P – P = K
P
P S
P
ρ B
H B
∆P
MC
H SP
Feed nozzles
ρ SP
P
∆P SV
H
RB
ρ
P
P – P =
= P – P H + SP H B ρ SP ρ B
R
Gas plant ∆P = P – Riser ∆P = P – P
∆P SV
P S
P = Riser ∆P + P + H RB H + SP H – Riser ∆P – H B RB ρ SP ρ B
ρ
RB
= P – P = K +
∆P SV
ρ
RB
Figure 3 Underflow standpipe
standpipe above the valve; times the standpipe density. The pressure below the valve is the reactor pressure; plus the riser pressure drop; plus the length of the riser from the regenerated catalyst inlet to the elevation of the feed noz- zles; times the catalyst density in that section of the riser. This relationship is shown by the following:
the regenerated catalyst slide valve depending on the regenerator limitations. Catalyst circulation limitation Any cat cracker that is at a catalyst circulation limit is losing money. This does not happen often because the incentive to fix the problem is very large. It should be noted that when it does occur, the reactor temperature cannot be raised, the feed preheat dropped, nor can more feed be added to the unit. Higher cat activity might help if the regenerator tem- perature is not too high. Conversion may be limited, and the opportunity to produce more light olefins could be lost. The reason for the limitation needs to be identified. Is it a low slide valve differential, is the slide valve wide open, or is something restricting catalyst flow? If the slide valve is wide open, it might be possible to raise the feed preheat temperature or increase the delta coke if the regenerator is not at its temperature limit. The most common obstruction limiting catalyst flow is a lump of coke entering the spent catalyst standpipe. Opening the spent catalyst slide valve to 100% open may allow the coke to pass into the regener- ator. To prevent this from happening, a grid can be installed on the spent catalyst standpipe entrance, or a stripper packing that has limited-size openings can be used. Several formulas are monitored to characterise the fluid - isation, such as the ratio of the minimum bubbling velocity
ΔP SV = P3 – P4 = K + H SP ρ SP + H B ρ B – Riser ΔP – H RB ρ RB
To increase the pressure drop (ΔP) across the valve, K can be increased, where K is the difference between the regen - erator and reactor pressures. The regenerator pressure is higher than the reactor in Figure 3, but a pressure balance can be made assuming the opposite is true. The higher regenerator pressure would require more horsepower (HP) for the air blower to be used and/or a higher blower rpm. Raising the bed level in the regenerator is another step in the right direction. Adding more lift gas at the bottom of the feed riser would lower the density in the section below the feed nozzles and could help. If it is possible to lower the suction pressure of the wet gas compressor (WGC), the reactor pressure would be lower, and the differential between the process vessels could be increased. Any increase in pressure drop in the reactor overhead line, the main column, or the overhead condensing equip - ment would also increase reactor pressure. It could affect
18
Revamps 2023
www.digitalrefining.com
Powered by FlippingBook