US2998379A - Hydrocarbon conversion process - Google Patents

Description

Aug. 29, 1961 J. K. MCKINLEY ET AL 2,998,379

HYDROCARBON CONVERSION PROCESS Filed April 2l, 1959 United States Patent O 2,998,379 HYDRO'CARBON CONVERSION PROCESS John K. McKinley, Hopewell Junction, NSY., Benjamin F. Smith, Groves, Tex., Samuel P. Dickens, Fishkill,

NX., and Wiley P. Ballard, Port Arthur, Tex., assignors to Texaco Inc., a corporation of Delaware Filed Apr. 21, 1959, Ser. No. 807,832 7 Claims. (Cl. 208-70) This invention relates to a method yand apparatus for the conversion of hydrocarbon oils to valuable products including high octane number gasoline fractions. In one of its more specific aspects it is directed to -a conversion -process wherein a hydrocarbon oil is subjected to thermal cracking conditions in a liquid phase cracking zone, products of cracking a vaporized and withdrawn from the cracking zone substantially Without vapor phase cracking, the products of cracking are subjected to fractional distillation separating a cracked fraction boiling within the gasoline boiling range, and said cracked fraction is catalytically reformed producing a high octane motor fuel fraction.

The manufacture of motor fuel fractions from higher boiling oils by thermal or catalytic cracking is Well known. In the cracking process, a hydrocarbon oil boiling above the gasoline boiling range, for example, gas oil, crude oil, topped crude, reduced crude and the like, is subjected to heat or heat and contact with a catalyst eifecting formation of hydrocarbons boiling within and below the boiling range of gasoline. The thermal cracking of heavy oils by the application of heat has been `supplanted in modern processing schemes by catalytic cracking largely as a result of the relatively low octane number of the thermally cracked gasoline fractions. Although the octane number of thermally cracked gasolines produced by conventional thermal cracking processes may be increased by catalytic refoming, the overall yield of gasoline of ya desired octane number is low in comparison with the yield of gasoline which may be produced by catalytic cracking. Although thermal cracking is a simple and economic process, the combination of thermal cracking with catalytic reforming has heretofore been less attractive than the use of catalytic cracking process. The term motor fuel or gasoline fraction is used herein to refer to a hydrocarbon fraction boiling Within the range of about 90 to 400 F. by ASTM distillation (ASTM method D86-56). Total butane retention gasoline is a mixture of hydrocarbons comprising butane and heavier hydrocarbons up to a specilied end point which is understood to be 400 F. if a lower temperature is not given.

In the development of thermal cracking it was discovered that a considerably higher octane number gasoline fraction is produced in cracking processes employing a large yamount of Vapor phase cracking. For tln's reason cracking apparatus was devised wherein the hydrocarbon oil is rapidly heated in a coil and then discharged into a drum called a soaking drum which is operated with a low level of liquid. As a result, a large part of the cracking in the soaking drum consists of vapor phase cracking. With the development of higher compression ratio engines, it has become impossible to produce gasolines of high enough octane number by thermal cracking even with a large amount of vapor phase cracking and so most hydrocarbon oils are now cracked in the presence of catalysts to produce higher octane number gasolines.

We have discovered that unexpectedly high yields of gasoline of high octane number can be produced by the integration of a thermal cracking process wherein vapor phase cracking is `avoided with a catalytic reforming process. In accordance with the process of this invention, hydrocarbon oils are subjected to heat and pressure while maintained in a liquid phase cracking zone. Upon crack- ICC ing, the heavy hydrocarbon is converted to low boiling hydrocarbons which vaporize from the liquid phase and separate therefrom. The hydrocarbon vapors are withdrawn from the cracking zone and cooled to temperatures below effective cracking temperatures, that is, below about 750 F. so that vapor phase cracking is substantially prevented. We have found that liquid phase cracking conditions may be advantageously maintained by heating a hydrocarbon oil to cracking temperatures and discharging the hot hydrocarbons into a soaking drum which is maintained substantially full of liquid, that is, at least percent full of liquid and preferably about percent full of liquid. Hydrocarbon vapors leaving the surface of the liquid in the soaking drum Iare passed directly to a cooling zone in which the hot vapors are contacter directly or in indirect heat exchange with cooling medium to reduce the temperature of the cracked products below 750 F. Heat is supplied to the cracking zone by heating the oil introduced into the soaking drum. The `fresh feed stream may be heated and additional heat may be introduced by heating a recycle stream of refractory oil as is well known in the thermal cracking art. Adequate cooling may be obtained by passing the Vapor products directly to a fractionating column operated `at bottom temperatures less than 750 F. The soaking drum may be heated externally to obtain maximum liquid phase cracking. Y.

Thermal cracking in the liquid phase is effected at temp peratures within the range of about 770 to 930 F., prefera-bly within the range of about 790 to 850 F. The pressure in the soaking drum may vary from about 50 to about 1000 pounds per square inch gauge or higher. Preferably the pressure is maintained within the range of about 250 to 400 pounds per square inch gauge. Thermal cracking occurs in the manner of a first order reaction and the rate for a given stock is governed mainly by time and temperature. Since the temperatures of the liquid and the vapor in a reactor substantially lilled with liquid are about the same, the relative amount of liquid and vapor phase cracking obtained `depends upon the relative liquid and vapor residence times. A liquid phase soaking time of about 60 to 300 minutes is employed preferably within the range of about 90 to 180 minutes. As a result of the relatively small vapor space in the reaction zone and thev volumetric expansion upon vaporization of the lcracked products, the vapor products 'are exposed to cracking temperatures for a relatively short time, for example, about 0.1 to 10.0 minutes, preferably about 1.0 to 5.0 minutes. Within the range of preferred residence times above, the amount of liquid phase cracking is within the range of about 18 to 180 times the amount of Vapor phase cracking, which for practical purposes may be said to be substantially Without vapor phase cracking. Desirably a portion of the liquid is continuously withdrawn from the liquid phase to prevent the Aaccumulation of coke in the soaking drum. This liquid stream may be stripped to separate gas, gasoline, and gas oil fractions and a heavy fuel oil fraction useful for fuel oil blending. The stripped gas oil fraction is highly refractory, that is, resistant to thermal cracking, and ymay be employed as 'a heat carrier. Recycled gas oil may be heated to a high temperature, for example, from about 900 to 0 F. or higher, and introduced into the soaking drum to supply heat and effect cracking of the fresh feed.

In order to separate cracked products from the liquid phase rapidly, a lifting vapor may be introduced into the liquid phase soaking drum. For example, a light distillate may be introduced at the bottom of the liquid phase which upon introduction into the hot cracking zone is vaporized and strips lower boiling components from the liquid phase. Other fluids such as recycle gas or steam may be employed as lifting agents. Alternatively cracked products may be released from the liquid phase by passing a portion of the liquid into a zone of lower pressure wherein the conversion products flash to vapor and may be separated from the heavier liquid which may be returned to the cracking zone.

The vapor conversion products from the liquid phase cracking zone as described above are cooled to a temperature below 750. The vapor may be cooled and condensed by contracting with liquid reflux in a fractional distillation tower. The cracked products are separated in the fractionator into gas, gasoline, and gas oil fractions. 'Ihe gas produced may be employed as a lifting agent or may be discharged from the system for the recovery of valuable constituents. The gas-oil fraction is desirably recycled as a heat carrier for introduction into the cracking zone. The gasoline fraction is converted to high octane gasoline by catalytic reforming preferably after pretreating in a mild hydrogenation process for the saturation of olefins and removal of sulfur. In the pretreating step the thermal cracked gasoline is contacted with a catalyst, for example cobalt molybdate, in the presence of hydrogen. The pretreated gasoline is then reformed in the presence of a catalyst, for example platinum on alumina, in the presence of hydrogen effecting isomerization and aromatization forming a high octane number gasoline fraction.

An advantage of the process of this invention is that high octane number gasoline is produced from heavy hydrocarbon oils.

Another advantage of the process of this invention is that high octane number gasolines are produced in unexpectedly high yields.

Another advantage of this invention is that heavy oils are converted to oils boiling within the gasoline boiling range by thermal cracking thereby avoiding the use of expensive catalyst and expensive catalyst regeneration facilities.

Another advantage of the process of this invention is that existing thermal cracking apparatus may be modiiied at little expense to produce high yields of gasoline which may be catalytically reformed to produce unexpectedly high overall yields of high octane number gasoline fractions.

The accompanying drawing diagrammatically illustrates one form of the process of this invention. Although the drawing illustrates one arrangement of apparatus in which the process of this invention may be practiced it is not intended to limit the invention to the particular apparatus or materials described.

Oil feed, for example, a topped crude oil, is introduced through line into heater 12. The oil feed is heated to a temperature within the range of about 820 to 870 F. and discharged through line 13. Cycle gas oil in line is passed through heater 17 wherein it is heated to a temperature of about l020 and discharged through line 18. Preheated oil feed and cycle gas oil are combined in line 20 and introduced into soaking drum 21. Hot liquid rises through soaking drum 21 to liquid level 22. Liquid overflows from soaking drum 21 through line 23 into soaking drum 24 having liquid level 25. Vapors produced in soaking drum 21 rise above liquid level 22 and are discharged through line 30. Vapors in soaking drum 24 rise above level 25 and are discharged through line 31. The vapors in lines 30 and 3l are combined in line 32 and are introduced into fractionator 35, where they are vcooled in contact with descending liquid. Gas oil is Withdrawn from the bottom of fractionator 35 through line 15 to provide the cycle gas oil heat carrier.

Heavy oil from the bottom of soaking drum 24 is withdrawn through line 45 and discharged to tar stripper 46. Tar stripper 46 comprises a fractional distillation tower operated at low pressure. Heavy fuel oil is Withdrawn from tar stripper 46 through line 50 and discharged to storage. Overhead from tar stripper 46 is passed to fractionator 35 to supply a part of the liquid reux therein.

Cracked gas is withdrawn from the top of fractionator 35 through line 60 for use not shown. Gasoline boiling range liquid is withdrawn from fractionator. 35 through line 61 and passed to mild hydrogenation facility 65. Hydrogen is introduced to facility 65 through line 66 and sulfur containing oi gas is discharged through line 67. Mild hydrogenation is employed to effect desulfurization and saturation of the cracked gasoline prior to charge to catalytic reforming. This step is employed to prolong the life of the reforming catalyst and may be omitted when mild reforming conditions are employed or when the oil feed is low in sulfur.

Hydrogen treated gasoline is passed from facility 65 through line 68 to catalytic reforming unit 70. Reformed gasoline is ldischarged through line 71 and hydrogen product gas is discharged through line 72.

Example A 20.8 API topped crude oil is processed in accordance with the process of this invention for the manufacture of motor fuel. The topped crude is preheated to a temperature of 774 F. and introduced into a cracking reactor in which a large body of liquid is maintained. The cracking reactor is substantially filled with liquid providing a high liquid level, the vapor space at the top being only that required for liquid vapor disengagement. The residence time of liquid in the cracking reactor is 134 minutes and the residence time of the vapor product is 2.7 minutes. Additionally, a clean oil recycle stream is heated to 949 F. and combined with the topped crude introduced into the cracking reactor maintaing the liquid phase in the reactor at a temperature of 840 F. The cracking reactor is maintained at a pressure of 128 p.s.i.g. Cracked products vaporize from the liquid phase in the reaction zone, pass through the disengagement section and are withdrawn to a quench section of a fractionating tower. The fractionating tower is operated to take 400 F. end point gasoline overhead and clean recycle oil from the bottom.

A portion of the liquid from the cracking reactor is continuously withdrawn to prevent the accumulation of coke. The separated liquid is stripped to remove clean oil and gasoline which are returned to an intermediate point in the fractionator. Tar, which may be used in fuel oil blending, is withdrawn from the stripper as bottoms. The overhead from the fractionator is stabilized to separate dry gas comprising propane and lighter and a total butane retention gasoline.

In comparison, conventional themal cracking of the topped crude of this example employing a coil and drum with substantially no accumulation of liquid in the drum, produces 44.5 volume percent butane retention gasoline and 50.0 volume percent fuel oil.

The liquid phase thermal cracked butane retention gasoline is fractionated to separate a to 400 F. fraction. The yield of 150 to 400 F. fraction is 23.6 volume percent of the original topped crude change. The foregoing 150 to 400 F. fraction is contacted with a cobalt molybdate catalyst in the presence of hydrogen at 700 p.s.i.g. pressure and an average reactor temperature of 727 F. and then contacted with a reforming catalyst comprising platinum on alumina with a small amount of combined chlorine `and uorine in the presence of hydrogen at an average reactor temperature of 918 F. and a pressure of 500 p.s.i.g. A catalytic reformate is obtained at a yield of 96.2 volume percent or 22.7 volume percent basis topped crude. The catalytic reformate has octane numbers of 90 clear and 99 with 3.0 cc. tetraethyl lead per gallon by ASTM method D908-56. This octane level is several units higher at the reforming yield employed than the octane number of reformate obtained by the catalytic reforming of conventionally thermal cracked gasolines.

In applicants combination liquid phase thermal cracking and catalytic reforming process, not only is a greater yield of gasoline produced in the thermal cracking step than in conventional thermal cracking but lthe liquid phase thermal cracked gasoline responds better to catalytic reforming than conventional thermal cracked gasoline so that the overall yield-octane relationship is unexpectedly high.

Obviously many modifications and variations of the invention, as hereinbefore set forth, may 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:

l. A process for the conversion of a hydrocarbon oil to high octane number motor fuel which comprises introducing said oil into a liquid phase reaction zone maintained at a temperature within the range of about 770 to 930 F. and at a pressure within the range of about 50 to 1000 pounds per square inch gauge, separating cracked products as a vapor from said liquid phase reaction Zone and cooling said vapor at an average vapor phase residence time above 750 F. of less than 10 minutes to a temperature below about 750 F., separating a fraction boiling within the boiling range of gasoline from said cracked products, and catalytically reforming said sepa rated fraction boiling within the boiling range of gasoline.

2. A process for the conversion of a hydrocarbon oil high octane number motor fuel which comprises subjecting said oil to a temperature within the range of about 770 to 930 F. and a pressure within the range of about 50 to 1000 pounds per square inch gauge in a liquid phase reaction zone for an average liquid phase residence time within the range of 60 to 300 minutes, separating cracked products as a vapor from said liquid phase reaction zone and cooling said vapor at an average vapor phase residence time above 750 F. of less than 10 minutes to a temperature below 750 F., separating =a fraction boiling within the boiling range of gasoline fromsaid cracked products, and catalytically reforming said separated fraction boiling within the boiling range of gasoline.

3. The process of claim 2 wherein the liquid phase residence time is Within the range of 18 to 180 times the vapor phase residence time.

4. The process of claim 2 wherein a gas oil fraction is separated from said cracked products, said gas oil is heated to a temperature within the range of 900 to 1050 F. `and passed to said liquid phase reaction zone to supply at least La part of the heat introduced into said liquid phase reaction zone.

5. The process of claim 2 wherein said liquid phase reaction zone is externally heated to supply at least a part of the heat introduced into said reaction zone.

6. A process for the conversion of a hydrocarbon oil to high octane number motor fuel which comprises subjecting said oil to a temperature within the range of about 790 to 850 F. and a pressure within the range of about 250 to 400 pounds per square inch gauge in a liquid phase reaction Zone lfor an average liquid phase residence time within the range of to 180 minutes, separating cracked products as a vapor from said liquid phase reaction zone and cooling said vapor at an average vapor phase residence time above 750 F. within the range of 1.0 to 5.0 minutes to a temperature below 750 F., separating a fraction boiling within the boiling range of gasoline from said cracked product, and catalytically reforming said separated fraction boiling Within the boiling range of gasoline.

7. The process of claim l, wherein the separated fraction boiling within the boiling range of gasoline is subjected to mild hydrogenation to effect desulfurization and saturation of the said gasoline fraction prior to the catalytic reforming step.

References Cited in the le of this patent UNITED STATES PATENTS

Claims (1)

1. A PROCESS FOR THE CONVERSION OF A HYDROCARBON OIL TO HIGH OCTANE NUMBER MOTOR FUEL WHICH COMPRISES INTRODUCING SAID OIL INTO A LIQUID PHASE REACTION ZONE MAINTAINED AT A TEMPERATURE WITHIN THE RANGE OF ABOUT 770 TO 930*F. AND AT A PRESSURE WITHIN THE RANGE OF ABOUT 50 TO 1000 POUNDS PER SQUARE INCH GAUGE, SEPARATING CRACKED PRODUCTS AS A VAPOR FROM SAID LIQUID PHASE REACTION ZONE AND COOLING SAID VAPOR AT AN AVERAGE VAPOR PHASE RESIDENCE TIME ABOVE 750*F. OF LESS THAN 10 MINUTES TO A TEMPERATURE BELOW ABOUT 750*F., SEPARATING A FRACTION BOILING WITHIN THE BOILING RANGE OF GASOLINE FROM SAID CRACKED PRODUCTS, AND CATALYTICALLY REFORMING SAID SEPARATED FRACTION BOILING WITHIN THE BOILING RANGE OF GASOLINE.

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