Effect of Catalyst Particle Size

The effect of the catalyst particle size on the rate of polymerization was first studied by Natta and Pasquon (1959), who found that for a constant concentration of the monomer the rate of polymerization changed with time depending on the particle size. While with ground TiCls (particle size 2 i) the rate of propylene polymerization quickly reached a maximum and then decreased gradually to an asymptotic stationary value (curve A in Fig. 9.6), with unground particles (size up to lOju) no maxima were found but the rate accelerated to approach the same asymptotic stationary value (curve B in Fig. 9.6). The former behavior is referred to as the decay type, whereas the latter is known as the build-up or acceleration type. Corresponding to the shapes of the rate curves, the different zones can thus be classified into build-up, decay, and stationary periods (see Fig. 9.6). 

Natta and Pasquon (1959) attributed the acceleration type behavior to an increase in surface due to break-up of catalyst particles under mechanical pressure of growing polymer chains (anchored to the catalyst active centers) in the early stages of polymerization. However, as the particle size becomes smaller, greater mechanical energy is required for further size reduction. Consequently, the particle size, and hence the specific surface area, would reach some asymptotic value. The observed stationary polymerization rate would correspond to this particle size of the catalyst. 

In some cases, a decay type behavior is observed which may be caused by active site destruction, such as by thermal deactivation or further reduction of the transition metal by the Group I-in metal component. The decay type kinetics is explained later in terms of the deactivation of active sites. 

The termination of a polymer chain growing at an active center may occur by various reactions (Young and Lovel, 1990 Odian, 1991), as shown below with propylene as the example. 

Chain transfer to an active hydrogen compound such as molecular hydrogen (external agent)  

Ziegler-Natta polymerizations have the characteristics of living polymerization with regard to catalyst active sites but not individual propagating chains. Thus the propagating chains have lifetimes of seconds or minutes at most, while active sites have lifetimes of the order of hours or days. Each active site produces many polymer molecules. The termination of a polymer chain growing at an active center may occur by various reactions, as shown below with propylene as an example. 

None of the above reactions terminates the kinetic chain. All are treated as chain transfer reactions since there is reinitiation of new propagating chains. The relative extents of the various termination reactions depend on the monomer, identity and concentrations of the initiator components, temperature, and other reaction conditions. There are considerable differences in the efficiencies of chain transfer to different Group I-III metal components for example, diethylzinc is much more effective in chain transfer compared to triethylaluminum. Molecular hydrogen is a highly effective chain-transfer agent and is commonly used for molecular weight control in the industrial production of polypropylene. 

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Finally, one must know the effect of catalyst particle size on Kw. For a pore diffusion-controlled reaction, activity should be inversely proportional to catalyst particle diameter, that is directly proportional to external catalyst surface area. 

Batchwise operating three-phase reactors are frequently used in the production of fine and specialty chemicals, such as ingredients in drags, perfumes and alimentary products. Large-scale chemical industry, on the other hand, is often used with continuous reactors. As we developed a parallel screening system for catalytic three-phase processes, the first decision concerned the operation mode batchwise or continuous. We decided for a continuous reactor system. Batchwise operated parallel sluny reactors are conunercially available, but it is in many cases difficult to reveal catalyst deactivation from batch experiments. In addition, investigation of the effect of catalyst particle size on the overall activity and product distribution is easier in a continuous device. 

The effect of catalyst particle size was investigated by two different catalyst particle size fractions 63-93 pm and 150-250 pm, respectively. The effect of the particle size is very clear as demonstrated by Figure 47.2. The overall hydrogenation rate was for smaller particles 0.17 mol/min/gNi while it was 0.06 mol/min/gNi, for the larger particles, showing the presence of diffusion limitation. This kind of studies can be used to determine the effectiveness factors. The conversion levels after 70 min time-on-stream were 21% and 3%, respectively, for these two cases. 

Figure 47.2. (a) Effect of residence time 156 s, fresh catalyst (solid symbol) 80 s, catalyst used once (open symbol) and (b) effect of catalyst particle size in citral hydrogenation at 25°C, 6.1 bar total pressure, residence time 156 s, solvent ethanol, 0.1 g catalyst Ni/Si02, initial citral concentration 0.02 M. 

No other papers have considered carefully the effects of catalyst particle size on activity. Comparisons of catalysts with different particle sizes could be misleading. Fortunately, most investigators have used a single batch of chloromethylated polystyrene to prepare their catalysts, and the subsequent comparisons of activities with different active site structures are likely valid. 

Fig. 49. Effect of catalyst particle size on vanadium deposition for an Arabian Heavy atmospheric residuum processed at 370° (700°F) under 12.59 MPa (1825 psia) of hydrogen (Tamm et al., 1981).

Figure 5-9 Effect of catalyst particle size on hydrodesulfurization.

Experimental data have shown that the first two items are factors of only secondary importance under conditions normally existing in commercial operations (73). Thus, conversion is not significantly affected by changing the vapor velocity (by altering the length/diameter ratio of the reactor, at constant volume), but is markedly influenced by temperature. Furthermore, the effect of catalyst particle size on cracking rate is ordinarily less pronounced than would be the case if mass transfer or diffusion were controlling. ... 

Fig. 9.12. Effect of catalyst particle size and the strength of the metal support interaction on the metalrsupport contact area.

Figure 5. Effect of Catalyst particle size on conversion of 2-MON catalyst Amberlyst-15 (0.4g), temperature 50°C, speed of agitation 1500 rpm, 2-MON 5g, ACzO 35g...

Example 3JS-1 Effect of Catalyst Particle Size mt Selectivity in Butene Dehydrogenation... 

Marangozis, J. Effect of Catalyst Particle Size on Performance of a Trickle-Bed Reactor. Ind. Eng. Chem. Process Des. Dev. 19 (1980) 326-328. 

A pillar structure of small rectangular posts was incorporated near the outlet of the reaction channel to retain the catalyst. The reaction was studied in the temperature range of 80-120 °C and at inlet pressures up to 5 bar. Benzyl alcohol conversion and benzaldehyde selectivity at 80 and 120 °C were very close to those from conventional glass stirred reactors. The best conversion of benzyl alcohol of 95.5% with selectivity to benzaldehyde of 78% was obtained for a micropacked bed reactors with catalyst sizes of 53-63 pm and a catalyst bed length of 48 mm at 120 °C and 5 bar. The effect of catalyst particle size on the reaction was examined with two ranges of particle size 53-63 pm and 90-125 pm. Lower conversion was obtained with particle sizes of 90-125 pm, indicating the presence of internal mass transfer resistances. In situ Raman measurements were performed and could be used to obtain the benzaldehyde concentration profile along the catalyst bed. 

Table 16.4 Experimentally observed effects of catalyst particle size and promotion on CO and methane residence times, conversion, and a, together with the explanation according to our model [46].

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