US2317053A - Alkali treatment of hydrocarbon oils - Google Patents

Description

April 20, 1943. L. M. HENDERSON ALKALI TREATMENT F HYDROCARBQN GILS Filed Feb. 2. 1939 Nw QQQQ "b =\n.m..

INVENTOR BZciwrenceMHenderaon ATroRNEY Patented Apr. 20, 1943 UNITED STATES PATENT OFFICE ALKALI TREATMENT F HYDROCARBON OILS Lawrence M. Henderson, Winnetka, Ill., assigner to The Pure Oil Company, Chicago, Ill., a corporation of Ohio 9 Claims.

This invention relates to an improved method oi treating hydrocarbon oils and more particularly to a process of sweetening and desulphurizing petroleum distillates such as are obtained by the distillation and/ or cracking of petroleum oils. and to the subsequent recovery of the treating reagent.

More specifically this invention relates to a novel method of sweetening and desulphurizing gasoline and naphthas containing undesirable malodorous sulfur compounds such as mercaptans and to the recovery of the treating reagent wherein an untreated distillate is iirst contacted with aqueous alkali solution to remove hydrogen sulde and low boiling mercaptans, and then contacted with alkali methanol to remove higher boiling mercaptans as alkali mercaptides. The alkali methanol after separation ,from the distillate may be recovered by distillation. `The higher boiling mercaptans may be recovered by heating the residual mercaptides in the presence of water whereby the mercaptides hydrolyze to mercaptans and distill oi. The aqueous as Well as the alcoholic alkali solution may be recovered and regenerated if desired. vFor reasons of economy, sodium hydroxide is the preferred alkali, although other alkalies may be used.

An object of the invention is to provide a method for sweetening and lowering/the sulphur content of mineral oils, particularly petroleum distillates.

Another object of the invention is to provide a method for sweetening and lowering the sulfur content of hydrocarbon oils and distillates in such manner that valuable mercaptans may be recovered.

A further object of the invention is to provide a method of sweetening hydrocarbon oils and distillates with alkali methanol in such manner that the methanol and alkali can be readily and economically recovered for further use.

Still another object of the invention is to provide a method of sweetening hydrocarbon oils and distillates by means of alkali methanol in which loss of methanol is reduced to a minimum, thereby lowering the cost of the process.

Other objects of the invention will become apparent from the following detailed description and accompanying drawing of which the single ligure is a diagrammatic elevational view of apparatus suitable for carrying out the invention. l

The benets obtained by treating petroleum distillates with either aqueous or alcoholic alkali are well known in the art, but the recovery of methanol and regeneration of the alkali by methods heretofore available, has been considered too expensive to permit commercial success of the operation.

Untreated petroleum distillates usually contain, among other compounds, a group of compounds known as mercaptans. These compounds cause the distillates to have a badV odor, which is a considerable detriment in marketing the products containing them.

A very sensitive test has been developed which Will show the presence of extremely small amounts of these compounds. This is the well known doctor test (Federal Test Specification 520.31). Distillates which do not show a rer action in the doctor test are said to be negative to the test, or sweet.

There are a number of Well known methods of eliminating mercaptans as such from petroleum distillates. In the case of the lighter distillates such as gasoline, kerosine and various special naphthas, mercaptans are frequently eliminated by conversion to less oiensive smelling sulfur compounds commonly presumed to be disuliides. While these methods do not remove the mercaptan sulfur from the distillates, there is a considerable improvement in odor as a result of the treating process. Materials frequently used to achieve these results are sodium plumbite (doctor solution), and calcium or sodium hypochlorite. The use of sodium plumbite is objectionable in that in a commercial operation control of the addition of elementary sulfur necessary to complete the'sweetening reaction is difficult, and excessive addition of sulfur results in a material degradation of the product being treated by causing the loss of octane number, lead susceptibility and frequently difficulty with meeting the usual corrosion tests. The use of hypochlorite is open to the objection that some distillates, especially cracked distillates, treated by thisy method seem to bel much less color stable than the untreated distillates;

I have now discovered that by my method of treating petroleum distillates with .alkaliV methanol, I am not only able to accomplish results that ordinarily are not possible with most treat- 'ing reagents, but in' addition, I am able to recover or regenerate the used methanol solution in a much more satisfactory and economical method than has heretofore been possible.

My discovery is based on the fact that petroleum distillates containing mercaptan sulfur compounds may be treated withaqueous alkali solutions to substantially ycompletely remove those mercaptans whose boiling points are approximately the same or lower than the boiling point of commercial anhydrous methanol and when these petroleum distillates which have been treated with the aqueous alkali solution for the removal of the lower boiling mercaptans are treated with alkali methanol, the higher boiling mercaptans may be substantially completely removed and due to the difference in boiling points, the resulting alkali mercaptides may be readily separated by distillation from the methanol. Distillates thus treated will be found to have the mercaptan sulfur substantially removed, will be sweet to the doctor test, c0101` stable and not only will pass the usual corrosion tests, but in addition will pass much more severe corrosion tests such as are frequently used in the paint and varnish industry. An example of such a severe corrosion test is described in H. A. Gardners book entitled Physical and Chemical Examination of Paints, Varnishes, Lacquers and Colors, eighth edition, page 801. and is hereinafter referred to as the varnish thinner corrosion test.

An examination of the boiling points of mercaptans commonly found in gasolines, kerosine and various special naphthas indicates that normal propyl mercaptan and mercaptans containing fewer carbon atoms boil approximately at or below the boiling point of commercial anhydrous methanol, which is 148 F. This is clearly shown in Table I:

Borgstrom, Dietz and Reid have shown in theirVV i article in Industrial and Engineering Chemistry, volume 22, page 245, that lower boiling mercaptans up to and including propyl and isopropyl mercaptans are substantially completely removed by an aqueous caustic soda solution of 8.72% by weight (2.18 normal) sodium hydroxide. I have conrmed the results of these authors by treating a grade of petroleum distillate known as textile spirits to which had been added 0.022% sulfur as normal propyl mercaptan, with solutions of aqueous sodium hydroxide. In one such experiment when 350 cc. of such textile spirits were shaken in a separatory funnel under an atmosphere of nitrogen with 88 cc. of 10% aqueous caustic solution and the treated spirits analyzed for mercaptan sulfur content, the treated material contained only 0.002% mercaptan sulfur. It is well known that mercaptans of lower molecular weight and lower boiling pointsthan propyl mercaptan, are correspondingly more acidic and are even more effectively removed by an aqueous alkali treatment. It is therefore apparent that propyl mercaptan and lower molecular weight mercaptans are quite readily removed from light petroleum distillates by means of aqueous alkali. Thus, all of the mercaptans whose boiling points are approximately equal to or lower than. the boiling point of methanol, may be removed with aqueous alkali. vIt is unnecessary to carefully lregulate the quantityof aqueous.v alkali used so long as suicient is used to react with the lower boiling mercaptans. A large excess may be used without interfering with the efficacy of the process.

Mercaptans may be substantially completely removed from commercial gasoline and a doctor sweet product obtained by a combination aqueous alkali-alkali methanol treatment as is clearly shown by the data in Table II:

Table II Pesresnt Pesrcent Treat RSH in y as No gasoline lxeat Egel; Doctor before treat treat #l 0.011 l (i-stage, 25% by Vol. 0.002 Positivo.

aqueous caustic.

#2 0.002 1-stage, 5% by vol. 0.000 Negative.

1 caustic-abs. methanol subsequent t0 treat 1.

#3 0.048 l 6-stage, 25% by vol. 0.012 Positive.

aqueous caustic. i

#i 0. 012 l l-stage, 15% by vol. 0.001 Negative. caustic methauol. Subsequent to treat 3.

l 12');7 by weight NaOH.

The gasoline treated by aqueous alkali followed by alkali methanol and shown in Table II were not only doctor negative, but were of much improved odor, color stable, and satisfactorily passed the severe varnish thinner corrosion test previously-described. While the alkali-methanol -used may contain varying amounts of water and still be reasonably effective for the removal of mercaptans, it has been found that mercaptans may be more completely removed when the methanol is substantially anhydrous material. Substantially anhydrous methanol as herein used is defined as methanol containing not substantially more than approximately 1% water.

It will be seen, therefore, that by first removing those mercaptans which boil near or below the'bo'ilig point of methanol, by means of aqueous alkali, the higher boiling mercaptans can then be removed with alkali-methanol and since these mercaptans and their corresponding mercaptides boil above the boiling point of methanol, the methanol can readily be separated by distillation and reused for sweetening further quantities of sour distillates.

Substantially anhydrous alkali methanol (12% NaOH) is slightly soluble in the distillates treated, the solubility increasing as the methanol becomes more anhydrous. This is shown in Table III:

Table III Percent by vol.

methanol soluble in l gasoline Percent by vol. methanol @meeuwen gaseoso Methanol remaining in gasoline after treating with alkali methanol can be substantially com pletely recovered by washing seven volumes of the gasoline with one volume of water. The same water may be used many times, that isun til the concentration of methanol in the water is approximately 50% by volume, and still elect a substantially complete removal of methanol from the gasoline. The methanol in the aqueous solution of methanol which may or may not contain small quantities of alkali may be recovered by distillation.

When alkali methanol has been used to remove only the higher boiling mercaptans from petroleum distillates, the methanol may be readily separated by distillation from the alkali mercaptides dissolved in the spent treating reagent, as is shown by the `following example. Anhydrous caustic methanol which had been used to treat distillates previously subjected to a preliminary aqueous caustic wash, was distilled and analyzed for mercaptan sulfur content. The methanol thus recovered was rfound to contain only 0.7% of the total amount of mercaptan sulfur which it had removed from the distillate treated. The balance of the mercaptan sulfur was left in the residue as sodium `rnercaptide together with the sodium hydroxide.

.It will be apparent from the foregoing data that I have discovered a method of effectively removing mercaptan sulfur compounds from light petroleum distillates whereby the treated distillate is not only doctor sweet and of improved odor, but in addition suffers no loss of octane number, is of lower sulfur content, and passes very severe corrosion tests. At the same time, the preliminary aqueous alkali treatment which removes the low boiling mercaptans makes possible the subsequent separation of high boiling mercaptides from methanol in the spent alkali methanol solution by means of simple distillalation, thus affording an economical and convenient means of recovering the treating materials for further use.

In the drawing, the numeral represents, generally, a pipe for conveying petroleum distillate such as gasoline or naphtha from a source of supply to pump 3 which discharges through pipe 5 into the lower portion olf mixing tower 1. In this tower the untreated distillate is thoroughly mixed with fresh aqueous alkali solution, conveyed from a suitable source of supply through pipe 9, pump and pipe I3 into the upper portion of tower 1. Spent aqueous alkali solution separates from the commingled products and may be drawn oir from the bottom of tower 1 through pipe I5 into a suitable storage tank |1. This solution may be transferred from storage tank |1 by means of pipe I9, pump 2| and .pipe 23, to be reused or regenerated or discarded, whichever may be desired. Treated naphtha from which the hydrogen sulfide and lower boiling mercaptans have been removed by the aqueous alkali treatment, is withdrawn from the top of tower 1 through pipe 25 which is connected to the lower portion of tower 21. Substantially anhydrous alkali methanol is conveyed from a suitable stor age through pipe 29, pump 3| and pipe 33 into the upper portion of mixing tower 21 and is therein 'commingled with the naphtha from tower 1 which enters through pipe 25. Alkali methanol containing therein high boiling alkali mercaptides may be withdrawn from the bottom of tower 21 through pipe 35 into a suitable storage tank 31. Naphtha from which the mercaptans have vbeen substantially completely remo-ved is withdrawn from the top of tower 21 by means of pipe 33 and discharged into the lower portion of tower 4| where methanol which has dissolved in the naphtha, is removed by means of washing with a small quantity of water. Water is conducted from a suitable source of supply through pipe 1&3, pump 45 and pipe 41 into the upper portion of tower 4| wherein it is commingled with the naphtha containing dissolved therein a small quantity of methanol. The wash water, which now contains methanol, may be withdrawn from the bottom of tower 4| by means of pipe 49 and recirculated through pipe 5| by means of pump 52 back into tower 4| for washing further quantities of naplitha; or when the methanol content of the wash water has reached the allowable maximum, it is withdrawn through pipe 49 and passed directly to a suitable storage tank 53 for accumulation prior to recovery of the methanol by distillation. Finished neutral naphtha is withdrawn from the top of tower 4| by means of pipe 55 and passes to suitable storage not shown.

Spent alkali methanol may be withdrawn from storage tank 31 by means of valve 5B, pipe 51 and pump 59, together with a suitable amount of al` kali methanol wash water from tank 53, conveyed through valve 1U, pipe '1| to pump 53, and transferred to still El by means of ,pipe B and fractionating tower 63. In still El the mixture of Wash water containing methanol and spent alkali methanol is heated to the minimum temperature required for distillation of methanol, which may be fractionated in tower 53, removed through pipe 65, condensed in condenser A51 and conveyed to suitable storage through pipe 69. This methanol is then available for making up additional alkali methanol for use in the process.

The temperature at the top of tower E3 while suliciently high to permit the distillation of methanol is not high enough to permit water and mercaptans or mercaptides to pass overhead. Water and mercaptans may be taken oi as a side stream at a lower point in fractionating tower 63 such as through pipe 12, cooler 13, pipe 1'5 and conveyed to a suitable accumulator tank not shown where the mercaptans and water readily separate. The mercaptans may be recovered as such by simple decantation.

The still bottoms or residue which consist of highly concentrated aqueous alkali and containing not more than a very small quantity of mercaptides, may be withdrawn from still 6| through pipe 11, valve 18 and pipe 19 into drum drier 8| and recovered as solid substantially anhydrous regenerated alkali suitably withdrawn at 33. Water vapor may be removed through pipe 35. As an alternative method the concentrated aqueous alkali may be withdrawn from still 6| through pipe 1.1, valve 8E and pipe 81 to storage tank` 89 forsubsequent use in making up aqueous alkali solution for possible use in the preliminary Wash in the process or for other suitable uses.

Thus it will be seen that; I havedevised a method-for not only sweetening hydrocarbon oils but for desulfurizing such oils. Mercaptans are removed in such a manner that the methanol can be continuously recovered and reused in a continuous cycle. The higher boiling mercaptans can likewise be recovered with ease and sold as a Valuable product of commerce.

No expensive, highly specialized equipment is required for carrying out the process and while the process as herein described is well adapted for carrying out the objects of the present invention, 4,

various modications and changes may be made without departing from the substance of the invention. My invention includes all modifications falling within the scope of the appended claims.

I claim:

l. A process of treating untreated hydrocarbon k alkali methanol solution, separating said oil from said alkali methanol, recovering the methanol from the spent alkali methanol by fractional distillation thereof and recycling the methanol to the process. l

2. A process of rendering untreated hydrocarbon oil containing sulfur compounds boiling above and below the boiling point of methanol sweet and non-corrosive whereby said oil is commingled with suilicient aqueous alkali in a primary treating zone whereby removal of substantially all weakly acidic sulfur compounds the boiling points of which are not substantially above the boiling point of methanol is effected, separating said oil and said aqueous alkali solution, commingling said oil with suicient substantially anhydrous alkali methanol in a second ,treating zone to remove as mercaptides in the alkali methanol solution sulfur compounds the boiling points of which are higher than that of methanol, separating said oil from said alkali methanol, recovering methanol substantially free of sulfur compounds from said separated alkali methanol by distillation thereof and recycling the recovering methanol.

3. Process in acco-rdance with claim 2 where the alkali is sodium hydroxide.

4. Process in accordance with claim 2where the anhydrous alkali methanol contains not substantially over 1% of water.

5. The process of removing sulfur compounds from petroleum distillate containing mercaptans boiling above and below the boiling point of methanol which comprises treating said distillate with suicient aqueous alkali whereby sub..-

.stantially all hydrogen sulfide and mercaptans boiling below the boiling point of methanol are removed, separating the aqueous caustic alkali from the distillate, then treating the distillate with anhydrous caustic alkali methanol in quantitles sufficient` tolremove substantially the reanimes maining mercaptans present in the'distillateas mercaptides in the alkali methanol solution, sepa-` rating the alkali methanol from the distillate, recovering methanol substantially free of mercaptans from said separated alkali methanol by fractional distillation thereof and recycling the recovered methanol.

6. In a process of treating a substantially methanol-immiscible hydrocarbon oil containing sulfur compounds boiling above the boiling point of methanol that is substantially free of hydrogen sulfide and from which substantially all mercaptans having boiling points not substantially higher than the boiling point of methyl alcohol have been removed, the steps of commingling said hydrocarbon oil with substantially an` hydrous alkali methanol to remove as mercaptides in the alkali methanol solution sulfur compounds the boiling points of which are higher than that of methanol, said alkali methanol being present in a quantity in ,excess of that required for reaction with said sulfur compounds, separating said hydrocarbon oil from said alkali methanol, recovering methanol substantially free of mercaptans from said separated alkali methanol by fractional distillation thereof and recycling the recovered methanol.

7. A continuous process of treating untreated hydrocarbon oil containing mercaptans boiling above and below the boiling point of methanol which comprises contacting said oil with suf.- flcient aqueous alkali solution whereby removal of substantially all n-propyl mercaptan and mercaptans having lower boiling points than said n-propyl mercaptan is effected, separating said oil and said alkali solution, contacting the oil with sufficient substantially anhydrous alkali methanol to effect removal asA mercaptides in the alkali methanol solution of mercaptans the boiling points of which are higher than the boiling point of n-propyl mercaptan, separating said oil from said alkali methanol, recovering the methanol substantially free of mercaptans from the spent alkali methanol by fractional distillation thereof and recycling the recovered methanol.

8. Process in accordance with claim 7 in which the oil is repeatedly washed with Water following separation of the alcoholic alkali solution until the alcohol content of the water is approximately 50% and the alcohol is then separated from the Water by distillation and recycled.

9. In a process for removing mercaptans from low boiling hydrocarbon oil containing mercaptans boiling above and below the boilingl point of methanol and which has been preliminarily treated with aqueous alkali solution to remove hydrogen sulfide and lower boiling mercaptans contained in the oil, the steps which comprise contacting the oil with caustic alkali methanol solutionto remove substantially all the remaining mercaptans in said 4oil as mercaptides in the alkali methanol solution, then contacting the oil with water to Wash residual methanol from the oil, separating the Water and the alkalimethanol solution from the oil and subjecting the mixed water and alkali-methanol solutions to fractional distillation to recover therefrom substantially mercaptan-free methanol and substantially mercaptan-free alkali solution and recycling the methanol fortreatment of further quantities of mercaptan-containing hydrocar bon oil. LAWRENCEM. HENDERSON.'

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