90
K. Igarashi and H. Ooshima
Table 5.1 Yields of glycine and l-alanine crystals obtained at different residence time
Methanol composition (%)
Average residence time, τ (s)
3.3
33
50
α
α
78
β
α
91
β
α + β
Fig. 5.7 SEM images of the alanine crystals
Figure 5.7 presents SEM images of the crystals of l-alanine obtained at various
residence times. The crystals were needle-like and those were bundled. The typical
shape of a l-alanine crystal is prismatic [10], which can be obtained by cooling
crystallization. In the present poor solvent crystallization, needle-like crystals were
obtained. However, since the XRD pattern for the needle-like crystals was the same
as that of the prismatic crystals (data not shown), the difference of shape was not due
to polymorphism. The crystals obtained at τ = 33 s were thick and easy to cleave.
The crystals obtained at short residence time were thin.
In the present study, the homogenizer was used for agitation. There is some fear
of breakage of crystals by high-speed mixing. Therefore, we checked the influence
of agitation for the crystal size and shape. 0.84 g of l-alanine crystals of 65 µm
in average size was suspended in 17 mL of saturated solution of l-alanine. The
suspension was agitated by the homogenizer at 24,000 rpm for 10 min. After that, the
crystals were recovered and observed by SEM. The shape and size of obtained crystals
were almost the same as those of the original crystals. Therefore, we concluded that
crystals with a size of less than at least 65 µm in average size were not broken by
short (second time-scale) high-speed agitation.
Figure 5.8 shows size distributions on the basis of the long axis length. At τ =
0.33 and 3.3 s, the size of crystals was small and the distribution was narrow. A broad
distribution was obtained at τ = 33 s. In a long mean residence time, some crystals
may stay in the crystallizer for long time and the supplied supersaturation may be
used for the growth of resident crystals, resulting in broad size distribution. The short
residence time should be advantageous to produce small crystals with a narrow size
distribution.
K. Igarashi and H. Ooshima
Table 5.1 Yields of glycine and l-alanine crystals obtained at different residence time
Methanol composition (%)
Average residence time, τ (s)
3.3
33
50
α
α
78
β
α
91
β
α + β
Fig. 5.7 SEM images of the alanine crystals
Figure 5.7 presents SEM images of the crystals of l-alanine obtained at various
residence times. The crystals were needle-like and those were bundled. The typical
shape of a l-alanine crystal is prismatic [10], which can be obtained by cooling
crystallization. In the present poor solvent crystallization, needle-like crystals were
obtained. However, since the XRD pattern for the needle-like crystals was the same
as that of the prismatic crystals (data not shown), the difference of shape was not due
to polymorphism. The crystals obtained at τ = 33 s were thick and easy to cleave.
The crystals obtained at short residence time were thin.
In the present study, the homogenizer was used for agitation. There is some fear
of breakage of crystals by high-speed mixing. Therefore, we checked the influence
of agitation for the crystal size and shape. 0.84 g of l-alanine crystals of 65 µm
in average size was suspended in 17 mL of saturated solution of l-alanine. The
suspension was agitated by the homogenizer at 24,000 rpm for 10 min. After that, the
crystals were recovered and observed by SEM. The shape and size of obtained crystals
were almost the same as those of the original crystals. Therefore, we concluded that
crystals with a size of less than at least 65 µm in average size were not broken by
short (second time-scale) high-speed agitation.
Figure 5.8 shows size distributions on the basis of the long axis length. At τ =
0.33 and 3.3 s, the size of crystals was small and the distribution was narrow. A broad
distribution was obtained at τ = 33 s. In a long mean residence time, some crystals
may stay in the crystallizer for long time and the supplied supersaturation may be
used for the growth of resident crystals, resulting in broad size distribution. The short
residence time should be advantageous to produce small crystals with a narrow size
distribution.