Page 371 - Chemical process engineering design and economics

350 Chapter 6

Table 6.32 lists the equations for sizing a Karr extractor, which is only a
rough approximation for a preliminary process design. Table 6.33 outlines the
calculating procedure. Tests using the actual solution, solvent, and equipment are
necessary to arrive at an accurate extractor size. Again, it is assumed that the solu-
tions are dilute so that the operating and equilibrium curves are linear. Thus, the
Kremser equation, Equation 6.32.5, can be used to calculate the number of equilib-
rium stages. The subscript V refers to the light phase and the subscript L to the
heavy phase. In Table 6.32, Equations 6.32.1 to 6.31.5 are for mass transfer from
the heavy phase to the light phase. Before using the Kremser equation, the operat-
ing solvent flow rate is required, which can be calculated from Equations 6.32.1
and 6.31.4. After specifying the recovery of the key component, the exit composi-
tion of the solvent stream is calculated from Equation 6.32.2. After calculating the
column diameter from Equations 6.32.7 and 6.32.8, use Equation 6.32.6 to calcu-
late the height of the extractor.
Although the size of the end sections of an extractor, where phase separation
occurs, could be calculated by a method similar to the one described in the section
on decanter sizing, a more approximate method will be used. Karr and Lo [62]
give the dimensions of the extractor used in their studies. The diameter of the end
section is 50 % greater than the column diameter, and its height is a little less than
the column diameter. For a pulsed-column extractor, Valle-Riestra '[53] used a
continuous-phase flux of 0.5 gal/min-ft 2 (3.40 m/min) and a height/diameter ratio
of 1.0 to size the end sections of an extractor. The cross-sectional area of the ex-
tractor, and hence, the diameter is calculated by dividing by the total volumetric
flow rate (the sum of the volumetric flow rates for both phases) by the total volu-
metric flow rate per unit of extractor cross-sectional area, obtained from Table
6.31. Then, add the diameter to the product of N e and HETS, as shown in Equation
6.32.6, to obtain the total column height.
The HETS for an extractor can be estimated by using the scaling rules de-
veloped by Karr and Lo [62] and experimental values of HETS summarized in
Table 6.31. First, determine if the extraction system is a low interfacial-tension
system or a high interfacial-tension system. Next, select a value of HETS from
Table 6.31 from the following systems:

low interfacial-tension - MIBK, acetic acid, water system
high interfacial-tension - o-xylene, acetic acid, water system
Then, scale this value of HETS for the extractor diameter using Equation 6.32.9. A
simpler procedure for obtaining HETS, however, is to use the correlation given by
Henley and Seader [31], shown in Figure 6.23. the correlation is acceptable for
both a low and high interfacial-tension system. The problem, however, is that in-
terfacial-tension data may not be available.
To complete sizing the Karr column requires sizing the electric motor. The
size of the electric motor to disperse one of the phases is small. Walas [51] states
that al.5hp(1.12 kW) motor is sufficient to agitate a Karr extractor 30 in (0.762

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