WO2011026972A1

WO2011026972A1 - Process for the removal of aromatic compounds from a mixture

Info

Publication number: WO2011026972A1
Application number: PCT/EP2010/063032
Authority: WO
Inventors: Gerrit Jan Harmsen; Martina Peters; Jan De With
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2009-09-07
Filing date: 2010-09-06
Publication date: 2011-03-10

Abstract

Aromatic compounds are removed from a mixture comprising such aromatic compounds and one or more aliphatic hydrocarbons in a process, wherein the mixture is contacted with liquid salt comprising a nitrogen-or phosphorus-containing cationic moiety and an anionic moiety comprising iron and halogen. The invention also provides a salt comprising an organic nitrogen- or phosphorus-containing cationic moiety and an anionic moiety comprising iron and halogen.

Description

PROCESS FOR THE REMOVAL OF AROMATIC COMPOUNDS FROM A

MIXTURE

The present invention relates to a process for the removal of aromatic compounds from a mixture comprising such aromatic compounds and one or more aliphatic hydrocarbons .

In refinery processes a number of streams become available that contain aromatic and aliphatic compounds. It is often desirable to separate the aromatic compounds from the aliphatic compounds. Separation of aromatic compounds may be desirable when the aromatic compounds contain heteroatoms such as sulphur or nitrogen.

Although removal of such heteroatom-containing compounds is commonly achieved by hydrodesulphurisation and hydrodenitrification processes, it is also possible to remove such compounds by extraction.

Such a process has been described in EP-A 1854786. This known process relates to the extraction of aromatic compounds, such as dibenzothiophene and derivatives thereof, from a mixture with at least one aliphatic hydrocarbon by contacting the mixture with an ionic liquid, which is a salt in the liquid state having a cation comprising an aromatic nitrogen-containing heterocyclic ring system. The ring system comprises an electron-withdrawing substituent, such as a cyano group. The nature of the anion is stated not to be crucial.

In WO-A 01/40150 the extraction of aromatics is disclosed in which extraction an ionic liquid is used, which ionic liquid must be non-neutral. Neutral has been defined on the basis of Lewis acid and Lewis base. At equimolar proportion in case of a nitrogen-containing cation and aluminium trichloride as function for the anion the resulting composition is neither acidic nor basic. Hence, an ionic liquid is neutral in the sense of WO-A 01/40150 if at equimolar proportions, which is the case for the tetrachloroaluminate species, where the mole fraction is 0.5, the resulting composition is neither acidic nor basic.

Various refinery streams may comprise mixtures of aliphatic and aromatic compounds. Examples include naphtha streams that have been subjected to reforming, e.g., with a platinum catalyst. It may be desirable to obtain pure aromatic compounds from such streams. US-B 7019188 relates to a process for the extraction of olefins and aromatics using an ionic liquid that

comprises a heterocyclic cationic group, e.g., 1-butyl-

3-methylimidazolium, and tetrafluoroborate as anion.

It has now been found that, contrary to the teachings of EP-A 1854786, the nature of the anion plays a

significant role in the performance of an ionic liquid in the extraction of aromatic compounds.

Accordingly, the present invention provides a process for the removal of an aromatic compound from a mixture comprising such aromatic compounds and aliphatic

hydrocarbons, wherein the mixture is contacted with liquid salt comprising a nitrogen- or phosphorus- containing cationic moiety and an anionic moiety

comprising iron and halogen.

The invention further provides novel salts suitable for use in the extraction of aromatic compounds from a hydrocarbon mixture comprising a nitrogen- or

phosphorus-containing cationic moiety and an anionic moiety comprising iron and halogen, and optionally one or more metals from Groups IB, IIA, IIB and IIIA of the Periodic Table of Elements.

The performance of anionic moieties that comprise iron and a halide has been found to be excellent in the extraction of aromatic compounds. In the present

invention, the extraction may be performed as a liquid- liquid extraction or as an extractive distillation.

Extractive distillation is preferred.

The aromatic compound that is removed from the mixture may be any aromatic organic compound. Examples include sulphur- and/or nitrogen-containing aromatic compounds that are found as contaminants in refinery streams. Such compounds can be benzothiophene compounds, more in particular alkyl-substituted dibenzothiophene compounds. Such compounds are sometimes referred to as refractory sulphides and are difficult to remove with mild hydrodesulphurisation . Therefore, such heteroatom- containing compounds may suitably be subjected to the process of the present invention in order to remove these contaminants from refinery streams. However, since the process is found to be excellently suitable to recover aromatic compounds in large proportions for use in, e.g., gasoline fuel or chemical conversions, it is preferred that the aromatic compound to be removed is a hydrocarbon that does not comprise a heteroatom.

Suitable compounds include hydrocarbon aromatics with 6 to 12 carbon atoms in the ring. Suitably, the aromatic compounds comprise one or more C₁-C6 substituents .

Although the substituents may be unsaturated, it is preferred that the C₁-C6 substituents are alkyl groups.

The present process is excellently suitable for the removal of benzene, toluene, xylene (o-, m-, p- and mixtures) and naphthalene from hydrocarbon mixtures. The liquid salt is a so-called ionic liquid.

Generally, these ionic liquids are known in the art. In addition to EP-A 1854786, WO-A 01/40150 and

US-B 7019188, reference is made to US-A 4359596 and the references cited in US-B 7019188. Ionic liquids are commonly known as molten salts with organic compounds that are liquid at relatively low temperature, suitably up to 100 °C, preferably up to 80 °C, more preferably up to 20 °C. Minimum melting points may start from -100 °C. Suitable lower melting points are -20 °C, preferably

0 °C. The liquid salt according to the present invention contains a nitrogen- or phosphorus-containing cationic moiety. It is preferred to have liquid salts that contain either a nitrogen-containing or a phosphorus- containing cationic moiety. In the process for the removal of aromatic compounds a mixture of nitrogen- containing and phosphorus-containing liquid salts may be used .

Suitable phosphorus-containing cationic moieties have been described in, e.g., US-A 4359596, and include tetrahydrocarbylphosphonium cations, in which the hydrocarbyl groups each may be an alkyl or an aromatic group, each containing from 1 to 12 carbon atoms.

Examples are tri-n-butyl-2-ethylhexyl phosphonium, tri- n—butyl-n-octylphosphonium, tri-n-butyl- phenylphosphonium and tri-n-butyl-3- methylbutylphosphonium cations.

Preferably, the liquid salt comprises a nitrogen- containing cationic moiety, since such liquid salts are easier to manufacture and provide better extraction results. Suitable moieties include trialkyl and

tetraalkyl ammonium groups. The alkyl groups in these moieties may contain from 1 to 12, preferably from 1 to 6 carbon atoms. Suitable ammonium groups include

trimethyl and tetramethyl ammonium, triethyl ammonium, tetraethyl ammonium, tetra-n-butyl ammonium. However, also guanidinium groups, optionally substituted by 1 to 6 methyl or ethyl groups can be used. It is preferred that the cationic moiety comprises a nitrogen-containing heterocyclic group. Such a heterocyclic group can be selected from pyrrole, pyrrolidine, imidazole, oxazole, piperidine, azepine, pyridine, pyrazine, pyrimidine, pyridazine, but also polycyclic rings can be used, such as quinoline, isoquinoline, pteridine, indole, purine and carbazole. It has been found that excellent

extracting properties can be obtained with cationic moieties wherein the nitrogen-containing heterocyclic group is aromatic. Hence, the groups advantageously comprise a pyridine, pyrimidine, pyrazine, quinoline, isoquinoline, pteridine, purine or imidazole moiety. Most preferably the cationic moiety comprises an

imidazolium or a pyridinium group.

When the cationic moiety comprises a nitrogen- containing heterocyclic group, this group is preferably substituted. Substituents are suitably positioned on the nitrogen atom, but advantageously, one or more

substituents are also located on one or more of the carbon atoms in the ring of the heterocyclic group. The substituents on the ring can be selected from a wide group of organic and inorganic groups, including

halides, cyano, nitro, trifluoromethyl , sulphide and alkylsulphide groups. Preferably, the substituents on the carbon atoms in the ring and those on the nitrogen or phosphorus atom are hydrocarbyl groups, optionally interrupted by a one or more of oxygen, nitrogen and sulphur atoms. Such substituents have suitably from 1 to 16 carbon atoms. If the substituent has too many carbon atoms the melting point of the salt may increase so that it may require too high a temperature to become

economically useful in the process of the present invention. Therefore, the substituent on the cationic moiety advantageously has from 1 to 6 carbon atoms. The substituent need not necessarily be saturated.

Especially, when a substituent is bound to a nitrogen atom in the ring, the substituent may be unsaturated, e.g., an allyl, butenyl, isobutenyl or pentenyl group.

However, advantageously the substituents are saturated, more preferably, the substituent is an alkyl group with 1 to 6 carbon atoms. Therefore, the cationic moiety is preferably selected from the group consisting of (N-Ri) - pyridinium and l-R₂-3-R₃-imidazolium, wherein Ri, R₂ and

R₃ are hydrocarbyl, more preferably alkyl, groups with 1 to 6 carbon atoms. When the alkyl substitutent contains more than 3 carbon atoms, the alkyl group is suitably linear. Such pyridinium and imidazolium moieties may bear one or more further substituents. Suitably the pyridinium moiety may contain one or more further hydrocarbyl, preferably alkyl substituents with 1 to 6 carbon atoms. In view of melting point considerations the pyridinium moiety preferably contains up to one further substituent. The further substituent may be positioned on the 0-, m-, or p-position, wherein the o- and m-positions are preferred, since such compounds tend to have the lower melting points. Imidazolium moieties may also comprise one or more hydrocarbyl, suitably alkyl substituents. The imidazolium moiety preferably not more than one further substituent. Preferably, the substituent is positioned on the 2-position since imidazolium compounds with an alkyl group on the 2- position tend to be more stable. As indicated above, the nature of the anionic moiety is significant for the performance of the ionic liquid. The anionic moiety may contain as halogen fluorine, chlorine, bromine or iodine. Since the application of bromine and iodine may result in high melting temperatures and since fluorine may be environmentally undesirable the anionic moiety preferably comprises chlorine.

The anionic moiety also comprises iron. That does not mean that iron is the only metal atom that is contained in the anionic moiety. It is also feasible that a mixed metal salt is being used. Examples of such mixed metal anionic moieties include combinations of iron with aluminium, gallium, zinc, copper and/or indium. Other metals from the Groups IB, IIA, IIB and IIIA of the

Periodic Table of Elements are also possible. Examples of such mixed metals moieties are FeZnCl6^~ and AlFeCl7^~. Preferably, the anionic moiety comprises FeCl₄ ^~.

Whereas WO-A 01/40150 teaches that neutral ionic liquids do not provide any extraction capability for aromatics, it has been found in the process of the present invention that removal of aromatic compounds from hydrocarbon mixtures can be excellently achieved when the Lewis acid and the Lewis base that make up for the liquid salt are combined in stoichiometric amounts; hence when the liquid salt is neutral.

The best results are obtained when the liquid salt is l-ethyl-3-methylpyridinium tetrachloroferrate .

The reaction conditions of the contact between the liquid salt and the mixture of aromatic compounds and aliphatic hydrocarbons can be selected between wide ranges. The skilled person will be able to determine the optimal reaction conditions based on temperature, concentration of the aromatic compounds in the mixture, the desired degree of purity and the nature of the liquid salt. Typically, the mixture is contacted with the liquid salt for a period of 1 s to 10 hr, preferably from 30 s to 1 hr, at a temperature of 20 to 100 °C, and a pressure of 0.1 to 20 bar. The skilled person may easily determine the best conditions dependent, e.g., on te feedstock, the desired purity of the products and the separation equipment.

Further, the process of the present invention may be carried out in the presence of a solvent wherein the liquid salt may be dissolved. Examples of such solvents are N-formyl-morpholine (NFM) , sulfolane (2,3,4,5- tetrahydrothiophene-1 , 1-dioxide) , N-methyl-2-pyrrolidone (NMP) , alkylene glycols such as triethylene glycol and dimethyl sulfoxide (DMSO) . An advantage of using such relatively cheap industrial solvents, is that a

relatively smaller amount of the relatively expensive ionic liquid may be used, while still obtaining a selective separation of the aromatic compounds from the aliphatic hydrocarbons. The liquid salt is suitably combined with such solvent before contacting with the mixture comprising aromatic compounds and one or more aliphatic hydrocarbons in accordance with the invention. In said embodiment, the relative amount of the solvent, based on total amount of ionic liquid and solvent, may be up to 99 wt.%, suitably at most 95 wt . % or at most 90 wt . % or at most 80 wt.%. For example, said relative amount of the solvent may be of from 5 to 95 wt.%, suitably of from 10 to 90 wt.% or of from 20 to 80 wt.%.

The present process may for example be a liquid- liquid extraction process or an extractive distillation process. An extractive distillation process is preferred in the present invention.

The invention can be applied to obtain the

desulphurisation of various refinery streams, such as gas oils, vacuum gas oils, naphtha, and cracked naphtha.

Alternatively, it may be applied to recover aromatic compounds from petroleum or coke-oven sources for the production of high purity aromatics or high octane aromatic concentrates for gasoline blending.

The preparation of the salts according to the

invention can be achieved on identical ways for

compounds of similar structure. Such a preparation method will be known to the person skilled in the art. Suitable methods include the heating of an ammonium or phosphonium halide with an iron halide, optionally preceded or followed by a heating step with a halide of another metal .

The invention will now be illustrated by reference to the following examples.

EXAMPLE 1

Preparation of ionic liquids

a) Pyridinium tetrachloroferrate compounds

A round-bottom flask with a magnetic stirrer was loaded with 5.00 g (31.7 mmol) of l-ethyl-3- methylpyridinium chloride and 5.15 g (31.7 mmol) iron

(III) chloride. The mixture was heated in vacuo (0.1 mbar) at 80 °C for 16 hr . Upon cooling a red-brown liquid consisting of l-ethyl-3-methylpyridinium

tetrachloroferrate (Et (3-MePy) . FeCl₄) was isolated.

In analogous procedures l-butyl-3-methylpyridinium tetrachloroferrate (Bu (3-MePy) . FeCl₄) and 1- allylpyridinium tetrachloroferrate ( (AlPy) . FeCl₄) were prepared . b) Imidazolium compounds

A round-bottom flask with magnetic stirrer was loaded with 6.99 g (40 mmol) of l-butyl-3-methylimidazolium chloride and 5.45 g (40 mmol) of zinc chloride. The mixture was heated in vacuo (0.1 mbar) at 60 °C for

16 hr . Upon cooling, a clear viscous liquid was obtained. Subsequently, 2.16 g (13.3 mmol) of iron(III) chloride was added and the mixture was heated in vacuo (0.1 mbar) at 80 °C for 16 hr . A brown-red liquid

( (BuMelm) FeZnCl₆/ZnCl₃, with a mole fraction of 33 mole% of the mixed metal anion FeZnCl6^~) was obtained.

EXAMPLE 2

Determination of distribution coefficient

In model experiments for the determination of the

distribution coefficients for the ionic liquids tested, 5 millilitres of a mixture of 0.25 mol/1 benzene and 0.25 mol/1 hexane in heptane solvent was contacted with 5 ml of the ionic liquid for a period of 4 hr at 40°C and at atmospheric pressure whilst being swirled at 500 rpm in a thermostatically controlled shaking apparatus. After separation of the phases obtained, the concentration of benzene in the two phases was determined by gas

chromatography. From the concentrations, the distribution coefficient (D) was calculated by the formula D =

Mo l ar f raCt l Onbenzene in ionic liquid/Mo l ar f raCt l Onbenzene in hexane ·

The results are shown in Table 1 for the following compounds :

Et (3-MePy) . FeCl₄;

Bu (3-MePy) . FeCl₄;

(AlPy) .FeCl₄; and

BuMelm. FeZnCl₆/ZnCl₃ (mole fraction FeZnCl₆ ^" is 33 mole%) . For comparison reasons the same procedure was followed for the compounds l-butyl-4-methylpyridinium tetrafluoroborate (Bu (4-MePy) .BF₄) (cf. US-B 7019188) and l-ethyl-3-methylpyridinium ethanesulphonate (Et(3- MePy) .EtS0₃) .

Table 1

From these experiments it is evident that the ionic liquids of the present invention show an advantageous distribution, and perform better than the ionic liquid US-B 7019188. The Table further shows that the nature the anionic moieties plays a significant role (cf.

experiments 1 and 6) .

EXAMPLE 3

The distribution coefficients in a commercial feedstock were determined for two ionic liquids.

The composition of the feedstock is shown in Table 2.

Table 2

The ionic liquid in an amount of 5 ml was contacted with 5 ml of the feedstock at a temperature of 40 °C, at atmospheric pressure for 4 hr . After separation of the phases obtained the following was determined:

distribution coefficient D_benzene wherein D =

Molar

i_n ionic liquid /Molar fraction_{benzene in} raffinate r

distribution coefficient D_aii_pnatics wherein D =

Molar fraction_ai_{iphatics in ionic} i_iquid/Molar fraction_ai_{iphatics in} raffinate (said "aliphatics" comprise all components mentioned in Table 2 except benzene) ; and

Selectivity S = D_benzene/Daliphatics ·

The results are shown in Table 3. Table 3 also shows the resultsfor a comparative experiment wherein no ionic liquid was used, but sulfolane instead. Table 3

EXAMPLE 4

The distribution coefficients in a commercial feedstock were determined for different mixtures

consisting of Et(3-MePy) .FeCl₄ as ionic liquid and N- formyl-morpholine (NFM) as solvent for the ionic liquid, wherein the weight percentage of the ionic liquid varied from 100 to 0 wt . % . The composition of the feedstock is shown in Table 4.

Table 4

The mixture of ionic liquid and NFM in a total amount of 5 ml was contacted with 5 ml of the feedstock at a temperature of 40 °C, at atmospheric pressure for 4 hr . After separation of the phases obtained the following was determined:

distribution coefficient D_benzene wherein D =

Molar f raCt l Oribenzene in mixture of ionic liquid and NFM/Molar

fraCtiOn_{kenzene n} raffinate

distribution coefficient D_aii_phatics wherein D =

Molar fraCtion_aii_phatics in mixture of ionic liquid and NFM/Molar fraction_aiip_hatics in raffinate (said "aliphatics" comprise all components mentioned in Table 4 except benzene) ; and

Selectivity S = D_benzene/Daliphatics ·

The results are shown in Table 5. Table 5

ExperiWt.% Wt.% NFM Dbenzene Daliphatics S ment No . Ionic

Liquid

9 100 0 1.76 0.045 39

10 75 25 1.13 0.032 35

11 50 50 1.02 0.042 24

12 25 75 0.75 0.034 22

13 0 100 0.79 0.124 6

Claims

C L A I M S

1. Process for the removal of aromatic compounds from a mixture comprising such aromatic compounds and one or more aliphatic hydrocarbons wherein the mixture is contacted with liquid salt comprising a nitrogen- or phosphorus-containing cationic moiety and an anionic moiety comprising iron and halogen.

2. Process as claimed in claim 1, wherein the aromatic compound is a hydrocarbon that does not comprise a heteroatom.

3. Process as claimed in claim 1 or 2, wherein the aromatic compound comprises one or more Ci to C6

substituents .

4. Process as claimed in any one of claims 1 to 3, wherein the cationic moiety comprises a nitrogen- containing heterocyclic group.

5. Process as claimed in claim 4, wherein the

heterocyclic group is aromatic.

6. Process as claimed in claim 5, wherein the cationic moiety comprises a pyridinium or an imidazolium group.

7. Process as claimed in claim 5 or 6, wherein the cationic moiety comprises one or more substituents with 1 to 6 carbon atoms .

8. Process as claimed in any one of claims 6 or 7, wherein the cationic moiety is selected from the group consisting of (N-Ri) -pyridinium and I-R₂-3-R3- imidazolium, wherein Ri, R₂and R3 are alkyl groups with 1 to 6 carbon atoms .

9. Process as claimed in any one of claims 1 to 8, wherein the anionic moiety comprises chlorine.

10. Process as claimed in claim 9, wherein the anionic moiety comprises FeCl₄ ^~.

11. Process as claimed in any one of claims 1 to 10, wherein the liquid salt is neutral.

12. Process as claimed in any one of claims 1 to 11, wherein the liquid salt is l-ethyl-3-methylpyridinium tetrachloroferrate .

13. Process as claimed in any one of claims 1 to 14, wherein the mixture is contacted with the liquid salt for a period of 30 s to 1 hr at a temperature of 20 to 100 °C, and a pressure of 0.1 to 20 bar.

14. Salt suitable for use in the extraction of aromatic compounds from a hydrocarbon mixture comprising a nitrogen- or phosphorus-containing cationic moiety and an anionic moiety comprising iron and a halide.

15. Salt as claimed in claim 14, which is as defined in any one of claims 4 to 12.