An Online Strategy To Increase the Average Crystal Size during

An Online Strategy To Increase the Average Crystal Size during ... The FTIR measurements of solute concentrations were used to develop a fines dissolu...
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Ind. Eng. Chem. Res. 2002, 41, 1321-1328

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An Online Strategy To Increase the Average Crystal Size during Organic Batch Cooling Crystallization F. Lewiner,† G. Fe´ votte,*,† J. P. Klein,† and F. Puel‡ LAGEP (Laboratoire d’Automatique et de Ge´ nie des Proce´ de´ s), UMR CNRS 5007, Universite´ Lyon 1, ESCPE, Baˆ t. 308G, 43 bld. du 11 novembre 1918, 69622 Villeurbanne Cedex, France, and Rhone-Poulenc Industrialization, CRIT, 24 avenue J. Jaure` s, 69153 Decines Charpieu Cedex, France

In situ attenuated total reflectance Fourier transform infrared (FTIR) measurements were shown to allow the online monitoring of supersaturation during solution crystallization processes, thus opening up new monitoring and control possibilities. With this aim in view, the monitoring of unseeded batch cooling solution crystallizations of two agrochemical products was investigated. The FTIR measurements of solute concentrations were used to develop a fines dissolution technique, based on a controlled heating-up procedure after primary nucleation. The strategy only requires the knowledge of the solubility curve and was deliberately designed to be robust and easy to implement. Two organic solute/solvent systems were studied. It is clearly demonstrated that the technique allows one to improve both the reproducibility of the final crystal size distribution and the mean crystal size. For Isoproturon, a well-known pesticide, the average final size is increased up to 90%. The efficiency of the monitoring procedure is also shown to depend on the solute/solvent system in question. 1. Introduction The manufacturing of high value-added specialty chemicals and pharmaceutical or agrochemical active ingredients often involves final or intermediate products in the solid form, where crystallization plays a key role as a separation and purification unit operation. The quality and end-use properties of the obtained particles are essentially determined by the habit of the crystals and the crystal size distribution (CSD) and, notably, by the amount of fines and/or large particles, the average size, and the width of the size distribution. It was earlier shown that the principal consequences of a bad control of crystallizers are the nonreproducibility and the low quality of the solid produced.1,2 Consequently, the control of industrial crystallizers or at least the optimization of operating conditions is of potentially great importance. However, as far as batch crystallization processes are concerned, many difficult control problems still have to be solved. Even the control targets and the operating variables, which could allow one to improve the quality and/or the reproducibility of the final products, are not always clearly identified.3-8 In particular, because the generation of supersaturation conditions in solution crystallization mainly depends on the cooling rate, a substantial research activity has been devoted to the computation of optimal temperature trajectories9-11 or optimal operating policies.12 Several review papers were focused on that topic.13-15 At least two statements arise from the reported theoretical and experimental results: 1. The efficiency of such control policies strongly depends on the accuracy of the nucleation and growth kinetic parameters, which are required to calculate an * To whom all correspondence should be addressed. E-mail: [email protected]. Tel: 33 4 72431839. Fax: 33 4 72431859. † Universite ´ Lyon 1. ‡ Rhone-Poulenc Industrialization, CRIT.

optimal temperature profile.15-18 Moreover, the assessment of the parameters requires cautious and complex experimental work, which is almost inconceivable in the context of industrial development. 2. The optimal strategies in question are basically open-loop control, and as such (i.e., they do not require any online measurement of the crystallization advancement) many possible drifts of the quality, productivity, and reproducibility are likely to happen because of the usual unavoidable industrial disturbances (i.e., batchto-batch variations of impurities, solid contents, drifts of the operating parameters, unexpected fouling, ...). An obvious solution to this problem lies in the closed-loop control of batch crystallizers, which is still a very active and open field of research. Many review papers were also published on this subject.8,14,15 Actually, although improvements in product CSD can be made by controlled operation or seeding,17 some fines can still be formed by secondary nucleation. Consequently, fines dissolution policies should also be considered as an efficient way of correcting the damage caused by excessive primary and/or secondary nucleation mechanisms. Fines dissolution can occur either just after nucleation or at any time during the crystallization operation13 and can be implemented through two different methodologies: 1. Once nucleation occurs, a fraction of the crystals in the slurry, of size lower than a selected size, is removed and passed through a heat exchanger to effect dissolution prior to return to the crystallizer.19 The dissolution can be performed using “open-loop control” or in response to an increase in the fines concentration.20,21 Such a procedure can lead to improvements in both the reproducibility and the product quality but is not necessarily easy to implement. In particular, the efficiency of the fines dissolution is strongly connected to the classification of particles which is a key modeling and control issue.22-25 2. A second possibility to dissolve fines is to heat the slurry after nucleation. For example, Heffels and de

10.1021/ie000962e CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002

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Figure 1. Experimental setup: bench-scale multipurpose temperature-controlled batch crystallizer equipped with an in situ ATR FTIR probe.

Jong26 have for objectives to keep the number of particles constant and so to increase the final CSD by dissolving the fines. They use a ParTec 100 probe to monitor the number of particles and use this information to decide when to start and stop the heating of the slurry. This method is easy to perform and very efficient in increasing the final average size and decreasing the width of the CSD. Even though various strategies have been proposed in the literature, it is obvious that time variations of relevant output variables such as CSD or supersaturation must be available through online measurements in order allow feedback control strategies to be implemented. Several research groups have shown that in situ attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy can be successfully applied to the monitoring of crystallization processes.15,27-34 To measure the solute concentration, a calibration approach was successfully used by Togkalidou et al.31 The authors analyze the FTIR spectral data using powerful chemometrics and compute supersaturation estimates in slurries of potassium dihydrogen phosphate (KDP). The accuracy is found to be on the order of (0.0005 kgKDP/kgsolvent for level of confidence 95%, which is quite a remarkable result. However, no online monitoring of the crystallization process was reported. Lewiner et al.32,33 reported a simplified calibration model which was successfully used for online measurements and compared with other models proposed in the literature.27-30 The aim of this paper is now to present a new online monitoring policy for batch crystallization processes, using the ATR FTIR measurements of supersaturation. Experimental results obtained for the crystallization of two agrochemical products (Bifenox and IPU) exhibiting significantly different behavior are reported. 2. Experimental Setup Figure 1 shows a schematic representation of the bench-scale evaporative crystallizer used for the present

study. The 5 L glass vessel is equipped with a jacket and a condenser. The jacket is baffled with a helicoidal ribbon, and a centrifugal pump forces the circulation. The stainless steel vessel lid is jacketed to limit heat losses. Stainless steel baffles are used in conjunction with a speed-controlled stirrer. A high-efficiency propeller (Mixel TT) is used to maintain a good homogeneity of particles of the slurry. The whole operating device is well instrumented and microcomputer (PC1)-controlled in order to allow the tracking of temperature trajectories. Cooling can be ensured by means of controlled evaporation and/or of heat transfer through the jacket wall. For this second cooling procedure, the temperature in the reactor is controlled by manipulating the setpoint temperature of a 2 kW heating bath containing water. Cold water circulating in coils is used for cooling the bath. The control range of the crystallizer temperature is 22-94 °C. More details on the reactor configuration and automation are given elsewhere.35 In situ measurements were performed using the infrared spectrometer Prote´ge´ 460 manufactured by Nicolet and two ATR immersion probes manufactured by Axiom Analytical Corp.: DPR-205 and DPR-207 equipped with a ZnSe and a ZnS conical internal reflection element, respectively. Because of its higher hardness, the ZnS crystal was finally found to be less sensitive to manipulations inherent to the laboratoryscale multipurpose use of the equipment. The spectrometer is equipped with a second PC dedicated to the Mid Infra Red measurements (PC2). The two PCs are connected via an ethernet link. 3. Crystallization of Isoproturon (IPU) in Ethanol 3.1. Crystallization System and Its Experimental Approach. IPU [C12H18N20, 3-(4-isopropylphenyl)-1,1dimethylurea] is a weed killer exhibiting platelike monocrystals which, according to industrial experience,

Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1323 Table 1. Batch Unseeded Cooling Crystallizations of IPU: Main Operating Parametersa batch Tn Theating ref (°C) (°C) N01 N02 N03 N04 N05 N06 N07 N09 N10 N11 N12 N13 N14 N15

55.1 56.5 57.7 52.8 57.1 57.4 57.6 56.8 54.1 54.2 49.9 53.8 52.6 57.4

59.7 59.8 60.0 61.6 58.3 59.4 62.4 59.9 56.5

T* - T* Tn Theating L h (°C) (°C) (µm)

hl (µm)

L hw (µm)

hlw (µm)

6.48 336.8 182.2 622.7 364.8 5.1 1.91 485.7 285.9 793.6 485.9 3.82 1.67 539.3 316.9 884.1 542.0 8.67 1.47 501.3 270.9 807.7 422.7 4.37 -0.18 615.7 350.3 1119.1 685.1 4 3.1 462.0 261.4 736.1 438.9 3.65 1.85 526.2 292.7 787.8 464.6 4.62 -0.99 439.3 254.2 1247.4 758.8 7.17 1.37 539.4 288.4 813.9 426.3 6.99 4.64 465.1 252.4 671.3 384.5 11.5 282.7 152.0 511.9 287.3 7.51 343.2 176.8 527.1 280.9 8.65 357.6 186.2 623.9 335.3 4.06 419.6 234.8 746.2 434.4

measured through image analysis, using the Visiolab 1000, Biocom software. Such measurements were performed manually for each crystal, from a minimum sample of 500 crystals. It was shown that a set of 500 crystals is sufficient to provide the same results as a larger number of crystals. To account for their plate shape (see Figure 2), the length and the width of the selected crystal were measured, and the results allowed the computation of both the number-average length and width of the sample (L h and hl, respectively) and the variation coefficient of the CSD in number (CVL and CVl, respectively). These values were calculated per crystal and not per class, as it is more usually the case, as follows: NT

L h)

For all of the experiments, the initial concentration is 0.2 ( 0.003 kgIPU/kgsolution and the cooling slopes before and after nucleation are respectively -30 and -22 °C/h. The stirring rate is 280 rpm. a

Li ∑ i)1 NT

(1)

NT

hl )

CVL )

li ∑ i)1 NT

x

(2)

NT

100

(Li - L h )2 ∑ i)1

L h

NT - 1

(3)

Given the crystal shape, the mass mi of a given crystal is expressed as follows, where ei is the thickness of the crystal: Figure 2. Microscopic pictures of table-like IPU crystals. Measurements of individual length and width (L and l).

mi ) FsLiWi ei

present a rather low growth rate. As explained in the sequel, unseeded crystallizations of IPU in ethanol were carried out with an initial concentration of 0.2 ( 0.003 kgIPU/kgSolution. Details and operating data from these runs are displayed in Table 1. Several batch runs were performed to estimate the final quality of IPU crystals obtained after primary nucleation and the “usual” cooling procedures (e.g., see N01 and N12-N15 in Table 1). After primary nucleation, a slight increase in the temperature was generally observed as an effect of the crystallization enthalpy. As explained below, for other runs (N02-N11), the suspension was deliberately heated after nucleation, between 2 and 3.4 °C, in order to study the effect of a possible fines dissolution on the final CSD. Initial solutions were prepared in the crystallizer using raw IPU crystals taken from an industrial bag, the dissolution in “technical grade ethanol” was achieved at 65 °C, and cooling was carried out according to the operating data specified in Table 1. Cooling was stopped at 22 °C. Depending on the variability of the nucleation point and on the decrease in the driving force for cooling the slurry at the end of the batch process, the total processing time was about 3 h for the “usual” crystallization operations. The duration of the dissolution procedure, according to the final heating setpoint temperature, was on the order of 20 min. In addition to the online FTIR measurement of supersaturation, the CSD of the final product was

Because ei cannot be measured through image analysis, the ratio b ) ei/Wi was assumed to be constant for all experiments, which finally appeared to be a reasonable approximation, except at very high cooling rates. Actually, b was assessed through scanning electron microscopy (SEM) and found to be on the order of 0.21. A mass balance based on the values of the initial and final solute concentrations was then applied to compute the final particle number, NT, and the weight mean sizes. More details are given by Lewiner et al.34 3.2. Experimental Results and Discussion. The solubility curve and the limit of the metastable zone were determined as described in a previous paper.33 The metastable zone was assessed for a cooling rate of -20 °C/h, and primary nucleation was not found to occur in a reproducible way. The effect of the cooling rate on the nucleation temperature was also investigated; it turned out that the cooling profile had no clear and logical effect on the width of the metastable zone. Typical concentration profiles reconstructed from the ATR FTIR measurement during batch unseeded cooling crystallizations of IPU in ethanol are displayed in Figure 3. After the solubility curve is crossed, a random primary nucleation occurs: the sharp decrease in concentration is associated with the growth of nuclei generated at the nucleation point. Two series of 10 identical crystallization experiments were performed to confirm the variability in the onset of primary nucleation. During these runs, which are not exhaustively

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Figure 3. Concentration profiles of dissolved IPU during unseeded batch crystallizations in ethanol.

Figure 5. Supersaturation profiles measured during the batch unseeded crystallization of IPU. A fines dissolution procedure was applied.

Figure 4. Variations of the average crystal sizes L h and hl against primary nucleation temperature, without fines dissolution. Measurements through image analysis (L, full lines; l, dashed lines).

Figure 6. Weight fraction divided by the width of class lengths of IPU obtained after cooling batch unseeded crystallizations where the fines dissolution procedure was applied.

presented in Table 1, the setpoint cooling rate was -20 °C/h and the initial solute concentrations were set to 0.1 and 0.2. Finally, primary nucleation was found to occur in the ranges of temperature of 26.5-34.9 and 48.4-55.1 °C, respectively. The system under consideration is, therefore, characterized by a random primary nucleation which, according to the level of supersaturation at which nucleation occurs, may impair the final size distribution. From that point of view, even though few final CSDs were measured, Figure 4 enlightens the decrease in the final mean size with decreasing nucleation temperatures. An in situ fines dissolution technique was then studied as a possible means of improving the quality and the reproducibility of the final CSD. Runs N02N11 were carried out to assess the efficiency of such an operating strategy. To optimize the heating of the suspension, the online supersaturation measurements were used to decide when to start and stop the heating. The following procedure was, therefore, applied: the solution was cooled until nucleation, and then the temperature was maintained constant until the concentration equilibrium was reached (detected using the FTIR measurements). The slurry was then heated until a setpoint temperature, Theating, where the temperature was maintained constant until the concentration equilibrium was reached (detected by the FTIR measurements). Cooling was then carried on. Such a procedure was expected to dissolve a possible excess of fine particles in order to reduce the batch-to-batch variations in the final particle number, NT, and average sizes.

From that point of view, Table 1 shows that heating the slurry after nucleation really improves the final CSD. Depending on the heating temperature, the average size can be increased by 90%. To optimize the quality of the product, the influence of the heating temperature was then studied for runs where nucleation took place around the same temperature (57-57.5 °C). With various nucleation temperatures, the same procedureswith the same setpoint heating temperatureswas performed in order to determine to which extent the reproducibility of the final product could be improved. Influence of the Heating Temperature. Figure 5 represents the supersaturation profiles obtained from the FTIR measurements. The different profiles show the variation of concentration loops, depending on the final heating temperature. The dissolution rate does not seem to be high. The concentration trajectory quickly joins the solubility after cooling is carried on. Figure 6 shows the CSD expressed in weight fraction divided by the class width (∆w/∆L) for different runs. A logarithmic axis is used in order to underline the fines content. It clearly appears that when the heating temperature increases, the main size population of the distribution is shifted to the bigger sizes. However, if the heating temperature is too high (run N09), the CSD is bimodal, showing many fine particles. The latter could be generated from an excessive dissolution. To confirm such a trend, five runs where primary nucleation took place at about 54 °C were selected to apply increasing final heating temperatures, Theating. The final particle

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Figure 8. Evolution of the mean length and width according to the nucleation temperature (L, full lines; l, dashed lines). Figure 7. Effect of the final heating temperature on the particle number for experiments where primary nucleation took place around 54 °C.

numbers were assessed using image analysis, as explained above. As one can see in Figure 7, even though few data are available, the effect of Theating is confirmed by the variations of the final particle number. Actually, NT decreases when the heating temperature increases in the neighborhood of the solubility temperature, and then the evolution is inverted for higher heating temperature. This is why, for the sake of clarity, the x axis indicates T* - Theating rather than Theating only. Such experimental results show that the final average size depends on the heating temperature. Heating up the slurry appears as a good means of improving the product quality, but it is necessary to keep a good compromise between a high average size and a narrow distribution, through the optimization of the fines dissolution procedure. More systematic experimental work, together with the design of a model relating primary nucleation, dissolution kinetics, and temperature profiles to the time variations of the CSD could allow the selection of optimal dissolution trajectories for any nucleation point. Influence of the Nucleation Temperature. As demonstrated above, when nucleation occurs at similar levels of supersaturation, heating the slurry to a constant prespecified setpoint temperature leads to a significant and controlled increase of the final average size. It is also an important issue to improve the reproducibility in the crystallization product for different nucleation temperatures. To test our strategy, the slurries obtained during operations exhibiting random nucleation temperatures (runs N02, N03, N04, N07, and N10) were heated in a similar way, until the same final dissolution temperature (around 59.7 °C). Figure 8 represents the evolution of the average sizes, L and l, depending on the nucleation temperature. It seems that the average lengths are fairly constant, while the average widths are very slightly increasing with the nucleation temperature. Despite such slight variation in the particle shape, it is clear that the procedure significantly improves the reproducibility of the CSD. Actually, it should be noticed that when the nucleation temperature decreases, the expected number of nuclei is higher. Consequently, the heating of the suspension should be more sustained in order to provide a similar number of particles after the dissolution. This was not the case for our experiments; anyway, the benefit of the fixed heating procedure which was applied is obvious.

Figure 9. Final CSD of IPU after applying the fines dissolution procedure with different nucleation temperatures: improvement of the reproducibility of the product.

The evolution of the final number of particles with the nucleation temperature, which is not represented here, confirms the earlier assumptions: the number increases as the nucleation is delayed. These results show that heating dissolves a certain amount of nuclei, but it is not surprising if the final particle number remains higher in the case of late primary nucleation. Figure 9 shows the size distributions measured through image analysis for the dissolution-controlled experiments performed with random nucleation temperatures. In comparison with the results displayed in Figure 4, it clearly appears that the control procedure increases the reproducibility of both the average size and the width of the size distribution of the final product. 4. Cooling Solution Crystallization of Bifenox in Methanol 4.1. Crystallization System and Its Experimental Approach. Bifenox [C14H9Cl2NO5, methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate] is a weed killer which is generally crystallized in methanol. The crystallization of this second solute/solvent system is known to be essentially dominated by agglomeration,32 a phenomenon which is not easy to model. Moreover, supersaturation remains very low after nucleation. Unseeded batch crystallizations of Bifenox in methanol were carried out. For these experiments, the initial concentration was 0.2 ( 0.004. The FTIR measurements were performed as described previously. Details and data from some of these runs are displayed in Table 2. For run N13, after primary nucleation, the temperature

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Table 2. Batch Unseeded Cooling Crystallizations of Bifenox: Main Operating Parameters (Initial Concentration ) 0.2 ( 0.004 kgBifenox/kgsolution) batch ref

dT/dt before nucleation (°C‚h-1)

Tn (°C)

N11 N12 N13 N14

-18 -21.3 -18.7 -19.74

51.9 52.77 52.6 51.64

dT/dt after Theating - nucleation SEM Tn (°C) (°C‚h-1) commentaries 2.6 3.4 0.78a 2.8

-11.8 -13.77 -21.6 -20.98

some fines many fines monocrystals some fines

a The slight increase in the temperature of the suspension is due to the crystalllization enthalpy.

Figure 11. Supersaturation profiles for batch unseeded crystallizations of Bifenox in methanol. A fines dissolution procedure was applied during run N12.

Figure 10. Concentration profiles of dissolved Bifenox during unseeded batch crystallizations in methanol.

Figure 12. SEM pictures of crystals obtained after the unseeded batch operations N12 (heating after primary nucleation) and N13 (without heating).

of the suspension was allowed to increase through the thermal effect of the crystallization. For other runs (N11, N12, and N14), the suspension after nucleation was deliberately heated (around 2-3.4 °C) in order to study the effect of the fines dissolution procedure on the final CSD. The final product was dispersed in a solution of methanol saturated with Bifenox, and the CSD was measured by laser diffraction, using a Malvern Mastersizer particle size analyzer. Because of the rather spherical shape of the final Bifenox particles, the CSD measurements were found to be suitable in this particular case. However, as explained in the following, the final particle shape was also observed after SEM pictures. 4.2. Experimental Results and Discussion. The experimental results about the solubility and the limit of the metastable zone have already been presented in a previous paper,32,33 where many experimental concentration and supersaturation profiles were shown and discussed. The goal is now to investigate the possibility of improving the quality and the reproducibility of the crystals through the manipulation of the temperature trajectory for this particular system. The influence on the final CSD of the heating procedure was first assessed, in comparison with the “usual” cooling batch crystallization operations. The comparison between runs N12 and N13 provides a typical example of the results obtained. Figures 10 and 11 show the ATR FTIR measurements performed during runs N12 and N13, where primary nucleation took place at similar levels of supersaturation. The trajectory of the relative supersaturation measured after the nucleation of batch N12 shows the concentration loop due to the heating of the suspension. Actually, because this was previously

the case, one could expect the heating period after nucleation performed during batch N12 to improve the CSD. However, it clearly appears that the fines dissolution procedure is not efficient. Table 2 indicates that a lot of additional fines was observed using both optical microscopy and SEM after runs N11, N12, and N14, with respect to run N13. Figure 12 shows representative pictures where it clearly appears that the final CSD is dramatically impaired after the heating loop during run N12. For batch N12, one can evaluate the amount of solid particles dissolved during the heating phase after primary nucleation (see Figure 10). The initial concentration of Bifenox in the solution is Ci ) 0.202. Following the nucleation period, the initial point of the loop (i.e., when the trajectory C(T) crosses the solubility curve) is (T1, C1) ) (53.7 °C, 0.158), and the point of the loop corresponding to the highest concentration is (T2, C2) ) (56.1 °C, 0.174). About 36% of the initial population of particles was thus dissolved so that it was reasonable to consider that almost no fines were remaining in the suspension when cooling was carried on. It should be noticed that past experimental works and industrial experience have already underlined the strong dependency of the CSD on agglomeration phenomena. This is why the unsuitability of the fines dissolution procedure was attributed to the burst of agglomerates due to thermal constraints and to the dissolution of crystal bridges inside the agglomerates: this assumption is consistent with other reported experimental results.26 To confirm such a hypothesis, the dissolution of final particles was investigated using online image analysis. As Figure 13 shows, heating the solvent results in the splitting of single final “big” particles into small ones which were already observed in Figure 12. Conse-

Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1327 L ) length of an IPU crystal, µm l ) width of an IPU crystal, µm L h ) number-average length, µm hl ) number-average width, µm L h w ) weight-average length, µm hlw ) weight-average width, µm NT ) total number of crystals T ) crystallizer temperature, °C T* ) solubility temperature corresponding to the initial solute concentration Ci, °C Theating ) final heating-up temperature, °C Tn ) nucleation temperature, °C W ) weight of crystals in a given size class Figure 13. Heating of a single particle of Bifenox in methanol. Real-time microscopic observation of the dissolution process and underlining of the burst into small particles.

quently, the dissolution procedure is unsuitable in the present case. 5. Conclusions and Perspectives The two studies reported in this paper aim at assessing the feasibility of improving the final CSD during batch organic cooling solution crystallizations by means of an “in situ” fines dissolution policy. The monitoring strategy is based on the use of online ATR FTIR measurements of supersaturation. After primary nucleation, the suspension is slightly heated and its trajectory in the undersaturated area is controlled to allow fine particles to dissolve. To maximize the final mean crystal size, appropriate operating parameters were searched. In the case of the crystallization of a first pesticide (IPU) in ethanol, the technique turned out to be efficient. The system exhibits large random variations of the metastable zone width leading to a poor reproducibility of the final CSD. With such a system, the fines dissolution strategy was proved to be efficient in reducing the batch-to-batch variations of the product and increasing the average sizes of the final platelike crystals. The cooling solution crystallization of a second weed killer (Bifenox) was also investigated. Such a particular product was known to be subject to agglomeration, which was confirmed through laboratory experiments. The comparison between the “usual” unseeded operations and ATR FTIR monitored batches showed that the final CSD was significantly impaired when heating was performed after primary nucleation. After cautious characterization of the final product, the irrelevance of the fines dissolution policy was attributed to the splitting of Bifenox agglomerates into smaller particles. The reported operating procedure is easy to implement and could thus be used to better operate industrial crystallizers without requiring any excessive development effort, provided that its relevance is demonstrated at the laboratory scale. However, it is worth noticing that the industrial robustness of the ATR probes, of the spectrometers, and of the required software tools (notably, the timelessness of calibration used to compute supersaturation online) is not fully guaranteed and still has to be improved. Notation C ) concentration, kgsolute/kgsolution C* ) solubility, kgsolute/kgsolution CV ) number variation coefficient of the CSD ei ) thickness of a given crystal, µm

Greek Letters σ ) relative supersaturation (C - C*)/C* Fs ) sensity of the solid, kg/m3 Abbreviations ATR ) attenuated total reflectance CSD ) crystal size distribution FTIR ) Fourier transform infrared spectroscopy IPU ) weed killer (Isoproturon) SEM ) scanning electron microscopy

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Received for review November 13, 2000 Revised manuscript received November 2, 2001 Accepted November 27, 2001 IE000962E