WO2024110449A1

WO2024110449A1 - Method and device for liquid-phase purification of trace impurities

Info

Publication number: WO2024110449A1
Application number: PCT/EP2023/082512
Authority: WO
Inventors: Laurent Benoit; Gaelle DUBOURG; Lynda BELKADI
Original assignee: Engie
Priority date: 2022-11-21
Filing date: 2023-11-21
Publication date: 2024-05-30
Also published as: FR3142104A1

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

DESCRIPTION

TITLE OF THE INVENTION: METHOD AND DEVICE FOR PURIFYING TRACES OF IMPURITIES IN LIQUID PHASE

TECHNICAL FIELD OF THE INVENTION

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

STATE OF THE TECHNIQUE

The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously designed or pursued. Therefore, unless otherwise noted, it should not be assumed that any of the approaches described in this section constitute prior art simply by virtue of their inclusion in this section.

The liquefaction of gases always requires extensive purification in order to avoid any crystallization of impurities (deposit of solid either in the exchanger carrying out the cooling or sub-cooling, 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”), that is to say a liquefied gas rich in methane, it is necessary to remove water to a level below 2 ppm to be able to store LNG without the risk of water ice forming; similarly, LNG cannot contain more than 50 ppm of carbon dioxide (“CO2”) at equilibrium atmospheric pressure without the risk of CO2 ice forming.

If we call “solvent” the fluid made up 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 the form of a trace 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 a thorough 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:

- methane solvent / water or CO2 impurities,

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

- CO2 solvent / water impurity.

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

- 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 as the degree of purity increases.

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 varying their solubility levels in the liquid (solubility which is directly controlled by the level of pressure and temperature). We can thus pool a certain amount of process equipment to reduce the overall cost of liquefied gas production. However, on the one hand, this technique has so far been based on processes which 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 stages of separation of impurities and condensation of the solvent, hence sometimes limited pooling of equipment.

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

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

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

- 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 drop in pressure and in particular using a vacuum pump, or by heat as for absorption,

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

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

- technologies where the formation of the solid impurity takes place 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 only useful 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 gases rich in methane, this separation is done at the final expansion at the end of subcooling liquefaction of the biomethane; the expansion causes the evaporation of 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 the US document 3,312,073, and

- in certain technologies, this three-phase state is caused by injecting liquid nitrogen into an air flow (mainly N2-O2 mixture) whose cooling causes the crystallization of the CO2.

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

The disadvantages of the previously mentioned polishing methods are:

- with regard to absorption polishing methods (amine washing): the small-scale investment cost (for biomethane polishing in particular) due to the complex boilermaking works 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 strong safety constraints when the heat comes from the combustion of part of the gas to be liquefied,

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

- with regard to polishing methods by cryogenic vapor phase 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; the problem arises here both in terms of design complexity, stability and lifespan - in fact, the system operating in batch, it alternates the phases of icing (therefore in cold) and defrosting (in hot), the fact of keeping the solvent vapor means that the stability of the flow is quickly impacted upon the formation of frost from the solid impurity (which gradually blocks the exchanger and therefore constrains the flow passage section), the phases icing and defrosting 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 necessary, therefore rare and complex and potentially expensive, to minimize these disadvantages and

- with regard to polishing methods by cryogenic separation using a three-phase route (impurity in solid form, solvent in both gaseous and liquid form): due to the presence of solvent in cold vapor, we find the same stability constraints flow 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 big problem with these cyclonic approaches is, beyond the complexity and cost of development, their lack of flexibility and therefore operational stability; the flow must precisely follow a very restricted operating range to allow good evacuation of solid particles. PRESENTATION OF THE INVENTION

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

To this end, according to a first aspect, the present invention aims at a process for purifying a gas flow in the liquid phase to achieve a molar level of impurity, called “Xj _mp , out”, associated with a solubility limit. of this impurity in the flow of liquid gas when this flow reaches saturation at an outlet pressure, called “P _or t”, comprising:

- a step of entering a gas flow comprising traces of impurity according to a molar rate Xjmp.in, the gas flow having an inlet temperature, called “Tj _n ”, and an inlet pressure, called “Pin”, the pressure couple Pj _n and Tj _n being configured to locate the gas flow in the vapor phase,

- 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 temperature of an equivalent pure gas flow without impurities, at high pressure,

- a step of cooling, according to the high pressure, the flow of compressed gas to an outlet temperature, called "T _or t", in a heat exchanger, so as to make the impurity captureable in the form of solid impurities, and capturing solid impurities in the heat exchanger, the temperature T _or t being selected so that, at the pair of pressure P _or t and temperature T _or t, the remaining impurity at the molar rate of impurity Xi _mp , out in the gas flow is soluble and an equivalent pure gas flow without impurities is completely liquefied, and

- an expansion step, depending on the outlet pressure P _or t of the gas flow in the liquid phase.

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

These provisions make it possible to obtain:

- a stable process using non-specific heat exchangers, with for example only a tolerance to the targeted operating pressure and a constant passage section,

- icing-defrosting periods greater than two hours and depending on the sizing of the exchangers which can even be much greater and

- 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 has thermal inertia, the latter further facilitates the speed of regeneration of the exchangers.

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 phenomenon of 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, 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 a restriction of passage section better and more easily and therefore can ice for much longer without destabilizing the flow and

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

In particular embodiments, the liquid phase purification process comprises a step which is a step of measuring a molar rate Xj _mp , in of an impurity in the gas flow and which precedes the compression step of the gas flow entered.

In particular embodiments, the cooling step comprises a vapor phase cooling step at the temperature Tj _n

In particular embodiments, the cooling step comprises:

- a first stage of cooling, according to the high pressure, of the compressed gas flow, to a temperature:

- in which the gas in its pure state equivalent to the gas entered is in the liquid phase, and

- which is greater than the crystallization temperature of the impurities at the determined molar rate of impurity called Xj _mp , in of the cooled gas flow,

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

- a second stage of cooling in the liquid phase, according to the pressure P _me d, of the gas flow up to the temperature All.

These embodiments make it possible to accelerate substitutions between heat exchangers in charge of impurity collection and heat exchangers undergoing regeneration. Indeed, the regeneration stages thus implemented consume less energy than in the variants presenting total expansion then cooling to the outlet conditions.

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 exit of the additional expansion stage, the gas is in the liquid phase.

These embodiments make it possible to guarantee the liquid state of the liquefied gas leaving the process.

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

In embodiments, the heat exchanger is configured to carry out the cooling step in the liquid phase, the method comprising a step of regenerating the heat exchanger.

These embodiments make it possible to implement the method continuously.

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

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.

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.

In embodiments, the process which is the subject of the present invention comprises a step of recycling the regeneration flow towards a stage of mass purification of the gas flow, located upstream of the inlet stage.

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.

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.

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

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

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

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

According to a second aspect, the present invention aims at a device for purifying a gas flow in the liquid phase to achieve a molar level of impurity, called "Xj _mp , out", associated with a limit of solubility of this impurity in the flow of liquid gas when this flow reaches saturation at an outlet pressure, called “P _or t”, which includes:

- a means (205) for entering a gas flow comprising traces of impurities at a molar rate Xjmp n, the gas flow having an inlet temperature, called "Tin", and an inlet pressure , called “Pj _n ”, the pressure couple 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 "High", 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 temperature of an equivalent pure gas flow without impurities, at the high pressure P,

- a heat exchanger (515, 516) for cooling, according to the high pressure, the flow of compressed gas at an outlet temperature, called "All", so as to make the impurity captureable in the form of solid impurities, and for capturing solid impurities, the temperature T _or t being selected so that, at the pair of pressure P _or t and temperature T _or t, the remaining impurity at the molar rate of impurity Xj _mp , out in the gas flow is soluble and that an equivalent stream of pure gas without impurities is completely liquefied,

- an expansion means (230), depending on the outlet pressure P _or t of the gas flow in the 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.

BRIEF DESCRIPTION OF THE FIGURES

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 the device which are the subject of the present invention, with reference to the appended drawings, in which:

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

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

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

Figure 4 represents, schematically, a graph representing curves representative of the passage from vapor to liquid as a function of pressure and temperature for a pure gas and a gas containing a defined level of impurities,

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

Figure 6 represents, schematically, a particular embodiment of a device for regenerating heat exchangers implemented by a device which is the subject of the present invention.

DESCRIPTION OF THE EMBODIMENTS

The present description is given on a non-limiting basis, each characteristic of an embodiment being able to be combined with any other characteristic of any other embodiment in an advantageous manner.

Note now that the figures are not to scale.

As can be understood from reading this description, various inventive concepts can be implemented by one or more methods or devices described below, several examples of which are provided here. The actions or steps carried out in the context of carrying out the method or device may be ordered in any appropriate manner. Accordingly, it is possible to construct embodiments in which 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.

The expression "and/or", as used herein and in the claims, should be understood to mean "either or both" of the elements thus conjoined, i.e. that is, elements that are present conjunctively in some cases and disjunctively in other cases. Multiple elements listed with "and/or" must be interpreted in the same way, i.e. "one or more" of the elements thus conjoined. Other elements may possibly be present, other than those specifically identified by the clause "and/or", whether or not they are linked 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 ( possibly including elements other than B); in another embodiment, to B only (possibly including elements other than A); in yet another embodiment, to A and B (possibly including other elements); etc.

As used in the present description and in the claims, the expression "at least one", with reference to a list of one or more elements, must be understood as meaning at least one element chosen from one or more multiple items in the item list, but not necessarily including at least one of each item specifically listed in the item list and not excluding any combination of items in the item list. 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 they are linked to these 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 l '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, at least one, optionally comprising more than one, B, without A present (and optionally comprising elements other than A); in yet another embodiment, at least one, optionally comprising more than one, A, and at least one, optionally comprising more than one, B (and optionally comprising other elements); etc.

In the claims, as well as in the description below, all transitional expressions such as "comprising", "including", "carrying", "having", "containing", "involving", "holding", "composed of ", and others, must be understood as open, that is, 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.

We call “gas flow containing traces of impurities” the set of solvent-liquid couples verifying all of the following criteria:

- 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 at most one bar,

- any gas containing an impurity in trace form, that is to say with an initial molar rate of impurity lower than a rate Ximp.max, this rate Xi _mp ,max corresponding to the solubility limit of the impurity in the gas, at the _max pressure and at a temperature called “T _ma x” corresponding to the dew point temperature of the pure gas (i.e. without impurities at the 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

- 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),

- 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 only on the temperature (within a few thousand Pascals); we also use the term “SLE domain” (for “Solid-Liquid Equilibrium”, translated as solid-liquid balance),

- 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, at less than 5°C and less than 1 bar around this point ), the temperature TsLVE(Xj _mp ) of 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 the pressure P and at the concentration Xj _mp (in practice, the solubility limit in the liquid phase does not depend on the value of the pressure P) - in practical, the conditions P andXj _mp are chosen so that this difference is at least 5°C,

- there is a maximum pressure, called “P _ma x”, equal (to within 1 bar) to the critical pressure of the flow of pure gas (without impurities) defining a range of pressures with the upper value P _max and the lower value at least 1 bara,

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

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

- air gases,

- carbon dioxide and

- natural gas and biomethane,

“Pure gas” is a gas flow comprising exclusively a given compound.

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

We observe, in this figure 4:

- a bubble curve 405 for a pure solvent, that is to say for a gas containing no impurities,

- a curve 410 representing the pressure and temperature conditions constituting the vapor phase solubility limit (SVE) of the impurity at the mole fraction Xj _mp , in in the solvent,

- a curve 415 representing the pressure and temperature conditions constituting the solubility limit in the liquid phase (SLE) of the impurity at the mole fraction Xj _mp , in in the solvent,

- a curve 420 representing the pressure and temperature conditions constituting the solubility limit in the liquid phase (SLE) of the impurity at the mole fraction Xj _mp , out in the solvent,

- a point “A”, designating a point of equilibrium of a gas flow entering a process which is the subject of the present invention comprising traces of impurities at a predetermined rate Xi _mp , in, at a pressure of input Pj _n and an input temperature Tin, - a point “B”, reached by compression then cooling of the inlet gas flow (compression generating undesirable heating of the gas), designating a point of equilibrium according to a pressure, called “High”, greater than the pressure necessary for _that , at the same time, a flow of theoretical pure gas and the flow of gas comprising the predetermined molar rate of _impurities equal to the pressure corresponding to the crossing of the boiling curves of the gas in its pure state 405, on the one hand, and of the gas containing traces of impurities 410, on the other hand,

- optionally, a point "C", reached by cooling to the gaseous state, the gas containing traces of impurities, designating a point of equilibrium according to the pressure implemented at point B and according to a temperature, called "Ti », equal to the dew point temperature of the gas in its pure state,

- a point "D'", reached by cooling to the liquid state, the gas containing traces of impurities, designating a point of equilibrium according to the pressure implemented 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 of the impurities are solid but at least a part,

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

- optionally, the transition from states “B” to “G” can be done as follows:

- a first cooling to the liquid state, from point “B” or “C” to a point “D”, of the gas containing traces of impurities, designating a point of equilibrium according to the pressure implemented 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,

- 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 high pressure, for which the gas in the pure state and the gas containing impurities are in the liquid state,

- a second cooling to the liquid state, towards a point “F”, of the gas containing traces of impurities, at 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 , out is in the solid state and the purified gas in the liquid state and

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

We observe, in Figure 1, which is not to scale, a schematic view of an embodiment of the method 100 which is the subject of the present invention. This process of purifying a gas flow in the liquid phase to reach a molar rate of impurity, called “Xj _mp , out”, associated with a limit of solubility of this impurity in the liquid gas flow when this flow reaches saturation at an outlet pressure, called “P _or t”, includes:

- a step 105 of entering _a gas flow comprising traces of impurities at a _molar rate input, called “Pin”, the pressure couple Pin and Tin being configured to locate the gas flow in the vapor phase,

- a step 110 of compressing the flow of gas entered at least according to a pressure, called "High", 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 temperature of a flow of pure gas without equivalent impurities, at high pressure,

- a step 11 1 of cooling, according to the pressure Phighe, the flow of compressed gas to an outlet temperature, called "T _or t", so as to make the impurity captureable in the form of solid impurities, the temperature T _or t being selected so that, at the pair of pressure P _or t and temperature Tout, the impurity remaining at the molar rate of impurity Xi _mp ,out in the gas flow is soluble and that a flow of pure gas without impurities equivalent is completely liquefied,

- a step 125 for capturing solid impurities and

- an expansion step 130, depending on the outlet pressure P _or t of the gas flow in the liquid phase.

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

Step 105 of entering the gas flow containing traces of impurities is carried out, for example, by the implementation of 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(s) to be separated from the solvent.

As can be understood, at the end of step 105 input, the following are known and predetermined:

- the inlet pressure of the gas containing impurities, Pj _n ,

- the inlet temperature of the gas containing impurities, Tin and

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

The couple of pressure Pj _n and temperature Tj _n being such that the solvent and the impurities are in the vapor state.

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.

As can be understood, at the end of the compression step 1 10, the gas containing impurities has the following characteristics:

- the temperature of the gas containing impurities is, unintentionally and due to compression, slightly higher than Tin,

- the molar rate of impurities of the gas containing impurities is unchanged and equal to Xj _mp , in and

- the pressure of the gas containing impurities is increased to a value called “Phaute” greater than Pj _n and less than P _max , selected so that at the temperature Tin and at least at the pressure Phaute, for the molar rate d '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.

As can be understood, at the end of the cooling step 1 11, the gas containing impurities has the following characteristics:

- the pressure of the gas containing impurities is equal to the high pressure,

- the temperature of the gas containing impurities, called “T _or t”, is chosen so that at the high pressure, the solvent is in the liquid phase and close, then, to the pure gas and so that the impurities are in the 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

- the molar rate of impurities in the gas falls and, to become the molar rate of impurities in the liquefied gas, says “Xi _mp , out”.

In variants, such as that shown in Figure 1, the cooling step 1 1 1 comprises a vapor phase cooling step 115.

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.

As can be understood, at the end of cooling step 1 15, the gas containing impurities has the following characteristics:

- the molar rate of impurities of the gas containing impurities is unchanged and equal to Xi _mp ,in,

- the pressure of the gas containing impurities is equal to the high pressure and

- 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 Phigh, the pure gas, the solvent and the impurities are at the interface of their vapor liquid equilibrium domains.

In variants, such as that shown in Figure 1, the cooling step 1 1 1 comprises a step 120 of cooling the gas in the liquid phase.

The cooling step 120 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 cooled in the liquid phase.

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.

As can be understood, at the end of the cooling step 120, the gas containing impurities has the following characteristics:

- the pressure of the gas containing impurities is equal to the high pressure,

- the temperature of the gas containing impurities, called "T _or t", is chosen so that at the high pressure, the solvent is in the liquid phase and close, then, to the pure gas and so that the impurities are in solid phase, the temperature T _or t thus being chosen to be lower than the solid liquid equilibrium temperature of the impurities at the molar rate of impurities Ximp n and

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.

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

As can be understood, at the end of the expansion step 130, the gas containing impurities has the following characteristics:

- the molar rate of impurities in the gas is equal to “Xi _mp ,out”,

- the gas temperature is equal to an outlet temperature, called “T _or t”, or has a slightly lower value due to expansion and

- the gas pressure is equal to the outlet pressure, called “P _or t”, less than Phaute.

In preferred embodiments, such as the method 300 shown in Figure 3, the cooling step 120 comprises:

- a first step 305 of cooling, according to the high pressure, the flow of compressed gas, to a temperature:

- a first step 310 of expanding the flow of liquid gas to a pressure at least equal to a pressure, called "P _me d", according to which the gas in its pure state without impurities equivalent to the gas entered is at its bubble point, and

- a second step 315 of cooling in the liquid phase, according to the pressure P _me d, of the gas flow up to the temperature T _or t.

The cooling step 305 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 cooled in the liquid phase.

As can be understood, at the end of the cooling step 305, the gas containing impurities has the following characteristics:

- the pressure of the gas containing impurities is equal to the high pressure,

- the temperature of the gas containing impurities, called “T2”, is chosen so that at the high pressure, the solvent and the impurities are in the liquid phase, the temperature T2 thus being chosen to be higher than the solid equilibrium temperature liquid 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-

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

As can be understood, at the end of intermediate expansion step 310, the gas containing impurities has the following characteristics:

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

- the molar rate of impurities of the gas containing impurities is thus unchanged and equal to Xjmpjn and

- the pressure of the gas containing impurities is reduced to the pressure P _me d.

The cooling step 315 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 cooled in the liquid phase.

As can be understood, at the end of the cooling step 315, the gas containing impurities has the following characteristics:

- the pressure of the gas containing impurities is equal to the pressure P _me d,

- the temperature of the gas containing impurities, called "All", is chosen so that at the pressure P _me d, the solvent is in the liquid phase and close, then, to the pure gas and so that the impurities are in the solid phase, the temperature T _or 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

- the molar rate of impurities in the gas falls and, to become the molar rate of impurities in the liquefied gas “X _im p, _or t”.

As can be understood, the process 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.

This makes it possible, in particular, to carry out the cooling steps to selectively remove impurities from the solvent, depending on the crystallization temperatures of these impurities.

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

In particular embodiments, such as that shown in Figure 3, the process 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 used in gas treatment processes and, preferably, in gas liquefaction processes. The exact nature of the regulator depends on the liquid gas to be expanded. Such a regulator is, for example, a Joule-Thomson valve or an expansion turbine.

In particular embodiments, such as that shown in Figure 3, the process 300 which is the subject of the present invention comprises a step 325 of storing the expanded liquefied gas.

The 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.

In particular embodiments, such as that shown in Figure 3, the heat exchanger is configured to carry out the liquid phase cooling step 120, the process comprising a step 330 of regenerating the heat exchanger.

In such embodiments, when a heat exchanger is used for a regeneration step 330, the entry of gas into the inlet step 105 is interrupted.

In variants, at least two heat exchangers are implemented, allowing the purification of the gas continuously, 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.

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

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

In such embodiments, such as that shown in Figure 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.

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

In preferred embodiments, the regeneration flow has a composition comprising the same compounds as the gas flow to be purified.

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

In particular embodiments, such as that shown in Figure 3, the process 300 which is the subject of the present invention comprises a step 340 of recycling the regeneration flow towards a step 345 of mass purification of the gas flow, located upstream of entry step 105. This recycling step 340 is carried out, for example, by implementing a pipe for transporting the regeneration flow.

In particular embodiments, such as that shown in Figure 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.

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 switching to the “frosting” mode again of the regenerated exchanger), the realization of one step at a time of adjustment pressure and adjustment of the composition of the fluid in the exchanger.

In variants, after possible evacuation of the regeneration flow, one possibility then consists of filling the exchanger with liquefied gas not yet purified (that is to say 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.

In particular embodiments, such as that shown in Figure 3, the process 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.

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 flow pre-filling to a regenerated heat exchanger. Such a flow is, for example, the regeneration flow or the gas flow containing impurities to be purified.

In particular embodiments, such as that shown in Figure 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.

The 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 containing impurities to be purified between heat exchangers.

We observe, in Figure 2, schematically, a particular embodiment of the device 200 which is the subject of the present invention. This device 200 for purifying a gas flow in the liquid phase to reach a molar rate of impurity, called "Xjmp.out", associated with a limit of solubility of this impurity in the liquid gas flow when this flow reaches saturation at an outlet pressure, called “P _or t”, includes:

- a step 205 of entering 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 entry pressure , called “Pj _n ”, the pressure couple Pj _n and Tj _n being configured to locate the gas flow in the vapor phase,

- a step 210 of compressing the gas flow entered at least according to a pressure, called "High", which corresponds to the lowest pressure for which the impurity according to the rate soluble, for a temperature equal to the dew point temperature of an equivalent pure gas flow without impurities, at high pressure,

- a step 21 1 of cooling, according to the pressure Phighe, the flow of compressed gas to an outlet temperature, called "T _or t", the temperature Tout being selected so that, at the pair of pressure P _or t and temperature Tout , the remaining impurity in the gas flow is soluble and an equivalent pure gas flow without impurities is completely liquefied,

- a step 225 for capturing solid impurities and

- an expansion step 230, depending on the outlet pressure P _or t of the gas flow in the liquid phase.

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

We observe, in Figure 5, 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:

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

- the gas in its pure state is in the liquid phase depending on the high pressure,

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

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

- the gas in its pure state and

- 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

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

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

We also observe, in Figure 5, that the device 500 can also include:

- a means 215 for cooling the gas entered, downstream of the compression means 210, to cool the gas to the temperature Tj _n ,

- a means 525 for storing purified liquefied gas,

- a means 530 for permutation between a frosted heat exchanger 515 and a regenerated heat exchanger 516,

- a cycle 535, closed or open, of refrigeration cooperating with the means 505 of heat exchange and/or

- a cycle 540, closed or open, of heating cooperating with a means of regeneration of a heat exchanger 516, the cycle 540 corresponding to the capture means 225. For example, when the device 500 is used for a mixture comprising methane, called "CP", as a solvent and carbon dioxide, called "CO2", as an impurity, starting from a rate of CO2 of 1.8%mol and to purify up to 0.04%mol:

- at point A of Figure 4 equivalent for this mixture, the mixture presents 18% mol of CO2, at a temperature of 20°C and a pressure of 1.1 bara,

- 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,

- at point D of Figure 4 equivalent for this mixture, the mixture is liquefied and subcooled without risk of crystallization for a molar rate of 18% mol of CO2, up to -112°C and always 35 bara,

- at point E of Figure 4 equivalent for this mixture, the mixture is relaxed up to approximately 17 bara,

- 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

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

We observe, in Figure 6, schematically, a particular embodiment of a circuit 600 for regenerating a heat exchanger of a device, 200 or 500, object of the present invention. This regeneration circuit 600 includes:

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

- an inlet 605 for the 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 thermal exchanger, 515 or 516, or to regenerate an exchanger, 515 or 516, to be regenerated,

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

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

- an outlet 615 for the regeneration flow coming 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

1. Process (100, 300) for purifying a gas flow in the liquid phase to achieve a molar rate of impurity, called "Xj _mp , out", associated with a solubility limit of this impurity in the flow of liquid gas when this flow reaches saturation at an outlet pressure, called “P _or t”, comprising:

- a step (105) of entering a gas flow comprising traces of impurity 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 couple 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 "High", which corresponds to the lowest pressure for which the impurity according to the rate Xjmp n is soluble, for an equal temperature at the dew point temperature of an equivalent pure gas flow without impurities, at high pressure,

- a step (1 11) of cooling, according to the high pressure, the flow of compressed gas to an outlet temperature, called "T _or t", in a heat exchanger (515, 516), so as to produce impurity capturable in the form of solid impurities, and capture of the solid impurities in the heat exchanger (515, 516), the temperature T _or t being selected so that, at the pair of pressure P _or t and temperature Tout, the remaining impurity at the molar rate of impurity Xi _mp ,out in the gas flow is soluble and an equivalent pure gas flow without impurities is completely liquefied, and

- an expansion step (130), depending on the outlet pressure P _or t of the gas flow in the liquid phase.

2. Method (100, 300) according to claim 1, wherein the cooling step (11 1) comprises a step (115) of cooling in the 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 high pressure, the flow of compressed gas, to a temperature:

- which is greater 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 expanding the flow of liquid gas to a pressure at least equal to a pressure, called "Pmed", according to which the gas in its pure state without impurities equivalent to the gas entered is at its bubble point, and

- a second step (315) of cooling in the liquid phase, according to the pressure P _me d, of the gas flow up to the temperature T _or 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 phase cooling step (120). liquid 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.

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 regenerating the 'heat exchanger.

7. Method (300) according to claim 6, wherein 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 input 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 1 1, which comprises a step (360) of replacing 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 dinitrogen (N2) and/or dioxygen (O2) as solvent or solvents in which the impurity is chosen from carbon dioxide and water, and

- a third mixture comprising carbon dioxide as 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 purifying a gas flow in the liquid phase to reach 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 _or t”, which includes:

- a means (210) for compressing the flow of gas entered at least according to a pressure, called "High", 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 equivalent impurities, at high pressure,

- a heat exchanger (515, 516) for cooling, depending on the high pressure, the flow of compressed gas to an outlet temperature, called “T _or t”, so as to make the impurity captureable in the form of solid impurities , and capture of solid impurities, the temperature Tout being selected so that, at the pair of pressure P _or t and temperature Tout, the remaining impurity at the molar rate of impurity Xi _mp , out in the gas flow is soluble and that an equivalent stream of pure gas without impurities is completely liquefied,

- an expansion means (230), depending on the outlet pressure P _or t of the gas flow in the liquid phase.