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Kinetics of fluidised bed melt granulation III: Tracer studies
Chemical Engineering Science 60 (2005) 3835 – 3845 www.elsevier.com/locate/ces
Kinetics of fluidised bed melt granulation III: Tracer studies H.S. Tan, A.D. Salman, M.J. Hounslow∗ Department of Chemical and Process Engineering, The University of Sheffield, UK Available online 18 April 2005
Abstract Previous work (Proceedings of World Congress on Particle Technology, Sydney 2002, 629–636.) has shown that granule breakage occurs during fluidised bed melt granulation (FBMG) and should not be neglected when it comes to addressing granulation kinetics. In the current work, we have developed and verified tracer experiments in FBMG, in an attempt to decouple the influence of granule size and age on breakage kinetics. The tracer data during granulation shows that granule breakage is occurring at a much slower rate than aggregation, while the breakage-only tracer experiments reveal the breakage selection rate to be independent of size at an approximate value of −0.01 s−1 . The observations allow us to deduce that the aggregation rate for smaller granules is actually faster than larger ones during granulation. 䉷 2005 Elsevier Ltd. All rights reserved. Keywords: Fluidised bed melt granulation; Tracer studies; Breakage; Kinetics
1. Introduction The first part of this series (Tan et al., 2004a) has investigated the influence of various operating conditions on the granule growth behaviours during fluidised bed melt granulation (FBMG). A series of rate processes were then identified for this type of granulation, which includes a range of aggregation activities coupled with binder solidification and breakage process. The second part of this series (Tan et al., 2004b) involved the use of an aggregation model in population balance modelling to describe the effective net growth during the process. The model was able to describe the data with great efficiency, but such an approach may not be mechanistically sufficient since breakage is also observed during FBMG (Biggs et al., 2002), and this prompts the need of a breakage model. It is therefore the purpose of this and the next in the series (Tan et al., 2004c) of our work to identify a physically sound breakage model, and subsequently quantify the breakage kinetics in FBMG by means of tracer experiments and population balance modelling. Fluidised bed granulation is a widely used industrial process in the manufacturing of a wide range of ∗ Corresponding author. Tel.: +44 114 222 7565; fax: +44 114 222 7566.
E-mail address: m.j.hounslow@sheffield.ac.uk (M.J. Hounslow). 0009-2509/$ - see front matter 䉷 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2005.02.009
granular products. This process generally works by spraying binder liquid onto a bed of fluidised particles, where upon wetting; the particles will be bound together by liquid bridges. Here we refer to the use of melt binder as the binding agent, where the binder liquid bridge will cool to solidify to form solid bridge at bed temperature lower than its melting point. The literature on identifying and quantifying the breakage kinetics during fluidised bed granulation is extremely limited. The latest work in estimating the breakage rate in FBMG was by Biggs et al. (2002), where they indirectly inferred the breakage process to be the reversal of the growth process based on the novel “spray-on”–“spray-off” experiment, and thereby deduced the breakage rate for a specific experiment. Such assumption may not be satisfactory from kinetics point of view, since the aggregation process is a second-order process in number concentrations while the breakage process is only a first-order process. This probably accounts for the discrepancies observed between their model prediction and experimental observation. Tracer studies are successfully employed in high shear granulation to understand the extent of breakage kinetics (Ramaker et al., 1998; Pearson et al., 2001; Van de Dries et al., 2003). These works reveal the potential of adding representative tracer granules into granulation experiment,
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thereby exploring the rate of breakage mechanism by tracking the flux of tracer granules across granule size during granulation. It is worth mentioning here that the approach taken here follows the work by Pearson et al. (2001), where tracer granules of known size and age are added to a standard experiment to decouple the effect of granule size and age on breakage rate. Our intention here is similar: to perform a well-defined tracer experiment in FBMG in order to gain an initial understanding on granule breakage mechanisms and kinetics in FBMG. In this work, a series of experiments were conducted to validate the behaviour of the tracer granules produced. A detailed study into the manufacturing of tracer granules and the method of tracer analysis will also be described fully. Additionally, a breakage-only tracer experiment will be reported to examine the degree of granule breakage when exposed to fluidisation in the absence of further binder addition.
2. Experimental 2.1. Materials The powder used in these experiments was glass ballotini supplied by Potters Europe (Type-AF 20409). The size distribution of these primary particles ranges from about 75 to 375 m with a volumetric median size of 175 m, while the density of glass ballotini is about 2500 kg m−3 . Flake polyethylene glycol (PEG) with an average molecular weight of 1500 Da was used as the binder. PEG1500 has a melting temperature range from 44 to 48 ◦ C with a viscosity of 0.065 Pa s measured at 70 ◦ C. A blue dye (Patent V80) was used to make the tracer granules. For all experiments described here, 600 g of glass ballotini and 90 g of PEG was used, corresponding to a binder to particle ratio (B/P ) of 0.15 (w/w). 2.2. Experimental setup The experiments reported here were carried out using a modified Strea-1 fluidised bed granulator (Niro Aeromatics) to improve the performance of granulation using melt binder. No filters have been used for these experiments, as the pressure drop across the filters affect the fluidisation behaviour which influences the reproducibility of the experiments. All the samples taken in this work were withdrawn using a sampling probe attached to the side of the conical vessel at approximately 10 cm from the bottom of the bed. Comparisons have been made to confirm that the granules withdrawn from the sampling probe are indeed comparable to the whole bed. Details on the functional operation of the granulator can be found in Tan et al. (2004a). The experimental conditions used for the manufacturing of the standard and tracer granules are summarised in Table 1.
Table 1 Standard operating conditions Superficial air velocity, Us , m s−1 Atomising air pressure, bar Bed temperature, ◦ C Binder spray rate, g min−1
0.97 (19.34a ) 1.5 (28.5 mb ) 32 8.4
a Superficial to initial minimum fluidising velocity, U /U . s mf bAverage median droplet size.
3. Experimental procedures 3.1. Standard experimental protocol The standard granules were manufactured according to the following experimental procedure: • The fluidising chamber was first heated by fluidising air to the designated operating bed temperature. • The fluidising air was then stopped and the glass ballotini (600 g) was weighed into the fluidising chamber. • The bed was fluidised for a short period (2–3 min) until the bed temperature had stabilised to the designated temperature. • The nozzle was inserted into the chamber, and the binder was pumped through for spraying. • The timing commenced once the spraying started and the fluidising air flow rate was immediately increased to the required flow rate. • The binder spray rate used in this work was calibrated at 8.4 g min−1 prior to spraying for each experiment. The mass of binder withdrawn per unit time was also continuously recorded every minute from the mass balance to ensure a consistent binder spray rate. 3.2. Preparation of tracer granules Tracer granules were produced in the same way as the standard granules above, but with the addition of a powdered blue dye. The step taken here is to premix the dye with powder before spraying the binder for granulation, which may seem to be a difficult mixing problem initially. However by comparison, this was found to be more efficient than spraying a dye-mixed binder solution onto the particles, which results in uneven dye distribution as a result of the maldistribution of binder across the powder bed. What follows describes the making of the tracer granules. • Approximately 1 g (corresponding to 0.2% by mass of ballotini) of patent V80 blue dye was manually premixed with 600 g of glass ballotini in a beaker. • The mixture was added into the fluidising chamber for further mixing by fluidisation. At this point, the whole bed turned blue within minutes and the uniformity of the dye distribution was examined by measuring the dye content of the mixture samples withdrawn from the bed (while
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Preparation of Tracer granules
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1g of Patent V blue dye + 600 g of glass ballotini Experiment conducted according to standard experiment protocol (Table 1) Experiment stopped at desire granule age (3, 6, 9, 10.45 minutes).
The whole bed of granules are sieved and stored according to desired sieve class
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Experiment conducted according to standard experiment protocol (Table 1)
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. Sieving into different sieve classes. . Granules per sieve class was weighed, dissolved, filtered, and diluted in 100ml volumetric flask. . The diluted solution was measured for its absorption using UV spectrophotometer
Fig. 1. Flow diagram for the tracer experiment.
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fluidising) using the UV spectrometer. Four samples have been measured in this case. The result shows that the average dye content on the particles is highly comparable and further inspection of individual particles under microscope confirms that they are highly uniformly dyed. We speculate that the surfaces of all the ballotini are probably saturated with dye and that the excess is blown away during fluidisation (since dye is found at the air outlet). This would naturally lead to uniform coverage. • The experiment was continued as normal according to the experiment described above by spraying the binder, and stopped when the granulation time reached the desired tracer granule age. For this work, tracer granules were produced at 3, 6, 9 and 10.75 min. • At this stage, all the granules in the chamber were removed and cooled to room temperature. They were then sieved into the required size fractions and retained for use in later experiments. In this study, tracer granules of 180, 500 and 1000 m were examined. To further inspect the dye distribution in the granules, granules of the investigated sieve class were divided into four portions and the concentration of the dye present in each portion was measured using the UV spectrometer. Again, the result does not reveal a strong sign of uneven dye distribution.
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Fig. 2. Comparison of the change in 4–3 mean size with time for standard and dyed experiment. Open symbol denotes the standard experiment; closed symbol denotes dyed experiment.
3.3. Standard tracer experiment For each tracer experiment, 6 g (that is, 1% by mass of glass ballotini) of previously sieved coloured granules of a certain age and size were added in. The sieved tracer
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granules were added after 3 min of the basic experiment, irrespective of their ages and sizes. The experiments in which these tracer granules were inserted into the system were carried out as follows: • Prior to addition of the prepared tracer granules, these granules were gently hand-sieved again to ensure any granules smaller than the desired size were omitted. • A granulation experiment was carried out following the basic recipe described in Table 1. After 3 min into the experiment, the sieved tracer granules were added into the bed from the top of the chamber, without stopping the experiment. The whole addition process took less than 10 s.
• The experiment was continued in the usual way with samples of about 27 g withdrawn each time at 4, 6, 8 and 10.75 min. The whole description of tracer experiment from the manufacturing to the addition of tracer granules into standard experiment is shown in the flow diagram in Fig. 1. 3.4. Breakage-only tracer experiment In this experiment, blue tracer granules are added to fluidise with a bed of normal granules at the standard experimental conditions (Table 1), but in the absence of further binder addition. Its purpose is to study the extent of gran-
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ule breakage under the fluidisation environment. Here, the bed of standard granules used were manufactured beforehand according to the standard experimental conditions in Table 1, but up to the point where B/P = 0.11. In this experiment, only the influence of granule size (180, 500 and 1000 m) is examined, and the tracer granules used are 3 min old. Similarly, only 6 g of the prepared tracer granules of known size and age were added to the fluidised granules. Samples of the granules were then withdrawn after 1, 2 and 5 min.
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4. Tracer validation 0.2
In order to justify the information obtained from the tracer experiment, it is essential for the tracer granules to be representative of the standard granules that they are mimicking during the granulation process. Therefore, various physical properties (e.g. size, binder content) of the tracer granules were compared with the standard granules in order to establish the functional equivalence of the tracer granules. 4.1. Mean size and evolution of granule size distribution Three standard experiments, and two for which the dye had been pre-mixed with the glass ballotini, were carried out in the Strea-1 granulator according to the experimental recipe and conditions described in Table 1 earlier. Samples were taken at several time intervals, and the four-three (or mass-weighted) mean sizes are compared in Fig. 2. The closed symbols represent those results from the dye containing experiments, and the open symbols are those for the standard experiments. The results here reveal little difference between the experiments conducted with and without dye, indicating the negligible effect of dye on the overall granule growth rate. For further comparison, the normalised mass-weighted granule size distribution (GSD) for the different samples withdrawn at different times are compared in Fig. 3. The dot symbol represents the standard experiment conducted without dye, while the lines are those with the presence of dye. Generally, it can be seen that the GSDs of the dye-containing granules are highly comparable to those of standard granules. The minor scatter of these experiments (with and without dye) suggests these tracer granules to behave closely to their standard counterparts, implying a negligible effect of dye on the overall granulation kinetics. 4.2. Stopped and resumed experiment The tracer granules prepared to be added into the standard tracer experiment were sieved and stored in a sample bag before the next use. It is therefore important to investigate if these granules still preserve the same properties as freshly prepared granules. For this purpose, a separate experiment
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Fig. 4. Comparison of the change in 4–3 mean size with time for normal dyed experiment and dyed “stopped and resumed” experiment. Open symbol denotes the normal dyed experiment; closed symbol denotes dyed “stopped and resumed” experiment.
using dye was conducted according to the following procedure: • Standard granulation was conducted with the dye added to the primary particles (similar to the preparation of the tracer granules). Sampling was conducted at the time interval similar to those in Fig. 2. • The experiment was stopped totally at 3 min, and the whole bed of granules was removed from the granulator. The mass of granules was evenly laid onto a tray, to prevent local agglomeration of the granules. These granules were covered under a closed environment with silica beads lying underneath to absorb the moisture in the stored environment. • The granules were left dried for a specific period of time. After this point, the fluidising chamber was reheated by the fluidising air to the designated temperature. The tray of granules that had been cooled was then gently poured into the chamber. The bed was slowly refluidised to the designated air flow rate, and spraying started again after this point. • Sampling of granules resumed as normal at the similar sampling time point shown in Fig. 2. The variation in mass-weighted mean size with time for this experiment can be seen Fig. 4. The open symbols represent the “stopped and resumed” experiment described, while the closed symbols represent the normal dye-containing experiments such as those shown in Fig. 2. It can be observed that the immediate size sampled at 3.5 min (or 0.5 min after the discontinuation of granulation) may be slightly smaller than those experiments performed without stopping. This is most probably due to granule breakage induced during the handling process from the time when the granulation was
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stopped till the point when it was resumed. Despite the slight initial decrement in size after the restarting of the experiment, the granules were still able to grow at a similar rate to those of a normal experiment. This simple experiment proves that despite the cooling and reheating of the granules, the granulation behaviour is not significantly affected.
results show that this particular internal characteristic of the standard granules is also well-represented by the tracer granules, indicating that the presence of dye on the particles does not interfere with the binder distribution with time.
5. Tracer analysis 4.3. Binder content The binder content within granules of different size classes and time was determined using a thermogravimetric technique similar to that used by Knight et al. (1998). Here, the granules of individual sieve classes were heated at 600 ◦ C for about 2 h to burn off the binder. The amount of granules used for each test is about 0.5–0.8 g, depending on the amount of the granules available in each sieve. The results of these experiments are shown in Fig. 5. Similarly, the closed symbols represent the dye-containing granules, and the open symbols represent those of standard experiments. It can be seen that the binder to particle (B/P ) ratio increases with time, and there is a slight tendency that bigger granules possess more binder than smaller granules initially, which gradually levels off with time. These
The method of tracer characterisation here is identical to that of Pearson et al. (2001). All the samples taken at different times during the experiment were sieved. Each sieve fraction was retained separately, and all the granules in each sieve size were dissolved in a known mass of distilled water. The resulting solution was vacuum-filtered through a 2 m filter membrane in order to remove the glass ballotini. Since the dye is soluble in water, it remained in the filtrate, resulting in a blue colour and the UV spectrophotometer was used to determine the intensity of colour in each sample. In order to calibrate the mass of dye in the spectrophotometer, solutions of known concentration were made based on a known amount of dye after several dilutions. With these solutions, it is possible to relate the absorption reading obtained from the spectrophotometer, to the known mass of
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Initial tracer size added Tracer mass distribution
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Fig. 7. TMD showing the relegated granules, .
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Dye mass fraction, x Fig. 6. Typical calibration plot for UV spectrophotometry illustrating the absorption of a solution as a function of the dye mass fraction.
dye concentration. The calibration chart was established by plotting the absorption values against the mass concentration, from which a linear fit can be made through all the points. The relation for the linear fit was then used to backcalculate the dye concentration in the experimental samples. Fig. 6 shows a typical calibration curve for spectrophotometry carried out for this work. In order to decide the wavelength used for the scanning of the samples, a broadspectrum scan was made on four dye solutions of different concentrations. It was evident that a major and minor peak appears in all cases, and the wavelength at which the major peak appeared was chosen for scanning. The major peak occurs at approximately 634–639 nm for the specific dye used. The analysis of dye concentration using the spectrophotometer is effective because both the PEG and the dye are water-soluble, and the presence of PEG in the filtered solution does not interfere with the absorption in the spectrophotometry measurement. From the concentration of dye in a given sieve interval, it is possible to calculate the total mass of dye in the granules that were used in the filtration experiments, mt, , Using this value, the overall mass of dye in a given size fraction, M¯ i , can be deduced with respect to the total mass of granules in that sieve interval: mt Wi M¯ i = × , (1) Wi ms where ms is the mass of granules used for the filtration, Wi is the mass of granules retained on the sieve interval, i, and Wi , is the total mass of granules sieved. The calculation above is normalised to 1 kg of granules. Using the tracer analysis described above, the evolution of the tracer mass distribution (TMD) with time can be obtained. The main purpose of this work is to extract the rate at which the granules of a certain size break, and how it breaks. Here, we characterised the extent of breakage using a similar
method to Pearson et al. (2001), in terms of the relegated granules. Effectively, we can obtain the mass fraction of dye in a given size fraction by normalising it to the total mass fraction of dye in a sample. As we are inserting tracer granules of known size into the system, the dye that appears in sizes smaller than the initial tracer size can be regarded as the “relegated” granules. Therefore the amount of relegated granules, or fraction of dye, can be essentially summed up over all sizes smaller than the initial tracer size. Here we use the same notation, , as in Pearson et al. (2001). This can be illustrated with a schematic TMD (Fig. 7), where denotes the area under the graph (i.e., mass fraction of tracer granules) smaller than the initial tracer size. It was proposed that the rate at which the dyed granules appear in the lower sizes is equal to the rate at which it is broken out of the larger sizes. If the rate at which the dye-containing particles are being selected for breakage is S0 (s−1 ) , then we can effectively represent the rate of change of the mass of tracer, M1− (larger than the minimum tracer size) by breakage is dM1− = −S0 (t)M1− . dt
(2)
It follows that d d (1 − ) = −S0 (t)(1 − ) or ln(1 − ) = −S0 (t). (3) dt dt So from a TMD, the initial decay of a plot of ln(1 − ) with time will have a slope that relates to the instantaneous breakage rate for size ranges larger than the input tracer size. Results obtained in this manner are correct in the absence of re-aggregation back into the original size classes. This is reasonable for the experiments conducted here, as the first sample was taken within a minute upon the addition of the tracer granules. To estimate the gradient of the initial slope, a first-order difference over the first two time points is used to obtain an underestimated selection rate constant.
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6. Results and discussions
transfer of dyed granules to smaller size classes is mainly due to particle attrition and breakage.
6.1. Tracer mass distribution 6.2. Impact of sieving on tracer results Fig. 8 shows a typical TMD from the tracer experiment described above. It is evident from the evolution of the TMD, that upon addition of tracer granules into the standard experiment, the dyed materials, in terms of tracer granules, both appear in lower and larger size classes than the initial tracer granules. This is as expected due to the nature of the experiment, where the dye is contained within granules greater than a certain size. With time, it is possible to observe a shift in TMD towards the larger size, suggesting the dominance of coalescence over breakage as the process proceeds. Pearson et al. (2001) mentioned that the appearance of dye in smaller size interval may be due to two reasons: (a) direct transfer of dye from larger granules to smaller ones, in terms of “wicking” of dye-containing binder upon granules collision, (b) breakage of granules. The first cause is very unlikely for FBMG due to the fast binder droplet solidification time (in the order of millisecond (Van de Scheur et al., 1998)), making it virtually impossible for the binder to stain the dye and transfer onto other particles in a short time. Thus we can deduce with confidence that the
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6.3. Effect of granule age To investigate the influence of tracer granule age on the breakage rate, four different tracer granules of age 3, 6, 9 and
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As the granules manufactured from FBMG are generally weak, there is the possibility that the granules may be broken during the process of sieving as part of the tracer analysis process. A separate sieving test was therefore conducted to examine this uncertainty. Here, only sieved tracer granules of 1 mm (1 g) were sieved together with 30 g of standard granules. The end result was taken for tracer analysis as described before. Fig. 9 shows the normalised TMD for such result, which reveals a negligible contribution to tracer granule breakage due to sieving, as observed from the tiny amount of dye materials found in size classes smaller than 1 mm. This proves the relegation of tracer granules shown earlier was indeed the consequence of granules breaking during granulation.
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Fig. 8. Typical evolution of TMD with time for tracer granules of 3 min old and 1 mm in size.
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10.45 min at a constant tracer size of 1000–1180 m were used. Fig. 10 shows the plot of variation of 1− with time for different ages. As mentioned, the main purpose of this work is to estimate the rate at which the granule breaks. This was done by making a first-order difference between first two time points, which estimates the initial slopes of granules of different ages to be at an approximate rate of −0.01 s−1 . From this plot, there is however no strong evidence proving a considerable effect of granule age on breakage rate. In high shear granulation (Pearson et al., 2001), the older granules are observed to be stronger than the younger ones due to increased granule consolidation with time, thereby leading to smaller breakage rate with older granules. The observation in the present work is to be expected due to the low shear environment in fluidised bed and short binder solidification time, leading to low granule densification and thus causing little variation in granule strength with time.
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6.4. Effect of granule size For size effect, three different sizes of tracer granules were used: 180–212, 500–600, 1000–1180 m. Fig. 11 illustrates the change in 1 − with time for different granule sizes, which clearly reveals the larger granules (1090 m) to experience more breakage than those of smaller ones. One obvious but misleading explanation is likely due to the presence of more flaws existing in larger granules, since the large granules are effectively the agglomeration of smaller granules with weaker bonds between these sub-granules. This will be further explained in the later section, as we are able to show that this is essentially because the smaller granules probably are growing faster than its larger counterparts as the granulation proceeds.
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Fig. 11. Fraction of tracer smaller than original tracer granules added for tracers of three different sizes at a constant age of 3 min.
6.5. Breakage mechanisms With the use of tracer granules, we also intend to identify unambiguously the different dominating mechanisms during the process. Fig. 12 shows the granules at the end of the process after the addition of 1 mm tracer granules of 3 min old. The coalescence between the blue and white granules observed in Fig. 12(a) is retrieved in a 1 mm sieve, and this is effectively due to the coalescence of a white granule with part of a broken tracer granule. In the same batch of
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Fig. 12. Microscopic pictures of tracer granules at the end of the process.
Breakage
Attrition
Fig. 13. Possible breakage mechanisms during fluidised bed granulation.
granules, smaller tracer granules can be found in the smaller sieve fraction of 212 m. Since the tracer granules added originate from 1 mm in size, these doublets and single tracer particles are likely the result of attrition occurring during the process. The result from the tracer size distribution (Fig. 8) provides further evidence that a granule may break to form a few larger fragments and some smaller fragments. Similar observations can also be found in Boerefijn et al. (1998) and Ning et al. (1997) while observing the breakage of granule upon impact. Fig. 13 summarises the breakage mechanisms observed in this work, illustrating granule upon impact, to break into a few larger fragments by breakage and some smaller fragments (primaries and doublets) by attrition process.
Compared to the spray-on tracer experiment (where the breakage rate for smaller granules is lower), the discrepancy in the observation here strongly suggests that the smaller granules seems to grow faster than the larger granules in the presence of binder during granulation, which would otherwise break at a similar rate as the larger granules in the absence of binder addition. On this basis, the smaller granules appear more likely to aggregate than the larger ones during granulation, which probably explains the subsequent decrease in span of the GSDs with time during granulation (Tan et al., 2004a). This behaviour is further supported by the success of the modelling work demonstrated in the previous paper (Tan et al., 2004b), where the net growth process in the FBMG experiment can be well described by the equipartition of kinetic energy (EKE) model which favours the interaction between small and large granules.
6.6. Breakage—only tracer experiment Fig. 14 shows the change in 1 − with time for different granule size. In the absence of binder addition, the initial breakage rate can be observed to be fairly similar for all the three sizes. The breakage rate can also be seen to have reduced after 2 min for 180 or 500 m, while it seems to continue for 1000 m. Compared to the spray-on experiment, the initial breakage rate for the 1000 m granule can also be estimated to be in the order of −0.01 s−1 , further suggesting that there is probably no re-aggregation of granules into the original size class. The result here reveals no significant effect of size, confirming the size independence of the breakage selection rate. However the observed reduction in the slope with time suggests a decrease in the breakage selection rate, indicating that the granules are probably heterogenous in nature as observed by Hounslow et al. (2001) in high shear granulation.
7. Conclusion In general, a similar tracer analysis method as Pearson et al. (2001) is employed here to reveal the effect of granule size and age on breakage during fluidised bed granulation. The first part of this work mainly concentrates on validating the reliability of tracer granules in mimicking the standard granules. Several experiments that involve the comparison of the change in mean size, size distribution and binder content with time of standard and tracer granulation have all demonstrated a comparable behaviour between the tracer and standard granules. The nature of spray-on tracer experiment is such that tracer granules of known size and age are added to the standard experiment during the process. The evidence provided by the photographic examination of the granule mixture and the tracer mass distribution allows us to deduce that a gran-
H.S. Tan et al. / Chemical Engineering Science 60 (2005) 3835 – 3845
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Abbreviation
1 0.8
FBMG PEG TMD GSD
0.6
1- χ
0.4
fluidised bed melt granulation polyethylene glycol tracer mass distribution granule size distribution
Acknowledgements 0.2 1000 µm 500 µm 180 µm
0.1 0
50
100
150
200
250
300
Fig. 14. Fraction of tracer relegated for tracers of three different sizes at a constant age of 3 min for spray-off experiment.
ule is likely to break into a few larger fragments and some smaller fragments (i.e., primaries). From the tracer relegation plot on spray-on tracer experiment, it can be preliminarily deduced that larger granules are more prone to break than smaller granules during granulation. However this is contradicted by the breakage-only tracer experiment, which reveals the granule breakage rate to be size independent in the absence of further binder addition. This result suggests that the smaller granules are growing faster than its larger counterparts during the granulation process. The analysis from the spray-on tracer experiment also depicts the granule breakage rate to be independent of granule age, which is probably due to the lack of granule consolidation in the low shear fluidising environment. Notation binder mass of dye in granules used for filtration, kg mass of granules used for filtration, kg overall mass of dye in a given size class, kg particle breakage selection rate, s−1 time, s superficial velocity, m s−1 mass of granules retained in a sieve interval
Greek letter
fraction of relegated granules (–)
References
350
Time (s)
B mt ms M P S0 t Us W
The authors would like to acknowledge EPSRC for funding this project and Unilever for supplying the materials.
Biggs, C.A., Boerefijn, R., Buscan, M., Salman, A.D., Hounslow, M.J., 2002. Fluidised bed granulation: modelling the growth and breakage kinetics using population balances. Proceedings of World Congress on Particle Technology, Sydney, 629–636. Boerefijn, R., Ning, Z., Ghadiri, M., 1998. Disintegration of weak lactose agglomerates for inhalation applications. International Journal of Pharm. 172, 199–209. Hounslow, M.J., Pearson, J.M.K., Instone, T., 2001. Tracer studies of highshear granulation II: population balance modeling. A.I.Ch.E. Journal 47 (9), 1984–1999. Knight, P.C., Instone, T., Pearson, J.M.K., Hounslow, M.J., 1998. An investigation into the kinetics of liquid distribution and growth in high shear mixer agglomeration. Powder Technology 97, 246. Ning, Z., Boerefijn, R., Ghadiri, M., Thornton, C., 1997. Distinct element simulation of impact breakage of lactose agglomerates. Advances in Powder Technology 8 (1), 1–23. Pearson, J.M.K., Hounslow, M.J., Instone, T., 2001. Tracer studies of high-shear granulation I: experimental results. A.I.Ch.E. Journal 47, 1978–1983. Ramaker, J.S., Jelgersma, M.A., Vonk, P., Kossen, N.W.F., 1998. Scaledown of a high shear pelletisation process: flow profile and growth kinetics. International Journal of Pharmaceutics 166, 89–97. Tan, H.S., Salman, A.D., Hounslow, M.J., 2004a. Kinetics of fluidised bed melt granulation I: effect of process variables. Chemical Engineering Science, submitted. Tan, H.S., Salman, A.D., Hounslow, M.J., 2004b. Kinetics of fluidised bed melt granulation II: modelling the net rate of growth. Chemical Engineering Science, submitted. Tan, H.S., Salman, A.D., Hounslow, M.J., 2004c. Kinetics of fluidised bed melt granulation IV: selecting the breakage model. Powder Technology 143–144, 65–83. Van de Dries, K., De Vegta, O.M., Girarda, V., Vromans, H., 2003. Granule breakage phenomena in a high shear mixer; influence of process and formulation variables and consequences on granule homogeneity. Powder Technology 133, 228–236. Van der Scheur, F.T., Goedendorp, P.L., Goot, A.J., van der Olieman, J.J., 1998. Fluidised bed agglomeration with polyethylene glycol melt binder: effects of bed temperature and droplet size. Proceedings of World Congress of Particle Technology 3, 103.
Kinetics of fluidised bed melt granulation III: Tracer studies H.S. Tan, A.D. Salman, M.J. Hounslow∗ Department of Chemical and Process Engineering, The University of Sheffield, UK Available online 18 April 2005
Abstract Previous work (Proceedings of World Congress on Particle Technology, Sydney 2002, 629–636.) has shown that granule breakage occurs during fluidised bed melt granulation (FBMG) and should not be neglected when it comes to addressing granulation kinetics. In the current work, we have developed and verified tracer experiments in FBMG, in an attempt to decouple the influence of granule size and age on breakage kinetics. The tracer data during granulation shows that granule breakage is occurring at a much slower rate than aggregation, while the breakage-only tracer experiments reveal the breakage selection rate to be independent of size at an approximate value of −0.01 s−1 . The observations allow us to deduce that the aggregation rate for smaller granules is actually faster than larger ones during granulation. 䉷 2005 Elsevier Ltd. All rights reserved. Keywords: Fluidised bed melt granulation; Tracer studies; Breakage; Kinetics
1. Introduction The first part of this series (Tan et al., 2004a) has investigated the influence of various operating conditions on the granule growth behaviours during fluidised bed melt granulation (FBMG). A series of rate processes were then identified for this type of granulation, which includes a range of aggregation activities coupled with binder solidification and breakage process. The second part of this series (Tan et al., 2004b) involved the use of an aggregation model in population balance modelling to describe the effective net growth during the process. The model was able to describe the data with great efficiency, but such an approach may not be mechanistically sufficient since breakage is also observed during FBMG (Biggs et al., 2002), and this prompts the need of a breakage model. It is therefore the purpose of this and the next in the series (Tan et al., 2004c) of our work to identify a physically sound breakage model, and subsequently quantify the breakage kinetics in FBMG by means of tracer experiments and population balance modelling. Fluidised bed granulation is a widely used industrial process in the manufacturing of a wide range of ∗ Corresponding author. Tel.: +44 114 222 7565; fax: +44 114 222 7566.
E-mail address: m.j.hounslow@sheffield.ac.uk (M.J. Hounslow). 0009-2509/$ - see front matter 䉷 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2005.02.009
granular products. This process generally works by spraying binder liquid onto a bed of fluidised particles, where upon wetting; the particles will be bound together by liquid bridges. Here we refer to the use of melt binder as the binding agent, where the binder liquid bridge will cool to solidify to form solid bridge at bed temperature lower than its melting point. The literature on identifying and quantifying the breakage kinetics during fluidised bed granulation is extremely limited. The latest work in estimating the breakage rate in FBMG was by Biggs et al. (2002), where they indirectly inferred the breakage process to be the reversal of the growth process based on the novel “spray-on”–“spray-off” experiment, and thereby deduced the breakage rate for a specific experiment. Such assumption may not be satisfactory from kinetics point of view, since the aggregation process is a second-order process in number concentrations while the breakage process is only a first-order process. This probably accounts for the discrepancies observed between their model prediction and experimental observation. Tracer studies are successfully employed in high shear granulation to understand the extent of breakage kinetics (Ramaker et al., 1998; Pearson et al., 2001; Van de Dries et al., 2003). These works reveal the potential of adding representative tracer granules into granulation experiment,
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thereby exploring the rate of breakage mechanism by tracking the flux of tracer granules across granule size during granulation. It is worth mentioning here that the approach taken here follows the work by Pearson et al. (2001), where tracer granules of known size and age are added to a standard experiment to decouple the effect of granule size and age on breakage rate. Our intention here is similar: to perform a well-defined tracer experiment in FBMG in order to gain an initial understanding on granule breakage mechanisms and kinetics in FBMG. In this work, a series of experiments were conducted to validate the behaviour of the tracer granules produced. A detailed study into the manufacturing of tracer granules and the method of tracer analysis will also be described fully. Additionally, a breakage-only tracer experiment will be reported to examine the degree of granule breakage when exposed to fluidisation in the absence of further binder addition.
2. Experimental 2.1. Materials The powder used in these experiments was glass ballotini supplied by Potters Europe (Type-AF 20409). The size distribution of these primary particles ranges from about 75 to 375 m with a volumetric median size of 175 m, while the density of glass ballotini is about 2500 kg m−3 . Flake polyethylene glycol (PEG) with an average molecular weight of 1500 Da was used as the binder. PEG1500 has a melting temperature range from 44 to 48 ◦ C with a viscosity of 0.065 Pa s measured at 70 ◦ C. A blue dye (Patent V80) was used to make the tracer granules. For all experiments described here, 600 g of glass ballotini and 90 g of PEG was used, corresponding to a binder to particle ratio (B/P ) of 0.15 (w/w). 2.2. Experimental setup The experiments reported here were carried out using a modified Strea-1 fluidised bed granulator (Niro Aeromatics) to improve the performance of granulation using melt binder. No filters have been used for these experiments, as the pressure drop across the filters affect the fluidisation behaviour which influences the reproducibility of the experiments. All the samples taken in this work were withdrawn using a sampling probe attached to the side of the conical vessel at approximately 10 cm from the bottom of the bed. Comparisons have been made to confirm that the granules withdrawn from the sampling probe are indeed comparable to the whole bed. Details on the functional operation of the granulator can be found in Tan et al. (2004a). The experimental conditions used for the manufacturing of the standard and tracer granules are summarised in Table 1.
Table 1 Standard operating conditions Superficial air velocity, Us , m s−1 Atomising air pressure, bar Bed temperature, ◦ C Binder spray rate, g min−1
0.97 (19.34a ) 1.5 (28.5 mb ) 32 8.4
a Superficial to initial minimum fluidising velocity, U /U . s mf bAverage median droplet size.
3. Experimental procedures 3.1. Standard experimental protocol The standard granules were manufactured according to the following experimental procedure: • The fluidising chamber was first heated by fluidising air to the designated operating bed temperature. • The fluidising air was then stopped and the glass ballotini (600 g) was weighed into the fluidising chamber. • The bed was fluidised for a short period (2–3 min) until the bed temperature had stabilised to the designated temperature. • The nozzle was inserted into the chamber, and the binder was pumped through for spraying. • The timing commenced once the spraying started and the fluidising air flow rate was immediately increased to the required flow rate. • The binder spray rate used in this work was calibrated at 8.4 g min−1 prior to spraying for each experiment. The mass of binder withdrawn per unit time was also continuously recorded every minute from the mass balance to ensure a consistent binder spray rate. 3.2. Preparation of tracer granules Tracer granules were produced in the same way as the standard granules above, but with the addition of a powdered blue dye. The step taken here is to premix the dye with powder before spraying the binder for granulation, which may seem to be a difficult mixing problem initially. However by comparison, this was found to be more efficient than spraying a dye-mixed binder solution onto the particles, which results in uneven dye distribution as a result of the maldistribution of binder across the powder bed. What follows describes the making of the tracer granules. • Approximately 1 g (corresponding to 0.2% by mass of ballotini) of patent V80 blue dye was manually premixed with 600 g of glass ballotini in a beaker. • The mixture was added into the fluidising chamber for further mixing by fluidisation. At this point, the whole bed turned blue within minutes and the uniformity of the dye distribution was examined by measuring the dye content of the mixture samples withdrawn from the bed (while
H.S. Tan et al. / Chemical Engineering Science 60 (2005) 3835 – 3845
Preparation of Tracer granules
. . .
1g of Patent V blue dye + 600 g of glass ballotini Experiment conducted according to standard experiment protocol (Table 1) Experiment stopped at desire granule age (3, 6, 9, 10.45 minutes).
The whole bed of granules are sieved and stored according to desired sieve class
3837
Standard experiment
Experiment conducted according to standard experiment protocol (Table 1)
3 min
Sampling time
Addition of tracer granules (6g)
4 min 6 min
Process time
8 min 10.75 min
Tracer analysis
. Sieving into different sieve classes. . Granules per sieve class was weighed, dissolved, filtered, and diluted in 100ml volumetric flask. . The diluted solution was measured for its absorption using UV spectrophotometer
Fig. 1. Flow diagram for the tracer experiment.
1
0.8 D4,3 (mm)
fluidising) using the UV spectrometer. Four samples have been measured in this case. The result shows that the average dye content on the particles is highly comparable and further inspection of individual particles under microscope confirms that they are highly uniformly dyed. We speculate that the surfaces of all the ballotini are probably saturated with dye and that the excess is blown away during fluidisation (since dye is found at the air outlet). This would naturally lead to uniform coverage. • The experiment was continued as normal according to the experiment described above by spraying the binder, and stopped when the granulation time reached the desired tracer granule age. For this work, tracer granules were produced at 3, 6, 9 and 10.75 min. • At this stage, all the granules in the chamber were removed and cooled to room temperature. They were then sieved into the required size fractions and retained for use in later experiments. In this study, tracer granules of 180, 500 and 1000 m were examined. To further inspect the dye distribution in the granules, granules of the investigated sieve class were divided into four portions and the concentration of the dye present in each portion was measured using the UV spectrometer. Again, the result does not reveal a strong sign of uneven dye distribution.
0.6
0.4
0.2 0
2
4 6 Time (min)
8
10
Fig. 2. Comparison of the change in 4–3 mean size with time for standard and dyed experiment. Open symbol denotes the standard experiment; closed symbol denotes dyed experiment.
3.3. Standard tracer experiment For each tracer experiment, 6 g (that is, 1% by mass of glass ballotini) of previously sieved coloured granules of a certain age and size were added in. The sieved tracer
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H.S. Tan et al. / Chemical Engineering Science 60 (2005) 3835 – 3845 t=2.min
t=0.min 0.4 l w(l) (kg/kg)
l w(l) (kg/kg)
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5 10.0
0.4
0.4
0.3 0.2 0.1 0.05 0.1
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0.05 0.1
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l (mm)
t=9.5min
t=10.75 min 0.4 l w(l) (kg/kg)
0.4 l w(l) (kg/kg)
0.5 1 l (mm)
0.3 0.2 0.1 0.05 0.1
0.5 1
5 10.0
l (mm)
0.3 0.2 0.1 0.05 0.1
0.5 1
5 10.0
l (mm)
Fig. 3. Comparison of mass-weighted size distributions with time between standard and dyed experiments. Closed symbol denotes the standard experiment; lines denotes dyed experiment.
granules were added after 3 min of the basic experiment, irrespective of their ages and sizes. The experiments in which these tracer granules were inserted into the system were carried out as follows: • Prior to addition of the prepared tracer granules, these granules were gently hand-sieved again to ensure any granules smaller than the desired size were omitted. • A granulation experiment was carried out following the basic recipe described in Table 1. After 3 min into the experiment, the sieved tracer granules were added into the bed from the top of the chamber, without stopping the experiment. The whole addition process took less than 10 s.
• The experiment was continued in the usual way with samples of about 27 g withdrawn each time at 4, 6, 8 and 10.75 min. The whole description of tracer experiment from the manufacturing to the addition of tracer granules into standard experiment is shown in the flow diagram in Fig. 1. 3.4. Breakage-only tracer experiment In this experiment, blue tracer granules are added to fluidise with a bed of normal granules at the standard experimental conditions (Table 1), but in the absence of further binder addition. Its purpose is to study the extent of gran-
H.S. Tan et al. / Chemical Engineering Science 60 (2005) 3835 – 3845
1
0.8 D4,3 (mm)
ule breakage under the fluidisation environment. Here, the bed of standard granules used were manufactured beforehand according to the standard experimental conditions in Table 1, but up to the point where B/P = 0.11. In this experiment, only the influence of granule size (180, 500 and 1000 m) is examined, and the tracer granules used are 3 min old. Similarly, only 6 g of the prepared tracer granules of known size and age were added to the fluidised granules. Samples of the granules were then withdrawn after 1, 2 and 5 min.
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0.6
0.4
4. Tracer validation 0.2
In order to justify the information obtained from the tracer experiment, it is essential for the tracer granules to be representative of the standard granules that they are mimicking during the granulation process. Therefore, various physical properties (e.g. size, binder content) of the tracer granules were compared with the standard granules in order to establish the functional equivalence of the tracer granules. 4.1. Mean size and evolution of granule size distribution Three standard experiments, and two for which the dye had been pre-mixed with the glass ballotini, were carried out in the Strea-1 granulator according to the experimental recipe and conditions described in Table 1 earlier. Samples were taken at several time intervals, and the four-three (or mass-weighted) mean sizes are compared in Fig. 2. The closed symbols represent those results from the dye containing experiments, and the open symbols are those for the standard experiments. The results here reveal little difference between the experiments conducted with and without dye, indicating the negligible effect of dye on the overall granule growth rate. For further comparison, the normalised mass-weighted granule size distribution (GSD) for the different samples withdrawn at different times are compared in Fig. 3. The dot symbol represents the standard experiment conducted without dye, while the lines are those with the presence of dye. Generally, it can be seen that the GSDs of the dye-containing granules are highly comparable to those of standard granules. The minor scatter of these experiments (with and without dye) suggests these tracer granules to behave closely to their standard counterparts, implying a negligible effect of dye on the overall granulation kinetics. 4.2. Stopped and resumed experiment The tracer granules prepared to be added into the standard tracer experiment were sieved and stored in a sample bag before the next use. It is therefore important to investigate if these granules still preserve the same properties as freshly prepared granules. For this purpose, a separate experiment
0
2
4 6 Time (min)
8
10
Fig. 4. Comparison of the change in 4–3 mean size with time for normal dyed experiment and dyed “stopped and resumed” experiment. Open symbol denotes the normal dyed experiment; closed symbol denotes dyed “stopped and resumed” experiment.
using dye was conducted according to the following procedure: • Standard granulation was conducted with the dye added to the primary particles (similar to the preparation of the tracer granules). Sampling was conducted at the time interval similar to those in Fig. 2. • The experiment was stopped totally at 3 min, and the whole bed of granules was removed from the granulator. The mass of granules was evenly laid onto a tray, to prevent local agglomeration of the granules. These granules were covered under a closed environment with silica beads lying underneath to absorb the moisture in the stored environment. • The granules were left dried for a specific period of time. After this point, the fluidising chamber was reheated by the fluidising air to the designated temperature. The tray of granules that had been cooled was then gently poured into the chamber. The bed was slowly refluidised to the designated air flow rate, and spraying started again after this point. • Sampling of granules resumed as normal at the similar sampling time point shown in Fig. 2. The variation in mass-weighted mean size with time for this experiment can be seen Fig. 4. The open symbols represent the “stopped and resumed” experiment described, while the closed symbols represent the normal dye-containing experiments such as those shown in Fig. 2. It can be observed that the immediate size sampled at 3.5 min (or 0.5 min after the discontinuation of granulation) may be slightly smaller than those experiments performed without stopping. This is most probably due to granule breakage induced during the handling process from the time when the granulation was
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0.20
0.20 6 minutes
4 minutes 0.15
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B/P
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0.5
1.0
1.5
2.0
2.5
Size (mm)
0.0 (d)
0.5
1.0
1.5
Size (mm)
Fig. 5. Binder to particle ratio in granules of different size classes at different times. Open symbol denotes standard granules; closed symbols denote tracer granules.
stopped till the point when it was resumed. Despite the slight initial decrement in size after the restarting of the experiment, the granules were still able to grow at a similar rate to those of a normal experiment. This simple experiment proves that despite the cooling and reheating of the granules, the granulation behaviour is not significantly affected.
results show that this particular internal characteristic of the standard granules is also well-represented by the tracer granules, indicating that the presence of dye on the particles does not interfere with the binder distribution with time.
5. Tracer analysis 4.3. Binder content The binder content within granules of different size classes and time was determined using a thermogravimetric technique similar to that used by Knight et al. (1998). Here, the granules of individual sieve classes were heated at 600 ◦ C for about 2 h to burn off the binder. The amount of granules used for each test is about 0.5–0.8 g, depending on the amount of the granules available in each sieve. The results of these experiments are shown in Fig. 5. Similarly, the closed symbols represent the dye-containing granules, and the open symbols represent those of standard experiments. It can be seen that the binder to particle (B/P ) ratio increases with time, and there is a slight tendency that bigger granules possess more binder than smaller granules initially, which gradually levels off with time. These
The method of tracer characterisation here is identical to that of Pearson et al. (2001). All the samples taken at different times during the experiment were sieved. Each sieve fraction was retained separately, and all the granules in each sieve size were dissolved in a known mass of distilled water. The resulting solution was vacuum-filtered through a 2 m filter membrane in order to remove the glass ballotini. Since the dye is soluble in water, it remained in the filtrate, resulting in a blue colour and the UV spectrophotometer was used to determine the intensity of colour in each sample. In order to calibrate the mass of dye in the spectrophotometer, solutions of known concentration were made based on a known amount of dye after several dilutions. With these solutions, it is possible to relate the absorption reading obtained from the spectrophotometer, to the known mass of
H.S. Tan et al. / Chemical Engineering Science 60 (2005) 3835 – 3845
Initial tracer size added Tracer mass distribution
0.20
0.16
Absorption, A
3841
0.12
0.08
χ
1-χ
Size
0.04
Fig. 7. TMD showing the relegated granules, .
0.00 0
4x10-9
8x10-9
12x10-9
16x10-9
Dye mass fraction, x Fig. 6. Typical calibration plot for UV spectrophotometry illustrating the absorption of a solution as a function of the dye mass fraction.
dye concentration. The calibration chart was established by plotting the absorption values against the mass concentration, from which a linear fit can be made through all the points. The relation for the linear fit was then used to backcalculate the dye concentration in the experimental samples. Fig. 6 shows a typical calibration curve for spectrophotometry carried out for this work. In order to decide the wavelength used for the scanning of the samples, a broadspectrum scan was made on four dye solutions of different concentrations. It was evident that a major and minor peak appears in all cases, and the wavelength at which the major peak appeared was chosen for scanning. The major peak occurs at approximately 634–639 nm for the specific dye used. The analysis of dye concentration using the spectrophotometer is effective because both the PEG and the dye are water-soluble, and the presence of PEG in the filtered solution does not interfere with the absorption in the spectrophotometry measurement. From the concentration of dye in a given sieve interval, it is possible to calculate the total mass of dye in the granules that were used in the filtration experiments, mt, , Using this value, the overall mass of dye in a given size fraction, M¯ i , can be deduced with respect to the total mass of granules in that sieve interval: mt Wi M¯ i = × , (1) Wi ms where ms is the mass of granules used for the filtration, Wi is the mass of granules retained on the sieve interval, i, and Wi , is the total mass of granules sieved. The calculation above is normalised to 1 kg of granules. Using the tracer analysis described above, the evolution of the tracer mass distribution (TMD) with time can be obtained. The main purpose of this work is to extract the rate at which the granules of a certain size break, and how it breaks. Here, we characterised the extent of breakage using a similar
method to Pearson et al. (2001), in terms of the relegated granules. Effectively, we can obtain the mass fraction of dye in a given size fraction by normalising it to the total mass fraction of dye in a sample. As we are inserting tracer granules of known size into the system, the dye that appears in sizes smaller than the initial tracer size can be regarded as the “relegated” granules. Therefore the amount of relegated granules, or fraction of dye, can be essentially summed up over all sizes smaller than the initial tracer size. Here we use the same notation, , as in Pearson et al. (2001). This can be illustrated with a schematic TMD (Fig. 7), where denotes the area under the graph (i.e., mass fraction of tracer granules) smaller than the initial tracer size. It was proposed that the rate at which the dyed granules appear in the lower sizes is equal to the rate at which it is broken out of the larger sizes. If the rate at which the dye-containing particles are being selected for breakage is S0 (s−1 ) , then we can effectively represent the rate of change of the mass of tracer, M1− (larger than the minimum tracer size) by breakage is dM1− = −S0 (t)M1− . dt
(2)
It follows that d d (1 − ) = −S0 (t)(1 − ) or ln(1 − ) = −S0 (t). (3) dt dt So from a TMD, the initial decay of a plot of ln(1 − ) with time will have a slope that relates to the instantaneous breakage rate for size ranges larger than the input tracer size. Results obtained in this manner are correct in the absence of re-aggregation back into the original size classes. This is reasonable for the experiments conducted here, as the first sample was taken within a minute upon the addition of the tracer granules. To estimate the gradient of the initial slope, a first-order difference over the first two time points is used to obtain an underestimated selection rate constant.
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6. Results and discussions
transfer of dyed granules to smaller size classes is mainly due to particle attrition and breakage.
6.1. Tracer mass distribution 6.2. Impact of sieving on tracer results Fig. 8 shows a typical TMD from the tracer experiment described above. It is evident from the evolution of the TMD, that upon addition of tracer granules into the standard experiment, the dyed materials, in terms of tracer granules, both appear in lower and larger size classes than the initial tracer granules. This is as expected due to the nature of the experiment, where the dye is contained within granules greater than a certain size. With time, it is possible to observe a shift in TMD towards the larger size, suggesting the dominance of coalescence over breakage as the process proceeds. Pearson et al. (2001) mentioned that the appearance of dye in smaller size interval may be due to two reasons: (a) direct transfer of dye from larger granules to smaller ones, in terms of “wicking” of dye-containing binder upon granules collision, (b) breakage of granules. The first cause is very unlikely for FBMG due to the fast binder droplet solidification time (in the order of millisecond (Van de Scheur et al., 1998)), making it virtually impossible for the binder to stain the dye and transfer onto other particles in a short time. Thus we can deduce with confidence that the
3min
6.3. Effect of granule age To investigate the influence of tracer granule age on the breakage rate, four different tracer granules of age 3, 6, 9 and
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Tracer Mass Fraction (%)
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Tracer Mass Fraction (%)
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Size (mm)
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10.75min
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Tracer Mass Fraction (%)
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Tracer Mass Fraction (%)
Tracer Mass Fraction (%)
As the granules manufactured from FBMG are generally weak, there is the possibility that the granules may be broken during the process of sieving as part of the tracer analysis process. A separate sieving test was therefore conducted to examine this uncertainty. Here, only sieved tracer granules of 1 mm (1 g) were sieved together with 30 g of standard granules. The end result was taken for tracer analysis as described before. Fig. 9 shows the normalised TMD for such result, which reveals a negligible contribution to tracer granule breakage due to sieving, as observed from the tiny amount of dye materials found in size classes smaller than 1 mm. This proves the relegation of tracer granules shown earlier was indeed the consequence of granules breaking during granulation.
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Fig. 8. Typical evolution of TMD with time for tracer granules of 3 min old and 1 mm in size.
H.S. Tan et al. / Chemical Engineering Science 60 (2005) 3835 – 3845
Slope estimation
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1 0.9 0.8 0.7 0.6
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Tracer mass distribution (%)
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0.6 0.7 0.8 0.9 1
3.0 min 6.0 min 9.0 min 10.75min
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Size (mm) Fig. 9. Tracer mass distribution by sieving tracer granules with standard granules.
0.1 2
4
6
8
10
12
Time (min) Fig. 10. Fraction of tracer granules smaller than original tracer granules added for tracers of four different ages at a constant size of 1000 m.
1 0.9 0.8 0.7 0.6 0.5 0.4
1-χ
10.45 min at a constant tracer size of 1000–1180 m were used. Fig. 10 shows the plot of variation of 1− with time for different ages. As mentioned, the main purpose of this work is to estimate the rate at which the granule breaks. This was done by making a first-order difference between first two time points, which estimates the initial slopes of granules of different ages to be at an approximate rate of −0.01 s−1 . From this plot, there is however no strong evidence proving a considerable effect of granule age on breakage rate. In high shear granulation (Pearson et al., 2001), the older granules are observed to be stronger than the younger ones due to increased granule consolidation with time, thereby leading to smaller breakage rate with older granules. The observation in the present work is to be expected due to the low shear environment in fluidised bed and short binder solidification time, leading to low granule densification and thus causing little variation in granule strength with time.
0.3
0.2 1000 µ m 500 µ m 180 µ m
0.1 2
4
6.4. Effect of granule size For size effect, three different sizes of tracer granules were used: 180–212, 500–600, 1000–1180 m. Fig. 11 illustrates the change in 1 − with time for different granule sizes, which clearly reveals the larger granules (1090 m) to experience more breakage than those of smaller ones. One obvious but misleading explanation is likely due to the presence of more flaws existing in larger granules, since the large granules are effectively the agglomeration of smaller granules with weaker bonds between these sub-granules. This will be further explained in the later section, as we are able to show that this is essentially because the smaller granules probably are growing faster than its larger counterparts as the granulation proceeds.
6 8 Time (min)
10
12
Fig. 11. Fraction of tracer smaller than original tracer granules added for tracers of three different sizes at a constant age of 3 min.
6.5. Breakage mechanisms With the use of tracer granules, we also intend to identify unambiguously the different dominating mechanisms during the process. Fig. 12 shows the granules at the end of the process after the addition of 1 mm tracer granules of 3 min old. The coalescence between the blue and white granules observed in Fig. 12(a) is retrieved in a 1 mm sieve, and this is effectively due to the coalescence of a white granule with part of a broken tracer granule. In the same batch of
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Fig. 12. Microscopic pictures of tracer granules at the end of the process.
Breakage
Attrition
Fig. 13. Possible breakage mechanisms during fluidised bed granulation.
granules, smaller tracer granules can be found in the smaller sieve fraction of 212 m. Since the tracer granules added originate from 1 mm in size, these doublets and single tracer particles are likely the result of attrition occurring during the process. The result from the tracer size distribution (Fig. 8) provides further evidence that a granule may break to form a few larger fragments and some smaller fragments. Similar observations can also be found in Boerefijn et al. (1998) and Ning et al. (1997) while observing the breakage of granule upon impact. Fig. 13 summarises the breakage mechanisms observed in this work, illustrating granule upon impact, to break into a few larger fragments by breakage and some smaller fragments (primaries and doublets) by attrition process.
Compared to the spray-on tracer experiment (where the breakage rate for smaller granules is lower), the discrepancy in the observation here strongly suggests that the smaller granules seems to grow faster than the larger granules in the presence of binder during granulation, which would otherwise break at a similar rate as the larger granules in the absence of binder addition. On this basis, the smaller granules appear more likely to aggregate than the larger ones during granulation, which probably explains the subsequent decrease in span of the GSDs with time during granulation (Tan et al., 2004a). This behaviour is further supported by the success of the modelling work demonstrated in the previous paper (Tan et al., 2004b), where the net growth process in the FBMG experiment can be well described by the equipartition of kinetic energy (EKE) model which favours the interaction between small and large granules.
6.6. Breakage—only tracer experiment Fig. 14 shows the change in 1 − with time for different granule size. In the absence of binder addition, the initial breakage rate can be observed to be fairly similar for all the three sizes. The breakage rate can also be seen to have reduced after 2 min for 180 or 500 m, while it seems to continue for 1000 m. Compared to the spray-on experiment, the initial breakage rate for the 1000 m granule can also be estimated to be in the order of −0.01 s−1 , further suggesting that there is probably no re-aggregation of granules into the original size class. The result here reveals no significant effect of size, confirming the size independence of the breakage selection rate. However the observed reduction in the slope with time suggests a decrease in the breakage selection rate, indicating that the granules are probably heterogenous in nature as observed by Hounslow et al. (2001) in high shear granulation.
7. Conclusion In general, a similar tracer analysis method as Pearson et al. (2001) is employed here to reveal the effect of granule size and age on breakage during fluidised bed granulation. The first part of this work mainly concentrates on validating the reliability of tracer granules in mimicking the standard granules. Several experiments that involve the comparison of the change in mean size, size distribution and binder content with time of standard and tracer granulation have all demonstrated a comparable behaviour between the tracer and standard granules. The nature of spray-on tracer experiment is such that tracer granules of known size and age are added to the standard experiment during the process. The evidence provided by the photographic examination of the granule mixture and the tracer mass distribution allows us to deduce that a gran-
H.S. Tan et al. / Chemical Engineering Science 60 (2005) 3835 – 3845
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Abbreviation
1 0.8
FBMG PEG TMD GSD
0.6
1- χ
0.4
fluidised bed melt granulation polyethylene glycol tracer mass distribution granule size distribution
Acknowledgements 0.2 1000 µm 500 µm 180 µm
0.1 0
50
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Fig. 14. Fraction of tracer relegated for tracers of three different sizes at a constant age of 3 min for spray-off experiment.
ule is likely to break into a few larger fragments and some smaller fragments (i.e., primaries). From the tracer relegation plot on spray-on tracer experiment, it can be preliminarily deduced that larger granules are more prone to break than smaller granules during granulation. However this is contradicted by the breakage-only tracer experiment, which reveals the granule breakage rate to be size independent in the absence of further binder addition. This result suggests that the smaller granules are growing faster than its larger counterparts during the granulation process. The analysis from the spray-on tracer experiment also depicts the granule breakage rate to be independent of granule age, which is probably due to the lack of granule consolidation in the low shear fluidising environment. Notation binder mass of dye in granules used for filtration, kg mass of granules used for filtration, kg overall mass of dye in a given size class, kg particle breakage selection rate, s−1 time, s superficial velocity, m s−1 mass of granules retained in a sieve interval
Greek letter
fraction of relegated granules (–)
References
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Time (s)
B mt ms M P S0 t Us W
The authors would like to acknowledge EPSRC for funding this project and Unilever for supplying the materials.
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