A Comparative Study of Aqueous Potassium Lysinate and Aqueous

Subscriber access provided by READING UNIV

Article

A Comparative Study of Aqueous Potassium Lysinate and Aqueous Monoethanolamine for Post-Combustion CO2 Capture Yue Zhao, Yangyang Bian, Hui Li, Hui Guo, Shufeng Shen, Jiangze Han, and Dongfang Guo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02800 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 8, 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.

Energy & Fuels 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 39

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

Energy & Fuels

A Comparative Study of Aqueous Potassium Lysinate and Aqueous Monoethanolamine for Post-Combustion CO2 Capture Yue Zhaoa, Yangyang Biana, Hui Lia, Hui Guoa, Shufeng Shena,*, Jiangze Hana and Dongfang Guob a

School of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology,

Shijiazhuang 050018, P.R. China b

Huaneng Clean Energy Research Institute, Changping District, Beijing 102209, P.R. China

ABSTRACT Aqueous monoethanolamine (MEA) is widely used for CO2 capture and has been demonstrated as an effective absorbent in many post-combustion capture plants. However, several disadvantages such as toxicity, high volatility, solvent degradation and high energy consumption were reported in the practical applications. Aqueous amino acid salts, particularly potassium lysinate (LysK), are considered as attractive alternative to MEA. A comparative study of absorbent characteristics of aqueous LysK (2.0M and 2.5M) and aqueous MEA (5.0M) was conducted in this study. The absorption and cyclic capacities, absorption and desorption rate, solubility of CO2 and heat of absorption were measured using a stirred batch-type reactor and a CPA201 reaction calorimeter under the similar post-combustion capture conditions. Thermal and oxidative degradation was also evaluated for aqueous 2.0M LysK at 383K and 423K under static N2 and O2 exposure conditions for 15 days. Samples were analyzed by total alkalinity and

13

C and 1H NMR spectra to provide insight into the

degradation products. The advantages of the comparable CO2 capture performance, high stability, and low solvent loss compared to the state-of-the-art solvent MEA, suggest LysK can be a potentially advantageous absorbent for industrial CO2 capture processes. Keywords: CO2 capture; Potassium lysinate; Monoethanolamine; Absorption; Desorption; Degradation

ACS Paragon Plus Environment

1

Energy & Fuels

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 39

1. INTRODUCTION The anthropogenic carbon dioxide (CO2) emissions from fossil fuel-fired industries contribute greatly to the increasingly worrying environmental impacts such as global warming.1,2 Post-combustion CO2 capture (PCC) has been proposed to be more suitable for the existing power-generation facilities that produce the largest amount of greenhouse gases emitted to the atmosphere. Capturing CO2 from the flue gas is technically difficult and expensive due to low CO2 partial pressures (about 5 – 15 kPa), which is considered as one of seven chemical separations to change the world.3 The widely accepted technique is solvent absorption using chemical absorbents that can react with CO2 in the flue gas to form CO2 rich solution in an absorber and then regenerate for solvent reuse by releasing CO2 with large energy consumption in a stripper.4,5 Accordingly, the performance of CO2 absorption and desorption is the most important factor for capture process. An absorbent with characteristics such as fast absorption rate, high absorption capacity, low heat of absorption, low solvent loss as well as better resistance to thermal and oxidative degradation, is highly desired. Nowadays, aqueous 30 mass % (5.0 M) monoethanolamine (MEA) is the most commercially applied and generally effective absorbent in many post-combustion capture plants due to its fast reaction rate with CO2.6 It is considered to be an industrial standard solvent to evaluate any potential absorbent. However, the use of MEA suffers several disadvantages such as high solvent loss due to high volatility and degradation at high temperature, toxicity, low saturated loading capacity (about 0.5 mol CO2/mol amine) and high regeneration energy consumption (typically 3.0–4.0 MJ/kg CO2).5–10 Therefore, solvent degradation and high-energy penalty related to the operating costs reminder researchers to undertake further improvement of this technology for a further large deployment.6,11,12 Recently, aqueous amino acid salts (AAS) have been considered as possible replacements to the conventional amine-based solvents for CO2 capture. Physicochemical properties, thermodynamics and kinetics of CO2 absorption in several AAS solutions have already been reported2,13–25 and a comprehensive list of aqueous AAS proposed for CO2 capture in the literature were summarized in our


2

Page 3 of 39

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

Energy & Fuels

previous work.15 Among the reported AAS absorbents, such as potassium salts of glycine, proline, lysine, sarcosine, in the literature,13–22 potassium lysinate (LysK, shown in Figure 1) was found to have higher equilibrium CO2 capacity and chemical reactivity toward CO2 than the industrial standard MEA and most of other amino acid salts.15,23–25 It has negligible vapor pressure at striping conditions due to its ionic nature. It was also generally reported that amino acid salts have more superior properties than MEA, such as better resistance to oxidative and thermal degradation and no environmental or toxic issues. However, in recent preliminary studies, some conflicting findings were reported that several AAS such as sodium of glycinate, sarcosinate, alaninate were found to have enhanced thermal degradation rates compared to MEA.26 Among these AAS studied, sodium sarcosinate had the slowest degradation rate, but the rate was still two times higher than that of MEA solvent. It was also pointed out that steric hindrance at the amine group plays a positive role in protecting the amino acid against degradation. LysK possesses an active α-amino group and a four-carbon aliphatic side chain with a ε-amino group, which involves different chemical structure from the reported amino acid salts. Therefore, further investigation and comparison with MEA at practical flue gas conditions needs to be performed before taking action to pilot demonstration step. To the best of our knowledge, detailed information of absorption features and degradation for aqueous LysK at CO2 capture conditions is still rare in literature and need to be revealed. In this study, we have designed batch experiments aimed at measuring the capture performance using aqueous 33.1 and 41.2 mass % LysK (2.0 M and 2.5 M, respectively) at post-combustion flue gas conditions. CO2 loading and cyclic capacity, the rates of CO2 absorption and desorption, the stability of solvent reuse and heat of absorption were determined. Thermal degradation was evaluated for aqueous unloaded LysK at 383K and 423K under static N2 and O2 exposure conditions. Samples of degradation and continuous cycle test were analyzed by total alkalinity and NMR spectra to provide insight into the degradation products. The results of these experiments were compared with those from the reference standard solvent, aqueous 30 mass % MEA.


3

Energy & Fuels

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 39

2. EXPERIMENTAL

2.1 Materials L-Lysine (Lys, ≥ 98% HPLC purity), MEA (99.1% GC purity), potassium hydroxide (KOH,

Semiconductor grade, 48.8 mass % aqueous solution) and 0.495 M sulfuric acid standard solution were purchased from Aladdin reagent, China, and used without further purification. The aqueous LysK solutions were prepared by neutralizing the lysine with an equimolar amount of KOH in a volumetric flask at 293 ± 1.0 K. Aqueous 30 mass % (about 5.0 M) MEA solutions that are the most common used in the industrial processes were also prepared and used for comparison in this study. An electronic analytical balance (OHAUS, CP214) was used for weight measurements with a precision of ± 0.1 mg. pH of solutions was determined by a Hanna HI2111 pH Meter (Hanna Instruments, Italy). N2 (99.99 vol %), O2 (99.5 vol %) and CO2 (99.995 vol %) were obtained from Shijiazhuang Xisanjiao oxygen generation station, China. Standard mixed gases, S1 (10.02 vol % CO2 using N2 as balance gas), were purchased from Nanjing Special Gas Factory Co., Ltd.. Pure N2, S1 and/or pure CO2 were used for calibration of non-dispersive infra-red (NDIR) CO2 gas analyzers (GXH-3011N, 0 – 20%, uncertainty 1.0% FS, Institute of Beijing Huayun Analytical Instrument; SKS-BA-CO2, 0 – 100%, uncertainty 1.0% FS, Guangdong Skesen Gas Detection Equipment Co., Ltd.). 2.2 Experimental setups and procedures

2.2.1 Absorption and desorption experiments The experiments were performed on a screening apparatus at near atmospheric pressure and temperatures in the range of 313K to 383K, as shown in Figure 2. This method can provide the information of the absorbents on the absorption capacity, the relative absorption and desorption rate, and cyclic capacity. For CO2 absorption experiment, N2 and CO2 flowing out of the compressed cylinders, controlled by mass flow controllers (D07-7C, Beijing Sevenstar, China, with ± 1.5% F.S. accuracy), are mixed in the mixing tank with a total flow rate of 540 mL/min. Then the gas mixture with CO2 partial pressure of about 13 kPa was sent to the reactor (a 250 mL flask) controlled at 313K


4

Page 5 of 39

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

Energy & Fuels

by thermostat bath (DF-101S, Yuhua, China, with ± 1.0% F.S. accuracy). The outlet gas stream was cooled by a Graham condenser (C544300, Synthware, Beijing). The CO2 concentration was determined by the CO2 gas analyzers (GXH-3011N) after an ice stainless cooler and a drying tube to eliminate water vapor. Aqueous unloaded LysK or MEA solution (100 mL) was preheated to 313K and quickly introduced into the reactor to start the absorption process, which was monitored by measuring the CO2 concentration of the outlet gas with time. The total absorption time was 2.5 hours. At this time, the CO2 partial pressure in the outlet gas stream can usually reach above 12.5 kPa. Liquid samples were taken for CO2 loading analysis. CO2 desorption apparatus is similar to that of CO2 absorption. A 100 mL flask was used and pure N2 with a flow rate of 200 mL/min was vented into the system. The temperature of regeneration was in the range of 363 to 383K. When the reactor was preheated to the desired temperature, 70 mL CO2-loaded solution was injected into the reactor to start the desorption process, and the CO2 concentration was monitored by the CO2 gas analyzer (SKS-BA-CO2) with time. The duration time was 1.5 hrs and then liquid samples were taken. 2.2.2 Absorption-desorption cycles In order to evaluate the reuse performance of the absorbents, the absorption-desorption cycles were carried out in a similar apparatus descried in Figure 2. The pure CO2 stream at a flow rate of 100 mL/min was bubbled through a 220 mL 2.0M LysK solution at 313K for 8.5 hrs. Then, the CO2-loaded solution were heated to 379 ± 1.0 K to release the captured CO2 in a 250 mL flask with a carry-over condenser (C351922, Synthware, Beijing) for 3.0 hrs to obtain a CO2 lean solution. One batch of complete cyclic experiment lasted about 12 hrs. The next absorption-desorption process started using CO2 lean loading of solution instead of zero CO2 loading. CO2 loading (α) and pH of solutions was measured before and after CO2 absorption of each cycle. 2.2.3 Solubility of CO2 Vapor–liquid equilibrium in CO2-LysK-H2O systems was performed in a thermostatted stirred cell reactor. A schematic diagram of the experimental set-up was presented in Figure 3. The method


5

Energy & Fuels

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 39

and procedure was similar to that for CO2 solubility measurement described in our previous work.24,27 The set-up consists of a vessel (Vv, 2.15 L) for storing the CO2 gas and a reactor (VR, 0.61 L) with pressure transducers (MIK-P300, 0 – 6 bar, MEACON China; GS4200-USB,0 – 6 bar, ESI). The temperatures (TV and TR) were recorded by PT-100 thermocouples (WZP-293) and a recorder (MIK200D) from MEACON China. In the run, a known mass and volume (Vs) of the absorbent solution was fed into the reactor and the system was allowed to come to the vapor-liquid equilibrium at a desired temperature. This gas pressure over the aqueous LysK solution was defined as the initial pressure P0. In these cases, the gas phase consists chiefly of solvent vapor and nitrogen in the reactor cell. Then, the CO2 was fed into the reactor and the total amount of added CO2 can be calculated from pressure changes (i.e. P1 down to P2) in the CO2 stored vessel by a modified Peng-Robinson equation of state. After the system can again reach VLE at which point the total pressure was called Pt. The CO2 partial pressure can be obtained (eq.1). By adding more CO2 into the reactor, new VLE in the reactor can be obtained. A set of VLE data, i.e. partial pressure of CO2 present in the gas phase vs the CO2 loading (α) in the liquid phase (eq.2), can be then obtained. PCO2 = Pt − P0

α=

(1)

VV  P1 P2  (VR − VS ) PCO2  − − RTV  z1 z2  zeq RTR

(2)

nLysK

where PCO2 is the partial pressure of CO2 present in the gas phase at vapor-liquid equilibrium. α is defined as the mole of CO2 per mole of LysK. z1 , z2 and zeq are the compressibility factors of gas at different conditions, respectively, which were calculated using the Peng-Robinson equation of state. 2.2.4 Heat of absorption Enthalpy measurements were performed using the CPA201 reaction calorimeter from ChemiSens AB, which can measure a true heat-flow from the reacting system. A schematic of the experimental setup used in the study is shown in Figure 4. A reactor of glass and stainless steel with an effective volume of 250 mL was used in a thermostated liquid bath during the experiments. The bottom plate of the reactor is the only path for heat transport, where a high precision Peltier element was used to


6

Page 7 of 39

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

Energy & Fuels

guarantee one-directional heat flow. Prior to each run, a known mass of solution (about 100 mL) was fed into the reactor and degassed under vacuum. The experiment was performed at a constant temperature until vapor-liquid equilibrium was reached. The vapor pressure of solvent in the gas phase of the reactor was assumed to be constant and equal to the total pressure before the injection of CO2. Approximately same amount of CO2 was added batch-wise via a BIOS mass flow controller into the reactor when equilibrium was achieved after the previous CO2 injection. The mass flow of CO2 and the heat flow from the reactor were logged continuously. An automation script was used to control the experiments, which can seek the stability in gas pressure and true heat-flow between each addition of CO2. When the deviation in the heat flow and pressure were within ± 0.02 W and ± 0.01 bar respectively within a period of 10 min, the system was considered to reach an equilibrium. The total amount of heat released during the absorption of CO2 was obtained by integration of the heat flow curve over the duration of the loading interval. The uncertainty in the amount of heat released was estimated to be ± 3.0%. 2.2.5 Thermal degradation evaluation The resistance to thermal degradation for chemical absorbents is a critical characteristic that helps to evaluate the potential absorbent for its operating costs, operational and environmental issues. Oxidative degradation is usually a slow process at absorption conditions, so extreme experimental conditions can speed up the degradation rates under high O2 concentration and high temperature to obtain results in a short-term. Thermal degradation experiments were carried out in PTFE tubes (about 50 ml) built in stainless steel cylinders. 20 ml of aqueous unloaded LysK solution was tested in each tube. Then different gases (i.e. N2 and O2) were injected into the tubes before closure to investigate the effect of oxygen-containing exposure. The sample tubes were then heated at 383K and 423K for 10, 120 and 360 h respectively. Tubes opened for sampling were not returned for further heating test. The total alkalinity of solutions, expressed as the consumed volume of sulfuric acid standard per kilogram of sample, was measured for the solutions after exposure to high temperature and compared to the as-prepared values. The measurements were performed by acid titration using an automatic potentiometric titrator (ZDJ-5, INESA Scientific Instrument Co., Ltd) and a standard equivalence


7

Energy & Fuels

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

point determination method. The 1H NMR and

13

Page 8 of 39

C NMR analysis was conducted for all samples in

deuterium oxide (D2O). 2.3. Analytical methods

Density and viscosity data of these absorbent solutions were obtained from a density meter (DMA 4100M, Anton Paar) with standard accuracy of ± 1.0 ×10-4 g·cm-3 and a digital rolling ball micro-viscometer (Lovis 2000ME, Anton Paar) with relative accuracy of ± 1.0 %. Based on the logged data of absorption process, the amount of CO2 in liter at standard temperature and pressure, VCO2, absorbed into the solutions at a given time is calculated by: In VCO2 = QCO − 2

Out QN2 yCO 2

(3)

Out 1 − yCO 2

where QCO2In and QN2 are the volumetric flow rates in standard liter per second (SLS) of CO2 and N2 fed into the reaction flask from mass flow controller, respectively and yCO2Out is the CO2 molar fraction in the outlet gas stream, which is logged as a function of time from the CO2 analyzer. Then the accumulated amount of CO2 in moles of CO2 , NCO2, absorbed into the absorbents can be obtained by integration over period of time, which can be expressed as: t

VCO2

0

22.40

N CO2 = ∫

(4)

dt

The absorption and desorption rate at a given time, rCO2, with units of mole /(kg solution• s), is calculated as:

rCO2 =

VCO2

(5)

22.40 × ms

where ms is the amount of CO2-unloaded (CO2-free) absorbent solution in the flask, kg. The CO2 cyclic capacity, αcyc, can be defined by the difference of CO2 rich loading after absorption and CO2 lean loading after desorption, with units in mole CO2 per mole absorbent or mole CO2 per kg CO2-free absorbent solution. The CO2 loadings can be obtained from the total amount of CO2 absorbed NCO2 by dividing with the total mass of absorbent solution or acid titration by a modified Chittick CO2 apparatus.24


8

Page 9 of 39

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

Energy & Fuels

α cyc = α rich -α lean

(6)

CO2 loading (α) of samples is defined in two ways. One is the moles of CO2 per mole of absorbent, mol CO2 / mole absorbent, the other is the moles of CO2 per kilogram of CO2-free absorbent solution for the samples, mol CO2 / kg. In the Chittick CO2 apparatus,24 the amount of captured CO2 in the sample of known volume and mass was measured by acid titration in a flask which is connected to a graduated gas measuring tube and an adjustable leveling bulb reservoir. The colored non-reactive liquid was kept at the same level to maintain the normal atmosphere pressure in the flask controlled by a digital pressure manometer (GM520, Shenzhen Jumaoyuan Science and Technology Co., Ltd.). Meanwhile, the concentrations of LysK and MEA were titrated using an automatic potentiometric titrator (ZDJ-5, INESA Scientific Instrument Co., Ltd). 1

H NMR and

13

C NMR experiments were carried out on a Bruker Avance 500 spectrometer. It

equipped with a 5 mm high-sensitivity triple-resonance TCI probe that maintained at 298.0 K. D2O was used for deuterium lock and deuterated (3-(trimethylsilyl)-2,2,3,3-tetradeuteropropionic acid sodium salt, TMSP-d4, was used as internal reference. The qualitative comparison for degradation evaluation was based on the reference NMR signals from the original prepared LysK solutions obtained with the same NMR method.

3. RESULTS AND DISCUSSION

3.1 CO2 capture performance with different absorbents The absorption and desorption profiles for three different absorbent solutions (i.e. 5.0M MEA, 2.0M and 2.5M LysK) as a function of time under the same conditions are presented in Figures 5 and 6. The absorption rate and desorption rate versus CO2 loading are also plotted in these Figures. Physicochemical properties such as density, viscosity and physical solubility of CO2 28–30 for these solutions were measured and listed in Table 1. Table 2 summarizes the CO2 absorption and desorption performance for aqueous LysK and MEA solvents in this work compared with the results from the references.31–33


9

Energy & Fuels

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 39

For absorption process, the CO2 concentration in the outlet gas stream is related to the removal efficiency, which reveals the absorption performance of any absorbent system. Generally speaking, the gas-liquid contacting time in a specific reactor corresponding to the removal efficiency higher than 80% (t80) is important reference data for comparison or absorber design. It can be observed from Figure 5a that the 5.0M MEA system maintained the highest removal efficiency and absorption rate in mole /(kg solution• s) during the first 80 min. t80 is about 60min, 70min and 75min for 2.0M, 2.5M LysK and 5.0M MEA, respectively, where all systems maintained high absorption rates. However, after 80 min, the 2.5M LysK system showed better absorption performance than 5.0M MEA (Figure 5 c). Moreover, the absorption rates for all systems decreased rapidly until the experiments were stopped. It was observed that the CO2 loadings turned stable and constant after 120 min for all systems. The CO2 loading from eq 4 for 2.5M LysK and 5.0M MEA was 2.47 and 2.56 mol CO2/kg solution respectively, which match well with the measured data within 3% error (Table 2). It is worth noting that the densities of the systems investigated are quite different, as shown in Table 1. The absorption rates of CO2 in mole /(L• s) were in the range of 48-51 x 10-5 for both 2.5M LysK and 5.0M MEA systems. Although the molar concentration of LysK is much lower than MEA, the comparable absorption capacity and absorption rate reveal that the 2.5M LysK system can be considered as an efficient absorbent. As can be seen from Table 2, the CO2 loading in mol CO2/mole absorbent for 2.5M LysK is double than that for 5.0M MEA, which was probably due to the structural feature of LysK with α-amino group and a ε-amino group in its side chain. For MEA, the formation of amine carbamate and protonated amine has a theoretical capacity of 0.5 mol CO2/mole absorbent, mainly due to low rate of hydrolysis of stable carbamate to bicarbonate. However, it was observed in LysK solution that the bicarbonate was formed as well as carbamate in a relatively low CO2 loading range and might become the main species at high CO2 loading.24 The strong basic amine group in side chain would be favorable for the carbamate hydrolysis and facilitate the CO2 absorption. Although LysK has high kinetics with CO2, which was reported in our previous work,15 the higher viscosity of its solution (3.5 mPa•s at 313K) could hurdle the mass transfer between the simulated gas and the liquid solution and then lower


10

Page 11 of 39

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

Energy & Fuels

the absorption rate. In the desorption experiments, CO2 desorption at 373K for the CO2 loaded MEA solution was measured and used as a reference curve for comparison, although 373K is lower than the traditional regeneration temperature (about 393K). Effect of temperature on the desorption efficiency was investigated for the LysK solutions. As can be seen from Figure 6, the CO2 loaded solutions can be heated rapidly up to the desired temperature within 10 min and then the flat-temperature zone was observed. It can be seen that CO2 concentration in the outlet gas stream increased rapidly to the peak and then decreased gradually with the elapsed time (Figure 6a). It was noted that CO2 was initially released and observed from the stripping gas stream when the temperature of solutions was above 353K. Then, when the temperature was up to the controlled temperature, the maximum CO2 concentration and desorption rate were obtained (Figure 6d). Similar behaviors were observed for these solutions: when CO2 loading decreases in the liquid phase, the desorption rate decreases. Moreover, with increasing the desorption temperature, the desorption rate increased and the more CO2 was released (Figure 6c). The calculated CO2 loadings from eq 4 matched well with the measured data for the resulting desorption solution. For 2.5M LysK, only 14.5% desorption efficiency was obtained at 367K. Desorption efficiency was found to be sensitive to the desorption temperature. CO2 loading changed from 2.425 to 1.188 mol CO2/kg solution at 379K, thus desorption efficiency could enhance up to about 51%, which is higher than that of the 5.0M MEA systems (about 43%) at 373K. It can be explained by the fact that at high temperature the generated water vapor can enhance CO2 stripping through the gas effect. The working capacity for 2.5M LysK and 5.0M MEA in these batch experiments was 1.237 and 1.079 mol CO2/kg solution respectively. The maximum desorption rate of CO2 was about 9.8 x 10-4 mole /(kg solution• s) for 2.5M LysK, which is greater than that of the 5.0M MEA systems. The decreasing desorption rate with the elapsed time can be explained by the decreasing driving force between the loaded solution and stripping nitrogen gas, since the equilibrium CO2 partial pressure decrease with the decreasing CO2 loading. 3.2 Continuous cycles of absorption-desorption experiment


11

Energy & Fuels

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 39

The feasibility of LysK absorbent was comprehensively investigated by means of several continuous cycles of absorption-desorption tests in order to give assessments of solvent behavior and CO2 cyclic capacity. As well known, chemical absorbents with high CO2 cyclic capacity as well as high absorption and desorption rates are favorable for large-scale applications. As the most commonly used absorbent, solvent behavior of 5.0M MEA has been extensively investigated in laboratory-scale and industrial application. In the practical post-combustion capture process (i.e. 10-15 mol% CO2 in the flue gas), the average CO2 rich and lean loadings are usually about 0.5 and 0.25 mol CO2/mole amine respectively. The maximum absorption capacity can reach about 2.5 mol CO2/kg fresh solution and the cyclic capacity is about 1.2 mol CO2/kg fresh solution. These values are roughly in agreement with the experimental data in this work and literature (Table 2) considering the industrial desorption temperature at about 393 K. Aqueous 2.0 M LysK solution was used to obtain the information about CO2 loading, cyclic capacity and pH of solution in eight runs of absorption-desorption at 313K-379K. The results were presented in Figure 7. The average cyclic capacity of 1.36 mol CO2/kg solution (~ 0.71 mol CO2/mole LysK) was seen, which is comparable with 5.0M MEA. It is clear that there is no significant change in solvent behavior after eight cycle runs when compared with the fresh absorbent, particularly considering the measuring uncertainty of CO2 loading and water loss during regeneration. It was noted that the absorption capacity has a little increase but the lean loading was kept relatively stable. This might be probably due to the fact that the volume of solution changed during the cycling runs due to sampling 8 mL each cycle. The cyclic capacity (△α) can also be estimated from the equilibrium solubility curves, which is defined as the difference of CO2 loadings under 10 kPa CO2 partial pressure at absorption and desorption temperature respectively. The comparison between LysK and MEA (data from literature 30) is presented in Figure 8. It can be seen that the solubility curves between 2.0M and 2.5M LysK overlapped each other at 313K and 383K in the CO2 partial pressure range investigated, indicating the little effect of LysK concentration on the CO2 solubility on the basis of mol CO2/mole LysK. The equilibrium–based method usually gives a much higher value for cyclic loading than batch


12

Page 13 of 39

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

Energy & Fuels

experimental data because of unlimited time to reach absorption equilibrium at the desired temperature. The estimated cyclic capacity in mol CO2/mole absorbent shows good agreement with the batch and multiple cycling runs for both absorbent systems. The comparable cyclic capacity in mol/kg solvent between LysK and MEA suggests LysK can be a promising alternative for industrial application. 3.3 Heat of absorption The enthalpy change (-∆Habs, kJ/mol CO2) can be directly measured by a reaction calorimetry when CO2 is absorbed into a liquid absorbent. In this work, differential heat of CO2 absorption (-∆Hdiff, kJ/mol CO2) has been determined using a CPA201 calorimeter at 313K by dividing the heat released from each addition of CO2 by the moles of CO2 absorbed during each addition. The heat of absorption of CO2 for 30 mass % MEA solution was measured and used as a reference case to compare with the literature and the LysK system investigated in this work. The results for 30 mass % MEA at 313K along with the literature data35,36 are shown in Figure 8a. As can be seen that the measured data fairly match with the reference data from Kim35and Luo36 when CO2 loading is above 0.2 mol CO2/mole amine, but show a little higher than the literature in the low CO2 loading range. The -∆Hdiff value of aqueous 30 mass % MEA was 84.1 kJ/mol CO2 at CO2 loading = 0.32. Furthermore, with increasing CO2 loading, the amount of heat released following CO2 addition became smaller. The heat of absorption of CO2 with 32 wt % LysK at various CO2 loading were obtained and graphical representations are shown in Figure 8b. In general, the enthalpy change is fairly constant, 110 ± 5 kJ/mol CO2, till a CO2 loading of about 0.8 is reached, which is higher than that for 30 mass % MEA. In our previous experiment,15 aqueous fresh LysK solution showed faster absorption kinetics, resulting in more stable carbamate. Therefore, it is not unexpected that a higher absorption heat than that of MEA. However, from the critical loading of about 0.8, a rapid decrease in the enthalpy value is visible with increasing CO2 loading. The -∆Hdiff value is about 60 kJ/mol CO2 at CO2 loading = 1.20. This phenomenon indicates that there are more than one ongoing reactions in aqueous LysK solution. According to

13

C NMR analysis in the CO2-loaded solutions,24 three new

additional carbon signals were observed in the chemical shift range of 160 - 165 ppm, which are


13

Energy & Fuels

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 39

assigned to the formation of two carbamate species (163 - 165 ppm) and bicarbonate (~ 160 ppm). It was also observed that the relative intensity among the new peaks changed greatly by increasing the CO2 loading from 0.3 to 1.4 mol CO2 /mol LysK. These suggest that the main species formed in the LysK-CO2-H2O systems might change from carbamate at low CO2 loading to bicarbonate/carbonate at high CO2 loading. As a result, the observed heat of absorption should be composed of the formation enthalpy of both carbamates and bicarbonate, and the former is higher than the latter. An approximately linear decrease was observed when CO2 loading is greater than 0.8. The heat of absorption of CO2 can also be estimated from vapor-liquid equilibrium (VLE) data by application of a simplified Gibbs-Helmholtz relationship ignoring the variation of the ratios of mole fractions and activity coefficients with temperature.   ∂ ln PCO2  ∂ 1 T 

( )

  = −∆H diff  R α

(7)

where PCO2 denotes the equilibrium partial pressure of CO2 in the gas over the 2.0M LysK solution, R is the universal gas constant and T is the temperature. As pointed out by Kim and Svendsen,35 this relatively simple method is proved to be helpful in an initial phase of the solvent selection and evaluation, but does not give an accurate description of the differential heat of CO2 absorption. With new VLE data this work and available data in our previous work24 for 32 wt % LysK at the temperature range of 313-383K, a semi-empirical model was obtained with R2 = 0.986.

ln PCO2 = 35.748 −

17990.1 α − 1.0439α − 8.5645α 2 + 9974.85 T T

(8)

Comparison of differential heat of absorption from calorimetric measurements with that obtained from solubility model is also shown in Figure 8b. The -∆Hdiff estimated from the solubility model for 32 wt % LysK gives a lower value over the entire loading range investigated within 25% difference from the direct measurement. 3.4 Degradation analysis by NMR spectroscopy As an industrial standard absorbent, thermal and oxidative degradation of MEA has been studied


14

Page 15 of 39

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

Energy & Fuels

extensively at laboratory, pilot and industrial scales and has been reviewed in detail.7-10 The main degradation products are volatile amines, aldehydes, and carboxylic acids, which are formed by two radical mechanisms: electron transfer or hydrogen abstraction.10 Although a low concentration of O2 (usually 3-5 vol %) is in the post-combustion flue gases, organic acids and heat-stable salts have been measured and reported in a pilot-scale CO2 capture plant.8 These oxidative degradation products can accumulate in the absorbent, probably resulting in foaming and corrosion in industrial columns and environmental issues due to waste disposal. It was also reported by Davis and Rochelle37 that thermal degradation rate of MEA roughly increase by a factor of four with a 17K increase in temperature or a doubling of the stripper pressure. At 408K, the degradation rate is about 2.5-6% per week. It is believed that the increase in O2 concentration in the flue gas and absorption temperature accelerates the MEA degradation. The MEA concentration had a reduction of about 12% using 6 v/v% O2 at 323K, whereas at 343K, about 10% reduction after 120 hours and about 30% reduction after 360 hours.38 Recently, sodium of glycinate, sarcosinate and alaninate were reported to have enhanced thermal degradation rates compared to MEA.26,39 The resistance of aqueous LysK solution to thermal and oxidative degradation is a critical characteristic that can help to evaluate its potential use for post-combustion capture. Since degradation is a relatively slow process under the post-combustion capture conditions, higher temperature (i.e. 423K) and O2 concentration (i.e. 100% O2 injection) than in the real industrial plant were applied in order to reduce the experiment time within 15 days. Thermal degradation was qualitatively studied to understand the effect of temperature without any reactive gas (CO2 or O2). Oxidative degradation was done in the similar conditions as thermal degradation using O2 instead of N2. Aqueous 2.0M LysK solutions were tested at 383K and 423K under different gas exposure. Samples were taken at 10, 120 and 360 h respectively for determination of total alkalinity and NMR spectroscopy. The results are shown in Table 3, Figures10 and 11. It was observed that the total alkalinity of the samples under nitrogen exposure at 383K had negligible change within 120 h. The similar results were seen even at higher temperature (423K). However, under O2 exposure over the absorbents at 383K, about 3% reduction in alkalinity was


15

Energy & Fuels

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 39

observed for 120 h and about 9% reduction for 360 h. The variation in total alkalinity increases with the increasing temperature and increases approximately linearly with exposure time. The total alkalinity decrease about 12% at 423K for 360 h. These suggest that carboxylic acids, aldehydes, ammonia or cyclic compounds might be formed as the oxidative degradation products for aqueous LysK solution as reported for aqueous MEA solution.10 Moreover, aldehydes are easily oxidised into acids even with air only. Thus, these possible degradation products could result in a decrease in total alkalinity and pH value. 13

C and 1H NMR experiments, shown in Figures 10, 11 (a-e), can qualitatively identify the change

of species in aqueous LysK solutions under various conditions. In these figures, A and B are referred as N2 and O2 exposure conditions, respectively.

13

C and 1H NMR spectra (a) were obtained for fresh

solutions as prepared in D2O. For the CO2-free LysK solution, carbon signals (C1-C6) appeared at 186.4, 58.7, 43.2, 37.3, 34.4 and 25.1 ppm. The chemical shifts of hydrogen attached to C2-C6 are identified at 3.23, 2.62, 1.58, 1.45 and 1.33 ppm. As it can be seen that, under N2 exposure conditions (A), the NMR signals did not change positions and no additional signals were observed in the 13C and 1

H NMR spectra within 120 h, even at 423K. These indicate that the thermal degradation of LysK is a

very slow process, and thus, the concentrations of degradation products are low or lower than the detection limit of NMR spectra analysis, which is consistent with the findings of total alkalinity determination. The stability of LysK absorbent was also investigated at the continuous absorption-desorption (313K-379K) runs. The CO2-lean samples were collected at the 4th and 8th runs for 13C NMR. In order to compare with the fresh absorbent, the sample from the 8th run was treated with acid to remove the absorbed CO2 and then added KOH to obtain the deprotonated LysK. The results are shown in Figure 12. The similar results were observed in the series runs for the CO2-lean samples. The peaks of bicarbonate and carbamate species are assigned in the chemical shift range of 160 – 170 ppm. The absorbent was kept stable after eight-cycle runs from 13C NMR spectra analysis. Under O2 exposure conditions (B), “doubled” carbon signals were found for heating 120 h at 423K, whereas only one additional peak at 3.00 ppm in 1H NMR spectra was observed. It was also noted that the intensity of C5 signal weakened greatly and a weak carbon signal at 167.7 ppm that is


16

Page 17 of 39

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

Energy & Fuels

assigned to the formation of carbamate appeared in the samples. Increasing the exposure time to 360 h, the 13C NMR signals became complex and strong change in chemical shift were also seen for 1H NMR spectra, which suggested that severe degradation happened at these conditions, resulting in a decrease in

total

alkalinity.

Possible

2-amino-hexano-6-lactam,

degradation

lysine

products,

dimmer,

as

shown

ammonia,

in

Figure

13,

including

5-aminopentanamide

and

6-Amino-2-oxohexanoate, were proposed by preliminary analysis from the NMR spectra. Apparently, further work needs to be done in this area in order to identify and quantify the degradation species by HPLC-MS and ion chromatography. It should be pointed out that these results were obtained under extreme O2 exposure conditions with high temperature and high O2 partial pressure. In practical, oxidative degradation usually happens in absorber conditions, i.e. 313K and 3-5 vol % O2, which are far away from the test conditions. Slow degradation rate is expected for LysK compared with MEA that has about 12% reduction of concentration under 6 vol % O2 at 323K for 120 h.

4. CONCLUSION

A relative comparison of the CO2 capture potentials of aqueous LysK and aqueous MEA was performed under the same experimental conditions. It was found that the CO2 absorption capacity about 2.5 mol CO2/kg solution, and absorption rates of CO2, 4.5 x 10-4 mole /(kg • s), for 2.5M LysK were comparable with 5.0M MEA. The working capacity for 2.5M LysK and 5.0M MEA in these batch desorption experiments was about 1.2 and 1.1 mol CO2/kg solution respectively, and the observed maximum desorption rate of CO2 was about 9.8 x 10-4 mole /(kg • s) for 2.5M LysK at 379K, which is greater than that of the 5.0M MEA systems at 373K. The average cyclic capacity of 1.36 mol CO2/kg solution (about 0.7 mol CO2/mole LysK) was obtained from continuous absorption-desorption cycles, which fairly match with the results based on equilibrium–based method. Moreover, absorption capacity and solvent behavior after eight cycle runs have no significant change. The -∆Hdiff value of 32 wt % LysK is about 110 ± 5 kJ/mol CO2 at CO2 loading below 0.8 mol CO2/mole LysK. A rapid decrease in the enthalpy value is visible when CO2 loading is greater than 0.8. These can be explained


17

Energy & Fuels

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

by the observed fact from

13

Page 18 of 39

C NMR spectra that heat of absorption should be composed of the

formation enthalpy of both carbamates and bicarbonate. Qualitative analysis from total alkalinity and NMR spectra at 423K for 120h shows that thermal degradation of LysK is a very slow process. Even under extreme O2 exposure conditions, only weak visible signals appeared at 423K for 120h. Increasing the exposure time to 360 h, severe degradation was observed. The advantages of the comparable CO2 capture performance, high thermal stability, and low solvent loss in comparison with MEA, suggest LysK can be applied for CO2 capture as a potentially advantageous absorbent.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected], [email protected]; Tel.: +86 311 88632183. Fax: +86 311 88632183. Funding The authors acknowledge Hebei Provincial Natural Science Foundation for Distinguished Young Scholars of China (B2015208067), and Hebei Provincial Science and Technology Research Project of College and University (QN2015070) for financial support. Notes The authors declare no competing financial interest.


18

Page 19 of 39

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

Energy & Fuels

REFERENCES (1)

Idem, R.; Supap, T.; Shi, H.; Gelowitz, D.; Ball, M.; Campbell, C.; Tontiwachwuthikul, P. Practical experience in post-combustion CO2 capture using reactive solvents in large pilot and demonstration plants. Int. J. Greenh. Gas Control. 2015, 40, 6–25.

(2)

Shen, S.; Yang, Y. Carbon dioxide absorption into aqueous potassium salt solutions of arginine for post-combustion capture. Energy Fuels. 2016, 30, 6585–6596.

(3)

Sholl, D. S.; Lively, R. P. Seven chemical separations to change the world. Nature. 2016, 532, 435–437.

(4)

MacDowell, N.; Florin, N.; Buchard, A.; Hallett, J.; Galindo, A.; Jackson, G.; Adjiman, C. S.; Williams, C. K.; Shah, N.; Fennell, P. An overview of CO2 capture technologies. Energy Environ. Sci. 2010, 3, 1645–1669.

(5)

Mumford, K. A.; Wu, Y.; Smith, K. H.; Stevens, G. W. Review of solvent based carbon-dioxide capture technologies. Front. Chem. Sci. Eng. 2015, 9, 125–141.

(6)

Liang, Z.; Rongwong, W.; Liu, H.; Fu, K.; Gao, H.; Fan, C.; Zhang, R.; Sema, T.; Henni, A.; Sumon, K.; Nath, D.; Gelowitz, G.; Srisang, W.; Saiwan, C.; Benamor, A.; Al-Marri, M.; Shi, H.; Supap, T.; Chan, C.; Zhou, Q.; Abu-Zahra, M.; Wilson, M.; Idem, R.; Tontiwachwuthikul, P. Recent progress and new developments in post-combustion carbon capture technology with amine based solvents. Int. J. Greenh. Gas Control. 2015, 40, 26–54.

(7)

Vega, F.; Sanna, A.; Navarrete, B.; Maroto-Valer, M. M.; Cortés, V. J. Degradation of amine-based solvents in CO2 capture process by chemical absorption. Greenhouse Gas Sci. Technol. 2014, 4, 707–733.

(8)

Reynolds, A. J.; Vincent Verheyen, T.; Adeloju, S. B.; Chaffee, A. L.; Meuleman, E. Monoethanolamine degradation during Pilot-Scale Post-combustion Capture of CO2 from a brown coal-fired power station. Energy Fuels. 2015, 29, 7441–7455.

(9)

Lepaumier, H.; Picq, D.; Carrette, P. L. New amines for CO2 capture. П. Oxidative degradation mechanisms. Ind. Eng. Chem. Res. 2009, 48, 9068–9075.

(10) Gouedard, C.; Picq, D.; Launay, F.; Carrette, P. L. Amine degradation in CO2 capture. I. A review. Int. J. Greenh. Gas Control. 2012, 10, 244–270. (11) Wang, L.; An, S.; Li, Q.; Yu, S.; Wu, S. Phase change behavior and kinetics of CO2 absorption into DMBA/DEEA solution in a wetted-wall column. Chem.Eng. J. 2017, 314, 681−687. (12) Wang, L.; An, S.; Yu, S.; Zhang, S.; Zhang, Y.; Li, M.; Li, Q. Mass transfer characteristics of CO2 absorption into a phase-change solvent in a wetted-wall column. Int. J. Greenh. Gas Control. 2017,64, 276−283. (13) van Holst, J.; Versteeg, G. F.; Brilman, D. W. F.; Hogendoorn, J. A. Kinetic study of CO2 with various amino acid


19

Energy & Fuels

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 39

salts in aqueous solution. Chem. Eng. Sci. 2009, 64, 59–68. (14) Muñoz, D. M.; Portugal, A. F.; Lozano, A. E.; de la Campa, J. G.; de Abajo, J. New liquid absorbents for the removal of CO2 from gas mixtures. Energy Environ. Sci. 2009, 2, 883–891. (15) Shen, S.; Yang, Y.; Bian, Y.; Zhao, Y. Kinetics of CO2 absorption into aqueous basic amino acid salt: potassium salt of lysine solution. Environ. Sci. Technol. 2016, 50, 2054 –2063. (16) Portugal, A. F.; Derks, P. W. J.; Versteeg, G. F.; Magalhães, F. D.; Mendes, A. Characterization of potassium glycinate for carbon dioxide absorption purposes. Chem. Eng. Sci. 2007, 62, 6534–6547. (17) Guo, D.; Thee, H.; Tan, C. Y.; Chen, J.; Fei, W.; Kentish, S. E.; Stevens, G. W.; da Silva, G. Amino acids as carbon capture solvents: chemical kinetics and mechanism of the glycine + CO2 reaction. Energy Fuels. 2013, 27, 3898–3904. (18) Sodiq, A.; Rayer, A. V.; Olanrewaju, A. A.; Abu Zahra, M. R. M. Reaction kinetics of carbon dioxide (CO2) absorption in sodium salts of taurine and proline using a stopped-flow technique. Int. J. Chem. Kinet. 2014, 46, 730–745. (19) Majchrowicz, M. E.; Kersten, S.; Brilman, W. Reactive absorption of carbon dioxide in L-prolinate salt solutions. Ind. Eng. Chem. Res. 2014, 53, 11460–11467. (20) Aronu, U. E.; Hartono, A.; Hoff, K. A.; Svendsen, H. F. Kinetics of carbon dioxide absorption into aqueous amino acid salt: potassium salt of sarcosine solution. Ind. Eng. Chem. Res. 2011, 50, 10465–10475. (21) Lim, J.; Kim, D. H.; Yoon, Y.; Jeong, S. K.; Park, K. T.; Nam, S. C. Absorption of CO2 into aqueous potassium salt solutions of L-alanine and L-proline. Energy Fuels. 2012, 26, 3910–3918. (22) Shen, S.; Yang, Y.; Zhao, Y.; Bian, Y. Reaction kinetics of carbon dioxide absorption into aqueous potassium salt of histidine. Chem. Eng. Sci. 2016, 146, 76–87. (23) Shen, S.; Yang, Y.; Wang, Y.; Ren, S.; Han, J.; Chen, A. CO2 absorption into aqueous potassium salts of lysine and proline: density, viscosity and solubility of CO2. Fluid Phase Equilibria. 2015, 399, 40−49. (24) Shen, S.; Zhao, Y.; Bian, Y.; Wang, Y.; Guo, H.; Li, H. CO2 absorption using aqueous potassium lysinate solutions: Vapor - liquid equilibrium data and modelling. J. Chem. Thermodynamics. 2017, 115, 209–220. (25) Zhao, Y.; Shen, S.; Bian, Y.; Yang, Y.; Ghosh, U. CO2 solubility in aqueous potassium lysinate solutions at absorber conditions. J. Chem. Thermodynamics. 2017, 111, 100–105. (26) Huang, Q.; Bhatnagar, S.; Remias, J. E.; Selegue, J. P.; Liu, K. Thermal degradation of amino acid salts in CO2 capture. Int. J. Greenh. Gas Control. 2013, 19, 243–250. (27) Bian, Y.; Zhao, Y.; Shen, S. Characteristics of potassium prolinate + water + ethanol solution as a phase changing absorbent for CO2 Capture. J. Chem. Eng. Data. 2017, 62, 3169–3177.


20

Page 21 of 39

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

Energy & Fuels

(28) Bian, Y.; Shen, S.; Zhao, Y.; Yang, Y. Physicochemical properties of aqueous potassium salts of basic amino acids as absorbents for CO2 capture. J. Chem. Eng. Data. 2016, 61, 2391–2398. (29) Mandal, B. P.; Madhusree, K.; Bandyopadhyay, S. S. Physical solubility and diffusivity of N2O and CO2 into aqueous solutions of (2-Amino-2-methyl-1-propanol + monoethanolamine) and (N-methyldiethanolamine + monoethanolamine). J. Chem. Eng. Data. 2005, 50 , 352–358. (30) Lee, J. I.; Otto, F. D.; Mather, A. E. Equilibrium between carbon dioxide and aqueous monoethanolamine solutions. J. Appl. Chem. Biotechnol. 1976, 26, 541–549. (31) Luo, X.; Liu, S.; Gao, H.; Liao, H.; Tontiwachwuthikul, P.; Liang, Z. An improved fast screening method for single and blended amine-based solvents for post-combustion CO2 capture. Sep. Pur. Technol. 2016, 169, 279–288. (32) Hartono, A.; Vevelstad, S. J.; Ciftja, A.; Knuutila, H. K. Screening of strong bicarbonate forming solvents for CO2 capture. Int. J. Greenh. Gas Control. 2017, 58, 201-211. (33) Li, K.; Cousins, A.; Yu, H.; Feron, P.; Tade, M.; Luo, W.; Chen, J. Systematic study of aqueous monoethanolamine-based CO2 capture process: model development and process improvement. Energy Sci. Eng. 2016, 4, 23–39. (34) Li, T.; Keener, T. C. A review: Desorption of CO2 from rich solutions in chemical absorption processes. Int. J. Greenh. Gas Control. 2016, 51, 290–304. (35) Kim, I.; Svendsen, H. F. Heat of absorption of carbon dioxide (CO2) in monoethanolamine (MEA) and 2-(aminoethyl)-ethanolamine (AEEA) solutions. Ind. Eng. Chem. Res. 2007, 46, 5803−5809. (36) Luo, W.; Yang, Q.; Conway, W.; Puxty, G.; Feron, P.; Chen, J. Evaluation and modeling of vapor−liquid equilibrium and CO2 absorption enthalpies of aqueous designer diamines for post combustion capture processes. Environ. Sci. Technol. 2017, 51, 7169−7177. (37) Davis, J.; Rochelle, G. T. Thermal degradation of monoethanolamine at stripper conditions. Energy Procedia. 2009, 1, 327–333. (38) Vega, F.; Sanna, A.; Maroto-Valer, M. M.; Navarrete, B.; Abad-Correa, D. Study of the MEA degradation in a CO2 capture process based on partial oxy-combustion approach. Int. J. Greenh. Gas Control. 2016, 54, 160–167. (39) Epp, B.; Fahlenkamp, H.; Vogt, M. Degradation of solutions of monoethanolamine, diglycolamine and potassium glycinate in view of tail-end CO2 absorption. Energy Procedia. 2011, 4, 75–80.


21

Energy & Fuels

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 39

Captions Table 1. Physicochemical Properties of Aqueous LysK and Aqueous MEA Solutions Table 2. Summary of CO2 Absorption and Desorption Performance for Aqueous LysK and MEA Solvents a. Table 3. Total Alkalinity and pH Values of Aqueous 2.0M LysK Solutions at Various Thermal Conditions.

Figure 1. Molecular structure of potassium lysinate (LysK) and monoethanolamine (MEA). Figure 2. Schematic diagram of experimental apparatus for CO2 absorption and desorption. Figure 3. Schematic diagram of high-temperature VLE apparatus. Figure 4. Schematic diagram of the CPA201 reaction calorimeter apparatus Figure 5. CO2 absorption performance using different absorbent solutions at 313 K. The CO2 concentration in the simulated flue gas was 13.1 ± 0.1 mol %. (a) the outlet CO2 concentration curves versus time; (b) integration curves of CO2 loading with measured data; (c) absorption rate with loading in mol CO2 kg-1 absorbent solution. Figure 6. CO2 desorption performance for different absorbent solutions. (a) outlet CO2 concentration curves versus time; (b) temperature of absorbent solutions; (c) integration curves of CO2 loading with measured data; (d) desorption rate with loading in mol CO2 kg-1 fresh absorbent solution. Figure 7. Cycling absorption-desorption runs of aqueous 2.0 M LysK solution for CO2 capture. (a) CO2 loading and working capacity in mol CO2 / mole absorbent; (b) CO2 loading and working capacity in mol CO2 / kg fresh solution; (c) pH values of solution. Figure 8. Equilibrium CO2 solubility and cyclic loading capacity for aqueous LysK and aqueous MEA solutions. Some data points were obtained from literature.24,25,30 Figure 9. Differential enthalpy of absorption of CO2. (a) Aqueous 30 mass % MEA solution compared with literature data,35,36 (b) aqueous LysK solution. Figure 10. 13C NMR spectra for LysK solution in D2O at N2 (A) and O2 (B) exposure conditions. A: (a) fresh solution; (b)383K, 10h; (c) 383K, 120h; , (d)423K, 10h; (e) 423K, 120h. B: (a) fresh solution; (b)383K, 120h; (c) 383K, 360h; , (d)423K, 120h; (e) 423K, 360h. Figure 11. 1H NMR spectra for LysK solution in D2O at N2 (A) and O2 (B) exposure conditions. A: (a) fresh solution; (b)383K, 10h; (c) 383K, 120h; , (d)423K, 10h; (e) 423K, 120h. B: (a) fresh solution; (b)383K, 120h; (c) 383K, 360h; , (d)423K, 120h; (e) 423K, 360h. Figure 12.

13

C NMR spectra of aqueous LysK solutions in D2O. (a) fresh CO2-free solution; (b) CO2-lean solution at

the 4th absorption-desorption runs (α = 0.35); (c) CO2-lean solution at the 8th runs (α = 0.34), (d) CO2-free solution after eight-cycle runs using acid and base treatment. Figure 13. Degradation products of aqueous LysK solution. Notes: (1) 2-Amino-hexano-6-lactam; (2) Lysine dimmer; (3) 5-Aminopentanamide; (4) 6-Amino-2-oxohexanoate


22

Page 23 of 39

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

Energy & Fuels

Table 1. Physicochemical Properties of Aqueous LysK and Aqueous MEA Solutions HCO2 a

Absorbent (mass %)

Molar concentration (mol L−1)

Temperature (K)

ρ (kg m−3)

η ( mPa s)

LysK 33.1

2.0

298

1116.8

3.557

5671

13.36

313

1109.1

2.351

6786

—

298

1141.9

5.544

6712

13.24

313

1134.6

3.518

7727

—

298

1010.8

2.391

3262

12.16

313

1003.6

1.578

4025

—

(kPa m3 kmol−1)

pH (-)

a

LysK 41.2

MEA 30

a

2.5

5.0

data from literature.28,29


23

Energy & Fuels

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

Page 24 of 39

Table 2. Summary of CO2 Absorption and Desorption Performance for Aqueous LysK and MEA Solvents a. Absorption

Desorption

Cyclic capacity (mol CO2/kg solution)

Absorbent (mass %)

α (mol CO2/mol absorbent)

α (mol CO2/kg solution)

rCO2, ·105 (mole kg−1 s−1)

α (mol CO2/mol absorbent)

α (mol CO2/kg solution)

rCO2, ·105 (mole kg−1 s−1)

Batch

Multiple

Equilibrium-based (mol CO2/mol absorbent)

Source

LysK 41.2

1.058

2.535

44.0

0.534

1.188

98.5

1.237 b

1.361 c

0.50 - 0.70

This work

MEA 30

0.473

2.510

51.0

0.293

1.431

65.9

1.079

—

0.21- 0.30

This work

0.487

—

45−50

0.328

—

70−90

0.795

0.75

—

Luo,2016 31,d

0.55

2.77

55

0.25

1.33

62

1.44

—

—

Hartono,2017 32,d

0.488

2.43

—

0.285

1.42

—

1.01

1.05

—

Li,201633,e

a

max

max

CO2 absorption-desorption at 313 K-373 K for MEA and 313 K-379 K for LysK. α is referred to the measured value by a modified Chittick CO2 apparatus in batch and

multiple experiments. Equilibrium-based cyclic capacity was estimated from the solubility curves at 313K and 383K. b

The initial CO2 loading of LysK solution was 2.425 mol CO2/kg solution for desorption at 379 K.

c

Aqueous 2.0 M LysK solution was used in multiple cycling runs.

d

CO2 desorption at 353 K.

e

CO2 absorption-desorption at 313 K-393 K at a PCC pilot plant treating real flue gas containing 11.0-13.5 mol% CO2.


24

Page 25 of 39

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

Energy & Fuels

Table 3. Total Alkalinity and pH Values of Aqueous 2.0M LysK Solutions at Various Thermal Conditions. Temperature / K

Injection gas

Exposure time / hours

Total alkalinity / mLg-1solution

Variation of alkalinity, %

pH (301.2 ± 0.5 K)

Variation of pH, %

303

—

—

3.593

—

12.85

—

383

100% N2

10

3.586

-0.18

12.83

-0.16

120

3.585

-0.20

12.88

0.23

120

3.470

-3.43

12.42

-3.35

360

3.255

-9.41

11.74

-8.64

10

3.599

0.16

12.94

0.70

120

3.600

0.21

12.96

0.86

120

3.415

-4.94

12.39

-3.58

360

3.158

-12.09

11.27

-12.30

100% O2

423

100% N2

100% O2


25

Energy & Fuels

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 39

Figure 1

Figure 1. Molecular structure of potassium lysinate (LysK) and monoethanolamine (MEA).


26

Page 27 of 39

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

Energy & Fuels

Figure 2

Figure 2. Schematic diagram of experimental apparatus for CO2 absorption and desorption.


27

Energy & Fuels

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 39

Figure 3

Figure 3. Schematic diagram of high-temperature VLE apparatus.


28

Page 29 of 39

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

Energy & Fuels

Figure 4

Figure 4. Schematic diagram of the CPA201 reaction calorimeter apparatus


29

Energy & Fuels

2.0M LysK 2.5M LysK 5.0M MEA

16

Out

CO2 molar fraction in the outlet gas stream, yCO2, (%)

Figure 5 (a) In

yCO2 12

8

4 80% removal

0 0

40

80 Time (min)

120

160


(b)

2.0

-1

CO2 loading α (mol CO2 kg fresh absorbent)

3.0

Measured loading data

1.0


0.0 0

60

40

80 Time (min)

120


160

(c)

40

5

-1

-1

rCO2 x 10 (mol kg s )

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 39

20

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

-1


Figure 5. CO2 absorption performance using different absorbent solutions at 313 K. The CO2 concentration in the simulated flue gas was 13.1 ± 0.1 mol %. (a) the outlet CO2 concentration curves versus time; (b) integration curves of CO2 loading with measured data; (c) absorption rate with loading in mol CO2 kg-1 absorbent solution.


30

Page 31 of 39

(a)

40

2.0M 2.5M 2.5M 2.5M 5.0M

Out

CO2 molar fraction in the outlet gas stream, yCO2, (%)

Figure 6

30

LysK, 367 K LysK, 367 K LysK, 373 K LysK, 379 K MEA, 373 K

20

10

In

0

yCO2 0

20

40

60

80

100

Time (min)

400

(b)

Τemperature (Κ)

380

360

2.0M 2.5M 2.5M 2.5M 5.0M

340


320 Heating-up zone

Flat-temperature zone

300 0

20

40

60

80

100

Time (min)

(c)

3.0

2.0M LysK, 367 K 2.5M LysK, 367 K 2.5M LysK, 373 K 5.0M MEA, 373 K 2.5M LysK, 379 K Measured loading data

2.5

-1


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

Energy & Fuels

2.0

1.5

1.0 0

20

40

60

80

100

Time (min)


31

Energy & Fuels

120

100

2.0M 2.5M 2.5M 2.5M 5.0M


(d)

80

-1

-1

rCO2 x 10 (mol kg s )

60

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 32 of 39

40

20

0 1.0

1.5

2.0

2.5

-1


Figure 6. CO2 desorption performance for different absorbent solutions. (a) outlet CO2 concentration curves versus time; (b) temperature of absorbent solutions; (c) integration curves of CO2 loading with measured data; (d) desorption rate with loading in mol CO2 kg-1 fresh absorbent solution.


32

Page 33 of 39

Figure 7

CO2 loading α (mol CO2/mol LysK)

1.5

(a)

Absorption at 313 K Regeneration at 379 K Cylic capacity

1.2

0.9

0.6

0.3

0.0 0

1

2

3

4

5

6

7

8

Cycling runs

CO2 loading α (mol CO2/kg fresh solution)

3.0

Absorption at 313 K Regeneration at 379 K Cylic capacity

2.5

(b)

2.0

1.5

1.0

0.5

0.0 0

1

2

3

4

5

6

7

8

7

8

Cycling runs

12

After absorption After regenration

(c)

11

10

pH

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

Energy & Fuels

9

8

7 0

1

2

3

4

5

6

Cycling runs

Figure 7. Cycling absorption-desorption runs of aqueous 2.0 M LysK solution for CO2 capture. (a) CO2 loading and working capacity in mol CO2 / mole absorbent; (b) CO2 loading and working capacity in mol CO2 / kg fresh solution; (c) pH values of solution.


33

Energy & Fuels

Figure 8 1000

100

CO2 partial pressure (kPa)

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 34 of 39

cyclic loading 10

2.0 M LysK, 313K 2.0 M LysK, 383K 2.5 M LysK, 313K 2.5 M LysK, 383K 5.0 M MEA, 313K 5.0 M MEA, 373K 5.0 M MEA, 393K

1

0.1 0.0

0.3

0.6

0.9

1.2

1.5

CO2 loading α (mol CO2 /mol absorbent )

Figure 8. Equilibrium CO2 solubility and cyclic loading capacity for aqueous LysK and aqueous MEA solutions. Some data points were obtained from literature.24,25,30


34

Page 35 of 39

Figure 9 150 30% MEA, this work 30% MEA, Kim 2009 30% MEA, Luo 2017

(a)

-∆Habs (kJ/mol)

120

90

60

30

0 0.0

0.2

0.4

0.6

0.8

α 150 32.9 % LysK, Run 1, this work 31.6 % LysK, Run 2, this work 33.2 % LysK, estimated from VLE model

(b) 120

-∆Habs (kJ/mol)

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

Energy & Fuels

90

60

30

0 0.0

0.4

0.8

α

1.2

1.6

Figure 9. Differential enthalpy of absorption of CO2. (a) Aqueous 30 mass % MEA solution compared with literature data,35,36 (b) aqueous LysK solution.


35

Energy & Fuels

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 36 of 39

Figure 10

A

B

Figure 10. 13C NMR spectra for LysK solution in D2O at N2 (A) and O2 (B) exposure conditions. A: (a) fresh solution; (b)383K, 10h; (c) 383K, 120h; , (d)423K, 10h; (e) 423K, 120h. B: (a) fresh solution; (b)383K, 120h; (c) 383K, 360h; , (d)423K, 120h; (e) 423K, 360h.


36

Page 37 of 39

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

Energy & Fuels

Figure 11

A

B

Figure 11. 1H NMR spectra for LysK solution in D2O at N2 (A) and O2 (B) exposure conditions. A: (a) fresh solution; (b)383K, 10h; (c) 383K, 120h; , (d)423K, 10h; (e) 423K, 120h. B: (a) fresh solution; (b)383K, 120h; (c) 383K, 360h; , (d)423K, 120h; (e) 423K, 360h.


37

Energy & Fuels

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 38 of 39

Figure 12

Figure 12. 13C NMR spectra of aqueous LysK solutions in D2O. (a) fresh CO2-free solution; (b) CO2-lean solution at the 4th absorption-desorption runs (α = 0.35); (c) CO2-lean solution at the 8th runs (α = 0.34), (d) CO2-free solution after eight-cycle runs using acid and base treatment.


38

Page 39 of 39

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

Energy & Fuels

Figure 13

Figure 13. Degradation products of aqueous LysK solution. Notes: (1) 2-Amino-hexano-6-lactam; (2) Lysine dimmer; (3) 5-Aminopentanamide; (4) 6-Amino-2-oxohexanoate


39

A Comparative Study of Aqueous Potassium Lysinate and Aqueous

Recommend Documents