Article pubs.acs.org/jced
Preparation and Characterization of Activated Carbon−Zeolite Composite for Gas Adsorption Separation of CO2/N2 System Masoume Rostami,† Masoud Mofarahi,*,† Ramin Karimzadeh,‡ and Davoud Abedi§ †
Chemical Engineering Department, Persian Gulf University, Bushehr, Iran Faculty of Chemical Engineering, Tarbiat Modares University, Tehran, Iran § Mahd-e-Tage Industrial Group, Qazvin, Iran ‡
ABSTRACT: In the present work, an alternative type of composite adsorbent has been prepared by adding a small amount of about 7 wt % of activated carbon into the zeolite 13X structure. Besides, zeolite 13X is synthesized as a pure adsorbent for operational comparison. The synthesized zeolite 13X, as well as the composite form, is produced by a hydrothermal method. Moreover, both of them are characterized using a variety of conventional analyzing procedures including X-ray diffraction, X-ray fluorescence, Brunauer− Emmett−Teller, and scanning electron microscopy analyses. Both adsorbents are tested in CO2/N2 separation via a standard adsorption process. Pure adsorption data of CO2 and N2 were determined experimentally at three temperatures of (283, 303, and 323) K and pressures up to 1600 mbar in a static volumetric method on both adsorbents. The results of comparing the activated carbon-zeolite composite with the crystalline structure of zeolite 13X revealed upgrades in some properties. Furthermore, the binary adsorption data of the system have been also measured on both adsorbents and evaluated by ideal adsorbed solution theory. Finally, the job is completed by the presentation of binary equilibrium diagrams as well as the experimental selectivities.
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zeolite composites from industrial wastes of coal fly ash and sawdust mixtures in a hydrothermal method to study the effective parameters on the preparation process. In addition, rice husk is another waste material that has been applied to carbon−zeolite composite production. That study has been investigated by Katsuki et al.7 using two methods of conventional and microwave hydrothermal. Another type of carbon−zeolite composite is prepared by Jha et al.8 from coal fly ash to remove toxic metal ions in aqueous solution. Preparation of ZSM-5 particles supported on several types of treated carbon is another alternative which is conducted by Ozaki et al.9 In addition, Martinez et al.10 synthesized MFI-type zeolites supported on some kinds of carbon materials to investigate the effect of the surface chemistry of carbon in zeolite preparation. Recently, Lakhera et al.11 studied preparation and characterization of carbon−zeolite composite with two weight percentage ratios of carbon and zeolite via the sol−gel method. In a different work, Lee et al.12 inspected some adsorbents including a commercial type of zeolite-activated carbon composite for adsorption of CO2 and compared it with zeolite 13X alone. Since the increase in carbon dioxide is the major reason for global warming issue, its removal has gained much more
INTRODUCTION Zeolite, the most important family in microporous materials with regular and uniform porous structures, has attracted growing interest as a molecular sieve in many scopes of industry for catalysis, adsorption, and separation processes, and has found new applications in electronics, magnetism, chemical sensors, and medicine.1,2 The well-known method of zeolite synthesis is hydrothermal which is basically carried out in the two stages of aging and crystallization in an alkali condition.2 To improve zeolite properties, many efforts toward its modification have been done, including, but not limited to, cation exchange, dealumination, surface treatment with cationic metal ions or ammonium ions, and acid.2 It is a fact that adding other components to the zeolite structure can change some physical properties in order to improve adsorption capacity. For example, involving carbon atoms in the zeolite structure is a way to improve the zeolite properties. This task can be done by various methods available in the literature. For example, Ma et al.3 prepared porous carbons by using zeolite Y as a template in a chemical vapor deposition (CVD) method. Carbon was impregnated in the zeolite structure, then the zeolite was removed and ordered porous carbon remained. In another work, Purnomo4 discussed the carbon−zeolite composite preparation from bagasse fly ash. Moreover, Ma et al.5 studied a kind of carbon−zeolite composite from elutrilithe as a source of silica, alumina, and carbon in its structure plus pitch to use in CO2 adsorption. In another work, Gao et al.6 prepared carbon− © XXXX American Chemical Society
Received: May 6, 2016 Accepted: June 16, 2016
A
DOI: 10.1021/acs.jced.6b00374 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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attention.5 It is good to know that adsorption can help this process effectively.13 Some varieties of adsorbents have been introduced as suitable ones for CO2 capture. However, performance improvement is expected, and thus composite adsorbent is a fast-moving scientific area. In addition, to the best of our knowledge, binary mixture equilibrium measurement and selectivity inspection of CO2 on activated carbon− zeolite composite in the separation system have not been investigated before and there are no data reported. Therefore, this issue is covered in our investigation. In the present work the preparation and characterization of synthesized zeolite 13X (SZX) and activated carbon−zeolite composite (ACZC) have been studied experimentally. Afterward, adsorption equilibrium data of pure CO2 and N2 and their binary mixtures within a certain range of temperatures and pressure on both the SZX and ACZC adsorbents are presented and compared with regards to their adsorption capacity and equilibrium selectivity. Finally, the predictions of binary equilibrium using ideal adsorbed solution theory have also been presented.
homogeneous phase. The synthesis condition was carried out in a Teflon autoclave and treated hydrothermally as follows: An overnight digestion time at room temperature followed by an 8 h crystallization time in an oven was performed at the temperature of 363 K with a ± 0.5 K uncertainty. After the crystallization period was finished in the oven, the product was filtered off, washed with deionized water until the filtrate pH reached around 8, and then dried at 373 K. 2.3. Composite Synthesis. To synthesize the composite, in the hydrogel preparation step, first, powdery carbon was added to sodium silicate solution with 15 min primary mixing followed by the drop-by-drop addition of of sodium aluminate solution under vigorous mixing for an efficient dispersion of carbon to create a homogeneous gel. It should be noted that in the hydrogel preparation step of ACZC synthesis, for each 1000 g of hydrogel, 8 g of powdery carbon was used. It is worth mentioning that the addition of a higher amount of carbon may cause insufficient dispersion of carbon in the hydrogel phase and of course a nonhomogeneous product. Therefore, effective dispersion of carbon is an issue that should be concerned because of the nonsoluble nature of the carbon particles in a prepared hydrogel. A hydrothermal synthesis was performed under the same condition of zeolite synthesis in an oven after aging overnight. At the end, the product was filtered off, washed, and dried at 373 K. The final amount of carbon involved in the zeolite structure is about 7 wt % (based on pure zeolite) which was measured by weighing a certain amount of resultant powdery composite after and before the burning of carbon from the composite structure. Both SZX and ACZC adsorbent crystals were produced in powder form, which is not appropriate for use in adsorption column, because of the adsorbent loss due to gas flow. Therefore, the adsorbent powder was combined with bentonite as a binder. However, the binder with lower specific area affects the adsorption capacity due to the reduction of surface area of samples. In this case, it is necessary to find minimum amount of binder to have both proper strength and less reduction in textual properties. Therefore, it started with small percentage of bentonite and finally a 10 wt % of binder was found as a suitable one to obtain enough strength for pellets when exposed to the conditions of the experimental measurements. In addition, a homemade extruder was applied to shape the adsorbents into a sufficient form. 2.4. Zeolite and Carbon−Zeolite Composite Characterization. The samples of SZX and ACZC were characterized by a variety of conventional techniques including X-ray diffraction (XRD), X-ray fluorescence (XRF), Brunauer− Emmett−Teller (BET), and scanning electron microscopy (SEM) analyses, and particle size analyzer (PSA). XRD patterns of primary SZX and ACZC were obtained to prove the crystalline structure of sample using an Inel X-ray diffractometer with Cu Kα radiation, operated at 30 mA and 40 kV. The diffraction patterns were collected in the 2θ range of −5.668° to 119.309°. In the XRD pattern reference of zeolite 13X, there are some main peaks that the pattern of adsorbents synthesized in this study should matched. Each peak and its corresponding angle demonstrate a certain plate in the crystalline structure of the zeolite. A BET analyzer (ASAP 2020, USA) has been applied to the granulated samples to gain information about textural properties which are important factors in the characterization of porous structures. The BET specific surface area was evaluated by N2 adsorption isotherms at 77 K and the total pore volume determined from the amount
2. MATERIALS AND EXPERIMENTAL PROCEDURE 2.1. Materials. The chemicals used for experiments included sodium silicate (Mahd-e-Tage, Iran, 98.6%), aluminum hydroxide (Iran Alumina, Iran, 99%), sodium hydroxide (Merck, Germany, 98%), activated carbon powder (Merck, Germany), bentonite powder, and deionized water. The pure gases used for adsorption tests including helium (>99.999%), carbon dioxide (>99.9%), and nitrogen (>99.999%) are all bought from Bushehr Lian Oxygen Aria company (Iran). 2.2. Zeolite Synthesis. All materials were prepared (with an accuracy of ±0.01 g for weight measurements) according to the molar ratio of 3.5 Na2O/Al2O3/2.9 SiO2/150 H2O chosen for the hydrogel formula.14 The details of synthesis procedure are as follows: by using the solid phase of sodium silicate, aluminum hydroxide, and sodium hydroxide, the required solutions of sodium silicate, sodium aluminate, and NaOH were prepared. These solution specifications have been defined as shown in Table 1: Table 1. Material Specifications Used in Zeolite Synthesis solution sodium aluminate sodium silicate NaOH water
puritya (balance water) 27.47%
ratio (weight) Na2O/Al2O3 = 1.42
29.70% 46%
needed amounts (for 100 g of hydrogel) 29.47 27.40
SiO2/Na2O = 2.78
0.29 42.84
a
For sodium aluminate solution: sum of Al2O3 and Na2O. For sodium silicate: sum of SiO2 and Na2O.
After weighing the needed amount of material, the first step was mixing the required deionized water and NaOH solution. Then the resulting solution was distributed in two equal volumes. Adding the sodium aluminate solution to one part and the sodium silicate to the other part was the next step. After that, the aluminate solution was poured into silicate solution drop by drop under mixing. When these chemicals were added to each other a thick gel was formed immediately. The resulting gel was gently stirred for 90 min at ambient temperature to break and get a B
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codes, and the model that best fit was chosen.17 These equations are used to fit and describe the pure component data by calculating the optimum parameters of each model. The root mean square deviation (RMSD) defined as shown is applied to investigate the suitability of these models in predictive situations.
of N2 adsorbed at P/P0 = 0.98. The volume and surface area of the micropores was also obtained using the t-plot method. The external surface area which includes the mesopores, macropores, and the external surface of the particles, was calculated from the difference between BET and the microporous surface area. In addition, the elemental Si/Al ratio of powdery zeolite can be determined using the XRF analyzer to demonstrate the NaX elemental structure. In this way, it is possible to detect a metal compound including SiO2 and Al2O3 in the zeolite structure in weight percent and then find out the Si-to-Al ratio. SEM (VEGA TESCAN, Czech Republic) was utilized to recognize morphologies of SZX and ACZC in high resolution. A PSA (ANALYSETTE 22, MicroTec Plus) was used for measuring the size of the powdery particles which are synthesized for use as adsorbents. 2.5. Adsorption Equilibrium Experiments. Pure and binary adsorption equilibrium data for the carbon dioxide/ nitrogen system were measured by a volumetric method. These tests carried out in a static volumetric set up has been shown elsewhere.15,16 Prior to each measurement, the granulated adsorbent was regenerated at 573 K under a 250 mbar helium flow as a purge gas provided by a vacuum pump with 0.05 mbar vacuum levels to be ready for adsorption experiments. The equilibrium measurements of pure CO2 and N2 on SZX and ACZC were performed on the basis of pressure, and volume changes at three temperatures of (283, 303, and 323) K within the pressure range of 0 to about 1600 mbar. The thermodynamic equilibrium was reached when the temperature and the pressure of the system were constant for at least 1 h for each equilibrium data point. The binary mixture adsorption measurements have also been performed in the same experimental setup. In this instance, three gas mixtures of 25, 53, and 74% of CO2 and balance N2 has been used. These tests were carried out in order to investigate the experimental selectivity of the system at two temperatures of (303 and 323) K and the constant pressure of 1.3 bar for both SZX and ACZC. For each adsorbent, six data points were measured at two temperatures and three gas mixtures. In binary tests, the time required to reach the equilibrium is higher than that in pure. After the equilibrium was established the composition of CO2 was measured using a CO2 analyzer (OXYBABY, Witt, Germany), and then the adsorbent was regenerated for next test. The procedure in more detail can be found elsewhere.15,16 In the separation of two or more species, it is worthy to know the inclination of each component to be separated by any agent. In the separation via the adsorption method, each component has a different trend to adsorb into the adsorbent. This fact would be defined as selectivity. Two kinds of selectivity can be considered: equilibrium and kinetic selectivity. The equilibrium kind refers to the adsorption capacity of each component in equilibrium conditions and the kinetic kind refers to the rate of each component to adsorption into a certain adsorbent. In this study, the focus is on the equilibrium selectivity measurement which is defined as selectivity =
n
Pyi = Pi 0xi
(2)
(3)
where P is the mixture pressure, yi and xi are the mole fractions of component i in the gas phase and adsorbed phase, respectively. Pi0 is the equilibrium gas phase pressure of the pure component in the mixture temperature and spreading pressure (π). The spreading pressure of component i can be calculated using the Gibbs adsorption isotherm as below: πi0A = RT
∫0
Pi0
qi Pi
dPi0
(4)
where A is the surface area of adsorption, R is the universal gas constant, T is the mixture temperature, and qi as well as Pi are the adsorption isotherm data. Here, the Langmuir−Freundlich isotherm equations have been applied to represent the pure equilibrium adsorption data. The total number of adsorbed moles (n) is determined by 1 = n
N
∑ i
xi ni0
(5)
where N is the number of components in the mixture and ni0 is the number of pure gas moles adsorbed at pressure Pi0. More details of this model and solution algorithm can be found elsewhere.18,19 By using this model and solving related equations at the same time, it is not only possible to predict the equilibrium mole fraction in the adsorbed phase but also possible to find the equilibrium selectivity.
3. RESULTS AND DISCUSSION The results of this paper are divided into two parts: characterization discussion and gas adsorption results which are presented as follows: 3.1. Characterization. Figure 1 depicts the XRD pattern of a powdery test zeolite. From the pattern, good crystallinity and good agreement have been found compared to the characteristic peaks of zeolite X in the literature.20 In fact, the characteristic peaks of SZX with high intensity are observed in this figure, indicating the successful formation of zeolite 13X.14 The XRD pattern of ACZC has also been presented in Figure 1. In this case, the XRD pattern again shows a crystalline
2
2
n
In this equation, qi is the loading and n is the number of data points. 2.6.1. Ideal Adsorbed Solution Theory (IAST). In this study, the ideal adsorbed solution theory (IAST) was chosen for selectivity prediction and multicomponent gas adsorption equilibria owing to its simplicity. The assumptions of this theory are the ideal manner of adsorbates in the adsorbed phase and the gas phase.18 Essential equations of this theory are as follows assuming that the surface potential of the mixture and that of all pure components are equal:
xCO2 /yCO x N2 /yN
∑i = 1 (qiexp − qical)2
RMSD =
(1)
where x and y are the mole fractions of corresponding gases in the adsorbed and gas phase, respectively. 2.6. Model Description. After collecting the experimental pure data, the results are compared to three predictive models: Langmuir, Freundlich, and Langmuir−Freundlich via Matlab C
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mole ratio chosen for zeolite synthesis in hydrogel preparation step. The analysis data of XRF has been shown in Table 3. Table 3. XRF Results of ACZC and SZX composition (wt %) SiO2 Al2O3 Fe2O3 CaO Na2O K2O MgO TiO2 MnO P2O5 S L.O.I.
Figure 1. XRD pattern of SZX and ACZC samples.
structure for the composite. It can be concluded that involving carbon in zeolite synthesis has no destructive effect on the crystalline structure of the zeolite. In the other words, according to Figure 1 the locations of main peaks are the same. This comparison shows that the chemical and crystalline structure of zeolite has remained unchanged and of course it is not expected to create any change in chemical structure of zeolite by the addition of carbon. The value of the specific area of an adsorbent is a representative of its micropore volume. For this reason, a BET analysis has been carried out, and the results are presented in Table 2. As seen, the surface area of both synthesized
ACZC
SZX
32.34 22.41 0.04 0.11 13.11 0.06 0.02 0.043 0.008 0.003 0.005 31.57
34.56 24.19 0.06 0.06 14.42 0.04 0.02 0.046 0.005 0.004 0.018 26.43
Meanwhile, to inspect the adsorbents morphology, the SEM image of SZX and ACZC are taken and illustrated in Figure 2
Table 2. Pore Structure Parameters of Adsorbents adsorbent synthesized zeolite 13X (SZX) activated carbon− zeolite composite (ACZC) zeolite 13X commercial (Zeochem)a zeolite 13X commercial (UOP)b a
BET surface (m2/g)
micropore volume (m3/g)
micropore area (m2/g)
ratio of micropore volume to total pore volume
677
0.296
634
0.899
656
0.267
573
0.847
564
0.250
537
0.823
624
0.260c
Figure 2. SEM image of SZX.
-
Reference 21. bReference 22. cTotal pore volume.
adsorbents is in the acceptable range of 500−800 m2/g measured with N2 for zeolites and is comparable to that of the commercial one.23 This high surface area and micropore volume are two promising properties for different industrial applications especially in the gas adsorption field.23 Also, in ACZC, the surface area and micropore volume were reduced owing to the nonhomogeneous area as a result of adding carbon particles in the zeolite structure. Also, the ratio of micropore volume to total pore volume indicates a significant volume of micropores compared to total pore volume. As mentioned earlier, the ratios of two main metallic elements existing in the zeolite structure, silicon and aluminum, are important indices. The XRF analyzer has measured the elemental ratio of Si to Al for both adsorbents and the results revealed that this ratio is about 1.21 and 1.22 for SZX and ACZC, respectively which are in the range of 1.0−1.5 considered for NaX zeolite23 showing the proper primary
Figure 3. SEM image of ACZC.
and Figure 3, respectively. As shown in Figure 2, a regular shape of SZX can be seen which is coincident with the results of PSA analysis that will be explained later. The SEM result of SZX is also compared with that in other works. The octahedral morphology of the zeolite particles depicted in the SEM images indicates formation of NaX crystals that is similar to previous results.24,25 According to the figures, the zeolite structure remains unchanged after adding carbon. Comparing both D
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equilibrium isotherms of N2 and CO2 on SZX and ACZC and their correlated parameters of adsorption models are presented. Figures 5 and 6 depict the isotherm data of N2 and CO2. The pressure is given in bar and the loading of adsorbent is in mole
illustrations of SEM, it is worth mentioning that the bigger particles with angular and slit shape are carbon particles which are observed in SEM image of ACZC and coincident with PSA results. These particles have dispersed in the zeolite structure and are associated with zeolite particles. A particle size distribution derived from PSA has been shown in Figure 4 for both SZX and ACZC. As per the figure, an
Figure 5. Pure adsorption isotherms on SZX at three temperatures.
Figure 4. Size distribution of synthesized particles.
appropriate particle size is seen for SZX with about 80% of the particles below 5 μm and 98% of the particles below 10 μm. It should be noted that particles larger than 2 μm, which are created due to aggregation, certainly exist in the sample as well, but the presented SEM image was taken from a relatively homogeneous and uniform area of sample. Therefore, the particles with the size of more than 2 μm might not be clear in Figure 2 and Figure 3. In Figure 4, particle size distribution of the ACZC sample is also depicted with two peaks. The first and second peaks demonstrate that the size of the particles is found below 10 μm for zeolite particles and below 100 μm for carbon particles. According to the data revealed from particle size analysis the mode value (defined as the most frequently occurring value) for SZX and ACZC are 2.9 and 3.8 μm, respectively, which are comparable to the size of commercial zeolite 13X particles found in the literature.26 In general, the selected ratios of materials and the experimental analysis results assured us that the resultant samples would be zeolite 13X and its composite form with carbon in experimental conditions of the hydrothermal method which is a conventional, simple, and more available one for zeolites synthesis.2 Another worthy point is that in the current experiment, the maximum possible amount of powdery carbon was obtained without any treatment to create a proper composite. Regarding the amount of carbon and binder added to the zeolite adsorbent, it should be mentioned that an extended investigation should be performed to find a more appropriate amount of both carbon and binder for the composite adsorbent. 3.2. Pure Adsorption Isotherm. As concluded from the characterization of both samples of SZX and ACZC, these materials are suitable for application in the gas adsorption process. In this section, the results of pure components
Figure 6. Pure adsorption isotherms on ACZC at three temperatures.
per kilogram of adsorbent. As seen and according to BET classification, both gases behave as type I, and CO2 is the most adsorbed gas in both adsorbents. The adsorption amount of CO2 and N2 increase with increasing pressure and decreasing temperature. In the case of CO2, the isotherms have a steep increase in the low-pressure region and a subsequent flattening of the adsorption toward 1.6 bar indicating that the system is close to the saturation limit. But in the N2 case, the adsorbed amount still increases when the pressure is raised to 1.6 bar. In Figures 5, and 6 a comparison of adsorption capacity of N2 and CO2 on both adsorbents can be also seen. It is worth noting that the adsorption capacity of CO2 is around 6 mol/kg, which is about seven times more than that of the N2 adsorbed. Therefore, it is expected to have a high equilibrium selectivity of CO2 in a mixture with N2 using both SZX and ACZC E
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Table 4. Equation Parameters and RMS Deviation of Adsorption Model of Langmuir−Frendlich CO2 equation 1/ n
Cμ = Cμs
(bP)
parameters
SZX
Cμs,0(mol/kg)
N2 ACZC
6.5
SZX
6.5
ACZC
3.6
2.6
χ (dimensionless)
0.1
−1.2
−0.3
0.3
⎛ Q ⎛ T0 ⎞⎞ ⎜ b = b0 exp⎜ − 1⎟⎟ ⎠⎠ ⎝ RT0 ⎝ T
b0 (bar−1)
13.6
18.4
0.1
0.2
Q/R (K)
3735.9
4774.0
2256.2
1499.3
⎛ T⎞ 1 1 = + α⎜1 − 0 ⎟ ⎝ n n0 T⎠
n0 (dimensionless)
1.8
1.5
1.1
1.0
α (dimensionless)
0.3
−0.5
0.2
0.2
RMSD
2.8 × 10−2
6.5 × 10−2
4.8 × 10−3
1.0 × 10−2
1 + (bP)1/ n
Cμs = Cμs,0
⎛ ⎛ T ⎞⎞ exp⎜⎜χ ⎜1 − ⎟⎟⎟ T0 ⎠⎠ ⎝ ⎝
adsorbents. Figure 5 shows that the SZX is a proper candidate for carbon dioxide separation from nitrogen as expected.27 The adsorption capacity of CO2 and N2 in zeolite 13X has been studied in many types of research.21,27−30 Although the results obtained in the current study show a good agreement with the literature, the priority of synthesized zeolite in some cases in this task are not the same as those in commercial ones. In Table 4 the regressed parameters of the Langmuir− Frendlich model obtained from fitting the pure experimental data have been represented for both SZX and ACZC adsorbents. The applicability of this model for such a system was assured, inasmuch as the low value of RMSD was ascertained. In Figures 5, and 6 the results of the adsorption isotherms model have been presented as well. By comparing the adsorbed amount of gases on SZX and ACZC, the higher adsorption capacity of ACZC can be seen against SZX for all temperatures. For example in the CO2 case, the percent of increase in ACZC capacity compared to that of SZX is about 8.57, 13.22, and 16.25% at the temperature of (283, 303, and 323) K, respectively. Compared to the activated carbon−zeolite composite in another work,5 the presented composite of the current study showed more increase in CO2 adsorption capacity. They improve the capacity of those adsorbents by modification of the adsorbent surface using an amine solution. Compared to that in another study12 that applied a commercial composite of the activated carbon−zeolite (zeocarbon), our synthesized composite reached a higher capacity of CO2 adsorption due to more surface area and higher pore volume. By equilibrium data of the isotherms of pure components, it is possible to predict the heat of adsorption. Therefore, the isosteric heat of adsorption or adsorption enthalpy of each component (ΔH) can be estimated from the Clausius− Clapeyron equation as follows: ⎛ d ln P ⎞ ΔH ⎜ ⎟ =− R ⎝ d(1/T ) ⎠q
Table 5. Enthalpy of Adsorption for Gases on Both Adsorbents (kJ/mol) SZX
ACZC
N2
CO2
N2
CO2
0.94
25.27
1.14
27.29
Additionally, from this table, the heat of adsorption for CO2 in ACZC is slightly higher than the value for SZX. It seems that this small increase is due to the affinity increase for CO2 as a result of carbon existence in zeolite structure. In the N2 case, it is obvious as well. 3.3. Binary Adsorption Data. The main superior indicator of an adsorbent is its high selectivity in a separation system. In this regards, the binary adsorption experiments have been carried out to gain experimental selectivity for the CO2/N2 system. The binary experimental data beside the results of IAST prediction have been presented in Figure 7a,b and Figure 8a,b, as well as Table 6 and 7 for both adsorbents. These results have been collected at a constant pressure of 1.3 bar and two temperatures of (303 and 323) K for three different gas mixtures. The experimental selectivity of adsorbents for this system has been obtained using equilibrium mole fraction of the gas phase and applying mole balance to calculate the mole fractions in the adsorbed phase. From these data, it is clear that the selectivity between CO2 and N2 on both SZX and ACZC adsorbents is high; as is expected from the pure component data. Our results suggest that the lower the temperature is the better composite adsorbent performs. Moreover, the IAST prediction describes the equilibrium data of CO2 on both SZX and ACZC adsorbents accurate enough. However, its prediction in the N2 case at conditions investigated here is only limited to the right trend. From Tables 6 and 7, at constant pressure, the equilibrium selectivities increase when the temperature decreases for both adsorbents. Additionally, according to the selectivity values presented, in some data points of experimental results the priority of selectivity for CO2 over ACZC adsorbent is obvious while the calculated ones using IAST are higher for composite adsorbent in all data points.
(6)
When the isotherm data are applied, the enthalpy value was determined by the slope of the plot of ln P vs the reciprocal absolute temperature at a constant loading. The results are presented in Table 5. As per this table, the heat of adsorption of CO2 is higher than that of N2 owing to higher adsorption capacity of CO2 due to more adsorption affinity of that gas. This fact exists in both SZX and ACZC adsorbents.
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CONCLUSIONS A new zeolite based structure adsorbent by adding a small amount of activated carbon during synthesis of zeolite 13X has F
DOI: 10.1021/acs.jced.6b00374 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 8. Binary adsorption equilibria of the CO2 and N2 mixture at 1.3 bar on ACZC at (a) 303 K and (b) 323 K: dashed lines, gas mixture concentration; symbols, the experimental values; solid lines, the IAST predictions.
Figure 7. Binary adsorption equilibria of the CO2 and N2 mixture at 1.3 bar on SZX at (a) 303 K and (b) 323 K: dashed lines, gas mixture concentration; symbols, the experimental values; solid lines, the IAST predictions.
Although the prediction of IAST showed that the ACZC adsorbent has a higher selectivity compared to SZX at the condition inspected here, the experimental results do not approve this priority. Therefore, from these deviations between experimental selectivities and the predicted ones using IAST, it is logical to conclude that applying IAST is not sufficient to predict the manner of such systems and the true controlling mechanism is still missing for the experimental data. After comparing SZX and ACZC results, it was revealed that by adding about 7 wt % of activated carbon to the zeolite structure, the zeolite properties can be slightly improved in the gas adsorption system. Moreover, it promises that an optimum amount of carbon can be added into the zeolite structure getting better results and a higher selectivity. In addition, this composite adsorbent could be expected to be a more suitable one than the conventional zeolite 13X in the CO2/N2
been introduced to have improved characteristics for the CO2/ N2 separation system. In addition, the XRF result of SZX revealed the accurate and in-range elemental ratio of Si and Al in the structure which indicates that the hydrogel formula can be a proper one leading to zeolite NaX. Moreover, from the comparison of the XRD pattern and the SEM illustration of SZX and ACZC, it can be concluded that involving activated carbon powder in the synthesis condition of zeolite does not cause disarrangement in the crystalline structure. Furthermore, BET analysis data revealed that our synthesized adsorbents with the appropriate specific surface area and micropore volume are comparable to the commercial zeolite 13X. Pure adsorption equilibrium measurements have shown that the isotherm for CO2 was much higher than that for N2 for both SZX and ACZC adsorbents indicating a preferential adsorption of CO2 over N2. G
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Table 6. Experimental Data of Binary Adsorption for SZX at 1.3 bara mixture composition (%)
a
yCO2b
qtot
qCO2
25 53 74
0.145 0.450 0.690
3.56 4.31 5.13
3.24 4.03 4.92
25 53 74
0.155 0.460 0.71
3.13 3.78 4.51
2.74 3.42 4.19
qN2 303 K 0.32 0.28 0.21 323 K 0.39 0.36 0.32
S
qCO2 (IAST)
qN2 (IAST)
S (IAST)
59.6 17.2 10.4
3.26 4.29 4.64
0.031 0.007 0.003
618 722 810
37.7 11.8 5.3
2.57 3.62 4.02
0.034 0.010 0.004
404 408 431
Measured with 0.05 bar uncertainty. bMeasured with 0.001 uncertainty.
Table 7. Experimental Data of Binary Adsorption for ACZC at 1.3 bara feed composition (%)
a
yCO2b
qtot
qCO2
25 53 74
0.13 0.43 0.685
3.63 4.81 5.44
3.19 4.57 5.31
25 53 74
0.155 0.45 0.70
3.15 4.44 5.16
2.62 4.04 4.75
qN2 303 K 0.44 0.24 0.13 323 K 0.53 0.40 0.41
S
qCO2 (IAST)
qN2 (IAST)
S (IAST)
48.6 23.9 17.5
3.53 4.85 5.28
0.029 0.004 0.001
806 1800 3000
27 9.9 4.9
2.92 4.18 4.70
0.034 0.006 0.002
475 814 1100
Measured with 0.05 bar uncertainty. bMeasured with 0.001 uncertainty.
separation system as a representative of flue gas flow which is one of the concerns in the topic of carbon capture.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +98 7733441495. Funding
The authors would like to thank Mahd-e-Tage Group Company for their financial support and kind assistance. Notes
The authors declare no competing financial interest.
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REFERENCES
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