Vax

Product ' f/Tl m Hsl fl9 1 Product

Fig. 5-2. Fractionation diagram.

composition as fractions 11 and 15. Samples 20, 21, and 22 can be redistilled in the same manner, and by a repetition of the procedure essentially all of the material will be given the desired separation.

The method outlined in the preceding paragraph can be made continuous. Thus by regulating the fractions vaporized in the various distillations it is possible to have fractions 5 and 13 of the same composition as the original mixture, and fresh feed can be added to these fractions before they are distilled.

Fractional Condensation. Instead of partially distilling a liquid into a distillate and a residue, a vapor can be partly condensed into a condensate and a residual vapor. The results obtained are exactly analogous to those for the successive distillation and, by a similar series of successive vaporization and partial condensation, similar separation can be effected. In fact, successive distillation and successive fractional condensation can be combined to increase the efficiency of the operation.

Multiple Distillation. Suppose an apparatus as in Fig. 5-3 consisting of a series of distilling kettles A, B, C, etc., each kettle containing a heating coil and necessary connection for vapors and liquid. Suppose that kettle A contains a liquid mixture of carbon disulfide and carbon tetrachloride of the composition x& as in Fig. 2-1; the kettle J3,

a liquid of composition xi; the kettle C, a composition of x2\ and so on. The liquid in A boils at 14, that in B at ¿2, and that in C at t&. Since the vapor leaving A is at a temperature 14 which is higher than the boiling temperature U in B, then, if the vapor from A is led into the heating coil of J5, it will give up its heat to the contents of B, boiling the liquid and itself being partly condensed. The vapor from B, if led into the heating coil of C will in the same way boil the liquid in C, the vapor being itself condensed as before. The condensed vapors in the coil may be drawn off into receiver D, E> and F, etc. However, since the composition of the liquid in B was selected to be the same as that of the vapor coming from the kettle A, from Fig. 2-1, the condensed v$por in the coil can be allowed to mix with the contents of B instead of being withdrawn into the receiver E. Now since the vapor from A is being mixed with the liquid in B and since there is a heat inter change between the two, it is much simpler to blow the vapor directly into the liquid thus dispensing with the coil.

The vapor leaving still B will be richer in carbon disulfide than the vapor from still A, and the liquid in B will therefore tend to become poor in carbon disulfide. The concentration of the liquid in B can be maintained constant by adding liquid from C which is rich in carbon

disulfide, and removing liquid from B and adding it to A. By a similar procedure the operation of the other stills can be maintained at a steady state. A little consideration of Fig. 2-1 will show that to make the system operate at the compositions indicated would require that all the vapor from C be condensed and returned through line L to C and that no liquid be withdrawn.

Rectification. An apparatus in which this direct interchange of heat, condensation, and evaporation can take place is called a rectifying tower, and the process carried on within it is called rectification.

Such a system is shown in Fig. 5-4, where 8 is the still body, or kettle. Resting on the outlet of the still is a column divided into com partments by plates perforated with small holes. Each plate has an overflow pipe discharging into a pool of liquid on the plate below. The layer of liquid on each plate is prevented from passing down through the holes by vapor which is rising up through these holes from the compartment next below. Any excess liquid accumulated on the plate flows down through the overflow pipe. The letters on the apparatus correspond to those of Fig 5-3. The vapor from the still at temperature h and composition X\ passes up and exchanges heat and molecules with the liquid in compartment B. A binary vapor of the composition xi is produced which bubbles up through the liquid on the next higher plate which is richer in carbon disulfide with the composition Xi. Here again exchange between the vapor and liquid takes place, and a vapor of composition x& even richer in the carbon disulfide is produced. This can be repeated any number of times, and the vapor finally issuing from the apparatus at the top and into the condenser is practically pure carbon disulfide. As in the previous case, each one of the compartments in the column may be considered a small still, in which the source of heat is the hot vapor coming from below and the cooling element is the cooler liquid from the plate above.

The quantitative relationships given are valid only in case the molal ratio of liquid overflowing from plate to plate to the vapor flowing through the plate is practically unity; i.e., the ratio of distillates to liquid vaporized is exceedingly small. In practice, less overflow must be employed to reduce the heat consumption, and the rate of enrichment is less rapid than that indicated in the explanation.

The analogy between this fractionating column and the series of kettles would be better if the vapor leaving the liquid on the plate had the equilibrium composition assumed. But, unfortunately, no design has been able to prevent some of the vapor from the plate below from passing through the liquid on the plate without coming into equilibrium with it. The vapor above any plate, therefore, will contain less of the volatile component than would be the case if complete equilibrium were reached. This ideal case is discussed here because it brings out clearly the nature of the underlying phenomena. The more practical cases will be considered in Chap. 7 on the Rectification of Binary Mixtures.

The interchange between the vapor bubbles and the liquid on the plate is a result of the fact that the two are not at equilibrium. Thus in the ideal case considered a vapor of composition X\ was bubbling through a liquid of the same composition. The vapor in equilibrium with a liquid of composition xi would have been Thus, as the sys tem tends to approach equilibrium, carbon tetrachloride molecules pass from the vapor to the liquid and carbon disulfide molecules will pass from the liquid to the vapor. The number of molecules passing in the two directions will be essentially equal since in most cases the energy released when one molecule of carbon tetrachloride goes into the liquid phase will be about equal to that required for vaporizing one molecule of the carbon disulfide. Thus the total number of molecules in the vapor tends to remain about constant. This interchange between the vapor and the liquid is governed by the usual mass-transfer mechanism, and the rate of exchange increases with the amount of interfacial area and the turbulence involved. A close approach to equilibrium is desired, and the equipment is designed to give intimate contact between the two phases. Besides the bubbling action already described, the process produces a considerable amount of spray, and there is also an interchange between the vapor and the liquid droplets above the main body of liquid that is helpful in obtaining a closer approach to equilibrium.

This process of countercurrent contact of a vapor with a liquid which has been produced by partial condensation of the vapor is termed rectification. Its result is equivalent to a series of redistillations with the consumption of no additional heat and is analogous in this"respect to multieffect evaporation. However, it is only the result that is similar and not the mechanism of obtaining it.

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