Insights into the Adsorption of Carbon Dioxide in the Presence of

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Insights into the Adsorption of Carbon Dioxide in the Presence of Water Vapor Utilizing a Low Molecular Weight Polyethylenimine-Impregnated CARiACT Silica Sorbent Esmail R Monazam, Ronald W Breault, Daniel J. Fauth, Lawrence J. Shadle, and Samuel Bayham Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01271 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 20, 2017

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Industrial & Engineering Chemistry Research

Insights into the Adsorption of Carbon Dioxide in the Presence of Water Vapor Utilizing a Low Molecular Weight Polyethylenimine-Impregnated CARiACT Silica Sorbent Esmail R. Monazam2, Ronald W. Breault∗1, Daniel J. Fauth1, Lawrence J. Shadle1, and Samuel Bayham1 1

National Energy Technology Laboratory, U. S. Department of Energy, 3610 Collins Ferry Rd. Morgantown, West Virginia 26507-0880, 626 Cochrans Mill Road, Pittsburgh, Pennsylvania 15236-0940 2 REM Engineering Services, PLLC, 3537 Collins Ferry Rd. Morgantown, West Virginia 26505 ABSTRACT Thermogravimetric analysis was employed to investigate the CO2 and H2O adsorption rates and water vapor equilibrium on anhydrous and pre-hydrate linear polyethylenimine (LPEI) sorbent impregnated within a commercially functional CARiACT G10 (HPV) silica support. Water vapor experiments utilizing specific humidity of 2%, 8%, and 16% in contact with an anhydrous PEI sorbent resulted in proportional quantities of water vapor uptake. Subsequently, both anhydrous and pre-hydrated PEI-impregnated sorbents were made available to identical humidified gaseous streams containing a CO2 concentration of 10% at 60oC. CO2 capacity increased dramatically in the presence of different levels of humidity. Various kinetic models were systematically employed to interpret the experimental data including single and multiplestep models. The rate data was best represented by a reaction mechanism pathway involving the interplay of CO2 with PEI-impregnated sorbents exhibited a quick adsorption phase followed by a slow approach to equilibrium. Moreover, a phenomenological rate model was developed to describe the dynamic H2O and CO2 uptakes at specific humidity levels studied. The kinetic study showed good agreement with experimental data. Furthermore, the effects observed during the adsorption and hydration are shown to be complementary to known chemical and physical transformations within the polyethylenimine’s macromolecule. INTRODUCTION Improved carbon management can increase efficiency, minimize consumption of natural resources, and reduce the environmental footprint for organic fuel conversion technologies. An enduring mission at NETL is to increase energy conversion efficiency. This reduces fuel consumption and thereby conserves our natural resources and lowers process operating costs. Another option for carbon management is capture and sequestration of carbon dioxide produced during combustion. The greatest immediate impact using this approach would be to deploy capture technologies on the existing fleet of utility scale power plants. In this application capture technologies scrub the carbon dioxide from a combustor’s flue gas at relatively low temperatures; this is referred to as post combustion carbon capture. Aqueous monothanolamine (MEA) is widely recognized as the benchmark solvent for the commercial application of post combustion carbon capture.3 Aqueous amine-based methodologies have fast CO2 absorption ∗

Corresponding author: Tel. 304-285-4486; fax: 304-285-4403; email: [email protected]

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rates; however, there are opportunities to improve upon such liquid based processes. Notwithstanding advancement in process optimization, the major technical challenges regarding this mature methodology include solvent losses and related toxicity problems, severe equipment corrosion, together with sizeable energy penalties associated with solvent regeneration.4,5 Taking this into consideration, CO2 adsorption employing solid amine sorbents has attracted much attention and is being acknowledged as a potential alternative for post combustion CO2 capture.68 In the development of these amine-based materials, disordered, amorphous or crystalline mesoporous silicas, along with silica foams are typically utilized as supports having high porosity, uniform pore dimensions, and large surface areas; upon which amines are incorporated physically by wet impregnation, or covalently either by post-grafting or co-condensation by aminosilanes and tetraalkoxysilanes.9-11 Ideally, high concentrations of amine moieties incorporated within these solid inorganic supports exhibit favorable CO2 adsorption properties, including high CO2 uptake performance, fast adsorption kinetics, along with helping to alleviate corrosion and more importantly, parasitic energy losses. Polyethylenimine is catalogued as being the most utilized aminopolymer, having a high amine content, about 33% nitrogen by weight.12 Hydroxyl entities present on the silica support surface construct strong amino-silica interactions, resulting in a uniform dispersion of PEI molecules throughout the pore space which considerably increases its CO2 uptake performance, along with accelerating CO2 sorption kinetics relative to pure polyethylenimine. Previous studies have utilize low molecular weight polyethylenimine as a model system for elucidating the effects of relative humidity, and therefore hydrogen bonding (water-water, water-amine, and amine-amine hydrogen bonding) on the surface structure of amino-containing polymers at the solid-vapor interface.13 The molecular structure of low molecular weight polyethylenimine (LPEI) is simplistic in nature, being comprised of repeating –(CH2-CH2-NH)- ethylenimine monomer units. LPEI has been reported to exhibit complex crystalline structures and phase behavior as a function of water content. In its anhydrate form, a segment of LPEI occurs as a parallel array of double-stranded helices stabilized by hydrogen bonds between the secondary amine groups of neighboring PEI chains. The remaining portion entails rigid amorphous domains that arise from the inevitable entanglement of the polymer chains. Upon the absorption of H2O, the anhydrate crystalline and rigid amorphous domains undergo transitions to a series of three distinct crystalline hydrates. Variations in the crystalline structure between the different hydrates are due to the differences in the hydrogen bonding network and the amount of water absorbed into the matrix. As such, each of the three hydrate crystalline states exhibits a distinct water structure defined by the number of amine-water molecules with which each water molecule coordinates. This characterization of LPEI properties led to significant interest in the behavior of, and the transitions between, bulk crystalline forms of LPEI as a function of temperature and water content.14-16 PEI is a blend of primary, secondary and tertiary amines which combine the higher equilibrium capacity of the tertiary amine with the higher reaction rate of the primary and secondary amine without appreciably changing the stripping characteristics.17 Hence, like its liquid amine counterparts, monoethanolamine (MEA), diethanolamine (DEA), and tertiary methyldiethanol amine (MDEA) in dry conditions, the main reaction between amine and CO2 is the formation of carbamate; 2 + ↔ + 2 + ↔ + (1)


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+ + ↔ + ′

where R = C2H4OH (CH2CH2OH), R = H for MEA, and R = R = C2H4OH for DEA. The carbamate formed by primary and secondary amines are generally stable to give a maximum stoichiometric loading of 0.5 mole CO2/mole amine. In other words, these amines bind CO2 at a 1:2 molar ratio. Tertiary amines do not result in carbamates and give stoichiometric loadings of 1 mole CO2/mole amine. Water is a significant component of power plant flue gas and is a proton transfer agent in the reaction of CO2 and amine groups.18 However, when water is present, further reaction of the carbamate ion (RNHCOO-) to form bicarbonate occurs:19 + 2 + ↔ +

(2)

Bicarbonate may also form directly from amine reacting with H2O and CO2: + + ↔ +

(3)

Therefore, in the presence of water, one mole of amine is effective in removing one mole of CO2 to form a carbamate ion. There are several reports available in the literature for the reactions of CO2 with aqueous blends of mixed amine systems and summarized by Choi et al.20 They categorized amine-silica composite materials into three classes, distinguishing synthesis by impregnation (class 1) as a realistic methodology for producing model adsorbents having favorable properties (i.e., high CO2 sorption capacity and selectivity, together with low energy consumption, relatively low cost, and ease of preparation) for potential large-scale CO2 capture. During the past decade, several amines have been employed for the synthesis and characterization of solid, amine-based materials, including polyethylenimine. Vaidya and Kenig21 found that the rate of carbamate formation was much faster than the rate of bicarbonate formation for CO2 adsorption with aqueous amines. Hiyoshi et al.22 studied humid adsorption with mono-, di-, and tri-amine silicas by FTIR and only detected formation of carbamates over the first 5 min of the adsorption experiment. This result led to the hypothesis that humid adsorption occurs by the quick formation of carbamates followed by the slow conversion to bicarbonates. Knowles et al.23 observed two phases during adsorption with monoamine silica using thermogravimetry. They attributed the first phase to the formation of carbamates, due to the similarity in rate to that of dry adsorption, which was rapid. The second adsorption phase was much slower and was postulated by the authors to be due to the formation of carbonates and bicarbonates. Utilizing a thermogravimetric analyzer, Serna-Guerrero and Sayari24 studied the adsorption of CO2 under humid conditions in pre-hydrated adsorbent by exposing it to humid inert gas before switching to a humid CO2 stream. However, they had difficulty in distinguishing the difference between the weight gain solely due to CO2 adsorption or due to simultaneous adsorption of CO2 and desorption of H2O. Serna-Guerrero and Sayari24 reported an increase in capacity of 20 to 79 % wt. when the relative humidity was increased from 27 to 74% for prehydrated monoamine silica. They concluded that the concentration of H2O during adsorption affects the synergistic increase in capacity compared to the dry case. Li et al.25 showed both PEI type and molecular weight soundly influenced the CO2 saturated sorption capacity of the polyamine-impregnated materials, having decreasing CO2



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uptake with increasing PEI molecular weight, together with having the branched PEI outperforming the linear compound owing to their greater mobility. Furthermore, they reported the sorption and desorption heat of LPEIs was lower than that of BPEIs. Li et al.25 also investigated the influence of PEI type and molecular weight regarding adsorbent stability during cyclical CO2 sorption-desorption studies, concluding PEI type influenced cycle sorptiondesorption stability greater than PEI molecular weight; and low molecular weight LPEI and BPEI made it easier to form the secondary product, urea at elevated temperature. They also narrated that moisture contained within the gas stream provided noteworthy improvement concerning the cycle sorption-desorption stability of formulated PEI materials. Zhang et al.26 investigated the reversible adsorption of CO2 in various gas mixtures, including dry air (CO2 concentration of 400 ppmv) in using a series of linear polyethylenimines having a number of several average molecular weights (Mn). The utilization of linear aminebased polymers (i.e., LPEI) containing only secondary amines in the polymer backbone bind CO2 less strongly than its primary counterpart; which could be beneficial in lowering the energy input regarding the endothermic desorption process. Zhang et al.26concluded linear PEIs supported on fumed silica are promising candidate for CO2 capture from various sources including air with desorption occurring rapidly at mild temperatures. Cyclical adsorption/desorption over hundreds of rotations conducted reported no noticeable decrease in CO2 adsorption capacity. They further reported the adsorption kinetics were similar for linear and branched PEI impregnated within the silica substrate, whereas the desorption rate was noticeably faster with the LPEI material. Zhang et al.26 also concluded the presence of moisture within the gas mixtures investigated had a positive effect on CO2 adsorption capacity. Zhang et al.27 probed both linear and branched polyethylenimines impregnated on hydrophilic (and hydrophobic) silica for evaluating moisture effects on CO2 sorption capacities. Enhancements were reported for these materials when subjected to specific humilities (3.8-11.5 mg H2O/g at 25oC). Increasing PEI loading resulted in an increase in water adsorption. Moreover, the branched variety of PEI was shown to be more hydrophilic than its linear counterpart. This significant finding holds the potential in energy savings during sorbent regeneration. Chaikittisilp et al.28 studied poly(allylamine) impregnated mesocellular silica foams for capturing CO2 from multiple gas mixtures, having CO2 partial pressures consistent with simulated flue gas (10%), and ambient air (400 ppmv CO2). Materials with low organic loading exhibited comparable utility to branched PEI-containing Class 1 adsorbents, which are viewed being the most promising adsorbents for stationary power plant applications. At high poly(allylamine) loadings, the CO2 adsorption capacities, together with amine efficiencies decreased. In comparison to linear PEI, these materials were described being more efficient under simulated ambient air conditions. The primary objective was to attain a better understanding of the rate parameters for CO2 capture for the immobilized LPEI adsorbent in the presence of water utilizing thermogravimetric analysis and kinetic analysis of the rate data. In addition, the kinetic results was interpreted in in light of the effects of humidity on the surface-specific structure and transitional phase change behavior of the LPEI-impregnated CARiACT silica particles. The CO2 uptake performance of the supported low molecular weight polyethylenimine (LPEI) sorbent was evaluated separately with three distinguishing water contents (2, 8 and 16% mass basis). EXPERIMENTAL


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In examining the fundamental rates for the fractional conversions of H2O and CO2, along with the rate of H2O and CO2 uptake; experimental data was initially collected employing a synthesized CARiACT G10 (HPV)-supported LPEI adsorbent prepared by a wet impregnation method. Methanol (reagent grade) and low molecular weight polyethylenimine (423) were employed in the synthesis of the immobilized sorbent procured from Aldrich Chemical Co. and used without purification. The silica substrate selected in this investigation was CARiACT procured from Fuji Silysia Chemical. The CARiACT G10 (HPV) substrate was selected based on suitability of the particle size for a fluidized bed reactor configuration, particles having a mean size of 80 µm. A nominal amount of 50% PEI by weight was immobilized within the silica substrate. Experimental details and material characteristics can be found elsewhere.29 Experiments were carried out using a Thermo Cahn Thermomax 300 thermogravimetric analyzer (TGA). In a typical experiment, a 30-mg sample was placed in the microbalance quartz sample holder heated to 105oC in N2 at a flow rate of 100 mL min-1, and held at temperature for 60 min until no weight change was observed. After the removal of pre-adsorbed CO2 and H2O, the temperature was adjusted to 60oC followed by manually switching the gas flow to a humidified 99.9% pure N2/H2O stream at 100 mL min-1 to perform pre-hydration process. The conditions were chosen based upon experimental work conducted in previous investigation showing a peak in the capacity at a temperature of 60 oC.8, 29 The adsorption of water onto the immobilized LPEI sorbent was conducted at 60oC under three different H2O concentrations (specific humidity): 2% (15% RH), 8% (55% RH), and 16% (100% RH) absolute water (mass%). The specific humidity was defined as the mass ratio of water to total gas. The soak time during the water adsorption was set at 120 minutes. After that, humidified CO2/N2 was introduced over the pre-hydrated immobilized LPEI sorbent for CO2 adsorption. The time during the CO2 adsorption was set at 120 minutes. In all experiments the flow rate of reactant gas (CO2 and H2O) and inert gas (N2) were set at 100 ml/min (at standard rate). The temperature and CO2 concentration during CO2 adsorption was kept constant at 60oC and 10% with balance nitrogen for all three different H2O concentrations. The weight change of the sorbent sample and the reaction temperature were recorded continuously at 1 Hz. A typical experimental observation for temperature and changes in weight during the TGA experiments is illustrated by Figure 1. RESULTS AND DISCUSSION Water adsorption mechanism The experimental weight gain curves for water uptake obtained in TGA experiments at different water contents (2, 8, and 16% absolute) in flowing inert gas using 30 mg of immobilized PEI are shown in Figure 2. As can be seen in Figure 2, the weight gain at the beginning of the uptake was fast, and then slower as time progressed. It is also seen from Figure 2 that the weight gain increased significantly with the water content. However, the rate of water uptake was found to be more complex. While a weight gain of 1.5% was achieved in 7.5 min at 2% water (Figure 2), the same weight gain was achieved after only 1.75 min in an atmosphere containing 16% water. At these water contents the weight gain may proceed per the following reactions: + → ,



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" + ⇄ " +

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(4)

To simplify the analysis, the degree of water uptake was normalized per the following equation: ((&) − ( ) ,( = (5) $= ,. )* − ( + ( where m(t) is the instantaneous weight of the solid during the exposure to water. Parameters m0 and mf are initial and final weights of the PEI sorbent, respectively. Parameters qt and qe are the water uptake at time “t” and equilibrium water uptake, respectively. Table 1 shows the equilibrium water uptake for different water contents. Increasing water content, increased the equilibrium water uptake. Figure 3 presents the extent of the normalized water uptake (qt/qe) as a function of time, with data points obtained at different water contents (2, 8, and 16% absolute). The results reveal that the degree of water uptake increased with water content and time. At the beginning, the uptake proceeded rapidly up to about 20 min, after which only gradual increases in uptake were observed. Figure 3 also shows a marked increase in the time needed (1.8 min to 7 min) to gain 50% of water uptake (X, H2O=0.5) when the water in the gas flow was raised from 2% to 8%. To evaluate the water uptake, an appropriate model is necessary to get a good prediction of the water uptake rates. The rates can be modeled using a basic empirical model (1st order, 2nd order, etc), or theoretical model, eg. Ficks’ law of diffusion. Empirical models predict the average water uptake as a function of soaking time, while the theoretical model predicts the water uptake as a function of time and space within the LPEI sorbent. In this study, the empirical models were chosen over the theoretical model. Previous tests with PEI impregnated sorbent demonstrated that empirical models provided good agreement with the data29. For example, considering second-order kinetics, the rate of water uptake at any time may be expressed as: ,= 0(,. − ,- ) (6) &

Upon integration with the initial conditions of qt = 0 at t = 0, the following equation is obtained: 0,. & (7) 1 + 0,. & By rearranging the above equation, one can obtained the following equation: ,- =

& & 1 = + (8) ,- ,. 0,. The slope and intercept of the linear plot between t/qt and t can be used to evaluate ‘k’ and ‘qe’, respectively. The variation of t/qt against time is plotted in Figure 4 for the different water


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content (2, 8, and 16% absolute). For all the cases, the theoretical (based on Figure 4) and experimental values of qe are in close agreement (Figure 4). Figure 5 displays the water uptake data of Figure 4 plotted in terms of equation (7). In all cases, there was poor agreement between qt/qe and time, indicating that the experimental results do not conform to the second-order kinetics rate, even though equation (8) provided theoretical equilibrium water uptake in close agreement with experimental values. Attempts were made to fit the water uptake data over the complete soaking time with a shrinking core model. For the reaction of A(gas) + B(solid) →product, if the gas diffusion through the initial gas film is the rate-controlling step, equation (9) can be applied as; &4 = 5678 $ (9) In general, gas film diffusion is rarely the rate-controlling step for these low temperature reactions. If diffusion through the product solid layer is the rate controlling step, equation (10) can be applied as

&4 = 5:;** (10) If chemical reaction at the surface is rate controlling, equation 11 can be applied as &4 = 5@A B1 − (1 − $)"/ D (11) where τgas, τdif, and τch are the characteristic times required for the conversion process due to gas diffusion, diffusion through the product layer, and chemical reaction, respectively. The experimental CO2 uptake data were compared to the ideal product diffusion control (Eq. 10) and chemical reaction (Eq. 11) models. The conversion−time data did not fit either model (see Figure 6), indicating that the water uptake cannot be described by the shrinking core model. Several articles also have been devoted to the kinetic analysis of experimental data using the following rate equation:30-32 $ = 0E($) (12) & where dX/dt represents the kinetic rate, t is the time, and f(X) is a mathematical function that depends on the kinetic model used.32 For reaction kinetics under isothermal conditions, equation (12) can be analytically integrated to yield: F($) = G

4

(

$ = 0& (13) E($)

where g(X) is an integral mathematical expression related to different mechanisms for solidphase reactions. When evaluated none of the expressions32 listed for g(X) provided straight linefitting parameters (R2), indicating that the water uptake cannot be described by the mechanisms defined in these models.



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Therefore, a frequently-used two-step model was employed to describe the adsorption rates of water uptake. This model is the generalized form for nucleation and growth processes in which the reactions occur in parallel and include both rapid and slow domains.33 Moreover, with the multiple domain adsorption mechanism, different water adsorption domains exhibited different adsorption characteristics. The multiple domain adsorption mechanism can be defined as; $ = $H + $I = JH )1 − 7K- K + + J8 )1 − 7M - M + (14) L

L

Where XR and XS are the levels of sorbate uptake for the rapid and slow processes. . Equation 14 correspond to nucleation or growth processes occurring in parallel with the relative contribution of each manifested by the value of the weight factors wR and wS where wR + wS= 1. To extract classical kinetic parameters, k, was defined as; "

0 = OP (15) As illustrated by Figure 7, the data from the rate model with parallel process were very compatible with the experimental data (R2>99.99). Therefore, in this study, a reaction model in parallel was used for analysis of water uptake. For a given water content in the gas stream (2%, 8% and 16%), values of wR aR, aS, nR, and nS were determined by curve fitting the rate data of Figure 7 with the parameters in equation (14) using TABLECURVE available from Statistical Package for the Social Sciences. The values of nR and nS define the type of rate mechanism for the process. The values determined for the shape parameters, nR, range from 0.97 to 1.01 for all water content. The values determined for the shape parameters, nS, range from 1 to 1.125 for all water content. Generally, when n is close to 1, the mechanism approaches first-order kinetic controlled, that is, kinetic controlled mechanisms. Therefore, in this study, a two- component first order rate was employed to describe the adsorption kinetics of water uptake. The values of wR, aR, and aS were recalculated based on the approximation of nR = nS = 1 for every set of conversion data obtained from the experiments at different water content. When the process is modeled using equation 14 with nR = nS= 1, the rate expression represents two parallel pseudo first-order reactions. The comparison of the experimental conversion data (X) and the conversion based on this model can be found in Figure 7 for different water concentrations in the sweep gas. The model and experimental data agree over the entire conversion time with overall variance explained (R2) greater than 99.99%. It should be noted that only few data points are shown initially at each gas water contents to clearly illustrate the overlap in the experimental and modeled data. The solid lines represent the model fit to experimental data. The parameters such as rate constants and the fractional contributions of the individual sorption processes are summarized in Table 2 and shown in Figure 8. The water adsorption rate constants of the rapid process (aR) were about 4.5 times greater than those of slow process (aS) for 8 and 16%; while the weight fractions of rapid adsorption (wR) decreased with increasing water contents in the sweep gas. It should be noted that model did not produce any slow process for 2% gas water content (wS=0). Therefore, the fast adsorption process may be thought of as corresponding to the nucleation of water adsorbing on surface sites, while the slow adsorption process can be thought to correspond principally to growth of those sites through the PEI within


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the mesopore. This slow growth process may be related to diffusion of water into the particle and so may exhibit a particle size dependence. However, the CARiACT support material is known to be mesoporous,29 and so unless the fast process includes adsorption in the mesopores, no effect would be expected until the particle size is reduced below that dimension. This investigation points to the importance of the initial water sorption kinetics in determining the overall dynamic process and how it relates to the water-induced solid state phase transitions of the synthesized LPEI sorbent in the hydration process. We propose that as the LPEI film is uniformly dispersed throughout the pore surface of the silica substrate. Upon exposure to elevated humidity conditions, several pronounced changes are known to occur. LPEI displays water-induced phase transitions among four distinct crystalline hydrates14. Within the hydration process, the chain conformation of LPEI changes from a dense packing of double-stranded helices in the anhydrate to a planar, zigzag conformation in the three hydrates, combined with water molecules by intermolecular hydrogen bonds. It is speculated that the LPEI used in this study behaves like the high molecular weight material used by Hashida et al.14 They reported changes in the hydration process from time resolved infrared measurements. As hydration increases the LPEI changes from the anhydrate with doubly stranded helices to the PEI-water complexes of hemihydrate (0.5), sesquihydrate (1.5), and dihydrate (2.0). Differences in the infrared spectra were observed among the hydrates despite their common chain conformations of planar-zigzag mode. Their conclusions show PEI-water interactions influence fundamentally the vibrational frequencies of the internal modes of the aminopolymer. This observation reveals the hydrogen bond strength is distinctive among the crystalline phases in the order of NH---N (anhydrate) > NH---O (hemihydrate) > NH---O (sesquihydrate) > NH---O (dihydrate). The amorphous domain region of LPEI also undergoes a partial change to the crystalline hydrates upon absorbing water. Furthermore, x-ray diffraction and time resolved infrared measurements performed by Hashida et al.14 summarize the hydration process at the studied temperature (60oC). The time dependence of the x-ray diffraction profiles during the hydration process at 60oC shows the LPEI sample in a molten state, perfectly dried and with the melting point of the anhydrate. Upon introduction of water vapor at 60oC, X-ray reflections characteristic of hemihydrate (0.5) and sesquihydrate (1.5) initially appear, followed by the peak intensity of hemihydrate decreasing, and the reflections of sesquihydrate increasing, indicating the transition from hemihydrate (0.5) to sesquihydrate (1.5) with increasing water content. Hence, the solid-state phase of PEI varied according to its water content. Moreover, a structural investigation of PEI utilizing a simultaneous WAXD/DSC measurement approach uncovered that the aminopolymer exhibits both water- and thermally-induced phase transitions among the four classes of crystalline hydrates. Their results suggest that upon heating (or hydration) the amino groups turn into more mobile entities, with the hydrogen bonds becoming much weaker. We advance from their studies; the LPEI anhydrate structure changes predominantly in the temperature region just below the melting point, resulting in the probability the double-stranded helices relaxes into single zigzag chains. CO2 Adsorption Mechanism The TGA experimental data representing the CO2 uptake performance based on the prehydrated polyethylenimine-impregnated CARiACT G10 (HPV) silica sorbent is illustrated in



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Figure 9 at different water contents (0, 2, 8 and 16%) under 1 atm of 10% CO2 (balanced with N2) gas stream at 60oC. Results show the CO2 uptake increased monotonically with CO2 exposure time until the aminopolymer material reaching near-saturation levels. The time required for the sorbent to achieve near-equilibrium levels were longer at the higher water content within the humidified 10% CO2 nitrogen balance gas stream under ambient pressure. It is important to emphasize that the rate of CO2 uptake decreased with increasing water content in the gas stream (Figure 10). The rates were derived by differentiating the experimental data in Figure 9.The maximum rate of process was obtained at t > 0 for all the water levels tested. In general, the CO2 uptake rate increased to a maximum and then decreased as the sorbent approached its near-equilibrium level (i.e., two-stage adsorption kinetics). Prior to the peak, the rate of CO2 uptake decreased with increasing water content in the gas stream. It is evident that the presence of water in the gas stream significantly influences the CO2 adsorption on PEI. The qe of the polyethylenimine-impregnated CARiACT G10 (HPV) increased linearly with content of water in the sweep gas when it was exposed to 10% CO2 on at 60 oC. Assuming that all the uptake was CO2, the qe values were 139, 160, 201.5, and 258 mg/g, at water contents of 0, 2, 8, and 16% (Figure 11), respectively. It was reasonable to assume that all of the uptake was due to CO2 since the sorbent was pre-hydrated for 120 min until it reached equilibrium hydration levels, prior to introducing CO2. The equilibrium sorption of CO2 increased by 6.46 mg CO2/g sorbent for each 1% increase in water vapor in the sweep gas over the experimental test range. Two possible scenarios explain the increase of qe with water content are presented.34 Foremost, low molecular weight polyethylenimine possesses primary and secondary amines which can directly react with CO2 to produce carbamate ions (Eq. 1). Water can then hydrolyze the carbamate ions to produce bicarbonate ions (Eq. 2). Alternatively, low molecular weight PEI can also directly react with CO2 and H2O to produce the bicarbonate ion species (Eq. 3). Both pathways effectively increase the number of available amine sorption sites by supplanting amine functional groups with water and freeing amine to adsorb additional sorbate. The direct reaction of CO2 with water also occurs, however, the reaction rate is reported to be negligible except at low pH values.35 Additionally, as previously reported by Danckwerts and McNeil,36 the carbamate to bicarbonate reaction (or carbamate hydrolysis) is very slow. Attempts were made to fit the CO2 uptake data over the complete range (Figure 9) with suitable rate expressions derived from existing models including a shrinking core model, together with various reaction models.32 These different rate models were systemically examined in fitting the CO2 sorption experimental data employing the pre-hydrated PEI-impregnated CARiACT G10 (HPV) sorbent. The results for every model displayed a significant lack of fit. Taking an approach similar to modeling the moisture adsorption process above, the simple multistep process described by equation (14) was tested for the adsorption of CO2 on prehydrated polyethylenimine-impregnated CARiACT G10 (HPV) particles. This model involves two parallel nucleation or growth processes. In Figure 12 the fit is presented comparing the experimental data to the parallel nucleation growth model. This model compared favorably . (R2>99.9%) with the experimental data (Table 3). For the diffusion controlled nucleation and growth models, the magnitude of the shape factor, nS or nR, directly reflects how the nucleation rate changes under isothermal conditions: 1.5 is identified as a zero-nucleation rate, 1.5–2.5 is a decreasing nucleation rate, 2.5 is a constant nucleation rate, and n values are greater than 2.5 for increasing nucleation rates.37 The values determined for the shape parameter nS range from 0.857 to 1.116 for all water contents; the


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average value of nS being 1.0 ± 0.13 (95% CL). As stated earlier, the observed value of nS = 1 defines the pseudo-first order rate expression. Thus, to simplify the analysis, value of nS = 1 was applied for all water contents which reflects the pseudo-first-order kinetics models. The observed value of nR, range from 2.267 to 2.85; the average value of nR was 2.48 ± 0.32 (95% CL) (see Table 3). Again, to simplify the analysis, a value of nR = 2.5 was exercised for all water contents. This indicated nuclei growth with a constant nucleation rate. The values of wR, aR, and aS, were recalculated based on the approximation of nS = 1 and nR = 2.5 for every set of adsorption data taken at different water content (see Table 3). Figure 13 illustrates how the initial portion of the reaction was dominated by rapid reaction. The combined or effective uptake, as represented by curves S + R, overlap the rapid rate portion in the first few minutes. Hence, slow reaction had little influence on the initial part of the reaction process. At longer times, the total reaction (S+R) was dominated by the slow reaction. Hence, it is concluded that the slow reaction has little influence on the initial part of the reaction process. As illustrated in Figure 14, the weight factors of the rapid reaction clearly show a decrease, together with an increase for the slow reaction with increasing water content. Figure 15 shows the effect of waters content on the kinetics rate constants. The rate constants for both the rapid and slow uptake processes decreased linearly with increasing water contents. An expression for the reaction rate, dX/dt, can be derived by differentiating equation 14 with respect to t at a constant temperature according to equations 16 and 17:

and

$H $H = Q0H (JH − $H ) R−ln U1 − VW & JH

" X" Y P

(16)

$I = 0I (JI − $I ) (17) & Therefore, the total rate is the summation of equations. (16) and (17) as; $HI $H $I $H = + = Q0H (JH − $H ) R−ln U1 − VW & & & JH

" X" Y P

+ 0I (JI − $I ) (18)

The rate–time (dX/dt versus t) data obtained at different water content (2-16%) using equation (18) are shown in Figure 16. The calculated rate-time agreed very favorably with the experimental rate-time data. In addition, the value of the maximum rate decreased with increasing water content (Figure 12). The proposed chemical reaction for post combustion carbon capture in the presence of water vapor utilizing the polyethylenimine-impregnated CARiACT G10 (HPV) sorbent is illustrated by equation 19 for the rapid formation of carbamate: 2 + ↔ + 2 + ↔ + (19) + + ↔ + ′ ACS Paragon Plus Environment


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Together with the reaction of the carbamate ion species in forming the bicarbonate entity which is considered as the slow reaction in this study: + 2 + ↔ +

(20)

In this present work, the interaction of carbon dioxide from the simulated flue gas with the prehydrated immobilized PEI sorbent produces the carbamate ions which principally establishes an “ionic” network on and within the surface and bulk layers of PEI. As illustrated here and in agreement with others, we postulate CO2 absorption on our pre-hydrated, PEI-impregnated CARiACT G10 (HPV) sorbent consists of two adsorption regimes; rapid chemisorption on the PEI surface, followed by bulk diffusion of CO2 within the multiple PEI layers. As CO2 adsorption proceeds, greater amounts of the carbamate “ionic” gel is formed through the complexation of CO2 with PEI molecules. This transformation significantly increases both the density and viscosity of the PEI layers which will impede further diffusion of CO2 within the interior pores or spaces between adsorbent cavities. At low temperature, the pre-hydrated amino polymer would have a higher viscosity and low molecular mobility. In contrast, at the reaction temperature of 60oC the pre-hydrate PEI molecules possesses a higher mobility, allowing for CO2 to diffuse deeper into the substrate’s interior pores and spaces; creating additional layers of carbamate. Moreover, the lower binding constant between the amino-bearing moieties and CO2 at 60oC; coupled with the lower cross-linking efficiency of the pre-hydrated PEI particles with increasing humidity promotes deeper CO2 penetration, resulting in a thicker carbamate layer.

SUMMARY The adsorption rate of water vapor on an anhydrous, PEI-impregnated CARiACT G10 (HPV) solid sorbent was investigated followed by a study of the rate of CO2 uptake on a prehydrate sorbent at various levels of specific humidity (2, 8, and 16%wt.). The effect of water vapor concentration on the adsorption capacities and rates for water and CO2 was examined at 60oC and ambient pressure. The pseudo equilibrium water uptake was found to be nearly proportional to water vapor concentration up to a saturated level of 60 mg H2O/g PEI. Our results show a decrease in the CO2 absorption rate together with an increase in H2O uptake with increasing water vapor concentrations. The interaction of water vapor and its subsequent uptake with the PEI-impregnated CARiACT G10 (HPV) solid sorbent was observed to be of two competing first-order kinetic rates, (1) rapid adsorption which corresponds to the surface adsorption and (2) a slower adsorption which corresponds to the mostly inner particle diffusion process. The relative contributions and kinetic parameters of rapid and slow uptake processes were quantified. At low levels of humidity, the water uptake was dominated by the rapid process; however, when the water vapor reached saturated levels approximately 50% of the uptake followed the slower rate path. The rapid water uptake process was about 4.5 times the slower one. At low humidity where the equilibrium uptake was quite low, there was no discernable contribution from the slow uptake process, and the rapid process was 60% higher than the rates measured at higher humidity. These two processes reflect known changes in the PEI from the tightly coupled and relatively rigid double helical structure in the anhydrate to the carbamate gel consisting of


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hydrogen bonded linear polymer chain. It is thought that the carbamate gel appears to form rapidly on the surface and, with sufficient driving force (higher partial pressure of H2O), penetrates and produces dihydrate complexes. These hydrate complexes apparently open the structure maximizing the adsorbates accessibility to NH moieties, though at a slower geldiffusion process rate. The kinetic analysis produce “n” values near 1 indicative of kinetic controlled rather than diffusion control (n ~ 0.5). As expected, hydrated the PEI impregnated sorbent improved the sorbent capacity to adsorb CO2. Nearly 130 mg CO2 was adsorbed on each gram of fully hydrated PEI with the uptake increasing about 3.5 mg CO2 for each percent increase in water vapor concentration. Furthermore, the reaction of CO2 with the pre-hydrated PEI sorbent was monitored as two competing processes which are described by (1) a rapid nucleation and growth process having an Avrami’s exponent of n = 2.5. This observed result reflects the growth of nuclei with a constant nucleation rate and (2) slow first order rates. A series of possible reaction mechanisms for these two processes were clearly identified. The rates of both slow and fast processes decreased as the humidity levels increased. This may reflect deeper penetration into the PEI layers facilitated by the formation of the hydrated carbamate ions. Similar to the changes observed in hydration rates, the CO2 uptake rates predominately followed the fast process at low levels of hydration, but as the water concentration increased, the number of accessible sites increased, and the slower process contributed a greater proportion of the uptake. When CO2 was adsorbed on anhydrous sorbent over 80% of the uptake followed the rapid pathway; however, the uptake represented only half of that achieved on the fully pre-hydrated sorbent. The amount of rapid CO2 uptake dropped off nearly 3% for each percentage increase in water content. The kinetic analysis produce “n” values near 1 and 2.5 indicative of twostep process, namely, kinetic controlled and nucleation and growth. Low molecular weight polyethylenimine possesses a hydroscopic feature characteristic in the formation of well-defined crystalline hydrates. As illustrated in previous investigations utilizing time-resolved infrared spectra measurements, the above-identified amino polymer chains seizes a completely extended arrangement in the sesquihydrate and dihydrate species, whereas a double-stranded helical appearance is attained in the anhydrate. The striking difference in chain conformation between anhydrate and hydrates lies in every NH-group and water molecule participating in hydrogen bonding. The hydrogen bond networks in the hydrates play a significant role in determining the molecular arrangements and properties, together with defining the amounts of water vapor uptake related to our PEI-impregnated CARiACT G10 (HPV) silica sorbent. We advance a scenario in which a gelled carbamate layer is established through the complexation of pre-hydrate LPEI moieties and CO2 molecules which opens access deeper within the mesopores, but at the same time slowing the diffusion of CO2 into the gel layer, influencing the CO2 adsorption kinetics. Notes The U.S. Department of Energy, NETL, and REM contributions to this paper were prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or



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represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors acknowledge the Department of Energy for funding the research.

REFERENCES 1. Stewart, C.; Hessami, M. A study of methods of carbon dioxide capture and sequestration–the Sustainability of a photosynthetic bioreactor approach. Energy Conv. and Manag. 2005, 46, 403. 2. Yang, H.; Xu, Z.; Fan, M.; Gupta, R.; Slimane, R. B.; Bland, A. E.; Wright, I. Progress in carbon dioxide separation and capture: A review, J. Env. Sci. 2008, 20, 14. 3. Rochelle, G. T. Amine scrubbing for CO2 capture. Science 2009, 325, 1652. 4. Li, K; Cousins, A.; Yu, H.; Feron, P.; Trade, M.; Luo, W.; Chen, J. Systematic study of aqueous monoethanolamine-based CO2 capture process: model development and process improvement. Energy Sci. & Eng. 2015, Open Access, 23-39. 5. Liu, Q.; Shi, J.; Zheng, S.; Tao, M.; He, Y.; Shi, Y. Kinetics studies of CO2 adsorption/desorption on amine-functionalized multiwalled carbon nanotubes. Ind. Eng. Chem. Res. 2014, 53, 11677. 6. Monazam, E.R.; Shadle, L.J.; Siriwardane, R.V. Equilibrium and absorption kinetics of carbon dioxide by solid supported amine sorbent. AIChE J. 2011, 57, 3153. 7. Monazam, E. R.; Shadle, L. J.; Siriwardane, R. V. Performance and kinetics of a solid amine sorbent for carbon dioxide removal. Ind. Eng. Chem. Res. 2011, 50, 10989. 8. Monazam, E. R.; Spenik, J.; Shadle, L. J. Fluid bed adsorption for the removal of carbon dioxide using immobilized polyethylenimine (PEI) on a mesoporous silica: kinetics analysis and breakthrough behavior. Chem. Eng. J. 2013, 223, 795. 9. Qi, G.; Fu, L.; Giannelis, E.P. Sponges with covalently tethered amines for highefficiency carbon capture, Nature Comm. 2014, 5, 5796. 10. Qi, G; Fu, L.; Choi, B. H.; Giannelis, E.P. Efficient CO2 sorbents based on silica foam with ultra-large mesopores. Energy Env. Sci., 2012 5, 7368. 11. Zhang, H.; Goeppert, A.; Czaun, M.; Surya Prakash, G.K.; Olah, G.A. CO2 capture on easily regenerable hybrid adsorbents based on polyamines and mesocellular silica foam. Effect of pore volume of the support and polyamine molecular weight. RSC Adv. 2014, 19403. 12. Zhao, J.; Simeon, F.; Wang, Y.; Luo, G.; Hatton, T. A. Polyethylenimine-impregnated siliceous mesocellular foam particles as high capacity CO2 adsorbents. RSC Adv., 2012, 2, 6509. 13. Lott, G.A.; King, M.D.; Scatena, L.F. Effects of relative humidity on the surface and bulk structures of linear polyethylenimine thin films. J. Phys. Chem. C 2014, 118, 17686.


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14. Hashida, T.; Tashiro, K.; Aoshima, S.; Inaki, Y. Structural investigation of water-induced phase transitions of Poly (ethylene imine). 1. Time-resolved infrared spectral measurements in the hydration process. Macromolecules 2002, 35, 4330. 15. Hashida, T.; Tashiro, K.; Inaki, Y. Structural investigation of water-Induced phase transitions of Poly (ethylene imine). III. The thermal behavior of hydrates and the construction of a phase diagram. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2937. 16. Hashida, T.; Tashiro K.; Structural study on water-induced phase transitions of Poly (ethylene imine) as viewed from the simultaneous measurements of wide-angle x-ray diffractions and DSC thermograms. Macromol. Symp. 2006, 242, 262. 17. Chakravarty, T.; Phukan, U. K.; Weiland, R. H. Reaction of acid gases with mixtures of amines. Chem. Eng. Prog. 1985, 81, 32. 18. Wang, J.; Chen, H.; Zhou, H.; Liu, X.; Qiao, W.; Long, D.; Ling, L. Carbon dioxide capture using polyethylenimine-loaded mesoporous carbons. J. Env. Sci. 2013, 25, 124. 19. Satyapal, S.; Filburn T.; Trela J.; Strange J. Performance and properties of a solid amine sorbent for carbon dioxide removal in space life support applications. Energy and Fuels 2001,15,250. 20. Choi, S.; Drese, J. H.; Jones, C. W. Adsorbent materials for carbon dioxide capture from large anthropogenic point sources, ChemSusChem. 2009, 2, 796. 21. Vaidya, P. D.; Kenig, E. Y. CO2‐Alkanolamine reaction kinetics: A review of recent studies. Chem. Eng. & Technol. 2007, 30, 1467. 22. Hiyoshi, N.; Yogo, K.; Yashima, T. Adsorption characteristics of carbon dioxide on organically functionalized SBA-15. Microporous Mesoporous Mater. 2005, 84, 357. 23. Knowles, G. P.; Graham, J. V.; Delaney, S. W.; Chaffee, A. Aminopropyl-functionalized mesoporous silicas as CO2 adsorbents. Fuel Process. Technol. 2005, 86, 1435. 24. Serna-Guerrero, R.; Da’na, E.; Sayari, A. New insights into the interactions of CO2 with amine-functionalized silica. Ind. Eng. Chem. Res. 2008, 47, 9406. 25. Li, K.; Jiang, J.; Yan, F.; Tian, S.; Chen, X. The influence of polyethyleneimine type and molecular weight on the CO2 capture performance of PEI-nano silica adsorbents. Applied Energy 2014, 136, 750-755. 26. Zhang, H.; Goeppert, A.; Prakash, G.K. S.; Olah, G. Applicability of linear polyethylenimine supported on nano-silica for the adsorption of CO2 from various sources including dry air. RSC Adv. 2015, 5, 52550. 27. Zhang, H.; Goeppert, A.; Olah, G.A.; Prakash, G.K.S. Remarkable effect of moisture on the CO2 adsorption of nano-silica supported linear and branched polyethylenimine. J. CO2 Utiliz. 2017, 19, 91-99. 28. Chaikittisilp, W.; Khunsupat, R.; Chen, T.T.; Jones, C.W. Poly(allylamine)— Mesoporous Silica Composite Materials for CO2 Capture from Simulated Flue Gas or Ambient Air. Ind. Eng. Chem Res. 2011, 50, 14203-14210. 29. Monazam, E.R.; Shadle, L.J.; Miller, D.C.; Pennline, H.W.; Fauth, D.J.; Hoffman, J.S.;. Gray, M.L Equilibrium and kinetics analysis of carbon dioxide capture using immobilized amine on a mesoporous silica, AIChE J. 2013, 59, 3, 923. 30. Monazam, E. R.; Breault, R. W.; Siriwardane, R.; Richards, G.; Carpender, S. Kinetics of the reduction of hematite (Fe2O3) by methane (CH4) during chemical looping combustion: a global mechanism. Chem. Eng. J. 2013, 232, 478.



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31. Monazam, E. R.; Breault, R.; Siriwardane, R.; Miller, D. Thermogravimetric analysis of modified hematite by methane (CH4) for chemical looping combustion: a global kinetics mechanism. Ind. Eng. Chem. Res. 2013, 52, 14808−14816. 32. Janković , B.; Adnađević, B.; Jovanović, J. Application of model-fitting and model-free kinetics to the study of non-isothermal dehydration of equilibrium swollen poly (acrylic acid) hydrogel: Thermogravimetric analysis. Thermochim. Acta 2007,452, 106. 33. Liu, W.X.; Li, W.B.; Hu, J.; Ling, X.; Xing, B.S.; Chen, J.L.; Tao, S. Sorption kinetic characteristics of polybrominated diphenyl ethers on natural soils. Env. Pollution 2010, 158 (9), 2815. 34. Wang, H. C.; Lu, C.; Bai, H.; Hwang, J. F.; Lee, H. H.; Chen, W.; Kang, Y.; Chen, S.-T.; Su, F.; Kuo, S-C.; Hu, F-C. Pilot-scale production of mesoporous silica-based adsorbent for CO2 capture. Appl. Surf. Sci. 2012, 258, 6943. 35. Pintola, T.; Tontiwachwuthikult, P.; Meisen. A. Simulation of pilot plant and industrial CO2-MEA absorbers, Gas Sep. & Pur. 1993, 7. 47. 36. Danckwerts, P. V.; McNeil, K. M. The absorption of carbon dioxide into aqueous amine solutions and the effects of catalysis. Trans. Inst. Chem. Eng. 1967, 45, T32 37. Málek, J. The applicability of Johnson–Mehl–Avrami model in the thermal analysis of the crystallization kinetics of glasses. Thermochim Acta. 1995, 267, 61.

Table 1. The effect of absolute humidity on water uptake on PEI-impregnated CARiACT G10 (HPV) solid sorbent Absolute Humidity, %

H2O Uptake, mol H2O/g of sorbent

H2O Uptake, mol H2O/g of PEI

2 8 16

1.03 4.42 6.67

2.06 8.84 13.34


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Table 2. Parameters obtained by fitting two-step generalized nucleation or growth process Equation (14) to experimental water adsorption H2O % wt.

wR

wS

aR

aS

nR

nS

wR/wS aR/aS

2

1

0

0.212

N/A

1

N/A

N/A

N/A

8

0.827

0.173

0.127

0.029

1

1

4.78

4.38

16

0.487

0.513

0.139

0.03

1

1

0.95

4.63

Table 3. Parameters obtained by fitting two-step generalized nucleation or growth process Equation (14) to experimental CO2 adsorption

H2O %

0

2

8

16

nR=1, nS=2.5

0

2

8

16

wS

0

0.249

0.433

0.665

wS

0

0.295

0.413

0.57

aS

N/A

0.077

0.046

0.071

aS

N/A

0.096

0.051

0.038

nS

N/A

1.036

1.116

0.857

nS

N/A

1

1

1

wR

1

0.751

0.567

0.335

wR

1

0.705

0.587

0.43

aR

0.112

0.042

0.033

0.012

aR

0.067

0.031

0.023

0.022

nR

2.031

2.267

2.306

2.849

nR

2.5

2.5

2.5

2.5

0

39.84

87.25

171.6 qe,S(mg/g)

0

47.2

83.22

147.1

qe,R(mg/g) 139.1

120.2

114.3

86.43 qe,R(mg/g)

139.1

112.8

118.3

110.9

N/A

0.096

0.051

0.038

qe,S(mg/g) kS(min-1)

N/A

0.0842 0.0634 0.0457

kS(min-1)

kR(min-1)

0.340 0.2470 0.2278 0.2117

kR(min-1)

2

R R=Rapid

1

1

1

0.999

R

2

0.3392 0.2492 0.2212 0.2173 0.997

S=Slow


1

0.997

0.998


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Figure 1. Typical experimental test. Three stages show activation, water uptake and humid CO2 uptake. Solid line – Temperature – Right axis. Broken line – Weight change – Left axis.


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Figure 2. Effect of different water concentrations during stage 2 (See Figure 1) on the sorbent weight gain.



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Figure 3. Effect of different water concentrations on the normalized rate of water uptake on dry sorbent during stage 2 (See Figure 1).


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Figure 4. Variation of t/qt (Equation 8) with time for water uptake during stage 2 at different water vapor concentrations.



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Figure 5. Examination of second-order kinetic rate on water uptake during stage 2 for different water concentrations. Symbol for experiment and solid line for the model.

Figure 6. Comparison of experimental water uptake data to data based on shrinking core model (diffusion and reaction control).


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Figure 7. Comparison of experimental water uptake data (symbol) with dry sorbent to the results from two parallel process model (solid lines) for different water concentrations during stage 2.

Figure 8. Predicted curves of water uptake at different water concentrations using two process model. Dashed lines for the slow (S) and solid lines for the rapid (R) uptake processes.



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Figure 9. Effect of water concentration on normalized CO2 uptake for pre-hydrated sorbent during stage 3 (See Figure 1)

Figure 10. Effect of water concentration with 10% CO2 on the rate of CO2 uptake (time derivative of the curves in Figure 9) during stage 3 .


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Figure 11. Effect of water concentration during stage 2 on the sorbent’s CO2 equilibrium capacity in stage 3

Figure 12. Comparison of experimental CO2 uptake data (symbol) for pre-hydrated sorbent (stage 2) with the results from two parallel process model (solid lines) at different water contents



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Figure 13. Predicted curves of CO2 uptake as a function of time using the two process model. Dashed lines for slow (S), solid lines for rapid (R), and heavy lines for the combined (S+R) uptake processes.

Figure 14. Effect of water concentration with 10% CO2 on the weight factor for the slow and rapid uptake processes by pre-hydrated sorbent.


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0.3 y = -0.3451x + 0.2539

kR

0.25 0.2 kR,S(min-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


0.15 0.1

y = -0.3767x + 0.0918

kS

0.05 0 0

5

10 water content (%)

15

20

Figure 15. Effect of water concentration with 10% CO2 on the kinetic rates of slow and rapid uptake processes using pre-hydrated sorbent.

Figure 16. CO2 uptake rate-time data for different water vapor concentrations for experiment (symbol) and two process model (solid line).



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TOC Graphic


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Insights into the Adsorption of Carbon Dioxide in the Presence of

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