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FCC Combustion Promoter Technology Enhancement
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Stoichiometric Catalytic Decomposition of Nitric Oxide over Cu-ZSM-5 Catalyst Removal of nitric oxide from the exhaust stream of various combustion sources has become increasingly important. Since, except at very high temperatures, nitric oxide is thermodynamically unstable with respect to oxygen and nitrogen, direct catalytic decomposition of nitric oxide should be possible. In a paper with the above title, Li and Hall (Yuejin Li and Keith Hall, J. Phys. Chem., 94(~990)6148~ have discussed an investigation of the decomposition of nitric oxide over Cu*+-exchanged ZSM-5 catalysts. At a contact time of 2 s and a temperature of 773 K, a 166% Cu*‘-exchanged ZSM-5 catalyst (Cu-ZSM-~-26-166~ converted over 90% of the nitric oxide contained (4%) in a helium stream. (Here 166% exchange means that more copper is exchanged into the zeolite than the base-exchange capacity of the zeolite.) At the beginning of the reaction, the products were nitrogen, oxygen and nitrous oxide. After 30 min the reaction reached steady state, wherein the nitrous oxide formation had dropped to undetectable amounts: however, nitrogen dioxide formation, which was negligible at the beginning, had increased. With a more active catalyst (Cu-ZSM-512-t40), 100% conversion of nitric oxide was achieved at 773 K and no nitrogen dioxide formation Occurred. From this and other experiments, it was concluded that nitrogen dioxide formation occurs homogeneously in the post-catalytic reactor lines at ordinary temperatures, The reaction 2 NO + 02-+2 NUn is thermodynamiGa~ly favoured by low temperatures and when the reaction is carried out
applied catalysis -
at temperatures above 773 K, the degree of nitrogen dioxide formation becomes limited. If the nitric oxide decomposition reaction is complete, nitrogen dioxide formation can be avoided completely. Otherwise, the secondary reaction (in the cold part of the system) is kinetically controlled and will remain low if the concentration of nitric oxide is low (e.g. ppm levels). In a practical application, nitrogen dioxide formation can be minimized by maximizing nitric oxide conversion and by a careful reactor design such that the mixture containing unconve~ed nitric oxide is quickly released to the atmosphere. The effects of the addition of oxygen and water vapour to the feed for the decomposition reaction were tested. Addition of 20% oxygen led to mild inhibition of the decomposition of nitric oxide. After elimination of oxygen in the feed, the conversion was restored to the original level in 30 min. Adding 2% water vapour also reduced the conversion. The recovery of the conversion took longer than with oxygen. One possible way to compensate for the decrease of the conversion due to oxygen and water might be to raise the reaction temperature. E.P.W. DE BEER
FCC Combustion Promoter Technology Enhancement
The Davison Chemical Division of W.R. Grace & Co. has announced the commercialization of its new maximum retention combustion promoter base, which will enable refiners to reduce promoter additions by up to 50%. Combustion promoters, typically platinum on alumina base, are used as additives to fluid cracking catalysts to promote complete combustion of
Volume 68 No. 1-2 - 22 January 1991
N5
carbon monoxide during catalyst regeneration. The enhanced promoter base has now been incorporated into all three Davison promoters: CP-3*, CP-5 and CPA. Since FCC catalysts and additives are always most active in their fresh state, the greater the percentage of the fresh catalyst retained when first introduced to the cat cracker, the better the activity maintenance. The retention of fresh catalyst is related to its attrition resistance. Davison Index (III) is one measure of attrition resistance. Due to environmental considerations, catalyst vendors have made significant strides over the past IO to 15 years, improving cracking catalyst attrition resistance. Today, essentially all FCC catalysts are produced with a DI of 10 or below compared with Dl’s of 15 to 25 prior to the 1980’s. However, until this time, carbon monoxide promoters have not kept pace with the improvements in catalyst retention, and promoter attrition indices remained in the 12-25 DI range. As a result, fresh additives losses remained excessively high. The introduction of alumina-based promoters during the 1980’s represented a significant improvement over first generation additives due to better activity maintenance versus silica-alumina based additives (reduced silica poisoning of platinum). However, alumina-based additives still did not achieve their full potential because an attrition resistant base was not available. Davison claims to have solved the retention problem by applying technology gained from extensive FCC experience to the manufacture of combustion promoter base. The CP-3, CP-5 and CPA promoters all now exhibit attrition properties comparable to the most attrition resistant
applied catalysis -
grades of cracking catalysts. Enhanced regenerator performance in commercial FCC units or reduced promoter usage for a given performance is expected in commercial units due to the use of attrition resistant combustion promoters.
Methane Coupling: 100% Selectivity to C2 Hydrocarbons
A recent short communication in Catalysis Letters (6 (1990) 255) by Pareira, Lee, Somorjai and Heinemann will send the methane couplers scuttling to their equipment to modify it, if necessary, to be able to add water to the feed-stream. The publication to which I refer shows that it is possible to obtain methane conversions of about 10% at hydrocarbon selectivities very close to 100%; this is achieved by operating at a temperature of about 600°C with methane:oxygen:water in the proportion 3:1:6.5 using a catalyst consisting of calcium, nickel and potassium oxides in the atomic ratio 2:l:O.l. A surprising feature of the results is that carbon deposition on the catalyst surface appears to be essential to good operation, there being a substantial induction period before a 100% carbon balance and 100% selectivity are achieved. The residence times used are very high in comparison to previously reported data for methane coupling. The authors reach a somewhat surprising conclusion concerning the mechanism: “Because of the lower temperatures and the fact that the reaction does not occur in the absence of a catalyst, we can rule out the gas phase reaction path”, i.e. the route accepted by many other authors. If such a conclusion can be reached, it could also have been reached by others for the many other catalyst systems which operate at temperatures below those at which gas-
Volume 68 No. l-2 - 22 January 1991
Stoichiometric Catalytic Decomposition of Nitric Oxide over Cu-ZSM-5 Catalyst Removal of nitric oxide from the exhaust stream of various combustion sources has become increasingly important. Since, except at very high temperatures, nitric oxide is thermodynamically unstable with respect to oxygen and nitrogen, direct catalytic decomposition of nitric oxide should be possible. In a paper with the above title, Li and Hall (Yuejin Li and Keith Hall, J. Phys. Chem., 94(~990)6148~ have discussed an investigation of the decomposition of nitric oxide over Cu*+-exchanged ZSM-5 catalysts. At a contact time of 2 s and a temperature of 773 K, a 166% Cu*‘-exchanged ZSM-5 catalyst (Cu-ZSM-~-26-166~ converted over 90% of the nitric oxide contained (4%) in a helium stream. (Here 166% exchange means that more copper is exchanged into the zeolite than the base-exchange capacity of the zeolite.) At the beginning of the reaction, the products were nitrogen, oxygen and nitrous oxide. After 30 min the reaction reached steady state, wherein the nitrous oxide formation had dropped to undetectable amounts: however, nitrogen dioxide formation, which was negligible at the beginning, had increased. With a more active catalyst (Cu-ZSM-512-t40), 100% conversion of nitric oxide was achieved at 773 K and no nitrogen dioxide formation Occurred. From this and other experiments, it was concluded that nitrogen dioxide formation occurs homogeneously in the post-catalytic reactor lines at ordinary temperatures, The reaction 2 NO + 02-+2 NUn is thermodynamiGa~ly favoured by low temperatures and when the reaction is carried out
applied catalysis -
at temperatures above 773 K, the degree of nitrogen dioxide formation becomes limited. If the nitric oxide decomposition reaction is complete, nitrogen dioxide formation can be avoided completely. Otherwise, the secondary reaction (in the cold part of the system) is kinetically controlled and will remain low if the concentration of nitric oxide is low (e.g. ppm levels). In a practical application, nitrogen dioxide formation can be minimized by maximizing nitric oxide conversion and by a careful reactor design such that the mixture containing unconve~ed nitric oxide is quickly released to the atmosphere. The effects of the addition of oxygen and water vapour to the feed for the decomposition reaction were tested. Addition of 20% oxygen led to mild inhibition of the decomposition of nitric oxide. After elimination of oxygen in the feed, the conversion was restored to the original level in 30 min. Adding 2% water vapour also reduced the conversion. The recovery of the conversion took longer than with oxygen. One possible way to compensate for the decrease of the conversion due to oxygen and water might be to raise the reaction temperature. E.P.W. DE BEER
FCC Combustion Promoter Technology Enhancement
The Davison Chemical Division of W.R. Grace & Co. has announced the commercialization of its new maximum retention combustion promoter base, which will enable refiners to reduce promoter additions by up to 50%. Combustion promoters, typically platinum on alumina base, are used as additives to fluid cracking catalysts to promote complete combustion of
Volume 68 No. 1-2 - 22 January 1991
N5
carbon monoxide during catalyst regeneration. The enhanced promoter base has now been incorporated into all three Davison promoters: CP-3*, CP-5 and CPA. Since FCC catalysts and additives are always most active in their fresh state, the greater the percentage of the fresh catalyst retained when first introduced to the cat cracker, the better the activity maintenance. The retention of fresh catalyst is related to its attrition resistance. Davison Index (III) is one measure of attrition resistance. Due to environmental considerations, catalyst vendors have made significant strides over the past IO to 15 years, improving cracking catalyst attrition resistance. Today, essentially all FCC catalysts are produced with a DI of 10 or below compared with Dl’s of 15 to 25 prior to the 1980’s. However, until this time, carbon monoxide promoters have not kept pace with the improvements in catalyst retention, and promoter attrition indices remained in the 12-25 DI range. As a result, fresh additives losses remained excessively high. The introduction of alumina-based promoters during the 1980’s represented a significant improvement over first generation additives due to better activity maintenance versus silica-alumina based additives (reduced silica poisoning of platinum). However, alumina-based additives still did not achieve their full potential because an attrition resistant base was not available. Davison claims to have solved the retention problem by applying technology gained from extensive FCC experience to the manufacture of combustion promoter base. The CP-3, CP-5 and CPA promoters all now exhibit attrition properties comparable to the most attrition resistant
applied catalysis -
grades of cracking catalysts. Enhanced regenerator performance in commercial FCC units or reduced promoter usage for a given performance is expected in commercial units due to the use of attrition resistant combustion promoters.
Methane Coupling: 100% Selectivity to C2 Hydrocarbons
A recent short communication in Catalysis Letters (6 (1990) 255) by Pareira, Lee, Somorjai and Heinemann will send the methane couplers scuttling to their equipment to modify it, if necessary, to be able to add water to the feed-stream. The publication to which I refer shows that it is possible to obtain methane conversions of about 10% at hydrocarbon selectivities very close to 100%; this is achieved by operating at a temperature of about 600°C with methane:oxygen:water in the proportion 3:1:6.5 using a catalyst consisting of calcium, nickel and potassium oxides in the atomic ratio 2:l:O.l. A surprising feature of the results is that carbon deposition on the catalyst surface appears to be essential to good operation, there being a substantial induction period before a 100% carbon balance and 100% selectivity are achieved. The residence times used are very high in comparison to previously reported data for methane coupling. The authors reach a somewhat surprising conclusion concerning the mechanism: “Because of the lower temperatures and the fact that the reaction does not occur in the absence of a catalyst, we can rule out the gas phase reaction path”, i.e. the route accepted by many other authors. If such a conclusion can be reached, it could also have been reached by others for the many other catalyst systems which operate at temperatures below those at which gas-
Volume 68 No. l-2 - 22 January 1991