Appendix D: Thermodynamic Data - Sources and Quality Considerations

In order to perform calculations using ChemApp, apart from the ChemApp object code you also need thermochemical data. These data are required in the form of thermochemical data-files in ChemSage-compatible format. The sample data-file cosi.dat supplied with ChemApp is used for the instruction contained in this part of the manual. Please contact GTT-Technologies if you need more information on thermochemical data, especially on non-ideal solution models.

D.1: Sources of thermochemical data

There are several sources of thermochemical data:

  1. Supplied as databases for use with ChemSage or FactSage (see Table 22), or as extracts of those databases in the form of ready-to-use data-files.

  2. If you are a user of ChemSage, FACT-Win, or FactSage, you can prepare an appropriate data-file from the databases you are using with these programs.

  3. Use your own data and an ASCII-editor to compile a data-file in the required format, if you are not a ChemSage user and wish to prepare your own data. However, note that you need to be extremely well familiar with the structure and contents of such data-files, especially since you do not have the interactive features of ChemSage available in order to check your data-file. Please contact GTT before you start preparing a file.

Table 22: Thermochemical data from GTT-Technologies

Form Contents
SGTE Pure Substance Data for over 4,000 pure substances
SGTE Solution Data for over 200 binary, ternary and quaternary non-ideal solution phases
FACT Pure Substance Data for over 4,300 pure substances
FACT Solution Data mainly for oxide and salt non-ideal solution systems
AEA Databases Data for pure substances and solution systems for nuclear applications
Standard 'Off-the-shelf' data-files for direct use
Customised Data-files compiled to user specification, for direct use, with data from the above databases and other sources

If you are interested in obtaining ready-to-use data-files from GTT, please view the Database Documentation.

D.2: Quality considerations

It is very important when applying thermodynamic data, especially for critical applications, that the user understands that certain limitations may exist regarding its quality and the generality of its use; this applies especially to complex data-files for multicomponent, multiphase systems. These potential limitations apply whether data is supplied by GTT, or whether you prepare it yourself. It is important that all data be verified against known facts, wherever possible.

Quality categories

The most comprehensive (and thus reliable) kind of data-file to use is one that is fully consistent; i.e. it contains the Gibbs energies of all phases of the chemical system given as functions of temperature, pressure, and composition based on the standard element reference state (SER). Such a data-file permits calculation of phase equilibria for all temperatures and pressures from room temperature upwards and over the entire composition range covered by the elementary components; also, the thermodynamic properties of any of the phases in the system - integral or partial values of specific heat, enthalpy, entropy, Gibbs energy or volume. However, such a data-file is an ideal case. The higher the number of components in a system the more likely it is that certain simplifications are necessary.

These simplifications relate to restrictions of the range of applicability, i.e. temperature and composition, and also to the range of thermodynamic properties that can be calculated. These restrictions are perfectly permissible when they match the scope of the application. The following examples demonstrate this point further:

D.3: Techniques for compiling a data-file

Preparation of any data-file implies that at least some, if not all, thermodynamic data are available in some form, preferably fully assessed and qualitatively approved. In these cases, compilation of a high quality, fully consistent data-file is relatively straightforward. However, it is frequently the case that these kind of data are not immediately at hand. In such cases, recourse has to be made to other approaches:

As may be understood from the above, preparation of a thermochemical data-file requires a high level of knowledge, skill and experience. Even when these attributes are present, it is not always possible to produce a data-file that immediately completely matches all a user's requirements.

The problems associated with the reliability of a data-file assume another level of significance when a person inexperienced in the field attempts to compile a new data-file from available thermochemical data, or even to compile a data-file for a more comprehensive system from two or more smaller data-files. A number of factors can be of major importance; for example, the presence of miscibility gaps and the relative importance of missing data for phases, etc. Hence, GTT-Technologies offers users a comprehensive data service, either to provide data-files of acceptable quality , or to provide assistance in their compilation and checking.

D.4: GTT data-file identification

If you receive data-files from GTT-Technologies, apart from the example data-files distributed with ChemSage and ChemApp , you will note that they are identified with a coded serial number made up of letters and digits; for example, 0135B03T.DAT. The code provides you with the following information:

Hence, from the serial number, it is possible for you to know the quality of a data-file, if and when it was upgraded, and the originator of the original or upgraded data-file. The code used to designate the quality of GTT data-files is shown in Table 23 below.

Table 23: Quality coding for GTT data-files

Quality Definition
A fully consistent thermodynamic data covering the entire range of temperature and composition
B partially consistent thermodynamic data covering only a specified range of temperature and/or composition of the system, a very high proportion of the data is reliably assessed
C partially consistent data covering only a specific range of temperature and/or composition, a significant amount of estimated data is used
Z data-files under development - a reliable quality definition is not yet possible

Example of a data-file serial number

0135B03T.DAT signifies that the original version of the data-file 0135 was produced in 1993, has the quality designation 'B', and was produced by co-workers in the (T)HERDAS group at the Institute for Theoretical Metallurgy at the RWTH Aachen.

0135A24G.DAT means that the same data-file as in the previous example has been upgraded by (G)TT to 'A' quality after a second revision, the last one being in 1994.

Documentation of data-files

In addition to the serial number, each GTT data-file is comprehensively documented. A commentary section below the thermochemical data provides details of the history, quality, area(s) of application, sources of data, literature references, details of the upgrading of the data-file, etc. In this way, a complete documentation of the data-file is provided for the user to judge the suitability of a data-file for particular applications.

Note: In contract to data-files in ASCII format (see Chapter 1.11.2), binary (see Chapter 1.11.3) and transparent data-files (see Chapter 1.11.4) only permit inspection of the commentary section when used with ChemSage. In order to permit ChemApp users who do not have ChemSage at their disposition to inspect the commentary, the appropriate text is delivered separately in ASCII form, using as file name the serial number of the data-file with the extension .CMT. Such a file contains a commentary section similar to the one shown at the end of Chapter E.1 . However, it is highly recommended that a ChemApp user also uses ChemSage, since this enables him to generate test outputs for his ChemApp applications in an easy-to-use interactive fashion.

ChemApp Programmer's Manual, Edition 3.6© GTT-Technologies, 2003