US2508817A - Sweetening light hydrocarbon oils - Google Patents

Description

May 23, 1950 L. T. DEVOL EI'AL SWEETENING LIGHT HYDROCARBGN OILS Filed April 18, 1947 3.52% 2 Ettatut Iv. E

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mia ifibomomk INVENTORS= LEE 7? DEYOL, WILLIAM J. Ann,

Patented May 23, 1950 2,508,817 SWEETENING LIGIIT HYDBOCARBON OILS Lee T. Devol and William J. Ayers, Associated, Calit, assignors to Tide Water Associated Oil Company, San Francisco, Calif., a corporation oi Delaware Application April 18, 1947, Serial No. 742,442

9 Claims. (Cl. 196-29) This invention relates to the treating of light hydrocarbon oils of the type suitable for use as motor fuels and containing objectionable amounts of mercaptans, for the purpose of reducing the mercaptan content. It has for its principal object the reduction of the mercaptan content, or "sweetening as this practice is commonly called, of light hydrocarbon oils in a simple and ellicient manner.

Another object is to provide a catalytic meth-- od for converting mercaptans contained in light hydrocarbon oils into-less objectionable comp unds.

An object of a particular embodiment of the invention is to provide a method whereby light hydrocarbon oils containing mercaptans may be simultaneously sweetened and inhibited against oxidation.

An object of another embodiment of the invention is to provide a finishing treatment whereby light oils which have been treated by certain conventional sweetening processes but which still contain small but objectionable amounts of mercaptans may be rendered satisfactory for market.

Other objects will be apparent from the following description.

In the manufacture of light hydrocarbon oils for motor fuels of the gasoline type by modern cracking methods, substantial amounts of mercaptans are formed when sulphur-containing heavy oils are used for cracking stocks. This is generally true whether the cracking is carried out in the presence or absence of cracking catalysts. In the subsequent fractionation oi the cracked products, the mercaptans become concentrated into the light fractions normally used for gasoline. Usually the mercaptans are mostly concentrated in that portion of the naphtha which has a boiling range of approximately 140 F. to 250 F. The mercaptans impart a highly objectionable odor to the naphtha and must be removed, or converted to less objectionable substances, before the naphtha is suitable for blending into a marketable'gasoline.

Various processes are in use for the purpose of removing the mercaptans or for converting them to less odorous disulfides. All of these processes have, to a greater or lesser degree, the disadvantage of requiring extensive equipment for handling large volumes of treating agent. In some of these processes relatively expensive treating or revivifying agents are used and occasionally large losses of these reagents are encountered. Some of the proceses involve the addition to the naphtha undergoing treatment of carefully controlled amounts of substances, such as free sulphur for example, which in excess would be injurious to the quality ot-the treated naph-. tha. These disadvantages are obviated by the present invention. a

In accordance with the invention mercaptans contained in light hydrocarbon oils are oxidized, principally to disulfides, by means of oxygen dissolved in the oil in the presence of small amounts of an oil-soluble N-alkyl derivative of paraphenylene diamine, which apparently acts as a catalyst for the reaction although the exact mechanism is not understood. Excellent results are obtained with NN'disecbutyl paraphenylene diamine and, accordingly, this is the preferred catalyst. However, even paraphenylene diamine itself, the oil-solubility of which is very limited, exhibits a catalytic effect commensurate with the minute amount which can be dissolved and thereby be made available for'catalytic action.

In practicing the invention suflicient oxygen is dissolved in the mercaptan-containing oil to-react with the mercaptans present. This is accomplished by contacting the oil with pure oxygen, air, or other free-oxygen containing gas. Either before, after, or simultaneously (but preferably before) contacting the oil with oxygen, th desired amount of catalyst is added to and thoroughly mixed with the oil. The 011 containing oxygen and catalyst in solution is then allowed to stand for a suillcient time to permit the desired amount of conversion ofv mercaptans by oxidation, after which it is passed to storage or blended with other components toform a finished fuel.

Depending upon the temperature, the amount of catalyst present, the concentration of oxygen dissolved'in the oil, the mercaptan content, and the degree of sweetening desired, the time of contact may vary from several minutesup to'a few ays.

Although the reaction proceeds with regularity at ordinary atmospheric temperatures, say "of about F., the time required for a given degree of sweetening may be reduced 40% to 60% by increasing the temperature of the oil to about F. Further reduction in time may be effecte'd at still higher temperatures, though added difllculties may be encountered due to the vapor pressures of the light naphthas.

The amount of catalyst to be used depends primarily on an economic balance between the cost ofthe catalyst and the gain accomplished by a reduction in the processing time. Ordinar- 11y, from about to about 250 pounds per 1000 barrels has been found to be effective. When the paraphenylene diamine derivative is a liquid, as is the case with the preferred disecbutyl compound, it may be added to the oil directly, or, if desired, in a concentrated solution in any desired solvent. If the derivative is a solid, it may conveniently be added in solution in alcohol, benzene or other suitable solvent.

The concentration of dissolved oxygen required for satisfactory operation may be attained by bringing the oil into approximate equilibrium with air. This may be accomplished. by injecting air into the oil or by permitting the oil to stand in storage in contact with air. Such concentration will ordinarily provide several times the quantity of dissolved oxygen theoretically required to complete the oxidation of all the mercaptans in most mercaptan bearing stocks. Greater speed of reaction may be obtained, however, by the use of pure oxygen or of a gas containing higher concentration of free oxygen than does air. In the use of high concentrations of oxygen, such as are obtained by saturating the oil with pure oxygen, certain fire hazards may be encountered, especially at elevated temperatures, which may render such concentrations undesirable.

The invention may be better understood by reference to the drawing which illustrates, in diagrammatic form, apparatus suitable for carrying out the invention. Sour naphtha (i. e. naphtha containing objectionable amounts of mercaptans) which may be taken directly from the fractionating system of the cracking plants or from storage, and which previously may have received a light wash with caustic soda solution to remove H28, and other substances more acidic than mercaptans, is fed to pump 2 through line I whence it is forced through lines 3 and 4 and through mixing device 8 (represented by mixing orifices 8') to tank 9 and thence through line It to storage. Prior to the entry of the naphtha stream into mixing device 8, the desired proportion of catalyst is added from tank 5 through valve 5 and proportioning pump I. Oxygen to effect the reaction is preferably supplied, in the form of pure oxygen, air or other gas containing free-oxygen, through line H immediately after the introduction of the catalyst in order that passage through the mixing device 8 may assist the solution of the oxygen in the naphtha. However, the oxygen may be added prior to the introduction of the naphtha into pump 2 (as is indicated by line I2) or it may be added to the gasoline in tank 9 (as is indicated by line l3). In the event that the naphtha has been in contact with oxygen prior to its entry into line I, as may be the case when the naphtha has been in storage, it is possible that it already contains sufiicient dissolved oxygen for satisfactory operation of the process. In such case the addition of oxygen at points ll, l2, or I3 may be unnecessary; however, the introduction of additional oxygen will probably prove advantageous in order to increase the quantity of dissolved oxygen and thereby increase the rate of reaction.

Tank 8 is of sufiicient capacity to permit the oxidation of the mercaptans to the desired degree. In practice tank 9 may conveniently be an intermediate storage tank from which stocks are removed for blending or for final storage.

When it is desired to carry out the process at a temperature above that of the naphtha entertraces of fatty acids,

ing pump 2, in order to increase the rate of the sweetening, heater H may be provided to which is supplied steam, or other heating medium, through line [5 and valve l6. Similarly, if it is desired to reduce the temperature of the naphtha leaving tank 9, cooler I! may be provided which is supplied with a cooling medium through line l8 and valve l9.

From the foregoing description it is apparent that the oil-soluble N'Ndialkyl paraphenylene diamine used as a catalyst is not recovered in the normal practice of the invention, but is retained in the sweetened naphtha. Although methods may be devised for its recovery, such recovery is not essential to the successful operation of the process. Due to the small amount of catalyst required for successful operation (from about 0.002% to about 0.06% by weight, though larger amounts can be used if desired) the cost of the catalyst is not excessive, and its presence is not detrimental to the finished gasoline. On the contrary, NN'disecbutyl paraphenylene diamine and related compounds are known to be excellent inhibitors against the formation of gum in gasoline, and the presence of such compounds in the gasoline in the concentrations used in the invention, after they have performed their purpose as oxidation catalysts for the conversion of mercaptans to disulfides, perform the added function of substantially inhibiting gum formation for extended periods of time. Thus, in the practice of the invention, cracked gasolines which normally tend to form gum in storage are simultaneously treated to reduce their mercaptan content and to inhibit the gum formation.

The conventional use of inhibitors of this type in the prevention of gum formation is not to be confused with their use in accordance with the invention. In the conventional practice the sour naphtha is first sweetened by one or more of the usual sweetening processes, and the inhibitor is added to the sweetened naphtha in sufficient amount to give the desired stability against gum formation. Contact of the naphtha with air is avoided, as far as possible, in order to prevent oxidation reactions tending to form gum. In contradistinction to the conventional practice, and in accordance with the invention, the inhibitor is added to the sour naphtha and the presence of dissolved oxygen is specifically provided for, in order to obtain the mercaptan reduction.

The invention is particularly adapted, though not restricted, to the treatment of naphthas containing relatively low (in the order of 0.002% of mercaptan sulphur) but objectionable amounts of mercaptans. Such naphthas may result from the cracking of low sulfur stocks, or they may result from the treatment of higher merca'ptan content naphthas by any one of several mercaptan removal processes which require a "finishing treatment in order to reduce the mercaptan content to an acceptable value An example of such a mercaptan removal process is the use of a caustic soda solution, with or without a so-called solutizer." Naphthas treated by such processes generally require a finishing treatment with the well-known doctor process, or other equivalent process, in order to obtain a satisfactory product. By the use of the present invention such low mercaptan naphthas are readily sweetened and simultaneously inhibited against gum formation.

Following are examples of the practice of the invention:

Example 1 A thermally cracked naphtha, derived from sulphur-bearing California crude oil residuum, with a boiling range of 100 F. to 250 F., and having a mercaptan content (as sulphur) of 0.02%, which had been first washed with a caustic soda solution of B. strength at the rate of 0.05 pound NaOH per 1000 barrels to remove traces of HzS and strong organic acids, was contacted with air, and 0.06% by weight of NN- disecbutyl paraphenylene diamine was added while the naphtha was at a temperature of about 125 F. The naphtha, thus treated, was passed to a receiving tank. After 24 hours the mercaptan content was reduced to 0.006%. After a second 24 hours the mercaptan content was 0.001%. Similar treatment of the same naphtha without the catalyst gave a final mercaptan content of 0.019% after 24 hours and 0.017% after 48 hours.

Example 2 larly to Example 1. After 24 hours the mercaptan content was reduced to 0.001%

Example 3 A thermally cracked naphtha similar to that in Example 1 but having an initial mercaptan content of 0.01% was contacted with pure oxygen and treated with 0.06% of the same catalyst at 120' F. After 6 hours the mercaptan content was 0.001%

In each of the foregoing examples the sweetened gasoline was tested by the ASTM method 1) 525-46 for gum stability and showed an induction period in excess of 300 minutes, indicating high resistance to gum formation in storage.

Example 4 5 diamine in a light hydrocarbon 011 containing mercaptans in the presence of oxygen dissolved in said oil.

2. The process according to claim 1 in which the light hydrocarbon oil is a cracked naphtha.

3. The process according to claim 1 in which the paraphenylene derivative is NN disecondarybutyl paraphenylene diamine.

4. The process of claim 1 in which the oil containing the dissolved paraphenylene diamine derivative and dissolved oxygen is maintained at a temperature above atmospheric until asubstantial amount of the mercaptans have become oxidized. Y

5. The step in the process of sweetening sour hydrocarbon oils which comprises dissolving a small amount of an oil-soluble NN dialkyl paraphen'ylene diamine in a light hydrocarbon oil and subsequently dissolving oxygen in said oil.

6. The method of accelerating the reaction of mercaptans with oxygen useful in the treatment of sour hydrocarbon oil which comprises oxidizing mercaptans dissolved in hydrocarbon oil with oxygen in the presence of a small amount of an oil-soluble NN dialkyl paraphenylene diamine dissolved in said oil.

'7. The process of simultaneously sweetening and inhibiting a sour hydrocarbon oil which comprises dissolving a small amount of an oil-soluble NN dialkyl paraphenylene diamine in a light hydrocarbon oil containing mercaptans and normally tending to form gum in storage, and storing the same in the presence of dissolved oxygen until the mercaptan content of said 011 has been reduced a substantial amount.

8. The process of oxidizing mercaptans and useful in the treatment of sour hydrocarbon oils which comprises: treating hydrocarbon oil containing mercaptans with NN dialkyl paraphenylene diamine while in the presence of dissolved oxygen, said amine being dissolved in said oil.

9. In the process of sweetening hydrocarbon oils which have been treated by conventional sweetening processes but which still contain small but objectionable amounts of mercaptans the finishing step which includes dissolving in the oil,

oxygen to oxidize the mercaptans and an oilsoluble NN dialkyl paraphenylene diamine to act as a catalyst for the oxidizing reaction, the catalyst being present in amount at least .002% by weight and being an excellent inhibitor against the formation of gum in oil, being suitable to remain in the sweetened oil.

1 LEE '1. DEVOL.

WILLIAM J. AYERS.

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

UNITED STATES PATENTS Number Name Date 2,015,038 Pevere Sept. 17, 1938 2,404,871 Van Ess July 80, 1948 2,426,087 Fetterly Aug. 19. 1947

Claims (1)

1. IN THE PROCESS OF SWEETENING LIGHT PETROLEUM HYDROCARBON OILS OF THE TYPE SUITABLE FOR MOTOR FUELS AND CONTAINING OBJECTIONABLE AMOUNTS OF MERCAPTANS, THE STEP OF DISSOLVING A SMALL AMOUNT OF AN OIL-SOLUBLE NN'' DIALKYL PARAPHENYLENE DIAMINE IN A LIGHT HYDROCARBON OIL CONTAINING MERCAPTANS IN THE PRESENCE OF OXYGEN DISSOLVED IN SAID OIL.

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