US2853439A - Combination distillation and hydrocarbon conversion process - Google Patents

Description

Sept. 23, 1958 v H. ERNST, JR

COMBINATION DISTILLATION AND HYDROCARBON CONVERSION PROCESS Filed Aug. 1, 1952 Hem'f Tfr-ast. Jr. 5m/anacr- United States Patent COMBINATION DISTILLA'IION AND HYDRO- CARBON CNVERSION PROCESS Henry Ernst, Jr., Fanwood, N. J., assignor to Esso Research and Engineering Company, a corporation of Delaware Application August 1, 1952, serial No. 302,050

6 claims. (ci. 19t-s2) cludes a series of integrated distillation and catalytic and/ or thermal conversion stages wherein produ-cts from all stages may be conducted to a single product fractionation stage and treated therein in such a manner that av topped crude obtained as liquid residue of a crude distillation stage is stripped of relatively high boiling constitu,- ents.

This invention is an improvement over that disclosed in Serial No. 153,372, filed April l, 1950, for Raymond E. Bittner and Channing C. Nelson, now abandoned, en.- titled Combination Crude Distillation and Oil Rening Process, and the continuation-in-part case Serial No. 259,654, led December 3, 1951, for the same inventors,V now Patent No. 2,777,801, dated January 15, 1957.

According to this invention there is eicient removal oil utilizing product vapors from a fluid catalytic cracking,` operation.

In practicing the invention broadly set forth above, fit1' has been found that with certain residual or crude petro leum oils there is a build up of condensed ring aromatic hydrocarbons in the recycle stock obtained on cracking and unexpected high yields of coke or carbonaceous ma,- terial have been obtained in catalytic cracking.; TheV present invention is concerned with reducing the 'amount'y of coke deposited on the catalyst by providing ahy-pass tower to remove a portion of the cracked products from.

the catalytic cracking Zone. The cracked vapors or prodi-l` ucts are partially condensed in 'a fractionator'to remove" cycle stock fractions andthe vapors boiling above the" rejected cycle stock are returned to the single product" fractionation stage.

The rest of the cracked products substantially at cracking temperature are passed to the product fractionator to act as stripping means for the reduced crude oil as will, be hereinafter described in greater detail.v y It is Well known in the art to produce topped or vreduced crudes by subjecting the crude to various distillation operations.,l This topped crude comprising' gas oil constituents boiling in the range from about 6509.11. to` ll F., and also some higher boiling constituents is then handled in a manner to secure a maximum segregation of these gas oil constituents. This is usually secured by the utilization of a vacuum distillation operation. The" gas oil fraction segregated in the vacuum distillation operation is then passed to a catalytic cracking process. The

of relatively high boiling constituents from a topped crude 2,853,439 Patented Sept. 23, 1958 ice vacuum distillation equipment used for working up the reduced crude is expensive with respect to investment, operation and maintenance. As a result, conventional type combination processes must be operated on a relatively large scale to be economical. Normally, refining capacities in excess of, say, about 20,000 bbl./ day of crude are required to make operations of this type pay while smaller refineries must be designed on the basis o-f an often undesirably high output of heavy fuel oil and other products of a relatively low commercial value.

In accordance with the present invention, an improved operation is secured wherein the vacuum distillation operation is eliminated. Also substantial savings in fractionating equipment are realized by supplying a large part` of cracked vapors and substantially all the vaporous products of the various stages of a combination process as well as the reduced crude oil from the crude distillation to a sinlge product fractionation stage and using these vapors to strip the reduced crude in the fractionation stage of its distillable constituents. For' this purpose, the crude oil may be subjected to distillation in a conventional crude distillation unit to produce an overhead stream of light virgin naphtha, a separate heavy naphtha stream, a still heavier stream of the kerosene and diesel oil range and reduced lcrude bottoms. All other fractions are supplied to a single product fractionator substantially as follows.

Reduced crude of the type just mentionedv may be passed directly to an upper portion of the lower contact,- ing section of a substantially conventional product. fractionating column. The heavy naphtha streammay be subjected to high temperature thermal or catalytic reforming or other conversion for the improvement of its motor fuel qualities. The total vaporized, efiluent from this conversion stage is fed to the product fractionating column at a point below the feedA point ofthe reduced crude, substantially at the temperature` of the conversion stage.

The light virgin naphtha may be introduced into the fractionating column ata point below the reduced crude feed point, and can be preheated. beforehand if. desired. Various final product streams may be recovered from the product fractionator which may include a fuel gas overhead, a low boiling fraction ofthe motor fuel boiling range, a heating oil fraction, a gas oil fraction, and v a heavy bottoms fraction of the fuel oil range. In addition, there' may be produced a gas oil fraction which contains the virgin gas oil fraction desired for feed toa further conversion step in admixture with a recycle or converted fraction in approximateoptimum ratio for preferably passed to a catalytic cracking stage to be conultiinate conversion operation. The gasr oil fraction is verted therein into additional amounts of motor fuel, diesel oil, gas oil range cycle stock and heavy bottoms.

The major portion` of the total effluent of this cracking stage is passed without substantial heat loss to the product fractionator at a point below the reduced crude feed. In this manner, the reduced crude oil is subjected to heating, vaporization and stripping action, with vapors from the gas oil cracking process and the naphtha vapors.

The rest of the eflluent from the catalytic cracking stage is passed to a by-p'ass tower to partially condense higher boiling cycle stock containing coke forming constituents which is withdrawn from the bottom of the tower and discarded from the process. The vapors boiling above the rejected cycle stock pass overhead from the by-pass tower and are returned to the single product fractionator at a point above the introduction of the re-y duced crude oil.

When operating substantially in the manner described above, extremely large volumes of processv vapors are available and utilized for reduced crude stripping with the effect that the volume of heavy fuel finally produced may be kept at a minimum.

llt will be appreciated that the invention in its broadest aspect is not limited to the type and number of intermediate conversion stages referred to'above which depend entirely on the character of the crude oil to be processed and the types, quality and relative proportions of the desired products. Other conversion stages may be added or substituted for those mentioned. Other conversion stages which may be added into the process of the invention include hydroforming and/or other octane improvement processes for the virgin naphtha. Thermal or suspensoid visbreaking, coking, etc. may be applied to the bottoms from the product fractionating tower. The product vapors of any of these stages can be supplied to the single product fractionation stage substantially as described above.

In certain casesythe process of the invention may be further simplified by eliminating the crude still and feeding the whole crude oil directly into the product fractionator. This may be possible if separate virgin naphtha and/or kerosene fractions are not required. When so operating, the whole crude may be fed to an upper portion of the lower contacting section of the product fractionator. A heavy naphtha stream and gas oil cracking stock` may be withdrawn from the fractionator and subjected to the conversions described above, particularly to reforming and catalytic cracking. The vaporized products of these conversions may then be returned to the product fractionator at a point below the crude feed point. Also light naphtha and overhead gases from the fractionator may be returned to the fractionator bottom to assist in crude stripping. In all other respects the operation of the process will be substantially analogous to that indicated above.

In the drawing the figure represents a schematic flow plan of a preferred modification of the combination process in accordance with the invention.

Referring now to the drawing, the reference character designates a crude still, the reference character 12 designates a naphtha reformer furnace and the reference character 14 designates a product fractionator. A cracking stage is schematically illustrated at 16. The functions and coaction of the apparatus referred to will be now explained using as an example the reiining of a medium gravity petroleum crude oil of the type of West Texas crude oil in a refinery having a capacity of about 35,000 bbl. of crude oil per day. It should be understood, however, that the apparatus may be used for the refining of different types of similar crude oils on a larger or smaller scale in a generally analogous manner.

` In a specific example, about 35,000 barrels a day of the crude oil are pumped through line to a heating coil located in furnace 26 wherein it is heated to a temperature suitable to vaporize a substantial portion of the oil. The oil so heated is passed through line 28 to a lower portion o-f the crude still 10 which it may enter at a temperature of about 650-800 F. and a pressure of about 10-70 p. s. i. g. Crude still 10 is preferably provided with a plurality of horizontal bubble cap plates or the like 32 to improve fractionation of the feed in a conventional manner. Reflux may be accomplished with the aid of one or more partial condensers 34 arranged in the top of still 10. For the purposes of the present example, still 10 may be so operated that three distillate streams and distillation bottoms are produced as follows.

All crude oil constituents boiling below about 250 F. are removed together as a vapor stream of light virgin naphtha overhead through line 36. This stream may amount to about 10-20% of the crude oil charged. All or a portion of this stream may be withdrawn from the process through line 38 and subsequently condensed in the conventional manner. A liquid stream `of heavy naphtha having a boiling range of about 250-400 F.

still 10 at a point below condenser 34. A portion of the heavy naphtha in line 40 may be withdrawn from the process through line 41. About l5-20% of the crude oil charged is recovered through line 40. A kerosene or diesel oil cut boiling within the range of about 400 F. up to about 700 F. and amounting to labout 20 to 30% of the crude oil is withdrawn from the process through line 42. The remainder of the charge, amounting to about30% to 50% and consisting predominantly of constituents boiling above 700-800 F. is withdrawn as reduced crude oil through line 44 from the bottom of still 10. The kerosene cut removed through line 42 is normally suitable for kerosene or diesel oil purposes after conventional treatment and it may be passed directly to storage. The other fractions may be treated in accordance with the present invention as will be presently described.

The light virgin naphtha vapors in line 36 may be passed directly to a lower portion of product fractionator 14. If desired, this vapor stream may be preheated to about 800 to 1000 F. to conform with the heat requirements of fractionator 14. This may be done by passing at least a portion of the vapors in line 36 through a heating coil located in a furnace 46. The heated light virgin naphtha vapors are passed through line 47 into the bottom portion of the product fractionator 14.

The heavy naphtha stream in line 40 may be pumped by pump 48 through the heating coil in furnace 12 at a pressure of about 200-1000 p. s. i. g. to be subjected to thermal reforming. This reforming stage may be of any conventional design well known in the Iart and may comprise a conventional tube furnace designed to provide for an oil residence time of about l0 liquid volumes per volume of reaction space per hour (v./v./hr.) at about l000ll00 F. and about 200-1000 p. s. i. g. pressure. At these conditions, the octane rating of the naphtha may be increased from about 30-50 to about 70-90 Research octane number without excessive cracking to normally gaseous hydrocarbons. The total effluent of the reformer furnace 12 is passed substantially at the reforming temperature and pressure through line S2 provided with pressure release device such as valve or venturi 54 into the lower portion of product fractionator 14 at a point close to but preferably below the feed point of line 47. Under the conditions of the reduced fractionator pressure of about 5-15 p. s. i. g., the naphtha is substantially completely vaporized when entering the lower portion of product fractionator 14.

The reduced crude in line 44 may be passed directly to the intermediate portion of fractionator 14, substantially at the temperature of its withdrawal from still 10 depending on heat requirements. Line 44 feeds into fractionator 14 at a point above the feed points of lines 47 and 52. In this manner, the vapors supplied through lines 47 and 52 pass upwardly through product fractionator 14 against the downwardly flowing reduced crude oil to strip the latter of vaporizable constituents. This effect and the operation of fractionator 14 will be described in greater detail hereinafter.

A side stream of gas oil boiling range hydrocarbons amounting to about 45% to 70% of the crude oil and boiling between about 600-700 F. and 900-1l00 F. which is suitable as a catalytic cracking stock, may be withdrawn through line 56 from an intermediate section of fractionator 14 by pump 58 and passed through line 56 to the catalytic cracking stage 16. This stream contains both cracked and virgin gas oil fractions. Any conventional cracking system adapted to convert gas oil range hydrocarbons into lower boiling oils, particularly of the motor fuel range, may be used. Continuous or batch operation may be employed in xed bed, moving bed, fluid or suspensoid systems. Heat required for cracking may be supplied as preheat yof process materials and/or as sensiblel heat of exothermicallyy regenerated catalyst or in any other conventional manner.

' Modied natural or synthetic clay or gel type catalysts such as activated montmorillonite clays, silicaalumina, silica-magnesia composites and other conventional cracking catalysts may be employed at temperatures of about 800-1000 F. and pressures of about atmospheric to 25 p. s. i. g., all in a manner known in the art. A cracking system offering particular advantages inconnection with the present invention is disclosed in Packie Patent No. 2,589,124 granted March 11, 1952.`

A major portion of the total hydrocarbon effluent of cracking stage'16 is passed substantially at the cracking temperature of about 850-1000 F. through line 62 to the lower portion of product fractionator 14, in some cases, at a point intermediate between the feed points of the reduced crude oil and of the reformed and virgin naphthas. If cracking stage 16 is operated at an elevated pressure, the pressure may be released by valve 64 to fractionator pressure. In most cases, the cracked material enters fractionator 14 in the vapor state to enhance the stripping atcion Iof the vapors supplied through lines 47 and 52. l

If desired, a portion of the gas oil stream withdrawn from product fractionator 14 through line 56 may be passed through line 65 and cooled in cooler 66 and returned as reflux through line 68 to an intermediate portion of product fractionator 14 below the withdrawal line 56 but preferably above feed line 44 for the reduced crude oil. Also a portion of the cooled gas oil may be withdrawn from the process through line 72, if desired.

As indicated in the drawing, vfractionator 14 comprises a lower stripping section '7"4 and an upper fractionation section 76. Both sections are provided with suitable means for improving the countercurrent contact between downowing liquid and upwardly flowing vapors. For the purpose of stripping, a bafe arrangement has been found to be efficient and such is shown schematically for section 74 by elements 78. Section 76 is illustrated to contain a number of bubble cap plates 82 to enhancethe eiciency of the fractionation process. Sections 74 and 76 may operate as follows.

Stripping section 74 receives, asidefrom the vapor and liquid streams supplied through lines 47, 52, 62 and 44, a liquid feed introduced at the top and comprising a gas oil cut removed from the bottom of section 76 via line 56 and supplied to section 74 via line 68. This gas oil is fed to section 74 to provide control over the reflux and heat removal in that section in order to obtain the desired end point and clean-up of the gas oil. All the heat required for stripping and fractionation in fractionator 14 is preferably supplied as sensible heat of the hydrocarbon streams entering section 74 to maintain a temperature'of, say, about 750-850 F. in the lowest portion of section 74. The vapors rising through section 74 strip the downwardly flowing gas oil cut, reduced crude and cracked liquid products of substantially al1 their distillable constituents andrthis vapor mixture passes on at a temperature of about 700 to 800 F. into fractionation section 76 to be treated as will be later described.

The reduced crude from line 44 which may contain as much as about 75% of gas oil suitable for feed to the catalytic unit is stripped and heated by the cracked vapors at, say, about 900 F. and 8 p. s. i. g. and by the virgin naphtha vapors at, say, about 800 F. and by the reformate at about 1025 F. The partial pressure effect of the other streams and the heat content thereof are suicient to cause the gas oil constituents of the reduced crude to vaporize. The net effect of the process in sec` tion 74 then is, (1) a bottom stream of unfluxed fuel oil amounting to about 15-20% on the crude oil and containing about 85-95% of flashed reduced crude, about 2% to 4% of'reformer tar, and about 3% to 6% of heavy oil from the catalytic operation, al1 blended auto- 6 maticallyso that-it may befluxed with lightfoilgblending stock for fuel oil viscosity correction and (2) vapors `containing all ofthe distillate products to be obtained from fractionator 14 ,-andfleaving section 74 overhead at about 775 F. l

A heavy material containing all the non-distillable constituents of the crude oil charged and of the fractionsl converted in-reformingrfurnace 12 and cracking zone-16col1ects at about 750-850 F. in the bottom zone of section 74 from which-it may be withdrawn via line 84. If desired, the temperature in the bottom of section 74 may be reduced to, say, about 700 F. byre'cycling heavy bottoms from line 84 by means of pump 86'via cooler k88 and line 92. The bottoms quenching may be desirableto prevent cracking and coking ofthe heavy liquid products. Combined reduced crude amounting. to about 15% to 20% on the crudel oil may be-recovered through line 94 tobe kfurther treated asv will appear hereinafter.

At the conditions of the present example about 3000 to 6000 mols/hr. of hydrocarbon vapors will be available to` strip about 300 to 600 mols/hr. of-liqu'id in section 74. This'favorable vapor-liquid ratio results in a substantially quantitative stripping effect being obtained in section 74. The number of baffles and the dimensions of section 74 depend largely on the character of the crude charged and the products desired. For the purposes of the present example, this section may be approximately 18 in diameter and 30' in height `and is equipped with`7 sets of disc-and-doughnut type contacting devices.

Referring now to cracking section or stage 16, extraneous gas oil maybe added to the feed line 56 to crackingsection 16 through line 96. In the specific eX? ample here used, about 5000 barrels per day of gas oil is introduced via line 96. The'amount of cracking feed stock withdrawn from product fractionator' 14 through line 56 andpassed to' cracking section16 is about 13,000 BLT/D. so that the total feed'to 'the cracking section 16 is about 18,000 B./D. Ofthe 13,000 B./D. feed, about 8,000 B./D. are flashed fresh feed'and about 5,000 B./D.l arerecycle stock.

The'major portion ofthe cracked vaporous yproducts leaving cracking section 16 is passed throughflin'e 62 a's above stated andthisv portion amounts to about 15,000 B./D. The rest of the cracked products amounting to aboutv3,000 B./D. are passed through line 98 to a bypass' tower 102 for separating and removing some of the undesirable cycle stock. The by-pass tower'102 has an enlarged upper fractionating sectionv 104 and a lower smallerstripping section 106. In-the specific example being described, the fractionating section 104 has-a di-' ameter of about 6 feet and a height of about 18 feet' whereas the stripping section'106 has a diameter of about 2 feet yand a height of about 15 feet. A steam inlet line 108 is'provided for introducing stripping steam into the bottom portion of lthe stripping section 106.` The cracked products from line 98 are preferably introduced into the lower portion of the upper fractionating section 104 of the tower 102 wherein they are fractionated to remove higher boiling refractory constituents. The tower 102 operates at a pressure of about 6-8 p. s. i. g. The temperature of the vapors leaving loverhead through 118`iS about 600-650 F;

Reux liquid for by-pass tower 102 is preferably withdrawn from product fractionator 14 through line 112 by pump`114 and passed through cooler 116 and then intro'- duced into the top of fractonating `section104 so that the upflowing vapors are fractionated as they pass upwardly countercurrent to the liquid condensate. The reliux liquid is withdrawn from the lower portion' of the fractionatingA section 76 of tower 14 but above the point of withdrawal of gas oil through line 56. The circuit 114-112-116-4 122 comprises primarily an intermediate heat removal cir` cuit for tower 14. The overhead vapors from tower 102 are passed through line 118and preferably returned t0' 7 the lower portion of the fractionating section 76 of tower 14 between the points of withdrawal of reflux liquid through line 112 and gas oil through line 56.

The amount of vapors being returned to tower 14 via line 118 is about 1500 B./D. measured as liquid. The liquid condensate passing from the lower portion of the fractionating section 104 of tower 102 into the upper portion of stripping section 106 is stripped of volatile constituents and a cycle stock amounting to about 1500 B./ D. and having a boiling point range above about 650 F. is withdrawn from the bottom of the stripping section 106 through line 124 and discarded. This cycle stock-may be used for fuel or may be cracked in extraneous cracking units.

The cracked vaporous products pass upwardly in the fractionating section 76 of tower 14 and are fractionated therein to separate gasoline and lighter hydrocarbons from higher boiling constituents. The vapors in fractionating section 76 of tower 14 may be at about 550 to 650 F. just above the withdrawal line 56 and the vapors are cooled and condensed by reflux oil cooled to about 130 F. and recycled through line 122 to tower 14. Heavier constituents are condensed and flow down the tower countercurrently to the vapors. The product fractionator tower 14 is maintained at a pressure of about 4 p. s. i. g. at the top and 8 p. s. i. g. at the bottom and therefore the overhead vapors leaving tower 14 through line 132 contain appreciable amounts of normally gaseous hydrocarbons. To recover gasoline hydrocarbons a system such as disclosed in the copending Rich et al. application Serial No. 153,332, filed April 1, 1950, and now abandoned, may be used. The overhead vapors from tower 14 passing through line 132 as shown in a simpler way on the drawing are condensed in condenser 134 and passed to a gas separator 136 for separating gas from liquid hydrocarbons. The liquid hydrocarbons comprise gasoline and are withdrawn from separator 136 through line 138 and a portion withdrawn as product through line 142 and the rest returned as refiux through line 144 to thetop yof fractionating section 76 of tower 14.

The gas passing overhead from gas separator 136 through line 146 contains gasoline constituents and may be passed to a compressor and absorber system of known or standard design to recover the gasoline constituents as product.

Heating oil may be withdrawn from tower 14 through line 148 at a temperature of about 450 F. to 500 F.

Final products may be recovered as follows. Gasoline of 400 F. end point amounting to about 50% to 60% on crude oil and having an octane rating of about 80-90 Research octane number is recovered via line 142. Heating or light diesel oil is recovered via line 148 at a rate of about -25% on crude oil.

Returning now to the combined reduced crude and heavy cracked bottoms withdrawn through line 84, they may, if desired, be blended with heating oil or lighter fractions supplied from line 148 to adjust their viscosity to meet specifications. The bottoms may be cooled to about 300 to 500 F. in cooler 8,8 and may be passed through line 152 to filtering facilities (not shown) or may be otherwise treated. The amount of bottoms removed via line 152 in the present example is about 8,500 B./D. Conventional sand filters, rotary or porous sintered ceramic filters may be used to remove from the combined residue all suspended or slurried solid particles, such as coke, catalyst carried over from cracking stage 16, etc. The solids removed during filtering may be discarded or if they comprise catalyst they may be passed to catalyst recovery means (not shown). A fuel oil grade residuum is recovered as the filtrate.

The system illustrated in the drawing permits of various modifications. As previously pointed out, one or more reduced crude visbreaking or coking stages may be inc luded. For example, part or all of the bottoms in line 84 may be subjected, for example, in furnace 154 to visbreaking and/or cokingto produce a heavy residuum and coke which is passed to a stripper 156 through line 158. Stripping steam is introduced into the bottom of stripper 156 through line 162. Heavy residuum and coke are withdrawn from the bottom of stripper 156 through line 164 and may be withdrawn from the system through line 166 and filtered or may be passed through line 168 and cooler 170 and fluxed with bottoms from tower 14 passing through line 152 and the mixture filtered, if desired. The lighter materials passing overhead from stripper 156 through line 172 are preferably passed to the stripping section 74 below feed inlet 44 to be used for stripping and be subjected therein to fractionation as described above.

Catalytic, rather than thermal, reforming may be employed in reforming stage 12 using such conventional catalysts as oxides and sulfides of groups V, VI or VIIIv metals, preferably supported on a suitable carrier, temperatures of about 850-1100 F. and pressures from atmospheric to about 400 p. s. i. g. in the presence or absence of extraneous hydrogen, all in a manner known per se. Other refining treatments such as bauxite treating, clay treating, etc. may follow reforming stage 12 provide that most of the hydrocarbon efliuent of such stages is supplied to fractionator 14 as described above.

While a variety of catalytic cracking systems may be used as cracking stage 16, fluid catalytic cracking involving the continuous production of cracked efiluent and continuous catalyst circulation between cracking and regeneration stages is most suitable for the purpose of the present invention.

The catalytic materials used in the fiuidized catalyst `cracking operation, in accordance with the present invention, are conventional cracking catalysts. A preferred catalyst comprises silica-alumina wherein the weight percent of the alumina is in the range from about 5 to 40% These catalysts may also contain other constituents, as for example, Th02, W03, M00, BeO, Bi2O2, CdO, U03, B203, Sn02, Fe203, V205, MnO, Cr202, CaO, T1203, MgO and Ce203 present in the concentration from 0.05% to 0.5%. The size of the catalyst particles is usually below about 200 microns. Usually at least 50% of the catalyst has a micron size in the range from about 20-80. Under these conditions with the superficial velocities as given in Packie Patent 2,589,124 above referred to, a dense fluidized bed is maintained in the lower section of the reactor and a disperse phase is present in the upper portion of the reactor.

The temperature and pressure conditions specified in the above example for the operation of fractionator 14, particularly of stripping section 74 are those best suited for the crude oil here specified. The conditions may be varied to a certain extent depending chiefly on the boiling characteristics of the crude oil charged, as will be readily understood by those skilled in the art. For example, for a lighter crude oil the yield of reduced crude from the bottom of section 74 may be somewhat lower and vice versa.

The present invention is broadly concerned with the effective removal of gas oil constituents from a topped or reduced crude containing the same by subjecting the reduced crude containing these gas oil constituents to the stripping action of the vaporous product produced in a fluid catalytic cracking operation. The topped or reduced crude is introduced into a lower section of a fractionation zone while the vaporous product from the fluid catalytic cracking operation is introduced into the fractionation zone at a point below the point of introduction of the topped crude. The cracking feedstock comprising constituents boiling mainly in the range from about 650 F. to 975 F. is removed from the fractionation zone at a point above the point of introduction of the topped crude.

By this process a desirable clean feed stock for the catalytic cracking operation is secured by actually employing catalyticallyf cracked vapors inthe fractionation zone as a strippingrmedium forremoving relatively high boiling'gas oil constituents fromlartopped crude containing; thei same. By` thepresent' operation it is possible to rremove from atoppedcrude, byl a method other than bya vacuum distillation operation, substantially all of the hydrocarbon constituents in ther'educed'crude boilingup to about 1000 andin some cases up to as high as` 1l00 depending upon temperature-and ratio of stripping vapors.

The improvement ofthe present invention comprises providing a by-pass tower' 102 into which part of the catalytically cracked products are lpassed to be fractionated into cycle stock of Isay 600' F. Yplus-to 800 F. plus boil-` ing range which is discarded-andlower boiling hydrocarbons which are returned-to the product fractionator 14. Bottoms from tower 14 are also removed from the process.

Byl providing the by-pass` tower some of the refractory highlyaromatic cycle'stoc k is removed and a better feed is provided forl the' catalytic cracking step-so that with this improvement less coke is `depositedon the catalyst for a givenfconversion. Or conversely, a higher conversion can be obtain'edlat'a given' cokeproduction. The rejection of la portion ofthe cycle stock is accomplished Without `losing the combination unit principle because the major'portion' ofthe hot cracked vaporous products are passed tothe bottom portion of the product fractionator. With' the present improvement there is a minimum Aloss` of flashedvirgin feed.

Alternate means of' removing this highly aromatic and high carbon-forming cycle stock from the unit substantially reduce the virgin-feed. This, of course, is undesirable in that this virgin material cracks with a much lower carbon'forming tendencyl and is'thedesirable material to crack... For. example,.in a combination unit, cycle stock .couldberemoved by'reducing the temperature, byappropri'ate conventionahmeans, in the lower section of tower. 14; This would-ofcourse :simultaneously reduce the boiling range. and-quantity of the virgin material ashed'. Or, it is possible to remove cycleV stock by partial' condensation on fractionation of the totalvapors from cracking stage 16. Due to the low temperature (700 F. or so) required to obtain substantial condensation of cycle stock this would render the remaining vapors unsuitable for use in flashing virgin material from the reduced crude.

Or it is possible to remove cycle stock from line 56. However, in this case an aliquot portion vof virgin feed is also removed.

It can be seen then that the use of this invention selectively removes cycle stock from the system with the minimum loss of heat and subsequent loss of flashed virgin feed, or loss of virgin feed directly.

The present invention is especially adapted for existing combination units which produce excessive coke yfor a given conversion during catalytic cracking and may be installed with minimum disturbance to existing circuits.

Another way of stating the unexpected results obtained with the present invention is that better product distribution is obtained, that is, for the same coke produced, 2 to 6% more gasoline on feed is obtained when the bypass tower is used than when it is not used. Also with the by-pass tower more heating oil is obtained.

The present invention is especially adapted for use with relatively poor quality feed stocks which contain relatively large amounts of condensed ring aromatic hydrocarbons or which on cracking from relatively large amounts of condensed ring aromatics hydrocarbons. Crude petroleum oil of this type comprises, for example, Middle East crudes such as Arabian and Kuwait and South American crudes such as Bachaquero, Lagunillas and Quiriquire.

As an example of the improved results obtained with this invention the following illustrates the advantage gained by its addition to a combination unit. In this '10 example, the crackinggcircuit was operated in both cases' at' maximum total feed rate andvfmaximum carbon burning `rate for maximum conversion. In one case cycle stock was removed in admix'turewithvirgin feed (corresponding to removal per line 5'6 in the' previous example) to give a balanced catalytic cracking operation and maximum gasoline. In the other case the principles of this invention were applied to remove cycle stock-with negligible loss of fresh virgin feed to give a balancedV catalyticcracking operation land maximum` gasoline.

Case A Gase B Mixed Cycle stock Withcycle drawn via' this' stock and invention fresh feed withdrawn Percent Withdrawn (Based ron parts of virgin feed to Case A):

Cycle Stock 4.4 Fresh Feed 9.0 Percent Conversion (to gasolineand 59.3

lighter based on 100 parts of virgin lfeed to Case A). Percent 'Heating Oil (Based on 100 24. 2

parts of virgin feed to Case A). Increased yields of gasoline and heating ol due to application of thisin- 4.1 (gasoline) vention (Based on 100 parts o virgin 0.8 l(Heating Oil) feedto Case A).

The aboveV example shows the application to anv existing combination unit'. It is plain that. this invention is also useful to reduce the initial size ofv the catalytic crackingregenerationiequipment if installed at the outset. The use of this inventiorrwill allow'a-size reduction ofl0-20% in thefregener'a-tion section for a givenvquantity of gasoline.v i

The ratiofof-cracked' products passedfthrough line 62 to the' cracked products passed through line.98 may vary between about 4 to 1i, to -2-*5`ito 1.

The. ratio of bottomslfrom by-pass tower 10-2- to total catalytic crackingjfeed fed through lines 96 plus 56 is about'l to 5, to' l toSO, preferably l tol0, to l to 25. "What'fis claimedis:

1. In a combination crude distillation and a catalytic cracking process wherein crude oil is distilled to produce a distillate and a reduced crude oil fraction containing a substantial quantity of hydrocarbon constitu-ents boiling in the gas oil boiling range and wherein said reduced crude oil fraction is passed to a product fractionation zone operated at a relatively low pressure and the reduced crude oil fraction is stripped with cracked hydrocarbon vapors produced in a catalytic cracking zone to remove said gas oil boiling range hydrocarbons and including constituents boiling in the motor fuel boiling range and lighter and wherein said gas oil boiling range hydrocarbons are segregated in said product fractionation zone and removed and cracked in said catalytic cracking zone operated at a relatively low pressure to produce cracked hydrocarbon vapors, the improvement which comprises passing a major portion of said cracked vapors into the bottom portion of said product fractionation zone for stripping said reduced crude oil fraction in said product fractionation Zone, passing the rest of the cracked vapors to a second separate fractionating zone to remove higher boiling constituents boiling above about 650 F. therefrom, discarding said last-mentioned higher boiling constituents from the process and passing the lower boiling hydrocarbons boiling below about 650 F. from the second fractionating zone to said first mentioned product perature above about 1000 F. and at a pressure above about 200 p. s. i. g., and vapors are passed from said thermal reforming step to the bottom portion of said product fractionation zone to strip said reduced crude oil introduced into an intermediate portion of said product fractionation zone and wherein product distillate fractions and reduced crude bottoms are recovered from said productV fractionation zone and wherein a distillate fraction of intermediate boiling range is withdrawn from said product fractionation zone and subjected to a catalytic low pressure cracking conversion at a temperature between about 800 F. and 1000 F. to produce cracked vapors containing lower boiling constituents, the improvement which comprises introducing a major portion of said catalytically cracked vapors into the bottom portion of said product fractionation Zone to assist in stripping said reduced crude oil in said product fractionation zone,

passing the rest of said catalytically cracked vapors into a separate smaller second fractionating zone to segregate higher boiling refractory hydrocarbons boiling above 650 F. from lower boiling hydrocarbons, discarding said segregated higher boiling hydrocarbons from the process and returning said lower boiling hydrocarbons passing overhead from said second smaller fractionating zone to said first-mentioned product fractionation zone.

3. In a process according to claim 1 wherein the ratio of the cracked vapors passing to the product fractionation zone to the cracked vapors passing to the second mentioned fractionating zone is at least 4 to 1.

4. In a process according to claim 2 wherein at least a'majorproportion of the heat required in said producty cracking unit, means for feeding said vgas oil condensate to said cracking unit, means for withdrawing cracked vapor products from saidcracking unit, the improvement which includes means for passing a major portion of the' cracked vapor products to the lower portion of said strippingv section to act as at least part of the gasiform stripping material, a second smaller fractionating tower, means for passing. the smaller portion of the cracked vaporproducts to said second fractionating tower wherein the cracked vapors are fractionated to separate higher boiling refractory hydrocarbon constituents from lower boiling hydrocarbons, means at the bottom of said second fractionating tower for removing higher boiling hydrocarbon constituents from said second fractionating tower, means for removing lower boiling hydrocarbons in vapor form from the top of said second fractionating tower and for passing them to a lower portion of said fractionating section of said first-mentioned fractionating tower but above the means for withdrawal of gas oil condensate.

6. An apparatus accordingto claim 5 wherein a pipe connects an intermediate portion of said upper fractionation sectionrof said main fractionating tower and the upper portion of said second smaller fractionating tower, said pipe being provided with a pump and a cooler whereby liquid withdraWn from said main fractionating tower is cooled and passed to the upper portion of said second smaller fractionating tower as reflux and a second pipe leading from said r'st mentioned pipe beyond said cooler for returning cooled liquid as renx to the upper'portion of said main fractionating tower.

References Cited in the le of this patent y vUNITED STATES PATENTS 2,174,858 Keith Oct. 3, 1939 2,203,825 Komarewsky June 11, 1940 2,238,860 `Kemp Apr. 15, 1941 2,246,959 Sweeney June 24, 1941 2,294,126 Ocon A-ug. 25, 1942 2,414,883 Martin Jan. 28, 1947 2,644,785 Harding et al. July 7, 1953

Claims (2)

1. IN A COMBINATION CRUDE DISTILLATION AND A CATALYTIC CRACKING PROCESS WHEREIN CRUDE OIL IS DISTILLED TO PRODUCE A DISTILLATE AND A REDUCED CRUDE OIL FRACTION CONTAINING A SUBSTANTIAL QUANTITY OF HYDROCARBON CONSITTUENTS BOILING IN GHE GAS OIL BOILING RANGE AND WHEREIN SAID REDUCED CRUDE OIL FRACTION IS PASSED TO A PRODUCT FRACTIONATION ZONE OPERATED AT A RELATIVELY LOW PRESSURE AND THE REDUCED CRUDE OIL FRACTION IS STRIPPED WITH CRACKED HYDROCARBON VAPORS PRODUCED IN A CATALYTIC CRACKING ZONE TO REMOVE SAID GAS OIL BOILING RANGE HYDROCARBONS AND INCLUDING CONSTITUENTS BOILING IN THE MOTOR FUEL BOILING RANGE AND LIGHTER AND WHEREIN SAID GAS OIL BOILING RANGE HYDROCARBONS ARE SEGRAGATED IN SAID PRODUCT FRACTIONATION ZONE AND REMOVED AND CRACKED IN SAID CATALYTIC CRACKING ZONE OPERATED AT A RELATIVELY LOW PRESSURE TO PRODUCE CRACKED HYDROCARBON VAPORS, THE IMPROVEMENT WHICH COMPRISES PASSING A MAJOR PORTION OF SAID CRACKED VAPORS INTO THE BOTTOM PORTION OF SAID PRODUCT FRACTIONATION ZONE FOR STRIPPING SAD REDUCED CRUDE OIL FRACTION IN SAID PRODUCT FRACTION ZONE, PASSING THE REST OF THE CRACKED VAPORS TO A SECOND SEPARATE FRACTIONATING ZONE TO REMOVE HIGHER BOILING CONSTITUENTS BOILING ABOVE ABOUT 650*F. THEREFROM, DISCARDING SAID LAST-MENTIONED HIGHER BOILING CONSTITUENTS FROM THE PROCESS AND PASSING THE LOWER BOILING HYDROCARBONS BOILING BELOW ABOUT 650*F. FROM THE SECOND FRACTIONATING ZONE TO SAID FIRST MENTIONED PRODUCT FRACTIONATION ZONE.

5. IN AN APPARATUS OF THE CHARACTER DESCRIBED INCLUDING A MAIN FRACTIONATING TOWER PROVIDED WITH AN UPPER FRACTIONATION SECTION AND A LOWER STRIPPING SECTION, MEANS FOR REMOVING VAPORS OVERHEAD FROM SAID TOWER, MEANS FOR INTRODUCING A REDUCED CRUDE OIL AT THE TOP OF SAID STRIPPING SECTION, MEANS FOR INTRODUCING STRIPPING GASIFORM MATERIAL INTO THE LOWER PORTION OF SAID STRIPPING SECTION, MEANS FOR WITHDRAWING A GAS OIL CONDENSATE FROM

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