Information Technology Reference
In-Depth Information
CHAPTER FOUR
PROCESSES WITH RECYCLE
Virtually all Aspen Plus blocks are designed to operate as part of a sequential-modular
(SM) simulator; that is, given the block's inputs and operating parameters, the block
will calculate internal conditions and its outputs. In most cases the block's operating
pressure and product stream pressures are specified in some way. This is illustrated by
the example of a Flash2 block shown in Figure 4.1.
Streams 1 and 2 are either specified or calculated by a block executed prior to block
B1. Stream 5 is a special case of free water, usually zero. The block's products, each
of streams 3 and 4, have
n
c
+
2 variables, where
n
c
is the number of components.
These are the flows for each of the components temperature and pressure, for a total of
2
n
c
+
4 variables. Counting equations for a Flash2 block, there are
n
c
material balances,
n
c
equilibrium equations, and two equations that satisfy the equilibrium requirement
that the temperature and pressure of the equilibrium streams, 3 and 4, are equal, for
a total of 2
n
c
+
2 equations with two degrees of freedom. The specification of any
of the possible two flash conditions — for example, the heat added and the vapor/feed
ratio — satisfies the block's requirements and permits calculation of the state of both
product streams. A process composed solely of SM models simulates the performance
of a process.
A variation on such a model poses the question: What temperature should the flash
operate at to give a desired product composition, within the constraints of thermody-
namic reality? As an example, if flashing a mixture of hydrogen and rocks, the basic
laws of nature will prohibit rocks from being in the overhead product. This mode
of simulation encompasses a design function. To accomplish this, one may employ a
