Process for desulphurisation of liquid hydrocarbon fuels

Abstract

The present invention provides a process for oxidative desulphurization of liquid hydrocarbon fuels, such as diesel fuel, gasoline, jet fuel, fuel oils, coal liquids and similar petroleum products to ultra low sulphur hydrocarbon fuels with sulphur content less than 10 ppm. In this process, the sulfur compounds present in hydrocarbon fuel are first oxidised to more polar sulphones/sulphoxides and then removed by solvent extraction with NMP containing antisolvent followed by final polishing by passing through adsorption column.

Description

FIELD OF THE INVENTION

The present invention relates to a process for the desulphurization of liquid hydrocarbon fuels. Particularly the invention relates to a process for oxidative desulphurization of liquid hydrocarbon fuels such as diesel fuel, gasoline, jet fuel, fuel oils, coal liquids and similar petroleum products to ultra low sulphur hydrocarbon fuels with sulfur content less than 10 ppm. More particularly, the present invention relates to oxidative desulphurization of liquid hydrocarbon fuels using carboxylic acid urea-H₂O₂ adduct, organosulphonic acid-H₂O₂ as oxidation system.

BACKGROUND OF THE INVENTION

Hydrocarbon fuels such as diesel fuel, gasoline, jet fuel, fuel oils and coal liquids are presently being consumed in vast quantities and their consumption continues to grow at alarming rates, due to their high energy densities and convenient physical form. This high consumption inevitably has a major impact on global environment. Most notably transport hydrocarbon fuels like diesel fuel, gasoline, jet fuels are receiving intense scrutiny due to increased environmental concerns. There is an increasing demand to reduce sulphur content in the hydrocarbon fuels to produce products, which have very low sulphur contents and are thereby marketable in the even more demanding market place.

Conventionally hydrodesulphurization of hydrocarbon fuels which involves contacting of hydrogen with hydrocarbon streams in presence of catalysts at elevated temperature and pressure to convert sulphur compounds present therein to hydrogen sulfide is used to produce hydrocarbon fuels with lower sulphur content. Hydrodesulphurization can easily remove sulphur from several common classes of sulphur compounds such as sulphides, disulphides and thiols present in hydrocarbon fuels, because these are easily accessible to contact with hydrodesulphurization catalyst, however sulphur compounds like 4,6-dimethyldibenzothiophene (4,6-DMDBT) and others similar thiophene species are rigid to hydrodesulphurization and therefore are difficult to remove. Conventional hydrodesulphurization of diesel for example which operates at moderate temperatures (315-400° C.) and hydrogen pressure 3.0˜9.0 MPa with Co/Mo/Al₂O₃ as catalyst can bring down their sulphur content 300-500 ppm easily. However to bring down sulphur content further below 100 ppm deep hydrodesulphurization of diesel requires very severe operating condition like the use of high temperature, high hydrogen pressure more active catalyst and long residence time. Deep hydrodesulphurization yield negative effects such as reduce catalyst life, high hydrogen consumption and high yield loss thereby resulting in higher operating cost. Apart from the cost involved, the energy requirement for implementation of hydro processing technology also leads to increased level of CO₂ emission from refinery itself. The US EPA has released new regulation that requires the sulphur content in the diesel fuel used in highway vehicle limited to 30 ppm by 2005 and 15 ppm effective by 2006. Similarly in European country the sulphur content in the diesel fuel will be limited to 30-50 ppm by 2005. The necessity has therefore been felt for a process complementary to hydrodesulphurization to remove condensed thiophene, benzothiophene and dibenzothiophene type sulphur compounds present in hydrodesulphurized hydrocarbon fuel to yield ultra low hydrocarbon fuels.

It is known that sulphur compound present in hydrocarbon fuels can be oxidized to sulphones, sulphoxides and subsequently removed by taking advantage of their different chemical and physical characteristics (Tetsuo Aeda et. al., 20^th American chemical society at San Diego Calif. March 13-17, 1994). U.S. Pat. No. 5,958,224 discloses a process for removal of refractory sulphur compounds from hydrotreater hydrocarbon stream by oxidizing them to sulphoxides and sulphones with peroxometal complexes like LMO(O₂)₂, where M is selected from the group consisting of Mo, W, Cr and L is alkyl phosphoric triamide like hexamethyl phosphoric triamide followed by adsorption on solid adsorbent like activated carbon, bauxite, clay, coke alumina or silica gel. The peroxometal complexes were prepared by reacting metal complexes with hydrogen peroxide. Collins et al (J. Mol. Catal A: Chem 117, (1977)397) reported their studies on oxidation of sulphur compounds present in gas oil with hydrogen peroxide using phosphotungstic acid as catalyst and subsequent removal of sulphones followed by solvent extraction using gamma-butyrolactone as solvent or by adsorption on silica gel to obtain ultra low sulphur gas oil. However large amounts of hydrogen peroxide were used for the oxidation of sulphur compounds present in gas oil.

Mei et al (Fuel 82(2003)405) reported their studies on ultrasound assisted oxidative desulphurization of diesel using hydrogen peroxide as oxidant, phosphotungstic acid as catalyst, methanol as solvent for extraction of sulphones and obtained 66.4% overall sulphur removal after 2.5 hr reaction. The sulphur removal efficiency could be improved to 91.8% by increasing the reaction time to 4 hours and using silica adsorption instead of solvent extraction to remove sulphones from diesel.

U.S. Pat. No. 6,368,495 discloses a process for oxidative desulfurization of petroleum fractions like gasoline, diesel fuel, kerosene which involves oxidation of thiophene and thiophene derivatives present in petroleum fraction to sulphones with alkyl hydroperoxide in presence of molybdenum on alumina as catalyst followed by decomposition of sulphones using catalyst like double layer hydroxides molecular sieves, inorganic metal oxides or a mixture thereof.

Ishihara et al. (Applied Catalysis A: Gen 279(2005)279) reported oxidative desulfurization of desulfurized light gas oil with sulphur content 39 ppm involving oxidation of the sulphur compounds present therein with tert-butyl hydroperoxide in presence of 16 wt % MoO₃/Al₂O₃ as catalyst followed by removal of the sulphones formed by adsorption over silica gel to obtain light gas oil with sulphur content less then 5 ppm.

WO 2004/029179 describes a process for oxidative desulfurization of hydrotreated hydrocarbon mixture boiling range 180-360° C. containing less than 350 ppm thiophenic sulphur involving oxidation of sulfur compounds with organic peroxide in presence of a catalytic composition containing completely amorphous micro and/or mesoporous mixed oxide comprising a matrix selected from silica, alumina, ceria, magnesia and mixture thereof where in one or more metal oxides selected from transition metal oxides and group IV a metal oxides are uniformly dispersed followed by separation of the sulfur oxygenated products from the hydrocarbon mixture.

Otsuki et al. (Energy & Fuels, 14 (2000) 1232) studied oxidation of model sulfur compounds with a mixture of hydrogen peroxide and formic acid and found that reactivity of dibenzothiophene derivatives increased with increase of electron density. Thus, reactivity order for oxidation was found to be 4,6-dimethyldibenzothiophene>4-methyl-dibenzothiophene>dibenzothiophene. This is reverse to the order of reactivity for hydrodesulfurization. They also studied oxidative desulfurization of light gas oil and vacuum gas oil using formic acid, hydrogen peroxide mixture as oxidant and N,N-dimethylformamide, acetonitrile, methanol, dimethylsulphoxide and sulpholane as solvents for extraction of sulphones.

EP 0565324A1 describes a method for recovering organic sulfur compounds from liquid oil which involves oxidation of sulphur compounds present in liquid oil by mixture of a number of oxidants including formic acid and peroxide followed by removal of organic sulphones by adsorption on alumina or silica.

WO2005/019386 discloses a process for desulphurization of hydrocarbonaceous oil by hydrodesulfurization to reduce sulphur to a relatively low level, oxidation of sulphur compounds present in hydrodesulphurized hydrocarbonaceous oil to sulphones with an aqueous oxidizing solution like acetic acid, H₂O₂ mixture, decomposing residual oxidizing agent on supported transition metal and removal of sulphones by adsorption.

U.S. Pat. No. 6,402,940 discloses a process for desulfurization of fuels such as diesel oil and similar products to reduce sulfur content to a range of from about 2 to 15 ppm which involves oxidation of sulphur compounds present in fuels with a oxidizing/extracting solution of formic acid, a small amount of commercial hydrogen peroxide and preferably not more than 14 wt % of water at slightly elevated temperature followed by separation of fuel from aqueous acid, flashed removal of residual acid water, neutralization of remaining acid with caustic solution or with anhydrous calcium oxide and removal of sulphones by adsorption on alumina.

It is known that of carboxylic acid such as formic acid, acetic acid, propionic acid containing 2-10 moles of active oxygen containing species such as commercial 30/50% aqueous hydrogen peroxide, urea hydrogen peroxide adduct or alkali metal peroxoborate per mole of sulfur forms corresponding peracid which are powerful oxidants for oxidation of various types of sulphur compounds including 4,6-dimethyldibenzothiophene and other condensed thiophene derivatives which are most refractory in hydrodesulphurization, present in hydrocarbon fuels to their corresponding sulphones.

It is also known that sulphones being more polar, can be removed from liquid hydrocarbon fuels by solvent extraction using polar solvents like N,N-dimethylformamide, acetonitrile, methanol, dimethyl sulphoxide or sulpholane followed by final finishing by passing through alumina, silica, clay or by adsorption on alumina, silica to obtain ultra low sulphur (below 30 ppm) hydrocarbon fuels.

In U.S. Pat. No. 6,402,940, oxidation of sulphur compounds present in hydrocarbon fuel is carried out with an oxidizing system consisting of a mixture of formic acid and aqueous hydrogen peroxide in a series of continuous stirred tank reactors (CSTR) followed by gravity separation of two phases in a settling tank. This type of system is not convenient for the handling of the bulk product like hydrocarbon fuel. Further this patent advocates the removal of oxidized sulphur compounds (sulphones, sulphoxides) from hydrocarbon fuel by adsorption on alumina in a packed bed column or circulating counter current fluidized alumina or mixer settler combination to obtain ultra low sulfur hydrocarbon fuel with sulphur content in the range 2-15 ppm. The removal of sulphones, sulphoxides from bulk oxidized hydrocarbon fuel by adsorption method will involve handling and regeneration of large amount of solid adsorbent and therefore is likely to be very inconvenient and practically difficult process. The oxidizing solution of formic acid and hydrogen peroxide should according to this disclosure, not contain more than 14 wt % water and beyond this, formic acid has to be purified by distillation for reuse. Further by using oxidative desulphurization process described in this U.S. Pat. No. 6,402,940, the cetane index of the hydrocarbon fuel which is a important characteristic of fuel like diesel can not be enhanced to an appreciable extent.

The oxidizing solution used for oxidation of sulphur compounds present in fuels is prepared by mixing formic acid and commercial hydrogen peroxide, which is normally 30% or 50% solution of hydrogen peroxide in water. Hydrogen peroxide solution of more than 50% strength is not safe to handle at industrial scale. With the use of this 30% or 50% solution of hydrogen peroxide large amount of water contained in it also goes to formic acid which necessitates frequent purification of formic acid to keep water content in oxidizing solution preferably below 14%. Further formic acid is a strong acid and its handling needs special metallurgy equipments. The weak acid like acetic acid when used in place of formic acid for preparation of oxidizing solution did not oxidize sulphur compounds present in hydrocarbon fuels due to the fact that peroxyacetic acid is not formed from acetic acid and commercial hydrogen peroxide (J. Prakt. Chem. 131 (1931) 357.

Alkali metal peroxoborates like sodium/potassium peroxoborate are white crystalline solids formed when sodium/potassium tetraborate is treated with commercial 30%/50% aq. hydrogen peroxide under alkaline conditions. Due to their ease of preparation, commercial availability and reasonably high stability alkali metal peroxoborates find wide applications in oxidations reactions as a substitute for highly concentrated unsafe hydrogen peroxide (J. Chem. Soc., Perkin Trans 1, (2000) 1471; Tetrahedron 51 (1995) 6145; Ullmann's Encyclopedia of Industrial Chemistry Vol. A 19, 177).

Organosulphonic peracids formed from organosulphonic acids like p-toluenesulphonic acid and commercial 30/50% aqueous hydrogen peroxide in polar solvents are powerful oxidants due to highly electrophilic active oxygen and are known to oxidize a variety of substrate (Tetrahedron 52 (1996) 5773).

Urea hydrogen peroxide adduct (UHP) is a white crystalline solid formed when urea is recrystallized from 30% or 50% aq. H₂O₂. Due to ease of preparation, commercial availability, relatively high portion of hydrogen peroxide (36.2%) and reasonable stability, UHP adduct find wide applications in oxidation reactions as a substitute for anhydrous unsafe hydrogen peroxide (Aldrichimica Acta 26 (1993) 35, U.S. Pat. No. 5,481,032).

N-Methylpyrrolidinone (NMP) a cyclic amine has excellent thermal stability is completely miscible with water in all proportions and can be handled in carbon, steel, nickel equipments. Although NMP has very high capacity but not high selectivity. However addition of antisolvent like water, ethylene glycol can increase the selectivity of NMP. In addition to selectivity the addition of water also decreases the boiling point of the solvent .NMP water mixture has been found to be the best solvent for dearomatization when raffinate is the product (E. Muller, Handbook of Solvent Extraction, Wiley Interscience 1983, p. 523). There are several commercial plants world wide manufacturing Lube Oil Base Stocks (LOBS), Benzene, Toluene, Xylene (BTX); food grade hexane and special boiling point solvents by using NMP water mixture for solvent extraction of aromatics. However, incidentally, there are no literature report and use of NMP antisolvent like water mixture for solvent extraction/removal of sulphones/sulphoxides from oxidized hydrocarbon fuels.

OBJECTS OF THE INVENTION

The main object of the present invention is to provide a process for desulphurization of liquid hydrocarbon fuels which obviates the drawbacks as detailed above.

Another object of the present invention is to provide a process for oxidative desulphurization of liquid hydrocarbon fuels such as diesel fuel, gasoline, jet fuel, fuel oil, coal liquid and similar petroleum products to ultra low sulphur hydrocarbon fuels with sulphur content less than 10 ppm.

It is another object of the invention to provide a process for oxidative desulphurization of liquid hydrocarbon fuels by using carboxylic acid-alkali metal peroxoborate as oxidation system which obviates the drawbacks as detailed above.

Another object of the invention is to provide a process for oxidative desulphurization of liquid hydrocarbon fuels such as diesel fuel, gasoline, jet fuel, fuel oil, coal liquids and similar petroleum products to ultra low sulphur hydrocarbon fuels with sulphur content less than 10 ppm.

Still another object of the invention is that it is capable of removing all types of sulphur such as thiols, disulphide, sulphide and thiophenic from the liquid hydrocarbon fuels, the process is more suitable for liquid hydrocarbon fuels containing condensed thiophene, benzothiophene, dibenzothiophene and alkylated dibenzothiophene type sulphur which is refractory to hydrodesulphurization.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a process for oxidative desulphurization of liquid hydrocarbon fuel comprising the steps of:

• • a) treating the hydrocarbon fuel with an oxiding solution consisting of a mixture of oxygen containing species and an organosulphonic or carboxylic acid to oxidize sulphur compound(s) present therein to corresponding sulphones or sulphoxides, wherein the oxidizing agent is used in an amount in the range of 2-10 mole times of sulphur present in hydrocarbon fuel, followed by separating treated hydrocarbon fuel from the oxidizing solution,
  • b) washing the separated hydrocarbon fuel obtained in step (a) with water to remove sulphones or sulphoxides therefrom and extracting the sulphones from hydrocarbon fuels to obtain desulphurised liquid hydrocarbon fuel.

In one embodiment, step (a) is carried out at a temperature in the range 20-150° C., at a pressure atmospheric to 10 kg/cm², for a period of 2 to 60 min in a batch manner.

In another embodiment of the invention, sulphones/sulphoxides are removed from the hydrocarbon fuel in step (b) by solvent extraction.

In yet another embodiment of the invention, the oxidation of sulphur compounds present in liquid hydrocarbon fuels to sulphones is carried out using an oxidizing solution consisting of carboxylic acid selected from the group consisting of formic acid, acetic acid, propionic acid and butyric acid, containing 2 to 10 moles of urea hydrogen peroxide adduct or alkali metal peroxoborate selected from the group consisting of sodium, potassium, magnesium, calcium, barium or strontium peroxoborate per mole of sulphur.

In yet another embodiment, the sulphur compounds present in hydrocarbon fuel are oxidized to polar sulphones/sulphoxides in a continuous counter current oxidation reactor by passing the hydrocarbon fuel as a continuous phase, and oxidizing solution containing carboxylic acid and active oxygen containing species as a dispersed phase, at a temperature in the range 20-150° C., at a pressure atmospheric to 10 kg/cm², with residence time of 2 to 60 minutes, washing oxidised diesel with alkaline water followed by extracting resultant polar sulphones/sulphoxides, with a mixture of N-methyl pyrrolidinone and about 2-20% anti solvent, in a continuous current column, at a temperature in the range of 20 to 80° C. to obtain the extract phase containing 4 to 5% hydrocarbons and a raffinate phase containing about 95% hydrocarbons, removing the solvent from the above said extract and raffinate phase by known method, followed by passing through a bed of alumina-silica or clay to obtain the desired sulphur free hydrocarbon fuel with 5-10 ppm sulfur.

In an embodiment of the present invention the liquid hydrocarbon fuel includes diesel fuel, gasoline, jet fuel, fuel oil, coal liquids and similar petroleum products.

In yet another embodiment the liquid hydrocarbon fuel used is untreated or hydrotreated containing thiophene, benzothiophene, dibenzothiophene type sulphur compounds which are refractory in nature in hydrodesulphurization (HDS), preferably hydrotreated liquid hydrocarbon fuels containing less than 500 ppm sulfur.

In yet another embodiment the carboxylic acid used is selected from formic acid, acetic acid, propionic acid and butyric acid, preferably formic and acetic acid.

In another embodiment the carboxylic acid used is 2 to 10 wt % of hydrocarbon fuel.

In another embodiment the organosulphonic acid used is selected from the group consisting of p-toluenesulphonic acid, benzenesulphonic acid, methanesulphonic acid, trifluoromethane sulphonic acid, and 3-nitrobenzene sulphonic acid, preferably p-toluene sulphonic acid.

In another embodiment the organo sulphonic acid is used as organic solution in an organic solvent.

In another embodiment the organic solvent used is selected from acetonitrile and N-methylpyrrolidinone.

In another embodiment oxidizing solution consists of organosulphonic or carboxylic acid and oxygen containing species is in the range of 3-5 mole times of sulphur present in hydrocarbon fuel.

In another embodiment the temperature used in the oxidation of sulphur compound present in hydrocarbon fuel is in the range of 30-100° C.

In another embodiment water content of the oxidizing solution containing carboxylic acid and alkali metal peroxoborate is in the range 0-14%.

In yet another embodiment the mole ratio of carboxylic acid or organosulphonic acid to alkali metal peroxoborate used in the oxidizing solution in the range of 5:1 to 50:1.

In another embodiment the mole ratio of carboxylic acid or organosulphonic acid to alkali metal peroxoborate used in the oxidizing solution used is in the range of 10:1 to 30:1.

In another embodiment, the reaction time period used for the oxidation of sulphur compounds present in hydrocarbon fuel is in the range of 7 to 20 minutes.

In one embodiment, solvent for extraction of sulphones/sulphoxides from oxidized hydrocarbon fuel is selected from N,N-dimethylformamide, acetonitrile, methanol, dimethyl sulfoxide, sulpholane, N-methylpyrolidinone containing 2-20% water as antisolvent.

In another embodiment the active oxygen containing species used is selected from 30/50 wt % aqueous H₂O₂, alkali metal peroxoborate, urea H₂O₂ adduct, alkali metal peroxoborate, alkali and alkaline earth peroxide and alkali and alkaline earth hydroperoxide.

In yet another embodiment the alkali metal peroxoborate used is selected from sodium, potassium, magnesium, calcium, barium, strontium peroxoborate.

In another embodiment the alkali and alkaline earth peroxide is selected from sodium, lithium, calcium, strontium, barium, magnesium and zinc peroxide.

In yet another embodiment the alkali and alkaline earth hydroperoxide used is selected from potassium and sodium hydroperoxide.

In yet another embodiment mole ratio of carboxylic acid or organosulphonic acid to active oxygen containing species is in the range of 5:1 to 50:1, preferably 10:1 to 30:1.

In yet another embodiment the use of continuous counter current oxidation reactor eliminate the need of settler as the settling zones are provided in the reactor itself.

In yet another embodiment the active oxygen containing species used is preferably 2 to 5 mole times of the sulphur present in hydrocarbon fuel.

In yet another embodiment, the oxidation of sulphur compounds present in hydrocarbon fuels is carried out preferably for 5 to 30 min residence time.

In yet another embodiment the washing of the oxidized hydrocarbon fuel to remove residual carboxylic acid present is carried out water containing 0.1 to 10% alkali or alkaline earth metal oxides, hydroxide, carbonate or bicarbonate.

In yet another embodiment the anti solvent used with NMP is selected from water, glycol and sulpholane, preferably water.

In yet another embodiment the anti solvent used in a mixture with NMP is preferably in the range of 5 to 15 wt %.

In another embodiment the recovery of NMP from NMP anti solvent mixture obtained as distillate is carried out by distillation of the solvent in drying column in the temperature range 80-250° C.

In another embodiment water is removed from carboxylic acid by distilling the overhead stream.

In yet another embodiment the sulphones/sulphoxides adsorbed on alumina, silica, clay are removed by desorption and solubilization using polar solvent selected from the group consisting of methanol, acetone, acetonitrile preferably methanol.

In still another embodiment the cetane index of the treated hydrocarbon fuel used is increased by 2-20 units.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a process for oxidative desulphurization of liquid hydrocarbon fuels to ultra low sulphur hydrocarbon fuels with sulphur content less than 10 ppm which comprise oxidation of sulphur compounds present in hydrocarbon fuels to more polar sulphones/sulphoxides in a continuous counter current oxidation reactor preferably in a temperature range 30-100° C. at atmospheric pressure preferably for 5-30 minutes with an oxidizing solution consisting of carboxylic acid containing 2 to 10 moles of active oxygen containing species like commercial aqueous hydrogen peroxide, alkali metal peroxoborate, alkaline earth peroxides per mole of sulphur, washing of oxidized hydrocarbon fuels with alkaline water to remove carboxylic acid present, followed by extraction of sulphones/sulphoxides in counter current extraction with N-methylpyrolidinone (NMP) containing 5-15 wt % water as antisolvent, water washing to remove entrained NMP, drying through salt filter and final finishing through a bed of alumina, silica or clay.

In oxidative desulphurization process herein, oxidation of sulphur compounds present in hydrocarbon fuels to more polar sulphones/sulphoxides is carried out in the temperature range 20-150° C. with a oxidizing solution consisting of carboxylic acid containing 2 to 10 moles of alkali metal peroxoborate per mole of sulphur, followed by removal of sulphones/sulphoxides from hydrocarbon fuel by solvent extraction and final finishing by passing through a bed of alumina/silica/clay or alternatively removing the sulphones/sulphoxides from hydrocarbon fuel by adsorption on solid alumina/silica/clay

The present invention provides a process for oxidative desulphurization of liquid hydrocarbon fuels by using carboxylic acid-alkali metal peroxoborate as oxidation system which comprises oxidation of sulphur compounds present in hydrocarbons fuels, in the temperature range 20-150° C. pressure atmospheric to 10 kg/cm² for 2 to 60 min in a continuous or batch manner with oxidizing solution consisting of carboxylic acid containing 2-10 moles of alkali metal peroxoborate per mole of sulphur, to sulphones/sulphoxides followed by separation of hydrocarbon fuel from oxidizing solution, water washing and removal of sulphones/sulphoxides from hydrocarbon fuel by solvent extraction and final finishing by passing through a bed of alumina/silica/clay or alternatively removing the sulphones from hydrocarbon fuels by adsorption on solid alumina/silica.

The present invention liquids hydrocarbon fuels such as diesel fuel, gasoline, jet fuel, fuel oil, coal liquids and similar petroleum products may be both untreated or hydrotreated containing condensed thiophene, benzothiophene and dibenzothiophene type sulphur compounds which are refractory in nature in hydro desulphurization (HDS) most preferably hydrodesulphurized diesel boiling range 167-376° C. containing 4,6-dimethyldibenzothiophene and other similar alkyl dibenzothiophene sulphur compounds with total sulphur content less than 500 ppm. Carboxylic acid used is selected from formic acid, acetic acid, propionic acid, butyric acid preferably formic and acetic acid. Alkali metal peroxoborate used is selected from sodium, potassium, magnesium, calcium, barium and strontium peroxoborate most preferable sodium peroxoborate. The oxidation of the sulphur compounds present in hydrocarbon fuels is carried out in the temperature range 20-150° C. preferably 30-100° C. at atmospheric pressure. The carboxylic acid used is preferably 2 to 20 wt % of the hydrocarbon fuel. In the mole ratio of carboxylic acid to alkali metal peroxoborate is 5:1 to 50:1 preferably 10:1 to 30:1. The consumption of alkali metal peroxoborate is dependent upon the sulphur present in the hydrocarbon fuel and cost of this material can have a negative effect on the economics of the process.

Therefore this process is most suitable for removal of small amounts of sulphur present in liquid hydrocarbon fuels preferable less than 500 ppm sulphur.

The desulphurized ultra low sulphur hydrocarbon fuel obtained contains total sulphur below 10 ppm. The process is capable of removing all types of sulphur such as thiols, disulphide, sulphide and thiophenic from the liquid hydrocarbon fuels, the process is more suitable for liquid hydrocarbon fuels containing condensed thiophene, benzothiophene, dibenzothiophene and alkylated dibenzothiophene type sulphur which is refractory to hydrodesulphurization.

The present invention also provides a process for oxidative desulphurization of liquid hydrocarbon fuels which comprises oxidation of sulphur compounds present in hydrocarbon fuels, to more polar sulphones/sulphoxides in a continuous counter current oxidation reactor in the temperature range 20-150° C. with an oxidizing solution consisting of carboxylic acid containing 2 to 10 moles of active oxygen containing species, per mole of sulphur, washing of the oxidized diesel with alkaline water to remove carboxylic acid present followed by extraction of sulphones/sulphoxides in a counter current extractor with N-Methylpyrrolidinone antisolvent mixture, water washing to remove entrained NMP, drying through a salt filter and final finishing by passing through a bed of alumina/silica/clay. The liquid hydrocarbon fuels such as diesel fuel, gasoline, jet fuel, fuel oil, coal liquids and similar petroleum products may be both untreated or hydrotreated containing condensed thiophene, benzothiophene and dibenzothiophene type sulphur compounds which are refractory in nature in hydrodesulphurization (HDS) most preferably hydrodesulphurized diesel boiling range 167-376° C. containing 4,6-dimethyldibenzothiophene and other similar alkyldibenzothiophene sulphur compounds with total sulphur content less than 500 ppm. The use of continuous counter current oxidation reactor eliminates the need of settler as the settling zones are provided in the reactor itself and increases the efficiency of the oxidation process by providing highest concentration of the oxidant to the tail feed and optimum utilization of the oxidant. The present process is very common in the petroleum refining for various extraction processes like aromatic extraction for special boiling point solvent and production of benzene, toluene and xylenes. The active oxygen containing species used is selected from commercial aqueous hydrogen peroxide, alkali metal peroxoborate like sodium, potassium, magnesium, calcium, barium, strontium peroxoborate, urea hydrogen peroxide adduct, alkali and alkaline earth peroxide like sodium, lithium, calcium, strontium, barium, magnesium, zinc peroxide, alkali and alkaline earth hydrogen peroxide like potassium hydrogen peroxide preferably commercial aqueous hydrogen peroxide, sodium peroxoborate and urea hydrogen peroxide adduct.

The final finishing of the diesel is carried out in adsorption column by passing diesel through a bed of adsorbent like alumina, silica, clay preferably nonactivated alumina. The sulphones/sulphoxides present in spent oxidized solution obtained from the continuous counter current oxidizer are separated from carboxylic acid and water by flashing. The present invention the invention as the consumption of active oxygen containing species is dependent upon the sulphur present in the hydrocarbon fuel and cost of this material can have a negative effect on the economics of the process. Therefore this process is most suitable for removal of small amounts of sulphur present in liquid hydrocarbon fuels preferably less than 500 ppm sulphur. This process is capable of removing all types of sulphur such as thiols, disulphides, thiophenic from the liquid hydrocarbon fuel, the process is more suitable for liquid hydrocarbon fuel containing condensed thiophene, benzothiophene, dibenzothiophene and alkylated dibenzothiophene type sulphur which is refractory to hydrodesulphurization.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a schematic flow sheet of the process and the invention wherein the sulphur removal is accomplished by oxidation, extraction followed by final finishing through adsorption.

The following description is illustrative of the process of the invention and should not be construed as limiting in any manner. Modifications and improvements of the process described below are possible without departing from the spirit and scope of the invention.

The sulphur containing hydrocarbon fuel for example HDS diesel fuel containing less than 500 ppm sulphur is introduced to the continuous counter current oxidation reactor B through line 1. The feed entering through line 1, if required is passed through the heat exchanger to bring the feed to desired temperature. The continuous counter current oxidation reactor B is a packed reactor filled with proper size rasig rings and the desired temperature is maintained by circulating suitable fluid at temperature slightly above the desired reaction temperature. The continuous counter current oxidation reactor B has the provision for feeding sulphur containing liquid hydrocarbon fuel near the bottom and oxidant solution from near the top with suitable compartments for separation of oxidized hydrocarbon fuel and the spent oxidizing solution containing oxidized sulphur compounds extracted from hydrocarbon fuel during oxidation, organic acid, unused active oxygen containing species, and water formed during the reaction and added along with the oxidant. The active oxygen containing species (oxidant) enter the admixing tank A through line 2 and the organic acid through line 3. In case of recovered organic acid, it enters the admixing tank A through the line 4 and the balance of the organic acid if required is added through the line 3. The oxidant solution thus turned is fed to counter current oxidation reactor B near the top of the reactor.

The carboxylic acid used for preparation of oxidant solution is selected from formic, acetic, propionic and butyric acids preferably formic acid, acetic acid. The active oxygen containing species used for preparation of oxidant solution is selected from commercial aqueous hydrogen peroxide, alkali metal peroxoborate, urea hydrogen peroxide adduct, and alkali, alkaline earth peroxide preferably commercial aqueous hydrogen peroxide, urea hydrogen peroxide adduct and sodium peroxoborate. The active oxygen containing species used is 2-10 mole times preferably 2-5 mole times of the sulphur present in hydrocarbon fuel. The oxidant solution fed to the reactor B is 2 to 10 wt % of the hydrocarbon fuel. The oxidation in the reactor B is carried out at temperature 20-150° C. preferably 30-100° C. at atmospheric pressure. The contact time between the hydrocarbon and oxidant solution is between 5 to 30 minutes. As determined by GC-SCD there was quantitative oxidation of sulphur compounds present in liquid hydrocarbon fuel to sulphones.

Oxidized hydrocarbon fuel with a sulphur content less than original feed goes out of continuous counter current oxidizer B through line 5 and spent oxidant solution coming out of reactor B through line 6 is recirculated directly to admixing tank A till water content of the spent oxidant solution exceeds a limit. This spent oxidant solution after adding makeup carboxylic acid through line 3 and active oxygen containing species through line 2 is again fed to the oxidizer B. When water content of spent oxidant solution coming out through line 6 exceeds a limit it is fed to flash distillation column C where sulphones fraction extracted by oxidant solution in oxidation reactor B is separated through line 7. Sulphones fraction thus obtained can find its way into coker, hot asphalt stream or any other suitable stream. Overhead stream from flash distillation column C exiting through line 8 is fed to azeotropic distillation column D where water is removed through overhead line 9 and the recovered carboxylic acid after passing through cooler is fed to admixing tank A through line 4.

The oxidized hydrocarbon fuel coming out of oxidizer B through line 5 is fed to acid neutralization column E where the acidic impurities present in oxidizing hydrocarbon fuel are neutralized and removed by washing with alkaline water stream 11. The washed oxidized diesel coming out of column E through line 10 is then taken to continuous counter current extraction column F where the sulphones present in oxidized liquid hydrocarbon fuel along with come of the polycyclic aromatic hydrocarbons are extracted by NMP containing 5-15 wt % antisolvent like water, which is fed through line 12 in the temperature range 20-80° C. The raffinate phase, low sulphur hydrocarbon fuel leaves the extractor F through line 13 and the extract phase coming out through line 14 is fed to solvent recovery column G. In the solvent recovery column G the sulphones and polycyclic aromatic hydrocarbon extracted by NMP antisolvent mixture are recovered by distillation in the temperature range 90-360° C. The sulphones fraction thus obtained through line 16 can be mixed with other sulphones fractions and can find its way into coker, hot asphalt stream or any other suitable stream. The overhead solvent fraction containing NMP and antisolvent coming out of column G through line 15 is taken to solvent dehydration column H, where antisolvent like water is removed from NMP by distillation in the range 80-250° C. The recovered NMP coming out from the bottom of solvent dehydration column H is recycled to the extraction column F after adding suitable amount of antisolvent like water.

Low sulphur liquid hydrocarbon fuel coming out from the extractor column F through line 13 is taken to low sulphur hydrocarbon fuel wash column I, where it is washed with water to remove the entrained NMP. The water phase containing NMP coming out of column I through line 19 is fed to solvent dehydration column H to recover NMP. The washed low sulphur hydrocarbon fuel coming out of column I through line 18 is taken to salt filter column J where water present in the low sulphur hydrocarbon fuel is removed in presence of dehydrating agents like calcium chloride, calcium oxide etc.

The dry low sulfur hydrocarbon fuel coming out of the salt filter column J through line 20 is passed through adsorption column K where the small amount of oxidized sulphur compounds contained in dry low sulphur hydrocarbon fuel are removed by adsorption on nonactivated alumina, silica or clay preferably nonactivated alumina and ultra low sulphur hydrocarbon fuel with sulphur content<10 ppm is obtained through line 21. In this process as the bulk of oxidized sulphur compounds present in oxidized hydrocarbon fuel are removed by solvent extraction with NMP containing antisolvent like water and the adsorption column is used only for final polishing to remove about 40 ppm sulphur present as sulphones, therefore there is very little load on the adsorption column. The adsorption cycle is carried out at ambient temperature and at pressure to ensure reasonable flow rate through the packed column. The desorption cycle is carried out after the adsorption column gets saturated and starts with washing of the adsorption column with a light hydrocarbon stream such as light naphtha to replace the remaining fuel in the adsorbent followed by passing of hot gas or steam to drive off naptha and then removal of oxidized sulfur compounds from bed by passing hot (50-80° C.) polar solvent like methanol.

The following examples are given by way of illustration and therefore should not be construed to limit the scope of this invention.

EXAMPLES

Studies were carried out by using HDS diesel obtained from two Indian refineries as feedstocks and their characteristics are given in table 1

TABLE 1
CHARACTERISTICS OF FEEDSTOCKS
	HDS Diesel	HDS Diesel
Characteristics	feedstock (I)	feedstock (II)
R.I at 20° C.	1.4658	1.4675
Density at 20° C. g/cc	0.8334	0.8418
Aromatics wt % (ASTM 2549)	24.5	27.4
Total sulphur	437 ppm	541 ppm
Cetane index	57.8	53.7
ASTM D 86
IBP	167.9	166.0
5% Recovered, ° C.	216.7	211.8
10% Recovered, ° C.	230.8	240.6
50% Recovered, ° C.	281.8	293.9
90% Recovered, ° C.	347.8	351.0
FBP	376.7	375
Class type analysis wt %
Total saturates	73.8	72.6
Mono Aromatics	20.5	ND
Poly Aromatics	4.0	ND
Total Aromatics	24.5	27.4
ND = not determined

Example 1

A series of experiments were carried out to optimize sulphur to oxidant mole ratio, carboxylic acid to hydrocarbon ratio and reaction time. All these experiments were carried out in a jacketed mixer settler equipped with mechanical stirrer, thermowell, neck for addition of reactant and drain valve. The temperature of reactants was maintained at desired level by circulating hot fluid in the jacket of the mixer settler. In the general experimental procedure, HDS diesel feedstock (I) (100 ml, 83.34 g) was added to the mixer settler and stirrer, hot fluid were started to keep the temperature of the reactant at 50° C. Desired amount of formic Acid (99-100%) was then added to the diesel followed by addition of fixed amount of 30% aqueous Hydrogen peroxide. The mixture was then stirred vigorously at 50° C. for a certain period and then taken out in the separating funnel through drain valves. The two layers namely hydrocarbon layer and formic acid layer were then separated. The diesel layer was then washed with water, aqueous sodium bicarbonate and water to remove entrained formic acid. The washed diesel layer was then dried on anhydrous sodium sulfate and analysed by G C SCD to determine the extent of oxidation of sulphur compounds present in it to sulphones. The results of these experiments are presented in Table 2. From these experiments it was established that for carrying out oxidation of HDS diesel at 50° C. by using mixture of formic acid, 30% aqueous hydrogen peroxide as oxidant.

TABLE 2
OXIDATION OF HDS DIESEL FEEDSTOCK (I)
BY USING FORMIC ACID 30% AQUEOUS HYDROGEN
PEROXIDE MIXTURE AS OXIDANT.
		Formic			Sulphur
	HDS diesel	acid,	Oxidant to	Reaction	compound
S No:	ml(gm)	gm	S mole ratio	time	oxidized(%)
1	100 (83.34)	20	20.00	30	100
2.	100 (83.34)	20	20.00	15	100
3.	100 (83.34)	20	20.00	10	100
4.	100 (83.34)	20	20.00	5	72
5.	100 (83.34)	20	20.00	8	97
6.	100 (83.34)	20	15.00	10	100
7.	100 (83.34)	20	10.00	10	100
8.	100 (83.34)	20	8.00	10	100
9.	100 (83.34)	20	5.00	10	100
10.	100 (83.34)	20	4.00	10	80
11.	100 (83.34)	15	5.00	10	100
12.	100 (83.34)	10	5.00	10	100
13.	100 (83.34)	8	5.00	10	100
14.	100 (83.34)	5	5.00	10	100
15.	100 (83.34)	4	5.00	10	78

In a mixer settler, the minimum requirements for quantitative oxidation of sulphur compounds present to sulphones are: reaction time about 10 min, oxidant (H₂O₂) to sulphur mole ratio about 5 and formic acid about 6% of the HDS diesel.

Example 2

After establishing the reaction parameters in mixer settler, experiments were carried out on oxidative desulphurization using HDS diesel feedstock (I) as feed as per the scheme shown in FIG. 1 in a blockout mode. The general discussion regarding FIG. 1 is given in Detailed description of the process. The oxidation of HDS diesel feedstock (I) was carried out in continuous counter current oxidation column B by using diesel as continuous phase at a flow rate 0.59 kg/hr and formic acid, 30% aqueous hydrogen peroxide mixture at a flow rate 0.29 kg/hr as disperse phase. The temperature of the continuous counter current oxidation column was maintained at 70° C. by passing hot water in the jacket of the column. The characteristics of the oxidized diesel obtained after acid neutralization column are given in Table 3. As seen from the Table 3 while there was no change in the cetane index of the HDS diesel after oxidation, the total sulphur present got reduced from 437 ppm to 365 ppm due to the partial extraction of oxidized sulphur compounds from oxidized diesel into the spent oxidant solution. Also there was slight decrease in aromatic content from 24.5 to 23.2 wt % as determined by ASTM 2549 on oxidation of HDS diesel.

TABLE 3
CHARACTERISTICS OF OXIDIZED DIESEL FEEDSTOCK (I)
                           Oxidized Diesel
Characteristics            feedstock (I)
R.I at 20° C.              1.4653
Density at 20° C. g/cc     0.8362
Aromatics wt % (ASTM 2549) 23.2
Total sulphur              365 ppm
Cetane index               57.8
ASTM D 86
IBP                        167.9
5% Recovered, ° C.         216.7
10% Recovered, ° C.        230.8
50% Recovered, ° C.        281.8
90% Recovered, ° C.        347.8
FBP                        376.7
Class type analysis wt %
Total saturates            73.8
Mono Aromatics             20.0
Poly Aromatics             3.2
Total Aromatics            23.2

The oxidized diesel thus obtained was extracted in the continuous counter current extraction column by using N-Methylpyrolidinone (NMP) containing 10% water as solvent at 50° C. at solvent to feed ratio (S/F)=1 and 2. The data obtained are presented in Table 4.

TABLE 4
CONTINUOUS COUNTER CURRENT
EXTRACTION STUDIES
	Feed:	Oxidized Diesel Feedstock (I)
	Solvent:	NMP + 10% water
	Temperature:	50° C.
TABLE 4
S. No:	Characteristics	S/F(wt/wt)
1.	S/F ratio(wt/wt)	1.0	2.0
2.	Flow rates(kg/hr)
	Feed:	2.983	2.423
	Solvent:	3.042	4.944
	Extract phase	3.087	5.036
	Raffinate phase	2.810	2.225
3.	Feed
	R.I, 20° C.	1.4653	1.4653
	Density, g/cc, 20° C.	0.8362	0.8362
	Total S, ppm	365	365
	Cetane index	57.8	57.8
	Aromatics, wt %	23.2	23.2
4.	Solvent
	R I, 20° c.	1.4637	1.4637
	Density, g/cc, 20° C.	1.0412	1.0412
5.	Extract Phase composition, wt %
	Hydrocarbons	5.13	4.6
	Water	9.2	9.4
	Solvent, wt %	85.6	86.0
6.	Raffinate phase composition, wt %
	Hydrocarbons	93.7	94.1
	Solvent, wt %	6.3	5.9
7.	Raffinate Hydrocarbons
	(Solvent free)
	R.I at 20° C.	1.4620	1.4590
	Density, g/cc at 20° C.	0.8310	0.8275
	Yield	88.3	86.4
	Non - aromatics	—	80.8
	Aromatics	—	19.2
	Total Sulphur, ppm	42	24
	Cetane Index	65.7	67.8
8.	Extract Hydrocarbons
	(Solvent free)
	R.I at 20° C.	1.5296	1.5219
	Density, g/cc at 20° C.	0.9449	0.9339
	Yield	11.7	13.6
	Non - aromatics	ND	21.4
	Aromatics	ND	78.6
	Total Sulphur, ppm	0.40	0.39

The extract phases coming out of the extraction column F were found to contain 85.6 and 86.0 wt % solvent (NMP), 9.2 and 9.4 wt % water, 5.13 and 4.6 wt % hydrocarbons at S/F=1 and S/F=2 respectively. These extract phases were fed to solvent recovery and solvent drying columns to recover sulphone fractions and NMP as described above.

Raffinate phases emerging from extraction column F were found to contain 93.7 and 94.1 wt % of hydrocarbons, 6.3 & 5.9 wt % solvent NMP at S/F=1 and S/F=2 respectively and were washed with water in low sulphur diesel wash column I to remove entrained NMP and then subsequently dried by passing through salt filter column J containing anhydrous calcium chloride. The dried low sulphur diesel samples thus obtained were found to contain 42 & 24 ppm sulphur at S/F=1 and S/F=2 respectively and their cetane index were found to be 65.7 and 67.8 respectively. Dry low sulphur diesel samples thus obtained were polished by passing through adsorption column K filled with 6 to 20 mesh size silica gel. Ultra low sulphur diesel samples obtained from the adsorption column were found to contain 5 and 4 ppm total sulphur respectively.

After saturation of adsorption column diesel trapped in adsorbent was taken out by light naptha and the column was regenerated as described above.

Example 3

Oxidative desulfurization of HDS diesel from another refinery (Feedstock II) was carried out in similar manner as described in Example 2 in a blockout mode. The oxidation of HDS diesel feedstock (II) was carried out in a continuous counter current oxidation column B by passing diesel as continuous phase at flow rate 0.59 kg/hr. and formic acid, 30% aqueous hydrogen peroxide mixture at a flow rate 0.29 kg/hr as discrete phase at 50° C. Characteristics of oxidised diesel obtained after and neutralization step is given in Table-5

TABLE 5
CHARACTERISTICS OF OXIDIZED DIESEL FEEDSTOCK(II)
                           Oxidized Diesel
Characteristics            Feedstock (IT)
R.I at 20° C.              1.4678
Density at 20° C., g/cc    0.8446
Aromatics wt % (ASTM 2549) 26.2
Total sulphur              452 ppm
Cetane index               53.7
ASTM D 86
IBP                        166
5% Recovered, ° C.         211.8
10% Recovered, ° C.        240.6
50% Recovered, ° C.        293.9
90% Recovered, ° C.        351.0
FBP                        375.0
Class type analysis wt %
Total saturates            72.6
Mono Aromatics             ND
Poly Aromatics             ND
Total Aromatics            27.4
ND = not determined

Again while there was no change in the cetane index of the HDS diesel after oxidation, the total sulphur got reduced from 541 ppm to 452 ppm due to partial extraction of the oxidized sulfur compounds from oxidized diesel into the spent oxidant solution. Also there was slight decrease in aromatic content from 27.4 to 26.2 an oxidation of HDS diesel. The oxidised diesel thus obtained was extracted in the continuous counter current extraction column by using NMP containing 10% water as solvent at 50° C. at S/F 1&2 and the data obtained is presented in Table-6. The extract phases obtained from extraction column F were found to contain 85.71 and 86.33 wt % NMP, 9.25 and 9.26 wt % water, 5.02 and 4.41 wt % hydrocarbons at S/F 1 and 2 respectively. These extract phases were processed further to recover sulphone fraction and NMP as described above. The raffinate phase obtained from the extraction column were found to contain 94.01 and 94.16 wt % hydrocarbons, 5.99 and 5.84 wt % NMP at S/F 1 & 2 respectively. These phases were washed with water in the low sulphur diesel wash column I to remove entrained NMP and then subsequently dried by passing through salt filter column J containing anhydrous calcium chloride. The dried low sulphur diesel samples thus obtained were found to contain 47 & 27 ppm total sulfur at S/F 1 & 2 respectively and their cetane index were found to be 63.2 and 64.8 respectively.

TABLE 6
CONTINUOUS COUNTER CURRENT
EXTRACTION STUDIES
	Feed:	Oxidized Diesel Feedstock (II)
	Solvent:	NMP + 10% water\
	Temperature:	50° C.
S. No:	Characteristics	S/F(wt/wt)
1.	S/F ratio(wt/wt)	1.0	2.0
2.	Flow rates(kg/hr)
	Feed:	3.018	2.431
	Solvent:	3.034	4.891
	Extract phase	3.095	5.023
	Raffinate phase	2.948	2.237
3.	Feed
	R.I, 20° C.	1.4678	1.4678
	Density, g/cc, 20° C.	0.8446	0.8446
	Total S, ppm	452	452
	Cetane index	53.7	53.7
	Aromatics, wt %	26.2	26.2
4.	Solvent
	R I, 20° c.	1.4637	1.4637
	Density, g/cc, 20° C.	1.0412	1.0412
5.	Extract Phase composition, wt %
	Hydrocarbons	5.02	4.41
	Water	9.25	9.26
	Solvent, wt %	85.71	86.33
6.	Raffinate phase composition, wt %
	Hydrocarbons	94.01	94.16
	Solvent, wt %	5.99	5.84
7.	Raffinate Hydrocarbons
	(Solvent free)
	R.I at 20° C.	1.4640	1.4610
	Density, g/cc at 20° C.	0.8380	0.8350
	Yield	87.5	85.6
	Non - aromatics	ND	77.6
	Aromatics	ND	22.4
	Total Sulphur, ppm	47	27
	Cetane Index	63.2	64.8
8.	Extract Hydrocarbons
	(Solvent free)
	R.I at 20° C.	1.5310	1.5298
	Density, g/cc at 20° C.	0.9450	0.9350
	Yield	12.5	14.4
	Non - aromatics	—	20.9
	Aromatics	—	79.1
	Total Sulphur, ppm	0.41	0.40

The dry low sulfur diesel samples thus obtained were polished by passing through adsorption column, filled with non-activated alumina. The ultra low sulphur diesel samples obtained from the adsorption column were found to contain 3 & 2 ppm sulfur respectively. After saturation of the adsorption column the diesel contained in it was recovered by displacement with light naphtha and the column was regenerated as described above.

Example 4

As the oxidizing solution consisting of carboxylic acid containing 2 to 10 moles of alkali metal peroxoborate adduct per mole of sulphur has role only in the oxidation of sulphur compounds present in liquid hydrocarbon fuels to sulphones/sulphoxides, experiments were carried out to access its efficiency. Experiments were carried out in a jacketed mixer settler equipped with mechanical stirrer, condenser, thermowel, neck for addition of the reactants and drain valve. The temperature of the reactants was maintained at desired level by circulating by hot fluid in the jacket of the mixer settler. HDS diesel feedstock (100 ml, 83.34 g) was added to the mixer settler and stirrer, hot fluid from circulatory bath were started to keep the temperature of the reactants at 50° C. Formic acid (5 g) was then added to the diesel in the mixer settler followed by addition of sodium perborate tetrahydrate (1.8 g, 10 mole times of sulphur).

The mixture was stirred vigorously at 50° C. for 30 minutes and then taken out in the separating funnel through drain valve. The two layers namely hydrocarbon layer and formic acid layer were then separated. The diesel layer was then washed with water, aqueous sodium bicarbonate and again with water to remove entrained formic acid. The washed diesel layer was then dried on anhydrous sodium sulphate and analyzed by GC SCD, which showed quantitative oxidation of sulphur compounds present to sulphones/sulphoxides. The oxidized diesel thus obtained was extracted with NMP containing 10% water as antisolvent at solvent to feed ratio 2 and the hydrocarbon phase after separation, washing with water, drying on anhydrous sodium sulphate gave sulphur content 45 ppm. The dry low sulphur diesel sample thus obtained on passing through adsorption column filled with silica gel yielded ultra low sulphur diesel containing 5 ppm total sulphur.

Example 5

As the oxidizing solution consisting of polar solvent, organosulphonic acid mixture containing 2 to 10 moles of species containing active oxygen per mole of sulphur has role only in the oxidation of sulphur compounds present in liquid hydrocarbon fuels to sulphones/sulphoxides, experiments were carried out to access its efficiency. Experiments were carried out in a jacketed mixer settler equipped with mechanical stirrer, condenser, thermowel, neck for addition of the reactants and drain valve. The temperature of the reactants was maintained at desired level by circulating by hot fluid in the jacket of the mixer settler. HDS diesel feedstock (100 ml, 83.34 g), acetonitrile (30 ml) were added to the mixer settler and stirrer, hot fluid from circulatory bath were started to keep the temperature of the reactants at 50° C. p-Toluenesulphonic acid monohydrate (1 g) was then added to the diesel to the mixture followed by addition of 50% aqueous hydrogen peroxide (0.839 g, 10 mole times of sulphur).

The mixture was stirred vigorously at 50° C. for 30 minutes and then taken out in the separating funnel through drain valve. The two layers namely hydrocarbon layer and acetonitrile layer were then separated. The diesel layer was then washed with water, aqueous sodium bicarbonate and again with water to remove entrained p-toluene sulphonic acid and acetonitrile. The washed diesel layer was then dried on anhydrous sodium sulphate and analyzed by GC SCD, which showed quantitative oxidation of sulphur compounds present to sulphones/sulphoxides. The oxidized diesel thus obtained was extracted with NMP containing 10% water as antisolvent at solvent to feed ratio 2 and the hydrocarbon phase after separation, washing with water, drying on anhydrous sodium sulphate gave sulphur content 35 ppm. The dry low sulphur diesel sample thus obtained on passing through adsorption column filled with silica gel yielded ultra low sulphur diesel containing 4 ppm total sulphur.

Example 6

As the oxidizing solution consisting of carboxylic acid containing 2 to 10 moles of urea-hydrogen peroxide adduct per mole of sulphur has role only in the oxidation of sulphur compounds present in liquid hydrocarbon fuels to sulphones/sulphoxides, experiments were carried out to access its efficiency. Experiments were carried out in a jacketed mixer settler equipped with mechanical stirrer, condenser, thermowell, neck for addition of the reactants and drain valve. The temperature of the reactants was maintained at desired level by circulating by hot fluid in the jacket of the mixer settler. HDS diesel feedstock (100 ml, 83.34 g) was added to the mixer settler and stirrer, hot fluid from circulatory bath were started to keep the temperature of the reactants at 50° C. Formic acid (5 g) was then added to the diesel in the mixer settler followed by addition of urea-hydrogen peroxide adduct (0.6477 g, 5 mole times of sulphur).

The mixture was stirred vigorously at 50° C. for 30 minutes and then taken out in the separating funnel through drain valve. The two layers namely hydrocarbon layer and formic acid layer were then separated. The diesel layer was then washed with water, aqueous sodium bicarbonate and again with water to remove entrained formic acid. The washed diesel layer was then dried on anhydrous sodium sulphate and analyzed by GC SCD, which showed quantitative oxidation of sulphur compounds present to sulphones/sulphoxides. The oxidized diesel thus obtained was extracted with NMP containing 10% water as antisolvent at solvent to feed ratio 2 and the hydrocarbon phase after separation, washing with water, drying on anhydrous sodium sulphate gave sulphur content 40 ppm. The dry low sulphur diesel sample thus obtained on passing through adsorption column filled with silica gel yielded ultra low sulphur diesel containing 4 ppm total sulphur.

Main Advantages of the Process

• 1. Use of continuous counter current oxidation reactor eliminates need of settler, as settling zones are provided in the reactor itself. This increases efficiency of oxidation process by providing highest concentration of oxidant to tail feed and optimum utilization of oxidant.
• 2. Oxidation step increases polarity of sulfur compounds present in the hydrocarbon fuel leading to their selective extraction by N-methyl pyrrolidinone antisolvent mixture
• 3. The process describes for the first time, use of N-methylpyrrolidinone antisolvent mixture as solvent for extraction of sulphones/sulphoxides from oxidised hydrocarbon fuels with minimum extraction of aromatics.
• 4. Extraction of sulphones/sulphoxides from oxidised hydrocarbon fuels reduces handling and regeneration of large amounts of adsorbent making the process convenient and more feasible
• 5. Cetane index of the hydrocarbon fuels used is increased by 2-20 units due to partial removal of aromatics in extraction.

Claims

1. A process for oxidative desulphurization of liquid hydrocarbon fuel comprising the steps of:

(a) treating hydrocarbon fuel with an oxiding solution consisting of a mixture of oxygen containing species and an organosulphonic or carboxylic acid to oxidize sulphur compound(s) present therein to corresponding sulphones or sulphoxides, wherein the oxidizing agent is used in an amount in the range of 2-10 mole times of sulphur present in hydrocarbon fuel, and separating treated hydrocarbon fuel from the oxidizing solution;

(b) washing separated hydrocarbon fuel obtained in step (a) with water to remove sulphones or sulphoxides therefrom, and extracting the sulphones from hydrocarbon fuels to obtain desulphurised liquid hydrocarbon fuel.

2. A process as claimed in claim 1, wherein step (a) is carried out at a temperature in the range 20-150° C., at a pressure in the range of atmospheric to 10 kg/cm2, and for a period of 2 to 60 min in a batch manner.

3. A process as claimed in claim 1, wherein the sulphones/sulphoxides are removed from the hydrocarbon fuel in step (b) by solvent extraction.

4. A process as claimed in claim 1, wherein in step (a) the carboxylic acid used is selected from the group consisting of formic acid, acetic acid, propionic acid and butyric acid, containing 2 to 10 moles of urea hydrogen peroxide adduct or alkali metal peroxoborate selected from the group consisting of sodium, potassium, magnesium, calcium, barium and strontium peroxoborate, per mole of sulphur to oxidize sulphur compounds to sulphones.

5. A process as claimed in claim 1, wherein the sulphur compounds are oxidized to polar sulphones/sulphoxides in a continuous counter current oxidation reactor by passing the hydrocarbon fuel as a continuous phase, and oxidizing solution containing carboxylic acid and active oxygen containing species as a dispersed phase, at a temperature in the range of 20-150° C., at a pressure in the range of atmospheric to 10 kg/cm2, with residence time of 2 to 60 minutes, washing oxidised fuel with alkaline water followed by extracting resulting polar sulphones/sulphoxides with a mixture of N-methyl pyrrolidinone and 2-20% anti solvent in a continuous current column, at a temperature in the range of 20 to 80° C. to obtain an extract phase containing 4 to 5% hydrocarbons and a raffinate phase containing about 95% hydrocarbons, removing solvent from the extract phase and raffinate phase, followed by passing through a bed of alumina-silica or clay to obtain the desired sulphur free hydrocarbon fuel with 5-10 ppm sulfur.

6. A process as claimed in claim 1, wherein the liquid hydrocarbon fuel is selected from the group consisting of diesel fuel, gasoline, jet fuel, fuel oil, coal liquids and similar petroleum products.

7. A process as claimed in claim 1, wherein the liquid hydrocarbon fuel comprises untreated or hydrotreated liquid hydrocarbon fuel containing thiophene, benzothiophene, dibenzothiophene type sulphur compounds which are refractory in nature in hydrodesulphurization (HDS), preferably hydrotreated liquid hydrocarbon fuels containing less than 500 ppm sulfur.

8. A process as claimed in claim 1, wherein the carboxylic acid used is selected from formic acid, acetic acid, propionic acid and butyric acid.

9. A process as claimed in claim 1, wherein the carboxylic acid used is 2 to 10 wt % of the hydrocarbon fuel.

10. A process as claimed in claim 1, wherein the organosulphonic acid used is selected from the group consisting of p-toluenesulphonic acid, benzenesulphonic acid, methanesulphonic acid, trifluoromethane sulphonic acid and 3-nitrobenzene sulphonic acid.

11. A process as claimed in claim 1, wherein the organosulphonic acid is used as organic solution in an organic solvent.

12. A process as claimed in claim 11, wherein the organic solvent used is selected from acetonitrile and N-methylpyrrolidinone.

13. A process as claimed in claim 1, wherein the oxidizing solution consists of organosulphonic or carboxylic acid and an oxygen containing species and is in the range of 3-5 mole times of sulphur present in hydrocarbon fuel.

14. A process as claimed in claim 1, wherein the temperature in oxidation of sulphur compound present in hydrocarbon fuel is in the range of 30-100° C.

15. A process as claimed in claim 4, wherein the water content of the oxidizing solution containing carboxylic acid and alkali metal peroxoborate is in the range 0-14%.

16. A process as claimed in claim 4, wherein the mole ratio of carboxylic acid or organosulphonic acid to alkali metal peroxoborate used in the oxidizing solution is in the range of 5:1 to 50:1.

17. A process as claimed in claim 4, wherein the mole ratio of carboxylic acid or organosulphonic acid to alkali metal peroxoborate used in the oxidizing solution is in the range of 10:1 to 30:1.

18. A process as claimed in claim 1, wherein the reaction time for oxidation of sulphur compounds present in hydrocarbon fuel is in the range of 7 to 20 minutes.

19. A process as claimed in claim 1, wherein the solvent used for extraction of sulphones/sulphoxides from oxidized hydrocarbon fuel is selected from the group consisting of N,N-dimethylformamide, acetonitrile, methanol, dimethylsulfoxide, sulpholane, and N-methylpyrolidinone containing 2-20% water as antisolvent.

20. A process as claimed in claim 1, wherein the oxygen containing species is selected from 30/50 wt % aqueous H2O2, alkali metal peroxoborate, urea hydrogen peroxide adduct, alkali metal peroxoborate, alkali and alkaline earth peroxide and alkali and alkaline earth hydroperoxide.

21. A process as claimed in claim 20, wherein the alkali metal peroxoborate used is selected from sodium, potassium, magnesium, calcium, barium and strontium peroxoborate.

22. A process as claimed in claim 20, wherein the alkali and alkaline earth peroxide is selected from the group consisting of sodium, lithium, calcium, strontium, barium, magnesium and zinc peroxide.

23. A process as claimed in claim 20, wherein the alkali and alkaline earth hydroperoxide is selected from potassium and sodium hydroperoxide.

24. A process as claimed in claim 1, wherein the mole ratio of carboxylic acid or organosulphonic acid to active oxygen containing species is in the range of 5:1 to 50:1, preferably 10:1 to 30:1.

25. A process as claimed in claim 5, wherein continuous counter current oxidation reactor is used to provide settling zones in the reactor in order to eliminate a settler.

26. A process as claimed in claim 1, wherein the active oxygen containing species is in the range of 2 to 5 mole times of the sulphur present in hydrocarbon fuel.

27. A process as claimed in claim 1, wherein the oxidation of sulphur compounds present in hydrocarbon fuels is carried out for 5 to 30 min residence time.

28. A process as claimed in claim 1, wherein the washing of the oxidized hydrocarbon fuel to remove residual carboxylic acid present is carried out with water containing 0.1 to 10% alkali or alkaline earth metal oxides, hydroxide, carbonate or bicarbonate.

29. A process as claimed in claim 5, wherein the antisolvent used with NMP is selected from water, glycol and sulpholane, preferably water.

30. A process as claimed in claim 29, wherein the anti solvent used in a mixture with NMP is in the range of 5 to 15 wt %.

31. A process as claimed in claim 5, wherein the recovery of NMP from NMP anti solvent mixture obtained as distillate is carried out by distillation of the solvent in drying column in the temperature range 80-250° C.

32. A process as claimed in claim 5, wherein the water is removed from carboxylic acid by distilling the overhead stream.

33. A process as claimed in claim 5, wherein the sulphones/sulphoxides adsorbed on alumina, silica, clay are removed by desorption and solubilization using polar solvent selected from the group consisting of methanol, acetone and acetonitrile, preferably methanol.

34. A process as claimed in claim 1, wherein the cetane index of the treated hydrocarbon fuel used is increased by 2-20 units.