PTQ Q4 2023 Issue

0.04 0.05

Basis 60,000 b / d 5 wt % coke Full burn (14 lb air/lb coke) Catalyst circulation 35 - 45 T/M

0.03

0.02

V = Eective supercial vapour velocity, fps e P = Particle density, lb/ft p P = Gas density, lb/ft p e = Entertainment, lb cat/ft vapour

Supercial velocity

20.6 46.7 Entrainment

0.01

2.5 3.5

3 4

10 5 6 7 8 9

e/P g

losses. The recovered catalyst from the downstream collec- tion equipment subtracted from the regenerator losses will give the amount leaving with the flue gas. This could corre - late with the stack opacity. If a representative sample of flue gas is taken at a point just downstream of the regenerator, the particle size distribution and the number of solids would also give an independent measure of the regenerator losses. Measuring regenerator cyclone efficiencies requires an estimate of the catalyst entrainment rate. The entrainment rate is shown in Figure 1 as a function of the gas density, the particle density and the effective gas velocity. The effective velocity equals the superficial gas velocity when the bottom of the primary cyclone is located above the transport disen- gaging height (TDH), and the air is well dispersed through the bed. In Figure 2 , the entrainment rates for a 60,000 b/d catalytic cracker are shown for various superficial velocities. A velocity of 2.5 ft/sec gives an entrainment rate about half the cata- lyst circulation rate, while at 3.3-3.5 ft/sec the two are about equal. Above this rate, the entrainment rate becomes very large. The TDH also increases with velocity. Even a 0.1-0.2 ft/sec increase may overload the cyclones when operating at 3.5 ft/sec. If increased losses are occurring, there are fundamen- tal questions that need to be answered. These are listed in Table 1 . The most serious of these is item 4, when the losses are high and have suddenly occurred. Causes of failure The causes of a sudden increase in catalyst losses can be the mechanical failure of a cyclone due to corrosion/oxidation, Figure 1 Measuring regenerator cyclone efficiencies using an estimate of the catalyst entrainment rate

Supercial velocity

Figure 2 The entrainment rates for a 60,000 b/d catalytic cracker for various superficial velocities

high-temperature stress/creep, and/or the high differential expansion of the metal components. Weld or metal failure can be caused by cooling sprays impinging on the cyclone system. Cyclones have been known to fall to the bottom of the regenerator, resulting in losses of up to 50 t/d. Water sprays are not usually included in modern designs due to the damage they can cause. Locations of the areas of the cyclone system that are sub- ject to the most problems are shown in Figure 3 . Holes in cyclones are not uncommon, and they can greatly reduce catalyst collection efficiency. Bypassing cyclones can hap - pen due to a hole in the plenum chamber or a weld crack. Full particle-size material will be lost; if this occurs, it will only worsen with time. If this is suspected, the cyclones need to be pressure checked to locate the crack, which contracts when the unit is cooled. Cyclone erosion has been greatly reduced with better refractories and operating within strict design parameters, as shown in Table 2 . The losses will noticeably increase if a cyclone dipleg gets plugged with either coke or refractory. A primary cyclone will dump into the secondary at a rate that will flood the cyclone and increase losses. A blocked secondary cyclone usually results in a loss of about a ton or two of catalyst/ day. This is manageable and might be fixed online. Bridging of the catalyst in the dipleg can also cause catalyst carry- over and is more common in the secondaries because the traffic is lower and the diplegs are much smaller. This is

Catalyst losses – questions to be answered

Weld cracks

Cross-over duct erosion

u When did the losses start? v Are they from the reactor, regenerator, or both? w Is the catalyst getting finer or coarser? x Magnitude of the losses? y Other events that may suggest a cause? z Are the losses increasing? { What was repaired at the last TAR and status?

Vortex termination erosion

Erosion catalyst bearing indrawn vapour

Table 1

Figure 3 Areas of the cyclone system subject to problems

PTQ Q4 2023