Bi-directional reactor and supported monoethenalamine for CO2 separation

The invention generally relates to a bi-directional reactor and supported amine sorbent, and more particularly to a method and system for carbon dioxide sequestration utilizing a bi-directional reactor and monoethenalamine (MEA) on a substrate. The bi-directional reactor is configured to reclaim the sorbent material as the sorbent is immobilized during the sorption phase, but is mobilized during desorption phased. The immobilized sorbent reacts with the desired contaminate to absorb and is transported to another reactor during desorption phase, thereby permitting reclamation of the sorbent.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to a bi-directional reactor and supported monoethenalamine (MEA), and more particularly to a method and system for carbon dioxide sequestration utilizing a bi-directional reactor and monoethenalamine (MEA) on a substrate.

2. Discussion of the Related Art

The atmospheric CO2concentration has increased by almost 38% since the beginning of the industrial revolution to a current level of about 386.8 ppm. Barrie, et al., “WMO Greenhouse Gas Bulletin,” World Meteorological Organization, Nov. 24, 2010, No. 6, pp. 1-4, which is hereby incorporated by reference as if fully set forth herein. More than 30% of all anthropogenic CO2emissions are estimated to have resulted from fossil fuel based electricity generation. “What is CCS?” IEA Greenhouse Gas R&D Programme, © 2010, printed Apr. 30, 2012, from http://www.ieaghg.org/index.php?/20091218110/what-is-css.html, 2 pages, which is hereby incorporated by reference as if fully set forth herein. These fossil fuels, including coal, oil and natural gas, will be used as major energy sources for the foreseeable future due to their low prices and abundance. However, people are concerned about the increase of CO2concentration in the atmosphere since CO2has been implicated as one of the main greenhouse gases leading to global climate changes. Accordingly, capture of CO2from flue gas streams in fossil-fuel based power plants has been considered as one of the major strategies for reduction of anthropogenic CO2emissions and thus the potential risks resulting from climate changes.

To date, all commercial CO2capture processes have been based on liquid amine compounds. Amine solutions are basic and can chemically remove many acid gases, including CO2, from flue gas. Danckwerts, “Reaction of CO2with ethanolamines,”Chemical Engineering Science,1979, 34, (4), pp. 443-446, which is hereby incorporated by reference as if fully set forth herein. Among the frequently used amine compounds is monoethanolamine (MEA). Aqueous amines along with membranes have been successfully used for separation of CO2from natural gas, however, they have not been used in fossil fuel based power plants since the overall costs associated with the current technologies are too high to be acceptable. The high costs are mainly due to the use of large concentrations of water in the aqueous amine solutions made for carbon dioxide separation. Typical amine solutions used by the natural gas industry for gas cleaning can contain as much as 70 wt % water. Abu-Zahra, et al., “CO2capture from power plants. Part I. A parametric study of the technical-performance based on monoethanolamine,”International Journal of Greenhouse Gas Control,2007, 1, (1), pp. 37-46; Feng, et al., “Reduction of Energy Requirement of CO2Desorption by Adding Acid into CO2-Loaded Solvent,”Energy&Fuels,2010, 24, pp. 213-219, each of which is hereby incorporated by reference as if fully set forth herein.

In recent years, people are increasingly interested in using solid sorbents synthesized with amines and solid supports or grafting materials for CO2capture in power plants. Different support materials have been used for immobilization of amines. Tanaka, “Comparison of thermal-properties and kinetics of decompositions of NAHCO3and KHCO3,” Journal of Thermal Analysis,1987, 32, (2), pp. 521-526; Glasscock, et al., “CO2absorption desorption in mixtures of methyldiethanolamine with monoethanolamine or diethanolamine,”Chemical Engineering Science,1991, 46, (11), pp. 2829-2845; Hagewiesche, et al., “Absorption of carbon-dioxide into aqueous blends of monoethanolamine and n-methyldiethanolamine,”Chemical Engineering Science,1995, 50, (7), pp. 1071-1079, each of which is hereby incorporated by reference as if fully set forth herein. Compared to aqueous amines, solid sorbents have several advantages when used for separation of CO2from flue gases in power plants. Mandal, et al., “Removal of carbon dioxide by absorption in mixed amines: modelling of absorption in aqueous MDEA/MEA and AMP/MEA solutions,”Chemical Engineering Science,2001, 56, (21-22), pp. 6217-6224; Liao, et al., “Kinetics of absorption of carbon dioxide into aqueous solutions of monoethanolamine plus N-methyldiethanolamine,”Chemical Engineering Science,2002, 57, (21), pp. 4569-4582; Ramachandran, et al., “Kinetics of the absorption of CO2into mixed aqueous loaded solutions of monoethanolamine and methyldiethanolamine,”Industrial&Engineering Chemistry Research,2006, 45, (8), pp. 2608-2616, each of which is hereby incorporated by reference as if fully set forth herein.

There is a need for an improved apparatus, process and sorbent.

SUMMARY OF THE INVENTION

Accordingly, the invention is directed to a bi-directional reactor and supported monoethanolamine for CO2sequestration that obviates one or more of the problems due to limitations and disadvantages of the related art.

An advantage of the invention is to provide a bi-directional reactor in which the sorbent material may be recycled.

Another advantage of the invention is directed away from using aqueous amines for carbon dioxide separation and using a supported amine, e.g., a supported monoethanolamine for carbon dioxide separation. Utilization of a supported amine minimizes operational energies since no energy is needed to heat and evaporate H2O, with its high specific-heat-capacity and latent heat of vaporization, in aqueous amine solutions during sorbent regeneration or CO2stripping processes. In addition, supported amines are easy to handle and transport and are less problematic than aqueous amine solutions from an operational viewpoint because they are less corrosive.

Another advantage of the invention is to provide a bi-directional apparatus configured to recycle a supported amine via bi-directional operation.

One aspect of the invention is directed towards a supported amine sorbent including a macroporous substrate and a monoethanolamine having a mass ratio to the substrate (monoethanolamine:substrate) ranging from about 40:60 to about 60:40.

Yet another aspect of the invention is directed towards a bi-directional reactor apparatus. The bi-directional reactor includes a first input, a first output, and a first reactor having a first reaction chamber which is in communication with the first reactor. The first input is coupled to the first reactor chamber. The bi-directional reactor further includes a second reactor having a second reaction chamber in communication with the first reactor. The second reactor is coupled to a second input and the first output. The first reactor is coupled to the second output. The first and second reactors include heating and cooling mechanisms as known in the art. The reactor is configured to operate bi-directionally. The first and second reaction chambers include fixed bed reactors as known in the art. Additional reactors may be added either in series or parallel to increase throughput or provide other advantages as known in the art.

Still another aspect of the invention is directed towards a method of sequestering carbon dioxide with a bi-directional apparatus. The method includes providing flue gas from a coal fired plant to the bi-directional reactor. The bi-directional reactor includes a first reactor and a second reactor and the flue gas is provided to the first reactor. Next, the method includes absorbing at least a portion of the carbon dioxide on a substrate and monoethanolamine having a mass ratio to the substrate (monoethanolamine:substrate) in a range from about 40:60 to about 60:40 in the first reactor.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Aspects of the invention are related to a bi-directional reactor, a supported amine configured to sequester a contaminate and a method of operating the same. Moreover, a pure MEA in a sorbent bed may be configured to sequester CO2in an embodiment of the invention. Moreover, the sorbent is configured to be reclaimed, such that it is immobilized during the sorption phase but mobilized during the desorption phase. The immobilized sorbent reactor with CO2in a sorption rector, but is transported to another reactor during the CO2desorption process due to the difference in sorption and desorption temperatures, thereby there are overall efficiencies in this process.

The apparatus can utilize a supported amine composition used for separating a contaminate, e.g., carbon dioxide, from an effluent. The supported amine composition is a non-aqueous sorbent or solid sorbent, which has several advantages, e.g., it requires less energy than an aqueous amine for separation of the same amount of carbon dioxide, is easy to handle and transport and is less problematic than an aqueous amine solution because it is less corrosive.

The supported amine which may used in the invention includes a substrate and a monoethanolamine having a mass ratio to the substrate (monoethanolamine:substrate) in a range from about 40:60 to about 60:40. The substrate is a supporting material for MEA, preferably, one that is inexpensive and widely available. In a preferred embodiment, the substrate has a low density in a range from about 0.3 g/cm3to about 1 g/cm3and is macroporous with an average pore size in a range from about 10 nm to about 1,000 nm, and more preferably with an average pore size in a range of about 300 nm to about 800 nm, and most preferably with average pore size in a range of about 50 nm to about 200 nm. The substrate used in the invention includes at least one of Ti, Al, Si, Fe, and combinations thereof. In a preferred embodiment, the substrate includes a macroporous material of TiO2, Al2O3, SiO2, FeOOH, TiO(OH)2and combinations thereof. In a preferred embodiment, the MEA has a purity in a range from about 70% to about 100%.

The invention also relates to a bi-directional reactor apparatus. The bi-directional reactor includes an input, an output, and a first reactor having a first reaction chamber in communication with the first reactor. The first input is coupled to the first reactor chamber. The bi-directional reactor further includes a second reactor having a second reaction chamber in communication with the first reactor. The second reactor is coupled to a second input and the first output. The first reactor is coupled to the second output. The first and second reactors include heating and cooling mechanisms as known in the art. The reactor is configured to operate bi-directionally. The first and second reactors are a fixed bed reactors as known in the art.

In operation, the bi-directional reactor can be run in a plurality of different operating modes: absorption (direction 1), desorption, absorption (direction 2), desorption, absorption (direction 1) and combinations thereof. Direction 1 is an opposite direction then direction 2.

The invention is also directed towards a method of sequestering carbon dioxide with a bi-directional apparatus. The method includes providing flue gas from a coal fired plant to the bi-directional reactor. The bi-directional reactor includes a first reactor and a second reactor and the flue gas is provided to the first reactor. Next, the method includes absorbing at least a portion of the carbon dioxide on a substrate and monoethanolamine having a mass ratio to the substrate (monoethanolamine:substrate) in a range from about 40:60 to about 60:40 in the first reactor.

FIGS. 1A and 1Bshow an apparatus according to an embodiment of the invention.

Referring toFIG. 1A, the apparatus is generally depicted as reference number100. The apparatus100is a bi-directional reactor apparatus configured to operate in either direction. The apparatus100is also configured to recycle a supported amine as described herein. The apparatus100includes an input102coupled to a first reactor chamber103and an output104coupled to a second reactor chamber105. Moreover, an input106is coupled to the second reactor chamber105and an output104is coupled to the first reactor chamber103. The heat source110and cooling source112are configured to heat and cool the first reactor chamber103. A heat source114and cooling source116are configured to heat and cool a second reactor chamber105. The first reactor chamber103may include a fixed bed reactor as known in the art. The second reactor chamber105may include a fixed bed reactor as known in the art. That is, the fixed bed reactor is configured to receive a sorbent bed to absorb/desorb desired constituents.

Referring toFIG. 1B, the reactor chamber103includes a lower support plate or screen120and an upper support plate or screen122. A supported amine124is placed between the upper plate122and lower plate120. In the reaction chamber sorption and desorption occurs. The second reactor chamber105is similar to the first reactor chamber103.

In operation, the reactor100can be run in a plurality of different operating modes, absorption (direction 1), desorption, absorption (direction 2), desorption, absorption (direction 1) and combinations thereof. This bi-direction operation allows for reclaiming, recycling or regenerating supported sorbent material, thereby reducing the overall cost of operation and increasing overall throughput. The terms reclaiming, recycling or regenerating are used interchangeably. Each mode of operation will now be described.

In absorption (direction 1), an effluent gas stream from an inlet102is directed towards to the reactor chamber103. The effluent may include any contaminated source that is configured to be sequestered with the sorbent bed, e.g., flue gas from a coal fired plant containing carbon dioxide. The reactor chambers103and105are configured as a fixed bed reactor as know in the art. In this embodiment, reactor chamber103includes an amine supported sorbent bed124, e.g., MEA-TiO2, and the supported sorbent bed and reactor chamber105includes a substrate, e.g., a macroporous material having an average pore size in a range from about 10 nm to about 1,000 nm. In a preferred embodiment, the substrate is TiO2. Absorption of carbon dioxide occurs in the reactor chamber103until the supported sorbent is saturated.

Next desorption of carbon dioxide is performed. This can be conducted by heating the reactor chamber103with the heat source110to release carbon dioxide and MEA vapor. The desorption temperature may be controlled with a temperature controller as known in the art. The MEA vapor is condensed and reclaimed in the second reactor chamber105. The condenser or cooler116is configured to a temperature to allow the MEA vapor to be condensed on the macroporous substrate, which is preloaded as the sorbent bed. The sorbent bed now includes a supported amine. The sorbent bed124is also now depleted of MEA, therefore, the sorbent bed124includes a substrate without MEA. Of course, some residual MEA may remain in the sorbent bed124. Carbon dioxide is released via outlet104.

Next, absorption (direction 2) is conducted. Direction 2 is an opposite direction of direction 1. That is, in direction 2 effluent gas stream from an inlet106is directed towards to the reactor chamber105. The effluent may include any contaminated source that is configured to be sequestered with the sorbent bed, e.g., flue gas from a coal fired plant containing carbon dioxide. The reactor chamber105includes an amine supported sorbent bed, e.g., MEA-TiO2, and supported sorbent bed and reactor chamber103includes a substrate, e.g., macroporous material having an average pore size in a range from about 10 nm to about 1,000 nm. Absorption of carbon dioxide occurs in the reactor chamber105until the supported sorbent is saturated.

Next desorption of carbon dioxide in reactor chamber105is performed. This can be conducted by heating the reactor chamber105with the heat source114to release carbon dioxide and MEA vapor. The desorption temperature may be controlled with a temperature controller as known in the art. The MEA vapor is condensed and reclaimed in the first reactor chamber103. The condenser or cooler112is configured to a temperature to allow the MEA vapor to be condensed on the macroporous substrate, which is preloaded as the sorbent bed. The sorbent bed124now includes a supported amine. The sorbent bed in reactor chamber105is also now depleted of MEA, therefore, the sorbent bed includes a substrate without MEA. Of course, some residual MEA may remain in the sorbent bed124. Carbon dioxide is released via outlet104. Now the process cycle can be repeated. That is, absorption (direction 1) can be conducted.

This process cycle is very beneficial as it can be easily realized by using an automatic switch to change the flow direction of flue gas for cyclic CO2sorption and desorption, and it can use low-temperature waste heat to provide the energy needed for CO2desorption.

Without intending to limit the scope of the invention, the following examples and embodiments illustrate how various aspects of the invention may be made and/or used.

TiO2Preparation and Characterization

In this example, a titanium oxide (TiO2) support material was prepared with Ti(OC2H5)4(99 wt %, Acros) containing 33-35 wt % TiO2. The first preparation step was to add a predetermined amount of Ti(OC2H5)4to water to obtain a H2O:Ti(OC2H5)4molar ratio of 26.3. Next, this mixture was stirred continuously for 1 hour. The resulting precipitate was filtered, washed with deionized water, and then dried at 393 K for 1.5 hours. The TiO2support material was obtained by calcining the resultant TiO(OH)2in air at 1,023 K for 3 hours.

The TiO2support material was characterized with a Micromeritics TriStar 3000 V6.04 A nitrogen physisorption analyzer in order to determine surface areas with the Brunauer, Emmett, and Teller (BET) method. The support structure had a BET surface area of 181.6 m2/g, an average pore size of 74.4 nm, and a volume of 3.3 cm3/g. Powder x-ray diffraction (XRD) of the TiO2support structure was also performed with a Philips X′Pert diffractometer using Cu—Kα radiation under the following operating conditions: voltage, 40 kV; current, 40 mA; start angle, 10°; end angle, 90°; step size, 0.01°; time per step, 0.05 s; and scan speed, 0.02. The experimental data was digitally collected and recorded.

FIG. 3illustrates the X-ray diffraction pattern of the prepared TiO2 support structure from the scan. The BET surface area, pore average size and volume of the sorbent support material, TiO2, are 5.68 m2/g, 66.4 nm and 0.11 cm3/g, respectively and TiO(OH)2corresponds to 123.2 m2/g, 4 nm and 0.08 cm3/g. Three major diffraction peaks appear at 2θ values of 27.5°, 36.2°, and 54.4°, corresponding to diffraction from the (110), (101), and (211) crystal planes, respectively, which is consistent with TiO2in the rutile phase. Wilska, “An X-ray diffraction study to determine the effect of the method of preparation upon the crystal structure of TiO2,” Acta Chemica Scandinavica,1954, 8, (10), pp. 1796-1801; Cheng, et al., “Optimizing preparation of the TiO2thin film reactor using the Taguchi method,”Materials&Design,2010, 31, (4), pp. 1749-1751, each of which is hereby incorporated by reference as if fully set forth herein.

Preparation of Supported Monoethanolamine

In this example, five supported monoethanolamine structures were prepared. Each MEA-TiO2(MT) structure or sorbent was prepared by loading a certain amount of as-purchased MEA (99 wt %, Acros) onto the prepared TiO2from example 1. Five MEA:TiO2mass ratios were used at 0.1, 0.25, 0.35, 0.4 and 0.45 or MEA loadings were used for preparing the MT sorbents tested for this research. The best loading of (40%) was determined and used for all subsequent tests. In addition, other materials and procedures may be utilized to prepare supported amines as described with reference to Fan, et al., “Supported Monothanolamine for CO2Separation,”Ind. Eng. Chem. Res.,2011, 50, pp. 11343-11349, which is hereby incorporated by reference as if fully set forth herein.

In this example, an experimental set-up was fabricated and used for the CO2separation or sequestration.FIG. 2shows a schematic diagram of the experimental set-up. Referring now toFIG. 2, the apparatus is generally depicted as reference number200. The apparatus200generally includes three parts: a gas preparation unit, a CO2sorption/desorption system, and gas-phase CO2concentration analysis equipment. Dilute CO2from cylinder202(1 mol % CO2in 99 mol % N2) was used for the sorption tests. N2from cylinder204(100 mol %) was used for CO2desorption tests and cleaning the apparatus. The flow rates of the inlet gases were controlled by two flow meters203and207(Matheson Tri-gas FM-1050). An additional flow meter209was used to measure the flow rate of the whole system.

Sorption tests were performed in the bottom reactor220, which has an inner diameter and length of 9 mm and 610 mm, respectively. The sorbent bed216was prepared by loading the sorbent prepared in example 2 between two bed holders214made from quartz wool. The bottom reactor was held in a tube furnace218, Thermo Corporation, TF55030A-1, where its temperature was controlled with a controller212, Yokogawa M&C Corporation, UT150. A syringe pump206was used to generate the water vapor used in moisture-containing gas streams. Temperature controller210, MiniTrol, Glas-Col Inc., thermo-tapes208heated the inlet gas tubes to prevent condensation of water vapor prior to entering the bottom reactor.

In operation, effluent gas stream from the bottom reactor220passed through a sorbent bed224. The sorbent bed224included a support material for MT sorbent, which was generally TiO2in the top reactor222. The top reactor222was configured with a condenser226including a spiral copper pipe and configured to condense the MEA vaporized from the bottom reactor220using cooling water circulating through a spiral copper pipe. The copper pipe had an inner diameter of 1.5 mm and was held at 12° C. by a small refrigeration unit228, MGW Lauda, RC-20 controller. The effluent gas from the top reactor224entered a water removal unit230and then an infrared gas analyzer232, ZRE, Fuji Electric System Co. Ltd. The sorption profiles were collected by a data collection computer234.

Each CO2desorption test was started immediately after the bed was saturated with CO2, as determined when the outlet CO2concentration during a sorption step became equal to the inlet CO2concentration. During a desorption step, pure N2from cylinder204was used as the carrier gas to bring the desorbed CO2from the bottom reactor220through the top reactor220and finally to the gas analyzer232. MEA vapor resulting from the CO2desorption in the bottom reactor220also flowed into the top reactor222and condensed there. Desorption temperatures were controlled by the bottom temperature controller212. When CO2desorption was completed, the material in the bottom reactor220was pure TiO2because all MEA was transported to the top reactor222and formed MT sorbent with the TiO2there due to the condensation of the MEA vapor from the bottom reactor220on the surface of pure TiO2originally in the top reactor222. Then the positions of the top and bottom reactors were switched to start the next sorption-desorption cycle.

FIG. 4shows a graphical representation of MEA on sorption capacity of MT sorbent according to an example of the invention. Referring toFIG. 4, the effect of MEA loadings on sorption capacity of MT sorbent (CO2: 1.0 vol %; N2: 99.0 vol %; gas flow rate: 0.3 L/min; sorption temperature: 45° C.) is shown. As shown, the CO2sorption capacity of the MT sorbent increases with increases in MEA loading and reaches 48.1 mg-CO2/g-MT when the MEA loading percentage is 45 wt %. The increasing trend inFIG. 4appears to result from the reaction between CO2and pure MEA instead of an aqueous amine solution and can be understood through the following equations, E1 and E2. Park, et al., “Sorption of carbon dioxide onto sodium carbonate,”Separation Science and Technology,2006, 41, (12), pp. 2665-2684, which is hereby incorporated by reference as if fully set forth herein.

βMEA=WMEA,0-wMEA,tWMEA,0(E3)
where wMEA,tis the quantity [g] of MEA consumed at reaction time [t].

According to the zwitterion mechanism for the reaction between MEA and CO2, both nCO2and mMEAin E1 and E2 should be 1. Aboudheir, et al., “Kinetics of the reactive absorption of carbon dioxide in high CO2-loaded, concentrated aqueous monoethanolamine solutions,”Chemical Engineering Science2003, 58, (23-24), pp. 5195-5210, which is hereby incorporated by reference as if fully set forth herein. Then, combining the integrated forms of E1 and E2 leads to equation 4 below. Park, et al., “Sorption of carbon dioxide onto sodium carbonate,”Separation Science and Technology,2006, 41, (12), 2665-2684, which is hereby incorporated by reference as if fully set forth herein. Equation 4 is as follows:
CCO2=CCO2,0exp{[1−exp(kWMEA,0(1−exp(−kdt))/Qg)]exp(−kdt)}/[1−exp(−kdt)]  (E4)
Equation E4 clearly shows that higher initial loading of MEA on TiO2results in lower outlet CO2concentration (CCO2) and thus higher CO2sorption capacity of MT. However, CCO2is also affected by other parameters, such as k and kdin E1, E2 and E4. The values of k and kd are determined by various factors including the surface area, particle size, and pore structure of TiO2, and the distribution of MEA on the TiO2. Therefore, it is believed that the characteristics of TiO2affect its CO2sorption profiles.

MEA is well-known for its reactivity with CO2, which was also observed. Typically, the MT sorbent could achieve one half of its total capacity within 10 minutes under any test conditions used. However, much longer periods of time were needed to attain the full capacity of an MT sample. The average CO2adsorption rate of the supported sorbent in the first 5 minutes was about 8 mg-CO2/g-MT/min, indicating that CO2is readily able to react with MEA on the surface of the sorbent. However, MEA molecules far away from the surface of the MT sorbent (close to the surface of the support TiO2particles) or condensed in the TiO2pores are not easily accessible to CO2due to diffusion limitations. This appears to explain why the CO2sorption capacity did not improve much when MEA loading on the MT sorbent increased from 40 to 45 wt %, as observed inFIG. 4. That is,FIG. 4clearly illustrates the effect of MEA loadings on sorption capacity of MT sorbent (CO2: 1.0 vol %; N2: 99.0 vol %; gas flow rate: 0.3 L/min; sorption temperature: 45° C.). Accordingly, 40 wt % MEA loading was used in other examples to evaluate the effect of other factors on CO2sorption.

Moisture

The MT sorbent was developed to overcome the shortcomings of conventional aqueous MEA-based CO2separation technologies by eliminating the use of water while maintaining its advantage of strong CO2absorption. However, the effect of water on the CO2sorption of MT has to be considered since flue gas from all combustion processes, including coal-fired power plants, contain water despite the MT sorbent being made without water. Therefore, a gas containing 0 vol % H2O, 1.0 vol % CO2and 99 vol % N2and another gas with 1.0 vol % CO2and 99.0 vol % N2were compared for their CO2sorption profiles. The results are shown inFIG. 5.

FIG. 5shows a graphical representation of effect of moisture and CO2concentrations according to an example of the invention [A (H2O: 0 vol %; MT: 40 wt % MEA loading; CO2: 1.0 vol %; N2: 99 vol %; gas flow rate: 0.3 L/min; sorption temperature: 45° C.), B (H2O: 1.0 vol %; MT: 40 wt % MEA loading; CO2: 1.0 vol %; N2: 98.0 vol %; gas flow rate: 0.3 L/min; sorption temperature: 45° C.)]. Referring toFIG. 5, it is shown that moisture has a positive effect on CO2sorption, especially in the initial CO2sorption period in which CO2outlet concentration is lower than 0.1 vol % (curve B). The performance of MT in this time period is important since it determines the breakthrough capacity of the sorbent. The CO2sorption mechanisms with and without the presence of water are expected to be different. Within a humid environment, the associated MEA-CO2reaction mechanism is based on the zwitterions theory proposed by Danckwerts and developed by others. Danckwerts, “Reaction of CO2with ethanolamines,”Chemical Engineering Science,1979, 34, (4), pp. 443-446; Ramachandran, et al., “Kinetics of the absorption of CO2into mixed aqueous loaded solutions of monoethanolamine and methyldiethanolamine,”Industrial&Engineering Chemistry Research,2006, 45, (8), pp. 2608-2616; Aboudheir, et al., “Kinetics of the reactive absorption of carbon dioxide in high CO2-loaded, concentrated aqueous monoethanolamine solutions,”Chemical Engineering Science,2003, 58, 23-24, pp. 5195-5210; Han, et al., “Understanding CO2Capture Mechanisms in Aqueous Monoethanolamine via First Principles Simulations,”Phys. Chem. Lett.,2011, 2, pp. 522-526, each of which is hereby incorporated by reference as if fully set forth herein.

The MEA-CO2reaction mechanism can be written in chemical reactions shown in R1 to R14 as follows:

where ki, k−i, and Kiare the forward reaction rate constant, the reverse reaction rate constant, and the equilibrium constant of the reversible reactions, i, respectively. The reaction rate of CO2can be expressed as follows:

rCO2-MEA=[CO2]⁡[R⁢NH2]-k-4k4⁡[R⁢NHCOO-]⁢∑k-b⁡[B⁢H+]∑kb⁡[B]1k4+k-4k4⁢∑kb⁡[B](E5)
where B represents the species which can abstract the proton from the zwitterion, including [H2O], and kband k−bare the forward and reverse reaction rate constants of the reverse reactions involving B. Ramachandran, “Kinetics of the absorption of CO2into mixed aqueous loaded solutions of monoethanolamine and methyldiethanolamine,”Industrial&Engineering Chemistry Research,2006, 45, (8), pp. 2608-2616; Aboudheir, et al., “Kinetics of the reactive absorption of carbon dioxide in high CO2-loaded, concentrated aqueous monoethanolamine solutions,Chemical Engineering Science,2003, 58, (23-24), pp. 5195-5210; Versteeg, et al., “On the kinetics between CO2and alkanolamines both in aqueous and non-aqueous solutions. An overview,”Chemical Engineering Communications,1996, 144, pp. 113-158, each of which is hereby incorporated by reference as if fully set forth herein. However, according to their experimental data and derivations, many researchers proposed that the zwitterion reaction scheme based on CO2sorption rate can be written as equation E6 as follows:
rCO2-MEA=k4[CO2][RNH2]  (E6)
where rCO2-MEAis not a function of water concentration. Versteeg, et al., “On the kinetics between CO2and alkanolamines both in aqueous and non-aqueous solutions. An overview,”Chemical Engineering Communications,1996, 144, pp. 113-158; Blauwhoff, et al., “A study on the reaction between CO2and alkanolamines in aqueous-solutions,”Chemical Engineering Science,1984, 39, (2), pp. 207-225, each of which is hereby incorporated by reference as if fully set forth herein. Ramachandran concluded that equation E5 is more representative than equation E6 for the kinetics of MEA based CO2sorption within a humid environment, although they demonstrated that equation E5 needs to be modified. Aboudheir, et al., “Kinetics of the reactive absorption of carbon dioxide in high CO2-loaded, concentrated aqueous monoethanolamine solutions,”Chemical Engineering Science,2003, 58, (23-24), pp. 5195-5210, which is hereby incorporated by reference as if fully setforth herein. The data inFIG. 5is in accordance with their finding. The results inFIG. 5also agree with the kinetic model of Crooks and Donnellan using a termolecular mechanism as follows:
rCO2-MEA=−{kRNH2[RNH2]+kH2O[H2O][RNH2][CO2]}  (E7)
in which kRNH2and kH2Oare the corresponding rate constants with respect to RNH2and H2O. Versteeg, “On the kinetics between CO2and alkanolamines both in aqueous and non-aqueous solutions. An overview,”Chemical Engineering Communications,1996, 144, pp. 113-158; Crooks, et al., “Kinetics and mechanism of the reaction between carbon-dioxide and amines in aqueous-solution,”Journal of the Chemical Society-Perkin Transactions,2, 1989, (4), pp. 331-333, each of which is hereby incorporated by reference as if fully set forth herein.

Furthermore, the degree to which water concentration affects rCO2-MEAmay need to be reconsidered. According to equation E7 the CO2-MEA reaction is first order with respect to both H2O and CO2. However, the data inFIG. 5does not support this conclusion since water did not show such a large positive effect. Actually, the effect decreases, disappears and finally becomes slightly negative as the sorption process proceeds. Therefore, the kinetics associated with the reactions in dry and wet environments may need further polishing.

Sorption Temperature

FIGS. 6A and 6Bshow a graphical representation of the effect of temperature on CO2sorption profile (FIG. 6A) and capacity (FIG. 6B) according to an example of the invention. More specifically,FIGS. 6A and 6Billustrate the effect of temperature on the CO2sorption profile (A) and capacity (B) (MT: 40 wt-% MEA loading; CO2: 1.0 vol %; N2: 99.0 vol %; gas flow rate: 0.3 L/min; sorption temperature: 45° C.). Effects of sorption temperature on the total CO2sorption capacity of MT sorbents were evaluated in the temperature range of about 25° C. to about 65° C. Referring toFIGS. 6A-6B, it is shown that the CO2sorption capacity increases with temperature in the range of about 25° C. to about 45° C., but decreases with the further increases of temperature from about 45° C. to 65° C.

The relationship between temperature and CO2sorption capacity can be understood from the thermodynamic and kinetic characteristics of R13. R13 is an exothermic reaction or its enthalpy change (ΔHR13<0) is negative under the experimental conditions. Palmeri, et al., “Carbon dioxide absorption by MEA—A preliminary evaluation of a bubbling column reactor,”Journal of Thermal Analysis and Calorimetry,2008, 91, (1), pp. 87-91; Kim, et al., “Heat of absorption of carbon dioxide (CO2) in monoethanolamine (MEA) and 2-(Aminoethyl)ethanolamine (AEEA) solutions,”Industrial&Engineering Chemistry Research,2007, 46, (17), pp. 5803-5809, each of which is hereby incorporated by reference as if fully set forth herein.

Based on the van't Hoff relationship, temperature increases do not favor R13 since equilibrium CO2sorption capacity (determined by KR13and associated with KR4and KR5) decreases due to the negative ΔHR13.

ⅆln⁢⁢KR⁢⁢13ⅆT=⁢ⅆln⁢⁢KR⁢⁢4⁢KR⁢⁢5ⅆT=⁢Δ⁢⁢HR⁢⁢13RT2.(E8)
The van't Hoff relationship is described in Smith, et al., “Introduction to Chemical Engineering Thermodynamics,” 6 ed., McGraw Hill, 2001. Two methods can be used for calculation of KR13for MT-based CO2sorption in a dry environment at a given temperature, T. The first is based on the thermodynamic properties of MEA, CO2, [(RNH3)+(R NHCOO)−] in R13 using:

Δ⁢⁢GR⁢⁢13o=⁢-RT⁢⁢ln⁢⁢KR⁢⁢13=⁢Δ⁢⁢H0,R⁢⁢13o-TT0⁢(Δ⁢⁢H0,R⁢⁢13o-Δ⁢⁢G0,R⁢⁢13o)+⁢Δ⁢⁢CPo⁡(T-T0)-T⁢⁢Δ⁢⁢CPo⁢ln⁢⁢TT0(E9)
where T0is reference temperature, ΔH0oand ΔG0oare the standard enthalpy and free Gibbs energy changes of R13 at the reference temperature, and
ΔCPo=CP,[(RNH3)+(R NHCOO)—]o−2CP,RNH2o−CP,CO2o(E10)
where CP,[(RNH3)+(R NHCOO)—]o, CP,RNH2oand CP,CO2orepresent the heat capacities of the three reactants and products at constant pressure. The second method is to combine E8 with the following relationship

KR⁢⁢13=⁢KR⁢⁢4⁢KR⁢⁢5=⁢kR⁢⁢4k-R⁢⁢4⁢kR⁢⁢5k-R⁢⁢5(E11)
where kR4, k−R4, kR5and k−R5are the forward and reverse rate constants of reactions R4 and R5, respectively. The forward reaction rate constants, kR4and kR5, increase with T according to the Arrhenius equation while KR13in E8 and E11 decreases with T. Fogler, “Elements of Chemical Reaction Engineering,”4 ed., Prentice Hall, PTR: 2006, which is hereby incorporated by reference as if fully set forth herein. Therefore, an optimal CO2sorption temperature exists that is a compromise between these kinetic and thermodynamic factors to obtain a reasonably high rate of R13 and yet large CO2sorption. In other words, the optimal sorption temperature for the MT based CO2sorption technology is defined as that which maximizes the CO2sorption capacity within a given reaction time period. The optimal temperature at which the maximum total CO2adsorption capacity was achieved under the given experimental conditions is 45° C.
Desorption Temperature

CO2desorption tests were performed at 80° C., 90° C., 100° C. and 110° C. to evaluate the effect of temperature on CO2sorption capacity of the MT sorbent regenerated for next cycle of sorption and desorption. The results are shown inFIG. 7.

FIG. 7illustrates the effect of desorption temperature (MT: 40 wt % MEA loading; CO2: 1.0 vol %; N2: 99 vol %; gas flow rate: 0.3 L/min; sorption temperature: 45° C.). The intermediate temperatures, 90° C. and 100° C., are better based on the sorption capacities obtained in the next sorption-desorption cycle. However, due to the higher energy consumption at 100° C., 90° C. was chosen as the CO2desorption temperature for all other MT evaluation tests. Moreover, desorption kinetics of MT are further described with reference to Fan, et al., “Desorption Kinetics of Monoethanolamine/Macroporous TiO2-Based CO2Separation Process,” Energy Fuels, 2011, 25, pp. 2988-2996, which is hereby incorporated by reference as if fully set forth herein.

Alternative Support Materials for MEA

An alternative Ti based support material is TiO(OH)2, which can be easily prepared at low temperatures compared to TiO2. It is stable even at 400° C. Lim, et al., “Carbothermal Reduction of TiO(OH)(2) in the Synthesis of (Ti,W)C,”Journal of the American Ceramic Society,2010, 93, (4), pp. 937-940. Its performance as a support for MEA is better than TiO2to some degree during most of the sorption period, as shown in the CO2breakthrough curves inFIG. 8. This might be explained with the kinetic model obtained by Ramachandran. Aboudheir, et al., “Kinetics of the reactive absorption of carbon dioxide in high CO2-loaded, concentrated aqueous monoethanolamine solutions,”Chemical Engineering Science,2003, 58, (23-24), pp. 5195-5210, which is hereby incorporated by reference as if fully set forth herein.

They found that the OH−increases the reaction rate between MEA and CO2. Therefore, TiO(OH)2can probably accelerate CO2sorption to some degree due to the OH−in its structure. Among many other possible highly porous and inexpensive MEA support materials is FeOOH. FeOOH starts to dehydrate at 213° C. or 490 K. Diakonov, II, “Thermodynamic properties of iron oxides and hydroxides. III. Surface and bulk thermodynamic properties of lepidocrocite (γ-FeOOH) to 500 K,”European Journal of Mineralogy,1998, 10, (1), pp. 31-41, which is hereby incorporated by reference as if fully set forth herein. Therefore, it is thermally stable under the operation conditions used in this research. It also has OH−in its structure and is less expensive than TiO2and TiO(OH)2.

FIG. 8shows a comparison of different support materials (A: TiO2; B: TiO(OH)2; C: FeOOH) for their effects on CO2sorption (MEA loading in each sorbent: 40 wt %; CO2: 1.0 vol %; N2: 99.0 vol %; gas flow rate: 0.3 L/min; sorption temperature: 45° C.). The sorption results with the pure MEA supported with FeOOH is also shown inFIG. 8. FeOOH is better than TiO2, but not as good as TiO(OH)2. When choosing support materials for MEA, other factors such as acidic components in the flue gas, cost and availability should also be considered. For example, acidic compounds in the flue gas, SOxand NOx, may affect the life spans of the support materials due to their potential reactions with the acidic compounds. Ti based compounds are better than FeOOH from the perspective of their corrosion-resistance abilities.

Industrial chemisorbents are required not only to be highly active and selective, but also regenerable. Therefore, five-cycle CO2sorption-desorption tests with MT sorbents were run under conditions with and without moisture. The results are presented inFIGS. 9A and 9B, disclosing CO2sorption capacities of MT during five sorption-desorption cycles [FIG. 9A(sorption gas: CO2: 1.0 vol %; N2: 99 vol %),FIG. 9B(sorption gas: H2O: 1.0 vol %; CO2: 1.0 vol %; N2: 98 vol %), sorption (MT: 40 wt % MEA loading; gas flow rate: 0.3 L/min; sorption temperature: 45° C.), desorption (N2: 100 vol %; gas flow rate: 0.3 L/min; sorption temperature: 90° C.)]. The average adsorption capacities for five-cycle tests at 45° C. under dry and humid (1 vol % H2O) sorption conditions are 45.8 and 48.1 mg-CO2/g-MT, respectively, indicating that MT can be used in both dry and wet environments for effective CO2separation.

The capacities of MT under the two different environments are higher than that of aqueous MEA, which can absorb 36 mg-CO2/g-aqueous-MEA. Chakma, “Methyldiethanolamine degradation—Mechanism and kinetics,”Canadian Journal of Chemical Engineering,1997, 75, (5), pp. 861-871, which is hereby incorporated by reference. In addition, they are also higher than the CO2sorption capacities of 21 sorbents among 24 evaluated by Sjostrom and Krutka in 2010. Khatri, et al., “Carbon dioxide capture by diamine-grafted SBA-15: A combined Fourier transform infrared and mass spectrometry study,”Industrial&Engineering Chemistry Research,2005, 44, (10), pp. 3702-3708; Sjostrom, et al., “Evaluation of solid sorbents as a retrofit technology for CO2 capture,”Fuel,2010, 89, (6), pp. 1298-1306, each of which is hereby incorporated by reference as if fully set forth herein. Most of those 24 sorbents tested by Sjostrom, et al., contain 40-50 wt % amines, which is equal to or higher than the MEA percentage (40 wt %) of the MT sorbent used in this research.

The regeneration temperatures of those sorbents varied from 80° C. to 120° C. and increased by 10° C. with each subsequent sorption-desorption cycle compared to the constant 90° C. used for the spent MT regeneration. The quantities of CO2immobilized on MT during the sorption period and CO2desorbed from spent MT during the desorption process, determined by integrating CO2concentration change profiles in each sorption-desorption cycle, are very close. In other words, the working capacity, as defined by Sjostrom and Krutka, is almost equal to the sorption capacity for the MT sorbent. Sjostrom, et al., “Evaluation of solid sorbents as a retrofit technology for CO2capture,”Fuel,2010, 89, (6), pp. 1298-1306, which is hereby incorporated by reference as if fully set forth herein. This is the reason that the CO2sorption capacities do not fluctuate considerably from one sorption-desorption cycle to another, as shown inFIGS. 9A and 9B.

The amount of energy needed for regeneration of a spent sorbent is an important consideration in its applicability, and can be evaluated by the following equation (E11):

Qmc=memc·Ce·Δ⁢⁢T+BL·Cs·Δ⁢⁢T+Cp·c·T2-Cs·T1+Qrmc
in this equation 1 and 2 stand for the CO2sorption and regeneration states, respectively; the subscripts, e, s, and c respectively represent the equipment, the sorbent, and the CO2; m is the mass, C is the specific heat [kJ/(kg·K)]; Cpis the constant pressure heat capacity for CO2; Q is the heat input [kJ]; Qris the heat of reaction [kJ/kg]; B is a constant of proportionality with dimensional units, and L is the CO2loading capacity, defined as mole-CO2/kg sorbent. Sjostrom, et al., “Evaluation of solid sorbents as a retrofit technology for CO2 capture,”Fuel,2010, 89, (6), pp. 1298-1306; Hoffman, et al., “Factors reactor for dioxide with solid, regenerable sorbents,”Proceedings of the International Technical Conference on Coal Utilization&Fuel Systems,2008; Vol. 2, pp 1139-1150, each of which is hereby incorporated by reference as if fully set forth herein. To reduce energy consumption needed for MT sorbent regeneration, more effort needs to be made to increase L, which can be realized by exploring better support materials and optimizing CO2sorption conditions.

The MT sorbent can be prepared using a simple method in an environmentally benign manner since no additional chemicals, such as organic solvents, are needed. The equipment requirements for separation of CO2with the MT based technology should not be as demanding as those associated with the majority of other CO2separation technologies since the spent sorbent regeneration temperature is 90° C., lower or much lower than those needed for other technologies. Sjostrom, “Evaluation of solid sorbents as a retrofit technology for CO2capture,”Fuel,2010, 89, (6), pp. 1298-1306, which is hereby incorporated by reference as if fully set forth herein. Moreover, this system allows for no external addition of water to the sorption system. Therefore, the capital equipment investment needed for the MT based CO2separation technology should be low.

Operational costs account for the majority of the overall CO2separation costs in all CO2capture technologies, with CO2desorption typically being the most expensive step. Avoidance of use of water and the reduction of the CO2desorption temperature is thought to contribute significantly to the total cost reduction of CO2separation.

The mechanism of the positive effect of OH— on CO2 sorption capacity needs to be further understood. In addition, studies on the thermodynamics and kinetics of R13 are still lacking, even though those of R14 are well-researched and many progresses have been made. R13 and R14 have different reactants and products. Therefore, the thermodynamic and kinetic study results reported in the literature for R14 can not be used for R13. Moreover, even for R14, some disagreements exist among the published papers regarding its thermodynamic and kinetic properties under the same CO2sorption conditions. For example, the enthalpy change of R14 during CO2sorption at 320 K is reported by Palmeri, et al., as ˜57 kJ/mole-CO2, while Mathonat, et al., report the value as ˜80 kJ/mole-CO2.

The inventions and methods described herein can be viewed as a whole, or as a number of separate inventions that can be used independently or mixed and matched as desired. All inventions, steps, processes, devices, and methods described herein can be mixed and matched as desired. All previously described features, functions, or inventions described herein or by reference may be mixed and matched as desired.