Effects of Transition Metal Oxide Catalysts on MEA ... - ACS Publications

May 22, 2017 - ... Regeneration for the Post-Combustion Carbon Capture Process ... University of Engineering and Technology, Karachi 74800, Pakistan...
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Effects of Transition Metal Oxide Catalysts on MEA Solvent Regeneration for the Post-Combustion Carbon Capture Process Umair H. Bhatti,†,§ Abdul K. Shah,‡,⊥ Jeong Nam Kim,§ Jong Kyun You,§ Soo Hyun Choi,§ Dae Ho Lim,§ Sungchan Nam,§ Yeung Ho Park,‡ and Il Hyun Baek*,†,§ †

University of Science and Technology, 217 Gajeong-ro Yuseong-gu, Daejeon 305-350, South Korea Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, South Korea ‡ Department of Fusion Chemical Engineering, Hanyang University, Seoul 04763, South Korea ⊥ Department of Chemical Engineering, Dawood University of Engineering and Technology, Karachi 74800, Pakistan §

S Supporting Information *

ABSTRACT: Large heat duty for amine regeneration in absorptionbased CO2 capture is one of the major drawbacks of this process. Along with a highly endothermic carbamate breakdown reaction in the stripper, the difficulty of proton transfer from protonated amines to water in the amine regeneration process is also considered a basic reason for high heat duty. Transition metal oxide catalysts can play a vital role in decreasing the required thermal energy for amine regeneration in the stripper by providing Bronsted acids and Lewis acids that would help break down the carbamate by direct attack. MEA saturated with CO2 at 35 °C, with initial loading of 0.56 mole CO2/mole amine, was used in this study. The performance of five different transition metal oxide catalysts, V2O5, MoO3, WO3, TiO2, and Cr2O3, was studied separately to investigate the effects of these catalysts on amine regeneration in the temperature range of 35−86 °C. It has been observed that MoO3 performance is much better as it regenerated almost double of the MEA solvent than noncatalytic amine regeneration systems, whereas other catalysts also showed considerable differences in amine regeneration in this temperature range. The amine regeneration performance trend was MoO3 > V2O5 > Cr2O3 > TiO2 > WO3 > blank test. The application of this work would mean that metal oxide catalysts could be used in strippers for a faster CO2 desorption rate at lower temperature, which would cause a significant reduction of the heat duty. KEYWORDS: Catalytic amine regeneration, MEA, Post-combustion CO2 capture, Transition metal oxides, Bronsted and Lewis acid sites

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INTRODUCTION Global warming is one of today’s most pressing issues. Increasing levels of CO2 emissions in the atmosphere, especially from coal-fired power plants, is one of the major reasons for climate change. Predominant power generation from sources other than coal-fired power plants does not seem likely to happen in the near future. Therefore, economically efficient technologies must be developed for capturing and sequestration of CO2 from large point sources.1,2 Post-combustion carbon capture using aqueous alkanolamine solvents is considered a mature, feasible, and relatively reliable technology that is also capable of producing ultrapure CO2. Ethanolamine (MEA) is the most widely used amine solvent for this purpose because of its low price, fast kinetics, high absorption capacity, and strong reactivity to CO2 even at very low CO2 partial pressures.3 Though some researchers have pointed out the drawbacks associated with the use of MEA for carbon capture on a global scale, still MEA is the most wellknown, readily available, and thoroughly studied solvent.4,5 © 2017 American Chemical Society

However, a large amount of thermal energy is consumed for MEA solvent regeneration in the stripper. With a system that captures 90% of CO2, the overall efficiency of the power plant might be reduced by 30−40%.6 Furthermore, the cost of the electricity produced could also increase by more than 35% due to the additional heat required for CO2 capture and compression. This huge energy penalty is attributed largely to the regeneration heat requirement for CO2 stripping, which accounts for almost 80% of the total operating cost.7 The large heat duty of amine solvent regeneration has posed a major challenge for implementation of carbon capture technology. Therefore, the heat duty of amine regeneration must be minimized to ensure worldwide acceptance of this technology without a considerable economic penalty. Received: February 26, 2017 Revised: May 15, 2017 Published: May 22, 2017 5862

DOI: 10.1021/acssuschemeng.7b00604 ACS Sustainable Chem. Eng. 2017, 5, 5862−5868

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that can accept an electron pair. The second type is Bronsted acidity, which occurs at sites that can donate a proton to a base.30 When metal oxide surfaces are exposed to water, water molecules are adsorbed on the surface as shown in eq 1. The interaction of water converts the surface oxide into hydroxyl groups that behave as Bronsted acids. Therefore, both roles (i.e., Lewis acid catalyst and Bronsted acid catalyst) can be predicted from transition metal oxide catalysts.

The heat duty of an absorption-based post-combustion carbon capture process can be minimized by process integration and solvent optimization. Various stripper configurations have been suggested to reduce the heat duty of the amine regeneration process such as multipressure stripping,8 stripper overhead compression,9 internal exchange stripper,10 interheated stripper column,11 heat-integrated strippers,12 improved split flow with vapor recompression,13 and heat pump distillation with a split flow process14 which claim to reduce the regeneration heat duty up to 50%. Many researchers have attempted to find a better amine solvent than MEA, with a lower heat of reaction and a higher CO2 absorption capacity. Chowdhury et al.15,16 synthesized amino alcohols that have a 20−30% lower heat of reaction than MEA. Puxty et al.17 analyzed 76 amines including primary, secondary, and tertiary amines; alkanolamines; polyamines; and amino acids to find better amine solvents, on the basis of absorption capacity and kinetics comparable to MEA. Recently, diethylenetriamine (DETA), 1-dimethylamine-2-propanol (1DMA2P), 1-diethylamino-2-propanol (1DEA2P), and triethylenetetramine (TETA) and its blend with MDEA have also been studied and found to have a lower regeneration heat duty than MEA.18−21 However, the vital factors required for commercialization of any solvent such as the availability, price, process economics, and annual productions of these solvents are yet to be evaluated. It has been widely known that blends of primary and secondary/tertiary amines can be used with a comparatively lower heat duty than primary amine only. Ma’mun et al.22 screened single and mixed-amine-based absorbents on the basis of their CO2 selectivity and cyclic capacity. Idem et al.23 studied MEA/MDEA blends and found a considerable reduction in heat duty. Other novel techniques to reduce the amine regeneration heat duty include addition of nanoparticles in the system and replacement of the conventional thermal regeneration process with an electrochemically mediated method.24,25 Though a great deal of research work has already been done in this area, so far, practically, it has not been reduced to the extent to make it a viable application. Recently, it has been reported that the heat duty for MEA solvent regeneration could be reduced significantly by adding solid acid catalysts in the stripper. Idem et al.26 investigated the effects of two solid acid catalysts − H-ZSM-5 (predominantly a Bronsted acid catalyst) and γ-Al2O3 (predominantly a Lewis acid catalyst) − in MEA solvent regeneration which caused a fair decrement in the regeneration heat duty. Liang et al.27 further investigated these two solid acid catalysts (H-ZSM-5 and γ-Al2O3) in the MEA solvent regeneration process to study the effects on heat duty. Both catalysts performed better than a noncatalytic system by desorbing a greater amount of CO2 and at faster rates. However, H-ZSM-5 exhibited better efficiency than γ-Al2O3. Presently, the effects of these two catalysts on heat duty of MEA solvent regeneration were evaluated on a bench-scale unit.28 The heat duty was reduced by 42 and 30% by using H-ZSM-5 and γ-Al2O3, respectively. Transition metal oxides have been used widely for their catalytic properties and could be a good option for providing Lewis acids and Bronsted acids in the stripper. The presence of defect sites on metal oxide surfaces is primarily responsible for many of their catalytic and chemical properties.29 Two types of acidity occur on solid acid surfaces. The first type is Lewis acidity, which occurs at coordinatively unsaturated metal atoms

MOsurf + H 2O → MO·H 2Osurf → MOH·OHsurf

(1)

Role of Acid Sites. H+ (Bronsted acid) and metal atoms (Lewis acid) can directly attack carbamate to rob it of its lone pair of electrons. In this way, the configuration of N atoms will be changed from sp2 to sp3, and the N−C bond strength would also be weakened by stretching. Consequently, the carbamate would be broken up by using less thermal energy, which will lead to faster CO2 stripping at lower temperatures.26 This changed mechanism of carbamate breakdown would make it possible to strip CO2 at faster rates than in conventional amine strippers. The major difference in the catalytic system is the availability of Bronsted acid sites and Lewis acid sites that assist in carbamate breakdown by lowering the activation energy of the desorption reaction. Thus, amine regeneration could be performed at lower temperatures, which would bring down the Qsen and Qvap. The main objective of this work was to investigate the effects of five transition metal oxides catalysts on the MEA solvent regeneration in terms of CO2 desorption rate and to determine the overall CO2 amount desorbed within a specified temperature range of 35−86 °C to assess the possibility of running the stripper at lower temperatures. The results are discussed below.

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EXPERIMENTAL SECTION

Materials. Monoethanolamine (MEA) (reagent grade) with purity of ≥99% was purchased from Acros Organics. MoO3, WO3, and Cr2O3 were purchased from Sigma-Aldrich in fine powder form with purity of >99.5%. V2O5 and Cr2O3 were purchased from Aldrich with purity of 98 and 99.8%, respectively. All materials were used without further purification. Experimental Apparatus and Procedure. MEA 5 M solutions were prepared with deionized distilled water. A continuous stirred-tank reactor (CSTR) was used for CO2 absorption in the MEA solution at 35 °C. Upon full saturation of MEA with CO2 at 35 °C, rich loading was checked by a TOC analyzer (Analytik Jena multi N/C 3100) and was found to be 0.56 mole CO2/mole MEA. In all experiments, 100 mL of CO2-loaded MEA solutions was used. A schematic diagram of the experimental apparatus is shown in Figure 1. A 250 mL four-necked flat-bottomed batch reactor was connected to an oil circulator (JeioTech HTC-10D) to maintain the desired temperature. A magnetic stirrer bar was used at 200 rpm to stir the solution. A thermocouple was inserted to measure the temperature of the amine solution, while a condenser was used to condense and recycle water vapors and also to prevent any amine loss. Moreover, a pressure gauge was also connected with the reactor to measure the inside pressure, and a vent valve was used to control the pressure. Pressure was maintained at 1.1−1.2 bar throughout the experiment. CO2 outgassed from the reactor was mixed with 50 cc/min of N2 gas, and this gas mixture was analyzed by using a gas chromatograph (Agilent Technologies 7890A). The values were recorded with respect to time and temperature. Before starting the experiment, 5 min of equilibration time was allowed, under stirring. There were two stages for all experiments: the temperature ramp-up stage, during which the temperature was raised constantly at 5 °C/min from 35 to 86 °C, and a 2 h isothermal stage, during which the temperature was kept constant at 86 ± 1 °C. Each catalyst (5 g of each) was added to 100 mL of MEA solution at room temperature. After 5 min of 5863

DOI: 10.1021/acssuschemeng.7b00604 ACS Sustainable Chem. Eng. 2017, 5, 5862−5868

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Figure 2. FT-IR spectra of pyridine-adsorbed MoO3, V2O5, Cr2O3, TiO2, and WO3. “B” and “L” denote Bronsted and Lewis acids, respectively.

highest number of Lewis and Bronsted acid sites. Moreover, Bronsted acid sites were detected on the surfaces of MoO3 and V2O5 only. The spectra of all catalysts are in accordance with the literature.31,32 (See the Supporting Information for other characteristics of the catalysts.)

Figure 1. Schematic diagram of the experimental apparatus. equilibration, the temperature was raised steadily at 5 °C/min until it reached 86 °C, where it was stabilized and kept constant for 2 h. The recording of values was started at 35 °C. A blank test was performed prior to all experiments, and the results were used as a baseline. The results are presented in Figure 3.

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RESULTS AND DISCUSSION Given the same conditions, all catalysts performed better by increasing the CO2 desorption rate and removing a greater amount of CO2 than the noncatalytic system. Figure 3 shows the CO2 desorption curve as a function of time and temperature for five metal oxide catalysts and a blank test, in the temperature range of 35−86 °C. It can be seen that MoO3 exhibited the best performance followed by V2O5 and then TiO2, Cr2O3, and WO3. Moreover, MoO3 and V2O5 started desorbing CO2 at temperature 65 °C, a better performance was shown by these metal oxides. WO3 stripped more CO2 than all other Lewis acid catalysts during the isothermal stage, and this trend continued until the end of the experiment. Furthermore, the performance of all five catalysts during the isothermal stage was not similar to their performance during the ramp-up stage. Also, as the duration of the experiment was extended, the desorption rate of CO2 from the catalytic systems decreased. The reason for this is the driving force for CO2 desorption was decreased

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CHARACTERISTICS OF THE TRANSITION METAL OXIDES Pyridine Adsorption FT-IR Spectra. The pyridine adsorption technique was used for detection of acid sites present on the catalysts’ surface. For the measurement of Lewis acid sites and Bronsted acid sites, self-supporting wafers (11 tons cm−2, 30 mg, and 1 cm2) were dehydrated in a specially designed stainless steel IR cell under vacuum (10−3 mbar) at 300 °C for 2 h. Hot pyridine vapors were introduced into the IR cell until the pyridine vapor pressure inside the IR cell reached 5 bar, and pyridine vapors were adsorbed on the catalyst surface at 100 °C for 2 h and further desorbed at 150 °C for 15 min under a continuous vacuum condition. After complete removal of physiosorbed pyridine, the IR cell was connected to an infrared spectroscope, and the IR spectra of the materials were recorded under wavenumber 600−4000 cm−1, 8 cm−1 optical resolution, and coaddition of 32 scans. The accurate IR spectra of materials were collected after subtracting the spectra of the dehydrated sample from the pyridine adsorbed spectra. The IR spectra of all metal oxide catalysts are shown in Figure 2. The infrared spectra show that all five catalysts possess Lewis acid sites, whereas MoO3 and V2O5 possess Bronsted acid sites as well. The characteristic absorption peaks of pyridine adsorbed on Lewis acid sites appear at 1440 cm−1 for all the catalysts. However, in the case of MoO3, an additional peak appears at 1451 cm−1 which also indicates the presence of Lewis acid sites. The peaks which appeared at 1482 cm−1, denoted by L+B, indicate the presence of Lewis acid sites and Bronsted acid sites at the metal oxides’ surfaces. At 1535−1540 cm−1 sharp peaks were recorded for MoO3 and V2O5, which indicate the presence of Bronsted acid sites over these two catalysts. Peaks which appeared at 1580−1585 cm−1 indicate Lewis acid sites. In addition, for MoO3, two peaks appeared at 1575 and 1585 cm−1. This shows that MoO3 contains the 5864

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Figure 3. CO2 desorption curves of MEA with and without catalysts as a function of time and temperature: 5 g of each catalyst was used. (a) CO2 desorption curves for the MEA blank test and WO3 and MoO3 catalysts. (b) CO2 desorption curves for the MEA blank test and Cr2O3, TiO2, and V2O5 catalysts.

Figure 4. CO2 desorption curves of MEA with and without catalysts as a function of time and temperature: 2 g of each catalyst was used. (a) CO2 desorption curves for the MEA blank test and WO3 and MoO3 catalysts. (b) CO2 desorption curves for the MEA blank test and Cr2O3, TiO2, and V2O5 catalysts.

Effect of the Catalyst/MEA Solvent Ratio. After obtaining positive results, it was necessary to investigate different MEA/catalyst ratios to study the effects of the catalyst amount. Therefore, different amounts of catalysts were used with a fixed amount of MEA solution. Two and 10 g of each catalyst were added with 100 mL of MEA, under the same conditions. The results are shown in Figures 4 and 5, respectively. As can be seen, the amount of the catalysts has a linear effect on the CO2 stripping rate and the total CO2 desorbed amount. The CO2 desorbed during the temperature ramp-up stage for MoO3 and V2O5 increased significantly because both types of acid sites are present on the surface of these two catalysts. Therefore, by using a higher amount of the catalysts, concentration of Lewis acids and Bronsted acids was increased. Hence, more collisions with carbamate and a consequent reduction of the activation energy of CO 2 desorption reaction is the reason for the improved CO2 stripping rate. In the case of TiO2, Cr2O3, and WO3, the CO2 desorption rate and the total amount of desorbed CO2 improved because of the presence of more Lewis acids in the

with a decrement in CO2 loading in the lean loading region where less carbamates were present in the system. The higher CO2 desorption rate and consequent large amount of desorbed CO2 is attributed to the change in the mechanism of CO2 desorption by the presence of Lewis and Bronsted acid sites. One important observation from catalytic regeneration is that CO2 is desorbed at much higher rates during the temperature ramp-up stage. However, as the experiment moves toward completion, MEA alone desorbs an almost equal or greater amount of CO2 as compared with catalytic systems. There are two reasons for this trend: 1) the presence of a larger number of acid sites is more effective in the rich loading region, because more carbamates are present, and thus more collisions and protonation of carbamate are possible. As a result of this, the CO2 desorption rate improved significantly; 2) because catalysts do not alter the reaction thermodynamics, rather the reduced activation energy of amine regeneration reaction results in faster CO2 stripping. Therefore, in a long run, an equal amount of CO2 would be desorbed from the catalytic and noncatalytic systems. 5865

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Figure 5. CO2 desorption curves of MEA with and without catalysts as a function of time and temperature: 10 g of each catalyst was used. (a) CO2 desorption curves for the MEA blank test and WO3 and MoO3 catalysts. (b) CO2 desorption curves for the MEA blank test and Cr2O3, TiO2, and V2O5 catalysts.

system. One difference between the catalysts with both types of acid sites and one type of acid site present on the catalyst surface is, with the presence of Bronsted acids, CO2 is desorbed at a much lower temperature as compared with only Lewis acid catalyst systems. MoO3 exhibited the best performance followed by V2O5 with a 45% and 28% increment in the CO2 desorption peak heights, respectively. Most importantly, the maximum amount of CO2 stripped from the MoO3 and V2O5 systems was at 78 and 84 °C, which are well below the conventional MEA regeneration temperature. This implies that catalysts with Bronsted acid sites are more effective for regeneration of amine solution at lower temperatures. Evaluation of Catalytic Solvent Regeneration Performance. The amount of total desorbed CO2 is one of the performance indicators in solvent regeneration experiments and is calculated by using eq 2 nCO2 = (αrich − αlean) ·C·V

Figure 6. Percent increment in the CO2 stripping amount from all five catalysts as compared with the noncatalytic system.

(2)

where nCO2 is the amount of CO2 desorbed in moles, and αrich and αlean are CO2 loading (mole CO2/mole MEA) before and after the experiment, respectively. C and V are the concentration and volume of the amine solutions in mol/L and L, respectively. A comparison of the amine regeneration in terms of the total amount of the CO2 stripped from noncatalytic and catalytic systems during the experiments is presented in Figure 6. The percent increment of the amount of CO2 stripped from all catalytic systems is shown, and the blank test is used as a reference. It can be seen that by using different amounts of catalysts, MoO3 and V2O5 stripped a significantly greater amount of CO2, 50−83 and 50−95%, respectively. The other three Lewis acid catalysts, Cr2O3, TiO2, and WO3, also desorbed up to 44% more CO2 during the experiment. Rate of CO2 Production. With the same amount of thermal energy provided, a higher rate of CO2 desorption results in smaller heat duty. The CO2 desorption rate was calculated by using eq 3 r=

(αrich − αlean) t

respectively, and t is the duration of the experiment in hours. The results are presented in Figure 7. The results prove that the addition of Bronsted acid (proton donor) or a Lewis acid

(3)

where αrich and αlean (mole CO2/mole MEA) are the CO2 loading of the MEA solution before and after the experiment,

Figure 7. CO2 desorption rate of the noncatalytic system and different amounts of each catalyst. 5866

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CONCLUSIONS In this work, the effects of five metal oxide catalysts were studied in the CO2-rich MEA solvent regeneration process. It has been observed that the acid sites present over transition metal oxides surfaces play an important role when used as a catalyst in the MEA solvent regeneration process. All of the catalysts improved the MEA solvent regeneration by stripping larger amounts of CO2 with faster desorption rates as compared with the noncatalytic MEA regeneration system. However, catalysts with both types of acid sites over them − MoO3 and V2O5 − had an exemplary effect on solvent regeneration as they desorbed a 94% and 84% greater amount of CO2 than the blank test, respectively. On the other side, catalysts with only Lewis acid sites − Cr2O3, TiO2, and WO3 − desorbed an almost 44% greater amount of CO2 than the blank test. Thus, it is clear that the presence of Bronsted acid sites is more effective in solvent regeneration especially in the rich loading region. Overall, during the experiments, a 1.4−2 times greater amount of CO2 was desorbed, at much faster rates, than was possible using a conventional noncatalytic stripping system which highlights the possibility of running the strippers at lower temperatures with metal oxide catalysts. Undesirably, some catalysts were dissolved in the MEA solvent which were then recovered by lowering the pH of the solvent. Moreover, we have witnessed that catalyst stability improves when supported on some other material. It would be valuable work to investigate the catalyst stability and effects of different support materials on catalysts’ stability, which will be carried out in our future work.

(electron acceptor) catalyst has a significant impact on the reaction rate. With the addition of 2 g of catalyst, the CO2 desorption rate was improved by 26−46%. Likewise, a 29−84% and 43−105% improvement of the CO2 desorption rate was observed with the addition of 5 g and 10 g of the catalysts, respectively. Heat Duty Reduction. In light of the obtained results, it is obvious that by using metal oxide catalysts, the MEA solvent could be regenerated at comparatively lower temperatures than used with conventional strippers. Conventional MEA strippers operate at temperature ≥120 °C, whereas metal oxide catalytic amine regeneration can be performed at ≤105 °C. Therefore, a considerable reduction in heat duty is possible due to a reduction in the use of sensible heat and heat of vaporization, because less steam is required for the catalytic stripping system. Sensible heat is directly proportional to the temperature difference of Treb and Tfeed. Thus, if we could lower the reboiler operating temperature and strip more CO2, we could significantly decrease the use of sensible heat. Moreover, under these conditions, less steam is required to maintain the temperature profile in the stripper column because the stripper would be running at lower temperature. Therefore, the heat of vaporization would also decrease, and this will cause a decrement in heat duty of amine regeneration. The sensible heat reduction can be calculated using eq 4 Q sen = Cp

Treb − Tfeed Msol 1 . . Δα MCO2 xsolv

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(4)

where Cp is the heat capacity of the rich solvent, Treb and Tfeed are the temperatures of reboiler and rich lean inlet, Δα is the difference between the rich and lean loading (mole CO2/mole amine), xsolv is the molar fraction of the MEA solvent, and Msol and MCO2 are the molar masses of MEA solution and CO2, respectively. Here, Treb is assumed to be 105 °C, and this is a reasonable assumption for a metal oxide catalytic regeneration system. The Cp values used for the MEA 30% solvent were reported by Weiland et al.33 The results are presented in Figure 8. The results indicate that by using catalysts with both types of acid sites present i.e. MoO3 and V2O5, 44−48% energy could be saved in terms of Qsen, while by using only a Lewis acid catalyst i.e. TiO2, Cr2O3, or WO3, 25−30% of Qsen could be reduced.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00604. Effects of the catalysts in CO2 absorption, catalyst recycling, and characteristics of metal oxide catalysts; XRD patterns and structural properties of the catalysts, total amount of CO2 gas (moles) desorbed from the catalytic and noncatalytic solutions during the experiments (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-42-860-3648. Fax: +82-42-860-3134. E-mail: [email protected]. ORCID

Il Hyun Baek: 0000-0002-6589-9737 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Education, ICT & Future Planning) (NRF2014M1A8A1049260).

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REFERENCES

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Figure 8. Sensible heat reduction by using different amounts of catalysts. 5867

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Research Article

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DOI: 10.1021/acssuschemeng.7b00604 ACS Sustainable Chem. Eng. 2017, 5, 5862−5868