LITERATURE REVIEW
2.5 THERMODYNAMIC AND PHYSICAL PROPERTIES
2.5.1 Structure of TP systems
TPs are generated from correlation equations or estimation equations such as an equation of state, a vapor pressure or activity coefficient equation, etc. Thus, during the calculation phase of the simulation only the estimation parameters of the methods employed for the specific set of chemical components appearing in the process need be known to the TP package.
A TP package is a multiple entry, multiple option sub-system available to an equipment module through a set of standard subroutine calls. Equipment modules can be developed without regard to the particular TP estimation procedures or components to be used since the preprocessing phase of the simulation run will condition the TP package to use the correct options and components when TP generation is invoked. Such an environment is referred to as autogeneration, and it frees the user from any coding of property-related data once a few simple options (one or two constants) and the component list have been identified.
TP packages contain two classes of options. The first relates to the use of alternate equations to predict the same property and the second to the road-map option which modifies the estimation sequence to compensate for various combinations of given or available and missing basic physical constants. In either case the options are resolved during the preprocessing phase, the first by user specification and the second by the TP preprocessor setting up the missing constants for direct use during auto generation. A useful adjunct to simulation programs is a version of the TP package which generates physical property reports such as Mollier charts and tabulated properties over limited ranges of temperature, pressure and composition to allow user inspection and validation
by independent means. Most simulation systems available do not provide this feature explicitly. An exception, however, is the FLOWTRAN system.
The most important and critical modules in any simulation program are the phase determination routines. These are not considered formally part of the TP package but there is an intimate relation, particularly in obtaining phase equilibrium ratios, which are composition dependent or in computing enthalpy or entropy for multiphase streams. Most simulators will handle two phase vapor-liquid equilibrium calculations. FLOWTRAN, in addition, considers three phase equilibria with partially miscible liquids. Experience, however, in these systems has been confined to mixtures well below their region. Little or no experience has been reported in the retrograde region.
Three phase vapor-liquid-solid equilibria may be very sensitive numerically and often .depend on reasonably good initial composition estimates. Although they have been generally approached from an activity coefficient point of view (Henley and Rosen,
1969), recent efforts have attempted to utilize an equation of state (Heidemann, 1974).
One of the values of process simulation with a built-in TP package in industry is the property consistency obtained within an entire project or within classes of similar projects. Where the system is widely used in the company, there is usually be a supporting group of several people responsible for supervising the accuracy and reliability of the estimation procedures used in the TP system. In the past, such a group would have been responsible for the compilation of company data handbooks and responding to requests for property information on new compounds. In the computer- based TP system environment, the best current knowledge and generalizations are coded and immediately accessible both to minor and major projects. The handbooks may still
exist as a resource for checking and validation and may TP experts generate themselves on the computer under careful control and supervision.
Nevertheless, the most difficult aspect of an effective TP package is integration and consistency. Estimation methods are not universal. Good K-value correlations do not bridge the entire domain of conditions and compositions from the critical temperature (or convergence pressure) to cryogenic temperatures. Hence, the user may have to choose an option, which is more valid in the dominant temperature region of the process. Ideally, one should be able to specify the technique to be used in evaluating phase behavior properties for each specific equipment module. Generally, errors in K-values (and relative volatilities) tend to be more serious than error in enthalpy. Current technology can usually predict K and enthalpy values over reasonable ranges of temperature and pressure adequate for process design. Errors in K-values may lead to serious design errors, especially when dealing with large recycles and or closely boiling components. Enthalpy errors can be handled more easily in the equipment design (for example by adding more tubes to a heat exchanger) than K-value errors (where for example more trays and a larger column design may be required).
Enthalpy estimation tends to be a patchwork quilt of methods. One may choose to rely on a conventional equation of state coupled with ideal gas heat capacity for the vapor enthalpy and a three-parameter corresponding states correlation for the liquid phase enthalpy via the heat of vaporization with the corresponding states of approach reserved for subcooled liquids. It may be noted that in most commercial systems many options for phase equilibrium calculations are offered, but no options for steam enthalpy are indicated. This suggests that a single correlation for enthalpy is adequate industrially for a broad spectrum of simulations.