US2389215A - Process for dehydrogenating a hydrocarbon - Google Patents

Description

1945. H. M. SINGLETON 2,389,215

PROCESS FOR DEHYDROGENATING HYDROCARBONS Filed Feb. 15, 1943 W INVENTOR. BY Mm ATTORNEY Patented Nov. 20, 1945 UNITEDVSTATES PATENT OFFICE. I

PROCESS FOR DEHYDBOGENATING A HYDROCABBON Henry M. Singleton, Goose Creek, Tex., assignor to Standard Oil Development Company,-a corporation of Delaware Application February 15, 1943, Serial No. 475,866 12 Claims. (Cl. 260-669) urated hydrocarbons. Olefins'for use in polymerization and alkylation are obtained by dehydrogenation of the corresponding paramns. Dioleflns for the production of rubber are obtained by further dehydrogenation of the oleflns. Dioleflns, such as butadiene, are also obtained from hydroaromatic hydrocarbons by a reaction which involves dehydrogenation. For example,

cyclohexane is first dehydrogenated to cyclohexene, which then breaks down into butadiene and ethylene. Polymerizable aromatic hydrocarbons, such'as styrene, are obtained from alkylated aromatics byaiehydrogenation.

From the foregoing discussion it will be evident that the process of dehydrogenation of hydrocarbons is one which has been intensively studied with a, view of increasing its economy and efficiency. Various catalysts have been suggested as promoters of dehydrogenation. The conditions best suited for dehydrogenation of various hydrocarbons in the presence of and in the absence of catalysts have been fairly well established after careful investigation. The efiect of pressure, including the partial pressure of hydrogen, on the reaction has been fully disclosed in the literature.

According to the present invention, the efficiency and economy of processes involving the dehydrogenation of hydrocarbons are favorably influenced by carrying out the dehydrogenation in the presence of certain organic compounds which possess the property of rapidly consuming large quantities of hydrogen, and in some cases cf undergoing conversion themselves to yield fur- 3-1181 quantities of the desired reaction product. "l'hese organic compounds may be referred to enerally a heterocyclic compounds,-which.in-'

clude in their ring structure at least one of the elements oxygen, nitrogen, and sulfur. Examples of such organic compounds are furan, thiophene, pyrrole and derivatives thereof. Readily available compounds ofthis type are furfural and furfuryl alcohol. The alkyl derivatives and the partial reduction compounds of the three general types specified above are typical of compounds which are contemplated for use in accordance with the present invention. These various compounds will be referred to hereinafter as hydrogen acceptors.

These organic compounds are characterized, in addition to their property of combining readily with hydrogen, by their susceptibility to decomposition into free radicals at elevated temperatures. The formation of these free radicals enhances the power of these organic compounds to consume hydrogen, particularly the nascent type, which results from the dehydrogenation of hydrocarbons. Under certain conditions these free radicals combine either with themselves or with the dehydrogenatedhydrocarbons to yield useful by-products. The generation of these free radicals can be controlled by the use of suitable addition agents. For example, small percentages of acid-reacting substances, such as strong mineral acids, tend to acceleratethe production of free radicals from these compounds, while the compoundsare stabilized and the formation of free radicals suppressed by the presence of basic substances such as ammonia or amino compounds. Thus, if, in a given reaction, the operating temperature is such as to promote the decomposition of the hydrogen acceptor and such decomposition is not desired, a certain percentage of ammonia, aniline, pyridine, or the like is included in the feed stock. On theother hand, if, underthe operating conditions, the hydrogen acceptor is relatively stable and decomposition thereof is desired, either for the purpose of increasing the hydrogen consumption or of producing additional amounts of desired end product, a small amount of an acidic substance, such as nitric acid, h-y- I drochloric acid, sulfuric acid, sulfur dioxide, or

the like is included in the feed.

The various dehydrogenation reactions contemplated by the present invention are well established in the art, and the operating conditions therefor are well known. The modification of these well known processes by the present invention does not necessarily alter these operating conditions, although the invention does afford the possibility of increasing the desired conversion at any given set. of conditions.v To put it another way, the present invention makes -possible the realization under milder conditions of yields heretofore requiring more severe conditions. As has previously been indicated, the present invention is applicable to dehydrogenation effected both by heat alone, as well as by heat and catalysts. Suitable catalysts for these reactions are numerous and fully disclosed .in the prior art. Especially effective catalysts are oxides and sulfides of metals of group VI of the periodic system, alone or in conjunction with supports such as activated aluminum. Molybdenum sulfide and chromium oxide may be mentioned as representative examples of group VI dehydroenation catalysts.

In general, dehydrogenation reactions contemplated by the present invention are carried out at temperatures ranging from about 600 to 1400 F. Reactions which involve the conversion of olefins to diolefins are ordinarily carried out in the upper end of this range, while the dehydrogenation of the paraflins to oleflns is ordinarily conducted in the lower end of the range. A higher operating pressure than ordinarily employed in dehydrogenation is permissible in the practice of the present invention because the reaction, when carried out according to the present invention, is not necessarily accompanied by an increase in volume. Pressures as high as 250 lbs/sq. in. may be employed, but lower pressures I are preferred. It is advantageous to dilute the reaction mixture with an inert gas, such as nitrogen, steam, molecular hydrogen, or natural gas. A ratio of diluent-to product as high as 50:1 may be employed, but preferably this ratio is maintained below :1 when diluent is used. The

presence of the diluent makes possible the employment of higher pressures without giving rise to excessive polymerization, to which there may be a considerable tendency under the operating conditions with certain hydrocarbons. The same effect as that realized with a diluent may be obtained by using a sufliciently high feed rate to provide a large amount of unreacted feed at all times in the reaction chamber. This type ation yields only a low conversion per pass, involves considerable recycling.

The nature of the present invention may be better understood by reference to the accompanying drawing, in which is shown, in front elevation in diagrammatic form, an apparatus of the type suitable for the practice of the method.

In referring to the drawing in detail, a specific embodiment of the process of the present invention will be discussed. This particular embodiment is the production of butadiene from cyclohexane, to the treatment of which the apparatus shown is particularly adapted. Itwill be understood, of course, that, with slight modification, the apparatus can be used forthe dehydrogenation or conversion of other materials.-

Numeral one designates a. fresh feed line, in which is arranged a heating coil 2, where the feed stock is supplied with suflicient heat to support the subsequent reaction. The feed is discharged from line 1 into the bottom of reactor 3, which, in the particular case illustrated, is provided at its lower end with a bed 4 of catalytic material effective for catalyzing dehydrogenation. Also included in the catalyst bed may be substances and which favor the splitting of molecules, such as free iron, nickel, cobalt, copper, and the like. In the upper part of reactor 3 is provided a heating element 5 which may be used to supply any heat required to complete the conversion of the cyclohexane to butadiene and ethylene. I

Arranged as close to the outlet of reactor 3 as convenient is an inlet line 6 for the introduction of cold fresh feed. The purpose of introducing cold feed at this point is to quench the reaction products so as to reduce them, as rapidly as possible, from reaction temperature to a temperature below that at which polymerization and other side of operthe hydrogen acceptor.

reactions may occur. After the process is once started, all of the fresh feed may be introduced by way of line 6. The mixture of reaction product and fresh feed is conducted by line I to a fractionating tower 8, ahead of which is arranged atemperature controlled unit 9 which may be utilized to impart heat to, or cool, the mix ture in line I, as required. The fractionator 8 is so operated as to give off, as an overhead stream through line I0, butadiene and lower boiling compounds. Unreacted cyclohexane is withdrawn. as bottom by way of line I I and recycled to line I.

The temperature inside reactor 3 is maintained preferably between about 1l00 and 1400" F. The pressure should preferably be less than about lbs/sq. in., 30 lbs/sq. in. being satisfactory. At this pressure the partial pressure of the reacting material is reduced by including in the reaction mixture an inert gas. For this purpose, steam may be introduced into line I by way of branch line I2. Nitrogen may be used if desired. Methane is also a useful diluent and in some cases may find its way into the final product, as hereinafter suggested.

In operating according to the present invention, a hydrogen acceptor is introduced into line I by branch line I3. In the particular embodiment shown, the hydrogen acceptor may be assumed to be furfural. This hydrogen acceptor may be used in an amount equal to, and preferably somewhat in excess of, the amount theoretically required to consume the hydrogen liberated in the conversion of cyclohexane to butadiene and ethylene. Of course, for a lesser effect a smaller amount of the hydrogen acceptor is emloyed. There is no objection to using a considerableexcess of the hydrogen acceptor, except that it reduces the capacity of the unit for the desired reaction. A slight excess over stoichiometrical requirements will ordinarily be satisfactory. Under the particular conditions employed furfural, in taking up hydrogen, is converted first to furfuryl alcohol, which, through further absorption of hydrogen, yields piperylene as an end product. Piperylene is a highly desirable by-product since it may be used as a raw material for the production of synthetic rubber, either by way of polymerization or copolymerization with isobutylene or butadiene or other rubber forming unsaturated hydrocarbons. Pipery lene is advantageously recovered from the system as a side stream from fractionator 8 by way of draw-oi! line I4. Any unconverted furfural or residual furfuryl alcohol or intermediate products boiling above piperylene and still capable of taking up hydrogen go into the bottoms with unclzlonvfrted cyclohexane and are returned to inlet It is believed to be evident that, instead of using furfural as the initial hydrogen acceptor, furfuryl alcohol may be employed with equal success. It may beobserved that, depending upon the specific conditions of working employed, the hydrogen acceptor may exhibit a greater or lesser tendency to decompose into free radicals. As has previously been suggested, this tendency may be enhanced by adding a small amount of an acidreacting substance to the feed. When an acidreacting substance is employed it may be used in an amount ranging from 0.01% to about 10% of Ordinarily, the concentration of the acid-reacting substance need not substantially .exceed 1% of the hydrogen acceptor. Particularly when methane is employed as a diluent, it is desirable to promote the femation of free radicals in the reactor. In this cue, a substantial excess of the hydrogen acceptor over that normally required to assimilate the hydrogen given up by the cyclohexane isadvantageous. With methane and free radicals present in the reactor, the yield of useful product is increased by including phosphoric acid either in the catalyst bed 4 or by arranging a separate bed of phosphoric acid on a suitable support, such as silica gel or activated alumina in the upper part of the reactor. In this case, a more careful fractionation of the product, or additional fractionation of the bottoms drawn oil. from fractionator 8 by line I I is advisable.

In like manner, if the operating conditions are such that an undesirably high conversion of the hydrogen acceptor into free radicals occurs in the reactor 3, there is added to the reaction mixture, by way of line II, a basic substance in addition to the hydrogen acceptor. A suitable basic substance for this purpose is aniline or pyridine, although ammonia itself may be employed.

When the apparatus shown in the drawing is utilized for the dehydrogenation of other hydrocarbons, such as the dehydrogenation of paraflins to the corresponding oleflns, it may be advantageously modified by either filling the reactor 3 entirely with catalyst or, if desired, leaving the reactor entirely empty. Likewise, in this case the quenching operation serves no particular function and may be omitted, the entire fresh feed being introduced by way of line I. When, however, the apparatus is used for the conversion of olefins into diolefins, the quenching step is highly desirable. It will be understood that the use of a catalyst is not necessary in any of these dehydrogenation reactions. When the apparatus is used for the production of styrene from ethylbenzene it will be understood that the fractionation step must be suitably modified. Here the degradation products of the hydrogen acceptor boiling below styrene'are taken oi! as an overhead stream and the styrene taken of! as a side stream, leaving unconverted ethylbenrene in the bottoms together with the hydrogen acceptor where the latter is furfural or furfuryl alcohol.

The nature and objects of the present invention having been thus described and illustrated, what is claimed as new and useful and is desired to be secured by Letters Patent is:

1. A method for subjecting a hydrocarbon to a conversion involving dehydrogenation which comprises adding an organic heterocyclic compound containing a plurality of double bonds in its ring structure to the hydrocarbon to be converted and subjecting the hydrocarbon in admixture with the organic heterocyclic compound to conditions suitable for said conversion.

2. A method for converting a hydrocarbon into a less saturated hydrocarbon which comprises adding an organic heterocyclic compound containing a plurality of double bonds in its ring structure to the hydrocarbon to be converted and subjecting the hydrocarbon in admixture with the organic heterocyclic-compound to a temmolecular weight of the initial hydrocarbon material.

3. A process for converting a hydrocarbon into a less saturated hydrocarbon which comprises subjecting the hydrocarbon in admixture with an organic heterocyclic compound containing a plurality of double bonds in its ring structure to t a temperature sufiicient to split 0!! hydrogen from said hydrocarbon and favoring formation of free radicals from said organic heterocyclic compound, the formation of free radicals being promoted by adding to the organic heterocyclic compound an acid reacting substance.

4- A process in accordance with claim 3 in which the acid reacting substance is a mineral acid.

5. A process for dehydrogenating a hydrocarbon which comprises subjecting the hydrocarbon in admixture with a compound of the furan series to a temperature between 600 F. and 1400' F., said dehydrogenation being promoted by addition of a mineral acid to said compound of the furan series.

6. A process in accordance with claim 5 in which the mineral acid added to said compound of the furan series is in an amount ranging between 0.01% to 10% of the compound of the furan series in admixture withthe hydrocarbon.

7. A process for converting; hydrocarbon into a hydrocarbon of less saturated nature which comprises subjecting the hydrocarbon in admixture with an organic heterocyclic compound containing a plurality of double bonds in its ring structure to a temperature sufiicient to split oi! hydrogen from said hydrocarbon and favoring the formation of free radicals, said formation of free radicals being inhibited by adding to said organic heterocyclic compound a basic substance.

8. A process in accordance with claim '7 in which the basic substance is a nitrogen compound. 9. A process for dehydrogenating a hydrocarbon which comprises adding to the hydrocarbon an organic heterocyclic compound containing a plurality of double bonds in its ring structure and subjecting the mixture to the action of a dehydrogenation catalyst at a temperature between about 600" F. and 1400" 1".

10. A method according to claim 9 in which the heterocyclic compound is used in an amount ranging from 0.01% to 10% of the amount of hydrocarbon employed.

11. A method for converting cyclohexane into unsaturated compounds which comprises perature suflicient to split ofl hydrogen from a cyclohexane in admixture with an organic heterocyclic compound containing a plurality of double bonds in its ring structure into contact with a dehydrogenation catalyst at a temperature between about 1100 F'. and 1400 F. fora limited period of time, quenching the reaction products and recovering unsaturated hydrocarbons therefrom 12. A process according to claim 11 in which the reaction products are quenched with fresh cyclohexane feed and the latter is recovered from the quenched reaction products and fed to the catalyst zone.

HENRY H; BINGIIION.

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