CA2364261A1 - Process for the production of aromatic hydrocarbons from c1-4 hydrocarbons - Google Patents

Abstract

The invention relates to a process for the production of aromatic hydrocarbo ns from a hydrocarbonaceous feedstock comprising C1-4 hydrocarbons. The process especially comprises subjecting the hydrocarbonaceous feedstock to specific electromagnetic energy, especially microwave energy or radio frequency energ y, in the presence of a catalyst comprising a metal and/or a metal oxide. It ha s been found that methane can be converted into especially benzene, toluene an d xylene at high methane conversion levels and at high selectivities.

Description

PROCESS FOR THE PRODUCTION OF AROMATIC HYDROCARBONS FROM
C1_4 HYDROCARBONS
The present invention relates to a process for the production of aromatic hydrocarbons from a hydro-carbonaceous feedstock comprising C1_4 hydrocarbons. The process especially comprises subjecting the hydro-carbonaceous feedstock to specific electromagnetic energy, especially microwave energy or radio frequency energy, in the presence of a catalyst comprising a metal and/or a metal oxide.
Processes for the conversion of coal and other hydrocarbon sources such as methane, e.g. from natural gas or recovered from coal bed or municipal landfill, to synthesis gas (i.e, a mixture of hydrogen and carbon monoxide) are well known. It is further known that the synthesis gas as mentioned above can be converted in high yields into highly valuable hydrocarbons such as hydrocarbon synthesis wax, motor gasoline, petroleum chemical feedstocks, liquefiable petroleum gas and/or aromatic hydrocarbons. These reactions are usually carried out in the presence of a suitable catalyst at temperatures between 100 and 400 °C and pressures between 1 and 100 bar. A well known example is the Fischer Tropsch reaction. This process produces a wide range of products including waxy materials, oxygenates, C2-4 olefins and liquid hydrocarbons. The types of catalysts that have been used for this and related processes include those based on metals or oxides of iron, cobalt, nickel and ruthenium, with and without promoters and/or supports. The products obtained in the Fischer Tropsch reaction may be converted by other processes (hydrocracking, aromatization over zeolites etc.) into different products.
Many attempts have been made to convert methane directly into valuable higher hydrocarbons. Several quite different approaches have been proposed, including the selective oxidation of methane into C2 hydrocarbons (ethane, ethene, ethyne) over metal oxide catalysts. The high temperature pyrolytic conversion of methane is another route. Still another class of reactions includes the low pressure electric discharge of methane. None of the above mentioned processes have been commercialised due to the either low product yields or unfavourable nature of the experimental conditions required for the reactions to proceed. Reactions which occur in the gas phase often require very low pressures while surface promoted reactions typically require high temperatures and sometimes high pressures to produce practical yields.
The oxidation of methane to C2's using gaseous oxidants often results in complete oxidation. There exists, however, still a clear interest in the direct conversion of methane into liquid products, especially in view of environmental (the flaring of associated gas) and economic/technical (stranded gas) considerations.
A potentially more useful and industrially applicable method for the direct conversion of methane into higher hydrocarbons is the use of high power pulsed energy, such as high power pulsed microwaves or high power pulsed radio frequency (Rf) energy. Such processes are known in the literature. These conversions are highly energy activated processes and are based on similar mechanisms, differing in wavelengths generated and hardware required.
Rf operates in the 30-100 MHz range, microwave in the 1000-3000 MHz range, although in general all high power waves (especially between microwave and Rf) may be used.

These technologies generate electromagnetic waves which cause molecular excitation resulting in local temperature increase and/or the liberation and energisation of free electrons.
In conventional pyrolysis methods, high temperature and elevated pressure are required to maintain the reaction on the catalytic surface. These conditions limit the maximum efficiency attainable for the process. Since conventional heating is the normal means to maintain the reaction temperature, the application is non-selective and both the reactants in the bulk and the catalyst are simultaneously heated. The high bulk temperature facilitates side reactions resulting in complex reaction mixtures as well as increased reaction rates of undesired back reactions. In contrast, the utilisation of e.g.
high-power millisecond microwave or radio frequency pulses to energise suitable metal surfaces can selectively and transiently for the desired forward reactions to occur, without simultaneously creating a high bulk temperature and possible undesired side products.
The application of e.g. a pulsed electric field via a train of high power, millisecond microwave or radio frequency pulses to the catalytic metal surface causes "excitation" of the conduction electrons. Usually the metals have only very shallow "skin depth" penetration of microwave or Rf irradiation. As a result the microwaves or Rf waves, which penetrate efficiently through any adsorbed organic layers, will directly generate an oscillating field at the metal surface. The energy absorbed by the metallic sites is transferred from a rapidly oscillating electric field into motion of electrons. These electrons may remain in the conducting band causing local heating or may be ejected into the gas phase where they can further interact with gas molecules thus initiating the desired chemical conversion reactions. Adsorbed reactants at such "hot" surface sites may acquire some of this energy and undergo reactions, otherwise the energy will then dissipate slowly into the matrix by conduction. Since only the surface is heated, there is a very large temperature gradient between the surface and the bulk of the gas phase, creating an efficient reactant circulation. Secondary reactions in the bulk will tend to be minimised by the much lower temperature of the gas phase. It has been shown that methane can be converted into mainly C2 and C3, using a reduced nickel catalyst. However, it will be appreciated that conversion of methane into higher hydrocarbons is an even more desired reaction.
It has now been found that C1_4 hydrocarbons, especially methane, can be directly converted in into aromatic hydrocarbons in a catalytic processes in which high power pulsed electromagnetic energy, e.g. microwave energy or radio frequency energy is used. It has been found that e.g. methane can be converted into especially benzene, toluene and xylene at high methane conversion levels and at high selectivities. Beside the aromatic hydrocarbons substantive amounts of hydrogen are formed.
In particular, the process uses intense coherent microwave or radio frequency radiation applied in pulses to a metal sponge metal/metal oxide catalyst with a high voidage. Thus, the present application relates to a process for the production of aromatic hydrocarbons from a hydrocarbonaceous feedstock comprising Cl_4 hydro-carbons, comprising subjecting the hydrocarbonaceous feedstock to high power pulsed electromagnetic energy ranging from microwave energy to radio frequency energy in the presence of a catalyst comprising a metal and/or a metal oxide.

The feedstock to be used may be every feedstock comprising mainly, e.g. more than 95 vol.o, of C1-4 hydrocarbons of the total amount of hydrocarbons present.
Beside hydrocarbons other gasses may be present, e.g.
nitrogen, carbon dioxide, noble gasses, or other gasses which are inert during the reaction. Suitably at least 80 vol. percent of the hydrocarbons in the feedstock are methane and ethane, preferably at least 90 vol. percent of the hydrocarbons are methane. Very suitably natural gas or associated gas may be used.
The metal or metal oxide used in the present invention is suitably derived from a transition metal.
Suitable transition metals or metal oxides are derived from a group I B, II B, V B, VI B, VII B or VIII metal, especially from a group I B, V B, VI B, VII B or VIII
metal, more especially from a group I B, V B, VI B, or VII B metal. Preferably the metal or metal oxide is derived from vanadium, tungsten, manganese, iron or copper, especially vanadium or tungsten. It will be appreciated that also mixtures can be used.
The metal or metal oxide can be used in many forms.
For instance metal powders, metal foils, wires and gauzes may be used. Also supported catalysts may be used. It is preferred to use a metal or metal oxide on a carrier. The carrier is especially a metal structure, e.g. a woven metal wire structure, small mesh wire netting, gauze wire cloth, a metal fibre structure or a metal sponge, a carbon fibre structure, a mineral fibre structure or a refractory oxide structure, especially a metal sponge.
Suitably any metal or transition metal can be used, especially metal fibres having a diameter between .05 and 2.5 mm, especially between .1 and 1 mm, preferably between .2 and .5 mm. Preferred are transition metals, especially copper or iron. The above described carrier containing catalysts combine the desired properties of strong interaction with microwave energy by virtue of their metallic sites, with a means of dispersion the thermal energy to the supported material. In addition, the large surface area of the porous catalyst facilitates the gaseous interaction and catalyses reactions further along the reaction sequence. In the case of metal carriers, the carrier as such may also catalyse the reaction, although it is preferred to add an additional catalyst. For this reason, the term catalyst as used in connection with the present application also covers the bare metal carrier as defined above.
A suitable surface area of the carrier is between 10 and 100000 mm2/g, especially between 50 and 50000 mm2/g, more especially between 500 and 10000 mm2/g, preferably between 1000 and 5000 mm2/g. The voidage of the catalyst (carrier or carrier and catalyst) is suitably at least 50 vol.o, especially at least 75 vol.o, suitably between 85 and 99.99 vol.o, especially between 90 and 99.9 vol.
percent, preferably between 96 and 99.7 vol. percent.
The amount of hydrocarbon feedstock used per minute per amount of carrier used is suitably between .Ol and 10 litre (273 K, 1 bar) of gaseous hydrocarbon feedstock per minute per gram of catalyst (carrier and catalyst when present)(but higher amounts, even up to 50 litres/g, are also possible), especially between 0.01 and 5 litre, preferably between 0.05 and 2 litre. In the case that a carrier is used in combination with a catalyst, the amount of catalyst is preferably between 1 and 300 wto percent of carrier, especially between 5 and 100 wt percent, preferably between 10 and 40 percent.
The aromatic hydrocarbons prepared in the process according to the present invention are suitably benzene or mono- or di-alkylated benzene, especially benzene, _ 7 _ toluene or xylene, but also ethyl benzene, diethyl benzene and trimetyl benzene.
The reaction according to the present invention may be carried out at all possible temperatures and pressures. The temperature is suitably carried out between -20 and 250 °C, especially between 0 and 160 °C, preferably between 10 and 100 °C, especially at ambient temperature. It is observed that the above indicated temperatures relate to the incoming gas stream. Due to the processes involved, the temperature will increase during the reaction. Usually the processed gas stream will have a temperature which is up to 160 °C higher than the incoming gas stream, especially up to 80 °C higher.
Further, it will be appreciated that much higher temperatures will exist at the surface of the catalyst.
The reaction is suitably carried out at a pressure between 0.2 and 30 bar, or even up to 50 bar or more, especially between 0.5 and 16 bar, preferably between .75 bar and 6 bar, especially at 1 to 2 bar. It is observed that during the reaction the pressure may increase due to an increase of the temperature and the formation of hydrogen.
The energy used in the process according to the present invention in a small scale experiment (about 40 ml CH4/min at 273 K and 1 bar) is suitably a continuous cycle of microwave power or radio frequency power between 200 and 1800 Watt, especially between 400 and 1400 Watt, preferably between 600 and 1000 Watt, for a period between .05 and 4 seconds, especially between .l and 2 seconds, preferably between 0.4 and 1.0 seconds, followed by a period between 1 and 20 seconds, especially between 1.5 and 12 seconds, preferably between 2 and 8 seconds, without microwave power or radio frequency power. The microwave power is emitted in the form of high energy pulses, suitably with a periodicity of between .5 _ g _ and 70 milliseconds, especially between 3 and 40 milliseconds, preferably between 5 and 20 milli-seconds. The pulses suitably last for a period between .1 and 12 milliseconds, especially between .3 and 7, preferably between .5 and 3 milliseconds. At increasing power, the pulse width as well as the amplitude may be increased. The above mentioned energy amounts relate to power generated during the first period of the cycle, i.e. when the energy is emitted in the form of high energy pulses.
An attractive way to carry out the process according to the present invention is to use less energy after the first period of energy addition. For instance, the second and following periods of energy supply may be carried out at a level of 15 to 80 percent of the level of the first period, especially at a level between 20 and 45 percent of the first pulse, preferably between 25 and 35 percent of the first pulse. In this way only a quarter to a third of the energy is required (when compared with the starting energy). If desired, the high energy used in the first period may be repeated after every 20 to 200 periods of energy supply, preferably after 30 to 100 periods a pulse train is used of about the same energy level as the first pulse. The thermal efficiency of the process is usually above 40 percent, and is especially between 50 and 80 percent.
The amount of energy to be used in the process according to the present invention is suitably between 15 and 200 Watt for the conversion of 1 mol methane/hour, preferably between 20 and 60 Watt, more preferably between 30 and 45 Watt. The amount of energy expressed as Watts per m3 of the catalyst is suitably between 20 and 4000 kW/m3, preferably between 50 and 2400 kW/m3, more preferably between 100 and 1600 kW/m3. It will be clear _ g _ that these figures will be dependant on e.g. the particular catalyst and the density of the catalyst. The amount of energy expressed in relation to the surface area of the carrier is suitably between 40 and 3200 kW/m2, especially between 80 and 1800 kW/m2, preferably between 120 and 1200 kW/m2.
The conversion of the hydrocarbonaceous feedstock comprising C1_4 hydrocarbons, especially the methane, is suitably at least 30 vol. percent, preferably between 50 and 99 percent, more preferably between 70 and 95 percent. It will be appreciated that as a general rule more energy will result in a higher conversion. The selectivities to aromatic hydrocarbons obtained in the process according to the invention is usually more than 30, but is often between 50 and 85 wto based on converted starting compound.
The aromatic compounds may be separated and worked up from the reaction mixture in the usual way. A suitable method is cooling the reaction product to a temperature between 15 and 25 °C, followed by distillation. Another way is to absorb the liquid hydrocarbons in a suitable liquid. The hydrogen and gaseous hydrocarbons formed may be separated, and used for separate purposes. If desired, the mixture may also be used for the generation of power, especially electricity, which can be used internally, e.g. for the generation of microwave or radio frequency power, or can be exported. The aromatic compounds can be used as such, e.g. as solvents or as fuel, or converted into other valuable chemical compounds.
A preferred use of the hydrogen formed in the process of the present invention is the use as an additional hydrogen source in a Fischer Tropsch process. Depending on the starting material for the synthesis gas preparation and the actual gasification process, the synthesis gas to be used in the Fischer Tropsch process may have a lower H2/CO ratio than the consumption ratio.
For instance in the case of coal gasification or the gasification of heavy oil, the H2/CO ratio is usually between 0.5 and 1.2, while in most Fischer Tropsch reaction, especially reactions using iron or cobalt catalysts, the consumption ratio is between 2.0 and 2.1.
Addition of the hydrogen produced in the process of this invention to coal- or heavy oil-derived synthesis gas may raise the H2/CO ratio to a ratio which is closer or equal to the consumption ratio. In the case of methane or natural gas as feed for the synthesis gas preparation, a very suitable process for the preparation of the synthesis gas is partial oxidation or catalytic partial oxidation. Such a process results in synthesis gas having an H2/CO ratio of 1.8-1.9. As described above, the consumption ratio is usually 2.0 to 2.1. Please note that the actual synthesis gas mixture which is contacted with the catalyst mixture may have a lower H2/CO ratio than the consumption ratio. For example, the use of a gas recycle may result in a lower H2/CO ratio. Also the use of a two-stage Fischer Tropsch process with addition of hydrogen between the stages may result in a low H2/CO
ratio for the first step.
It will be appreciated that the hydrogen produced in the present process may also be used in the synthesis of oxygenates, especially methanol and dimethylether, which oxygenates optionally may be converted into aliphatic hydrocarbons or aromatic hydrocarbons.
A preferred Fischer Tropsch process is a fixed or slurry bed process, using iron or cobalt based catalyst, preferably cobalt based catalysts. Especially cobalt on an alumina, silica or titania carrier is used, optionally promoted by rhenium, platinum, manganese, zirconium or scandium.
The excess hydrogen produced in the present process may also be used for the upgrading of the hydrocarbons made in the Fischer Tropsch process. It may be used in any hydrocracking process, hydrogenation process or hydrotreatment process (e. g. hydrofinishing). It may also be used for the purification (e.g. desulphurization) of hydrocarbonaceous feed used for the preparation of the synthesis gas.
Examples A Cober variable power (max. 3 kW), 2.4 GHz magnetron, Model S3F/4091, was used as a power source. A
pulse controller was connected to the power supply to control the pulse period and the duty cycles that were simultaneously monitored by a digital oscilloscope. The quartz tube chemical reactor (25 cm long and 1.7 cm in diameter) housed in the waveguide cavity was placed perpendicular to the direction of the microwave propagation and to the electric field so as to maximise the exposure to the incident microwave irradiation.
Controlled microwave experiments were performed in a batch reactor system in which the reactant gas was circulated through the catalytic bed with a peristaltic pump. A dry-ice condenser was connected downstream of a GC sampling valve to trap liquid product.
Fe sponge based catalysts were prepared by physical mixing of Fe sponge with commercial; or custom made metal or metal oxide powders. The weight ratio of Fe sponge to metal powder was about 0.3 to 1. Activated carbon and silica supported metal catalysts were prepared by impregnation methods, and then subjected to H2 reduction at 773 K for 2 h. The loading amount was 10 wto. In addition, Cu wire and Cu net catalysts were also used for comparison. The typical reaction conditions were: pure CH4 at atmospheric pressure, flow rate 40 ml/min, 0.25 g catalyst, incident microwave power 800 W, total irradiation time 1.8 s (15 pulses, 35 s rest time).
Optimum reaction conditions were achieved by varying the incident microwave power, the reaction pressure and the pulse sequence.
On-line analysis of products was carried out using a Hewlett Packard 5890A gas Chromatograph equipped with both a flame ionisation detector and a thermal conductivity detector. Products were separated using a Porapak-Q packed column. The selectivity to aromatics was calculated based on C-containing products, which represents the fraction of methane that was converted to specific hydrocarbon products taking into account the number of carbon atoms in the molecule.
Results for metal catalysts (on Fe sponge) are given in table 1. Hydrocarbonaceous feedstock: CH4; pressure:
1 bar; power: 800 Watt; pulse time: 0.5 s (20 pulses);
rest time: 7.5 s.
Results for metal oxide catalysts (on Fe sponge) are given in table 2. Hydrocarbonaceous feedstock: CH4;
pressure: 1 bar; power: 800 Watt; pulse time: 1.5 s (10 pulses); rest time: 3.5 s.
Results for metal oxide catalysts (on Cu wire carrier) are given in table 3. Hydrocarbonaceous feedstock: CH4; pressure: 1 bar; power: 800 Watt; pulse time: 1.5 s (10 pulses); rest time: 3.5 s.

Table 1 Catalyst (Fe Fe Cu/C Cu/Fe Mn/Fe Fe/Fe sponge) Methane conversion 81.1 63.0 85.5 84.5 80.7 Selectivity to aromatics 11.5 17.6 56.6 18.4 16.4 C1 41.2 63.7 33.7 35.5 41.7 C2 33.1 28.7 23.6 36.1 43.3 C3 3.1 3.0 4.6 4.8 1.5 C4 16.2 1.7 12.6 15.6 4.7 C5 1.7 0.0 0.4 1.1 2.2 C6+ 1.3 0.0 0.2 0.4 0.9 benzene 3.4 2.6 24.1 5.9 3.2 toluene 0.0 0.0 0.7 0.0 0.8 xylene 0.0 0.0 0.0 0.0 0.0 Table 2 Catalyst Fe Mn- Fe- Co- Ni- Cu- V205/ W03/
(Fe 0/Fe 0/Fe O/Fe O/Fe O/Fe Fe Fe sponge) Methane 90.5 86.3 87.3 81.0 82.2 79.7 91.4 90.5 cony.

Sel. to 66.5 75.7 75.9 45.3 32.7 62.2 88.3 87.2 arom.

C1 31.0 32.3 30.4 41.3 39.4 43.4 22.1 24.0 C2 22.8 21.6 15.6 28.3 7.8 20.2 15.5 13.4 C3 5.7 2.0 5.4 1.2 23.2 2.1 2.5 1.8 C4 3.5 2.0 1.9 1.6 2.3 3.8 0.7 1.2 C5 2.0 0.3 0.8 1.1 1.5 2.6 0.4 1.9 C6+ 1.7 2.6 3.6 9.5 12.9 3.7 0.8 1.1 benzene 20.3 34.0 34.0 7.4 3.4 15.6 41.0 42.2 toluene 6.1 2.1 4.1 4.9 3.4 3.1 5.8 5.6 xylene 7.0 2.9 4.3 4.8 6.1 5.6 11.1 8.6 Table 3 Catalyst Cu only Ni+W/A1203/Cu Co+Mo/A1203/Cu Ni/Cu (Cu wire) Methane 90.0 93.8 90.2 82.2 conv.

Sel. to 62.8 70.8 76.1 5.4 arom.

C1 25.1 16.5 24.5 39.4 C2 26.6 12.7 20.6 27.1 C3 6.6 5.7 6.3 12.1 C4 8.9 7.3 2.9 15.6 C5 1.5 6.4 1.4 1.5 C6+ 1.0 4.4 1.1 2.7 benzene 15.9 25.4 35.8 1.3 toluene 4.3 9.5 3.l 0.1 xylene 10.1 12.1 4.3 0.2

Claims (14)

1. Process for the production of aromatic hydrocarbons from a hydrocarbonaceous feedstock comprising C1-4 hydrocarbons, comprising subjecting the hydrocarbonaceous feedstock to high power pulsed electromagnetic energy ranging from microwave energy to radio frequency energy in the presence of a catalyst comprising a metal and/or a metal oxide.

2. Process according to claim 1, in which at least 80 vol. percent of the hydrocarbons in the feedstock are methane and ethane, preferably at least 90 vol. percent of the hydrocarbons are methane.

3. Process according to claim 1 or 2, in which the metal or metal oxide is derived from a transition metal, preferably the metal or metal oxide is derived from a group IB, IIB, VB, VIB, VIIB or VIII metal, more preferably the metal or metal oxide is derived from vanadium, tungsten, copper or manganese.

4. Process according to any one of claims 1 to 3, in which the catalyst is a metal or a metal oxide on a carrier, preferably in which the carrier is a metal structure, e.g. a woven metal wire structure, a metal fibre structure or a metal sponge, a carbon fibre structure, a mineral fibre structure or a refractory oxide structure, especially a metal sponge, more preferably in which the carrier metal is copper or iron.

5. Process according to any of claims 1 to 4, in which the aromatic hydrocarbon is benzene or mono- or di-alkylated benzene, especially benzene, toluene or xylene.

6. Process according to any of claims 1 to 5, in which the reaction is carried out at a temperature between 0 and 160 °C, preferably between 10 and 100 °C, especially at ambient temperature, and at a pressure between 0.5 and 16 bar, preferably between .75 bar and 6 bar, especially at 1 bar.

7. Process according to any of the preceding claims, in which a continuous cycle of microwave power or radio frequency power between 400 and 1400 Watt, preferably between 600 and 1000 Watt, is used for each 40 ml methane/min (273 K, 1 bar) for a period between .1 and 2 seconds, preferably between 0.4 and 1.0 second followed by a period between 1.5 and 12 seconds, preferably between 2 and 8 seconds, without microwave power or radio frequency power.

8. Process according to any of the preceding claims, in which the following pulses are at a level between 20 and 45 percent of the first pulse, preferably between 25 and 35 percent of the first pulse, more preferably a process in which after each 30 to 100 pulses a pulse is used of about the same energy level as the first pulse.

9. Process according to any of the preceding claims in which energy is used having a frequency between 20 MHz and 2.4 GHz, preferably microwave frequency between 2.1 and 2.7 GHz, preferably around 2,4 GHz.

10. Process according to claim 9 in which radio frequency energy is used having a frequency between 20 and 100 MHz.

11. Process according to any of the preceding claims, in which the amount of energy used for the conversion of 1 mol methane/hour is between 20 and 60 Watt.

12. Process according to any of the preceding claims, in which the amount of energy is between 50 and 2400 kW/m3, preferably between 100 and 1600 kW/m3.

13. Process according to any of the preceding claims, in which the amount of energy is between 80 and 1800 kW/m2, preferably between 120 and 1200 kW/m2.

14. Process according to any of the preceding claims, in which the hydrogen which is formed in the process is used as an additional hydrogen source in a Fischer Tropsch hydrocarbon synthesis process, preferably a Fischer Tropsch process using a cobalt based catalyst.