The combustion of propane and air as catalyzed by platinum in a fluidised bed of hot sand

Abstract

Electrically heated beds of silica sand have been fluidized by mixtures of propane and air, which attain the temperature of the bed very quickly. With the bed below 760°C, these gases burn only in the bubbles bursting through the upper surface of the sand. With beds slightly hotter than 760°C combustion occurs in bubbles during their ascent through the fluidized sand. At ≈900°C, the gas mixture burns in the small bubbles formed at the distributor of the bed: below 1000°C, no combustion of C 3 H 8 appears to occur in the particulate phase, that is, between the sand particles, but only in the bubble phase. However, when catalyst pellets (of Al 2 O 3 coated with Pt) are added to fluidized sand below 760°C, measurements of [CO 2 ] in the bed reveal that C 3 H 8 oxidizes on the Pt. The measured temperature difference between a catalyst particle and the cooler sand shows that the catalyzed burning of C 3 H 8 on Pt is first order in C 3 H 8 and zeroth order in O 2 . The reaction is that producing CO and H 2 O from C 3 H 8 : actually the CO tends to diffuse well away from the catalyst before burning, most probably in a bubble. However, some limited oxidation of CO to CO 2 can occur on the platinum. The overall rate of oxidation is controlled by mass transfer of C 3 H 8 from the particulate phase to a catalyst particle. Thus, sand inhibits the combustion of C 3 H 8 in air, whereas platinized alumina particles catalyze it, thereby enabling more of the heat of combustion to be released inside a fluidized bed. The above considerations are affected by some of the off gases being recycled back into the bed, when U/U mf >6.