Article pubs.acs.org/IECR
Carbon Dioxide Capture with Polyethylenimine-Functionalized Industrial-Grade Multiwalled Carbon Nanotubes Qing Liu,† Junjie Shi,† Qiannan Wang,‡ Mengna Tao,† Yi He,*,† and Yao Shi*,† †
Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemical and Biological Engineering, Zhejiang University, 310027 Hangzhou, China ‡ Institute of Environmental Science & Engineering, Hangzhou Dianzi University, 310018 Hangzhou, China ABSTRACT: Industrial grade multiwalled carbon nanotubes (IG-MWCNTs) were functionalized with polyethylenimines (PEIs) for CO2 capture from simulated flue gas. Ethylenediamine end-capped PEI (PEI-EC) impregnated IG-MWCNTs was found to exhibit a significantly higher adsorption capacity than other branched PEIs impregnated IG-MWCNTs. The PEI-EC impregnated IG-MWCNTs were characterized with various experimental methods including N2 adsorption/desorption isotherms, X-ray diffraction, and thermogravimetric analysis. The CO2 adsorption behavior of the PEI-EC impregnated IGMWCNTs is influenced by PEI loading and adsorption temperature. The PEI-EC impregnated IG-MWCNTs exhibit a CO2 adsorption capacity as high as 2.538 mmol/g at 343 K, and can be completely regenerated higher than 403 K. The CO2 adsorption/desorption kinetics of the PEI-EC impregnated IG-MWCNTs was investigated with Avrami’s fractional order kinetic model. The activation energy of the CO2 adsorption/desorption was calculated from Arrhenius equation and used to evaluate the performance of the adsorbent. aircraft.24 Xu et al. developed a novel “molecular basket” to capture CO2 by using a PEI modified MCM-41.25 MCM-41PEI adsorbent achieves a higher CO2 adsorption capacity than pure PEI or MCM-41 because a synergetic effect takes place inside the pore structure of the sorbent.25 To enhance the adsorption capacity of MCM-41-PEI, PEI supported on poreexpanded (PE) MCM-41 was studied by Heydari-Gorji et al.26 The well-dispersed PEI inside the PE-MCM-41 pores exhibits a CO2 adsorption capacity as high as 4.68 mmol/g at 348 K for 55 wt % PEI loading.27 Sanz et al. investigated the adsorption of pure CO2 on SBA-15 impregnated with branched PEI, and found the maximum adsorption value close to 2.05 mmol/g at 348 K and 1 bar.28 In their further study, PEI impregnated grafted pore-expanded SBA-15 yields the CO2 uptake of 5.34 mmol/g at 318 K and 0.15 bar.29 Lin et al. reported PEI incorporated MIL-101 with different PEI loading for CO2 adsorption. The CO2 adsorption capacity is significantly enhanced after modification, and reaches the maximum of 4.2 mmol/g at 298 K; the selectivity of CO2/N2 is up to 1200 at 323 K.20 The PEI functionalized other sorbents, such as layered silicate sorbent,30 glass fiber matrix,31 and monolith,32 were also studied for CO2 adsorption. In this work, IG-MWCNTs was selected among various porous sorbents because of its low cost, large pores, good thermal conductivity, hydrophobicity, as well as high chemical, thermal, and mechanical stability required to operate in flue gas stream.22,33,34 We systematically studied the CO2 adsorption behavior of IG-MWCNTs impregnated with branched PEI. The PEI-impregnated IG-MWCNTs were characterized with
1. INTRODUCTION There is a growing concern to the global warming caused by increased emission of carbon dioxide in recent years. This has motivated researchers to develop efficient technologies for CO2 capture from large point sources, such as power plants, iron and steel industries, gas processing industries, etc.1 Conventionally, aqueous solutions of amines are used to capture CO2 in stationary emission sources.2 However, the absorption processes are usually accompanied by a large amount of energy consumption and severe corrosion issues.3 As an alternative approach, adsorption processes using porous solid adsorbents for CO2 capture have attracted much attention recently because they can overcome the shortcomings of traditional absorption processes.4−8 Among the porous solid adsorbents, amine-functionalized adsorbents have gained increasing attention for CO2 capture from flue gas stream owing to their high adsorption capacity and selectivity.9 Based on the type of support, the amine functionalized adsorbents can be further classified into following major categories: carbonaceous materials,10,11 zeolite-based sorbents,12,13 silica-supported sorbents,14−16 polymer-based sorbents,17,18 and metal−organic frameworks.19−21 Industrial grade multiwalled carbon nanotubes (IG-MWCNTs) are a low-cost support (one-tenth the price of MWCNTs) for the amine functionalized adsorbents. An adsorbent using IG-MWCNTs as support can reach an adsorption capacity of 2.145 to 3.088 mmol/g.22 Polyethylenimine (PEI) has been widely used in the preparation of amine-functionalized adsorbents because it has more amino groups per molecule than other amine-containing compounds.23 Linear PEI only has secondary amines, while branched PEI contains primary, secondary, and tertiary amino groups. Both linear and branched PEI have been used for CO2 adsorption. The first use of PEI for CO2 capture was conducted by Satyapal et al. to improve the CO2 removal in space © 2014 American Chemical Society
Received: Revised: Accepted: Published: 17468
August 6, 2014 September 26, 2014 October 9, 2014 October 9, 2014 dx.doi.org/10.1021/ie503118j | Ind. Eng. Chem. Res. 2014, 53, 17468−17475
Industrial & Engineering Chemistry Research
Article
where q is the adsorption capacity of CO2 (mmol/g), M is the mass of adsorbent (g), Q is the gas flow rate (cm3/min), c0 and c are influent and effluent CO2 concentrations (vol %), t denotes time (min), T0 is 273 K, T is the gas temperature (K), and Vm is 22.4 mL/mmol. In this work, c0 was 10 vol % at atmospheric pressure, and Q was kept at 50 cm3/min. The adsorbent was regenerated using TSA-N2-stripping method in the same experimental apparatus. The adsorption column was kept at the desired desorption temperature, and the adsorbent was fed into the column. The inlet of the adsorption column was switched to a N2 flow. The CO2 concentration after desorption was measured by the GC once in a minute, thus the volume of pure CO2 was calculated according to the CO2 concentration and N2 flow rate. Therefore, the efficiency of CO2 desorption could be calculated at the given time. The adsorbent was considered complete regeneration when the mass of the regenerated adsorbent equaled that of the adsorbent before CO2 adsorption experiment. 2.4. Adsorption/Desorption Kinetic Model. The CO2 adsorption/desorption kinetics of PEI-impregnated IGMWCNTs are appropriate to evaluate the performance of adsorbents and to understand the overall mass transfer in the CO2 adsorption/desorption process.37 In addition, it is necessary to predict CO2 adsorption/desorption kinetics for the rational simulation and design of gas-treating units.38 Avrami’s fractional order kinetic model was recently developed to simulate phase transition and crystal growth of materials.39 It has been applied to describe the adsorption of CO2 on aminefunctionalized adsorbents.40 The general form of the model is as follows.
various experimental methods including N2 adsorption/ desorption isotherms, X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The effects of different PEI, PEI loading capacity, and adsorption temperature for CO 2 adsorption were investigated. The optimal PEI loading capacity of the adsorbent was determined. The adsorption temperature was evaluated by analyzing breakthrough curves, and a deactivation model was applied to simulate the breakthrough curves. The temperature swing adsorption (TSA) combined with N2 stripping regeneration method was conducted to ascertain the optimum regeneration temperature. In addition, the adsorption/desorption kinetics of CO2 was investigated to obtain insight into the underlying mechanisms. The activation energy Ea of CO2 adsorption/desorption calculated from Arrhenius equation was used to evaluate the performance of the adsorbent.
2. EXPERIMENTAL DETAILS 2.1. Preparation of Adsorbents. Branched PEI (Mw = 300, 600, from Xiya Reagents Co., Ltd., China; Mw = 1800, 10 000, 70 000, from Aladdin Reagents Co., Ltd., China; ethylenediamine end-capped, Mn ≈ 600 by GPC, from Sigma-Aldrich) was incorporated into the IG-MWCNTs (TNIM8, Organic Chemical Co., Ltd., China) supports by wet impregnation.22 A PEI was dissolved in 50 g of ethanol (99.7%, Sinopharm Chemical Reagent Co., Ltd., China) and stirred for 30 min at room temperature, before the addition of 2 g of IG-MWCNTs supports. After stirring for 3 h, the mixture was evaporated at 353 K and subsequently dried at 373 K in open air for 1 h. These samples were denoted as IG-MWCNTsPEI(X)-Y, where X, Y represented the molecular weight of PEI and the weight percentage of PEI in the composites, respectively. As for ethylenediamine end-capped PEI (PEIEC) impregnated IG-MWCNTs, the sample was denoted as IG-MWCNTs-PEI-EC-Y. 2.2. Characterizations of Adsorbents. The surface area and pore volume were measured with static volume adsorption system (Model-ASAP 2020, Micromeritics Inc., USA) by obtaining the N2 adsorption/desorption isotherms at 77.4 K. Prior to the adsorption measurement, the samples were outgassed at 423 K for 24 h. The N2 adsorption/desorption data were recorded at the liquid nitrogen temperature (77 K) and then used to determine the surface areas with the Brunuer−Emmett−Teller (BET) equation. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out with a thermogravimetric analyzer (SDT Q600, TA Instruments, Inc., New Castle, DE) under a dynamic N2 atmosphere from 300 to 900 K, with a heating rate of 10 K/ min. The crystal phase of sorbents were characterized by a powder X-ray diffractometer (XRD, Rigaku D/Max 2550/PC, Rigaku Co., Ltd., Japan) using Cu Kα radiation (40 kV, 30 mA). 2.3. Adsorption/Desorption Experiments. The experiment for CO2 adsorption was carried out by following the procedure described previously.35 Adsorbents were treated under a N2 flow at 423 K for 90 min and then cooled to the desired temperature. The concentration of CO2 was measured by a gas chromatograph (GC). The adsorption capacity of CO2 at a given time was calculated by eq 136 q=
1⎡ ⎢ M⎣
∫0
t
Q
c 0−c ⎤ T0 1 dt ⎥ 1 − c ⎦ T Vm
∂qt ∂t
= kAnAt nA − 1(qe − qt )
(2)
where kA is the Avrami kinetic constant, and nA is the Avrami exponent. The Avrami exponent nA reflects mechanism changes that may take place during the adsorption process.41 The nA is the dimensionality of growth of adsorption sites: nA = 2 for one-dimensional growth, nA = 3 for two-dimensional growth, and nA = 4 for three-dimensional growth.42 For a homogeneous adsorption in which the probability of the adsorption to occur is equal for any region for a given time interval, nA = 1.43,44 An Avrami exponent of exactly 2 indicates perfect one-dimensional growth from adsorption sites which form continuously and at a constant rate.42 The integrated form for the above-mentioned boundary conditions is nA
qt = qe(1 − e−(kAt ) )
(3)
Considering that desorption of CO2 is the decomposition of carbamate/bicarbonate,45 and Avrami’s fractional order kinetic model can well describe the adsorption process, it is possible to reproduce the majority of the decomposition processes by the Avrami model.46 y = 1 − exp( −(kAt )nA )
(4)
where y represents the desorption partion.
3. RESULTS AND DISCUSSION 3.1. Characterizations of Adsorbents. The N2 adsorption/desorption isotherms of IG-MWCNTs-PEI-EC-40 are presented in Figure 1. IG-MWCNTs-PEI-EC-40 shows a smaller adsorption capacity of N2 than unmodified IGMWCNTs.22 The amino groups occupy the inner space of
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dx.doi.org/10.1021/ie503118j | Ind. Eng. Chem. Res. 2014, 53, 17468−17475
Industrial & Engineering Chemistry Research
Article
Figure 3. TGA and DSC profiles of IG-MWCNTs-PEI-EC-40. Figure 1. N2 adsorption (hollow symbols) and desorption (solid symbols) isotherms of IG-MWCNTs-PEI-EC-40.
channels inside IG-MWCNTs, leading to a comparatively lower surface, pore volume, and average pore diameter of 29.17 m2/g, 0.078 mL/g, and 10.70 nm, respectively.22 The XRD patterns of IG-MWCNTs and IG-MWCNTs-PEIEC-40 in Figure 2 shows that the strong diffraction peak at 2θ =
Figure 4. Adsorption capacity of IG-MWCNTs-PEI(X)-Y and IGMWCNTs-PEI-EC-Y at 338 K.
adsorption capacity of IG-MWCNTs-PEI-EC-Y is significantly larger than that of IG-MWCNTs-PEI(X)-Y, which is almost invariant at 338 K. The existence of amino groups at the ends of PEI molecules, as shown in Figure 5, seems to be the major reason accounting for the large difference in adsorption capacity between the two types of adsorbents. The adsorption capacity of PEI-EC impregnated IG-MWCNTs varies considerably with PEI-EC loading capacity. With increasing amine loading, the adsorption capacity of CO2 increases first, reaching the maximum capacity 2.141 mmol/g at amine loading of 40 wt %, and then decreases. When the PEI-EC loading is low, the IG-MWCNTs can be considered to be homogeneously covered with PEI-EC, as PEI-EC molecules are relatively far away from one another which allows the accessibility of CO2 molecules to internal amino sites.28 However, for high PEI-EC loading, the increase of the number of these molecules within the channel structure of IG-MWCNTs can lead to a blocking effect, thus reducing the approachability of CO2 into the inner adsorption sites.28 3.2.2. Effects of Temperature. The effects of adsorption temperature for CO2 adsorption onto IG-MWCNTs-PEI-EC40 were evaluated by analyzing breakthrough curves at different temperature, as shown in Figure 6. The increase of breakthrough time with increasing temperature is possibly related with a chemical adsorption of CO2 with IG-MWCNTs-PEI-EC40. The adsorption capacity of IG-MWCNTs-PEI-EC-40 versus time at varying temperatures is given in Figure 7. The
Figure 2. XRD patterns of IG-MWCNTs and IG-MWCNTs-PEI-EC40.22
26° and weak diffraction peaks at 2θ = 44, 57, and 78° of adsorbents correspond to the graphite (002), (100), (004), and (110) lattice planes.22 The structure of IG-MWCNTs-PEI-EC40 has been retained by comparing the diffraction pattern of both adsorbents. Nevertheless, the peak intensity decreases after PEI-EC impregnation, suggesting that PEI-EC had been impregnated into IG-MWCNTs. TGA and DSC profiles of IG-MWCNTs-PEI-EC-40 are given in Figure 3. The first weight loss region (
