US2868717A - Production of gasoline by catalytic cracking - Google Patents

Description

Jan. 13, 1959 I E. L. CLARIDGE PRODUCTION oF GASOLINE BYCATALYTIC cRAcxING Filed Nov. 19, 1956 INVENTORI ELMOND L. CLARIDGE BY' MQ( WW HIS ATTORNEY mesmo United States atnt PRODUCTION OF GASLINE BY CATALYTHC CRACKING Elmond L. Claridge, Houston, Tex., assigner` to Shell Development Company, New York, N. Y., a corporation of Delaware Application November 19, 1956, serial Ns. 623,255

' s Claims. k(ci. 20s-94) This invention relates to the production of gasoline from petroleum residues through catalytic cracking.

Although other methods are sometimes employed, the general practice in producing gasoline from petroleum residues through catalytic cracking involves separation of a suitable catalytic cracking feed stock from the petroleum residue by a flashing operation. In this operation, the petroleum residue is heated in a suitable coil in a furnace to a flashing temperature which is generally on the order vof 750-95G F. and the heated and partially vaporized residue is then passed to a flashing chamber in which the pressure is lower than that prevailing in the heating coil. The pressure in the flashing chamber may be up to about 50 p. s. i. g. but is usually maintained below atmospheric pressure by a suitable source of vacuum. The pressure may be as low as -20 mm. Hg. In some cases, two stages of flashing are applied with the rst Stage being under a slight super-atmospheric pressure, e. g. 40 p. s. i. g., with the second flashing stage under vacuum, e.. g. 100 mm. Hg., either with or without heating between the hashing stages. Under these conditions and especially when the pressure is below atmospheric, the vapor velocities in the flashing zone are quite lhigh and the residence time is quite low. As a consequence, there is considerable tendency for the flashed vapors to entrain part of the residue. Even traces of entrained residue exert a very detrimental effect on the quality of the overhead product as catalytic cracking feed stock. Also, the petroleum residue normally contains appreciable amounts of organo-metallic compounds which tend to pass overhead with the flashed product and these two are extremely detrimental even in very small concentrations.

In order to reduce the carryover of residue and reduce somewhat the concentration of organo-metallic compounds (which are of low volatility), it has been the practice in some cases to pass the flashed vapors through a relatively inefficient rectification zone to which part of the condensed flashed product is supplied as knocle back or reux. Usuallyfonly from 3 to 10 trays are provided. An ecient rectication cannot be applied here since the pressure drop required for such rectification would be prohibitive and the temperatures are in the range where the increased residence time required for e'cient rectification would lead topundesired thermal reactions and coking.

.The bottoms from the flashing operation may be further flashed as described but are generally withdrawn as a product of the process and sent to thermal cracking, asphalt manufacture, Bunker C fuel oil, or the like, depending upon the particular situation at hand. The reflux from the `rectication zoneis sometimes recycled to the furnace but is normally combined with the asher bottoms and are hence withdrawn This reflux stream contains the tarry material which, except for the reux stream, would be entrained in the asher overhead prodtuct.

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The flasher overheadproduct after being cooled to condense normally liquid hydrocarbons is passed as the feed stock to the catalytic cracking unit.

In the catalytic cracking unit, the said oil is cracked in the vapor phase at temperatures upward of 850 F. to about 10.50 F. with a suitable solid cracking catalyst. The catalytic cracking step is effected in the conventional manner which is well known and therefore requires no detailed description. The vapors from the catalytic cracking reactor consist of normally gaseous materials as well as gasoline, gas oil and partially converted and unconverted feed oil, and therefore boil over the whole temperature range up to about .1000 F. This material, containing for example 30% gasoline, is passed to a fractionation zone and enters the fractionation zone at a temperature which is close to that in the catalytic cracking reactor, i. e., it is vastly superheated. It is introduced into the fractionation zone near the bottom below a gas oil drawoff tray. In this bottom section there is little or no rectification. In order to effect efficient fractionation of the oil vapors from the catalytic cracking unit into the desired products in the upper portion of the fractionation column, a part of the bottom product from the fractionator is withdrawn, cooled, and reintroduced into the fractionation column at a point in the bottom section above the feed inlet point but below the said gas oil drawoff tray. A large amount of this oil is recycled in this way to effect a sucient desuperheating of the feed entering the fractionation section of the fractionating column. The net make of this oil is removed as bottom product from the fractionation column.

The vapors passing from the catalytic cracking reactor to the fractionator normally carry small though appreciable amounts of catalyst fines and these fines collect in the bottom product of the fractionator. Thus, this bottom product is usually called slurry oil. This slurry oil is of exceptionally poor quality as a catalytic cracking feed stock. lt is a heavy, highly aromatic and highly refractionary oil which, when included in catalytic cracking feed stock, greatly increases the coke make. It is therefore normally withdrawn from the system and either thermally cracked, used as a .cutter stock in producing Bunker C fuel oil, or used for the production of carbon black. in some cases, however, it contains considerable amounts of cracking catalyst, and in order to recover this catalyst the oil is returned to the catalytic cracking unit in spite of its poor quality. In these cases, however, the slurry oil is settled to produce a thickened slurry thereby allowing return of the catalyst with a minimum recycle of the oil vehicle to the catalytic cracking unit. The relatively catalyst-free oil from such thickening operation,

. generally called clarified oil, is withdrawn as a product and used as mentioned above.

The process of the present invention follows in broad outline the above described operations. However, in the process of the invention, better use is made of the slurry oil, thickened slurry and/ or claried oil while at the same time improving the feed stock to the catalytic cracking unit.

In spite of the fact that the slurry oil is an exceptionally poor catalytic cracking feed stock, it contains valuable feed stock components. These are concentrated in the front end of the oil. This is due to the fact that in the application of the slurry oil as a means for desuperheating the cracked vapors fed to the fractionator, this slurry oil receives practically no rectification.' Consequently it boils over a very wide range (in spite of its high temperature origin inthe fractionating column) and generally has a quite low Engler initial boiling point,

heatedl condition.

'J3 e. g. 450 F. On the other hand, at the temperatures normally encountered in the rectification section of the flashing operation, e. g. 60G-800 F., the cracking catalyst is capable of acting favorably on the removal of coke forming constituentsin the catalytic-cracking feed.

'In the process of my invention,'the net make of the slurry oil 'or a part of it at tan "adjusted temperature (depending upon"tlie"'volurne) is used as a knockback in the rectification section ofthe Vasher used for the catalytic cracking 'feed stock preparation whereby the desirable catalytic cracking feed stock components in the slurry oil are vaporized by the catalytic cracking feed stock vapors in this section andare thus returned tothe catalytic cracking unit. In the preferred embodiment of the invention, the catalytic cracking feed stock is improved by 'contactwith the'cracking catalyst in this section in which case two advantageous results are realized.

The improved process of the present invention will be described as to its 'detailsin the following in which reference will bemade to a specific preferred embodiment illustrated in the'ow diagram which constitutes the accompanying drawing. In this flow diagram, the few apparatus indicated are illustrated diagrammatically.

Referring to the drawing, reduced crude, i. e., crude petroleum whichV has been topped to remove lower boiling straight-run material, is, passed by line 1 to the coils 2 in a suitable heater 3 wherein it is heated to an outlet temperature of'about 800 F. The heated and partially vaporized reduced crude is then passed by the transfer line 4 to the vacuum asher vessel 5 in which the'pressure is maintained at about 70 mm. `ll-lg. The material enters the flash chamber tangentially so that the centrifugal action of the swirling vapors tends to throw out the major part of the suspended residue droplets. The residue, which in this case is pitch, is withdrawn from the flash chamber by line 6.

The ashed vapors pass through the overflow of the ash chamber to the bottom section of the rectifier 7. Rectifier 7 is provided in this case with four grid trays. Bubble trays may not be used since they would be clogged with catalyst. Shower decks may be used, but are less e'fcient than grid trays. A stream of liquid oil containing finely divided catalyst in suspension is introduced into the upper portion of the rectifier 7. This material passes downward through the rectification zone countercurrent to the rising vapors from the flasher. The vapors entering the rectification zone in this case are at a temperature of about 710 F. and in appreciably super- Thcy are in relatively large amount compared to the amount of slurry oil introduced. For example, in the-particular case, the ratio of these two materials introduced is about 10:1.

The overhead-vapors from the rectifier 7 are cooled in condenser 8 and passed to separator 9. The vapors are drawn od to the vacuum source via line 10, and the liquid condensate, called flashed distillate, is passed to the catalytic cracking unit 11.

ln a typical case, the flashed distillate is cracked in the catalytic cracking unit at a temperature of 950 F. to a 55% conversion. The vapors from the catalytic cracking reactor are passed via line 12 to the bottom section of fractionator 13. These vapors carry some catalyst fines in suspension. The catalyst collects in the heavy oil at the bottom of the fractionator and is withdrawn in suspension with the oil. A large amount of this bottom oil, i. e., slurry oil, is withdrawn and recycled by line 14 after cooling in cooler 15 in order to desuperheat the hydrocarbon feed entering the upper portion of the fractionator. The net make of slurry oil is passed byline 16 to. the `topof rectifier7 as'described.

In two typical-casesf'thelvapors entering the rectifier 7fhave the.followingzEnglerdistillations shown incolumns A and B:

TABLE 1 Vacuum erzgler distillation corrected to atmospheric pressure, F.

A B C D E F 159 134 se? ses v901 919 914 972 948 1,027 96s 1,081 60% ...991 70%-- 1,016 1, 054.v 900 1,112 E. P 945 1,000

The slurry oil in two typical cases 'has the distillation shown in columns C and D in the above table. The slurry oil removed from the bottom of the rectifier 7 has, in two typical cases, the distillation shown incolumns E and F inthe above table.

It will be seen that a sizable amount of the front end of the slurry oil is vaporized and removed with the flashed vapors. It is generally desirable to remove only this portion of the front end of the slurry oil since, if the flashing and rectification are carried out under conditions to cut deeply into the slurry oil, ythe quality of the` catalytic cracking feed stock is impaired. Generally, the amount volatilized should not be more .than 50% of the slurry oil introduced. T he amount of the slurry oil removed in the rectification step is regulated by regulating the amount and temperature of the slurry oil supplied to the rectification Zone as described. If the net make of slurry oil is excessive, the excess is removed by line 17. Ifvthe net make of slurry oil is insufficient, the slurry oil may be recycled. Thus, in the caseillustrated, the feed to the flasher is 30,700 bbl/day. 'The flashed distillate vapors passing to therectifier 7 amounts to 24,420 bbl./ day. VThe net make of slurry -oil in the fractionator 13 is 1,110 bbl./day. The net make ofslurry oil withdrawn from the system by line 18 is 880 bbl./ day. This net .make of* slurry oil from the fractionator 13 is insufficient for the optimum result. Therefore, the slurry oil withdrawn from the rectifier 7 byline 19'is passed to a thickener 20. A thickened slurry is withdrawn from the thickener by line 21 and recycled to the rectier 7 in an amount of 1,800 bbl./ day, which, with the slurry oil from ,thefractionator, gives a ratio of flashed distillate to slurry oil in the rectifier 7 of about 10.5. It will be appreciated that in order to maintain the system in balance, the slurry oil removed by line 18 as net make will contain the same amount vof'catalyst as that'withdrawn fromfractionator 13.

The temperature ofthe slurry oil leaving the bottom of fractionator 13 pis normally between about 600-700 F., and inthe particular caseiit is about 687 F. If 'this slurry oil were passed to the rectier`7 `at this temperature, more than 50% of the slurry oil'would be vaporized and combined with the flashed distillate. The slurry oil is therefore generally cooled somewhat, e.`g., to S50-'550 F. by the cooler 22 in line 16. In some cases, this cooler may be eliminated bymaking use'of the cooler 15. "Ihus, instead of removing'net slurry oil'make as shown, it may be removed from the stream in line 14 which has-been cooled in cooler 15.

The improvement in the quality of the catalytic cracking feed stock due to contacting it with the cracking catalyst in the rectifier 7 increases as the concentration of catalyst in the slurry in this section is increased. "It is not essential that advantage be' taken of 'this additional improvement. Thus,claried oil, rather than slurry oil, can be used in the rectifier 7 whilestill retaining the former advantage. The 'slurry oil passed from thefractionator 13 normally'contains'from' smallamounts up to about 3 lbs. of 'crackingcatalystper barrel. "This concentration may be increased in the rectifier 7 by initial most desirable feed stock components of the oil.

thickening of the slurry oil (not shown) or preferably by recycling thickened slurry as illustrated and described above. The concentration cracking catalyst should, however, be limited such that the concentration in the slurry in line 19 does not exceed about 30 lbs./bbl. Additional cracking catalyst or clay may be added by line 23 if desired.

The effect of the suspended cracking catalyst on the quality of the resulting flashed distillate is illustrated in the following table wherein the properties of flashed distillates produced with and without suspended cracking catalyst in the rectification sectionv are given. The concentration of cracking catalyst in the slurry was in this case 20 lbs./ bbl.

The invention is not limited to the specic embodiment illustrated and just described. For instance, while the use of slurry oil thickened as to its catalyst content affords best results as regards improvement in the quality of the catalytic cracking feed stock, beneficial results may be obtained without such thickening, i. e., using the net slurry oil from the catalytic cracking fractionator without thickening. In some cases this may be even more advantageous since the concentration of desirable catalytic cracking feed components in the slurry feed to the rectification zone is higher. Additional knockback may be made up by the conventional method of supplying overhead product condensate to the rectication zone.

If desired, clarified oil rather than slurry oil may be used. This removes the improvement in the feed stock quality due to contact with the catalyst in the slurry oil, but still leaves the advantage of recovering as catalytic cracking feed in a simple and economical manner the If desired, other finely divided solids which may be more active in removing undesirable catalytic cracking feed stock components may be added to the oil to the rectification section either to replace the cracking catalyst missing in the clarified oil or to augment the cracking catalyst in the slurry oil. In any case, the cracking catalyst or other material passed to the rectification section in the flashing operation is withdrawn from the system and not recycled to the catalytic cracking unit where, due to the extremely bad effects of the heavier portion of the carrier oil, the advantages of the invention would be erased.

I claim as my invention:

1. In the production of gasoline from petroleum residues by catalytic cracking, 'Tie improved process which comprises in combination the steps of heating petroleum residue to flashing temperature, flashing the heated petroleum residue in a zone of reduced pressure thereby liberating flashed petroleum vapors, passing said vapors up through a rectification zone in countercurrent contact with a slurry of finely divided cracking catalyst in liquid oil produced as hereinafter specified, withdrawing a slurry from said rectification zone as a product of the process, condensing the vapors after said contact in a separate condensing zone, catalytically cracking the resulting condensate, passing the vapors from the catalytic cracking at essentially the cracking temperature to a fractionation zone at an inlet thereto, wthdrawing slurry oil from the bottom of said fractionation zone, cooling a first part of said withdrawn slurry oil and returning said cooled first part back to said fractionation zone at a point above said inlet thereby to desuperheat the catalytically cracked vapor feed entering said fractionation zone, and passing another part of said withdrawn slurry oil to the above said rectification zone to provide said slurry of finely divided cracking catalyst in liquid oil whereby the vapors of the catalytic cracking feed are contacted With'the catalyst in the slurry oil prior to removing the latter from the system in said product of the process and at the same time hydrocarbons in the slurry oil which are present due to the use of a slurry oil to effect desuperheating are vaporized by the said vapors of the catalytic cracking feed in said rectification zone and passed with said feed to the catalytic cracking zone.

2. Process according to claim 1 further characterized in that said slurry withdrawn from said rectification zone as a product of the process is passed to a partial settling zone and a part of said slurry more concentrated in finely divided catalyst is recycled from said partial settling zone to the top of said rectification zone to increase the concentration of finely divided cracking catalyst in suspension in the liquid oil in said rectification zone.

3. Process according to claim 1 further characterized in that the slurry oil passed to said rectification zonev constitutes the total net make of said slurry oil in said fractionation zone and said slurry oil after being withdrawn from said fractionation zone is cooled to a temperature such that the slurry oil removed from said rectification zone as a product of the process is at least 50% of the net slurry oil introduced into said rectification zone.

References Cited in the file of this patent UNITED STATES PATENTS 2,252,729l Packen et a1. Aug. 19, 1941 2,429,247 Van Dornick Oct. 21, 1947 2,447,149 wier Aug. 17, 194s

Claims (1)

1. IN THE PRODUCTION OF GASOLINE FROM PETROLEUM RESIDUES BY CATALYTIC CRACKING, THE IMPROVED PROCESS WHICH COMPRISES IN COMBINATION THE STEPS OF HEATING PETROLEUM RESIDUE TO FLASHING TEMPERATURE, FLASHING THE HEATED PETROLEUM RESIDUE IN A ZONE OF REDUCED PRESSURE THEREBY LIBERATING FLASHED PETROLEUM VAPORS, PASSING SAID VAPORS UP THROUGH A RECTIFICATION ZONE IN COUNTERCURRENT CONTACT WITH A SLURRY OF FINELY DIVIDED CRACKING CATALYST IN LIQUID OIL PRODUCED AS HEREINAFTER SPECIFIED, WITHDRAWING A SLURRY FROM SAID RECTIFICATION ZONE AS A PRODUCT OF THE PROCESS, CONDENSING THE VAPORS AFTER SAID CONTACT IN A SEPARATE CONDENSING ZONE, CATALYTICALLY CRACKING THE RESULTING CONDENSATE, PASSING THE VAPORS FROM THE CATALYTIC CRACKING AT ESSENTIALLY THE CRACKING TEMPERATURE TO A FRACTIONATION ZONE AT AN INLET THERETO, WITHDRAWING SLURRY OIL FROM THE BOTTOM OF SAID FRACTIONATION ZONE, COOLING A FIRST PART OF SAID WITHDRAWN SLURRY OIL AND RETURING SAID COOLED FIRST PART BACK TO SAID FRACTIONATION ZONE AT A POINT ABOVE SAID INLET THEREBY TO DESUPERHEAT THE CATALYTICALLY CRACKED VAPOR FEED ENTERING SAID FRACTIONATION ZONE, AND PASSING ANOTHER PART OF SAID WITHDRAWN SLURRY OIL TO THE ABOVE SAID RECTIFICATION ZONE TO PROVIDE SAID SLURRY OF FINELY DIVIDED CRACKING CATALYST IN LIQUID OIL WHEREBY THE VAPORS OF THE CATALYTIC CRACKING FEED ARE CONTACTED WITH THE CATALYST IN THE SLURRY OIL PRIOR TO REMOVING THE LATTER FROM THE SYSTEM IN SAID PRODUCT OF THE PROCESS AND AT THE SAME TIME HYDROCARBONS IN THE SLURRY OIL WHICH ARE PRESENT DUE TO THE USE OF A SLURRY OIL TO EFFECT DESUPERHEATING ARE VAPORIZED BY THE SAID VAPORS OF THE CATALYTIC CRACKING FEED IN SAID RECTIFICATION ZONE AND PASSED WITH SAID FEED TO THE CATALYTIC CRACKING ZONE.

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