US2396697A - Conversion of normally gaseous hydrocarbons - Google Patents

Description

Federated Mar E9, 1946 one!) stares 'TNT PHE- CONVERSION OF NORMALLYZ GASEOUS HYDROCARBONS Everett Gorin, Ballast, Ten,

assignor to Socony- Vacuum @il Company, Incorporated, a corporation of New York No llirawing. Application June Serial No. 400,296

, 1e Claims. This invention relates to the pyrolysis of normally gaseous hydrocarbons for the production duction of benzene from methane.

The formation of benzene, acetylene and light olefins, such as ethylene, by the pyrolysis of normally gaseous hydrocarbons, such as methane, ethane and propane, is known. However, for example, in the strictly thermal conversion of methane to benzene, the benzene onl becomes quantitatively measurable at temperatures above about 00 C. By increasing the temperature the yield will show a gradual rise up to 6-10% at 1200-2300 0., and then it begins to fall of as the reaction temperature is raised still higher.

Naturally, numerous attempts have been made to discover catalysts or other means for aiding the pyrolysis of methane to benzene in order that lower reaction temperatures may be used and/or the yield of benzene increased. Nevertheless, insofar as I am aware, these attempts have been rather uniformly unsuccessful.

It, therefore, is an object of my invention to provide an improved process of converting normally gaseous hydrocarbons to light aromatic hydrocarbons. Another object is to afford a process of converting normally gaseous hydrocarbons to aromatic and unsaturated aliphatic hydrocarbons, which is an improvement over strictly thermal processes for accomplishing the same results, due to my use of certain reaction promoters. These and other objects will be apparent from the following description of my invention.

I now have found that by carrying out the pyrolysis of normally gaseous hydrocarbons, to produce aromatic and light unsaturated aliphatic hydrocarbons, in the presence of a certain class of metal halides under pyrolyzing conditions that, compared to a strictly thermal process, substantially lower temperatures maybe used to obtain at least as high yields or substantially higher yields may be obtained at the same temperature.

The metal halides used under the conditions of my process are not catalysts because they enter into the reaction and are chemically changed, viz., reduced, by the reaction. Thus, for example, in the conversion of methane to benzene using stannic chloride as the promoter and assuming, for sake of simplicity, that benzene is the 1 only hydrocarbon formed, the equation is as follows:

SCI-1'4 9511014: CsHs +9S nC12 18HC1 out feasibly at high temperatures some conven-' lent way of removing hydrogen must be found. Furthermore, I discovered that not'only must hydrogen acceptors be provided but also inductors for removing hydrogen from the gaseous hydrocarbons should be present. It appears that the metal halides or my invention perform these functions under the pyrolyzing conditions of the process whereby a substantial improvement over a strictly thermal process is obtained. Accordingly, I have designated these halides as reaction promoters.

The reaction promoters which I have discovered are the easil reducible, heavy metal halides,

halides of mercury, lead, and tin. During the pyrolyzing reaction the metal halides are re= duced to a lower halide or to the metal itself. Therefore, metal halides should not be used, which, either themselves .or the products they form when reduced in the presence of hydrogen, strongly promote cracking of the hydrocarbons to carbon and hydrogen under the pyrolyzing conditions. This, in general, means that metal halides which are solids under the conditions of the process, and metal halides which form products of reduction that are solids under the conditions or the process, should not be used. It is to be noted, however, that a compound such as ferricchloride, which upon completereduction will form metallic iron which is a strong cracking catalyst for methane, may be used if the process is regulated so that the ferric chloride is reduced only to ferrous chloride. On the other hand, cuprous chloride, although not particularly desirable, might be used even though it is reduced to metallic copper (solid below 1083? 0.) because metallic copper is only a mild cracking catalyst, and any handling dlfilculties may be largely ch- 40 viated, for example, by working with a large excess of the molten salt so that metallic copper is retained in colloidal suspension. In the broader aspects of the invention, it might be said that my reaction,promoters are metal halides which are reduced under the operating conditions of my process to lower metal halides or to the metals themselves, and which neither by themselves nor through their reduction products substantially promote cracking of the hydrocarbons being treated. "It appears that any such metal halide will be an effective reaction promoter in my process. However, as stated above, the presently preferred reaction promoters are the chlorides of mercury, tin and lead. These compounds and the products they yield, when reduced with hydrogen, are all liquids or vapors above 501 0. Furthermore, they show no tendenc to form carbides and are noncatalytic toward cracking of the hydrocarbons. In the case of some metal bromides the amount and ofv these halides I particularly prefer the rine or bromine also are reaction promoters for the pyrolysis of normally gaseous hydrocarbons. This fact affords the opportunity of using a metal halide of the above-mentioned type in combination with chlorine or bromine for promoting the pyrolysis reaction whereby the combined eflect of these two'reagents is obtained. It is to be understood my invention includes the use of metal halides in such combinations.

The actual operating procedure for carrying out my pyrolysis of methane in the presence of metallic halides may be quite similar to the technique used heretofore for the strictly thermal pyrolysis of methane. The only necessary difference is that instead of the hydrocarbons being subjected to the high temperatures only, they are subjeotedto these high temperatures while in the presence of my metal halide reaction promoters. Thus, for

example, when employing a stannic chloride promoter, the methane, after removal of sulfur, if

necessary or desired, may be bubbled through a stannic chloride saturator, which is suitablv heated, and then the resulting reaction mixture of hydrocarbon and stannic chloride vapors may be passed through a conventional thermal pyrolysis converter, such as a quartz, Monel metal, or porcelain reactor. The temperature within the converter, however, may be substantially less for my process, as set forth hereinafter, than for straight thermal pyrolysis of methane. Further, it is to be understood that the total yield of benzene may be increased by further pyrolysis of product gases from which benzene has been removed.

The temperature of operation should be above about 500 C., in fact, when converting methane I recommend a temperature above about 600 C. In my preferred operation I employ a temperature between about 600 and about 1200" C. However, if desired, higher temperatures may be used, particularly when interested in acetylene produc tion. Thus, for instance,'at temperatures up to 1200 C. the yields obtained by my process are in every instance substantially superior to those obtained at corresponding temperatures in the absence of a promoter. Therefore, from a practical standpoint, it is important to note this increase in yield is obtained and that my process also makes possible, for the first time, the production of substantial amounts of benzene from methane at temperatures below 1000 C. v

I have found that when, for example, a mixture of methane and a fluid metal halide is pyrolyzed the metal halide is reduced and the'methane is simultaneously converted to benzene, naphthalene, anthracene and other condensed aromatic hydrocarbons, acetylene and ethylene, while some carbon is formed in addition. The methane conversion may be regarded as the superposition of two reactions. First of all, the methane reduces the halide and is simultaneously converted to.

benzene and the other products listed-above.

I Then, in general, the occurrence of the first reac-' tion induces an additional decomposition of the methane wherein hydrogen is split out and any or all of the above products, such as benzene, etc., are

formed.

In the first reaction, the metal halide acts as a reagent for the conversion of methane. This re- 5 hydrocarbons.

assess? action usually proceeds at a very much higher rate than the direct thermal conversion of methane.

Thus, in my experiments I have found that I am able in my process to obtain a substantial conversion of the methane which results in a complete reduction of the metal halide by the methane under reaction conditions where methane alone sufiered practically no decomposition. The amount of the induced decomposition relative to the amount of this direct reaction of methane with the metal halide is greater the higher the temperature and the higher the ratio of methane to metal halide used.

This same relationship between induced and non-induced reactions holds also for similar reactions wherein liquid aromatic hydrocarbons, acetylene and olefins aresimultaneously produced from other normally gaseous hydrocarbons.

Since the induced decomposition of methane always involves the formation of hydrogen while the non-induced r. action does not, a quantitative measure of the ratio of the induced to the noninduced reactions may be obtained by taking the ratio of the number of mols of free hydrogen produced to the number of mols of hydrogen accepted by the metal halide.

Thus. for example, I found that when a methane-mercuric chloride mixture containing 15 mol percent of mercuric chloride was passed through a pyrolysis tube, the outer periphery of which was maintained at 925 C., at a rate suflicient to cause complete reduction of the halide to hydrogen chloride that the ratio of the induced to the non-induced reaction was 1:3. When lower concentrations of the halide were used the ratio increased. Accordingly, I prefer to use a low con-.

centration of metallic halide promoter, say, be-

tween about 0.1 and about 20 mol per cent, and, particularly those ranges below about 10 mol per cent. However, it is to be understood that amounts of promoter above 20 mol per cent may be out by these data.

Table 'Mol percent i {52 Pezroienit; ggs? T 1) Total E n hto il ca a ys pres. main y liter/hr. secs. N benzene) Atmospheres 15.0 None 890 Trace 13.5 3%SnCl4" 69 876. l 5.5 r [60.0 21%SnCh- 25 925 1 7.5

21. 4 PbCh l 63 925 1 3. 1'

11.8 PbCh 76 923 1 4.6

10.0 l 20% HgCh 113 775 1 6.9

1 H161, was used in the form of a liquid bath containing :11: egcgfi! of PbCl and the methane was bubbled through When liquid halides which are non-volatile under the operating conditions are used, it is often desirable to employ a large excess of the halide in the form of a liquid bath as was done with PbCl above. In this way the excess halide not reduced, as well as the'liquid reduction product, may serve as a direct heat exchange medium for the In the present process, commercial feasibility is ing.or regenerating the reaction promoter, just as catalyst regeneration is essential in the usual catalytic process. This regeneration may be conducted in any suitable manner. For instance, no difliculty is experienced in the regeneration of the metal halides of my process whose reaction with hydrogen is reversible. In these cases, the halide can be regenerated, for example, by bubbling the off gas, containing HCl but freed of light and heavy oil, into the molten metal either under pressure or at a lower temperature than is used in the reaction on methane. It is interesting also to note that this reaction is exothermic, and that the pyrolysis reactions of converting methane to benzene are endothermic. Therefore, it is desirable to carry out these two operations within heat transfer relationship of each other. Halides, such as mercuric chloride, whose reduction with methane is not reversible and which do not form oxychlorides, may beregenerated, for example, by the exothermic reaction of burning the off gas, containing HCl but freed of light and heavy oil, in admixture with the metal in question.

A third regeneration method generally applicable to all my metal halide reaction promoters involves separation of the halogen acid from the pyrolysis off gases followed by catalytic oxidation of the halogen acid to the free halogen. Any of the known methods may be used to effect this catalytic oxidation. The free halogen obtained may then be contacted with the reduced form of the metal halide whereby the original metal halide is produced.

The invention has been described with particular reference to the conversion of methane to benzene for the sake of simplicity. However, it is to be understood the invention is applicable to the treatment of normally gaseous hydrocarbons generally. Moreover, it should be understood clearly that my process produces, just as does a strictly thermal process of this type, unsaturated aliphatic hydrocarbons, particularly ethylene and acetylene, as well as aromatic hydrocarbons, and that the relative ratios of these compounds in the reaction products can be varied by altering the operating conditions analogously to the manner known in the art for accomplishing this result in strictly thermal processes.

I claim:

l. The process for manufacturing light aromatic hydrocarbons and normally gaseous unsaturated hydrocarbons from normally gaseous paramnic hydrocarbons which comprises admixing with the normally gaseous paramn from 0.1 to 20.0 mol percent of a fluid, easily reducible heavy metal halide, maintaining the temperature halide being selected so that under the conditions of the process its reduction product will be a fluid and so that neither it nor its reduction product will promote substantial cracking of the hydrocarbons.

2. A continuous process for the manufacture of light aromatic hydrocarbons and normally gaseous unsaturated hydrocarbons which comprises passing a normally gaseous paraflinic hydrocarbon in admixture with from 0.1 to 20 mol percent of an easily reducible metal halide gas at a temperature between 500 C. and about 1200 C. through a pyrolysis zone at a rate such that the contact time is at least sufficient to obtain substantially complete reduction of the highest valence state of the metallic halide, cooling the product gases, and recovering the aromatic hydrocarbons and unsaturated hydrocarbons from unconverted paraflins, hydrogen halide and metallic halide reduction product.

3. A continuous process for the manufacture of light aromatic hydrocarbons and normally gaseous unsaturated hydrocarbons which comprises passing methane in admixture with from 0.1 to 20 mol percent of an easily reducible metal halide gas at a temperature between 600 C. and about 1200 C. through a pyrolysis zone at a rate such that the contact time is at least suflicient to obtain substantially complete reduction of the highest valence state of the metallic halide, cooling the product gases, and recovering the aromatic hydrocarbons and unsaturated hydrocarbons from unconverted methane, hydrogen halide and metallic halide reduction product.

4. The process for manufacturing light aromatic hydrocarbons and normally gaseous unsaturated hydrocarbons from methan which comprises admixing with the methane from 0.1 to 20.0 mol per cent of fluid, easily reducible heavy metal halide, maintaining the temperature of the mixture between 600 C. and about 1200 C. until reduction of said halide occurs, said halide being selected so that under the conditions of the process its reduction product will be a fluid and so that neither it nor its reduction product will promote substantial cracking of the hydrocarbons.

5. The process of claim 1 in which the metallic halide is a halide of mercury.

6. The process of claim 1 in which the metallic halide is a halide of tin.

7. The process of claim 1 in which the metallic halide is a halide of lead.

8. The process of claim 2 in which the metal halide gas is a gaseous halide of mercury.

9. The process of claim 2 in which the metallic halide gas is a gaseous halide of tin.

10. The process of claim 2 in which the metal halide gas is a gaseous halide of lead.

EVERETT GORIN.