Revamps 2025 Issue

Regenerator temperature profiles

Bed level

Dense

Dilute

Normal (A)

7 8

(B) (A) (B)

11 27 22

Low

33 33

Table 1

provides easy access to the internals. The pressure balance can be revisited, and changes might be made to anticipate future feeds and operating modes. Spent catalyst distribution Even distribution of the spent catalyst and air is essential for lowering afterburn and meeting the regulations for emissions of CO, NO, and SOx. Refiners with a design that transfers the spent catalyst into a single quadrant of the regenerator have temperature variations of up to 100ºF in the bed. Often, uneven cyclone temperatures are observed, and afterburn occurs in the cyclones, plenum chamber, and flue gas line. The high CO concentration above the bed where the catalyst enters and the high O₂ in the other sec - tions above the bed make the afterburn of CO inevitable. Changing the air distribution has not been successful in alleviating afterburn when all the spent catalyst is put into a single quadrant. Dispersing the catalyst over the top of the bed has eased the problem. One such distributor is shown in Figure 3 . In the regenerator, the air and catalyst move vertically, up in the middle and down at the walls, with little radial mixing. Adding multiple distribution arms has worked well, as has a single distributor across the entire regenerator.

Figure 3 Regenerator air and spent catalyst distributors

held a lot of catalyst, because the carbon steel available had a limit of 1,200ºF, which limited the coke burning rate. Bed depths of 10 to 15 ft were adequate since the superficial velocities were around 1.5-2.5 ft/sec. Side-by-side designs (reactor/regenerator) were used, and the spent catalyst line entered the regenerator from the bottom with carrier air, which pushed the catalyst through plates that contained holes. The plates were pie-shaped and sealed at the walls with a catalyst seal. If the air broke through the seal at the wall, a hot spot would form and could progress all the way around the regenerator. These were boxed in and cooled with steam until the unit was shut down for a turn-around. An even flow of the gas through the holes was main - tained by ensuring the pressure drop across the plate grid was at least 30% of the bed delta P. Lower pressure drops would allow the grid to weep nearer the walls, and the resulting catalyst flow would cause erosion of the grid due to higher velocities in the active holes. The holes have to be large enough so that they do not plug, and the plate needs to have the required thickness for mechanical considera- tions. Hole spacing needs to provide good coverage while also not allowing the coalescence of gas bubbles above the plate. Air rings could also be applied in these applications, but more than one would have to be used with larger-diameter regenerators. The nozzles used to distribute the air have two orifices. The inner orifice is sized for even flow to all the nozzles, while the outer nozzle controls the velocity into the bed. Velocities need to be high enough to provide the needed coverage of the cross-sectional area, yet not so high as to cause catalyst attrition. The number of rings depends on the maximum allowable jet penetration from the rings. Pipe grids (Figure 3) are also used in these smaller designs,

Even distribution of the spent catalyst and air is essential for lowering afterburn and meeting the regulations for emissions of CO, NO, and SOx

Counter-current catalyst and air flow have proven to be effective in reducing catalyst deactivation and controlling NOx and CO concentrations. Control of excess O₂ in the flue gas is better, which improves burning efficiency. Air distribution The air distribution system is an area that might improve operations and profitability. Hardware used in this service includes flat plates with holes, pipe grids, and air rings. Each design had to overcome issues that caused operating problems, high maintenance costs, uneven air distribution, and short run lengths. The size of the FCCU dictated the type of distributor. Large-capacity units had large-diameter regenerators that

Revamps 2025