WO2010041113A1

WO2010041113A1 - Process for the direct oxidation of methane to methanol by means of non-thermal plasma in a reactor with the gliding arc in tornado (gat) technology

Info

Publication number: WO2010041113A1
Application number: PCT/IB2009/007019
Authority: WO
Inventors: Giuseppe Bellussi; Alberto De Angelis; Paolo Pollesel
Original assignee: Eni S.P.A.
Priority date: 2008-10-06
Filing date: 2009-09-25
Publication date: 2010-04-15
Also published as: IT1391370B1; ITMI20081769A1

Abstract

The present invention relates to a process for the direct oxidation of methane to methanol in which the reaction is carried out in a gliding arc in tornado (GAT) reactor with the non-thermal plasma, also known as cold plasma, technology. Said process comprises the following phases: injecting into said GAT reactor in a tangential direction, alternatively one of the following gaseous mixtures comprising methane and pure oxygen, or methane and air, or methane and enriched air, injecting steam into said GAT reactor, generating non- thermal plasma by means of two electrodes positioned inside the GAT reactor, collecting the methanol produced, condensing it and separating it. The internal walls of the GAT reactor act as anode and an electrode is inserted in its interior which acts as cathode in spiral form coaxial to the reactor.

Description

PROCESS FOR THE DIRECT OXIDATION OF METHANE TO METHANOL BY MEANS OF NON-THERMAL PLASMA IN A REACTOR WITH THE GLIDING ARC IN TORNADO (GAT) TECHNOLOGY The present invention relates to a process for the direct oxidation of methane to methanol in which the reaction is carried out in a gliding arc in tornado (GAT) reactor with the non-thermal plasma, also known as cold plasma, technology. This method can be used for the conversion of any gaseous stream comprising pure methane, mixtures of methane which comprise acid gas, for example by the presence of CO₂, and natural gas.

It is known that methanol is industrially produced by means of the steam reforming process starting from methane and steam. This process uses two reactors with two different catalysts. In the first reaction step the methane is converted to syngas, in the second reaction step the syngas is converted to methanol. The process is very endothermic, consequently the process burns considerable quantities of methane to generate the heat necessary for sustaining it. It also requires a great effort in terms of investments as it uses two reactors and two different catalysts. The methane-to-methanol reaction is a partial oxida- tion and as such must be controlled to prevent the complete oxidation of the methane to CO₂ and the formation of by-products such as formaldehyde and formic acid, deriving from the further oxidation of the methanol . Alternative technologies are known for the direct oxidation of methane to methanol. Among these innovative technologies there are non-thermal plasma technologies. These can be divided into conventional and non- conventional technologies. Non-thermal plasma systems comprise a tubular reactor in which an electric arc or discharge is generated by means of a power generator. This electric discharge brings only a part of the electrons present into a state of excitation, i.e. into a state of high energy, whereas the average temperature of the circulating gas phase is kept low or even close to room temperature. This thermal conditions defines nonthermal plasma. Non-thermal plasma is by nature nonstable as it is not in thermodynamic equilibrium. This allows a gas phase to be maintained at relatively low temperatures, whereas the electrons are at much higher temperatures. Excited electrons are extremely reactive and can effectively promote various reactions, avoiding decomposition reactions of methanol which would take place in a thermal plasma, where the temperature of a single electron is higher than 10,000 K. There are vari- ous methods for generating non-thermal plasma such as radio frequencies or electric discharges; various forms of electrodes can be used for forming different types of plasma, such as for example a plasma crown, a pulsating crown, the gliding arc and gliding arc in tornado.

Applications of non-thermal plasma to partial oxidation processes of natural gas and methane to give syngas (mixture of carbon monoxide and hydrogen) or methanol, are known. The following references can be classified as non- conventional non-thermal plasma technologies.

A.M. Czernichowski, European Patent 1012113 (28.08.1998), World Energy System Corporation effected the oxidation of methane CH₄ to syngas using a gliding arc for generating plasma. The reactor used is a cylinder divided into two areas separated by a perforated membrane. The plasma is generated in the upper area by means of two divergent electrodes, the reaction is completed in the lower thermally insulated area and the product is re- moved in order to avoid the formation of ash and tar. The gas flows from the head of the reactor at a rate higher than 10 m/s consequently moving the electric arc generated. The reagent system comprises: • natural gas and oxygen, • natural gas with oxygen and steam, • natural gas with enriched air and steam.

The total volume of the reactor is 1.9 litres. Promising results are obtained when the O₂/hydrocarbon and H₂0/hydrocarbon molar ratios are equal to about 1. A H₂/CO molar ratio equal to 2.04 is in fact reached with an energy consumption equal to 0.24 KWh/Nm³ of syngas produced. By-products such as ethane or ethylene are present at 0.1% molar, whereas the average content of CO₂ ranges from 10% to 20% molar in relation to the tempera- ture of the thermally insulated area.

CS. Kalra, A.F.Gutsol, A.A.Friedman, IEEE Trans. On Plasma Science, 33 (1), 2005, 32 - 41 apply the gliding arc in tornado technology to the partial oxidation of methane to syngas . The experiment was carried out by feeding methane and air in different molar ratios. The reactor is a glass cylinder in which a spiral electrode is positioned, which acts as cathode, the internal surface of the reactor is the anode. The total volume of the reactor is 0.2 litres. The gas enters in a tangential di- rection from a mouth situated in the upper part of the side surface of the reactor, whereas air is fed from the bottom of the reactor. The reaction products are discharged from the head. The internal flow is in the form of an inverted vortex and this characteristic generates very high circulation velocities of the gas which are such, as to move the gliding arc and ensure an effective exchange of mass and heat. The residence times of the gas in the reactor are extremely low. The cost of the electric energy reaches a minimum value of 0.09 KWh/Nm³ of syngas produced and the molar ratio of syngas produced is 1.4-1.5. There is no formation of ash and there is no consumption or erosion of the electrode. The experiment did not analyze the influence of water or carbon dioxide. T.Nozaki, A.Hattori, K.Okazaki, Cat. Today, 98, 2004, 607 - 16 effected the oxidation of methane to methanol with a new plasma micro-reactor. This reactor consists in a very thin glass tube 60 mm long and with an internal diameter of 1 mm in which a twisted metallic wire is placed. The mixture fed consists of methane and oxygen in a molar ratio O₂/CH₄ equal to 0.5. The reaction products are separated and condensed (methanol, formaldehyde, formic acid and water) in a cold trap, the non- condensed gases go for gas-chromatography. A methanol yield of 17% molar and a yield of formaldehyde of 3% mo- lar are obtained with the almost complete conversion of O₂. A technical problem encountered during the experimentation is the condensation of the oxygenated liquid fractions inside the reactor which block it and cause an unstable pulsating flow. The following references are known as conventional non-thermal plasma technologies. D.W.Larkin, T. A. Caldwell, L.L.Lobban, R.G.Mallison, Energy & Fuels, 12, 1998, 740 - 44 and D.W.Larkin, L.Zhou, L.L.Lobban, R.G.Mallison, Ind.Eng. Chem.Res . 40, 2001, 5496 - 5506 effected the partial oxidation of meth- ane to methanol by generating silent electric discharges in a dielectric glass reactor interpositioned between two metallic electrodes. The gas flows axially between the electrodes. The reactor is cooled with water and the oxygenated organic liquid fractions are condensed and sepa- rated. The gas fed is a mixture of methane and oxygen, in a molar ratio CH₄/O₂ equal to 3/1. With a conversion of O₂ of 67% molar and methane lower than 20% molar, the oxygenated liquid fractions obtained have a selectivity of 57% molar and methanol 15% molar. Air and enriched air were not tested. The same CH₄/O₂ ratio equal to 3/1 was tested with 10% molar of carbon monoxide CO, obtaining similar results.

S.L.Yao, T.Takemoto, F.Ouyang, A.Nakayama, E.Suzuki, A.Mizuno, M.Okumoto, Energy and Fuels, 14, 2000, 459 - 63 and Y.Wang, C.Tsai, M.Shih, L.hsieh, W.Chang, Aerosol and Air Quality res., 5 (2), 2005, 204-210 effected the partial oxidation of methane using a conventional dielectric plasma reactor, made of Pyrex glass with an internal diameter of 12 mm. The reactor is covered with an aluminum strip used as cathode. Inside the reactor, there is a wire arranged axially and used as anode. A gas mixture with a molar ratio CH₄/O₂ equal to 94/6 is fed from above at 100 ml/min. During the experimentation, the influence of the discharge power and intensity was analyzed. The best results were obtained at 5W with a conversion of the methane of 1.9% molar and a selectivity of methanol of 47% molar (the yield to methanol is 0.9%) . The intensity of the discharge has no effect on the selectivity of the methanol. The energy efficiency of the process is 19% based on the conversion of oxygen and 9% based on the production of methanol. These values are too low for allowing this technology to substitute the traditional one. Non-conventional plasma technologies generate a greater concentration of excited electrons with respect to con- ventional technologies. Furthermore, in non-conventional technologies, the electric arc is moveable and lasts longer. The mobility of the arc in non-conventional technologies guarantees a gas phase circulating at relatively lower temperatures with respect to what takes place in conventional technologies. Unfortunately, often, the data obtained cannot be reproduced. In the present state of the art a stable non-conventional plasma reactor does not exist .

Conventional plasma technologies provide very low yield values to methanol which are not of great interest if compared with traditional industrial processes. In general, the technical problems associated with the controlled partial oxidation process of methane to methanol are the tendency of the methane to completely oxidize to carbon dioxide and the further oxidation of the methanol to formaldehyde or formic acid.

The objective of the present invention is to use a gliding arc in tornado (GAT) reactor in the controlled partial oxidation process of methane to methanol, which generates non-thermal plasma, in which the reaction temperature and the residence times of the gas phase are kept low. Controlling the reaction temperature means being able to prevent the complete oxidation of the methane. Very low residence times are allowed with a gas cir- culation rate which is such as to remove the freshly formed methanol and therefore prevent undesired secondary reactions. A possible advantage of non-conventional plasma technologies is that they avoid the use of current steam reforming industrial technologies, with a signifi- cant saving from the point of view of energy and plant investments .

In one of its embodiments, the present invention relates to a process for the direct oxidation of methane to methanol in a tubular reactor which comprises the follow- ing phases: • injecting into said tubular reactor in a tangential direction, alternatively, one of the following gaseous mixtures comprising

• methane and pure oxygen, • methane and air,

• methane and enriched air

• injecting steam into said tubular reactor,

• generating non-thermal plasma by means of two electrodes positioned inside said tubular reactor, • collecting the methanol produced, condensing it and separating it.

In a further embodiment, the present invention relates to said direct oxidation process of methane to methanol, wherein steam is injected together with said gaseous mixture into said tubular reactor or from the bottom of said tubular reactor.

In another embodiment, the present invention relates to said direct oxidation process of methane to methanol, wherein said reaction takes place in a Gliding Arc in Tornado (GAT) tubular reactor by means of non-thermal plasma.

The process claimed has the main advantage of controlling the reaction temperature, thus preventing the complete oxidation of the methane . Furthermore, said process allow very low residence times to be maintained in order to prevent undesired secondary reactions. A possible advantage of non- conventional plasma technologies is that they avoid the use of industrial steam reforming technologies, with a significant saving from the point of view of energy and plant investments .

Further objectives and advantages of the present invention will appear more evident from the following description and enclosed drawings, provided for purely il- lustrative and non- limiting purposes, wherein:

Figure 1 shows a scheme of the controlled partial oxidation reactor with the gliding arc in tornado (GAT) technology. Detailed description The process, object of the present invention, comprises the phases described hereunder. The reagent mixture which comprises methane, steam and an oxidant alternatively selected from pure oxygen, air or enriched air, is first injected. The presence of steam is fundamental for enabling a significant yield to methanol for the development of the process . The steam is preferably in- jected together with the gaseous reagent mixture or separately from the bottom of the tubular reactor. Nonthermal plasma is then generated by means of two elec- trodes positioned inside the reactor and an external cur- rent generator.

With reference to Figure 1, the reactor is preferably tubular and of the gliding arc in tornado (GAT) type and preferably comprises a spiral electrode (4) which acts as cathode and is situated coaxially with respect to the cylindrical volume of the reactor. The internal walls of the reactor preferably act as anode. The helix angle of the spiral electrode is calculated so that it is identical to the flow of the gaseous vortex inside the cylin- drical reactor, to prevent slowing down the gas rate.

The gaseous mixture comprising methane and oxidant (1) is injected in a tangential direction through a mouth (5) situated in the upper part of the side surface of the reactor. The steam preferably enters from the bottom of the reactor (2), and the reaction products in gas phase leave the head of the reactor (3) . The reaction products are subsequently condensed and separated.

During the reaction in the GAT reactor, an inverse vortex flow is generated and this ensures an extremely high rate of the gases at the inlet. In this way, the electric arc can move and is maintained fluent, thus guaranteeing a sufficient heat disposal and maintaining the gaseous mass at relatively low temperatures. The temperature of the non-thermal plasma preferably ranges from 100⁰C to 700⁰C and the pressure of the plasma preferably ranges from 0.1 barg to 2 barg. A high circulation of the gas also allows very low residence times to be reached in order to remove the methanol formed and prevent it from further oxidizing to CH₂O and HCOOH. The gas mixture comprises oxygen preferably from 0% in moles to 30% in moles and steam preferably from 0% in moles to 10% in moles. The sum of oxygen and water is preferably at least 5% in moles of the reagent gas. The molar ratio between methane and oxygen CH₄/O₂ preferably ranges from 3:1 to 2:1 and the molar ratio between methane and water CH₄/H₂0 preferably ranges from 10:1 to 2:1.

The gases fed have a temperature preferably ranging from O⁰C to 100⁰C, more preferably from 2O⁰C to 50⁰C, and a total pressure preferably ranging from 0.1 barg to 10 barg, more preferably from 0.1 barg to 2 barg.

The cathode can also have a circular and moveable form, situated coaxially with respect to the cylindrical volume of said reactor. The material of which the cathode is produced is preferably selected from a group of mate- rials consisting of stainless steel, nickel alloys, metals of the second and third row of group VIIIB of the periodic system as such or in an alloy with the metals of said group .

Claims

CLAIMS .

1. Process of direct oxidation of methane to methanol inside a tubular reactor which comprises the following steps : • injecting in tangential direction into said tubular reactor alternatively one of the following gas mixtures comprising:

^■ methane and pure oxygen,

^■ methane and air, ■ methane and enriched air,

• injecting steam into said tubular reactor,

• generating non thermal plasma by means of two electrodes placed inside said tubular reactor,

• collecting the produced methanol, condensing it and separating it.

2. Process according to claim 1, wherein steam is injected together with said gas mixture or from the bottom of said tubular reactor.

3. Process according to claim 1, wherein said direct oxidation reaction is carried out in a tubular Gliding Arc in Tornado reactor (GAT) by means of non thermal plasma .

4. Process according to claim 1, 2 and 3, wherein the inside surface of said reactor works as anode and wherein an electrode which works as cathode is placed inside said reactor.

5. Process according to claim 1, 2, 3 and 4 wherein the cathode is fixed and spiral shaped or mobile, circu- lar shaped and coaxial to said reactor.

6. Process according to claim 1, 2 and 3, wherein the reagents concentration comprises O₂ between 0% molar and 30% molar, H₂O between 0% molar and 10% molar, so that the sum of oxygen and water must be at least 5% molar of reagent gas.

7. Process according to claims 1, 2 and 3 wherein the molar ratio between methane and oxygen CH₄/O₂ is from 3:1 to 2:1 and the molar ratio between methane and water CH₄/H₂O between from 10:1 to 2:1.

8. Process according to claims 1, 2 and 3 wherein the non thermal plasma temperature is comprised between 100⁰C and 700⁰C and wherein the non thermal plasma pressure is comprised between 0.1 barg and 2 barg.

9. Process according any of the previous claims, wherein the reagent gas has an inlet temperature T varying from 0⁰C and 100⁰C and an inlet total pressure varying from 0.1 barg and 10 barg.

10. Process according to claim 9, wherein the reagent gas has an inlet temperature T varying from 20⁰C and 5O⁰C and an inlet total pressure varying from 0.1 barg and 2 barg .

11. Process according to claims 1, 2, 3, 4 and 5, wherein the cathode is selected among a group of material consisting of stainless steel, nickel alloy, metals from the second and third line of VIIIB group in the periodic system as single or as alloys of metals of said group.