US2723948A - Catalytic cracking heat exchange process - Google Patents

Description

NOV l5, W N. MCCURDY JR CATALYTIC CRACKING HEAT EXCHANGE PROCESS Filed Aug. l, 1950 Clbborrle United States Patent O l CATALYTIC CRACKING HEAT EXCHANGE PROCESS William N. McCurdy, Jr., Linden, N. J., assignor to Esso Research and Engineering Company, a corporation of `Delaware Application August 1, 1950, Serial No. 176,979 2 Claims. (Cl. 196-52) This invention generally concerns a catalyticcracking operation in which a gas oil fraction of a petroleum crude oil is cracked to yield lower boiling constituents. In particular the present invention concerns the specific problem of operating heat exchangers in conjunction with a fluidized catalytic cracking operation.

The principal` object of this invention is -to overcome certain problems of heat exchanger fouling ordinarily encountered in processing the streams connected with a liuidized catalytic cracking operation. The particular heat exchanger referred to is a heat exchanger conventionally used to preheat the oil fed to the catalytic cracking reactor. Thus, it is necessary to preheat the oil feed to a temperature of 500 to 700 F. to secure the `desired temperature conditions before injection of the oil into the catalytic reaction vessel. A large portion of this heating requirement can be met by passing the oil feed in heat exchange relationship with certain streams of the catalytic cracking system. However, it has been found that relatively rapid fouling of the heat exchanger in which this is carried out occurs. The fouling isapparently due to deposition on the heat exchange coils of gummy material; presumably caused by polymerization of constituents usually occurring in the presence of oxygen dissolved in the oil feed, or possibly due tocarbonization of portions of the feed. It appears that these fouling constituents deposit `on heat exchange tubes when the oil feed has been raised to temperatures in the range of 250 to 400 F.

The fouling of these heat exchange tubes is clearly disadvantageous. First, it results in the lowering of heat exchange co-eftcients so that even after a very short period of use, new or cleaned heat exchange tubes will lose some portion of their heat transfer capabilities. The result is `that heat exchange to the oil feed will become ineicient and temperature requirements in the entire catalytic cracking system may become maladjusted. At times the failure to preheat the oil feed sufficiently has been responsible for directly affecting the throughput at which a catalytic cracking unit could be operated. To keep this diiiculty within limits it consequently becomes necessary to frequently clean the heat exchange surfaces.

Heretofore the cleaning of heat exchangers employed in the indicated Service has been a diiiicult and expensive problem. While the use of solvents has been attempted, no solvent has been found which is suitable for adequately removing the gummy constituents so as to properly clean the heat exchange surfaces. It has consequently been the general practice to partially, or completely dismantle the heat exchange equipment to permit mechanical cleaning of the tube surfaces. As the heat exchangers are generally arranged in the form of bundles of a large number of tubes, it is difficult, or impossible to adequately clean many of the tubes of the bundle except at their points of tangency with a cleaning bar which may be inserted through the bun- 21,723,948! Patented Nov. 15, 1955 dle. Again, when the tube bundle has become slightly deformed it is often impossible to introduce a cleaning bar through the bundle. Similarly, it has been found that Sandblasting is an expensive and time-consuming method to clean the tube bundles, as the sand does not penetrate to the inner tubes unless great care is used, but simply cleans the outer tubes of the bundle.

For these reasons, therefore,` adequate means to sm` ply and effectively clean the tubes of a heat exchanger associated with a catalytic cracking unit has been aV serious operational problem. As will be disclosed, the process of this invention adequately solves this problem in an advantageous manner.

The specific nature of the problem involved, and the manner in which the process of this invention solves this problem may be appreciated by reference to the accompanying drawing, and the following description. The drawing to which the description relates, diagrammatically illustrates one ow plan employed in catalytic cracking systems.

Referring now to the drawing, the numeral 1 identifies a catalytic cracking reactor in which the cracking operation may be conducted. The numeral 2 represents the associated catalyst regenerator and the remaining processing units shown in the drawing identify the slurry equipment and associated apparatus required. The catalytic cracking process conducted in reactor l is of the nature identified as a liuidized catalytic cracking operation. The term fluidized identities the manner in which the solid catalyst particles are maintained as a dense boiling bed by passing gas through the particles to maintain a high degree of turbulence. Thus, if oil feed which already is vaporized, or will become vaporized upon contact with the hot catalyst, depending on the oil temperature, is introduced through line 3 into the lower portion of reactor 1 together with solid particles of catalyst, by critically adjusting the rate of ow of the oil feed, and the proportion of solid catalyst particles, it is possible to maintain the catalyst par ticles in reactor 1 in a fluidized state. The catalyst particles will under these conditions assume the characteristics of a liquid so that an upper level 4 may be maintained in the reactor. The catalyst particles below the level 4 will beA in the form of a dense bed of turbulent particles visually appearing as a boiling liquid-like mass. The gaseous oil feed will pass upwardly through this mass of catalyst particles to be removed from the top of the reactor through line 5.

As the process described is a continuous process, catalyst and oil feed are continuously introduced to the lower portion of reactor 1 as described, while cracked products are removed at the top of the reactor through line 5 as described, and catalyst is continuously removed from the bottom of reactor 1 through draw-o 6. The catalyst Withdrawn through draw-off 6 may be said to be spent catalyst, having been retained in the reactor 1 for sufficient periods of time to have become fouled with carbon and gummy material. This catalyst may be regenerated by the process of burning the fouling constituents from the catalyst. For this purpose air may be passed through line 7 to carry the spent catalyst upwardly through line 8 to regeneration zone 2. In zone 2, as in reactor 1, the catalyst is maintained in a fluidized condition by the air. By suitably controlling the air to catalyst ratio, and the temperature of the system, it is possible to oxidize or burn the catalyst impurities so as to regenerate the catalyst. Regenerated catalyst may then be Withdrawn from the regenerator through standpipe 9 for recycle to reactor 1 as formerly described.

The gas velocities employed to secure the desired fluidization of the catalyst in reactor 1 and regenerator 2 are a slurry, and is generally identified as slurry oil.

generally in the range of aboutV l to 3 feet per second. While the density of the catalyst particles in the gas under uidized conditions will depend upon the catalyst particle size, and the particulargas velocity employed, in general the density of the fluidized bed will be about 10 to 25 pounds per cubicV foot. Gas and catalyst ow rates are adjusted to-secure this density. Reactor 1 is maintained at temperatures in the range of 900 to 1000 F. while regenerator 2 is maintained at temperatures of about 1050 to 1150 F. The catalyst employed may be selected from vany of the known cracking catalysts, such as alumina supported on Group 6 metals. Similarly, the catalyst may be cobalt, nickel, iron or compounds of Group 6 oxides, with nickel, cobalt or iron. VThe catalyst is preferably in powdered form having a size of about 200 to 400 mesh, although particles outside this range may be em- Y ployed.

Insofar as the cracking process heretofore generally identified is well known to the art no further description of these elements of the present invention will be given. To understand the process of this invention, the description will now be given of the manner in which the product stream from reactor 1 is processed, and the manner in which the oil feed passing through line 3 is preheated.

Referring first to the processing of the cracked product stream of line 5, it may be noted that this stream will consist principally of cracked constituents originally present in the oil feed. As conversion is not 100%, the stream of line 5 will also contain some portion of the uncracked original constituents of the oil feed. ln addition, some catalyst particles will be carried from reactorV 1 in line 5.

In this connection it may be noted that a cyclone separator or several stages of cyclone separators may be employed to minimize catalyst carry over in line 5. However, it is impossible to avoid some catalyst entrainment. For this reason the stream of line 5 will contain about 2% of the catalyst introduced to the bottom of reactor 1. The stream of cracked products from the reactor 1 is then conducted through line 5 to the fractionator 10. As illustrated, the fractionator may consist of a conventional type of fractionator employing a bottom pump around circuit. The fractionator may contain any desired type of fractionating plates, or packing material, or as illustrated, the upper part of fractionator 10 may contain fractionating plates 11, while the lower part of fractionator 10 may contain the disc and donut packing diagrammatically illustrated and-designated by numeral 12. The products of line 5 are introduced to the bottom of fractionator 1t), and the fractionated constituents are then removed from fractionator 10 through overhead line 13, and side stream withdrawal lines 14 and 15. The heavier boiling constituents existing in the material introduced to the fractionator are withdrawn from the bottom of fractionator 16, through line 16. These heavier boiling constituents will contain catalyst carried over from reactor 1. As this catalyst has now been Vconcentrated in the lower boiling fractions, the catalyst concentration will be about 0.2 pound per gallon to about l pound per gallon, although normally the catalyst concentration is about 0.4 pound per gallon. This concentration of catalyst in the heavy constituents of line 16 provides a heavy oil appearing as The stream of slurry oil withdrawn through line 16 is then subdivided to pass in part through the waste heat boiler 17 and the feed preheater 18. The portion of the slurry oil passed through the waste heat boiler 17 is passed in heat exchange relationship with water so as to remove heat from the slurry oil and simultaneously produce useful steam. The cooled slurry oil is then re-introduced to fractionator 10 through line 19. The portion of the slurry oil pumped through the feed preheater 18 is similarly passed in heat exchange relationship with oil feed so as to preheat the oil feed. This is generally conducted, as shown by the solid lines, by passing theslurry oil of line 16 through the coils 20 of the preheater 18. The oil feed pumped through line 1 is circulated around the outer portion of the coils 20 for removal from the heat exchanger through line 3. The slurry oil withdrawn from fractionator 10 has a temperature of above 500 F., or preferably about 600 or 650 F., and in passing through preheater 18 is generally cooled to about 450 F. This cooling of the slurry oil is effective to heat-oil feed, by heat exchange, to a temperature of about 400 F; and preferably to above 500 F. As formerly indicated, operation of this feed preheater results in the deposition of fouling materials on the coils 2,0. In a typical commercial installation, ithas been found that heat exchange coefficients can drop fromabout 50 to 30 in a period of l5 days, as a result of this fouling. Ordinarily this necessitates cleaning of coils 2t) after about l5 days of operation.

in accordance with this invention, the undesired drop in heat transfer co-efficients, and the necessity for removiug the heat exchanger Yfrom the system for cleaning, may be Vavoided by periodically altering the flow Q the slurry oil and oil feed through the heat exchanger. The neCessary alteration in heat exchange operation is indicated by the dashed. lines in the drawing. Thus. Periodically, slurry oil is cutoff from passage through coils 20 and is passedl through line 22 so as topess around, rather than through Coil v20. Theslurry oil may theo be removed from the heet. exchanger through line 23. Simultaneously oil feed through the shell of the preheater is discontinued, and the oil is passed-through line 24 for introduction directly into coil 20 and yfor removal from coil 20 through line 25. For simplicity, the valves required in the lines associated with the preheater have not been illustrated. `It is apparent that by these changes the normal ow of slurry oil through coils 20 and oil feed around coils 20 is reversed. In the reversed condition, oil feed is passed through coils 20 while slurry oil is passed through the shell around coils 20. It has been found that by periodically reversing the slurry oil and oil feed flows in this manner, fouling of heatvexchanger 2,0 may be substantially eliminated. At the same time the undesired drop in the heat transfer co-eflicients of the heat exchanger is substantially `eliminated.

As a specific example of a process employing this invention, a commercial catalytic cracking operation was conducted embodying this invention. During the test period slurry oil was introduced to the heat exchanger at a temperature of about 600 F. and was withdrawn at a temperature of about 45.0 F- Oil feed, was .introduced to the heat exchanger at a temperature of about 250 F. and was withdrawn at a temperature of about 450 F. About 60,00() gallons per hour of slurry oil and about 35,000 gallons per hour of oil feed were pumped through the heat exchanger. The Catalyst coutent of the slurry oil was 0.4 lb. of catalyst per gallon o f oil. At the beginning of the test period, the outside of coils 20 were fouled. Upon interchange of the two fiuids for 48 hours, the slurry cleaned the outside of the tubes. This was proven because when the uids were again interchanged to their original flow courses, it was found that the overall heat transfer coeicients ofthe heat exchanger were improved an average of 15.7 B. t. u.,/ hr./ sq. foot/ F. and the oil feed outlet temperature had been increased an average of 36V F. as a result of this operation. Furthermore, it was found that the heat transfer coefficients were maintained equal to those secured in fresh mechanically cleaned heat exchange bundles.

As described, therefore, the process of this invention specifically Concerns the operation of the oil feed preheater associated with a catalytic cracking operation. The oil feed preheater passes an oil feed in heat exchange relationship with hot slurry oil from thecatalytic cracking operation. By periodically reversing the paths of flow of the oil feed, and slurry oil through the heat exchanger, material operational advantages are securedr as described. lt is apparent that the frequency of how reversal in this manner may be chosenas desired. Experience has shown that flow reversal about every 15 days is desirable, and that this flow reversal must be maintained for about 48 hours to secure effective cleaning by the slurry of the outside of the tubes. While the invention has been described with regard to a particular type of fluidized cracking operation, it is apparent that the invention is equally applicable to other embodiments of tluidized cracking processes. For example, while the process described to exemplify the present invention consists of what has been described as a downflow catalytic cracking operation, this invention is equally applicable to upllow cracking operations.

What is claimed is:

1. A catalytic cracking operation which comprises heating a gas oil feed to a temperature in the range of 400 to 650 F. in a preheating zone, catalytically cracking said heated gas oil in a fluid catalyst cracking zone, passing the cracked products from said cracking zone together with entraincd solid catalyst to a fractionation zone, separating from the cracked products a hot high boiling hydrocarbon oil slurry fraction containing about 0.2 to 1% of solid catalyst particles per gallon of oil, passing the said hot high boiling hydrocarbon slurry fraction from said fractionation zone in indirect heat exchange relationship with the said gas oil feed stream in said preheating zone, and periodically reversing the flow of the feed gas oil and high boiling hydrocarbon slurry fraction while substantially maintaining flow of said liquids so as to bring the gas oil feed and high boiling slurry fraction on opposite sides of the heat exchange surfaces of the said preheating zone and maintaining such reversed ow for an extended period of time whereby the solid particles in the slurry remove fouling material from said preheating zone deposited from said gas oil feed and to improve the heat transfer coefficient between the two oil streams.

2. A catalytic cracking operation which comprises heating a gas oil feed to a temperature in the range of 400 to 650 F. in a preheating zone, catalytically cracking said heated gas oil in a fluid catalyst cracking zone, pass ing the cracked products from said cracking zone to# gether with entrained solid catalyst to a fractionation zone, and separating from the cracked product a hot high boiling hydrocarbon oil slurry fraction containing about 0.2 to 1% of solid catalyst particles per gallon of oil, passing the said hot high boiling hydrocarbon slurry fraction at a temperature of about 650 Fn from said fractionation zone in indirect heat exchanger relationship with the said gas oil feed stream in said preheating zone, and periodically reversing the flow of the feed gas oil and hot high boiling hydrocarbon slurry fraction while substantially maintaining ow of said liquids so as to bring the gas oil feed and high boiling slurry fraction on opposite sides of the heat exchange surfaces of the said preheating zone and maintaining such reversed flow for about 48 hours whereby the solid particles in the slurry remove fouling material from said preheating zone deposited from said gas oil feed and to improve the heat transfer coeflcient between the two oil streams.

References Cited in the flc of this patent UNITED STATES PATENTS 2,210,257 Pyzel et al. Aug. 6, 1940 2,353,399 Herthel July l1, 1944 2,423,833 Hirsch July 15, 1947 2,493,494 Martin Jan. 3, 1950 2,576,843 Lockman Nov. 27, 1951

Claims (1)

1. A CATALYTIC CRACKING OPERATION WHICH COMPRISES HEATING A GAS OIL FEED TO A TEMPERATURE IN THE RANGE OF 400* TO 650*F. IN A PREHEATING ZONE, CATALYTICALLY CRACKING SAID HEATED GAS OIL IN A FLUID CATALYST CRACKING ZONE, PASSING THE CRACKED PRODUCTS FROM SAID CATALYST CRACKING ZONE TOGETHER WITH ENTRAINED SOLID CATALYST TO A FRACTIONATION ZONE, SEPARATING FROM THE CRACKED PRODUCTS A HOT HIGH BOILING HYDROCARBON OIL SLURRY FRACTION CONTAINING ABOUT 0.2 TO 1% OF SOLID CATALYST PARTICLES PER GALLON OF OIL, PASSING THE SAID HOT HIGH BOILING HYDROCARBON SLURRY FRACTION FROM SAID FRACTIONATION ZONE IN INDIRECT HEAT EXCHANGE RELATIONSHIP WITH THE SAID GAS OIL FEED STREAM IN

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