Muliple Reflux ?

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  • #377 Reply

    Krunal Shah

    Can there be more than one reflux , I.e, in case of multiple component distillation can there be reflux on the distillation column other than at the top. If yes, what are the consequences regarding efficiency and feasibility of the distillation process…?

  • #378 Reply

    Yes, It is possible in multicomponent distillation.
    It is very common practice in fractionation process also. it is termed as Pump Around in fractionation process.

    In multicomponent distillation if the column design is such a way that you want getting intermediate stream or you are using zonal distillation with molecular sieves then you will have to provide reflux at more than one places.

    You should keep following things in mind regarding to reflux.:
    1. Increase reflux for fix composition of distillate will reduce the tower height required, if the tower height is fixed and you want to increase the purity of distillate you should increase the reflux.
    2.If reflux is increase then the cost of cooling of distillate and the reboiler duty will increase in other words operation cost will increase but the purity of distillate will also increase. So we have to find the optimum value at which we get our desire purity at minimum possible operating cost.
    Hope you get the answer abt the efficiency.

    Now about feasibility…if you are planning to separate individual components from the mixture in a single column then it is difficult and the area of research, I have not heard any commercial plant working such way, for multicomponet distillation if N=number of components in mixture then (N-1) numbers of distillation columns will be required to separate out each.

    So there are some scenarios when the most light key component and Other components boiling points have significant difference and we decide to separate the heavy key as distillate and rest as top products from the first column, In such cases, reflux can be entered in may be 2nd or 3rd stage (from top) but such arrangements are very rare in practice in distillation process as we want pure components. But it is very common in fractionation process as we are separating mixture according to the boiling point range in fractionation.

    Hope you get your answer.


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