530
The Chemistry and Technology of Petroleum
(Gerber et al., 1999). Nickel, vanadium, iron, and copper compounds contained in catalytic cracking feedstocks are deposited on the catalyst during the cracking operation, thereby adversely
affecting both catalyst activity and selectivity. Increased catalyst metal contents affect catalytic
cracking yields by increasing coke formation, decreasing gasoline and butane and butylene production, and increasing hydrogen production.
The recent commercial development and adoption of cracking catalyst-treating processes definitely improve the overall catalytic cracking process economics.
19.5.2.1 Demet
A cracking catalyst is subjected to two pretreatment steps. The first step effects vanadium removal;
the second effects nickel removal, to prepare the metals on the catalyst for chemical conversion to
compounds (chemical treatment step) that can be readily removed through water washing (catalyst
wash step). The treatment steps include use of a sulfurous compound followed by chlorination with
an anhydrous chlorinating agent (e.g., chlorine gas) and washing with an aqueous solution of a chelating agent (e.g., citric acid, HO 2 CCH 2 C(OH)(CO 2 H)CH 2 CO 2 H, 2-hydroxy-1,2,3-propanetricarboxylic acid). The catalyst is then dried and further treated before returning to the cracking unit.
19.5.2.2 Met-X
This process consists of cooling, mixing, and ion-exchange separation, filtration, and resin regeneration. Moist catalyst from the filter is dispersed in oil and returned to the cracking reactor in a
slurry. On a continuous basis, the catalyst from a cracking unit is cooled and then transported to
a stirred reactor and mixed with an ion-exchange resin (introduced as slurry). The catalyst-resin
slurry then flows to an elutriator for separation. The catalyst slurry is taken overhead to a filter, and
the wet filter cake is slurried with oil and pumped into the catalytic cracked feed system. The resin
leaves the bottom of the elutriator and is regenerated before returning to the reactor.
19.6 PROCESS PARAMETERS
Catalytic cracking is endothermic and, that being the case, heat is absorbed by the reactions and
the temperature of reaction mixture declines as the reactions proceed and a source of heat for the
process is required. This heat comes from combustion of coke formed in the process. Coke is one
of the important, though undesirable, products of cracking, since it forms on the surface and in the
pores of the catalyst during the cracking process, covering active sites and deactivating the catalysts. During regeneration, this coke is burned off the catalyst to restore catalytic activity and, like
all combustion processes, the process is exothermic, liberating heat.
Most fluid catalytic cracking units are operated to maximize conversion to gasoline and LPG.
This is particularly true when building gasoline inventory for peak season demand or reducing
clarified oil yield due to low market demand. Maximum conversion of a specific feedstock is usually
limited by both FCCU design constraints (i.e., regenerator temperature, wet gas capacity, etc.) and
the processing objectives. However, within these limitations, the FCCU operator has many operating and catalyst property variables to select from to achieve maximum conversion.
Conversion usually refers to the mount of fresh feedstock cracked to gasoline and lighter products and coke:
Conversion
L CO HCO CO
=
-
+
+
100 (
)
where
LCO is light cycle oil
HCO is heavy cycle oil
CO is clarified oil
All are expressed as percentage of the fresh feedstock.
The Chemistry and Technology of Petroleum
(Gerber et al., 1999). Nickel, vanadium, iron, and copper compounds contained in catalytic cracking feedstocks are deposited on the catalyst during the cracking operation, thereby adversely
affecting both catalyst activity and selectivity. Increased catalyst metal contents affect catalytic
cracking yields by increasing coke formation, decreasing gasoline and butane and butylene production, and increasing hydrogen production.
The recent commercial development and adoption of cracking catalyst-treating processes definitely improve the overall catalytic cracking process economics.
19.5.2.1 Demet
A cracking catalyst is subjected to two pretreatment steps. The first step effects vanadium removal;
the second effects nickel removal, to prepare the metals on the catalyst for chemical conversion to
compounds (chemical treatment step) that can be readily removed through water washing (catalyst
wash step). The treatment steps include use of a sulfurous compound followed by chlorination with
an anhydrous chlorinating agent (e.g., chlorine gas) and washing with an aqueous solution of a chelating agent (e.g., citric acid, HO 2 CCH 2 C(OH)(CO 2 H)CH 2 CO 2 H, 2-hydroxy-1,2,3-propanetricarboxylic acid). The catalyst is then dried and further treated before returning to the cracking unit.
19.5.2.2 Met-X
This process consists of cooling, mixing, and ion-exchange separation, filtration, and resin regeneration. Moist catalyst from the filter is dispersed in oil and returned to the cracking reactor in a
slurry. On a continuous basis, the catalyst from a cracking unit is cooled and then transported to
a stirred reactor and mixed with an ion-exchange resin (introduced as slurry). The catalyst-resin
slurry then flows to an elutriator for separation. The catalyst slurry is taken overhead to a filter, and
the wet filter cake is slurried with oil and pumped into the catalytic cracked feed system. The resin
leaves the bottom of the elutriator and is regenerated before returning to the reactor.
19.6 PROCESS PARAMETERS
Catalytic cracking is endothermic and, that being the case, heat is absorbed by the reactions and
the temperature of reaction mixture declines as the reactions proceed and a source of heat for the
process is required. This heat comes from combustion of coke formed in the process. Coke is one
of the important, though undesirable, products of cracking, since it forms on the surface and in the
pores of the catalyst during the cracking process, covering active sites and deactivating the catalysts. During regeneration, this coke is burned off the catalyst to restore catalytic activity and, like
all combustion processes, the process is exothermic, liberating heat.
Most fluid catalytic cracking units are operated to maximize conversion to gasoline and LPG.
This is particularly true when building gasoline inventory for peak season demand or reducing
clarified oil yield due to low market demand. Maximum conversion of a specific feedstock is usually
limited by both FCCU design constraints (i.e., regenerator temperature, wet gas capacity, etc.) and
the processing objectives. However, within these limitations, the FCCU operator has many operating and catalyst property variables to select from to achieve maximum conversion.
Conversion usually refers to the mount of fresh feedstock cracked to gasoline and lighter products and coke:
Conversion
L CO HCO CO
=
-
+
+
100 (
)
where
LCO is light cycle oil
HCO is heavy cycle oil
CO is clarified oil
All are expressed as percentage of the fresh feedstock.