Unformatted text preview: converge the material balance for the recycle
process. Exercise 11.20 (continued)
The resulting material balance was as follows:
D1 D3 L2 V2 B2 D2 B3 39.6
39.7 From the above material balance, the isopropanol product purity is 99.9 mol% and the
water product purity is 99.6 mol%. The benzene loss is negligible. The material balance lines
are shown on the triangular diagram on the following page. The separation in column 1 is in
distillation region 1, but the decanter sends the distillate, D2, into region 2 where the separation
in column 3 occurs.
The composition profiles for columns 2 and 3 are shown in two subsequent pages, where
it is seen that no pinch zones are evident. Exercise 11.20 (continued)
Analysis: (continued) Exercise 11.20 (continued)
Analysis: (continued) Exercise 11.21
Subject: Separation of a mixture of acetic acid (A) and water (W) into nearly pure acetic acid
and pure water, using a two-column sequence that includes heterogeneous azeotropic distillation
with n-propyl acetate (NPA).
Given: 1,000 kmol/h of 20 mol% acetic acid in water.
Find: A suitable two-column design.
Analysis: The phase behavior of the acetic acid/water/n-propyl acetate system at 1 atm was
explored with the UNIQUAC equation for liquid-phase activity coefficients, noting that the
normal boiling points in oC are 118.1 for A, 100 for water, and 101.4 for NPA. Using the
ChemCad and Aspen Plus programs, the following data were obtained, including a residue curve
map for the vapor-liquid system at 1 atm and a ternary liquid-liquid diagram for the system at
80oC, both shown on the next page. Acetic acid and water form a minimum-boiling azeotrope at
90 mol% water that boils at 99.47oC. Acetic acid and n-propyl acetate do not form an azeotrope.
Water and n-propyl acetate form a heterogeneous azeotrope at approximately 40 mol% NPA and
80oC. This azeotrope separates into a mixture of almost pure water and 83 mol% NPA. The two
diagrams show that a ternary mixture, when distilled will tend to give a bottoms of almost pure
acetic acid and an overhead that when condensed will split into two liquid phases, one of near
pure water, and the other of concentrated NPA that can be recycled to the column as entrainer.
Consider a separation sequence that omits the preconcentator column shown in Fig. 11.31
because the feed is already concentrated in water. The feed is sent directly into the
heterogeneous azeotropic distillation column with a decanter to split the two liquid phases
resulting from the condensation of the overhead vapor. An initial geometrical construction for
determining the separation in that column is included in the ternary diagram on the next page.
The feed, F, of 800 kmol/h of W and 200 kmol/h of A enters the column along with an entrainer,
R, of saturated NPA-rich phase containing 1...
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