Make sure your extrapolations are reasonable

November 2018

Good physical property and safety data are a prerequisite for a reliable risk evaluation and assessment. Modelling and simulation of chemical processes and scenarios require numerous and, most importantly, consistent data. When only limited experimental information is available, estimation of not available pro­perty data or extrapolations are required.

Physical and chemical properties of pro­cessed and handled chemicals are vital basic information. Several classes of data can be distinguished:

• Thermodynamic properties
• Transport properties
• Safety information
• Toxicological data
• Ecotoxicological data

Many times properties need to be extrapolated well beyond the ranges where data have been originally measured. Figure 1 shows an example where vapour pressure data obtained at higher tempe­ratures were extrapolated to ambient tem­perature. This kind of extrapolation is often  necessary for transport classification (in accordance with GHS/CLP) as well as for the creation of material safety data sheets (MSDS).

The extrapolation with an inappropriate approach or the wrong correlation equa­tion can lead to discrepancies of several orders of magnitude as shown on Figure 1 for the vapour pressure extra­polation of dicyclohexyl methylphospho­nate.

Figure 1: Extrapolations can be tricky 

The propagation of errors is another problem. E.g. when sizing a safety valve, an error in the vapour pressure would result in a wrong estimated temperature at relief. This would either lead to an oversized or undersized valve with subsequent issues, e.g. valve shattering, undersized downstream treatments or even failure of the mechanical integrity of the protected vessel. 

Powerful tools and concepts are available so that data can be safely extra­polated to regions not accessible by experiment.

Various possibilities exist to estimate physical property data:

• Empirical correlations and rules
• Group contribution methods
• Corresponding states approaches
• Structure-property relationships
• Thermodynamic consistency tests
• Interpolation and extrapolation from similar compounds
• Homologues series analysis
• Chemical family plots and trends
• Analogous series methods
• Multi property correlations
• Patterns of property behaviour
• Combinations of above methods

The well-known and comprehensive TÜV SÜD physical property data base software PPDS can be used for retrieving pure component and mixture data. Critically evaluated data and relations for tempe­rature-dependent properties as well as regression programs creating new cor­relations are also available.

In addition, experimental as well as extra­polated or estimated data must always be rigorously checked and validated for correctness, plausibility, accuracy and precision before use.

The following services can be provided:

• Selection of property methods
• Choice of correlation equations
• Regressions and statistical analysis
• Error propagation and visualisation
• Interpretation of results

Our laboratories and experts can help you to measure, estimate or predict safety information. We evaluate and assess physical property data to improve the safety of your processes.

Contact : TÜV SÜD Process Safety