indicator of uneven distribution that can lead to afterburn and CO breakthrough. Combustion promoters to mitigate afterburn Use of CO promoters is the most common and effective way to mitigate afterburn. Combustion promoters help increase the rate of reaction of CO to CO 2 through the following equation:
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CO + ½O 2 → CO 2
CO promoters, present in the dense phase, help move this reaction into the dense phase. A good guideline is to start by adding 1 to 2 ppm platinum or platinum equivalent based on total catalyst replacement (for example, a CO promoter con - taining 500 ppm used at a rate of 2 lb per 1000 lb total addi- tions results in 1 ppm Pt at steady state). However, actual CO promoter requirements can vary significantly based on regenerator conditions, changes in feed, and contaminant metals. CO promoter can be added via a small shot pot loader, an additive loader (preferred), or pre-blended with the catalyst. Ideally, spreading the addition of the promoter throughout the day as opposed to a small amount when afterburn begins to increase is much more effective. This allows the regenerator temperature and promotion level to be more constant and creates an environment for consistent CO promotion. Platinum was originally chosen as an oxidation catalyst because it was the only metal that could be used in low enough concentrations to catalyse CO oxidation without catalysing undesirable de-hydrogenation reactions (other metals yielded higher amounts of hydrogen and methane). However, platinum also promotes nitrogen oxide (NOx) for - mation in the flue gas from nitrogen in coke. Platinum-based promoters can increase NOx emissions by up to four to five times base emissions. NOx emissions are harmful to human health and the envi - ronment. They cause harmful effects on the human respi- ratory system. Besides, NO 2 reacts with water to produce nitric acid and contributes to acid rain. NOx is also consid - ered a greenhouse gas. Nitrous oxide has about 300 times the warming power of carbon dioxide and stays in the atmo - sphere for more than 100 years. 2 For these reasons, NOx emissions regulations are becoming more stringent. In geographical areas with fewer restrictions on NOx emis - sions, platinum-based promoters are still widely used due to their affordability and high performance. For geographical areas where NOx emissions are strongly regulated, palla - dium-based promoters are often used in place of platinum. Switching from a platinum-based promoter to a non-plati- num-based (palladium) promoter usually leads to a reduc- tion of NOx emissions by 60 to 70%, as shown in Figure 2 . In recent years, many refiners using palladium-based promoters have started to switch back to platinum-based promoters due to increased palladium costs. These refin - ers either have some room in their NOx emissions limits or a post-treater for NOx, such as SCR (selective catalytic reduction), SNCR (selective non-catalytic reduction), or LoTOx (low-temperature oxidation for NOx control). For refiners that have these options available, understanding
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Dense bed temperature, ˚F
Figure 1 Afterburn vs dense bed temperature
individual grids or rings can be adjusted or lift air relative to the main air blower, and regenerator bed levels can be opti- mised. CO promoter can be added and will be effective if there is enough CO and O 2 present. However, sometimes the only method of resolution for this type of afterburn is a shut- down with modification to the air grid or catalyst distribution. Importance and mitigation of afterburning Afterburning has two major consequences in the FCC. First, the dilute phase temperature limits the feed rate and flex - ibility to run opportunity feeds. Second is potential serious damage to cyclones and flue gas systems from operating at excessively high temperatures, which can lead to premature shutdowns and costly repairs. Higher dense bed temperature increases the rate of com- bustion of CO to CO 2, thereby avoiding CO breakthrough to the dilute phase. Figure 1 shows a common example of after - burn decreasing with increasing dense bed temperature. Regenerator bed level can have a major impact on CO breakthrough and afterburn. Higher bed levels often help minimise CO breakthrough because of increased residence time. Understanding how different regenerator levels impact CO, NOx, and O2 can help refiners define their optimum operating window. It is also important to observe CO, NOx, and O 2 when undertaking catalyst withdrawals. If CO or afterburn are increasing significantly at lower bed levels or during catalyst withdrawals, it is generally advised to oper- ate at a higher bed level and decrease time intervals between withdrawals. Higher bed levels can also cause increases in catalyst carryover, especially if transport disengaging height (TDH), where the catalyst concentration in the flue gas stays constant, is not maintained. 1 Other afterburn indicators can be observed in the regen- erator by looking at dense, dilute, and cyclone tempera- tures. Typically, these temperatures should trend with one another. Dense phase temperatures increase together, dilute phase temperatures increase together and so on, and the delta between temperatures in similar axial locations should trend together. If one or more of the temperature indicators (TIs) start to divert, this can be a sign of uneven distribution or damage. Regenerated e-cat should be uni- form in colour; if there are very dark-coloured particles mixed with very white particles (salt and pepper), this is an
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Catalysis 2023
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