Distillation column design packing efficiency

Determining the number of theoretical and actual trays in a distillation column is only part of the design necessary to ensure system performance. The interpretation of distillation, absorption, or stripping requirements into a mechanical vessel with internal components (trays or packing, see Chapter 9) to carry out the function requires use of theoretical and empirical data. The costs of this equipment are markedly influenced by the column diameter and the intricacies of the trays, such as caps, risers, weirs, downcomers, perforations, etc. Calcvdated tray efficiencies for determination of actual trays can be lost by any unbalanced and improperly designed tray. 

Two different approaches have evolved for the simulation and design of multicomponent distillation columns. The conventional approach is through the use of an equilibrium stage model together with methods for estimating the tray efficiency. This approach is discussed in Chapter 13. An alternative approach based on direct use of matrix models of multicomponent mass transfer is developed in Chapter 14. This nonequilibrium stage model is also applicable, with only minor modification, to gas absorption and liquid-liquid extraction and to operations in trayed or packed columns. 

Apparatus may be discussed in terms of the distillation flask, the distillation column, the condenser, and the collecting flask(s). By far the most effort has been expended in the design and operation of the distillation column, which is at the heart of the separation efficiency. The form of the column, its size, and the packing used are very influential upon the results that are achievable. A summary of some different types of columns is given in Table 1 and of packing in Table 2. 

In addition, these depths apply to packed beds with uniform liquid and gas distribution similar to that used in distillation columns. In many designs of commercial cooling towers, distribution is of a substantially lower quality. Thus, the packed depth specified must make allowance for these less efficient distributor designs. For large size dumped tower packings, the value of Hqg inay be 25% to 40% greater than that from Table 6-1 when commercial liquid distributors are used. 

---