Application of preferential crystallization for racemic compound integrating thermodynamics, kinetics and optimization 5

Chapter Systematic preferential crystallization process of mandelic acid 120 6.1 Introduction According to the introduction in Chapter 2, the applicability of preferential crystallization to racemic compounds would significantly widen the potential of usually cheap crystallization based techniques for enantioseparation (Jacques et al., 1981; Brock and Dunitz, 1994; Kinbara et at., 2001; Lorenz et al., 2001, 2006; Profir et al., 2002). During the crystallization process, it is crucial to keep the freedom of supersaturation of the racemate in its metastable zone to avoid its spontaneous nucleation and to control the supersaturation of the target enantiomer as the spontaneous nucleation of the target enantiomer may easily initiate the spontaneous nucleation of its racemate. The model compound in this chapter is mandelic acid, which is a widely used reagent in classical resolution. It has attracted some efforts to study the crystallization process and relevant data measurement due to its relatively cheap price and favorable phase diagram characteristics for preferential crystallization. For example, the coupling process of liquid chromatography and preferential crystallization was suggested for efficient enantioseparation of the enantiomers of mandelic acid in aqueous solution by Lorenz et al. (2001). The principle based on the thermodynamic phase diagram for the crystallization was introduced in this work. The binary and ternary phase diagrams of mandelic acid enantiomer in water system were constructed and DSC was applied successfully for the solubility determination ((Lorenz and Seidel-Morgenstern, 2002; Mohan et al., 2002). Profir and Rasmuson (2004) reported that metastable conglomerate mandelic acid crystals can be formed upon primary nucleation in water and acetic acid. After a time-lag, the conglomerate was transformed into the stable form racemic compound. The time-lag range depended on 121 the operation conditions and decreased at increasing concentration or temperature and in the presence of micrometer-size particles (Profir et al., 2002). They also studied the influence of solvent and the operation conditions, such as filtration, cooling rate and stirring rate, on the crystallization of mandelic acid (Profir and Rasmuson, 2004). In addition, for the isothermal batch crystallization of the mandelic acid in water, different analytical techniques were evaluated to determine the solute concentration in the liquid phase and the metastable zone width measurement were presented, obtaining the basis for growth kinetics investigation (Perlberg et al., 2005). Preferential crystallizations of enantiomers were also discussed for the two enantiomeric systems, conglomerate threonine and racemic compound mandelic acid by Lorenz et al. (2006a). From this work, the 99.3% purity of (S)-MA was obtained. As well, the effects of the presence of the counter-enantiomer on the growth rate and crystal shape were illustrated in this work. Furthermore, a cyclic crystallization process, which provided alternating the pure mandelic acid enantiomer and the racemic compound, was proposed (Lorenz et al., 2006b). During this process, the online polarimetry and online density measurement were used in application of preferential crystallization for mandelic acid. Recently, the potential crystallization inhibitors were used on the chiral separation for mandelic acid (Mughal et al., 2007). The results showed that while none of the additives dramatically inhibit the crystallization of (S)-MA, they significantly inhibit the crystallization of racemate MA. This may lead a new way to a crystallization process for the chiral separation. However, very few attempts have been directed in preferential crystallization process itself combining the aspects of thermodynamics, kinetics, optimal operation and in-situ monitoring. This chapter presents a systematic study on solubility, 122 metastable zone, kinetics and supersaturation control profile to obtain crystal product with good quality. In this chapter, the certain enantiomerically enriched mandelic acid was obtained from a racemic composition by using a HPLC with a semi-preparative chiral column. Based on the solubility and metastable zone data derived in the chapter and kinetic data obtained in chapter 5, we demonstrated a systematic approach, in which a modified strategy and optimal operation profile were proposed for a racemic compound system. Three different cooling profiles were applied to the subsequent crystallization process. The final product’s optical purity, yield and crystal size distribution were examined and compared. 6.2 Experiment 6.2.1 Semi-preparative HPLC separation of mandelic acid Semi-preparative HPLC separation of mandelic acid was performed using Chiralcel AD-H semi-preparative HPLC column (dimension 250mm L x 10mm I.D). The mobile phase is Heptane/TFA/IPA (95/0.1/5 v/v) at 25°C column temperature, with flow rate of 4.5ml/min and UV-Vis detection at 210nm. 6.2.2 Direct crystallization operation The crystallization experiments were carried out in an automatic laboratory reactor system equipped with an l-L glass jacketed crystallizer as described in the Fig. 4.2. Three types of cooling profiles, controlled cooling, linear cooling and forced cooling, were used respectively on the three batch direct crystallization operation of 123 mandelic acid. The start point of all experiments was the same solution which was the 80% mole percent (S)-MA saturated solution at 35 oC. The crystallizer was kept oC higher than that of the saturated solution in order to assure that no crystal exited in the solution prior to crystallization. After 0.5h, the temperature of the crystallizer was reduced to the saturation temperature. The batch cooling crystallization experiment was started. When the temperature decreased slightly, the pure (S)-MA seeds prepared by fast cooling from the solution of pure (S)-MA were added into the solution. Then the temperature was cool down according to the different cooling profile with the range of 35-20 oC. During these experiments, several samples were taken at definite time intervals to observe the optical purity of crystal products, solute concentration in liquid phase, and crystal size distribution. The optical purity of crystal products were measured by using DSC, Polarimeter, and HPLC. In this work, the polarimeter was equipped with a sodium vapor lamp emitting light with a wavelength of 589.3 nm and a quartz cell of 50 mm path length. HPLC was an Agilent 1100 series HPLC system with Chiralcel AD-H analytical column (dimension 250mm L x 4.6 mm I.D.) under separation conditions: Heptane /TFA/IPA (95/0.1/5 v/v) at 25°C column temperature, flow rate of 1.0ml/min and UV-Vis detection at 210nm. The crystal size distribution was measured using a Malvern Mastersizer 2000 and the solute concentration was measured using Shimadzu 2450 UV-visible spectrophotometer. 6.3 Results and discussion 6.3.1 Semi-preparative HPLC separation of mandelic acid 124 The loading capacity of Mandelic acid on Chiralcel AD-H was determined by injecting different amount of sample onto the column. It was found that mandelic acid shows baseline separation at 8.07mg loading, while partial separation of mandelic acid are observed when sample loading increases to 16.15mg, shown in Fig. 6.1. Fig 6.1 Partial separation of MA on Chiralcel AD-H semi-preparative HPLC column (dimension 250mm L x 10mm I.D) at different loadings 8.07mg and 16.15mg per injection using Heptane/TFA/IPA (95/0.1/5 v/v) as mobile phase, at 25°C column temperature, flow rate of 4.5ml/min and UV-Vis detection at 210nm. 125 The sample loading was determined as 16.15 mg per analysis. Through semipreparative chiral HPLC separation, we have successfully isolated the 80% mole percent S enantiomer and pure R enantiomer from its racemate, by collecting two different fractions at two different retention time, t =27-37 mins and t =37-43 mins. It is presented in Fig. 6.2. Fig 6.2 Fraction collection under semi-preparative HPLC separation of MA on Chiralcel AD-H column (dimension 250mm L x 10.00 mm I.D.) under separation conditions: Heptane/TFA/IPA (95/0.1/5 v/v) at 25°C column temperature, flow rate of 4.5ml/min and UV-Vis detection at 210nm. Fraction (a) collected at retention time 2737 minutes and fraction (b) is collected at 37-43 minutes. 126 The optical purities of these two fractions were analyzed on analytical chiral column, Chiralcel AD-H using Heptane /TFA/IPA (95/0.1/5 v/v) as the mobile phase and a flow rate of 1.0ml/min. The results are shown in Fig. 6.3. These two fractions could be accumulated until a certain volume enough for the preferential crystallization process by using continuous HPLC separation with an antosampler and automated fraction collector. Fig 6.3 Chromatogram of two fractions (a) and (b) obtained through semi-preparative HPLC separation of mandelic acid on Chiralcel AD-H analytical column (dimension 250mm L x 4.6 mm I.D.) under separation conditions: Heptane/TFA/IPA (95/0.1/5 v/v) at 25°C column temperature, flow rate of 1.0 ml/min and UV-Vis detection at 210nm. 6.3.2 Preferential crystallization operation for mandelic acid 6.3.2.1 The progression of preferential crystallization 127 A typical loading of preferential crystallization of mandelic acid was the saturated solution with 80% mole percent (S)-MA at 35 oC. According to the results of solubility and MSZWs of MA in chapter 4, this saturated solution was saturated in the eutectic composition of MA (Eu-MA) at 25.5 oC and saturated in the (RS)-MA at 23.3 o C. Six batches direct crystallization experiments were carried out starting from the same solution with different modes, which are (a) Exp_01: with seeding and final temperature at 28 oC; (b) Exp_02: with seeding and final temperature at 25.5 oC; (c) Exp_03: with seeding and final temperature at 24 oC; (c) Exp_04: with seeding and final temperature at 23.3 oC; (e) Exp_05: with seeding and final temperature at 22.5 o C; (f) Exp_06: without seeding until nucleation occurred. As described in Fig. 6.4, a saturation solution with 80% mole percent (S)-MA at 35 oC is repented by point A. The saturated point of Eu-MA and (RS)-MA are represented at point B and C respectively. When the controlled cooling profile was used, the solution seeded with (S)-MA was cooled in different runs to 28, 25.5, 24, 23.3 and 22.5 oC, shown in Fig. 6.4 as well. The optical purity of the final crystal products with different cooling degree was analyzed by HPLC and polarimeter and the results are listed in Table 6.1. The calibration curve of optical rotation with concentration for pure (S)-MA will be shown in section 6.3.2.3. 128 0.92 0.82 0.72 Concentration (g/ml) Exp S-MA solubility Eu-MA solubility RS-MA solubility S-MA supersolubility Eu-MA supersolubility RS-MA supersolubility 0.62 Exp Exp Exp Exp Exp 0.52 A 0.42 0.32 B 0.22 C 0.12 0.02 -10 10 20 Temperature(oC) 30 40 Fig 6.4 Progression of direct crystallization of mandelic acid. 129 by well designed and controlled procedure (Wang and Ching 2006). However, for the racemic compound, it was found here that only excess S enantiomer can be obtained by the preferential crystallization process. When the solution was cooled to 21.5 oC without seeding as Exp_06, the primary nucleation occurred by exceeding the metastable zone of initial composition mandelic acid. The products were always in the form of (RS)-MA and (S)-MA. This could be due to the crucial characteristic of a racemic compound: no selectivity of nucleation between the pure enantiomer and racemate for a racemic compound. So, the optical purities of these experiments results show that almost pure S mandelic acid crystal product was obtained from the preferential crystallization with seeding within the safe supersaturation critical limit, while both S and RS were crystallized out when the system temperature exceeded the saturated temperature of racemate. It indicated that the key factor to get the pure enantiomer product is its solubility characteristic for direct crystallization of racemic compound. It was also found that though the end temperature reached the Eu-MA saturated temperature 25.5 oC, the eutectic component crystal of mandelic acid did not come out from the solution and the pure S enantiomer was still obtained as a product. It may be explained that the eutectic mandelic acid is still in the metastable zone area, though the end temperature already surpass its saturated temperature. Or it could be due to that the eutectic mandelic acid can not come out as a stable compound from the solution. So during the course of the whole direct crystallization process, the competition could be only between (S)- and (RS)- mandelic acid crystal in the solution. 131 6.3.2.2 Optimal operation profile In order to realize the supersaturation control in preferential crystallization process, an optimal operation profile should be used. In this work, the classical simplified equation derived by Nyvlt et al. (1973) for optimal operation of batch crystallization with seed was used with assuming constant crystal growth rate G and constant nucleation rate B (Eq. 6-1). As discussed in Wang and Ching’s work (2006), the thermodynamics and crystallization kinetics should be combined together to apply this equation to control the supersaturation (T0 − T ) /(T0 − T f ) = (t / t c ) (6-1) From the thermodynamics point of view, the 80% mole percent (S)-MA saturated solution at 35 oC was used as the starting point for the preferential crystallization operations. According to the solubility data, this saturated solution was saturated in the (RS)-MA at 23.3 oC. That is to say that only S enantiomer is supersaturated in the temperature range of 35 to 23.3 oC. After continually cooling, the spontaneous nucleation of the racemic RS may be carried out. According to MSZW data in chapter 4, the supersaturation should be kept within circa oC to avoid spontaneous nucleation of both enantiomers. Also, the applicable supersaturation for RS racemate is 5.5-25 oC. Therefore it is acceptable to set the final targeted temperature at 20 oC. Because the MSZW should be narrower in the solution with seed crystals and the supersaturated solution was unstable in a relatively higher second metastable region where many nuclei with opposite chirality of seed could form (Hongo et al., 1981; Qian and Botsaris, 1998), the supersaturation should be 132 kept lower than it measured under homogenous condition. In this work, a critical supercooling was chosen to control at around 2.5 oC. The corresponding ∆C should be ca. 0.027g/ml water. Then, the crystal growth kinetics would be considered. According to Eq.5-15, when the ∆C=0.027g/ml, the G would be 0.35µm/min. It could be acceptable to assume constant G during the operation when the supersaturation is well controlled within a critical region ca. 2.5oC. It is also reasonable to assume constant B on the curse of the whole process because the MT used in this work is low. Therefore, the equation 6-1 can be directly used as cooling profile to control the supersaturation in the preferential crystallization of mandelic acid. As the mean size was circa 230 µm for the seed and 527 µm for a target product, the operation time can be calculated as tc = (527-230)/0.35=848 min. For the comparison purpose, the linear and forced cooling profiles were used as well. The temperature of the solutions inside the crystallizer was controlled by the LabMax system and the corresponding real temperature was recorded. These three profiles are presented in Fig. 6.5. 133 Temperature( C) 40 controlled forced linear 35 o 30 25 20 200 400 600 Time (min) 800 1000 Fig 6.5 Three different cooling profiles. The concentration of both enantiomers in the liquid phase and corresponding process trajectory for controlled, forced and linear cooling process are illustrated in Figs. 6.6, 6.7 and 6.8. It was found that all these crystallization processes can be divided into two stages: stage (a to b) and stage (b to c). In the stage 1, the concentration of (S)-MA decreased properly while the concentration of (R)-MA remained almost constant. This suggests that the product crystals obtained from stage should be in form of pure (S)-MA. However, when the processes came into stage 2, the concentration of (R)-MA began to decrease clearly. (R) form enantiomer would appear in the product crystal accordingly. Therefore, if the (S) form enantiomer of mandelic acid is desired, the operation range should be within the stage 1. The temperature at the end of the stage for the controlled cooling profile was about 23.3oC, which was the saturated temperature of (RS)-MA. In addition, it can be proposed that when the operation line of stage was longer and the end point of stage 134 deviated more from the eutectic line, the potential to get the pure (S) products should be increased. By the comparison of three figures, it was found that the longest operation stage was for the controlled cooling profile, while the operation stage for the forced cooling profile was shortest one. It implies that the controlled profile is the most favorite operation to get the high yield product. Indeed, based on the difference between initial and final solution concentration, circa 65% yield was obtained for the controlled operation, while circa 58% was for forced operation and 62% was for linear operation. 0.7 35ºC 30ºC 25ºC 20ºC racemic line eutectic line experiment data R-MA (g/ml H2O) 0.6 0.5 0.4 0.3 0.2 b 0.1 a c 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 S-MA (g/ml H2O) Fig 6.6 The concentration of both enantiomers in the liquid phase and corresponding process trajectory for controlled cooling process. 135 0.7 35ºC 30ºC 25ºC 20ºC racemic line eutectic line experiment data R-MA (g/m l H2 O) 0.6 0.5 0.4 0.3 0.2 b 0.1 a c 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 S-MA (g/ml H2O) Fig 6.7 The concentration of both enantiomers in the liquid phase and corresponding process trajectory for forced cooling process. 136 0.7 35ºC 30ºC 25ºC 20ºC racemic line eutectic line experiment data 0.6 R-MA (g/ml H2O) 0.5 0.4 0.3 0.2 b 0.1 c a 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 S-MA (g/ml H2O) Fig 6.8 The concentration of both enantiomers in the liquid phase and corresponding process trajectory for linear cooling process. 6.3.2.3 Optical purity and crystal size distribution The optical purity of product crystal was measured by HPLC and Polarimenter, verified by DSC as mentioned in introduction. At first, a calibration curve of optical rotation with concentration for pure (S)-MA was demonstrated in Fig. 6.9. The relationship of concentration of pure R-enantiomer C with optical rotation α is expressed as Equation 6-2. α = 0.7448*100 C (6-2) 137 0.8 data Optical rotation 0.7 linear regression 0.6 0.5 0.4 0.3 0.2 y = 0.7448x 0.1 0 0.2 0.4 0.6 0.8 Concentration (g solute/100ml water) Fig 6.9 Calibration curve of optical rotation with concentration for pure (S)-MA. In addition, another calibration curve of melting temperature with different mole percent (S)-MA was constructed in Fig. 6.10. The DSC thermograms and HPLC results of final crystal products from the experiments under controlled, forced and linear cooling profiles are presented on Figs. 6.11 and 6.12. Finally, the Table 6.2 shows the optical purities results of the final crystal products, which were derived by the methods mentioned above. 138 Melt temperature ( oC) 134 Liquidus line 132 Eutectic invariant 130 128 126 124 122 120 118 116 114 70 80 90 100 Mole percent of S-MA Fig 6.10 Calibration curve of melting temperature with the mole percent (S)-MA. Table 6.2 The optical purity of final crystal product Experiment Polarimeter sample DSC concentration Temperature (g/100ml o ( C) water) α Mole percent of S-MA from HPLC (%) Mole percent of S-MA from polarimete r (%) Controlled cooling 132.5 0.2845 0.2119 100 100 Forced cooling 121.8 0.2089 0.0598 80.6 80.6 Linear cooling 129.9 0.3992 0.2706 95.5 95.5 139 Fig 6.11 DSC results for the final products and pure (S)-MA. Fig 6.12 HPLC results of final products and pure (S)- and (RS)-MA. 140 From all results, it was obviously found that there was just one peak observed in Fig. 6.11 for DSC and Fig. 6.12 for HPLC. The pure S enantiomer products were obtained only by controlled cooling experiments. This suggests the controlled operation was successful and induced nucleation of racemate was effectively inhibited by controlling the supersaturation of S enantiomer within the critical value. By comparison, only 80.6% and 95.5% mole percent of (S)-MA can be obtained by forced and linear crystallization operation respectively. In the Figs. 6.11 and 6.12, the two peaks were shown for DSC curves and HPLC separation results. It indicates the crystal nucleation and growth of racemic RS can occur in these two operations. At our operation temperature, the racemic RS did not reach its metastable zone. This indicates that the nucleation of RS racemate was induced by S enantiomer nucleation. It could be explained by two reasons: a) the supersaturation of Senantiomer surpassed a critical limit that its secondary nucleation would breed the nucleation of RS racemate though the supersaturation of S enantiomer was still within its metastable zone (Qian and Botsaris, 1998); b) the spontaneous nucleation of both RS racemate and S enantiomer when the supersaturation of S enantiomer surpassed its metastable zone. The crystal size distributions of the final products under different crystallizations were presented in Fig. 6.13 and the CSD of seeds prepared by fast cooling were shown as well. It was easy to found that final products obtained by the controlled cooling operation showed larger particles with narrower distribution. The two most representative parameters describing a crystal distribution are the weight mean size and the coefficient of variation (CV) (wideness of the distribution) (Randolph and Larson, 1988). The crystal product weighted mean size of controlled operation was 527 µm with 53% coefficient of variation, while the product crystals 141 were 362 µm weighted mean size with 70% coefficient of variation for the forced operation and 457 µm weighted mean size with 60% coefficient of variation for the linear operation. 12 seed forced cooling Volume percentage (%) 10 controlled cooling linear cooling 10 100 1000 10000 Crystal size (µm) Fig 6.13 Crystal size distribution of (S)-MA seeds and crystal products from different cooling profiles. Based on final product optical purities and crystal size distribution, it was indicated that the proposed controlled cooling profile was successful in mandelic acid resolution to get the pure S enantiomer. During this preferential crystallization process, it is essential to control the supersaturation of S enantiomer within a critical range to prevent the nucleation of RS racemate. 142 6.3.2.4 Seed size effect on crystal size distribution In the presence of seed crystals, secondary nucleation can be suppressed. The amount of required seeds to overcome the secondary nucleation depends on the seed mean size, quantity and quality of the seeds, as well as the cooling policy (Hojjati and Rohani, 2005). When the optimal cooling profile is determined, the seed size has an effect on the CSD of final product. The effect of seed size have been experimental investigated on the final CSD of potassium sulphate, ammonium alum and potassium alum (Hlozny et al., 1992; Jagafesh et al., 1996, 1999; Doki et al., 1999, 2001; Kubota et al., 2001). In this work, the effect of seed size was studied as well. The two different sizes of (S)-MA seed crystal (75-90 µm and 125-150µm) were prepared using Glisonic Autosiever. They were used respectively as seed crystals in the same optimal preferential crystallization process as described above. The final products CSD were compared with the products of last section (seed size around 225-245µm). The results are shown in Fig. 6.14. 143 Volume percentage (%) 12 225-245µm 125-150µm 10 75-90µm 10 100 1000 10000 Crystal size (µm) Fig 6.14 Effect of seed size on the final CSD of preferential crystallization for MA. It was found that the weight mean size of final product increased with the seed size increasing. It increased from 435µm for seed size 75-90µm to 527µm for seed size of 225-245µm. In these experiments, the seed load was same and the yield was constant because of same crystallization operation. When the seed size increases, the seed crystal surface will decrease. The amount of solid deposited on crystal surfaces is constant at constant yield. Therefore, the final crystals mean size will increase with the seed crystal surface decrease. However, the final crystal mean size did not increased significantly at high seed size. The weight mean size of final crystals for the seed size 125-150µm was 502µm, which is little smaller than that for seed size 225245µm. Therefore, the crystal seed size 225-245µm is suitable for the current studied preferential crystallization process. 144 6.4 Conclusions The 80% mole percent S enantiomer enantiomerically enriched mandelic acid was obtained from a racemic composition by using a HPLC with a semi-preparative chiral column. Through the study on the direct crystallization progression for the mandelic acid system, it was found that the optical pure product could be obtained by direct crystallization with seeding within certain safe supersaturation limit, which suggests that there was no selectivity of crystal growth and nucleation of the pure enantiomer and racemate for a racemic compound. The relative solubility and critical supersaturation control of the pure enantiomer and racemate were found essential to obtain the pure enantiomer. In addition, a systematic study on solubility, metastable zone, kinetics and supersaturation control profile to obtain crystal product with good quality was extended to mandelic acid, the racemic compound system. Based on the thermodynamic and kinetic consideration, the mandelic acid is favorable for the preferential resolution. An optimal temperature control profile was proposed to control the critical supersaturation in order to inhibit the induced nucleation of the racemate. Two other cooling profiles, namely forced and linear cooling profile, were applied to the direct crystallization process for comparison. The final product’s optical purity, yield and crystal size distribution were examined and compared for these profiles. The results show that the final crystal products of our proposed control cooling profile were almost optical pure with high yield and good crystal size distribution. Seed size effect on crystal size distribution was studied simply as well. The weight mean size of final crystal product increased with the seed size increasing because the amount of solid deposited on crystal surfaces is constant at constant yield. 145 However, the final product mean crystal size did not increased significantly at high seed size. 146 [...]... (%) 12 2 25- 2 45 m 1 25- 150 µm 10 75- 90µm 8 6 4 2 0 1 10 100 1000 10000 Crystal size (µm) Fig 6.14 Effect of seed size on the final CSD of preferential crystallization for MA It was found that the weight mean size of final product increased with the seed size increasing It increased from 4 35 m for seed size 75- 90µm to 52 7µm for seed size of 2 25- 2 45 m In these experiments, the seed load was same and the... designed and controlled direct crystallization However, when the crystallization final temperature was lower than 23.3 oC, such as Exp_ 05 at 22 .5 oC, (RS)-MA began to supersaturate and the product crystals were in the form of mixture of (RS)-MA and (S)-MA This may suggest that there is no selectivity of crystal growth of the pure enantiomer and racemate for a racemic compound when both (S) and (RS)... size and the coefficient of variation (CV) (wideness of the distribution) (Randolph and Larson, 1988) The crystal product weighted mean size of controlled operation was 52 7 µm with 53 % coefficient of variation, while the product crystals 141 were 362 µm weighted mean size with 70% coefficient of variation for the forced operation and 457 µm weighted mean size with 60% coefficient of variation for the... cooled to 21 .5 oC without seeding as Exp_06, the primary nucleation occurred by exceeding the metastable zone of initial composition mandelic acid The products were always in the form of (RS)-MA and (S)-MA This could be due to the crucial characteristic of a racemic compound: no selectivity of nucleation between the pure enantiomer and racemate for a racemic compound So, the optical purities of these experiments... The weight mean size of final crystals for the seed size 1 25- 150 µm was 50 2µm, which is little smaller than that for seed size 2 252 45 m Therefore, the crystal seed size 2 25- 2 45 m is suitable for the current studied preferential crystallization process 144 6.4 Conclusions The 80% mole percent S enantiomer enantiomerically enriched mandelic acid was obtained from a racemic composition by using a HPLC with... inhibited by controlling the supersaturation of S enantiomer within the critical value By comparison, only 80.6% and 95. 5% mole percent of (S)-MA can be obtained by forced and linear crystallization operation respectively In the Figs 6.11 and 6.12, the two peaks were shown for DSC curves and HPLC separation results It indicates the crystal nucleation and growth of racemic RS can occur in these two operations... (Wang et al 2007) For the preferential crystallization of a conglomerate, the crystallization rates of two opposite enantiomers can be controlled to some extent and certain resolution can be achieved 130 by well designed and controlled procedure (Wang and Ching 2006) However, for the racemic compound, it was found here that only excess S enantiomer can be obtained by the preferential crystallization. .. 2.5oC It is also reasonable to assume constant B on the curse of the whole process because the MT used in this work is low Therefore, the equation 6-1 can be directly used as cooling profile to control the supersaturation in the preferential crystallization of mandelic acid As the mean size was circa 230 µm for the seed and 52 7 µm for a target product, the operation time can be calculated as tc = (52 7-230)/0. 35= 848... was for the controlled cooling profile, while the operation stage 1 for the forced cooling profile was shortest one It implies that the controlled profile is the most favorite operation to get the high yield product Indeed, based on the difference between initial and final solution concentration, circa 65% yield was obtained for the controlled operation, while circa 58 % was for forced operation and. .. forced operation and 62% was for linear operation 0.7 35 C 30ºC 25 C 20ºC racemic line eutectic line experiment data R-MA (g/ml H2O) 0.6 0 .5 0.4 0.3 0.2 b 0.1 a c 0 0 0.1 0.2 0.3 0.4 0 .5 0.6 0.7 S-MA (g/ml H2O) Fig 6.6 The concentration of both enantiomers in the liquid phase and corresponding process trajectory for controlled cooling process 1 35 0.7 35 C 30ºC 25 C 20ºC racemic line eutectic line experiment . in the form of mixture of (RS)-MA and (S)-MA. This may suggest that there is no selectivity of crystal growth of the pure enantiomer and racemate for a racemic compound when both (S) and (RS). ml/min and UV-Vis detection at 210nm. 6.3.2 Preferential crystallization operation for mandelic acid 6.3.2.1 The progression of preferential crystallization 128 A typical loading of. crucial characteristic of a racemic compound: no selectivity of nucleation between the pure enantiomer and racemate for a racemic compound. So, the optical purities of these experiments results