US3470084A - Method of separation of gaseous hydrocarbons from gasoline - Google Patents

Description

N. H. SCOTT sept. 3o, 1969 METHOD 0F SEPARATION OF' GASEOUS HYDROCARBONS PROM GASOLINE Filed Nov.' 20, 1967 nk aux YN .s

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/0 A N mw United States Patent O U.S. Cl. 208-101 5 Claims ABSTRACT OF THE DISCLOSURE Method for recovering gasoline and normally gaseous hydrocarbons from the effluent of a hydrocarbon conversion zone, such as a coking unit. The method utilizes a combination of low and high pressure separation Zones operating in conjunction with a dual absorption zone system. Gasoline and LPG are recovered as separate products.

BACKGROUND O-F THE INVENTION This invention relates to a separation process. It also relates to a method for separating the efiluent from a hydrocarbon conversion zone into normally gaseous products and normally liquid products. It particularly relates to a method for recovering gasoline and normally gaseous hydrocarbons from the eiiiuent of a hydrocarbon coking unit.

It is yknown in the prior art to subject relatively heavy hydrocarbonaceous materials to heat soaking or thermal cracking conditions in order to convert the materials to coke and upgraded hydrocarbon products, such as LPG, gas oil, and gasoline.

The prior art processes of coking generally take the form of delayed coking, fluid coking, etc. Since the standard coking processes are well known in the prior art, it is not considered necessary to show the coking process in detail. The units presently available for coking operate by heating a feed to a temperature from 750 F. to 950 F. and under a pressure of from about l0 to 100 p.s.i.g. Sufficient residence times from a few seconds to perhaps several hours areutilized to convert the heated material to coke and lower molecular weight products. The coke is removed from the coking drums and the uid efiiuent is sent to hydrocarbon recovery facilities.

The prior art schemes for separating the eiuent `from a conversion process, such as a coking unit, include broadly the use of a fractionation column, an absorption column, and a stabilizer column. Conventionally, coker gasoline, coker gas oil, and normally gaseous hydrocarbons are obtained in these prior art processes.

The present invention provides improvement over the prior art schemes by utilizing a combination of processing techniques which achieve a substantially greater recovery of desired normally gaseous hydrocarbons in high purity while at the same time maintaining the quality of the coker gasoline at a desirably high level.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide `a method for separating the effluent of a hydrocarbon conversion zone into normally gaseous hydrocarbons and normally liquid hydrocarbons.

It is another object of this invention to provide an improved method for separating the fluid effluent of a coking conversion zone into gasoline and normally gaseous hydrocarbons.

Therefore, the present invention provides an improved method for recovering gasoline and normally gaseous hydrocarbons from the ffuid efiiuent of a hydrocarbon conversion unit which comprises the steps of: (a) frac- 3,470,084 Patented Sept. 30, 1969 ice zone a first rich oil fraction and a second gaseous stream;

(e) passing said second gaseous stream into a second absorption zone maintained under absorption conditions including the presence of at least a portion of said gas oil fraction as absorption medium; (f) removing from said second absorption zone a second rich oil stream and a first product stream comprising C2 and lighter components; (g) passing said first rich oil into said separation zone of Step (b) and passing said second rich oil stream to said fractionation zone of Step (a); and, (h) introducing the remaining liquid stream of Step (b) into a second fractionation zone and recovering therefrom a second product stream comprising normally gaseous hydrocarbons and a third product stream comprising gasoline.

Another embodiment of this invention includes the method hereinabove wherein a portion of said third product stream is passed into said first absorption zone as absorption medium.

Thus, it is seen from the broad embodiments presented hereinabove that the present invention provides a facile and economical separation process for recovering gasoline and normally gaseous hydrocarbons, such as C3 and C4 hydrocarbons, from the eiuent of a hydrocarbon conversion zone. The critical features of the invention include, in combination, a first fractionating column, a liquid-gas separation zone, dual absorption zones, and a second fractionation zone.

It is to be noted that the present invention has been described as being broadly applicable to separating the fluid effluent from a hydrocarbon conversion unit. More particularly, however, the present invention is uniquely applicable to separating the fluid effluent from a hydrocarbon coking unit. However, the present invention is not to be limited to the coking operation; rather, it is to be limited only by the scope of the appended claims since such separation scheme can be utilized to separate any uid effluent containing the various components to be separated herein.

To produce a fiuid efiluent which is separated according to the practice of the preferred embodiment of the present invention, a high boiling hydrocarbon charge, such as crude bottoms from the vacuum distillation of reduced crude, vacuum tar, reduced crudes, topped crudes, blends thereof, etc. is charged into a conventional conversion operation which is maintained -under conventional conversion conditions, such as coking conditions of either the delayed type or uid type. The charge material is first preheated and the preheated charge is fed into the bottom portion of, for example, a coker drum or plurality of drums which is subjected to elevated temperatures and usually moderate pressures. Under coking conditions, destructive distillation of the charge material occurs and results in the formation of lighter boiling hydrocarbons and coke. The lighter boiling hydrocarbons are withdrawn from the coker drum while the coke is usually caused to remain therein. When the desired amount of coke is formed, the conventional manner of operation at that point discontinues the introduction of the preheated charge material and the deposited coke is removed from the drum. The lighter boiling hydrocarbon materials which have been withdrawn from the coker drum comprise the preferred fluid effluent to be separated according to the present DESCRIPTION OF THE DRAWING Referring now to the drawing, a suitable high boiling charge material is passed Avia line into hydrocarbon conversion zone 11 which may be a conventional coking unit. Fluid eiiluent from the conversion zone is withdrawn vialine 12 and passed intofractionation zone 13. Suitable fractionation conditions are maintained inzone 13 to separate the fluid efiiuent, initially, into a light distillate fraction which is Withdrawn vialine 14, a side-cut gas oil fraction which is withdrawn vialine 15, and a heavy gas oil fraction which is withdrawn from the lower portion ofzone 13 vialine 16. Depending upon the characteristics of the fluid effluent inline 12, there may be more or less product streams withdrawn fromzone 13.

The light distillate fraction containing gasoline boiling range material and normally gaseous hydrocarbons such as C2, C3, and C4 hydrocarbons admixed with other gases, such as hydrogen, is passed vialine 14 into first separation zone 17 which is maintained under relatively low pressure. Suitable separation conditions are maintained in zone `17 in order to separate the light distillate fraction into a first gaseous stream which is removed vialine 18 and a first liquid stream which is withdrawn via line 19.

The material inline 18 comprises a broad mixture of normally gaseous hydrocarbons, including the previously mentioned hydrogen and acid gases, if any, and is at a relatively low pressure. According to the preferred embodiment of this invention, the first gaseous stream inline 18 is passed into compressingmeans 20 for raising the pressure thereof to a relatively high level. The cornpressed gaseous stream is removed from compressing means 20 via line 18 (a), admixed with hereinafter specilied liquid stream fromline 21, and the admixture passed vialine 22 intoseparation zone 23 which is maintained under a relatively high pressure. Other suitable operating conditions are maintained inzone 23 to separate the admixture into a second gaseous stream which is withdrawn vialine 24 and a second liquid stream which is withdrawn vialine 25.

The second gaseous stream is introduced vialine 24 intofirst absorption zone 26, preferably, at the lower end thereof.Absorption zone 26 is maintained under absorption conditions including the presence of at least a major portion of the rst liquid stream withdrawn from separation zone 17 as at least one part of the absorption medium therein and which is introduced intoabsorption zone 26 via line 19. As more fully discussed hereinafter, an additional part of the absorption medium may also be introduced intoabsorption zone 26 vialine 35. Other operating conditions are maintained inabsorption zone 26 in order to absorb substantially all of the C3 and C4 hydrocarbons as Well as some C2 hydrocarbons into the absorption medium. Thereafter, the first rich absorption oil s withdrawn fromzone 26 vialine 21 and passed into admixture with the compressed gaseous material from line 18(a), as previously mentioned.

A third gaseous stream comprising C2 and lighter gaseous components contaminated with C3 and C, hydrocarbons is withdrawn fromabsorption zone 26 vialine 27 and passed intosecond absorption zone 28. The absorption medium utilized inabsorption zone 28 is a portion of the lighter gas oil stream which was removed fromfractionation zone 13 vialine 15. This lighter gas oil absorption medium is passed vialine 29 into the upper portion ofabsorption zone 28 for contact therein with the gaseous material being introduced vialine 27. A first product stream comprising C2 and lighter components including hydrogen and acid gas, if any, is `withdrawn fromabsorption zone 28 vialine 30 and, preferably, sent to fuel. The second rich absorption oil is withdrawn fromabsorption zone 28 vialine 31 and returned tofractionation zone 13, preferably, as reux therein at a locus above the withdrawal locus for the lighter gas oil material inline 15.

Returning now toseparation zone 23, the second liquid stream inline 25 is now passed intosecond fractionation zone 32 which is maintained under fractionation conditions suflicient to produce a gaseous stream comprising, preferably, C, hydrocarbons contaminated with C3 and C., hydrocarbons which is withdrawn vialine 33. It is distinctly preferred, in order to maximize the recovery of C3 and C4 hydrocarbons, that the material inline 33 be sent vialine 37 intoseparation zone 23 in admixture with the previously mentioned first rich absorption oil inline 21. However, if desired, another product stream comprising these light hydrocarbons may be removed from the system vialine 33. A second product stream comprising C3 and C4 hydrocarbons is removed fromzone 32 via line 36 and a third product stream comprising a conversion gasoline is withdrawn from the system vialine 34. As previously mentioned, it is preferred that a portion of the third product stream inline 34 be passed vialine 35 intofirst absorption zone 26. However, a portion of the material inline 35 may also be sent tozone 13, orzone 18, orzone 23, by means not shown, if desired. A still further preferred embodiment of this invention is characterized Iby having the material inline 35 introduced intoabsorption zone 26 at a locus above the locus of introduction of the first liquid stream in line 19.

By operating in the manner set forth hereinabove it was found that extremely high recoveries (i.e., quantity and quality) of the normally gaseous hydrocarbons, such as C3 and C4 hydrocarbons, was achieved. In addition, a desrably stabilized gasoline product and, in the preferred embodiment, two gas oil fractions, were obtained as products.

'It is further noted that the preferred embodiment is characterized by having the rich oil from the rst absorption zone being passed into the second separation zone maintained at relatively high pressure thereby providing enrichment of the liquid with normally gaseous components which are subsequently separated infractionation zone 22. Similarly, the preferred embodiment is characterized by having not only two absorption zones, but by having also two distinct absorption mediums which are introduced into the first absorption zone. By operating in this manner, excellent recovery of C3 and C4 hydrocarbons is obtained.

The description of the drawing has not included specific operating details for each of the pieces of equipment contained in the combination method. It is deemed within the skill of those experienced in this art to choose the proper operating conditions to effectuate the various separations required by the description of the invention. The fractionation steps, the absorption steps and the separation steps are basically conventional operations which have been combined in a novel manner to produce an improved result. Those skilled in the art will also recognize that the required separating conditions will be, of course, influenced to a considerable extent lliy the characteristics of the fluid to be separated in line However, the following example is furnished to set forth the best mode contemplated for practicing the preferred embodiment of the invention.

EXAMPLE A commercial size coking unit was operated accordmg to conventional practice. The fluid eluent, after separation of the coke, was passed into apparatus schematically arranged as in the attached drawing.

The charge to the coking unit (line was a vacuum reduced crude having the following properties:

Gravity, API 19.11 Sulfur, wt. percent 0.16 Con Carbon, wt. percent 10.9 UOP K factor 12.25

After suitable pre-heat, the feedstock Was charged at a rate equivalent to 6000 barrels per stream day into the coking zone at a transfer temperature of about 900 F. and a coke chamber pressure of about 75 p.s.i.g. After separation of the coke, the fluid effluent was ultimately passed vialine 12 intofractionation zone 13.

The iluid effluent or vapors from the cake chamber are passed into the lower section offractionator 13 at a temperature of 820 F. and a pressure of about 75 p.s.i.g. An overhead stream comprising normally gaseous material and gasoline is withdrawn vialine 14 at a temperature of about 302 F., a side-cut stream comprising light gas-oil is withdrawn vialine 15 at a temperature of about 585 F., and a bottoms stream comprising heavy gas-oil is withdrawn vialine 16 at a temperature of about 750 F. Under these conditions the following -product streams were separated (composition dla-ta are in mols per hour):

Line No. Line No.

14 Component Component Total 798. 56

The material inline 14 is cooled by condensing means, such as an air-iin condenser, to a temperature of about 100 F. and passed into separator 17 which is under a pressure of about 60 p.s.i.g. A gaseous stream is Withdrawn from separator 17 vialine 18 and a liquid stream withdrawn via line 19. These withdrawn streams had the following composition (data are in mols per hour):

The gaseous stream inline 18 is passed into centrifugal compressor means 20 wherein the pressure is raised from about 60 p.s.i.g. to about 225 p.s.i.g. and then admixed, 'as previously mentioned, with recycle streams fromlines 21 and, preferably fromline 37, respectively. The admixture is then introduced intoseparator 23 at a temperature of about 100 F. and a pressure of about 215 p.s.i.g. Under these conditions a gaseous stream is separated and withdrawn vialine 24 and a remaining liquid stream is withdrawn vialine 25. The composition of these withdrawn streams fromseparator 23 had the following composition (mols per hour):

Line No.

Component 22 24 25 The liquid stream inline 25 is introduced intofractionation zone 32 which preferably is divided into two separate distillation columns, the iirst one being typically a gasoline stripper for the removal of C2s and lighter material from the feedstock and the second one being a debutanizer column for the recovery of C3 and C4 hydrocarbons and gasoline as product streams. For convenience sake, however, the dual fractionation zones have been shown in the attached drawing as a single column. By operating the fractionation zones under conventional conditions of temperature and pressure, an overhead stream comprising the C2 and lighter components contaminated with C3 and C4 hydrocarbons is withdrawn vialine 33. In the preferred embodiment of the invention the material inline 33 is passed vialine 37 and 21 into admixture with the `compressed vapors in line 18(a) as previously mentioned. However, if desired, the material inline 33 may be Withdrawn from the system and no recycle tohigh pressure separator 23 being employed. Obviously, a small amount of material or large amount of material or no amount of material may be .Withdrawn from the system vialine 33. Typically, a gasoline stripper would operate with a ash zone temperature of about F. and a pressure of about 242 p.s.i.g. Similarly, the debutanizer column could operate with a ash zone temperature of about 400 F. and a flash zone pressure of about p.s.i.g. Under these conditions a stream comprising C3 and C4 hydrocarbons is withdrawn fromfractionation zone 32 via line 36 and a gasoline product stream is withdrawn from the bottom of the debutanizer column vialine 34. As illustrative of the type of separation which may be accomplished infractionation zone 32, the following composition data is provided (mols per hour):

Line No.

C6533SF Total 11o. 02 96. 54 201. 09

Referring again tohigh pressure separator 23, the gaseous material inline 24 is passed into aiirst absorption zone 26 at a temperature of about 100 F. and a pressure of about 212 p.s.i.g. Sufficient operating conditions are maintained inabsorption zone 26 to dissolve substantially all of the C3 and C4 hydrocarbons into the lean oil which comprises at least in part the material in line 19 the composition of which has previously been given. The lean oil is introduced intoabsorption zone 26 so that effective counter-current contact may be obtained between the liquid and vapor in the zone. If desirable or deemed necessary, suitable packing material may also be placed inzone 26, the type and quantity of which are well known to those skilled in the art. Additionally, another portion of the absorption medium comprises gasoline fromline 34 which is introduced intozone 26 via line 35 (the composition of the material inline 34 has also been previously given). Under the operating conditions imposed inzone 26 the C2 and lighter materials are effectively rejected from this zone and are withdrawn fromabsorber 26 vialine 27. The combined rich oil having C3 and C., hydrocarbons absorbed therein is similarly withdrawn fromzone 26 vialine 21, admixed with the material coming fromline 37, and further admixed with thegas leaving compressor 20 via line 18(11) for recycle and liquid enrichment inhigh pressure separator 23. Illustrative of the separation obtained inabsorber 26 are the following composition data (mols per hour):

The rejected C2 and lighter material contaminated with C3-icomponents is passed vialine 27 intosecond absorber 28 at a temperature of about 118 F. and a pressure of about 205 p.s.i.g. The gas inline 27 is introduced into the lower portion ofabsorber 28 and contacted in counter-current manner therein with lean oil introduced into the upper portion ofabsorber 28 vialine 29. As previously mentioned, the absorber oil or medium for use inzone 28 is a portion of the light gas-oil product which was removed from fractionatingcolumn 13 vialine 15, the composition of which has previously been given. The C2 and lighter material is ultimately rejected vialine 30 and sent preferably to fuel. The rich oil fromsecond absorption zone 28 is withdrawn vialine 31 and preferably returned to the upper portion offractionating column 13 as additional reflux thereon. A typical composition data of the separation obtained inabsorber 28 is shown as follows (mols per hour):

8 PREFERRED EMBODIMENT Therefore, in summary, the preferred embodiment of the present invention includes the method for separating the fluid effluent from a coking reaction zone which comprises: (a) passing said efiluent into a first fractionation zone under conditions sufficient to produce a first distillate fraction comprising hydrogen, normally gaseous hydrocarbons, and gasoline, a second distillate fraction comprising light gas-oil, and a third fraction comprising heavy gas-oil; (b) introducing said first distillate into a first separation zone maintained under conditions including relatively low pressure sufficient to produce a first gaseous stream and a first liquid stream; (c) compressing said first gaseous stream and admixing the compressed stream with a hereinafter specified rich oil stream; (d) passing the admixture of Step (c) into a second separation zone maintained under conditions including relatively high pressure sutlicient to produce a second gaseous stream and a second liquid stream; (e) passing said second gaseous stream into a first absorption zone maintained under conditions suficient to absorb Ca-ihydrocarbons into an absorption medium comprising at least in part said first liquid stream of Step (b); (f) introducing a third gaseous stream comprising C2 and lighter gaseous components contaminated with C3. and C4 hydrocarbons into a second absorption zone maintained under conditions sufficient to absorb said contaminants into an absorption medium comprising at least in part a portion of said light gasoil from Step (a); (g) removing from said second absorption zone a first product stream comprising hydrogen, and C2 and lighter hydrocarbons; (h) passing said second liquid stream into a second separation zone under conditions sufiicient to produce a second product stream comprising C3 and C4 hydrocarbons and a third product stream comprising gasoline; (i) introducing the rich oil containing absorbed C3 and C4 hydrocarbons from Step (c) into admixture with said compressed gaseous stream as specified in Step (c); and, (j) returning a portion of said third product stream to said first absorption zone as another part of said absorption medium.

The invention claimed:

1. Method for recovering gasoline and normally gaseous hydrocarbons from the fluid eflluent of a hydrocarbon conversion unit which comprises the steps of:

(a) fractionating said efliuent in a first fractionation zone into a light distillate fraction and gas-oil fraction, said light distillate fraction containing gasoline and normally gaseous hydrocarbons;

(b) separating said light distillate fraction in a separation zone into a first gaseous stream and a first liquid stream;

(c) introducing at least a portion of said first gaseous stream into a first absorption zone maintained under absorption conditions including at least a portion of said first liquid stream as absorption medium;

(d) removing from said first absorption zone a first rich oil fraction and a second gaseous fraction;

(e) passing said second gaseous stream into a second absorption zone maintained under absorption conditions including the presence of at least a portion of said gas-oil fraction as absorption medium;

(f) removing from said second absorption zone a second rich oil stream and a first product stream comprising C2 and lighter components;

(g) passing said first rich oil into said separation zone of Step (fb) and passing said second rich oil to said fractionation zone of Step (a); and,

(h) introducing the remaining liquid stream of Step (b) into a second fractionation zone and recovering therefrom a second product stream comprising normally gaseous hydrocarbons and a third product stream comprising gasoline.

2. Method according to claim 1 wherein a portion of said third product stream is passed into said -iirst absorption zone as absorption medium.

3. Method according to claim 1 wherein said third product stream comprises C3 and C4 hydrocarbons.

4. Method according to claim 1 wherein said hydrocarbon conversion unit comprises a coking reaction zone.

5. Method for separating the fluid efliuent from a coking reaction zone which comprises:

(a) passing said etiiuent into a rst fractionation zone under conditions sufficient to produce a irst distillate fraction comprising hydrogen, normally gaseous hydrocarbons, and gasoline, a second distillate fraction comprising light gas-oil, and a third fraction comprising heavy gas-oil;

(b) introducing said rst distillate into a irst separation zone maintained under condition including relatively low pressure suicient to produce a iirst gaseous stream and a first liquid stream;

(c) compressing said rst gaseous stream and admixing the compressed stream with a hereinafter specified rich oil stream;

(d) passing the admixture of Step (c) into a second separation zone maintained under conditions including relatively high pressure suicient to produce a second gaseous stream and a second liquid stream;

(e) passing said second gaseous stream into a Ifirst absorption zone maintained under conditions sufficient to absorb C3+ lhydrocarbons into an absorption medium comprising at least in part said first liquid stream of Step (b);

(f) introducing a third gaseous stream comprising C2 and lighter gaseous components contaminated with C3 and C4 hydrocarbons into a second absorption zone maintained under conditions sufficient to afbsorfb said contaminants into an absorption medium comprising at least in part a portion of said light gas-oil from Step (a);

(g) removing from said second absorption zone a first product stream comprising hydrogen, and C2 and lighter hydrocarbons;

(h) passing said second liquid stream into a second separation zone under conditions sulicient to produce a second product stream comprising C3 and C4 hydrocarbons and a third product stream comprising gasoline;

(i) introducing the rich oil containing absorbed C3 and C4 hydrocarbons from Step (e) into admixture with said compressed -gaseous stream as `specified in Step (c); and,

(j) returning a portion of said third product stream to said -rst absorption zone as another part of said absorption medium.

References Cited UNITED STATES PATENTS 2,182,536 12/1939 Eaton 208-101 2,745,889 5/1956 Johnston et al. 208--101 2,908,625 10/ 1959 Melder et al 208--101 2,939,834 6/ 1960 Evans 208-101 2,985,583 5/ 1961 Gilmore 208-101 HERBERT LEVINE, Primary Examiner U.S. C1. X.R.

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