Use of a Robust and Inexpensive Nanoporous TiO2 for Pre

Article pubs.acs.org/EF

Use of a Robust and Inexpensive Nanoporous TiO2 for Precombustion CO2 Separation Saeed Danaei Kenarsari,† Maohong Fan,*,†,‡ Guodong Jiang,†,§ Xiaodong Shen,§ Yuqian Lin,∥ and Xin Hu∥ †

Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071, United States School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States § College of Materials Science and Engineering, Nanjing Universty of Technology, Nanjing 21009, People’s Republic of China ∥ College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, People’s Republic of China ‡

ABSTRACT: The objective of this research is to study the performance of an inexpensive high-surface-area nanoporous titanium oxide (TiO2) on the CO2/H2 separation and resulting pre-combustion CO2 capture. The experiments were carried out at different temperatures (25, 50, 75, 100, and 125 °C) and pressures (5, 10, 15, 20, 25, 30, and 35 bar) using a fixed-bed adsorber. The data obtained for the pure component isotherms and binary gas mixtures were correlated using Sips and Langmuir−Freundlich binary-component-expanded isotherm adsorption (LFBE) models, respectively. Also, the deactivation model was used to simulate the observed CO2 sorption breakthrough curves. Experimental results show that the capture capacities of the sorbent for both H2 and CO2 were improved with the increase in the pressure and decrease in the temperature. The maximum sorption capacities for pure CO2 and H2 were found to be 14.4 and 5.2 mmol/g of TiO2 at 35 bar and 25 °C, respectively. The increase in the temperature and decrease in the pressure improve the sorption selectivity of TiO2 for CO2. The selectivity value of TiO2 reached 9.87 at 125 °C and 5 bar for a CO2/H2 molar ratio of 50:50. TiO2 also shows great stability and regenerability. This study indicates that nanoporous TiO2 is potentially a cost-effective and robust CO2/H2 separation agent and provides the knowledge needed for further demonstration of the nanoporous TiO2-based pre-combustion CO2 separation technology.

1. INTRODUCTION It has been largely accepted that the increase of the CO2 concentration in the atmosphere contributes to the greenhouse effect and, thus, causes considerable global climate change.1−8 Because hydrogen is a clean and efficient energy source, its demand has been growing rapidly in recent years.3,9,10 Currently, more than 95% of H2 is produced by combining steam methane reforming (SMR) with water−gas shift (WGS).11 The resulting gas mixture is mainly composed of H2 and CO2. Also, in the integrated gasification combined cycle (IGCC) technology, a fossil energy carrier, such as coal, is gasified using an oxidizer (O2 and steam), thus yielding CO and H2. Then, CO is converted to H2 and CO2 with the WGS process, and additional H2 is produced.12 The produced gas mixture containing 40% CO2 and 60% H2 leaves the reformer at a high pressure (35 bar). Lastly, to produce high-purity H2 as a clean fuel, H2 needs to be separated from the H2/CO2 mixture. Multiple approaches have been proposed, such as cryogenic distillation, absorption with liquids, membrane purification, and sorbent adsorption.3,11 The adsorptive separation of gases, where one component interacts more strongly with the internal surfaces of a porous material, is a promising approach for CO2/H2 separation.13 This cyclical process exposes a gas mixture into a bed filled with porous sorbents, through which one gaseous component advances quickly to the other end, while the other gas is adsorbed onto the internal surface of the solid material and can be removed by either vacuum/pressure swing adsorption © 2013 American Chemical Society

(VSA/PSA) or temperature swing adsorption (TSA) to regenerate the bed.14 PSA is an economic and appealing option for CO2 separation because of its low energy requirements and relative simplicity. PSA is a mature technology used to separate gas species from a mixture of gases according to the adsorption affinity and/or kinetics of the species with regard to the sorbent material. It can be a highly effective process, provided that sorbents with high selectivity and high capacity are available. Examples of the most widely used CO2 sorbents are carbon,15−18 zeolites,3,19 and metal−organic frameworks (MOFs).13,20−22 Low selectivity or regenerability are the main disadvantages of carbon and zeolite sorbents.23 Drawbacks of MOFs are higher costs, thermal instability, and a drastic drop in the selectivity because of the residual presence of even a small amount of H2O in the gas mixture.24 The exceptional porosity and thermal stability of nanoporous sorbents (i.e., silica or titania based) make this class of sorbents exciting candidates to replace or augment the current suite of available sorbents. Titanium-based materials have been employed successfully in various fields of science and technology.25 Over the past decade, materials derived from TiO2 have been broadly investigated for numerous applications, including solar cells/batteries,26,27 electroluminescent hybrid Received: September 21, 2013 Revised: October 21, 2013 Published: October 24, 2013 6938

dx.doi.org/10.1021/ef4019004 | Energy Fuels 2013, 27, 6938−6947

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Figure 1. Schematic of the high-pressure adsorption apparatus: (1) hydrogen cylinder, (2) carbon dioxide cylinder, (3) argon cylinder, (4) nitrogen cylinder, (5−8) MFC, (9−12, 22, and 24) back-pressure regulator, (13−15) three-way valve, (16) temperature controller, (17) thermocouple, (18) adsorber, (19) glass wool, (20) heating tape, (21) filter, (23) buffer tank, (25) MS, (26) computer, (27) MFC power supply/control module, and (28) hydrogen analyzer.

devices,28 and photocatalysis,29 because of their remarkable chemical and physical characteristics. In addition, titaniumbased materials with high specific surface area and ion exchangeability have shown exciting prospects for postcombustion CO2 capture.30 Nevertheless, TiO2 has not been evaluated for its performance on pre-combustion CO 2 separation. To fill the above-mentioned gap, in this study, the new titania-based sorbent (C8-Ti), a type of nanoporous TiO2, was evaluated for CO2/H2 separation. C8-Ti was prepared by the templating method.31

Quantachrome instrument (model ASIQC0100-4). The sample was first degassed in vacuum conditions at 150 °C for at least 4 h before the adsorption analysis was performed. The specific surface area of the sorbent was determined by the standard Brunauer−Emmett−Teller (BET) equation at a relative pressure range from 0.06 to 0.3. The highest relative pressure (p/p0 = 0.995) data were used to calculate the total pore volume of the sorbents. The pore size distribution (PSD) of the sorbent was obtained by the density functional theory (DFT) model. X-ray diffraction (XRD) data for the TiO2 sorbent before and after multiple adsorption/desorption cycles were collected using a Rigaku smartlab automated XRD system, equipped with a θ−θ XRD goniometer and a solid-state X-ray detector. The operating current and voltage were set at 40 mA and 40 kV, respectively. For XPS, spectra were obtained using a Kratos Axis Ultra DLD X-ray photoelectron spectrometer (XPS) with a monochromated Al Kα source (1486 eV) running at 150 W. Survey scans were performed using the pass energy of 80 eV and a step size of 1 eV. High-resolution scans were performed using the pass energy of 40 eV and a step size of 0.1 eV. Further characterization was performed for TiO2 using Hitachi TM1000 field emission scanning electron microscopy (SEM). For transmission electron microscopy (TEM), micrographs were obtained under bright field illumination using a FEI Tecnai G2 F20 S-Twin transmission electron microscope with a field emission gun (FEG) running at 200 kV accelerating voltage. 2.2. CO2/H2 Separation Setup and Operation Procedure. All experiments are conducted in the one-column adsorption apparatus, as shown in Figure 1. The apparatus was constructed with stainless-steel tubes. All parts were connected with 1/8 in inner diameter tubing and Swagelok three-way valves. The stainless-steel adsorber has a length of 22 cm and an inner diameter of 1/2 in. The adsorber is wrapped by an electric heating tape, and the temperature is monitored using a K-type thermocouple. The adsorber is insulated to allow for conducting

2. EXPERIMENTAL SECTION 2.1. Nanoporous TiO2 Preparation. 2.1.1. Synthesis. TiO2 was synthesized using the ligand-assisted templating method. Titanium(IV) isoproxide (15 g), obtained from Aldrich, was blended with octylamine (3.41 g), purchased from J&K Scientific, to obtain a homogeneous, colorless solution. Then, 15 mL of distilled water was added to the homogeneous solution while continuously being stirred, which led to an immediate precipitation. HCl (2.6 mmol), obtained from Quzhou Reagent Juhua Co., Ltd., was then added to this mixture and set at room temperature for 12 h before the precipitant was aged at 40 °C for 2 days, 60 °C for 2 days, and then 80 °C for 4 additional days. The produced white solid material was placed into a sealed tube in an oven at 100 °C for 2 days, 120 °C for 2 more days, and then 140 °C for 2 additional days. Then, the white solid was consecutively washed with a mixture of methanol (375 mL) and diethyl ether (125 mL) and with 500 mL of methanol, supplied by Sinopharm Chemical Reagent Co., Ltd. The resulting TiO2 was put into an oven at 150 °C for 2 days before it was used for CO2/H2 separation in this study. All of the chemicals were used as received without any further purification. 2.1.2. Characterization. The pore structure of the TiO2 sorbent was analyzed with nitrogen adsorption at 77 K, using the 6939


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experiments at a variety of temperatures with minimum heat loss. The system pressure is preserved by Swagelok back-pressure regulators (9, 10, 12, 14, 22, and 24). The mass flow rate controllers (MFCs) (5, 6, 7, and 8) and the CM4MFC power supply/control module (27) were purchased from Parker. The volume of the buffer tank (23) is 300 cm3. The buffer tank was used to ensure the stability of the pressure and flow rate of the stream before entering the mass spectrometer. The mass spectrometer (MS, Agilent 5975C) and the H2 analyzer (Horiba TCA-300) were used to monitor the outlet gas composition online. The pressure of gases past the back-pressure regulator is slightly higher than atmospheric pressure, which would allow for the mixing of Ar as the trace gas with them. A combination of manual and automatic valves allowed for numerous options of flow. The adsorption/ desorption operation is controlled through the LabVIEW software. Prior to experiments, the MFCs, the MS, and the H2 analyzer were calibrated. In the calibration process of MFC, the inlet and outlet pressures of MFC for all of the gases (H2, CO2, and N2) were kept at 20 and 15 bar using corresponding pressure regulators. However, the flow rate of Ar, as the trace gas, was kept at a constant value, and the inlet and outlet pressures of MFCs were kept at 80 and 20 psi, respectively. A regulator (24) was used to ensure that the inlet pressure of MS was kept stable at 1.1 bar. All of the MFCs were calibrated with a wet test meter (Elser American Meter). Nanoporous TiO2 was mixed with sand to avoid an excessive pressure drop of the gas stream along the adsorber (16). The CO2/H2 mixture, with varying CO2/H2 mole ratios, was fed into the adsorber using MFC (5) and MFC (6). The CO2/H2 separation was performed through two consecutive steps, adsorption and desorption. Adsorption started with introducing N2 into the adsorber until the pressure in the adsorber elevated to a set value, followed by closing the back-pressure regulator (22) and simultaneously venting the incoming N2 through two three-way valves (13 and 14). In this manner, the pressure inside the adsorber was maintained at the constant set value, while the concentration of N2 entering MS gradually decreased, as shown in Figure 2a. The flow conditions were varied as designed: CO2 from its cylinder (1), H2 from its cylinder (2), or a CO2/H2 mixture prepared with two MFCs [H2 MFC (5) and CO2 MFC (6)] was introduced into the adsorber to displace the filled N2. The gas sorption step proceeded until equilibrium concentration profiles of CO2 and H2 were observed on the MS, and the apparent mole decrease(s) of the gas(es) during the whole sorption period, Mj,a, was calculated. The next step required the regulator (22) to shut off the flow from the filter (21) into the buffer tank (23). The volume(s) of CO2 or H2 in the space between the regulator (22) and MS, Mj,bpr‑MS, was then measured. The flow rate of H2 was calculated using the H2 analyzer readings, according to the following equation: H 2 (%) =

FH2 FH2 + FCO2 + FAr + FN2

(1)

where F is the molar flow rate. The sorption capacities of TiO2 for CO2 and H2 were calculated using the following equation:32 Qj =

1 ⎢⎡ mso ⎢⎣

∫0

ts

(Fj ,in − Fj ,out)dt −

∫0

t

F′j ,out dt −

yj ,in PVeff ⎤ ⎥ ZjRT ⎥⎦ (2)

Figure 2. (a) Mole flow rate profile of gases during N2 filling, (b) H2− CO2 sorption, and (c) N2 evacuation periods (sorbent, TiO2; inlet CO2/H2 mole ratio, 50:50; temperature, 50 °C; and pressure, 10 bar).

where j is either CO2 or H2, Qj stands for the sorption capacities of the sorbent for the two gases, mso is the mass of the sorbent, Fj,in and Fj,out refer to the inlet and outlet molar flow rates of CO2 and H2, respectively, ts is the time needed to reach sorption equilibrium, as ′ represents the molar flow rate of CO2 or H2 shown in Figure 2b, Fj,out entering the MS at the end of the sorption, t is the time required for purging all of the gas(es) existing between the gas back-pressure regulator (24) and the MS (25) at the end of adsorption, as shown in Figure 2c, yj,inlet is the inlet mole fraction of CO2 or H2, P is the sorption pressure, T is the sorption temperature, Zj is the compressibility factor of CO2 or H2, R is the universal gas constant, and Veff is the effective volume used for gas sorption and calculated using the following equation:

Veff = Vtot −

msa m − so ρsa ρso

(3)

where Vtot (30.0 cm3) is the adsorber total volume, msa (22.0 g) and mso (3.0 g) are the weights of sand and sorbent, respectively, ρsa (2.63 g/cm3) and ρso (3.8 g/cm3) are the true densities of sand and TiO2, respectively. On the basis of all known values in eq 3, the calculated Veff was 20.8 cm3. 6940


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In this study, experiments were conducted at five different temperatures (25, 50, 75, 100, and 125 °C) and seven different pressures (5, 10, 15, 20, 25, 30, and 35 bar) with pure gases and a CO2/H2 mixture (50:50 molar ratio).

3. RESULTS AND DISCUSSION 3.1. Characteristics of the Sorbent. The BET surface area, pore total volume, and pore radius of TiO2 are 1234 m2/g, 0.68 cm3/g, and 16 Å, respectively. Figure 3 depicts the pore

Figure 5. SEM image of TiO2.

adsorption, particularly in a binary-component mixture of CO2/H2, will be discussed below. As shown in Figures 6 and 7,

Figure 3. Pore size distribution of TiO2 with the DFT method and total pore volume versus the pore radius.

size distribution of TiO2, the sorbent used for this research. The obtained TEM image of TiO2 (Figure 4) indicates that TiO2

Figure 6. Experimental and theoretical adsorption equilibrium isotherms of single-component CO2 and H2 on TiO2 at different temperatures (symbols, experimental; solid lines, theoretical).

Figure 4. TEM image of TiO2.

consists of an unfragmented aggregated nanoporous structure. From SEM image (Figure 5), it is visible that the TiO2 sorbent has a relatively uniform particle distribution with an average particle size of 10 μm. BET, TEM, and SEM images show that TiO2 has a nanoporous netlike cluster structure. 3.2. Effects of the Temperature and Pressure on CO2/ H 2 Separation. The important factors affecting CO 2

Figure 7. Experimental and theoretical adsorption equilibrium isotherms of binary CO2/H2 (molar ratio of 50:50) on TiO2 at different temperatures (symbols, experimental; solid lines, theoretical). 6941


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used in eq 5. The variation of the isosteric heat with the amount of CO2 and H2 adsorbed is displayed in Figure 8.

regardless of single or binary components, CO2 and H2 adsorption capacities are improved with increasing the pressure up to 35 bar. At 25 °C and 35 bar, sorption capacities for pure CO2 and pure H2 reach 14.4 and 5.2 mmol/g of TiO2, respectively. TiO2 as a metal oxide has a polar surface because of the mixed ionic and covalent bonding.33 H2 has a weak quadrupole moment, whereas CO2 has a much stronger quadrupole moment, and thus, the electrostatic interaction between CO2 and TiO2 is much stronger than that between H2 and TiO2. Hence, the CO2 sorption capacity is more than that of H2. As shown in Figure 6, sorption capacities for CO2 and H2 decrease with increasing the temperature from 25 to 125 °C, because of dominant physical adsorption on the surface of nanoporous TiO2. For pure CO2 and pure H2 adsorption, the most widely used analytical isotherms are Langmuir and Freundlich isotherms. The Langmuir model describes monolayer adsorption (the adsorbed layer is one molecule in thickness) on an ideal and flat surface assuming surface homogeneity, localized adsorption on the solid surface, and energetically equivalent adsorption sites.18 Freundlich describes the non-ideal and reversible adsorption. This empirical model can be applied to multilayer adsorption, with non-uniform distribution of adsorption heat and affinities over the heterogeneous surface. Furthermore, Sips isotherm34 is a combined form of Langmuir and Freundlich expressions applied to predict the heterogeneous adsorption systems and avoid the limitation of lacking a fundamental thermodynamic basis at vanishing concentrations with the Freundlich isotherm model.34 At low adsorbate concentrations, it reduces to the Freundlich isotherm, while at high concentrations, it predicts a monolayer adsorption capacity characteristic of the Langmuir isotherm.34 The Sips isotherm equation can be written in the following general form: q (Kp)1/ n = qe 1 + (Kp)1/ n

Figure 8. Isosteric heat of adsorption of CO2 and H2 as a function of the amount of gas adsorbed.

Adsorption processes in porous materials are governed by the interplay between the strength of fluid−wall and fluid−fluid interactions as well as the effects of the confined pore space on the state and thermodynamic stability of fluids confined to narrow pores. These governing interactions manifest themselves in the shape or type of the adsorption isotherm.36 In this study, we used the Sips isotherm to model the singlecomponent CO2 and H2 adsorption data at different temperatures, as shown in Figure 6. The adsorbed amount of H2 and CO2 increased continuously, while pressure increased and a gradual flattening was observed for H2 when the pressure was sufficiently high; however, the CO2 isotherms were almost linear. According to the International Union of Pure and Applied Chemistry (IUPAC),37 the adsorption isotherms shown in Figure 6 belong to a type I isotherm class, where the uptake is governed by the accessible pore volume rather than the internal surface area. The Sips isotherm parameters of the single-component CO2 and H2 are shown in Table 1. The Sips model provides a good description of the experimental data.

(4)

where q and qe are the number of moles adsorbed at a given pressure and the number of moles adsorbed at saturation, respectively, p is the pressure, and K and n are constants. The constant n is often regarded as the heterogeneity factor, with values greater than 1 indicating a heterogeneous system. Values close to 1 indicate a material with relatively homogeneous binding sites. For n = 1, the Sips model becomes equivalent to the Langmuir equation.18 Application of the models to the experimental data was performed with the optimization toolbox in MATLAB, where the values of the different fitting parameters were found by minimizing the sum of the squared relative errors (SSE) between the predicted and experimental adsorption amounts at all conditions. The isosteric heat of adsorption can be obtained using the Clausius−Clapeyron equation as follows:35 Q st = −RT 2

∂ ln p ∂T

q

Table 1. Parameters of the Sips Model for Pure H2 and CO2 temperature (°C) CO2

H2

(5)

25

50

100

qCO2,e (mmol/g)

43.9

29.4

19.5

KCO2 (×10−2, bar−1)

1.29

0.85

0.78 1.31

nCO2

1.06

1.13

qH2,e (mmol/g)

6.72

5.37

5.11

KH2 (×10−2, bar−1)

6.87

6.08

5.70

n H2

0.75

0.90

1.02

In this paper, the adsorption of the CO2/H2 mixture with the molar ratio of 50:50 was studied (Figure 7). Two models are commonly used to describe the binary-component adsorption: Langmuir binary-component isotherm adsorption and Langmuir−Freundlich binary-component isotherm adsorption (LFB). Particularly, LFB can be described as

The use of this equation is based on the assumption that the gas bulk phase behaves like an ideal gas and that the volume of the adsorbed phase can be neglected.12 On the basis of the experimental isotherms at different temperatures, the isosteric heat can be obtained by plotting ln p versus 1/T for a constant q. These points are fitted with a straight line, whose slope is 6942


Energy & Fuels qCO

2

qCO ,e

=

2

qH

2

qH ,e

Article

(K CO2pCO )1/ n 1 + (K CO2pCO )1/ n + (K H2pH )1/ n 2

=

2

of 50:50 are shown in Figure 9. The selectivity of TiO2 toward CO2 improves as the temperature increases or the pressure

2

(6)

2

(K H2pH )1/ n 2

1 + (K CO2pCO )1/ n + (K H2pH )1/ n 2

(7)

2

where pCO2 and pH2 are the partial pressures, KCO2 and KH2 are the adsorption equilibrium constants, qCO2,e and qH2,e are the binary-component equilibrium adsorption constants, qCO2 and qH2 are the adsorption capacity of CO2 and H2, respectively, and n expresses the adsorption order of the heterogeneous surface. In eqs 6 and 7, order n of CO2 is the same as that of H2 but the effects of the sorbent surface on the different adsorbates are not the same; therefore, order n of CO2 and H2 is different and needs to be defined as nCO2 and nH2, respectively. Therefore, the Langmuir−Freundlich binary-component-expanded isotherm adsorption (LFBE) model can be obtained via eqs 8 and 9, and the parameters of TiO2 between 25 and 125 °C for CO2/ H2 molar ratios of 50:50 are shown in Table 2. qCO

2

qCO ,e

=

2

qH

2

qH ,e 2

decreases. The selectivity value of TiO2 reaches 9.87 at 125 °C and 5 bar for a CO2/H2 molar ratio of 50:50. Although there are some studies using other types of adsorbents (MOFs,21 activated carbon,38 and Zeolite 13X39) reporting higher CO2 capture capacities and selectivities in the literature, nanoporous TiO2 can potentially be a promising sorbent for pre-combustion CO2 capture because of its significant advantages, i.e., low cost and remarkable thermal stability and robustness under harsh conditions (125 °C and 35 bar), while offering relatively high CO2 capture capacity and selectivity. 3.4. CO2 Sorption Breakthrough Analysis. Dependent upon the characteristics of attractive forces between the sorbent and the adsorbate, there are two types of adsorption. Physical sorption or physisorption occurs when a gas is brought into contact with a solid and involves only relatively weak intermolecular van der Waals forces (long-range London dispersion forces and the short-range intermolecular repulsion).36 Because of weak van der Waals forces, the heat of physical adsorption is low and the activation energy of physisorption is zero. High pressures and low temperatures favors more physical sorption, while desorption is favored at low pressures and high temperatures. Conversely, chemical adsorption or chemisorption is driven by the interaction between the molecules of the adsorbent and those of the adsorbate. Generally, chemisorption is exothermic, releasing high heat energy. Chemisorption may be strongly repressed at low temperatures. To identify the type of sorption and obtain the activation energy of CO2 sorption onto the TiO2 surface, the following deactivation model was used. Equation 11 shows the adsorption reaction between CO2 and a vacant site of the TiO2, resulting in the formation of the activated site (CO2··· TiO2). In addition, equation 12 demonstrates the CO2 desorption, which results in the formation of a vacant site and CO2. A gradual deactivation of the sorbent occurs because of the formation of activated sites and change of the pore structure and active surface area.

(K CO2pCO )1/ nCO2 2

1 + (K CO2pCO )1/ nCO2 + (K H2pH )1/ nH2 2

=

Figure 9. Selectivity of TiO2 at binary-component CO2/CH4 with a molar ratio of 50:50.

2

(8)

(K H2pH )1/ nH2 2

1 + (K CO2pCO )1/ nCO2 + (K H2pH )1/ nH2 2

2

(9)

Table 2. Parameters of the LFBE Model for CO2/H2 with a 50:50 Molar Ratio temperature (°C)

25

50

75

100

125

qCO2,e (mmol/g)

36.3

32.5

25.5

21.0

16.1

KCO2 (×10−2, bar−1)

1.5

1.1

1.0

0.9

0.7

nCO2

1.08

1.11

1.21

1.28

1.54

qH2,e (mmol/g)

4.04

2.99

2.74

1.80

1.70

KH2 (×10−2, bar−1)

6.6

5.7

5.0

4.5

3.9

n H2

0.83

0.77

0.78

0.82

0.99

3.3. CO2 Sorption Selectivity. In separation processes, a good indication of the potential for successful separation is the selectivity of a porous material for one component over other components in the mixture. In most of the experimental work available in the literature, the selectivity is calculated on the basis of single-component adsorption of H2 and CO2 but the selectivity analysis was based on binary adsorption of a simulated mixture of CO2/H2 in the present study. The selectivity for CO2 over H2 is defined as: ⎛ xCO ⎞⎛ yH ⎞ 2 ⎜ 2 ⎟ ⎟⎟ S = ⎜⎜ ⎟ ⎜ ⎝ x H2 ⎠⎝ yCO2 ⎠

(10)

where xCO2 and xH2 are the mole fractions of CO2 and H2, respectively, in the absorbed phase, and yCO2 and yH2 are the corresponding mole fractions in the bulk phase. Three-dimensional (3D) diagrams of TiO2 selectivity of CO2 over H2 for the binary-component CO2/H2 with a molar ratio 6943

CO2 + TiO2 → CO2 ··· TiO2

(11)

CO2 ··· TiO2 → CO2 + TiO2

(12)


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In the deactivation model, an activity term is considered to manifest the deactivation because of the decrease in the vacant cite concentration. This model assumes a pseudo-steady-state and ignores the axial dispersion term. Thus, the species conservation equation for CO2 and the rate equation for the activity of the solid reactant can be written as eqs 13 and 14, respectively.40−42

−Q

dCA − k 0CAα = 0 dms

(13)

dα = −kdCAn α m (14) dt where Q (mL/min) is the volumetric flow rate, CA (vol %) is the CO2 concentration in the exit flow, ms (g) is the sorbent mass, k0 (mL min−1 g−1) is the initial adsorption rate constant, α is the activity of the TiO2, t (min) is the time, and kd (min−1) is the deactivation rate constant. The analytical solution of the deactivation model (eq 15) is obtained by assuming n = 0 and m = 1 and the initial activity of the solid as 1.9,43 −

⎤ ⎡ km CA = exp⎢ − 0 s exp( −kdt )⎥ ⎦ ⎣ Q CA0

Figure 11. Determination of parameters of the Arrhenius form of initial CO2 sorption (sorption temperature, 25−125 °C; sorption pressure, 20 bar).

(15)

This solution assumes a fluid-phase concentration that is independent of the deactivation process along the adsorber. Figure 10 depicts the experimental CA/CA0 data as a function of

Figure 12. Determination of parameters of the Arrhenius form of TiO2 deactivation (sorption temperature, 25−125 °C; sorption pressure, 20 bar).

relatively small obtained activation energies, the sorption of CO2 on TiO2 appears to be a physisorption and chemisorption process rather than a simple physisorption or chemisorption process. The activation energy values obtained in this study for CO2 sorption onto TiO2 are larger than the activation energy values for sorption of CO2 onto Na2CO3 (E0 value of 2.856 kJ/ mol and Ed value of 3.092 kJ/mol)43 and smaller than the activation energy for sorption of CO2 onto Mg2SiO4 (E0 value of 43.2 kJ/mol and Ed value of 48.5 kJ/mol).42 3.5. Mass Transport and Diffusion of CO2. The fractional uptake curves of CO2 on TiO2 versus the sorption time at different temperatures and pressures, measured experimentally, are plotted in Figures 13 and 14, respectively. When the higher sorption temperature or lower sorption pressure was applied, a shorter time was required for the equilibrium to be reached. To obtain the diffusivity for CO2 through TiO2 sorbent, a classical pore diffusion model was employed.44 Assuming that the heat transfer is sufficiently rapid relative to the sorption rate and the particles are spherically shaped, the diffusion equation in spherical coordinates is written as44,45

Figure 10. Experimental data and numerical model of CO 2 breakthrough curves (pressure, 20 bar).

time at different temperatures accompanied with the deactivation model predictions. Through nonlinear regression analysis of the experimental CO2 sorption breakthrough data at different temperatures, the two parameters k0 and kd were obtained. Thus, the Arrhenius forms of CO2 sorption with TiO2 of the present experiments are: k 0 = A 0 exp[−E0 /RT ]

(16)

kd = Ad exp[−Ed /RT ]

(17)

Plots of ln(k0) versus 1/T and ln(kd) versus 1/T are shown in Figures 11 and 12, respectively. Through regression, an E0 value of 4.81 ± 0.30 kJ/mol and an A0 value of 1.12 × 105 mL min−1 g−1 are found, respectively. In addition, regression found an Ed value of 9.39 ± 0.94 kJ/mol and an Ad value of 52.14 min−1, respectively. The prediction of the deactivation model shows good agreement with the experimental data. Considering the

∂q ∂q ⎞ 1 ∂⎛ = 2 ⎜r 2Dc ⎟ ∂t ∂r ⎠ r ∂r ⎝ 6944

(18)


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diffusion time constants Dc/r2c . Tables 3 and 4 list the diffusion time constants for CO2 on TiO2 sorbent at different Table 3. Summary of Diffusion Time Constants of CO2 on TiO2 for Different Temperatures T (°C)

25

50

75

100

125

Dc/r2c (×10−4 s−1)

2.09

2.70

3.44

4.30

7.42

Table 4. Summary of Diffusion Time Constants of CO2 on TiO2 for Different Pressures P (bar)

5

10

15

20

25

30

Dc/r2c (×10−4 s−1)

11.55

9.17

5.21

3.20

2.70

2.01

temperatures and pressures. It can be seen that the diffusivity of CO2 falls in the range of 2−12 × 10−4 s−1. The diffusion coefficient increases with increasing temperature and decreases with increasing pressure, which is consistent with the Chapman−Enskog theory.46 3.6. Sorbent Regeneration. For practical application and prolonged operation, the sorbent should possess a high adsorption capacity for CO2 and high selectivity toward CO2 and exhibit high stability. The stability of TiO2 was investigated through consecutive adsorption−desorption cycles in the current apparatus using pressure swing regeneration. In a typical adsorption−desorption cycle, the sample was introduced to the binary-component CO2/H2 with a molar ratio of 50:50 at 25 °C and 25 bar, followed by regeneration using the flow of nitrogen for 1 h. For each cycle, the adsorption capacity was calculated. Figure 15 shows the CO2 uptake for 10 such

Figure 13. Fractional uptake profiles of CO2 on TiO2 at different temperatures at 25 bar.

Figure 14. Fractional uptake profiles of CO2 on TiO2 at different pressures at 50 °C.

where Dc is the diffusion constant and q(r,t) is the adsorbed phase concentration at time t and radial position r. Although the sorbent particles are not exactly spherical, representation as an equivalent sphere is a reasonable assumption for most particle shapes.41 If the volume of the bulk gas is sufficiently large, so that the uptake of a single adsorbent particle is small, relative to the total sorbate introduced to the system, the initial boundary and conditions can be given as follows: q(r , 0) = q0′

q(rc , 0) = q0

∂q ∂r

=0 r=0

Figure 15. CO2 and H2 uptake of TiO2 under 50 vol % CO2 and 50 vol % H2 at 25 °C and 25 bar as affected by the number of adsorption/ desorption cycles (desorption conditions: temperature, 25 °C; carrier gas, N2 with a flow rate of 76 mL/min).

(19)

where rc is the particle radius. The solution of eq 18 is given by q ̅ − q0′ q0 − q0′

=

mt 6 =1− 2 m∞ π

∞

∑ n=1

⎛ n2π 2D t ⎞ 1 c ⎜− ⎟ exp 2 n2 r ⎝ ⎠ c

(20)

adsorption−desorption cycles. Under these conditions, TiO2 exhibited remarkable stability during extensive pressure swing cycling over multiple adsorption/desorption cycles. Table 5 shows the BET surface area, the total pore volume, and the pore size of TiO2, for both fresh sorbent and after multiple cycles. The BET surface area and the total pore volume reduced because of compression at 25 bar after 10 cycles, but these values are still remarkably high. XRD spectra (Figure 16) of the sorbent after multiple cycles in comparison to the fresh sorbent showed that the characteristic bands exhibited identical intensities. This

where q(t) ̅ is the average adsorption amount in the particle, q′0 is the initial adsorption amount in the particle, q0 is the equilibrium uptake in the particle, and mt/m∞ is the fractional adsorption uptake. At short times, eq 18 is approximated by44 1/2 mt Dt 6 ⎛ Dct ⎞ ⎜ 2 ⎟ − 3 c2 ≈ m∞ π ⎝ rc ⎠ rc

(21) 44

This expression is accurate to within 1% for mt/m∞ < 0.85. In this study, eq 21 is used as the diffusion equation to predict the 6945


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adsorption capacities were improved with increasing the pressure in the range of 5−35 bar and decreasing the temperature within the range of 25−125 °C. Sips and LFBE models were used to model adsorption of pure gases and a CO2/H2 mixture (with the CO2/H2 molar ratio of 50:50) on TiO2, respectively. The deactivation model was found to fit the CO2 breakthrough curves well. Also, TiO2 shows good regenerability over multiple adsorption/desorption cycles, which was confirmed by BET, XRD, and XPS analysis results. Therefore, TiO2 is a promising cost-effective material for CO2/ H2 separation, although more work needs to be performed before the feasibility of using the material as a sorbent becomes a reality. For example, life-cycle analysis of the nanoporous TiO2-based CO2/H2 separation and the impacts of the presence of H2O in the stream need to be performed. Modification of TiO2 with alkyl compounds, such as amines, to improve the selectivity of TiO2 could be studied as well.

Table 5. Structural Properties of Nanoporous TiO2 (Fresh and after 10 Adsorption/Desorption Cycles) samples

BET (m2/g)

total pore volume (cm3/g)

DFT pore radius (Å)

fresh after 10 cycles

1234 980

0.68 0.56

16 12.5

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +1-307-766-5633. Fax: +1-307-766-6777. E-mail: [email protected]. Notes

Figure 16. XRD spectra of TiO2.

The authors declare no competing financial interest.

■

indicates the stability of the crystal structure of this sorbent. Diffraction peaks at 25.3° and 48° indicate that TiO2 is in the anatase phase.47 The intensities of XRD peaks of the samples reflect that the nanoparticles are crystalline, and broad diffraction peaks are an indication of amorphous structure. Figure 17 shows the XPS signal of fresh TiO2 and the sorbent after 10 cycles. There is no shift in binding energy,

ACKNOWLEDGMENTS This work was funded by the Wyoming Clean Coal Program, the National Natural Science Foundation of China (21106136), and the National Undergraduate Training Programs for Innovation and Entrepreneurship of China (201310345011).

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REFERENCES

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Figure 17. XPS spectra of Ti in TiO2.

which indicates that the oxidation states of the titanium did not change after multiple cycles. Both XRD and XPS results are consistent with the observed high stability (Figure 15), which indicates that TiO2 is not altered under cycling conditions. It is worth noting that the same sorbent was used for all of the experiments in this study; therefore, this sorbent went through hundreds of adsorption/desorption cycles at different conditions and maintained its capture capacity.

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CONCLUSIONS Nanoporous TiO2 was used for separation of CO2/H2 under the conditions relevant to pre-combustion CO2 capture. Regardless of single or binary components, CO2 and H2 6946


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