Distillation, differential column height

Choose materials of construction based on corrosion considerations. Column diameters are determined by specifying linear velocities for the two phases. Column heights are determined by estimating the actual number of stages based on the theoretical stage requirements and average stage efficiency. Internals in pulse columns are very similar to those in distillation towers, especially for sieve trays. Therefore, distillation correlations can be used to estimate FOB purchased and installed costs for continuous differential contactors, if they are assumed to be pulse columns. 

Calculation of the pressure drop and flooding rate is particularly important for vacuum columns, in which the pressure may increase severalfold from the top to the bottom of the column. When a heat-sensitive liquid is distilled, the maximum temperature, and hence the pressure, at the bottom of the column is limited and hence the vapour rate must not exceed a certain value. In a vacuum column, the throughput is very low because of the high specific volume of the vapour, and the liquid reflux rate is generally so low that the liquid flow has little effect on the pressure drop. The pressure drop can be calculated by applying equation 4.15 over a differential height and integrating. Thus ... 

HTU. Abbreviation for height of a transfer unit the height of a distillation column or fractionating tower in which unit separation is achieved by transfer from liquid to vapor or vice versa, of the materials being separated. Unit separation is defined by the differential equation that takes into account the varying concentrations along the column. HTU is also applied to extraction and other countercurrent separation processes. 

Consider a differential height of a packed distillation column of unit cross section as shown in Fig. 5.6 1, For component i. 

Furthermore, a distillation operation can also be described differentially in terms of a continuum as presented, for instance, by Hoffman. This representation is more suitable for, say, packed columns vis-a-vis plate columns, although the information is sometimes interchangeable for example, in terms of the height equivalent of a theoretical plate (HETP). Other concepts used include what is referred to as the height of a transfer unit (HTU) and the number of transfer units (NTUs). 

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