Many commercially important chemicals are produced by the reaction of a volatile and a nonvolatile component on a supported catalyst. A large fraction of these actions are oxidations and hydrogenations of organic compounds over precious metal and nickel catalysts. Such reactions are an important step in the production of specialty chemicals (for example aniline and crotonaldehyde) and biochemicals (for example hydrogenated vegetable oils and xylose). In addition to being multiphase in nature, these reactions are typically fast (mass transport limited) and quite exothermic. The reaction heat can lead to liquid-phase vaporization both within the catalyst pellet pores and on the external surface. The gas-phase catalytic reaction occurring in the gas-filled pores can result in a large rise in the intraparticle temperature and pressure. The overall rate may also increase substantially as a result of the phase change and gas-phase catalytic reaction. Undesirable high temperature excursions (hot spots) within a catalytic reactor may result. These single pellet phenomena complicate the design and control of multiphase catalytic reactors. With a better understanding of these systems new types of multiphase reactor operations and designs can be developed in which the beneficial influence of the nonisothermal phenomena is exploited or the detrimental impact is avoided. The PI is planning a fundamental experimental and modelling study of the behavior of a single supported catalytic pellet in an exothermic multiphase reaction system. The single pellet or even single pore level often dictates the overall reactor behavior. Two experiments involving cyclohexene hydrogenation on a supported palladium pellet are planned. In the first experiment the pellet is suspended in a tube and contacted by a flowing liquid film and annular gas flow. The overall, rate, degree of external wetting, pellet mass, and the pellet temperature will be monitored as various bulk conditions are changed. The dependence of the overall rate, temperature rise, and the degree of internal and external wetting on the bulk conditions will be determined as phase change occurs. In the second experiment a novel multiphase diffusion and reaction cell will be developed to study in more detail the intraparticle temperature profile. The single pellet is wetted on one face by the liquid and on the other by the gas. The large differences of temperature rise in gas- and liquid-filled catalyst pellets provides a means of probing the location of gas- or liquid-fill pores. Development and numerical solution of several pertinent models of the single, partially wetted and filled catalytic pellet will be carried out. The research should provide fundamental data to be ultimately used in multiphase reactor design.
