EP2338955A1

EP2338955A1 - Selective removal of aromatics

Info

Publication number: EP2338955A1
Application number: EP09252723A
Authority: EP
Inventors: designation of the inventor has not yet been filed The
Original assignee: BP Oil International Ltd
Current assignee: BP Oil International Ltd
Priority date: 2009-12-03
Filing date: 2009-12-03
Publication date: 2011-06-29

Abstract

According to the present invention there is provided a process for reducing the concentration of one or more aromatic compounds in a feedstock comprising one or more aromatic compounds and one or more olefinic compounds, which process comprises contacting the feedstock with an ionic liquid comprising an anionic component and a cationic component to produce a product with a higher mole ratio of olefinic compounds to aromatic compounds than the feedstock, and an ionic liquid phase with a lower mole ratio of olefinic compounds to aromatic compounds than the feedstock, wherein the surface charge profile of the cationic component has a maximum value at a charge density value (σ) in the range -0.0085 < σ < -0.0040 e/Å², and wherein from 25% to 65% of the molecular surface area of the anionic component has a charge density value (σ) in the range -0.0085 < σ < +0.0085 e/Å², where e represents the charge of an electron.

Description

This invention relates to the selective removal of aromatics from a hydrocarbon mixture, more specifically to the selective removal of aromatics from a hydrocarbon mixture additionally comprising olefins.
Legislation is increasingly requiring the reduction of benzene concentrations in gasoline. As gasoline is predominantly produced from the refining of crude oil, then refining processes need to be modified and/or new processes developed in order to keep pace with such legislative requirements.
The major sources of benzene and other aromatic compounds in gasoline are refinery streams derived from catalytic reforming, and also naphtha streams derived from fluid catalytic cracking (cracked naphtha).
In catalytic reforming, a straight run naphtha (i.e. naphtha taken directly from the crude oil distillation unit (CDU) of a refinery), or after further distillation to remove lighter components, predominantly comprises linear alkanes which typically have poor octane ratings. To improve octane rating, they are often subjected to catalytic reforming processes, in which reactions such as the conversion of n-paraffins to iso-paraffins and cyclic alkanes to aromatics occur. The UOP Platforming™ process is an example of such a catalytic reforming process (Platinum Metals Review, 1961, 5 (1), 9-12). Other reforming processes are described in the Kirk-Othmer Encyclopaedia of Chemical Technology, .
In fluid catalytic cracking, a heavy crude oil fraction, for example vacuum gas oil, is contacted with a solid acid catalyst, typically zeolite Y or USY, which breaks alkanes into molecules with fewer carbon atoms. Aromatic hydrocarbons and olefins are also produced. Catalytic cracking processes are described in Kirk-Othmer Encyclopaedia of Chemical Technology, .
One process for reducing the concentration of aromatics in a hydrocarbon mixture is by hydrogenation, for example hydrodearomatisation processes in which the aromatic-containing composition and hydrogen are contacted with a catalyst comprising a Group VIB and Group VIII element, for example as described in US 1,965,956 , or a catalyst additionally comprising boron and a carbon support, as described in US 5,449,452 .
However, although such processes convert aromatic compounds to the corresponding naphthenic compounds, olefins that are also present in the initial hydrocarbon mixture are also hydrogenated to the corresponding paraffins. This is undesirable because olefins have a beneficial effect on octane rating, and hence their presence in a gasoline fuel can be advantageous.
Another method of removing aromatic compounds is to use distillation. However, to separate aromatics from olefins and alkanes in a gasoline composition, or in one or more naphtha feeds that can be blended to produce gasoline, a large distillation column with a large number of trays would be required, which would be highly energy and capital intensive. Additionally, the formation of azeotropes between some components can limit the extent of separation that is possible by distillation.
A further method is to use solvent extraction. Solvent extraction may be performed using a solvent such as N-methyl-2-pyrrolidine, as described by Sergeant et al in Fuel Processing Technology, 41, 1995, 147-157. Another such solvent is sulfolane (2,3,4,5-tetrahydrothiophene-1,1-dioxide). GB 1,008,921 discloses a process in which sulfolane is used to selectively dissolve aromatic compounds in a hydrocarbon stream. A problem with this process, however, is that the solvent recovery and recycle is energy intensive; see Meindersma et al in Chem. Eng. Res. Design, 86 (2008), 745-752. Additionally, solvent is continually lost from the process, and therefore requires continuous replenishment.
Ionic liquids are non-volatile liquid salts. They have found application in separation technology, for example in the separation of paraffinic molecules from aromatic molecules or olefinic molecules.
For instance, Meindersma et al in Fuel Processing Technology, 87 (2005), 59-70 describe a process for the separation of C₄ to C₁₀ aliphatic alkanes and aromatic compounds such as benzene, toluene, ethyl benzene and xylenes using ionic liquids. It was found that the ionic liquids [Mebupy]BF₄, [Mebupy]CH₃SO₄, [BMIM]BF₄ and [EMIM]tosylate can achieve superior toluene/heptane separation compared to sulfolane.
Meindersma et al have also shown, in American Institute of Chemical Engineers Annual Meeting, November 2004, that the above-mentioned ionic liquids, as well as the ionic liquids [EMIM]HSO₄, [MMIM]CH₃SO₄ and [EMIM]C₂H₅SO₄, can be used to separate toluene/heptane mixtures. In Chem. Eng. Res. Design, 86 (2008), 745-752, Meindersma et al describe how toluene/heptane separation using [Mebupy]BF₄ can be performed in a rotating disc separation column.
The same authors further describe, in Fluid Phase Equilibria, 247 (2006), 158-168, that the ionic liquids [Mebupy]BF₄, [EMIM]C₂H₅SO₄, [EMIM]CH₃SO₄ and [BMIM]CH₃SO₄ achieve greater separation of heptane and toluene compared to sulfolane.
Meindersma et al, in Chem. Eng. Comm., 193(11), 1384-1396, 2006, additionally state that [EMIM]HSO₄ also achieves superior toluene/heptane separation compared to sulfolane, and that [Mebupy]BF₄, [Mebupy]CH₃SO₄ and [BMIM]BF₄ are the most suitable at 40 °C, with [EMIM]tosylate also being most suitable at 75 °C.
The use of the ionic liquid [BMIM]PF₆ in the separation of ethylbenzene from octane is described by Zhu et al in Separation and Purification Technology, 56 (2007), 237-240.
The separation of nonane or undecane from benzene, toluene or m-xylene using [BMIM]PF₆ is described in Maduro et al in Fluid Phase Equilibria, 265 (2008), 129-138.
The extraction of aromatic hydrocarbons from aromatic/aliphatic mixtures using chloroaluminate ionic liquids is described by Zhang et al in Energy & Fuels, 2007, 21, 1724-30.
Anjan, in Chem. Eng. Progress, December 2006, 102(12), 30-39, also discusses alkane (paraffin)/aromatic separation, in particular hexane/benzene separation using different ionic liquids.
EP-A-1 854 786 describes the use of an ionic liquid to extract aromatic compounds from a mixture containing at least one aliphatic hydrocarbon. The ionic liquid comprises a cation having an aromatic nitrogen-containing heterocyclic ring system, in which one of the nitrogens in the aromatic ring is quaternised, and the ring has at least one electron-withdrawing substituent.
US 7,019,188 describes the use of ionic liquids in separating one or more of olefins, diolefins and aromatics from mainly paraffinic hydrocarbon streams.
US 6,623,659 describes a process for separating olefinic from non-olefinic hydrocarbons such as paraffins, cycloparaffins, oxygenates and aromatics using metal ions, in particular copper or silver ions, dissolved in ionic liquids, in which the metal salts are used to form a complex with the olefinic compounds and retain them in the ionic liquid phase.
DE 10154052 describes the use of ionic liquids for separating aromatics from other hydrocarbons, and exemplifies benzene/cyclohexene separation using the ionic liquid [EMIM](CF₃SO₂)₂N. However, very low selectivity was achieved.
There remains a need for an improved process for selectively separating aromatics from a feedstock which additionally comprises olefins.
According to the present invention, there is provided a process for reducing the concentration of one or more aromatic compounds in a feedstock comprising one or more aromatic compounds and one or more olefinic compounds, which process comprises contacting the feedstock with an ionic liquid comprising an anionic component and a cationic component to produce a product with a higher mole ratio of olefinic compounds to aromatic compounds than the feedstock, and an ionic liquid phase with a lower mole ratio of olefinic compounds to aromatic compounds than the feedstock, wherein the surface charge profile of the cationic component has a maximum value at a charge density value (σ) in the range -0.0085 < σ < -0.0040 e/Å², and wherein from 25% to 65% of the molecular surface area of the anionic component has a charge density value (σ) in the range -0.0085 < σ < +0.0085 e/Å², where e represents the charge of an electron.
The feedstock comprises one or more aromatic compounds. In an embodiment, the one or more aromatic compounds comprise one or more monoaromatic compounds, for example selected from benzene, toluene, ethylbenzene, propylbenzene, isopropylbenzene, xylenes, ethyltoluenes, trimethylbenzenes, diethylbenzenes, n-butyl benzene and tetramethylbenzenes, and/or one or more compounds with more than one aromatic ring, for example selected from naphthalene, methylnaphthalenes, fluorene, methylfluorenes, phenanthrene, anthracene, methylphenanthrenes, methyl-isopropyl-phenanthrenes, dimethylphenanthrenes, methylanthracenes, fluoranthrene, pyrene, methylpyrenes, benzofluoranthene, cyclopentopyrene, benzanthracene, benzopyrenes, perylene, indenopyrene and benzoperylene. Such aromatic compounds are often found in refinery streams for use in the production of fuels such as gasoline and/or diesel. In an embodiment, the one or more aromatic compounds are selected from benzene and toluene.
The feedstock also comprises one or more olefinic compounds. In an embodiment, the one or more olefinic compounds are selected from linear, cyclic and branched alkenes, for example selected from C₄ to C₁₀ olefins such as butenes, pentenes, hexenes, hexadienes, heptenes, octenes, cyclohexene, cyclooctene, and/or larger molecules such as C₁₁ to C₂₀ olefins, for example hexadecene or octadecene. In an embodiment, the one or more olefinic compounds include 1-hexene. The olefinic compound can have one carbon-carbon double bond or a plurality of carbon-carbon double bonds. Examples of olefinic compounds containing a plurality of carbon-carbon double bonds include 1,3-butadiene and dicyclopentadiene.
The feedstock can be a process stream derived from the refining of crude oil, or a mixture of two or more process streams from crude oil refining. Typically, the feedstock is predominantly comprised of hydrocarbons, but it may also contain heteroatom-containing organic molecules such as organo-nitrogen and organo-sulphur compounds.
Ionic liquids are generally defined as low melting point salts, i.e. compounds having an anionic component and cationic component, and which are typically liquids at temperatures below 150 °C. In the present invention, the ionic liquids are preferably liquid at temperatures below 100 °C. Individual ionic liquids or mixtures of two or more ionic liquids can be used.
The surface charge profile of a molecule can be calculated using quantum mechanics calculations, based for example on density functional theory. By way of illustration, calculations may be performed using the software "COSMO-RS" (Conductor-like Screening Model for Real Solvents), the principles of which are described in detail by Klamt in "COSMO-RS, From Quantum Chemistry to Fluid Phase Thermodynamics and Drug Design", published by Elsevier (1st Edition, 2005). In essence, COSMO-RS theory considers all molecular interactions to consist of local pair-wise interactions of segments of molecular COSMO-surfaces. Quantum chemical COSMO calculations provide a discrete surface around a molecule embedded in a virtual conductor. Each segment, i, of the surface is characterised by its area (a_i) and the screening charge density (σ_i), which takes into account the electrostatic screening of the molecule by its surrounding, and the back-polarization of the molecule. In a virtual conductor, screening is considered to be perfect, and so the σ_i value is related predominantly to back-polarization. The total energy of the ideally screened molecule (E_cosmo) is also provided. Within COSMO-RS theory, a liquid is considered to be an ensemble of closely packed ideally screened molecules. Thermodynamic properties of compounds are calculated from statistical averaging in the ensemble of interacting surface segments. To describe the composition of the molecular ensemble with respect to the interactions, the probability distribution of σ has to be known for all compounds, X_i. Such probability distributions, p^x(σ) are termed "σ-profiles". The σ-profile of the whole system or mixture, p_s(σ), is a sum of the σ-profiles of the components X_i weighted with the mole fraction in the mixture, x_i.
The surface charge profile, or σ-profile, of a molecule or ion can be represented graphically with charge density on the abscissa, and a frequency value on the ordinate related to the surface area of the molecule or ion having that charge density. The charge density is often represented by the term sigma (σ), and is expressed in units of charge per unit area, for example e/Å², where e represents the charge of an electron, or the negative of the charge of a proton, otherwise termed the "elementary charge". The elementary charge, is taken to be 1.602176 x 10^-19 C. The surface area of a molecule or ion associated with a particular charge density value can be expressed in a variety of ways, for example as a surface area value itself, i.e. the surface area of the molecule or ion having that charge density, or as a relative value such as the percentage of the total surface area of the molecule or ion having that charge density.
The anionic and cationic components of the ionic liquid are preferably selected so as to match the surface charge profile of the one or more aromatic compounds in the feedstock. Matching the surface charge profiles enables better interaction between the ionic liquid components and the one or more aromatic compounds, which improves their solubility in the ionic liquid and provides a more efficient selective removal from the feedstock. Preferably, the surface charge profile differs significantly from that of the one or more olefinic compounds.
It has been found that effective separation can be achieved by using a cationic component having a surface charge profile with a maximum at charge density values, σ, in the range -0.0085 < σ < -0.0040 e/Å², and an anionic component in which at least 25% and up to 65% of its molecular surface area has a charge density value in the range -0.0085 < σ < +0.085 e/Å². Below 25%, the capacity for the aromatic tends to be insufficient. Above 65%, selectivity tends to be too poor for effective separation. Preferably, from 30 to 60% of the anion molecular surface area has a charge density value in the range -0.0085 < σ < +0.085 e/Å².
Examples of cationic components falling within the above definition include imidazolium, pyrrolidinium and pyridinium cations having one, two or three alkyl substituents, the alkyl substituents typically having from 1 to 6 carbon atoms, for example 1 to 4 carbon atoms. In an embodiment, the cationic component is selected from dialkyl-substituted imidazolium cations such as 3-butyl-1-methyl-imidazolium [BMIM], dialkyl-substituted pyridinium cations such as 3-methyl-N-butyl pyridinium [3-Mebupy], and dialkyl-substituted pyrrolidinium cations, such as 1-methyl-3-butyl pyrrolidinium [Mebupyrr]. Additional examples include N-containing organic cations with two aromatic rings, for example quinolinium and guanidinium cations and alkyl-substituted analogues thereof. [3-Mebupy] and [BMIM] are particularly effective as the cationic component.
Examples of suitable anionic components include dicyanamide [N(CN)₂; DCA], tricyanomethanide [C(CN)₃; TCM], SCN and B(CN)₄.
The performance of an ionic liquid in separating aromatic compounds from olefinic compounds in a feedstock can be expressed using two parameters. The distribution coefficient, D, of a component is defined as the ratio of the concentration of that component in the ionic liquid, C^IL, compared to the concentration of that component remaining in the product, C^Prod. Concentrations can be expressed in any units, for example on a molar basis such as mol/L, on a weight basis (w/w), such as g/g or kg/kg, on a volume basis (v/v), such as L/L or mL/mL, or a weight per volume basis (w/v), such as g/mL or kg/L
For an aromatic compound, the distribution coefficient, D_arom, can be represented as shown in Equation 1: $D_{arom} = {C_{arom}}^{IL} / {C_{arom}}^{Prod}$
Similarly, the equivalent distribution coefficient for an olefin, D_olefin, can be represented as shown in Equation 2: $D_{olefin} = {C_{olefin}}^{IL} / {C_{olefin}}^{Prod}$
Where the D value is high, the relative concentration of the component in the ionic liquid phase is high compared to the product phase, and represents high solubility of that component in the ionic liquid phase. For selective separation of aromatic compounds from a hydrocarbon feedstock comprising olefinic compounds, preferred ionic liquids will have a high D_arom value, and a low D_olefin value. High D values also indicate that the ionic liquid has a high capacity for the relevant component.
The selectivity, S, of one component over another can be expressed in terms of the ratio of the distribution coefficients for the two components to be separated. The selectivity, S, for aromatic/olefin separation can be calculated according to Equation 3: $S = D_{arom} / D_{olefin}$
Thus, a high S value represents relatively higher aromatic solubility in the ionic liquid compared to olefin.
Preferred ionic liquids combine the benefits of high distribution coefficients with high selectivity, i.e. have high D_arom and S values.
The ionic liquid preferably has an S value greater than 0.75 times that of the corresponding S value for sulfolane, and more preferably has an S value greater than that of the corresponding S value for sulfolane. Typically the S value is greater than 5.25, for example greater than 5.5 or greater than 10, wherein the corresponding D_arom and D_olefin values are calculated from the respective C^IL and C^Prod values expressed on a w/w basis, typically g/g. Higher D_arom values represent high capacity for an aromatic compound, whereas higher S values are advantageous in that the selectivity towards aromatic separation compared to olefinic separation is greater. Therefore, a combination of high D_arom and high S values is preferred, as this may reduce the size of separation vessels and the quantities of ionic liquids required for achieving a sufficient level of separation. In addition, high S values may correspond to a reduced quantity of olefin removed from the feedstock, which is beneficial towards products that are used as or in the production of gasoline fuels for maintaining high octane rating, and/or reducing the negative impact on octane rating through removal of the aromatics. Additionally, the purity of aromatics that can be recovered from the ionic liquid phase may be higher.
Although dependent on the D_arom and S values of the ionic liquid, the volume ratio of ionic liquid to hydrocarbon feedstock fed to the separator is typically in the range of from 10:1 to 0.001: 1.
Preferred ionic liquids that have shown particularly good separation activity, in terms of capacity and selectivity, include [3-Mebupy][C(CN)₃]_, [3-Mebupy][N(CN)₂], [BMIM][C(CN)₃] and [Mebupyrr][N(CN)₂].
According to a process of the present invention, a feedstock comprising one or more aromatic compounds and one or more olefinic compounds is contacted with an ionic liquid to produce a product with a lower concentration of the one or more aromatic compounds. The ionic liquid and the product typically form separate phases, the ionic liquid generally being denser, for example when the feedstock and product predominantly comprise hydrocarbons. After contact with the ionic liquid, the mole ratio of olefinic compounds to aromatic compounds is higher in the product compared to the initial feedstock, while in the ionic liquid phase the mole ratio is lower than that of the feedstock. Thus, the concentration of one or more aromatic compounds in the feedstock is reduced, yielding a product with a lower concentration of aromatic compounds. This is advantageous, for example, in the production of gasoline from refinery process streams, in that the aromatic components can be selectively removed, thereby reducing toxicity of the gasoline and enabling compliance with regulatory aromatics content limits. Moreover, removal of olefinic compounds from the feedstock may be minimised, which is beneficial in relation to gasoline octane rating.
The net effect of the ionic liquid separation process is a relative decrease in the concentration of aromatic compounds compared to the concentration of olefin compounds in a feedstock, resulting in a product having a lower concentration of aromatic compounds compared to the feedstock, with a lower reduction of the olefin content of the product compared to the feedstock. As mentioned above, it is advantageous to minimise the removal of olefinic compounds from the hydrocarbon feedstock by the ionic liquid. Preferably more than 10% of the aromatic compounds are removed from the hydrocarbon feedstock, for example in the range of from 10 to 100%. Preferably, greater than 50% of the aromatic compounds are removed from the hydrocarbon feedstock.
It has been found that a process of the present invention is particularly effective at selective removal of aromatic compounds where their content in the feedstock is low, for example less than 20% by volume. At such concentrations, the D_arom exceeds that of sulfolane as a solvent.
In one embodiment of the process, the ionic liquid and feedstock are both fed to a separation vessel. In a particular embodiment, the ionic liquid and feedstock are fed counter-currently into a static vessel, wherein the denser phase, typically the ionic liquid, is fed to the upper portion of the vessel and the less dense phase, typically the feedstock, is fed to the lower portion of the vessel. This ensures good mixing between the two phases, and improves the extraction efficiency of aromatic compounds from the feedstock. Such a process can operate continuously, with the resulting product being removed from the separation vessel at a point preferably above the inlet point of the ionic liquid, and the aromatic-containing ionic liquid being removed from a point below the inlet for the feedstock.
In another embodiment, the ionic liquid and feedstock are stirred or otherwise agitated in a first separation vessel. They can then be allowed to settle after mixing before portions of each phase are removed from the vessel. Alternatively, the mixed phases can be transferred to a second separation vessel, where the two phases are allowed to separate. This process can also operate continuously, such that a continuous supply of ionic liquid and feedstock can be fed to the first vessel, and the mixed phases can be continuously extracted from the first separation vessel to the non-agitated second separation vessel, the resulting product phase and ionic liquid phase from which can be separately and continuously extracted.
In a further embodiment, the ionic liquid and feedstock are fed to a rotating disc separation column which acts as the separation vessel, in which a series of separation plates are rotated, providing agitating mixing of the ionic liquid and feedstock phases, the ionic liquid being fed towards the top of the column, and the feedstock being fed into the column at a point nearer the base of the column. The extracted product phase, which contains a reduced concentration of aromatic compounds, is removed from a point towards the top of the separation column, while ionic liquid with extracted aromatic compounds can be removed from a point towards the bottom of the column. An example of a suitable apparatus is described by Meindersma et al in Chem. Eng. Res. Design, 86 (2008), 745-752, the contents of which are incorporated herein by reference.
The ionic liquid can be processed to remove the aromatic compounds. This can be achieved by distillation or flash separation techniques, due to the extremely low volatility of the ionic liquid. The aromatic compounds can then be used for chemicals manufacture, for example in toluene or xylene production. Alternatively, they can be hydrogenated to produce cyclic alkanes, typically naphthenes such as cyclohexane, alkylcyclohexanes and polycyclic naphthenes. Aromatic hydrogenation can be achieved, for example, by contacting the aromatic-containing composition and hydrogen with a catalyst comprising a Group VIB and Group VIII element, for example as described in US 1,965,956 or US 5,449,452 , the latter of which involves the use of a catalyst additionally comprising boron and a carbon support. Pressures of 3 MPa or more are typically employed, for example in the range of from 10 to 25 MPa, and temperatures are typically in the range of from 200 to 450 °C.
Where the feedstock is a gasoline or one or more refinery streams that can be used in gasoline production, the hydrogenated aromatics can be added to the product of the initial ionic liquid extraction process, i.e. the product with reduced aromatics content. In this case, the product is a gasoline or can be used in the production of gasoline. The naphthenes (cyclic alkanes) from the aromatics hydrogenation process can be useful components of gasoline fuel, and do not suffer the same toxicity and regulatory constraints of the corresponding aromatic compounds from which they can be produced. By adding them to the initial product of the extraction, fuel yields may be improved.
The product, with reduced aromatic content after the ionic liquid extraction, can be used as a fuel, for example diesel or gasoline depending on the boiling point of the product. Gasoline is produced from hydrocarbon mixtures that typically boil within the range of from 40 to 200 °C. In a crude oil refinery, sources of gasoline blending stock include straight run light or heavy naphtha, isomerised straight run naphtha, cracked naphtha, pyrolysis gasoline, and products of catalytic reforming and alkylation processes. The feedstock of the present invention may comprise one or more of these refinery streams. Alternatively, the feedstock can be a fuel formulation, for example a gasoline fuel or a diesel fuel, that requires a reduction in aromatic concentration.
The process of the present invention can be applied in the selective removal of aromatics in the production of diesel fuel. Typical refinery streams used in the production of diesel include straight run middle distillate, light cycle oils, heavy cycle oils, vacuum gas oils and cracked gas oils, any one or more of which can be used as a feedstock in the process of the present invention. Typically, suitable process streams for diesel fuels boil within the range of from 150 to 400 °C. Refinery process streams used in the production of diesel fuel typically comprise higher concentrations of aromatics with more than one aromatic ring, compared to streams used for gasoline production for example. Aromatic concentrations in a diesel fuel are regulated, and are preferably less than 11% by volume in the final fuel, more preferably less than 8% by volume. One source of aromatics in diesel fuels is light cycle oil, produced from fluidised catalytic cracking, and in one embodiment the process is used in the production of diesel fuel, or a product that can be blended with diesel, and the feedstock is or comprises light cycle oil.
In a further embodiment of the invention, the ionic liquid is also capable of removing heteroatom-containing organic compounds that may be present in the feedstock. Thus, when contacted with an ionic liquid, the product phase that is produced will have a lower concentration of one or more heteroatom-containing organic compounds compared to the feedstock. Additionally, the olefin to heteroatom-containing organic compound molar ratio will be higher in the product phase compared to the feedstock, and the olefin to heteroatom-containing organic compound molar ratio will be lower in the ionic liquid phase compared to the feedstock.
Thus, the ionic liquid may be used to separate aromatics and heteroatom-containing compounds from a hydrocarbon feedstock. Heteroatom-containing compounds typically include sulphur and/or nitrogen-containing compounds, such as organo-amines and organo-sulphides, for example heterocyclic compounds or compounds comprising one or more alkyl and/or aryl groups. When contacted with the ionic liquid in accordance with the present invention, at least a portion of the heteroatom-containing compounds will be extracted into the ionic liquid phase to produce a product with a reduced concentration of heteroatom-containing and aromatic compounds, which product can be used as or used in the production of a fuel, in particular low heteroatom and low aromatics-containing fuel. This reduces the need to carry out processes such as hydrodesulphurisation, hydrodenitrification or hydrocracking in order to reduce sulphur and nitrogen levels in a fuel, which reduces the quantity of hydrogen consumed in producing such fuels.
In a further embodiment, the feedstock comprises sulphur-containing organic compounds. Typical sulphur-containing organic compounds associated with refining include mercaptans, sulphides, di-sulphides, thiophenes, benzothiophenes and dibenzothiophenes, at least a portion of which are extracted in the ionic liquid phase during the extraction process. The product comprises reduced concentrations of sulphur-containing compounds and aromatic compounds, and can be used as, or in the production of, low sulphur and low aromatic fuels, such as gasoline or diesel. The ionic liquid is removed from the separation vessel, and the separated compounds from the feedstock are separated, for example by flash separation or distillation. The extracted compounds may then be subjected to a desulphurisation reaction, for example a hydrodesulphurisation reaction. Typically, hydrodesulphurisation reaction involve the use of temperatures in the range of from 200 to 430 °C, for example 230 to 400 °C or 280 to 400 °C, and pressures in the range of from 20 to 200 bara (2 to 20 MPa), for example 25 to 130 bara (2.5 to 13 MPa). The desulphurisation preferably takes place before any benzene hydrogenation stages, as sulphur can act as a catalyst poison for some aromatic hydrogenation catalysts. A suitable desulphurisation process is described in US 2007/0227948 , the contents of which are incorporated herein by reference.
In another embodiment, the feedstock comprises nitrogen-containing organic compounds. Typical nitrogen-containing organic compounds associated with refining include heterocyclic aromatic compounds such as pyridines, quinolines and pyrroles. Their removal, for example by hydrodenitrification, tends to require more hydrogen compared to sulphur removal. Conditions similar to those of hydrodesulphurisation reactions can be used. For example, temperatures in the range of from 200 to 430 °C, such as 230 to 400 °C or 280 to 400 °C, and pressures in the range of from 20 to 200 bara (2 to 20 MPa), for example 25 to 130 bara (2.5 to 13 MPa) may be used.
After any aromatics hydrogenation and desulphurisation/denitrification reactions, the resulting composition can be added to the product from the initial extraction, which reduces any loss in yield of products such as fuels from the feedstock.
In a further embodiment of the invention, the feedstock is a coker naphtha. Coker naphtha derives from coking processes such as delayed coking, which are typically carried out on a vacuum residue, i.e. the residue remaining after vacuum distillation. The aromatics content of coker naphtha, depending on the boiling point, is typically in the range of from 1 to 50% by volume, and the olefins content is typically in the range of from 15 to 50 % by volume. Removing aromatics from a coker naphtha leaves an olefin-rich product, which can be oligomerised to produce hydrocarbons that can be used as or in the production of fuels such as gasoline, diesel or aviation fuel, in particular diesel fuel. Additionally, as coker naphtha is typically quite rich in heteroatom-containing compounds, such as sulphur- or nitrogen-containing compounds, which can also be removed by the ionic liquid during extraction, then the removal of aromatics and heteroatom-containing compounds can be achieved in a single processing step.
Embodiments of the present invention will now illustrated with reference to the accompanying drawings, in which:

Figure 1 illustrates a process according to the present invention in which gasoline is produced from a naphtha feedstock;
Figure 2 illustrates a process according to the present invention in which a product comprising an increased olefin content is produced from a coker naphtha;
Figure 3 shows the surface charge profiles of toluene, 1-hexene and various other aromatic compounds;
Figures 4a and 4b show the surface charge profiles of toluene, 1-hexene and various ionic liquid cationic components;
Figure 5 shows the surface charge profiles of toluene, 1-hexene and various ionic liquid anionic components; and
Figure 6 is a graph showing the relationship between D_toluene and toluene content of the feedstock for sulfolane and the ionic liquid [3-Mebupy][N(CN)₂].

Referring to the Figures, Figure 1 illustrates a process for producing gasoline, in which an ionic liquid 1 and a full range naphtha feedstock 2 comprising aromatic compounds, olefinic compounds and sulphur-containing compounds is fed to an extraction vessel 3. The resulting mixture 4 is then fed to separation/recovery vessel 5, where an olefin-rich product stream 6 with reduced aromatic and sulphur content is obtained. Regenerated ionic liquid 7 and a process stream 8 that is rich in aromatic and sulphur-containing compounds are also obtained.
The olefin-rich product stream 6 is fed to a sulphur removal unit 9. Prior to desulphurisation, the sulphur content of the olefin-rich product stream may be less than 1000 ppm expressed as elemental sulphur, more preferably less than 200 ppm. The sulphur removal unit may comprise an adsorbent, such as zinc oxide, which adsorbs the sulphur-containing compounds. A suitable desulphurisation process is described in US 2007/0227948 . Preferably, the resulting product stream 10 has a sulphur content of less than 50 ppm sulphur, expressed as elemental sulphur. In the embodiment that is illustrated in Figure 1, the reduced sulphur product stream 10 is then fractionated in fractionator 11 to produce ultra low sulphur gasoline fraction 12 and ultra low sulphur aviation fuel fraction 13. Alternatively, the whole product can be used to produce gasoline.
The regenerated ionic liquid 7 is removed from the base of separation/recovery vessel 5 and recycled to separation vessel 2.
Process stream 8 that is rich in aromatic and sulphur-containing compounds is fed to a hydrodesulphurisation unit 14 to reduce the sulphur concentration therein, typically to a value less than 200 ppm, and more preferably less than 50 ppm. The resulting desulphurised stream 15 is then fed to a hydrodearomatisation unit 16, where it is contacted with hydrogen in the presence of a hydrodearomatisation catalyst. The resulting dearomatised stream 17 comprising cyclic alkanes is then fed to a fractionator 18 to remove any unconverted aromatics 19. The resulting low sulphur dearomatised product stream 20, comprising mainly cyclic alkanes, may then be blended with the ultra low sulphur gasoline fraction 12, thereby enhancing the gasoline yield.
Figure 2 illustrates another process according to the present invention, in which a coker naphtha feedstock 1 and ionic liquid 2 are fed to a separation vessel 3. A product stream 4 comprising high olefin content, and reduced sulphur, nitrogen and aromatics levels, is removed from separation vessel 3. The olefins, optionally after further separation or purification, may then be used as a feedstock for an oligomerisation process, to produce hydrocarbons that can be used as, or used in the production of diesel fuel.
An ionic liquid phase 5 containing extracted aromatics and nitrogen- and sulphur-containing compounds is also removed from the base of separation vessel 3. This phase is then fed to an ionic liquid recovery column 6, where the extracted components 7 are separated from the ionic liquid by distillation and, if desired, subjected to further processing. The extracted components 7 and the ionic liquid 8 are removed from the recovery column, and the ionic liquid is recycled to the separation vessel 3.
Figure 3 shows the surface charge profiles of toluene and 1-hexene, together with some other aromatic compounds. As can be seen, the profile for the olefin shows a predominant charge density centred on σ = 0, with a shoulder at positive σ value relating to the negative π-electron density associated with the double bond. For the aromatic compounds, there are two distinct peaks on the charge profile, relating to the negatively charged π-electron density above and below the aromatic ring, which corresponds to the peak at positive σ values, and the positively charged portion of the molecular surface in the plane of the ring, around the hydrogen atoms, with a peak at negative σ values.
Figures 4a and 4b show the surface charge profiles for toluene, 1-hexene and a number of ionic liquid cations. As the ions are positively charged, the surface charge profile is strongly weighted towards the negative σ values. It can be seen that the surface charge profiles for [3-Mebupy], [BMIM] and [Mebupyrr] have their maximum charge density at σ values in the range -0.0085 < σ < -0.004, whereas the tetra-methyl-ammonium, tri-methyl-sulfonium and tetra-methyl-phosphonium cations, have surface charge profiles outside this range, around σ = -0.01 e/Å². Preferred cations include those based on pyridinium and imidazolium ions.
Figure 5 shows the surface charge profiles for a toluene, 1-hexene and a number of ionic liquid anions. The anions generally have between 25 and 65% of their surface area with a charge density σ in the range of from -0.0085 to +0.0085. This includes anions such as DCA, TCM, B(CN)₄, SCN and CH₃SO₄. It has been found that the extraction performance of ionic liquids for aromatics from olefins with these anions follows the order B(CN)₄ ≈ TCM > DCA > SCN > CH₃SO₄. Improved extraction is correlated to a larger overlap of the anion surface charge profile with the whole toluene surface charge profile. The superior extraction performance of B(CN)₄ and TCM is in agreement with the fact that the largest relative part of the surface charge area overlaps, which corresponds to increased interaction and therefore a higher distribution coefficient.
The following non-limiting Examples illustrate the present invention.
In the Examples, surface charge profiles of the cations and molecules were generated by the program COSMO-RS. The charge density is calculated as the inverse of the actual charge on the molecular surface. Thus portions of the molecule with a positive charge appear in the profile as a negative σ value and a negative charge appear as a positive σ value.
The following abbreviations are used in the Examples:

[BMIM] = 1-methyl-3-butylimidazolium
[EMIM] = 1-ethyl-3-methylimidazolium
[Mebupy] = 4-methyl-N-butyl pyridinium
[MMIM] = 1,3-dimethylimidazolium methylsulfate
[3-Mebupy] = 3-methyl-N-butyl pyridinium
[Mebupyrr] = 1-methyl-3-butyl pyrrolidinium
S222 = triethyl-sulfonium
N1888 = methyl-tri-octyl-ammonium

Example 1

This Example demonstrates the extraction selectivity of ionic liquids for aromatics over olefins over prior art solvents.
20 ml of a synthetic FCC gasoline feed comprising 1 wt% benzene, 3 wt% toluene, 40 wt% 1-hexene and 56 wt% hexane was stirred for 15 min at 30 °C in a glass vessel with 20 ml of sulfolane. After stirring, the two phases were allowed to settle for 1h. Samples were taken from both phases and analyzed by gas chromatography. The resulting composition data of both phases were used to calculate the olefin and aromatic distribution coefficients, D_1-hexene, D_benzene, and D_toluene. The aromatic/olefin extraction selectivities, S_{benzene/1-hexene} (= D_benzene/D_1-hexene) and S_{toluene/1-hexene} (= D_toluene/D_1-hexene) were also calculated.
The procedure was then repeated using the following ionic liquids in place of sulfolane: [3-Mebupy][C(CN)₃]; [3-Mebupy][N(CN)₂]; [BMIM][C(CN)₃]; [Mebupyrr][N(CN)₂]; S222 (CF₃SO₂)₂N; and N1888 (CF₃SO₂)₂N.

Performance results (in the form of distribution coefficients and selectivities) for these ionic liquids and sulfolane in relation to benzene / 1-hexene and toluene/1-hexene separations are shown in Table 1. The units of the distribution coefficients are expressed on a weight basis, in g aromatic or olefin per g solvent or ionic liquid.

Table 1

Ionic Liquid / Solvent	D_benzene	D_toluene	D_1-hexene	S_{benzene/1-hexene}	S_{toluene/1-hexene}
[3-Mebupy] [C(CN)₃]	0.70	0.56	0.04	20.00	16.00
[3-Mebupy][N(CN)₂]	0.6	0.39	0.03	20.00	13.00
[BMIM][C(CN)₃]	0.61	0.43	0.04	14.23	10.00
[Mebupyrr][N(CN)₂]	0.31	0.16	0.01	28.86	15.30
Sulfolane	0.25	0.21	0.03	8.33	7.00
S222 (CF₃SO₂)₂N	0.28	0.20	0.02	17.71	12.63

Ionic Liquid / Solvent	D_benzene	D_Toluene	D_1-hexene	S_{benzene/1-hexene}	S_{toluene/1-hexene}
N1888 (CF₃SO₂)₂N	1.11	1.03	0.83	1.34	1.24

It can be seen from Table 1 that the Mebupy- and BMIM-containing ionic liquids show significant advantages in terms of D_arom values, and hence have a high capacity for aromatic compounds. Their selectivities are also significantly higher than the reference solvent sulfolane and the S222 and N1888 cations. Thus, compared with e.g. sulfolane, these ionic liquids selectively increase the distribution coefficient of benzene as well as toluene while leaving the distribution coefficient of 1-hexene nearly unaffected. As a result the extraction selectivity for both aromatics over 1-hexene is significantly higher compared with sulfolane.
Mebupyrr also confers significant advantages in terms of selectivity towards aromatics, as it has very low D_olefin values and therefore high S-values, significantly higher than the sulfolane reference and the S222 and N1888 cations. This is due, in part, to its comparatively much lower D_olefin value.
Ionic liquids comprising Mebupy, BMIM and Mebupyrr also exhibited higher extraction selectivity for aromatics from a synthetic FCC-derived gasoline feed comprising 1 wt% benzene, 3 wt% toluene, 40 wt% 1-hexene and 56 wt% hexane compared to sulfolane.

Example 2

In this Example, ionic liquids were used to extract aromatics at low aromatic concentrations.
20 ml of a toluene/1-hexene mixture containing various concentrations of toluene was contacted for 15 min at 30 °C in a stirred glass vessel with 20 ml of solvent. Sulfolane was used as the solvent. After stirring, the two phases were allowed to settle for 1h. Samples were taken from both phases and analyzed by gas chromatography. The resulting composition data of both phases were used to calculate the distribution coefficient of toluene, D_toluene.
The procedure was then repeated using [3-Mebupy] [N(CN)₂] in place of sulfolane.
As shown in Figure 6, at low toluene (aromatics) concentration the capacity of the ionic liquid for the aromatics is significantly higher than that of the reference solvent sulfolane. Additionally, with sulfolane, the extraction capacity (D_arom) decreases with decreasing aromatics content while with the ionic liquid the D_arom increases with decreasing aromatics content. This makes ionic liquids such as [3-Mebupy][N(CN)₂] especially suitable for the extraction of aromatics present at low concentrations from olefin containing feeds.

Example 3

A model FCC cracked naphtha stream comprising 1wt% benzene, 3wt% toluene, 40wt% 1-hexene and 56wt% hexane was continuously fed to a rotating disc extraction column, together with the ionic liquid [3-Mebupy][N(CN)₂]. A temperature of 40°C was maintained. The rotating disc apparatus has been described previously by Meindersma et al in Chem. Eng. Res. Design, 86 (2008). The extraction column had an internal diameter of 60mm, and was 3.8m tall. The rotating element comprised numerous discs, and was rotated at 700rpm. The top and bottom of the extraction column comprised settlers to ensure good phase separation. Stators were present on the inner wall of the extraction columns.
16.8 kg/h of the model cracked naphtha feedstock was fed towards the top of the column. Ionic liquid and hydrocarbon product were continually removed from the column. Gas chromatography was used to analyse the hydrocarbon product removed from the extraction column. 86% of the benzene initially present in the feedstock had been removed, together with 64% of the toluene. Only 5% of the 1-hexene initially present was extracted by the ionic liquid.

Example 4

A conversion model based upon the process scheme illustrated in Figure 1 was derived using Aspen Plus® software. The model was based upon a full range naphtha composition comprising, by weight, 3% paraffins, 21% isoparaffins, 10% naphthenes, 30% olefins, 31% aromatics, and 5% high boilers (i.e. non-aromatic components having 12 or more carbon atoms). Additional components included 74.5 µg/g aliphatic thiols, 128.5 µg/g aliphatic sulphides (aliphatic mercaptans), 15.0 µg/g aliphatic disulphides (total 219 µg/g aliphatic sulphur-containing compounds) and 319.2 µg/g thiophenic sulphur, giving a total sulphur content of 537.2 µg/g, wherein these values refer to the weight of elemental sulphur included in the various compounds.
The flow rate of the naphtha feedstock to the process was 100,000 barrels per day (100 kbpd), where one barrel is equal to 158.9873 litres.
The separation characteristics of the ionic liquid were taken to be those of [3-mebupy][N(CN)₂], which resulted in a calculated hydrocarbon product after extraction as comprising (by weight) 3.9% paraffins, 26.5% isoparaffins, 12.6% naphthenes, 37.7% olefins, 13.0% aromatics, 6.4% high boilers, and 300 ppm total sulphur. After sulphur absorption, the sulphur content is reduced to 10 ppm.
The hydrocarbons extracted in the ionic liquid phase comprise (by weight) 0.5% paraffins, 4.5% isoparaffins, 2.0% naphthenes, 8.3% olefins, 83.6% aromatics, 1.0% high boilers, and 1231ppm total sulphur. After hydrodesulphurisation, the sulphur content is reduced to 10ppm.

These quantities are listed in Table 2 for comparison.

Table 2

Component	Feed	Product	Recovered from Ionic Liquid
Paraffins (wt%)	3	3.9	0.5
Isoparaffins (wt%)	21	26.5	4.5
Naphthenes (wt%)	10	12.6	2.0
Olefins (wt%)	30	37.7	8.3
Aromatics (wt%)	31	13.0	83.6
High Boilers (wt%)	5	6.4	1.0
Total sulphur (ppm)	537.2	299	1231

Octane performance data for the various hydrocarbon streams from the process are listed in Table 3.

Table 3

Parameter	Naphtha Feed	HA-HS^a Stream	HO-LS^b Stream	HA-LS^c Stream	LA-LS^d Stream	Gasoline Pool^e
Flow (kbpd)	100	23	77	26	25	102
RON	116.9	114.8	117.5	101.6	110.2	115.7
MON	108.4	104.6	109.5	89.6	100.1	107.2
Benzene (wt%)	12.0	40.6	2.2	40.5	0.0	1.6
Sulphur (ppm)	537.2	1234	299	10	9	10

^a Process stream 8 in Figure 1
^b Process stream 6 in Figure 1
^c Process stream 15 in Figure 1
^d Process stream 17 in Figure 1
^e The gasoline pool was produced by blending the LA-LS stream with the HO-LS stream after it has been subjected to sulphur adsorption.

As can be seen from Table 3, the hydrocarbon product of the ionic liquid separation process has improved motor octane number (MON) and research octane number (RON) compared to the initial full range naphtha feed, even though it is low in sulphur and low in aromatics, at least in part because the olefins content has not been reduced after the ionic liquid extraction process. To mitigate against the substantial losses in gasoline yield that occur due to removal of the aromatic components by the ionic liquid, the hydro-desulphurised and dearomatised composition produced from the components extracted with and subsequently recovered from the ionic liquid is blended with the ultra low sulphur (ULS) gasoline product, to produce a gasoline product with low aromatics and sulphur content, but with comparable MON and RON characteristics to the naphtha feed.

Claims

A process for reducing the concentration of one or more aromatic compounds in a feedstock comprising one or more aromatic compounds and one or more olefinic compounds, which process comprises contacting the feedstock with an ionic liquid comprising an anionic component and a cationic component to produce a product with a higher mole ratio of olefinic compounds to aromatic compounds than the feedstock, and an ionic liquid phase with a lower mole ratio of olefinic compounds to aromatic compounds than the feedstock, wherein the surface charge profile of the cationic component has a maximum value at a charge density value (σ) in the range -0.0085 < σ < -0.0040 e/Å², and wherein from 25% to 65% of the molecular surface area of the anionic component has a charge density value (σ) in the range -0.0085 < σ < +0.0085 e/Å², where e represents the charge of an electron.
A process according to claim 1, wherein the feedstock is a coker naphtha, a gasoline stream or comprises one or more refinery streams that can be used in the production of gasoline.
A process according to claim 1 or claim 2, wherein the one or more aromatic compounds are selected from benzene, toluene, ethylbenzene, propylbenzene, isopropylbenzene, xylenes, ethyltoluenes, trimethylbenzenes, diethylbenzenes, n-butyl benzene, tetramethylbenzenes, naphthalenes, methylnaphthalenes, fluorene, methylfluorenes, phenanthrene, anthracene, methylphenanthrenes, methyl-isopropyl-phenanthrenes, dimethylphenanthrenes, methylanthracenes, fluoranthrene, pyrene, methylpyrenes, benzofluoranthene, cyclopentopyrene, benzanthracene, benzopyrenes, perylene, indenopyrene and benzoperylene.
A process according to any preceding claim, wherein the amount of aromatic compounds in the feedstock is less than 20% by volume of the feedstock.
A process according to any preceding claim, wherein the one or more olefinic compounds are selected from butenes, pentenes, hexenes, dienes, hexadienes, heptenes, octenes, cyclohexene, cyclooctene and C₁₁ to C₂₀ olefins.
A process according to any preceding claim, wherein from 30 to 60% of the anion molecular surface area has a charge density value in the range -0.0085 < σ < +0.085 e/Å².
A process according to any preceding claim, wherein the cationic component is selected from dialkyl-substituted imidazolium cations, dialkyl-substituted pyridinium cations, dialkyl-substituted pyrrolidinium cations, and N-containing organic cations with two aromatic rings.
A process according to claim 7, wherein the cationic component is selected from 3-butyl-1-methyl-imidazolium [BMIM], N-alkyl-3-methyl-N-butyl pyridinium [3-Mebupy], and 1-methyl-3-butyl pyrrolidinium [Mebupyrr].
A process according to any preceding claim, wherein the anionic component is selected from dicyanamide [N(CN)₂; DCA], tricyanomethanide [C(CN)₃; TCM], SCN and B(CN)₄.
A process according to any of claims 1 to 6, wherein the ionic liquid is selected from [3-Mebupy][C(CN)₃], [3-Mebupy][N(CN)₂], [BMIM][C(CN)₃] and [Mebupyrr][N(CN)₂].
A process according to any preceding claim, further comprising separating the aromatic compounds from the ionic liquid phase.
A process according to claim 11, wherein the process further comprises hydrogenating the separated aromatic compounds.
A process according to claim 12, wherein the process further comprises combining the resulting hydrogenated aromatic compounds with said product or a derivative thereof.
A process according to any preceding claim, wherein the feedstock comprises one or more heteroatom-containing organic compounds and the process produces a product with a higher mole ratio of olefinic compounds to heteroatom-containing organic compounds than the feedstock, and an ionic liquid phase with a lower mole ratio of olefinic compounds to heteroatom-containing organic compounds than the feedstock.
Use of an ionic liquid for reducing the concentration of one or more aromatic compounds in a feedstock comprising one or more aromatic compounds and one or more olefinic compounds, wherein the ionic liquid comprises an anionic component and a cationic component, in which the surface charge profile of the cationic component has a maximum value at a charge density value (σ) in the range -0.0085 < σ < -0.0040 e/Å² , and from 25% to 65% of the molecular surface area of the anionic component has a charge density value (σ) in the range -0.0085 < σ < +0.0085 e/Å², where e represents the charge of an electron.