Method for removing acid compounds from a gaseous effluent using a tertiary amine-based absorption solution

Abstract

The present invention relates to a method for removing acid compounds contained in a gaseous effluent, which comprises contacting the gaseous effluent in an absorption column with an absorption solution containing water, 20% to 28% by weight of pentamethyldipropylenetriamine, and 5% to 35% by weight of N-methyldiethanolamine.

Description

Technical Field

[0001] The present invention relates to the field of methods for the deacidification of gaseous effluents. The present invention is advantageously applied in the treatment of gases of industrial origin, natural gas and gases resulting from biomass fermentation.

Background Art

[0002] For removing acid compounds present in the gas, in particular carbon dioxide (CO2), hydrogen sulfide (H2S), carbonyl sulfide (COS), carbon disulfide (CS2), sulfur dioxide (SO2) and mercaptans (RSH) such as methyl mercaptan (CH3SH), ethyl mercaptan (CH3CH2SH) and propyl mercaptan (CH3CH2CH2SH), a gas deacidification process using an aqueous amine solution is generally used. The gas is deacidified by contacting it with an absorption solution, which is then regenerated thermally.

[0003] These processes for the deacidification of acid gases are generally known as "solvent scrubbing" with "chemical" solvents, as opposed to the use of "physical" solvents for absorption, which are based on solubility and which depend directly on and are proportional to the partial pressure of the entities in the gas phase, without being based on the implementation of chemical reactions. The partial pressure is the product of the concentration of the entity in the phase multiplied by the total operating pressure. "Chemical" solvents advantageously benefit from two effects to increase the solubility of the entities.

[0004] The chemical solvent corresponds to an aqueous solution containing reactants that preferentially react with acid compounds (such as H2S, CO2, COS, CS2, etc.) present in the gas to be treated in order to form salts without reacting with other non-acid compounds of the gas. Subsequently, the treated gas, after contacting the solvent, is depleted in acid compounds and selectively transferred into the solvent in the form of salts. The chemical reaction is reversible, whereby the solvent filled with acid compounds can be subsequently deacidified, for example under the action of heat, on the one hand, to release the acid compounds in gaseous form, which can then be stored, converted, or used for various applications, and on the other hand, the solvent can be regenerated, and the solvent returns to its initial state and can thus be reused in a new reaction stage with the acidic gas to be treated. The stage of the reaction between the solvent and the acidic gas is generally known as the absorption stage, and the stage where the solvent is deacidified is known as the regeneration stage of the solvent.

[0005] Generally, in this context, the performance quality of the separation of acid compounds from the gas mainly depends on the nature of the reversible reaction selected. Conventional processes for the deacidification of acidic gases are generally "amine" processes, i.e., they are based on the reaction between amines in aqueous solution and acid compounds. These actions fall within the general context of acid / base reactions. H2S, CO2, or COS, for example, are acid compounds, especially in the presence of water, while amines are basic compounds. The mechanism of the reaction and the nature of the resulting salts generally depend on the structure of the amines used.

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

[0007] The performance quality of the acid gas removal process by scrubbing with amines directly depends on the nature of the amines present in the solvent. These amines can be primary, secondary or tertiary. They can exhibit one or more equivalent or different amine functional groups per molecule.

[0008] To improve the performance quality of the acid gas removal process, more effective amines are constantly sought, especially those that can absorb a larger amount of acid gas and be regenerated with less energy.

[0009] One limitation for absorption solutions currently used in "selective" acid gas removal applications is the insufficient absorption selectivity of H2S over CO2. This is because in the case of natural gas sweetening, it may be required to selectively remove H2S while minimizing CO2 absorption. This constraint is particularly important when dealing with gases that already contain amounts of CO2 below the desired specification. Next, the maximum absorption capacity of H2S with the maximum H2S absorption selectivity over CO2 is required. This selectivity allows maximizing the amount of gas to be treated and recovering the acid gas at the regenerator outlet where the H2S concentration is as high as possible. This limits the size of the units in the sulfur line downstream of the treatment and ensures better operation of the units. In some cases, a unit for concentrating H2S may be required to concentrate the acid gas to H2S. This type of amine scrubbing unit installed to treat the acid gas also requires the most selective amines.

[0010] It is well known that tertiary or secondary amines with severe steric hindrance have a slower rate for capturing CO2 than primary or secondary amines with less hindrance. On the other hand, amines, especially tertiary or secondary amines with severe steric hindrance, generally have instantaneous kinetics for capturing H2S. As a result, selective removal of H2S can be performed using these tertiary and secondary amines with severe steric hindrance based on well-defined kinetic performance qualities.

[0011] Therefore, tertiary amines such as MDEA, or hindered secondary amines that exhibit slow reaction rates with CO2, are commonly used. However, their selectivity is limited when the acid gas is filled at a high level.

[0012] Another limitation for many absorption solutions commonly used in total deacidification applications is the extremely slow absorption rate of CO2 or COS. When the desired specifications for CO2 or COS are extreme, i.e., close to complete purification from these compounds, it is generally required to have the fastest possible reaction rate in order to lower the height of the absorption column. This is because if the reaction is slow, it is necessary to assume the use of a very tall column to give sufficient time for the chemical reaction to occur. In fact, the absorption column, especially when pressurized equipment is involved, accounts for a large portion of the capital cost of the process.

[0013] Regardless of whether the maximum rate theory of CO2 and COS absorption in the total deacidification of all acid contaminants or the minimum rate theory of CO2 capture in selective deacidification applications (selective absorption of H2S with respect to CO2) is required, it is always desirable to use an absorption solution with the largest possible circulation capacity with respect to the gaseous effluent to be treated, for example, the contaminants desired to be removed from natural gas. This circulation capacity, denoted by Δα, corresponds to the difference in the filling levels (α represents the number of moles of acid compound n absorbed per kilogram of absorption solution) between the absorption solution withdrawn at the bottom of the absorption column and the absorption solution supplied to the column. This is because the larger the circulation capacity of the absorption solution, the more restricted the flow rate of the absorption solution required to deacidify the gas to be treated. In a gas treatment process, a decrease in the flow rate of the absorption solution generally has a strong impact on reducing the capital cost, especially in the sizing of the column when it is possible to reduce the column diameter, but also by reducing the size of other items of equipment such as heat exchangers, pumps, and flash drums. acid gas The decrease in the flow rate of the absorption solution in a gas treatment process generally has a strong impact on reducing the capital cost, especially in the sizing of the column when it is possible to reduce the column diameter, but also by reducing the size of other items of equipment such as heat exchangers, pumps, and flash drums.

[0014] Another important aspect of the operation for gas treatment or for the scrubbing of industrial flue gases with solvents is the regeneration of the separating agent for purifying the solvent from the contaminants accumulated in the absorption stage. Depending on the type of absorption (physical and / or chemical), regeneration by means of reduced pressure and / or by distillation and / or by entrainment with a vaporized gas known as "stripping gas" is generally envisaged. The energy consumption required for the regeneration of the solvent can be very high, which is particularly true when the partial pressure of the acid gas is low or when the chemical binding force is high, and can represent a significant operating cost of the deacidification process.

[0015] It is well known to those skilled in the art that the energy required for the regeneration by distillation of an amine solution can be broken down according to three different headings: the energy required to heat the absorption solution between the top and the bottom of the regenerator, the energy required to reduce the partial pressure of the acid gas in the regenerator by evaporation of the stripping gas, and finally the energy required to break the chemical bond between the amine and the acid compound. These first two headings are proportional to the flow rate of the absorption solution that needs to circulate within the unit in order to achieve a given purification performance, i.e. specification, of the solvent to be regenerated. Therefore, in order to reduce the energy consumption associated with the regeneration of the solvent, it is further preferred to maximize the circulation volume of the solvent. This is because the larger the circulation volume of the absorption solution, the more the flow rate of the absorption solution that needs to be used to deacidify the gas to be treated can be limited.

[0016] In the search for more effective amines that make it possible to reduce the circulation flow rate and the regeneration energy, Patent US6267939 and Patent Application WO09 / 156273 provide absorption solutions based on specific polyamines such as N,N,N’,N’-tetramethyldipropylenetriamine (TMDPTA) or pentamethyldiethylenetriamine (PMDETA).

[0017] Also, for the purpose of reducing the renewable energy of the deacidification process, documents FR2877858, FR2895273, FR2900843, FR2898284, FR2900842, FR2986441 and FR2986442 provide the use of an absorption solution that forms two phases when absorbing a certain amount of acid gas so that only the phase filled with acid gas requires regeneration. However, this solution has the drawback of not being compatible with conventional high-pressure deacidification plants because it requires additional steps and items of equipment as well as multiple preventive measures to keep the operation under control. This is because the separation of a single-phase liquid absorption solution into two liquid phases, also known as the demixing phenomenon, can cause significant operational problems during absorption or at higher temperatures, upstream or during the regeneration stage, if the process and items of equipment are not suitable for handling two phases. These problems are described in particular in documents FR3014101 and WO2015 / 177333. Under the operating conditions of an absorber, also known as an absorption column, this liquid / liquid phase separation can interfere with the transfer of acid gas into the absorption solution and destabilize the column. It can also disrupt the liquid / gas phase equilibrium, which is very important for the implementation of the deacidification cycle, especially the regeneration stage. It can further cause sudden changes or irregular flows in the composition of the liquid stream, resulting in an unstable process, impossible control, and random performance.

[0018] Another problem lies in the stability of the absorption solution, particularly the thermal stability of the amine. The absorption solution may decompose under the influence of temperature, thereby restricting the operating conditions of the process, particularly the temperature at which solvent regeneration is carried out. As an example, increasing the temperature of the regenerator by 10 °C doubles the rate of thermal decomposition of monoethanolamine (MEA). Thus, the regeneration of an aqueous solution of an alkanolamine such as MEA is carried out at the bottom temperature of the regenerator on the order of 120 °C, and in practice, even at 130 °C in the case of a more stable amine such as MDEA. As a result of these bottom temperatures of the regenerators, acidic gases (CO2, H2S, COS, CS2, etc.) are obtained at moderate pressures of 0.1 - 0.3 MPa. Depending on the nature and use of the regenerated acidic gas, the acidic gas can be sent to a treatment unit or compressed for reinjection and isolation. In particular, for the purpose of overcoming this thermal stability problem, patent application WO04 / 082809 provides the use of an absorption aqueous solution containing, for example, a high concentration of a tertiary polyamine, typically exceeding 60%, such as pentamethyldipropylenetriamine (PMDPTA).

[0019] Still other problems are commonly encountered, such as the corrosiveness of the absorption solution or the foaming of the absorption solution.

[0020] This is because absorption solutions based on amines, such as alkanolamine-based absorption solutions like MEA, diethanolamine (DEA), or MDEA, are known to be corrosive with respect to items of steel equipment used in the process for the deacidification of gaseous effluents. These corrosion risks require fairly limited measures, such as the item of equipment being made of a corrosion-resistant alloy, which is more expensive or has lower mechanical resistance, or increasing the cost of the solution and using corrosion inhibitors that accumulate irreversibly and potentially in the form of fouling, the presence of which is recommended to be continuously monitored with effective content.

[0021] Foaming of the absorption solution is a known problem in the deacidification of gaseous effluents, which can lead to various harmful consequences such as premature blockage of the absorption column or the regeneration column, resulting in a reduction in production capacity, treated gas outside the target specifications, loss of amine due to entrainment of droplets into the treated gas or the acid gas, and in fact, even shutdown of the unit. Amine absorption solutions are known to have a tendency to foam, especially when they are in contact with liquid hydrocarbons. This problem is often solved by adding an antifoaming agent, which further increases the operating cost and complicates the process (necessity for monitoring of the content, addition of filters, etc.).

[0022] In this regard, it is difficult to find a formulation of an absorption compound that enables the removal of acid compounds in any type of effluent and enables the deacidification process to be operated at lower operating costs (including regeneration energy) and capital costs (including the cost of the absorption column), while meeting the requirements of absorption capacity, selectivity, especially chemical stability with respect to temperature, low corrosivity, and limitation of foaming.

Prior Art Documents

Patent Documents

[0023]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

[0024] Object and Summary of the Invention Accordingly, an object of the present invention is to meet the needs of the prior art and overcome one or more of the above-mentioned drawbacks of the prior art. [Means for Solving the Problems]

[0025] Accordingly, the present invention is a method for removing acid compounds such as CO2, H2S, COS, CS2 or mercaptans from a gas using an absorption aqueous solution containing a specific combination of at least two specific amines, which method unexpectedly exhibits at least one of the following effects: - Limitation of foaming compared to conventional solutions, especially in the presence of hydrocarbons and under decomposition conditions such as those in an operating unit; - Lower corrosivity compared to conventional solutions; - Greater stability compared to absorption solutions based on polyamines according to the prior art, i.e., reduction of decomposition especially in the presence of molecular oxygen.

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

[0027] The absorption solution according to the present invention can also limit the flow rate of the absorption solution used in this process as a result of good performance quality regarding the circulation capacity for the absorption of acid gases, particularly CO2 and H2S, and absorption selectivity for H2S. These performance qualities are superior to those of MDEA in terms of the circulation capacity for the absorption of acid gases, and also superior to those of the polyamines cited in the prior art, especially in the latter case.

[0028] According to the present invention, the absorption solution is preferably in a single-phase form under the operating conditions of this process, more specifically, at least under the absorption conditions and until the solution enters the regenerator, thereby making it possible to omit the separation step after absorption, particularly by sedimentation.

[0029] Thus, according to a first aspect, the present invention provides a method for removing acid compounds contained in a gaseous effluent, which includes carrying out a step of absorbing the acid compounds by bringing the gaseous effluent into contact with an absorption solution containing the following: - water; - 20% to 28% by mass of pentamethyldipropylenetriamine; - 5% to 35% by mass of N-methyldiethanolamine.

[0030] According to one or more embodiments of the present invention, the absorption solution contains 10% to 30% by mass of N-methyldiethanolamine and preferably 42% to 70% by mass of water.

[0031] According to one or more embodiments of the present invention, the absorption solution contains 37% to 75% by mass of water.

[0032] According to one or more embodiments of the present invention, the absorption solution contains the following: - 5% to 20% by mass of N-methyldiethanolamine; and At least one activating compound containing a primary or secondary amine functional group selected from the group consisting of the following, in an amount of 0.5% by mass to 20% by mass: - 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.

[0033] In this case, the absorption solution can contain 5% by mass to 15% by mass of N - methyldiethanolamine, preferably 10% by mass to 15% by mass of N - methyldiethanolamine.

[0034] Preferably, the activating compound is piperazine.

[0035] Advantageously, the absorption solution contains 0.5% by mass to 10% by mass of the at least one activating compound, preferably 0.5% by mass to 6% by mass of the at least one activating compound, more preferably 1% by mass to 6% by mass of the at least one activating compound.

[0036] According to one or more embodiments of the present invention, the absorption solution further comprises at least one physical solvent selected 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, 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 tributyl phosphate.

[0037] According to one or more embodiments of the present invention, the absorption solution does not contain an antifoaming additive.

[0038] According to one or more embodiments of the present invention, the absorption stage of the acid compound is carried out at a pressure of 0.1 MPa to 20 MPa and a temperature of 20 °C to 100 °C.

[0039] According to one or more embodiments of the present invention, an absorption solution filled with an acid compound is obtained after the absorption stage, and the method includes carrying out at least one regeneration stage of the absorption solution filled with the acid compound at a pressure of 0.1 MPa to 1 MPa, preferably 0.1 MPa to 0.5 MPa, and a temperature of 100 °C to 180 °C, preferably 110 °C to 140 °C.

[0040] According to one or more embodiments of the present invention, the absorption solution is a single-phase solution during the absorption stage in the absorption column and at least up to the inlet of the regeneration column where the regeneration stage is carried out, and preferably has a temperature of 110 °C or lower in the absorption column and at least up to the inlet of the regeneration column.

[0041] According to one or more embodiments of the present invention, the gaseous effluent is selected from natural gas, synthesis gas, combustion flue gas, refinery gas, acid gas resulting from an amine unit, tail gas resulting from a unit for converting H2S to sulfur by the Claus process, gas resulting from biomass fermentation, gas from a cement factory, or incinerator flue gas.

[0042] The method according to the present invention can be used to selectively remove H2S with respect to CO2 from a gaseous effluent containing H2S and CO2, preferably from natural gas.

[0043] The method according to the present invention can also be used for the decarbonization of biogas.

[0044] Other subjects and advantages of the present invention will become apparent by reading the following description, which is given by way of non-limiting example, following specific exemplary embodiments of the present invention, and this description is made with reference to the accompanying drawings described below.

Brief Description of the Drawings

[0045]

Figure 1

Mode for Carrying Out the Invention

[0046] Description of Embodiments The present invention provides for the removal of acid compounds from an aqueous effluent by using an absorption aqueous solution, the composition of which is described in detail below. This method will be described in more detail following the details regarding the composition of the absorption solution.

[0047] [Composition of the Absorption Solution] The absorption solution used for the removal of acid compounds contained in the gaseous effluent comprises the following: - water; - 20% to 28% by mass of pentamethyldipropylenetriamine (PMDPTA); and - 5% to 35% by mass of N-methyldiethanolamine (MDEA).

[0048] MDEA is a tertiary monoamine used in aqueous solution and constitutes a reference absorption solution for selectively absorbing H2S with respect to CO2 contained in the gas, particularly in the field of gas deacidification.

[0049] PMDPTA is a tertiary polyamine of the following formula, more specifically, a tertiary triamine.

[0050]

Chemical formula

[0051] It is known in the prior art that polyamines offer advantages over monoamines by having the potential to increase the absorption capacity, thereby enabling a greater number of reaction sites per unit mass of the product. However, polyamines are not equivalent with respect to the absorption capacity of acid gases, the selective removal performance of H2S, and the chemical stability under the conditions of the acid gas treatment process. In addition, some polyamines used in aqueous solution may cause a demixing phenomenon, i.e., the separation of a single-phase liquid absorption solution into two liquid phases, under specific temperature conditions and acid gas filling level conditions.

[0052] The combination of PMDPTA and MDEA at the indicated concentrations in aqueous solution enables the prevention of the separation of the absorption solution into two liquid phases, i.e., the demixing phenomenon, under the temperature and filling level conditions typically encountered during the stages of circulation to the absorber and regenerator, while achieving a particularly high capacity for the absorption of acid gases.

[0053] The inventors have also unexpectedly demonstrated that a specific combination of these two amines in an aqueous solution exhibits limited foaming in the presence of hydrocarbons and under the decomposition conditions of the absorption solution, excellent chemical stability characteristics, reduction of corrosion with respect to items of metal equipment used during deacidification, and very good performance quality for the selective absorption of H2S with respect to CO2 contained in the gas effluent to be treated.

[0054] According to one embodiment, the absorption solution contains 10% to 30% by mass of MDEA.

[0055] The absorption solution can contain 37% to 75% by mass of water.

[0056] When the absorption solution contains 10% to 30% by mass of MDEA, the absorption solution preferably contains 42% to 70% by mass of water.

[0057] Anywhere in this specification, the sum of the mass fractions expressed in mass% for the various compounds of the absorption solution is equal to 100% by mass of the virgin absorption solution, i.e., the absorption solution that does not take into account the absorbed acid gas, or other co-absorbed products or decomposition products.

[0058] Unless otherwise specified, the concentration ranges are understood to include the limits.

[0059] In this specification, unless otherwise noted, the pressure is expressed as an absolute value.

[0060] According to one embodiment, the aqueous absorption solution contains 20% to 25% by mass of PMDPTA and 25% to 35% by mass of MDEA.

[0061] According to a specific embodiment, the absorption solution consists of an aqueous solution composed of 20% to 28% by mass of PMDPTA and 5% to 35% by mass of MDEA, preferably an aqueous solution composed of 20% to 25% by mass of PMDPTA and 25% to 35% by mass of MDEA.

[0062] According to a specific embodiment, the absorption solution includes the following: - Water; - 22% to 28% by mass of PMDPTA; and - 5% to 35% by mass of MDEA.

[0063] In this case, the absorption solution can be an aqueous solution composed of 22% to 28% by mass of PMDPTA and 5% to 35% by mass of MDEA, preferably an aqueous solution composed of 22% to 25% by mass of PMDPTA and 25% to 35% by mass of MDEA.

[0064] According to another embodiment, the absorption solution includes the following: - Water; - 20% to 28% by mass of PMDPTA; - 5% to 20% by mass of MDEA; and - 0.5% to 20% by mass of at least one activating compound selected from the group consisting of the following primary or secondary amine functional groups: - 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.

[0065] According to this embodiment, the absorption solution can include a mixture of activating compounds as listed above, as understood by the expression "at least one activating compound".

[0066] An activating compound is understood to mean a compound that can accelerate the absorption rate of CO2 and optionally COS contained in the gas to be treated. In this specification, an "activating solution" is referred to to indicate an absorption solution containing such an activating compound.

[0067] According to the present invention, such an activating solution is used, in particular, for the non-selective deacidification of the gas to be treated, for example, for the total deacidification of the gas, i.e., to achieve very strict specifications, i.e., high purification performance quality, when it is desired to remove CO2 and COS.

[0068] Preferably, according to this embodiment regarding the activating solution, the absorption solution contains 0.5% to 10% by mass of the at least one activating compound, preferably 0.5% to 6% by mass of the at least one activating compound, and more preferably 1% to 6% by mass of the at least one activating compound.

[0069] According to this embodiment regarding the activating solution, the concentration of MDEA in the absorption solution is preferably 5% to 15% by mass, preferably 10% to 15% by mass. In this case, the absorption solution can contain 0.5% to 10% by mass of the at least one activating compound, preferably 0.5% to 6% by mass of the at least one activating compound, and more preferably 1% to 6% by mass of the at least one activating compound.

[0070] Preferably, according to this embodiment regarding the activating solution, the absorption solution contains at least one activating compound that is piperazine. More preferably, the absorption solution contains piperazine as the activating compound.

[0071] According to another preferred embodiment, the absorption solution contains and can consist of the following: - water; - 20% to 28% by mass of PMDPTA, preferably 22% to 27% by mass of PMDPTA; -5 mass% to 20 mass% of MDEA, preferably 5 mass% to 15 mass% of MDEA; and -0.5 mass% to 20 mass% of piperazine, preferably 0.5 mass% to 10 mass% of piperazine, more preferably 1 mass% to 6 mass% of piperazine.

[0072] The concentration of water is variable and represents the mass complement with respect to the total of the other compounds contained in the absorption solution.

[0073] According to the present invention, the concentration of water is variable.

[0074] Preferably, the absorption solution contains at least 37 mass% and at most 75 mass% of water.

[0075] According to one embodiment, the absorption solution contains an organic compound that is generally known as a "physical solvent" and is non-reactive with respect to acid compounds, which makes it possible to increase the solubility of at least one or more acid compounds in the gaseous effluent. Thus, according to this embodiment, the absorption solution can contain 5 mass% to 50 mass% of the physical solvent.

[0076] For example, the absorption solution can contain at least one physical solvent such as alcohols, ethers, ether alcohols, glycols and ethers of polyethylene glycol, glycol thioethers, esters and alkoxy esters of glycols and polyethylene glycol, glycerol esters, lactones, lactams, N-alkylated pyrrolidones, morpholin-3-one, derivatives of morpholine, imidazoles and imidazolidinones, N-alkylated piperidones, cyclotetramethylene sulfone, N-alkylformamides, N-alkylacetamides, ether ketone alkyl carbonates or alkyl phosphates and their derivatives. As non-limiting examples, the physical solvent can be 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, 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 or tributyl phosphate.

[0077] [Properties of the gaseous effluent] According to the present invention, the absorption solution can be used to deacidify the following gaseous effluents: natural gas, synthesis gas, combustion flue gas, refinery gas, acid gas resulting from an amine unit, tail gas resulting from a unit for converting H2S to sulfur by the Claus process, biogas such as gas resulting from biomass fermentation, gas from a cement factory or incinerator flue gas. These gaseous effluents contain one or more of the following acid compounds: CO2, H2S, mercaptans (e.g., methyl mercaptan (CH3SH), ethyl mercaptan (CH3CH2SH), propyl mercaptan (CH3CH2CH2SH)), COS, CS2 or SO2.

[0078] Combustion flue gas is produced, in particular, by the combustion of hydrocarbons, biogas, or coal in a boiler or, for example, for a combustion gas turbine for the purpose of generating electricity. As an example, the deacidification process according to the present invention can be carried out to absorb at least 70%, preferably at least 80%, and in fact even at least 90% of the CO2 contained in the combustion flue gas. Such removal of CO2 can be referred to as "decarbonization" of the gas. These flue gases generally have a temperature of 20°C to 60°C and a pressure of 0.1 MPa to 0.5 MPa, and contain 50% to 80% by volume of nitrogen (N2), 5% to 40% by volume of CO2, 1% to 20% by volume of oxygen (O2), and, if not removed upstream of the deacidification process, a small amount of impurities such as SO x compounds and NO x compounds. In particular, the deacidification process according to the present invention is particularly suitable for absorbing CO2 contained in combustion flue gas having a low CO2 partial pressure, for example, less than 0.02 MPa.

[0079] The acid removal process according to the present invention can be used to remove acid from synthesis gas. The synthesis gas contains carbon monoxide CO, hydrogen H2 (usually, the H2 / CO ratio is equal to 2), steam (usually in a saturated state at the temperature where scrubbing is carried out), and CO2 (on the order of about 10% by volume). The pressure is usually 2 - 3 MPa, but can reach up to 7 MPa. It can further contain sulfur-containing impurities (such as H2S, COS, etc.), nitrogen-containing impurities (NH3, HCN), and halogen-containing impurities.

[0080] The acid removal process according to the present invention can be used to remove acid from natural gas. Natural gas is mainly composed of gaseous hydrocarbons, but can contain some of the following acid compounds: CO2, H2S, mercaptan, COS, or CS2. The content of these acid compounds can vary very easily, up to 70% by volume for CO2 and up to 40% by volume for H2S. The temperature of the natural gas can be 10°C - 100°C. The pressure of the natural gas to be treated can be 1 - 20 MPa. The present invention can be used to achieve the specifications generally imposed on the acid-removed gas, such as less than 2% by volume of CO2, actually less than 50 ppm by volume of CO2, less than 4 ppm by volume of H2S, and less than 50 ppm by volume, and further less than 10 ppm by volume of total sulfur, in order to continue the liquefaction of natural gas.

[0081] The deacidification process according to the present invention can be used to deacidify biogas, typically a gas that is generally pretreated to remove impurities such as H2S, mercaptans or siloxanes, which results from biomass fermentation. These gases generally have a temperature of 5°C to 60°C and a pressure of 0.1 MPa to 2 MPa, and can contain 30% to 75% by volume of methane, 0% to 40% by volume of nitrogen (N2), 15% to 50% by volume of CO2, and 0% to 10% by volume of oxygen (O2). In particular, the deacidification process according to the present invention is advantageously used to remove CO2 from biogas, which has the prominent feature of generally containing a large amount of oxygen, for example, several tenths to several percent by volume of oxygen. Amines are generally sensitive to oxygen and are known to decompose more easily in the presence of oxygen. Therefore, the present invention is particularly suitable for the treatment of biogas as a result of the good chemical stability of the absorption solution in the presence of oxygen.

[0082] [Method for removing acid compounds in gaseous effluent] The method for removing acid compounds from a gaseous effluent according to the present invention includes a step of absorbing the acid compounds by bringing the gaseous effluent into contact with an absorption solution.

[0083] As shown in the scheme of Figure 1, a regeneration step follows the absorption step.

[0084] In the present invention, various ranges of parameters for a given step, such as a pressure range and a temperature range, can be used alone or in combination. For example, in the present invention, a certain range of preferred pressure values can be combined with a more preferred range of temperature values.

[0085] Referring to FIG. 1, a plant for the deacidification of gaseous effluents includes an absorption column C1 equipped with means for bringing the gas and the liquid into contact, such as random packing, structured packing or plates. The gaseous effluent to be treated is carried by a pipe 1 that appears at the bottom of column C1. A pipe 4 enables the introduction of the absorption solution at the top of column C1. A pipe 2 enables the discharge of the treated (deacidified) gas, and a pipe 3 enables the absorption solution rich in acid compounds after absorption to be carried to a regeneration column C2. This regeneration column C2 is equipped with an internal mechanism for bringing the gas and the liquid into contact, such as plates, random packing or structured packing. At the bottom of column C2, there is a reboiler R1 that contributes the heat necessary for regeneration by evaporating a part of the absorption solution. The solution rich in acid compounds is introduced at the top of regeneration column C2 via a pipe 5. A pipe 7 enables the discharge of the gas rich in acid compounds released during regeneration at the top of column C2, and a pipe 6 located at the bottom of column C2 enables the regenerated absorption solution to be sent back to absorption column C1. A heat exchanger E1 enables the heat of the regenerated absorption solution generated from column C2 to be recovered in order to heat the absorption solution rich in acid compounds coming out of absorption column C1.

[0086] The absorption stage consists of bringing the gaseous effluent arriving via pipe 1 into contact with the absorption solution arriving via pipe 4. During contact, the amine functional groups of the molecules of the absorption solution react with the acid compounds contained in the effluent to obtain a gaseous effluent depleted in acid compounds, which is discharged via pipe 2 at the top of column C1, and an absorption solution rich in acid compounds, which is discharged via pipe 3 at the bottom of column C1 for regeneration.

[0087] The absorption stage of the acid compounds can be carried out at a pressure in column C1 of 0.1 MPa to 20 MPa, preferably 2 MPa to 10 MPa, for the treatment of natural gas, and preferably at 0.1 MPa to 0.3 MPa for the treatment of industrial flue gas.

[0088] The absorption stage of the acid compound can be carried out at a temperature within the column C1 of 20°C to 100°C, preferably 30°C to 90°C, and in practice 30°C to 60°C.

[0089] The regeneration stage consists, in particular, of heating the absorption solution rich in acid compound, optionally under reduced pressure, in order to release the acid compound in gaseous form. The absorption solution rich in acid compound coming out of column C1 is introduced into heat exchanger E1, where it is heated by the flow circulating in pipe 6 coming from the regeneration column C2. The solution heated at the outlet of E1 is introduced into the regeneration column C2 via pipe 5 generally at a temperature of 110°C or less.

[0090] In the regeneration column C2, the absorption solution reaching via pipe 5 is brought into contact with the vapor generated by the reboiler, under the effect of which the acid compound is released in gaseous form and discharged at the top of column C2 via pipe 7. The regenerated absorption solution, i.e., the absorption solution depleted of acid compound, is discharged via pipe 6, cooled in E1, and then recirculated to the absorption column C1 via pipe 4.

[0091] The regeneration stage can be carried out by thermal regeneration and optionally supplemented with one or more reduced pressure stages. For example, the absorption solution rich in acid compound discharged via pipe 3 can be sent to a first flash drum (not shown) after optional heating and before entering the heat exchanger E1. In the case of natural gas, by reducing the pressure, it is possible to obtain the gas discharged at the top of the drum, which contains most of the aliphatic hydrocarbons co-absorbed in the absorption solution. This gas can be optionally washed off by a part of the regenerated absorption solution, and the gas thus obtained can be used as fuel gas. The flash drum 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 set according to the use conditions of the fuel gas and is typically on the order of 0.2 to 1.5 MPa, preferably 0.5 to 1.5 MPa. The flash drum operates at a temperature substantially the same as the temperature of the absorption solution obtained at the bottom of the absorption column C1.

[0092] Subsequently, the absorption solution contains a reduced amount of acid gas after the depressurization step. It can be partially recycled to the absorption column C1, optionally at a different level than the regenerated absorption solution resulting from column C2, for a new absorption cycle in a particular embodiment of the present invention (not shown). When the absorption solution is partially recycled to the absorption column, the other part of the absorption solution resulting from the depressurization step is introduced into the regeneration column C2.

[0093] In a particular embodiment of the present invention (not shown), the absorption solution is completely recycled to the absorption column C1 after the depressurization step, and the regeneration column C2 is not used.

[0094] Regeneration can be carried out at a pressure within the column C2 of 0.1 MPa to 0.5 MPa, preferably 0.1 MPa to 0.4 MPa, and actually up to 1 MPa, and a temperature within the column C2 of 100 °C to 180 °C, preferably 100 °C to 140 °C, more preferably 110 °C to 140 °C, even more preferably 115 °C to 140 °C, and even more preferably 115 °C to 130 °C. For example, the regeneration temperature within the column C2 is 115 °C to 130 °C when the acid gas is sent to the atmosphere or to a downstream treatment process such as the Claus process or a tail gas treatment process.

[0095] According to a preferred embodiment, regeneration is carried out at a pressure within the column C2 of 0.1 MPa to 0.5 MPa, preferably 0.1 MPa to 0.4 MPa, and a temperature of 110 °C to 140 °C, more preferably 115 °C to 130 °C.

[0096] According to the present invention, the liquid absorption solution preferably remains a single-phase solution during the absorption stage in the absorption column C1 and during the circulation of the acid compound-rich solution up to at least the inlet to the regeneration column C2. This is because a specific formulation of the absorption solution can minimize the risk of demixing of the absorption solution during absorption and circulation to the regeneration column, as such demixing can cause operational problems. This is because when the solution is in the form of two separate phases in the absorption column, the flow of acid compounds moving from the gas to the absorption solution is strongly affected, and as a result, it is generally necessary to adjust the height of the column (increase in column size). Therefore, this phenomenon presents implementation difficulties and is difficult to model considering the complexity of the system. Furthermore, a solution in the form of two separate liquid phases requires the installation of a separation device (e.g., for separation by sedimentation) upstream of the regeneration column, which complicates the process and increases costs. Such problems are thus avoided in the process according to the present invention.

[0097] According to the present invention, the absorption solution minimizes corrosion of items of equipment of a plant for the deacidification of a gaseous effluent, the surface of which comprises steel and which is adapted to be in contact with said solution.

[0098] These items of equipment can be made of low-alloy steel, also known as carbon steel, or stainless steel. Low-alloy steel is understood to mean steel mainly composed of iron, in particular at least 90% by mass of iron, and from 0.01% to 2.5% by mass of carbon, and at least one alloying element selected from aluminum, chromium, cobalt, copper, manganese, molybdenum, nickel, niobium, phosphorus, silicon, titanium, tungsten or 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 within the limit of 1% by mass. Stainless steel, which is less susceptible to corrosion, differs in composition from low-alloy steel in that its chromium content exceeds 11.5% by mass.

[0099] Items of the associated apparatus include, but are not limited to, absorption and regeneration columns where gas / liquid contact occurs; pipes connecting various elements of the plant; elements placed within a chamber, such as plates, structured or random packings, heat exchangers, pump and valve bodies, or storage tanks.

[0100] According to the present invention, items of the apparatus include a surface mainly composed of steel, but this does not exclude the presence of other materials used in the manufacture of such items of the apparatus.

[0101] Due to its limited corrosive power, the absorption solution makes it possible to limit the corrosion of items of the apparatus of the deacidification plant and also to limit or render unnecessary the use of corrosion inhibitors in the method according to the present invention.

[0102] Another advantage provided by the use of the absorption solution for the deacidification of the gaseous effluent according to the method of the present invention is the limitation of the foaming phenomenon. Foaming generally impairs the performance quality of the process and can even lead to the shutdown of the unit. Foaming is a phenomenon that is still difficult to control today, yet it is important for the operation of the deacidification unit.

[0103] Unexpectedly, the absorption solution makes it possible to limit foaming in the presence of aliphatic or aromatic hydrocarbons that may initially be present in the gas to be treated and / or under specific decomposition conditions of the solution, reflecting its use conditions. Thus, it is possible to limit or even dispense with the use of antifoam additives conventionally used to overcome the problem of foaming of amine solutions. Thus, according to one embodiment, the absorption solution does not contain antifoam additives.

[0104] The antifoam additive is understood to mean any additional compound or mixture of additional compounds in the absorption solution other than the above-mentioned compounds that can participate in the composition of the absorption solution, which can prevent the foaming of the absorption solution or remove already formed bubbles.

[0105] Conventionally, antifoaming additives have been used by an initial metering consisting of the periodic injection of the antifoaming solution into the absorption solution to achieve a few mass percent, and then this metering is adjusted according to the level of foaming observed on the unit. Conventionally, silicon-based organic compounds such as polydialkylsiloxanes or silicone resins are generally used as emulsions in water. It should be noted that the excessive or inappropriate use of antifoaming additives is known to exacerbate foaming problems, which makes it difficult to manage the use of such additives.

[0106] To characterize the foaming power of the solution and / or the need to use an antifoaming additive, a test that can be advantageously used in the laboratory consists of determining the height of the foam generated in a graduated cylinder-shaped cylindrical glass container following the stirring of the solution, and the height of the foam is expressed as a percentage of the initial height of the solution as a function of time.

[0107] It is carried out, for example, at 20 °C on a 50 ml volume of solution, placed in a 250 ml beaker and stirred for 4 minutes at 1200 revolutions per minute using a five-blade stirrer. Immediately after stopping the stirring, the height of the foam generated on the surface of the solution is measured immediately (t0) and at 30 seconds and expressed as a percentage of the initial height. The uncertainty in the measurement corresponds to 2.5% of the initial height.

[0108] According to the present invention, the absorption solution shows resistance to decomposition, especially in the presence of oxygen, and thus limits the formation of by-products that promote corrosion by phenomena well known to those skilled in the art. In the present specification, mention is also made in particular of the chemical stability for representing this resistance to decomposition at high temperatures such as during the regeneration of the absorption solution.

[0109] The method according to the invention can advantageously selectively remove H2S from the gas to be treated, for example natural gas, with respect to CO2. This is because the absorption solution has excellent performance quality for this type of application, in particular a high absorption capacity for H2S and a relatively slow absorption rate for CO2, which enables effective selective removal and also allows the flow rate of the absorption solution used to be reduced.

[0110] The method according to the invention can advantageously be used, in particular for the non-selective deacidification of gaseous effluents, such as the decarbonization of combustion flue gases or biogas, not only from the point of view of the ability to absorb acid gases, in particular CO2, but also from the point of view of the rate of absorption of CO2 and the point of view of chemical stability, due to the good performance quality of the activated absorption solution.

Examples

[0111] The following examples illustrate, in a non-limiting manner, the performance quality of the absorption solutions used according to the invention for removing acid compounds such as CO2 or H2S contained in gaseous effluents.

[0112] As the absorption solutions in these examples, an aqueous solution of PMDPTA combined with MDEA is used. The composition of these solutions used in the method according to the invention is summarized in Table 1 below.

[0113] Some absorption solutions also contain piperazine (Pz) used as an activator.

[0114]

Table 1

[0115] In the first step (Examples 1 and 2), it has been shown that the specific physico-chemical properties of absorption solutions A and B, in particular the miscibility of the components, are very different from those of aqueous solutions of PMDPTA according to the prior art, regardless of the presence or absence of Pz.

[0116] The compositions of the absorption solutions (absorption solutions C, D, E) according to the prior art using PMDPTA are shown in Table 2 below. Also shown in Table 2 are absorption solution F based on pentamethyldiethylenetriamine (PMDETA), which is another polyamine (according to document WO04 / 082809 or US6267939), and two other absorption solutions M and N based on N,N,N’,N’-tetramethyldipropylenetriamine (TMDPTA), which is another polyamine disclosed, for example, in document US6267939 or WO09 / 156273A2.

[0117]

Table 2

[0118] In Examples 3 to 5, the advantages of the absorption solutions used according to the present invention are explained in terms of chemical stability, corrosion and limitation of foaming problems, in comparison with various aqueous solutions of MDEA, with or without Pz (the absorption solutions are shown in Table 3 below). The absorption solutions shown in Table 3 constitute reference solvents for the treatment of acid gases. Some characteristics of the solutions used in the method according to the present invention are also compared with the characteristics of a 40% by mass aqueous solution of diethanolamine (DEA) (absorption solution x shown in Table 3), which is also a reference solvent for the treatment of acid gases.

[0119]

Table 3

[0120] In the absorption solutions shown in Tables 1 to 3, the sum of the mass fractions expressed as the mass % of the various compounds and water is equal to 100% by mass of the virgin absorption solution, i.e., the absorption solution without considering the absorbed acid gas, or other co-absorption products or decomposition products.

[0121] In Example 3, the chemical stability of the absorption solution B is compared with the chemical stability of the absorption solution K according to the prior art based on MDEA and Pz, and the chemical stability of the absorption solution F according to the documents WO04 / 082809 and US6267939. The chemical stability of the absorption solution B is also compared with the chemical stability of the absorption solution M according to the prior art containing 50% TMDPTA, and the chemical stability of the absorption solution N according to the prior art (which is the same as the absorption solution B according to the present invention, but PMDPTA is replaced by TMDPTA) containing 25% TMDPTA, 15% MDEA and 5% Pz.

[0122] In Example 4, the corrosivity of the absorption solutions A and B is compared with the corrosivity of the absorption solution J according to the prior art based on MDEA, the corrosivity of the absorption solution L according to the prior art based on MDEA and Pz, and also the corrosivity of a 40% by mass aqueous solution of DEA.

[0123] Finally, in Example 5, the foaming characteristics of the absorption solutions A and B are compared with the foaming characteristics of the absorption solution J according to the prior art based on MDEA and the foaming characteristics of the absorption solution K according to the prior art based on MDEA and Pz.

[0124] [Example 1: Miscibility at 40°C, influence of CO2 filling level] As described in the document FR2877858, the demixing phenomenon observed in an aqueous solution of PMDPTA filled with CO2 can be eliminated by combining PMDPTA and MDEA in a ratio defined according to the present invention. This absorption solution consists of replacing a specific mass ratio of PMDPTA with an equivalent mass ratio of MDEA according to the present invention.

[0125] Depending on the composition of the absorption solution based on PMDPTA and the composition of the gas to be treated, particularly the CO2 partial pressure, liquid / liquid separation known as demixing can be carried out. For a given absorption solution (i.e., a given concentration of amine and water), the conditions for demixing to occur at 40°C are to gradually increase the CO2 partial pressure, thereby the filling level of CO2 in the physicochemical equilibrium state (α = n acid gas / namine , n acid gas is the number of moles of acid gas in the solution, and n amine is the number of moles of amine in the solution) is determined by laboratory tests (in a well-stirred gas / liquid reactor) by gradually increasing

[0126] According to the results of these laboratory tests, the absorption solution can be used in the acid removal process as described in FIG. 1. The absorption solution based on PMDPTA and MDEA defined by the present invention remains a single-phase solution under operating conditions corresponding to the operating conditions of the absorption column (i.e., generally 40° C.), and is particularly suitable for this type of process.

[0127] Here, the liquid / liquid equilibrium at 40° C. is involved, which corresponds to the low temperature in the absorption column. Laboratory tests are carried out on absorption solutions A and C having a tertiary amine concentration of 50% by mass. The results obtained for the various absorption solutions are summarized in Table 4 below.

[0128] [Table 4]

[0129] According to the results in Table 4, it can be seen that solution A enables a single-phase absorption solution to be obtained under operating conditions representative of the operating conditions of the absorption column (in the example, a CO2 partial pressure of 0.3 MPa and 40° C. corresponding to a CO2 filling level of 1.24).

[0130] On the other hand, it can be seen that the absorption solution (solution C) containing 50% PMDPTA according to the prior art shows a demixing phenomenon at a filling level of 0.7 to 1.5.

[0131] [Example 2: Influence of temperature on miscibility] The demixing phenomenon observed in an aqueous solution of PMDPTA can be eliminated by combining PMDPTA with MDEA at a ratio defined according to the present invention. This specific combination also makes it possible to prevent any demixing phenomenon observed in PMDPTA solutions, without an activator or in the presence of an activator such as piperazine.

[0132] Depending on the composition of the absorption solution based on PMDPTA, the presence of any primary or secondary amines, and the temperature of the absorption solution, liquid / liquid phase separation can occur (demixing phenomenon).

[0133] For a given absorption solution (i.e., a given concentration of amine and water), the temperature at which demixing occurs is determined by laboratory tests in a thermostatically controlled oil bath by gradually increasing the temperature of the bath in which a sample of the absorption solution is immersed. The demixing phenomenon is detected by observing a change in the transparency of the mixture, i.e., when demixing occurs, the mixture becomes turbid.

[0134] According to the results of these laboratory tests, the absorption solution can be used in a deacidification process as described in Figure 1. The absorption solution based on PMDPTA and MDEA in the ratio defined by the present invention is particularly suitable for this type of process because it makes it possible to remain single-phase under the temperature conditions (generally below 110 °C) after passing through the filling / effluent exchanger.

[0135] Laboratory tests are carried out on activated absorption solutions (B and G) having a tertiary amine concentration of 40% by mass and a Pz concentration of 5% by mass. The results obtained for the various absorption solutions are summarized in Table 5 below. The solutions are filled with CO2 at a filling level of 1 mol / mol amine, which represents the state at the outlet of the filling / effluent exchanger.

[0136]

Table 5

[0137] According to the results in Table 5, it can be seen that the absorption solutions (Solutions A and B) used according to the present invention can completely eliminate the demixing phenomenon observed in Solutions C and G according to the prior art. Therefore, Absorption Solutions A and B are advantageous because they enable a single-phase absorption solution under conditions of a CO2 filling level and a temperature corresponding to the outlet of the filling / effluent exchanger (generally 110 °C or less).

[0138] On the other hand, it can be seen that the CO2-filled solutions containing 50% PMDPTA (Solution C according to the prior art) or 30% PMDPTA and 5% Pz (Solution G according to the prior art) exhibit demixing phenomena at temperatures below 110 °C, i.e., 95 °C for Solution G and 40 °C for Solution C.

[0139] [Example 3: Stability of Absorption Solution] The amines of the absorption solutions used according to the present invention exhibit a prominent feature of being particularly resistant to decomposition that can occur in the deacidification unit.

[0140] The decomposition test is carried out on the absorption solution in a sealed reactor whose temperature is controlled by a regulating system. For each solution, a liquid volume of 100 cm 3 is injected into the reactor for testing. This solution is degassed in advance from all dissolved contaminants by maintaining a vacuum before injecting the gas, and then the reactor is placed in a heating jacket at the set temperature and under magnetic stirring. Thereafter, the liquid volume is flashed overnight at 60 °C and a total pressure of 0.2 MPa with a gas flow of 23 Sl / h consisting of a mixture of CO2 and nitrogen at a partial pressure of 0.17 MPa to achieve a filling level representing the solution filled at the bottom of the absorber. After saturation, the absorption solution saturated with CO2 is heated in a sealed reactor at 140 °C for 15 days. Then, the solution is cooled to 80 °C and then flashed at this temperature with a nitrogen flow of 30 Sl / h at a pressure of 0.2 MPa to remove CO2. Subsequently, the solution is analyzed to measure the concentration of the residual amine.

[0141] The influence of oxygen on the decomposition of the absorption solution can also be studied by replacing part of the nitrogen with air during the saturation stage at 60 °C and extending this stage for 3 days. During the saturation stage, the flow rates of air and nitrogen and the total pressure are adjusted to achieve the desired oxygen partial pressure while maintaining the CO2 partial pressure at 0.17 MPa.

[0142] During the filling stage, the relative decomposition rates due to the decomposition of solution B, prior art solution M based on TMDPTA, prior art solution N (similar to solution B but with PMDPTA replaced by TMDPTA), prior art solution F containing 50% PMDETA, and reference absorption solution K containing 39% by mass of MDEA and 6% by mass of Pz under CO2 in the absence of oxygen over 15 days are shown in Table 6 below. The relative decomposition rate is calculated by dividing the decomposition rate of the amine in the absorption solution by the decomposition rate of absorption solution K under the same experimental conditions.

[0143] The decomposition rate (DR) of the amine is calculated by the following formula.

[0144]

Equation

[0145] Here, [A] is the total concentration of the amine in the decomposed solution, [A]° is the total concentration of the amine in the non-decomposed solution, and w initial and w final are the masses of the solution before and after the decomposition test. The concentrations [A] and [A]° are determined by gas chromatography.

[0146]

Table 6

[0147] In the saturation filling stage, in the presence of oxygen, the relative decomposition rate DR when the oxygen partial pressure is maintained at 0.02 MPa for 3 days under a CO2 pressure for 15 days is shown in Table 7 below.

[0148] [Table 7]

[0149] According to Table 6, this example shows that the polyamine described in the prior art is not equivalent in terms of chemical stability compared to the reference solution and the solution used according to the present invention.

[0150] The absorption solution used according to the present invention, unlike the polyamine solution according to the prior art, exhibits chemical stability comparable to that of the reference solution under a CO2 pressure and in the absence of oxygen.

[0151] Furthermore, according to Table 7, the absorption solution used according to the present invention shows increased resistance to oxygen compared to reference solution K.

[0152] This is particularly advantageous for amine units for the treatment of synthesis gas (syngas) and natural gas, which generally prevent decomposition by oxygen by means of specific preventive measures or in the case of CO2 capture in post-combustion, or in the decarbonization applications for the treatment of biogas resulting from biomass fermentation, which can contain from a few volume percent to a few volume percent of oxygen.

[0153] [Example 4: Corrosiveness of the absorption solution] The corrosion resistance of the metallurgy used in the process is an important point to be confirmed when introducing new technologies.

[0154] The corrosivity of the absorption solution was studied by autoclave tests under conditions more severe than the application conditions. The results could be compared with tests carried out on different reference absorption solutions: a solution of MDEA (solution J), a solution containing a mixture of MDEA and Pz (solution L), and an aqueous solution of DEA (40 wt% DEA and 60 wt% water).

[0155] As the corrosivity of amine solutions is known to be greater in the presence of decomposition products, the first step consisted of decomposing solutions A and B for 1 week under 140 °C and 3.5 MPa of CO₂.

[0156] These decomposed solutions were then used to carry out corrosion tests on the lengths of AISI 1020 carbon steel and AISI 316L stainless steel, and their chemical compositions are shown in Table 8 below (values given as wt%). Iron is not shown in Table 8 and is the main element in all these alloys.

[0157]

Table 8

[0158] The first test was carried out at 110 °C and 3.5 MPa of CO₂ to represent a solution filled under conditions of temperature and CO₂ filling level that represent the extreme conditions that could be encountered between the bottom of the absorber and the inlet of the regenerator.

[0159] The second test was carried out at 135 °C under nitrogen after extracting CO₂ (by stripping) from the previous solution to represent the state of the absorption solution at the regenerator outlet (solution regenerated at a filling level of less than 0.1 mol / mol).

[0160] In each test, carbon steels of two lengths and stainless steels of two lengths are placed on a rotating mount (peripheral speed close to 0.4 m / s). These lengths are provided in the form of sheets with a side length (lateral length) of 26 mm and a thickness of 1 mm. Before each test, each sample is polished with grade 600 sandpaper, then carefully degreased in ethanol, rinsed with deionized water, and weighed.

[0161] To remove deposits of corrosion products, at the end of the test and before weighing, appropriate surface cleaning is carried out on each sample according to the method described in ASTM G1.

[0162] The corrosion rate is calculated from the change in mass and is expressed in μm / year.

[0163] The corrosion rates obtained during these tests are shown in Table 13 below (data from the first test) and Table 14 (data from the second test), and are compared with those obtained during similar tests carried out in Solutions J and L.

[0164] Table 9 shows the corrosion rates (μm / year) measured after a 4-week test at high filling conditions (saturated under 3.5 MPa of CO2) and 110 °C.

[0165] Table 10 shows the corrosion rates (μm / year) measured after a 4-week test under lean conditions (filling level less than 0.1 mol / mol) and at 135 °C (Solutions A and B) or 120 °C (reference solution).

[0166] It has been found that the absorption solution used according to the present invention is less corrosive than the reference solution.

[0167]

Table 9

[0168]

Table 10

[0169] [Example 5: Foaming of an absorption solution containing a compound saturated with a hydrocarbon or resulting from the decomposition of an amine] By conducting a foaming test, the foaming power of the absorption solution can be evaluated. The foaming test consists of measuring the height of the foam generated after stirring the solution, and the test is expressed as a percentage of the initial height of the solution as a function of time.

[0170] The test is carried out at 20 °C on a 50 ml solution, placed in a 250 ml beaker, and stirred for 4 minutes at 1200 revolutions per minute using a 5 - blade stirrer.

[0171] Immediately after stopping the stirring, the height of the foam generated on the surface of the solution is measured immediately (t0) and at 30 seconds, and expressed as a percentage of the initial height of the liquid.

[0172] The uncertainty in the measurement is evaluated at 2.5% of the initial height.

[0173] This study is carried out on reference absorption solutions J and K, and absorption solutions A and B according to the present invention.

[0174] In absorption solutions A and J, the effect on the foaming of specific anionic products generally resulting from the oxidative decomposition of the amine is observed. To represent the composition of the aged solutions, various carboxylic acids were added to absorption solutions A and J in the following amounts, expressed in ppm with respect to the fresh absorption solution. That is, glycolic acid 3000 mass ppm, oxalic acid 500 mass ppm, formic acid 40000 mass ppm, acetic acid 6000 mass ppm. That is, the total amount of carboxylic acids added corresponds to 4.95 mass% of the fresh absorption solution.

[0175] To observe their foaming power on the absorption solution of acid gas in a gaseous effluent containing hydrocarbons, absorption solutions B and K activated with Pz are saturated with n-hexane or toluene by contacting the absorption solution with several milliliters of these compounds at ambient temperature. Subsequently, the hydrocarbon-saturated solutions B and K are recovered by separation by sedimentation before being subjected to a foaming test.

[0176] Finally, the foaming power of the absorption solution B according to the present invention is compared with the reference absorption solution K and the absorption solution G according to the prior art, which are decomposed under the described conditions in the absence of oxygen in Example 7, and a mixture of carboxylic acids corresponding to 4.95% of the mass of the decomposed absorption solution (the composition of which is shown above) is added.

[0177] Table 11 shows the foam height expressed as a percentage of the height before stirring, observed at the stop of stirring and 30 seconds later, in absorption solutions A and J in the presence of carboxylic acids.

[0178]

Table 11

[0179] Under the test conditions, in the presence of acid compounds representing oxidative decomposition, the absorption solution A according to the present invention does not show a significant foaming effect, while the reference absorption solution J has been found to have a foaming tendency under the same stirring conditions.

[0180] Table 12 shows the foam height expressed as a percentage of the height before stirring, observed at the stop of stirring and 30 seconds later, in absorption solutions B and K saturated with n-hexane or toluene.

[0181]

Table 12

[0182] Under test conditions, solution B according to the invention saturated with n-hexane does not show a significant foaming effect, whereas reference absorption solution K based on MDEA and Pz saturated with n-hexane was found to show a foaming tendency under the same stirring conditions.

[0183] Also, in the presence of toluene, absorption solution B according to the invention was found to exhibit a foaming coefficient that is substantially half that of reference solution K based on MDEA and Pz.

[0184] This remarkable effect makes it possible, for example, to assume a delay in blockage on the absorption column by using the absorption solution according to the invention as compared to the reference absorption solution, and also, all other things being equal, to assume 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 processed for a given flow rate of the solvent.

[0185] For constructing a new plant for treating acidic gas containing hydrocarbons, such as in the case of natural gas, and using the absorption solution according to the invention, for a given gas flow rate and a given absorption solution flow rate, it is also possible to assume a reduction in the diameter of the absorption column, and thus a reduction in the cost of the absorption column, as compared to the normal sizing rules in the reference absorption solution.

[0186] In absorption solutions B, G and K, which were decomposed under the decomposition conditions in the absence of oxygen as defined in Example 7 and to which a mixture of carboxylic acid and its composition corresponding to 4.95% by mass of the decomposed solution was added, the foam height expressed as a percentage of the height before stirring, observed at the time of stopping stirring and 30 seconds later, is shown in Table 13.

[0187]

Table 13

[0188] Under test conditions, it was found that the solution decomposed starting from Solution B according to the present invention exhibited a foaming effect reduced by half compared to the absorption solution decomposed under the same conditions starting from the reference solution K based on MDEA and Pz. Surprisingly, the absorption solution decomposed starting from Absorption Solution B according to the present invention containing 27% PMDPTA also showed a foaming tendency reduced by half 30 seconds later compared to the absorption solution decomposed starting from the prior art Absorption Solution G containing 30% PMDPTA, which indicates a delaying effect in the appearance of foaming.

Claims

1. Performing a step of absorbing an acid compound by bringing a gaseous effluent into contact with an absorption solution, wherein the absorption solution comprises: - water; - 20% to 28% by mass of pentamethyldipropylenetriamine; - 5% to 35% by mass of N-methyldiethanolamine; A method for removing an acid compound contained in a gaseous effluent.

2. The method according to claim 1, wherein the absorption solution comprises 10% to 30% by mass of N-methyldiethanolamine.

3. The method according to claim 1, wherein the absorption solution comprises 37% to 75% by mass of water.

4. The absorption solution comprises: - 5% to 20% by mass of N-methyldiethanolamine; and - 0.5% to 20% by mass of at least one activating compound comprising a primary or secondary amine functional group 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; The method according to claim 1.

5. The method according to claim 4, wherein the absorption solution comprises 5% to 15% by mass of N-methyldiethanolamine.

6. The method according to claim 4 or 5, wherein the activating compound is piperazine.

7. The method according to any one of claims 4 to 6, wherein the absorption solution comprises 0.5% to 10% by mass of the at least one activating compound.

8. The method according to any one of claims 1 to 7, wherein the absorption solution further comprises at least one physical solvent selected 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, 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 tributyl phosphate.

9. The method according to any one of claims 1 to 8, wherein the absorption solution does not contain an antifoaming additive.

10. The method according to any one of claims 1 to 9, wherein the step of absorbing the acid compound is carried out at a pressure of 0.1 MPa to 20 MPa and a temperature of 20 °C to 100 °C.

11. An absorption solution filled with an acid compound is obtained after the absorption step, and the method includes carrying out at least one regeneration step of the absorption solution filled with the acid compound at a pressure of 0.1 MPa to 1 MPa and a temperature of 100 °C to 180 °C. The method according to any one of claims 1 to 10.

12. The method according to claim 11, wherein the absorption solution is a single-phase solution during the absorption step in the absorption column and at least up to the inlet of the regeneration column where the regeneration step is carried out.

13. The method according to claim 12, wherein the absorption solution has a temperature of 110 °C or lower within the absorption column and at least up to the inlet of the regeneration column.

14. The gaseous effluent is selected from natural gas, synthesis gas, combustion flue gas, refinery gas, acid gas resulting from an amine unit, tail gas resulting from a unit for converting H ２ S to sulfur by the Claus process, gas resulting from biomass fermentation, gas from a cement plant, or incinerator flue gas, the method according to any one of claims 1 to 13.

15. H ２ S and CO ２ from the gaseous effluent containing CO ２ for selectively removing H ２ S with respect to CO, the method according to any one of claims 1 to 14.

16. A method for decarbonizing biogas using the method according to any one of claims 1 to 14.

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