WO2014194376A1

WO2014194376A1 - Benzylamine, benzylamine derivatives and benzylamine mixtures for co2 removal from gas streams

Info

Publication number: WO2014194376A1
Application number: PCT/AU2014/000859
Authority: WO
Inventors: Graeme PUXTY
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2013-08-29
Filing date: 2014-08-29
Publication date: 2014-12-11

Abstract

A process for removing a target acidic gas from a gas stream rich in the target acidic gas comprising contacting the gas stream with a solution to absorb a target gas from a gas stream, the solution comprising a solvent and at least one absorbent compound dissolved in the solvent, said at least one absorbent compound comprising a compound of formula (1):

Description

BENZYLAMINE, BENZYLAMINE DERIVATIVES AND BENZYLAMINE MIXTURES FOR C0₂ REMOVAL FROM GAS STREAMS

Field of the invention

[0001] The present invention is directed to an absorbent solution for absorbing an acidic gas, such as carbon dioxide, from a gas stream.

Background of the invention

[0002] Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.

[0003] Waste gas emissions are of significant concern, and the presence of certain gaseous constituents in a waste gas stream can result in air pollution. There is significant research into methods for treating waste gas streams to remove these gaseous constituents from waste gas streams. Carbon dioxide (C0₂) emissions, in particular, attract a great deal of attention and the discussion of waste gas emissions that follows will largely be in respect of carbon dioxide. However, the skilled addressee will appreciate that much of this discussion is also applicable to other waste gases.

[0004] There is growing pressure for stationary producers of greenhouse gases to dramatically reduce their atmospheric emissions. Of particular concern is the emission of carbon dioxide (CO₂) into the atmosphere. One method of reducing atmospheric CO₂ emissions is through its capture and subsequent storage in geological or deep sea reservoirs.

[0005] The process for capturing CO₂ from power station or combustion device flue gases is termed post combustion capture. In post combustion capture, the CO₂ in flue gas is first separated from nitrogen and residual oxygen using a suitable solvent in an absorber. The CO₂ is then removed from the solvent in a process called stripping (or regeneration), thus allowing the solvent to be reused. The stripped CO2 is then liquefied by compression and cooling, with appropriate drying steps to prevent hydrate formation. Post combustion capture in this form is applicable to a variety of stationary CO₂ sources including power stations, steel plants, cement kilns, calciners and smelters.

[0006] Aqueous amine solutions and alkanolamine solutions in particular, have been investigated as solvents in post combustion CO₂ capture. The capture process involves a series of chemical reactions that take place between water, the amine and carbon dioxide. Amines are weak bases, and may undergo acid-base reactions. Once dissolved into the amine solution, the aqueous CO₂ reacts with water and the neutral form of the amine react to generate carbonic acid (H₂CO₃), aqueous bicarbonate (HCO₃ ^") ions and aqueous carbonate (CO₃ ^2") ions, according to the generally acknowledged equations described below:

CO₂ + 2H₂O→ HCO3^" + H₃O⁺ (equation 1

CO₂ + OH^"→ HCO3^" (equation 2

CO₃ ^2" + H₃O⁺→ HCO3^" + H₂O (equation 3

HCO3^" + H₃O⁺→ H₂CO₃ + H₂O (equation 4

OH^" + H₃O⁺→ 2H₂O (equation 5

R^aR^bR^cN + H₃O⁺ <→ R^aR^bR^cNH ⁺ (equation 6

[0007] If the amine contains a primary (R^aR^bNH, R^b = H) or secondary amine (R^aR^bNH, R^b≠ H), an additional reaction pathway becomes available, where carbon dioxide and the primary or secondary amine react to generate a carbamate (R^aR^bNCOO^"). The carbamate may also then participate in acid-base chemistry, according to the generally acknowledged reactions described below. Tertiary amines (R^aR^bR^cN, R^a, R^b R^c≠ H) cannot form carbamates.

CO₂ + R^aR^bNH + H₂O <→ R^aR^bNCOO^" + H₃O⁺ (equation 7)

R^aR^bNCOO^" + H₃O⁺ <→ R^aR^bNCOOH (equation 8) [0008] It is generally acknowledged that the molar absorption capacity of an aqueous amine solution, as measured by the number of moles of CO2 absorbed per mole of amine functionality in solution (a), is dependent upon the pH equilibria that operate in the amine solution and the formation of carbamate species.

[0009] Carbamate formation by primary and secondary amines is a direct reaction between the amine nitrogen and CO2. This reaction consumes two moles of amine per mole of C0₂ absorbed. One mole is amine that is converted to carbamate, and the second is amine that accepts the proton released by carbamate formation. Carbamate is also a base, as illustrated by the reaction of equation 8 to form carbamic acid. However, it is a much weaker base than an amine and as such does not contribute as a proton acceptor at typical CO₂ absorption conditions. The stability of the carbamate species is influenced only weakly by temperature (the reaction enthalpy is small).

[0010] CO2 desorption is achieved by heating of an aqueous amine solution containing CO₂. The two major effects of heating are to reduce the physical solubility of CO2 in the solution, and to reduce the pK_a of the amine resulting in a concomitant reduction in pH and in CO2 absorption capacity, the net effect of which is CO2 release. The extent of the reduction in pK_a is governed by the enthalpy of the amine protonation reaction which in turn is governed by the amine chemical structure. All the other reactions, including carbamate formation, have small reaction enthalpies and are insensitive to temperature. Typically, the enthalpy of amine protonation is four to eight times larger than the enthalpies of the carbonate reactions and two to four times larger than the enthalpy of carbamate formation. It is the lowering of the pH upon heating that drives the reversal of carbamate and carbonate/bicarbonate formation during desorption, rather than any significant reduction in stability.

[0011] The cyclic capacity (a_cyciic) of an aqueous amine solution is defined as the moles of CO2 that can be absorbed and released per mole of amine by cycling the absorbent between low temperature (a_riCh) and high temperature

a_cyciic = a_riCh - ctiean- In terms of chemistry, this cyclic capacity is primarily governed by the change in amine pK_a with temperature. The larger this cyclic capacity, the more efficient the amine. 30 wt% monoethanolamine (MEA, HO-CH2-CH2-NH2), which is currently employed in industrial CO2 capture, possesses an undesirable cyclic capacity of approximately a_cyciic = 0.1 1 (40°C-80°C)

[0012] In summary, there exists a relationship between the change in amine pK_a as a function of temperature, and the cyclic capacity of CO2 absorption and release of an aqueous amine.

[0013] Identification of the problem with C0₂ absorption cyclic capacity has prompted efforts aimed at seeking amines with improved cyclic capacities. However, amines used for industrial CO2 capture that achieve larger CO2 cyclic absorption capacity than MEA have poor rates of C0₂ absorption. Slow C0₂ absorption rates are undesirable because to achieve the requisite absorption of C0₂ longer gas-liquid contact times are required which means larger absorption columns and greater capital cost. The benefits gained through increased cyclic capacity are thus offset by the disadvantages associated with decreased rates.

[0014] Amines such as MEA also suffer from oxidative and thermal degradation due to exposure to molecular oxygen in flue gas, and heating to release absorbed CO2. This degradation requires ongoing reclamation and replenishment of the absorbent solution at considerable cost.

[0015] Amines such as MEA are also limited in the maximum concentration that can be used due to corrosion and viscosity. MEA is limited to 30 wt% as at higher concentrations it becomes too corrosive for use in contact with carbon steel. Another amine 2-amino-2-methyl-propanol (AMP, HO-CH2-C(CH₃)2-NH₂), which is also currently employed in industrial C0₂ capture, is also limited to around 30 wt% due to high viscosity.

[0016] There thus exists a need to identify amines whose aqueous solutions possess improved properties for application in gas capture technologies, such as C0₂ capture technologies. Summary of the invention

[0017] In one aspect of the invention there is provided a process for absorbing target acidic gas from a gas stream rich in the target acidic gas comprising contacting the gas stream with a solution to absorb a target gas from a gas stream, the solution comprising a solvent and at least one absorbent compound dissolved in the solvent, said at least one absorbent compound comprising the compound of formula (1 ):

wherein R¹ to R⁵ are each independently selected from the group consisting of: H, hydroxyl, alkyl hydroxyl, alkoxy, substituted or unsubstituted Ci to C₂0 alkyl, substituted or unsubstituted C₂ to C₂₀ alkenyl, substituted or unsubstituted C₂ to C₂₀ alkynyl, substituted or unsubstituted aryl, substituted or unsubstituted heterocycle, substituted or unsubstituted carbonyl, aldehyde, substituted or unsubstituted carboxylate, substituted or unsubstituted ester, substituted or unsubstituted Ci to C₂o alkoxy, substituted or unsubstituted carboxamide, substituted or unsubstituted imine, NR⁶R⁷, N=CR⁸R⁹, and halo; and

R⁶ and R⁷ are each independently selected from the group consisting of: H, substituted or unsubstituted Ci to C₂o alkyl, substituted or unsubstituted C₂ to C₂o alkenyl, and substituted or unsubstituted C₂ to C₂o alkynyl; wherein R⁸ and R⁹ are independently selected from the group consisting of H, substituted or unsubstituted Ci to C₂₀ alkyl, substituted or unsubstituted C₂ to C₂₀ alkenyl, and substituted or unsubstituted C₂ to C₂o alkynyl substituted or unsubstituted aryl, substituted or unsubstituted heterocycle, substituted or unsubstituted carbonyl, aldehyde, substituted or unsubstituted carboxylate, substituted or unsubstituted ester, substituted or unsubstituted alkoxy, substituted or unsubstituted carboxamide, substituted or unsubstituted imine, NR⁶R⁷, and halo; and when any of R¹ to R⁵ is a substituted Ci to C₂o alkyl, the substituted Ci to C₂₀ alkyl is selected from the group consisting of: haloalkane, alkylsulfide, hydroxyalkyl, alkyl thiol, alkyl phosphine, alkyl phosphate, alkyl phosphonate, alkyl ether, alkyl alkanoate, alkyl hydroperoxide alkyl peroxide, alkyl cyanate, alkyl isocyanate, alkyl nitrate, alkyl cyanide, alkyl amide, alkyl imine, alkyl imide, alkyl azide, alkyl diazine, and alkyl nitrite.

[0018] Preferably, no more than one of R¹ to R⁵ is an alkyl amine. In one embodiment, none of R¹ to R⁵ are alkyl amine. Preferably, none of R¹ to R⁵ is an alkyl aryl. Preferably, no more than three, more preferably not more than two and even more preferably no more than one of R¹ to R⁵ comprises a substituted or unsubstituted Ci to C₂o alkyl. In another embodiment, no more than three, more preferably not more than two and even more preferably no more than one of R¹ to R⁵ comprises a substituted or unsubstituted Ci to C₅ alkyl. Examples of substituted Ci to C₅ allyl include substitution with a substituent selected from the group consisting of hydroxyl, thiol, amino, alkoxy and carboxyl.

[0019] The group substituted or unsubstituted carbonyl may be acyl such as a hydrocarbyl-carbonyl where the hydrocarbyl may be selected from group consisting of aryl, Ci to C₂₀ alkyl, C₂ to C₂₀ alkenyl and C₂ to C₂₀ alkynyl.

[0020] The group substituted or unsubstituted carboxyl may be carboxyl or hydrocarbyloxycarbonyl wherein the hydrocarbyl is selected from the group consisting of aryl, Ci to C₂₀ alkyl, C₂ to C₂o alkenyl and C₂ to C₂o alkynyl.

[0021] The group substituted or unsubstituted ester may be an acyloxy wherein the group acyl is hydrocarbyl-carbonyl where the hydrocarbyl may be selected from group consisting of aryl, Ci to C₂o alkyl, C₂ to C₂o alkenyl and C₂ to C₂o alkynyl.

[0022] Preferably R¹ and R⁵ are selected to enhance the solubility of the absorbent in the solvent. In embodiments, in which the solvent comprises water, R¹ to R⁵ are at least one (preferably at least 2 and more preferably at least 3) of R¹ to R⁵ are selected from substituents which are capable of hydrogen bonding with the aqueous solvent. Examples of substituents capable of forming hydrogen bonding with aqueous solvents may be selected from the group consisting of hydroxyl, hydroxyalkyi, alkoxy, thiol, amino (i.e. -NH₂) carboxyl and aryloxy. In further embodiments, preferably at least three, and more preferably at least four, of R¹ to R⁵ are selected from the group consisting of H, hydroxyl, hydroxyalkyi, alkoxy, thiol or amino. In one set of embodiment the one of the substituents R¹ to R⁵ is Ci to C₆ alkyl and the remaining substituents are selected from the group consisting of H, hydroxyl, hydroxyl-Ci to C5 alkyl, alkoxy, thiol and amino. In one set of embodiments R¹ to R⁵ are H or one of R¹ to R⁵ is Ci to C₄ alkyl (such as methyl) and the others are H. In a further set of embodiments the nitrogen substituents R⁶ and R⁷ are each hydrogen (H). Compounds wherein R⁶ and R⁷ are both hydrogen generally exhibit improved solubility in the relevant solvents such as water and protic and/or a polar aprotic solvents.

[0023] Advantageously, absorbent compounds of formula (1 ) have lower susceptibility to thermal and oxidative degradation than a 30 wt% MEA solution due to the inherent chemical stability imparted by the aromatic ring structure. Preferably the cyclic absorption capacity of the solution for the target gas is comparable to that of a tertiary or sterically hindered amine solution and the rate of absorption of the target gas is comparable to or better than a 30wt% MEA solution.

[0024] The solution is preferably a single phase liquid solution prior to the absorption of the acid gas as well as after the absorption of the acid gas (i.e. no precipitation of the reactants of the absorption process). The absorbent may be dissolved or disperse in one or more solvents. The solvent is typically an organic solvent, water or a combination thereof. The organic solvents are preferably a protic and/or a polar aprotic solvent. Suitable solvents that may be used include, but are not limited to, methanol, ethanol, propenol, glycols, carbonates (e.g. propylene carbonate), N-methyl-2-pyrrolidone, acetonitrile, dimethyl sulfoxide, dioxane, sulfolane, dimethylformamide, pyridine, acetone, dichloromethane, methyl ethyl ketone, chloroform, tetrahydrofuran, ethyl acetate, 2-butanone, toluene. [0025] Ideally the target gas is an acidic gas. Preferably the target gas is selected from the group consisting of CO₂, NO_x (where x is between 0.5 and 2), SO₂, H₂S, carbonyl sulphide, carbon disulfide, thiols or a halogen gas, such as Cl₂, F₂, l₂, or Br₂. More preferably the target gas is C0₂ or S0₂. Most preferably the target gas is C0₂. In an embodiment a number of target gases may be absorbed from a gas stream using the solution of the present invention, the target gases being selected from various combinations of C0₂, NO_x, S0₂, H₂S or a halogen gas, such as Cl₂, F₂, l₂, or Br₂.

[0026] In an embodiment the target gas is C0₂.

[0027] Preferably the solution is an aqueous solution. The use of an aqueous solution is advantageous for both economic and environmental reasons. The solvent may also be any protic solvent such as methanol, n-butanol or glycol or polar aprotic solvent such as ethylacetate or dimethylsulfoxide in which the acid gas and amine are jointly soluble. Furthermore, the target gas to be absorbed from the gas stream needs to be at least partially soluble in the solvent, so that the target gas is able to interact with the various constituents of the solution. Many of the gases of interest are at least partially soluble in water. In a preferred embodiment, the solvent comprises a mixture of water and a protic solvent (e.g. water and an alcohol) or water and a polar aprotic solvent. The selection of a co-solvent to replace part of the water as a solvent may influenced by improved characteristics of the co-solvent, resulting in a solvent mixture with increased acidic gas solubility, lower heat capacity or a higher boiling point.

[0028] In an embodiment R¹ to R⁵ are each independently selected from the group consisting of: H, hydroxyl, or Ci to C₁₀ alkyl; R⁶ and R⁷ are independently selected from the group consisting of H, methyl, or ethyl. More preferably the absorbent compound is benzylamine or a benzylamine derivative. Most preferably the compound is benzylamine.

[0029] In one set of embodiments, the compound of formula (1 ) is present in the solution in an amount of at least 1 % (such as 1 % to 70 %) by weight based on the weight of the solution. The compound of formula 1 is preferably present in an amount between 1wt % and 50wt%, more preferably 1 .5wt% and 40wt%, even more preferably between 2wt% and 30wt% and even more preferably between 5wt% and 20wt% based on the total weight of the solution. In a preferred embodiment, the upper limit of the compound of formula (1 ) is limited by the concentration which results in the precipitation of the reactants of the absorbent(s) and the absorbed acid gases. The upper limit of the compound of formula (1 ) is preferably 70wt%, more preferably 65wt%, even more preferably 60wt% and yet even more preferably 55wt% of the total weight of the solution.

[0030] The total concentration of acidic gas absorbant compounds including the compounds of formula (1 ) is preferably at least about 10 weight %. More preferably the concentration is at least about 20 weight %. Even more preferably the concentration is at least about 30 weight %. Yet even more preferably the concentration is at least about 40 weight %. Most preferably the concentration is at least about 50 weight % or above 50 weight %.

[0031] Advantageously, the solution has a low viscosity and low corrosion potential. This allows the solution to contain the compounds at high concentration while still being able to maintain effective operating conditions. This provides for a solution with a large target gas absorption capacity and target gas absorption rate. Preferably the viscosity of the solution measured at 40°C is less than 3mPa.s. More preferably, the viscosity of the solution is less than 2.75mPa.s. Even more preferably, the viscosity is less than 2.5mPa.s.

[0032] The at least one absorbent compound dissolved in the solvent will comprise the compound of formula (1 ) which may constitute the total of the gas absorbent compound or may be present in solution with other acidic gas absorbent compounds so that the total gas absorbent compounds comprise one or more gas absorbent compounds in addition to the compounds of formula (1 ). In one embodiment the solution contacted with the gas stream comprises one or more acidic gas absorbing compounds selected from amines and imidazoles in addition to the compound of formula (1 ). The one or more additional amines may be selected from primary, secondary and tertiary amines. [0033] Examples of suitable amines include primary amines such as monoethanolamine, ethylenediamine, 2-amino-2-methylpropanol, 2-amino-2-methyl- ethanolamine and benzylamine; secondary amines such as N-methylethanolamine, piperazine, piperidine and substituted piperidine, diethanolamine, diglycolamine and diisopropanolamine; and tertiary amines such as N-methyldiethanolamine, and amino acids such as taurine, sarcosine and alanine.

[0034] In an embodiment, the solution further includes an additional amine compound, such as a tertiary or sterically hindered amine. The additional amine compound helps to avoid precipitation of the absorbent compound out of solution, which may be an issue at high weight loadings of the absorbent compound, and/or depending on the chemical environment of the solution. Suitable compounds may include for example: 2-amino-2-methyl-1 -propanol (AMP), 3-piperidinemethanol, 3-piperidineethanol, 2- piperidinemethanol, 2-piperidineethanol, N-piperidinemethanol, N-piperidineethanol, 2-methylaminoethanol, Ν,Ν-dimethylaminoethanol and 3-quinclidinol. In some embodiments it is preferred that the compound is not monoethanolamine, diethanolamine, aminoethylethanolamine, Diglycolamine, piperazine, N- Aminoethylpiperazine, N-(2-hydroxyethyl) piperazine and morpholine

[0035] In a further embodiment, the solution comprises an imidazole and more preferably an N-functionalised imidazole. Suitable N-functionalised imidazoles may be found in US8741246, which in incorporated herein by reference.

[0036] The suitable N-functionalised imidazoles disclosed in US 8,741 ,246 are of formula (2):

(2) wherein

R¹ is substituted or unsubstituted Ci_-2o alkyl, substituted or unsubstituted C_2-20 alkenyl, substituted or unsubstituted C_2-2o alkynyl, substituted or unsubstituted Ci_-2o heteroalkyl, substituted or unsubstituted C_2-2o heteroalkeroalkyl, substituted or unsubstituted C_2-20 heteroalkenyl, substituted or unsubstituted C_2-20 heteroalkynyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted heterocycloalkyi, substituted or unsubstituted aryl, substituted or unsubstituted heteroaryl, substituted or unsubstituted thio, substituted or unsubstituted amino, substituted or unsubstituted alkoxyl, substituted or unsubstituted aryloxyl, silyl, siloxyl, cyano, or nitro; and

R², R³, and R⁴ are each independently selected from hydrogen, halogen, hydroxyl, substituted or unsubstituted Ci_-2o alkyl, substituted or unsubstituted C_2-2o alkenyl, substituted or unsubstituted C_2-2o alkynyl, substituted or unsubstituted Ci_-2o heteroalkyl, substituted or unsubstituted C_2-20 heteroalkenyl, substituted or unsubstituted C_2-20 heteroalkynyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted heterocycloalkyi, substituted or unsubstituted aryl, substituted or unsubstituted heteroaryl, substituted or unsubstituted thio, substituted or unsubstituted alkoxyl, aryloxyl, substituted or unsubstituted amino, cyano, or nitro.

[0037] Specific examples of such compounds include the 1 -N-(Ci to C₂₀ alkyl) imidazoles such as 1 -butyl imidazole

[0038] In some, embodiment the solution comprises a combination of N-functionalised imidazoles and one or more amines. [0039] The one or more amines which may be used in addition to the N-functionalised imidazoles may be selected from the group consisting of primary, secondary and tertiary amines including the specific examples of such amines referred to above.

[0040] The total wt% of the at least one absorbent compound in solution is preferably at least 20wt%, more preferably at least 25 wt %, still more preferably at least 30wt%, even more preferably at least 40wt% and yet even more preferably at least 50wt% relative to the total weight of the solution. This component will typically consists of the compound of formula (1 ) and optionally one or more compounds selected from amines and N-functionalised-imidazoles. The compound of chemical formula (1 ) preferably comprises at least 1 % (e.g. 1 % to 70%) more preferably between 1wt% and 50wt%, still more preferably between 1.5wt% and 40wt%, even more preferably between 2 wt% and 30wt% and even more preferably between 5wt% and 20wt% relative to the total weight of the solution.

[0041] In another aspect of the invention there is provided a process for removing a target gas from a gas mixture including: contacting a gas mixture that is rich in target gas with an absorbent solution, as described above, to form a target gas rich solution and a gas mixture that is lean in target gas; and desorbing the target gas from the target gas rich solution. In yet a further set of embodiments there is further provided use of a compound of formula (1 ) in solution in a solvent at a concentration of at least 20% by weight, based on the total weight of the solution, for absorbing an acidic gas from a gas stream.

[0042] In one set of embodiments there is further provided a composition comprising a solution for an acidic gas comprising:

A. a solvent;

B. at least one absorbent compound comprising a compound of formula (1 )

wherein R¹ to R⁵ are each independently selected from the group consisting of: H, hydroxyl, substituted or unsubstituted Ci to C₂o alkyi, substituted or unsubstituted C₂ to C₂o alkenyl, substituted or unsubstituted C₂ to C₂o alkynyl, substituted or unsubstituted aryl, substituted or unsubstituted heterocycle, substituted or unsubstituted carbonyl, substituted or unsubstituted aldehyde, substituted or unsubstituted carboxylate, substituted or unsubstituted ester, substituted or unsubstituted alkoxy, substituted or unsubstituted carboxamide, substituted or unsubstituted imine, NR⁶R⁷, N=CR⁸R⁹, and halo; and

R⁶ and R⁷ are each independently selected from the group consisting of: H, substituted or unsubstituted Ci to C₂₀ alkyi, substituted or unsubstituted C₂ to C₂₀ alkenyl, and substituted or unsubstituted C₂ to C₂o alkynyl (more preferably R⁶ and R⁷ are both hydrogen); wherein R⁸ and R⁹ are independently selected from the group consisting of H, substituted or unsubstituted Ci to C₂₀ alkyi, substituted or unsubstituted C₂ to C₂₀ alkenyl, and substituted or unsubstituted C₂ to C₂₀ alkynyl substituted or unsubstituted aryl, substituted or unsubstituted heterocycle, substituted or unsubstituted carbonyl, substituted or unsubstituted aldehyde, substituted or unsubstituted carboxylate, substituted or unsubstituted ester, substituted or unsubstituted alkoxy, substituted or unsubstituted carboxamide, substituted or unsubstituted imine, NR⁶R⁷, and halo; and when any of R¹ to R⁵ is a substituted Ci to C₂o alkyi, the substituted Ci to C₂o alkyi is selected from the group consisting of: haloalkane, alkyi sulfide, alkyi thiol, alkyi sulfonate, alkyi phosphine, alkyi phosphate, alkyi phosphonate, alkyi ether, alkyi alkanoate, alkyi hydroperoxide alkyi peroxide, alkyi cyanate, alkyi isocyanate, alkyi cyanide, aikyi nitrate, alkyi nitrite, alkyl amide, aikyi imine, alkyi imide, alky! azide, alkyi diazine, and alkyi nitrite; and

C. an absorbed acidic gas, preferably carbon dioxide, at a concentration above the equilibrium concentration when the solution is exposed to air at below the boiling point of the solvent.

Preferably, the concentration of the absorbed acidic gas is more than two times (and even more preferably five times) the equilibrium concentration when the solution is exposed to air at belo the boiling point of the solvent, thus representing the absorbed acidic gas concentration in the solvent during the absorption process as previously described. The background amount of acidic gas, such as CO2, will generally be less than 0.1% by weight based on the total weight of the solution. In one embodiment the absorbed acidic gas will constitute at least 0.2% by weight based on the total weight of the solution on absorption of the gas more preferably at least 1 % and still more preferably at least 10% absorbed acidic gas by weight based on the total weight of the solution.

[0043] in one embodiment the solution comprises one or more amines in addition to the compound of formula (1 } which additional amines may, for example, be selected from primary, secondary and tertiary amines optionally including N-functionalised imidazoles such as those of formula (2).

[0044] Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings.

Brief description of the drawings

[0045] Figure 1 illustrates a process for removing a target gas from a gas mixture according to the present invention.

[0046] Figure 2 is a graph showing CO2 loading per kg of absorbent solution at different C0₂ partial pressures for a range of different solutions. [0047] Figure 3 is a graph showing the evolution of the CO₂ partial pressure with time.

[0048] Figure 4 is a graph showing the viscosity of absorbent solutions containing varying amounts of benzylamine or MEA at different temperatures.

[0049] Figure 5 is a graph showing the overall mass transfer coefficient for various CO₂ loadings/mole amine for a range of amines including mixtures of benzylamine and 2-amino-2-methyl-1 -propanol (AMP) compared with MEA.

[0050] Figure 6 is a graph showing the overall mass transfer coefficient for various C0₂ loadings/mole amine for mixtures of benzylamine and MEA compared with MEA above.

Detailed description of the embodiments

[0051] The invention relates to the use of a solution including benzylamine, a benzylamine derivative, a benzylamine mixture, a benzylamine derivative mixture, or a combination thereof in a solution for absorbing a target gas from a gas stream.

[0052] Figure 1 provides an illustration of an embodiment of a process for capture of a target gas from a flue gas stream. In this particular embodiment, the target gas is C0₂. The process 100 includes an absorption reactor 102, for absorbing C0₂ from a flue gas stream, and a desorption reactor 104 for desorbing CO₂.

[0053] The absorption reactor 102 includes a first inlet 106, a second inlet 108, a first outlet 110, and a second outlet 1 12, and a gas absorption contact region 114. The first inlet 106 of the absorption reactor 102 is a flue gas inlet through which a C0₂ rich flue gas enters the absorption column 102. The second inlet 108 is an absorbent solution inlet through which a CO₂ lean absorbent enters the absorption column 102. The CO₂ rich flue gas and the CO₂ lean absorbent contact in the gas absorption contact region 114. In this region the C0₂ in the C0₂ rich flue gas is absorbed into the absorbent solution where it is bound in solution to form a C0₂ lean flue gas and a CO₂ rich absorbent solution.

[0054] The absorbent solution includes an absorbent molecule. In this particular embodiment, the absorbent molecule is benzylamine, e.g.:

[0055] However, it wiii be understood that other suitable benzylamine derivatives may also be employed, the benzylamine derivatives having the general structure below, where the R¹ to R^? are as previously defined:

[0056] The local environment of the soiution may be altered in the absorption column to favour the absorption reaction, e.g. to increase absorption of COa into solution where it is bound to the benzylamine. Such alterations of the local environment may include a change in pH, a change in solution temperature, a change in pressure etc. Alternatively, or additionally, the soiution may include other compounds which assist in the absorption of COa. These compounds may alter the affinity or absorption capacity of the benzylamine for CO2, or these compounds may be also absorb CO2.

[0057] If additional compounds are added to the absorbent solution in the absorption reactor 102, the process may additionally include means to remove these compounds.

[0058] The absorption of CO₂ from the CO2 rich f!ue gas into the absorbent solution results in a C0₂ lean gas and a CO₂ rich absorbent soiution. The CO₂ lean gas may still include some C0₂, but at a lower concentration than the C0₂ rich flue gas, for example a residual concentration of C0₂.

[0059] The CO₂ lean gas leaves the absorption column 102 through the first outlet 110, which is a C0₂ lean gas outlet. The C0₂ rich absorbent solution leaves the absorption column through the second outlet 1 12, which is a CO2 rich absorbent outlet.

[0060] Desorption reactor 104 includes an inlet 1 18, a first outlet 120, a second outlet 122, and a gas desorption region 124. The C0₂ rich absorbent outlet 1 12 of the absorption column 102 forms the inlet 1 18 of the desorption column 104. Desorption of C0₂ from the C0₂ rich solution occurs in the gas desorption region 124.

[0061] Desorption of C0₂ from the C0₂ rich solution may involve the application of heat or a reduction in pressure to favour the desorption process. Furthermore, additional compounds may be added to the C0₂ rich solution to enhance the desorption process. Such compounds may alter the solution environment, for example by changing solution pH or altering another parameter to favour the desorption reaction.

[0062] Removal of C0₂ from the C0₂ rich solution results in the formation of a C0₂ gas stream and a C0₂ lean absorbent solution. The C0₂ lean absorbent solution may still include some C0₂, but at a lower concentration than the C0₂ rich solution, for example a residual concentration of C0₂.

[0063] The C0₂ gas stream is taken off via the first outlet 120, which is a C0₂ outlet. The C0₂ lean absorbent solution is taken off via the second outlet 122, which is a C0₂ lean absorbent solution outlet. The C0₂ lean absorbent is then recycled and fed through the second inlet 108 to the absorption column 102.

[0064] The invention will now be described with reference to the following Examples. It is to be understood that the examples are provided by way of illustration of the invention and that they are in no way limiting to the scope of the invention.

EXAMPLES Example 1

[0065] The C0₂ mass absorption capacity, expressed in kg C0₂/kg solvent (water) of several concentrations of benzylamine and a mixture (30 wt% and 50 wt% BA, and 50 wt% BA +10 wt% AMP) were tested at different partial pressures of C0₂ in a vapour- liquid-equilibrium (VLE) apparatus including a 160 mi glass vessel, a thermal bath, and a CO2 dosing unit between 40°C and 80°C. The method was validated by comparing measurements of CO2 absorption by MEA 30 wt% with literature data. Results of this experiment are presented in Figure 2. The amount of solvent circulating in a PCC process is linearly related to cyclic CO₂ mass absorption capacity of the solvent between the absorption (rich) and desorption (iean) column temperatures. Concentrated benzylamine solvents show superior absorption capacity compared to MEA 30 wt%, but the concentration of BA is limited by precipitation. The optimal concentration of BA, is at the limit of precipitation when at equilibrium with 15 kPa COs. At this concentration, the cyclic capacity is estimated to be 0.04 kg C0₂/kg solvent, compared with 0.021 kg CO2/ kg solvent for MEA 30 wt%. Precipitation can be avoided by the use of a second amine such as AMP.

Exampte 2

[0066] The enhancement factor, E, in amine mixture was determined by C0₂ initial rate absorption at 40°C in a pressurised stirred-vessei with a flat gas-liquid interface with MDEA-BA and compared to MEA- DEA and DEA-MDEA. The initial partial pressure in the vessel was about 5 bar C0₂. The evolution of the C0₂ partial pressure with time is shown in Figure 3.

Kg,, proms - sr

E

g, pramster

[0067] Benzylamine performed as well as or better than MEA and DEA as a rate promoting agent. A larger amount of benzylamine in aqueous MDEA resulted in a higher enhancement of the C0₂ absorption rate. Due to the favourable physical properties of benzylamine use at higher concentrations is possible.

Example 3

[0068] Thermodynamic characterisation of the protonation reaction of benzylamine and some benzylamine derivates by titration revealed that the enthalpy is much larger than MEA, piperazine, as well as several piperazine derivatives, and other alkanolamines. A large enthalpy is favourable in terms of minimising the total energy input required for solvent regeneration.

[0069] Also assessed was the impact of different substituents on aqueous solubility. The addition of methyl groups did not impact upon solubility. However, the addition of hydroxyl groups resulted in relatively poor solubility making them less suitable for use in aqueous solvent media in the absence of a cosolvent. This does not preclude their use in a mixed aqueous/organic solvent media.

Amine Enthalpy of protonation, Δ /-/_prot Water Solubility kJ/mol

Benzylamine -60 soluble

Monoethanolamine -41 soluble

Piperazine -36 limited solubility

1 -Methylpiperazine -37 limited solubility

2-Amino-2-methyl-1 -propanol -47 limited solubility

2-Methylbenzylamine -48 soluble

3-Methylbenzylamine -46 soluble

4-Methylbenzylamine -55 soluble

2-Hydroxybenzylamine NA poor

3-Hydroxybenzylamine NA poor

4-Aminophenol NA poor

Example 4

[0070] Viscosities, η in Pa.s, of concentrated benzylamine aqueous solutions were measured between 25°C and 80°C with an Anton Paar AMVn viscometer. Corresponding densities are required for the determination of η and were measured with a benchtop density meter (Anton Paar DMA 38). [0071] At 40°C, the maximum viscosity (2.51 mPa.s) was measured with BA 75wt%. This value is reasonably low compared to that of MEA 80wt% (10.2 mPa.s) as depicted in Figure 4. Therefore, viscosity of concentrated benzylamine aqueous solutions would not raise operability issues such as solvent's pumpability and pourability. Moreover low viscosity is also favourable for the diffusion rate of a C0₂ in the solvent as the diffusion rate is inversely proportional to the liquid viscosity (Stokes-Einstein equation).

[0072] It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Example 5

[0073] The mass transfer coefficient of C0₂ absorption {Kg, mmol.m^"2s^"1kPa^"1) in benzylamine and benzylamine mixtures with 2-amino-2-methyl-1 -propanol (AMP) and monoethanolamine (MEA) have been measured at 40°C. The C0₂ loading (mol C0₂ / mol amine) of the liquid was also varied. The measurements were made using a wetted-wall contactor in which the rate of C0₂ absorption is measured into a falling liquid film of known surface area at atmospheric pressure. This device mimics the gas-liquid contacting of a packed column. Details of the device and experimental procedure can be found in G. Puxty, et al., Chem. Eng. Sci., 65 (2010), 915-922.

[0074] The compositions are tabulated below. Figure 5 compares BZA and AMP to results for MEA alone. All compositions show enhanced mass transfer compared to MEA, due the combined effects of comparable reaction kinetics and low viscosity. Figure 6 compares BZA and MEA to MEA alone. Again mass transfer benefits are seen for all compositions relative to MEA alone.

[mmol.m^'V.kPa ¹]

CO₂ loading

Solution 0 0.1 0.2 0.3 0.4

5.0M MEA 2.57 2.15 1.41 0.88

6.0M MEA 2.59 2.28 - 1.43 0.89

4.5M BZA 1.5M 4.22 3.45 2.87 1.63 0.8 AMP

3.5M BZA 2.5M 3.22 2.98 2.02 1.44 0.53 AMP

3.0M BZA 3.0M 3.57 2.69 1.8 1.32 0.54 AMP

3.0M BZA 3.0M 3.43 3.08 1.34

MEA

4.5M BZA 1.5M 4.55 3.08 (0.37)* 0.54 (0.43)*

MEA

3.0M BZA 4.2M 4.00 1.82 (0.34)* 0.86 (0.42)*

MEA

^* C0₂ loading in parenthesis

Claims

1 . A process for removing a target acidic gas from a gas stream rich in the target acidic gas comprising contacting the gas stream with a solution to absorb a target gas from a gas stream, the solution comprising a solvent and at least one absorbent compound dissolved in the solvent, said at least one absorbent compound comprising a compound of formula (1 ):

wherein R¹ to R⁵ are each independently selected from the group consisting of: H, hydroxyl, substituted or unsubstituted Ci to C₂0 alkyl, substituted or unsubstituted C₂ to C₂₀ alkenyl, substituted or unsubstituted C₂ to C₂₀ alkynyl, substituted or unsubstituted aryl, substituted or unsubstituted heterocycle, substituted or unsubstituted carbonyl, substituted or unsubstituted aldehyde, substituted or unsubstituted carboxylate, substituted or unsubstituted ester, substituted or unsubstituted alkoxy, substituted or unsubstituted carboxamide, substituted or unsubstituted imine, NR⁶R⁷, N=CR⁸R⁹, and halo; and

R⁶ and R⁷ are each independently selected from the group consisting of: H, substituted or unsubstituted Ci to C₂o alkyl, substituted or unsubstituted C₂ to C₂o alkenyl, and substituted or unsubstituted C₂ to C₂o alkynyl; wherein R⁸ and R⁹ are independently selected from the group consisting of H, substituted or unsubstituted Ci to C₂₀ alkyl, substituted or unsubstituted C₂ to C₂₀ alkenyl, and substituted or unsubstituted C₂ to C₂o alkynyl substituted or unsubstituted aryl, substituted or unsubstituted heterocycle, substituted or unsubstituted carbonyl, substituted or unsubstituted aldehyde, substituted or unsubstituted carboxylate, substituted or unsubstituted ester, substituted or unsubstituted alkoxy, substituted or unsubstituted carboxamide, substituted or unsubstituted imine, NR⁶R⁷, and halo; and when any of R¹ to R⁵ is a substituted Ci to C₂o alkyl, the substituted Ci to C₂₀ alkyl is selected from the group consisting of: haloalkane, alkyl sulfide, hydroxyalkyl, alkyl thiol, alkyl sulfonate, alkyl phosphine, alkyl phosphate, alkyl phosphonate, alkyl ether, alkyl alkanoate, alkyl hydroperoxide alkyl peroxide, alkyl cyanate, alkyl isocyanate, alkyl cyanide, alkyl nitrate, alkyl nitrite, alkyl amide, alkyl imine, alkyl imide, alkyl azide, alkyl diazine, and alkyl nitrite; and wherein the at least one absorbent compound is in the solution at a concentration of at least about 20% by weight based on the total weight of the solution.

2. The process of claim 1 , wherein the solvent is water.

3. The process of claim 1 or 2, wherein R¹ to R⁵ are each independently selected from the group consisting of: H, hydroxyl amine, thiol or Ci to Cio alkyl; and R⁶ and R⁷ are independently selected from the group consisting of H, methyl, or ethyl.

4. The process of claim 3, wherein R¹ to R⁵ are each independently selected from the group consisting of H, hydroxyl, amine, thiol and Ci to C5 alkyl; R⁶ and R⁷ are independently selected from the group consisting of H, methyl and ethyl; and wherein no more than one of R¹ to R⁵ are selected from the group consisting of Ci to C5 alkyl.

5. The process of claim 3, wherein R¹ to R⁵ are each independently selected from the group consisting of: H, hydroxyl, amine, thiol and Ci to C₃ alkyl; R⁶ and R⁷ are independently selected from the group consisting of H, methyl, and ethyl; and wherein no more than one of R¹ to R⁵ are selected from the group consisting of Ci to C5 alkyl and at least one of R¹ to R⁵ are hydroxyl.

6. The process of claim 1 or 2, wherein R¹ to R⁵ are each independently selected from the group consisting of H, hydroxyl, amine and thiol; and R⁶ and R⁷ are independently selected from the group consisting of H, methyl and ethyl.

7. The process of claim 1 or 2 wherein the absorbent compound comprises benzylamine.

8. The process of any one of the preceding claims, wherein the at least one absorbent compound is in the solution at a concentration of at least about 30 weight %.

9. The process of any one of the preceding claims, wherein the at least one absorbent compound is in the solution at a concentration of at least about 40 weight %.

10. The process of any one of the preceding claims, wherein the at least one absorbent compound is in the solution at a concentration of at least about 50 weight %.

1 1 . The process of any one of the preceding claims, wherein the viscosity of the solution at 40°C is less than about 3mPa.s.

12. The process of claim 1 1 , wherein the viscosity of the solution at 40°C is less than about 2.75mPa.s.

13. The process of claim 12, wherein the viscosity of the solution at 40°C is less than about 2.5mPa.s.

14. The process of any one of the preceding claims, where the at least one absorbent compound further comprises an additional amine compound.

15. The process of claim 14, wherein the additional amine compound is a tertiary or sterically hindered amine.

16. The process of claim 14 or claim 15, wherein the concentration of the compound of formula (1 ) is in the range of 1 wt% and 65 wt%.

17. The process of any one of the previous claims further comprising desorbing the target gas from the target gas rich solution.

18. A composition comprising a solution for absorbing an acidic gas comprising: a solvent; at least ooa absorbent oompound comprising a compound of formyla (1 )

wherein H' t R^s are each independeritiy sefeotad from the group consisting of: H_!: do y!, syfcsstifytad or ynsubststyfed C* to G_2¾ alkyb sybsityied or ynsubstltyied 2 t £¾ø alk l^ su^litut^d: or ynsuhstfioted C¾ to C alkynyi, subst¾yted or ynsybstltytad ary, su isted or ynsybststofed hotaroeyolo, sybstluta or ynsubstity ted carfeonyl, substituted or unsubstituted aldehyde, substituted or unsybstityted eafboxyiate_s subsliutad or unsubstituted eater, substituted or ynsubstltuted alkoxy, substituted or ynsubstituied ear oxamdo, substituted or unsubstituted imirse, NRR^'\ -C ^R⁸, andhaio; and;

R^B and ^? are aaeh Independently selected from the grou oonsisfiMj of: B, substituted or ynsybsfityted .G$ t €¾ a!ky sobsiiutod or unsybstltuted (¾ to ¾ a!kenyi, apd substituted or urssybstityted C_s to C₂o alkynyi; vvf elh ^$ nd ^ are In e endently selected from the groyp consisting of H, sy sifuted of mi&siiui«<i C to C½ aikyl, ¾ »s¾fut^- or uosybsfiuted C¾ to C¾ al!senyl, and substituted or un ybstifytad ¾ to ¾ alkynyi sybsiiuted or unsyostiiyted aryi, substituted or un$u^t ute haterocyole, substituted or yrmiibstituted oar onyS, sybstityfed or ynsybsiityted aldehyde, sybsifuled or ynsubstityted earhoxylate, ^u sS «d or unsubetrtuted eatar, suost&ted or unsu sityf ad at koxy, aubsfityta o unsybstituted oarbo amlde, sybstitued or unsubsfituied Woo, NR- , and halo; and when an of to Is a substituted C- to alkyl_f the substituted C to Cs$ alley! Is selected from the grou consisting of: haloalkane, aikyl sulfide, a!kyl thiol, hydroxyaik !, aikyl sulfonate, aikyl phospfiine, a!kyf phosphate, atkyf phosphonata. alkyl ether, alkyl alkanoate, alkyl hydroperoxide alkyl peroxide, alkyl cyanate, alkyl isocyanate, alkyl cyanide, alkyl nitrate, alkyl nitrite, alkyl amide, alkyl imine, alkyl imide, alkyl azide, alkyl diazide and alkyl nitrite; andalkyl diazine, and alkyl nitrite; and wherein the at least one absorbent compound is in the solution at a concentration of at least about 20 weight %; and

C. an absorbed acidic gas at a concentration above the equilibrium concentration when the solution is exposed to air at below the boiling point of the solvent.

19. A composition according to claim 18 further comprising an additional compound selected from the group consisting of amines N-functionalised imidazoles and mixtures thereof.

20. A composition according to claim 18 or claim 19 wherein the absorbent compound of formula (1 ) is present in an amount between 1 % by weight and 50% by weight based on the total weight of the solution.