US2393476A - Sweetening of gasoline - Google Patents

Description

Jan. 22, 1946. I-I. E. MESSMORE 2,393,476

SWEETENING OF GASOLINE Filed July 1, :1944

STRAIGHT RUN PETROLEUM DISTIl-LATE DISTILLATION ZONE HIGH-BOILING LOW-BOILING FRACTION FRACTION PRIMARY TREATING LONE SWEETENED DISTILLATE INVENTOR H. E. MESSMORE Patented Jan. 22, 1946 SWEETENING GASOLINE Harold E. Messmore, nauemue, om, assignor to Phillips Petroleum Company, a corporation of Delaware Application July 1, 1944, Serial No. 543,134

8 Claims. (01. 196-30) This invention relates to a method of refinin hydrocarbon mixtures containing mercaptans and more particularly to improvements in methods of sweetening "sour hydrocarbon distillates.

Mosthydrocarbon distillates obtained from the fractionation of petroleum, either from crude or refined stocks, contain a sufficient amount of sulfur as mercaptans to render the distillate corrosive and malodorous, or sour." Extensive research and study have been devoted to the problem of extracting these compounds or converting them into compounds which are inoffensive and non-corrosive as shown by the extensive prior art on such "sweetening processes. Conversion in the latter method is accomplished by some oxidizing agent which converts the mercaptans present in the petroleum mixture principally to disulfides.

One of the methods employed, for example, is to treat the hydrocarbon distillate with so-called doctor solution which is a solution of lead oxide, or litharge, in an aqueous solution of sodium hydroxide. On mixing the hydrocarbons containing mercaptans with this solution, the mercaptans are converted to lead mercaptides. To this mixture is added a reagent adapted to form insoluble lead compounds and convert the sulfur compounds to disulfides. This may be accomplished by the addition of finely divided sulfur which reacts with the lead mercaptides to form insoluble lead sulfide and the corresponding disulfides. Another well-known method is the copper sweetening process which comprises bringing the hydrocarbons containing mercaptans in contact with a bed of solid, or a solution of, cupric chloride. Mercaptans are directly oxidized to disulfides in an oxidation-reduction reaction involving the reduction, of the cupric chloride to cuprous chloride and hydrochloric acid from which the cupric compound is regenerated with oxygen.

Methods as described above are ordinarily more or less efficacious in treating petroleum distillates to remove or destroy malodorous and corrosive mercaptans by sweetening processes, however, some petroleum stocks have been found which are relatively difficult to sweeten, that is, relatively slow to sweeten. Upon investigation, it is found that this slowness to sweeten is due to the relative inertness of some higher boiling mercaptans. Recognition of this fact and attempts to overcome the difficulty are shown in the prior art which describes processes of separating the petroleum stocks into relatively high-boiling and low-boiling fractions and treating each fraction separately. Thus the treatment of the higher boiling and less reactive fractions with a larger proportion of treating solution than is necessary for the lower boiling fraction has been described. Also it is known to treat the separate fractions however, are satisfactorily eflective to readily sweeten some hydrocarbon stocks which are dimcult to sweeten.

It is, therefore, an object of this invention to provide a method of rapidly and effectively sweetening hydrocarbon mixtures which are relatively difficult to sweeten.

Another object of this invention is to provide a novel methodfor the conversion of malodorous sulfur compounds occurring in hydrocarbon mixtures into less obnoxious substances.

A further object is to accomplish said conversion by a novel method which comprises fractionating the hydrocarbon mixture into relatively high-boiling and low-boiling fractions, separately treating the high-boiling fraction, combining the highand low-boiling fractions, and further treating the mixture.

Other objects and advantages of the invention will be apparent from the description which follows.

In accordance with the present invention I have discovered a mode of operation which more effectively and efliciently sweetens petroleum distillates which contain at least some high boiling, slow to sweeten, mercaptans. In accomplishing the sweetening process according to my invention the distillate is separated into two fractions, the higher boiling of which contains the mercaptans more resistant to sweetening as well as some mercaptans which are commonly termed easy to sweeten, and the lower boiling of which contains 'mercaptan compounds substantially all of which are easil and rapidly sweetened. The higher boiling fraction is subjected to a conventional sweetening process whereby a major portion of the mercaptans therein are sweetened. The low-boiling fraction is then mixed with the partially sweetened high-boiling fraction and the mixture is subjected to a secondary sweetening step to yield an eilluent which is negative to the well-known doctor test for soumess. When the process is carried out in accordance with my invention as described herein, it has been found that the net sweetening effect is greater than that obtained by either separately treating the two fractions and combining the products or by subjecting the total feed to sweetening in a single step or in successive steps.

With no intention of limiting th scope of the invention, a possible explanation of the beneficial effects obtained in the practice of the invention may be found in the mechanics of the reactions involved. Examination has shown that the mercaptans which are present in the hydrocarbon distillate may include various molecular weight 60 aliphatic, aromatic or cyclic mercaptans and that some of these may exist in the primary, secondary and/or tertiary form. It is i'urtherknown that primary aliphatic mercaptans oxidize to disulfldes instantaneously, that secondary mercaptans oxidize more slowly but still rather rapidly and that tertiary mercaptans, especially those of higher molecular weights, oxidize relatively slowly. In the sweetening of hydrocarbons containing a mixture of such mercaptans various combinations of disulfides may be formed. For example, two molecules 01 similar mercaptans may react to form a symmetrical disulfide, such as dimethyl disulfide, diethyl disulfide and others, or molecules of different mercaptans may react to form a mixed disulfide containing unlike radicals, as methyl-ethyl disulfide or other disulfide depending on the hydrocarbon radicals involved. Similarly, a primary mercaptan may combine with a secondary or a tertiary mercaptan to form a mixed disulfide and, based upon the kinetic converted mercaptans is withdrawn by a line 6 and passed to a mixing pump 1 where it is mixed with the low-boiling fraction from line 3. From mixing pump I the mixture is passed by a line 8 to a secondary treating zone 9 in which the mixture is subjected to a conventional sweetening theory, when such a reaction occurs it 'may do so more rapidly than the reaction between two secondary and two tertiary mercaptans as the case may be.

In Iractionating the feed stock preliminary to carrying out the sweetening operation by the process of my invention it is only necessary to separate the raw hydrocarbon stock into a relatively high-boiling fraction and a relatively lowboiling fraction. The high-boiling fraction must be so cut that it contains substantially all the high molecular weight tertiary mercaptans which are normally slow to sweeten. This fraction may also contain a considerable proportion of primary and secondary mercaptans as well. The lowboiling fraction will normally contain primary,

secondary and tertiary mercaptans, but these tertiary compounds being of relatively low molecular weight are usually not diificult to sweeten.

Upon subjecting the higher boiling fraction to a sweetening operation substantially all of the primary and secondary mercaptans are fully oxidized to disulfides. In addition, an appreciable proportion of the normally slow to react tertiary mercaptans may also be sweetened in this first operation. Usually the amount of sourness remaining is relatively small but rather refractory to further sweetening as conventionally practiced. When I mix the separated low-boiling fraction containing its easy to sweeten" mercaptans with the partially sweetened high-boiling fraction, the entire stock may then be easily and rapidly sweetened. Apparently the relatively large excess of easy to sweeten mercaptans in relation to the remaining "difllcult to sweeten mercaptans, fosters the ultimate formation of mixed disulfides with the result that the combined stock is then fully and completely sweetened.

The accompanying figure illustrates schematically one embodiment of the present invention. A mercaptan-containing stock is introduced by a line I into a distillation zone 2 in which it is fractionated into a relatively low-boiling fraction which is removed by means of a line 3, and into a relatively high-boiling fraction which is removed .by means of a line 4. The high-boiling fraction is introduced into a primary treating zone 5 in which most of the mercaptan content is converted into disulfldes by a conventional sweetening process. The sweetening may be carried out using conventional methods with well known reagents such as cupric chloride solution, plumbite solutions or solutions of polysulfides of the alkali and alkaline earth metals. The hydrocarbon fraction containing the disulfides and unprocess and from which the sweetened distillate is withdrawn by a line Ill. The treating zones include a reactor in which the hydrocarbons contact the treating reagents and a separator in which the hydrocarbons and treating reagents may be separated, as by gravity settling.

The primary sweetening zone may include a centrifugal pump type mixing device, a Jet type mixer, a packed conduit type or substantially any mixing device adapted to mix intimately two immiscible liquids. Similarly, mixer 1 may be substantially any type contactor, but I have found the centrifugal pump type mixer to give superior results.

If desirable, the sweetening process may be conducted in batch operations in which case the lower boiling untreated fraction of hydrocarbons may be added to the treated higher boiling fraction in the primary treating zone after the initial comparatively rapid conversion has subsided. Additional sweetening reagent may be introduced with the low-boiling fraction as required. Operation in this manner eliminates the necessity of the secondary treating zone, the sweetening of the hydrocarbon being completed upon the addition of the lower-boiling fraction.

In view of the well-known fact, already mentioned, that certain of the higher boiling mercaptans are slow to sweeten, the lower boiling fraction should be selected to have a maximum boiling point between approximately 250-350 F., depending upon the characteristics of the hydrocarbon stock and of the particular mercaptans or sourness to be treated. Since mercaptans are usually present in small amounts and comprise principally those of lower boiling points, the fractionation should be controlled to furnish such proportion of mercaptans in each fraction as to accomplish most effectively the desired sweetening.

The invention is easily applicable to doctor or copper sweetening or processes using other sweetening reagents, such as a polysulfide solution. In copper sweetening by the liquid modification the high-boiling fraction is contacted with copper treating solution, separated therefrom, mixed with the low-boiling fraction and the mixture sweetened. Following the final sweetening operation the hydrocarbon stock may be treated with an alkaline sulfide solution to remove minute traces of copper if present and to give color and gum stability to the treated stock. For application of the invention to doctor sweetening, the high-boiling fraction is mixed with doctor solution and sulfur in the primary treating zone, the treated material combined with the low-boiling fraction and the mixture treated with further doctor solution and sulfur in the secondary treating zone. If desired, a separation may be effected between the treated high-boiling fraction and the treating agents in the primary zone, prior to the combination with the low-boiling fraction and treatment in the secondary treating zone.

In treating a hydrocarbon mixture by the method described herein, utilization may be made of one sweetening reagent in the primary treating zone and of another sweetening reagent in the secondary treating zone.

Example I A straight run petroleum stock boiling from approximately 100 to 425 F. and containing mercaptans norma ly slow to sweeten was fractionated into two fractions. The separation was so made that the low-boiling fraction contained naphtha boiling from 100 to about 325 F., while the high-boiling fraction contained the remainder of the material, that is, the fraction boiling from about 325 to 425 F. The high-boiling fraction was contacted with a quantity of cupric chloride treating solution and the mixture separated into its two phases. The upper oil phase, tested for sweetness according to the "doctor test, was slightly positive. The doctor test solution was made and the test carried out according to the standard method as given on pages 180-181, Chemical Refining of Petroleum, by Kalichevsky and Stagner, Copyright, 1942, by Reinhold Publishing Company. The doctor test is also described in the U. S. Bureau of Mines Technical Paper 3233.

To this partially sweetened high-boiling fraction was added the low-boiling fraction, and the mixture treated in a conventional manner with a copper chloride treating solution. The treated mixture was tested for sourness by the doctor test and found to be negative.

Another quantity of the same straight run petroleum stock was treated, without fractionation,by the copper chloride treating solution according to conventional methods. A sample of this material when tested by the doctor test showed slightly positive.

Still another quantity of the same straight run petroleum stock was separated into two fractions of the boiling ranges stated above and each fraction treated separately with the cupric chloride treating solution. Upon combining the separately treated oil streams, and applying the doctor test, the mixture was found to be slightly positive.

Example II Another straight run petroleum naphtha boiling from 100 to 450 F. was fractionated into two reagent. A sample of the treated fraction tested sour" to the standard doctor tests This partially sweetened stock was then mixed with the sour low boiling fraction and the mixture treated with a copper chloride treating solution. The treated mixture was fully sweet to the doctor test.

The process of the invention is not to be limited by any theories or modes of operations but only by the claims which follow.

I claim:

1. A process for refining petroleum oils containing mercaptans which comprises fractionating the said petroleum oils into relatively highboiling and low-boiling fractions, contacting said high-boiling fraction with an oxidizing sweetening reagent, combining the treated high-boiling fraction with said low-boiling fraction, contacting the mixture with an oxidizing sweetening reagent, and separating the treated oils from the treating agents.

2. A process for refining petroleum oils containing mercaptans which comprises fractionating the said petroleum oils into a first fraction with a maximum boiling range of about 250 to 350 F. and a second fraction boiling above about 350 F., contactin said higher boiling fraction with an oxidizing sweetening reagent, combining the treated higher boiling fraction with the lower boiling fraction, contacting the mixture with an fractions, one boiling from 100 to about 350 F.

and the second from 350? to 450 F. This highboiling fraction was treated according to conventional methods of doctor sweetening. A delayed doctor test on the so-treated stock showed some sourness. This treated high-boiling stock was then mixed with the low-boiling sour fraction and the mixture given a normal plumbite treating. A sample of this latter treated stock was found to be fully sweet.

A "delayed doctor test is a sodium plumbite Another straight run petroleum naphtha was fractionated into two fractions boiling from 100 to 300 F. and 300 to 400 F.. respectively. The high-boiling fraction was treated with a solution of strong base and sodium polysulflde sweetening oxidizing sweetening reagent, and separating the treated oils from the treating agents.

3. The process or claim 2 in which the sweetening reagent comprises a solution of cupric chloride.

4. The process of claim 2 in which the sweetening reagent comprises an aqueous alkaline plumbite solution.

5. The process of claim 2 in which the sweetening reagent comprises a solution of a strong base and a polysulfide of the group consisting of the alkali and alkaline earth metals.

6. A process for refining hydrocarbon mixtures containing mercaptans which comprises fractionating the said hydrocarbon mixtures into relatively high-boiling and low-boiling fractions, contacting said high-boiling fraction with an oxidizing sweetening reagent, combining the treated high-boiling fraction with said low-boiling fraction, contactin the mixture with an oxidizing sweetening reagent, and separating the hydrocarbons from the treating agents.

7. A process of refining hydrocarbon mixtures containing mercaptans which comprises fractionating the said hydrocarbon mixture into relatively high-boiling and low-boiling fractions, contacting said high-boiling fraction with an oxidizing sweetening reagent to substantially sweeten said fraction, introducing said low-boilin fracing fraction, contacting the mixture with a sec ond oxidizing sweetening reagent, and separating the hydrocarbons from the treating agents.

HAROLD E. MESSMORE.

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