Catalytic mass transfer characteristics

Slurry reactors are widely used in the chemical process industry due to their superior mass transfer characteristics. Catalytic hydrogenation of unsaturated fatty oils and catalytic oxidation of olefines are among practical examples in which slurry reactors are utilized. 

The effectiveness of the PPR and LFR as catalytic reactors or adsorbers can be high if they are designed with due consideration given to the flow and mass transfer characteristics. Scale up and reactor modeling benefit from the modular construction and the well-defined geometry of these reactors. 

Y Zheng, X Xu. Study on catalytic distillation processes. Part 1. Mass transfer characteristics in catalyst bed within the column. Trans Inst Chem Engrs. Part A, 70 459-464,1992. 

Mass Transfer in Catalytic Microstructured Reactors 247 Table 6.3 Mass transfer characteristics for different channel geometries [53],... 

Fluidized bed reactors (FBRs) are chemical reactors in which (catalytic) particles interact with a gas stream that is fed from the bottom, such that the mixture (emulsion phase) behaves as a fluid. This type of reactors is often used in the chemical and process industries, where they have gained their popularity due to their excellent heat and mass transfer characteristics. FBRs are used for instance for gas-phase polymerization reactions for polyolefin production (polyethylene, polypropylene), chemical looping combustion or reforming processes, and gas-phase Fischer—Tropsch synthesis. 

Multiphase fixed bed reactors have complex hydrodynamic and mass transfer characteristics (see also Section 4.9). Thus, the modeling and scale-up are difficult. As an instructive example, we inspect the catalytic 1-octene hydrogenation as a model reaction (Battsengel, Datsevitch, and Jess, 2002 Battsengel, 2002). Table 4.11.4 lists the characteristics of the commercial Ni-catalyst (NISAT, Siidchemie) used for the experiments, data on chemical media, and the parameters that determine the mass transfer. 

Ryan M.J., E.R. Becker, and K. Zygourakis (1991) Light-off performance of catalytic converters the effect of heat/mass transfer characteristics , SAE Paper 910610. 

Trickle-bed reactors, wherein gas and liquid reactants are contacted in a co-current down flow mode in the presence of heterogeneous catalysts, are used in a large number of industrial chemical processes. Being a multiphase catalytic reactor with complex hydrodynamics and mass transfer characteristics, the development of a generalized model for predicting the performance of such reactors is still a difficult task. However, due to its direct relevance to industrial-scale processes, several important aspects with respect to the influence of external and intraparticle mass transfer effects, partial wetting of catalyst particles and heat effects have been studied previously (Satterfield and Way (1972) Hanika et. al., (1975,1977,1981) Herskowitz and Mosseri (1983)). The previous work has mainly addressed the question of catalyst effectiveness under isothermal conditions and for simple kinetics. It is well known that most of the industrially important reactions represent complex reaction kinetics and very often multistep reactions. Very few attempts have been made on experimental verification of trickle-bed reactor models for multistep catalytic reactions in the previous work. 

Applications of microreactors to biphasic catalytic reactions constitute a topac of interest. The benefits of having an exceedingly high surface-to-volume ratio and efficient mass-transfer in microchannels have led many researchers to study continuous flow systems using microreactors for catalytic reactions. The excellent mass transfer characteristics within and between the catalyst carrier phase and reaction medium, together with the minimal catalytic pore diffusion resistances at the micrometer scale, make such biphasic catalysis an attractive alternative to conventional catalysis operation (Wieflmeier, 1996 Rahman et al., 2006). 

Steps 1 and 7 are highly dependent on the fluid flow characteristics of the system. The mass velocity of the fluid stream, the particle size, and the diffusional characteristics of the various molecular species are the pertinent parameters on which the rates of these steps depend. These steps limit the observed rate only when the catalytic reaction is very rapid and the mass transfer is slow. Anything that tends to increase mass transfer coefficients will enhance the rates of these processes. Since the rates of these steps are only slightly influenced by temperature, the influence of these processes... 

This simplified description of molecular transfer of hydrogen from the gas phase into the bulk of the liquid phase will be used extensively to describe the coupling of mass transfer with the catalytic reaction. Beside the Henry coefficient (which will be described in Section 45.2.2.2 and is a thermodynamic constant independent of the reactor used), the key parameters governing the mass transfer process are the mass transfer coefficient kL and the specific contact area a. Correlations used for the estimation of these parameters or their product (i.e., the volumetric mass transfer coefficient kLo) will be presented in Section 45.3 on industrial reactors and scale-up issues. Note that the reciprocal of the latter coefficient has a dimension of time and is the characteristic time for the diffusion mass transfer process tdifl-GL=l/kLa (s). 

The HTE characteristics that apply for gas-phase reactions (i.e., measurement under nondiffusion-limited conditions, equal distribution of gas flows and temperature, avoidance of crosscontamination, etc.) also apply for catalytic reactions in the liquid-phase. In addition, in liquid phase reactions mass-transport phenomena of the reactants are a vital point, especially if one of the reactants is a gas. It is worth spending some time to reflect on the topic of mass transfer related to liquid-gas-phase reactions. As we discussed before, for gas-phase catalysis, a crucial point is the measurement of catalysts under conditions where mass transport is not limiting the reaction and yields true microkinetic data. As an additional factor for mass transport in liquid-gas-phase reactions, the rate of reaction gas saturation of the liquid can also determine the kinetics of the reaction [81], In order to avoid mass-transport limitations with regard to gas/liquid mass transport, the transfer rate of the gas into the liquid (saturation of the liquid with gas) must be higher than the consumption of the reactant gas by the reaction. Otherwise, it is not possible to obtain true kinetic data of the catalytic reaction, which allow a comparison of the different catalyst candidates on a microkinetic basis, as only the gas uptake of the liquid will govern the result of the experiment (see Figure 11.32a). In three-phase reactions (gas-liquid-solid), the transport of the reactants to the surface of the solid (and the transport from the resulting products from this surface) will also... 

When comparing film flow monolithic reactors with conventional catalytic packed reactors, one can conclude that the critical hydrod)mamic characteristics (hydraulic capacity, pressure drop, and volumetric mass transfer rates) are similar, but monoliths have distinct advantages greater flexibility, easier scale-up, the susceptibility of fhe surface to coating procedures, and advances in control of flooding—all allowing the use of very small channels and therefore efficienf cafalysf ufilizafion. 

While the above criteria are useful for diagnosing the effects of transport limitations on reaction rates of heterogeneous catalytic reactions, they require knowledge of many physical characteristics of the reacting system. Experimental properties like effective diffusivity in catalyst pores, heat and mass transfer coefficients at the fluid-particle interface, and the thermal conductivity of the catalyst are needed to utilize Equations (6.5.1) through (6.5.5). However, it is difficult to obtain accurate values of those critical parameters. For example, the diffusional characteristics of a catalyst may vary throughout a pellet because of the compression procedures used to form the final catalyst pellets. The accuracy of the heat transfer coefficient obtained from known correlations is also questionable because of the low flow rates and small particle sizes typically used in laboratory packed bed reactors. 

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