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Effect of initial dextrose concentration, seeding and cooling profile on the crystallization of dextrose monohydrate
food and bioproducts processing 9 0 ( 2 0 1 2 ) 406–412
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Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp
Effect of initial dextrose concentration, seeding and cooling profile on the crystallization of dextrose monohydrate Abhay Markande a,d , John Fitzpatrick a,∗ , Amale Nezzal b , Luc Aerts c,d , Andreas Redl d a
Department of Process and Chemical Engineering, University College Cork, Cork, Ireland Nezzal Powder Consulting, 55 rue Bermond, 81000 Albi, France c UCB Pharma, Avenue de l’Industrie, Braine l’Alleud, Belgium d Innovation Center, Syral N.V., Aalst, Belgium b
a b s t r a c t A batch seeded cooling crystallizer was used to study dextrose monohydrate crystallization. Experiments were conducted to investigate how a 2% increase in the initial dextrose concentration (from 65.5 to 67.5%) would influence final crystal yield and size. The crystallizations were performed for three different seed masses and cooling profiles, consequently the influence of these parameters was also investigated. The parameters were varied in accordance with an industrial scale process. An in-line focused beam reflectance measurement probe and an in-line process refractometer were used to continuously monitor the crystallizations. The experimental results showed that the 2% increase in initial dextrose concentration had a major influence on the rate of crystallization and yield over a 24 h crystallization period, and only a minor influence on the median crystal size. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Crystallization; Dextrose
1.
Introduction
Industrial scale continuous crystallizers often exhibit periodic changes of supersaturation, solid phase content, crystal size, and production rate. In continuous crystallizers, crystal size distribution deviates from the desired distribution due to the presence of external disturbances, such as changes in the concentration of solute in the feed stream, and due to the randomly occurring changes in the operating conditions. In batch processes, batch crystallizer suffers from a changing level of supersaturation during their operation. In sugar manufacturing, dextrose monohydrate is produced by crystallization of high dextrose equivalent (DE) syrup. At the start of crystallization, the syrup is seeded with massecuite from a previous batch. The massecuite is a slurry containing grown crystals suspended in the syrup at the end of crystallization. During production, the mixing ratio of massecuite to syrup is often varied and hence the concentration of dextrose at the start of crystallization process changes. The increase or decrease in initial dextrose concentration may influence the crystallization rate and yield of the process as
∗
well as the final crystal size distribution resulting from nucleation and growth phenomena. In relation to the dextrose crystallization, the amount of crystals produced is very sensitive to variations in the initial dextrose concentration and the temperature profile (Parisi et al., 2007). A key variable during batch crystallization processes is the solution supersaturation which significantly determines the development of nucleation and growth phenomenona (Srisanga et al., 2006) and consequently, the final crystal yield and size. It is well established that the rate of cooling directly affects both nucleation and growth kinetics. For instance, a fast cooling regime results in the build-up of supersaturation to levels that have preference for nucleation over growth, consequently leading to a large population of smaller sized crystals. On the other hand, a controlled cooling regime can control supersaturation so as to grow the nucleated crystals rather than produce new nuclei, resulting in larger sized crystals (Jones and Mullin, 1974). Besides, Kubota et al. (2001) showed that seeding plays a key role in crystallization to control crystal size distribution (CSD). If the seed mass is insufficient, then secondary nucleation will be important and the
Corresponding author. E-mail address: j.fi[email protected] (J. Fitzpatrick). Received 16 February 2011; Received in revised form 25 August 2011; Accepted 23 November 2011 0960-3085/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fbp.2011.11.010
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final CSD will be dispersed. On the other hand, a large quantity of seeds reduces productivity (Doki et al., 2001). Overall minor changes in these crystallization parameters can produce significant changes in crystal yield and size. In-line monitoring can be applied in trying to interpret how changes in process variables influence the performance of a crystalliser. For example in a batch crystallization, in-line monitoring can be applied to measure the component concentration in solution and CSD continuously over time. This can be applied to investigate how process parameters, such as seed size or cooling rate, influence the final crystal yield and final CSD. In sugar crystallization, the sugar concentration in the mother liquor can be estimated by measuring the refractive index of the sugar slurry as a whole since the presence of crystals hardly affects the measurement. The refractive index measurement is used to give the brix, from which the total dissolved solids content can be evaluated. If the initial purity of the syrup is known then the sugar concentration can be evaluated. In line process refractometers exist to measure the refractive index continuously, such as the K-Patents process refractometer. Focus beam reflectance measurement (FBRM) has emerged as a widely used technique for the in situ particle size characterization of high-concentration particulate slurries, such as occur in many crystallizations. FBRM is a probe based measurement, which is installed directly in the crystallizer without the need for sample dilution or manipulation (Barrett and Glennon, 1999). FBRM has been successfully applied as a useful tool for detecting nucleation and characterizing the metastable zone width (Barrett and Glennon, 2002). FBRM measures a chord length distribution, which can be related to a particle size distribution (Worlitschek et al., 2005). Secondary nucleation and particle growth can be attributed to the time evolution of chord length distribution (Kail et al., 2009). Markande et al. (2009) utilized an in-line K-Patents refractometer and a FBRM probe to investigate the influence of seeding and cooling profile on the crystallization of dextrose monohydrate with an initial dextrose concentration of 65.5%. As already stated above, dextrose crystallization is very sensitive to variations in the initial dextrose concentration and the temperature profile. At an industrial scale, fluctuations in initial dextrose concentrations do occur, consequently the main objective of this paper was to investigate how a typical range of variation in initial dextrose concentration (i.e. from 65.5% to 67.5%) influences the final dextrose crystal yield and size, and rate of progression of the crystallization. Inline K-Patents refractometer and FBRM probe, as described by Markande et al. (2009), were used during the experimentation. This experimentation was carried out for three different seed masses and cooling profiles, consequently, the influence of these parameters was also investigated. Furthermore, the in-line measurements provided data throughout the crystallizations (and not just at the beginning and end) and these data were used to help in the interpretation of the results.
2.
Materials and methods
2.1. Dextrose monohydrate seed crystals and dextrose syrups d-glucose, commercially known as dextrose, can be crystallized to either anhydrous or hydrate form from an aqueous solution. It has three different crystal forms: ␣ monohydrate, ␣ anhydrous and  anhydrous dextrose. In this work, the studied
Table 1 – Dry substance composition of dextrose syrup by HPLC. Component
% (w/w)
Dextrose Maltose Maltotriose Higher sugars Others (fructose)
93.3 3.62 1.62 1.29 0.17
crystalline dextrose is the dextrose monohydrate (DMH) form. The seeds of dextrose monohydrate crystals of 99.5% purity were supplied by Syral Belgium N.V., Belgium as industrial grade quality. The seeds were in a size range of 125–150 m, as measured by sieve analysis. Dextrose monohydrate was produced by crystallization of high dextrose equivalent (DE) syrup. Two syrups were supplied by Syral Belgium N.V., Belgium, having dextrose concentrations of 65.5% and 67.5%. Dextrose concentration is defined as mass of dextrose per unit mass of solution excluding impurities. Both syrups had the same purity of 93.3% (w/w of total sugar), as measured by HPLC, and the dry substance composition of the syrup is presented in Table 1.
2.2. Experimental set up for in-line monitoring of the batch crystallizer The batch cooling crystallization experiments were performed in a 3.2 L jacketed glass crystallizer with a U-shaped bottom. The diameter of the crystallizer was 150 mm with a height of 200 mm. The batch vessel was equipped with an anchor type agitator. The agitator of diameter 100 mm, height 60 mm and width 22 mm was used. An agitator was placed at a height 15 mm from the bottom of the crystallizer. All experiments were performed at 38 rpm to ensure all the crystals were maintained in suspension. The temperature in the crystallizer was controlled with a programmable thermostat. A constant batch size of 2 kg of slurry was used in all experiments. The set up was equipped with two in-line measurement instruments. A K-Patents process refractometer (K-Patents PR23-AP) and a FBRM D600 probe were used to monitor the liquid phase and chord length distribution, respectively. The position of the probes was chosen in the high mixing zone, i.e. near the agitator. The measurements from both probes were recorded every 2 min. As the measurement by FBRM and in-line refractometer change in a continuous way, the data at every 1 h is considered for the comparison. The complete experimental setup used in this study is shown in Fig. 1.
2.3.
Evaluation of yield of crystals produced
The yield is defined as the mass of dextrose crystals produced during a crystallization trial divided by the mass of dextrose in solution at the beginning of the trial. This is expressed mathematically in Eq. (1). Yield =
MC MF Cd0
(1)
where MC is the mass of crystals produced during crystallization, MF is the mass of feed solution and Cd0 is the initial dextrose concentration in solution at the beginning of the crystallization (expressed as a mass fraction). Both MF and Cd0 exclude the small amount of impurities present.
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Fig. 1 – Experimental batch crystallization set-up. The K-Patents in-line refractometer was used in the evaluation of dextrose concentration over time, including the initial concentration and the final concentration at the end of a crystallization trial. The refractometer measures refractive index as a Brix value and this was converted to dextrose concentration using a calibration table (details of this procedure are provided in Markande, 2009). Once again, the dextrose concentrations are expressed as mass of dextrose per unit mass of solution excluding impurities. The evaluated final dextrose concentration (CdF ), expressed as a mass fraction, was used to calculate the mass of crystals using the following Eq. (2).
In this work, yields obtained from 67.5% initial dextrose concentration trials (Yield67.5% ) is compared with those from corresponding 65.5% initial dextrose concentration trials (Yield65.5% ). In addition to comparing the difference in yields, the percentage increase in yield of the 67.5% trials over the corresponding 65.5% trials is presented. This percentage increase in yield is defined in Eq. (4). % Increase in yield = 100 ×
2.4. MF (Cd0 − CdE ) MC = 1 − CdE
(2)
Substituting Eq. (2) into Eq. (1) and expressing yield as a percentage gives Eq. (3), and this equation was used to calculate yields. Yield = 100 ×
(Cd0 − CdE ) (1 − CdE )Cd0
(3)
Controlled cooling
o
Crystallizations were performed by cooling down the seeded syrup from 42 ◦ C to 33 ◦ C in 24 h. During the crystallization trials, seeding was performed at 42 ◦ C. Crystallization trials were performed at each of the initial dextrose concentrations of 65.5% and 67.5% for three seed masses of 5%, 12.5% and 20% and three cooling profiles. The cooling profiles used were either natural, linear or controlled (illustrated in Fig. 2) as given by the following temperature (T)–time (t) relationship in Eq. (5).
n t tf
(5)
Linear cooling
42
Temperature( C)
(4)
Batch cooling crystallizations
T(t) − Ti = Tf − Ti
44
Yield67.5% − Yield65.5% Yield65.5%
where n = 1 is linear; n = 3 is controlled and n = 1/3 is natural (subscripts i and f are initial and final, respectively).
Natural cooling
40 38
Table 2 – Crystal yield for duplicate trials. Cooling profile
36 34
Initial dextrose concentration (kg/kg)
32 0
2
4
6
8
10
12
14
16
18
20
22
24
Time (hrs) Fig. 2 – Cooling profiles used in batch crystallizations.
26
Linear Linear Natural
65.5 67.5 67.5
Crystal yield (%)
Trial 1
Trial 2
22.89 32.82 36.19
23.00 33.06 36.85
409
18
69
15
67
12 9 6 3 0 1
10
100
1000
Dextrose Concentration (%)
Number frequency (%/micron)
food and bioproducts processing 9 0 ( 2 0 1 2 ) 406–412
65 63 61 59 57
Chord length (microns)
55
Fig. 3 – Comparison of duplicate frequency chord length distributions (linear cooling profile; 12.5% seed mass). The reproducibility of the in-line monitoring techniques and the crystallization procedure was assessed by performing duplicate trials with linear and natural cooling profiles and two initial dextrose concentrations. Comparison of crystal yield obtained for each of the duplicate trials is presented in Table 2 and a comparison of the chord length distribution for one of the duplicate trials is presented in Fig. 3.
at 67.5% dextrose concentration at 65.5% dextrose concentration Solubility curve
30
32
34
36
(a)
Crystallizations were performed at the two initial dextrose concentrations of 65.5% and 67.5%. With a difference of only 2% between the two concentrations it may intuitively be expected that there may not be much difference between the crystallizations, however it does depend on the solubility diagram and the solubility at the temperature at the end of crystallization in particular. This temperature was 33 ◦ C and the corresponding dextrose solubility is 0.568 g dextrose per g of solution or 56.8% (Markande, 2009). The maximum yield for each of the initial concentrations can be obtained by applying Eq. (3) with a final dextrose concentration equal to the solubility limit. From this, the maximum yields for the initial concentrations of 65.5% and 67.5% were 30.8% and 36.8%, respectively. Consequently, the 2% variation in initial dextrose concentration leads to a 6% difference in yield of the 67.5% trial over the 65.5% trial, if equilibrium were to be achieved at the end of crystallization. Expressing this as a percentage increase in yield (Eq. (4)), this represents a 19.5% increase in yield. Furthermore, the difference in initial concentration may affect the rate of progression of the crystallization. The higher concentration may be expected to take longer because there is more dextrose in solution, however the higher initial supersaturation may counteract this. Overall, it is intuitively difficult to predict what may happen. Fig. 4 displays a comparison of the soluble dextrose concentration – temperature profiles for two trials carried out at 65.5% and 67.5% initial dextrose concentration. Both trials were performed with a natural profile and at 5% seed mass. The rate of crystallization is faster at the higher initial dextrose concentration of 67.5% resulting in the final soluble dextrose concentration at the end of crystallization being lower for the 67.5% trial. The corresponding yields for the 65.5% and 67.5% trials were 26% and 36%, respectively. This is a major difference but it needs to be considered in the context of the maximum yields of 30.8% and 36.8% for the 65.5% and 67.5%
Supersaturation
3.1. Effect of initial dextrose concentration on the crystallization
42
44
1.35 at 65.5% dextrose concentration at 67.5% dextrose concentration
1.25 1.20 1.15 1.10 1.05 1.00 0
2
4
6
8
10 12 14 16 18 20 22 24 26
Time(hrs)
(b) 2.2.E+04
Particle counts (#/sec)
Results and discussion
40
Fig. 4 – Effect of initial dextrose concentration on the soluble dextrose concentration–temperature profile (both trials were performed with a natural cooling profile and at 5% seed mass).
1.30
3.
38
Temperature (°C) °
Particle counts of 1-112 µm
2.0.E+04 1.8.E+04 1.6.E+04 1.4.E+04 1.2.E+04 1.0.E+04
Natural cooling at 65.5% concentration Natural cooling at 67.5% concentration
8.0.E+03 6.0.E+03 0
2
4
6
8
10 12
14
16
18 20
22
24
26
Time (hrs)
Fig. 5 – Time evolution of (a) supersaturation and (b) FBRM counts per second in the 1–112 m size range during crystallization (natural cooling profile; 5% seed mass). trials, respectively. The 67.5% trial is near its maximum of 36.8% and is thus near equilibrium. The in-line measurements of dextrose concentration and chord length distribution (CLD) taken throughout the crystallization can be used to interpret the rate of crystallization progression in both trials. Converting the dextrose concentration data into how supersaturation varies with time is presented in Fig. 5a. Fig. 5b shows the corresponding FBRM counts per second data in the size range from 1 to 112 m. As this is below the size of the seeds, these data are used as an indicator of the generation of nuclei. It is important to note that in a chord length distribution the number of shorter chords does not directly correspond to the number of the smaller crystals. For example, a short chord can be also
food and bioproducts processing 9 0 ( 2 0 1 2 ) 406–412
3.2. yield
Effect of initial dextrose concentration on crystal
Further trials were undertaken to investigate how the change in initial dextrose concentration from 65.5 to 67.5% influenced crystallizations performed with the three different seed masses (5%, 12.5%, 20%) and three different cooling profiles (linear, natural, controlled). The results from these trials are presented in Table 3. It is clearly observed from Table 3 that the 2% change in initial dextrose concentration strongly influenced the yield of the process. For example, during natural cooling at 67.5% concentration, the percentage increase in yields (Eq. (4)) were 38%, 33%, and 24% for the 5%, 12.5%, and 20% seed masses, respectively. In the case of linear cooling, the percentage increase in yields were larger at 55%, 43% and 34% for the 5%, 12.5 and 20% seed masses, respectively. However, this must be considered in the context of an inherent 19.5% increase in yield that would be obtained if equilibrium was achieved by both 67.5% and 65.5% initial dextrose concentration trials. Consequently, the higher yields were in-part obtained by both the faster rate of crystallization during the 24 h crystallization period and the inherent higher maximum equilibrium yield obtainable by the higher initial dextrose concentration trials.
Particle counts (#/sec)
45 Particle counts of 194-233 µm
40 35 30 25 20 15 10 5
Particle counts (#/sec)
0 2.2.E+04 Particle counts of 1-112 µm
2.0.E+04 1.8.E+04 1.6.E+04 1.4.E+04 1.2.E+04 1.0.E+04
Natural cooling at 65.5% concentration Natural cooling at 67.5% concentration
8.0.E+03 6.0.E+03 0
Effect of initial dextrose concentration on crystal
Even though a major increase in yield was observed at higher initial dextrose concentration, there was little influence observed on the final median crystal size (Table 3). A decrease in crystal size at 5% seed mass was observed during natural cooling. However, at 5% seed mass, linear and controlled cooling showed an increase in crystal size. The final crystal size at 12.5% and 20% showed no significant influence upon change in the initial dextrose concentration. In fact, it might be expected that the higher dextrose consumption would produce larger crystals. Actually, despite the high dextrose consumption and the increase in solid concentration, the additional crystal mass produced was due to the newly generated crystals and their subsequent growth.
4
6
8
10
12
14
16
18
20
22
24
26
Fig. 6 – Comparison of particle counts in the 1–112 m and 194–233 m size ranges for natural cooling (5% seed mass and initial dextrose concentrations of 65.5% and 67.5%). 45 Particle counts of 194-233 µm 40 35 30 25 20 15 10 5 0 2.2.E+04 Particle counts of 1-112 µm
2.0.E+04 1.8.E+04 1.6.E+04 1.4.E+04 1.2.E+04 1.0.E+04
Linear cooling at 65.5% concentration Linear cooling at 67.5% concentration
8.0.E+03 6.0.E+03 0
3.3. size
2
Time (hrs)
Particle counts (#/sec)
generated by a larger crystal if the beam intersects it close to its border (Worlitschek et al., 2005). Furthermore, the chords in the range of 1–112 m can be also generated by the seeds, however, if the number of the shorter chords increases during the crystallization, the increase can be used as an indicator of nucleation. For the higher initial dextrose concentration of 67.5%, Fig. 5a shows the highest increase in supersaturation at around 6 h, after which there is a very rapid decrease in supersaturation. The particle counts data in Fig. 5b shows the highest counts for the higher initial dextrose concentration of 67.5%, inferring that the highest levels of nucleation occurred during this trial. One explanation for the higher nucleation observed in the 67.5% trial is that the higher levels of supersaturation caused higher levels of secondary nucleation (as opposed to primary nucleation as these levels are still within the metastable zone for dextrose monohydrate). The high levels of nucleation created many new crystals which greatly increased the surface area available for crystal growth and it is this additional crystal surface area that resulted in the increased crystallization rate and rapid reduction of dextrose in solution.
Particle counts (#/sec)
410
2
4
6
8
10
12
14
16
18
20
22
24
26
Time (hrs)
Fig. 7 – Comparison of particle counts in the 1–112 m and 194–233 m size ranges for linear cooling (5% seed mass and initial dextrose concentrations of 65.5% and 67.5%). FBRM chord length data for natural and linear cooling profiles are presented in Figs. 6 and 7, respectively. Both show a more rapid increase in the counts measured in the 1–112 m range for the 67.5% concentration. The high increase of particle counts of 1–122 m at 67.7% concentration indicates high nuclei formation through secondary nucleation. As already mentioned, the secondary nucleation may be caused by the higher levels of supersaturation, however this may not necessarily be the only dominant mechanism. For example, there is also the possible influence of secondary nucleation by the
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Table 3 – Effect of initial dextrose concentration, seed mass and cooling profile on crystal yield and size. Cooling
Seed mass (%)
D50 Natural Natural Natural Linear Linear Linear Controlled Controlled Controlled a
5 12.5 20 5 12.5 20 5 12.5 20
Concentration 67.5%(%)
Concentration 65.5 (%) 1
Yield
144 130 121 134 131 122 129 126 107
D50
26 27 29 20 23 26 11 19 21
a
129 130 122 149 129 123 146 128 121
Yield 36 36 36 31 33 35 23 28 31
Median chord length in m.
collision mechanism, where the magma density plays a major role in the nucleation rate (Myerson and Ginde, 2002). For the natural cooling profile, Fig. 6 shows that the increase in larger crystals (194–233 m particle counts) at 67.7% concentration was lower than at 65.5% concentration. A high amount of fine crystals (1–112 m particle counts) generated and a slow asymptotic increase in larger crystals at 67.5% dextrose concentration resulted in a lower median crystal size. The same behaviour was observed at higher seed masses (data not shown) for the natural cooling profile giving an increase in fine counts with increased dextrose concentration, although, there was no significant difference in the counts measured for 194–233 m. The observed asymptotic increase in the longer chord counts suggests that the number density of larger crystals does not increase at the higher magma densities. This may be due to secondary nucleation by collision mechanism, where the magma density plays a dominant role in the nucleation rate (Myerson and Ginde, 2002). Since the collision mechanism induces crystal breakage and a reduction of larger crystals, this may offset their growth rate. During linear cooling, the influence of initial dextrose concentration on the large particle counts (194–233 m) was different. For example, at 5% seed mass, Fig. 7 shows that the large counts observed were greater at the 67.5% initial dextrose concentration and this resulted in a larger median particle size. Comparing both sets of trials, the higher initial dextrose concentration resulted in more fine crystals being produced in the 1–112 m range. This demonstrated the formation of a higher number of nuclei, which is true for each of the three cooling profiles and seed masses. This produced an increase in crystal surface area which increased the rate of dextrose consumption. There was no clear trend observed between the initial dextrose concentration and the large particle counts in the 194–233 m range, with lower counts being recorded for natural cooing at the higher dextrose concentration and the opposite occurring during linear cooling. The lower counts during natural cooling may be explained by the greater dominance of secondary nucleation, with more nuclei being produced leading to smaller final crystal sizes.
3.4. Effect of seed mass and cooling profile on yield and crystal size Table 3 also shows the influence of seed mass and cooling profile on yield and crystal size. Increasing seed mass caused an increase in yield. This trend would be expected, as a greater
amount of seeds leads to higher crystallization rates during the 24 h trials. Cooling profile also influenced crystal yield with the natural profile providing the highest yields and the controlled profile providing the lowest for both initial dextrose concentrations. This is due to the higher crystallization rates occurring during the natural cooling profile over the 24 h trials. The influence of seed mass and cooling profile on crystal median size is less clear, although lower seed mass appears to produce larger median crystal sizes. Markande et al. (2009) discusses these trends in more detail for the lower initial dextrose concentration. Overall, Table 3 demonstrates the important influence of the initial dextrose concentration on yield during the 24 h trials, where the yields obtained for the 67.5% initial dextrose concentration were consistently much higher, irrespective of the seed mass and cooling profile used.
4.
Conclusions
The initial dextrose concentration of the inlet stream entering an industrial continuous crystallizer can fluctuate and this can influence the performance of dextrose crystallization. This paper investigated the influence of a typical industrial variation in initial dextrose concentration and how it affected the rate of progression of the crystallization, crystal yield and final crystal size in a batch cooling crystallizer. The crystallization trials showed that a 2% increase in initial dextrose concentration showed a significant increase in dextrose crystal yield, with percentage increase in yields ranging from about 24% to 55% for the seed masses and cooling profiles tested. The main contributor to the increased percentage yields is the inherent higher percentage increase in equilibrium yield of 19.5% obtainable by the higher initial dextrose concentration. The remainder is due to faster crystallization rates being achieved during the higher concentration trials, moving these trials closer to equilibrium by the end of the 24 h crystallization time. In-line measurements were applied to continuously monitor and help interpret the crystallizations. Higher levels of supersaturation were observed during the higher initial dextrose concentration trials using data obtained from the in-line refractometer. The higher supersaturation levels led to greater levels of nucleation, as monitored by the in-line FBRM probe. The greater nucleation caused an increased rate of dextrose consumption due to increased crystal surface area. Both seed mass and cooling profile influenced the rate of crystallization at both initial dextrose concentrations. As expected, higher seed mass resulted in faster crystallizations.
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The natural cooling profile trials were the fastest while the controlled cooling profile trials were the slowest for both initial dextrose concentrations. Contrary to the expectation that the higher dextrose consumption, at the higher initial dextrose concentration, would produce larger crystals, there was little influence observed on the final median crystal size. This is an indication of the importance of secondary nucleation and how it influences final crystal sizes. Secondary nucleation was greater at the higher initial dextrose trials which counteracted the greater dextrose consumption, as the crystal mass produced was mainly due to newly generated crystals and their subsequent growth. Overall, the crystal yields obtained and the amount of crystals produced were very sensitive to variations in the initial dextrose concentration.
References Barrett, P., Glennon, B., 1999. In-line FBRM monitoring of particle size in dilute agitated suspensions. Particle & Particle Systems Characterization 16, 207. Barrett, P., Glennon, B., 2002. Characterizing the metastable zone width and solubility curve using Lasentec FBRM and PVV. Chemical Engineering Research Design 80, 799–805. Doki, N., Kubota, N., Sato, A., Yokota, M., 2001. Effect of cooling mode on product crystal size in seeded batch crystallization of potassium alum. Chemical Engineering Science 81, 313–316.
Jones, A.G., Mullin, J.W., 1974. Programmed cooling crystallization of potassium sulfate solutions. Chemical Engineering Science 29 (1), 105–118. Kail, N., Marquardt, W., Briesen, H., 2009. Process analysis by means of focused beam reflectance measurements. Industrial Engineering Chemistry Research 48, 2936–2946. Kubota, N., Doki, N., Yokota, M., Sato, A., 2001. Seeding policy in batch cooling crystallization. Powder Technology 121 (1), 31–38. Markande, A., 2009. Dextrose crystallization – control of crystal growth and crystal size. PhD thesis, University College Cork, Ireland. Markande, A., Nezzal, A., Fitzpatrick, J., Aerts, L., 2009. Investigation of the crystallisation kinetics of dextrose monohydrate using insitu particle size and supersaturation monitoring. Particulate Science and Technology 27, 373–388. Myerson, A.S., Ginde, R., 2002. Crystal growth and nucleation. In: Myerson, A.S. (Ed.), Handbook of Industrial Crystallization. Elsevier, pp. 33–65. Parisi, M., Terranova, A., Chianese, A., 2007. Pilot plant investigation on the kinetics of dextrose cooling crystallization. Industrial & Engineering Chemistry Research 46, 1277. Srisa-nga, S., Flood, A.E., White, E.T., 2006. The secondary nucleation threshold and crystal growth of ␣-glucose monohydrate in aqueous solution. Crystal Growth & Design 6 (3), 795. Worlitschek, J., Hocker, T., Mazzotti, M., 2005. Restoration of PSD from chord length. Particle & Particle Systems Characterization 22 (2), 81.
Contents lists available at SciVerse ScienceDirect
Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp
Effect of initial dextrose concentration, seeding and cooling profile on the crystallization of dextrose monohydrate Abhay Markande a,d , John Fitzpatrick a,∗ , Amale Nezzal b , Luc Aerts c,d , Andreas Redl d a
Department of Process and Chemical Engineering, University College Cork, Cork, Ireland Nezzal Powder Consulting, 55 rue Bermond, 81000 Albi, France c UCB Pharma, Avenue de l’Industrie, Braine l’Alleud, Belgium d Innovation Center, Syral N.V., Aalst, Belgium b
a b s t r a c t A batch seeded cooling crystallizer was used to study dextrose monohydrate crystallization. Experiments were conducted to investigate how a 2% increase in the initial dextrose concentration (from 65.5 to 67.5%) would influence final crystal yield and size. The crystallizations were performed for three different seed masses and cooling profiles, consequently the influence of these parameters was also investigated. The parameters were varied in accordance with an industrial scale process. An in-line focused beam reflectance measurement probe and an in-line process refractometer were used to continuously monitor the crystallizations. The experimental results showed that the 2% increase in initial dextrose concentration had a major influence on the rate of crystallization and yield over a 24 h crystallization period, and only a minor influence on the median crystal size. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Crystallization; Dextrose
1.
Introduction
Industrial scale continuous crystallizers often exhibit periodic changes of supersaturation, solid phase content, crystal size, and production rate. In continuous crystallizers, crystal size distribution deviates from the desired distribution due to the presence of external disturbances, such as changes in the concentration of solute in the feed stream, and due to the randomly occurring changes in the operating conditions. In batch processes, batch crystallizer suffers from a changing level of supersaturation during their operation. In sugar manufacturing, dextrose monohydrate is produced by crystallization of high dextrose equivalent (DE) syrup. At the start of crystallization, the syrup is seeded with massecuite from a previous batch. The massecuite is a slurry containing grown crystals suspended in the syrup at the end of crystallization. During production, the mixing ratio of massecuite to syrup is often varied and hence the concentration of dextrose at the start of crystallization process changes. The increase or decrease in initial dextrose concentration may influence the crystallization rate and yield of the process as
∗
well as the final crystal size distribution resulting from nucleation and growth phenomena. In relation to the dextrose crystallization, the amount of crystals produced is very sensitive to variations in the initial dextrose concentration and the temperature profile (Parisi et al., 2007). A key variable during batch crystallization processes is the solution supersaturation which significantly determines the development of nucleation and growth phenomenona (Srisanga et al., 2006) and consequently, the final crystal yield and size. It is well established that the rate of cooling directly affects both nucleation and growth kinetics. For instance, a fast cooling regime results in the build-up of supersaturation to levels that have preference for nucleation over growth, consequently leading to a large population of smaller sized crystals. On the other hand, a controlled cooling regime can control supersaturation so as to grow the nucleated crystals rather than produce new nuclei, resulting in larger sized crystals (Jones and Mullin, 1974). Besides, Kubota et al. (2001) showed that seeding plays a key role in crystallization to control crystal size distribution (CSD). If the seed mass is insufficient, then secondary nucleation will be important and the
Corresponding author. E-mail address: j.fi[email protected] (J. Fitzpatrick). Received 16 February 2011; Received in revised form 25 August 2011; Accepted 23 November 2011 0960-3085/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fbp.2011.11.010
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final CSD will be dispersed. On the other hand, a large quantity of seeds reduces productivity (Doki et al., 2001). Overall minor changes in these crystallization parameters can produce significant changes in crystal yield and size. In-line monitoring can be applied in trying to interpret how changes in process variables influence the performance of a crystalliser. For example in a batch crystallization, in-line monitoring can be applied to measure the component concentration in solution and CSD continuously over time. This can be applied to investigate how process parameters, such as seed size or cooling rate, influence the final crystal yield and final CSD. In sugar crystallization, the sugar concentration in the mother liquor can be estimated by measuring the refractive index of the sugar slurry as a whole since the presence of crystals hardly affects the measurement. The refractive index measurement is used to give the brix, from which the total dissolved solids content can be evaluated. If the initial purity of the syrup is known then the sugar concentration can be evaluated. In line process refractometers exist to measure the refractive index continuously, such as the K-Patents process refractometer. Focus beam reflectance measurement (FBRM) has emerged as a widely used technique for the in situ particle size characterization of high-concentration particulate slurries, such as occur in many crystallizations. FBRM is a probe based measurement, which is installed directly in the crystallizer without the need for sample dilution or manipulation (Barrett and Glennon, 1999). FBRM has been successfully applied as a useful tool for detecting nucleation and characterizing the metastable zone width (Barrett and Glennon, 2002). FBRM measures a chord length distribution, which can be related to a particle size distribution (Worlitschek et al., 2005). Secondary nucleation and particle growth can be attributed to the time evolution of chord length distribution (Kail et al., 2009). Markande et al. (2009) utilized an in-line K-Patents refractometer and a FBRM probe to investigate the influence of seeding and cooling profile on the crystallization of dextrose monohydrate with an initial dextrose concentration of 65.5%. As already stated above, dextrose crystallization is very sensitive to variations in the initial dextrose concentration and the temperature profile. At an industrial scale, fluctuations in initial dextrose concentrations do occur, consequently the main objective of this paper was to investigate how a typical range of variation in initial dextrose concentration (i.e. from 65.5% to 67.5%) influences the final dextrose crystal yield and size, and rate of progression of the crystallization. Inline K-Patents refractometer and FBRM probe, as described by Markande et al. (2009), were used during the experimentation. This experimentation was carried out for three different seed masses and cooling profiles, consequently, the influence of these parameters was also investigated. Furthermore, the in-line measurements provided data throughout the crystallizations (and not just at the beginning and end) and these data were used to help in the interpretation of the results.
2.
Materials and methods
2.1. Dextrose monohydrate seed crystals and dextrose syrups d-glucose, commercially known as dextrose, can be crystallized to either anhydrous or hydrate form from an aqueous solution. It has three different crystal forms: ␣ monohydrate, ␣ anhydrous and  anhydrous dextrose. In this work, the studied
Table 1 – Dry substance composition of dextrose syrup by HPLC. Component
% (w/w)
Dextrose Maltose Maltotriose Higher sugars Others (fructose)
93.3 3.62 1.62 1.29 0.17
crystalline dextrose is the dextrose monohydrate (DMH) form. The seeds of dextrose monohydrate crystals of 99.5% purity were supplied by Syral Belgium N.V., Belgium as industrial grade quality. The seeds were in a size range of 125–150 m, as measured by sieve analysis. Dextrose monohydrate was produced by crystallization of high dextrose equivalent (DE) syrup. Two syrups were supplied by Syral Belgium N.V., Belgium, having dextrose concentrations of 65.5% and 67.5%. Dextrose concentration is defined as mass of dextrose per unit mass of solution excluding impurities. Both syrups had the same purity of 93.3% (w/w of total sugar), as measured by HPLC, and the dry substance composition of the syrup is presented in Table 1.
2.2. Experimental set up for in-line monitoring of the batch crystallizer The batch cooling crystallization experiments were performed in a 3.2 L jacketed glass crystallizer with a U-shaped bottom. The diameter of the crystallizer was 150 mm with a height of 200 mm. The batch vessel was equipped with an anchor type agitator. The agitator of diameter 100 mm, height 60 mm and width 22 mm was used. An agitator was placed at a height 15 mm from the bottom of the crystallizer. All experiments were performed at 38 rpm to ensure all the crystals were maintained in suspension. The temperature in the crystallizer was controlled with a programmable thermostat. A constant batch size of 2 kg of slurry was used in all experiments. The set up was equipped with two in-line measurement instruments. A K-Patents process refractometer (K-Patents PR23-AP) and a FBRM D600 probe were used to monitor the liquid phase and chord length distribution, respectively. The position of the probes was chosen in the high mixing zone, i.e. near the agitator. The measurements from both probes were recorded every 2 min. As the measurement by FBRM and in-line refractometer change in a continuous way, the data at every 1 h is considered for the comparison. The complete experimental setup used in this study is shown in Fig. 1.
2.3.
Evaluation of yield of crystals produced
The yield is defined as the mass of dextrose crystals produced during a crystallization trial divided by the mass of dextrose in solution at the beginning of the trial. This is expressed mathematically in Eq. (1). Yield =
MC MF Cd0
(1)
where MC is the mass of crystals produced during crystallization, MF is the mass of feed solution and Cd0 is the initial dextrose concentration in solution at the beginning of the crystallization (expressed as a mass fraction). Both MF and Cd0 exclude the small amount of impurities present.
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Fig. 1 – Experimental batch crystallization set-up. The K-Patents in-line refractometer was used in the evaluation of dextrose concentration over time, including the initial concentration and the final concentration at the end of a crystallization trial. The refractometer measures refractive index as a Brix value and this was converted to dextrose concentration using a calibration table (details of this procedure are provided in Markande, 2009). Once again, the dextrose concentrations are expressed as mass of dextrose per unit mass of solution excluding impurities. The evaluated final dextrose concentration (CdF ), expressed as a mass fraction, was used to calculate the mass of crystals using the following Eq. (2).
In this work, yields obtained from 67.5% initial dextrose concentration trials (Yield67.5% ) is compared with those from corresponding 65.5% initial dextrose concentration trials (Yield65.5% ). In addition to comparing the difference in yields, the percentage increase in yield of the 67.5% trials over the corresponding 65.5% trials is presented. This percentage increase in yield is defined in Eq. (4). % Increase in yield = 100 ×
2.4. MF (Cd0 − CdE ) MC = 1 − CdE
(2)
Substituting Eq. (2) into Eq. (1) and expressing yield as a percentage gives Eq. (3), and this equation was used to calculate yields. Yield = 100 ×
(Cd0 − CdE ) (1 − CdE )Cd0
(3)
Controlled cooling
o
Crystallizations were performed by cooling down the seeded syrup from 42 ◦ C to 33 ◦ C in 24 h. During the crystallization trials, seeding was performed at 42 ◦ C. Crystallization trials were performed at each of the initial dextrose concentrations of 65.5% and 67.5% for three seed masses of 5%, 12.5% and 20% and three cooling profiles. The cooling profiles used were either natural, linear or controlled (illustrated in Fig. 2) as given by the following temperature (T)–time (t) relationship in Eq. (5).
n t tf
(5)
Linear cooling
42
Temperature( C)
(4)
Batch cooling crystallizations
T(t) − Ti = Tf − Ti
44
Yield67.5% − Yield65.5% Yield65.5%
where n = 1 is linear; n = 3 is controlled and n = 1/3 is natural (subscripts i and f are initial and final, respectively).
Natural cooling
40 38
Table 2 – Crystal yield for duplicate trials. Cooling profile
36 34
Initial dextrose concentration (kg/kg)
32 0
2
4
6
8
10
12
14
16
18
20
22
24
Time (hrs) Fig. 2 – Cooling profiles used in batch crystallizations.
26
Linear Linear Natural
65.5 67.5 67.5
Crystal yield (%)
Trial 1
Trial 2
22.89 32.82 36.19
23.00 33.06 36.85
409
18
69
15
67
12 9 6 3 0 1
10
100
1000
Dextrose Concentration (%)
Number frequency (%/micron)
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65 63 61 59 57
Chord length (microns)
55
Fig. 3 – Comparison of duplicate frequency chord length distributions (linear cooling profile; 12.5% seed mass). The reproducibility of the in-line monitoring techniques and the crystallization procedure was assessed by performing duplicate trials with linear and natural cooling profiles and two initial dextrose concentrations. Comparison of crystal yield obtained for each of the duplicate trials is presented in Table 2 and a comparison of the chord length distribution for one of the duplicate trials is presented in Fig. 3.
at 67.5% dextrose concentration at 65.5% dextrose concentration Solubility curve
30
32
34
36
(a)
Crystallizations were performed at the two initial dextrose concentrations of 65.5% and 67.5%. With a difference of only 2% between the two concentrations it may intuitively be expected that there may not be much difference between the crystallizations, however it does depend on the solubility diagram and the solubility at the temperature at the end of crystallization in particular. This temperature was 33 ◦ C and the corresponding dextrose solubility is 0.568 g dextrose per g of solution or 56.8% (Markande, 2009). The maximum yield for each of the initial concentrations can be obtained by applying Eq. (3) with a final dextrose concentration equal to the solubility limit. From this, the maximum yields for the initial concentrations of 65.5% and 67.5% were 30.8% and 36.8%, respectively. Consequently, the 2% variation in initial dextrose concentration leads to a 6% difference in yield of the 67.5% trial over the 65.5% trial, if equilibrium were to be achieved at the end of crystallization. Expressing this as a percentage increase in yield (Eq. (4)), this represents a 19.5% increase in yield. Furthermore, the difference in initial concentration may affect the rate of progression of the crystallization. The higher concentration may be expected to take longer because there is more dextrose in solution, however the higher initial supersaturation may counteract this. Overall, it is intuitively difficult to predict what may happen. Fig. 4 displays a comparison of the soluble dextrose concentration – temperature profiles for two trials carried out at 65.5% and 67.5% initial dextrose concentration. Both trials were performed with a natural profile and at 5% seed mass. The rate of crystallization is faster at the higher initial dextrose concentration of 67.5% resulting in the final soluble dextrose concentration at the end of crystallization being lower for the 67.5% trial. The corresponding yields for the 65.5% and 67.5% trials were 26% and 36%, respectively. This is a major difference but it needs to be considered in the context of the maximum yields of 30.8% and 36.8% for the 65.5% and 67.5%
Supersaturation
3.1. Effect of initial dextrose concentration on the crystallization
42
44
1.35 at 65.5% dextrose concentration at 67.5% dextrose concentration
1.25 1.20 1.15 1.10 1.05 1.00 0
2
4
6
8
10 12 14 16 18 20 22 24 26
Time(hrs)
(b) 2.2.E+04
Particle counts (#/sec)
Results and discussion
40
Fig. 4 – Effect of initial dextrose concentration on the soluble dextrose concentration–temperature profile (both trials were performed with a natural cooling profile and at 5% seed mass).
1.30
3.
38
Temperature (°C) °
Particle counts of 1-112 µm
2.0.E+04 1.8.E+04 1.6.E+04 1.4.E+04 1.2.E+04 1.0.E+04
Natural cooling at 65.5% concentration Natural cooling at 67.5% concentration
8.0.E+03 6.0.E+03 0
2
4
6
8
10 12
14
16
18 20
22
24
26
Time (hrs)
Fig. 5 – Time evolution of (a) supersaturation and (b) FBRM counts per second in the 1–112 m size range during crystallization (natural cooling profile; 5% seed mass). trials, respectively. The 67.5% trial is near its maximum of 36.8% and is thus near equilibrium. The in-line measurements of dextrose concentration and chord length distribution (CLD) taken throughout the crystallization can be used to interpret the rate of crystallization progression in both trials. Converting the dextrose concentration data into how supersaturation varies with time is presented in Fig. 5a. Fig. 5b shows the corresponding FBRM counts per second data in the size range from 1 to 112 m. As this is below the size of the seeds, these data are used as an indicator of the generation of nuclei. It is important to note that in a chord length distribution the number of shorter chords does not directly correspond to the number of the smaller crystals. For example, a short chord can be also
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3.2. yield
Effect of initial dextrose concentration on crystal
Further trials were undertaken to investigate how the change in initial dextrose concentration from 65.5 to 67.5% influenced crystallizations performed with the three different seed masses (5%, 12.5%, 20%) and three different cooling profiles (linear, natural, controlled). The results from these trials are presented in Table 3. It is clearly observed from Table 3 that the 2% change in initial dextrose concentration strongly influenced the yield of the process. For example, during natural cooling at 67.5% concentration, the percentage increase in yields (Eq. (4)) were 38%, 33%, and 24% for the 5%, 12.5%, and 20% seed masses, respectively. In the case of linear cooling, the percentage increase in yields were larger at 55%, 43% and 34% for the 5%, 12.5 and 20% seed masses, respectively. However, this must be considered in the context of an inherent 19.5% increase in yield that would be obtained if equilibrium was achieved by both 67.5% and 65.5% initial dextrose concentration trials. Consequently, the higher yields were in-part obtained by both the faster rate of crystallization during the 24 h crystallization period and the inherent higher maximum equilibrium yield obtainable by the higher initial dextrose concentration trials.
Particle counts (#/sec)
45 Particle counts of 194-233 µm
40 35 30 25 20 15 10 5
Particle counts (#/sec)
0 2.2.E+04 Particle counts of 1-112 µm
2.0.E+04 1.8.E+04 1.6.E+04 1.4.E+04 1.2.E+04 1.0.E+04
Natural cooling at 65.5% concentration Natural cooling at 67.5% concentration
8.0.E+03 6.0.E+03 0
Effect of initial dextrose concentration on crystal
Even though a major increase in yield was observed at higher initial dextrose concentration, there was little influence observed on the final median crystal size (Table 3). A decrease in crystal size at 5% seed mass was observed during natural cooling. However, at 5% seed mass, linear and controlled cooling showed an increase in crystal size. The final crystal size at 12.5% and 20% showed no significant influence upon change in the initial dextrose concentration. In fact, it might be expected that the higher dextrose consumption would produce larger crystals. Actually, despite the high dextrose consumption and the increase in solid concentration, the additional crystal mass produced was due to the newly generated crystals and their subsequent growth.
4
6
8
10
12
14
16
18
20
22
24
26
Fig. 6 – Comparison of particle counts in the 1–112 m and 194–233 m size ranges for natural cooling (5% seed mass and initial dextrose concentrations of 65.5% and 67.5%). 45 Particle counts of 194-233 µm 40 35 30 25 20 15 10 5 0 2.2.E+04 Particle counts of 1-112 µm
2.0.E+04 1.8.E+04 1.6.E+04 1.4.E+04 1.2.E+04 1.0.E+04
Linear cooling at 65.5% concentration Linear cooling at 67.5% concentration
8.0.E+03 6.0.E+03 0
3.3. size
2
Time (hrs)
Particle counts (#/sec)
generated by a larger crystal if the beam intersects it close to its border (Worlitschek et al., 2005). Furthermore, the chords in the range of 1–112 m can be also generated by the seeds, however, if the number of the shorter chords increases during the crystallization, the increase can be used as an indicator of nucleation. For the higher initial dextrose concentration of 67.5%, Fig. 5a shows the highest increase in supersaturation at around 6 h, after which there is a very rapid decrease in supersaturation. The particle counts data in Fig. 5b shows the highest counts for the higher initial dextrose concentration of 67.5%, inferring that the highest levels of nucleation occurred during this trial. One explanation for the higher nucleation observed in the 67.5% trial is that the higher levels of supersaturation caused higher levels of secondary nucleation (as opposed to primary nucleation as these levels are still within the metastable zone for dextrose monohydrate). The high levels of nucleation created many new crystals which greatly increased the surface area available for crystal growth and it is this additional crystal surface area that resulted in the increased crystallization rate and rapid reduction of dextrose in solution.
Particle counts (#/sec)
410
2
4
6
8
10
12
14
16
18
20
22
24
26
Time (hrs)
Fig. 7 – Comparison of particle counts in the 1–112 m and 194–233 m size ranges for linear cooling (5% seed mass and initial dextrose concentrations of 65.5% and 67.5%). FBRM chord length data for natural and linear cooling profiles are presented in Figs. 6 and 7, respectively. Both show a more rapid increase in the counts measured in the 1–112 m range for the 67.5% concentration. The high increase of particle counts of 1–122 m at 67.7% concentration indicates high nuclei formation through secondary nucleation. As already mentioned, the secondary nucleation may be caused by the higher levels of supersaturation, however this may not necessarily be the only dominant mechanism. For example, there is also the possible influence of secondary nucleation by the
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Table 3 – Effect of initial dextrose concentration, seed mass and cooling profile on crystal yield and size. Cooling
Seed mass (%)
D50 Natural Natural Natural Linear Linear Linear Controlled Controlled Controlled a
5 12.5 20 5 12.5 20 5 12.5 20
Concentration 67.5%(%)
Concentration 65.5 (%) 1
Yield
144 130 121 134 131 122 129 126 107
D50
26 27 29 20 23 26 11 19 21
a
129 130 122 149 129 123 146 128 121
Yield 36 36 36 31 33 35 23 28 31
Median chord length in m.
collision mechanism, where the magma density plays a major role in the nucleation rate (Myerson and Ginde, 2002). For the natural cooling profile, Fig. 6 shows that the increase in larger crystals (194–233 m particle counts) at 67.7% concentration was lower than at 65.5% concentration. A high amount of fine crystals (1–112 m particle counts) generated and a slow asymptotic increase in larger crystals at 67.5% dextrose concentration resulted in a lower median crystal size. The same behaviour was observed at higher seed masses (data not shown) for the natural cooling profile giving an increase in fine counts with increased dextrose concentration, although, there was no significant difference in the counts measured for 194–233 m. The observed asymptotic increase in the longer chord counts suggests that the number density of larger crystals does not increase at the higher magma densities. This may be due to secondary nucleation by collision mechanism, where the magma density plays a dominant role in the nucleation rate (Myerson and Ginde, 2002). Since the collision mechanism induces crystal breakage and a reduction of larger crystals, this may offset their growth rate. During linear cooling, the influence of initial dextrose concentration on the large particle counts (194–233 m) was different. For example, at 5% seed mass, Fig. 7 shows that the large counts observed were greater at the 67.5% initial dextrose concentration and this resulted in a larger median particle size. Comparing both sets of trials, the higher initial dextrose concentration resulted in more fine crystals being produced in the 1–112 m range. This demonstrated the formation of a higher number of nuclei, which is true for each of the three cooling profiles and seed masses. This produced an increase in crystal surface area which increased the rate of dextrose consumption. There was no clear trend observed between the initial dextrose concentration and the large particle counts in the 194–233 m range, with lower counts being recorded for natural cooing at the higher dextrose concentration and the opposite occurring during linear cooling. The lower counts during natural cooling may be explained by the greater dominance of secondary nucleation, with more nuclei being produced leading to smaller final crystal sizes.
3.4. Effect of seed mass and cooling profile on yield and crystal size Table 3 also shows the influence of seed mass and cooling profile on yield and crystal size. Increasing seed mass caused an increase in yield. This trend would be expected, as a greater
amount of seeds leads to higher crystallization rates during the 24 h trials. Cooling profile also influenced crystal yield with the natural profile providing the highest yields and the controlled profile providing the lowest for both initial dextrose concentrations. This is due to the higher crystallization rates occurring during the natural cooling profile over the 24 h trials. The influence of seed mass and cooling profile on crystal median size is less clear, although lower seed mass appears to produce larger median crystal sizes. Markande et al. (2009) discusses these trends in more detail for the lower initial dextrose concentration. Overall, Table 3 demonstrates the important influence of the initial dextrose concentration on yield during the 24 h trials, where the yields obtained for the 67.5% initial dextrose concentration were consistently much higher, irrespective of the seed mass and cooling profile used.
4.
Conclusions
The initial dextrose concentration of the inlet stream entering an industrial continuous crystallizer can fluctuate and this can influence the performance of dextrose crystallization. This paper investigated the influence of a typical industrial variation in initial dextrose concentration and how it affected the rate of progression of the crystallization, crystal yield and final crystal size in a batch cooling crystallizer. The crystallization trials showed that a 2% increase in initial dextrose concentration showed a significant increase in dextrose crystal yield, with percentage increase in yields ranging from about 24% to 55% for the seed masses and cooling profiles tested. The main contributor to the increased percentage yields is the inherent higher percentage increase in equilibrium yield of 19.5% obtainable by the higher initial dextrose concentration. The remainder is due to faster crystallization rates being achieved during the higher concentration trials, moving these trials closer to equilibrium by the end of the 24 h crystallization time. In-line measurements were applied to continuously monitor and help interpret the crystallizations. Higher levels of supersaturation were observed during the higher initial dextrose concentration trials using data obtained from the in-line refractometer. The higher supersaturation levels led to greater levels of nucleation, as monitored by the in-line FBRM probe. The greater nucleation caused an increased rate of dextrose consumption due to increased crystal surface area. Both seed mass and cooling profile influenced the rate of crystallization at both initial dextrose concentrations. As expected, higher seed mass resulted in faster crystallizations.
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The natural cooling profile trials were the fastest while the controlled cooling profile trials were the slowest for both initial dextrose concentrations. Contrary to the expectation that the higher dextrose consumption, at the higher initial dextrose concentration, would produce larger crystals, there was little influence observed on the final median crystal size. This is an indication of the importance of secondary nucleation and how it influences final crystal sizes. Secondary nucleation was greater at the higher initial dextrose trials which counteracted the greater dextrose consumption, as the crystal mass produced was mainly due to newly generated crystals and their subsequent growth. Overall, the crystal yields obtained and the amount of crystals produced were very sensitive to variations in the initial dextrose concentration.
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Jones, A.G., Mullin, J.W., 1974. Programmed cooling crystallization of potassium sulfate solutions. Chemical Engineering Science 29 (1), 105–118. Kail, N., Marquardt, W., Briesen, H., 2009. Process analysis by means of focused beam reflectance measurements. Industrial Engineering Chemistry Research 48, 2936–2946. Kubota, N., Doki, N., Yokota, M., Sato, A., 2001. Seeding policy in batch cooling crystallization. Powder Technology 121 (1), 31–38. Markande, A., 2009. Dextrose crystallization – control of crystal growth and crystal size. PhD thesis, University College Cork, Ireland. Markande, A., Nezzal, A., Fitzpatrick, J., Aerts, L., 2009. Investigation of the crystallisation kinetics of dextrose monohydrate using insitu particle size and supersaturation monitoring. Particulate Science and Technology 27, 373–388. Myerson, A.S., Ginde, R., 2002. Crystal growth and nucleation. In: Myerson, A.S. (Ed.), Handbook of Industrial Crystallization. Elsevier, pp. 33–65. Parisi, M., Terranova, A., Chianese, A., 2007. Pilot plant investigation on the kinetics of dextrose cooling crystallization. Industrial & Engineering Chemistry Research 46, 1277. Srisa-nga, S., Flood, A.E., White, E.T., 2006. The secondary nucleation threshold and crystal growth of ␣-glucose monohydrate in aqueous solution. Crystal Growth & Design 6 (3), 795. Worlitschek, J., Hocker, T., Mazzotti, M., 2005. Restoration of PSD from chord length. Particle & Particle Systems Characterization 22 (2), 81.