Equilibrium catalyst activity

It can be seen from Table I that, in order to maintain high equilibrium catalyst activity and to reduce fresh catalyst makeup, the trend is to increase CBI and to decrease CRC, so as to minimize catalyst inventory in the reactor-regenerator system. To this end, the superiority of FFB regeneration has become quite evident. 

Two examples of commercial cases are shown in Figure 6. The two regimes can be encountered in a single unit depending on the (equilibrium) catalyst activity, as in unit A. Obviously, this makes it very difficult to decide which catalyst to select if a unit is operating in the transition zone between the two regimes. 

Steam treatment severity has been varied to match commercial equilibrium catalyst activities and other properties. One measure of a suitably steam-aged catalyst is the surface area which should be in the range of 51 to 200 m /g (Ritter, 1985). Perhaps a more meaningful measure is to use a bulk property of the zeolite, the unit cell size, which is measured by X-ray diffraction. Typically, the unit cell size of USY zeolites are reduced to below 24.26 A whereas RE-USY zeolites equilibrate to 24.26-24.32 A and REY zeolites to 24.5 A in the FCC unit (Scherzer,... 

Metals are most active when they first deposit on the catalyst. With time, they lose their initial effectiveness through continuous oxidation-reduction cycles. On average, about one third of the nickel on the equilibrium catalyst will have the activity to promote dehydrogenation reactions. 

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

The equilibrium values are not reached at a rhodium catalyst on a micro structured reactor within the limits of the experimental conditions and the constructional constraints [3]. As possible explanations post-catalytic reactions at lower temperatures or, more likely, insufficient catalyst activity concerning the short residence times are seen. 

Enolization and ketonization kinetics and equilibrium constants have been reported for phenylacetylpyridines (85a), and their enol tautomers (85b), together with estimates of the stability of a third type of tautomer, the zwitterion (85c). The latter provides a nitrogen protonation route for the keto-enol tautomerization. The two alternative acid-catalysed routes for enolization, i.e. O- versus Af-protonation, are assessed in terms of pK differences, and of equilibrium proton-activating factors which measure the C-H acidifying effects of the binding of a proton catalyst at oxygen or at nitrogen. 

ZSM-5 has been used successfully in commercial operations when processing high boiling range feedstock and resids. This is principally due to its ability to maintain activity despite the presence of a high concentration of feed metals. ZSM-5 s excellent metals tolerance has been demonstrated commercially at equilibrium catalyst metals levels up to 10,000 ppm nickel plus vanadium and 6,000 ppm sodium with very little detrimental effect. Laboratory tests show that ZSM-5 is far less affected by metals than Y-zeolite catalysts. Metals were introduced, as follows ... 

The control of the activity and selectivity of cracking catalyst is the key to optimum yields and profitability. Currently, refineries employ two different methods of control the addition of fresh catalyst and the addition of good quality equilibrium catalyst. Onsite FCCU catalyst demetalization, called Demet, is a third alternative which was originally developed by ARCO and then improved by ChemCat Corporation workers (1). The Demet procedures are used to remove active metal contaminants from the surface of equilibrium catalysts, thus improving catalyst activity and selectivity. Demet procedures are applicable to all types of amorphous and zeolitic catalysts. 

This paper gives an example of the response of one equilibrium catalyst to the three basic Demet procedures and to modified versions of these procedures. The catalysts are evaluated by elemental analysis and by their cracking performance, as determined by the micro activity test (MAT). 

Cracking performance. Micro activity test results for the different preparations are given in Table II. Variants of all three basic Demet procedures (see Fig. 1-3) can be used to improve the performance of this equilibrium catalyst. 

In Table II, the product yields of REY-PILC are compared with PILC, a commercial equilibrium catalyst, and with the same commercial catalyst that had been deactivated in the laboratory to near constant conversion. The addition of REY to PILC maintained activity in the presence of steam while coke yield was reduced and the LCO/HCO ratio was slightly higher than for either of the commercial catalysts. This suggests that the microstructure of the PILC after pretreatment D will still convert large molecules into gasoline range products instead of generating coke as seen in PILC alone. 

It is often necessary to employ more than one adiabatic reactor to achieve a desired conversion. The catalytic oxidation of SOj to SO3 is a case in point. In the first place, chemical equilibrium may have been established in the first reactor and it would be necessary to cool and/or remove the product before entering the second reactor. This, of course, is one good reason for choosing a catalyst which will function at the lowest possible temperature. Secondly, for an exothermic reaction, the temperature may rise to a point at which it is deleterious to the catalyst activity. At this point, the products from the first reactor are cooled prior to entering a second adiabatic reactor. To design such a system, it is only necessary to superimpose on the rate contours the adiabatic temperature paths for each of the reactors. The volume requirements for each reactor can then be computed from the rate contours in the same way as for a... 

Natural gas is reacted with steam on an Ni-based catalyst in a primary reformer to produce syngas at a residence time of several seconds, with an H2 CO ratio of 3 according to reaction (9.1). Reformed gas is obtained at about 930 °C and pressures of 15-30 bar. The CH4 conversion is typically 90-92% and the composition of the primary reformer outlet stream approaches that predicted by thermodynamic equilibrium for a CH4 H20 = 1 3 feed. A secondary autothermal reformer is placed just at the exit of the primary reformer in which the unconverted CH4 is reacted with O2 at the top of a refractory lined tube. The mixture is then equilibrated on an Ni catalyst located below the oxidation zone [21]. The main limit of the SR reaction is thermodynamics, which determines very high conversions only at temperatures above 900 °C. The catalyst activity is important but not decisive, with the heat transfer coefficient of the internal tube wall being the rate-limiting parameter [19, 20]. 

Biodiesel can be produced by a sustainable continuous process based on catalytic reactive distillation. The integrated design ensures the removal of water byproduct that shifts the chemical equilibrium to completion and preserves the catalyst activity. The novel alternative proposed here replaces the liquid catalysts with solid acids, thus dramatically improving the economics of current biodiesel synthesis and reducing the number of downstream steps. The key benefits of this approach are ... 

Catalyst activity at optimum amine level decreased with increasing basicity. This effect is also explained in terms of Reaction 3 where IR data indicate a pronounced shift to the left. If it is assumed that equilibrium increasingly favors the rhodate anion with the stronger bases, it can be readily shown that the maximum fraction of species II decreases with increasing basicity. In other words, the amount of species II present at optimum amine levels is lower for TEA than for DMBA. 

IR data supported the formation of rhodium carbonyl anions on addition of amine suggesting a mechanism in which these anions are in equilibrium with the active species, a hydrido rhodium carbonyl containing coordinated amine. Such a mechanism provides an explanation for the variation in catalyst activity with amine concentration and basicity and affords a method for predicting the effect of other amine ligands. 

It appears that some ferrous chloride is formed but not in the amount that would be expected if equilibrium had been established in Reaction 2. Reactions analogous to Reactions 4 and 5 with ferrous chloride substituted for calcium chloride may be the reason for this. In any case, the equilibrium constant indicates that the amount of ferrous chloride that can be formed is limited to one mol per nine mols of zinc chloride. We have unpublished data that indicates that ferrous chloride does not affect catalyst activity. 

When the chemical treatment is applied to a sample previously steamed at a higher temperature (725 C), which would simulate the zeolite in an equilibrium catalyst, only a very small, if any, decrease in activity is observed (Fig. lb). The fact that when the steaming temperature is high (> 700flC), the Bronsted sites of enhanced acidity (bands at 3600 and 3525 cm"1) are not observed (8), and only the HF and LF bands are present (8), will explain the activity behaviour observed for samples USY-2 and U2F-35. 

Coke Deposition. The properties of catalyst fractions separated in coked condition from spent equilibrium catalyst are summarized in Tables III and IV. The distribution of catalyst fractions along with the percent carbon found on each coked fraction is given in Figure 2. The activity for coke deposition falls off sharply with increase in density. Only the three lightest fractions show a coke make that is significantly above the minimum coke make exhibited by the heavier fractions. The fact that the lightest fractions are the most active is consistent with the notion that they are the youngest. The distribution of catalyst... 

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