Packed Columns For Distillation

In bubble cap and perforated plate columns, a large interfacial area between the rising vapour and the reflux is obtained by causing the vapour to bubble through the liquid. An alternative arrangement, which also provides the necessary large interfacial area for diffusion, is the packed column, in which the cylindrical shell of the column is filled with some form of packing. A common arrangement for distillation is as indicated in Figure 11.58, where the packing may consist of rings, saddles, or other shaped particles, all of which are designed to provide a high interfacial area for transfer. These are referred to in Chapter 4. In packed columns the vapour flows steadily up and the reflux steadily down the column, giving a true countercurrent system in contrast to the conditions in bubble cap columns, where the process of enrichment is stagewise.

Packed Column Distillation
Figure 11.58. A packed column for distillation

11.11.1. Packings

The selection of a suitable packing material is based on the same arguments as for absorption towers considered in Chapter 12, although for industrial units the most usual packings are rings, and the material of construction is determined by the corrosive nature of the fluids, or otherwise. It is important to note that in a distillation system operating at high reflux ratios the mass of reflux is approximately equal to the mass of vapour, although at low reflux ratios the flow of the liquid is only a small fraction of that of the vapour. Since the vapour has a much lower density than the liquid, the process is really one in which a small quantity of liquid passes through the vapour, and the establishment of good distribution of the liquid is more difficult than in absorption towers, where the two streams are more nearly balanced.

In Chapter 4 the characteristics of packings and their influence on column hydraulics are considered, and in Chapter 12 the mass transfer aspects are covered. The packing for a particular application may be selected using this information, although it may be noted that, in the case of vacuum distillation for example, pressure-drop considerations may be of overriding importance, and there may be problems associated with the wetting of the packing because of the low liquid loadings. For distillation in packed towers, it is normal practice to increase the calculated height of packing by 40 per cent to allow for liquid mal-distribution and wetting problems.

11.11.2. Calculation of enrichment in packed columns

With plate columns, the vapour leaving an ideal plate is richer in the more volatile component than the vapour entering the plate, by one equilibrium step. Peters (70) suggests that this same enrichment of the vapour will occur in a certain height of packing, which is termed the height equivalent of a theoretical plate (HETP). As all sections of the packing are physically the same, it is assumed that one equilibrium stage is represented by a given height of packing. Thus the required height of packing for any desired separation is given by HETP x (No. of ideal stages required).

This is a simple method of representation which has been widely used as a method of design. Despite this fact, there have been few developments in the theory. Murch(71) gives the following relationships for the HETP from an analysis of the results of a number of workers. Columns 50-750 mm diameter and packed over heights of 0.9-3.0 m with rings, saddles, and other packings have been considered. Most of the results were for conditions of total reflux, with a vapour rate of 0.18-2.5 kg/m2s which corresponded to 25-80 per cent of flooding. The relationship is:

where the values of C1 , C2, C3 varied with packings as given in Table 11.4.

It may also be noted that the mixtures considered were mainly hydrocarbons with values of relative volatilities only slightly in excess of 3. In equation 11.143:

dc = column diameter (m), Z = packed height (m), a = relative volatility,

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