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Unformatted text preview: content of greater than 25 wt% A is impossible because the operating line would have
a slope less than an overlapping tie line, making it impossible to step off stages in the right
direction. Exercise 8.14
Subject: Extraction of trimethylamine (TMA) from benzene (C) with water (S).
Given: Three equilibrium stages. Solventfree extract to contain 70 wt% TMA. Solventfree
raffinate to contain 3 wt% TMA. Liquidliquid equilibrium data.
Find: Using the HunterNash method with a righttriangle diagram, find feed composition and
watertofeed ratio.
Analysis: The given phase boundary liquid compositions and the equilibrium liquidliquid
compositions (as dashed tie lines) are plotted on the righttriangle diagram below. Included on
the diagram are the final extract and raffinate compositions. The final extract composition is
obtained by locating a point, P, for 70 wt% TMA, 30 wt% benzene, and 0% water (given solventfree composition), and drawing a straight line from P toward the point S (pure water) to where
the line intersects the phase boundary. This is point E for the extract. In a similar manner, the
raffinate composition, R, is determined. Exercise 8.14 (continued)
On a second diagram, shown below, a trial and error procedure is used to find the operating
point, P' , that will result in the stepping off of three equilibrium stages, as illustrated in Fig. 8.19,
to obtain the specified final extract and final raffinate. The final trial is shown on the diagram,
where M is the mixing point for extract + raffinate, and for feed + solvent. Assuming waterfree
feed, the resulting feed composition is at F, with 57.5 wt% TMA and 42.5 wt% benzene. The
ratio of mass of solvent to mass of feed is given by the ratio of line lengths = line FM/line MS =
0.56. Exercise 8.15
Subject: Extraction of diphenylhexane (DPH) from docosane (C) with furfural (U) at 45 and
80oC
Given: Feed, F, of 500 kg/h of 40 wt% DPH in C. 500 kg/h of solvent, S, containing 98 wt% U
and 2 wt% DPH. Raffinate to contain 5 wt% DPH. Liquidliquid equilibrium data.
Find: Number of theoretical stages. DPH in kg/h in the extract.
Analysis:
Case of 45oC: The given liquidliquid equilibrium data are plotted in the righttriangle
diagram below. Included on the diagram are composition points F for the feed, R for the
raffinate on the equilibrium curve, and S for the solvent. A straight line extends from point F to
point S. Because the mass flow rates of the feed and solvent are equal, the mixing point, M, is
located at the midpoint of this line. Another straight line extends from point R to point M, and
then to an intersection with the equilibrium curve at point E, which is the final extract. Using the
inverse leverarm rule on line RME, the mass ratio of R to E is 0.445. Combining this with an
overall material balance: F + S = 500 + 500 = 1,000 = R +E
gives R = 308 kg/h and E = 692 kg/h. From the diagram, the mass fraction of DPH in the
extract is 0.281. Therefore, the DPH in the extract is 0.281(692) = 194.5 kg/h, which is 92.6% of
the total DPH...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
 Spring '11
 Levicky
 The Land

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