Revamps 2023 Issue

Resolving low slide valve differentials and catalyst circulation problems

Troubleshooting catalyst circulation problems that lead to unscheduled shutdowns and reduced income

Warren Letzsch Warren Letzsch Consulting PC

F luid catalytic cracking, or ‘cat cracking’, was the first and is the most important circulating fluid bed pro - cess with more than 16 million B/D of capacity. In larger cat crackers, more than 100 tons per minute of cata- lyst pass through the unit. Smooth, safe flow of the catalyst is essential to the operation. Low slide valve delta Ps (ΔPs) and ragged catalyst circulation can lead to unscheduled shutdowns and reduced income. Low slide valve differentials The fluid catalytic cracker (FCC) has three slide valves (SVs) to control catalyst circulation and unit pressure balance. These are the flue gas SV, regenerated catalyst SV, and spent catalyst SV. While the spent catalyst slide valve reg - ulates catalyst flow to the regenerator, it controls the bed level in the reactor/stripper. The regenerator is the swing vessel regarding catalyst inventory changes. A flue gas slide valve controls regenerator pressure but is normally set to control the pressure difference between the reactor and regenerator. It is important to control the difference (ΔP); otherwise, catalyst flow can become erratic or a flow rever - sal could occur. The regenerated catalyst slide valve regulates hot catalyst flow to the reactor and maintains the set reactor tempera - ture. If the catalyst flow is variable, the reactor temperature will fluctuate. Bridging of the catalyst in the standpipe that interrupts catalyst flow can cause sudden large drops in

reactor temperature. If these are severe enough, the feed could be shut off due to the low-temperature safety proto - col built onto the control system. Regenerated catalyst standpipes operate as underflow standpipes. More head (typically measured in psi) is gener - ated above the slide valve to ensure a positive seal between the reactor and regenerator, and the excess pressure is burned up across the regenerated (‘regen’) catalyst slide valve. FCC units are laid out to provide a differential of 5-6 psi across the regenerated catalyst slide valve. As the pres- sure drop through the reactor system increases, the slide valve pressure drop will decline. Increases in feed rate, cat- alyst (‘cat’) circulation, and reactor temperature will cause this reduction in the slide valve ΔP. A number of older FCC units operate with a 3 psi differential across their regen cat slide valve, but this is considered the minimum safe oper - ating level. Abnormal catalyst flow through the standpipe occurs if the catalyst defluidises or if excess gas is present in the standpipe. Additional friction occurs in the standpipe when fluidisation is lost in a portion of the standpipe, causing the pressure above the slide valve to decrease. Many of the old 1942-1960 vintage FCC units were designed with long 75-140 ft standpipes when catalysts had fresh apparent bulk densities (ABDs), ranging from 0.38 to 0.50 gm/cc. These equilibrated at values of 0.60 to 0.70 gm/cc. Aeration was a necessary component of the design, and the loss of fines even prompted the inclusion of catalyst attritors to aid the catalyst circulation. Aeration is still important, however, because larger units tend to have longer standpipes. Aeration is added to the standpipe to prevent the loss of fluidisation. The gas is added at various taps located down the standpipe about 5-11 ft apart. These taps are normally rotated around the standpipe as the elevation drops to avoid having the gas go up one side. Variations in catalyst flow rates and or changes in the catalyst properties can cause defluidisation. If the standpipe has a large diam - eter, more than one tap may be used at a specific location. Internals are also being used in some units to break up any large bubbles that might develop. Aeration tap velocities need to be high enough to penetrate the standpipe, or the gas will flow up the standpipe wall. The design of the aeration system has been done using

Pressure (psia)

150

Standpipe

150

Gas source

Figure 1 Aeration piping

Revamps 2023