Information Technology Reference
In-Depth Information
CHAPTER ELEVEN
MULTISTAGE EQUILIBRIUM SEPARATORS
Distillation and extraction are the most commonly used separation methods in chemical
processing, and models of these are at the heart of Aspen Plus. All the Aspen Plus
distillation and extraction models may be found in the model library under the tab
Columns.
For a distillation column, simulation implies the existence of a fully configured
column (i.e., a number of theoretical stages) and the feed and sidestream locations.
When the state of the feeds, and the overhead, bottoms, and sidestream specifications
are provided, the model calculates the state of the products, and the reboiler and
condenser heat loads, if they were not explicitly specified. For an extraction column
the same ideas apply except that no overhead and bottoms specifications are required.
There is limited capability in Aspen Plus to design a column directly to yield products
of specified composition. Thus, the technique is to search the solution space for a trial
column with the characteristics desired, and therefore many rating solutions may be
required until one that matches the desired design specifications is obtained.
11.1 BASIC EQUATIONS
All models of distillation and extraction processes involve the solution of the material,
equilibrium, and energy balance equations or an applicable subset. All models are
based on the analysis of a single stage, such as that shown in Figure 11.1. There are
several ways to write the describing equations, based on the choice of independent
variables. For the set of equations below, the choices
v
i
and
l
i
, the component
i
vapor
and liquid leaving stage
j
;
f
i
, the componential feed to stage
j
;
s
i
, a componential
sidestream leaving stage
j
,
T
j
and
P
j
, the temperature and pressure of stage
j
,and
Q
j
,
the total energy entering or leaving stage
j
, are used. The phase equilibrium constant
