US2727850A

US2727850A - Sweetening process

Info

Publication number: US2727850A
Application number: US349707A
Authority: US
Inventors: Jr William E Stanley; John E Kaufman
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1953-04-20
Filing date: 1953-04-20
Publication date: 1955-12-20
Anticipated expiration: 1972-12-20

Description

Dec. 20, 1955 w. E. STANLEY, JR., ETAL 2,727,850

SWEETENING PROCESS Filed April 20.

.QUUQ

vv\ Enl Hummm; bmkwwm,

W. www y 0 N m u E Taf N m R V$.K m m55. m .MM .mw W

United States Patent C SWEETEMNG PRoCEss William E. Stanley, Jr., Park Forest, and John E. Kaufman, Chicago, Ill., assignors to Standard (Eli Company, Chicago, lll., a corporation of Indiana Applicants Aprii ze, 1953, serial NQ. 349,767

s exams. (ci. 19e-2s) This invention relates to the sweetening of sour petroleum distillates. More particularly it relates to the reduction in sulfur content of sour naphthas which contain phenolic compounds and H25.

Many methods are known for the treatment of sour naphthas to convert the mercaptans to disultldes and thereby improve the odor of the naphtha. Generally these processes do not decrease the sulfur content of the naphtha and may even increase the sulfur content. The increased use of high sulfur crude oils has forced the reners to remove sulfur compounds in order to produce naphthas of desirably low sulfur contents.

Virtually all naphthas contain dissolved hydrogen sulfide which is usually removed by washing the naphtha with an aqueous caustic solution. This solution is ordinarily recycled to the washing operation until the caustic is exhausted and the solution is then discarded. In one method or' sweetening, caustic solution from the H25 washing operation is used as the aqueous caustic medium for the free-oxygen conversion of mercaptans to disuliides. This method reintroduces into the naphtha the mercaptan sulfur, in the form of disultldes, which had been extracted by the caustic solution during the washing step.

A well known method for decreasing the mercaptan content of naphtha involves the contacting of sour naphtha with an aqueous caustic solution containing phenolic compounds derived from the naphtha undergoing treatment. This process does not produce a sweet product naphtha; usually the naphtha is slightly sour, i. e., a mercaptan number of about 2; at times the mercaptan number of the effluent naphtha may be as high as 1G. The fat solution of aqueous caustic, phenolates and mercaptides is regenerated by oxidizing the mercaptides to disuliides with free-oxygen, usually in the presence of an added oxidation catalyst. The slightly sour naphtha is then sweetened by one of the conventional methods, such as, the doctor process.

It is an object of this invention to sweeten sour naphthas under conditions to reduce the sulfur content by physical removal of the mercaptans present therein. Another object is a process for economizing on aqueous caustic solution usage in the sweeter-ling of sour naphthas by utilizing aqueous caustic solution from an HzS prewash in a subsequent air oxidation sweetening step, wherein said caustic solution has been treated to remove most of the mercaptides absorbed in said prewash step. A particular object is a combination process for the sweetening of a sour, thermally cracked petroleum naphtha containing B2S, which process involves an aqueous caustic prewash, solutizer treatment of the washed naphtha, regeneration of the fat prewash caustic solution and the fat solutizer solution in a common regenerator to reduce the mercaptide content and sweetening of the extracted naphtha using the regenerated caustic solution. Other objects will become apparent in the course of the detailed description of the invention.

It has been found that a sour thermally cracked naphtha can be sweetened with a decrease in sulfur content substantially equal to the mercaptan sulfur originally present therein by a combination process which comprises (l) contacting said naphtha with an aqueous caustic solution under conditions to remove appreciable amounts of mercaptans and phenolic compounds, (2) separating naphtha from an aqueous solution comprising caustic, mercaptides and phenolates, (3) contacting the naphtha from step (2) with au aqueous solution containing caustic and phenolates under conditions to produce a slightly sour naphtha, (4) separating a fat solution comprising caustic, mercaptides and phenolates from slightly sour naphtha, (5) combining the aqueous solution of steps (2) and (4) and contacting the combined solution with free-oxygen under conditions to convert the major portion of the mercaptides to disullides, (6) removing disulfides from the regenerated solution of step (5) to produce a lean solution, (7) recycling a portion of said lean solution to the contacting zone of step 3), (8) contacting the slightly sour naphtha from step (4) with a portion of the lean solution in the presence of free-oxygen under conditions to convert essentially all of the rnercaptans to disuldes, and (9) separating an essentially sweet naphtha from aqueous caustic solution.

he invention is described in detail in connection with the description of a preferred embodiment thereof set out in the annexed drawing which forms a part of this specification. The embodiment presented in the drawing is schematic in nature and many items of process equipment have been omitted as these may readily be added thereto by those skilled in the art.

Any sour petroleum naphtha may be the feed to this process. However, it is preferred to use naphthas coni taining appreciable amounts of phenolic bodies, such as,

phenols, cresols and xylencls. Naphtha derived from the thermal cracking of gas oils is the preferred feed to the process.

ln 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 l2 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 i3. The aqueous caustic solution comprises essentially water and alkali metal hydroxide such as sodium hydroxide or potassium hydroxide. Although solutions containing as little as 5 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 coni.- pounds from the raw naphtha. In general between about l0 and 35 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. ln this embodiment the aqueous caustic solution contains 25 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. ln general the volume ratio of aqueous solution to raw naphtha feed is between about 1:5 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 3 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.

'Ihe temperature of prewashing is normally slightly above atmospheric temperatures.

An aqueous solution comprising NaOH, sodium mercaptides and sodium phenolates is withdrawn from the bottom of vessel 13 and is passed by way of line 18 into line 19. 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 phenolates which act as solutizers for the mercaptans. In this illustration these phenolates` have been extracted from the naphtha feed to the process. Aqueous caustic-phenolate solution is introduced into an upper portion of extractor 23 by Way of valved line 24.

The aqueous caustic-phenolate solution may contain between about 5 and 15 Weight percent of uncombined, i. e., free alkali metal hydroxide. More than this amount may be present. The solution also contains an amount of phenolic compounds in the form of alkali metal phenolates sufiicient to improve the solubility of mercaptans in the caustic solution. In general the solution will contain between about and 30 volume percent of phenolic compounds in the form of phenolates; preferably the solution should contain between about and 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 phenolic compounds derived from thermally cracked naphtha. These phenolic compounds are commonly known as petroleum cresols.

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 1:2.7.

Extractor 23 is operated at a slightly elevated temperature. In general the extraction is carried out at ternperatures 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. In general the naphtha in line 26 will be slightly sour, i. e., have a mercaptank number of about 2. In some instances the mercaptan number may be as much as 10.

A fat solution comprising water, tree NaOH, phenolates and rnercaptides 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 and the prewash solution from line 18 meet in line 19 and are passed into regenerator 31. Regenerator 31 is provided with an internal heat exchanger 32.

The mercaptide-containng aqueous caustic solution is regenerated by oxidizing the rnercaptides to the corresponding dsuliides. These disulfides are only moderately soluble in aqueous caustic solution and may be decanted away from the aqueous phase; the aqueous phase normally contained an appreciable amount of entrained disuldes. The oxidation of the rnercaptides to disuliides is obtained by treating the aqueous solution in regenerator 31 with atmospheric oxygen. Air from source 33 is regeneration is carried out at a temperature between` about and 160 F. The rnercaptides cannot be completely converted to disultides without destroying the hydroquinone catalyst. To prevent oxidation of the hydroquinone catalyst, a small amount of rnercaptides 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. M. Air and vapors are discharged from regenerator 31 by way of vent 38. l

A mixture of aqueous caustic-phenolate solution and disuliides is withdrawn from regenerator 31 and is passed by way of line 39 into an upper portion of disulfide extractor 41. Disultide extractor 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 the disulfide-rich oil by burning. In this embodiment a light catalytically cracked cycle oil in the gas oil boiling range from source 42 is introduced by way of line 43 into extractor 41. The amount of liquid hydrocarbon solvent used should be at least sufficient to remove substantially all of the disulides present in the eiuent from regenerator 31. This amount will vary with operating conditions. In general the volume ratio of liquid hydrocarbon solvent to regenerated solution will be between about 1:4 and 1:1. In 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 refinery fuel -by way of line 44.

A lean solution comprising water, NaOH, rnercaptides and phenolates is Withdrawn from extractor 41. A portion of this solution is passed by way of valved line 46 to line 27 where it meets the slightly sour naphtha from extractor 23. 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..

The mixture of naphtha and lean solution in line 27 is passed into sweetening vessel 51 which is provided with internal heat exchanger 52. Vessel 51 is similar in construction to regenerator 31. In vessel 51 the naphtha is sweetened by air oxidation of the mercaptans present in the naphtha. Free-oxygen for this purpose is introduced into vessel 51 from source 53 by way of line 54.

An elevated temperature is maintained in vessel 51 in order to accelerate the oxidation. Temperatures between about 100 and 160 F. are suitable. In this embodiment a temperature of 140 F. is used.

The amount of lean solution used in vessel 51 must be at least a catalytic amount. The volume ratiol of lean solution to slightly sour naphtha may be between about 1:50 and 1:10.

Aqueous caustic phenolate solution is Withdrawn from the bottom of vessel 51 by way of line 57 and recycled by way of valved line 58 to line 46. This caustic solution may be sent to waste disposal by way of valved line 59; Makeup caustic solution may be added to line 58 directly instead of via line 46. n

An essentially sweet product naphtha is withdrawn from vessel 51 by Way of line 61. Normally this naphtha will be dehazed by conventional means not shown.

The major portion of lean solution in line 46 is passed by way of valved line 24 to the upper portion of mercaptan extractor 23 for reuse in the mercaptan extraction operation.

Some feed naphthas have such a low mercaptan content that a substantially sweet product naphtha may be obtained without the use of mercaptan extractor 23. In such a case the naphtha is passed from vessel 13 by way of line 21, by-pass line 66 and line 27 to vessel Si. in this method of operation mercaptan extractor 23 is isolated by closing the valves in

lines

22, 24, 26, 28 and 29. Thus aqueous solution from vessel 13 is regenerated in vessel 31 and is Washed in vessel 41 and the lean solution is then passed to vessel 51.

By the process described above, it is apparent that it is possible to remove physically substantially all the mercaptans and all the HzS from a E28-containing, sour thermally cracked naphtha with maximum economy in the use of aqueous caustic solution.

Thus having described the invention, what is claimed is:

l. A combination process for sweetening a sour naphtha, which process comprises (l) contacting a feed naphtha which contains H25 mercaptans and phenolic compounds With a solution comprising essentially water and caustic under conditions to remove H28 and appreciable amounts of said mercaptans and said phenolic compounds, (2) separating HzS-free naphtha from an aqueous solution comprising caustic, mercaptides and phenolates, (3) contacting the naphtha from step (2) with an aqueous solution containing caustic and phenolates under conditions to produce a slightly sour naphtha, (4) separating a fat solution comprising caustic, mercaptides and phenolates from slightly sour naphtha, (5) combining the aqueous solutions of steps (2) and (4) and contacting the combined solution with free-oxygen under conditions to convert the .major portion of the mercaptides to disuldes, (6) removing disuliides from the regenerated solution from step (5) to produce a lean solution, (7) recycling a portion of said lean solution to the contacting zone of step (3), (8) contacting the slightly sour naphtha from step (4) with a portion of the lean solution in the presence of free-oxygen under conditions to convert essentially all of the mercaptans to disuldes, and (9) separating an essentially sweet naphtha from aqeous caustic phase.

2. The process of claim l wherein the aqueous caustic solution of step (1) contains between about 20 and 30 weight percent of alkali metal hydroxide and the volume ratio of said solution to feed is between about 1:5 and 1:1.

3. The process of claim 1 wherein (a) the aqueous solution of step (3) contains between about 5 and 15 weight percent of free-alkali metal hydroxide and between about 10 and 30 volume percent of phenolic compounds and (b) the volume ratio of solution to naphtha in step (3) is between about 1:10 and 1:1.

4. The process of claim 1 wherein step (6) comprises contacting the disulfide-caustic solution mixture with a liquid hydrocarbon solvent for disuliides in an amount suflicient to remove substantially all of said disuldes.

5. A sweetening process which comprises (A) contacting a sour, thermally-cracked petroleum naphtha containing HzS with a solution containing between about 20 and 30 weight percent of NaOH and the remainder essentially water, in a volume ratio of solution to naphtha between about 1:4 and 1:2, (B) separating a sour naphtha from an aqueous solution containing NaOH, mercaptides and phenolates, (C) contacting the naphtha from step (B) with a lean aqueous solution containing between about 5 and 15 weight percent of NaOH and between about 15 and 25 volume percent of phenolic compounds, derived from the thermally cracked naphtha, in a volume ratio of solution to naphtha between about 1:4 and 1:2 at a temperature between about and 110 F. for a time suicient to remove the major proportion of the inercaptans present in the naphtha, (D) separating a fat solution containing NaOH, mercaptides and phenoiates from a slightly sour naphtha, (E) combining the aqueous solution from step (B) with the fat solution of step (D), (F) regenerating said combined solution by treatment with free-oxygen at a temperature between about and F. in the presence of an added mercaptan-oxidation catalyst, thereby converting a major proportion of said mercaptides to disuldes, (G) contacting said regenerated solution with a petroleum distillate to produce a lean solution containing NaOH, phenolates and minor amounts of mercaptides and disuldes, wherein the volume ratio of distillate to regenerated solution is between about 1:4 and 1:1, (H) recycling a major portion of said lean solution to step (C), (l) contacting the sour naphtha from step (D) with a portion of Said lean solution in the presence of free-oxygen at a temperature between about 100 and 160 F. for a time suflicient to essentially sweeten said naphtha and (I separating an essentially sweet product naphtha from an aqueous caustic-phenolate phase.

References Cited in the le of this patent UNITED STATES PATENTS 2,556,438 Parlier s June 12, 1951 2,560,178 Krause et al. July l0, 1951 2,570,278 Ryder Oct. 9, 1951

Claims

1. A COMBINATION PROCESS FOR SWEETING A SOUR NAPHTHA, WHICH PROCESS COMPRISES (1) CONTACTING A FEED NAPHTHA WHICH CONTAINS H2S MERCAPTANS AND PHENOLIC COMPOUNDS WITH A SOLUTION COMPRISING ESSENTIALLY WATER AND CAUSTIC UNDER CONDITIONS TO REMOVE H2S AND APPRECIABLE AMOUNTS OF SAID MERCAPTANTS AND SAID PHENOLIC COMPOUNDS, (2) SEPARATING H2S-FREE NAPHTHA FROM AN AQUEOUS SOLUTION COMPRISING CAUSTIC, MERCAPTIDE AND PHENOLATES, (3) CONTACTING THE NAPHTHA FROM STEP (2) WITH AN AQUEOUS SOLUTION CONTAINING CAUSTIC AND PHENOLATES UNDER CONDITIONS TO PRODUCE A SLIGHTLY SOUR NAPHTHA, (4) SEPARATING A "FAT" SOLUTION COMPRISING CAUSTIC, MERCAPIDES AND PHENOLATES FROM SLIGHTLY SOUR NAPHTHA, (5) COMBINING THE AQUEOUS SOLUTIONS OF STEPS (2) AND (4) AND CONTACTING THE COMBINED SOLUTION WITH FREE-OXYGEN UNDER CONDITIONS TO COVERT THE MAJOR PORTION OF THE MERCEPTIDES TO DISULFIDES, (6) REMOVING DISULFIDES FROM THE REGENERATED SOLUTION FROM THE STEP (5) TO PRODUCE A "LEAN" SOLUTION, (7) RECYCLING A PORTION OF SAID "LEAN" SOLUTION TO A CONTACTING ZONE OF STEP (3), (8) CONTACTING THE SLIGHTLY SOUR NAPHTHA FROM STEP (4) WITH A PORTION OF THE "LEAN" SOLUTION IN THE PRESENCE OF FREE-OXYGEN UNDER CONDITIONS TO CONVERT ESSENTIALLY ALL OF THE MERCAPTANS TO DISULFIDES, AND (9) SEPARATING AN ESSENTIALLY SWEET NAPHTHA FROM AQUEOUS CAUSTIC PHASE.