Catalyst, contacting efficiency

Tracer methods proposed by Schwartz et al. (19) and Colombo et al. (21) were used to determine total and external catalyst contacting efficiency. These techniques have been described elsewhere (22). Total contacting efficiency, r)c defined as the fraction of total (external and internal) catalyst area contacted by liquid can be obtained by ... 

The dispersion model with particle diffusion always assumes complete external contacting of particles by liquid which may not be the case in trickle flow. This means that the effective diffusional time constant is increased in trickle flow resulting in a reduced apparent effective diffusivity which is based on the total external surface area. Using this diffusivity in the expression for the Thiele modulus, and equating it to the modulus defined for trickle-bed operation by Dudukovic (130) results in the following estimate of the external catalyst contacting efficiency, UcE ... 

None of the available studies in the literature dealing with dispersion directly relate efficiency of catalyst utilization to dispersion. However, a conservative basis of design would be to assume that in the zone where a uniform distribution is being established, catalyst contacting efficiency is only partial. 

Catalyst mass flowrates exceeding about 1600 Ib/ft -min (7800kg/m -min) results in poor steam/catalyst contacting, flooded trays, insufficient catalyst residence time, and increased steam entrainment to the spent catalyst standpipe. This is reflected by the stripper efficiency and catalyst density shown in Figure 7.10. The primary concern is hydrocarbon entrainment to the regenerator leading to loss of product, increased catalyst deactivation, increased delta coke and potential loss of conversion and total liquid yield, and feed rate limitation. A rapid decrease in stripper bed density is an indication that... 

The effectiveness of the gas-solid mass transfer in a circulating fluidized bed (see Chapter 10) can be reflected by the contact efficiency, which is a measure of the extent to which the particles are exposed to the gas stream. As noted in Chapter 10, fine particles tend to form clusters, which yield contact resistance of the main gas stream with inner particles in the cluster. The contact efficiency was evaluated by using hot gas as a tracer [Dry et al., 1987] and using the ozone decomposition reaction with iron oxide catalyst as particles [Jiang etal., 1991], It was found that the contact efficiency decreases as the particle concentration in the bed increases. At lower gas velocities, the contact efficiency is lower as a result of lower turbulence levels, allowing a greater extent of aggregate formation. The contact efficiency increases with the gas velocity, but the rate of increase falls with the gas velocity. 

An appropriate model for trickle-bed reactor performance for the case of a gas-phase, rate limiting reactant is developed. The use of the model for predictive calculations requires the knowledge of liquid-solid contacting efficiency, gas-liquid-solid mass transfer coefficients, rate constants and effectiveness factors of completely wetted catalysts, all of which are obtained by independent experiments. 

Since dissolved gas concentrations in the liquid phase are more difficult to measure experimentally than the liquid reactant concentration, Equation 8 evaluated at the reactor exit 5=1 represents the key equation for practical applications involving this model. Nevertheless, the resulting expression still contains a significant number of parameters, most of which cannot be calculated from first principles. This gives the model a complex form and makes it difficult to use with certainty for predictive purposes. Reaction rate parameters can be determined in a slurry and basket-type reactor with completely wetted catalyst particles of the same kind that are used in trickle flow operation so that DaQ, r 9 and A2 can be calculated for trickle-bed operation. This leaves four parameters (riCE> St gj, Biw, Bid) to be determined from the available contacting efficiency and mass transfer correlations. It was shown that the model in this form does not have good predictive ability (29,34). 

The second portion of El-Hisnawi s study consisted of evaluating liquid-solid contacting efficiency and liquid holdup using impulse response tracer experiments. Experiments were performed using the same catalyst packing and solvents employed... 

Figure 3 shows the results obtained when the above correlations developed using the more active 2.5% Pd catalyst are applied to data obtained on the less active 0.5% Pd catalyst. It can be seen that the agreement between the model predictions and experimental values is of lesser quality than the previous case. The deviations are especially noticeable with increasing L/us or decreasing external contacting efficiency since the predicted lines cross each other at an intermediate value which is opposite to the data. On a positive note, the predictions are more accurate than those that would be obtained using the literature correlations as shown earlier in Figure 1. Further work is needed to determine the underlying reason for this behavior, however.

All the correlations for the contacting efficiency qc (= total catalyst area contacted by liquid/total catalyst surface area) reported in the literature are for nonporous packing. These are summarized by Schwartz et al.84 and Table 6-6... 

Two factors would be expected to restrict the conversion efficiency of the fluidized bed back-mixing of the gas (141), and occurrence of bubbles of reaction vapors that pass up through the bed with a relatively short residence time and poor contacting efficiency with catalyst (56,230). The back-mixing might also be expected to result in some penalty in product distribution because of over-cracking of some portions of the feed and under-cracking of others. However, these factors have not proved to be of any serious consequence. 

Ikeda (14) paid attention to the effect of the fines fraction (

optimal size range to obtain good fluidization and good gas-solid contact efficiency from the results of industrial operations (OlO, V12, Z5 see Table III). His criterion for good fluidization is indicated as A in Fig. 3, a target in developing or selecting catalysts for FCB. 

The main differences between the models lies in whether or not some fraction of the catalyst is in direct contact with the bubble gas, and in the extent of axial mixing in each phase. Properties of various models have been discussed extensively by Gilliland and Knudsen (G7) in relation to the extent of reaction in experimental fluidized bed reactors, considering that allowance for direct contact between bubble gas and a certain amount of catalyst in it is the sole way to account for the contact efficiency. Unless a fraction of the catalyst particles is assumed to be entrained in the bubble gas, the bubble size calculated to fit the reaction data is found to decrease with increasing catalyst activity at otherwise identical fluidization conditions, in which the bubble size should remain constant. Essentially the same decrease in bubble size was observed by Miyauchi and Morooka (M29) in their analysis of the data by Lewis et al. (LI 2), and by Furusaki (F14) in his fluidized bed data for the Deacon reaction. 

The contact in the dilute phase and the transition zone is different from that in the dense phase. The former is related to mass transfer between the gas phase and an agglomerate of solid particles, whereas the latter is mainly related to mass transfer between bubbles and emulsion including a certain amount of directly contacting catalyst. Upcm consideration of hindered settling of the swarm of particles (S16, Z7), we also find contact efficiency to be a function of the population density of particles. Normalizing by the Ig of the main dense phase, eg.de, e/ e.de is chosen as a variable... 

The equation shows that the strong interaction of particles affects the contact efficiency. The interaction seems to increase rapidly with increasing particle population. Figure 78 can be used to estimate local contact efficiency when the density distribution of solid catalyst is known. 

The Sohio process is considered one of the most successful applications of FCB. Problems in industrial application of the reaction arose from the strong exothermicity of propylene ammoxidation and from the intermediate production of acrylonitrile in the consecutive reactions (V9). It is particularly noticeable that the catalyst gives high selectivity, and the reactor design aims at better fluidization and higher contact efficiency than in the FCC process. 

Another difficulty of this process was low ccxiversion in the commercial plant, which could not be predicted from the results of the pilot plants. Grekel et al. (G15) reported the effects of particle size distribution, gas inlet devices, and internals, cm contact efficiency. Volk et al. (VI2) also emphasized the effect of bed internals. These developmental studies became a very useful guide for applications of fluidized catalyst beds. 

However, approximate treatment is possible. Ikeda and Tashiro (19) report an optimization of catalytic reactions in fluid beds. They And that the maximum yield of the intermediate product decreases, and that the optimum contact time increases for first-order consecutive and parallel reaction systems if contact efficiency in the reactor decreases. They also showed the most economical equilibrium activity and the optimal size distribution of catalyst. 

Another measure of the efficiency of ammonia conversion is the space velocity which may be used. Space velocity refers to the volume of reactants fed to a reactor per hour, divided by the volume of the reactor. For liquid reaction streams this relationship is straightforward. For gases, however, the space velocity is defined as being the volume of gases corrected to 0°C and 760 mm Hg (1 atm) passing through the reactor (or catalyst) volume/hour. This amounts to a measure of the gas-catalyst contact time for heterogeneous reaction (Eq. 11.7). 

The high surface-to-volume ratio. In addition to an efficient illumination, in the case of photocatalytic reactions there is an efficient catalytic exposure to radiation and the reagent/catalyst contact is maximized [132, 133]. The small size of the channel also provides better control over variables such as temperature and fiow rates, due to the fast heat and mass transfer. 

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