Page 408 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

5. Select the equipment specifications (parameters) for each piece of equipment in the process.
6. Select the way in which the results are to be displayed.
7. Select the convergence method and run the simulation.

Step 3 is achieved by constructing the flowsheet using equipment icons and connecting the icons with
process streams. Sometimes, it is convenient to carry out this step first.

The interaction between the elements and steps and the general flow of information is shown by the lines
on the diagram. Of the seven input steps given above, steps 2, 5, and 7 are the cause of most problems
associated with running process simulations. These areas will be covered in more detail in the following
sections. However, before these topics are covered, it is worth looking at the basic solution algorithms
used in process simulators.

There are basically three types of solution algorithm for process simulators [1]: sequential modular,
equation solving (simultaneous nonmodular), and simultaneous modular.

In the sequential modular approach, the equations describing the performance of equipment units are
grouped together and solved in modules—that is, the process is solved equipment piece by equipment
piece. In the equation solving, or simultaneous nonmodular, technique, all the relationships for the
process are written out together and then the resulting matrix of nonlinear simultaneous equations is
solved to yield the solution. This technique is very efficient in terms of computation time but requires a lot
of time to set up and is unwieldy. The final technique is the simultaneous modular approach, which
combines the modularizing of the equations relating to specific equipment with the efficient solution
algorithms for the simultaneous equation solving technique.

Of these three types, the sequential modular algorithm is by far the most widely used. In the sequential
modular method, each piece of equipment is solved in sequence, starting with the first, followed by the
second, and so on. It is assumed that all the input information required to solve each piece of equipment
has been provided (see Section 13.2.5). Therefore, the output from a given piece of equipment, along with

specific information on the equipment, becomes the input to the next piece of equipment in the process.
Clearly, for a process without recycle streams, this method requires only one flowsheet iteration to
produce a converged solution. The term flowsheet iteration means that each piece of equipment is solved
only once. However, there may be many iterations for any one given piece of equipment, and batch units
require time–series calculations to match the required scheduling of operations for the given unit. This
concept is illustrated in Figure 13.2.

Figure 13.2 Solution Sequence Using Sequential Modular Simulator for a Process Containing No
Recycles