I.FLUID CATALYTIC CRACKING: www,wissenschaftler-avh.in
B.CATALYST/ADDITIVES(Contd.):
Q-53
How Heavy Metals contaminate the FCC Catalyst?
A-53
Heavy Metal contamination on FCC Catalyst
All cat cracker feedstock contain heavy metal contaminantsin the ppm range, the most common metals being nickel (Ni),vanadium (V), iron (Fe) and copper (Cu), all of which promote dehydrogenation and condensation reactions.
Extensive investigation of their relative activity for coke and hydrogen production indicate that nickel is four times as active as vanadium (4Ni + V) in producing hydrogen and coke[1] as studied by Arco back in the 1970s. Iron was not considered significant since this metal is usually present as “tramp” iron and is not catalytically active.
A distinction must be made between tramp Fe and Fe deposited on the cracking catalyst. Tramp Fe is composed of Fe particles in the catalyst stream that originate from erosion of pipes, vessels and other hardware. To the extent that these particles do not break up in very fine particles that can attach
themselves to the cracking catalyst, they have little effect on catalyst activity and selectivity. However, they could affect CO oxidation and SOx emissions. Iron deposited on the catalyst is in most cases the result of organic, colloidal or other finely dispersed Fe in feed. It has been recently recognized that this latter form of Fe is an important factor causing FCC catalyst deactivation. Most often, loss of activity and bottoms
cracking has been observed. Decreases in average bed density (ABD) have also been reported. In general, the more finely dispersed the depositing Fe is, the more effective it is in causing catalyst deactivation. Fe compounds present in crude oil, and as minute impurities in cracking catalyst, have some dehydrogenation activity but at orders of magnitude less than Ni or V.
Grace Davison generally uses a catalyst Ni equivalent index of Ni + Cu + V/4, which assumes that Cu and Ni are equal in dehydrogenation activity. It should be noted that an NPRA paper dating back to 1979 by Ashland /UOP has indicated the Ni or V equivalent factors may not be accurate in assessing the relative contribution of metals, especially at high concentrations, because of a non-linear response of the ontaminants. It should be stressed that the previously noted Ni equivalent indices do not necessarily reflect the relative
effects on catalyst activity, but rather only the effects on coke and gas make.
Gas Production & Compressor Limits
Both pilot plant and commercial data have shown a number of undesirable yield shift changes occurring as metals accumulate on the catalyst. The most obvious result of metals poisoning is a sharp increase in hydrogen production. Although the wt% increase is not usually significant, the volume of gas can
rise dramatically and limit compressor capacity A common means of tracking H2 make is via the H2/C1,
or H2/(C1 + C2) molar ratio. With this method the ratio remains fairly constant with conversion as long as reactor temperature and feedstock remain constant, so changes should reflect differences in metal activity. A relatively metals-free commercial operation may operate with a 0.2-0.3 H2/CH4 ratio, whereas resid operations encounter ratios well above 1.0. Levels up to 3.0 have been reported to Grace Davison.
At typical FCC catalyst addition rates, less than one-third of the total metals on commercial equilibrium catalyst are active dehydrogenation catalysts themselves. It should be understood that the activity of metals deposited on the catalyst in a commercial FCCU is much less than an equal amount of artificially deposited metals due to the passivation effect brought about by the continuous reaction regeneration cycles
encountered in the commercial unit Concurrent with the increased hydrogen, metals increase contaminant coke yield. The metals that catalyze H2 formation also catalyze condensation/polymerization reactions,which form coke. .
B.CATALYST/ADDITIVES(Contd.):
Q-53
How Heavy Metals contaminate the FCC Catalyst?
A-53
Heavy Metal contamination on FCC Catalyst
All cat cracker feedstock contain heavy metal contaminantsin the ppm range, the most common metals being nickel (Ni),vanadium (V), iron (Fe) and copper (Cu), all of which promote dehydrogenation and condensation reactions.
Extensive investigation of their relative activity for coke and hydrogen production indicate that nickel is four times as active as vanadium (4Ni + V) in producing hydrogen and coke[1] as studied by Arco back in the 1970s. Iron was not considered significant since this metal is usually present as “tramp” iron and is not catalytically active.
A distinction must be made between tramp Fe and Fe deposited on the cracking catalyst. Tramp Fe is composed of Fe particles in the catalyst stream that originate from erosion of pipes, vessels and other hardware. To the extent that these particles do not break up in very fine particles that can attach
themselves to the cracking catalyst, they have little effect on catalyst activity and selectivity. However, they could affect CO oxidation and SOx emissions. Iron deposited on the catalyst is in most cases the result of organic, colloidal or other finely dispersed Fe in feed. It has been recently recognized that this latter form of Fe is an important factor causing FCC catalyst deactivation. Most often, loss of activity and bottoms
cracking has been observed. Decreases in average bed density (ABD) have also been reported. In general, the more finely dispersed the depositing Fe is, the more effective it is in causing catalyst deactivation. Fe compounds present in crude oil, and as minute impurities in cracking catalyst, have some dehydrogenation activity but at orders of magnitude less than Ni or V.
Grace Davison generally uses a catalyst Ni equivalent index of Ni + Cu + V/4, which assumes that Cu and Ni are equal in dehydrogenation activity. It should be noted that an NPRA paper dating back to 1979 by Ashland /UOP has indicated the Ni or V equivalent factors may not be accurate in assessing the relative contribution of metals, especially at high concentrations, because of a non-linear response of the ontaminants. It should be stressed that the previously noted Ni equivalent indices do not necessarily reflect the relative
effects on catalyst activity, but rather only the effects on coke and gas make.
Gas Production & Compressor Limits
Both pilot plant and commercial data have shown a number of undesirable yield shift changes occurring as metals accumulate on the catalyst. The most obvious result of metals poisoning is a sharp increase in hydrogen production. Although the wt% increase is not usually significant, the volume of gas can
rise dramatically and limit compressor capacity A common means of tracking H2 make is via the H2/C1,
or H2/(C1 + C2) molar ratio. With this method the ratio remains fairly constant with conversion as long as reactor temperature and feedstock remain constant, so changes should reflect differences in metal activity. A relatively metals-free commercial operation may operate with a 0.2-0.3 H2/CH4 ratio, whereas resid operations encounter ratios well above 1.0. Levels up to 3.0 have been reported to Grace Davison.
At typical FCC catalyst addition rates, less than one-third of the total metals on commercial equilibrium catalyst are active dehydrogenation catalysts themselves. It should be understood that the activity of metals deposited on the catalyst in a commercial FCCU is much less than an equal amount of artificially deposited metals due to the passivation effect brought about by the continuous reaction regeneration cycles
encountered in the commercial unit Concurrent with the increased hydrogen, metals increase contaminant coke yield. The metals that catalyze H2 formation also catalyze condensation/polymerization reactions,which form coke. .
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