CA1141321A

CA1141321A - Extended cycle regenerative reforming

Info

Publication number: CA1141321A
Application number: CA000362266A
Authority: CA
Inventors: Charles S. Mccoy
Original assignee: Chevron Research and Technology Co
Current assignee: Chevron USA Inc
Priority date: 1980-10-28
Filing date: 1980-10-09
Publication date: 1983-02-15
Also published as: JPH0260713B2; NL8005882A; FR2492837B1; BE885884A; DE3039723A1; GB2085912B; FR2492837A1; JPS5776086A; GB2085912A

Abstract

ABSTRACT OF THE DISCLOSURE

A multiple zone catalytic reforming process in which the C5+ yield or the catalyst run length in all reaction zones downstream of the first is increased by maintaining less than 1 weight percent coke on the catalyst in the first reaction zone by adjusting the frequency of regeneration or replacement.

Description

002 EXTENDED CYCLE REGENERATIVE REFORM~NG

004 FIELD OF T~E INVENTION
005 The present invention is dlrected to the catalytic 006 reforming of hydrocarbon fractions. More speciflcally, the 007 present invention is concerned with reforming in a plurality of 008 reaction zones in series to improve the octane rating of the 009 feed.
010 BACXGROUND OF T~E INVENTI~N
011 Reforming of a naphtha fraction is generally 012 accomplished by passing the naphtha through a plurality of 013 reaction zones in series, each zone containing a catalyst com-014 prising a hydrogenation-dehydrogenation component supported on 015 a porous solid carrier. Typlcal catalysts include platinum on 016 alumina with or without such promoters such as rhenium, tin, 017 iridium, etc. The naphtha fraction to be reformed is contacted 018 in the first reaction zone with a platinum-containing catalyst 019 at reaction conditions to convert principally naphthenes to 020 aromatics. In addition to naphthene dehydrogenatlon, side 021 reactions such as isomerization, hydroisomerization and hydro-022 cracking may also occur. Typically, the effluent from the 023 first reaction zone is heated prior to being introduced to a 024 subsequent reaction zone.
025 After a period of use in reforming, the catalyst 026 becomes gradually deactivated due to the deposition of coke on 027 the surface of the catalyst and consequently a decrease of the 028 octane values of the reformate product is observed.
029 If the octane requirements imposed upon the par-030 ticular reforming system are to be continuously met, the 031 reaction temperature of the catalyst must be increased ln order 032 to compensate for the loss in activity due to the coke deposl-033 tion. The fastest catalyst deactivation occurs in the reactor 034 where paraff in dehydrocyclization and hydrocracking are the 035 principal reactions. Consequently, even with a constant inlet 036 temperature, the average reaction temperature increases with 037 each successive reactor because the reactions in each 002 successive reactor are not as endothermic as in the preceding 003 reactor.
004 Coke deposltion on the catalyst not only decreases 005 the activity of the catalyst -but also results in a decrease in 006 the yield of C5+ gasoline product produced. Thus,.the yield of 007 C5+ gasoline product generally decllnes throughout the 008 reforming process untll it reaches an unacceptable level, at 009 which point common practice is to regenerate all or part o the 010 catalyst. Typical coke levels on the catalyst at the tlme of 011 regeneration are 10 to 12 weight percent or more on the 012 catalyst in the last reactor and 5 or 6 weight percent on the 013 catalyst in the first reactor. Coke levels on catalysts in 014 intermediate reactors will generally fall between these two 015 figures.
016 Regeneration procedures, whether continuous or batch 017 operations, are generally well known to the art. Such 018 procedures generally involve several steps: a car5On burn-off 019 by contacting the catalyst with oxygen-containing gas at an 020 elevated temperature until substantially all of the carbon is 021 removed; subsequent contacting of the catalyst with an 022 oxygen-containlng gas to redistribute the platlnum group metal;
023 a halogen adiustment and optlonally a reduction of the 024 regenerated catalyst with a hydrogen-containing gas prlor to 025 returning the catalyst to the reactor. See, for instance, U.S.
026 Patent No. 3,496,096.
027 SUMMARY OF T~E INVENTION
028 It is an object of this inventlon to provide a 029 reforming process having an extended operating cycle between 030 regenerations when compared with ordinary reforming processes.
031 It is another object of this invention to provide a method for 032 extending the effective life of a reforming catalyst which is 033 nearing the end of the run.
034 In accordance with one embodiment of the present 035 invention there is provided for a reforming process where~n a 036 naphtha feedstock is contacted at reforming conditions in the 037 presence of hydrogen with a reforming catalyst in a plurality 001 _3_ 002 of reaction zones in series, a product of improved octane 003 rating is recovered from the effluent of the last reaction 004 zone, and during the course of said reforming coke deposits 005 upon sai_ catalyst thereby deactivating and necessitating 006 eventual regeneration or replacement thereof, the method of 007 extending the length of service of the catalyst in all reaction 008 zones downstream of the first whlch comprises maintaining the 009 level of coke on the catalyst in said first reaction zone at 010 less than 1~ by weight of the catalyst by adjusting the 011 frequency at whch the catalyst in said first reaction zone is 012 regenerated or replaced with fresh or regenerated catalyst.
013 In accordance with another embodiment of the present 014 invention, there is provided a multiple stage process for 015 catalytically reforminq a naphtha charge stock which comprises:
016 (a) reacting said charge stoc~ in the presence of 017 catalyst and hydrogen at catalytic reforming conditions in a 018 moving bed reaction zone through which the catalyst is movable 019 via gravity flow and recovering the resulting hydrocarbonaceous 020 effluent from the moving bed reactlon zone;
021 (b) maintaining the amount of coke deposited on the G22 catalyst in said moving bed reaction zone below 1% by weight Dy 023 at least periodically introducing fresh or regenerated catalyst 024 into the upper end of said moving bed reaction zone and at 025 least periodically withdrawing an equivalent amount of coke-con-026 taminated catalyst from the lower end of said moving bed 027 reaction zone;
028 (c) further reacting the effluent of said moving bed 029 reaction zone at catalytic reforming conditions in a flxed bed 030 reactor system containing at least one fixed bed reaction zone;
031 (d) recovering a normally li~uid catalytically reformed 032 product from the effluent withdrawn from the last fixed bed 033 reaction zone.
034 In accordance with yet another embodiment of the 035 present invention, there is provided a catalytic reformlng 036 process in which a naphtha fraction is contacted at reformlng ~37 conditions in the presence of hydrogen with a reforming 3Zl 001 -~-002 catalyst in a p`lurality of reaction zones in series, a product 003 of improved octane rating is recovered from the effluent of the 004 last reaction zone, and during the course of said reforming 005 coke deposits upon said cata~yst and deactivates thereby 006 decreasing decreasing the yield of said product, tne method of 007 increasing said yield whlch comprises maintaining the level of 008 coke on the catalyst in said first reaction zone at less than 009 1~ by weight by adjusting the frequency at which the catalyst 010 in said first reaction zone is regenerated or replaced with a 011 fresh or regenerated catalyst.

013 FIG. 1 illustrates the effect on reaction tempera-014 ture, CS+ yield and hydrogen production of a catalyst bed 015 containing 0, 10, 20 and 30% fresh catalyst on top of a layer 016 of coked catalyst. FIG. 2 illustrates yield losses due to 017 variable levels of coke on catalyst at the top of a bed of 018 coked catalyst. FIG. 3 illustrates the results of a comparlson 019 between reforming with coked catalyst and with 10~ fresh 020 catalyst on top of coked catalyst.

022 The present invention is applicable to those 023 reforming systems wherein a plurality of reation zones in 024 series are used. Preheaters are preferably present between 025 reaction zones so that the temperature of the feed to each 026 reaction zone may be controlled. Preferably, each reaction 027 zone will be located in a separate reactor. The present 028 invention is concerned with those systems wherein at least two 029 reactors and preferably from 3 to 5 reactors are in series.
030 Although some or all of the reactors may be moving bed 031 reactors, the most preferred system is for all reactors to be 032 fixed bed systems or all reactors but the first to be fixed bed 033 systems with the first being a moving bed system.
034 In multi-reaction zone reforming systems, the 035 catalyst may vary in composltion in the different reforming 036 zones, although generally the catalyst is the same in all 037 zones. ~owever, the volume of catalyst generally differs from , ~l~i;3'Zl 002 one reaction zone to the next. A typical catalyst loading ln a 003 three-reactor system may employ one-quarter of the total charge 004 of catalyst in the first reactor, one-quarter in the second 005 reactor and one-half in the last reactor. The first reactor 006 generally contains less catalyst because the highly endothermic 007 reaction taking place therein results in the rap~d coollng of 008 the feed. If a large volume of catalyst were present in the 009 first reactor, the temperature of the feed in the lower portion 010 of the catalyst bed would be too low for significant dehydro-011 genation reactions to occur and thus the lower portion of the 012 catalyst bed would not be used effectively.
013 To achieve the benefits of the present invention, the 014 catalyst in the first reaction zone should be replaced or 015 regenerated with sufficient frequency to maintain a level of 016 coke in the catalyst less than 1% by weight preferably less 017 than 0.7% and still more preferably less than 0.5~ by weight.
018 The first reaction zone may contain up to 30% by volume of the 019 total catalyst in the reactor system, although preferabiy it 020 will contain only up to 20~ and still more preferably only up 021 to 15% of the total catalyst mass in the reactor system.
022 The temperature in each of the reactlon zones can be 023 the same or different, but generally it will fall in the range 024 from 700~F to 1050F and preferably within the range of about 025 850F to 1000F. The terminal reaction zone generally has the 026 highest average catalyst bed temperature. The pressure in each 027 of the reaction zones will usually be the same, eitner 028 atmospheric or superatmospheric. Preferably, the pressure will 029 be in the range of 25 to 1000 psig and more preferably between 030 50 and 750 psig. The temperature and pressure can be 031 correlated with the liquid hourly spaced velocity (L~SY) to 032 favor any particularly desirable reforming reactions and will 033 generally be from 0.01 to 10 and preferably from 1 to 5. It is 034 apparent that with different catalyst loading and different 035 reaction zones, the space velocities in the individual reactlon 036 zones can vary considerably.
~;

ll.~i;~l 002 Although reforming generally results in the 003 production of hydrogen, it is common to recycle hydrogen 004 separated from the effluent of any of the reaction zones, 005 usually the terminal reaction~ zone, to the first or subsequent 006 reaction zones. The hydrogen can be admixed with the feed 007 prior to contacting catalyst or simultaneously with the intro-00~ duction of the feed to the reaction zone. The presence of 009 hydrogen serves to reduce formation of coke which tends to 010 poison the catalyst. ~ydrogen is preferably introduced into 011 the reforming reaction zone at a rate which varies from 0.5 to 012 20 mols of hydrogen per mol of feed. Hydrogen can be an ad-013 mixture with light gaseous hydrocarbons.
014 The catalyst used in the reaction zones comprises a 015 platinum group component in association with a porous solid 016 carrier. Preferably the platinum group component is platinum 017 and the preferred porous solid carrier is a porous refractory 018 in organic oxide, for example, alumina. The platinum group 019 component will be present in an amount of from 0.01 to 3 weight 020 percent and preferably 0.01 to 1 weight percent.
021 Other components in addition to the platinum group 022 component can be present on the porous solid carrier. It is 023 particularly preferred that rhenium be present, for example in 024 an amount of 0.01 to S weight percent and more preferably 0.01 025 to 2 weight percent. Rhenium significantly improves the yields 026 obtained using a platinum-containing catalyst, and a 027 platinum-rhenium catalyst is more fully described in U.S.
028 Patent 3,415,737. Generally, the catalyst will be promoted for 029 reforming by the addition of a halide, particularly fluorlde or 030 chloride. The halide provides a limited amount of acldity to 031 the catalyst which is beneficial to most reforming operations.
032 The catalyst promoted with hallde preferably contains 0.1 to 3 033 weight percent total halide content and the preferred hallde is 034 chloride.
035 The catalyst within the first reactlon zone may be 036 replaced or regenerated in situ or ex sltu, with sufficient 037 frequency to maintain less than 1 weight percent coke on the 002 catalyst, and preferably less tAan 0.7 welght percent and more 003 preferably, below 0.5 weight percent. The amount of coke on 004 the catalyst should be calculated as the average amount of coke 005 on the entire volume of catalyst in the first reaction zone.
006 The catalyst in the first reaction zone may e1ther be disposed 007 in a fixed or moving bed. If the reactlon zone is a flxed bed, 008 it may be radial flow, upflow or downflow, and it may be 009 desirable for a swing reactor to be present and ready to be put 010 in service when the first fixed bed reaction zone is removed 011 from service for regeneration. Of course, if the first 012 reaction zone has a moving catalyst bed, the reaction zone c n 013 be maintained onstream while fresh or regenerated catalyst is 014 added to the top of the reaction zone and used coke-deactivated 015 catalyst withdrawn from the bottom. The remaining reaction 016 zones in the system can be either fixed bed or moving bed 017 reaction zones, b~t for the purposes of this invention, it is 018 advantageous if they are fixed bed reaction zones. The fixed 019 bed reaction zones can be either upflow, downflow or radial 020 flow with radial flow being preferred.
021 To determine the level of coke or carbon on the 022 catalyst of the first reactor, the catalyst can be sampled as 023 it is removed from the reactor in a moving bed reaction zone or 024 catalyst samples may be withdrawn from the reaction zone itself 025 without disrupting a normal reforming process. A variety of 026 means are available for removing catalyst samples from reactors 027 without involving shutdown of the reactor, for example, see 028 U.S. Patents 3,129,5gO and 3,319,469. The level of coke or 029 carbon deposited on the catalyst sample may be determined by 030 any suitable means such as combustion of the carbon and 031 measurement of the quantity of CO2 produced or by determining 032 the rate at which a combustion zone progresses through a bed of 033 coke-contaminated catalyst, such as described in ~.S. Patent 034 3,414,382.
035 The hydrocarbon feedstock employed in the retorming 036 operation of the present invention may be any suitable hydro-037 carbon capable of being catalytlcally reformed at the stated ~14~3'Zi oo1 ~

002 conditions. Preferably the feedstock is a naphtna fraction, 003 which is a light hydrocarbonaceous oil generally boiling within 004 the range from 70 to 550F and preferably from 150 to 450F.
005 The feedstoc~ may be, for example, either a straight-run 006 naphtha, a thermally cracked or catalytically cracked naphtha 007 or blends thereof. Generally the naphtha feed will contain 008 from about 25~ to 75% and preferably about 35% to 60~
009 paraffins, about 15~ to 65% and preferably about 25% to 55%
010 naphthenes and about 5% to 20% ~romatics, calculated on a 011 volume percent basis.
012 Catalyst regeneration procedures are well known to 013 the art and will generally include burning the carbon from the 014 catalyst by contacting the catalyst to an oxygen-containing gas 015 at an elevated temperature from about 700 to 1100F.
016 Preferably the oxygen concentration and temperature are 017 increased in stages as the regeneration progresses. Following 018 the carbon burn-off, a gaseous halogen may be introduced into 019 contact with the catalyst. Subsequently, the catalyst may be 020 dried and reduced before being returned to the reforming zone.
021 Examples of suitable regeneration processes are illustrated in 022 U.S. Patents 3,134,732 and 3,496,096.

024 The present invention will be further clarifled by 025 consideration of the following examples which are intended to 026 be purely exemplary and not limiting of this invention.
027 Example 1 shows that the presence of a small amount of fresh 028 catalyst on top of a bed of catalyst containing 10.9% carbon 029 acts to substantially increase the C5+ yield and hydrogen 030 production as well as to decrease the average catalyst 031 temperature required to ~ake a product of a predetermined 032 octane. These results indicate that the presence of a small 033 amount of fresh catalyst upstream of a larger mass of 034 coke-contaminated catalyst serves to significantly extend the 035 length of time which the total mass of catalyst can be 03~ maintained in reforming service before being regenerated.
037 Example 2 shows the adverse effect on yield due to the presence 3Zl 001 _9_ 002 of an increasing amount of coke on a small mass of catalyst 003 situated above a larger mass of coke deactivated catalyst.
004 These results indicate that the less carbon that is present on OOS the catalyst in the first reaction zone, the better the overall 006 yield. Example 3 is a side-by-side comparison of reformlng 007 with a catalyst bed containing 10% fresh catalyst on top of a 008 bed of test catalyst containing various levels of co~e which 009 illustrates the activity and yield advantages of having a small 010 layer of fresh catalyst present in the top of the catalyst bed.
011 Example 1 012 A mid-continent naphtha having the characteristics 013 shown in Table I was passed through a series of foul reactors 014 containing a platinum-rhenium reforming catalyst at reforming OlS conditions including a pressure of 200 psig, a liquid hourly 016 space velocity of 2, a hydrogen to hydroccarbon mol ratio of 3 017 and a temperature adjusted to obtain a reformate product having 018 a research octane number of 98 clear.

022 Mid-Continent Naphtha a24 Gravity API 55.0 025 D-86 Distillation 027 10% - F 214 028 30% - F 239 029 50% - F 263 030 70% - F 2~4 031 90% - F 342 032 EP - ~F 390 034 % Paraffins 43.1 035 % Naphthenes 46.8 036 % Aromatics 10.0 002 After 55 days onstream, a portion of the catalyst was 003 removed from the last reactor in the series. The catalyst, 004 averaging 10.9 weight percent coke, was tested ln a micro-sized 005 pilot plant reformer in three separate tests in which the top 006 10%, 20% and 30% of the used catalyst replaced by an equlvalent 007 amount of fresh catalyst. The results, as represented ln FIG.
008 1 and Table II show that by placing a layer of fresh catalyst 009 on top of a larger mass of coked catalyst, (1) the activity of 010 the total mass of catalyst increased significantly, by 19F for 011 10% fresh catalyst, by 22F for 20~ fresh catalyst, and by 32F
012 for 30% fresh catalyst; (2) the C5+ yield increased by 1.9 013 liquid volume percent, from approximately 78.6 to 80.5 LV
014 percent; (3) the hydrogen production increased by about li~, 015 from 1217 standard cubic feet per barrel of feed to 1356-1347 016 standard cubic feet per barrel of feed; and (4) CH4 production 017 decreased 23-26~, from 108 standard cubic feet per barrel of 018 feed to 80-83 standard cubic feet per barrel of feed. Thus, 019 the presence of a small amount of fresh catalyst on top of a 020 larger amount of coked catalyst serves to substantially 021 increase the activity, C5+ liquid yield and rate of hydrogen 022 production, far more than would ~e predicted just from the 023 small amount of fresh catalyst added.

027 Layered-Bed Tests on End-of-Run Catalyst 029 ~ C5+,LV% H2, SCF/B ~ , SCF/B
031 End-of-Run (EOR) 032 Catalyst, 10.9% C, 955 78.6 1217 108 033 10% Fresh over 034 90~ EOR Catalyst 936 80.6 1356 82 035 20% Fresh over 036 80% EOR Catalyst 933 80.5 1347 80 037 30% Fresh over 038 70% EOR Catalyst 923 80.5 1349 83 039 Example 2 040 A study was made to determine the effect on yield of 041 a varying amount of coke on the top 10% of c~talyst in a 1141;~Zl 002 cataiyst bed. FIG. 2 shows the effect on C5+ yield and H2 003 yield associated with an increasing carbon content on the 004 catalyst in the top 10% of the catalyst bed. Using as the 005 standard a catalyst bed containing 10~ fresh catalyst on top of 006 90% catalyst containing 13.3 weight percent carbon, a catalyst 007 bed with the top 10% of catalyst containing 2 weight percent 008 carbon loses 1% by volume of C5+ yield; a catalyst bed with the 009 top 10% of catalyst containing about 6 weight percent carbon 010 loses 2% by volume of C5+ yield; and a catalyst bed with the 011 top 10% of catalyst containing about 12 weight percent carbon 012 loses about 3% by volume of C5+ yield. Hydrogen yield loss 013 also increases in the same manner with increasing carbon 014 content in top 10% of the catalyst in the catalyst bed. Thus, 015 to obtain the maximum yield benefit from the process of this 016 invention, the amount of carbon on the catalyst in the top of 017 the catalyst bed should be kept as low as poss1ble.
018 Example 3 019 A test was conducted to compare the performance of a 020 bed of co~e-deactivated platinum-rhenium catalyst with an 021 equivalent volume of catalyst comprislng 10 volume ~ fres~
022 catalyst on top of 90% of the deactivated catalyst. Samples of 023 catalyst from a commercial reformer were obtained at approxi-024 mately 0, 28, 61, 89 and 122 days on stream. One portion of 025 each catalyst sample was tested on the feedstock shown in Table 026 I at reforming conditions including a pressure of 200 psig, a 027 liquid hourly space velocity of 2, a hydrogen to hydrocarbon 028 mol ratio of 3 and a temperature adjusted to obtain a reformate 029 product having a research octane number of 98, clear. A layer 030 of 10~ fresh catalyst was put on top of a 9o% layer of catalyst 031 from the 61, 89 and 122-day samples, respectively, and then 032 tested under the same reforming conditions.
033 The results, as shown in Figure III, demonstrate that 034 the catalyst beds containing 10% fresh catalyst are far more 035 active (20F after 120 hours) and more selectlve (4% C5+ yleld 036 after 120 hours) than the beds containing only coked catalyst.
037 The fouling rate for the beds containing 10% fresh catalyst is 002 less than that for the bèds of coked catalyst -- 0.15Ftday vs.
003 0.33F/day, indicating that the effect of the fresh catalyst is 004 far out of proportion to its volumetric presence.
OOS From the foregoing, it may be seen that the pr,esent 006 invention operates in a novel and effective manner to increase 007 the activity, selectivity (C5+ yield) or both of the total mass 008 of catalyst in a reforming reaction zone, and thus it ~er~.its 009 the service life of the bulk of catalyst to be extended before 010 regenera~tion or replacement is necessary.
011 Although only specific arrangements and modes of 012 operation of the present invention have been descr~bed, 013 numerous changes can be made in those arrangements without 014 depar.ing from the spirit of the invention and also changes ' 015 that fall within the scope of the appended claims are intended 016 to be embraced thereby.

Claims

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

1. In a reforming process wherein a naphtha feedstock is contacted at reforming conditions in the presence of hydrogen with a reforming catalyst in a plurality of reaction zones in series, a product of improved octane rating is recovered from the effluent of the last reaction zone, and during the course of said reforming coke deposits upon said catalyst thereby deactivating said catalyst and necessitating eventual regeneration or replacement thereof; the method of extending the length of service of the catalyst in all reaction zones downstream of the first which comprises maintaining the level of coke on the catalyst in said first reaction zone at less than 1% by weight by adjusting the frequency at which the catalyst in said first reaction zone is regenerated or replaced with fresh or regenerated catalyst.

2. The process of Claim 1 wherein said first reaction zone contains not more than 30% by volume of the total mass of catalyst in said plurality of reaction zones.

3. The process of Claim 1 wherein said first reaction zone comprises a moving bed of catalyst and at least periodically a portion of coke-contaminated catalyst is with-drawn from a lower portion of said moving bed, regenerated, and returned to the upper portion of said moving bed.

4. A multiple stage process for catalytically reforming a naphtha charge stock in a plurality of reaction zones in series which comprises:
(a) reacting said charge stock in the presence of catalyst and hydrogen at catalytic reforming conditions in a moving bed reaction zone through which said catalyst is movable via gravity flow and recovering the resulting hydrocarbonaceous effluent from said moving bed reaction zone;
(b) maintaining the amount of coke deposited on the catalyst in said moving bed reaction zone below 1% by weight by at least periodically introducing fresh or regenerated catalyst into the upper end of said moving bed reaction zone and at least periodically withdrawing an equivalent amount of coke-contaminated catalyst from the lower end of said moving bed reaction zone;
(c) further reacting said effluent of said moving bed reaction zone at catalytic reforming conditions in a fixed bed reactor system containing at least one fixed bed reaction zone;
(d) recovering a normally liquid catalytically reformed product from the effluent withdrawn from the last fixed bed reaction zone.

5. The process of Claim 4 wherein said coke-contaminated catalyst withdrawn from said moving bed reaction zone is regenerated and returned to the top of said moving bed reaction zone as said regenerated catalyst.

6. The process of Claim 4 wherein said first reaction zone contains less than 30% by volume of the total catalyst in said reaction zones.

7. In a reforming process wherein a naphtha feedstock is contacted at reforming conditions in the presence of hydrogen with a reforming catalyst in a plurality of reaction zones in series, a product of improved octane rating is recovered from the effluent of the last reaction zone, and during the course of said reforming coke deposits upon and said catalyst deactivates thereby decreasing the yield of said product, the method of increasing said yield which comprises maintaining the level of coke on the catalyst in said first reaction zone at less than 1% by weight by adjusting the frequency at which the catalyst in said first reaction zone is regenerated or replaced with a fresh or regenerated catalyst.

8. The process of Claims 1, 4 or 7 wherein said catalyst comprises an alumina support having disposed thereon in intimate admixture 0.01 to 3 weight percent platinum and 0.01 to 5 weight percent rhenium.