EP3978851A1 - Anlage und verfahren zur gewinnung von flüssigem methan von einem methanhaltigen gemisch - Google Patents

Anlage und verfahren zur gewinnung von flüssigem methan von einem methanhaltigen gemisch Download PDF

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EP3978851A1
EP3978851A1 EP21200140.8A EP21200140A EP3978851A1 EP 3978851 A1 EP3978851 A1 EP 3978851A1 EP 21200140 A EP21200140 A EP 21200140A EP 3978851 A1 EP3978851 A1 EP 3978851A1
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Prior art keywords
component
methane
gas flow
liquefaction
temperature
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English (en)
French (fr)
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Vittorio DELL'ACQUA
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Sites Impianti Termici Elettrici E Strumentali Srl Soc
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Sites Impianti Termici Elettrici E Strumentali Srl Soc
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
    • F25J1/0022—Hydrocarbons, e.g. natural gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
    • F25J1/0032—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration"
    • F25J1/004—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using the feed stream itself or separated fractions from it, i.e. "internal refrigeration" by flash gas recovery
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
    • F25J1/0047—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle
    • F25J1/0052—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by vaporising a liquid refrigerant stream
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/006—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the refrigerant fluid used
    • F25J1/0097—Others, e.g. F-, Cl-, HF-, HClF-, HCl-hydrocarbons etc. or mixtures thereof
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0211—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle
    • F25J1/0219—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process using a multi-component refrigerant [MCR] fluid in a closed vapor compression cycle in combination with an internal quasi-closed refrigeration loop, e.g. using a deep flash recycle loop
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0244—Operation; Control and regulation; Instrumentation
    • F25J1/0245—Different modes, i.e. 'runs', of operation; Process control
    • F25J1/0248—Stopping of the process, e.g. defrosting or deriming, maintenance; Back-up mode or systems
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J5/00—Arrangements of cold exchangers or cold accumulators in separation or liquefaction plants
    • F25J5/002—Arrangements of cold exchangers or cold accumulators in separation or liquefaction plants for continuously recuperating cold, i.e. in a so-called recuperative heat exchanger
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2205/00—Processes or apparatus using other separation and/or other processing means
    • F25J2205/30—Processes or apparatus using other separation and/or other processing means using a washing, e.g. "scrubbing" or bubble column for purification purposes
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2210/00—Processes characterised by the type or other details of the feed stream
    • F25J2210/66—Landfill or fermentation off-gas, e.g. "Bio-gas"
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2220/00—Processes or apparatus involving steps for the removal of impurities
    • F25J2220/60—Separating impurities from natural gas, e.g. mercury, cyclic hydrocarbons
    • F25J2220/66—Separating acid gases, e.g. CO2, SO2, H2S or RSH
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2230/00—Processes or apparatus involving steps for increasing the pressure of gaseous process streams
    • F25J2230/30—Compression of the feed stream
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2260/00—Coupling of processes or apparatus to other units; Integrated schemes
    • F25J2260/80—Integration in an installation using carbon dioxide, e.g. for EOR, sequestration, refrigeration etc.

Definitions

  • the present invention relates to the field of gas separation, and more particularly, it relates to a plant and a method for obtaining liquid methane from a methane-containing mixture.
  • the present invention finds application mainly, though not exclusively, in obtaining liquid methane downstream of biogas production facilities using fermentation of organic waste from vegetal or animal residues, for example originating from waste disposal.
  • Residues useful for biogas production may have several origins: agroindustrial waste, food industry waste (waste flours or expired products), livestock industry waste (effluents from animals or carcasses); crops can also be used specially devised for being harvested and chopped in order to produce a "biomass" from which biogas is obtained in appropriate facilities.
  • Biomasses commonly used for this purpose are, for example, corn, sugar sorghum, wheat, giant cane, beets.
  • the degradation process preferably takes place in the absence of oxygen and at a controlled temperature thereby producing carbon dioxide, hydrogen and methane.
  • biogas and digestate i.e. natural liquid fertilizer.
  • Biogas is then converted into electric power by means of a generator, whereas the digestate is used as fertilizer for land plots for farming.
  • biogas produced with known facilities and technologies is usually employed for producing electric power. Instead, the obtainment of liquid methane, or biomethane from biogas produced by said facilities has to face a series of problems.
  • a first problem lies in that the facilities for producing biogas have to be equipped with appropriate systems for separating methane from the other gases forming the gas mixture produced by the facility.
  • a further drawback of known systems based on the use of cryogenic fluids results from the fact that they are suitable especially for use in treating large flows of biogas and are therefore unsuitable to operate with reduced flows of incoming biogas for obtaining liquid methane.
  • the known systems are not suitable to treat biogas flows with flow rates below 1000 m 3 /h.
  • An object of the present invention is therefore to overcome the drawbacks of prior art, by providing a plant and corresponding method for obtaining liquid methane from a methane-containing mixture, such as, for example, a biogas coming from degradation of organic residues.
  • a further object of the invention is to provide a plant and corresponding method for obtaining methane that are capable of operating with low energy consumption and reduced environmental impact.
  • Another object of the invention is to provide a plant and corresponding method for obtaining liquid methane that can be easily stored and transported and is ready to be used as a fuel.
  • a still further object of the invention is to provide a plant and corresponding method for obtaining liquid methane that can be made industrially in a cost-effective manner.
  • the invention relates to a plant and corresponding method for obtaining liquid methane from a methane-containing mixture.
  • the plant for obtaining liquid methane according to the invention comprises a set of stations mutually interlinked in series, wherein a methane-containing mixture is subjected to a set of treatments aimed at obtaining liquid methane, which can then be used as a fuel.
  • the plant for obtaining methane according to the present invention is arranged to be associated with a source of gas mixture comprising a set of gas components, among which at least methane.
  • the source of gas mixture comprises a plant for biogas production in which a fermentation system, or digester, for producing biogas by degradation of organic residues is housed.
  • the biogas coming from the fermentation system, or digester usually comprises, besides methane, also carbon dioxide, sulphur dioxide and other components, among which ammonia and trace oxygen, hydrogen and nitrogen in traces and also various types of dusts.
  • the plant according to the invention is preferably a small-sized plant, or micro-plant, capable of operating with extremely reduced incoming biogas flows in order to obtain liquid methane.
  • the plant according to the invention is capable of liquefying methane flows smaller than 300 m 3 /h, more preferably with methane flows between 125 m 3 /h and 250 m 3 /h.
  • a separation zone comprising a first pre-treatment station for pre-treating the mixture containing methane and coming from the source of gas mixture, said pre-treatment station being arranged to purify said gas flow, and a second treatment station arranged to separate the components of the gas flow.
  • the plant for obtaining liquid methane according to the invention further comprises a liquefaction station located downstream of said treatment station and configured to carry out liquefaction of the least one gas flow component obtained as a result of separation, wherein the liquefaction station is configured to operate at low temperature, and wherein the component to be liquefied comprises a methane-containing gas flow for obtaining liquid methane.
  • the second treatment station is configured to separate the components CO 2 and methane CH 4 contained in said mixture exiting the first pre-treatment station.
  • the separation zone is adapted to separate the various components of the mixture, a gas flow containing a high concentration of methane is obtained from said separation.
  • the gas flow containing methane is then subjected to a liquefaction process for obtaining liquid methane, which can be easily stored and transported and is intended for subsequent use as a fuel.
  • the second treatment station comprises a plurality of separation units comprising spraying towers associated with a circuit arranged to deliver an incoming liquid to said towers and to appropriate extraction membranes allowing separation of carbon dioxide, in order to obtain the gas flow containing a high concentration of methane.
  • said incoming liquid supplied to the spraying towers is water. Due to the fact that the plant comprises a plurality of spraying towers associated with appropriate extraction membranes, separation of carbon dioxide takes place by means of a first step of absorption with water and a subsequent step of extraction of carbon dioxide by means of the extraction membranes.
  • the spraying towers and appropriate extraction membranes are arranged in series, in a chain-like manner.
  • the configuration according to which the spraying towers and the extraction membranes are arranged in series, in a chain-like manner it is possible to obtain separation of carbon dioxide with a gradually decreasing percentages, whereby the last spraying tower will effect absorption of a carbon dioxide percentage much smaller than the one absorbed and extracted by means of the first spraying tower of the chain.
  • the load losses upon passage of the gas flow from a spraying tower to a subsequent one are minimal, and thus a remarkable energy saving is obtained.
  • a volumetric compressor and a corresponding heat exchanger which allow the methane-containing mixture to reach optimal conditions, i.e. certain pressure and temperature levels, for starting the separation process carried out in said separation zone arranged downstream.
  • the gas flow obtained from separation, or residual gas is a gas having a high concentration of methane, i.e. rich in methane, which gas, according to the invention, is then subjected to a liquefaction process.
  • a liquefaction process In order to obtain liquefaction of the methane-rich gas flow it is necessary to ensure favorable conditions for this process.
  • Such favorable conditions mainly provide that the methane-rich gas flow reaches a very low temperature, preferably between -125°C and -150°C, more preferably a temperature of -130°C.
  • the liquefaction station of the plant according to the invention is configured to carry out a series of steps of cooling the methane-rich gas flow originating in the separation zone.
  • the liquefaction station is configured to carry out cooling of the methane-rich gas flow for subsequent steps, until the desired temperature in the above-mentioned range is reached, a reduction in the energy consumption of the whole plant is achieved.
  • the invention provides for indirect liquefaction of methane by using a chain of refrigeration units in order to allow methane to change from the gaseous state to the liquid state.
  • the change of methane from the gaseous state to the liquid state takes place by exploiting the frigories of the state change from the gaseous state to the liquid state of the refrigerant fluid used in the last cooling step of the chain of refrigeration units employed.
  • the liquefaction station comprises corresponding compression means allowing the methane-rich gas flow to reach certain pressure levels favorable to the liquefaction process.
  • said pressure levels are preferably about 30 bar.
  • the liquefaction station comprises a set of heat exchangers arranged to facilitate reaching of the liquefaction temperature during carrying out of the series of cooling steps.
  • the heat exchangers are suitably devised for reaching certain values of heat exchange coefficient in the various steps of cooling the methane-rich gas flow.
  • said heat exchangers comprise cylindrical elements, or tubes, having a hollow structure and a plurality of radial fins arranged concentrically along respective generatrices both inside and outside said hollow structure.
  • the fins extend radially from the center of said cylindrical elements and, in addition, they may be at least partially interrupted along said generatrices, i.e. they have interruptions in the axial direction, in order to create more turbulences during the heat exchange step, i.e. said fins may be interrupted along the corresponding generatrices.
  • the cylindrical elements are arranged coaxially in series and preferably rotated relative to one another, so as to cause a discontinuity in the radial fins in order to generate a corresponding greater turbulence effect over the methane-rich gas flow.
  • the liquefaction station further comprises pump means arranged to distribute the liquefied gas flow, i.e. the liquid methane, and suitable storage tanks for subsequent transport and use thereof as a fuel.
  • pump means arranged to distribute the liquefied gas flow, i.e. the liquid methane, and suitable storage tanks for subsequent transport and use thereof as a fuel.
  • the liquefaction station comprises a safety system, equipped in turn with a plurality of expansion vessels, or reservoirs, to be associated with each cooling step.
  • the expansion vessels or reservoirs preferably contain hydraulic oil and are provided each with a safety valve.
  • the expansion vessels are also associated with a main tank containing hydraulic oil and are configured to convert a pressure change above a threshold value into an increase in volume inside said expansion vessel, this causing exit of the oil towards the main tank.
  • the safety system further comprises respective pump means configured to bring the hydraulic oil back from the main tank towards the corresponding expansion vessels in order to allow the gas flow to reach its working pressure in a corresponding cooling step.
  • the method for obtaining liquid methane according to the invention comprises the steps of:
  • the separation step provides for a first pre-treatment step in which the gas mixture is subjected to a purification step in order to eliminate traces of polluting components present in the gas mixture.
  • the liquefaction step provides for subjecting the gas flow rich in methane to a compression step adapted to bring the gas flow to a certain pressure of about 30 bar and cool down said component by means of several subsequent cooling steps, until a temperature between -125°C e -150°C, more preferably closer to -130°C, is reached.
  • the plant and corresponding method for obtaining liquid methane according to the invention preferably find application in the field of the production of biogases from degradation of organic residues of animal or vegetal origin. More preferably, the plant according to the invention is configured to produce certain amounts of liquid methane, by operating with even very low biogas flows, typically below 1000 m 3 /h, and flows of methane to be liquefied lower than 300 m 3 /h, preferably ranging from 125 m 3 /h to 250 m 3 /h.
  • the plant for obtaining liquid methane according to the invention comprises a set of mutually interlinked stations in which a methane-containing mixture is subjected to a set of treatments in order to obtain liquid methane, which can then be used as a fuel.
  • the plant for obtaining methane of the present invention is associated with a source of gaseous mixture of gases including, among others, methane.
  • the source of gas mixture comprises a fermentation system, or digester, (not shown) and the gas mixture comprises a biogas flow F1 originating from the degradation of organic residues that has taken place in said digester.
  • the biogas flow F1 coming from the fermentation system comprises methane, carbon dioxide, sulphur dioxide, ammonia and other components such as, for example, oxygen, hydrogen and nitrogen.
  • the plant comprises a separation zone 100, schematically shown in Fig. 1 , in which separation of the various carbon dioxide component from the biogas flow F1 coming from the source of gas mixture takes place.
  • the separation zone 100 comprises, in turn, a first pre-treatment station 101 for pre-treating the biogas flow F1 coming from the fermentation system, said pre-treatment station being arranged to purify the gas flow F1, and a second treatment station 120 arranged to separate the components of the purified gas flow F1.
  • the first pre-treatment station 101 comprises at least one active carbon filter 103, in the illustrated embodiment two filters 103 arranged in parallel, equipped with active carbon filtration means and arranged to purify the biogas flow F1 by removing the sulphur dioxide component and, at least partially, other polluting components such as, for example, ammonia.
  • Ammonia in particular is removed only partially by means of the filter 103, as said filter is specific for sulphur dioxide, whereas the remaining part is dissolved in the liquid of the treatment station 120, typically water, as will become more apparent from the following description.
  • the first pre-treatment station 101 further comprises compression means 105 and heat exchangers 107, 109, which allow the biogas flow F1 to reach certain pressure and temperature condition, which will promote the separation process carried out by the second treatment station 120.
  • the heat exchangers 107, 109 may be of the air-gas type.
  • the heat exchanger 109 is preferably suitably devised, as will be described in the present description here below.
  • the heat exchanger 107 may be of the conventional type, or it may also be devised similar to the heat exchanger 109.
  • the second treatment station 120 is configured to separate the carbon dioxide and methane components from the biogas flow F1 by means of a spraying or wet process.
  • the second treatment station 120 comprises a plurality of separation modules 121 comprising spraying towers 123 associated with a circuit for delivering an incoming liquid L1 to said towers 123.
  • the separation modules 121 further comprise suitable extraction membranes 125 allowing separation of carbon dioxide F2 in order to obtain a methane-rich gas flow F3.
  • the liquid L1 supplied by said circuit consists mainly of water.
  • the membranes 125 are selective for CO 2 and the ammonia present in the flow F1 is dissolved in the water L1 and, as it accumulates, the pH value of said liquid L1 rises.
  • a pH meter 124 connected to each tower, or reactor, 124 opens a water purge circuit for purging water from each tower 123 when the pH value exceeds a predetermined threshold, and directs the recirculated liquid to the osmotic membrane separating ammonia inside the tower 123.
  • the biogas flow F1 coming from the fermentation system or digester follows a specific path aimed at separating the biogas flow F1 into its various components such as, for example, the carbon dioxide component F2 and the methane component F3, as will become more apparent from the following description.
  • the biogas flow F1 entering the separation zone 100 has a temperature between 36°C and 46°C, and the pressure value is substantially equal to the value of the atmospheric pressure.
  • the biogas flow F1 in the illustrated embodiment, is subjected to a preliminary filtration step to eliminate dusts, which step is carried out by means of appropriate mechanical separators 102, and thereafter it is subjected to a compression step carried out by means of a volumetric compressor 105 in order to obtain a pressure value of about 5 bar for said biogas flow F1.
  • a compression step carried out by means of a volumetric compressor 105 in order to obtain a pressure value of about 5 bar for said biogas flow F1.
  • the temperature of the biogas flow F1 rises until it reaches a value of about 170°C.
  • the biogas flow F1 is subjected to a cooling step carried out by means of the air-gas heat exchanger 107.
  • the temperature of the biogas flow F1 is brought again to a temperature around the value of the ambient temperature, which in this particular case is about 35°C.
  • the biogas flow F1 After the biogas flow F1 has reached the temperature and pressure levels favorable to the separation process, it is subjected to a purification step through the filter 103 provided with active carbon filtration means and arranged downstream of the air-gas heat exchanger 107.
  • the step of purification of the biogas flow F1 it is necessary to cool down the biogas flow F1 until a temperature closer to 0°C, more preferably of about 1°C, is reached. Cooling of the purified biogas flow F1 is carried out by means of the heat exchanger 109 in which a refrigerating fluid such as, for example, R448A circulates, cooled by a chiller 112 associated with a further air-gas heat exchanger 110.
  • a refrigerating fluid such as, for example, R448A circulates
  • the solubility of carbon dioxide F2 in the liquid L1 is ensured. It is thus possible to subject the biogas flow F1 to the separation process carried out by the second treatment station 120 by means of the separation modules 121.
  • the purified gas flow F1 exiting the first pre-treatment station 101 flows into the second treatment station 120, which is adapted to separate the purified biogas flow F1 essentially into two main components: the carbon dioxide component F2 and the methane component F3.
  • the second treatment station 120 comprises three separation modules 121 arranged in series.
  • Each of the separation modules 121 comprises in turn a spraying tower 123 associated with a corresponding extraction membrane 125 and further associated with a shared circuit adapted to deliver an incoming liquid L1 to said spraying towers 123.
  • the liquid L1 entering said towers essentially consists of water.
  • the purified gas flow F1 exiting the first pre-treatment station 101 flows through the separation modules 121, in succession one by one, gradually effecting extraction of the carbon dioxide component F2 by means of the corresponding membranes 125 associated with the spraying towers 123.
  • the purified biogas flow F1 enters the first separation module 121, i.e. the first spraying tower 123, where separation of a part of the carbon dioxide component F2 from said biogas flow F1 takes place by means of a water L1 spraying process. Said CO 2 component is further extracted by means of the corresponding extraction membrane 125 arranged downstream of said spraying tower 123.
  • the gas flow F3 resulting from extraction of a part of the carbon dioxide component F2 is a gas flow comprising a high concentration of methane.
  • the gas flow exiting the first separation module 121 enters the second separation module 121 where it is subjected to the same separation process, i.e., inside the respective spraying tower 123, a further part of the carbon dioxide component F2 is separated by means of spraying or wet separation process, which further part is further extracted by means of the corresponding membrane 125 arranged downstream of the spraying tower 123.
  • the gas flow exiting the second separation module 121 consists of a gas flow with a methane concentration even higher than that of the gas flow exiting the first separation module 121. Subsequently, the gas flow passes through the third separation module 121, where, by means of a spraying process with water L1, there is carried out a further separation of the carbon dioxide component F2, which is extracted by means of the corresponding extraction membrane 125 associated with the tower 123. Therefore, at the outlet of the third separation module 121, two flows exiting the spraying tower 123 are obtained, namely a flow of the carbon dioxide component F2 on one hand, and a gas flow F3 rich in methane, i.e. with very high methane concentrations, on the other hand.
  • the temperature of the water L1 used by the separation modules 121 is preferably 1°C and once the carbon dioxide CO 2 component has been separated by means of the membranes 125, the water L1 is advantageously re-circulated, this being of advantage for the management of the plant.
  • the methane component F3 also comprises a part of moisture that will be discarded by means of a dehydration filter, as described below.
  • the plant for obtaining liquid methane according to the invention comprises a liquefaction station 200, 300 configured to carry out liquefaction of at least one component F2, F3 of the biogas flow F1, which component has been obtained from the separation of the various components.
  • the liquefaction station 200, 300 is advantageously configured to operate at low temperature.
  • the component that is subjected to the liquefaction step is the methane component F3.
  • FIG. 2A there is schematically illustrated the process of liquefaction of the carbon dioxide component F2.
  • the flow of carbon dioxide F2 exiting the separation zone 100 is at a temperature of about 0°C and a pressure of 5 bar.
  • the carbon dioxide component F2 is subjected to a first compression step carried out by means of an appropriate compressor 201 according to the invention.
  • a volumetric compressor 201 is provided, the operation of which will be described in detail below.
  • the compressor 201 allows bringing the carbon dioxide component F2 to a pressure of about 20 bar.
  • the temperature of the carbon dioxide component F2 rises until it reaches a temperature value of essentially 40°C.
  • the carbon dioxide component F2 is subsequently cooled down by means of an appropriate heat exchanger 203 arranged downstream of the compressor 201 associated with a gas-air heat exchanger 205 to obtain a temperature of about -20°C. At the outlet of said heat exchanger 203 there is obtained liquid carbon dioxide F2'.
  • liquefaction station 300 used to carry out the process of liquefaction of the methane component F3 exiting the separation zone 100.
  • liquefaction temperature In order to obtain liquefaction of the methane component F3 it is necessary to ensure favorable conditions for said process. Such favorable conditions provide for the methane component F3 to reach a very low temperature of about -130°C, called liquefaction temperature.
  • the liquefaction station 300 of the plant according to the invention is configured to effect a series of cooling steps for cooling the methane component F3.
  • the plant according to the invention will be capable of producing liquid methane with an energy consumption between 0.3 and 0.35 kW/kg of liquid methane, remarkably lower than the consumptions of current technologies, which are about 0.5 kW/kg of liquid methane.
  • the invention is thus particularly advantageous in the making of small-sized plants, or micro-plants, i.e. plants capable of operating with extremely small incoming biogas flows, for example smaller than 1000 m 3 /h, and flows of methane to be liquefied smaller than 300 m 3 /h.
  • the liquefaction station 300 comprises a dehydration filter 302 arranged at the inlet of the station 300, in order to separate especially moisture from the flow F3. Moisture is discarded from the dehydration filter 302, which is a molecular sieve with activated alumina, with hot air regeneration. Downstream of the filter 302, in the flow F3 there will remain traces of oxygen, hydrogen and nitrogen, which are gases that are incondensable at the temperature of liquefaction of methane and will therefore be found in the dome of an expansion vessel, or reservoir, 317 located upstream of the outlet of the station 300. Said residual gases are preferably sent to a small stripping column for recovering gaseous methane, present in small quantities in the mixture, whereas the remaining gases are evacuated through a torch.
  • compression means 301 Downstream of the filter 302 there are provided compression means 301 allowing the methane component F3 entering the liquefaction station 300 to reach certain pressure levels favorable for the liquefaction process.
  • the liquefaction station 300 further comprises, according to the invention, heat exchangers 305, 307, 313, 315 arranged in series and adapted to carry out the series of cooling steps according to the invention.
  • the process of liquefaction of the methane component F3 provides for carrying out substantially five subsequent cooling steps. Still referring to Fig. 2B , the process of liquefaction of the methane component F3 will now be explained.
  • methane takes no active part in the liquefaction process, but it is liquefied indirectly by using the frigories of a set of refrigeration units employed for obtaining said series of cooling steps.
  • the methane component F3 When entering the liquefaction station 300, the methane component F3 has a temperature of about 1°C and a pressure of 5 bar, which are substantially the temperature and pressure values reached by the methane component F3 exiting the separation zone 100.
  • the methane component F3 entering the liquefaction station 300 will be subjected to a compression step by means of an appropriate compressor 301 in order to reach a pressure value of about 30 bar.
  • the compressor 301 is a volumetric compressor.
  • the methane component F3 reaches a temperature of about 130°C, which is then brought to ambient temperature, here substantially of about 35°C, by means of a gas-air heat exchanger 303 located downstream of the compressor 301.
  • the subsequent cooling steps are carried out, which provide for reaching gradually lower temperatures, until the methane component F3 reaches the liquefaction temperature of about -130°C.
  • the first cooling step provides for bringing the methane component F3 from ambient temperature, i.e. from 35°C, to a temperature lower than 0°C, namely to a temperature of about -30°C.
  • the liquefaction station 300 comprises a first heat exchanger 305 using a refrigerating fluid, for example of the type R448A, said first heat exchanger being arranged downstream of the heat exchanger 303 and associated with a corresponding air-gas heat exchanger 306.
  • the methane component F3 will be subjected to a second cooling step providing for bringing said methane component F3 from the temperature of about -30°C to a temperature of about -60°C.
  • the liquefaction station 300 comprises a second heat exchanger 307, arranged downstream of the heat exchanger 305, which, by using a refrigerating component, such as, for example, ethylene at the gaseous state, at a pressure of about 0 relative bar and a temperature level of about -30°C, reduces the temperature level of the methane component F3.
  • a refrigerating component such as, for example, ethylene at the gaseous state
  • the second cooling step further comprises a step of liquefaction of the refrigerating ethylene component used, which will be recirculated in order to exploit the sensible heat of said ethylene component in a subsequent cooling step for the methane component F3, i.e. in the fourth cooling step of the series of cooling steps.
  • the liquefaction station 300 comprises a third heat exchanger 309 associated with an air-gas heat exchanger 311, which heat exchangers promote the cooling process in the second and fourth cooling steps by using the sensible heat of ethylene.
  • a compressor 308 arranged to effect compression of ethylene from 0 bar to 30 bar.
  • the compressor 308 is preferably a compressor of the scroll type.
  • the temperature of ethylene is brought back to ambient temperature by means of the gas-air heat exchanger 311 and thereafter, by means of the heat exchanger 309 containing refrigerating fluid R448A, the ethylene will be brought to the temperature of -30°C, thus liquefying it, and subsequently it will be recirculated for being used in the fourth cooling step illustrated below.
  • ethylene changes its state from gaseous to liquid at a constant temperature by condensation and the refrigerating fluid R448A also changes its state at a constant temperature from liquid to gaseous by evaporation.
  • the third cooling step following the second cooling step, provides that the methane component F3 reaches an even lower temperature level, changing from -60°C to -80°C.
  • the liquefaction station 300 comprises a corresponding heat exchanger 313 arranged downstream of the heat exchanger 307 and further promoting recovery of the sensible heat of the methane component F3 evaporated in the fifth cooling step before liquefaction and is recirculated inside said heat exchanger 313. Therefore, the temperature of the methane component F3 in the third cooling step is lowered by recovery of the sensible heat of said component, by recirculating said component from the fifth cooling step to the third cooling step.
  • the fourth cooling step provides for lowering the temperature of the methane component F3 from -80°C to -100°C.
  • the liquefaction station 300 comprises a fourth heat exchanger 315, arranged downstream of the heat exchanger 313 of the third cooling step, said fourth heat exchanger, by recirculating the ethylene component from the second cooling step, lowers the temperature of the methane component F3 to a value of about -100°C.
  • the methane component F3 changes from the gaseous state to the liquid state at a constant temperature by condensation, whereas the ethylene component changes from the liquid state to the gaseous state at constant temperature by evaporation.
  • the fifth cooling step provides for effecting further cooling in order to bring the methane component F3 from the temperature value of -100°C to the liquefaction temperature of about -130°C.
  • the liquefaction station 300 comprises an expansion vessel 317 which is arranged downstream of the heat exchanger 315, and in which expansion of the methane component F3 from a pressure value of 30 bar to a pressure value of 8 bar is effected.
  • the pressure value of 8 bar is usually the pressure value at which the liquefied methane is stored in storage tanks or reservoirs for subsequent transport
  • this fifth cooling step about 30% of the methane component F3 is evaporated and recirculated, in order to recover the sensible heat of said methane component F3, necessary to effect cooling in the third cooling step.
  • operation of the heat exchangers 309 and 315 effecting liquefaction of the methane component and ethylene is based on latent heat rather than sensible heat.
  • the liquefaction station 300 further comprises pump means 319 associated with the expansion vessel 317 and arranged to distribute the liquefied gas flow F4 exiting the expansion vessel 317, i.e. the liquid methane, to appropriate storage tanks for subsequent transport and use of the same as a fuel.
  • pump means 319 associated with the expansion vessel 317 and arranged to distribute the liquefied gas flow F4 exiting the expansion vessel 317, i.e. the liquid methane, to appropriate storage tanks for subsequent transport and use of the same as a fuel.
  • the compressor 201, 301 used in the liquefaction station 300.
  • the compressor 201, 301 is preferably a volumetric compressor provided with a hydraulic feeding system arranged to receive an incoming gas flow with flow rates between 100 and 500 m 3 /h.
  • the compressor 201, 301 comprises a first compression cylinder 211 and a second compression cylinder 311 which are arranged opposite to each other and have each a corresponding compression piston 221, 223, and in which compression of the biogas flow entering the liquefaction station 200, 300 is effected.
  • the first compression cylinder 211 and the second compression cylinder 213 are coaxial to each other and are arranged opposite to each other.
  • the compressor 201, 301 comprises a third cylinder 215 also provided with a corresponding piston 225, said third cylinder being arranged centrally between the first compression cylinder 211 and the second compression cylinder 213 and coaxial to said first 211 and second 213 compression cylinders.
  • the hydraulic feeding system of the compressor 201, 301 comprises a reserve tank 217 containing hydraulic oil and arranged so as to define a substantially "T"-shaped configuration together with the coaxial cylinders, i.e. the first compression cylinder 211, the second compression cylinder 213 and the third central cylinder 215.
  • the hydraulic feeding system of the compressor 201, 301 further comprises adjusting means 219 and pump means 220 for the hydraulic oil flow, said pump means being arranged between the reserve tank 217 and the coaxial cylinders 211, 213, 215.
  • the adjusting means 219 are a proportional valve and the pump means 220 are a variable flow rate hydraulic pump.
  • the gas flow Fi entering the compressor 201, 301 has a pressure of substantially 5 bar and can be formed either by the methane component F3 or by the carbon dioxide component F2, depending on the component to be subjected to the liquefaction process in the respective liquefaction station 200, 300.
  • the gas flow Fi entering the compressor 201, 301 enters the first compression cylinder 211 and actuates, by means of its pressure, the piston 221 of said first cylinder 211.
  • the piston 225 of the central cylinder 215 is actuated by the oil flow coming from the reserve tank 217.
  • the thrust of the piston 225 of the central cylinder 215 is substantially reduced because the counter-pressure of the gas flow Fi is minimal.
  • the flow rate of the oil coming from the reserve tank 217 will be high, in accordance with the flow rate capacity of the pump means 220.
  • the flow rate adjusting means perform a function of adjusting the flow rate of the pump means 220 so as to maintain the energy consumption thereof constant.
  • the exiting flow gas Fu subjected to compression in the first compression cylinder 213 or in the second compression cylinder 215 and intended to be subjected to the liquefaction process will have a pressure of about 30 bar.
  • the compressor 201, 301 further comprises detection means such as a sensor arranged to detect the end of the stroke of the piston 225 of the central cylinder 215 and generate an electric control signal for reversing the oil flow towards the reserve tank 217.
  • the piston 225 comprises a small magnet which cooperates with an inductive sensor, arranged on the wall of the cylinder 215 housing said piston 225, to generate a signal indicative of the end of the stroke of the piston 225 in order to allow reversing the direction of the linear movement of said piston 225.
  • FIG. 5 and 6 there is illustrated a heat exchanger 305, 307, 313, 315 according to the invention.
  • the heat exchangers 305, 307, 313, 315 used especially in the liquefaction station 300 are configured to reach high values of exchange coefficient in the various cooling steps.
  • the heat exchanger comprises a tubular cylindrical element 3 having a hollow structure and a plurality of radial fins arranged concentrically along respective generatrices both inside and outside said hollow structure.
  • the fins 5 extend radially from the center of the cylindrical element 3 and at least some of them 5 can be interrupted in order to create more turbulences during the heat exchange step, i.e. said fins may be interrupted along the corresponding generatrices.
  • the diameter of the inner part of the cylindrical element 3 is between 60 cm and 70 cm, more preferably of 65 cm, whereas the outer diameter of the heat exchanger is between 80 cm and 95 cm, more preferably of 90 cm.
  • said cylindrical elements 3 can be associated in series along the longitudinal axis in order to attain the desired length for the heat exchanger 305, 307, 313, 315.
  • the ends 7,9 of a heat exchanger 305, 307, 313, 315 are suitably configured to allow interlinking with other similar heat exchangers in order to attain the desired length.
  • the cylindrical element 3 has a length of about 200 cm.
  • the cylindrical elements 3 are arranged coaxially in series and are preferably rotated with respect to one another, so as to cause discontinuity in the radial fins 5 in order to generate a corresponding greater turbulence effect on the methane-rich gas flow touching said radial fins. More preferably, a first cylindrical element 3 is rotated relative to a following second cylindrical element 3 so that a fin 5 of the first cylindrical element 3 is substantially sandwiched between two fins of an adjacent cylindrical element 3.
  • the liquefaction station 300 further comprises a safety system 400 schematically shown in Fig. 7 .
  • the safety system 400 is preferably applied to all the refrigerating circuits of the plant in which a liquid phase is present.
  • the safety system is necessary to remedy the detrimental effects due to a sudden pressure increase caused by boiling of the liquid phase, in case of sudden stoppage of the plant.
  • the safety system 400 is applied to all refrigerating circuits of the plant, because a liquid phase is present in them all.
  • the safety system 400 mainly comprises a plurality of expansion vessels, or tanks, 401 preferably containing hydraulic oil balancing the cooling pressure during the various cooling steps.
  • the safety system further comprises a main tank 403 containing in turn hydraulic oil.
  • the expansion vessels 401 are further provided with a safety valve 402 arranged downstream of the expansion vessels.
  • the valve 402 connects said expansion vessels 401 with the main tank 403.
  • the expansion vessels 401 are further configured to turn a pressure change, above a threshold level, into a corresponding increase in volume within the expansion vessel 401, this resulting in the oil being transferred from the expansion vessel 401 towards the main tank 403, through the valve 402 which is opened when said threshold is exceeded.
  • the safety system 400 further comprises corresponding pump means 405, arranged between said expansion vessels 401 and the main tank 403 and configured to bring back the hydraulic oil from the main tank 403 towards the corresponding expansion vessels 401, thus allowing the gas flow to reach the correct operating pressure.
  • the valve 402 is calibrated to intervene at a pressure slightly higher than that of the refrigerating circuit with which it is associated. For example, in the case of the refrigerating circuit in which ethylene circulates, described in Fig. 2B , the valve 402 is calibrated to intervene when the pressure in the circuit exceeds 32 bar, the maximum pressure of the gas in the circuit being 30 bar.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Mechanical Engineering (AREA)
  • Thermal Sciences (AREA)
  • General Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
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EP21200140.8A 2020-10-01 2021-09-30 Anlage und verfahren zur gewinnung von flüssigem methan von einem methanhaltigen gemisch Withdrawn EP3978851A1 (de)

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Citations (2)

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Publication number Priority date Publication date Assignee Title
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US5642630A (en) * 1996-01-16 1997-07-01 Abdelmalek; Fawzy T. Process for solids waste landfill gas treatment and separation of methane and carbon dioxide
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