US2999062A - Scrubbing fluid coking effluent - Google Patents

Description

Sept. 5, 1961 H. B. HENDERSON 2,999,062

SCRUBBING FLUID COKING EFFLUENT Filed Sept. 12, 1958 2 Sheets-Sheet 1 56.2 FIG. I

FRACTIONATOR FRACTlONATOR (ATMOSPHERIC) (ATMOSPHERIC) LIGHT GAS OIL GAS oIL DRIOR ART CATALYTIC CRACKER 4 FEED 4,2 (SCRUBBER 23 SCRUBBER TEMPERATURE TEMPERATURE GRADIENT GAS OIL GRADENT ".1112: GAS OIL ssownooT) soowwsm) 26 GAS oIL i6 1 L a 2 259g M COKE CHARGE BURNER CHARGE URNER STEAM 33111111"; 37

TO COKE BURNER STEAM To COKE BURNER VACUUM STILL 39 E28 (BELOW SOMMHG) RECYCLE STREAM RECYCLE STREAM INVENTOR.

HUGH B. HE NDE RSON AGENT Sept. 5, 1961 H. B. HENDERSON 2,999,062

SCRUBBING FLUID COKING EFF'LUENT Filed Sept. 12, 1958 2 Sheets-Sheet 2 FIG. 3

RELATIONSHIP BETWEEN RESIDUUM IN HEAVY GAS OIL PRODUCT AND GAS OIL IN RECYCLE STREAM I5 A PERCENT RESIDUUM INVENTION IOOOF) IN HEAVY GAS CONVENTIONAL oII PRODUCT |O 0 IO 20 3O 4O 5O 6O 7O 80 PERCENT GAS OIL( IOOOF) IN RECYCLE FIG. 4

RELATIONSHIP BETWEEN PER INVENTION TO CONVENTIONAL TOTAL REFLUX RATE AS FUNCTION OF RESIDUUM CONTENT OF HEAVY GAS OIL PRODUCT PERCENT PER INVENTION OF CONVENTIONAL TOTAL RECYCLE RATIO 6 o 5 IO I5 INVENTOR. PERCENT RESIDUUM IN HUGH a HENDERSON HEAVY GAS OIL PRODUCT AGENT United States Patent 2,999,062 SORUBBING. FLUID COKING 'EFFLUENT Hugh B. Henderson, Pleasant Hill, -Calif., assignor to Tidewater Oil Company, San Francisco, Calif., a corporatlon of Delaware Filed Sept. 12,.1958, Ser. No. 760,634

2 Claims; (Cl. 208-102) This invention relates to the coking of heavy mineral oils and pitches to produce lighter oils and coke. More particularly, it relates to a novel treatment ,of residuum foundin the efiluentjresultmg from'the process of fiuid ized coking. Still more particularly, it relates to a fluid coking process for decreasing the recycle rate required to totally destroy the residuum charged to the coking proc ess, with a simultaneous upgrading ofheavy coker gas oil as a catalytic crackingcharge' stock; conversely, the recycle rate may" be held constant, and an increase in the yield of catalytic cracking charge stock obtained, with a simultaneous upgrading of'this stock. I i

In the so -calle d fluidized cokingprocess, heavy mineral oils from the refinery (usually containing large amounts of residuum, which term forthe purposes of this specification and the appended claims will, be used toide note' heavy mineral oil constitutents having boiling'po-ints above -100 0 Pi) is sprayed upon heated particles of coke that are maintainedin a fluidized condition 'in a reactor by thepas'sage'of steam therethrough. 0 1i con: tact with the'heated cake, the residuum is thermally de: composed, or "cr'acked"(ciommonly referred to as coked), into lighter "oils and'a'dditional coke, the latter depositinglarg'elyasi a layer'on the original coke particles; A'streamof the coke'part'icles'is continuously withdrawn from thejre'action zone and passed 'to a burning zone, wheresome of it is burned to heat the remaind er, and heated cokeis continuously recirculated to the reactor. a v v As conventionally practiced, the vapors from the cob ing zone, consisting of hydrocarbons and steam, are passed to a scrubberwher'ein the heavier constituents are condensed.- The liquid vapor equilibrium" coiiditionsinthe scrubber ar'e'i'n the range of '4' to 8 p.s.i.a. hydrocarbon partial pressure, with corresponding equilibrium temperatures. From this scrubber,"'the'heaviestconstituents are withdrawn and recycle to the "coking reactor for further treatment, since theycontain aconsiderable amount' of residuum, which it is desired to process to totaljextinc tion. A gas 'oil is also withdrawn from the scrubber, being normally intended for use as charge stock to a catalytic crack er'. 'Still lighter constituents pass through the scrubber'asa vapor and'enter a fractionatorfwhere they are subsequently-condensed and removed as liquid and gaseous products,'some"of the liquid also being suitable as cha rge'stock for the catalytic cracker. The presentinv'ention relates to'proc'es'sing the constituents removed as liquid in the scrubber, particularly those heretofore sent 'to the recycle stream.

Conventionally, the liquid stream returned to the reactor as recycle orspillback'has included a considerable amount of gas oil that would be suitable as charge stock for a catalytic cracking unit. Returning this gas-oil to the reactor, along with the residuum, has resulted in its degradation as a catalytic'crackin'g charge stock, since additional residencetim'e at high temperature results in dehydrogenation, thermal cracking, and other reactions considered unfavorable as compared to catalytic crack ing. Bearing manna-mane that one of the prime'oh jects of coking is toobtain large amounts of suitable catalytic cracking chargestock, it will seen'thatcom ventional recycle practice lowers the quality of the catalytic cracker charge stock by recycling this gas oil, and that some of the recycled gasoil is'consumedin the coker on recycle, resulting ina loss of highquality'charge stockl 2,999,062 Patented Sept. 5, 1961 Moreover, the conventional process sends a considerable proportion of understroyed residuum into the 'gas oil st-rean'i instead of recycling it to extinction. Residuum has very detrimental elfects on catalytic cracking, for 'it is laden with coke for-ming components and a significant amount ofheavy metals. The coke-forming components form coke or carbon in the catalytic cracker, reducing the conversion, or gasoline yield of the catalytic cracking plant, since the yield is a function of the carbon that can be burned off the catalyst in a regenerating operation. The heavy metals permanentlyreduce the value of the catalyst, for they cannot be burned off. Some of them act as dehydrogenation catalysts 'and' affect the distribution of cracked products. They cause the catalytic cracker to make more gas' and more coke and less gasoline, and so substantially reduce the capacity of the catalytic cracking plant.

Because of the difiiculties caused by the presence of large amounts of residuum in the gas oil stream from the reactor 'e'fiiuent, some refineries have been unable (as a practical matter) to charge this gas oil directly to the catalytic cracker, as was initially the prime object of the fluidized coking process. Instead, the gas oil has had tobeprocessed to' eliminate residuum, such as by charging it first to atherm-al cracker and then to a fractionator before charging to the catalytic cracker. Such procedure has resulted in a rise in costs.

Attemptsto' remove residuum from the gas oil have heretofore proven unsatisfactory. "When the scrubber was cooled, the amount of residuum in the gas oil stream was reduced somewhat, but as the same time the amount of gas oil in the steam was reduced even more; furthermore, a larger'am'ount of gas oil' went'back into the recycle stream and was degra'dedor consumed in the coker. When the scrubber was cooled enough to eliminate substantially all the residuum from the gas oil (i.e'., to reduce the residuum content in the gas oil] stream below 5%), the gas oil stream itself was reduced by an even greater percentage, and the degradation resulting from recycle of the gas oil was just as harmful as the residuum formerly was. 7

By the present invention, the recycle rate required to totally destroy the residuum 'fed to the coker, may be substantially decreased. Simultaneously, the heavy coker gas oil is upgraded and becomes a betterchargestock for catalytic cracking units. To do this, the inyention calls for (1) removing substantially all of the residuum from the gasoil stream by lowering the temperature in the scrubber and (2) shifting the liquid vapor equilibrium by lowering the hydrocarbon partial pressure, thereby increasing the relative volatility between the gas oil components and the residuum components. This shift may be accomplished by vacuum distillation of all those streams from the reactorfelfiuent containing significant amounts of key components of gas oil and-residuum. The distillations is carried out at total pressures'below fifty millimeters of mercury absolute pressure and prefferably in the range of 10-50 mmlHg.

In summary then, the present invention removes practically all of the residuum fr-om the gas oil stream by low ering the temperature of the scrubber, even'v-though this means that the gas oilstream obtained directly from the scrubber is reduced. It then subjects the recycle stream to a special vacuuindistillation, at a pressureblow SO mm. Hg, to produce an overhead and a bottoms stream. The vacuum tower overhead stream contains the heavy gas oils'tha't are desirable as catalytic. cracking charge stock, and this overhead streamfis not recycled to .the reactor and so is saved from degradation; instead it is then added to the direct gas oil stream. The vacu-um tower bottoms containing practically all the remaining residuum, are returned to the reactor'as recycle stock, so thatthc 3 residuum can be almost totally destroyed, ending up either as cracked gas oil, gas, or coke.

By processing in accordance with the invention, the following advantages are obtained:

(1) A superior catalytic cracking charge stock is produced, for the gas oil stream (a) contains a very low percentage, if any, of high-boiling (residuum) constituents, tending to form coke on the catalyst and so reducing the gasoline yield of the catalytic cracker; (b) contains a lower percentage of heavy metals, tending to permanently alfect the distribution of cracked products from the cracker; contains much less gas oil that is degraded by recycling it to the coker and its high-temperature conditions.

(2) There is less gas oil in the recycle stream; so the recycle rate for destroying a given percentage of residuum is lowered.

(3) No separate heat source (requiring other fuel) is needed for the vacuum distillation of this invention, for the heat required may be furnished in the coking reactor by the by-product coke and transferred as sensible heat in the feed to the vacuum tower.

The invention may be more readily understood by reference to the drawing wherein:

FIGURE 1 is a diagrammatic flow sheet of a portion of prior-art, conventional, fluid coker.

FIGURE 2 is adiagrammatic flow sheet of a corresponding portion of a fluid coker operated in accordance with the invention.

FIGURE 3 is a graph showing the relationship between the percent of residuum in the gas oil product stream and the percent of gas oil in the recycle stream, in both the conventional process and the process of this invention.

FIGURE 4 is a graph showing the relationship in the present invention between the percentage of the conventional recycle rate used and the percentage of the residuum in the heavy gas oil product.

Referring to FIGURE 1, a coking reactor 1 is connected to a scrubber 2 by cyclones 3 in the conventional manner. Conduits 4 and 5 respectively lead to and from a coke burner (not shown). The coke is maintained in a fluidized condition by steam 6 being introduced at various locations. 1 through line 7. In the reactor 1 it contacts the hot fluidized coke and, as a result, some of it is converted to coke, while much more of it is cracked and passes with the etfluent through the cyclones 3 into the scrubber 2.

In the scrubber 2, preliminary fractionation takes place at a temperature gradient of about 600-750 F., with the following effects: 7

(a) Uncondensed vapors pass up through line 8 into a fractionator 9, from which various fractions are withdrawn at atmospheric pressure, at lines 10, 11 and 12, for example. The lightest stream, line 12, is cooled by a heatexchanger 14 and passes through a reflux accumulator 15. There the remaining stream (wet gas) passes through line 16, while raw distillate leaves through line 17, part 2 for use as a quenching oil; the remainder is recycled by line 28 to the input line 7 of the coking reactor 1.

The purpose of recycling through line 28 is to crack the residuum to extinction. Corresponding to the physical equilibrium conditions, this recycled residuum contains a proportionate amount of gas oil. This concentration of gas oil is a function of the liquid-to-vapor ratio, the number of equilibrium contacts, and the equilibrium temperature and pressure. The return of this gas oil to the reactor 1 results in its degradation, as previously outlined. If the recycle rate is increased by lowering the end point of the gas oil, in order to decrease the amount of residuum in the gas oil stream 23, the concentration of gas oil in the recycle line 28 increases at a rate corresponding to the adjusted equilibrium temperature. The result is that the recycle rate of actual residuum increases asymptotically as compared to the total recycle rate. The corresponding increase in recycle of gas oil, which is exponential in magnitude as compared to the total recycle rate, causes degradation of this stock as a catalytic cracking charge stock, since the recycle rate of gas oil contained in the total recycle stream 28 increases faster than the recycle rate of the residuum.

The present invention, illustrated in FIGURE 2, provides a vacuum tower or still 30 fed from a stream 31 containing the bottoms withdrawn from the scrubber 2 through the line 25. Substantially all the residuum material is forced into line 25 by adjustment of the scrubber temperature gradient to about 550 to 700 F. by regulation of the amount of reflux in the streams 21 and 27. By this means, the amount of residuum remaining in the gas oil stream 23 is reduced to between 0% and 5%, normally to between 1% and 3%.

Residuum is charged to the reactor 7 A steam jet ejector 32 may be used to provide the necessary vacuum conditions (i.e., a pressure below 50 mm. of Hg) in the tower 30, which contains conventional liquid-vapor contacting devices. The overhead from the tower 30 thus passes through line 33 and overhead condenser 34 to a reflux accumulator 35. Steam is introduced to the vacuum tower 30 through line 39 to reduce hydrocarbon partial pressure and lower temperatures, resulting in more favorable relative volatilities of the components to be separated. Reflux from accumulator 35 is re-introduced to vacuum tower 30 through line 36 to obtain more favorable separation. After withdrawal of the reflux portion through the line 36, overhead gas oil is passed through line 37. This gas oil may be introduced into the line 23 for use as charging stock for the catalytic cracker (not shown). The bottoms from the tower 30 are recycled to the coking reactor 1 through lines 38 and 7.

Thus, in this invention, the approximately 50 F. drop in temperature in the scrubber 2 substantially reduces being used as reflux through line 18, the remainder being 7 taken off as product through line 19. A heavier fraction may be passed to the catalytic cracker through line 11, while a still heavier fraction is returned through line 10 to serve as scrubbing oil.

(b) Heavy gas oil passes out from the scrubber 2 through line 20. conventionally, some of this material is withdrawn into a sidestream 21, cooled by a heat exchange 22 and used (together with cool bottoms from the fractionator 9 through line 10) as scrubbing oil. The remainder. of the gas oil is sent out through a line 23 for use as desired, the ideal being to send it to a catalytic cracking unit (not shown), if the gas oil is really suitable for catalytic cracking. Usually, however, it is not suitable without further processing.

(c) Recycle stock, the heaviest of all, passes out from the scrubber 2 through line 25. Some of'this is passed by line 26 and cooling heat exchanger 27 back to the Scrubber the volume of the gas oil stream 20 to force substantially all of the residuum therein into the line 25. This means that much additional gas oil is forced into the line 25, but (after withdrawal of quenching oil as usual through the line 26), the stream of gas oil and residuum is sent by the line 31 to the vacuum still 30. The heat needed in the still 30 is carried there by the streamitself from the scrubber 2 through lines 25 and 31. Initially, this heat is produced in the coker burner (not shown) by a very small increase in the amount of coke burned therein, is transmitted to the reactor 1 by the line 5, and is then transferred to the effluent. So there is little cost for this heat and no separate heating installation is used.

Within the still 30, the vacuum conditions, usually are 10-50 mm. Hg. The gas oil isfractioned off as vapor through the line 33, while the bottoms and recycle line 38 contains nearly all the residuum. The reflux of some of the gas oil into the still 30 by line 36 does not harm it, and maintains better conditions in the still 30. As a result, the gas oil passing through line 37 is almost residuum-free, and the net residuum in line 23 after the stream from line 32 enters it, is reduced.

below 5%, usually between 1% and 3%, but depending in part upon the composition of the original residuum of line 7. Coke and heavy metals are no longer a substantial problem in the catalytic cracking unit, and the gas oil in lines 23 and 37 may be charged directly to the catalytic cracker without further treatment to reduce the residuum content. Furthermore, the residuumban be cracked (coked) to extinction, while the quality of the gas oil in lines 23 and 37 is upgraded so that it can be directly charged to a catalytic cracker.

Two specific examples are presented, one in which the recycle rate is raised to destroy substantially all the residuum and one which the conventional recycle rate is used. 7

EXAMPLE 1 The feed stock or residuum having the properties shown in Table I is introduced at line 7 into the coking reactor 1. Table l.-Prperties of feed stock residuum charged to coking reactor API gravity 6.9 Conradson carbon, wt. percent 17.5

Sulfur, Wt. percent 3.95 A.S.T.M. distillation (D-1160), F.:

IBP 529 828 909 975 30% 1019 40% 1061 The conditions inthe coking reactor 1 are shown in Table H.

Table Il.0perating conditions in coking reactor A. CONVENTIONAL Reactor temperature:

Dense phase, F 955 Dilute phase, F 980 Cyclones, F 1010 Total reactor steam, wt. percent of fresh feed 10.1 Scrubber temperature gradient, F 600-750 B. PER INVENTION All factors same as above, except:

Scrubber temperature gradient, F 550-700 To be more specific, at the very bottom of the scrubber 2 the vapor enters from the cyclone 3 at about 1000 F. Vapor leaves the scrubber 2 through line 8 at about 550 F. The present invention is concerned with contacting apparatus 40, which may be trays, as shown, or shed decks, in the central part of the scrubber 2, i.e., lying between the gas oil line 20 and the quenching oil reflux line 26. The apparatus 40 may be termed the effective fractionating section or zone of the scrubber 2. The lower contacting trays or sheds 41 are primarily for the purpose of heat exchange between the quench oil and the hot vapors from the cyclones 3. Similarly, the upper contacting sheds or trays 42 are primarily for the purpose of heat exchange between the scrubbing oil and the vapors arising from the effective fractionating zone 40.

When the scrubber temperature gradient is spoken of in either the specification or claims, the gradient in the effective fractionating zone 40 is meant. The conventional scrubber temperature gradient is 600 F. to 750 F. In other words, in the conventional process, the

temperature at the bottom of the zone 40 is about 750 F., and the temperature at the top of the zone 40 is about 600 F.

In the present example, as shown in Table II, part B, the temperature gradient is 550 F. to 700 R, which means that it is about 700 F. at the bottom of the zone 40 and about 550 F. at the top. This forces much more material into the line 25, including practically all the residuum, as well as more gas oil than usual. However, the use of the vacuum still 30 achieves further separation of gas oil from residuum. The still 30 in this example is operated at about mm. Hg.

Table III contrasts conventional yields against those ofthis example of the present invention: I

Table lII.Fluid coking yields 7 Conventional Per Invention l3.p.s.d. v01. B.p.s.d. v01.

percent percent Fresh Feed (line 7):

650-l,000 F I5, 348 27.0 6,348 27. 0 above, 1,000 F 21, 252 78.0 21, 252 73.0

Total charge 21, 600. 100.0 21,000 100.0

Recycle (line 38):

6501,000 F 3, 520 33.1 6, 132 40. 5 above 1,000 I 7; 120 66. 9 8, 973 59.6

Total 10, 640 100.0 15,105 100.0

Hgavy Gas Oil Product (line i ciao-1,000 F 0, 243 87.2 7, 000 99.6 above 1,000 F. (Residuum) 1, 360 12. 8 32 0. 4

Total 10, 603 100.0 7,932 100.0

Heavy Gas Oil Pr0duct,'90%- 1 I i i I I ASIM 11-1160:

Distillation Temperature, F 900 s 5 Whilethere appears to be a reduction in the heavy gas oil product, it should be noted that it contains only 0.4% residuum, whereas the conventional gas oil contained 12.8% residuum and could not economically be charged directly to the catalytic cracking unit. The product of this invention makes an excellent charge stock. In fact, it is better than the bare table indicates because more of the gas oil near 1000 F. is sent to recycle and less of that near 650 F. is sent there. But the most important fact may well be that there is no need to give any special treatment to the 7,932 barrels'per day of heavy gas oil product before sending it directly to the catalytic cracker, whereas, some treatment other than, or prior to, catalytic cracking must be given to the 10,603 barrels per day of conventional heavy gas oil product.

The residuum content of theheavy gas oil-product has been reduced by 1,328 barrels out of 1,360 or about 97.6%.

EXAMPLE 2 With the feed and the conditions in the reactor '1 and still 30 the same as in Example 1, but with the temperature gradient of the zone 40 changed to about560 F. to 710 F. inorder to provide a recycle rate in line 38 the same as in line 28 ofFIG. l (as shownin Table III) the present invention still obtains substantial reduction of the amount of residuum in the gas oil line. Table IV shows the results.

Table IV.-Fluid coking yields where recycle rate equals conventional recycle rate B.p.s.d. Vol. Percent;

Fresh Feed (Line 7):

6501,000 F 6, 348 27.0 above 1,000 F 21, 252 73.0

Total 27, 600 100. 0

Recycle (Line 38): T

6501,000 F; 2, 766 26.0 above 1,000 F 7, 874 74. 0

Total 10, 640 100. 0

Heavy Gas Oil Product (Lines 23 and 37):

$504,000 F 9, 507 95.5 above 1,000 F h 448 4.0 Total .Q 0,955 I 100.0

Heavy Gas 011 Product, ASTM D-1160:

Distillation Temperature, F 910 Here, the residuum content in the gas oil has been reduced to 4.0%, well below the limit above which it becomes impractical to charge the gas oil directly to a catalytic cracking unit.

Moreover, it will be noted that the 10,640 barrels per day of recycle (the same total amount as in the conventional process) is 74% residuum and only 26% gas oil, whereas in the conventional process, 33.1% of the recycle is gas oil. In actual barrels, this example shows that by following the present process of reducing the temperature in the scrubber and then vacuum distilling the normally-recycled product, the invention sends 754 more barrels of residuum and that much less of gas oil to recycle.

So far as the heavy gas oil product is concerned, it contains 912 barrels less of residuum, a reduction of about 67%. This reduction lowers the content below the critical limit, so that the product can economically be charged to the catalytic cracker.

EXAMPLE 3 EXAMPLE 4 Another way of raising the hydrocarbon partial pressure is to change the steam content of the effluent. Results substantially identical to those of Example 1 are obtained by lowering the steam content to 5%, expressed as weight percent of fresh feed. The temperature and pressure conditions otherwise remain as in Table HA.

FIGS. 3 and 4 may help in understanding the significance of this invention. FIG. 3 shows two curves, one resulting from practicing the conventional process of FIG. 1, the other from practicing the invention as in FIG. 2. In both curves the percent of the residuum in the gas oil product is plotted against the percent of the gas oil in the recycle stream. Note that the conventional recycle stream would contain 75% gas oil it the residuum content of the heavy gas oil product were to be reduced to the same level as that attained by this invention in Example 1, where the recycle stream contains only 40.5% gas oil.

Since the total recycle rate is proportional to (100-percent of gas oil in the recycle stream) It is readily seen that the total recycle rate obtained by this invention is less than half the conventional recycle rate. This fact is illustrated in FIG. 4, where the recycle rate is plotted against the amount of residuum in the heavy gas oil product.

Conversely, the percentage of gas oil in the recycle stream, when using the present invention to produce a heavy gas oil product containing the same amount of residuum as that in the conventional process, drops from 33% to 3%, as shown in FIG. 3, lines A and B. This is equivalent to a 69% reduction in the total recycle rate. Moreover, as FIG. 4 shows, the lower the residuum content of the heavy gas oil product desired, the greater the contrast between the results of the present invention and those of the conventional process.

Similarly, by referring to lines A and C, one can see the importance of the invention. The drop in residuum is huge, while the gas oil recycled remains the same.

To those skilled in the art to which this invention relates, many changes in constructions and widely differing embodiments and applications of the invention will suggest themselves without departing from the spirit and scope of the invention. The disclosures and the description herein are purely illustrative and are not intended to be in any sense limiting.

As stated hereinbefore, the term residuum is used herein and in the claims to denote those constituents of hydrocarbon oils which have boiling points above 1000 F. at atmosphere pressure. These constituents are generally of an asphaltic or resinous nature and, in ordinary distillation, will crack before they distill. Under the partial pressure conditions obtained in a fluid coker, however, a considerable portion of these constituents vaporize and appear as components of the vapor efliuent.

I claim:

1. In a fluid coking process wherein a charge of highboiling constituents is coked and the effluent is scrubbed to separate out a gas-oil stream normally containing objectionable amounts of residuum, and a heavy stream, normally recycled, the improvement comprising introducing the effluent into the lower end of a scrubbing zone having a temperature gradient lying between temperatures low enough to substantially free said gas-oil stream from residuum, said gas-oil stream leaving from above said zone, and to drive some gas-oil and substantially all of said residuum down into said heavy stream, which leaves from below said zone; distilling said heavy stream in a distillation zone at substantially its withdrawntemperature by means of its heat content when withdrawn and at a pressure below 50 mm. Hg while simultaneously introducing steam in a quantity sufiicient to further reduce the hydrocarbon partial pressure and lower the temperature gradient in the distillation zone, thus making additional sensible heat available, using said additional heat to effect an adequate reflux of overhead distillate, thereby obtaining a substantially residuum free gas-oil stream as overhead product and a bottoms stream containing a minimum of gas-oil; and charging said bottoms stream back as part of said coking charge.

2. In a fluid coking process wherein a charge of highboiling constituents is coked and the efiiuent is introduced directly into the bottom of a scrubbing zone and scrubbed to separate out a gas-oil stream containing objectionable amounts of residuum and a heavy stream, normally recycled, the improvement comprising scrubbing at a temperature gradient of about 700 F. in the bottom of the scrubbing zone to about 550 F. at the top of said zone, to provide a hydrocarbon partial pressure sufficiently high to substantially free said gas-oil stream from residuum and to drive some of said gas-oil and substantially all of said residuum into said heavy stream, distilling said heavy stream at substantially its withdrawn temperature and in the substantial absence of added heat, utilizing its own heat content at a pressure below 50 mm. Hg to obtain a substantially residuum-free gas-oil stream as overhead, and a bottoms stream; and charging said bottoms stream back to said coking reactor.

References Cited in the file of this patent UNITED STATES PATENTS 2,203,930 Smith June 11, 1940 2,217,385 Schulze et al Oct. 8, 1940 2,644,785 Harding et al July 7, 1953 2,734,852 Moser Feb. 14, 1956 2,780,586 Mader Feb. 5, 1957 2,843,529 Hill et al July 15, 1958 2,905,623 Moser Sept. 22, 1959

Claims (1)

1. IN A FLUID COKING PROCESS WHEREIN A CHARGE OF HIGHBOILING CONSTITUENTS IS COKED AND THE EFFLUENT IS SCRUBBED TO SEPARATE OUT A GAS-OIL STREAM NORMALLY CONTAINING OBJECTIONABLE AMOUNTS OF RESIDUUM, AND A HEAVY STREAM, NORMALLY RECYCLED, THE IMPROVEMENT COMPRISING INTRODUCING THE EFFLUENT INTO THE LOWER END OF A SCRUBBING ZONE HAVING A TEMPERATURE GRADIENT LYING BETWEEN TEMPERATURES LOW ENOUGH TO SUBSTANTIALLY FREE SAID GAS-OIL STREAM FROM RESIDUUM, SAID GAS-OIL STREAM LEAVING FROM ABOVE SAID ZONE, AND TO DRIVE SOME GAS-OIL AND SUBSTANTIALLY ALL OF SAID RESIDUUM DOWN INTO SAID HEAVY STREAM, WHICH LEAVES FROM BELOW SAID ZONE; DISTILLING SAID HEAVY STREAM IN A DISTILLATION ZONE AT SUBSTANTIALLY ITS WITHDRAWN TEMPERATURE BY MEANS OF ITS HEAT CONTENT WHEN WITHDRAWN AND AT A PRESSURE BELOW 50MM. HG WHILE SIMULTANEOUSLY INTRODUCING STREAM IN A QUANTITY SUFFICIENT TO FURTHER REDUCE THE HYDROCARBON PARTIAL PRESSURE AND LOWER THE TEMPERATURE GRADIENT IN THE DISTILLATION ZONE, THUS MAKING ADDITIONAL SENSIBLE HEAT AVAILABLE, USING SAID ADDITIONAL HEAT TO EFFECT AN ADEQUATE REFLUX OF OVERHEAD DISTILLATE, THEREBY OBTAINING A SUBSTANTIALLY RESIDUUM FREE GAS-OIL STREAM AS OVERHEAD PRODUCT AND A BOTTOMS STREAM CONTAINING A MINIMUM OF GAS-OIL; AND CHARGING SAID BOTTOMS STREAM BACK AS PART OF SAID COKING CHARGE.

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