Method for Reducing Mercaptans in Hydrocarbons

Abstract

A method for reducing mercaptan concentration in a liquid hydrocarbon, comprising: contacting a mercaptan-rich liquid hydrocarbon having a first concentration of mercaptan sulfur with a composition comprising an oxidizing agent and water wherein the molar ratio of the oxidizing agent to mercaptan sulfur in the mercaptan-rich liquid hydrocarbon is from 3:1 to 10:1; and separating the water from the liquid hydrocarbon to yield a mercaptan-depleted liquid hydrocarbon having a second concentration of mercaptan sulfur, the second concentration being less than the first concentration; wherein a major amount of mercaptan compounds in the mercaptan-rich liquid hydrocarbon are converted to at least one sulfur oxoacid or salt thereof, having the formula: [RSOx]nY wherein R is a hydrocarbyl group; x is an integer from 1 to 3; n is 1 or 2; and Y is hydrogen, an alkaline metal, or alkaline earth metal.

Description

TECHNICAL FIELD

The invention relates generally to methods for reducing mercaptan concentration in liquid hydrocarbons.

BACKGROUND

Some hydrocarbons, such as crude oil and jet fuel, contain significant amounts of mercaptans which may have an impact on the value of these hydrocarbon streams. As a result, such hydrocarbon streams are usually sold at a discount in the market. Thus, reducing the mercaptan content could substantially improve both the marketability and the value of such hydrocarbons.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to a method for reducing mercaptan concentration in a liquid hydrocarbon, comprising: contacting a mercaptan-rich liquid hydrocarbon having a first concentration of mercaptan sulfur with a composition comprising an oxidizing agent and water wherein the molar ratio of the oxidizing agent to mercaptan sulfur in the mercaptan-rich liquid hydrocarbon is from 3:1 to 10:1; and separating the water from the liquid hydrocarbon to yield a mercaptan-depleted liquid hydrocarbon having a second concentration of mercaptan sulfur, the second concentration being less than the first concentration; wherein a major amount of mercaptan compounds in the mercaptan-rich liquid hydrocarbon are converted to at least one sulfur oxoacid or salt thereof, having the formula:

[RSO_x]_nY

wherein R is a hydrocarbyl group; x is an integer from 1 to 3; n is 1 or 2; and Y is hydrogen, an alkaline metal, or alkaline earth metal.

DETAILED DESCRIPTION

The following terms will be used throughout the specification and will have the following meanings unless otherwise indicated.

“Liquid hydrocarbon” refers to crude oil or jet fuel.

“Crude oil” refers to all types of mineral oils found in nature. Crude oil includes oils obtained from wells, shale, rock and/or sand among others. The term “crude” or “crude blend” is used interchangeably and each is intended to include both a single crude and blends of crudes.

“Jet fuel” refers to hydrocarbons having a boiling range between 280° F. and 572° F. (138° C. and 300° C.).

“Mercaptan” refers to compounds of the general formula R—SH wherein “R” means a hydrocarbyl group and “SH” means a mercapto group.

“Hydrocarbyl” refers to hydrocarbyl radicals containing 1 to 48 carbon atoms including branched or unbranched, cyclic or acyclic, saturated or unsaturated species, such as alkyl groups, alkenyl groups, or aryl groups.

“Mercaptan-rich liquid hydrocarbon” refers to a liquid hydrocarbon having a mercaptan sulfur content of at least 200 ppm.

The unit “ppm” means parts per million.

The invention effectively reduces the level of mercaptan sulfur in a liquid hydrocarbon. A mercaptan-rich liquid hydrocarbon having a first concentration of mercaptan sulfur is contacted with a composition comprising an oxidizing agent and water wherein the molar ratio of the oxidizing agent to mercaptan sulfur in the mercaptan-rich liquid hydrocarbon is from 3:1 to 10:1; and separating the water from the liquid hydrocarbon to yield a mercaptan-depleted liquid hydrocarbon having a second concentration of mercaptan sulfur, the second concentration being less than the first concentration; wherein a major amount of mercaptan compounds in the mercaptan-rich liquid hydrocarbon are converted to at least one sulfur oxoacid or salt thereof, having the formula:

[RSO_x]_nY

wherein R is a hydrocarbyl group; x is an integer from 1 to 3; n is 1 or 2; and Y is hydrogen, an alkaline metal, or alkaline earth metal.

The Composition: The composition for oxidizing the mercaptans comprises an oxidizing agent and water. Examples of suitable oxidizing agent include, but are not limited to, hydrogen peroxide, hypochlorite salts, organic peroxyacids, percarbonate salts, and persulfate salts and mixtures thereof. The oxidizing agent is chosen such that it is soluble in water or in a water and low molecular weight alkanol mixture. Typical low molecular weight alkanols include methanol and ethanol. In one embodiment, the oxidizing agent is an alkali or alkaline earth metal hypochlorite salt. Examples of such salts include sodium hypochlorite, potassium hypochlorite, calcium hypochlorite, and magnesium hypochlorite. In one embodiment, the oxidizing agent is sodium hypochlorite. Aqueous sodium hypochlorite solutions are widely available commercially in varying concentration ranges (typically 1-15 wt. %).

The pH of the composition may be adjusted by adding a suitable base to the composition. Generally, the base is selected from alkali metal hydroxides and alkaline earth metal hydroxides. In one embodiment, the pH of the composition is from 7 to 14; in a second embodiment, from 8 to 12.

Method for Reducing Mercaptan Concentration in a Liquid Hydrocarbon: The concentration of mercaptan sulfur in a liquid hydrocarbon is dependent on the source. In one embodiment, the liquid hydrocarbon is a crude oil. In another embodiment, the liquid hydrocarbon is a jet fuel.

In one embodiment, the liquid hydrocarbon contains at least 200 ppm mercaptan sulfur; in a second embodiment, at least 300 ppm mercaptan sulfur; in a third embodiment, at least 400 ppm mercaptan sulfur; in a fourth embodiment, at least 500 ppm mercaptan sulfur; in a fifth embodiment, at least 600 ppm mercaptan sulfur; in a sixth embodiment, no more than 3000 ppm mercaptan sulfur.

A mercaptan-rich liquid hydrocarbon having a first concentration of mercaptan sulfur is contacted with a composition comprising an oxidizing agent and water wherein the molar ratio of the oxidizing agent to mercaptan sulfur in the mercaptan-rich liquid hydrocarbon is from 3:1 to 10:1, by means known in the art, such that a major amount of mercaptan compounds in the mercaptan-rich liquid hydrocarbon are converted to at least one sulfur oxoacid or salt thereof, having the formula: [RSO_x]_nY wherein R is a hydrocarbyl group; x is an integer from 1 to 3; n is 1 or 2; and Y is hydrogen, an alkaline metal, or alkaline earth metal. The term “major amount” is understood to mean greater than 50 percent by weight of mercaptan compounds in the mercaptan-rich liquid hydrocarbon. The composition can be introduced continuously or intermittently into operating gas or fluid pipelines. Alternatively, batch introduction can be used particularly for offline pipelines. The contact can be from 20° C. to 300° C.; in a second embodiment, from 20° C. to 100° C.: in a third embodiment, at room temperature. The amount of the composition to the mercaptan-rich liquid hydrocarbon is from 5:95 to 95:5 volumetric.

In one embodiment, at least 60 percent by weight of the mercaptan compounds in the mercaptan-rich liquid hydrocarbon are converted to the at least one sulfur oxoacid or salt thereof; in a another embodiment, at least 70 percent by weight of the mercaptan compounds in the mercaptan-rich liquid hydrocarbon are converted to the at least one sulfur oxoacid or salt thereof; in yet another embodiment, at least 80 percent by weight of the mercaptan compounds in the mercaptan-rich liquid hydrocarbon are converted to the at least one sulfur oxoacid or salt thereof.

The contact is for a sufficient time such that a major amount of mercaptan compounds in the mercaptan-rich liquid hydrocarbon are converted to at least one sulfur oxoacid or salt thereof having the formula: [RSO_x]_nY, wherein R is a hydrocarbyl group; x is an integer from 1 to 3; n is 1 or 2; and Y is hydrogen, an alkaline metal, or an alkaline earth metal. In one embodiment, the contact time is at least 30 seconds; in a second embodiment, at least one minute; in a third embodiment, at least five minutes; in a fourth embodiment, at least one hour; in a fifth embodiment, at least two hours; in a sixth embodiment, less than 24 hours.

Vigorous mixing is desired to minimize the formation of disulfides. Conventional methods in the prior art for removing mercaptans from hydrocarbons typically involve “sweetening” wherein mercaptans are oxidized to form disulfides. Light mercaptans (C₁-C₄) may be removed in an aqueous wash in this process but removal of heavy mercaptans (C₄₊) is less effective due to the poor water solubility of heavy mercaptans. Disulfides which are derived from heavy mercaptans may decompose back to mercaptans at high temperatures, for example, during the distillation process. In the present invention, a major amount of mercaptan compounds in the mercaptan-rich liquid hydrocarbon are converted to at least one sulfur oxoacid or salt thereof which are highly water soluble and therefore easily removed from the hydrocarbon stream.

The water can be separated from the liquid hydrocarbon in a phase separation device known in the art, resulting in a mercaptan-depleted liquid hydrocarbon having a second concentration of mercaptan sulfur, the second concentration being less than the first concentration. Suitable phase separation devices include, but are not limited to, cyclone devices, electrostatic coalescent devices, gravitational oil-water separators, and centrifugal separators.

In one embodiment, the mercaptan-depleted liquid hydrocarbon contains less than 50 ppm mercaptan sulfur; in a second embodiment, less than 40 ppm mercaptan sulfur; in a third embodiment, less than 30 ppm mercaptan sulfur; in a fourth embodiment, less than 20 ppm mercaptan sulfur; in a fifth embodiment, less than 10 ppm mercaptan sulfur; in a sixth embodiment, less than 5 ppm mercaptan sulfur; in a seventh embodiment, less than 1 ppm mercaptan sulfur.

Method for Reducing Halide Concentration: The use of halide-containing oxidizing agents, such as sodium hypochlorite, may produce a halide treated product having an organic halide impurity content, typically from about 40 to 4000 ppm. The presence of organic halides in such treated products may be undesirable. In one embodiment of the method for reducing mercaptan concentration in a liquid hydrocarbon, wherein the oxidizing agent is a halide-containing oxidizing agent, the mercaptan-depleted liquid hydrocarbon is further contacted with an aqueous caustic solution under conditions to reduce the halide concentration in the mercaptan-depleted liquid hydrocarbon.

In one embodiment, the aqueous caustic solution is selected from alkali and alkaline earth metal hydroxide solutions, and mixtures thereof. Examples include lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, and magnesium hydroxide solutions. In another embodiment, the aqueous caustic solution is a sodium hydroxide solution. In yet another embodiment, the concentration of caustic ranges from 0.001 N to 10 N. In one embodiment for mercaptan removal or reduction, the molar ratio of caustic to halogen-containing oxidizing agent is in the range of 0.05:1 to 1:1.

The mercaptan-depleted liquid hydrocarbon is contacted with an aqueous caustic solution by means known in the art. Methods may be conducted as batch, semi-continuous or continuous processes, as described above. In one embodiment, the contact is from 20° C. to 300° C.; in a second embodiment, from 25° C. to 200° C.; in a third embodiment, from 30° C. to 150° C.; in a fourth embodiment, from 70° C. to 100° C. Contact may be done with vigorous mixing. In one embodiment, the contact time is at least one minute; in another embodiment, at least five minutes; in yet another embodiment, at least one hour; in still yet another embodiment, at least two hours; in one embodiment, less than 24 hours.

In one embodiment, the amount of aqueous caustic solution to the mercaptan-depleted liquid hydrocarbon is from 5:95 to 95:5 volumetric from which halide is removed.

In one embodiment, the aqueous caustic solution further comprises a low molecular weight alkanol as a co-solvent. The low molecular weight alkanol may have straight or branched chain alkyl groups containing 1 to 4 carbon atoms. Examples of suitable alkanols include methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol and tert-butanol and mixtures thereof. Typically, the low molecular weight alkanol is ethanol. In one embodiment, the amount of lower alkanol is from 1-49 percent by weight based on the total weight of the solution; in another embodiment, from 5-25 percent by weight based on the total weight of the solution.

In one embodiment, the aqueous caustic solution further comprises a phase transfer catalyst. Examples of suitable phase transfer catalysts include, but are not limited to, quaternary ammonium salts, phosphonium salts and pyridinium salts. If used, the phase transfer catalyst may be present in an amount from 0.001 to 0.5 mole equivalents of caustic agent. In one embodiment, the phase transfer catalyst is a quaternary salt. In one embodiment, the phase transfer catalyst is cetyltrimethylammonium chloride.

In one embodiment, after caustic treatment, the mercaptan-depleted liquid hydrocarbon is recovered resulting in a mercaptan-depleted liquid hydrocarbon with a reduced halide concentration. Any means of separating the aqueous caustic solution from the mercaptan-depleted liquid hydrocarbon may be used. Examples include decantation, gravity separation, settler based on gravity, extractor and others that are known in the art.

EXAMPLES

The following examples are given to illustrate the present invention. It should be understood, however, that the invention is not to be limited to the specific conditions or details described in these examples.

Examples 1-5

A high mercaptan sulfur crude blend (RSH=400 ppm) was treated with an aqueous sodium hypochlorite solution for several minutes with vigorous stirring. The layers were allowed to separate and the treated crude oil was collected. The mercaptan sulfur concentration in the treated crude oil was determined by UOP Method 163-67. The results are set forth in Table 1.

	TABLE 1
		NaClO
		Concen-		Reaction	Total RSH in
	NaClO/RSH	tration	Temperature	Time	treated crude
	mole ratio	(wt. %)	(° C.)	(min)	(ppm)
Ex. 1	3	2	23	2	<1
Ex. 2	6	2	23	2	<1
Ex. 3	6	1	23	5	<1
Ex. 4	6	1	70	5	<1
Ex. 5	6	1	70	10	<1

As shown, the mercaptan sulfur content of crude oils can be effectively reduced using the disclosed method.

Examples 6-9

A light jet fuel (boiling point range=350-450° F.) having mercaptan sulfur content of 644 ppm and a heavy jet fuel (boiling point range=450-550° F.) having a mercaptan sulfur content of 408 ppm were each treated with aqueous sodium hypochlorite solutions for 5 minutes at room temperature with vigorous stirring. The layers were allowed to separate and the treated jet fuel was collected. The mercaptan sulfur concentration in the treated jet fuel was determined by UOP Method 163-67. The results are set forth in Table 2.

	TABLE 2
		NaClO		RSH Content
		Concentration	NaClO/RSH	After Oxidation
	Description	(wt. %)	mole ratio	(ppm)
Ex. 6	Light Jet Fuel	1	6	<1
Ex. 7	Light Jet Fuel	5	6	<1
Ex. 8	Heavy Jet	1	6	<1
	Fuel
Ex. 9	Heavy Jet	5	6	<1
	Fuel

As shown, the mercaptan sulfur content of jet fuels can be effectively reduced using the disclosed method.

Aqueous Phase Analysis: The sulfur content in the treated oil and in the aqueous layer may be analyzed to determine the extent of conversion of mercaptan sulfur to sulfur oxoacids or salts thereof in the aqueous phase.

Example 10

A high mercaptan sulfur crude blend (RSH=644 ppm) was treated with a 1% aqueous sodium hypochlorite solution for several minutes at room temperature with vigorous stirring. The layers were allowed to separate and were collected. The total sulfur concentrations of the feed and the treated oil were determined by X-ray fluorescence. The total sulfur concentration in the aqueous was determined by ICP. The results are set forth in Table 3 as an average of five runs.

TABLE 3
Total sulfur in feed             0.24 g
Total mercaptan sulfur in feed   0.026 g
Total sulfur in treated oil      0.214 g
Total sulfur in water phase      0.0207 g
Total sulfur recovered           0.23 g
Overall sulfur recovery rate     97%
Sulfur in water as % of mercaptan sulfur in feed 80%

As shown, the results demonstrate that the total sulfur in the water represent an 80% reduction of mercaptan sulfur.

Examples 11-12

A high mercaptan sulfur crude blend (RSH=644 ppm) was treated with an aqueous sodium hypochlorite solution with vigorous stirring. The layers were allowed to separate and the aqueous layer was collected. Phenol was added into the aqueous phase to reduce sodium hypochlorite left in water solution. The aqueous layer was then evaporated at 80° C. under vacuum. The solids were collected and dried at room temperature under vacuum for three days. The relative amounts of sulfur oxoacids, or salts thereof, in the aqueous layer were determined by X-ray photoelectron spectroscopy (XPS). The results are set forth in Table 4.

	TABLE 4
			[RSO3]nY Content
	Reaction Time	NaClO/RSH	After Oxidation
	(min)	mole ratio	(wt. %)
Ex. 11	0.5	3:1	>89
Ex. 12	5	6:1	>90

As shown, the results indicate that over 89 wt. % of the sulfur species in the aqueous phase are sulfur oxoacids having the formula [RSO₃]_nY.

Reduction of Chloride Content by Caustic Treatment

Comparative Example A

A halide-containing liquid hydrocarbon was prepared by treating a high mercaptan sulfur crude oil blend with a 1% aqueous sodium hypochlorite solution (6:1 NaClO:RSH molar ratio) for 5 minutes at room temperature with vigorous stirring. The layers were allowed to separate. The crude oil treated with the halide-containing oxidizing agent was collected and then washed several times with water. The organic chloride content in the washed oil was determined according to ASTM D4929.

Examples 13-16

A halide-containing liquid hydrocarbon was prepared by treating a high mercaptan sulfur crude oil blend with a 1% aqueous sodium hypochlorite solution (6:1 NaClO:RSH molar ratio) for 5 minutes at room temperature with vigorous stirring. The layers were allowed to separate. The crude oil treated with the halide-containing oxidizing agent was collected and then washed with an aqueous caustic solution. The organic chloride content in the washed oil was determined according to ASTM D4929.

The chloride reduction results are set forth in Table 5.

	TABLE 5
		Chloride
		Content
	Washing Conditions	(ppm)
	Comp. Ex. A	Water, 25° C., 5 min.	71
	Ex. 13	1N NaOH, 25° C., 0.5 min.	22
	Ex. 14	1N NaOH, 20° C., 5 min.	16
	Ex. 15	1N NaOH/EtOH, 40° C., 30 min.	11
	Ex. 16	1N NaOH, 90° C., 60 min.	<1

As shown, aqueous caustic solutions are effective in reducing the chloride content of a crude oil treated with the halide-containing oxidizing agent using the disclosed method.

For the purposes of this specification and appended claims, unless otherwise indicated, all numbers expressing quantities, percentages or proportions, and other numerical values used in the specification and claims, are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by the present invention. It is noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the,” include plural references unless expressly and unequivocally limited to one referent. As used herein, the term “include” and its grammatical variants are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that can be substituted or added to the listed items.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope is defined by the claims, and can include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

Claims

1. A method for reducing mercaptan concentration in a liquid hydrocarbon, comprising:

(a) contacting a mercaptan-rich liquid hydrocarbon having a first concentration of mercaptan sulfur with a composition comprising an oxidizing agent and water wherein the molar ratio of the oxidizing agent to mercaptan sulfur in the mercaptan-rich liquid hydrocarbon is from 3:1 to 10:1; and

(b) separating the water from the liquid hydrocarbon to yield a mercaptan-depleted liquid hydrocarbon having a second concentration of mercaptan sulfur, the second concentration being less than the first concentration;

wherein a major amount of mercaptan compounds in the mercaptan-rich liquid hydrocarbon are converted to at least one sulfur oxoacid or salt thereof, having the formula: [RSOx]nY

wherein R is a hydrocarbyl group; x is an integer from 1 to 3; n is 1 or 2; and Y is hydrogen, an alkaline metal, or alkaline earth metal.

2. The method of claim 1, wherein the liquid hydrocarbon is a crude oil.

3. The method of claim 1, wherein the liquid hydrocarbon is a jet fuel.

4. The method of claim 1, wherein the liquid hydrocarbon contains at least 200 ppm mercaptan sulfur.

5. The method of claim 1, wherein the oxidizing agent is an alkali or alkaline earth metal hypochlorite salt.

6. The method of claim 1, wherein at least 60 percent by weight of the mercaptan compounds in the mercaptan-rich liquid hydrocarbon are converted to the at least one sulfur oxoacid or salt thereof.

7. The method of claim 1, wherein at least 70 percent by weight of the mercaptan compounds in the mercaptan-rich liquid hydrocarbon are converted to the at least one sulfur oxoacid or salt thereof.

8. The method of claim 1, wherein the mercaptan-depleted liquid hydrocarbon contains less than 50 ppm mercaptan sulfur.

9. The method of claim 1, wherein the mercaptan-depleted liquid hydrocarbon contains less than 10 ppm mercaptan sulfur.

10. The method of claim 1, wherein the mercaptan-depleted liquid hydrocarbon contains less than 1 ppm mercaptan sulfur.

11. The method of claim 1, wherein the oxidizing agent is a halide-containing oxidizing agent and the mercaptan-depleted liquid hydrocarbon is further contacted with an aqueous caustic solution under conditions to reduce the halide concentration in the mercaptan-depleted liquid hydrocarbon.

12. The method of claim 11, wherein the aqueous caustic solution is selected from alkali and alkaline earth metal hydroxide solutions, and mixtures thereof.

13. The method of claim 11, wherein the aqueous caustic solution further comprises a low molecular weight alkanol.

14. The method of claim 11, wherein the aqueous caustic solution further comprises a phase transfer catalyst.