Ethanol-H20

Butene-butane

Butadiene-butene

Isoprene-pentene

Toluene-paraffinic hydrocarbons.. .

Extractive Agent h2s04

h2so4

Glycerine Acetone, furfural Acetone, furfural Acetone Phenol

Acetone-methanol Water means, such as extraction, must be employed to separate the agent. The bottoms from the tower are treated to separate the agent and the bottoms product. Occasionally, a portion of the agent is added with the feed in order to maintain its concentration essentially the same above and below the feed.

Top product

Top product

Bottom product

Fig. 10-1. Schematic diagram of extractive distillation system.

Bottom product

Fig. 10-1. Schematic diagram of extractive distillation system.

The design calculations for such systems are straightforward, assuming that the physical-chemical data on the systems are available. In the limiting case of an essentially nonvolatile extractive agent, the problem reduces to a standard binary or multicomponent problem depending on the feed to the unit, except for the modification of the volatilities. In case the agent is volatile, the problem is more complicated, but it can be handled by methods analogous to those used foi regular distillations.

Total Reflux. This limiting condition is not equivalent to that for regular rectification because to maintain a given concentration of the extractive solvent in the liquid phase would require the addition of an infinite amount per unit of feed. Thus the concentration of the components being separated would approach zero in the bottoms from the

extraction tower. It would require an infinite number of plates to reduce the concentrations from the finite values in the upper part of the column to zero at the bottom, except for the case where the solvent was nonvolatile. This limit of an infinite number of plates and infinite heat consumption does not aid in orienting the design calculations. A useful limit for orientation purposes can be based on the desired separation of the key components from the overhead to bottoms. Thus the calculations should be carried down the column until the desired bottoms ratio of the key components is obtained. Actually the system can be considered as shown in Fig. 10-2. Column A is the usual extractive distillation unit, which obtains the desired degree of separation of the key components, and produces a bottoms containing a finite concentration of the key components in the desired ratio. B is some type of unit that produces the vapor for column A and reduces the concentration of the key components without changing their ratio. Column C is the tower that separates the solvent from the bottom product. In case the solvent is nonvolatile, unit B is a still. The number of theoretical plates for tower A is the desired answer. The minimum number of plates at total reflux _

will therefore be calculated on the basis of jl: a the desired ratio of the key components in the overhead and bottoms. If the relative volatility is reasonably constant over the concentration range involved, Eq. (7-52) should be satisfactory. In any case, the plate-to-plate calculation methods of Chap. 9 can be applied.

Minimum Reflux Ratio. The calculation for this case is similar to that for the usual multicomponent mixture, but with the extractive agent included at both the pinched-in regions. The asymptotic value is calculated in the same manner as Eqs. (9-13) and (9-15). Thus for Fig. 10-3, the asymptotic concentration above the feed is

Feed

Fig. 10-3. Extractive distillation diagram.

Oihk

and, below the feed,

where Xsn) XSm = asymptotic values of solvent above and below the feed plate respectively Sxs = solvent added to system at top of column In many cases, the mixture to be separated is a binary, since it is usually desirable to separate all but two components by regular distillation and then subject them to the extractive operation. Generally, the amount of solvent added at the top of the tower is varied with the reflux ratio in order to maintain a constant mol fraction of solvent in the total liquid returned to the top region of the column. An alternative method of operation is to employ a given solvent rate independent of the reflux ratio, which would give a solvent concentration in the tower that varied widely with the reflux ratio. The first method of operation appears to be more desirable for most cases, in order to obtain a relative constant concentration of solvent in the tower that approximates the desired value. The equations for the minimum reflux ratio have therefore been derived to be most convenient for the first method of operation. For the general case in which the feed contains light and heavy components in addition to the key components, it is recommended that the minimum reflux ratio be calculated by

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