Low yields of LPG including C= and C=
C =
LCO C–C
ZSM-5 Zeolite
C =
Y zeolite metal traps, optimised unit cell size + matrix
Feedstock Vacuum gas oil or Resid (C –C)
Naphtha C –C (includes C =, C = andC=)
Higher octane
Coke
Bottoms
Catalyst technology Preserve activity in presence of metals, minimise hydrogen transfer, minimise dry gas
ZSM-5 technology
Figure 1 Roles of Y-zeolite-based catalyst and ZSM-5 technology in the generation of propylene and butylene
ZSM-5 technology is key to increasing propylene yield from the FCC. This shape- selective zeolite cracks gasoline-range molecules predominantly into LPG olefins (Dwyer & Degnan, 1993). In general, to maximise propylene and FCC gasoline RON, the preferred base catalyst generates and preserves the maximum yield of gasoline-range olefins so these can be further cracked by the ZSM-5 technology into propylene. The concept is illustrated in Figure 1 , showing the catalytic approach to maximise LPG olefins from the FCC. The use of ZSM-5 has two key advantages over operational parameters to increase the yield of propylene from the FCC: It has no effects on the coke yield in the FCC, so it has a negligible effect on process CO 2 emissions It has an outsized impact on propylene yields relative to changing operational parameters. This last point is exemplified by the data in Figure 2 . For this comparison between a base catalyst with and without a high-activity ZSM-5 additive, the propylene yield is observed to increase ~2 wt% on a fresh feed basis (ff) when the riser temperature is increased by ~20°C, whereas the use of only 2 wt% ZSM-5 additive increases the C 3 = yield by nearly this amount at a constant temperature. Generally, operational shifts that reduce CO₂ emissions from the FCC unit, accomplishing the objective of reduced Scope 1 CO₂ emissions, also reduce the production of LPG olefins from the FCC unit. To regain the lost LPG olefins
from these operating moves, ZSM-5 additive rates could be increased until the desired LPG production rates are met. Figure 3 depicts a commercial example where the adaptability of the FCC was exploited to reduce the riser outlet temperature (ROT) while recovering the valuable C 3 = yield using ZSM-5 technology (Brandt, 2010). As shown in Figure 3, the ROT in this commercial example was reduced by ~25ºC, which resulted in a decrease in Scope 1 emissions of approximately 35,000 MT of CO₂/year while maintaining the same propylene yield. This example highlights the flexibility of the FCC unit to achieve different yield and CO₂ emission profiles.
12
10
8
6
4
2
0
65
70
75
80
85
Conversion, wt%
B ase B ase
521˚C
B ase 543˚C B ase + 2% ZSM-5 521˚C B ase + 2% ZSM-5 566˚C
566˚C B ase + 2% ZSM-5 543˚C
Figure 2 Davison Circulating Riser (DCR) data comparing propylene yields between a base catalyst (open symbols) and the same base catalyst + 2 wt% of a high-activity ZSM-5 additive (filled symbols) at various temperatures between 521°C and 565°C
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