WO2010069582A1

WO2010069582A1 - Process for the conversion of methane, contained in gas flows, into hydrogenated liquid hydrocarbons

Info

Publication number: WO2010069582A1
Application number: PCT/EP2009/009108
Authority: WO
Inventors: Luigina Maria Flora Sabatino
Original assignee: Eni SpA
Current assignee: Eni SpA
Priority date: 2008-12-19
Filing date: 2009-12-16
Publication date: 2010-06-24
Anticipated expiration: 2011-06-19
Also published as: ITMI20082277A1; IT1392390B1

Abstract

Methane is converted, alone or in a blend with other gases, into liquid hydrocarbons, essentially aromatic hydrocarbons, in the presence of a catalytic system comprising a H-ZSM-5 zeolite having an SiO₂/Al₂O₃ molar ratio ranging from 20 to 100, from 0.5 to 15% by weight of molybdenum and from 0.1 to 5% by weight of cerium (IV), at a temperature of 500-900°C, with the production of hydrogen as reaction by-product.

Description

PROCESS FOR THE CONVERSION OF METHANE, CONTAINED IN GAS FLOWS, INTO HYDROGENATED LIQUID HYDROCARBONS

The present invention relates to a process for the conversion of methane, contained in gaseous streams, into hydrogenated liquid hydrocarbons.

More specifically, the present invention relates to a process for the preparation of hydrogenated hydrocarbons, consisting in treating a gas containing methane, of both a fossil and biological origin, at a high temperature, with a suitable catalyst, in order to prepare a mixture of higher hydrocarbons essentially consisting of cyclic and/or aromatic hydrocarbons, and subjecting the mixture obtained to hydrogenation. The process object of the present invention is particularly useful for the conversion of gas containing methane, present close to oil fields or deriving from biological processes, for the collection, sorting and transportation of the product. The same process is par- ticularly advantageous for the recovery and use of so- called associated gas and gas of a biological origin.

Furthermore, as is known, the objectives of refinery and chemical plant activities comprise the transformation of hydrocarbons having a relatively low value into more valuable hydrocarbon streams, or into actual chemical products. Methane, for example, the simplest of saturated hydrocarbons, can often be found in large quantities as by-product in mixtures with other hydrocarbons having a higher molecular weight, or as gas component at the out- let of process units. Although methane is effectively used in some chemical reactions (such as the production of methanol and formaldehyde) , it is not as useful as hydrocarbons with a higher molecular weight . For this reason, process streams containing methane are normally burnt as fuel.

Industrially developed processes for effecting these transformations pass through the production of syngas (a mixture of CO and H₂) which can be subsequently converted to methanol or paraffin waxes via Fischer-Tropsch synthe- sis (indirect conversion) .

As far as direct conversion is concerned, i.e. the transformation of natural gas into liquid hydrocarbons in a single step, several methods have been proposed among which oxidative coupling (pyrolysis carried out in the presence of oxygen on suitable catalysts) and non- oxidative pyrolysis which leads to the production of light olefins (ethylene and propylene) , aromatic compounds (BTX, naphthalene, etc.) and hydrogen.

The production of higher hydrocarbons starting from methane becomes significant at high temperatures and in any case, when the conversion is carried out in the presence of a catalyst which can guarantee high synthesis rates of the higher hydrocarbons, also under thermody- namically unfavourable conditions. The presence of a suitable catalyst can also significantly influence the distribution of the products in the final mixture.

European patent application 192,289 describes the direct conversion of natural gas into aromatic hydrocarbons, at high temperatures, on a catalyst based on alka- line silicate and also includes aluminium and/or gallium. International patent application WO 92/01656 describes the same conversion in the presence of a transition metal supported on a refractory material, essentially consisting of a metal oxide, at temperatures of up to 300⁰C. European patent application 858,987 relates to a process for the conversion of a light hydrocarbon blend, in gaseous phase under normal temperature and pressure conditions and containing methane, into higher hydrocarbons, liquid under normal temperature and pressure conditions. This process includes an absorption step of the mixture in question on a supported metal catalyst comprising a metal or combination of metals, at least one of which belonging to Group VIII of the Periodic System of Elements, followed by a desorption phase. The first step is ef- feeted at temperature values not lower than 300⁰C. The catalyst consisting of molybdenum supported on H- ZSM- 5 zeolite is of interest for the conversion of methane to aromatic compounds, as described in "Journal of Catalysis 181, 175-188 (1999)". The methods for the production of higher hydrocarbons which the above-mentioned state of the art refers to, i.e. the disclosure for effecting said methods, as appears possible from reading the state of the art, seem to underline the importance of the concentration of methane in the gaseous mixture to be fed to the high- temperature treatment, almost proclaiming the possibility of industrial advantages only in the case of the conversion of methane alone (substantially pure methane) . As this solution must start from blends containing methane under dif- ferent percentages, it would imply onerous and difficult pre-treatment for the separation and removal of the other components of the blend of interest, and would therefore make the industrial application of these methods problematical . It has now been found that it is possible to prepare blends of higher hydrocarbons by directly subjecting a gas containing methane, for example associated gas already in its formation site and without the necessity of previously enriching the relative blend in methane gas, to high- temperature treatment, in order to produce aro- matic hydrocarbons with 1 to 3 condensed rings, and/or variously substituted by alkyl groups, and hydrogen, which can be recovered by means of appropriate storage tanks . The aromatic products can be initially mixed with the oil (petroleum) produced. Subsequently, when the quantity of stored hydrogen is sufficient, the same can be used to hydrogenate part or all of the aromatic products in order to produce hydrocarbon compounds having a high hydro- gen/carbon ratio. In this way, the oil produced can be enriched with fractions of distillates useful for the production of fuels and/or intermediate products for the petrochemical industry.

On the whole, the process proposed allows certain streams of natural gas to be upgraded, which could otherwise not be used or which would be difficult to transport as gas, into liquid hydrocarbon products, compatible with the oil itself. The possibility of transforming associated gas into liquid hydrocarbons also enables oil fields to be exploited which, for logistic reasons or environment regulations (restrictions on gas flaring) could not be put into production as they are not capable of treating the gas component .

The conversion into industrially interesting products of a mixture which is normally sent to combustion is thus obtained, at the same time obtaining a precise upgrading of the same mixture .

An object of the present invention therefore relates to a process for the direct conversion of methane, con- tained in a blend with other gases, into liquid hydrogen- ated hydrocarbons, which can be obtained with a high selectivity, using a catalyst comprising Mo supported on a zeolite. The process is described in the enclosed claims. All the catalysts mentioned in the state of the art are suitable for the preliminary conversion of methane (natural, associated, biological, etc.) into liquid products . The use can therefore be envisaged of the supported catalyst described in the above-mentioned European patent application EP 858,987, consisting of one or more metals selected from those of Group VIII of the Periodic System of Elements, deposited on a carrier represented by an oxide. The nickel/copper mixture is particularly preferred. The catalytic composition described in International patent application WO 99/03949 can also be used, consist- ing of a mixture including a zeolite and zinc aluminate, previously treated with a reducing gas at a high temperature .

From a completely general point of view, the following compositions can be used in the process according to the present invention: a catalyst consisting of a zeolite and one or more of the following components: oxides of alkaline and alkaline-earth metals; oxides of Group IIIA; metals of Groups IVB, VB, VIB, VIIB, VIIIB, IB, HB, IHB and a binder consisting of refractory oxides, or a mixture thereof, selected from SiO₂, Al₂O₃, ZrO₂, TiO₂, MgO.

The zeolite is selected from:

Structural types of MFI, MTT, TON, NES, MTW, MWW (ERB- 1) , BEA, FER, MOR, FAU, ERS, OFF; Zeolites selected from those mentioned above, wherein part of the silicon is substituted by Ti;

Zeolites selected from those mentioned above, wherein part or all of the Al is substituted by B, Fe, Ga, V, Cr;

Zeolites impregnated or exchanged with metals of Groups IVB, VB, VIB, VIIB, VIIIB, IB, HB, IHB with a weight percentage of 0.01 to 30%;

Zeolites exchanged with a metal of Groups VIII, IB, HB and impregnated with V, Mo, W, treated with a promoter such as Ru, Rh, Pd, Pt in quantities ranging from 0.001 to 5% preferably from 0.01 to 2% by weight.

The catalyst can be formed by one or more of the above-mentioned components, impregnated with an alkaline metal .

The metal (s) can be deposited on zeolite by ionic ex- change in solution, ion exchange in the solid state, im- pregnation, precipitation. Various metals can be deposited in a single step or following a step procedure. In the step preparation, the intermediates can be simply dried or dried and calcined. The final catalyst is dried and calcined in the presence of air.

According to a preferred embodiment, the process according to the present invention includes a first conversion step of associated gas into liquid products through dehydro-aromatization with the formation of aromatic products (mainly benzene and naphthalene) followed by hy- drogenation, which allows naphthenic products to be obtained.

The gas conversion can be effected at a temperature value of 350 to 800⁰C, depending on the gas composition and catalyst selected, at a pressure ranging from 0.2 to 5 bar and GHSV ranging from 500 to 50,000 h^'1.

The aromatization reaction of methane can be carried out in a reaction area comprising one or more catalytic beds in one or more reactors . The arrangement of reactors can include one or more fixed or mobile bed reactors into which the catalyst, suitably formed, is introduced. All the systems can consist of various beds in series. For mobile bed reactors, the gas/solid mixing can be favoured by mechanical stirring or through forced recirculation of the reaction fluids. The circulation of the reagent gas can be effected in countercurrent with respect to the feeding direction of the catalyst particles.

The reactor configuration can consist of a reaction area, in turn consisting of one or more catalytic beds, and a regeneration area: portions of exhausted catalyst are periodically extracted from the reaction area to be transferred to the regeneration area. The regeneration is effected by means of a regeneration gas and has the purpose of totally or partially removing the carbonaceous deposits possibly present on the catalyst. The regeneration can be effected in a fixed bed, fluid or ebullated bed. The regeneration beds can be one or more. Before being brought back to the reaction area, the catalyst, in relation to the regeneration gas used, can be subjected to carburization in an area different from the regeneration and reaction area. The carburization can be effected in one or more beds. The bed can be fixed, fluid or ebullated.

The aromatic compounds obtained from the dehydro- aromatization reaction can be separated and possibly hy- drogenated.

The hydrogenation can be effected on catalysts consisting of supported metals of Group VIII (Pt, Pd, Ru, Ni) . If noble metals supported on zeolites are used, the hydrogenation effected at 260 to 315°C, at a pressure of 4.2 MPa, an LHSV of 3 to 4 h^"1, proves to be equal to or higher than 95%.

The hydrogenation can be effected by feeding hydrogen to the blend produced during the preliminary high- temperature treatment or, according to a preferred embodiment, the same blend is removed during its formation to be then subjected to hydrogenation on the part of the same hydrogen gas produced and accumulated during the same thermal treatment . The most critical aspect of this process consists in the rapid deactivation of the catalyst. The conversion, in fact, drastically diminishes with the reaction time due to the formation of carbonaceous deposits on the same catalyst. The deactivation can be slowed down by reducing the acidity of the H-ZSM-5 (de-alumination or silanation) or by introducing small quantities of CO, CO₂, H₂, H₂O, in the feed. The effects obtained are extremely limited and unsatisfactory from a technological point of view {Krzysztof Skutil, Marian Taniewski, Fuel Proc . Techn. , 87 (2006) 511-521) .

It has now been surprisingly found that the selection of a suitable promoter, added to one of the above- mentioned catalysts, is capable of preventing the formation of coke through a continuous action. The promoter selected belongs to the group of lanthanides, in particu- lar, Cerium was selected. The capacity of Cerium oxide of being transformed from CeO₂ (+4) to Ce₂O₃ (+3) through a relatively low activation energy, at least compared to other oxides, is in fact known. The Ce₂O₃ ( +3) can be easily re-oxidized to CeO₂ (+4) in an oxidizing environment. Even the loss of a considerable quantity of oxygen and therefore the formation of a large quantity of oxygen vacancies, does not cause variations in the structure of the crystal. As there are no phase variations in the crystal structure in the release and re-absorption of oxygen, the CeO₂ can be used as an oxygen container: 2

CeO₂ <-> Ce₂O₃ + 0.5 O₂. The Cerium oxide can also be used as a carrier, in the preparation of a mixed carrier, added during the catalyst formation phase. The process for the preparation of the hydrogenated hydrocarbons according to the present invention can be illustrated by the block scheme of Figure 1. It consists of the following units:

A. unit for the conversion (dehydro-aromatization) of methane alone or in a mixture with other gases such as gaseous alkanes (ethane, propane, etc..) or inert gas such as nitrogen and carbon dioxide, into liquid hydrocarbons essentially consisting of aromatic and light hydrocarbons (paraffins and C₄-Ci₀ cyclo- paraffins) ; B. first gas- liquid separation unit (H₂ + liquid hydrocarbons) ;

C. second separation unit of H₂ from liquid hydrocarbons dissolved therein. The hydrogen thus separated in the two separation units is used for the hydrogena- tion of the aromatic compounds, whereas the light hydrocarbons are recycled and, together with methane or its blends, represent the dehydro-aromatization feedstock; D. hydrogenation unit of the aromatic compounds, using the hydrogen produced in the methane conversion units;

E. hydrogen separation unit from the hydrogenated compounds. The hydrogen recovered is recycled to the hydrogenation unit.

The process can additionally include the removal of at least part of the catalyst present in the reaction zone and its regeneration with a suitable regeneration reagent in a regeneration area which can consist of one or more reactors. The regeneration can be effected in a fixed or mobile bed regime. The regeneration can be followed by a re-carburization phase of the catalyst effected in the carburization area which can consist of one or more reactors with a regime of the fixed or mobile bed type. The regenerated catalyst, possibly also re-carburized, can be totally or partially recycled, and introduced into the reactor in which the dehydro-aromatization reaction of methane takes place. As is well known to experts in the field, the carburization can be effected with the use of various carburization agents consisting of a hydrocarbon (methane, ethane, propane, butane, isobutane) alone or in a mixture with hydrogen, or a mixture consisting of hydrogen and CO and/or CO₂. The carburization can be effected at 300 to 900⁰C, and at a pressure of 1-10 bar. According to the present invention, a fixed quantity of catalyst is charged into a conversion reactor. The activation of the catalyst is then effected, which can be carried out at a temperature selected within the range of 350-800⁰C, through a gas flow (for a duration of 15-120 minutes) . The activation can be carried out with an inert gas (He, N₂, Ar), with a reducing or oxidizing agent. After treating the catalyst in an oxidizing or reducing environment, the flow is substituted by an inert gas (15-20 minutes) . Once the reaction temperature has been reached, the reagent mixture is sent onto the catalyst.

For the catalytic conversion of methane to higher hydrocarbons, a sample was used, under non-oxidizing conditions, consisting of Mo-ZSM-5, with a SiO₂/Al₂O₃ molar ratio within the range of 20-100, Mo (w/w) of between 0.5- 15%. The zeolite was prepared according to what is de- scribed in US patent 3,702,886, the Molybdenum was introduced by impregnation, using a solution of ammonium hep- tamolybdate, whereas the promotion with cerium (0.1-5% w/w) was effected by impregnation of Mo/ZSM-5 with a ce- rium salt.

A tubular quartz reactor having an inner diameter of 20 mm, kept inside an electric oven in the constant temperature area, was used for determining the catalytic activity. In the initial phase, the reactor is heated with a slope of 10°C/min to the operating temperature (650- 800⁰C) maintained for 30 minutes under an air flow (100 ml/min) , then for 10 minutes in Argon (100 ml/min) .

After this initial phase, the reactor is fed with pure methane having a GHSV (Gas Hourly Space Velocity) within the range of 750-1,500 ml/ (g_cat h) .

The characterization of the products in gas phase is effected with a Varian CP-3800 gas chromatograph, using two TCDs as detector. The characterization of the liquids is effected with an HP 5890 gas chromatograph, using a FID as detector.

The reaction products initially consist of CO and CO₂. After about 60 minutes from the beginning of the feeding, the presence of CO and CO₂ is no longer registered and the first products appear, contemporaneously. The conversion is calculated by the ratio: (CH₄I_n- CH₄OUt) /CH₄I_n. The selectivity is calculated by considering the ratio of the number of carbon atoms contained in the single molecule of product, with respect to the total carbon converted. The performances of the catalyst can be at least partially restored (oxidation of the carbonaceous residues and polycondensates) by using a regeneration process which includes a cooling phase to 100⁰C in argon and a heating phase (5°C/min, final temperature 580-600⁰C) in air.

The method according to the present invention will now be further illustrated by some examples which are naturally only for applicative and non- limiting purposes. In these examples reference will be made to process units and procedures well-known to experts in the field. EXAMPLES

The data of three samples are provided. A reference sample Mo/H-ZSM-5 (Mo = 2% w/w; SiO₂/Al₂O₃ = 56) , an optimized sample Mo/HZSM-5 (Mo = 4% w/w; SiO₂Ml₂O₃ = 35) and a promoted sample Ce-Mo/HZSM-5 (Ce = 0,5% w/w, Mo = 4,2% w/w, SiO₂/Al₂O₃ = 35) .

Figure 2 shows the conversion of methane for the reference sample and for the optimized sample. The optimization leads to a material having a higher activity and stability. Regeneration processes also allow optimum per- formances to be obtained by the optimized material. The conversion of methane with respect to the time obtained for the fresh optimized sample and after regeneration, is shown in Figure 3. The stability of the catalyst can be increased by introducing CO₂ in the feed. Activity tests in the presence of CO₂ indicate, mainly after regeneration and for quite long reaction times, an increase in activity in terms of methane conversion, deactivation rate and also a higher stability in the distribution of the aromatic products.

A gas having the following composition was subsequently used:

Methane 84-87% (v/v) , ethane 10% (v/v) , carbon dioxide 3-6% (v/v), to reproduce natural gas. The experimental conditions relating to the pre- treatment of the catalyst, space velocity and the analysis method are analogous to those relating to the conversion of pure methane. The reaction temperature varied from 550 to 800⁰C. The total conversion of hydrocarbons, for a feed consisting of natural gas, is higher with respect to that of pure methane. In the first 25 hours the conversion passes from 25 to 23%. After 25 hours of reaction, the deactivation becomes more pronounced and the total conversion of carbon tends towards the value observed for the CH₄/CO₂ mixture .

A higher yield to aromatic products (intended as a ratio between moles of C in liquid products and moles of

C fed) , also corresponds to the higher conversion of the hydrocarbons observed passing from a feed consisting of

CH₄ to CH4/CO2 and to CU₄ZC₂H₆ZCO₂.

A variation in the product distribution can also be observed, mainly passing from pure methane to the methane/carbon dioxide mixture. There is an increase in the selectivity to naphthalene, a decrease in the selectivity to benzene, whereas the selectivity to toluene remains unchanged. The presence of ethane causes a further slight increase in the selectivity to naphthalene. Conversion of methane in the presence of the promoted catalyst

The experimental conditions relating to the pre- treatment of the catalyst, space velocity and analysis method are described in the paragraph relating to the methane conversion. The reaction temperature varied from 550 to 800⁰C.

The data obtained with a sample promoted with Cerium indicate, as main effect of the promoter, a different distribution of products with a lesser formation of coke and greater formation of aromatic compounds, as indicated in the following table.

By using a mixture containing CH₄ and 1% by volume of CO₂, after 44 hours of reaction, the quantity of coke deposited on the sample promoted with cerium proves to be 22% less with respect to the non-promoted sample. The decrease in the quantity of coke is accompanied by a higher yield to aromatic compounds, the same passes from 4% for Mo/H-ZSM-5 to 6% for Ce-Mo/HZSM-5. Separation and hydrogenation of the aromatic compounds Separation

The aromatic compounds can be easily separated from the methane and other light gases, due to the consistent difference in relative volatility. It is possible to ef- feet the separation through compression, at room temperature. As an alternative to compression, resort can be made to a washing/extraction with a heavy hydrocarbon compound followed by fractionation. Using C₃₀ as absorbing liquid and simulating an absorption column of 10 theoretical steps, a recovery of 95% of benzene is obtained. Hydrogenation of the aromatic compounds

The aromatic compounds, essentially consisting of benzene, toluene, naphthalene and methyl -naphthalenes, are hydrogenated to naphthenic products, after separation from the lower hydrocarbons .

The hydrogenation is carried out in a fixed bed reactor under the following conditions:

P = 62 bar, T = 315⁰C; H₂/aromatics = 712 Nm³/m³ , LHSV = 1 h^"1, catalyst = Y zeolite impregnated with Pt (0.5 kg/1) .

Under these conditions, the conversion to hydrogen- ates proves to be 98%.

Claims

1. Process for the conversion of methane, contained in gas flows, into hydrogenated liquid hydrocarbons which comprises: a. conversion of methane into essentially aromatic liquid hydrocarbons in the presence of a catalytic system containing cerium (IV) with production of hydrogen as by-product; b. separation of the liquid hydrocarbon fraction from the gas phase consisting of non reacted gas and hydrogen; c. separation of the hydrogen from the gas mixture and accumulation of the same (H₂) in dedicated containers; d. recycle of the remaining gas mixture at the conversion stage (a) ; e. hydrogenation of the essentially aromatic liquid hydrocarbons by using the reaction by-product hydrogen; f. regeneration of the catalyst in the presence of a regeneration gas in a suited regeneration area; g. optionally, carburization of the regenerated catalyst in a carburization zone with a carburization mixture; h. recycle of the catalyst, after regenera- tion/carburization, in the reaction zone.

2. Process for the preparation of hydrogenated hydrocarbons according to claim 1, wherein the gas supplied to treatment (a) is associated gas or biogas .

3. Process according to claim 1 or 2, wherein methane is in mixture, at 20-90% in volume, with other gases chosen between ethane, propane, butane, nitrogen, inert gases and/or carbon dioxide .

4. Process according to any one of the previous claims, wherein treatment (a) is carried out by sending the gas containing methane through a catalytic composition at a temperature not lower than 350⁰C and a pressure not lower than 10 bar.

5. Process according to any one of the previous claims, wherein treatment (a) is carried out in one or more fixed bed reactors .

6. Process according to any one of the previous claims from 1 to 4 , wherein treatment (a) is carried out in one or more fluid bed reactors.

7. Process according to any one of the previous claims from 1 to 4 , wherein treatment (a) is carried out in one or more slurry reactors wherein the catalyst is formed into microspheres and dispersed into the reaction medium and the mixing is achieved by means of mechanical stir- ring or by means of forced recirculation of the reaction fluids .

8. Process according to any one of the previous claims, wherein the reactor or reactors of treatment (a) are run with the recirculation of part of the effluents.

9. Process according to claim 8, wherein the recycling ratio, or the share of recirculated fraction in relation to the fresh load, ranges from 0.5 to 5 weight/weight.

10. Process according to any one of the previous claims, wherein said treatment (a) is carried out at a temperature comprised between 350 and 900⁰C.

11. Process according to any one of the previous claims, wherein treatment (a) is carried out at a pressure comprised between 0.2 and 10 bar.

12. Process according to any one of the previous claims, wherein treatment (a) is carried out at a GHSV comprised between 500 and 50.000 h^"1.

13. Process according to any one of the previous claims, wherein the hydrogenation of the mixture obtained in treatment (a) is carried out at a temperature comprised between 250 and 350⁰C.

14. Process according to any one of the previous claims, wherein hydrogenation is carried out in the presence of a catalyst consisting of a metal belonging to the VIII group of the Periodic Table, appropriately supported.

15. Process according to any one of the previous claims, wherein treatment (a) is carried out in the presence of a catalytic system comprising a zeolite H-ZSM-5, having a SiO₂/Al₂O₃ molar ratio comprised between 20 and 100, from 0.5 to 15% by weight of molybdenum and from 0.1 to 5% by weight of cerium (IV) .