Method and device for liquid-phase purification of trace impurities

Abstract

The method (100) for liquid-phase purification of trace impurities of a gas stream comprises: - a step (105) of introducing a gas stream comprising trace impurities, - a step (110) of compressing the gas stream introduced, - a step (111) of cooling the gas stream and capturing the solid impurities, and - a step (130) of expanding the gas stream.

Description

[0001] DESCRIPTION

[0002] TITLE OF THE INVENTION: METHOD AND DEVICE FOR LIQUID PHASE PURIFICATION OF TRACES OF IMPURITIES

[0003] TECHNICAL FIELD OF THE INVENTION

[0004] The present invention relates to a method and a device for the liquid phase purification of traces of impurities. It applies, in particular, to the field of the production of natural gas and liquid biomethane.

[0005] STATE OF THE ART

[0006] The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been conceived or pursued previously. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section constitute prior art solely because of their inclusion in this section.

[0007] Gas liquefaction always requires extensive purification to avoid any crystallization of impurities (deposition of solids either in the exchanger performing cooling or subcooling, or in the final or intermediate storage of the liquid). This extensive purification is sometimes called "polishing". For example, in the case of Liquefied Natural Gas ("LNG"), i.e. a liquefied gas rich in methane, it is necessary to remove water to a level below 2 ppm to be able to store the LNG without the risk of water ice formation; similarly, LNG cannot contain more than 50 ppm of carbon dioxide ("CO2") at atmospheric equilibrium pressure without the risk of CO2 ice formation.

[0008] If we call "solvent" the fluid consisting of the majority compounds of the starting gas and which are intended to be stored in liquid form at the end of the process, and if we call "impurity" an element in trace form which risks crystallizing, this need for polishing consists of purifying the solvent to a sufficiently low level of impurity to avoid any risk of crystallization on the one hand during cooling of the solvent and on the other hand during its storage in liquid form. It is therefore in a way the finishing of the purification or an advanced purification. This step is common to all liquefied gases for a list of impurities (by definition never exhaustive). We can simply cite as examples the following solvent / impurity pairs to illustrate the potential applications:

[0009] - methane solvent / water or CO2 impurities,

[0010] - solvent nitrogen (“N2”) or oxygen (“O2”) / impurities water or CO2 and

[0011] - CO2 solvent / water impurity.

[0012] This so-called polishing purification represents a specific cost which is always significant (i.e. the energy expended and the price expended to achieve this level of advanced purification) for two main reasons:

[0013] - depending on the purity level to be achieved, the number of purification technologies is reduced and therefore limits the choices and - the separation efficiency of most technologies drops when the degree of purity increases.

[0014] An attractive approach in the case of gas liquefaction is to use the cold required for liquefaction to both condense the solvent and separate the impurities from the solvent by adjusting their solubility levels in the liquid (solubility which is directly controlled by the pressure and temperature level). It is thus possible to share a certain number of process equipment to reduce the overall cost of producing liquefied gas. However, on the one hand, this technique has so far relied on processes that require very fine sizing and technical adjustments and whose operational stability is complex and difficult to ensure; and on the other hand, these processes sometimes require different operating points (in particular the optimal pressure level of the gas to be liquefied) between the impurity separation and solvent condensation stages, hence the sometimes limited sharing of equipment.

[0015] To thoroughly purify a gas to be liquefied, there are two main approaches (themselves possibly using various specific phenomena and technologies):

[0016] - a first approach, the most widespread and the most expensive, consists of carrying out the polishing stage upstream of the cooling-liquefaction of the solvent; we then use techniques such as:

[0017] - absorption-based systems, for example amine washing in the case of natural gas polishing); regeneration of amines or absorbent compound requiring a significant amount of heat or

[0018] - systems based on adsorption, for example: adsorption on molecular sieves or activated carbons); the regeneration of the sieves or carbons is then carried out either by reducing pressure and in particular using a vacuum pump, or by heat as for absorption,

[0019] - a second approach consists of using cold to separate the impurities from the solvent and to crystallize the impurity during the cooling of the solvent:

[0020] - the most widespread technology is based on crystallization of CO2 in the vapor phase of the solvent (for example methane); the CO2 (and even water) is deposited in the exchanger where the cooling of the gas to be liquefied takes place and, to maximize this deposition phenomenon, this crystallization takes place at low pressure, i.e. at a pressure where the impurity passes directly from the vapor phase to the solid phase and where the solvent remains vapor during the separation process; this technique assumes alternating or batch operation, called "batch": an exchanger captures the solid impurity and therefore "freezes" progressively until the solvent can no longer flow; there is then a switch to a virgin exchanger while the exchanger full of the solid deposit "thaws" and therefore regenerates or

[0021] - technologies where the formation of the solid impurity occurs in a three-phase state: the solid impurity is produced in a two-phase mixture of the gaseous solvent (see, for example, document CN 102 636 002A where a gas-liquid separator referenced 7 in the single figure is useful only if the natural gas is only partially liquefied in the exchanger referenced 4, in which the state is therefore a three-phase state): - in certain technologies, applied to the liquefaction of methane-rich gases, this separation occurs at the final expansion at the end of liquefaction in sub-cooling of the biomethane; the expansion causes the evaporation of a part of the liquid biomethane while the CO2 crystallizes mainly due to the excess cooling generated and must be separated from a sludge by a separator such as the separator referenced 32 in figure 1 of document US 3,312,073, and

[0022] - in certain technologies, this triphasic state is caused by injecting liquid nitrogen into an air flow (therefore mainly a N2-O2 mixture), the cooling of which causes the crystallization of the CO2.

[0023] In these technologies, the use of cyclonic systems sometimes makes it possible to continuously evacuate the solid impurity without resorting to an alternative / batch process.

[0024] The disadvantages of the previously mentioned polishing methods are:

[0025] - with regard to absorption polishing methods (amine washing): the small-scale investment cost (for biomethane polishing in particular) due to the complex boilermaking work that the method requires, as well as the energy and environmental cost: high heat requirement for the regeneration of the active ingredient, which can also pose significant safety constraints when the heat comes from the combustion of part of the gas to be liquefied,

[0026] - regarding adsorption polishing methods (molecular sieves or activated carbons): cost of the regeneration system either by vacuum pump or by heat (with then the same problems as for amines),

[0027] - regarding polishing methods by cryogenic vapor separation): the impurity (CO2 or H2O) solidifies directly from the gaseous state to the solid state (anti-sublimation) and the solvent remains gaseous; the system operates in batch mode; the problem arises here both in terms of design complexity, stability and lifetime - in fact, the system operating in batch mode, it alternates the frosting (therefore in cold) and defrosting (in hot) phases, the fact of keeping the vapor solvent means that the stability of the flow is quickly impacted as soon as the frost of the solid impurity forms (which gradually blocks the exchanger and therefore constrains the flow passage section), the frosting and defrosting phases must therefore be reduced to maintain a certain stability of the flow; this can therefore generate significant thermomechanical constraints (cooling / heating) and accelerate the fatigue of the exchangers;moreover, in all cases, exchangers with a specific design are required, therefore rare and complex and potentially expensive to minimize these drawbacks and;

[0028] - with regard to polishing methods by cryogenic separation in a three-phase process (impurity in solid form, solvent in both gaseous and liquid form): due to the presence of solvent in cold vapor, the same flow stability constraints are found if the solid impurity accumulates - this is why most of the time in these approaches, the use of a cyclonic type separator is used to evacuate the solid impurity, the major problem with these cyclonic approaches is, beyond the complexity and the cost of development, their lack of flexibility and therefore of operational stability; the flow must precisely follow a very restricted operating range to allow the good evacuation of the solid particles. PRESENTATION OF THE INVENTION

[0029] The present invention aims to remedy all or part of these drawbacks.

[0030] To this end, according to a first aspect, the present invention aims at a liquid phase purification process of a gas flow to achieve a molar rate of impurity, called "Xj mp ,out”, associated with a solubility limit of this impurity in the liquid gas flow when this flow reaches saturation at an outlet pressure, called “P ou t”, including:

[0031] - an entry step of a gas flow comprising traces of impurity according to a molar rate Xjmp.in, the gas flow having an entry temperature, called "Tj n ", and an input pressure, called "Pin", the pressure torque Pj n and Tj n being configured to locate the gas flow in the vapor phase,

[0032] - a step of compressing the flow of gas entered at least according to a pressure, called "Phaute", which corresponds to the lowest pressure for which the impurity according to the rate Ximp. is soluble, for a temperature equal to the dew point of a flow of pure gas without impurities equivalent to the pressure Phaute,

[0033] - a cooling step, depending on the pressure Phaute, of the compressed gas flow to an outlet temperature, called "T ou t", in a heat exchanger, so as to make the impurity capturable in the form of solid impurities, and to capture the solid impurities in the heat exchanger, the temperature T ou t being selected so that, at the pressure torque P ou t and temperature T ou t, the remaining impurity at the molar rate of impurity Xi mp ,everything in the gas stream is soluble and an equivalent pure gas stream without impurities is completely liquefied, and

[0034] - an expansion stage, depending on the outlet pressure P ou t of the gas flow in liquid phase.

[0035] Thanks to these provisions, cooling is used as a principle for separating impurities from the solvent but with a specific arrangement and applied at specific operating points which make the overall liquefaction process more stable and less restrictive in terms of operability than existing polishing solutions.

[0036] These provisions make it possible to obtain:

[0037] - a stable process using non-specific heat exchangers, for example with only a tolerance to the target operating pressure and a constant flow section,

[0038] - frosting-defrosting periods exceeding two hours and depending on the size of the exchangers which may even be significantly longer and

[0039] - an exchange line dedicated to polishing which is very compact because the exchange is done in liquid at the level of the solvent-impurity mixture, which minimizes the size and weight of the exchange line used and therefore if there is thermal inertia, the latter makes it possible to further facilitate the regeneration speed of the exchangers.

[0040] The concept of the present invention is based on cryogenic separation in the liquid phase: pressure and temperature parameters are chosen such that the solvent is already completely liquid when the crystallization of the impurity begins. Thus, this approach retains the advantages of existing cryogenic separation, namely the possibility of pooling the polishing and liquefaction phases, while avoiding these main defects, namely: - on the one hand, the rapid alternations of the icing-defrosting phases because in the liquid phase, the solvent tolerates better and more easily a restriction of the passage section and can therefore ice for much longer without destabilization of the flow and

[0041] - on the other hand, the low flexibility of the separation system when it operates in three-phase state and with cyclonic separator.

[0042] In particular embodiments, the liquid phase purification method comprises a step which is a step of measuring a molar rate Xj mp ,in of an impurity in the gas stream and which precedes the compression step of the incoming gas stream.

[0043] In particular embodiments, the cooling step comprises a vapor phase cooling step at temperature Tj n

[0044] In particular embodiments, the cooling step comprises:

[0045] - a first stage of cooling, according to the Phaute pressure, of the compressed gas flow, to a temperature:

[0046] - in which the gas in the pure state equivalent to the gas entered is in the liquid phase, and

[0047] - which is higher than the crystallization temperature of the impurities at the determined molar rate of impurity called Xj mp ,in of the cooled gas flow,

[0048] - a first stage of expansion, of the liquid gas flow up to a pressure at least equal to a pressure, called “P me d”, according to which the pure gas without impurities equivalent to the entered gas is at its bubble point, and

[0049] - a second stage of cooling in the liquid phase, according to the pressure P me d, from the gas flow to the temperature Tout.

[0050] These embodiments make it possible to accelerate substitutions between heat exchangers responsible for collecting impurities and heat exchangers undergoing regeneration. Indeed, the regeneration steps thus implemented consume less energy than in the variants having total expansion then cooling to outlet conditions.

[0051] In embodiments, the method which is the subject of the present invention comprises an additional expansion step, downstream of the expansion step, the liquid phase cooling step being configured to cool the gas flow to a temperature such that at the outlet of the additional expansion step, the gas is in the liquid phase.

[0052] These embodiments make it possible to guarantee the liquid state of the liquefied gas at the outlet of the process.

[0053] In embodiments, the method which is the subject of the present invention comprises a step of storing the expanded liquefied gas.

[0054] In embodiments, the heat exchanger is configured to perform the liquid phase cooling step, the method comprising a step of regenerating the heat exchanger.

[0055] These embodiments allow the method to be implemented continuously.

[0056] In embodiments, the regeneration step comprises a step of inserting a regeneration flow into the heat exchanger to be regenerated.

[0057] These embodiments make it possible to optimize the regeneration of the heat exchanger. In embodiments, the regeneration stream has a composition comprising the same compounds as the gas stream to be purified.

[0058] These embodiments avoid the risks of contamination of the gas flow to be purified by third-party compounds when the heat exchanger is used to carry out the cooling step.

[0059] In embodiments, the method which is the subject of the present invention comprises a step of recycling the regeneration flow to a step of mass purification of the gas flow, located upstream of the inlet step.

[0060] These embodiments make it possible to enhance the regeneration flow, the composition of which is similar to the composition of the gas to be purified.

[0061] In embodiments, the method which is the subject of the present invention comprises, downstream of a regeneration step, a step of temporary storage of the regeneration flow.

[0062] These embodiments make it possible to make the implementation of the process more flexible, particularly with a view to pre-filling the temperature exchangers before they are put into operation.

[0063] In embodiments, the method which is the subject of the present invention comprises, downstream of a regeneration step, a step of pre-filling a regenerated heat exchanger.

[0064] These embodiments make it possible to condition a regenerated heat exchanger before this exchanger is used for the collection of impurities.

[0065] In embodiments, the method which is the subject of the present invention comprises a step of substituting a heat exchanger in parallel with a step of regenerating another heat exchanger.

[0066] According to a second aspect, the present invention relates to a device for liquid phase purification of a gas flow to achieve a molar rate of impurity, called "Xj mp ,out”, associated with a solubility limit of this impurity in the liquid gas flow when this flow reaches saturation at an outlet pressure, called “P ou t”, which includes:

[0067] - a means (205) for entering a gas flow comprising traces of impurities according to a molar rate Xjmp n, the gas flow having an inlet temperature, called "Tin", and an inlet pressure, called "Pj n ", the pressure torque Pj n and Tj n being configured to locate the gas flow in the vapor phase,

[0068] - a means (210) for compressing the flow of gas entered at least according to a pressure, called "Phaute", which corresponds to the lowest pressure for which the impurity according to the rate Xi mp ,in is soluble, for a temperature equal to the dew point of a flow of pure gas without equivalent impurities, at the high pressure P,

[0069] - a heat exchanger (515, 516) for cooling, according to the pressure Phaute, the compressed gas flow to an outlet temperature, called "Tout", so as to make the impurity capturable in the form of solid impurities, and to capture the solid impurities, the temperature T ou t being selected so that, at the pressure torque P ou t and temperature T ou t, the remaining impurity at the molar rate of impurity Xj mp ,everything in the gas stream is soluble and an equivalent pure gas stream without impurities is completely liquefied,

[0070] - a means (230) of relaxation, according to the outlet pressure P ou t of the gas flow in liquid phase. The device which is the subject of the present invention has the same advantages as the method which is the subject of the present invention.

[0071] BRIEF DESCRIPTION OF THE FIGURES

[0072] Other advantages, aims and particular characteristics of the invention will emerge from the following non-limiting description of at least one particular embodiment of the method and device which are the subject of the present invention, with reference to the appended drawings, in which:

[0073] Figure 1 represents, schematically, and in the form of a flowchart, a first particular succession of steps of the method which is the subject of the present invention,

[0074] Figure 2 schematically represents a first particular embodiment of the device which is the subject of the present invention,

[0075] Figure 3 represents, schematically, and in the form of a flowchart, a second particular succession of steps of the method which is the subject of the present invention,

[0076] Figure 4 schematically represents a graph showing curves representing the transition from vapor to liquid as a function of pressure and temperature for a pure gas and a gas containing a defined level of impurities,

[0077] Figure 5 schematically represents a second particular embodiment of the device which is the subject of the present invention and

[0078] Figure 6 schematically represents a particular embodiment of a heat exchanger regeneration device implemented by a device which is the subject of the present invention.

[0079] DESCRIPTION OF EMBODIMENTS

[0080] This description is given without limitation, each characteristic of an embodiment being able to be combined with any other characteristic of any other embodiment in an advantageous manner.

[0081] Please note that the figures are not to scale.

[0082] As understood from the present description, various inventive concepts may be implemented by one or more methods or devices described below, several examples of which are provided herein. The actions or steps performed in carrying out the method or device may be ordered in any suitable manner. Accordingly, it is possible to construct embodiments in which the actions or steps are performed in a different order than illustrated, which may include performing certain acts simultaneously, even if they are presented as sequential acts in the illustrated embodiments.

[0083] The expression "and / or", as used herein and in the claims, is to be understood to mean "either or both" of the elements so conjoined, i.e., elements which are present conjunctively in some cases and disjunctively in other cases. Multiple elements listed with "and / or" are to be interpreted in the same way, i.e., "one or more" of the elements so conjoined. Other elements may optionally be present, other than the elements specifically identified by the "and / or" clause, whether or not they are related to these specifically identified elements.Thus, by way of non-limiting example, a reference to "A and / or B", when used in conjunction with open language such as "comprising" may refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

[0084] As used in this specification and in the claims, the expression "at least one", with reference to a list of one or more elements, is to be understood to mean at least one element selected from one or more elements in the list of elements, but not necessarily including at least one of each element specifically listed in the list of elements and not excluding every combination of elements in the list of elements. This definition also allows for the optional presence of elements other than the specifically identified elements in the list of elements to which the expression "at least one" refers, whether or not related to those specifically identified elements.Thus, by way of non-limiting example, "at least one of A and B" (or, equivalently, "at least one of A or B", or, equivalently, "at least one of A and / or B") may refer, in one embodiment, to at least one, optionally including more than one, A, without B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, without A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

[0085] In the claims, as well as in the description below, all transitional expressions such as "comprising", "including", "carrying", "having", "containing", "involving", "holding", "consisting of", and the like, are to be understood as open, i.e., as meaning including but not limited to. Only the transitional expressions "consisting of" and "consisting essentially of" are to be understood as closed or semi-closed transitional expressions, respectively.

[0086] The term "gas flow containing traces of impurities" refers to the set of solvent-liquid pairs that meet all of the following criteria:

[0087] - a gas flow with impurity whose pressure is lower than a maximum pressure called “Pmax” corresponding to the critical pressure of pure gas (i.e. without impurities) with a margin of one bar at most,

[0088] - any gas containing an impurity in trace form, i.e. with an initial molar impurity rate lower than a rate Ximp.max, this rate Xi mp ,max corresponding to the solubility limit of the impurity in the gas, at the pressure ma x and at a temperature called “T ma x » corresponding to the dew point temperature of the pure gas (i.e. without impurities at pressure P ma x in practice, this rate is therefore less than 5%mol (Xj mp , m ax) and preferably less than 3% mol and

[0089] - the evolution of the solubility of this impurity in the solvent is such that for a given concentration of impurity Xi mp < Ximp.max: - in the pure vapor phase of the solvent, the solubility limit temperature of this impurity at isoconcentration Xjmp corresponds to an “SVE domain” (for “Solid-Vapor Equilibrium”, translated as solid-vapor equilibrium),

[0090] - in the liquid phase of the solvent, the solubility limit temperature of this impurity at isoconcentration Xjmp is constant to within plus or minus one degree Celsius depending on the pressure; in other words, in the liquid phase of the solvent, the solubility of the impurity depends almost exclusively on the temperature (to within a few thousand Pascals); the term "SLE domain" (for "Solid-Liquid Equilibrium") is also used,

[0091] - for a given pressure P (lower than the critical pressure of the solvent), in the zone close to the liquid-vapor equilibrium point of the pure solvent (typically, less than 5°C and less than 1 bar around this point), the temperature TsLVE(Xj mp ) of the solubility limit of the impurity (therefore for the solvent in the two-phase state at this pressure P) at the concentration Xi mp is lower than the temperature TsLE(Xj mp) corresponding to the solubility limit temperature when the solvent is completely in the liquid phase of the impurity at pressure P and concentration Xj mp (in practice, the solubility limit in the liquid phase does not depend on the value of the pressure P) - in practice, the conditions P and Xj mp are chosen so that this difference is at least 5°C,

[0092] - there is a maximum pressure, called “P ma x », equal (to within 1 bar) to the critical pressure of the pure gas flow (without impurities) defining a pressure range with the upper value P ma x and for value less than at least 1 bara,

[0093] - at the solubility limit of the impurity in the gas flow, at pressure Pmax and at a temperature equivalent to the dew point temperature of the pure gas flow (therefore without impurity) at pressure Pmax.

[0094] In a non-limiting manner, gas flows thus considered can be:

[0095] - air gases,

[0096] - carbon dioxide and

[0097] - natural gas and biomethane,

[0098] A "pure gas" is a gas flow containing only a given compound.

[0099] The general concept of the present invention is shown in Figure 4.

[0100] We observe, in this figure 4:

[0101] - a 405 bubble curve for a pure solvent, i.e. for a gas containing no impurities,

[0102] - a curve 410 representing the pressure and temperature conditions constituting the vapor phase solubility limit (VPL) of the impurity at the molar fraction Xj mp ,in in the solvent,

[0103] - a curve 415 representing the pressure and temperature conditions constituting the liquid phase solubility limit (SLE) of the impurity at the molar fraction Xj mp, in the solvent,

[0104] - a curve 420 representing the pressure and temperature conditions constituting the liquid phase solubility limit (SLE) of the impurity at the molar fraction Xj mp , out in the solvent,

[0105] - a point “A”, designating an equilibrium point of a gas flow at the inlet of a process which is the subject of the present invention comprising traces of impurities according to a predetermined rate Xi mp ,in, according to an inlet pressure Pj n and an inlet temperature Tin, - a point "B", reached by compression then cooling of the inlet gas flow (the compression generating an undesirable heating of the gas), designating an equilibrium point according to a pressure, called "Phaute", higher than the pressure necessary so that, at the same time, a theoretical pure gas flow and the gas flow comprising the predetermined molar rate of impurities Xj mp ,i nswitch from a vapor state to a liquid state, in other words, according to a pressure greater than or equal to the pressure corresponding to the intersection of the boiling curves of the gas in the pure state 405, on the one hand, and of the gas containing traces of impurities 410, on the other hand,

[0106] - optionally, a point “C”, reached by cooling to the gaseous state, of the gas containing traces of impurities, designating an equilibrium point according to the pressure applied at point B and according to a temperature, called “Ti”, equal to the dew point temperature of the gas in the pure state,

[0107] - a point "D'", reached by cooling to the liquid state, of the gas containing traces of impurities, designating a point of equilibrium according to the pressure applied at points B and C and according to a temperature for which the gas in the pure state is in the liquid state and the impurities contained in the gas containing traces of impurities begin to crystallize (solubility limit of the impurity reached), which does not assume that all the impurities are solid but at least some,

[0108] - a point “G”, reached by expansion of the purified gas, designating an equilibrium point according to an outlet pressure P ou t and a temperature Tout, chosen so that the purified gas is in the liquid state,

[0109] - optionally, the transition from states “B” to “G” can be achieved as follows:

[0110] - a first cooling in the liquid state, from point “B” or “C” to a point “D”, of the gas containing traces of impurities, designating an equilibrium point according to the pressure applied at points B and C and according to a temperature, called “T2”, higher than the crystallization temperature of the impurities and for which, for the chosen temperature, the gas in the pure state and the gas containing impurities are in the liquid state,

[0111] - an intermediate expansion in the liquid state, towards a point “E”, of the gas containing traces of impurities, according to the temperature T2 or according to a temperature close to the temperature T2 at the target pressure conditions, and according to a target pressure, called “PSLE”, lower than the Phaute pressure, for which the gas in the pure state and the gas containing impurities are in the liquid state,

[0112] - a second cooling to the liquid state, towards a point “F”, of the gas containing traces of impurities, according to a temperature close to the outlet temperature Tout and at the pressure PSLE, for which at least part of the impurities in concentration greater than Xj mp ,everything is in the solid state and the purified gas in the liquid state and

[0113] - relaxation towards the “G” point, as mentioned above.

[0114] Figure 1, which is not to scale, shows a schematic view of an embodiment of the method 100 which is the subject of the present invention. This method of liquid phase purification of a gas flow to achieve a molar rate of impurity, called "Xj mp ,out”, associated with a solubility limit of this impurity in the liquid gas flow when this flow reaches saturation at an outlet pressure, called “P ou t”, includes:

[0115] - a step 105 of entry of a gas flow comprising traces of impurities according to a molar rate Xj mp ,i n , the gas flow having an inlet temperature, called “Tin”, and an inlet pressure, called “Pin”, the pressure couple Pin and Tin being configured to locate the gas flow in the vapor phase,

[0116] - a step 110 of compression of the flow of gas entered at least according to a pressure, called "Phaute", which corresponds to the lowest pressure for which the impurity according to the rate Xj mp ,in is soluble, for a temperature equal to the dew point of a flow of pure gas without impurities equivalent, at the pressure Phaute,

[0117] - a step 11 1 of cooling, according to the pressure Phaute, of the compressed gas flow to an outlet temperature, called "T ou t", so as to make the impurity capturable in the form of solid impurities, the temperature T out being selected so that, at the pressure torque P ou t and temperature Tout, the remaining impurity at the molar rate of impurity Xi mp ,everything in the gas stream is soluble and an equivalent pure gas stream without impurities is completely liquefied,

[0118] - a step 125 for capturing solid impurities and

[0119] - a 130 relaxation step, depending on the outlet pressure P ou t of the gas flow in liquid phase.

[0120] The method 100 shown in Figure 1 represents a minimalist embodiment of the method which is the subject of the present invention, this method 100 comprising a limited number of steps.

[0121] The step 105 of inputting the gas flow comprising traces of impurities is carried out, for example, by implementing a pipe configured to be connected to a storage of gas to be purified or to, directly or indirectly, a mass purification device, configured to remove most of the impurities from the gas flow. Such a mass purification device is widely known in the field of gas production, particularly gas in the liquid state. The nature of the mass purification device depends on the impurity(ies) to be separated from the solvent.

[0122] As understood, at the end of step 105, the following are known and predetermined:

[0123] - the inlet pressure of the gas containing impurities, Pj n ,

[0124] - the inlet temperature of the gas containing impurities, Tin and

[0125] - the molar rate of impurities at the inlet of the gas containing impurities, Ximp n.

[0126] The pressure torque Pj n and temperature Tj n being such that the solvent and impurities are in the vapor state.

[0127] The compression step 110 is carried out by any type of compressor ordinarily used in gas treatment processes and, preferably, in gas liquefaction processes. The exact nature of the compressor depends on the gas to be liquefied.

[0128] As understood, at the end of compression step 110, the gas containing impurities has the following characteristics:

[0129] - the temperature of the gas containing impurities is, involuntarily and due to compression, slightly higher than Tin,

[0130] - the molar rate of impurities in the gas containing impurities is unchanged and equal to Xj mp ,in and

[0131] - the pressure of the gas containing impurities is increased to a value called “Phaute” greater than Pj n and less than P ma x, selected so that at temperature Tin and at least at pressure Phaute, for the molar rate of impurity Xi mp ,in, the impurities and the solvent are in the vapor phase. The cooling step 1 1 1 is carried out, for example, by any heat exchanger ordinarily used in gas treatment processes and, preferably, in gas liquefaction processes. The exact nature of this heat exchanger depends on the gas to be liquefied.

[0132] As understood, at the end of cooling step 1 11, the gas containing impurities has the following characteristics:

[0133] - the pressure of the gas containing impurities is equal to the pressure Phaute,

[0134] - the temperature of the gas containing impurities, called “T out », is chosen so that at pressure Phaute, the solvent is in liquid phase and close, then, to pure gas and so that the impurities are in solid phase, the temperature Tout being thus chosen to be lower than the solid-liquid equilibrium temperature of the impurities at the molar rate of impurities Xj mp ,in and

[0135] - the molar rate of impurities in the gas falls and, to become the molar rate of impurities in the liquefied gas, called "Xi mp ,out ».

[0136] In variants, such as that shown in FIG. 1, the cooling step 111 comprises a vapor phase cooling step 115.

[0137] The cooling step 1 15 is carried out, for example, by any heat exchanger ordinarily used in gas treatment processes and, preferably, in gas liquefaction processes, for gases in the vapor phase. The exact nature of this heat exchanger depends on the gas to be liquefied.

[0138] As understood, at the end of the cooling step 115, the gas containing impurities has the following characteristics:

[0139] - the molar rate of impurities of the gas containing impurities is unchanged and equal to Xi mp ,in,

[0140] - the pressure of the gas containing impurities is equal to the pressure Phaute and

[0141] - the temperature of the gas containing impurities, called “Ti”, is chosen so that at the molar rate of impurity Xj mp,in and at the pressure Phaute, the pure gas, the solvent and the impurities are at the interface of their vapor-liquid equilibrium domains.

[0142] In variants, such as that shown in FIG. 1, the cooling step 111 comprises a step 120 of cooling the gas in liquid phase.

[0143] The cooling step 120 is carried out, for example, by any heat exchanger ordinarily implemented in gas treatment processes and, preferably, in gas liquefaction processes. The exact nature of this heat exchanger depends on the gas to be cooled in the liquid phase.

[0144] In variants, the cooling step 115 and the cooling step 120 are carried out in a unitary heat exchanger, these two cooling steps, 115 and 120, thus being combined.

[0145] As understood, at the end of the cooling step 120, the gas containing impurities has the following characteristics:

[0146] - the pressure of the gas containing impurities is equal to the pressure Phaute,

[0147] - the temperature of the gas containing impurities, called “T ou t », is chosen so that at pressure Phaute, the solvent is in liquid phase and close, then, to pure gas and so that the impurities are in solid phase, the temperature T ou t being thus chosen to be lower than the solid-liquid equilibrium temperature of the impurities at the molar rate of impurities Ximp n and

[0148] - the molar rate of impurities in the gas falls and, to become the molar rate of impurities in the liquefied gas, called "Xi mp ,out ».

[0149] The capture step 125 is carried out in conjunction with the liquid phase cooling step 120, by “frosting” the heat exchanger carrying out the cooling step 120.

[0150] The expansion step 130 is carried out by any type of expander commonly used in gas treatment processes and, preferably, in gas liquefaction processes. The exact nature of the expander depends on the gas to be liquefied. Such an expander is, for example, a Joule-Thomson valve or an expansion turbine.

[0151] As understood, at the end of the expansion step 130, the gas containing impurities has the following characteristics:

[0152] - the molar rate of impurities in the gas is equal to "Xi mp ,out »,

[0153] - the gas temperature is equal to an outlet temperature, called “T ou t”, or has a slightly lower value due to the relaxation and

[0154] - the gas pressure is equal to the outlet pressure, called “P ou t » , less than Phaute.

[0155] In preferred embodiments, such as the method 300 shown in FIG. 3, the cooling step 120 comprises:

[0156] - a first step 305 of cooling, according to the Phaute pressure, of the compressed gas flow, to a temperature:

[0157] - in which the gas in the pure state equivalent to the gas entered is in the liquid phase, and

[0158] - which is higher than the crystallization temperature of the impurities at the determined molar rate of impurity called Xj mp ,in of the cooled gas flow,

[0159] - a first stage 310 of expansion of the liquid gas flow to a pressure at least equal to a pressure, called “P me d”, according to which the pure gas without impurities equivalent to the entered gas is at its bubble point, and

[0160] - a second stage 315 of cooling in liquid phase, according to the pressure P me d, of the gas flow up to temperature T ou t.

[0161] The cooling step 305 is carried out, for example, by any heat exchanger ordinarily implemented in gas treatment processes and, preferably, in gas liquefaction processes. The exact nature of this heat exchanger depends on the gas to be cooled in the liquid phase.

[0162] As understood, at the end of the cooling step 305, the gas containing impurities has the following characteristics:

[0163] - the pressure of the gas containing impurities is equal to the pressure Phaute,

[0164] - the temperature of the gas containing impurities, called "T2", is chosen so that at pressure Phaute, the solvent and the impurities are in the liquid phase, the temperature T2 being thus chosen to be higher than the solid-liquid equilibrium temperature of the impurities at the molar rate of impurities Xjmp.in and - the molar rate of impurities of the gas containing impurities is thus unchanged and equal to jmp.in-

[0165] The intermediate expansion step 310 is carried out by any type of expander commonly used in gas treatment processes and, preferably, in gas liquefaction processes. The exact nature of the expander depends on the gas to be liquefied. Such an expander is, for example, a Joule-Thomson valve or an expansion turbine.

[0166] As understood, at the end of the intermediate expansion step 310, the gas containing impurities has the following characteristics:

[0167] - the gas temperature is equal to, or close to, T2, remaining as at the target pressure, the solvent and the impurities of the gas containing impurities remaining in the liquid phase,

[0168] - the molar rate of impurities in the gas containing impurities is thus unchanged and equal to Xjmpjn and

[0169] - the pressure of the gas containing impurities is reduced to the pressure P me d.

[0170] The cooling step 315 is carried out, for example, by any heat exchanger ordinarily implemented in gas treatment processes and, preferably, in gas liquefaction processes. The exact nature of this heat exchanger depends on the gas to be cooled in the liquid phase.

[0171] As understood, at the end of the cooling step 315, the gas containing impurities has the following characteristics:

[0172] - the pressure of the gas containing impurities is equal to the pressure P me d,

[0173] - the temperature of the gas containing impurities, called "All", is chosen so that at pressure P me d, the solvent is in liquid phase and close, then, to the pure gas and for the impurities to be in solid phase, the temperature T ou t being thus chosen to be lower than the solid-liquid equilibrium temperature of the impurities at the molar rate of impurities Xj mp ,in and

[0174] - the molar rate of impurities in the gas falls and, to become the molar rate of impurities in the liquefied gas "X im p, ou t ».

[0175] As understood, the method 300 which is the subject of the present invention can implement a plurality of successions of cooling and expansion steps, until the elimination of impurities from the gas flow.

[0176] This allows, in particular, the cooling steps to be carried out to selectively remove impurities from the solvent, depending on the crystallization temperatures of these impurities.

[0177] In the case where several impurities are present in the gas flow, the Phaute pressure considered is the maximum Phaute pressure for all the molar rates of the different impurities.

[0178] In particular embodiments, such as that shown in Figure 3, the method 300 which is the subject of the present invention comprises an additional expansion step 320, downstream of the expansion step 130, the liquid phase cooling step 120 being configured to cool the gas flow to a temperature such that at the outlet of the additional expansion step 320, the gas is in the liquid phase. The additional expansion step 320 is carried out by any type of expander ordinarily implemented in gas treatment processes and, preferably, in gas liquefaction processes. The exact nature of the expander depends on the liquid gas to be expanded. Such an expander is, for example, a Joule-Thomson valve or an expansion turbine.

[0179] In particular embodiments, such as that shown in FIG. 3, the method 300 which is the subject of the present invention comprises a step 325 of storing the expanded liquefied gas.

[0180] Storage step 325 is carried out, for example, by cryogenic storage operating at pressure and temperature conditions allowing the gas to be maintained in the liquid state.

[0181] In particular embodiments, such as that shown in FIG. 3, the heat exchanger is configured to carry out the liquid phase cooling step 120, the method comprising a step 330 of regeneration of the heat exchanger.

[0182] In such embodiments, when a heat exchanger is operated for a regeneration step 330, the gas inlet in the inlet step 105 is interrupted.

[0183] In variants, at least two heat exchangers are implemented, allowing continuous gas purification, and the supply of the gas to be cooled alternates between these at least two heat exchangers. This alternation is carried out periodically or depending on the operating state of the heat exchangers. Such an operating state can be determined based on the capture, by a sensor, of a value of a physical quantity representative of the operating state of the heat exchanger. Such a physical quantity is, for example, the operating temperature of the heat exchanger.

[0184] The regeneration step 330 is carried out, for example, by active or passive heating of the heat exchanger. Active heating is carried out, for example, by inserting a regeneration flow into the heat exchanger.

[0185] The regeneration step 330 may include a step (not shown) of discharging the liquid contained in the heat exchanger to be regenerated. This discharge may be carried out into the atmosphere or by recycling this liquid upstream of the process 300.

[0186] In such embodiments, such as that shown in FIG. 3, the method 300 which is the subject of the present invention comprises a step 335 of inserting a regeneration flow into the heat exchanger to be regenerated.

[0187] The regeneration flow can be any fluid with a temperature higher than the outlet temperature T2. For example, this temperature is at least 5°C higher than the outlet temperature T2.

[0188] In preferred embodiments, the regeneration stream has a composition comprising the same compounds as the gas stream to be purified.

[0189] For example, the regeneration flow is formed from gas containing impurities, in trace amounts or more present (before mass purification, for example).

[0190] In particular embodiments, such as that shown in FIG. 3, the method 300 which is the subject of the present invention comprises a step 340 of recycling the regeneration flow to a step 345 of mass purification of the gas flow, located upstream of the inlet step 105. This recycling step 340 is carried out, for example, by implementing a pipe for transporting the regeneration flow.

[0191] In particular embodiments, such as that shown in FIG. 3, the method 300 which is the subject of the present invention comprises, downstream of a regeneration step 330, a step 350 of temporary storage of the regeneration flow.

[0192] The storage step 350 is carried out, for example, by a storage tank adapted to the composition and operating conditions of the regeneration flow. The implementation of this storage step 350 allows, at the end of the regeneration (and before the regenerated exchanger switches to the “frosting” mode again), the carrying out of a step of both adjusting the pressure and adjusting the composition of the fluid in the exchanger.

[0193] In variants, after possible evacuation of the regeneration flow, one possibility then consists of filling the exchanger with liquefied gas which has not yet been purified (i.e. still containing the impurity with a rate of at most Xi mp ,in) so that the composition in the regenerated exchanger at the start of its use in purification corresponds to the conditions in nominal operation.

[0194] In particular embodiments, such as that shown in FIG. 3, the method 300 which is the subject of the present invention comprises, downstream of a regeneration step 330, a step 355 of pre-filling a regenerated heat exchanger.

[0195] The pre-filling step 355 is carried out, for example, by the implementation of an automaton or a control and command device, such as a computer for example, associated with a hydraulic circuit configured to provide a pre-filling flow to a regenerated heat exchanger. Such a flow is, for example, the regeneration flow or the gas flow comprising impurities to be purified.

[0196] In particular embodiments, such as that shown in FIG. 3, the method 300 which is the subject of the present invention comprises a step 360 of substituting a heat exchanger in parallel with a step 330 of regenerating another heat exchanger.

[0197] Substitution step 360 is carried out, for example, by the implementation of an automaton or a control and command device, such as a computer for example, associated with a hydraulic circuit configured to alternate the supply of gas comprising impurities to be purified between heat exchangers.

[0198] Figure 2 shows schematically a particular embodiment of the device 200 which is the subject of the present invention. This device 200 for liquid phase purification of a gas flow to achieve a molar rate of impurity, called “Xjmp.out”, associated with a solubility limit of this impurity in the liquid gas flow when this flow reaches saturation at an outlet pressure, called “P ou t”, includes:

[0199] - a step 205 of entry of a gas flow comprising traces of impurities according to a molar rate Ximp.in, the gas flow having an entry temperature, called “Tj n", and an inlet pressure, called "Pj n ", the pressure torque Pj n and Tj n being configured to locate the gas flow in the vapor phase,

[0200] - a step 210 of compressing the flow of gas entered at least according to a pressure, called "Phaute", which corresponds to the lowest pressure for which the impurity according to the rate Ximp n is soluble, for a temperature equal to the dew point of a flow of pure gas without impurities equivalent to the pressure Phaute,

[0201] - a step 21 1 of cooling, according to the pressure Phaute, of the compressed gas flow to an outlet temperature, called "T ou t », the temperature Tout being selected so that, at the pressure torque P ou t and temperature All, the remaining impurity in the gas stream is soluble and an equivalent pure gas stream without impurities is completely liquefied,

[0202] - a step 225 for capturing solid impurities and

[0203] - a 230 expansion step, depending on the outlet pressure P ou t of the gas flow in liquid phase.

[0204] Embodiments and variants of the characteristic means of the device 200 which is the subject of the present invention are described with reference to figures 1, 3, 5 and 6.

[0205] Figure 5 shows schematically a particular embodiment of the device 500 which is the subject of the present invention. In this device 500, the liquid phase cooling means 220 comprises:

[0206] - a first means 505 for cooling in liquid phase, according to the pressure Phaute, of the flow of compressed gas at least to a temperature, called “T2” for which:

[0207] - the gas in the pure state is in the liquid phase according to the pressure Phaute,

[0208] - the impurities of the cooled gas flow, according to the determined molar rate of impurity called Ximp n, are in liquid phase according to the pressure Phaute,

[0209] - a first means 510 of expansion, according to the temperature T2 of the liquid gas flow at least at a pressure, called “P me d”, for which:

[0210] - the gas in its pure state and

[0211] - the gas flow containing traces of impurities according to the determined molar rate of impurity called Xjmp, in, are in the liquid phase at temperature T2 and

[0212] - a second means 515 of cooling in liquid phase, according to the pressure P me d, of the compressed gas flow at most at a temperature Tout, according to which:

[0213] - the gas in the pure state is in the liquid phase according to the pressure Phaute,

[0214] - the impurities of the cooled gas flow, according to the determined molar rate of impurity called Ximpjn, are in solid phase according to the pressure Phaute.

[0215] We also observe, in figure 5, that the device 500 can also comprise:

[0216] - a means 215 for cooling the incoming gas, downstream of the compression means 210, to cool the gas to the temperature Tj n ,

[0217] - a means 525 of storing purified liquefied gas,

[0218] - a means 530 of permutation between a frosted heat exchanger 515 and a regenerated heat exchanger 516,

[0219] - a closed or open refrigeration cycle 535 cooperating with the heat exchange means 505 and / or

[0220] - a closed or open heating cycle 540 cooperating with a means of regenerating a heat exchanger 516, the cycle 540 corresponding to the capture means 225. For example, when the device 500 is implemented for a mixture comprising methane, called “CP”, as solvent and carbon dioxide, called “CO2”, as impurity, starting from a CO2 level of 1.8 mol% and to purify down to 0.04 mol%:

[0221] - at point A of figure 4 equivalent for this mixture, the mixture presents 1.8%mol of CO2, according to a temperature of 20°C and a pressure of 1.1 bara,

[0222] - at point B of figure 4 equivalent for this mixture, the mixture has a pressure of 35 bara (or 36 bara to take a margin linked to pressure losses) and a temperature of 20°C,

[0223] - at point D of figure 4 equivalent for this mixture, the mixture is liquefied and sub-cooled without risk of crystallization for a molar rate of 1.8%mol of CO2, down to -1.12°C and still 35 bara,

[0224] - at point E of figure 4 equivalent for this mixture, the mixture is expanded to approximately 17 bara,

[0225] - at point F of figure 4 equivalent for this mixture, the mixture is subcooled to -161 °C and 17 bara for example: the liquid is purified to at least 0.04%mol CO2 and

[0226] - at point G of figure 4 equivalent for this mixture, the mixture is expanded to 1 bara for final storage at the same pressure.

[0227] In Figure 6, we observe schematically a particular embodiment of a circuit 600 for regenerating a heat exchanger of a device, 200 or 500, which is the subject of the present invention. This regeneration circuit 600 comprises:

[0228] - a set of valves, allowing the selective supply of different parts of the circuit 600,

[0229] - an inlet 605 for regeneration flow, this flow being able to come from the gas to be purified, upstream or downstream of a mass purification device, this flow being intended to prefill a heat exchanger, 515 or 516, or to regenerate an exchanger, 515 or 516, to be regenerated,

[0230] - a circuit 540 for regenerating at least one heat exchanger, 515 and / or 516, the circulation of which in regeneration flow is controlled by the set of valves,

[0231] - a tank 610 for temporary storage of regeneration flow from the regeneration circuit 540 and / or

[0232] - an output 615 for regeneration flow from the regeneration circuit 540, for example for supply to a mass purification device positioned upstream of the device 500 which is the subject of the present invention.

Claims

CLAIMS 1. Method (100, 300) for liquid phase purification of a gas flow to achieve a molar rate of impurity, called “Xj mp ,out”, associated with a solubility limit of this impurity in the liquid gas flow when this flow reaches saturation at an outlet pressure, called “P ou t”, including: - a step (105) of entry of a gas flow comprising traces of impurity according to a molar rate Xjmp n, the gas flow having an entry temperature, called “Tin”, and an entry pressure, called “Pj n ", the pressure torque Pj n and Tj n being configured to locate the gas flow in the vapor phase, - a step (1 10) of compressing the flow of gas entered at least according to a pressure, called "Phaute", which corresponds to the lowest pressure for which the impurity according to the rate Xjmp n is soluble, for a temperature equal to the dew point temperature of a flow of pure gas without impurities equivalent to the pressure Phaute, - a step (1 11) of cooling, according to the pressure Phaute, the flow of compressed gas to an outlet temperature, called “T ou t", in a heat exchanger (515, 516), so as to make the impurity capturable in the form of solid impurities, and capture of the solid impurities in the heat exchanger (515, 516), the temperature T ou t being selected so that, at the pressure torque P ou t and temperature Tout, the remaining impurity at the molar rate of impurity Xi mp ,everything in the gas stream is soluble and an equivalent pure gas stream without impurities is completely liquefied, and - a stage (130) of relaxation, according to the outlet pressure P ou t of the gas flow in liquid phase.

2. Method (100, 300) according to claim 1, in which the cooling step (11 1) comprises a step (115) of cooling in vapor phase to the temperature Tj n .

3. Method (300) according to one of claims 1 or 2, in which the cooling step (1 1 1) comprises: - a first step (305) of cooling, according to the Phaute pressure, of the compressed gas flow, to a temperature: - in which the gas in the pure state equivalent to the gas entered is in the liquid phase, and - which is higher than the crystallization temperature of the impurities at the determined molar rate of impurity called Xi mp ,in of the cooled gas flow, - a first step (310) of expansion of the liquid gas flow to a pressure at least equal to a pressure, called “Pmed”, according to which the gas in the pure state without impurities equivalent to the gas entered is at its bubble point, and - a second stage (315) of cooling in the liquid phase, according to the pressure P me d, of the gas flow up to temperature T ou t.

4. Method (300) according to one of claims 1 to 3, which comprises an additional expansion step (320), downstream of the expansion step (130), the cooling step (120) in phase liquid being configured to cool the gas flow to a temperature such that at the outlet of the additional expansion stage, the gas is in the liquid phase.

5. Method (300) according to one of claims 1 to 4, which comprises a step (325) of storing the expanded liquefied gas.

6. Method (300) according to one of claims 1 to 5, in which the heat exchanger is configured to carry out the step (120) of cooling in the liquid phase, the method comprising a step (330) of regeneration of the heat exchanger.

7. Method (300) according to claim 6, in which the regeneration step (330) comprises a step (335) of inserting a regeneration flow into the heat exchanger to be regenerated.

8. Method (300) according to claim 7, in which the regeneration flow has a composition comprising the same compounds as the gas flow to be purified.

9. Method (300) according to one of claims 7 or 8, which comprises a step (340) of recycling the regeneration flow to a step (345) of mass purification of the gas flow, located upstream of the inlet step (105).

10. Method (300) according to one of claims 7 to 9, which comprises, downstream of a regeneration step (330), a step (350) of temporary storage of the regeneration flow.

11. Method (300) according to one of claims 7 to 10, which comprises, downstream of a regeneration step (330), a step (355) of pre-filling a regenerated heat exchanger.

12. Method (300) according to one of claims 7 to 11, which comprises a step (360) of substituting a heat exchanger in parallel with a step (330) of regenerating another heat exchanger.

13. Method (100, 300) according to one of claims 1 to 12, in which the gas flow is a flow of a mixture chosen from: - a first mixture comprising methane as solvent in which the impurity is chosen from carbon dioxide and water, - a second mixture comprising nitrogen (N2) and / or oxygen (O2) as solvent or solvents in which the impurity is chosen from carbon dioxide and water, and - a third mixture comprising carbon dioxide as a solvent in which the impurity is water.

14. Method (100, 300) according to one of claims 1 to 13, implemented for a mixture comprising methane as solvent and carbon dioxide as impurity.

15. Device (200) for liquid phase purification of a gas flow to achieve a molar rate of impurity, called “Xj mp ,out”, associated with a solubility limit of this impurity in the liquid gas flow when this flow reaches saturation at an outlet pressure, called “P ou t”, which includes: - a means (205) for entering a gas flow comprising traces of impurities according to a molar rate Xjmp n, the gas flow having an inlet temperature, called "Tin", and an inlet pressure, called "Pj n ", the pressure torque Pj n and Tj n being configured to locate the gas flow in the vapor phase, - a means (210) for compressing the flow of gas entered at least according to a pressure, called "Phaute", which corresponds to the lowest pressure for which the impurity according to the rate Ximp n is soluble, for a temperature equal to the dew point temperature of a flow of pure gas without impurities equivalent to the pressure Phaute, - a heat exchanger (515, 516) for cooling, depending on the pressure Phaute, the flow of compressed gas to an outlet temperature, called “T out", so as to make the impurity capturable in the form of solid impurities, and to capture solid impurities, the temperature Tout being selected so that, at the pressure couple P ou t and temperature Tout, the remaining impurity at the molar rate of impurity Xi mp ,everything in the gas stream is soluble and an equivalent pure gas stream without impurities is completely liquefied, - a means (230) of relaxation, according to the outlet pressure P ou t of the gas flow in liquid phase.

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