Transformation of carbonaceous feedstocks at high temperature catalytic conditions necessary to achieve economical production of fuels, chemicals and materials often introduces the complexity of transport limitations. Momentum, diffusion and heat transfer will alter the behavior of catalysts, the thermal conversion of particles, and the chemical pathways of carbohydrate chemistry, as shown in the following examples.
Diffusion in Microporous Materials
Catalysis of organic molecules in porous materials introduces the complexity molecular diffusion as a potential rate limitation. In the past few decades, the development of new hierarchical zeolites with controlled distributions of micropores and mesopores has introduced a large ratio of surface-to-volume and the potential for significant rate limitations derived from surface. In this work, we have probed the effect of the surface by comparing experimental measurements of hydrocarbon diffusion with kinetic monte carlo simulations to demonstrate the conditions that lead to dramatic surface limitations and a path foward for complex microporous particle design. More information available in ACS Nano (link), Chemistry of Materials (link), and Journal of Physical Chemistry C (link).
Reactive Leidenfrost Effect
Ablative pyrolysis (direct contact heating) permits immense heat transfer (>MW per m2) to drive biopolymer decomposition. Under these extreme thermal condtiions, cellulose rapidly degrades to short-chain oligomers and eventually vapors. Bubbles of cellulose-derived vapors push the particle off the surface with violent motion (left animation). When the surface temperature reaches 750 deg C, vapor generation is so aggressive that the molten cellulose droplet lifts off the surface and dramatically reduces the rate of heat transfer. This "reactive Leidenfrost" effect can be suppressed by heating particles using porous surfaces, which allow generated vapors to pass through the heating surface and keep the particle in good thermal contact (right animation). More information in Nature Scientific Reports (link).
A Heat Engine Biomass Conveyor
The reactive Leidenfrost effect of pyrolyzing cellulose provides a mechanism to propel biomass fibers and particles using spontaneously generated vapors. As cellulose decomposes to organic products, the violent flow of vapors will randomly propel a particle in any direction. This skittering motion can be controlled by the introduction of a ratcheted hot surface which directs organic vapors and drags a particle in the orientation of the long surface of the ratchet. More information available in Energy & Environmental Science (link).
Aerosol Generation from Biomass
The thermal decomposition of cellulose above 467 deg C produces a bubbling liquid intermediate which exists for a fraction of a second. On the timescale of microseconds, collapsing molten cellulose bubbles generate liquid jets which fragment into ejected aerosols of molten cellulose. Ejected aerosols oscillate before freezing into a hard particle. Spontaneous ejection can transport nonvolatile cellulose oligomers into the gas phase as well as inorganic material such as calcium or magnesium. This phenomenon transports nonvolatile material into the atmosphere in forest fires. In cigarette smoking, this transport mechanism can transfer heavy tars into the lungs. More information published in Energy & Environmental Science (link) and ChemSusChem (link).