Page 423 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

unknown, they in turn can be estimated from one vapor pressure and one liquid density. Group-
contribution models require even less information: merely the chemical structure of the molecule.
However, these estimations can never be as accurate as experimental data. In thermodynamics, as
elsewhere, you get only what you pay for—or less!

Compounding this problem is the development and implementation of expert systems to help choose the
thermodynamic model. To date, these features merely offer false hope. Human thermodynamics experts do
not recommend them.

A safe choice of thermodynamic model requires knowledge of the system, the calculation options of the
simulator, and the margin of error. In this section, guidance on choosing and using a thermodynamic model
is given. In an academic setting, the choice of thermodynamic model affects the answers but not the ability
of the student to learn how to use a process simulator—a key aspect of this book. Therefore, the examples
throughout this book use simplistic thermodynamic models to allow easy simulation. In any real problem,
where the simulation will be used to design or troubleshoot a process, the proper choice of
thermodynamic model is essential. This section focuses on the key issues in making that choice, in using
experimental data, and in determining when additional data are needed.

It has been assumed that the reader understands the basics of chemical engineering thermodynamics as
covered in standard textbooks [4,5,6]. As pointed out before, it is extremely important that the chemical
engineer performing a process simulation understand the thermodynamics being used. In a course, the

instructor can often provide guidance. The help facility of the process simulator provides a refresher on
details of the model choices; however, these descriptions do not include the thermodynamics foundation
required for complete understanding. If the descriptions in the help facility are more than a refresher, the
standard thermodynamics textbooks should be consulted.

If the thermodynamic option used by the process simulator is a mystery, the meaning of the
results obtained from the simulation will be equally mysterious.

13.4.1 Pure-Component Properties

Physical properties such as density, viscosity, thermal conductivity, and heat capacity are generally not a
serious problem in simulation. The group-contribution methods are reasonably good, and simulator
databanks include experimental heat capacity data for more than a thousand substances. Although these

correlations have random and systematic errors of several percent, this is close enough for most purposes.
(However, they are not sufficient when you are paying for a fluid crossing a boundary based on
volumetric flowrate.) As noted in Section 13.2.2, one should always be aware of which properties are
estimated and which are from experimental measurements.

13.4.2 Enthalpy

Although the pure-component heat capacities are calculated with acceptable accuracy, the enthalpies of
phase changes often are not. Care should be taken in choosing the enthalpy model for a simulation. If the