Patent Grant
8936719
The present invention relates to the removal of sulfur compounds from hydrocarbon materials, in particular an oxidation and extraction process using water and/or an ionic liquid (IL) as an extractant.
The removal of sulfur compounds from fossil hydrocarbon (HC) mixtures down to ppm levels is of major technical importance at various levels in industry and society due to the fact that sulfur compounds or derivatives thereof can have negative effects on technical operations and the environment. Legislation in the European community currently limits the sulfur level in fuels such as gasoline and diesel to 50 ppm and below.
The necessity for refineries to furnish ultra low sulfur fuels challenged established desulfurisation technologies, e.g. hydro desulfurisation (HDS), and lead to development of new deep desulfurisation (DDS) processes. The existing HDS technologies have a number of shortcomings in the application of DDS due to very high operating temperatures and pressures and, more importantly, the use of unsustainable large quantities of hydrogen.
New DDS processes comprise of contacting fuels after conventional desulfurisation (HDS, Merox, etc.) with a sulfur selective extractant and in many cases a supporting additive which are immiscible with the fuel phase.
Technologies other than HDS for the reduction of sulfur levels in HC fuels, include (i) oxidative desulfurisation (ODS) and (ii) extraction with ionic liquids. Both areas focus on the DDS of liquid HCs such as fuel oils, diesel fuel, jet fuel, gasoline, and crude with contents of ≦1500 ppm.
The area of ODS involves in the first step the oxidation of S-contaminants to sulfoxides and/or sulfones which exhibit low solubility in HCs, and are thus available for extraction into a suitable polar solvent in a subsequent step. Oxidants in this process typically consist of peroxides, in most cases aqueous hydrogen peroxide solutions. For example, several patents (U.S. Pat. Nos. 5,310,479; 6,402,940; EP 0565,324 A) and publications (T. Kabe et al. Energy & Fuels 2000, 14, 1232; Zannikos et al. Fuel Process Technol. 1995, 42, 35; T. Aida et al. Prep.-Am. Chem. Soc., Div. Pet. Chem. 1994, 39, 623; T. Hirai et al. Ind. Eng. Chem. Res. 2002, 41, 4362; Stournas et al. Fuel Process Technol. 1995, 42, 35; A. R. Lucy et al. J. Mol. Catal. A: Chemical 1997, 117, 397) disclose the application of organic carboxylic acids, e.g. formic acid, in conjunction with aqueous hydrogen peroxide.
Although these processes are effective they have a number of shortcomings namely: (i) the use of the peroxide oxidant in stoichiometric excess (2.5 to 3.5 times in U.S. Pat. No. 6,402,940; up to 1000 times in T. Hirai et al. Ind. Eng. Chem. Res. 2002, 41, 4362); (ii) the use of large amounts of flammable and volatile organic compounds; (iii) difficulty in rendering these processes both economically and environmentally benign; and (iv) occasional difficulties in recovering additives such as oxidiser/extractants; e.g. formic acid forms an azetrop when mixed with water which is difficult to break and requires additional process steps.
The second area of IL technology comprises of contacting ionic liquids with HCs such as diesel fuels, in which they are immiscible (US Pat Appl. 20050010076A1; Wasserscheid et al. Chem. Commun. 2001, 2494; Zng et al. Green Chem. 2002, 4, 376, U.S. Pat. No. 7,001,504 Schoonover). After gravity separation of the S-laden IL extractant and repeated extraction steps, model fuels with S-levels<50 ppm are obtained. A similar technology uses a combination of ILs and hydrogen peroxide as an oxidiser for the DDS of light oil (Wei et al. Green Chem. 2003, 5, 639).
Whilst ionic liquids have been known for many years, they have only recently attracted great interest as versatile materials due to their unique properties. They are defined as being liquids which consist of ions only and are also referred to as molten salts. Their attractive properties include, amongst others, a very low vapour pressure, good electrical conductivity, high chemical robustness and solubility characteristics which can easily be controlled by varying the nature of either the cation or anion (P. Wasserscheid, W. Keim Angew. Chem. 112 (2000) 3926; T. Welton, Chem. Rev. 99 (1999) 2071; J, d. Holbrey, K. R. Seddon, Clean Products and Processes, 1 (1999) 223).
The present invention provides a process for reducing the sulfur content of a hydrocarbon material containing sulfur compounds, the process comprising:
contacting the hydrocarbon material with an oxidant selected from organic peroxy acids, organic peroxides, inorganic peroxides and mixtures thereof, in at least a stochiometric amount and for a time sufficient to oxidise a sulfur compound to a sulfone compound;
contacting the hydrocarbon material with an aqueous extractant for a time and under conditions sufficient to allow at least a portion of the oxidised sulfur compounds to be extracted into the aqueous extractant, and
Optionally, the process may include a second and subsequent extractions with the aqueous extractant to further reduce sulfur content. A final extraction with an ionic liquid (IL) may be conducted.
The present invention also provides for substitution of the aqueous extractant with an IL in one or more of the other extraction steps.
The step of contacting the hydrocarbon material with the oxidant may be conducted prior to contacting with the extractant or concurrently with contacting with the extractant.
The aqueous extractant may be brine or water, preferably water.
When the hydrocarbon material comprises naphtha or a diesel fraction, the step of contacting the hydrocarbon material with the oxidant may be conducted after an initial extraction of the naphtha or diesel fractions with an ionic liquid extractant in order to selectively remove dienes which may otherwise deactivate the oxidation step.
The IL extractant may be an IL of the general composition Q+A−, where Q+ is a quarternary ammonium or phosphonium cation and A− is an inorganic or organic anion, selected such that the IL is in a liquid state at the operating temperature and pressure of the process. For example, the ionic liquid can have a Q+ cation selected from an alkyl pyridinium cation, an alkyl pyrrolidinium cation, an alkyl piperridinium cation, a di-alkyl imidazolium cation, a tri-alkyl imidazolium cation, tetra-alkylphosphonium and a tetra alkyl ammonium cation, and a A− anion selected from the group consisting of a halide anion, nitrate anion, alkylsulfate anions, alkylsulfonate anions, alkylsubstituted aryl sulfonates such as the p-toluene sulfonate anion, a triflate anion, a thiocyanate anion, a hexafluorophosphate anion, a tetrafluoroborate anion, dicyanamide anion, a bis(trifluormethanesulfonyl)imid anion, a halogenoaluminate anion, an organohalogenoaluminate anion, and mixtures thereof. More particularly, the ionic liquid can be those listed in table 2 below.
Preferably, the IL is selected so it has a miscibility gap when in contact with the hydrocarbon phase sufficient to minimise undesired losses of hydrocarbon from the hydrocarbon phase into the ionic liquid phase. It is also preferable that the selected ionic liquid has a miscibility gap when in contact with the hydrocarbon phase sufficient to minimise settling times for phase separation and dispersion of the ionic liquid into the hydrocarbon phase.
Suitable oxidisers include: organic peroxy acids such as carboxylic peracids, preferably carboxylic per acids having 2 or more carbon atoms, more preferably peracetic acid; organic peroxides such as t-butyl hydrogen peroxide; inorganic peroxides such as hydrogenperoxide, perborates, persulfates; and mixtures thereof such as carboxylic acid hydrogenperoxide mixtures. Preferably, the oxidiser is selected from peracetic acid, or a mixture of acetic acid and hydrogen peroxide. The amount of oxidiser is preferably a near stochiometric amount, more preferably one to two mol equivalent of peroxy acid or peroxide compound for the conversion of a sulfur compound to a sulfone.
In the oxidation of hydrocarbons comprising diesels, the amount of oxidant is preferably about 10 to about 20 mol equivalent of peroxy acid or peroxide compound. More specifically, when it is desired to reduce the amount of sulphur in hydrocarbon materials comprising, in particular, diesel, to low levels (eg below about 15 ppm), an additional oxidation step may be included in the process to oxidise the sulphur in compounds that are difficult to oxidise, for example thiophenes and benzothiophenes. This additional oxidation step may comprise one or a combination of two or more techniques selected from, but not limited to, ultrasonication, microwave irradiation and catalysis for deep oxidation. In the catalysis technique, the catalyst materials may comprise typical compounds known to promote such oxidations, including, but not limited to, catalyst systems based on early transition metal oxides, such as polyoxometalates and heteropolyoxometalates and catalyst systems based on late transition metals such as iron, ruthenium, rhodium, nickel, palladium and platinum. The oxidising agents used in the deep oxidation step may be selected from those described earlier and may be combined with the catalyst and the hydrocarbon in one single step, or be combined with the catalyst prior to contacting with the hydrocarbon for a time sufficient to generate the catalytically active species from the two components.
The extraction may be conducted at ambient temperature and atmospheric pressure. For removal of more complex sulfur compounds, slightly elevated temperatures may be beneficial. Extraction into water may, for example, be conducted up to the boiling point of water at a given pressure. A person skilled in the art would appreciate that for a volatile hydrocarbon, such as a natural gas condensate, an increase in pressure will be required under elevated temperatures to keep the NGC in the liquid phase.
The ratio of hydrocarbon to extractant may be about 10:1 or higher, preferably about 8:1, more preferably about 5:1. Smaller ratios are also viable, however, with smaller ratios the cost of the extractant for the process will be commensurately higher.
The process of the present invention is suitable for reducing the sulfur content of a range of hydrocarbons including natural gas condensates, light oils, diesel, gasoline, petroleum, jet fuels, and products of coal gasification and liquidification. The process has been found to be highly effective when used on hydrocarbons from actual oil refinery streams. Such hydrocarbons contain a variety of sulfur compounds of varying complexity and resistance to oxidation, depending on the source. This is in strong contrast to laboratory hydrocarbon model compositions which may include only limited selected sulfur compounds and where the limited selected composition of hydrocarbons impacts on the effectiveness of the process.
The innovation of the present invention offers several advantages over existing technologies: it is, in terms of economics and sustainability, superior to HDS technology since no hydrogen is involved and operations can be carried out under mild conditions, thus minimising capital investment and operational costs.
The consumption of the oxidising agent is maintained at a minimum due to the process of the present invention being effective with near stoichiometric amounts of oxidiser, whereas prior art processes operate with large excess amounts of oxidiser. Since peroxide oxidising agents represent a large cost factor, the present invention delivers considerable economic benefit in comparison to prior art ODS processes operating with excess amounts of oxidising agents.
Although the process according to this invention is not limited to the use of the oxidiser peracetic acid, the use of this agent has the benefit of generating acetic acid (AA) as a non-toxic and environmentally soft by-product of the reaction.
After complete oxidation of the S-compounds, the present invention may use water as the extracting solvent instead of frequently used volatile, flammable, and harmful organic solvents (such as DMF, ACN, DMSO, NMP). At the same time the water also serves to remove trace amounts of acid. Therefore additional amounts of bases such as hydroxide solutions are not needed.
A final polishing step can be carried out with an IL, which is, like water, an environmentally unproblematic extraction medium.
The extractant of the present invention can be separated and regenerated from the S-compounds in a simple manner by distillation techniques, thus avoiding large volume waste streams and, in case of IL extractants, also allows for economic operation.
In the present invention, distillative recovery of the AA stemming from the oxidation step is unproblematic, because AA, unlike formic acid, does not form an azeotrop with water. Thus after recovery, the AA can be re-used as a raw material for the generation of the oxidiser PA.
Therefore the method used in the present invention for the reduction of S-levels in liquid HC can be operated in a simple and economically viable manner with very low and easy to handle waste streams.
The present invention will now be further described by way of preferred embodiments which are intended to be illustrative only and not restrictive.
A number of experiments using NGC hydrocarbon material to illustrate the present invention were conducted as described below. For all experiments, gas chromatographic analysis was performed to confirm the identity of the NGC before and after extraction. A Shimadzu GC2014 was used under the following conditions:
All tests were conducted on refinery streams rather than sample models created in a laboratory, in particular on NGC streams containing 400 to 500 ppm S. It will be appreciated that NGC generally comprise a mixture of linear and branched saturated hydrocarbons with a low content of aromatic and olefinic (unsaturated) hydrocarbon. The major constituents of NGC are C5 and C6 fractions (n/iso-pentanes, n/iso-hexanes). An analytical report on a sample of NGC after treatment to remove inorganic sulphur and prior to treatment in the present process is provided in Table 1.
General Procedure for Oxidative Extraction of NGC with Water
Initial S-content of NGC was determined by using a S-sensitive X-ray Fluorescence detector. A stoichiometric amount (based on initial S-content) of the oxidiser peracetic acid (PAA) was added to a 5:1 by volume mixture of NGC and water (typically 100:20 ml, several up-scaling experiments were also carried out on a multi litre scale) at ambient temperature under vigorous stirring in a sealed glass reaction vessel. Thorough mixing could be achieved either mechanically or, more efficiently, by ultrasonication. Contact times can vary from 0.25 to 48 h. The biphasic mixture was allowed to settle until clear separation into two layers was observed. The lower aqueous layer containing AA and the majority of the oxidised S-compounds was separated and transferred to recycling. The upper NGC layer was sampled for sulfur analysis (S-sensitive X-ray Fluorescence detector).
The same procedure for mixing and separating but without prior addition of PAA oxidiser (except in entry 9, Table 2) was applied in subsequent (multiple) extractions.
General Procedure for Oxidative Extraction of NGC with Ionic Liquids
Initial S-content of NGC was determined by using a S-sensitive X-ray Fluorescence detector. A stoichiometric amount (based on initial S-content; e.g. 415 ppm) of the oxidiser PAA was added to a 5:1 by volume mixture of the NGC and an IL (typically 100:20 ml, several up-scaling experiments were also carried out on a multi litre scale) at ambient temperature under vigorous stirring in a sealed glass reaction vessel. Thorough mixing of the biphasic system could be achieved either mechanically or, more efficiently, by ultrasonication. Contact times can vary from 0.25 to 48 h. Alternatively, the NGC can be contacted with the PAA for a set period of time prior to the addition of IL. The biphasic mixture was allowed to settle until clear separation into two layers was observed. The lower IL layer containing AA and the majority of the oxidised S-compounds was separated and transferred to recycling. The upper NGC layer was sampled for sulfur analysis (S-sensitive X-ray Fluorescence detector).
The same procedure for mixing and separating but without prior addition of PAA oxidiser was applied in subsequent (multiple) extractions.
Results of the oxidation/extraction experiments are summarised in Tables 2 and 3. A key to symbols in these tables is provided below:
Key
“Extr.capab.” is extraction capability; defined as (1−ppm Sulfur after extraction/ppm Sulfur prior to extraction).
On the Anion side X: MeSO3 is Methyl sulfonate, OTos is p-toluene sulfonate, SCN is thiocyanate, DCN is dicyanamide, Br is bromide, and BTA and NTf2 is bis(trifluormethanesulfonyl)imid.
Discussion of Results for Experiments Using NGC Hydrocarbon Material
Extending contact times in the oxidation step beyond 6 h does not significantly impact on the result (Table 2, entries 13-17).
In the first oxidation and extraction step the S-level is already reduced by 72 to 87% (Table 2, entries 1, 3, and 8). The NGC having been desulfurised once is contacted in subsequent multiple steps (up to 5) with 1/5 volumes of water under the above conditions to achieve further reduction of the S-levels and to remove trace amounts of residual acid. After the 2nd step, extractions enter into a saturation phase approaching S-levels of 50 ppm (
The difficulty in further removing residual amounts of S is probably not due to incomplete oxidation. In a separate test (see
Oxidation/Extraction experiments were also carried with a selection of ILs. In initial tests (Table 3) ILs were screened for their extraction capability towards 400 to 500 ppm sulfur samples of NGC, and the ILs EMIM-SO3Me and Bu3MeP-Tos were found to be the most effective in terms of extraction capability, phase separation, and stability towards aqueous media (Table 3, entries 1-23).
However, this method of extraction is only effective in conjunction with the use of an oxidiser, e.g. PAA (Table 3, entries 24, 25)
Employing Bu3MeP-OTos as the extractant allowed reducing S-levels in the liquid HC below 30 ppm (Table 3, entries 36-40,
In case of Bu3MeP-OTos, settling times of the biphasic mixture were considerably longer than with other ILs, e.g. EMIM-SO3Me. Hence differences in the sharpness of phase separation impact on the recovery of NGC after multiple extractions. When EMIM-SO3Me was employed 1.6 wt % NGC were found in the IL, whereas 8 to 10% were detected in for Bu3MeP-OTos.
For an economical viable process it is vital that the S-laden extractant can be regenerated in a simple manner. The present invention employs economical steam and mild vacuum distillation techniques to remove S-compounds from the extractant. Due to their high boiling point and stability ILs remain behind and unaffected with recoveries of ˜95%, whereas the S-compounds move into the steam phase. This generates a simple to handle waste stream.
Other suitable techniques for regeneration of the extractants include but are not limited to, flash distillation with a stream of inert gas such as nitrogen, steam distillation with a steam stripper column and fraction distillation.
Thus EMIM-SO3Me and Bu3MeP-OTos which had been regenerated by steam distillation showed reproducible extraction capabilities in subsequent tests similar to those observed for the fresh material. No degradation of the ionic liquid extractants was observed under these conditions and the identity of the ionic liquid extractants was established by suitable analytical methods (e.g. NMR, HPLC).
The ability of water and ILs for the DDS of said HCs was combined in a new oxidation/extraction process comprising of the steps (i) oxidation in an PAA/water medium, (ii) multiple extractions/washings with water, and (iii) a final polishing step for further reducing the S-content with an IL. According to test results (Table 2, entry 22-23;
In addition, the use of water as an extractant following oxidation with peracetic acid compared well with the results using an ionic liquid extract (Table 2, entries 18-23 compared to Table 3, entries 31-35 and 36-40). For these entries, 8 mmol (1.68 ml) of peracetic acid was added to 400 ml of NGC with an initial sulfur content of 415 ppm. Subsequent extractions were conducted with water or ionic liquid as indicated. The use of water as the sole extractant (or minimal use of an IL as the extractant for the final polishing step—eg entry 23) is advantageous from both an economic and environmental perspective.
A number of experiments were also conducted using diesel, pre-extracted diesel and naphtha hydrocarbons. These will now be discussed further below.
General Procedure for the Oxidative Extraction of Diesel, Pre-Extracted Diesel and Naphtha Hydrocarbons
Tests were conducted on real diesel, from refinery streams which were collected before the subjection to any deep desulfurisation processes. The tests on these materials were carried out in a similar manner as described for the NGC and illustrated in
The diesel sample and an excess stoichiometric amount (Table 4, based on initial S-content) of the oxidiser (e.g. peracetic acid [32 wt % solution in dilute acetic acid]; H2O2 [30 wt % in dilute acetic acid]) were contacted for a defined time under vigorous stirring at a defined temperature (Table 4) in a reaction vessel equipped with a reflux condenser system of a capacity sufficient to prevent losses of any volatile components of the diesel material. Thorough mixing of the biphasic system was achieved either mechanically, or, more efficiently when operating on a larger scale, by means of a counter current mixing system, rotating mixer, microwave radiation, or by ultrasonication.
The tests were conducted on a scale of a few 100 ml up to several litres. The oxidation step was carried out with either PAA or a mixture of hydrogen peroxide and acetic acid, the latter allowing for a more economical operation by avoiding the use of expensive premanufactured PAA. Alternatively, the oxidation may be carried out in presence of a catalyst selected from typical compounds known to promote such oxidations, including, but not limited to, catalyst systems based on early transition metal oxides, such as polyoxometalates and heteropolyoxometalates and catalyst systems based on late transition metals such as iron, ruthenium, rhodium, nickel, palladium and platinum. In some of the tests the diesel phase was immediately contacted with the oxidiser/catalyst mixture, whereas in some other tests the catalyst was pre-treated with the oxidiser for a set period of time.
After completion of the oxidation step, a first water wash step was conducted in which water was added under stirring to give a mixture of the diesel and aqueous phase. IL extractions were conducted after separation of the aqueous phase. After completion of all (typically 6) IL extractions, a final water wash followed.
The biphasic mixture was allowed to settle until separation into two layers was observed. The lower aqueous layer containing AA and oxidised S-compounds was separated and transferred to recycling. The upper diesel layer was sampled for sulfur analysis.
The same procedure for mixing and separating but without prior addition of the oxidiser was applied in subsequent (multiple) extractions with an ionic liquid followed by a final water wash.
Results are shown in Table 4.
Discussion of Results for Experiments Using Diesel, Pre-Extracted Diesel, or Naphtha Hydrocarbon Material
As can be seen from the data in Table 4, the best results are achieved using 20 times the stoichiometric amount (ie 20 mol equivalents) excess of PAA relative to the initial S-content (ca. 400 ppm). Under these conditions final total sulphur contents approximating 10 ppm are achieved.
It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.
The present application claims priority from United States of America Provisional Patent Application No. 60/784,472 filed on 22 Mar. 2006, the contents of which are incorporated herein by reference.
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