Effects of vaporization, diffusion and condensation on the combustion of reactive fluids in hot porous media

A number of major industrial accidents have been attributed to leakage of r eactive fluids into porous insulation material surrounding a hot pipe. We r eport here on a numerical and simplified analytical study of the behaviour of volatile reactive liquids dispersed within a hot inert matrix, with a pa rticular focus on the extent to which spontaneous ignition may be delayed o r prevented by the loss of liquid through evaporation. The motivation for this theoretical and numerical study was the experimenta l identification of a 'plateau condition', in which the temperature at the centre of a uniformly heated cube of insulation material, doped with a vola tile liquid and located in an oven, remained constant, below the set oven t emperature for a considerable period. These experimental observations are d escribed in the introductory sections. A physico-chemical model is then set -up and numerical results are derived in which the plateau development is s imulated. The point is also made that, as the block of material warms up, v apour may diffuse to, and condense in, the cooler central region. This may have a considerable bearing on the subsequent behaviour. The numerical mode l is related to experimental studies performed using organic liquids of var ying reactivity and volatility. A simple analytical approach is then presen ted, in which the key physical processes involved are exposed, and the rate of erosion of the plateau is interpreted. The importance of the rate of di ffusion of vapour through the block and the mechanisms involved, the parame ters that control how readily the liquid evaporates and the relation of the plateau to the propensity for combustion to take place are discussed.