Process for regenerating liquid absorbent

Abstract

The present invention provides a process for regenerating a liquid absorbent, the process comprising: contacting the liquid absorbent with a hydrophobic medium wherein the liquid absorbent comprises at least one amine of formula (I) and a degradation product thereof, the degradation product comprising at least one imine of formula (II) wherein each Ar is independently an aromatic group, and each R is independently selected from the group consisting of hydrogen, an organic group, and NH2; and selectively extracting the degradation product into a hydrophobic medium or selectively extracting the degradation product through a hydrophobic medium. # imgabs0 #

Description

[0001] This application is a divisional application of an application filed on November 20, 2019, with application number 201980079870.3 and invention name “Process for regenerating liquid absorbent”. Technical Field

[0002] The present invention relates to a process for regenerating a liquid absorbent comprising an amine and its imine degradation products. The process comprises contacting the liquid absorbent with a hydrophobic medium and selectively extracting the degradation products into or through the hydrophobic medium. The process is particularly suitable for regeneration of amine-based liquid absorbents for capturing CO2 from gas streams, and it will be convenient to disclose the invention in the context of this exemplary application. Background of the Invention

[0004] Carbon dioxide (CO2) emissions are considered the main cause of the greenhouse effect and global warming. In the Kyoto Protocol, the United Nations Framework Convention on Climate Change has set targets for reducing greenhouse gas emissions.

[0005] One way to reduce atmospheric CO2 emissions is through the capture and subsequent geological storage of atmospheric CO2. In post-combustion capture, the CO2 in the flue gas is first separated from the nitrogen and residual oxygen using a suitable liquid absorbent in an absorber. The CO2 is then removed from the absorbent in a process known as stripping, thereby allowing the absorbent to be reused. The stripped CO2 is then liquefied by compression and cooling, with appropriate drying steps to prevent hydrate formation. This form of post-combustion capture is applicable to a variety of CO2 sources, including power stations, steel mills, cement kilns, calciners, biogas plants, natural gas processing, methane reforming and smelters.

[0006] A variety of amines and alkanolamine aqueous solutions have been studied as reactive absorbents in post-combustion CO2 capture. The capture process involves a series of chemical reactions between water, amines and CO2. Amines are weak bases and can undergo acid-base reactions. Once dissolved in the amine solution, CO2 reacts with water and the neutral form of the amine (R) according to the generally accepted equation described below. a R b R c N) to produce protonated amines, carbonic acid (H2CO3), aqueous bicarbonate (HCO3 - ) ions and aqueous carbonate (CO3 2- )ion:

[0007]

[0008]

[0009] If the amine is a primary amine (R a R b NH, R b =H) or secondary amine (R a R b NH, R b ≠H), then an additional reaction pathway with favorable absorption kinetics becomes available, in which carbon dioxide and amine react to form carbamate (R a R b NCOO - ). The carbamate group can then also participate in acid-base chemistry according to the generally accepted reactions described below.

[0010]

[0011] CO2 desorption during stripping is achieved by heating the aqueous amine solution containing the absorbed CO2. Heating reduces the physical solubility of CO2, but more importantly reduces the pK of the amine. a , resulting in a concomitant decrease in pH and CO2 absorption capacity. pK a The degree of reduction is mainly determined by the enthalpy of the amine protonation reaction. All other reactions, including the formation of carbamates, have small reaction enthalpies and are therefore relatively insensitive to temperature. Therefore, the circulation capacity of the aqueous amine solution, which is defined as the number of moles of CO2 that can be absorbed and released per mole of amine by cycling the absorbent between low and high temperatures, depends strongly on the molecular structure of the amine and in particular on its sensitivity to protonation.

[0012] Tertiary amine (R a R b R c N, R a , R b , R c ≠H) and certain sterically hindered primary or secondary amines have high cyclic absorption capacity, but cannot form appreciable amounts of carbamate groups. Therefore, the absorption kinetics of these systems are relatively unfavorable. Aromatic amines (R a R b R c N, R a =aromatic) are not basic enough to form carbamate or protonated amine species and are generally not suitable for CO2 capture.

[0013] One consequence of the thermal cycles inherent in amine-based CO2 capture processes is degradation of the amines, and particularly oxidative degradation when O2 is present in the gas stream being treated (as is typical in combustion flue gases or biogas). This results in a loss of performance over time, with high costs associated with absorbent regeneration and replenishment. Unhindered primary and secondary amines and alkanolamines are particularly susceptible to oxidative degradation. For example, oxidative degradation of 30 wt% monoethanolamine (MEA) in a coal flue gas environment results in approximately 1.5 kg of amine losses per metric ton of captured CO2. The degradation products typically include organic acids (e.g., formic acid and oxalic acid) and other polar molecules that are highly soluble in the absorbent aqueous solution. Therefore, regeneration must be performed by separating the starting amine from its degradation products via distillation, ion exchange, or electrodialysis. The recovered degradation products are discarded as waste.

[0014] Therefore, there is a need for amine-based liquid absorbent systems having improved stability against degradation and / or regeneration suitability, but still providing satisfactory gas absorption capacity. In addition, it is desirable to provide improved methods for regenerating amine-based liquid absorbents when they have been degraded in use.

[0015] The reference herein to a patent document or other matter as prior art is not to be taken as an admission that the document or matter was known as at the priority date of any claim, or that the information it contains was part of the common general knowledge. SUMMARY OF THE INVENTION

[0017] According to a first aspect, the present invention provides a process for regenerating a liquid absorbent, comprising:

[0018] A liquid absorbent is contacted with a hydrophobic medium, wherein the liquid absorbent comprises at least one amine of formula (I) and its degradation products, the degradation products comprising at least one imine of formula (II):

[0019]

[0020] wherein each Ar is independently an aromatic group, and each R is independently selected from hydrogen, an organyl group, and NH2; and

[0021] The degradation products are selectively extracted into or through a hydrophobic medium.

[0022] In some embodiments, the hydrophobic medium is an organic solvent.The organic solvent may be selected from the group consisting of: an aromatic hydrocarbon solvent, an oxygenated solvent, and a halogenated solvent.

[0023] In some embodiments, the process further comprises separating the liquid absorbent from the hydrophobic medium and recycling the liquid absorbent to the liquid absorbent stream in the gas absorption process.

[0024] In some embodiments, the process further comprises converting the extracted degradation products to form at least one regenerated amine of formula (I).The regenerated amine may optionally be recycled to the liquid absorbent stream in the gas absorption process.

[0025] Conversion of the degradation products may include hydrolyzing the imine to form a first amount of a regenerated amine of formula (I) and a carbonyl compound of formula (III):

[0026]

[0027] The aqueous solution containing the first amount of regenerated amine and the organic solution containing the carbonyl compound can then be separated by liquid-liquid separation. The carbonyl compound can then be converted to form a second amount of regenerated amine of formula (I) by reductive amination. Reductive amination can include: i) reacting the carbonyl compound with hydroxylamine to form an oxime; and ii) reducing the oxime to form a second amount of regenerated amine.

[0028] In some embodiments, the liquid absorbent is an aqueous composition comprising at least 10 wt % water.

[0029] In some embodiments, the liquid absorbent comprises at least one additional base selected from tertiary amines, hindered amines, carbonates, amino acid salts, and mixtures thereof, the additional base having a pK greater than that of the amine of formula (I): a . pK a Preferably, the pK of the amine of formula (I) is a As used herein, the pK of an amine or other base is at least 0.25 units higher. a Refers to the pK of the corresponding conjugate acid a .

[0030] In some embodiments, the liquid absorbent comprises at least one aliphatic amine as an absorbent. The aliphatic amine may be selected from unhindered primary or secondary amines and alkanolamines. In such embodiments, the aliphatic amine may include monoethanolamine and / or diethanolamine. The molar ratio of the aliphatic amine to the combined sum of the compound of formula (I) and the compound of formula (II) in the liquid absorbent may be greater than 2:1, such as greater than 5:1, or greater than 10:1.

[0031] In some embodiments, the liquid absorbent comprises absorbed carbon dioxide. In some such embodiments, the liquid absorbent for contacting with the hydrophobic medium can be obtained from the liquid outlet stream of the carbon dioxide absorber tower. Optionally, after extracting the degradation products, the liquid absorbent can be returned to the outlet stream of the carbon dioxide stripping tower.

[0032] In some embodiments, the liquid absorbent comprises the amine of formula (I) in an amount of at least 10 wt%, such as from 10 wt% to 80 wt%.

[0033] In some embodiments, the liquid absorbent comprises the imine of formula (II) in an amount of at least 1 wt % prior to contacting with the hydrophobic medium.

[0034] In some embodiments, extracting the degradation products into or through a hydrophobic medium reduces the amount of the imine of formula (II) in the liquid absorbent to less than 0.5 wt%.

[0035] In some embodiments, each R in formula (I) and formula (II) is hydrogen. In some embodiments, each Ar in formula (I) and formula (II) comprises a monocyclic six-membered aromatic group. The monocyclic six-membered aromatic group may comprise at least one nitrogen ring atom. In some embodiments, each Ar is selected from the group consisting of: 2-pyridyl, 3-pyridyl, 4-pyridyl and phenyl. In some embodiments, the liquid absorbent comprises 3-aminomethylpyridine.

[0036] According to a second aspect, the present invention provides a method for absorbing carbon dioxide from a gas stream comprising carbon dioxide, the method comprising:

[0037] The gas stream is contacted with a liquid absorbent comprising at least one amine of formula (I), wherein the amine of formula (I) degrades to form degradation products comprising at least one imine of formula (II):

[0038]

[0039] wherein each Ar is independently an aromatic group, and each R is independently selected from hydrogen, an organyl group, and NH2; and

[0040] The liquid absorbent is regenerated by a process according to any embodiment disclosed herein.

[0041] According to a third aspect, the present invention provides a method for absorbing carbon dioxide from a gas stream comprising carbon dioxide and molecular oxygen (O2), the method comprising contacting the gas stream with a liquid absorbent, the liquid absorbent comprising:

[0042] i) at least one amine of formula (I):

[0043] as well as

[0044] ii) at least one aliphatic amine,

[0045] wherein the compound of formula (I) is selectively degraded by O2 to form degradation products comprising at least one imine of formula (II):

[0046]

[0047] Wherein, in the amine of formula (I) and the imine of formula (II), each Ar is independently an aromatic group, and each R is independently selected from hydrogen, an organic group and NH2.

[0048] The aliphatic amines are degraded to a lesser extent than if the process were carried out with an equivalent liquid absorbent lacking the at least one amine of formula (I).

[0049] In some embodiments, the aliphatic amine is selected from unhindered primary or secondary amines and alkanolamines. The aliphatic amine may include monoethanolamine and / or diethanolamine.

[0050] In some embodiments, the molar ratio of the aliphatic amine to the combined sum of the compound of formula (I) and the compound of formula (II) in the liquid absorbent is greater than 2:1, such as greater than 5:1 or greater than 10:1.

[0051] In some embodiments, the method further comprises regenerating the liquid absorbent by a process according to any embodiment disclosed herein.

[0052] Where the terms "comprise", "comprises" and "comprising" are used in this specification (including the claims), they should be interpreted as specifying stated features, integers, steps or components, but not excluding the presence of one or more other features, integers, steps, components or groups thereof.

[0053] As used herein, the terms "first," "second," "third," etc., in connection with various features of the disclosed devices are arbitrarily designated and are intended only to distinguish two or more such features that the devices may incorporate in various embodiments. The terms themselves do not indicate any particular orientation or order. Furthermore, it should be understood that the presence of a "first" feature does not imply the presence of a "second" feature, the presence of a "second" feature does not imply the presence of the "first" feature, and so on.

[0054] Further aspects of the invention are presented below in the detailed description of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Embodiments of the invention will herein be described, by way of example only, with reference to the accompanying drawings, in which:

[0057] Figure 1 is a schematic diagram of a system for regenerating a liquid absorbent according to an embodiment of the present invention, the system being integrated into a process for removing CO2 gas from a gas mixture.

[0058] Figure 2 is a graph showing the concentrations of 3-AMPy and imine dimers formed from the degradation of 3-AMPy as determined by IR spectroscopy (solid dots) and HPLC (circles) in a pilot plant with an aqueous absorbent initially containing 6 M of 3-AMPy during 1500 hours of CO2 capture.

[0059] Figure 3 is a graph showing the concentrations of 3-AMPy, AMP, and imine dimers formed from the degradation of 3-AMPy in a 200+ day pilot plant CO2 capture using an aqueous absorbent initially containing 3M of 3-AMPy and 3M of AMP.

[0060] Figure 4 is a GC-MS chromatogram of the organic phase obtained after extraction with dichloromethane of a sample of an aqueous absorbent initially containing 3 M of 3-AMPy and 3 M of AMP and used for pilot plant CO2 capture for 158 days.

[0061] Figure 5 is a GC-MS chromatogram of the aqueous phase remaining after extracting a sample of the aqueous absorbent with dichloromethane, which initially contained 3M of 3-AMPy and 3M of AMP and was used for pilot plant CO2 capture for 158 days.

[0062] Figure 6 The aqueous solution of benzylamine was exposed to an atmosphere of CO2 and O2 at 55°C for 10 days. 13 C-NMR spectroscopy.

[0063] Figure 7 Describes the composition of the aqueous absorbent 13 C-NMR spectra of an aqueous absorbent initially containing 3 mol / L 3-AMPy and 3 mol / L 2-amino-2-methyl-1-propanol (AMP) and used for pilot plant CO2 capture for 208 days, (a) before absorption with Amberlite XAD-2 beads, (b) after absorption with beads, and (c) components extracted from the absorbent by the beads.

[0064] Figure 8 Depicted the following 13 C-NMR spectra: (d) fresh absorbent solution of diethanolamine, (c) absorbent solution of diethanolamine after exposure to an atmosphere of CO2 and O2 at 55°C for 115 hours, (b) absorbent solution of diethanolamine and benzylamine after exposure to an atmosphere of CO2 and O2 at 55°C for 115 hours, and (a) absorbent solution of diethanolamine and 3-aminomethylpyridine after exposure to an atmosphere of CO2 and O2 at 55°C for 115 hours. Detailed Description

[0066] The present invention relates to gas absorption using amine-based liquid absorbents, for example in CO2 capture processes. Liquid absorbents comprising amines of formula (I), such as aminomethylpyridine or benzylamine, provide advantageous CO2 capture performance. The cycle absorption capacity is similar to that of tertiary or sterically hindered amines, but the absorption kinetics are more similar to monoethanolamine (MEA). In formula (I), Ar is an aromatic group, and R is selected from hydrogen, an organic group, and NH2.

[0067]

[0068] The stability of liquid absorbents containing amines of formula (I) has been studied and it has been found that these materials are less susceptible to thermal degradation than 30 wt% MEA due to the inherent chemical stability imparted by the aromatic ring structure. In addition, the oxidative degradation that occurs when the absorbent is contacted with an oxygen-containing gas stream at high temperatures primarily involves the formation of imine dimer degradation products of formula (II).

[0069]

[0070] The oxidative degradation of the imine dimer is understood to originate from the protonated amine and follows the scheme shown below (for the specific case of 3-aminomethylpyridine):

[0071]

[0072] The dominance of this degradation pathway under representative CO2 gas absorption conditions is considered surprising because the purposeful imine formation reaction is usually carried out under non-aqueous conditions, wherein the water reaction product is removed from the solution and / or a catalyst is used to obtain a considerable reaction rate and conversion rate. The selective degradation mode shown in the scheme is considered to be unique to the amine of formula (I) because the resulting imine is arranged via a stable conjugated π-bond but is favorable. This stabilization effect does not extend to aromatic compounds substituted with aminoalkyls whose linking groups are ethylene or longer chain alkylenes, or in fact does not extend to conventional aliphatic amines or alkanolamine absorbents such as MEA. Such amines undergo the same initial loss of protonated amines to form aldehydes, but further degradation forms a variety of products, including carboxylic acids, aldehydes and amino acids (C Gouedard, DPicq, F Launay, PL Carrette; Int. J. Greenh. Gas Con., 10, 244 (2012)).

[0073] The inventors have recognized that the unique degradation properties of amines of formula (I) can be exploited to regenerate liquid absorbents containing these amines. Accordingly, disclosed herein is a process for regenerating a liquid absorbent comprising at least one amine of formula (I) and its imine degradation products, the imine degradation products comprising at least one imine of formula (II):

[0074]

[0075] In the molecules of formula (I) and formula (II), each Ar is independently an aromatic group, and each R is independently selected from hydrogen, an organic group and NH2. The process comprises contacting the liquid absorbent with a hydrophobic medium, and selectively extracting the degradation products into or through the hydrophobic medium.

[0076] The imine dimer degradation products are sufficiently non-polar to preferentially migrate into or through a hydrophobic medium, while the undegraded amine absorbent is retained in the polar and generally aqueous liquid absorbent. Thus, highly selective removal of the degradation products can be provided with little concomitant loss of undegraded amine molecules. This regeneration method is not feasible for many other amine-based absorbents and alkanolamine-based absorbents because the degradation products themselves are polar molecules that are preferentially soluble in the liquid absorbent.

[0077] The removal of the degradation products provides a regenerated liquid absorbent which can then be recycled back to the gas capture process. Advantageously, such a regeneration process can be easily integrated into a gas capture facility. As a result of the removal of the degradation products, the amine absorbent concentration of the absorbent solution in the process and the gas absorption capacity thereof can be maintained at a suitable high level, for example via the addition of a supplementary amine of formula (I). In addition, the deleterious effects of imine dimers on the physical properties of the absorbent solution, such as increased viscosity, can be mitigated.

[0078] As will be discussed in more detail below, the regeneration process may optionally further include converting the imine dimer degradation products back to amines of formula (I). Thus, at least a portion of the degradation products may be recovered and reused for gas absorption, which is not feasible when using many other amine absorbents.

[0079] Extraction with hydrophobic media

[0080] The hydrophobic medium may generally include any liquid or solid material that is substantially immiscible or insoluble in water but is capable of dissolving or absorbing the imine of formula (II) thereby extracting it from the liquid absorbent.

[0081] In some embodiments, the hydrophobic medium is a hydrophobic liquid, and in particular an organic solvent. It will be appreciated that suitable organic solvents need not have zero capacity for dissolved water, but should be substantially immiscible with the liquid absorbent so that distinct phases are formed upon contact.

[0082] During contact, the two phases can be well mixed to ensure that the degradation products are transferred to the organic solvent phase. Subsequent phase separation then allows the organic phase to be separated from the regenerated liquid absorbent by liquid-liquid separation. The organic solvent can be contacted with the liquid absorbent and then separated from the liquid absorbent using conventional methods and equipment (including non-dispersed membrane contactors) for liquid-liquid extraction. Liquid-liquid extraction can be a batch process or a continuous process, and can include a single stage or optionally multiple stages to provide improved selectivity. The hydrophobic liquid and the liquid absorbent can be contacted in any suitable ratio, for example, with a weight ratio of from 1:10 to 10:1, or from 1:5 to 5:1, or from 1:3 to 3:1 such as about 1:1. The hydrophobic liquid and the liquid absorbent can be contacted at any suitable temperature, but particularly at a temperature below the boiling point of the hydrophobic liquid, for example 20°C to 70°C, or 20°C to 50°C. Optionally, the hydrophobic liquid and the liquid absorbent can be stirred or sheared to improve contact, and thereby improve the extraction of the imines of formula (II).

[0083] The choice of organic solvent is not considered to be particularly limited, as long as the organic solvent is sufficiently hydrophobic so that the imine degradation products are preferentially distributed into the organic solvent relative to the liquid absorbent. In some embodiments, the organic solvent is a non-polar solvent or a polar aprotic solvent that is immiscible with water. In some embodiments, the organic solvent is volatile, for example, having a boiling point (at atmospheric pressure) below 120°C, such as from 25°C to 120°C, or below 100°C, or below 80°C. Higher volatility allows the organic solvent to be more easily separated from the dissolved components by evaporation in the downstream processing of the degradation products, and can also prevent the accumulation of solvent in the working amine absorbent.

[0084] Suitable organic solvents may include aromatic hydrocarbons such as toluene, oxygenated solvents such as ethyl acetate, and halogenated solvents such as dichloromethane (DCM). Such solvents may provide excellent phase separation properties and highly selective extraction of degradation products when in contact with aqueous amine absorbents, and also allow for easy conversion of dissolved imine dimers via subsequent acid hydrolysis steps, as will be discussed below. Highly non-polar solvents such as aliphatic hydrocarbons are also expected to be effective for the extraction of imines.

[0085] In some embodiments, the organic solvent includes at least one selected from the group consisting of C2-C6 linear and branched alkyl esters of acetic or formic acid, C3-C6 linear or branched aliphatic ketones, methyl halides, ethyl halides, and aromatic hydrocarbons.

[0086] It has been found that the use of hydrophobic organic solvents provides high selectivity for the extraction of imine dimer degradation products, and is considered to be advantageous due to ease of implementation. However, it is also contemplated that other hydrophobic media can be used. In some embodiments, the liquid absorbent is contacted with a hydrophobic solid such as a polymer bead, and the relatively non-polar imine dimer degradation product is preferentially absorbed into the solid. This can be done, for example, in a packed absorption tower through which the liquid absorbent passes. Suitable hydrophobic solids can include porous hydrophobic polymer resins. A class of suitable hydrophobic solids is a polyaromatic absorbent resin, such as a cross-linked styrene polymer, such as a styrene-divinylbenzene copolymer. Amberlite XAD-2 beads available from Sigma-Aldrich and Purosorb PAD500 available from Purolite are such materials. Another class of hydrophobic solids considered suitable are acrylic absorbent resins, such as aliphatic methacrylate polymers. Purosorb PAD950 available from Purolite is such a material.

[0087] In other embodiments, the liquid absorbent is contacted with a hydrophobic membrane that allows the imine dimer degradation products to selectively permeate through the membrane while retaining the amine absorbent and other polar species.

[0088] Liquid absorbent

[0089] In some embodiments, the liquid absorbent is an aqueous composition.The aqueous liquid absorbent may comprise at least 10 wt% water, such as at least 20 wt% water.

[0090] In some embodiments, the liquid absorbent comprises an amine of formula (I) in an amount of at least 10 wt%, such as 10 to 80 wt%, 15 to 80 wt%, 20 to 80 wt%, or 25 to 80 wt%, relative to the total weight of the composition.

[0091] In some embodiments, in addition to the amine of formula (I), the liquid absorbent also contains an additional absorbent for CO2. The weight ratio of the amine of formula (I) to the additional absorbent can be, for example, from 99:1 to 1:99, or from 10:1 to 1:10, or from 5:1 to 1:5. In some embodiments, the liquid absorbent contains one or more additional CO2 absorbing compounds selected from amines, alkanolamines, carbonates, amino acid salts and imidazoles. The one or more additional amines can be selected from primary amines, secondary amines and tertiary amines.

[0092] Examples of suitable additional CO2 absorbing compounds include primary amines such as monoethanolamine, ethylenediamine, 2-amino-2-methyl-1-propanol, 2-amino-2-methyl-ethanolamine; secondary amines such as N-methylethanolamine, piperazine, piperidine and substituted piperidines, 3-piperidinemethanol, 3-piperidineethanol, 2-piperidinemethanol, 2-piperidineethanol, diethanolamine, diglycolamine and diisopropanolamine; tertiary amines such as N-methyldiethanolamine, N-piperidinemethanol, N-piperidineethanol, N,N-dimethylaminoethanol and 3-quinuclidinol; potassium carbonate; sodium glycinate, potassium taurine, sodium alanine; imidazole and N-functionalized imidazoles such as those disclosed in US Pat. No. 8,741,246, and amino acids such as taurine, sarcosine and alanine.

[0093] In some embodiments, the liquid absorbent comprises an amine having a pK greater than that of the amine of formula (I) in the composition. a For example, another base with a higher pK of at least 0.25 units. a The base is preferably selected from tertiary amines, hindered amines, carbonates, amino acid salts and mixtures thereof, and most preferably selected from tertiary amines, hindered amines and mixtures thereof. We have found that in the presence of such a base, the formation of imine dimers by the amines of formula (I) is inhibited, and thereby the overall rate of degradation is also inhibited. Without wishing to be bound by any theory, it is believed that the additional high pK a The amine or other base is preferentially protonated by the protons released when the acid gas is absorbed, and in particular by the protons generated by the rapid reaction of CO2 with the amine of formula (I) to form carbamates. Thus, the preferential absorption of these protons inhibits the initial steps in the degradation process of the amine of formula (I). In addition, the favorable kinetics of CO2 absorption are not unacceptably impaired by the additional base, because CO2 still reacts directly with the amine of formula (I) to form carbamates. The liquid absorbent may contain an additional high pK in an amount of at least 10 wt%, such as at least 20 wt%. a In some embodiments, the additional high pK a The base and the amine of formula (I) are present in similar molar amounts, such as from 2:1 mol / mol to 1:2 mol / mol or about 1:1 mol / mol.

[0094] In the case of (aminomethyl)pyridine, the pK values ​​of 2-(aminomethyl)pyridine, 3-(aminomethyl)pyridine, and 4-(aminomethyl)pyridine at 25°C are a is 8.6. Preferably, the additional high pK a A base, in particular a sterically hindered amine or a tertiary amine, having a pK of at least 8.85 a , such as a pK of at least 9 a , pK from 8.85 to 11.5 a or pK between 9 and 11.5a .

[0095] As used herein, the term "sterically hindered amine" is defined as those compounds comprising at least one primary amino group or secondary amino group attached to a secondary or tertiary carbon atom. In one embodiment, the sterically hindered amine is a secondary amino group attached to a secondary or tertiary carbon atom or a primary amino group attached to a tertiary carbon atom.

[0096] Examples of suitable sterically hindered amines, tertiary amines, carbonates and amino acid salts include those shown in the table below having corresponding conjugate acid pK at 25°C: a Those.

[0097] Alkali <![CDATA[At 25 °C, pK a > 2-Amino-1-propanol 9.5 2-Amino-2-methyl-1-propanol 9.7 Piperidine 11.1 2-Piperidinylmethanol 10.1 3-Piperidinylmethanol 10.4 4-Piperidinylmethanol 10.6 2-Piperidinylethanol 10.5 4-Piperidinylethanol 10.6 2-(Dimethylamino)ethanol 9.2 Potassium bicarbonate 10.3 Sodium Glycinate 9.6 Potassium Taurate 9.1 Sodium Alanine 9.7

[0098] Other suitable tertiary and sterically hindered amines of the required basicity will be readily apparent to those skilled in the art in view of the above reference to the degradation mechanism and methods of inhibiting imine formation.

[0099] In some embodiments, the liquid absorbent comprises:

[0100] An amine of formula (I) in an amount of 10 wt % to 80 wt %, such as 15 wt % to 80 wt %, 20 wt % to 80 wt % or 25 wt % to 80 wt % relative to the total weight of the solution;

[0101] Optionally, up to 70 wt%, such as from 10 wt% to 70 wt%, of an additional amine selected from tertiary amines, hindered amines and mixtures thereof, the additional amine having a pK greater than that of the amine of formula (I): a ;as well as

[0102] Water in an amount of at least 10 wt%, such as from 10 wt% to 90 wt% or 20 wt% to 80 wt%.

[0103] In some embodiments, the liquid absorbent comprises one or more aliphatic amine absorbents. In some embodiments, the aliphatic amines are selected from unhindered primary or secondary amines and alkanolamines. Such amines, which include some of the most commonly used absorbents in CO2 capture applications, are susceptible to oxidative degradation, forming a range of degradation products, such as organic acids and other polar molecules. We have surprisingly found that the presence of compounds of formula (I) in the liquid absorbent solution inhibits this degradation, even when included only as a minor component of the total amines. Therefore, the compound of formula (I) itself acts as an effective co-absorbent, but also acts as a sacrificial amine to inhibit the irreversible loss of additional (and usually the most abundant) aliphatic amine absorbent molecules. Advantageously, the liquid absorbent can then be regenerated as disclosed herein, thereby removing the imine degradation products.

[0104] As used herein, the term "unhindered primary or secondary amine" is defined as those compounds comprising at least one primary or secondary amino group attached to a primary carbon atom. The aliphatic amine may include, or consist of, monoethanolamine or diethanolamine. The aliphatic amine may be present as the primary amine absorbent compound in the liquid composition, such as more than 50% of the total absorbent. For example, the molar ratio of the aliphatic amine to the combined sum of the compound of formula (I) and the compound of formula (II) may be greater than 2:1, or greater than 5:1, or greater than 10:1, or greater than 20:1, or greater than 50:1, and the aliphatic amine is preferably selected from unhindered primary or secondary amines and alkanolamines.

[0105] Liquid absorbents do not need to include ionic liquids or organic salts such as imidazolium cations or quaternary ammonium salts, and may be substantially free of such components. If desired, compositions may include solvents other than water to change the solubility of the amine of formula (I) and / or other absorbents that may be present. Examples of cosolvents may, for example, be selected from the group consisting of: glycols, glycol derivatives selected from the group consisting of glycol ethers, glycol ether esters, glycol esters, short-chain aliphatic alcohols such as C1 to C4 alkanols, long-chain aliphatic alcohols, long-chain aromatic alcohols, amides, esters, ketones, phosphates, organic carbonates and organic sulfur compounds. Additional components may be present, such as solutes or other materials.

[0106] The liquid absorbent may include an imine of formula (II) in an amount greater than 1 wt% or greater than 2 wt% before contact with the hydrophobic medium. In the case of a cyclic absorption process such as post-combustion CO2 capture, the amount of degradation products in the liquid absorbent will generally be maintained at a steady-state concentration, which is determined by the rate of degradation, the rate of removal of the absorbent for regeneration and recycling, and the efficiency of the regeneration process (i.e., the percentage of imine removal). Therefore, it will be understood that the preferred concentration range of the imine of formula (II) can be determined by balancing the necessary conditions of competition to minimize both the level of degradation products in the working absorbent and the size of the stream removed for regeneration. In practice, the steady-state amount of the imine of formula (II) can be maintained below the level at which operational problems (e.g., loss of mass transfer or foaming due to increased viscosity) and the reduction in CO2 absorption capacity due to the loss of the active amine of formula (I) become unacceptable. Therefore, the liquid absorbent may contain an imine of formula (II) in an amount less than 10 wt%, or less than 5 wt%, or less than 3 wt%, such as less than 2 wt%, before contact with the hydrophobic medium.

[0107] The liquid absorbent may still contain the imine of formula (II) after contact with the hydrophobic medium, but at a lower concentration than before contact. It will again be appreciated that the maximum concentration reduction that is technically achievable may not be preferred in practice due to economic constraints. In some embodiments, the extraction reduces the amount of imine in the liquid absorbent to less than 1 wt%, or less than 0.5 wt%, or less than 0.1 wt%, such as less than 0.05 wt%.

[0108] The liquid absorbent may contain absorbed gas, such as CO2, when in contact with the hydrophobic medium. The concentration of absorbed CO2 may be at least two times (and even more preferably at least five times) the equilibrium concentration obtained when the absorbent is exposed to air below the boiling point of the aqueous solvent. In one embodiment, the absorbed CO2 constitutes at least 0.2% by weight, or at least 1% by weight, or at least 10% by weight of the absorbent.

[0109] Amines of formula (I) and imines of formula (II)

[0110] The liquid absorbent to be regenerated comprises at least one amine of formula (I) and its degradation product, and the degradation product comprises at least one imine of formula (II). The composition can comprise only one amine molecule of formula (I), and therefore comprises a single corresponding imine molecule of formula (II). However, it is contemplated that a mixture of amine absorbent molecules can be used, such as a mixture of two or more aminomethylpyridine isomers. Therefore, it will be understood that the resulting degradation product can include a variety of imine molecules, wherein two Ar and two R groups can be the same or different, as determined by the two starting molecules of formula (I).

[0111] The R groups of the amine molecules and the imine molecules are selected from hydrogen, organic groups and NH2. In some embodiments, the R groups are selected from hydrogen, alkyl, cycloalkyl and aromatic groups, such as hydrogen, C1 to C 10 In some embodiments, the R group is hydrogen.

[0112] The Ar group of the amine molecule and the imine molecule is selected from an aromatic group. In some embodiments, the Ar group includes a monocyclic aromatic group, in particular a six-membered monocyclic aromatic group, which is optionally substituted with one or more non-aromatic substituents such as C1 to C 10 In some embodiments, the monocyclic six-membered aromatic group contains 0, 1, 2 or 3 nitrogen ring atoms, wherein the other ring atoms are carbon. In some embodiments, the Ar group is selected from 2-pyridyl, 3-pyridyl, 4-pyridyl and phenyl.

[0113] In some embodiments, the liquid absorbent comprises 2-aminomethylpyridine, 3-aminomethylpyridine, 4-aminomethylpyridine and / or benzylamine, and in particular 3-aminomethylpyridine, as the amine of formula (I).

[0114] The extracted degradation products were converted into

[0115] In some embodiments, the regeneration process includes converting the extracted degradation products to form a regenerated amine of formula (I). The regenerated amine can then be recycled to the gas absorption process as make-up amine, thereby reducing waste formation and overall amine consumption.

[0116] According to the following representative scheme, the conversion may include the step of hydrolyzing the imine of formula (II) to form one equivalent of the regenerated amine of formula (I) and one equivalent of the carbonyl compound of formula (III):

[0117]

[0118] The imines of formula (II) are found to be unstable under acidic aqueous conditions. For the case where the degradation products are extracted from the liquid absorbent into an organic solvent, hydrolysis can therefore be achieved by contacting the separated organic phase with an acidified aqueous phase, the acidified aqueous phase for example comprising a catalytic amount of acetic acid. After suitable mixing and hydrolysis time, the two phases are separated by conventional liquid-liquid separation technology, wherein the regenerated amine of formula (I) is distributed in the aqueous phase, and the carbonyl compound of the formula (III) of lower polarity is retained in the organic phase. The aqueous phase of the first part of the regenerated amine of formula (I) can then be recycled to the gas absorption process. It will be understood that the degraded amine of a theoretical maximum of 50mol% can be recovered by this process step, because each imine molecule is incorporated into two amine molecules.

[0119] Optionally, the carbonyl compound can then be reductively aminized to form a second amount of regenerated amine of formula (I). The reductive amination of the carbonyl compound to amine can be carried out via many different routes, usually requiring the use of both a reducing agent and a nitrogen source. In some embodiments, reductive amination is by reacting the carbonyl compound with hydroxylamine to form an oxime, and then reducing the oxime to form an amine of formula (I) to achieve, as depicted in the following scheme:

[0120]

[0121] In other embodiments, reductive amination can be achieved using ammonia, H2 and a transition metal catalyst (e.g., as disclosed in Organic Letters, 2002, 4(12), 2055-2058), or using ammonium formate (Leuckart reaction, as disclosed in Journal of Organic Chemistry, 1944, 9(6), 529-536).

[0122] The regenerated amine produced by the conversion of the carbonyl compound can then also be recycled to the gas absorption process. In theory, this allows substantially all of the imine dimer degradation products to be recovered and recycled as amines, although it will be appreciated that the decision to implement one or both amine recovery steps (i.e., hydrolysis and reductive amination) in the regeneration process may be influenced by the cost compared to the cost of fresh amine purchase.

[0123] Post-combustion CO2 capture

[0124] The regeneration process disclosed herein is believed to be particularly useful in CO2 capture processes such as post-combustion capture, and may optionally be integrated into such processes. Figure 1 A schematic process flow diagram showing one such embodiment is depicted in . The process (100) includes an absorption reactor (102) for absorbing CO2 from a flue gas stream and a desorption (stripper) reactor (104) for desorbing CO2. The absorption reactor (102) includes a first inlet (106), a second inlet (108), a first outlet (110), a second outlet (112) and one or more gas absorption contact areas (114). The first inlet (106) of the absorption reactor (102) is a flue gas inlet through which flue gas rich in CO2 enters the absorption tower (102). The total pressure of the flue gas rich in CO2 can vary widely depending on the application. The CO2 partial pressure in the flue gas can be, for example, between 0.1 kPa and 100 kPa. The second inlet (108) is an absorbent inlet through which a CO2-depleted aqueous absorbent comprising an amine of formula (I) and its degradation products including an imine of formula (II) (as described above) enters the absorption tower (102). The CO2-rich flue gas and the CO2-depleted absorbent are contacted in the gas absorption contact zone (114). In these zones, CO2 in the CO2-rich flue gas is absorbed into the absorbent to form a CO2-depleted flue gas that leaves the tower (102) via the first outlet (110) and a CO2-rich absorbent that leaves via the second outlet (112). The CO2-depleted flue gas may still contain some CO2, but at a lower concentration than the CO2-rich flue gas.

[0125] In the absorption tower (102), the absorbent can have a temperature between 25°C and 50°C, thus allowing a range of significant temperature rise during stripping and thus allowing high circulation capacity. The local environment of the absorbent can be changed in the absorption tower (102) to facilitate the absorption reaction, for example to increase the absorption of CO2 into the solution. Such changes in the local environment can include changes in pH, changes in temperature, changes in pressure, etc. Alternatively or in addition, the solution can contain other compounds that help absorb CO2. These compounds can change the affinity or absorption capacity of the amine absorbent molecules (including amine absorbent molecules of formula (I)), or these compounds can also absorb CO2. If additional compounds are added to the absorbent solution in the absorption reactor (102), the process can additionally include means for removing these compounds.

[0126] The desorption reactor (104) includes an inlet (118), a first outlet (120), a second outlet (122) and one or more gas desorption zones (124). The CO2-rich absorbent exiting through the second outlet (112) of the absorption tower (102) enters the desorption tower (104) through the inlet (118). Then, desorption of CO2 from the CO2-rich absorbent occurs in the gas desorption zone (124).

[0127] The desorption of CO2 may involve the application of heat and / or a reduction in pressure to facilitate the desorption process. In addition, additional compounds may be added to the CO2-rich solution to enhance the desorption process. Such compounds may alter the solution environment, such as by changing the pH of the solution or changing another parameter to facilitate the desorption reaction.

[0128] Removing CO2 from the CO2-rich absorbent results in the formation of a CO2-rich gas stream and a CO2-depleted absorbent. The CO2-depleted absorbent may still contain some CO2, but at a lower concentration than the CO2-rich absorbent entering the desorber (104). The CO2-rich gas stream is discharged via a first outlet (120) as a CO2 outlet, and can subsequently be compressed, cooled and liquefied for injection into a geological formation (not shown). The CO2-depleted absorbent is discharged via a second outlet (122) and is recycled to the absorber (102) via the second inlet (108) of the absorber (102). The required temperature of the absorbent in the desorber (104) is typically higher than in the absorber (102), and therefore the CO2-depleted stream and the CO2-rich stream circulating between the two towers can be thermally contacted in a heat exchanger (126) to improve the overall energy efficiency of the process.

[0129] The aqueous absorbent circulating between the two towers is typically exposed to high temperatures, particularly in the desorber (104), and is also exposed to the presence of molecular oxygen (O2) entering with the flue gas. As a result, a portion of the amine of formula (I) is converted into degradation products including imines of formula (II), as disclosed herein. Without any intervention, the amount of imine will increase over time, reducing the circulating CO2 capacity of the aqueous absorbent and eventually also causing operational problems due to the increased viscosity of the absorbent solution, such as foaming or loss of mass and transfer.

[0130] The process (100) thus comprises a regeneration section comprising a mixer (128) and a phase separation unit (130). A portion of the CO2-rich absorbent leaving the absorber (102) via the second outlet (112) is transferred to the regeneration unit as a regeneration stream (132) where it is contacted with an organic solvent (134) in the mixer (128). The organic solvent is hydrophobic and is selected based on its capacity to selectively extract the imine of formula (II) from the aqueous absorbent, as disclosed herein. The mixer (128) provides sufficient turbulence and contact time to allow a high degree of extraction of the imine into the organic solvent. The mixed stream (136) leaving the mixer is then transferred to a phase separation unit (130), such as a continuously operated decanter, where the immiscible organic and aqueous phases are separated. The aqueous phase of the CO2-rich absorbent, depleted for the imine extracted into the organic phase, leaves the separation unit as a regenerated stream (138). The regenerated stream (138) may contain residual imine, but at a lower concentration than in the regenerated stream (132). The regenerated stream is combined with the CO2-depleted absorbent leaving the desorber (104) via its outlet (122) and is thereby rejoined to the main loop of absorbent solution circulating in the process. Figure 1 As depicted in FIG. 1 , the aqueous phase is the lower density phase that separates to the uppermost layer in the phase separation unit (130), as would be expected when an organic solvent having a specific gravity greater than water (such as dichloromethane) is used. However, it will be understood that if an organic solvent having a specific gravity lower than water (such as toluene) is used, the aqueous phase may alternatively be the higher density phase.

[0131] Extracting CO2-rich absorbent for regeneration is considered to be advantageous because the greater ionic strength caused by CO2 absorption will increase the polarity difference between the phases and thereby increase the effectiveness of imine extraction. In addition, the regenerated absorbent can preferably be added to the CO2-poor solution leaving the desorption reactor (104). This avoids any residual organic solvent present in the regenerated absorbent from being carried over into the CO2-rich gas stream product. Instead, at least some of any residual organic solvent can be stripped from the absorbent in the absorber (102). However, it will be understood that the absorbent for regeneration can in principle be extracted from any suitable position on the main loop and returned to any suitable position on the main loop.

[0132] It will also be appreciated that the required size of the regeneration stream (132) relative to the stream of absorbent circulating in the main loop of the process will depend on the expected degradation rate and the level of degradation products that can be tolerated in the absorbent. By minimizing the degradation rate, the size of the stream separated for regeneration can be reduced. It is therefore particularly preferred that, in addition to the amine of formula (I), the aqueous absorbent comprises a further base, such as a base having a greater pK than the amine of formula (I). a tertiary amine or hindered amine, as disclosed herein. Therefore, the proportion of aqueous absorbent separated for regeneration (per pass) relative to the total circulating absorbent can be small, such as less than 5%, or less than 2%, or less than 1%. Based on the estimated degradation rate obtained in the pilot-scale CO2 absorption experiment, it is believed that the concentration of the imine of formula (II) in the working absorbent can be maintained below 2 wt% by continuously regenerating about 0.1% of the absorbent circulating in the main loop.

[0133] The organic stream (140) containing imines leaves the phase separation unit (130). Optionally, the stream is treated in an integrated re-amination unit (142) to convert at least a portion of the contained imines of formula (II) into amines of formula (I). The amines thus recovered are then recycled to the main absorbent loop via an amine return stream (144). Alternatively, the organic stream (140) containing imines can be processed to recover only the organic solvent, processed in a batch mode (or off-site) to regenerate the amines, or discarded as waste.

[0134] If present, the integrated re-amination unit (142) may include a hydrolysis unit in which the organic stream (140) containing the imine is contacted with an acidified aqueous phase. The imine is thereby hydrolyzed to form one equivalent of an amine of formula (I) and one equivalent of a carbonyl compound of formula (III), as disclosed herein. After phase separation, the aqueous phase, now containing the regenerated amine, is returned to the main absorbent loop via an amine return stream (144). The organic phase containing the carbonyl compound may be further processed on-site or off-site in a batch mode or a continuous mode to convert the carbonyl compound into an amine of formula (I) by reductive amination and / or to recover the organic solvent, or may be discarded as waste.

[0135] In an aspect of the present invention, a process for removing CO2 gas from a gas mixture is provided, comprising: i) circulating a liquid absorbent comprising at least one amine of formula (I) between a CO2 absorption step and a CO2 desorption step, wherein: (a) the absorption step comprises contacting a gas mixture comprising CO2 with a liquid absorbent in a CO2-lean form to form a liquid absorbent in a CO2-rich form and a CO2-lean gas mixture, (b) the desorption step comprises desorbing CO2 from the liquid absorbent in a CO2-rich form to form a liquid absorbent in a CO2-lean form and a CO2-rich gas mixture, and (c) the at least one amine of formula (I) is degraded so that the liquid absorbent comprises the at least one amine of formula (I) and its degradation products, which include at least one imine of formula (II); and ii) regenerating the liquid absorbent according to any embodiment disclosed herein.

[0136] Other applications

[0137] It is expected that the selective degradation chemistry of the amines of formula (I) can also be applied to processes for absorbing gases other than CO2, for example other acid gases such as H2S or SO x Such gases can be absorbed together with CO2 or from a CO2-free gas stream.

[0138] In one set of embodiments, the amine of formula (I) is used to remove molecular oxygen. Therefore, the amine of formula (I) is added to the liquid composition in an amount sufficient to react with the unwanted molecular oxygen entering the composition, thereby preventing or inhibiting other undesirable oxidation reactions in the liquid composition. Once the liquid composition contains an elevated level of the imine of formula (II), the composition can be regenerated by the method disclosed herein. The liquid composition to which the amine of formula (I) is added can be an amine-containing absorbent for CO2 capture applications, including conventional absorbents such as absorbents based on monoethanolamine (e.g., 30% MEA). Example

[0139] The present invention is described with reference to the following examples. It should be understood that these examples are illustrative of the invention described herein and are not limiting.

[0140] The chemical abbreviations used in this specification have the following meanings:

[0141] AMPy: (aminomethyl)pyridine

[0142] 2-AMPy: 2-(aminomethyl)pyridine

[0143] 3-AMPy: 3-(aminomethyl)pyridine

[0144] 4-AMPy: 4-(aminomethyl)pyridine

[0145] MEA: Monoethanolamine

[0146] AMP: 2-amino-2-methyl-1-propanol.

[0147] Example 1: Pilot Plant Test Using an Aqueous Absorbent Containing 6 mol / L 3-AMPy

[0148] An extended pilot plant trial has been conducted using a 0.4 metric tonne / day CO2 capture plant located at a lignite power station, the CO2 capture plant being configured with an absorber (102), a desorber (104) and a heat exchanger (126), substantially as Figure 1 The capture device is based on 80m 3 / h flow rate of flue gas slipstream operation.

[0149] The campaign using 6 mol / L aqueous 3-AMPy (61 wt% 3-AMPy and 39 wt% water) was operated for a duration of approximately 1500 hours (63 days). During operation, the performance of the equipment was evaluated in terms of reboiler energy requirements, and the degradation of the amines was monitored. The single major degradation product formed was also identified and characterized. Minimum reboiler loads of 2.9 GJ / metric ton CO2 and 2.6 GJ / metric ton CO2 were achieved without and with the rich cold split process configuration, respectively. This is compared to 3.4 GJ / metric ton CO2 for 5 mol / L monoethanolamine (MEA) in both configurations. In the rich cold split configuration, a portion of the CO2-rich absorbent leaving the absorber (102) bypasses the heat exchanger (126) and enters the stripper (104) at a level higher than the inlet of the heated portion of the CO2-rich absorbent. This reduces water and amine vapors and reduces the condenser load on the stripper.

[0150] In laboratory tests under accelerated degradation conditions, the major degradation product formed was found to be the imine dimer of 3-AMPy. During exercise, the loss of the amine and the formation of the imine were monitored by infrared (IR) spectroscopy and high performance liquid chromatography (HPLC). 13 C-NMR and 1 Additional analysis of device samples by H-NMR spectroscopy confirmed that the previously identified imine was the major degradation product in the device.

[0151] Figure 2 is a graph of the trends in amine and imine concentrations during the pilot plant campaign.

[0152] During pilot plant testing, the degradation reaction mechanism was studied in the laboratory by breaking down the overall reaction into possible individual chemical transformations and testing whether they occur. The determined mechanism proceeds via loss of protonated 3-AMPy molecules and ammonium ions, followed by oxidation. The mechanism for the complete degradation of 3-AMPy to imines via reaction with oxygen is shown in the following scheme.

[0153]

[0154] Example 2. Pilot Plant Test Using an Aqueous Absorbent Containing 3 mol / L 3-AMPy and 3 mol / L AMP

[0155] Based on the degradation mechanism of the above scheme, it is believed that if 3-AMPy is formulated with a stronger base to reduce the formation of protonated 3-AMPy during CO2 absorption, its degradation can be inhibited. 2-Amino-2-methyl-1-propanol (AMP) was selected as the amine for formulation because it has the required basicity and is known to be robust in CO2 capture applications. AMP does not react directly with CO2, but acts as a base to preferentially accept the protons released when 3-AMPy reacts. Simulations indicate that the concentrations used in the aqueous absorbent of 3 mol / L of 3-AMPy and 3 mol / L of AMP are optimal for reducing degradation and maintaining capture performance.

[0156] A pilot plant campaign was conducted for 5000 hours with an aqueous absorbent containing 3 mol / L 3-AMPy and 3 mol / L AMP (32 wt% 3-AMPy, 26 wt% AMP and 42 wt% water). Because this absorbent degrades much more slowly than the absorbent of Example 1, it was possible to conduct a much longer campaign. The same reboiler energy requirement as in Example 1 was achieved. Figure 3The concentrations of 3-AMPy, AMP, and imines are shown over the duration of the exercise. The rate of imine formation is several orders of magnitude lower than observed in the experiments of Example 1. In addition, the overall degradation rate was found to be 10 times slower than expected for 5 mol / L MEA (see Example 3).

[0157] The formation of imine dimers and the inhibition of degradation by using the more basic amine co-absorbents that this allows is a unique property of amino(C1)aromatic systems and especially aminomethyl-substituted heteroaromatics. (Aminoalkyl)pyridines with longer bridging chains such as ethyl and propyl between the amino group and the pyridine group do not form imines and degrade via chain loss and more traditional mechanisms that form products that cannot be easily recovered or regenerated as the original amine.

[0158] Example 3. Comparison of the absorbents of Example 1 and Example 2 with MEA

[0159] The pilot plant was also operated with 5 mol / L (30 wt%) aqueous monoethanolamine (MEA) for approximately 500 hours. This allowed the optimum reboiler duty for each absorbent to be determined via parameter studies. These optimum reboiler duties and amine degradation rates are shown in the table below and are for a standard plant configuration (no rich split). It should be noted that the MEA degradation information was obtained from the literature as MEA was only run for a short duration in the pilot plant.

[0160]

[0161] *Degradation rate was obtained from the book P. Feron, Absorption-Based Post-Combustion Capture of Carbon Dioxide, Elsevier (2016).

[0162] It is known that the oxidative degradation of monoethanolamine produces organic acids (such as formic acid and oxalic acid) and a wide variety of other products, many of which are highly soluble in the aqueous absorbent solution. Therefore, the degradation products are not easily recoverable or regenerated to form monoethanolamine.

[0163] Example 4. Other bases that inhibit the degradation of (aminomethyl)pyridine

[0164] As shown in Example 2, the presence of AMP in the absorbent composition inhibits the formation of imines resulting from dimerization of AMPy derivatives formed upon absorption of CO2. Other bases, including those having a higher conjugate acid pK than (aminomethyl)pyridine aAmines of this type can also be used for this purpose. Preferred bases are tertiary amines and sterically hindered amines, which are stable and provide proton acceptance during CO2 absorption. The average pK values ​​of 2-(aminomethyl)pyridine, 3-(aminomethyl)pyridine and 4-(aminomethyl)pyridine at 25°C are a It is 8.6.

[0165] Suitable bases generally have a pK lower than (aminomethyl)pyridine. a At least 0.25 units higher pK a , i.e. about 0.25 units higher than 8.6 (this represents a 2.5× increased selectivity for protons). Examples of suitable tertiary and sterically hindered amines, carbonates and amino acid salts include those specified in the table below, together with the pK of the conjugate acid at 25°C a .

[0166] Alkali <![CDATA[pK at 25 °C a > 2-Amino-1-propanol 9.5 2-Amino-2-methyl-1-propanol 9.7 Piperidine 11.1 2-Piperidinylmethanol 10.1 3-Piperidinylmethanol 10.4 4-Piperidinylmethanol 10.6 2-Piperidinylethanol 10.5 4-Piperidinylethanol 10.6 2-(Dimethylamino)ethanol 9.2 Potassium bicarbonate 10.3 Sodium Glycinate 9.6 Potassium Taurate 9.1 Sodium Alanine 9.7

[0167] Example 5. Extraction of Imine Dimer into Organic Solvent

[0168] A sample of an aqueous absorbent which initially already contained 3 mol / L 3-AMPy and 3 mol / L 2-amino-2-methyl-1-propanol (AMP) was taken after 3800 hours (about 158 ​​days) of operation in the pilot plant campaign described in Example 2. Despite the improved stability of the solution due to AMP, Figure 3 It is apparent that the imine dimer degradation product is present in large amounts by this time (about 0.2 mol / L).

[0169] Equal volumes of sample and dichloromethane were shaken vigorously in a separatory funnel and allowed to undergo phase separation. The two phases were then collected and analyzed separately by GC-MS. Figure 4 and Figure 5 Gas chromatograms of the organic and aqueous phases are depicted separately, with peaks identified by mass spectrometry via comparison with a library of reference spectra. It was found that the imine degradation products were present almost exclusively in the organic phase, whereas the amine absorbent remained almost exclusively in the aqueous phase.

[0170] Samples of the same aqueous absorbent were also extracted with an equal volume of toluene or ethyl acetate. The organic phase after extraction with dichloromethane, toluene and ethyl acetate was passed through 13 C NMR analysis. In all three cases, the spectra confirmed successful and selective extraction of the imine dimer into the organic phase.

[0171] Example 6. Effect of regeneration on viscosity

[0172] Another sample of the aqueous absorbent, which had initially contained 3 mol / L of 3-AMPy and 3 mol / L of 2-amino-2-methyl-1-propanol (AMP) and which was taken after about 5000 hours (about 208 days) of operation in the pilot plant campaign described in Example 2, was extracted with dichloromethane as described in Example 5. As a result of the extraction, the viscosity of the sample decreased from 9.9 mPa.s to 6.0 mPa.s (measured at 40° C. using an Anton Parr LOVIS 2000ME instrument). In comparison, the viscosity of the fresh CO2-free absorbent was 4.7 mPa.s. The effect of the viscosity reduction is to improve the performance of the absorbent in the plant by better flow properties and reduced mass transfer resistance.

[0173] To demonstrate the effect of imine degradation products on viscosity, imines were artificially synthesized and isolated according to the scheme shown below.

[0174]

[0175] The synthesized imine (structure confirmed by IR spectroscopy) was added to fresh absorbent and the effect on viscosity was measured (measured using Anton Parr LOVIS2000ME at 40° C.). The viscosity of the solution increased with increasing concentration of the synthesized imine.

[0176] Example 7. Regeneration of 3-AMPy from Imine Degradation Products

[0177] A sample of an aqueous absorbent that initially already contained 3 mol / L of 3-AMPy and 3 mol / L of 2-amino-2-methyl-1-propanol (AMP) was obtained after about 3800 hours (about 158 ​​days) of operation in the pilot plant campaign described in Example 2. The sample was shaken in a separatory funnel with an equal volume of dichloromethane (DCM) for 1 minute. The phases were then allowed to separate, and each phase was collected separately. The imine degradation product, now in DCM, was converted to 3-AMPy by a two-step procedure, which in turn provided the first and second equivalents of 3-AMPy per mole of imine. In the first step, two equivalents of acetic acid and excess water were added to a DCM solution of the imine dimer and stirred vigorously at room temperature for 24 hours. The imine dimer is unstable under acidic aqueous conditions and is therefore hydrolyzed according to the following scheme to form molecules of 3-AMPy and aldehyde.

[0178]

[0179] The resulting water / DCM mixture is placed in a separatory funnel, and after phase separation, the aqueous and DCM phases are collected separately. 3-AMPy remains in the aqueous phase, and the aldehyde remains in the DCM phase. The aldehyde is then converted into an additional portion of 3-AMPy by stirring the DCM / aldehyde solution in ethanol with hydroxylamine and hydrochloric acid (HCl) at room temperature for 2 hours to form an oxime. A small amount of zinc (Zn) powder is then added, and stirring is continued for 2 hours, followed by reduction of the oxime to an amine. The reaction is carried out according to the following scheme. The Zn powder is then removed by filtration, and the DCM and ethanol are removed by rotary evaporation. The formation of pure 3-AMPy is accomplished by 1 H-NMR confirmed.

[0180]

[0181] Example 8. Oxidative degradation of benzylamine

[0182] The oxidative degradation of another amine of formula (I), benzylamine (BZA), was studied using accelerated degradation conditions designed to simulate long-term degradation in a gas capture process. 10 mL of a 1 mol / L solution of BZA was placed in a Fischer-Porter vessel. The vessel was sealed and the remaining headspace of the vessel was filled with CO2 (1 bar) and O2 (9 bar). The liquid phase in the vessel was stirred via a magnetic stir bar and heated to 55°C by immersion in a water bath for 10 days. Samples were then taken and the reaction mixture was stirred for 10 days by immersion in a water bath. 13 C-NMR spectroscopy (Bruker Avance 400). Figure 6 The resulting spectrum depicted in clearly shows the formation of a considerable amount of the corresponding imine of formula (II) from the characteristic C=N peak at 160.57 ppm and the NC peak at 93.66 ppm. Based on the relative peak areas, about 41% of the BZA has been converted to the imine.

[0183] Example 9. Extraction of Imine Dimers by Solid Absorption

[0184] A sample of an aqueous absorbent which had initially contained 3 mol / L 3-AMPy and 3 mol / L 2-amino-2-methyl-1-propanol (AMP) was taken after 5000 hours (about 208 days) of operation in the pilot plant campaign described in Example 2. Despite the improved stability of the solution due to AMP, the imine dimer degradation product was present in large quantities by this time (about 0.35 mol / L).

[0185] Amberlite XAD-2, a hydrophobic cross-linked polystyrene copolymer resin, was washed 3 times in deionized water and then 3 times in dichloromethane (DCM). The clean beads were mixed with an absorbent sample that had been exposed to flue gas for 5000 hours. The mixture was stirred for 2 hours to allow the hydrophobic component to adhere to the polymer resin. The mixture was filtered to remove the water-soluble part of the degraded amine absorbent. The beads were washed with water, and the water was added to the filtrate. The beads were then washed with 3 parts of DCM, and the collected DCM was partially evaporated.

[0186] Figure 7 Shown are (a) the sample before contact with Amberlite, (b) the aqueous filtrate and (c) the DCM wash residue. 13 C NMR spectrum. The characteristic imine peak around 160 ppm is clearly present in the DCM wash and is essentially absent in the aqueous filtrate, indicating that most of the imine is extracted by Amberlite. Very little amine is extracted into the beads and carried over into the DCM wash, as evidenced by the absence of peaks around 42 ppm (these peaks are present in the spectra of the sample and the aqueous filtrate). In the case of washing the adsorbed imine from Amberlite into DCM, the imine can then be further processed in the same manner as the imine extracted by liquid-liquid extraction.

[0187] Example 10. BZA and 3-AMPy as sacrificial oxygen scavengers

[0188] Diethanolamine (DEA) is a low-cost alkanolamine used industrially for CO2 separation applications, which is known to degrade in the presence of O2. The effects of 3-aminomethylpyridine (3-AMPy) and benzylamine (BZA) on the degradation of DEA were studied using accelerated degradation conditions designed to simulate long-term degradation in gas capture processes. DEA solutions were prepared as follows: i) 6 mol / L DEA; ii) 5.9 mol / L DEA and 0.1 mol / L BZA; and iii) 5.9 mol / L DEA and 0.1 mol / L 3-AMPy. Each solution (10 ml) was placed in a Fischer-Porter container. The container was sealed and the remaining headspace of the container was filled with CO2 (1 bar) and O2 (9 bar). The liquid phase in the container was stirred via a magnetic stir bar and heated to 55°C by immersion in a water bath for 115 hours.

[0189] Then take the sample and pass 13 C-NMR spectroscopy (Bruker Avance 400). Figure 8 The resulting spectrum is depicted in . The spectrum clearly shows that, compared with the undegraded solution ( Figure 8 Compared with (d) in the above figure, for the DEA-only solution ( Figure 8 (c) in Figure 5) shows considerable degradation of DEA (peak at 67 ppm). This degradation did not occur for solutions containing small amounts of BZA or 3-AMPy (respectively Figure 8 (b) and Figure 8 The absence of DEA degradation in the presence of 0.1 mol / L BZA or 0.1 mol / L 3-AMPy indicates that these amines have been selectively degraded, thereby preventing the degradation of DEA.

[0190] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It should be understood that the invention includes all such variations and modifications that fall within the spirit and scope of the invention.

[0191] Future patent applications may be filed in Australia or overseas based on or claiming priority from the present application. It will be appreciated that the accompanying provisional claims are provided by way of example only and are not intended to limit the scope of what may be claimed in any such future application. Features may be added to or omitted from the provisional claims at a later date in order to further define or redefine one or more inventions.

Claims

1. A method for absorbing carbon dioxide from a gas stream comprising carbon dioxide and molecular oxygen (O2), the method comprising contacting the gas stream with a liquid absorbent, the liquid absorbent comprising: i) at least one amine of formula (I): as well as ii) at least one aliphatic amine, wherein the compound of formula (I) is selectively degraded by O2 to form degradation products comprising at least one imine of formula (II): in, In the amine of formula (I) and the imine of formula (II), each Ar is independently an aromatic group, and each R is independently selected from hydrogen, an organic group and NH2.

2. The method according to claim 1, wherein the aliphatic amine is selected from the group consisting of unhindered primary or secondary amines and alkanolamines.

3. The method of claim 1 or claim 2, wherein the aliphatic amine comprises monoethanolamine and / or diethanolamine.

4. The method according to claim 1 or claim 2, wherein the aliphatic amine consists of monoethanolamine or diethanolamine.

5. The method of any one of claims 1 to 4, wherein the aliphatic amine comprises monoethanolamine.

6. The method of any one of claims 1 to 4, wherein the aliphatic amine comprises diethanolamine.

7. The method according to any one of claims 1 to 6, wherein the molar ratio of the aliphatic amine to the combined sum of the compound of formula (I) and the compound of formula (II) in the liquid absorbent is greater than 2:1, preferably greater than 5:

1.

8. The method according to any one of claims 1 to 6, wherein the molar ratio of the aliphatic amine to the combined sum of the compound of formula (I) and the compound of formula (II) in the liquid absorbent is greater than 10:

1.

9. The method according to any one of claims 1 to 6, wherein the molar ratio of the aliphatic amine to the combined sum of the compound of formula (I) and the compound of formula (II) in the liquid absorbent is greater than 20:1, preferably greater than 50:

1.

10. The method of any one of claims 1 to 9, wherein each R is hydrogen.

11. The method according to any one of claims 1 to 10, wherein each Ar comprises a monocyclic six-membered aromatic group, preferably wherein each Ar is selected from the group consisting of 2-pyridyl, 3-pyridyl, 4-pyridyl and phenyl.

12. The method of claim 11, wherein the monocyclic six-membered aromatic group comprises at least one nitrogen ring atom.

13. The method according to any one of claims 1 to 12, wherein the liquid absorbent is an aqueous composition comprising at least 10 wt% water.

14. The method according to any one of claims 1 to 13, further comprising regenerating the liquid absorbent by contacting the liquid absorbent with a hydrophobic medium and selectively extracting the degradation products into or through the hydrophobic medium.

15. The method according to claim 14, wherein the hydrophobic medium is an organic solvent, preferably wherein the organic solvent is selected from the group consisting of: aromatic hydrocarbon solvents, oxygenated solvents and halogenated solvents.

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