Energy & Fuels 2006, 20, 959-963
959
HCl Removal and Chlorine Distribution in the Mass Transfer Zone of a Fixed-Bed Reactor at High Temperature 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 January 13, 2006. ReVised Manuscript ReceiVed February 25, 2006
The breakthrough curves and mass transfer zone (MTZ) of the fixed-bed are of great importance. That they are influenced by a number of factors makes the prediction of these a difficult problem. In this study, the HCl removal using sorbent self-prepared has been studied in the fixed-bed reactor. The breakthrough curves near 1 mg/m3 were measured in the 2-10 cm depth of the fixed-bed. The results show that solid sorbents with active species in the fixed-bed reactor are capable of reducing the HCl to very low level at 550 °C, and the breakthrough time is proportional to the depth of the fixed-bed. The critical bed depth is dependent on initial concentration, flow velocity, and chemical reaction parameter. The fixed-bed model based on the assumption for the breakthrough pattern to be constant through the bed is very reasonable for HCl removal. There are three distinct zones within the fixed-bed processes including the saturated zone, mass transfer zone (MTZ), and blank zone. The chlorine distribution in the MTZ can be predicted by the combination of the fixed-bed constant pattern and the grain surface reaction model, which fits the experimental data well.
1. Introduction The present scrubber technologies for removing HCl at low temperature (below 300 °C) are relatively simple, easy to operate, and have low capital costs.1 However, this technology may cause a side reaction by which polychlorinated dibenzop-dioxins (PCDDs) and dibenzofurans (PCDFs) can form by de novo synthesis, especially in the municipal waste incineration process, and can cause hot corrosion of the body of the incinerator.2 The removal of HCl vapor from the feed-gas can be beneficial in any power plant, especially in the hightemperature molten carbonate fuel cells (MCFC) and the integrated coal gasification combined cycle (IGCC) because of the great corrosion potential of the vapor in contact with metal components. A number of processes are available for removing HCl vapor from industrial and incinerator waste gases. These processes scavenge HCl by adsorption onto activated carbon or alumina or by reaction with alkali oxides. Each process has its advantages and disadvantages, but using the sorbent in a fixed-bed reactor is the most direct method of producing the highest quality fuel gas. The commercial sorbents for removal HCl are relatively expensive. Some kinds of alkali or alkali earth metal have traditionally been the most widely used sorbent for this process.2-11 Krishnan et al.5 evaluated several natural carbonate minerals as HCl scavengers of simulated coal gas in a laboratory fixed-bed reactor from 400 to 600 °C. All of the * To whom correspondence should be addressed. Tel.: 86-21-65497221. E-mail:
[email protected]. † Shanghai University of Electric Power. ‡ ECUST. (1) Nygaard, S. K. H.; Johnsson, J. E. Chem. Eng. Sci. 2002, 57, 347. (2) Fujita, S.; Suzuki, K.; Ohkawa, M.; Shibasaki, Y.; Mori, T. Chem. Mater. 2001, 13, 2523. (3) Dou, B.; Chen, B.; Gao, J.; Sha, X. Energy Fuels 2005, 19, 2229. (4) Gao, B. D. J.; Baek, S. W.; Sha, X. Energy Fuels 2003, 17, 874.
tested sorbents reacted rapidly with HCl vapor. Mura and Laliai6 studied the kinetics of the reaction between CaO and HCl and determined the activation energy for the chemical reaction as 45 kJ mol-1 and for the diffusion in the solid phase as 37 kJ mol-1. Weineu and Jensen7 studied 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 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. Daoudi and Walters9,10 carried out the reaction of HCl and CaO using a recording thermobalance. Based on the method of initial reaction rate, 22.8 kJ mo1-1 was found to be the activation energy for the system. So far, removal efficiency or reaction kinetic for HCl removal has been well considered for most studies. However, there are far fewer applications of fixed-bed modeling for the chlorine distribution along the bed depth. In this paper, a predictive correlation has been developed to correlate the breakthrough time and bed depth, and then a simple mathematical model with chemical reaction and diffusivity parameters obtained from the experiments predicts with good accuracy the MTZ of the fixed-bed. (5) Krishnan, G. N.; Wood, B. J.; Canizales, A.; et al. Development of disposable sorbent for chloride removal from high-temperature coal-derived gases. Processing of the Coal-fired Power Systems 94-AdVances in IGCC and PFBC ReView Meeting; DOE/METC-94/1008. (6) Mura, G.; Lallai, A. Chem. Eng. Sci. 1992, 47, 2407. (7) Weineu, G. E.; Jensen, P. I. Ind. Eng. Chem. Res. 1992, 31, 164. (8) Wang, W. Y.; Ye, Z. C.; Bjerle, I. Fuel 1996, 75, 202. (9) Daoudi, M.; Walters, J. K. Chem. Eng. J. 1991, 47, 1. (10) Daoudi, M.; Walters, J. K. Chem. Eng. J. 1991, 47, 11. (11) Dou, B.; Gao, J.; Sha, X. Fuel Process. Technol. 2001, 72, 23.
10.1021/ef060018g CCC: $33.50 © 2006 American Chemical Society Published on Web 03/30/2006
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2. Experimental Section
Table 1. The Properties of the Fresh Sorbent
2.1. Preparation of Sorbent. Sorbent was prepared by pelletizing the powder of alkali and alkali earth metal substance with binders and texturing agents. The mixture was placed in the temperaturecontrolled oven where the heating rate was controlled so that it increased from room temperature to 100 °C in 2 h, and calcined at 550 °C for 6 h. During calcination, NaHCO3, Ca(OH)2, and Mg(OH)2 decomposed into Na2CO3, CaO, and MgO, releasing CO2 and H2O. The evolution of these gases produced a favor of sorbents, but at higher calcination temperature, the activity of sorbents decreased due to sintering of the solid. Surface area of fresh sorbent was determined with the BET method using a Micrometric Acusorb 2100E apparatus. The results in previous papers4 have indicated the superior activity of this sorbent for removal of HCl from fuel gas under high-temperature conditions, and the properties were shown in Table 1. 2.2. Chlorine Capability of Sorbent. The chlorine capability 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
main composition saturation chlorine capability (%) bulk density (g/m3) surface area (m2/g) pore volume (mL/g) average pore diam (µm)
NaHCO3, Mg(OH)2, Ca(OH)2, 85 wt % 44.02 0.66 32.2305 0.1018 147.82
Table 2. Typical Operation Conditions in the Experiments parameter
value
particle size of sorbent bed porosity inlet HCl concentration space velocity operating pressure
0.25-0.35 mm 0.40 1000 mg/m3 3000 h-1 1 atm
CaCl2, and NaCl were produced, and they 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, and thus the reaction capability of sorbent will decrease. The operating temperature of 500-600 °C is also an optimization temperature in practical process of IGCC,5 which has been selected in our study.3,4,11,13
(1)
where wHCl and w0 represent the amount of HCl and fresh sorbent, respectively. The saturation chlorine capability of sorbent, q0, is the maximum chlorine capability, which represents the saturated absorbing condition of the sorbent. Thus, the conversion of sorbent, x, is defined as: x)
q × 100% q0
(2)
2.3. Apparatus and Procedure. The experimental system for the fixed-bed reactor consists of a gas manifold, a fixed-bed reactor, and a chlorine analysis section. The details of the experimental setup were presented in a previous paper.11 In the gas manifold, the gas mixture is prepared by entraining HCl vapor with N2. When N2 passes through a vessel containing 20-30% concentration HCl solution, a simulated gas with an acceptably stable HCl concentration can be obtained. The sorbent is supported by quartz wool in the center of the tube in the fixed-bed reactor. The sample temperature was indirectly measured by a thermocouple. In a typical experiment, first the composition of a gas and then the temperatures of the reactor and the HCl concentration were stabilized. During the start-up stage, the gas flow was directed away from the sample to prevent any reaction. To start the removal process, the gas flow switched to the sample section of the reactor. The gaseous effluent from the sorbent bed is analyzed for HCl content by dissolving the HCl vapor in a solution of NaOH. At the end of the experiment, the sorbent is cooled, removed from the reactor, and analyzed to determine its chlorine capability. The HCl concentration in a solution of NaOH is analyzed chemically for Cl- by AgNO3 titration. The chlorine capabality of sorbent is determined according to the method of determination of chlorine in coal.12 In this study, the typical experimental conditions are summarized in Table 2.
3. Results and Discussion 3.1. Effect of the Temperature. First, the chlorine capability of sorbent at different temperatures was shown in Figure 1. As seen from Figure 1, the chlorine capability of sorbent selfprepared increases with increasing temperature. At the temperatures of 350-650 °C, some phases corresponding to MgCl2, (12) GB 3558-83. State Standard of China, 1983.
Figure 1. The chlorine capability of sorbent at different temperatures.
3.2. 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 (cb). The widely applied models for the relationship of the time and bed depth in fixed-bed adsorption are based on emptybed time and the adsorbent exhaustion rate, and also developed for gas adsorption on carbon or phenol on carbon. Although the original work was done for the gas-charcoal adsorption system, its overall approach can be applied in the quantitative description of other systems. The fundamental equations describing the relationship between breakthrough time (t) and bed depth (z) can be established by some parameters.14
ln
( )
c0 - 1 ) ln(e(ksq0/u(z)) - 1) - ksc0t cb z)
∫cc
u q0ks
b
0
dc c - c*
(3) (4)
where c* is the HCl equilibrium concentration in the fixedbed. The concentration field is considered to be low, for example, effluent concentration c < 0.15c0, and for t f ∞, q f q0. When the differential equation systems were solved, a linear model between the bed depth and breakthrough time for HCl removal in a fixed-bed reactor was obtained by eq 3, which enables the (13) Dou, B.; Zhang, M.; Gao, J.; Shen, W.; Sha, X. Ind. Eng. Chem. Res. 2002, 41, 4195. (14) Lee, V. K. C.; Porter, J. F.; McKay, G. Ind. Eng. Chem. Res. 2000, 39, 2427.
Mass Transfer Zone of a Fixed-Bed Reactor
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Figure 2. The bed depth versus the breakthrough time. Table 3. Parameters for the Relationship of Breakthrough Time and Bed Depth m 0.731
ks (s-1)
b -1.118
6.18 ×
10-3
Figure 3. The relationship of q and c.
z0 × 10-2 (m) 0.23
breakthrough time (t) of the bed to be determined by a specified bed depth (z).
t ) mz + b
(5)
where
q0 m) c0u
Figure 4. Model for chlorine capability of sorbent along axis of a fixed-bed.
(6)
and
b)-
( )
c0 1 ln - 1 ksc0 cb
(7)
The parameter ks can be determined from the tangent slope of the z versus t curve. The slope parameter, m, and intercept, b, are found from a trial-and-error method incorporating a minimum sum of the error-squared method by minimizing the following terms:
SSE )
∑(tcal - texp)2
(8)
The critical bed depth (z0) is also an important parameter, which represents the theoretical depth of adsorbent sufficient to prevent HCl concentration from exceeding cb at t ) 0. z0 can be obtained by eqs 3 and 4.
z0 )
( )( ) c0 u ln - 1 qks cb
(9)
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 also react with the molten carbonate electrolyte. The allowable HCl concentration in the feed gas must be less than 1 mg/m3. Thus, when HCl concentration in reactor effluent gas reaches 1 mg/ m3, the sorbents are regarded as spent. The effects of the breakthrough curves were obtained at 550 °C at z ) 2, 4, 6, 8, and 10 cm near 1 mg/m3. A relationship of bed depth and breakthrough time is shown in Figure 2 with parameters shown in Table 3. Experimental errors from breakthrough curves were estimated to be around 4.8%.4 It is interesting to find that the breakthrough time is proportional to the depth of the fixedbed. Some researchers also obtained similar results. For example,
Guo et al.15 studied the adsorption of sulfur dioxide onto activated carbon, and they obtained the linear relationship of breakthrough time and length. However, for the activated carbons adsorption of sulfur dioxide, the processing is dominated on physical adsorption, and the removal of HCl using sorbent is focused on the chemical reaction of HCl vapor with solid sorbent and mass transfer.2-11 As compared to the length of the fixed-bed, the critical bed depth (z0) is a more important parameter. It is the minimum bed depth, which can be used to optimize the practical operation and economic analysis. The results indicate that a higher initial concentration and/or flow velocity will lead to a higher critical bed depth, which is dependent on initial concentration, flow velocity, and other parameters. The value of the critical bed depth z0, in this study, is about 0.23 cm. In addition, the breakthrough curves are characterized by a sharp change in HCl concentration and long breakthrough time for longer bed depth at the temperature of 550 °C. The solid-phase mass transfer resistance and chemical reaction may dominate the overall process rate, and this kinetic behavior has been investigated in previous papers.3,4,11 Figure 3 shows that q is proportional to c in the fixed-bed reactor at the reaction time of 12 and 15 h. There are the same 0.47 (g/m3)-1 of the slope parameters of two straight lines in Figure 3, which shows the value for the operating parameter ξ of the fixed-bed reactor obtained by q/c under the conditions. This result also indicates that the constant pattern model for the fixed-bed reactor in this study is very reasonable to describe the fixed-bed behavior. A typical constant pattern model for the fixed-bed reactor can be shown by eq 10:8,11
c q ) c0 q0
(10)
3.3. The Chlorine Distribution in MTZ. Generally, there are three distinct zones within the practical fixed-bed processes: a zone nearest the inlet in which the sorbent is fully saturated. Further down the bed is a zone in which the chlorine (15) Guo, J.; Lua, A. C. Sep. Purif. Technol. 2003, 3, 265.
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The mathematical model used to describe the distribution of chlorine of sorbent assumes that the reaction rate is first order with respect to HCl and chlorine capability of sorbent. This statement is very consistent with the Adams-Bohart model for the gas-charcoal adsorption system, which assumes that the adsorption rate is proportional to both the residual capacity of the activated carbon and the concentration of the sorbing species. Heat effects, in particular, can be ignored under the experimental conditions investigated because of the low concentration of HCl (
