US3284526A

US3284526A - Process for thermal hydrodealkylation

Info

Publication number: US3284526A
Application number: US306598A
Authority: US
Inventors: James A Frayer; Robert F Mansfield; Rodney E Peterson; Eldon M Sutphin
Original assignee: Gulf Research and Development Co
Current assignee: Gulf Research and Development Co
Priority date: 1963-09-04
Filing date: 1963-09-04
Publication date: 1966-11-08
Anticipated expiration: 1983-11-08
Also published as: GB1048112A; DE1283212B; NL6410268A; DK112938B

Description

United States Patent 3,284,526 PROCESS FOR THERMAL HYDRODE- ALKYLATION James A. Frayer, Pittsburgh, Robert F. Mansfield, New Kensington, Rodney E. Peterson, Oakmont, and Eldon M. Sutphin, Pittsburgh, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware No Drawing. Filed Sept. 4, 1963, Ser. No. 306,598 9 Claims. (Cl. 260672) This invention relates to a process for the thermal hydrodealkylation of alkyl aromatics, particularly to a process for the thermal hydrodealkylation of alkyl aromatics without the formation of appreciable amounts of carbon or coke deposits.

Alkyl aromatics, such as toluene, can be dealkylated to lighter aromatics, such as benzene, by subjecting such alkyl aromatic in the presence of hydrogen to an elevated temperature and an elevated pressure for a controlled length of time. As a result of such reaction conditions the alkyl group is cleaved from the alkyl aromatic and combines with the hydrogen present to form a saturated aliphatic hydrocarbon. The desired aromatic can be separated from the saturated aliphatic hydrocarbon and unreacted alkyl aromatic, if present, in any convenient way.

In order to effect the desired thermal hydrodealkylation of the alkyl aromatic, it is imperative that the temperature of the mixture of alkyl aromatic and hydrogen during such reaction be maintained within a range of about 1150 to about 1800 F., preferably about 1250 to about 1350 F. for about one to about 400 seconds, preferably about 10 to about 100 seconds. The initial step in the process, therefore, involves heating the mixture of alkyl aromatic and hydrogen to the reaction temperature. Sufificient hydro-gen must be present to replace the alkyl chain cleaved from the aromatic ring at the elevated reaction temperatures and also to combine with the alkyl chain to form therewith a saturated aliphatic hydrocarbon. In the general molar ratio of hydrogen to alkyl, aromatic charge can be from about 1.5 to about 20, preferably from about three to about eight. Any alkyl aromatic can be so treated, for example, toluene, xylenes, tri methyl benzene isomers, alkyl nap'hthalenes and mixtures thereof, alkyl phenols, etc. There is a tendency, however, during the preheating step, particularly in start-up procedures, for the alkyl aromatic to be converted to the corresponding saturated compound. Toluene, for example, has a tendency during the initial heating period to be converted to methylcyclohexane. When these saturated materials are then introduced into the hydrodealkylation zone, wherein the elevated temperature defined above exists, they tend, in the presence of hydrogen, to crack, forming both carbon, or coke, and gases which are predominantly methane. Such conditions lead to lower yields of the desired dealkylated aromatic and require frequent shutdowns because of coke deposits in the thermal hydrodealkylation reaction zone.

We have found that the above difficulties can be avoided and the desired thermal hydrodealkylation of alkyl aromatics can be effected with little or no coke iorrnation or undesirable methane production by maintaining the mixture of alkyl aromatic and hydrogen during the preheating period at a temperature below about 85 0 to about 950 F., preferably below about 900 F., for a time no greater than about 50 seconds, preferably for about one to about 10 seconds. Contact or residence time herein is defined as the length of time a differential volume of material remains in the preheating zone or the thermal hydrodealkylation zone at the defined temperature. Under such conditions the charge alkyl aromatic is not saturated to any appreciable extent during the preheating period and the only substantial reaction occurring during hydrodealkylation period is the desired hydrodealkylation of the alkyl aromatic.

This procedure is particularly applicable during the start-up. For example, initially hydrogen can be passed through the system in a closed cycle, and suitable heat can be added thereto in the preheating zone to raise the temperature of the system to a defined temperature level, for example, about 1000 to about ll00 F. Cold alkyl aromatic is then introduced into the preheating zone with the hot circulating hydrogen. Since the addition of cold alkyl aromatic to the pre'heater will have a tendency to lower the temperature level thus obtained in the preheating zone, additional heat is added thereto in order to quickly raise the temperature therein again to the defined level. This procedure will avoid excessive residence time of the reactants at the undesired low temperature level and thereby eliminate the defined hydrogenation in the preheating zone and the cracking of the saturated ring compounds in the thermal hydrodealkylation zone to form undesirable gases and coke.

The mixture of hydrogen and alkyl aromatic when introduced into the preheating zone is at a temperature of about to about 800 F., preferably at a temperature of about 700 to about 800 F. When the mixture of alkyl aromatic and hydrogen is at a temperature above about 75 F. when it is introduced into the preheating zone, it can be heated to such temperature in any convenient manner, for example, by indirect heat exchange relationship with the heated products from the thermal hydrodealkylation reaction zone. In any event the time the reaction mixture is maintained above about 75 F., but below about 850 to about 950 F., preferably below about 900 F. during the defined preheating period, whether all or only a portion of the heat acquired by said mixture was obtained in the preheating zone, must be about one to about 10 seconds, but in no event more than about 50 seconds.

The preheating zone can be heated in any suitable manner, for example indirectly by gas-fired heaters. The mixture leaving the preheating zone is at a temperature of about 1150 to about 1250 F. Therefore the temperature therein can range from a low of about 70 F. to a high of about 125 0 F., but the average temperature in the preheating zone will be from about 1000 to about 1100 F., preferably about 1050 F. Under these conditions no more than about five percent, and generally less than about two percent, by Weight of the alkyl aromatic is dealkylated therein.

The heated mixture is then introduced into the hydrodealkylation zone, at which point the desired hydrodealkylation reaction takes place. Since this reaction is exothermic it is accompanied with release of heat. The temperature can be maintained therein at any temperature level in any suitable manner or the heat resulting from hydrodealkylation can be permitted in large measure to remain therein, resulting in an appreciable temperature rise of the products in the hydrodealkylation reaction zone. In any event the temperature in the hydrodealkylation reaction zone will remain Within a range of about 1150 to about 1800 F., preferably about 1250 to about 1350 F. The residence period Will be about one to about 400 seconds, preferably about 10 to about seconds.

The pressure drop throughout the system will not be appreciable and therefore the pressure will be substantially the same throughout. Thus the pressure can be from about 100 to about 1000 pounds per square inch gauge, preferably about 400 to about 600 pounds per square inch gauge.

Upon removal from the hydrodealkylation reaction zone, the product resulting from the reaction therein will comprise principally dealkylated alkyl aromatic, unreacted charge aromatic, methane and excess or unreacted hydrogen. In a matter of about one to about five seconds, for example, the reaction mixture is cooled by any convenient means, for example, by indirect heat exchange relationship with fresh alkyl aromatic and hydrogen charge, to a temperature below about 600 F. and after further cooling through heat exchangers to ambient temperature. Hydrogen and other gases are then vented from the reaction mixture and the remainder is separated into its component parts by any suitable means, preferably by distillation at a temperature of about 175 to about 250 F. and a pressure of about one to about 10 pounds per square inch gauge.

The invention can further be illustrated by the following. Four runs were made on a mixture of toluene and hydrogen wherein the temperature of the preheater and the hydrodealkylation reactor were varied and the residence times in the preheater were also varied. The results obtained are set forth below in the table. The mixture of toluene and hydrogen was in each run introduced into the preheater at a temperature of 75 F.

Table Run No 1 2 3 4 Operating Conditions:

Average PreheaterTemperature,

700 815 955 1, 175 Average Reactor Temperature,

600 1, 280 1, 277 1, 280 Unit Pressure, Pounds per Square Inch Gauge 460 460 460 460 Hydrogen to Toluene Mole Ratio 8. 8. 0 8. 0 4. 0 Contact Time in Preheater,

Seconds 166 151 136 6 Contact Time in Reactor, Seconds 113 68 68 50 Product Gas Composition: Mole Percent:

Hydrogen-- 41. 4 16. 3 51. 4 57. 1 Methane" 53. 2 75. 4 46. 7 41. 4 Ethane"- 1.7 7. 2 1.8 1 O to O 3.7 1.1 0.0 Yield Oi Total Liquid Product:

Percent By Volume 01 Charge... 68. 2 24. 5 51.1 83. 5 Inspection on Total Liquid Product:

Gravity, API 55.1 27.8 28.6 28.0 Analysis, Percent by Volume of urge:

l2. 8 23. 7 1. 7 20.0 10. 3 1. 8 0.8 0.2 0. 0 1. 0 0.0 0.5 0.0 0.2 saturates 54. 4 0.8 Methylcyclohexane 33. 2 0.5 Cyclohexane 5. 6 0.2 Methylcyclopentane. 2. 3 Trace Isopentane. 2. 2 Trace Other 0 -01 10. 7 0.1 Olefins 1.0 0.0 Aromatic Ring Balance:

Percent Aromatic Rings Recovered 11. 4 28. 6 00. 0 98.6 Percent Rings Lost Due to Saturation and/or Cracking 88. 6 71. 4 40. O 1. 4

A study of the data in the table illustrates the advantages of operating in accordance with the process defined and claimed herein. In run No. 1 the average preheater temperature was 700 F. and the reactor temperature was 600 F. In the second and third runs the reactor temperature was essentially the same, 1280" F. and 1277" F., respectively, but the preheater temperature in run No. 2 was 815 F. and in run No. 3 955 F. The contact time in the preheater in the first three runs was approximately the same, 166, 151 and 136 seconds, respectively.

In run No. 1 the yield of saturates was 54.4 percent by volume, of which 33.2 percent was methylcyclohexane. Very little hydrodealkylation occured in the hydrodealkylation reactor. Very little aromatics were recovered, and 88.6 percent were lost due to saturation and/or cracking. In run No. 2 saturation was again occurring in the preheater, but the saturated compounds so formed were at the elevated temperature existing in the hydrodealkylation reactor cracked to methane and ethane. Some benzene was produced. Hydrogen consumption was high, since saturation, cracking and hydrodealkylation were all occurring and consuming the same in run No. 2. The percent rings lost due to saturation and/ or cracking was still high. Merely raising the preheater temperature in run No. 3 but reducing only slightly the contact time in the preheater was sufiicient to improve the operation but not satisfactorily. The API gravity of the liquid product indicates that the product is essentially benzene. However, only 60 percent of the benzene rings were recovered and 40 percent of the rings were lost due to saturation and/or cracking. Operation in accordance with the process of the invention as exemplified by run No. 4 shows that most of the toluene went to benzene. Only 1.4 percent of the rings were lost due to saturation and/or cracking.

Obviously many modifications and variations of the invention, as hereinabove set forth, can be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. A process for the hydrodealkylation of an alkyl aromatic which comprises heating a mixture of said alkyl aromatic and hydrogen to a temperature of about 1150 to about 1250 F., maintaining the temperature of said mixture during said heating period at a temperature below about 950 F. for a time less than about 50 seconds, and thereafter subjecting the resultant heated mixture to a temperature of about 1150 to about 1800" F. and a pressure of at least about pounds per square inch gauge for about one to about 400 seconds to effect dealkylation of said alkyl aromatic.

2. A process for the hydrodealkylation of an alkyl aromatic which comprises heating a mixture of said alkyl aromatic and hydrogen to a temperature of about 1150 to about 1250 F., maintaining the temperature of said mixture during said heating period at a temperature below about 950 F. for about one to about 10 seconds, and thereafter subjecting the resultant heated mixture to a temperature of about 1150 to about 1800 F. and a pressure of at least about 100 pounds per square inch gauge for about one to about 400 seconds to effect dealkylation of said alkyl aromatic.

3. A process for the hydrodealkylation of toluene which comprises heating a mixture of said toluene and hydrogen to a temperature of about 1150 to about 1250 F., maintaining the temperature of said mixture during said heating period at a temperature below about 950 F. for a time less than about 50 seconds, and thereafter subjecting the resultant heated mixture to a temperature of about 1150 to about 1800 F. and a pressure of at least about 100 pounds per square inch gauge for about one to about 400 seconds to etfect dealkylation of said toluene.

4. A process for the hydrodealkylation of toluene which comprises heating a mixture of said toluene and hydrogen to a temperature of about 1150 to about 1250 F., maintaining the temperature of said mixture during said heating period at a temperature below about 950 F. for about one to about 10 seconds, and thereafter subjecting the resultant heated mixture to a temperature of about 1150 to about 1800 F. and a pressure of at least about 100 pounds per square inch gauge for about one to about 400 seconds to effect dealkylation of said toluene.

5. A process for the hydrodealkylation of an alkyl aromatic which comprises heating a mixture of said alkyl aromatic and hydrogen to a temperature of about 1150" to about 1250 F., maintaining the temperature of said mixture during said heating period at a temperature below about 850 F. for a time less than about 50 seconds, and thereafter subjecting the resultant heated mixture to a temperature of about 1250 F. to about 1350 F. and a pressure of at least about 100 pounds per square inch gauge for about 10 to about 100 seconds to effect dealkylation of said alkyl aromatic.

6. A process for the hydrodealkylation of an alkyl aromatic which comprises heating a mixture of said alkyl aromatic and hydrogen to a temperature of about 1150 to about 1250 F., maintaining the temperature of said mixture during said heating period at a temperature below about 850 F. for about one to about 10 seconds, and thereafter subjecting the resultant heated mixture to a temperature of about 1250 to about 1350 F. and a 10 pressure of at least about 100 pounds per square inch gauge for about 10 to about 100 seconds to effect dealkylation of said alkyl aromatic.

7. A process for the hydrodealkylation of toluene which comprises heating a mixture of said toluene and hydrogen to a temperature of about ll50 to about 1250 F., maintaining the temperature of said mixture during said heating period at a temperature below about 850 F. for a time less than about 50 seconds, and thereafter subjecting the resultant heated mixture to a temperature of about 1250 F. to about 1350 F. and a pressure of at least about 100 pounds per square inch gauge for about 10 to about 100 seconds to effect dealkylation of said toluene.

8. A process for the hydrodealkylation of toluene which comprises heating a mixture of said toluene and hydrogen 25 to a temperature of about 1150 to about 1250" F., maintaining the temperature of said mixture during said heating period at a temperature below about 850 F. for about one to about 10 seconds, and thereafter subjecting the resultant heated mixture to a temperature of about 1250 F. to about 1350 F. and a pressure of at least about 100 pounds per square inch gauge for about 10 to about 100 seconds to effect dealkylation of said toluene.

9. A process for the hydrodealkylation of toluene which comprises heating a mixture of toluene and hydrogen to a temperature of about 1175 F. in less than about six seconds and thereafter subjecting the resultant heated mixture to a temperature of about 1280 F. and a pressure of about 460 pounds per square inch gauge for about 50 seconds to effect dealkylation of said toluene.

References Cited by the Examiner UNITED STATES PATENTS 3,193,595 7/1965 Kenton et al 260672 FOREIGN PATENTS 790,595 2/ 1958 Great Britain.

OTHER REFERENCES Silsby et al.: Journal of Applied Chemistry, vol. 6, August 1956, pp. 347-356.

DELBERT E. GANTZ, Primary Examiner.

C. R. DAVIS, Assistant Examiner.

Claims

1. A PROCESS FOR THE HYDRODEALKYLATION OF AN ALKYL AROMATIC WHICH COMPRISES HEATING A MIXTURE ON SAID ALKUL AROMATIC AND HYDROGEN TO A TEMPERATURE OF ABOUT 1150* TO ABOUT 1250* F., MAINTAINING THE TEMPERATURE BELOW MIXTURE DURING SAID HEATING PERIOD AT A TEMPERATURE BELOW ABOUT 950* F. FOR A TIME LESS THAAN ABOUT 50 SECONDS, AND THEREAFTER SUBJECTING THE RESULTANT HEATED MIXTURE TO A TEMPERATURE OF ABOUT 1150* TO ABOUT 1800* F. AND A PRESSURE OF AT LEAST ABOUT 100 POUNDS PER SQUARE INCH GAUGE FOR ABOUT ONE TO ABOUT 400 SECONDS TO EFFECT DEALKYLATION OF SAID ALKYL AROMATIC.