Distribution, spent catalyst

Both high CRC and afterburn in partial combustion regenerators can be overcome by proper design of the spent catalyst distributor and the air grid. Figure 6 shows one example of a modern spent catalyst distributor designed by Khouw et al. (1994), which distributes spent catalyst laterally through several horizontal arms. 

Greater afterburn, particularly with an uneven air or spent catalyst distribution system... 

The CO promoter is added to most FCC units to assist in the combustion of CO to COj in the regenerator. The promoter is added to accelerate the CO combustion in the dense phase and to minimize the higher temperature excursions that occur as a result of afterburning in the dilute phase. The promoter allows uniform burning of coke, particularly if there is uneven distribution between spent catalyst and combustion air. 

There be a uniform distribution of air and spent catalyst. Air/ catalyst mixing in the regenerator can significantly affect the SO, pick-up efficiency. 

Air and spent catalyst distribution. Modifications to the air and spent catalyst distributors permit uniform dispersion of air and spent catalyst into the regenerator. Improvements are lower carbon on the catalyst and less catalyst sintering. The benefits are a cleaner and higher-activity catalyst, which results in more liquid products and less coke and gas. 

The main purpose of the regenerator is to produce a clean catalyst, minimize afterbum, and reduce localized sintering of the catalyst. For efficient catalyst regeneration it is very important that the air and the spent catalyst are evenly distributed. Although, in recent years, the design of air distributors has improved significantly, the same cannot... 

Selective plugging/addition of air distributor jets to match the spent catalyst distribution... 

The regenerator review will include spent catalyst distribution, air distribution, and cyclones. If the test run with heavy feed indicates a temperature limitation, catalyst coolers, partial combustion, or riser quench should be considered. 

The activity trend obtained with Cui xCoxFe204 catalysts is well supported by the surface metal ions composition determined from XPS analysis. Figure 8 displays the Cu/(Co+Fe) (Co/Fe for X = 1) ratio calculated from XPS results in the left panel and phenol conversion with products selectivity for all catalyst compositions in the right panel. This exercise is mainly to imderstand the distribution of metal ions and their heterogeneity on the smface, as it directly influences catalytic activity. On fresh catalysts, relative Cu-content decreases linearly with decreasing Cu-content and it is in good correlation with bulk Cu-content measmed by x-ray fluorescence. A high Cu/Fe ratio is found on spent catalysts at 0.25 < x < 0.75. It is to be... 

Improve spent catalyst distribution to reduce afterburn... 

Spent catalyst being introduced into the regenerator should be distributed as evenly as possible across the catalyst bed. It is the nature of a fluidized bed that mixing... 

Another commercial nnit was revamped to improve spent catalyst distribution. This particular unit has a shallow bed of 8-10 deep with a bed L/D ratio of 0.4. The revamp resulted in a reduction of afterburn from 190 to 100°F and decrease in COP addition from 45 to 30 Ib/d (Figure 15.5). 

FIGURE 15.5 Spent catalyst distribution revamp performance. 

Fleisch et al. (1984) measured the catalyst surface area and pore volume changes that occurred after severe deactivation of a 100- to 150-A pore catalyst. The results of these measurements are shown in Table XXVIII for various positions in the reactor bed. Catalyst surface area and pore volume are substantially reduced in the top of the bed due to the concentrated buildup of metals in this region. The pore volume distribution of Fig. 44 reveals the selective loss of the larger pores and an actual increase in smaller (<50-A) pores due to the buildup of deposits and constriction of the larger pores. Fleisch et al. (1984) also observed an increase in the hysteresis loop of the nitrogen adsorption-desorption isotherms between fresh and spent catalysts, which reflects the constrictions caused by pore... 

Fig. 44. Pore volume distribution of fresh and spent catalyst after processing residuum (Fleisch et al., 1984).

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... 

The relative number of active sites as deduced from thiophene HDS (of spent catalysts) is also shown in Figure 4 for the Mo/Si02 catalysts. Fully in line with the XPS results, independent of the coke level the number drops to about 25% of that of the fresh catalyst. Comparison of the coverage of 50% and the remaining number of active sites (25%) suggests that the coke is not randomly distributed over the catalyst but is to some extent primarily deposited onto the active phase. The latter conclusion can also be deduced from the results of the XPS study [7J. 

Comparison of Active Metal Pairs. The catalysts were evaluated at a temperature of 780°-850°F at a 2000°-2800°F pressure with a space velocity of 0.5 vol/vol/hr. The range of product distribution from this evaluation is summarized in Table IV. The quality of liquid product as defined by gravity, and by residuum (975°F +), hydrogen, nitrogen, sulfur, and oxygen contents, and by carbon on spent catalysts is presented... 

It had a monomodal pore distribution in the range of 4.4 to 12 nm pore diameter, with a most probable pore size of 5.4 nm. The pore volume and surface area of the catalyst were 0.17 cnr/g and 106 mz/g respectively. The spent catalyst also contained significant amounts of silica fouling, which had deposited on the catalyst from de-foaming agents added to the oil during the industrial use of the catalyst. 

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