The Design of Disposal Systems for Runaway Chemical Reaction Relief

In the event of thermal runaway in a chemical plant, it is necessary to relieve the pressure generated in the reactor. Venting of the reactor will normally give rise to a two-phase mixture which requires treatment before any atmospheric discharge. In many cases, dump tanks are used to separate the two phases before the gas is passed on for further treatment. These vessels provide separation by reducing the velocity of the gas phase and allowing the liquid to fall out due to the force of gravity. Considerable research has centred on the design of reactor vent systems, but very little information is available concerning the correct design of disposal systems. This research programme has developed a relatively simple design methodology to design dump tanks by relating the methods used in vent sizing to those used in designing separator vessels. In order to evaluate the methods, a small scale test rig was designed and built around a sealed Dewar reaction vessel which was vented to a series of different sized dump tanks. This allowed the effect of using both oversized and undersized dump tanks to be studied. Initial experimentation using water/steam as a two-phase mixture provided results which gave good agreement with the predictions. Further tests using runaway reaction systems also provided good agreement with the predictions. Consideration.has been given to scale up of the small scale to full scale apparatus, where worst case scenarios need to be incorporated, showing that larger reactors need proportionately larger dump tanks. The effects of vent sizing design considerations have been evaluated, showing that small alterations to the reactor operating conditions can reduce the required dump tank volume by an order of magnitude. The methodology developed during this research programme has been compared to methods given in literature, which give a range of dump tank sizes. Further development of the methodology has produced a simplified method and the errors of this compared to the original methods are quantified. The simplified method provides an undersizing of the dump tank, in the worst case 15% undersized. Typically this method gives a more accurate estimation of the required volume. An amendment to the simplified design equation ensures that the dump tank will always be sufficiently sized.