Process of separation in a packed column

In addition, the separating behaviour of a packed column cannot be completely described from a physical point of view by only one parameter, such as the HETP or the HTU value. Sizniann [159] has shown that material transfer in a packed column is the more intense the easier it is for the components to get from inside one phase to the liquid-vapour interface and from there to the interior of the other phase. Hence, two resistances to transfer, Wp and IFp, for material transport in the vapour and liquid phases, respectively, have always to be considered. The magnitudes of these resistances depend on the mean path travelled in the respective phase, the frequency of mixing at certain points along the flow path, the formation of turbulence and other factors which have been discussed in chap. 4.2. The distribution of the resistances to mass transfer for 7 different packings as determined by Sizmann is given in Table 18. It is seen that the proportion of the resistance in the vapour phase may vary from 9 to 96% of the total resistance. 

F = Deterioration factor for number of stages, N = Number of theoretical stages, y = Liquid composition in head 

Packing type Size (mm) Material Free column volume/ packing volume ) Load HETF (cm / (cm) cm X h) Wol IFp Wnl Flooded (Wd+IFf)  

In spite of a frequent redistribution of the liquid it may occur that the packing is not unlforndy wetted the film itself may also have different forms. Stuke [161] has investigated this subject in detail, and has examined the influence of the nature of the packing, the properties of the liquid, the column load and the distillation pressure (with its associated temperature) on irregularities of film formation. His conclusion.s can be summarized as follows (see also chap. 4.2). 

Channelling on the packing will occur principally at low loads a large amount of liquid is distributed better on the surface of the packing. 

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