Process For Reducing The Quantity Of Carbon Dioxide Produced In A Fluid Catalytic Cracking Regeneration Off Gas

Abstract

The present invention relates to a process for the reduction of CO2 emissions from the flue gas of a cracking catalyst regenerator that is part of a fluidized catalytic cracking system which cracks petroleum feedstocks such as petroleum distillates of residual or crude oil which, when catalytically cracked, provide either a gasoline or a gas oil product. This process may also be utilized with regard to the cracking of synthetic feeds having boiling points of from 400° F. to about 1000 as exemplified by oils derived from coal or shale oil. By reducing the CO2 emissions in the regeneration step of catalytic cracking, the further goal of maximizing the production of CO in the flue gas is achieved, the CO being further utilized as a fuel in the refinery or further processed to produce hydrogen.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/350,715, filed Jun. 2, 2010, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a process for reducing the quantity of carbon dioxide emissions during the fluidized catalytic cracking of hydrocarbons and catalyst regeneration. The present invention further relates to a process for reducing carbon dioxide (hereinafter “CO2”) emissions in the off gas of a cracking catalyst regenerator during catalyst regeneration. The present invention further relates to a process for reducing the quantity of CO2 in a fluid catalytic cracking regeneration off gas that is derived from the combusting of coke on spent catalyst in a fluid catalytic cracking regeneration zone of a fluidized catalytic cracking unit.

BACKGROUND

A fluid catalytic cracker (FCC) is a refinery processing unit designed to make gasoline and other refined products from a petroleum distillate feedstock. Catalytic cracking of petroleum distillates in a FCC unit allows for the molecular weight of relatively high molecular weight hydrocarbons to be reduced to lower molecular weight hydrocarbons which have a broader range of use. The cracking is performed in a catalytic cracking reactor in the presence of catalyst particles. In the process of converting the feedstock to products, a portion of the feedstock (on the order of approximately 5 weight percent) is converted to coke and remains on the catalyst. Due to the costs associated with replacing the catalyst, it is desirable to regenerate the catalyst for further use. Since the cracking catalyst is fluidized, this allows for the circulation of the catalyst within the FCC reactor system, from the FCC reactor to the regeneration zone and back. The catalyst is separated from the reactor products, stripped, and sent to a regenerator in the regeneration zone for the regeneration. In a regenerator, the coke is burned off of the catalyst and a regenerated catalyst is returned (hot) to the FCC reactor. Typically, the coke is burned to reduce CO2 using air. In some cases, oxygen is used to supplement the air to increase conversion or throughput.

In recent years, there has an increasing concern throughout the world regarding air pollution from industrial emissions of noxious oxides of nitrogen, sulfur and carbon. In response to these concerns, various government agencies have sought to place limits on allowable emissions of one or more of these pollutants by placing stringent regulations in place to govern these emissions. The operators of FCC units have faced this concern particularly with regard to the emissions associated with the regenerator off gas of a FCC unit. The actual cracking process results in the depositing of coke on the catalyst which makes the catalyst non-functional. As a result, the catalyst is regenerated so that it can be further used. The coke is removed by burning it off of the catalyst in a regenerator. The burning of the coke on the cracking catalyst results in an off gas that includes a variety of species that are considered pollutants such as NOX, CO, CO2, and SOX.

In view of the recent push to limit the release of CO2 due to pollution concerns, there is a need to provide a process for the regeneration of the fluid catalytic cracker catalyst without producing an excess amount of CO2. It is the object of the present invention to meet this need.

SUMMARY OF THE INVENTION

The present invention is concerned with the reduction of CO2 emissions from the flue gas of a cracking catalyst regenerator that is part of a fluidized catalytic cracking system which cracks petroleum feedstocks such as petroleum distillates of residual or crude oil which, when catalytically cracked, provide either a gasoline or a gas oil product. In addition, the present invention may be utilized with regard to the cracking of synthetic feeds having boiling points of from 400° F. to about 1000° F. as exemplified by oils derived from coal or shale oil. By reducing the CO2 emissions in the regeneration step of catalytic cracking, the further goal of maximizing the production of CO in the flue gas is achieved, the CO being further utilized as a fuel in the refinery or further processed to produce hydrogen.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates the process of the present invention for reducing the quantity of CO2 produced in fluid catalytic cracking regeneration off gas derived from the combusting of coke on catalyst in the regeneration zone of the fluid catalytic cracking unit.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a process for reducing the quantity of CO2 (hereinafter “CO2”) produced in a fluid catalytic cracking (hereinafter “FCC) regeneration off gas that is derived from the combusting of coke on catalyst in a FCC regeneration zone 2 of a FCC unit 1. Catalytic cracking of petroleum distillates in a FCC unit 1 allows for the molecular weight of relatively high molecular weight hydrocarbons to be reduced to lower molecular weight hydrocarbons.

Catalytic cracking, as well as FCC units 1 for catalytic cracking, are well known in the art. Accordingly, the present invention is not limited by the actual structure of the FCC unit 1. Typically these types of units include the hydrocarbon cracking zone which include a catalyst known to assist in hydrocarbon cracking and a catalyst regeneration zone 3 which includes a reactor 4 where the spent catalyst is regenerated in a regenerator 5. As used herein, when reference is made to the fact that a catalyst is “spent” this refers to be an activation or low activity of the catalyst utilized in the FCC cracking zone 3 is inactive or whose reactivity is low due to the deposit of coke on the surface of the catalyst.

While any type of catalyst known in the art for use in FCC units 1 may be utilized, catalysts that are particularly suited for the present process of catalytic cracking include any variety of catalyst that are known to include, but are not limited to, siliceous inorganic oxides, such as silica, alumina, or silicon-containing cracking catalyst including crystalline, aluminosilicate zeolite associated with a porous refractory oxide matrix, such as a clay or the like. Zeolites suitable for these types of systems include an X zeolite or a Y zeolite having a very low sodium content. Cracking catalyst can also comprise a silica-alumina mixture. Note that the catalyst utilized in the FCC units 1 are in a fluidized state. As used herein, the phrase “fluidized state” refers to the state of the catalyst. More specifically, it refers to the instance when the catalyst particles are placed under the conditions which cause them to behave as if they were a fluid (in a solid/fluid mixture when present in the reactor 4/regenerator 5 vessel).

In addition to the hydrocarbon cracking zone 3, catalyst regeneration zone 2, and catalyst, the FCC unit 1 also includes a means to transfer the spent catalyst to the regenerator 6 and the regenerated catalyst back to the reactor 7.

The cracking conditions employed during the conversion of the petroleum distillates may be any conditions that are known in the art for such conversions. For example, the cracking conditions will typically function at a temperature from about 600° F. to about 1000° F., at a catalyst-to-hydrocarbon weight ratio of from about 3 to about 10, and a weight hourly space velocity of from about 5 to about 50 per hour for the hydrocarbon conversion. Note that while these conditions are included, they are simply included by way of example and are not meant in any way to limit the present invention. As a result of the hydrocarbons involved as well as the high temperatures and involved, the byproduct of the cracking of the hydrocarbons is coke. The coke that is formed as a result of the hydrocracking winds up being deposited on the surface of the catalyst. Since the coke is deposited on the surface of the catalyst, the reactive sites of the catalyst are blocked thereby resulting in the inactivation and/or poisoning of the catalyst. While the amount of coke deposited on the surface of the cracking catalyst can vary, it will typically be from about 0.5% w to about 15% w depending on the composition of the feedstock. In many instances it will be possible to regenerate the catalyst for further use.

With regard to the present invention, the cracking catalyst utilized in the hydrocarbon cracking zone is regenerated in the catalyst regeneration zone 2 by utilizing a process in which the coked or spent catalyst is combusted in the presence of a minimum amount of pure oxygen or relatively pure oxygen in order to produce an off gas from the regeneration reaction which contains a minimum amount of CO2 while at the same time maximizing the production of carbon monoxide (hereinafter “CO”) in the off gas. This off gas, which in the past would typically be released into the atmosphere, can now because of the high amount of CO be used for a variety of different uses within associated processes.

In the standard cracking procedure, after the hydrocarbons are cracked, the hydrocarbon products are separated from the catalyst. As noted above, as a result of the cracking coke is deposited on the surface of the catalyst. Over a period of time, the activity of the catalyst begins to be diminished due to the amount of coke that is present on the surface of the catalyst. Once the catalyst begins to reach a level of activity which is considered to be inefficient (efficiency being determined by the particular FCC structure being utilized as well as process conditions and product recovery), this spent catalyst is circulated to the catalyst regeneration zone 2 where in the prior art the catalyst would undergo the regeneration in a regenerator 5 in the presence of air, sometimes supplemented with oxygen. This regeneration typically takes place at a temperature which ranges from about 500° C. to about 800° C., preferably from about 600° C. to about 650° C. at a pressure that ranges from about 1 psig to about 100 psig, preferably from about 20 psig to about 40 psig. Note that with regard to the prior art processes of regeneration of catalyst from a FCC unit 1, the result was an off gas from the regeneration zone which included a large amount of CO2. In the present process, the objective is to remove a substantial portion or all of the coke from the catalyst (regenerate the catalyst) while at the same time minimizing the production of CO2 and maximizing the production of CO. In order to maximize the quantity of CO present in the off gas from the regeneration zone 2 while at the same time minimizing the quantity of CO2 present, the amount of oxygen added to the regeneration zone is the minimum quantity required to produce a CO quantity in the off gas that is greater than 35 volume percent during combustion of the coke. Furthermore, as used herein, the phrase “maximizing the production of CO”, refers to the production of an off gas in which the amount of CO found in the final off gas is greater than 35 volume %, preferably greater than 40 volume %, typically from about 35 volume % to about 65 volume %. As used herein with regard to the present invention, the phrase “minimizing the production of CO2” refers to the production of an off gas in which the amount of CO2 found in the final off gas is less than 50 volume %, typically from about 0 volume % to about 20 volume %.

As used herein, the phrases “maximizing the production of CO” and “minimizing the production of CO2” means the least quantity or amount by weight of oxygen that is necessary to carry out the regeneration process with the shift of the reaction being towards the production of CO rather than CO2. In other words, the amount of oxygen utilizes will drive the reaction towards the production of CO rather than CO2 as there will not be sufficient oxygen present to drive the reaction to the production of CO2. This quantify of oxygen results in all of the oxygen being consumed in the reaction thereby leaving very little oxygen to further react to produce CO2.

In order to achieve these levels of CO and CO2, pure oxygen or relatively pure oxygen is supplied to the catalyst regeneration zone during the regeneration step of the process. As used herein, the phrase “pure oxygen” refers to an oxygen stream which includes 95 volume % oxygen to 100.0 volume % oxygen. Furthermore, as used herein, the phrase “relatively pure oxygen” or “substantially pure oxygen” refers to and oxygen stream that contains from 23 volume % oxygen to 99.9 volume % oxygen.

More specifically, the oxygen is supplied in a substoichiometric relationship to the amount of coke on the catalyst which is to undergo regeneration. This oxygen is added to the regeneration zone by any suitable means such as, but not limited to, a sparging device in the bottom of the regeneration zone. The pure or relatively pure oxygen is introduced into the combustion zone in an amount from about 40% to about 60% of the stoichiometric amount required to convert carbon into CO2 and introduced independently of the hydrocarbon cracking catalyst. More specifically, the amount of oxygen introduced into the system is from about 40 to about 60% of the stoichiometric amount required to convert carbon into CO2, preferably from about 45 to about 55%.

When the carbon on the surface of the catalyst is burned, it will produce a syngas that comprises CO in an amount greater than 35 volume % of the syngas total, a minimum amount of CO2, and resultant H2 and some H2O from the conversion of the coke to CO. Any sulfur oxides produced would then be scrubbed from the syngas. By carrying out the process of the present invention, it is possible to produce an off gas which can be further utilized within the present process or with regard to another industrial process rather than burning the resulting off gas in the FCC regenerator and/or venting the resulting off gas. More specifically, in one embodiment of the present invention, the off gas stream may be burned as a fuel in any number of industrial processes. In another embodiment, the off gas stream can be subjected to a water gas shift reaction to produce a stream that is high in hydrogen. The water gas shift reaction would typically include compressing the off gas stream, combining the compressed off gas stream with steam and then contacting the combined off gas/steam stream with a catalyst to produce a water gas shift effluent that is high in hydrogen. This hydrogen rich water gas shift effluent stream may then be further purified in a pressure swing absorption unit such as those known in the art to produce a high purity hydrogen stream (99.9% purity or higher) and a pressure swing absorption tail gas that contains the remaining hydrogen and methane as well as some additional components depending on the makeup of the original feedstream. This pressure swing absorption tail gas may then be burned to make steam and a CO2 stream that may be utilized in any number of manners with regard to a variety of industrial processes.

In a still a further embodiment, CO2 can be adsorbed in an amine contactor to produce a pure CO2 stream and a hydrogen stream with from 95 to 99% hydrogen purity. The pure CO2 could then be recycled to the catalyst regeneration unit of the FCC to control temperature in the regeneration unit.

In yet another embodiment of the present invention, the syngas can be compressed and introduced into a steam reformer (hydrogen plant for example) downstream of the main reforming furnace to join the syngas from the steam reformer. It would then be fed along with syngas from the steam reformer to the water gas shift converter where the CO is shifted to hydrogen. The hydrogen would be co-produced from both syngas streams and purified in the steam reformer (either by CO2 removal or by pressure swing adsorption).

Still further, the syngas can be compressed, combined with steam and contacted with a catalyst to produce hydrogen via the water gas shift reaction. CO2 could be adsorbed in an amine contactor to produce a pure CO2 stream and a hydrogen stream with from 95 to 99% hydrogen purity. A portion of the syngas is bypassed around the shift reactor and contacted with a second catalyst to form methane and water. The methanation reaction is exothermic and a significant quantity of steam can also be produced from the methanation product. The syngas can be compressed, combined with steam and contacted with catalyst to produce hydrogen via the water gas shift reaction. CO2 could be adsorbed in an amine contactor to produce a pure CO2 stream and a hydrogen stream with from 95 to 99% hydrogen purity. A portion of the syngas is bypassed around the shift reactor contacted with a second catalyst to form longer chain molecules via the Fischer Tropsch reaction. The product from the Fischer Tropsh reaction is further fractionated to produce diesel, vacuum gas oil and naphtha fractions which can be produced as final products or fed to other process units in the refinery, such as hydrocracker to further refine the vacuum gas oil and reformers and isomerization units to upgrade the naphtha into gasoline. Other products of the Syngas are possible such as methanol, ethanol, etc.

The FCC is typically one of the larger producers of CO2 in a refinery. The advantage of the present invention is that CO2 emissions would be severely reduced if not completely eliminated from the FCC. By introducing pure oxygen or substantially pure oxygen instead of air, the heat balance of the FCC is maintained and a useful byproduct (syngas) can be produced from the FCC and CO2 production reduced or eliminated. (The amount of CO2 reduction will be determined by the control of the regeneration reaction to limit CO2 production and maximize CO production. Using the present invention, it is envisioned that the typical syngas quality will be as follows: H2O—9%, H2—33%, CO—44%, CO2—12%, COS—0%, and CH4—2%.

Typically, hydrogen is produced in a refinery by reaction of steam and methane to produce hydrogen and CO2. Net refinery CO2 production can be reduced by reducing hydrogen production (and related CO2 emissions) from the reformer and substituting hydrogen production from FCC regenerator off gas.

Claims

1. A process for reducing the quantity of CO2 produced in a fluid catalytic cracking regeneration off gas that is derived from the combusting of coke on catalyst in a fluid catalytic cracking regeneration zone of a fluidized catalytic cracking unit, said process comprising carrying out the combustion of the coke on catalyst under regeneration conditions using a pure or relatively pure oxygen gas that is introduced in an amount sufficient to produce an off gas stream that contains at least 35 volume % of carbon monoxide.

2. The process of claim 1, wherein the fluidized catalytic cracking unit comprises a hydrocarbon cracking zone and a catalyst regeneration zone and the catalyst utilized in the hydrocarbon cracking zone, once contacted with hydrocarbon in the cracking zone where a lower molecular hydrocarbon is produce and some coke deposited on the catalyst is circulated to the catalyst regeneration zone where the catalyst undergoes regeneration before being recycled back to the hydrocarbon cracking zone for further use.

3. The process of claim 1, wherein the regeneration conditions comprise a temperature from about 500° C. to about 800° C. and a pressure from about 1 psig to about 100 psig

4. The process of claim 1, wherein the purity of oxygen introduced to the combustion zone is from about volume 23% to about 99.9 volume %.

5. The process of claim 4, wherein the amount of oxygen introduced is from about 80 volume % to about 99.5 volume %.

6. The process of claim 4, wherein the carbon monoxide content in the off gas stream produced as a result of regeneration of the spent catalyst is at least 40 volume %.

7. The process of claim 4, wherein the carbon monoxide content in the off gas stream produced as a result of regeneration of the spent catalyst is from 35 volume % to 65 volume %.

8. The process of claim 1, wherein the off gas stream that contains at least 35 volume % of carbon monoxide is burned as fuel.

9. The process of claim 1, wherein the off gas stream that contains at least 35 volume % of carbon monoxide is subjected to a water gas shift reaction that includes compressing the off gas stream, combining the compressed off gas stream with steam and then contacting the combined stream with a catalyst to produce water gas shift effluent that is high in hydrogen.

10. The process of claim 9, wherein the hydrogen in the water gas shift effluent is further purified in a pressure swing adsorption unit to produce a high purity hydrogen stream and a pressure swing adsorption tail gas.

11. The process of claim 10, wherein the pressure swing adsorption tail gas is burned to make steam and CO2.

12. The process of claim 9, wherein the water gas shift effluent is subjected to an amine contactor to produce a pure CO2 stream and a high purity H2 stream, the pure CO2 stream being recycled to the fluid catalytic cracking regenerator to control the temperature in the regenerator.

13. A process of minimizing carbon dioxide from a combustion off gas stream from a fluid catalytic cracking unit comprising a FCC reactor for converting a stream of petroleum feedstock in the presence of a fluidized bed of cracking catalyst into a stream of cracked product and a FCC regenerator for combusting coke buildup on the catalyst to thereby regenerate the catalyst and provide the combustion off gas stream, comprising the steps of combusting the coke buildup deposited on the spent catalyst in the FCC regenerator by contacting the spent catalyst with an amount of pure or relatively pure oxygen gas under regeneration conditions in order to minimize the conversion of carbon monoxide to carbon dioxide and thus form an off gas stream that contains at least 35 volume % of carbon monoxide wherein the amount of pure or relative pure oxygen ranges from about 40% to about 80% of the stoichiometric amount required to convert carbon into carbon dioxide.

14. The process of claim 13, wherein the regeneration conditions comprise a temperature from about 500° C. to about 800° C. and a pressure from about 1 psig to about 99 psig.

15. The process of claim 13, wherein the amount of oxygen introduced is from about 40 to 60%.

16. The process of claim 15, wherein the amount of oxygen introduced is from about 45% to about 55_%.

17. The process of claim 16, wherein the carbon monoxide content in the off gas stream produced as a result of regeneration of the spent catalyst is at least 40 volume %.

18. The process of claim 16, wherein the carbon monoxide content in the off gas stream produced as a result of regeneration of the spent catalyst is from 35 volume % to 65 volume %.

19. A process for cracking hydrocarbons in a cracking system employing a catalytic cracker, a catalyst regenerator having a dilute phase and an inventory of circulating particulate solids including cracking catalyst, wherein said hydrocarbons are cracked in contact with said cracking catalyst and coke is formed on said cracking catalyst at cracking conditions in said catalytic cracker and wherein said regeneration is carried out under regeneration conditions in the presence of pure or relatively pure oxygen to regenerate thus forming an off gas that contains at least 35.0% by volume of carbon monoxide.

20. The process of claim 19, wherein the fluidized catalytic cracking unit comprises a hydrocarbon cracking zone and a catalyst regeneration zone and the catalyst utilized in the hydrocarbon cracking zone, once spent, is circulated to the fluid catalytic cracking regeneration zone where the spent catalyst undergoes regeneration before being recycled back to the hydrocarbon cracking zone for further use.

21. The process of claim 19, wherein the regeneration conditions comprise a temperature from about 500° C. to about 800° C. and a pressure from about 1 psig to about 99 psig

22. The process of claim 19, wherein the amount of oxygen introduced is from about 40% to about 60% of the stoichiometric amount required to convert carbon into carbon dioxide.

23. The process of claim 22, wherein the amount of oxygen introduced is from about 45 to about 55%.

24. The process of claim 22, wherein the carbon monoxide content in the off gas stream produced as a result of regeneration of the spent catalyst is at least 40 volume %.

25. The process of claim 22, wherein the carbon monoxide content in the off gas stream produced as a result of regeneration of the spent catalyst is from 40 volume % to 65 volume %.

26. The process of claim 19, wherein said cracking catalyst comprises a silica-alumina catalyst or a zeolite-containing catalyst.

27. The process of claim 26, wherein said zeolite-containing catalyst is selected from the group consisting of an X zeolite and a Y zeolite.

28. A process for reducing CO2 emissions during the cracking of hydrocarbons and catalyst regeneration comprising a cracking catalyst that is recycled between a hydrocarbon cracking zone and a catalyst regeneration zone operated at a temperature of from 1050° F. to about 1300° F. wherein the regeneration off-gas is formed by burning coke off said catalyst, in the presence of pure or relatively pure oxygen gas, at regeneration conditions, in said regeneration zone wherein during the coke-burning the pure or relatively pure oxygen is injected in an amount sufficient to minimize the conversion of carbon monoxide to carbon dioxide and thus form an off gas stream that contains at least 35 volume % of carbon monoxide.

29. The process of claim 28, wherein the amount of pure or relative pure oxygen ranges from about 40% to about 60% of the stoichiometric amount required to convert carbon into carbon dioxide.

30. The process of claim 28, wherein said catalyst inventory circulates between said hydrocarbon cracking and said regeneration zone and wherein said catalyst inventory is stripped, in the presence of steam, to remove hydrocarbons from said catalyst inventory.

31. A process for regenerating a hydrocarbon cracking catalyst having coke deposits thereon in a FCC unit comprising a hydrocarbon cracking zone and a catalyst regeneration zone comprising the steps:

(a) contacting the catalyst having coke deposits thereon with pure oxygen or relatively pure oxygen in the hydrocarbon cracking zone at an average temperature in the range of about 500° C. to about 800° C., the pure oxygen or relatively pure oxygen introduced into the catalyst regeneration zone in an amount from about 40% to about 60% of the stoichiometric amount required to convert carbon into carbon dioxide and introduced independently of the introduction of the spent catalyst into the catalyst regeneration zone, thereby combusting at least part of the coke on the catalyst and effectively minimizing the conversion of at least part of the carbon monoxide formed as a result of the combusting of the coke to carbon dioxide to produce an off gas having greater than 35 volume % of carbon monoxide.

32. The process of claim 31, wherein said catalyst is in the fluidized state.

33. The process of claim 32, wherein said catalyst is cycled between said catalyst regeneration zone and the hydrocarbon cracking zone.

34. The process of claim 33, wherein said catalyst is selected from the group consisting of type X zeolites, type Y zeolites and mixtures of these.

35. The process of claim 1, wherein at least part of the carbon monoxide in said combustion gas is converted to carbon dioxide by water-gas shift reaction.

36. The process of claim 1, wherein said oxygen-inert gas mixture entering said combustion zone comprises oxygen and a gas selected from nitrogen, carbon dioxide or mixtures of these.