Composite Fouling Characteristics of Calcium Oxalate Monohydrate

evaporation setup to simulate the operation cycle in latter effects of sugar mill evaporators. Simulated solutions with various COM to SiO2 supersatur...
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Ind. Eng. Chem. Res. 2002, 41, 3379-3388

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MATERIALS AND INTERFACES Composite Fouling Characteristics of Calcium Oxalate Monohydrate and Amorphous Silica by a Novel Approach Simulating Successive Effects of a Sugar Mill Evaporator H. Yu,† R. Sheikholeslami,*,† and W. 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 amorphous silica (SiO2) were investigated in a dynamic system under subcooled flow-boiling and a novel but simple continuous evaporation setup to simulate the operation cycle in latter effects of sugar mill evaporators. Simulated solutions with various COM to SiO2 supersaturation (SS) ratios as well as those of pure COM and SiO2 were tested for comparative studies. The results demonstrated that COM fouling resistances began to rise shortly after the experiment started with the highest extent of fouling exhibited at an initial COM concentration of 50 ppm (initial COM SS ∼ 2.6). The fouling resistance of SiO2 (500 ppm) began to rise at a theoretical silica supersaturation level of 5.3. SiO2 fouling involved the deposition of two silica species, that is, dissolved monomeric silica and colloidal silica particles, with colloidal silica particles being more prevalent at SiO2 supersaturation. In the binary systems, the synergistic effect of COM on composite fouling occurred at an intermediate concentration of COM (50 ppm) whereas antagonism was obtained at either low or high COM concentration (20 or 100 ppm). The observed variations in the extent of composite fouling may be partly attributed both to the changes in the magnitude of interfacial energy barrier between the surface of the particle and the wall and to the physical properties of the fouling species such as particle size. Instrumental analysis (SEM-EDS and XRD) was used to investigate structure and composition of scale. The presence of COM and its cementing effect in the composite scale from [COM] ) 50 ppm were confirmed. Introduction Scale formation in evaporators causes significant processing problems in sugar mills. Calcium oxalate and amorphous silica constitute the most intractable scale components formed on the calandria tubes of Australian sugar mill evaporators. The later vessels of a multipleeffect evaporator station are mostly affected1 as a consequence of high supersaturations during juice evaporation. The scale mainly consists of silica (SiO2) and calcium oxalate in either of its two crystalline forms, that is, calcium oxalate monohydrate (COM) and calcium oxalate dihydrate (COD), as shown in Table 1, with COM being the most stable form under high temperatures.2 The deposit formation results in increased energy consumption in order to maintain the operating requirements. Once the energy input becomes uneconomical to maintain the required evaporation rate, the sugar mill is forced to shut down so that the scale can be removed by chemical and/or mechanical means. Also, if the scale * Corresponding author. Telephone: +61 2 9385 4343. Fax: +61 2 9385 5966. E-mail: r.sheikholeslami@ eng.unsw.edu.au. † The University of New South Wales, Sydney. ‡ Sugar Research Institute.

Table 1. Typical Scale Composition in the Fifth Vessel of a Multieffect Evaporator Station in an Australian Sugar Mill component

conc (wt %)

calcium oxalate (as COM and COD) silica hydroxyapatite others

51.3 33.3 7.8 7.6

is not periodically removed, sucrose degradation due to extended residence time occurs.1,3 Previous works on evaporator fouling of COM and SiO24,5 have focused on the deposition of a single component, despite the coexistence of these two species in the evaporator scale. The presence of multiple compounds may give rise to interactive effects, which are not predicted by studying individual compounds. Also, the presence of more than one compound may affect scale removal by altering the structure and strength of the scale.6,7 Thus, it is necessary to study the effect of composite fouling and how interactions between the two components affect the scale property if a better understanding and mitigation of an actual fouling process is intended. In batch experiments, interactions between the COM and SiO2 have been demonstrated to affect both the kinetics and thermodynamics of COM precipitation and

10.1021/ie0110134 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/13/2002

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Figure 1. Schematic diagram of the dynamic test unit.

SiO2 polymerization under controlled pH (6-8) and temperature (60-80 °C) conditions.8 The presence of aqueous SiO2 was found to slightly inhibit the crystal growth of COM and increase the observed solubility for COM, whereas the presence of COM significantly increased the rate of SiO2 polymerization but had little effect on SiO2 solubility. The primary mechanism for COM and SiO2 coprecipitation was proposed to involve both the formation of a complex between COM and SiO2 and the specific adsorption of SiO2 or COM-SiO2 species onto COM crystal faces. The results indicated that while SiO2 was likely to control the solubility of COM during coprecipitation, the kinetics of coprecipitation might be controlled by COM. To gain a further understanding of the mechanisms governing the composite fouling of COM and SiO2 and to develop an effective predictive model, fouling experiments were performed in a dynamic system by a simple but novel approach. This approach allowed the simulation of the effect of feed concentration of successive stages of the evaporator system within one experiment. The dynamic system consists of a circulating fouling loop under forced convective and subcooled nucleate boiling heat transfer and evaporation, resulting in a gradual increase in the feed concentration. The experiments were conducted in aqueous solutions of COM or SiO2 alone and their binary mixtures. Characterization of deposits obtained from the dynamic tests was also carried out, as information on the physicochemical properties of scale is essential for better process control and the development of novel treatment procedures for scale control and removal. Experimental Section Test Apparatus. The experimental apparatus used for the dynamic runs is shown in Figure 1. It is comprised of a closed-loop circulation system which includes a storage tank, a circulation pump (Model CH420, Gundfos), a double-pipe heat exchanger, a cooling heat exchanger, series of pressure, temperature, and flow measuring and control devices, optional microfilters (20 µm pore size, Cuno Pacific Pty. Ltd.) located before and after the annular test section for removal of crystals/particulates in the solution, and a PC for control and data acquisition. The annular test section consists of a central heating tube made of stainless steel (dimensions 19.1 mm o.d.,

1.6 m length; Sandvik Australia P/L) and a glass outer wall (i.d. 38.1 mm; Pegasus, Canada) to allow visual observation of the deposition process during the experiment. All other pipings and fittings in the water systems were manufactured from stainless steel to prevent corrosion. The flow rate was measured with a magnetic flow meter (Model COPA-XE 400, Elsag Bailey). The test solution was heated with a countercurrent flow of steam (100-200 kPa) obtained from a regulated steam supply system. An in line cooling unit (Diecon, Marine Products Inc.) was used to obtain the heat balance within the flow loop and keep the bulk temperature of the feed constant (80 °C) throughout the run. All fouling experiments were operated at a constant water flow rate (0.35 L/s) and heat flux (7.0 kW) (steam temperature varied between ∼120 and 135 °C) using two automatic control valves operated with a data acquisition and control system (Genie, American Advantech Co.), which also collected the inlet and outlet temperatures of both streams and the bulk temperature from the embedded thermocouples and the flow rates from the magnetic flow meter. The results were constantly displayed on the computer screen and recorded into the computer’s hard disk at given time intervals. To reduce the magnitude of data fluctuation, all experimental measurements were averaged over every 10-20 min prior to further analysis. Test Procedure. Dynamic tests were performed as described in the following. Distilled water in the feed tank (250 L) was first heated by circulation through the annular test section, and the system was allowed to stabilize at predetermined operating conditions before known amounts of calcium (as CaCl2), oxalate (as Na2C2O4), and silica (as Na2SiO3‚9H2O, see ref 8 for details of solution preparation procedures) predissolved in portions of distilled water (2.5-10.0 L) were added successively with 5 min intervals to allow complete mixing in the bulk solution and let the system restabilize. HCl or NaOH (0.1-10 M) was used to adjust the initial pH of the concentrated solutions to 6.5 (sodium silicate is present in solution as monosilicic acid which dimerizes and then polymerizes to form silica at this pH level9,10) before adding the chemicals to the feed tank. All salts and chemicals used in this study were analytical grade reagents. Data collection was then initiated after all the chemicals were mixed in the tank, this point being taken as t ) 0. The changes in COM

Ind. Eng. Chem. Res., Vol. 41, No. 14, 2002 3381 Table 2. Operational Conditions of Evaporators in a Sugar Mill11,12 in Comparison to Those of This Work sugar mill evaporator conditions

3rd effect

concentration factor (CF) initial (feed) supersaturation COMb silicac overall range of supersaturation (3rd effect feed - 5th effect exit) COM silica initial COM/SiO2 SS ratio (3rd effect feed) temp (°C) pressure (kPa) superheat (°C)

1.5a

4th effect 2.0a

5th effect 3.0a

this work 1-10

2-7 1-5

4-12 2-6

5-22 3-11

1-5.3 1.7

100 98 12

2-66 1-33 0.5-3.5 80 50 20

54 15 26

1-53 1-17 0.6-3.0 80 101 24

a Approximate values based on the changes in the sucrose concentration (wt %) in a sugar mill evaporator. b The COM supersaturation is calculated as [COM]/19 (19 is the solubility (ppm) of COM at 80 °C). c The silica supersaturation is calculated as [SiO2]/290 (290 is the solubility (ppm) of SiO2 at 80 °C).

and SiO2 concentrations during the run were monitored through periodic sample withdrawal from the test solution. The withdrawn samples were either immediately filtered through 0.22 µm syringe filters (Millipore Co.) and acidified to pH < 2 using concentrated HCl for later analysis of COM or were diluted without filtration for SiO2 analysis. Calcium, oxalate, total SiO2, and reactive SiO2 contents were determined using ICP-AES (Varian Vista AX) and UV-visible spectroscopy (Varian Cary 1E). Details of the sample preparation and analysis have been reported elsewhere.8 After each run, the scale deposited on the heated tube was removed and characterized by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS, Hitachi S4500) and X-ray powder diffraction (XRD, Siemens D5000 diffractometer). To simulate various effects of the evaporation cycle in a sugar mill evaporator, the test solution was concentrated by evaporation (∼5-6 L/h) without passing through either of the in line filters during the run and the key operating conditions such as superheat, COM/ SiO2 supersaturations (SSs), and concentration factors (CFs) were maintained in the ranges comparable to those usually encountered in the later effects of a sugar mill evaporator. Single and binary solutions were tested using identical experimental conditions. In the binary systems, COM was used as the controlling species, the initial supersaturation of which was varied between 1 and 5.3 to determine the maximum degree of composite fouling from various feed solutions, in the presence of a fixed initial supersaturation of SiO2 (1.7). Table 2 presents a summary of the operational parameters of later effects of a sugar mill evaporator in comparison to the experimental conditions of this work. The continuous evaporation process was used to determine which stage of the evaporator resulted in the onset of COM/SiO2 fouling and the corresponding critical SS values. In addition, this will enable further investigations on the effect of other factors such as surface temperature and velocity to be carried out in future work. Calculation of Fouling Resistance. The instantaneous fouling resistance, Rf(t) ((m2 K)/kW), is calculated (eq 1) from the overall heat transfer coefficient (U0) at the beginning of each run and at any given time, t, when fouling has taken place. The overall heat transfer coefficient, U0 (W/(m2 K)), is determined from information on the heat duty, q (W), the heat transfer surface area, A (m2), and ∆Tm, the log-mean-temperaturedifference (°C) for a countercurrent arrangement (eq 2).

Rf(t) )

1 1 U0(t) U0(t)0)

(1)

q A∆Tm

(2)

U0 ) Results and Discussion

Single Systems. (a) COM Fouling. Parts a and b of Figure 2 show the fouling curves of COM and the corresponding COM SS curves at COM initial concentrations of 20, 50, and 100 ppm. The onset of COM fouling occurred almost immediately after the experiments began (CF above 1.0). The instantaneous fouling resistance from an initial COM concentration of 50 ppm was found to be the highest at any given CF level, probably due to the formation of metastable colloidal COM particles in solution, which were not separated by filtration with the 0.22 µm pore size membrane filter before ICP analysis and therefore contributed to the higher degree of COM supersaturation observed in solution (Table 3). This hypothesis was verified by passing COM solutions through a series of membrane filters (pore size 2.5-0.005 µm) and analyzing the resulting COM concentrations after each filtration (data not shown). For the other two runs, the instantaneous fouling resistance from an initial [COM] of 100 ppm was found to be lower than that from 20 ppm COM. Lencar and Watkinson13 also found that the initial fouling rate of calcium oxalate in a double pipe cooling heat exchanger decreased as the initial relative supersaturation of COM in the feed increased from 1 to 14.5. The authors indicated that the reason for this behavior was unclear.13 The instantaneous fouling resistances for all the COM single systems obtained in this work were in the range 0.05-0.1 ((m2 K)/kW), much lower than those of Lencar and Watkinson, and as a result very few COM scales were observed on the heat exchange surface. This difference may be explained by a lower range of initial COM supersaturation used in this study and the fact that our solutions were concentrated without further chemical additions, whereas Lencar and Watkinson13 used constant feed conditions by injecting makeup chemicals periodically without evaporation. Also, in this work, bulk precipitation of COM in the feed tank was observed because of its normal solubility and high rate of precipitation under the existing operating conditions, in agreement with our previous batch tests.8 (b) SiO2 Fouling. The SiO2 fouling resistance and the SiO2 supersaturation curves are shown in Figure

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Figure 2. COM fouling in single systems. (a) Fouling resistance profiles at different initial COM concentrations: (O) COM 20 ppm; (3) COM 50 ppm; (9) COM 100 ppm; (1) conc. factor. (b) Changes in COM supersaturations in solution. Dissolved SS: (b) COM 20 ppm; (1) COM 50 ppm; (9) COM 100 ppm. Theoretical SS: (O) COM 20 ppm; (3) COM 50 ppm; (0) COM 100 ppm. Table 3. Effect of Supersaturation Ratio on Fouling of COM Single Systems initial COM conc (ppm)

initial SS ratio

equil SS in solution8

Rf(t) at CF ) 4.0a (×102 m2 K/kW)

20 50 100

1.1 2.6 5.3

1.0 1.2 1.0

5.2 8.4 3.8

a

Figure 3. Silica fouling at the initial concentration 500 ppm: (a) (b) silica fouling resistance and (9) conc. factor; (b) variations in the (2) total silica supersaturation (measured by ICP) and (4) theoretical silica supersaturation; (c) variation in the (b) dissolved silica supersaturation (measured by UV).

Estimated from linear regression analysis.

3. The fouling resistance of the SiO2 single system remained almost negligible at low CF levels (1.0-3.0) before rising sharply when the CF reached above 3.0 (theoretical silica SS ∼ 5.3). This is apparently due to a drastic increase in the difference between total and theoretical SiO2 supersaturations (supersaturations based on total and “theoretical” silica concentrations, that is, the silica concentration in the absence of precipitation, respectively; also see Table 2) in solution from this theoretical SS level (Figure 3b), which represented the proportion of SiO2 removed from solution (either adhered to the surface or precipitated in the tank) as evaporation continued. Additional information on silica fouling was obtained by monitoring the changes in the dissolved SiO2 supersaturation levels in solution during the experiment (Figure 3c). It was found that the dissolved SiO2 supersaturation ratios in solution was mainly below 2.0 at CFs 1.0-3.0 and then markedly increased from 2.0 to 2.5 as the fouling resistance steeply rose to ∼1.0 ((m2 K)/kW). It may be inferred from these results that direct deposition of molecular silica (surface polymerization) may have occurred at low CFs, which was then followed by particulate deposition of colloidal SiO2 species, resulting in the steep rise in the fouling resistance.6,8,14 The above two fouling mechanisms are known to yield SiO2 deposits of different physical characteristics: mon-

Figure 4. Reproducibility of dynamic experimental data.

omeric SiO2 species form an impervious film while colloidal silica particles produce a porous layer.14 This change may alter the thermophysical properties (e.g., thermal conductivity) of silica scales and could well explain the fouling resistance curve in Figure 3a. Binary Systems. (a) Reproducibility. The reliability of the experimental data was evaluated by having a duplicate run for one of the binary systems (Figure 4). The results show that the fouling resistance data were reproducible with 90% of the data points having less than 10% deviation, a low magnitude considering the sources of errors expected.

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Figure 5. Composite fouling of COM/SiO2 at the initial SiO2 concentration 500 ppm and the initial COM concentration 50 ppm: (a) (O) silica single system, (0) COM single system, (]) binary system, (2) conc. factor; (b) (b) dissolved silica SS, (O) total silica SS, (1) theoretical silica SS.

(b) Effect of COM/SiO2 Ratio. Figures 5 and 6 present the data obtained for COM/SiO2 composite fouling in the binary systems together with those of the corresponding single systems. The data for an initial COM concentration of 50 ppm (Figure 5) showed that composite fouling resistance started to rise at a lower degree of theoretical SiO2 supersaturation (∼2.0) than that observed for pure SiO2 fouling (∼5.3), indicating a synergistic effect between the two scale components. This phenomenon may be explained on the basis of the batch test results reported in a previous paper in which the presence of COM enhanced the rate of SiO2 polymerization.8 This in turn would increase the particulate deposition of colloidal SiO2 species during the fouling process, leading to a decrease in the degree of supersaturation required for composite fouling. However, such a synergistic behavior was not observed for composite fouling resistances at initial COM concentrations of 20 and 100 ppm (Figure 6), both of which remained lower than those of the comparative single systems up to a theoretical SiO2 SS of 9.0 (CF ∼ 5.0) even though under those conditions SiO2 was expected to have high polymerization rates, as shown in the previous work of batch tests.8 Thus, it is possible that other mechanisms may also occur during composite fouling of COM and SiO2. The effect of precipitating species on particulate fouling has previously been reported.15 The deposition rate of silt and hematite particles, for example, was enhanced in the presence of calcium carbonate precipitation.15 The increased deposition rate of silt may be accounted for by a reduced energy barrier between negatively charged silt particles and the negatively charged stainless steel wall because of the incorporation of crystallizing calcium carbonate (calcite) into the silt particles. A similar interaction has also appeared in membrane fouling.16 Hong and Elimelech16 examined the fouling of nanofiltration membranes by natural organic matter (NOM). The rate of fouling was significantly increased in the presence of divalent cations such

Figure 6. Composite fouling of COM/SiO2 at the initial SiO2 concentration 500 ppm and the initial COM concentrations 20 and 100 ppm. (a) (O) Silica single system. COM single systems: (0) COM 20 ppm and (]) COM 100 ppm. Binary systems: (4) COM 20 ppm and (3) COM 100 ppm. (2) Conc. factor. (b) Changes in silica supersaturations in solution. COM 20 ppm: (b) dissolved SS; (1) total SS; (9) theoretical SS. COM 100 ppm: (O) dissolved SS; (3) total SS; (0) theoretical SS.

as calcium, owing to a decrease in the overall surface charge of NOM and the membrane. Thus, the high composite fouling rate observed for the solution containing 50 ppm COM and 500 ppm SiO2 in this study may be due to the lower interfacial energies between the surface of the heat exchanger tube and COM/SiO2 colloidal particles as a result of charge-neutralization between positively charged COM particles and negatively charged SiO2 particles.17,18 Accordingly, the low fouling rates observed at initial COM concentrations of 20/100 ppm were probably due to the formation of colloidal COM/SiO2 particles with surface charges that remained sufficiently high to prevent particle deposition. Moreover, the presence of a high COM concentration (100 ppm) may have led to rapid aggregation and precipitation of COM/SiO2 complexes in the bulk solution because the kinetics of COM precipitation increased as the initial COM supersaturation in solution increased.8 On the other hand, the physical properties of fouling particles such as size and shape may also have an impact on particulate deposition, as they determine particles’ transport regime as well as their attachment efficiency to the wall.19,20 Hence, the different composite fouling rates of COM and SiO2 observed in the binary systems may also be related to the variations in the size distribution of particles formed at different initial COM supersaturation levels.21 Thus, the approach used and the dynamic test results described above demonstrated that the fouling resistance of COM started to rise at an earlier stage of the evaporation cycle (theoretical COM SS g 1.0) than that observed for SiO2 fouling (theoretical SiO2 SS g 5.3), albeit to a much smaller extent than that of the SiO2 fouling resistance. The composite fouling resistance in the presence of 50 ppm COM started to rise from a theoretical SiO2 SS level of ∼2.0 corresponding to the

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Figure 7. Scales from COM/SiO2 binary systems with various initial COM concentrations: (a) 20 ppm; (b) 50 ppm; (c) 100 ppm; (d) SiO2 single system.

third effect of a sugar mill evaporator station because of the synergistic effect of COM on composite scale formation. Scale Deposits. (a) Visual and Tactile Examination. The amount of scales deposited onto the surface of the heat exchange tube was found to be small for all the COM single systems, in accordance with the low COM fouling resistances observed. Visual examination of the heat exchange surface during the later stage of the SiO2 fouling experiment found that the silica scales had a rippled surface (Figure 7d) similar to that described for SiO2 in another study.22 This rippled layer of deposits contained aggregates of large particles and was partially swept away upon draining the test section to remove the tube, uncovering a uniform inner layer of hard scales (Figure 7d). This inner layer adhered to the tube wall and could only be removed by mechanical means. These observations corresponded well with the proposition that the deposition of monomeric silica and colloidal silica species occurred at various stages of fouling. For the composite scales, only the one obtained at the initial COM concentration 50 ppm led to a complete coverage of the tube surface (Figure 7b). As was noticed when scraping off the deposits, these scales were more tenacious than the inner layer scales formed in the SiO2 single system. Composite deposits formed at the initial COM concentrations 20 and 100 ppm (Figure 7a and c) were composed of large isolated clusters, which appeared to be less tenacious than the scales obtained with 50 ppm COM. (b) SEM-EDS. Scanning electron microscopy was used to evaluate the structural and morphological

features of the scale deposits. Figures 8 and 9 present the electron micrographs of SiO2 and composite scales at various magnifications. It was evident from Figure 8a and b that the inner layer scales of SiO2 have a different morphology from those of the outer layer of SiO2 scales and the three composite scales. The inner layer scales appeared to be more compact than the outer layer scales, which had a porous structure. As mentioned earlier in fouling data analysis, this difference in the structure would be expected to result in a higher thermal resistance for the outer layer deposit, in comparison to the dense inner layer deposit under the flow-boiling conditions as the pores of the deposit were filled with a mixture of water and steam, which had a lower thermal conductivity than water.23 The composite deposits (Figure 9), on the other hand, exhibited a layered structure and certain surface fragmentation indicating periodic growth of different fouling species on the solid phase and changes in the crystallinity of scales.3 A closer look at the cross sections of the scales showed that the composite scales from 50 ppm COM (Figure 10b) were more densely packed in comparison to the SiO2 inner layer scales, which appeared to contain a matrix of spheroids (Figure 10a). This difference might explain why this deposit was more tenacious than the SiO2 scales. Previous studies24,25 on CaSO4 and CaCO3 fouling also found that the presence of CaCO3 led to an increase in the strength of the CaSO4 scales. The authors suggested that the coprecipitated CaCO3 might act as a bonding agent, cementing the structure of the CaSO4 scale layer. It is likely that a similar effect may have occurred in this study. Energy-dispersive X-ray microanalysis (EDS) was used to identify the main elements in the scale samples. A typical elemental profile for the composite scales, shown in Figure 11a, revealed the presence of Ca in the deposit, which was later confirmed by XRD to be associated with COM (see next section) in addition to Si and O as the major components of both silica and composite scales (Figure 11). Small levels of Na and Cl were also present, possibly because of the entrapment and/or absorption of NaCl from the solution. To obtain further information on the composition of composite scales, samples were either treated with resin setting (to examine the particle cross section) or mixed with potassium bromide (KBr) powder and made into KBr pellets in a die (to examine the particle surface) to give polished sections for X-ray analysis. As indicated by the high molar ratios between SiO2 and COM at the cross sections of the sample particles (Table 4), COM

Figure 8. Scanning electron micrographs of SiO2 scales: (a) inner layer; (b) outer layer.

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Figure 9. Scanning electron micrographs of composite scales: (a) COM 20 ppm; (b) COM 50 ppm; (c) COM 100 ppm.

Figure 10. SEM cross sections of SiO2 scales and composite scales: (a) pure SiO2 scales; (b) composite scales at [COM] ) 50 ppm.

was located mainly in the interior of the particle while large amounts of SiO2 were present at the particle surface. The uneven distribution of COM might be explained by the initial formation of a COM/SiO2 complex or COM particles, which served as additional

nucleation sites or a growth center for subsequent SiO2 polymerization and adsorption.8 (c) XRD. X-ray powder diffraction analysis was conducted on the SiO2 and composite scales in order to identify the crystalline phases involved in the composite

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Figure 11. (a) EDS spectrum for the composite scales with the initial COM concentration 100 ppm. (b) EDS spectrum for the silica scales (inner layer). Table 4. Summary of Experimental Data and Molar Ratiosa of SiO2 and COM for the Composite Scales Obtained from Different Initial COM Concentrations initial COM/SiO2 conc (ppm)

final COM conc (ppm)

final reactive SiO2/ total SiO2 conc (ppm)

initial molar ratio SiO2/COM in soltn

molar ratio SiO2/COM at particle surface

molar ratio SiO2/COM at particle cross sectionb

0/500 20/0 50/0 100/0 20/500 50/500 100/500

0 22.8 31.8 27.1 76.3 77.5 83.9

723.1/1500 0 0 0 925.1/1687.5 698.2/782.1 810.4/1789.3

61:1 24:1 12:1

95:1 50:1 34:1

5:1 4:1 4:1

a

Quantitative results of the overall deposit composition are not available. b Average of at least 10 measurements.

fouling. Figure 12 shows the diffraction patterns of the composite scales together with that of the SiO2 scales. The XRD spectra of all the scales gave a smooth broad peak or “halo” with a 2θ value of approximately 23° caused by the presence of amorphous silica.26 However, this peak appeared to be narrower for the composite scales compared to that of the pure SiO2 scales, probably because of the reorganization of SiO2 molecules into a less disordered arrangement in the presence of COM since the halo width represented the variation of the average distances of atoms in the amorphous Si-O-Si structural unit.26 The spectra of composite scales at 50 and 100 ppm COM were also found to contain additional sharp peaks (2θ values of 14.9°, 24.4°, 31.7°, 38.3°, and 45.4°) due to the presence of COM.27 The spectrum for 20 ppm COM composite scales did not show COM crystalline peaks, probably because of the low amount of COM formed (Table 4). Effect of Boiling Heat Transfer. Under the forced convective and subcooled boiling heat transfer, bubble formation on certain nucleation sites of a heat exchange

surface could be observed, especially at the upper end of the test tube, which was closer to the steam inlet. The increased heat transfer associated with bubble formation under boiling is primarily based on two mechanisms: microlayer evaporation and microconvection.28 Microlayer evaporation tends to promote scale deposition because of the local concentration effect as a thin liquid film, microlayer, beneath the bubble quickly evaporates. The detaching bubble then leaves behind a ring of deposit around the nucleation site from which the following bubbles can gradually build up a diskshaped deposit,29,30 as was observed in this study (Figure 13). Microconvection, on the other hand, can increase scale removal as a result of the extra stirring of the liquid boundary layer by detaching bubbles.31 It appeared that microconvection was the prevailing mechanism in this study, since deposit accumulation was generally found to be thicker close to the steam outlet where bubble formation and bubble induced convection were less intense.

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which began at a lower theoretical SiO2 SS level (∼2.0) than that occurring for the SiO2 single system because of increased particulate deposition. In contrast, the composite fouling resistance in the presence of COM at the initial concentration either 20 or 100 ppm was found to be lower than that of pure SiO2. Part of this difference may be related to the changes in the magnitude of the interfacial energy barrier between the surface of the particle and the wall and the physical properties of the fouling species such as particle size. The composite scales obtained from 50 ppm COM were found to be more tenacious than SiO2 scales, probably because of the cementing effect of COM on the deposit structure, as revealed by SEM analysis. The composite scales from 20 and 100 ppm COM were less tenacious than the 50 ppm COM composite scales because of the incomplete coverage on the tube surface. The boiling conditions might affect the scale deposition through microlayer evaporation and microconvection with microconvection being the prevailing factor. Further work is underway to study the composite fouling behavior of COM and SiO2 in the presence of sugar in solution, which is expected to affect the rate of composite fouling and the characteristics of the composite scale. Figure 12. X-ray diffraction patterns of (a) SiO2 scales and composite scales; (b) initial [COM] ) 20 ppm; (c) initial [COM] ) 50 ppm; (d) initial [COM] ) 100 ppm.

Acknowledgment The financial support of Australian Research Council and The Sugar Research Institute (SRI) of Australia as well as the fruitful research collaboration with SRI are gratefully acknowledged. Literature Cited

Figure 13. Scanning electron micrograph of bubble nucleation sites for the composite scales (lower end).

Conclusions The composite fouling of COM and SiO2 has been studied under forced convective subcooled nucleate boiling heat transfer and simulated evaporative conditions comparable to those in the later effects of sugar mill evaporators from both single and binary solutions with various COM/SiO2 ratios. The results showed that the COM fouling resistance began to rise immediately after the experiment started with the highest extent of fouling occurring at the initial COM concentration 50 ppm (initial COM SS ∼ 2.6). The fouling resistance of pure SiO2 from the initial concentration 500 ppm began to rise at the theoretical SS level ∼5.3. The overall fouling resistances of COM were found to be much less than that of SiO2 because of rapid bulk precipitation. The process of SiO2 deposition may involve different SiO2 species at various degrees of evaporation and SiO2 supersaturation, giving rise to the formation of two types of SiO2 deposits with different physical characteristics. The presence of COM at the initial concentration 50 ppm led to a synergistic effect on composite fouling,

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Received for review December 14, 2001 Revised manuscript received May 9, 2002 Accepted May 13, 2002 IE0110134