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Packed Towers 357
Figure 9-76. Partial pressure
of ammonia over aqueous
solutions of ammonia. Used
by permission of Leva, M.,
Tower Packings and Packed
Tower Design, 2nd Ed., U.S.
Stoneware Co. (now, Norton
Chemical Process Products
Corp.) (1953).
Detemzine Number of Transjk Units, N This problem steps off six full transfer units and approx-
imately % of the seventh. Report as 6.33 units.
The number of transfer units is determined graphical-
ly by: Performance Interpretation
1. Bisecting the vertical distance (line C) of Figure 9-7’7 The point X1 represents 0.092 mol NHs/mol H20,
between the 8% aqua operating line and the equilib- which is equivalent to 8 \vt% ammonia solution. If a weak-
rium curve B. er product were desired fewer transfer units would be
2. Starting at Y2, draw horizontal line to line C. Extend required.
this horizontal to right of C far enough to make “r, Point (e) is the intersection of the equilibrium curve
to line C” equal to “line C to end,” then step vertical- and the operating line and represents the equilibrium
ly to operating line, D. Move horizontally to line C, so condition for tower outlet liquor, and the maximum
that (a) (b) = (b) (c). Continue moving up the tower liquor concentration.
in this stepwise fashion. Note that the 10% (wt) aqua product operating line is
3. At end, XlY,, if steps do not end at exact point, esti- shown on the diagram, but such a concentration cannot
mate fraction of vertical step required and report as be reached when operating at 150 psig. A greater pressure
fractional transfer unit. is required in order to lower the equilibrium curve.