Dewar scale-up for reactive chemical waste handling
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The use of non-pressurised Dewar flasks has been proposed by some parts of the chemical waste treatment industry to determine the exothermic reaction incompatibility of mixtures. Temperature rises of between 6-10C in the Dewar vessel over a period of 10 minutes have been suggested by the industry as criteria to indicate an exothermic reaction of concern. This paper reports work sponsored by the Health and Safety Executive (in discussion with the Environment Agency) to investigate the limits of this method for scale-up to vessels typically used in the waste treatment industry. A literature review of the specific heat losses from Dewar flasks and large-scale vessels is compared to specific heat losses of Dewar flasks measured experimentally. Typical values of thermal characteristics of large-scale vessels used in the waste industries have also been assessed. The specific heat loss in the Dewar flask and large vessels are very different. Scale-up limits of four types of Dewar have been calculated for different values of overall heat transfer coefficients for large-scale vessel. Thermal behaviour of exothermic reactions in a Dewar flask has been compared to that predicted in large vessels using reaction kinetics and heat transfer models. For fast and highly energetic reactions the reaction energy release rate can be significant compared to the heat losses and the Dewar flask can detect runaway reactions. However, for low energy reactions or reactions with long induction time, the heat losses can be significant compared to the heat release rate and the Dewar test can then miss exotherms or give non-conservative results. It appears that the 6-10C criterion proposed by the waste treatment industry might be observed when the heat losses do not have a significant importance compared to the reaction heat release rate. However, the reaction completion time at large scale would be shorter than at the Dewar scale. In some cases, 10 minutes might be sufficient to detect the exotherm but not the runaway reaction. The test should therefore be run to reaction completion in order to fully detect exotherms. Reliable conclusions about the scale-up of Dewar data can be obtained when the chemical reaction kinetics are well known. Unfortunately this is not generally the case in the waste-treatment industry. So, unless the specific heat loss of the Dewar has been shown to be less than large-scale vessels, this method in isolation is likely to be unreliable for scale up to large vessels. 2008 Crown Copyright.
