Investigation of CO2 Regeneration in Single and ... - ACS Publications

Subscriber access provided by TUFTS UNIV

Article

Investigation of CO2 regeneration in single and blended amine solvents with and without catalyst Helei Liu, Xin Zhang, Hongxia Gao, Zhiwu Liang, Raphael O. Idem, and Paitoon Tontiwachwuthikul Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00778 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Investigation of CO2 regeneration in single and blended amine solvents with and without catalyst Helei Liu, Xin Zhang, Hongxia Gao, Zhiwu Liang*, Raphael Idem* and Paitoon Tontiwachwuthikul Joint International Center for CO2 Capture and storage (iCCS), Provincial Hunan Key Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing Carbon-dioxide Emissions, Department of Chemical Engineering, College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, P.R. China

* Corresponding authors: Tel.: +86-13618481627; fax: +86-731-88573033; E-mail address: [email protected](Z. Liang). Tel.: +1-306-585-4770; Fax: +1-306-585-4855; Email address: [email protected](R. Idem).

1

ACS Paragon Plus Environment


TOC graphic

0.52

CO2 equilibrium Loading(mol CO2/mol amine)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 33

Blank 25g H-ZSM-5 25g MCM-41 225g SO4 /ZrO2

0.50 0.48 0.46 0.44 0.42 0.40 0.38 0.36 0

100

200

300

400

500

600

Time(min)

2


Page 3 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Abstract Various single and blended amines (namely, MEA(monoethanolamine), MEA-DEEA

(2-Diethylamino

ethanol),

MEA-MDEA(methyldiethanolamine),

MEA-1DMA2P (1-Dimethylamino-2-propanol)) with three types of catalysts (H-ZSM-5, MCM-41 and SO42-/ZrO2) were studied to determine the respective roles of catalyst and solvent in heat duty and CO2 desorption rate with an initial CO2 loading of 0.5 mol CO2/mol amine at 371 K. The results show that performance of the three catalysts in all the four investigated aqueous solution systems followed the trend: H-ZSM-5 > MCM-41 > SO42-/ZrO2. These results highlight the fact that even though HZSM-5 has moderate acidic sites as compared to MCM-41 and SO42-/ZrO2, its large B/L acid sites ratio coupled with mesopore surface area had the best performance. Furthermore, based on this study, the blended system of aqueous 5M MEA+1M MDEA with H-ZSM-5 provided the best approach for solution regeneration because the strong electron withdrawing chemical structure of MDEA.

Keywords：：Tertiary amine structure, single and blended amines, solvent regeneration, solid acid catalyst, Brϕnsted and Lewis acid sites, mesopore surface area, heat duty

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 33

1. Introduction The increasing emissions of carbon dioxide (CO2), the most abundant greenhouse gas (GHG) produced through human activities, is blamed for the increasing global warming and climate change issues. CO2 is produced mainly due to the intensive use of fossil fuels for power generation and other industrial activities.1 CO2 capture and storage (CCS) is considered to be one of the most efficient ways as well as a major option to control CO2 emissions, and as such, global warming and climate change. As is well known, post combustion CO2 capture technology is mostly based on the use of aqueous amine solution to absorb CO2 from its mixture with other components such as from flue gases of power plants. The gas-liquid reaction mechanism has been researched in 1960 by Bird R et al.2 This process has its drawbacks in terms of high heat duty for solvent regeneration. A lot of work has been reported in the literature which attempts to solve this problem. For example, Feng et al.3 studied the reduction of the energy requirement for CO2 desorption by adding an acid to a CO2-loaded solvent. Eisaman et al.4 demonstrated that the CO2 desorption energy requirement from aqueous bicarbonate solutions under high pressure operation can be reduced by 29% as compared with ambient-pressure operation. Zhang et al.5 reported that an intensified TBS (Thermomorphic biphasic solvent) process with agitation, nucleation and ultrasonic methods offers significant advantages for improving the capture efficiency and reducing process costs. In 1876, Kim et al.6 had researched the catalyst performance in carbon capture process. Recently, Idem et al.7, showed that adding solid acid catalysts to the amine in 4


Page 5 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the regeneration process is an effective method to reduce solvent regeneration heat duty. A variety of promising Brϕnsted acid catalysts and Lewis acid catalysts were listed in their work, together with many experiments based on MEA solution to compare

the

performance

of

different

catalysts.

Among

these

catalysts,

H-ZSM-5(which is a framework type aluminosilicate zeolite) was shown to be the best with 5M MEA solution in reducing the heat duty for solvent regeneration. As a predominantly proton donor(i.e. Brϕnsted acid) catalyst8, H-ZSM-5 facilitatesCO2 desorption by breaking down the carbamate and reducing the activation enthalpy for proton transfer. MCM-41(one type of Lewis acid catalyst) is widely used in the CO2adsorption process due to its high specific surface area9. Thus, the study of MCM-41 for use in the CO2 desorption process is necessary since desorption is the reverse of adsorption. In addition, Idem et al.7 reported that MCM-41,being one type of Lewis acid catalyst, could be applied to solvent regeneration as an electron acceptor. It is seen that proton transfer plays an important role in theCO2 desorption process. Therefore, any suitable Brϕnsted acid catalysts(which is a proton donor) and Lewis acid catalysts(which is an electron acceptor) can be used to reduce solvent regeneration heat duty. In order to explore this concept in the CO2desorption process, catalysts with differing amounts of proton donor acidity and electron acceptor acidity need to be tested. One type of super solid acid catalyst- sulfated zirconia(SO42-/ZrO2) with a unique acid catalyst activity, was also used in this work. A super solid acid such as SO42-/ZrO2 is a type of acid whose acidity is stronger than 100% sulfuric acid which has been used as a heterogeneous catalyst in many chemical reactions10. It has 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 33

been shown to be an excellent material for catalyzing reactions that need strong acid sites11,

12

. Thus, in this work, three types of catalysts with contrasting catalyst

characteristics were employed in order to obtain possible relationships between their characteristics and performance in CO2 desorption from CO2-loaded amine solutions. It is known that another method to reduce the energy consumption in CO2 desorption is the use of blended amines. Sakwattanapong et al.13 reported that a tertiary amine (e.g. MDEA) blended with MEA can reduce the energy requirements as well as improve the capture efficiency. Zhang et al.5 used blended amine N,N-Dimethylcyanoacetate and (Hexylamine, Cycloheptylamine or Dipropylamine)to improve the CO2desorption performance. As is well known, MEA is a primary alkanolamine and it has fast kinetics for CO2 absorption. Besides MDEA, there is another tertiary amine, namely, DEEA, which is a promising candidate for use inbulk CO2 removal as it can be prepared from renewable and cheap resources (i.e. ag ricultural products and residues)14. Also, 1DMA2Pis another promising amine which exhibits faster kinetics in capturing CO2,a nd larger CO2 absorption capacity as compared with MEA15. Thus, the two potential tertiary amines blended with MEA can bring considerable savings in the regeneration energy consumption as well as improving CO2desorption rate. Consequently, these blended amines with catalyst need to be tested to demonstrate their potential as promising alternative solvents and catalyst to reduce the total cost in the CO2 adsorption/desorption process. In this work, four different solvent systems (5M MEA, 5M MEA and 1M MDEA,5M MEA and 1M DEEA, and 5M MEA and 1M 1DMA2P) with and without 6


Page 7 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


catalysts were tested. First ， the four types of solution without catalyst were experimented to compare their CO2cyclic capacity, rate of CO2 desorption and heat duty starting from the same initial loading of 0.5 molCO2/mol amine over different desorption temperatures in the range from 90 to 105 oC. Finally, CO2 desorption study was performed by testing three different catalysts(i.e. H-ZSM-5,MCM-41and SO42-/ZrO2) separately to the 4 solvent systems. The aim of this work was to find the best catalyst and solvent system combination, which can produce the necessary synergistic effect to reduce the energy consumption for CO2 desorption as compared with existing catalysts and solvent combinations. 2.Experimental Section 2.1 Chemicals and materials MEA of analytical grade (of purity>99.9%) was purchased from Hengxing Chemical Preparation Co. Ltd., China. Reagent grade DEEA of purity of 99% was purchased from Tianjin Xiya Chemical Reagent Co. Ltd., China. Reagent grade MDEA was purchased from Tianjin Kermel Chemical Reagent Co. Ltd., China with purity of ≥99%. 1DMA2Pwas purchased from Shanghai Macklin Chemical Reagent Co. Ltd., China with purity of 99%. Commercial grade Carbon dioxide (CO2) was supplied by Changsha Rizhen Gas Co. LTD., China with a purity of ≥ 99.9%. H-ZSM-5 and MCM-41 molecular sieve catalysts were purchased from Tianjin Nankai University Catalyst Co. Ltd., China. SO42-/ZrO2 was synthesized according to the method reported recently by Cristian et al.16 The super solid acid catalyst was prepared by mixing fresh zirconia and 1 mol/L 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 33

H2SO4 solution. The fresh zirconia, support of the catalyst, was prepared by precipitation of zirconium oxychloride (ZrOCl2·8H2O, Macklin 98%) hydrate solution (with mass ratio of ZrOCl2/H2O = 1:8) with ammonia (NH4OH, Mallin 28%) at pH of 9. After aging overnight at room temperature, the resulting precipitate was thoroughly washed with purified water until there was no Cl- left, as detected with 0.5 mol/L AgNO3, dried at 110℃ overnight, and ground into powder under 100 mesh. Then, the fresh zirconia was sulfated by impregnation with 20 mL of 1M H2SO4 per gram of zirconia for 12h, followed by drying at 110oCovernight and calcination in air at 600℃ for 3h to obtain a super solid acid catalyst. 2.2 Catalyst Characterization All

the

catalysts

were

characterized

by

specific

surface

area

(Brunauer–Emmett–Teller (BET) theory), NH3-temperatureprogrammed desorption (NH3-TPD), pyridine adsorption-Fourier-transform infrared (Py-IR). Additionally, SO42-/ZrO2 was characterized by powder X-ray diffraction (XRD) and Infrared Spectroscopy (IR). 2.2.1 N2 adsorption-desorption experiments Specific surface area measurements were used to study the surface area and pore structure of the catalysts. These were carried out on a Micromeritics TriStar II 3020 instrument, using the adsorption of N2 at the temperature of liquid nitrogen. Prior to measurement, each sample was degassed at 523 K for 16h and finally out gassed to 10-3 Torr. 2.2.2 TPD of NH3 8


Page 9 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NH3-temperature programmed desorption (NH3-TPD) measurements were used to obtain the strength and amount of acid sites in the solid acid catalysts with a Micromeritics Auto Chem II 2920 V3.05 instrument. Prior to the analysis, the catalyst samples were heated at a rate of 10℃ per minute from room temperature to 200℃while passing helium gas and kept at 200℃for 30 min. The samples were cooled to 60℃ in helium gas flow, and then NH3 gas was adsorbed on the catalyst for 60 min. NH3 adsorption was performed passing 5% NH3-He at 100oC for 2 h over the catalyst that was earlier pretreated at 400℃. Then, TPD of NH3 was performed by passing helium (60ml/min) over the NH3 adsorbed catalyst while heating the catalyst chamber at a programmed rate of 10℃/min, to 800℃ end. After then, the NH3-TPD signal was detected using an online thermal conductivity detector (TCD). 2.2.3 Pyridine adsorption – Fourier-transform infrared (Py-IR) Pyridine adsorption – Fourier-transform infrared (Py-IR) measurements of the catalyst samples were recorded with a Brucker Vector 22 spectrometer in the absorption mode with a resolution of 4 cm-1. A self-supporting wafer of each sample was made and loaded in an IR cell. The wafer was pretreated at 200oC under flowing oxygen for 30 min, and a background spectra were recorded after the sample was cooled down to 25oC. Adsorption of pyridine was then conducted until saturation. Py-IR spectra were recorded after degassing for 30 min h at 150oC.The inner ring deformation vibration absorption bands of pyridine molecules are 1580 cm-1 and 1572 cm-1. If pyridine is adsorbed to a Brϕnsted acid site (B), the characteristic peak appears at 1540 cm-1, and the peak that appears at 1450 cm-1 is for the adsorption of 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 33

pyridine to the Lewis acid site (L). Therefore, the absorption bands in the vicinity of 1540 cm-1 and 1450 cm-1 were used to characterize the B acid and L acid, respectively. The peak height of infrared spectrum is proportional to the amount of catalyst acid site. Then the acid site amounts of Brϕnsted acid, Lewis acid and total acid of catalyst were obtained through quantitative analysis. 2.2.4 Fourier transform infrared spectrometer (FT-IR) The unsulfated zirconia support and the sulfated catalysts were both characterized by Fourier transform infrared (FT-IR) spectrometer using the Bruker vector 22 FT-IR spectroscopy in the wavelength range of 800–4000 cm-1 in order to verify the effective sulfation of zirconia by the appearance of the band in the range of 1000–1250cm-1. This is the characteristic band of the bond of the sulfate groups and zirconium oxide. 2.2.5 X-Ray Diffraction (XRD) X-ray diffraction (XRD) was performed on a D8-Advance with a Bruker diffractometer using Cu Ka radiation (40 kV, 40 mA) in the range between 2° and 30°and Cu Kαradiation (40 kV, 40 mA) in the range between 10o and 80o. Diffraction data were recorded with a scanning speed of 0.5°of 2fs-1 and 2θangle of 0.02°at 25℃.

2.3 Experimental apparatus and procedure for solvent regeneration The device CO2 desorption is shown in Figure S6. About 2L of the amine solutions with the desired CO2 loading and amine concentration were poured into a 3L four-necked flask. The concentrations of both the single and blended amines were confirmed by titration using 1M standard hydrochloric acid (HCl) solution. Then, CO2 10


Page 11 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


was bubbled into the solution to obtain CO2-rich loading solvents. The initial CO2-rich loading for all experiments was controlled in the range of 0.495-0.505 mol CO2/mol amine. In order to obtain the desired CO2 loading, the simple of amine was taken ever certain minutes to measure and monitor the CO2 loading. Once the overloading appears, the fresh amine solution was added into loaded amine to obtained the amines solution with desired CO2 loading. Exactly 25g of the prepared catalyst giving an approximate weight ratio of 1/80 to the amine solution were placed in the flask as the batch reactor5. A mechanical agitator set at 400 rpm was placed in the middle hole of the flask to ensure the temperature of the liquid uniform. A thermometer was set up in one hole to measure the temperature of the liquid. The whole flask was immersed in a NC thermostat oil bath pot (DF-101T) to maintain the desired temperature of the reaction mixture. The energy consumption of the CO2 desorption was measured by an electric energy meter (digital display single-phase meter, Shandong Lichuang Science and technology Co. Ltd., China, accuracy of 0.001kW/h, and an uncertainly of +0.1kW/h). Catalytic effects of the different catalysts on CO2 desorption were evaluated by analyzing the CO2 loading of liquid samples taken at 0, 1, 1.5, 2, 4,6 and 9 h after the temperature of the solutions in the flask had reached 343K. The samples were collected in vials and were immediately put on an ice bath in order to maintain the CO2 loading of the samples. Then CO2 loading of each sample was obtained by titration17 performed at least two times to ensure repeatability and reliability of the data. 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 33

2.4 Calculations The CO2 loading, α (alpha), is calculated byequation (1) : =

(1)

The amount(n/mol) of desorbed CO2 was calculated from the CO2 liquid phase loading-α(mol CO2/mol amine) of samples as shown in equation (2). αrich represents the CO2 loading of the initial amine solution, and αlean represents the CO2 loading of the sample. C(mol/L) and V(L) are the concentration and volume of amine solutions,respectively. nCO2 = (α rich − α lean ) ⋅ C ⋅ V

( 2)

The energy consumption of the solvent regeneration process was calculated by equation (3). The regeneration energy is one of the important indicators recorded by an electric energy meter: heat input/time electricity（ MJ） = amount CO 2 / time n（ CO2） (mol) The rate of desorption could be calculated by using the following equation: Qreg =

Qreg =

heat input/time electricity（ MJ） = amount CO 2 / time n（ CO2）(mol)

(3)

(3)

3.Results and Discussion 3.1 Catalyst characteristics 3.1.1 N2 adsorption-desorption isotherms The N2 adsorption and desorption isotherms obtained from the surface area measurements are shown in Figure S2. These isotherms can be classified as type IV according to IUPAC18. The appearance of the specific hysteresis loop shows that all 12


Page 13 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the three catalysts have mesopores, which are preferable for most industrial adsorbents. In addition, taking into account the mass transfer limitations of micro porous materials as well as a higher BET specific surface area, mesopore size adsorbents exhibit a superior performance as compared to micropore size adsorbents19-21. 3.1.2 BET surface area and pore size/distribution The pore properties of the catalysts(e.g. pore size, pore-size distribution and pore volume) as well as the BET surface area have substantial influences on their catalytic performances. The obtained structural data and pore size distributions are shown in Table 1.The results showed that the BET surface area as well as the mesopore surface area of the catalysts both decreased in the following order: MCM-41 > H-ZSM-5 > SO42-/ZrO2. The Pore volume also followed the same trend:MCM-41 > H-ZSM-5 > SO42-/ZrO2. Furthermore, for all the three catalysts, the proportion of mesopores was largest, followed by micropores and then by macropores. Table 1:

Comparison of structural properties of various catalyst.

3.1.3 Total acid site and acid site strength The acidity data of the three catalysts obtain from NH3-TPD is shown in Table 2. The table shows that SO42-/ZrO2 has the largest amount of total acid sites, followed by MCM-41and then by H-ZSM-5. The same trend was exhibited for strong acid sites. However, the weak and moderate acid sites of the three catalysts did not exactly follow the same trend. The trend was:SO42-/ZrO2> H-ZSM-5> MCM-41. Figure 1 shows the acid site strength of the three catalysts. The peak that appears near 100℃ 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 33

represents the weak acid site, while the one near 220℃ represents the moderate acid site. On the other hand, the peak near 480℃ represents the strong acid sites. It can be seen in the figure that H-ZSM-5 and MCM-41 have almost no strong acid peaks whereas SO42-/ZrO2 has an obvious peak implying that only SO42-/ZrO2can be classified as having strong acid sites. Table 2: NH3-TPD amount of acidic sites (mmol g-1) Figure 1. NH3-TPD-curves of SO42-/ZrO2，MCM-41 and H-ZSM-5. 3.1.4B/L acid ratio The amounts for the different types of acid sites for the three catalysts obtain from Py-IR are shown in Table 3. These include Brϕnsted acid sites, Lewis acid sites, total acid sites and the ratios of B/L acid sites. The table shows that the ratios of B/L acid sites for H-ZSM-5, MCM-41 and SO42-/ZrO2are 1.5116, 0.7505 and 0.5834, respectively. Table 3:Acid properties of various solid acid catalysts 3.1.5 XRD spectrum The XRD spectrum which indicates whether the sulfate ion (SO42-) was indeed impregnated into zirconia is shown in Figure2. The spectrum of SO42-/ZrO2 not only contains the characteristic peak of ZrO2, but also contains the characteristic peak of SO42- thereby confirming the incorporation of SO42- in ZrO2. Figure 2. X-ray diffraction patterns. 3.1.6 FT-IR spectrum The FT-IR spectrum of SO42-/ZrO2 is shown in Figure 3. In the figure, the peak 14


Page 15 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with a round circle shows the characteristic peak of sulfate. The spectrumthus confirms that sulfate (SO42-) was indeed impregnated into zirconia. Figure 3. FT-IR spectra of sulfate-free and sulfate-doped zirconia samples. 3.2 CO2 Desorption Performance 3.2.1 CO2 desorption performance without catalyst 3.2.1.1 Effect of blending MEA with a tertiary amine The typical CO2desorption profile (CO2 loading versus time curve) of the four amines (i.e. 5M MEA, 5M MEA+1M DEEA, 5M MEA+1M 1DMA2P, and 5M MEA+1 MDEA) with an initial CO2 loading of 0.5 mol CO2/mol amine at the desorption temperatures of 98℃ is shown in Figure 4. In this work, CO2 desorption performance was evaluated in terms of rate of CO2 desorption, amount of CO2 desorbed and heat duty within the first 90 min of the desorption experiment since it was within this timeframe that CO2 desorption profile for all the conditions had the steepest slopes as shown in Figure 4.The CO2 desorption performance of the four amines with an initial CO2 loading of 0.5 mol CO2/mol amine at the desorption temperatures of 90, 95, 100, 105℃ is shown in Table4. The figure shows that, at any temperature, the fastest rate of CO2 desorption and the largest amount of CO2 desorbed were from 5M MEA+1M MDEA. This was followed by 5M MEA+1M 1DMA2P > 5M MEA+1M DEEA > 5M MEA. It was observed that at the end of the specified 90 min timeframe of the CO2 desorption process for all the temperatures for the four amine solutions, the CO2 loading in the remaining solutions increased in the order: 5M MEA+1M MDEA＜5M MEA/1M 1DMA2P＜5M MEA/1M DEEA＜5M 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 33

MEA. In contrast, the CO2cyclic capacity was of the opposite order. Figure 4: CO2 desorption in single and blended amines at 98℃ The better performance of blended amines than MEA is explained below. It is well known that a tertiary alkanolamine is the easiest amine to regenerate because it is much easier for the available bicarbonate to accept H+ than H2O as a stronger base. According to Caplow22, the solvent regeneration mechanism for a primary amine such as MEA involves two steps: (a) carbmate breakdown with protons; (b) deprotonation of AmineH+. However, for tertiary amines, it is only the second step that is involved because there is no possibility of formation of carbamate, and so, there is no carbamate available in the solution. The reactions are as shown below: a) CO2-MEA-water system 1. Carbamate breakdown: MEA − COO − + H 3O + ←→ MEA − H + − COO − ( zwitterion ) + H 2 O ←→ MEA + H 2 O + CO2 ↑

The process includes two steps: proton-transfer and N-C bond breaking. 2. AmineH+ deprotonation: MEAH + + H 2 O ←→ MEA + H 3O +

（5）

b) When a tertiary amine is added, reaction (5) is divided into two further steps as in Eqns(6) and (7) which require smaller activation energies. MEAH+ can more easily transfer the proton to the tertiary amine (e.g. DEEA, MDEA , 1DMA2P) than directly to water thereby creating protonated tertiary amines (e.g. DEEAH+, MDEAH+ or 1DMA2PH+).This reduces the overall activation energy of the reaction.CO2-blended

amine

system

(MEA-DEEA-CO2-H2O,

or 16


(4)

Page 17 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MEA-MDEA-CO2-H2O or MEA-1DMA2P-CO2-H2O). 1.Blended AmineH+ deprotonation (1) Reaction process using MEA-DEEA solution (as an example) without bicarbonate:

MEAH + + DEEA ←→ MEA + DEEAH +

(6)

+

DEEAH + H 2O ←→ DEEA + H 3O

(7)

Another feature of alkanolamines is the availability of carbonate (CO32-) and bicarbonate ions (HCO3-) in the aqueous amine mixture. Both ions can be detected in all types of amines whether single or blended, as has been observed by Jakobsen et al.23 and Shi et al.19in amine solutions at the high CO2 loading region using nuclear magnetic resonance (NMR) analysis. Three amine systems: (i)5M MEA(one single aqueous primary amine) and (ii) two examples of aqueous primary-tertiary amine blends (5M MEA+1M MDEA and 5M MEA+1M 1DMA2P) were tested for their bicarbonate concentrations, and the results are shown in Figure 5. It can be seen from the figures that the bicarbonates of the single MEA system were generated starting at the CO2 loading of 0.42 mol CO2/mol amine whereas for the blended amine systems, the bicarbonate ions were generated at lower CO2 loadings of 0.36 and 0.39 mol CO2/mol amine for MEA-1DMA2P and MEA-MDEA systems respectively. Figure 5: Concentration of bicarbonate in MEA-CO2-H2O, MEA-MDEA-CO2-H2O and MEA-1DMA2P-CO2-H2O systems at 293.15K using NMR analysis Reaction process for MEA-DEEA solution (as an example) with bicarbonate: MEA − COO − + H 2 O ←→ MEA + HCO3−

(8)

The deprotonation reactions of tertiary amine with bicarbonate can be explained 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 33

by Eqns.9, 10 and 11. MEAH + + HCO3− ←→ MEA + H 2CO3

(9)

MEAH + + DEEA ←→ MEA + DEEAH +

(10)

+

− 3

DEEAH + HCO ←→ DEEA + H 2CO3

(11)

So there are two proton acceptors, namely, HCO3- and H2O for single and blended amines. However, there is only one proton donor MEAH+ for the single MEA amine whereas for blended amine, there are two types: MEAH+ and DEEAH+ (or MDEAH+ or 1DMA2PH+). Therefore, the addition of a tertiary amine to a primary amine promotes the desorption of CO2 as shown in the following pathway. MEAH+

H2O HCO3-

MEAH+

H2O

DEEAH+/MDEAH+/1DMA2P+

HCO3-

HCO3- performs two roles in the CO2 desorption process. One is to alter the reaction path but does not change the products nor the thermodynamics, just like a “catalyst”. The other is to accept a proton from protonated amine (AmineH+) to generate H2CO3 and then release CO2 directly as shown by Eq.(12). (3)CO2 release from protonated bicarbonate/carbonic acid: A min eH + + HCO3− ←→ A min e + H 2CO3 ←→ A min e + H 2O + CO2 ↑

(12)

The energy consumption for CO2 desorption in blended amines is lower than that of single primary amine MEA because the initial loading of the blended amine at which the bicarbonate (HCO3-) exists is lower than that of single primary amine MEA. Thus, in the same lean CO2 loading region, the HCO3- concentration in the blended 18


Page 19 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


amine is higher and the carbamate concentration is lower than the corresponding values of the single amine, MEA19. From a thermodynamic point of view, the thermal stability of carbamate is higher than that of HCO3-, implying that the energy required to break down the carbamate is larger than that for HCO3-. Therefore, the energy consumption for CO2desorption in the lean loading region for a primary-tertiary amine blend is less than that of the single primary amine MEA. This implies that for the same energy supply the rate and amount of CO2 desorbed will be larger for primary-tertiary amine blends than the single primary amine, MEA. 3.2.1.2 Effect of the structure/characteristics of the tertiary amine in the amine blend The tertiary amines used in this work were MDEA, 1DMA2P and DEEA. Their chemical structures are as shown in Figure S3. Table 4 shows that for all temperatures, the largest rate and amount of CO2 desorbed were from 5M MEA+1M MDEA. This was followed by 5M MEA+1M 1DMA2P, then by 5M MEA+1M DEEA and finally by 5M MEA.

Table 4: CO2 desorption in single and blended amines at 90℃-105℃ This performance trend can be attributed to the difference in the contributions of each tertiary amine to the amine blend in terms of rate/amount of CO2 desorption and heat duty can be explained on the basis of their chemical structures. Each tertiary amine has bulky functional groups attached to the N atom of the amino group. For MDEA, it is one methyl group and 2 ethanol (OH) groups. On the other hand, for 1DMA2P, it is two methyl groups and 1 iso-propanol (OH) group whereas for DEEA, it is two ethyl groups and one ethanol (OH) group. The presence of bulky groups helps to facilitate 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 33

HCO3- production and its eventual reaction to produce CO2.The type of bulky functional groups determines the degree of facilitation. For MDEA, the two ethanol groups are strong electron withdrawing groups which provide additional positive modifying effect in facilitating HCO3- production whereas for 1DMA2P, the two methyl groups (one extra methyl group and one less OH group) are electron donating which tend provide a negative modification to a typical tertiary amine characteristics. In the case of DEEA, there are two ethyl groups which lead to a stronger electron donating group than the methyl group, thereby providing a stronger negative modification. The consequence of this is the trend that is observed in Table 4 in terms of CO2 desorption amount and rate. 3.2.1.3 Effect of CO2 desorption temperature The other observation is the increase of the amount and rate of CO2 desorbed as the temperature increased for both blended amines and the single amine MEA. CO2 desorption is an endothermic process involving carbamate breakdown as well as bicarbonate decomposition. Thus, an increase in temperature introduces more heat energy which increases the rate of reaction thereby increasing the amount of CO2 produced as the temperature increases. Specifically, Table 4 shows that with an increase in temperature, the initial loading in which the bicarbonate (HCO3-) first appears is reduced because a higher temperature facilitates bicarbonate generation via carbamate exchange6, as in reaction (5). With a higher fraction of HCO3- in the solution, more CO2 is produced through an easier HCO3- pathway. 3.2.2 CO2 desorption with catalyst 20


Page 21 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.2.2.1 Amount and rate of CO2 desorption The CO2 desorption curve of 5M MEA with and without catalyst are shown in Figure 6. And the other CO2 desorption curves with the same trend were shown in Figure S4. These curves reflect the performance of three catalysts (two molecular sieves H-ZSM-5 and MCM-41, and one super solid acid SO42-/ZrO2) in four different solvents systems: MEA, MEA-DEEA, MEA-MDEA and MEA-1DMA2P for the whole 9 h duration performed at 98℃. It can be seen that the lean loading value of each amine solution system is achieved regardless of whether or not a catalyst is added. The catalysts can only change the reaction rate but does not affect the chemical equilibrium. In addition, Figures 6 show that the CO2 loading decreased sharply in the first 90 minutes, and then the slope turned less sharply in the next 180 minutes, and then nearly tended to be constant after 8 h of the experiments. Consequently, all analyses were done for the first 90 min of the test runs. The introduction of each of the three catalysts facilitated amine regeneration over the blank run (i.e. without any catalyst). Among these catalysts, H-ZSM-5 provided the best performance in terms of amount and rate of CO2 desorption followed by MCM-41, and then SO42-/ZrO2 being the worst.

Figure 6: CO2 desorption in 5M MEA with and without catalysts at 371K. 3.2.2.2 Heat duty for CO2 desorption The heat duty, amount of CO2 regenerated and CO2 desorption rate for single and blended amine solvents with and without catalysts at 371K based on the first 90 min are shown in Table 5. The relative energy requirement of these sixteen tests are 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

plotted in Table 6 for the first 90min to determine the most energy-effective approach. 5M MEA without catalyst was taken as the baseline at 100%. It can be seen that heat duty was reduced with both catalysts for the same solvent systems. The H+ from Brønsted acid or Al atom from Lewis acid can attack the N atom of the carbamate to rob it of its lone pair of electrons, thereby providing the avenue for significant reduction of the heat duty for solvent regeneration. The first three sets are 5M MEA solution (single), second three sets are 5M MEA+1M DEEA, then 5M MEA+1M 1DMA2P and finally 5M MEA+1M MDEA. H-ZSM-5 performed better than both MCM-41 and SO42-/ZrO2. The CO2 desorption performance of different system was presented in terms of relative heat duty. The relative heat duty was defined and used in this work. The relative heat duty, the heat duty for 5M MEA without catalyst regeneration is considered as baseline (Hbaseline, kJ/mol) was taken as 100%. The relative heat duty is defined as the rate of the different system regeneration heat duty (Hi, kJ/mol) and the baseline heat duty Hbaseline. The relative heat duty of blended amine (5M MEA/1M MDEA) was obtained as 58.3%. The blended amine with three types catalysts of H-ZSM-5, MCM-41 and SO42-/ZrO2 were calculated as 52.5%, 50% and 48.9%, respectively. All the results were shown in Table 6. It can be seen that MEA-MDEA with H-ZSM-5 gave a higher heat duty.

Table 5: Heat duty, amount of CO2 regeneration and CO2 desorption rate for single and blended amine solvents with and without catalysts at 371K based on the first 90 minutes. 22


Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 6: CO2 desorption heat duty for the first 90 minutes. In the CO2 desorption process, four critical characteristics of the solid acid catalysts can affect their desorption performance in solvent regeneration. These are: (a) the total acid site, (b) mesopore surface area, which determines the proportion of acid sites that can be used for the reaction, (c) the B/L ratio, which determines the predominant mechanism, and (d) the amount of Brϕnsted acidic sites which determines the amount of proton (H+) that the catalyst can provide. The 5M MEA/1M MDEA blended amine system was taken as an example for studies of the relationship between the catalyst characteristics and the heat duty and rate of CO2 desorption. The total acidic sites for H-ZSM-5, MCM-41 and SO42-/ZrO2 were:1.1927, 1.3084 and 2.3815 mmol/g, respectively(Table 2). Also, based on Table 1 and Table 3, the mesopore surface areas were 151.56, 963.19 and 72.53m2/g while the ratios of acid site were 1.5116, 0.7505 and 0.5834, respectively. The relationship between these catalyst characteristics (the total acid site, mesopore surface area and B/L ratio)and the heat duty and rate of CO2 desorption in the first 90 min are presented in Figure S5-S7. This results shows that the three catalyst characteristics are not independently responsible for the performance observed in the catalysts. So it became necessary to explore the performance of the solid acid catalysts in terms of the a combined impact of these factors. One of those factors is the product of B/L and mesopore surface area. A plot between the rate of CO2 desorption and heat duty against the product of B/L and mesopore surface area is shown in Figure 7. It is observed in the figure that an increase in the product of B/L and mesopore surface area leads to a decrease in the 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

heat duty as well as an increase in the rate of CO2 desorption. This implies that the more Brønsted acid sites relative to Lewis acid sites and larger mesopore surface area, the better the performance of the catalyst for CO2 desorption. This explains the poor catalytic performance of MCM-41 because it has the largest mesopore surface area, but B/L is relatively small. It also explains the poor performance of SO42-/ZrO2which had the lowest mesopore surface area and the smallest B/L ratio even though it had the strongest acid sites of the three catalysts. In contrast, H-ZSM-5 with a large mesopore surface area and the largest B/L ratio in the three catalysts showed the best catalytic performance both for single and blended amines. It is known that MDEA is a tertiary alkanolamine that generates protonated amines rather than carbamate directly, and then transfers the proton to HCO3-to release CO2, which reduces the energy required for regeneration greatly. Consequently, the blended amine MEA-MDEA with H-ZSM-5 provided the best option out of all the options examined in this study.

Figure 7: Influence of (mesopore surface area)*(B/L) on heat duty and CO2 desorption rate.

3.Conclusions The catalytic performance of the three catalysts in terms of heat duty efficiency and CO2 desorption rate in single or blended amine decreased in the order: H-ZSM-5 > MCM-41 > SO42-/ZrO2. Based on the research by Yu24, the characteristic of the catalyst that played a significant role in catalyst performance was probably the product of mesopore surface area and B/L acid sites ratio of catalysts. That is to say, the larger amount of Brϕnsted acidic sites of the catalysts, the larger the value (B/L) 24


Page 25 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and the larger the mesopore surface area combined may led to a better performance of the catalysts in terms of heat duty and the CO2 desorption rate in the solvent regeneration process. The performance of the amines in terms of rate/amount of CO2 desorption and heat duty followed the trend: 5M MEA+1M MDEA > 5M MEA+1M 1DMA2P > 5M MEA+1M DEEA > 5M MEA. The observed trend followed the electron withdrawing or donating influence of the functional groups attached to the N atom. Consequently, the strong electron withdrawing chemical structure of MDEA made the solution most prone to CO2 desorption as compared with the chemical structures of 1DMA2P and DEEA. The best combination of catalyst and amine was H-ZSM-5 in combination with MEA-MDEA solvent. This combination produced the strongest catalyst and amine structure effects for the largest rate/amount for CO2 desorption as well as the least heat requirement for CO2 desorption.

Acknowledgements The financial support from the National Natural Science Foundation of China (NSFC-Nos.U1362112, 21376067, 21476064, 21406057, 21536003 and 51521006), National Key Technology R&D Program (MOST-No.2014BAC18B04), Innovative Research Team Development Plan (MOE-No.IRT1238), Specialized Research Fund for the Doctoral Program of Higher Education (MOE-No.20130161110025), and China Outstanding Engineer Training Plan for Students of Chemical Engineering & Technology in Hunan University (MOE-No.2011-40) are gratefully acknowledged.

Supporting Information Figure S1, S2, S3, S4, S5, S6, S7 were given in Supporting Information

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

Reference (1) Knuutila, H.; Svendsen, H. F.; Anttila, M., CO2 capture from coal-fired power plants based on sodium carbonate slurry; a systems feasibility and sensitivity study. Int. J.Greenhouse Gas Control 2009, 3, 143-151. (2) Bird, R. B.; Stewart, W. E.; Lightfoot, E. N., Transport phenomena. (Revised Seconded.). John Wiley ＆ Sons 2007. (3) Feng, B.; Du, M.; Dennis, T. J.; Anthony, K.; Perumal, M. J., Reduction of Energy Requirement of CO2 Desorption by Adding Acid into CO2-Loaded Solvent. Energ. Fuel. 2009, 24, 213-219. (4) Eisaman, M. D., CO2 desorption using high-pressure bipolar membrane electrodialysis. Energ.Environ. Sci. 2011, 4, 4031-4037. (5) Zhang, J.; Yu, Q.; Agar, D. W., Intensification of low temperature thermomorphic biphasic amine solvent regeneration for CO2 capture. Chem. Eng. Res. Des. 2012, 90, 743-749. (6) Shi, H.; Naami, A.; Idem, R.; Tontiwachwuthikul, P., Catalytic and non catalytic solvent regeneration during absorption-based CO2 capture with single and blended reactive amine solvents. Int. J.Greenhouse Gas Control 2014, 26, 39-50. (7) Idem, R.; Shi, H.; Gelowitz, D.; Tontiwachwuthikul, P., Catalytic method and apparatus for separating a gaseous component from an incoming gas stream. In US: 2013. (8) Seo, S.; Simoni, L. D.; Ma, M.; Desilva, M. A.; Huang, Y.; Stadtherr, M. A.; Brennecke, J. F., Phase-Change Ionic Liquids for Postcombustion CO2 Capture. Energ. Fuel. 2014, 28, 5968-5977. (9) Liu, Z. L.; Yang, T.; Zhang, K.; Cao, Y.; Pan, W. P., CO2 adsorption properties and thermal stability of different amine-impregnated MCM-41 materials. Journal of Fuel Chemistry & Technology 2013, 41, 469-475. (10) Reddy, B. M.; Sreekanth, P. M.; Lakshmanan, P., Sulfated zirconia as an efficient catalyst for organic synthesis and transformation reactions. J. Mol. Catal. A Chem. 2005, 237, 93-100. (11) Ecormier, M. A.; Wilson, K.; Lee, A. F., Structure-reactivity correlations in sulphated-zirconia catalysts for the isomerisation of alpha-pinene. J. Catal. 2003, 215, 57-65. (12) Yadav, G. D.; Pathre, G. S., Novel mesoporous solid superacids for selective C-alkylation of m -cresol with tert -butanol. Micropor. Mesopor. Mat. 2006, 89, 16-24. (13) Sakwattanapong, R.; Adisorn Aroonwilas, A.; Veawab, A., Behavior of Reboiler Heat Duty for CO2 Capture Plants Using Regenerable Single and Blended Alkanolamines. Ind. Eng. Chem. Res. 2005, 44, 4465-4473. (14) And, P. D. V.; Kenig, E. Y., Acceleration of CO2 Reaction with N,N-Diethylethanolamine in Aqueous Solutions by Piperazine. Ind. Eng. Chem. Res. 2007, 47, 34-38. (15) Liu, H.; Liang, Y.; Liang, Z.; Liu, S.; Fu, K.; Sema, T.; Rongwong, W., Solubility, Kinetics, Absorption Heat and Mass Transfer Studies of CO2 Absorption into Aqueous Solution of 1-Dimethylamino-2-propanol. Energy Procedia 2014, 63, 659-664. (16) Fu, B.; Gao, L.; Lei, N.; Wei, R.; Xiao, G., Biodiesel from Waste Cooking Oil via 26


Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Heterogeneous Superacid Catalyst SO22−/ZrO2. Energ. Fuel. 2009, 23, 569-572. (17) Wen, L.; Liu, H.; Rongwong, W.; Liang, Z.; Fu, K.; Idem, R.; Tontiwachwuthikul, P., Comparison of Overall Gas-Phase Mass Transfer Coefficient for CO2 Absorption between Tertiary Amines in a Randomly Packed Column. Chem. Eng. Technol. 2015, 38, 1435-1443. (18) Sing, K. S. W., Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Provisional). Pure Appl. Chem. 2009, 54, 2201-2218. (19) Chal, R.; Gerardin, C.; Bulut, M.; Donk, S. V., ChemInform Abstract: Overview and Industrial Assessment of Synthesis Strategies Towards Zeolites with Mesopores. Chemcatchem 2011, 3, 67-81. (20) Possato, L. G.; Diniz, R. N.; Garetto, T.; Pulcinelli, S. H.; Santilli, C. V.; Martins, L., A comparative study of glycerol dehydration catalyzed by micro/mesoporous MFI zeolites. J. Catal. 2013, 300, 102-112. (21) Qin, Z.; Shen, B.; Yu, Z.; Deng, F.; Zhao, L.; Zhou, S.; Yuan, D.; Gao, X.; Wang, B.; Zhao, H., A defect-based strategy for the preparation of mesoporous zeolite Y for high-performance catalytic cracking. J. Catal. 2013, 298102-111. (22) Caplow, M., Kinetics of carbamate formation and breakdown. J. Am. Chemi. Soc. 2002, 90, 6795-6803. (23) Jakobsen, J. P.; Jostein Krane, A.; Svendsen, H. F., Liquid-Phase Composition Determination in CO2-H2O-Alkanolamine Systems: An NMR Study. Ind. Eng. Chem. Res. 2005, 44, 9894-9903. (24) Liang, Z.; Idem, R.; Tontiwachwuthikul, P.; Yu, F.; Liu, H.; Rongwong, W., Experimental study on the solvent regeneration of a CO2℃loaded MEA solution using single and hybrid solid acid catalysts. AIChE J 2015, 62, 753-765.

27



2-

-1.01

SO4 /ZrO2

-1.02

MCM-41 H-ZSM-5

-1.03 TCD Signal（ a.u.）

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

-1.04 -1.05 -1.06 -1.07 -1.08 -1.09 0

100

200

300 400 Temperature(℃ )

500

600

700

Figure 1. NH3-TPD-curves of SO42-/ZrO2，MCM-41 and H-ZSM-5.

2-

1200

a. SO4 /ZrO2 b. ZrO2

1000

800

600

b

400

200

a 0 10

20

30

40

50

60

70

80

2Theta

Figure 2. X-ray diffraction patterns.

28


Page 29 of 33

a.ZrO2 2-

b.ZrO2-SO4

SO4

4000

3500

3000

2500

2000

1500

2-

1000

500

Figure 3. FT-IR spectra of sulfate-free and sulfate-doped zirconia samples.

0.52 5M MEA 5M MEA+1M 1DMA2P 5M MEA+1M DEEA 5M MEA+1M MDEA

0.50 CO2 Loading(mol CO2/mol amine)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.48 0.46 0.44 0.42 0.40 0.38 0.36 0.34 0

100

200

300

400

500

600

Time(min)

Figure 4: CO2 desorption in single and blended amines at 98℃

29



1.0 5M MEA 5M MEA+1M MDEA 5M MEA+1M 1DMA2P

Concentration(mol/L)

0.8

0.6

0.4

0.2

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

CO2 Loading(mol CO2/mol amine)

Figure 5: Concentration of bicarbonate in MEA-CO2-H2O, MEA-MDEA-CO2-H2O and MEA-1DMA2P-CO2-H2O systems at 293.15K using NMR analysis

0.52

CO2 equilibrium Loading(mol CO2/mol amine)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

Blank 25g H-ZSM-5 25g MCM-41 225g SO4 /ZrO2

0.50 0.48 0.46 0.44 0.42 0.40 0.38 0.36 0

100

200

300

400

500

600

Time(min)

Figure 6: CO2 desorption in 5M MEA with and without catalysts at 371K.

30


Page 31 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7: Influence of (mesopore surface area)*(B/L) on heat duty and CO2 desorption rate. For Table of Contents Only

Sample

H-ZSM-5 MCM-41 SO42-/Zr O2

Table 1: Comparison of structural properties of various catalyst. BET surface Pore Average Pore size distribution area(m2/g) volu pore me Microp Mesop Macrop diamete Micro Mesop r Total (cm3/ ore ore ore(100 pore ore g) (nm) (100%) (100%) %) 128.4 279.9 0.292 151.56 28.06% 69.97% 1.97% 4.1732 0 7 1 963.1 0.966 0 963.19 17.80% 81.54% 0.66% 3.8783 9 2 0.174 2.25 72.53 74.78 3.57% 91.87% 4.56% 9.3357 5

Table 2: NH3-TPD amount of acidic sites (mmol g-1) 31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

catalyst H-ZSM-5 MCM-41 SO42-/ZrO2

Total 1.1927 1.3084 2.7961

Weak 0.4727 0.3693 1.0008

Moderate 0.7082 0.5772 0.4691

Page 32 of 33

Strong 0.0117 0.3619 1.3263

Table 3: Acid properties of various solid acid catalysts catalyst

Brϕnsted (mmol g-1)

Lewis (mmol g-1)

Total (mmol g-1)

H-ZSM-5 MCM-41 SO42-/ZrO2

0.2100 0.0083 0.0103

0.1389 0.0110 0.0177

0.3489 0.0193 0.0280

Brϕnsted / Lewis (B /L) 1.5116 0.7505 0.5834

Table 4. CO2 desorption in single and blended amines at 90℃-105℃ Amine solvents 5M MEA Temperature(

5M

5M

5M

MEA+1M

MEA+1M

MEA+1M

MDEA

DEEA

1DMA2P

CO2 desorption rate(%)，9h

11.36

17.49

12.48

15.23

0.5679

1.0495

0.7490

0.9140

16.21

22.39

16.72

19.46

0.8105

1.3434

1.0032

1.1677

24.25

30.99

25.59

27.29

1.2125

1.8596

1.53516

1.6375

31.21

42.74

33.23

36.50

1.5604

2.5644

1.9939

2.19

90 Desorption quantity (mol), first 90 min CO2 desorption rate(%)，9h 95 Desorption quantity (mol), first 90 min CO2 desorption rate(%)，9h 100 Desorption quantity (mol), first 90 min CO2 desorption rate(%)，9h 105 Desorption quantity (mol), first 90 min 32


Page 33 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 5: Heat duty, amount of CO2 regeneration and CO2 desorption rate for single and blended amine solvents with and without catalysts at 371K based on the first 90 minutes. Amount of CO2 Heat duty CO2 Desorption Solution Catalyst (MJ/kg regeneration rate CO2) (mol) (*10-2mol/min) No catalyst 20.7 0.85 0.94 25g H-ZSM-5 15.6 1.10 1.23 5M MEA 25g MCM-41 16.9 1.03 1.05 25g SO42-/ZrO2 18.7 0.94 1.15 No catalyst 12.1 1.45 1.61 5M 25g H-ZSM-5 10.1 1.76 1.95 MEA+1M 25g MCM-41 10.3 1.70 1.83 MDEA 225g SO4 /ZrO2 10.9 1.64 1.89 No catalyst 14.1 1.27 1.42 5M 25g H-ZSM-5 10.3 1.64 1.82 MEA+1M 25g MCM-41 11.8 1.49 1.60 1DMA2P 225g SO4 /ZrO2 12.2 1.44 1.66 No catalyst 15.0 1.17 1.30 5M 25g H-ZSM-5 12.1 1.44 1.59 MEA+1M 25g MCM-41 13.3 1.32 1.41 DEEA 225g SO4 /ZrO2 13.8 1.27 1.47

Table 6: CO2 desorption heat duty for the first 90 minutes. Catalyst Percentag （ % e ） Amine

Blank

H-ZSM-5

MCM-41

SO42-/ZrO2

5M MEA

100

75.2

81.6

90.2

5M MEA+1M MDEA

58.3

48.9

50

52.5

5M MEA+1M DEEA

72.6

58.7

64.1

66.6

5M MEA+1M 1DMA2P

67.9

49.6

57.1

59

33


Investigation of CO2 Regeneration in Single and ... - ACS Publications

Recommend Documents