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Environmental and Carbon Dioxide Issues 2
3
Kinetic Absorption of CO into blended Ammonia (NH) solutions with a new Cyclic Amine 4-aminomethyltetrahydropyran (4-AMTHP) Lichun Li, Graeme Puxty, Marcel Maeder, Robert C. Burns, Hai Yu, and William Owen Conway Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019
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Introduction Aqueous alkanolamine solutions based post combustion capture process is the most mature and widely used technique for the chemical removal of carbon dioxide (CO2) emission sources.1-3 Exploring alternatives and even better solvents to replace the traditional aqueous monoethanolamine (MEA) absorbent have attracted research attentions in recent decade, with a particular focus on reduction of both capital (upfront) and operational (ongoing) cost. Aqueous ammonia (NH3) as the capture agent is one of such processes and has been developed rapidly. Using aqueous ammonia as absorbent has several advantages compared to the benchmark MEA process including low-cost chemical absorbent, high CO2 absorption capacity,4 no oxidative or thermal degradation issues,5 potential for the combined removal of all acid gases present in the flue gases like NOx and SOx,6, 7 lower energy requirement for solvent regeneration,8 and the potential for production of value added by-products i.e. ammonium bicarbonate 4. Despite the benefits outlined above, the NH3 based CO2 capture process suffers from high NH3 solvent loss rate and low CO2 absorption rates, limiting its commercial deployment at large scale
9-12.
CO2 absorption rate is a critical parameter that
defines the dimensions of the absorber column, ultimately affecting the capital cost of the CO2 capture process. Enhancement of the CO2 absorption rate is required to cutback the substantial capital cost associated with the absorption equipment.
13, 14
Many efforts have been made to
increase the CO2 absorption rate of aqueous NH3 solutions through the addition of rapidlyreacting primary or secondary amines in small quantities, often referred to as “promoters”. Examples include sarcosine,15 piperazine,10 and MEA9 which have all been demonstrated to be attractive additives. It is generally accepted that the chemical properties, including fast kinetics and high absorption capacity, are desirable when selecting amine absorbents. The cyclic amine absorbents i.e. piperazine (PZ), 3- and 4-piperidinealkanols (3-PM, 4-PM) have been recognised to be an attractive category of absorbents due to their fast kinetics towards CO2. 16-20
The large second-order rate constants of the cyclic amines make them attractive for CO2
absorption processes both independently and as promoters by blending with aqueous ammonia solutions. Our group has recently investigated a new heterocyclic amine 4-AMTHP which was found to have similar kinetic behaviour to MEA while possessing a larger and more favourable protonation enthalpy.21 The combination of rapid kinetics and a highly exothermic protonation enthalpy strongly position 4-AMTHP as a promising candidate to increase CO2 absorption rate towards aqueous NH3 solutions. The current work is focussed on the evaluation of the
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behaviour of 4-AMTHP as a kinetic promoter in aqueous NH3 solutions and determination of the underlying chemical mechanism and pathways for CO2 in the blends. The kinetics of CO2 absorption into blended 4-AMTHP / NH3 solutions have been investigated here by stoppedflow spectrophotometry at 25.0oC and a comprehensive reaction mechanism describing the suite of reactions involved in the 4-AMTHP-NH3-CO2-H2O system developed by simple combination of the independent reaction mechanism a of the respective individual amine systems of 4-AMTHP-CO2-H2O and NH3-CO2-H2O. The reaction mechanism determined from a homogenous condition was further employed to develop a mass transfer model. And this mass transfer model has been successfully employed to rationalize the CO2 absorption measurement on a wetted-wall column. 1.
Experimental section
1.1
Chemicals High-purity CO2 gas (BOC), N2 (Coregas), 4-AMTHP (99%, Suzhou Vosun Chemical
Co., Ltd), ammonium bicarbonate (>99%, Sigma-Aldrich) and thymol blue sodium salt (Sigma-Aldrich) were all used as obtained without further purification. Ultra-high-purity MilliQ water was boiled to remove CO2 and was used to prepare all solutions for stopped-flow kinetic and wetted-wall column measurements. 1.2
Kinetic measurements - stopped-flow spectrophotometry Kinetic measurements of CO2 absorption into blended 4-AMTHP/NH3 solutions were
carried out on an Applied Photophysics DX-17 stopped-flow spectrophotometer using a J&M Tidas MCS 500-3 diode-array detector. The measurements were operated via reacting a blended 4-AMTHP/NH3 solution containing small amount of acid base indicator, in a 1:1 ratio with a solution containing dissolved CO2 (CO2(aq)) in water on the stopped-flow spectrophotometer at 25 oC. The reaction kinetics and pH changes were tracked by monitoring the visible spectrum of the coloured acid-base indicators over the wavelength range 400 700 nm. All measurements were carried out at 25.0 °C and thermostatted to within ± 0.1 °C via a circulating Julabo F20 water bath. The real-time temperatures of the reacting solutions were measured by a thermocouple installed in side of the cell apparatus. Full details of the stoppedflow procedure can be found in our previous work.22 The reaction kinetics between CO2(aq) and a series of blended 4-AMTHP/NH3 solutions with various concentrations of 4-AMTHP and NH3 were investigated. The initial concentration of the dissolved CO2, [CO2]0, after mixing is set at 4.7 mM. The compositions of the blended 4-AMTHP/NH3 solutions after
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mixing include 5.0 mM NH3 with 0.0 - 5.0 mM 4-AMTHP and 5.0 mM 4-AMTHP with 0.0 5.0 mM NH3. 12.5 µM thymol blue indicator was added into the blended 4-AMTHP/NH3 solutions. 1.3 Mass transfer measurements – wetted-wall column A bench scale wetted-wall column was employed for the examination of the overall mass transfer coefficients which describes the absorption rate of CO2 into the blended NH3/4AMTHP solutions. On the wetted-wall column apparatuses, the liquid form a thin film and falling down on the surface of the column located in the centre of the reacting chamber while the gas stream counter-current contact with the liquid by flowing thorough from bottom to top of the reaction chamber. The dimensions of the wetted-wall column as well as the experimental procedures have been detailed in our previous publication.13 The CO2 absorption flux for each of the blended NH3/4-AMTHP solution investigated was measured over a series of CO2 partial pressures, ranging from 1 to 10 kPa. With a total gas flow rate of 5 L minT , the different CO2 partial pressures were attained by changing the ratio of the gas flow rates between CO2 and N2. To get a thin, even film on the surface of the column, the liquid flow rates were kept between 100 and 120 mL.minT . The amount of CO2 absorbed into NH3/4-AMTHP solutions were determined by measuring the CO2 content of the gas stream before and after counter-current contact with the liquid film via a Horiba VA3000 CO2 analyzer. 2.
Kinetic and equilibrium model The reaction mechanisms of the 4-AMTHP-CO2-H2O and NH3-CO2-H2O systems have
been fully resolved which can be sourced from previous publication from our group. To develop the reaction mechanism of the blended 4-AMTHP-NH3-CO2-H2O system, an assumption has been made that the mixed amine-CO2 reaction system is a simple combination of the individual amine-CO2 reaction mechanisms including the 4-AMTHP-CO2-H2O and NH3-CO2-H2O systems. It is also assumed that there are no syngeneic or catalytic involved that two amines collaborate with each other only via proton transfer and therefore buffering the pH. Figure 1 demonstrates the reaction scheme which describes all the interactions and species involved in the 4-AMTHP-NH3-CO2-H2O system. In the mechanism, and herein, RNH2 represents 4-AMTHP.
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K8
NH2CO2-
NH2CO2H k-7
K4
H2CO3 k1 k-1
NH4
K5
K6
CO2(aq)+H2O
NH3
K3
CO32-
k2 k-2
k7 +
HCO3-
CO2(aq)+OH-
K12 k-9 k9 K11
RNHCO2-
RNHCO2H K10
RNH3+
RNH2
Figure 1. Full reaction scheme illustrating the reactions and species involved in the NH3RNH2-CO2-H2O system. All the values of equilibrium constants and reaction rates can be obtained from open literature. When dissolve into H2O, CO2 as a reactive gas undergoes reactions with both H2O and OH-, showing in equations (1) and (2). Subsequent protonation reactions including the protonation of carbon species (CO32-, HCO3-) and OH- are also incorporated in the mechanism, equations (3)-(5). The reaction rate and equilibrium constants for equations (1)-(5) were sourced from open publications. 23-25 CO 2(aq) +H 2O CO 2(aq) +OH CO32- +H + HCO3- +H + H + +OH -
K3 K4 K5
k1 ,K1 k -1 k 2 ,K 2 k -2
H 2CO3
(1)
HCO3-
(2) (3)
HCO3H 2CO3
(4) (5)
H 2O
When amine is added to the CO2-H2O system, deprotonation of the amine undertakes consequently. Amine also rapidly react with CO2 and form carbamic acid, which deprotonates to carbamate species simultaneously. This applies for both NH3 and 4-AMTHP, as shown in equations (6) to (11). Values of K6, k7, k-7, K7, and K8 for NH3 and values of k9, k-9, K9, K10, K11 and K12 for RNH2 have been determined and sourced from our previous work.22, 26 NH3 +H +
K6
CO 2(aq) +RNH 2 RNHCO-2 +H +
CO 2(aq) +NH3 RNH 2 +H +
K10
(6)
NH 4+ k 7 ,K 7
RNHCO 2H
k -7 K8
k 9 ,K 9 k -9
RNHCO 2 H
NH2CO 2H
RNH3+
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(7) (8) (9) (10)
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NH 2CO-2 +H +
K11
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(11)
NH2CO 2 H
Activity coefficients have been estimated for all the ionic species to cope the non-ideal behaviour caused by the species charges. For all the ionic species, the Debye-Hückel equation was used to estimate their activity coefficients, equation (12). (12) log =
In the above equation,
1+ represent the activity coefficient of the ionic species i, zi
represents the charge of species i and
represents the ionic strength of the solution. The
parameter A was taken from literature 27. 3. Data analysis 3.1 Stopped-flow spectrophotometric kinetic analysis The experimental data obtained from stopped-flow spectrophotometry was analysed using an in-house extended version of Reactlab Kinetics (www.jplusconsulting.com/reactlabkinetics). The in-house version incorporates the activity coefficient corrections for all charged species. All of the kinetic and equilibrium reactions are coupled with each other, and with the pH of the solution, so that they cannot be analysed independently. To overcome this, global analysis of a series of kinetic measurements acquired at a single temperature but under different concentration conditions was used to obtain the kinetic behaviour of CO2 absorption into aqueous 4-AMTHP/NH3 solutions. To analyse the kinetics of CO2 absorption into blended 4-AMTHP/NH3 solutions, the liquid reaction model for the 4-AMTHP-NH3-CO2-H2O system was employed generated by simple combination of the respective reactions of the individual amine systems, NH3-CO2-H2O and 4-AMTHP-CO2-H2O. All reactions for the 4-AMTHP-NH3-CO2-H2O system are listed in equations (1) to (12), where RNH2 represents 4-AMTHP. All reaction rate and /equilibrium constants were sourced from open publications. During the evaluation of the kinetic data from the stopped-flow measurements, the parameter representing the free CO2 concentration in solution, [CO2]aq was regressed as part of the analysis. The value was found to be within the errors of the experimental technique (±5.0%) which is validation of the robustness of measurement and analysis procedure.
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3.2 wetted-wall column mass transfer analysis 3.2.1 Overall CO2 mass transfer coefficient, KG The CO2 absorption rate into an aqueous amine solution measured on a wetted wall column apparatus can be correlated to effective surface area and CO2 driving force, shown in equation (13). rCO2 = KGA(PbCO2
PCO2) = KGA × PbCO2
KGA × PCO2
(13)
In equation (13), rCO2 stands for CO2 absorption flux, mol.s-1.m-2, K G represents the b overall mass transfer coefficient (mol.s-1.m-2.kPa-1), PCO2 is the average partial pressure of CO2 * in the bulk flue gas (kPa), PCO2 represents the equilibrium CO2 partial pressure of the mixed
amine solutions (kPa), and A represents the effective surface area (m2). According to equaition b
(13), the slope of the plot of the CO2 absorption flux against CO2 driving force (PCO2
* PCO ) 2
subject to the overall mass transfer coefficient, K G . The CO2 partial pressures in the bulk flue b
gas, PCO2 , is calculated from the natural logarithm mean average of inlet (PCO2) and outlet (Pout CO2 ) CO2 partial pressure, shown below in equation (14). PbCO2
=
Pin CO2 ln
Pout CO2 Pin CO2
(14)
Pout CO2
3.2.2 CO2 mass transfer model The complete reaction mechanism for the homogeneous NH3-4-AMTHP-CO2-H2O system was further employed for the development of a robust overall CO2 mass transfer model for the purpose of explaining the experimental data measured from the wetted-wall column apparatus. For the CO2 mass transfer model in this work, it is proposed that CO2 diffuses via the gas-liquid interface into the liquid phase where all the chemical reactions adopted from the homogeneous NH3-4-AMTHP-CO2-H2O system take place. In other words, it is assumed that there are no reactions occurring at the bulk gas phase and the gas and liquid interphase. An in-house developed software tool based on Matlab®
28
has been used to describe the
proposed overall CO2 mass transfer model and simulate the mass transfer performance of CO2 absorption into the blended NH3/4-AMTHP solutions on the wetted-wall column. The software tool is capable of solving partial differential equations and nonlinear simultaneous equations
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which define the diffusion, reaction and equilibrium processes processing within the thin liquid film as a function of reaction time and the thickness of the thin liquid film. Physical properties of the blended amine solutions including viscosity and density are assumed to be constant throughout the CO2 absorption process. The partial differential equations can be described as a combination of Fick’s law (diffusion) and chemical reaction in equation (15) 29, 30: ! "
2
=#
!
%
$2
(15)
In equation (15), Ci is the concentration of species i (M), t is time (s), Di is the diffusion coefficient of species i (m2.s-1), x represents the distance from the gas-liquid interface (m), and ri is the rate of formation or destruction of species i with chemical reaction (M.s-1). For a given gas-liquid exposure time (te), the average CO2 absorption flux at the gas-liquid interface (x = 0) can be calculated according to equation (16). %&'(& !)2 =
#!)2
"&
"*
0
+
!!)2 -" "
(16)
The diffusion coefficients for CO2 and all other species involved, together with the exposure time, were calculated as based on viscosities and densities of the liquids, and the wetted-wall operating conditions. Details can be found in our previous publication.31 The concentration of CO2 molecular at the gas-liquid interface (x=0) was estimated from the Henry’s Law constant. The Henry’s Law constant of CO2 calculation is adapted from open literature 32, describing in equation (17) HCO2 = 2.82 × 106e
2044/T
(17)
The CO2 absorption flux then can be estimated for various CO2 partial pressures. Hence, the overall CO2 mass transfer coefficient can be regressed from the slope of plotting CO2 absorption flux against the CO2 partial pressure (driving force), according to equation (14). 4.
Results and discussion
4.1 Homogeneous reaction mechanism development The homogeneous reaction behaviour in the mixed NH3/4-AMTHP solutions were measured by stopped-flow spectrophotometry at 25.0 oC. Various blend concentrations were evaluated by fixing one amine concentration at 5.0 mM in the solution while varying the concentration of the second amine from 0.0 to 5.0 mM. Thus, two sets of kinetics experiments were performed on the blended NH3 /4-AMTHP solutions with NH3 as the amine in excess
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(5.0 mM NH3 with 0.0 - 5.0 mM 4-AMTHP) and 4-AMTHP as the dominant amine (5.0 mM 4-AMTHP with 0.0 - 5.0 mM NH3). The measured stopped-flow data was analysed using the chemical mechanism incorporating equations (1) – (12) without regression of any kinetic parameters to the data. Representative experimental and calculated absorbance traces at 590 nm for amine blends containing 5.0 mM NH3 and 0.0 - 5.0 mM 4-AMTHP respectively, and vice versa (5.0 mM 4-AMTHP and 0.0 - 5.0 mM NH3) reacting with 4.7 mM CO2(aq) are shown in Figure 2. Due to the large amount of measured data and their excellent agreement with fitting results, it is difficult to distinguish between the experimental and calculated absorbance traces in Figure 2. Alternatively, absorbance traces displayed with a logarithm time scale showing detailed comparisons between the experimentally measured and calculated traces can be found in the Supplementary Material section. 0.22
0.22
(a)
(b)
5mM NH6+ 5mM 4-AMTHP 5mM NH6+ 4mM 4-AMTHP
0.2
5mM 4-AMTHP+ 5mM NH6 5mM 4-AMTHP+ 4mM NH6
0.2
5mM 4-AMTHP+ 3mM NH6
5mM NH6+ 3mM 4-AMTHP 5mM NH6+ 2mM 4-AMTHP
0.18
5mM 4-AMTHP+ 2mM NH6
0.18
5mM 4-AMTHP+ 1mM NH6
5mM NH6+ 1mM 4-AMTHP 5mM NH6
0.16
Absorbance at 590 nm
Absorbance at 590 nm
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0.14
0.12
0.1
0.14
0.12
0.1
0.08
0.08
0.06
0.06
0.04
5mM 4-AMTHP
0.16
0.04
0
0.5
1
1.5
2
2.5
0
0.5
Time (s)
1
1.5
2
2.5
Time (s)
Figure 2. Measured and calculated absorbance traces over time at 590 nm for the reaction of 4.7 mM CO2(aq) (a) 5.0 mM NH3 blended with various 4-AMTHP concentrations from 0.0 to 5.0 mM) (b) 5.0 mM 4-AMTHP blended with various NH3 concentrations (0.0 to 5.0 mM), both in the presence of 12.5 [
thymol blue indicator. Markers are the measured data and
calculated traces are displayed as solid lines. The overall time required for CO2(aq) reaction is a direct indicator of the overall rate of CO2 absorption at a given CO2(aq) concentration, where shorter reaction times indicate a faster reaction kinetics towards CO2(aq). With an increase in concentration of 4-AMTHP from 0.0 to 5.0 mM, the overall time required for CO2 reaction in the 5.0 mM NH3 solutions progressively shifts from ~1.5 s to ~ 0.5s, indicating significant enhancement of the CO2 reaction kinetics. When 4-AMTHP is added into aqueous NH3 solutions, 4-AMTHP behaves as a direct chemical competitor for CO2(aq) absorption, owing to the significantly faster rate constant for 4-AMTHP carbamic acid formation (k9 = 54 × 102 M-1s-1) compared to the corresponding value for
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ammonia, (k7 = 4.5 × 102 M-1s-1). Conversely, from Figure 2(b) the reaction kinetics do not change significantly with varying amounts of additional NH3 in the presence of 5.0 mM 4AMTHP where the overall time required for CO2 reactions consistently around ~0.5 s. This phenomena can be explained by the different roles of the amines in the solutions depending on pH and their concentration, respectively. In the latter case, NH3 contributes to the kinetics by acting as a buffer to accept H+ in the solutions while 4-AMTHP, owing to the faster kinetic constant with CO2, chemically binds CO2 as carbamic acid which deprotonates to carbamate. As shown in both Figure 2 (a) and (b), the good agreement between experimental and calculated absorbance data validates the chemical model developed here for the complete CO24-AMTHP-NH3-H2O system without any requirement for regression of the reaction rates or equilibrium constants. This further justifies the assumptions that a simple combination of reaction mechanisms of the two independent amine systems, CO2-NH3-H2O and CO2-4AMTHP- H2O respectively, can be applied and successfully represents the comprehensive reaction mechanism for the CO2-4-AMTHP-NH3-H2O system without any catalytic or syngeneic effects, under the homogeneous conditions and low concentrations in the solutions herein. 4.2 Mass transfer model development To further investigate the impact of 4-AMTHP as a rate promoter in NH3 solutions, and to obtain detailed data for these effects at representative operating conditions experienced in a realistic CO2 absorption process, the effect of operating parameters including CO2 loading and 4-AMTHP concentration on the mass transfer of CO2 absorption into blended NH3/4-AMTHP solutions were investigated using a wetted-wall column at 25.0oC. It should be noted that only homogeneous solutions were evaluated herein and the presence of solid precipitates was not observed in any CO2 loaded absorbent solutions or during the wetted-wall measurements. The following is a discussion of the measured and predicted overall CO2 mass transfer co-efficients, KG for a series of mixed NH3/4-AMTHP solutions. 4.2.1 CO2 absorption into blended solutions with varying CO2 loadings Measured and predicted CO2 overall mass transfer coefficients (KG), from CO2 absorption into aqueous 3.0 M NH3/0.3 M 4-AMTHP solutions, as a function of CO2 loading are shown in Figure 3(a). It can be observed from the Figure 3(a) that CO2 mass transfer into the blended NH3/4-AMTHP solution decreases gradually with the increasing of CO2 loading. This effect stems from two phenomena. Firstly, from the equilibrium point of view, as the CO2
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loading of the solution increases the concentration of free NH3 and 4-AMTHP decreases, showing in Figure 3(b). Secondly, the availability of the free forms of NH3 and 4-AMTHP at the gas-liquid interface decreases due to the slow diffusion of reacted species, NHCO2-, 4AMTHPCO2-, NH4+, and 4-AMTHPH+, away from the interface to the bulk liquid due to higher viscosities. This is demonstrated in the calculated equilibrium concentrations for the CO2 preloaded NH3 4-AMTHP solutions in Figure 3(b) where both NH3 and 4-AMTHP species exist in various forms.
3 M NH3
Concentration of NH3 species (M)
3
0.80 Experimental data 0.60 Simulation data k2 = 205 M-1.s-1
0.40
NH9 NH
