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In-Depth Information
The key to solving such problems is a thorough understanding of the liquid - liquid
phase equilibrium. The binary parameter values that determine the decanter perfor-
mance are critical and must be checked carefully to be sure that the calculations yield
the result expected. Excellent sources of liquid - liquid equilibrium data are Arlt et al.
(1979 - 1987) and Stephen and Stephen (1964). When ternary liquid - liquid equilibrium
data for a particular component pair is not available, it may be necessary to determine
its binary parameters from vapor - liquid equilibrium data.
It is very difficult to merely specify the appropriate blocks in Aspen Plus and expect
the process to converge. As an example, an analysis of Figure 14.9 shows that two tear
streams are required. The streams about which one potentially knows the most about
are the decanter products, streams C and F. It is likely that Aspen Plus's choice of tear
streams will need to be changed. The design and simulation of such a system begin
with the development of a ternary diagram which shows the various tie lines. The use
of a selected tie line and performance specifications with simple blocks permits the
development of a material balance. An example is given at Examples/ExampleFour.
Starting from this point, rigorous blocks are piecewise inserted into the process, and
each of the columns is designed separately. The complete simulation requires consider-
able refinement, starting with loose specifications and will probably require an increase
in the number of theoretical stages required and the reflux ratio as the specifications are
tightened. An exact solution to tighter specifications becomes more and more difficult
to achieve and should be approached by starting with loose specifications, high reflux
ratios, and excessive number of stages.
14.5 WORKSHOPS
Workshop 14.1 A saturated liquid feed of 50 mol % methylcyclohexane, the remain-
der being n -heptane, is to be separated by extractive distillation using phenol as the
extracting agent. It is required that the methylcyclohexane product be at least 95 mol
% pure and the toluene product be at least 95 mol % pure. The phenol recycle stream
is to be at least 99.5 mol % pure. The columns are to operate at 760 mmHg. The
binary parameter values stored in the Aspen Plus data bank are adequate. Design the
system.
Workshop 14.1a Determine the optimum amount of phenol to be used as an auxil-
iary feed to the first column, Dist1. Determine the number of theoretical stages required
and the location of the feeds.
Workshop 14.1b Place the second column, Dist2, on the process flowsheet of Work-
shop 14.1a. Use the bottoms product of Dist1 as the feed to Dist2. Design Dist2.
Workshop 14.1c Connect the bottoms of Dist2 to Dist1. Be sure to provide a
makeup feed stream to account for the phenol losses from each column. Execute the
completed model calculations until convergence is achieved.
Workshop 14.2 A mixture containing 28.1 mol % methanol, 70.9 mol % water, and
1 mol % mesity oxide is to be distilled in a column designed to produce a distillate
with at least 99.5 mol % methanol and a bottoms with at least 99.5 mol % water.
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