US2346642A - Process for the production of olefins and aromatic hydrocarbons - Google Patents

Description

April 1944.. D. F. BABCOCK EI'AL I ,346,6

PROCESS FOR THE PRODUCTION OF OLEFINS AND AROMATIC HYDROOARBONS Filed Dec. 30, 1937 2 Sheets-Sheet 1 Da//T Babcock Jrfhurl MLgr -h INVENTORS BY g 2 g ATTORNEY .April 18, 1944. v D. F.- BABCOCK ETAL PROCESS FOR THE PRODUCTION OF OLEFINS AND AROMATIC HYDROCARBONS Filed Dec. 30, 1937 -2 Sheets-Sheet 2 Patented Apr. 1a, 1944 FIN 8 AND AROMATIC HYDBOC ABBONS Dale F. Babcock, Elmhurlt, and Arthur W; Lax-char, Wilmington, Del, aaslgnors to E. I. du Pont de Nemours @Company, Wilmington, Del., a corporation ofDelaware Application December 30, 1937, Serial No.'182,408

Claims. (Cl. 280-668) This invention relates to the manufacture of low molecular weight oleiins and aromatics from petroleum, and more particularly, pertains to the production of normally gaseous oieflns such as ethylene and normally liquid aromatics such as benzene, toluene, and xylenes.

The fundamental principlesapplied by petroleum refiners in producing gasoline and related products from hydrocarbon oils by pyrolysis at elevated temperatures have been known for a long time. It is also well known that most processes which have previously been proposed for making olefins and aromatics by oil cracking, convert relatively small proportions of the oil to olefins or aromatics and yield the latter in a very impure state. It is well known that the effectiveness of many of these processes is based on cracking conditions which are so severe that sustained continuous operation is impossible because of failure of materials of construction or due to blocking of vital equipment partswith carbon. This invention constitutes an improvement over' existing methods. I

One object of this invention is to provide an eii'ective method for obtaining high yields 'of olefins, especially ethylene and/or propylene. Another object is the simultaneous conversion of an appreciable portion of a hydrocarbon oil. to aromatic compounds and especially to benzene, toluene, and xylenes. A further object is the production of these aromatic compounds in a relatively pure state and in relatively high yields, from which undesirable contaminants can be readily removed. A'still further object is the achievement of the above ends under such conditions that continuous operation is possible for long periods of time. Other objects will appear hereinafter.

This invention relates particularly to processes in which a hydrocarbon is cracked at high temperature in the vapor phase. In such processes the conversion products are usually separated into gas, light distillate, recycle, and refractory fuel oil fractions. The above mentioned objects are accomplished by the following invention which comprises recovering C5 or mixtures of C3 with C4 and/or higher'molecular weight hydrocarbons from the reaction products and espepreferred arrangements of apparatus for carrying out the invention are illustrated diagram-. matically. Figure 1 is a flow sheet in diagrammaticai form of a process and apparatus illus-v trating one embodiment of the invention. Figure 2 is a flow sheet in diagrammatical form of the preferred process and apparatus illustrative of the invention. It is to be understood that details of this equipment which are familiar to those versed in the art have been omitted for the sake of clarity, that modifications in constructional details and in the arrangement of parts may be made without departing from the spirit of the invention, and that said invention is not limited to such factors except as may be required by the claims.

Referring more particularly to the drawing designated as Figure No. 1, raw material, which may for example be gas oil or other petroleum oil, is delivered from a suitable storage through line i to pump IA by means of which it is transferred-via pipe line 2 to work tank 3. The fresh, raw material is mixed in work tank 3 with other oil fractions which will be subsequently described. The mixture leaves tank 3 through line 4 and is delivered by pump 5 through line 6 to preheating coil I. This and other coils are suitably arranged in banks in a furnace setting. The preheating bank is preferably heated by the waste gases discharged from a combustion chamber of the furnace in order that the gases vented to the stack will be at a minimum temperature. The charging stockis forced serially through the tubes of preheating bank I in order that the oil maybe heated to a vaporizing temperature of approximately 325 to 385 C. Ordinarily, little Y lighter oils .which are in a vaporized state in cially from the gas fraction, and'returning the The invention will be more thoroughly understood by reference to the following specificationand to the accompanying drawings in which the evaporator 9, pass overhead from the latter through outlet line II and are introduced into drying bank l3, which is situated in an intermediate temperature zone in the furnace setting,

and. like preheating coil 1, is largely heated by' waste products. of combustion.

Theoil passes serially through the tubes which comprise the drying bank and its temperature is raised to about 460 to 490 C. Thetemperature-time relationship in this bank is so adjusted that extensive cracking does not occur. Owing to ineflicient separation of mist and vapor in the evaporator and to condensation in transfer line l2, a small amount of liquid oil is sometimes charged to the drying coils. This liquid, however, is eflectively evaporated in the drying bank, which discharges a substantially dr vapor.

After being thoroughly dried in coil II, the hot vapors are introduced into a bank of cracking or conversion tubes ll. These tubes are arranged serially in the radiant section of the furnace setting. preferably around the sides and top wall of the main combustion chamber. It is desirable to raise the temperature of the vapors leaving the drying zone as rapidly as possible to the active conversion temperature, i. e. at least 650 C. and preferably about 690 to about 710' C. The vapors are maintained at this high temperature level in the latter portion or soaking section of bank II. It will of course be realized that at this high reaction temperature, precautions must be taken to avoid the deposition of carbon in the reaction tubes and toprevent injuries to them. These difficulties are avoided by careful location of the tubes with respect to the furnace setting and burner flames and by maintaining high velocities of oil vapor through the tubes in order that the heat applied to them may be promptly absorbed and carried away.

The hot reaction products, which have been held at a substantially constant temperature in the soaking section of II, are discharged through pipe ll into arrester II. The arrester is simply a chamber into which a cold, relatively volatile hydrocarbon fraction is introduced through a suitable spray nozzle. The hot reaction products and the cold spray are intimately mixed; the latter is largely evaporated, and the temperature of the reaction products is dropped almost instantaneously from about 700 to about 250 C. This quenching procedure terminates the cracking reaction abruptly. The mixture of quenched reaction products and vaporized quenching medium leaves the arrester by means of line H and passes to the base of flash tower it. The function of this tower, which may be of any well known design suitable for distillation of hydrocarbon products. is to separate the high boiling constituents formed during the cracking reaction, from unconverted charge and lower boiling materials. The heavy oils and tarry materials, which may find use as fuel oils, are discharged from the base of the tower it through outlet I! and cooler and are finally sent to storage vessels through pipe line 2|. The vapors which separate from the hot liquid at the base of tower it are scrubbed in the upper portion of the tower by descending, relatively volatile oil which has been introduced into the top of the column as reflux through line 28. Theterminal temperature conditions of tower I! are adjusted by varying the rates of quench liquid to arrester l8 and of reflux to the tower so that the vapors emerging from the top 01' the column to line 22 are free from heavy, tarry constituents and that the fuel oil fraction discharged at the base is substantially free from volatile constituents which might have further value as cracking stock.

The vapors issuing from the top of tower I. pass by means of line 22 to the base of tower 22. Its purpose is to separate unconverted raw material or constituents in essentially the same boiling range from lower boiling liquids and fixed fractionaflng tower. A light distillate having anend point of perhaps 150' to 200 C. is pumped into the top of tower 23 through line It to serve as reflux. Suflicient reflux is added in this manner so that the vapors. if cooled to condense constituents boiling in the gasoline range, give a condensate which has an end point of about 100 C. The higher boiling materials descend to the base of the fractionating tower where they are discharged by means of outlet line 24 through cooler 2|. Aportion of the cooled bottoms from the fractionating tower 23 passes to pump 2| through line 21 and is introduced by way of line 20 into the top of tower it to serve as reflux there. The remainder returns by means of lines 28 and 2 to work tank 2 where it is mixed with fresh charge and the steps outlined above repeated to efiect further cracking.

The eiiluent vapors leave tower 23 through outlet pass into condenser 2i, and are there cooled to about 30 C. The pressure at this point is about 10 lbs. per sq. in. gauge. Under these conditions an appreciable amount of liquid is formed in the condenser and passes together with uncondensed vapors to a gas-liquid separator It by means of line 32. Light distillate is withdrawn from the base of this separator through outlet II. A portion is delivered through line It to pump 80 which forces the material through line I! and thence through lines 21A and 20 to furnish quenching medium and reflux to the arrestor It and fractionating tower 23, respectively. The light distillate made above that used for the purposes Just mentioned is withdrawn from the system to suitable storage tanks through line As much as 60% of the vaporized raw material may be converted to gas. Much of this gas is withdrawn from the top of separator 33 through line I! as a complex mixture containing hydrogen. methane, ethylene, ethane, propylene, propane, butylene, butane, and higher boiling constituents.

The complex gas mixture is processed in recovery plant 40 to separate the high boiling hydrocarbons from lighter constituents. The equipment for achieving this separation is not indicated diagrammatically since it is not a part of this invention. It may be mentioned, however, that the higher boiling materials can be separated from the complex gas mixture by a series of steps involving compression. liquefaction, and rectification procedures or by compression, absorption, and rectification operations. The higher boiling constituents of the gas, which may consist of Cs and higher hydrocarbons, are returned from recovery plant 40 to work tank 3 via lines ll and 2. This recovered light distillate is here mixed with fresh charging stock and recycle stock and the mixture subjected to the cracking procedure outlined above.

It has been found that 18 to 20% of the vaporized fresh charge can be converted to ethylene by following the steps indicated above. At the same time, high yields of propylene and of ethane and C4 hydrocarbons can also be obtained from the complex gas mixture sent to the gas recovery plant. Concurrently, high yields of light distillate containing principally benzene, toluene, and xylenes may be achieved. It has been found that there is a marked increase in yields of normally gaseous products comprising hydrogen to C: hy-

drocarbons, inclusive, and especially of the more valuable constituents, ethylene and/or propylene, when constituents of higher molecular weight are separated from the gas fraction and recycled to the oil cracking zone, the cracked products being combined with the cracked products from the oil cracking zone. It has been observed that this method of operating not only increases the yields of benzene. toluene. and xylene but also greatly increases their concentration in the light distillate fraction. It is, for example, possible to produce a light distillate by these means in which the benzene, toluene, and xylenes aggregate in excess of 80%.

Is has also been found that this invention can be advantageously applied to a multistage oil and gas cracking process for manufacturing low molecular weight oleflns and aromatics. A better understanding of this application of the invention may be obtained by reference to the drawing designated as Figure No. 2 in which a preferred arrangement of equipment is illustrated diagrammatically. Raw material, which again. for example, may be "gas oil is subjected to the treatment previously described in detail with reference to Figure No. 1. A complex gas mixture containing hydrogen, methane. ethylene. ethane, propylene, propane, butylene, and higher boiling hydrocarbons is withdrawn from the top of separator I33 through line I39 and passed to recovery plant I 40. Here the mixture is separated by liquefaction means into the following fractions: l hydrogen and methane. I2) ethylene. i 3) a mixture of ethane, C3 and C4 hydrocarbons, and 14) higher boiling hydrocarbons. The hydrogen and methane fraction, which may serve as fuel. is discharged from the recovery plant through line I42. Ethylene is released via line I43 to suitable storage or directly to processes in which it may be used as a raw material.

The fraction comprising ethane and C3 and C4 hydrocarbons is delivered to pump I44, by means of which'it' 'is forced through pip line I45 and thence into preheating coil I46. This and other gas treating coils are arranged in a furnace setting designed for high temperature service and the preheating coil is preferably situated so that it is heated largely by waste combustion gases. Such portion of the charge as may be in a liquid state under inlet temperature and pressure conditions is completely vaporized as it passes through the tubes of preheating bank I46 in which a final temp rature of 650 to 700 C. is attained. The hot gases are then introduced into a bank of cracking or conversion coils I41 and are held at 760 to 790 C. in the latter portion of these coils which are preferably situated in a. radiant section of the furnace setting. The time-temperature relationship in this coil is so regulated that about 50% of the incoming ethane, C3, and C4 hydrocarbons are converted to other compounds, for example, hydrogen. methane, ethylene, and hydrocarbons in the motor spirit boiling range. The 50% conversion mentioned above refers to the average of ethane. Ca. and C4 conversions and not to a. 50% conversion of each constituent. High vapor velocities are employed in the conversion tubes to prevent excessive tube wall temperatures and to minimize carbon deposition.

The hot. reaction products are discharged from conversion zone I41 through line I 48 into arrester I49. Arresters I49 and IIS are similar in design. The products of gas cracking are reduced instantly from reaction temperature to about 120 C, by quenching with a volatile fraction which sear CI I KOOITI is withdrawn-from line I 31, through line I54 to the arrester spray nozzle. The mixture of quenched reaction products and largely vaporized quenching stock leaves arrester I 48 by means of exit line I 5|, and joins a similar mixture discharged from oil cracking arrester H8. The combined, quenched products from oil and gas cracking pass to the base of flash tower II8 via line III. From this point on the equipment for processing the reaction products is common to both cracking stages and with the exception of recovery plant I40, functions in the manner previously described for the case in which oil only is cracked.

The constituents comprising the higher boiling hydrocarbon fraction which is separated in recovery plant I40 are now obtained from the gas fractions produced by oil and gas cracking operations. This higher boiling mixture is discharged from recovery plant I40 through line Hi to line I02 in which it is returned to work tank 3. It is then mixed and recycled to the oil cracking zone with recycle stock and fresh feed.

As in the case in which oil only is cracked, it has been found that there is a marked increase in yields of gaseous products from hydrogen to C3 hydrocarbons, inclusive, and of benzene, toluene. and xylenes when Cs and higher molecular weight constituents are separated from gas produced by combined oil and gas cracking and recycled to the oil cracking zone. It has also been found that this procedure greatly increases the concentrations of benzene, toluene, and xylenes in the aromatic fraction.

Example A hydrocarbon distillate with characteristics similar to those of the petroleum fraction known to refiners as gas oil" was used as a cracking stock. This raw material was mixed in a work tank with partially cracked or recycle fractions in the ratio of 52.7 parts by weight of fresh charge to 47.3 parts of recycle stocks. The manner in which these recycle fractions were obtained will be described in detail later.

The above mixture was pumped at the rate of 355 lbs. per hour through a. bank of preheating or vaporizing tubes arranged in series. These tubes were so mounted in a furnace setting as to be heated largely by waste gases passing from the main combustion chamber of the furnace to its stack. The oil mixture was heated to 345 to 350 C. in the preheating section and was discharged at this temperature into the lower part of an evaporator. This was a vertically mounted steel vessel with a segment above the oil inlet filled with packing rings. Eighty-two lbs. per hour of steam were also admitted to the evaporator. The high boiling constituents of the hot oil mixture dropped to the base of the evaporator and were withdrawn and cooled at the rate of 33 lbs. per hour. Since the constituents of the recycle stock were relatively low boiling, the evaporator bottoms were mostly uncracked high boiling ends from the fresh charge. The vaporized oil and steam passed overhead through the bed of packing. left the evaporator at 280 C., and then passed serially through the tubes of a drying coil in which the temperature of the vapors was raised to 470 C. After leaving the drying coil, the vapors were transmitted to a bank of cracking or conversion tubes which were arranged in series around the top wall of the main combustion chamber. The temperature of the vapors was raised rapidly to 700 C. in the cracking zone and was maintained at substantially this temperature in a final or soaking bank which was also situated in the radiant section of the furnace.

The hot reaction products from the soaking zone were discharged to an arrester chamber and there intimately mixed with a cold spray of about 550 lbs. per hour of a light, distilled, hydrocarbon fraction. This procedure gave a temperature of about 270 C. at the arrester exit from which the mixture of quenched reaction products and vaporized quenching stock passed to the base of a. fractionating tower. The quenched products from a second or gas cracking furnace, to be described later, were also received by the same tower. The heavy, tarry products of oil and of gas cracking accumulated at the bottom of thetower, and 44.9 lbs. per hour, suitable for fuel oil, were withdrawn and cooled. The lower boiling materials, including gas, steam, constituents in the gasoline boiling range, and unconverted charging stock were taken off overhead a vapor and passed into the base of a second fractionating column. Gas, light distillate, and steam were taken off the top of the second tower while higher boiling materials were discharged from its base and cooled. Three hundred and twenty pounds per hour of these cooled bottoms were pumped into the top of the first fractionating tower to serve as reflux there. The remainder was mixed with incoming fresh feed oil and other recycle stock and recharged to the preheating zone of the oil cracking furnace. The overhead vapor from the second fractionator was cooled in a condenser and the resulting mixture of vapor and condensate was discharged to a gas-liquid separating chamber. The two towers and the separator were operated at a pressure of 8 to lbs. per sq. in. gauge. Water, condensed from steam admitted earlier in the process, was released from the base of the separator. Light distillate was pumped from this separator to the top of the second tower as reflux, as well as to the arrester of the oil cracking unit described above and also to that of a gas cracking unit to be described below. The excess, or make of arcmatic distillate, was removed from the separator at the rate of 35.5 lbs. per hour.

Gaseous products of oil and gas cracking, amounting to 2,550 cu. ft./hr. (on water free basis at 0 C. and l atm. pressure) were vented from the top of the gas-liquid separator and conveyed to a gas recovery plant. The crude gas had approximately the following composition.

The gas was separated in this plant into the following fractions: (1) hydrogen and methane, (2) ethylene, (3) a mixture of ethane, Ca, and C4 hydrocarbons, and (4) C4 and higher boiling hydrocarbons. Fraction 4, consisted chiefly of C5, C0, and higher hydrocarbons but also contained butylenes and butadiene. This fraction was sent to the work tank holding charging stock for the oil cracking operation and was continuously mixed there with fresh feed oil and recycle stock from the base of the second fractionating tower. The composite of the above three stocks made up the charge to the oil cracking unit and was processed in the manner previously outlined. The unsaturated constituents of fraction 4 produced additional gas and liquid products and also under weni pply r n t ammatics when recycled through the oil crackingprocess. The aromatic compounds in this recycle fraction, however, were more refractory than the olefinic materials and passed through the cracking process substantially unchanged, thus favoring the production of a distillate predominantly aromatic in character.

Fraction 3 above, contained most of the ethane and Ca and C4 hydrocarbons present in the raw gas admitted to the gas recovery plant, as well as small percentage of C5 hydrocarbons. This fraction was vaporized and forced, together with 28 lbs. per hour of steam. into a bank of preheating tubes in a separate gas cracking furnace. The mixture of steam and hydrocarbon vapor was heated to 700 C. in the preheating zone and was then passed serially through the tubes of a cracking or conversion coil in which the vapors reached a temperature of 770 to 780 C. As in the case of the oil cracking furnace, the preheating coil was heated largely by convection and the conversion coil chiefly by radiation. Approximately 50% of the aggregate charge was converted to products other than C2He. C3, and C4 hydrocarbons in a single passage through the gas cracking furnace. The temperature of the hot cracked products was dropped almost instantaneously to 115 C. by quenching with cold distillate pumped from the gas-liquid separator into an arrester at the exit of the gas cracking coil. The quenched products joined a similar mixture from the oil cracking operation and both were fractionated into gas, light aromatic distillate, recycle stock, and refractory fuel oil in a common separation system.

It has been previously mentioned that the bottoms obtained from the evaporator used in the oil cracking operation were substantially unvaporized fresh feed oil. The following yields were obtained based on parts by weight of vaporized fresh feed: I

Cal-I4 28.3

Aromatic distillate 23.0

Refractory fuel oil 29.1

Prior to losses in gas recovery plant.

The aromatic distillate contained approximately 37% benzol, 26% toluol, and 20% xylenes.

It should be understood that the successful application of this invention is not limited to the use of a single raw material. The use of gas oil has been mentioned but other stocks such as crude petroleum and topped or reduced crude may also be employed. A viscosity breaking" operation may be used to advantage in conjunction with the oil cracking step if the raw material has a high pour point and high viscosity.

Steam was used to adjust the contact time in the cracking sections of the oil and gas furnaces mentioned in the foregoing example. The process, however, will function adequately without steam and its use is optional.

The invention pertains to improving the yield of oleflns and aromatic hydrocarbons and is applicable to any cracking process where these products are obtained. In the cracking of hydrocarbons' at temperatures in excess of 600 C. oleflns and aromatic hydrocarbons are produced in substantial quantities, this invention is particularly applicable in such instances. The upper temperature limit or the cracking reactions in which oleflns and aromatic hydrocarbons are produced depends upon the length of time the hydrocarbons to be cracked remain under the cracking conditions. The shorter the time of cracking the higher the temperature that can be used thus temperatures of about 900 C. have been used and it is possible to use even higher temperatures if the length of time of cracking is properly adjusted. In the tubular type cracking unit it is preferable to operate the gas cracking step at higher temperatures than the oil cracking step, usually at least 50 C. higher. When the oil vapor is cracked in the tubular type units described herein the cracking step may be carried out at 650 to 750 C., preferably at 690 to 710 C. and the time-temperature relationship is so adjusted that 40 to 60% of the total vaporized oil charge is cracked in a single passage through the cracking zone. It is also desirable to operate the gas cracking step in such a way that a per pass conversion of the same order of magnitude is achieved. It is possible that the gas cracking step could be carried out at a temperature the same as or lower than the oil cracking step provided that the length of time of cracking in the gas cracking step is properly adjusted to produce oleiins and aromatic hydrocarbons.

It is desirable to maintain high vapor velocities in the high temperature zones to avoid overheating oi cracking tubes and deposition of coke. It is also important from the standpoint of continuity of operation to efiect an efllcient separation between cracked tar and lower boiling materials. The necessity of avoiding carbonaceous material in recycle stocks is familiar to those versed in the cracking art.

The production of ethylene has been emphasized in the earlier description but other hydrocarbons, such as the lower olefins, can also be readily produced. For example, any part or all of the propylene in the raw gas may be separated in the gas recovery plant and withdrawn from the process. In this case, C2Hc and C4 hydrocarbons, and any C3 hydrocarbon not removed serve as charging stock for the gas cracking operation. The process is equally flexible as regards the production of ethane or of C4 hydrocarbons. The production of an aromatic distillate rich in benzene, toluene, and xylenes has been particularly mentioned but higher aromatics, including naphthalene, may be obtained by operating the fractionation equipment so that distillate of a higher end point is produced.

It has been pointed out that yileds of aromatics and of gaseous products of lower molecular weight than C4 hydrocarbons can be improved by recracking C5 and higher hydrocarbons recovered from the raw as produced by oil or oil and gas cracking. It is also advantageous to include some C4 hydrocarbons in the recycle mixture, especially if these are rich in butylenes and butadiene. Another satisfactory method is as follows: (1) the high boiling hydrocarbons are recovered from the raw gas, such as obtained from the gas and liquid separator 33 or 3, (2) the Co and higher constituents, which are predominantly aromatic in character, are separated and withdrawn from the process, and (3) the Cs or C5 and some C4 hydrocarbons which are predominantly oleflnic, are recycled for further cracking and polymerization. An alternative procedure is to recover the high molecular weight constituents of the raw gas, such as obtained from the gas and liquid separators 33 or I 33, and separate from this fraction and from light distillate from the gas-liquid separator, a C6 and higher fraction rich in aromatics and a lower boiling fraction rich in olefins. The latter, which may containchieiiy C5 or C5 and C4 hydrocarbons, is recycled to the oil cracking operation. The former fraction is withdrawn and processed to recover benzene, toluene, xylenes, etc. These modifications in character and methods of obtaining the light recycle fraction are equally applicable to single stage oil cracking or multiple. stage oil and gas cracking.

The recracking of C5 or mixtures of C5 with Cl and/or higher molecular weight hydrocarbons recovered from products of cracking increases the yields of gaseous oleflns and of aromatics.

It also greatly increases the concentration of benzene, toluene, and xylenes in the crude arcmatic product and produces this fraction in a relatively pure state from which undesirable contaminantssian be easily removed by well known" methods. A very important advantage of this invention is that the above results are obtained under such conditions that continuous operation is possible for long periods of time. It ha s,'for example, been found extremely difiicult, when the recycling feature outlined above is not employed, to obtain an aromatic fraction in equivalent yields and quality without encountering one or more of the following difficulties: (1) choking of equipment with coke, (2) overheating and burning out of cracking tubes, (3) an excessive production of hydrogen and methane at the expense of more valuable olefins, and (4) the production of heavy tar which is unsuitable for fuel oil. This invention effects marked improvements in yeilds and quality of valuable products and, at the same time, avoids all or the above, well known difiiculties.

As many-apparently widely difierent embodiments of this invention may be made without departing from the spirit and scope thereof, it is to the hydrocarbons to be cracked a hydrocarbon fraction comprising essentially C5, C6, and C7 hydrocarbons.

2. In the process of cracking the vapors of a hydrocarbon oil boiling within the range of gas oil so as to produce lower olefins and aromatic hydrocarbons by passing hydrocarbon oil vapor through a cracking zone at a temperature between 650 and 750 C., the method of improva mixture of C4,

ing the yield of said lower olefins and aromatics which comprises separating from the cracked gaseous product a fraction consisting of a mixture 0104, C5, C6, and C1 hydrocarbons, and returning at least a portion of same to the oil cracking zone.

3. In the process oi? cracking at temperatures between 650' and 750' C. the vapors of hydrocarbonoilboilingwithintherangeotgasoilso astoproducelowerolefinsandaromatichydrocarbons, the method of improving the yield of saidlower olefins and aromatics which oomprisesseparatingthedesiredlowerolefinsand aromatics from the cracked gases and then separating two hydrocarbon fractions, one comprising ssentiallyamixture oiCaCa. and Cihydrocarbons, and the other comprising essentially a mixture of C4, Cl, Ca, and C1 hydrocarbons, cracking at least a portion oi the C: to C fraction at a temperature higher than the temperature used in the first cracking step and returning at least a portion of the Ca to C1 hydrocarbon fraction to the iirst cracking step. t

4. A process for the production 01 ethylene and aromatic hydrocarbons which comprises cracking at a temperature between 650' and 150' C. normally liquid hydrocarbons of petroleum oil boillngwithintherangeorgasoilsoastoproduce ethylene and aromatic hydrocarbons, cracking in a separate step at a higher temperature ethane and 0s and C4 hydrocarbons, separating into separate fractions from the products of both cracking steps a hydrocarbon traction consistingoiamixtureofethaneandcs and C4 hydrocarbons and another traction consisting or a mixture of 04, Ca, Cs, and C1 hydrocarbons, renirning at least a portion of the first-mentioned fraction to the last-mentioned higher temperature cracking step and recycling at least a portion of the assess:

last-mentioned traction by admixing m with the fresh feed to the first-mentioned materials in approximately the same boiling range, also separating a light aromatic distillate traction, and finally separating the resulting cracked gaseous products into the following tractions-a fraction containing hydrogen and methane, a second fraction containing ethylene, a third fraction containing the aromatic hydrocarbons, a

iourth fraction containing predominantly ethane and Cs and C4 hydrocarbons, and a filth traction consistingofamixtureoiC4.Cs,Ca,andC-|aliphatic hydrocarbons; cracking at a higher temperature the fourth traction and recovering from said cracked fraction additional ethylene and aromatic hydrocarbons, and returning to the first cracking step the filth fraction together with the unconverted raw material and constituents in approximately the same boiling range.

DALE I". BABCOCK.

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