Thermodynamics - General Information
Overview
Selecting appropriate thermodynamic methods is an important and necessary step in the solution of flowsheet simulations. It is your responsibility as the user to ascertain that appropriate methods are used. SimSci provides many help mechanisms to guide you in your choices, not the least of which is direct hotline support with simulation experts. PRO/II provides a broad range of methods that allow solution of the wide variety of systems encountered in the chemical process industries.
Thermodynamic properties are an integral part of the flowsheet calculations. The equilibrium K-value (VLE, LLE and SLE) is used in phase separation calculations. Enthalpies are used to determine the energy required to take a system of components from one set of thermal conditions to another. Entropies are used for calculation of compressors, expanders and Gibbs free energy minimization reactors. Densities are used in heat transfer, pressure drop calculations and column tray sizing. Solid solubilities are used in calculating dissolvers and crystallizers.
Transport property methods may also be selected for use in conjunction with the thermodynamic system. Transport properties include liquid and vapor viscosities, liquid and vapor thermal conductivities, and liquid diffusivities. While not technically a transport property, vapor-liquid surface tension is also included with this group. Transport properties find utility in rigorous heat transfer calculations, pressure drop calculations, column sieve tray calculations and packed column calculations. Transport properties are also reported in the stream properties and distillation tray liquid and vapor properties reports.
Organization of Thermodynamic Methods
SimSci has made it easy to select a complete set of thermodynamic methods for a simulation with the thermodynamic system concept. A thermodynamic system consists of methods to calculate the following: K-values (VLE, LLE and SLE); liquid, vapor and solid enthalpies; liquid and vapor entropies; and liquid, vapor and solid densities.
Numerous thermodynamic systems are provided for your convenience. Furthermore, they are grouped into logical categories to facilitate location of the appropriate system(s) for solution of a flowsheet simulation. Thermodynamic systems are always designated by the K-value method, even though several methods may be used in any given system to evaluate the various thermodynamic properties. See Thermodynamic Property Calculation Categories for more information on the available thermodynamic systems.
Many of the systems support three phase (vapor-liquid-liquid) equilibrium calculations. When you select a system which supports vapor-liquid-liquid calculations, you are asked to confirm the calculation option for the simulation: vapor-liquid calculations only, vapor-liquid-water decant calculations, or rigorous vapor-liquid-liquid calculations. The water decant option is adequate for many hydrocarbon-water systems. See Water Decant Options for more information.
Modification of Thermodynamic Systems
After a thermodynamic system has been selected, it can be modified as desired (including the system name). For example, you may select a particular system such as Soave-Redlich-Kwong and wish to use the COSTALD method to evaluate the liquid densities instead of the method preselected in the Soave-Redlich-Kwong system. This may be easily accomplished through use of the Thermodynamic System - Modification Window. In this way, a custom system may be created and saved for use with a particular flowsheet application.
The combinations of methods for the various standard systems provided by PRO/II are enumerated in the individual category descriptions. See Thermodynamic Property Calculation Categories for more information.
It is also possible to modify specific data for a thermodynamic method. For example, an amine residence-time correction-factor may be provided for the Amine system when MDEA or DGA is used. The databanks to use with a particular method may also be selected. Thus, you can use the SimSci LIBMGR Program to create a special databank of parameters for a particular thermodynamic method and then use the databank selection feature to request that the special databank be used.
The methods used for the water decant calculation may also be modified. See Thermodynamics - Water Options for more information.
Selection of Transport Calculation Methods
The Thermodynamic System - Modification Window is also used to select the transport methods to be used in conjunction with the thermodynamic system which has been selected for modification in the Thermodynamics - Main Data Entry Window. Transport properties may be selected on an overall level, in which all transport properties are provided by selection of a single transport system. See Overall Transport Specifications for more information on this option. In similar fashion to thermodynamic properties, a system of methods may be chosen and the method for any particular transport property subsequently revised. See Thermodynamics - Transport Properties for more details on transport properties.
Description of the Thermodynamic Methods
Because of the large number of thermodynamic methods in PRO/II, they are not listed in this discussion. However, a descriptive help text for any of the methods may be accessed from the Thermodynamic System - Modification Window. Select the property type in the first list box, the method name in the second list box and push the Help button on the Toolbar.
Complete descriptions for all of the thermodynamic methods may be found in topics under Thermodynamic Methods in Chapter 2, Volume 1 of the PRO/II Reference Manual.
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