METHOD FOR THE REMOVAL OF ACID COMPOUNDS FROM EXHAUST FUME USING AN ABSORBENT SOLUTION BASED ON TERTIARY AMINATES

Description

technical field

[0001] The present invention relates to the field of gaseous effluent deacidification processes. The invention is advantageously applicable to the treatment of industrial gases, natural gas, and gas from biomass fermentation. Previous technique

[0002] Gas deacidification processes commonly employ aqueous amine solutions to remove acidic compounds from gases, including carbon dioxide (CO₂), hydrogen sulfide (H₂S), carbon oxysulfide (COS), carbon disulfide (CS₂), sulfur dioxide (SO₂), and RSH mercaptans such as methyl mercaptan (CH₃SH), ethyl mercaptan (CH₃CH₂SH), and propyl mercaptan (CH₃CH₂CH₂SH). The gas is deacidified by contacting it with the absorbent solution, which is then thermally regenerated.

[0003] These acid gas deacidification processes are also commonly called "solvent scrubbing" using a "chemical" solvent, as opposed to the use of a "physical" solvent for absorption, which is not based on chemical reactions but on solubility, which is directly dependent on and proportional to the partial pressure of the gaseous species. Partial pressure is the product of the concentration of the species in the phase multiplied by the total operating pressure. A "chemical" solvent advantageously benefits from both effects to increase the solubility of the species.

[0004] A chemical solvent is an aqueous solution containing a reagent that reacts preferentially with acidic compounds (H₂S, CO₂, COS, CS₂, etc.) present in the treated gas to form salts, without reacting with other non-acidic compounds in the gas. After contact with the solvent, the treated gas is depleted of acidic compounds, which are selectively transferred as salts into the solvent. The chemical reactions are reversible, allowing the solvent, now laden with acidic compounds, to be subsequently deacidified, for example, by heating. This releases the acidic compounds as gases, which can then be stored, processed, or used for various applications. The solvent is also regenerated, returning to its initial state and thus ready for reuse in a new reaction with the acidic gas being treated.The phase of reaction of the solvent with the acid gas is commonly called the absorption phase, and the one where the solvent is deacidified is called the solvent regeneration phase.

[0005] In general, the performance of separating acidic compounds from gas in this context depends primarily on the nature of the chosen reversible reaction. Conventional acid gas deacidification processes are generally so-called "amine processes," meaning they rely on the reaction of acidic compounds with amines in aqueous solution. These reactions fall under the general category of acid-base reactions. H₂S, CO₂, and COS, for example, are acidic compounds, particularly in the presence of water, while amines are basic compounds. The reaction mechanisms and the nature of the salts obtained generally depend on the structure of the amines used.

[0006] For example, US6852144 describes a method for removing acidic compounds from hydrocarbons using an aqueous absorbing solution of N-methyldiethanolamine (MDEA) or triethanolamine containing a high proportion of at least one compound belonging to the following group: piperazine and / or methylpiperazine and / or morpholine.

[0007] The performance of acid gas deacidification processes using amine scrubbing is directly dependent on the nature of the amine(s) present in the solvent. These amines can be primary, secondary, or tertiary. They can have one or more equivalent or different amine functional groups per molecule.

[0008] In order to improve the performance of deacidification processes, there is a continuous search for ever more efficient amines, in particular those capable of absorbing a larger quantity of acid gases and being regenerated with less energy.

[0009] A limitation of absorbent solutions commonly used in so-called "selective" acid removal applications is insufficient H₂S absorption selectivity relative to CO₂. Indeed, in some natural gas acid removal applications, the goal is to selectively remove H₂S while minimizing CO₂ absorption. This constraint is particularly important for gases already containing a CO₂ content equal to or lower than the desired specification. The aim is then to maximize H₂S absorption capacity with maximum H₂S absorption selectivity relative to CO₂. This selectivity maximizes the amount of gas treated and recovers acid gas from the regenerator outlet with the highest possible H₂S concentration, thus limiting the size of the sulfur chain units downstream of the treatment and ensuring their improved performance.In some cases, a hydrogen sulfide (H₂S) enrichment unit is required to concentrate the acid gas with hydrogen sulfide. On these types of amine scrubbing units installed to treat acid gas, the most selective amine is also sought.

[0010] It is well known that tertiary or secondary amines with severe steric hindrance have slower CO₂ uptake kinetics than less sterically hindered primary or secondary amines. Conversely, amines, and particularly tertiary or secondary amines with severe steric hindrance, generally have instantaneous H₂S uptake kinetics. Consequently, it is possible to achieve selective H₂S removal with these tertiary and secondary amines with severe steric hindrance based on their distinct kinetic properties.

[0011] Tertiary amines, such as MDEA, or hindered secondary amines with slow reaction kinetics with CO2 are thus commonly used. However, they exhibit limited selectivity at high acid gas loading rates.

[0012] Another limitation of many absorbent solutions commonly used in total acidification applications is their excessively slow CO₂ or COS absorption kinetics. When CO₂ or COS requirements are very stringent, approaching complete removal of these compounds, the fastest possible reaction kinetics are generally sought to minimize the height of the absorption column. Indeed, if the reactions are slow, taller columns must be considered to allow sufficient time for the chemical reaction to occur. However, absorption columns represent a significant portion of the process's capital investment costs, especially when pressure vessels are involved.

[0013] Whether the goal is maximum CO₂ and COS absorption kinetics in a total acid removal application targeting all acidic contaminants, or minimum CO₂ capture kinetics in a selective acid removal application (selective absorption of H₂S relative to CO₂), the objective is always to use an absorbent solution with the highest possible cycle capacity for the contaminants being removed from the treated gaseous effluent, e.g., natural gas. This cycle capacity, denoted Δα, corresponds to the difference in loading rate (α representing the number of moles of acidic compounds absorbed per kilogram of absorbent solution) between the absorbent solution drawn from the bottom of the absorption column and the absorbent solution feeding the column.Indeed, the higher the cyclic capacity of the absorbent solution, the lower the flow rate of absorbent solution required to deacidify the gas being treated. In gas treatment processes, reducing the absorbent solution flow rate also has a significant impact on reducing investment, particularly in terms of column sizing if their diameter can be reduced, but also by reducing the size of other equipment such as heat exchangers, pumps, and flash tanks.

[0014] Another crucial aspect of gas treatment or industrial flue gas cleaning operations using solvents is the regeneration of the separating agent to purify the solvent of contaminants accumulated during the absorption phase. Depending on the type of absorption (physical and / or chemical), regeneration is generally considered through expansion, distillation, and / or entrainment by a vaporized gas known as "stripping gas." The energy consumption required for solvent regeneration can be very high, particularly when the partial pressure of acidic gases is low or when chemical bonding forces are strong, representing a considerable operating cost for the deacidification process.

[0015] It is well known to those skilled in the art that the energy required for the regeneration of an amine solution by distillation can be broken down into three distinct components: the energy required to heat the absorbent solution between the head and the bottom of the regenerator, the energy required to lower the partial pressure of acidic gases in the regenerator by vaporizing a stripping gas, and finally, the energy required to break the chemical bond between the amine and the acid compound. The first two components are proportional to the flow rates of absorbent solution that must be circulated through the unit to achieve a given purification performance, i.e., a specificity, of the solvent to be regenerated. To reduce the energy consumption associated with solvent regeneration, it is therefore preferable to maximize the solvent's cycle life.Indeed, the higher the cyclic capacity of the absorbing solution, the more the flow rate of absorbing solution that needs to be implemented to deacidify the gas to be treated can be restricted.

[0016] In the search for more efficient amines to reduce circulation flow and regeneration energy, US patent 6267939 and patent application WO09156273 propose an absorbent solution based on particular polyamines, such as N,N,N',N'-Tetramethyldipropylenetriamine (TMDPTA) or pentamethyldiethylenetriamine (PMDETA).

[0017] With the aim of also reducing the energy required for regeneration in the acidification process, documents FR2877858, FR2895273, FR2900843, FR2898284, FR2900842, FR2986441, and FR2986442 propose using an absorbent solution that forms two phases when it absorbs a quantity of acid gases, such that only the phase containing the acid gases requires regeneration. However, this solution has the disadvantage of not being compatible with a conventional high-pressure acidification plant because it requires additional steps and equipment, as well as numerous precautions to maintain control of the operation.Indeed, the separation of a single-phase liquid absorbent solution into two liquid phases, also known as demixing, can cause serious operational problems during absorption or at higher temperatures, upstream or during the regeneration stage, if the process and equipment are not designed to handle both phases. These problems are described in particular in documents FR3014101 and WO2015177333. Under the operating conditions of the absorber, also called an absorption column, this liquid-liquid phase separation can disrupt the transfer of acid gas to the absorbent solution and destabilize the column. It can also disrupt the liquid-vapor phase equilibrium, which is crucial for the performance of the acidification cycle, especially during the regeneration stage.It can also cause sudden changes in the composition of liquid flows, irregular flows such that the process becomes unstable, its control impossible and its performance random.

[0018] Another problem lies in the stability of the absorbing solution, particularly the thermal stability of amines. The absorbing solution can degrade under the influence of temperature, which limits the operating conditions of the process, especially the temperature at which solvent regeneration is carried out. For example, increasing the regenerator temperature by 10°C doubles the thermal degradation rate of monoethanolamine (MEA). The regeneration of aqueous alkanolamine solutions, such as MEA, is therefore carried out at regenerator bottom temperatures of around 120°C, or even 130°C for more stable amines, such as MDEA. Due to these bottom temperatures, the acid gases (CO₂, H₂S, COS, CS₂, etc.) are obtained at moderate pressures, from 0.1 to 0.3 MPa.Depending on the nature of the regenerated acid gas and the applications, the acid gas can be sent to a treatment unit or compressed for reinjection and sequestration. To overcome this thermal stability problem, patent application WO04082809 proposes, for example, the use of aqueous absorbent solutions containing a high concentration, typically greater than 60%, of a tertiary polyamine, such as pentamethyldipropylenetriamine (PMDPTA).

[0019] Other problems are also commonly encountered, such as the corrosivity of absorbent solutions or the foaming of absorbent solutions.

[0020] Absorbent solutions based on amines, for example those based on alkanolamines such as MEA, diethanolamine (DEA), or MDEA, are indeed known to be corrosive to steel equipment used in gaseous effluent deacidification processes. These corrosion risks necessitate rather stringent measures, such as the use of corrosion-resistant alloys for equipment, which may be more expensive or less mechanically robust, or the use of anti-corrosion additives that increase the cost of the solution, accumulate irreversibly, and can cause fouling, requiring continuous monitoring to ensure their presence at the correct levels.

[0021] Foaming of absorbent solutions is a known problem in gaseous effluent deacidification, potentially leading to various adverse consequences, such as premature clogging of absorption or regeneration columns resulting in reduced production capacity, treated gas outside of specified limits, amine losses through entrainment in the treated or acidic gas, or even unit shutdown. Amine-based absorbent solutions are known to foam, particularly when in contact with liquid hydrocarbons. This problem is often addressed by adding antifoaming agents, which, again, increases operating costs and complicates the process (requiring monitoring of concentration, the addition of filters, etc.).

[0022] In this context, it is difficult to find a formulation of absorbent compounds that can remove acidic compounds from any type of effluent, allowing the deacidification process to operate at lower operating costs (including regeneration energy) and investment costs (including the cost of the absorption column), and that meet the requirements of absorption capacity, selectivity, chemical stability, particularly at temperature, low corrosivity and limiting foaming.

[0023] Furthermore, patent EP-4 010 106 discloses a process for removing acidic compounds contained in a gaseous effluent, in which an adsorption step of said acidic compounds is carried out by contacting the gaseous effluent with an adsorbent solution comprising a mixture of a tertiary polyamine, a tertiary amine and a secondary polyamine, such as a mixture of pentamethyldipropylenetriamine PMDPTA, N-methyldiethanoamine MDEA and piperazine. Objectives and Summary of the Invention

[0024] The present invention therefore aims to meet the needs of the prior art and to overcome one or more of the disadvantages of the prior art mentioned above.

[0025] The present invention thus proposes a method for removing acidic compounds, such as CO2, H2S, COS, CS2, mercaptans, from a gas using an aqueous absorbing solution comprising the particular combination of at least two specific amines, which unexpectedly exhibits at least one of the following effects: greater resilience to foaming compared to traditional solutions, particularly in the presence of hydrocarbons and in degraded conditions such as those of operating units; less corrosivity compared to traditional solutions; greater stability, i.e. less degradation, particularly in the presence of dioxygen, compared to polyamine-based absorbent solutions according to the prior art.

[0026] Furthermore, the inventors have demonstrated that the use of this particular combination of specific amines in aqueous solution, with an activator such as piperazine, can improve the cyclic absorption capacity and the CO2 absorption kinetics compared to a reference formulation such as a mixture of MDEA and piperazine.

[0027] The absorbent solutions used in the process according to the invention can also limit the flow rate of absorbent solution required in the process, due to their good performance in terms of cyclic absorption capacity of acid gases, particularly CO₂ and H₂S, and their absorption selectivity with respect to H₂S. This performance is superior to that of MDEA, and also to that of polyamines mentioned in the prior art, particularly for the latter in terms of cyclic absorption capacity of acid gases. According to the present invention, the absorbent solution is advantageously in a monophasic form under the operating conditions of the process, more particularly at least under the absorption conditions and until the solution enters the regenerator, which notably eliminates the need for a post-absorption settling step.

[0028] Thus, the present invention proposes, according to a first aspect, a method for removing acidic compounds contained in a gaseous effluent, in which an absorption step of the acidic compounds is carried out by contacting the gaseous effluent with an absorbent solution comprising: water; between 20% and 28% by weight of pentamethyldipropylenetriamine; between 5% and 35% by weight of N-methyldiethanolamine; between 5% and 50% by weight of a physical solvent chosen from the group consisting of methanol, ethanol, 2-ethoxyethanol, triethyleneglycoldimethyl ether, tetraethyleneglycoldimethyl ether, pentaethyleneglycoldimethyl ether, hexaethyleneglycoldimethyl ether, heptaethyleneglycoldimethyl ether, octaethyleneglycoldimethyl ether, diethylene glycol butoxyacetate, glycerol triacetate, sulfolane, N-methylpyrrolidone, N-methylmorpholin-3-one, N,N-dimethylformamide, N-formylmorpholine, N,N-dimethylimidazolidin-2-one, N-methylimidazole, ethylene glycol, diethylene glycol, triethylene glycol, thiodiglycol, propylene carbonate, tributylphosphate.

[0029] According to one or more embodiments of the invention, the absorbent solution comprises between 10% and 30% by weight of N-methyldiethanolamine, and preferably between 42% and 70% by weight of water.

[0030] According to one or more embodiments of the invention, the absorbent solution comprises between 37% and 75% water by weight.

[0031] According to one or more embodiments of the invention, the absorbent solution comprises: between 5% and 20% wt of N-methyldiethanolamine; and between 0.5% and 20% wt of at least one activating compound comprising a primary or secondary amine function selected from the group consisting of: piperazine; 1-methylpiperazine; homopiperazine; N-(2-hydroxyethyl)piperazine; 3-(methylamino)propylamine; N,N'-dimethyl-1,6-hexanediamine; N-methyl-1,6-hexanediamine; N,N',N'-trimethyl-1,6-hexanediamine; 2-amino-2-methyl-1-propanol.

[0032] In this case, the absorbing solution may contain between 5% and 15% by weight of N-methyldiethanolamine, and preferably between 10% and 15% of N-methyldiethanolamine.

[0033] Preferably, the activating compound is piperazine.

[0034] Advantageously, the absorbing solution comprises between 0.5% and 10% by weight of said at least one activating compound, preferably between 0.5% and 6% by weight of said at least one activating compound, and even more preferably between 1% and 6% by weight of said at least one activating compound.

[0035] According to the invention, the absorbing solution further comprises at least one physical solvent selected from the group consisting of methanol, ethanol, 2-ethoxyethanol, triethyleneglycoldimethyl ether, tetraethyleneglycoldimethyl ether, pentaethyleneglycoldimethyl ether, hexaethyleneglycoldimethyl ether, heptaethyleneglycoldimethyl ether, octaethyleneglycoldimethyl ether, diethylene glycol butoxyacetate, glycerol triacetate, sulfolane, N-methylpyrrolidone, N-methylmorpholin-3-one, N,N-dimethylformamide, N-formylmorpholine, N,N-dimethylimidazolidin-2-one, N-methylimidazole, ethylene glycol, diethylene glycol, triethylene glycol, thiodiglycol, propylene carbonate, tributylphosphate.

[0036] According to one or more embodiments of the invention, the absorbent solution does not include an anti-foaming additive.

[0037] According to one or more embodiments of the invention, the step of absorption of acidic compounds is carried out at a pressure between 0.1 MPa and 20 MPa, and at a temperature between 20°C and 100°C.

[0038] According to one or more embodiments of the invention, an absorbing solution loaded with acid compounds is obtained after the absorption step, and at least one regeneration step is carried out on said absorbing solution loaded with acid compounds at a pressure between 0.1 MPa and 1 MPa, preferably between 0.1 MPa and 0.5 MPa, and at a temperature between 100°C and 180°C, preferably between 110°C and 140°C.

[0039] According to one or more embodiments of the invention, the absorbing solution is monophasic during the absorption step within an absorption column and until at least the entry into a regeneration column in which the regeneration step is carried out, and preferably has a temperature less than or equal to 110°C in the absorption column and until at least the entry into the regeneration column.

[0040] According to one or more embodiments of the invention, the gaseous effluent is chosen from natural gas, synthesis gas, combustion fumes, refinery gases, acid gases from an amine unit, tail gases from a Claus process H2S to sulfur conversion unit, gases from biomass fermentation, cement plant gases, incinerator fumes.

[0041] The process according to the invention can be implemented for the selective removal of H2S with respect to CO2 from a gaseous effluent containing H2S and CO2, preferably natural gas.

[0042] The process according to the invention can also be implemented for the decarbonation of biogas.

[0043] Other objects and advantages of the invention will become apparent from the following description of particular embodiments of the invention, given by way of non-limiting examples, the description being made with reference to the attached figure described below. List of figures

[0044] There figure 1This illustration represents a schematic diagram of the process for treating acid gas effluents. It does not include all the components necessary for implementing the process, such as heat exchangers, pumps, mixers, etc. Only the elements necessary for understanding the invention are shown; a person skilled in the art could complete this representation to implement the invention. Description of the implementation methods

[0045] The present invention proposes a method for removing acidic compounds from a gaseous effluent by using an aqueous absorbent solution, the composition of which is detailed below. The process is described in more detail following the explanation of the absorbent solution's composition. Composition of the absorbent solution

[0046] The absorbent solution implemented for the removal of acidic compounds contained in a gaseous effluent comprises: water; between 20 and 28% by weight of pentamethyldipropylenetriamine (PMDPTA); between 5 and 35% by weight of N-methyldiethanolamine (MDEA); and between 5% and 50% by weight of a physical solvent chosen from the group consisting of methanol, ethanol, 2-ethoxyethanol, triethyleneglycoldimethyl ether, tetraethyleneglycoldimethyl ether, pentaethyleneglycoldimethyl ether, hexaethyleneglycoldimethyl ether, heptaethyleneglycoldimethyl ether, octaethyleneglycoldimethyl ether, diethylene glycol butoxyacetate, glycerol triacetate, sulfolane, N-methylpyrrolidone, N-methylmorpholin-3-one, N,N-dimethylformamide, N-formylmorpholine, N,N-dimethylimidazolidin-2-one, N-methylimidazole, ethylene glycol, diethylene glycol, triethylene glycol, thiodiglycol, propylene carbonate, tributylphosphate.

[0047] MDEA is a tertiary monoamine, which, when used in aqueous solution, constitutes a reference absorbing solution in the field of gas deacidification, particularly for the selective absorption of H2S compared to CO2 contained in the gas.

[0048] PMDPTA is a tertiary polyamine, more precisely a tertiary triamine, with the following formula:

[0049] It is known in the prior art that polyamines offer an advantage over monoamines by allowing a higher number of reactive sites per unit mass of product, with the potential for increased absorption capacity. However, polyamines are not equivalent in terms of acid gas absorption capacity, performance for the selective removal of H₂S, and chemical stability under acid gas treatment process conditions. Furthermore, some polyamines, when used in aqueous solution, can lead to demixing, i.e., separation into two liquid phases of the monophasic liquid absorbing solution, under certain temperature and acid gas loading conditions.

[0050] The combination of PMDPTA and MDEA in aqueous solution, in the indicated concentrations, makes it possible to avoid a separation of the absorbing solution into two liquid phases, i.e. a demixing phenomenon, under the temperature and loading conditions usually encountered during the absorption and circulation phase up to the regenerator, while achieving particularly high acid gas absorption capacities.

[0051] The inventors also highlighted that, unexpectedly, the specific combination of these two amines in aqueous solution exhibits limited foaming in the presence of hydrocarbons and under degraded conditions of the absorbing solution, excellent chemical stability properties, less corrosivity towards metallic equipment used during deacidification, and very good performance for the selective absorption of H2S compared to CO2 contained in a gaseous effluent to be treated.

[0052] According to one embodiment, the absorbent solution comprises between 10% and 30% by weight of MDEA.

[0053] The absorbent solution can contain between 37% and 75% water by weight.

[0054] The absorbent solution preferably contains between 42% and 70% by weight of water when the solution contains between 10% and 30% by weight of MDEA.

[0055] Throughout the description, the sum of the mass fractions expressed as % weight of the different compounds of the absorbing solution is equal to 100% by weight of the virgin absorbing solution, i.e. the absorbing solution without taking into account absorbed acid gases or other co-absorbed products or degradation products.

[0056] Concentration ranges are understood to be inclusive of limits, unless otherwise specified.

[0057] In this description, pressures are expressed in absolute values ​​unless otherwise stated. According to one embodiment, the aqueous absorbent solution comprises 20% to 25% by weight of PMDPTA and 25% to 35% by weight of MDEA.

[0058] According to an embodiment which is not part of the claims, the absorbent solution consists of an aqueous solution composed of 20% to 28% weight of PMDPTA and 5% to 35% weight of MDEA, preferably composed of 20% to 25% weight of PMDPTA and 25% to 35% weight of MDEA.

[0059] According to an embodiment not included in the claims, the absorbent solution comprises: water; between 22% and 28% by weight of PMDPTA; and between 5% and 35% by weight of MDEA.

[0060] In this case the absorbing solution may consist of an aqueous solution composed of 22% to 28% weight of PMDPTA and 5% to 35% weight of MDEA, and preferably composed of 22% to 25% weight of PMDPTA and 25% to 35% weight of MDEA.

[0061] According to another embodiment which is not part of the claims, the absorbent solution comprises: water; between 20% and 28% by weight of PMDPTA; between 5% and 20% by weight of MDEA; and between 0.5% and 20% by weight of at least one activating compound comprising a primary or secondary amine function selected from the group consisting of: piperazine; 1-methylpiperazine; homopiperazine; N-(2-hydroxyethyl)piperazine; 3-(methylamino)propylamine; N,N'-dimethyl-1,6-hexanediamine; N-methyl-1,6-hexanediamine; N,N',N'-trimethyl-1,6-hexanediamine; 2-amino-2-methyl-1-propanol.

[0062] According to this embodiment, the absorbing solution may comprise a mixture of activator compounds as listed above, as is implied by the expression "at least one activator compound".

[0063] An activating compound is defined as a compound that accelerates the absorption kinetics of CO₂, and in some cases COS, contained in the gas being treated. In this description, an "activated solution" refers to an absorbing solution containing such an activating compound.

[0064] According to the invention, such an activated solution is implemented in particular for a non-selective deacidification application of the gas to be treated, for example a total deacidification application of the gas, i.e. when it is sought to eliminate CO2 and COS to achieve very high specifications, i.e. high purification performance.

[0065] Preferably, according to this embodiment relating to an activated solution, the absorbing solution comprises between 0.5% and 10% by weight of said at least one activating compound, preferably between 0.5% and 6% by weight of said at least one activating compound, and even more preferably between 1% and 6% by weight of said at least one activating compound.

[0066] According to this embodiment relating to an activated solution, the concentration of MDEA in the absorbing solution is preferably between 5% and 15% by weight, preferably between 10% and 15% by weight. In this case, the absorbing solution may contain between 0.5% and 10% by weight of said at least one activating compound, preferably between 0.5% and 6% by weight of said at least one activating compound, and even more preferably between 1% and 6% by weight of said at least one activating compound.

[0067] Preferably, according to this embodiment involving an activated solution, the absorbent solution comprises at least one activating compound, which is piperazine. More preferably, the absorbent solution comprises an activating compound, which is piperazine.

[0068] According to another embodiment which is not part of the claims, the absorbent solution comprises: water; between 20% and 28% by weight of PMDPTA, preferably between 22% and 27% by weight of PMDPTA; between 5% and 20% by weight of MDEA, preferably between 5% and 15% by weight of MDEA; and between 0.5% and 20% by weight of piperazine, and preferably between 0.5% and 10% by weight of piperazine, more preferably between 1% and 6% by weight of piperazine.

[0069] The water concentration is variable and represents the complement, by weight, of the sum of the other compounds included in the absorbing solution.

[0070] According to the invention, the water concentration is variable.

[0071] Preferably, the absorbent solution comprises at least 37% water by weight. According to the invention, the absorbent solution contains organic compounds that are non-reactive with respect to acidic compounds, commonly called "physical solvents," which increase the solubility of at least one or more acidic compounds in the gaseous effluent.Thus, according to the invention, the absorbent solution comprises between 5% and 50% by weight of a physical solvent such as methanol, ethanol, 2-ethoxyethanol, triethyleneglycoldimethyl ether, tetraethyleneglycoldimethyl ether, pentaethyleneglycoldimethyl ether, hexaethyleneglycoldimethyl ether, heptaethyleneglycoldimethyl ether, octaethyleneglycoldimethyl ether, diethylene glycol butoxyacetate, glycerol triacetate, sulfolane, N-methylpyrrolidone, N-methylmorpholin-3-one, N,N-dimethylformamide, N-formylmorpholine, N,N-dimethylimidazolidin-2-one, N-methylimidazole, ethylene glycol, diethylene glycol, triethylene glycol, thiodiglycol, propylene carbonate, and tributylphosphate. Nature of gaseous effluents

[0072] According to the invention, absorbent solutions can be used to deacidify the following gaseous effluents: natural gas, synthesis gas, combustion fumes, refinery gases, acid gases from an amine unit, tail gases from a Claus process H2S to sulfur conversion unit, biogas such as gas from biomass fermentation, cement plant gases, and incinerator fumes. These gaseous effluents contain one or more of the following acidic compounds: CO2, H2S, mercaptans (e.g., methyl mercaptan (CH3SH), ethyl mercaptan (CH3CH2SH), propyl mercaptan (CH3CH2CH2SH)), COS, CS2, and SO2.

[0073] Combustion fumes are produced, in particular, by the combustion of hydrocarbons, biogas, or coal in a boiler or gas turbine, for example, for the purpose of generating electricity. By way of illustration, a deacidification process according to the invention can be implemented to absorb at least 70%, preferably at least 80%, or even at least 90%, of the CO₂ contained in the combustion fumes. Such CO₂ removal can be referred to as "decarbonation" of the gas. These fumes generally have a temperature between 20°C and 60°C, a pressure between 0.1 MPa and 0.5 MPa and can contain between 50% and 80% by volume of nitrogen (N2), between 5% and 40% by volume of CO2, between 1% and 20% by volume of oxygen (O2), and some impurities such as SOx and NOx, if they have not been removed upstream of the deacidification process.In particular, the deacidification process according to the invention is particularly well suited to absorbing CO2 contained in combustion fumes having a low partial pressure of CO2, for example a partial pressure of CO2 less than 0.02 MPa.

[0074] The deacidification process according to the invention can be implemented to deacidify synthesis gas. Synthesis gas contains carbon monoxide (CO), hydrogen (H₂) (generally in an H₂ / CO ratio of 2), water vapor (generally saturated at the temperature where the scrubbing is performed), and CO₂ (on the order of ten percent by volume). The pressure is generally between 2 and 3 MPa, but can reach up to 7 MPa. It may also contain sulfur (H₂S, COS, etc.), nitrogen (NH₃, HCN), and halogenated impurities.

[0075] The deacidification process according to the invention can be used to deacidify natural gas. Natural gas consists mainly of gaseous hydrocarbons, but may contain several of the following acidic compounds: CO₂, H₂S, mercaptans, COS, and CS₂. The content of these acidic compounds is highly variable and can reach up to 70% by volume for CO₂ and up to 40% by volume for H₂S. The temperature of the natural gas can range from 10°C to 100°C. The pressure of the natural gas to be treated can range from 1 to 20 MPa. The invention can be implemented to achieve specifications generally imposed on deacidified gas, which are less than 2% by volume of CO2, or even less than 50 ppm by volume of CO2 to then achieve liquefaction of natural gas, less than 4 ppm by volume of H2S, and less than 50 ppm, or even less than 10 ppm, by volume of total sulfur.

[0076] The deacidification process according to the invention can be implemented to deacidify biogas, typically gases resulting from biomass fermentation and generally pretreated to remove impurities such as H₂S, mercaptans, or siloxanes. These gases generally have a temperature between 5°C and 60°C, a pressure between 0.1 and 2 MPa, and may contain between 30% and 75% by volume of methane, between 0% and 40% by volume of nitrogen (N₂), between 15% and 50% by volume of CO₂, and between 0% and 10% by volume of oxygen (O₂). In particular, the deacidification process according to the invention is advantageously implemented to remove CO₂ from biogas, which typically contains a significant amount of oxygen, for example, several tenths to several percent by volume of oxygen. It is known that amines are generally sensitive to oxygen and degrade more easily in the presence of oxygen.The present invention is therefore particularly well suited to the treatment of biogas, due in particular to the good chemical stability of the absorbing solution in the presence of oxygen. Process for removing acidic compounds from a gaseous effluent

[0077] The process for removing acidic compounds from a gaseous effluent according to the invention includes a step of absorbing the acidic compounds by bringing the gaseous effluent into contact with the absorbing solution.

[0078] The absorption stage is followed by a regeneration stage, as shown in the diagram of the figure 1 .

[0079] In the present invention, the different parameter ranges for a given step, such as pressure ranges and temperature ranges, can be used alone or in combination. For example, in the present invention, a preferred pressure range can be combined with a more preferred temperature range.

[0080] With reference to the figure 1The gaseous effluent deacidification system comprises an absorption column C1 equipped with means for contacting the gas and liquid, such as bulk packing, structured packing, or trays. The gaseous effluent to be treated is conveyed through a line 1 opening at the bottom of column C1. A line 4 allows the introduction of the absorbent solution at the top of column C1. A line 2 allows the treated (deacidified) gas to be discharged, and a line 3 conveys the absorbent solution, enriched in acidic compounds following absorption, to a regeneration column C2. This regeneration column C2 is equipped with internals for contacting the gas and liquid, such as trays, bulk packing, or structured packing. The bottom of column C2 is equipped with a reboiler R1, which provides the heat necessary for regeneration by vaporizing a portion of the absorbent solution.The acid-enriched solution is introduced at the top of the regeneration column C2 via a line 5. A line 7 allows the acid-enriched gas released during regeneration to be discharged from the top of column C2, and a line 6 located at the bottom of column C2 sends the regenerated absorbent solution to the absorption column C1. A heat exchanger E1 recovers heat from the regenerated absorbent solution in column C2 to heat the acid-enriched absorbent solution exiting absorption column C1.

[0081] The absorption step consists of bringing the gaseous effluent arriving through line 1 into contact with the absorbent solution arriving through line 4. During contact, the amine functions of the molecules of the absorbent solution react with the acidic compounds contained in the effluent in order to obtain a gaseous effluent depleted in acidic compounds which is discharged through line 2 at the top of column C1 and an absorbent solution enriched in acidic compounds discharged through line 3 at the bottom of column C1 to be regenerated.

[0082] The acid compound absorption step can be carried out at a pressure in column C1 of between 0.1 MPa and 20 MPa, preferably between 2 MPa and 10 MPa for the treatment of natural gas, and preferably between 0.1 MPa and 0.3 MPa for the treatment of industrial fumes.

[0083] The absorption step of acidic compounds can be carried out at a temperature in column C1 between 20°C and 100°C, preferably between 30°C and 90°C, or even between 30°C and 60°C.

[0084] The regeneration stage consists primarily of heating and, possibly, expanding the acid-enriched absorbent solution to release the acid compounds in gaseous form. The acid-enriched absorbent solution exiting column C1 is introduced into heat exchanger E1, where it is heated by the flow circulating in line 6 from regeneration column C2. The heated solution exiting E1 is then introduced into regeneration column C2 via line 5, at a temperature generally less than or equal to 110°C.

[0085] In the regeneration column C2, under the effect of the contact of the absorbing solution arriving via line 5 with the steam produced by the reboiler, the acidic compounds are released in gaseous form and discharged at the top of column C2 via line 7. The regenerated absorbing solution, i.e. depleted in acidic compounds, is discharged via line 6 and is cooled in E1, then recycled into the absorption column C1 via line 4.

[0086] The regeneration stage can be carried out by thermal regeneration, possibly supplemented by one or more expansion stages. For example, the absorbent solution enriched in acidic compounds, discharged through line 3, can be sent, after possible heating, to a first expansion vessel (not shown), before passing through the heat exchanger E1. In the case of natural gas, the expansion produces a gas discharged from the top of the vessel containing most of the aliphatic hydrocarbons co-absorbed by the absorbent solution. This gas can optionally be scrubbed with a portion of the regenerated absorbent solution, and the resulting gas can be used as fuel. The expansion vessel preferably operates at a pressure lower than that of the absorption column C1 and higher than that of the regeneration column C2.This pressure is generally determined by the operating conditions of the fuel gas and is typically in the range of 0.2 to 1.5 MPa, preferably 0.5 to 1.5 MPa. The expansion vessel operates at a temperature substantially identical to that of the absorbing solution obtained at the bottom of absorption column C1.

[0087] The absorbing solution then contains a reduced amount of acidic gas after the expansion step. It can be partially recycled to absorption column C1 for a new absorption cycle in a particular embodiment of the invention (not shown), possibly at a different level than the regenerated absorbing solution from column C2. In this case, where the absorbing solution is partially recycled to the absorption column, the remaining portion of the absorbing solution from the expansion step is introduced into regeneration column C2.

[0088] In a particular implementation of the invention (not shown), the absorbing solution after the expansion step is fully recycled into the absorption column C1 and no regeneration column C2 is implemented.

[0089] Regeneration can be carried out at a pressure in column C2 of between 0.1 MPa and 0.5 MPa, preferably between 0.1 MPa and 0.4 MPa, or even up to 1 MPa, and at a temperature in column C2 of between 100°C and 180°C, preferably between 100°C and 140°C, more preferably between 110°C and 140°C, and even more preferably between 115°C and 140°C, and even more preferably between 115°C and 130°C. For example, the regeneration temperature in column C2 is between 115°C and 130°C in cases where the acid gas is sent to the atmosphere or to a downstream treatment process, such as a Claus process or a tail gas treatment process.

[0090] According to a preferred embodiment, regeneration is carried out at a pressure in column C2 between 0.1 MPa and 0.5 MPa, preferably between 0.1 MPa and 0.4 MPa, and at a temperature between 110°C and 140°C, more preferably between 115°C and 130°C.

[0091] According to the invention, the liquid absorbent solution advantageously remains monophasic during the absorption phase in the absorption column C1 and during the circulation of the solution enriched with acid compounds, at least until it enters the regeneration column C2. The specific formulation of the absorbent solution minimizes the risk of demixing during absorption and circulation to the regeneration column. Such demixing could indeed cause operational problems. If the solution is in the form of two separate phases in the absorption column, the flow of acid compounds transferred from the gas to the absorbent solution is significantly impacted, and the column height generally needs to be adjusted accordingly (increasing the column size).This phenomenon presents implementation challenges and, given the system's complexity, is difficult to model. Furthermore, a solution in the form of two distinct liquid phases may require the installation of separation devices (e.g., sedimentation) upstream of the regeneration column, which adds to the complexity and cost of the process. Such problems are thus avoided in the process according to the invention.

[0092] According to the invention, the absorbent solution minimizes the corrosion of the equipment of the gaseous effluent deacidification installation, whose surfaces, comprising steel, are brought into contact with said solution.

[0093] These components can be made of low-alloy steel, also called carbon steel, or stainless steel. Low-alloy steel is defined as steel composed primarily of iron, specifically at least 90% iron by mass, and 0.01% to 2.5% carbon by mass, and at least one alloying element chosen from among aluminum, chromium, cobalt, copper, manganese, molybdenum, nickel, niobium, phosphorus, silicon, titanium, tungsten, and vanadium, the individual content of said at least one alloying element being less than 5% by mass. This definition does not exclude the presence of other elements up to a limit of 1% by mass. Stainless steels, which are less susceptible to corrosion, are distinguished from low-alloy steels by a chromium content exceeding 11.5% by weight.

[0094] The equipment concerned includes, but is not limited to, absorption and regeneration columns where gas / liquid contact is made, conduits connecting the different elements of the installation, elements positioned inside enclosures such as trays, structured or bulk packings, exchangers, pump and valve bodies, and storage tanks.

[0095] According to the invention, the equipment comprises surfaces mainly composed of steel, which does not, however, exclude the presence of other materials used for the manufacture of such equipment.

[0096] Due to its limited corrosion power, the absorbent solution makes it possible to limit the corrosion of the equipment in the deacidification installation, and also to limit or eliminate the use of anti-corrosion additives in the process according to the invention.

[0097] Another advantage of using the absorbent solution for the deacidification of a gaseous effluent according to the process of the invention is the reduction of foaming. Foaming generally impairs process performance and can even lead to unit shutdown. Foaming remains a difficult phenomenon to control, yet it is critical for the operation of the deacidification unit.

[0098] Unexpectedly, the absorbent solution limits foaming in the presence of aliphatic or aromatic hydrocarbons, which may initially be present in the gas being treated, and / or under certain degradation conditions of the solution, reflecting its conditions of use. It is therefore possible to consider reducing, or even eliminating, the use of antifoaming additives, traditionally used to address the problem of foaming in amine solutions. Thus, in one embodiment, the absorbent solution does not contain any antifoaming additive.

[0099] An anti-foaming additive is any compound or mixture of compounds added to the absorbent solution, other than the compounds mentioned, which may be part of the absorbent solution, and which is capable of preventing foaming of the absorbent solution or eliminating foam already formed.

[0100] Traditionally, the use of antifoaming additives is achieved through an initial dosage consisting of a steady injection of an antifoam solution into the absorbent solution to reach a concentration of a few tenths of a percent by weight. This dosage is then adjusted according to the level of foaming observed on the unit. Silicon-based organic compounds such as polydialkylsiloxanes or silicone resins, generally emulsified in water, are commonly used. It should be noted that the excessive or inappropriate use of antifoaming additives is known to exacerbate the foaming problem, making the use of such additives difficult to control.

[0101] A test which can be advantageously used in the laboratory to characterize the foaming power of a solution and / or the need to use an anti-foaming additive consists of determining the height of foam generated in a cylindrical glass container of the graduated cylinder type, following agitation of the solution, the height of foam being expressed as a percentage relative to the initial height of the solution, as a function of time.

[0102] For example, it is performed at 20°C on a 50 ml volume of solution placed in a 250 ml beaker and stirred using a 5-bladed stirrer for 4 minutes at 1200 rpm. As soon as stirring stops, the height of the foam generated above the surface of the solution is measured immediately (t0) and again at 30 seconds and is expressed as a percentage of the initial height. The uncertainty of the measurement is 2.5% of the initial height.

[0103] According to the invention, the absorbent solution exhibits resistance to degradation, particularly in the presence of oxygen, thus limiting the production of by-products that promote corrosion through phenomena well known to those skilled in the art. This resistance to degradation, especially at high temperatures such as those encountered during the regeneration of the absorbent solution, is also referred to in this description as chemical stability.

[0104] The process according to the invention advantageously allows for the selective removal of H₂S from CO₂ in the gas to be treated, for example, natural gas. The absorbent solution exhibits good performance for this type of application, notably a relatively slow CO₂ absorption rate combined with a significant H₂S absorption capacity, enabling efficient selective removal and allowing for reduced flow rates of the absorbent solution used.

[0105] The process according to the invention can be advantageously implemented for non-selective deacidification of a gas effluent, for example for the decarbonation of combustion flue gas, or the decarbonation of biogas, in particular because of the good performance of the activated absorbing solutions, in terms of the capacity to absorb acid gases, in particular CO2, but also in terms of CO2 absorption kinetics, and chemical stability. Examples

[0106] The examples below illustrate, in a non-limiting way, the performance of absorbent solutions for removing acidic compounds, such as CO2 or H2S, contained in a gaseous effluent.

[0107] The examples below are not part of the claimed invention.

[0108] In these examples, aqueous solutions of PMDPTA in combination with MDEA are used as absorbent solutions. Table 1 below summarizes the compositions of these solutions.

[0109] Some absorbent solutions also contain piperazine (Pz) used as an activator. Table 1 Absorbent solution A PMDPTA (22% by weight) + MDEA (28% by weight) + H₂O Absorbent solution B PMDPTA (27% by weight) + MDEA (13% by weight) + Pz (5% by weight) + H₂O

[0110] Initially (examples 1 and 2), we show that certain physico-chemical properties of the absorbing solutions A and B, in particular the miscibility of the constituents, are very different from those of aqueous solutions of PMDPTA according to the prior art with or without Pz.

[0111] The compositions of the prior art absorbent solutions used, based on PMDPTA (absorbent solutions C, D, E), are given in Table 2 below, as well as an absorbent solution F based on another polyamine, pentamethyldiethylenetriamine (PMDETA) (according to document WO04082809 or US6267939), and two other absorbent solutions M and N based on another polyamine, N,N,N',N'-Tetramethyldipropylenetriamine (TMDPTA), disclosed for example in documents US6267939 or WO09156273A2. Table 2 Absorbent solution C PMDPTA (50% by weight) + H2O Absorbent solution D PMDPTA (45% by weight) + H2O Absorbent solution E PMDPTA (30% by weight) + H2O Absorbent solution F PMDETA (50% by weight) + H2O Absorbent solution G PMDPTA (30% by weight) + MDEA (10% by weight) + Pz (5% by weight) + H₂O Absorbent solution M TMDPTA (50% by weight) + H2O Absorbent solution N TMDPTA (27% by weight) + MDEA (13% by weight) + Pz (5% by weight) + H₂O

[0112] Examples 3 through 5 illustrate the advantages of absorbent solutions in terms of chemical stability, reduced corrosion, and foaming, compared to various aqueous solutions of MDEA with and without Pz, the absorbent solutions for which are listed in Table 3 below. The absorbent solutions given in Table 3 are reference solvents for acid gas treatment. Certain characteristics of the solutions used in the process are also compared to those of a 40 wt% aqueous diethanolamine (DEA) solution (absorbent solution x given in Table 3), which is also a reference solvent for acid gas treatment. Table 3 Absorbent solution H MDEA (50% by weight) + H2O Absorbent solution I MDEA (47% by weight) + H2O Absorbent solution J MDEA (45% by weight) + H2O K absorbent solution MDEA (39% by weight) + Pz (6% by weight) + H₂O Absorbent solution L MDEA (40 wt.) + Pz (3.3 wt.) + H2O Absorbent solution x DEA (40% by weight) + H2O

[0113] In the absorbent solutions given in Tables 1 to 3, the sum of the mass fractions expressed as % weight of the different compounds and water is equal to 100% by weight of the virgin absorbent solution, i.e. the absorbent solution without taking into account absorbed acid gases or other co-absorbed products or degradation products.

[0114] In example 3, the chemical stability of absorbent solution B is compared to that of the prior art absorbent solution K based on MDEA and Pz, as well as to that of absorbent solution F according to documents WO04082809 or US6267939. The chemical stability of absorbent solution B is also compared to that of the prior art absorbent solution M containing 50% TMDPTA, and that of the prior art absorbent solution N containing 25% TMDPTA, 15% MDEA and 5% Pz, a solution similar to absorbent solution B but in which PMDPTA is replaced by TMDPTA.

[0115] In example 4, the corrosivity of absorbent solutions A and B is compared to that of the prior art absorbent solution J based on MDEA, to that of the prior art absorbent solution L based on MDEA and Pz, and to that of an aqueous solution of DEA at 40 wt%.

[0116] Finally, in example 5, the foaming characteristics of absorbent solutions A and B are compared to those of the prior art absorbent solution J based on MDEA and the prior art absorbent solution K based on MDEA and Pz. Example 1: Miscibility at 40°C. Effect of CO2 concentration

[0117] The demixing phenomenon observed in aqueous solutions of PMDPTA charged with CO2, as described in document FR2877858, can be eliminated by combining PMDPTA with MDEA in the proportions defined according to the invention. This absorbent solution consists of substituting a certain weight percentage of PMDPTA with an equivalent weight percentage of MDEA according to the invention.

[0118] Depending on the composition of the PMDPTA-based absorbing solution and the composition of the gas to be treated, particularly the partial pressure of CO2, a liquid-liquid phase separation can occur, known as demixing. Laboratory tests (in perfectly stirred gas-liquid reactors) determine the conditions under which demixing occurs for a given absorbing solution (i.e., given concentrations of amines and water) at 40°C, by progressively increasing the partial pressure of CO2 and therefore the CO2 loading rate (α = n acid gas / n amine, where n acid gas is the number of moles of acid gas and n amine is the number of moles of amine in the liquid solution) at physicochemical equilibrium.

[0119] According to the results of these laboratory tests, the absorbent solution can be implemented in a deacidification process as described in the figure 1An absorbing solution based on PMDPTA and MDEA as defined according to the invention is particularly suitable for this type of process, because it remains monophasic under the operating conditions corresponding to those of the absorption column (i.e. generally 40°C).

[0120] This study focuses on liquid-liquid equilibria at 40°C, which corresponds to the low temperatures found in an absorption column. Laboratory tests were conducted using absorbent solutions A and C with a tertiary amine concentration of 50 wt%. Table 4 below summarizes the results obtained for the different absorbent solutions. Table 4 Absorbent solution [PMDPTA] (%weight) [MDEA] (%weight) Load rate at which we enter the two-phase zone Charge rate at which we exit the two-phase zone A 22 28 no mixing by raising the load ratio for PPCO2 < 0.3 MPa (i.e., up to a load ratio of 1.24) C 50 0 0,7 1,5

[0121] According to the results in Table 4, we can see that solution A allows us to have a single-phase absorbing solution under operating conditions representative of those of the absorption column (in the example 0.3 MPa of partial pressure in CO2 and 40°C, corresponding to a CO2 loading rate of 1.24).

[0122] On the other hand, we see that an absorbing solution containing 50% of PMDPTA according to the prior art (solution C) exhibits a demixing phenomenon for a charge ratio between 0.7 and 1.5. Example 2: Effect of temperature on miscibility

[0123] The demixing phenomenon observed in aqueous solutions of PMDPTA can be eliminated by combining PMDPTA with MDEA in the proportions defined according to the invention. This specific combination also prevents any demixing phenomenon observed in PMDPTA solutions without an activator or in the presence of an activator such as piperazine.

[0124] Depending on the composition of the PMDPTA-based absorbing solution and the possible presence of a primary or secondary amine, and the temperature of the absorbing solution, a liquid-liquid phase separation may occur (demixing phenomenon).

[0125] The temperature at which demixing occurs for a given absorbent solution (i.e., given concentrations of amines and water) is determined by laboratory tests in a thermostatically controlled oil bath. This is achieved by progressively increasing the temperature of the bath in which a sample of said absorbent solution is immersed. The demixing phenomenon is detected by observing the change in the transparency of the mixture: the mixture becomes cloudy when demixing occurs.

[0126] According to the results of these laboratory tests, the absorbent solution can be implemented in a deacidification process as described in the figure 1An absorbing solution based on PMDPTA and MDEA in the proportions defined by the invention is particularly suitable for this type of process, because it allows to remain single-phase under the temperature conditions after passing through the charge-effluent exchanger (generally less than or equal to 110°C).

[0127] Laboratory tests were performed on activated absorbent solutions (B and G) with a tertiary amine concentration of 40 wt% and a phosphorus concentration of 5 wt%. Table 5 below summarizes the results obtained for different absorbent solutions. The solutions were charged with CO₂ at a loading ratio of 1 mole per mole of amine, representative of the conditions at the outlet of the charge-effluent heat exchanger. Table 5 Absorbent solution [PMDPTA] (%weight) [MDEA] (%weight) [Pz] (%weight) Temperature (T) at which separation occurs (°C) A 22 28 0 No demixing by temperature rise for T ≤ 110°C C 50 0 0 40 B 27 13 5 No demixing by temperature rise for T ≤ 110°C G 30 10 5 95

[0128] According to the results in Table 5, the absorbent solutions implemented (solutions A and B) completely eliminate the demixing phenomenon, which is observed with the prior art solutions C and G. Absorbent solutions A and B are therefore interesting because they allow for a single-phase absorbent solution under conditions of CO2 loading rate and temperature corresponding to the outlet of the charge-effluent exchanger (generally less than or equal to 110°C).

[0129] On the other hand, we see that a solution loaded with CO2 containing 50% of PMDPTA (solution C according to the prior art) or 30% of PMDPTA and 5% of Pz (solution G according to the prior art) exhibit a demixing phenomenon for a temperature below 110°C, i.e. 95°C for solution G, and 40°C for solution C. Example 3: Stability of absorbing solutions

[0130] The amines in the absorbent solution implemented according to the invention are particularly resistant to degradation that can occur in a deacidification unit. A degradation test is performed on absorbent solutions within a closed reactor whose temperature is controlled by a regulation system. For each solution, the test is carried out with a liquid volume of 100 cm³ injected into the reactor. The solution is first degassed of all dissolved contaminants by maintaining a vacuum before any gas injection, and the reactor is then placed in a heating chamber at the set temperature and subjected to magnetic stirring.The liquid volume is then swept at 60°C overnight under a total pressure of 0.2 MPa by a gas flow of 23 NL / h consisting of a mixture of nitrogen and CO₂ at a partial pressure of 0.17 MPa, in order to achieve a loading rate representative of a solution loaded at the bottom of an absorber. After saturation, the CO₂-saturated absorbing solution is heated in a closed reactor to 140°C for 15 days. The solution is then cooled to 80°C and swept at this temperature by a nitrogen flow of 30 NL / h at a pressure of 0.2 MPa to remove the CO₂. The solution is then analyzed to determine the residual amine concentration.

[0131] The influence of oxygen on the degradation of the absorbing solution can also be studied by replacing some of the nitrogen with air during the saturation phase at 60°C and extending this phase for 3 days. The air and nitrogen flow rates, as well as the total pressure, are adjusted to achieve the desired partial pressure of oxygen while maintaining the partial pressure of CO₂ at 0.17 MPa during the saturation phase.

[0132] Table 6 below shows the relative degradation rate, by CO2 degradation, of solution B, prior art solution M based on TMDPTA, prior art solution N (similar to solution B but in which PMDPTA is replaced by TMDPTA), prior art solution F containing 50% PMDETA, and the reference absorbent solution K containing 39 wt% MDEA and 6 wt% Pz), for a duration of 15 days under CO2 pressure in the absence of oxygen during the loading phase. The relative degradation rate is calculated by dividing the degradation rate of the amines in the absorbent solution by that of absorbent solution K under the same experimental conditions.

[0133] The degradation rate (TD) to amines is calculated using the equation below: TD % = A m final − A ° m initial A ° m initial

[0134] Or : [A] is the total concentration of amines in the degraded solution, [A]° is the total concentration of amine in the non-degraded solution, and m initial and m final are the masses of solution before and after the degradation test.

[0135] The concentrations [A] and [A]° are determined by gas chromatography. Table 6 Absorbent solution Concentrations of amines (% weight) of the absorbing solution (with added water) TD related B 27% PMDPTA + 13% MDEA + 5% Pz 1 N 27% TMDPTA + 13% MDEA + 5% Pz 1,3 F 50% PMDETA 6 M 50% TMDPTA 6,3 K 39% MDEA + 6% Pz 1

[0136] Table 7 below gives the relative TD degradation rate, for a period of 15 days under CO2 pressure in the presence of oxygen in the saturation loading phase, the partial pressure of oxygen being maintained at 0.02 MPa for 3 days. Table 7 Absorbent solution [PMDPTA] (%weight) [MDEA] (%weight) [Pz] (%weight) TD related B 27 13 5 0,8 K 0 39 6 1

[0137] This example, according to Table 6, shows that the polyamines described in the prior art are not equivalent in terms of chemical stability compared to the reference solutions and the solutions implemented.

[0138] The absorbent solution used exhibits chemical stability under CO2 pressure and in the absence of oxygen comparable to that of a reference solution, unlike polyamine solutions according to the prior art.

[0139] Furthermore, the absorbent solution used exhibits increased resistance to oxygen compared to the reference solution K, according to Table 7.

[0140] This is particularly interesting for amine units processing synthesis gas (syngas) and natural gas which generally protect themselves from degradation by oxygen through special precautions, or in the case of CO2 capture in post-combustion, or decarbonation applications in the treatment of biogas from biogas fermentation which may contain several tenths to several percent by volume of oxygen. Example 4 : corrosivity of absorbent solutions

[0141] The corrosion resistance of the metallurgies used in the processes is an important point to check when introducing new technologies.

[0142] The corrosivity of absorbent solutions was studied through autoclave tests under more severe conditions than those encountered in the application. The results were compared to tests conducted on various reference absorbent solutions: an MDEA solution (solution J), a solution containing a mixture of MDEA and Pz (solution L), and an aqueous DEA solution (40% DEA by weight and 60% water by weight).

[0143] As it is known that the corrosivity of amine solutions is greater in the presence of degradation products, a first phase consisted of degrading solutions A and B at 140°C under 3.5 MPa of CO2 for one week.

[0144] These degraded solutions were then used to perform corrosion tests on AISI 1020 carbon steel and AISI 316L stainless steel coupons, the chemical compositions of which are given in Table 8 below (values ​​given as mass percentages). Iron, not shown in Table 1, is the major element in all these alloys. Table 8 Fe Cr Neither Mo C Mn If S AISI 1020 ball. 0,01 0,01 <0,005 0,17 0,75 0,01 0,003 AISI 316L ball. 16,86 10,30 2,18 0,02 1,34 0,45 0,002

[0145] A first test was carried out at 110°C under 3.5 MPa of CO2 to represent the charged solution under the temperature and CO2 charge rate conditions representative of extreme conditions that can be encountered between the bottom of the absorber and the inlet of the regenerator.

[0146] A second test was carried out, after extraction (by stripping) of CO2 from the previous solution, at 135°C under nitrogen, in order to represent the condition of the absorbing solution at the outlet of the regenerator (solution regenerated at a charge rate of less than 0.1 mole / mole).

[0147] For each test, two carbon steel coupons and two stainless steel coupons are placed on a rotating setup (peripheral speed close to 0.4 m / s). These coupons are in the form of 26 mm square plates, 1 mm thick. Before each test, each sample is polished with 600 grit sandpaper, then thoroughly degreased in ethanol, rinsed with deionized water, and weighed.

[0148] At the end of the test, each sample is adequately cleaned before weighing to remove corrosion product deposits, following the method described in ASTM G1. The corrosion rate is calculated from the mass change and is expressed in µm / year.

[0149] The corrosion rates obtained during these tests are given in Table 13 (data from the first test) and Table 14 (data from the second test) below and are compared to those obtained during similar tests carried out in solution J and solution L.

[0150] Table 9 shows the corrosion rates (µm / year) measured after 4 weeks of testing under rich loaded conditions (saturated under 3.5 MPa of CO2) and at 110°C.

[0151] Table 10 presents the corrosion rates (µm / year) measured after 4 weeks of testing under lean conditions (load rate less than 0.1 mol / mol) and at 135°C (solutions A and B) or 120°C (reference solutions).

[0152] It turns out that the absorbent solutions implemented are less corrosive than the reference solutions. Table 9 Absorbent solution Concentrations of amines (% weight) of the absorbing solution (with added water) Corrosion rate on AISI 1020 Carbon Steel (µm / year) Corrosion rate on AISI 316L stainless steel (µm / year) A 22% PMDPTA + 28% MDEA 6 <1 B 27% PMDPTA + 13% MDEA + 5% Pz 10 <2 J 45% MDEA 21 <5 L 40% MDEA + 3.3% Pz 12 <5 X 40% DEA 145 15 Table 10 Absorbent solution Concentrations of amines (% weight) of the absorbing solution (with added water) Corrosion rate on AISI 1020 Carbon Steel (µm / year) Corrosion rate on AISI 316L stainless steel (µm / year) A 22% PMDPTA + 28% MDEA <2 <1 B 27% PMDPTA + 13% MDEA + 5% Pz <1 <2 J 45% MDEA 11 <5 L 40% MDEA + 3.3% Pz <5 <5 X 40% DEA 2040 15 Example 5: Foaming of absorbent solutions saturated with hydrocarbons or containing compounds resulting from the degradation of amines

[0153] The foaming test is used to evaluate the foaming power of absorbent solutions. The foaming test consists of determining the height of foam generated after agitation of the solution; this is expressed as a percentage of the initial foam height of the solution, as a function of time.

[0154] It is carried out at 20°C on a volume of 50 ml of solution, placed in a 250 ml beaker, and stirred using a 5-blade stirrer for 4 minutes at 1200 rpm.

[0155] As soon as stirring stops, the height of foam generated above the solution surface is measured immediately (t0) and at 30 seconds and is expressed as a percentage of the initial liquid height. The measurement uncertainty is estimated at 2.5% of the initial height.

[0156] The study is carried out on the reference absorbing solutions J and K, and on the absorbing solutions A and B.

[0157] On absorbent solutions A and J, the effect on foaming of certain anionic products, generally resulting from the oxidative degradation of amines, is observed. In order to represent the composition of an aged solution, various carboxylic acids were added to absorbent solutions A and J in the following quantities, expressed in ppm by weight relative to the fresh absorbent solution: 3,000 ppm by weight of glycolic acid, 500 ppm by weight of oxalic acid, 40,000 ppm by weight of formic acid, and 6,000 ppm by weight of acetic acid, representing a total quantity of added carboxylic acids of 4.95% of the mass of the fresh absorbent solution.

[0158] To observe their foaming power on acid gas absorbent solutions in hydrocarbon-containing gases, absorbent solutions B and K, activated with Pz, are saturated with n-hexane or toluene by contacting the absorbent solution with a few milliliters of these compounds at room temperature. The hydrocarbon-saturated solutions B and K are then collected by decantation before being subjected to the foaming test.

[0159] Finally, the foaming power of absorbent solution B is compared with the reference absorbent solution K and the absorbent solution G according to the prior art, degraded under the conditions described in the absence of oxygen in example 7 and in which a mixture of carboxylic acids representing 4.95% of the mass of the degraded absorbent solution whose composition is indicated above has been added.

[0160] Table 11 shows the foam height, expressed as a percentage of the height before stirring, observed when stirring stopped and after 30 seconds in absorbing solutions A and J in the presence of carboxylic acids. Table 11 Absorbent solution Concentrations of amines (% weight) of the absorbing solution (with added water) Foam height generated (%) after agitation as a function of time(s) t = 0 t = 30 J + 4.95% acids 45% MDEA 7,5 0 A + 4.95% acids 22% PMDPTA + 28% MDEA < 2,5 0

[0161] It turns out that under the test conditions, in the presence of acidic compounds representative of oxidative degradation, the absorbing solution A does not exhibit a significant foaming effect in the presence of the reference absorbing solution J, which exhibits a tendency to foam under the same stirring conditions.

[0162] Table 12 shows the foam height, expressed as a percentage of the height before stirring observed when stirring stopped and after 30 seconds in absorbent solutions B and K saturated with n-hexane or toluene. Table 12 Absorbent solution Concentrations of amines (% weight) of the absorbing solution (with added water) Foam height generated (%) after agitation as a function of time(s) t = 0 t = 30 K saturated with n-hexane 39% MDEA + 6% Pz 9,5 2,4 B saturated with n-hexane 27% PMDPTA + 13% MDEA + 5% Pz 2,4 2,4 K saturated with toluene 39% MDEA + 6% Pz 12 2,4 B saturated with toluene 27% PMDPTA + 13% MDEA + 5% Pz 7 1,2

[0163] It turns out that under the test conditions, the n-hexane saturated solution B does not exhibit a significant foaming effect, while the reference absorbing solution K based on MDEA and Pz saturated with n-hexane exhibits a tendency to foam under the same stirring conditions.

[0164] It also turns out that in the presence of toluene, the absorbent solution B has a foaming factor almost half that of the reference solution K based on MDEA and Pz.

[0165] This notable effect makes it possible, for example, to consider, with the use of the absorbing solution according to the invention, a delay in clogging compared to the reference absorbing solution on an absorption column and, all other things being equal, an increase in the capacity of an existing gas treatment unit, i.e., an increase in the flow rate of acidic gas that can be treated for a given solvent flow rate.

[0166] For the construction of new acid gas treatment facilities containing hydrocarbons such as in the case of natural gas and implementing the absorbing solution according to the invention, it is also possible to consider a reduction in the diameter of the absorption columns and therefore the cost of the latter compared to the usual sizing rules for the reference absorbing solution, for a given flow rate of gas and absorbing solution.

[0167] Table 13 shows the foam height, expressed as a percentage of the height before stirring, observed when stirring stopped and after 30 seconds in absorbent solutions B, G and K degraded under the degradation conditions in the absence of oxygen, defined in Example 7, and to which a mixture of carboxylic acids representing 4.95% by weight of the degraded solution and whose composition is described above have been added. Table 13 Absorbent solution Concentrations of amines (% weight) of the absorbing solution (with added water) Foam height generated (%) after agitation as a function of time(s) t = 0 t = 30 K degraded + 4.95% acids 39% MDEA + 6% Pz 21 17 B degraded + 4.95% acids 27% PMDPTA + 13% MDEA + 5% Pz 10 10 G degraded + 4.95% acids 30% PMDPTA + 10% MDEA + 5% Pz 7 19

[0168] It turns out that under the test conditions, the solution degraded from solution B exhibits a foaming effect reduced by a factor of 2 compared to the absorbent solution degraded under the same conditions from the reference solution K based on MDEA and Pz. Surprisingly, it is also observed that the absorbent solution degraded from absorbent solution B containing 27% PMDPTA shows, after 30 seconds, a foaming tendency also reduced by a factor of 2 compared to the absorbent solution degraded from the prior art absorbent solution G containing 30% PMDPTA, which exhibits a delayed foaming effect.

Claims

1. Process for the removal of the acid compounds contained in a gaseous effluent, in which a step of absorption of the acid compounds is carried out by bringing the gaseous effluent into contact with an absorbent solution comprising: - water; - between 20% and 28% by weight of pentamethyldipropylenetriamine; - between 5% and 35% by weight of N-methyldiethanolamine; - between 5% and 50% by weight of at least one physical solvent chosen from the group consisting of methanol, ethanol, 2-ethoxyethanol, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, pentaethylene glycol dimethyl ether, hexaethylene glycol dimethyl ether, heptaethylene glycol dimethyl ether, octaethylene glycol dimethyl ether, diethylene glycol butoxyacetate, glyceryl triacetate, sulfolane, N-methylpyrrolidone, N-methylmorpholin-3-one, N,N-dimethylformamide, N-formylmorpholine, N,N-dimethylimidazolidin-2-one, N-methylimidazole, ethylene glycol, diethylene glycol, triethylene glycol, thiodiglycol, propylene carbonate, tributyl phosphate.

2. Process according to Claim 1, in which the absorbent solution comprises between 10% and 30% by weight of N-methyldiethanolamine, and preferably between 42% and 70% by weight of water.

3. Process according to Claim 1, in which the absorbent solution comprises between 37% and 75% by weight of water.

4. Process according to Claim 1, in which the absorbent solution comprises: - between 5% and 20% by weight of N-methyldiethanolamine; and - between 0.5% and 20% by weight of at least one activating compound comprising a primary or secondary amine function chosen from the group consisting of: - piperazine; - 1-methylpiperazine; - homopiperazine; - N-(2-hydroxyethyl)piperazine; - 3-(methylamino)propylamine; - N,N'-dimethyl-1,6-hexanediamine; - N-methyl-1,6-hexanediamine; - N,N',N'-trimethyl-1,6-hexanediamine; - 2-amino-2-methyl-1-propanol.

5. Process according to Claim 4, in which the absorbent solution comprises between 5% and 15% by weight of N-methyldiethanolamine, and preferably between 10% and 15% of N-methyldiethanolamine.

6. Process according to either of Claims 4 and 5, in which the activating compound is piperazine.

7. Process according to one of Claims 4 to 6, in which the absorbent solution comprises between 0.5% and 10% by weight of said at least one activating compound, preferably between 0.5% and 6% by weight of said at least one activating compound, and more preferentially still between 1% and 6% by weight of said at least one activating compound.

8. Process according to one of the preceding claims, in which the absorbent solution does not comprise an anti-foaming additive.

9. Process according to one of the preceding claims, in which the step of absorption of the acid compounds is carried out at a pressure of between 0.1 MPa and 20 MPa, and at a temperature of between 20°C and 100°C.

10. Process according to one of the preceding claims, in which an absorbent solution loaded with acid compounds is obtained after the absorption step, and at least one step of regeneration of said absorbent solution loaded with acid compounds is carried out at a pressure of between 0.1 MPa and 1 MPa, preferably of between 0.1 MPa and 0.5 MPa, and at a temperature of between 100°C and 180°C, preferably of between 110°C and 140°C.

11. Process according to Claim 10, in which the absorbent solution is a single-phase solution during the absorption step within an absorption column and up to at least the entry into a regeneration column in which the regeneration step is carried out, and preferably has a temperature of less than or equal to 110°C in the absorption column and up to at least the entry into the regeneration column.

12. Process according to one of the preceding claims, in which the gaseous effluent is chosen from natural gas, synthesis gas, combustion flue gases, refinery gases, acid gases resulting from an amine unit, tail gases resulting from a unit for the conversion of H2S to give sulfur by the Claus process, gases resulting from biomass fermentation, gases from cement works or incinerator flue gases.

13. Process for the selective removal of H2S with respect to CO2 from a gaseous effluent comprising H2S and CO2 according to one of the preceding claims, preferably for the selective removal of H2S with respect to CO2 from natural gas.

14. Process for the decarbonization of a biogas according to one of Claims 1 to 12.