US 2795531 A
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June 11, 1957 G. HY. MEGUERIAN IQTAI. 2,795,531
CRACKED NAPHTHA SWEETENING Filed sept. 28, 1954' ESR JNVENTORS.- 'Garbs H. Mega/erin By 'rheaab're ram g t) United States PatentO CRACKED NAPHTHA SWEETENING Garbis H. Megueran, Park Forest, Ill., and Theodore B.
Tom, Grilith, Ind., assignors to Standard Oil Company, Chicago, lll., a corporation of Indiana Application September 28, 1954Serial No. 458,848`
5 Claims. (Cl. 196-29) This invention relates to the sweetening of sour petroleum distillates. More particularly, the sour cracked naphthas. In particular, the invention relates tothe sweetening of sour cracked naphthas utilizing free oxygen and a polyhydroxybenzene catalyst. It is common prac tice to sweeten, i. e., remove mercaptans from -sour petroleum distillates by contacting the sour distillate Withfree oxygen in the presence of aqueous caustic and' a poly-k hydroxybenzene catalyst. The most commonly used polyhydroxybenzene catalyst is hydroquinone. In some instances, petroleum cresols and xylenols derived from petroleum distillates by extraction with aqueous caustic solution have been used as catalysts in the oxidation of mercaptans to disullides. When using either hydroqui none or petroleum cresols as the oxidation catalyst, side reactions take place which result in the production of a sweet oil of poorer color than the sour oil. Frequently, the degradation in color is so great that sweet naphtha cannot be used for the production of water-white gasoline.
An object of the invention is a process for sweetening sour petroleum distillates lby the oxidation of mercapt'ans in the presence of aqueous caustic and a polyhydroxybenzene catalyst Without serious color degradation of` the distillate. Another object is a sweetening process wherein phenolic compounds derived from petroleum distillates may be used as catalysts in the oxidation of mercaptans without serious loss of color of the mercaptan-containing distillate. Other objects will become apparent in the course of the detailed description.
A sour petroleum distillate is sweetenedwithout.signicant loss of color 'by removing phenolic compounds from said distillate prior to oxidizing mercaptans to disuldes with free oxygen in the presence of aqueous caustic solution containing a catalytically eliective amount of gallic acid.
Some sour naphthas are sweetened without significant loss of color by removing phenolic compoundstherefrom prior to oxidation of the mercaptans in the presencefof aqueous caustic solution containing petroleum cresols and/or xylenols, which cresols and xylenols have been treated to remove color-forming bodies.
It has been discovered that dihydroxybenzenes, suchas hydroquinone, catechol, alkylhydroquinone, such as t-'butylhydroquinone, and trihydroxybenzenes, -such as pyrogallol, in the presence yof aqueous caustic solution and free oxygen, react with mercaptans to form mercaptyl compounds. Under the conditions of aqueouscaustic concentration and mercaptan amounts present in conventional sweetening operations utilizing aqueous caustic, free oxygen, and hydroquinone, tetramercaptylquinone is formedby the reaction of hydroquinone, mercaptans and oxygen. The mercaptyl quinones are oil-solubleand highly colored; these mercaptyl quinones are extracted from the aqueouscaustic phase by the oil and appear in the sweet oil with consequent degradation incolor of the oil relative to the color of the sour oil.
The phenolic compounds present in cracked distillates and' manyvirgin heavier-than-,gasoline distillates include ice not only' alkylmonohydroxybenzene-s, Ibut also alkylpolyhydroxybe'nzenes. Thus, the so-colled petroleum cresols and petroleum xylenols which have been obtained by extraction of' a petroleum distillate with aqueous caustic solution arev av mixture of alkylmonohydroxybenzenes and alkylpolyhydroxybenzenes; it appears that only a small amountof polyhydroxybenzene materials are present. The raw cresols and xylenols are most commonly used as solutizers to promote' the solubility of mercaptans in aqueous-caustic: sol'utiouin processes which physically remove' crease the oil solubility. Gallic acid (3,4,5-t'rihydroxybenzoic acid) can 'be used as aV catalyst for the sweetening ofr sour distillates-in the presence of aqueous caustic solution: andfree oxygen without significant deleterious effect on the= color of the sweetdistillate. However, cracked distillatesl andl Virgin distillates which contain phenolic compounds' undergo al color degradation when the raw 30` sour distillate is-r sweetened with aqueous c-austic and'free oxyg'en'finithevpresence ofgallic acid catalyst. It has been found that'the polyhydroxybenzene compounds present in the sour distillate react in the sweeteningzone with mer- `captans to'form the highly-colored oil-soluble mercaptyl quinones,A which degradate the color of the distillate. Therefore, in the process of this invention, sour cracked distillates and virgin distillates containing phenolic compounds' are treated to-remove phenolic compounds prior tothe-oxidation of the mercaptans to disulides 'by treating' the dephenolized distillate with aqueous caustic solution and free oxygen in the presence of gallic acid catalyst.
Although in general monohydroxybenzenes, such as cresol andrxylenol -are relatively ineffective as mercaptan oxidationrcatalysts, these particular phenols are sutliciently active to permit their use in the sweetening of sour catalytically cracked naphthas. Thus, in the sweetening of sour catalytically cr-ackedl naphtha, a sweet naphtha is obtainable.` without appreciable color loss by iirst removing. phenolicv compounds from the sour cat naphtha and thensweetening the dephenolized sour naphtha by treatingfwith-aqueous caustic solution and free oxygen in the presence of petroleum cresols. and/or xylenols which have been treated to removev polyhydroxybenzenes. The petroleum cresols and/or xylenols` which are to be used as-mercaptan oxidation catalysts are treated as follows: An` aqueous caustic-cresylate-mercaptide solution which has-beenobtained by treatment of a sour cracked naphtha withaqueous caustic solution is contacted with free oxygenand additionalmercaptan, if necessary, in the presence oftapetroleumdistillate until all of the polyhydroxybenzene4 compounds have been converted to mercaptyl equinones which pass into the distillate; the distillate is decanted,V leaving a rened aqueous caustic-cresylate solution containing. essentially no polyhydroxybenzenes; the
.refined aqueous caustic-cresylate solution is then utilized .embodiment setout in theA drawingfis-schematic in nature and many items of process equipment have been omitted, as these may readily be added by those skilled in the art.
rthe petroleum distillate feed to the process contains sufficient mercaptan to be sour to the doctor test. In addition, the petroleum distillate contains phenolic compounds which are commonly known as cresols and/or xylenols. cracked naphthas, either thermal or catalytic naphthas and catalytic gas oils boiling in the heavier-than-gasoline range, e. g., from about 330 F. to about 625 F.
In the illustrative embodiment, the feed consists of a raw thermally cracked heavy naphtha having a mercaptan number of about 25 which contains HzS and phenolic compounds. The raw naphtha from source 11 is introduced by way of line 12 to a point near the bottom of a prewash vessel 13. Prewash vessel 13 is a vertical cylindrical vessel provided with means for intimately contacting two immiscible liquids.
Aqueous caustic solution from source 16 is passed by way of line 17 into an upper portion of prewash vessel 13. The aqueous caustic solution comprises essentially water and alkali metal hydroxide such as sodium hydroxide or potassium hydroxide. Although solutions containing as little as weight percent of caustic may be utilized, it is preferable to operate with solutions containing a sufficient concentration of caustic to extract an appreciable amount of mercaptans and phenolic compounds from the raw naphtha. In general, between about and 3S weight percent of caustic, i. e., alkali metal hydroxide, should be present in the aqueous solution. However, higher concentrations may be used. It is preferred to operate with an aqueous caustic solution containing between about 20 and 30 weight percent of NaOH. In this embodiment, the aqueous caustic solution contains weight percent of NaOH.
The amount of aqueous caustic solution used in prewash vessel 13 will be dependent upon the type of naphtha and the degree of contacting attained in the vessel. In general, the volume ratio of aqueous solution to raw naphtha feed is between about l:5 and 1:1 or in other terms, between about 20 and 100 volume percent of aqueous caustic solution are used, based on raw naphtha feed; preferably between about 25 and 50 volume percent,i. e., a volume ratio of solution to naphtha of between about 1:4 and 1:2. In this embodiment, the volume ratio of aqueous caustic solution to raw naphtha is 1:3.
The temperature of prewashing is normally slightly above atmospheric temperatures.
An aqueous solution comprising NaOH, sodium mercaptides and sodium cresylates is withdrawn from the bottom of vessel 13 and is passed by way of line 18 into valved line 19, for recycle to line 17. Periodically, solution is withdrawn and disposed of by way of valved line 29. HzS-free naphtha is removed from the upper portion of vessel 13 and is passed by way of line 21 and valved line 22 into mercaptan extractor 23.
Mercaptan extractor 23 is a vertical cylindrical vessel provided with means for intimately contacting two immiscible liquids. In extractor 23 the sour naphtha is contacted with an aqueous caustic solution containing cresylates which act as solutizers for the mercaptans. In this illustration, these cresylates have been extracted from the naphtha feed to the process. Aqueous caustic-cresylate solution is introduced into an upper portion of extractor 23 by way of line 24.
suicient to improve the solubility of mercaptans in the l caustic solution.v In general, the solution will contain between about l0 and 30 volume percent of phenolic compounds in the form of cresylates; preferably, the solu- The process is particularly useful with sour tion should contain between about 15 and 25 volume percent of phenolic compounds which have been derived from the thermally cracked naphtha undergoing treatment. In addition to these the solution charged to vessel 23 will contain some alkali metal mercaptides. In this embodiment, the lean aqueous caustic solution contains about 7 weight percent of free NaOH and about 20 volume percent of cresols derived from thermally cracked naphtha.
The amount of lean solution used in extractor 23 is dependent upon the type of naphtha charged thereto. In general, the volume ratio of lean solution to naphtha will be'between about 1:10 and 1:1. Usually the volume ratio of solution to naphtha will be between about 1:4 and 1:2. In this embodiment, the volume ratio is 112.7.
Extractor 23 is operated at a slightly elevated temperature. In general, the extraction is carried out at temperatures between about 60 and 110 F.
Extracted naphtha is withdrawn from extractor 23 by way of valved line 26 and line 27. The mercaptan number of the naphtha from extractor 23 will vary somewhat depending on the quality of the charge naphtha and the type and amount of lean solution used. The naphtha in line 26 will be slightly sour, i. e., have a mercaptan number of about 2. In some instances, the mercaptan number may be as much as l0.
A fat solution comprising water, free NaOH, cresylates and mercaptides is withdrawn from the bottom of extractor 23 by way of valved line 28. This fat solution may be passed to waste disposal periodically by way of valved line 29. The fat solution from line 28 is passed by way of line 29 into regenerator 31. Regenerator 31 is provided with an internal heat exchanger 32.
The conversion of the mercaptides present in the fat solution is catalyzed by gallic acid. A catalytically etective amount of gallic acid from source 30 is passed by way of line 30a into line 29. The amount of gallic acid introduced is dependent upon the conditions of operation of regenerator 31. In general, between about 0.05 and about 0.5 weight percent of gallic acid is present in regenerator 31, based on fat solution.
The mercaptide-containing aqueous caustic solution is regenerated by oxidizing the mercaptides to the corresponding disuldes. These disul-des are only moderately soluble in aqueous caustic solution and may be decanted away from the aqueous phase; the aqueous phase normally contains an appreciable amount of entrained disuldes. The oxidation of the mercaptides to disuldes is obtained by treating the aqueous solution in regenerator 31 with atmospheric oxygen. Air from source 33 is passed by way of line 34 into the lower portion of regenerator 31.
The rate of oxidation is also increased by carrying out the regeneration at elevated temperatures; preferably the regeneration is carried out at a temperature between about and 160 F. The mercaptides can not be completely converted to disuliides without destroying the gallic acid catalyst. To prevent oxidation of the gallic acid catalyst, a small amount of mercaptides are permitted to remain in the regenerated solution.
In this illustration, regenerator 31 is operated at about F. and at an air introduction rate of about 135 S. C. F. VM. Air and vapors are discharged from regenerator 31 by way of vent 38.
A mixture of aqueous caustic-cresylate solution and disuldes is withdrawn from regenerator 31 and is passed by way of line 39 into an upper portion of disul-de extractor 41. Disultideextractor 41 is a vertical cylindrical vessel similar in construction to extractor 23. A liquid hydrocarbon which is an excellent solvent for disultides is introduced into the lower part of extractor 41. While virtually any liquid hydrocarbon oil may be used for this purpose, it is preferred to use an oil which is cheap in order to permit disposal of thedisulde-rich oil by burning. In this embodiment, alight catalytical- Iy' cracked' cycle oil in the gas oil boiling range from source l42 is introduced by way of line 43 into extractor 41'. The amount of liquid hydrocarbon solvent used should beat least suicient to remove substantially all of the disulides present in the effluent from regenerator 31. This amount will vary with operating. conditions. In general, the volume ratio of liquid hydrocarbon solvent to regenerated solution willbe between about 1:4 and 1:1. I-n this embodiment, the volume ratio of LCCO to regenerated solution is 1:2.
The disulfide-containing light cat cycle oil is withdrawn from the top of extractor 41 and. is sent to disposal as relinery fuel by way of line 44. Y Any oil-soluble mercaptyl quinones which are formed in regenerator 31 are removed along with disuliides by the LCCO in extractor 41, thereby no color bodies are introduced intoextractor 23.
A lean solution comprising water, NaOH, mercaptides and cresylates is withdrawn from extractor 41` and a portion of this solution is passed by way of line 46 to line 24. Some loss of caustic solution is inevitable. Makeup aqueous caustic solution from source 47 is introduced by way of valved line 48 into line 46. Periodically aqueous caustic-cresylate solution is removed from the system and passed to disposal by way of valved line 49.
The extracted naphtha in line 26` contains occlud'ed aqueous caustic-cresylate solution. Sulhcient polyhydroxybenzene material is present in this solution to result in a significant degradation in color of the product naphtha. The hazy extracted naphtha is passed by way of line 26 and 27 into coalescer 51. Coalescer 51 isv a vertical cylindrical vessel provided with a bed of material suitable for agglomerating the droplets of aqueous caustic-cresylate solution to a size suihcient to drop out of the naphtha. Coalescer 51 may be provi-ded with bre glass, steel wool, excelsior, or gravel. In this illustraton, coalescer 51 is packed with` crushed rock. Other methods of removing the aqueous caustic-cresylate haze may be utilized, for example, water washing. Aqueous caustic-cresylate solution is removedl from the bottom of coalescer 51 by way of valved line 5-2 and is recycled to extractor 23 by way of line 24. Or it may be passed to `disposal by way of lines not shown.
The dephenolized extracted naphtha is passed by way of line 56 into sweetening vessel 57. Sweeteningvessel 57` is a vertical cylindrical vessel provi-ded with an internal heat exchanger 58. Aqueous caustic solution from source 61 is passed by way of valved line 63 and line 64 into line 56 where it meets the extracted sour naphtha. The free oxygen which is needed in the sweetening reaction is provided by air from source 67 which is passed by way of line 68 into line 64. It has been found that introducing the free oxygen into the circulating caustic stream has a beneficial effect on the sweetening reaction and on the amount of gallic acid catalyst which must be used. Introduction of free oxygen along with the sour naphtha frequently results in excessively high concentration of free oxygen in line 56 with the result that the catalyst is necessarily oxidized and lost. Sufficient free oxygen must be introduced to con-vert. the mercaptan to the corresponding disullides.
Gallic acid catalyst is introduced into vessel 57 from source 7l by way of line 72. Introduction of the gallic acid into the vessel decreases the loss of catalyst; The
amount of gallic acid catalyst present in sweetening vessel 57 is determined primarily by the sourness of the naphtha charged therein. In general, from about 0.1 to about 0.6 pound of gallic acid are needed per 1000 barrels of sour naphtha per mercaptan number of the sour naphtha. The mercaptan number is the number of milligrams of mercaptan sulfur present per l() ml. of sour naphtha. By careful control of the arenas;
amount of air and temperature in vessel 57, the` amount end of the range. Normally the usage of gallic acid's between about 0.2 and 0.3 lb./ 1000 bbls./mercaptan number ofthe sour oil. A
The amount 'o f free oxygen present in vessel 57 is 'more than'fthe theoretical required for the conversion of the mercapt'ans. In general, between about 1 and 5 times the theoretical requirement of oxygen is utilized. Preferably, the oxygen usage isY between about 1.5 and 2.5 times the theoretical requirement. The rateofmercaptan oxidationis. increased by operation at elevated temperatures. Temperatures between about and F. are suitable. In this particular embodiment, utilizing thermally cracked naphtha, the temperature in vessel'57 is about 140 F. The volume ratio-of aqueous caustic solution to sour naphtha in vessel 57 is between about 1:50 and 1:1/0and preferably is about 1:20. Aqueous caustic-cresylate solution is withdrawn from the bottom of vessel 57 by way of line 76 and is recyclediby way of valved line 63 and line 64I to line 56. Periodically, the aqueous caustic solution is passed t disposal vby way' of valved-f. line 77.
An essentially sweet product naphtha is withdrawn from vessel 57 by way of line 81- and is passed to storage not shown. If this sweet naphtha is hazy, owing to Vocclusion of aqueous caustic solution, it may be dehazed 'by conventional methods, such as passage through a coalescer or a salt drum.
Although in the embodiment no solutizer has been present in the aqueous caustic solution utilized in vessel 57, when operating with a naphtha having a higher mercaptan number, it is desirable to utilize a solutizer. This solutizer may be an alcohol, such as methanol, or a fatty acid salt such as potassium isobutyrate, or preferably, a refined petroleum cresol or xylenol. For example, aqueous caustic-cresylate solution from line 46 may be utilized in vessel 57 if the regeneration treatment andv the extraction have been sutlicient to remove essentially all of the naturally occurring polyhydroxybenzenes present in the cresylates removed from the naphtha in extractor 23. If this .aqueous caustic-cresylate solution is not `sutiiciently refined, further treatment of the causticcresylate solution With free oxygen in the presence of sour light cat cycle oil should be carried out. A sour light cat cycle oil is utilized in order to insure the presence of suflicien't mercaptan to convert all of the natural polyhydroxybenzenes to oil-soluble mercaptyl quinones and to remove these oil-soluble mercaptyl quinones. The simplest procedure for determining that the aqueous cau-stic-cresylate solution has been suiiiciently rened is the use of a portion of the treated solution as a catalyst in the sweetening of a dephenolized cat naphtha or a `sour naphtha using gallic acid catalyst.` When no appreciable color degradation results from the use of the treated aqueous caustic-cresylate solution, the aqueous caustic-cresylate solution is suiciently reiined for use as a solutizer in vessel 57 or as 'a catalyst in the treatment vof cat naphtha.
Some feed naphthas have Isuch a low mercaptan content that a substantially sweet product naphtha may be obtained without the use of mercaptan extractor 23. In suoh a case, the naphtha is passed from vessel 13 by way of line 21, by-pass line 83 and line 27 to vessel 51. In this method of operation, mercaptan extractor 23 is isolated by closing the valves in lines 22, 24, 26, and 28.
The results obtainable with the processY of the invention are illustrated by the following examples. These examples are not to be considered as limiting the scope of the invention. In the examples, 100 ml. of catalytically cracked heavy naphtha having a mercaptan number of 4 was shaken in a 250 ml. volumetric flask with 20 ml. of aqueous solution at room temperature of about 75 F. for a time of l0 minutes. The only oxygen which entered the ysystem was that obtained by exposing the jagitated materials to atmospheric air.
a color of +21 Saybolt.
In each example, at the end of minutes, thel sour naphtha was sweet to the doctor test. The sour naphtha 'feed to the sweet- 'ening zone had been prewashed with 25% aqueous NaOH solution to remove phenolic materials; the prewashed naphtha was then washed with' water to remove aqueous caustic-cresylate haze. The sour cat heavy naphtha had Example I i The aqueous solution in this example Awas an aqueous caustic-cresylate mercaptide solution obtained in the renery by contacting a sour thermally cracked naphtha. The aqueous caustic-cresylate solution contained about 10% of tree NaOHand about 10% of petroleum cresols. The color of the sweet-naphtha after contacting with the aqueous caustic-cresylate solution and air was +3 Saybolt; a degradation of 18 units. v
Example 2 Aqueous caustic solution containing NaOH and 1% of hydroquinone was utilized in this example as the aqueous solution. The color of the sweet naphtha was 2 NPA-a medium tan.
Example 3 In this example, aqueous 25% NaOH containing 1% gallic acid, CP grade, was utilized as the aqueous solution. The color of the sweet naphtha was +21. Thus, when using gallic acid, no color degradation resulted during the sweetening operation.
Example 4 A reiined aqueous caustic-cresylate solution was prepared by contacting 20 ml. of aqueous caustic-cresylate solution such as was used in Example l with 50 ml. of sour light catalytically cracked cycle oil while blowing air through the mixture. The contacting was continued for l hour at 75 F. At the end of this time, the oil was decanted from the treated aqueous caustic-cresylate The treated aqueous caustic-cresylate solution was utilized as the aqueous solution for the treatment of sour cat naphtha. ment had a Sayoolt color of +15. This indicates that the aqueous caustic-cresylate solution had not been suiciently refined.
Thus having described the invention, what is claimed is:
l. A sweetening process which comprises (l) contacting a sour cracked naphtha containing phenolic cornpounds, including polyhydroxybenzenes free of carboxyl substituents, with aqueous caustic solution and separating sour naphtha from an aqueous caustic solution phase, said phase containing substantially all of the phenolic Vcompounds present in the sour cracked naphtha charged,
loxygen in an amount at least suicient to convert all the mercaptans contained in said phenolic compoundfree sour naphtha, said other aqueous caustic solution containing an amount of gallic acid at least .sufficient to catalyze the mercaptan oxidation, said gallic acid being essentially the only polyhydroxybenzene present in said other aqueous caustic solution, and `(4) separating the aqueous caustic solution from said cracked naphtha,
r*said sweet cracked naphtha being characterized by a The sweet naphtha from this treatcolorsubstantially the same as the color of the ythe naphtha in step (2) by water washing said naphtha.
3. A sweetening process which 'comprises (a) contacting a sour cracked naphtha containing phenoliccornpounds, including polyhydroxybenzenes free of carboxyl substituents, with aqueous caustic solution, said aqueous caustic solution being capable of removing a substantial amount of the mercaptans present in said sour cracked naphtha and essentially all of the phenolic compounds present in said sour cracked naphtha, (b) separating aqueous caustic solution containing extracted mercaptans and phenolic compounds from a nap-htha phase which is sour to the doctor test, and is essentially free of dissolved phenolic compounds and contains occluded aqueous caustic solution phase, (c) treating the separated sour naphtha of step (b) to remove said occluded aqueous cans-tic solution phase, thereby producing'a sour naphtha essentially tree of phenolic compounds, (d) sweetening said phenolic compound-free naphtha by contacting it with another aqueous caustic solution in the presence of an amount of free oxygen at least suicient to convert the mercaptans contained in the naphtha, said other aqueous caustic solution containing an amount of gallic acid at least suiicient to catalyze the mercaptan conversion, said gallic acid being essentially the only polyhydroxybenzene present in said other aqueous caustic solution and separating other aqueous caustic solution phase from a sweet naphtha, said sweet naphtha being characterized by a color substantially the same as the color of the sour cracked naphtha charged to the process, and (e) regenerating the aqueous caustic solution phase separated in step (b) by (e i) contacting said solution with free oxygen in an amount suicient to convert at least the major portion of said mercaptans to polysullides, said separated aqueous solution containing an amount of gallic acid at least sufficient to catalyze the conversion of said mercaptan, (e ii) washing the aqueous caustic solution of step (e i) with a liquid hydrocarbon oil under conditions to remove said polysuldes and color bodies formed in said free oxygen treatment, and (e iii) separating liquid hydrocarbon oil from washed 4regenerated aqueous caustic solution, and (f )recycling said regenerated solution to step (a).
4. The process of claim 3 wherein said naphtha is a thermally cracked naphtha.
5. The process of claim 3 wherein said naphtha is a catalyticallly cracked naphtha.
References Cited inthe le of this patent UNITED STATES PATENTS 2,164,851 Yabrol et al July 4, 1939 2,369,771 Bond Feb. 20, 1945 2,427,250 Bond Sept. 9, 1947 2,462,810 Ferguson Feb. 22, 1949 2,645,602 Tom et al July 14, 1953 FOREIGN PATENTS 502,401 Canada May 11, 1954 OTHER REFERENCES A. S. T, M. Standards on Petroleum Products and Lubricants, American Society for Testing Materials, November 1954, pages 88-98.