Resolving low slide valve differentials and catalyst circulation problems
Troubleshooting catalyst circulation problems that lead to unscheduled shutdowns and reduced income.
Warren Letzsch Consulting PC
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Fluid catalytic cracking, or ‘cat cracking’, was the first and is the most important circulating fluid bed process with more than 16 million B/D of capacity. In larger cat crackers, more than 100 tons per minute of catalyst 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 regulates 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 reversal could occur.
The regenerated catalyst slide valve regulates hot catalyst flow to the reactor and maintains the set reactor temperature. 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 protocol built onto the control system.
Regenerated catalyst standpipes operate as underflow standpipes. More head (typically measured in psi) is generated 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 pressure drop through the reactor system increases, the slide valve pressure drop will decline. Increases in feed rate, catalyst (‘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 operating 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 diameter, 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 one of two methods. The first is a high-pressure gas source with a control valve feeding the fluidisation taps, which have upstream restriction orifices regulating the flow evenly to the taps. As shown in Figure 1, this system relies on sonic velocities through the orifices to regulate the aeration. If the supply gas pressure drops below the minimum requirement for this system, the flow to the individual taps will become uneven. Plugged taps can also cause problems. The second method uses individual flow indicators and controllers that allow each tap to operate independently of the others. This costs more but gives greater control of the unit.
Either plant air or superheated steam can be used in the unit for aeration. These streams must be dry. Any water entering the unit will flash, causing pressure surges and irregular catalyst flow. Wet steam going anywhere into the unit can cause pressure surges and possibly damage the metallurgy and refractory.
A single gauge pressure survey can locate where the defluidisation is occurring in the standpipe. Flow rate adjustments to the taps should be based on the expected direction of the gas being injected to improve flow. Figure 2 shows the desired pressure profile down the standpipe and the result of defluidisation.
If catalyst circulation changes significantly, the catalyst may be over-aerated. Small bubbles may coalesce, forming larger bubbles, reducing the density (ρ) of the catalyst in the standpipe. If the bubbles get large enough, they may partially block catalyst flow. The rapid build-up and breaking of these bubbles may cause rapid slide valve (and temperature) fluctuations.
Additional start-up and turnaround concerns
At design conditions, the catalyst typically circulates at 5-6 ft/sec down the standpipes, while the bubbles want to rise between 1-3 ft/sec, depending on their size. While a 6-inch bubble is the reported maximum stable bubble size for cracking catalysts, it is possible that bubbles can reach the size of the standpipe if the bubbles become stagnant. This type of behaviour occurs when the feed rate is low during start-ups or turndowns.
If the catalyst is flowing down at 3 ft/sec, it is inevitable that the coalescence of the aeration gas into large bubbles will result in unstable catalyst flow. On start-ups, the solution is to get the feed rate high enough to avoid this phenomenon. When troubleshooting a cat circulation problem, calculate the velocity of the catalyst going down the standpipe to ensure it is above the bubble rise velocities.
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