Page 117 - Process simulation and control using Aspen

CHAPTER

Aspen Plus Simulation of

Distillation Models

3 . 1 BUILT-IN DISTILLATION MODELS

An Aspen simulation package has nine built-in unit operation models for the separating
column. In the Aspen terminology, these packages are named as DSTWU, Distl, RadFrac.
Extract. MultiFrac, SCFrac, PetroFrac, RateFrac and BatchFrac. Under these categories,
several model configurations are available. Note that Extract model is used for liquid-
liquid extraction. Among the built-in column models, DSTWU, Distl and SCFrac
represent the shortcut distillation and the rest of the distillation models perform rigorous
calculations.
DSTWU model uses Winn-Underwood-Gilliland method for a single-feed two-product
fractionating column having either a partial or total condenser. It estimates minimum
number of stages using Winn method and minimum reflux ratio using Underwood
method. Moreover, it determines the actual reflux ratio for the specified number of
stages or the actual number of stages for the specified relux ratio, depending on which
f
is entered using Gilliland correlation. It also calculates the optimal feed tray and reboiler
as well as condenser duty. Remember that this model assumes constant molar overflow
and relative volatilities.
Distl model includes a single feed and two products, and assumes constant molar
overflow and relative volatilities. It uses Edmister approach to calculate product
composition. We need to specify a number of stages, e.g. feed location, reflux ratio,
pressure profile and distillate to feed iD/F) ratio. Actually, when all the data are
provided, we can use this column model to verify the product results.
RadFrac is a rigorous fractionating column model that can handle any number of
feeds as well as side draws. It has a wide variety of appUcations, such as absorption,
stripping, ordinary distillation, extractive and azeotropic distillation, reactive distillation, etc.
MultiFrac is usually employed for any number of fractionating columns and any
number of connections between the columns or within the columns. It has the ability to
simulate the distillation columns integrated with flash towers, feed furnaces, side
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