Combustion, spent catalyst regeneration

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. 

As discussed in Chapter 1, a portion of the feed is converted to coke in the reactor. This coke is carried into the regenerator with the spent catalyst. The combustion of the coke produces H2O, CO, CO, SO2, and traces of NOx. To determine coke yield, the amount of dry air to the regenerator and the analysis of flue gas are needed. It is essential to have an accurate analysis of the flue gas. The hydrogen content of coke relates to the amount of hydrocarbon vapors carried over with the spent catalyst into the regenerator, and is an indication of the rcactor-stripper performance. Example 5-1 shows a step-by-step cal culation of the coke yield. 

If a reliable spent catalyst temperature is not available, the stripper is included in the heat balance envelope (II) as shown in Figure 5-4. The combustion of coke in the regenerator satisfies the following heat requirements ... 

It is important that combustion of the coke in the spent catalyst occur in the dense bed of catalyst. Without the catalyst bed to absorb this heat of combustion, the dilute phase and flue gas temperatures increase rapidly. This phenomenon is called afterburning. It is critical that spent catalyst and combustion/lift air are being introduced into the regenerator as evenly as possible across the catalyst bed. It is also important to note that vertical mixing is much faster than lateral mixing. 

Other more capital-intensive modifications include installing a dedicated air blower for the spent catalyst riser. The spent catalyst riser often requires a higher back-pressure than the main air blower to deliver the catalyst into the regenerator. Therefore, less total combustion air would be available if one common blower is used to transfer spent catalyst and provide combustion air to the air distributors. 

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. 

FCC units, and in particular the catalyst regenerating section, may give rise to significant pollution. Sulfur in the coke oxidizes to SO2 and SO3, while the combustion also generates NOx compounds. In addition, the flue gas from the regenerator contains particulate matter from the catalyst. The FCC process is also the major source of sulfur in gasoline. Of all the sulfur in the feed, approximately 50% ends up as H2S in the light gas-LPG fraction, 43% in the liquid products and 7% in the coke on the spent catalysts. 

The types of reactions involving fluids and solids include combustion of solid fuel, coal gasification and liquefaction, calcination in a lime kiln, ore processing, iron production in a blast furnace, and regeneration of spent catalysts. Some examples are given in Sections 8.6.5 and 9.1.1. 

The above work concentrated most of its attention on the use of zinc chloride as the molten halide and on the use of bituminous coal extract as feed to the process. Hydrocracking of the extract (1) and regeneration by a fluidized-bed combustion technique of the spent catalyst melt (2) from the process were both demonstrated in continuous bench-scale units. 

The catalyst/oil disengaging system is designed to separate the catalyst from the reaction products and then rapidly remove the reaction products from the reactor vessel. Spent catalyst from the reaction zone is first steam stripped, to remove adsorbed hydrocarbon, and then routed to the regenerator. In the regenerator all of the carbonaceous deposits are removed from the catalyst by combustion, restoring the catalyst to an active state with a very low carbon content. The catalyst is then returned to the bottom of the reactor riser at a controlled rate to achieve the desired conversion and selectivity to the primary products. 

The FFB regenerator shown in Fig. 7a is used as the basic regeneration reactor. Coke-laden spent catalyst and recirculated regenerated catalyst enter the bottom of the FFB vessel and are fluidized by the combustion air in the operating regime of fast fluidization. The near regenerated catalyst flows cocurrently with the flue gas out from the top of the FFB vessel. After gas/solid separation, the catalyst falls into the bottom of a disengager situated on top of the FFB vessel and maintained in a mildly fluidized state by air... 

The major drawback of these Pd/MgO catalysts is, however, related to the practical impossibility of regenerating spent catalysts by conventional O2 treatments at high temperatures. Under these oxidative conditions, sublimation of PdCU takes place from the reaction between metallic Pd and CI2 evolved from combustion of hydrochlorocarbons. As the consequence, extensive leaching of palladium results, and the catalytic activity is lost. 

---