Mercaptan extraction

Patented Aug. 19, 1947

2,425,776

UNITED STATES PATENT OFFICE

2,425,776

MERCAPTAN EXTRACTION '

Chester E. Adams, Highland, and Theodore B.

Tom, Hammond, Ind., assignors to Standard Oil Company, Chicago, Hl., a corporation ol

Indiana

No Drawing. Application August 22, 1945, Serial No. 612,126

Claims.

(CI. 196—30) alkylene radical. We may also employ as solutizers compounds having a. plurality of thioether radicals and/or a plurality of carboxyl groups, examples of which may be written as follows:

R

R— c.OzM.

S— Ri—SR

R

R—C—COaM

Ri—SR

R R-i-COiM S-Ri-OR (RS)8—Rs—OOjM _E__S_R₁-S-Ri-&-R_I-CO_JM ! C0,M- Bi—S—Ri— CO2M

In this formula N is a whole number and R an aikyi radical or hydrogen, Ri is an alkylene radical and R2 is an aliphatic residue having any number of free valences depending on the value

In general, we have found that the thioether acids having from 3 to .6 carbon atoms are most effective although certain thioether acids of 8 to 10 carbon atoms may be employed, particularly when the acids are dibasic. An example of such is butyl thio succinic acid. Examples are methioaeetic acid, methiopropionic acid, methiobutyric acid, ethioacetic acid and ethiobutyric acid. It should be understood, of course, that in actual operation the acids are present as their alkali metal salts and we have found that ^the P°^ta®’ slum salts are in general more effective than the sodium salts. ,

In the extraction of mercaptans from light petroleum distillates, straight-run or cracked gasoline, heavy naptha, light naptha, absorption naptha, etc., we have found that the thioether acids above described, when used in combination with caustic alkali, exhibit a high extraction efficiency as indicated by the mercaptan distribution coefficient as hereinafter described. We have also found that the extraction_t efficiency is considerably increased by employing the thioethei acids in combination with other ^^ell'^k,^E0V7“ > tizers including the lower alcohols, the amines

This invention, relates to the extraction of weakly acidic substances from hydrocarbon solutions and more particularly it relates to the extraction of mercaptans from hydrocarbon oils such as petroleum distillates,, naptha, gasoline, kerosene, etc. In the manufacture of gasoline and other petroleum distillates known in refinery practice as “light oils,” it has long been a problem to remove therefrom ill-smelling sulfur compounds which are primarily mercaptans. Cer- : tain processes such as the doctor treating process have been employed to convert mercaptans into other sulfur compounds such as disulfides which are less objectionable from the standpoint of odor. However, this conversion does not decrease the sulf ur content of the gasoline and it has been found that the disulfides themselves are quite objectionable from the standpoint of gasoline detonation characteristics. .

Numerous processes have been devised for extracting the mercaptans completely, thus producing a gasoline of higher knock rating ana higher susceptibility to knock rating improvement resulting from the addition of tetraethyl lead. In general, these processes have employed strong alkali solutions in conjunction with certain mercaptan solubilizing agents known as solutizers.

One of the objects of this invention is to provide an improved solutizer for the extraction of weakly acidic substances from petroleum distillates. Another object of the invention is to provide a process of removing mercaptans more effectively from gasoline and other petroleum distillates by means of a solution which can be readily regenerated and recycled in the extraction operation. A further object of the invention is to provide a solutizer which is inexpensive and at the same time highly effective and which may be employed in combination with other solutizers. .

According to our invention we employ as a solutizer in our process the alkali metal salts of the thioether acids which may also be termed carboxythioethers. These compounds have the following structural formula:

RiS—R2—CO2—M in which M is an alkali metal, generally sodium or potassium, Ri is a hydrocarbon radical generally alkyl, and R2 is an alkylene or substituted

2,425,776 and certain of the organic acids, e. g. butyric acid, but we prefer to employ our new solutizer in combination with the phenols including the cresylic acids, etc.

In carrying out our process, the hydrocarbon distillate containing mercaptans or other weakly acidic material is contacted preferably in a countercurrent extraction tower with a strong alkaline solution of the thioether acid solutizer. For this purpose, sodium or potassium hydroxide may be used, preferably the latter, in a concentration of about 2 to 6 normal free caustic above that combined with the solutizer agent. The concentration of solutizer employed will usually be within the range of about 5 to 40 per cent on the basis of free thioether acid in the alkali solution. Usually the concentration of thioether acid falls in the range of 1 to 3 molar. The extraction step is preferably carried out at ordinary temperature, for example 40 to 100° F„ and the solutizer solution containing dissolved mercaptans or other weakly acidic substance is withdrawn from the extractor and separately regenerated after which it is recycled to the extraction step. The regeneration operation consists in removing from the caustic solutizer solution the dissolved mercaptans or other weakly acidic substances, and is preferably carried out by blowing with air, for example at a slightly elevated temperature, e. g. 120 to 130° F. It is desirable to avoid subjecting the solutizer solution to a temperature sufficiently high to accelerate materially hydrolysis of the thioether acid employed.

In the regeneration operation, part of the mercaptans may be removed by dissociation and evaporation and part by the oxidizing action of the air converting them to disulfides which are no longer soluble in the solutizer solution. These disulfides may be removed as an upper oily layer and, if desired, a hydrocarbon or other immiscible solvent may be employed to assist in their removal from the regenerated solutizer solution. The solutizer solution is then recycled to the continuous extraction step of the process for the accumulation of more mercaptans.

Test No.

1.

2.

3..

4..

5..

6..

7..

8..

9..

10.

or oxygen. One very significant benefit resulting from the use of these phenolic catalysts is that they enable regeneration to be carried out at a lower temperature, for example room temperature, thereby saving the cost of heating and cooling the solution and reducing corrosion.

In the treatment of certain petroleum stocks, particularly cracked gasoline containing small amounts of phenolic compounds, especially cresols, it is desirable to precede the extraction with an alkali washing step to remove part of the phenols, cresols, etc., thus preventing an excessive amount of such phenolic substances accumulating in the solutizer solution. In the treatment of such phenol-containing stocks, however, there is a distinct advantage to permitting a controlled amount of such phenols, cresols, etc., accumulating in the solutizer solution, and we have found that the combination of cresols and our thioether acids provides a very effective reagent for extracting mercaptans. We prefer to control the amount of cresols in the range of about 1 to 3 normal concentration. Inasmuch as the distillate extracted by such a solution has been found to carry away in solution a small amount of cresols in the concentration which is in equilibrium with that in the solutizer solution, and inasmuch as cresols exert a valuable antioxidant effect on the treated gasoline, particularly in the case of cracked stocks, it is clearly desirable to employ a solutizer solution containing cresols. Other solutizers besides cresols may be employed with our thioether acids and a suitable proportion is a molar ratio to the other solutizer of 0.2 to 4.

The following table gives the results obtained in the extraction of a sour naptha containing primarily butyl mercaptan as an example of a weakly acidic substance. Removal of the butyl mercaptan is indicated by the copper number of the naptha. Extraction was carried out in the ratio of one volume of solutizer solution to five volumes of naptha in a single batch extraction step. The composition of the solutizer solution is as indicated. The naphtha, before extraction, had a copper number of 100.

Composition of Solutizer Solution

Copper No. After Extraction

Butyl Mercaptan Distribution Coefficient, Kq

5.35N KOH, 1.65N C2H5SCH2COOK_______________

S2S’ ·83Ν C2H5SCH2COOK, 2N k cresylate-...

C2H5SCH2COOK, N K cresylate____

3N_K0H, 2N C2H5SCH2COOK, 2N K cresylate________

5ζ1·θ2Ν C2H5SCH2COOK, .96N K cresylate..

Na cresylate

5.2N KOH, 2.6N CH3SCH2COOK_______________

IS ^25’ CH3SCH2COOK, N K cresylate___________

ΛΧΤ ^θ^^Ν CH3SCH2OOOK, 1.5N K cresylate________

4N NaOH, 2N CHsSCHsCOONa N Na cresylate

2

5.5

162 245 245

162

162

245

The regeneration reaction may be accelerated by employing catalysts which have the ability to. speed the oxidation of the mercaptans. For this purpose certain alkali-soluble metal oxides may be employed, such as the oxides of lead, tin, etc., but we prefer to employ compounds of the character of phenols and polyphenols in accordance with the teaching of the U. S. patent of Pevere, 2,015,038 (September 17, 1935). According to this method there is added to the solutizer solution a small amount of a phenol or polyphenol such as hydroquinone, catechol, pyrogallol, gallic acid, tannic acid, etc., using a concentration of about 0.1 to 1 per cent, thereby greatly increasing the rate of regeneration in the presence of air

The efficiency of extraction is indicated by the coefficient, Kq, which is the concentration of mercaptans in the caustic solution divided by the concentration remaining in the naptha. The ^,J use of higher concentrations of the thioether acids was found to result in separation or salting out from the solution owing to insufficient solubility. Thus 6NKOH—3N thioether acid salted out in one test. When employed in combination 70 with cresylic acid, however, this difficulty is avoided and the distribution coefficient is still further increased.. Very little difficulty with separation and emulsification of gasoline by the alkaline solution was noted.

In countercurrent operation, it is easily possi5 ble to reduce the mercaptan concentration to a point at which the treated gasoline will react sweet to the doctor test, i. e. the standard test with sodium plumbite solution. It may often be more economical, however, to remove the major । part of the mercaptans from the hydrocarbon stock by extraction with mercaptan solutizer and subsequently complete the sweetening of the stock in a separate operation with doctor solution, hypochlorite treatment, etc. to produce a J stock which will pass the doctor test.

The copper number referred to above is determined by titrating a 100 ml. sample of naptha with standard ammoniacal copper solution, 1 ml. of which is equivalent to 1 mg. mercaptan sulfur per 100 ml. (1 copper number). The end point is observed when the blue color fails to be discharged after shaking.