164
Ind. Eng. Chem. Res. 1992,31, 164-171
Hydrogen Chloride Reaction with Lime and Limestone: Kinetics and Sorption Capacity Claus E. Weinell, Peter I. Jensen, Kim Dam-Johansen,* and Hans Livbjerg Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Lyngby, Denmark
The capacity of solid slaked lime and limestone for binding HC1 from a gas phase has been investigated in the temperature range 60-1OOO "C. The binding capacity is largest in the range 500-600 "C. However, for slaked lime in the presence of water, a large binding capacity is observed also below 150 "C. This is ascribed t o the formation of a partially liquid product phase. At temperatures exceeding 500 "C the binding capacity is limited by chemical equilibrium between gas and solid. For the range of properties studied the binding capacity is independent of particle size and only depends slightly on specific surface area. The kinetics of the binding reaction is governed by diffusion in the solid phase which is proved to follow an unreacted grain-core model. Diffusion coefficient for mass transport in the grain is reported for the temperature range 80-250 "C.
Introduction The reactions between hydrogen chloride and solid lime (Ca(0H)J or limestone (CaC03)to form calcium chloride are potentially important for various technical processes such as the removal of HC1 from waste combustion flue gases or hot gas in pressurized coal combustion and gasification units. The present investigation was undertaken with the aim of studying the kinetics and the mechanisms of the binding reactions and to examine how process conditions affect the HC1-binding capacity of the solid reactants. Petrini et al. (1979) studied HC1 binding on porous limestone particles in a laboratory fixed bed reactor in the range 350-600 "C. The ultimate conversion of CaC03 to CaClzis rather low but strongly increasea with temperature to about 30% at 600 "C. Increasing particle diameter in the range 0.2-2 mm and HC1 concentration in the range 1.3-4 vol 90leads to a smaller conversion of CaC03 to CaC1,. The progress of reaction is influenced by blocking of the pores in the low-porosity solid by the product components and at higher temperatures by a molten phase of CaC03 and CaC12. In a similar laboratory investigation of HC1 binding on limestone, Ketov et al. (1968) found that there is an optimal temperature in the range 450-550 "C, depending on lime type, for which a maximum conversion of CaC03 to CaC12is obtained. Schuch (1979) reports results from a pilot plant unit with injection of porous slaked lime particles into a tube section of a streaming flue gas. In the temperature range 26e400 "C the retention of HCI varies in the range 40-loo%, increasing with temperature, water content in the gas, and with decreasing particle diameter in the range 11-39 pm. High HC1 retention (>90%) requires a lime feed rate 3-6 times in excess of that needed stoichiometrically. Karlsson et al. (1981) studied HCl binding on small porous particles of slaked lime (Ca(OH)2)in a laboratory fixed bed reactor from 150 to 400 "C. To achieve proper gas/solid contact, the particles were dispersed in a layer of sand. They estimated the value of kinetic parameters in a rate expression which is first order in HCl concentration and in the amount of unconverted, available Ca(OH)2. A maximum of 55% Ca(OH)2 was found to be available for the binding reaction, At high temperatures the rate of reaction is slowed down by the presence of COz and HzO. Recently Jozewicz et al. (1990) observed that the reactivity of reagent grade calcium hydroxide with HC1 strongly depends on the relative humidity of the gas.
Rasch (1980) in a theoretical survey discusses various technological aspects of using lime or limestone for binding acidic gases including the influence of chemical equilibria, mass-transfer resistance, and gas/solid contact.
Reactions and Chemical Equilibria The following binding reactions are of importance: Ca(OH)2(s) + 2HCl(g) s CaC12(s)+ 2H20(g) (1) CaCO,(s) + BHCl(g) F? CaC12(s)+ HzO(g) + C02(g) (2)
CaO(s) + 2HCl(g) e CaC12(s)+ H20(g)
(3)
At high temperatures CaC03 and Ca(OH), decompose: CaC03(s) * CaO(s) + C02(g) (4) Ca(OH),(s) e CaO(s) + H20(g) (5) The gas composition range of practical interest is 0-1% HCl, 5 2 0 % H20, and 1-15% COz. The equilibrium pressures of HzO and COz for (4) and (5), respectively, are shown on Figure 1. In the range of water concentration encountered in practice Ca(OH)2is stable below -400 "C where the equilibrium pressure of HC1 by (1)is extremely low. At higher temperatures Ca(OH)zdecomposes to CaO, which may react further to form CaC03 if COz is present. At temperatures above 500 "C conversion of lime to CaC12 may be limited by the chemical equilibria of either (2) or (3). The equilibrium pressures of HC1 for these reactions are shown in Figure 2. The equilibrium calculations can serve only as qualitative guidelines for selecting appropriate reaction conditions. They are based on the free energy functions of the pure, crystalline statm of the solids. Under actual reaction conditions, some of the reactions may not proceed completely to equilibrium within the reaction time of practical use.
Apparatus The laboratory reactor equipment is shown schematically in Figure 3. A synthetic flue gas containing HC1 was mixed from gas cylinders via a flow panel with Matheson electronic flow controllers. A high-precision Kontron Instruments 420 HPLC pump adds water to the gas. The water passes through an evaporator which is a stainless steel cylinder with a volume of 0.4 L, filled with glass beads and heated externally to 150 "C by an electric heating element. To damp small pulsations of the water flow, caused by the pump and by irregular evaporation, the
0888-5885/92/2631-Ol64$03.OO/O 0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 165 Table I. Properties of the Different Solid Reagents Used in the HC1-Binding Investigation Faxe Solid Slaked Lime (Ca(OH)z) fraction BET area, m2/g mean particle size, pm I 19.7 2.4 I1 12.1 2.1 I11 8.8 4.8 IV 13.6 20.5 V not measured 115.0 Stevns Chalk (Limestone) (-99 w t % CaCOJ particle d i m ,. .um BET area. m2/e 850-1000 1.2-1.85 ,I
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Table 11. Final Lime Conversion (% ) as a Function of Particle Size (d.)" temp, d, = 20.5 pm; d, = 2.12 pm; K = 13.6 m2/g SBET = 12.1 m2/g 373 5.2 f 1.3 1.7 f 1.1 413 5.7 f 1.3 6.0 f 1.4 473 20.6 f 2.2 21.9 2.3 513 25.5 f 2.5 29.5 f 2.8 ~
Figure 1. .Equilibrium pressure of COz by reaction 4 and of H 2 0by reaction 5. The figure is based on thermodynamic data from Barin (1989).
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Figure 2. Equilibrium pressure of HCl by reactions 2 and 3. The figure is based on thermodynamic data from Barin (1989).
water vapor and a purge stream of nitrogen are expanded from about 4.5 atm through a needle valve. During steady-state operation of the equipment the temperature and pressure of the evaporator remain completely constant. The quartz glass reactor (Figure 4) is similar to that applied by Dam-Johansen and 0stergaard (1986, 1991). The inlet section, filled with quartz rings, serves to mix and heat the gas. The solid particles are placed in a small, internal tube section in close contact with the thermometer pocket with a Pt/PtRh thermocouple. The replaceable internal reactor tube is connected to the outlet with a conical, ground joint. The reactor assembly is placed in a vertical tubular furnace with a three-zone temperature control which allows the reactor to be thermostated precisely at any temperature in the range 60-1200 "C. The effluent gas is analyzed chemically by absorbing HC1 in water by means of an absorption system similar to that used by Petrini et al. (1979). The gas bubbles up through one compartment of the absorber and causes the water to recirculate rapidly so that the HCl concentration can be measured with a very small time lag in the other, bubble-free compartment by a pH electrode or a Cl--selective electrode. The aqueous HCl concentration data as a function of time from the start of an experiment is transferred on-line to a computer where the data are filtered by calculating the average HC1 concentration for small (5 s) time intervals. By simple mass balance these primary data are transformed to effluent gas HCl concentration and fractional lime conversion as functions of time.
After an experiment the lime particles were dissolved in acid and analyzed chemically for C1- by Volhart titration which usually agrees with the on-line measurement of lime conversion within a range of a few percent. HC1 was recovered quantitatively by the absorption system except for some runs where the absorption efficiency was reduced due to the formation of a hydrochloric acid aerosol, easily recognizable as white fumes in the exit gas of the absorption system. Since part of the acid aerosol escapes the absorber, the gas analysis data of these runs have been discarded. Aerosol formation was apparently provoked by a low water concentration and a high oven temperature (>250 "C in the absence of water and >500 "C for PHz0 > 0.05 atm). The aerosol formation can often be reduced by increasing the absorber temperature. Our theory is that the aerosol formation is caused by traces of submicron nuclei of a condensed volatile solid component of the lime particles or the reactor walls which affect the condensation of hydrochloric acid in the absorber system.
Experimental Section The experiments were performed with commercially available particles of solid slaked lime or limestone. Data for the solid sorbents are shown in Table I. The powder particles were placed in the reador in a very thin, even layer on a support of fine quartz wool. The conversion vs time curve as shown in Figure 5 has a characteristic shape which is qualitatively the same for all runs. At the start of the run the sorption rate is very high and most of the HC1 of the gas is consumed. The rate of sorption gradually decreases, and after some time which varies with the experimental conditions, but is usually less than 15 min, there is no longer any observable change of the HC1 concentration in the gas when it is passed through the reactor. The lime conversion thereafter remains constant-at least on a time scale of practical significance. The form of the curve depends on the kinetics of the sorption reaction and is further analyzed in the last part of the paper. The ultimate, constant level of conversion obtained after about 15 min is called the final conversion in the following. It varies in the range 4-90% and depends significantly on temperature, water content, and lime type. By repeating some of the runs, we found a -5% standard deviation for the variation between different samples with respect to both sorption capacity and sorption rate. This was normally by far the largest contribution
166 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992
Figure 3. Experimental apparatus schematically: 1, reagent gases; 2, gas flow controllers; 3, evaporator; 4, needle valve; 5, temperature controller; 6, water feed pump; 7, manometer; 8, reactor; 9, tubular oven; 10, three-zone temperature controller; 11, HCl absorber; 12, pH or C1- electrode; 13, electronic measuring system for electrode; 14, computer.
to experimental error. A few exceptions to this rule are specially noted in the following. Specific Surface Area and Particle Size. Table I1 shows the final lime conversion a t four temperatures for two lime samples with almost the same specific surface area, but with a 10-fold variation of particle size. There is obviously little or no influence of particle size on the final lime conversion for these runs. As a consequence, we conclude that the limitation in the final lime conversion is not caused by macroscopic phenomena such as blocking of the pores in an outer shell of the particle. On a macroscopic level the particles are hence homogeneously converted which is confirmed by energy-dispersive X-ray (EDAX) analysis of polished fracture surfaces of reacted particles which always show an even radial distribution of chloride in the particle even for much larger particles than those shown in Table I (Figure 6). This is true, not only for particles at the final conversion state but also for particles examined at intermediate levels of conversion. The influence of specific surface area on the final lime conversion is less clear from the data. However, several runs seem to indicate that under certain conditions the surface area only exerts a minor influence on the final conversion. This is shown on, e.g., Figure 7 a t high temperatures and for SBm> 12 m2/g. Exceptions to this rule are the particles with the lowest value of SBET on Figure 7 which yields a consistently lower final conversion and the low-temperature results with very low final conversion. Temperature and Water Content. Figures 8 and 9 show how the concentration of water in the gas affects the final lime conversion from 60 to 250 "C. At temperatures below 150 "C the conversion, in the absence of water, is very low (
