PURIFICATION AND LIQUEFACTION OF BIOGAS BY COMBINATION OF A CRYSTALLIZATION SYSTEM WITH A LIQUEFACTION EXCHANGER

Abstract

Plant and process for the production of liquid methane from a feed gas stream comprising at least methane and carbon dioxide. A feed gas stream is injected into a CO.sub.2 crystallizer in countercurrent fashion against a stream of predominantly liquid methane, thereby crystallizing amounts of carbon dioxide from the feed gas stream. Gaseous methane recovered from the CO.sub.2 crystallizer is liquefied at a liquefaction exchanger.

Claims

1. A plant for the production of liquid methane from a feed gas stream comprising at least methane and carbon dioxide, said plant comprising, in the direction of circulation of the gas stream: a continuously operating system for the crystallization of carbon dioxide in which a stream predominantly comprising liquid methane is circulated countercurrentwise to the feed gas stream, thereby producing a methane-enriched gas stream; and a liquefaction exchanger adapted and configured to liquefy the produced methane-enriched gas stream exiting from the crystallization system.

2. The plant of claim 1, further comprising, upstream of the crystallization system, at least two heat exchangers adapted and configured to bring the feed gas stream to a temperature of between 50 C. and 85 C.

3. The plant of claim 2, wherein the at least two exchangers are arranged in parallel and each following a cycle comprising a production stage and a regeneration stage, with, at each moment of the cycle, an exchanger in the production stage and an exchanger in the regeneration stage.

4. The plant of claim 2, further comprising, upstream of the at least two heat exchangers a water condensation exchanger adapted and configured to bring the feed gas stream to a temperature of between 0 C. and 20 C.

5. The plant of claim 1, further comprising: a pipe connected to a bottom of the crystallization system; and a filter disposed in the pipe adapted and configured to separate liquid and solid phases, thereby enabling recovery of CO.sub.2 crystals.

6. A process for production of liquid methane from a feed gas stream that comprises methane and carbon dioxide using the plant of claim 1, said process comprising the steps of: continuous crystallization of carbon dioxide contained in the feed gas stream by injection of the feed gas stream into the continuously operating system for the crystallization of carbon dioxide in which a stream predominantly comprising liquid methane is circulated countercurrentwise to the feed gas stream; recovering gaseous methane in an upper part of the continuously operating system for the crystallization of carbon dioxide; liquefying the recovered gaseous methane with the liquefaction exchanger; and recovering liquid methane at an outlet of the liquefaction exchanger.

7. The process of claim 6, wherein the feed gas stream further comprises water and said process further comprises, before the crystallization stage, the steps of: reducing a concentration of the water in the feed gas stream to produce a water-depleted feed stream by lowering a temperature of the feed gas stream to a temperature of between 57 C. and 75 C.; and recovering the water-depleted feed gas stream.

8. The process of claim 7, wherein the temperature of the feed gas stream is lowered in a first step during which the temperature of the feed gas stream is lowered to a temperature of between 0 C. and 10 C. and in a second step during which the temperature of the feed gas stream is lowered to a temperature of between 57 C. and 75 C.

9. The process of claim 6, wherein, during said continuous crystallization, a ratio of a flow rate of the feed gas stream to a flow rate of the countercurrentwise liquid methane stream is such that solid CO.sub.2 particles formed during said continuous crystallization are entrained towards the lower bottom of the continuously operating system for the crystallization of carbon dioxide.

10. The process of claim 8, further comprising the steps of: recovering a mixture of liquid methane and solid CO.sub.2 particles at the lower bottom of the crystallization system; and separating the solid CO.sub.2 particles from the liquid methane in the recovered mixture, thereby producing a stream of recovered liquid methane downstream of said separation that has a carbon dioxide content of between 2 ppm and 1000 ppm.

11. The process of claim 10, further comprising the step of recycling the stream of recovered liquid methane in the continuously operating system for the crystallization of carbon dioxide.

12. The process of claim 10 further comprising the step of recycling the stream of recovered liquid methane in the liquefaction exchanger.

13. The process of claim 10, wherein the separated solid CO.sub.2 particles are used to contribute cold to a cooling cycle of the liquefaction exchanger.

14. The process of claim 6, wherein the stream of recovered liquid methane is partially returned to the continuously operating system for the crystallization of carbon dioxide.

15. The process of claim 6, wherein the feed gas stream is biogas.

16. The process of claim 6, wherein the feed gas stream is at a pressure of between atmospheric pressure and 20 bar.

17. A process for production of liquid methane from biogas that comprises methane, water, and carbon dioxide, said process comprising the steps of: condensing amounts of water from a biogas feed stream by lowering a temperature of the biogas feed gas stream in a condensing exchanger to a temperature of between 57 C. and 75 C.; recovering a water-depleted feed gas stream from the condensing exchanger having a water content lower than that of the raw feed gas stream; injecting the water-depleted feed gas stream into a bottom portion of a CO.sub.2 crystallizer system; feeding a stream predominantly comprising liquid methane into a top portion of the CO.sub.2 crystallizer countercurrentwise to the injected feed gas stream, thereby crystallizing amounts of CO.sub.2 contained in the injected water-depleted feed gas stream; recovering gaseous methane from the upper portion of the CO.sub.2 crystallizer; liquefying the recovered gaseous methane with liquefaction exchanger, a cooling power of the liquefaction exchanger being supplied by a refrigerant; recovering a stream of liquid methane at an outlet of the liquefaction exchanger; and filtering solid CO.sub.2 particles from the recovered stream of liquid methane to produce a CO.sub.2-depleted stream of liquid methane having a CO.sub.2 content of between 2 ppm and 1000 ppm.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0036] FIG. 1 is a schematic of an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0037] The invention will now be described in more detail using FIG. 1.

[0038] The water of the feed gas stream is first of all condensed. To do this, the feed gas stream 1 is, in a first step, brought to a temperature of between 0 and 10 C. (1 C., for example) by means of an exchanger 2, in order to condense a part of the water contained in the biogas and to remove it in the liquid state. This stage makes it possible to reduce the amount of solid to be removed in the downstream exchangers.

[0039] In a second step, the biogas is brought to a temperature of between 57 C. and 75 C. in order to remove the majority of water present and the impurities or contaminants of COV type present in the feed gas stream. Two exchangers 3a and 3b are placed in parallel in order to cool the biogas down to this temperature. The exchangers are regenerated once the solid is deposited on the walls. The cycle time of each exchanger comprises a production phase and a regeneration phase of between 10 min and 10 h, preferably of between 30 min and 2 h.

[0040] In the context of the invention, the first step and the second step are combined in a stage known as reducing the concentration of water.

[0041] A gas stream 4 depleted in water and in impurities is recovered at the outlet of the exchangers. This gas stream 4 is subsequently injected into a crystallization system 5 countercurrentwise to a mixture 6 mainly comprising liquid methane. The gas in contact with the liquid will bring about the crystallization of the CO.sub.2 contained in the gas as it is cooled (cooling between 50 C. and 163 C.). It should be noted that it will be possible to observe, at the same time but to a lesser extent, the crystallization of the H.sub.2S contained in the feed stream.

[0042] The ratio of the flow rate of the feed gas stream to the flow rate of the countercurrentwise liquid methane stream is such that the entrainment of the solid CO.sub.2 particles towards the lower bottom of the crystallization system is ensured and that the gas at the top of the crystallization system contains virtually no CO.sub.2. The excess liquid flow rate ensures in particular very good wetting of the entire surface present inside the column and used as contactor between the liquid and the gas. For example, it will be possible to use structured packings as contactor.

[0043] Gaseous methane 7 is thus recovered in the upper part of the crystallization system, which gaseous methane is sent to a liquefaction exchanger 8 so as to produce liquid methane. A part 14 of this liquid methane will be returned to the crystallization system and another part 15 of this liquid methane will be sent to the production of liquid methane.

[0044] A mixture 10 of liquid methane and of solid CO.sub.2 particles is recovered at the outlet of the lower bottom of the crystallization system. The phases of the said mixture are separated so as to recover the solid CO.sub.2 particles, and a stream of liquid methane 11 comprising between 2 ppm and 1000 ppm of CO.sub.2, preferably between 5 ppm and 200 ppm of CO.sub.2, is recovered at the outlet of the separation. It should be noted that it will be possible to carry out the phase separation by means of a filter or of several filters in parallel and in alternating operation, or any other liquid/solid separation system.

[0045] The liquid methane stream 11 recovered at the outlet of the filtration is, on the one hand, recycled 12 to the top of the crystallization system and, on the other hand, sent 13 to the exchanger for liquefaction of the methane in order to ensure wetting of the exchange surface and to prevent deposits of CO.sub.2 crystals in the liquefaction exchanger.

[0046] The process according to the invention requires a frigorific power contribution in order to operate. This contribution can be produced in several ways (depending on the amount of liquid biomethane to be produced, for example). By way of example but not exclusively: 1. From a liquid nitrogen source 2. By a process of reverse Brayton type: In the latter case, a refrigerant (nitrogen or a nitrogen/helium mixture) is compressed, cooled and expanded in a turbine. This refrigerant is subsequently reheated countercurrentwise to the hot fluids (including the biogas) in the exchangers. It contributes the cold necessary for its cooling down to 75 C., on the one hand, and for the liquefaction of the methane vapour which exits from the crystallization system.

[0047] From a thermodynamic viewpoint, when the mixture mainly composed of CO.sub.2 and methane is cooled, the CO.sub.2 begins to solidify from the gas phase below a certain temperature threshold (direct change from vapour CO.sub.2 to solid CO.sub.2). The temperature for the appearance of the first solid CO.sub.2 crystals is estimated at approximately 87 C. for 1 bara and 50 mol % of CO.sub.2 (i.e. temperature lower than the outlet temperature of the 2 exchangers in parallel). As liquid outflow from the crystallization system, the solid CO.sub.2 and the liquid phase are close to thermodynamic equilibrium. The solid is then filtered off upstream of the pump in order to retain only the liquid phase. Nevertheless, a small amount of solid CO.sub.2 may again be formed in the presence of liquid methane in the liquid reflux exchanger. If need be, either two exchangers in parallel (one in production and another in regeneration) or a solid/liquid phase separation system may then be placed on the liquid biomethane product line.

[0048] It should be noted that the solid CO.sub.2 recovered in the phase separation system can be used to contribute cold to the cooling cycle and to thus reduce the specific consumption of the cryo-solidification cycle.

[0049] It should be noted that the process described is designed to operate at low pressure between atmospheric pressure and 20 bar, preferably between atmospheric pressure and 5 bar.

[0050] While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

[0051] The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

[0052] Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing i.e. anything else may be additionally included and remain within the scope of comprising. Comprising is defined herein as necessarily encompassing the more limited transitional terms consisting essentially of and consisting of; comprising may therefore be replaced by consisting essentially of or consisting of and remain within the expressly defined scope of comprising.

[0053] Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

[0054] Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0055] Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

[0056] All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.