US2952627A - Desulfurization of petroleum hydrocarbons with aqueous caustic soda and formaldehyde - Google Patents

Description

C. W. RIPPIE OF PETROLEUM HYDROCARBONS WITH AQUEOUS CAUSTICSODA AND FORMALDEHYDE Filed March 18, 1958 Sept. 13, 1960 DESULFURIZATION am N NSDWMQ United States Patent yOffice 2,952,627 Patented Sept. 13, 1060 DESULFURIZATION OF .PETROLEUM HYDRO- CARBONS WITH AQUEOUS CAUSTIC SODA AND FORMALDEHYDE Charles W. Rippie, Ashland, Ky.,

Oil & Refining Company, of Kentucky Filed Mar. 18, 1958, Ser. No. 722,212

6 Claims. (Cl. 208-231) This invention relates to the purication of vpetroleum hydrocarbons and is directed particularly to the desulfurization thereof.

A principal objective of this invention has been to provide a process by means of which sulfur compounds present in liquid hydrocarbons such as gasoline may be removed in an easy and inexpensive way.

A further objective of the invention has been to provide a process for limproving the lead susceptibility and stability of hydrocarbon motor fuels such that a given octane number may be -achieved by the incorporation of a relatively smaller amount of tetraethyl lead and such that the amount of inhibitors conventionally added may be reduced.

The adverse affect of small quantities of sulfur compounds contained in a hydrocarbon motor fuel upon the lead suscept-ibility 4thereof is Well-understood, and various processes are now used to reduce such sulfur content to a minimum consistent with the cost of treatment and the savings in lead cost to be obtained thereby. However, it is generally impractical or impossible for the reflner to endeavor to obtain complete desulfurization because the processing cost would exceed the savings that would be derived from the improvement in lead susceptibility. While the sulfur content of liquid hydrocarbon motor fuels varies according to the method by which they are obtained, it is not unusual for catalytically cracked gasoline or motor fuel fractions to contain from 0.02 to 0.40% sulfur depending upon feed stock, catalyst, severity of processing, and other -conditions and for straight-run gasolines to contain still more.

It has long been known that treatment of a liquid petroleum hydrocarbon containing organic sulfur with caustic soda in aqueous solution is capable of converting some of the sulfur compounds into substances which are more soluble in the caustic solution than in the hydrocarbon and which therefore are removed from the hydrocarbon upon separation of the vaqueous alkali therefrom. The reaction and dissolution are rapid, and the cost of treatment is reasonable inasmuch as the caustic soda solution, after use, may be regenerated yfor reuse. However, caustic soda treatment is effective in removing only those sulfur compounds which are of the RSH type, e.g., mercaptans. Organic suliides of the R28 type remain substantially unaffected, yet they constitute `a substantial percentage of the total sulfur content and impair the lead susceptibility of the fuel in which they are present.

An objective of the present invention has been to provide a desulfurizing process of the caustic soda type but modified to effect removal of organic sulides of the R28 type simultaneously with removal of sulfur compounds of the RSH type contained in the pertoleum hydrocarbon. More specifically, an objective of the invention has been to provide a desulfurizing process of the caustic soda type wherein the total sulfur content may be reduced to a greater degree than with the use of caustic soda alone.

Briefly, this invention is directed to a process of desulfurization wherein liquid petroleum hydrocarbon Yconassignor to Ashland Ashland, Ky., a corporation taining sulfur compounds, but which is free or substantially free of hydrogen sulfide, is contacted conjointly with a strong aqueous solution of caustic soda and a small `amount of formaldehyde, whereby the total sulfur is reduced materially and the lead susceptibility of the product .is substantially improved in comparison with the lead susceptibility of the same hydrocarbon feed stock treated with caustic soda only.

' Although the use of formaldehyde previously has been .proposed as a desulfurizing agent in processes wherein strong acid, high temperature, or molten sodium has been employed, it Ihas now been found that a substantial reduction of total sulfur readily can be achieved in a much simpler Way and Vat low or ordinary processing temperature by contacting the organic sulfur-beating hydrocarbon simultaneously with caustic soda and a small quantityrof formaldehyde, for instance l part formaldehyde to 15,000 .to 40,000 parts by volume of hydrocarbon being treated. In fact, an important feature of the present invention resides in the use of an amount of formaldehyde which, even Ywhen in commercial aqueous solution is so small that it does not dilute the caustic soda solution `by a significant amount or adversely affect the regenerability thereof.

Peculiarly, treatment of hydrocarbon feed stock with caustic soda and formaldehyde simultaneously or conjointly provides a greater reduction of total sulfur content than that obtained by contacting the feed stock successively with caustic soda only, then formaldehyde without caustic soda, `or vice versa. The chemical mechanism by which such results are obtained is not fully understood; it is possibly that the caustic soda, While converting mercaptans into mercaptides in the usual manner, promotes reaction of the formaldehyde with compounds of the R28 type with which formaldehyde alone does not possess the `ability to react. In lany event, the addition of an -apparently negligible quantity of formaldehyde in the presence of caustic soda has been found consistently to decrease the total sulfur content by an amount which is 20u-40% greater than is furnished by caustic soda alone, and permits 0.5 to 0.6 cc. per gallon less tetraethyl lead to be added to the motor fuel than is otherwiserequired to produce a given octane number in the `same stock desulfurized by caustic soda only.

Formaldehyde and caustic soda in aqueous admixture react with one `another at ordinary temperatures to produce nsodium formate and alcohol, neither of which possesses desulfurizing properties. Therefore, in the practice Yof the present invention it lis desirable to introduce the formaldehyde linto contact With the `caustic: soda and hydrocarbon in such manner that desulfurization of the hydrocarbon -by the conjoint .action of the caustic soda and formaldehyde is accomplished before Ithe formaldehyde is destroyed ythrough reaction with the caustic soda. This may be accomplished conveniently by introducing the formaldehyde :into the caustic soda solution at a point just ahead of that :at ywhich contact with the hydrocarbon is established, or by introducing the formaldehyde -at the lsite wherein the caustic soda is brought into contact 'with the hydrocarbon. In either manner the yformaldehyde is caused to react with organic sulfur components before undesirable pre-desulfurizing reaction occurs :between the formaldehyde and the caustic soda. In general, however, the process of this invention, except as yto `addition of formaldehyde, may be the `same as a conventional caustic soda desulfurization process as practiced in ,the various ways known :to the The invention is further predicated upon the discovery and determination that a spent caustic soda `solution in which formaldehyde has Ibeen incorporated to promote greater desulfurization of hydrocarbon in the treatment introduced into the system,

is more easily regenerated than when no formaldehyde has been incorporated. For example, in a conventional caustic soda regeneration process wherein the suliide content of the spent solution is converted Iinto removable disuldes by blowing with air,.the spent solution must be held at appreciable .temperature and must be blown for an appreciable period of time in order to provide a suitable degree of regeneration. In contrast with these requirements, the sultdes contained in spent caustic s-oda as produced in the desulfurization process of the present invention are converted into removable sulfur compounds 4in a rapid andeflicient manner Without increase in temperature, for` example, merely by incorporating air into the -spent caustic solutionY through use of an eductor or the like. The spent caustic solution produced in Va desulfurizing process conducted according to the present invention readily may be regenerated by other known regeneration procedures.

One typical process of desulfurization followed by vregeneration `of the spent caustic solution is illustrated in the accompanymg drawing. Sulfur-containing gasoi Yline (or other hydrocarbon to be desulfurized) but from 4 uct, passes through line 8 to storage.V Desulfurization of the gasoline occurs in the period of transit of the stream to the knock-out drum and during'the time of residence therein while separation of the caustic soda solution from the gasolineoccrs. Spent vaqueous caustic soda is removed from the bottom .layer in the knockout drum through line 9 which connects to receiver 10.

For regeneration of the spent caustic solution, a pump 12 having its intake line connected to tank 10, discharges through line 13 to a tank 14. However, to facilitate introduction of air into the stream, the pump 12 operates in conjunction with a recirculation Ysystem 15 to which air is introduced in quantity suicient to cause conversion of organic suliides in the spent caustic soda solution into disulfdes which may be removed by washing as later described. As shown, Kthe recirculation system comprises a line 16 connected to the pump discharge line 13 and .which hydrogen sulfide has been removed by any suitable means enters the system through =line 1 wherein it mixes with caustic soda solution from the line 2. Aqueous Vformaldehyde Ysolution is admitted to the stream .through line 3. The caustic soda solution provided in line 2 may be fresh aqueous caustic soda solution or may be a solu- 'tion which has been regenerated in various ways, one of which subsequently described.

' In typical caustic sodav desulfurization, the aqueous caustic. solution used may contain approximately 10% by weight of caustic soda (or caustic potash) in water, and the ow rate is adjusted to provide approximately one partby` volume yof caustic soda solution to five parts by volume of gasoline, but it is runderstood that substantial variations in the causticgasoline ratio may be made to suit the requirements of `given feed stocks. In the preferred practice of the invention, the formaldehyde employed is a commercial 37% aqueous formaldehyde solution known in the trade as formalin. .This solution may be metered into the caustic soda stream solution of ,line 2 at a rate of one part by volume of formalin to about 15,000-18,000 parts by volume of sour `gasoline charged into the system through line 1. However, in place of using Iformaldehyde as a 37% aqueous solution, which was chosen because of its commercial availability and cheapness, other concentrations may be used. VIn general, no economic benefit has been found in using formaldehyde in greater proportion to the gasoline or caustic soda than that indicated. In the system shown, the formaldehyde flow rate :is controlled by means of arproportioner 4 or suitable pump andY metering valve. As subsequently explained, the pressure in iline 2 may be approximately 200' p.s.i.

The stream containing sour gasoline, caustic soda solution, and formaldehyde' flowing in line 5 passes through a diiferential pressure control valve indicated generally at 6, beyond which the stream passes -into a knock-out or settling drum 7. A-t the pressure control valve 6 the pressure on the stream is reduced to approximately psi., and the valve functions as a mix'uing valve bringing the components of the stream into intimate contact with one another. It will be seen that the point at which formaldehyde is introduced into the system Ais relatively closelyY `adjacent the point lat which sour lgasoline is whereby the formaldehyde and caustic soda contact the Vgasoline before any signicant reaction occurs Vbetween thecaustic soda and the formaldehyde. If desired, however, the lformaldehyde may be introduced `directly into line 11 or all three components may be commiugled at a single point eig. the mixing valve 6. Y

At the knock-out dru-m 7, aqueous caustic soda settles from the gasoline which, as a now desulfurized proda line 17 which is connected to the line 16 and which leads to the intake side of pump 12. Line 16 is valved as at 18 to cause a predetermined portion of the pump eiuent to recirculate through the line 17, and air may be introduced into the recirculating stream by means of a conventional eductor 19 before the stream is returned to the system. In place of the eduetor, air may be introduced into the caustic soda stream in any other suitable manner. 'Ihe significant point is that conversion of suliides to disulfides in the spent caustic soda solution Vis very rapid. Some of the caustic soda solution from pump 12 bypasses the eductor system, but the air absorbed into the recirculated stream is adequate to effect conversion to disulfides of the entire spent caustic soda solution through the commingling which occurs in the pipes, pump and tank 14.

A pump 20 delivers the air-treated caustic soda solution from tank 14 through a line 21 to a disulfide scrubber 22 wherein the solution is washed with a solvent, such as the conventionally used naphtha, capable of dissolving the disuldes therefrom. Y

In the scrubber system shown, a column of naphtha is maintained within tank 22 above a lower layer of caustic soda solution. To facilitate washing, the naphtha phase is recirculated into contact with the caustic soda stream owing through pipe 21; for this purpose the re- Ycirculating system includes an outlet line 23 which is located above the caustic-naphtha interface and which leads to a pump 24. The pump discharges'through a line 2S,

into line 21, and both streams pass through a mixer 26, Y

The naphtha phase containing disuliides may also be treated for reuse. In the system disclosed, naphtha is withdrawn from the upper part of the disulfide scrubber 22 through a line 30 which leads to a knock-out drum 31, wherein entrained caustic soda solution is allowed to settle from the naphtha as a lower layer which may be withdrawn as necessary." The solvent is discharged from the knock-out drum through aline 32 leading to a ash tower 33 which may be suitably packed. VTo facilitate vaporization of naphtha, the lower portion of the ash tower 33 may be maintained at a temperature of approximately 275 F. by recirculation of naphtha through a steam reboiler34 which is supplied from line 35 and which returns heated Yliquid to thentiash tower through line 36. By vaporization, the upper portion of the flash tower is kmaintained at a suitably lower temperature, for example,

200 F. whereby desulfurized naphtha vapors are generated which leave the tower through line 37 and pass to a condenser 38.

Condensed naphtha is collected in an accumulator tank 39 from which a line 40 returns the desulfurized naphtha solution rto the disulfide scrubber. In the system shown, the accumulator tank is provided with a liquid level controller indicated generally at 41 which controls the motor of a pump 42 installed in line 40 whereby the pump delivers naphtha from the accumulator to the disulfide scrubber in accordance with the rate at which it collects in the accumulator. Fresh naphtha may be charged into the system through the line 43.

Itis also to be noted that the system for feeding naphtha to the reboiler 34 includes an outlet 44 which is valved as at 45. Since disulides collect at the bottom of the flash tower, the higher heat of vaporization is employed as a means of maintaining the desired temperature diiferential in the ash tower. By the manipulation of valve 45, disuliides at the bottom of the ash tower readily may be discharged from the system.

In the practice of the process of this invention desulfurization of the hydrocarbon proceeds rapidly at or about room temperature, for example from 70 to 100 F. While the rate of oxidation of sulfur compounds contained in the spent caustic may be increased by increasing the solution temperature during regeneration, that procedure is not required when the spent caustic solution contains spent formaldehyde since the oxidation rate is sufficiently rapid for practical purposes without temperature increase. Also, while the system has been shown for use in desulfurizing straight-run or cracked gasolines,

f it is to be understood that the latter term is intended to include motor fuels produced by alkylation, reforming, isomerization, or other methods. It is also to be understood that the desulfurizing advantages afforded by the use of formaldehyde with caustic soda may be utilized in nonregenerative methods of caustic treatment where the organic sulfur content is suciently low to permit wasting of the spent treating solution. Similarly, the invention is adapted to be used for removing sulfur from hydrocarbons heavier than fractions burning in the gasoline range, such as fuel oils, in which organic suliides seriously interfere with utilization.

In typical operation of the process for desulfurizin-g gasoline, the average research octane number of the treated i.e. desulfurized gasoline with 3 cc. of tetraethyl lead added per gallon and `adjusted to 8# RVP, was 84.35 when caustic soda only was used for desulfurization, while an octane number of 85.80 (based on the same TEL addition) was obtained when caustic soda and formaldehyde were used for desulfurization of the same hydrocarbonan increase of 1.45 octane. Thus, the conjoint use of formaldehyde and caustic permits a saving of 0.6-0.7 cc. or more of tetraethyl |lead per gallon of gasoline. In terms of cost at the current price of tetraethyl lead, the amount thereof required to produce the given octane number was $56.55 per thousand barrels, while the cost of formaldehyde required for the treatment of 1000 barrels was $2.75. The cost of regeneration is the same in either case.

The following table shows the comparison of total sulfur content of gasolnes desulfurized with caustic soda and with caustic soda and formaldehyde in accordance with the present invention:

6 Extensive tests have shown further that on an average treatment with caustic soda and formaldehyde reduces RSH sulfur by 35.5%, RZS surfur by 23.8%, and total sulfur by 23.4%, and less inhibitor, therefore, is required to meet specifications for gums of the treated gasoline.

Tests Vof gasoline desulfurized according to the invention also show that stability, as measured on copper and glass dishes, is improved, and less inhibitor therefore is required to meet specifications for gums of the treated gasoline.

In place of using naptha to wash disuliides from the caustic soda solution, other suitable solvents may be used as readily will be understood by those skilled in the art. Similarly, as previously explained, it is to be understood that the recirculation system herein disclosed is merely typical and that other systems for recirculating spent caustic solution readily may be used. Thus, the introduction of formaldehyde into the caustic soda treating solution not only increases the degree of desulfurization over that which could be obtained by the use of caustic soda ondy, but renders the spent caustic soda solution more amenable to regeneration.

Having described my invention, I claim:

1. The process of desulfurizing liquid petroleum hydrocarbon containing alkyl sulde type sulfur compounds which method comprises, contacting the petroleum hydrocarbon with aqueous caustic soda solution and introducing into contact with both components, while excludingoxygen, a quantity of formaldehyde in the proportion of 1 part thereof to each l5,000-l8,000 parts of said hydrocarbon thereby to effect conjoint reaction of the caustic soda and formaldehyde with alkyl sulfide type sulfur compounds present in the hydrocarbon, then separating from the petroleum hydrocarbon the said aqueous caustic soda solution and sulfur compounds dissolved therein derived from said alkyl suliides, and regenerating the separated caustic soda solution for reuse by contacting the same with air.

2. A method of treating a petroleum hydrocarbon to reduce the amount of alkyl sulfide type sulfur compound impurities therein which method comprises, bringing streams of aqueous caustic soda solution and the said petroleum hydrocarbon into liquid contact with one another and, which excluding oxygen, introducing formaldehyde into the stream of Iaqueous caustic soda solution at fa point just ahead of the point of liquid contact thereof with the hydrocarbon stream in the amount of approximately one part of formaldehyde to 15,000-18,000 parts by volume of the said hydrocarbon whereby the formaldehyde reacts with sulfur compounds in the said hydrocarbon before substantial reaction thereof with the caustic soda occurs.

3. A method of treating a petroleum hydrocarbon containing RZS and RSH type sulfur compounds to reduce the amounts thereof which method comprises, continuously, while excluding oxygen, bringing streams of aquecus caustic soda solution and the said petroleum hydrocarbon into liquid contact with one another and with a small quantity of formaldehyde in the amount of vapproximately one part of formaldehyde to 15,00040,000 parts by volume of petroleum hydrocarbon whereby the formaldehyde and caustic soda yact conjointly to remove RgS and RSH type sulfur compounds from the hydrocarbon.

4. The method of claim 3 wherein the treatment is conducted at approximately room temperature.

5. The method of claim 3 wherein the treatment is conducted at approximately room temperature and wherein spent caustic soda solution produced in the treat'- nient is subsequently regenerated for reuse by introduction of air therein.

6. The method of removing objectionable alkyl sulfide compounds from hydrocarbon motor fuel which method in whichY reagentthe formaldehyde constitutes approxmately 1 pan by weight per 15,000 to 40,000 parts of 5 UNITED STATES PATENTS hydrocarbon being treated,V then regenerating the 'caustic 2,616,832 Browder et ral Nov. 4, 1952 comprises, treating said motor fuel, While excludingV soda solution for reuseV by introducing air therein, then" oxygen, with adesulfurizing reagent consisting of aqueous separating the reagent from the hydrocarbon motor 11501., caustic soda solution and aqueous formaldehyde solution References Cited in the me of thigpatet

Claims (1)

1. THE PROCESS OF DESULFURIZING LIQUID PETROLEUM HYDROCARBON CONTAINING ALKYL SULFIDE TYPE SULFUR COMPOUNDS WHICH METHOD COMPRISES, CONTACTING THE PETROLEUM HYDROCARBON WITH AQUEOUS CAUSTIC SODA SOLUTION AND INTRODUCING INTO CONTACT WITH BOTH COMPONENTS, WHILE EXCLUDING OXYGEN, A QUANTITY OF FORMALDEHYDE IN THE PROPORTION OF 1 PART THEREOF TO EACH 15,000-18,000 PARTS OF SAID HYDROCARBON THEREOF TO EFFECT CONJOINT REACTION OF THE CAUSTIC SODA AND FORMALDEHYDE WITH ALKYL SULFIDE THPE SULFUR COMPOUNDS PRESENT IN THE HYDROCARBON, THEN SEPARATING FROM THE PETROLEUM HYDROCARBON THE SAID AQUEOUS CAUSTIC SODA SOLUTION AND SULFUR COMPOUNDS DISSOLVED THEREIN DERIVED FROM SAID ALKYL SULFIDES, AND REGENERATING THE SEPARATED CAUSTIC SODA SOLUTION FOR REUSE BY CONTACTING THE SAME WITH AIR.

Images