US2168610A - Method of treating hydrocarbon oils - Google Patents

Description

Aug. 8, 1939. H. c. scHuTT METHOD OF TREATING HYDROCARBON OILS Filed June 24, 1935 QMS QMS Patented Aug. 8, 1939 PATENT or-'Flcr-z y 2,168,610 Mn'rnop or matr-lisse mnooAnnoN Hermann c. schuit, North Tarrytown, N. Y.. f signor to Gyro Process Company, Detroit, Mich.,

a corporation of Michigan Application 4June 24,

8 Claims.

My invention relates tothe treatment of hydrocarbon oils for the production of hydrocarbons of lower boiling ran'ge and more particularly to the production of a normally gaseous n hydrocarbon product of narrow boiling range by cracking of relatively heavy hydrocarbon oils.

It is an object of my invention to provide a process for thermally treating hydrocarbon oil and recovering in an economic manner a sbstantially pure, normally gaseous hydrocarbon product of relatively narrow boiling range from the products of a cracking operation. J

It is another object of my `invention to produce continuously from `a relatively heavy hydrocarbon oil by means of a primary cracking operation, a cracking stock amenable to high temperature conversion without carbon deposition or other impairment of the high temperature conversion coil. A

It is still another object of my invention to provide a process for the treatment of hydrocarbon oil in which oil separated from the products of a cracking operation for` recycling'has higher hydrogen to carbon ratios than either the lighter or heavier rejected portions of such products.

It is a furtherobject of my invention to provide a process for the treatment of hydrocarbon oil in which the heat content in vapors from a vapor phase cracking operation.

It is also an object of my invention to provide a process for the treatment of hydrocarbonoil in which a sudden cooling of high temperature gases issuing from a gas cracking coil may be accomplished with a minimum of coking.

Other and further objects of my invention will appear from the following description and appended claims. A

The accompanying drawing which forms part of the instant specification and is to be read in conjunction therewith is a schematic showing in elevation, with parts in section, of one form of apparatus capable `of carrying out the process of my invention. y

In general, a relatively heavy hydrocarbon oil, such as reduced crude,or gas oil, is heated to vaporize a small amount of the lower boiling constituents, the remainder of the oil in conjunction with recycle stock then being subjected to a relatively low temperature primary cracking operation productive of vapors of lower boiling range. A portion of the unvaporized oil is re- `5.', turned to the primary cracking coil as part of 1935, Serial No. 28,059

the recycle stock, the residue being withdrawn from the system as fuel oil.

A, separation from the vapors of a vapor fraction having characteristics 4suitable for high temperature vapor phase cracking is carried out l by cooling and condensation of higher boiling constituents of the :total quantity of vapors, the condensate being recycled to the primary cracking coil. The cooling and condensation of these vapors is effected by means of oil having a rela- 10` tively high hydrogen to carbon ratiorecovered from the products'of the vapor phase cracking operation and used to reflux the vapors evolved from the low temperature primary cracking operation. It will be observed that I am thus 1| enabled to utilize the heat of these vapors for vaporizing the recycle oil which then passes in vapor form with the uncondensed vapor fraction to the high temperature vapor phase 'cracking coil. y

The products fof the high temperature vapor phase cracking operation are united with gaseous products of substantially the same temperature containing that normally gaseous hydrocarbon fraction whose recovery is an object of my invention. 'I'he gas vapor mixture is then quenched by means of a cooler hydrocarbon oil to a temperatureH below that. at which further reaction continues. 'I'his oil is preferably of the same boiling range as recycle oil for the vapor phase cracking operation and is substantially completely vaporizable at the quenching temperature. The quenched products are then fractionated to remove fuel oil of relatively. low hydrogen to carbon ratio as a bottom product, low boiling vapors and gases being 'removed as an overhead product. The recycle stock recovered as an intermediate fractional cndensate by this operation will` preferably have a hydrogen .to carbon ratio higher than that of n the fuel oil or the distillate recovered from the vapors in the overhead product. The overhead product is cooled and condensed to recover a quantity of distillate of motor fuel boiling range. The residual gases are liqueed in part by cooling-and compressing them. The liquids recovered, after stabilization for the establishment of the desired vapor pressure specifications, are removed from the stabilizer and fractionator, respectively, as finished distillate of motor fuel boiling range. This dis`` tillate is unsuited for recracking to produce ethylene, due to its low hydrogen to carbon ratio and is highly aromatic in character.

The uncondensed gaseous constituents rel maining after compression and cooling are first dehydrated and then fractionated at a very low temperature to remove the very light gases such as hydrogen and methane. They are then sub- Jected tofurther low temperature fractionation from which operation there is recovered, as a final overhead product. a very low boiling normally gaseous hydrocarbon fraction such as etlwlene, for example. Those constituents boiling higher than the final 'fraction sought are delivered to a gas cracking coil where they are subjected to conditions of cracking at low pressure and high temperature sumcient to convert a substantial quantity of the several constituents into a lighter constituent such as is desired in the flnal normally gaseous fraction. The gas mixture issuing from the high temperature gas cracking coil is immediately cooled to substantially the temperature of the vapors issuing from the vapor phase cracking coil before being mixed with these vapors. This immediate Dre-cooling is carried out to prevent the objectionable coking which would take place if the high temperature gases were immediately mixed with the low temperature liquid oil in the final quenching operation and to prevent any extended reaction with its possibilities of losing the desired unsaturated hydrocarbons due to polymerization. The pre-cooling vis carried out by means of the quantity of vapors and steam of relatively low temperature separated from the fresh feed passing to the low temperature primary cracking operation. As previously pointed out, the precooled gas mixture and the vapors from the vapor phase cracking operation are then quenched with a cooler hydrocarbon oil and fractionated. y

Referring now more particularly to the drawing, a relatively heavy hydrocarbon oil such as a Mid-Continent reduced crude oi' 24-26 A. P. I. flows through the pipe I -to the pump 2. The oil is charged by the pump 2 through the line 3 and the heat exchanger 4 to the top tray of a plurality of trays 5 in a flash tower 8. This tower may be operated at a pressure of from. 8 to 18 pounds per square inch gauge. The oil in passing through the heat exchanger is raised to a temperature of from 250 to 350 F. and is further heated in the flash tower, as will be more fully described hereinafter, to a temperature sumcient to vaporize those lighter fractions of the oil boiling, for example, between 475 to 525 F; These vapors, together with stripping steam, are discharged from the tower through the pipe 1. The unvaporized oil together with a quantity of recycle oil is withdrawn from a collecting pan .8, which forms a vapor riser 8 in the flash tower I, through the pipe I by means of a pump II which discharges the oil at a temperature of from 575 to 625 F. through the pipe I2 to a low temperature primary cracking coil I3 diagrammaticallyvshown in a furnace I4. This furnace as shown is by way of example only since it will be apparent that the furnace may consist of several coils in progressively hotterfheating zones so arranged as to create the most-suitable temperature gradient in the cracking coil for the particular stock being processed.

kThe cracking furnace I4 is so operated as to convert a certain amount of the higher boiling hydrocarbons into lower boiling hydrocarbons. For example, if the oil charged to the cracking furnace were of a boiling range of from 525 F. to 1000 the oil issuing therefrom may have a boiling range oi from 300 F. to 1100 F. The

products of the primary cracking operation issue from' the furnace I4 at a temperature ranging from 850 to 925 F. depending upon the cracking coil temperature gradient chosen for the stock in this phase of the operation and pass through the pipe I5 to an evaporator I8 maintained at a pressure of, for example, 90--100 pounds per square inch gauge.

The vapors evolved in the flash zone of the evaporator will have a boiling range of from approximately 300 F. to 700 F., although it will be understoodthat a minor portion of lower boiling constituents and gases will have been formed in the primary cracking coil. These vapors rising upwardly through a plurality of trays I1 countercurrent to downflowing reflux will be condensed in part so that the remaining vapors will be composed of hydrocarbons having boiling points up to 550-575 F. This condensate collecting in the receiver I8 flows at a temperature of from 750 to 825 F. through a pipe I9 controlled by valve 20 to a suitable ytray in the flash tower 6 where it aids in vaporizing the fresh feed and from which tower it is recycled to the primary cracking coil. The unvaporized oil from the flash zone of the evaporator I6 flows at a temperature of from 800 to 875 F. through a pipe 2l controlled by a valve 22 to the base of the flash tower 6 below the collecting pan 8. This oil lis stripped with steam introduced through pipe 23, the vapors stripped from the oil rising upwardly through the vapor riser 3 and ultimately being condensed and recycled to the cracking coil through the pipe I'Il. The residual oil in the ash tower flows through the pipe 24 to the pump 25 by which it is forced through the pipe 28 and cooler 21 and discharged from the system as fuel oil.

Vapors discharged from the evaporator I6 through a pipe 28 pass under the evaporator pressure to a high temperature vapor phase cracking coil 29 in a furnace 30. Although I have diagrammatically shown but a single coil, it will be obvious that the furnace 30 may be provided with a plurality of coils so arranged as to permit the cracking operation to be accomplished in a manner understood by those skilled in the art.

The products of the high temperature vapor phase cracking operation discharge at a temperature of from 1175 F. to 1225 F. through the pipe 3| to an arrester 32, the vapors in their passage thereto being commingied with tempered gases from a high temperature gas cracking operation. These gases are fed,into the pipe 3| by means of the pipe |38. The gas vapor mixture in the arrester 32 is quenched by means of a cool hydrocarbon oil to a temperature of from 600 to 650 F., the quenched products discharging through a pipe 33 in a separating chamber 34 of a fractionating tower 35.

The fractionating tower is preferably operated at 5-15 pounds per square inch gauge, the oil in the separating chamber 34 being stripped with steam introduced through a pipe 38. The stripped oil discharges from. the fractionating tower through the pipe 31 controlled by the valve 38 to the pump 39v by which it is forced through the pipe 40 into the pipe 26 where it is mingled with the fuel oil from the flash tower 6 for withdrawal from the process.

The vapors in the separating chamber 34 pass upwardly through a plurality of fractionating trays 4I countercurrent `to downiiowing reflux introduced above the top tray through a pipe 42 is that intermediate fraction of the products of the high temperature vapor phase cracking operation which I have discovered is `most suitable for recycling to the vapor phase cracking coil. This fraction has a higher hydrogen to carbon `ratio than either the heavieroil removedl fromA the base of the fractionating tower or the' heav- "iest fraction removed as an overhead product.

It is, therefore, most suited for recycling and is withdrawn from the collecting pan `through fthe pipe 41 by means of the pump 48 which forces this recycle oil through the 'pipe 49 controlled by the valve to above the top tray in the evaporator I6. This recycle oil in the evaporator serves as reflux for condensing these vapors from the low temperaturel cracking coil not suitable for the high temperature vapor phase cracking operation, and is simultaneously vaporized by the heat liberated in the condensation of those vapors. Although the quantity of heat liberated by those vapors condensed will generally be sufllcient to vaporize the recycle oil, I may, nevertheless, provide a supplementary heating coil 5| through which the oil may be passed from the pipe 49 by opening the valve 52 and closing the valve 50. In this manner a considerable degree of ilexibility is supplied. y

The condensate collecting in the pan 45 of the fractionating tower is also used as a quenching medium, being withdrawn at a temperature of i from 400 to 500 F. through a pipe 53 by means of a pump 54 which discharges the oil through the pipe 55 controlled by a valve 56, through the heat exchanger 4, pipe 51, cooler 56, and pipe 59 from which it may be returned through the pipe 60 controlled by the valve 6| into the fractionating tower above a suitable one of the upper trays `46. A portion of this oil may be also diverted throughthe pipe 42 to a suitable one of the trays 4| in the separating chamber 34 of the tower. The quench oil passes from the pipe 59 through kthe pipe 62 controlled by the valve 63 into the arrester -32 where it is completely vaporized in quenching the products of the cracking operationsn ,t If desired, a portion of the oil in the pipe 55 `may be conducted to a heat exchanger 64 by opening valves 65 and 66 in pipes 61 and 69 respectively, and closing the valve 56. The heat in this oil obviously maybefurther utilized for supplying reboil heat at `other needed points of the system as indicated.

The vapors and gases leaving the fractionating tower 35 at a temperature of from 180 to 230 F. pass overhead through the pipe 69 through the condenser 10 and pipe 1|, and at a temperature I of from 60`90 F. pass' with the condensate into an accumulator 12 maintained at a pressure of trolling the endpoint of the overhead product.y The remainder of the condensate in the accumulator 12 is withdrawn therefrom through a pipe 19 by means of a pump 19 and discharged thereby through a pipe 19 by means of a pump 19 and discharged thereby through a pipe" into an accumulator 9|. The gaseous products in the ac cumulator I2 pass overhead through a pipe 92 to a treating chamber 43 wherein they may be suitably treated for theremoval of certain objectionable sulphur bearing compounds.` The desulphurized gases upon being discharged from the treating zone 93 pass through the pipe 96 to the suction side of a compressor 91. The gases are discharged from the compressor at a pressure of approximately 400-450 pounds per square inch gauge through the pipe 09 to a cooler 99 from which condensate and uncondensed gases are discharged through the pipe 90 into the stream nowin`g through the pipe 90 to the accumulator 9|.

The condensate in the accumulator 9| discharges therefrom under the existing pressure and at a. temperature of from 60 to 90 F.

through the pipe 9| into the heat exchanger 64 from which it discharges through the pipe 92 controlled by the valve 93 into a fractionating or'v stabilizing tower 94. Additional heat may be supplied to the condensate in this tower bydany suitable heating medium circulated through a reboiling coil generally indicated as 95. The condensate stripped to secure a close fractionation consistent with the separation desired is discharged from the tower 94 through a pipe 95' controlled by a valve 96, passes through the cooler 91 from which it is carried through the pipe 99 to storage tanks (not shown).

It will be noted that this distillate recovered from the overhead product of the fractionating tower 35 has a hydrogen to carbon ratio lower than that of the intermediate fraction withdrawn from the fractionating tower 35 through the pipe 41 for recycling to the high temperature vapor phase cracking coil.

The gaseous constituents in the accumulator 9| lare discharged through the pipe 99 and dehydrator 94 for the removal of the last traces of water and then pass through pipe 05 into a high pressure fractionator |00 whose primary function is to eect a separation from the gaseous mixture of those gaseous constituents lower boiling than the normally gaseous hydrocarbon of narrow boiling range to be recovered as a desired final product. Specifically, where ethylene is the desired final product, gases would pass upwardly from the fractionator |00 through the reflux return pipe |0|, through the reflux condenser |02 whose temperature would be controlled by heat exchange with a suitable cooling medium to so control the temperature that the residual gases through a pipe |04 leading from the reflux condenser, would consist of hydrogen and methane. The pipe |04 is provided with a valve |00 for maintaining a pressure in the fractionator |00 consistent `with the overhead product and the top tower temperature. Heat is supplied to the condensate in the base of the fractionator |00 by any suitable heating medium circulated through a reboiler coil |06 at a temperature such as will secure the desired close fractionation. The condensate is discharged from the fractionator through the pipe |01 controlled by valve |08.` I'he gases in the fractionator 94 pass upwardly throughia reflux return pipe |09 through a reflux condenser ||0 operating to maintain a temperature in the top of the fractionator of from 175 to 225 F. The gases yleave the reflux condenser through a pipe controlled by a valve ||2, are commingled with the condensate discharged from fractlonator through the pipe |01 and pass into a fractionator ||3, maintained at a lower pressure and temperature than the fractionator |00.

Gases in the fractionator ||3 pass upwardly through a reflux return pipe III into a reflux condenser I 5 in which refrigeration is effected by. means of heat exchange with liquefled'normally gaseous hydrocarbons fed thereto lthrough a pipe ||1 controlledv by a valve H8, are permitted to vapoxize and then discharged through a pipe IIS controlled by a valve |20. The reux condenser ||5 is designed', for example, to maintain a temperature in the top of the fractionator ||3 of from 0 to -50 F. Heat may be supplied to the condensate in the base of the fractionator ||3 by any suitable heating medium circulated through a reboiler coil |2|, the condensate being discharged from the fractionator through a pipe |22 controlled by a valve '|23. This condensate under the operating conditions described would consist of propane, propylene, butane, butylene,.

pentan, and amylene. The gases uncondensed in' the reflux condenser ||5 are discharged therefrom through a pipe |24 controlled by a valve |25 into a fractionator |26.

In the fractionator |23 gases rise upwardly through the reflux return pipe |21 into a reflux,

condenser |28 into heat exchange with a suitable refrigerating medium. When ethylene is the desired flnal product this condenser will be operated to maintain a temperature in the top of the fractionator |26 of from 25 to 75 F., the ethylene being discharged from the reflux condenser through a pipe |30 provided with a'control valve 3|. Heat may be supplied to the condensate in the base of the fractionator by any suitable heating medium circulated through a reboiler coil |32. Condensate is discharged from the fractionator through the pipe |33 which joins the pipe |22 from the fractionator 3. Under the operating conditions described, when ethylene is to be the final product, condensate flowing through the pipe |33 will consist primarily of ethane, propane, and propylene.

Refrigeration in the reilux condenser ||5 may be supplied by closing a valve |34 in the pipe 22 and opening the valves ||l and in the pipes ||1 and ||9 respectively, thus permitting the normally gaseous liquid in the pipe |22 to vaporize' to supply refrigeration. It may be desirablel to use only the colder liquid from the tower for refrigeration. In such case, the liquid in pipe |22 may be bypassed by opening a valve |23a in a pipe I 23b,and closing the valve |23.

The condensate, after expansion, flows through a pipe |35 as a gas at a pressure bf from 100-125 pounds'per square inch' gauge to a gas cracking coil |36 located in a furnace |31. Although I have diagrammatically shown but a single coil in this furnace, it will be obvious to those skilled in the art that any number of coils may be provided as are necessary yto secure the desired temperaturelcharacteristic in the coil. The gases pass through the coil |35 issuing' therefrom at a pressure consistent with the pressure in the fraction- 'ator 35 and at a temperature of from 1400 to 1450 F. The higher molecular weight gases -will be progressively cracked to produce lower molecular weight gases of the desired characteristics. The products of the high temperature gasl cracking operation are discharged from the coil |36 through a pipe |33 and limmediately commingled at the furnace outlet with relatively cool vapors and steam discharged from the ilash tower 6 through the pipe 1. The quantity of vapors commingled with the gases is sufllclent to lower the temperature to 1150 to 1200 F., which is substantially the temperature of the products passing from the high temperature vapor phase cracking `coil 29 through the pipe 3| to the arrester 32. This immediate precooling prevents cold surface cooling and accompanying polymerization productive of heavy polymers Aof coke. As has been previously pointed out, 'the tempered products of the gas cracking operation then pass into the pipe 3| following which quenching of the combined gas vapormixture to a temperature between 600 and 650 F. is carried out in the arrester 32.

It will be seen that I have accomplished the objectsof my invention and have provided a process for the treatment of relatively heavy hydrocarbon oil which provides a novel economic mode of producing a pure normally gaseous hydrocarbon fraction of narrow boiling range. The time of reaction is so limited that the products leaving the'reaction coil still contain from 25% to 40% of the same kind of constituents'as passed to the cracking coil. 'I'his is consistent with the optimum reaction time permissible without having excessive secondary reactions, principally polymerization, with a resulting loss of unsaturated hydrocarbons whose recovery is desired. I have provided in a process for treating hydrocarbon oils for the production of lower boiling hydrocarbons a method by which only that fraction will be recycled which has the most favorable hydrogen to carbon ratio. I have provided a process for the treatment of hydrocarbon oils in which a stock most suitable for high temperature vapor phase cracking is continuously produced from the fresh oil by a primary low temperature cracking or viscosity breaking operation. I have also provided a process in which the heat capacity of the oil subjected to a primary low temperature cracking operation is gainfully employed invaporizing recycle oil for the high temperature vapor phase cracking operation.

It is to be understood that the recitation of temperatures and pressures in the foregoing illustration of the practice of my process is by way of example only and that my invention is to be limited only as defined by the appended claims.

It will be understood that certain features and sub-combinations are of utility and may be employed without reference to other features and sub-combinations. This is contemplated by and is within the scope of my claims. It is further obvious that various changes may be made in details within the scope of my claims. This is contemplated by and is within the scope of my claims. It is further obvious that various changes may be made in details within the scope of my claims without departing from the spirit of my invention. It is therefore, to be understood that my invention is not to be limited to the specific details shown and described.

Having thus described the invention, what is claimed is:

1. Inca process for converting normally liquid hydrocarbons into normally gaseous hydrocarbons, in which vapors of hydrocarbon oil are cracked in the vapor phase, quenched, dephlegmated, incondensable gases removed from the dephlegmating zone and separated into low molecular weight normally gaseous hydrocarbom desired, and higher molecular weight normally gaseous hydrocarbons; the step of cracking the higher molecular Weight normally gaseous hydrocarbons and admixing them with the vapor phase cracked hydrocarbons before said quenching step.

2. In a process for converting norm`ally liquid hydrocarbons into normally gaseouswnydrocarbons including the steps of heating a hydrocarbon oil to vaporizing temperatures, separating the heated oil into vapors and unvaporized .oil, heating the vapors to active vapor phase cracking temperature, stopping the reaction by quenching the highly heated vapors with a cool hydrocarbon oil, fractionating the quenched vapors, removing the incondensable gases from the fractionating zone, separating said gases into a fraction containing the desired normally gaseous hydrocarbons of low molecular weight, and normally gaseous hydrocarbons of higher molecular weight, cracking the normally gaseous hydrocarbons of higher molecular weight and admixing them with the cracked hydrocarbon oil vapors leaving the vapor phase cracking zone and then subjecting the mixture to said quenching operation.

3. In a process of converting normally liquid hydrocarbons into normally gaseous hydrocarbons, including the steps of subjecting liquid hydrocarbons to a liquid phase cracking operation, separating the cracked hydrocarbons into vapors and unvaporized oil, cracking the vapors in the vapor phase, quenching the cracked vapors with a cool hydrocarbon oil, withdrawing the hydrocarbons from the quenching operation and fractionating them, removing uncondensed gases from the fractionating zone and separating them into a fraction comprising low molecular weight normally gaseous hydrocarbons desired, and low boiling hydrocarbons of higher molecular weight than the desired product, cracking said higher molecular weight hydrocarbons,

withdrawing said cracked hydrocarbons from said last mentioned cracking zone and reducing their temperature by admixing them with the products from said vapor phase cracking operation, and then passing said mixture to said quenching zone.

4. In a process for converting normally liquid hydrocarbons of high molecular weight into low molecular weight hydrocarbons, including the steps of subjecting said high boiling hydrocarbons to a 'lrst cracking operation at temperatures in the vicinity of 900 F., separating the cracked hydrocarbons into vapors and unvaporized oil, cracking the vapors in a second cracking operation at a temperature in the vicinity of l200 F., reducing the temperature of said cracked hydrocarbon vapors by mixing a hydrocarbon oil therewith, fractionating said quenched hydrocarbon mixture, withdrawing low molecular welghthydrocarbons from said fractionating zone, and separating them into a fraction of low molecular` weight hydrocarbons desired as the iinal product, and a low boiling hydrocarbon fraction of. higher molecular weight than the molecular weight of the desired product, subjecting said last mentioned hydrocarbon fraction to a cracking operation at a temperature in the vicinity of 1400 F. and immediately admixing said last mentioned cracked hydrocarbons with the cracked hydrocarbons passed to said quenching zone.

5. In a process for converting normally liquid hydrocarbons of high molecular weight into low molecular weight hydrocarbons, including the steps of subjecting said high boiling hydrocarbons .to a first cracking operation at temperatures in the vicinity of 900 F., separating the cracked hydrocarbons into vapors and unvaporized oil, cracking the vapors in a second cracking operation at a temperature in the vicinity of 1200 F., reducing the temperature of said cracked hydrocarbon vapors by mixing a hydrocarbon oil therewith, fractionating said quenched hydrocarbon mixture, withdrawing low molecular weight hydrocarbons from said fractionatingzone, and separating them into a fraction of low molecular weight hydrocarbons desired as the final product, and a low boiling hydrocarbon fraction of higher molecular weight than the molecular weight of the desired product, subjecting said last mentioned hydrocarbon fraction to a cracking operation at a temperature in the vicinity of 1400u F., immediately admixing said last mentioned cracked hydrocarbons with the cracked hydrocarbons passed to said quenching zone, and introducing a low boiling hydrocarbon oil into the rst separating zone whereby the sensible heat of the hydrocarbons from the rst cracking step will vaporize said oil.

6. A method as in claim 5 in which said lowboiling hydrocarbon oil is withdrawn from the fractionatin'g zone.

7. In a process for converting normally liquid hydrocarbons of high molecular weight into low molecular weight hydrocarbons, including the steps of subjecting said. high boiling hydrocarbons to a first cracking operation at temperatures in the vicinity of 900 F., separating the cracked hydrocarbons into vapors' and unvaporized oil, cracking the vapors in a second cracking operation at a temperature in the vicinity of 1200* F., reducing the temperature of said cracked hydrocarbon vapors by mixing a hydrocarbon oil therewith, fractionating said quenched hydrocarbon mixture, withdrawing low molecular weight hydrocarbons from said fractionating zone, and separating them into a fraction of low molecular weight hydrocarbons desired as the final product, and a low boiling hydrocarbon fraction of `higher molecular weight than the molecular weight of the desired product, subjecting said last menf tioned hydrocarbon fraction to a cracking operation at a temperature in the vicinity of 1400" F., immediately admixing said last mentioned cracked hydrocarbons with the cracked hydrocarbons passed to said quenching zone, fractionating unvaporized oil from said first separating step in a second fractionating zone, withdrawing heavier liquid hydrocarbons from the process from said fractionating zone, withdrawing the lighter liquid hydrocarbons from said second fractionating zone and recycling them to the iirst cracking step.

8. A method as in claim 'I in which the fresh hydrocarbon oil to be processed is introduced into said second fractionating zone, vapors withdrawn from said second fractionating zone and admixed with the cracked hydrocarbons leaving the third cracking zone.

HERMANN C. SCHU'I'I.

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