US2904510A

US2904510A - Process for treating catalytic cracking recycle fractions

Info

Publication number: US2904510A
Application number: US596892A
Authority: US
Inventors: Jr Willis J Service
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1956-07-10
Filing date: 1956-07-10
Publication date: 1959-09-15
Anticipated expiration: 1976-09-15

Description

S 15, 1959 w. J. SERVICE, JR

2,904,510 PROCESS FOR TREATING CATALYTIC CRACKING RECYCLE FRA CTIONS Filed July 10, 1956 FIG. 'Q

, CATALYTIC l2 CRACKING FRESH FEED /6 UNIT 20 wwuuu 22 64s 5 6 -/TOWER Pnonucrs j-Nnunm 50 1 RECOVERY 5 L. n:A r/m; OIL

ZONE 2 54 ATMOSPHERIC rows 65 FREHEATER 7 STEAM 7.9 4 PREHEATER 6/ 46 I03 F/G. 2. 1 tzz zgu SPLITTING TOWER //0 mi I 51 5a INVENTOR.

Willis J. Service, Jr., BY

WJMQA A TTORNEY.

United States Patent PROCESS FOR TREATING CATALYTIC CRACK- ING RECYCLE FRACTIONS Willis J. Service, Jr., Baytown, Tex., assignor, by mesne assignments, to Esso Research and Engineering Company, Elizabeth, N.J., a corporation of Delaware Application July 10, 1956, Serial No. 596,892 14 Claims. (Cl. 208-366) This invention relates to a process for treating catalytic cracking recycle fractions. More particularly, this invention relates to an improved distillation process for obtaining recycle fractions of reduced aromaticity to be utilized in conducting catalytic cracking operations.

In the catalytic cracking of suitable petroleum feed stocks, such as feed stocks comprising petroleum gas oil fractions, it is the customary practice to fractionate the hydrocarbon products obtained from catalytic cracking of the feed stock in order to recover desired gasoline boiling range product components and also to obtain a heavier fraction for recycle to the catalytic cracking operation. A serious problem is encountered with respect to the obtaining of such recycle fractions in that undesirably large quantities of aromatic components are normally present therein.

Accordingly, an object of the present invention is the provision of a process for providing catalytic cracking recycle fractions of reduced aromaticity.

Another object is the provision of an improved distillation process for the treatment of catalytic cracking hydrocarbon products in order to obtain recycle fractions of reduced aromaticity.

A further object is the provision of a distillation process for the recovery of a recycle stock from the hydrocarbon products of a catalytic cracking operation Wherein the recycle stock is countercurrently contacted during distillation with a hydrocarbon contacting agent pro duced during the distillation process.

These and other objects are attained, in general, by fractionating the hydrocarbon products obtained from a catalytic cracking operation in a distillation zone under conditions to provide at least a distillate fraction boiling in the range between about -600 and 1000 F. and a bottoms fraction boiling above about 850 F., next obtaining from the bottoms fraction a contact fraction of reduced volatility and then countercurrently contacting at least a portion of the components of said distillate fraction in said distillation zone with said contact fraction, whereby the aromaticity of the said distillate fraction is reduced.

The practice of the present invention will now be described in greater detail in conjunction with the accompanying drawings wherein:

Fig. 1 is a schematic flow sheet illustrating a preferred manner in which the process of the present invention may be practiced; and

Fig. 2 is a fragmentary flow sheet with parts broken away illustrating another manner in which the process of the present invention may be practiced.

Turning now to Fig. 1, there is schematically shown a catalytic cracking unit which is designated generally by the numeral 10. It will be understood that the catalytic cracking unit may be of any suitable construction utilizing, for example, a fixed bed of cracking catalyst or a fluidized cracking catalyst.

A suitable feed stock, such as, for example, a petroleum hydrocarbon feed stock comprising gas oil compo- 2,904,510 Patented Sept. 15, 1959 2 nents, is charged to the catalytic cracking unit 10 by way of a charge line 12. A recycle fraction obtained in a manner to be described is charged to the line 12 by way of a line 14 and is passed to the catalytic cracking unit 10in admixture with the fresh feed.

The mixture of fresh feed stock and recycle fraction is catalytically cracked in the catalytic cracking unit 10 under catalytic conversion conditions including, for example, a temperature within the range of about 800 to about 1150 F. The hydrocarbon products obtained by catalytic cracking of the feed mixture are discharged from the catalytic cracking unit 10 by Way of a discharge line 16 leading to a products recovery zone 18 of any suitable construction. For purposes of simp1ifi cation, the products recovery zone 18 has been schematically shown. It will be understood, of course, that the products recovery zone 18 in actual practice will normally comprise at least one, and preferably a plurality of, fractionating columns provided with suitable vapor liquid contact means, heating means, etc. for fractionating the products delivered to the zone 18 by way of the line 16. By way of illustration, the zone 18 may be operated so as to provide for the separation of gaseous hydrocarbon product components discharged therefrom by Way of a line 20. In similar fashion, a naphtha fraction including desirable gasoline boiling range components may be discharged from the zone 18 by way of a line 22 and heating oil components may be discharged from the zone 18 by way of a line 24.

In accordance with the present invention, there is also recovered in the zone 18 a hydrocarbon product stream from which the recycle fraction and contact fraction of the present invention are to be derived. Thus, for example, a fraction boiling above the heating oil range and preferably comprising from about to volume percent of components boiling above 600 F. is discharged from the zone 18 by way of a line 26 containing a pump 28, the line 26 leading to a preheater 30 of any suitable con struction. l

On passage through the preheater 30, the heavy fraction 26 is brought to a temperature within the range of about 500 to about 800 F. and is discharged from thence by way of a line 32 to a suitable fractionating tower such as an atmospheric distillation tower 34.

Operations in the atmospheric tower 34 are preferably conducted so as to provide a distillate fraction boiling in the range between about 600 and about 800 F. Such a fraction may be discharged from the atmospheric tower 34 by way of the line 36 containing a pump 37, the line 36 leading to the recycle line 14. Lighter components of the charge stock 26, such as components boiling above about 400 F. and a heating oil fraction consisting primarily of components boiling between about 400 and 600 F. are discharged from the tower 34 by way of the

lines

38 and 40, respectively. A heavier bottoms fraction containing a major amount of components boiling above about 800 F. is discharged from the atmospheric tower 34 by way of a line 42 containing a pump 44.

The bottoms fraction 42 is charged to a second preheater 46 of any suitable construction wherein such bottoms fraction is brought to a temperature within the range of about 600 to about 800 F. The thus preheated bottoms fraction is discharged from the preheater 46 by way of the line 48 leading to a vacuum distillation tower 50 which is operated, for example, at a vacuum of about A to 15 inches of mercury, absolute. It will be understood that the vacuum tower 50 will be of any suitable construction comprising, for example, a boot zone 52 and

product recovery zones

54 and 56, the tower 50 being provided with suitable vapor-liquid fractionating contact means such as hell cap trays, etc. The vacuum tower 50 may be operated, for example, to provide for the recovery of a distillate fraction boiling in the range between about 600 and 1000" F., such distillate fraction being discharged from the tower 50 by way of a line 58 containing a pump 60, the line 58 also leading to the recycle line 14.- Lighter components of the charge stock 48 to the tower 50 may be removed overhead therefrom by way of a line 61 and, if desired, by suitable side stream draw-oil? lines (not shown).

In accordance with the present invention, there is also obtained from the vacuum tower 50 a bottoms fraction having an initial boiling point within the range of about 850 to about 900 F. and comprising a substantial portion of components boiling above 900 F. A bottoms fraction of this nature is discharged from the bottom of the boot zone 52 by way of a line 62 containing a pump 64 and controlled by a valve 66.

A contact fraction to be utilized in extractively distilling recycle components for the catalytic cracking operation is derived from the bottoms fraction 62. It is necessary that the contact fraction have a reduced volatility equivalent to the reduction in volatility obtainable by cooling the bottoms fraction by about 50 to about 300 F. The volatility of the bottoms fraction 62 may be reduced to provide the contact fraction of the present invention by any suitable means. Thus, for example, the steam introduced into the boot zone 52 of the tower 50 as a stripping medium by way of a steam line 65 controlled by a valve 67 may be introduced in an amount sufficient to reduce the volatility of the bottoms fraction by the desired amount. When such is the case, the bottoms fraction 62 as withdrawn from the tower 50 may be utilized as a contacting agent without further treatment. In this situation, for example, all or a suitable portion of the bottoms fraction 62 may be returned from the line 62 by way of a line 6% controlled by a valve 70 to the vacuum tower 50 and introduced therein in a manner to be described in greater detail as the contacting agent of the present invention. In similar fashion, all or a portion of the bottoms fraction 62 may be charged by way of a line 72 controlled by a valve 74 to the atmospheric tower 34 for use as a contact agent therein. When a portion of the bottoms fraction 62 is not to be recycled, such portion may be discarded by way of a line 77 controlled by a valve 79.

When steam stripping does not produce the desired degree of cooling, the valve 66 may be closed and a valve 76 in a branch line 78 passing through a cooler 80 may be open whereby further cooling of the bottoms fraction 62 may be obtained. It will be understood that the contact fraction passing through the cooler 80 by way of the line 78 may be charged to the vacuum tower 50 by way of the line 68, to the atmospheric tower 30 by way of the line 72, or both, as described above.

A still further manner in which a contact fraction of reduced volatility may be obtained is shown in Fig. 2. Turning now to Fig. 2, there is shown a suitable distillation tower of the type shown in Fig. 1, such tower being designated generally by the numeral 50', and comprising a boot zone 52 and fractionating zones 54- and 56, respectively. A feed stock 48 of the type obtained in the manner described in connection with Fig. 1 is charged to the tower 50 at a suitable point above the boot zone 52. As in the case of Fig. 1, an overheads fraction 61', a recycle fraction 58', and a bottoms fraction 62 are obtained in the vacuum tower 50. However, in this instance the bottoms fraction in the line 62 is pumped by means of a suitable pump 64 through a charge line 100 leading to a splitting tower 102 containing a heating coil 105 wherein a minor amount of the bottoms fraction is removed overhead by way of the line 103. The remainder of the bottoms fraction is discharged from the splitting tower 102 by way of a line 104 containing a pump 406 and is charged by way of the line 104 to the tower 4 50' as the contacting agent of reduced volatility of the present invention.

In Fig. 2 there is also illustrated a manner in which a contacting agent of the present invention is preferably utilized to countercurrently contact recycle components while such recycle components are in a vaporized condition. Thus, in Fig. 2 there are shown adjacent

bubble cap trays

110 and 112 positioned in the fractionating portion 54 of the tower 50' above the feed inlet line 48. It will be understood, of. course, that the

zones

52, 54' and 56 of the tower 50' may each contain a plurality of suitable fractionating devices and that only the

bubble cap plates

110 and 112 have been shown in order to simplify the drawing. It will be noted that, as is shown in Fig. 2, the contact agent 104 is charged to the distillation tower 50 by suitable distributing means such as a spider 114 which is positioned in the vapor space of that tray which is the highest tray above the feed injection point which is still below the lowest side stream draw-oil 58. It will be understood that the contacting agent will preferably be charged to the vacuum tower 50 and the atmospheric tower 34 of Fig. l in a similar fashion so as to be introduced in the vapor space of the highest tray which is still below the lowest side stream draw-off line.

It will be further understood that the manner in which the recycle fraction 14 is obtained from a sequentially operated atmospheric tower 34 and vacuum tower 50 as shown in Fig. 1 constitutes one preferred manner in which such a recycle fraction may be obtained, but that other suitable distillation recovery systems may also be utilized if desired, provided only that at least a portion (e.g., at least the heaviest 50%) of the components of the recycle fraction for a catalytic cracking operation are countercurrently contacted in vapor phase with a contact agent of the present invention.

Generally speaking, it is desirable that the amount of contact fraction to be employed in distilling recycle components comprise at least about 5 volume percent of the material charged to the distillation tower wherein distillation is to be efiectuated. Preferably, from about to 300 volume percent of contacting agent is employed.

The total amount of recycle material charged by way of the line 14 to the line 12 may be varied widely. Thus, for example, from about 5 to 70 volume percent of recycle material, based on the fresh feed, may be employed. However, if desired, lesser or greater amounts of recycle material may be employed.

What is claimed is:

1. In a process wherein a heavy hydrocarbon product fraction containing a major amount of components boiling above about 800 F. and obtained by the catalytic cracking of a petroleum hydrocarbon feed stock is fractionated in a vacuum distillation zone to provide at least a distillate fraction boiling in the range between 600 and 1000 F. and a bottoms fraction having an initial boiling point above about 850 F. and comprising a substantial portion of components boiling above 900 F., the improvement which comprises the steps of recovering said bottoms fraction exteriorly of said vacuum distillation zone and physically treating said thus-recovered bottoms fraction to obtain therefrom a contact fraction having a reduced volatility and returning said contact fraction to said vacuum distillation zone and countercurrently contacting at least the portion of said vaporized components of said distillate fraction boiling above 800 F. with said contact fraction of reduced volatility in said distillation zone whereby said portion of said vaporized components will be countercurrently contacted while the volatility of said contact fraction is reduced whereby the aromaticity of said thus-contacted portion of said distillate fraction is reduced.

2. A process as in claim 1 wherein said contact fraction is obtained by cooling said bottoms fraction by at least about 50 F.

3. A process as in claim 1 wherein said contact fraction is obtained by removing the lightest components of said bottoms fraction, the remainder of said bottoms fraction comprising said contact fraction.

4. In a process wherein the hydrocarbon products obtained by the catalytic cracking of a petroleum hydrocarbon feed stock are fractionated in 21 products recovery zone and wherein at least one of the fractions obtained in said products recovery zone is a primary firaction containing at least about 80 volume percent of components boiling above about 600 F., the improvement which comprises distilling said primary fraction in a first distillation zone under conditions to provide a first distillate fraction boiling in the range between about 600 and 800 F. and to provide a first bottoms fraction having an initial boiling point above about 850 F. and comprising a substantial portion of components boiling above 900 F., separately distilling said first bottoms fraction in a vacuum distillation Zone under conditions to provide at least a second distillate fraction boiling in the range between about 600 and 1000 F. and a second bottoms fraction boiling above about 850 F., recovering said bottoms fraction exteriorly of said vacuum distillation zone and physically treating said thus-recovered bottoms fraction to obtain therefrom a contact firaction having a reduced volatility and returning said contact fraction to said vacuum distillation zone and countercurrently contacting at least the portion of said vaporized components of said second distillate fraction boiling above 800 F. with said contact fraction of reduced volatility in said distillation Zone whereby said portion of said vaporized components will be countercurrently contacted while the volatility of said contact fraction is reduced whereby the aromaticity of said thus-contacted portion of said second distillate fraction is reduced.

5. A method as in claim 4 wherein said first and second distillate fractions are combined and utilized as a recycle fraction during said catalytic cracking operations.

6. In a process wherein a hydrocarbon product stream comprising a majority of components boiling above about 600 F. is obtained by the distillation of the hydrocarbon products derived from a petroleum hydrocarbon catalytic cracking operation, the improvement which comprises charging said distillate fraction to a vacuum distillation tower operated under conditions to provide a heaviest distillate fraction boiling in the range between about 600 and 1000 F. and a bottoms fraction boiling above at least about 850 F., recovering said bottoms fraction exteriorly of said vacuum distillation zone and physically treating said thus-recovered bottoms fraction to obtain therefrom a contact fraction having a reduced volatility, charging said contact fraction of reduced volatility to said vacuum distillation zone in the vapor space of the highest tray below the draw-01f point for said heaviest distillate fraction to thereby countercurrently contact at least the portion of the vaporized components of the said distillate fraction boiling above 800 F. with said contact fraction of reduced volatility in said distillation zone whereby said portion of said vaporized components will be countercurrently contacted while the volatility of said contact fractionis reduced whereby the aromaticity of said thus-contacted portion of said distillate fraction is reduced.

7. A process as in claim 6 wherein said contact fraction is obtained by cooling said bottoms fraction by at least about 50 F.

8. A process as in claim 6 wherein said contact fraction is obtained by removing at least about the lightest components of said bottoms fraction from said bottoms fraction, said remainder of said bottoms fraction comprising said contact fraction.

9. In a process wherein a heavy hydrocarbon product fraction containing a major amount of components boiling above about 800 F. and obtained by the catalytic cracking of a petroleum hydrocarbon feed stock is heated to a temperature Within the range of 600 to 800 F. and fractionated in a vacuum distillation zone to provide at least a distillate fraction boiling in the range between 600 and 1000 F. and a bottoms fraction having an initial boiling point above about 850 F. and comprising a substantial portion of components boiling above 900 F., the improvement which comprises the steps of recovering said bottoms fraction eXteriorly of said vacuum distillation zone and physically treating said thus-recovered bottoms fraction to obtain therefrom a contact fraction having a reduced volatility and returning said contact fraction to said vacuum distillation zone and countercurrently contacting at least the portion of said vaporized components of said distillate fraction boiling above 800 F. with said contact fraction of reduced volatility in said vacuum distillation zone whereby said portion of said vaporized components will be countercurrently contacted while the volatility of said contact fraction is reduced whereby the aromaticity of said thuscontacted portion of said distillate fraction is reduced, said physical treatment being sufiicient to provide a contact fraction having a volatility after physical treatment equivalent to the reduction in volatility obtainable by cooling said bottoms fraction by about 50 to 300 F.

10. A process as in claim 9 wherein said contact fraction is obtained by cooling said bottoms fraction by at least about 50 F.

11. A process as in claim 9 wherein said contact fraction is obtained by removing the lightest components of said bottoms fraction, the remainder of said bottoms fraction comprising said contact fraction.

12. In a process wherein a hydrocarbon product stream comprising a majority of components boiling above about 600 F. is obtained by the distillation of the hydrocarbon products derived from a petroleum hydrocarbon catalytic cracking operation, the improvement which comprises heating said product stream to a temperature within the range of about 600 to about 800 F charging said heated product stream to a vacuum distillation tower operated under conditions to provide a heaviest distillate fraction boiling in the range between about 600 and 1000 F. and a bottoms fraction boiling above at least about 850 F., recovering said bottoms fraction eXteriorly of said vacuum distillation zone and physically treating said thusrccovered bottoms fraction to obtain therefrom a contact fraction having a reduced volatility, charging said contact fraction of reduced volatility to said vacuum distillation zone in the vapor space of the highest tray below the draW-oif point for said heaviest distillate fraction to thereby countercurrently contact at least the portion of the vaporized components of the said distillate fraction boiling above 800 F. with said contact fraction of reduced volatility in said distillation zone whereby said portion of said vaporized components will be countercurrently contacted while the volatility of said contact fraction is reduced whereby the aromaticity of said thuscontacted portion of said distillate fraction is reduced, said physical treatment being sufi'icient to provide a contact fraction having a volatility after physical treatment equivalent to the reduction in volatility obtainable by ggglinlg said bottoms fraction by about 50 to about 13. A process as in claim 12 wherein said contact fraction is obtained by cooling said bottoms fraction by at least about 50 F. I

14. A process as in claim 12 wherein said contact fraction is obtained by removing at least about the lightest components of said bottoms fraction from said bottoms fraction, said remainder of said bottoms fraction comprising said contact fraction.

Kraft et al May 30, 1939 Owen et a1. July 24, 1956

Claims

1. IN A PROCESS WHEREIN A HEAVY HYDROCARBON PRODUCT FRACTION CONTAINING A MAJOR AMOUNT OF COMPONENTS BOILING ABOVE ABOUT 800* F. AND OBTAINED BY THE CATALYTIC CRACKING OF A PETROLEUM HYDROCARBON FEED STOCK IN FRACTIONATED IN A VACUUM DISTILLATION ZONE TO PROVIDE AT LEAST A DISTILLATE FRACTION BOILING IN THE RANGE BETWEEN 600* AND 1000* F. AND A BOTTOMS FRACTION HAVING AN INITIAL BOILING POINT ABOVE ABOUT 850* F. AND COMPRISING A SUBSTANTIAL PORTION OF COMPONENTS BOILING ABOVE 900* F., THE IMPROVEMENT WHICH COMPRISES THE STEPS OF RECOVERING SAID BOTTOMS FRACTION EXTERIORLY OF SAID VACUUM DISTILLATION ZONE AND PHYSICALLY TREATING SAID THUD-RECOVERED BOTTOMS FRACTION TO OBTAIN THEREFROM A CONTACT FRACTION HAVING A REDUCED VOLATILITY AND RETURNING SAID CONTACT FRACTION TO SAID VACUUM DISTILLATON ZONE AND COUNTERCURRENTLY CONTACTING AT LEAST THE PORTION OF SAID VAPORIZED COMPONENTS OF SAID DISTILLATE FRACTION BOILING ABOVE 800* F. WITH SAID CONTACT FRACTION OF REDUCED VOLATILITY IN SAID DISTILLATION ZONE WHEREBY SAID PORTION OF SAID VAPORIZED COMPONENTS WILL BE COUNTERCURRENTLY CONTACTED WHILE THE VOLATILITY OF SAID CONTACT FRACTION IS REDUCED WHEREBY THE AROMATICITY OF AID THUS-CONTACTED PORTION OF SAID DISTILLATE FRACTION IS REDUCED.