Process for the extraction of mercaptans from hydrocarbon oil

Patented Aug. 19, 1947

2,425,777

UNITED STATES PATENT OFFICE

2,425,777

PROCESS FOR THE EXTRACTION OF MERCAPTANS FROM HYDROCARBON OIL

Chester E. Adams, Highland, and Theodore B.

Tom, Hammond, Ind., assignors to Standard Oil Company, Chicago, Ill., a corporation of Indiana

No Drawing. Application August 22, 1945, Serial No. 612,127

Y is a simple whole number. Examples are the ethers of glycol- and polyglycol substituted aliphatic acids, and their corresponding thio and thioxy derivatives. For convenience, these compounds are termed herein “polyether acids,” their common characteristic being a chain of at least two ether or thioether linkages in combination with an organic acid radical. The simplest compound of the class is methoxymethoxy-acetic acid and its corresponding thio derivative, methiomethio-acetic acid; likewise methiomethoxyacetic acid and methoxymethio-acetic acid may be employed. Ethioethoxy-acetic acid and ethoxyethoxy-acetic acid are both excellent solu15 tizers. These solutizers generally contain from to 10 carbon atoms.

Polyethoxy acids may be conveniently prepared from glycol and its derivatives. Thus, glycol monomethyl ether (methylcellosolve) is readily 20 condensed with monochloracetic acid by first treating with sodium methylate and driving off the methanol in a vacuum..

Besides the polyethers and thioethers of acetic acid, we may also employ similar derivatives of 25 higher molecular weight acids, e. g. propionic and butyric acids, as well as the dibasic acids such as malonic and succinic. We may also employ the polysubstituted polyethers, the simplest of which are bismethoxy succinic acid and bismethoxy 30 acetic acid. Likewise we may employ the dibasic acids wherein the polyether linkage joins the two acid radicals as in glycol diacetic acid

C₂H4(OCH2CO₂H)2 _3r which may be prepared by condensation of the ' '' sodium derivative of glycol with sodium monochloracetate followed by liberation of the acid from its salt. Also we can use such compounds as the following:

⁴⁰ CO2H—CH2O—C2H4—O—C2H4—O—CH2CO2H the generic formula of which is

CO2H—RO—(R—O) n—RCO2H where R is an alkylene radical and 11 is a whole number.

In carrying out our process, the hydrocarbon distillate containing mercaptans or other weakly acidic material is contacted preferably in a 50 countercurrent extraction tower with a strong alkaline solution of the polyether 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 55 that combined with the solutizer agent. The

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, naphtha, gasoline, 5 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- 10 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 sulfur 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 and 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 substantially nonvolatile and which will not be lost from the treating solution by dissociation and evaporation. A still further object of the invention is to provide a solutizer which is sufficiently effective in removing mercaptans from napthas to produce a naptha which will pass the doctor test without further treatment.

According to our invention, we employ for the treatment of hydrocarbon distillates alkaline solutions of solutizers having the following generic formula:

RiX—(R2X) y—R3CO2H in which Ri is an alkyl, substituted alkyl, or hydrogen radical, R2 is an alkylene radical, Rs is an alkylene or substituted alkylene radical, X is either oxygen or sulfur, preferably oxygen, and

2,425,777 concentration of solutizer employed will usually be within the range of about 5 to 40 per cent on the basis of free polyether acid in the alkali solution. Usually the polyether concentration falls within the range of 1.5 to 4 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 carboxy ether 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.

The regeneration reaction may be accelerated by employing catalysts which have the ability to speed the oxidation of the mercaptans. For this 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 5 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 10 and our polyether 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 15 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, par20 ticularly 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 polyether acids and a suitable proportion of polyether acids is a molar 25 ratio to the other solutizer of 0.2 to 4.

The following table gives the results obtained in the extraction of a soui’ naptha containing primarily butyl mercaptan as an example of a weakly acidic substance. Removal of the butyl 30 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 particular solutizers employed are in35 dicated in the table. The copper number of the naptha treated was 100 before extraction.

Test No.

Composition of Solutizer Solution

Cu No. of Naptha After Extraction

Butyl Mercaptan Distribution Coefficient, Kq

Comments

2.

3.

4.

5.

6N KOH, 3N CHjOCHjCOOK________________

2N CH3OCH2COOK, N K cresyiate

BN KOH, 3N C2H5OCH2CH2OCH2COOK

4N KOH, 2N C2HSOCH2CH2OCH2COOK, NK cresyiate””

3N KOH, 2N C2H₅OCH2CH₂OCH2COOK, 1.5 N K cresyiate

6_______

7_______

8_______

9_______

10______

6N NaOH, 2N C2H₅OCH2CH2OCH2COONa

4N NaOH, 2N OiHiOOHzCHjOOHiCOONa, N Na'cresvia'te-----BN KOH, 2N C2HSSCH2CH2OCH2COOK

4N KOH, 2N O₂HsSCH₂OH₂OCH₂OOOK NK’cresyiate’

3N KOH, 2N O₂H_S8CH₂CH₂OCH₂COOK 1.5 N K:Ue_SytateJ”””

Reuse: Cu No. 100 -----> 1.5.

Reuse: Cu No. 100 -----, 2.5.

1% entrainment. Emulsion broken with Tretolite. Reuse: Cu No. 100 -----> 2.

Reuse: Cu No. 100 -----> 4

Reuse: Cu No. 100 -----> 3.5.

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 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 avoiding the cost of heating and cooling large volumes of solution. Corrosion of equipment is also reduced.

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

The efficiency of extraction is indicated by the coefficient, Kq which is the concentration of mer55 captans in the caustic solution divided by the concentration remaining in the naptha.

The copper number referred to above is determined by titrating a 100 ml. sample of naptha with standard ammoniacal copper solution, 1 ml. 60 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.

As a general rule when the concentration of ⁶⁵ mercaptans in a naptha stock falls below about one copper number it will exhibit a negative doctor reaction and therefore may be technically considered “sweet.” It will be noted that several of the extractions described hereinabove effected 70 a reduction of copper number below one illustrating the high efficiency of our new solutizers and their usefulness in producing acceptably sweet stocks on extraction in only a single stage. In general, however, multistage extraction is em75 ployed, for example, countercurrent tower ex2,425,777 tracting said distillate countercurrently with a solution of an alkali metal hydroxide containing about 3 to 6 normal free alkali and about 1 to 3 normal polyether acid in the form of its alkali metal salt.

10. The process of removing mercaptans from a sour hydrocarbon distillate which comprises extracting said distillate with a solution of an alkali metal hydroxide containing glycol diacetic 10 acid in the form of its alkali metal salt in a concentration of at least 0.5 normal.

11. In the process of removing mercaptans from a sour petroleum distillate wherein said distillate is contacted with a solution of an alkali 15 metal hydroxide in which the mercaptans are dissolved, the alkali solution is separated from the distillate and regenerated by the action of an oxygen-containing gas whereby the mercaptans are converted to disulfides insoluble in the said 20 alkali solution, the said disulfides are separated from the alkali solution and the resulting solution is returned to the contacting step for removal of mercaptans from additional amounts of petroleum distillate, the improvement comprising 25 increasing the solubility of mercaptans in said alkali solution by adding thereto a solutizer material comprising an alkali metal salt of a polyether acid.

12. In the process of removing mercaptans 30 from a hydrocarbon distillate wherein said distillate is contacted with a solution of an alkali metal hydroxide containing a small amount, of traction, in which case acceptably sweet stocks are obtainable by the use of our new solutizer over a wide range of conditions either alone or in combination with other solutizers such as the phenols exemplified by cresylic acid. It some- 5 times may be more economical practice to remove the major portion of the mercaptans from the stock by solutizer extraction and then complete the sweetening in a further separate sweetening operation with lead plumbite doctor, hypochlorite, or other well-known sweetening method to produce a stock which will pass the doctor test where that is desired.