532
The Chemistry and Technology of Petroleum
which were not stripped) occurs with the liberation of heat. Regenerator temperatures are typically 705°C–760°C (1300°F–1400°F). Heat exchangers and the circulating catalyst capture the heat
evolved during regeneration to be used in preheating the reactor feed to appropriate cracking temperatures (usually in the range, 495°C–550°C (925°F–1020°F).
During operations, the entire catalyst inventory is continually circulated through the unit.
Catalyst residence time in the riser reactor section is typically 1–3 s (with current trends to even
shorter residence times), and the entire reactor–stripper–regenerator cycle is less than 10 min.
To achieve cycle times of this order, catalyst circulation rates as high as 1 ton/s in large units are
required. To withstand such movement, the catalyst must be sufficiently robust to withstand the
operational stress.
Process temperatures are high, coke is repeatedly deposited and burned off, and the catalyst particles are moving at high speed through steel reactors and pipes. Contact between the catalyst particles
and the metal walls and interparticle contact is unavoidable. Thus, catalyst loss from the unit caused
by poor attrition resistance can be a serious problem, since the quantities lost must be replaced by fresh
catalyst additions to maintain constant unit performance. Catalyst manufacturers work hard to prevent
inordinate losses due to attrition, and refineries keep a close watch on catalyst quality to be sure the
produce conforms to their specifications. Therefore, the robustness of the catalyst is carefully monitored and controlled to a high attrition resistance that is determined by rigorous test methods that
place a semi-quantitative evaluation on attrition resistance, which is generally related to breakdown
with time in commercial units.
As described earlier, in some units, cracking does not always take place in the reactor and reaction often occurs in the vertical or upward sloped pipe called the riser (giving credence to the
name riser reactor and riser pipe cracking) forming products, including coke (e.g., Bartholic, 1989).
Preheated feedstock is sprayed into the base of the riser via feed nozzles where it contacts extremely
hot fluidized catalyst at 1230°F–1400°F (665°C–760°C). The hot catalyst vaporizes the feed and
catalyzes the cracking reactions that break down the high-molecular-weight oil into lighter components including LPG, constituents, gasoline, and diesel. The catalyst–hydrocarbon mixture flows
upward through the riser for just a few seconds and then the mixture is separated via cyclones.
The catalyst-free hydrocarbons are routed to a main fractionator for separation into fuel gas, propane and butanes, gasoline, light cycle oils used in diesel and jet fuel, and heavy fuel oil.
19.6.2 CokIng
The formation of coke deposits has been observed in virtually every unit in operation, and the
deposits can be very thick with thicknesses up to 4 ft have been reported (McPherson, 1984). Coke
has been observed to form where condensation of hydrocarbon vapors occurs. The reactor walls
and plenum offer a colder surface where hydrocarbons can condense. Higher boiling constituents in
the feedstock may be very close to their dew point, and they will readily condense and form coke
nucleation sites on even slightly cooler surfaces.
Unvaporized feed droplets readily collect to form coke precursors on any available surface, since
the high boiling feedstock constituents do not vaporize at the mixing zone of the riser. Thus, it is
not surprising that residuum processing makes this problem even worse. Low residence time cracking also contributes to coke deposits, since there is less time for heat to transfer to feed droplets and
vaporize them. This is an observation in line with the increase in coking when short contact time
riser crackers (q.v.) were replacing the longer residence time fluid bed reactors.
Higher boiling feedstocks that have high aromaticity result in higher yields of coke. Furthermore,
polynuclear aromatics and aromatics-containing heteroatoms (i.e., nitrogen, oxygen, and sulfur) are
more facile coke makers than simpler aromatics (Speight, 1987). However, feed quality alone is
not a foolproof method of predicting where coking will occur. However, it is known that feedstock
hydrotreaters rarely have coking problems. The hydrotreating step mitigates the effect of the coke
formers and coke formation is diminished.
The Chemistry and Technology of Petroleum
which were not stripped) occurs with the liberation of heat. Regenerator temperatures are typically 705°C–760°C (1300°F–1400°F). Heat exchangers and the circulating catalyst capture the heat
evolved during regeneration to be used in preheating the reactor feed to appropriate cracking temperatures (usually in the range, 495°C–550°C (925°F–1020°F).
During operations, the entire catalyst inventory is continually circulated through the unit.
Catalyst residence time in the riser reactor section is typically 1–3 s (with current trends to even
shorter residence times), and the entire reactor–stripper–regenerator cycle is less than 10 min.
To achieve cycle times of this order, catalyst circulation rates as high as 1 ton/s in large units are
required. To withstand such movement, the catalyst must be sufficiently robust to withstand the
operational stress.
Process temperatures are high, coke is repeatedly deposited and burned off, and the catalyst particles are moving at high speed through steel reactors and pipes. Contact between the catalyst particles
and the metal walls and interparticle contact is unavoidable. Thus, catalyst loss from the unit caused
by poor attrition resistance can be a serious problem, since the quantities lost must be replaced by fresh
catalyst additions to maintain constant unit performance. Catalyst manufacturers work hard to prevent
inordinate losses due to attrition, and refineries keep a close watch on catalyst quality to be sure the
produce conforms to their specifications. Therefore, the robustness of the catalyst is carefully monitored and controlled to a high attrition resistance that is determined by rigorous test methods that
place a semi-quantitative evaluation on attrition resistance, which is generally related to breakdown
with time in commercial units.
As described earlier, in some units, cracking does not always take place in the reactor and reaction often occurs in the vertical or upward sloped pipe called the riser (giving credence to the
name riser reactor and riser pipe cracking) forming products, including coke (e.g., Bartholic, 1989).
Preheated feedstock is sprayed into the base of the riser via feed nozzles where it contacts extremely
hot fluidized catalyst at 1230°F–1400°F (665°C–760°C). The hot catalyst vaporizes the feed and
catalyzes the cracking reactions that break down the high-molecular-weight oil into lighter components including LPG, constituents, gasoline, and diesel. The catalyst–hydrocarbon mixture flows
upward through the riser for just a few seconds and then the mixture is separated via cyclones.
The catalyst-free hydrocarbons are routed to a main fractionator for separation into fuel gas, propane and butanes, gasoline, light cycle oils used in diesel and jet fuel, and heavy fuel oil.
19.6.2 CokIng
The formation of coke deposits has been observed in virtually every unit in operation, and the
deposits can be very thick with thicknesses up to 4 ft have been reported (McPherson, 1984). Coke
has been observed to form where condensation of hydrocarbon vapors occurs. The reactor walls
and plenum offer a colder surface where hydrocarbons can condense. Higher boiling constituents in
the feedstock may be very close to their dew point, and they will readily condense and form coke
nucleation sites on even slightly cooler surfaces.
Unvaporized feed droplets readily collect to form coke precursors on any available surface, since
the high boiling feedstock constituents do not vaporize at the mixing zone of the riser. Thus, it is
not surprising that residuum processing makes this problem even worse. Low residence time cracking also contributes to coke deposits, since there is less time for heat to transfer to feed droplets and
vaporize them. This is an observation in line with the increase in coking when short contact time
riser crackers (q.v.) were replacing the longer residence time fluid bed reactors.
Higher boiling feedstocks that have high aromaticity result in higher yields of coke. Furthermore,
polynuclear aromatics and aromatics-containing heteroatoms (i.e., nitrogen, oxygen, and sulfur) are
more facile coke makers than simpler aromatics (Speight, 1987). However, feed quality alone is
not a foolproof method of predicting where coking will occur. However, it is known that feedstock
hydrotreaters rarely have coking problems. The hydrotreating step mitigates the effect of the coke
formers and coke formation is diminished.