GB2185754A - Process for producing gasoline - Google Patents

Abstract

A process for producing gasoline of improved octane number comprises oligomerising one or more alkenes, and optionally one or more alkanes, having from 2 to 6 carbon atoms over either silicalite (whether stabilized or not) or zeolite, optionally in the presence of water, etherifying the resulting hydrocarbon mixture in the presence of methanol over an acid cation exchange catalyst, extracting the residual methanol with water and distilling the methanol-free etherified effluent.

Description

SPECIFICATION Process for producing gasoline The present invention relates to a process for the production of gasoline having an improved octane number.

An increasing number of countries are requesting that either lead-free gasoline or gasoline having a reduced lead content has to be used. Accordingly, it has been necessary to investigate means of improving the octane number of gasoline.

Gasoline range materials are generally prepared by standard oligomerisation of olefinic hydrocarbons in the presence of phosphoric acid containing catalysts. These materials essentially consist of dimers. They are upgraded by the addition of lead-free gasoline anti-knock additives, among which methyl tert-butyl ether (MTBE) is most often used. Other additives, such as tert-amyl methyl ether (TAME), and other types of additives, such as alcohols, have also been tried.

More recently, gasoline products have been prepared by oligomerisation processes which consist in converting normally gaseous olefins (either alone or in admixture with paraffins) into an olefinic gasoline blending stock by passage over various catalysts of the "intermediate pore size silicaceous crystalline molecular sieve" type, as disclosed in U.S. 4,417,088. These catalysts comprise two classes of materials: - crystalline aluminosilicates, usually called "zeolites", which contain significant amounts of alumina in addition to silica; and - crystalline silica polymorphs, essentially free of alumina, e.g. silicalite.

One of the main interests of these recent oligomerisation processes lies in the higher liquid hourly space velocities (LHSV), typically 3040, which may be used, whereas standard oligomerisation processes over phosphoric acidcontaining catalysts typically only allow LHSV values of 1--4. Another advantage is that siliceous crystalline molecular sieves can be regenerated, whereas phosphoric acid-containing catalysts cannot be reused.

Many oligomerisation processes over zeolites have been disclosed e.g.: over ZSM-5 of controlled acidity and reduced aromatization activity (U.S. 3,960,978); over ZSM-4, ZSM-12, ZSM-18, chabasite or zeolite beta (U.S. 4,021,502); over ZSM-5 type catalysts in the presence of cofed water (U.S. 4,150,062; and overZSM-12 (U.S. 4,254,295).

Crystalline silica polymorphs have also been used for oligomerisation processes: over silicalite (U.S. 4,414,423 and U.S. 4,417,088); and over crystalline borosilicate (U.S. 4,451,685).

However the octane number of the olefinic gasoline blending stock obtained by any of the hearabove cited processes is not sufficient and therefore requires addition of lead-free gasoline additives such as methyl tert-butyl ether and/or tertamyl methyl ether.

There is therefore a need in the art for a process for the production of gasoline having an improved octane number, which would require a lesser amount of anti-knock additive or even no anti-knock additive at all.

An object of the invention is to provide a process for the production of gasoline having an improved octane number.

An other object is to provide an integratedmulti- step process for the production of gasoline having an improved octane number.

Still another object is to provide a process for producing gasoline which requires a lesser amount of anti-knock additive or even no anti-knock additive atall.

The process of the present invention for producing gasoline comprises the steps of: (i) contacting under oligomerisation conditions a feed essentially comprising one or more alkenes, and optionally one or more alkanes, having from 2 to 6 carbon atoms with a catalyst consisting of an intermediate pore size siliceous crystalline molecular sieve, optionally in the presence of water; (ii) separating the water from the effluent, if water was used in step (i); (iii) optionally, removing substantially all or part of the normally gaseous hydrocarbons from the effluent; (iv) mixing the effluent with methanol; (v) contacting the mixture from step (iv) under etherification conditions with an acid cation exchange catalytic material; (vi) extracting the residual methanol with water from the etherified effluent; (vii) distilling the methanol-free etherified effluent; and (viii) recovering a gasoline effluent having an improved octane number.

The Applicant has now unexpectedly found that, by producing gasoline according to the process of the invention, the octane number of the resulting stock is greatly improved.

The process of the invention has the advantage of giving gasoline which has a highly improved octane number over that of gasoline produced by oligomerisation processes over intermediate pore size siliceous crytalline molecular sieve catalysts.

Under preferred conditions, the process of the invention has even yielded gasoline which has an octane number at least as good as that of gasoline obtained by standard oligomerisation processes over phosphoric acid-containing catalysts. This is more unexpected since etherification of this latter gasoline does not improve its octane number.

As will be shown hereinafter, the advantages of the process of the invention result from the combination of steps as described.

The feed used for the process of the invention is a feed containing one or more alkenes having from 2 to 6 carbon atoms, optionally in the presence of one or more alkaneswhose number of carbon atoms falls in the same range.

The feed used for the process of the invention preferably consists of a mixture of butanes and butenes, comprising: isobutane from 0 to about 40 wt % n-butane from about 5 to about 30 wt % 1-butene from about 10 to about 30 wt % 2-butenes from about 15 to about 35 wt % isobutene from 0 to about 50 wt % and up to about 5% of lighter and/or heavier hydrocarbons.

The composition of the feed depends on its origin.

Typical feeds are isobutene-free, C4-feeds from MTBE production units, C4-cuts from catalytic cracking plants, or even C4-cuts from steam cracking (after removal of butadiene). However, the last two types of feeds are increasingly used for the production of MTBE by the reaction of isobutene with methanol, whereby residual mixtures are obtained which essentially comprise butanes and n butenes in approximately equal weight ratio. Such mixtures are most preferably used as feed for the process of the invention.

The oligomerisation reaction is carried out in a known manner on an intermediate pore size siliceous crystalline molecular sieve.

These materials have the ability of sorting molecules based on the size and/or on the shape of the molecules. Intermediate pore size siliceous crystalline molecular sieves have the unique characteristics of being able to differentiate between large molecules and molecules containing quaternary carbons on the one hand, and smaller molecules on the other. By intermediate pore size, as used herein, is meant an effective pore aperture in the range of about 0.5 to 0.65 nm when the molecular sieve is in the H-form, the preferred effective pore size range being from about 0.53 to 0.62 nm.

While the oligomerisation reaction may be carried out in the presence of silicalite, it is preferable to use as catalyst a stabilized halogenated silicalite. Such catalysts and their preparation are disclosed in our European Patent Application No. filed 23rd January 1987 claiming priority from Luxembourg Patent Application No. 86278 filed 29th January 1986; the process described in this patent application for stabilising crystalline polymorph catalysts of the silicalite type comprises passing over the said catalyst a gaseous stream comprising one or more organic, aliphatic chlorinated, brominated or fluorinated compounds, having a vapour pressure of at least 13 KPA at 200--230"C, and a halogen/carbon ratio of at least 1, and a gaseous vehicle (non-reducing), at a temperature of 200-500"C, for sufficient time to fix 0.1 to 5% by weight of halogen onto the silicalite.

The oligomerisation step may also be carried out over zeolites, as described in the prior art, zeolites having a lowAl:Si ratio being preferred. However, the improvement in the octane number of the gasoline recovered at the end of the process of the invention is lower when zeolites are used in the oligomerisation step.

Water may be added in the oligomerisation step, because it usually improves the stability of the catalyst used. Also, the octane number of the gasoline obtained by the process of the invention is usually improved by the use of water in the oligomerisation step.

The oligomerisation reaction is carried out under well known oligomerisation conditions. Typically, the pressure may be from atmospheric to about 60 bars (6 MPa), preferably from 3 to 20 bars (0.3 to 2 MPa), and most preferably of about 15 bars (1.5 MPa). Higher pressures tend to give higher gasoline yields, while lower pressures tend to improve the octane number of the gasoline. The Applicant has found that the highest pressures, up to 60 bars (6 MPa), should only be used if the catalyst is stabilized halogenated silicalite. Another consideration is that the magnitude of the pressure used in the oligomerisation step should from an economical point of view preferably be equal to that of the pressure used in the etherification step.

The temperature in the oligomerisation step is typically from 200"C to 5000C, but preferably from 250"C to 450"C and most preferably of about 350 C if silicalite (whether stabilized or not) is used, and preferably from 200"C to 350"C and most preferably of about 280"C if zeolite is used.

The liquid hourly space velocity of the feed (LHSV) is typically from 2 to 20, but the Applicant has found that the LHSV can be as high as 50.

When water is used in the oligomerisation step, it is usually added in a molar water:feed ratio of 0.5 to 1.5, preferably of 0.5 to 1.0 and most preferably of about 0.7. However, the advantages resulting from the addition of water must be balanced against the cost disadvantages, and the lowest possibie amount of water is therefore preferred.

The oligomerisation step of the present invention is more efficient with small crystallite sieve particles than with larger crystalline particles. Preferably the molecular sieve crystals or crystallites are less than about 10 micrometres. Methods for making molecular sieve crystals in different physical size ranges are known to the art.

The molecular sieves crystallites can be composited with inorganic matrix materials, or they can be used with an organic binder. It is preferred to use an inorganic matrix because the molecular sieves, owing to their large internal pore volumes, tend to be fragile, and to be subject to physical collapse and attrition during normal loading and unloading of the reaction zones as well as during the oligomerisation. Where an inorganic matrix is used, it is highly preferred that the matrix be substantially free of hydrocarbon conversion activity. It can be appreciated that if an inorganic matrix having hydrogen transfer activity is used, a significant portion of the oligomerswhich are produced by the molecular sieve may be converted to paraffins, and to a large degree the benefits of the invention will be lost.

if water was used in the oligomerisation step, it must be removed from the effluent in the next step.

Removal of the water is best achieved by first decanting the water, then drying the hydrocarbon mixture. No hydrocarbon separation is normally allowed to take place during this step. Nevertheless, if such is desired, normally gaseous hydrocarbons may be totally or partially separated from normally liquid hydrocarbons and removed from the effluent, for example, to recover isobutene which may e.g. be transformed into poiyisobutene; the octane number of the gasoline obtained by the process of the invention is however not as good if such separation has been carried out.Normally gaseous hydrocarbons, as used herein, are all hydrocarbons up to C4, which are gaseous at room temperature under atmospheric pressure; partial separation thereof, as used herein, may be as well total separation of the hydrocarbons having from 1 to 3 carbon atoms as partial separation of all hydrocarbons up to and including C4.

Methanol is then added to the hydrocarbon mixture in essentially an equimolar amount, relative to the isoolefins contained in the hydrocarbon mixture. Examples which may be mentioned of this are a molar ratio of methanol to isoolefins of 0.9 to 1 to 1 to 0.9, preferably 0.95 to 1 to 1 to 0.95, particularly preferably about 1 to 1. For a given feed and given oligomerisation conditions, the amount of isoolefins in the effluent may be estimated experimentally from an analytical determination of C4to C7 isoolefins in the mixture.

The etherification step is carried out in a known manner on an acid cation exchange material, for example on a styrene/divinylbenzene copolymer containing sulphonic acids groups, in a solid or suspended layer at a temperature of 30"C to 120"C, preferably 40"C to 90"C, and under a pressure of 1 to 50 bar (0.1 to 5 MPa), preferably 3 to 20 bar (0.3 to 2 MPa) and most preferably of about 15 bar (1.5 MPa) with a liquid hourly space velocity (LHSV) of 0.05 to 10 1 of total feed materials per litre of cation exchange material per litre of cation exchange material per hour, and preferably of about 1. At high values of LHSV, the etherification yield tends to worsen, while significant amounts of dimethyl ether are produced at low values of LHSV.In this reaction, the pressures and temperatures mentioned are so adjusted relative to one another that the etherification reaction proceeds in the liquid phase.

After passing through the etherification stage, the residual methanol is extracted by adding water to the effluent. Said methanol may be recovered by distillation of the methanol-water mixture and/or recyclied by introducing the mixture instead of or in addition to the water of the oligomerisation step.

The methanol-free effluent is fed into a distillation column where the gasoline is separated from the remaining C3 and C4 hydrocarbons. This column has, for example, 30 to 80, preferably 40 to 70 plates.

It can be designed as a bubbie cap column or as a sieve tray column or any other distillation column with trays or packing materials which provides sufficient separating performance. This distillation column is operated under a pressure of 1 to 10 bar (0.1 to 1 MPa), preferably 3 to 7 bar (0.3 to 0.7 MPa) and particularly preferably 4 to 6 bar (0.4 to 0.6 MPa).

The gasoline thus recovered has an improved octane number.

A preferred form of plant operation according to the process of the invention will now be described, by way of example only, with reference to the sole figure of the accompanying drawings which shows a flow diagram of an installation for carrying out the process of the invention.

Referring to the Figure, the feed (1) and water (2) are introduced into the oligomerisation reactor (3).

This reactor can be for example a fixed-bed reactor or a tube reactor, and it can be used upflow or downflow but preferably upflow as shown. The resulting mixture is fed through pipe (4) into a separator (5), where most of the condensed water (6) is recovered. The separated product is then fed through pipe (7) to a drying unit (8).

The dried hydrocarbon mixture coming out through pipe (9) is then mixed with methanol (10) and introduced into the etherification reactor (11).

This reactor can be for example a fixed-bed reactor or a tube reactor, and it can be used upflow or downflow but preferably upflow as shown.

The etherified product of reactor (11) is fed through pipe (12) where it is mixed with water (16) and introduced into a separator (17). The hydrocarbon mixture is fed through pipe (18) into a distillation column (13). Gasoline is recovered as the bottom product (14) of the distillation column, while the top product (15) consists essentially of a mixture of C3 and C4 hydrocarbons.

In the separator (17), the water extracts the methanol, and the water-methanol mixture is fed through pipe (19) and mixed with fresh water (20) before being recycled into the oligomerisation reactor (3).

The invention will now be described further in the following Examples which are intended to be illustrative and are not intended to limit the scope of the present invention.

EXAMPLE 1 Silicalite was loaded in a reactor and heated at 500"C for 3 hours under a nitrogen flow at a gaseous hourly space velocity (GHSV) of 500. The temperature was then lowered to 280"C while maintaining a flow of nitrogen which was saturated with CCI4 for 4 hours. Stabilized halogenated silicalite was thus obtained.

An hydrocarbon feed, mixed with water in a molar ratio water:feed of 7:10, was passed over stabilized halogenated silicalite at a temperature of 380"C, under a pressure of 15 bars (1.5 MPa) and at LHSV 30.

The composition of the hydrocarbon feed was as follows: (weight %) propane 1.71 propylene 0.09 iso butane 27.87 n-butane 12.16 butenes 58.09 higher hydrocarbons 0.08 After water was decanted off, the resulting hydrocarbon mixture was dried and then mixed with methanol in a weight ratio methanol: hydrocarbon mixture of 3:10. The mixture was then passed in the etherification reactor containing as catalyst a Duolite ES-276 resin (strongly acidic total sulfonated porous styrenedivinylbenzene copolymer beads with a total exchange capacity of 1.8 mol. H+ per litre; Duolite is a Trade Mark of Diamond Shamrock Corp.) art a temperature of 80"C, under a pressure of 15 bars (1.5 MPa) and at LHSV 1.

The etherified effluent was distilled, giving as the bottom fraction a gasoline having the following characteristics: ethers in gasoline MTBE 16.7wt% TAME(t-amyl methyl ether) 19.7 wt % methyl tert-hexyl ether 6.0 wt % methyl tert-heptyl ether 3.1 wt % octane numbers: RON 97.2 MON 82.9 density 0.745 COMPARATIVE EXAMPLE 1 The procedure of Example 1 was repeated up to the decanting of the water and drying after the oligomerisation step. The gasoline recovered was stored at atmospheric pressure until it was tested.

The following results were obtained: octane numbers: RON 89.0 MON 78.2 density 0.710 COMPARATIVE EXAMPLE 2 The feed of Example 1 was submitted to polymerisation over a phosphoric acid-containing catalyst. The RON of the gasoline obtained was 96.5.

This gasoline was submitted to an etherification under the conditions described in Example 1. After etherification, the RON of the product was 96.0.

This comparative example shows that etherification of gasoline obtained by classical procedures does not improve its octane number.

EXAMPLE 2 Stabilized halogenated silicalite was prepared as described in Example 1.

The feed of Example 1, mixed with water in a molar ratio water/feed of 0.72, was passed over stabilised halogenated silicalite at a temperature of 341"C under a pressure of 14.9 bars (1.49 MPa) and at LHSV 30.7.

The process of Example 1 was repeated, and the gasoline recovered had the following characteristics: ethers in gasoline MTBE 9.5 wt % TAME 4.9 wt % heavier ethers 11.0 wt % Octane numbers RON 98.0 MON 82.7 COMPARATIVE EXAMPLE 3 The procedure of Example 2 was repeated up to the decanting of the water and drying after the oligomerisation step. The gasoline which was obtained was stored at atmospheric pressure until it was tested. The following octane numbers were obtained: RON 93.4 MON 80.0 EXAMPLE 3 Silicalite was loaded in a reactor and heated at 500"C for 3 hours under a nitrogen flow with a GHSV of 500. The temperature was then lowered to 285"C while maintaining a flow of nitrogen (saturated with CCI4) for 110 minutes. Stabilized halogenated silicalite has thus been obtained.

An hydrocarbon feed, mixed with water in a molar ratio water:feed of 7:10, was passed over stabilized halogenated silicalite at a temperature of 320"C, under a pressure of 15 bars (1.5 MPa) and at LHSV 30.

The composition of the hydrcarbon feed was as follows: (weight %) propane 0.77 propylene 0.17 isobutane 40.03 n-butane 11.77 butenes 46.98 higher hydrocarbons 0.28 Water was decanted off, the product was dried and normally gaseous hydrocarbons were separated. The resulting hydrocarbon mixture was then mixed with methanol in a weight ratio of methanol:hydrocarbon mixture of 1:10, and passed over an amberlyst 15 resin (strongly acidic sulfonated porous styrene - divinylbenzene copolymer beads; Amberlyst is a Trade mark of Rohm & Haas) at a temperature of 75"C, under a pressure of 15 bars (1.5 MPa) and at LHSV 1.

The etherified effluent was distilled, giving as the bottom fraction a gasoline having the following characteristics: ethers in gasoline MTBE 0.4 wt % TAME 4.9 wt % C7 and higher ethers 11.0 wt % octane numbers RON 95.8 MON 81.5 EXAMPLE 4 An hydrocarbon feed, mixed with water in a molar ratio water:feed of 1:1, was passed over silicalite at a temperature of 31 00C, under a pressure of 2 bars (0.2 MPa) and at LHSV 40.

The composition of the hydrocarbon feed was as follows: (weight %) C3 hydrocarbons 0.9 isobutane 32.5 n-butane 11.8 n-butenes 53.8 isobutene 1.0 After removal of water by decanting, and drying, the resulting hydrocarbon mixture was mixed with methanol in a weight ratio of methanol: hydrocarbon mixture of 15:100, then passed in the etherification reactor containing Amberlyst 15 (as described in Example 3) at a temperature of 75"C, under a pressure of 15 bars (1.5 MPa) and at LHSV 1.

The etherified effluent was distilled, giving as a bottom fraction a gasoline having the following characteristics: ethers in gasoline MTBE 14.4wt% TAME 4.4wt% higher ethers 6.5 wt % octane numbers RON MON 82.3

Claims (23)

1. Process for producing gasoline, comprising the steps of (i) contacting under oligomerisation conditions a feed comprising one or more alkenes, and optionally one or more alkanes, having from 2 to 6 carbon atoms with a catalyst consisting of an intermediate pore size siliceous crystalline molecular sieve; (ii) mixing the effluent with methanol; (iii) contacting the mixture from step (ii) under etherification conditions with an acid cation exchange catalytic material; (iv) extracting the residual methanol with water from the etherified effluent (v) distilling the methanol-free etherified effluent; and (vi) recovering a gasoline effluent having an improved octane number.

2. Process according to Claim 1, wherein the oligomerisation step is carried out in the presence of water, and, in an additional step, said water is separated from the oligomerisation effluent stream and said effluent is dried before mixing it with methanol.

3. Process according to Claim 2, wherein the methanol-water mixture obtained in step (iv) is recycled by introducing it instead of or in addition to the water used in the oligomerisation step.

4. Process according to Claim 2 or 3, wherein water is used in a molar ratio water:feed of 0.5 to

1.5, preferably of 0.5 to 1.0, and most preferably of about 0.7.

5. Process according to Claim 1, comprising the additional step of removing substantially all or part of the normally gaseous hydrocarbons from the effluent from the oligomerisation step before mixing said effluent with methanol.

6. Process according to any one of Claims 1 to 5, wherein the feed comprises a mixture of butanes and butenes.

7. Process according to any one of Claims 1 to 6, wherein the feed comprises a mixture of butanes and n-butenes.

8. Process according to any one of Claims 1 to 7, wherein the oligomerisation step is carried out at a temperature of from 200"C to 500 C, under a pressure of from 1 to 60 bars (0.1 to 6 MPa), and art a LHSV of from 2 to 50.

9. Process according to any of Claims 1 to 8, wherein the oligomerisation step is carried out under a pressure of from 3 to 20 bars (0.3 to 2 MPa).

10. Process according to any one of Claims 1 to 9, wherein the intermediate pore size siliceous crytalline molecular sieve is stilicalite.

11. Process according to Claim 10, wherein the silicalite has been stabilized by halogenation.

12. Process according to Claim 10 or 11, wherein the oligomerisation step is carried out at a temperature of from 250 to 450"C, and preferably at about 350"C.

13. Process according to any one of Claims 1 to 9, wherein the intermediate pore size siliceous .crystalline molecular sieve is zeolite.

14. Process according to Claim 13, wherein the oligomerisation step is carried out at a temperature of from 200 to 350"C, and preferably at about 280"C.

15. Process aceording to any one of Claims 1 to 14, wherein the pressure in the oligomerisation step and tbe pressure in the etherification step are substantially equal to about 15 bars (1.5 MPa).

16. Process for improving the octane number of gasoline produced by contacting under oligomerisation conditions, optionally in the presence of water, a feed comprising one or more alkenes, and optionally one or more alkanes, having from 2 to 6 carbon atoms with a catalyst consisting of an intermediate pore size siliceous crystalline molecular sieve, comprising the steps of: (i) mixing the gasoline with methanol; (ii) contacting said mixture under etherification conditions with an acid cation exchange catalytic material; (iii) extracting the residual methanol with water from the etherified effluent; (iv) distilling the methanol-free etherified effluent; and (v) recovering a gasoline effluent having an improved octane number.

17. Process according to Claim 16, wherein the feed comprises a mixture of butanes and butenes.

18. Process according to any one of Claims 1 to 16, wherein methanol is added in essentially an equimolar amount relative to the isoolefins.

19. Process according to any one of Claims 1 to 18, wherein the etherification step is carried out at a temperature of from 30 to 1200C, under a pressure of from 1 to 50 bars (0.1 to 5 MPa), and at a LHSV of from 0.05 to 10.

20. Process according to any one of Claims 1 to 19, wherein the etherification step is carried out at a temperature of from 40 to 90"C, under a pressure of from 3 to 20 bars (0.3 to 2 MPa), and at a LHSV of about 1.

21. Process for producing gasoline substantially as hereinbefore described with reference to the accompanying drawings.

22. Process for producing gasoline substantially as hereinbefore described in any one of the foregoing Examples 1 to 4.

23. Gasoline whenever prepared by a process as claimed in any one of the preceding claims.