, Description:GAS-LIQUID ABSORPTION IN MICROCHANNEL
Field of the invention:
The present invention relates to Gas-liquid absorption in microchannel and generally the process of gas absorption in aqueous solution of amino acid salts within a microchannel gas-absorber device.
The invention particularly relates to absorption of gases such as carbon dioxide, carbon monoxide, sulphur dioxide, hydrogen sulphide or flue gases in solution of amino acids or combination(s) thereof. The process and device is useful in prevention and control of emission of undesired gases in the environment. Moreover, it can have industrial applications in automobiles, petroleum industries, specialised chemical production reactors, space technology, biomedical and lab on the chip devices.
Background and Objective of the invention
Continuous growing demand of energy leads to greater need of processing and burning of fossil fuels. These processes release and accumulate carbon dioxide, carbon monoxide, gaseous hydrocarbons, sulphur dioxide, hydrogen sulphide or flue gases or their mixtures into atmosphere. Most of these gases are associated with global warming, climate changes and health problems in humans. Strict environmental regulations are being adopted and implemented globally to mitigate these harmful gases. Emission of these gases, especially carbon dioxide, into environment can be regulated and controlled efficiently through chemical & physical absorption.
Among absorption solvents, alkanolamines such as monoethanolamine, diethanolamine, diisopropanolamine, N-methyldiethanolamine etc. are recognized as most effective solvents. Alkanolamines are being extensively used in removal of acidic gases such as carbon dioxide and hydrogen sulphide from variety of industrial and non-industrial gas streams and discharges. However alkanolamine solutions undergo degradation in oxygen rich conditions or environments and generate toxic products.
The aqueous alkaline salts of amino acids are being used as potential alternative for alkanolamines in certain areas of gas treatment, although these are more expensive than alkanolamines. The solution of amino acids possesses low volatility, high surface tension and ionic nature which make them more stable toward oxidative degradation.
Several investigators [Kumar et al. (2003), Portugal et al. (2009)] studied absorption of carbon dioxide in amino acid salt solutions and reported similar absorption characteristics as shown by alkanolamine solutions. Along with proper and efficient solvent such as amino acids, process intensification can be implemented to improve efficiency of absorption processes.
In past decade, microfluidic devices have emerged as best process intensification solution for conventional chemical equipments. These devices are generally referred as microchannel, microreactor, microabsorber or droplet devices. Microfluidic devices offer enhanced rate of heat and mass transfer in single phase system with high surface to volume ratios as compared to conventional equipments.
In the multiphase microfluidic devices, these systems further improve Taylor dispersion and mixing limitations of single phase. Furthermore, higher safety and control features make these devices more attractive for gas-liquid operations such as chemical absorptions.
Yue et al. (2007) investigated the mass transfer characteristics of gas–liquid with the absorption of pure carbon dioxide into water, buffer solution and sodium hydroxide solution in a rectangular microchannel having hydraulic diameter of 667 micrometre. They studied the characteristics of liquid side mass transfer coefficient and interfacial area in different two-phase flow regime, namely slug, slug-annular and churn flow. It was demonstrated that liquid side volumetric mass transfer coefficient and interfacial area in microchannels were at least one or more orders of magnitude higher than those in traditional gas–liquid contactors.
Niu et al. (2009) studied the absorption of carbon dioxide into an aqueous solution of piperazine activated N-methyldiethanolamine in 0.5, 1 and 2 mm circular channels. They reported that the removal efficiency increased with an increasing superficial liquid velocity and decreases with an increasing superficial gas velocity. The addition of Piperazine to an aqueous solution of N-methyldiethanolamine significantly enhanced the rate of absorption and removal efficiency.
Gao et al. (2011) investigated the absorption of carbon dioxide using microporous tube-in-tube microchannel reactor, using aqueous solution of monoethanolamine. They reported that carbon dioxide removal efficiency increased with increasing concentration of monoethanolamine, reducing channel width, decreasing superficial gas velocity, increasing superficial liquid velocity and increasing the absorbent temperature.
Chinese Patent CN103861424A refers to the process of capturing carbon dioxide in hydrophobic microchannel having hydraulic diameter 300-1000 micrometres using absorbent monoethanolamine, diethanolamine, piperazine, diethylene glycol amine, diisopropanolamine, 2-amino-2- or N- methyl-1-propanol diethanolamine, and organic amines or mixture between two or more.
Another Chinese Patent CN101612510B refers to the process of absorption of carbon dioxide in monoethanolamine, diethanolamine or methyldiethanolamine or an organic solvent, polyethylene glycol dimethylether, propylene carbonate, methanol, ethanol or polyethylene glycol; or solution of potassium carbonate; or mixture in microchannel having hydraulic diameter 50-3000 micrometres.
One other Chinese Patent CN102019129B refers to the process of absorption of carbon dioxide in water, sodium carbonate-sodium bicarbonate buffer, or sodium hydroxide in double tube micro-reactor having annular microchannel radial spacing of 250-1000 micrometres and pore diameter 5-200 micrometer.
The present invention is intended to propose a method of gas absorption in aqueous solution of amino acids within microchannel. The present invention particularly discloses a method for absorption of carbon dioxide in amino acid salt solution in a microchannel gas-absorber device having hydraulic diameter in the ranges of 50-1000 micrometer.
Prime object of the present invention is to propose a method of absorption of gases such as carbon dioxide, carbon monoxide, sulphur dioxide, hydrogen sulphide or flue gases in amino acids solution, amine solution or combination thereof within microchannel.
Another object of the present invention is to propose the method of gas absorption in aqueous solution of amino acid salts within microchannel.
Another object of the invention is to propose a suitable solution for intensified and effective absorption of gases such as carbon dioxide, carbon monoxide, sulphur dioxide, hydrogen sulphide or flue gases, particularly to be performed in a microchannel having hydraulic diameter in the ranges of 1-100 micrometer or 150-1000 micrometer.
Another object of the present invention is to propose an embodiment of the microchannel gas-absorber device for conducting the process of intensified and effective absorption of gases such as carbon dioxide, carbon monoxide, sulphur dioxide, hydrogen sulphide or flue gases in amino acids solution, amine solution or combination thereof, wherein said microchannel/microreactor/microabsorber or droplet device comprises of hydraulic diameter in the ranges of 1-100 micrometer or 150-1000 micrometer.
Another object of the present invention is to propose industrial applications of said proposed method of gas absorption in aqueous solution of amino acids within microchannel, particularly in automotive industries, petroleum industries, specialised chemical production reactors, space technology, biomedical and lab on the chip devices.
Summary of the invention:
The present invention discloses a method of absorption of gases such as carbon dioxide, carbon monoxide, sulphur dioxide, hydrogen sulphide or flue gases in amino acids solution, amine solution or combination thereof.
(i) Constructional aspects of most preferred embodiment of the proposed Microchannel gas-absorber device:
Types of the microchannel/microreactor/microabsorber or droplet device can be opted from prior art due to Hessel et al. (2005), Shui et al. (2007), Mansur et al. (2008) and Zhao and Middelberg (2011), with diameter in range of 1-100 micrometres (µm) or 150-1000 micrometres (µm) but not between 100-150 micrometres (µm). While, the micromixer type can be opted from T-shaped, cross shaped, parallel lamination micromixers, sequential lamination micromixers, flow focusing enhanced mixers, chaotic advection micromixers, droplet micromixers, right angled shaped confirmation (Capretto et al. 2011) excluding Y-type micromixer.

Novel functional features of the Microchannel gas-absorber device, generally proposed for conducting gas absorption in amino acids solution, amine solution or combination thereof, are as under:
a) The hydraulic diameters or specific diameters of microchannel are in range of 1-100 micrometres (µm) or 150-1000 micrometres (µm) but not between 100-150 micrometres (µm).
b) The gas and liquid used for absorption process are co-flowing or counter-flowing with each other.
c) The channel wall can be partially or completely coated with catalyst and may be hydrophilic or hydrophobic (i.e. wall contact angle is between 10 to160 degree).
d) The microchannel is of optional shape, preferably of circular, rectangular, triangular, trapezoidal or as square cross-sectional.
e) The device has periodic converging-diverging cross-section (Chandra et al. 2016) for enhancement of flow and mass transfer. Further, microchannel could be serpentine flow channel with circular or noncircular bent. The present invention could be applied of parallel or nonparallel microchannel flow system.
f) The microchannel can be complex, and highly integrated microfluidic networks.
g) The microchannel is made up of metals, alloys, glass, and polymeric materials. The metals comprise aluminium, iron, glass, copper, silver, gold, silicon and platinum as substrate or body or coating materials. The alloys comprise steel, stainless steel, brass, bronze or duralumin as substrate or body or coating materials.
h) The polymeric microchannel refers to Polytetrafluoroethylene (PTFE), Poly-vinyl choride, Polydimethylsiloxane (PDMS), but not limited to, as substrate or body or coating materials.
i) The inlets for the mixing or contacting gas and liquid with each other in the microchannel is T-shaped, cross shaped, parallel lamination micromixers, sequential lamination micromixers, flow focusing enhanced mixers, chaotic advection micromixers, droplet micromixers, right angled shaped confirmation.
j) The channel may be tube-in tube channel or single microchannel or concentric channel type or corrugated channel type.
k) Further, the flow of fluid(s) can be divided in any number of channels or integrated from multiple channels to single or multiple streams during absorption operation.
(ii) Effective formulations of liquid absorbents, proposed for conducting the process of intensified and effective absorption of gases such as carbon dioxide, carbon monoxide, sulphur dioxide, hydrogen sulphide or flue gases within the proposed and preferred embodiment of the microchannel:
(a) The liquid absorbent is an aqueous solution of amino acids or amino acids salts alone or combination(s) thereof or mixtures of amino acids with amines or piperazine or combination(s) thereof.
(b) The amino acids are alkaline salts or pure solution of at least one of the amino acids namely histidine, alanine, isoleucine, arginine, leucine, asparagine, lysine, aspartic acid, methionine, sarcosine, cysteine, phenylalanine, glutamic acid, threonine, glutamine, tryptophan, glycine, valine, taurine, proline, selenocysteine, serine, tyrosine or amino acid solution neutralized with an inorganic bases or solution of mixture of two or more amino acids salts.
(c) The amines may be selected from aqueous solutions of alkanolamines such as primary, secondary, or tertiary amine, viz. monoethanolamine (MEA), diethanolamine (DEA), dissoppropanolamine (DIPA), and N-methyldiethanolamine (MDEA), diethylene glycol amine, 2-amino-2- or N- methyl-1-propanol diethanolamine,
(d) The overall concentration of amino acid solution is between 0.001N to 3N.
(e) The gases to be treated within the proposed microchannels are selected from carbon dioxide, carbon monoxide, sulphur dioxide, hydrogen sulphide or flue gases or combination of two or more of them and can be mixed with inert gases such as nitrogen and oxides of nitrogen to dilute or adjust the concentration or quality of the selected gas(es).
(iii) Novel aspects of the proposed method of absorption of gas in the proposed liquid absorbent(s), in general:
(a) The flow rate of gas and liquid used for absorption are in ranges of 0.05 to 10 meter per second. The flow patterns formed inside microchannel during gas absorption process are bubble, slug, annular, churn and stratified.
(b) The absorption process may be physical absorption due to solubility or chemical absorption which involves chemical interaction between two fluid phases (gas-solvent).
(c) The gas and liquid stream may be subjected to filtration processes before introduction to microchannel. The flow rates of fluids (gases and liquid) may be regulated and controlled through means of pumps, mass flow controllers, pressure regulators and sensors.
(d) The absorption process and devices in claim 1 may be combined with conventional absorbers such as packed bed absorption or used separately.
(e) The absorption process may be employed for selective and complete removal or absorption or separation of carbon dioxide, hydrogen sulfide, sulfur oxide, carbon monoxide or flue gases or mixture.
With regard to the construction and assembly of said optionally preferred embodiment of the microchannel gas-absorber device and its effective functioning, following schematic drawings are depicted in Figure 1 and Figure 2 hereinbelow.

Figure1: One prospective view of the preferred embodiment of the microchannel gas-absorber device;

Figure 2: Indicative scheme/flow-chart of functioning of the microchannel gas-absorber towards carrying out the gas absorption in the liquid absorbents.

Illustrations towards conducting gas absorption on one selected liquid absorbent formulation within the preferred embodiment of the microchannel (as depicted in the schematic drawings):

Example 1
The carbon dioxide enters from one of inlet while the aqueous solutions of amino acids enter from other inlet of T- or cross- shaped microchannel. Both of fluids enter into microchannel at rate of 0.05 meter per second. The concentration of carbon dioxide was adjusted to 10% v/v by supplying nitrogen and controlling by mass flow controller. The aqueous glycine having concentration 1.0 kmol/m3 may be used as absorbent. The gas and liquid mixed in microchannel and slug flow patterned was formed. The carbon dioxide absorbed into aqueous solution of amino acid through mass transfer in region of forward and backward hemispherical caps and walls. The salts of amino acids may be obtained by neutralising the amino acid with metal hydroxide. The neutralised salts of amino acids react in similar ways as amines reacts with CO2, i.e. by carbamate and bicarbonate formation.

Example 2
The carbon dioxide enters from two inlets while the aqueous solutions of amino acids enter from other inlet of a cross shaped microchannel. The gases enter into microchannel at flow rate of 0.1 meter per second while liquid enters at flow rate of 0.05 meter per second. The concentration of carbon dioxide was adjusted to 10% v/v by supplying nitrogen and controlling by mass flow controller. The aqueous glycine having concentration 1.0 kmol/m3 may be used as absorbent. The gas and liquid mixed in microchannel and slug formation was formed. The carbon dioxide absorbed into aqueous solution of amino acid through mass transfer in region of forward and backward hemispherical caps and walls. The salts of amino acids may be obtained by neutralising the amino acid with metal hydroxide. The neutralised salts of amino acids react in similar ways as amines reacts with CO2, i.e. by carbamate and bicarbonate formation.

Example 3
The carbon dioxide enters from one of inlet while the aqueous solutions of amino acids mixed with amine(s) enter from other inlet of T- or cross-shaped microchannel. Both of fluids enter into microchannel at rate of 0.05 meter per second. The concentration of carbon dioxide was adjusted to 10% v/v by supplying nitrogen or oxides of nitrogen and controlling by mass flow controller. The amine amino acid salts (AAAS), (formed by mixing equinormal amounts of amino acids; e.g. glycine, ß-alanine and sarcosine, with an organic base; 3(methylamino)propylamine (MAPA)) of concentration 1.0 kmol/m3 used as absorbent. The gas and liquid mixed in microchannel and slug flow patterned was formed. The carbon dioxide absorbed into aqueous solution of amino acid through mass transfer in region of forward and backward hemispherical caps and walls. The salts of amino acids may be obtained by neutralising the amino acid with metal hydroxide. The neutralised salts of amino acids react in similar ways as amines reacts with CO2, i.e. by carbamate and bicarbonate formation.

References:
1. Capretto, L., Cheng, W., Hill, M., & Zhang, X., Micromixing within microfluidic devices. In Microfluidics (2011) (27-68), Springer Berlin Heidelberg.
2. Chandra A. K., Kishor K., Mishra P. K., Alam M. S., Numerical Investigations of Two-phase Flows through Enhanced Microchannels, Chem. Biochem. Eng. quarterly, 2016, 30(2) 149-159.
3. Gao Na-Na, Wang Jie-Xin, Shao Lei, and Chen Jian-Feng, Removal of Carbon Dioxide by Absorption in Microporous Tube-in-Tube Microchannel Reactor, Ind. Eng. Chem. Res., 2011, 50, 6369–6374.
4. Hessel V., Panagiota A., Gavriilidi A. and Löwe H., Gas-Liquid and Gas-Liquid-Solid Microstructured Reactors: Contacting Principles and Applications, Ind. Eng. Chem. Res. 2005, 44, 9750-9769
5. Kumar P. S., Hogendoorn J. A., Feron P. H. M, and Versteeg G. F., Equilibrium Solubility of CO2 in Aqueous Potassium Taurate Solutions: Part 1. Crystallization in Carbon Dioxide Loaded Aqueous Salt Solutions of Amino Acids, Ind. Eng. Chem. Res., 2003, 42, 2832-2840.
6. Mansur E. A., Mingxing YE, Wang Y. and Dai Y., A State-of-the-Art Review of Mixing in Microfluidic Mixers, Chinese Journal of Chemical Engineering, 2008, 16(4) 503—516
7. Niu H., Pan Liwei, Su Hongjiu, and Wang Shudong, Effects of Design and Operating Parameters on CO2 Absorption in Microchannel Contactors., Ind. Eng. Chem. Res. 2009, 48, 8629–8634.
8. Portugala A.F., Sousab J.M., Magalhãesa F.D., Mendesa A., Solubility of carbon dioxide in aqueous solutions of amino acid salts, Chemical Engineering Science, 64 (2009) 1993-2002.
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11. Zhao C. X., Middelberg A. P. J., Two-phase microfluidic flows, Chemical Engineering Science, 2011, 66, 1394–1411

Claims:We Claims:
1. Microchannel absorber for gas absorption in novel amino acid based liquid absorbents and the method involved thereof; wherein said microchannel comprises hydraulic diameter in the range of 1-100 micrometres or 150 to 2000 micrometres or (µm) but not between 100-150 micrometres (µm), the liquid absorbent is aqueous solution of amino acid or amino acid salts or combination(s) of different amino acid salts or aqueous solution of amino acid salts with amines with or without activator(s) thereof, wherein the gas for absorption within the said microchannel absorber is selected from carbon dioxide, carbon monoxide, sulphur dioxide, hydrogen sulphide or flue gases or combination of two or more whereas inlets or micro-mixers for the mixing or contacting gas and liquid with each other in the microchannel is a passive mixer excluding Y-shaped micro-mixers.
2. Microchannel absorber for gas absorption in novel amino acid based liquid absorbents and the method involved thereof, as claimed in claim 1; wherein the preferred embodiment of said microchannel absorber is characterized in following constructional and/or functional features:
(a) The hydraulic diameters or specific diameters of microchannel are in range of 1-100 micrometres or 150 to 2000 micrometres or (µm) but not between 100-150 micrometres (µm);
(b) The gas and liquid used for absorption process are co-flowing or counter-flowing with each other;
(c) The channel wall can be partially or completely coated with catalyst and may be hydrophilic or hydrophobic (i.e. wall contact angle is between 10 to160 degree);
(d) The microchannel is of optional shape, preferably of circular, rectangular, triangular, trapezoidal or as square cross-section;
(e) The device has periodic converging-diverging cross-section for enhancement of flow and mass transfer. Further, microchannel could be serpentine flow channel with circular or noncircular bent. The present invention could be applied of parallel or nonparallel microchannel flow system;
(f) The microchannel can be complex, and highly integrated microfluidic networks;
(g) The microchannel is made up of metals, alloys, glass, and polymeric materials. The metals comprise aluminium, iron, glass, copper, silver, gold, silicon and platinum as substrate or body or coating materials. The alloys comprise steel, stainless steel, brass, bronze or duralumin as substrate or body or coating materials;
(h) The polymeric microchannel refers to Polytetrafluoroethylene (PTFE), Poly-vinyl choride, Polydimethylsiloxane (PDMS), but not limited to, as substrate or body or coating materials;
(i) The inlets for the mixing or contacting gas and liquid with each other in the microchannel is selected from active and passive mixer which are selected from T-shaped, cross shaped, parallel lamination micromixers, sequential lamination micromixers, flow focusing enhanced mixers, chaotic advection micromixers, droplet micromixers, right angled shaped confirmation;
(j) The channel may be tube-in tube channel or single microchannel or concentric channel type or corrugated channel type.
3. Microchannel absorber for gas absorption in novel amino acid based liquid absorbents and the method involved thereof, as claimed in claims 1 and 2, wherein the said microchannel absorber is provisioned with employing/storage/maintenance and regulation of the liquid absorbent therein by way of flow control devices viz.. pumps, sensor and mass flow controllers.
4. Microchannel absorber for gas absorption in novel amino acid based liquid absorbents and the method involved thereof, as claimed in claims 1 and 2, the process for absorption of gas in a liquid absorbent in the microchannel having hydraulic diameter in the range of 1-100 micrometres or 150 to 2000 micrometres or (µm) but not between 100-150 micrometres (µm), wherein, the gas is selected from carbon dioxide, carbon monoxide, sulphur dioxide, hydrogen sulphide or flue gases or combination of two or more and the liquid absorbent is pure amino acids solution, aqueous solution of pure amino acids, amino acids neutralized with inorganic bases or salts of amino acids or amine amino acid salts solution or may be mixture of amino acid with amines or piperazine or combination thereof or diluted mixture.
5. Microchannel absorber for gas absorption in novel amino acid based liquid absorbents and the method involved thereof, as claimed in any of the preceding claims, wherein the gases to be treated within the said microchannel gas-absorber by said method are selected from carbon dioxide, carbon monoxide, sulphur dioxide, hydrogen sulphide or flue gases or combination of two or more of them.
6. Microchannel absorber for gas absorption in novel amino acid based liquid absorbents and the method involved thereof, as claimed in any of the preceding claims, the gas selected for absorption may be mixed with inert gases such as nitrogen and oxides of nitrogen to dilute or to adjust the concentration of the gases.
7. Microchannel absorber for gas absorption in novel amino acid based liquid absorbents and the method involved thereof, as claimed in any of the preceding claims, wherein the effective formulations of liquid absorbents for conducting the process of intensified and effective absorption of gases such as carbon dioxide, carbon monoxide, sulphur dioxide, hydrogen sulphide or flue gases within the preferred embodiment of the said microchannel absorber is characterized in:
(a) The liquid absorbent is aqueous solution of amino acids or salts of amino acids alone or may be mixed with amines or piperazine or combination thereof in specific proportion.
(b) The amino acids are alkaline salts or pure solution of at least one of the amino acids namely histidine, alanine, isoleucine, arginine, leucine, asparagine, lysine, aspartic acid, methionine, sarcosine, cysteine, phenylalanine, glutamic acid, threonine, glutamine, tryptophan, glycine, valine, taurine, proline, selenocysteine, serine, tyrosine or amino acid solution neutralized with an inorganic bases or solution of mixture of two or more amino acids salts.
8. Microchannel absorber for gas absorption in novel amino acid based liquid absorbents and the method involved thereof, as claimed in any of the preceding claims, wherein said method of absorption of gas in the liquid absorbent(s), in general is characterized in:
(a) The flow rate of gas and liquid used for absorption are in ranges of 0.05 to 10 meter per second;
(b) The flow patterns formed inside microchannel during gas absorption process are bubble, slug, annular, churn and stratified;
(c) The absorption process may be physical absorption due to solubility or chemical absorption which involves chemical interaction between two fluid phases (gas-solvent);
(d) The gas and liquid stream may be subjected to filtration processes before introduction to microchannel. The flow rates of fluids (gases and liquid) may be regulated and controlled through means of pumps, mass flow controllers, pressure regulators and sensors;
(e) The absorption process and devices in claim 1 may be combined with conventional absorbers such as packed bed absorption or used separately; and
(f) The absorption process may be employed for selective and complete removal or absorption or separation of carbon dioxide, hydrogen sulfide, sulfur oxide, carbon monoxide or flue gases or mixture.
9. Microchannel absorber for gas absorption in novel amino acid based liquid absorbents and the method involved thereof, as claimed in claim 1, wherein said microchannel absorber and the said process of gas absorption in the liquid absorption formulation is characterized in their industrial applications in automobiles, petroleum industries, petrochemical process, chemical processes environmental emissions control devices, heat transfer devices, specialised chemical production reactors, sensor, gas purification, gas separation, space, biomedical and lab on the chip devices.