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feature

March/April 2008
InFocus

A Cat of a Different Color

CHETAH Helps Predict Energy Release Hazards

chetah 8.0 book and cdLike the big cat in the wild, CHETAH 8.0, the ASTM computer program for Chemical Thermodynamic and Energy Release Evaluation, works quickly. It’s a rapid method for professionals to screen a chemical or combination of chemicals for explosive potential early in the design and development stage.

CHETAH calculates three important impact sensitivity indicators: a chemical’s maximum enthalpy — a calculation of energy plus pressure and volume — of decomposition, overall energy release, and a net plosive density. Chemical and pharmaceutical companies use these calculations for evaluating chemicals used in industry reactors, to assess new chemicals being produced in laboratories and to check new or modified process operations. Government agencies and others look at safety with CHETAH. In addition, industry and academia put it to work in investigating the environmental fate of chemicals.

B. Keith Harrison, Ph.D., P.E., associate vice president of academic affairs and dean of the graduate school at the University of South Alabama, has helped refine CHETAH over many years, and he comments that the program is weighted toward safety. “CHETAH has been designed to err on the side of caution and is more likely to identify that something is a hazard when it in fact is not, than to make the error of wrongly reporting something is safe.”

The CHETAH manual introduction, in boldface and capital letters, stresses that CHETAH does not replace the actual physical testing of materials. Rather, CHETAH points to the need for and further guides impact sensitivity and flammability testing. “If CHETAH indicates that a chemical represents a possible hazard, further experimental testing can determine if this is indeed the case and the extent of the hazard. CHETAH is just one tool that can be used for reactive hazard evaluation, but a very useful one,” Harrison says.

Using methodology developed by S.W. Benson, University of Southern California, the program predicts thermodynamic properties based on the molecular structure of components. Users can either enter the Benson groups for a molecule or molecules, or construct the molecule with ChemDraw software and paste it into CHETAH to enter the needed Benson group.

CHETAH does more than predict chemical reactive hazards, Harrison emphasizes, and possesses five additional uses:

  • Thermodynamic property prediction, the program’s most-used feature;
  • Energy release evaluation to predict the danger of reactive decompositions;
  • Heat of reaction evaluation that results from a user-specified chemical reaction;
  • Flammability hazard evaluation; and
  • Heat of combustion calculations.

Since its first release in 1974, CHETAH has evolved from its FORTRAN antecedents to Windows interfaces for easier use. In addition, the Benson group database expansion has made the program even more useful for evaluating newly developed chemicals. Another notable change is the inclusion of flammability calculations.

More enhancements are in store for the next CHETAH release, planned for late this year or early 2009. “There are several popular ways of predicting energy release hazards: heat of decomposition, oxygen balance, adiabatic temperature rise and methods based on the nature of functional groups,” Harrison says. “CHETAH 8.0 makes all these types of calculations except adiabatic temperature rise. CHETAH 9.0 will include that.”