EP0145466A2

EP0145466A2 - Process for catalytic cracking of metal-contaminated hydrocarbons in which the cracking catalyst is passivated

Info

Publication number: EP0145466A2
Application number: EP84308523A
Authority: EP
Inventors: Bernie John Pafford; Terry Allan Reid
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1983-12-09
Filing date: 1984-12-07
Publication date: 1985-06-19
Also published as: EP0145466A3; JPS60139344A; JPH051059B2

Abstract

A process is described for passivating metal contaminants present in hydrocarbon feedstock (16) which contaminants are deposited on cracking catalyst. The process involves contacting the regenerated catalyst from the regeneration zone (30) with H₂-containing gas (70, 74, 76, 78, 80, 96) in the transfer zone (42, 44) communicating with the regeneration zone and the reaction zone (12). The H₂-containing gas may be added at a plurality of spaced apart locations to the transfer zone and preferably passes through a control means (72) which is regulated by a sensor means (82) analyzing a cracked product and/or by a sensor means (84) monitoring the catalyst circulation rate.

Description

BACKGROUND OF THE INVENTION

The present invention is directed at a process for catalytic cracking of hydrocarbon feedstocks. More specifically, the present invention is directed at a method for reducing the detrimental effects of metal contaminants such as nickel, vanadium and/or iron, which typically are present in the hydrocarbon feedstock processed and are deposited on the cracking catalyst.
In the catalytic cracking of hydrocarbon feedstocks, particularly heavy feedstocks, nickel, vanadium and/or iron present in the feedstocks become deposited on the cracking catalyst promoting excessive hydrogen and coke makes. These metal contaminants are not removed by conventional catalyst regeneration operations, which convert coke deposits on the catalyst to CO and C0₂. As used hereinafter, the term "passivation" is defined as a method for decreasing the detrimental catalytic effects of metal contaminants such as nickel, vanadium and/or iron which become deposited on the cracking catalyst.
Several patents disclose the use of a reducing atmosphere to passivate cracking catalyst. U. S. Patent No. 2,575,258 discloses the addition of a reducing agent to regenerated catalyst at a plurality of locations in the transfer line between the regeneration zone and the cracking zone for countercurrent flow of the reducing gas relative to the flow of the regenerated catalyst. This patent also discloses the addition of steam to the transfer line downstream of the points at which reducing gas is added to the transfer line to assist in moving regenerated catalyst from the regeneration zone to the reaction zone. Countercurrent flow of the reducing gas relative to the catalyst flow is not desirable, particularly at relatively high catalyst circulation rates, since the catalyst and reducing gas will tend to segregate into two oppositely flowing phases. This would result in poor catalyst contacting. Moreover, it is possible that bubbles of countercurrently flowing reducing gas intermittently could interrupt the recirculation of the catalyst.
International Patent Application (PCT) No. WO 82/04063 discloses in the processing of metal-contaminated hydrocarbons, the addition of reducing gas to a stripping zone disposed between the regeneration zone and the reaction zones to strip the catalyst. This patent also discloses the addition of reducing gas to a separate vessel and/or to the riser downstream of the flow control means to reduce at least a portion of the oxidized nickel contaminants present.
European Patent Publication No. 52,356 also discloses that metal contaminants can be passivated utilizing a reducing atmosphere at an elevated temperature. This publication discloses the use of carbon monoxide, hydrogen, propane, methane, ethane, and mixtures thereof as reducing gases for passivating regenerated catalyst before the catalyst is returned to the reaction zone. This publication also discloses that the contact time of the reducing gas with the catalyst may range between 3 seconds and 2 hours, preferably between about 5 and 30 minutes. This patent publication further discloses that the degree of passivation is improved if antimony is added to the cracking catalyst.
U. S. Patent No. 4,377,470 discloses a process for catalytic cracking of a hydrocarbon feed having a significant vanadium content. Reducing gas may be added to the regenerator and to the transfer line between the regenerator and the reactor to maintain the vanadium in a reduced oxidation state.
U. S. Patent Nos. 4,280,895; 4,280,896; 4,370,220; 4,372,840; and 4,372,841 disclose that cracking catalyst can be passivated by passing the catalyst through a passivation zone, having a reducing atmosphere maintained at an elevated temperature for a period of time ranging from 30 seconds to 30 minutes, typically from about 2 to 5 minutes.
U. S. Patent No. 4,298,459 and 4,280,898 describe processes for cracking a metals-containing feedstock where the used cracking catalyst is subjected to alternate exposures of up to 30 minutes of an oxidizing zone and a reducing zone maintained at an elevated temperature to reduce the hydrogen and coke makes. This patent describes the use of a transfer line reaction zone disposed between a regeneration zone and a stripping zone.
U. S. Patent No. 4,268,416 also describes a method for passivating cracking catalyst in which metal contaminated cracking catalyst is contacted with a reducing gas at elevated temperatures to passivate the catalyst.
U. S. Patent No. 3,408,286 discloses the addition of a liquid hydrocarbon to regenerated catalyst under cracking conditions in a transfer line before the regenerated catalyst is recharged to the cracking zone. The cracking of the liquid hydrocarbon prior to entering the cracking zone operates to displace entrained regenerator gases from the regenerated catalyst entering the cracking zone.
In many existing catalytic cracking systems it may not be desirable to add a separate free standing passivation or reduction zone due to space limitations particularly where the passivation zone is to be retrofitted in an existing cracking facility.
It also may be desirable to avoid the expense associated with the construction and installation of a separate free-standing passivation zone.
It also may be desirable to provide a process which regulates the rate of addition of reducing gas to the passivation zone.
It also may be desirable to provide a process which does not have a significantly adverse effect on the catalyst circulation rate.
The present invention is directed at the incorporation of the reduction or passivation zone into a transfer line. The passivation zone preferably is disposed in a transfer line between the regeneration zone and the reaction zone, such that at least a portion of the metal contaminated catalyst circulating between the regeneration zone and the reaction zone passes through the passivation zone. The passivation zone preferably is located in the transfer line through which regenerated catalyst passes from the regeneration zone to the reaction zone. Depending upon the particular operating conditions in the reaction zone and regeneration zone and the desired operating efficiencies, it may be desirable to dispose a stripping zone before the passivation zone to minimize the amount of oxygen carried into the passivation zone by the regenerated catalyst.

SUMMARY OF THE INVENTION

The present invention is directed at an improved method for passivating catalyst used to crack hydrocarbon feedstock to lower molecular weight products in a reaction zone where the feedstock contains a metal contaminant selected from the group consisting of nickel, vanadium, iron and mixtures thereof, where at least some of the metal contaminant becomes deposited on the catalyst and where coke becomes deposited on the cracking catalyst. Coke and metal contaminated catalyst is transferred from the reaction zone to a regeneration zone for removal of coke therefrom. Regenerated catalyst is circulated from the regeneration zone to the reaction zone through a transfer zone. The improvement comprises adding a H₂-containing reducing gas to the transfer zone to thereby provide a passivation zone wherein cracking catalyst passing therethrough is at least partially passivated.
The H₂-containing reducing gas (hereinafter referred to as "reducing gas") preferably is selected from the group consisting of hydrogen and mixtures with CO and/or hydrocarbons. A stripping gas may be added at the point where the catalyst enters the transfer zone, the stripping gas removing at least a portion of the oxygen and/or oxygenated compounds present on the catalyst. To maximize the contact time of the reducing gas in the transfer zone, reducing gas preferably is added to the transfer zone upstream of the flow control means. Preferably the reducing gas is added to the standpipe from the regeneration zone. The rate of addition of reducing gas to the passivation zone preferably is regulated. This may be accomplished by monitoring the composition of the cracked product stream from the reaction zone and/or the catalyst circulation rate. When the reducing gas added to the passivation zone comprises hydrogen, the rate of reducing gas added to the passivation zone may be regulated to minimize the hydrogen concentration in the cracked product. The reducing gas addition rate may be regulated to maintain the catalyst circulation rate within a predetermined range.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a simplified schematic drawing of one embodiment for practicing the subject invention.
Figure 2 is a simplified schematic drawing of an alternate embodiment for practicing the subject invention.
Figure 3 presents plots of hydrogen yield and coke yield as a function of the residence time of metals contaminated catalyst in a passivation zone.
Figure 4 presents a plot of the Gas Producing Factor as a function of catalyst residence time in a passivation zone, the reducing gas utilized and the temperature of the passivation zone.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figure 1, one method for practicing the subject invention is shown. In this drawing pipes, valves, instrumentation, etc. not essential to an understanding of the invention have been deleted for simplicity. Reaction or cracking zone 10 is shown containing a fluidized catalyst bed 12 having a level at 14 in which a hydrocarbon feedstock is introduced into the fluidized bed through line 16 for catalytic cracking. The hydrocarbon feedstock may comprise naphthas, light gas oils, heavy gas oils, residual fractions, reduced crude oils, cycle oils derived from any of these, as well as suitable fractions derived from shale oil kerogen, tar sands, bitumen, processing, synthetic oils, coal hydrogenation, and the like. Such feedstocks may be employed singly, separately in parallel reaction zones, or in any desired combination. Typically, these feedstocks will contain metal contaminants such as nickel, vanadium, and/or iron. Heavy feedstocks typically contain relatively high concentrations of vanadium and/or nickel. Hydrocarbon gas and vapors passing through fluidized bed 12 maintain the bed in a dense, turbulent, fluidized condition.
In reaction zone 10, the cracking catalyst becomes spent during contact with the hydrocarbon feedstock due to the deposition of coke thereon. Thus, the terms "spent" or "coke-contaminated" catalyst as used herein generally refer to catalyst which has passed through a reaction zone and which contains a sufficient quantity of coke thereon to cause activity loss, thereby requiring regeneration. Generally, the coke content of spent catalyst can vary anywhere from about 0.5 to about 5 wt.% or more. Typically, spent catalyst coke contents vary from about 0.5 to about 1.5 wt.%.
Prior to actual regeneration, the spent catalyst is usually passed from reaction zone 10 into a stripping zone 18 and contacted therein with a stripping gas, which is introduced into the lower portion of zone 18 via line 20. The stripping gas, which is usually introduced at a pressure of from about 10 to 50 psig, serves to remove most of the volatile hydrocarbons from the spent catalyst. A preferred stripping gas is steam, although nitrogen, other inert gases or flue gas may be employed. Normally, the stripping zone is maintained at essentially the same temperature as the reaction zone, i.e., from about 450°C to about 600°C. Stripped spent catalyst from which most of the volatile hydrocarbons have been removed, is then passed from the bottom of stripping zone 18 through U-bend 22 and connecting vertical riser 24, which extends into the lower portion of a regeneration zone. Air is added to riser 24 via line 28 in an amount sufficient to reduce the density of the catalyst flowing therein, thus causing the catalyst to flow upward into regeneration zone 26 by simple hydraulic balance.
In the particular configuration shown, the regeneration zone is a separate vessel (arranged at approximately the same level as reaction zone 10) containing a dense phase catalyst bed 30 having a level indicated at 32, which is undergoing regeneration to burn-off coke deposits formed in the reaction zone during the cracking reaction, above which is a dilute catalyst phase 34. An oxygen-containing regeneration gas enters the lower portion of regeneration zone 26 via line 36 and passes up through a grid 38 in the dense phase catalyst bed 30, maintaining said bed in a turbulent fluidized condition similar to that present in reaction zone 10. Oxygen-containing regeneration gases which may be employed in the process of the present invention are those gases which contain molecular oxygen in admixture with a substantial portion of an inert diluent gas. Air is a particularly suitable regeneration gas. An additional gas which may be employed is air enriched with oxygen. Additionally, if desired, steam may be added to the dense phase bed along with the regeneration gas or separately therefrom to provide additional inert diluents and/or fluidization gas. Typically, the specific vapor velocity of the regeneration gas will be in the range of from about 0.8 to about 6.0 feet/sec., preferably from about 1.5 to about 4 feet/sec.
In regeneration zone 26, flue gases formed during regeneration of the spent catalyst pass from the dense phase catalyst bed 30 into the dilute catalyst phase 34 along with entrained catalyst particles. The catalyst particles are separated from the flue gas by a suitable gas-solid separation means 54 and returned to the dense phase catalyst bed 30 via diplegs 56. The substantially catalyst-free flue gas then passes into a plenum chamber 58 prior to discharge from the regeneration zone 26 through line 60. Where the regeneration zone is operated for substantially complete combustion, the flue gas typically will contain less than about 0.2, preferably less than 0.1 and more preferably less than 0.05 volume % carbon monoxide. The oxygen content usually will vary from about 0.4 to about 7 vol.%, preferably from about 0.8 to about 5 vol.%, more preferably from about 1 to about 3 vol.%, most preferably from about 1.0 to about 2 vol.%.
Regenerated catalyst exiting from regeneration zone 26 preferably has had a substantial portion of the coke removed. Typically, the carbon content of the regenerated catalyst will range from about 0.01 to about 0.6 wt.%, preferably from about 0.01 to about 0.1 wt.%. The regenerated catalyst from the dense phase catalyst bed 30 in regeneration zone 26 flows through the transfer zone comprising standpipe 42 and U-bend 44 to reaction zone 10.
In Figure 1, passivation zone 90 extends for substantially the entire length of standpipe 42 and U-bend 44 to the point where hydrocarbon feedstock enters through line 16 to gain substantially the maximum possible residence time. If a shorter residence time is desired, passivation zone 90 could comprise only a fraction of the length of standpipe 42 and/or U-bend 44. Conversely, if a greater residence time were desired, the cross-sectional area of standpipe 42 and/or U-bend 44 could be increased. Stripping gas streams optionally may be added at the inlet of passivation zone 90 to minimize the intermixing of regeneration zone gas with the passivation zone reducing gas. The stripping gas may be any which will not adversely affect the passivated catalyst and which will not hinder the processing of the feedstock in the reaction zone. A preferred stripping gas is steam although reducing gas and other gas also may be satisfactory. In this embodiment, line 92 is disposed upstream of passivation zone 90, to minimize intermixing of the reducing atmosphere in passivation zone 90 with the gas stream from regeneration zone 26 by stripping out entrained oxygen from the regenerated catalyst.
Since the catalyst residence time in standpipe 42 and U-bend 44 typically may range only from about 0.1 to about 2 minutes, often it may be advantageous to maximize the effectiveness of the catalyst residence time in passivation zone 90 by injecting increasing quantities of reducing gas into the passivation zone until the additional reducing gas ceases to produce benefits in the cracking process. This may occur if the addition of reducing gas adversely affects the catalyst flow rate through the passivation zone. This also may occur when the incremental increase in the rate of reducing gas addition to the passivation zone does not result in a decrease in the hydrogen and/or coke make in reaction zone 10. In Figure 1, the reducing gas flow rate through line 70 is regulated by a control means, such as control valve 72. Reducing gas passing through control valve 72 in line 70 subsequently passes through a plurality of lines such as 74, 76, 78, 80, and 96 to distribute the reducing gas into passivation zone 90. Control valve 72 is shown being regulated by a cracked product monitoring means, such as analyzer 82. Analyzer 82 may be adapted to monitor the content of one or more products in stream 52. Since the hydrogen content of the cracked product is a function of the degree of catalyst metals passivation, in a preferred embodiment, analyzer 82 may be a hydrogen analyzer. Alternatively, since the rate of coke production also is a function of the degree of catalyst metals passivation, the rate of reducing gas addition also could be regulated by monitoring the rate of coke production. This may be accomplished by monitoring the heat balance around reaction zone 10 and/or regeneration zone 26.
The rate of addition of reducing gas to passivation zone 90 also must be maintained below the point at which it will cause a significant fluctuation in the catalyst circulation rate. In the embodiment shown in Figure 1, the rate of catalyst circulation through passivation zone 90 may be monitored by a sensing means, such as sensor 84, shown communicating with regeneration zone 26, standpipe 42 and control valve 72.
In the commercial operation of this embodiment, the concentration of hydrogen in product stream 52 may be monitored by analyzer 82, which adjusts the rate of addition of reducing gas through control valve 72 to minimize the hydrogen content in stream 52. Sensor 84 operates as a limit on control valve 72, by decreasing the rate of addition of reducing gas to passivation zone 90, when the rate of addition of reducing gas begins to adversely affect the catalyst circulation rate.
Referring to Figure 2, an alternate embodiment for practicing the subject invention is disclosed. The operation of this embodiment is generally similar to that previously described in Figure 1. In this embodiment, riser reaction zone 110 comprises a tubular, vertically extending vessel having a relatively large height in relation to its diameter. Reaction zone 110 communicates with a disengagement zone 120, shown located a substantial height above regeneration zone 150. The catalyst circulation rate is controlled by a valve means, such as slide valve 180, located in spent catalyst transfer line 140, extending between disengagement zone 120 and regeneration zone 150. In this embodiment hydrocarbon feedstock is injected through line 112 into riser reaction zone 110 having a fluidized bed of catalyst to catalytically crack the feedstock. Steam may be injected through lines 160 and 162 in a second transfer zone, such as return line 158, extending between regeneration zone 150 and reaction zone 110 to serve as a diluent, to provide a motive force for moving the hydrocarbon feedstock upwardly and for keeping the catalyst in a fluidized condition.
The vaporized, cracked feedstock products pass upwardly into disengagement zone 120 where a substantial portion of the entrained catalyst is separated. The gaseous stream then passes through a gas-solid separation means, such as two stage cyclone 122, which further separates out entrained catalyst and returns it to the disengagement zone through diplegs 124, 126. The gaseous stream passes into plenum chamber 132 and exits through line 130 for further processing (not shown). The upwardly moving catalyst in reaction zone 110 gradually becomes coated with carbonaceous material which decreases its catalytic activity. When the catalyst reaches the top of reaction zone 110 it is redirected by grid 128 into stripping zone 140 in spent catalyst transfer line 142 where it is contacted by a stripping gas, such as steam, entering through line 144 to partially remove the remaining volatile hydrocarbons from the spent catalyst. The spent catalyst then passes through spent catalyst transfer line 142 into dense phase catalyst bed 152 of regeneration zone 150. Oxygen containing regeneration gas enters dense phase catalyst bed 152 through line 164 to maintain the bed in a turbulent fluidized condition, similar to that in riser reaction zone 110. Regenerated catalyst gradually moves upwardly through dense phase catalyst bed 152 eventually flowing into overflow well 156 communicating with return line 158. Return line 158 is shown exiting through the center of dense phase catalyst bed 152, and communicating with riser reaction zone 110.
Flue gas formed during the regeneration of the spent catalyst passes from the dense phase catalyst bed 152 into dilute catalyst phase 154. The flue gas then passes through cyclone 170 into plenum chamber 172 prior to discharge through line 174. Catalyst entrained in the flue gas is removed by cyclone 170 and is returned to catalyst bed 152 through diplegs 176, 178. Regenerated catalyst is returned to reaction zone 110 from regeneration zone 150 through a transfer zone comprising overflow well 156 and return line 158.
As previously indicated for the embodiment of Figure 1, a passivation zone, such as passivation zone 190, may be disposed in or may comprise substantially all of overflow well 156 and/or return line 158. Additional reducing gas may be added to passivation zone 190 through lines 160 and 162 into return line 158. If the quantity of reducing gas added through lines 160, 162 to passivate the catalyst is not sufficient to adequately aerate the regenerated catalyst particles, it may be desirable to dilute the reducing gas added through lines 160, 162 with steam or other diluent. As shown for the embodiment of Figure 1, it may be desirable to add a stripping gas, such as steam, through line 192'to overflow well 156 to remove entrained oxygen from the regenerated catalyst.
The reducing gas preferably is added to passivation zone 190 at a plurality of locations through branched lines, such as lines 202, 204, 206, 208, 210, and 212 extending from reducing gas header 200. As previously described in Figure 1, a control means, such as control valve 220 is disposed in reducing gas header 200 to regulate the rate of addition of reducing gas to passivation zone 190. A cracked product monitoring means, such as analyzer 230 is shown communicating with cracked product line 130 and with control valve 220 to maintain the sampled cracked product component within the desired limits by regulation of the rate of addition of reducing gas to passivation zone 190. Since hydrogen is one of the products produced by the adverse catalytic properties of the metal contaminants, hydrogen may be the preferred component to be regulated. Since the metal contaminant also catalyzes the formation of coke, the rate of reducing gas addition also could be regulated by the monitoring of the rate of coke production, such as by monitoring the heat balance around regeneration zone 150, as previously described. As in the embodiment of Figure 1, the rate of catalyst circulation may be monitored by a sensing means, such as sensor 240, communicating with valve 220, to control the maximum rate of addition of reducing gas to passivation zone 190. The commercial operation of this embodiment would be substantially similar to that previously described for the embodiment of Figure 1. A component in the product stream such as hydrogen is monitored by analyzer 230, which directs control valve 220 to adjust the rate of addition of reducing gas to passivation zone 190 to minimize the hydrogen content in stream 130. Sensor 240, communicating with regeneration zone 150 and line 158, monitors the catalyst circulation rate and operates as an over-ride on control valve 220, to reduce the rate of addition of reducing gas if the reducing gas has or is about to have an adverse effect on the catalyst circulation rate.
The metals concentration deposited on the catalyst is not believed to differ significantly whether the embodiment of Figure 1 or the embodiment of Figure 2 is used. Thus, the amount of reducing gas which is consumed in passivation zone 90, 190 of the embodiments of Figure 1, 2, respectively should not differ greatly.
While the reducing gas consumption rate in passivation zones 90, 190, of Figures 1, 2, respectively, will be a function, in part, of the metal contaminant levels on the catalyst, the desired degree of passivation and the amount of reducing gas infiltration into the regeneration zone, it is believed that the overall rate of consumption of the reducing gas will range from about 0.5 to about 260 SCF, preferably from about 1 to about 110 SCF for each ton of catalyst passed through passivation zones 90, 190, if hydrogen is utilized as the reducing gas.
In the embodiments of Figures 1 or 2 it is believed that the combustion of coke in regeneration zones 26 or 150, respectively, will heat sufficiently the cracking catalyst subsequently passed through passivation zones 90, 190, respectively. A temperature of at least 500°C, preferably above about 600⁰C, is necessary for adequate passivation of the catalyst. If the temperature of the catalyst entering passivation zones 90 or 190 is not sufficiently high, additional heat may be added to the passivation zone either directly, such as by the preheating of the reducing gas, or by adding steam, or indirectly, such as by the addition of a heat exchange means prior to, or within the passivation zone.
Reaction zones 10, 110 and regeneration zones 26, 150, of Figures 1, 2, respectively, may be of conventional design and may be operated at conditions well-known to those skilled in the art. Regeneration zones 26, 150 may be operated in either a net oxidizing or a net reducing mode. In a net oxidizing mode, oxidizing gas in excess of that required to completely combust the coke to C0₂ is added to the regeneration zone. In a net reducing mode insufficient oxidizing gas is added to completely combust the coke to C0₂. It is believed that the regeneration zones 26 and 150 preferably should be operated in a net reducing mode, since carbon monoxide is a reducing gas which will decrease the adverse catalytic properties of the metal contaminants on the catalyst prior to the catalyst entering passivation zones 90, 190.
The required residence time of the catalyst in passivation zones 90, 190 may be dependent upon many factors including the metal contaminant content of the catalyst, the degree of passivation required, the con- cenration of reducing gas in the passivation zone, and the passivation zone temperature. If the residence time required is greater than that available, certain changes may be made to increase the passivation zone capacity and/or increase the rate at which the catalyst is passivated. This may be accomplished by the addition to the catalyst of effective amounts of passivation zone rate enhancers, such as cadmium, germanium, indium, tellurium, zinc, and tin or by the addition of passivation promoters such as antimony, tin, bismuth and manganese.
Laboratory tests presented in the examples below have demonstrated that it may be possible to achieve effective metals passivation in continuous operation utilizing a transfer zone for metals passivation.

Example I

These tests demonstrated that effective metals passivation could be achieved in a passivation zone having a relatively low hydrogen partial pressure. These tests were conducted in a continuous circulatory pilot unit with an integral passivation zone operated at 1300°F (704°C) using an equilibrium Super DX cracking catalyst manufactured by Davison Chemical Co. a division of W. R. Grace and Co. The catalyst, which had 190 wppm nickel, 220 wppm vanadium, 50 wppm copper, and 5500 wppm iron was impregnated with an additional 500 wppm nickel and 1500 wppm vanadium. The impregnated catalyst had been utilized for several hours in a reaction zone followed by regeneration prior to these tests. The test results are presented in Table 1 below.
In these tests the effectiveness of varying hydrogen-nitrogen gas mixtures on metal passivation was measured. The samples first were exposed to a simulated net oxidizing regeneration zone atmosphere having about 2.5 to about 3.5 vol.% excess oxygen. The samples subsequently were exposed to the indicated passivation atmosphere maintained at 1300°F (704°C) for about 3 minutes.
From Table I it may be seen that even the use of a reducing gas having a hydrogen partial pressure of only 0.20 atmosphere was able to significantly passivate the adverse catalytic effects of the metal contaminants present on the catalyst. This demonstrates that significant passivation may be realized even at relatively low reducing gas concentrations in the passivation zone.

Example II

These tests demonstrated that passing metal contaminated catalyst through a passivation zone maintained at an elevated temperature and having a relatively short residence time was effective in passivating the catalyst. These tests were conducted in a continuous circulating pilot unit with an integral passivation zone operated at 1300°F (704°C) using the Super DX equilibrium cracking catalyst which had been impregnated as before to the same metal contaminant content. The results are presented in Figure 3.
From Figure 3 it may be seen that the hydrogen make and coke makes may be reduced by passing of the catalyst through a passivation zone for even relatively short periods of time, such as the residence time typically available in regenerated catalyst transfer zones.

Example III

Additional tests were conducted in a micro catalytic cracking (MCC) unit to determine if a significant amount of the passivation achieved on one pass through the passivation zone is retained, or whether circulation through the reaction and/or regeneration zones reactivates the catalyst. Table 2 below indicates that the degree of passivation of metal contaminated catalyst is, to some extent, cumulative. This further demonstrates that relatively short residence periods in a passivation zone will be effective for catalyst passivation. These tests were conducted in a batch gas treatment vessel operated at 1300OF using an equilibrium Super DX catalyst impregnated with 1000 wppm nickel and 4000 wppm vanadium. Changes in the catalytic activity of the metal contaminants were monitored with the microactivity test gas producing factor (MAT GPF).
In Table 2 test data are shown in which the catalyst was exposed to only a pure hydrogen atmosphere for the indicated period. Also shown are test data in which the catalyst alternately was exposed to a pure hydrogen atmosphere for 30 seconds and to a blend of gases comprising 8% CO, 12% C0₂ and 80% N₂ for 9 minutes. This latter atmosphere was designed to approximate the conditions in a regeneration zone operated in a net reducing manner. It may be seen that for comparable hydrogen treat times, the pure hydrogen atmosphere produced a catalyst having a lower gas producing factor than the catalyst exposed to the alternate passivation zone - regeneration zone atmospheres. However, it should be noted that as the cumulative hydrogen treat time increased, the gas producing factor declined with time. This further indicates that a short residence time passivation zone such as a passivation zone disposed in a transfer zone may be effective particularly over a prolonged period of operation.
In a typical commercial cracking system such as that shown in Figure 1 catalyst residence time in the regenerated catalyst transfer zone, comprising standpipe 42 and _U-bend 44, typically is about 0.1 to about 2 minutes. Similarly, for a typical commercial cracking system similar to that shown in Figure 2, average catalyst residence time in second transfer zone 190 typically ranges between about 0.1 and about 1.0 minutes. The temperature of the regenerated catalyst in the regenerated catalyst transfer zones of Figures 1 and 2 typically ranges between about 600°C and about 790⁰C. Thus, the regenerated catalyst transfer zones of Figures 1 and 2 typically have sufficient residence time and catalyst at a sufficiently high temperature to passivate catalyst upon the introduction of reducing gas.
It is believed that commercial grade CO and process gas streams containing H₂ and/or CO can be utilized as the reducing agent in passivation zone 90. Hydrogen or a reducing gas stream comprising hydrogen is preferred, since this produces the highest rate of metals passivation and achieves the lowest levels of metal contaminant potency. This is shown by the MCC unit data presented in Figure 4. Preferred reducing gas streams containing hydrogen include catalytic cracker tail gas streams, reformer tail gas streams, spent hydrogen streams from catalytic hydroprocessing, synthesis gas, steam cracker gas, flue gas, and mixtures thereof. The reducing gas content in the passivation zone should be maintained between about 2% and about 100%, preferably between about 10% and about 75% of the total gas composition depending upon the hydrogen content of the reducing gas and the rate at which the reducing gas can be added without adversely affecting the catalyst circulation rate.
The stripping gas, if any, added through line 92 of Figure 1 and line 192 of Figure 2 will be a function in part of catalyst flow rate. Typically, the stripping gas flow rates through each of these lines may range between about 0.1 SCF and about 80 SCF, preferably between about 8 and about 25 SCF per ton of catalyst circulated.
Passivation zones 90, 190 may be constructed of any chemically resistant material capable of withstanding the relatively high temperature and the erosive conditions commonly associated with the circulation of cracking catalyst. The materials of construction presently used for transfer piping in catalytic cracking systems should prove satisfactory.
The pressure in passivation zones 90, 190, of Figures 1, 2, respectively, will be substantially similar to or only slightly higher than the pressures in the regenerated catalyst transfer zones of existing catalytic cracking systems. When the embodiment of Figure 1 is used, the pressure in passivation zone 90 may range from about 5 to about 100 psig, preferably from about 15 to about 50. When the embodiment of Figure 2 is used the pressure may range from about 15 psig to about 100 psig, preferably from about 20 psig to about 50 psig.
In general, any commercial catalytic cracking catalyst designed for high thermal stability could be suitably employed in the present invention. Such catalysts include those containing silica and/or alumina. Catalysts containing combustion promoters such as platinum also can be used. Other refractory metal oxides such as magnesia or zirconia may be employed and are limited only by their ability to be effectively regenerated under the selected conditions. With particular regard to catalytic cracking, preferred catalysts include the combinations of silica and alumina, containing 10 to 50 wt.% alumina, and particularly their admixtures with molecular sieves or crystalline aluminosilicates. Suitable molecular alumino-silicate materials, such as faujasite, chabazite, X-type and Y-type aluminosilicate materials and ultra stable, large pore crystalline aluminosilicate materials. When admixed with, for example, silica-alumina to provide a petroleum cracking catalyst, the molecular sieve content of the fresh finished catalyst particles is suitably within the range from 5-35 wt.%, preferably 8-20 wt.%. An equilibrium molecular sieve cracking catalyst may contain as little as about 1 wt.% crystalline material. Admixtures of clay-extended aluminas may also be employed. Such catalysts may be prepared by any suitable method such as by impregnation, milling, co-gelling, and the like, subject only to the provision that the finished catalysts be in a physical form capable of fluidization.
In this patent specification, the following apply:

Length expressed in feet is converted to m by multiplying by 0.3048.
Gauge pressure in pounds per square inch gauge (psig) is converted to equivalent kPa by multiplying by 6.895.
SCF is an abbreviation for standardized cubic feet. 1 standardized cubic foot is 28.316 liters referred to 0°C and 0 kPA gauge pressure.

Claims

1. A process for passivating a fluid bed cracking catalyst utilized to crack metal contaminated hydrocarbon feedstocks to lower molecular weight products wherein a hydrocarbon feedstock containing a metal contaminant selected from the group consisting of nickel, vanadium, iron and mixtures thereof is passed into a reaction zone containing therein a fluid bed cracking catalyst to produce cracked products and fluid bed cracking catalyst contaminated with deposited coke and said metals, said coke being removed from said fluid bed cracking catalyst in a regeneration zone from which at least a portion of the said coke depleted metal contaminated fluid bed cracking catalyst is circulated to said reaction zone through a transfer zone communicating with said regeneration zone and said reaction zone, said process being characterized by the step of introducing a H₂-containing reducing gas into said transfer zone substantially co-currently with the flow of catalyst through a plurality of locations to form a passivation zone and at least partially passivate said metal contaminant deposited on said fluid bed cracking catalyst while adjusting the rate of addition of said H₂-containing reducing gas to maintain the said cracking catalyst circulation rate above a predetermined lower limit.

2. The process of claim 1 further characterized in that the said transfer zone includes a catalyst flow control means, and at least a portion of the said H₂-containing gas is added upstream of the control means.

3. The process of claim 1 or claim 2 further characterized in that catalyst exiting from the regeneration zone is contacted with a stripping gas prior to entering the passivation zone.

4. The process of any one of claims 1 to 3 further characterized in that the hydrogen content of the cracked products stream is monitored and the rate of addition of H₂-containing gas is adjusted in response thereto.

5. The process of claim 4 further characterized in that the rate of addition of H₂-containing gas is adjusted to substantially minimize the hydrogen content in the cracked products stream.

6. The process of any one of claims 1 to 5 further characterized in that the said passivation zone is maintained at a temperature of at least 500°C.

7. The process of any one of claims 1 to 6 further characterized in that the rate of addition of the H₂-containing gas to the transfer zone is in the range between about 0.5 and about 260 SCF (about 14.16 and about 7362.68 liters, measured at 0°C and 0 kPa gauge) per ton of catalyst circulated through the transfer zone.

8. The process of any one of claims 1 to 7 further characterized in that the H₂-containing gas is selected from hydrogen and mixtures of hydrogen with carbon monoxide and/or hydrocarbons.

9. The process of any of claims 1 to 8 further characterized in that the residence time of catalyst in the passivation zone is at least 2 minutes.