Equilibrium/equilibria catalyst effects

Figure 3.23 Hydrogen plant. Temperature approach to equilibrium and catalyst effectiveness factors.

Wliile this definition does not address the question of how catalysts effect rate increases, it does ensure that a catalyst cannot cause the equilibrium composition to deviate from that of the uncatalyzed reaction. 

The circulating catalyst in the FCC unit is called equilibrium catalyst, or simply E-cat. Periodically, quantities of equilibrium catalyst are withdrawn and stored in the E-cat hopper for future disposal. A refinery that processes residue feedstocks can use good-quality F-cat from a refinery that processes light sweet feed. Residue feedstocks contain large quantities of impurities, such as metals and requires high rates of fresh catalyst. The use of a good-quality E-cat in conjunction with fresh catalyst can be cost-effective in maintaining low catahst costs. 

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. 

The conversion to equilibrium is effected in the presence of a catalyst at a pressure of 300 bar. Assuming the behavior to remain ideal, calculate the fractional conversion for each reaction and hence the volume composition of the equilibrium mixture. 

The reaction can, however, be made preparative for (91) by a continuous distillation/siphoning process in a Soxhlet apparatus equilibrium is effected in hot propanone over solid Ba(OH)2 (as base catalyst), the equilibrium mixture [containing 2% (91)] is then siphoned off. This mixture is then distilled back on to the Ba(OH)2, but only propanone (b.p. 56°) will distil out, the 2% of 2-methyl-2-hydroxypentan-4-one ( diacetone alcohol , 91, b.p. 164°) being left behind. A second siphoning will add a further 2% equilibrium s worth of (91) to the first 2%, and more or less total conversion of (90) — (91) can thus ultimately be effected. These poor aldol reactions can, however, be accomplished very much more readily under acid catalysis. The acid promotes the formation of an ambient concentration of the enol form (93) of, for example, propanone (90), and this undergoes attack by the protonated form of a second molecule of carbonyl compound, a carbocation (94) ... 

A catalyst has no effect on the position of equilibrium. A catalyst increases the rate at which equilibrium is attained. As discussed in the Reaction Rates chapter, a catalyst provides an alternative route of lower activation energy. Because the rates of both the forward and backward reactions are increased, there is no change in the position of equilibrium. In industry, the presence of a catalyst allows a process to be carried out at a lower temperature (thereby reducing heat energy costs) whilst maintaining a viable rate of reaction. 

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 catalyst intraparticle reaction-diffusion process of parallel, equilibrium-restrained reactions for the methanation system was studied. The non-isothermal one-dimensional and two-dimensional reaction-diffusion models for the key components have been established, and solved using an orthogonal collocation method. The simulation values of the effectiveness factors for methanation reaction Ch4 and shift reaction Co2 are fairly in agreement with the experimental values. Ch4 is large, while Co2 is very small. The shift reaction takes place as direct and reverse reaction inside the catalyst pellet because of the interaction of methanation and shift reaction. For parallel, equilibrium-restrained reactions, effectiveness factors are not able to predict the catalyst internal-surface utilization accurately. Therefore, the intraparticle distributions of the temperature, the concentrations of species and so on should be taken into account. 

Laboratory steam deactivations represent a significant compromise in the effort to simulate equilibrium catalyst. Since hydrothermal deactivation of FCC catalysts is not rapid in commercial practice, deactivation of the fresh catalyst in the laboratory requires accelerated techniques. The associated temperatures and steam partial pressures are often in substantial excess of those encountered in commercial units. In some instances, the effect of contaminant metals is measured by an independent test not affiliated with steam deactivation. In subsequent yields testing, interactions between different modes of deactivation may be overlooked. Finally, single mode deactivation procedures can not reproduce the complex profile of ages and levels of deactivation present in equilibrium catalyst. 

Figure 8 illustrates the effect of temperature on vanadium mobility (measured on equilibrium catalysts from three FCC units) and the large impact of certain unit variables. For these measurements the coarse equilibrium catalyst was tested in a mobility test using the method described above by substituting sieved coarse equilibrium catalyst for vanadium impregnated catalyst. 

There are several differences between conventional steam reforming and UMR cost of reformer, heat transfer limitations, reaction equilibrium limitations, effectiveness of catalyst and feed stock limitations. 

Rajagopalan et al [5] brought experimental evidence that fresh catalyst is diffusionally limited when cracking West Texas heavy gas oil at 773°K. But, it is not clear whether this limitation remains after the steaming, which simulates the hydrothermal deactivation of fresh catalyst to the equilibrium catalyst. Moreover, since the result of activity in M.A.T. is the average performance of a decaying catalyst, it is impossible to determine whether the effectiveness factor of uncoked catalyst is less than 1. 

Fig. 5.9 Effect of a V-trap on a commercial FCC catalyst (Ecat = Equilibrium Catalyst, MA = Microactivity test (7))...

By way of example, Figure 3 shews the effect of steaming severity on zeolitic surface area (ZSA) for catalyst A and C. Also identified are typical values for equilibrium catalysts. What is seen is that the conditions needed to deactivate A to typical equilibrium ZSA are different than for C. If C is deactivated using the preferred conditions for A, then activity and surface areas are not in line with commercial experience. If the reverse is true, then A is deactivated too severely. 

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