Novel promoters for carbon dioxide absorption in potassium carbonate solutions By Guoping Hu, BEng, MEng Submitted in total fulfilment of the requirements of the degree of Doctor of Philosophy January 2018 Department of Chemical Engineering Melbourne School of Engineering The University of Melbourne Australia
Abstract Carbon dioxide is a major driver for climate change and carbon capture and storage (CCS) is widely recognized as an effective way to reduce CO2 emissions to mitigate climate change. However, managing the cost of carbon capture to an acceptable level is of vital importance to deploy it at an industrial scale. Potassium carbonate solvent (K2CO3) shows promise as a solvent for carbon capture due to its low cost, low corrosivity, low degradation rates and low environmental impact. However, as the absorption rate of CO2 using K2CO3 is relatively slow, improving its absorption kinetics via adding rate promoters is crucial for reducing the capital cost of absorption equipment required to build the carbon dioxide capture plant. In this study, a series of promoters were investigated to improve the absorption kinetics. Different promoters including organic promoters, inorganic promoters and enzymatic promoters have been reported in the literature. From the literature review, a good promoter should be economically acceptable, stable, non-toxic, non-corrosive, highly efficient, environmentally benign, recyclable, and have a low vapour pressure. It was recommended that more efforts should be focused on carbonic anhydrase enzyme and different amino acids, which is the focus of the present study. A carbonic anhydrase (NZCA) was first examined as a promoter in potassium carbonate solutions. The catalysis kinetics of this promoter were tested via the stopped flow technique and a wetted wall column (WWC). The Michaelis-Menten catalysis parameter (kcat/km) was determined to be 2.7 10 7 M -1 s -1 at 298 K, resulting an activation energy of 51±1 kj/mol at 298 328 K. The promoting coefficient of the NZCA was determined to be 5.3 10 7 M -1 s -1 using a WWC in 30 wt. % potassium carbonate solutions (ph ~ 11 12) at 323 K. Furthermore, the NZCA kept more than 70% I
of its initial catalysis efficiency after continuously running for 8 hours in 30 wt. % K2CO3 solutions at ph of 10.6 10.8 and temperature of 323 K. Then, histidine was investigated as a promoter for CO2 absorption as this is an important component in carbonic anhydrase. Results showed that histidine anion ions (His ) are the main species reacting with CO2 in basic conditions (ph>9) with a reaction order of 1.18±0.08 across the temperature range of 298 313 K. The zwitterion mechanism was used to fit the kinetic data and it showed that both protonation and deprotonation reactions contributed to the overall reaction rate. Ionic strength was also shown to have a significant influence on the reaction kinetics when the histidine concentration is high ( 0.2 M). The reaction rate between histidine and CO2 was shown to be slower than that of glycine and proline and slightly faster than that of taurine at low concentrations (<0.1 M). A range of different amino acids were next investigated as promoters. The amino acids investigated in this study were 2-piperazinecarboxylic acid, asparagine, aspartic acid, glycine, leucine, lysine, proline, sarcosine, serine and valine. Furthermore, proline, sarcosine, glycine, leucine and lysine were tested as rate promoters in potassium carbonate solvent for carbon dioxide absorption using a wetted wall column. Results showed that the anions of the amino acid salts are the major species reacting with carbon dioxide. Therefore, the promoting effect of amino acid salts is sensitive to changes in ph due to changes in species distribution. Sarcosine and proline were the most effective promoters among the amino acid salts tested in this study with comparable promoting performance at higher ph values ( 12.5) but with sarcosine more effective at lower ph values (<12.5). Compared to 0.5 M monoethanolamine (MEA), sarcosine and proline showed faster rate promotion effects for carbon dioxide absorption into 30 wt% potassium carbonate II
solvents at high ph (>12.0), while the promoting performance of MEA was comparable with that of proline and slightly poorer than that of sarcosine at low ph (<12.0) conditions. Lastly, a carbonic anhydrase mimicking polymer was synthesized and characterized as a catalyst for the CO2 hydration reaction. Results showed that the lower critical solution temperature (LCST) of PNiPAm-co-CyclenZn is 33.7 o C which is close to the physiological temperature. Above the LCST, PNiPAm-co-CyclenZn undergoes a phase transition from a swollen hydrated state to a shrunken dehydrated state. This property can potentially enable easy separation of PNiPAm-co- CyclenZn from the CO2 loaded solution exiting the absorber column so that it does not enter the high temperature stripping column. In the reaction between CO2 and H2O, the catalysis coefficient at 298 K of PNiPAm-co-CyclenZn was determined to be 380±20 M 1 s 1 at a ph of 7.36 and 2330±40 M 1 s 1 at a ph of 9.06. Arrhenius fitting of the catalysis coefficients showed an activation energy of 60±2 kj/mol at ph of 7.36. This study presents the first example of a temperature responsive polymeric catalyst for carbon dioxide absorption. Results from this study can guide recommendations for choosing promoters for industrialized CO2 capture process using a potassium carbonate aqueous solution and will allow for a CO2 capture process with lower costs. III
Declaration This is to certify that: i) The thesis comprises only my original work except where indicated in the preface; ii) iii) Due acknowledgement has been made in the text to all other material used; The thesis is fewer than 100 000 words in length, exclusive of tables, maps, bibliographies and appendices.. Guoping Hu November 2017 IV
Preface Results in Chapters 2, 4, 5, 6, 7 have been published in peer reviewed journals (listed below) and the contents have been modified to fit the purpose of this thesis. Chapter 2 Guoping Hu, Nathan Nicholas, Kathryn Smith, Kathryn Mumford, Sandra Kentish, Geoff Stevens. Carbon dioxide absorption into promoted potassium carbonate solutions: A review. International Journal of Grenhouse Gas Control, 2016 (53), 28 40. Guoping Hu, Kathryn Smith, Yue Wu, Sandra Kentish, Geoff Stevens. Recent progress on the performance of different rate promoters in potassium carbonate solvents for CO2 capture. Energy Procedia, 2017 (114), 2279 2286 Chapter 4 Guoping Hu, Kathryn Smith, Nathan Nicholas, Joel Yong, Sandra Kentish, Geoff Stevens. Enzymatic carbon dioxide capture using a thermally stable carbonic anhydrase as a promoter in potassium carbonate solvents. Chemical Engineering Journal, 2017 (307), 49 55. Chapter 5 Guoping Hu, Kathryn Smith, Liang Liu, Sandra Kentish, Geoff Stevens. Reaction kinetics and mechanism between histidine and carbon dioxide. Chemical Engineering Journal, 2017 (307), 56 62. Chapter 6 Guoping Hu, Kathryn Smith, Yue Wu, Sandra Kentish, Geoff Stevens. Screening amino acid salts as rate promoters in potassium carbonate solvent for carbon dioxide absorption. Energy & Fuels, 2017 (31), 4280 4286 V
Chapter 7 Guoping Hu, Zeyun Xiao, Kathryn Smith, Sandra Kentish, Luke Connal, Geoff Stevens. A carbonic anhydrase inspired temperature responsive polymer based catalyst for accelerating carbon capture. Chemical Engineering Journal, 2018 (332), 556 562 VI
Acknowledgements This work presented in the thesis was conducted with the assistance of a number of people and organizations to whom I would like to express my sincere gratitude: Professor Geoff Stevens, Professor Sandra Kentish, Dr. Kathryn Smith and Dr. Nathan Nicholas, for their supervision, advice, encouragement and support throughout the project. Dr. Gabriel Da Silva, my advisory committee chair, for his suggestions and support. Department of Chemical Engineering, the CO2 solvent group members Frank Wu, Dr. Nouman Mirza, Indrawan, Alita Aguiar, Siming Chen, Thomas Moore and Dr. Kathryn Mumford, engineering workshop member Justin Fox, general office staff Dr. Michelle de Silva, Tabitha Cesnak, Cara Jordan and Louise Baker, academics and fellow postgraduates Dr. Qi Zheng, Sam Law, Fan Wu, Hiep Lu, April Li, Hongzhan Di, Dr. Yong Wang, Dr. Jinguk Kim, Dr. Zheng Li, Dr. Shinji Kanehashi, Dr. Lina Wang, Dr. Liang Liu and Dr. Colin Scholes etc., who provided assistance and support throughout the project and my study, either spiritual or knowledgable. My collabrators, Dr. Joel K. Yong, Dr. Zeyun Xiao and Dr. Luke Connol, for providing research materials and inspiring discussions. Peers and senior researchers for hosting my visit and inspiring discussions on scientific research topics. Particulate Fluids Processing Centre, a Special Research Centre of the Australian Research Council, for infrastructure and funding support. Peter Cook Centre for CCS research and CO2CRC, for infrastructure support and funding support for travelling. VII
Melbourne School of Engineering, for financial suport to travelling. The University of Melbourne, for financial support during my study. The Gordon Conference Committee, for providing travel funds sponsoring my attendancy to the conference. Finally to all my friends and family not mentioned above, who were always there to provide support and encouragement, through the many chanllenges and triumphs encountered while completing this project. VIII
Table of Contents Abstract... I Declaration... IV Preface... V Acknowledgements... VII Table of Contents... IX List of Figures... VII List of Tables... XI Nomenclature... i Chapter 1 Background...- 1-1.1 Reducing carbon dioxide from the atmosphere...- 1-1.2 Techniques for carbon dioxide reduction...- 2-1.2.1 Solvent absorption for capturing carbon dioxide...- 3-1.2.2 Sorbent adsorption for capturing carbon dioxide...- 4-1.2.3 Membranes for capturing carbon dioxide...- 5-1.2.4 Mineral carbonation for capturing carbon dioxide...- 6-1.2.5 Cryogenics distillation...- 7-1.2.6 Others...- 8-1.3 Potassium carbonate solvent absorption system for carbon dioxide capture...- 8-1.4 Current challenges with the potassium carbonate absorption process...- 10-1.5 Aim of this study...- 11 - Chapter 2 Literature Review...- 12-2.1 Inorganic Promoters...- 12-2.1.1 Arsenite...- 12-2.1.2 Boric acid...- 13-2.1.3 Vanadate...- 16-2.1.4 Other inorganic promoters...- 17-2.2 Organic Promoters...- 18-2.2.1 Amines...- 18-2.2.1.1 Monoethanolamine...- 18 - IX
2.2.1.2 Diethanolamine... - 25-2.2.1.3 Piperazine... - 25-2.2.1.4 Other amines... - 27-2.2.2 Amino acid salts... - 27-2.3 Enzymatic promoters... - 29-2.3.1 Carbonic anhydrase... - 29-2.3.2 Metal compounds mimicking carbonic anhydrase... - 32-2.4 K2CO3 pilot plant studies with rate promoters... - 34-2.5 Lessons learnt from the literature... - 35-2.5.1 Promoting mechanisms... - 35-2.5.2 Comparison of different promoters and remarks... - 37 - Chapter 3 Experimental... - 41-3.1 Stopped flow technique... - 41-3.1.1 Stopped flow... - 41-3.1.2 Stopped flow methods... - 45-3.1.3 Stopped flow validation... - 46-3.2 Wetted wall column technique... - 47-3.2.1 Wetted wall column... - 47-3.2.2 Wetted wall column methods... - 50-3.2.2.1 Gas film mass transfer coefficient... - 50-3.2.2.2 Liquid physical mass transfer coefficient... - 51-3.2.2.3 Enhancement factor... - 52-3.2.2.4 Surface renewal model... - 52-3.2.2.5 Pseudo first order reaction constant... - 53-3.3 Polymer Characterization... - 54-3.4 Materials... - 55 - Chapter 4 A Thermally Stable Carbonic Anhydrase as a Promoter in Potassium Carbonate Solvents for Carbon Dioxide Capture... - 58-4.1 Introduction... - 58-4.2 Results and discussion... - 59-4.2.1 Stopped flow experiments... - 59 - X
4.2.2 Wetted wall column experiments... - 65-4.2.3 Comparison of results from stopped flow and wetted wall column... - 68-4.3 Conclusions... - 69 - Chapter 5 Reaction Kinetics between Histidine and Carbon Dioxide... - 70-5.1 Introduction... - 70-5.2 Results and discussion... - 73-5.2.1 The reaction contribution of different histidine species... - 73-5.2.2 Determination of corrected reaction pseudo-first-order rate constants (kobs )... - 74-5.2.3 Zwitterion mechanism fitting with the experimental data... - 78-5.2.4 Influence of ionic strength on the reaction kinetics... - 80-5.2.5 Comparison of histidine with other amino acids... - 83-5.3 Conclusions... - 84 - Chapter 6 Screening of Amino Acids as Promoters for CO2 Absorption... - 86-6.1 Introduction... - 86-6.2 Results and discussion... - 87-6.2.1 Speciation and reaction kinetics of amino acid salts with CO2... - 87-6.2.2 Promotion performance of amino acid salts in potassium carbonate solvent... - 90-6.2.3 Effect of ph on the absorption kinetics... - 92-6.2.4 Comparison of amino acids and monoethanolamine (MEA) as rate promoters for CO2 absorption in potassium carbonate solvent... - 94-6.3 Conclusions... - 95 - Chapter 7 A Carbonic Anhydrase Mimicking Polymer for Accelerating Carbon Capture... - 96-7.1 Introduction... - 96-7.2 Results and discussion... - 98-7.2.1 Synthesis and characterization... - 98-7.2.1.1 4-Vinylbenzyl Cyclen... - 99-7.2.1.2 PNiPAm-co-Cyclen... - 104-7.2.1.3 PNiPAm-co-CyclenZn... - 104-7.2.1.4 LCST Determination... - 107-7.2.2 Carbon dioxide hydration catalysis efficiency of PNiPAm-co-cyclenZn... - 110-7.3 Conclusions... - 113 - XI
Chapter 8 Conclusions and Recommendations...- 115-8.1 Conclusions...- 115-8.2 Recommendations...- 117 - References...- 119 - XII
List of Figures Figure 1.1. Technologies for carbon dioxide capture...- 3 - Figure 1.2. A solvent absorption process for CO2 capture (picture sourced from CO2CRC)..- 4 - Figure 1.3. A typical sorbent adsorption process for capturing CO2 (picture sourced from CO2CRC)...- 5 - Figure 1.4. Membrane separation mechanisms (picture sourced from CO2CRC)...- 6 - Figure 1.5. A carbonation process reported by Wang et al. [55]...- 7 - Figure 1.6. A simplified diagram of cryogenics distillation...- 8 - Figure 1.7. A flow diagram of a traditional absorption process...- 9 - Figure 2.1. Structural formula of arsenite ions...- 13 - Figure 2.2. Simplified equilibrium diagram for borate speciation [18, 112, 113]...- 14 - Figure 2.3. Mechanism for borate-catalysed hydration of CO2 proposed by Guo et al. [101]- 15 - Figure 2.4. Simplified equilibrium diagram for vanadium (V) speciation in basic to ph neutral water [96]...- 17 - Figure 2.5. Kinetics research results for the reaction CO2-MEA...- 20 - Figure 2.6. Promoting mechanism of MEA for CO2 absorption in potassium carbonate solutions...- 22 - Figure 2.7. Structures of piperazine in aqueous solutions...- 26 - Figure 2.8. Simplified equilibrium diagram for PZ speciation...- 26 - Figure 2.9. Different ligands used for mimicking carbonic anhydrase...- 33 - Figure 2.10. Pilot plant results with different promoters compared with unpromoted solvent[85] (A: 35 wt. % K2CO3 with L/G of 4; B: 35 wt. % K2CO3 with 3% boric acid; C: 36 wt. % K2CO3 with 9% glycine with L/G of 3; D: 40 wt. % K2CO3 with 10% glycine with L/G of 5; E: 41 wt. % K2CO3 with 9.1% glycine with L/G of 4)...- 35 - Figure 2.11. Intermediate formulas of different promoters [101, 152, 159, 175, 180]...- 37 - Figure 3.1. Schematic of stopped-flow technique[194]...- 42 - Figure 3.2. SX.17 MV flow diagrams...- 43 - Figure 3.3. Experimental absorbance versus time for CO2 hydration at different CO2 concentrations (wavelength of 400 nm, temperature of 303 K and ph of 7.5)...- 46 - VII
Figure 3.4. The structure diagram of the wetted wall column used in the study...- 48 - Figure 4.1. Turnover numbers (a) and Michaelis-Menten constants (b) for NZCA (298 328 K) - 61 - Figure 4.2. Catalysis scheme of CO2 hydration by carbonic anhydrase[99, 217]...- 62 - Figure 4.3. Effect of ph on the NZCA activity at 298 K...- 64 - Figure 4.4. Effect of CO2 loading and ionic strength on the catalysis efficiency of the NZCA at 298 K...- 65 - Figure 4.5. Promotion effect of the NZCA for CO2 absorption with 30 wt. % K2CO3 solvents (0.04 0.32 loading) at 323 K using WWC...- 66 - Figure 4.6. Thermal stability of the NZCA in 30 wt. % K2CO3 (0.1 loading) at 323 K...- 68 - Figure 5.1. Transformation among different forms of histidine...- 71 - Figure 5.2. Distribution of Histidine species under different acidity at 298 K[230]...- 71 - Figure 5.3 Distribution of histidine formations at different temperatures (a: 298 K, b: 303 K, c: 308 K, d: 313 K)...- 73 - Figure 5.4. Corrected pseudo first order reaction rate constants at different ph and temperatures...- 74 - Figure 5.5. Corrected pseudo first order reaction rate constant between CO2 and His at the temperatures of 298 313 K...- 75 - Figure 5.6. Double log coordinate plot of observed pseudo-first-order rate constants versus the concentration of His -...- 76 - Figure 5.7. Determination of reaction constant to His at different temperatures...- 77 - Figure 5.8. Arrhenius plot of the reaction of His with CO2...- 78 - Figure 5.9. Zwitterion mechanism fitting of the reaction between CO2 and His -...- 79 - Figure 5.10. Comparison of extrapolating results in this study with experimental results by Shen et al.[229] at high histidine concentrations...- 81 - Figure 5.11. Comparison of extrapolating results from this study with WWC results from literature[229] using a b value of 0.44 representing the ionic strength impact...- 82 - Figure 5.12. Comparison of results extrapolated from stopped flow experiments using b=0.67 at 298 K, b=0.65 at 303 K, b=0.46 at 313 K to correct for ionic strength with experimental WWC results by Shen et al.[229] at high histidine concentrations... - 83 - VIII
Figure 5.13. Comparison of kinetics results between amino acids and CO2 at low ionic strength (<0.05 M)...- 84 - Figure 6.1. Transformation of different species of amino acid salts with ph...- 88 - Figure 6.2. Distribution of valine ionic species at various ph values...- 88 - Figure 6.3. Reaction rate between CO2 and amino acid salt solutions (~5 mm) at neutral (7.3±0.2) and basic ph values (around pka values, lysine: ph~pka1, lysine*: ph~pka2) at 298 K...- 89 - Figure 6.4. Pseudo first order reaction constants between different amino acid anions and CO2: Lysine # is the lysine species with negative two valency while all other amino acids have negative one valency (the results for glycine agree with previous research[193], while the data for histidine was extracted from our previous research[242])...- 90 - Figure 6.5. Enhancement factors using 30 wt. % potassium carbonate solvents with and without amino acid salts (0.5 M) in a WWC at ph of 12.5 and temperature of 323 K...- 91 - Figure 6.6. Enhancement factors using 30 wt. % potassium carbonate solvents with and without amino acid salts (0.5 M) in a WWC over a range of ph values at 323 K...- 93 - Figure 6.7. Enhancement factors using proline, sarcosine and MEA (0.5 M) as promoters in 30 wt% potassium carbonate solvent. Results were obtained using a WWC over a range of ph values at 323 K...- 94 - Figure 7.1. Proposed mechanism for the hydration of CO2 by carbonic anhydrase.[243]...- 97 - Figure 7.2. Synthesis of the cyclenzn pendant PNiPAm and the small molecule of cyclenzn- 99 - Figure 7.3. 1 H NMR of 4-vinylbenzyl cyclen...- 101 - Figure 7.4. 13 C NMR of 4-vinylbenzyl cyclen...- 102 - Figure 7.5. Electrospray ionization-mass spectrometry (ESI-MS) of 4-vinylbenzyl cyclen - 103 - Figure 7.6. 1 H NMR spectra of PNiPAm-co-cyclen and PNiPAm-co-cyclenZn. The proton signals from the cyclen moieties are enlarged...- 105 - Figure 7.7. SEC diagram of PNiPAm-co-Cyclen...- 106 - Figure 7.8. ICP-OES measurement at the wavelengths of 202.548 and 206.200 nm with four standard solutions of 0, 4, 10, 20 ppm (9.34 mg of PNiPAm-co-CyclenZn dissolved in 10 ml solution)....- 107 - IX
Figure 7.9. (a) LCST study of the PNiPAm-co-cyclenZn in water (10 mg/ml). (b) Variable temperature 1 H NMR of PNiPAm-co-cyclenZn in D2O. As the temperature increases, the polymer separated from the solution as evidenced by the loss of signal....- 109 - Figure 7.10. Arrhenius fitting of Michaelis-Menten catalysis coefficients of the PNiPAm-co- CyclenZn...- 111 - Figure 7.11. Activity of the PNiPAm-co-CyclenZn catalyst for CO2 hydration reaction showing the thermal stability and recyclability. Each cycle represents a catalytic assay after heating the polymer catalyst to 328 K and then cooling and repeating the kinetic assay at 298 K. No measurable decrease in activity of the polymer catalyst was observed....- 113 - X
List of Tables Table 2.1. Some amine promoters in potassium carbonate solutions...- 18 - Table 2.2. Kinetic research about CO2 absorption in MEA aqueous solutions...- 21 - Table 2.3. Promoting performance of amines in potassium carbonate solutions...- 24 - Table 2.4. Promotion performances of different AAS under different conditions...- 29 - Table 2.5. Promotion performances of carbonic anhydrase under different operating conditions - 30 - Table 2.6. Comparison of different inorganic promoters... - 38 - Table 2.7. Kinetics data of different promoters from the literature...- 39 - Table 3.1. Buffers and indicators used in this study...- 45 - Table 3.2. Information on reagents used in this work...- 56 - Table 4.1. Catalysis coefficient of the NZCA at different ph values...- 60 - Table 4.2. Comparison of carbonic anhydrase catalytic coefficients for CO2 hydration...- 63 - Table 5.1 Thermodynamic properties of histidine...- 72 - Table 5.2. Reaction rate constants with respect to His at different temperatures...- 77 - Table 6.1. pka values of amino acid salts at 298 K in diluted solutions...- 87 - Table 7.1. Comparison of catalysis coefficients for PNiPAm-co-CyclenZn and other carbonic anhydrase mimics... - 112 - XI
Nomenclature General notations a Regression constant A Central column part of the WWC Aa m 2 Contact area b mm Optical pathlength B Absorption chamber of the WWC c Regression constant C Bathing chamber of the WWC d m Diameter DCO2 m 2 s 1 Diffusivity of CO2 E Enhancement factor Ea kj mol 1 Activation energy f Regression constant g m s 2 Gravity constant G m 3 s 1 Gas volumetric flow rate h m Height of WWC HCO2 Pa m 3 mol 1 Henry constant k Reaction constant kb kb-x Rate constant of deprotonation reaction Rate constant for deprotonation by x kcat s 1 Turn over number kcat/km M 1 s 1 Catalysis efficiency kg KG mol Pa 1 m 2 s 1 gas mass transfer coefficient mol Pa 1 m 2 s 1 Overall mass transfer coefficient kl o m s 1 Liquid phase physical mass transfer coefficient i
Km M Michaelis-Menten constant kobs s 1 Observed first order reaction rate constant kobs ' s 1 Corrected observed first order reaction rate constant L m Length of wetted wall column NCO2 mol m 2 s 1 Absorption flux P * CO2, b Pa CO2 equilibrium partial pressure PCO2, b Pa CO2 partial pressure in the gas phase Q Buffer factor Ql m 3 s 1 Liquid flowrate R J mol 1 K 1 Gas constant r m s 1 Reaction rate Re Sc Sh Reynolds number Schmidt number Sherwood number T K Temperature t s Reaction time Tr K Reference temperature v m s 1 Linear velocity of the gas V m 3 s 1 Θ Volumetric flowrate of the liquid W m Circumference of the column α Molar fraction Γ kg m 1 s 1 Mass rate of flow per unit width δ m Thickness of a layer ε M 1 cm 1 Extinction factor μg Pa s 1 Gas viscosity ρg kg m 3 Gas density ρl kg m 3 Liquid density τ s Surface contact time A dimensionless driving force ii
[CO2] M Concentration of CO2 [H2O] M Concentration of H2O [MEA] M Concentration of MEA rcp o kj mol 1 K 1 Heat capacity changes rg o kj mol 1 Standard molar Gibbs energy rh o kj mol 1 Standard molar enthalpy Abbreviations AAS Abs CA DEA His LCST MEA NG NZCA PZ WWC Amino acid salt Absorbance Carbonic anhydrase Diethanolamine Histidine Lower critical solution temperature Monoethanolamine Not given Novozymes carbonic anhydrase Piperazine Wetted wall column iii
Chapter 1 Background The emission of carbon dioxide into the atmosphere is recognized as a significant driver for climate change. Carbon capture and storage (CCS) techniques are efficient and effective ways to reduce carbon dioxide emissions to the atmosphere. However, the cost of any carbon capture technique has to be reduced to manageable levels before it can be deployed at an industrial scale[1]. 1.1 Reducing carbon dioxide from the atmosphere The total amount of carbon on earth is relatively constant and its distribution among the lithosphere, the atmosphere and the biosphere remained relatively constant until the industrial era[2]. The subsequent emission of CO2 to the atmosphere from human activities is recognized as the main reason for climate change including global warming, changes in sea levels, extreme hot summers and cold winter, and agricultural problems[3-7]. With an exponentially increasing global population, there are many basic human needs such as food, water and energy to be met, which may result in higher quantities of carbon dioxide being emitted into the atmosphere[8]. Therefore, there has been an increasing focus on the development of new energy resources as well as cleaner and more efficient energy systems to reduce overall carbon dioxide emissions[5], though there is still a long way to go before emission targets are met. Fossil fuels, coals and natural gas will remain as the main sources of energy in the near future as they remain cheap and abundant while also experiencing much security and stability in their utilization systems[9]. It is inevitable that the emissions of carbon dioxide to the atmosphere will continue, which will lead to even more climate change effects. Therefore, we must devise economical, stable, environmentally friendly ways to reduce these effects. - 1 -
1.2 Techniques for carbon dioxide reduction Many methods for the reduction of CO2 atmospheric levels or CO2 emissions have been investigated, such as re-forestation, ocean fertilization and CO2 mineral carbonation[10-14]. These processes are able to simultaneously capture and sequester CO2 simultaneously at a low energy cost. However, these processes alone are not efficient enough to significantly reduce the quantity of CO2 being emitted to limit climate change. Carbon dioxide capture and storage (CCS)[15] is an efficient way to reduce carbon dioxide emissions into the atmosphere. However, there is a high capture cost[16, 17] associated with these capture options and appropriate storage is also required for preventing the captured CO2 from entering the atmosphere[9]. Carbon capture from combustion processes can be classified into three configurations depending on at which stage the CO2 is being captured: pre-combustion, oxyfuel combustion and post-combustion. However, the capture technologies (Figure 1.1) are similar for all these configurations and include absorption, adsorption, membrane, cryogenic separation (a single process of capture and compression), mineralization and a combination of these techniques[18-25]. - 2 -
Figure 1.1. Technologies for carbon dioxide capture 1.2.1 Solvent absorption for capturing carbon dioxide The absorption of carbon dioxide into an aqueous solvent has been investigated for decades, and was first used for purifying gases, such as hydrogen gas, natural gas and synthesis gas[26], and more recently for reducing CO2 emissions[19]. A typical solvent absorption process is presented in Figure 1.2, in which a mixture gas flows through an absorber and CO2 is captured in the solvent, then the CO2 loaded solvent is heated to regenerate the solvents and obtain pure CO2. Many solvents have been investigated for their efficiencies in the absorption of CO2, including monoethanolamine (MEA), diamines and ternary amines[27], piperazine and its derivatives[28-30], ammonia[31], amino acid salts[32], ionic liquids[33, 34], and their blends[30, 35-37]. MEA is the most widely used solvent. However, the use of MEA brings about some disadvantages such as a high energy penalty for solvent regeneration, its high degradation rate and corrosivity [38-40]. Some research has been conducted from the perspective of reducing energy costs, such as utilising - 3 -
solar energy in power plants to supplement the total energy requirements[41]. However, much time is needed for scaling up this technique to the industrial level. In addition, the drawbacks of solvent degradation and corrosion have to be addressed as well. Figure 1.2. A solvent absorption process for CO2 capture (picture sourced from CO2CRC) 1.2.2 Sorbent adsorption for capturing carbon dioxide In an sorbent adsorption process, CO2 is adsorbed from a process stream using solid materials (Figure 1.3) and then released via a thermal swing (TSA) or pressure swing (PSA)[42]. Both physical adsorption and chemical adsorption can be used for CO2 capture and a range of sorbent materials have been investigated such as activated carbon[43], amine sorbents[44, 45], metal oxides[46], metal-organic frameworks (MOFs)[47] and carbonates[48]. - 4 -
Figure 1.3. A typical sorbent adsorption process for capturing CO2 (picture sourced from CO2CRC) 1.2.3 Membranes for capturing carbon dioxide Membrane separation is a technology that can selectively sieve different components from a mixture of gases and liquids via thin film materials. Membrane materials can be organic (ex. cellulose acetate, polysulfone and polyimide), inorganic (ex. ceramic and metallic membranes) or a mixture of both (ex. metal-organic framework (MOF) supported polymeric membranes)[49-51]. In a CO2 separation process, the major driving force is usually CO2 partial pressure (i.e. CO2 concentration). Permeability and selectivity are both important parameters in membrane - 5 -
technology. The separation mechanisms of gas molecules through a membrane can be categorised into five models: Knudsen diffusion, molecular sieving, solution-diffusion model (Figure 1.4), surface diffusion and capillary condensation[52]. Figure 1.4. Membrane separation mechanisms (picture sourced from CO2CRC) 1.2.4 Mineral carbonation for capturing carbon dioxide Mineral carbonation is a method for capturing and storing CO2 at the same time. Figure 1.5 showed a research using calcium oxide for capturing carbon dioxide to form calcium carbonate. Mineral carbonation can also provide a pathway for capturing CO2 and releasing the captured CO2 with a high temperature regeneration process in a process known as chemical looping[53]. The mineral materials for capturing CO2 can be mineral wastes (ex. metallurgy wastes) and metal oxides (ex. CaO, MgO or a mixture of both)[11, 54]. The major barrier for this technology is the slow reaction kinetics. However, there has been recent research on enhancing its kinetics via a carbonic anhydrase enzyme in aqueous solution[55]. - 6 -
Figure 1.5. A carbonation process reported by Wang et al. [55] 1.2.5 Cryogenics distillation Cryogenics (low temperature distillation) is a method to condense CO2 under low temperature to produce concentrated liquid CO2 for transport and storage, and the other gases (mainly N2) flow through to the atmosphere[23, 56]. The advantage of cryogenics distillation is that the CO2 can be captured and compressed in one step. - 7 -
Figure 1.6. A simplified diagram of cryogenics distillation 1.2.6 Others There are some other technologies reported for carbon capture including algae cultivation[57] or a combination of the different technologies mentioned above[58]. However, more efforts are needed to make these technologies competitive with thoes mentioned above (1.2.1 1.2.5). 1.3 Potassium carbonate solvent absorption system for carbon dioxide capture Potassium carbonate (potash solution) is a good solvent for carbon dioxide capture because of its low regeneration energy, low degradation and low corrosivity. It was first developed to absorb carbon dioxide as an impurity from synthesis gas, natural gas, hydrogen gas in a process known as the Hot Potassium Carbonate (Benfield) Process [59, 60]. - 8 -
8 7 2 3 4 5 1 6 Figure 1.7. A flow diagram of a traditional absorption process (1 Flue gas, 2 Absorber, 3 Reducing valve, 4 Flashing vessel, 5 Desorber, 6 Closed steam coil, 7, 8 Condenser) The potassium carbonate solution was widely used in later research on CO2 absorption[61] and a traditional absorption process is shown in Figure 1.7[60]. The main parts are the absorber and desorber. The flue gas is fed into the absorber counter-currently to the solvent for absorption. The loaded solvent is then sent into a desorber, where CO2 is stripped from the solvent by increasing the temperature and/or decreasing the pressure of the desorber. This desorbed CO2 will then be compressed and liquefied for storage or utilization, and the regenerated solvent can be channelled back to the absorber for reuse in the absorption process. - 9 -
1.4 Current challenges with the potassium carbonate absorption process Corrosion is an important problem caused by the acidic nature of the flue gas and from the degradation of solvent. This problem can be mitigated to some degree by adding corrosion inhibitors into solvents. Potassium dichromate[62], vanadium (V)[63], EDTA[64], CuCO3[63] and so on have been reported to act as corrosion inhibitors in different equipment. Another challenge is the degradation of solvents by forming heat stable salts such as potassium sulphate. Due to the low solubility of these heat stable salts, precipitation of the weakly soluble potassium sulphate can be employed to minimize the effect of these impurities when H2S and SOx are present in the flue gas[63]. The precipitation of potassium bicarbonate was also considered as a problem for leading to pipe blockages in the system. However, a novel precipitation technique for the absorption of CO2 with highly concentrated potash solution was proposed by Mumford et al.[65]. The crystallization of the solvent should be investigated in detail to manage possible problems caused by solids. The major shortcoming of the potassium carbonate absorption system is its slow reaction kinetics in comparison to the commonly used MEA absorption system. Research has indicated that physical mass transfer can be enhanced by the chemical reactions (Reactions 1.5.1 1.5.4) when the absorption temperature is higher than 318 K, but that the chemical reactions are not fast enough to be instantaneous even at a temperature of 378 K[66, 67], indicating that largely improving absorption kinetics cannot be obtained by solely increasing temperature. CO 2 (g) CO 2 (aq) CO 2 (aq) + OH (aq) HCO 3 (aq) (Fast) 1.5.1 HCO 3 (aq) + OH (aq) CO 2 3 (aq) + H 2 O(aq) (Instantaneous) 1.5.2 CO 2 (g) + H 2 O(aq) H 2 CO 3 (aq) (Slow) 1.5.3-10 -
H 2 CO 3 (aq) + OH (aq) HCO 3 (aq) + H 2 O(aq) (Instantaneous) 1.5.4 As can be seen from the reaction regime, reaction 1.5.1 is fast but not fast enough to be treated as instantaneous. When the ph of the solvent is greater than 9.0, Reaction 1.5.3 is negligible in comparison with Reaction 1.5.1[67-69], hence the rate limiting step of the absorption process is Reaction 1.5.1. Since Reaction 1.5.1 is not fast enough, the absorption kinetics is slow. Therefore, a tall absorber is needed to get high absorption efficiency, leading to a very high capital investment and operation penalty. Adding rate promoters and improving the mass transfer efficiency in the absorption column[70, 71] are recognised as good options for improving the slow kinetics. 1.5 Aim of this study The aim of this project is to accelerate the carbon dioxide absorption rate using potassium carbonate solvent via exploring different rate promoters. By doing this, the scale of carbon capture absorption columns can be potentially decreased and the overall costs for carbon capture can be minimized. - 11 -
Chapter 2 Literature Review A promoter can be classified into one of the three classes: inorganic, organic or enzymatic promoters. Much research has been conducted on the addition of promoters into potassium carbonate solutions for carbon dioxide absorption, such as hypochlorite[72], [73], bromine and hypobromite[74], sulphite, selenite and tellurate[75]. However, the most widely studied promoters in potassium carbonate solutions have been amines such as monoethanolamine (MEA), diethanolamine (DEA) and piperazine (PZ) [26, 76-81], amino acids [82-86], arsenious acid [87-90], boric acid [91-93], vanadates [94-96] and carbonic anhydrase [97-101]. 2.1 Inorganic Promoters 2.1.1 Arsenite Arsenious acid (H3AsO3) is one of the best promoters for hydration of CO2 and the promotion of CO2 absorption in potassium carbonate solutions. Arsenious acid has been researched widely because of its excellent promoting performance, high stability, favourable ionization constant, high solubility, availability and low cost [102-107]. It was used in industrial absorption of CO2 as a promoter in potassium carbonate solutions more than 50 years ago [88] and was also used as a promoter in amine and sodium carbonate-bicarbonate solutions [87-89]. A packed column for CO2 absorption using arsenite promoted potassium carbonate solution was designed by Kumar in 1989 [90]. The arsenite ion (Figure 2.1) is recognized as the promoting species as it has a single lone pair of electrons, which can serve to neutralize the Lewis acidity of CO2, and it also has a pyramidal structure similar to NH3, which allows for direct and facile interaction between CO2 and the base to form a CO2 base complex [67]. Almost all research performed has shown that arsenite is a good catalyst for CO2 hydration. However, as it is toxic and carcinogenic [83, 108, 109], it is no longer used as a promoter in commercial applications. - 12 -
Figure 2.1. Structural formula of arsenite ions 2.1.2 Boric acid Boric acid has been studied extensively in the laboratory as a promoter for CO2 absorption in potassium carbonate solutions as it is environmentally benign, economically affordable and tolerant to oxidative and thermal conditions. It also has no significant influence on the vapour liquid equilibria (VLE) of CO2 at low concentrations and no interaction with other minor components such as sulphur oxide in the flue gas [18, 69, 91-93, 109, 110]. In aqueous solutions, the speciation of boric acid is influenced by ph. At low concentrations, B(OH)4 is dominant in basic solutions (ph 9.3) [18, 93, 101, 111]. However, when the concentration of boric acid is higher than 0.025 M, polyborates will form (Figure 2.2) and the concentration of B(OH)4 may be restricted [18]. This is also shown in the study by Ahmadi et al. [91], in which the CO2 absorption did not change significantly beyond a certain boric acid concentration. - 13 -
Figure 2.2. Simplified equilibrium diagram for borate speciation [18, 112, 113] The promoting mechanism of boric acid was first proposed by Guo et al. (Figure 2.3) [101], in which the boric acid-water complexes deprotonate to form the active species B(OH)4 (Equation 2.1.1, Step 1, Figure 2.3), then B(OH)4 reacts with CO2 to form an intermediate B(OH)3 HCO3 (Equation 2.1.2, Step 2, Figure 2.3), and the HCO3 in the intermediate is replaced with water forming HCO3 and regenerating the promoter (Equation 2.1.3, Step 3, Figure 2.3). B(OH) 3 H 2 O B(OH) 4 + H + 2.1.1 B(OH) 4 + CO 2 B(OH) 4 CO 2 2.1.2 B(OH) 4 CO 2 + H 2 O B(OH) 3 H 2 O + HCO 3 2.1.3-14 -
Figure 2.3. Mechanism for borate-catalysed hydration of CO2 proposed by Guo et al. [101] However, when boric acid was used in a pre-combustion pilot plant demonstration in 2012 by Smith et al. [114], the promoting performance was not evident. This was attributed to the decrease of ph when adding boric acid to the potassium carbonate solution. It was reported [114] that the ph value of potassium carbonate solution with a loading of 0 decreased from 12.3 to 10.9 upon an addition of 3 wt.% boric acid. Therefore, the rate of Reaction 1.5.2 could counteract the improvements provided by boric acid when the loading is high. However, the reduction in ph due to the addition of boric acid can be overcome by the changing the addition of boric acid to its salts. Another possible reason is the CO2 solubility decrease in carbonate solutions due to the addition of boric acid[93], which will reduce the driving force for CO2 absorption, and therefore, influence the promoting performance of boric acid. - 15 -
2.1.3 Vanadate Vanadium (V) compounds were initially used as additives in amine systems to solve corrosion problems [115-117]. It was later found that vanadate also has a promoting effect on CO2 hydration [94] and was used as a promoter together with sodium or potassium borate in potassium carbonate solutions [110]. However, the addition of vanadate may reduce the CO2 solubility in potassium carbonate solution [95]. Recently, it was found that the promoting species of vanadium were HVO42 and HV2O73 (Figure 2.4) and the catalysing performance of both active species was comparable to MEA at low concentration [96]. The catalysis performance of HVO42 was more efficient than arsenites including HAsO3 2 and H3AsO3 [108]. However, as vanadium species are 2 comparatively sensitive to ph values and vanadium concentrations. The concentrations of HVO4 and HV2O73 decrease with a decrease in ph (due to CO2 being absorbed). Further, polyvanadates species forming as the total concentration of vanadium increases. This limits the effectiveness of vanadium (V) as it can only be used in small concentration, thus vanadium is not recognised as a suitable promoter in industrialized CO2 absorption process. Therefore, vanadium (V) is more suitable as a corrosion inhibitor rather than a rate promoter. - 16 -
Figure 2.4. Simplified equilibrium diagram for vanadium (V) speciation in basic to ph neutral water [96] 2.1.4 Other inorganic promoters Other inorganic promoters have been investigated for improving CO2 absorption rates in K2CO3 including phosphate and silicate [102], hypochlorite [88, 102], selenite and tellurate [102]. However, they all have various drawbacks including poor promoting performance, instability, corrosiveness and toxicity. Phan [108, 109] concluded that all species that feature O or OH groups, or that act as Lewis bases with CO2 as a Lewis acid, or that have a pyramidal or tetrahedral structure to facilitate the CO2 molecule approaching the base site, could potentially act as a catalyst. This is a similar conclusion to that of Dennard and Williams [72] who stated that the oxyanions of promoters should have a lone pair of electrons and have the ability to act as acceptors for promoting CO2 absorption. Most inorganic promoters are thermally stable and resistant to degradation, but they may switch between different speciation at different concentrations, temperatures or ph values. This is the - 17 -
main reason for some promoters such as boric acid and vanadate not demonstrating good performance over a range of industrial operating conditions. 2.2 Organic Promoters 2.2.1 Amines Research on the absorption of CO2 into amine promoted potassium carbonate mixtures was first conducted in 1967 by Danckwerts and McNeil [105]. Amines can be classified as primary, secondary and tertiary depending on the number and orientation of carbon atoms bonded to the amine functional group. Primary and secondary amines are usually used as promoters in potassium carbonate solutions, while tertiary functional group amines are seldom found as promoters as they do not have a significant promoting effect [69, 76, 118]. The most widely used amines in the literature are monoethanolamine (MEA), diethanolamine (DEA) and piperazine (PZ) (Table 2.1). Table 2.1. Some amine promoters in potassium carbonate solutions Amine Abbreviation Formula 2-aminoethanol MEA (monoethanolamine) 2,2'-iminodiethanol DEA (diethanolamine) Piperazine PZ 2.2.1.1 Monoethanolamine Monoethanolamine (MEA) has been the main solvent used for CO2 absorption for many years as it has fast absorption rate, high absorption capacity and good selectivity for CO2 [118, 119]. - 18 -
Research investigating the reaction kinetics between CO2 and MEA is summarized in Table 2.2. In the present case, recent kinetic data has been included and the techniques used to measure reaction kinetics have been provided. As shown in Figure 2.5 and Table 2.2, the kinetic results [76, 118-127] vary between different investigations. This discrepancy may result from the use of different physical properties, measurement techniques and reaction regime assumptions. However, the limitations associated with pure MEA as a solvent has led to the use of mixtures of potassium carbonate and MEA as an alternative solvent which can simultaneously, overcome the slow CO2 absorption kinetics of potassium carbonate solutions while minimising the drawbacks of MEA including evaporation and degradation[26, 120, 128]. MEA promotes the absorption of CO2 via two pathways. The first pathway is the increase in OH - concentration in the solvent when MEA is added and the carbamate forms. The second pathway is the zwitterion mechanism pathway (Figure 2.6), which is the dominant promoting mechanism for MEA. - 19 -
Figure 2.5. Kinetics research results for the reaction CO2-MEA According to published data [22, 129-131], MEA has been shown to have good absorption and promoting performance (Table 2.3). MEA reacts with CO2 in aqueous solution via the zwitterion mechanism (Figure 2.6) [76, 77], in which MEA first reacts with CO2 to form a zwitterion intermediate, and then the intermediate reacts with a base to form bicarbonate ions and regenerate MEA. In this reaction, the zwitterion formation (Step 1, Figure 2.6) is the rate-determining step as the zwitterion (HO(CH2)2HNH + COO, Figure 2.6) is not stable. - 20 -
Table 2.2. Kinetic research about CO2 absorption in MEA aqueous solutions CAmine /mol/l Amine Additives Apparatus Reaction order with respect to amine and CO2 T/K kmea/ L/(mol s) Reference 0.0091-0.06 MEA - Stopped flow method 1 278-303 log e k MEA = 25.53 5076 ± 180 T - MEA - Stirred cell 1 298-353 log e k MEA = 24.98 4775 T - MEA - Review 1 313K log e k MEA = 26.81 5400 T 0.013-1.5 MEA - Stopped flow method 1 278-298 log e k MEA = 27.47 5617 T 0-2.2 MEA 30 wt.% K2CO3 Wetted wall column 1 316-356 log e k MEA = 22.17 3825 T 0-0.4 MEA 1.5, 1.7 M AMP Wetted wall column 1 303-313 log e k MEA = 36.63 8534 T 0.1-0.5 MEA 0.5, 1.0 M TEA Wetted wall column 1 303-313 log e k MEA = 26.43 5376 T - MEA - Stirred cell 1 318-333 log e k MEA = 29.23 6354 T - MEA - Stirred cell 1 278-353 log e k MEA = 25.30 4955 T 3.0-9.0 Loaded MEA 1.0-5.0 Loaded MEA - Laminar jet 1 293-333 log e k MEA = 22.25 4412 T - Wetted wall column 1 298-343 * Superscript T based on concentration-based model; superscript γ based on activity based model * { log T ek MEA = 23.64 4112 T γ log e k MEA = 23.86 4742 T [127] [120] [118] [126] [76] [125] [124] [123] [119] [122] [121] - 21 -
Figure 2.6. Promoting mechanism of MEA for CO2 absorption in potassium carbonate solutions The promoting reaction rate can be determined by Equation 2.2.1, where r is the reaction rate, [CO2], [MEA] and [B] are the concentrations of CO2, MEA and basic species, respectively, kmea and k-mea are the rate constants of the reverse zwitterion formation reaction (Step 1, Figure 2.6), and kb is the rate constant of deprotonation reaction (Step2, Figure 2.6). r = k MEA [CO 2 ][MEA] 1+ k MEA k B [B] 2.2.1 B can be hydroxyl ions (OH ), MEA itself, and other basic species (such as H2O, HCO3, and CO3 2 in MEA promoted potassium carbonate solutions), by which the reactions are as follows (Equation 2.2.2 2.2.6), where k B x is the rate constant for deprotonation by x (H2O, MEA, HCO3, OH and CO3 2 ) and is a characteristic constant of solvent. HO(CH 2 ) 2 HNH + COO + H 2 O k B H2O HO(CH 2 ) 2 HNCOO + H 3 + O 2.2.2 HO(CH 2 ) 2 HNH + COO + CO 3 2 k B CO 3 2 HO(CH 2 ) 2 HNCOO + HCO 3 2.2.3 HO(CH 2 ) 2 HNH + COO + HCO 3 k B HCO 3 HO(CH 2 ) 2 HNCOO + H 2 CO 3 2.2.4 HO(CH 2 ) 2 HNH + COO + OH k B OH HO(CH 2 ) 2 HNCOO + H 2 O 2.2.5-22 -
AmH + COO + AmH k B MEA HO(CH 2 ) 2 HNCOO + HO(CH 2 ) 2 HNH 2 2.2.6 Therefore, the reaction rate can be written as Equation 2.2.7. r 1 = k MEA [CO 2 ][MEA] k 1+ Am1 k B H 2O [H 2 2O]+k B CO3 2 [CO 3 ]+kb HCO3 [HCO 3 ]+kb OH [OH ]+k B MEA [MEA] 2.2.7 As the zwitterion (HO(CH2)2HNH + COO, Figure 2.6) is not stable, the zwitterionic formation is the rate limiting step, 1 k MEA, thus, the overall reaction order dependency on amine and CO2 k B [B] is unity (Equation 2.2.8, n=1)[118]. However, Dugas[132] has reported a second order dependency on MEA (Equation 2.2.8, n=2) at greater amine concentration (7 M), in which 1 k MEA k B [B] and the deprotonation step can be the rate limiting step. r 1 = k MEA [CO 2 ][MEA] n 2.2.8-23 -
Table 2.3. Promoting performance of amines in potassium carbonate solutions Amines K2CO3 Amine Temperature Acceleration* Reference con. wt.% con. wt.% K MEA 1.8 0.5 291 0.2 [26] MEA 30 5 336 15 [76] MEA 30 10 336 45 [76] MEA 25 5 294 6 [78, 131] DEA 25 5 294 2.6 [78, 131] DEA NG NG 363 4-5 [133] DEA 20 2 353 1.6 [134] DEA 25 2 323-363 ~3 [135] DEA 25 5 323-363 ~6 [135] PZ 20 5 333 10 [64, 136] MDEA 25 5 294 1 [78, 131] *Acceleration = NG Not given CO 2 absorption rate in promoted K 2 CO 3 solutions CO 2 absorption rate in unpromoted K 2 CO 3 solutions at same absorption conditions 1 Amine promoted potassium carbonate solutions are the most mature promoted systems for CO2 absorption. They have been used in industrial plants for CO2 scrubbing of synthesis gas and pilot plants for CO2 capture from power plant flue gases [78, 137]. However, the addition of MEA increases the regeneration energy requirement for solvent regeneration and the degradation and corrosion problems still exist, thus, a search for alternative promoters is still required. - 24 -
2.2.1.2 Diethanolamine Diethanolamine (DEA) has also been widely studied as a promoter for CO2 absorption in potassium carbonate solutions [62, 77, 115, 120, 135, 138]. The kinetic data was well reviewed by Blauwhoff et al. [119] and Versteeg et al. [118], and is summarized in Table 2.3. As shown in Table 2.3, the CO2 absorption rate into potassium carbonate solutions can be enhanced by adding a small amount of DEA (2 wt.% to 5wt.%). Recently, a DEA promoted potassium carbonate solution was used for post-combustion CO2 capture in a tray column and showed good absorption performance [139]. The promoting pathways of DEA are often assumed to be similar to MEA with a reaction order of one with respect to CO2. However, the reaction order in terms of DEA is not always the same and research results showed that the reaction order can range between one and two under different conditions (OH and DEA concentrations) [118, 119]. The zwitterion mechanism is also widely used to interpret the kinetics data [140]. However, some researchers have proposed a termolecular mechanism for the interpretation of the kinetic data [141], in which the intermediate is not a zwitterion, but a loosely-bound complex [142]. 2.2.1.3 Piperazine Piperazine (PZ) has recently become very popular as a promoter due to its low vapour pressure, good promoting performance, low degradation and low corrosivity [136, 143-150]. It is reported to have better promoting performance than DEA [151] and be comparable to or even better than that of MEA [151]. Piperazine in aqueous solutions can have several forms (Figure 2.7 and Figure 2.8) depending on the ph value and CO2 concentration. These forms are protonated PZ (PZH + ), diprotonated PZ (H + PZH + ), PZ carbamate (PZCOO ), protonated PZ carbamate (H + PZCOO ) and PZ dicarbamate - 25 -