Page 365 - Chemical process engineering design and economics

344 Chapter 6

Solving these equations simultaneously, NU = 11.93 and N L = 13.07 - founding
off, NU =12 trays above the feed point and NL = 13 trays below the feed point.

Liquid-Liquid Extractors

Several liquid-liquid extractors have been reviewed by Lo [61]. Extractors are
divided into two classes: unagitated, and agitated. Among the unagitated extrac-
tors there are the packed and sieve plate designs, which are similar to fractionators
and absorbers. Examples of agitated extractors, shown in Figure 6.20, are the
rotating disc and Oldshue-Rushton extractor. Another agitated extractor is the Karr
reciprocating-plate extractor. For all these extractors, backmixing, which reduces
the column efficiency, is a problem. Agitation is needed to increase mass transfer
by dispersing one of the phases and increasing turbulence in the continuous phase.
In the rotating-disc extractor, the disc is the agitator, in the Oldshue-Rushton col-
umn it is flat blade turbine impellers, and in the reciprocating-plate extractor, it is
the up-and-down motion of the plate stack. Horizontal stator rings above and be-
low each disc or impeller, shown in Figure 6.20, reduces backmixing.

Extractor Sizing
As for absorbers and strippers, the height of the extractors can be calculated sim-
ply by calculating the number of equilibrium stages and multiplying by HETS.
Additional height is needed at the top and bottom of the extractor for phase separa-
tion. Figure 6.21 shows that the Karr reciprocating-plate extractor is one of the
more efficient based on both HETS and throughput. Karr and Lo [62] developed a
procedure for scaling reciprocating-plate extractors from small-scale tests. The
Karr extractor will be used to illustrate a procedure for sizing extractors.
In Table 6.31, Lo [61] has tabulated the minimum HETS for the methyliso-
butylketone (MIBK), acetic-acid, water system and the o-xylene, acetic acid water
system. The minimum HETS is measured by fixing the geometry of the extractor,
holding the throughput constant, and then varying the reciprocating-plate fre-
quency. At low frequencies, the dispersed phase drop size is large and therefore
the mass-transfer rate is small, resulting in a large HETS. As the frequency in-
creases, the drop size decreases, and the mass-transfer rate increases decreas-
ing HETS. As shown in Figure 6.22, HETS decreases until flooding occurs. The
operating frequency must be less than the mininium frequency to avoid flooding.

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