US2423637A - Process and apparatus for the catalytic cracking of a hydrocarbon oil - Google Patents

Description

E. F. CHANDLER PROCESS AND APPARATUS FOR THE CATALYTIC CRACKING OF A HYDROCARBON OIL Filed March 17, 1943 2 Sheets-Sheet 1 July 8, 1947.

INVENTOR w a July 8, 1947. E. F. CHANDLER PROCESS AND APPARATUS FOR CATALYTIC.

CRACKING .OF A HYDROCARBON OIL 2 Sheets-Sheet 2 Filed March 17, 1945 v INVENTO W Patented July 8, 1947 UNITED STATES PATENT OFFICE PROCESS AND APPARATUS FOR THE CATALYTIC CRACKING OF A HYDRO- CARBON OIL 2 Claims.

This invention pertains to methods of, and apparatus for, treating mineral oil. 'Ih'e invention further relates to methods of, and apparatus for, improving the yield of desirable products obtained from petroleum hydrocarbons. More specifically, the invention provides an improved process for the conversion of higher boiling fraction into lower boiling constituents suitable for use in the production of motor distillates, motor fuels, and the like. Preferably, the desired molecular rearrangements of the oil compositions are brought about thermally, in the presence of a suitable catalyst. The reactions or inter-actions are desirably caused to take place in the vapor phase, and with or without pressure within the conversion zone. However, the present invention is applicable to operations in the liquid or vapor phase, and in the absence of a catalytic agent, chemical promotor, or the like.

An important feature of the invention resides in preparing the oil that is fed to the process in such a manner as to insure the deliveryto the conversion zone of an optimum composition selectively prepared for the most economical and efiicient treatment under the controlled conditions which have been predetermined and established within the conversion zone. The original oil charged to the process, for example, may be composed of fractions having boiling points ranging from approximately 96 degrees F., to possibly 790 degrees F., or thereabouts.

In its simplest form, the present process includes the step of dividing the charging oil into a plurality of oil streams, each comprising a substantially related but relatively narrow range of boiling points. For example, stream (A) may be composed of the fractions ranging from 96 deg. F., to 342 deg. F. Stream (B) might contain the fractions having boiling points ranging from about 381 deg. F. to 453 deg. F. Stream (C) may comprise the fractions boiling at from 586 deg. F., to 688 deg. F., and stream (D) might be composed of the fractions between 714 deg. F. and 790 deg. F.

For the purpose of establishing the characteristics of the several streams, as above, the original charging oil may be vaporized and delivered to a suitable fractionating tower which is adapted to deliver, as side streams, the cuts indicated as A, B, C and D, in a manner Well known to the art. These streams, in turn, preferably are vaporized and the vapors separately supplied to individual conversion zones wherein the vapors ar submitted to treatment which has been selectively established as most suitable for each set of stream characteristics. By this means, a method is afforded for the continuous treatment of a specific charging stock, and at all times during the operation, each fractional group, composing the whole, may be kept under close process control with respect to temperature, pressure, etc.; individual conversion zone conditions may be selectively modified and adjusted, manually or automatically, during the operation.

The vapors issuing from each of the several conversion zones may be individually dephlegmated for the purpose of condensing and collecting the lower boiling hydrocarbons produced, or the vapors issuing from the several conversion zones may b all delivered to a single dephlegmating or fractionating tower, and the lower boiling hydrocarbons resulting from th several individual conversion reactions mixed and. recovered. Preferably, while the lower boiling hydrocarbons would logically be removed after each pass through a conversion zone, the heavier hydrocarbons which condense in the dephlegmator or fractionating column are re-cycled for further treatment. Depending upon the method of operation and/ or upon the results desired, the hydrocarbon to be re-cycled may be delivered to, and mixed with, the original charging oil; may then be selectively again passed through the same conversion zone as an addition to the hydrocarbons being delivered thereto, or the re-cycle stock from one conversion zone operation may be delivered to, and re-cycled through, one of the other conversion zones.

For example, one section of the system, may be operated continuously on virgin charging stock to produce only converted or cracked hydrocarbons of a certain type, while the re-cycle stock from this operation is blended with, and passed to, another conversion zone to produce a mixture of cracked products, etc. Preferably, as pointed out, the vapors are thermally treated in the presence of a suitable catalytic agent or agents within the conversion zones. It will be understood that any suitable catalyzer or chemical promotor best suited for producing the particular results desired may be employed.

It has been pointed out herein that the process gives good results when operated in the vapor phase, without the aid of a catalytic agent, as compared with the usual vapor phase operating methods. The improved system of selectively treating the oil results in increased yields of lower boiling hydrocarbons with reductions in the generation of fixed gases and reduced deposits of troublesome carbon. However, in the preferred form, the invention is particularly important as affording an improved method of operating processes in which catalytic agents are employed. In this connection, the several advantages of catalytic hydrocarbon conversion are fully realized. The process may be economically and efficiently operated at relatively low temperatures, the average of the several conversion zOne temperatures ranging between 800 and 950 deg. F., more or less. An opportunity for a more complete catalytic action i afforded without increasing the over-all speed of the process, and losses due to under and over treating certain portions of the charge may be practically eliminated.

By maintaining optimum conditions within the conversion zone for the most eficient treatment of a certain hydrocarbon group, selectively assur- 4 or carbon which is freed, due to slight maladjustment of the several predetermined optimum conditions of either the group of fractions being fed to the conversion zone, or of conditions within the zone itself, will exist in the nascent state and is readily re-combined under the thermocatalytic conditions of the conversion zone to form new hydrocarbons, with a minimum loss in the form of non-condensable gaseous products. This building-up effect can be further enhanced,

if required, by enriching the oil being delivered to a particular conversion zone, by the addition ing the presence of this group as the optimum charge being delivered to the particular conversion zone, improvements are obtained in the qualityand the quantity of the yield of low boiling hydrocarbons which indicate new and important advances in the art of catalytic hydrocarbonrconversion. The method of selectively staging the thermo-catalytic treatment of the charging stock in predetermined cuts having substantially optimum characteristics, with respect to the particular treatment within each said stage, affords a degree of flexibility of process control far beyond anything heretofore obtainable in thoseprocesses in which it is customary to subject a comparatively wide hydrocarbon fractional range to the relatively limited reactive scope of the catalyst.

It is well known that when hydrocarbon vapors comprising a wide range of fractions of different boiling points. are contacted with a catalyzer within a heated zone, only a limited few of these fractions fall within the optimum range of thermo-catalytic conversion activity, with the re sult thatany fractions above or below this relatively limited range do not respond to the'desired treatment. The more refractory constituents may pass through the zone substantially unchanged, while the less refractor fractions are converted largely into free hydrogen and cokeforming products of carbon. It is to avoid these and other well-known objections to processes of the presently used types that this method of multi-selective conversion has been devised.

While the process herein described has been designed to overcome the objections outlined, actual experience has shown that the broad method employed aifords inherent operating advantages heretofore obtainable only by the employment of separate, independent means. For example, experience has shown that no matter how carefully the effort is made to adjust the conditions of optimum charging stock to predetermined optimum thermo-catalytic treating conditions, there will be some products of side reactions. The present process, by the method described, reduces the results of these eflectsto the minimum, but which nevertheless results in the release of hydrogen from some of the hydrocar bons with a resulting dropping out of carbon. However, it has been found that in the present process, losses due to these causes are practically absent in the conversion zone handling the least refractory group of hydrocarbon fractions, probably because of the greater ease with which optimum operating conditions can be established and maintained.

In the conversion zones which handle the more refractory groups of hydrocarbons, any hydrogen thereto of suitable hydrocarbon fractions selectively obtained from the virgin stock being initially delivered to the process, and preferably having a sufficiently high hydrogen content to meet the deficiency mentioned. A similar result may also be obtained by utilizing a re-cycle stock from one of the conversion zones found to possess the desired composition.

As in thermal cracking, long-chain hydrocar" bons above the gasoline boiling range are broken down into smaller molecules which boil within the gasoline range. Cracking, which takes place in the presence of a catalyst, is generally accomplished at relatively lower temperatures, and either at atmospheric or relatively low pressures above atmospheric. A suitable catalyst serves not only to facilitate the ruptured hydrocarbon chains, but also controls the mechanism of their destruction along certain paths to form less olefinic materials and more materials of the branched-chain and aromatic types, which dis- 7 play improved anti-knock characteristics.

In catalytic processes of the usual type, a gasoline yield of about 45% is obtained, based on the charge to the catalyst. Also, from practically any stock employed, the octane number will range from about 71 to 81. In operating the process herein described, the average yield is considerably increased, ranging between 45% and 52%, and having an octane number of the order of from 78 to 82. The increase both in the yield of gasoline and anti-knock rating is attributed to the greater economy and eificiency of the selective system of operation wh ch likewise affords other improvements in performance, as, for example, a relatively longer cycle of continuous, uninterrupted operation and a reduction of the usual quantity of fixed gas andlow-grade end products.

Further economies and advantages in the overall performance may be obtained when the presently-described process, operating preferably catalytically, is employed in conjunction with the system described in my co-pending application Serial No. 455,553, filed August 21, 1942, Conversion of Petroleum, in which case the heavier liquid hydrocarbons resulting in the first said process would be delivered to the second said process for further treatment and the production of additional lower boiling constituents by the method described therein. This second process,

may be operated as a straight vapor: phase, thermal-cracking unit, or, as pointed out therein, as a catalytic conversion process.

The operation of this process, in the broad sense, is not necessarily predicated on the use of any specific form or type of catalyzer, many kinds of which are well known as suitable for employment in connection with the treatment of hydrocarbons. It is desirable, however, in view of the method described in which relatively specific groups of hydrocarbons may be selectively set apart and submitted to predetermined,

closely-controlled conditions, that the catalyzer decided upon, or the conditions under which a specific catalyzer is used, be governed by a full consideration of the characteristics of the hydrocarbons to be treated within a given reaction zone and with careful respect to the ultimate results to be obtained.

Probably, for the first time, this pro zess affords the means for continuously running a stream of hydrocarbon oil of highly complex molecular structure, under practically ideal conditions in so far as the several groups of hydrocarbons, separated from the whole, while actually subject to treatment, represent substantially optimum qualities selectively adapted to the treating range of a given set of thermal and chemical conditions called for by the catalyzer or chemical promotor decided upon. For example, in the operation of the process for the conversion of higher boiling hydrocarbons into hydrocarbons boiling Within the gasoline distillate range, good results have been obtained by the use of a catalyst in pellet form so packed into the conversion tubes as to allow the free passage therethrough of the hydrocarbon vapors. These pellets were composed of kieselguhr to afford a base having ultramicroscopic capillary pores combined with an active agent such as hydrogen-reduced. iron particles. Mixed, for example, with sodium hydrate and water to form a paste, the pellets are formed and baked to hardness. When completed, they are extremely porous and absorptive at elevated temperatures and in the presence of hydrocarbon vapors.

The composition, particularly with respect to the active material, responds to wide modification to meet any required catalytic or chemical promotional effect. During these operations, good results were obtained using the same type of catalyzer in each selective stage, by modifying the temperature and timing of the reaction period in favor of the particular group of hydrocarbons being passed therethrough. The catalyzer has been found to be highly resistant to poisoning, will operate efficiently over long periods, and is easily and quickly re-activated in an atmosphere of super-heated steam. It should be understood that all phases of operation of the process should be accurately and closely governed. Careful control of. temperatures is important and should be automatically maintained. The rate and quantity of oil feed should be carefully regulated, both with respect to the charging stock as well as the proportioning and distribution of side stream and re-cycle stocks.

Usually, in operating cracking processes of the continuous type, the oil is pumped through the system at a relatively constant, uniform rate. Experience with the present system has shown that while this method may be employed, certain important advantages can be obtained by pumping the oil through the system in uniform impulses or waves. In this manner, a distinct dwell is afforded to the vapors in the conversion zones, and while they are in contact with the catalytic agents. This dwell, while relatively brief, affords an opportunity for the completion of the reaction period, the products of the reaction being ejected, as it were, by the next impulse bringing fresh hydrocarbons into the conversion zone. The timing of the impulses may be such that the over-all rate of feed is not reduced.

In the drawings, which illustrate schematically several alternative forms of the method of the present invention,

Fig. 1 shows schematically one arrangement of apparatus for carrying out the present invention; and

Figs. 2, 3 and 4 show alternative forms.

In Fig, l the charging stock is delivered by pipe 2 to the pre-heater coil 3 positioned at the upper end of tower ll. From the lower end of pre-heater coil 3, the oil is delivered to vaporizer 4 by pipe 5. Then by pipe 6 the charging stock is conveyed to expansion chamber '1 at the lower end of tower H. The lighter non-condensing products pass through opening 8 in a vapor state, and some of the higher boiling fractions condense in tower H, fall back upon tray 9, and are delivered by pipe It to a heater coil 82 where the same are vaporized and delivered to the heated catalyzer zone 3. The products which are converted in zone l3 are delivered by pipe M to heat exchanger l5 Where, preferably by indirect heat exchange, as shown, the oil passing through pipe l is vaporized. Lower boiling products escape from tower ll, through pipe l0 and are delivered to tower ll at a point above tray l8 where they are filmed on suitable plates !9.

The heavier products collect on the tray l8 and are drawn off through pipe 20, and thus may be passed through heat exchanger 2! to be carried off by pipe 22 to oil heater or vaporizer 23.

From this vaporizer, the vapors are delivered to the, second catalyzer zone 2%. The heated, converted products of this reaction pass through pipe 25 to heat exchanger l5, which also supplies heat to coil 4. The mixed and blended heated products from heat exchanger 55 then pass through pipe 26 into the lower end of tower H, and thence upwardly through perforated tray I8. The lighter vaporous constituents issue through tray I 8 to heat and vaporize the oil being delivered into the tower by pipe l6, and are aided by the filming action of plates I9. The lower boiling hydrocarbons which may be within the motor distillate range leave the tower l! in vapor form through pipe, 28, and thence pass through suitable condensing means 29 in a manner well known in the art;

In this: arrangement, it will be noted that conversion zone i3 is operated exclusively upon by drocarbons which have been condensed out of the originalv charging stock and hence are substantially virgin uncracked oil, whereas conversion zone 24 is operated on a mixture of cracked and uncracked hydrocarbons. It is understood that some thermal cracking may take place during the vaporization within the tube 4. The products which, pass through zone l3 being less refractory, may be subjected to a milder thermo-catalytic action, while the products passing through zone 24' being more refractory, may be subjected to a more intense thermo-catalytic action. For example, during the passage through zone l3 of the vapors from the oil vaporizing coil l2, sufficient heat would be delivered thereto by a furnace (not shown) to afford a vapor phase conversion temperature of the order of from 850 to 950 deg. 9., while the heat delivered to the oil vapors passing through zone 24 would be sufficient to raise their temperature to the order of 1,000 to 1,050 deg. F., the exact temperatures depending, as herein pointed out, upon the kind of oil being treated, the type of catalyzer employed (if any), and other recognized factors. The process has been successfully operated at substantially atmospheric pressure. Pressure, however, may be employed should the kind of oil and other operating conditions so require. Pressures of the order of from 2 to 5 atmospheres have been employed, and higher pressures are obviously possible. Preferably, only such pressure is retained on the system as results from the oil being delivered thereto through pipe 2 by suitable pumping means (not shown), which may not be substantially above atmospheric, as set forth. An important advantage of the method herein described employing two or more selectively adjusted thermo-catalytic Zones to which the vapors of predetermined oil cuts are delivered, resides in the fact that the cracking action in each zone is under conditions conducive to the maximum efliciency. There is a minimum of fixed gas and polymerized material formed. Depending upon the kind of oil being treated, the feed rate may be of such order that the vapors are under treatment within the respective zones for from 5 to 300 seconds, more or less. The conversion in each vapor phase zone is conducted under conditions ascertained by test or experience to be best for the particular cuts, and may be so adjusted as to produce cracked products having an average molecular weight ranging from approximately 30% to 75% of the average molecular weight of the out from which the cracked products are derived. In the preparation of the cuts for thermocatalytic action in the several selectively adjusted zones, suitable tubular heaters, I2 and I3 for example, may be employed in which said oil cuts are subjected preferably only to sufficient heating to vaporize the oil substantially without cracking the same. preferably preheated and vaporized at temperatures calculated to produce a minimum of precracking. The presence of some virgin oil constituents in the vapors which are subjected to more intense action in zone 24 affords, in the presence of a suitable catalyst, the slight excess of hydrogen that may be required to maintain a hydrocarbon molecular balance and prevent the undue release of carbon.

In the arrangement shown in Fig. 2, the charging stock enters by a pipe 35, passes through preheater coil 36 in tower 4|, and then by pipe 31 enters the vaporizing coil 38 in heat exchanger 42. The oil leaves the vaporizing coil by a pipe 39 and enters as vapor in a suitable fractionating tower 40 which is equipped to develop a plurality of side streams 44, 45 and 46, etc. To indicate the possibility of adjusting the characteristics of said side streams, valve outlets 41, 48 and 49 are connected with suitable tray levels within tower 40. It is understood that the purpose of these valved outlets is to permit the selection of side streams of the desired boiling point groups, For the purpose of removing from tower 40 uncondensed vapors of lower boiling hydrocarbons, pipe 50 is shown as leading to a suitable condenser 5|. However, it would be understood that these latter products may, if desired, be fed to a suitable reforming unit for the purpose of improving the anti-knock rating of the distillate and recovery of other desired products.

Side stream 44 is fed to a suitable oil-vaporizing furnace coil 52, and then to conversion zone 53 where it is delivered by pipe 54 to heat exchanger 42. Likewise, side stream 45 is fed to vaporizing coil 56, then to conversion zone 51, and then by pipe 58 to heat exchanger 42. Side stream 46 is similarly fed to heater 60, thence to conversion zone 6|, and by pipe 62 the heated conversion products are delivered to heat exchanger 42. In this manner, the heated products of the several reaction zones blend and serve to The incoming charging stock also is' be employed for this purpose.

8. heat the oil in coil 38 and also the oil in coil 63 as the products pass therethrough and leave the pipe 64, which conveys the oil into tower 4|. The heavier hydrocarbons which condense and collect in the lower part of tower 4| are removed through pipe 65, and are then delivered to coil 63 wherein they are vaporized. Leaving coil 63, the vapors pass to tower 40 by pipe 66, in which latter tower they mix and blend with the vapors delivered by pipe 39.

Tower 4|, like tower 40, is provided with an outlet pipe 61 by means of which the lower boiling hydrocarbons may be removed, condensed and collected, or otherwise treated as desired. In this arrangement, the heating furnaces and sources of heat for coils 52, 56 and 60, being well known in the art, have been omitted. Likewise, the means for selectively heating the conversion zones 53, 51 and 6| are not shown. It is understood, of course, that any well known means may The valved outlet 69 at the lower end of tower 4|] may serve to remove the end products, preferably in liquid form, from the system.

In the arrangement shown in Fig, 3, the charging stock is fed through pipe 10 and enters oilheating furnace 1!, for which 12 represents a suitable source of heat for vaporizing the oil within the furnace coil, and from which, by means of pipe 13, the vapors are discharged into tower 14. This tower may be of any suitable type and is adapted to deliver the selectively chosen side streams 15, 16 and 11, which are composed of hydrocarbons condensed at different levels in the fractionating system of the tower. These side streams are delivered, respectively, to the vaporizing coils 18, 19 and 80, and from these coils the vaporized products are delivered to the conversion zones 8!, B2 and 83, respectively. It will be understood, of course, that these conversion zones are heated or are otherwise selectively conditioned for the optimum handling of the respective hydrocarbon groups being passed therethrough.

By means of the delivery pipes 84, and 8B, the heated products of the several conversion zones are delivered, respectively, to the expansion towers 81, B8 and 89. At the top of each of these expansion chambers or towers, there are provided take-off means 95, and 91, respectively. In this manner, vapors of lower boiling hydrocarbons may be removed from said towers, the means preferably being such that the same may be adjusted to deliver simultaneously from all of such towers, hydrocarbon vapors having a substantially similar boiling point range. They may, on the other hand, constitute selectively different groups of boiling point ranges. This may be governed by the use to be made of the overhead products from the expansion chambers or towers. For example, they may be mixed, blended, reprocessed, reformed, etc.

The higher boiling hydrocarbons which condense and collect in the towers are removed from the lower part of the respective towers through the draw-ofi pipes 98, 9| and 92, all of which are shown as feeding into pipe 93 leading to sump 94. The mixed processed hydrocarbons which collect in this portion are pumped over through pipe I00, and are delivered to the oil-vaporizing unit lili. From thence, the vapors are delivered to the conversion zone |02 where, in contact with a suitable catalytic agent and under predetermined thermal conditions, they are treated and delivered to the dephlegmating or fractionating tower I03 and permitted to expand. The lower boiling hydrocarbons areremoved from tower I03 as vapor through pipe I34, and the heavier condensed hydrocarbons are removed from the tower through 1 pipe I and thence returned to the sump from be :those of straight run gasoline, may be withdrawn from the tower through the pipe I I5, and may then be condensed and collected or otherwise treated. The vapors of higher boiling hydrocarbons arepreferably fractionated within the tower and removed selectively as side streams H3, Ill and H8, which, in turn, are vaporized in the oil heating furnaces II 9, I25 and I2I, respectively, at appropriate temperatures, the vapors being individually supplied to the conversion zones I22, I23 and I24, respectively, for treatment under predetermined thermal conditions. This treatment is preferably done in the presence of a suitable catalyzer conditioned for each zone with particular respect to the characteristics of the hydrocarbons passing therethrough.

The heated vapors from zone I22 are delivered by pipe I25 to the expansion chamber I3I, and the vapors from zone I23 are delivered by pipe 126 to the expansion chamber I32. Vapors from the zone I24, leaving by pipe I21, enter the heat exchanger :23 to supply heat to the coil I30 therein. The vapors leaving heat exchanger I28 by pipe I29 enter the expansion chamber I33. It will be noted that each expansion chamber is provided with an overhead outlet pipe through which the vapors of low boiling hydrocarbons may be removed. By means of pipe I35, higher boiling fractions condensing in chamber I3I are delivered into the expansion chamber I32, preferably in direct and in indirect heat exchange relationship with the incoming heated conversion vapors from zone l23. Likewise, the higher boiling fractions which condense in the chamber I32 are delivered into chamber I33 in heat-receiving relationship to the heated vapors from I23.

The higher boiling hydrocarbons which condense and accumulate in I33 are removed and delivered through pipe I43 to the oil heater I4I where they are vaporized and delivered to the reaction zone I42 for thermo-catalytic conversion. The heated products from 142 then enter the tower I 43, from which tower lower boiling hydrocarbon vapors may be removed through pipe I44. Higher boiling fractions which condense in tower I43 are removed through pipe I45, delivered to the coil I30 for preheating, and then through pipe I43 delivered into the tower I I4 in heat-exchange relationship with the incoming heated vapors from the coil III. By means of the pipe I48, some of the condensed hydrocarbons from I43 may be returned to the system for further treatment through the valved inlet I49 leading to chamber I3I Also, by means of the valved pipe branches I53 and I5I of the pipe I52, some or all of these heavier fractions may be delivered to one or both of the chambers I32 and I33, in a similar manner, for further'heating and treatment.

The several arrangements illustrated in the drawings for carrying out the process of the present invention are illustrative only, and these arrangements may obviously be modified without departing from the spirit or the invention, as defined by the appended claims; These several a-rran'gements have been selected to'show some of the methods of carrying the broad principle into practice.

An underlying principle has been clear- 'ly" set forth in the foregoingspecification; and it will beseen that the several variants each accomplishes in its specific way the important feamm of the inventive idea. It will beu'nders't'ood, of course, that any suitable catalyst may be selected for use in the several heat-treating zones, said catalysts or chemical promotors preferably being adaptedto the particularoil being processed and/or to the particular fractions selectively submitted to said thermal treatment. It will befurther understood that in carrying out the process, suitable means will be employed for temperature control, pressure control, or such combinations'of the .same as may be required.

1. In a process of selectively cracking a hydrocarbon oil stock, the steps comprising preheating and vaporizing liquid hydrocarbon charge oil by indirect heat exchange with hot cracked vapors issuing from first and second thermal catalytic cracking zones, submitting the vapors of said charge oil to expansion and fractionation in a first fractionating zone, removing a lower boiling fraction as a vapor from said first fractionating zone, condensing a higher boiling fraction in said first fractionating zone, and subjecting the condensate, while still hot, to further treatment of direct evaporation and selective cracking in the first catalytic cracking zone for optimum conversion, expanding and dephlegmating the cracked Vapors in a second fractionating zone while in admixture with the previously separated lower boiling point fraction removed from the first fractionating zone, removing motor fuel vapors from said second fractionating zone and condensing them, condensing a heavier fraction in said second fractionating zone, and vaporizing and subjecting to further separate selective cracking said heavier fraction from the second fractionating zone in the second catalytic cracking zone, 00- mingling and expanding the cracked vapors therefrom with those obtained from the first cracking zone, fractionating the mixed cracked hydrocarbons in the second fractionating zone, and recycling the heavier condensate therefrom through the second cracking zone.

2. In a system for selectively cracking a hydrocarbon oil stock, the combination of a first catalytic cracking chamber and a second catalytic cracking chamber, a first heating element and a second heating element associated with said first and said second catalytic cracking chambers, respectively, for vaporizin oil stock prior to its entrance into each chamber, a heat-exchange chamber receiving vapors from such cracking chambers, a heat-exchange element positioned within said chamber, pipe means for delivering the charging stock to the heat-exchange element whereby the charging stock will be preheated and vaporized by indirect heat exchange from the cracked vapors from the catalytic chambers, a first fractionating tower for receiving the preheated and vaporized charging stock, a second fractionating tower, pipe means connecting the first fractionating tower with the second fractionating tower for delivery thereto of the lowboiling hydrocarbons from the first tower, means for withdrawing and condensing such lower-boiling hydrocarbons from said second tower, means connecting the first tower with the first of said 11 heating elements and the first cracking chamber, means connecting the second tower with the second heating element and the second cracking "chamber, and means connecting the heat-exchange chamber with the second tower, whereby lower-boiling hydrocarbons from the heat-exchange chamber are removed and condensed, and higher-boiling hydrocarbons are recycled to the second heating element and second catalytic chamber.

EDWARD F. CHANDLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,972,149 Keith Sept. 4, 1934 Houdry June 8, 1937 20 OTHER REFERENCES Eaton et a1., Critical Temperatures Of Petroleum Oils; Ind. and Eng, Chem; vol. 24, No. 7; July 1932, pp. 819-822.

Sachanen, Chemistry and Tech. 01 Cracking; 1932; pages 32 and 33. (Copy 1n Div. 31, U. S. Patent Office.)

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