Nonuniform catalyst activity

If the catalyst active centers are nonuniform, a time variation of the average value of Kp may be caused by the change of the proportion between the centers with various reactivity during polymerization. However, in the case of chromium oxide catalysts the experimental data show that the... 

An important aspect concerning catalytically active membrane reactors, is the distribution of the active phase within the membrane system. Modem modification techniques (van Praag et al. 1989, Lin, de Vries and Burggraaf 1989) allow control over the catalyst distribution and preferential deposition of the active phase at different places in the membrane (top layer/support) system. Studies on conventionally used plate-shaped and cylindrically-sha-ped catalytically active pellets (Vayenas and Pavlou 1987a, b, Dougherty and Verykios 1987) have shown that nonuniformly activated catalysts (catalysts with nonuniform distribution of active sites according to a certain profile)... 

F(c/c,) denotes the dimensionless form of an arbitrary rate expression./(x) is a nonuniform, normalized catalyst activity distribution inside the pellet. A(x) is an auxiliary function, subject to the following linear differential equation ... 

As time progresses, the catalyst in the HDS reactor decays because of metal (vanadium and nickel) and coke depositions. The deposition of these metals occurs nonuniformly along the length of the reactor (more deposits occur near the reactor inlet than at the reactor outlet). In normal plant operations, the catalyst activity decline is counterbalanced by a rise in feed temperature, a reduction in the amount of quench fluids fed to the reactor or both, so as to achieve the same quality product. The process is terminated upon the attainment of a maximum allowable temperature (MAT) anywhere in the reactor. The catalyst bed is then regenerated. The time required to achieve the MAT is often called the reactor cycle life. 

We conclude that the dispersion of the elements in the oxide-form catalysts depends not only on the phosphorus content but also on the preparation procedures. In any case, large amounts of phosphorus favor the agglomeration of metal oxides. The discrepancies in the literature suggest that both the preparation conditions and the activation conditions (such as the nature of the presulfiding mixture and the sulfidation temperature) may also affect the textural properties and morphology of the sulfided catalysts. The nonuniform distribution of molybdenum in the catalyst particles at low phosphorus loadings may affect the XPS results discussed in terms of phase dispersion. 

CONVECTIVE DIFFUSION IN CATALYTIC REACTORS OF NONCIRCULAR CROSS SECTION AND NONUNIFORM CATALYST ACTIVITY... 

The objective here is to simulate duct reactor performance with nonuniform catalyst activity and identify optimal deposition strategies when reactant diffn-sion toward the active surface is hindered, particularly in the corners of the flow channel. Both types of power-function profiles, listed in Table 23-3, are evaluated for n = 1,2,4, 8. The delta-function distribution has been implemented by Varma (see Morbidelli et al., 1985) to predict optimum catalyst performance in porous pellets with exothermic chemical reaction. Nonuniform activity profiles for catalytic pellets in fixed-bed reactors, in which a single reaction occnrs, have been addressed by Sznkiewicz et al. (1995), and effectiveness factors for... 

TABLE 23-3 Normalized Catalyst Activity Profiles Nonuniform Distribution of Catalyst on the Inner Walls of Straight Channels with Rectangular Cross Section"... 

Twelve normalized shape functions presented in Table 23-2 are analyzed at two different Damkohler numbers (i.e., P = 1,10 ) and five aspect ratios (i.e., Ar = 1,4, 8, 20,100). Each nonuniform catalyst activity profile is identified by number, as indicated in Table 23-9. 

We will first discuss methods with whieh the stmeture of the surface is determined, and then those that determine the ehemieal eomposition of the surface (catalyst and substrate). Finding relationships between the struetures of material and the catalyst activity requires high-resolution investigation of the microstructure of the catalyst. Since heterogeneous eatalysts are often highly nonuniform solids, correct sampling, sample preparation, and choice of the appropriate method are important if meaningful results are to be obtained. 

Kotter, M. and L. Riekert, Impregnation type catalysts with nonuniform distribution of the active component on inert carriers. Chem. Eng. Fund., 2 pp. 31-38, 1983. 

Many commercial heterogeneous catalysts are not impregnated in a uniform fashion. For example, various precious-metal catalysts consist of an exterior active shell and an inert core in order to enhance the effectiveness factor. Several automobile-muffler catalysts have a carbon-monoxide-oxidation catalyst in one shell and an NOx-reducing catalyst in another shell. Our understanding of the reaction-diffusion interaction facilitated this rational design of the optimal profile of catalyst-activity distribution and shape. It would be of both practical importance and academic interest to develop a rational procedure for enhancing the performance of metallocenes by their nonuniform impregnation on the support. 

So far in this chapter, the catalyst activity has been assumed uniform throughout a pellet. Nonuniform activity can result from poor impregnation, deliberate partial impregnation, or from deactivation by poisoning species. In such cases, the intrinsic kinetics can be represented by ... 

Due to the widespread usa of alumimrm alkyls in olefin polymerization reactions (e.g., as water scavenger or for catalyst alkylation), it is often difficult to assess the influence of changes in the reaction conditions on a specific observance in the copolymetization behavior. The choice of an appropriate aluminum compound is cmcial for a satisfartory incorporation of polar monomers into polyolefins. MAO, often used for catalyst activation, is not always able to proted the catalyst from deactivation.This can be attributed to insufficient com-plexation " and a nonuniform composition of the commerdal MAO solutions. The contained trimethylalumi-num (TMA) can also lead to increased chain transfer from the catalyst to the present aluminum compoimds. Hence, isobutyl modified MAO, for example, exhibits better protective characteristics. Therefore, and due to the difficult examination of MAO containing reaction mixtures, the addition of further well-defined aluminum compounds like TMA, triethy-laluminum (TEAL), tri-n-butylaluminum (TNBA), TIBA, and tri-/i-octylaluminum (TOA) has been examined. 

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. 

One of the main reasons for a lower specific activity resides in the fact that electrodes with disperse catalysts have a porous structure. In the electrolyte filling the pores, ohmic potential gradients develop and because of slow difiusion, concentration gradients of the reachng species also develop. In the disperse catalysts, additional ohmic losses will occur at the points of contact between the individual crystallites and at their points of contact with the substrate. These effects produce a nonuniform current distribution over the inner surface area of the electrode and a lower overall reaction rate. 

Shyr Y-S and Ernst WR (1980) Preparation of Nonuniformly Active Catalysts, J Catal, 63 426. 

Partially Crystalline Transition Metal Sulphide Catalysts. Chiannelli and coworkers (6, 7, 8) have shown how, by precipitation of metal thio-molybdates from solution and subsequent mild heat-treatment many selective and active hydrodesulphurization catalysts may be produced. We have shown (18) recently that molybdenum sulphide formed in this way is both structurally and compositionally heterogeneous. XRES, which yields directly the variation in Mo/S ratio shows up the compositional nonuniformity of typical preparations and HREM images coupled to SAED (see Figure 2) exhibit considerable spatial variation, there being amorphous regions at one extreme and highly crystalline (18, 19) MoS at the other. 

Active centers, nature of, 10 96 Active site, 27 210-221 in catalysts, 17 103-104, 34 1 for olefin chemisorption, 17 108-113 dual-site concept, 27 210 electrical conductivity, 27 216, 217 ESCA, 27 218, 219 ESR, 27 214-216 infrared spectroscopy, 27 213, 214 model, 27 219-221 molybdena catalyst, 27 304-306 Mdssbauer spectroscopy, 27 217, 218 nonuniform distribution, transport-limited pellets, 39 288-291... 

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