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How to estimate a final temperature after a reaction
Predicting an adiabatic-temperature rise is useful to chemical engineers for a number of purposes. One use is with combustion reactions whereby it is useful for determining the adiabatic-flame temperature. The adiabatic-temperature rise can be calculated for any reaction of interest, and thus it is often useful in the analysis and design of reactors. The adiabatic-temperature rise is also useful to evaluate reaction hazards. In such cases, possible hazardous reactions are evaluated as part of a safety assessment. It is even possible to postulate possible decomposition reactions for chemicals and calculate an adiabatic-temperature rise as a predictor of a possible decomposition-reaction hazard [1].
Unfortunately, calculating the adiabatic-temperature rise conventionally requires a tedious iterative solution, which is cumbersome to do by hand, There are software products available, such as popular commercial flowsheet-simulation programs that do this calculation quite nicely. However, such software packages are quite expensive and are not readily available to many practicing chemical engineers. Because of this, we present here a means of performing this calculation that makes use of much less-expensive technical software. The software used here is not specifically designed to make this calculation but does so quite adequately and also has the benefit of documenting the calculations rather well.
Calculating adiabatic-temperature rise
The adiabatic-temperature rise is simply the increase in temperature caused by a specified reaction yielding specified products in a circumstance such that all the heat released by the reaction (assuming an exothermic reaction for such calculations) is retained in the system and is used to heat up the products - there is no heat transfer into or out of the system. Such a process can be represented by the line AC shown in Figure 1, where this example assumes that we start with reactants at 25°C. As the reaction proceeds, the reaction mass heats up, eventually reaching a final temperature at the completion of the specified reaction.
The calculation procedure for the final adiabatic temperature and the associated adiabatic rise in temperature is also shown schematically in Figure 1. The usual calculation path takes advantage of the thermodynamic principle (enthalpy is a state function) that allows calculation of an actual process by use of a convenient alternative path having the...