US2728712A - Reforming paraffinic naphthas employing lithium, calcium, barium, or strontium - Google Patents

Description

Dec. 27, 1955 l. MAYER 2,728,712

REFORMING PARAFFINIC NAPHTHAS EMPLOYING LITHIUM, CALCIUM, BARIUM OR STRONTIUM Filed July 17, 1952 2 Sheets-Sheet 1 STAm'zEnz. GAS 4- QaFLux DQIJM 'rAblLvzera NAPHTHA FEED NAPHTHA Paoouc-r 29 -T'ZI 7.-1

IVAM MAYEQ, inventar @ES W Swan CLtGoraen Dec. 27, 1955 l. MAYER 2,728,712

REFORMING PARAFFINIC NAPHTHAS EMPLOYING LITHIUM, CALCIUM, BARIUM OR STRONTIUM Filed July 17, 1952 2 Sheets-Sheet 2v Hvorzoad HEM/Y MAW-:THA

United States Patent O REFORMING PARAFFINIC NAPHTHAS EMPLOY- 'lrNG LITHIUM, CALCIUM, BARIUM, 0R STRON- Ivau Mayer, Summit, N. J., assignor to Esso Research and Engineering Company, a corporation of Delaware Application July 17, 1952, Serial No. 299,310

11 Claims. (CI. 196-50) This invention relates to a method of improving antiknock characteristics of gasolines. It relates particularly to a method of dehydrogenating paraiinic constituents of naphthas, especially normal Cs to Ca alkanes, to the corresponding mono-olelins with a minimum degradation of the naphtha to lighter hydrocarbons. Speciiically the invention relates to a dehydrogenation process wherein liquid lithium or lithium-containing mixtures are used as the dehydrogenating agent.

The petroleum industry is increasingly concerned with upgrading the anti-knock characteristics of naphthas in order to improve their eiciency in internal combustion engines. With heavier naphthas fair results have been obtained in this respect with the aid of thermal or catalytic reforming or hydroforming. However, in the case of light parainic naphthas consisting essentially of normal hexane, heptane and octane and the corresponding isoparains, the octane improvement has been quite unsatisfactory, Accordingly, it has been proposed to dehydrogenate the light paraflins to form the corresponding mono-oleins which have appreciably better antiknock characteristics. However, no satisfactory method for doing this has been known heretofore, as generally such previous processes employed such high temperatures that appreciable cracking and hence a substantial loss in naphtha volume resulted. Thermal as well as most catalytic dehydrogenation processes have been found unecononiical because of excessive coke yields, while selective oxidation has been found impractical to date because of difficulties in keeping the reaction from proceeding too far.

It has been proposed heretofore to treat various hydrocarbons with alkali metals such as sodium for various purposes. For instance, mineral oils have been sweetened by contact with sodium under mild conditions, to remove sulfur from the oil by reaction of the sodium with any mercaptans and organic suldes. Catalytically cracked stocks of the gas oil and lighter types have likewise been treated with sodium under mild conditions, e. g. at temperatures below about 300 F., for purposes of desulfurization and removal of gum forming constituents. This removal has been presumed to involve a mechanism similar to polymerization commonly used in the preparation of buna type rubbers and drying oils. In still another known process oxidized oil has been treated with sodium metal under quite drastic conditions of temperature and high pressure whereby presumably the oxygenated compounds are reduced and also desulfurized. However, none of these prior processes has accomplished any appreciable dehydrogenation of naphtha-range parains, which dehydrogenation is the principal object of the present invention. Indeed, with the most common alkali metals and even with some alkaline earth metals dehydrogenation of paraflins through the mechanism of metal hydride formation has been found to be impossible except at such low temperatures that the reaction rate becomes entirely impractical. On the other hand, as the reaction temperature is raised, these metals lose their ability to accept hydrogen and instead the reaction becomes increasingly one of ordinary cracking, the alkali metal serving principally as a sweetening agent.

It has now been discovered that only four metals possess characteristics of such a nature that they are suitable for selective dehydrogenation of Cs to Cs parafiins, with the formation of a corresponding metal hydride from which the original metal can be regenerated in a practical manner. These metals are calcium, barium, strontium and, in the preferred embodiment of the invention, especially lithium or mixed metals or alloys, such as lithium-aluminum, which contain lithium in sufcient quantity to be liquid at the critical reaction conditions required.

In the process of the invention light virgin naphtha boiling between about to 250 F. and having a research octane number of less than about 65 or 70 (clear) is reformed by dehydrogenation with the aid of molten lithium or a molten mixture of metals which contains lithium. In this dehydrogenation reaction the parans are extensively converted into corresponding mono-olens and the split off hydrogen forms a hydride with the lithium. The critical reaction conditions include reaction temperatures of about 700 to 1000* F., preferably 850 to 950 F., and substantially atmospheric pressure, that is, about 0 to 10 p. s. i. g. Within the limits stated it is preferred to employ as low reaction temperatures as are consistent with practical dehydrogenation rates.

The spent metal in the form of lithium hydride, which as such is a solid below about 1250 F., is withdrawn from the reaction Zone and separately dehydrogenated back to the original state, to permit reuse. To avoid deposition of solid hydride anywhere in the system, it may be desirable to keep the concentration of hydride within its limits of solubility in metallic lithium. For instance, the hydride concentration may be kept below about l5 weight percent on metallic lithium, preferably between about 8 to 11 weight percent, which mixture has a melting point in the range of about 400 to 500 F. The regeneration of the hydride proceeds at commercially practical rates in the temperature range between about 1300 and 2000 F. and at atmospheric pressure or lower, e. g. at 5 to 15 p. s. i. absolute. The lowest practical pressures are generally preferred since they require lower temperatures for regeneration. High purity hydrogen is produced in this regeneration reaction.

The invention will now be described and illustrated with reference to the attached drawing.

The single gure of the drawing is a diagrammatic flow plan showing equipment suitable for carrying out the various steps of the process.

Referring to the drawing, a predominantly parainic light virgin naphtha having a boiling range between about 150 and 250 F. is introduced through line 1 into bubble cap tower 2 where it is dried by distillation to prevent subsequent loss of lithium by reaction with water present in the feed. This virgin feed naphtha may have a research octane number of about 70 or less (clear), a gravity of about 55 to 75 API and may contain as much as 100 volume percent, and preferably at least 30 to 50 weight percent, of paraflinic hydrocarbons, any other constituents present being essentially naphthenic or aromatic in character.

Tower 2 may be operated at 180 to 210 F. at a pressure of 5 to 25 p. s. i. g., e. g. at a bottoms temperature of about 192 F., and at a pressure of about 10 p. s. i. g. The overhead from tower 2 may be passed through line 3 and condenser 4 to a separator 5 where separated water 3 may be drawn olf through line 6 while a portion of the hydrocarbon condensate may be returned as reflux to the top of the tower 2 through line 7, as is well known.

The dried naphtha in line 8 is preferably preheated to about 600 to 750 F. by heat exchange with hot process streams, e. g. with spent metal stream 40 sub sequently to be described, and passed into dehydrogenation chamber 10 which may be maintained at about 900 F and substantially atmospheric pressure, e. g. 2 p. s. i. g. Chamber 10 contains a pool of liquid lithium through which the preheated naphtha is bubbled, or with which it is otherwise contacted. as by forced circulation through inert packing material, at a rate of about 0.1 to 0.5 liquid volume per hour per volume of lithium. In this step both the parafiinic and naphthenic constituents of the naphtha are dehydrogenated by transfer of hydrogen to the liquid lithium. This exothermic reaction may be represented by the following typical equation.

A cool scrubbing oil is used in scrubber 11 to quench the overhead products from reactor 10 totheir dew point, e. g. 150 to 250 F., to recover entrained liquid lithium. The scrubbing oil may be a condensed portion of the naphtha product, preferably the higher boiling fraction thereof, or an extraneous stream such as heavy naphtha may be introduced at 1S for this purpose. The scrubbing oil may be pumped around scrubber 11 through line 13 after cooling in cooler 12 to a suitable temperature, e. g. tol25 F. Since this recycle stream 13 contains the recovered lithium in the form of an oil slurry, a portion of stream 13 may be branched off and returned through lines 14 and 8 to the dehydrogenation chamber 10.

The overhead vapor from scrubber 11 may be at least partially condensed in condenser 20 to permit use of a liquid pump instead of having to compress all of the product in a gas compressor. After compression in pump 21 and/or compressor 22 the product is then passed to stabilizer tower 25 where undesirably light fractions such as propane and lighter gases are removed overhead from the product naphtha. Generally this gaseous fraction will be small, e. g. less than weight percent on naphtha feed. The stabilizer tower may be operated, for instance, at a still temperature of about 450 F., a reflux temperature of about 100 F. and a pressure of about 335 p. s. i. g., so as to leave less than about 0.5 percent C3 and lighter hydrocarbons in the stabilized bottoms, and less than about l percent C4 hydrocarbons in the overhead gas. ln order to supply heat to stabilizer 25, a portion of the oil is withdrawn through line 26, thence passed through a trap-out drum, thence passed via line 27 into furnace 43 wherein it is heated and returned to the bottom of stabilizer 25 through line 29. In other words, the system just now referred to acts as a reboiler to supply heat to the stabilizer. Operating conditions, including reux drum pressure, which are best suited to accomplish this may be determined in each case according to well-known engineering principles and will depend on various local variables such as the temperature level of available cooling water, etc.

Stabilized naphtha product is withdrawn from tower 25 through bottoms line 28. Heat from the naphtha product may be recovered by heat exchange with some of the cooler process streams such as the condensate feed to the stabilizer, or the hot naphtha product may be employed in a heat exchanger serving as a reboiler for the bottoms of drying tower 2 and so on. The recovered naphtha product may amount to about 85 to 95 or more volume percent based on virgin naphtha feed, depending on feed composition and degree of cracking. The SubSiaDiial mprovement of the naphtha is indicated by the following specific example:

It is seen that a high yield of product of excellent antiknock characteristics is obtained, largely as a result of the very extensive conversion of parans to mono-olefins, naphthenes to aromatics and cyclopentanes to cyclopentenes. An appreciable yield of substantially pure hydrogen can also be recovered as a valuble by-product from the regeneration stage.

The lithium hydride formed during the dehydrogenation reaction is maintained below about 5-15 weight percent, preferably at about 10 weight percent on lithium by continuous withdrawal of a portion of the molten bath from dehydrogenator 10. Accordingly a lithium-lithium hydride stream may be pumped from dehydrogenator 10 through line 40, heat exchangers 41 and 42 and furnace 43 where it is heated to 1450-1700" F., preferably about 1540" F. in order to effect decomposition of the lithium hydride into metallic lithium and molecular hydrogen. The hydrogen thus formed is separated from the molten lithium in separator 45 substantially at atmospheric pressure or lower. Separated lithium is returned to dehydrogenator 10 through line 46 and the hydrogen is cooled and scrubbed n scrubber 50 with an extraneous heavy naphtha or a portion of the scrubbing oil stream 14. The separated high-purity hydrogen is withdrawn from scrubber 50 through line 51 and may then be compressed, for instance, to about 450 p. s. i. g. for further use in various hydrogenation or other processes as desired. A portion of the liberated hydrogen may be recycled to furnace 43 via line 52 in order to obtain good mixing in the heating coil 43. Net hydrogen production may amount to about 1000 to 2000 Astandard cubic feet per barrel of virgin naphtha feed, depending on the type of hydrocarbon feed.

The lithium recovered from the hydrogen stream is withdrawn from scrubber 50 through line 55 and returned as a slurry in scrubbing oil to dehydrogenator 10 through line 8. A portion of the scrubber stream 55 may be recycled through line 58 for reuse in scrubber 50 after sufficient cooling, e. g. at 100 F.

From the foregoing general description, it will be apparent that the invention is not limited to the particular illustrative embodiment described. On the contrary various modifications and variations may be made without departing from the scope or spirit of the present invention. For instance, while lithium has been described as the preferred dehydrogenation agent, calcium, barium, or strontium may be used similarly, provided that they are alloyed with enough lithium or other metal so as to reduce their melting point below the temperature at which the dehydrogenation is carried out. For instance, a metal mixture containing about to 90 weight percent lithium and 10 to 20 weight percent calcium can be used. On the other hand, sodium, potassium and cesium, while having sutlciently low melting points, are not suitable because they do not form stable hydrides at the temperatures necessary for sufciently rapid dehydrogenation of the saturated naphtha feed.

Likewise, while drying of feed by distillation has been specifically described, it is apparent that the feed may be dried to the required degree by alternate means, as by contacting with sulfuric or phosphoric acid, activated alumina, silica gel, calcium chloride, or other well known chemical drying agents.

One of the principal advantages of this invention is that the dehydrogenation is operable at sufficiently low temperatures to avoid decomposition of normal paratlins containing more than five carbon atoms, e. g. hexane or octane, to methane or other undesirable gases. At the saine time the formation of aromatics from the paralin is kept desirably low, thus avoiding any appreciable volumetric yield losses which otherwise accompany such aromatics formation. Another major advantage is that the contact material is in a liquid state both in the reaction and the regeneration zones. As a result excellent heat transfer coeicients are obtained and relatively small heat transfer areas are required.

The need for continuous circulation of the liquid or solid metallic hydrogen acceptor between separate reaction and regeneration zones may be eliminated by employing a cyclic semi-batch process wherein a plurality of two or more reactor vessels are connected in parallel and desirably provided with internal heating or cooling coils or other heat transfer means. In such an arrangement one reactor may be on stream for carrying out the desired dehydrogenation while the hydrogen acceptor charge of the other reactor is being regenerated. When the hydride content of the first reactor reaches the maximum permissible concentration, one of the other reactors is used for dehydrogenation while the lirst reactor is put on a regeneration cycle, and so on.

The invention for which patent protection is sought is particularly pointed out in the appended claims.

I claim:

1. A continuous process for reforming light virgin naphtha containing a major proportion of Cs to Ca alkanes which comprises passing essentially water-free naphtha through a closed reaction zone maintained at a temperature between about 800 and 950 F. and a pressure of about 0 to 5 p. s. i. g. and containing a pool of metallic liquid which comprises a major proportion of lithium, thereby obtaining a dehydrogenation of the alkanes and forming lithium hydride, removing the resulting dehydrogenated naphtha product vapors from the reaction zone, also withdrawing a portion of the metallic liquid which contains lithium hydride formed in the process, passing the withdrawn metallic liquid into a separation zone maintained at a temperature between about 1300 and 2000 F. and at a pressure not in excess of about 2 p. s. i. g., removing a stream of liberated hydrogen from the separation zone, and also removing regenerated metallic liquid from the separation zone for reuse in the reaction zone.

2. A process according to claim 1 wherein the virgin naphtha is dried to a water content of not more than 0.0005 weight percent by fractional distillation prior to contacting with the lithium.

3. A process according to claim l wherein the metallic liquid in the reaction zone contains about to 90 weight percent of lithium and 20 to 10 weight percent of calcium.

4. A process according to claim 1 wherein the naphtha product vapors eliiuent from the reaction zone are cooled to the dew point of the hydrocarbon vapors by scrubbing with a cold inert scrubbing liquid to form a liquid slurry which contains lithium previously entrained from the reaction zone.

5. A process according to claim 4 wherein the scrubbing liquid is a condensed portion of the naphtha product vapors.

6. A process according to claim 4 wherein the lithium slurry is recycled from the scrubbing zone to the reaction zone.

7. A process according to claim 1 wherein the stream of hydrogen removed from the separation zone is scrubbed with a scrubbing oil to cool the hydrogen and to separate out entrained lithium in the form of a slurry.

8. A process according to claim 7 wherein the scrubbing oil is a heavy naphtha.

9. A process of producing lithium metal from lithium hydride which comprises forming a mixture containing up `to about "15j Vweight percent of lithium hydride in lithium, flowing the mixture at a temperature above its melting point into a decomposition zone, heating the mixture in the decomposition zone to a temperature between about 1500and 2000 F. at a pressure not higher than atmosphgrjvcgmwllereby tug lithium hydride is decomposed nto gen charging the hydrogen and lithium to a separation zone, withdrawing a stream 4of hydrogen gas from the separation zone, and also withdrawing a liquid stream of high-purity lithium from the separation zone.

10. A process according to claim 9 wherein the liquid lithium product is used as solvent for fresh lithium hydride.

1l. A process according to claim 1 wherein the virgin naphtha is dried to a water content of not more than 0.0005 weight percent of contact with a chemical drying agent.

References Cited in the tile of this patent UNITED STATES PATENTS 1,805,686 Cross May 19, 1931 1,859,028 Cross May 17, 1932 1,865,235 Cross June 28, 1932 2,002,747 Morrell May 28, 1935 2,052,812 Wait Sept. 1, 1936

Claims (1)

1. A CONTINUOUS PROCESS FOR REFORMING LIGHT VIRGIN NAPHTHA CONTAINING A MAJOR PROPORTION OF C6 TO C8 ALKANES WHICH COMPRISES PASSING ESSENTIALLY WATER-FREE NAPHTHA THROUGH A CLOSED REACTION ZONE MAINTAINED AT A TEMPERATURE BETWEEN ABOUT 800 AND 950*F. AND A PRESSURE OF ABOUT 0 TO 5 P. S. I. G. AND CONTAINING A POOL OF METALLIC LIQUID WHICH COMPRISES A MAJOR PORTION OF LITHIUM THEREBY OBTAINING A DEHYDROGENATION OF THE ALKANES AND FORMING LITHIUM HYDRIDE, REMOVING THE RESULTING DEHYDROGENATED NAPHTHA PRODUCT VAPORS FROM THE REACTION ZONE, ALSO WITHDRAWING A PORTION OF THE METALLIC LIQUID WHICH CONTAINS LITHIUM HYDRIDE FORMED IN THE PROCESS PASSING THE WITHDRAWN METALLIC LIQUID INTO A SEPARATION ZONE MAINTAINED AT A TEMPERATURE BETWEEN ABOUT 1300 AND 200* F. AND AT A PRESSURE NOT IN EXCESS OF ABOUT 2 P. S. I. G., REMOVING A STREAM OF LIBERATED HYDROGEN FROM THE SEPARATION ZONE, AND ALSO REMOVING REGENERATED METALLIC LIQUID FROM THE SEPARATION ZONE FOR REUSE IN THE REACTION ZONE.

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