Determination of Reaction Heat: A Comparison of Measurement and Estimation Techniques

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Abstract

The heat of reaction is an important parameter in the safe, successful scale-up of chemical processes. Reaction heat data is used to calculate the potential adiabatic temperature rise of the desired reaction, providing a worst-case scenario for rapid reaction of the entire batch with no heat loss to the surroundings. The data is used in parallel with information regarding the thermal stability of reaction mixtures/components and an intimate knowledge of the process to analyze the risk associated with running it on-scale. If the level of risk is judged to be unacceptable, the analysis can be used to make rational process changes in order to reduce the risk to an acceptable level. The Pfizer global process safety network provides a heat of reaction for all processes run in our kilo laboratories, pilot plant, and manufacturing facilities. In general, there are two methods used to determine reaction heats: (1) experimental measurement using some form of calorimetry, or (2) estimation techniques. Since experimental measurement is not always practical, accurate, or necessary, we set out to show that estimation techniques could be used reliably and efficiently to provide heat of reaction data for a wide range of chemistry. To gain confidence in our ability to accurately predict reaction heats, we carried out a comparative study of measured versus estimated values. The results of this study will be discussed in detail, including rationalization of any significant disparity through further analysis to more fully understand the limitations/advantages of both techniques. To help ensure consistent application of measurement/estimation across the Pfizer global process safety network, we developed a decision tree to determine whether estimation or measurement should be considered for a particular reaction. In order to maximize the efficiency gain and to ensure accuracy in our estimations, we have created a heat estimation database that allows for (1) rapid archival/retrieval of model compounds, (2) calculation of reaction heat and adiabatic temperature rise, and (3) reporting/documentation of the results.

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