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Model experiments of autoxidation reaction fouling Wilson, David Ian


Chemical reaction fouling of heat exchangers is a severe problem in the petrochemicals industry, where deposits can be formed by a wide range of undesirable reactions. Autoxidation has been identified as a prime source of deposit formation in oxygenated process streams and fuel storage systems but the fouling mechanism has not been fuily investigated. The fouling of heat exchangers subject to autoxidative fouling was studied using model solutions of an active alkene, indene, in inert solvents saturated with air. The heat exchangers were operated at moderate surface temperatures (180-250°C) and at turbulent flow velocities. The experiments featured air pressures of 342-397 kPa and heat fluxes of 90-280 kW/m². The effects of chemical reaction rate, surface temperature and flow velocity were investigated and compared with existing chemical reaction fouling models. Chemical initiators were used to eliminate the chemical induction periods observed under ‘natural’ thermal initiation and permitted the study of the chemical reaction rate and surface temperature as separate variables. The chemistry of the physical system and the complex reaction mechanism prevented extensive model development. Two fouling probes were used: an annular probe which allowed visual inspection of deposit formation and a novel tubular heat exchanger constructed during this work which allowed inspection of the deposit in situ after an experiment. The same fouling mechanism was found to generate deposit in both probes. Chemical analyses were developed to monitor the autoxidation reaction during the batch fouling experiments. The results confirmed that fouling was caused by the deposition of insoluble polyperoxide gums generated by the reaction of indene and oxygen. The gums aged on the heat exchanger surface to form complex oxygenated solids which were not easily removed. These results confirmed the hypotheses of Asomaning and Watkinson (1992). The kinetics of indene autoxidation was studied in a separate series of semi-batch stirred tank experiments. The rate of formation of polyperoxides was influenced by the solvent nature, temperature, oxygen concentration and mode of initiation. The aromatic polyperoxides exhibited limited solubility in aliphatic solvents. The kinetics of indene autoxidation could not be described by the schemes reported in the literature and were found to be subject to oxygen mass transfer effects. The fouling resistance behaviour was controlled by conditions in the bulk fluid. The initial, linear fouling rate decreased with increasing flow velocity and increased with bulk reaction rate and surface temperature. A simple fouling model, involving generation of deposit in the reaction zone next to the heat transfer surface and an attachment factor related to the mean fluid residence time, was fitted to the experimental data. Once the solubility limit was reached, the fouling resistance showed increasing rate behaviour, caused by the deposition of globules of insoluble gum. The effect of an antioxidant on the fouling process was studied. The efficiency of the antioxidant, di-t-butyl-4-methylphenol, was found to be severely reduced under the enhanced thermal conditions in the heat exchanger. Simulated ageing experiments were performed to investigate the fate of polyperoxides exposed to the enhanced temperatures on the heat exchanger surface. The studies confirmed that the insoluble polyperoxide gums undergo ageing processes after deposition.

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