CA1240946A - Passivation of cracking catalyst - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

PASSIVATION OF CRACKING CATALYST

A process for passivating the adverse cata-lytic effects of metal contaminants, such as nickel, vanadium and iron, in a cracking system com-prising a reaction zone, a regeneration zone and a passivation zone and wherein a passivation promoter selected from the group consisting of cadmium-tin mix-tures, cadmium, germanium, indium, tellurium and zinc and mixtures thereof is added to the cracking system.
The passivation zone is maintained at an elevated temperature and may also have a reducing atmosphere.

Description

3~i 1 BACKGROUND OF_T~IE INVENTION

2 The present invention is directed at a

3 process for catalytic cracking of hydrocarbon feed-

4 stocks. More specifically, the present invention is directed at a method for reducing the detrimental 6 effects of metal contaminants such as nickel, vanadium 7 and/or iron, which typically are present in the 8 hydrocarbon feedstock processed and are deposited on 9 the cracking catalyst.

In the catalytic cracking of hydrocarbon 11 feedstocks, particularly heavy feedstocks, nickel, 12 vanadium and/or iron present in the feedstocks become 13 deposited on the cracking ca-talyst promoting excessive 14 hydrogen and coke makes. These metal contaminan-ts are not removed by conventional catalyst regeneration 16 operations, which convert coke deposits on the catalyst 17 to CO and CO2. ~s used hereinafter, the term "passi-18 vation" is defined as a method for decreasing the 19 detrimental catalytic effects or metal contaminants such as nickel, vanadium and/or iron which become 21 deposi-ted on the cracking catalyst.

22 Several patents disclose the use of a re-23 ducing atmosphere to passivate cracking catalyst. U.S.
24 Patent Nos. 4,280,895; 4,280,896; 4,370,840;
4,372,841; 4,370,220; and 4,409,093 disclose that 26 cracking catalyst can be passivated by passing the 27 ca-talyst through a passivation zone having a reducing 28 atmosphere maintained at an elevated temperatùre for a 29 period of time ranging from 30 seconds to 30 minutes, typically from about 2 to about 5 minutes. These 31 patents disclose that tin, antimony, bismuth and man-32 ganese may be added to improve the degree oE
33 passivation.

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1 European Patent Publication No. 1,642; U.S.
2 Patent Nos. 4~257,919; 4,326,990; and 4,324,648 also 3 disclose the use of tin for metals passivation. These 4 patent publications also disclose the sequential use of a reducing atmosphere at elevated temperature and the 6 use of a hydrogen atmosphere at elevated temperature to 7 simulate aging of the catalyst prior to testing. U.S.
8 Patent No. 4,235,704 also discloses the use of tin for 9 decreasing the adverse catalytic activity of metal contaminants.

11 Several patents disclose the use of a 12 reducing atmosphere to passivate cracking catalyst. U.
13 S. Patent No. 2,575,258 discloses the addition of a 14 reducing agent to regenerated catalyst at a plurality of locations in the transfer line between the regen-16 eration zone and the cracking zone for countercurrent 17 flow of the reducing gas relative to the flow of the 18 regenerated catalyst. This patent also-discloses the 19 addition of steam to the transfer line downstrèam of the points at which reducing gas is added to the 21 transfer line to assist in moving regenerated catalyst 22 from the regeneration zone to the reaction zone.
23 Countercurrent flow oE the reducing gas relative to the 24 catalyst flow is not desirable, particularly at rela-tivel~ high catalyst circulation rates, since the 26 catalyst and reducing gas will tend to segregate into 27 two oppositely flowing phases. This would result in 28 poor catalyst contacting. Moreover, it is possible that 29 bubbles of countercurrently flowing reducing gas intermittently could interrupt the recirculation of the 31 catalyst.

32 International Patent Application (PCT) No.
33 WO 82/04063 discloses in the processing of metal-34 contaminated hydrocarbons, the addition of reducing gas to a stripping zone disposed between the regeneration 1 zone and the reaction zones to strip the catalyst. This 2 patent also discloses the addition of reducing gas to a 3 separate vessel and/or to the riser downstream of the 4 flow control means to reduce at least a portion of the oxidized nickel contaminants present.

6 European Patent Publication No. 52,356 also 7 discloses that metal contaminants can be passivated 8 utilizing a reducing atmosphere at an elevated tempera-9 ture. This publication discloses the use of reducing gases for passivating regenerated catalyst before the 11 catalyst is returned to the reaction zone. This publi-12 cation also discloses that the contact time of the 13 reducing gas with the catalyst may range between 3 14 seconds and 2 hours, preferably between about 5 and 30 minutes. This patent publication further discloses that 16 the degree of passivation is improved if antimony is 17 added to the cracking catalyst.

18 U. S. Patent No. 4,377,470 discloses a 19 process Eor catalytic cracking of a hydrocarbon feed having a significant vanadium content. Reducing yas may 21 be added to the regenerator and to the transfer line 22 between the regenerator and the reactor to maintain the 23 vanadium in a reduced oxidation state.

24 U.S. Patent Nos. 4,153,535; 4,221,677;
~5 4,153,534; 4,206,039; 4,218,344; 4,267,072;
26 4,146,463; 4,233,276; 4,300,977; U.K. Patent Nos.
27 1,s75,018i 1,575,019; and Canadian Patent No.
28 1,048,951 also disclose the use of cadmium in a cata-29 lytic cracking process to absorb the sulfur oxides released.

31 U.S. Patent Nos. 4,298,459 and 4,280,898 32 describe processes for cracking a metals-containing 33 feedstock where the used cracking ca-talyst is subjected 34 to alternate eexposures of up to 30 minutes of an oxi-1 dizing zone and a reducing zone maintained at an ele-2 vated temperature to reduce the hydrogen and coke 3 makes. These patents describe the use of a transfer 4 line reaction zone disposed between a regeneration zone and a stripping zone. The '898 patent discloses that a 6 metallic reactant, such as cadmium, zinc, sodium, scan-7 dium, titanium, chromium, molybdenum, manganese, co-8 balt, nickel, antimony, copper, the rare earth metals, 9 and compounds of these metals may be added to adsorb the sulfur oxides produced.

11 U. S. Patent Nos. 4,280,859; 4,280,896, 12 4,370,220; 4,372,840; 4,372,841; and 4,409,093 dis-13 close that cracking catalyst can be passivated by pass-14 ing the catalyst -through a passivation zone, having a reducing atmosphere maintained at an elevated tempera-16 ture for a period of time ranging from 30 seconds to 30 17 minutes, typically from about 2 to 5 minutes.

18 U. S. Patent Nos. 4,298,459 and ~,280,898 19 describe processes for cracking a metals-containing feeds-tock where the used cracking catalyst is subjected 21 to alternate exposures of up to 30 minutes oE an 22 oxidizing zone and a reducing zone maintained at an 23 elevated temperature to reduce the hydroyen and coke 24 makes. ~hese patents describe the use of a transfer line reaction zone disposed between a regeneration zone 26 and a stripping zone. The '898 patent discloses that a 27 metallic reactant, such as cadmium, zinc, sodium, 28 scandium, titanium, chromium, molybdenum, manganese, 29 cobalt, nickel antimony copper, the rare earth metals, and compounds of these metals may be added to adsorb 31 the sulfur oxides produced.

~Z ~ 3 1 U. S. Patent No. 4,268,416 also describes a 2 method for passivating cracking catalyst in which metal 3 contaminated cracking catalyst is contacted with a 4 reducing gas at elevated temperatures to passivate the catalyst.

6 U. S. Patent No. 3,408,286 discloses the 7 addition of a liquid hydrocarbon to regenerated 8 catalyst under cracking conditions in a transfer line 9 before the regenerated catalyst is recharged to the cracking zone. The cracking of the liquid hydrocarbon 11 prior to entering the cracking zone operates to dis-12 place entrained regenerator gases from the regenerated 13 catalyst entering the cracking zone.

14 Several patents describe the addition of elements or compounds to passivate the adverse 16 catalytic effects of iron, nickel and vanadium which 17 may be present i-n the hydrocarbon feedstock.

18 U. S. Patent No. 2,901,419 discloses the use 19 of additives selected from groups III and IV of the Periodic Table, preferably from the right side 21 sub-groups or from -the right side sub-groups of groups 22 I and II. Preferred compounds include copper, silver, 23 yold, zinc, cadmium and mercury and compounds of these 24 metals. Included in the specifically disclosed com-pounds were cadmium fluoride, cadmium formate, cadmium 26 oxalate and cadmium oxide. The group III metals 27 include indium, while the group IV metals include 28 germanium.

29 PCT Patent Publications Nos. WO 82/03225 and W0 82/03226 disclose the use of several metals, their 31 oxides and salts, and their organometallic compounds to 32 immobilize vanadium in a catalytic cracking operation.
33 The metals include indium, tellurium, magnesium, 34 calcium, strontium, barium, scandium, yttrium, 1 lanthanum, titanium, zirconium, hafnium, niobium, 2 tantalum, manganese, iron, thallium, bismuth, the rare 3 earths and the Actinide and Lanthanide series of 4 elements~

U. S. Patent No. 4,386,015 discloses the use 6 of germanium and yermanium compounds to passivate metal 7 contaminants in a catalytic cracking operation.

8 European Patent Application No. 38,047 9 discloses the use of germanium and germanium compounds for passivating metal.

11 U. S. Patent No. 4,238,317 is directed at a 12 method for decreasing the carbon monoxide and sulfur 13 oxide emissions from a catalytic cracking system. A
14 metallic oxidation promoter may be used to oxidize the carbon monoxide and sulfur oxides. The oxidation 16 promoter may include cadmium, zinc, magnesium, 17 strontium, barium, scandium, titanium, chromium, 18 molybdenum, manganese, cobalt, nickel, antimony, 19 copper, lead, the rare earth metals, and compounds thereof.

21 U. S. Patent Nos. 4,208,302 and 4,256,564 22 disclose the use of indium and indium compounds for 23 passivating the adverse catalytic effects of metal 24 contaminants. The patents both indicate that -the catalyst was aged prior to use by exposure to alternate 26 high reducing and oxidizing cycles prior to use.

27 U. S. Patent No. 4,257,919 discloses the use 28 of indium, tin, bismuth, and compounds thereo~ for pas-29 sivating metal contaminants.

1 U. 5. Patent Nos. 4,169,042 and 4,218,337 2 disclose the use of elemental tellurium, tellurium 3 oxides, and compounds convertible to elemental tel-4 lurium, or tellurium oxide to passivate the adverse catalytic effects of metal contaminants.

6 The addition of reducing gas to the transfer 7 line between the regeneration zone and the reaction 8 zone would obviate the necessity for installing a 9 separate passivation vessel in the cracking system. The use of the transfer line as a passivation zone would be 11 of particular utility in existing cracking systems 12 where space limitations would preclude the addition of 13 a separate passivation vessel. However, the residence 14 time of the cracking catalyst in the transfer line is rather limited.

16 It would, therefore, be advantageous to have 17 a method for increasing the rate of passivation of the 18 metal contaminants in the transfer line.

19 It also would be advantageous to have a method for passivating the metal contaminants on the 21 cracking catalyst without the addition of a separate 22 passivation vessel.

23 The present invention is directed at a 2~ method for increasing the rate of me-tal con-taminant passivation in a passivation zone disposed in a crack-26 ing system by the addition to the cracking system of a 27 passivation promoter. The passivation promoter prefer-28 ably is selected from the group consisting oE
~9 cadmium-tin mixtures, cadmium, germanium, indium, tel-lurium, zinc, and mixtures thereof.

~ ~ --2 The present invention is directed to a pro-3 cess for passivating cracking catalyst in a cracking 4 system comprising a reaction zone, a regeneration zone, and a passivation zone, wherein a hydrocarbon feedstock 6 containing a metal contaminant selected from the group 7 consisting of nickel, vanadium, iron and mixtures there-8 of is passed into a reaction zone of said cracking 9 system containing therein a cracking catalyst to pro-duce cracked products and cracking catalyst contam-11 inated with deposited coke and said metals, said co~se 12 being removed from said cracking catalyst in a regen-13 eration zone from which at least a portion of the said 14 coke depleted metal contaminated cracking catalyst is circulated to said reaction zone through a passivation 16 zone maintained under passivation conditions prior to 17 returning said catalyst to said reaction zone, said 18 process being characterized by the step of adding an 19 effective amount of a passivation promoter to the crack-ing system, said passivation promoter being selected 21 from the group of metals consisting of cadmium-tin 22 mixtures, cadmi~m, germanium, indium, tellurium, zinc, 23 compounds thereof and mixtures thereof.

24 In a preferred embodiment, the passivation zone is disposed at least partially in the transfer 26 zone communicating with the regeneration zone and 27 reaction zone. The temperature in the transfer zone 28 preferably is maintained in the range of about 700C to 29 about 850C. The concentration of the passivation promoter in the system preferably is maintained between 31 about 0.005 and about 0.20 weight percent of the 32 cracking catalyst present in the cracking system, and 33 more preferably within the range of about 0.025 and 34 about 0.10 weight percent. Particularly preferred ~Lz~

1 passivation promoters comprise cadmium-tin, germanium, 2 zinc, cadmium, and compounds thereof, with cadmium and 3 cadmium compounds being most preferred. The residence 4 time of the catalyst in the passivation zone preferably is maintained between about 0.1 and about 20 minutes, 6 more preferably between about 0.S and about 2 minutes.
7 Passivation promoter preferably is added to the feed or 8 deposited on the catalyst, with the more preferred 9 method comprising the addition of the promoter with the feed.

12 Figure 1 is a simplified schematic drawing 13 of one embodiment for practicing the subject invention.

14 Figure 2 is a simplified schematic drawing of an alternate embodiment for practicing the subject 16 invention.

17 Figure 3 is a plot of the degree of passi-18 vation for various metal contaminated cracking catalyst 19 samples as a function of cumulative residence time in a passivation zone.

21 DETAILED DESCRIPTION_ OF THE INVENTION

22 Referring to Figure 1, one method for prac-23 ticing the subject invention is shown. In this drawing 24 pipes, valves, instrumentation, etc. not essential to an understanding of the invention have been deleted for 26 simplicity. Reaction or cracking zone 10 is shown 27 containing a fluidized catalyst bed 12 having a level 28 at 14 in which a hydrocarbon feedstock is introduced 29 into the fluidized bed through line 16 for catalytic cracking. The hydrocarbon feedstock may comprise 31 naphthas, light gas oils, heavy gas oils, residual 32 fractions, reduced crude oils, cycle oils derived from ~L2~

1 any of these, as well as suitable fractions derived 2 from shale oil, kerogen, tar sands, bitumen processing, 3 synthetic oils, coal,hydrogenation, and the like. Such 4 feedstocks may be employed singly, separately in parallel reaction zones, or in any desired combination.
6 Typically, these feedstocks will contain metal con~
7 taminants such as nickel, vanadium and/or iron. Heavy 8 feedstocks typically contain relatively high concen-9 trations of vanadium and/or nickel. Hydrocarbon gas and vapors passing through fluidized bed 12 maintain the 11 bed in a dense, turbulent, fluidized condition.

12 In reaction zone 10, the cracking catalyst 13 becomes spent during contact with the hydrocarbon 14 feedstock due to the deposition of coke thereon. Thus, the terms "spent" or "coke-contaminated" catalyst as 16 used herein generally refer to catalyst which has 17 passed through a reaction zone and which contains a 18 sufficient quantity of coke thereon to cause activity 19 loss, thereby requiring regeneration. Generally, the coke content of spent catalyst can vary anywhere from 21 about 0.5 to about 5 wt.% or more. Typically, spent 22 catalyst coke contents vary ~rom about 0.5 to about 1.5 23 wt.%-2~ Prior to actual regeneration, the spent catalyst is usually passed from reaction zone 10 into a 26 stripping zone 18 and contacted therein with a strip-27 ping gas, which is introduced into the lower portion of 28 zone 18 via line 20. The stripping gas, which is 29 usually introduced at a pressure of from about 10 to 50 psig, serves to remove most of the volatile hydro-31 carbons from the spent catalyst. A preferred stripping 32 gas is steam, although nitrogen, other inert gases or 33 flue gas may be employed. Normally, the stripping zone 34 is maintained at essentially the same temperature as the reaction zone, i.e., from about 450C to about 36 600C. Stripped spent catalyst from which most of the ~z~

1 volatile hydrocarbons have been removed, is then passed 2 from the bottom of stripping zone 18 through U-bend 22 3 and connecting vertical riser 24, which extends into 4 the lower portion of a regeneration zone. Air is added to riser 24 via line 28 in an amount sufficient to 6 reduce the density o~ the catalyst flowing therein, 7 thus causing the catalyst to flow upwardly into regen-8 eration zone 26 by simple hydraulic balance.

9 In the particular configuration shown, regeneration zone 26 is a separate vessel ~arranged at 11 approximately the same level as reaction zone 10) 12 containing a dense phase catalyst bed 30 having a level 13 indicated at 32, which is undergoing regeneration to 14 burn-oEf coke deposits formed in the reaction zone during the cracking reaction, above which is a dilute 16 catalyst phase 34. An oxygen-containing regeneration 17 gas enters tlle lower portion of regeneration zone 26 18 via line 36 and passes up through a grid 38 in the 19 dense phase catalyst bed 30, maintaining said bed in a turbulent fluidized condition similar to that present 21 in reaction zone 10. Oxygen-containing regeneration 22 gases which may be employed in the process of the 23 present invention are those gases which contain 24 molecular oxygen in admixture with a substantial por-tion of an inert diluent gas. Air is a particularly 26 suitable regeneration gas. An additional gas which may 27 be employed is air enriched with oxygen. Additionally, 28 if desired, steam may be added to the dense phase bed 29 along with the regeneration gas or separately therefrom to provide additional inert diluents and/or fluidiza-31 tion gasO Typically, the specific vapor velocity of the 32 regeneration gas will be in the range of from about 0.8 33 to about 6.0 feet/sec., preferably from about 1.5 to 34 about ~ feet/secO

1 In regeneration zone 26, flue gases formed 2 during regeneration of the spent catalyst pass from the 3 dense phase catalyst bed 30 into the dilute catalyst 4 phase 34 along with entrained catalyst particles. The catalyst particles are separated from the flue gas by a 6 suitable gas-solid separation means 54 and returned to 7 the dense phase catalyst bed 30 via diplegs 56. The 8 substantially catalyst-free flue gas then passes into a 9 plenum chamber 58 prior to discharge from the regener-ation zone 26 through line 60. Where the regeneration 11 zone is operated for substantially complete combustion 12 of the coke, the flue gas typically will contain less 3.3 than about 0.2, preferably less than 0.1 and more pre-14 ferably less than 0.05 volume % carbon monoxide. The oxygen content usually will vary from about 0.4 to 16 about 7 vol.%, preferably from about 0.8 to about 5 17 vol.%, more preferably from about 1 to about 3 vol.%, 18 most preferably from about 1.0 to about 2 vol.%.

19 Regenerated catalyst exiting from regen-eration zone 26 preferably has had a substantial 21 portion of the coke removed. Typically, the carbon 22 content of the regenerated catalyst will range from 23 about 0.01 to about 0.6 wt.%, preferably from about 24 0.01 to about 0.1 wt.%. The regenerated catalyst from the dense phase catalyst bed 30 in regeneration zone 26 26 flows through a transfer zone comprising standpipe 42 27 and U-bend 44 to reaction zone lO.

28 In Figure 1 passivation zone 90 extends for 29 substantially the entire length of standpipe 42 and U-bend 44 to gain substantially the maximum possible 31 residence time. If a shorter residence time is desired, 32 passivation zone 90 could comprise only a fraction of 33 the length oE standpipe 42 and/or U-bend 44. Converse-34 ly, if a greater residence time were desired, the crosssectional area of standpipe 42 and/or u-bend A4 ~4~

could he increased. .Stripping gas streams, optionally 2 may be added at the inlet of passivation zone 90 to 3 minimize the intermixing of regeneration zone gas with 4 the passivation zone reducing gas. The stripping gas S may be any non~oxidizing gas, such as steam, which will 6 not adversely affect the passivated catalyst and which 7 will not hinder the processing of the feedstock in the 8 reaction zone. In this embodiment, line 92 is disposed 9 upstream of passivation zone 90, to minimize inter-mixing of the reducing atmosphere in passivation zone 11 go with the gas stream from regeneration zone 26 by 12 stripping out entrained oxygen from the regenerated 13 catalyst.

14 Since the catalyst residence time in stand-pipe 42 and U-bend 44 typically may range only from 16 about 0.1 to about 2 minutes, it may be necessary to 17 increase the rate at which the metal contaminant 18 present on the cracking catalyst is pas~ivated. It has 19 been found that the addition of passivation promoters selected from the group consisting of cadmium-tin mix-21 tures, cadmium, germanium, indium, -tellurium, zinc, 22 compounds thereof and mixtures thereof increases -the 23 rate of passivation of the metal contaminants/ particu-24 larly where the residence time of the cracking catalyst in a passivation zone is less than about 5 minutes. The 26 combination of cadmium-tin increases the passivation of 27 the metal contaminants above that which would be 28 realized with comparable quantities of cadmium or tin 29 alone. Often it may be advantageous to maximize the effectiveness of the catalyst residence time in 31 passivation zone 90 by injecting increasing quantities 32 of reducing gas into the passivation zone until the 33 additional reducing gas ceases to produce benefits in 34 the cracking process. This may occur if the addition of reducing gas adversely affects the catalyst Elow rate 36 through the passivation zone. This also may occur when 37 the incremental increase in the rate of reducing gas 1 addition to the passivation zone does not result in a 2 corresponding decrease in -the hydrogen and/or coke make 3 in reaction zone 10. In Figure 1, the reducing gas flow 4 rate through line 70 is regulated by a control means, such as control valve 72. Reducing gas passing through 6 control valve 72 in line 70 subsequently passes through 7 a plurality of lines such as 74, 76, 78 and 80 and 96 8 to distribute the reducing gas into passivation zone 9 90. Control valve 72 is shown being regulated by a cracked product monitoring means, such as analyzer 82.
11 Analyzer 82 may be adapted to monitor the content of 12 one or more products in stream 52. Since the hydrogen 13 content of the cracked product is a function of the 14 degree of catalyst metals passivation, in a preferred embodiment, analyzer 82 may be a hydrogen analyzer.
16 Alternatively, since the rate of coke production also 17 is a function of the degree of catalyst metals passi-18 vation, the rate of reducing gas addition also could be 19 regulated by monitoring the rate of coke production.
This may be accomplished by monitoring-the heat balance 21 around reaction zone 10 and/or regeneration zone 26.

22 The rate of addition of reducing gas to 23 passivation zone 90 also must be maintained below the 2~ point at which lt will cause a significant fluctuation in the catalyst circulation rate. In the embodiment 26 shown in Figure 1, the rate of catalyst circulation 27 through passivation zone 90 may be monitored by a 28 sensing means, such as sensor 84, shown communicating 29 with regeneration zone 26, standpipe 42 and control valve 72.

31 In the commercial operation of this embodi-32 ment~ the concentration of hydrogen in product stream 33 52 may be monitored by analyzer 82, which adjusts the 34 rate of addition of reducing gas through control valve 72 to minimize the hydrogen content in stream 52.
36 Sensor 84 operates as a limit on control valve 72, by 12~0~3~
~ 15 -1 decreasing the rate of addition of reducing gas to 2 passivation zone 90, when the rate of addition of 3 reducing gas begins to adversely affect the catalyst 4 circulation rate.

Referring to Figure 2, an alternate embodi-6 ment for practicing the subject invention is disclosed.
7 The operation of this embodiment is generally similar 8 to that previously described in Figure 1. In this 9 embodiment, riser reaction zone 110 comprises a tubular, vertically extending vessel having a rela-11 tively large height in relation to its diameter.
12 Reaction zone 110 communicates with a disengagement 13 zone 120, shown located a substantial height above 14 regeneration zone 150. The catalyst circulation rate is controlled by a valve means, such as slide valve 180, 16 located in spent catalyst -transfer line 140, extending 17 between disengagement zone 120 and regeneration zone 18 150. In this embodiment, hydrocarbon feedstock is 19 injected through line 112 into riser reaction zone 110 having a fluidized bed of catalyst to catalytically 21 crack the feedstock. Steam may be injected -through 22 lines 160 and 162 in a second transfer zone, such as 23 return line 158, extending between regeneration zone 24 150 and reaction zone 110 to serve as a diluent, to provide a motive force for moving the hydrocarbon feed-26 stock upwardly and for keeping the catalyst in a 27 fluidized condition.

28 The vaporized, cracked feedstock products 29 pass upwardly into disengagement zone 120 where a substantial portion of the entrained catalyst is 31 separated. The gaseous stream then passes through a 32 gas-solid separation means, such as two stage cyclone 33 122, which further separates out entrained catalyst and 34 returns it to the disengagement zone through diplegs 124, 126. The gaseous stream passes into plenum chamber 36 132 and exits through line 130 for further processing ~.240~

l (not shown). The upwardly moving catalyst in reaction 2 zone 110 gradually becomes coated with carbonaceous 3 material which decreases its catalytic activity. When 4 the catalyst reaches the top of reaction zone 110 it is redirected by grid 12B into stripping zone 140 in spent 6 catalyst transfer line 142 where it is contacted by a 7 stripping gas, such as steam, entering through line 144 8 to partially remove the remaining volatile hydrocarbons 9 from the spent catalyst. The spent catalyst then passes through spent catalyst transfer line 142 into dense 11 phase catalyst bed 152 of regeneration zone 150. Oxygen 12 containing regeneration gas enters dense phase catalyst 13 bed 152 through line 164 to maintain the bed in a 14 turbulent fluidized condition, similar to that in riser reaction zone 110. Regenerated catalyst gradually moves 16 upwardly through dense phase catalyst bed 152 even-17 tually flowing into overflow well 156 communicating 18 with return line 158. ~eturn line 158 is shown exiting l9 through the center of dense phase catalyst bed 152, and communicating with riser reaction zone 110.

21 Flue gas formed during the regeneration of 22 the spent catalyst passes from the dense phase catalyst 23 bed 152 into dilute catalyst phase 154. The flue gas 24 then passes through cyclone 170 into plenum chamber 172 prior to discharge through line 174. Catalyst entrained 26 in the flue gas is removed by cyclone 170 and is 27 returned to catalyst bed 152 through diplegs 176, 178.

28 As previously indicated for the embodiment 29 of Figure 1, a passivation zone, such as passivation zone 190, may be disposed in or may comprise substan-31 tially all of overflow well 156 and/or return line 158.
32 If passivation zone 190 comprises substantially all of 33 return line 158, the fluidizing gas injected through 34 lines 160 and 162 may comprise reducing gas. TO avoid excess reducing gas consumption while providing suffi-36 cient quantities of gas to adequately fluidize the - ~z~

1 regenerated particles in line 158, it may be desirable 2 to dilute the reducing gas with steam and~or othe 3 diluent gas added through lines 160 and 162. The 4 residence time of catalyst in overflow well 156 and return line 158 typically ranges between abouc 0.1 and 6 about 1 minute. Here also it may be necessary to 7 increase the rate at which metal contaminant on the 8 catalyst is passivated. As shown for the embodiment of 9 Figure 1, it may be desirable to add a stripping gas, such as steam through line 192 to overflow well 156 to 11 remove entrained oxygen from the regenerated catalyst.

12 The reducing gas preferably is added to 13 passivation zone 190 at a plurality of locations 14 through branched lines, such as lines 202, 204, 206, 208, and 210 extending from reducing gas header 200. As 16 previously described in Figure 1, a control means, such 17 as control valve 220 is disposed in reducing gas header 18 200 to regulate the rate of addition of reducing gas to 19 passivation zone 190. A cracked product monitoring means, such as analyzer 230 is shown communicating with 21 cracked product line 130 and with control valve 220 to 22 maintain the sampled cracked product component within 23 the desired limits by regulation of the rate of addi 2~ tion of reducing gas to passivation zone 190. Since hydrogen is one of the products produced by the adverse 26 cataly-tic properties of the metal contaminants, 27 hydrogen may be the preferred component to be regu-28 lated. Since the metal contaminant also catalyzes the 29 formation of coke, the rate of reducing gas addition also could be regulated by -the monitoring of the rate 31 of coke production, such as by monitoring the heat 32 balance around regeneration zone 150, as previously 33 described. ~s in the embodiment of Figure 1, the rate 34 of catalyst circulation may be monitored by a sensing means, such as sensor 240, communicating with valve 36 220, to control the maximum rate of addition of 37 reducing gas to passivation zone 190. The commercial 1 operation of this embodiment would be substantially 2 similar to that previously described for the embodiment 3 of Figure 1. A component in the product stream, such as 4 hydrogen, is monitored by analyzer 230, which directs control valve 220 to adjust the rate of addition of 6 reducing gas to passivation zone 190, such as to 7 minimize the hydrogen content in stream 130. Sensor 240 8 monitors the catalyst circulation rate and operates as 9 an over-ride on control valve 220, to reduce the rate of additio~ of reducing gas if the reducing gas has, or 11 is about to have, an adverse effect on the catalyst 12 circulation rate.

13 The metals concentration deposited on the 14 catalyst is not believed to differ significantly whether the embodiment of Figure 1 or the embodiment of 16 Figure 2 is used. Thus, the amount of reducing gas 17 which is consumed in passivation zones 90, 190 of the 18 embodiments of Figures 1, 2, respectively, and the 19 amount of passivation promoter which is added should not differ greatly. Since the catalyst must be 21 fluidized in the embodiment of Figure 2, and need not 22 be fluidized in the embodiment of Figure 1, it is more 23 likely that~ in practicing the embodiment of Figure 2, 2~ a diluent gas will be added with reducing gas to pas-sivation zone 190 to Eluidize the catalyst.

26 The rate of addition of the passivation 27 promoter will be a function, in part, of the residence 28 time of the cracking catalyst ln the passivation zone, 29 the particular passivation promoter utilized, the metals level on the catalyst, the desired degree of 31 passivation and the passivation zone and temperature.

32 Typically, the passivation promoter concentration may 33 range between about 0.005 and about 0.20 weight percent 34 of the catalyst present in the cracking system and preferably between about 0.025 and about 0.10 weigh-t 36 percent of the cracking catalyst present.

Q ~ ~ ~
-- lg 1 While the reducing gas consumption rate in 2 passivation zones 90, 190, of Figures 1, 2, respec-3 tively, will be a function, in part, of the metal 4 contaminant levels on the catalyst, the desired degree of passivation and the amount of reducing gas infil-6 tration into the regeneration zone, it is believed that 7 the overall rate of consumption of the reducing gas 8 will range from about 0.5 to about 260 SCF, preferably 9 from abou-t 1 to about 110 SCF, for each ton of catalyst passed through passivation zones 90, 190 if hydrogen is 11 used as the reducing gas.

12 In the embodiments of Figures 1 or 2, it is 13 believed that the combustion of coke in regeneration 14 zones 26 or 150, respectively, will heat sufficiently the cracking catalyst subsequently passed through pas-16 sivation zones 90, 190, respectively. The required 17 temperature in passivation zones 90, 190 will be a 18 function of the desired degree of passivation, the 19 particular passivation promoter utilized and the pas-sivation zone residence time. If the temperature of 21 the catalyst entering passivation zones 9Q or 190 is 22 not sufficiently high, additional heat may be added to 23 the passivation zone either directly, such as by the 24 preheating of the reducing gas, or by adding steam, or indirectly, such as by the addition of a heat exchange 26 means prior to, or within the passivation zone.

27 Reaction zones 10, 110 and regeneration 28 zones 26, 150, of Figures 1, 2, respectively, may be of 29 conventional design and may be operated at conditions well-known to those skilled in the art. Regeneration 31 zones 26, 150 may be operated in either a net oxidizing 32 or a net reducing mode. In a net oxidizing mode, 33 oxidizing gas in excess of that required to completely 34 combust the coke to CO2 is added to the regeneration zone. In a net reducing mode insufficient oxidizing gas lX~

1 is added to completely combust the coke to CO2 2 Regeneration zones 26 and 150 preferably should be 3 operated in a net reducing mode, since carbon monoxide ~ is a reducing gas which will help decrease the adverse catalytic properties of the metal contaminants on the 6 catalyst prior to the catalyst entering passivation 7 zones 90, 190.

8 The required residence time of the ca-tal~st 9 in the passivation zone may be dependent upon many factors, including the metal contaminant content of the 11 catalyst, the degree of passivation required, the con-12 centration of reducing gas in the passivation zone, and 13 the passivation zone temperature. The present invention 14 is of particular utility where the passivation zone residence time is limited, such as where the passiva-16 tion zone is disposed in the transfer zone communi-17 cating with the regeneration zone and reaction zone as 18 shown in Figures 1 and 2. It is to be understood, how-19 ever, that the present invention may be utilized where the passivation zone is not located in the transfer 21 line.

22 The utility of the present invention may be 23 seen from the following examples in which the effec-2~ tiveness of cadmium-tin mixture, cadmium, germanium, indium, tellurium, and zinc is demonstrated, particu-26 larly when combined with the use of a passivation zone 27 having a relatively short residence time.

28 Samples of previously used Super-DX cracking ,~,. .
29 catalyst, a silica alumina catalyst manufactured by Davison Chemical Company, a division of W. R. Grace and 31 Company, was impregnated with 1000 wppm nickel and 4000 32 wppm vanadium. Samples were passivated at 704C without 33 the addition of any passivation promoter. The Gas Pro-34 ducing Factor (GPF), a direct measure of the metal con-taminant activity, obtained by a microactivity test rr~e /71~

~0~3~6 1 (MAT) as described in ASTM D3907-80, was measured with 2 samples having differing passivation zone residence 3 times. The results are shown in Table I. The GPF is 4 described in detail, by Earl C. Gossett, "When Metals Poison Cracking Catalyst", Petroleum Refiner, Vol. 39, 6 No. 6, June 1980, pp. 177-180.

7 Table I

Catalyst Residence 11 Time in Hydrogen Degree of 12 Passivation Zone Gas Producing Passivation 13 (min) Factor (GPF) (GPF/GPFo) ~ . . . .
14 0 19.0 (GPFo) 1.0 15.6 0.82 16 8 13.9 0.73 17 10 12.9 0.68 18 20 9.5 0 50 19 40 7.5 0.39 6.5 0.34 21 90 5.8 0.31 22 2 hr 5~5 0.29 23 3 hr 5.3 0.28 24 4 hr 5.0 0.26 Separate samples of this same metal con-26 taminant-impregnated Super-DX catalyst were impregnated 27 with 2000 wppm of cadmium, germanium, indium~ tellurium 28 and zinc. These results are reported in Tables II, III, 29 IV, V and VI, respectively.

~LO~

Example 1 2 Samples of the Super-DX metal contaminated 3 cracking catalyst having 2000 wppm of each of the 4 above-noted passivation promoters were placed in a passivation zone maintained at 704C for varying resi-6 dence times after which the GPF of the passivated 7 catalysts was determined. Tables II, III, IV, V and VI
8 present the gas producing factors and degree of passi-9 vation for the passivated catalyst samples impregnated with cadmium, germanium, indium, tellurium, and zinc, 11 respectively. Tables II - VI also present the GPF pre-12 dicted from the additive effect of hydrogen passivation 13 and the use of passivation promoters. The degree of 14 passivation from Table I was ~sed to estimate the passivation achieved by hydrogen alone. The GPF for 16 the promoted samples without hydrogen passivation 17 denoted as GPFo was used to estimate the individual 18 contribution from the passivation promoter alone. The 19 predicted combination of these effects for metal passivation was calculated as follows: GPF predicted =
21 ~Individual effect of hydrogen passivation at each 22 residence time) ~ (GPF for promoted sample with no 23 hydrogen passivation)~ The deyree of passiva-tion 24 attributable to hydrogen passivation at each residence time is 276 (GpF ) at residence time (GPFol bas3 28 The degree of passivation attributable to the passi-29 vation promoter is 31 ( G O additive) ~ ~ Pass) 1 where GP~o base = GPF with no hydrogen passivation and 2 no passivation promoter GPFo~ additive = GPF with no hydro-4 gen passivation, but with the pas-sivation promoter present 6 GPFpaSs = GPF measured for hydrogen 7 passivation at indicated time with 8 no passivation promoter present 9 As may be seen from Tables II-VI, at short passivation zone residence times, i.e., less than about 10 minutes, 11 when each of the passivation promoted catalyst samples 12 is passivated, the reduction in the gas producing 13 factors ls greater than the additive effect for the 14 individual reductions in the gas producing factor for hydrogen passivation at a given passivation zone resi-16 dence time and temperature and the effect of the metal 17 passivation additive.

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- 2g 1 Another sample of Super-DX metal contami-2 nated cracking catalyst having 1000 wppm Ni and 4000 3 wppm V was passivated at 704C without the addition of 4 any passivation promoter. This catalyst exhibited 5 higher metal contaminant activity as compared with -that

6 used in the previous tests. The Gas Producing Factor

7 again was measured at different passivation zone resi-

8 dence times to measure the metal contaminant activity.

9 The results are shown in Table VII.

Example II

11 A sample of this second Super-DX metal con-12 taminated catalyst was impregnated with only 250 wppm 13 of cadmium. The catalyst sample was passivated for 14 varying residence times, after which the GPF of the 15 passivated sample was measured. The results are also 16 presented in Table VII. As may be seen from Table VII, 17 at short passivation zone residence times, i.e., less 18 than about 30 minutes, the red~ction in the Gas Pro-19 ducing Factor for the passivation promoted sample is 20 greater than the additive effect for the individual 21 reductions in the GPF for hydrogen passivation at a 22 given pas~ivation zone residence time and temperature 23 and the metals passivation additive..

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~L0~6 1 Thus, Tables I - VII demonstrate that -the 2 present invention is of particular utility in situa-3 tions where the passivation zone residence time is 4 relatively short, such as when a transfer line passi-vation zone is utilized.

6 Tables VIII and IX demonstrate that the 7 unexpected reduction in the Gas Producing Factor may be 8 affected by the passivation zone temperature.

9 ~ third sample of Super-DX metal contami-nated cracking catalyst having 800 wppm NI and 2400 11 wppm V was placed in a passivation zone for varying 12 times at 593C and 649C to determine the GPF at dif-13 ferent passivation zone residence times.

14 Example II

These catalyst samples also were impregnated 16 with 1000 wppm cadmium and the tests repeated. From 17 Table VII it may be seen that the unexpected reduction 18 in the GPF shown in Table II for cadmium at 704C not 19 realized at 593C, or 649C. This illustrates that, at short residence times, it may be necessary to maintain 21 the passivation zone above a predetermined temperature 22 ~or effective metals passivation.

1 Table VIII

3 CADM~UM; PASSIVATION ZONE TEMPERATURE 593C

4 Cracking Catalyst Residence Time No Cadmium 1000 wppm Cadmium 6 in Hydrogen Measured Gas Measured Gas 7 Passivation Zone Producing Factor Producing Factor 8 (min) tGPF Meas) (GPF Meas) 9 0 14.7 15.9 11.8 14.6 11 10 12.3 15.8 12 30 11.5 15.7 13 60 11.2 15.1 14 Table IX

C~ACKING CATALYST IMPREGNATED WITH 1000 WPPM
16 _ CADMIUM, PASSIVATIOM TEMPERATURE 649C

17 Cracking CatalySt 18 Residence Time No Cadmium 1000 wppm Cadmium 19 in Hydroyen Measured Gas Measured Gas Passivation Zone Producing Factor ~roducing Factor 2]. (min) (GPF Meas) (GPF Meas) 22 o 14.7 14.8 ~3 5 13.1 15.6 24 10 12.2 15.2

10.6 12.4 26 60 8.5 10.1 27 Example IV

28 Samples of the Super-DX metal contaminated 29 cracking catalyst having a combination of 1000 wppm tin and 1000 wppm cadmium also were placed in a passivation 31 zone maintained at 704C for varying residence times 32 after which the GPF of the passivated catalysts was 33 determined. The GPF data for the combination of tin ~2~

1 and cadmium also is presented in Table X. As may be 2 seen from Table X, the use of a passivation zone and no 3 passivating agent reduced the GPF of the cracking cata-4 lyst. The addition of cadmium, tin, and particularly a combination of cadmium and tin all reduced the GPF
6 still further. However, it should be noted that the 7 combination of cadmium and tin reduced the GPF below 8 that of equivalent weights of either cadmium or tin 9 alone, particularly at short residence times, i.e., about five minutes or less.

11 Example V

12 A sample of Super DX cracking catalyst con-

13 taminated with 1000 wppm Ni and 4000 wppm V was exposed

14 alternately to one minute in a hydrogen passivation lS zone and to ten minutes in a regeneration zone com-16 prising 2~ oxygen to simulate a commercial cracking 17 system. Gas Producing Factors were obtained at various 18 cumulative residence times in the passivation zone and 19 also are presented in Figure 3. As shown in Figure 3, the combination of 1000 wppm tin and 1000 wppm cadmium 21 produced a higher degree of passivation than either 22 2000 wppm tin, 2000 wppm cadmium or catalyst without 23 passivation promoter.

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~24~9a~i 1 Separate samples of metal contaminant im-2 pregnated Super DX ca-talyst were impregnated with 2000 3 wppm of cadmium, or with 2000 wppm of tin. The GPF of 4 the unpassivated catalyst was determined and is presented in Table X and Figure 3. Samples of these 6 catalysts also were placed in a passivation zone main-7 tained at 70AOC for varying residence times after which 8 the GPF of the passivated catalyst was determined.
9 These results also are presented in Table X. In addition, tests were conducted in which the indicated 11 catalyst samples alternately were exposed to a hydrogen 12 passivation zone for one minute and to a typical regen-13 eration zone atmosphere comprising 2~ oxygen for ten 14 minutes to simulate a commercial cracking system. Gas Producing Factors were obtained at various cumulative 16 residence times in the passivation zone. Plots of the 17 degree of passivation 19 GPFo for Super DX catalyst without impregnated passivation 21 promoter, with 2000 wppm tin, and with 2000 wppm cad-22 mium are presented in Figure 3. GPFo is the Gas Pro-23 duciny Factor obtained with no residence time in a 24 passivation zone. Use of the term GPF
26 GPFo 27 serves to minimize any inherent differences in contam-28 inant metal activity of the catalyst samples, and per-29 mits comparison of the relative degrees of passivation as a function of cumulative hydrogen passivation 31 residence time.

32 The present invention is of particular 33 utility in situations where the passivation zone 34 residence time is relatively short, such as where a transfer line passiva-tion zone is utilized.

~L2~
_ 36 _ 1 The passivation promoters may be added to 2 the cracking system or impregnated on-to the cracking 3 catalyst in elemental form or as a compound which may 4 decompose to deposit the passivation promoter on the catalyst. The particular passivation promoter which is 6 utilized will be dependent on many factors, including 7 availability, process economics, corrosion, and desired 8 degree of passivation.

9 Among the preferred cadmium, germanium, indium, tellurium and zinc compounds are metal organic, 11 organic or inorganic complex salts, with metal organic 12 oil soluble compounds being particularly preferred. The 13 particular passivation promoter which is utilized will 14 be dependent on many factors, including availability, process economics, corrosion, and desired degree of 16 passivation. Particularly preferred passivation pro-17 moters include cadmium-tin mixtures, cadmium, german-18 ium, zinc and compounds thereof, with cadmium-tin mix-19 tures and compounds thereof being especially preferred.
When cadmium-tin mixtures are used, the cadmium-tin 21 ratio/ on an elemental metal basis, may change from 22 about 0.1:1 to about 9:1.

23 From the data presented above~ it can be 24 seen that the combination of reducing gas passivation at elevated temperature and the use of the previously 26 enumerated passivation promoters was more effective 27 than either treatment alone, particularly at passi-28 vation zone residence times of about 5 minutes or less, 29 which would be greater than typical residence times for cracking catalyst in a transfer line passivation zone.
31 The combination oE the use of one or more passivation 32 promoters and the reducing zone operated at elevated 33 temperature to passivate metal contaminants present on 34 cracking catalyst is of particular utility where the 1 passivation zone i5 disposed in the transfer zone, such 2 as passivation zones 90, 190 of Figures 1 and 2, 3 respectively.

4 The amount of passivation promoter which is utilized will be dependent on several factors, 6 including the particular promoter utilized, the metal 7 contaminant content on the catalyst, the desired degree 8 of passivation, the average catalyst residence time in 9 the passivation zone, and the conditions in the passi-vation zone. The amount of passivation promoter which 11 is used typically will range between about 0.005 and 12 about 0.20 weight percent of the catalyst, preferably 13 between about 0.025 and about 0.10 weight percent of 14 the catalyst.

The method by which the passivation promoter 16 is added to the catalyst is not believed to be criti-17 cal. The passivation promoter may be impregnated 1~3 directly into the catalyst before use, or it may be 19 added to the cracking system during operation. To main-tain the desired degree of passivation, a preferred 21 method is to add the passivation promoter directly to 22 the cracking system, preferably by adding a slip stream 23 of the passivation promoter in a suitable carrier to 24 the reaction zone.

In a typical commercial cracking system such 26 as that shown in Figure 1 catalyst residence time in 27 the transfer zone, comprising standpipe 42 and U-bend 28 44, typically is about 0.1 to about 2 minutes. Simi-29 larly, for a typical commercial cracking system similar to that shown in Figure 2, average catalyst residence 31 time in transfer zone 190 typically ranges between 32 about 0.1 and about 1.0 minutes. Thus, the transfer 33 zones of Figures 1 and 2 typically have sufficient 34 residence time to passivate catalyst upon the intro-duction of reducing gas.

~Z~ 6 _ 38 _ 1 The reducing agent utilized in the passi-2 vation zone is not critical. It is believed that com-3 mercial grade CO and process gas streams containing H2 4 and/or CO can be utilized. Hydrogen or a reducing gas stream comprising hydrogen is preferred, since this 6 achieves the highest rate of metals passivation and the 7 lowest level of metal contaminant potency. Preferred 8 reducing gas streams containing hydrogen include 9 catalytic cracker tail gas streams, reformer tail gas streams, spent hydrogen streams from catalytic hydro-11 processing, synthesis gas, steam cracker gas, flue gas, 12 and mixtures thereof. The reducing gas conten-t in the 13 passivation zone should be maintained between about 2%
14 and about 100%, preferably between about 10% and about 75% of the total gas composition depending upon the 16 hydrogen content of the reducing gas and the rate at 17 which the reducing gas can be added without adversely 18 affecting the catalyst circulation rate.

19 The stripping gas, if any, added -through line 92 of Figure 1 and line 192 of Figure 2 will be a 21 function in part of catalyst flow rate. Typically, the 22 stripping gas Elow rates through each of these lines 23 may range between about 0.1 SCF and about 80 SCF, 2~ preEerably between about 8 and about 25 SCFM per ton of catalyst circulated.

26 Passivation zones 90, 190 may be constructed 27 of any chemically resistant material capable of with-2~ standing the relatively high temperature and the 29 erosive conditions commonly associated with the cir-culation of cracking catalyst. The materials of con-31 struction presently used for transfer piping in 32 catalytic cracking systems should prove satisfactory.

1~0~

_ 39 _ 1 The pressure in passivation zones 90, 190, 2 of Figures 1, 2, respectively, will be substantially 3 similar to or only slightly higher than the pressures 4 in the regenerated catalyst transfer zones of existing catalytic cracking systems. When the embodiment of 6 Figure l is used, the pressure in passivation zone 90 7 may range from about 5 to about 100 psig, preferably 8 from about 15 to about 50. When the embodiment of 9 Figure 2 is used the pressure may range from about 15 psig to about 100 psig, preferably from about 20 psig 11 to about 50 psig.

12 In general, any commercial catalytic 13 cracking catalyst designed for high thermal stability 14 could be suitably employed in the present invention.

15 Such catalysts include those containing silica and/or

16 alumina. Catalysts containing combustion promoters such

17 as platinum also can be used. Other refractory metal

18 oxides such as magnesia or zirconia may be e~ployed and l9 are limited only by their ability to be efEectively 20 regenerated under the selected conditions. With par-21 ticular regard to catalytic cracking, preferred cata-22 lysts include the combinations of silica and alumina, 23 containing 10 to 50 wt.% alumina, and particularly 24 their admixtures with molecular sieves or crys-talline 25 aluminosilicates. Suitable molecular alumino-silicate 26 materials, such as faujasite, chabazite, X-type and 27 Y-type aluminosilicate materials and ultra stable, 28 large pore crystalline aluminosilicate materials. When 29 admixed with, for example, silica-alumina -to provide a 30 petroleum cracking catalyst, the molecular sieve con-31 tent of the fresh finished catalyst particles is 32 suitably within the range from 5-35 wt.%, preferably 33 8-20 wt.%. An equilibrium molecular sieve cracking 34 catalyst may contain as little as about 1 wt.~
35 crystalline material. Admixtures of clay-extended 36 aluminas may also be employed. Such catalysts may be 37 prepa~ed by any suitable method such as by impregna~

_ 40 _ 1 tion, milling, co-gelling, and the like, subject only 2 to the provision that the finished catalysts be in a 3 physical form capable of fluidization.

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS
FOLLOWS:

1. A process for passivating a cracking catalyst utilized to crack metal contaminated hydro-carbon feedstocks to lower molecular weight products in a cracking system wherein a hydrocarbon feedstock con-taining a metal contaminant selected from the group consisting of nickel, vanadium, iron and mixtures thereof is passed into a reaction zone of said crack-ing system containing therein a cracking catalyst to produce cracked products and cracking catalyst contam-inated with deposited coke and said metals, said coke being removed from said cracking catalyst in a regen-eration zone from which at least a portion of the said coke depleted metal contaminated cracking catalyst is circulated to said reaction zone through a passivation zone maintained under passivation conditions prior to returning said catalyst to said reaction zone, said process being characterized by the step of adding an effectual amount of a passivation promoter to the crack-ing system, said passivation promoter being selected from the group of metals consisting of cadmium-tin mixtures, cadmium, germanium, indium, tellurium, zinc, compounds thereof and mixtures thereof.

2. The process of claim 1 further char-acterized in that the cracking system is further com-prised of a transfer zone communicating with the re-generation zone and the reaction zone, and wherein the passivation zone is at least partially disposed in the said transfer zone.

3. The process of claim 1 fur-ther characterized in that a reducing gas is added to the passivation zone.

4. The process of any one of claims 1-3 further characterized in that the temperature of the passivation zone is maintained above about 700°C.

5. The process of any one of claims 1-3 further characterized in that the temperature in the passivation zone is maintained within the range of about 700°C to about 850°C.

6. The process of any one of claims 1-3 further characterized in that the concentration of the passivation promoter in the cracking system ranges between about 0.005 and about 0.20 weight percent of the cracking catalyst present.

7. The process of any one of claims 1-3 fur-ther characterized in that the average residence time of the cracking catalyst in the passivation zone ranges between about 0.1 and about 20 minutes.

8. The process of any one of claims 1-3 further characterized in that the passivation pro-moter is added to the hydrocarbon feed to the reaction zone.

9. The process of any one of claims 1-3 further characterized in that the passivation promoter is impregnated onto the catalyst prior to its intro-duction to the cracking system.

10. The process of any one of claims 1-3 further characterized in that when a cadmium-tin mix-ture is used as the passivation promoter, the cadmium to tin ratio in the mixture or an elemental metal basis, ranges between about 0.1:1 and about 9:1.