can you please explain me in detail.
      V = W x (359/M) x (1/60), by this we will be getting in Cu.ft/sec,

      And also what are the units you have considered for W and M like gm/mol or Kg/mol.

      1. Hey venkat, glad you read my post,

        (1/60) is a time conversion factor,
        359 is standard molecular wt.
        W is the mass flowrate that you want to generate

    4. Hi Ajay

      Every solvents has individual vapour velocity,In above calculation why r we using standard vapour velocity for normal or vaccum distillation...?

      1. Under vacuum, there wont be any considerable change in vapour velocity,
        But coming to normal distillation, the velocity depends upon the following factors [Directly & Indirectly],
        1. Vapour density, [Directly]
        2. Heat supply for evaporation, [Directly]
        3. Vapour line sizing, [Directly]
        4. Vapour pressure developed inside reactor, [Directly]
        5. Utility supply to condenser. [Indirectly]

        To make the concept clear, i've given it as constant.

        AJAY K


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