WO2009098264A1

WO2009098264A1 - Process for oxidative desulphurization of a hydrocarbon fuel

Info

Publication number: WO2009098264A1
Application number: PCT/EP2009/051333
Authority: WO
Inventors: Eri Ito; Michiel Makkee; Jacob Adriaan Moulijn; Rudy Wilhelmus Peter Wagemans; Xiaoding Xu
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2008-02-06
Filing date: 2009-02-05
Publication date: 2009-08-13

Abstract

The present invention provides a process for the oxidative desulphurization of organic sulphur compounds in a hydrocarbon fuel, comprising providing a heterogeneous mixture by mixing a hydrocarbon fuel phase and a lactone phase, and contacting the heterogeneous mixture with oxygen to oxidize the organic sulphur compounds to obtain a desulphurised hydrocarbon fuel phase and an oxidized organic sulphur compound- comprising lactone phase The invention further provides a hydrocarbon fuel.

Description

PROCESS FOR OXIDATIVE DESULPHURIZATION OF A HYDROCARBON

FUEL

The present invention provides a process for the oxidative desulphurization of organic sulphur compounds in a hydrocarbon fuel and a hydrocarbon fuel.

Hydrocarbon fuels often contain various quantities of organic sulphur compounds, which may contribute to the formation of acid rain when oxidized to SO_x. Furthermore, this SO_x can deactivate exhaust catalyst in cars. Therefore, recent regulations on fuel formulations limit the amount of organic sulphur compounds . The conventional process to remove organic sulphur compounds from hydrocarbon fuels is hydrogen desulphurization. However, some organic sulphur compounds remain after hydrogen desulphurization. These remaining organic sulphur compounds can be removed from the hydrocarbon fuel by an oxidative desulphurization process. Optionally, the hydrogen desulphurization process can be entirely replaced by an oxidative desulphurization process.

Processes based on the oxidative desulphurization of hydrocarbon fuels can be separated into process using a peroxide as the oxidant and processes using air or oxygen as the oxidant. Using air or oxygen as the oxidant has the advantage that the use of air is significantly cheaper than peroxides such as hydrogen peroxide, t-BuOOH or peracids.

In current air based oxidative desulphurization processes, hydrocarbon fuel comprising an organic sulphur compound is contacted with air in the presence of a catalyst. In Ito et al . (Ito, E et al . , Catalysis Today, issue 116, 2006, p 446 to 460), desulphurization of diesel is discussed. In Ito et al . several processes are described including oxidative desulphurization using among others oxygen as the oxidant . In one of the oxidative desulphurization process of Ito et al . the sulphur components in the diesel are catalytically oxidized using for instance a Co or Fe comprising catalyst. Subsequently, the oxidized sulphur components are extracted from the diesel with a polar solvent. In WO 2005/116169, a process is disclosed for removing sulphur- containing compounds from a fuel by contacting the fuel with air to oxidize the sulphur-containing compounds in the presence of a transition metal oxide catalyst. In WO 2005/116169, the sulphur-containing compound is dibenzothiophene (DBT) . The fuel is contacted with the air for 18 hours before the maximum conversion of DBT is obtained. In addition, WO 2005/116169 mentions that no conversion of DBT is observed in the absence of a catalyst. Following the oxidation, a polar organic solvent is added to the treated fuel to extract the oxidized sulphur-containing compounds from the treated fuel.

We have now found that it is possible to oxidize organic sulphur compounds in a hydrocarbon fuel with air by contacting hydrocarbon fuel, air and specific organic solvents in the absence of an additional catalyst.

Accordingly, the present invention provides a process for the oxidative desulphurization of organic sulphur compounds in a hydrocarbon fuel, comprising providing a heterogeneous mixture by mixing a hydrocarbon fuel phase and a lactone phase, and contacting the heterogeneous mixture with oxygen to oxidize the organic sulphur compounds to obtain a desulphurised hydrocarbon fuel phase and an oxidized organic sulphur compound-comprising lactone phase.

An advantage of the process according to the present invention is that the lactone, besides being an organic solvent, also acts as a catalyst for the oxidation of the organic sulphur compound. As a consequence, no additional catalyst is required, preferably no other catalyst is present.

Furthermore, we have found that the use of lactones significantly shortens the time required to oxidize organic sulphur compounds .

Additionally, the oxidation and oxidized organic sulphur compound solvent extraction can be performed in a single step. Reference herein to a desulphurised hydrocarbon fuel is to a hydrocarbon fuel that has a sulphur content based on the total weight of the hydrocarbon fuel of less than 0.05 wt%(500 ppm by weight), preferably less than 0.005 wt% {50 ppm by weight), more preferably less than 0.001 wt% (10 ppm by weight) .

In the process according to the present invention hydrocarbon fuels comprising organic sulphur compounds are oxidatively desulphurised. Organic sulphur compounds typically found in hydrocarbon fuels include heterocyclic sulphur-containing compounds such as thiophene, benzothiophene (BT) , dibenzothiophene (DBT) , 4 -methyl dibenzothiophene (mDBT) , 4, 6 -dimethyl -dibenzothiophene (dmDBT) and tribenzothiophene .

Generally, these organic sulphur compounds are difficult to remove and can still be found in the hydrocarbon fuel after a hydrogen desulphurization treatment . In the present process the hydrocarbon fuel is mixed with a lactone. Due to the polar nature of the lactone, the hydrocarbon and lactone do not form a homogeneous mixture . A heterogeneous mixture is formed comprising a hydrocarbon fuel phase and a lactone phase. It will be appreciated that some lactone may dissolve in the hydrocarbon fuel phase and form a homogeneous mixture, however the bulk of the lactone, such as 90wt% or more, will form a heterogeneous mixture. The obtained mixture is contacted with oxygen to oxidize the organic sulphur compounds present in the hydrocarbon fuel . In the process according to the invention a desulphurised hydrocarbon fuel phase and oxidized organic sulphur compound- comprising lactone phase are obtained, either in the form of a heterogeneous mixture or as two separate phases.

During the oxidation, the organic sulphur compounds are typically oxidized to their corresponding sulphones . During the oxidation process monoxides of the organic sulphur compounds are formed as intermediate products. However, it was found that these quickly reacted further to sulphones, because the dioxide (i.e. sulphone) is more stable than the monoxide. Although other oxidized organic sulphur compounds may be formed, most of the organic sulphur compounds are typically oxidized to sulphones . Therefore, for simplicity we will refer only to sulphones herein below. It will be appreciated that any such reference includes other oxidized organic sulphur compounds formed .

The formed sulphones are polar and dissolve readily in the polar lactone phase. As a result a desulphurised hydrocarbon fuel phase is obtained together with a sulphone-comprising lactone phase. Preferably, the desulphurised hydrocarbon fuel phase is separated from the sulphone-comprising lactone phase. Due to the polar nature of the lactone the heterogeneous mixture of desulphurised hydrocarbon fuel and lactone is unstable. When the mixing is discontinued, two separate layers are formed which can be separated by means known in the art.

Preferably, the sulphone- comprising lactone is further treated after separation to remove the sulphone. The sulphone-comprising lactone can be treated to remove the sulphones by any suitable process known in the art, including catalytic distillation or catalytic extraction. The sulphone -comprising lactone can be contacted with hydrogen to form H₂S and hydrocarbon. Alternatively, it can be heat treated to form SO₂ and hydrocarbon. The lactone may, optionally after removing the hydrocarbon, be recycled for desulphurization of further hydrocarbon fuel .

In the oxidative desulphurization process according to the invention the heterogeneous mixture comprising the hydrocarbon fuel phase and the lactone phase is contacted with oxygen under oxidation conditions.

Preferably, the heterogeneous mixture is contacted with oxygen at a temperature in the range of from 80⁰C to 25O⁰C, more preferably from 130⁰C to 23O⁰C. Even more preferably, the heterogeneous mixture and oxygen are contacted at a temperature in the range of from 130 to 160 ⁰C. It will be appreciated that the choice of the reaction temperature is typically influenced by factors such as the properties of the hydrocarbon fuel being treated and the desired level of conversion.

The pressure at which the heterogeneous mixture is contacted with oxygen is typically low. However, it is preferable that the pressure is high enough to prevent significant evaporation of the fuel. Preferably, the contacting takes place at pressures in the range of from 0.8 to 5 bar, more preferably 1 to 2.5 bar, even more preferably 1 to 1.5 bar, still more preferably ambient pressure is used.

It has been found that the process may also be operated at higher temperatures and/or pressures. Organic sulphur compound conversion rates have been found to increase under such conditions, however severe coke production may be observed at conditions of high temperature and/or pressure.

In the process according to the invention the heterogeneous mixture comprising the hydrocarbon fuel phase and a lactone phase is contacted with oxygen for a time sufficient to achieve the desired conversion of organic sulphur compounds. It will be appreciated that the required contact time is influenced by the method of contacting. Preferably, the mixture is contacted with oxygen for a time in the range of from 2 to 10 hours, more preferably of from 5 to 8 hours.

The hydrocarbon fuel to be desulphurised in the process according to the invention may any organic sulphur compound-comprising hydrocarbon fuel. Preferably the fuel is liquid under the oxidation conditions.

Suitable hydrocarbon fuels include straight run fuels from an atmospheric crude distiller, the effluents from a thermal cracker, visbreaker, hydrocracker or FCC. Such fuels may for example be heavy cycle oil, diesel, gasoline, kerosene or fuel oil. Optionally, the hydrocarbon fuel has been subjected to a hydrodesulphurization (HDS) prior to feeding it to the oxidative desulphurization process according to the invention. Hydrocarbon fuels suitable for the process according to the invention may comprise in the range of from 0.001 wt% {10 ppm by weight) to 5 wt% sulphur or sulphur compound, preferably of from 0.002 wt% (20 ppm by weight) to 2 wt% sulphur, based on the total weight or the hydrocarbon fuel. It will be appreciated that some of the above mentioned suitable hydrocarbon fuels already fall within the definition of desulphurised hydrocarbon fuel given herein above. However, the sulphur or sulphur compound contents of these fuel may be reduced further, preferably by at least 10wt%, more preferably by at least 50wt%.

The lactone may be any suitable polar lactone, which can dissolve sulphones . Preferably, the lactone is a butyrolactone or valerolactone, more prefereably, γ- butyrolactone (GBL) , γ-valerolactone (GVL) or δ- valerolactone (DBL) . These lactones combine high sulphone solubility with high thermal stability. γ-Butyrolactone is especially preferred as it is cheap. Oxygen solubility in the lactone is low, typically below IxIO^"3 raol 0₂/mol lactone. This ensures that the oxidation takes place under oxygen- lean conditions, reducing oxidation side reactions such as the oxidation of the lactone. The lactone itself does not act as an oxidant, even if no oxygen is present.

Preferably, the lactone is obtained from biomass. The oxygen may be provided as pure oxygen or in the form of air or oxygen-enriched air. Preferably, air is used as this is a cheap source or oxygen and is readily available.

In the process according to the invention a mixture comprising hydrocarbon fuel and a lactone is provided. Preferably, the mixture comprises in the range of from 20 to 80 wt%, more preferably, 40 to 60 wt% of lactone based on the weight of the mixture. If desired other solvents, such as paraffinic solvents, may be included in the mixture. Such paraffinic solvents may improve the separation of the obtained desulphurised hydrocarbon fuel phase and the sulphone-comprising lactone phase. Preferably, hydrocarbon fuel and lactone are provided in a 1:1 weight ratio. The presence of small amounts of water, e.g. less than lwt% based on the total weight of the mixture, in the mixture results in a slightly lower conversion compared to the water- free conversion. However, water was not found to be detrimental to the process.

As mentioned herein before, the hydrocarbon fuel and the lactone do not readily mix. Therefore, the mixture should be continuously mixed. The oxygen, e.g. air, should be contacted with the heterogeneous mixture. It will be appreciated that the contact area of the hydrocarbon fuel phase, the lactone phase and the oxygen is of influence on the mass-transfer and reaction rates. Also it will equally be appreciated that mixing rates and the method of contacting the heterogeneous mixture with the oxygen influence the contact area of the hydrocarbon fuel phase, the lactone phase and the oxygen. However, it has been observed that although the mixing rate and method of contacting the heterogeneous mixture with oxygen are important, they are not critical to the operation of the process.

The desulphurised hydrocarbon fuel and sulphone- comprising lactone may be separated by any means known in the art.

Suitably, mixing of the hydrocarbon fuel phase and the lactone phase and separation of the obtained - S - desulphurised hydrocarbon fuel and sulphone~comprising lactone can be performed using known mixer- settler devices comprising means to additionally allow for contacting the mixture with oxygen, for instance by bubbling air thought the heterogeneous mixture during mixing.

The obtained desulphurised hydrocarbon fuel phase may be further treated, preferably after separation form the sulphone-comprising lactone phase, to remove any remaining sulphone or lactone. Typically, the remaining lactone and any dissolved sulphone can be separated from the desulphurised hydrocarbon fuel by gravity separation or centrifuging. After removal of the sulphone, the lactone can subsequently be recycled. The desulphurised hydrocarbon fuel can be further processed, such as by washing with water or adsorption using silica gel or alumina, to remove traces of the solvent.

In an alternative process for oxidative desulphurization of a hydrocarbon fuel, the hydrocarbon fuel is first contacted with a lactone to remove at least part of the organic sulphur compounds by solvent extraction, whereby a desulphurised hydrocarbon fuel and an organic sulphur compound-comprising lactone is obtained. The desulphurised hydrocarbon fuel and organic sulphur compound- comprising lactone are subsequently separated and the organic sulphur compound- comprising lactone is contacted with oxygen to oxidize the organic sulphur compounds in the lactone. The obtained lactone with dissolved therein oxidized organic sulphur compounds, i.e. sulphones, can be further treated as described herein above. This alternative process for oxidative desulphurization of a hydrocarbon fuel can be performed under the same process conditions as described herein above .

In another aspect the inventions relates to a desulphurised hydrocarbon fuel, in particular diesel, obtainable by the oxidative desulphurization process according to the invention. As mentioned herein above some of the lactone is oxidized during the oxidative desulphurization. These oxidized lactone components may be soluble in hydrocarbon fuels and especially suitable as fuel additives, e.g. in diesel fuel. In case the lactone is derived from biomass, the oxidized lactones compounds are bio-additives and reduce the carbon footprint of the obtained hydrocarbon fuel .

In Figure 1 a schematic representation is given of an embodiment of the process according to the invention. In figure 1, hydrocarbon fuel 1 is provided together with lactone 3 to oxidative desulphurization reactor 5. Optionally, a mixture of hydrocarbon fuel 1 and lactone 3 is provided to the reactor. Hydrocarbon fuel 1 and lactone 3 are continuously mixed in reactor 5, while air 7 is supplied to reactor 5 to provide the oxygen required for the oxidation of organic sulphur compounds to sulphones . Oxygen depleted air is obtained from reactor 5 as exhaust 9. If desired, exhaust 9 can be further treated to remove any hydrocarbon compounds, which evaporated in reactor 5. Mixture 11, comprising desulphurised hydrocarbon fuel and sulphone-comprising lactone, is withdrawn from reactor 3 and supplied to settler 13, where is separates into a layer comprising predominantly desulphurised hydrocarbon fuel 15 and a layer comprising predominantly sulphone-comprising lactone 16. After the separation is complete, the desulphurised hydrocarbon fuel 17 is removed from settler 13 and may be further treated if desired. Sulphone - comprising lactone 18 is withdrawn from settler 13 and supplied to separation unit 19, which may for instance be a distillation of extraction unit, to separate the sulphone and the lactone. Sulphone 21 can be heat treated or hydrotreated to convert sulphone 21 to hydrocarbons and respectively SO₂ or H₂S. Optionally, the separation of the lactone and the sulphone can be combined with the treatment of the sulphone by replacing separation unit 19 by a reactive separation unit such as catalytic distillation. Lactone 23 obtained form separation unit 19 can be recycled to reactor 3 to from part of the lactone provided to reactor 3.

The present invention is illustrated by the following non- limiting examples.

The oxidation behavior of organic sulphur compounds was measured in several solvents (n-tetradecane and γ - butyrolactone) and in a real diesel fuel. In addition a comparison was made with reactions in which further catalysts were added besides lactone. Additionally, the stability of y-butyrolactone (GBL) was determined under oxidation conditions.

Oxidative desulphurization (ODS) experiments were performed in a glass, stirred reactor with bubbling air through the liquid at 140 ⁰C with or without a catalyst. Ambient pressures were applied. For ODS of DBT GC analysis was conducted using a Varian CP-3380 GC with FID, a 6 meter CP Sil-5 GC column and a Varian autosampler CP- 8400. The integral GC peak area percentages of organic sulphur compound, the solvent and the products were used to calculate the conversions.

For ODS of real diesel, fuel samples were subjected to Sulphur-sensitive GC analysis using a HP 5890 series II GC, equipped with the same type of column with a Sievers 335 Sulphur Chemiluminescence Detector (SCD) . The integral peak area is proportional to the amount of sulphur in the sample . In Example 1, the effect of the choice of solvent and the presence of a catalyst on the oxidation of organic sulphur compounds was measured. A solution of 0.2 wt% DBT in n-tetradecane {C14) , γ -butyrolactone (GBL) or a mixture thereof was contacted with oxygen, by bubbling air through the solution, at a temperature of 14O⁰C. As model catalysts 10% V on SiO₂ and 5% MnO₂/3% Co₃O₄ on Al₂O₃ were used (0.1 gram of catalyst per 50 gram of solvent) . The DBT conversion (X_DBT) was measured in time for each solution. The results are shown in Table 1.

Table 1

* not according to the invention Table 1 shows that organic sulphur compounds can effectively be oxidized in the presence of a lactone. It can be seen from Table 1 that the best results are obtained when besides lactone, no additional catalyst is present at all. The experiment using a paraffin solvent (C14) in the absence of either a lactone or another catalyst showed low conversion of organic sulphur compounds .

In Example 2, the conversion of 4 , 6~dimethyl~ dibenzothiophene (dmDBT) was measured in a solution of 0.167 wt% dmDBT in GBL. The solution of dmDBT in GBL was contacted with oxygen, by bubbling air through the solution, at a temperature of 140⁰C. The conversion of dmDBT in time is shown in Table 2.

Table 2

In Example 3, the simultaneous conversion of thiophene, benzothiophene (BT) , dibenzothiophene (DBT) , 4-methyl dibenzothiophene (tnDBT)and 4 , 6-dimethyl- dibenzothiophene {dmDBT) was measured in a solution comprising a mixture of equimolar {2.22 XlO^"3 M (mol per litre) } amounts of thiophene, BT, DBT, MDBT, dmDBT in GBL. The solution of the mixture of organic sulphur compounds in GBL was contacted with oxygen, by bubbling air through the solution, at a temperature of 14O⁰C. The observed conversions in the mixture in time are shown in Table 3. Table 3

In Example 4, GBL solutions with different DBT concentration were desulphurised using oxidative desulphurization. Oxidative desulphurization was performed in the absence of a catalyst. Table 4 shows the obtained DBT conversions (X_DET) versus the reaction time.

Table 4

*extrapolated value

For oxidative desulphurization of 0.02 wt . % or 0.2 wt. % DBT in GBL at 140 ⁰C, complete conversion was obtained within 3 hours and 5 hours, respectively. For the sample comprising 2 wt . % DBT in GBL, conversion increased at a constant rate between 2 and 7 hours reaction time. By extrapolating the measured conversion between 2 and 7 hours it can be predicted that full conversion is obtained with in 10 hours of reaction time.

In Example 5, the oxidative desulphurization process according to the invention was also performed using a real diesel fuel. The diesel fuel was first hydrodesulphurised to remove raercaptans and other relatively easy removable sulphur compounds . The resulting hydrocarbon fuel is a diesel fuel comprising 0.0044 wt% sulphur (44 ppm by weight) .

30 gram of the diesel was mixed with 30 gram of GBL. The mixture was contacted with oxygen at a temperature of 140⁰C. The depletion of the sulphur content in the diesel was followed for a period of 7 hours as shown in Table 5. After 4 hours no sulphur compounds are detected in the diesel phase sulphur.

Table 5

In Example 6, the stability of GBL under oxidation conditions was determined. A sample of 60 grams of GBL was continuously contacted with air, by air bubbling through the GBL, for a period of 80 hours at a temperature of 14O⁰C. The observed consumption of GBL in time is shown in Table 6. The results shown in Table 6 show that after SO hours only 12 wt% of the GBL was consumed, i.e. converted by oxidation. Experiments using N-methylpyrrolidone (NMP) , which is a good solvent for both the organic sulphur compounds and sulphone showed an immediate distinct color change of the NMP, indicating that the NMP reacted with the oxygen.

Table 6

Claims

C L A I M S

1. Process for the oxidative desulphurization of organic sulphur compounds in a hydrocarbon fuel, comprising providing a heterogeneous mixture by mixing a hydrocarbon fuel phase and a lactone phase, and contacting the heterogeneous mixture with oxygen to oxidize the organic sulphur compounds to obtain a desulphurised hydrocarbon fuel phase and an oxidized organic sulphur compound- comprising lactone phase.

2. Process according to claim 1, wherein the heterogeneous mixture is contacted with oxygen in the absence of an additional catalyst.

3. Process according to claim 1 or 2 , wherein the heterogeneous mixture is contacted with oxygen at a temperature in the range of from 80 to 25O⁰C, preferably 130 to 230⁰C.

4. Process according to any one of the preceding claims, wherein the lactone is a butyrolactone or valerolactone, prefereably, γ -butyrolactone, γ -valerolactone or δ- valerolactone .

5. Process according to any one of the preceding claims, wherein the heterogeneous mixture of hydrocarbon fuel and lactone comprises in the range of from 20 to 80 wt% of lactone, preferably of from 40 to 60 wt% of lactone based on the weight of the total heterogeneous mixture.

6. Process according to any one of the preceding claims, wherein the oxygen is comprised in air.

7. Process according to any one of the preceding claims, wherein the heterogeneous mixture is contacted with oxygen in the range of from 2 to 10 hours, preferably of from 5 to 8 hours .

8. Process according to any one of the preceding claims, further comprising separating the desulphurised hydrocarbon fuel phase from the oxidized organic sulphur compound-comprising lactone phase.

9. Process according to any one of the preceding claims, wherein the oxidized organic sulphur compound-comprising lactone phase is treated to remove the oxidized organic sulphur compounds .

10. Process according to claim 9, wherein the oxidized organic sulphur compounds are converted to hydrocarbons and SO₂ or H₂S.

11. Process according to any one of the preceding claims, wherein the lactone is obtained from biomass.

12. Desulphurised hydrocarbon fuel obtainable by a process according to claim 11, preferably desulphurised diesel obtainable by a process according to claim 11.