US2457975A - Removing mercaptans - Google Patents

Description

Jan. 4, E949. J. A. BOLT REMOVING MERCAPTANS 2 Sheets-Sheet 1 Filed March 9, 1944- wm wwmwmmw NIN mmvmmws MNA@ Jan. 4, 1949.

J. A. BOLT REMOVING MERCKAPTANS 2 sheets-sheet 2 Filed March 9, v1944 Patented Jan. 4, 1949 REMOVING MERCAPTAN S l John A. Bolt, Chicago, I11.,'assigncr to` Standard Oil Company, Chicago, Ill., a corporation of Indiana Application March 9, 1944, .serial No. 525,663

7 Claims.

This invention relates to the process of removing mercaptans from sour petroleum distillates such as gasoline and naphthas. In particular, the invention relates to the treatment of petroleum distillates containing, besides mercaptans, certain phenolic constituents having a beneficial effect on maintaining the oxidation stability of the distillate and supplying solutizer needed for extraction of mercaptans by caustic solutions.

One of the objects of the invention is to eiect sweetening or removal of mercaptans by extraction with caustic alkali solutions while retaining in the treated distillate the desirable phenols. Another object of the invention is to provide a combination process for simultaneously removing mercaptans from two stocks, one of which is substantially free of phenols and incapable of supplying the phenolic solutizer desired in the process. Other objects of the invention will be apparent from the following description:

According to my invention, a stock rich in phenols is extracted with caustic alkali simultaneously with the extraction of a stock poor in phenols and the phenols are transferred from the phenols-rich stock to the phenols-poor stock by dissolving them in the caustic solution which is recycled in both sweetening operations. I have also provided a method for sweetening gasoline containing hydrogen sulfide in which the gasoline is separated into a heavy fraction and a light fraction from which the hydrogen sulfide is removed by stabilization, after which each fraction is processed separately for removal of mercaptans and finally recombined to reconstitute the desired sweet gasoline.

My invention is illustrated by drawings in which Figure 1 shows diagrammatically a simplified apparatus for carrying out the process and Figure 2 shows a modified flow diagram of a plant for treating two stocks. Referring to Figure 1, sour gasoline which may be ar cracked gasoline, a straight-run gasoline or a mixture thereof, usually containing hydrogen sulfide, is charged by line I to fractionating column II wherein the light naphtha fraction is separated by vapor line I2 condensed in condenser I3 leading to receiver I4, whence the light naphtha is withdrawn by line I5 and forced into stabilizing tower I6 wherein xed gases including propane and hydrogen sulfide are removed by line I1. The stabilized light naphtha is then conducted by lines I8 and I9 to mercaptan extractor 20.

In mercaptan extractor 2D, the naphtha ows upward countercurrent to a stream of caustic solution containing dissolved phenols which increase the solubility of mercaptans in the caustic. For this purpose a solution containing about 10 to 30 per cent of caustic soda or caustic potash may be employed having a phenols concentration of about 5 to 30 per cent, preferably about 15 to 20 per cent. The phenols extracted by the caustic from the distillates are mainly cresols and xylenols.- The caustic solution is introduced into extractor 2U by line 26 and withdrawn by line 21, while the extracted naphtha is discharged by line 28, whence it may be withdrawn from the process by line 29, or blended with heavy naphtha in line 30 as hereinafter described.

The heavy naphtha fraction from fractionating tower II is conducted by lines 3| and 32 to mercaptan extractor 33 wherein it is intimately contacted countercurrently with a caustic solution containing phenols similar to the caustic solution employed in extractor 20 hereinabove described. Extractors 20 and 23 may be provided with suitable baiiles or packing to give effective contact between the liquid phases therein. Obviously, a series of countercurrent stages may be used in lieu of tower extractors. Mercaptans are dissolved by -the caustic-phenols solution which is withdrawn at the bottom of the extractor by line 34, while the sweetened naphtha is withdrawn by line 35 and may be discharged from the system by line 36 or blended with the treated light naphtha frac-tion in line 30 hereinabove described.

The alkali solution withdrawn from extraction towers 20 and 33 is conducted through lines 21, 34, and 31to caustic regenerator 38, wherein mercaptans are removed and the caustic solution is restored to a condition suitable for the extraction of further` quantities of mercaptans from the naphtha streams in extractors 20 and 33. Regenerator 38 may be a baiiled or packed tower, an agitated Vessel, or a series of vessels arranged for countercurrent treatment of the caustic solution. Various methods of regeneration may be employed. One method is to distil off the mercaptans in a current `of steam introduced by line 39, the mercaptans escaping by line 4B. Another methodl is to oxidize the mercaptans at a lower tempera-ture by introducing air or oxygen through line 4I. On oxidation the mercaptans are converted to disuldes which are insoluble in the caustic and separate therefrom as an oil. When using this method of regeneration, it is preferred to separate the disulfldes from the caustic solution and prevent them from being returned to the naphtha streams in'which they are soluble. In this case it is desirable to warm the solution, e. g., to -150 F. and to employ an oxidation catalyst to accelerate the oxidation reaction. For this purpose polyphenols may be used, such as hydroquinone, pyrogallol, catechol, tannic acid, etc., the amount of catalyst being of the order of 0.1 to 2 per cent. Oxides of certain metals may also be used as promoters such as the oxides of lead, copper, nickel, cobalt and vanadium, these oxides being suspended or dissolved in the caustic solution.

After regeneration, the caustic solution is conducted by line 42 to storage 43, whence it is withdrawn las needed by pump 44, discharged through line 26 to the top of extractor 20, and by pump 45 discharged through line 4B to extractor 33. Fresh caustic solution may be added as make-up to the system from supply tank 41.

Where a gasoline stock charged at I contains an excessive amount of phenols, as in the case of certain cracked gasolines and heavy naphtha from special crudes, it is sometimes found that the phenols accumulate in the caustic solution to an excessive amount, interferingwith liquid separation and requiring that some of the solution be discarded from the system from time to time. In order to avoid this, it is desirable in these cases to pretreat the heavy naphtha fraction rich in phenols to remove part of the phenols therefrom before charging it tothe mercaptan extractor 33. The pretreatment is conveniently accomplished by conducting the heavy naphtha through valved line 3l to mixer 48, wherein it is contacted with an alkaline solution introduced by line 49. The alkali is settled in settler 50 and recycled by line 5I back to mixer 48. From time to time excess accumulation of alkali solution is withdrawn from the system by line 52. This solution may be substantially neutral, the alkaline constituents having been largely neutralized by phenols and/or organic acids such as naphthenic acids contained in the heavy naphtha stream charged by line 3l. The naphtha is withdrawn from settler 5D by line 53 leading to mixer 54, werein it i-s contacted with an alkaline stream introduced by line 55. For this purpose fresh -caustic soda introduced by line 58 may be employed or, if desired, a portion i of the caustic solution may be withdrawn from the mercaptan extraction system by pump 51. The mixture of caustic solution and naphtha from mixer 54 is settled in settler 58, the caustic being recycled to the mixer by line 59, while the naphtha is conducted to the mercaptan extractor by line 60.

If desired, other phenols-containing stocks may be treated in mercaptan extractor 33 simultaneously with the heavy naphtha fraction from frac# tionator I I or in lieu thereof. Cracked heavy naphtha from either the thermal or catalytic cracking process may be supplied for this purpose by line 6I and this stock may be charged directly to the extractor 33 by line A62 or vit may be given a preliminary alkali treatment to reduce the concentration of phenols and acids While charging it to mixer 48 by line 63. Likewise naphtha stocks poor in phenols may be charged to mercaptan extractor 28 from an independent source, indicated by line 64. Light virgin naphtha, casinghead gasoline, stabilized cracked naphtha, etc., may be treated in this way. Liquid hydrocarbons absorbed from refinery gases or natural gas, commonly known as absorption naphtha, comprised principally of C4 and C5 hydrocarbons, may be thus treated, They contain substantially no phenols and, after treatment with the phenol-caustic solution in my process, 'they absorb suicient phenols from the solution to exert a beneficial antioxidant eiect, thereby protecting the hydrocarbon distillate against gum formation resulting from atmospheric oxidation in storage and han* dling. Before charging to extractor 2U, it is usually desirable to wash with alkali to remove B2S and some mercaptans as shown in washer 22, caustic being supplied by line 23 and discarded by line 24, the stock being transferred by line 25 to tower 20.

Referring now to Figure 2, a light naphtha stock substantially free of phenols is charged by line I8 to Wash tower or caustic scrubber TI, the temperature being regulated by by-passing a portion of the stream through heat exchanger 72.

In scrubber 1I any hydrogen sulde present'isV absorbed by a suitable alkali solution and discarded by line 13, While the naphtha flows by line 14 to extraction tower l5. Here it is contacted countercu-rrently with a caustic solution containing a mercaptan solvent or 'solutizer as hereinabove described. The naphtha substantially free of mercaptans is discharged from the system by line 'I8 while the fat solution is withdrawn by line Tl, forced through heater 18 and coalescer 19, thence into regenerator 8U. Coalescer 'i9 is simply a chamber with suitable contacting surface, e. g., metal wool or other packing, to provide for more complete separation of naphtha and caustic.

A supply of air is injected into regenerator 88 from compressor 8I and line 82, the temperature in the regen-ator suitably being about to 150 F. as hereinbefore described in connection with Figure l. A regeneration temperature of 120 to 130 F. is usually satisfactory, The regeneration reaction may be accelerated by operating under pressure, for example, 1 to 10 atmospheres gauge. Mechanical agitation is provided in the regenerator to obtain rapid and intimate contact between air and caustic solution. The regeneration reaction is substantially as hereinbefore described, mercaptans being largely oxidized to disuldes which separate from the caustic solution and which are subsequently removed in regenerated solution accumulator 83, the caustic being transferred thereto by line 84. Regeneration catalyst, for example, tannin, is introduced by line as needed. Unused air is discarded by vent 36 While di-sulfides are withdrawn as an oil from settler 83 by line 81.

Regenerated caustic solution passes by line 88 to rcoalesc'er 89 where entrained disulfides are vremoved by line 98 and returned to the accumulator 83. Fresh caustic may be added as makeup to the system by line 9 I. The regenerated lean caustic solution now passes by line 92 through coo1er'93 and line 94 back to the extraction tower 'i5 for the treatment of further quantities ci naphtha. `If it is desired to discard a portion of the caustic solution from the system for any reason, it may be passed by line 95 to scrubber 'Il Vwhere the unused alkali content may be exhausted before discarding through line i3. Otherwise fresh caustic solution supplied by line 96 may be used for the operation of scrubber 1l.

Simultaneously with the `operation just described, the heavy naphtha feed stock containing phenols, particularly cresols and xylenols, is charged by line IUD through mixer I 0I and thence t'o settler 02. In mixerY IUI it is intimately contacted with a caustic alkali solution, e. g., caustic soda supplied by line ID3 and recycled `from the bottom of settler |02 by pump I04. The purpose of the pretreatment is, as hereinabove indicated, to remove from the heavy naphtha excess amounts of phenolic substances which would oth- Yerw'ise` accumulatein the subsequent extraction stage.' It is satisfactory to operate the caustic pretreater |02 in a semi-continuous fashion, withdrawing a part of the caustic solution at intervals through line |05 when the free alkali content becomes quite low and recharging make-up to the pretreater byline |06. Washed naphtha then passes by line'slill and |08 to extraction tower |09 or, if necessary, entrained caustic solution of regenerated caustic-phenols vsolution supplied by lines 92 and I2.` 'Ihe extracted naphtha iiowsfrom the top of tower |03 byline ||3 leading to settler ||4 and thence by line ||5 to product line IIB. If necessary to remove Ventrainedcaustic solution or solutizer solution, the stream may be diverted through coalescer as indicated in the drawing.

The spent caustic solution from the extractor |09 or so-called fat solution is withdrawn by line IIB through heater ||9 where it is heated to a suitable regeneration temperature and thence by line |20 to regenerator |2| If desired, the caustic solution may be diverted by line |22 through coalescer |23 to remove entrained naphtha. The operation of regenerator 12| is vsubstantially the same as that oflregenerator 80, a suitable regeneration catalyst being introduced as needed by line |20. Air is supplied to regenerator |2| from compressor |25 and line |26 leading to CO2 scrubber |27 where carbon dioxide is removed by a suitable alkali and thence by line |28 to the regenerator, The regenerated caustic solution is conducted by line |29 to solution accumulator and disulde separator |30, unused air bein-g vented by line |3| and disulfide oil being withdrawn by line |32. Regenerated caustic solution is conducted by line |33 to coalescer |34 where any entrained disuldes are separated and returned by line |35 to separator |30. The lean caustic solution passes by line |36, |31, |38 and |39 to extraction towers 15 and |09 for the treatment of further amounts of light naphtha and heavy naphtha stocks. If desired, the lean solution may be by-passed through cooler |40 to assist in maintaining the temperature of the extractors at about room temperature or below, e. g., from 30 to 80 F., the extraction efficiency being greater at the lower temperature. Fresh caustic and make-up solutizer, if needed, can be supplied to the system by line |4| from which they are introduced to the regenerator |2| or to the caustic accumulator |30.

In the operation of this system regenerated caustic solution from accumulator |30 may be continuously or intermittently charged to extraction tower 'l5 while the regenerated caustic solution from accumulator 83 may be similarly transferred to extraction tower |09. It is preferred to operate the syste-m by recycling the caustic solution between accumulator 83 and extractor l5 simultaneously while recycling caustic solution from accumulator |30 and extractor |09, and then to periodically transfer caustic solution between the light naphtha and the heavy naphtha systems. In this way each system may be accurately controlled with respect to the rate of feeding and amount of regeneration required. In extractor the light naphtha stock containing substantially no phenols absorbs a part of the phenols from the caustic solution charged by line |38,

' 6 providing'a treated light product containing suicient phenols to exert a beneficial oxidation stablllzing en 'ect thereon. In extractor |09 exactly the 'reverse action-occurs, the phenols contained in the stock charged accumulating in the caustic solution in an amount sufficient to make up for those phenols lost by absorption in extractor l5. The amount of prewashing in caustic pretreater "|02 is regulated either continuously or intermittently to maintain in the system the desired concentration of phenols for effective mercaptan 'ex- -traction,^e. g., about 5% to 30% by volume of the caustic solution employed.

The concentration of phenols employed in the caustic solution in extractor 15 may be the same as that employed in extractor |09 but because the mercaptans contained in the light naphtha are more readily extractable than those contained in vthe heavy naphtha, I may operate extractor l5 with a lower phenols concentration than extractor |09. For example, extractor 'l5 may suitably be operated with acaustic solution having a phenols concentration of about 5 to 12 per cent by volume whereas extractor |00 may suitably operate at a' phenols concentration of 8 to 25 per cent by Volume, the concentration being regulated by the yamount of phenols removed in pretreater |02.

The addition of solutizer by line |4| is usually practiced only in starting up the system or in case of accidental loss of phenols from the system. In addition to the transfer of phenols from the heavy stock to the caustic solution to enhance its effectiveness for extracting mercaptans from the light stock, there is also the advantage of accumulating light mercaptans, especially methyl and ethyl mercaptans from the light stock. These light mercaptans are more soluble in the caustic solution than the heavy mercaptans in the stock supplied to extractor |09 and are also less readily oxidized. Consequently, when the caustic solution is regenerated by oxidation, they remain in the caustic solution while the heavier mercaptans are converted to disuldes and eliminated. The light mercaptans remaining in the solution serve to protect the oxidation catalyst from over-oxidation.

The terms light naptha and heavy naptha used in the claims are employed in the sense usual in the petroleum refining industry to mean low boiling naptha and high boiling naptha, both napthas boiling largely within the gasoline boiling range.

Having thus described my invention what I claim is:

1. The process of removing mercaptans from a scure light naphtha substantially free of phenols which comprises continuously contacting said light naphtha with a caustic alkali solution containing phenols as a solutizer suicient to substantially increase the solubility of mercaptans therein, thereby extracting mercaptans from said naphtha and simultaneously decreasing the concentration of phenols in said caustic solution owing to the distribution of phenols between said caustic solution and said naphtha, separating said caustic solution from said naphtha, removing mercaptans from said caustic solution thereby regenerating it for further use, recycling said regenerated caustic solution to the naphtha extraction step and controlling the phenols concentration of said caustic solution by contacting a regulated portion of it with a stream of heavy naphtha containing a substantially higher concentration of phenols than said light naphtha Ygen in .the;presenc.e.of anloxdation .catalyst 4tirareby-convertingmeroaptans to insoluble disuldes, and said -disulfldes lare separated `from 1saidfna11s- .tic solution -before recycling ,to said `.extraction step.

74. The process aof claim ,1 wherein the `concentration of phenolsiin saidzcaustic solution ismaintained in ,the range of about -5 to v12% by'vol-ume.

.5. The process .of yremoving .mercaptans ifrom sour qheavy naphtha containing a .substantial amount .of phenols vcomprising :contacting said heav;7 naphtha with a caustic solution vicontaining phenols asasolutizer.ionmercaptans wherebyniercaptans andphenols are extracted bysaid caustic solution, separating -`said `caustic solution from fsaid heavy naphtha, removing lmercaptans from said caustic solution `thereby regenerating A itfor furtheruserecycling the yregenerated caustic solution `to the :extraction step of the process .and preventing .excessive accumulation of phen- .01s ,in said .caustic solution -by contacting sa controlled portion `thereof with a `,stream of A-sour ,Y .light .naphtha Ysubstantially free .whereby .excess ,-.phenols .are transferred to ,said light `naplcltha `as .a result Aoi. .the distribution .of .phenols .between said naphtha ,and said .caustic solution, .then returningsaidkportion 4to tnelieavy A naphtha `contacting .cycle lNumber 'L8 1.16.. The process of cla-1m 5 wherein fthe seid heavy maphtha vis .subjeeted fito a preliminary alkali l.extractiori treatment to ,remove Ipax-1; .of

@the phenolsrand assist controlling the Vplieliols eoncentreton ,of said .caustic solution.

.The process .of -claim `5 wherein the fooncen- 1tration of phenols lin :said caustic solution Ais .maintained .1i-n the range fof .about '8 to 25% ;b y

wolum-e.

JOHN ,A. BOLT.

REFERENCES CITED The following references lare .not record inthe -ill-e aof 1this patent:

TUNITED STATES .PA'I'ENIIS @THE-R REEERENCES yGrusc Vet al. iChemica'l Technology of Petro- 1eum,^'McGraw-Hill 'Book =Co. (1942) page 95.

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