Preparation of polyphenyls
United States Patent Brown et al.  
 [111 3,876,719 1 Apr. 8, 1975 PREPARATION OF POLYPHENYLS Inventors: Jerry L. Brown, Heflin; James T.  
 Bell, Anniston, both of Ala.  
 Assignee: Monsanto Company, St. Louis. Mo.  
 Filed: July 29, 1971 Appl. No.: 167,491  
 U.S. Cl. 260/670; 260/668 R Int. Cl. C07c 15/12 Field of Search 260/670, 668 R References Cited UNITED STATES PATENTS 1/1939 Conover et al. 260/670 3,009,970 11/1961 Odioso et al 260/670 Primary E.\&#39;aminerCurtis R. Davis Attorney, Agent, or FirmN. E. Willis; J. E. Maurer; F. D. Shearin [57] ABSTRACT 12 Claims, No Drawings 1 PREPARATION OF POLYPHENYLS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an improved&#39;p&#39;rocess for the pyrolysis of benzene and other aromatics to prepare biphenyl and other polyphenyls. and to promoters which effectively increase the degree of conversion.  
 2. Description of Prior Art Pyrolytic processes for converting benzene and other aromatic hydrocarbons into polyphenyls are well known. These processes normally involve vaporizing the aromatic. heating the vapors to pyrolyzing temperatures and subsequently separating the product from unconverted feed material. The reaction is usually executed in industrial installations by passing the hydrocarbon vapors through tubes. conduits. or passageways which are externally heated to pyrolytic temperatures.  
 , The apparatus employed in such operations generally includes a hydrocarbon vaporizer together with one or more pyrolyzing tubes or passageways extending from the vaporizer to a suitable condenser or other conventional means in which the product is separated from the more volatile starting material. Any suitable heat source can be used to provide the requisite temperature for the vaporizer and the pyrolysis tubes.  
  In the normal operation of the pyrolytic process, from about 10 to 20 percent of the aromatic starting material is dehydrogenated and condensed to form polyphenyls in each pass through the converter. This low conversion rate has been a subject of concern and various efforts have been employed to increase conversion and productivity of the converter units. One of the most successful means to increase conversion was by the addition of certain promoters to the aromatic feed stream as described in US. Pat. No. 2.l43,509. This reference discloses that certain partially oxidized or reacted aromatic or aliphatic hydrocarbons. and particularly those containing one or more oxygen atoms. exerted a favorable influence on conversion of benzene or alkylated benzene to polyphenyls when the promoter was premixed with the feed prior to vaporization. Specifically mentioned promoters included alcohols. aldehydes, ketones. aliphatic acids. acid esters and ethers. Particularly preferred promoters included lower aliphatic alcohols and acetone premixed at concentrations of from about 0.1 to 2.0 percent by weight of the aromatic feed.  
  Although significant improvements in conversion have been realized by the use of such promoters, economic advantages remain to be realized by yet further improvements in the conversion rate. It is accordingly an object of the present invention to provide a novel class of reaction promoters to increase the pyrolytic conversion of aromatics to polyphenyls. It is a further object of this invention to provide a method for increasing the degree of conversion by introducing one or more promoters directly into the pyrolysis zone. It is yet a further object of this invention to provide a class of reaction promoters which are suitable for injection into the pyrolysis zone and which may be used in conjunction with promoters of the prior art which are premixed with the aromatic feed, thereby to achieve further increases in the conversion rate.  
 SUMMARY The conversion of aromatics to polyphenyls by pyrogenie dehydrogenation is increased by introducing into the pyrolysis zone at one or more points a reaction promoter selected from the group consisting of oxygenfree lower alkyl-substituted aromatic and aliphatic hydrocarbons and mixtures thereof. The promoters are preferably introduced at each point at a rate of from about 0.05 to 0.5 percent by weight of the aromatic hydrocarbon feed. Particularly preferred promoters include C2 to C alkyl-substituted aromatics. e.g. isopropyl benzene. and C to C alkyl-substituted aliphatics. e.g. trimethyl pentane.  
  Pyrolytic conversions of aromatic hydrocarbons are normally conducted in the vapor state under a gauge pressure of between about 10 and 200 pounds per square inch and at between about 600C. and 950C. The rate of conversion is generally uneconomically slow at temperatures less than about 600C. whereas excessive degradation occurs at temperatures above about 950C. In accordance with the conventional process. an aromatic hydrocarbon such as benzene is vaporized and the vapors heated to a temperature of about 600C. by any conventional means. The heated vapors are then passed into a pyrolysis zone maintained between about 600C. and 950C., and more preferably between 700and 800C. The vapors exiting the pyrolysis zone are condensed and the polyphenyls separated from the unconverted hydrocarbon feed.  
  Preferred apparatus employed in accordance with conventional procedures includes a hydrocarbon vaporizer. a pyrolyzing element or tube and a product collecting means such as a condenser. These units are provided with suitable connecting means to permit fluid flow from the vaporizer through the pyrolyzing element into the condenser. Any conventional means may be employed to obtain proper operating temperatures throughout&#39;the apparatus. One or more pyrolytic tubes can be incorporated in the apparatus. the size and number of tubes depend upon the relative size of the vaporizer and upon the rate of production desired. The pyrolytic conduits are preferably tubular and straight but can assume any convenient cross-sectional configuration and can also follow curved. tortuous or helical paths provided that the flow of the vapor stream is not impeded. The flow rate through the tubes is preferably at a sufficiently high rate to provide turbulent flow. A recycle circuit is normally provided to conduct the unconverted hydrocarbon feed from the condenser to the vaporizer.  
  An alternative process which may be employed is the molten bath process wherein vapors of the aromatic hydrocarbon are passed directly through a heat transfer medium comprising a suitable relatively inert and nonvolatile molten material. such as molten lead or lead alloys or molten salt. In this process the promoters of the instant invention are introduced into the vaporized aromatic feed immediately preceding introduction of that feed into the molten bath. In general. a tubular pyrogenic converter is preferred to the molten bath because it allows greater flexibility of operation and provides for multiple injections of promoter at various stages of the pyrolysis zone. Accordingly, the following detailed description of the preferred embodiments of the instant invention are directed to the use of a tubular converter.  
  The term polyphenyf is used herein in its broadest sense to designate polymerized benzene and substituted benzenes. Thus, it encompasses biphenyl, ter- 3 phenyl, quaterphenyls. alkyl-substituted polyphenyls, naphthylenes. phenyl-substituted naphthylenes and the like.  
  The expression pyrolysis&#34; is used herein to designate high temperature reactions wherein promoters activate or accelerate the process and not in the more restrictive sense of the term sometimes found in texts which excludes the presence of catalytic bodies.  
  In carrying out the present invention. the rate or degree of conversion of the aromatic hydrocarbon to the polyphenyl is increased by introducing one or more reaction promoters into the pyrolysis zone at one or more points within the zone. The promoter is preferably injected into the pyrolysis zone at points spaced to provide substantially equal incremental reaction zones between points of injection and between the final point of injection and the exit of the converter. In one embodiment for example. in a converter having a 6 seconds total dwell time, promoters are advantageously injected at points located at three-sixths, four-sixths. and fivesixths of the distance from the entrance to the exit of the converter corresponding to incremental reaction zones of l second dwell time and to total dwell times within the converter of 3, 2, and 1 second respectively. Incremental dwell times are preferably from about 0.5 to 1.5 seconds, but may range from about 0.1 to 3 seconds or longer depending upon the size of the converter. the total dwell time, and the like.  
  The amount of promoter injected at each point of introduction into the pyrolysis zone is preferably from about 0.05 to 0.5 percent by weight of the aromatic feed, and more preferably from 0.1 to 0.2 percent, the exact and optimum amount depending upon such conditions as operating temperature and pressure, operating rate and dwell time within the pyrolysis zone, the incremental dwell times between points of promoter introduction. and the composition of the aromatic hydrocarbon feed. Amounts of promoter in excess of 0.5 percent can. of course. be introduced at any point, but there is generally little or no process advantage to be gained thereby. When injecting the promoter at multiple points it is generally satisfactory to inject equal amounts at each point although variations in rate of injection are permissible if desired or required to optimize the effect of the promoter.  
  The reaction promoters of the present invention are broadly defined as oxygen-free C to C alkylsubstituted aromatic hydrocarbons such as ethyl benzene, propyl benzene, butyl benzene, methylethyl benzene, dimethylethyl benzene, and ethyl naphthylene; and C, to C alkyl-substituted aliphatic hydrocarbons such as trimethyl pentane. trimethyl hexane, dimethyl octane, methylethyl pentane, methylethyl hexane, and dimethylethyl hexane. Particularly preferred promoters are isopropyl benzene and trimethyl pentane.  
  The aromatic hydrocarbon feeds suitable for use in accordance with this invention include benzene and the lower alkyl-substituted benzenes, such as toluene and xylene, and also naphthylene and lower alkylsubstituted naphthylenes. Also the simpler nuclearsubstituted derivatives of benzene, toluene, xylene and naphthylene such as monoand dichloro and bromosubstituted compounds may be employed in conducting the reaction. The feed may also include minor amounts, and generally less than about 25 percent by weight, of biphenyl or terphenyl where the preparation of higher polyphenyls is desired.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be more fully understood by reference to the following detailed description and examples. In these examples and throughout the specification all parts and percentages are by weight unless otherwise specified.  
  The following examples illustrate the method of the present invention by describing the pyrolysis of benzene to produce simple polyphenyls, primarily diphenyl, terphenyl and quaterphenyl. In these examples benzene was vaporized and heated to a temperature of about 500C. at a rate of about 26 grams per minute, and the vapors passed into a pyrolytic converter maintained at 750C. and comprised of a single coiled /8 inch tube having a length of 50 feet. The retention time of the benzene in the pyrolytic converter section was approximately 6 seconds. Individually controlled promoter injection ports were installed at one-half, twothirds, and five-sixths of the distance from the entrance to the exit of the converter tube. Selected promoters were metered into the pyrolysis zone at one, two or all three points as desired and at predetermined rates. The material exiting the pyrolysis zone was collected to separate and recover the polyphenyl products, and unreacted benzene was recycled to the vaporizer. The effect of various promoters and promoter injection rates was determined by ascertaining the degree of conversion of benzene to polyphenyl with and without promoter injection.  
  The details of the various tests and the test results are presented in the following examples:  
 EXAMPLE I The effect on the conversion of benzene to polyphenyls of injecting 0.1 and 0.2 percent of isopropyl benzene into the pyrolysis zone at each of one, two and three points was determined as follows:  
  Total 7( &#39;71 Promoter Injected Promoter Conversion Increase None 0 15.8 0.17: at Point 1 0.171 17.4 10.2 +01% at Point 2 0.2 18.2 15.2 401% at Point 3 0.3 18.7 18.4 0.27: at Point l 0.27: 18.6 17.7 +0.2&#39;7 at Point 2 0.4 20.5 29.8  
 Point 1 5/6 distance from entrance to exit Point 2 36 distance from entrance to exit Point 3 /2 distance from entrance to exit The above data illustrate that an increase in conversion of approximately 30 percent is easily obtained by the method of this invention.  
 EXAMPLE II Total 0. 171 Acetone Feed 02% Acetone Feed Promoter Injected Promoter 7: Conv. &#39;71 Inc. 71 Conv. 7: Inc.  
 None 0 18.2 20.0 0.171 at 0.1 19.4 6.6 20.4 2.0 Point 1 +0.1 at Point 2 0 2 20.4 12.1 21.5 7.5 +0.17: at Point 3 0.3 21.5 18.1 22.2 11.0 0.2% at Point 1 0.2 20.2 11.0 21.3 6.5 +0.27: at Point 2 0.4 21.6 18.7 22.1 10.5 +0.2% at Point 3 0 6 22.6 24.2 23.0 15.0  
 EXAMPLE Ill Total Promoter Premixed Conversion /r Increase None 15.8 0.171 15.8 0 (1.271 16.0 1.3  
  The remarkable effect of the promoters of this invention of increasing the conversion of aromatics to polyphenyls is especially surprising in view of the prior art Total 0% Acetone Feed 0.171 Acetone Feed Promoter Injected Promoter &#39;7: Conv. Inc. 7: Conv. &#39;7: Inc.  
 None 0 14.6 16.7 0.1% at Point 1 0.1 15.6 6.9 17.3 3.6 +0.17! at Point 2 0.2 17.9 22.6 18.8 12.6 +0.17: at Point 3 0.3 19.0 30.2 19.8 18.5 0.2% at Point 1 0.2 17.2 17.8 17.9 7.2 +0.27: at Point 2 0.4 18.5 26.8 19.5 16.8 +0.2% at Point 3 0.6 20.4 39.8 20.9 25.2  
 EXAMPLE IV which teaches for example in U.S. Pat. No. 2.143.509  
  The method of Example I was repeated by injecting various promoters into the entrance of the pyrolysis zone only. The following data show the effect of these promoters on conversion and illustrate that trimethyl pentane is substantially equivalent to isopropyl benzene while ethyl benzene, although somewhat less active than the others, is still an effective promoter.  
 Total Promoter Injected 71 Conversion &#39;7: Increase None 16.1  
 0.47t Ethyl Benzene 17.1 6.2  
 0.2% Trimethyl Pentane 18.5 14.9  
 0.27: Isopropyl Benzene 18.7 16.2  
 EXAMPLE V ample IV.  
 that the addition of hydrocarbons. notably aliphatic and heterocyclic hydrocarbons. has a repressing or inhibiting effect on the formation of polyphenyls. Contrary to this teaching in the art we have found that premixing the promoters of the present invention has little beneficial effect on conversion but that the injection of aliphatic and aromatic hydrocarbons into the pyrolysis zone effects a significant improvement in conversion.  
  Although the invention has been described in considerable detail in the foregoing for purpose of illustration it is to be understood that such detail is solely for that purpose and many variations can be made without departing from the spirit and scope of the invention. Specifically. aromatic hydrocarbons other than benzene and promoters of the class defined other than those specifically enumerated above may be employed in the practice of this invention as hereinbefore described.  
 Also, mixtures of promoters and mixtures of aromatic hydrocarbon feeds may be employed where desired. Variations in pyrolysis conditions including rates. temperatures and pressures are contemplated and will be apparent to those skilled in the art. Thus. the scope of the invention is not to be limited by the details of the preceding examples.  
  The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:  
  l. A method of converting benzene to polyphenyls which comprises passing benzene through a pyrolysis zone maintained at a temperature of from about 700C to about 800C, and introducing into said pyrolysis zone at one or more points a compound selected from the group consisting of isopropylbenzene, trimethylpentane and ethylbenzene. wherein the amount of compound introduced at each point is from about 0.05% to about 0.5% by weight of the hydrocarbon feed.  
  2. A method of claim 1 wherein the compound is isopropylbenzene.  
  3. A method of claim 1 wherein the compound is trimethylpentane.  
  4. A method of claim 1 wherein the compound is ethylbenzene.  
  5. A method of claim 2 wherein a catalytic amount of a volatile aliphatic hydrocarbon containing at least one oxygen atom is premixed with the benzene prior to the pyrolysis zone.  
  6. A method of claim 5 wherein from 0. l 7( to 2.0 percent acetone by weight of benzene is premixed with the benzene prior to the pyrolysis zone.  
  7. In a method of converting aromatic hydrocarbons to polyphenyls which comprises passing an aromatic hydrocarbon through a pyrolysis zone maintained at pyrolyzing temperatures, the improvement comprising introducing into said pyrolysis zone at one or more points a catalytic amount of a compound selected from the group consisting of oxygen-free C to C alkylsubstituted aliphatic hydrocarbons, C to C 4 alkylsubstituted aromatic hydrocarbons, and mixtures thereof.  
  8. In a method of claim 7 wherein the pyrolysis zone is maintained at a temperature of from about 600C to about 950C.  
  9. In a method of claim 8 wherein the amount of compound introduced at each point is from about 0.05 to 0.5 percent by weight of the aromatic hydrocarbon feed.  
  10. In a method of claim 9 wherein the pyrolysis zone is maintained at a temperature of from about 700C to about 800C.  
  11. In a method of claim 7 wherein a catalytic amount of a volatile aliphatic compound containing at least one oxygen atom is premixed with the aromatic hydrocarbon feed prior to the pyrolysis zone.  
  12. In a method of claim 7 wherein the incrimental reaction zones existing between the points of introduction and between the final point of introduction and the exit of the pyrolysis zone are substantially equal and include incrimental reaction dwell times of from about 0.1 to about 3.0 seconds.  
  UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,876,719 DATED April 8,1975 |NVENTOR(S) I Jerry L. Brown et al.  
  it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 Column 2, line 15, after &#34;at&#34; insert temperatures Column 5, 1st Table, the second line below the column headings reads: 0.1% at 0.1 19.4 6.6 20.4 2.0  
 Point 1 should be 0.1% at Point 1 0.1 19.4 6.6 20.4 2.0  
 gignfid and Sealed this [SEAL] ywinth D3) of July 1975 Arrest.-  
 RUTH C. MASON Arresting Officer