US2935462A - Selective segregation of hydrocarbon constituents in a combination crude distillation process - Google Patents

Description

May 3, 1960 R. P. CAHN 2, 35,462

SELECTIVE SEGREGATION OF HYDROCARBON CONSTITUENTS IN A COMBINATION CRUDE DISTILLATION PROCESS Filed Aug. 2, 1954 HH mtDwI Robert P Cahn Inventor By dX/K Attorney United States Patent SELECTIVE SEGREGATION or HYDROCARBON CONSTITUENTS m A COMBINATION CRUDE DISTILLATION rnocnss Robert P. Cahu, Elizabeth NJ assignor to Esso Research and Engineering Company, a corporation of Delaware Application August 2, 1954, Serial No. 441,014

9 Claims. c1. 208-81) [This invention relates to the refining of mineral oil, and more particularly to an improved refining process involving integrated hydrocarbon cracking and fractionating steps, as well as to improved apparatus in which said fractionating step is carried out.

In the past, it has been well known in the petroleum refining art to produce topped or reduced crudes by subjecting the crude to various distillation operations. This topped crude, comprising gas oil constituents boiling in the range of from about 6501100 F., and higher boiling constituents, is then handled in a manner to secure a maximum segregation of these gas oil constituents. Normally this is secured by the utilization of a vacuum distillation operation. The gas oil fraction segregated in the vacuum distillation operation is then usually passed to a catalytic or thermal cracking process. Such prior art processes have a number of disadvantages, among these being the necessity for providing a plurality of product fractionators yielding several streams of prodnets of desirable boiling ranges. For economical heat recovery, large numbers of heat exchange apparatus are required, both within each unit and in combination between units. Vast tank facilities must be provided to permit storage of the various products prior to blending in desired proportions. The vacuum distillation equipment used for working up the reduced crude is expensive with respect to investment, operation, and maintenance. As a result of these complications, conventional type combination processes must be operated on a relatively large scale to be economical. Normally, refining capaci ties in excess of about 20,000 barrels per day of crude are required to make operations of this type pay, while smaller refineries must be ,designed on the basis of an often undesirably high output of heavy fuel oil and other products of a relatively low'commercial value.

It is an object of the present invention, therefore, to provide a combination crude distillation and cracking process wherein the vacuum still for reduced crude distillation'is eliminated.

It is a further object of the present invention to provide a process permitting substantial savings in fractionating equipment and tankage facilities.

1 It is a still further object of the invention to provide a process whereby heavy bottoms from reduced crude may be recovered substantially uncontaminated by gas oilfractions.

I It is a still further object of the invention to provide a process whereby cracked gas oil may be recovered for recycling to a cracking unit. 7

- It is a still further object of the invention to provide a process-whereby a portion of the cracked gas oil can berecovered essentially uncontaminated with virgin gas oil in the same boiling range, for withdrawal from the cracking process or for recycling to the cracking unit after suitable treatment.

It is another object of the present invention to provide an.-improved fractionating tower in which the desired separation of hydrocarbons may be carried out.

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These and other objects of the present invention are accomplished by employing a combination crude distillation and cracking process which comprises providing a fractionating column having an upper stripping section, a lower stripping section, and a condensing section positioned between the said stripping sections. Crude oil is subjected to distillation to produce a reduced crude, which is then introduced into the upper stripping zone of the fractionating column. The distillate from the distillation operation is subjected to a cracking operation and the product of this treatment is introduced in, vapor form into the fractionating column at a point below the point of introduction of the reduced crude. These vapors have the eflfect of stripping the reduced crude in the upper stripping section.

where it is further stripped with a vapor of higher temperature which has been introduced into the fractionating column at a point below the point of introduction of the said stripped product. This second stripping operation produces a vapor containing cracked gas oil. The vapor containing the cracked gas oil is passed to a condensing zone in which asubstantial amount of the cracked gas oil is condensed to a liquid. All of the condensed gas' oil is then withdrawn from the fractionating column, a portion of it being withdrawn from the process or recycled to the catalytic cracking stage and a further portion being cooled and recycled to the said eondensin zone.

By providing a process of this nature, all of the condensed gas oil is removed from the column without becoming mixed with the heavy bottoms from the reduced crude and from the heavy product of the cracking stage. Once these bottoms become mixed with the gas oil, separation isvery difficult, but such a problem is obviated by the use of the process of the present invention. The process also has the advantage of utilizing large volumes of hot process vapors for the purpose of stripping the reduced crude of gas oil with the effect that the volume of heavy fuel finally produced may be kept at a minimum and lower than may be accomplished in conventional operation involving atmospheric distillation. A single heavy fuel fraction is obtained in which all heavy constituents uncontaminated by gasoil formed in the various stages are combined, and which may, if necessary, be subected to a single filtering treatment to recover a final low sediment fuel oil of best quality. Alternatively, this oil mayalso be fed to a visbreaking or coking operation.

These advantages are in addition to the obvious saving of heat exchange equipment resulting from the elimination of various intermediate heating and cooling operations 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 light gas oil with a boiling range of about 430650- F., and reduced crude bottoms. The reduced crude-bottoms at this stage may still contain the gas oil fraction intended in part at least for feed stock to a gas oil cracking step. The kerosene fraction may be recovered as product directly from the crude still, because further conversion will not substantially enhance its value as kerosene or diesel oil.

In certain cases, the process of the present invention may be 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 The stripped product is passed from the: upper stripping section to the lower stripping section so operating, the whole crude may be fed to an upper portion of the lower stripping section of the product fractionator. A heavy naphtha stream and gas oil cracking stock may be withdrawn from the fractionator and the latter subjected to various conversions, particularly 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 reheated and returned to the fractionator to assist in crude stripping. Steam and other hot gases may also be added at the bottom of the fractionator to aid in the stripping operation.

The invention may be best understood fromthe more detailed description hereinafter, which refers to the accompanying drawings, in which:

Figure I is a schematic flow plan of a preferred modification of the combination process in accordance with the. invention; and

Figure II is a view in section showing in detail the structure of the product fractionator wherein a series of 'fractionating trays or equivalent means is illustrated.

Referring now to the drawings, and particularly to Figure I, there are illustrated a crude still 10, a reforming unit 11, a catalytic cracking stage 12, and a product fractionator 13. The functions and co-action of these elements will be forthwith explained, using as an example the refining of a medium gravity crude of the type of Arabian Qatar crude in a refinery having a capacity of about 10,000 barrels of crude per day. It should be understood, however, that the system may be used for the refining of difierent types of crude on a larger or smaller scale in a generally analogous manner.

In operation, the crude oil enters the crude still through line 14, usually at a temperature of about 750-800 F. and at a pressure of about 40-70 p.s.i.g. The still 10 may be provided with a plurality of horizontal bubble cap plates to improve fractionation of the feed'in a conventional manner. For the purposes of the present invention, still 10 may be operated to produce three distillate streams and distillation bottoms.

All crude constituents boiling below about 2500 F. are removed as a vapor stream of light virgin naphtha overhead through line at a temperature of about 300- 350 F. This stream may amount to about 25% of the crude charged. A liquid stream of heavy naphtha having a boiling range of about 250-500 F. is removed through line 16 from another portion of still 10. About 20-25% of the crude charged is recovered through line 16. A kerosene or diesel oil out boiling within the range of about 500-700 F. and amounting to about 17-23% of the crude is drawn 011 through line 17. The remainder of the charge, amounting to about 30-50% and consisting predominantly of constituents boiling above 600-700 F., is withdrawn as reduced crude through line 18 from the bottom of still 10. The kerosene cut removed through line 17 is normally suitable for kerosene or diesel oil purposes without further treatment, and it may be passed directly to storage. The other fractions may be treated in. accordance with the present invention, as will be subsequently described. The light virgin naphtha vapors in line 15 may be passed directly to a lower portion of product fractionator 13. If desired, this vapor stream may be preheated to about 1000 1200 F. to conform with the heat requirements of fractionator 13. This may be done by by-passing at least a portion of the vapors in line 15 through a coil 19 located in connection with reforming furnace 11.

The heavy naphtha stream is passed through line 16 at a pressure of about 900-1100 p.s.i.g. to the thermal reformer 11. This reforming stage may be of any conventional design well known in the art. As indicated in Figure I, it may comprise a conventional furnace 11 designed to provide an oil residence time of about 18-25 liquidvolumes per volume of reaction space per hour the fractionator 13 at a point substantially identical withthe feed point of line 15. In Figure II of the drawings these two streams are shown as entering thebottom of Y fractionator 13 at the same point through line 15.

The reduced crude in line 18 may be passed directly to the fractionator 13, substantially at the temperature of its withdrawal from the still 10. Line 18 feeds into the fractionator 13 at a point 18a above the common feed point of lines 15 and 20. In this manner the vapors supplied through lines 15 and 20 pass upwardly'through'.

fractionator 13 against the downwardly flowing reduced crude to strip the latter of vaporizable constituents.

This effect and the operation of fractionator 13 will be" described in greater detail subsequently.

Gas oil hydrocarbons amounting to about 45% to 60% of reduced crude and boiling between about 600 F. and 1100 F. are withdrawn, preferably at a temperature of about 650-750 F., as a sidestream from the fractionator 13 at a point above the reduced crude feed inlet 18a through line 22. A portion of this withdrawn gas oil may be recycled through line 23 and heat exchanger 24- to a point higher or lower in the column in order to provide reflux and temperature control. A" portion of this gas oil, however, passes through line 25 to the cracking stage 12. Any conventional cracking" system adapted to convert gas oil range hydrocarbons into lower boiling oils, particularly of the motor fu'elrange, may be used. While a thermal cracking operation may be used, it is generally preferred to provide'a catalytic process, which may be of a continuous or batch operation in fixed bed, moving bed, fluid or suspensoid systems. supplied as preheat of process materials and/or as sensible heat of exothermically regenerated catalyst, or' in' any other conventional manner. Modified natural or synthetic clay or gel type catalysts such as silica-alumina" composites or 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 to the prior art.

The total hydrocarbon efiluent from cracking stage 12 is passed substantially at the cracking temperature of about 900 F. through line 26 to the fractionator'13 either at a point above the condensing section 21, or below it, or both. In most cases the cracked material enters the fractionator 13 in the vaporous state to enhance the stripping action of the vapors supplied through lines 15' and 20.

" As indicated in Figures I and II, the fractionator 13 comprises an upper fractionating section 13a, an upper stripping section 27, a lower stripping section 28, and an intermediate condensing section 21. Relatively low boiling constituents such as' gas and naphtha and heating oil are taken off via the upper fractionating section 13a as shown. Reduced crude introduced near the upper part of the upper stripping section is contacted'counter currently therein with rising hot vapors from the cracking unit introduced at 26. The reduced crude descending through the tower exchanges its virgin gas oil content with the cracked gas oil introduced through line- 26. By

the time the reduced crude reaches the cracked product feed point, all the virgin gas oil has been stripped or replaced by cracked gas oil in the same boiling range; The stripped reduced crude containing cracked constituents in the gas oil boiling range is accumulated-on total draw-off tray 29 and flows around the'condensingsece- Heat required for cracking may be tion' 21 through line 30 and re-enters the column at the top of the bottom stripping section 28. In this section the condensed portion of the cracked gas oil, together with the high boiling constituents of the reduced crude introduced through line 18 and any heavy constituents introduced from the cracking stage, is contacted with rising vaporsintroduced through line 15. These are the gaseous products of the reformer 11 and the preheated light naphtha. taken from the top of crude still and reheated in coil 19, and also some catalytic cracking productv vapors; This action strips the cracked gas oil from the heavy bottoms, which are removed from the bottom of the fractionator through line 31.

The. cracked gas oils stripped from the bottoms then pass upwardly through the total draw-off tray 32, which defines thebottom of the condensing section 21. The draw-ofi trays 29 and 32 are so constructed as to allow vapors to pass upwardly through them, while preventing liquid from falling down to the next lower tray.

In the condensing section 21 a substantial percentage and preferably all of the cracked gas oil stripped from the liquid in stripping section 28 is condensed on plates 33 and 34, while the uncondensed vapors pass upwardly through the trap-out tray 29Iand are combined with cracking product vapors introduced via line 26 into the tower 13.' 'Although in. the preferred 'form of the invention substantially all of the cracked gas oil is condensed in the-condensing section '21, the. fractionator may be operated in such a way that a portion of the cracked gas oil willpass through the trap-out tray 29 in admixture with the reformer product and will exchange with the virgin gas oil entering with the reduced crude at line 18. The condensed gas oil is removed from the lower trap-out tray 32 through line 35, where it may either be recycled through condenser 36 and line 37 to return to condensing section 21, or may pass through line 38 to subsequent or intermediate processing. A portion of the condensed gas oil may be fed to another treating process such as a phenol treating plant and then recycled to the cracking section. Suflicient cracked gas oil is recycled to the condensing zone 21 in order to maintain the temperature therein in the range of about 600-750 F.

A mixture of virgin and cracked gas oils is removed from fractionator 13 through line 22 as previously described and recycled to the cracking stage.

By the provision of the condensing zone 21 a partial recycle of a so-called heart-cut gas oil fraction to the cracking zonemay be obtained. This permits closer control over the operating conditions of the cracking stage and improved cracking yields. In addition, the heavy crude bottoms removed through line 31 are uncontaminated by any gas oil. As pointed out, the system also provides a method for the selective withdrawal of a catalytically cracked gas oil which may be withdrawn entirely from the system and used for any desired purpose. Also, this cracked gas oil can be subjected to a treating process to improve its catalytic cracking qualities before return to the cracking section.

In some embodiments of the system it may be desirable to have a portion of the reformer products introduced into the fractionator 13 at the point where the cracked products normally enter, i.e., through line 26. Conversely, in some cases it might also be desirable to introduce a portion of the products of the cracking stage at the point where the effluent from the reformer enters, i.e., through line in the bottom stripping zone 28 rather than at the bottom of upper stripping zone 27 as shown by the drawing.

In a typical operation utilizing the present invention, 100 volumes of reduced crude boiling above about 650 F. and containing about 48 volumes of gas oil boiling in the range of from 650-975 F. are fed to the fractionator at the point indicated on the drawings as 18a. The reduced crude may be introduced at a temperature of about 700 F. Catalytic cracking reactor products'containing'72 volumes of recycle stock in the 650975 F. gas oil boiling range are fed to the fractionator through lines 26 and 15. Eighty percent of this material is introduced through line 26, with the remaining 20% introduced at point 15, the latter in admixture with thermal reformer product. The thermal reformer product has a temperature of about 1050 F. so that the gas mixture entering the'bottom of the fractionator is 975 1000 F. The cracked material enters through line 26 at a temperature of about 925 F.

If the condensing section 21 is not operated, the gas oil sidestream 25 will consist of about volumes of material, of which 48 volumes will be virgin material and 72 volumes cracked material, i.e., a 2/3 ratio of virgin cracked gas oil. However, when the condensing section 21 is operatedby pumparound cooling through the cooler 36, a sidestream of 8 volumes of gas oil in the 650-975 F. boiling range can be withdrawnthrough the line 38. This sidestream will consist of 1-2 volumes of virgin material and 7-6 volumes of cracked material, or a ratio of virgin to cracked material of between 1/7 and l/ 3. Under these conditions, sidestream 25 will consist of 112 volumes of material, of which 47-46 volumes will be virgin material and 65-66 volumes will be cracked material. Hence, in this operation the withdrawal of 7 volumes. ofcracked gas oil from the system results in the loss of only 1 2 volumes of virgin gas oil, which is a valuable catalytic cracking feed stock. Without the operation of. the condensing section, 4-7 volumes of virgin material would have to have been withdrawn with the 7 volumes of cracked gas oil through sidestream 25.

' During such an operation the temperature at the top of the condensing section 21 is maintained at 700 F., while at the bottom a temperature of 800 F. is maintained. Condensed gas oil withdrawn at 35 has a temperature of about 750 F., and the portion recycled through cooler 37 and reintroduced through pipe 38 has a temperature of about 550 F. Relatively high boiling material, i.e., material boiling above about 975 F., is withdrawn from the bottom of the fractionator at 31 at a temperature of about 825 F. Relatively low pressure in the order of 8 p.s.i.g. is maintained in the bottom stripping section 28.

While there has been described herein a specific embodiment of the present invention, together with certain equivalent processes, other equivalent steps and processes will occur to those skilled in the prior art. The process of the present invention is intended to cover such a range of equivalents, and is not to be limited to the specific embodiments herein described.

- What is claimed is:

1. In a combination fractionating and cracking process, the improvement which comprises providing a first upper stripping zone, a second lower stripping zone, and an intermediate condensing zone, introducing crude oil con-.

taining virgin constituents in the gas oil boiling range into I said first upper stripping zone, introducing the product of a hydrocarbon cracking treatment in vapor form containing cracked constituents in the gas oil boiling range into said first stripping zone, at a point below the point of introduction of said crude oil containing virgin constituents, stripping said crude oil with said vapors in said first stripping zone to remove most of the said virgin gas oil constituents from said crude oil, passing the stripped product containing some condensed cracked gas oil constituents to said second stripping zone and bypassing said intermediate condensing zone, further stripping said stripped product in said second lower stripping zone with a vapor of higher temperature to produce a vapor containing cracked gas oil constituents, passing the cracked gas oil-containing vapor to said intermediate condensing zone, condensing said cracked gas oil constituents in said condensing zone, withdrawing all of the condensed cracked gas oil from said condensing zone, recycling a portion of saidcondensed gas oil to said condensing zone,

using the unconde'nsed vapors from the condensing' zone" as additional stripping vapors in said first stripping :zone

andwithdrawinga sidestream' from the fractionator at the top of said first shipping zone comprising a mixture of cracked and virgin gas oil constitutents and bypassing at least a portion of said stream to a cracking zone to produce said product of a hyrocarbon cracking treatment.

2. The process according to claim 1 wherein said vapor of higher temperature introduced into said lower stripping section is the product of a hydrocarbon reforming treat-' ment.

3. The process according to claim 1 wherein said reduced crude in said first stripping zone isstripped by a mixture of said, product of said cracking treatment and said vapor of higher temperature.

4 The process according to claim f1 wherein the stripped product in said second stripping section is further stripped by a mixture of said product of said cracking treatment and said vapor of higher temperature.

SLThe process according to claim 1 whereinthe stripping vapors in each of said stripping sections comprise a mixture of said product of said cracking treatment and said vapor of higher temperature.

6. In a combination fractionating and cracking process, the improvement which comprises providing a fractionating column having an upper stripping section, a lower' stripping section, and a condensing: section positioned between said stripping sections, subjecting crude oil'to' form into said fractionating column at a point below the point of introduction of said reduced crude, stripping saidreduced crude with said vapors in said upper stripping section, passing the stripped product to said lower stripping section while bypassing said condensing section, further stripping said product in said lower stripping section with" the product of said reforming treatment in vapor form,- introduced into said lower stripping section at a point below the point of introduction of said stripped product, to produce a vapor containing cracked gas oil, passing the gas oil-containing vapor to said condensing zone, condensing said cracked gas oil in said condensing zone,- passing vapor containing uncondensed gas oil from said condensing zone to said upper stripping section, withdrawing all of the condensed gas oil from said condensing.-

zone, recycling a portion of said condensed gas oilto said cracking treatment, recycling a further portion of said condensed gas oil to said condensing zone, and

withdrawing said uncondensed gas oil from said upper stripping section.

7. The process according to claim 6 wherein said product of the cracking treatment is introduced at'a" temperature of from about 800 F. to about 1000 F. and

the product from the reforming treatment is introduced at a temperature of from about 1000 F. to about 1200' F.

8. The process according to claim 6wherein a portion of the condensed gas oil withdrawn from said condensingzone of said fra'ctionating column is subjected to an additional treating step prior to recycling'to said crackin'g' treatment.

9. The process according to claim 6 wherein a portion of the condensed gas oil withdrawn from said con-' densing zone of said fractionating column is subjected to. phenol treatment prior to recycling to said cracking treat-' ment.

References Cited in the tile of this patent UNITED STATES PATENTS 2,018,309 Jones Oct. 22, 1935- 2,082,224 Snow June 1, 1937 2,130,596 Ocon Sept. 20, 1938 2,644,785 Harding et al. July 7, 1953' 2,777,801 Bittner et a1. Ian.- 1, 1957

Claims (1)

1. IN A COMBINATION FRACTIONATING AND CRACKING PROCESS, THE IMPROVEMENT WHICH COMPRISES PROVIDING A FIRST UPPER STRIPPING ZONE, A SECOND LOWER STRIPPING ZONE, AND AN INTERMEDIATE CONDENSING ZONE, INTRODUCING CRUDE OIL CONTAINING VIRGIN CONSTITUENTS IN THE GAS OIL BOILING RANGE INTO SAID FIRST UPPER STRIPPING ZONE, INTRODUCING THE PRODUCT OF A HYDROCARBON CRACKING TREATMENT IN VAPOR FORM CONTAINING CRACKED CONSTITUENTS IN THE GAS OIL BOILING RANGE INTO SAID FIRST STRIPPING ZONE, AT A POINT BELOW THE POINT OF INTRODUCTION OF SAID CRUDE OIL CONTAINING VIRGIN CONSITTUENTS, STRIPPING SAID CRUDE OIL WITH SAID VAPORS IN SAID FIRST STRIPPING ZONE TO REMOVE MOST OF THE SAID VIRGIN GAS OIL CONSTITUENTS FROM SAID CRUDE OIL, PASSING THE STRIPPED PRODUCT CONTAINING SOME CONDENSED CRACKED GAS OIL CONSTITUENTS TO SAID SECOND STRIPPING ZONE AND BYPASSING SAID INTERMEDIATE CONDENSING ZONE, FURTHER STRIPPING SAID STRIPPED PRODUCT IN SAID SECOND LOWER STRIPPING ZONE WITH A VAPOR OF HIGHER TEMPERATURE TO PRODUCE A VAPOR CONTAINING CRACKED GAS OIL CONSTITUENTS, PASSING THE CRACKED GAS OIL-CONTAINING VAPOR TO SAID INTERMEDIATE CONDENSING ZONE, CONDENSING SAID CRACKED GAS OIL CONSTITUENTS IN SAID CONDENSING ZONE, WITHDRAWING ALL OF THE CONDENSED CRACKED GAS OIL FROM SAID CONDENSING ZONE, RECYCLING A PORTION OF SAID CONDENSED GAS OIL TO SAID CONDENSING ZONE, USING THE UNCONDENSED VAPORS FROM THE CONDENSING ZONE AS ADDITIONAL STRIPPING VPORS IN SAID FIRST STRIPPING ZONE AND WITHDRAWING A SIDESTREAM FROM THE FRACTIONATOR AT THE TOP OF SAID FIRST STRIPPING ZONE COMPRISING A MIXTURE OF CRACKED AND VIRGIN GAS OIL CONSTITUTENTS AND BYPASSING AT LEAST A PORTION OF SAID STREAM TO A CRACKING ZONE TO PRODUCE SAID PRODUCT OF A HYDROCARBON CRACKING TREATMENT.

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