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The Deposition of Calcium Oxalate and Amorphous Silica Scale under Dynamic Conditions which Simulate sugar mill Evaporators.

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Dev. Chem. Eng. Mineral Process. 12(3/4), pp. 309-322, 2004.
The Deposition of Calcium Oxalate and
Amorphous Silica Scale under Dynamic
Conditions which Simulate Sugar Mill
Evaporators
H. Yu, R. Sheikholeslami* and W.O.S. Doherty'
School of Chemical Engineering and Industrial Chemistry, University
of New South Wales, Sydney, New South Wales 2052, Australia
'Sugar Research Institute, Mackay, Queensland 4 740, Australia
Composite fouling of calcium oxalate monohydrate (COM) and amorphous silica
( S O 3 in sugar mill evaporators poses a major processing problem. The significance
of COM-Si02 interactions during composite scale formation has been recognized
previously and their effects on the fouling behaviour of both species have been
demonstrated in batch tests. This work investigates the mechanisms of composite
fouling of COM and SiO, in a dynamic system under subcooled flow-boiling and
continuous evaporation. The experimental approach used in this study is novel. It
simulates the operation cycle in the latter efects of sugar mill evaporators within one
experimental run whilst maintaining a relatively simple process. The composite
fouling behaviour of COM and SiO- has been tested in aqueous solutions with a
range of COM/SiO, supersaturation ratios to determine the critical feed composition
at which the maximum degree of compositefouling occurred. The synergistic efect 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 and 100 ppm). This may be due to changes in the magnitude of the interfacial
energy barrier between the surface of the particles and the wall, and to the
diferences in the physical properties of thefouling species such as particle size.
* Author for correspondence (r.sheikholeslami@eng.unsw.edu.au).
309
H. Yu, R. Sheikholeslamiand W.O.S.Doherty
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 multiple-effect evaporator station are mostly affected [I] as a
consequence of high supersaturations during juice evaporation. The scale mainly
consists of silica (SiOz) and calcium oxalate in either of its two crystalline forms, i.e.
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 andor mechanical means. Also if the scale is not
periodically removed, sucrose degradation due to extended residence time and
corrosion of the tubes occurs [ 1,3].
Component
Concentration (wt%)
- Calcium oxalate (as COM and COD)
Silica
Hvdroxvauatite
Others
51.3
33.3
I
7.8
7.6
I
Previous works on evaporator fouling of COM and SiOz [4, 51 have focused on
the deposition of a single component, despite the co-existence 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 ] .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 lunetics and thermodynamics of COM precipitation
and SiOz polymerization under controlled pH (6-8) and temperature (60-80°C)
conditions [7]. The presence of Si02 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 Si02 polymerization but had little effect on Si02
solubility. The primary mechanism for COM and Si02 co-precipitation was proposed
to involve both the formation of a complex between COM and SiOz and the specific
adsorption of Si02 or COM-Si02 species onto COM crystal faces. The results
indicated that while SiOz was likely to control the solubility of COM during coprecipitation, the kinetics of co-precipitation might be controlled by COM.
To gain a further understanding of the mechanisms governing the composite
fouling of COM and Si02 and to develop an effective predictive model, fouling
experiments were performed in a dynamic system by a simple but novel approach.
310
Deposition of Scale under Conditions which Simulate Sugar Mill Evaporators
This approach allowed the simulation of the effect of feed concentration in successive
stages of the evaporator system within one experiment. The dynamic system consists
of a circulating fouling loop under forced convective and sub-cooled nucleate boiling
heat transfer and evaporation resulting in gradual increase in the feed concentration.
The experiments were conducted in aqueous solutions of COM or SiOt alone and
their binary mixtures. Characterization of deposits obtained from the dynamic tests
was also carried out, as mformation 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 Details
The experimental apparatus for dynamic runs is shown in Figure 1. It includes a
closed-loop circulation system with a storage tank, a circulation pump (Model CH420, Gundfos), a double-pipe heat exchanger, a cooling heat exchanger, and a series of
pressure, temperature and flow measuring and control devices. There are optional
micro-filters (20 pm 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.
Cont roI
Valce
Annular
Test Section
Condensate
to drain
Figure 1. A schematic diagram of the dynamic test unit.
The annular test section consists of a central heating tube made of stainless steel
(Sandvik Australia Pty) and a glass outer wall (Pegasus, Canada) to allow visual
observation of the deposition process during the experiment. All other piping and
fittings in the water systems were manufactured from stainless steel to prevent
corrosion. The flow rate was measured by a magnetic flowmeter (Model COPA-XE
400, Elsag Bailey). The test solution was heated by counter-current flow of steam
(100-200 kPa) obtained fiom 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.
311
H.Yu,R. Sheikholeslami and W.O.S.Doherty
All experiments were conducted at a constant water flow rate (0.35 L/s) and heat flux
(7.0 kW) using two automatic control valves operated by a data acquisition and
control system (Genie, American Advantech Co.), which also recorded the inlet and
outlet temperatures of both streams and the bulk temperature from embedded
thermocouples and flow rates from the magnetic flowmeter. These data were
constantly displayed on the computer screen and recorded on the computer hard disk
at given time intervals. To reduce the magnitude of data fluctuation, all experimental
measurements were averaged over every 10-20 min period prior to further analysis.
Dynamic tests were performed as described as follows. Distilled water in the feed
tank (250 L) was first heated by circulating through the annular test section and the
system allowed to stabilize at predetermined operating conditions before known
amounts of calcium (as CaC12), oxalate (as Na2C2O4)and silica (as Na2Si03.9H20)
pre-dissolved in distilled water (2.5-10.0 L) were added successively with 5 minute
intervals. This allowed complete mixing in the bulk solution and let the system restabilize. HCl or NaOH (0.1-10M) was used to adjust the initial pH of the
concentrated solutions to 6.5 before they were added 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 and SiOzconcentrations during the run were monitored through
periodic sample withdrawal from the test solution. The withdrawn samples were
either immediately filtered through 0.22 pm syringe filters (Millipore Co.) and
acidified to pH < 2 using concentrated HCI for later analysis of COM, or were diluted
without filtration for SiOz analysis. Calcium, oxalate, total Si02 and reactive SiOz
contents were determined using ICP-AES (Inductively Coupled Plasma Atomic
Emission Spectrometer; Varian Vista AX) and UV-visible spectroscopy (Varian Cary
1E spectrophotometer). Details of the sample preparation and analysis have been
reported elsewhere [7]. 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 without passing through either of the
in-line filters during the run. The key operating conditions such as superheat,
COM/Si02 supersaturations (SS) and concentration factors (CF) were maintained in
the range 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 vaned between 1-5.3 to determine the maximum degree
of composite fouling from various feed solutions, in the presence of a fixed initial
supersaturation (1.7) of SiOz (see Table 2).
Thus the continuous evaporation approach was used to determine which stage of
the evaporator resulted in the onset of COM/SiOz composite fouling, and the critical
COM/Si02 SS ratio at which the maximum composite fouling occurred. Knowledge
of these aspects will provide a basis for further investigation on the effect of other
factors such as surface temperature and velocity to be carried out in future work.
The instantaneous fouling resistance (Rqo, mz WkW) was calculated (from
Equation 1 below) from the overall heat transfer coefficient (U,) at the beginning of
312
Deposition of Scale under Conditions which Simulate Sugar Mill Evaporators
each run and at any given time (t) when fouling has taken place. The overall heat
transfer coefficient (U",W/m2 K), was determined from the heat duty (4, W), the heat
transfer surface area (A, m2) and AT,, the log-mean-temperature-difference ("C) for a
counter current arrangement (see Equation 2):
U,
=4
AAT,
q =mc,AT,
(3)
where m is the mass flow rate; cp is the specific heat of heating fluid; and aq is the
temperature difference of fluid along the annulus. The measurement uncertainties of
temperature and velocity were less than 0.5%. The uncertainties involved in the heat
flux and heat transfer coefficient were less than 9%, a usual range for this type of
experiment.
Table 2. Operational conditions of evaporators in a sugar mill [8, 91 in comparison
to this work.
Sugar mill evaporator
41h
I 51h
Conditions
I
OGrall Gnge ofsuDersaturation
(3rdeffect feed - 5' effect exit)
COM
IL
I
I
I
I
I
2-66
1-33
A
Silica
nitial COM/Si02 ss ratio (3rd
effect feed)
Temperature ("C)
Pressure ( H a )
Superheat ("C) **
I
This work
0.5-3.5
100
98
12
80
50
20
1-53
1-17
1
0.6-3.0
54
15
26
80
101
24
* Approximate values based on the changes in the sucrose concentration (wt %) in a
sugar mill evaporator.
* * Difference between steam and liquid saturation temperature.
a COM supersaturation is calculated as: [COM]/19 (19 is the solubility (ppm) of
COM at 80°C).
Silica supersaturation is calculated as: [SiOJ/290 (290 is the solubility (ppm) of
SiO?at 80°C).
313
H. YU, R. Sheikholeslami and W.O.S. Doherty
Results and Discussion
(i) Single Systems
Figures 2a and 2b 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 fouiing resistance from an initial COM concentration of 50 ppm was
found to be the highest at any given CF level. This was probably due to the formation
of metastable colloidal COM particles in solution which were not separated by
filtration with 0.22 pm pore size membrane filter before ICP analysis, and therefore
contributed to a higher degree of COM supersaturation observed in solution (see
Table 3). 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
Watkinson [ 101 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 [lo].
0 20
11
[COM]=ZOpprn
v [COMj=SOppm
0
[COM]=IOOppm
+Conc factor
0 15
3
5
mE
9
7
0:
v
5
0
Y
d
0 05
-=
3
60
5
v)
-f Dissohed SS,[COM]=Mppm
-0- Theore. SS,[COM]=ZOpprn
50
y Dissolved SS,[COM]=Spprn
Q- Theore. SS,[COM]=50ppm
-8- Dissohed SS, [COM]=lM)ppm
U Theore. SS,[COM]=IOOpprn
40
a
30
s=
3
c
.-0 000
3!
g
2
010
1
P% 2o
a
;
10
8
0
0
500
1000
1500
2000
2500
3000
Time (min)
Figure 2. COM fouling in single systems at initial concentrations of 20, SO and 100
PPm.
The instantaneous fouling resistances for all the COM single systems obtained in
this work were in the range 0.05 - 0.1 (m2 WkW), much lower than those of Lencar
and Watkinson and as a result very little COM scale was observed on the heat
314
Deposition of Scale under Conditions which Simulate Sugar Mill Evaporators
Initial 'OM
conc. (ppm)
20
50
100
Initial ss ratio
1.1
2.6
5.3
Equilibrium SS
in solution (71
1.o
1.2
1.o
*Rt,) at CF = 4.0
(m2 K/k w ) x I d
5.2
8.4
3.8
1.2
-
13
- 11
/q
/ :
- 9
- 7
ti
.2
- 5
s
U
- 3
- 1
- 0
500
1000
1500
2000
2503
3000
Time (min)
Figure 3. Silica fouling at initial conc. of 500 ppm.
315
H. Yu,R. Sheikholeslarniand W.O.S.Doherty
The SiO2 fouling resistance and the SiOz supersaturation curves are shown in
Figure 3. The fouling resistance of the SiOz single system remained almost negligible
at low CF levels (1 .O-3.0), before rising sharply when CF exceeded 3.0 (theoretical
silica SS -5.3). This is apparently due to a drastic increase in the difference between
total and theoretical S O 2 supersaturations (supersaturations based on total and
‘theoretical’ silica concentrations, i.e. silica concentration in the absence of
precipitation, respectively; also see Table 2) in solution from this theoretical SS level
(see Figure 3b), which represented the proportion of SiOz 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 SiOz supersaturation levels in solution during the experiment (see
Figure 3c). It was found that the dissolved SiOz supersaturation ratios in solution were
mainly below 2.0 at CF 1.0-3.0, and then markedly increased from 2.0 to 2.5 as the
fouling resistance steeply rose to -1.0 (mz KkW). It may be inferred from these
results that direct deposition of molecular silica (surface polymerization) may have
occurred at low CF, which was then followed by particulate deposition of colloidal
SiOz species resulting in the steep rise in the fouling resistance [6, 7, 1 I]. The above
two fouling mechanisms are known to yield Si02 deposits of different physical
characteristics; monomeric S O z species forms an impervious film while colloidal
silica particles produces a porous layer [ll]. This change may alter the
thermophysical properties (e.g. thermal conductivity) of silica scale and could well
explain the fouling resistance curve in Figure 3a.
(ii) Binary Systems
Figures 4 and 5 present the data obtained for COM/Si02 composite fouling in the
binary systems, together with those of the corresponding single systems. The data for
an initial COM concentration of 50 ppm (see Figure 4) showed that composite fouling
resistance started to rise at a lower degree of theoretical SiO2 supersaturation (-2.0)
than that observed for pure SiOz 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 SiOz polymerization [7]. This in turn would increase the
particulate deposition of colloidal SiOz species during the fouling process leading to a
decrease in the degree of supersaturation required for composite fouling. However,
such a synergistic behaviour was not observed for composite fouling resistances at
initial COM concentrations of 20 and 100 ppm (see Figure 5). Both resistances
remained lower than those of the comparative single systems up to a theoretical S O z
SS of 9.0 (CF -5.0),even though under those conditions SiO, was expected to have
high polymerization rates as shown in previously in batch tests [7]. Thus it is possible
that other mechanisms may also occur during composite fouling of COM and SO2.
The effect of precipitating species on particulate fouling has previously been
reported [IZ].The deposition rate of silt and hematite particles, for example, was
enhanced in the presence of calcium carbonate precipitation [ 121. The increased
deposition rate of silt may be accounted for by a reduced energy barrier between
negatively charged silt particles and a negatively charged stainless steel wall due to
the incorporation of crystallizing calcium carbonate (calcite) into the silt particles.
316
Deposition of Scale under Conditions which Simulate Sugar Mill Evaporators
Similar interaction has also appeared in membrane fouling [ 131. Hong and Elimelech
[ 131 examined the fouling of nanofiltration membranes by natural organic matter. The
rate of fouling was significantly increased in the presence of divalent cations such as
calcium, due to a decrease in the overall surface charge of natural organic matter and
the membrane. Thus, the high composite fouling rates observed for solutions
containing 50 ppm COM and 500 ppm SiO2 in thn study may be due to the lower
interfacial energies between the surface of the heat exchange tube and C0WSiO2
colloidal particles as a result of charge-neutralization between positively charged
COM particles and negatively charged SOzparticles [ 14, 151.
1.2
- 9
-7
z
t
-5
0:
6
- 3
- 1
+ Dissotved silica SS
Total Silica SS
+ Theore silicaSS
0
500
1000
1500
2000
2500
3000
Time (rnin)
Figure 4. Composite fouling of COM/SiO2 at initial S O , concentration of 500 ppm
and initial COM concentration of SO ppm.
The low fouling rates observed at initial COM concentrations of 20 and 100 ppm
were probably due to the formation of colloidal COM/Si02 particles with surface
charges that remained sufficiently high to prevent particle deposition. Moreover, the
presence of lugh COM concentration (100 ppm) may have led to rapid aggregation
and precipitation of COM/SiO2 complexes in the bulk solution. This is due to the
lunetics of COM precipitation increasing as the initial COM supersaturation in
solution increased [7].
Alternatively, the physical properties of fouling particles such as size and shape
may also have an impact on particulate deposition as they determine the partide
transport regime, as well as their attachment efficiency to the wall [16, 171. Hence,
317
H. Yu. R. Sheikholeslamiand W.O.S.
Doherty
the different composite fouling rates of COM and SiOz 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 [181.
1.2
,
Silica single system
COM single system (20ppm)
COM single system (1OOppm)
A Binarysystem, [COM]=2Opprn
v Binary system, [COM]=lOOppm
--C Conc. factor
0
1.O -
=$ o,8
2
.
0.6 -
'"E
0
0
u
-. ._
0.4 I
02
5 00
4-
14 -
:: l2E
.g
+o_
2
7
,,,8
-
0
-t Dissolved silica SS ([COM]=2Oppm)
-G- Dissolved silica SS ([COM]=lWppm)
-v-
TOW silica ss ([~0~]=20ppm)
+Total silica SS ([COMj-IOOppm)
-t Theore silica SS ([COM]=2Oppm)
+IF Theore silica SS ([COMj=100ppm)
500
loo0
1500
2000
25M
3000
Time @in)
Figure 5. . Composite fouling of COM/SiOz at initial Si02 concentration of 500 ppm
and initial COM concentration of 20 and 100 ppm.
Thus the dynamic test results described above demonstrated that COM fouling
started at an earlier stage of the evaporation cycle, albeit to a much smaller extent
than that observed for Si02 fouling. Both synergistic and antagonistic effects by COM
occurred in the binary systems tested, depending on the initial SS ratio between Si02
and COM. This was due to variations in the interfacial energies between the surface
of the heat exchange tube and the COM/SiOt colloidal particles, as well as different
physical characteristics of the particles.
(iii) Scale Deposits
Studies were carried out to examine the effect of COM on the physicochemical
properties of composite scales, as would be required for an effective process control
and pre-treatment technology. The amounts of scale deposited onto the surface of a
heat exchanger tube were 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 SiOz fouling experiment found that
318
Deposition of Scale under Conditions which Simulate Sugar Mill Evaporators
the silica scale had a rippled surface similar to that described for SiOz in another study
[19]. 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 scale. This inner layer adhered to the tube wall and could
only be removed by mechanical means. These observations supported 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 an initial COM concentration of
50 ppm led to a complete coverage of the tube surface. As observed when scraping
off the deposits, this scale was more tenacious than the inner layer scale formed in the
SiOz single system. Composite deposits formed at initial COM concentrations of 20
and 100 ppm were composed of large isolated clusters, which appeared to be less
tenacious than the scale obtained with 50 ppm of COM.
Scanning electron microscopy was used to evaluate the structural and
morphological features of the scale deposits. Figures 6 and 7 present the electron
micrographs of SiOz and composite scales at various magnifications.
Figure 6. Scanning electron micrographs of SiO, scales: (a) inner layer; (b) outer
layer.
It was evident from Figures 6 (a) and (b) that the inner-layer scale of SiOz has a
different morphology from the outer layer of SiOz scale and the three composite
scales. The inner-layer scale appeared to be more compact than that of the outer-layer
scale, which had a porous structure. As mentioned earlier in fouling data analysis, this
difference in the structure would be expected to result in a hgher thermal resistance
for the outer-layer deposit, compared to the dense inner-layer deposit under the flowboiling conditions. Then the pores of the deposit were filled with a mixture of water
and steam which had a lower thermal conductivity than water [20]. However, the
composite deposits (see Figure 7a) 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]. Elemental composition of the
scale samples was obtained using energy dispersive x-ray microanalysis (EDS). EDS
spectra of the composite scales (such as the one shown in Figure 7b for 100 ppm
COM) revealed the presence of Ca in the deposit. This was later confirmed by XRD
319
H. Yu. R. Sheikholeslami and W O.S. Doherty
to be associated with COM (see next section) in addition to Si and 0 as the major
components of both silica and composite scales. Small levels of Na and C1 were also
present, possibly because of the entrapment and/or absorption of NaCl from the
solution.
Figure 7. SEM-EDS analysis of composite scale at COM of 100 ppm: (a) SEM
micrograph; (b) EDS spectrum.
Closer inspection o f the cross sections of the scales showed that the composite
scale from 50 ppm COM (Figure 8b) was more densely packed compared to the Si02
inner-layer scale, which appeared to contain a matrix of spheroids (Figure pa). Thls
difference might explain why t h s deposit was more tenacious than the Si02 scale.
Previous studies [2 1,221 on CaS04 and CaC03 fouling also found that the presence of
CaCO, led to an increase in the strength of CaS04 scale. The authors suggested that
the co-precipitated CaC03 might act as a bonding agent, cementing the structure of
the CaS04 scale layer. It is likely that a similar effect may have occurred in this study.
Figure 8. SEM cross-sections of SiOr scale and composite scale: (a) pure SiOI scale;
(b) composite scale at cokfof 50 ppm.
320
Deposition of Scale under Conditions which Simulate Sugar Mill Evaporators
X-ray powder diffraction analysis was conducted on the Si02 and composite
scales in order to identify the crystalline phases involved in the composite fouling.
The XRD spectra of all the scales gave a smooth broad peak or “halo” with a 26 value
of approximately 23’ caused by the presence of amorphous silica [23]. However, ths
peak appeared to be narrower for the composite scales compared to that of the pure
Si02scale. This was probably due to the re-organization of SiOz 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-0% structural unit
[23]. The spectra of composite scales at 50 and 100 ppm COM were also found to
contain additional sharp peaks (28 values of 14.9’, 24.4’, 31.7’, 38.3’ and 45.4’) due
to the presence of COM [24]. The spectrum for 20 ppm COM composite scale did not
show COM crystalline peaks, probably because of the low amount of COM formed.
Conclusions
The composite fouling of COM and Si02 has been studied under forced convective
sub-cooled 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/Si02 ratios. The results showed that the COM
fouling resistance began to rise immediately after the experiment started, the highest
extent of fouling occurred at an initial COM concentration of 50 ppm (initial COM S S
-2.6). The fouling resistance of pure Si02 from an initial concentration of 500 ppm
began to rise at a theoretical S S level of about 5.3. The overall fouling resistances of
COM were found to be much less than that of Si02 due to rapid bulk precipitation.
The process of Si02 deposition may involve different Si02 species at various degrees
of evaporation and Si02supersaturation, giving rise to the formation of two types of
Si02 deposits with different physical characteristics.
The presence of COM at an initial concentration of 50 ppm led to a synergistic
effect on composite fouling, which began at an earlier stage of evaporation
(theoretical SiOz S S level -2.0) than occurred for Si02 single system due to increased
particulate deposition. The presence of COM at an initial concentration of either 20 or
100 ppm, in contrast, shifted the onset of composite fouling to a later stage of the
evaporation cycle than that observed for pure Si02. Part of this difference may be
related to the changes in the magnitude of an interfacial energy barrier between the
surface of particle and wall, and the physical properties of the fouling species such as
particle size. The composite scale obtained from 50 ppm COM was found to be more
tenacious than Si02 scale, probably due to the cementing effect of COM on the
deposit structure, as shown by SEM analysis. The composite scales from 20 and
100 ppm COM were less tenacious than the 50 ppm COM composite scale due to the
incomplete coverage on the tube surface. Further tests on composite fouling of COM
and S O 2 in sucrose media is underway to provide a better understanding of the effect
of sucrose on composite fouling mechanisms, and the composite-scale characteristics.
Acknowledgements
Financial support from the Australian Research Council and The Sugar Research
Institute (SRI) of Australia, as well as research collaboration with SRI, are gratehlly
acknowledged.
321
H. Yu, R. Sheikholeslamiand W.O.S.Doherty
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scala, deposition, oxalate, mill, sugar, simulated, dynamics, calcium, conditions, silica, amorphous, evaporators
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