US2734021A

US2734021A - Preparation of catalytic feed stocks

Info

Publication number: US2734021A
Authority: US
Publication date: 1956-02-07
Anticipated expiration: 1973-02-07

Description

Feb. 7, 1956 H. z. MARTIN ETAL PREPARATION OF CATALYTIC FEED STOCKS FROM HEAVY HYDROCARBONS 4 Sheets-Sheet 1 Filed Sept. 4, 1951 .d ...um m m m h a@ o m ukn|w Ama AN Mm @muy .TILU H Tm, a |v e nu, Nm ow Iv .mdmQ 4/ 1CL www im Ji mw# ic.. Al om@ N @N m k o mm A@ 2 J @N .XW Huma A m: ma HHMTQ/ J -mw Iv j |v |1L down/Bd A| W.. .F.(IP I u uw .Ii

Feb. 7, 1956 H z MAR-rm ETAL 2,734,021

PREPARATION 0F CATALYTIC FEED STOCKS FROM HEAVY HYDROCARBONS 4 Sheets-Sheet 2 Filed Sept. 4, 1951 Trey-"Z STEAM REcLYciLE Feb. 7, 1956 H. z. MARTIN ErAL. 2,734,021 Y PREPARATION oF CATALYTIC FEED sTocKs FROM HEAVY HYDROCARBONS 4 Sheets-Sheet 3 Filed Sept. 4, 1951.

Feb. 7, 1956 H. z. MARTIN ETAL 2,734,021

PREPARATION OF CATALYTIC FEED STOCKS FROM HEAVY HYDROCARBONS Filed Sept. 4. 1951 4 Sheets-Sheet 4 f @As Qasmuum w+- 2 fia?.

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United States Patent O rnErARArIoN or cArALyTrc FEED sTocKs rnoM nnAvY HYDnoCARBoNs Homer Z. Martin, Cranford, .lames W. Brown, Elizabeth, and Harvey E. W. Bumsde, Locust, N. 3., assignors to Esso Research and Engineering Company, a corporation of Delaware Application September 4, 1951, Serial No. 244,968

3 Claims. (Cl. 196--49) This invention relates to a process for the production of valuable distillates by partial evaporation and cokng of heavy or residual hydrocarbons and by subsequent catalytic cracking of the vaporized products. More specifically the invention relates to a combination process wherein a vapor fraction suitable as a catalytic cracking feed stock is obtained by briefly contacting a residual oil with a het medium under conditions conducive to minimizing thermal cracking. The resulting vapors are promptly passed to a catalytic cracking zone before they become substantially degraded by undesirable polymerization reactions or by thermal cracking, while kadditional amounts of distillate are obtained from the unvaporized portion of the residual oil as well as from any recycle stock by drying or cokng in a dense iluidized bed of finely divided solids.

Continually increasing demand for distillates such as high quality gasoline and gas oil has stimulated efforts toward adapting relatively undesirable heavy residues such as topped or reduced crude or various analogous pitches for eventual use in catalytic cracking processes. However, because of the excessive carbon forming tendencies of these materials andtheir high content of catalyst contaminants, their direct use in catalytic processes has proved impractical. Accordingly, efforts to utilize these materials have largely consisted of segregating therefrom thermal gasoline and relatively ,light gas oil fractions by various applications of vacuum flashing or cokng or both, only the gas oils having been considered as satisfactory feeds for catalytic conversion. However, conventional vacuum ashing has been only moderately successful to date since at the usual vacuum. still temperatures of about 700 F. the yield of low value bottoms product is quite high, whereas at higher distillation temperatures the problem of maintaining a satisfactory vacuum becomes most difficult. Moreover such high temperature distillate product has been found to be of substantially poorer quality because of fairly extensive thermal cracking which unavoidably occurs in conventional high temperature distillations.

The various techniques previously proposed for the conversion of residual oils by cokng also have left much to be desired. For instance, cokng at about 9506 F. has been found to produce a fairly desirable feed stock for catalytic cracking, but the rate of reaction is very slow so that it is necessary to provide large, costly reactor vessels. Also, where the usual.' fluid solidsV technique is employed, a large solids hold up for fluidization is also required and-occasionalfluidization difficulties may be encountered. On the other hand, when higher cokng temperatures such as ll'OOf F. are employed so as to obtain an increased rate of reaction, difficulties are encountered in devising a practical method for removing the resulting vapors from the coking zone before they become degraded'by'various after-reactions. Specifically, it has been determined that an excellent feed stock for catalytic cracking can be obtained by cokng residual oils at 11'00" F. nalluidbed, if the vapori'zed feed is removed- "ice from the fluid bed and quenched, and preferably introduced in less than one second into the catalytic cracking zone. However, it is apparent that this is not feasible in conventional operation with deep uidized beds which hold a considerable amount of the vaporspfor up to 15 or more seconds in a dilute phase above the bed before the vapors are disengaged and withdrawn. The dilute phase is incorporated in the usual fluid bed reactor to decrease entrainrnent of powdered solids in the gaseous reaction products.

t has also been proposed previously ,to minimize undesirable thermal cracking in the conversion of residual oils by employing a two-step process wherein the feed is' first subjected to high temperature for a brief period, followed by separation of vapors and further cokng of the'unvaporized portion of the feed, all vapors being finally fractionated to yield the desired distillate fractions such as thermal gasoline and gas oil for use as a catalytic cracking feed stock. However, even this process has had certain shortcomings since the resulting products', Aand especially the catalytic cracking feed stocks, have been found to degrade appreciably not only during distillation but even on standing at room temperature.

lt has now been found that residual oils can be converted to valuable products in a process in which they are treated rst under conditions leading to very rapid volatilization with a minimum of thermal degradation, followed by thermal cracking of the unvaporized portion of the oil to produce additional vapors suitable as catalytic feed stock; and further followed by prompt Vcatalytic cracking of the overhead' products thus formed. Minimizing of the time involved in transporting' the volatilize'd feed to the catalytic cracking zone is one of the major points of this invention.

The first step of thisk process may take several forms, depending on the means chosen for the attainment of maximum rapid volatilization. One convenient way is treatment at relatively high temperature, say 1050 to ll50 F. at substantially atmospheric pressure, in a socalled transfer line uid solids contacter. The time of exposure ofthe bulk `ofthe vapors to thisv temperature is limited to a maxim-um of about 1/10 of a second at temperatures of about 1-'150l F. or about l to 3 seconds at lower temperatures such as 10506 F'. In this' step the residuum is contacted with inert powdered solids on which the usual carlcgnaczeousv material, which is formed as' one of the products of thiskk kind of operation, is deposited. ln addition, the metallic constituents which cause considerable deactivation of the catalyst in the' subsequent operation are also deposited on the inert solid and, in this way, are effectively prevented from entering .the catalytic conversion zone. The inert s'olid material in this invention is generally circulated along with the residuum, and has the further purpose of supplying the he'at for the vaporization and mild' cracking ofthefeed stock.

The second form ofthe first step of this invention consists ofy a lower temperature treatment kof the' residuum, say 8,50 to l000 F. Although the time of Contact is maintained low, as in the higher temperature treatment, it is not necessary tov maintain it quite as low as in the higher temperature treatment, and contact times of' up to about 5 seconds may be permissible' at about 850' F. However, in order to attain theV desired high degree and high rate of vaporization, large quantities of insert gas are used to reduce the partial pressure of the hydrocarbon feed or instead a relatively high vacuum, which may range from about 0.5 to' 5 inches Hg', may be applied to the vaporizingf zone. This secondmethod of treatment may include the circulation of solid particles onfwhich cokel and metallic impurities may be deposited, or in some cases this coke circulation may be dispensed with.

In both of these .vporizing operations, a residual amount of unvaporized oil will remain, which may range from about 2 to 20 weight percent of the feed in the case of the so-called high temperature treatment and may range up to about 50 or 60% of the feed in the so-called W temperature treatment. In most of these schemes, since coke or a similar inert solid is circulated, this residual oil will be deposited on this solid. It is then necessary and desirable to provide equipment for further converting the unvaporized oil and for drying this oily coke so that the latter may be reused in the process. This is done by providing a so-called coker or coke-drying vessel in which the coke may be held for a sufficiently long time to take care of this function. It will be seen then that the present invention has a dual aspect, the principal one being the vaporization of the residuum under relatively mild conditions to obtain a catalytic feed stock and the other being the drying of the coke required in the process. However, since the former aspect requires a short vapor contact time while the latter requires a long holding time for the coke particles, part of this invention consists in the separation of these two operations whose requirements are almost mutually exclusive.

It has been found in addition that the relatively large coke-drying vessel may simultaneously serve another useful function, namely, the thermal cracking of other more refractory stocks. In practically every refinery materials of this kind are available, which could be transformed to useful final products by severe thermal treatment. One of these is the so-called clarified oil, the bottoms product from catalytic cracking of gas oils. ther materials might be the recycle streams from a coking operation, or the bottoms product of thermal cracking.

In particular it has been found that by passing the entire vaporized portion of the feed, including relatively heavy fractions, directly to the catalytic cracking zone without allowing any appreciable thermal cracking, a catalytic cracking feed stock is obtained which, in spite of the very high end point of the feed stock charged to the catalytic zone, surprisingly results in low enough cat alytic coke production therein to be commercially attractive. Also, somewhat unexpectedly, the catalytic feed stock obtained in accordance with this invention produces higher yields of higher octane number gasoline than when only a gas oil fraction is conventionally fed to the catalytic cracking zone after fractional distillation of the vaporized feed into various fractions ranging from fixed gas to heavy bottoms. lt seems that not only does such intervening fractionation harm the stock by allowing certain polymerization and thermal crackn ing reactions to occur, but the presence of the xed gas and gasoline in the catalytic zone apparently improves the antiknock characteristics of the catalytic gasoline produced from the gas oil fraction present in the vaporized feed. Moreover, especially good results may be obtained in this connection by injecting cracking catalyst into the aforesaid vaporized hydrocarbon fraction as soon as practicable after removal of the latter from the vaporization line and before the vaporized fraction is introduced into the catalytic cracking zone proper. A further advantage can be obtained by simultaneously feeding different feeds at proper points in the novel process. For example, a readily crackable feed such as reduced crude may be fed to the quick-conversion transfer line while a more refractory stock such as clarified catalytic cycle oil may be fed directly to the fluid coking zone along with the unvaporized portions of the reduced crude.

Accordingly, it is an object of the present invention to improve the yield and quality of gasoline and gas oil from residual oils. A more specific object is an efficient vaporization of residual oils to produce high yields of valuable distillates suitable for catalytic cracking while only the heaviest residual materials are cracked thermal- 1y.

Still other objects as well as the general nature of the 4 l invention will become apparent from the subsequent description wherein reference is made to the accompanying drawing. In all figures of the drawing the same numerals are used to designate the same or analogous pieces of equipment.

Fig. l is a semi-diagrammatic illustration of a system suitable for carrying out one preferred embodiment of the invention wherein hot solids are briefly mixed in a transfer line with heavy feed, partially to volatilize the latter rapidly by straight evaporation and some very mild thermal cracking, The resulting vapors are charged to catalytic cracking equipment. The solids, containing some unvaporized oil, are sent to a coke drying vessel.

Fig. 2 shows a similar system wherein the hot solids are removed from the uidized zone by overhead entrainment into a superposed transfer line.

Fig. 3 shows an alternative system employing a red coil and a vacuum transfer line to accomplish the partial evaporation with a minimum of coking.

Fig. 4 shows a system wherein the partial evaporation and incidental mild coking is obtained by contacting the feed with hot ue gases.

Referring now in detail to Fig. l of the drawing as an example, a pitch or reduced crude such as an 8% bottoms fraction characterized by a Conradson carbon of about 10 to 30 weight percent and about 5 A. P. l. gravity, and obtained by vacuum distillation of a West Texas crude is used as feed. This feed is supplied at about 300 to 800 F., or preferably at about 650 to 750 F. through line Il into transfer line vaporizer 2 which is maintained at a temperature of about 1050 to ll00 F. Finely divided inert solids such as petroleum coke parti cles having an average diameter up to about 500 microns, preferably between about and 300 microns and heated to a temperature between about 1100 and l500 F. are

also introduced into the vaporizer line Z through line 3 in a ratio of about 5 to 50 lbs. of solids per pound of pitch. As a result, the mixture of solids and hydrocarbon feed in line 2 will attain a temperature of about i050 to 1100 F., and is allowed to remain in the vaporizer line 2 for a period not in excess of about 2 to 5 seconds, preferably between 0.5 and l second, as otherwise an uneconomical amount of the feed would be converted to dry gas.

The effect of vapor residence time and temperature on conversion of West Texas vacuum pitch to dry gas is shown in Table I below. In general, residence times are chosen so as to yield less than l0 weight percent, and preferably less than 5 Weight percent of dry gas on feed, somewhat longer residence times being permissible at lower temperatures as shown in Table I.

TABLE I Time t0 Yield, Time to Yield Typical vapor velocity through vaporizer line Z may range between about 10 and 100 feet per second, and an inert diluent gas such as steam may be introduced through line 9 simultaneously with the feed in order to aid in the control of vaporization and of contact time.

In vaporizer line 2 a substantial portion of the liquid feed is vaporized While the unvaporized portion remains in contact with the solid particles. From vaporizer line 2 the partially vaporized mixture of feed and solids is passed through a dust separator such as cyclone 4 wherein the solid particles surrounded by unvaporized feed are separated from the vaporous portions of the mixture. The total vaporized product is passed Vthrough line 5 to a catalytic cracking unit 6 where, under conditions well known per se, it is converted to desirable distillate fractions such as vgasoline and 'heating oil. 'I-he desired fractions may be isolated from the cracked vapors in distillation tower 7 and other conventional recovery equipment (not shown). `The bottoms fraction obtained in the fractionation tower, and usually referred to as clarified cycle oil after settling or other lseparation of the entrained catalyst therefrom, may be returned to the process and coked as later described inrgreater detail. The catalytic cracking unit may preferably be a conventional fluid bed, though some of the important advantages of the present invention may be obtained also with other known types of catalytic units suc'h as thev so-called suspensoid u nits wherein a finely divided catalyst is employed in the feed vapors in the form vof a dilute suspension; or with a moving bed or a fixed bed unit.

However, it is essential to the success of the present invention not only to keep the residence time in the transfer line 2 in the neighborhood of one second or less, but also to prevent thereafter any undesirable thermal reactions from occurring in the liberated vapors before the latter are brought into the catalytic conversion zone. To this end it has been found especially advantageous to inject finely divided cracking catalyst through line 8 'into the vapors in line 5 as soon as the latter are separated in cyclone 4 from the inert solids and unvaporized feed, so that Whatever cracking occurs in the hot vapors during passage to the catalytic cracking unit is of the desirable catalytic type rather than the inferior thermal type. This procedure has been found particularly convenient in systems employing so-called fluid catalytic cracking units, in which case some active catalyst, preferably catalyst lfreshly regenerated in a conventional fluid combustion unit (not shown), may be readily introduced through line 8 and mixed with the hydrocarbon vapors in line 5 in a ratio of about 5 to 20 lbs. of catalyst per pound of feed.

Where injection of solid catalyst through line 8 may be impractical or inconvenient, as when the catalytic cracking unit is of the fixed bed type, it is important to take other steps for minimizing degradation of the hot vapors prior to the catalytic step. This can be accomplished at least to some extent by prompt quenching of the vapors to at least rl000 F., but preferably to 900 F. or lower, which can be obtained by direct injection of cooling Water or other quenching liquid through line 8, or by indirect heat exchange. However, it must be pointed out that these alternatives are not true equivalents of the aforementioned catalyst injection, promptly after vaporization of the feed. On the contrary, even if the volatilized feed portion is promptly quenched, mere standing or storage of such a quenched feed at room temperatures has been found to cause a loss of `some of its favorable qualities as a catalytic cracking feed stock, especially with regard to dry gas formation and gasoline yield.

This is illustrated in Table II wherein catalytic cracking data are summarized for three different test runs. In each run the same feed, produced by coking a vacuum pitch at 950 F., was passed over a bed of fresh conventional silica-alumina cracking catalyst at 950 F. at 2 v./hr./v., the length of storage of the feed between its production by coking and its introduction into the catalytic cracking test being the only variable.

'TABLE II Eject of feed storage on catalytic cracking yields Test No Feed Storage (Hours) at Room Temp...

Yields from Cat. Cracking It will be seen-that, -as feed storage prior to catalytic conversion is increased, less and less of the feed is catalytically converted to products boiling below 430 F., indicating that a more refractory feed is produced during storage. Furthermore, increased length of storage of this type of feed also significantly reduces the gasoline yields at a given conversion. Consequently, -it is again pointed out that while prompt quenching of the transfer line vapors is vuseful if catalyst injection is impractical at that stage, quenching alone gives no assurance against feed degradation as compared with the effective protection obtained by prompt catalyst injection.

The inert solids containing unvaporized oil, which oil may be equal to about 2 to 20 Weight percent of the original residuum, are passed from cyclone 4 through line 10 to a fluid vessel 1i where they are dried by thermally converting the unvaporized feed to dry coke and vapors at temperatures between about 1050 and ll00 F. Steam or other inert gas is introduced into coking v essel 11 through line 13 at a rate sufficient to produce an upward superficial gas velocity of about 0.5 to 5, preferably about 1.5 to 3 feet per second so as to keep the solid particles in the form of a dense turbulent fluidized bed having an upper level 12. The average residence time of the solid particles in coking vessel 11 may range from about l to l0 minutes. The liberated vapors may be Withdrawn from vessel lll. through line 14 and Acombined with the vapors flashed off in Vaporizer line 2 before introduction into cyclone 4. However, especially if clarified catalytic oil or a similar refractory cycle stock is fed into vessel 1l through line 16, Vit may .be preferable to pass the cracked vapors from vessel 11 through cyclone 17 and line il@ to a separate recovery system, rather than combining them with the hydrocarbon vapors from line 2. The `high-temperature, longer time coking in vessel 11 results in a high conversion and yields a product containing more gasoline of relatively high octane number than in the transfer line, but the heavier fractions constitute a poorer catalytic feed stock. Also, diolefins such as butadiene are produced, which it is better to exclude from the catalytic cracker and subsequent refining equipment.

Some of the hot, relatively dry, coke-containing particles may be withdrawn from vessel 11 through well 30, where residual feed is stripped off by steam or the like introduced at 3i., and finally the stripped, hot solids are passed back to vaporzer line 2 through line 3 as previously described. Excess coke produced in the process may be removed through line 32 or at any other convenient point.

Furthermore, coke-containing solid particles are also withdrawn from vessel 11 through well 19 and standpipe 20, preferably after being stripped of residual volatiles by introduction of steam or the like through line 21. From standpipe 20 the coked solids are passed to a heating zone 23 where the temperature of the circulating solids is raised to about 1100 to l500 F., `or sufficiently to supply the heat required by the reactions proceeding in vaporizer line 2 and coking vessel 11. This can be accomplished in any known manner as, for instance, by introducing an oxygen-containing gas such as air at 26 so that the coke from standpipe 20 is at least partially burned in transfer line 23 and the temperature of the remaining solids is correspondingly raised. Alternatively, or in addition to supplying heat by combustion of the coke produced in the process, the required heat may also be produced in an auxiliary burner 24 in which an extraneous fuel such as fuel gas introduced at 25 is burned with air introduced at 26, and the resulting hot ue gas is then contacted with the solids in line 23. AS a further alternative, the transfer line burner 23 illustratcd in Fig. l may be replaced by a fluid unit similar to vessel 11 wherein the process coke may be fluidized and burned in air in a manner well known per se. 'Some burning of the circulating coked solids has been found generally beneficial since this increases the surface area of the solids and consequently facilitates feed vaporization thereon.

The heated solids from line 23 may be passed to a cyclone 27 where they are separated from the flue gases and returned through dip leg 23 to coking vessel it at a proper rate to maintain the desired coking temperature. Also a part of the heated solids may be aliowed to bypass coking Vessel if and introduced directly through lines 29 and steam line 9 into vaporizer line 2., which arrangement is particularly necessary when it is desired to operate the vaporizer line 2 at a higher temperature than coking vessel 11.

In Fig. 2, another suitable system is shown wherein the transfer line vaporizer is placed directly above the coking vessel so that the inert gas used for fluidizing the solids in the coking vessel can be used effectively in assisting vaporization of the feed in the superimposed transfer line. Referring specifically to Fig. Z, the residual oil is injected through feed injector i into vaporizer line 2 which is again maintained at a temperature between about 1050 and ll F. with the aid of a finely divided solid heat carrier. This heat carrier, together with steam or other inert gas and with the hydrocarbon vapors produced in vessel il, is conveniently supplied to the vaporizer line 2 by entrainment from the coking vessel lll located below, the total gas being sufficient to raise the superficial gas velocity in the relatively small-diameter line Z to a value of at least about ft./ sec., preferably to about to 50 ft./sec. Besides helping to propel the hot solids and oil feed upwardly at the desired rate, the inert gas serves the further useful function of reducing the partial pressure of the oil feed in line 2 and thereby facilitates the evaporation of the feed. in the processes illustrated in Figs. l and 2, about 5 to 25 weight percent of steam on residuum feed to the ecker is typical. In Fig. 2, when it is desired to avoid undue condensation of the coker vapors on contact with reiatively cool feed in feed vaporizing line 2, additional heat may also be supplied to line Z by hot solids from line 29.

Following a coking period of less than 2, and preferably less than one second, the stream comprising diluent gas as well as vaporized feed and the unvaporized portions thereof which get deposited on the inert solids, is withdrawn from vaporizer 2 into a separating device such as cyclone 4. The separated vapors, which represent the desired feed stock for catalytic cracking, are withdrawn from the cyclone through line 5, and after injection of catalyst or quenching liquid at 8, are passed to the catalytic cracking unit 6 and processed in a manner analogous to that already described in connection with Fig. l.

The solids containing the unvaporized portions of the feed, after separation from the vapors in cyclone d, are passed through standpipe it? to the aforementioned coking vessel 11 having a relatively large diameter. There the solids are uidized by means of steam or a similar inert gas introduced at t3 at a rate sufficient to give a superficial upward gas velocity preferably of about 2 to 3 ft./sec., or up to about 5 ft./sec. as is well known per se. Under substantially the same conditions as described in connection with Fig. l, the unvaporized portions of the feed are coked in the resulting dense fiuid bed in vessel il and the resulting vapors, together with the inert gas, are passed from the fiuid bed directly upward into the relatively small-diameter vaporizer line 2 previously described. En this manner, the coke or other solid particles entrained in the aforesaid vapors serve conveniently as a heat carrier for the .f'aporizing step proceeding in line 2. T he degree of entrainment may be adjusted by varying the fluid level il?. as well as by variation of the amount of aeration steam added at i3. in general it is deesirable to have a solids/oil weight ratio of at least 10:1 in the transfer line, as otherwise the solids separated in cyclone 4 tend to be too wet for permitting the desired fluidity in line 10. However, because of this high ratio of solids to oil, in the systems shown in Figs. 1 and 2, it is impractical to remove any substantial amounts of heat from the reaction mixture between the vaporizer line and the coker, and consequently both of these zones are aimost necessarily operated at very similar temperatures. Besides introducing the main feed stock at 1, other hydrocarbon feed such as recycle stock may be introduced directly into the coking vessel il.

Relatively dry solids may be withdrawn from vessel ,til through stripping well 19 and, after mixing with air introduced at Z2, some of the coke deposited in the process may be burned in combustion zone 23 and reheated solids returned through lines Z and 29 to coking vessel il. and/or Vaporizer line 2 in substantially the same fashion as described in connection with Fig. l. Excess process coke may be stripped of vapors in well Si) and withdrawn through line 32. The advantage of this process as compared with that illustrated in Fig. l, is a considerably simpler ow plan while one of its drawbacks is the impossibility of treating the coker overhead separately from the directly vaporized feed portions which latter normally constitute a more desirable catalytic feed stock.

Fig. 3 shows still another system adapted for carrying out of the present invention. According to this embodiment the preheated pitch or residuum at about 700 E. is fed through line l to a fired coil 33 located in furnace Since in this instance substantially all of the required process heat may be supplied to the liquid iced indirectiy from furnace 3d, only a low concentration of inert solids, such as 0.1 to l lb. of coke or the like per gailon of liquid feed, needs to be injected through line 3 as in established techniques of so-called suspensoid cracking, the main purpose of the inert solids being to prevent coke deposition on the coil,

When the desired temperature of vaporization, which may be between about h` and l000 F. is reached in the coil, the mixture of oil and solids is flashed into a vacuum transfer line 2 where about the lightest 35 to 75 weight percent of the feed vaporizes and the non-vapor- 'Zed portion is immediately separated in cyclone 4. Vacuum may be created by a steam ejector 35' connected to the cyclone overhead line 5 or by other conventional means such as vacuum pumps. The absolute pressure in the vacuum line may be of the order of 0.1 to l0, or preferably 0.5 to 5 inches of mercury. The heavy distiliate in line L? is pumped to about atmospheric pressure by pump d, and otherwise treated in substantially the same manner as described in connection with Fig. l. Cracking cataiyst may be added through line 8. The solids concentration in the unvaporized feed portion in line l@ is maintained sufficiently low to permit pumping of the resulting suspension or slurry back to about atmospheric pressure and it is introduced into coking vessel 11 wherein the unvaporized feed is substantially completely coked or thermally cracked in a dense turbulent bed of inert solids fiuidized with the aid of steam or other inert gas introduced at i3. Unlike in the previously described systems, the low solids/oil ratio in line l@ of Fig. 3 permits a fairly independent adjustment of the respective temperatures in line 2 and Coker li. in alternate methods of operation sufcient solids may be circulated so that the residual unvaporized oil forms a relatively low proportion of the mixture being passed to Coker 1i and may be handled by a screw conveyor, or if the mixture is actually free flowing, the solids may be circulated from v ssel #i to vessel by means of iluidized standpipes.

From coker vessel il solid coke, after stripping in withdrawal well 3d, may again be mixed with steam introduced at 9 and circulated back through line 3 for admiring with fresh feed. Net coke may be withdrawn at 32 while coking products may be withdrawn through line lid and processed 'further as previously described with reference to Fig. l. Also, where desired, additional heat` may be supplied to the uid bed incoker vessel. 1I. by

burningand recirculating some ofthe coke i`n.lines.20l

23 and 28` in the manner described in detail in connection with Fig. 1. In fact, furnace 34 may be entirely replaced by this heating apparatus. Alternatively, heat may also be supplied in this or. the other herein described embodirnentsby direct or indirect heat exchange of the fluidized solids in vessel 1i with hot regeneratedcatalyst from. the catalytic cracking unit, as described and claimed incopending applications Serial No. 227,169 (tiled May 19., 1951). and Serial No. 230,746 (liledllune9, 1951).

Still/another embodiment of the inventionV is represented in Fig. 4 wherein a hot extraneous gas ismixed withv the .pitch feed so as tovaporize the latter, the presence of the extraneous gas having `an effect equivalent to. vacuum ashing. Specifically, pitch or a vacuum residuum from. line 1 are introduced into line 2 and mixed there with hot iiue gases taken from cyclone 27 at about 150,0 to 2000 F. in an amount sucient to vaporize about 20 to 75% of the pitch. The amount of flue gases required may be such as to decrease the hydrocarbon partial pressure in line 2 to about 0.03 to 0.2 atmospheres when line 2 is operated at about atmospheric pressure. The resulting mixture of flue gas andvaporized portion of feed is immediately separated from the unvaporized feed portion in cyclone 4 and the separated gas passing through line 5 is promptly cooled by means offheat exchanger 8`to about 400"A F. or lower, at whichl temperature substantially all of the vaporized hydrocarbon product is condensed for recovery. The liquid hydrocarbon condensate is separated from the ue gas in drum 41 andV finally converted into the desired products in! catalytic cracking zone 6. Any hydrocarbons contained inthe uncondensed gas coming from drum 41 may be recovered by further cooling or by passing the gas streamvthrough a charcoal adsorption plant, by oil scrubbing, or other conventional recovery system.

The unvaporized portion of the feedis removed from cyclone 4l through pipe 10 and introduced into the iiuidized bed in coking vessel 11 where thermal conversion of the heavy feed into light products and coke is carriedto substantial completion at about 950 to 1000" F. In the illustrated arrangement it is possible to operate coking vessel 11 at a substantially higher temperature than vaporizing line 2, since heat to the uid bed is supplied not only in the unvaporized feed stream passing through standpipe 10, but additional heat may also be brought in directly by the circulating solids. As shown, coke may be withdrawn from the fluid bed through stripping well 19 and line 20 whereuponA they are mixed in line 23 with the hot flue gas from burner 24 which may bey at, atemperature between about 2-500 and" 3500 F. or higher. The resulting direct heat exchange4 between the relatively cool solids and the hot flue gas desirably lowers the temperatures of the latter to about- 1500 to 2000 F., thereby minimizing any local overheating when the oilY feed is eventually injected into the hot ue gas in vaporizing line 2. rThe mixture of hot. coke and flue gas in line 23 may be passed through linel 37 to cyclone 27 and the separated coke returned from the cyclone through leg 23 to the iiuid bed in coking vessel 11 while the separated gas stream is passed through line 36 to vaporizing line 2 where it is mixed with fresh feed as previously described.

Alternatively, the system illustrated in Fig. 4 may be operated so that the coke and ue gas mixture in line 23 is discharged directly into vaporizing line 2, lay-passing cyclone 27 and returning the coke to the fluid bed only after separation in cyclone 4, together with the unvaporized feed portion. In this embodiment, a high rate of coke circulation may be desirable and consequently the flue gas may be diluted with steam introduced at 38 so as to provide sufficient gas for lifting. In such an instance, the temperature of the coke and gas mixture in line 23 may accordingly be about 950 to 1100 F. prior to injection into the; .vaporizing line 2. This further minimizes local' overheating, of. feed and also provides considerable surface .area for the evaporation of hydrocarbons.4 Vaporization in transfer line. 2 andt thermal cracking in vessel 11 wouldl occurl here at approximately the same temperature.

A still simplermoditication of the system illustrated in Fig. 4 may omit all coke circulation and pass hot line gas fromY burner 24' through. line 23 directly into vaporizing line 2, without any adrnixing. with solids. Where the vaporization of feed in line 2 is thus carried out at about 7501000 F. or higher, the sensible heat in the unvaporized portion of the feed will be suiiicient to operate coker 11' at or above 850 F. without further addition of heat. Of course, a somewhat greater danger of overheatingis present when hot flue gases coming from the burner are thus injected directly into` the vaporizing line, than when the temperature of the gases is first reduced by heat exchange with the coker solids..

The various processes just described with reference to Fig. 4 not only provide a cheap alternative for vacuum ilashing followed by coking of the` vacuum pitch, but also permit a larger fraction ofthe crude to be taken overhead as distillate product for use as feed stock in catalytic crack* ing. For instance, it willbe seen by consulting a hydrocarbon vvapor pressure chart thata temperature of 950 F. and a partial hydrocarbonpressure of 0.064 atmospheres in vaporizing line` 2 corresponds. to a distillate having a normalboiling-point of over 1200 F. Use of a transfer line allows such-vaporization toV becarried out so quickly that. the amount of undesirable` thermal cracking is negli-4 gible.

tion. of several specific embodimentsV has been given for purposes of illustration rather than limitation. Statedv more generally, the invention is broadly applicable to heavy hydrocarbons unsuited fordirect feed to a catalytic crackingv zone because of their excessive carbon forming and/ or other` catalyst contaminating characteristics. Ac

cordingly, the feed stock may be a reducedcrude petroleum residue obtained by atmospheric or vacuum distillation and may represent the. bottom-2 to.25 volume percent ofV the virgin crude distilled, or it may be a whole crude, or

tar. from visbreaking operations or similar tars or pitches. Prior to feeding into the vaporizing.zone,such heavy feed' stocltsare usually preheated to temperatures ranging from 200- to 10.00 F., preferably 60.0 to.- 800 F., and their Fig. 4,.,up to. densities. as high as about 5 or l0 lbs/cu. ft.. wherev vaporization of the feed is carried out in the pres-- ence. of a substantial amount of solids as illustrated in Fig. 1.

In the cokingvessels. the'y existing. conditions are those characteristic of', the maintenanceof a dense, turbulent, iiuidized bed, that is, the superficial gas velocity may range between about 0.5 to 5, or preferably 2 to 3 feet per second, and the apparent density of the dense bed may range between about 10 and 50 lbs./ cu. ft. The residence time of the unvaporized feed in the Coking vessel may range frornl about l to 150 minutes depending on temperature and the refractory nature of the stock.

The inert contact solids used in the coking vessels, and in the vaporizing lines Where indicated, preferably are finely divided particles of petroleum coke whose diameter may range up to about 500 microns, preferably between and 300 microns. Instead of coke, finely divided inorganic inerts such as sand, spent clays, pumice and the like may be used similarly, except that in such cases it will be preferable to burn all the coke produced in the process it will befunderstood. that the foregoing detailed descrip-1 arsenal,

and use any excess heat for steam generation or the like whereas in the case of petroleum coke, net coke product may be readily recovered in substantially pure form. The ratio of unvaporized feed to solids in the coking vessels is maintained sufficiently low to allow proper iluidization, desirable ratios being preferably not in excess of about l wt./wt./hr. at about 900 F. coking temperatures, or 3 wt./wt./hr. at 1000 F. coking temperatures.

Where a catalytic cracking zone of the fluid type is used, the velocity and density conditions prevailing therein are also substantially within the same range as those described above with respect to the coking vessel, as is well known per se. Moreover, despite the fact that the feed introduced into the catalytic cracking Zone contains a very substantial proportion of constituents boiling well above the gas oil range, the temperature conditions are essentially the same as those used in conventional gas oil cracking, i. e., between about 800 and 1000 F. The catalyst used in the catalytic cracking zones may be any conventional cracking catalyst such as activated clay, activated alumina, synthetic composites of silica withua minor proportion of alumina, magnesia or boria, and so on.

Furthermore, the physical arrangement specifically described herein may be changed or modified by those skilled in the art without departing from the spirit of the present invention. For instance, while the unvaporized portion of the feed separated in cyclones 4 may be suliiciently fluid to allow transfer to the coking vessels by ordinary pumps as shown in Fig. 3, or it may contain enough dry solids to be transportable to the coking vessels in a duid state through standpipes and the like as shown in Figs. 1, 2 and 4, relatively viscous mixtures containing unvaporized feed and inert solids may also be transported to the coking vessels with the aid of screw conveyors or the like. Also, while no specic mention thereof has been made earlier herein, when a standpipe is used to transport mixtures of unvaporized feed and inert solids from the cyclone to the coking vessel, it may be beneficial to operate with a positive pressure difference across such a cyclone standpipe so as to force the flow of the wet solids in the desired direction, thus preventing any vexatious clogging.

Finally, whenever the volatilization of liquid hydrocarbon feed in transfer line 2 has been referred to herein simply as vaporizatiom it will be understood that at the temperatures employed such vaporization is inherently accompanied by some mild cracking or coking, and consequently that any hydrocarbon vapor described herein as vaporized feed includes not only simply evaporated feed constituents but the incidental cracking products as well.

Having given a full description of the invention and of the manner of using it, the scope of the invention is particularly pointed out and distinctly claimed in the appended claims.

We claim:

1. A process for converting a heavy hydrocarbon feed stock into lighter products which comprises passing the feed stock through a heating and vaporizing zone at a temperature in the range of 750 F. to 1150 F., whereby a portion of the feed stock is vaporized, separating the vapors from the unvaporized portion of the feed stock, immediately injecting nely divided cracking catalyst into the separated vapors andimmediately passing the mixture of vapors and cracking catalyst'through a catalytic cracking zone maintained under cracking conditions, recovering cracked products including gas and gasoline from the cracking zone, introducing the unvaporized portion of the feed to a coking zone containing a dense, turbulent bed of inert solids maintained at a temperature in the range of about 850 F. to 1150o F. whereby the unvaporized feed undergoes thermal cracking in the coking zone during an average residence time of at least 1/2 minute, withdrawing thermally cracked vapors from the coking zone, passing the withdrawn thermally cracked vapors through a second catalytic cracking zone and recovering valuable distillate fractions therefrom, withdrawing solids containing coke deposited thereon from the coking zone, burning the withdrawn solids in a free oxygen-containing gas thereby raising the temperature of said solids to a temperature above 1150c F. and returning the heated solids to the coking zone.

2. A process according to claim l in which the heating and vaporizing zone constitutes a transfer line system and wherein the feed stock is contacted with hot solids during a period of time less than one second.

3. A process for converting a heavy hydrocarbon feed stock into lighter products, which comprises passing vthe feed stock through a heating and vaporizing zone at a temperature in the range of 750 to 1150" F., whereby a portion of the feed stock is vaporized, separating the vapors from the unvaporized portion of the feed stock, immediately injecting iinely divided cracking catalyst into the separated vapors and immediately passing the mixture of vapors and cracking catalyst through a catalytic cracking zone maintained under cracking conditions, recovering cracked products including gas and gasoline from the cracking zone, introducing the unvaporized portion of the feed to a coking zone containing a dense, turbulent bed of inert particulate solids maintained at a temperature in the range of about 850 to 1150 F. and for a time period sufdcient to thermally crack the unvaporized feed, withdrawing thermally cracked vapors from the coking zone, passing said thermally cracked vapors through a catalytic cracking zone containing cracking catalyst at cracking temperature, withdrawing solids carrying coke deposits from the coking zone, burning at least part of said coke to reheat said solids, and returning reheated solids to the coking zone to supply thermal cracking heat requirements in said zone.

References {2i-ted in the file of this patent UNITED STATES PATENTS 2,223,192 Swartwood Nov. 26, 1940 2,380,897 Murphree et al July 31, 1945 2,382,755 Tyson Aug. 14, 1945 2,388,055 Hemminger Oct. 30, 1945 2,399,050 Martin Apr. 23, 1946 2,420,145 McAfee May 6, 1947 2,436,160 Blanding Feb. 17, 1948 2,437,222 Crowley et al. Mar. 2, 1948 2,543,884 Weikart Mar. 6, 1951

Claims

1. A PROCESS FOR CONVERTING A HEAVY HYDROCARBON FEED STOCK INTO LIGHER PRODUCTS WHICH COMPRISES PASSING THE FEET STOCK THROUGH A HEATING AND VAPORIZING ZONE AT A TEMPERATURE IN THE RANGE OF 750* F. TO 1150* F., WHEREBY A PORTION OF THE FEED STOCK IS VAPORIZED, SEPARATING THE VAPORS FROM THE UNVAPORIZED PORTION OF THE FEED STOCK, IMMEDIATLEY INJECTING FINELY DIVIDED CRACKING CATALYST INTO THE SEPARATED VAPORS AND IMMEDIATELY PASSING THE MIXTURE OF VAPORS AND CRACKING CATALYST THROUGH A CATALYTIC CRACKING ZONE MAINTAINED UNDER CRACKING CONDITIONS, RECOVERING CRACKED PRODUCTS INCLUDING GAS AND GASOLINE FROM THE CRACKING ZONE, INTRODUCING THE UNVAPORIZED PORTION OF THE FEED TO A COKING ZONE CONTAINING A DENSE, TURBULENT BED OF INERT SOLIDS MAINTAINED AT A TEMPERATURE IN THE RANGE OF ABOUT 850* F. TO 1150* F. WHEREBY THE UNVAPORIZED FEED UNDERGOES THERMAL CRACKING IN THE COKING ZONE DURING AN AVERAGE RESIDENCE TIME OF AT LEAST 1/2 MINUTE, WITHDRAWING THERMALLY CRACKED VAPORS FROM THE COKING ZONE, PASSING THE WITHDRAWN THERMALLY CRACKED VAPORS THROUGH A SECOND CATALYTIC CRACKING ZONE AND RECOVERING VALUABLE DISTILLATE FRACTIONS THEREFROM, WITHDRAWING SOLIDS CONTAINING COKE DEPOSITED THEREON FROM THE COKING ZONE, BURNING THE WITHDRAWN SOLIDS IN A FREE OXYGEN-CONTAINING GAS THEREBY RAISING THE TEMPERATURE OF SAID SOLIDS TO A TEMPERATURE ABOVE 1150* F. AND RETURNING THE HEATED SOLIDS TO THE COKING ZONE.