WO1990013616A1

WO1990013616A1 - Isomerization process

Info

Publication number: WO1990013616A1
Application number: PCT/US1990/002309
Authority: WO
Inventors: Peter Kokayeff
Original assignee: Union Oil Company Of California
Priority date: 1989-04-28
Filing date: 1990-04-26
Publication date: 1990-11-15

Abstract

Yields of isomerized hydrocarbons are improved by a process in which a C5-rich stream and a C6-rich stream (13, 18) are separately and serially treated under isomerization conditions (10, 20). The process optimizes the yield of C5+ hydrocarbons at any given octane number.

Description

ISOMERIZATION PROCESS

Field of the Invention

This invention relates to the isomerization of hydrocarbons.

In one of its more particular aspects it relates to an isomerization process characterized by improved yields of branched chain C₅ and Cg hydrocarbons. In another aspect, it relates to improving the octane number of gasoline.

BACKGROUND OF THE INVENTION

Various refinery streams make up the gasoline pool. Of these, the naphtha streams, frequently used in gasoline production, contain normal paraffins (n-paraf- fin) which have relatively low octane numbers. Lead additives were formerly added to low octane gasolines to boost their octane numbers. However, with the phasing out of the use of lead additives in gasoline, it has become increasingly important to increase the octane number of the gasoline by further refining and processing the various streams that make up the gaso¬ line pool. The light naphtha stream, which boils in the 60° to 160° F. range, is composed predominantly of C₅ and C₆ hydrocarbons, such as n-pentane, i-pentane, n-hexane, 2-methyl pentane, 3-methyl pentane and the dimethyl butanes. The branched isomers have higher octane numbers than the straight chain isomers. In the past, catalytic isomerization processes have been used to treat the light naphtha stream in order to convert the straight chain paraffins, such as n-hexane and n- pentane, to equilibrium mixtures of branched chain isomers, and to produce a product having a higher octane number than the light naphtha stream. The isomerization process consisted of mixing the light naphtha stream with hydrogen, heating it to a tempera¬ ture high enough to promote isomerization and passing the heated mixture over an isomerization catalyst. The effluent from the isomerization process was then sepa¬ rated into a stream rich in normal paraffins, which was recycled, and an isomerizate rich in branched hydrocar¬ bons which was blended into the gasoline pool.

Since isomerization reactions are equilibrium limited, the effluent from the isomerization process consisted of an equilibrium distribution of all possi¬ ble isomers. Following isomerization, the effluent was separated into straight chain paraffins and branched chain paraffins either by a distillation step or a selective absorption step. The straight chain paraf¬ fins were then recycled for further isomerization.

During isomerization, various side reactions can occur. One particularly troublesome side reaction is the cracking of the feed to C₄ - components, that is, to products of lower molecular weight than pentane. Such cracking represents a loss of desired C₅ and C₆ branched chain paraffins. Prior art processes were therefore modified to minimize the loss of feed to C₄- product. Since a balance exists between acceptable rates of isomerization and rates of cracking, various methods have been explored to reach a favorable compro¬ mise between the two processes. Higher temperatures. for example, which favor faster isomerization, also result in increased cracking of the feed. Therefore, moderate temperatures were utilized.

Various catalysts have been employed in the isom¬ erization process. Acid-promoted aluminum chloride catalysts, Y-type crystalline zeolite catalysts con¬ taining a metal of Group VIII of the Periodic Table (U. S. Patent Nos. 3,236,761 and 3,263,762), synthetic mordenite catalysts containing highly dispersed plati¬ num or palladium in the presence of hydrogen (U. S. Patent Nos. 3,527,835 and 3,299,153), stabilized Y- sieve hydrogen zeolite compositions (U. S. Patent No. 3,354,077), and others have been used. However, the yields of C₅+ products, that is, paraffinic hydrocar¬ bons of C₅ or longer chain length have been lower than desired and have resulted in only a limited improvement in octane number. Accordingly, an isomerization proc¬ ess capable of producing higher yields of C₅+ products is desired.

SUMMARY OF THE INVENTION

The present invention provides an isomerization process capable of producing higher yields of C₅+ products. The process comprises providing C₅ - rich and C₆ - rich hydrocarbon streams and separately treat¬ ing the streams under isomerization conditions to produce an isomerized product having a higher C₅ + product yield for any given octane number. In a pre¬ ferred embodiment, a C₅ - rich hydrocarbon stream is isomerized and the isomerizate is mixed with a C₆ - rich hydrocarbon stream, and the combined stream is isomerized resulting in a yield of C₅ + hydrocarbon of greater than about 97 percent by weight.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a schematic flow diagram illustrating the process of the present invention.

Figure 2 is a graph showing the relationship between yield of C₅+ paraffins and octane number.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a process for increasing yields of C₅+ paraffins in an isomerization process. In the process the feed is separated into a C₅ rich stream and a C₆ rich stream and the two streams are treated separately in two reactors in series in order to provide a total C₅+ yield, which is greater than when the mixed feed is treated in a single isomeriza¬ tion reactor.

The feeds isomerized in the present invention are hydrocarbon streams, such as straight run petroleum distillates obtained from crude oil, hydrocracked feedstocks, coker products and the like. Examples include straight run naphthas, coker naphthas, hydro¬ cracked naphthas, thermally cracked or catalytically cracked naphthas and blends thereof. The feedstock to the process may boil in the range from about 50° F. to about 200° F. and preferably entirely boils at tempera- tures less than about 180° F. The feedstock will usually comprise pentane and hexane, predominantly in the forms of n-pentane and n-hexane. Usually the feedstock contains a mixture of at least 20 and typi¬ cally at least about 35 weight percent n-pentane or n- hexane and may consist only of pentane and/or hexane. The feedstock can contain any proportion of C₅ and C paraffins from zero C₅ to 100 percent C₅. For example, 60 percent C₅ and 40 percent Cg paraffins, or 40 per¬ cent C₅ and 60 percent Cg paraffins. A highly preferred feedstock is a light gasoline fraction (i.e., boiling in the range from about 50° F. to about 185° F.) having an octane rating needing improvement.

The isomerization is carried out in a fixed bed reactor in which the catalyst is a typical isomeriza¬ tion or hydroisomerization catalyst.

The isomerization catalyst employed in the process of the invention includes any catalyst having the property of promoting the isomerization or hydroisomer¬ ization of n-paraffins in a hydrocarbon feedstock, and more particularly the isomerization of n-pentane and/or n-hexane compounds. Typical isomerization catalysts contain a noble metal such as platinum and/or palladium on an acidic component. The acidic component has sufficient acidity to impart activity for isomerizing a hydrocarbon feedstock, particularly a feedstock con¬ taining n-pentane and/or n-hexane. Suitable acidic components include silica-aluminas and crystalline molecular sieves having isomerizing activity. Crystal¬ line molecular sieves are preferred acidic components. The term "crystalline molecular sieve" as used herein refers to any crystalline component which has suffi- cient activity for isomerization and is capable of separating atoms or molecules based on their respective dimensions. Crystalline molecular sieves may be zeo- litic or nonzeolitic. The term "nonzeolitic" as used herein refers to molecular sieves whose frameworks are not formed of substantially only silica and alumina tetrahedra. The term "zeolitic" as used herein refers to molecular sieves whose frameworks are formed of substantially only silica and alumina tetrahedra such as the framework present in ZSM-5 type zeolites, Y zeolites, and X zeolites. Examples of zeolitic crys¬ talline molecular sieves which can be used as an acidic component of the catalyst include Y zeolite, fluorided Y zeolites, X zeolites, zeolite beta, zeolite L, morde- nite and zeolite omega. Examples of nonzeolitic crys¬ talline molecular sieves which may be used as an acidic component of the catalyst include silicoaluminophos- phates, aluminophosphates, ferrosilicates, titanium aluminosilicates, borosilicates and chromosilicates.

A preferred Y zeolite for the isomerization cata¬ lysts is one prepared by first ammonium exchanging a Y zeolite to a sodium content between about 0.6 and 5 weight percent, calculated as Na₂0, calcining the ammonium exchanged zeolite in the present of at least 0.2 psi water vapor partial pressure at a temperature between 600° F. and 1650° F.to reduce the unit cell size to a value in the range between 24.40 and 24.64 Angstroms, and then ammonium exchanging the zeolite once again to replace at least 25 percent of the resid¬ ual sodium ions and obtain a zeolite product of less than 1.0 weight percent sodium and preferably less than 0.6 weight percent sodium, calculated as Na₂0. Such a Y zeolite is highly stable and maintains a high activi¬ ty. The zeolite is described in detail in U. S. Patent No. 3,929,672, the disclosure of which is hereby incor¬ porated by reference in its entirety.

In addition to the zeolitic crystalline molecular sieves disclosed herein, other examples of acidic components that may be combined with the active metals and their combinations include non- crystalline acidic materials such as silica-alumina or silica-alumina in an alumina dispersion. The latter is described in U. S. Patent No. 4,097,365, the disclosure of which is incorporated by reference in its entirety.

An example of non-zeolite crystalline molecular sieves also useful as an acidic component in the isom¬ erization catalysts useful in the process of the present invention is a silicoaluminophosphate, known by the acronym "SAPO," described in detail in U. S. Patent No, 4,440,871, the disclosure of which is hereby incor¬ porated by reference in its entirety. Another useful class of nonzeolitic crystalline molecular sieves is generally referred to as crystalline aluminophosphates, designated by the acronym "A1P0₄." The structure and preparation of the various species of aluminophosphates are discussed in U. S. Patent Nos. 4,310,440 and 4,473,663, the disclosures of which are hereby incorpo¬ rated by reference in their entirety. Yet another class of nonzeolitic molecular sieves suitable for use is known as ferrosilicates, designated by the acronym "FeSO." A preferred ferrosilicate denominated as FeSO- 38 is disclosed in European Patent Application No. 83220068.0 filed October 12, 1982 and published on May 16, 1984 as Publication No. 0 108 271 A2, the disclo- sure of which application is hereby incorporated by reference in its entirety. Still other examples of nonzeolitic sieves include borosilicates, chromosili- cates and crystalline silicas. Borosilicates are described in U. S. Patent Nos. 4,254,247, 4,264,813 and 4,327,236, the disclosures of which are hereby incorpo¬ rated by reference in their entireties. Chromosili- cates are described in detail in U. S. Patent No. 4,405,502, the disclosure of which is also hereby incorporated by reference in its entirety. A preferred crystalline silica, essentially free of aluminum and other Group IIIA metals, is a silica poly orph, i.e., silicalite, which may be prepared by methods described in U. S. Patent No. 4,061,724, the disclosure of which is hereby incorporated by reference in its entirety.

In preparing the isomerization catalyst, the acidic component may be combined with a binder or matrix material comprising a porous, inorganic refrac¬ tory oxide component having essentially no cracking or isomerizing activity, to produce support particulates. The acidic component is combined with the porous, inorganic refractory oxide component, or a precursor thereof. Illustrative inorganic oxides are alumina, silica, titania, magnesia, zirconia, borilia, silica- magnesia, silica-titania, other such combinations and the like, with alumina being the most highly preferred. Examples of precursors that may be used include pep- tized alumina, alumina gel, hydrated alumina, hydro- gels, and silica sols. Normally, the porous, inorganic refractory oxide component or its precursor is mixed or comulled with an acidic

>ent in amounts such that the final dry catalyst mixture will comprise (1) be¬ tween about 2.5 weight percent and about 95 weight percent acidic component, preferably between about 15 weight percent and about 80 weight percent, and (2) between about 5 weight percent and about 98 weight percent of porous, inorganic refractory oxide, prefera¬ bly between about 10 weight percent and about 40 weight percent. The comulled mixture is then formed into particulates, usually by extrusion through a die having openings of a cross sectional size and shape desired in the final catalyst particles. For example, the die may have openings therein in the shape of three-leaf clo¬ vers so as to produce an extrudate material similar to that shown in Figures 8 and 8A of U. S. Patent No. 4,028,227, the disclosure of which is hereby incorpo¬ rated by reference in its entirety. Among preferred shapes for the die openings are those that result in particles having surface-to-volume ratios greater than about 100 reciprocal inches. After extrusion, the support particulates or the finished catalyst particles are cut into lengths of from 1/16 to 1/2 inch. The resulting particles are subjected to a calcination at an elevated temperature, normally between about 600° F. and about 1,600° F. , to produce support particulates or catalytic particles of high crushing strength.

Typical isomerization catalysts contain a noble metal such as platinum and/or palladium on a cracking or acidic component as hereinbefore described. A preferred acidic component for the isomerization cata¬ lyst is a zeolitic crystalline molecular sieve. An especially preferred isomerization catalyst is a plati¬ num or palladium-containing Y zeolite, for example. those disclosed in U. S. Patent No. 3,929,672, referred to above. Another preferred isomerization catalyst contains platinum and hydrogen mordenite. Examples of other useful isomerization or hydroiso erization cata¬ lysts are disclosed in U. S. Patent Nos. 4,232,181 and 4,182,692 issued to Kiovsky et al., U. S. Patent No. 3,925,503 issued to Parthasarathy, and U. S. Patent No, 4,238,319 issued to Hauschildt et al., all of which are incorporated by reference in their entireties herein. Preferred isomerization catalysts include those promot¬ ing the isomerization reactions in the presence of added hydrogen (hydroisomerization) and particularly those catalysts promoting isomerization of a gaseous hydrocarbon feedstock.

The conditions employed to isomerize (or hydroi- somerize) a hydrocarbon feedstock will vary depending upon the particular process embodiment in which the isomerization catalyst is used and the nature of the feedstock. Most usually, the feedstock is passed through a fixed bed of the isomerization catalyst in a reactor operated at a temperature less than about 700° F. , usually in the range from about 250° F. to about 650° F. , preferably from about 450° F. to about 600° F. , and most preferably from about 525° F. to about 575° F. The feedstock contacted with the isomerization catalyst may be in the liquid phase, but preferably is in the gaseous phase when it contacts the isomerization catalyst. Typical pressures maintained during such contacting are in the range from about 200 psig to about 600 psig, preferably about 250 psig to about 350 psig. The weight hourly space velocity (WHSV) of the feedstock over the isomerization catalyst is about 0.1 to about 15.0 hr -"^■ preferably about 0.5 to about 3.0 hr. ~¹. In the presence of hydrogen, the hydrocarbon feed contacts the isomerization catalyst under hydroi¬ somerization conditions wherein a hydrogen recycle rate of usually about 1,000 to about 8,000, and preferably about 1,500 to about 4,500 standard cubic feed per barrel (scf/bbl) is maintained. Typically the hydrogen to hydrocarbon molar ratio (Hyd./HC) is about 2/1 to about 8/1, perferably about 2/1 to about 4/1.

As pointed out above the improvement in the present invention resides in the separate treatment of C₅ and Cg paraffin streams.

Referring to Figure 1, isomerization reactors 10 and 20 are connected in series by means of a conduit 15. The feed to reactor 10 is provided by means of a conduit 11 which is fed from conduits 12 and 13. Conduit 11 is fitted with a gauge 14. The feed to reactor 20 is provided through a conduit 16 which is fed by conduits 17, 18 and 15. A gauge 19 is provided for conduit 16. Reactor 20 is provided with an efflu¬ ent conduit 21. A back pressure regulator 22 is pro¬ vided in conduit 15 and a back pressure regulator 23 is provided in conduit 21.

In operation a liquid feed is introduced to reac¬ tor 10 via conduit 13. Hydrogen is added via conduit 12 and is mixed with the liquid feed in conduit 11 and introduced into reactor 10. The effluent from reactor 10, which is an isomerized product of the liquid feed, is removed from reactor 10 via a conduit 15 and intro¬ duced into a conduit 16 where it is mixed with a second liquid feed via conduit 18 and hydrogen via conduit 17. The combined feeds are then introduced into reactor 20. The effluent of reactor 20, which is an isomerized product of the feeds comprising the liquid feed intro¬ duced via conduit 18 and the effluent from reactor 10, is removed from reactor 20 via a conduit 21. Both conduits 15 and 21 are equipped with back pressure regulators to control the direction of feed of the various streams.

The process of the present invention operates to isomerize the C₅ and C_g streams separately and in separate reactors. For example, a C₅ stream can be isomerized in the first reactor i.e., reactor 10, and the product of isomerization thereof fed to the second reactor i.e., reactor 20, along with a C stream, or a C_g stream can be isomerized in the first reactor i.e., reactor 10, and the product of isomerization thereof fed to the second reactor, i.e., reactor 20 along with a C₅ stream. Reaction conditions in both reactors may be the same or different. For example, either reactor may operate at a temperature differential of up to about 100° F. from the other. Catalyst quantities may also be varied. Although advantageous results are realized, regardless of the order of treatment of a C₅ and Cg streams, it has been found more advantageous to treat the C₅ stream in the first reactor and to feed its isomerization product along with the Cg stream to the second reactor than to feed the Cg stream to the first reactor and to feed its isomerization product along with the C₅ stream to the second reactor. However, both methods are preferred over feeding a mixture of C₅ and C₆ paraffins to a single isomerization reactor or to a series of isomerization reactors because treating the C₅ stream first results in higher yields of C₅ + hydrocarbons while treating the C₆ stream first results in higher yields of isomerized Cg hydrocarbons.

The invention will be better understood by refer¬ ence to the following examples which are included for purposes of illustration and are not to be construed as in any way limiting the scope of the present invention, which is defined in the appended claims.

In the following examples, the experimental condi¬ tions were standard throughout the runs and are shown in Table 1.

TABLE 1

Pressure 300 psig WHSV 3.0/hr Hyd/HC 4/1 molar Feed equimolar mixtures of n-C5/n-C6 paraffins Catalyst : A palladium-containing Y zeolite

(Catalyst A in Examples 16-19 of U. S. Patent No. 3,929,672)

The examples represent the results of isomeriza¬ tion runs and list for each run the specific product analyses obtained using gas chromatography. The runs were conducted using two different temperatures, 536° F. and 573° F. as stated in the examples. The feeds to the two reactors differed in each series of runs as follows. The n-C₅ stream was fed to the first reactor, hereinafter Reactor A, and the n-C₆ stream was fed to the second reactor, hereinafter Reactor B, in Runs l, 4, 7 and 10; the n-Cg stream was fed to Reactor A and the n-C₅ stream was fed to Reactor B in Runs 2, 5, 8 and 11; and a mixture of n-C₅ and n-Cg hydrocarbons was fed to Reactor A and the isomerized effluent from Reactor A was fed to Reactor B in runs 3, 6, 9 and 12.

In the specification of the results of the runs, abbreviations used are shown in Table 2.

TABLE 2

HYDROCARBON PRODUCTS i-C5 Mole percent of pentane isomer product, i.e., 2-methyl butane and 2,2-dimethyl propane, the total pentane product

2.2 DMB Mole percent of 2,2-dimethyl butane product in the total hexane product

2.3 DMB Mole percent of 2,3-dimethyl butane product in the total hexane product

2 MP Mole percent of 2-methyl pentane product in the total hexane product

3 MP Mole percent of 3-methyl pentane product in the total hexane product n-Cg Mole percent of n-hexane product remaining from the total hexane product

Wt % C₅+ Percent by weight of C₅ and higher paraffins in the total product

Vol % C₅+ Percent by volume of C₅ and higher paraffins in the total product

RONC Calculated research octane number of the C₅ + isomers in the product

EXAMPLE 1

The feedstocks described above were isomerized in Reactor A at 536° F. antl Reactor B at 572° F. In each case, the effluent from Reactor A was fed to Reactor B along with the indicated feed. The results are shown in Table 3.

TABLE 3

Run ^L-^C5 2.2 DMB 2.3 DMB 2MP 3MP

1 64.7 15.5 6.9 38.2 20.3

2 52.8 16.3 7.3 39.0 20.2

3 59.2 16.1 7.2 38.1 20.2

1 19.0 98.1 99.4 75.1

2 18.3 97.5 99.0 74.1

3 18.4 97.7 99.1 74.4

EXAMPLE 2

The temperature in Reactor A was 536 F. and the temperature in Reactor B was 536° F. The results are shown in Table 4.

TABLE 4

Run i^s 2.2 DMB 2.3 DMB 2 MP

4 60.1 11.7 4.9 38.1

5 27.9 16.8 7.1 38.4

6 47.2 15.9 6.7 38.7

Run 3 MP zC₆ Wt % Cg + Vol % C₅ + RONC

4 19.9 25.3 99.5 100.7 72.1

5 19.9 17.8 98.5 99.8 70.3

6 20.2 18.5 98.9 100.2 72.8 EXAMPLE 3

The temperature in Reactor A was 572° F. and the temperature in Reactor B was 536° F. The results are shown in Table 5.

TABLE 5

Run ^L- 5 2.2 DMB 2.3 DMB 2 MP 3 MP

7 67.6 11.9 5.0 38.1 19.8

8 29.0 17.2 7.3 38.1 19.8

9 61.8 16.9 7.1 38.3 19.9

Run n-C₆ wt % c₅ + Vol % c₅ + RONC

7 25.2 98.9 100.0 73.1

8 17.6 95.5 96.8 70.7

9 17.8 96.9 98.2 75.3

EXAMPLE 4

The temperature in Reactor A was 572° F. and the temperature in Reactor B was 572° F. The results are shown in Table 6.

TABLE 6

Run ^' 5 2.2 DMB 2.3 DMB 2 MP 3 MP

10 67.6 15.8 7.1 38.0 20.3

11 52.9 16.4 7.3 37.9 20.2

12 65.3 16.3 7.3 38.0 20.3

Run Wt %

S=£^β ^C<S ⁺ Vol % ^C + RONC

10 18.9 97.6 98.9 75.7

11 18.2 94.4 95.8 74.1

12 18.2 95.7 97.1 75.7 It can be seen from these results and from the graph of the data in Figure 2 that the yield of C,-+ paraffins in the configuration where the n-C₅ stream was fed to Reactor A and the n-C_ stream to Reactor B

6

(Runs 1, 4, 7 and 10 and CONF #1 in Figure 2) was higher at each temperature arrangement than the yield where the n-C„ stream was fed to Reactor A and the n-C.. 6 5 stream is fed to Reactor B or where the combined n-C₅ and n-C6 streams were fed to Reactor A and the isomeri- zation product thereof was fed to Reactor B (Runs 3, 6, 9 and 12 and CONF #3 in Figure 2). Further, it can be seen from Example 4 that for a given research octane number (RONC = 75.7), the yield obtained (Wt % C₅ +) using the configuration in which n-Cr paraffins were fed to Reactor A and n-C₆ paraffins were fed to Reactor B (Run 10) was almost 2 percent higher than the yield obtained from feeding a combined n-C^ and n-C_β stream to Reactor A and then feeding the product of isomeriza-

tion thereof to Reactor B (Run 12) . Thus, it has been shown that the separate treatment of n-C₅ paraffins and n-Cg paraffins in an isomerization process produces significantly higher yields than the prior art method of isomerizing a mixed n-C₅ and n-C₆ stream.

The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. For example, other temperatures than those specifically illustrated and other reaction conditions as well as other catalysts may be utilized in practic¬ ing the present invention. Consequently, the present embodiments and examples are to be considered only as being illustrative and not restrictive, with the scope of the invention being indicated by the appended claims. All embodiments which come within the scope and equivalency of the claims are, therefore, intended to be embraced therein.

Claims

I Claim:

1. A process for producing an isomerization product rich in C₅+ hydrocarbons which comprises:

providing a C₅-rich hydrocarbon stream and a Cg- rich hydrocarbon stream;

treating one of said C₅-rich hydrocarbon stream an^d sa^{id c}β-_ i_c_ι hydrocarbon stream under isomerization conditions to produce an isomerizate thereof;

treating a mixture of said isomerizate and the other of said Cg-rich hydrocarbon stream and said Cg- rich hydrocarbon stream under isomerization conditons; and

recovering an isomerization product rich in C₅+ hydrocarbons.

2. A process according to claim 1 wherein said C₅-rich hydrocarbon stream and said C₈-rich hydro¬ carbon stream together comprise a light gasoline frac¬ tion boiling in the range of about 50° F. to about 185° F.

3. A process according to claim 1 wherein said isomeriztion product comprises a mixture of 2- methyl butane, 2,2-dimethyul propane, 2,2-dimethyl butane, 2,3-dimethyl butane, 2-methyl pentane and 3- methyl pentane.

4. A process for producing an isomerization product rich in C₅+ hydrocarbons which comprises:

treating a C₅-rich hydrocarbon stream under isom¬ erization conditions to produce an isomerizate thereof;

treating a mixture of said isomerizate and a Cg_ rich hydrocarbon stream under isomerization conditions; and

recovering an isomerization product rich in C₅+ hydrocarbons.

5. A process for producing an isomerization product rich in C₅+ hydrocarbons which comprises:

treating a Cg-rich hydrocarbon stream under isom¬ erization conditions to produce an isomerizate thereof;

treating a mixture of said isomerizate and a Cg- rich hydrocarbon stream under isomerization conditions; and

recovering an isomerization product rich in C-+ hydrocarbons.

6. A process for producing an isomerization product rich in C₅+ hydrocarbons which comprises:

mixing a C₅-rich hydrocarbon stream with hydrogen and passing the resulting combined stream over an isomerization catalyst in a first reaction zone at a temperature of about 250° F. to about 650° F. , a pres¬ sure of about 200 psig to about 600 psig, a weight hourly space velocity (WHSV) of about 0.1 hr. ^_1 to about 5.0 hr. ^-1, and a hydrogen to hydrocarbon molar ratio (Hyd./HC) of about 2/1 to about 8/1 to produce an isomerizate of said C₅-rich hydrocarbon stream;

combining said isomerizate with a Cg-rich hydro¬ carbon stream and hydrogen and passing the resulting combined stream over an isomerization catalyst in a second reaction zone separate from said first reaction zone at a temperature of about 250° F. to aout 650° F., a pressure of about 200 psig to about 600 psig, a WHSV of about 0.1 hr. ^-1 to about 5.0 hr. ^_1 amd a Hyd./HC of about 2/1 to about 8/1; and

recovering from said second reaction zone an isomerization product rich in C₅+ hydrocarbons.

7. A process according to claim 6 wherein said isomerization catalyst is a zeolitic crystalline molecular sieve containing platinum or palladium.

8. A process according to claim 7 wherein said zeolitic crystalline molecular sieve is a stream stabilized Y zeolite.

9. A process according to claim 6 wherein said isomerization catalyst is a hydrogen mordenite containing platinum.

10. A process according to claim 6 wherein said temperature is about 450° F. to about 600° F.

11. A process according to claim 6 wherein said temperature is about 525° F. to about 575° F.

12. A process according to claim 6 wherein said pressure is about 250 psig to about 350 psig.

13. A process according to claim 6 wherein said WHSV is about 0.5 hr. ^"-¹- to about 3.0 hr. ^_1.

14. A process according to claim 6 wherein said Hyd./HC is about 2/1 to about 4/1.

15. A process according to claim 6 wherein said C₅ + hydrocarbon product is receovered from said second reaction zone in a yield greater than about 97 weight percent.

16. A process according to claim 6 wherein said isomerization catalyst is a platinum-containing hydrogen mordenite, said temperature is about 450° F. to about 600° F. , said pressure is about 250 psig to about 350 psig, said WHSV is about 0.5 hr. ^_1 to about 3.0 hr. ^_1, and said Hyd./HC is about 2/1 to about 4/1.

17. A process according to claim 16 wherein said C₅+ hydrocarbon product has a research octane number (RONC) of about 72 to about 76 and is recovered from said second reaction zone in a yield of about 97.5 weight percent to about 99.5 weight percent.

18. A process according to claim 4 wherein said isomerization conditions are the same for treating said Cg-rich hydrocarbon stream and said Cg-rich hydro¬ carbon stream.

19. A process according to claim 6 wherein the temperature in said second reaction zone is up to 100° F. higher than the temperature in said first reaction zone.

20. A process according to claim 6 wherein the temperature in said first reaction zone is up to about 100 ° F. higher than the temperature in said second reaction zone.