Light or prime olefins, defined herein as ethylene and propylene, serve as feeds for the production of numerous chemicals. Olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Alternative feedstocks for the production of light olefins are oxygenates, such as alcohols, particularly methanol, dimethyl ether, and ethanol. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
The catalysts used to promote the conversion of oxygenates to olefins are molecular sieve catalysts. Because ethylene and propylene are the most sought after products of such a reaction, research has focused on what catalysts are most selective to ethylene and/or propylene, and on methods for increasing the life and selectivity of the catalysts to ethylene and/or propylene.
The conversion of oxygenates to olefins (OTO), particularly the conversion of methanol to olefins (MTO), in a hydrocarbon conversion apparatus generates and deposits carbonaceous material (coke) on the molecular sieve catalysts used to catalyze the conversion process. Excessive accumulation of these carbonaceous deposits will interfere with the catalyst's ability to promote the reaction. In order to avoid unwanted build-up of coke on molecular sieve catalysts, the OTO and MTO processes incorporate a second step comprising catalyst regeneration. During regeneration, the coke is at least partially removed from the catalyst by combustion with oxygen, which restores the catalytic activity of the catalyst and forms a regenerated catalyst. The regenerated catalyst then may be reused to catalyze the conversion of methanol to olefins.
The combustion of the carbonaceous deposits from molecular sieve catalyst compositions during catalyst regeneration is an exothermic process. The exothermic nature of catalyst regeneration presents a problem in OTO regeneration systems because the amount of coke formed on the molecular sieve catalyst compositions utilized in OTO reaction systems preferably is maintained at higher levels in order to maintain a high prime olefin (ethylene and propylene) selectivity. As a result, the amount of heat liberated from the OTO molecular sieve catalyst compositions during catalyst regeneration is significantly great.
The tremendous amount of heat liberated during the regeneration of heavily coked catalyst particles, such as coked OTO catalyst particles, may exceed the metal tolerances of the metals used to form the catalyst regenerator, particularly of the separation vessels, e.g., cyclone separators, contained therein as well as the conduits used to transport regenerated catalyst back to the hydrocarbon conversion apparatus. The creation of localized “hot spots” in catalyst regenerators also poses a significant problem in that catalyst is not regenerated uniformly throughout the regeneration zone. The heat also can damage and/or deactivate the catalyst particles themselves.
U.S. Publication No. 2006-0135348 A1, filed Mar. 2, 2005, discloses a regeneration system and a process for regenerating catalyst. The invention relates to efficiently regenerating catalyst particles by minimizing the formation of localized “hot spots” and “cold spots” in a regeneration zone. The document discloses that the invention includes mixing spent catalyst from a reactor and cold catalyst from a catalyst cooler and directing the mixed catalyst to the regeneration zone in a fluidized manner with a fluidizing medium. In the regeneration zone, the mixed catalyst contacts an oxygen-containing regeneration medium under conditions effective to regenerate the spent catalyst contained therein.
U.S. Patent Application Publication No. 2006/0149108 discloses a process of converting oxygenates to olefins wherein a catalyst cooler is attached to the reactor to cool the spent catalyst before it is reintroduced to fresh oxygenate feedstream. The publication discloses withdrawing all of the spent catalyst that enters the catalyst cooler from the bottom of the catalyst cooler. The publication also disclosures a catalyst cooler with a plurality of cooling tubes, a fluidizing gas distributor and a catalyst inlet above the catalyst outlet, the outlet being located at the bottom of the catalyst cooler.
It would also be desirable to reduce the catalyst hold-up in this OTO system. This would result in a reduction in the total amount of expensive catalyst required in the system as well as reducing the time in which the catalyst is exposed to deactivating conditions in the regenerator.
Improved processes are sought for regenerating highly coked catalyst particles, such as coked catalyst particles derived from OTO reaction systems, while maintaining desirable temperature characteristics in the OTO catalyst regenerator through improved mixing of the catalyst and higher catalyst circulation rates between the regenerator and the catalyst cooler. In particular there is a need to reduce/minimize the amount of catalyst in the OTO reaction system, e.g., by decreasing the amount of catalyst that is held up in the system.