Revamps 2023 Issue

Bed fluidity vs catalyst entrainment

Flux values for standpipes of various sizes (lb/ft² sec)

Superficial velocity,

Fluidity

Entrainment,

Head

10%W

20%W

30%W

ft/sec

lb/ft³ gas

(ft of catalyst) 25

68 81

135 162 189 217

203 243 284 325

678 814 949

2.0 2.0 2.4 2.4

Good Poor Good Poor

0.175 0.340

36 49 64

0.30 0.54

108

1,085

Table 2

Table 1

The capacity of standpipes depends on the design of the unit. The flow through a standpipe without any restrictions is given by the equation:

to the minimum fluidisation velocity or the F factor shown here: EXP (0.508 F) F factor = –––––––––––––––––– (ABD) 0.663 (APS) 0.568 The F in the numerator is either the 0-40 or 0-45 micron catalyst content in the inventory. It is more convenient to use the 0-40 micron number since the F factor can be cal- culated from the data on an equilibrium catalyst sheet and is plotted vs time. When the F factor falls, there may be a threshold where the flow becomes erratic. Below this value, the refiner may choose to take steps to improve the situation, such as adding fines back to the unit. The catalyst should be 20-80 microns, but very fine material like precipitator fines should be avoided. Some units run successfully with no 0-40 micron catalyst. When this happens, the quality of fluidisation is reduced, and the gas bubbles tend to be larger, which may impact both the regeneration of the catalyst and the stripper operation even though there are no circulation concerns. A loss of catalyst fines can occur due to cyclone erosion, faulty catalyst loading , holes in cyclones, a plugged dipleg, high superficial velocities, which overload the cyclones, or a high bed level in the regenerator. A damaged grid can cause poor fluidity, and Table 1 shows the impact on catalyst entrainment. The higher entrainment will not be equally distributed to the cyclones and can cause localised cyclone flooding. It is important to check bed densities, cyclone temperatures, and the catalyst analyses after any upset to determine if a problem has occurred with the gas distributor.

W = 3.57 (Standpipe Density) (L) O.5

where W is the flow rate of catalyst as lb/ft² sec, the density is the flowing density of the catalyst as lb/ft³, and L is the height in feet of the catalyst above the valve. This flow is controlled by the restriction at the bottom of the standpipe, namely the slide valve. Flow is usually limited to 35% or less of the W value, and the slide valve opening should be centred. Table 2 gives the calculated fluxes for standpipes of various standpipe lengths. Most FCC units can operate with cat/oil ratios from 4 to 9 as designed. Typical cat/oils are 5-6, but this can be turned down or up as necessary to meet processing objectives. This translates to designed flux rates from 160 to 230. The catalyst velocity down the standpipe is the flux divided by the density, which translates to velocities of about 5 to 6 ft/ sec. Standpipes can operate over the range of 130 to 300. Higher velocities would require the redesign of the inlet hopper because too much gas would be carried with the catalyst, leading to poor differentials. Note that some units were designed with lower standpipe densities, and the pre- vious table would be slightly different for their cat cracker. Data for the range of flux rates in more than 35 com- mercial FCC units were given by Matsen (Exxon) and are shown in Figure 4 . These were the operating ranges and not necessarily the limits for the units monitored. Improving circulation The role of the hopper above the standpipe is to deaerate the catalyst in the regenerator bed before it enters the top of the standpipe. It is presumed the catalyst is well fluid- ised entering the hopper. If the bed level is too high or low, depending on the regenerator design, the catalyst may not flow smoothly into withdraw wells. If the air distributor gets plugged or damaged in a way that affects catalyst fluidi- sation, the poorly fluidised catalyst will avalanche into the standpipe and only be partly fluidised. It is extremely diffi- cult to refluidise the catalyst once it is in the standpipe. In one instance, the plate grid air distributor was modi- fied to get more air into the bed by enlarging the holes. The grid ΔP was reduced to 20% of the bed ΔP. The resulting poor circulation occurred because the outer portion of the distributor did not get air, and catalyst wept through holes. The standpipe was located near the regenerator wall, and there was no chance of a smooth operation.

300

200

50 60 70

100

Standpipe diameter (inches)

Figure 4 Circulation rates in catalytic crackers

Revamps 2023