Abstract:
The concentration of sulfur trioxides in an FCC unit regenerator is maintained within environmentally accepted limits, while maintaining an adequate amount of gas for fluidizing conditions in the regenerator, by admixing the regenerator oxygen-containing gas with an inert gas. The quantity of the inert gas is controlled by a control loop measuring the pressure drop in the regenerator, and adjusting the amount of the inert gas to maintain the pressure drop within the predetermined limits.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of a copending application, Ser. No. 298,404, filed Sept. 1, 1981, now U.S. Pat. No. 4,395,325. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an improved method of operating a regeneration zone of catalytic cracking units. 
     2. Description of the Prior Art 
     Environmental limitations imposed by state and federal regulatory agencies are becoming increasingly important considerations in the operation of catalytic cracking units (e.g., fluid catalytic cracking --FCC units). In many areas of the country, and even in some foreign countries, economic penalties, (e.g., reduced throughput, more expensive raw materials) are being paid for the excessively high levels of pollutants produced in the catalytic cracking operations. 
     A typical FCC unit comprises a reactor zone or vessel filled with a catalyst, and a regenerator vessel wherein spent catalyst is regenerated. Feed is introduced into the reactor vessel, and is converted therein over the catalyst. Simultaneously, coke forms on the catalyst and deactivates the same. The deactivated (spent) catalyst is removed from the reactor zone and is conducted to the regenerator zone, wherein coke is burned off the catalyst with an oxygen-containing gas (e.g., air), thereby regenerating the catalyst. The regenerated catalyst is then recycled to the reactor vessel. Some of the catalyst is fractionated into fines and lost during the process because of constant abrasion and friction thereof against the various parts of the apparatus. Most of the gaseous pollutants, formed in a catalytic cracking operation, are produced in the regenerator zone or vessel. 
     The efficiency of the regenerating operation is dependent on several operating parameters, the most important of which are regeneration temperature and oxygen availability. In recent years most operators have concentrated on raising regenerator temperature to increase the efficiency of the regenerator zone through a complete or almost complete combustion of carbon monoxide in the regenerator vessel. This is most commonly accomplished by operating at air rates exceeding those required to burn the coke off the catalyst, and by introducing a carbon-monoxide (CO) combustion promoter, usually comprising at least one of the following metals: platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), osmium (Os), and rhenium (Re). Some new regenerator designs have incorporated better mixing methods for mixing coked catalyst with a CO combustion promoter and oxygen (e.g., fast fluidized bed regenerator of Gross et al, U.S. Pat. No. 4,118,338, the entire contents of which are incorporated herein by reference). However, while these new methods of operation of the regenerating vessel decrease the amount of carbon monoxide exiting with the flue gas and improve the overall efficiency of the regeneration process, they may contribute to an increased level of production of other pollutants, e.g., sulfur oxides, particularly sulfur trioxide (SO 3 ), and nitrogen oxides (see for example Luckenbach, U.S. Pat. No. 4,235,704). 
     Simultaneously with the improved methods of operation of the regeneration zone, which alone may contribute to the increased production of sulfur oxides in the flue gases of the regenerator, sulfur feed levels in petroleum crudes available for cracking have been steadily increasing over the past few years. In the past, due to overall low levels of sulfur in FCC feeds, SO 3  levels in flue gases were low and generally only total SO x  levels were monitored without an SO 2  /SO 3  breakdown, or without regard to the SO 3  levels. With the combination of the high sulfur feed levels, the high temperatures in the regeneration zone, and excessive air rates used in the regenerator, the SO 3  concentration in the flue gas can be high enough to cause condensation in the flue gas which can result in a visible plume. The presence of a visible plume may violate local opacity requirements. In addition, the absence of a plume indicates, in a vast majority of cases, that the SO 3  emissions have not reached the maximum allowable limit. 
     The excessively high levels of SO x  and SO 3  are particularly experienced when it is necessary to reduce throughput, as required, for example, by seasonal shifts in demand for FCC products, or shifts due to upstream or downstream processing problems. The SO 3  levels are particularly high under those circumstances because the modern regenerator designs (e.g., those of U.S. Pat. No. 4,118,338) require gas velocities in the combustor section sufficiently high to provide entrainment rates greater than the catalyst circulation rate. However, as mentioned above, the operation of the regenerator at such high oxygen-containing gas velocities, at reduced throughputs, results in excessive levels of sulfur oxides (SO x ), and especially sulfur trioxide (SO 3 ), in the flue gas. Such high levels of SO x  may violate current environmental regulations. 
     SUMMARY OF THE INVENTION 
     An inert gas is admixed with the regeneration gas stream prior to the introduction of the latter into the regenerator. The additional volume of the inert gas maintains the catalyst fluidized to a desired degree at the low throughput conditions. However, the formation of excessive SO x  levels is prevented because the flow rate of oxygen-containing regeneration gas can be decreased without the loss of fluidization conditions in the regenerator bed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of one exemplary embodiment of the present invention using externally-supplied inert gas to fluidize the catalyst bed in the regenerator. 
     FIG. 2 is a schematic representation of an alternative exemplary embodiment of the present invention using flue gas recycle as an inert gas to fluidize the catalyst bed in the regenerator. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The inert gas is any gas which does not chemically react with, or have any adverse physical effect on, the catalyst, the carbon monoxide combustion promoter admixed with the catalyst, the feed, or the products of the FCC process. Suitable inert gases are, for example, nitrogen, flue gas obtained from the FCC regenerator, helium or argon. If the inert gas is a flue gas obtained from the FCC regenerator, it can be admixed with the oxygen-containing regeneration gas either before or after it undergoes physical (e.g., cyclone), or any other (e.g., electrostatic precipitator), cleaning treatment. The point from which the regenerator flue gas is taken to be recycled for admixture with the regeneration gas (i.e., before or after undergoing cleaning treatment) will depend to a large extent on individual unit economics and operating conditions. Thus, the flue gas which has been passed through the regenerator cyclone system, but not through an electrostatic precipitator, has a larger proportion of fines and a higher temperature than the flue gas which has undergone the electrostatic precipitator treatment. Conversely, the flue gas which has been conducted through both, the cyclone system and the electrostatic precipitator system, has a lower solids fines content, but it also has a lower temperature. Thus, an FCC operator, as will be obvious to those skilled in the art, has a choice, depending on the particular process conditions, of using either a high temperature, relatively high solids content gas, or a lower temperature, relatively low solids content gas, for admixture with the oxygen-containing gas introduced into the regenerator. In any event, the recycle of the flue gas into the regeneration vessel decreases the sulfur oxide emissions from the regenerator. Repeated contacting of the sulfur oxides (SO x ) with the catalyst in the regenerator increases the driving force for the SO x  capture onto the catalyst which, after the regeneration cycle is completed, is returned to the reactor portion of the FCC installation. In the FCC reactor, the entrapped SO x  is released as hydrogen sulfide (H 2  S) to a sulfur recovery section. Insofar as H 2  S is routinely recovered as sulfur in a conventional sulfur recovery process, the release of the sulfur in the form of hydrogen sulfide from the reactor vessel is preferred to increased SO x  or SO 3  emissions from the regenerator vessel. 
     The amount of inert gas admixed with the oxygen-containing regeneration gas depends on the amount of gas that is necessary in the regenerator for maintaining the fluidized bed conditions, and, at the same time, on the concentration of sulfur oxides in the flue gas exiting the regenerator. The catalyst bed in the regenerator is fluidized, so that apparent catalyst density in the regenerator is about 10 to about 30 pounds per cubic feet (lb/ft 3 ), preferably about 12 to about 20 lb/ft 3 , and most preferably about 14 to 16 lb/ft 3 . The apparent density can be measured either directly by an appropriate density measuring instrument, or indirectly, e.g., by measuring pressure differential or pressure drop in the regenerator at an appropriate location. The data from the means measuring the degree of fluidization (e.g., the density or the pressure sensor) may be relayed to an operator who manually adjusts the flow rate of an inert gas in a direction to minimize the deviation of the measured parameter from the predetermined value thereof. Alternatively, the sensor measuring a parameter relating to the degree of fluidization may be a part of a control loop comprising a controller regulating the flow rate of the inert gas into the regenerator. In response to the measurement of a control parameter (e.g., ΔP or density) different from the preset value thereof, the controller compares the measured value to the preset value to arrive at the control parameter deviation. The controller then adjusts the flow rate of the inert gas in a direction to minimize the deviation. The control parameter may be set at any value, depending on the mode of operation of the regenerator. The appropriate location for measuring the catalyst bed density depends on the type of the regenerator used in the FCC installation, process conditions and operator preference. It will be obvious to those skilled in the art that any convenient location may be chosen for measuring the bed density, and that the density of the regenerator bed can be maintained at any convenient level in accordance with this invention. 
     The SO 3  concentration can be monitored by any convenient means, e.g., directly by an SO 3  analyzer or, indirectly, by an oxygen (O 2 ) analyzer in the flue gas line. Preferably, however, the SO 3  concentration is controlled indirectly by an oxygen analyzer in the flue gas line. The analyzer measures excess oxygen in the flue gas line. The amount of excess oxygen in the flue gas is maintained at a minimum (e.g., 0.0% to 1.0% by mole) to substantially assure that the SO 3  concentration is maintained at or below environmentally acceptable limits to assure that there is no visible plume in the regenerator flue gas. The data from the oxygen analyzer can be relayed to the process operator, who in turn manually adjusts the amount of the regeneration gas conducted into the regenerator to maintain the oxygen level in the flue gas within the predetermined limits. Alternatively, the analyzer may be a part of a control loop comprising, in addition to the analyzer, a controller regulating the flow rate of the regeneration gas conducted into the regenerator. In response to an excessive oxygen concentration level in the flue gas, the controller decreases the flow rate of the regeneration gas. Conversely, in response to a lower oxygen concentration level than that preset in the controller, the controller increases the flow rate of the regeneration gas. 
     The process of this invention can be used with any regenerator design which requires a substantial amount of gas for maintaining fluidization conditions in the regenerator. However, the process is particularly applicable to the operation of riser type regenerators used in FCC units, processing high sulfur feedstocks, e.g., those disclosed in Gross et al, U.S. Pat. No. 4,118,338, the entire contents of which are incorporated herein by reference. 
     The process of this invention can also be used with any conventionally-used catalytic cracking feeds, such as naphthas, gas oils, vacuum gas oils, residual oils, light and heavy distillates and synthetic oils. Suitable catalysts are conventionally used catalytic cracking catalysts, e.g., those containing silica and silica-alumina or mixtures thereof. Particularly useful are higher and lower activity zeolites, preferably low coke-producing cyrstalline zeolite cracking catalysts comprising faujasite crystalline zeolite and other zeolites known in the art. The carbon monoxide burning promoter optionally used in the process is any conventionally used carbon monoxide burning promoter, such as platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), osmium (Os), or rhenium (Re). The amount of the carbon monoxide burning promoter in the bed of catalyst is maintained at a conventional level, as disclosed e.g., by Schwartz in U.S. Pat. No. 4,072,600. The regenerator procedure for the catalyst containing the promoter is preferably that particuarly promoting the recovery of available heat generated by the burning of carbonaceous deposits produced in hydrocarbon conversion, such as that disclosed in U.S. Pat. No. 3,748,241 and 3,886,060, the entire contents of both of which are incorporated herein by reference. 
     The invention will now be described in conjunction with two exemplary embodiments thereof illustrated in the FIGURES. 
     Referring to FIG. 1, a gas oil feed is introduced through a conduit 2 into a riser reactor 4 along with a regenerated catalyst conducted to the reactor from the regenerator 6 by a conduit 5. The feed volatilizes almost instantaneously, and it forms a suspension with the catalyst which proceeds upwardly in the riser. The suspension then passes into a generally wider section of the reactor which contains solid-vapor separation means 9, such as conventional cyclones. The catalyst is separated from the products of the reaction, and is then conducted to a stripping section 13, wherein entrained gases are removed from the catalyst by steam. 
     Stripped catalyst containing carbonaceous deposits (i.e., coke) is withdrawn from the bottom of the stripping section through a conduit 7 and conducted to a regeneration zone or vessel 6. In the regeneration zone, the catalyst undergoes preliminary regeneration in a relatively narrow combustion zone 11 by passing oxygen-containing gas, such as air, into the combustion zone and burning the coke off the catalyst. The catalyst suspension then proceeds into a relatively wider section 17 of the regenerator, wherein residual carbon and CO are combusted, and, finally, into a solids-gas separation section 21 containing separation means, e.g., cyclones, 19. In the separation section 21, the catalyst is separated from the flue gas. The flue gas is then conducted to an optional power recovery section 8, then to a cooler 10, and finally to an electrostatic precipitator 12 before it is discharged into the atmosphere. Inert gas, e.g, nitrogen, can be optionally supplied to the regenerator by a conduit 18 equipped with a valve 20. The inert gas may be admixed with the oxygen-containing regeneration gas, e.g., air, by a conduit 24, before the regeneration gas is introduced into a blower 26, or through a conduit 22 after the regeneration gas is discharged from the blower 26. The amount of the inert gas admixed with the air is regulated by a control loop comprising a pressure sensor 25, a controller 27 and a valve 20. The pressure sensor 25 measures the pressure drop in the combustor section 11, and relays this information to the controller 27, equipped with a setpoint 29. The controller 27 also controls the operation and the degree of opening of the valve 20. The pressure sensor 25 measures the pressure drop in the combustor section across a distance of 62.4 inches. The measured value of the pressure differential across that distance corresponds directly to apparent density of the catalyst bed in the combustor section. Thus, optimum pressure differential across the 62.4 inches vertical distance in the combustor section is 14-16 inches of water, corresponding to an apparent catalyst density of 14-16 pounds per cubic foot (lb/ft 3 ). If the pressure differential detected by the sensor 25, and thus the apparent catalyst density, exceeds tht level, thereby indicating an excessive amount of inert gas introduced into the regenerator, controller 27 decreases the opening of the valve 20 to decrease the deviation between the optimum pressure differential value and the measured value. Conversely, if the measured pressure differential value is lower than the optimum value, controller 27 increases the opening of the valve 20 to introduce more inert gas into the regenerator to maintain the fluidized catalyst bed at the desired density. Accordingly, the deviation (defined as the difference between the measured pressure differential value and the preset value) is decreased. 
     The amount of the oxygen-containing regeneration gas (air in this example) is controlled by a controller 16, having a set point 14. The amount of the air introduced into the regenerator is controlled by valve 31, which is controlled by the controller 16 in response to the oxygen concentration in the flue gas monitored by an oxygen sensor 17. The optimum amount of oxygen (O 2 ) concentration in the flue gas is set in the setpoint 14 at about about 0 to about 1 mole percent, preferably about 0 to about 0.7 mole percent, and most preferably about 0 to about 0.5 mole percent. Control of the flue gas oxygen content within the aforementioned limits enables operator of the process to keep the SO 3  emissions at such a level that the molar ratio of SO 3  /SO x  is less than 5 percent. If the concentration of O 2  detected by the detector 17 in the flue gas exceeds the level set at the setpoint 14, thereby indicating an excessive amount of oxygen being introduced into the regeneration zone, controller 16 decreases the opening of the valve 20 to decrease the intake of air into the regenerator. Accordingly, the concentration of oxygen in the regenerator will decrease, and so will the concentration of SO 3  in the flue gas. Conversely, if the concentration of oxygen detected by the detector 17 is less than the limit set in the setpoint 14, the controller 16 sends a signal to valve 20 to increase the opening thereof, thereby increasing the intake of air into the regenerator. Valve 20 may also be commonly controlled by operator intervention to control the rate of air flow, and thus the SO 3  content of the flue gas. However, it is preferred to operate the valve 20 by means of an automatic control loop described above. 
     In an alternative embodiment, illustrated in FIG. 2, the inert gas admixed with air (or any other oxygen-containing gas) is recycled flue gas. The flue gas may be recycled from three different alternative positions shown in the drawing and discussed in detail below. 
     The operation of the reactor and the regenerator section of the apparatus of the embodiment of FIG. 2 is identical to that of the embodiment shown in FIG. 1, discussed above. The respective parts of the apparatus of FIG. 2 are numbered identically to those of the embodiment of FIG. 1, with a prefix of 100. Thus, for example, the gas oil feed line 102 of FIG. 2 corresponds to the gas oil feed line 2, and the riser reactor 104 of FIG. 2 corresponds to that of the riser reactor 4 of FIG. 1. Accordingly, it is believed that the operation of the respective parts of the embodiment of FIG. 2 will be obvious to those skilled in the art from the above description of the embodiment of FIG. 1. Flue gas is recycled in this embodiment from the exit of the regenerator vessel to the conduit 123 carrying oxygen-containing regeneration gas. The flue gas may be recycled into the conduit 123 from one of three different locations shown in FIG. 2. Thus, the flue gas may be recycled from the outlet of the regenerator cyclone separation system via line 128 (shown as a phantom line in FIG. 2) to the suction side of the combustion air blower 126. If the flue gas is recycled directly from the outlet of the cyclone separation system, it provides a high pressure, relatively high temperature gas with a relatively high catalyst fines content. 
     Alternatively, the recycle flue gas may be recycled from the inlet of the electrostatic precipitator through a conduit 129 (also shown as a phantom line in FIG. 2) to the intake side of blower 126. In this embodiment, the recycled gas has lower temperature than that recycled through a conduit 128, because it passed through a power recovery section 108 and a cooler 110. 
     In yet another embodiment, the recycled flue gas may be recycled from the outlet of the electrostatic precipitator 112 through a conduit 130 to the blower 126. In this case, the recycled flue gas also has a relatively low temperature and a relatively low solids catalyst fines content because of its passage through the electrostatic precipitator. The proper source for the recycled flue gas may be chosen by an individual operator based on individual unit economics. However, as mentioned above, the recycle of the flue gas from any of the three aforementioned points in the process aids in the reduction of the emission of oxides of sulfur (SO x ) from the regenerator. Repeated contact of the SO x  from the flue gas with the catalyst increases the driving force for the SO x  capture onto the catalyst which, eventually, is returned to the riser of the reactor, wherein the SO x  is released as hydrogen sulfide. 
     An optional blower 132 may be placed in the flue recycle line 122 in order to increase, if needed, the capacity of the main blower 126. It will be obvious to those skilled in the art that the control of the amount of flue gas admixed with the oxygen-containing regeneration gas is accomplished in the same manner as in the embodiment of FIG. 1 by a control loop comprising a controller 127 with a setpoint 129, a pressure sensor 125 and a valve 120 controlled by the controller 127. 
     It will also be obvious to those skilled in the art that the catalytic cracking process and apparatus of this invention may be conventionally equipped with a number of other control loops normally used in catalytic cracking installations, and the operation of these conventional loops can be integrated and/or can be kept independent of the operation of the control loops discussed above. Such conventionally used control loops, and other details of FCC processes, are fully disclosed in the following patents and publications: U.S. Pat. Nos. 2,383,636 (Wurth); 2,689,210 (Leffer); 3,338,821 (Moyer et al); 3,812,029 (Snyder, Jr.); 4,093,537 (Gross et al); 4,118,338 (Gross et al); Venuto et al, Fluid Catalytic Cracking with Zeolite Catalysts, Marcel Dekher Inc. (1979). The entire contents of all of the above patents and publications are incorporated herein by reference. 
     It will be apparent to those skilled in the art that the above general description of the apparatus, the process and of the specific embodiments thereof can be successfully repeated with apparatus and ingredients equivalent to those generically or specifically set forth above and under variable process conditions. 
     From the foregoing specification, one skilled in the art can readily ascertain the essential features of this invention and without departing from the spirit and scope thereof can adopt it to various diverse applications.