Low catalyst activity

An important conclusion from these simulations is that neither the FFB nor the MAT test simulates riser behavior, since neither of these tests reflects the intrinsic catalyst activity and decay. These intrinsic parameters are masked by backmixing in the FFB test and by a high temperature drop in the adiabatic MAT unit. In addition, the time averaged values of the long contact time FFB and MAT tests overemphasize the low catalyst activity at long contact times, while the short contact time riser is only exposed to initial activity and catalyst decay. 

During the first 2h of reaction, a decrease in AcOH conversion (from 48 to 43 %) for benzene acetylation at 523 K with an increase in selectivity to the monoacetylated product (from 80 to 90%) can be observed. The only problem involves the low catalyst activity 1.5 mmolh 1g 1 of acetophenone, which corresponds to a TOF value of 2.2 h-1. This means that less than 0.2 g of this acetylated arene can be produced per hour and per gram of catalyst under the operating conditions (i.e. 10 times less than in the liquid phase acetylation of anisole with AA). The kinetic study of the reaction shows an increase in the selectivity with the substrate/acetic acid ratio, but no increase in yield, an increase in acetic acid conversion with the reaction temperature with a significant decrease in selectivity due to a greater formation of diacetylated products.[62,63] HFAU and RE-FAU zeolites do... 

Although there is dispute about the exact oxidation state of titanium in the active species [Ti(III) or Ti(IV)], it was suggested, from the results of ESR measurements, that Ti(III) species form highly active sites for producing syndiotactic polystyrene in styrene polymerisation systems with the TiBz4—[Al(Me)0]x catalyst [50]. The moderately low catalyst activity is attributable to the stability of the benzyl transition metal derivatives towards reduction. 

As can be seen from Table 9 an increase in the ( i/ Nd-ralio from 0.5 to 3.0 results in an increase of the cis- 1,4-content. A further increase of the ci/ Nd-ratio to 4.0 and 10.0 decreases the cis- 1,4-content. BR which is obtained without halide donor (at a very low catalyst activity) exhibits a unique mi-crostructural composition 71.9% czs-1,4, 21.2% trans-1,4 and 6.9% 1,2. This observation corroborates the fact that high-czs-l,4-poly(butadiene) products only can be obtained in the presence of halide donors. 

As can be seen from Table 14 the addition order affects reaction rate, molar mass and PDI. The authors suggested that in-situ activation results in the formation of two types of active species the relative concentrations of which were governed by the addition sequence. Addition order (1) EASC + NdV + DIBAH promotes the formation of insoluble species which produced polymer with a broad, bimodal MMD (PDI = 7.5) at low catalyst activity. Addition order (3) DIBAH + NdV + EASC leads to the formation of more soluble catalyst species which exhibit increased catalyst activity and produce BR with a monomodal MMD (PDI = 3.4). The influence of the addition order on cis-1,4-content is negligible. 

The low catalyst activity in toluene and the found non-linear Mn-con-version characteristics are explained by chain-transfer reactions. A reaction scheme that accounts for the abstraction of a proton from toluene by the allyl-end of the growing poly(butadiene) chain is given in Scheme 13. 

Figure 5 shows the dependence of the effectiveness factor on the Thiele modulus for the different pellet shapes. At small values of 4> the effectiveness factor approaches unity in all cases. Here, the chemical reaction constitutes the rate determining step—the corresponding concentration profiles over the pellet cross-section arc flat (sec Fig. 4). This situation may occur at low catalyst activity (k is small), large pore size and high porosity (Dc is large), and/or small catalyst pellets (R is small, i.c. in fluidized bed reactors R is typically around 50 /im).

In non-supported catalysts, most active centers (>95%) become encased within the growing polymer particle and thereby become unavailable for additional polymer formation. This results in low catalyst activity. A major improvement occurred in the early 1970s when supported Ziegler-Natta catalysts began to emerge. Leading polyethylene producers of the time (Shell, Solvay Cie, Hoechst, Mitsui and Montecatini Edison) developed many of these catalysts (8). Of course, most have morphed into present-day companies, such as LyondellBasell and INEOS. 

Under conditions of insufficient P-NH, protonolysis becomes too slow and therefore rate-determining. Such a situation can exist at low acid and/or low free ligand concentration, but also in a basic solvent when the protons preferentially reside on the basic solvent molecules. This explains why 2-pyridylphos-phines with electron-withdrawing pyridyl substituents, (e. g., halides) in particular, lead to low catalyst activities in solvents like NMP. 

The Phillips STAR process also regenerates the catalyst on a cyclic basis, but while the Houdry regeneration is actually a mechanism to provide the heat for the reaction even when coke buildup is still very low, the catalyst in the isothermal STAR process is only regenerated after coke has accumulated to appreciable levels that result in low catalyst activity. 

In 1993, Perez et al. reported norbornene polymerization in aqueous emulsion at 70 °C using PdCl2 as a catalyst precursor [92, 93]. A stable latex consisting of low-molecular-weight oligomeric material (degree of polymerization DP ca. 10) was obtained with low catalyst activities (70 TO h ). Very small latex particles of 10 to 20 nm diameter were reported. In the free radical polymerization of olefinic monomers such small particles are only obtained by microemulsion polymeriza-... 

The pyrolysis of hydrocarbons follows the thermal cracking mechanism (4). Apart from the pressure, the conditions in the tubular steam reformer and in the preheater are not far from that of a steam cracker in an ethylene plant. With low catalyst activity, the pyrolysis route may take over. This is the situation in case of severe sulphur poisoning or in attempts to use non-metal catalysts so far showing very low activity (1). Non metal catalysts have mainly been based on alkaline oxides being active for gasification of coke precursors. However, it has been difficult to avoid the formation of olefins and other pyrolysis products (1,2,5). In fact, it was demonstrated (2,4) that co-production of syngas and light olefins was possible from heavy gas oil and naphtha over a potassium promoted zirconia catalyst. 

Preliminary experiments demonstrated that the stirring speed of 1000 ipm was necessary to overcome possible impact of external mass transfer, therefore throughout the text this value is inplicitly assumed, if not stated otherwise. Application of 2-propanol as a solvent resulted in extremely low catalyst activity, tos only 1-propanol was further utilized. It was also noticed, that preheating and stirring of the mixture of 1-propanol and sitosterol was essential to improve conversion of sitosterol. 

A low catalyst activity (Figure 19) in the polymerization of a-olefins with metallocene and halfsandwich complexes can be due to many reasons the central metal plays the most important part in... 

Another aspect for low catalyst activity is its limited lifetime due to decomposition processes such as the reduction of the metal atom in the catalyst. 

Due to the initial low catalyst activity, no attention was placed on a quick separation of catalyst and hydrocarbons in the early days of FCC. However, after the development of zeolite-based catalysis and riser cracking combined with improved catalyst stability, riser outlet temperatures of 970°F and greater were observed. It was observed that quick disengaging of hydrocarbons reduced dry gas and delta coke yields. 

In Figure 2.25 the effectiveness factor as function of the Thiele modulus for different pellet shapes is shown. For small values of the Thiele modulus the effectiveness factor reaches unity in all cases. The reaction rate is controlled by the intrinsic kinetics, and the reactant concentration within the pellet is identical to the concentration at the outer pellet surface. This situation may be observed for low catalyst activity or very small particles as used in fluidized beds or suspension reactors. For large values of the Thiele modulus the dependency of r p approaches an asymptotic solution tjp = micp with w = 1, 2, 3 for a slab, a cylinder, and a sphere, respectively. This situation may occur for very fast reactions or large catalyst particles. The concentration in the center of the catalyst particles approaches zero for rip < 0.2. 

With low catalyst activity, the thermal cracking route (pyrolysis) may also take over in the reformer tube [389]. This is the situation in case of severe sulphur poisoning or in attempts to use non-metal catalysts with low activity. The risk of carbon formation depends on the type of hydrocarbon with the contents of aromatics being critical. Ethylene formed by pyrolysis results in rapid carbon formation on nickel (refer to Section 5. 2). Ethylene may also be formed by oxidative coupling if air or oxygen is added to the feed - or by dehydration of ethanol. 

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