US2848522A - Process for recovery of cracking products - Google Patents

Description

Aug. 19, 1958 F. E. GILMORE 2,848,522

PRocEss FoR RECOVERY oF cRAcKING RRonucTs Filed Jan. 4. 1954 v United States Patent O M' PROCESS FOR RECOVERY OF CRACKIN G PRODUCTS Forrest E. Gilmore, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Application January 4, 1954, Serial No. 401,954

3 Claims. (Cl. 260683) This invention relates generally to the cracking of hydrocarbons. In one aspect, it relates to the recovery of gaseous products of the thermal cracking of a normally gaseous hydrocarbon. In another aspect, it relates to the prevention of polymer deposits in a system in which the gaseous products of isobutane cracking are resolved into their components.

The thermal cracking of normally gaseous products, such as isobutane, to obtain feed materials for catalytic alkylation processes is well known in the art. In the prior art, it has been customary to cool the effluent from such a cracking process to such a temperature that only normally liquid constituents are liquefied. The normally gaseous products have customarily been resolved by a conventional fractionation process, such as ordinary fractional distillation and/or absorption in a hydrocarbon oil or other solvent followed by stripping. I have found that, in the practice of such a process, diiculties have arisen in connection with the operation of the gas-separation steps as a result of the formation and deposition of polymeric products in absorbers and in fractional distillation columns. The nature and the constitution of these deposits are not completely understood. However, it appears that they are composed, at least in part, of aromatic and/or olefnic or diolefinic polymers which appear to be formed by the polymerization of aromatic or unsaturated materials which are present in the cracking products and which are readily polymerizable under the separation conditions.

This invention provides a process wherein the formation'of such polymeric products and the consequent fouling of gas-separation equipment is eliminated or minimized.

According to this invention, the effluent from a gascracking process, such as a thermal, isobutane cracking process, is partially liquefied under conditions of temperature and pressure such that substantially all of the C., and heavier constituents of the efiluent are liquefied. There .are thus obtained a gaseous phase and a liquid phase which are processed separately for the recovery of the several constituents of each.

Further, according to this invention, it is preferred that the above described liquefaction be conducted under such temperature and pressure conditions that at least 60 percent of the C3 and heavier components of the effluent are liquefied, and it is further preferred that the temperature and the pressure of liquefaction be such that at least 80 percent of the C3 and heavier constituents are liquefied.

The temperature of liquefaction can vary within rather broad limits, and, generally, any combination of temperature and pressure which will produce the desired extent of partial liquefaction can be used. It is generally preferred, however, to conduct the liquefaction by cooling the cracked effluent to a temperature in the range 40 to 70 F. at a pressure in the range 80 to 120 p. s. i. g. A temperature in the range 55 to 65 F. and a pressure in the range 95 to 105 p. s. i. g. are still more desirable.

2,848,522 Patented Aug. 19, 1958 ICC The efluent from the cracking step will ordinarily be at a temperature in the range 900 to 1500 F. and is generally cooled first by the direct injection of water as a quench to lower the temperature to a point at which no further cracking occurs, e. g. below 900 F., and the effluent is further cooled, for example, to a temperature of from 300 to 500 F., preferably 350 to 400 F., by indirect heat exchange. If desired, the cooled effluent can then be contacted with a hydrocarbon wash oil to remove oil-insoluble impurities, such as carbon, coke, asphalt, or tar. The cooled and washed eluent can then be partially liquefied, according to this invention.

The drawing is a diagrammatic flow sheet illustrating one embodiment of this invention wherein isobutane is thermally cracked to obtain a mixture comprising butanes, butylenes, propane, propylene, ethane, ethylene, methane, hydrogen, and C5 and heavier hydrocarbons together with small, but deleterious, amounts of easily polymerizable impurities, which have previously caused the deposition of heavy polymeric deposits in gas-fractionating equipment. i

As shown in the drawing, an isobutane-containing fraction enters cracking zone 18 through inlet 19. In cracking zone 18, the isobutane fraction is subjected to a combination of high temperature and at least moderately elevated pressure, as .a result of which' olefins are formed. The cracked efiiuent passes through conduit 1 to oil separation zone 4. In conduit 1, the effluent is quickly cooled to below reaction temperature by the direct injection of water through inlet 2. Further cooling is effected in indirect heat exchanger 21. A wash oil is added through inlet 3 to remove oil-insoluble impurities from the cracked material.

Generally, the wash oil introduced through inlet 3 can be any normally liquid hydrocarbon oil. However, it is usually preferred to use `a heavy oil, such as an aromatic and/or naphthenic recycle gas oil obtained in a cracking process. A very satisfactory oil for this purpose is a recycle gas oil having an aniline number of approximately 138 F. and an initial boiling point above 400 F.

The mixture of wash oil and cracked eiuent is passed to oil separation zone 4, wherein the liquid oil separates from the gas. The oil is passed through conduit 22 to settling zone 5 wherein insoluble materials removed from the cracked effluent settle and are withdrawn as an oil slurry through outlet 24. Clarified oil is recycled through conduits 23 and 3.

The gas separated in oil-separation zone 4 is passed through conduit 25 and heat exchanger 26, therein being cooled to such a temperature that the steam resulting from the water quench is condensed. The condensed water is removed in water removal Zone 27 and withdrawn through outlet 27A. The cracked effluent then passes through conduit 28 and refrigeration means'29 to phase-separation z one 7. Refrigeration means 29 can be any refrigeration means known in the art, for example, one employing a liquefied petroleum gas as a refrigerant. The cracked efluent, cooled to a temperature in the range 55 to 60 F. at a pressure in tlie range to 125 p. s. i. g., separates in phase-separation zone 7 into a gaseous phase and a liquid phase. The liquid phase obtained under the liquefaction conditions, according to this invention, contains substantially all of the polymer-forming materials present in the efiluent. Thus, the gas phase withdrawn through conduit 31 for resolution into its components contains substantially no polymer-forming materials. Thus, the deposition of polymers in the subsequenty gas-separation steps is substantially eliminated.

The gas phase is passed through conduit 31 to demethanizer 6 wherein it is contacted, with an absorption oil introduced through inlet 9 and conduit 48. Demethanizer 6 is operated at a top temperature in the range 100 to 110 F., a top pressure in the range 110 to 120 p. s. i. g., a bottom temperature in the range 150 to 160 F., and a bottom pressure in the range 115 to 125 p. s. i. g. Demethanizer 6 is also provided with auxiliary cooling means 33l to facilitate temperature control. Demethenizer 6 is also provided with a socalled donut tray 34 positioned in an upper part there-of. Tray 34 is ordinarily an annular tray provided with a central opening 57 in chimney 57A.

The absorption oil introduced into demethanizer 6 through conduit 48 removes most of the C3 and heavier constituents from the gas. However, some of its lighter constituents are evaporated into the gas. In order to recover these constituents, aswell as additional C3 and heavier gas constituents, a heavy hydrocarbon sponge oil is introduced into the top of demethanizer 6 through inlet 11. The sponge oil contacts the gas entering through opening 57 and absorbs some of the C3 and most of the C4 and heavier material. The enriched sponge oil is collected on tray 34 and passed through conduit 36 to stripper wherein dissolved materials are stripped from the sponge oil by any known procedure, such as the use remainder is withdrawn through conduit 56 and is preferably recycled to demethanizer 6 through conduit 47. The overhead product can be withdrawn through outlet 56 and used for feed for ethylene recovery or in an alkylation system, such as one in which diisopropyl alkylate or ethylbenzene is produced.

The liquid phase obtained in phase -separation zone 7 is passed through conduit to debutanizer 8. An overhead fraction comprising butane, butylenes, and lighter materials is withdrawn through conduit 50 and condenser Si to accumulator 15. Part of this fraction is returned as reflux through conduit 52 and the remainder is passed through conduit 53 to accumulator 13 and is therein processed together with corresponding materials obtained during the fractionation of the gaseous phase.

The kettle fraction comprising C5 and heavier products and including substantially all of the polymer-forming impurities previously described is withdrawn as a byproduct through conduit 17. This product or the heavier fractions thereof can be used as a drying oil constituent or as a rubber softener.

The following table illustrates the compositions of several of the streams in a plant of the type shown in the drawing. In the table, the stream compositions, ex-

0: 1 of increased temperature and/or reduced pressure. Ma- 0 elusive of water, are given 1n pound-moles per hour.

Reference Numeral ln Drawing-- 28 30 31 47 57 48 37 40 42 43 49 17 53 151.3 0.6 150.7 0.6 151.3 0.5 0.5 0.6 230.2 8.5 221.7 8.7 230.2 0.2 7.2 7.0 8.5 36.5 4.9 31.6 0.6 36.5 5.7 14.3 8.6 4.9 7.0 1.3 5.7 3.9 7.0 2.6 5.8 3.2 1.3 119. 6 45. 6 74. 0 9.1 3. 5 89. 6 199. 6 110. 0 45. 6 67. 3 29. 3 38.0 3. 4 1. 4 40. 0 96. 2 56. 2 29. s 142.1 97. 0 45. 1 45. 1 160. 2 115.1 97. 0 377. 1 252. 8 124. s 124. 3 429. 8 305. 5 252. s 16.6 12.2 4.4 4.4 17.8 13.4 12.2 10.9 10.6 0.3 0.3 0.5 0.2 642.0 642.0

Total 1,158.6 462.8 695.8 46.3 429.9 642.0 954.2 931.9 619.7 452.2

terials removed by the stripping are passed through conduit 20 and combined with the cracking eluent in conduit 1. Stripped sponge oil is cooled and recycled through conduit 54 and inlet 11.

The absorption oil introduced into demethanizer 6 through inlet 9 and conduit 48 is preferably a debutanized gasoline fraction, such as a straight run gasoline fraction boiling within the range 100 to 400 F.

The sponge oil introduced into the upper part of demethanizer 6 is preferably a parainic or straight-run gas oil or similar straight-run hydrocarbon material and preferably boils within the range 250 to 700 F.

A gaseous fraction comprising chiefly ethane, ethylene, methane, and hydrogen is withdrawn from demethanizer 6 as an overhead fraction through outlet 35. Enriched absorption oil is withdrawn from demethanizer 6 through conduit 37 and passed to debutanizer 12 in which C4 and lighter materials are removed by fractional distillation. A kettle fraction can be withdrawn from debutanizer 12 through outlet 39. At least part of this fraction is preferably recycled .through conduit 48 to demethanizer 6.

An overhead fraction comprising butanes, butylenes, and lighter hydrocarbons is passed through conduit 40 and condenser 41 to accumulator 13. Part of this material is returned to debutanizer 12 as reflux through conduit 42. .Gaseous material can be vented from the system through outlet 55 if desired. The remainder of the overhead liquid fraction is passed through conduit 43 to deethanizer 14. A mixture of propane, propylene, butanes, and butylenes is withdrawn from deethanizer 14 as a keetle product through outlet 49 and can be used as a feed to an alkylation or polymerization system. An overhead product comprising chiefly C2 products, i. e. ethane and ethylene, is withdrawn through conduit 44 and condenser 45 to accumulator 16. Part of this fraction can be returned as reflux through conduit-46. The

It is evident from the table that substantially all of the C5 and heavier materials present in the cracked effluent are eliminated from the gas-separation system shown in the drawing. Thus, 10.6 pound-moles of the 10.9 pound-moles -of the C5 and heavier materials tlowing through conduit 28 are removed through outlet 17. This includes substantially all of the polymer-forming materials present in the cracked efuent. As a result, these materials do not enter demethanizer 6, debutanizer 12, or deethanizer 14 and cannot form deposits therein.

Debutanizer 8, debutanizer 12, and deethanizer 14 can be operated under conditions known in the art, and the following are examples of such conditions:

Debutanizer 8 can be operated at a top temperature in the range 145 to 155 F., a top pressure in the range 180 to 200 p. s. i. g., a bottom temperature in the range 365 to 375 F., and the bottom pressure in the range l to 205 p. s. i. g.

Debutanizer 12 can be operated at a top temperature in the range to 130 F., a top pressure in the range to l50 p. s. i. g., a bottom temperature in the range 400 to 410 F., and a bottom pressure in the range 135 to p. s. i. g.

Decthanizer 14 can be operated at a top temperature in the range 100 to 120 F., a top pressure in the range 450 to 505 p. s. i. g., a bottom temperature in the range 220 to 240 F., and a bottom pressure in the range 455 to 510 p. s. i. g.

Under the above conditions, accumulator 13 will be operated at a temperature in the range 95 to 105 F. and a pressure in the range 130 to 140 p. s. i. g. Accumulator 15 is operated at a temperature in the range 95 to 105 F. and a pressure in the range to 195 p. s. i. g. Also, under the above conditions, accumulator 16 is operated at a temperature in the range 95 to 105 F. and a pressure in the range 450 to 500 p. s. i. g.

It will be understood by those skilled in the art that columns 6, 8, 10, 12 and 14 are `equipped with suitable devices and elements such as reboilers, insulation, pumps, and the like, not shown in the drawing.

While certain process steps, structures, and examples have been given for purposes of illustration, it is clear that the invention is not limited thereto.

Variation and modication are possible within the scope of the disclosure and the claims of this invention. Thus, in the system shown in the drawing, the use of wash oil introduced through inlet 3 can be eliminated and the cooled eiiiuent can be passed directly to water removal Zone 27 for further processing as described. Also, the invention can be applied to cracking operations other than ordinary thermal cracking, for example, catalytic cracking, particularly that type of catalytic cracking wherein a gaseous or vaporous cracking catalyst, such as a simple organic halogen compound, is used.

From the foregoing, it will be seen that the basic concept of this invention lies in the elimination of polymer formation in the gas-separation steps of a cracking process by partial liquefaction of the cracking efuent under conditions such that substantially all of the C4 and heavier constituents are liquefied and subsequent resolution of the resulting gas and liquid phases separately.

I claim:

1. A process which comprises subjecting a normally gaseous hydrocarbon to cracking to obtain a composite product comprising hydrogen and C1 to C5 olens and parains together with minor amounts of readily polymerizable aromatic and unsaturated impurities; cooling said composite product to a temperature such that the cracking reaction ceases; removing oil-insoluble impurities from said composite product; subjecting said product to refrigeration at a temperature in the range 40 to 70 F. and at a pressure in the range 115 to 125 p. s. i. g., whereby a gaseous phase and a liquid phase are obtained; recovering from said gaseous phase a C3-C4 fraction, a C2 fraction, and a fraction comprising methane and hydrogen; and recovering from said liquid phase a C4 and lighter fraction and a C5 and heavier fraction containing substantially all of said readily polymerizable impurities.

2. A process which comprises subjecting a normally gaseous hydrocarbon to cracking to obtain a composite product comprising hydrogen and C1 to C5 olens and parains together with minor amounts of readily polymerizable aromatic and unsaturated impurities; cooling said composite product to a temperature such that the cracking reaction ceases; removing oil-insoluble impurities from said composite product; subjecting said product to refrigeration at a temperature in the range to 65 F. and at a pressure in the range 115 to 125 p. s. i. g., whereby a gaseous phase and a liquid phase are obtained; recovering from said gaseous phase a C3-C4 fraction, a C2 fraction, and a fraction comprising methane and hydrogen; and recovering from said liquid phase a C4 and lighter fraction and a C5`and heavier fraction containing substantially all of said readily polymerizable impurities.

3. A process which comprises subjecting isobutane to thermal cracking at a pressure of at least 105 p. s. i. g. to obtain a composite product comprising hydrogen and C1 to C5 parafhns and oleins together with minor amounts of readily polymerizable aromatic and oleiinic impurities; cooling said product; contacting the cooled product with a heavy recycle gas oil, whereby oil insoluble impurities are removed; removing Water from the thus-treated product; refrigerating the resulting product at a temperature in the range 55 to 65 F. and at a pressure in the range 95 to 105 p. s. i. g., whereby at least percent of the C3 and heavier constituents are liqueiied and a liquid and a gas phase are obtained; subjecting said gas phase to contact with a debutanized straight-run gasoline, whereby a hydrogen-methane fraction containing small amounts of C2 and heavier compounds and an enriched gasoline fraction are obtained; contacting said hydrogen-methane fraction with a parafiinic gas oil to remove therefrom said C3 and heavier compounds and recovering said compounds; fractionating said enriched gasoline fraction to recover a C3-C4 fraction and a C2 fraction; fractionating said liquid phase to obtain a C4-and-lighter fraction and a C5-and-heavier fraction containing substantially all of said readily polymerizable impurities, whereby said impurities are prevented from forming polymeric deposits during the recovery of said C2 and said C5-C4 fractions.

References Cited in the le of this patent UNITED STATES PATENTS 1,215,732 Snelling Feb. 13, 1917 2,427,954 Frey Sept. 23, 1947 2,543,742 Evans Feb. 27, 1951 2,621,216 White Dec. 9, 1952 2,661,812 Gilmore Dec. 8, 1953 2,672,489 Holland Mar. 16, 1954 2,689,625 Davis Sept. 21, 1954 2,745,889 Johnston et al May 15, 1956

Claims (1)

1. A PROCESS WHICH COMPRISES SUBJECTING A NORMALLY GASEOUS HYDROCARBON TO CRACKING TO OBTAIN A COMPOSITE PRODUCT COMPRISING HYDROGEN AND C1 TO C5 OLEFINS AND PARAFFINS TOGETHER WITH MINOR AMOUNTS OF READILY POLYMERIZABLE AROMATIC AND UNSATURATED IMPURITIES; COOLING SAID COMPOSITE PRODUCT TO A TEMPERATURE SUCH THAT THE CRACKING REACTION CASES; REMOVING OIL-INSOLUBLE IMPURITIES FROM SAID COMPOSITE PRODUCT; SUBJECTING SAID PRODUCT TO REFRIGERATION AT A TEMPERATURE IN THE RANGE 40 TO 70*F. AND

Images