Page 193 - Chemical engineering design

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CHEMICAL ENGINEERING
But, computational difficulties can arise due to the iterative methods used to solve recycle
problems and obtain convergence. A major limitation of modular-sequential simulators is
the inability to simulate the dynamic, time dependent, behaviour of a process.
Equation based, dynamic, simulators require appreciably more computing power than
steady-state simulators; to solve the thousands of differential equations needed to describe
a process, or even a single item of equipment. However, with the development of fast
powerful machines this is no longer a restriction. By their nature, equation based programs
do not experience the problems of recycle convergence inherent in sequential simulators.
But, as temperature, pressure and flow-rate are not fixed and the input of one unit is not
determined by the calculated output from the previous unit in the sequence, as with steady-
state simulators, equation based programs are more time demanding on computer time.
This has led to the development of hybrid programs in which the steady-state simulator
is used to generate the initial conditions for the dynamic simulation.
The principal advantage of equation based, dynamic, simulators is their ability to model
the unsteady-state conditions that occur at start-up and during fault conditions. Dynamic
simulators are being increasingly used for safety studies and in the design of control
systems.
The structure of a typical simulation program is shown in Figure 4.4.

Data input

Equipment
sub-routines
Library and
specials
Thermodynamic
sub-routines
Data
Convergence output
promotion
sub-routines Executive program (organisation of the problem)
Physical
property
data files
Cost data
files

Figure 4.4. A typical simulation program

The program consists of:

1. A main executive program; which controls and keeps track of the flow-sheet calcu-
lations and the flow of information to and from the sub-routines.