Patent Publication Number: US-7914666-B1

Title: Low NOx FCC catalyst regeneration process

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to a process for catalytic cracking of hydrocarbons. 
     DESCRIPTION OF THE PRIOR ART 
     Fluid catalytic cracking (FCC) is a catalytic conversion process for cracking heavy hydrocarbons into lighter hydrocarbons by bringing the heavy hydrocarbons into contact with a catalyst composed of finely divided particulate material. Most FCC units use zeolite-containing catalyst having high activity and selectivity. 
     The basic components of the FCC process include a riser, a reactor vessel, a catalyst stripper, and a regenerator. In the riser, a feed distributor inputs the hydrocarbon feed which contacts the catalyst and is cracked into a product stream containing lighter hydrocarbons. Catalyst and hydrocarbon feed are transported upwardly in the riser by the expansion of the lift gases that result from the vaporization of the hydrocarbons, and other fluidizing mediums, upon contact with the hot catalyst. Steam or an inert gas may be used to accelerate catalyst in a first section of the riser prior to or during introduction of the feed. Coke accumulates on the catalyst particles as a result of the cracking reaction and the catalyst is then referred to as “spent catalyst.” The reactor vessel disengages spent catalyst from product vapors. The catalyst stripper removes absorbed hydrocarbon from the surface of the catalyst. The regenerator removes the coke from the catalyst and recycles the regenerated catalyst into the riser. 
     The spent catalyst particles are regenerated before catalytically cracking more hydrocarbons. Regeneration occurs by oxidation of the carbonaceous deposits to carbon oxides and water. The spent catalyst is introduced into a fluidized bed at the base of the regenerator, and oxygen-containing combustion air is passed upwardly through the bed. After regeneration, the regenerated catalyst is returned to the riser. 
     Oxides of nitrogen (NO x ) are usually present in regenerator flue gases but should be minimized because of environmental concerns. Regulated NO x  emissions generally include nitric oxide (NO) and nitrogen dioxide (NO 2 ), but the FCC process can also produce N 2 O. In an FCC regenerator, NO x  is produced almost entirely by oxidation of nitrogen compounds originating in the FCC feedstock and accumulating in the coked catalyst. At FCC regenerator operating conditions, there is negligible NO x  production associated with oxidation of N 2  from the combustion air. Production of NO x  is undesirable because it reacts with volatile organic chemicals and sunlight to form ozone. 
     The two most common types of FCC regenerators in use today are a combustor style regenerator and a bubbling bed regenerator. Bubbling bed and combustor style regenerators may utilize a CO combustion promoter comprising platinum for accelerating the combustion of coke and CO to CO 2 . The CO promoter decreases CO emissions but increases NO x  emissions in the regenerator flue gas. 
     The combustor style regenerator has a lower vessel called a combustor that burns the nearly all the coke to CO 2  with little or no CO promoter and with low excess oxygen. The combustor is a highly backmixed fast fluidized bed. A portion of the hot regenerated catalyst from the upper regenerator is recirculated to the lower combustor to heat the incoming spent catalyst and to control the combustor density and temperature for optimum coke combustion rate. As the catalyst flue gas mixture enters the combustor riser, the velocity is further increased and the two-phase mixture exits through symmetrical downturned disengager arms into upper regenerator. The upper regenerator separates the catalyst from the flue gas with the disengager the followed by cyclones and return it to the catalyst bed which supplies hot regenerated catalyst to both the riser reactor and lower combustor. 
     A bubbling bed regenerator carries out the coke combustion in a dense fluidized bed of catalyst. Fluidizing combustion gas forms bubbles that ascend through a discernible top surface of a dense catalyst bed. Only catalyst entrained in the gas exits the reactor with the vapor. Cyclones above the dense bed to separate the catalyst entrained in the gas and return it to the catalyst bed. The superficial velocity of the fluidizing combustion air is typically less than 1.2 m/s (4 ft/s) and the density of the dense bed is typically greater than 480 kg/m 3  (30 lb/ft 3 ) depending on the characteristics of the catalyst. The mixture of catalyst and vapor is heterogeneous with pervasive vapor bypassing of catalyst. The temperature will increase in a typical bubbling bed regenerator by about 17° C. (about 30° F.) or more from the dense bed to the cyclone outlet due to combustion of CO in the dilute phase. The flue gas leaving the bed may have about 2 mol-% CO. This CO may require about 1 mol-% oxygen for combustion. Assuming the flue gas has 2 mol-% excess oxygen, there will likely be 3 mol-% oxygen at the surface of the bed and higher amounts below the surface. Excess oxygen is not desirable for low NO x  operation. 
     A regeneration process to burn off essentially all of the coke on the catalyst is called a “full burn” and requires excess oxygen, typically at amounts between about 0.5 and 4 mol-%. There is a need for an FCC process that lowers NO x  emissions while ensuring the catalyst is regenerated to be essentially free of coke. 
     SUMMARY OF THE INVENTION 
     An FCC process producing lower NO x  emissions during regeneration by using excess oxygen levels at less than or equal to about 0.5 mol-% and a plenum temperature above about 730° C. (about 1350° F.). The process may further include limiting the Pt content in the catalyst to less than or equal to about 0.5 ppm. NO x  emissions produced through this process may be below 20 ppmv. The process may also include adjusting the metal content of the feedstock for such metals as antimony (Sb), nickel (Ni), or vanadium (V). Additional variables for reducing NO x  emissions that may be used in conjunction with this process may include increasing the flue gas residence time, injecting NH 3  into the flue gas, adding or using NO x -reducing catalysts, increasing stripping of the catalyst, and increasing the catalyst zeolite to matrix ratio. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is an elevational diagram showing an FCC unit. 
         FIG. 2  is a graph showing NO x  increasing with the addition of platinum. 
         FIG. 3  is a graph showing NO conversion to N 2  increasing with increased temperature. 
     
    
    
     DETAILED DESCRIPTION 
     This invention relates generally to an improved FCC process. Specifically, this invention may relate to an FCC process with lower NO x  emissions. NO x  reacts with other chemicals in the air to produce hazardous materials for the environment. 
     The FCC process may use an FCC unit  10 , as shown in  FIG. 1 . Feedstock enters a riser  12  through a feed distributor  14 . Feedstock may be mixed with steam in the feed distributor  14  before exiting. Lift gases, which may include inert gases or steam, enters through a steam distributor  16  in the lower portion of the riser and creates a fluidized medium with the catalyst. Feedstock contacts the catalyst to produce cracked hydrocarbon products and spent catalyst. The hydrocarbon products are separated from the spent catalyst in the reactor vessel  18 . 
     In the reactor vessel  18 , the blended catalyst and reacted feed vapors enter through a riser outlet  20  and separated into a cracked product vapor stream and a collection of catalyst particles covered with substantial quantities of coke and generally referred to as spent catalyst or “coked catalyst.” Various arrangements of separators to quickly separate coked catalyst from the product stream may be utilized. In particular, a swirl arm arrangement  22 , provided at the end of the riser  12 , may further enhance initial catalyst and cracked hydrocarbon separation by imparting a tangential velocity to the exiting catalyst and cracked product vapor stream mixture. The swirl arm arrangement  22  is located in an upper portion of a separation chamber  24 , and a stripping zone  26  is situated in the lower portion. Catalyst separated by the swirl arm arrangement  22  drops down into the stripping zone  26 . 
     The cracked product comprising cracked hydrocarbons including gasoline and light olefins and some catalyst may exit the separation chamber  24  via a gas conduit  28  in communication with cyclones  30 . The cyclones  30  may remove remaining catalyst particles from the product vapor stream to reduce particle concentrations to very low levels. The product vapor stream may exit the top of the reactor vessel  18  through a product outlet  32 . Catalyst separated by the cyclones  30  returns to the reactor vessel  18  through diplegs into a dense bed  34  where catalyst will pass through chamber openings  36  and enter the stripping zone  26 . The stripping zone  26  removes adsorbed hydrocarbons from the surface of the catalyst by counter-current contact with steam over the optional baffles  38 . Steam may enter the stripping zone  26  through a line  40 . A spent catalyst conduit  42  transfers spent catalyst to a regenerator  50 . 
     As shown in  FIG. 1 , the regenerator  50  receives the spent catalyst and typically combusts the coke from the surface of the catalyst particles by contact with an oxygen-containing gas. The oxygen-containing gas enters the bottom of the regenerator  50  via a regenerator distributor  52  and passes through a dense fluidizing bed of catalyst. Flue gas consisting primarily of N 2 , H 2 O, O 2 , CO 2  and perhaps containing NO x  and CO passes upwardly from the dense bed into a dilute phase of the regenerator  50 . A primary separator, such as a tee disengager  54 , initially separates catalyst from flue gas. Regenerator cyclones  56 , or other means, remove entrained catalyst particles from the rising flue gas. Flue gas enters a plenum  58  before exiting the vessel through a plenum outlet  60 . Combustion of coke from the spent catalyst particles raises the temperatures of the catalyst. The catalyst may pass, regulated by a control valve, through a regenerator standpipe  62  which attaches to the bottom portion of riser  12 . 
     At FCC regenerator operating conditions, studies indicate there is negligible NO x  production associated with oxidation of N 2  from the combustion air. Rather, most of the NO x  produced results from the combustion of the coke on the spent catalyst during the regeneration part of the FCC process. 
     Most NO x  appears to be formed in the initial stages of spent catalyst regeneration from organic nitrogen compounds cracked or desorbed from the spent catalyst upon heating to regenerator temperature. Sampling the combustion gases at increasing elevations in a combustor style regenerator also indicates that NO x  are at their maximum during the early portion of regeneration by showing NO x  concentrations are greater in the lower and middle part of the regenerator, early in the regeneration process, than at the upper portion of the regenerator. Laboratory experiments show that preheating spent catalyst to regenerator temperature with the inert gas helium before adding helium with oxygen mixture produces less NO x , indicating that preheating without oxygen present drives off volatile, organic nitrogen compounds that are readily oxidized to NO x . Also pilot plant experiments show that increasing the temperature of spent catalyst stripper to drive off volatile organics reduces NO x  emissions. 
     Many variables affect the production of NO x . The addition of platinum-based CO combustion promoters increases NO x  emissions and may be one of the most important variables in driving NO x  production. For example, pilot plant data indicates that 1 ppm of fresh platinum in the inventory can increase NO x  production by five-fold, and 2-4 ppm fresh platinum can increase NO x  production by ten-fold. The impact of added fresh platinum seemed to level off after the 2 ppm amount. 
     Platinum, which is known to catalyse oxidation of NH 3  to oxides of nitrogen, may be oxidizing volatile nitrogen compounds, such as NH 3 , HCN and larger organic nitrogen compounds, to NO x  in high yield with low yields of elemental N 2 . Platinum may also decrease CO, afterburn, and temperature of the regenerator dilute phase, all three of which correlate with decreased NO x  production. 
     Another variable, in addition to platinum, in NO x  production is excess oxygen. Increased excess oxygen in the regenerator, has been shown to result in increased NO x  production. In a combustor regenerator typically about 98% of the total combustion air is fed to the combustor and only about 2% of the air is fed to the regenerator to maintain fluidization. The 2% air fed to the regenerator corresponds to about 0.4% excess oxygen in flue gas if none of it was consumed. Therefore, when a combustor style regenerator is operated at flue gas-excess oxygen levels below 0.5%, the combustion gases leaving the combustor are enriched in CO, HCN, and other NO x -reducing species and low in oxygen. These species are then burned at low oxygen concentrations in the upper regenerator resulting in very low NO x  emissions. 
     An additional variable is the regenerator plenum  58 , or flue gas, temperature. When operating at low platinum levels and low excess oxygen levels, temperatures increase for the regenerator dilute phase, regenerator cyclones  56 , plenum  58 , and flue gas. Historically this has been considered undesirable for cyclone life and refiners often increase excess oxygen or increase platinum promoter additions, or both, to cool the regenerator cyclones  56 . Therefore, it was unexpected to learn in the development of this process that NOx may decrease strongly with increasing regenerator dilute phase and plenum temperatures. This is counter-intuitive because “thermal” NO (NO produced by oxidation of N 2  by O 2 ) increases with combustion temperature. High combustion temperatures are known to make very high levels of thermal NO x  in CO boilers and conventional furnaces. Here, however, NO x  production may decrease with increased plenum  58  or flue gas temperature. In this situation, NO x  may decrease by about 1% per about 0.5° C. (1° F.). In general, NO x  at 0% excess oxygen decreased from 40 ppmv at about 675° C. (1250° F.) to about 20 ppmv at about 730° C. (1350° F.). This finding appears to be opposite to conventional wisdom for FCC processing. 
     The role of the transition metals nickel, vanadium and iron present in FCC feedstocks on NO x  formation appears to be complex. In an oxidizing environment, feed nickel and vanadium deposited on the catalyst increase NO x  formation. In pilot plant testing, increasing catalyst vanadium from 930 to 1540 ppm by adding organic vanadium compound to the feedstock increased NO x  emissions from 20 ppmv to about 35 ppmv at 1.5 mol-% excess oxygen. Similarly, increased NO x  levels occur with higher nickel content feedstock. For example, in pilot plant experiments a high nickel content catalyst at 8400 ppm, produced 55 ppmv NO x  at 1.5 mol-% excess oxygen. However, also in an oxidizing environment, nickel and vanadium may reduce high levels of NO. 
     For example, when 0.09 to 0.11 gm/hr of NO was added to the air feed to a pilot plant regenerator containing platinum at conditions that produced about 0.11 gm/hr of NO, only about 60 to 70% of the added NO reported to the flue gas for an effective conversion of 30-40% of the added NO. With no platinum present, all of the additional NO was reduced. From these data, it appears metals on FCC catalyst may reduce high levels of NO in oxidizing conditions (1% excess oxygen) or that NO formation from organic nitrogen compounds by these metals is suppressed by high NO levels. 
     In a reducing environment, as shown in laboratory testing, (helium+CO or helium+Coke on catalyst), nickel, vanadium, and iron on FCC catalyst can reduce NO with CO or Carbon, so it appears that these feed metals catalyze may either formation or reduction of NO x  depending upon the local concentrations of oxygen, NO x  reductants, and NO. Commercially, reducing, weakly oxidizing and highly oxidizing environments all probably exist because the large diameters may cause mixing non-uniformities. Nickel, vanadium, and iron may, on balance, catalyze net NO x  reduction in low oxygen areas of the regenerator. 
     For many years antimony has been injected into the FCC feed to suppress H 2  and coke formation catalyzed by feed nickel deposited on the catalyst. Antimony has been thought to form a mixed Ni/Sb oxide with lower dehydrogenation activity. It is generally accepted that the maximum suppression of H 2  occurs when Sb is injected at 0.5 times the feed nickel content and excess Sb provides little or no further benefit. Furthermore, excess antimony may increase NO x  emissions. Frequently, when refiners begin to inject feedstock with greater nickel content, they sometimes “base load” by injecting antimony in excess of the optimal 0.5 Sb/Ni ratio. The excess antimony can result in a 2 to 5-fold increase in NO x  emissions when the injected antimony ratio to nickel content of feed is about 2.0 and the ratio of Sb/Ni on catalyst was under 0.1. 
     Flue gas residence time increases the reduction in NO with increasing gas contact time with the catalyst. The NO decreases about 10% per second of residence time in the combustor or about 4% per second in the regenerator  50 . This is also consistent with early formation by NO x  followed by its subsequent reduction in a weakly oxidizing environment. 
     Additional variables for reducing NO x  emissions that may be used in conjunction with this process may include increasing the flue gas residence time, injecting NH 3  into the flue gas, adding or using NO x -reducing catalysts, increasing stripping of the catalyst, and increasing the catalyst zeolite to matrix ratio. Commercial data shows reductions in NO with increasing gas contact time with the catalyst. The NO decreases about 10% per second of residence time in the combustor or about 4% per second in the larger regenerator vessel. NH 3  injection into the flue gas decreases NO x  1% per 1 ppm of NH 3  injection, consistent with 20%-40% conversion of NH 3  by reaction with NO x , assuming a 1:1 stoichiometry. Multiple vendors sell NO x -reducing catalysts that have been shown to decrease NO x  emissions. Increasing the steam during the stripping step may remove greater amounts of nitrogen-containing hydrocarbon which then will not enter the regenerator for combustion. Increasing the zeolite to matrix ratio of the cracking catalyst may also decrease NO x  emissions. 
     In summary, an FCC process to produce lower NO x  emissions may include regenerating spent catalyst with an excess oxygen level less than or equal to about 0.5 mol-%, preferably less than or equal to about 0.2 mol-%, and a plenum temperature above about 730° C. (1350° F.), preferably about 750° C. (1375° F.). Furthermore, the process may include limiting the platinum in the catalyst to about 0.5 ppm or less, preferably 0.2 ppm or less. NO x  emissions from this FCC process may be less than or equal to about 25 ppmv NO x , preferably less than or equal to about 20 ppmv NO x . Modifications to this process to lower NO x  emissions may include selecting a feedstock having an antimony content less than about 0.5 times, preferably about 0.2 times, its nickel content. CO combustion promoters may be used, preferably substantially free of platinum, and further a NO x -reducing catalyst may be used. The regenerating step of the process may use a combustion regenerator or a bubbling bed regenerator. Ammonia may also be injected into the flue gas, preferably at an amount approximately equal to or in excess of the amount of NO x  in the flue gas, before exiting the regenerator. 
     EXAMPLE 1 
     As shown in  FIG. 2 , NO emissions increase as platinum containing promoters are added. This example shows an FCC pilot plant regenerator versus flue gas Oxygen concentration at three levels of platinum in catalyst. The oxygen source was air used for catalyst regeneration and the platinum source was a commercial CO combustion promoter with approximately 850 ppm Pt. The data show a strong interaction between O 2  concentration (measured on a dry basis in the flue gas) and added platinum on NO emissions. The addition of even 1 ppm of Pt increases NO x  at least 5-fold at 0.5% vol % O 2 . 
     EXAMPLE 2  
     As shown in  FIG. 3 , NO conversion to N 2  increases with increased flue gas temperature. This example shows the extent of conversion of NO by excess CO in Helium over a regenerated (&lt;0.01 wt % carbon) commercial equilibrium catalyst. The data show the interaction of CO concentration and regenerator temperature on the rate of NO reduction. This commercially important reaction requires temperatures in excess of 700° C. and preferably in excess of 730° C. and CO concentrations greater than 1000 ppm entering the dilute phase to provide substantial NO x  reductions. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. It should he understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the invention.