Hydrodynamics of multi-phase packed bed micro-reactors

Why to use packed bed micro-reactors for catalyst testing? Miniaturized packed bed reactors have a large surface-to-volume ratio at the reactor and particle level that favors the heat- and mass-transfer processes at all scales (intra-particle, inter-phase and inter-particle or reactor level). If the mass-transfer processes are fast in respect to the reaction-rate, then the reaction-rate is under kinetic control over the entire range of conversion and it is possible to measure intrinsic kinetics, from differential to integral conditions, eliminating extrapolation and improving reliability. For reaction mixtures where the catalyst-deactivation phenomena should be included in the reaction-rate expression to predict the reactor performance, these continuous reactors allow the measurement of intrinsic kinetics and, independently, catalyst deactivation over long periods of time. When using several reactors in parallel, as in parallel multi-flow experimentation units, a fast evaluation of the reaction-rate dependency on the process variables can be performed. Why to study the hydrodynamics of multi-phase packed bed micro-reactors? Established correlations to evaluate hydrodynamic parameters are based on large-scale trickle beds (Re>1, dparticle >1mm), where inertia and gravity play a prominent role. For multi-phase packed bed reactors the Reynolds number Re is calculated based on the liquid properties and the particle diameter Re= ?LuLdp/?L where ?L is the liquid density, uL is the liquid velocity, dp is the particle diameter, and ?L is the liquid viscosity. There are important differences in flow behavior upon downscaling. When using small volumes and flow velocities (Re 0.5), the gas-liquid interaction can be boosted by increasing flow-rates, the size of void-channels (inter-pillar or inter-particle space) has a major impact not only on the bed porosity but also on the distribution of the phases inside the bed, and changing from a full wetting condition to a partial wetting condition produces a significant change in flow pattern. It is still difficult to predict accurately the multi-phase flow pattern for micro-fabricated and miniaturized packed beds because many variables (Ca number, contact angle, pore-connectivity, and feed-section design) seem to be important. It is worthwhile to perform visualization or residence time distribution experiments in connection with hot flow experiments to establish the relation between bed packing characteristics and the gas-liquid-solid reaction performance as well as analyzing trends with the parameters. What taught us the mathematical modelling? In chapter VI we described the effect of volatility on residence time distribution and conversion in multiphase reactors. This is relevant for the many processes where substantial vaporization of the liquid feed occurs. The typical situation is that the evaporated molecules not only lower the concentration in the liquid phase but also travel faster through the reactor. This has direct importance in understanding multiphase reactors with light reactants and products (such as HDS), offering insight into practical issues such as bypassing and stripping of relevant compounds. Our complete model uses two mobile zones, one for the liquid phase and one for the gas phase, with dispersion in each zone and mutual mass transfer. We explored the entire parameter space for our model numerically. We learned that the relevant parameters are the velocities and volumetric fluxes of the phases, the thermodynamic equilibrium between the phases, the dispersion behavior of the individual phases and the mass transfer rate between the phases. We described quantitatively how the mean residence time of a component decreases when it significantly evaporates to a faster-moving gas phase. We explored how slow mass transfer contributes to the broadening of the residence time distribution. Experimentally, we validated the model in a more limited parameter space in a gas-liquid micro-packed bed with volatile and non-volatile compounds in different solvents. Additionally, using reactor simulations we developed a simple model to predict the conversion for systems with a volatile reactant at mass-transfer rate values relevant for micro-reactors and trickle-bed reactors. We used a liquid-only reaction to represent the case of a fully wetted heterogeneous catalyst or a homogeneous non-volatile catalyst in the liquid phase. Our results indicate that: for multi-phase continuous micro-reactors with high mass-transfer rates (St ? 100, St=(ka)glØL/uL, where (ka)gl is the gas-liquid mass-transfer coefficient, Ø is the liquid hold-up, L is the reactor length, and uL is the liquid velocity), the volatile compounds have a lower concentration in the reactive phase and the overall residence time for such compounds is lower. These two effects, lower concentration and shorter residence time, are related manifestations of the same phenomenon. Accounting for only the change of concentration reliably predicts the conversion for all reaction orders higher than zero. Liquid by-passing via the gas phase is not important since thermodynamic equilibrium is already reached in the top of the bed. For large scale trickle-beds the typical range of St is 5-50, at these intermediate mass-transfer rates the conversion is further reduced due to a significant evaporation of the reactant to the gas phase without re-condensation before leaving the reactor. At even lower mass-transfer rates (St ? 1), too few molecules evaporate before leaving the reactor and the effect of the volatility vanishes. Wherever possible, we extracted from the detailed numerical model practical engineering correlations for average residence time and conversion. The results presented in this work teach whether reactant volatility should be considered in a reactor design. Wrap up\u85 The whole thesis work is summarized in more detail in chapter VII, where also the impact of the micro-reactor hydrodynamic characteristics on catalyst kinetic testing is discussed and recommendations for future work are given.DelftChemTechApplied Science