Dry Scrubbing of Gaseous HCl with Solid Lime in a ... - ACS Publications

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Ind. Eng. Chem. Res. 2001, 40, 304-313

Dry Scrubbing of Gaseous HCl with Solid Lime in a Cyclone Reactor at Low Temperatures Ana M. Fonseca,† Jose´ J. O Ä rfa˜ o,‡ and Romualdo L. Salcedo*,‡ Universidade Fernando Pessoa, Departamento de Cieˆ ncia e Tecnologia, Praç a 9 de Abril, 4100 Porto, Portugal, and Faculdade de Engenharia da Universidade do Porto, Departamento de Engenharia Quı´mica, R. Bragas, 4099 Porto, Portugal

This paper describes a theoretical and experimental study on the use of a reverse-flow cyclone for the dry scrubbing of HCl. Solid lime particles are injected at the cyclone inlet at low temperatures. The inlet gas composition varied in HCl concentration up to 550 ppmv and in water content up to 3.4 vol %. The experimental results show complete HCl removal for solid reactant feeds corresponding to 2-3 times the stoichiometric ratio and for very short residence times (0.05 s). The presence of humidity in the gas was found to be very important for obtaining high HCl removal efficiencies. Significant differences were found in the morphology of the solid products obtained with and without humidification. The experimental results were modeled by considering plug flow for the solid phase and plug flow or perfectly mixed flow for the gaseous phase. In both cases, there is good agreement with the experimental observations. The results suggest that cyclones can be effectively and economically used for acid gas cleaning purposes. Introduction Acid gases, namely, SOx, HF and HCl, are gaseous pollutants that can be found in the effluent gases of several combustion processes, such as the burning of fossil fuels and the incineration of domestic and hospital wastes. The removal of these toxic acid gases is performed through reaction with alkaline sorbents in wet, semi-dry, and dry processes. Dry scrubbing consists of the injection of an alkaline reactant, in the form of a finely divided dry powder, into the flue gas stream. The removal of the acid components takes place through adsorption and reaction at the solid’s surface. The solid product formed is then removed in the dust removal equipment, usually bag filters or electrostatic precipitators. Bag filters have been favored because the deposited dust layers contain unreacted sorbent that further reduces the acid gases content in the exhaust gases. Calcium-based sorbents (namely, CaCO3, Ca(OH)2, and CaO) are widely used because of their low cost and high efficiency for acid gas scrubbing.1-3 The removal efficiency in dry-scrubbing processes has traditionally been low, but recent studies indicate that efficiencies higher than 90% can easily be achieved.3 Scrubbing at low temperatures allows for the recovery of energy from the flue gases, decreases the volume of gas to be treated and enables the capture of some compounds (namely, heavy metals) as they condense. Also, a lower temperature increases the adsorption capacity of the solid sorbent.1,3-8 However, there is a lower limit for temperature so that problems with the buoyancy of the gas and plume formation can be avoided. Cyclones are centrifugal dedusters, used industrially since 1886, characterized by simple designs, low investment and maintenance costs, and the ability to operate at high loadings, high temperature, and pressure. It would be of great practical and economic relevance if * Author to whom correspondence should be addressed. Tel.: +351 22 5081644. Fax: +351 22 2000808. E-mail: [email protected].

the dry scrubbing of acid gases and the dust removal could take place in the same equipment. Cyclones seem particularly suited to conjugating this double function of reactor and gas-solid separator for several types of reactions.9-11 For typical HCl concentrations (∼500 ppmv) and solid sorbent/gas ratios (2-3 times the stoichiometric ratio), the increase in loading at the cyclone inlet is below 3.5 × 10-3 kg m-3, and this should not pose any problem for any cyclone.12 As examples of industrial applications of the cyclone reactor we can refer to solid and liquid combustion reactions, mineral treatments, nonferrous metal production, polymerization reactions, and the drying of granular materials.10,13 In particular, wood sawdust pyrolysis was studied by Le´de´ et al.9 and Le´de´11 in small cyclones with internal diameters ranging from 0.028 to 0.04 m for particle sizes ranging from 2 × 10-4 to 1 × 10-3 m, and practically complete conversions were obtained. These authors observed the high efficiency and stability of operation obtained in their cyclone reactor. Li10 made a scientific study of the hydrodynamics of the gaseous and solid phases in cyclones with internal diameters varying from 0.028 to 0.40 m at Reynolds numbers of the gas at the cyclone inlet from 150 to 40000. The solid phase was again constituted by particles with dimensions between 2 × 10-4 and 1 × 10-3 m, which are similar to those found on most laboratory-scale circulating bed reactors.11 According to the model of Villermaux et al.,14 in this range of conditions, the gas flow in the cyclone can be described by an entry zone with plug flow, followed by a partially short-circuited perfectly mixed zone. For the solid phase, the results show a plug flow, in agreement with the results of Le´de´ et al.,15 which were obtained in a 0.125 m internal diameter cyclone. Very high heat and mass transfer coefficients in cyclone reactors are also reported by some authors.10,16,17 Li10 and Le´de´ et al.18 reported results of thermal decomposition reactions in cyclone reactors, indicating high conversions of reactants coupled with very high production capabilities.

10.1021/ie000634e CCC: $20.00 © 2001 American Chemical Society Published on Web 11/21/2000

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 305

Figure 1. Schematic diagram of the experimental equipment.

The general conclusions of these studies indicate that the cyclone reactor allows, in a single continuous operation, for heating of the reactants and for gas-solid separation in very short times (less than 1 s). However, scientific studies focusing on cyclones as acid gas cleaning reactors are, to the authors’ knowledge, unavailable in the literature, and this may be a limiting factor to a more widespread use of these devices for concomitant gas cleaning and dust removal. The study of the application of this technology in the dry scrubbing of acid gases is thus the main purpose of the present paper. This study focuses on the removal of hydrogen chloride emissions by continuous injection of solid particles of Ca(OH)2 at low temperatures and high relative humidity levels, because of the promising reacting capacity between these two compounds under these operating conditions.19-22 Experimental Section The capacity to remove HCl from a gaseous effluent using solid particles of Ca(OH)2 was studied in a cyclone reactor. Two sets of experiments were carried out at 50 and 130 °C, with the inlet HCl concentration varied from 0.56 to 2.1 mol m-3, the humidity level from 0 to 3.4 vol %, and the amount of solids injected from 0.2 to 6 times the stoichiometric ratio. Figure 1 shows a schematic diagram of the experimental equipment, which basically promotes the continuous contact between a gaseous stream of HCl in N2 with the solid particles of Ca(OH)2 in a small laboratory-scale allPTFE cyclone reactor.

The solid particles are fed to the reactor using N2 as the carrier gas, and the two streams are joined together immediately before the cyclone inlet. The main portion (>90% by mass) of the solid particles is captured in the cyclone hopper. The particle size distribution of the lime feed is log-normal, with MMD ) 1.37 µm and σg ) 2.26.23 Because of the particles’ small dimensions, some of them are carried with the cyclone exit gas. They are captured by two GFA filters with 0.047 m diameter, placed in series in a common PTFE filter holder. This arrangement was necessary to avoid some small losses when a single GFA filter was used.24 The effluent gas concentration is measured by bubbling a known fraction of this stream in water by means of an appropriate external circulation airlift absorption vessel.19,21 The HCl is completely absorbed in the water,24 and the pH of the resulting solution is monitored online with a computerized data acquisition system. The HCl removal efficiency (η) is then calculated through material balances. For a cross check, at the end of the experiment, the solids captured in the cyclone are dissolved in distilled water and chemically analyzed for chlorides by an appropriate spectrophotometric method.25 The same procedure is followed for the solids captured in the back filters at the exit of the cyclone. The conversions of the solid reactant in the cyclone (χF) and in the filter (χC) are then obtained through material balances. A detailed description of the most important parts of the experimental equipment is presented below. Cyclone Reactor. The cyclone reactor is basically a

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Figure 2. Dimensions of the cyclone reactor.

modified Stairmand HE design, with a narrower inlet and vortex finder, to increase collection efficiency (Figure 2). PTFE was chosen as the material for the cyclone reactor because of the extremely high corrosivity of gaseous HCl. The reactor is placed inside a highprecision gas-chromatographic temperature-controlled oven. The gas flow rate through the cyclone was kept at 2.3 × 10-4 Nm3 s-1. Preliminary experiments were carried out to evaluate the particle collection efficiency of the cyclone in the absence of reaction, over the range of experimental conditions. The solid flow rate at the inlet of the cyclone was maintained at 1.3 × 10-7 kg s-1, giving loads lower than 1 g/Nm3. The results indicate that the global collection efficiency is mostly independent of the operating conditions in the studied range.24 The average collection efficiency in the absence of reaction was 91.6% ( 3.2% for a 5% significance level. Because the molar volume of the solid product is 2.37 times larger than that of the solid reactant, the chemical reaction with HCl will increase the particles’ dimensions and consequently may increase the cyclone’s collection efficiency. To quantify this increase, the Mothes and Loffler26 model for predicting cyclone performance was coupled with an empirical estimate of the particles turbulent dispersion coefficient,27 and the model results indicate that the predicted increase should be less than 1% in all cases. Because the global collection predicted from the model in the absence of chemical reaction agrees well with the global collection experimentally observed, the global collection in the absence of chemical reaction is considered representative of the global collection with chemical reaction. Test Gas. The contaminated gas stream results from the mixing of commercial HCl type N26 with a minimum purity of 99.6% (maximum H2O content ) 10 ppm) with N2 with a minimum purity of 99.95% (maximum H2O content ) 10 ppm). The HCl flow rate is varied with appropriate needle valves and coarsely measured by a PTFE microrotameter. This value is accurately

calculated by the slope of the H+ curve against time in the absence of lime reactant injection.24 The N2 stream is optionally diverted to the humidification system, which is basically a bubbling column with a height and internal diameter of 0.38 and 0.05 m, respectively, filled with distilled water. The effluent gas temperature and humidity are measured in situ with a hygrometer (Protimeter I-220 Rotronic). The humidification column and the hygrometer are placed inside a thermostatted water bath with the temperature set according to the desired humidity level. All of the tubing leading from the humidification column to the inlet of the oven is thermally insulated to prevent any water condensation. Inside the oven, before entering the cyclone reactor, the test gas is passed through an 8 m coil of PTFE tube to promote homogenization both in temperature and composition. Solid Reactant. Calcium hydroxide pro-analysis (Riedel-de-Haen) was chosen to avoid any competing reaction with impurities. A complete characterization of this solid reactant was done through SEM analysis, X-ray sedimentometry, mercury porosimetry, and N2 adsorption at 77 K and is available elsewhere.21,24 The results obtained indicate that the solid particles are essentially submicrometric and nonporous. The solid particles’ dimensions are well represented by spheres with a mean number diameter estimated at 0.30 µm. The Ca(OH)2 solid particles are continuously fed to the cyclone reactor through a Wright MK2 dust feeder using N2 as the carrier gas with a flow rate of 2.3 × 10-4 Nm3 s-1. This dust-feeder was chosen because of its special ability to feed fine dusts. However, a significant amount of agglomerated particles could still be found in the solids fed to the cyclone reactor. To minimize this, a modified Stairmand HE type cyclone with a 0.70 m internal diameter was placed upstream of the test cyclone.23 A controlled fraction of the carrying gas containing the solid particles that exits the deagglomerating cyclone was then fed to the cyclone reactor. This stream passes through copper tubing (0.01 m internal diameter) wrapped with a temperaturecontrolled heating coil, so that the gaseous stream temperature is set at the reaction temperature before entering the cyclone reactor. HCl Detection System in the Effluent Gas. The HCl concentration of the effluent gas at the outlet of the reactor is determined by absorbing a controlled fraction of this gas in a special gas absorber that avoids undesirable interaction between gas bubbles and the pH electrode.19,21 The gas flows through a two-compartment external circulation airlift absorber filled with 2.5 × 10-4 m3 of distilled water. The gas bubbles up through the main chamber of the absorber, causing a rapid recirculation of the liquid because of the different densities of the solutions present in the two compartments. A pH electrode immersed in the bubble-free lateral compartment is connected to a data acquisition unit (signal transducer, A/D acquisition interface, and microcomputer) that continuously reads 350 pH values per second and registers the average value corresponding to each second. The absorption system is immersed in a thermostatted bath kept at a temperature 10 °C above that of the humidification system. This avoids unwanted condensations since, at lower temperatures, an aqueous phase is formed because of the occurrence of a maximum boiling point azeotrope for the system HCl/H2O.28


Figure 3. Experimental results for the HCl removal efficiency of a typical experiment (T ) 50 °C, [HCl]in ) 0.829 × 10-2 mol m-3, [H2O] ) 3.4 vol %).

Figure 5. Comparison of the model proposed in Figure 4 with the experimental results (T ) 50 °C, [HCl]in ) 0.829 × 10-2 mol m-3, [H2O] ) 3.4 vol %).

Figure 4. Model for the evolution of the HCl outlet gas concentration with time.

Preliminary experiments showed that the HCl detection system’s response is well-described by a first-order dynamic model with a time delay. The parameters of this model were experimentally obtained, as 9.7 ( 2.7 s for the time constant and 11.5 ( 1.2 s for the time delay, both at a 5% significance level.24 Experimental Results HCl Removal Efficiency. In the presence of humidity, very high HCl removal efficiencies were experimentally obtained in the cyclone reactor. The results of the evolution of the removal efficiency with time for a typical experiment are given in Figure 3. The removal efficiency curves show two different phases: in the first moments of reaction, the removal efficiency increases with time, followed by an apparently stationary phase. This behavior was found in all of the experiments performed. It can be shown that the initial transient phase is due to the response of the HCl detection system, mentioned above, and not to the reaction.24 Figure 4 shows the proposed model for the variation with time of the HCl concentration in the gas at the outlet of the cyclone reactor: when the Ca(OH)2 feeding is initialized (ts), the HCl concentration in the effluent gas changes instantaneously to a constant value, corresponding to the steady-state removal efficiency observed in the second phase of Figure 3. The effect of the dynamics of the response of the HCl detection system in this model can be seen in Figure 5, where the good agreement between the experimental results and the proposed model is patent. Thus, practically no transient state can be detected by this data acquisition system, indicating that a very fast reaction is taking place in the cyclone reactor. The amount of solid reactant fed to the cyclone reactor is an important variable as it is directly related to the

Figure 6. Influence of the amount of solid reactant on the HCl removal efficiency. b: T ) 50 °C, [HCl]in ) 1 × 10-2 mol m-3, [H2O] ) 3.4 vol %. 4: T ) 50 °C, [HCl]in ) 2 × 10-2 mol m-3, [H2O] ) 3.4 vol %. O: T ) 130 °C, [HCl]in ) 0.7 × 10-2 mol m-3, [H2O] ) 3.4 vol %.

operating costs of this process. The ratio between the amount of Ca(OH)2 fed and the corresponding to the stoichiometric quantity is represented by R/SR. It was experimentally observed that, in the presence of humidity, the HCl removal efficiency increases with R/SR, as can be seen in Figure 6. Nevertheless, at 50 °C, the HCl removal efficiency reaches 100% for R/SR values of 2-3, indicating that, under these operating conditions, obviously no improvement would result from further increasing the amount of solid reactant. Figure 6 also shows that the HCl concentration in the inlet gas, in the studied range, does not seem to have a significant influence on the HCl removal efficiency. However, more experimental data at the higher inlet gas concentrations are necessary to support this conclusion. Despite the scatter in the experimental data, it seems that the HCl removal efficiency is higher at 50 °C than at 130 °C. This may be due to the relative humidity of the test gas,19,21,22 which is higher at the lower temperature. Other pulished works29 also refer to the importance of the relative humidity for HCl removal with calcium silicate solids, although not to the extent that occurs with lime. Conversion of the Solid Reactant. The results of the solid reactant’s conversion obtained with humidified gas at 50 °C are presented in Figure 7. The values obtained indicate that the usage of the solid reactant is low since the final conversions are on average lower

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Figure 7. Experimental results of the conversion of the solid reactant (T ) 50 °C, [HCl]in ≈ 1 × 10-2 mol m-3, [H2O] ) 3.4 vol %). b, χC; O, χF.

Figure 8. Evolution with time of the conversion of the solid captured in the cyclone reactor (T ) 50 °C, [H2O] ) 3.4 vol %). b: [HCl]in ) 1 × 10-2 mol m-3, R/SR ) 2.5. 4: [HCl]in ) 1 × 10-2 mol m-3, R/SR ) 5.5. O: [HCl]in ) 2 × 10-2 mol m-3, R/SR ) 0.5.

than 30%. The conversion of the solid captured in the cyclone (χC) is systematically lower than the conversion of the solid captured in the filter (χF). This is expected because the solid particles in the filter are in permanent contact with the gas, whereas the contact between the gas and the solid particles in the cyclone is restricted to the solid’s residence time. Figure 7 also shows that the differences between χC and χF are lower at the higher values of R/SR. Once again, this is expected because higher values of R/SR correspond to higher HCl removal efficiencies in the cyclone reactor. The concentration of the gas that passes through the filter is therefore lower under the latter condition, resulting in a smaller reaction extent in the filter. The values of χC are apparently lower when R/SR increases, since for the same amount of HCl removed, increasing the amount of the solid reactant will result in a smaller value for its fractional conversion. Because solid reactant particles deposit on the cyclone walls, some particles within the cyclone reactor may contact the flowing gas for a much longer time than estimated from the particle residence time. To ascertain this effect on the reaction extent, experiments under the same experimental conditions but with different time spans were carried out. Figure 8 shows that the conversion of the solid captured in the cyclone does not change with time, independent of the experimental conditions. This result excludes the possibility that the solid particles deposited on the cyclone walls are responsible

Figure 9. Influence of the presence of humidity on HCl removal in the cyclone reactor (T ) 50 °C, [HCl]in ) 1 × 10-2 mol m-3, [H2O] ) 3.4 vol %).

for the HCl removal. Therefore, the observed conversions for the solid particles are obtained only for particles reacting at the cyclone core and not at the cyclone walls, once more indicating a very fast reaction. The presence of the filter at the outlet of the cyclone reactor increases the HCl removal efficiency as part of this gas is being removed through the reaction that takes place in the filter. It is thus important to verify the extent to which the reaction is due either to the cyclone or to the backfilter. Assuming that the solid particles captured in the filter are converted in the cyclone to the same extent as the average value obtained for the particles captured in the cyclone, the difference between χF and χC is directly related to the amount of HCl that was removed through reaction in the filter. The HCl removal efficiency due to the cyclone reactor alone can thus be estimated. It was found that the increase in the HCl removal efficiency due to the filter in our experiments is on average lower than 0.5% and reaches a maximum of 1.5%. These results indicate that the HCl removal efficiency obtained in the cyclone reactor is not dependent on the presence of the filter and is mostly due to the cyclone itself. Humidity Level in the Test Gas. The importance of the presence of humidity in the gaseous effluent was evaluated through a simple experiment: the humidification of the gas was stopped in the middle of an experiment, with all other operating conditions maintained. Figure 9 shows the evolution of the amount of H+ present in the absorbing solution at the outlet of the cyclone reactor as a function of time. At the beginning, when the HCl feeding begins, the amount of H+ in the absorbing solution increases at an approximately constant rate (corresponding to the HCl feeding flow rate). When the feeding of Ca(OH)2 is initiated, the amount of H+ remains constant, indicating that all of the HCl is being removed in the cyclone reactor. Stopping the humidification of the test gas has an almost immediate effect of increasing the amount of H+ in the absorbing solution, with a slope similar to that observed at the beginning of the experiment. This indicates that no HCl is being removed in the cyclone reactor in the absence of humidity. In fact, for all of the experiments performed in the absence of moisture, the solid particles obtained in the cyclone and in the filter at the outlet of the cyclone after 20 min of reaction time showed conversions smaller than 4%. These results are in agreement with other published works19,21 in which the reaction of


chemical composition of both samples is the same, the presence of water seems to affect the organization of the crystalline structure. These findings support the hypothesis advanced by Duo et al.30 and by Fonseca et al.21 Modeling of the Experimental Results

Figure 10. SEM photographs of the reacted solid particles captured in the cyclone (T ) 50 °C, [HCl]in ) 2.09 × 10-2 mol m-3). (a) [H2O] ) 0%; (b) [H2O] ) 3.4 vol %.

gaseous HCl and solid Ca(OH)2 was studied in fixedbed laboratory reactors. To explain the abrupt stopping of this reaction in the absence of humidity, Duo et al.30 proposed the “crystallization and fracture model”. These authors assumed that the reaction progresses only when the fracture of the crystalline structure of the solid product occurs, since its molar volume is higher than that of the reactant. Accordingly, the reaction stops when the energy necessary to break the product’s crystalline structure is higher than the energy released by the reaction. Fonseca et al.21 consider the complementary hypothesis of formation of an aqueous phase that breaks the crystalline structure of the solid product and enables the progress of the reaction. In an attempt to explain the observed strong influence of the humidity in the gas, the solid products obtained in two experiments at 50 °C with and without humidity were analyzed through X-ray diffraction and electron scanning microscopy (SEM). The chemical composition was found to be the same:24 unreacted Ca(OH)2 and a solid solution identified as CaClOH‚2H2O. Jozewicz and Gullett31 have also identified this compound as the main product of the reaction between gaseous HCl and solid Ca(OH)2. These authors verified that this compound is the result of a solid solution of the original reaction product, CaCl2‚2H2O, with unreacted Ca(OH)2. The results of the SEM analysis are presented in Figure 10. The differences in the solid’s morphology are very clear in these photographs: the solid obtained in the experiments without moisture presents a much more “closed” crystalline structure as essentially no openings in its surface can be found. The laminar crystals that can be seen in this solid suggest a rigid structure. On the contrary, the solid obtained in the presence of humidity has visible openings in its structure. Because the

Previous kinetic studies indicate that, under the present operating conditions, the reaction between gaseous HCl and solid Ca(OH)2 is controlled by diffusion through the solid product layer.19,21,24,30,32 Nevertheless, the very high conversions obtained in the cyclone reactor for very short contact times are incompatible with solid product diffusion limitations. This can be explained taking into account the fact that, in our experiments, the gas is always in contact with totally unconverted solid reactant, and therefore, the limitation to diffusion through the solid product layer does not exist since no product has yet been formed. Because of the high velocities of the gas achieved in the cyclone, the external and internal mass transfer limitations are also negligible.24 Therefore, in the modeling of the experimental results, it will be assumed that the reaction is in the chemical regime. In the absence of conclusive results regarding the reaction order in previous kinetic studies,24 first-order kinetics relative to the HCl concentration will be assumed. The rate of consumption of the gaseous reactant per unit surface area of the solid is given by

-rA ) ksCA

(1)

where ks is the kinetic constant of the reaction and CA is the HCl concentration in the cyclone. The reaction under study will be represented by the general expression

2A(g) + B(s) f D(s) Solid Phase. In accordance with previous studies,10 it will be assumed that the solid particles flow in the cyclone with a plug flow. The solid particles are captured along the cyclone, resulting in a decrease in the solid’s concentration throughout its path. A simple way to simulate this behavior is to represent the solid’s flow by a cascade of N CSTRs, considering that a fraction of the solid will be captured between the outlet of each CSTR and the inlet of the one immediately following it. The solid’s capture efficiency in each CSTR (ηcap) will be defined as the ratio between the captured mass in every unit of time and the solid’s mass flow rate throughout the CSTR. It will also be assumed that ηcap remains constant along the cyclone. Taking into account that the global collection efficiency of the solid particles in the cyclone was experimentally determined to be 92%, the value of ηcap can be obtained through the following expression: N

ηcap(1 - ηcap)i-1 ) 0.92 ∑ i)1

(2)

The determination of the mean residence time for the solid particles in the cyclone (trs) is not within the scope of this study. To the authors’ knowledge, only the work of Li10 has focused on the determination of trs. This author obtained 0.4 s for the residence time of 2 × 10-4 m diameter particles in a cyclone with a 0.28 m

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diameter, at operating conditions similar to those of the present work. The extrapolation of this result to the present study is not valid as the solid particles here are 1000 times smaller. Therefore, and in the absence of other results, the value of 0.4 s will be considered as an upper limit for the solid’s residence time in the cyclone. A lower limit for this parameter could be the residence time of the gas, which is calculated as the ratio between the volume of the cyclone and the gas flow rate, giving 0.04 s. Taking into account that the rate of consumption of the solid reactant B is half that of the gaseous reactant A, given by eq 1, a material balance to the solid reactant in the ith CSTR leads to the equation

N˙ Bei ) N˙ Bi + 0.5SksCAi

(4)

where SB is the specific surface area of the solid reactant and MB is its molar mass. The reaction that takes place in each CSTR produces D in the same molar amount as that of B consumed. Therefore, there is also a molar flow rate of product D inside each reactor (N˙ Di), calculated by

N˙ Di ) N˙ Dei + (N˙ Bei - N˙ Bi)

(5)

The conversion of the solid captured in the cyclone predicted by this model (χCmod) is obtained through the following equation: N

χCmod )

ηcapN˙ Di ∑ i)1 0.92N˙ Be1

(6)

Gaseous Phase. Previous studies indicate that the gas flow in a cyclone is well-described by a model considering an entry zone with plug flow followed by a perfectly mixed zone partially short-circuited to the outlet.10,14 Li10 proposed some correlations to predict the parameters of this model, which, when applied to the present situation, indicate that the plug flow occupies 42% of the total volume of the cyclone and that 35% of the total gas flow rate is short-circuited to the outlet of the cyclone. Taking into account that this short-circuit is incompatible with the complete HCl removal efficiencies obtained experimentally, the application of this model to the present situation is not reasonable. Therefore, two limit situations will be considered to simulate the gas flow in the cyclone: perfectly mixed flow and plug flow. In the case where perfectly mixed flow is assumed, the concentration of the gaseous reactant in the cyclone (CA) is uniform. The equation describing the mass balance of the gaseous reactant takes the form

QCAe ) QCA + SBMBnBtotksCA

T (°C) 50 130

trs (s) 0.04 0.4 0.04 0.4

gaseous phase in piston-type flow

gaseous phase in perfectly mixed flow

ks -2 (m3gas msolid s-1)

ks -2 (m3gas msolid s-1)

33 3.3 16 1.6

(7)

61 6.1 21 2.1

where Q is the total gas flow rate, CAe is the HCl concentration at the entrance of the cyclone reactor, and nBtot is the total molar amount of solid reactant present inside the cyclone, which is given by

(3)

where N˙ Bei and N˙ Bi are, respectively, the molar flow rate of B at the inlet and outlet of this CSTR; CAi is the gaseous reactant concentration in this reactor; and S is the surface area of B available for reaction. S is proportional to the average amount of B present in this CSTR, estimated as the product of N˙ Bi with the average residence time of the solid in each CSTR (trs/N)

S ) SBMBN˙ Bitrs/N

Table 1. Results Obtained in the Nonlinear Least-Squares Analysis

N

nBtot )

∑ i)1

N˙ Bi

trs

(8)

N

The HCl removal efficiency in the cyclone predicted by this model (ηmod) is, in this case, calculated as

ηmod ) 1 -

CA CAe

(9)

The calculation of CA is done iteratively through eqs 7 and 8, where N˙ Bi is obtained by solving eqs 3 and 4. Assuming that the gas flows in the cyclone with plug flow, the same cascade of CSTRs used to represent the solid’s flow will be considered here. The mass balance to the gaseous reactant in the ith CSTR takes the form

QCAi-1 ) QCAi + SBMBN˙ Bi

trs kC N s Ai

(10)

The values of CAi can be obtained for each CSTR solving the second-order equation resulting from the combination of eqs 3, 4, and 10. The HCl removal efficiency in the cyclone predicted by this model (ηmod) is, in this case, calculated as

ηmod ) 1 -

CAN CAe

(11)

The predictions of the proposed model were obtained to investigate the sensibility of the model to the parameters that describe it, namely, N, trs, and ks. It was found that the values of ηmod and χCmod decrease as N increases but remain constant for values of N higher than 30. All of the simulations were then performed considering N ) 50. As expected, the results indicate that ηmod and χCmod increase with trs and ks. Because two limiting values were defined above for the residence time of the solid particles in the cyclone (trs), the only parameter of the proposed model is the kinetic constant of the chemical reaction (ks). Its value was determined through nonlinear least-squares analysis,33 by minimizing the residuals between the experimental results and those predicted by this model simultaneously for η and χC. The results obtained for 3 -2 msolid s-1 at 50 ks (Table 1) are between 3.3 and 61 mgas 3 -2 °C and 1.6 and 21 mgas msolid s-1 at 130 °C. Such values are in agreement with previous kinetic studies on this reacting system.21 The smaller values of ks obtained for the higher temperature can be explained by taking into account that the limitation to diffusion through the product layer was not considered in this model and that


Figure 11. Comparison between the experimental results of the HCl removal efficiency and the predictions of the proposed model (T ) 50 °C, [HCl]in ) 1 × 10-2 mol m-3, [H2O] ) 3.4 vol %). s, Gaseous phase in plug flow. ‚‚‚, Gaseous phase in perfectly mixed flow.

Figure 12. Comparison between the experimental results of the solid conversion in the cyclone and the predictions of the proposed model (T ) 50 °C, [HCl]in ) 1 × 10-2 mol m-3, [H2O] ) 3.4 vol %). s, Gaseous phase in plug flow. ‚‚‚, Gaseous phase in perfectly mixed flow.

the importance of this limitation increases with temperature.19,21,24 The comparison between the experimental results and those predicted by the model for the HCl removal efficiency (Figure 11) and for the conversion of the solid captured in the cyclone (Figure 12) indicate that the proposed model can reasonably describe the experimental observations. The similarities between the predictions of the model for the two limiting situations considered to describe the flow of the gaseous phase are also apparent in these figures. Conclusions Complete HCl removal was obtained in a cyclone reactor with a continuous and steady-state process with dry injection of solid fine particles of lime in the presence of humidity in the effluent gas, especially at the lowest temperature studied (50 °C), at 2-3 times the stoichiometric quantity of the solid reactant. The observed solid usage was low as the conversions of this reactant were on the average lower than 30%. The re-utilization of this partially reacted solid by mixing it with fresh reactant could eventually reduce the amount of Ca(OH)2 consumed without compromising the HCl removal efficiency of the process. However, this re-utilization

should be studied carefully as the presence of product layer diffusion limitations will reduce the reactivity of this partially converted solid.19,21,30,32 The experimental results show that the presence of humidity is a very important factor in the efficiency of this system. In the absence of humidity, this dryscrubbing process is not adequate for the removal of HCl as the removal efficiencies obtained were lower than 3%. These results are in agreement with fixed-bed experiments for a similar system.19,21 The relative humidity seems to be the variable influencing the reactivity of this system because higher removal efficiencies were obtained at the lowest temperature. Analysis comparing the solid products obtained in experiments carried out in the presence and absence of humidity revealed that their chemical composition is the same. However, SEM analysis showed that the crystalline structure of the solid product obtained without humidity was apparently more rigid and closed, eventually constituting a strong limitation to the progress of reaction. In the modeling of the experimental results, the solid flow in the cyclone was considered to be plug flow and was simulated by a cascade of CSTRs. The residence time of the solid was estimated to be between 0.04 and 0.4 s. The flow of the gas was simulated under two limiting situations: perfectly mixed flow and plug flow. Chemical reaction control was assumed, with first-order kinetics relative to the HCl concentration. The proposed model can reasonably describe the experimental results both for the HCl removal efficiency and for the solids conversion. The only parameter of this model is the kinetic constant of the chemical reaction, whose value 3 -2 msolid s-1 was estimated to be between 3.3 and 61 mgas 3 -2 -1 at 50 °C and between 1.6 and 21 mgas msolid s at 130 °C, depending on the residence time of the solid and the hydrodynamics of the gas. These high values obtained for ks indicate a very fast chemical reaction between gaseous HCl and solid particles of Ca(OH)2, which is in agreement with previous kinetic studies on this reacting system.21 The main conclusion of this study is that, at a laboratory scale, the use of a cyclone reactor in a continuous dry-scrubbing process using solid particles of Ca(OH)2 is very efficient in the purification of gaseous emissions containing HCl. This equipment allows for the simultaneous highly efficient scrubbing of the gas and the removal of the solid particles in very short residence times. Using a cyclone reactor for the dry scrubbing of gases containing HCl (and possibly other acid pollutants such as HF and SO2) may thus be an interesting alternative approach for industrial-scale exploration. However, this process can only be effective and economical at an industrial scale if partial recirculation of the solid reactant is achieved. Furthermore, because the reaction proceeds mostly on the cyclone and not on the backfilter, any industrial process will require extremely high efficiency cyclones if bag filters are to be avoided. Very high efficiency cyclones have been designed by solving a numerical optimization problem,34 have been extensively tested at a laboratory scale,35 and are presently undergoing pilot-scale tests at a large Portuguese chemical manufacturer. Recycling part of the gaseous stream exiting the cyclone has recently been shown by Le´de´11 to produce complete gasification of wood sawdust by pyrolysis. Thus, in a similar way, very high efficiency recirculation cyclones, which have demonstrated at a laboratory scale a clear superiority over single reverse-flow cyclones for

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the capture of submicron particles,36 could be used profitably for the simultaneous acid gas cleaning and particulate capture. In addition, it is important to extend this work to higher temperatures and to the simultaneous removal of SO2 and HCl using a simulated flue gas. Acknowledgment The authors acknowledge financial support from JNICT (Contracts PEAM/C/SEL/482/95 and P/EQU/ 12003/98) and from the scientific programs CIE ˆ NCIA and PRAXIS XXI (scholarships BD/2238/92-RN and BD/ 5474/95). Nomenclature CA ) HCl concentration in the cyclone reactor (mol m-3) CAe ) HCl concentration at the inlet of the cyclone reactor (mol m-3) CAi ) HCl concentration in the ith CSTR (mol m-3) CSTR ) Continuously stirred tank reactor ks ) Reaction rate constant per unit surface area of the -2 s-1) solid (m3gas msolid M ) Molar mass of the solid reactant (kg mol-1) MMD ) Mean mass diameter (µm) nBtot ) Total amount of solid reactant present inside the cyclone reactor (mol) N˙ ei ) Molar flow rate at the inlet of the ith CSTR (mol s-1) N˙ i ) Molar flow rate at the outlet of the ith CSTR (mol s-1) R/SR ) Ratio between the amount of Ca(OH)2 fed and that corresponding to the stoichiometric quantity S ) Surface area of solid reactant available for reaction (m2) SB ) Specific surface area of the solid reactant (m2 kg-1) Q ) Gas flow rate (m3 s-1) t ) Time (s) ts ) Instant of time when the feeding of Ca(OH)2 starts (s) trs ) Mean residence time of the solid particles in the cyclone (s) T ) Temperature (°C or K) Greek Symbols χC ) Conversion of the solid captured in the cyclone reactor χCmod ) Conversion of the solid captured in the cyclone reactor predicted by the model χF ) Conversion of the solid captured in the filter η ) HCl removal efficiency ηcap ) Solid particle capture efficiency in each CSTR ηmod ) HCl removal efficiency predicted by the model σg ) Geometric standard deviation

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Received for review July 6, 2000 Revised manuscript received September 20, 2000 Accepted September 21, 2000 IE000634E

Dry Scrubbing of Gaseous HCl with Solid Lime in a ... - ACS Publications

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