Field of the invention

The present disclosure relates to a method and system for producing liquid methane, for example for processing a process fluid created from biowaste material.

Background

The environmental impact to human health of fossil fuel sources of energy is an acknowledged and growing problem. Fossil fuel sources result in the release of greenhouse gases to the environment and are in finite supply. However, there still remains a need to transport goods and people in a cost effective and efficient manner. Renewable energy sources are quite rightly receiving a lot of attention as alternative energy sources. However, when it comes to the issues of propulsion and transportation, there still remains a need for dense & efficient energy storage that can also be fuelled onto a vehicle quickly and conveniently. Examples of such alternative sources of energy may comprise liquid hydrogen and electrical/battery power. However, both require significant infrastructure development - in the case of electrical/battery power there requires a significant investment in a charging infrastructure, and in the case of liquid hydrogen there is a high cost and time associated with hydrogen production.

However, it would be greatly beneficial if alternative energy sources are not only easy to refuel and implement, but also contribute to carbon capture and thereby reduce the level of greenhouse gasses in the environment.

Summary of the invention

Aspects of the invention are as set out in the independent claims and optional features are set out in the dependent claims. Aspects of the invention may be provided in conjunction with each other and features of one aspect may be applied to other aspects.

In summary, aspects of the disclosure relate to a method and system for producing liquid methane, for example from biowaste material. Intercepting environmental methane from waste streams may advantageously supply fuel with an environmental benefit. Because methane has a much greater global warming potential (GWP) than carbon dioxide (approximately 86 times greater over the course of 20 years), intercepting methane in this manner could reduce the level and global warming potential of greenhouse gasses.

Aspects of the disclosure may use, for example, grass as a biowaste material. Of course, other suitable biowaste materials may be used such as slurry and food waste. Grass is cheap and sustainable method of fixing carbon dioxide in the soil and collecting solar energy. By extracting useful energy from biowaste material in this way, this can provide a chemical vector for energy storage that has been converted in a “clean” way that has a lower environmental impact than, for example, fossil fuels and/or e le ct rif i cati o n/b atte rif i cati o n .

When harvesting biogases produced from biowaste material, the resultant gas mixture may comprise impurities and a high degree of carbon dioxide that renders it less useful for a source of combustion. Embodiments of the disclosure may advantageously remove the carbon dioxide and/or other impurities and convert the biogas into liquid methane, which can be cheaply and conveniently stored and transported and also represents a dense source of energy storage. By using cryogenic techniques described herein, to separate, purify and liquefy component species it is possible to obtain > 99.5% Chk (molar).

While biowaste has been described as a possible source of the biogases, it will be understood that the systems and methods described herein may be used for processing other gases. For example, the systems and methods disclosed herein may be used for processing:

- Natural gas such as: o Flare gas from oil & gas processing; o Leaking gas from abandoned natural gas wells; o Natural gas for LNG production; o Associated gas from oil production; o Gas released from fracking; o LNG fuelling stations;

- Bio-gas such as: o Landfill gas; o anaerobic digestion; o Process ventilation; o Brewing I food production; o Pyrolysis; o Sewer gas, drain gas, water treatment processes.

- Gasses produced from farming/agriculture proceses, such as: o Haylage I sileage I feed production, o Slurry pits o Enteric fermentation

- Gasses produced from other processes, such as: o Sabatier process; o Industrial processing; o Power generation exhaust; o Palm oil mill effluent (POME); and o Domestic cesspits.

Furthermore, it is envisioned that the methods and systems described herein may be implemented on a small or micro-scale - that is, close to the consumer. This means there are reduced transportation/infrastructure costs associated with it.

A basic operating principle of embodiments of the disclosure is based on the idea of performing a cryogenic separation of component gas species from a gas such as a biogas by utilising phase change properties. Nitrogen may be used as a working fluid to perform a refrigeration cycle such as a reverse Brayton cycle, and freeze the carbon dioxide out of the biogas and create liquid methane via a cryogenic distillation process, although it will be understood that other substances may be used. The working fluid may be a cryogenic liquefied gas (a “cryogen”) or a refrigerated liquefied gas. For example, neon or argon may be used as the working fluid, as well as helium and hydrogen. In some examples a mixture of cryogens may be used as the working fluid. The components of the system are readily available and minimal pre-treatment of the biogas is required - in some examples the biogas may only be dried and/or passed through an activated carbon filter. Furthermore, the use of a separate working fluid (liquid nitrogen) to the process fluid (biogas) reduced issues of contamination that may otherwise occur. Accordingly, in a first aspect there is provided a system for producing liquid methane, for example from biowaste material. The system comprises an inlet for receiving a process fluid comprising carbon dioxide and methane, both in a gaseous phase, from an anaerobic digestion process of the biowaste material; and a separator comprising a cooling means for cooling the process fluid to a temperature whereby carbon dioxide in the process fluid freezes to a solid phase and whereby methane in the process fluid condenses to a liquid phase. The separator comprises a first stage for receiving the process fluid via the inlet, the first stage having a collector for collecting frozen carbon dioxide in a solid phase, and a second stage downstream of the first stage for collecting condensed methane in a liquid phase. The cooling means is arranged to cool the second stage to a lower temperature than the first stage so as to create a temperature gradient through the separator.

It will be understood that in some examples the system also comprises an anaerobic digestor for creating the process fluid. The inlet may be coupled to the anaerobic digestor for receiving the incoming process fluid from the anaerobic digestor.

The system may further comprise a working fluid for cooling the separator, the working fluid configured to cool the process fluid to a cryogenic temperature. The working fluid may comprise, for example, nitrogen, neon or argon.

The system may additionally comprise a compressor for compressing the working fluid, an accumulator in fluid communication with the compressor for receiving the compressed working fluid, and an expander in fluid communication with the accumulator for expanding the working fluid to cool it further. The expander may be in fluid communication with the cooling means for cooling the separator. The compressor may be a liquid piston compressor and is arranged to feed compressed working fluid to the accumulator before feeding it to the cooling means.

The liquid piston compressor may comprise a water-glycol mixture to compress the working fluid in a near-isothermal process. The liquid piston compressor may comprise a plurality of liquid piston compressors, each of the plurality of liquid piston compressors being contained within a respective cylinder. The plurality of liquid piston compressors can be operated to scale capacity and even out variations in pressure. The system may further comprise a cooler for cooling the liquid piston compressor and/or the liquid in the liquid piston compressor, wherein the cooler is coupled to the anaerobic digester and arranged to use waste heat extracted from cooling the liquid piston compressor and/or the liquid in the liquid piston compressor to heat the anaerobic digestor.

The system may further comprise a heat exchanger arranged to cool working fluid being fed to the separator by heating working fluid leaving the separator.

In some examples the system comprises a controller for controlling the separator. The separator may comprise at least two vessels, each comprising respective first and second stages. The separator vessels may be arranged to be used in counter phase such that a first vessel is operated by the controller in a first mode while a second vessel is operated by the controller in a second mode for a first selected time period, and then the first vessel is operated by the controller in the second mode while the second vessel is operated in the first mode by the controller for a second selected time period. In the first mode (“CO2 collection”) the controller is configured to operate the separator so that the corresponding vessel receives process fluid and is cooled by the cooling means to a temperature to collect, by the collector of the first stage, frozen carbon dioxide from the process fluid in a solid phase inside the vessel. In the second mode (“CO2 regeneration”) the controller is configured to operate the separator so that the corresponding vessel is heated such that the collected frozen carbon dioxide in a solid phase on the collector in the first stage of the vessel sublimates from the collector surface to a gaseous phase and vents from the first stage of the vessel. The first and second selected time periods may be the same, for example.

The controller may be configured to reduce the pressure in the corresponding vessel in the second mode, for example to less than 5 bar, for example to 1 bar.

The cooling means may comprise a pair of cooling coils, with a respective pair wrapped around each corresponding separator vessel, wherein one of the pair of cooling coils is configured to permit the flow of working fluid in an alternate direction to the other of the pair of cooling coils in a contra-flow arrangement, and wherein the cooling means are arranged to flow working fluid through one of the pair of coils in a first direction to cool the corresponding separator vessel in the first mode, and flow working fluid through the other of the pair of coils in a second direction opposite to the first direction to warm the corresponding separator vessel in the second mode. The pair of cooling coils may be arranged around each separator vessel such that, in the first mode, cold working fluid is received in a region adjacent to the second stage before passing around the second stage and then passing around the first stage, such that the second stage is cooled to a lower temperature than the first stage.

However, it will be understood that the cooling means may take another form to perform the function of cooling the separator. For example, the cooling means may be arranged to provide a cooling jacket or bath around the separator, for example such that the or each separator vessel is submerged in a bath of coolant.

The system may be arranged to collect carbon dioxide vented from each separator vessel and flow it around the second stage of another separator vessel to further cool the second stage of another separator vessel.

The system may further comprise a heat exchanger for cooling the process fluid entering the separator, and wherein the vented gaseous carbon dioxide is arranged to be fed through the heat exchanger to cool the incoming process fluid to the separator.

In some examples the collector of the first stage of the separator comprises a grid or mesh to provide an enlarged internal surface area. The second stage may additionally or alternatively comprise cooling fins on an interior surface therefore to improve cooling.

The system may further comprise a compressor for compressing the process fluid before passing to the separator, the compressor coupled to a cooler for cooling the process fluid during the compression process, wherein the cooler is coupled to the anaerobic digester and arranged to use waste heat extracted from cooling the process fluid during the compression process to heat the anaerobic digestor.

In another aspect there is provided a method for producing liquid methane from a process fluid, for example from a biogas produced from biowaste material. The method comprises: receiving a stream of process fluid comprising carbon dioxide and methane, both in a gaseous phase, created for example from an anaerobic digestion process of biowaste material; feeding the stream of process fluid into a separator having a first stage, a second stage downstream of the first stage, and a cooling means for cooling the separator; and cooling the process fluid in the first stage with the cooling means to a first temperature such that carbon dioxide in the process fluid freezes to a solid phase, and collecting frozen carbon dioxide in a solid phase on a collector surface; and cooling the process fluid in the second stage with the cooling means to a second temperature cooler than the first temperature, such that methane in the process fluid condenses to a liquid phase.

The method may further comprise cooling the process fluid with a working fluid, the working fluid configured to cool the process fluid to a cryogenic temperature. The working fluid may comprise nitrogen.

The method may further comprise compressing the working fluid with a liquid piston compressor, feeding the compressed working fluid to an accumulator, and then expanding the working fluid to cool it further. The liquid piston compressor may comprise a water- glycol mixture for compressing the nitrogen working fluid in a near-isothermal process. The liquid piston compressor may comprise a plurality of liquid piston compressors, each of the plurality of liquid piston compressors being contained within a respective cylinder, and wherein the method comprising collecting the compressed working fluid from each of the plurality of liquid piston compressors in the accumulator to minimise variations in pressure of the compressed working fluid. The plurality of liquid piston compressors can be operated to scale capacity and even out variations in pressure.

The method may further comprise cooling the liquid piston compressor and/or the liquid in the liquid piston compressor and using waste heat extracted from cooling the liquid piston compressor and/or the liquid in the liquid piston compressor to heat the anaerobic digestion process. The method may further comprise using working fluid that has been used to cool the process fluid to cool working fluid that is to be used to cool the process fluid.

The separator may comprise at least two vessels, each having respective first and second stages, and each arranged to be used in counter phase such that a first vessel is operated in a first mode while a second vessel is operated in a second mode for a first selected time period, and then the first vessel is operated in the second mode while the second vessel is operated in the first mode for a second selected time period. In the first mode (“CO2 collection”) the corresponding vessel may receive process fluid and is cooled by the cooling means to a temperature to collect frozen carbon dioxide in the process fluid in a solid phase on the collector inside the first stage of the vessel. In the second mode (“CO2 regeneration”) the corresponding vessel may be heated such that frozen carbon dioxide in a solid phase on the collector in the first stage of the vessel sublimates from the collector surface to a gaseous phase and vents from the vessel.

The method may further comprise comprising reducing the pressure in the corresponding vessel in the second mode, for example to less than 5 bar, for example to 1 bar.

The cooling means may comprise a pair of cooling coils, with a respective pair wrapped around each corresponding separator vessel, wherein one of the pair of cooling coils is configured to permit the flow of working fluid in an alternate direction to the other of the pair of cooling coils in a contra-flow arrangement. The method may comprise flowing working fluid through one of the pair of coils in a first direction to cool the corresponding separator vessel in the first mode, and flowing working fluid through the other of the pair of coils in a second direction opposite to the first direction to warm the corresponding separator vessel in the second mode.

The method may further comprise receiving cold working fluid via the cooling means in a region adjacent to the second stage before passing the working fluid around the first stage, to cool the second stage to a lower temperature than the first stage.

The method may further comprise collecting gaseous carbon dioxide vented from each separator vessel and flowing it around the second stage of another separator vessel to further cool the second stage of another separator vessel.

The method may further comprise cooling the process fluid entering the separator, and/or cooling the incoming process fluid to the separator with the vented gaseous carbon dioxide.

The method may further comprise comprising (i) compressing the process fluid before cooling it to a temperature whereby carbon dioxide in the process fluid freezes to a solid phase and whereby methane in the process fluid condenses to a liquid phase, (ii) cooling the compressor and/or the compressed process fluid, and (iii) using waste heat extracted from the cooling process to heat the anaerobic digestion process.

In another aspect there is provided a computer readable non-transitory storage medium comprising a program for a computer configured to cause a processor to perform the method of the aspect described above.

Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 shows a functional schematic diagram of a system for producing liquid methane from biowaste material;

Fig. 2 shows a perspective view of an example render of a system for producing liquid methane, such as the system of Fig. 1 ;

Fig. 3 shows an alternative perspective view of the example system of Fig. 2;

Figs. 4A and 4B show plots of working fluid and process fluid temperature against entropy that may be experienced using the system of the present disclosure, such as the system of Figs. 1 to 3;

Fig. 5 shows a functional schematic of an example working fluid compressor for use with a system for producing liquid methane from biowaste material, such as the example system of Figs. 1 to 3;

Fig. 6 shows a schematic of a cross-section through an example vessel of a separator for use with a system for producing liquid methane from biowaste material, such as the example system of Figs. 1 to 3;

Fig. 7 shows a perspective view of an example separator for use with a system for producing liquid methane from biowaste material, such as the example system of Figs. 1 to 3; and

Fig. 8 shows a cross-section through one of the vessels of the example separator of Fig. 7.

It will be understood that like reference numbers indicate the same features or features with the same or similar functionality.

Specific description

Embodiments of the claims relate to system and method for producing liquid methane, for example from biowaste material. An example of a such a system for producing liquid methane from biowaste material is shown in Figs. 1 to 3. As noted above, while biowaste has been described as a possible source of the biogases, it will be understood that the systems and methods described herein may be used for processing other gases.

The system 100 show in Figs. 1 to 3 comprises an anaerobic digestor 101 in fluid communication with a separator 113. In between the separator 113 and the anaerobic digestor 101 is, in fluid communication with each other in series, an optional dryer 103, an optional activated carbon filter 105, an optional process fluid compressor 107, a process fluid cooler 109, and an optional process fluid heat exchanger 111. Although not shown in Fig. 1 , the separator 113 comprises two vessels 113a, 113b as can be seen in Figs. 2 and 3, although it will be understood that in other examples the separator 113 may comprise more or less vessels. The two vessels 113a, 113b each comprise an inlet 180 which may be coupled via a common valve 181 , as shown in Fig. 7 and described in more detail below.

The separator 113 comprises a cooling means 145 (shown in more detail in Figs. 6 to 8 and described in more detail below) that is arranged to cool the separator 113 and, in the examples shown, surrounds the separator 113. In the example shown, the cooling means 145 comprises a jacket of a pair of coils that surrounds and is wrapped around each vessel of the separator 113, although it will be understood that the cooling means 145 may take other forms in other examples and need not be limited to a pair of coils. The cooling mean s 145 may be enclosed with a separator housing, for example as shown in Fig. 7 and described in more detail below. The cooling means 145 is coupled to a source of working fluid, in this case nitrogen 117. The source of nitrogen 117 is coupled to the cooling means via an optional working fluid compressor 119, an optional accumulator in fluid communication with the compressor (not shown in Fig. 1 but shown in Fig. 5); and an optional expander 125 in fluid communication with the accumulator. The expander 125 is in fluid communication with the cooling means. In the example shown in Figs. 1 to 3 there is also an optional regenerative heat exchanger 123 located between the working fluid compressor 119 and the expander 125. In the example shown the compressor 119 is a liquid piston compressor 119 and may comprise a water-glycol mixture.

As shown in Figs. 6 and 8 as described in more detail below, each vessel of the separator 113 comprises respective first and second stages, the first stage 190 for receiving the process fluid from the anaerobic digestor 101 , and the second stage 195 downstream of the first stage 190 for collecting condensed methane in a liquid phase. The first stage 190 has a collector for collecting frozen carbon dioxide in a solid phase. The collector may be the internal walls of the first stage 190 of each vessel, and/or may comprise otherfeatures configured to provide an enlarged surface area, such as a grid or mesh, or even copper or steel wool, for example.

A cooler 115 is coupled to the source of nitrogen 117 and the working fluid compressor 119. In the example shown in Figs. 2 and 3, the cooler 115 is provided as a cooling jacket comprising a repeating series of cooling pipes that surround and are in thermal communication with the source of nitrogen 117 and/or the compressor 119 and form part of a housing enclosing the source of nitrogen 117 and the compressor 119. The cooler 115 is coupled to a chiller 121 , and the chiller 121 is coupled to and in thermal communication with the anaerobic digestor 101. In some examples the cooler 115 may comprise a cooling jacket or vessel (effectively a large tank of coolant such as water-glycol) and the compressor 119 and/or accumulator may be submerged in this vessel. The cooling jacket or vessel may be cooled by a repeated series of cooling pipes as described above. This cooling jacket or vessel may serve as a reservoir for the liquid piston compressor fluid. The system may also comprise a controller (not shown) for controlling the system.

The system is designed to have a relatively small footprint (for example, for loading on the back of a pick-up truck), and the system shown in Fig. 2 and 3 has maximum dimensions of 2m x 1 m x 1.5m.

The anaerobic digestor 101 is arranged to create a process fluid comprising carbon dioxide and methane, both in a gaseous phase, from an anaerobic digestion process of the biowaste material. For example, the anaerobic digestor 101 may be configured to receive an input waste stream of, for example, grass. The anaerobic digestor 101 may be configured to receive up to 500kg/day in waste and the anaerobic digestion process may create up to 32 kg of liquid biomethane per day. The gas flow output by the anaerobic digestor 101 and input into the separator 113 may be 83 m ³ /day, with 59% CFk by volume. Once processed by the separator 113 the purity of the liquid methane may be up to 99.5% CH4 (molar).

The dryer 103 is arranged to cool and dry the process fluid (in the examples shown, biogas) output by the anaerobic digestor 101 and remove water and optionally some contaminants. The optional activated carbon filter 105 is arranged to remove contaminants such as hydrogen sulphide, siloxanes and volatile organic compounds (VOCs). The process fluid compressor 107 is then arranged to compress the filtered process fluid to a pressure of around 4 bar. Once the process fluid has been compressed by the process fluid compressor 107 it is cooled again by the process fluid cooler 109 and then cooled further by the process fluid heat exchanger 111. The process fluid heat exchanger 111 is arranged to receive waste CO2 from the separator 113 and use it to cool the incoming process fluid to the separator 113. In this way, the process fluid heat exchanger 111 is configured to recover cold from the exhaust CO2 to pre-cool the process fluid. The CO2 will be of a relatively high purity (for example, >97%) and may be captured and re-used for agriculture or industry.

The cooling means 145 is configured to act as a cooling jacket to each vessel of the separator 113 and is arranged to cool the process fluid in the separator 113 to a temperature whereby carbon dioxide in the process fluid freezes to a solid phase and whereby methane in the process fluid condenses to a liquid phase. In particular, the cooling means 145 is arranged to cool the second stage to a lower temperature than the first stage so as to create a temperature gradient through the separator 113, such that the carbon dioxide freezes in the first stage at a first temperature and then liquid methane condenses from the process fluid in the second stage at a second temperature lower than the first temperature. To do this a working fluid is provided to the cooling means 145 for cooling the separator 113, the working fluid configured to cool the process fluid in the separator 113 to a cryogenic temperature.

The source of the working fluid is nitrogen as described above, as it is simple to service and replenish and has no global warming potential (GWP). The working fluid compressor 119 is a liquid piston compressor (as will be described in more detail below with reference to Fig. 5) and is arranged to achieve efficient near-isothermal compression of the working fluid. This is achieved by the fluid in the piston (which may, for example, be awater-glycol mix) receiving and absorbing any heat generated in the compression process. The working fluid compressor 119 may be configured to compress the working fluid to a pressure of about 40 bar and is arranged to feed compressed working fluid to the cooling means.

The regenerative heat exchanger 123 is configured to recover cold from returning working fluid that is returning from the separator 113 to cool the working fluid flowing to the separator 113. The expander 125 is arranged to expand this cooled working fluid to cool itfurtherand feed the expanded and cooled working fluid to the cooling means surrounding the separator. Once the working fluid has passed through the cooling means, it is returned to the working fluid compressor 119 via the regenerative heat exchanger 123.

As shown in Figs 1 to 3, the working fluid cooler 115 is arranged to cool the working fluid compressor 119. The chiller 121 is arranged to use waste heat extracted from the working fluid cooler 115 by cooling the working fluid compressor 119 to heat the anaerobic digestor 101.

The separator vessels are arranged to be used in counter phase such that a first vessel is operated by the controller in a first mode (a CO2 collection stage) while a second vessel is operated by the controller in a second mode (a CO2 regeneration stage) for a first selected time period, and then the first vessel is operated by the controller in the second mode while the second vessel is operated in the first mode by the controller for a second selected time period.

In the first mode the controller is configured to operate the separator 113 so that the corresponding vessel receives process fluid and is cooled by the cooling means 145 to a temperature to collect, by the collector of the first stage 190, frozen carbon dioxide from the process fluid in a solid phase inside the vessel 113a, 113b, and condensed liquid methane in the second stage 195 of the vessel.

In the second mode the controller is configured to operate the separator 113 so that the corresponding vessel 113a, 113b is heated such that the collected frozen carbon dioxide in a solid phase on the collector in the first stage 190 of the vessel sublimates from the collector surface to a gaseous phase and vents from the first stage 190 of the vessel.

It will be understood that in some examples the first and second selected time periods may be the same. In this way, a first vessel 113a may be operating in the first mode while a second vessel 113b is operating in the second mode, and then the two vessels 113a, 113b may switch so that the first vessel 113a is operating in the second mode while the second vessel 113b is operating in the first mode. However, in other examples the first selected time period may be longer than the second selected time period.

In some examples the controller is configured to reduce the pressure in the corresponding vessel 113a, 113b in the second mode, for example such that the pressure in the vessel is less than 5 bar. The pressure may be reduced in both the first stage 190 and the second stage 195, or just in the first stage 190.

The cooling means 145 is configured to permit the flow of working fluid in each coil of the pair of cooling coils in alternate directions (in a contra-flow arrangement). For example the cooling means 145 are arranged to flow working fluid through one of the pair of coils in a first direction to cool the corresponding separator vessel 113a, 113b in the first mode, and f low working fluid through the other of the pair of coils in a second direction opposite to the first direction to warm the corresponding separator vessel 113a, 113b in the second mode.

The process fluid compressor 107 is arranged to compress the process fluid before passing to the separator 113. In the example shown, the process fluid compressor 107 is arranged to receive cleaned/filtered and dried process fluid from the anaerobic digestor 101 and provide compressed process fluid to the process fluid cooler 109. The process fluid cooler 109 is arranged to receive the compressed process fluid from the process fluid compressor 107 and cool the process fluid before passing it to the process fluid heat exchanger 111 where it is cooled further. The process fluid heat exchanger 111 is arranged to cool the process fluid entering the separator 113 and is arranged to feed the vented gaseous carbon dioxide through the heat exchanger to cool the incoming process fluid to the separator 113.

In use, an input waste stream is fed into the anaerobic digestor 101. The input waste stream may comprise, for example, grass. The anaerobic digestor applies heat to the input waste stream and digests the input waste in an anaerobic (i.e. without the presence of oxygen) process. This results in the production of a process fluid (a “biogas”) comprising gaseous methane (CH4) and carbon dioxide (CO2) as well as trace amounts of other gases (such as N2, O2, Ar) as well as water (H2O).

The process fluid is fed from the anaerobic digestor 101 to the dryer 103. The dryer 103 receives the process fluid from the anaerobic digestor 101 and cools and dry the process fluid (in the examples shown, biogas) output by the anaerobic digestor 101 to remove water and optionally some contaminants. The process fluid is then fed from the dryer 103 and passed through the optional activated carbon filter 105 which removes contaminants such as hydrogen sulphide, siloxanes and volatile organic compounds (VOCs). The process fluid then passes through compressor 107 which compresses the filtered process fluid to a pressure of around 4 bar. Once the process fluid has been compressed by the compressor 107 it is fed to the optional process fluid cooler 109 where it is cooled and then passed on to the process fluid heat exchanger 111 where it is cooled again by passing heat to gaseous CO2 that is extracted from the separator 113. In this way, the process fluid heat exchanger 111 recovers cold from the exhaust CO2 to pre-cool the process fluid. The gaseous CO2 will be of a relatively high purity (for example, >97%) and may be captured and re-used for agriculture or industry.

The process fluid is then fed from the process fluid heat exchanger 111 to the separator 113. The separator 113 is cooled by the cooling means 145 that operates by passing a working fluid through the cooling means 145. As discussed above, the cooling means 145 may comprise a jacket of a series of coils that surrounds each vessel of the separator 113. The inlet of at least one of the coils may be proximate to the second stage 195, such that the coldest working fluid enters the coils proximate to the second stage 195 first, resulting in the second stage 195 of each vessel being cooled to a lower temperature than the first stage 190.

The source of the working fluid is nitrogen as described above, and the working fluid is fed from the source 117 to the working fluid compressor 119 achieves efficient near-isothermal compression of the working fluid. The working fluid compressor 119 may compress the working fluid to a pressure of about 40 bar. The compressed working fluid is then fed to the regenerative heat exchanger 123 which recovers cold from returning working fluid that is returning from the separator 113 to cool the working fluid flowing to the separator 113. The cooled working fluid is then fed to expander 125 which expands this cooled working fluid to cool it further. The expanded working fluid is now at a cryogenic temperature and is fed to the cooling means 145 surrounding the separator 113 to cool the process fluid flowing into and through a corresponding one of the vessels 113a, 113b of the separator 113. Because the cooling means 145 cools the second stage 195 first, any methane flowing through the process fluid at the second stage condenses into a liquid form where it is then collected, for example in a dewar. The cooling means also cools the first stage 190 but to a higher temperature than the second stage 195, however this temperature is still low enough to freeze any carbon dioxide in the first stage 190 such that it collects in a solid form on a collector in the first stage 190.

Once the working fluid has passed through the cooling means 145, it is returned to the working fluid compressor 119 via the regenerative heat exchanger 123 where cold is extracted to the cool the incoming working fluid. The working fluid cooler 115 cools the working fluid compressor 119 and/or the source of the working fluid 117. The chiller 121 extracts waste heat from the working fluid cooler 115 by cooling the working fluid compressor 119 to heat the anaerobic digestor 101 and thereby help to drive the anaerobic digestion process.

Once a sufficient amount of frozen carbon dioxide has collected on a collector surface in a first stage 190 of a separator 113 vessel, the frozen carbon dioxide needs to be removed otherwise the system will eventually block and/or cease to function effectively. To do this, the separator 113 is run in a second mode of operation, whereby the vessel is heated. In the case of there being two or more separator 113 vessels, the incoming process fluid is switched from a first vessel to a second vessel. Once the switch has occurred, so that process fluid is no longer being fed into the first vessel, the first vessel can be operated in the second mode. In the second mode, the flow of cool working fluid to the first vessel is halted (instead the flow of cool working fluid is diverted and fed to the coils surrounding the second vessel), and instead a warm working fluid is delivered through the other one of the pair of coils of the cooling means. The warm working fluid may come, for example, from the anaerobic digestor -forexample, the chiller 121 may warm the anaerobic digestor 101 with a second working fluid, and once this second working fluid has warmed the anaerobic digestor 101 it may then be fed to the second set of coils of the cooling means to warm each vessel of the separator 113. It will also be understood that the warm working fluid may come from a number of other waste heat streams. At the same time, the pressure in the first vessel may be reduced (for example, to 5 bar) to encourage the sublimation of carbon dioxide from the collector.

The warm working fluid is fed in counter phase to the cool working fluid, such that the first stage 190 of the vessel is warmed to a higher temperature than the second stage 195. The warm working fluid is sufficiently warm to raise the temperature in the first stage 190 such that the frozen carbon dioxide collected on the collector surface in the first stage 190 sublimates and is converted into a gaseous form. This gaseous carbon dioxide is extracted from the separator 113 vessel, fed through the process fluid heat exchanger 111 to cool the incoming process fluid, and then collected, for example for use in agriculture or industry. The first and second vessels of the separator 113 may be run in their respective first and second modes for a selected time interval, for example based on previous measurements of when the collector surface is full or starts to become covered in a layer frozen carbon dioxide. Additionally, or alternatively, the vessels may comprise a sensor, for example on the collector, that senses when the collector surface is covered in frozen carbon dioxide and/or when the pressure rises, a restriction and blockage occurring.

While the anaerobic digestor 101 is shown as part of the system 100 described above in relation to Figs. 1 to 3, it will be understood that in some examples the anaerobic digestor 101 need not be present and that a process fluid (e.g. biogas) may be provided from an external source - for example, the process fluid may be supplied via a supply line ora grid or may be bottled. In this way, the process fluid may be captured elsewhere and transported to a facility for processing the process fluid and extracting useful carbon dioxide and liquid methane.

While the separator 103 has been described as having two stages 190, 195, it will also be understood that in other examples the separator may have more stages such that the separator has a plurality of stages. For example, the separator 103 may have further stages for separating further gases and/or contaminants from the process fluid. For example, the separator 113 may have an intermediate stage (between the first stage 190 and second stage 195) cooled to an intermediate temperature to separate out another gas, such as carbon monoxide. Additionally, or alternatively, the separator 103 may comprise stages for separating out ammonia and/or hydrogen sulphide.

Moreover, while the first and second stages 190, 195 of the separator 103 are shown in the examples as being part of the same vessel/the same physical entity, it will be understood that in other examples the first and second stages 190, 195 (as well as any additional stages) may be provided as separate physical vessels, each representing a separate self-contained physical entity, that are in fluid communication with each other.

The temperatures and entropy of the working fluid and the process fluid are shown in Figs. 4A and 4B respectively. Figs. 4A and 4B show how the cold working fluid takes heat from the cold, post-separated process fluid (biogas), causing a phase change in the methane in the process fluid to collect liquid biomethane (LBM). As can be seen from Figs. 4A and 4B, the working fluid cools the process fluid by operating in a reverse version of the Brayton cycle.

In some examples (as shown, for example, in Fig. 5) the liquid piston compressor 119 comprises a plurality of liquid piston compressors, each of the plurality of liquid piston compressors being contained within a respective cylinder. The example shown in Fig. 5 comprises four liquid piston compressors, 519a-d. The four liquid piston compressors 519a-d are coupled to a common accumulator 520. Each cylinder 519a-d comprises a working fluid inlet valve 536a-d, and working fluid outlet valve 530a-d, a coolant drain port 534a-d and a coolant inlet port 531 a-d. A pump 537a-d is coupled to each coolant inlet port 531 a-d to regulate the flow of coolant to each cylinder 519a-d. However, it will be understood that in other examples there may be fewer pumps than there are cylinders - for example, two cylinders may share a common pump.

The working fluid outlet valves 530a-d between each liquid piston compressor 519a-d and the common accumulator 520 can regulate the flow of compressed working fluid to/from each cylinder 519a-d and the common accumulator 520. The coolant inlet ports 531 a-d and drain ports 534a-d can be controlled to regulate the pressure in each cylinder 519a-d and may be coupled to a coolant reservoir (not shown). The coolant reservoir may be provided as an insulating or cooling jacket around each cylinder 519a-d, for example in a manner similar to the cooling means for the separator 113. The accumulator 520 also comprises a coolant inlet valve 535 for regulating the flow of coolant to the accumulator 520 to regulate the pressure in the accumulator 520. The coolant inlet valve 535 may be coupled to a reservoir of coolant via an optional pump, and/or may be coupled to each cylinder 519a-d via each coolant drain port 534a-d. A pressure flow control valve 533 can also regulate the flow of working fluid to/from each cylinder 519a-d and/or from the accumulator 520.

The common accumulator 520 is arranged to maintain a constant pressure by varying the volume of the accumulator 520 using an additional liquid piston that is not cycled. The control of the liquid piston compressors 519a-d, the flow valves and the accumulator 520 may be performed by a controller, for example the same controller that controls the separator 113.

The liquid piston compressors 519a-d use a high-pressure water-glycol mix as a coolant to compress the nitrogen working fluid along a near isothermal process path. During the compressions stroke, heat generated through the compression process is transferred into the compressing water-glycol mix. This results in a very low temperature rise in the nitrogen working fluid and enables the near-isothermal compression path. Furthermore, a high internal surface area structure increases heat transfer.

The compression cycle of each liquid piston compressor 519a-d operates as follows:

1. Nitrogen working fluid at 3 bar fills the cylinder. Water-glycol in the cylinder is displaced via a coolant drain port 534a-d into a coolant reservoir (not shown);

2. The coolant drain 534a and working fluid inlet port 536a-d are closed;

3. Water-glycol is pumped via a pump 537a-d from the coolant reservoir into the cylinder, acting as a piston and compressing the working fluid;

4. Once the working fluid achieves the target pressure of 40 bar, the outlet flow valve 530a-d is opened; and

5. Water-glycol continues to be pumped into the cylinder, displacing the working fluid at 40bar into a high-pressure working fluid reservoir in the accumulator 520.

As shown in Fig. 5, the liquid piston assembly 519 can comprise multiple cylinders to scale capacity. The compression and drain stroke do not need to be symmetrical, for example greater time may be available for the compression stroke to maximise heat transfer.

The working fluid outlet of the liquid piston compressors 519a-d are run into the common accumulator 520 which is a high-pressure accumulator. A constant 40 bar is maintained in the accumulator 520 by varying the volume of the reservoir in the accumulator 520 using an additional liquid piston that is not cycled. This element acts to damp out pressure waves due to the opening and closing of the working fluid outlet valves 530a-d. This element also acts as a water trap. As the entire system is kept at 273K or just below, all water vapour is condensed out of the working fluid at this stage. The dry, high pressure working fluid is then released from the accumulator 520 via a pressure flow control valve 533. As described above, the cryogenic separation system 100 shown in Figs. 1 to 3 comprises a separator 113 with two separation vessels 713a, 713b, each having two internal stages 190, 195 forming respective chambers. A cross-section of one of the vessels is shown in Fig. 6. In the example shown in Fig. 6 each vessel is cylindrical in nature and comprises a first stage 690 positioned above a second stage 695. The first and second stages 690, 695 are formed as part of the same vessel (i.e. they have the same external dimensions) and may be formed by subdividing the vessel. However, the first stage 690 is larger (in internal volume) than the second stage 695, and in the example shown the first stage 690 is about 2/3 the volume of the vessel whereas the second stage 695 is about 1/3 the volume of the vessel. The first stage 690 of the vessel may be larger than the second stage 695 to enable a larger surface area for frozen carbon dioxide to collect on and thereby act as a collector.

As shown in Fig. 6, the first stage 690 comprises a process fluid (e.g. biogas) inlet 680 at one end thereof feeding into the first stage 690 of the vessel (in the example shown, the inlet 680 is through a top surface of the first stage 690 of the vessel). As shown in Fig. 7, the inlet 780 of each vessel 713a, 713b is coupled to the other via a valve 781 , that also couples the vessels 713a, 713b to the process fluid heat exchanger 111.

The first stage 690 is in fluid communication with the second stage 695 of the vessel, which comprises a liquefied drain 685 (fordraining liquefied methane) and a non-liquefied outlet 682 for extracting un-liquefied process fluid. The liquefied drain 685 and non-liquefied outlet 682 are at an opposite end of the vessel to the inlet 680 on an opposing face of the vessel to the inlet 680. In the example shown, the liquefied drain 685 is provided at a bottom surface of the second stage 695 such that liquefied methane can drain from the second stage 695 under gravity. The non-liquefied outlet 682 extends some way into the second stage such that it stands significantly proud of the bottom surface of the second stage 695. The non-liquefied outlet 682 may be coupled to inlet 680 such that the process fluid is recycled through the vessel to improve the distillation of liquid methane from the process fluid.

As can be seen, the cooling means 645 comprises a pair of cooling coils, with a respective pair wrapped around each corresponding separator vessel to provide a cooling “jacket” to each vessel. The cooling means 645 may further be insulated and/or enclosed in a separator housing 760 as shown in Fig. 7 (Fig. 7 only shows a separator housing 760 on one of the vessels 713a but it will be understood that a separator housing 760 may be provided on both vessels 713a, 713b). In the example shown in Fig. 6, the cooling means 645 are arranged to provide a cooling circuit 670 and a warm-up circuit 675. The cooling circuit 670 is arranged to cool the second stage 695 of the vessel first before then cooling the first stage 690 of the vessel. The warm-up circuit 675 is arranged to warm only the first stage 690 of the vessel. In the example shown, the second stage 695 also comprises a second circuit: a process fluid circuit 673. In the example shown, rather than the first stage 690 being directly coupled to the second stage 695, the first stage 690 is in fluid communication with the second stage 695 via the process fluid circuit 673 (although it will be understood that in other examples the process fluid may pass directly from the first stage 690 to the second stage 695, for example though an aperture in the lower surface of the inside of the first stage 690/top surface of the inside of the second stage 695).

In this example, process fluid which has already been cooled to the extend that carbon dioxide has been removed is passed through the process fluid circuit 673 and then around the second stage 695 to cool the second stage (in addition to the cooling provided by the cooling circuit 670) before then flowing into the second stage 695. The process fluid circuit 673 delivers the process fluid adjacent to the bottom surface of the second stage 695, such that any liquid methane already in the fluid can flow directly out through the liquefied drain 685, and any process fluid still in a gaseous phase has to travel upwards through the second stage 695 (where it will be cooled further, condensing yet more of the methane contained therein) before flowing through the non-liquefied outlet 682.

In the first stage 690 CO2 is frozen as dry-ice and collected on a collector 691 , which in the example shown is a mesh or grid located inside the first stage 690. However, it will be understood that in other examples the collector 691 may comprise the internal walls of the first stage 690 of the vessel. The collector691 is configured to provide an enlarged internal surface area to aid in the collection of frozen carbon dioxide extracted from the process fluid. In the second stage 695, methane is liquified and removed for collection in a sperate dewar container. The second stage 695 may be configured to provide an enlarged internal area to increase the efficacy of cooling by the cooling means 645 - in the example shown in Fig. 6, the second stage 695 comprises cooling fins 696 arranged on an interior surface of the vessel, to help facilitate heat transfer between the process fluid in the second stage 695 of the vessel and the cooling means 645.

Each vessel is operated in counter-phase to a corresponding second vessel - whilst one vessel is actively cooling the process fluid, separating the CO2 and liquifying the methane, the other vessel is being regenerated to remove the accumulated dry ice. In the example shown in Fig. 7, the first vessel 713a may be operated in counter phase to the second vessel 713b. The regeneration process results in a stream of cold CO2 gas, which is used to improve the process efficiency to optionally help cool the second stage 695 (for example, of the corresponding second vessel) and/or the incoming process fluid to the first stage 690.

In use in the first mode (the CO2 collection stage) the process fluid (biogas) enters the separator 113 from the top of the vessel into the first stage 690 which is a CO2 separation chamber. Cold nitrogen working fluid enters the cooling means 745 adjacent to the first stage 690 and flows upwards in a coil around the first stage 690. CO2 gas in the incoming process fluid transfers heat to the cold working fluid, warming it and freezing out to form dry ice. The volume of the first stage 690 has a sufficiently large surface area and sufficient void space to allow ice formation without blocking. Remaining gasses in the process fluid (predominantly methane) pass through this chamber as very cold gas at 139K and flow into the second stage 695 which is the CH4 distillation chamber.

In use in the second mode (the CO2 regeneration stage), once the first stage 690 become full with solid CO2 (for example as determined with a sensor inside the first stage 690 and/or after a predetermined time interval and/or a predetermined volume of flow of process fluid into the first stage 690), the operation of the full separation vessel is switched to regeneration mode. The process fluid inlet 680 is switched over from receiving flow from the process fluid heat exchanger 111 to providing flow to the process fluid heat exchanger 11 (for example, by way of a valve proximate to the inlet 680 such as valve 781 ). The pressure in the first stage 690 of the separator 113 is also dropped to below 5 bar, for example to 1 bar. The cold working fluid fed into the cooling circuit 670 is halted, and instead warm working fluid is fed warm up circuit 675. The combination of the warm working fluid and pressure drop cause the frozen CO2 to sublimate into CO2 vapour, which flows through the process fluid heat exchanger 111 to pre-cool process fluid entering the second (active) separation vessel.

Additionally or alternatively, the first stage 690 may comprise a CO2 outlet valve, whereby when the vessel is operated in the second mode or being “regenerated”, the CO2 gas vents through the CO2 outlet valve (for example by a controller opening the outlet valve and closing the inlet 680). As described above, the vented CO2 can then be used to cool the incoming process fluid, for example by passing through process fluid heat exchanger 111 as shown in Figs. 1 to 3.

Once the process fluid has passed through the first stage 690, it flows into the second stage 695. Here, cold process fluid (gas) exiting the first stage 690 (the CO2 separation chamber) undergoes counter flow heat exchange with more nitrogen working fluid via a coupled coil arrangement. The process fluid transfers more heat to the cold working fluid. The resulting temperature is reduced to 125K, at which temperature the methane is a liquid. The liquid methane condenses and is drawn off into a dewer vessel via liquefied process fluid drain 685 for storage. Other trace species (Nitrogen, Oxygen) may be fractionally distilled out. Any remaining process fluid may then be recycled, for example back into the inlet 680 of the first stage 690.

Although the examples shown in Figs. 1 to 3 and 5 and 6 show the cooling means 145, 645, 745 comprising a pair of coils, it will be understood that the cooling means may be provided in other ways sufficient to cool the first and second stages 690, 695 to cryogenic temperatures. For example, the cooling means may comprise a single coil wrapped around each vessel. In such examples the coil may be configured to have either cold or warm working fluid flowing through it, for example such that when the separator 113 is operating in the second or “regeneration” mode, the cold working fluid is replaced with warm working fluid. Fig. 8 shows a cross-section through one of the separator vessels 713a, 713b of Fig. 7. It can be seen that the separator vessel is cylindrical in nature with a subdivision between the first stage 890 and the second stage 895. The first stage 890 has a larger internal surface area than the second stage 895. As shown in Fig. 8, the first stage 890 comprises a process fluid (e.g. biogas) inlet 880 at one end thereof feeding into the first stage 890 of the vessel (in the example shown, the inlet 880 is through a top end of the first stage 890 of the vessel). The first stage 890 is in fluid communication with the second stage 895 of the vessel, which comprises a liquefied drain 885 (fordraining liquefied methane) and a non-liquefied outlet 882 for extracting un-liquefied process fluid. The liquefied drain 885 and non-liquefied outlet 882 are at an opposite end of the vessel to the inlet 880 on an opposing face of the vessel to the inlet 880. In the example shown, the liquefied drain 885 is provided at a bottom surface of the second stage 895 such that liquefied methane can drain from the second stage 895 under gravity. The non-liquefied outlet 882 extends some way into the second stage such that it stands significantly proud of the bottom surface of the second stage 895. The non-liquefied outlet 882 may be coupled to inlet 880 such that the process fluid is recycled through the vessel to improve the distillation of liquid methane from the process fluid.

As can be seen, the cooling means 845 comprises a pair of cooling coils, with a respective pair wrapped around each corresponding separator vessel to provide a cooling “jacket” to each vessel. The cooling means 845 may further be insulated and/or enclosed in a separator housing 760 as shown in Fig. 7 (Fig. 7 only shows a separator housing 760 on one of the vessels 713a but it will be understood that a separator housing 760 may be provided on both vessels 713a, 713b). As with the example shown in Fig. 6, in the example shown in Fig. 8 the cooling means 845 are arranged to provide a cooling circuit 870 and a warm-up circuit 875. The cooling circuit 870 is arranged to cool the second stage 895 of the vessel first before then cooling the first stage 890 of the vessel. The warm-up circuit 875 is arranged to warm only the first stage 890 of the vessel. In the example shown, the second stage 895 also comprises a second circuit: a process fluid circuit 873. In the example shown, rather than the first stage 890 being directly coupled to the second stage 895, the first stage 690 is in fluid communication with the second stage 695 via the process fluid circuit 873 (although it will be understood that in other examples the process fluid may pass directly from the first stage 890 to the second stage 895, for example though an aperture in the lower surface of the inside of the first stage 890/top surface of the inside of the second stage 895).

As described above in relation to Fig. 6, the second stage 895 may be configured to provide an enlarged internal area to increase the efficacy of cooling by the cooling means

845 - in the example shown in Fig. 8, the second stage 895 comprises cooling fins 696 arranged on an interior surface of the vessel, to help facilitate heat transfer between the process fluid in the second stage 895 of the vessel and the cooling means 845. Although not shown, in some examples the first stage 890 of the separator is also configured to provide an enlarged internal surface area to aid in the collection of frozen carbon dioxide extracted from the process fluid, for example such as a grid or mesh to provide an enlarged internal surface area and to act as a collector

It will be appreciated from the discussion above that the embodiments shown in the Figures are merely exemplary, and include features which may be generalised, removed or replaced as described herein and as set out in the claims. In the context of the present disclosure other examples and variations of the apparatus and methods described herein will be apparent to a person of skill in the art.