Stagewise distillation

Rate equations of chemical change are usually available at moderate temperatures for roost of the reactions of interest. The same applies to reaction equilibrium constants. Sufficient information is needed to extrapolate both the equilibrium and rate constants to the boiling point. This is normally performed by an Arrhenius-type equation for which data are not difficult to acquire. It should be noted that for work on stagewise distillation reactors no reaction kinetic data were needed, since theoretical plates were assumed throughout. When such information was required, the reaction was assumed to be of pseudo-first order. 

The distillation operation is embodied in a vertical column to take advantage of the difference in density between the liquid and vapor phases. Thus, in stagewise distillation, the more-dense liquid phase L moves down vertically via downcomers at the side of the plate and next to the column wall. The liquid phase is thereby introduced at or on each plate and flows across the plate also by gravity for example, by virtue of the hydraulic gradient that builds up in the downcomer. Meanwhile, the less-dense vapor phase V moves upward, passing up and through the liquid on the plate via holes or bubble caps (or nozzles) and overall moves countercurrently to the liquid phase, as represented schematically in Figure 4.1. 

The principle of the perfectly-mixed stirred tank has been discussed previously in Sec. 1.2.2, and this provides essential building block for modelling applications. In this section, the concept is applied to tank type reactor systems and stagewise mass transfer applications, such that the resulting model equations often appear in the form of linked sets of first-order difference differential equations. Solution by digital simulation works well for small problems, in which the number of equations are relatively small and where the problem is not compounded by stiffness or by the need for iterative procedures. For these reasons, the dynamic modelling of the continuous distillation columns in this section is intended only as a demonstration of method, rather than as a realistic attempt at solution. For the solution of complex distillation problems, the reader is referred to commercial dynamic simulation packages. 

Chapter 3 concerns the dynamic characteristics of stagewise types of equipment, based on the concept of the well-stirred tank. In this, the various types of stirred-tank chemical reactor operation are considered, together with allowance for heat effects, non-ideal flow, control and safety. Also included is the modelling of stagewise mass transfer applications, based on liquid-liquid extraction, gas absorption and distillation. 

Mass transfer separation processes, e.g., distillation, gas absorption, etc., are normally treated in terms of stagewise or differential procedures. In a stagewise procedure, concentration changes are taken to occur in distinct jumps, as, for... 

Revision of the nomenclature for the underflow streams of washing thickeners to bring it into line with that used for other stagewise processes, including distillation and absorption (Chapter 10). 

A distillation process. The behaviour of liquid and vapour streams in any stagewise process can usually be approximated by a number of non-interacting first order systems in series. For example, Rose and Williams021 employed a first order transfer function to represent the dynamics of liquid and vapour flow in a 5-stage continuous distillation column. Thus for stage n in Fig. 7.17 ... 

From the usual plate-type distillation and absorption columns, it was but natural to attempt to devise stagewise equipment for G/S processing, for better heat economy and better utilization of the contacting media. But instability of the downcomer for solids transfer between stages remained for many years an unsolved hydrodynamic problem, and even to this data, the stable operation of many solids downcomers still depends on mechanical devices. 

The transfer of mass from one phase to another is involved in the operations of distillation, absorption, extraction, humidification, adsorption, drying, and crystallization. The principal function of the equipment used for these operations is to permit efficient contact between the phases. Many special types of equipment have been developed that are particularly applicable for use with a given operation, but finite-stage contactors and continuous contactors are the types most commonly encountered. A major part of this chapter, therefore, is devoted to the design aspects and costs of stagewise plate contactors and continuous packed contactors. 

NTU (Number of Transfer Units) The NTU required for a given separation is closely related to the number of theoretical stages or plates required to carry out the same separation in a stagewise or plate-type apparatus. For equimolal counterdiffusion, such as in a binary distillation, the number of overall gas-phase transfer units Nog required for changing the composition of the vapor stream from y to t/2 is... 

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