Ind. Eng. Chem. Res. 2003, 42, 6343-6347
6343
Nucleation, Growth, and Composition of Crystals Obtained from Solutions of Na2CO3 and Na2SO4 Bing Shi,† W. James Frederick, Jr.,‡ and Ronald W. Rousseau* School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100
The formation of crystals from solutions containing sodium carbonate and sodium sulfate is a complex process that often results in fouling of heat-transfer surfaces. Complexity results from a dependence of the composition of crystals formed on the composition of the solution, and the solution composition changes with crystallization. In the present work, experiments involving evaporation at 115 °C were used to identify these species and the specific regimes in which they are formed. Crystals produced in these experiments were sodium sulfate, burkeite, a new species referred to as dicarbonate, and either sodium carbonate monohydrate or anhydrous sodium carbonate. Introduction Solutions of sodium carbonate and sodium sulfate are found in natural brines and in numerous process streams. One of the largest uses of such systems is in the kraft pulping process, which produces aqueous solutions that are then concentrated in multiple-effect evaporators and burned for chemical and energy recovery. The large quantities of sodium carbonate and sodium sulfate in this stream present serious problems in maintaining evaporator operation; they must be concentrated to very high solids content and tend to form scale on the evaporator surfaces, which reduces heat-transfer rates and product throughput.1,2 In the present research, we address the formation of crystals from aqueous solutions designed to simulate liquor streams found in pulp mills. These streams actually contain a myriad of components, but sodium salts appear to be of most significance, especially from the perspective of causing scale on heat-transfer surfaces.3 Compositions of crystals initially formed through the isothermal evaporation of solvent were determined as a function of the carbonate-to-sulfate mole ratio, C:S. Moreover, the solution composition changed with growth of the original crystals formed, and conditions were determined under which a second salt species was formed. In addition to the possibility of forming crystals of the pure species from the solutions, sodium carbonate and sodium sulfate may also crystallize as hydrates or double salts. One of these is burkeite, a double salt with a sodium carbonate-to-sodium sulfate mole ratio (C:S) of approximately 1:2.4 A recently identified second double salt with a C:S value that varies from slightly below to slightly above 2:1 can also be formed,5 and for convenience we shall refer to this species throughout the present manuscript as dicarbonate. Several properties of the crystalline species formed contribute to the problem of evaporator scaling. Among * To whom correspondence should be addressed. Fax: 404384-0185. E-mail:
[email protected]. † Current address: Pfizer Global Research and Development, 10777 Science Center Dr., San Diego, CA 92121. ‡ Current address: Forest Products and Chemical Engineering, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden.
these are (1) a reduced solubility with increased temperature in the operating range of interest and (2) the solutions and slurries exhibit high viscosities at the high solids content of interest. Since the highest temperatures in the evaporator are on the heat-transfer surfaces, the solutions are more supersaturated and have the tendency to form nuclei on these surfaces. Experimental Equipment and Procedures Aqueous solutions having sodium carbonate-to-sodium sulfate molar ratios from C:S ) 1:2 to C:S ) 12:1 were prepared from ACS-grade reagents. The initial solution compositions were usually 30 wt % solutes, although other compositions were used occasionally. After the solids had been completely dissolved, the solution was decanted into the crystallizer that was assembled immediately to conduct the experiment. Crystallizations were carried out in a 1-L Parr reactor. Details of the apparatus are given elsewhere.6 A PID controller adjusted a heater to provide the desired system temperature (generally, 115 °C) and evaporation rate (3-5 g/min). Evaporated water was condensed and weighed so that the composition of the residual mixture in the reactor could be estimated by mass balance. A Lasentec FBRM (focused beam reflectance measurement) D600L system was used to detect the formation of crystals in the system and it gave a real-time estimate of the crystal dimensions.7,8 Results Initial Salt Formation. A series of experiments was performed at fixed evaporation rates, but varying initial C:S ratios, and were continued until formation of crystals was indicated by the FBRM. The experiments produced the data shown in Figure 1. Clearly, the composition of the initial crystals formed was highly dependent on the ratio of carbonate to sulfate in the solution: sodium sulfate (thenardite) was produced from solutions with C:S molar ratios less than about 1:5 (sodium carbonate mole fraction, xNa2CO3, less than ∼0.2 on a solvent-free basis); burkeite from solutions with 1:5 < C:S < 4:1; dicarbonate from solutions with 4:1 < C:S < 7:1; and sodium carbonate (thermonatrite) from solutions with C:S > 7:1.
10.1021/ie030054s CCC: $25.00 © 2003 American Chemical Society Published on Web 08/01/2003
6344 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003
Figure 1. Compositions of the crystals formed (excluding residual or structural water) in the sulfate (S), burkeite (B), dicarbonate (D), and carbonate (C) regions as a function of solution composition. b experimental data at 115 °C from the present study.
Figure 2. FBRM data obtained during evaporation of a solution initially containing a carbonate-to-sulfate mole ratio of 3:1. Note the two nucleation events at about 35 wt % and about 42 wt % total solids.
Table 1. Total Solids Contents at the First Nucleation Points for Solutions Evaporated at 115 °C C:S in the solutions (mole ratios) 1:2 1:1 2:1 3:1 4:1 5:1 6:1 7:1 9:1 12:1 a
solid species crystallized burkeite burkeite burkeite burkeite burkeite dicarbonate dicarbonate dicarbonate thermonatrite thermonatrite
concentration ((0.005) at the first nucleation point EDTA added
no EDTA
0.285a
0.314 0.321 0.335 0.351 0.348 0.353
0.294a 0.308a 0.332 0.338 0.345 0.341 0.343 0.337 0.335
0.351
400 ppm EDTA added, others 1000 ppm.
It is well-known that primary nucleation is affected by variables that include mixing intensity, the rate at which supersaturation is generated (evaporation in the present instance), and the presence of impurities in the system. In the present research, mixing intensity and evaporation rate were held constant in all runs. However, prior studies had found that small concentrations of calcium, which was brought into the system as a contaminant in the reagent sodium carbonate, affected the nucleation kinetics of burkeite.9 To evaluate the effect in the present work, ethylenediamine tetra acetic sodium salt (EDTA) was added to some of the solutions to sequester calcium and prevent it from inhibiting nucleation. Table 1 shows how the addition of EDTA influenced the composition of the crystals formed when it was present in the initial solutions at either 400 or 1000 ppm. Interestingly, all runs without EDTA produced crystal compositions higher in sodium carbonate content than did those with EDTA. Secondary Species Formation. The carbonate-tosulfate ratio in the solution changes during the crystallization of burkeite or another double salt whose C:S ratio is different from the solution. With the crystallization of burkeite, for example, approximately 2 mol of sodium sulfate are removed from solution for every mole of sodium carbonate. Accordingly, as burkeite is crystallized, the solution composition moves from left to right along the horizontal axis of Figure 1. This means that a transition from burkeite to dicarbonate
Figure 3. Photomicrographs of crystal samples taken from a run in which the initial solution had a carbonate-to-sulfate mole ratio of 7:1. The cores of the large, well-formed crystals are agglomerates of crystals of the dicarbonate species.
should occur as the C:S mole ratio in solution moves through a value of about 4:1 (xNa2CO3 ≈ 0.8). Figure 2 shows chord-length distributions measured during a run that began with a carbonate-to-sulfate mole ratio in solution of 3:1. Evidence of two nucleation events was detected, one at about 35% total solids and a second at about 42% total solids. Samples of the crystals taken from the system were analyzed by X-ray diffraction, which confirmed that the first nucleation was of burkeite and the second was of dicarbonate. Similar results were obtained in a run beginning with a 2:1 carbonate-to-sulfate mole ratio in solution. A second transition can occur as the carbonate-tosulfate mole ratio in solution moves through a value of about 7:1, and the crystals formed go from being the dicarbonate species below C:S ) 7:1 to sodium carbonate above that C:S ratio. This was verified in a run for which the initial C:S mole ratio in solution was 7:1. Dicarbonate crystals formed as water was evaporated, and these clustered into agglomerates. As the solution became richer in sodium carbonate relative to sodium sulfate (because the C:S mole ratio in the crystals is less than that in the solution), the agglomerates eventually were incorporated into large crystals of sodium carbonate monohydrate, as shown in Figure 3.
Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 6345
Figure 4. FBRM results obtained during the evaporation of a solution of sodium carbonate and sodium sulfate in the mole ratio of 9:1. Two nucleation events have been observed: sodium carbonate monohydrate (thermonatrite) and the dicarbonate species.
Figure 5. Clear crystals of sodium carbonate monohydrate and the agglomerates (black spots) of the dicarbonate species formed subsequently in the solution of sodium carbonate and sodium sulfate in the mole ratio of 9:1.
Finally, a transition can occur when a solution begins with a carbonate-to-sulfate ratio greater than 9:1, and crystals of sodium carbonate (thermonatrite) are produced. As these crystals do not contain sodium sulfate, their formation reduces the carbonate-to-sulfate mole ratio in solution, forcing it to the left in Figure 1. Therefore, the crystal composition will change to dicarbonate when the C:S ratio in the solution falls below C:S ) 9:1. Figure 4 shows FBRM output for a run in which the initial solution had a carbonate-to-sulfate ratio of 9:1. The first nucleation event at about 34% total solids was followed closely by nucleation of the dicarbonate species. The number of the latter crystals was much greater than the number of sodium carbonate monohydrate crystals, which is reflected by the large number of counts during the second nucleation event. These crystals formed agglomerates and could be easily distinguished from thermonatrite crystals, as is shown in Figure 5. Calcium Inhibition. It was shown in earlier work that the presence of calcium could inhibit the nucleation of burkeite.9 Consequences of such behavior are reflected in Table 1, which shows that without the addition of EDTA (a calcium scavenger), higher supersaturation, as manifested by a higher nucleation temperature, was necessary for crystallization of both burkeite and dicarbonate. Additional unusual behavior could also be attributed to the presence of calcium. When EDTA was added to solutions in the dicarbonate region, the X-ray diffraction patterns of recovered crystals showed no noticeable amounts of burkeite or sodium carbonate. However, if
no EDTA was added, the samples contained varying amounts of sodium carbonate monohydrate, as is illustrated with the X-ray diffraction patterns in Figure 6. Such behavior is a clear indication of inhibition of nucleation of the dicarbonate species, and this behavior was further examined in two runs in which 100 ppm of calcium carbonate powder was added to the solutions in the burkeite and in the dicarbonate regions. In these two cases, the calcium concentration in the solutions was approximately 50 ppm. In the first experiment, nucleation in a solution with a carbonate-to-sulfate mole ratio of 2:1 was delayed from a solids concentration of approximately 33.5-37.0 wt %. In the second experiment, the initial solution had a carbonate-to-sulfate mole ratio of 6:1. In the presence of calcium ions, nucleation of sodium carbonate nucleation monohydrate, followed by subsequent nucleation of the dicarbonate species, was observed. Scaling Tendencies. Operation of the laboratory crystallization unit encountered no difficulties with the formation of scale on the crystallizer surfaces when the system was operated in the burkeite region. However, the addition of calcium carbonate caused severe scaling. Operations in the dicarbonate region produced scaling problems quite different from those found in the burkeite region. For example, during an experiment in which the initial solution had a carbonate-to-sulfate mole ratio of 3:1, the solids content was increased from about 30 to 48 wt %. The process spanned the burkeite and dicarbonate regions, and consequently two nucleation events occurred, one for burkeite and the second from dicarbonate. When the crystallizer was dismantled at the end of the experiment, the bottom of the unit was covered with a thin layer of scale that was uniform in thickness and brightness. Analysis by XRD showed that it consisted of both dicarbonate and burkeite species. A similar experiment in which the initial solution composition was in the dicarbonate region resulted in scale comprised only of the dicarbonate species. Discussion Crystallization of a solid mixture that contains two components in a ratio different from that of the solution always results in a change in the solution composition, and the change in solution composition is increased as more crystal mass is generated. Figure 7 contains a smoothed representation of the data from Figure 1, and it shows how the direction of composition change with crystallization depends on the prevailing solution composition. In the discussion that follows, it is assumed that crystals are formed at saturation; that is, there is no supersaturation required for crystal nucleation and growth. Although this assumption makes it possible to predict the composition of crystals formed and the concomitant changes in solution composition, caution should be exercised in overlooking the potential impact of the rates of nucleation and growth on these predictions. The composition range along the horizontal axis of Figure 7 is divided into regions corresponding to the four species that can be formed by crystallization: sodium sulfate, burkeite, dicarbonate, and sodium carbonate. During crystallization, the solution composition changes in the directions indicated by arrows at the boundaries of the regions. However, shown in Figure 7 are three labeled points (1, 2, or 3) at the intersections of vertical
6346 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003
Figure 6. XRD patterns of crystals obtained from solutions of sodium carbonate and sodium sulfate in a mole ratio of 7:1 with or without EDTA. The latter case is compared with the XRD patterns of sodium carbonate monohydrate (JCPDS PDF #08-0448).
Figure 7. Relationship between solution composition and the composition of crystals formed.
lines with the diagonal and which are called invariant points; crystallization at these points produces crystals of constant composition during the course of the process. Such behavior results because the solvent-free composition in the solution is the same as the composition of crystals formed. Invariant Point 1 is on the boundary between the sulfate and burkeite regions. Solids formed from a solution whose composition falls at Point 1 consist of a mixture of sodium sulfate and burkeite crystals. To the left of this point, the carbonate content of a solution will increase (on a solvent-free basis) with evaporation of solvent and concomitant crystallization of sodium sul-
fate; eventually, the composition will reach the boundary and burkeite will begin to be formed along with additional sodium carbonate crystals. To the right of Point 1 but prior to Point 2, the curve is above the diagonal, which means that crystallization of burkeite due to solvent evaporation will decrease the carbonate content of the solution (relative to sulfate). The second invariant point (Point 2) is located at a carbonate-to-sulfate mole ratio around 1:3 (xNa2CO3 ≈ 0.25), and the vertical line through this point in Figure 7 divides the burkeite region into two parts. Because crystallizations from solutions on either side of this line result in their compositions moving away from Point 2, the point is referred to as a metastable condition. A solution whose composition (on a solvent-free basis) is at Point 2 will produce burkeite crystals whose composition is the same as the solution. Crystallization of burkeite due to evaporation from a solution with a composition to the right of Point 2 will result in an increase of the carbonate-to-sulfate mole ratio in the solution; that is, the solution composition moves to the right from Point 2. When the solution composition reaches the boundary between burkeite and the dicarbonate species, crystals of the latter begin to be formed. The third invariant point is on the boundary between the dicarbonate region and the carbonate region. Crystallization from a solution whose composition is to the left of Point 3 produces dicarbonate species and moves the solution composition toward Point 3. Sodium carbonate crystals are formed upon crystallization from a solution whose composition is to the right of Point 3,
Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 6347 Table 2. Comparisons of Predicted and Experimentally Determined Conditions at the Second Nucleation Point solid species crystallized
second nucleation point (total solids content)
C:S in the solution (mole ratio)
first
second
prediction
experimentala
1:2 1:1 2:1 3:1 4:1
burkeite burkeite burkeite burkeite burkeite
dicarbonate dicarbonate dicarbonate dicarbonate dicarbonate
0.93 0.692 0.492 0.414 0.372
too high to detect too high to detect 0.493, 0.483,b 0.494c 0.415 0.355, 0.372d
a No additives unless indicated. b With 400 ppm EDTA. c With 100 ppm CaCO3. d With 1000 ppm EDTA.
and upon such crystallization the solution composition moves toward Point 3. Once the solution reaches the boundary between the two regions, moving either from left to right or from right to left, crystals are formed of both sodium carbonate and the dicarbonate species. The ratio of carbonate to sulfate in solution and the resulting composition of crystals formed has been the focus in the preceding discussion. However, from a processing perspective it is equally important to relate the formation of salts to the total solids content of the solution. For example, if a solution of sodium carbonate and sodium sulfate has a C:S ratio that places it in the burkeite region, burkeite crystals will be formed when evaporation increases the total salt content above the solubility limits at the given conditions. What happens to the C:S ratio with further evaporation depends on whether the solution composition is to the left or right of Point 2. In either case, the above treatment shows that the C:S mole ratio in the solution changes toward that leading to crystallization of a second species: sodium sulfate if the initial composition is to the left of Point 2 and dicarbonate if it is to the right of Point 2. To illustrate, consider the evaporation of a burkeite solution at 115 °C having an initial C:S mole ratio of 3:1. The data in Table 1 indicate that crystallization of burkeite begins when the solids content (sodium carbonate and sodium sulfate) is between 33.2 and 35.1 wt %. The sodium carbonate mass fraction in the saturated solution is 0.691 (on a solvent-free basis). Further evaporation leads to crystallization of burkeite through growth of crystals formed by nucleation. At any point during the process and with the earlier assumption that the solution and crystals are in equilibrium, the composition of the solid formed is given by the fit to the portion of the data in Figure 1 in the burkeite region: that is,
y ) 0.949x3 - 1.1785x2 + 0.7882x + 0.0824 (0.2 e x e 0.79) (1) where x and y are weight fractions of sodium carbonate in the liquid and solid on a solvent-free basis. Continued evaporation results in additional crystallization and increases in the C:S mole ratio in solution. The dissolved solids content of the saturated solutions does not change greatly with changes in the C:S mole ratio, but when the solution reaches a C:S mole ratio of about 5 (weight fraction sodium carbonate ) 0.789 on a solvent-free basis), nucleation and growth of the dicarbonate species occurs. Table 2 shows comparisons of predictions, following the methodology outlined above, with experimental
results. Although favorable, such results clearly depend on the protocol followed in the crystallization process. Conclusions During evaporative crystallizations of solutions containing sodium carbonate and sodium sulfate, the solution compositions can span more than one region and crystallize multiple solid species in sequence. The nucleation point of the first species in these regions has been obtained experimentally. On the basis of these experimental data and theoretical analysis, the nucleation point of the second species can be predicted. Examples show that the predicted nucleation point of the dicarbonate species (as the second nucleating species) in solutions starting from the burkeite region compared favorably with the experimental data. Crystallization in the system Na2CO3-Na2SO4-H2O was greatly influenced by calcium. Calcium ions inhibited the nucleation of burkeite and the dicarbonate species, resulting in higher supersaturation and, in some circumstances, reversing the order of the nucleating species. Finally, the experiments encountered scaling problems in two cases: (1) with the addition of calcium carbonate to solutions of high carbonate content in the burkeite region and (2) when the dicarbonate species was crystallized from the solution. Acknowledgment The authors are grateful for the financial support provided by the U.S. Department of Energy, the Georgia Research Alliance, Andritz, Inc., International Paper Company, MeadWestvaco Corporation, Potlatch Corporation, and Weyerhaeuser Company. Literature Cited (1) Grace, T. M. Evaporator Scaling. South. Pulp Pap. Manuf. 1977, 42 (8), 16. (2) Frederick, W. J.; Grace, T. M. Scaling in Alkaline Spent Pulping Liquor Evaporators. International Conference on the Fouling of Heat Transfer Equipment; ASME Press: New York, 1979. (3) Novak, L. Sodium Salt Scaling in Connection with Evaporation of Black Liquors and Pure Model Solutions. Ph.D. Thesis, Chalmers Tekniska Ho¨gskola, Go¨teborg, 1979. (4) Green, S.; Frattali, F. The System Sodium CarbonateSodium Sulfate-Sodium Hydroxide-Water at 100 °C. J. Am. Chem. Soc. 1946, 68, 1789. (5) Shi, B.; Rousseau, R. W. Structure of Burkeite and A New Crystalline Species Obtained from Solutions of Sodium Carbonate and Sodium Sulfate. J. Phys. Chem., in press. (6) Shi, B. Crystallization of Solutes that Lead to Scale Formation in Black Liquor Evaporation. Ph.D. Dissertation, Georgia Institute of Technology, 2002. (7) Law, D. J.; Bale, A. J.; Jones, S. E. Adaptation of Focused Beam Reflectance Measurement to in-situ Particle Sizing in Estuaries and Coastal Waters. Marine Geol. 1997, 140, 47. (8) Ruf, A.; Worlitschek, J.; Mazzotti, M. Modeling and experimental analysis of PSD measurements through FBRM. Part. Part. Syst. Charact. 2000, 17, 167. (9) Shi, B.; Frederick, W. J.; Rousseau, R. W. Effects of Calcium and other Ionic Impurities on the Primary Nucleation of Burkeite, Ind. Eng. Chem. Res. 2003, 42, 2861.
Received for review January 22, 2003 Revised manuscript received May 8, 2003 Accepted May 8, 2003 IE030054S
