Measurement of Gas Flow Rates from Small-Scale Reactions

The rate and total volume of noncondensable gas generation are important parameters in the safe, successful scale-up of chemical processes. Information regarding the evolution of noncondensable gas is used to (1) ensure the gas can be vented from the reactor without overpressurization, (2) calculate the concentration of flammable gases (or oxygen) to avoid creating an explosive mixture in the equipment, and (3) size the scrubber to ensure the capacity and heat removal rate are sufficient. The data are used in parallel with information regarding the heat of reaction from the desired chemistry, thermal stability of reaction mixtures/components, and an intimate knowledge of the process to analyze the risk associated with scaling up. 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. Several techniques have been developed to study the gas hazards associated with a particular reaction. In general, these techniques suffer from at least one of two primary limitations: (1) the volume is not measured directly, causing the condition and/or composition of the gas to influence the accuracy of the measurement, or (2) the sensitivity is too low to take reliable measurements from small-scale reactions. The Pfizer Global Process Safety Network set out to develop a new device that could solve both of these problems and be used in conjunction with a microcalorimeter to provide heat of reaction data in parallel. This paper describes a new device that is used in series with a thermal mass flow meter to accomplish this goal. Detailed discussion of the error bounds on the flow rate from this device is also included.