US2550091A - Mercaptan removal from hydrocarbons - Google Patents

Description

April 2\4, 1951 J. E. SEEBOLD MERCAPTAN REMOVAL FROM HYDROCARBONS Filed Aug. ll, 1948 mw Ems for being recycled.

Patented Apr. 24, 1951 UNITED STATES PATENT OFFICE MERCAPTAN REMOVAL FROM HYDROCARBONS James E. Seebold, Chicago, Ill., assignor to Standard Oil Company, Chicago, Ill., a corporation of Indiana Application August 11, 1948, SerialNo. 45,041

'l Claims. (Cl. IBG-32) tion comprising a lower aliphatic alcohol and a caustic base, such as sodium hydroxide. More particularly, it relates to an improvement in the aforesaid process whereby the alcohol used in the extraction operation may be substantially freed from mercaptans and rendered suitable In an especially advantageous embodiment, my invention may be used to improve the burning characteristics of sour heater oils derivedl by distillation of crude petroleum or cracked petroleum products.

Many types of crude petroleums contain substantial quantities of sulfur in the form ofy mercaptans, disuldes, heterocyclic sulfur compounds, and the like. These materials are in general highly objectionable in rened petroleum products, owing to their unpleasant odor, corrosive qualities, and other undesirable characteristics. In heater oils,`for example, the presence of small proportions of high-boiling sulfur compounds results in the rapid deposition of carbon or coke when burned, especially when burned in domestic sleeve-type space heaters, in which the vaporizing grooves of the burner ten-:l to ll with 'the carbon. so that the oil flou7 is retarded, and

more frequent shut-downs for cleaning are required. Heater oils derived from. sour crudes are ofthis type, since, they ordinarily contain substantial proportions of mercaptans, which result in unsatisfactory burning qualities. Moreover, when such oils are doctor-sweetened to remove the mercaptans, a part or all of the mercaptans are converted to disulides, which have a considerably higher boiling range than that of the oil, from which, they were derived. These high boiling disuliides are particularly undesirable in heating oils destined for use in critical space heaters, as the following tests demonstrate:

Fifty-gallon quantities of several heater oils of varying disulfide content were 1burned in a Jungers Model C sleeve-type space heater at a draft equivalent to 0.03 inch of water pressure and a iiow rate of 0.3 gallon per hour. Under these conditions, an oil which deposits 6 grams or. less. of carbon on the heating surfaces in the course of a 50-gallon run is considered accept- 2 able. The results of the tests are set forth in the following table:

` gig/tlml Refined Oil Treatment 1,1 er' calgan Mercap- Disul' Carbon tan No. fide No. deposited Grams None 90 0 4.0 Doctor-sweetened 89 0 89 20. 3 Caustic-extracted to Mercaptan No. of 7; then doctoredswectened 66 0 7 5. 9

In the table, the mercaptan number (sometimes called the copper number) is the milligrams of mercaptan sulfur per milliliters of oil, generally determined by titration with a standardized copper-salt solution. The disuliide number is the milligrams of disulfide sulfur per 100 milliliters of oil; this number is determined by reducing the disulfides to mercaptans with .zinc and hydrochloric acid, and then determining the mercaptan number. The foregoing data demonstrate that it is necessary either to remove the disulfldes from the oil or to prevent their formation during refining operations and even during storage, where disuli'ldes may be produced by atmospheric oxidation ofV mercaptans.

Similar or related problems are encountered with other sulfur-containing hydrocarbon fractions obtained from sour crudes. This invention is concernedV with a process for avoiding such diiiiculties by extraction of mercaptans, RSI-I from hydrocarbon stocks before the mercaptans have become oxidized to disuliides, RSSR, where R represents the same or diiierent hydrocarbon radicals.

The prior art discloses numerous methods for extracting mercaptans from petroleum oils. In an especially effective process, the petroleum oil is extracted with an aqueous solution comprising a lower aliphatic alcohol such as methanol and a caustic alkali such as sodium hydroxide or potassium hydroxide. Mercaptans are freely soluble in such extractant solutions, and are readily removed thereby from hydrocarbon charging stocks comprised thereof. The resulting extraction solution may subsequently be stripped of all or thev greater part of its alcohol content. After removal of the mercaptans, the regenerated caustic liquor may be recycled, in combination with the recovered alcohol.

A major difficulty with the caustic-alcohol process lies in the fact that lower aliphatic alcohols form azeotropes with mercaptans, and the recovered alcohol is therefore contaminated with a substantial proportion of mercaptans. Alcohols contaminated in this manner are of course unsuitable for recycling, since they could not be used to produce a substantially mercaptan-free refined stock. I have now devised a means for avoiding this difficulty by fractionally distilling, in the presence of an oxidizing gas or oxygen-afording gasiform stream, the alcohol separated from the mercaptan-rich extract. VIn an especially advantageous embodiment of my invention, I distill the mercaptan-contaminated alcohol in a fractionating column while introducing into an upper section thereofran oxidizing gas. Any mercaptans within the upper section of the column are converted thereby into disuldes, from which the alcohol readily distills in purified form. The

- resulting disuliides flow downward through the column and further purify the ascending alcohol vapors by extracting mercaptans therefrom. If

desired, this conversion of rnercaptans to disul- Vdes may be facilitated by introducing a small quantity of an alkaline agent, such as caustic or ammonia, to the treating zone.

One object of my invention is to provide an improved method for sweetening hydrocarbons. Another object of my invention is to separate mer- Acaptans from lower aliphatic alcohols in an especially ei'icient and convenient manner. A further object of my vinvention is to purify a causticlalcohol sweetening solution by means of an oxidative fractional distillation method. Other objects of my invention and its advantages over the prior art will be apparent from the description and claims.

It is to be understood that my invention is applicable broadly to the separation of mercaptans vfrom hydrocarbons, including aliphatic, aromatic, and Ynaphthenic hydrocarbons, and mixtures thereof. My invention is especially useful in connection with the purification of petroleum fractions, such as kerosene, gas oil, fuel oil, gasoline, naphthas, and the like.

It is also to be understood that my process is applicable broadly to the purification and regeneration of simple caustic-alcohol sweetening solutions and to various modifications thereof, as disclosed in the prior art, including solutions which contain variousv solutizers such as salts of phenols, cresols, carboxylic acids, and other organic acids. Suitable caustic alkalies include the class of alkali-metal hydroxides, preferably sodium Yhydroxide or potassium hydroxide. The alcohol should be a lower aliphatic alcohol, preferably. methanol. Ethanol, isopropyl alcohol, or n-propyl alcohol can be used.

In the sweetening of sour hydrocarbon streams with caustic-alcohol solutions, the sour charging stock is rst extracted with an aqueous causticalcohol solution wherein the proportion of caustic in the aqueous caustic portion of the solution is preferably within the range of about 35 to 60 percent by weight and the proportion of aqueous caustic in the total solution is preferably between about and 80 percent by weight, the remainder being substantially all alcohol, exclusive of solutizer. Somewhat smaller proportions of water may be employed with potassium hydroxide than for removing mercaptans than sodium hydroxide solutions. Precautions should be taken to prevent contacting the caustic solution with carbon dioxide, since carbonates are formed thereby, unbalancing the composition of the solution and necessitating auxiliary lter equipment for their removal.

From the extraction step is withdrawn a refined hydrocarbon phase, lean in mercaptans, and an aqueous caustic-alcohol extract, rich in mercaptans. The latter may then be exhaustively stripped to separate substantially all mercaptans and alcohol therefrom, and the vaporous stream of mercaptans and alcohol is distilled in a fractionating column in the presence of an oxidizing gas, whereby the mercaptans are converted wholly or partly into disuldes, and the alcohol is withdrawn overhead in puried form, substantially free from mercaptants, or containing a greatly reduced proportion thereof. I prefer to use air as the oxidizing gas, but I may use oxygen, with or without diluents, and I may also use other oxidizing agents capable of converting mercaptans to disuliides, such as hydrogen peroxide. The distillation is preferably carried out at ordinary pressure and at the temperature required to operate the column at such pressure, generally from l0 to 25 F. above the boiling point of the alcohol. However, somewhat elevated pressures and temperatures may also be used, for example pressures up to around 200 pounds per square inch or somewhat higher, and corresponding temperatures.

Alternatively, I may fractionally distill the mercaptan-rich aqueous caustic-alcohol solution and separate therefrom all or substantiallly all of the alcohol, contaminated with mercaptans. The alcohol overhead stream may then be purified by oxidative distillation as described above. Thereafter, the aqueous caustic phase is freed from mercaptans by a further stripping operation, and is recycled t0 the extraction step in combination with the purified alcohol.

Ordinarily, in the stripping or fractionation of the mercaptan-rich aqueous caustic-alcohol solution, it is desirable to operate under rigidly controlled conditions such as will produce a substantially anhydrous overhead stream, since the proportion of mercaptans in the overhead stream is very greatly increased by operating under conditions which permit as litte as one percent of Water to be distilled simultaneously therewith. My process, however, makes it unnecessary to employ such conditions, since I am able to remove even much larger proportions of mercaptans with savings in costs and reduction of operating diiculties.

My invention will be more fully understood from the attached owsheet, in which I have carefully set forth and described an advantageous embodiment thereof, together with illustrative alternatives.

A sour hydrocarbon stream, such as a sour heater oil boiling in the range of about 350 to 600 F. is charged to process by way of line I0 leading to mixer II and settler I2, in which an aqueous solution of an alkali is employed to remove hydrogen sulfide and carbon dioxide. For this purpose, a dilute solution of caustic soda is satisfactory. The caustic solution canbe recycled by pump I3 from settler I2 to mixer Il and discarded when spent.

The washed hydrocarbon stream, now free from hydrogen sulide and carbon dioxide, is conducted by line I4 to mixer I5 and settler I6, the rst mercaptan extraction stage. The extraction may be carried out in a baffled tower employing countercurrent flow of hydrocarbon and causticalcohol solution; but satisfactory extraction can be carried out batchwise with as few as two t0 five stages, or with only two to four stages where potassium hydroxide solutions are used. The drawing illustrates a process employing two extraction stages.

In mixer I5, caustic-alcohol solutions are introduced by lines Il and I8. Therein, vthey are commingled with the hydrocarbon, and the mixture is allowed to stratify in settler I6. The quantities of fresh and recycled caustic-alcohol solutions are preferably adjusted to give a ratio of approximately 1:1 by volume of caustic-alcohol solution to hydrocarbon. Vigorous and intimate mixing should be carried out in mixer I5, suitably by use of an orifice-type mixer, a rapid agitator, or the like. The mixing and settling should preferably be carried out at a temperature below about 100 F., preferably around 90 F., and at autogenous or slightly elevated pressures, suitably from around atmospheric pressure to about 200 pounds per square inch. The causticalcohol solution from settler I6 maybe recycled by pump I9 to mixer I5.

Treated hydrocarbon from the first extraction stage is conducted by line to mixer 2|, where it is commingled with another batch of causticalcohol solution, supplied through line 23 and recycle line 24. The mixture then flows into settler 22, where stratification takes place, and the separated caustic-alcohol phase is recycled by pump 25.

The treated hydrocarbon phase from settler 22, now substantially free from mercaptans, is conducted by line 26 to mixer 2l, where the hydrocarbon stream is washed -with water to remove the small proportion of alcohol dissolved therein. The quantity of dissolved alcohol is ordinarily quite small, usually from `about .2 to 5 percent .by volume or less, owing to the essentially oleophobie nature of the alcohols employed in the extraction process; however, it will be apparent that the recovery thereof is an important economic consideration. Wash water is supplied to mixer 21 by line 29, and a portion of the wash water from settler 28 may be recycled by pump 30 to mixer 21, if desired, to increase the volume of `water for contacting the hydrocarbon stream. A rened, substantially mercaptan-free hydrocarbon stream is `withdrawn from settler 28 through line 3 I.

The mercaptan-rich caustic-alcohol solution, sometimes called the spent caustic solution, is withdrawn from the first extraction stage by line 32 through heat exchanger 33 and heater 3, where its temperature is raised to around 210 F. or somewhat above. The hot caustic-alcohol solution is then introduced through line 35 into an intermediate point of stripper column 3B, where the alcohol and mercaptans are substantially completely removed by the action of reboiler '31 and taken overhead as a vaporous stream, ordinarily with a, substantial quantity of water. Water or free steam, preferably superheated steam, may be added at a low point in the stripper if desired to assist in removingfthe mercaptans from the caustic solution. For this purpose, the ywash water from settler 28, containing recovered alcohol, may be supplied by line 33 to reboiler 31, where it is vapori'zed and the alcohol is stripped out. The temperature of the liquid in the base of stripper 36 is preferably within the range of about 250 to 350 F. From the bottom of 'the stripper, a hot'stream of lean aqueous caustic solution emerges-now substantially free from mercaptans, and is recycled through line 39, heat exchanger 33, coolerA, and line 23 to mixer 2I. .y

From the top of stripper 36 vapors of alcohol, mercaptans, and water are withdrawn by line 4I, and are introduced into an intermediate point of alcohol purier column l|12, equipped with reboiler 43. Therein, the stream is fractionally distilled, the water and the major proportion of the mercaptans passing downward, and the alcohol, contaminated with a small proportion of mercaptans, for example around .2 to 5 percent, passing upward. In order to purify the alcohol fraction, a stream of an oxidizing gas such as air is introduced through line 44 into an upper intermediate section of column 42, above the entry point of line 4I, and preferably 'at the lowest point in the column at which the mercaptanalcohol azeotrope composition exists. The oxidizing gas serves to convert substantially all mercaptans reaching the upper section of column 42 into the corresponding disulfides, from which the alcohol readily distills in pure form, because of the fact that the disuldes have much higher boiling points than the corresponding mercaptans. The disuldes therefore flow downward within column 42, wh-ere they perform the additional function of extracting mercaptans from the rising vapor stream, owing tothe high solubility of mercaptans in disulfides. Y`

The purified alcohol vapors emerge overhead through condenser l5 into reflux drum 46, from which a portion is reuxed to the top of column 42 through line M, and the remainder is withdrawn and recycled through lines 48 and 23..to mixer 2l. Gases are vented rfrom reflux drum `46 through line 49, optionally through a scrubber or an activated charcoal adsorber to recover alcoholl vapors contained therein.

From the bottom of column 42 emerges a heterogeneous mixture of mercaptans, disulfides, and water. This mixture flows through line 50 into decanter 5I, from which an organic phase comprising substantially all of the mercaptans and disuldes is withdrawn through line` 52. This phase may be subjected to further proces-sing, such as by fractional distillation, to separate the components thereof in substantially pure form. The aqueous phase from decanter 5I is substantially free from sulfur compounds, owing to the low solubility of the latter in water. The aqueous phase may therefore be withdrawn and recycled through line '29 to mixer 21. Excess water may be withdrawn and discarded through line 53.

It will be apparent that the process set forth in the foregoing flowsheet may be modified in numerous Ways without departing from the spirit of my invention. As an illustration, the mercaptans and disuldes may be withdrawn via line 54 from column 42 at trapout plate 55, located in the lower section of the column above or below the entry point of line 4I. The trapout plate Should preferably be located at a point sufficiently low in the column that substantially all of the alcohol has been removed from the descending liquid stream. This embodiment has the advantage that the aqueous stream reaching the bottom of column 42 may be stripped entirely free of mercaptans by reboiler 43, and decanter 5| may be dispensed with.

As a further alternative, caustic stripper 36 may be operated under suitable conditions to 7 produce a vaporous overhead stream comprising essentially alcohol with only a small proportion ofV mercaptans, for examplel around 1 to 5 percent. This can be done vconveniently by withdrawing mercaptans from trapout plate 56 through line 51. The'vapor vstream emerging overhead through Vline 4"! is purified in column 42 as hereinbefore described. In this embodiment, however, only a comparatively small proportion of the total sulfur compounds derived from the sour charging stock emerge from the bottom of column 42; consequently the load on column 42 is substantially reduced.

Typical heater oils, reiined by caustic methanol extraction, have the following properties:

Gravity, API 39 to 43 Color, Saybolt Universal 8 to 18 Flash, TCC 115 to 150 Mercaptan No Copper Strip Corrosion, 3 hours at Y 212 F negative Distillation, ASTM:

Initial boiling point F 332 10% F- 360 50% F 438 90% F 527 Maximum F 574 In a specic application of my invention, the total extract obtained in the caustic-methanol extraction of a sour heater oil was distilled in a column. When such an extract was fractionally distilled. without treatment with an oxidizing gas according to my invention, a methanol fraction was obtained overhead having a mercaptan number around 300, corresponding to approximately 2 percent by volume of mercaptans. When 1863 parts by weight of such an extract were fractionally distilled while injecting approximately cubic feet of air per hour per square foot of column cross section into the middle portion of the column, a puried methanol stream was obtained having a mercaptan number of only 51, corresponding to a mercaptan content of only 0.33 percent by volume.

While I have described my invention with reference to a specic example, it is to be distinctly understood that I am not limited to the charging stock, operating conditions, and manipulative steps set forth therein. My invention is to be construed broadly within the limits of my disclosure, andY any modifications or -equivalents that would ordinarily occur to one skilled in the art are to be considered as lying within the scope ofmy'invention.

8 In accordance with the foregoing description, I claim as my invention:

l. A process for removing mercaptans from a mercaptan-containing lower aliphatic alcohol obtained by distillation from a caustic-alkali solution spent in desulfurizing a sour petroleum hydrocarbon which comprises fractionally distilling said mercaptan-containing lower aliphatic alcohol in lcontact with an oxidizing gas stream and withdrawing a vaporous alcohol stream of substantially reduced mercaptan content. Y

2. The process of claim 1 wherein said lower aliphatic alcohol is methanol.

3. The process of claim 1 wherein said lower aliphatic alcohol is ethanol.

4. The process of claim 1 wherein said lower aliphatic alcohol is isopropyl alcohol.

5. The process of claim 1 wherein said oxidizing gas is air.

6. A process for removing mercaptans from a mercaptan-containing lower aliphatic alcohol obtained by distillation from a caustic-alkali solution spent in desulfurizing a sour petroleum hydrocarbon which comprises distilling said mercaptan-containing lower aliphatic alcohol in a fractionating zone, introducing into an upper portion of said zone a stream of an oxidizing gas, and withdrawing from the top of said fractionating zone a vaporous alcohol stream of substantially reduced mercaptan content.

7. A process for removing mercaptans from mercaptan-contaminated methanol obtained by distillation from an aqueous caustic-alkali solution spent in desulfurizing a sour petroleum hydrocarbon which comprises distilling said mercaptan-containing methanol in a fractionating column, introducing into an upper portion of said column a stream of air, and withdrawing from the top of said column a vaporous stream of methanol substantially reduced in mercaptan content.

JAMES E. SEEBOLD.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,437,348 Brown et al Mar. 9, 1948 2,464,019 Bond et al. Mar. 8, 1949 2,488 O00 Bernard Nov. 15, 1949

Claims (1)

1. A PROCESS FOR REMOVING MERCAPTANS FROM A MERCAPTAN-CONTAINING LOWER ALIPHATIC ALCOHOL OBTAINED BY DISTILLATION FROM A CAUSTIC-ALKALI SOLUTION SPENT IN DESULFURIZING A SOUR PETROLEUM HYDROCARBON WHICH COMPRISES FRACTIONALLY DISTILLING SAID MERCAPTAN-CONTAINING LOWER ALIPHATIC ALCOHOL IN CONTACT WITH AN OXIDIZING GAS STREAM

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