A New Technique to Remove Hydrogen Chloride Gas at High

Jan 24, 2003 - Ceramic Research Institute, National Institute of Advanced Industrial ..... Plantech; Toyo Denka; Shimadzu; and Kawasaki Heavy Industri...
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Ind. Eng. Chem. Res. 2003, 42, 1023-1027

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A New Technique to Remove Hydrogen Chloride Gas at High Temperature Using Hydrogrossular Satoru Fujita,*,† Kenzi Suzuki,† Toshiaki Mori,‡ and Yasuo Shibasaki† Ceramic Research Institute, National Institute of Advanced Industrial Science and Technology, 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan, and Department of Materials Science and Technology, Faculty of Science and Technology, Hirosaki University, 3, Bunkyo-cho, Hirosaki, 036-8561, Japan

The reaction of HCl gas with hydrogrossular [Ca3Al2(SiO4)0.8(OH)8.8] was performed using a fixedbed flow-type reactor above 400 °C. It was found that, upon heating, the hydrogrossular was dehydrated to mayenite phases [Ca12Al10Si4O32(OH)6 and Ca12Al10Si4O35], which reacted with the HCl gas in the temperature range of 400-950 °C to finally produce wadalite (Ca12Al10Si4O32Cl6) and CaCl2. Thus, HCl gas could be efficiently removed from effluent gases above 400 °C. The amount of chlorine fixed per gram of wadalite and CaCl2 was 0.22 g. An apparent activation energy of 24.2 kJ/mol was obtained for the reaction between HCl and hydrogrossular, which was consistent with the values found in the literature for the reaction between HCl and CaO, i.e., 20.9, 22.8, or 28.1 kJ/mol. Introduction Because of its high solubility and corrosive nature, HCl gas is one of the most troublesome materials among acidic gases, although it is rather a modest air pollutant. Therefore, HCl must be removed from flue gases before its emission into the atmosphere. Refuse combustion and hazardous waste incineration are a main source of HCl. The HCl gas emitted from an incinerator mainly results from the destruction of toxic halogenated organic wastes including pesticides, various chlorinated hydrocarbons such as poly(vinyl chloride) (PVC) in solid refuse, and waste polychlorinated biphenyls (PCBs) from industrial uses.1 In recent years, a great deal of attention has been paid to the problem of reducing the level of atmospheric pollution caused by HCl gas. A solid sorbent such as CaO is used to control acid gas emission from combustion processes, most notably municipal waste combustion and hazardous waste incineration. The present scrubber technology to remove HCl using CaO or Ca(OH)2 is relatively simple and easy to operate, and as a result, the capital costs can be reduced.2-4 The reaction of HCl gas with a hydrogrossular sorbent is also interesting for controlling acid gas emission from combustion processes, most notably municipal waste combustion and hazardous waste incineration.5 Hydrogrossular is a possible new solid sorbent for HCl gas. It is a hydration product in solidified cement pastes6-9 and can also be synthesized by the hydrothermal treatment of byproducts such as coal ash or molten slag.10,11 It was found that hydrogrossular sorbents in a fixed-bed reactor are capable of reducing the level of * Corresponding author: Satoru Fujita. Ceramic Research Institute, National Institute of Advanced Industrial Science and Technology, 2266-98 Anagahara, Shimoshidami, Moriyamaku, Nagoya 463-8560, Japan. Tel.: 81-52-736-7258. Fax: 8152-736-7405. E-mail: [email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Hirosaki University. Tel. and fax: 81-172-39-3565. Email: [email protected].

HCl gas emission to near zero in the temperature range of 400-950 °C.5 The reaction with HCl converts hydrogrossular to wadalite and CaCl2 above 400 °C.5 Chloride ions in wadalite still remain after heating at high temperatures. Such high-temperature removal of HCl can lead to the control of the downstream formation of hazardous byproducts such as polychlorinated dibenzop-dioxins (PCDDs) and polychlorinated dibenzofrans (PCDFs).12 Mayenite (Ca12Al10Si4O35), formed by the calcination of hydrogrossular above 680 °C, was found to combust volatile organic compounds (VOCs) such as benzene, toluene, and propylene above 400 °C and also to decompose other VOCs (chlorobenzene) into carbon oxides (CO2 and CO) and H2O.13 Chloride ions formed from the decomposition of chlorobenzene were captured by mayenite, resulting in the formation of wadalite.13,14 Therefore, hydrogrossular is more promising than the conventional HCl sorbents because of its interesting performance for both HCl fixation and hydrocarbon combustion. The objective of the present study is to determine the kinetics of HCl removal using hydrogrossular and to estimate the fixation of HCl gas using real gas phases for application in a combustion system. Experimental Section Materials. The hydrogrossular sorbent material was synthesized by the hydrothermal reaction of a stoichiometric mixture of alumina sol, amorphous silica, and CaO. The mixture was put in a Teflon-lined stainless steel autoclave (25-mL volume) with distilled water and then heated under rotation at 50 rpm. The water-tosolid ratio was 12:1 w/w. The autoclave was placed in a temperature-controlled oven, the temperature of which was programmed to increase from room temperature to 200 °C in 2 h. The mixture was then kept at 200 °C for 15 h. The solid products were separated by filtration and dried at 110 °C for 24 h. Fixation of HCl in a Fixed-Bed Reactor. Hydrogrossular pellets having a particle size of 300-500 µm were prepared after pressing the powder sample and

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Table 1. Experimental Conditions for HCl Fixation in a Fixed-Bed Reactor test no.

acidic conditions

1 2 3

HCl (1000 ppm) + N2 (balance) HCl (1000 ppm) + H2O (10%) + N2 (balance) HCl (1000 ppm) + H2O (10%) + CO2 (10%) + O2 (10%) + N2 (balance) HCl (1000 ppm) + H2O (10%) + CO2 (10%) + O2 (10%) + SO2 (10 ppm) + CO (100 ppm) + NO (200 ppm) + N2 (balance) HCl (1000 ppm) + CO2 (10%) + O2 (10%) + SO2 (10 ppm) + CO (100 ppm) + NO (200 ppm) + N2 (balance)

4 5

sieving. The length of the packed bed was 13 mm for 1 g. The packed bed was placed in the center of a quartz glass. The experiment was started from room temperature to a temperature between 400 and 950 °C under a N2 gas flow rate of 500 mL/min. A mixture gas of HCl (1000 ppm) and dry N2 was then pumped into the fixedbed reactor at the same flow rate. The experiments were also carried out using a real effluent gas composed of H2O, CO2, SO2, CO, and NO. The reaction gas was adjusted by maintaining the same flow rate with the following composition: 1000 ppm HCl, 10% H2O, 10% CO2, 10 ppm SO2, 100 ppm CO, 200 ppm NO, and N2 as the balance (see Table 1). The solid products after reaction with HCl were cooled from each reaction temperature to room temperature in flowing N2. Measurement. X-ray powder diffraction data of the solids were obtained using a diffractometer (Rigaku, RINT2000) with Cu KR radiation (30 kV, 40 mA). The surface area of the sample was measured by N2 adsorption-desorption at 77 K using the BET method (Bell Japan, BELSORP 28SP). The conversion of hydrogrossular to chlorine-containing compounds was determined by quantifying the theoretical content of chlorine in wadalite and CaCl2. The content of Cl- ions in the solid samples was determined by titration with 0.1 N AgNO3 solution using K2CrO4 as an indicator after dissolution of the solid. Results and Discussion Reaction with HCl at High Temperatures. The reaction of hydrogrossular with HCl at high temperatures is expressed by eq 15

5Ca3Al2(SiO4)0.8(OH)8.8 + 12HCl f Ca12Al10Si4O32Cl6 + 3CaCl2 + 19H2O (1) According to this equation, the total chlorine content per gram of wadalite and CaCl2 is 0.22 g, with half the amount in wadalite (0.11 g) and the other half in CaCl2. These amounts were also experimentally obtained.5 Without HCl, it was confirmed that hydrogrossular decomposed to hydromayenite [Ca12Al10Si4O32(OH)6] and CaO at 320 °C (see eq 2), followed by further dehydration to mayenite at 680 °C13

5Ca3Al2(SiO4)0.8(OH)8.8 f Ca12Al10Si4O32(OH)6 + 3CaO + 19H2O (2) The surface areas of hydrogrossular were 43 and 7 m2 g-1 before and after calcination at 700 °C, respectively. The structure of mayenite consisted of a framework of (Al,Si)O4 tetrahedra, with superoxide anions (O2-) and peroxide species (O22-) located in the large cavity with a diameter of 4 Å in the framework.13,15-17 Such a structure is similar to that of anhydrous mayenite, the

Figure 1. Crystal structure of wadalite (Ca12Al10Si4O32Cl6), showing the links of tetrahedra. T(1) and T(2) sites are occupied by Al and Al1/3Si2/3, respectively. The large and small spheres represent chloride and calcium ions, respectively.

cavity of which is occupied by OH- ions. Therefore, it was considered that the fixation process of Cl- ions involved the dehydration of hydrogrossular, followed by conversion through the mayenite phases of Ca12Al10Si4O32(OH)6 and Ca12Al10Si4O35, finally to wadalite and CaCl2.5 Figure 1 shows the structure of wadalite. It was produced by substituting O2-, O22-, or OH- ions in the cavity of the framework with Cl- ions.13 Experiments were also conducted to elucidate the rate of the reaction of hydrogrossular with HCl. Under the condition of 1000 ppm HCl (see test no. 1 in Table 1), the conversion of hydrogrossular to wadalite and CaCl2 is plotted in Figure 2 versus the reaction time at various temperatures. Here, the conversion was determined from the ratio between the total amount of Cl- ions in wadalite and CaCl2 and the theoretical amount fixed in hydrogrossular. The initial reaction rate was determined from the slope of the curves in Figure 2. Figure 3 shows the results in the fashion of the Arrhenius equation. The activation energy was 24.2 kJ/mol, which is close to published data of 20.9, 22.8, or 28.1 kJ/mol for the reaction between CaO and HCl.2,18,19 This is reasonable because HCl is preferentially fixed with CaO during the initial period of the reaction and our activation energy was determined from the initial reaction rate. The determined activation energy is much smaller than that for a conventional chemical reaction, namely, ca. 100 kJ/mol, which suggests that the reaction is controlled by the diffusion of HCl, as discussed in the literature.2,18,19 Estimation of HCl Capture for the Real Effluent Gas Composition. Figures 4 and 5 show the effect of coexisting gases on the conversion of hydrogrossular to wadalite and CaCl2 at 700 and 800 °C, respectively. It is obvious that H2O present in the effluent gas significantly suppressed the reaction, whereas other gases, such as CO2, O2, SO2, and CO, did not. These findings lead us to conclude that HCl fixation with hydrogrossular is more difficult from a wet effluent gas than from a dry effluent gas. At 800 °C, HCl fixation was terminated at the level of 50% conversion, which is understood in terms of the idea that, at 800 °C, H2O

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Figure 2. Conversion-reaction time curves for the reaction of hydrogrossular to wadalite and calcium chloride at various temperatures under 1000 ppm HCl. 9, 400 °C; O, 500 °C; 2, 600 °C; 4, 700 °C; [, 800 °C; 0, 900 °C; b, 950 °C.

Figure 4. Conversion-reaction time curves for the reaction of hydrogrossular to wadalite and calcium chloride at 700 °C under various gas compositions. b, test no. 1 (see Table 1); 0, 2; [, 3; ×, 4; 4, 5.

Figure 3. Arrhenius plot for hydrogrossular.

Figure 5. Conversion-time curves of hydrogrossular to wadalite and calcium chloride at 800 °C under various gas compositions. b, Test no. 1 (see Table 1); 0, 2; [, 3; ×, 4; 4, 5.

suppresses only CaCl2 formation but not wadalite formation, as the XRD data show (see below). Figures 6 and 7 show the XRD patterns after reaction at 700 and 800 °C, respectively. Diffraction peaks attributed to wadalite and CaCl2‚2H2O were observed at 700 °C and under the condition of a dry effluent gas at 800 °C. For the wet effluent gas at 800 °C, on the other hand, wadalite and CaO were formed, but CaCl2

was not, which supports the idea that CaO did not react with HCl to yield CaCl2 under wet conditions at 800 °C. In other words, part of the hydrogrossular converted to wadalite was responsible for HCl fixation under wet conditions at 800 °C. It should be noted that CaCO3 and CaSO4, possible products from the respective reactions

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Figure 6. XRD patterns of hydrogrossular after reacting with various gas compositions at 700 °C. b, Wadalite; 4, CaCl2‚2H2O. (a) Test no. 1 (see Table 1); (b) 2; (c) 3; (d) 4; (e) 5.

Figure 8. Chemical free energy changes for chlorine reaction. 1000 ppm HCl and 1 atm total pressure for all plots.

Conclusions Hydrogrossular packed in a flow-type reactor was found to be capable of reducing HCl from the effluent gas in the temperature range of 400-950 °C. HCl gas was fixed as wadalite and CaCl2. The apparent activation energy was 24.2 kJ/mol, which suggests that the reaction was controlled by HCl diffusion. HCl fixation was suppressed in the presence of H2O, especially at 800 °C, probably as a result of inhibition of the reaction between CaO and HCl to yield CaCl2. Figure 7. XRD patterns of hydrogrossular after reacting with various gas compositions at 800 °C. b, Wadalite; 4, CaCl2‚2H2O; 2, CaO. (a) Test no. 1 (see Table 1); (b) 2; (c) 3; (d) 4; (e) 5.

Acknowledgment

between CaO and CO2 and between CaO and SO2, were not produced at all at 700 and 800 °C. In addition to wadalite formation, the following reaction occurs to fix HCl gas at 700 °C and also at 800 °C in the absence of H2O

The authors are grateful to Dr. S. Velu (Pennsylvania State University) for his many fruitful suggestions. They also acknowledge the financial support of Japan Insulation Co., Ltd.; Ueda Lime; NGK Insulators; TYK Corporation; Hitachi Metals; Shinko Plantech; Toyo Denka; Shimadzu; and Kawasaki Heavy Industries.

CaO (s) + 2HCl (g) S CaCl2 (s) + H2O (g) + ∆H ∆H ) -183.640 kJ/mol at 800 °C According to equilibrium calculations, the forward reaction is favored.20 ∆Gc in both the absence and presence of H2O is plotted in Figure 8 as a function of temperature.21 At temperatures from 300 to 1300 K, ∆Gc increases from -183 to -70 kJ/mol in the absence of H2O, whereas ∆Gc > 0 above 1040 K (or 767 °C) in the presence of 10% H2O.22 Therefore, in the presence of 10% H2O, CaCl2 formation does not occur at all above 1040 K, which is in accord with the fact of no CaCl2 formation occurred under the condition of a wet effluent gas at 800 °C (see Figure 8). It was reported that the optimum temperature was 500-600 °C for the reaction of CaO with 1000 ppm HCl in the presence of 5 and 15% H2O,22 which is in accord with the present results.

Nomenclature K ) absolute temperature (K) ∆Gc ) chemical free energy change (kJ/mol) E ) activation energy (kJ/mol) ∆H ) enthalpy chemical free energy change (kJ/mol) X ) fractional conversion of solid t ) time (s)

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Ind. Eng. Chem. Res., Vol. 42, No. 5, 2003 1027 (3) Mula, G.; Lallai, A. On the kinetics of dry reaction between calcium oxide and gas hydrochloric acid. Chem. Eng. Sci. 1992, 47, 2407. (4) Weinell, C. E.; Jensen, P. I.; Dam-Johansen, K.; Livbjerg, H.. Hydrogen chloride reaction with lime and limestone: Kinetics and sorption capacity. Ind. Eng. Chem. Res. 1992, 31, 164. (5) Fujita, S.; Suzuki, K.; Ohkawa, M.; Shibasaki, Y.; Mori, T. Reaction of Hydrogrossular with Hydrogen Chloride Gas at High Temperature. Chem. Mater. 2001, 13, 2523. (6) Kobayashi, S.; Shoji, T. Infrared analysis of the grossularhydrogrossular series. Mineral. J. 1883, 11 (7), 331. (7) Lager, G. A.; Armbruster, T.; Rotella, F. J.; Rossman, G. R. OH substitution in garnets: X-ray and neutron diffraction, infrared, and geometric-modeling studies. Am. Mineral. 1989, 74, 840. (8) Passaglia, E.; Rinaldi, R. Katoite, a new member of the Ca3Al2(SiO4)3-Ca3Al2(OH)12 series and a new nomenclature for the hydrogrossular group of minerals. Bull. Mineral. 1984, 107, 605. (9) Sacerdoti, M.; Passaglia, E. The crystal structure of katoite and implications within the hydrogrossular group of minerals. Bull. Mineral. 1985, 108, 1. (10) Fujita, S.; Suzuki, K.; Shibasaki, Y. The Mild Hydrothermal Synthesis of Hydrogrossular from Coal Ash. J. Mater. Cycle., & Waste Manag. 2002, 4, 41. (11) Fujita, S.; Suzuki, K.; Shibasaki, Y.; Mori, T. Synthesis of Hydrogarnet from Molten Slag and its Hydrogen Chloride Fixation Performance at High-temperature. J. Mater. Cycles Waste Manage. 2002, 4, 70. (12) Stieglitz, L.; Zwick, G.; Beck, J.; Bautz, H.; Roth, W. Carbonaceous particles in fly ashsA source for the de-novo synthesis of organochlorocompounds. Chemosphere 1989, 19, 283. (13) Fujita, S.; Suzuki, K.; Ohkawa, M.; Mori, T.; Iida, Y.; Miwa, Y.; Masuda, H.; Shimada, S. Oxidative Destruction of Hydrocarbons on a New Zeolite-like Crystal of Ca12Al10Si4O35 including O2and O22- Radicals. Chem. Mater., in press.

(14) Suzuki, K.; Fujita, S.; Shibasaki, Y.; Ogawa, N.; Yamasaki, T.; Fukuda, T.; Sataka, S. A new technique to control dioxins formation; Combustion of precursor compounds and removal HCl gas using hydrogrossular minerals at high temperature. Organohalogen Compd. 2001, 50, 480. (15) Lacerda, M.; Irvine, J. T. S.; Glasser, F. P.; West, A. R. High oxide ion conductivity in Ca12Al14O33. Nature 1988, 332, 525. (16) Hosono, H.; Abe, Y. Occurrence of superoxide radical ion in crystalline 12CaO‚7Al2O3 prepared via solid-state reactions. Inorg. Chem. 1987, 26, 1192. (17) Hayashi, K.; Hirano, M.; Matsuishi, S.; Hosono, H. Microporous Crystal 12CaO‚7Al2O3 Encaging Abundant O- Radicals. J. Am. Chem. Soc. 2002, 124 (5), 738. (18) Walters, J. K.; Daoudi, M. The removal of hydrogen chloride from hot gases using calcined limstone. In Management of Hazardous and Toxic Wastes in the Process Industries; Kolaczkowski, S. T., Crittenden, B. D., Eds.; Elsevier: London, 1987; p 574. (19) Brian, K. G.; Wojciech, J.; Leonard, A. S. Reaction kinetics of Ca-based sorbents with HCl. Ind. Eng. Chem. Res. 1992, 31, 2437. (20) Barin, I.; Knaeke, O.; Kubasehewski, O. Thermochemical Properties of Inorganic Substances; Springer-Verlag: Berlin, 1977; Supplement, p 861. (21) Barin, I.; Sauert, F.; Shultze-Rhonhof, E.; Wang, S. Thermochemical Data of Pure Substances; VCH Publishers: Weinheim, Germany, 1989. (22) Duo, W.; Sevill, J. P. K.; Kirkby, N. F.; Clift, R. Formation of product layers in solid-gas reaction for removal of acid gases. Chem. Eng. Sci. 1994, 49 (24A), 4429.

Received for review February 26, 2002 Revised manuscript received October 22, 2002 Accepted December 2, 2002 IE020158N