US2909582A - Isomerization process - Google Patents

Description

1959 L. BLEICH ETAL 2,909,582

ISOMERIZATION PROCESS Filed Dec. 51, 1956 2o SPLITTER 'SUPERFRACTl0NATOR 2 L -a FRESH z; FEED f h 6 ADSORBER o ISOMERIZER Leon Bleich Howard G. Codet Inventors Charles E. Hemminger By WM -MAflorney United States PatentO search and Engineering Company, a corporation of Delaware a J Application December 31, 1956, Serial No. 631,785

7 Claims. (Cl. 260-68353) The present invention relates to the conversion of hydrocarbons, particularly the straight chain paraflins, to produce the corresponding branchchained paraifins by means of a catalytic isomerization reaction. More particularly, the present invention relates to the production of very high octane-motor fuel by an improved isomerization process wherein normal pentanes and hexanes are isomerized in the presence of a Friedel-Crafts catalyst, either with or without the use of a suitable carrier such as Porocel, a well-known brand of porous alumina, and in the presence of promotional amounts of a halogencontaining promoter such as a hydrogen halide under isomerization reaction conditions. Stillmore particularly, the present invention relates to a process for recovering a higheroctane fuel from such an operation than has hitherto been found possible.

The isomerization of normal paraflins to iso paraflins in the; presence of Friedel-Crafts type catalyst, of which A101 is a typical example, and in the presence ofpromotional amounts of halogen-containing promoters is old. Numerous processes have tbeen devise-d, both vapor phase and liquid phase, for isomerizing normal paraffins to the corresponding'isoparaifin. However, the application of this technique to isomerizing light hydrocarbons, in particular the pentane and hexane fractions, has been beset by real difliculties.

A problem that has long confronted the refineries is the utilization ofthe C -18O F. naphtha stream obtained from distilling virgin naphtha or light naphtha from the hydroforming or reforming operation. This stream is rich in pentanes and hexanes as well as containing significant amounts 'of aromatics, such as benzene, and naphthenes such as methyl-cyclopentane and cyclo-hexane. In normal operation, this stream is either passed directly to the gasoline pool, or the heavier portion thereof is reformed. The latter represents a substantial economic loss, in that reforming destroys the paraflinic components present. Among the reasons why processing this stream is difficult, is that there are presently no economic means for upgrading it. isomerization in particular has not been feasible because of the high separation cost involved, particularly of the C fraction.

With the increased emphasis on high octane gasolines to provide fuel for the high compression engines in modern automobiles, a handy tool has been the isomerization of light normal hydrocarbons to the corresponding iso-hydrocarbons. Thus, normal butane, normal pentane, normal hexane have all been converted to their corresponding isomers to provide high octane gasoline. However, when it is attempted to isomerize the naphtha fraction boiling in the C 180 .F. range, severe difficulties are. encountered. The stream consists not only of normal pentane and normal hexane, but also isopentane, the methyl pentanes, thedimethyl butanes, cyoloparaflins and aromatics. The presence of more than a critical upper limit, that is about 0.5% of aromatics in the isomerization unit, results in retarding the conventional liquid phase isomerization re action. Furthermore, when itis attempted to remove aro- 2,909,582 Patented Oct. 20, 1959 matics and naphthenes from the paraflinic hydrocarbons, severe losses are encountered because of the azeotroping effects of normal hexane with the benzene andmethyl cyclopentane. Still further, the processing by isomerization of vfeedscontaining a substantial amount of isoparafilns is not attractive economically. Substantial amounts of the'dimethyl but-anes and isopentanes during their residence inthe isomerization unit are reconverted to less branch chain material, thus lowering the product octane value. F

It is therefore, theprincipal purpose of the present invention to provide the art with an improved method for isomerizing and upgrading light naphthas.

It is also an object of the present invention to provide a simple and efiective method of upgrading light petroleum naphthas boiling in the range of from about 60 to 180 F. to form high octaneprodncts with substantially no yield loss.

These and other objects will appear more clearly from the detailed specification and claims which follow.

It hasnow been found that the troublesome C /C light naphtha fraction can be converted into very high octane light naphthaswith an exceptionally high yield advantage by separating the light naphtha feed with molecular sieves having pore openings of about 5 A. into a non-normal paraifin/cyclohydrocarbon product and a normal paraffin product, isomerizing the latter, fractionati'ng the isoparaffins from the cyclic hydrocarbon, further separating the methyl pentane from isopentane and-dimethyl butanes, passing methylpentane to the isomerization plant along with the normal pentane and hexane. The isomerizate is recycled to the adsorption zone. By this particular combination of process steps, it is possible to convert substantially completely normal pentane and hexane to isoparafiins and simplify, and thus make commercially attractive, the hitherto extremely costly and difiicult sepa ration of naphthenes and aromatics from isoparaflins, due to previous removal of N-hexane, which 'azeotropes with these cyclic compounds.

It has, of course, been known for some time that certain zeolites, both naturally occurring and synthetic, and sometimes termed molecular sieves, have the property of separating straight chain from branched chain hydrocarbon isomers as well as from cyclic and aromatic compounds. These zeolites have innumerable pores of uniform size and only molecules small enough to enter the pores can be adsorbed. The pores may vary in diameter from 3 or 4 A. to 15 or more, but it is a property of these zeolites or molecular sieves that any particular product has pores of substantially uniform size. The scientific and patent literature contains numerous references to the adsorbing action of natural and synthetic zeolites. Among the natural zeolites having this sieving property may be mentioned chabazite. A synthet-ic zeolite with molecular sieve properties is described in US. Patent 2,442,191. Zeolites may vary somewhat in composition but generally contain the elements silicon, aluminum and oxygen as well as an alkali metal and/or an alkaline earth metal, e.g. sodium and/ or calcium. The naturally occurring zeolite analcite, for instance, has the empirical formula NaAlSi O ,H O. Barret US. Patent 2,306,610 teaches that all or part of the sodium is replaceable by calcium to yield on dehydration a molecu lar sieve having the formula (Ca,Na )Al Si O .2H O. Black U. S. Patent 2,522,426 describes a synthetic molecular sieve zeolite having a formula 4CaO.Al O .4SiO A large number of other naturally occurring zeolites having molecular sieve activity, i.e. the ability to adsorb a- 3 ly reviews, vol. III, pages 293 to 320 1949), published by the Chemical Society (London).

Referring now to the drawing, a fresh feed consisting of hydrocarbons boiling in the C to 180 F. range, and consisting principally of normal pentane, isopentane, normal hexane, the methyl-pentanes, the dimethyl butanes, cyclo paraflins such as methyl-cyclo pentane, and aromatics such as benzene is passed through line 2 to molecular sieve bed 4. The feed is preferably preheated to a sufficiently high temperature to vaporize it. A temperature of about 200 to 400 is particularly effective. The naphtha feed is passed, preferably in vapor phase, at temperatures of about 200 to 400 through the adsorption zone or tower. The adsorbent, which may be any natural or synthetic zeolite or other adsorbent of the molecular sieve type heretofore described, and having pore diameters of about SA. units is arranged in any desired manner in the adsorption zone or tower 4. It may for example, be arranged on trays or packed therein with or without supports. Conditions maintained in the molecular sieve treatment stage in tower 4 are flow rates of 0.1 to 2.0 v./v./hr., temperatures of about 200 to 400 F., and pressures from atmospheric to about p.s.i.g. With molecular sieves of the indicated pore size, the normal pentane and hexane constituents of the feed are readily adsorbed, while the isoparaffins, naphthenes and aromatics are not adsorbed but pass overhead as a sieve raflinate from the molecular sieve treatment zone through line 12, and are passed to the fractionation system described below.

When the molecular sieves in adsorption zone 4 become saturated with normal paratfins, as may readily be determined hy conventional means such as refractive index, gravity, or spectrographic analysis of the effluent, the flow of naphtha feed to the adsorption zone is stopped and the desorption cycle or regeneration of the sieves begins.

In a preferred embodiment of the invention, a plurality of adsorption zones is employed and the hydrocarbon stream merely switched from the saturated moleclar sieve adsorption zone or a fresh regenerated adsorption zone. Desorption is effected by any one of a number of conventional means such as by raising the temperamm of the zone to 450 to 800 F. with or without addition of an inert gas such as carbon dioxide, methane, ammonia, and with or without the reduction of pressure to atmospheric or below. An excellent means of desorption also has been found to be the replacement of the adsorbed normal hydrocarbons isothermally by an olefinic gas, preferably one containing a substantial proportion of propylene; for example, cracked refinery gas containing a major proportion of propylene and minor proportions of ethane and propane is a satisfactory stripping gas. The desorbed normal pentane and hexane is now passed through line 6 to isomerization plant 8, wherein the hydrocarbons are isomerized, preferably in the liquid phase and in the presence of a conventional Friedel-Crafts catalyst which may be supported on a porous aluminum oxide support such as Porocel. The isomerization plant is of conventional design and normal conditions prevail in the isomerization step. If desired, a promotional amount of a hydrogen halide promoter such as HCl may be added. Hydrogen gas also may be added if desired to the reaction zone 8. Temperatures within isomerization zone 8 may be in the range of about 100 to 275 F., preferably 150 to 225 F., and the promoter (HOl) may be added in amounts of 0.1 to 6.0 wt. percent, preferably 1.0 to 2.0 wt. percent, based on light naphtha charged to the reactor. A pressure of 100 to 450 p.s.i.g., preferably 150 to 250 p.s.i.g. normally obtains, as well as a hydrogen partial pressure of 0.1 to 3.0 mol percent H preferably 1.0 to 1.3 mol percent H and a v./v./ hr. of 0.3 to 3.0, preferably 0.5 to 1.0 v./v./hr.

It is also highly desirable to maintain in the isomerization zone a critically small proportion of a cyclic hydrocarbon, preferably benzene and methyl-cyclo pentane or cyclo hexane. The presence of benzene in amounts of from 0.1 to 0.5% prevents cracking of feed and sludging of catalyst. However, more than this critical amount of benzene retards the process. As will be seen more clearly below, in a preferred embodiment of the invention, these cyclic hydrocarbons are supplied as an integral part of the process from the reaction product.

The effluent from isomerization unit 8 is a mixture of unreacted normal hexane and pentane as well as an isomerizate consisting of isopentane, methyl pentanes, dimethyl butanes, and high boiling naphthenic by-products. In accordance with the present invention, the total effluent from the isomerization zone is neither distilled nor otherwise fractionated, but is passed via lines 10 and 2 to the molecular sieve adsorption zone along with further amounts of fresh feed. The non-normal constituents of the total recycle product as well as the non-normal constituents of the fresh feed are thus removed as raffinate from adsorption zone 4, while the unreacted normal constituents of the total recycle are re-adsorbed and recycled to the isomerization .zone. carried out to extinction.

The rafiinate from molecular sieve adsorption zone 4, consisting essentially of isoparaifins, naphthenes and aromatics is passed through line 12 to splitter tower 14 where simple fractionation yields an overhead product of isoparafiins and a bottoms stream of naphthenes and aromatics. Tower 14 may be operated at a pressure of 0 to p.s.i.g., and a temperature of to 275 F. Such a separation isv not possible when normal hexane is present, because of its strong azeotroping characteristics with cyclic hydrocarbons.

A bottoms stream 16 consisting of large proportions ofnaphthenes and aromatics may be passed, if desired, to a reforming process for conversion to a highly aromatic high octane premium fuel component. Advantageously, a portion of this bottoms stream is employed as the benzene comprising additive for the isomerization zone 8, as pointed out above. In this embodiment, an amount sufficient to provide 0.1 to 0.5% of benzene based on naphtha feed to isomerization unit 8 is passed to that unit via line 18.'

The overhead stream from splitter 14 consisting essentially of isopentane, the dimethyl butanes and the methyl pentanes is now passed via line 20 to super-fractionation tower 22. This vessel, because of the relative closeness of the boiling points of these constituents is provided with a relatively large number of theoretical plates, in the neighborhood of 50 to 100, and is operated to separate an overhead fraction 24 containing high proportions of isopentane and 2,2-dimethyl butane. These products have very high octane ratings, in the neighborhood of 102 to 106 (with 3 cc. lead); the bottoms product, the methyl pentanes, however, having significantly lower octane value and it is one of the important objects of the present invention to convert these to the higher octane value dimethyl butanes. In accordance with this objective, the methyl pentanes are taken off as bottoms product through line 26 and are passed to the isomerization unit 8 along with the adsorbate from the molecular sieve adsorption zone. Because of the relatively high concentration of normal hexanes in the isomerization zone, the methyl pentanes are preferably converted to dimethyl butanes, and are recovered in a manner already described above. Any 2,3-dimethyl butane present in the fresh feed or formed in the isomerization unit is either removed as overhead product from the superfractionator or is removed from that 'vessel along with the methyl pentanes and recycled to the isomerization unit for further conversion to 2,2-dimethyl butane.

The process of the present invention results in substantial yield advantages over previously proposed processes. Because of the recycling to extinction of the normal pentane and normal hexane and the methyl pentanes, until they are completely converted to high octane fuels,

This process may be maximum utilization of feed is realized. The process of the present invention results not only in a premium octane motor fuel, but in a premium gasoline having the critical volatility necessary in a premium gasoline. This feature is unique with the process described. Furthermore, the isomerization and recycling is accomplished without necessity of distillation and necessary segregation of product from overhead. The process of the present invention results in a product that cannot be produced by any presently known steps or sequence of steps or combination of steps from the C to 180 F. naphtha fraction. It is an important feature of the present invention that the cut be restricted to exclude substantially all normal heptane. This hydrocarbon in a liquid phase isomerization process tends to foul the catalyst.

The process of the present invention may be modified in a manner obvious to those skilled in the art. Thus, the splitter tower may follow rather than precede the super-fractionator. Substantially, any naphtha fraction boiling in the range specified and from whatever source, may be employed as fresh feed. However, it is highly desirable to remove or to bring to a very low concentration olefins in the fresh feed. A guard chamber, which may be any conventional mean-s for olefin removal, or even a molecular sieve bed, may be used for this purpose. The presence of olefins may cause premature exhaustion of the molecular sieve adsorption zone, and it is definitely deleterious in the isomerization zone. Furthermore, if the fresh feed initially is low in isoparafiins and particularly low in aromatics, it may be fed at least in part directly to the isomerization zone. Still further, instead of employing a part of the bottoms product to the splitter to supply the small amount of aromatics promoter for the isomerization zone, a portion of the fresh feed equivalent to 0.1 to 0.5% of benzene may be passed directly to the isomerization vessel. Furthermore, though the invention has its utility in connection with a liquid phase isomerization process, the process may also be carried out in connection with a conventional vapor phase isomerization operation using conventional vapor phase catalysts.

A particularly advantageous method for preparing a molecular sieve material having 5 Angstrom unit pore openings suitable for the adsorption step is the mixing of an aqueous solution of sodium metasilicate and a solution of sodium aluminate having a ratio of Na O to A1 of 1:1 to about 3:1 at a temperature of about 160 to 250 F., in proportions giving a ratio of Si0 to A1 0 in the reaction mixture of about 1:1 to 2:1. The molecular sieve material is obtained in good yields after a heat soaking period of about 30 minutes or longer. The crystalline material, having an approximate composition of Na O.Al O .2SiO .XI-I O is filtered, base exchanged with a calcium salt such as calcium chloride and calcined at a temperature of about 400 to 900 F. to yield a composition having uniform pore openings of 5 Angstrom units.

It is further to be understood that instead of the natural or synthetic zeolites described as suitable for this purpose, other adsorbents having uniform pore openings of about 5 Angstrom units may be employed. Thus, certain activated carbons have been found to have uniform pore openings of this size and may be used for this service.

What is claimed is:

1. An improved process for upgrading naphthas boiling in the range of C to 180 F., and comprising normal pentane and hexane as well as substantial portions of aromatics and the other cyclic hydrocarbons which comprises passing a vaporized stream of said naphtha to an adsorption zone, adsorbing straight chain hydrocarbons from said stream and rejecting branched chain and aromatics and other cyclic hydrocarbons, passing unadsorbed branched chain pentanes and hexanes and aromatics and other cyclic hydrocarbons to a multiple fractionation zone, desorbing and withdrawing straight chain hydrocarbons from said adsorption zone, passing said desorbed hydrocarbons to an isomerization zone, withdrawing an isomerizate comprising branched chain pentanes and hexanes from said zone, passing said isomerizate to said adsorption zone for adsorption of unreacted straight chain hydrocarbons, fractionating aromatics and cyclic hydrocarbons from isoparaffins in one portion of said multiple fractionation zone and fractionating isopentane and dimethyl butanes from methyl pentane in another portion of said zone, and withdrawing separate streams comprising aromatics and other cyclic hydrocarbons, isopentane and dimethyl butanes, and methyl pentanes, respectively, from said last named zone.

2. The process of claim 1, wherein said methyl pentane comprising stream is passed to said isomerization zone.

3. The process of claim 1, wherein said isomerization zone contains a Friedel-Crafts isomerization catalyst.

4. An improved process for upgrading naphthas boiling in the range of C to 180 F. and comprising substantial proportions of normal pentane and hexane as well as cyclic hydrocarbons, including aromatics, which comprises passing a vaporized stream of said naphtha to an adsorption zone, maintaining in said zone an adsorbent having uniform pore openings of about 5 Angstroms, adsorbing normal pentane and normal hexane from said stream, passing unadsorbed hydrocarbons including branched chain pentanes and hexanes and aromatics and other cyclic hydrocarbons to a fractionation zone, desorbing said normal hydrocarbons, passing said desorbed hydrocarbons to a liquid phase Friedel-Crafts isomerization zone, withdrawing an isomerizate from said Zone, passing said isomerizate to said adsorption zone for adsorption of unreacted normal pentane and hexane, fractionating aromatics and other cyclic hydrocarbons from isoparafiins in said fractionation zone, withdrawing a stream rich in cyclic hydrocarbons from said fractionation zone, withdrawing a stream rich in branched chain pentanes and hexanes from said zone, passing said last named stream to a second fractionation zone, withdrawing a stream rich in high octane isopentane and dimethyl butanes from said zone, separately withdrawing a stream rich in methyl pentane from said zone and passing at least a portion of said last named stream to said isomerization zone.

References Cited in the file of this patent UNITED STATES PATENTS 2,306,610 Barrer Dec. 29, 1942 2,395,022 Sutton et al. Feb. 19, 1946 2,425,535 Hibshman Aug. 12, 1947 2,433,482 Roberts Dec. 30, 1947 2,443,607 Evering June 22, 1948 2,818,449 Christensen et a1. Dec. 31, 1957 OTHER REFERENCES Chemical and Engineering News, vol. 32, p. 4786, Nov.

Claims (1)

1. AN IMPROVED PROCESS FOR UPGRADING NAPHTHAS BOILING IN THE RANGE OF C5 TO 180* F., AND COMPRISING NORMAL PENTANE AND HEXANE AS WELL AS SUBSTANTIAL PORTIONS OF AROMATICS AND THE OTHER CYCLIC HYDROCARBONS WHICH COMPRISES PASSING A VAPORIZED STREAM OF SAID NAPHTHA TO AN ADSORPTION ZONE, ADSORPTION STRAIGHT CHAIN NAPHTA TO AN FROM SAID STREAM AND REJECTING BRANCHED CHAIN AND AROMATICS AND OTHER CYCLIC HYDROCARBONS, PASSING UNADSORBED BRANCHED CHAIN PENTANES AND HEXANES AND AROMATICS AND OTHER CYCLIC HYDROCARBONS TO A MULTIPLE FRACTIONATION ZONE, DESORBING AND WITHDRAWING STRAIGHT CHAIN HYDROCARBONS FROM SAID ADSORPTION ZONE, PASSING SAID DESORBED HYDROCARBONS TO AN ISOMERIZATION ZONE, WITHDRAWING AN ISOMERIZATE COMPRISING BRANCHED CHAIN PENTANES AND HEXANES FROM SAID ZONE, PASSING SAID ISOMERIZATE TO SAID ADSORPTION ZONE FOR ADSORPTION OF UNREACTED STRAIGHT CHAIN HYDROCARBONS, FRACTIONATING AROMATICS AND CYCLIC HYDRO-

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