Catalysts activity index

Figure 6 (7). The catalyst activity index, a, is the isomerization rate calculated from the reaction time and the octane numbers of the product, a first-order reaction being assumed.

The effects of non-uniform distribution of the catalytic material within the support in the performance of catalyst pellets started receiving attention in the late 60 s (cf 1-4). These, as well as later studies, both theoretical and experimental, demonstrated that non-uniformly distributed catalysts can offer superior conversion, selectivity, durability, and thermal sensitivity characteristics over those wherein the activity is uniform. Work in this area has been reviewed by Gavriilidis et al. (5). Recently, Wu et al. (6) showed that for any catalyst performance index (i.e. conversion, selectivity or yield) and for the most general case of an arbitrary number of reactions, following arbitrary kinetics, occurring in a non-isothermal pellet, with finite external mass and heat transfer resistances, the optimal catalyst distribution remains a Dirac-delta function. 

The conventional faujasite catalysts were steamed at 1400 F in 2 atmospheres steam at varying times in order to achieve the same activity as measured by our standard Fluid Activity Index (FAI). Table I summarizes the steaming conditions employed. 

Fig, 51. Super active 3rd generation catalyst Isotactic index vs. polymerization time. Polymerization in hexane at 70 °C and 7 bar... 

It is not easy to compare the activity of the V-W-Ti catalysts here tested with the lot of chromia, Pt and Pd based catalysts previously used because they have different shapes (monoliths and spheres) and because very different particle sizes arc involved (having thus very different effectiveness factors). For conqiarison purposes, all X-T curves were adjusted to a simple fust order kinetic model (with rate based on overall volume of catalyst, both for monoliths and for fixed beds). From the kinetic constants so obtained (see details of the method in ref 7), the preexponential factors (ko) of the Arrhenius law and the apparent energies of activation (E, p) were calculated for all catalysts. One example is shown in Figure 17. By the well Imown compensation effect between ko and E,pp, the kg values so obtained were recalculated for a given E.pp value of 44 kJ/mol. Such new ko value was used [7] as an activity index of the catalyst. 

Friedel and Crafts themselves observed that aluminum chloride is by no means the only specific catalyst in the Friedel-Crafts reaction. A number of other acidic metal halides could also be employed however, these were less reactive. The strength or coordinating power of different Lewis acids can vary widely against different Lewis bases. Hence it is extremely difficult to establish a scale of strength of Lewis acids in a manner analogous to that used for Brpnsted acids. Despite the difficulties, a number of qualitative orders of reactivity have been proposed. A comparative study of the activity of various Friedel-Crafts catalysts was performed by Olah and coworkers. Thus the activity index (the lowest temperature at which reaction occurs) of a large number of Lewis acid halides was measured using the benzylation reaction as the probe. 

By now this basic formulation has had many interpretations. For example (Activity) has been used to refer to coke-on-catalyst, amine index of the material, reference to conversion in some specific chemical test who knows what else. The value of n, reported in various studies as ranging from 0 to 12, has been represented to indicate diffusion control (0.5) up to essentially "... we don t know what is going on here. .. (12). .. ". The factor is a proportionality constant specific to catalyst, operating conditions and chemical reaction. Voorhies model, based on time-on-stream observations, is obviously not general, but it is a good place to start. 

Since the measurement of on-line catalyst activity is difficult, we found it convenient to follow an on-line "reaction index" (RI), which is a selectivity ratio. The complex MTO reaction scheme can be presented schematically as A —> B —> C, where A represents methanol and DME, B - olefins, and C -aromatics and paraffins (Fig. 3). One particularly useful RI is the propane/propene ratio. Propene is the primary light olefin and propane represents paraffins. The propane/propene RI can be easily monitored by an on-line GC. We found that hydrocarbon selectivities correlate well with RI. For fixed hydrodynamics, it also correlates well with methanol conversion. 

FIGURE 77 Impact resistance of films as a function of the polymer melt index. The addition of poisons to the reactor affects LCB levels in the polymer. In contrast to 02, CO diminishes elasticity, which in turn results in less orientation in the blown film, and therefore improved impact and tear resistance. (Cr/silica-titania catalyst, activated at 650 °C, polymer density of 0.938 g mL, film thickness 25 pm). 

Melt index potential means the maximum MI that can be obtained with a catalyst activated at maximum temperature (871 °C), and making homopolymer at maximum reaction temperature (110 °C). Catalystshaving a high-MI potential need not be used to make high MI resins, but they have the capability. Low-MI polymers are typically manufactured more easily with catalysts having a high-MI potential. 

FIGURE 82 Response to shear stress, shown here as the polymer HLMI/MI ratio, as a function of catalyst activation temperature for polymers made in the slurry process with Cr/silica-titania catalyst. Reaction temperature was varied (102-110 °C) to produce three series of polymers of constant melt index. (Compare with Figure 83.)... 

Activity data of catalysts subjected to high temperature treatments at 700°C and 800°C and that of hydrothermally treated samples are given in Table 2. The percentage loss of catalyst activity during deactivation tabulated in Table 2 is a helpful index to rank catalysts with respect to their resistance to deactivation. 

The dependenee of polymerization rates on the ratio of the eatalyst to amine was investigated next. The obtained data show that varying this ratio from 1 1 to 1 4 has no inflnenee on polymerization rate. Polydispersity indexes and MW do not depend on the eoneentration of amine. These resnlts allow ns to propose that amine does not partieipate in the ehain transfer reactions, but interacts with ruthenium complex to generate in sitn catalyst active for ATRP. 

The recent interest in electronic factors in catalysis has produced two significant theories. The first is that with the metals the electronic configuration, in particular of the d-band, is an index of catalyst activity. The second is that with the oxides, activity may be controlled by the semiconducting property. Hitherto, these theories have been regarded as unrelated to one another. 

Titration acidity of Houdry type S (SiOj-AhOs) catalyst as a function of time. Numbers refer to activity index as determined by CAT-A. 

The ethylene polymerization of this catalyst was carried out in an autoclave reactor at 221°F in isopentane as the slurry solvent in the presence of triisobutylaluminum as cocatalyst and 50 psig of hydrogen and sufficient ethylene to achieve a total reactor pressure of 550 psig. The catalyst activity was 10,540 g of PE/g of catalyst/ hr, which corresponded to an activity of 146,000 g PE/g Ti/hr. The granular polyethylene product obtained was considered suitable for a particle-form slurry process such as the Phillips slurry process. The polyethylene sample displayed a Melt Index (I value of 0.70 and a High Load Melt Index ) value (HLMI) of 3 1 with a HLMI/MI ratio of 45, which indicates tfiat the polyethylene molecular weight distribution was of an intermediate value. 

Figure 3.12 Relative Melt Index potential (RMIP) vs secondary catalyst activation temperature. RMIP is the melt index of the polyethylene sample normalized by the Melt Index of the standard Phillips catalyst containing 1 wt% Cr and activated with one thermal treatment in air at 870°C. Melt Index is inversely proportional to polymer MW. Reprinted from [12] with permission from Elsevier Publishing.

The data in Table 4.6 show that catalyst activity increases with polymerization temperature from 1,150 g PE/g catalyst at 65 C to 5,100 g PE/g catalyst at 77.5°C. However, very importantly, the polyethylene molecular weight may be controlled over the range necessary for the manufactme of commercial grades of polyethylene for industrial appHcations, as shown by Melt Index (MI values from 4.1 to 0.6. Isopentane or isopentane containing 0.3 ppm oxygen was needed to produce the relatively higher molecular weight products. 

Figura 9.2 Effect of catalyst activation temperature on melt flow index. (From Ref. 1.)...

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