Energy & Fuels 2005, 19, 2229-2234
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Reaction of Solid Sorbents with Hydrogen Chloride Gas at High Temperature in a Fixed-Bed Reactor Binlin Dou,*,† Bingbing Chen,† Jinsheng Gao,‡ and Xingzhong Sha‡ Department of Environment Engineering, Shanghai University of Electric Power, Pingliang Road 2103#, Shanghai 200090, China, and Department of Energy Chemical Engineering, ECUST, Meilong Road 130#, Shanghai 200237, China Received May 18, 2005. Revised Manuscript Received July 19, 2005
The gas-solid reaction and breakthrough curves in the fixed-bed reactor are of great importance, and being influenced by a number of factors makes the prediction of these factors a difficult problem. In this study, the reaction rate between solid sorbents and hydrogen chloride gas at high temperature was first investigated. On the basis of a fixed-bed reactor, the experimental results were analyzed by the shrinking core model of diffusion and surface chemical reaction control. The results showed that reaction rates of two sorbents with hydrogen chloride gas were controlled by the combination of the surface chemical reaction and diffusion of product layers, and the reaction rates nearly keep constant within 15 h of the initial reaction period and then decrease gradually. The results of the breakthrough curves show that solid sorbents in the fixedbed reactor are capable of reducing the HCl level to near-zero levels at 550 °C. The experimental results and prediction for breakthrough curves are in good agreement for two sorbents.
Introduction The reaction of solid sorbents with hydrogen chloride (HCl) gas at high temperature is one of the most important problems in developing the cleanest power plant including the high-temperature molten carbonate fuel cells (MCFC) and the integrated coal gasification combined cycle (IGCC).1-4 In addition, incineration may be an environmentally sound option for hazardous waste disposal. Various harmful pollutants, such as hydrogen chloride, sulfur oxides (SOx), nitrogen oxides (NOx), alkali metals, heavy metals, and other contaminants, concentrate in these processes. The present scrubber technologies for removing HCl at low temperature (below 300 °C) are relatively simple, easy to operate, and have low capital costs.5 However, this technology may cause a side reaction by which polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) can form by de novo synthesis, especially in the municipal waste incineration process and can cause hot * Author to whom correspondence should be addressed. Tex: 8621-65497221. E-mail:
[email protected]. † Shanghai University of Electric Power. ‡ East China University of Science and Technology (ECUST). (1) Dou, B.; Gao, J.; Baek, S. W.; Sha, X. High-temperature HCl removal with sorbents in a fixed-bed reactor. Energy Fuels 2003, 17, 874. (2) Krishnan, G. N.; Wood, B. J.; Canizales, A.; et al. Development of disposable sorbent for chloride removal from high-temperature coalderived gases, In processing of the Coal-fired Power Systems 94Advances in IGCC and PFBC Review Meeting, DOE/METC-94/1008. (3) Krishnan, G. N.; Wood, B. J.; Canizales, A.; et al. Development of disposal for chloride removal from high-temperature coal-derived gases, DOE/MC/30005-96/C0545. (4) Harrison, D. Control of gaseous contaminants in IGCC processes, An overview in twelfth annual international Pittsburgh coal conference, Pittsburgh, September 11-15, 1995; p 1047. (5) Nygaard, S. K. H.; Johnsson, J. E. Simulation studies of the influence of HCl absorption on the performance of a wet fue gas desulphurisation pilot plant. Chem. Eng. Sci. 2002, 57, 347.
corrosion of the body of the incinerator.6 In fact, longterm exposure to a very small amount of HCl gas has deleterious effects because it can lead to severe corrosion of hardware and deactivation of the catalyst in MCFC; HCl also can react with the molten carbonate electrolyte to form halides.2,3,7 In recent years, a great deal of attention has been paid to the removal of HCl at high temperature in a fixed reactor in order to achieve the cleanest gas cleaning and the highest efficiency power plant. Weineu and Jensen8 studied the hydrogen chloride reaction with lime and limestone in the temperature range 60-1000 °C. The capacity of solid slaked lime and limestone for binding HCl is the largest in the range 500-600 °C. At temperatures exceeding 500 °C, the binding capacity is limited by the chemical equilibrium between gas and solid. The kinetics of the binding reaction is governed by diffusion in the solid phase, which is proved to follow an unreacted grain-core model. Krishnan et al.2,3 evaluated several natural carbonate minerals as HCl scavengers for simulated coal gas from 400 to 600 °C. All the tested sorbents reacted rapidly with HCl vapor, and nahcolite was superior in its reaction capacity. Mura and Laliai9 studied the kinetics of the reaction between CaO and HCl and determined the reaction kinetics parameters under certain condi(6) 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. (7) Lisi, L.; Lasorella, G.; Malloggi, S.; Russo, G. Single and combined deactivating effect of alkali metal and HCl on commercial SCR catalyst. Appl. Catal. B: Environ. 2004, 50, 251. (8) Weineu, G. E.; Jensen, P. I. Hydrogen chloride reaction with lime and limestone: kinetics and sorption capacity. Ind. Eng. Chem. Res. 1992, 31, 164. (9) Mura, G.; Lallai, A. On the kinetics of dry reaction between calcium oxide and gas hydrochloric acid. Chem. Eng. Sci. 1992, 47 (911), 2407.
10.1021/ef050151t CCC: $30.25 © 2005 American Chemical Society Published on Web 08/27/2005
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Table 1. Properties of Two Sorbents
sorbent
particle diam (mm)
bulk density (g/cm3)
surface area (m2/g)
pore volume (mL/g)
average pore diam (µm)
CN1 CN2
0.45-0.90 0.45-0.90
0.60 0.83
32.2305 120.8831
0.1018 0.2503
147.82 67.54
tions. Satoru et al.6 evaluated Hydrogrossular sorbents in a fixed-bed reactor and determined that they are capable of reducing the HCl level to zero levels above 400 °C. So far, removal efficiency or reaction kinetics for HCl removal has been well considered for most studies.8-13 However, there are far fewer applications of fixed-bed modeling to predict the reaction rate and fixed-bed breakthrough with time. The aim of this paper is to develop new control techniques having higher removal efficiencies of HCl gas in the temperature range 350-650 °C using new sorbents with a fixed-bed reactor to clarify the reaction rate control that occurs between solid sorbent and HCl gas, and predictive relationships to correlate the reaction rate and the breakthrough curve with time were put forward.
Figure 1. Schematic diagram of experimental apparatus. 1, simulated gas supply; 2, mixer; 3, reactor system; 4, automatic temperature control; 5, analysis system; 6, wetmeter.
Experimental Section Preparation of Sorbents. Sorbent CN1 was prepared hydrothermally from a stoichiometric mixture of alkali metal and alkaline earth metal, including NaHCO3, Ca(OH)2, Mg(OH)2, etc., with binders and texturing agents. The mixture was placed in the temperature-controlled oven where the heating rate was controlled so that it increased from room temperature to 100 °C in 2 h and was calcined at 550 °C for 6 h. Sorbent CN2 was prepared with 12.5 wt % alkali metal compound on γ-Al2O3 by wet impregnation. During calcaination, NaHCO3, Ca(OH)2, and Mg(OH)2 decomposed into Na2CO3, CaO, and MgO, releasing CO2 and H2O; the empirical formula calculated from the chemical compositions corresponds to Na0.98Mg0.25Ca2.3O3.04 and Ca0.32[Al2O3]5.8O0.32 for CN1 and CN2, respectively. The surface area of fresh sorbent was determined with the BET method using a Micrometric Acusorb 2100E apparatus, and the properties are shown in Table 1. Apparatus. The laboratory reactor system is shown schematically in Figure 1. The system consists of a gas manifold, a reactor, and a chloride analysis section. In the gas manifold, the gas mixture is prepared by entraining HCl vapor with dry N2. The reaction gas, HCl at a concentration of 1000 mg/m3 in dry N2 gas, was introduced into the fixed-bed reactor. The concentration of HCl (1000 mg/m3) is similar to that of average composition including the system gas of MCFC and IGCC processes and the exhaust gas of incinerators. The second section is the reactor system including the fixed-bed reactor and temperature-control device. The tube in the center of the fixed-bed reactor, which has a diameter of 10 mm and a length of 500 mm, is made of quartz. The tube was placed in an oven which can be operated at temperatures between 20 and 1000 °C. The sample temperature was indirectly measured by a thermocouple placed at sample height between the quartz tube and the inner wall of the oven. A test had shown that the difference between the temperature at this location and the (10) Daoudi, M.; Walters, J. K. A thermogravimetric study of the reaction of HCl gas with calcined limestone. Chem. Eng. J. 1991, 47, 1. (11) Daoudi, M.; Walters, J. K. The reaction of HCl gas with calcined limestone: the effect of particle size. Chem. Eng. J. 1991, 47, 11. (12) Dou, B.; Gao, J.; Sha, X. Study on the reaction kinetics of HCl removal from a high-temperature coal gas. Fuel Process. Technol. 2001, 72 (1), 23. (13) Wang, W. Y.; Ye, Z. C.; Bjerle, I. The kinetics of the reaction of hydrogen chloride with fresh and spent Ca-based desulfurization sorbents. Fuel 1996, 75 (2), 202.
Figure 2. The chlorine capacity of two sorbents under different temperatures. temperature of the sample was not more than 2 °C. The sorbent of 1 g is supported by quartz wool in the center of the tube. The gaseous effluent from the sorbent bed is analyzed for HCl content by dissolving the HCl vapor in a solution of NaOH. The chlorine concentration in a solution of NaOH is measured by AgNO3 titration. At the end of the experiment, the sorbent is cooled, removed from the reactor, and analyzed to determine its chlorine capacity according to the method of determination of chlorine in coal. The outlet HCl concentration in the fixed-bed reactor was measured by titration online in the experimental system. It is also useful to indicate the utilization of sorbent by analyzing chlorine content in the sorbent after the experiment. The chlorine capacity of sorbent, q, is based on the amount of fresh sorbent and the amount of HCl absorbed by the sorbent, which is kilograms of chlorine absorbed per kilograms of sorbent. Thus, q was calculated by the following equation:
q)
wHCl × 100% w0
(1)
where wHCl and w0 represent the amount of HCl and fresh sorbent, respectively. The saturation chlorine capacity of sorbent, q0, is the maximum chlorine capacity, which represents the saturated absorbing condition of the sorbent. Thus, the conversion of sorbent, x, is defined as
x)
q × 100% qo
(2)
Results and Discussion Effect of Temperature. The chlorine capacity of two sorbents after exposure to the reaction gas for 15 h at the space velocity of 3000 h-1 and inlet HCl concentration of 1000 mg/m3 is shown in Figure 2. Figure 2 shows that the chlorine capacity of two sorbents increased with increasing temperature and
Reaction of Solid Sorbents with HCl Gas in a Reactor
Figure 3. The chlorine capacity of two sorbents under different reaction time. Table 2. Breakthrough Time and Chlorine Capacity at Three Space Velocities 1000/h-1
2000/h-1
3000/h-1
sorbent
t (h)
q (%)
t (h)
q (%)
t (h)
q (%)
CN1 CN2
20.1 16.5
9.1 7.3
12.0 9.2
6.0 5.8
6.3 4.8
4.1 2.9
indicates that temperature has a significant influence on the reaction of HCl removal. At temperatures of 350650 °C, some phases corresponding to Na0.98Mg0.25Ca2.3CI6.08 and Ca0.32Cl0.64[Al2O3]5.8 were produced. MgCl2, CaCl2, and NaCl have the melting point of 701, 770, and 801 °C, respectively. Above 701 °C, MgO is stable and does not react with HCl from the standpoint of thermodynamics; thus the reaction capacity of sorbent will decrease. The reaction between sorbent and HCl from 350 to 650 °C can be represented by R-1 and R-2 as follows:
Na0.98Mg0.25Ca2.3O3.04 + 6.08HCl f Na0.98Mg0.25Ca2.3CI6.08 + 3.04H2O (R-1) Ca0.32[Al2O3]5.8O0.32 + 0.64HCl f Ca0.32Cl0.64[Al2O3]5.8 + 0.32H2O (R-2) On the basis of a practical operating process in MCFC and IGCC and our present analysis result, 550 °C should be the optimization temperature,1,2,3,11 which has been selected in this study. Then the experiments were arranged at 550 °C, and the chlorine capabilities of two sorbents were analyzed after exposure to the reaction gas for various reaction times. The results are shown in Figure 3. The data show that the initial chlorine capacity increased rapidly, but after 15 h it slows considerably. Although two sorbents are able to react with a considerable amount of HCl, the time required for absorption saturation is rather long. Effect of Space Velocity. The breakthrough time (t) and chlorine capacity (q) of two sorbents after exposure to the HCl reaction gas of 1000 mg/m3 at three space velocities and 550 °C are shown in Table 2. Table 2 shows that higher space velocity corresponds to smaller breakthrough time and chlorine capacity of sorbent. Reaction Kinetics. The model developed in the present study consisted of the mass conservation equations for the gas and solid phases. Consider a fixed-bed reactor packed randomly with fresh sorbent particles, with a gas containing HCl pollutant being fed to the top of the bed at a constant flow rate. The governing
Energy & Fuels, Vol. 19, No. 6, 2005 2231
Figure 4. Schematic diagrams of the gas solid reaction control; (a) chemical reaction at grain surface, and (b) reactant diffusion through the product layer.
equation for predicting the fixed-bed dynamics is
[∂z∂c] + [∂c∂t] + F(1 - )[∂q∂t ] ) 0
u
t
z
z
(3)
where F is the bed density, is the void fraction of the bed, u is the interstitial velocity of the carrier fluid, t is the time, z is the distance from the inlet of the mobile phase, and c represents HCl concentrations in the mobile phases. Equation 1 is basically the unsteadystate mass balance of the adsorbate, with initial and boundary conditions as follow:
when t ) 0 and z ) 0, c ) c0; when t ) 0 and z > 0, c ) 0 and q ) 0 In the language of wave propagation theory, the introduction of the feed generates a self-sharpening wave. Because of the finite mass-transfer rate, the selfsharpening wave will eventually evolve into a constant pattern traveling at a constant velocity, u. In the constant-pattern wave, the ratio of HCl concentrations in the stationary and mobile phases is constant. Assuring the breakthrough pattern to be constant through the bed, a simple correlation of the gas and the solid can be obtained:12
c(ct - co) co(ct - c)
)
q qo
(4)
where co is the inlet HCl concentration, and ct represents overall gas concentration. The observed rate of the HCl and sorbent reaction may be governed either by chemical steps with finite rates or by a combination of chemical reaction and mass transfer steps. The process was assumed to be isothermal. The influence of external diffusion is very low compared to that of the other steps since the flow rate of gas around the particle was fixed. The possibility of reaction control by film mass transfer is minimized by the small particle size; likewise, pore diffusion limitations are minimized by the low porosity of sorbents, high gas concentration or velocity, and so on. The particles of sorbent were assumed to be made up of spherical grains. Two mechanisms of surface chemical reaction and reactant diffusion through the product layer are important,1,8,9,12,13 which is shown in Figure 4. If reaction is controlled by the chemical reaction at the grain surface, the data can be modeled through use of the shrinking core model expression:
g(x) ) 1 - (1 - x)1/3 ) t/τg
(5)
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Dou et al.
where τg represents the time required for complete conversion (x ) 1); it can be related to the intrinsic chemical reaction rate constant, ks, by
ks )
rF 1 bcom τg
(6)
If the reaction is controlled by reactant diffusion through the product layer, it will follow the expression
p(x) ) 1 - 3(1 - x)2/3 + 2(1 - x) ) t/τp
(7)
where τP also represents the time required for complete conversion (x ) 1); it can be related to the effective diffusion coefficient De by
De )
r2 F 1 6bcom τp
Figure 5. x versus t with model (5).
(8)
Reactions are likely to be controlled by the combination mechanism of surface chemical reaction and reactant diffusion through the product layer.8 In this case, it is rational to get the following expression:
g(x) + δ2p(x) ) t/τg δ2 )
τp k sr ) τg 6De
(9) (10) Figure 6. x versus t with model (7).
The value of δ2 represents the ratio of product layer diffusion resistance to chemical reaction resistance and is a form of the shrinking core reaction modulus. Obviously, the parameter, δ2, which indicates the effect of the chemical reaction rate and diffusion rate on the gas-solid reactions, is an important parameter describing the process characteristics. When δ2 , 1, the reaction can be assumed to be controlled by the rate of the intrinsic chemical reaction; when δ2 > 10, which means that the chemical reaction resistance is much smaller than the diffusion resistance, and it is reliable to assume that the reaction is under product layer diffusion control, intermediate values of δ2 suggest that the reaction is controlled by both chemical and product layer diffusion.9,12 Equations 5, 7, and 9 are used to model the x-t data; τg, τp, and δ2 can be estimated by minimizing:
Table 3. Kinetic Parameters sorbent model CN1
N
[τgg(xi) + τpp(xi) - ti]2 ) Q(τg,τp) ∑ i)1
Figure 7. x versus t with model (9), and reaction rate for two sorbents.
(11)
The sorbents studied in this work are far from pure substances, and thus the HCl removal reaction by sorbents is rather complicated. The rate-limited process is difficult to elucidate by single physical and/or chemical control. We have to clarify the controlling mechanism of the reaction and the relation between the reaction rate and removal performance of sorbent at high temperature. The value of c/co is obtained by the fractional conversion of sorbent in the fixed bed. The g(x) and p(x) rate expressions were applied both singly (eq 5) and (eq 7) by minimizing (eq 11) with respect to either τg or τp by setting the other to zero and in combination (eq 9) by minimizing (eq 11) with respect to both τg and τp simultaneously, to determine the expression for x-t data presented in Figures 5-7. The value of the effective
CN2
(5) (7) (9) (5) (7) (9)
SSE
τg
0.189 0.675 0.028 0.097 0.485 0.061
171.5
τp
293.4 150.5 80.8 125.7 293.4 119.0 22.0
ks (h-1)
De (m2/h)
δ2
9.8 × 10-2
3.2 × 10-6 1.3 × 10-3 7.8 × 10-5 0.538 5.5 × 10-3 5.2 × 10-5 6.5 × 10-3 2.3 × 10-3 0.185
diffusivity and chemical reaction rate constant can be obtained through the optimization data processing, and the parameters are shown in Table 3. The results show the fact that the SSE for the chemical model reaction control is less than that for the product layer diffusion model, and that the SSE for the combined model is less than that for the chemical model. It suggests that the whole process is governed by combination of the chemical reaction and product layer diffusion with fitting by a model derived from the fixedbed reactor and shrinking core models. Both the chemical reaction and product layer diffusion are important. The reaction rate R is also shown in Figure 7, and the
Reaction of Solid Sorbents with HCl Gas in a Reactor
results indicate that the reaction rate remains nearly constant within 15 h of the initial reaction period, and then the reaction rate decreases gradually. In fact, as the reaction proceeds, the solid particle reaction capacity is progressively consumed and the unreacted core of the particle shrinks, thus increasing the difficulty of the HCl molecules to reach the fresh sorbent. The chemical reaction and diffusion inside the particle are, at any temperature, essentially dependent on porosity. This parameter can be affected by temperature and reaction degree. Sorbents were prepared by calcination at 550 °C. However, our experimental work was carried out at 550 °C so that thermal sintering could not occur. Thus, porosity changes are solely due to the effect of the chemical reaction. This may be the result of the great difference existing between the molar volumes of the solid reagent and the product. The decrease in the overall reaction rate is due to the decrease of particle porosity owing to the high molar volume of the solid product in comparison with that of the solid reagent. This similarity in behavior has been widely studied for CaO sulfation. The dependence of diffusivity on porosity was explained by referring to the “random pore model”. Breakthrough Curves. The objective of fixed-bed operations is to reduce the concentration in the effluent so that it does not exceed a predefined breakthrough value. During the reaction process, the HCl concentration in the effluent gas rises rapidly when the sorbent has reached a certain capacity. The rapid change is termed breakthrough. The widely applied models for the relationship of the time and bed depth in fixed-bed adsorption are based on empty-bed time and the adsorbent exhaustion rate, and also are developed for gas adsorption on carbon or phenol on carbon. However, there is still only limited literature on the application of these models in a combination of chemical reaction and adsorption. The concentration field is considered to be low, that is, effluent concentration c < 0.15c0, and in fact, a very simple equation will be developed; eq 12 can be obtained from eq 4 by differential equation:
c0 dq dc )dt q0 dt
(12)
Equation 3 is coupled with equations given by eqs 4 and 9, and all are integrated with the initial and boundary conditions; at time equal to zero, the amount of HCl in the vapor phase and loaded on the solid sorbent is zero. When the HCl-laden gas is suddenly passed through the bed, at times greater than zero, the gaseous HCl concentration at the inlet of the bed is equal to the baseline HCl concentration, c0. It leads to the following equation that can be used to predict the breakthrough curve:
t ) t1/2 +
c0 3q0
∫cc/2f(c) dc 0
(13)
0
where t1/2 is the half-time,
(
f(c) ) τg 1 -
)
c c0
-2/3
(
+ 6τp 1 -
c c0
)
-1/3
-1
(14)
Experimental breakthrough data were obtained at a constant HCl inlet concentration of 1000 mg/m3 at the
Energy & Fuels, Vol. 19, No. 6, 2005 2233
Figure 8. The breakthrough curves of two sorbents.
Figure 9. The breakthrough curves of two sorbents in the initial period.
temperature of 550 °C and the space velocity of 3000 h-1. The breakthrough curves of two sorbents for the experiment and the model are shown in Figure 8. In addition, for sustained and efficient operation of the IGCC or MCFC process, the feed gas must be free of contaminants such as chloride species. HCl is especially deleterious to MCFC because it can lead to severe corrosion of cathode hardware, and it can also react with the molten carbonate electrolyte. The allowable HCl concentration in the feed gas must be less than 1 mg/ m3.1-3 Thus, when HCl concentration in the reactor effluent gas reaches 1 mg/m3, the sorbents are regarded as spent. Figure 9 shows the results of breakthrough curves near 1 mg/m3. The fixed-bed experimental results indicated that two sorbents could react rapidly with HCl vapor at 550 °C. After the breakthrough of outlet HCl concentration, the HCl concentration in the reactor effluent gas increased rapidly. Sorbent CN2 of CaO on support has better utilization; however, the amount of CaO is very limited. Sorbent CN1 having a very simple preparation process is a very cheap sorbent. It is found that CN1 sorbent with active species is the better sorbent for HCl removal, having a longer breakthrough time, which shows that the better reaction capacity may be due to a combination of the necessary amount of reactive component and favorable structure. However, the variations in reactivity between two sorbents were minor. A large surface
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area enhances the effectiveness of sorbent for the removal of HCl from a gas stream, and a larger amount of reactive component in sorbent could facilitate gassolid reaction at high temperature. This result is very useful and important for the development and improvement of solid sorbents for removal of HCl vapor. In addition, the results indicate that the sorbents based on the mixed solid oxides in the fixed-bed reactor are capable of reducing the HCl level to near-zero levels at high temperatures, and the HCl gas can be efficiently removed from the inlet gas at 550 °C. It is expected that the achievable HCl concentrations are as low as the tolerance of a specific application such as high-temperature fuel cells. At the same time, an excellent agreement exists between the experimental HCl breakthrough data and the model predictions. The breakthrough curves simulated by our methods show that the reaction of HCl and sorbents is strongly favorable as reflected by the sharpness of the curves, which is in agreement with the results of the fixed-bed constant pattern. The breakthrough times obtained from experiments are 6.3 and 4.8 h for CN1 and CN2, respectively, which are very close to the theoretical breakthrough times computed from the equation. The validity, which shows that the reaction rate is especially controlled by a combination of surface chemical reaction and reactant diffusion through the product layer at high temperature, is also revisited in breakthrough curves. Although the method for HCl removal reported in this paper is costlier than the scrubber method, there are several advantages. For instance, there is a very high efficiency technique and no possibility for the formation of hazardous byproducts such as PCDDs and PCDFs, which are usually generated in the scrubber technology. Further studies are necessary under more realistic conditions to establish the feasibility for commercial application. Conclusions The reaction between solid sorbents and hydrogen chloride gas at high temperatures was investigated in a fixed-bed reactor. The higher temperature corresponds
Dou et al.
to the higher chlorine capacity of sorbent from 350 to 650 °C. Under the considered operating conditions, the reaction rates of two sorbents with hydrogen chloride gas were controlled by the combination of the surface chemical reaction and diffusion of product layers, and the results indicate that the reaction rates remain nearly constant within 15 h of the initial reaction period, and then the reaction rate decreases gradually. The numerical models of the reaction rate and breakthrough curves were developed with good agreement by fitting the experimental data set of two sorbents. Acknowledgment. This work is sponsored by the NSFC (No.59776017) and State Key Fundamental R&D Project of China (G1999022104). Nomenclature c co ct De g(x) ks m p(x) q qo r R SSE t T u x
HCl concentration in the bed (mg m-3) HCl concentration at inlet (mg m-3) overall HCl concentration (mg m-3) effective diffusion coefficient in a porous structure (m2 s-1) conversion function under chemical control intrinsic reaction rate constant, n ) 1 (cm s-1) molecular weight of reactant (g mol-1) conversion function under diffusion control chlorine capacity of sorbent (%) saturation chlorine capacity of sorbent (%) radius of particle (µm) reaction rate (kg kg-1 h-1) sum of the squares of the errors time (h) temperature (°C) bed velocity (m s-1) conversion (%)
Greek Letters F reactant density (kg m-3) δ2 ratio of diffusion to chemical reaction resistance bed porosity τg characteristic time for chemical reaction control (h) characteristic time for diffusion control (h) τp EF050151T
