Patent Grant
7914666
This invention relates generally to a process for catalytic cracking of hydrocarbons.
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 (NOx) are usually present in regenerator flue gases but should be minimized because of environmental concerns. Regulated NOx emissions generally include nitric oxide (NO) and nitrogen dioxide (NO2), but the FCC process can also produce N2O. In an FCC regenerator, NOx 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 NOx production associated with oxidation of N2 from the combustion air. Production of NOx 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 CO2. The CO promoter decreases CO emissions but increases NOx 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 CO2 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/m3 (30 lb/ft3) 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 NOx 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 NOx emissions while ensuring the catalyst is regenerated to be essentially free of coke.
An FCC process producing lower NOx 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. NOx 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 NOx emissions that may be used in conjunction with this process may include increasing the flue gas residence time, injecting NH3 into the flue gas, adding or using NOx-reducing catalysts, increasing stripping of the catalyst, and increasing the catalyst zeolite to matrix ratio.
This invention relates generally to an improved FCC process. Specifically, this invention may relate to an FCC process with lower NOx emissions. NOx 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
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
At FCC regenerator operating conditions, studies indicate there is negligible NOx production associated with oxidation of N2 from the combustion air. Rather, most of the NOx produced results from the combustion of the coke on the spent catalyst during the regeneration part of the FCC process.
Most NOx 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 NOx are at their maximum during the early portion of regeneration by showing NOx 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 NOx, indicating that preheating without oxygen present drives off volatile, organic nitrogen compounds that are readily oxidized to NOx. Also pilot plant experiments show that increasing the temperature of spent catalyst stripper to drive off volatile organics reduces NOx emissions.
Many variables affect the production of NOx. The addition of platinum-based CO combustion promoters increases NOx emissions and may be one of the most important variables in driving NOx production. For example, pilot plant data indicates that 1 ppm of fresh platinum in the inventory can increase NOx production by five-fold, and 2-4 ppm fresh platinum can increase NOx 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 NH3 to oxides of nitrogen, may be oxidizing volatile nitrogen compounds, such as NH3, HCN and larger organic nitrogen compounds, to NOx in high yield with low yields of elemental N2. Platinum may also decrease CO, afterburn, and temperature of the regenerator dilute phase, all three of which correlate with decreased NOx production.
Another variable, in addition to platinum, in NOx production is excess oxygen. Increased excess oxygen in the regenerator, has been shown to result in increased NOx 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 NOx-reducing species and low in oxygen. These species are then burned at low oxygen concentrations in the upper regenerator resulting in very low NOx 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 N2 by O2) increases with combustion temperature. High combustion temperatures are known to make very high levels of thermal NOx in CO boilers and conventional furnaces. Here, however, NOx production may decrease with increased plenum 58 or flue gas temperature. In this situation, NOx may decrease by about 1% per about 0.5° C. (1° F.). In general, NOx 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 NOx formation appears to be complex. In an oxidizing environment, feed nickel and vanadium deposited on the catalyst increase NOx formation. In pilot plant testing, increasing catalyst vanadium from 930 to 1540 ppm by adding organic vanadium compound to the feedstock increased NOx emissions from 20 ppmv to about 35 ppmv at 1.5 mol-% excess oxygen. Similarly, increased NOx levels occur with higher nickel content feedstock. For example, in pilot plant experiments a high nickel content catalyst at 8400 ppm, produced 55 ppmv NOx 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 NOx depending upon the local concentrations of oxygen, NOx 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 NOx reduction in low oxygen areas of the regenerator.
For many years antimony has been injected into the FCC feed to suppress H2 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 H2 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 NOx 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 NOx 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 NOx followed by its subsequent reduction in a weakly oxidizing environment.
Additional variables for reducing NOx emissions that may be used in conjunction with this process may include increasing the flue gas residence time, injecting NH3 into the flue gas, adding or using NOx-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. NH3 injection into the flue gas decreases NOx 1% per 1 ppm of NH3 injection, consistent with 20%-40% conversion of NH3 by reaction with NOx, assuming a 1:1 stoichiometry. Multiple vendors sell NOx-reducing catalysts that have been shown to decrease NOx 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 NOx emissions.
In summary, an FCC process to produce lower NOx 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. NOx emissions from this FCC process may be less than or equal to about 25 ppmv NOx, preferably less than or equal to about 20 ppmv NOx. Modifications to this process to lower NOx 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 NOx-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 NOx in the flue gas, before exiting the regenerator.
As shown in
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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.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3245920 | Keith et al. | Apr 1966 | A |
| 4778665 | Krishnamurthy et al. | Oct 1988 | A |
| 4784752 | Ramamoorthy et al. | Nov 1988 | A |
| 4812430 | Child | Mar 1989 | A |
| 4851374 | Yan et al. | Jul 1989 | A |
| 4986897 | Chin | Jan 1991 | A |
