Device intended to extract carbon dioxide from a gaseous mixture mainly composed of methane and carbon dioxide, particularly in the case of biogas.

Abstract

Apparatus intended to extract carbon dioxide from a gaseous mixture composed of methane and carbon dioxide, by condensation at low temperature and high pressure, and to supply methane at a pressure sufficient for supplying an internal combustion engine and for other uses.

Description

Title of the invention: Apparatus for extracting carbon dioxide from a gas mixture mainly composed of methane and carbon dioxide, particularly in the case of biogas.

[0001] Throughout the world, many families operate small farms far from large cities, having neither gas nor electricity at their disposal, so their food is often cooked on wood fires or other fuels, often lit in the main room of their home. The atmosphere of the room is thus heavily polluted by carbon monoxide fumes, soot particles or other combustion elements which are very dangerous for health.

[0002] Small biogas production plants, using household waste from one or more families, livestock excrement and farm waste, bring undeniable progress to the situation described, as biogas can be used as fuel for cooking. Note that biogas is a mixture composed essentially of methane CH4 (most often 50 to 60%, in % of the number of moles), carbon dioxide CO2 (40 to 50%), water vapor (there 3%) and traces of hydrogen sulfide (H2S).

[0003] In the context of these small installations, significant additional progress would therefore be made if the biogas, after the elimination, already commonly carried out, of water vapor and hydrogen sulfide, were also freed from the majority of the carbon dioxide. Reduced to methane, with traces of residual carbon dioxide, the biogas thus purified would then have a much better energy yield. In addition, since its combustion only releases carbon dioxide and water vapor, the health risks mentioned above would be considerably reduced.

[0004] The subject of the present patent application is an apparatus for extracting the carbon dioxide present in the biogas previously purified of water vapor and hydrogen sulfide H2S, with an energy expenditure much lower than the energy potential of the biogas installation concerned, the methane being delivered at a pressure sufficient to supply, for example, a gas cooker, a gas boiler or an internal combustion engine.

[0005]

[0005] Carbon dioxide CO2 is present in a proportion, expressed in percentage of the number of moles, often around 40% to 50% but which can vary from one day to the next depending on the waste used.

[0006] The elimination of carbon dioxide in current installations is most often done by filtering the CO2 using membranes designed for this purpose or by "washing" the biogas with water, the CO2 dissolving in the water. These two processes are effective but expensive and are unsuitable for small installations located far from water and electricity supply networks. There are other processes for removing CO2: washing with organic solvents, washing with amines, adsorption by pressurized modules (PSA). But these are also unsuitable for small isolated installations and are also expensive.

[0007]

[0007] The Apparatus, the subject of the present patent application, is based on the liquefaction, by lowering the temperature combined with high pressure, of most of the carbon dioxide present in the biogas.

[0008] The conditions for liquefaction and solidification of pure carbon dioxide are known, the triple point where the three phases, solid, liquid and gas, coexist being established at the temperature of Tt = -56.6°C under a pressure of Pt = 5.11 atmospheres (5.18 bars) and the critical point at Tc = 31.3°C under Pc = 72.9 atmospheres (73.9 bars). The triple point and the critical point of methane are respectively at -182.5°C and -82.6°C, i.e. at temperatures much lower than those of the corresponding points of carbon dioxide.

[0009] It emerges from experimental and theoretical studies of the CO2-CH4 mixture, made public by different laboratories, that the critical point and the liquefaction conditions of carbon dioxide mixed with methane are very significantly modified and vary according to the proportion of methane. Thus, with a proportion of methane of 20%, calculated as a percentage of the number of moles of the mixture, the critical temperature Tc and the critical pressure Pc of condensation of carbon dioxide become respectively 13°C and 85 bars. With a proportion of 40% methane, Tc is approximately -8°C, while Pc has a maximum at approximately 90 bars. Indeed, the critical pressure of carbon dioxide in the mixture for the other proportions of methane is systematically lower than that measured with 40% methane. Thus, with a proportion of 50%, the critical temperature is established at Tc=-20.3°C, the critical pressure Pc being at 88.2 bars.With a proportion of 60% methane, a fairly common value in biogas, the critical temperature Tc is close to -33.7°C and the critical pressure Pc is 82 bars. If the proportion of methane is 70%, Tc and Pc are respectively -46.4°C and 72 bars. At 75% methane, Tc becomes close to -52°C and Pc close to 68 bars; at 80% methane, Tc is close to -58.5°C and Pc is 65 bars. The evolution of the critical point as a function of the proportion of methane is given in figure-1, which also highlights the evolutions of the dew points and boiling points for different percentages of methane in the mixture.

[0010]

[0010] In figure-1- the case of the percentage of 60% of methane in the mixture is explained in the case of a mixture cooled to -50°C, the pressure being gradually increased. From about 15 bars, the carbon dioxide begins to condense (Dew Point). When the pressure reaches about 68 bars the Boiling Point is exceeded and all the carbon dioxide is therefore liquefied. At 60% methane, but at different temperatures, the dew and boiling points are distributed over two curves A and B which meet at the critical point. In Figure 1, the corresponding curves are also shown for 40%, 50% and 80% methane in the mixture.

[0011]

[0011] Note that the triple point of the mixture, for a given proportion of CH4 is is found at a lower temperature than the triple point of CO2 and at a higher pressure, the triple points of the mixtures, depending on the proportion of CH4, being approximately aligned on a line segment A marking the region where CO2 can exist in solid form. Line segment A is indicated in Figure 1.

[0012] In the various installations in operation, the biogas, previously purified of water, hydrogen sulfide, ammonia and other volatile organic compounds, most often contains from 25% to 50% of CO2, or from 50 to 75% of methane, with traces of hydrogen, it being recalled that the above percentages are established from the number of moles of each of the gases considered. Given the changes, according to the percentage of methane, of the critical point, the dew point and the boiling point of carbon dioxide, by cooling the mixture to a temperature close to -53°C and simultaneously maintaining its pressure at a value greater than 90 bars, the conditions required for the liquefaction of CO2 are therefore obtained for all methane concentrations up to approximately 77%, these conditions also making it possible to avoid the formation of dry ice.In order to obtain condensation of carbon dioxide for methane contents above 77%, it is necessary to cool to lower temperatures, below -53°C, close to -56°C, adjusting the pressure to avoid the formation of dry ice.

[0013] The Apparatus, the subject of the present patent application, makes it possible to extract the majority of the carbon dioxide from the biogas previously freed from hydrogen sulfide H2S, water vapor, ammonia and other volatile organic compounds, thus reduced to a gaseous mixture essentially composed of methane and carbon dioxide. The Apparatus in question mainly comprises a tank intended to collect the liquid carbon dioxide and an enclosure comprising two heat exchangers connected together by pipes allowing the circulation of the gaseous mixture, the enclosure filled with a fluid, most often dry air at atmospheric pressure, being cooled to a temperature close to -60°C (213.15°K),

[0014] The mixture is previously compressed to a chosen pressure of more than 90 bars by a compressor placed upstream of the Device. Then the tubing, at the outlet of the compressor, in which the mixture circulates under pressure, which is substantially at ambient temperature, then enters the cooled enclosure, then directs the mixture into the first exchanger of the Device, sized to cool it by exchange of heat between the mixture and the fluid filling the cold enclosure, at a minimum temperature chosen up to -56°C (217.15°K), the minimum temperature being chosen according to the methane content of the mixture. The chosen pressure is maintained by the compressor, which is preferably controlled by a pressure sensor placed at the outlet of the Device. At the outlet of the first exchanger, a pipe leads the cooled mixture into the second exchanger. Therefore, as specified above (Paragraphs

[0009] and

[0010] and [Fig.l]), the conditions for liquefaction (also called condensation) of carbon dioxide are met for all methane contents, up to more than 75%. The methane, remaining in the gaseous form under these temperature and pressure conditions, is de facto separated from the carbon dioxide which has become liquid, the heat released by the condensation of said gas being evacuated by the second exchanger sized for this purpose.At the outlet of the second exchanger, a pipe leads the methane, possibly with residual carbon dioxide added, to the outlet of the Device where it is recovered, at the pressure chosen by the user of the Device. The second exchanger is designed and positioned in such a way that the liquid carbon dioxide flows by gravity from any point of the exchanger to the lower part of the latter, where a suitable pipe allows the pressurized liquid gas to reach, still under the effect of gravity, the tank placed vertically below the two exchangers. The inlet orifice of the tank, located at its top, is equipped with a tap, open during the filling phases and closed during the emptying phases of said tank. Emptying is carried out by a drain orifice, located under the tank, and also equipped with a tap, which is closed during the filling phases and open during the emptying phases.The liquid carbon dioxide is then recovered for the intended use by the user of the Device. Since condensation of the carbon dioxide may, if necessary, begin to occur in the first exchanger, for example in the case where the ambient temperature is low, the first exchanger is also designed and positioned in such a way that the liquid carbon dioxide which would form there flows by gravity from any point of the exchanger to the lower part of the latter, this part having to be, like that of the second exchanger, connected by a tube to the inlet orifice of the tank described above. Note that the two exchangers, the tanks and the tubes connecting them together, must be sized to withstand the chosen pressure.

[0015] The apparatus, the subject of the patent application, intended to extract carbon dioxide from a mixture of methane and carbon dioxide, is remarkable in that placed downstream of a compressor delivering said mixture at a chosen pressure, greater than 90 bars, this pressure being maintained throughout the carbon dioxide extraction process, it consists of an enclosure cooled to a temperature temperature up to -60°C (213.15°K), filled with a fluid, most often dry air at atmospheric pressure, two exchangers, located in said enclosure and in which the compressed mixture circulates, and a reservoir intended to receive the liquid carbon dioxide, the two exchangers and the reservoir being connected together by pipes in which the compressed mixture circulates, the first exchanger, receiving the compressed mixture directly from the compressor, is intended to cool said mixture to a temperature up to -56°C (217.15°K), by heat exchange with the fluid filling the enclosure, the conditions for condensation of the carbon dioxide being then reached, the second exchanger, receiving the compressed mixture after its passage through the first exchanger, being intended to evacuate the heat released by the condensation of the carbon dioxide, also by heat exchange with said fluid,the outlet of the first exchanger being located vertically below its inlet, the two exchangers and said pipes being designed so that the liquid carbon dioxide obtained is conveyed by the effect of gravity to the tank, the inlet of which, located vertically above the drain, is equipped with a tap, open during the filling phases of said tank and closed during the emptying phases, during which the liquid carbon dioxide is collected, the drain of the tank also being equipped with a tap, closed during the filling phases and open during the emptying phases, the quasi-pure methane under pressure being collected in parallel at the outlet of the second exchanger and conveyed by a pipe to the outlet of the Device, located vertically above the inlet in the tank, and then treated according to the intended use.

[0016] The operation of the Device will be better understood with the help of figure-2-describing an example of embodiment whose energy balance is explained after the descriptions.

[0017] In this example, the mixture of carbon dioxide and methane, substantially at room temperature and atmospheric pressure, is conveyed, by means of the tubing (11) to a compressor (1) capable of raising its pressure to more than 250 bars. It leaves the compressor, due to the compression, at a higher temperature via the tubing (12) and enters an exchanger (2) intended to lower its temperature to around room temperature. The compressed mixture, conveyed via the tubing (22), then enters the embodiment of the Apparatus, the enclosure (3) being cooled to -55°C (218.15°K), a non-return valve (310) being placed on the tubing (22) in the vicinity of the point of penetration of said tubing into the Apparatus.After said valve, the gas mixture is conveyed into the first exchanger (31) intended to cool the mixture, which consists of a stainless steel tube wound in a helix, the axis (312) of the helix being vertical and the inlet (313) in said exchanger being located vertically above the outlet orifice (311) of said exchanger, all the . points of passage of said mixture inside the exchanger being located, by construction of the propeller, vertically below the previous point of passage, thus allowing the natural flow of the liquefied carbon dioxide which would come to be formed in this exchanger, towards the outlet orifice (311) of the exchanger, which is vertically substantially at the same level as the inlet orifice (321) in the second exchanger (32), to which it is connected by a tube (35) in which the gas under pressure circulates.The second exchanger is also made up of a stainless steel tube wound in a helix, the axis (322) of the helix being vertical, the outlet orifice (323) of this second exchanger being vertically above the inlet orifice (321), the role of this second exchanger being, taking into account the temperature and pressure conditions chosen, to obtain the condensation of the carbon dioxide, all the points of passage of said mixture inside the exchanger being located vertically below the next point of passage, so that as the carbon dioxide condenses, the liquid gas, by the effect of gravity, collects in the tube (35), which is also connected by a tube (36), provided with a tap (34), to the tank (33) intended to receive the liquid carbon dioxide.The inlet orifice (351) of said tank is located vertically below the inlet orifice (321) of the “second exchanger”, the tap (34) being open during the tank filling phase and closed when the latter is full. The tank is provided with a drain orifice (352), located vertically below the tank and provided with a tap (353), which is closed during the filling phase and then open, when the filling tap (34) is closed, thus allowing the liquid carbon dioxide to be collected. Note that the devices for measuring the level of liquid CO2 in the tank and for controlling the taps (34) and (353) are not shown, several technical solutions, not falling within the scope of the invention, can be implemented to do this. The quasi-pure methane under pressure is collected at the outlet orifice (323) of the second exchanger, i.e. in this exemplary embodiment, at the outlet of the Device.The pressurized methane thus collected is then processed according to the intended use. In the particular case of this embodiment, the pressurized methane of approximately 250 bars is conveyed to an exchanger (41) intended to heat it to ambient temperature. It is then directed to a storage tank (43) allowing methane cylinders (45) to be filled under pressure of approximately 200 bars, which can be used, for example, to supply vehicles running on natural gas. Note that pressure sensors (332), (333) and (334) allow the operation of the Device to be regulated according to the use made of it. Furthermore, non-return valves (331) and (421) are positioned on the circuit used by the methane as well as taps (335), (42) and (44) allowing the different parts of the installation described to be isolated, in particular during start-up. A process management microprocessor connected to the various . sensors, check valves and taps, can, if necessary, manage the operation of the Device. Not shown in Figure-2-, it is not mandatory.

[0018]

[0018] Figure-3- shows another example of embodiment of the Apparatus. As in the example of figure-2-, the compressed mixture, substantially at room temperature, conveyed by the tubing (22), enters the enclosure (3) cooled to -55°C, a non-return valve (310) being placed on the tubing (22) in the vicinity of the point of penetration in this embodiment of the Apparatus.It is conveyed into the first exchanger (31), which is made up of a stainless steel tube wound in a helix, the axis (312) of the helix being vertical and the inlet (313) in said exchanger being located vertically above the outlet orifice (311) of said exchanger, all the points of passage of said mixture inside the exchanger being located, by construction of the helix, vertically below the previous point of passage, thus allowing the natural flow of liquefied carbon dioxide, which would occur in this phase of cooling of the mixture, towards the outlet orifice (311) of the exchanger, which is vertically substantially at the same level as the inlet orifice (321) in the second exchanger (32), to which it is connected by a tube (35) in which the gaseous mixture circulates.The second exchanger is also made up of a stainless steel tube wound in a helix, the axis of the helix (312) being the same as that of the first exchanger, the outlet orifice (323) of this second exchanger being vertically below the inlet orifice (321), the role of this second exchanger being, taking into account the chosen temperature and pressure conditions, to obtain the condensation of the carbon dioxide, all the points of passage of said mixture inside the exchanger being located vertically below the previous point of passage, so that as the condensation of the carbon dioxide progresses, the liquid gas, by the effect of gravity, gathers, in the tube (329), which is also connected by a tube (36), to a tank (33) intended to receive the liquid carbon dioxide operating in the same way as the tank (33) of the embodiment example of figure-2- and provided with the same devices (351), (34), (352) and (353).The pressurized methane is collected at the outlet of the tube (329) and conveyed through the tube (330) outside the cold zone, then treated according to the intended use. In order only to show another possibility of producing the first exchanger and the second exchanger according to the invention, the taps, the non-return valves, the pressure sensors necessary for the operation of the Device are not all shown in figure-3-.

[0019] In the previous examples of embodiment of an Apparatus, that of figure-2- and that of figure-3-, the first and the second exchanger are both made of a stainless steel tube wound in a helix. This is an interesting technical solution because it makes it possible to limit as much as possible the number of junctions or welds between the different parts of each of the two exchangers and of the latter with the other parts of the Device. In the case of figure-3, the exchangers (31) and (32), whose functions are well defined, can be made in the same steel tube wound in a helix, which simplifies the manufacture of the Device and avoids junctions and welds between the two exchangers.

[0020] Figure-4- shows another example of embodiment of the Apparatus. The first exchanger (51) is made from tubes (500) placed vertically and connected in a sealed manner, in the upper part, to a substantially horizontal pipe (510) conveying the gas mixture into said first exchanger from the inlet (513) in the Apparatus. Said tubes are connected in a sealed manner in the lower part to a pipe (511) inclined at a small angle (a) of 3 or 4 degrees to the horizontal allowing the carbon dioxide which would condense in this first exchanger (51) to be brought back by the effect of gravity, towards the reservoir (53) intended to receive the liquid carbon dioxide, operating in the same way as the reservoir (33) of the example of embodiment of figure-2- and equipped with the same devices. In this embodiment of the Device, the second exchanger (52) is made in the image of the first.The gas mixture enters the second exchanger (52) using the pipe (521), inclined to the horizontal at a small angle (|3), of 3 or 4 degrees. The different vertical pipes (600) are connected in a sealed manner, in the lower part to the pipe (521) and in the upper part to the pipe (520) which brings the compressed methane to the outlet (523) of the second exchanger. In order only to show another possibility of realizing the first and second exchangers according to the invention, the taps, the non-return valves, the pressure sensors necessary for the operation of the Device are not shown in figure-4-. .

[0021]

[0021] In these examples of embodiment of the Apparatus, that of Figure-2- or variants of Figure-3- and Figure-4-, the temperature of the cold enclosure is -55°C (218.15°K) the pressure to be reached and maintained being obviously greater than 90 bars. The pressure must be fixed according to the use that will be made of the methane produced.

[0022] To clarify the energy and therefore economic aspects of the Device, a concrete case was analyzed. The choice was made for a small installation, producing on average 108 m3, at atmospheric pressure, of biogas per day, obtained by the methanization of approximately 400 kg to 500 kg of various waste, i.e. the case of a large farm or a village, the average methane content being approximately 60%, a commonly encountered value, this gives approximately 64.7 m3 of methane per day. The energy expended at each stage of the process described is below.

[0023] Since the elimination of hydrogen sulfide H2S is obtained chemically (for example adsorption on activated carbon) at the outlet of the biogas reactor, this desulfurization operation does not require any energy input.

[0024] As for the elimination of water vapor, for example by cooling to a temperature of around 2 to 3 °C, it consumes little energy. It has been calculated at approximately 10320 kJ / 24h, or an average power of 120 W for the chosen installation.

[0025] The compression stage at 250 bars requires more power, approximately 1750 W on average, an assessment confirmed by the high pressure compressors currently available on the market.

[0026] For cooling and maintaining the biogas at -53°C and removing the CO2 condensation heat, a very average-performing refrigeration unit, with a coefficient of performance of 2.0, consuming 220 W, is sufficient.

[0027] Furthermore, the consumption of the possible microprocessor for managing the assembly is approximately 50W.

[0028] In total, adding the energy required for the operation of each of the phases, we obtain an energy consumption of approximately 2.1 kW, which must be related to the instantaneous power produced by the installation, i.e. 64.7 m3 of methane per day, with a lower calorific value of 9.96 kWh / m3, providing 2.32 MJ / 24h, i.e. an instantaneous power of 26.89 kW. Thus, the energy required for the operation of the installation represents approximately 7.8% of the potential power delivered.

[0029] The process described above, studied mainly for the purification of biogas from small methanization installations, can be used to extract carbon dioxide CO2 from any mixture of carbon dioxide CO2 and methane CH4, regardless of the origin of said mixture. Furthermore, said process can also be sized for large methanization installations.

Claims

[Claim 1] Claims Apparatus for extracting carbon dioxide from a mixture of methane and carbon dioxide, notable in that placed downstream of a compressor delivering said mixture at a chosen pressure, greater than 90 bars, this pressure being maintained throughout the carbon dioxide extraction process, it consists of an enclosure cooled to a temperature of up to -60°C (213.15°K), filled with a fluid, most often dry air at atmospheric pressure, two exchangers, located in said enclosure and in which the compressed mixture circulates, and a reservoir intended to receive the liquid carbon dioxide, the two exchangers and the reservoir being connected together by pipes in which the compressed mixture circulates, the first exchanger, receiving the compressed mixture directly from the compressor, is intended to cool said mixture to a temperature of up to -56°C (217.15°K), by heat exchange with the fluid filling the enclosure,the conditions for condensation of the carbon dioxide being then reached, the second exchanger, receiving the compressed mixture after its passage through the first exchanger, being intended to evacuate the heat released by the condensation of the carbon dioxide, also by heat exchange with said fluid, the outlet orifice of the first exchanger being located vertically below its inlet orifice, the two exchangers and said pipes being designed in such a way that the liquid carbon dioxide obtained is conveyed by the effect of gravity to the tank, the inlet orifice of which, located vertically above the drain orifice, is equipped with a tap, open during the filling phases of said tank and closed during the emptying phases, during which the liquid carbon dioxide is collected, the drain orifice of the tank, also being equipped with a tap, closed during the filling phases and open during the emptying phases,the quasi-pure methane under pressure being collected in parallel at the outlet of the second exchanger and conveyed by a pipe to the outlet of the Device, located vertically above the inlet into the tank, and then treated according to the intended use.,

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