UBC Theses and Dissertations
The role of olefins in fouling of heat exchangers Asomaning, Samuel
Chemical reaction fouling is one of several categories of fouling of heat exchangers. It is encountered mostly in petroleum, petrochemical, and food processing industries, where it results in severe economic penalties. Olefins have been associated with fouling during heating of organic mixtures, and gum formation during storage and use of hydrocarbon fuels. In this work, thermal fouling studies are reported for a number of olefins, present at 10 % wt. in kerosene, undergoing sensible heating in the liquid phase at relatively high heat fluxes. Experimental work was done on an available fouling rig consisting of an annular probe and a coiled wire probe mounted in parallel. The annular probe with its heated central core operated in turbulent flow whilst the coiled wire, with flow normal to it, was in the laminar flow regime. Runs were conducted both under oxygenated (air-saturated) and deoxygenated conditions. The range of bulk temperatures was from 70 - 85 ℃, the initial wall temperatures were 180 - 205 °C, with a system pressure of 410 kPa (abs.). The range of heat fluxes was 150 - 350 kW/m². Only minor differences were noted between the extent or rate of fouling on the two different probes. Runs at heat fluxes below 180 kW/m² and bulk temperatures below 80 °C generally showed no measurable fouling with any of the olefins tested. Linear and falling rate fouling curves were observed at more severe conditions over 45 hours of typical runs. Under air saturated conditions, straight chain terminal olefins of C₈ - C₉ showed little or no measurable fouling. The longer chain length hexadecane-1, showed a significant increase in fouling. Moderate fouling was observed for 4-vinylcyclohexene. The cyclic olefins, dicyclopentadiene and indene, yielded the greatest R[formula omitted] values, being about 30 - 50 times those of the straight chain terminal olefins. Under deoxygenated conditions, typical R[formula omitted] values were a factor of about 30 below the corresponding values for air saturated conditions. R[formula omitted] generally increased with increasing heat flux. Where the antioxidant initially present in the olefin was not removed before use, very little fouling occurred. The effects of deoxygenation, heat flux and species effects are discussed and a probable fouling mechanism involving formation of polymeric peroxides by autoxidation of the olefins suggested. The fouling rates over the linear portions of the fouling curves have been calculated and the deposit thermal conductivity based on the maximum fouling resistance and deposit thickness have been estimated. Analyses of selected deposits have been presented and compared with both theoretical calculations for the expected polymeric peroxides and values in the literature.
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