Classification of Distillation Processes

There are many criteria under which one can classify distillation: type of accessories, operating mode, design calculation assumptions, etc. Distillation can either be binary or multicomponent. According to the type of accessories used to increase the mass transfer in the separation process, a distillation column can be packed (use of packing) or staged (use of plates). It can be batch or continuous. Also, according to the assumptions made and accuracy expected in a distillation design calculation, a calculation technique can either be a shortcut method or a rigorous method.

Packed columns and staged columns Although simple distillation in a still historically represents the start of the distillation process, a complete separation of the components of the mixture using this process is not possible. Therefore, the application of these stills is restricted to laboratory-scale distillation, where high purities are not required or when the mixture is easily separable.

One can look at simple distillation as consisting of one equilibrium stage where a liquid and a vapour are in contact with one another and mass and heat transfers take place between the two phases. If N such stages are stacked one above the other, and are allowed to have successive vaporization and condensation, that results in a substantially richer vapour and weaker liquid (in terms of the more volatile component) in the condenser and the reboiler, respectively. This multistage arrangement is representative of a distillation column, where the vapour from the reboiler rises to the top and the liquid from the condenser is refluxed downwards (see Figure 2). The contact between the liquid and the vapour phase is established through accessories such as packing or plates. When the accessory is a stack of plates, then the result is a column of trays. Similarly, if the accessory is packing, the result is a packed column.

Continuous distillation and batch distillation The basic difference between a batch column and a continuous column is that in continuous distillation the feed is continuously entering the column, while in batch distillation the reboiler is normally fed at the beginning of the operation. Also, while the top products are removed continuously in both batch and continuous operations, there is no bottom product in a conventional batch distillation. Since in a continuous operation the total product flow equals that of incoming feed or feeds, the process reaches a steady state. In batch distillation, on the other hand, the reboiler becomes depleted over time, so the process is unsteady. Such differences are illustrated in Figure 3.

Batch distillation is a direct extension of the simple distillation still, where the Rayleigh equation

Figure 2 Equilibrium processes. stage.

Figure 2 Equilibrium processes. stage.

(eqn [1]) is applicable. However, in both batch and continuous distillation, multistage mass transfer and thermodynamic equilibrium stage calculations are used for obtaining the steady-state relationship be tween the product composition (instantaneous in case of batch) and feed composition.

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