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Complete this implementation and run the workshop. The results are given at
Workshops/Twelve-2a.
Workshop 12.2b Replace all of the heaters in Workshop 12.2a with countercurrent
heat exchangers, using the shortcut method to implement the design specifications
given above. Run Aspen Plus to determine the tear streams. Compare the number of
tear streams to those in Workshop 12.2a. Where necessary, substitute the appropriate
values obtained from Workshop 12.2a to aid in converging the model. The results are
given at Workshops/Twelve-2b.
Workshop 12.2c Convert the heat exchanger design from above to shortcut simula-
tion models and rerun the simulation. The results are given at Workshops/Twelve-2c.
Workshop 12.2d Design the distillation column using RadFrac at twice the min-
imum reflux ratio and a distillate rate specification such that the column meets the
specifications above. Select a suitable design to prepare for using it to replace the
existing DSTWU model. The results are given at Workshops/Twelve-2d.
Workshop 12.2e Replace the DSTWU model with the RadFrac model from
Workshop 12.2d. Make the necessary Aspen Plus run(s) required to develop a
complete material and energy balance. If necessary, substitute the appropriate values
obtained from Twelve-2a to aid in converging the model. The results are given at the
Workshops/Twelve-2e.
Workshop Notes
Workshop 12.2a The placement of a pump is appropriate because of the large
changes in pressure.
Workshop 12.2b The shortcut design method does not provide any design details but
merely estimates the area required. It uses a constant overall heat transfer coefficient
with a default of about 150 Btu/(hr-ft 2 - F). The best of poor choices would be to obtain
an estimate of U from a literature source such as Perry and Green (1999). A proper
design would involve use of the EDR models referred to in Chapter Nine.
Workshop 12.2c Conversion of the design to a rating model consists merely of
providing the area calculated in 12.11. Convergence of a small exchanger will proba-
bly require the specification of a smaller area convergence tolerance than the default
value.
Workshop 12.2d Using the DSTWU as an approximate solution, the RadFrac model
solution space was searched with a few trial runs settling on about 35 trays, feed
onstage 18, and a reflux ratio of about 1.3. For a duplicator block fed a DSTWU (for
comparison) and a RadFrac block with the same feed, after settling on a D / F value of
0.641, a sensitivity study varying the reflux ratio was made. Results show that a reflux
ratio of 1.2 meets the recovery specifications.
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