US2320274A - Conversion of normally gaseous hydrocarbons - Google Patents

Description

Patented May 25, 1943 CONVERSION OF NORMALLY GASEOUS HYDROCARBON S Everett Gorin, Dallas, Tex., assignor to Socony- Vacuum Oil Company, Incorporated, New York, N. Y., a corporation of New York No Drawing. Application June 26, 1941,

Serial No. 399,943

11 Claims.

This invention relates to the pyrolysis of normally gaseous hydrocarbons for the production of aromatic hydrocarbons and may include the formation of light unsaturated aliphatic hydrocarbons, such as acetylene and ethylene. The invention is concerned particularly with the production of benzene from methane.

Theformation of benzene, acetylene and light olefins, such as ethylene, by the pyrolysisof 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 only becomes quantitatively measurable at temperatures above about.900 C. By increasing the temperature the yield will show a gradual rise up to 640% at 12001300 C., andthen it begins to fall off 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, in so far as I am aware, these attempts have been rather uniformly unsuccessful.

.It, therefore, is anobject of my invention to provide an improved process of converting normally gaseous hydrocarbons to light aromatic hydrocarbons. Another object is to afford a process ofconverting normally gaseous hydrocarbons to aromatic and unsaturated aliphatic hydrocarbons, which is an improvement over strictly thermal same results.

Another object is to afford an improvedprocess of converting normally gaseous paraffins to light aromatics which operates at temperatures below 1000 C. A more specific object is to provide a process of converting methane to benzene in substantial amounts at temperatures below 1000 C. These and other objects will be apparent from the description of my invention.

I have found now that the difficulties of prior art processesv may be eliminated by employingprocesses for accomplishing the sible by the direct pyrolysis of methane itself.

The first step in my process, namely, the halogenation of the parafiin hydrocarbon, may be carried out by methods well known to those versed in the art. It is not essential to the success of the operation, however, that the initial hydrocarbon gas be completely converted into the corresponding alkyl halide. Nevertheless, in all cases, the alkyl halide must be present in substantial amounts, and I prefer to operate with at least about 20-30% by volume of alkyl halides in the charge stock for the pyrolysis reaction. It also is to be noted that purification of the halogenated reaction product, such as, by the removal of the hydrogen chloride and higher halides, prior to charging such-product to the pyrolysis stage is not essential. These latter factors are of a practical significance to my invention, because many halo genation processes, as, for example, those for preparing methyl chloride are conducted usually so as to obtain about 20 to 30% by volume of alkyl halide in the reaction product.

The second step in my process, namely, the pyrolysis of the alkyl halides, may be conducted in the same manner known to the art for pyrolysis of, methane alone. However, much lower temperatures may be used in my process. Thus, temperatures as low as about 500 C. may be used, and in all cases the temperature is preferably below about 1000" C. It is to be definitely understood, nevertheless, that temperatures above 1000 C., say, up to 1200 C. or higher, may be used,

An important feature of my process from the practical standpoint is that exothermic reactions or substantially thermoneutral reactions are involved. In the prior art, the thermal pyrolysis of methane and the like to produce aromatics and unsaturated aliphatics, involves strongly endothermic reactions. Therefore, the problem of supplying the heat requirements to these prior art reactions at the high temperatures necessary to obtain substantial yields of the desired products presents a major difiiculty in conducting such operations.

On the other hand, for example, the equilibrium for the pyrolytic conversion of methyl chloride to benzene according to the equation:

(1) 6CH3Cl=CeH6+6HCl+3Hz is very favorable at all temperatures above about 500 C. and is very nearly thermoneutral; therefore, this reaction is very easy to control in large scale installations.

I have found further that appreciable quantities of methane also are formed so that superimposed on reaction (1), a reaction of the type of benzene according to (1) are even more favorable than is above indicated.

I also have found that appreciable quantities of acetylene also are formed in the pyrolysis of methyl chloride, and by a proper choice of' conditions the process may be directed; towards ob.- taining acetylene as the principal product. The reaction is rather strongly endothermic requiring about 50.0 kg. cal. per mol of acetylene, formed, But, inasmuch as the reaction which occurs simultaneously is exothermic to the'extent of 20 kg. cat, the overall heat require ments of the reaction are considerably less than is the case in the direct processing of methane to acetylene.

The normally gaseous hydrocarbons higher than methane may be treated in an analogous manner to the procedures outlined above with particular reference to treating methane. Thus, for example, in the processing of ethane, the hydrocarbon is first halogenated to either ethyl chloride or ethyl bromide which is then subjected to thermal treatment in a separate operation. Just as in the case of methyl chloride, the pyrolysis of ethyl chloride to form benzene is thermodynamically favorable at relatively low temperatures. The reaction is endothermic to the extent of 27 kg. cal. per mol of benzene. The reaction which occurs simultaneously to. a considerable extent is, however, exothermic to the extent of 1'7 kg. cal. per mol of hydrogen so that. the overall reaction is very nearlythermoneutral.

In the co-pending applications, Serial No. 400,296 and Serial No, 399,942, filed June 28, 1941 and June 26, 1941, respectively, it is disclosed that certain metal halides and free chlorine and free bromine are efiective reaction promoters for the pyrolysis of normally gaseous hydrocarbons. It is to be understood that reaction promoters of this typemay be used in my present process for the pyrolysis of alkyl; halides, Thus, the pyrolysis of the alkyl halide may be carried out in the presence of an easily reducible halide of a heavy metal orin the presence of chlorine or bromine or in the presence of a combination of these reaction promoters.

The hydrogen halide obtained in the product gases of my process may be used as the source of'halcgen for preparing the alkyl halides where by the process may be operated without appreciable loss of halogen. In case halogenation with a free halogen is preferred to the. use of a hydrogen halide as the halogenating agent, the 1786 halogen may be obtained in known manner from the hydrogen halide by-product. A convenient way, however, of preparing the alkyl halides with a halogen halide comprises the catalytic oxidation of a hydrocarbon-hydrogen halide mixture, for example, according to the following equation:

In order to illustrate the invention further, the following specific examples are given:

Example I Methyl chloride formed in the conventional manner from methane was pyrolyzed at 925 C. and atmospheric pressure for a contact time of 11 seconds. In this operation 9.2 percent of the methyl chloride was converted'to benzene, 9.3 to tar, and 6.9 to acetylene and ethylene. Approximately, 25 per cent of the methyl chloride remained undecomposed by this treatment.

Example III Methyl. bromide formed by conventional treatment of methanewas pyrolyzed at 925 C. and atmospheric pressure for a contact time of 27.8 seconds. By this operation 5.8 mol percent of the methyl bromide was converted to benzene, 2.5 to tar, and 3.5 to acetylene.

It is to be understood that in these reactions, higher yields of benzene may be obtained by further pyrolysis of the product gases after removing benzene contained therein. Furthermore, if desired, the pyrolysis may be effected in the pres-'- ence of a dehalogenating catalyst, such as activated alumina or silica. v

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, as known in the art, the relative ratios of these compounds in the reac: tion products can be varied by altering the operating conditions.

I". cl im! 1. The process of manufacturing benzene, ethylene and acetylene from normally gaseous parafiins which comp-rises halogenating the parafiins in a first stage to form an alkyl halide charge stock containing at least about 20% by volume of alkyl halides. and then in a second separate stage pyrolyzing said alkyl halide charge stock at a temperature above about 500 C.

2. The process of manufacturing benzene, thylene, and acetylene which comprises pyrolyzing. a charge stock consisting, essentially of light alkyl halides at a temperature above about 600 C.

3. The process of claim 1 wherein the pyrolyzing temperature is below 1000 C.

4. The process of claim 2 wherein the"pyrolyz ing temperature is below 1000 C.

5. The process of manufacturing benzene, ethylene and acetylene from methane which comprises halogenating the methane in a first stage to form a charge stock containing at least about 20%, by volume of methyl halides and then pyrolyzing said charge stock in a second separate stage at a temperature above about 600 C.

6. The process of manufacturing benzene which comprises pyrolyzing a charge stock consisting essentially of a methyl halide at a temperature above about 500 C.

7. The process of claim 5 wherein the pyrolyzing temperature is below 1000 C.

8. The process of claim 6 wherein the pyrolyzing temperature is below 1000 C.

9. The process which comprises halogenating a normally gaseous hydrocarbon to form an alkyl halide charge stock, and. then in a second, separate step pyrolyzing the alkyl halide charge stock at a temperature above about 500 C. to convert the alkyl halide in the charge stock to normally liquid, aromatic hydrocarbons.

10. The process which comprises halogenating methane to form a methyl halide charge stock, and then in a second, separate step pyrolyzing the methyl halide charge stock at a temperature above about 500 C. to convert the methyl halide in the charge stock to normally liquid aromatic hydrocarbons.

11. The process which comprises pyrolyzing a charge stock consisting essentially of light alkyl halides at a temperature above about 600 C. to produce normally liquid aromatic hydrocarbons, olefins and acetylene, condensing the normally liquid hydrocarbons, and recycling the uncondensed olefins, acetylene and unreacted alkyl halides to produce additional normally liquid aromatic hydrocarbons therefrom.

EVERE'I'I GORIN.