GB2484095A

GB2484095A - Carbon dioxide recovery method

Info

Publication number: GB2484095A
Application number: GB1016345.9A
Authority: GB
Inventors: Kristian La Vda
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-09-29
Filing date: 2010-09-29
Publication date: 2012-04-04
Also published as: GB201016345D0; WO2012044178A1; EP2621875A1

Abstract

A process for recovery of carbon dioxide by conversion to methane, comprising reacting CO2 with hydrogen to form methane and water via free radical reaction, where hydrogen gas is dissolved in supercritical CO2 at a ratio of 2:1, reaction is initiated with hydrogen peroxide and optionally oxygen gas at a ratio of 1:0 to 1:1, and microwaves provide energy for free radical initiation. The process may be continuous flow, and may be at 300 - 500 oC. Energy for maintaining the reaction may comprise microwaves, laser light, or electricity. The process may further comprise free radical precursors e.g. N2O, NO, NO3, HNO2, HClO, and perchlorate. A system for the recovery of CO2 comprises a geological facility providing energy for electrolysis of water to oxygen and hydrogen, where the oxygen is used in free radical initiation and the hydrogen is reacted with CO2. The system may convert methane to longer chain hydrocarbons.

Description

CO2 RECOVERY METHOD The present invention relates to a process for the recovery of carbon dioxide to methane.

Field of the invention

A lot of focus has been aimed at CO2 recovery due to its undesirable environmental impact, and many methods have been developed to address this problem, for example by capturing and storing CO2. In some of these sequestering methods CO2 is separated from other flue gases either pre-or post-combustion, compressed to the supercritical state, and injected into geological storage or oil fields. In other methods the carbon is recycled into a solid phase, by use of the Bosh reaction, where CO2 gas is reacted with hydrogen gas to CO gas and water, and a second step where the CO gas is then reacted with more hydrogen gas to form solid carbon and more water. The solid carbon may then be combusted or refined to for instance methanol.

Alternatively, the Bosh reaction can be stopped after step one, and the CO gas can be reacted with hydrogen gas in a Fisher Tropsch reaction to form more complex hydrocarbons.

The present invention addresses this problem in a novel manner, by reacting CO2 in a supercritical phase with hydrogen gas (the H2 is dissolved in the supercritical CO2) in a free-radical reaction with H2O2 and optionally oxygen as radical-initiator. Thus, the process according to present invention is characterized by reacting carbon dioxide with hydrogen to form methane and water in a free radical reaction, where hydrogen gas is dissolved in supercritical carbon dioxide at a ratio of 2:1, and said reaction is initiated with hydrogen peroxide and optionally oxygen gas at a ratio of 1:0 to 1:1, and microwaves provide energy for the free racial initiation.

Reaction parameters of the method according to the present invention At STP (standard conditions for temperature and pressure) CO2 is a gas, and it can be frozen to a liquid (dry ice). If the temperature and pressure are both increased to at or above the critical point for CO2, it becomes a supercritical fluid. The critical temperature and pressure for CO2 is 31.1°C and 73 atm, respectively.

When the CO2 is in a supercritical phase, it is neither gas nor liquid, but rather a fluid with properties of both. It can expand to fill a container much like a gas, but has the density of a liquid, It can diffuse through solids like a gas, but dissolve materials like a liquid. Thus the hydrogen gas is easily dissolved in the supercritical CO2 in accordance with the present invention.

Thus, in a supercritical phase the CO2 will be present in a very dense phase, where there exists no phase separations that may slow the reaction between supercritical CO2 fluid and hydrogen gas. In the method according tot he present invention, the following reaction then takes place: CO2(supercritical) + 2 H2(gas) -> CH4 (gas)+ 2H2O(Iiquid) In order for this reaction to take place a catalyst is needed, and this catalyst is comprised of small amounts of H2O2 and optionally 02. Said catalyst is present as a radical-initiator, i.e. it initiates free radical formation. The catalyst is required only for the initiation, then a cascade reaction will start, and the reactions will continue even without the presence of the catalyst until there are no reactant or stabile forms of molecules left. Since they act only as initiators/catalyst, only very small amounts of H2O2 and 02 are necessary for the process to work.

Preferably the catalyst is comprised of both H2O2 and 02. The combination is optimal for radical formation. H202 is contributes to the formation of the advantageous H& radicals, which hydrogenate easily, while 02 also readily produces other free radicals. It is however possible to run the reaction without 02, and as it is very explosive this might be preferable in some instances. If 02 is used, the most preferred ratio between H202 and 02 is 2:1. One can however use up to equal amounts of H2O2 and 02 and still get usable if not optimal results. Thus, the catalyst comprises H2O2 and 02 at a ratio of 1:0 to 1:1, with an optimum of 2:1.

In addition microwaves are used continually during the process as an energy source.

Eventually a water phase will usually build up in the bottom of the reactor. The process is then done, and C02 or other gasses are then not in a supercritical phase anymore.

The reaction is capable of running at quite low temperatures due to free-radical mechanisms. Preferably the reaction temperature is maintained at between 300 and 500 °C. The optimal temperature will depend on other reaction parameters, such as the pressure. The reaction may be run at different temperatures and pressures, as long as the CO2 remains in the supercritical phase.

In the following detailed reaction scheme, radicals (also often referred to as free radicals) are labeled with a dot (j), and are uncharged, hyper reactive molecules, atoms or ions with unpaired electrons on an open shell configuration. The labels (g) and (I) refers to gas and liquid phase, respectively. The presence of a catalyst comprising of H202 and preferably 02, as well as microwaves, is required for free radical initiation.

Reaction scheme of the method according to the present invention 1. Main/maior reaction: C02(g) + 2H2(g) -* Heat/Microwaves -* C02(super critical) + 2H2(dissolved in C02) -÷ HeaUMicrowaves -* CH4(g) + 2H20(l) The microwaves create free radicals by release of energy to the vibrating molecules.

H202 and 02 form free radicals easily/rapidly.

2. Side reactions/free radical reactions:

Examples H202:

H202(l) -* H0 + & H202(l) -> 2H0 Hydrogenation is most usual with the H0 radical (Li et al 1991)

Examples C02/02:

02(g) -÷ 20 C0+0-C0+02 C&+HO-CH-÷H20 Possible radical formation of OH4: CF++3H0-> CH4+30 The reaction will continue until there are no more reactants/stabile molecules present.

3. Cascade reaction initiated by free radical reactions: CO2+2H -> COH+OH HO is the most stabile of these radicals, and will be formed and continue to hydrogenate (Li et al 1991), the reaction itself will "run amuck" until there is no possibility for any further free radicals, and because a water phase will be formed.

The main reaction (point 1 above) will therefore produce the major/main product.

4. Consequence of the main reaction (point I above)j When the reaction is done, that is, when the reactants have been used up or no more stabile radicals can be formed, there is a water phase (liquid phase) left in the reactor, and a gas phase of methane (CH4).

The process may of course be carried out as a continuous reaction, in a continuous flow reactor.

Advantages and preferred embodiments of the method according to the present invention There are some distinct advantages to using CO2 in a supercritical phase in accordance with the present invention. By using CO2 in a supercritical phase much lower temperatures may be used when compared to the traditional Bosh reaction method. The reaction is quite safe since the reactants are in very close proximity to each other (in near liquid phase). Use of free radical initiation allows the reactions the reactions to run at said lower temperatures, and low pressure. As a consequence, the method according to the present invention is run at a lower temperature and pressure, in a less complicated reactor, and is cheaper to carry out.

In a preferred embodiment of the present invention, the process of CO2 recovery is used as part of a complete system, where the energy and other components needed as well as further processing of the resulting methane is all part of one integrated system.

The CO2 can be of any origin, but in the above mentioned preferred embodiment of a system of the present invention it is obtained from industry or recycled from the atmosphere by aid of existing membrane technology. The CO2 is used in the present invention as a carbon source via free-radical mechanisms as described herein.

The hydrogen needed for the process of 002 recovery is, in the preferred embodiment or a complete system, acquired from a geometric facility. Such facilities harvest energy from geometric heat in the ground either directly by heat differentials, or by heat exchangers. This energy is used for electrolysis of H20 to 02 and H2. The oxygen and hydrogen gasses can also be sold as bi-products, and the oxygen can be used as free-radical initiator in accordance with the invention. The H202 used as free-radical initiator together with the oxygen can be purchased or made onsite. The methane produced by the 002 recovery may in turn be converted by water gas reactions! Fisher Tropsch reactions to higher order hydrocarbons.

The system according to said preferred embodiment does not cover large areas for production of bio-methanol, and will be very economical in areas with easily available geometrical energy, such as Iceland.

Please see figure 1 for a flow chart overview of the system according to the preferred embodiment of the present invention.

Although a preferred embodiment of the invention is described above as a complete system, it will be apparent to anybody skilled in the art that numerous substitutions to this system can be made. This system is a preferred embodiment because coupling the 002 recovery with a geometric facility has several advantages, not the least of which is cost efficiency and clean production.

In accordance with the present invention, H202 and preferably 02 as well, is used as a catalyst!radical formation initiator. In addition, according to a preferred embodiment of the present invention, other materials including all forms of gasses, liquids or solid materials that may be used as precursors for free radical formation may be used with the present invention in order to jump start the reaction between supercritical 002 and dissolved H2. For example, N20 forms free radicals very readily and therefore is very environmentally damaging (it is 200 times more damaging than 002), which makes it an excellent choice for improving the free radical formation of the present invention, if that is desired. Although any free radical formation precursors may be used in accordance with the present invention, preferred examples include NO, NO3, HNO2, HOlO, and perchlorate.

The main source of energy for the process in accordance with the present invention is microwaves. Microwaves in accordance with the present invention are electromagnetic waves of 300MHz to 300GHz. Preferably, the invention uses microwaves of a frequency of 2,5 to 6 GHz, and most preferably a frequency of 4 to 5 GHz. In addition to microwaves, other forms of energy sources can be employed in order to form free radicals in accordance with the present invention. Such energy sources may be any kind of energy, including laser or electricity; static or not.

Claims

Patent claims 1. Process for recovery of carbon dioxide to methane, c h a r a c t e r i z e d b y reacting carbon dioxide with hydrogen to form methane and water in a free radical reaction, where hydrogen gas is dissolved in supercritical carbon dioxide at a ratio of 2:1, and said reaction is initiated with hydrogen peroxide and optionally oxygen gas at a ratio of 1:0 to 1:1, and microwaves provide energy for the free racial initiation.
2. Process in accordance with claim 1, c h a r a c t e r i z e d b y that the ratio of hydrogen peroxide to oxygen gas is ratio 2:1.
3. Process in accordance with claim 1, c h a r a c t e r i z e d b y that the process is carried out as a continuous flow reaction.
4. Process in accordancewith claim 3, characterized by that the temperature of the process is maintained at between 300 and 500 00.
5. Process in accordance with claim 3, c h a r a c t e r i z e d b y that energy for maintaining the reaction is provided from microwaves and/or other forms of energy including laser or electricity, including static electricity.
6. Process in accordance with claim 1, c h a r a c t e r i z e d b y that precursors for free radical formation are used to further increase the initiation of free radical formation.
7. Process in accordance with claim 6, c h a r a c t e r i z e d b y that said precursors are selected form the list comprising N20, NO, NO3, HNO2, HCIO, and perch lorate.
8. System for recovery of carbon dioxide to methane, c h a r a c t e r i z e d b y a geometric facilety providing the energy for electrolysis of water to oxygen gas and hydrogen gas, where said oxygen gas is used for free radical initiation in the process in accordance with claim 1, and said hydrogen gas is used as the reactant hydrogen gas in the process in accordance with claim 1.
9. System in accordance with claim 8, c h a r a c t e r i z e d b y converting the methane produced to higher order hydrocarbons.