US2413945A

US2413945A - Treating petroleum distillates

Info

Publication number: US2413945A
Application number: US525664A
Authority: US
Inventors: John A Bolt
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1944-03-09
Filing date: 1944-03-09
Publication date: 1947-01-07
Anticipated expiration: 1964-01-07

Description

Jm 7, 1947. .1.A. BOLT TING PETROLEUM D ISTILLATES Filed March 9. 1944 dcription.

'rac Per Jann A. it, chicago, m.,

'lx- DISLATE r f or Stand 0i! Company, Chicagorlll., a 'coration oro India V `Application This invention relates to a process o! removingY mercaptans from petroleum distillates and particularly from sour" gasoline and napthas.

More especially the invention relates to the ex-v traction of mercaptans from gasoline with caustic generally. known in the artes Vmercaptari solvents or solutizers." l

One object :ofthe invention is to provide a process bywbiciisasolineand napthas-.may be `readily treated by the caustic extraction process to produce a' product which is sweet or substantially sweetto the doctor test and of satisfactory marketable character. Another object ol' .the invencion is to reduce the number of extraction stages by a combination extraction-oxidation process. Still another object of the invention is to reduce the cost of regenerating the caustic solution used in the extraction process and avoid losses of regeneration catalyst. Other objects of 'captans in the caustic. These c'es are the invention will be apparent from the following l processes have been proposed for removins metcaptans from gasoline in order to improve I thc'odor and .the knocking characteristics. It has l been thepractice .to convert the mercaptans to unite .by treatment with alkaline lead oxide, lineorite, and numerous other has been found 4that the disuliides have an undesirable eiect on the knock rating of Ithe gasoline and they particularly reduce the lead susceptibility-by which is meant that the response to treatment with lead tetraethyl is lessened by A s, met. serial No, scacci- 6 cls. (ci. 19e-2s) g oxygen. This oxidation of mercaptans may be carried out either in ,the presence of the gasoline or in a separatestep. In the former case the vdisuliii'les which result dissolve in the gasoline while in the latter case theyl may be separated and discarded from the process and thus jprevented from contaminating the treated gasoline. The oxidation may be accelerated by various catalysts, particularly the polyphenols, tannic acid, hydroquinone, etc. Where an oxidation catalyst is used it is desirable to control the air blowing or oxidation of the caustic solution to avoid oxidizing the catalyst itself to a point where its effectiveness is destroyed.

In practice, it has been found necessary in the regeneration of the caustic mercaptan solution 'to halt the oxidation short of complete mercaptan removal in order to prevent destruction oi the regeneration catalyst. As a result appreciable amounts of mercaptans are left in the extracted gasoline owing to .the inability of the partially regenerated solution to remove all the mercaptans. Furthermore, in order to obtain an extracted gasoline of low mercaptan content, an extremely eillcient extraction process is required.

' According to my invention, I control the air regeneration of the caustic solution so that the regenerated caustic solution retains a small but oxidize the remaining mercaptans in the gasoline the presence of organic disuldes. Extraction of mercaptans with caustic solutions, generally caustic soda or caustic potash, has the advantage oi actuallyremoving the mercaptans from the gasoline instead of converting lthem to disulfides, and the extraction is considerably improved by the presence of phenols, organic acids, amines, alcohols, etc. Naphthenic acids, isobutyric acid, cresols and those phenols inherently present in gasoline. particularly cracked gasoline, are espe-` cially eective and the presence of thesesocalled solutizers is especially desirable in the extraction of the mercaptans of higher molecular weight.

When employing these caustic solutions it has been the practice to regenerate them either by steaming out the merca-ptans or by oxidizing the inercamans todisulildes by ineens of atmospheric at the point of air introduction in order to render the gasoline sweet or, substantially sweet .to the doctor test. In carrying out the process in the above manner, the heavier mercaptans which are the most difficult to extract and yet most readily oxidized are converted to disuldes in the presa ence of the regenerated solution without materially reducing its' mercaptan content. The caustic solution is then employed in the usual way to extract the gasoline after which it is recycled to the regeneration step of the process.

Referring to the drawing, gasoline, either straight-run or cracked gasoline or a naptha v fraction suitable for gasoline blending, is intro duced to the process by line it leading to inert captan extractor il wherein it iiows upwardly countercurren-t to a stream of caustic solution iri-J troduced by line I2. Extractor ii is provided 3 with suitable contacting elements, bale plates, ceramic packing, etc., or the extractor il may be substituted by a series of extraction stages arranged for countercurrent operation in the manner well known in the art. If .the ieed stock contains a considerable amount of phenolic material or hydrogen suliide, it is desirable to extract the hydrogen suliide and part of the phenolic material by a preliminary causticwashing treatment in pretreater I3 supplied by a caustic solution thru line I4 w'hich may be withdrawn at intervals or continuously by line I6. The amount of pretreating may be controlled by valves in lines I 8 and l1. If the feed stock has been properly stabilized, i. e. freed of propane, then it will be substantially free of hydrogens sulfide also and it .vill only be necessary to remove from it phenols and naphthenic acids which may be present insucient quantity to interfere with the subse-A quent operation of the caustic extractor where phenols and acids tend to accumulate by solution in the caustic.

The mercaptan extractor Il is suitably supplied with a caustic solution containing about l to 30% of the anhydrous caustic, generally caustic soda or caustic potash` along with 5 to 30% of cresols, xylenols or other phenols, isobutyric acid, naphthenic acids or other suitable solvent for mercaptans. In case of cracked stocks and gasoline from certain crudes, there is usually enough phenols present to supply solutizer needed in the caustic solution. These cresols, xylenols, etc., build up in the caustic owing to their weakly acidic nature. In addition to the solutizer, the caustic solution contains a catalyst for accelerating regeneration by oxidation. Thus for example, about 0.1 to 2% of a polyphenol such as catechol, hydroquinone or tannic acid (tannin) may be used. Tanning is particularly desirable because of its relatively low cost.

As the gasoline flows upwardly thru extractor i l the mercaptans are selectively dissolved in the caustic solution. The temperature is commonly maintained at ordinary temperature although temperatures of 40 to .100 F. may be employed satisfactorily, Lower temperatures are favorable to extraction of mercaptans.

The caustic solution with dissolved mercaptans is withdrawn by line I8 from the base of the extractor and flows thru heater I9 to caustic regenerator 20 which is suitably a packed or baiiled tower. Alternatively an agitated pressure vessel suitable for pressures of 100 to 500 pounds per square inch may be used. Sufcient heat is introduced at i9 to provide a temperature in the regenerator of about 100 to 150 F., e. g. 130 F.,

which would.. destroyV the regeneration catalyst, ionfexam'ple termin.` The reaction in regenera- 20 consists-essentiallyin converting mercap- 'tansto organic disulndes which are oily substances insoluble in the caustic solution. The

v'-iolliiwir'ig equation' in which R is a hydrocarbon radical represents the reaction:

ZRSH O2=RSSR H Q +y2 Disulde+ a From the regenerator 20 the causticl solution ilows into separator 24 from which the disuldes are withdrawn continuously or periodically by line 25. The partially regenerated caustic salu-- tion should contain about to 400 mgs. per 100 m1. of residual mercaptan sulfur. If the mercaptan sulfur is reduced to an amount appreciably lower than this, a substantial loss in regeneration catalyst will result. If the mercaptan sulfur ofl the partially regenerated caustic is allowed to appreciably exceed 400 ings. per 100 ml., the subsequent sweetening operation will not be satisfactory and an excessive amount of disulfide may be introduced into the gasoline product. It is preferred to'control the regeneration in 20 toproduce a vpartially regenerated caustic solution having a residual mercaptan sulfur content of 200 to 300 mgs. per 100 m1.

From vseparator '24 the regenerated caustic solution goes by line l2 and cooler I2a back to the top of the mercaptan extractor Il. In the upper section of the mercaptan extractor the caustic solution cornes in contact with the gasoline from which most of the mercaptans have been extracted. A stream of air is introduced by line 26 in the upper section of the mei-captan extractor just sufficient to convert the mercaptans -remaining in the gasoline to disulildes without appreciably lowering the mercaptan content of the caustic. When operating with a multistage extraction unit the required amount of air may be introduced ahead of the last extraction stage. It is desirable to reduce the mercaptan sulfur content of the gasoline' in extractor Ii to less than 10 mgs. per 100 ml. and preferably to less than 5 rngs. per 100 ml. when the gasoline reaches the point where air is introduced into the' extraction system. On conversion of these residual mercaptans to disuldes by air oxidation in contact with the caustic solution, a sweet or substantially sweet gasoline ls readily obtained.

The presence of the gasoline serves to speedV the reaction between air and mercaptans dissolved in the caustic-sclutizer solution, possibly shifting the equilibrium in the direction of disulfide formation and also perhaps by its action in supplying dissolved oxygen to the caustic solution. There are indications that the oxidation reaction may proceed largely in the oil phase. It the caustic solution supplied to the extractor by line l2 has been heated it is usually desirable to cool it before passing it to the lower section of the extractor in order to obtain a more favorable equilibrium for mercaptan extraction. This may be accomplished by trapping out thev caustic solution on trapout plate v2l, passing it by line 2B to cooler 29 andthence back to the extractor by line 30.

Gasoline is withdrawn from the top of extractor Il by line 3i leading to water wash tower 32 where any entrained caustic 'solution is removed by a water spray introduced by line 33, the waste Water being withdrawn by line 3l. Any excess of caustic solution entrained in the treated gasoline may be coalesced by passing it thru metal Wool or other porous material, then collected and returned to the system, if desired,

before the final washing step. The'nished gaso-- line is withdrawn to storage by line 3l, and undissolved air, principally nitrogen, which wasY introduced by line 28 is vented from the wash tower by line 36.

Fresh caustic for the process may be supplied from tank 31 and line 38. Additional mgeneration catalyst, for example a solution oi' tannin.

- removed readily by the air oxidation in extractor Il; thus they serve to protect the regeneration catalyst without interfering with the removal of mercaptans from the gasoline. Where the gasoline stock treated is decient in mercaptans of low molecular Weight, I may add them in small ethyl, or, less desirably, propyl mercaptan to the caustic solution iiowing to the regenerator or,l

preferably by line 4I and line i2 leading to the in two stages, adding the light mercaptans to the second stage and oxidizing -therein to convert substantially all the heavy mercaptans todisulfides which are removed before recycling tothe gasoline treater. Where stage regeneration is practiced, the light mercaptans can be added to the last stage. Also, the stream of partially regenerated caustic solution can be spilt and a portion only sent to the sweetening zone with air blowing, the other portion being added to a lower tatedof the fractionator Il b'y line 42 shown o e is Having thus described vmy process what I claim 4. The process of removing mercaptans from sour gasoline to provide a gasoline substantially sweet to the doctor test which comprises extracting a major proportion of the mercaptans from said gasoline in an extraction stage, oxidizing remaining mercaptans to disulfides in an oxidation stage in the presence of a phenolic mercaptan oxidation catalyst, conducting a strong caustic alkali solution containing a solutizer thru said oxidizing stage and then thru sai(J extraction stage in series, countercurrent to the flow of gasoline, separating said caustic solution from gasoline in said extraction stage, regenerating said caustic solution by oxidizing a major proportion of dissolved 'mercaptans to disulfides infsoluble in said caustic solution, separating disul- -treaten Alternatively, I may regenerate caustic 1. In the process of sweetening a sour petrol leum distillate wherein mercaptans are extracted from said distillate by contacting in an extraction zone with a caustic alkali solution containing a solutizer and a regeneration catalyst comprising a phenolic hydroxy compound, separating caustic solution from distillate and partially regenerating the caustic solution by oxidation with an oxygencontaining gas, limiting the oxidation oi said caustic solution to prevent destruction of l said amount of residual mercaptans to remain unoxif disulfides from partially regenerated caustic solution, and recycling the partially regenerated caustic solution from the regeneration zone to said extraction zone, the improvement comprising intimately contacting 'said partially regenerated caustic solution in a sweetening zone with distillate from said extraction zone, injecting air into said sweetening zone in an amount sufiicient to complete the oxidation of mercaptans in the presence of said distillate and produce a substantially sweet distillate product, then conducting the caustic solution from said sweetening zone to said extraction zone.

2. The process of claim 1 wherein said organic regeneration catalyst is a polyphenol.

3. The process o1' claim 1 wherein said caustic regenerator is operated at a temperature oi' about 100 to 150 F. and the caustic solution is cooled before returning to said extraction zone.

` fldes from'said caustic solution, returning said amount for the purpose just described. Thus I l may add, continuously or intermittently, methyl,

solution to said oxidation stage and supplying an oxygen-containing gas to said oxidation stageto complete the conversion of residual mercaptans contained in said caustic solution and lsaid gasoline.

5. The' process oi" claim 4 wherein said solutizer is a mixture of phenolic substances present in 25' smallv amounts in the gasoline being treated and `which accumulate in the caustic solution recycled in the process.

6. In the process of removing mercaptans from a sour petroleum distillate wherein said distillate is extracted countercurrently in an extraction stage with a caustic alkali solution containing about 5 to 30% of a solutizer having the effect of increasing the solubility of mercaptans in said caustic solution sufiiciently to extract substantially all of the light mercaptans and a major part of the heavy mercaptans contained in said sour distillate leaving less than about 10 mg. mercaptan sulfur per 100 ml. of distillate, wherein the said caustic alkali solution containing dissolved mercaptans is separated from the gasoline and regenerated by oxidation with an oxygen containing gas at about 100 to 150 F. in the presence oi' about 0.1 to 2% of a regeneration catalyst .comprising a polyphenol soluble in said caustic solution, substantially all the heavy mercaptans being converted to organic disuliides by the re- A generation reaction, leaving about 200 to 300 mg.

regeneration catalyst by allowing a substantial j J of ylight mercaptan sulfur per ml. unoxidized in said caustic solution to protect said regenera tion catalyst from oxidation, and wherein said organic `disulfides are separated from said regenlerated caustic solution and the said solution is recycled for treating additional distillate, the improvement comprising intimately contacting with an oxygen-containing gas in an oxidation stage, said regenerated caustic solution and extracted distillate from said extraction stage thereby selectively converting the residual heavy mercaptans in said extracted distillate to disulfides substantially without oxidizing said light mercap tans remaining in said regenerated caustic solution, and thereafter conducting the caustic solution from said oxidation stage to said extraction stage, where it is employed for the extraction of further quantities of sour petroleum distillate.

JOHN A. BOLT.