Ind. Eng. Chem. Res. 1997, 36, 2141-2148
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High Surface Area Calcium Carbonate: Pore Structural Properties and Sulfation Characteristics S.-H. Wei, S. K. Mahuli, R. Agnihotri, and L.-S. Fan* Department of Chemical Engineering, The Ohio State University, Columbus, Ohio 43210
Limestone or calcium carbonate (CaCO3) used as a sorbent in the removal of acid gas precursors (SO2) from combustion systems suffers from poor pore structural properties which causes low reactivity and incomplete utilization. The surface area and pore size distribution of CaCO3, if tailored appropriately, can considerably enhance its reactivity for SO2. This study focuses on the optimization of pore properties of CaCO3 particles and the enhancement of SO2 reactivity and ultimate utilization. The carbonate is produced by precipitation from an aqueous suspension of calcium hydroxide by injecting CO2. The influence of operating parameters, suspension concentration, gas flow rate, and additives (surfactant) on the surface area and pore volume is investigated. The surface area of the carbonate powder can be controlled in the range of 10-70 m2/g by varying the operating parameters. The SO2 reactivity and the ultimate utilization of the calcium carbonate indicate a dramatic improvement and can be correlated with the surface area and pore volume characteristics of the particles. Introduction Limestone is extensively used in the in-situ removal of acid gas species such as SO2 from pulverized and fluidized-bed coal combustors and H2S from advanced dual-cycle gasification systems. The SO2 removal process involves the injection or fluidization of dry calciumbased powders in the high-temperature environment (800-1150 °C) of the combustor, calcination of the sorbent to produce CaO, and reaction with SO2 to form the higher molar volume CaSO4.
CaCO3(s) T CaO(s) + CO2(g) CaO(s) + SO2(g) + 0.5O2(g) T CaSO4(s) The optimum particle size and the temperature range of operation is different for each of the above applications. The initial reactivity of the sorbent and the ultimate sulfur capture are strongly influenced by its surface area and pore size and volume characteristics. The commercial mined limestone powder used in the SO2 capture suffers from a very low surface area (less than 3 m2/g) and negligible porosity. As a result, it suffers from loss of internal pore volume due to pore filling/pore plugging by the higher molar volume product CaSO4. Further, Ghosh-Dastidar et al. (1996) have shown that the pores generated by the calcination of the limestone lie in the less than 50 Å range which are very susceptible to pore blockage and plugging, leading to premature termination of sulfation and incomplete utilization of sorbent. They indicated that the carbonate powders with high surface area and porosity can exhibit very high reactivity and conversion, compared to high surface area hydrates. Fan et al. (1995) have developed a novel sorbent based on optimization of the surface area and pore size distribution of the calcium carbonate powder. Such a calcium carbonate powder with maximization of the surface area and of the surface area to pore volume leads to generation of CaO with maximization of pores in the 50-200 Å size range. This powder is generated by a precipitation process in a three-phase reactor system by bubbling CO2 through a Ca(OH)2 suspension in the presence of certain additives. * To whom correspondence should be addressed. S0888-5885(96)00768-3 CCC: $14.00
Precipitated calcium carbonate is extensively produced for industrial uses such as fillers, extenders, diluents, and inerts in the paper, paints, cosmetics, pesticides, pharmaceuticals, and plastics manufacturing industries. The behavior of CaCO3 precipitation using CO2 bubbling through a Ca(OH)2 solution has been reported in the literature. However, most of the studies were directed toward optimizing the properties of the carbonate that are crucial for the above applications, such as the fineness of particle size, uniformity of product size distribution, optical properties, and product purity. Moreover, a number of studies are conducted in a stirred-tank mixed-suspension, mixed-product removal (MSMPR) crystallizer which is based on a liquidliquid phase reaction (Maruscak et al., 1971; Swinney et al., 1982; Hostomsky and Jones, 1991; Tai and Chen, 1995). Most of the previous studies have been for low concentrations of Ca(OH)2 (saturated solution or lower). Further, the investigations have focused on morphological characteristics of the precipitate, such as crystalline/ amorphous form and their transformations, and on particle shape and size characteristics. Studies on the surface area and porosity of the precipitated product are very scarce; those available are mostly for saturated solutions. Ramachandran and Sharma (1969) analyzed the absorption kinetics and controlling mechanisms for CO2 absorption in a lime slurry. They showed that the rate of absorption (and hence precipitation of CaCO3) in a Ca(OH)2 suspension is considerably higher than that in a saturated Ca(OH)2 solution containing no solids. Furthermore, the rate of absorption is proportional to the square root of the amount of solids for a fixed particle size using the film theory model. Uchida et al. (1975) also used the film theory to predict the enhanced rate of absorption in a solid suspension containing fine particles compared to a saturated solution containing no solids. Nieh et al. (1991) used a bubble column to study the morphology of calcium carbonate generated from a calcium hydroxide solution. There have been studies on rate-controlling phenomena during precipitation. Juvekar and Sharma (1973) showed that, for the carbonation of lime, the gas film contribution to the total mass-transfer resistance is negligible. They developed the analytical equations for © 1997 American Chemical Society
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Figure 1. Schematic of the experimental setup.
rate of dissolution of CO2 under various conditions of solids loading, CO2 concentration in the gas phase, etc. Yagi et al. (1988) studied the size enlargement of CaCO3 particles produced in a sparged stirred vessel. They observed that the product particles formed flocs after formation and the average size of the flocs increased with increasing product concentration. They observed floc sizes to be larger than 20 µm, but the size reduced with increasing agitation. Jones et al. (1992) studied the effect of liquid mixing rate on the transient mean crystal sizes during precipitation of CaCO3 from an aqueous lime solution. They observed that the mean crystal size increases with increasing agitation rate consistent with their model predictions. Hostomsky and Jones (1995) applied the penetration theory for gasliquid mass transfer combined with the mass and population balances to predict the effect of mass-transfer rates on the precipitation of CaCO3. Uebo et al. (1992) conducted experiments for suspension concentration up to 10 wt % and broadly studied the crystal shape and size. Chapnerkar and Badgujar (1994) studied the surface area of CaCO3 produced by precipitation from a slurry containing 15-20% Ca(OH)2 between temperatures of 50 and 70 °F. They observed that the surface area is highly sensitive to temperature changes; it exhibits an increasing trend with temperature but remains unchanged at higher temperatures above 65 °F. They demonstrated that the addition of a small quantity of sugar stabilizes the surface area variation with temperature. Although the influence of different operating parameters on the properties of the precipitated calcium carbonate has been previously studied, an understanding of the surface area and porosity aspects of the precipitated product is generally lacking. Experimental Section A schematic of the setup used for carbonation is shown in Figure 1. The Pyrex reactor is 64 mm in diameter and 380 mm high. A sintered-glass plate with a pore opening of 25-50 µm (ASTM Por C) is used as the gas distributor. An aqueous suspension of calcium hydroxide (industrial grade) of various concentrations is prepared and reacted batchwise with pure CO2 gas which is introduced from the bottom of the reactor. Dispex N40V, Dispex A40 (Allied Colloids), and Lignosite 100 (Georgia Pacific) are the surfactants used,
Figure 2. Schematic of the differential bed reactor system for sulfation studies.
ranging in concentrations from 2 to 16 wt % (based on the weight of calcium hydroxide used). The particle size distributions of carbonation products are measured using a Sedigraph 5100 particle size analyzer (Micromeritics). The particles sampled from the reactor are filtered by No. 1 filter paper and dried in a vacuum oven at 75 °C for 24 h. A thermogravimetric analyzer (Perkin Elmer) is used to determine the conversions of calcium hydroxide during the carbonation reaction. The surface areas are measured by the lowtemperature nitrogen adsorption using the BET technique (Quantachrome). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) are used to study the crystal structure and composition of the precipitated CaCO3. Sulfation investigations are conducted in a differential fixed-bed reactor assembly shown in Figure 2. The main components include a 1.0-in.-o.d. ceramic reactor tube housed in a single-zone vertical-hinge furnace (Thermcraft). The reactor accommodates a customdesigned sorbent bed holder (1/8 in. i.d.) which enters from the bottom. The sulfation experiments are conducted by dispersing a very small amount of sorbent (about 20 mg) on quartz wool and using a high reactant gas flow rate of 1.6 L/m through the sorbent bed. This ensures minimal external transport resistances. The sorbent is precalcined by maintaining the sorbent at the desired reaction temperature (800-1000 °C) inside the reactor under an inert nitrogen flow for 10 min before exposing to SO2. The composition of the reactant gas used is 3900 ppm SO2, 5.45% O2, and balance N2. The extent of sulfation is determined (from SO42- concentration) using ion chromatography (Alltech). Results and Discussion The pore structural properties and their evolution is linked to the rate of formation of carbonate in the solution. The rate of formation is studied by analyzing the time course of either the solution pH or the Ca2+ ion concentration or the solution conductivity. In this study, the time course of pH is investigated for rate
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Figure 3. Time course of pH and Ca(OH)2 conversion. Initial Ca(OH)2 concentration: 2.56 wt %. CO2 flow rate: 2.38 L/m.
analysis and is further corroborated with TGA analysis of the sample at various time intervals. Rate of Formation of CaCO3 and Its Particle Size Distribution. Figure 3 shows the changes in pH and Ca(OH)2 conversion with bubbling time for a 2.56 wt % Ca(OH)2 suspension with a 2.38 L/m CO2 gas flow rate. The pH of the suspension starts from a high basic value of about 12 which is maintained for a period of about 5 min before dramatically dropping to a weak acidic value of 6. A number of researchers have observed this typical pH behavior which exhibits two distinct regions: a constant rate of absorption period and a falling rate period. During the constant rate period, the aqueous phase is always kept saturated with OH- ions and, as can be seen in Figure 3, more than 75% of the Ca(OH)2 conversion to carbonate is completed during this period. Thus, the point of drastic pH reduction on the inverse S-shaped pH profile corresponds to near completion of CaCO3 formation. The effect of suspension concentration and the gas flow rate on the formation rate is studied by recording the time courses of pH under various operating conditions. In Figure 4, at a fixed CO2 gas flow rate of 2.38 L/m, the time for complete CaCO3 formation is seen to increase from about 1 to 5 min as the Ca(OH)2 concentration is increased from 0.16 to 2.56 wt %. A similar phenomenon is observed in Figure 5 with increasing CO2 flow rate for a fixed Ca(OH)2 concentration. The time required for completion of the reaction decreases with increasing CO2 flow rate for a 2.56 wt % Ca(OH)2 suspension. The crystal form of the CaCO3 is also investigated as a function of carbonation time. This is performed by withdrawing a small amount of sample from the reactor, drying the sample, and conducting XRD studies. As can be seen in Figure 6, following the first minute of carbonation, there is some unconverted Ca(OH)2 which is consumed by the 4th minute. Moreover, the CaCO3 is always calcitic in form. The crystal form of the calcium carbonate formed under other solids concentration and gas flow rate conditions is also observed to be entirely calcite. Uebo et al. (1992) have suggested that the calcium carbonate formed in the initial 1-3 min may be amorphous in nature. In this work, investigations of carbonate formed in the first 4 min indicated the crystal form to be calcite after drying.
Figure 4. Influence of suspension concentration on the pH variation with time. CO2 flow rate: 2.38 L/m.
Figure 5. Influence of CO2 flow rate on the pH behavior. Ca(OH)2 suspension concentration: 2.56 wt %.
The particle size and its distribution in a precipitation system is known to be dominated by shear stress effects (Nieh, 1990) as dictated by the gas flow rate and the solids concentration. The influence of CO2 flow rate and suspension concentration on the ultimate particle size is studied by measuring the size distribution after 20 min of carbonation. The suspension concentration has a pronounced effect on the particle size distribution, as can be seen in Figure 7. The lower suspension concentration (1.28 wt %) leads to much larger particle sizes and a wider size distribution in the range of 2-20 µm with an average (d50) size of about 9 µm. The particle size distributions of lower concentration suspensions could not be ascertained using the Sedigraph technique. The particle size is not affected by the CO2 flow rate in the range 1.43-3.8 L/m and lies in the narrow range between 1 and 2 µm. Surface Area of CaCO3 and Influence of Surfactant. The role of the operating parameters in determining the surface area properties of the product is not well understood. Figure 8 shows the influence of the suspension concentration on the ultimate surface area of the calcium carbonate at a fixed CO2 flow rate of 2.38
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Figure 8. Effect of solids concentration on the surface area. CO2 flow rate: 2.38 L/m.
Figure 6. X-ray diffractograms of the partially carbonated samples following CO2 bubbling for (a) 1, (b) 2, (c) 3, and (d) 4 min.
Figure 7. Effect of suspension concentration on the product particle size distribution.
L/m. The CaCO3 from the saturated solution of Ca(OH)2 exhibits a surface area of only about 6 m2/g and negligible pore volume. The surface area increases with increasing supersaturation, with 2.56 wt % concentration showing a surface area of 31 m2/g. In order to analyze the temperature change associated with the
carbonation process, the reaction temperature is recorded and showed less than a 10 °C rise for 2.56 wt % concentration. For a fixed suspension concentration, the CO2 flow rate has little influence on the BET surface area of the product. Ramachandran and Sharma (1969) have shown the existence of fine solid particles enhances the mass-transfer rate between the gas and liquid phases. This enhancement may lead to increased local supersaturation which will introduce a higher nucleation rate and form finer primary particles which are the components of the agglomerate formed later (Jones et al., 1992). Surfactants and other additives are used in the precipitation process to modify certain properties of the final product. Some anionic surfactants are known to produce stabilizing and dispersing action by ionizing in water to produce a cation together with a polyanion which adsorbs onto the particle surface and maintains a state of dispersion between adjacent particles. In this work, two types of anionic surfactants, which have been used previously by researchers studying high surface area hydrated lime and carbonate (Kirchgessner and Jozewicz, 1989; Bjerle and Ye, 1991), are investigated. Dispex consists of sodium (N40V) or ammonium (A40) salt of a polycarboxylic acid, while calcium lignosulfonate, which is a byproduct of the paper industry, is a sulfonated anionic surfactant. As can be seen in Figure 9, the surface area increases from 31 m2/g in the absence of surfactant to between 45 and 60 m2/g with addition of 2 wt % surfactant. Dispex N40V and Dispex A40 each improve the surface area to about 60 m2/g, while lignosulfonate shows about 47 m2/g. However, increasing the surfactant concentration further drastically reduces the ultimate surface area, as shown in Figure 9. In order to understand the mechanism by which surfactant leads to high surface area, the development of pH, the particle size, and the product surface area is investigated further. The rate of pH change in the absence and presence of surfactant (2 wt %) indicates little difference in the rate of carbonation and precipitation. Moreover, all of the CaCO3 formed in the presence of 2 wt % surfactant is also calcitic. The carbonate formation is also completed by the 4th minute, which, as shown in Figure 5, is also comparable to that without
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Figure 11. Evolution of product surface area with and without surfactant. Figure 9. Influence of surfactant and its concentration on the carbonate surface area. Solids concentration: 2.56 wt %. CO2 flow rate: 2.38 L/m.
Figure 10. Influence of surfactant on particle size distribution after 20 min of bubbling time. Solids concentration: 2.56 wt %. CO2 flow rate: 2.38 L/m.
surfactant. The ultimate particle size distribution also shows a similar pattern with or without 2 wt % surfactant. Figure 10 shows that all the particles have a narrow distribution between 1 and 2 µm. The evolution of surface area is studied by withdrawing a small quantity of sample from the reactor at various times and analyzing its surface area after drying. As can be seen in Figure 11, for precipitation without surfactant, the surface area increases in the initial 3-4 min to a maximum value of nearly 70 m2/g and then decreases continually to about 31 m2/g after 20 min. On the other hand, the surface area for carbonate with 2% Dispex N40V increases steadily until 61 m2/g. Thus, the surfactant does not influence the surface area by altering the rate of formation or by changing the particle size. Sedigraph results show that the particle size increases slightly from d50 ) 1.0 to 1.5 µm for the initial 4-10
min without surfactant, while it remains unchanged at 1.5 µm with surfactant. Figure 12 shows the SEM photomicrographs of the carbonate sample prepared under various operating conditions. Figure 12a shows the large, well-defined cubical crystals of the carbonate produced from 0.16 wt % solution, while Figure 12b shows the granular, high surface area particles of carbonate produced under 2.56 wt % concentration. The cauliflower-like granular and porous structure of the high surface area carbonate produced under surfactant conditions is seen to be more porous than the carbonate sample without surfactant. Moreover, the surfactant-based carbonate is also seen to possess a higher porosity from the pore volume distributions shown in Figure 13. SO2 Reactivity of CaCO3 and Effect of Pore Properties. The reactivity of the precipitated CaCO3 is investigated and compared with commercially available carbonates and hydrates under identical conditions. The chemical composition and internal pore properties of three precipated CaCO3 sorbents (MC1, MC2, and MC3), commercial Linwood calcium carbonate (LC), and Linwood hydrate (LH) are given in Table 1. The sorbent ultilization following 30 min of sulfation at 900 °C is presented in Figure 14 for smaller than 45 µm particles (-45 µm). The precipitated carbonates exhibit conversion much higher than the commercial sorbents. The MC1 shows nearly complete sorbent conversion of 98%, while the commercial hydrate and carbonate show only 51% and 42%, respectively. The MC3 carbonate, with surface area and pore volume similar to those of the Linwood hydrate, exhibits much higher conversion than the commercial sorbents. Thus, for similar pore properties, carbonate shows a higher conversion than hydrate, which can be explained as follows. Borgwardt and Bruce (1986) have shown that, at high temperatures, CaO derived from carbonate exhibits less surface area loss than hydrate-derived CaO due to a lower sintering rate. Ghosh-Dastidar et al. (1996) compared the calcination and sulfation characteristics of Linwood carbonate and hydrate and showed greater surface area retention by carbonate at 1080 °C. However, most of the surface area of the calcined carbonate resides in the small pores (
