Simulation programs provide process engineers with an effective tool for process development besides experiments and trial plants. Modern simulation programs allow engineers to simulate individual units as well as networks of units. There are advantages to using simulation tools such as better understanding safety aspects, time and cost savings, and optimization of process control. Simulation programs can be divided into two groups: stationary programs suitable for steady state processes, and instationary programs for dynamic systems. Common simulation programs and the basic approaches of sequential modular and equation-oriented are described.

1. Simulation Programs
Besides experiments and trial plants, simulation using
computer programs furnishes the process development engineer
with a very effective tool.
Development of very fast, large computers & effective physical
models has now made it possible to simulate individual units.
E.g. networks of units of the same type (e.g., a stirred tank
cascade, a heat exchanger network) & networks of dissimilar
units (e.g., chemical plants).

2. The advantages of using suitable simulation tools in process
development are:
Better understanding of the process (including safety aspects)
Saving time
Increasing the scale-up factor
Reducing costs due to fewer pilot plant tests
Optimization of process control

3. Simulation Programs
Mathematically, simulation can be divided into 2 large groups:
stationary & instationary processes.
Stationary Programs: more suitable for simulation of steady
state process.
Instationary Programs: used for simulation of dynamics
systems.

4. Selected Simulation Programs for
Stationary Processes

5. Selected Simulation Programs
for Dynamic Systems

6. There are 2 basic approaches to calculating stationary processes:
Sequential modular & Equation-oriented approach.
Sequential Modular Approach: the solution strategy of
classical calculation by hand is transferred to the flow-sheeting
program. The program treats one unit operation after another.
Equation-Oriented Approach: combine the equations for the
linkage of the flow sheet and the apparatus model in a matrix,
which is then simultaneously solved.

7. Sequential Modular Approach
Advantages:
Large flow sheets can be prepared, since the blocks are calculated
individually.
Simulation can be understood in terms of process engineering.
Start-up behavior is generally well behaved.
Disadvantages:
Convergence problems in flow sheets with many recycle streams.
Incomprehensible convergence behavior in large flow sheets.
Slow for large flow sheets with recycle streams & complex
specifications.

8. Equation-Oriented Approach
Advantages:
Optimum convergence behavior in flow sheets with recycle streams
Certain convergence behavior
Very fast
Disadvantages:
Requires large storage space
Errors difficult to find
No insight into the units
Specification of initial values necessary

9. Example of Flow Sheet
with Recycling

10. Components of
Simulation Program
Modern commercial simulation program contains the following building blocks:
User interface.
Physicochemical data model (material systems such as conventional systems,
solids, electrolytes, polymers, etc.; physicochemical data of pure substances &
mixtures; equilibrium, etc.).
Unit operation model (black box models such as mixers, separators, component
splitters, etc.; models of phase separation and relaxation, heat transfer model,
multistage models, pumps and compressors, reactor models such as equilibrium
reactor, stoichiometric reactor, tubular reactor, etc.).
General simulation functions (sequencing, tearing, feed-forward/feedback
controller, convergence, optimization, etc.).
Additional programs.

11. Procedure Recommended for
Simulation Problems
Formulation of task, for example: CSTR with recycling
Choice of software suitable for the case
Choice of chemical components
Choice of thermodynamic model
Plotting of flow sheet
Linkage of the streams & units
Physicochemical data of pure substances
Specification of the streams
Specification of the unit operations

12. Procedure Recommended for
Simulation Problems
Formulation of task, for example: CSTR with recycling
This is a kind of drawing a flow chart summarizing the input, the
unknowns & the sequence of the process. That is done by the
user itself.

13. Procedure Recommended for
Simulation Problems
Choice of software suitable for the case
Some programs are recommended for steady state process, while
others are better in case of dynamic process as listed before.
There are programs specified for certain types of process, e.g.
Prosim for fertilizers, Pro/II for oil industries & HYSYS for gas
processing.
Some programs are only used in design or simulation of certain
type of equipment, e.g. Pipe simulation, power generation plants.

14. Procedure Recommended for
Simulation Problems
Choice of chemical components
The 1st step in the simulation is selecting the components
handled in the case. This includes any input, output & utilities.
Choice of thermodynamic model
It’s called fluid package & it has 2 functions:
1- Specifying the suitable equation of state
2- Calculating different thermodynamic & physical properties for
any component mixture, e.g. H, S , , , ……

15. Procedure Recommended for
Simulation Problems
Linkage of the streams & units
This step differs from program to another, for some programs
there is a tool used to connect the different unit operations
involved the process.
Specifications of the streams
i.e. enter the known physical properties, flow rates & phases for
the streams.

16. Procedure Recommended for
Simulation Problems
Specifications of the unit operations
i.e. specifying the type of the equipment, the operating
conditions, sizing the equipment