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Ind. Eng. Chem. Res. 2003, 42, 904-910
GENERAL RESEARCH Composite Fouling of Calcium Oxalate and Amorphous Silica in Sugar Solutions Hong Yu,† Roya Sheikholeslami,*,† and William O. S. Doherty‡ School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, Sydney, New South Wales 2052, Australia, and Sugar Research Institute, Mackay, Queensland 4740, Australia
The composite fouling of calcium oxalate monohydrate (COM) and SiO2 in the multi-effect evaporators is known to cause a variety of problems in sugar mills with serious technicoeconomical consequences. A batch experimental study has been conducted to characterize the COM-SiO2 interactions in both single- and binary-foulant systems in the presence of sucrose under conditions relevant to those in the sugar mill evaporators. The presence of sucrose increased the rates of COM precipitation and SiO2 polymerization and decreased the solubilities of COM and SiO2 in solution. Colloidal silica particles were formed at a lower threshold initial silica supersaturation (SS) of 1.8 in a single-foulant sucrose solution than that formed in water (2.0). Both promotory and inhibitory effects on the coprecipitation of COM-SiO2 thermodynamics and kinetics in the binary-foulant systems were observed, depending on the concentrations of COM and sucrose. The threshold silica SS at which silica colloids formed was further decreased (from SS ) 1.8 to 1.3) by the presence of COM. These findings may be attributed to a number of factors such as adduct formation between COM, SiO2, and sucrose, adsorption of COM-sucrose complexes by polymeric SiO2-sucrose species, and the availability of water molecules. Introduction Multiple-effect evaporation of cane juice leads to the accumulation of organic and inorganic nonsugar scales on the calandria tubes of sugar mill evaporators. The scales that are formed reduce the heat-transfer efficiency and the capacity of the evaporator station. Chemical cleaning of the evaporator not only is costly but also adds to effluent discharges, thereby posing environmental problems. Additional costs of evaporator scaling are associated with the reduced sugar quality from the extended residence time of juice during evaporation and equipment corrosion.1-3 Calcium oxalate monohydrate (COM) and amorphous silica (SiO2) are found to be the primary components of a tenacious composite scale formed in the later vessels (i.e., fourth and fifth effects) of a multi-effect evaporator station due to concentration and temperature effects during juice evaporation. Special chemical cleaning reagents (e.g., ethylenediaminetetraacetic acid) are often needed, in addition to conventional chemical reagents, to effectively remove the deposits from the vessel.2 The precipitation of oxalate is due to either the existence of oxalic acid in the sugar cane plant or decomposition of other components of the juice to oxalic acid during processing. Silica is an essential trace element in the cane plant tissue and is present in the dirt carried into the sugar factory with the cane supply.
Calcium is introduced into the mill via the cane plant and by lime addition during juice clarification.4,5 Previously, studies of potential interactions between COM and SiO2 have been published using water.6,7 COM-SiO2 coprecipitation was shown to affect both the kinetics and thermodynamics of COM precipitation and SiO2 polymerization due to the formation of a COMSiO2 complex and specific adsorption of silica particles onto COM crystal faces.6 A synergistic effect of COMSiO2 interactions on the rate of composite fouling occurred at an intermediate COM-SiO2 supersaturation (SS) ratio (COM, 50 ppm), whereas an antagonistic effect was obtained at either low or high SS ratio (COM, 20 and 100 ppm). The composite scale obtained from 50 ppm COM was of higher tenacity than that of the pure SiO2 scale because of the cementing effect of COM on the deposit structure.7 Little work has been conducted which involves COMSiO2 interactions in sugar (i.e., sucrose) solutions. This work is aimed to investigate how the presence of sucrose affects the coprecipitation thermodynamics and kinetics of COM and SiO2 under conditions similar to those of sugar mill evaporators. A batch experimental study using synthetic juice solutions is carried out for both single- and binary-foulant systems, and the results are compared with findings for water. Experimental Section
* Corresponding author. Tel: +61 2 9385 4343. Fax: +61 2 9385 5966. E-mail:
[email protected]. † The University of New South Wales. ‡ Sugar Research Institute.
Analytical-grade salts dissolved in CO2-free distilled water were used to make up the calcium, oxalate, and silica solutions (CaCl2‚2H2O, Na2C2O4‚H2O, and Na2-
10.1021/ie020602m CCC: $25.00 © 2003 American Chemical Society Published on Web 01/22/2003
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SiO3‚9H2O). Synthetic juice solutions were prepared from a commercial sugar (purity 99.5%) kindly supplied by Sugar Australia, Pty, Ltd., New South Wales, Australia. All other reagents were analytical grade. COM precipitation and SiO2 polymerization reactions were initiated by mixing supersaturated solutions of COM (20-200 ppm) and SiO2 (50-600 ppm)6 with sucrose solutions [10-40% (w/w) sucrose in final mixtures, a range comparable to those in the sugar mill evaporator], and the initial pH of the mixtures was adjusted to 6.0-8.0 using HCl or NaOH as required. Samples of mixtures were then transferred to screwcap plastic tubes and left to equilibrate at 60-80 °C in a water bath for at least 24 h (unless stated otherwise). Previous studies indicated that COM precipitation equilibrium was reached in much less time than this period (e.g., 4-6 h for COM at pH 8.0, 60 °C),6 and so 24 h would be sufficient to ensure equilibrium. The concentrations of Ca2+ and Ox2- were measured by filtering sample solutions through a 0.22 µm membrane filter (Millipore Co.) and analyzing the filtrate using ICP-AES (Varian Vista AX spectrometer) and UV-vis spectroscopy (Varian Cary 1E spectrophotometer). To avoid further precipitation of COM on cooling after the filtration, the samples were acidified to pH < 1.5 using concentrated HCl. The reactive and total silica (including reactive and nonreactive or colloidal forms of silica) concentrations were determined in unfiltered samples using UV-vis spectroscopy and ICP, respectively, as previously described.6 Equilibria of COM precipitation and SiO2 polymerization were established through measurements at predetermined reaction times until constant solution concentrations of COM and SiO2 were achieved. The pH of the solution was also monitored with an Orion 370 pH meter and combination pH electrode. Comparative tests of COM-SiO2 coprecipitation in sucrose solutions were performed by mixing appropriate volumes of stock calcium, oxalate, silica, and sucrose solutions and following sample preparation and analytical procedures similar to those of COM and SiO2 single systems. For comparison of the effect of sucrose on COM precipitation kinetics, the experimental data obtained for COM precipitation in both single- and binary-foulant systems were fitted to the simple rate equation:8
dC ) kσn dt
(1)
σ ) (qt1/2 - qe1/2)/qe1/2
(2)
where C is the molar concentration of COM, k is a rate constant (mol/L‚h), σ is the relative SS, and n is the order of reaction related to the mechanisms on which the crystallization process is based [i.e., values of n near unity suggest a bulk diffusion process during crystal growth, and n close to 2 indicates a surface-controlled mechanism; higher order (n > 2) polynuclear mechanisms may also occur].8 In eq 2, qt and qe are the ionic products (mol2/L2) of Ca2+ and Ox2- at time t and at equilibrium, respectively. The observed solubility of calcium oxalate is expressed in terms of the concentration solubility product, Kcsp (mol2 L-2), as given by
Kcsp ) [Ca2+][Ox2-]
(3)
Figure 1. Changes in the rate constant (k) and order of reaction (n) for COM precipitation in single-foulant sucrose solutions as a function of the sucrose concentrattion: initial COM concentration, 150 ppm; initial pH, 8.0; temp, 60 °C; R2 (correlation coefficient), >0.995.
or written as
C* ) xKcsp
(4)
where [Ca2+] and [Ox2-] are the concentrations of calcium and oxalate ions (mol/L) at equilibrium and C* is the solubility concentration (mol/L) of calcium oxalate. The rate equation used to describe the changes in the reactive SiO2 concentration during SiO2 polymerization is expressed as
-
d(SiO2) ) k[(Ct - Ce)/Ce]n dt
(5)
where Ct and Ce are the reactive silica concentrations (ppm) at time t and at equilibrium, respectively. k and n are the rate constant (ppm/h) and order of reaction, respectively. Results and Discussion Single Systems. Experiments involving the precipitation of COM and the polymerization of SiO2 from single-foulant sucrose solutions were conducted for a range of sucrose concentrations. COM showed higher precipitation rates in sucrose solutions than that in water with higher k and lower n values (see Figure 1) according to the rate equation (eq 1). The extent of the increase was clearly dependent on the sucrose concentration (Figure 1). The reduced n values at high sucrose concentrations seemed reasonable because hindered diffusion was expected to be more significant with increasing viscosity in solution. The effect of sucrose on the solubility concentrations of COM at different initial COM concentrations is shown in Figure 2. The solubility of COM was decreased by the presence of sucrose in solution in comparison to its solubility in water (i.e., from 13.6 ppm in water to 6.3 ppm in a 40% sucrose solution), and the extent of decrease increased with increasing sucrose concentration. These observations corresponded well to the literature results, which showed a reduced oxalate concentration with increasing sucrose concentration.4 A solubility concentration peak at an initial COM concentration of ca. 50 ppm was also found on the solubility curves of COM for all sucrose solutions, in accordance with an enhanced composite fouling rate possibly due to colloidal deposition achieved at this
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Ind. Eng. Chem. Res., Vol. 42, No. 4, 2003 Table 2. Silica Polymerization Kinetics Data in Single-Foulant Sucrose Solutions (Initial Silica SS Ratio, 1.8; pH, 6.0; Temp, 60 °C) system designation
log(k)
n
induction timea (h)
R2 b
water 10% sucrose solution 25% sucrose solution 40% sucrose solution
-1.16 -1.10 -0.87 -0.32
2.3 2.1 2.1 1.8
216 192 192 48
0.912 0.920 0.872 0.980
a A period when supersaturated silica solutions remain temporarily metastable (without significant drops in dissolved silica concentrations). b Correlation coefficient.
Figure 2. Effect of sucrose on the solubility concentration of COM: initial pH, 8.0; temp, 60 °C. Symbols: (b) water; (O) 10% sucrose solution; (1) 25% sucrose solution; (3) 40% sucrose solution. Table 1. COM Solubility Concentration Data in Water and Sucrose Solutions system designation
peak concn (ppm) at initial [COM] of 50 ppma
water 10% sucrose solution 25% sucrose solution 40% sucrose solution
5.3 4.6 3.0 2.8
a Difference between the equilibrium concentration value at 50 ppm and the average value at higher initial COM concentration.
Figure 4. Amount of colloidal silica in water and sucrose solutions: initial pH, 8.0; temp, 60 °C.
Figure 5. Sucrose molecule.
Figure 3. Kinetics of silica polymerization in single-foulant sucrose solutions: initial pH, 6.0; temp, 60 °C. Symbols: (9) water; (O) 10% sucrose solution; (1) 25% sucrose solution; (b) 40% sucrose solution.
initial concentration level in water.7 The presence of sucrose reduced the heights of these solubility peaks (Table 1), which gave an indication of the amount of colloidal COM species or a different type of calcium oxalate hydrate formed.6,9,10 A similar kinetic enhancement was observed for silica polymerization in sucrose solutions. Figure 3 shows the solution concentration of reactive silica expressed in terms of the SS ratio as a function of the reaction time to allow comparison of polymerization rates based on different silica solubilities. (The solubilities of silica in sucrose solutions were found to decrease with increasing sucrose concentrations.5,11) Again, the rate of silica polymerization was increased by the presence of sucrose, although in this case the extent of increase was much smaller at lower sucrose concentrations (e25%; Table 2).
Additional experimental evidence of the effect of sucrose on SiO2 polymerization thermodynamics was found by measuring the equilibrium reactive and total silica concentrations over a range of initial silica SS ratios (see Figure 4). In general, the differences between the equilibrium reactive and total silica concentrations imply the formation of the colloidal (nonreactive) form of SiO2 in solution.6,12 Colloidal silica occurred in sucrose solutions at an initial silica SS ratio of 1.8 or above, whereas little or no silica colloids could be found at an initial silica SS of 1.8 in water. Thus, the presence of sucrose reduced the threshold initial silica SS for producing colloidal SiO2 species from 2.0 to 1.8. Furthermore, the amounts of colloidal silica were reduced by the presence of sucrose at an initial silica SS of 2.0 (see Figure 4). The above results demonstrated that the presence of sucrose in solution had a considerable effect on the kinetics and thermodynamics of both COM precipitation and SiO2 polymerization. This may be due to several factors including complex formation with calcium oxalate and silica species by sucrose molecules (Figure 5) via coordination to the hydroxyl groups of sucrose without deprotonation of hydroxyl groups,13-15 as supported by the little changes in the solution pH during the reaction, and the availability of water molecules. The sucrose complexes promoted COM precipitation and SiO2 polymerization by providing additional nucleation
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Figure 6. Changes in the rate constant (k) and order of reaction (n) for COM precipitation in binary-foulant sucrose solutions as a function of the initial silica concentrattion at 25% sucrose concentration (w/w): initial COM concentration, 150 ppm; pH, 8.0; temp, 60 °C; R2 (correlation coefficient), >0.995.
and growth sites, resulting in the formation of colloidal silica at a lower initial silica SS ratio. These polymeric compounds might continue to aggregate the colloidal species of SiO2 by further complexation or adsorption. Therefore, the amounts of SiO2 colloids would be generally lower in sucrose solutions than those found in water (see Figure 4). Additionally, the intermolecular associations between sucrose and water16 may have influenced the solubilities of COM and SiO2 by reducing the number of water molecules available for the solvation of solutes (COM and SiO2).17 Binary Systems. (a) Kinetics. The results obtained in the kinetic studies of COM precipitation from binaryfoulant solutions containing 25% sucrose are shown in Figure 6, which are representative of the kinetic behavior of COM precipitation in sucrose solutions of various concentrations. It was observed that COM precipitation was accelerated by the presence of SiO2 in sucrose solution, in contrast to the situation with binary-foulant mixtures in water.6 The difference might be associated with the formation of various polymeric species involving sucrose and SiO2 as previously described. At high SiO2 concentrations, these colloids might flocculate and form highly gellike particles, which not only act as new growth centers but also induce aggregation by trapping COM crystallites in the matrix, as has been demonstrated for other biological macromolecules.18,19 The order of reaction was found to decrease (from 3.0 to 2.2) with increasing SiO2 concentration, a trend similar to that observed for singlefoulant sucrose solutions of increasing sucrose concentrations (see Figure 1 and Table 2). This again could be attributed to a hindered diffusion effect as a result of increased polymeric complex formation between SiO2 and sucrose, which induced an increase in the solution viscosity. Figures 7 and 8 show the effect of COM on the rate of SiO2 polymerization at sucrose concentrations of 25 and 40%, respectively. In a less concentrated sucrose solution (Figure 7), the rate of SiO2 polymerization was significantly increased by the presence of COM in comparison to that of the corresponding single-foulant sucrose solution, although higher COM concentrations caused little further increase in the polymerization rate constant and the order of reaction, which was largely between 1 and 2 (Table 3). Similar effects of COM were also observed for the binary-foulant mixtures in water.6
Figure 7. Effect of COM on the kinetics of SiO2 polymerization in binary-foulant sucrose solutions containing 25% sucrose at initial pH 6.0 and 60 °C: (b) [COM] ) 0; (O) [COM] ) 50 ppm; (1) [COM] ) 100 ppm; (3) [COM] ) 150 ppm.
Figure 8. Effect of COM on the kinetics of SiO2 polymerization in binary-foulant sucrose solutions containing 40% sucrose at initial pH 6.0 and 60 °C: (b) [COM] ) 0; (O) [COM] ) 50 ppm; (1) [COM] ) 100 ppm; (3) [COM] ) 150 ppm.
On the other hand, the rate of SiO2 polymerization in a more concentrated sucrose solution (Figure 8) was slightly increased at low COM concentration (50 ppm) but then decreased with increasing COM concentrations. As shown in Table 3, the induction period increased by a factor of 32 when the COM concentration was raised from 50 to 150 ppm. Therefore, the presence of COM either promoted or inhibited the rate of SiO2 polymerization in sucrose solutions depending on its concentration as well as the concentration of sucrose. The role of COM as either a polymerization promotor or inhibitor of silica in sucrose solutions may be attributed to the following: (i) the formation of sucrose adducts/complexes with SiO2 and COM and (ii) subsequent adsorption of COM-sucrose species onto SiO2 polymeric complexes (and vice versa).20,21 At low concentration of either COM or sucrose, SiO2 polymerization was accelerated because of a surface charge neutralization of colloidal SiO222,23 by the incorporation of COM-sucrose complexes. This hypothesis is supported by the result that the ζ potential of a COM-SiO2 composite deposit was lower than that of a pure SiO2 particle.24 With increasing amount of COMsucrose complexes adsorbed from solution, as could be expected at high sucrose and COM concentrations, silica polymerization was suppressed because of an overbalancing of surface charges of silica particles, resulting in the stabilization of silica colloids. Moreover, the
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Table 3. Summary of Silica Polymerization Kinetics Data in Single- and Binary-Foulant Sucrose Solutions with Different Sucrose Concentrations (Initial Silica SS Ratio, 1.8; pH, 6.0; Temp, 60 °C) 25% sucrose solution system designation
log(k)
[COM] ) 0 [COM] ) 50 ppm [COM] ) 100 ppm [COM] ) 150 ppm
-0.87 0.36 0.38 0.40
a
40% sucrose solution
n
induction time (h)
R2
log(k)
n
induction time (h)
2.1 1.5 1.2 1.5
168 5 2 2
0.872 0.996 0.997 0.998
-0.32 -0.07 -0.49 n/aa
1.8 1.5 1.9 n/a
48 24 72 770
R2 0.980 0.968 0.991
Not available.
Figure 9. Interpolated solubility surface for COM in binaryfoulant sucrose solutions: sucrose concentration, 25% (w/w); initial pH, 8.0; temp, 60 °C.
COM-sucrose macromolecules adsorbed might also stabilize the colloidal silica by steric forces,25 especially at high COM concentrations (e.g., 150 ppm) when sucrose complexes mainly consisted of branched or cross-linking structures.15 (b) Thermodynamics. To evaluate the precipitation equilibrium of a binary-foulant system, three-dimensional (3D) precipitation equilibrium surfaces can be constructed by plotting the solubility concentration of either one of the precipitating species as a function of the initial concentrations of two species. This approach is considered necessary, in particular because there is no control over the final equilibrium concentration of the precipitants resulting from the coprecipitation experiments. The computer software MATLAB (V6.0) was used in this study to connect randomly generated experimental data by a 3D interpolating mesh. Alternatively, it can fit a smooth surface to data. The latter approach, although more desirable, requires the MATLAB user to choose an appropriate equation representing the solubility surface. Figures 9 and 10 show examples of COM and SiO2 solubility surfaces, respectively, for a given sucrose concentration of 25%; similar results were achieved from different sucrose solutions. There are lumps and irregularities on the surfaces possibly because of experimental errors associated with those data in space. To quantitatively assess the interfering effect between two species, a solubility surface could be cut by selected “iso-concentration” planes at chosen initial concentrations of the interferring compound, yielding a series of two-dimensional solubility curves for the main compound as a result of the increasing presence of the interferring compound. The solubility curves in Figures 11 and 12 represent the respective “iso-cuts” of COM and SiO2 solubility surfaces from Figures 9 and 10.
Figure 10. Interpolated solubility surfaces for SiO2 in binaryfoulant sucrose solutions: sucrose concentration, 25% (w/w); initial pH, 8.0; temp, 60 °C. Labels: (A) total SiO2 concentration; (B) reactive SiO2 concentration.
Figure 11. Effect of SiO2 on the solubilities of COM at 25% (w/w) sucrose concentration, initial pH 8.0, and 60 °C, slicing through the surface in Figure 9 by x-z planes representing an initial [SiO2] of 50, 200, 400, and 600 ppm, respectively.
Figure 11 shows that the solubilities of COM in the binary-foulant sucrose solutions were much higher at low initial COM concentration range (
