METHANE, HYDROGEN, CARBON OXIDE AND CARBON DIOXIDE"

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FIELD OF THE INVENTION

Object of the present invention is a process for the treatment of a gas mixture comprising methane, hydrogen, carbon oxide and carbon dioxide.

In particular, such mixture can derive from biogas and/or syngas, considered as mixtures of gases such as carbon monoxide (CO), hydrogen (Hj), methane (CH4) and carbon dioxide ( CO ₂ ), in variable amounts.

Specific object of the present invention is a process for the treatment of syngas comprising hydrogen, methane, carbon dioxide and carbon oxide, as well as other trace gases such as hydrogen chloride, organic acids with low molecular weight and nitrogenous bases (ammonia).

Syngas is a gas mixture obtained by thermal dissociation of the organic matter in absence or in deficiency of oxygen. The components of such gaseous mixture can be separated into gases of economic interest and polluting gaseous compounds which reduce the fuel and commercial potentialities of the gases of economic interest; the former are: hydrogen, methane, carbon oxide and carbon dioxide, whereas the latter are: hydrochloric acid, organic acids with low molecular weight and nitrogenous bases (ammonia).

KNOWN PRIOR ART

Methane is at the moment an essential energy source, e.g. in the district heating, in the cogenerators for the electric power, in the automotive, in general in the gas network, etc.; if it is mixed with carbon oxide, it is a fuel mixture of tested use in civil and industrial heat generators (town gas).

In particular, recently it is of significant interest the production of biomethane and methane obtained by thermal process, i.e. of a gas containing at least the 95% of methane and produced starting from renewable sources. For this reason, several plants and processes for the recovery and isolation of methane from gases obtained from organic/polymeric matrices from waste, both biodegradable (biogas) and generic (syngas), are emerging. This because the methane obtained from renewable sources, which is chemically identical to the methane from extraction and therefore has the same uses, is produced starting, e.g. from OFMSW (Organic Fraction of Municipal Solid Waste), agricultural by-products and animal wastes, rubbers and plastics. In particular, the most biodegradable materials tend to naturally release decomposition products into the environment, with high environmental impact and greenhouse effect.

The increase in the number of plants and processes for the production of biomethane is as well supported by various public incentives.

For these reasons, scientific and industrial studies to produce natural fuels are continuously developed. Pyrolysis is a particularly studied process because, at the moment, is the only one that would allow the treatment and the energy recovery of not necessarily biodegradable organic material; it is clear that the studies concern with the management of the pyrolytic process from an economic and environmental point of view, and are object of specific inventions and patents.

Moreover, recovery processes of methane from gaseous fluxes from waste, deriving, e.g., from industrial processes, are very common.

Object of the present invention is to provide a process and a plant which allow the production of methane, carbon monoxide, hydrogen and carbon dioxide, avoiding high costs and consumptions and which do not bring about severe damages to the environment, with a resulting high environmental impact.

Further object of the present invention is to obtain pure carbon dioxide from biogas and/or syngas with acceptable costs, to be used in the food Held (beverages, packaging under modified atmosphere, etc.); this is possible because the separation process is based on two phase transitions:

- deposition (gas - solid);

- sublimation (solid - gas).

The process causes a drastic reduction of environmental impact due to the recovery of C0 ₂ , carbon monoxide and hydrogen, which determines a reduction of the industrial processes actually used for the production these gases. As a matter of fact, the process allows an optimization of the productions and significant ecological advantages, since it would be recovered: • CO ₂ with reduction of the industrial processes currently used for the production of this gas,

• gaseous hydrogen usable to feed fuel cells or internal combustion engines, as well as to meet the request of the chemical industry, which at the moment makes use of production systems with high environmental impact based on the cracking of the petroleum tractions,

• liquid carbon monoxide with reduction of the exhausts of this gas in the atmosphere.

BRIEF SUMMARY OF THE INVENTION

These and other objects are achieved by a process for the treatment of a gas mixture mainly consisting of methane and carbon dioxide, wherein the method comprises the following steps:

- purifying the gas mixture;

- cooling the purified gas mixture to obtain carbon dioxide in the solid state;

- separating the solid carbon dioxide from methane in the gaseous state, wherein the above mentioned steps are carried out substantially at ambient pressure;

- sublimating the solid carbon dioxide.

An advantage of such implementation is that it allows, starting from gas mixtures, e.g. obtained by anaerobic digestion, pyrolysis or gasification processes, to obtain: - methane with purity higher than the starting one, up to 99%;

- solid carbon dioxide at a temperature lower than -80 °C;

- gaseous carbon dioxide with high purity level.

A further advantage of the invention is due to the separation of CO ₂ and to the production of biomethane at ambient pressure, thus reducing the plant and operating costs. Moreover, liquid nitrogen is used as exchange fluid in the cooling, of which there is a significant abundance on the market of cryogenic fluids.

Specific object of the invention is a process for the treatment of syngas comprising methane, hydrogen, carbon monoxide and carbon dioxide, wherein the process comprises the following steps:

- purifying the gas mixture (scrubber: char or tar);

- fractionally cooling the purified gas mixture to obtain: gaseous carbon dioxide with high purity level, methane in the liquid state, hydrogen in the gaseous state, carbon monoxide in the liquid state;

- separating the solid carbon dioxide from the syngas in the gaseous state;

- sublimating the solid carbon dioxide;

wherein the above mentioned steps are substantially carried out at ambient pressure. An advantage of such implementation is that it allows, starting from gas mixtures obtained for example by pyrolysis or gasification processes, to obtain:

- liquid methane with purity higher than the starting one, up to 99%;

- gaseous carbon dioxide with high purity level obtained by sublimating the separated carbon dioxide in the solid state within the scope of the process;

- gaseous hydrogen;

- carbon monoxide in the liquid state.

Object of the invention is as well a specific plant for the treatment of a gas mixture comprising methane, hydrogen, carbon oxide and carbon dioxide, wherein the plant comprises a scrubber to eliminate impurities and undesired gases present in the mixture, a liquid-nitrogen heat exchanger downstream of the scrubber, to cool the purified gas mixture and to obtain carbon dioxide in the solid state, a cyclone, downstream of the heat exchanger, to separate the solid carbon dioxide from a gaseous fuel mixture consisting of methane+hydrogen+carbon oxide, a sublimation system for the carbon dioxide in order to obtain it in the gaseous state and with high purity level, a liquid-nitrogen heat exchanger to obtain methane in the liquid state, a reverse cycle Stirling engine to obtain the fractionation of the gaseous hydrogen and the liquid carbon monoxide.

Additional features of the invention can be deduced from the dependent claims. BRIEF DESCRIPTION OF THE FIGURES

Further features and advantages of the invention will become evident from the reading of the following description provided for illustration purposes and without limitation, with the aid of the figures depicted in the attached tables.

• Figure 1 is a general block diagram for the production of biomethane and carbon dioxide according to an embodiment of the invention; it is an option wherein the sublimation of solid CO ₂ (aiming at recovering pure gaseous CO ₂ in a gas cylinder) occurs through the absorption of heat from the fluid evolving in a Bryton engine, wherein the hot source consists of the combustion smokes coming from an electricity-generating set fed with biomethane; also in this case, mechanical work and electric power are generated with the coupling of the Bryton engine to an alternator.

• Figure 2 is an indicative plant lay out for the production of biomethane and carbon dioxide according to an embodiment of the invention; it is a more immediate option, with the recovery of CO ₂ with high purity level, by means of its sublimation and storage in gas cylinders.

· Figure 3 is a specific plant lay out of the invention: in this it is reported the option of separating the CO ₂ , as in figure 2, and the single components of the gaseous fuel, by means of a fractionated cooling; in a liquid-nitrogen exchanger the following are obtained: H2 and CO in the gaseous state and methane (T< -162 °C) with high purity level and in the liquid state. Thereafter, the still gaseous mixture is sent to a reverse cycle Stirling engine, from which carbon oxide (T< -192 °C) in the liquid slate and gaseous hydrogen are obtained.

DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE PRESENT INVENTION

Referring at first to figure 1, it is highlighted a general block diagram representing the production of biomethane and carbon dioxide.

It is premised that, with the process of the invention, starting from gas mixtures to be purified, it is possible in any case to obtain:

- methane with purity higher than the starting one, up to 99%;

- solid carbon dioxide at a temperature lower than -80 °C;

- gaseous carbon dioxide with high purity level.

Together with carbon dioxide other gases present in the mixture can condense.

When it is desired to obtain pure carbon dioxide, the other gases must be separated before condensation or the solid condensate must undergo sublimation (as already described above).

With reference to figure 3, the specific process proposed in the invention, starting from gas mixtures to be purified and obtained with pyro lysis processes, allows to obtain:

- methane with purity higher than the starting one, up to 99%, i.e. gaseous fuel consisting of an extremely pure mixture of hydrogen, methane and carbon oxide;

- gaseous carbon dioxide with high purity level;

- carbon oxide (T< - 192 °C) in the liquid state (as an option);

- gaseous hydrogen.

Together with carbon dioxide other gases present in the mixture can condensate. Generally, the obtained methane has purity greater than 95% V/V, with the real possibility of reaching 99%.

As it is known, in the biogas production process (block 10) a by-product called digestate is also produced (block 20), which can be used variously, such as for example as an organic fertilizer.

By means of the process of the invention, the biogas or the syngas can be separated into solid carbon dioxide (block 30) and methane in the gaseous state (block 40), the whole process as better indicated in the following, substantially occurring at ambient pressure.

Purified solid carbon dioxide, or even not purified, can have different uses:

- not purified (refrigeration industry), as refrigerant fluid with low environmental impact, for the food storage or where low temperatures are required;

- not purified, coupled to the combustion smokes of an electricity-generating set fed with biomethane, can generate electric current by means of a Bryton cycle engine powering an alternator, thus converting the thermal energy into electric power;

- purified: in the food industry, e.g. for beverages.

Figure 2 is a general block diagram of a plant for the production of biomethane and carbon dioxide, according to an embodiment of the invention.

In the diagram of figure 2, the gas mixture to be purified mainly comprising methane and carbon dioxide, can consist of pre-cooled biogas and is fed to a wet separator, e.g. a scrubber 140, wherein it is pre-treated by removing most of impurities and undesired gases, e.g. small percentages of FfoS and NFh or the like. Thereafter, the purified mixture is introduced into a rapid-cooling heat-exchanger 1 10, which makes use of liquid nitrogen as exchange fluid, conveniently designed to avoid solid CO ₂ deposits. A specific pneumatic system pushes the mixture into a cyclone 120. The cyclone 120, conic in the bottom part, communicates with:

- thermostated storage chamber 150 of the solid C02;

- thermally insulated storage chamber 130 for the CO + CH4 + H2 mixture.

Figure 3 is a lay-out of a plant for the production of methane, hydrogen, carbon monoxide and carbon dioxide, according to a specific embodiment of the invention. In the diagram of figure 3, the gas mixture to be purified comprising hydrogen, methane, carbon oxide and carbon dioxide consists of pre-cooled syngas and is sent to a wet separator, e.g. a scrubber 140, wherein it is pre-treated by removing most of impurities and undesired gases, e.g. small percentages of H ₂ S and NH ₃ , HC1, organic acids or the like. Thereafter, the purified mixture is introduced into a rapid-cooling heat-exchanger 1 10, which makes use of liquid nitrogen as exchange fluid, conveniently designed to avoid solid CO ₂ deposits. A specific pneumatic system pushes the mixture into a cyclone 120. The cyclone 120, conic in the bottom part, commun icates with:

- thermostated storage chamber 150 of the solid CO ₂ ;

- thermally insulated storage chamber of the gas mixture.

The solid CO ₂ condensate undergoes sublimation at controlled temperature, thus obtaining pure carbon dioxide introduced into gas cylinders.

By analyzing the phase diagram of CO ₂ in the gas/solid region, from which the values in the table have been extrapolated, it can be noted that it is possible to separate CO ₂ in the solid state below the temperature of -78.5 °C at ambient pressure.

If the CO ₂ is in a mixture with other gases, then the vapor pressure should be seen as partial pressure, therefore, if a nearly quantitative separation is desired (greater than 99%), by keeping the overall pressure of the system at 1 atin (therefore without compression), going down to - 120 °C is necessary.

The option of obtaining pure hydrogen in the gaseous state, liquid methane and liquid carbon oxide, provides for going below - 162 °C (down to -172 °C) for the methane and below -192 °C (down to -202 °C) for the carbon oxide.

Preliminary laboratory tests have been performed by cooling the biogas (CH4-CO ₂ mixture) with liquid nitrogen, reaching temperatures below -160 °C. The condensation involved all the substances with a freezing point greater than the working temperature; for example: water, hydrogen sulfide, organic acids.

If pure CO ₂ has to be obtained, the foreign substances must be eliminated before deposition; alternatively, it is possible to subject the solid to sublimation in a controlled temperature tower, thus obtaining the separation of the CO ₂ only.

The invention allows to take advantage, in many of its applications, of nondestructive processes (e.g. the anaerobic digestion) with respect to the combustion thermal processes, which are more conservative (reduced increase of entropy and therefore reduced overall environmental impact) and to create an optimal process for the production of methane with low costs, also considering that methane is the energy carrier of the future, particularly in the liquid state.

The invention would also facilitate the implementation in the industrial production system of combined natural cycles (e.g. thermal energy from methane combustion or from a solar paraboloid, refrigeration energy etc.) for processes aiming at improving the following ratios: (Emitted carbon dk>xide)/(Energy of produced materials), (Energy of produced materials)/ (Energy of used materials) and facilitates also the possibility of developing local "network" systems for the energy and materials.

Obviously, modifications or improvements may be added to the invention as described as a result of contingent or particular motivations, but without deviating from the scope of the invention claimed hereunder.