Page 410 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

13.2.1 Selection of Chemical Components

Usually, the first step in setting up a simulation of a chemical process is to select which chemical
components are going to be used. The simulator will have a databank of many components (more than a
thousand chemical compounds are commonly included in these databanks). It is important to remember
that all components—inerts, reactants, products, by-products, utilities, and waste chemicals—should be
identified. If the chemicals that you need are not available in the databank, then there are usually several
ways that you can add components (user-added components) to your simulation. How to input data for
user-added components is simulator specific, and the simulator user manual should be consulted.

13.2.2 Selection of Physical Property Models

Selecting the best physical property model is an extremely important part of any simulation. If the wrong
property package or model is used, the simulated results will not be accurate and cannot be trusted. The
choice of models is often overlooked by the novice, causing many simulation problems down the road.
Simulators use both pure component and mixture properties. These range from molecular weight to
activity-coefficient models. Transport properties (viscosity, thermal conductivity, diffusivity),

thermodynamic properties (enthalpy, fugacity, K-factors, critical constants), and other properties (density,
molecular weight, surface tension) are all important.

The physical property options are labeled as “thermo,” “fluid package,” “property package,” or

“databank” in common process simulators. There are pure-component and mixture sections, as well as a
databank. For temperature-dependent properties, different functional forms are used (from extended
Antione equation to polynomial to hyperbolic trigonometric functions). The equation appears on the
physical property screen or in the help utility.

For pure-component properties, the simulator has information in its databank for hundreds of compounds.
Some simulators offer a choice between DIPPR and proprietary databanks. These are largely the same,
but the proprietary databank may contain additional components, petroleum cuts, electrolytes, and so on.
DIPPR is the Design Institute for Physical Property Research (a part of AIChE), and sharing of process
data across different simulators (e.g., ASPEN Plus, CHEMCAD, HYSYS, PRO/II, SuperPro Designer)
can be enhanced by using that databank. (Note that some proprietary databanks may not be supplied in the
academic versions of these simulators.) All simulators also have built-in procedures to estimate pure-
component properties from group-contribution and other techniques. The details of these techniques are
covered in standard chemical engineering thermodynamics texts [4,5,6] and are not described here.
However, one must be aware of any such estimations made by the simulator. Any estimation, by
definition, increases the uncertainty in the results of the simulation. The entry in the databank for each
component should indicate estimations. For example, many long-chain hydrocarbons have no

experimental critical point because they decompose at relatively low temperatures. However, because
critical temperatures and pressures are needed for most thermodynamic models, they must be estimated.
Although these estimations allow the use of equation-of-state and some other models, one must never
assume that these are experimental data.

Heat capacities, densities, and critical constants are the most important pure-component data for