Article pubs.acs.org/EF
Effects of the Adsorbent Preparation Method for CO2 Capture from Flue Gas Using K2CO3/Al2O3 Adsorbents Surajit Sengupta,† Vinay Amte,† Rajeshwer Dongara,† Asit Kumar Das,*,† Haripada Bhunia,‡ and Pramod Kumar Bajpai‡ †
Reliance Technology Group, Reliance Industries Limited, Jamnagar, Gujarat 361140, India Department of Chemical Engineering, Thapar University, Patiala, Punjab 147004, India
‡
ABSTRACT: The effect of the preparation method on the CO2 adsorption capacity of K2CO3/Al2O3 adsorbents is examined. The multi-step impregnation (MI) method enables uniform dispersion of active species (K2CO3) in the broad macropores without blocking narrower mesopores. This facilitates higher loading of accessible K2CO3 for CO2 adsorption and, hence, higher adsorption capacity. The single-step impregnation (SI) method suffers from blockage of narrower mesopores by excessive growth of K2CO3. This limits the CO2 accessibility toward active species in the porous structure because of the formation of larger active species aggregates. For 50 wt % K2CO3/Al2O3 prepared by MI and SI methods, the maximum CO2 adsorption capacity at CO2 partial pressure of 8 kPa is found to be 3.12 and 2.1 mmol/g, respectively. The regeneration efficiency of 50MI and 50SI are observed to be nearly 65 and 56%, respectively, at 130 °C in multi-cycle testing. The experimental data for CO2 adsorption were described by the Langmuir isotherm, and the isosteric heat as a function of fractional coverage of the adsorbent was evaluated by means of the van’t Hoff equation. The isosteric heat showed a decreasing trend with an increase in the surface coverage of the adsorbent. From the results, it is concluded that the adsorbent prepared by the MI method shows better performance because of its tunable textural and morphological properties to achieve higher CO2 adsorption capacity.
1. INTRODUCTION Various post-combustion CO2 capture processes, including absorption, membrane separation, cryogenic techniques, etc., are well-known for capturing CO2 from flue gas streams. However, these processes have some demerits, such as higher regeneration energy, operational and reliability issues, and high cost for CO2 capture.1,2 Hence, the demand for a more sustainable and effective CO2 capture process has driven renewed interest in the adsorption route. The solid adsorption process offers potential for selective uptake and release of CO2. The unique features that make the adsorption process more receptive to CO2 capture are (i) high sorption capacity, (ii) excellent tunable physical properties, (iii) lower regeneration energy and temperature, (iv) tolerance to impurities, (v) low cost, (vi) minimal reliability issues, e.g., no corrosion/ degradation, etc. Although the adsorption route possesses an alternative to conventional capture processes, it also invites improvements for its applicability. Various solid adsorbents, including zeolites 13X, activated carbon, metal organic framework (MOF), alkali metal carbonate, etc., are reported for effective separation of CO2. MOFs have shown significant CO2 adsorption capacity (3−33 mmol/g at 25 °C and 40 bar).3 These materials are extensively used for adsorptive separation of CO2 using a pressure swing adsorption (PSA) technique. However, no study on the cost of MOF adsorbent regeneration has yet been performed because no MOFs have been tested in industrial-scale CO2 capture applications. On the other hand, alkali metal carbonate-based solid adsorbents are widely studied as a capture media for CO2 from dilute flue gas streams4−14 by temperature swing adsorption. The adsorbents, e.g., sodium carbonate (Na2CO3) and potassium carbonate (K2CO3), follow carbonate−bicar© 2014 American Chemical Society
bonate reaction chemistry to form alkali hydrogen carbonate,12,15,16 as given below. M 2CO3(s) + CO2 (g) + H 2O(g) ⇔ 2MHCO3(s)
Previous studies on K2CO3-based adsorbents have shown better CO2 adsorption capacity than Na2CO3.8,17 The CO2 adsorption capacity for K2CO3-based adsorbents is reported in the range of 0.23−2.70 mmol of CO2/g of adsorbent. MgOpromoted K2CO3 supported on Al2O3 showed high adsorption capacity but requires a much higher temperature (400 °C) for complete regeneration.5−7 Lee et al.13 reported improvement in sorbent regeneration using modified alumina and α-Al2O3. It was found that K2CO3-based sorbents can be regenerated between 130 and 200 °C depending upon the type of alumina.13,14 In the series of work published by Zhao et al.,17−26 the focus was on the carbonation and regeneration behaviors of supported K2CO3. K2CO3 supported on the γAl2O3 adsorbent showed higher adsorption capacity among other sorbents. Sengupta et al.27 investigated the improvement in regeneration temperature for K2CO3/Al2O3 adsorbents by applying support modifications. Zhao et al.17−26 and Sengupta et al.27 reported that K2CO3·1.5H2O is the active precursor for adsorption, while the formation of intermediate stable species [K4H2(CO3)3·1.5H2O and KAl(CO3)2(OH)2] requires a higher regeneration temperature. The CO2 capture process performance using different alkali-metal-based adsorbents is investigated with respect to the effect of water pretreatment, reaction kinetics, carbonate loadings, multi-cycle stability, Received: August 11, 2014 Revised: October 30, 2014 Published: December 8, 2014 287
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Figure 1. Flow sheet of the adsorbent preparation method.
etc.5,15,16,27,28 Lei et al.29 reported a multi-step impregnation (MI) method for an Al2O3-supported K2CO3/MgO adsorbent and achieved improved adsorbent capacity (2.4 mmol of CO2/ g) with a composite adsorbent of 30 wt % K2CO3 and 20 wt % MgO at 60 °C and regeneration at 300 °C. Wei and Mo30 investigated incorporation of a high amount of titania into SBA15 for the removal of estrogen using a MI method and showed its superiority over single-step impregnation (SI). Park et al.37 demonstrated a pilot-scale CO2 capture system from a slip stream of the real flue gas from a coal-fired circulating fluidizedbed combustor using dry regenerable sorbents. The overall performance of a 0.5 MWe pilot plant showed very promising CO2 removal efficiency of 50−80%. The effect of operating parameters, such as carbonation and regeneration temperature, moisture content of flue gas, solid holdup in a carbonator, and solid circulation rate, was studied. Yi et al.38 investigated the feasibility of the dry sorbent CO2 capture process in the continuous solid circulation mode between a fast fluidized-bed carbonator and a bubbling fluidized-bed regenerator. The CO2 removal of 18−43% was reported with a decrease in gas velocity, lower carbonation temperature, and increased solid circulation rate. It is worth mentioning that the effect of the adsorbent preparation method on the physicochemical proper-
ties and performance of the K2CO3/Al2O3 adsorbents has not been reported in the literature. This work focuses on the effect of adsorbent preparation by SI and MI of K2CO3 on an alumina support and their adsorption/regeneration performance in a fixed-bed reactor system. It also highlights the role of physicochemical properties of adsorbents prepared by both methods on adsorption and regeneration characteristics.
2. EXPERIMENTAL SECTION 2.1. Adsorbent Development. The adsorbents were prepared by single- and multi-step incipient wetness impregnation methods. Reagent-grade potassium carbonate (K2CO3) was supplied by Merck India, Ltd. Commercially available spray-dried γ-Al2O3 supplied by Saint Gobain was used to prepare the support material, which was calcined at 550 °C for 4 h. A flow sheet for the adsorbent preparation method is shown in Figure 1. In a SI method, K2CO3 was dissolved in a required volume of water such that it corresponds to a water pore volume of the support. The support was poured into the solution and mixed thoroughly for 30 min, so that the K2CO3 solution was allowed to enter into the pores of the support material by capillary action. On the other hand, MIs were followed for higher loading of metal carbonates (10−60 wt %). For example, 10 wt % K2CO3 was impregnated at the successive stage to attain a higher loading. After 288
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Figure 2. Schematic of the experimental setup.27
Table 1. Structural Characteristics of Prepared Adsorbents adsorbent fresh γ-Al2O3 K2CO3/γ-Al2O3
preparation method multi-step impregnation
single-step impregnation
adsorbent namea
K2CO3 loading (wt %)
BET surface area (m2/g)
total pore volume (cm3/g)
average pore diameter (nm)
γ-Al2O3 10MI 20MI 30MI 40MI 50MI 60MI 50MIb 10SI 20SI 30SI 40SI 50SI 37SI
0 10 20 30 40 50 60 50 10 20 30 40 50 37
172 158 127 98 55 44 23 29 151 102 75 52 42 65
0.482 0.402 0.350 0.278 0.156 0.126 0.063 0.074 0.393 0.317 0.239 0.142 0.117 0.165
10.2 10.5 10.7 11.3 10.3 10.5 10.8 10.3 10.7 11.0 10.9 11.2 11.4 10.1
a
Abbreviations were used to designate the adsorbent, such as 10MI for 10 wt % K2CO3 impregnated on Al2O3 using the multi-step impregnation method. SI, single-step impregnation; MI, multi-step impregnation (up to five-step impregnation). bTwo-step impregnation (25 + 25 wt %).
every impregnation stage, adsorbent was dried at 120 °C for 24 h and the water pore volume was then measured and, accordingly, subsequent stages were performed to complete the desired K2CO3 loading. 2.2. Apparatus and Method. The adsorption−regeneration studies have been carried out in a stainless-steel fixed-bed reactor (Figure 2) with a height of 42 cm and shell internal diameter of 3.5 cm. The experimental setup mainly consists of three sections: gas injection, fixed-bed adsorption, and CO2 analysis in effluent stream. The diameter and height of the adsorbent zone are 1.7 and 8.3 cm, respectively. The temperatures along the reactor were measured by thermocouples at three different positions: 4, 8, and 12 cm from the bottom. About 8 g of solid adsorbent was used for each experimental run. A simulated flue gas composition of 8 vol % CO2, 0−15 vol % H2O, and rest N2 was used for the adsorption study. The simulated gas mixture of N2 and CO2 was passed through a temperature-controlled water saturator to saturate the stream at a given temperature. The CO2 concentration in the treated effluent stream was measured continuously by an online infrared (IR) analyzer (Servomex 1440 series, measurement limit of 0−10 vol %). The total flow rate of the
feed gas stream was 150 ± 2 mL/min at a temperature between 45 and 65 °C and atmospheric pressure. The temperature of the saturator was kept between 30 and 70 °C to study the effect of water in the CO2 adsorption. Then, the simulated gas mixture was passed through the adsorbent bed. The adsorbent was pretreated with moisture for 30 min to achieve higher adsorption capacity.7 The regeneration was carried out for some selected adsorbents at a temperature range between 120 and 300 °C under N2 atmosphere. The details of the experimental procedure are described elsewhere.27 2.3. Adsorbent Characterization. The textural properties, such as Brunauer−Emmett−Teller (BET) surface area and total pore volume of the adsorbents, were determined using nitrogen adsorption−desorption experiments (Micromeritics ASAP 2020). The total pore volume was calculated from the amount of N2 adsorbed at a relative pressure (P/Po) of 0.99. The total surface area was calculated using a multi-point BET surface area method. The morphologies of the adsorbent particles were observed by field emission scanning electron microscopy (SEM, JEOL JSM-6100). 289
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The structure of the alumina support and adsorbents is examined by transmission electron microscopy (TEM, Technai-20, Phillips) operated at 200 kV. The phase characteristics of the adsorbent materials were performed by X-ray diffraction (XRD, X’Pert Pro, PANalytical) using Cu Kα radiation at room temperature. The thermal stability and decomposition of selected adsorbents were carried out using thermogravimetric analysis (TGA, Universal V4.5A TA Instruments) using N2 as the carrier gas, and the heating rate was maintained at 10 °C/min.
3. RESULTS AND DISCUSSION 3.1. Effect of K2CO3 Loading on Physicochemical Properties of Adsorbents. 3.1.1. Textural Analysis. Table 1 shows the structural properties of supported adsorbents prepared by 10−60 wt % K2CO3 impregnation using SI and MI methods. The results show that both the BET surface area and pore volume decrease with the increase in the metal carbonate loading. In the impregnation method, the active component (i.e., K2CO3) and the support material are two separate phases and their interaction does not lead to any significant changes in the mesoporous structure of the support.31 Hence, the specific surface area (SA) and total pore volume (TPV) decrease with increasing K2CO3 loading as K2CO3 covers the pore walls and sequentially fills up the pores during the MI method. From Table 1, it is seen that, for a given loading, SA and TPV are higher in MI than in SI. For example, at 40 wt % K2CO3 loading, the SA and PV of the multi-step method are 55 m2/g and 0.156 cm3/g versus 52 m2/g and 0.142 cm3/g, respectively, in the single-step method. The pore size distribution (PSD) was performed by the Barrett−Joyner−Halenda (BJH) method. K2CO3-based adsorbents show uniform distribution of pores with a maximum average mesopore diameter of 10 nm, as shown in Figure 3a. The adsorbent prepared by both methods exhibits broader maxima at slightly increasing mesopore diameters (ranges from 10.3 to 11.3 nm), but the remaining total pore volume in the mesopores was significantly decreasing at higher K2CO3 loading than that in the fresh alumina. This suggests that K2CO3 residing in the pores was attributed to the decrease in the mesoporosity. On the other hand, the average pore diameter of the various adsorbents is slightly increasing with the K2CO3 loading. In fact, the smaller pores are completely filled during loading of K2CO3 solution. Therefore, upon averaging the pore size, there is no contribution of very small pores. Thus, the calculated average pore size becomes increased, but physically, none of the pore becomes widened. All of the adsorbents (prepared by both MI and SI methods) depict a type IV isotherm and exhibit H2 hysteresis loops (Figure 3b), generally associated with the interconnected pores of mesoporous materials.35 In all cases, hysteresis loops result in complete filling of the mesopores at 0.4 < P/Po < 1. From the average pore diameter data in Table 1 and also from PSD in Figure 3a, it is noted that, for the same carbonate loading, the MI method leads to a lower average pore diameter but higher pore volume and surface area compared to those of the SI method. It may be due to the homogeneous dispersion of K2CO3 in the longitudinal direction inside the broad mesopores without blocking narrower mesopores.33,34 On the other hand, the SI method suffers from blockage of narrower mesopores by excessive growth of K2CO3. This limits the accessibility of CO2 to active species in the porous structure because of the formation of larger active species aggregates. It
Figure 3. (a) PSD of γ-alumina and supported K2CO3-based adsorbents and (b) adsorption isotherm of γ-alumina and 10MI− 50MI adsorbents with N2 at 77 K.
is, therefore, interesting to observe MI attributes on the CO2 adsorption capacity at a similar loading, as discussed in subsequent sections. 3.1.2. SEM Analysis. The particle morphology images for K2CO3-based adsorbents were established by SEM (Figure 4). Figure 4a shows the uniform distribution of K2CO3 on the γAl2O3 support prepared by the MI method. The 50% K2CO3/ Al2O3 samples consisted of particles with regular spherical morphology and a smooth surface. In the MI method, the formation of the K2CO3 layer is supposed to be increased with K2CO3 loading on the γ-Al2O3 support. The higher loading (such as 60 wt % K2CO3) results in the formation of large agglomeration of the active component on the surface, which leads to a decrease in active sites for adsorption (shown in Figure 4b). Figure 4c shows some aggregates of support materials with K2CO3, which is prepared using SI, providing less active sites for CO2 adsorption. This might be due to the inhomogeneity of metal salt dispersion in the support induced by capillary action. A similar observation was reported by Zhao et al. and Sengupta et al. at higher K2CO3 loading (K2CO3 of >45 wt %).24,27 Hence, the results showed that the preparation method strongly affected the morphology of the K2CO3/Al2O3 adsorbents, as confirmed by XRD and BET results. Panels a and b of Figure 5 show the SEM with energydispersive spectroscopy (EDS) cross-section images of 50MI and 50SI particles. It is seen that K2CO3 is uniformly distributed in 50MI compared to 50SI. EDS intensity line profiles were extracted from the spectrum images along the green line drawn on the cross-section of adsorbents prepared by MI and SI methods. Figure 5c also shows the distribution of 290
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Figure 4. SEM images taken at the magnification of 250× for (a) 50MI, (b) 60MI, and (c) 37SI.
particles of γ-Al2O3. The selected area electron diffraction (SAED) pattern of the support sample (see the insets of Figure 6) shows concentric rings corresponding to (400) and (440) planes of γ-Al2O3, which are the reflections at 2θ of 46.3° and 67.2°, respectively.36 However, these concentric and diffused rings in the electron diffraction pattern emphasize the characteristics of the polycrystalline structure and amorphous phase in γ-Al2O3. In the case of 50MI, K2CO3 is quite uniform and distributed homogeneously compared to 50SI and 60MI, as shown in Figure 6b. In the MI method, K2CO3 is finely dispersed on γAl2O3, as indicated by a considerably low concentration of rodlike structures in the TEM image. Furthermore, the SAED image did not show a dots pattern corresponding to the crystalline K2CO3 phase. However, K2CO3 present inside the mesopores is not clearly visible because of the low resolution of the TEM instrument. TEM images of the 50SI adsorbent depict a large amount of 0.4−0.5 μm long rods of uniform thickness of ∼0.1 μm spread over the external surface of γ-Al2O3 particles (Figure 6c). These rod-like structures of K2CO3 on the support surfaces are due to the excessive growth of K2CO3 during the SI method, which may block some of the mesopores of γ-Al2O3. Interestingly, K2CO3 rods displayed globular pores of varying size of 10−20 nm under high magnification. The globules inside the pores may be attributed to the elimination of air entrapped inside the pores because of the higher capillary pressure.32 These kind of porous rods are more visible in the 60% K2CO3/Al2O3 adsorbent (60MI). The absence of these structures in the fresh alumina sample clearly implies that this porous globular structure is originated because of the K2CO3 phase. However, these kind of globular pores were not observed in 50MI, indicating that K2CO3 is finely dispersed in the pores of γAl2O3. The SAED images taken at the rod-like structures contained a dotted pattern in between the concentric rings of γAl2O3. These dots can be correlated to diffraction from crystalline phases of K2CO3 (SAED image inset in Figure 6c). In the case of 60MI, a significant amount of K2CO3 was found on the external surface (Figure 6d). Herein, K2CO3 beyond 50 wt % occupies the remaining pores of the adsorbent (remaining pore volume is 0.063 cm3/g) and deposits on the external surfaces as excess metal carbonate (Figure 4b). Hence, the reduced pore volume may exhibit a reduction in the CO2 adsorption capacity because of the reduced accessibility to CO2 diffusion, as discussed in the subsequent section. 3.1.4. XRD Analysis. The XRD results as shown in Figure 7a reveal the presence of the γ phase in γ-Al2O3, K2CO3 in pure
Figure 5. (a and b) Cross-section of 50MI and 50SI and (c) line scans across the cross-section of 10MI−50MI and 50SI showing the distribution of K2CO3 inside the particle.
K2CO3 active ingredient from the K intensity line scans of the cross-section for 10MI−50MI and 50SI adsorbents. For 10MI− 50MI adsorbents, one can see an increase in intensity of K inside the porous structure with an increase in K2CO3 loading. This suggests a uniform distribution of K2CO3 during MI of the active ingredient. From the comparison between the line scans for 50MI and 50SI, it suggests that K2CO3 is uniformly distributed inside the pores in the case of 50MI compared to 50SI because the K intensity is higher for 50MI. It is also interesting to see that the K intensity near the particle surface for 50SI is high enough, which demonstrates the bulk deposition of K2CO3 at the surface during the SI method. Hence, it is worth mentioning why the CO2 adsorption capacity is higher in the case of MI adsorbents compared to SI adsorbents, which is discussed in the subsequent sections. 3.1.3. TEM Analysis. The structure of the support material and arrangement of K2CO3 on γ-Al2O3 was studied to determine the effect of the adsorbent preparation method. In Figure 6a, the TEM image indicates the presence of about 0.5 μm size agglomerates of γ-Al2O3 particles, in which the mesopores of 300 °C). This is in agreement with the observation reported by Zhao et al.23 It is seen from Figure 7b that, the higher the γ-Al2O3 content in the adsorbents (e.g., 37SI), the higher the stable species formation because of the support−active phase interaction. 3.1.5. TGA. TGA for some selected adsorbents, viz., γ-Al2O3, 40SI, 40MI, 50MI, and 50MI, after adsorption is shown in Figure 8. It is observed that there was a total loss of 12 wt %
Figure 9. Comparison between the CO2 adsorption capacity reported in the literature13,22 and the present study using SI and MI methods (conditions: 55 °C and atmospheric pressure with 8 vol % CO2).
of various K2CO3 adsorbents prepared by both methods and also the comparison to the adsorption capacity of CO2 reported in the literature by the SI method. In both SI and MI methods, the adsorption capacity increases gradually with the increase in the metal carbonate loading. In MI-prepared adsorbents, the CO2 adsorption capacity achieved was 3.12 mmol/g until K2CO3 loading reaches 50 wt %. However, at higher loading (60 wt %), the adsorption capacity abruptly drops. This is due to a lower pore volume of 60MI adsorbent at higher K2CO3 loading. It signifies K2CO3 deposition on the surface of particles instead of inside the pores, which exhibits the chance of blockage of some pores (Figure 4b). Therefore, accessibility of active K2CO3 sites is important for CO2 adsorption. On the other hand, for 50 wt % loading using the SI method, the adsorption capacity was limited to 1.93 mmol/g because of blockage of narrower mesopores by excessive growth of K2CO3. 3.3. Effect of Water on Adsorption Capacity. The effect of water vapor on CO2 adsorption capacity with varying concentrations in the simulated gas mixture was studied using 37SI and 40MI adsorbents. The adsorption capacity increases with the increase in the H2O concentration, as shown in Figure 10. In the absence of water, for the 37SI adsorbent, the adsorption capacity was 0.30 mmol/g. The poor adsorption capacity is also validated by XRD analysis (Figure 7b). For 37SI, as the water vapor concentration in the simulated gas
Figure 8. TGA for γ-Al2O3, 40SI, 40MI, 50MI (fresh), and 50MI (after the adsorption/regeneration step).
pure γ-Al2O3 during TGA. Primarily, these losses are due to removal of free and associated water at higher temperatures. It is also observed that, with the increase in K2CO3 loading from 10 to 60 wt % in fresh multi-step-impregnated adsorbents, the total weight loss increases from 13.3 to 28.2 wt %. In the first stage (∼200 °C), there was a weight loss of K2CO3·1.5H2O (decomposition temperature of 100−200 °C)17 along with free residual surface water. In the subsequent stages II (200−400 °C) and III (400−800 °C), there were weight losses because of the removal of stable components, such as KAl(CO3)2(OH)2, along with associated water.27 Figure 8 also shows that the total weight loss of 40MI was more compared to that of 40SI. This is due to the method of stagewise impregnation, which leads to more associated water present in the form of K2CO3·1.5H2O and KAl(CO3)2(OH)2. It should be noted that, up to 200 °C, the total weight loss for 40MI is higher than that for 40SI. This suggests that dispersion 293
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Figure 10. Effect of the H2O concentration on the adsorbent performance at 55 °C using MI- and SI-based adsorbents.
mixture is increased to 15 vol %, the adsorption capacity increased to 1.85 mmol/g. It is interesting to note that, as the H2O concentration in the feed gas stream increases, the breakthrough time for CO2 adsorption increases. This is due to the formation of active species (K2CO3·1.5H2O) with the increase in the H2O concentration in simulated gas,7,22 resulting in a higher CO2 adsorption capacity. The above observations are more pronounced for the 40MI adsorbent (2.39 mmol/g) compared to the 37SI adsorbent (1.85 mmol/ g). 3.4. Effect of the Adsorbent Preparation Method on Adsorption Breakthrough Time. The effect of SI and MI methods on the CO2 adsorption at 55 °C is shown in Figure 11a. For a given adsorption temperature and CO2 concentration in the feed, 40MI and 50MI adsorbents showed better adsorption characteristics than their counterparts, 40SI and 50SI (Figure 5c). The possible explanation is as follows: in the MI method, a limited amount of K2CO3 (10 wt %) is impregnated sequentially, followed by drying. During every step of impregnation, followed by drying, K2CO3 deposited inside the pores provides porosity for exhibiting accessibility to CO2 diffusion because of expulsion of water of crystallization, while, during the SI method, excessive growth of K2CO3 limits the accessibility toward CO2 diffusion in the porous structure. Moreover, MI allows for uniform distribution of the active K2CO3 ingredient into the pores of the support material compared to SI (formation of larger crystallites/aggregates as shown in Figures 4 and 6c). Hence, the MI method provides an adsorbent with a higher loading of K2CO3 with more active sites for CO2 adsorption, which shows a longer breakthrough time (Figure 11a). In addition, TGA and XRD results reveal the presence of active species, such as K2CO3·1.5H2O, in multistep-impregnated adsorbents, which exhibit higher CO 2 adsorption than those prepared by SI. Figure 11b shows the CO2 concentration profile with respect to time for a single adsorption−regeneration cycle of 50MI and 50SI adsorbents. It is seen that the total cycle time for single adsorption−regeneration is higher in the case of 50MI compared to that in the case of 50SI. This is due to the active sites available for CO2 adsorption as well as regeneration. 3.5. Regeneration of Adsorbents and Their Multicycle Stability. The regeneration studies have been carried out for 50MI, 50SI, and 37SI adsorbents in the presence of N2 as sweep gas. The effect of the regeneration temperature and multi-cycle stability for 50MI, 50SI, and 37SI adsorbents
Figure 11. (a) Breakthrough curves for single- and multi-stepimpregnated adsorbents at 55 °C and atmospheric pressure (CO2, 8 vol %; H2O, 15 vol %) and (b) cyclic study for the 50MI adsorbent.
regenerated at 120, 130, and 300 °C are shown in Figure 12. The regeneration characteristics of these adsorbents are
Figure 12. Effect of the regeneration temperature and multi-cycle study for 37SI and 50MI (sweep gas flow of 60 mL/min).
represented as percent regeneration, which is the ratio of cyclic capacity (i.e., capacity for the regenerated adsorbent) with respect to fresh adsorption capacity. It was observed that nearly 65 and 56% regeneration were achieved for 50MI and 50SI, respectively, at 130 °C. As explained by XRD and TGA, after regeneration at 130 °C, the bicarbonate species is significantly converted back to carbonate species. For the 294
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Figure 13. (a) CO2 adsorption isotherms on the 50MI adsorbent at 318, 328, and 338 K with best fitting Langmuir theoretical isotherms and (b) isosteric heat of CO2 adsorption on the 50MI adsorbent as a function of the fractional coverage of the adsorbent.
37SI adsorbent, the regeneration achieved was 43 and 80% at 120 and 300 °C, respectively. The decrease in regeneration efficiency at a low temperature is due to the formation of stable component KAl(CO3)2(OH)2, which only regenerates at high temperatures (>300 °C).17,27 The regeneration efficiency of the adsorbent prepared by the MI method is higher than that of the SI method, which suggests that the regenerated MI adsorbent has more vacant sites than the SI adsorbent. It is due to the blockage of some mesopores during the SI method, which contributes less active sites for CO2 adsorption as well as regeneration. This is in good agreement as observed from TEM images. It is clear from Figure 12 that all MI and SI adsorbents exhibit stable multi-cycle stability. 3.6. Isosteric Heat Calculations for CO2 Adsorption. Isosteric heat of adsorption is defined as the ratio of the infinitesimal change in the adsorbate enthalpy to the infinitesimal change in the amount adsorbed. The estimation of the heat released is important in the kinetic studies because the heat released because of adsorption is partly absorbed by the adsorbent and partly released to the surroundings. The released heat portion absorbed by the adsorbent increases the particle temperature, and that slowdown of adsorption kinetics because the mass uptake is controlled by the rate of cooling of the particle in the later course of adsorption. The isosteric heat also indicates the temperature sensitivity of the equilibrium constant and thereby provides insight into of the variation of adsorption and desorption kinetics with the temperature. Hence, it is a critical design variable in estimating the performance of the CO2 adsorption process. It also gives some indication about the surface energetic homogeneity and heterogeneity in accordance with variation with surface coverage. The dependence of isosteric heat of adsorption with surface coverage signifies the strength of interaction between adsorbent and adsorbate, followed by the adsorbate− adsorbate interaction.39 Using a Langmuir isotherm shows that the dependence of the adsorbed amount q from the pressure p is shown by eq 1 q = qm
Kp 1 + Kp
⎡ Q ⎛ To ⎞⎤ ⎜ K = Ko exp⎢ − 1⎟⎥ ⎠⎦ ⎣ RTo ⎝ T
(2)
⎡ ⎛ T ⎞⎤ qm = qm,o exp⎢φ⎜1 − o ⎟⎥ ⎣ ⎝ T ⎠⎦
(3)
where Q is the heat of adsorption, qm,o = qm at To (=298 K), and φ is a dimensionless parameter, which may be ∼0 or 85% of theoretical capacity) compared to the SI method because of uniform dispersion of K2CO3 in the pores. The adsorbents with higher loading require higher temperatures (>300 °C) for regeneration because of the formation of a stable component on the adsorbed species. The regeneration efficiency was also improved for MI-prepared adsorbents because of the limited chance of blockage of mesopores. Therefore, the MI method is effective to impregnate a high amount of K2CO3 in a porous alumina support to achieve a higher CO2 adsorption capacity. The derived isosteric heat of adsorption as a function of the fractional coverage showed that the heat was reduced with an increase in the surface coverage of the adsorbent.
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REFERENCES
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dx.doi.org/10.1021/ef501792c | Energy Fuels 2015, 29, 287−297
