Using the ASTM Computer Program for the Prevention of Reactive and Flammability Hazards:
Since 19741 some form of the computer program now named CHETAH®2 has been provided by ASTM Committee E27 on Hazard Potential of Chemicals for predicting the energy release hazards of pure chemicals and mixtures. CHETAH is used routinely by a number of chemical and pharmaceutical companies to predict the potential for deflagration or detonation of a pure chemical or a mixture of chemicals as a part of their hazard evaluation process for the synthesis of new chemicals and for reactor designs. The emphasis is on the evaluation of chemicals or mixtures of chemicals before synthesis or manufacture to prevent accidents. In addition, CHETAH can be used to predict the flammability of chemicals and to predict the enthalpy of combustion and combustion products. The combustion calculations can be accomplished for chemicals composed of a very wide range of elements. CHETAH can also be used to predict the enthalpy change for a user-specified chemical reaction or the equilibrium constant for a reaction, whether the reaction involves a hazardous situation or not.
CHETAH is a unique tool for predicting both thermochemical properties and certain reactive chemical hazards associated with a pure chemical, a mixture of chemicals, or a chemical reaction. The calculations are made using only information concerning the molecular structure of the components, using the well-accepted Benson’s second order group contribution technique3 to predict important thermodynamic properties. The database of molecular fragments (Benson’s groups) used to describe the molecules is believed to be the largest such database in existence (with 880 groups currently), allowing a very large number of possible molecules to be calculated. Also, CHETAH has an extensive database of molecules for which the complete necessary thermochemical data are available from the literature for immediate calculations.
CHETAH also is a useful tool for predicting a number of parameters associated with the flammability in air of a pure material or in some cases for mixtures. The following flammability related quantities are predicted:
• Lower Flammable Limit (LFL) (by Bothwell’s Method2 and Britton’s Method4);
• Limiting Oxygen Concentration (LOC);
• Lower Limit Flame Temperature (LLFT);
• Maximum Flame Temperature (Tmax);
• Fundamental Burning Velocity (Su) of Single and Mixed Fuels;
• Quenching Distance (qd) Lowest Minimum Ignition Energy (LMIE);
• Lower Flammable Limit (LFLT) at Temperatures Other Than 298 K.
Use of CHETAH for Prediction of Reactive Hazards
CHETAH makes a prediction of which chemicals or chemical mixtures might be an explosive hazard (impact sensitivity). It does this by applying a series of thermodynamic and structure-based correlations and using the results of the calculations to provide an overall indicator of the potential hazard.
Several methods have been developed in the scientific literature for rapidly screening chemicals for possible reactive hazards. Examples include: maximum heat of decomposition (CHETAH), calculated adiabatic temperature rise (CART),5 and quantification of energetic groups (for example, the CHETAH Plosive Density Method). However, all these methods have limitations.6, 7, 8, 9 These predictive methods are not meant to replace the physical testing of materials. They are, however, useful for rapid screening to help identify the need for further testing.
CHETAH calculates three major impact sensitivity indicators:
1. The maximum enthalpy of decomposition. This assumes the test composition decomposes into a set of products so that the maximum energy possible is released.
2. An overall energy release evaluation which is composed of largely the indicator of the maximum enthalpy of decomposition, but modified by other factors that may be appropriate such as the oxygen balance and the number of peroxide bonds in a molecule.
3. Net plosive density, which is a group contribution index weighting stabilizing and destabilizing molecular fragments.
An example of CHETAH being used for the rapid screening of chemicals is provided by Cardillo and Nebuloni.10 Two thousand mixtures relative to waste streams from vapor degreasing, dry cleaning, and solvent extraction were screened using CHETAH. These authors were able to predict the proportions of certain waste chemicals that when combined will yield or not yield a reactive hazard. Calculations were made as to the amount of an inert compound required to maintain certain mixtures as non-reactive hazards. The authors point out that an experimental effort surveying all of these mixtures would be prohibitively expensive and time-consuming. CHETAH provided helpful guidance for the experimental program.
Another example of the possible utility of the use of CHETAH for chemical reactive hazard screening is provided in Wei, et al.11 A study was done on the application of screening tools to a list of reactive incidents in a report by the U.S. Chemical Safety and Hazard Investigation Board. The authors made CHETAH calculations for the chemicals involved in 33 incidents in reactors. In 24 of the 33 incidents, CHETAH indicated at least one chemical with a “high” hazard. The argument was made that the use of screening tools would help prevent many reactive chemical incidents.
Accuracy of CHETAH for the Prediction of Reactive Hazards
The calculations of CHETAH have been compared to an experimental database to provide an assessment of the accuracy of the predictions. The Committee E27 reactive chemicals database was assembled over a number of years with the cooperation of several chemical and pharmaceutical companies. CHETAH was tested on approximately 500 compositions from the database. Approximately one half of the sample was composed of impact-sensitive compositions with the rest being insensitive compositions.
As shown in Figure 1, CHETAH correctly identified the sensitive compositions in 83 percent of the cases using the maximum enthalpy of decomposition. The criterion termed the overall energy release evaluation improved the prediction slightly to 89 percent. The net plosive density method correctly identified 82 percent of the test cases but was only able to make calculations for about 60 percent of the test cases due to the limitations of the parameters needed for the method.
A sample output page for CHETAH for a calculation of the impact sensitivity of chemicals is shown in the second example below. Note that information is furnished other than the three indicators discussed. This additional information is useful in certain specialized situations for further analysis.
CHETAH has been biased to avoid, as much as is feasible, the type of error in which an actual explosive composition is predicted to be non-sensitive. When looking at the database composed of one-half sensitive compositions and one-half insensitive compositions, the overall energy release evaluation indicator will only incorrectly identify a sensitive composition as being insensitive in about 5 percent of the test cases. However an insensitive composition will be labeled sensitive in about 30 percent of the cases. CHETAH is obviously conservative to help prevent reactive accidents. The net plosive density method does the best job discriminating between sensitive and non-sensitive cases with about 15 percent of the insensitive cases labeled as sensitive.
Examples of Using CHETAH
For molecules not already in the database, the molecule is described by combining molecular fragments or groups using Benson’s method. For the sample molecule in Figure 2, the groups that make up the molecule are indicated. Note each group is made up of a primary part before the dash and a secondary part after the dash describing the neighbors of the primary group.
If you do not feel comfortable specifying the groups yourself, there is an alternative. By using the popular ChemDraw software, you can draw the molecule and then save it and paste it into CHETAH. CHETAH will then automatically determine the Benson groups.
An example of a Thermodynamic Table calculation follows in which the user makes use of the ChemDraw program to enter the needed Benson groups into CHETAH.
After starting CHETAH the user will see the following screen:
The user could directly enter Benson groups for a molecule or several molecules on this page, but for our example let’s use the ChemDraw program to get our Benson groups. First draw the molecule of interest (for example, toluene) using ChemDraw: