Enhancement in CO2 Adsorption on Hydrotalcite-based Material by

Oct 15, 2015 - (10) The most common HTlc used for CO2 sorption at high .... Mg/Al ratios for selected samples (Thermo-iCAP 6000 Series). ..... In othe...
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Enhancement in CO2 Adsorption on Hydrotalcite-based Material by Novel Carbon Support Combined with K2CO3 Impregnation Lakshminarayana K. G. Bhatta,*,† Seetharamu Subramanyam,‡ Madhusoodana D. Chengala,§ Umananda M. Bhatta,† and Krishna Venkatesh† †

Centre for Emerging Technologies, Jain University, Ramangaram District, 562112, India Central Power Research Institute, Bangalore, 560080, India § Ceramic Technological Institute, Bharat Heavy Electricals Ltd., Bangalore, 560012, India ‡

S Supporting Information *

ABSTRACT: In recent years, a great deal of interest has been shown for high-temperature adsorption of CO2 on hydrotalcitelike compounds (HTlcs). Numerous efforts have been undertaken to enhance the CO2 capture property of HTlcs, including alkali-metal impregnation, use of support materials, and modification of chemical composition. The present work demonstrates the applicability of coal-derived graphitic material (CGM) as an effective support for neat as well as K2CO3-promoted Mg−Al HTlc, enhancing the CO2 adsorption capacity. Both surface area and basic site density affect the adsorption capacity. The K2CO3promoted CGM-supported Mg−Al HTlc exhibited a fresh adsorption capacity of 1.10 mmol g−1 at 300 °C under a total pressure of 1 bar. After the initial drop, it maintained an average working capacity of 0.42 mmol g−1 during nine cycles of adsorption− desorption in the temperature range of 300−400 °C. The Yoon−Nelson kinetic model fit well with the experimental data.

1. INTRODUCTION

of support materials, and modification of composition of Mg− Al HTlc. Even though alkali metal carbonates exhibit good CO2 adsorption capacity, they are not very attractive sorbents due to low kinetics, higher regeneration energy, difficulty in heat control, and poor durability. However, they can be used to enhance the CO2 capture properties of other sorbents such as HTlcs, MgO, CaO, and alkaline ceramics.10 Among alkali metal carbonates, K2CO3 has been widely used as a promoter for HTlcs due to its beneficial thermodynamic/kinetic properties. It is believed that K2CO3 impregnation increases the sorption capacity of HTlcs due to an increase in the number of active sites on the surface, despite a reduction in Brunauer−Emmett− Teller (BET) surface area and pore volume. However, high loading of K2CO3 can cause pore or surface blockage owing to formation of bulk K2CO3 aggregates and thus can reduce the sorption capacity.14,15 On the other hand, several studies have shown that CO2 sorption capacity and multicycle stability of HTlcs can also be enhanced by using supports such as zeolites, carbon nanofibers (CNFs), multiwall carbon nanotubes (MWNTs), and graphene oxide (GO).16−19 However, the support material employed in these studies is either expensive or used in large amounts, which results in lower HTlc loadings. The presence of a large amount of inert material in a sorbent could increase the bed volume due to a low weight fraction of the active material, leading to higher capital and operating costs. Hence, the use of inexpensive support material with high HTlc loading is always desirable. In this regard, the present work aims

Carbon dioxide capture and storage (CCS), the most practical option to reduce greenhouse gases, is best applied to large point sources including fossil fuel power plants, fuel processing plants, and allied industrial plants. Among various technical options for CO2 separation and capture, adsorption of CO2 on solid sorbents has been widely investigated as a means of an alternative to benchmark absorption technology, which is having many formidable problems.1 In recent years, sorption enhanced reaction (SER) concepts for H2 production and removal of CO2 from hot flue gas/syngas, two emerging areas of CCS technology, have been receiving attention due to their potential energy efficiency and cost-effectiveness.2−6 Hydrotalcite-like compounds (HTlcs) and metal-based oxides (alkaline ceramics, CaO-based sorbents, and MgO-based sorbents) are the most promising candidates among many state-of-the-art solid sorbents for these applications. Several studies have demonstrated the applicability of HTlcs in the sorption enhanced water−gas shift (SEWGS) process for effective CO2 capture.7−9 In general, HTlcs offer many advantages, including high selectivity for CO2, good regenerability, excellent hydrothermal stability, adequate mechanical strength, good compatibility with SER catalysts, inexpensiveness compared with lithium ceramics, and fast kinetics.10 The most common HTlc used for CO2 sorption at high temperature is (Mg)1−x(Al)x(OH)2(CO3)x/2·mH2O (Mg−Al HTlc).11,12 However, the low sorption capacity of Mg−Al HTlc ( KCH > GCH > CH. CO2 is a weak Lewis acid. Its adsorption takes place at specific locations on surface of HTlcs called active sites, which are strongly governed by basic properties of HTlcs. The active sites on the adsorbent can be enhanced by either increasing the total surface area or by modifying the chemistry of the exposed surface area.32,33 The CO2 adsorption capacity of GCH is higher than that of CH. This enhancement of sorption capacity could be attributed to random dispersion of HTlc particles on a compatible support, leading to an increase in the effective HTlc surface area, which is evident from BET analysis (Table 2). A high-resolution image of GCH (Figure 5c) showed polycrystalline grains with lattice planes corresponding to (200) planes of MgO periclase distributed randomly, separated by amorphous regions. On MgO, CO2 adsorbs as monodentate on edge sites and bidentate on corner sites.34 Thus, the number of active sites exposed to acidic CO2 molecules is larger in GCH, compared with CH. In order to explain the effect of alkali-metal impregnation, particularly K2CO3, on CO2 capture properties of HTlcs, many research groups have proposed different mechanisms.35−40 Even though the positive effect of K2CO3 impregnation has been widely accepted, the CO2 adsorption mechanism under different conditions is still being investigated. However, it is generally agreed that K+ ions interact with Mg−O and/or Al− O centers on the surface of the sorbent. Consequently, more basic sites are generated, enabling CO2 adsorption at high temperatures.41 In the present study, KCH exhibited better sorption capacity compared with GCH and CH. The partial reconstruction of the original layered structure upon wet impregnation will cause an ion-exchange process, presumably resulting in the formation of a thin layer of KOH. Apparently, the subsequent calcination of the sorbent results in chemically bound Al(Mg)−O−K surface species on the external surface. Consequently, more basic sites are generated, which are related

A.S =

∑ TCDsignal × CA

(2)

where A.S is the amount of CO2 (mmol) and CA is the calibration factor (mmol CO2/area unit). The basic site densities for GKCH and GCH were found to be 0.95 mmol g−1 (9.9 μmol m−2) and 0.68 mmol g−1 (3.1 μmol m−2), respectively. These results are in accordance with the explanation offered for observed sorption capacities of sorbents. 3.3. Cyclic Stability. The lifetime of adsorbents, which determines the frequency of their replacement, has a significant impact on the economics of any commercial scale operation. Hence, the ideal sorbent should exhibit excellent cyclic stability. As GKCH exhibited the highest fresh adsorption capacity among the tested samples, its cyclic stability was assessed by continuous adsorption−desorption cycles. The sorbent showed a loss of 52% of its fresh capacity in the second cycle. This

Table 3. CO2 Adsorption Data and Yoon−Nelson Kinetic Parametersa

a

sorbent

qBT (mmol g−1)

qSA (EXPER) (mmol g−1)

KYN (min−1)

τ (min) (EXPER)

τ (min) (EMD)

qSA (EMD) (mmol g−1)

R2

CH KCH GCH GKCH

0.26 0.52 0.49 0.80

0.48 0.83 0.75 1.10

0.232 0.198 0.209 0.243

7.04 14.38 14.15 18.53

6.40 14.62 13.85 18.51

0.41 0.82 0.71 1.00

0.9947 0.9703 0.9619 0.9981

qBT, breakthrough adsorption capacity; qSA, saturation adsorption capacity; EXPER, experimental; EMD, empirical. 10880

DOI: 10.1021/acs.iecr.5b02020 Ind. Eng. Chem. Res. 2015, 54, 10876−10884

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Figure 5. TEM micrographs of (a) CH showing hydrotalcite like nanostructures, (b) KCH showing micron-sized structures, higher magnification images of (c) GCH and (d) GKCH, showing (200) fringes of periclase MgO, (e and f) MgO nanocrystals in GKCH showing defects (planar defect and twin boundaries).

Figure 7. TPD profiles of GCH and GKCH. Figure 6. Effect of K2CO3 loading on adsorption capacity of CH at 300 °C, 1 bar.

investigate the kinetics of adsorption in a fixed bed column.42 The linear form of the Yoon−Nelson model is represented by relation 3.43

might be due to either incomplete desorption of CO2 in the selected temperature window or an irreversible chemisorption. As shown in Figure 8, the sorbent maintained an average working capacity of 0.42 mmol g−1 up to 10 cycles after the initial drop in capacity, in the temperature range of 300−400 °C. This indicates good thermal stability of the sorbent in the selected temperature window. 3.4. CO2 Adsorption Kinetics. The Yoon−Nelson model is a relatively less complicated model that can be used to

⎛ C ⎞ ln⎜ ⎟ = k YNt − τk YN ⎝ C0 − C ⎠

(3) −1

where kYN is the Yoon−Nelson rate constant (min ), τ is the time required for 50% of adsorbate breakthrough (min), t is the sampling time (min), C0 is the initial concentration of CO2, and C is the concentration of CO2 at any time during the evaluation. The model parameters kYN and τ were reckoned 10881

DOI: 10.1021/acs.iecr.5b02020 Ind. Eng. Chem. Res. 2015, 54, 10876−10884

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adsorption capacity of HTlcs.45,46 Future work will involve optimization of support material synthesis to achieve higher SSA, complete characterization of the support material, and a CO2 capture study under SERP relevant conditions.

4. CONCLUSION Numerous efforts have been undertaken to enhance the CO2 capture property of HTlcs, including alkali metal carbonate impregnation, use of support materials, and modification of chemical composition. The use of inexpensive support material with high HTlc loading is always desirable for practical applications. The present work demonstrated the applicability of coal-derived graphitic material (CGM) as an effective support for Mg−Al HTlc, enhancing the CO2 adsorption capacity. The sorption capacity of CGM-supported Mg−Al HTlc was found to be further increased upon K2 CO 3 impregnation. The K2CO3-promoted CGM-supported Mg−Al HTlc exhibited a fresh adsorption capacity of 1.10 mmol g−1 at 300 °C under a total pressure of 1 bar. After the initial drop, it maintained an average working capacity of 0.42 mmol g−1 during nine cycles of adsorption−desorption in the temperature range of 300−400 °C. The adsorption behavior of sorbents could be interpreted on the basis of surface area and basic site density. The addition of support material increased the surface area of neat Mg−Al HTlc, leading to enhanced sorption capacity. Alkali metal carbonate impregnation on CGMsupported Mg−Al HTlc generated high basic site density due to increased defects concentration, resulting in further enhancement of sorption capacity. The Yoon−Nelson kinetic model fit well with the experimental data.

Figure 8. Normalized CO2 adsorption capacity on GKCH over 10 adsorption−desorption cycles (adsorption 300 °C; desorption 400 °C).

from the linear dependence of ln[C/(C0 − C)] versus time t. The empirical saturation adsorption capacity [qSA(EMD)] in terms of mmol g−1 was calculated using the following relation: qSA =

F × τ(EMD) × CCO2 (4)

W × 22.4

where F is the feed flow rate in normal milliliters per minute, τ(EMD) is the time required for 50% of adsorbate breakthrough (min) based on the Yoon−Nelson model, CCO2 is the mole % of CO2 in the feed and W is the weight of the adsorbent in grams.44 These values for different adsorbents are presented in Table 3. The values of model parameters are high for GKCH. Further, the values of τ obtained by the model for all sorbents are close to the experimental results, and high values of correlation coefficients indicate that Yoon−Nelson model fit well to the experimental data. The primary objective of the present study was to find an inexpensive support material for neat as well as K2CO3promoted Mg−Al HTlc and investigate their CO2 capture properties at high HTlc loading. For comparison, the CO2 adsorption capacities of HTlcs, reported in the literature, are presented in Table 4. The adsorption capacity of GKCH is on par with sorption capacities of adsorbents reported in the literature, taking into account the variation in synthesis and adsorption measurement conditions. The selected experimental conditions have a certain relevance for postcombustion CO2 capture under hot conditions using the TSA technique. The presence of moisture is known to increase significantly the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02020.

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The pore size distribution and N 2 adsorption− desorption isotherms for adsorbents studied (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: 91-80-27577250. Fax: 91-80-27577246. E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

Table 4. CO2 Uptake on HTlcs from the Literature and Present Work material CNF-supported (90 wt %) K-promoted Mg−Al HTlc GO-supported (6 wt %) Na-promoted Mg−Al HTlc MWCNT-supported (38 wt %) Mg−Al HTlc K-promoted Mg−Al HTlcc K-promoted Mg−Al HTlcc 18.5% K and 1.5% Na-promoted Mg−Al HTlcd K-promoted gallium substituted HTlc Mg3 Al1−stearate LDH 10% Ga and 20% K-promoted Mg−Al HTC CGM-supported (5 wt %) K-promoted Mg−Al HTlc a

b

Tads (°C)

Ptotal (bar)

gas compositiona

sorption capacity (mmol g−1)

ref

250 300 300 350 350 300 200 300 300 300

1.1 1 1 13 20 1.34

5% CO2 + 12% H2O 15% CO2 20% CO2 52% CO2 CO2 + H2O 30% CO2 70% CO2 100% CO2 100% CO2

2.2f 0.54 ∼0.4 1.60 1.60 1.21 1.40 1.25 1.82 1.10

18 17 16 31 47 24 48 49 50 This work

1 1.08e 1

Balanced by N2/He/Ar. Calcination temp in °C: b500, c450, d650. Rest-400; epCO2. fmmol g−1 HTlc. 10882

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ACKNOWLEDGMENTS The authors are grateful to Dr. V. Raghavendra, Mayura Analytical Pvt. Ltd., Bangalore and Dr. P. S. Sai Prasad, IICT, Hyderabad for discussions and help in experimental configuration. The authors would like to thank Prof P. V. Satyam and his students, Institute of Physics, Bhubaneswar, for helping in electron microscopy measurements. The authors would also like to acknowledge the authorities of Bangalore Institute of Technology, Bangalore; St. Joseph’s College, Bangalore; and Centre for Nano Science and Engineering and Spectroscopy/ Analytical Test Facility at Indian Institute of Science, Bangalore for helping in characterization of samples (XRD, BET, SEM, and Raman spectra). Thanks are also due to Sharon Olivera, Omkar Powar, Narayan Pandith, and Vivek for assistance in experimental work.

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DOI: 10.1021/acs.iecr.5b02020 Ind. Eng. Chem. Res. 2015, 54, 10876−10884