Feed Hydrotreating for Improved Thermal Cracking

This invention relates to thermal cracking (pyrolysis) and more particularly to a process for improving the yields of light olefins and BTX (Benzene-Toluene-Xylene) from a thermal cracking feedstock.

In accordance with the present invention, a thermal cracking feedstock is hydrogenated in the presence of a bimetallic catalyst which includes added chlorine and/or fluorine followed by thermal cracking (non-catalytic) thereof. It has been found that pre-hydrogenation in the presence of such a promoted catalyst increases yields of light olefins and BTX, as compared to pre-hydrogenation in the presence of an unpromoted bimetallic catalyst.

The catalyst employed in the present invention is a bimetallic catalyst including at least one metal from Group VI and at least one metal from Group VIII of the Periodic Table. The Group VI metal is generally tungsten and/or molybdenum and the Group VIII metal is generally nickel and/or cobalt. The active form of the catalyst is the sulfided form and such sulfiding can be effected prior to use of the catalyst or, in situ, in that the feeds which are treated generally include sulfur. As representative examples of such catalysts, there may be mentioned tungsten-nickel sulfide, cobalt-molybdenum sulfide, nickel-molybdenum sulfide, etc. It is to be understood that the term bimetallic is employed in a generic sense to indicate at least two metals and, accordingly, encompasses catalysts which employ more than two metals.

The bimetallic catalyst may be supported on a suitable support, as known in the art. As representative examples of such supports, there may be mentioned: silica-alumina, various activated aluminas, silica-alumina containing zeolites, activated aluminas containing zirconium dioxide, titanium dioxide, etc; and the like.

The chlorine and/or fluorine may be added to the bimetallic catalyst to effect promotion thereof by contacting the catalyst with fluorine and/or chlorine, hydrogen chloride or a chloro-and/or fluoro-substituted organic compound. The chloro-and/or fluoro-substituted organic compound is preferably a chloro-and/or fluoro-substituted hydrocarbon. The chloro-and/or fluoro-substituted hydrocarbon may be an aliphatic, cycloaliphatic or aromatic hydrocarbon, and is preferably a saturated aliphatic hydrocarbon. As representative examples of such halo-substituted compounds, there may be mentioned: carbon tetrachloride, chloroform, dichloroethane, dichloro-difluoromethane, monochlorotrifluoromethane, chlorocyclopropane, tetrafluoromethane, etc. The selection of a particular compound is deemed to be within the scope of those skilled in the art from the teachings herein.

The bimetallic catalyst may be pretreated to effect the addition of chlorine and/or fluorine thereto; however, the preferred procedure is to treat the bimetallic catalyst, in situ, by adding a chloro-and/or fluoro-substituted hydrocarbon to the hydrocarbon feedstock. Thus, for example, a chloro and/or fluoro substituted hydrocarbon such as carbon tetrachloride, may be added to the pyrolysis feedstock which is to be pre-treated, and the feedstock, including the carbon tetrachloride introduced into the pretreatment reactor. The reactor may be operated at a temperature slightly below the temperature to be employed in the subsequent hydrotreating and pretreatment is continued until the catalyst in the reactor retains the desired amount of chlorine. At this point, the reactor temperature is raised to the hydrotreating temperature for initiation of the hydrotreating operation. The feed to the hydrocracking reactor may also include the chloro and/or fluro substituted hydrocarbon in small amounts; for example, in parts per million, to compensate for any small amount of chlorine and/or fluorine which may be removed from the catalyst during the hydrocracking operation.

The bimetallic catalyst is treated to provide the catalyst with a promoting amount of chlorine and/or fluorine. In general, the catalyst is treated to provide a chlorine and/or fluorine content of at least 0.5 weight percent and preferably at least 1 weight percent, with the chlorine and/or fluorine content most preferably being at least 2.5 weight percent. In general, the chlorine and/or fluorine content does not exceed about 15 weight percent and preferably does not exceed about 10 weight percent. In most cases, it is not necessary for the chlorine and/or fluorine content to exceed 7.5 weight percent to obtain the desired results.

The feedstock which is treated in accordance with the present invention is a feedstock which when subjected to pyrolysis produces light olefins and mononuclear aromatics. In general, such a feedstock is characterized by an initial boiling point of at least 350.degree. F. and a final end point of no greater than about 1100.degree. F. The preferred feedstock is a gas oil fraction, with such gas oil fractions generally being derived from either petroleum, athabasca tar, shale or coal sources.

The hydrogenation is effected at a temperature of from 640.degree. F. to 950.degree. F., preferably 650.degree. F. to 750.degree. F. and a total pressure of from 750 to 2500 psig, preferably 900 to 1500 psig. In accordance with the present invention, it is possible to employ pressures which are lower than those employed with the untreated catalysts. Thus, although it is possible to use the higher pressures, such higher pressures are not required. Similarly, in accordance with the present invention, it is possible to employ lower temperatures than heretofore employed in the art. However, it is also possible to use the higher temperatures, although in general such higher temperatures are not required. In general, hydrogen is employed in an amount of from 1000 to 15,000 SCF/bbl, preferably from 3000 to 10,000 SCF/bbl. The liquid hourly space velocity is generally from 0.5 to 3.0 V/H/V, and preferably from 0.8 to 1.7 V/H/V.

The purpose of the hydrogenation is to alter the feed to increase the yield of mononuclear aromatics (in particular BTX) and ethylene in the subsequent thermal cracking, and such alteration is effected in a manner to prevent excessive hydrocracking. Some hydrocracking is effected to break naphthenic rings of polynuclear aromatics; however, such hydrocracking is limited in order to prevent excessive gas production. Thus, the hydrogenation is effected at mild conditions to saturate polynuclear aromatics and cause some ring rupture to mononuclear aromatics, without excessive gas formation. It has been found that the use of the promoted catalyst results in an improved hydrogenation to alter the feed structure to provide improved yields of BTX and light olefins in the subsequent thermal cracking, without effecting excessive hydrocracking to lighter molecular weight hydrocarbons. In most cases, the 400.degree.-800.degree. F. fraction present in the hydrogenated feed is no less than 85% of that present in the feed to the hydrogenation.

The hydrotreated feedstock is then thermally cracked at conditions known in the art. In general, such thermal cracking is effected at temperatures of greater than 1400.degree. F., generally in the order of 1500.degree. F. to 1800.degree. F., and at residence times in the order of from 0.01 to 5 seconds, most generally 0.1 to 1 second. The thermal cracking is generally effected in the presence of steam, with the steam to hydrocarbon weight ratio generally being from 0.5:1 to 2.0:1 . Thermal cracking procedures ae known in the art and no further details in this respect are deemed necessary for a complete understanding of the present invention.

The invention will be further described with respect to the following example; however, the scope of the invention is not to be limited thereby:

A raw Nigerian gas oil with inspections as shown under column I of Table I was pyrolyzed with a steam dilution of 1:1 by weight at maximum cracking severity . From the pyrolysis coil effluent, the following valuable products were obtained:

Ethylene: 20.2 wt%

Benzene: 5.1 wt%

Toluene: 3.1 wt%

C.sub.8 Aromatics: 2.25 wt%

Also recovered is heavy +550.degree. F. fuel oil amounting to 16.6 wt%.

The same raw Nigerian gas oil was hydrotreated over a nickel molybdate on alumina catalyst under the following operating conditions:

Total pressure: 950 psig

Liquid hourly space velocity: 1.0

H.sub.2 /Oil SCF/bbl: 2500

Reactor average temp.: 685.degree. F.

A 340.degree. F. to END point hydrotreated gas oil was recovered. It has inspections shown under column II of Table I. The polynuclear aromatics content is reduced from 19.2 wt.% to 9.7% and the contents of paraffins and cycloparaffins are increased.

The hydrotreated gas oil is then pyrolyzed with a steam dilution of 1:1 by weight at maximum cracking severity. From the pyrolysis coil, the following valuable products are obtained:

Ethylene: 21.0 Wt%

Benzene: 6.8 Wt%

Toluene: 4.4 Wt%

C.sub.8 Aromatics: 2.7 Wt%

The recovered +550.degree. F. heavy fuel oil is 13.7 Wt%.

The same raw Nigerian gas oil as used in Example 1 was hydrotreated over a nickel molybdate on alumina catalyst with the catalyst loaded with a halogenated promoter such that the catalyst contained 2.5 Wt% chlorine. The operating conditions for the hydrotreating were as follows:

Total pressure: 950 psig

Liquid hourly space velocity: 1.0

H.sub.2 /Oil SCF/Bbl: 2500

Reactor average temp.: 689.degree. F.

CCl.sub.4 in feed, wt.%: 0.1

The hydrogreated gas oil recovered has inspections as shown under Column III of Table I. Compared with Example 2, the hydrotreating is done under the same conditions except with the CCl.sub.4 promoter. The product has higher paraffins and lower polynuclear aromatics.

The recovered hydrotreated gas oil is then pryolyzed at a steam dilution of 1:1 by weight at the maximum severity. From the pyrolysis coil, the following valuable products are obtained:

Ethylene: 21.7 Wt.%

Benzene: 6.9 Wt.%

Toluene: 4.4 Wt.%

C.sub.8 Aromatics: 2.7 Wt.%

The recovered +550.degree. F. heavy fuel oil is reduced to 12.5 Wt.%.

The same Nigerian gas oil as used in example 1 was hydrotreated over a nickel molybdate on alumina catalyst with the catalyst loaded with a halogenated promoter such that the catalyst contained 4.4 Wt.% of chlorine. The operating conditions for the hydrotreating were as follows:

Total pressure: 950 psig

Liquid hourly space velocity: 1.0

H.sub.2 /Oil SCR/ggl: 2500

Reactor average temp.: 685.degree. F.

CCl.sub.4 in feed, wt%. 0.5

The hydrotreated gas oil recovered has inspections as shown under column IV of Table I. Compared with Example 2 and 3, the product has the greatest paraffin and lowest polynuclear aromatics contents.

The recovered hydrotreated gas oil is then pyrolyzed at a steam dilution of 1:1 by weight at the maximum severity. From the pyrolysis coil, the following valuable products are obtained:

Ethylene: 21.8 Wt.%

Benzene: 6.9 Wt.%

Toluene: 4.4 Wt.%

C.sub.8 Aromatics: 2.7 Wt.%

The recovered +550.degree. F. heavy fuel oil is 12.5 Wt.%

The above examples show the advantages for using the promoter during hydrotreating.

Numerous modifications and variations of the present invention are possible in light of the above teachings and, therefore, within the scope of the appended claims the invention may be practised otherwise than as particularly described.