Page 422 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Often, a series of case studies will need to be run using a base-case simulation as a starting point. This is
especially true when performing a parametric optimization on the process (see Chapter 14). When
performing such case studies, it is wise to make small changes in input parameters in order to obtain a
converged simulation. For example, assume that a converged simulation for a reactor module at 350°C
has been obtained, and a case study needs to be run at 400°C. When the equipment temperature in the
reactor module is changed and the simulation is rerun, it may be found that the simulation does not
converge. If this is the case, then, for example, go back to the base-case run, change the reactor
temperature by 25°C, and see whether it converges. If it does, then the input can be changed by another
25°C to give the desired conditions, and so on. The use of small increments or steps when simulating
changes in flowsheets often produces a converged simulation when a single large change in input will not.

Often when simulating a process, it is the flowrate of products (not feeds) that is known—for example,
production of 60,000 tonne/yr of chemical X, with a purity of 99.9 wt%. Assume that a converged
solution has been found in which all the product specifications have been met except that the flowrate of

primary product is not at the desired value. For this case, it is a simple matter to multiply all the feeds to
the process by a factor to obtain the desired flowrate of the product; that is, just scale the solution up or
down by a constant factor and rerun the simulation to get the correct equipment specifications.

For more advanced simulation applications, such as optimizing or simulating existing plants, it may be
necessary or useful to use controller modules in the simulation to obtain a desired result. For example, in
a recycle loop it might be required that the ratio of two components entering a reactor be set at some fixed
value. A controller module could be used to adjust the purge flowrate from the recycle stream to obtain
this ratio. The use of controller modules introduces additional recycle loops. The way in which
specifications for controllers are given can cause additional convergence problems, and this topic is
covered in detail by Schad [10].

13.4 Choosing Thermodynamic Models

The results of any process simulation are never better than the input data, especially the thermodynamic
data.

Everything from the energy balance to the volumetric flowrates to the separation in the
equilibrium-stage units depends on accurate thermodynamic data.

If reaction kinetics information is missing, the simulator cannot calculate the conversion from a given
reactor volume. Because such a calculation is not possible, only equilibrium reactor modules and those
with specified conversions can be used.

Unfortunately, process simulators have default thermodynamics packages, which will—without
warning—blindly miscalculate the entire flowsheet.

Only a few, readily available data are required to estimate the parameters in simple thermodynamic

models. If the critical temperature and critical pressure are known for each pure component, the
parameters for simple, cubic equations of state can be estimated. Even if these critical properties are