Extra particle mass-transfer limitations

Slurry Process Extra Particle Mass-Transfer Limitations. In a slurry process, very fine catalyst is suspended in a liquid medium, which in the case of an FT process may be the heavier fraction of the liquid product itself. The slurry mode of operation allows effective removal of reaction heat and good temperature control. Because of the small catalyst particles (particle diameter a few hundred micrometers or less), intraparticle diffusion limitation is not a constraining factor for reaction rate. However, compared with a fixed-bed mode of operation the advantage of better catalyst utilization may be offset by the reduced volume of catalyst that can be accommodated in the reactor space. 

The basic challenges for parallel test reactor development for high-throughput experimentation are, apart from technological challenges, related to technical demands that arise with the special issues for parallel test reactors, which are identical with the demands for conventional test reactors for gas-phase reactions. The criteria that must be fulfilled to obtain intrinsic catalyst properties from experimental data relate mainly to mass and heat transfer. A sufficient contact between the reactants and the catalyst must be insured to avoid mass transfer limitations inside and outside of the catalyst particles. Isothermal operation under laboratory conditions and avoidance of heat transfer limitations are also crucial. As a general quality check prior to operation intra- and extra-particle limitations should be... 

The use of nonporous supports for protein immobilization will eliminate the problems associated with the intraparticle diffusion limitations. With columns regularly packed with small spherical particles of limited size distribution, the mass transfer effects associated with a nonuniform flow distribution will be minimized. Extra column effects due to diffusion in the stagnant liquid pockets will be reduced by using short and properly designed connecting lines to the injector and the detector. 

For most zeoUtes, when NH3 is used as the probe molecule at a given temperature, the time needed to estabhsh thermal equihbrium after each dose at first increases with increasing adsorbed amount, passes through a maximum, then decreases rapidly and finally reaches a value close to the time constant of the calorimeter. For example. Fig. 3 shows the time constant (in seconds) versus the amount of NH3 adsorbed for samples of H-ZSM-5 (Si/Al = 10.3) pretreated at 673 K or 1073 K and possessing a very small particle size (0.02 to 0.05 xm). The maximum time constant is higher for the sample pretreated at 1073 K than for the sample pretreated at 673 K, because increasing the pretreatment temperature causes dealumination, i.e. creates extra-framework aluminum species which restrict the access to the channels and creates diffusional limitations. The time constant of the calorimeter was close to 300 s. The heat transfer is determined by the mass transfer which becomes slower. 

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