Separation of Chlorobenzoic Acids through Hydrotropy - Industrial

1956

Ind. Eng. Chem. Res. 1998, 37, 1956-1969

Separation of Chlorobenzoic Acids through Hydrotropy Ede´ sio J. Coloˆ nia,† Ashok B. Dixit,‡ and Narayan S. Tavare* Department of Chemical Engineering, University of Bradford, West Yorkshire BD7 1DP, U.K.

The separation of o- and p-chlorobenzoic acids from their mixtures was investigated using hydrotrope solutions. The aqueous solubilities of o- and p-chlorobenzoic acids in different concentrations of hydrotrope (i.e., sodium butyl monoglycol sulfate) at different temperatures were determined by the weight disappearance method. The ternary solid-liquid-phase equilibrium diagram for the o-chlorobenzoic acid-p-chlorobenzoic acid-water system with the same hydrotrope, incorporating several hydrotrope isoplethal curves and two-component saturation and eutectic tie-lines, was constructed. The sensitivity and feasibility of the proposed process were examined by carrying out solubilization and equilibrium precipitation experiments with the mixtures of various compositions, including the eutectic composition. The precipitation kinetics of pure o- and p-chlorobenzoic acids from their hydrotrope solutions were studied in a laboratory-scale agitated precipitator. The nucleation and growth rates are correlated in terms of significant process variables using power law models. With this hydrotrope it was possible to enrich p-chlorobenzoic acid significantly in the product crystalline phase from the eutectic composition in a single stage. Highly pure (>99.0%) chlorobenzoic acid crystals were obtained from precipitation of noneutectic mixtures of o- and p-chlorobenzoic acids in their primary precipitation region of the ternary phase equilibrium diagram. Introduction In the manufacture of chemical and pharmaceutical products, a variety of isomeric or closely related mixtures of organic compounds are encountered and often require their separation into nearly pure components. In most cases the constituents have very close chemical and physical properties due to their similar molecular sizes and are difficult to separate by conventional methods such as distillation, extraction, and crystallization. For an effective separation only the methods exploiting properties based on the differences in microproperties of molecular structure of the component to be separated rather than the macrostructure have a good chance. Several possible techniques are available; most depend on the addition of a third component which augments the differences in physical properties such as boiling points, melting points, and solubilities, thus increasing the ease of separation. Extractive or adductive crystallization, reactive distillation or extraction, and extraction with supercritical fluids have been advocated. Creating a third component by complex formation as in an adductive crystallization process or adding a second phase and then distributing differentially these components between the resultant phases as in a process of dissociation extraction can be employed in some cases. The purpose of this paper is to illustrate the efficacy of a novel crystallization (or precipitation) technique for isomer separations using the phenomenon of hydrotropy. Hydrotropes, a class of organic compounds, significantly increase the aqueous solubility of certain sparingly soluble compounds and can be used for a * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +44 1274 23 5700. Phone: +44 1274 23 36 81. † Deceased. ‡ Present address: Edinburgh Petroleum Services Ltd., Research Park, Riccarton, Edinburgh EH14 4AP, U.K.

variety of applications. Sharma and co-workers (Jankiraman and Sharma, 1985; Gaikar and Sharma, 1986; Gaikar et al., 1988; Sadvilkar et al., 1995a,b) used hydrotropes to enhance the rates of multiphase chemical reactions as well as to separate selectively close-boiling compounds by either extractive distillation or liquidliquid extraction. Tavare and co-workers (Tavare and Gaikar, 1991; Geetha et al., 1991; Raynaud-Lacroze and Tavare, 1993; Coloˆnia et al., 1993; Coloˆnia and Tavare, 1994; Jadhav et al., 1995) exploited the phenomenon of hydrotropy for the separation of isomeric mixtures. For several binary mixtures, they found that the enhancement in the solubility with the hydrotrope concentration was significantly higher for one of the components and used this observation to selectively precipitate one of the components by controlled dilution with water. This technique avoids the use of inflammable and expensive organic solvents as are often used in dilution crystallization or extreme temperatures and multistage operations required in melt crystallization. They demonstrated its successful application for the separation of o- and p-chloronitrobenzenes, 1- and 2-naphthols, and m- and p-aminoacetophenones, in some cases even at eutectic compositions. The specificity in solubilization has been successfully employed not only for the separation of close boiling and both close boiling and close melting components from binary mixtures forming simple eutectics but also for achieving improved quality and purity of pharmaceutical intermediates such as 6-aminopenicellanic acid (Tavare and Jadhav, 1996). Phatak and Gaikar (1993) reported the solubilities of pure o- and p-chlorobenzoic acids (CBAs) in various hydrotropes in order to look at the feasibility of their separations through hydrotropy. The present research was motivated to look systematically at the use of the phenomenon of hydrotropy for the separation of o- and p-CBAs from their mixtures using the suitable hydrotrope sodium butyl monoglycol sulfate (NaBMGS) and to demonstrate how it can be

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Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1957

Figure 1. Binary phase equilibrium diagram for the o- and p-CBA system.

approached using sound analysis based on crystallization principles. CBAs, used as intermediate chemicals, can be synthesized by the oxidation of a mixture of chlorotoluenes; therefore, the end product will contain an isomeric mixture of mainly o- and p-CBAs and cannot be separated by the conventional separation techniques. The melting points of o- and p-CBAs are 142 and 243 °C, respectively. These two components form a simple eutectics system as shown in Figure 1. Laddha and Sharma (1978) reported the dissociation leaching (or extraction) method on a laboratory scale to separate selectively p-CBA from o-CBA. They observed that both these isomers, particularly the p-isomer, are sparingly soluble in most organic solvents. They were able to enrich a mixture starting from 60-87 and 100% pisomer by treating it with 0.2 M NaOH in a single stage at 27 and 65 °C with high separation factors, respectively. The initial experimental study concerned the determination of the relevant solubility data, the construction of the ternary phase equilibrium diagram for the oCBA-p-CBA-water system with the chosen hydrotrope, and the characterization of precipitation kinetics of both o- and p-CBAs from their respective hydrotrope solutions (Coloˆnia et al., 1996, 1998; Dixit et al., 1996). In the present study, the separation of o- and p-CBA from their mixtures using NaBMGS as a hydrotrope is reported in detail. The sensitivity and feasibility of the separation to the process variables along with the influence on precipitation kinetics were examined by performing a series of precipitation experiments with various mixture compositions and process conditions. In addition, supersaturation profiles for the experiments with a single-component system are evaluated. Phase Equilibrium Relations Solubility Data. The solubilities of pure o- and p-CBAs in aqueous NaBMGS solutions were determined by the weight disappearance method (Geetha et al., 1991; Coloˆnia et al., 1996; Dixit et al., 1997). The solubility curves of o- and p-CBA in aqueous NaBMGS solution at 25 and 45 °C are shown in Figure 2. Although hydrotropes significantly increase the aqueous solubility of certain sparingly soluble compounds, they themselves are freely soluble in water and appear to self-aggregate in an aqueous solution as organized assemblies in a stacklike fashion and solubilize the solute by a similar associative mechanism above a minimum hydrotrope concentration. Above this thresh-

Figure 2. Solubility curves.

old called the critical hydrotrope concentration (CHC), the solubilization rises appreciably and eventually levels off to a plateau, thus resulting in a sigmoidal solubility versus hydrotrope concentration curve. For NaBMGS, the commercially available neat and critical hydrotrope concentrations are about 100 and 20 g/100 g of water, respectively. At 25 °C, although the relative enhancement in solubility for both components is the same (∼12) from the critical to neat hydrotrope concentration, the absolute increase for o-CBA (from 0.9 to 10.7 g/100 g of water) is significantly higher than that for p-CBA (from 0.1 to 1.2 g/100 g of water). Since the absolute increases in solubility for the two components are significantly different, both dilution and thermal effects can be exploited to achieve improved performance of the separation process (see also Phatak and Gaikar, 1993). Figure 2 also demonstrates that the absolute increases at 45 °C are higher by a factor of about 2 than those at 25 °C. The solubilities of o- and p-CBA in water at 25 °C are 0.30 and 0.01 g/100 g of water, respectively. Slightly lower values (0.21 and 0.008 g/100 g of water for o- and p-CBA, respectively) were reported at 30 °C by Laddha and Sharma (1978). The values reported by both Osol and Kilpatrick (1933) (∼0.21 and 0.007 g/100 g of water) and Phatak and Gaikar (1993) (∼0.21 and 0.008 g/100 g of water for o- and p-CBAs, respectively, at 30 °C) were similar to those of Laddha and Sharma (1978). The ratio of the solubility of two CBAs at 25 °C in a neat solution of NaBMGS to that in distilled water is 36 for o- and 109 for p-CBA, respectively. Trends in solubilities of o- and p-CBAs in NaBMGS solutions reported in this study and by Phatak and Gaikar (1993) are similar; the precise comparison though is difficult. The solubility of o-isomer is much higher (about 1 order of magnitude) than that of p-isomer in most hydrotrope solutions. The absolute enhancement in the o-CBA solubility may be attributed to its molecular structure. Generally, the p-component has a higher melting point and a lower boiling point than the corresponding ocomponent. The aggregation structure of a hydrotrope in aqueous solution can better accommodate the ocomponent into mobile aggregates than its symmetrically structured p-component. Ternary Phase Equilibrium Diagram. The construction of the ternary phase equilibrium diagram is essential for the prediction of the efficacy of a given hydrotrope as the location of the initial composition and the path followed by the separation process on the ternary phase diagram decide the purity of the products separated. In addition to pure single-component solu-

1958 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998

Figure 4. Experimental setup.

Figure 3. Ternary phase diagram of the o-CBA, p-CBA, and water system. T ) 25 °C (all component concentrations are in wt %, cH ) hydrotrope concentration, g/g of water).

bility data, datapoints on a saturation line with respect to both these components were determined from the analysis of the concentrations of o- and p-CBAs in the equilibrated solutions and solid phases by HPLC. The solution phase concentrations were used to construct the two-component saturation tie-line. The solubility of a given mixture of o- and p-CBAs in a specific NaBMGS solution was determined by the crystal disappearance point method (Coloˆnia et al., 1997), and the resulting data were used to construct the hydrotrope isoplethal curves at different concentrations, namely, 0.3, 0.5, 0.7, and 1.0 g/g of water at 25 °C on a ternary phase diagram, with each apex representing one of the three components, viz., o-CBA, p-CBA, and water. The twocomponent saturation tie-line was obtained by joining the water apex and datapoints representing the solutions saturated with respect to both components. The eutectic tie-line was drawn by simply joining the water apex and the datapoint representing the eutectic composition, i.e., 86% o-CBA in the binary system. For the present system, NaBMGS was influential in shifting the two-component saturation tie-line away from the eutectic tie-line, thus providing the possibility of separation of the eutectic composition. For the sake of clarity, only the enlarged top water apex region of the ternary phase equilibrium diagram, with a base representing the relative composition of CBAs, along with four hydrotrope isoplethal curves and eutectic and twocomponent saturation tie-lines, is given in Figure 3. The intersections of the isoplethal curves with the two axes represent the solubilities of single-component o- and p-CBAs in the specified hydrotrope concentration. For a given hydrotrope concentration, the saturation concentration of o-CBA was significantly higher than that for p-CBA. Also, the solubility of o-CBA augments more rapidly with the hydrotrope concentration than that of p-CBA. The solubility of p-CBA is much influenced by the presence of o-CBA in the solution phase. The twocomponent saturation tie-line is very important as it divides the ternary phase equilibrium diagram into two primary crystallization regions for these two components of the binary system. Depending upon the initial composition of the mixture, one of the two-components would selectively precipitate out on dilution with water, provided that the process path followed during the separation remains within the primary crystallization region. If the initial composition of the mixture lies on

the left-hand side of the two component saturation tieline in the o-CBA primary crystallization region of the ternary phase equilibrium diagram, then o-CBA would primarily precipitate out and similarly p-CBA from the p-CBA primary crystallization region on the right-hand side. Thus, the location of the initial composition and the path followed by the process on the ternary phase equilibrium diagram decide the precipitating component. Precipitation Kinetics Experimental Section. Solubilization-precipitation experiments were conducted in a 2-L laboratoryscale isothermal agitated and jacketed batch crystallizer in order to investigate the kinetics of the precipitation process of o- or p-CBA from its aqueous hydrotrope solution by addition of water at a known rate. A schematic diagram of the experimental setup is shown in Figure 4. The crystallizer used was a jacketed glass vessel, with an internal diameter of 10 cm, equipped with four stainless steel full baffles and a 35-mm sixbladed stainless steel turbine impeller with 20-mm clearance for the agitation. The temperature of the crystallizer was maintained at a desired value within (0.1 °C by circulating a constant-temperature water from the bath through the jacket of the crystallizer. The precipitant, i.e., distilled water, was also maintained at the same constant temperature. A programmable peristaltic pump (Watson Marlow model 505 Du) was employed for the accurate addition of water to the crystallizer at the desired rate. In a typical experiment with pure components, a nearly-saturated solution of either o- or p-CBA (usually ∼90% of the precipitating component at the saturation point) in 500 g of a neat NaBMGS solution (50% w/w) was prepared at the desired temperature. The solution was then filtered through a filter paper (Whatman no. 1) at constant temperature to remove any solid-phase impurities. The filtrate was then immediately transferred to the crystallizer whose temperature was already kept at the required value. The agitation was started, and the stirrer speed was measured with the help of a tachometer and adjusted to the set value (∼20 Hz). The precipitant, i.e., water (∼1000 g, i.e., about twice the initial amount of hydrotrope), was added to the crystallizer at the desired flow rate using a programmable peristaltic pump for the predecided time ranging from 40 to 240 min. The entire scontent of the crystallizer was then filtered. A sample of the final filtrate was used for the solution phase analysis. The product crystals were dried in an oven below 70 °C, and a sample from the resulting dry product crystals was used to study the


crystal morphology using a scanning electron microscope. In the case of experiments with mixtures, initially, the requisite amounts of o- and p-CBA (∼80% of the solubility of p-CBA in the p-CBA primary crystallization range for the p-CBA composition g 14% being the eutectic composition and ∼80% of the solubility of o-CBA otherwise in the o-CBA primary crystallization range) were dissolved in a neat NaBMGS solution (50% w/w). The rest of the procedure employed was identical with that for a pure component. Small suspension samples (3-5 g) were withdrawn periodically from a fixed location near the impeller diametrically opposite to the feed inlet in the crystallizer and were filtered immediately at the crystallizer temperature using a Bu¨chner funnel. The filtrate was weighed and used for the determination of o- and p-CBA concentrations in the solution phase by HPLC (Varian model 5020). For the HPLC analysis, a S5 ODS2 (2 m × 4.6 mm) column was used. The supersaturation value at any time was calculated by subtracting the solubility from the actual concentration in the case of a singlecomponent system. The crystals on the filter paper were then resuspended in the Isoton solution, an electrolyte solution supplied by Coulter Electronics, saturated with o-CBA and/or p-CBA depending upon the precipitating component in an experiment and analyzed using a multichannel Coulter counter (model: Multisizer II) fitted with a 400-µm orifice tube. The measurements were made in the range of 8-268 µm. In the case of experiments with mixtures, samples of the solids retained by the filter paper were also analyzed by HPLC to determine the composition of the solid phase. Results and Discussion For the sake of simplicity, the precipitation kinetics of single pure components were initially investigated and the results were reported in terms of relative nucleation kinetics alone (Coloˆnia et al., 1996; Dixit et al., 1996). In this study further solution-phase details are reported along with the precipitation experiments with mixtures. Precipitation of o- or p-CBA from Their SingleComponent Hydrotrope Solutions. A series of 10 experimental runs for o-CBA was performed to study the influence of three important variables, viz., temperature, stirrer speed, and water addition rate, on the nucleation and growth rates. Initially, the temperature was varied from 15 to 45 °C, while the water addition rate and stirrer speed were kept almost constant at ∼13 g/min and ∼14 Hz, respectively. The flow rate was varied from 4 to 24 g/min in the next four runs whereas the stirrer speed was maintained at ∼10 and 17 Hz in the last two runs in order to examine the influence of stirrer speed. Further, four experiments (runs 11-14) were carried out for p-CBA. In the first two runs, the temperature was kept constant and the flow rate of water was varied from 12 to 4 g/min, whereas the remaining two experiments were conducted at 45 °C with two different water flow rates (∼12 and 4 g/min). As the change in the p-CBA solubility with water addition was negligible, an amount of water (400-600 g) at the operating temperature was initially added to the crystallizer in order to achieve a sufficient level of supersaturation for p-CBA. The remaining water was added at a constant flow rate using a peristaltic pump. A sample was withdrawn for size analysis at time zero to check whether any precipitation has taken place. In

Figure 5. Typical o-CBA solubility and concentration profiles.

all the experiments, no precipitation was detected for these samples at time zero. The solubility and concentration profiles for typical runs of o-CBA (runs 2, 3 and 8) are given in Figure 5. runs 2 and 3 were performed at temperatures of 25 and 35 °C, respectively, with water addition rates of ∼13 g/min, while run 8 was performed at a water addition rate of ∼24 g/min and 25 °C. The experimentally determined concentrations of both these components from samples from runs 1-14 were correlated in the form of an exponentially decaying function of time using nonlinear regression analysis. All the correlations were satisfactory, yielding good regression correlation coefficient (r > 0.95) and small relative standard error in coefficient estimates (e < 0.05). The solubility data at different temperatures employed were well correlated into a linear function of hydrotrope concentration by a linear regression analysis over the experimental range (with linear regression correlation coefficient r > 0.99 at all temperatures). The concentration and solubility values at any time were estimated from the correlations developed for both o- and p-CBA. The supersaturation value at any time was determined by subtracting the calculated solubility from the estimated actual concentration, and its variation with time can be evaluated. The estimated actual concentration at the time from the correlated component concentration profile for the run and the calculated solubility at any time was determined at prevailing hydrotrope concentration from the appropriate solubility-hydrotrope concentration correlation at crystallizer temperature. The hydrotrope concentration (g/g of water) was determined from the material balance calculations using the initial hydrotrope concentration and the amount of water added up to the sample time. Figure 6 demonstrates the variation of supersaturation with time and also the effect of process variables such as water addition rate, temperature, and stirrer speed on the supersaturation profiles of o- and p-CBA. For any water addition rate, the initial slightly undersaturated solution first became saturated and then supersaturated as a result of the reduction in the solute solubility as the hydrotrope concentration decreased due to dilution. Since both nucleation and growth processes depleted the level of supersaturation, this important process variable passed through a maximum and then decreased to a constant value close to zero (i.e., the saturation point). The peak position of the supersaturation curve and the extent of supersaturation were found to vary significantly with significant process


Figure 6. CBA supersaturation profiles.

parameters. At a lower water addition rate, the supersaturation profile was wider while at a higher flow rate it had a sharper and earlier peak. Consequently, a smaller number of large crystals were produced at lower flow rates. The flatter supersaturation profile for run 5 performed at the lowest water addition rate (∼4 g/min) and the sharper and earlier peak in the supersaturation profile for run 8 performed at the highest water addition rate (∼24 g/min) in the present experimental program are apparent in Figure 6, with both these runs being carried out at 25 °C. For run 4 (performed at a temperature of 45 °C and a water addition rate of ∼13 g/min), the supersaturation increases rapidly to a very high value of ∼20 mg/g of water whereas for other runs the maximum level of supersaturation was observed to remain almost constant (∼10 mg/g) even when the water addition rate was varied from 4 to 24 g/min. The peak position, however, shifts depending on the water addition rate. In some cases at longer times (say, for example, after ∼30 min in the case of run 4), the negative supersaturation was estimated due to an experimental error and fitting procedure employed. Thus, qualitative observations indicated that both the water addition rate and the temperature had a significant influence on both the supersaturation profiles and crystal size distributions. A small change in the supersaturation profile was, however, observed when the stirrer speed was changed over the range from 10.3 (run 9) to 15.3 (run 2). For the case of p-CBA experiments (runs 11-14) supersaturation values were evaluated using a procedure similar to o-CBA experiments (runs 1-10) as described above. Included in Figure 6 are the supersaturation profiles for runs 12 and 13 performed using the water addition flow rate of 4 g/min at 25 and 45 °C, respectively. Since the crystallizer was operated for a short period (ranging from 65 to 150 min) at low magma concentration for p-CBA in all runs, the solutions remained supersaturated at low levels and the supersaturation profiles did not pass through a distinct maximum over the period of batch time. Comparatively low values of supersaturation and magma concentrations were encountered for all p-CBA experiments (runs 11-14). Typical variations of population density based on the total solvent capacity at any time with crystal size at four sampling times in the precipitation of o- and p-CBA for runs 2 and 11 from the hydrotrope solutions are presented in Figure 7, respectively. The solid and open symbols represent samples of o-CBA (run 2) and p-CBA

Figure 7. Typical population density plots for CBAs (runs 2 and 11).

Figure 8. Final product size distributions (see Figure 6).

(run 11), respectively. In both these runs the population density function decays with crystal size at any sample time and increases with time and hence with the amount of diluent. In almost all these runs there is a suggestion that the population density data on the population density plots (e.g., Figure 7) at early sample times (typically below 3000 s) show a slight hyperbolic curvature, while they tend to become linear later, reflecting the supersaturation profile and sensitivity of kinetic events. The variation in the population density with time is less distinct in p-CBA than in o-CBA. This might be due to low levels of both supersaturation and magma concentration in p-CBA experiments. The population density curves appear to follow the expected trends in batch crystallizers with varying solvent capacity (Tavare, 1995b). The crystal size distributions of the final products are reported in Figure 8. The variation of the population density curves for o-CBA with water addition rate showed the expected trend. A small number of small crystals in run 5 with a water addition rate of 4 g/min are due to a widespread flat supersaturation profile (Figure 6). Run 8 was performed at a water addition of 24 g/min having a large number of small crystals, reflecting earlier and sharper peaks in the supersatu-


ration profile. For the remaining water addition rates, the population density curves lie between these two limits. The number of smaller crystals in these runs appear to be almost constant, and an appreciable variation with the water addition rate is apparent for large crystals. At a high temperature of 45 °C in run 4, a small number of large crystals were observed, while at a low temperature of 15 °C in run 1 (not shown), a large number of small crystals were observed. This might be due to the sensitivity of kinetic events, especially that of the crystal growth rate, to the temperature. The effect of water addition rate on the final crystal size distribution for p-CBA was similar. At 25 °C, a small number of large crystals were produced with 4 g/min (run 11) whereas a large number of small crystals were formed with 12 g/min (run 12). Similarly, a large number of large crystals were found in run 13 (45 °C) as compared to that in run 11 (25 °C). This might be again due to different sensitivities of kinetic rates to both supersaturation and temperature. The final product population density variations for two runs with p-CBA (run 12 with 4 g/min at 25 °C and run 13 with ∼12 g/min at 45 °C) are also included in Figure 8. The curvature for these product population density data may be attributed to low levels of supersaturation and magma concentrations encountered over the short period of batch times for a relatively slow growing system. Although the experimentally measured crystal size distributions show generally large scatter, the final product crystal size distributions for all runs appear reasonably well behaved. The crystal size distribution and hence product crystal size can be improved by proper manipulation of the significant process variables. Large size crystals can be obtained by controlling the level of supersaturation at a desirable value. Generally, controlled low water addition rate, high stirrer speed, and temperature would favor the production of largesize, good-quality crystals (see also Tavare and Gaikar, 1991; Raynaud-Lacroze and Tavare, 1993). The transient population densities based on the total solvent capacity at any time, estimated using the crystal size distribution data obtained from a Coulter counter and typically shown for four samples at different times in run 2 for o-CBA and in run 11 for p-CBA as in Figure 7, were used to determine the nucleation and growth rates by the method of s-plane analysis from the two successive population densities (Tavare and Garside, 1986; Dixit et al., 1996). In this method, a plot of the time rate of change of the Laplace-transformed population density, defined on the basis of the total solvent capacity, against the product of the Laplace transform variable and the average Laplace-transformed population density based on the total solvent capacity over an optimal range of the Laplace transform variable should yield a straight line with a slope equal to the negative average growth rate and the intercept of the nucleation rate based on the total capacity of the solvent. The growth and nucleation rates represent average values over the time interval, and so all other state variables incorporated in the kinetic correlations should correspond to an average time. All the results of the nucleation and growth rates obtained from experimental runs were used to correlate an empirical power law kinetic expression in terms of significant variables. In all, 44 for o-CBA (from runs 1-10) and 17 for p-CBA (from runs 11-14), datapoints evaluated at different times under different process

conditions were used for the estimation of growth and nucleation rate expressions. Various process parameters such as temperature, T (K), stirrer speed, N (Hz), component concentration in the solution phase, co or cp (g/g of water), hydrotrope concentration, cH (g/g of water), mean crystal size, L h (µm), supersaturation, ∆c (g/g of water), and magma concentration, MT (g/g of suspension), were considered for their incorporation in the nucleation and growth rate correlations. The regression analyses were carried out using Sigma Plot nonlinear curve-fitting software. The best-fit correlations are given as

G ) 0.29∆c0.88N2.08L h -0.96 and B ) 3300G0.52 for o-CBA (1) B ) 58G0.82

for p-CBA

(2)

where G and B represent the growth rate (µm/s) and the nucleation rate (no./(g of water‚s)), respectively. The supersaturation, stirrer speed, and mean crystal size were found to be the significant parameters of the growth rate correlation for o-CBA. The growth rate correlation was not evaluated due to the limited number of experimental observations in the case of p-CBA. It was thought desirable to correlate the nucleation rate in terms of growth rate rather than supersaturation as these types of correlations are commonly derived from kinetic studies of a continuous MSMPR (mixed-suspension mixed-product removal) crystallizer. These offer advantages over the correlations in terms of supersaturation as a variable as they eliminate the use of the difficult-to-measure variables and therefore of the unreliably measured variable, viz., supersaturation, and the consideration of the segregation effects (Tavare, 1995b). Nevertheless, the possibility of segregation does exist in the present experiment but can be substantially minimized by using a high stirrer speed and low water addition rate. With a limited number of observations, only the growth rate was found to be a significant variable in the nucleation rate correlation (eq 1) for o-CBA. The comparison of the experimental and estimated values for nucleation and growth rates suggested that these rates can be estimated with reasonable accuracy by these empirical correlations. The scatter in growth rate correlations is generally high when such an approach is employed (Tavare and Garside, 1986; Tavare and Gaikar, 1991). The comparisons of the experimental and estimated nucleation rates for both these components are presented in Figure 9. The nucleation rates of o-CBA are much higher than those of p-CBA over similar growth rate ranges as observed in precipitation experiments performed with the singlecomponent hydrotrope solution. All the exponents in both the kinetic correlations (eqs 1 and 2) are reasonable. The relative kinetic orders for o- and p-CBA lie within the range reported in the literature and are estimated with relative standard deviations of 25 and 13%, respectively. The negative exponent of the mean particle size in the growth rate correlation might be due to a comparatively smaller variation in its value with changes in the process conditions. The orders of magnitude of the exponents are concurrent with those reported by Tavare and Gaikar (1991) and RaynaudLacroze and Tavare (1993). Separation of o- and/or p-CBA from Their TwoComponent Hydrotrope Solutions. To examine the feasibility and sensitivity of the proposed separation


Figure 9. Relative nucleation kinetics.

technique using the phenomenon of hydrotropy for the separation of o- and p-CBAs from their mixtures, a series of experiments was conducted with a hydrotrope solution containing mixtures of these two components at various compositions. In all, 18 additional experiments as listed in Table 1 were conducted, varying most significant variables such as the initial composition of o- and p-CBA, water addition rate, temperature, and degree of seeding: six experiments with 97% o-CBA in the o-CBA primary crystallization region (runs 15-20), two at eutectic composition (i.e., 86% o-CBA and 14% p-CBA) (runs 21 and 22), and the remaining 10 in the p-CBA primary crystallization region (runs 23-32) (eight for the representative industrial reactor product composition (i.e., 58% o-CBA and 42% p-CBA) (runs 2330), one at the composition used by Laddha and Sharma (1978) (i.e., 40% o-CBA and 60% p-CBA) (run 31), and the last one at 10% o-CBA and 90% p-CBA in order to cover the complete composition range (run 32)). As mentioned, solubilization-precipitation experiments for mixtures were conducted in the same 2-L laboratory-scale crystallizer using procedure similar to that used for single-component solutions. In a typical precipitation experiment, the predecided amount of the o- and p-CBA mixture (usually corresponding to ∼80% of the saturation point with respect to the precipitating component for a given composition) was dissolved in ∼500 g of a neat NaBMGS solution (50% w/w) at the desired temperature. The insoluble impurities from the solution were removed by filtering the solution, and the filtrate was then immediately transferred to the crystallizer at a desired temperature. The agitation was adjusted to the set value (∼20 Hz). The precipitant, i.e., water (∼1000 g), was added to the crystallizer at the desired flow rate using a programmable peristaltic pump for the predecided time ranging from 40 to 240 min. Several small suspension samples (3-5 g) were withdrawn periodically and filtered immediately at the crystallizer temperature using a Bu¨chner funnel. The o- and p-CBA concentrations in the solution and solid phases were determined by analyzing the filtrate and the solids on the filter paper with the help of HPLC. A sample from the resulting final product crystals was used to study the crystal morphology using a scanning electron microscope. The population densities of the precipitated solids were evaluated using a multichannel Coulter counter (model Multisizer II) fitted with a 400µm orifice tube in the size range from 8 to 268 µm.

The initial experiment (run 15) in a series of six experiments with a mixture containing 97% o-CBA at 25 °C was performed at a high water addition rate (12 g water/min) and a low stirrer speed (∼13 Hz), and both the sampling and subsequent size analysis during this experiment were difficult as the resulting product crystals were severely agglomerated. The minimization of the agglomeration effect was unsuccessful as indicated by the next two experiments (runs 16 and 17), performed at a higher stirrer speed (∼20 Hz) and a lower water addition rate (4 g of water/min). In addition, seed loading (g0.5% of the amount of hydrotrope) was effective in eliminating the agglomeration in product as shown by the three experiments (runs 18-20) performed at different seed loadings. Product characteristics were improved in experiments over these levels of seed loading. Thus, the seed loading can be used as an important manipulating variable as it can provide adequate surface area to deplete the generated supersaturation to a desirable level. This will minimize undesirable nucleation and agglomeration rates and promote crystal growth at a desirable level. The recovery and purity of the final product are the important parameters in all the separation experiments, and their values are reported in Table 1. In the six experiments with the initial 97% o-CBA composition, the recovery and purity varied from 50 to 70% and 95 to 100%, respectively. It was difficult to comment on the improvement in purity for the experiments due to quality of the product and accuracy of the experimental analysis. In six experiments (runs 15-20) performed in the o-CBA primary crystallization region, pure o-CBA as expected was crystallized out. The initial composition was close to 100%, and the separation was not practically noticeable. The final composition was almost the same as that of the initial composition in almost all the experiments. Two experiments were performed with the initial mixtures at the eutectic composition (86% o-CBA and 14% p-CBA), and the product contained ∼70% p-CBA at ∼80% recovery. This confirmed that the technique could be used for the separation of the eutectic composition. The purity of the final product in experiments performed at the eutectic composition (runs 21 and 22) suggested that both o-CBA and p-CBA precipitated out in these experiments but at different compositions. The precipitation experiments with the eutectic composition provide significant enrichment with respect to p-CBA in the solid phase for both hydrotropes. These two experiments confirmed that the eutectic composition could be separated through the phenomenon of hydrotropy. The separation of eutectic mixtures is generally difficult, requiring unconventional techniques such as adductive or extractive crystallization and strategy of selective reversible reactions (Tavare, 1995b). Since the technique of selective precipitation from hydrotrope solutions for separation purposes is relatively simple, it can be employed for eutectic mixtures. Raynaud-Lacroze and Tavare (1993), however, showed that the eutectic composition of 1- and 2-naphthols could not be broken with sodium cumene sulfonate (NaCS), while Coloˆnia et al. (1993) demonstrated the feasibility of separating compositions of o- and pchlorobenzoic acids and o- and p-chloronitrobenzenes (CNBs) using NaCS. Tavare and Coloˆnia (1997) explored further the possibility of separation of mixtures of o-, m-, and p-CNBs at the eutectic composition (i.e., the three binary and the ternary mixtures) using

Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1963 Table 1. Experimental Program Details and Ranges

run no.

precipitation of CBA

initial binary composition of CBA

N, Hz

T, °C

1-10 11-14 15-20 21-22 23-30 31 32

o p o o and p p p p

100% o 100% p 97% o 86% o 42% p 60% p 90% p

∼10-17 ∼12-14 ∼13-32 ∼20 ∼20 ∼20 ∼20

15-45 25-45 25 25 25-45 25 25

a

range of variables Q, W0, g/g of g/min initial soln 4-24 4-12 4-12 4 4-17a 4 4

S0, g of water 400-600

0.0-0.015 0.0-0.025

120-240b 200

RCBA

PCBA

50-80% o 60-95% p 45-75% o ∼80% p 68-91% p 83% p 23% p

99+% o 99+% p 95+% o ∼70% p ∼100% p ∼100% p ∼100% p

Variable for some runs. b For variable Q.

commercially available NaBMGS hydrotrope. With the help of three ternary phase equilibrium diagrams constructed using the one- and two-component equilibrium compositions of binary mixtures at different hydrotrope concentrations together with the eutectic and two-component saturation tie-lines, they demonstrated that it was possible to separate two of the three binary simple and the ternary eutectics because NaBMGS was able to shift their equilibrium curves away from the eutectic tie-lines. It was decided to study the effect of various significant process parameters with the representative industrial reactor product composition, i.e., 42% o-CBA in the binary system, which will be used for the separation. Initially, the constant water addition rate was decreased from 12 to 4 g/min (runs 23 and 24). The effects of programmed water addition profiles on the product recovery and purity were examined in the next three runs (runs 25-27). An experiment was carried out with a seed loading of 0.25% of the initial hydrotrope mass (run 28). The temperature was varied in the next two runs (runs 29 and 30). Thus, it was possible to obtain good-quality, pure p-CBA crystals from the industrial reactor product mixture containing 42% p-CBA by this technique. To compare the effectiveness of the proposed process with the dissociation leaching (or extraction) reported by Laddha and Sharma (1978), an experiment was conducted with 60% p-CBA (run 31). In the last run (run 32), the amount of p-CBA was increased to 90% in order to cover the complete composition range. In all, the experiments in the p-CBA primary crystallization region resulted in good-quality and pure (g99%) p-CBA. In addition to Table 1, the initial compositions used in the experimental program are denoted in the binary (Figure 1) and ternary (Figure 3) phase equilibrium diagrams. Three runs (runs 24, 29, and 30) were conducted at 25, 35, and 45 °C with a constant water addition rate of 4 g/min. The water addition rate was varied with time in the other three runs (runs 25-27), while the crystallizer temperature was kept constant at 25 °C. A fixed amount (960 g) of water was added in each experiment. To achieve the initial low level of supersaturation, in runs 25-27, a predecided quantity of water was added in a batch mode (at t ) 0) with subsequent slow and variable addition with time as in a semibatch mode. Different initial supersaturation levels were effected by adding 240, 150, and 120 g of water initially to the crystallizer in runs 25-27, respectively. Also, in runs 25-27, water was added at variable flow rates with very low flow rate (1 g/min) during the initial nucleation period. The water addition and its rate profiles for this series of experiments are shown in Figure 10. Dotted curves for the water addition rate profiles indicate the set values on the programmable

Figure 10. Water addition and its rate profiles.

peristaltic pump, while solid curves represent the smooth curves drawn using the Harvard graphic package. The water addition rate was programmed in steps at appropriate times on the peristaltic pump as shown in the top section of Figure 10. These profiles were designed on an ad hoc basis for this system in such a way that the addition rate would increase with time in order to control the supersaturation generation rate in a desirable manner. It is known that for constant water addition rate profiles (such as runs 23-24 and 28-30) the supersaturation passes through the maximum (see also Figure 6). Using these programmed addition profiles, it was anticipated that the sharpness of the supersaturation peak could be minimized. Ideally with proper manipulation it should be possible to run the crystallizer at a constant level of supersaturation. The precise programmed water addition profiles could be derived for a well-characterized system from the theoretical analysis (Tavare, 1995b). In the present case, these were decided heuristically from the past analysis on the other system. In all these runs, the amount of water added is the same and therefore the yield, i.e., the amount of crystalline material produced, would be more or less the same, but the way in which the amount of water was added was different, naturally reflecting the different distributions of solid crystalline material along the crystal size. Typical solution-phase component concentration profiles determined from solution samples withdrawn during the experiments by HPLC analysis are shown in Figure 11 for both the components in four experiments. All runs show significant depletion in concentrations due to the dilution effect. For run 19 performed with the initial 97% o-CBA composition, o-CBA crystallizes out, while for runs 24, 25, and 27 employing the typical industrial reactor product composition, p-CBA crystallizes out. Run 24 was performed using a constant addition rate of 4 g of water/min, and different variable water addition rate profiles were used in the other two


Figure 11. Component concentration profiles.

runs (runs 25 and 27). The slight hump in o-CBA concentration profiles may be due to experimental errors in both sampling and chemical analysis. The initial o-CBA, p-CBA, and water compositions used in runs 24, 25, and 27, respectively, were in the p-CBA primary crystallization region of the ternary phase equilibrium diagram (Figure 3), indicating that p-CBA would selectively precipitate out on dilution with water. This was confirmed by analyzing the precipitated solids with the help of HPLC analysis. The final p-CBA product purity was greater than 99% in all the experiments. The present technique appears to give a superior quality product in comparison with previous studies. The separation of sparingly water-soluble isomeric and nonisomeric mixtures of organic acidic or basic mixtures by dissociation extraction (or leaching) was reported by Laddha and Sharma (1978). In the case of CBAs, the solid mixture was directly treated with aqueous solutions of sodium hydroxide and at 65 °C a single-stage operation gave essentially pure p-CBA. Dissociation extractive crystallization is an extension of dissociation extraction and a two- or three-phase separation technique which invokes differences in the molecular structure and chemical affinity of the components to be separated and the solubility of complexes in a suitable solvent. Lasanizadegan and Tavare (1996a,b) extended the efficacy of this technique to different classes of chemical group and media in the area of difficult separations and explored this technique for the separation of o- and p-chlorobenzoic acids. On the basis of the screening procedure, methanol and piperazine were selected as solvent and reagent, respectively. Piperazine forms a sparingly soluble complex with o-chlorobenzoic acid in methanol, and the complex precipitates out. A ternary equilibrium phase diagram for the o-chlorobenzoic acid-p-chlorobenzoic acid-methanol system was constructed depicting three isotherms. To establish the separation feasibility, a series of batch equilibrium precipitation experiments was performed at three different temperatures. Improved separation can be achieved at low temperatures. The recovery of o-chlorobenzoic acid, defined as the ratio of the amount of the component separated in the form of complex to the initial amount, can be achieved in excess of 80% in a single stage, while the separation factor, defined as the ratio of o-CBA to p-CBA in solid to that in liquid phase, provides a measure for the ease of separation and can usually approach infinity. Dissociation extractive crystallization can thus be successfully employed

Figure 12. Typical population density plots for p-CBA (runs 24 and 25): Influence of water addition profiles.

for the separation of o- and p-CBAs and requires an additional recovery step for the component from its complex. Experimental population density plots at four sampling times in the precipitation of p-CBA for two runs (runs 24 and 25) are presented in Figure 12. The solid filled symbols were used for the population density data obtained in a run (run 25) performed with variable water addition rate profiles (Figure 10), while the hollow open symbols were used for that in a run (run 24) performed at a constant water addition rate of 4 g/min. Both the runs appear to show expected trends in the population density plots with respect to time. The population density curves appear to pass through a hump for samples taken after an hour or so during run 24. For experiments performed at constant water addition rates the supersaturation values generally pass through a maximum (see, for example, Figure 6 for single components). Under such conditions a possibility of segregation does exist and the hump in the population density curves may be attributed to the segregation effects (see also Tavare, 1989). At high levels of supersaturation, nucleation rates are high. There appears to be a high probability of producing high levels of supersaturation locally and subsequent local production of nuclei. Both generation of supersaturation due to added diluent water and its depletion due to kinetic events, mainly nucleation, may cause spatial variation of these quantities at a certain time. Such segregation effects can be minimized using programmed water addition similar to run 25. The population density profiles followed patterns similar to those reported previously (see, for example, Figure 7) without indicating the influence of segregation effects. The final product population density plots for runs 23-30 are shown in Figure 13. These runs were performed starting with the representative industrial reactor product composition, varying water addition rate profiles, temperature, and seed loading. Runs 23 and 24 were performed at constant water addition rates of 12 and 4 g/min, respectively. A plateau or hump in the population density curves may be attributed to a localized increase in supersaturation as a result of competition between generation and depletion of supersaturation and convective flow characteristics. Run 23 was performed at a higher water addition rate than run 24;


Figure 13. Final product size distributions of p-CBA crystallized from the industrial reactor product composition: Influence of water addition profile, temperature, and seed loading.

determine the nucleation and growth rates from a pair of successive population density data obtained from runs 23-30 performed for the industrial reactor product (i.e., 58% o-CBA and 42% p-CBA). The results of nucleation and growth rates of all seven runs are included in Figure 9. The results from runs 24, 29, and 30 indicated that nucleation rates were practically independent of the crystallizer temperature, although there was a suggestion that both growth and nucleation rates were slightly higher at higher temperature. This observation is consistent with earlier results from the precipitation of p-CBA from its nearly saturated aqueous NaBMGS solution. The nucleation rates evaluated from runs 24, 29 and 30 (experiments with a constant water addition rate) were lower than those from runs 25-27 (experiments with variable water addition rates with a low water addition rate during the initial nucleation period). Thus, the water addition rate appears to be a dominant variable as it influences the supersaturation level and hence the nucleation rate. Nucleation rates determined from runs 25-27 operated with the variable water addition rates are higher than those operated at the constant addition rate (runs 24, 29, and 30) as the average level of supersaturation during runs 25-27 is expected to be higher than that in runs 24, 29, and 30. Consequently, two separate power law relative nucleation kinetics in terms of growth rates were developed, one for the experiments with the constant water addition rate as given by

B ) 175G0.47

(3)

and the second for the variable water addition rate as

B ) 10200G0.86

Figure 14. Final product size distribution of o-CBA crystallized from 97% o-CBA: Influence of seed loading.

naturally it will have an earlier and sharper supersaturation peak, resulting in a sharper and earlier hump. This effect can be eliminated by programming the water addition rate profiles as shown in Figure 10 for runs 25-27. An increase in temperature in runs 29 and 30 was found to improve the product population density profile slightly due to beneficial influence of temperature on kinetic events. Alternatively, the influence of the segregation effect can also be minimized by the addition of seeds as in run 28. In the case of precipitation of o-CBA from a 97% o-CBA and 3% p-CBA mixture also, as mentioned above, the process of seeding played an important role in minimizing both segregation and agglomeration effects, as indicated by the final product size distributions shown in Figure 14 for runs 18-20. The crystal size distributions are not influenced significantly with seed loading over the relatively small range, and there appears a suggestion that the number concentration increases with seed loading. Due to difficulties associated with the sampling and subsequent crystal size analysis, it was difficult to infer any results from other runs (runs 14-17). The method of s-plane analysis was employed to

(4)

The results of a least-squares regression analysis are given in Figure 9. Low values of the nucleation rate from a previous correlation (eq 2) may be the reflection of low magma concentration produced and the higher values in this work may be due to some influence of the presence of the other component. Nevertheless, the kinetic correlations appear reasonable, yielding the relative kinetic orders ranging between 0.5 and 0.9. These correlations are, however, based on only a limited number of experimental observations. The methods employed to evaluate the levels of supersaturation for single-component precipitation experiments were found unsatisfactory for the case of precipitation experiments with mixtures and therefore the supersaturation profiles were not determined in these series. Typical scanning electron micrographs of the final dry product crystals for eight samples are shown in Figure 15. Two SEMs in the first row represent the product crystals of o-CBA (run 2) and p-CBA (run 11) resulting from precipitation from single-component solutions and altogether different kinds of morphologies. The product crystals of o-CBA shown in the next row for runs 15 and 17 performed with the initial 97% o-CBA mixture at water addition rates of 12 and 4 g/min, respectively, indicate their influence on product morphologies. Relatively large-size and well-defined crystals are produced in run 17. The first left-hand photograph in the third row shows a sample of the product crystals resulting from run 22 performed with the eutectic composition perhaps depositing both these components, while the second on the right-hand side shows a sample of welldefined p-CBA product crystals resulting from the initial


Figure 15. Scanning electron micrographs of product CBA crystals.

42% p-CBA binary composition mixture with the programmed water addition in run 27. Samples of p-CBA product crystals from runs 30 and 32 are shown in the final row. Runs 30 and 32 were performed at a water addition rate of 4 g/min with the initial 42 and 90% p-CBA binary composition mixtures at 45 and 25 °C, respectively. Large-size crystals were obtained at 45 °C. This paper concentrates on the process identification studies and not on process simulations. Mixing can influence significantly the overall performance of a crystallizer. Extensive experimental and theoretical studies have been reported on the influence of mixing mainly on the barium sulfate precipitation system in a variety of process configurations. The general topic of mixing and mathematical analysis of continuous-flow

systems is of considerable interest, and a variety of micromixing models using the approaches developed mainly in chemical reaction engineering have been used for reactive precipitation processes (Pohorecki and Baldyga, 1979, 1983, 1985, 1988; Garside and Tavare, 1984, 1985; Tavare, 1986, 1992, 1993, 1994, 1995a,b; Farmer and Beckman, 1991; Fitchett and Tarbell, 1990; Mydlarz et al., 1990; Baldyga, 1993; Mehta and Tarbell, 1983; Muhr et al., 1996; Tavare and Borissova, 1997). Most recent studies appear to deal with batch or semibatch reactive precipitation processes (Kuboi et al., 1986; Pohorecki and Baldyga, 1988; Tosun, 1988; Baldyga et al., 1990, 1995; Tovstiga and Wirges, 1990; Marcant and David, 1991, 1993, 1994; Aslund and Rasmuson, 1992; Podgorska et al., 1993; Iyer and Przbycien, 1994; Jones et al., 1992). The analysis presented in the past appears to have inadequate physical insight into the interaction of mixing, reaction, and subsequent crystallization but has rather provided a means of evaluating and subsequently predicting the performance for a given crystallizer configuration. In addition, attempts are being made to integrate computationally fluid dynamic and precipitation processes into commercial CFD codes in order to define both spatial distributions of instantaneous and local values of intensive properties of all species. On many occasions, detailed three-dimensional, time-dependent direct valid numerical evaluations may not be feasible, and other viable approaches may be advocated (Van Leeuwen et al., 1996a,b; Wei and Garside, 1997a,b; Baldyga and Orciugh, 1997). In most experimental studies, the mean product crystal size was used by many investigators as an important variable because of its variation with easily observable but significant variables such as stirrer speed, feed concentration, feed flow rates, and feed locations, etc. It is, however, important to note that such variables have different effects in different modes of operations, i.e., batch and single- and double-feed semibatch and continuous precipitators. Pohorecki and Baldyga (1983) observed a slight decrease in mean product crystal size with an increase in stirrer speed. Using micromixing models they predicted the opposite trends of mixing influence on the product crystal size in batch and continuous units. In a single-feed semibatch precipitator Tosun (1988) found a decrease at low stirrer speed and an increase at high stirrer speed at various feed locations, while he found little influence in a doublefeed semibatch operation. On the other hand, both Tovstiga and Wirges (1990) and Baldyga et al. (1990) reported a decrease in the mean product crystal size in a single-feed semibatch unit. Baldyga et al. (1995) also observed an extensive double-feed semibatch precipitation study in an agitated Rushton tank reactor, delineating the effects of addition mode, feed rate, concentration and volume ratios, and intensity of mixing on crystal size distribution and morphology, and predicted the possibility of a minimum in the product crystal mean size with stirrer speed. Several investigators have also demonstrated the influence of feed locations of the reactant solutions. Kuboi et al. (1986) obtained smaller crystals when two reactant solutions were fed close together on the liquid surface than when they were fed on the opposite sides of a stirrer shaft on the liquid surface in a double-feed semibatch precipitation. In his single- and double-feed semibatch experiments, Tosun (1988) found the smallest product size when the feed solutions were fed to the suction zone of the stirrer,


while the largest product size was obtained when the solutions were fed at the discharge zone of the stirrer. On the contrary, Tovstiga and Wirges (1990) obtained smaller crystals when the solution was fed at the discharge zone of the impeller than when it was fed on the liquid surface in a single-feed semibatch precipitator. Taguchi (1993) in his ∼150-mL double-feed semibatch precipitator noticed a sharper and earlier peak in the thermodynamic supersaturation ratio and slightly later development of number concentration when the reactant streams are fed nearer to the impeller than when they were fed at the top liquid surface. Marcant and David (1991, 1993, 1994) pointed out that a change in feed location showed a stronger influence than that in stirrer speed on product performance characteristics. In their double-feed semibatch Rushton tank precipitator (∼10 L), Baldyga et al. (1995) observed a larger product mean size when two reactant streams were fed through vertical tubes located diametrically apart midway between the impeller shaft and the vessel wall and just below the initial liquid level than when two feed tubes were at an angle of inclination (52°) to the liquid level, located 3 cm above the impeller disk at 1-cm distance diametrically apart and pointing toward the central axis. For the present studies a single-feed stream, i.e., diluent water, was added through the glass tube at a fixed point very close to the impeller level midway between the impeller shaft and the crystallizer wall in all precipitation experiments performed at relatively high stirrer speeds. In most cases partial segregation effects were minimized. Simulations of such a precipitator may provide useful information on the influence of mixing on the product size distribution characteristics. Conclusions The separation of o- and/or p-CBA from their mixture and pure single-component aqueous NaBMGS solutions was achieved. The aqueous solubilities of o- and p-CBAs in different concentrations of NaBMGS solutions were determined at 25 and 45 °C by the weight disappearance method. The experimental results showed that o-CBA has a higher aqueous solubility than p-CBA under otherwise similar conditions. A ternary phase equilibrium diagram for the system o-CBA-p-CBA-water at 25 °C was constructed. Several hydrotrope isoplethal curves and two-component saturation and eutectic tielines were included in the ternary diagram. Precipitation experiments for o- and p-CBAs by water dilution from their NaBMGS solutions were conducted in a 2-L, agitated, jacketed crystallizer. Solution- and solidphase concentrations by HPLC and crystal size distributions by a multichannel Coulter counter were determined. The effects of significant process variables, namely, initial binary composition, water addition flow rate, temperature, and speed of agitation on the crystal size distributions, were investigated in a series of batch experiments. The method of s-plane analysis was used to determine the crystal growth and nucleation rates from transient population density data. The power-law rate models were employed to correlate the nucleation and growth rates in terms of significant process variables. The nucleation rates were significantly higher for o-CBA than for p-CBA. The nucleation rates of p-CBA for the initial 42% p-CBA binary composition were relatively high in experiments with a variable water addition rate probably due to higher levels of

supersaturation throughout the runs and were practically independent of the crystallizer temperature. The power-law relative nucleation kinetic correlations in terms of growth rate with the relative kinetic orders between 0.5 and 0.9 were estimated for o- and p-CBAs. These experiments demonstrate the possible separation of o- and p-CBA from their mixture through hydrotropy. Solubilization-equilibrium precipitation experiments confirm the promising potential of hydrotropes for the purpose of separation of binary solid mixtures even at the eutectic composition. Acknowledgment The experimental work reported in this paper was performed at Department of Chemical Engineering, UMIST, Manchester M60 1QD, England. Free supply of hydrotropes from Hu¨ls (U.K.) Ltd. and Hydrior AG is appreciated. A.B.D. thanks the Leverhulme Trust for the award of visiting fellowship. Nomenclature B ) nucleation rate, no./(g of water‚s) c ) component concentration in the solution phase, g/g of water cH ) hydrotrope concentration in the solution phase, g/g of water ∆c ) supersaturation driving force, mg/g of water e ) relative standard error G ) overall linear crystal growth, µm/s L ) crystal size, µm L h ) mean crystal size, µm MT ) magma concentration, g/g of water n ) population density function, no./m‚g of water; population density function based on the total solvent capacity, no./m N ) stirrer speed, Hz P ) purity, composition of the component in the solid phase, % Q ) water addition rate, g/min r ) correlation coefficient R ) recovery, i.e., percent ratio of mass of the component recovered as a product to that charged initially, % S0 ) amount of water added initially, g T ) temperature, K, °C W0 ) seed loading, g/g of initial hydrotrope solution, % Subscripts CBA ) chlorobenzoic acid o ) ortho p ) para

Literature Cited Aslund, B. L.; Rasmuson, A. C. Semibatch reaction crystallization of benzoic acid. AIChE J. 1992, 38, 328. Baldyga, J. Micromixing and precipitation. In Proceedings of the 12th Symposium on Industrial Crystallization; Rojkowski, Z. H., Ed.; Politechnika: Warszawska, Poland, 1993. Baldyga, J.; Oricuch, W. Closure problem for precipitation. Trans. Inst. Chem. Eng. 1997, 75A, 160. Baldyga, J.; Pohorecki, R.; Podgorska, W.; Marcant, B. Micromixing effects in semi-batch precipitation. In Proceedings of the 11th Symposium on Industrial Crystallization; Mersmann, A., Ed.; Europeon Federation of Chemical Engineering: GarmischPartenkirchen, Germany, 1990. Baldyga, J.; Podgorska, W.; Pohorecki, R. Mixing-precipitation model with application to double feed semibatch precipitation. Chem. Eng. Sci. 1995, 50, 1281. Coloˆnia, E. J.; Tavare, N. S. Separation of eutectics through hydrotropy. Proceedings of the 1994 IChemE Research Event;

1968 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 University College London, London, 1994; Institute of Chemical Engineers: London, 1994. Coloˆnia, E. J.; Raynaud-Lacroze, P. O.; Tavare, N. S. Separation of isomers: Hydrotropy and precipitation. In Proceedings of the 12th Symposium on Industrial Crystallization; Rojkowski, Z. H., Ed.; Politechnika: Warszawska, Poland, 1993. Coloˆnia, E. J.; Dixit, A. B.; Tavare, N. S. Separation of o- and p-chlorobenzoic acids: hydrotropy and precipitation. J. Cryst. Growth 1996, 166, 976. Coloˆnia, E. J.; Dixit, A. B.; Tavare, N. S. Phase relations of o- and p-chlorobenzoic acids in hydrotrope solutions. J. Chem. Eng. Data 1998, 43, 220. Dixit, A. B.; Coloˆnia, E. J.; Tavare, N. S. Selective precipitation of p-chlorobenzoic acid. In Proceedings of the 13th Symposium on Industrial Crystallization; Biscans, B., Garside, J., Eds.; PROGEP: Toulouse, France, 1996; Vol. 1. Dixit, A. B.; Coloˆnia, E. J.; Tavare, N. S. Use of a hydrotrope in selective precipitation of o- or p-chlorobenzoic acids. Proceedings of the 1997 Jubilee IChemE Research Event; University of Nottingham, 1997; Inst. Chem. Eng.: London, 1997; Vol. 2. Farmer, R. W.; Beckman, J. R. Evaluation of crystal size distribution enlargement in a multistage column crystallizer. Ind. Eng. Chem. Res. 1991, 30, 196. Fitchett, D. E.; Tarbell, J. M. Effect of mixing on the precipitation of barium sulfate. AIChE J. 1990, 36, 511. Gaikar, V. G.; Sharma, M. M. Extractive separations with hydrotropes. Solvent Extr. Ion Exch. 1986, 4, 839. Gaikar, V. G.; Mahapatra, A.; Sharma, M. M. New strategies in extractive distillations: use of aqueous solution of hydrotrope and organic bases as solvent for organic acids. Sep. Sci. Technol. 1988, 23, 429. Garside, J.; Tavare, N. S. Mixing, reaction and precipitation in an MSMPR crystallizer: Effects of reaction kinetics on the limits of micromixing. In Industrial Crystallization 84; Jancic, S. J., de Jong E. J., Eds.; Elsevier: Amsterdam, The Netherlands, 1984. Garside, J.; Tavare, N. S. Mixing, reaction and precipitation: limits of micromixing in an MSMPR crystallizer. Chem. Eng. Sci. 1985, 40, 1485. Geetha, K. K.; Tavare, N. S.; Gaikar, V. G. Separation of o- and p-chloronitrobenzenes through hydrotropy. Chem. Eng. Commun. 1991, 102, 211. Iyer, H. V.; Przybycien, T. M. Protein Precipitation: effects of mixing on protein solubility. AIChE J. 1994, 40, 349. Jadhav, V. K.; Dixit, B. A.; Tavare, N. S. Solubilities of m- and p-aminoacetophenones in hydrotrope solutions. J. Chem. Eng. Data 1995, 40, 669. Janakiraman, B.; Sharma, M. M. Enhancing rates of multiphase reactions through hydrotropy. Chem. Eng. Sci. 1985, 40, 2156. Jones, A. G.; Hostomsky, J.; Zhou, L. On the effect of liquid mixing rate on primary crystal size during the gas-liquid precipitation of calcium carbonate. Chem. Eng. Sci. 1992, 47, 3817. Kuboi, R.; Harada, M.; Winterbottom, J. M.; Anderson, A. J. S.; Nienow, A. W. Mixing effects in double-jet and single-jet precipitation. Proceedings of World Congress III Chemical Engineers; Society of Chemical Eng.: Tokyo, 1986; Paper 8g302. Laddha, S. S.; Sharma, M. M. Separation of close boiling organic acids and bases by dissociation extraction: Chlorophenols, N-alkylanilines and chlorobenzoic acids. J. Appl. Chem. Biotechnol. 1978, 28, 69. Lashanizadegan, A.; Tavare, N. S. Dissociation extractive crystallization: Separation of di- and trichloroacetic acids. Chem. Eng. Res. Des. 1996a, 74, 773. Lashanizadegan, A.; Tavare, N. S. Dissociation extractive crystallization: Separation of o- and p-chlorobenzoic acids. Proceedings of IChemE Research Event; Inst. Chem. Eng.: London, 1996b; Vol. 1. Marcant, B.; David, R. Experimental evidence for and prediction of micromixing effects in precipitation. AIChE J. 1991, 37, 1698. Marcant, B.; David, R. Influence of micromixing on precipitation in several crystallizer configurations. In Proceedings of the 12th Symposium on Industrial Crystallization; Rojkowski, Z. H., Ed.; Politechnika: Warszawska, Poland, 1993; Vol. 1. Marcant, B.; David, R. Prediction of micromixing effects in precipitation: Case of double-jet precipitators. AIChE J. 1994, 40, 424.

Mehta, R. V.; Tarbell, J. M. Four environment model of mixing and chemical reaction. I: Model development. AIChE J. 1983, 29, 320. Muhr, H.; Penicot, P.; Plasari, P.; Villermaux, J. Influence of feed point locations and draft tube geometry on size distribution and morphology of barium sulphate crystals obtained in a continuous precipitator. In Proceedings of the 13th Symposium on Industrial Crystallization; Biscans, B., Garside, J., Eds.; PROGEP: Toulouse, France, 1996. Mydlarz, J.; Reber, J.; Briedis, D.; Voigt, J. A. Kinetics of zinc oxalate precipitation in a mininucleator-MSMPR crystallizer system. AIChE Symp. Ser. 1990, 87, No. 284, 158. Osol, A.; Kilpatrick, M. The salting-out and salting-in of weak acids. The activity coefficients of the molecules of ortho, meta and para chlorobenzoic acids in aqueous salt solutions. J. Am. Chem. Soc. 1933, 55, 4430. Phatak, P. V.; Gaikar, V. G. Solubility of o- and p-chlorobenzoic acids in hydrotrope solutions. J. Chem. Eng. Data 1993, 38, 217. Podgorska, W.; Baldyga, J.; Pohorecki, R. Micromixing effects in double feed semibatch precipitation. In Proceedings of the 12th Symposium on Industrial Crystallization; Rojkowski, Z. H., Ed.; Politechnika: Warszawska, Poland, 1993; Vol. 1. Pohorecki, R.; Baldyga, J. The influence of intensity of mixing on the rate of precipitation. In Industrial Crystallization 78; de Jong, E. J., Jancic, S. J., Eds.; North-Holland: Amsterdam, The Netherlands, 1979. Pohorecki, R.; Baldyga, J. The use of a new model of micromixing for determination of crystal size in precipitation. Chem. Eng. Sci. 1983, 38, 79. Pohorecki, R.; Baldyga, J. The effect of micromixing on the precipitation process in a premixed feed continuous stirred tank crystallizer (CSTC). Proceedings of 5th Conference on Mixing; VDI: Wuzburg, Germany, 1985. Pohorecki, R.; Baldyga, J. The effects of micromixing and the manner of reactor feeding on precipitation in stirred tank reactors. Chem. Eng. Sci. 1988, 43, 1949. Raynaud-Lacroze, P. O.; Tavare, N. S. Separation of 2-naphthol: Hydrotropy and Precipitation. Ind. Eng. Chem. Res. 1993, 32, 685. Sadvilkar, V. G.; Khadilkar, B. M.; Gaikar, V. G. Aqueous solutions of hydrotropes as effective reaction media for the synthesis of 4-aryl-1,4-dihydropyridines. J. Chem. Technol. Biotechnol. 1995a, 65, 33. Sadvilkar, V. G.; Samant, S. D.; Gaikar, V. G. Claisen-Schmidt reaction in hydrotropic medium. J. Chem. Technol. Biotechnol. 1995b, 62, 405. Taguchi, K. Precipitation of barium sulphate: Kinetics of nucleation and growth and the influence of mixing. Ph.D. Thesis, University of Manchester, Manchester, U.K., 1993. Tavare, N. S. Mixing in continuous crystallizers. AIChE J. 1986, 32, 705. Tavare, N. S. Micromixing limits in an MSMPR crystallizer. Chem. Eng. Technol. 1989, 12, 1. Tavare, N. S. Mixing, reaction and precipitation: environment micromixing models in continuous crystallizers-I. Premixed feeds. Comput. Chem. Eng. 1992, 16, 923. Tavare, N. S. Mixing, reaction and precipitation: Micromixing models in continuous crystallizers. In Proceedings of 12th Symposium on Industrial Crystallization; Rojkowski, Z. H., Ed.; Politechnika: Warszawska, Poland, 1993; Vol. 1. Tavare, N. S. Mixing, reaction and precipitation: an interplay in continuous crystallizers. Chem. Eng. Sci. 1994, 49, 5193. Tavare, N. S. Mixing, reaction and precipitation: Interaction by exchange with mean micromixing models. AIChE J. 1995a, 41, 2537. Tavare, N. S. Industrial Crystallization: Process Simulation Analysis and Design; Plenum: New York, 1995b. Tavare, N. S.; Garside, J. Simultaneous estimation of crystal nucleation and growth kinetics from batch experiments. Chem. Eng. Res. Des. 1986, 64, 109. Tavare, N. S.; Gaikar, V. G. Precipitation of salicylic acid: Hydrotropy and reaction. Ind. Eng. Chem. Res. 1991, 30, 722. Tavare, N. S.; Jadhav, V. K. Solubilities of 6-aminopenicillanic acid and phenoxyacetic acid in hydrotrope solutions. J. Chem. Eng. Data 1996, 41, 1196. Tavare, N. S.; Coloˆnia, E. J. Separation of eutectics of chloronitrobenzenes through hydrotropy. J. Chem. Eng. Data 1997, 42, 631.

Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1969 Tavare, N. S.; Borissova, A. Mixing, reaction and precipitation: environment micromixing models in continuous crystallizers with unpremixed feeds. Proceedings of the 1997 Jubilee Research Event; East Midland Conference Centre, April 1997; Inst. Chem. Eng.: London, 1997; Vol. 2. Tosun, G. An experimental study of the effects of mixing intensity on particle size distribution in barium sulphate precipitation. Proceedings of the 6th European Conference on Mixing; European Federation of Chemical Engineering: Pavia, May 1988. Tovstiga, G.; Wirges, H.-P. The effect of mixing intensity on precipitation in a stirred tank reactor. In Industrial Crystallization ‘90; Mersmann, A., Ed.; European Federation of Chemical Engineering: Garmisch-Partenkirchen, Germany, 1990. Van Leeuwen, M. L. J.; Bruinsma, O. S. L.; Van Rosmalen, G. M. Influence of mixing on the product quality in precipitation. Chem. Eng. Sci. 1996a, 51, 2595.

Van Leeuwen, M. L. J.; Bruinsma, O. S. L.; Van Rosmalen, G. M. Precipitation and mixing: Influence of hydrodynamics on the precipitate. In Proceedings of the 13th Symposium on Industrial Crystallization; Biscans, B., Garside, J., Eds.; PROGEP: Toulouse, France, 1996b. Wei, H.; Garside, J. Pumped, stirred and maybe precipitated: Simulation of precipitation processes using CFD. Acta Polytech. Scand., Chem. Technol. Ser. 1997a, No. 244, 9. Wei, H.; Garside, J. Application of CFD modelling to precipitation systems. Trans. Inst. Chem. Eng. 1997b, 75A, 219.

Received for review September 24, 1997 Revised manuscript received January 30, 1998 Accepted February 17, 1998 IE970686C

Separation of Chlorobenzoic Acids through Hydrotropy - Industrial

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