Catalyst profiling

Radial density gradients in FCC and other large-diameter pneumatic transfer risers reflect gas—soHd maldistributions and reduce product yields. Cold-flow units are used to measure the transverse catalyst profiles as functions of gas velocity, catalyst flux, and inlet design. Impacts of measured flow distributions have been evaluated using a simple four lump kinetic model and assuming dispersed catalyst clusters where all the reactions are assumed to occur coupled with a continuous gas phase. A 3 wt % conversion advantage is determined for injection feed around the riser circumference as compared with an axial injection design (28). 

Some studies of potential commercial significance have been made. For instance, deposition of catalyst some distance away from the pore mouth extends the catalyst s hfe when pore mouth deactivation occui s. Oxidation of CO in automobile exhausts is sensitive to the catalyst profile. For oxidation of propane the activity is eggshell > uniform > egg white. Nonuniform distributions have been found superior for hydrodemetaUation of petroleum and hydrodesulfuriza-tion with molybdenum and cobalt sulfides. Whether any commercial processes with programmed pore distribution of catalysts are actually in use is not mentioned in the recent extensive review of GavriUidis et al. (in Becker and Pereira, eds., Computer-Aided Design of Catalysts, Dekker, 1993, pp. 137-198), with the exception of monohthic automobile exhaust cleanup where the catalyst may be deposited some distance from the mouth of the pore and where perhaps a 25-percent longer life thereby may be attained. 

Cova (Cl 1) has examined the vertical distribution of catalyst concentration as a function of gas and liquid flow rates for systems with finite net liquid flow. A theoretical model is presented which predicts the catalyst profile as a function of physical properties and operating conditions, and which adequately represents observations for both laboratory and pilot-scale operations. 

Figure 1 Illustration of the concept for catalyst profiling based on a set of sensitive test reactions ...

The statistical similarity analysis was performed based on determination of Euclidean distance between hypothetical and catalyst profile according to the following formula ... 

From 1995 to 2000, catalyst profiles of several ruthenium catalysts bearing pyridine-diimide 1 [13], diiminocarbene 2 [14], diamine-arene 3 [15],phos-phino-arene 4 [16], and substituted cyclopentadienyl 5 and 6 [17, 18] were shown to have good activity for the cydopropanation (Fig. 1). At the relatively high reaction temperature of 60-100 °C,they also gave moderate-to-high yields over 90%. It is interesting in that the dipyridine-diimide complex 1 and the p-cymene-carbene complex 2 show high trans selectivity, 86 14 and 82 18, respectively. 

The performance indexes, which define an optimal catalyst distribution, include effectiveness, selectivity, yield and deactivation rate. The key parameters, affecting the choice of the optimal catalyst profile, are the reaction kinetics, the transport resistances, and the production cost of the catalyst. An extensive review of the theoretical and experimental developments in this area is available [20]. Two typical examples to demonstrate the importance of an appropriate distribution of the active components are now described. 

When we are approaching a nonuniform and sinusoidal, or valley and mountain, catalyst profile, we need other spatial coordinates. This is the case of the cycloid curves, described by a point in a circumference, which rolls without slipping along a rectilinear line. Mathematically speaking, the representation of the crystal growth in the catalyst is the one of a trochoid (shortened cycloid) as depicted in Figure 13.11. The equations in a parametric form are... 

In Fig. 2, TPR profile of the unreduced dried catalyst is compared with the profile of a sample prepared from this catalyst by calcination at 500°C in air. The reduction peak at 320°C in the profile of the calcined sample may characterize the reduction of Co O /7/ while that at 470°C is caused by the reduction of mixed oxides,e.g. xCoO.yMgO.The peak at 320°C in the catalyst profile originates very probably from CO evolved during the decomposition of cobalt hydroxycarbonate (compare with Fig. 3b). The peak at 370°C may be ascribed to the reduction of cobalt oxides originating from the decomposed hydroxycarbonate. From the comparison of both TPR profiles it clearly follows that the composition of unreduced forms differs and that the metallic phase originated from different precursors. 

Again we carried out competition reactions with a 1 1 mixture of (S,5)-6a and R,R)-lik under the two stirring regimes. Not unexpectedly the reaction profiles were identical to the single catalyst profiles. In both cases the product had a small enantiomeric excess of 10-12 % of the (/ ) isomer produced by (S,5)-6a. 

Figure 15.20 Concept for catalyst profiling based on a set of sensitive test reactions (a) determination of particular performance data for an individual catalyst (activity and selectivity values in different test reactions) (b) visualization of performance profiles from the particular performance values as fingerprints for individual catalysts as a basis of similarity analysis.

While this study is mainly concerned with the influence of the catalyst concentration profile on the reactor performance, additional calculations were done under identical conditions with a uniform catalyst profile having the reactor mean value purpose... 

For reactor diameters larger or equal to Dr = 30 cm and particle sizes up to 200 urn the catalyst settling can be neglected in as far as the catalyst profile does not affect the overall conversion. 

In general, catalyst sedimentation has to be accounted for in slurry reactors. The distribution of the catalyst along the reactor can be computed using the sedimentation-dispersion model. As to the results of Kato et al. (73), the solid dispersion coefficients do not differ much from those of the liquid phase. From the data provided by Cova (74), Imafuku et al (75), and Kato et al. (73), the solids concentration profiles can be calculated. As in the FT process the catalyst particles are usually small, according to Kolbel and Ralek (35) the diameter should be less than 50 um, the catalyst profiles are not very pronounced, in accordance to the measurements of Cova (74). 

Reduction of the oxidic precursors resulting from the oxidation of the supported complex cyanides leads to metal or alloy particles. Figure 4 shows temperature-programmed reduction (IPR) profiles measured with the thermal conductivity detector for the oxidic precursors of iron, copper-iron and nickel-iron catalysts. With the pure iron catalyst profiles for the alumina and for the titania supported ones are presented. The reduction profiles of the iron-copper and... 

The TPR profiles for caleined Ni and PtNi catalysts are presented in the Fig 1. The NiM catalyst profile shows two reduction peaks at 524 and 670 K associated to isolated NiO... 

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