US2049423A - Process of refining petroleum distillate - Google Patents

Description

Aug. 4, 1936. E. 5. BROWN ET AL 2,049,423

PROCESS OF REFINING PETROLEUM DISTILLATES Filed July 9, 1932 SET/25g Donald/55W i dilarmy Patented Aug. 4, 1936 PATENT] OFFICE PROCESS OF REFINING PETROLEUM DISTILLATE Ernest S. Brown, Inglewood, and Donald B. Nutt, El Segundo, Calif., assignors to Standard Oil Company of California, San Francisco, Calif a corporation of Delaware Application July 9, 1932, Serial No. 621,578

19 Claims.

This invention relates to a process of treating petroleum distillates and particularly distillates containing mercaptans, for the purpose of sweetening them. The invention is particularly directed toward a process of sweetening petroleum distillat'es by means of a novel reagent, whereby mercaptans are readily converted into other unobjectionable compounds and the distillates otherwise improved in a rapid and economical manner without the use of extraneous air or oxygen.

The invention is also directed to a method of treating petroleum distillates and maintaining the treating agent in the most effective condition.

One of the customary methods of treating petroleum distillates involves the use of what is known as the doctor solution. Doctor solution is ordinarily prepared by dissolving litharge in a caustic soda solution, sodium plumbite being thus formed. Free sulfur is generally used in addition. The doctor solution appears to sweeten petroleum distillates by converting certain types of sulfur bodies to other compounds which are sweet to the doctor test and do not have an objectionable odor. There are numerous disadvantages to the use of doctor solution, however. For example, one of the disadvantages of the conventional doctor treatment is that when treating petroleum distillates of relatively unstable color (such as cracked distillats), the doctor solution can not be reused without sacrificing the color of the distillate being treated. Forthis reason, it is difficult, if not impossible, to adapt the doctor treatment to a continuous operation and the chemical consumption is great, adding materially to the cost. Furthermore, as one of the steps of the doctor treatment is the addition of sulfur, the treating process is difilcultly controlled in that it is substantially impossible to add only the precise and exact amount of sulfur required. Moreover, there is an appreciable loss of stock through the discarding or attempted revivification of the spent doctor solution.

The invention not only distinguishes from and obviates the prior difficultiesencountered in the sweetening of petroleum distillates with doctor solution, but also obviates all of the difficulties hitherto encountered when attempts were made to use suspensions of lead sulfide in caustic. It is known that a suspension of lead sulfide in caustic is capable of sweetening certain distillates and such suspensions have been used heretofore while being subjected to the action of air brought into intimate contact with the oil and treating solution. Such treatment, however, could only be applied to certain distillates and during such treatment, appreciable concentrations of dissolved lead compounds and soluble sulfides formed in the treating solution, rendering the same ineffective and moreover making it impossible to reuse the treating solution on cracked distillates. The presence of dissolved lead in the treating solution has a marked deteriorating efliect upon the color of the treated distillate and although such soluble lead compounds are formed under all conditions, the use of air during the treatment merely aggravates the condition and renders this treating process commercially impractical.

As stated before, one of the disadvantages of the conventional doctor treatment is that, when treating petroleum distillates of relatively unstable color (particularly cracked distillates), the doctor solution can not be reused without sacrificing the color of the distillate being treated, unless practically complete removal of lead sulfide is accomplished before reuse. A suspension of lead sulfide is also likely to cause color reversion unless the treating process is controlled in accordance with this invention.

It has been found, however, that a suspension of lead sulfide in an alkalinemedium can be satisfactorily and economically employed in the treatment of all distillates, including cracked.distil-' lates, either with or without the use of extraneous air or oxygen provided, however, that the treatment be carried out in accordance with the controlled conditions forming the subject matter of this invention. Moreover, by conducting the treating process in accordance with this invention, the treating agent or treating solution may be-reused successfully for prolonged periods of time and therefore large quantities of distillatc treated thereby with exceptionally good results.

More specifically. it has been found that the addition of carefully controlled amounts of either soluble lead compounds or soluble sulfides, to the caustic suspension of lead sulfide (used as a treating agent) and the use of either no air or a minimum quantity of air, will eliminate all of the difiiculties commonly experienced in lead sulfide treatment and result in a process which has marked advantages over either the common doctor treatment or the previously attempted treat- It is an object of this invention to disclose andprovide a method of treating petroleum distillates for the purpose of producing distillates which do not react to the doctor test, that is, which do not contain mercaptans.

. An object of this invention is to disclose and provide a method of controlling the treatment of petroleum distillates with a reagent consisting of an alkaline suspension of lead sulfide, whereby said reagent may be used in a continuous manner even on distillates of relatively unstable color, without sacrificing the color of the distillate being treated.

Another object of this invention is to disclose and provide a process of treating petroleum distillates by means of a reagent substantially free from soluble lead salts or compounds.

Another object is to disclose and provide a continuous process of treating petroleum distillates to produce doctor negative distillates without deterioration in regard to color.

Another object is to disclose and provide a method of sweetening petroleum distillates by means of a specific reagent'and to disclose a method of maintaining said reagent in an effective condition during continuous use thereof.

, A still further object of the invention is to disclose and provide a method of treating distillates for the purpose of producing doctor negative distillates without the use of extraneous air or oxygen.

An object of the invention is to disclose and provide a novel treating agent for use in sweetening petroleum distillates.

Another object is to disclose and provide a novel treating agent capable of being continuously employed in the sweetening of petroleum distillates.

A still further object is to disclose and provide a method of operation whereby a treating agent may be continuously revivified during continued use in the sweetening of distillates.

These and other ,objects, uses and advantages of the invention will become apparent to those skilled in the art from the following detailed discussion of the conditions, steps and elements embraced by the process of this invention.

In order to facilitate understanding of the invention, reference will be had to the appended drawing, in which Fig. 1 diagrammatically represents one arrangement of elements whereby the process may be placed into operation.

Fig. 2 diagrammatically represents a modified arrangement of elements whereby, the sweetening of petroleum distillates may be carried out continuously in accordance with this invention.

As has been stated hereinbefore, the invention is particularly directed towards a process which employs an aqueous alkaline'solution containing lead sulfide in suspension as a treating agent. This treating agent or reagent should be essentially free from soluble lead salts or compounds and/or soluble sulfides, andathe process of this invention includes steps and means for maintaining the reagent substantially free from these interfering components.

A suitable reagent for use in this process may be prepared from any caustic soda or pota'sh solution having a gravity of between about 1 B. and the strongest solution available, although solu tions of between 8 B. and 30 B. are generally to be employed. Preferably, a caustic soda'solution of between about 20 and 30 B. is employed.

Reagents prepared with caustic soda solutions weaker than about 12 B.-are relatively slow in effecting the sweetening reaction, while those prepared with solutions above about 30 B. have a tendency to cause emulsification with some petroleum oils undergoing treatment, particularly when severe agitation is employed. In order to maintain the reagent in most eifective condition for rapid refining operations, it is found desirable to withdraw the solution or reagent when it has been diluted to a point materially below 20 B. and to replace it with a fresh 30 B. solution.

The reagent may contain in suspension from between about 0.1 and 2.0 lbs. and preferably between 0.5 and 1.0 pounds of lead sulfide per gallon of reagent; a reagent containing about 0.75 pounds of lead sulfide per gallon has been found to give excellent results. The range of concentrationmay be materially varied, however, without rendering the process inoperative. Lead sulfide may be prepared by precipitation from lead acetate-or other soluble lead salt with a solution of sodium or other soluble sulfide in carefully regulated amounts. As a matter of fact, a suitable reagent or treating solution may be prepared by adding sodium sulfide to an ordinary doctor solution, the sodium sulfide being added in quantity sufiicient to precipitate the lead in the form of lead sulfide. Care should be taken, however, that the-reagent is substantially free from any soluble lead compounds or from an excess of soluble sulfides. In actual practice, it has been found that the sweetening reaction of a fresh reagent mixture is very greatly improved after some, at least, of the lead sulfide hasabecome oil-wetted. For this reason, it has been found desirable in preparing a reagent for use, to recirculate the re-v agent after the precipitation of lead sulfide there-- in in contact with an oil or distillate, such as a previously purified distillate in a closed system until the sulfide becomes oil-wetted, in part at least.

The process of treating petroleum distillates consists of the steps of contacting the sour distillate with the treating solution or reagent in any suitable manner, contact being maintained for an appropriate period of time. As the time of contact required is influenced by the character, gravity and source of the petroleum distillate and the alkalinity and lead sulfide content of the reagent, it is impossible to state any definite contact times.

It is to be understood that the process is applicable to a wide variety of petroleum oils and distillates. Primarily, the process is applicable to petroleum distillates of the kerosene and gasoline type but any sour distillate may be treated by the process and by the reagent described hereinabove, irrespective of the sulfur content of the distillate.

The process and reagent are particularly adapted Distillates containing larger quantities of mercaptan sulfur generally require the introduction of extraneous air or oxygen during the contacting step, but only a very small amount of air or oxygen is required. For example, cracked gasoline distillates containing 0.1 to 0.12% of mercaptan sulfur should be treated with not more than about 0.02-0.03 cubic feet of air per gallon of oil. The amount of air or. oxygen introduced during the treating step should be carefully controlled since any excess of air over that required to effect the sweetening reaction develops the soluble plumbite ion in the reagent and the presence of plumbite or of soluble lead compounds in general has been found to have a marked deteriorating effect upon the color of the treated distillate. In no case should there be present in the reagent more than traces of soluble lead compounds for, as has been stated hereinabove, the presence of soluble lead compounds decreases not only the rate of sweetening but also causes a drop in the color of the treated oil. For thisreason, it has been found desirable to test the reagent being used at frequent intervals and to remove soluble lead or soluble sulfides by the addition of suitable chemicals.

One arrangment of elements in which the process may be effectively placed in operation is illustrated in Fig. 1. The naphtha or petroleum distillate to be treated may be admitted to the system by a line I provided with a valve 2, the line leading to the inlet by a suitable pump-3 provided with a discharge line 4 leading to a treating tank 5. The discharge line 4 may terminate near the bottom of the treating tank 5 in a spray head 6. The discharge line 4 may be provided with a valve 1. A level of aqueous caustic soda suspension of preformed lead sulfide is maintained in the treating vessel 5, as indicated at 8. A pressure drop across the sprays 6 provides turbulent contact between the oil and the reagent and the sweetening reaction may be effected merely by this contacting and by the rising of the produced fine droplets of oil through the reagent. Further contacting of the distillate with the reagent may be provided by returning the oil as through the swing pipe 9 and line l0 to the pump 3 and circulating it repeatedly through the reagent in the treating vessel 5, the valve 2 being closed and the valves 1 and it being open. Still more vigorous and eflicient agitation of the distillate with the reagent may be provided by circulating a stream of the reagent through the pump 3 as through lines l2 and I3, the valve l4 being closed and valves l5 and It being open to a pointgiving the desired flow.

The valve 2, during this recirculation, may be partially open so that the stream leaving the pump 3 by the line 4 may suitably carry 10%, 20%, 30% or 40% by volume of recirculated reagent, the remainder being oil undergoing treatment. high in mercaptan content, the amount of reagent recirculated in this manner may constitute 60% of the stream. The contacting in the pump 3 or in the treating vessel 5 should not be severe enough to cause the formation of difiicultly broken emulsions of oil and reagent and the agitation in the pump and acrossthe sprays may be governed accordingly. The contact of gasoline with reagent is continued in this manner until the contents of the treating vessel 5 or the gasoline at the discharge of the pump 3, does not react to the doctor test.

then be removed as through the swing pipe 9, line l0, pump 3, line 4 and discharge line H, valves I i In treating cracked distillates extremely The circulating pump 3 may be then shut down and the contents of the treating .vessel 5 allowed to settle. The treated oil may and I8 being opened and valves 1, 2 and I5 being closed. I

Upon the withdrawal of the treated oil, the system is ready to receive a fresh charge of sour oil. As noted above, distillates containing relatively small amountsof mercaptans, are sweetened merely by agitating them with a suspension of lead sulfide, but oils containing relatively large amounts of mercaptans (more than about 0.03% mercaptan sulfur) require in addition to this treatment, the presence of a small amount of extraneous air or oxygen during the contacting with the reagent. Such air or oxygen-may be supplied at anyconvenient point in the treating or contacting system. It has been found that such air oroxygen may be most suitably added at the suction side of the charging or mixing. pump 3, as, for example, through line I!) provided with a valve 20. In the event that the addition of air at this point causes cavitation of the pump, the line l9 may be placed at the discharge rather than the suction side of the pump.

Air should not be used during the sweetening reaction unless it is found to be necessary and in any case, the addition of air or oxygen should be carefully controlled since any excess air develops the soluble plumbite ion in the reagent. The removal of dissolved lead from the reagent may be eifected bythe addition of carefully controlled amounts of sodium sulfide to the reagent, or by bubbling a gas containing hydrogen sulfide through the reagent in controlled amounts. Thisremoval of soluble lead may be done between 'the periods of discontinuous contacting of the reagent with oil or sodium sulfide or other soluble sulfide, or hydrogen sulfide containing gas may be added to the reagent while it is being agitated with the oil.

For example, it may be introduced to the reagent in the treating vessel itself or it may be added to the suction of the charging and mixing pump 3 through the line H controlled by the valve 22.

The apparatus disclosed in Fig. 1 is primarily adapted for batch operation and includes means for'introducing oil to be treated and contacting it with a reagent, means for circulating any desired proportions ofoil and/or reagent through the treating vessel, and means for maintaining the reagent in efficient condition by permitting the introduction of corrective chemicals. A closed system is here employed so that losses are reduced to a minimum. Y

In Fig. 2, an arrangement of elements particularly adapted for continuous operation is disclosed. A body of the reagent is carried in the treater 25 as indicated by the level line 26. a A doctor positive distillate to be treated is continuously admitted by line 27, provided with a valve 28, into the suction line 29 of a pump 30 adapted to discharge by line 34 into one or more spray lines 32 and 32' positioned near the bottom of the treater 25. The discharge line 3i is shown provided with two'spray heads so as to afford adequate pressure drop across the sprays at widely differing treating rates.

A discharge line 33 leads from near the top of the treater 25 to a line 3 8 provided with a valve 35, the line 34 discharging into a settling tank 36. The line 33 may also connect with a line 31, having a valve 38 therein, and a branch 39 communicating with the suction side of the pump 30. The line 39 may be provided with a valve 49.

The lower or bottom portion of the settling a discharge line 45 leading to a suitable means tank 36 is provided with a discharge line 4| having a valve 42 therein, the line 4| being connected by line 43 having a valve 44 therein, with the bottom of'the treating tank 25. The line 4| may also connect with the suction line of the pump as, for example, by communicating with the branch line 39.

The top of the settling tank 36 is provided with adapted to separate solids from liquid as, for example, a filter 46. The line 45 may be provided with a valve 41 and a branch line 48 having a valve 49 therein, said branch line 48 being adapted to by-pass the filter 46 and to place the line 45 in communication with the line 31.

In actual operation, the distillate to be treated is continuously admitted through line21 to the suction line 29 of the pump 30 and discharged by said pump into the spray heads 32 and 32' in the bottom of the treater 25. It is to be understood that a suitable body of reagent is maintained in the treater. A desired proportion of the reagent may be continuously withdrawn from the treater and recirculated as by lines 43, 4|, and 39, valve 44 being opened the desired amount so as to permit a desired proportion of the reagent from the treater 25 to be continuously withdrawn and recirculated by the pump 30. The treated distillate, together with some reagent, is continuously discharged from the top of the treater 25 by line 33. If valve 33' is closed, then the treated distillate, together with the proportion of the reagent, will pass into line 34 (valve 35 being open) into the settling tank 36 wherein a separation between the distillate and the aqueous caustic soda solution containing lead sulfide in suspension, is eifected. The reagent settles to the bottom of the settler 36 and may be continuously withdrawn therefromby line 4l,-the valve 42 being open. This settled reagent is thus discharged by lines 4| and 39 into the suction line 29 of the pump 30. 1

The treated distillate is. discharged from the top of the settling tank 36 as by line 45, and may be eliminated from the system by lines 48 and 31, valves 49 and 38 being open and valve 40 being closed. A separation between the reagent and the distillate being treated occurs in the treating tank 25 and the treated gasoline can bevdischarged from the top of the treating tank 25 by line 33 into line 31 and removed from the system through the valve 38. Generally, however, when a large amount of recirculation is being employed and when the rate of treating is high, it is desirable to employ the auxiliary settling tank. 36 as described hereinabove Furthermore, when distillates high in mercaptan sulfur are being treated and it is necessary to introduce air oroxygen into the treater as, for example, by the valved line 50, then it is desirable to employ the auxiliary settling tank.

When air is added, its admission should be carefully controlled so that no more is added than is absolutely necessary to effect the sweetening reaction. It is difiicult to avoid the formation of soluble lead compounds, however, and as has been avoid the gradual accumulation of soluble lead compounds, it is convenient to add, either intermittently or continuously, small and controlled amounts of sodium or other water soluble sulfide, in order to precipitate the soluble lead as lead sulfide. This may be conveniently done in continuous operation at the suction side of the pump 30, as through the valved line 5|. The reagent should be sampled at various points in the system, as from the bottom of the treater 25, and 5 tested periodically for the presence of soluble sulfides or soluble lead compounds in order to make certain that the additions of corrective agents are merely sufiicient to maintain the reagent in substantially balanced condition. The addition of soluble sulfides or soluble lead salts, when and as required, may also be made directly to the body of the reagent in the treating tank 25 as, for example, by the valved line 52.

As has been mentioned hereinabove, the lead 15 sulfide reagent used in the process ofthis invention, exhibits an improved sweetening reaction when some part, at least, of the lead sulfide has become oil-wetted. When a large proportion of the lead sulfide in the reagent is oil-wetted, there is a tendency for some of the lead sulfide to be carried over by the treated distillate. For example, some of the distillate being discharged by line from the settling tank 36, may contain oil-wetted lead sulfide in suspension. In order 25 to prevent the loss of this lead sulfide, it may be desirable to pass the distillate passed from the settler 36 through a suitable means for removing solids therefrom as, for example, a filter press 46. When it is foundthat the distillate being 30 discharged from the settler 36 contains lead sulfide in suspension, valve 49 may be closed, valve 41 opened, and the distillate passed through the filter, clarifier, thickener, settling tank or other suitable means 46, before being discharged by 5 line 31.

The apparatus of the character illustrated in Fig. 2 has satisfactorily and continuously treated California cracked gasoline containing an average of 0.02 mercaptan sulfur without the necessity of the addition of any'air whatever to the treating system. In this particular example, the vertical treating vessel 25 was 10 feet in diameter and 30 feet high, agitation being effected both in the treating pump 30 and by the sprays 32 and 32 at a'treating rate of 500 barrels of 50 gallons each of gasoline per hour. In another example of the process, one volume of reagent has been repeatedly used in the treatment of more than 2,000 volumes of sour gasoline containing about 0.02 mercaptan sulfur without the addition of air or oxygen.

This continued reuse was accomplished without loss of lead and without the necessity of adding appreciable quantities of caustic soda over that mechanically lost from the system. When 7 the caustic soda solution has been diluted to materially below 20 B. by continued use and the addition of sodium sulfide solution, it is.

desirable to remove a portion of the aqueous liquor from the solids and to replace it with fresh concentrated caustic soda solution, such as a 30 B. caustic. This procedure increases the activity ,of the reagent and promotes operation.

It will thus be seen that a simple and eifective method of rendering oils negative to the doctor test, has been disclosed. As repeatedly emphasized hereinabove, the'reagent should under no circumstances contain more than traces of soluble lead compounds or of soluble sulfides. If the soluble lead compounds appear in the reagent, they should be precipitated by the addition of carefully regulated amounts of hydrogen sulfide or any soluble sulfide. In case. soluble sulfide appears in the reagent, this should be removed by the addition of carefully regulated amounts of a soluble lead salt. Sour distillates containing large quantities of dissolved hydrogen sulfide may be preliminarily treated'before the sweetening treatment for the removal of the dissolved hydrogen sulfide, as the constant additionof this dissolved gas to the reagent would cause the gradua accumulation of soluble sulfides and the presence of soluble'sulfides causes a retardation and finally a cessation of the sweetening reaction.

Complete sulfide removal is not necessary, however, it is desirable only that the HzS content of the sour distillate entering the treating system be no greater than that necessary to reprecipitate the soluble lead compounds which are formed inthe reagent during treatment.

The following is a typical example illustrating the necessity for maintaining a balanced reagent. A typical mercaptan containing California cracked gasoline was contacted with 8% of a suspension of lead sulfide in 8 B. gravity caustic gasoline had dropped to +16.

An exact duplicate of the original reagent was then used for a second series of treatments and the procedure followed was identical except that a lesser quantity of air was present during treatment and after sweetening each portion of gasoline, sodium sulfide solution was added in quantities just sufficient to precipitate the soluble lead compounds formed in the reagent during the previous treating operation. It was found that the second and third portions of gasoline were completely sweetene after but a 5 minute agitation and the colors 0 the finished gasolines were +30 and +29 respectively. In other words, the time of treatment required when the process was carried out in accordance with this invention was 14% and 5% respectively of the time required .ordinarily in treating the gasoline. The additlonal advantage was the improved color of the finished product.

Although certain specific concentrations, ten'iperatures, times, conditions and operations have been described in detail, it isto be understood that such conditions, times and ingredients are merely illustrative of the process and are not to be understood as limiting. Numerous changes and modifications can be made as will become apparent to those skilled'in the art and all such changes and modifications as come within the scope of the appended claims are embraced thereby.

We claim: 1

1. In a process of refining petroleum distlllates, the steps of contacting a distillate with an aqueous caustic treating solution containing lead sulfide in suspension in a mixing zone, without the addition of oxygen-containing gas thereto, said caustic treating solution being substantially free from dissolved lead compounds and added free sulfur, maintaining said treating solution substantially free from dissolved lead compounds by the regulated addition thereto of an agent adapted to precipitate said lead compounds from solution, forcing the mixture of distillate and treating solution into a treating and settling zone, continuously withdrawing a portion of the treating solution from the treating and settling zone, recirculating said withdrawn treating solution through said mixing zone and back into said treating and settling zone, and continuously withdrawing a portion of the treated distillate from the treating and settling zone and from the system. l

2. In a process of refining petroleum distillates, the steps of contacting a distillate with an aqueous caustic treating solution containing lead sulfide in suspension in a mixing zone, forcing the mixture of distillate and treating solution into a treating and settling zone, continuously withdrawing a portion of the treating solution from the treating and settling zone, recirculating said withdrawn treating solution through said mixing zone and back into said treating and settling zone, continuously withdrawing a portion of the treated distillate from the treating and settling zone and from the system, and maintaining said aqueous caustic treating solution substantially free from dissolved lead compounds by adding water-soluble sulfides thereto during said recirculation in quantities sufilcient to combine with dissolved lead com.- pounds which may be formed during contacting to form lead sulfide.

3. In a process of refining petroleum distillates, the steps of contacting a distillate with an aqueous caustic; treating solution containing from about 0.1 to 2.0 pounds of lead sulfide per gallon in suspension in a mixing zone without the addition 'of oxygen-containing gas, said treating solution being substantially free from water-soluble sulfides, dissolved lead compounds and added free sulphur; maintaining said treating solution substantially free from dissolved lead compounds which may be formed during said contacting by adding quantities of a watersoluble sulfide to said solution to precipitate dissolved lead compounds in the form of lead' sulfide, regulating said addition in accordance with the quantity of soluble lead compounds formed in said treating solution; maintaining said treating solution substantially free from water-soluble sulfides which may be formed during said contacting by adding quantities of soluble lead compounds to said solution, regulating said addition in accordance with the quantity of water-soluble sulfides formed in said treating solution; forcing the mixture of distillate and treating solution into a treating and settling zone, continuously withdrawing a portion of the treating solution from the treating and settling zone, recirculating said withdrawn treating solution through said mixing zone and back into said treating and settling zone, and continuous- 1y withdrawing a portion of the treated distillate from the treating and settling zone and from the system.

4. In a process of refining petroleum distillates, the steps of contacting a distillate having less than about 0.02%-0.03% mercaptan sulfur with an aqueous caustic treating solution containing from about 0.1 to 2.0 pounds of lead solfide per gallon in suspension in a mixing zone 70 without the addition of oxygen-containing gas, forcing the mixture of distillate and treating solution into a treating and settling zone, continuously withdrawing a portion of the treating solution from the treating and settling zone, re- 7 circulating said withdrawn treating solution through said mixing zone and back into said treating and settling zone, continuously with- 8 and 30 B., agitating the treating solution with a purified petroleum oil to'wet the lead sulfide with oil, separating the aqueous caustic solution and oil-wetted lead sulfide from the purified petroleum oil, then contacting the aqueous caustic solution containing oil-wetted lead sulfide with a sour petroleum distillate, maintaining said caustic solution substantially free from dissolved lead compounds by the'reguiated addition thereto of an agent adapted to precipitate such lead compounds from solution in the form of a sulfide, agitating the mixture, and finally separating the aqueous caustic solution and lead sulfide suspended therein from the treated distillate.

6. In a process of refining petroleum distillates, the steps of repeatedly contacting a caustic solution stronger than 8 B., containing lead sulfide in suspension, with petroleum distillates, said caustic solution being substantially free from water-soluble sulfides, dissolved lead compounds and added free sulfur, maintaining said solution free from dissolved lead compounds by adding a water-soluble sulfide in minimum qauntity required for substantially complete conversioniof said soluble lead compounds into lead sulfide, and separating said solution containing lead sulfide in suspension from the treated distillate after said contacting.

'7. A process of refining hydrocarbon distillates containing less than about 0.02% to 0.03% mercaptan sulfur to produce distillates which are negative to the doctor test, comprising contacting such hydrocarbon distillates, without the additionpf oxygen-containing gas, with an aqueous alkali solution containing lead sulfide in suspension, and maintaining said solution substantially free from dissolved lead compounds by converting any sodium plumbite which may be formed into lead sulfides by the regulated addition of watersoluble sulfides to the solution during said contacting.

8. A process of refining hydrocarbon distillates containing less than about 0.02% to 0.03% mercaptan sulfur to produce distillates which are negative to the doctor test, comprising contacting such hydrocarbon distillates, without the addition of oxygen-containing gas, with an alkali solution containing from 0.1 .to 2.0 pounds of lead sulfide per gallon in suspension, and maintaining said solution substantially free from dissolved lead compounds by converting any sodium plumbite which may be formed into lead sulfides by the regulated addition of water soluble sulfides to the solution during said contacting.

9. In a process of refining hydrocarbon distillates containing less than about 0.02% to 0.03% mercaptan sulfur to produce distillates which are negative to the doctor test, the steps of contacting such hydrocarbon distillates, without the addition of oxygen-containing gas, with a caustic solution of between 8" and 30 B., and containing 0.1 to 2.0 pounds of lead sulfide per gallon in suspension, and maintaining said solution substantially free from dissolved lead compounds by converting any dissolved lead compounds which may be formed into lead sulfides by the regulated addition of water soluble sulfides to the solution.

a 10. In a process of refining petroleum distillates containing less than about 0.02% to 0.03% mercaptan sulfur, the steps of recirculating a mixture of such petroleum distillate and an aqueous caustic solution containing lead sulfide 1 in suspension, without the addition of oxygencontaining gas, into and out of a treating zone until the distillate is doctor negative, and maintaining said caustic treating solution substantially free from dissolved lead compounds by the regulated addition of water soluble sulfides to such treating solution in quantity sufiicient to convert all dissolved lead compounds which may be formed during such recirculation into lead sulfide.

11. In a process of refining petroleum distillates containing more than about 0.02% to 0.03% mercaptan sulfur to produce distillates which are negative to the doctor test, the steps of recirculating a mixture of such petroleum distillate and an aqueous caustic solution containing lead sulfide in suspension into and out of a treating zone until the distillate is doctor negative, introducing not more than about 0.02 to 0.03 cubic feet of air per gallon of distillate during such recirculation, and maintaining said caustic treating solution substantially free from dissolved lead compounds by introducing regulated quantities of water soluble sulfides into the treating solution in quantity suflicient to convert all sodium plumbite which may be formed during such recirculation into lead sulfide.

12. In a process of treating hydrocarbon distillates containing more than 0.02% to 0.03% of mercaptan sulfur, the steps of contacting such distillate with an alkali solution containing lead sulfide in suspension and free from dissolved lead compounds, introducing not more than about 0.02 to 0.03 cubic feet of air per gallon of distillate during said contacting, and maintaining said soadding a water soluble sulfide to the solution in regulated quantities sufiicient to keep all sodium plumbite which may be formed, in the form of lead sulfide.

13. A process of refining hydrocarbon distillates containing less than about 0.03% mercaptan sulfur to produce distillates which are negative to the doctor test, consisting ofcontacting hydrocarbon distillates containing less than about 0.03% mercaptan sulfur with a caustic solution of between 8 and 30 B., said solution containing lead sulfide in suspension and being substantially freefrom water-soluble sulfides and dissolved lead compounds; maintaining said solution substantially free from dissolved lead compounds which may be formed during said contacting by adding quantities of a water-soluble sulfide to said solution to precipitate dissolved lead compounds in the. form of lead sulfide, regulating said addition in accordance with the quantity .of soluble lead compounds existing in said solution; and finally separating the caustic solution from the treated distillate.

14. In a process of sweetening petroleum dlstillates, the steps of repeatedly contacting ansoluble sulfide to the solution, during said contacting, in quantity sufficient to precipitate dissolved leadcompounds in the form of lead sulfide, regulating the quantity of water-soluble sulfide thus added in accordance with the quantity of dissolved lead compounds formed during said contacting, whereby said alkali solution is maintained substantially free from elemental sulfur and dissolved lead compounds, and then separating said alkali solution from the petroleum distillate after such contacting.

15. In a process of refining petroleum distillates, the steps of repeatedly contacting an alkali solution containing lead sulfide in suspension with petroleum distillate, said solution being substantially free from water-soluble sulfides and dissolved lead compounds; maintaining said 'solution free from water-soluble sulfides, during said contacting, by adding a soluble lead compound thereto, during said contacting, in quantity sufficient to combine with water-soluble sulfides formed during treatment of the distillate, to form lead sulfide therefrom; and separating the alkali treating solution from the petroleum .distillates after such contactin 16. In a process f refining petroleum distillates, the steps of: recirculating a mixture of petroleum distillate and an aqueous caustic solution containing from'about 0.1 to 2.0 pounds of lead sulfide per gallon in suspension, into and out of a treating zone until the distillate is doctor negative, maintaining said caustic treating solution substantially free from dissolved lead compounds which may be formed during such recirculation by introducing into the circulating solution water-soluble sulfides in quantity sufiicient to convert all such dissolved lead compounds into lead sulfides; and separating the treated distillates from the treating solution.

17. In a process of sweetening petroleum distillates to produce oils which are negative to the doctor test, the steps of contacting-a petroleum distillate containing. mercaptans with an alkali treating solution containing lead sulfide in suspension, said solution being substantially free from dissolved lead compounds, and maintaining said solution substantially free from dissolved lead compounds during such contacting by the regulated addition thereto of a suitable chemical agent adapted to precipitate dissolved lead compounds from said treating solution.

18. In a process of sweetening petroleum distillates to produce oils which are negative to the doctor test, the steps of: contacting a petroleum distillate containing mercaptans with an alkali treating solution containing lead sulfide in suspension, said solution being substantially free from dissolved lead compounds and water-soluble sulfides; maintaining said treating solution substantially free from dissolved lead compounds which may be formed during said contacting by adding quantities of a water-soluble sulfide to said solution to precipitate dissolved lead compounds in the form of lead sulfide; regulating said addition in accordance with the quantity of soluble lead compounds formed; maintaining said treating solution substantially free from watersoluble sulfides which may be present therein by adding quantities of water-soluble lead compounds to said solution, and regulating said addition of water-soluble lead compounds in accordance with the quantity of water-soluble sulfides present in said solution.

19. In a process of sweetening petroleum distillates to produce oils which are negative to the doctor test, the steps of contacting a petroleum distillate containing mercaptans with an alkali treating solution containing lead sulfide in suspension, said contacting being performed in the substantial absence of free sulfur, said solution being substantially free from dissolved lead compounds, and maintaining said solution substantially free from dissolved lead compounds during such contacting by the regulated addition thereto of a suitable chemical agent adapted to precipitate dissolved lead compounds from said treating solution.

ERNEST S. BROWN.

DONALD B. NUTT.

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