Vanadium deposition

The equivalent nickel content of the feed to the FCCU can vary from <0.05 ppm for a weU-hydrotreated VGO to >20 ppm for a feed containing a high resid content. The nickel and vanadium deposit essentially quantitatively on the cracking catalyst and, depending on catalyst addition rates to the FCCU, result in total metals concentrations on the equiUbrium catalyst from 100 to 10,000 ppm. 

Fig. 5 Infrared spectra of 0.03 ML vanadium deposited on silica as a function of CO exposure. Preparation and measurements were done at 90 K...

Pazos et al. (1983) proposed a consecutive HDM reaction mechanism similar to the model compound studies in which part of the vanadium deposition occurs from species V) not originally present in the oil ... 

Fig. 42. Catalyst activity after vanadium deposition (Takeuchi et al., 1985).

Fig. 45. Effect of hydrogen partial pressure on vanadium deposition for an Arabian Heavy atmospheric residuum at a reaction temperature of 371°C (700°F) using a -in. extrudate catalyst (Tamm et at., 1981).

Fig. 48. Experimental vanadium deposit distributions in a microporous catalyst (100 A micropore diameter/204 m2/g SA) macroporous catalyst (1300 A pore diameter/14.5 m2/g SA) and bimodal catalyst (120 A micropore, 25,000 A macropore diameters/200 m2/g SA) (Plumail e al., 1983).

Fig. 49. Effect of catalyst particle size on vanadium deposition for an Arabian Heavy atmospheric residuum processed at 370° (700°F) under 12.59 MPa (1825 psia) of hydrogen (Tamm et al., 1981).

Since the method of metal impregnation will also have a large impact on the profile of vanadium deposition of the catalyst, we clearly need to review this aspect of catalyst testing. Via cyclic metal impregnation and catalyst deactivation, we can approach "Real-World" conditions far better. (Table VII.)... 

The plot of the apparent kinetic constants, ka, as a function of Wvr the vanadium deposited on catalyst, at different temperatures levels, for catalyst C, is shown in Figure 5. A linear relationship can be observed (interrupted lines in Figure 5) between both variables, for same temperature level, however the interception, at the start of run, differs from the experimental values for ka. The dot-and-segment lines in Figure 5 correspond to the interpolation between the experimental values for ka, obtained during the long deactivation test, and the ka data for the fresh catalyst. 

The assumption is less critical than it appears under a first inspection. Pazos et.ai. [1] found that, for Bachaquero crude, the amount of vanadium deposited on catalyst was sometimes highest at the reactor exit maximum percent difference of vanadium concentration, with reference to the average value in the bed, was 25%. 

Only the vanadium deposit hinders diffusion inside the catalyst particles. 

Catalyst porosity, which changes with time, can be written, then, in terms of the vanadium deposit ... 

Figures 7 and 8 show that the linear decay predicted by Equation (10) is followed consistently by the two catalysts, A and C. The rule of linear decay was followed with 11.8% and 7,9% of average deviation for catalysts A and C, respectively. For calculating a, the rate of decay in (10), values of s 1,95 and pvc 4,72 g/cu,cm. were taken. The latter is the density of vanadium sesquioxide (S3V2) which Is usually considered the prevalent product in the vanadium deposit.

Wv = g of vanadium deposited per unit mass of fresh catalyst, ). 

This conclusion can be drawn because the small amount of vanadium deposited is almost uniformly distributed in the grain volume. A vanadium radial concentration gradient induced by a diffusion limitation may be one reason for the discrepancies found in the literature. In such cases, the active phase inside the grain is less affected by the vanadium, but can be affected by the carbon deposit. 

Coke on the catalyst is, thus, largely responsible for catalyst deactivation by loss of surface area, and this could be minimized by increasing the hydrogen pressure. However, increasing pressure has been reported to increase vanadium deposition more near the exterior surface of the catalyst pellet (13,14). In essence, an increase in the hydrogen pressure has a beneficial effect in suppressing coke formation, but can lead to shorter catalyst life due to rapid accumulation of vanadium at pore mouths. 

Table n. Concentration of Carbon and Vanadium Deposited on Different Catalysts... 

Figure 6 shows the influence of the axial position of the catalyst pellet in the reactor. As can be observed, the vanadium deposition profiles shift from a M-shaped profiles to a more U-shaped profile. This is due to the increasing presence of intermediates outside the catalyst pellet when going further downstream in the reactor. Experimental work by Ware Wei (<5) showed a similar shift in the shape of the metal deposition profile. 

Figure 7 shows the influence of the initial pore radius, in the case of a wide- and narrow-pore silica catalyst, on the vanadium deposition profiles at an average axial position in the reactor. Both cases show the presence of deposition maxima, indicating that the deposition process is diffusion rate-limited. In the case of the narrow-pore silica the core volume of the pellet potentially available for vanadium deposition cannot be reached by reactant and intermediates and is lost for vanadium deposition. 

Figure 5. Influence of the bulk difftision coefficient, D, on the vanadium deposition profile in a wide pore silica catalyst pellet at reactor inlet conditions. (Model compound VO-TPP, 673 K, 10 MPa H2, initial pore radius 30 nm, catalystpellet radius 0.85 mm)...

Figur. Influence of axial position in the reactor on the vanadium deposition profile in a wide pore silica catalyst pellet.

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