WO1993022404A1

WO1993022404A1 - Process and apparatus for catalytic reforming of naphtha

Info

Publication number: WO1993022404A1
Application number: PCT/EP1993/001066
Authority: WO
Inventors: John Mcinnes Bauld
Original assignee: Exxon Research And Engineering Company
Priority date: 1992-05-01
Filing date: 1993-04-27
Publication date: 1993-11-11
Also published as: DE69311548D1; EP0567700A1; ES2103084T3; EP0638115B1; MY131195A; DE69311548T2; EP0638115A1; CA2134548A1

Abstract

A catalytic naphtha-reforming process and apparatus comprises at least two reactors (18, 22, 25), and the operating temperature of at least the downstream reactor (25) is progressively raised to maintain the quality of the reformate as catalyst activity diminishes due to the deposition of coke thereon. When the maximum operating temperature of at least the downstream reactor (25) is attained, naphtha is diverted therefrom to a substitute reactor (40) containing active reforming catalyst, and naphtha-reforming operation is continued until the operating temperature of at least the substitute reactor (40) has been attained. The naphtha-reforming process is thereupon interrupted and catalyst in all reactors (18, 22, 25, 40) is regenerated. Thereafter, the naphtha-reforming process is restarted employing the original downstream reactor (25) in place of the substitute reactor (40). The regeneration-time is a reduced proportion of the naphtha-reforming time compared to the proportion for prior processes and apparatus.

Description

Process and Apparatus for Catalytic Reforming of Naphtha

The present invention relates to a process and to

-*^:ι an apparatus for the catalytic reforming of naphtha.

Background

Catalytic reforming, or hydroforming, is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight-run gasolines. In reforming, a multi¬ functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Platinum metal catalysts, or platinum to which one or more additional metal promoters has or have been added to form polymetallic catalysts are currently employed, reforming being defined as the total effect of the molecular changes, or hydrocarbon .reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics. isomerization of n-paraffins; isomerization cf alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably, carbonaceous material, (or "coke"), the latter being deposited on the catalyst. During the course of a reforming operation, the activity of the catalyst becomes reduced, due to factors which include the progressive deposition of coke on the catalyst. The reduction in catalyst activity is apparent as a reduction in the amount of reformed high-octane naphtha. The octane number of the product naphtha can be maintained by raising the temperature of contact between the naphtha and catalyst, but this expedient is limited by the temperatures at which there is an unacceptable level of risk of damage to equipment used for the reforming ^' process. When the maximum desirable operating temperature has been reached, it is necessary to regenerate the activity of the catalyst in order to restore the operation of the reforming process to economically useful levels.

Regeneration processes in fixed-bed catalytic reforming operations are basically of two types. In a so-called semi-regenerative process, the entire unit is operated by gradually and progressively increasing the temperature to maintain the activity of the catalyst, (which activity is reduced by coke deposition) until finally the entire unit is shut down for regeneration, and reactivation, of the catalyst. After regeneration and reactivation of the catalyst, the unit is put back on-oil. In a cyclic regeneration process, the reactors are individually isolated, or in effect swung out of line by various manifolding arrangements, motor operated valving and the like. The catalyst is regenerated to remove the coke deposits, and then reactivated while the other reactors of the series remain on stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, until it is put back in series. The cyclic method of regeneration offers advantages over the semi- regenerative type process in that regenerative type process in that regeneration and reactivation of the catalyst is effected in the isolated reactor without shutting down the unit and there is no loss of production. Moreover, because of this advantage, the unit can be operated at higher severities (that is, lower pressures and higher temperatures) to produce higher C§⁺ liquid volume yields of high octane gasoline than semi-regenerative refoπning units. However, reforming units adapted for cyclic operation are considerably more costly than units adapted for semi-regenerative operation, and the costs involved in converting a semi-regenerative unit for cyclic regeneration are prohibitively high.

Present semi-regenerative reforming units are usually operative to provide a reformate having an octane rating or octane number of less than 102 because the production of higher rated reformates accelerates catalyst deactivation and hence leads to unacceptably short periods of operation between shut-downs for regeneration. One way of dealing with the resulting reduced cycle- length time (i.e., the minimum time between similar phases of contiguous operations of the unit) is to increase the amount of catalyst in the reactors of the unit. Unfortunately such an expedient results in conversion of the naphtha feed to low value gases and reduces the yield of high octane naphtha.

It will be understood from the foregoing that there is a strong incentive to provide a means of modifying existing semi-regenerative reforming units so that they can produce high yields of high octane naphtha. The incentive is all the greater as legislation is enforced against the addition of octane-boosting lead compounds to naphtha. The present invention provides a catalytic reforming process performed in a reforming unit comprising a plurality of catalyst-containing reaction zones or reactors connected in series, wherein feed naphtha is passed into the first (or upstream) reactor of the series of reactors in the unit and product naphtha having a higher octane rating than the feed naphtha is recovered from the last (or downstream) reactor of the series of reactors in the unit, the process comprising, in sequence, the steps αf:

(a) passing naphtha into and through the series of reactors of the unit and- progressively increasing the temperature of at least the last reactor of the series of reactors to maintain the octane rating of the product naphtha at least at a given value;

(b) substituting a spare reactor containing active reforming catalyst in place of the last reactor of the series when the temperature of at least the last reactor of the series attains a specified maximum temperature;

(c) passing feed naphtha through the reactors of the unit and recovering a naphtha product having at least the given octane rating from the last reactor of the series;

(d) progressively raising the temperature of at least . the last reactor of the series of reactors to maintain at least the given octane of the product naphtha;

(e) optionally repeating steps (b) , (c) and (d) ; (f) interrupting the passage of naphtha into and through the reactors of the unit, and regenerating catalyst (by in situ and/or ex situ regeneration) in at least the last reactor of the series; and

(g) optionally repeating steps (a) to (f).

The amount of catalyst in the last reactor of the series in the_. unit may comprise from 50 to 90 weight percent of the total catalyst in the unit. The first reactor of the series in the unit may contain from 5 to 20 weight percent of the total catalyst in the unit.

In step (a), the temperature of the first reactor may be progressively raised to maintain the octane rating of the naphtha product.

The unit may comprise at least two, and usually three reactors connected in series.

In step (a), the temperature of other reactor(s) between the first and last reactors may be progressively raised to maintain the octane rating of the naphtha product.

The amount of catalyst contained in the reactors connected between the first and last reactors of the series may be in a range of from 5 to 45 weight percent of the total catalyst in the unit.

Step (f), may be effected by in situ regeneration and reactivation of catalyst in the reactors of the unit and at least one spare reactor, the spare reactor(s) being connected or connectable to form part of the unit for step (f). Following the regeneration and reactivation, the spare reactor(s) may be disconnected from the unit, and step .(a) may be started.

The present invention also provides a catalytic reforming unit for the catalytic re orming of naphtha comprising a plurality of catalyst-containing reactors connected in series for the passage of naphtha through the unit from the first reactor of the series to the last reactor thereof, means for heating naphtha passing from one reactor to the next reactor in the series, means for heating naphtha passing to the first reactor of .the series, recovery means for recovering reformed naphtha from the last reactor of the series, a substitute catalyst-containing reactor (which may contain substantially the same amount of catalyst as the said last reactor) , means or diverting naphtha to circulate from the last-but-one reactor of the series to the substitute reactor whereby the substitute reactor functions as the last reactor of the. series.

The last reactor and the substitute reactor may be connected in parallel with each other to receive naphtha from the last-but-αne reactor of the series and to discharge naphtha to the said recovery means, there being valves which may be operable to cause naphtha to pass through only one or other, at a time, of the said last reactor and the substitute reactor.

At least some of the said valves may be operable to cause catalyst regenerating/reactivating vapors to pass through the said last reactor and the said substitute reactor at the same time during a catalyst- regeneration/reactivation operation of the unit. Preferably, the regenerating/reactivating vapors pass through the last reactor and the substitute reactor in series. The invention is now further described with reference to non-limitative embodiments thereof and with reference to the accompanying drawing. The embodiments are given by way of example and are not limitative of the scope of the invention.

The drawing is a simplified flow sheet of a reforming unit according to the invention, and the drawing shows those features which are important for illustration of the principle of the process and apparatus of the invention. Items of equipment which are not shown, but which would be appreciated by a person skilled in the act, have been omitted.

Referring to the drawing, feed naphtha from a storage facility 11 is passed via pipes 12 and 13 to a preheat furnace 14 wherein the temperature of the naphtha is raised to a suitable or conventional naphtha- reforming temperature. Catalytic reforming of naphtha is effected in the presence of hydrogen, and although hydrogen is often a by-product of reforming, it is preferred to add a hydrogen-containing gas to the naphtha feed passing to the reformer-reactors. A hydrogen-containing gas in a pipe 15 is circulated by a circulation fan 16 into pipe 13 so that the naphtha passes to the furnace 14 in admixture with hydrogen.

The preheated naphtha and hydrogen mixture is circulated from the furnace 14 via a conduit 17 to a first-stage reactor 18 containing a naphtha reforming catalyst, e.g., a catalyst comprising platinum dispersed on a halogen-containing alumina support and optionally containing promoter metals and other components, such as sulfur. The reforming reactions which occur in the first stage reactor 18 include the conversion of substituted and unsubstituted cyclo-paraffins to aromatics with the production of hydrogen and the isomerization of n-paraffins to iso-paraf ins, inter alia. The aforesaid, and other reactions, are endothermic in sum, and the effluent stream leaving the reactor 18 is at a lower temperature than the stream entering the reactor 18.

The effluent stream from the first-stage reactor 18 is recovered in conduit 19 and passed via a re-heat furnace 20 into conduit 21 which conducts the reheated stream to a second-stage reactor 22. The re-heat furnace raises the temperature of the effluent stream from reactor 18 to a temperature suitable for the second-stage reforming reactions in the second-stage reactor 22.

The second-stage reactor 22 contains a naphtha reforming catalyst which may be the same as, or different to, the catalyst in the reactor 18. The second-stage reactor 22 operates at a temperature which is similar to or generally somewhat higher than the temperature in the first reactor 18. The principal naphtha-reforming reactions in the second-stage reactor 22 include the conversion of paraffins to aromatic hydrocarbons. The foregoing reactions are, in sum, endothermic and the effluent from the reactor 22 is at a lower temperature than the feed thereto.

The effluent from the second-stage reactor 22 is recovered in line 23 and passed through a reheat furnace 24 where it is heated to a temperature suitable for the naphtha reforming reactions in a third-stage reactor 25. Reheated effluent from the furnace 24 passes to the third-stage reactor 25 via line 26. The third-stage reactor 25 contains a naphtha- reforming catalyst which may be similar to, or the same as, the catalyst in one or both of the other reactor stages, and the third stage reactor 25 is operated at a temperature which is similar to or slightly higher than the temperature in the second-stage reactor 22. The principal naphtha-reforming reactions in the third-stage reactor 25 include the conversion of paraffins to aromatics and low-molecular weight products and by-products.

The aforesaid reactions may, in toto, be either endothermic or exothermic.

The effluent from the third stage reactor 25 is recovered in line 27, and conducted via product line 28 and a cooler 29 to a separator 30. The cooler 29 condenses reformed naphtha liquids (e.g., C4~hydro- carbons) so that reformed naphtha can be separated from the gases (H2 and light hydrocarbons) formed during the reforming reactions. The cooler 29 may be a heat exchanger which serves to preheat feed naphtha passing via line 13 from line 12 with hydrogen-containing recycle gas from line 15.

Liquid reformate is recovered from the separator 30 via line 31, and the gases pass into line 32. At least some of the gases are circulated by the fan 16 (which may be optionally supplemented by another circulation fan 33, which could comprise an additional compression stage of fan 16) to line 15 for use in the reforming process as described. Excess gases are recovered in line 34 and stored and/or discharged in and/or from facility 35.

The amounts of reforming catalyst in the first, second and third stage reactors are usually different from each other. Typically, the first-stage reactor may contain from 5 to 12 w% (e.g., about 10 w%) of the total reforming catalyst in the three reforming stages; the second stage reactor may contain from 10 to 25 w% (e.g., about 20 w%) of the total catalyst, and the third-stage reactor may contain from 63 to 85 wt (e.g., about 70 w%) of the total catalyst. These different quantities of catalyst and the different operating temperatures of the re orming stages are to take account of the different types of reforming reactions which are significant in each stage.

During the course of a naphtha-reforming operation, a carbonaceous material, termed "coke", deposits progressively on the catalyst and reduces its activity. The amount of coke deposition is lowest in the first-stage reactor, highest in the third (or last) stage reactor, and intermediate in the second (or intermediate) stage reactor or reactors. Eventually, the amount of coke deposition, and corresponding reduction in catalyst activity, is such that it is economic to interrupt the reforming process to restore the reforming activity of the catalyst by a so-called 'regeneration¹ process.

When the amount of coke on the catalyst in the last reforming stage is in the range of from 10 to 50 w% or higher (relative to the weight of catalyst in the last reforming stage) , it is usually considered economic to interrupt reforming and initiate catalyst regeneration operations. The catalyst in the first- reforming stage (e.g., reactor 18) would typically have a coke content in the range of up to 5 w%, relative to the weight of catalyst in the first stage, and the coke content of catalyst in the intermediate stage (e.g. , reactor 22) would typically be in the range of from 10 to 25 w%. The coke levels in the first and intermediate reforming stages are not so high as to reduce the reforming capability of the catalyst.in the first and intermediars stages to uneconomic levels. In accordance with the invention, when the catalyst in the last reforming stage (i.e., in reactor 25) requires regenerating, the de¬ activated catalyst is replaced by substitute active reforming catalyst without substantially interrupting the reforming process. The substitute active catalyst is disposed in a substitute or redundant reactor 40, and the amount of substitute active catalyst therein may be less than, the same as or more than the amount in the reactor 25, but is preferable about the same.

The reactor 40 may be brought on stream in the naphtha- reforming operation in the following manner:-

Firstly, the fuel supply to the furnaces 14, 20 and 24 is reduced to reduce their heat outputs, and the naphtha feed to the reforming unit from the storage facility 11 is interrupted. The reactor 25 is taken out of service by closing the valves 43 and 44. Preferably, the valves 63 and 65 are also closed. The substitute reactor 40 is made ready for service by opening valves 61, 41 and 42, and then the supply of naphtha feed from the storage facility is resumed by the operation of suitable pumps and valves (not shown), and the supply of fuel to the furnaces 14, 20 and 24 is increased so that the temperature of the naphtha feed and any hydrogen circulating therewith is progressively increased to naphtha- reforming temperatures. The heated naphtha (and any hydrogen- containing gas) raise the temperature of catalyst in the reactors to naphtha-reforming temperatures, and the reactor 40 then operates in place of reactor 25. The time during which the reforming unit is out of service while the foregoing actions are implemented to substitute the reactor 40 for the reactor 25 is typically about 24 hours, although the actual time may vary depending upon the configuration and layout of the reforming unit, the components involved in the reactor substitutions, and the number of human operatives available. However, this non- service time is considerably shorter than the period usually taken to reactivate the catalyst in a semi-regenerative reforming unit. The latter period is usually in the range of from (approximately) 7 to 14 days, and may be approximately 10 days for a typical, competently- operated unit.

Thus the invention reduces the non-operational time of a reformer unit and thereby increases its profitability.

During the ref arming operation using reactor 40 as the last naphtha-reforming stage, the first and intermediate reforming stages (i.e. , reactor 18 and 22) .are operated at slightly higher temperatures than during corresponding periods of the reforming operation employing the reactor 25 in order to compensate for the coke deposits on the catalysts therein. However, due to the reactions which occur in the last naphtha- reforming stage, the catalyst in reactor 40 becomes deactivated by coke-deposition more quickly than the other reactors. The operating temperatures of the reactors 18 , 22 and 40 are raised by appropriate amounts to compensate for their respective progressive catalyst deactivation by coke deposits and when the operation of reactor 40 has attained the maximum desirable level for economic naphtha-reforming, either the reactor 40 may be substituted by another reactor containing active catalyst, in accordance with the invention, or the naphtha-reforming process may be interrupted for regeneration of the catalyst, in accordance with the invention. In most instances, the latter option will be preferred since less investment in the relatively expensive reforming catalyst is required. The regeneration part of the process cycle is effected employing the following principal steps. The supply of naphtha from source 11 to the reactors is interrupted by suitable valves (not shown), and naphtha and other combustible materials are removed and are purged from the system using, inter alia, a relatively inert gas, such as a flue gas. The inert purge gas is supplied via conduit 45, and purge gas is circulated to all parts of the system by circulation fans 16 (and 33, if present). The valves 43 and 44 upstream and downstream of the reactor 25 are opened so that purge gas passes through the reactor 25. Valves 65, 41 and 42 are open and valves 61 and 63 are closed for the passage of inert purge gas via reactor 40.

After the reforming unit has been thoroughly purged, an oxygen-containing gas is passed into the reformer unit from line 47. The oxygen-containing regeneration gas may contain a low concentration (e.g., about 1 v%) of oxygen to avoid and/or prevent physical and/or chemical damage to the reforming catalyst. A suitable regeneration gas is a flue gas from a combustion process.

The regeneration gas is passed into the reactor 18 from line 17 at a suitable temperature to cause removal of carbonaceous material therefrom by oxidation, and then passes via lines 21 and 23 to the reactor 22 for coke removal therein at a suitable temperature. The regeneration gas is then passed via line 26 into reactors 25 and 40 to regenerate the catalyst in both reactors. Used regeneration gas is recovered in line 28, and at least part may be discharged from the unit via line 50, either directly to atmosphere or indirectly via suitable purifiers (not shown) . Any remaining regeneration gas is passed via cooler 29, line 51 and valve 52 to line 15, where it mixes with added regeneration gas from line 47.

The regeneration gas may pass via the reactors 25 and 40 in series in any order (i.e., either first via reactor 25 and then via reactor 40, or vice versa). In order to regulate the passage of regeneration gas via the reactors 25 and 40, suitable pipes with valves are provided which interconnect the reactors. These interconnecting pipes include first pipe 60, having at least one valve 61, which connects the normally-upstream ends of the reactors 25 and 40, a second pipe 62, having at least one valve 63, which interconnects the normally- downstream ends of the reactors 25 and 40, and a by-pass pipe 64 having a valve 65 which provides a connection between the first pipe 60 and the second pipe 62.

During a regeneration operation, valves 43, 44, 41, 42 and 65 are open, and valves 61 and 63 are closed, so that regeneration gas passes first via reactor 25 and then via by-pass pipe 64 into the reactor 40 until carbonaceous deposit has been adequately removed from the catalyst.

The regeneration gas is heated as necessary or desirable, and maintained within a suitable temperature band by cooling. The heating and cooling may be effected by any convenient means, e.g., including any of those already mentioned herein. when the carbonaceous deposit has been adequately removed from the catalyst in all the reactors by the action of the regeneration gas, the catalyst is reactivated by appropriate reactivation procedures. The reactivation procedure may involve adding chlorine to the catalyst, since the catalyst tends to be stripped of chlorine during regeneration. Chlorine (or a chemical precursor thereof) is, supplied from a source 54 and a conduit 55 into each reactor 18, 22, 25 and 40 via respective open valves 57, 58, 59, 43, 44, 65, 41 and 42, until the catalyst in each reactor attains a desired level of chlorination, as determined by known techniques. Thereafter, the supply of chlorine is interrupted, and excess chlorine is purged by inert gas. The catalyst may ^'similarly be treated with other known activation and/or pretreating agents until restored to a condition suitable for use in a naphtha-reforming process. Thereupon, the reforming unit is purged with inert gas from line 45 until substantially free of reactive chemical entities and then prepared for a further cycle of reforming operations. The valves 43 and 44 remain open so that reformer effluent from reactor 22 and reheater 24 passes into reactor 25, and valve 63 is opened so that reformer effluent from reactor 25 passes to line 28. The valves 41 and 42 are closed so that the reactor 40 is isolated until required for use as a substitute .reactor, and the valve 65 is closed to prevent flow via the by-pass pipe 64. For preference, valve 61 may be closed. Thereafter, naphtha and hydrogen-containing gas are circulated to the reactors 18, 22 and 25 in a first part of the reforming operation, and then to the reactors 18, 22 and

40 in a second part of the reforming operation (in which the reactor 40 is employed as a substitute for the reactor 25) as described. During this second part of the reforming operation, valves 43, 44, 63 and 65 are closed and valves 61, 41 and 42 are open so that feed material in line 26 passes via reactor 40. 1 \J

Although the embodiment of the drawing has a single substitute reactor 40, it is within the scope of the invention to employ more than one substitute reactor so that the length of each naphtha-reforming run between regenerations can be further extended. For the embodiment described, the naphtha-reforming operation employing reactor 25 would operate for from 3 to 12 months (e.g., 7 months), and the naphtha-reforming operation employing the substitute reactor 40 would operate for a further 2 to 12 months (e.g., 5 months), before catalyst regeneration and reactivation would be necessary. The foregoing periods and ranges of periods are illustrative, and not to be regarded as limitative. Thus, the process and apparatus of the invention enable an existing semi-regenerative naphtha-reforming plant almost to double its operating cycle-length without incurring relatively high capital expenditure.

Claims

Claims ;

1. A catalytic reforming process performed in a reforming unit comprising a plurality of catalyst- containing reaction zones connected in series, wherein feed naphtha is passed into the first (or upstream) reaction zone of the series of reaction zones in the unit and product naphtha having a higher octane rating than the feed naphtha is recovered from the last (or downstream) zone of the series of reaction zones in the unit, the process comprising, in sequence, the steps of:

(a) passing naphtha into and through the series of reaction zones of the unit and progressively increasing the temperature of at least the last zone of the series of zones to maintain the octane rating of the product naphtha at least at a given value;

(b) substituting a spare reaction zone containing active reforming catalyst in place of the last reaction zone of the series when the temperature of at least the last reaction . zone of the series attains a specified maximum temperature;

(c) passing feed naphtha through the reaction zones of the unit and recovering a naphtha product having at least the given octane rating from the last reaction zone of the series;

(d) progressively raising the temperature of at least the last reaction zone of the series of reaction zones to maintain at least the given octane rating of the product naphtha; (e) optionally repeating steps (b) , (c) and (d) ;

(f) interrupting the passage of naphtha into and through the reaction zones of the unit, and regenerating catalyst (by in situ and/or ex situ regeneration) in at least the last reaction zone of the series; and

(g) optionally repeating steps (a) to (f) .

2. The process of claim 1 wherein the amount of catalyst in the last zone of the series in the unit comprises from 50 to 90 weight percent of the total catalyst in the unit.

3. The process of claim 1 or claim 2 wherein the first reaction zone of the series in the unit contains from 5 to 20 weight percent of the total catalyst in the unit.

4. The process of any one of claims 1 to 3 wherein in step (a) , the temperature of the irst zone is progressively raised to maintain the octane rating of the naphtha product.

5. The process of any one of claims 1 to 4 wherein the unit comprises at least three reaction zones connected in series.

6. The process of any one of claims 1 to 5 wherein in step (a), the temperature of other reaction zone(s) between the first and last zones is/are progressively raised to maintain the octane rating of the naphtha product.

7. The process of claim 5 or claim 6 wherein the amount of catalyst contained in the reaction zones connected between the first and last zones of the series is in a range of from 5 to 45 weight percent of the total catalyst in the unit.

8. The process of any one of claims 1 to 7 wherein step (f) is effected by in situ regeneration and reactivation of catalyst in the reaction zones of the unit and at least one spare reaction zone, the spare reaction zone(s) being connected to form part of the unit for step (f) .

9. The process of claim 8 wherein, following the regeneration and reactivation, the spare reaction zone(s) is/are disconnected from the unit, and step (a) is started.

10. A catalytic reforming unit for the catalytic reforming of naphtha comprising a plurality of catalyst- containing reactors or reaction zones connected in series for the passage of naphtha through the unit from the first reactor or zone of the series to the last reactor or zone thereof, means for heating naphtha passing from one reactor or zone to the next reactor or zone in the series, means for heating naphtha passing to the first reactor or zone of the series, recovery means for recovering reformed naphtha from the last reactor or zone of the series, a substitute catalyst-containing reactor or zone (which may contain substantially the same amount of catalyst as the said last reactor or zone) , means for diverting naphtha to circulate from the last-but-one reactor or zone of the series to the substitute reactor or zone whereby the substitute reactor or zone functions as the last reactor or zone of the series.

11. A unit as in claim 10 wherein the last reactor or zone and the substitute reactor or zone are connected or connectable in parallel with each other to receive naphtha from the last-but-one reactor or zone of the series and to discharge naphtha to the said recovery means, there being valves operable to cause naphtha, to pass through only one or other, at a time, of the said last reactor or zone and the substitute reactor or zone.

12. A unit as in claim 11 wherein at least some of the said valves are operable to cause catalyst regenerating/ reactivating vapors to pass through the said last reactor or zone and the said substitute reactor or zone at the same time during a catalyst-regeneration/reactivation operation of the unit.