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Reaction Kinetics of Hydrogen Chloride with Calcium Oxide by Fourier Transform Infrared Spectroscopy Maosheng Li and Henry Shaw New Jersey Institute of Technology, University Height, Newark, New Jersey 07012
Chen-Lu Yang* Beltran Associates, Inc., 1133 East 35th Street, Brooklyn, New York 11210
The chemical kinetics of the HCl reaction with CaO was investigated over the temperature range of 148-400 °C. A thin film averaging 0.1 ( 0.05 µm CaO was deposited on quartz wool in order to minimize the effects of pore diffusion for this heterogeneous reaction. The decrease in HCl and increase in H2O concentration were continuously monitored in a Fourier transform infrared (FTIR) cell with ZnSe windows. The HCl and H2O spectra were collected automatically at 1.2 s intervals using gas chromatography software. The gaseous reactants and products were circulated continuously between the reactor and FTIR cell using a pump in order to minimize bulk masstransfer effects on the CaO surface. In this way, rapid data collection at constant temperature could be achieved without sampling problems. The results were in excellent agreement with first-order reaction kinetics with respect to the HCl concentration. The Arrhenius activation energy of 31.1 ( 3.0 kJ/mol is slightly larger than that reported in the literature due to minimization of pore diffusion by study of a very thin layer of CaO and circulation of the reactants at a high rate to minimize mass-transfer effects. Introduction While various types of Ca-based sorbents were studied for hydrogen chloride (HCl) removal from flue gases, limited fundamental kinetic information is available to assist process design and system optimization. Karlsson et al.1 studied the reaction of HCl with calcium hydroxide [Ca(OH)2] in a temperature range between 150 and 400 °C and determined that the reaction rate is firstorder with respect to HCl. At higher temperatures, the reaction was found to slow because of the release of H2O from Ca(OH)2 and the existence of CO2. Weinell et al.2 studied the reaction of 1000 ppm HCl with a Ca(OH)2 fixed bed in a temperature range between 60 and 1000 °C. A maximum 60% utilization of the sorbent was reported at a temperature of about 600 °C. Reaction kinetics were controlled by gas diffusion through the solid product. The utilization was found to be independent of particle size in the range of 2-20 µm. Because H2O and CO2 inhibit the chemical reaction of HCl with Ca-based sorbents, it makes sense to calcine Ca(OH)2 and calcium carbonate (CaCO3) before using them for HCl removal. Daoudi and Walters3 studied the direct reaction of a calcium oxide (CaO) particle with 0.5-5% HCl in the temperature range of 310-670 °C. At the upper half of the temperature range, the apparent activation energy was found to be 22 kJ/mol. The reaction rate decreased as the sorbent particle size increased from 10 to 725 µm. Mura and Lallai4 passed 1000 ppm HCl through a fixed bed of CaO particles in an electric furnace at temperatures of 200-600 °C. An activation energy of 45 kJ/mol was reported with a particle size between 180 and 920 µm. Gullet et al.5 studied the kinetics of the reaction between HCl and CaO at HCl concentrations between 1000 and 7500 ppm * Corresponding author.
Figure 1. Schematic diagram of the reaction system.
and temperatures between 150 and 350 °C. First-order reaction rates were observed with an apparent activation energy of 28.1 kJ/mol. In the temperature range of 150-350 °C, the reaction is controlled by gaseous diffusion through the developing product layer. The objective of this research is to determine the gas/ solid reaction kinetics of HCl and CaO under a minimum influence of bulk mass transfer and pore diffusion. Bulk mass transfer was improved by employing a circulation pump in the reaction apparatus. A very thin film of CaO was coated on quartz wool to minimize pore diffusion. The gas-phase reactants were automatically and continuously analyzed by Fourier transform infrared spectroscopy (FTIR) to avoid sampling errors. Apparatus The preparation of CaO and the reaction of HCl with CaO were carried out in a 2.45-cm-i.d. and 26.7-cm-long quartz tubular reactor as illustrated in Figure 1. A quartz fritted disk was installed to keep the CaO-coated
10.1021/ie990628m CCC: $19.00 © 2000 American Chemical Society Published on Web 04/20/2000
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Figure 2. IR spectrum of 0.5% NO2 in N2.
Figure 3. Full IR spectrum of 4.9% HCl in N2.
quartz wool in the middle of the reactor. The reactor was housed in a one-zone Lindberg tubular furnace. The desired reaction temperature was measured with a K-type thermocouple which was inserted into the center of the quartz wool section in the reactor. A bypass coil was installed to compensate for the additional volume of the FTIR cell in the circulation loop. Through the circulation between the reactor and the FTIR cell, concentrations of gaseous reactants and products were continuously and automatically measured by FTIR. The system was designed to perform the following tasks: (1) preparation of CaO, (2) quantification of the initial concentration of HCl, and (3) monitoring of the reaction progress of HCl and CaO. IR Identification and Analysis The FTIR spectrometer was used to conduct the online measurements. A personal computer with a BIO3200 data acquisition system allowed measurement of spectroscopic changes as a function of time. Spectra were collected through an entire experiment at a rate that resolved the changes of interest. A GC-32 software package allowed the FTIR to be used as a detector to acquire chromatograms for NO2 quantification. NO2 Identification and Analysis. A half percent NO2 in nitrogen was passed through the FTIR cell to obtain the standard IR spectrum as shown in Figure 2. The spectrum has a split peak at a wavelength of about 1600 nm. The valves were then positioned to allow N2 to purge the FTIR cell. After the purge, the flow rate of N2 was set at 438 cm3/min and a background chromatogram was collected. A volume of 0.5% NO2 was injected into the flow system to obtain a corresponding chromatogram. The experiment was repeated with NO2 of 219, 438, 657, and 876 cm3. A calibration curve was prepared by plotting the volumes of NO2 injected into the flow system versus the peak areas. HCl Identification and Analysis. A cylinder of HCl in N2 was supplied by Matheson Gas Co. Its concentration was determined as 4.9% by mass spectroscopy, when the cylinder was shipped. To check the labeled concentration, the HCl/N2 gas mixture was passed through the FTIR cell followed by a sodium hydroxide aqueous scrubber. The chloride concentration in the scrubbing solution was then determined by an ionselective electrode. While the gas mixture was passed through the FTIR cell, a standard IR spectrum was taken for HCl identification. Figure 3 shows the full spectrum of 4.9% HCl in N2. Only two bands, νp(2) at 2842 nm and νp(3) at 2821 nm, were chosen for kinetic study.
Figure 4. Time-dependent FTIR spectra of H2O vapor in the region from 1665 to 1555 cm-1 during the reaction of HCl with CaO at 400 °C.
Identification of H2O. Water vapor can promote the backreaction of CaCl2 to HCl and CaO and can interfere with the measurement of NO2 in the FTIR by aqueous absorption. Therefore, it is important to identify and even quantify its presence in the reaction system. According to eq 1, the amount of H2O increases during
CaO + 2HCl f CaCl2 + H2O
(1)
the progress of the reaction. Figure 4 shows how the intensities of a series of H2O bends in the region 16651555 cm-1 increase with time during the progress of the reaction at 400 °C. Similar results were obtained for reactions at other temperatures. CaO Preparation Approximately 0.5 g of quartz wool (Alltech Associates, Inc.) was immersed into a 0.1 M calcium nitrate [Ca(NO3)2] aqueous solution. The wet quartz wool was then removed from the solution and spread on a platter. The quartz wool with the platter was placed in an oven to dry at about 150 °C. In this way, Ca(NO3)2 was uniformly deposited on the quartz wool. After drying, the quartz wool was placed in the reactor, where the temperature was maintained at 400 °C for 1 h to remove moisture. The system was set to allow oxygen (O2) to flow through the bypass coil, reactor, circulation pump, rotameter, FTIR cell, and scrubber. H2O tends to adsorb on the surface of quartz and can be removed at temperatures over 300 °C. Therefore, while O2 was passed through the quartz wool, the reactor temperature was kept at about 400 °C to make sure that the quartz wool was dehydrated completely. After the removal of mois-
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Figure 6. Time-dependent FTIR spectra of HCl bends νp(2) at 2842 cm-1 and νp(3) at 2821 cm-1 during the reaction of HCl with CaO at 400 °C. Figure 5. Chromatogram of NO2 determination.
ture, the valves to the reactor were closed and the temperature was increased to 700 °C and kept for about 3 h to quantitatively convert Ca(NO3)2 to CaO. After the calcination, the reactor was cooled to room temperature to determine the conversion. Based on eq 2, 2 mol of NO2 is generated for every 1
Ca(NO3)2 f CaO + 2NO2 + 1/2O2
(2)
mol of CaO produced. Therefore, the amount of NO2 given off from the decomposition of Ca(NO3)2 can be used to determine the quantity of CaO deposited on the quartz wool.6 During the calcination, O2 was passed through the FTIR cell directly from the cylinder to purge the cell and to collect the IR background spectrum. After the reactor was cooled to room temperature, the valves were quickly switched to allow O2 to flow through the reactor and to carry NO2 from the reactor to the FTIR cell for analysis. The switching valves were operated just like a gas chromatography (GC) injection. The GC32 software allows the FTIR to produce a NO2 peak as a gas chromatogram for quantification. Figure 5 is the gas chromatogram of NO2 from the decomposition of Ca(NO3)2 deposited on 0.5 g of quartz wool. The IR spectrum was consistent with that of 0.5% NO2 in N2, shown in Figure 2. The area of the peak was used to calculate the amount of NO2 from the conversion of Ca(NO3)2 to CaO. It was found that 56 mg of CaO/g of quartz wool was deposited. Because the average quartz fiber diameter was 9 ( 2 µm, the average uniform CaO layer was estimated to be on the order of 0.1 µm. Thus, the system primarily involves the surface reaction and minimizes pore diffusion. Bulk mass transfer through the product layer is minimized by maintaining a large recirculation rate. HCl Conversions After the amount of CaO deposited on the quartz wool was determined, the whole system was purged with N2 to remove NO2 and possible contaminants. The purge was continued until the baseline of the chromatogram became stable. When the system was stabilized, the valves were positioned to allow the pump to circulate nitrogen between the reactor and the bypass coil. The 12.5 dm3/min circulation dropped the reactor temperature quickly to a steady state which can be adjusted to the desired reaction temperature by manipulating the initial reactor temperature. While the reactor temperature was equilibrated, 4.9% HCl in N2 was passed through the rotameter and FTIR cell. After the spec-
Figure 7. Temperature dependency on the reactivity of HCl toward CaO at temperatures of 148-203 °C.
trum of HCl became stable, the two three-way valves were positioned to allow the circulation among the reactor, circulation pump, rotameter, and FTIR cell. The reaction of HCl and CaO was carried out in the reactor at a specified temperature. A series of HCl IR spectra at the desired reaction time interval were obtained after the GC collection, as shown in Figure 6. The absorbance at 2842 nm was used to quantify the HCl concentration and calculate the conversion. The first drop of intensity at an absorbance of 2842 nm was due to dilution. Finally, a dilution factor of 0.355 was determined for the system. Because the intensity of an IR absorbance band is proportional to the concentration of the component absorbing the radiation, the expression is written as
A ) aC
(3)
where A is the intensity of the absorbance, C the concentration of the component of interest, and a the ratio constant. The fractional conversion is written as
x)
A0 - At A0
(4)
where A0 is the intensity of HCl absorbance at 2843 nm at time zero and At the intensity of HCl at time t. Figures 7-9 are plots of HCl conversion with respect to reaction time in minutes. A parabolic increase of conversion with reaction time at temperatures between 148 and 400 °C was found. Conversions also increased with increasing temperature. This suggests that the
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Figure 8. Temperature dependency on the reactivity of HCl toward CaO at temperatures of 248-300 °C.
Figure 10. Plot of ln(1 - x) with time from temperatures of 140275 °C.
Figure 9. Temperature dependency on the reactivity of HCl toward CaO at temperatures of 330-400 °C.
Figure 11. Plot of ln(1 - x) with time from temperatures of 300400 °C.
process is controlled by chemical reaction rather than adsorption. Reaction Kinetics of HCl and CaO The overall reaction under study is described in eq 1. The stoichiometry of the reaction was verified by X-ray diffraction analysis and by verification that the maximum weight gain corresponds quantitatively to 100% conversion of CaO to calcium chloride (CaCl2).7 Firstorder kinetics in HCl was found in three previous studies.2,3,5 Because CaO is in large excess to that needed for the stoichiometry of the reaction, it does not enter the rate expression. However, because this is a heterogeneous reaction the overall rate constant has units of (g min)-1. The first-order rate equation can be expressed as Figure 12. Arrhenius plot for the reaction of HCl with CaO.
CA ln ) -kt C0
(5)
where CA is the concentration of A at time t, C0 the initial concentration of A, and k the rate constant in 1/time, or
ln(1 - x) ) kt
(6)
where x ) conversion ) (C0 - CA)/C0. Figures 10 and 11 are the plots of logarithms of 1 x versus reaction time. The experimental results were
consistent with the first-order reaction with respect to the HCl concentration at temperatures from 148 to 400 °C. The Arrhenius equation was applied to correlate the rate constants derived from these experimental data. A plot of the results is shown in Figure 12. The activation energy obtained from the slope of the bestfit line was found to be 31.1 kJ/mol with a corresponding preexponential factor of 0.241 (g min)-1, leading to the following relation expressing the effect of temperature on the rate constant as
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(-31100 RT )
k ) 0.241 exp
(7)
where T is the absolute temperature in Kelvin and R ) 8.314 J/(mol K). The standard deviation of the activation energy is 31.1 ( 3.0 kJ/mol. The activation energy reported here is larger than that reported by other authors for reaction of Ca-based sorbents with HCl. Among these authors, Daoudi and Walters.3 calculated an activation energy of 22.2 kJ/mol for the system of HCl/CaCO3 under the chemical reaction and mass-transfer control. Weinell et al.2 reported an activation energy in the range from 10.0 to 15.0 kJ/ mol for the system of HCl/Ca(OH)2 under the control of pore diffusion and mass transfer. Gullett et al.5 investigated the reaction of HCl and CaO over the temperature range from 148 to 350 °C under the conditions that minimized bulk mass transfer and pore diffusion limitation. The activation energy they reported is 28.1 kJ/mol. The activation energy obtained in this research indicates that the efforts to minimize the influences of bulk mass transfer and pore diffusion were effective and 31.1 kJ/mol is probably more representative of a true surface reaction. Conclusions The reaction kinetics of HCl with CaO at temperatures between 148 and 400 °C were studied with the use of FTIR spectroscopy under conditions minimizing mass transfer and pore diffusion limitations. The reaction was carried out in a closed system with a pump to circulate the gas-phase reactant and product between the reactor and FTIR cell. The initial concentration of HCl was about 1.74 vol %, and CaO was 28 mg deposited on 0.5 g of quartz wool. The average calculated thickness of CaO on the quartz wool is less than 0.1 µm.
The reaction rate of HCl with CaO is first-order with respect to HCl concentration. The Arrhenius equation is adequate to express the effect of temperature on the reaction rate with a preexponential factor of 0.241 (min mg)-1 and an activation energy of 31.1 kJ/mol. The FTIR spectrum is suitable for NO2, HCl, and water vapor identification and analysis. The FTIR with GC-32 software can be effectively applied to real-time monitoring of a gas-solid reaction. The real-time data allow the determination of kinetics with good precision and avoid sampling errors. Literature Cited (1) Karlsson, H. G.; Klingspor, J.; Bjerle, I. Adsorption of Hydrochloric Acid on Solid Slaked Lime for Flur Gas Clean Up. J. Air Pollut. Control Assoc. 1981, 31, 1177. (2) 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. 1992, 31, 164. (3) Daoudi, M.; Walters, J. K. The Removal of Hydrogen Chloride from Hot Gases Using Calcined Limestone. Management of Hazardous and Toxic Waste in the Process Industries; Elsevier: London, 1987. (4) Mura, G.; Lallai, A. The Kinetics of Dry Reaction between Calcium Oxide and Gas Hydrochloric Acid. Chem. Eng. Sci. 1992, 47, 2407. (5) Gullett, B. K.; Jozewicz, W.; Stefanski, L. A. Reaction Kinetics of Ca-Based Sorbent with Gaseous HCl. Ind. Eng. Chem. Res. 1992, 31, 2437. (6) Gopalakrishnan, R. A Novel Application of FT-IR Spectroscopy for the Kinetics of High-Temperature Reaction of SO2 with CaO. Appl. Spectrosc. 1990, 2, 310. (7) Daoudi, M.; Walters, J. K. Thermogravimetric Study of the Reaction of Hydrogen Chloride Gas with Calcined Limestone: Determination of Kinetic Parameters. Chem. Eng. J. 1991, 47, 1.
Received for review August 19, 1999 Revised manuscript received March 7, 2000 Accepted March 9, 2000 IE990628M
