Different catalyst particle sizes

Although hydrogenation of 4-CBA over Pd/C is very fast, there is strong diffusion resistance. Furthermore, apparent kinetic equations on different catalyst particle sizes have been obtained from experimental data. 

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. 

Fig.4. Deactivation of CH4 steam reforming catalyst (Tops e R-67). Variation of the tar conversion in exit gas with time-on-stream for different catalyst particle sizes at 740 C (space-time=0.45kg.h/kq). ...

Different catalyst particle sizes are considered. The diameters of these particles are 1.5, 2.0 and 2.3 cm. Figure 7.4 shows the effect of dp on the inlet optimal feed temperature policy for the reactor. It is evident from this figure that for dp s 2.0 the optimal dimensionless feed temperature policy >y(T) (m(t)) is an increasing temperature profile with r until the upper constraint y is attained. 

Table 2 XPS results corresponding to different catalyst particle sizes...

The assumption of neglecting the terms related to the intra- and extra-particle diffusion has been checked by carrying out kinetic runs with different catalyst particle size and different reactant flow rate as well as by theoretical estimates. 

Exclusion of pore diffusion tests with different catalyst particle sizes and pressures... 

The calculation procedure described above and in [gJ gives as a result a complete description of the conditions in the reactor including temperature and concentration profiles, pressure drop, reaction rates, gas enthalpies, equilibrium temperatures, effectiveness factors, etc. Furthermore, radial temperature and concentration profiles in catalyst particles and across the gas film surrounding the particles may printed for selected levels in the catalyst bed. Fig. 10 and 11 show some results obtained by simulating the performance of an adiabatic catalyst bed for the same inlet and outlet conditions (cfr. Table 2, first example) specifying two different catalyst particle sizes. 

Liquid holdup has been established to depend on liquid viscosity and superficial liquid mass raised to the power 1/3 although other studies have correlated it with superficial mass velocity divided by liquid viscosity both raised to the power 1/3. In any case, liquid viscosity affects liquid holdup. Also, the size of catalyst particle plays an important role since liquid holdup depends inversely on it raised to the power 2/3 (Satterfield et al., 1969). Hence, significant effects by using different catalyst particle size and type of liquid (different viscosities) can be expected, and those coefficients of correlations for feed physical properties, operating variables, and size of catalyst against liquid holdup can also be different. 

Much care should be taken into account when different catalyst particle sizes are mixed and the fines generated by the catalyst attrition are present, since problems to control and stabilize the catalyst bed level in the EBR at a predetermined oil-to-gas ratio can be caused. This can also result in major operational problems such as higher... 

It was shown in laboratory studies that methanation activity increases with increasing nickel content of the catalyst but decreases with increasing catalyst particle size. Increasing the steam-to-gas ratio of the feed gas results in increased carbon monoxide shift conversion but does not affect the rate of methanation. Trace impurities in the process gas such as H2S and HCl poison the catalyst. The poisoning mechanism differs because the sulfur remains on the catalyst while the chloride does not. Hydrocarbons at low concentrations do not affect methanation activity significantly, and they reform into methane at higher levels, hydrocarbons inhibit methanation and can result in carbon deposition. A pore diffusion kinetic system was adopted which correlates the laboratory data and defines the rate of reaction. 

Minimize the effects of transport phenomena If we are interested in the intrinsic kinetic performance of the catalyst it is important to eliminate transport limitations, as these will lead to erroneous data. We will discuss later in this chapter how diffusion limitations in the pores of the catalyst influence the overall activation energy. Determining the turnover frequency for different gas flow velocities and several catalyst particle sizes is a way to establish whether transport limitations are present. A good starting point for testing catalysts is therefore ... 

Figure 3.8. Kinetic data from molecular beam experiments with NO + CO mixtures on a Pd/MgO(100) model catalyst [70]. The upper panel displays raw steady-state C02 production rates from the conversion of Pco = PN0 = 3.75 x 10-8 mbar mixtures as a function of the sample temperature on three catalysts with different average particle size (2.8, 6.9, and 15.6 nm), while the bottom panel displays the effective steady-state NO consumption turnover rates estimated by accounting for the capture of molecules in the support. After this correction, which depends on particle size, the medium-sized particles appear to be the most active for the NO conversion. (Reproduced with permission from Elsevier, Copyright 2000).

The results obtained with nickel raised the question whether the relation found between rate of exchange and particle size holds also for other metals of group VIII. We therefore carried out the benzene-D2 reaction on some iridium catalysts widely differing in particle size. We chose iridium because we knew from earlier experiments that iridium black gives a very characteristic cyclohexane isotopic distribution pattern with a maximum for C6H4Ds, whereas the patterns of Ni, Ru, Pd, and Pt show a maximum for the d6 compound. 

In addihon to shape selechvity and acid-site strength, other catalyst characteristics that influence the catalyhc performance of SAPO-34 have also been idenhfied. Variahon in the SAPO-34 gel composition and synthesis condihons have been were used to prepare samples with different median particle sizes and Si contents (Tables 15.3 and 15.4) [104]. In these samples the median parhcle size was varied from 1.4 to 0.6 xm, and the Si mole frachon in the product was varied from 0.14 down to 0.016. A comparison of samples B and E (which have similar parhcle size distributions) shows that reducing Si content decreases propane formation and increases catalyst life. A comparison of samples B and C (which have similar Si contents) illushates an increase in catalyst life with a reduchon in parhcle size. 

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. 

This sieve effect cannot be considered statically as a factor that only determines the amount of accessible acid groups in the resin in such a way that the boundary between the accessible and non-accessible groups would be sharp. It must be treated dynamically, i.e. the rates of the diffusion of reactants into the polymer mass must be taken into account. With the use of the Thiele s concept about the diffusion into catalyst pores, the effectiveness factors, Thiele moduli and effective diffusion coefficients can be determined from the effect of the catalyst particle size. The apparent rates of the methyl and ethyl acetate hydrolysis [490] were corrected for the effect of diffusion in the resin by the use of the effectiveness factors, the difference in ester concentration between swollen resin phase and bulk solution being taken into account. The intrinsic rate coefficients, kintly... 

From the foregoing dicussion it is apparent that residuum hydroconversion processes can be influenced adversely by pore diffusion limitations. Increasing the catalyst porosity can alleviate the problem although increased porosity is usually accompanied by a decrease in total catalytic surface area. Decreasing the catalyst particle size would ultimately eliminate the problem. However, a different type of reaction system would be required since the conventional fixed bed would experience excessive pressure drops if very fine particles were used. A fluidized system using small particles does not suffer from this limitation. However, staging of the fluidized reaction system is required to minimize the harmful effects that backmixing can have on reaction efficiency and selectivity. 

For pure Ti02> the q.y. is higher, for the more concentrated suspension ho wever for the case of both iron-doped TiC>2, the higher q.y. is obtained for the lower concentration of catalyst. The differences can be explained in terms of differences in particle size of the catalysts. Recent results by Navio et al. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

PtRu catalysts with controlled atomic ratios were prepared by adjusting the nominal concentrations of platinum and ruthenium salts in the solution, whereas different mean particle sizes could be obtained by adjusting some electric parameters of the deposition process, e.g., ton (during which the current pulse is applied) and toff (when no current is applied to the electrode), as determined by different physicochemical methods (XRD, EDX, and TEM) [40], Characterization by XRD led to determine the crystallite size, the atomic composition and the alloy character of the PtRu catalysts. The atomic composition was confirmed using EDX, and TEM pictures led to evaluate the particle size and to show that PtRu particles formed small aggregates of several tens of nanometers (Figure 9.10). 

Mossbaucr spectroscopy In order to obtain more information about the change in chemical composition of the catalyst during operation, Mdssbauer spectroscopy was performed on catalyst A and B. before and after use (figure 4). The spectra of the fresh catalysts clearly illustrate the difference in particle size of the catalysts. At 77 K catalyst B gives rise to a number of superimposed... 

An ideal study of support effects requires model catalysts with metal particles that are identical in size and shape (so that only the support oxide varies). This is difficult to achieve for impregnated catalysts, but identical metal particles can be prepared via epitaxial model catalysts [36]. Well-faceted Rh nanocrystals were grown on a 100-cm area NaCl(OOl) thin film at 598 K. One half of a Rh/NaCl sample was covered with Al Oj, and the other half with TiO. The preparation of Rh particles for both Al Oj- and TiO -supported model catalysts in a single step prevents any differences in particle size, shape, and surface structure which could occur if the samples were prepared in separate experiments. Three model catalysts were prepared, with a mean Rh particle size of 7.8, 13.3, and 16.7 mn (the films were finally removed from the NaCl substrate by flotation in water). Activation was performed by O /H treatments, with the structural changes followed by TEM (Fig. 15.6). Oxidation was carried out in 1 bar O at 723 K prodncing an epitaxially grown rhodium oxide shell on a Rh core (cf Fig. 15.5e), whereas the hydrogen reduction temperature was varied. 

One further difference exists between HDS and HDM. Bridge [37] has shown, very clearly, that HDS is not limited by diffusion while HDM is. Using a nickel-molybdate based catalyst with a unimodal microporous size distribution, the demetalation of Arabian heavy atmospheric residuum was found to be affected by catalyst particle size, while HDS was not. As the diameter of the pore was decreased, the maximum in the metals deposition profile moved closer to the external surface of the pellet, agmn indicating difiusional limitations for FIDM. 

Natta and Pasquon [12) were the first to study the effect of the catalyst particle size on the rate of polymerization. For a constant concentration of the monomer, they found that the rate of polymerization changed with time. With ground Ti02 (particle size < 2/i) the rate of propylene polymerization quickly reached a maximum and then decreased gradually to an asymptotic stationary value. In the case of unground particles (size up to 10/i), however, there were no maxima but the rate accelerated to approach the same asymptotic stationary value. The former behavior is referred to as the decay type, whereas the latter is known as the build-up or acceleration type. Figure 9.7 shows typical rate curves of these types and the different zones, termed build-up, decay, and stationary periods, into which they can be classified. 

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