WO2007068008A2

WO2007068008A2 - Alkylation process

Info

Publication number: WO2007068008A2
Application number: PCT/ZA2006/000145
Authority: WO
Inventors: Arno De Klerk; Reinier Johannes Jacobus Nel; Tebogo Molapisane Sakuneka
Original assignee: Sasol Technology Pty Ltd
Current assignee: Sasol Technology Pty Ltd
Priority date: 2005-12-09
Filing date: 2006-12-08
Publication date: 2007-06-14
Anticipated expiration: 2008-06-09
Also published as: WO2007068008A3

Abstract

The invention provides a process for the alkylation of one or more streams containing one or more aromatics selected from the group including benzene, toluene, o-xylene, p-xylene, m-xylene, and ethyl benzene with olefins wherein the process is carried out at an aromatics to olefin ratio of from 1:6 to 1:30 on a molar basis, said process being carried out under oligomerisation conditions in the presence of a solid phosphorous catalyst at a temperature of from 140°C to 240°C and a pressure of from 1.5 MPa to 10 MPa whereby both oligomerisation and alkylation occur together.

Description

ALKYLATION PROCESS Field of the Invention The invention provides a process for the alkylation with olefins of tar-derived material and of other mixed-aromatics containing materials.

Background to the Invention Olefinic motor-gasoline, which is used as high-octane blending stock, is typically produced by the oligomerisation of low molecular weight olefins over an acid catalyst. Catalytic polymerisation specifically is a process for the polymerisation of low molecular weight olefins over a solid phosphoric acid catalyst. This process for the upgrading of low molecular weight olefins to high-octane gasoline blending stock is well known and is, for example, disclosed in U.S. Pat. Nos. 2,596,497 and 2,909,580. Although this process is most ideally suited to the production of high-octane naphtha, middle distillate is also formed. The middle distillate production from a unit can be increased by recycling a fraction of the naphtha produced (referred to as the "tetramer-mode of operation").

The alkylation of aromatics with olefins is also well known and a number of commercial processes are commercially available (Hydrocarbon Processing, 82:3, 2003, 88, 89 & 92). The reaction is catalysed by an acidic catalyst which is typically a zeolite in the newer generation processes, but solid phosphoric acid, aluminium chloride and many other acid catalysts such as sulphuric acid and hydrofluoric acid can also be used (Ullman's Encyclopedia of Industrial Chemistry, 1985, 5^th Edition, A1.190). Typically these processes use pure aromatics, like benzene, and short chain olefins as feed. Side-reactions, such as polymerisation of the olefins and dialkylation of the aromatics, are minimised by using an excess of the aromatic compound, i.e. by operating at a high aromatic to olefin ratio. Selectivity and per pass conversion are typically increased by inter-bed olefin injection. Selectivity can also be increased by catalytic distillation.

High purity aromatics, like benzene, are required by the alkylation processes to minimize the build-up of impurities in the recycle. In this case, the purity of the benzene ultimately determines the size of the purge stream from the benzene recycle and the overall production cost associated with the process. The aromatic feed to an alkylation process is therefore typically purified before use. Some coal gasifiers result in the co-production of pyrolysis products (tar). In coal-to-liquids processes making use of such units, this tar material is a valuable source of compounds with a high octane number and high density, but contains a lot of benzene. Thus, in a coal-based Fischer-Tropsch process having an associated tar refinery, most benzene in the fuel is derived from the tar. The reduction of benzene could be obtained by minimising its formation or by elimination of the benzene through conversion. However, benzene extraction is costly and reduces the octane value of the fuel. Benzene extraction is therefore normally only considered by refiners if the benzene is not too diluted and they are also involved in the petrochemical market (benzene is a commodity chemical).

Based on what is known in the art, it is expected that benzene could be alkylated with a solid phosphoric acid catalyst. The alkylation mechanism is known to proceed via a carbocation intermediate. However, • Benzene is less reactive for alkylation than alkyl benzenes like toluene, xylenes, ethylbenzene, etc. In a mixed aromatic stream, such as a coal tar or fluid catalytic cracking naphtha product, it is therefore expected that the alkyl benzenes will preferentially be alkylated.

• Catalysts capable of alkylation can also be active for transalkylation. During the alkylation of a mixed-aromatic stream, containing benzene and some alkyl aromatics, this could potentially cause more benzene to be formed than is being consumed by alkylation.

• Alkylation in the presence of a high concentration of olefins may lead to olefin on olefin oligomerisation rather than alkylation of the aromatics.

• High concentrations of olefins and longer chain olefins may cause repeated alkylation and/or the formation of very heavy products. Such products may foul the catalyst and also require further refining before being used as transportation fuel.

• Low per pass aromatics conversion may require use of a recycle stream in the process. This would not only increase the operating cost of the process, but also the capital cost, especially when dealing with a mixed aromatic feed that will be difficult to separate from the alkylated product.

Summary of the Invention

Thus, the present invention provides an alkylation process wherein purification of the aromatics is not required and alkylation of a mixture of aromatics may be done as part of an oligomerisation process. This is especially aimed at reducing the amount of benzene, although the invention is applicable to all aromatics capable of undergoing alkylation, like toluene, phenol, etc.

The invention thus provides a process for the alkylation of mixed-aromatics containing streams, like partially hydrogenated tar-derived material, fluid catalytic cracker naphtha, reformate and coker naphtha, with olefins in an olefin-rich stream i.e. one which has a low mixed aromatics to olefins ratio or, optionally, in a conventional olefin oligomerisation process, which results in a significant reduction of the benzene content by upgrading of both the aromatics containing feed and the olefin feed. Thus, the invention extends to a process for the alkylation of one or more streams containing one or more aromatics selected from the group including benzene, toluene, o-xylene, p-xylene, m-xylene, and ethyl benzene with olefins wherein the process is carried out at an aromatics to olefin ratio of from 1 :6 to 1 :30 on a molar basis, said process being carried out under oligomerisation conditions in the presence of a solid phosphorous catalyst at a temperature of from 140°C to 24O⁰C and a pressure of from 1.5 MPa to 10 MPa whereby both oligomerisation and alkylation occur together.

The olefins may be from an olefin-rich stream. If the aromatics containing stream already contains sufficient olefins, there is no need to mix it with an olefin-rich stream, although this can be done should a different aromatics to olefin ratio be preferred.

The one or more streams may contain two or more aromatics. The aromatics to olefin ratio may be from 1 :8 to 1 :26 on molar basis.

Typically the aromatics to olefin ratio may be 1 : 14 to 1 :26 on a molar basis.

Typically the aromatics to olefin ratio may be 1 :14 on a molar basis.

The process may be carried out at a temperature of from 15O⁰C to 200⁰C.

The process may be carried out at a temperature of from 17O⁰C to 18O⁰C. The process may be carried out at a pressure of from 3.5 to 4 MPa.

The aromatics containing stream may contain from 5 mol% to 95 mol% aromatics and the remainder including one or more waxes, dienes, oxygenates, paraffins, olefins, and heteroatomic compounds. Thus the process may operate between 5 mass% and 50 mass% benzene in the aromatics stream.

The process combined feed streams may have in excess of 30 mass% paraffin.

The combined feed streams to the process may have in excess of 50 mass% paraffin.

The high amount of paraffin in the combined feed may permit elimination of paraffin recycle normally associated with olefin oligomerisation processes. This is typically for heat management reasons.

The one or more aromatics stream may be selected from the group including:

- partially hydrogenated tar-derived material;

- fluid catalytic cracker naphtha;

- reformate;

- coker naphtha;

- products of aromatisation units; and

- hydrocracker product.

The benzene in the process may be in a 1 :8 to 1 :29 molar ratio to the olefins.

The toluene in the process may be in a 1 :8 to 1 :29 molar ratio to the olefins. The degree of alkylation may be controlled by parameters including at least the aromatics to olefin ratio.

The process may include alkylation of a tar-derived mixed-aromatic stream as part of an oligomerisation process.

The product of the process may include less than 5 mass% C₁₅+ material. The product of the process may include less than 1 mass% C₁₅+ material. The olefins in the olefin-rich stream may be predominantly C₂ to C₅. Typically said olefins are C₃ to C₄ olefins.

In a fuels refining context, benzene alkylation can be used to convert benzene to high-octane fuel components. This is not only desirable due to the octane value of alkylbenzenes, like cumene (RON=113, MON=99), but also because it removes benzene from the fuel pool. Benzene is known to be a powerful carcinogen and its content in gasoline is regulated to be less than 1.0 vol %, in line with Euro-4 specifications.

The degree of alkylation may be controlled by the aromatics to olefin ratio and operating conditions of the process.

This is believed to have economic benefit, since it upgrades the mixed-aromatics stream without the need for prior purification, addresses fuel specification concerns with respect to benzene content and octane number and requires no additional processing units to be built for purification and/or conversion.

The invention extends to alkylation of a tar-derived mixed-aromatic stream as part of an oligomerisation process. This process can be used with any solid phosphoric catalyzed oligomerisation process by introducing a mixed-aromatics containing stream to the reaction without influencing the olefin conversion. The feed used in the commercial solid phosphoric acid catalysed olefin oligomerisation process can be propylene, butylenes, pentenes or a mixture of the olefins. The highest concentration of propylene is preferred. The results are surprising because of the known propensity for side-reactions at low aromatics to olefin ratios, as well as the classic organic chemistry teaching that in a mixture of alkylated mono-aromatics benzene is the least reactive aromatic molecule (lowest probability of being alkylated).

Both tar- and crude-derived material contains heteroatoms like nitrogen and sulphur. Although the bulk of these compounds can be removed with conventional hydrotreating, some low levels (=10ppm) always remain present. These can only be removed through additional processes like washing or guard bedding. The fact that this process is able to be used with these partially hydrogenated materials is even more surprising because of the reasonable possibility of catalyst deactivation due to heteroatom contaminants from these materials. The present invention further provides the operating conditions required for selective benzene alkylation over a solid phosphoric acid catalyst as part of an oligomerisation process and in the presence of other aromatics.

An olefin oligomerisation process may be operated whereby a mixed-aromatics stream is contacted with olefins over a solid phosphoric acid catalyst, at temperatures of 150 - 250⁰C₁. ideally at 170 - 180C.

In the process, with a mixed C₃ZC₄ olefin feed, the 6₃ ¹S may preferably alkylate, while the C₄ ¹S may preferably oligomerise.

This is beneficial because C₃-BZ (cumene) has a higher octane value than C₄-Bz, while C₄-oligomers have a higher hydrogenated octane value than C₃-oligomers.

Surprisingly it was found that,

• The benzene selectivity to alkylation was higher than that of the alkyl benzenes at similar aromatic concentration. • The alkylation and oligomerisation product was very clean, with less than 1 mass% C-_I5+ material being formed, despite the olefin concentration being higher than that used in commercial benzene alkylation processes. Further transalkylation was inhibited and the benzene selectivity was substantially uninfluenced by such reactions.

As the process temperature is increased above 180°C, the alkylation of benzene becomes less selective in relation to the other aromatics. This finding was also unexpected.

Detailed description of the invention

The benzene content in a mixed-aromatics stream can be reduced by co-feeding this stream with an olefin-rich stream to a catalytic reactor unit. The benzene, which is an undesirable gasoline component because of its carcinogenity, is converted by alkylation to less volatile, highly desirable fuel and/or chemicals components. The commercial reaction conditions for the alkylation of benzene to produce cumene over solid phosphoric acid catalysts are almost identical to those employed for oligomerisation (150-250⁰C, pressure sufficient for liquid phase reaction). The solid phosphoric acid will thus catalyse the polymerisation of olefins as well as the alkylation of benzene and other aromatics with olefins.

The alkylation reaction occurs in parallel with the polymerisation reaction. Similar olefin conversions are obtained with and without the addition of the mixed- aromatic stream (compare results shown in examples 2 and 3). The product yield from the plant is thus not reduced. This form of process intensification has an economic benefit, since it upgrades the mixed-aromatic material, addresses fuel specification concerns and requires no additional processing units to be built for extraction and/or conversion.

The solid phosphoric acid catalyst used in this process can be selected from any of the solid phosphoric acid catalysts that are commercially available. The preparation and properties of conventional solid phosphoric acid catalysts are set forth in Appl. Catal. A₁ 308 (2006) 204-209 (Coetzee, et al.); U.S. Pat. No. 2,120,702 (Ipatieff et al.); U.S. Pat. No. 3,050,472 (Morrell); U.S. Pat. No. 3,050,473 (Morrell) and U.S. Pat. No. 3,132,109 (Morrell); and also in British Patent No. 863,539. A solid phosphoric acid catalyst is typically prepared by mixing phosphoric acid with a support, extruding the resulting paste, and calcining the extruded material. The support is preferably a naturally occurring, porous silica-containing material such as, for example, kieselguhr, kaolin, infusorial earth or diatomaceous earth. A minor amount of additives can be added to the support to increase the strength and hardness. Phosphoric acid consists of a family of acids which exist in equilibrium with each other and differ from each other in their degree of condensation. These acids include ortho- phosphoric acid (H₃PO₄), pyro-phosphoric acid (H₄P₂O₇), triphosphoric acid (H₅P3O10), and polyphosphoric acids, and the precise composition of a given sample of phosphoric acid will be a function of the P₂O₅ and water content of the sample.

The solid phosphoric acid catalyst used in the examples is a cylindrical extrudate having the following properties: a nominal diameter of 6 mm, a loaded density of 1 g/cm³, and free phosphoric acid content of 20 to 25 mass %.

In normal cumene production processes, the benzene conversion is limited to about 12% per pass by the amount of olefin added. The olefin conversion is essentially complete and the unconverted benzene is recycled. Since in the present invention this reaction is performed in an excess of olefin, high benzene conversion per pass was obtained, typically 85% or more . The present invention therefore allows the use of mixed-aromatics and especially benzene-containing streams without prior purification of the aromatics and/or of the benzene.

A further benefit of the present invention pertains to heat management. The catalytic polymerisation reaction is highly exothermic. The reaction heat is typically controlled by adding a saturated hydrocarbon to the olefin feed stream and/or as a quench between multiple catalyst beds within the reactor. A recycle from the reactor effluent, a propane-butane mixture, is conventionally used for this purpose. In instances where the mixed aromatic feed stream contains non- reactive molecules, like paraffins, these molecules will act as heat sink, reducing the quench needed to control the reaction heat generated by the two parallel reactions.

A potential concern that was overcome by the present invention relates to catalyst life time. The deactivation of solid phosphoric acids can be the consequence of coking or excessive hydration. Coking is the formation of heavy compounds that remain on the catalyst surface, coating the catalyst particles and plugging the pores, thereby causing catalyst deactivation. Excessive hydration causes the catalyst to physically soften, resulting in high pressure drops in fixed bed reactors. Alkylation in the presence of a high concentration of olefins and longer chain olefins may cause repeated alkylation and/or the formation of very heavy products. In our work, therefore, the rate of deactivation of the catalyst used for alkylation at low aromatics to olefin ratio was similar to that found in an olefin oligomerisation process. It was also found that the addition of a mixed aromatic feed, like a partially hydrogenated tar-derived stream, does not increase the boiling point of the product. The final boiling point and distillation range of the product was very similar with and without the co-feeding of the mixed aromatic stream. It was also found that the addition of a mixed aromatic feed containing nitrogen bases, like a partially hydrogenated tar-derived stream, deactivates the catalyst very slowly, not markedly reducing the normal lifetime of the catalyst. Another surprising benefit was found when using a mixed olefin feed for the alkylation. Since a mixture of propene and butene is normally used as feed for catalytic polymerization of olefins, both propene and butene alkylation would be expected. Propene alkylation is however favoured, irrespective of the feed composition. This is advantageous, as cumene has a higher octane number than the butylbenzenes (RON=I 04-107 and MON=92-96).

The laboratory reactor system used for examples 1 -7 was operated at a Peclet number less than 100, causing inefficient hydrodynamic performance. This resulted in lower conversions being obtained compared to that found in pilot plant testing (see example 8) and that expected in a commercial plant.

Example 1

A partially hydrogenated coal tar naphtha product (86 mass%) and olefin rich mixture of C₃ and C₄ hydrocarbons, Fischer-Tropsch condensate-derived feed (14 mass%) was contacted over a solid phosphoric acid catalyst The operating pressure was 3.8 MPa and the temperature was varied between 180 and 200⁰C. The tar-derived feed contained 58 mass% aromatics (25 mass% benzene, 16 mass% toluene and the rest heavier aromatics). The Fischer-Tropsch condensate derived feed contained about 87 mass% olefins (19 mass% propene, 59 mass% butene and 9 mass% pentene). The ratio of tar derived feed to Fischer-Tropsch derived feed was 7:1 on a mass basis (or 1 :10.7 on a molar basis of benzene to C₃-C₄ olefins). The liquid hourly space velocity was 1.6 h^"1. The liquid hydrocarbon stream was analysed in each case and the results obtained are reported in Table 1. Table 1.

Example 2

A solid phosphoric catalyst similar to that used for oligomerisation of olefins, was used under conditions similar to those commercially practiced in olefin oligomerisation (3.8 MPa). A Fischer-Tropsch condensate-derived feed containing about 52 mass% olefins (35 mass% propene and 17 mass% butene) was fed to a reactor at a liquid hourly space velocity of 1.0 h^"1.

The liquid hydrocarbon product stream was analysed and the results obtained are summarised in Table 2.

Table 2.

Example 3

Example 2 was repeated except that a fractionated tar-derived feed containing 58 mass% aromatics (57 mass% benzene, 1 mass% toluene) was co-fed with the Fischer-Tropsch condensate-derived feed to the reactor. The ratio of tar-derived feed (mixed aromatics) to Fischer-Tropsch condensate-derived feed (olefins) was 1 :9.4 on a mass basis (or 1 :14 on a molar basis of benzene to C₃-C₄ olefins). The liquid hydrocarbon product stream was analysed and the results obtained are summarised in Table 3.

Table 3.

Example 4 Example 2 was repeated except that the ratio of tar-derived feed (mixed aromatics) to Fischer-Tropsch condensate-derived feed (olefins) was 1 :4.9 on a mass basis (or 1 :7.4 on a molar basis of benzene to C₃-C₄ olefins).

The liquid hydrocarbon product stream was analysed and the results obtained are summarised in Table 4.

Table 4.

Example 5

Example 2 was repeated except that the ratio of tar derived feed (mixed aromatics) to Fischer-Tropsch condensate-derived feed (olefins) was 1 :16.8. on a mass basis (or 1 :25.8 on a molar basis of benzene to C₃-C₄ olefins), and the liquid hourly space velocity was 2.0 h^"1.

The liquid hydrocarbon product stream was analysed and the results obtained are summarised in Table 5. Table 5.

Example 6

The feed used in the commercial solid phosphoric acid catalysed olefin oligomerisation process can be propylene, butylenes or a mixture of the two. Example 2 was repeated except that a fractionated tar-derived feed containing 57 mass% aromatics (56 mass% benzene, 1 mass% toluene) was co-fed with Fischer-Tropsch condensate-derived feed with different propylene to butylenes ratios. The ratio of tar-derived feed (mixed aromatics) to Fischer-Tropsch condensate-derived feed (olefins) was 1 :9 on a mass basis (or 1 :13 on a molar basis of benzene to C₃-C₄ olefins).

The liquid hydrocarbon product stream was analysed and the results obtained are summarised in Table 6.

Table 6.

Example 7

L5 The middle distillate yield from catalytic polymerisation plants can be increased by recycling a fraction of the gasoline produced ("tetramer mode of operation"). Example 2 was repeated except that an olefinic gasoline fraction, obtained from a commercial catalytic polymerisation plant, was co-fed with the Fischer-Tropsch condensate-derived feed to the reactor. The gasoline co-feeding was done to simulate the recycle employed commercially to boost distillate production. The gasoline fraction was an olefin mixture (bromine number >100) boiling between 40 and 200⁰C. The ratio of gasoline to Fischer-Tropsch condensate-derived feed was 1 :0.8 on a mass basis. The liquid hourly space velocity was 1.0 h^"1.

The liquid hydrocarbon product stream was analysed and yielded a 91% conversion of the mixed C₃ and C₄ olefins.

Example 8

Example 7 was repeated except that a tar derived feed containing 58 mass% aromatics (57 mass% benzene, 1 mass% toluene) was mixed with the gasoline fraction obtained from the commercial plant. The ratio of tar-derived feed (mixed aromatics) to gasoline to Fischer-Tropsch derived feed was 1 :β.7:5.4 on a mass basis (or 1 :8.2 on a molar basis of benzene to C₃-C₄ olefins). The liquid hourly space velocity was 1.0 h^'1.

The liquid hydrocarbon product stream was analysed and the results obtained are summarised in Table 7.

Table 7

Butylbenzene 33 37

Example 9 Both tar- and crude-derived material contains heteroatoms like nitrogen and sulphur. Although the bulk of these compounds can be removed by hydrotreating, low levels of these compounds always remain present.

Example 2 was repeated except that an unfractionated tar-derived feed containing 40 mass% aromatics (23 mass% benzene, 17 mass% toluene and 5ppm nitrogen) was co-fed with Fischer-Tropsch condensate-derived feed to the reactor. The ratio of tar-derived feed (mixed aromatics) to Fischer-Tropsch condensate-derived feed (olefins) was 1 :11 on a mass basis (or 1 :20 on a molar basis of benzene to C₃-C₄ olefins).

To determine if the heteroatoms deactivate the catalyst, the reaction was run for an extended period. Over a period of 86 days, the benzene conversion declined with only 3%, while the olefin conversion declined by only 1%. The liquid hydrocarbon product stream was analysed and the results obtained are summarised in Table 8.

Table 8.

Claims

1. A process for the alkylation of one or more streams containing one or more aromatics selected from the group including benzene, toluene, o- xylene, p-xylene, m-xylene, and ethyl benzene with olefins wherein the process is carried out at an aromatics to olefin ratio of from 1 :6 to 1 :30 on a molar basis, said process being carried out under oligomerisation conditions in the presence of a solid phosphorous catalyst at a temperature of from 140⁰C to 24O⁰C and a pressure of from 1.5 MPa to 10 MPa.

2. A process as claimed in claim 1 , wherein the olefins are from an olefin-rich stream.

3. A process as claimed in claim 1 or claim 2, wherein the one or more streams contain two or more aromatics.

4. A process as claimed in any one of the preceding claims, wherein the one or more aromatics stream contains from 5 mol% to 95 mol% aromatics and the remainder including one or more waxes, dienes, paraffins, olefins, oxygenates, and heteroatomic compounds.

5. A process as claimed in any one of the preceding claims, wherein the aromatics to olefin ratio is from 1 :8 to 1 :26 on molar basis.

6. A process as claimed in any one of claims 1 to 5, said one or more aromatics stream being selected from the group including:

- partially hydrogenated tar-derived material;

- fluid catalytic cracker naphtha;

- reformate;

- coker naphtha;

- products of aromatisation units; and - hydrocracker product.

7. A process as claimed in any one of claims 1 to 6, wherein benzene is in a 1 :8 to 1 :29 molar ratio to the olefins.

8. A process as claimed in claim 1 to 6, wherein toluene is in a 1 :8 to 1 :29 molar ratio to the olefins.

9. A process as claimed in any one of claims 2 to 8, wherein the degree of alkylation is controlled by parameters including at least the aromatics to olefin ratio.

10. A process as claimed in any one of claims 6 to 9, which includes alkylation of a tar-derived mixed-aromatic stream as part of an oligomerisation process.

11.A process as claimed in any one of claims 1 to 10, wherein the product of the process includes less than 5 mass% C₁₅+ material.

12. A process as claimed in claim 11 , wherein the aromatics to olefin ratio is

1 : 14 to 1 :26 on a molar basis.

13. A process as claimed in any one of the preceding claims, which process is carried out at a temperature of from 15O⁰C to 200⁰C.

14. A process as claimed in claim 13, which process is carried out at a temperature of from 17O⁰C to 18O⁰C.

15. A process as claimed in any one of the preceding claims, wherein the process is carried out at a pressure of from 3.5 to 4 MPa.