Distillation countercurrent

This modification of the continuous reactor (cf. benzoylacetanilide, p. 2) with countercurrent distillation is preferred for reactions in which a large amount of solvent or excess of one reactant is essential, but increase of total volume is undesirable. It is especially useful if the substances involved are heat-sensitive with this apparatus the reactants are heated for only a few minutes at most. It is particularly applicable to the preparation of acid chlorides from carboxylic acids and thionyl chloride (cf. Notes 3 and 4). An indefinite amount of product can be prepared by replenishing the reactants as they are consumed. 

Since the form of the transformed mass balances and the operating equations for the kinetically controlled reactive distillation are identical to the corresponding equations for conventional countercurrent distillation, we therefore can apply the same... 

Allen and co-workers at the Eastman laboratories developed a procedure for the preparation of oleoyl chloride and other acid chlorides sensitive to heat in a continuous reaction with countercurrent distillation such that the heat-sensitive acid chloride is heated for only a few minutes. Oleoyl chloride of satisfactory purity was obtained in yield of 97-98%. 

The equipment for continuous distillation can only separate one stage in the equilibrium diagram. Countercurrent distillation, also called rectification, has found widespread application with normal pressure and coarse vacuum distillation when complex mixtures or components with small relative volatility factor are to be separated. The fundamentals are discussed above (2.1.3.3.2) the technical side will be dealt with here [41-45]. 

The attention of many workers has been given to the equilibrium-limited case (proportionate-pattern) of multiple adsorption (D2, Wl, W3, W7, among others). In the constant-pattern case, Fujita s work has already been discussed (Section III, B, 2b). Also, using the theoretical-plate approximation to a packed column, plate-by-plate calculations can be carried out in the constant-pattern case exactly as for continuous (countercurrent) distillation this treatment is suggested from work on chromatographic and displacement problems by Mair (M2), Spedding (S6), and Glueckauf (G3). Moreover the linear-equilibrium result can be extended, in a nearly trivial fashion, to any number of components. 

Like countercurrent distillation, the countercurrent chromatographic systems are binary separators thar is. they can make a cut between two key components. Most industrial applications have separated binary mixtures. Then the purpose of zone I in Fig. 14.3-1 is to remove the more strongly adsorbed solute B from A. Thus, in this zone solute A shonld go up and solute B should go down. In terms of the average solute velocities,... 

Continuous distillation, or fractionation, is a multistage, countercurrent distillation operation. For a binary solution, with certain exceptions, it is ordinarily possible by this method to separate the solution into its components, recovering each in any state of purity desired. 

The reader is reminded that the word distillation can be translated as separation drop by drop (see Chap. 2). It can therefore be used as a collective term for processes in which mixtures of mutually soluble liquids can be separated by evajmration and condensation of the liquid, the condensed part becoming richer in the most volatile component. The word gives no indication of the technique adopted in the sejjarating process. The terms simple distillation and countercurrent distillation , however, define the inode of operation (Fig. 23). In a strictly physical sense distillation need not produce any separation we also speak of distillation when a pure liquid is eva-jiorated, the vapour is condensed and the condensate is removed. 

In simple distillation the molecules emerging from the evaporating surface move uniformly until they reach the condensing surface. In countercurrent distillation part of the condensed vapour, termed reflux , returns to the boiler, meeting in its passage the rising vapour. Provision is made for intimate contact between the liquid and... 

Countercurrent distillation enables components to be separated having differences in boiling point of about 0.5 deg C, whilst this figure can be as low as 0.05 °C if extremely efficient columns are employed, as in the separation of isotopes. By the use of selective methods and, in difficult cases, by combination with other methods of separation such as extraction, countercurrent distribution and gas chromatography, separations have been performed with mixtures previously r arded as inseparable. 

In the following sections the complex processes taking place in countercurrent distillation will be discussed further (cf. Kuhn and Kuhn and Ryffel [3]). 

Difference in boiling point Simple or countercurrent distillation 4.1 4.7.5.1... 

The influence of column diameter and surface tension on the HTU in packed columns in the countercurrent distillation of binary mixtures was studied by Gomez and StrumUlo [8a]. They found the relation... 

On the basis of the Raleigh equation von Weber [138] developed a diagram from which the concentration of the liquid in the still pot after distilling a charge B down to a residue B-, can be read off for ideal mixtures if x is known. A method of calculating the stepwise countercurrent distillation of ternary systems was evolved by Vogel-pohl [139]. A mathematical model was constructed by Domenech et al. [139a]. A... 

An unambiguous influence of the distillation pressure has not as yet been established. Some investigators have reported an optimum pressure in countercurrent distillation, others have stated that no such influence exists. 

Another point of interest is the dynamic behaviour of packed columns. Heise, Hiller and Wagner [162] investigated the response of a packed column to marked disturbances of the liquid-vapour ratio in the countercurrent distillation of the ternary, almost ideal mixture ethyl acetate-benzene-trichloroethylene (see chap. 4.2, 4.15). 

What has been said so far, however, by no means mentions all the complicated processes taking place in a packed column. Krell (see chap. 4.2) had previously pointed out the effects of surface energies and had summarized the factors operative in countercurrent distillation with packed columns as follows. 

It is apparent from this summary that countercurrent distillation is such a complex process that very great difficulties are often involved in determining the influence of a certain factor on the separation by experiment. Owing to this fact, contradictory statements are frequently encountered in the literature. 

The hold-up is an essential factor in a countercurrent distillation and has an important influence on the time required for carrying it out (see chap. 4.10.5). Whilst the theoretical plate number and the reflux ratio increase in proportion to lAog , the time expended increases as the square of lf og tx. It is hence the time consumed that best characterizes the difficulty of a separation. If we calculate the quotient... 

Billet [176 c] shows that deviations from optimum column lay-out may lead to a drastic increase in cost. The connexion of main and side columns and the use of partial condensation in direct and countercurrent operation, where appropriate are also discussed. These principles should be considered in laboratory and semitechnical work. Comparative studies of the applicability of heuristic rules and approximate methods in the selection of appropriate column connexions for nmlticomponenl countercurrent distillation were made by Jobat et al. [176d]. 

Investigations of Hiibner and Schliinder [103a] show that the Murphree efficiency can be calculated sufficiently exactly with the turbulent diffusion model if the material transfer and the intensity of turbulent mixing in the vapour and liquid phases are known. The efficiencies in the countercurrent distillation of ternary mixtures are discussed by Vogelpohl and Ceretto [103b]. 

Another experimental approach is to carry out a batch countercurrent distillation with the test mixture at the selected load and reflux ratio small samples of the distillate (about 1% of the stiU charge) are regularly taken and also — together with every second sample — a few drops of the liquid from the still pot the compositions are then determined. By plotting the concentrations of the distillate and still pot... 

The plate equivalent is the minimum number of plates required at infinite reflux ratio to attain the same enrichment (xB- -Xg) as in a countercurrent distillation with a finite reflux ratio. All distillation conditions except the reflux ratio remain the same. Thus, in the McCabe-Thiele diagram the separating stages are drawn between the diagonal and the equilibrium curve v = oo). 

---