Page 412 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Table E13.1 shows the results for the two outlet streams from the absorber—Streams 12 and 13—for two
simulations, each using a different thermodynamic model for the vapor-liquid equilibrium calculations.

The second and third columns in the table show the results using the SRK (Soave [7], Redlich and Kwong
[8]) model, which is the preferred model for many common components. The fourth and fifth columns
show the results using a model that is specially designed to deal with ionic type compounds (HCl) that
dissolve in water and then dissociate. The difference in results is remarkable. The HCl-water system is
highly non-ideal, and, even though the absorption of an acid gas into aqueous solutions is quite common,
the SRK model is not capable of correctly modeling the phase behavior of this system. With the SRK
model, virtually all the HCl leaves the absorber as a gas. Clearly, if the simulation were done using only
the SRK model, the results would be drastically in error. This result is especially disturbing because
SRK is the default thermodynamics package in many simulators.

Table E13.1 Results of Simulation of HCl Absorption Using Two Different Physical Property
Models

More details of model selection are given in Section 13.4.

The importance of thermodynamic model selection and its impact on the validity of the results of a
simulation are discussed at length by Horwitz and Nocera [9], who warn

“You absolutely must have confidence in the thermodynamics that you have chosen to
represent your chemicals and unit operations. This is your responsibility, not that of the
software simulation package. If you relinquish your responsibility to the simulation package, be
prepared for dire consequences.”

13.2.3 Input the Topology of the Flowsheet

The most reliable way to input the topology of the process flow diagram is to make a sketch on paper and
have this in front of you when you construct the flowsheet on the simulator. Contrary to the rules given in
Chapter 1 on the construction of PFDs, every time a stream splits or several streams combine, a simulator
equipment module (splitter or mixer) must be included. These “phantom” units were introduced in
Chapter 5 and are useful in tracing streams in a PFD as well as being required for the simulator. Most