A deracemization technique using periodic temperature fluctuations on a conglomerate forming system undergoing a swift racemization in solution is demonstrated. The method uses heating and cooling periods of the suspension in order to create cycles of partial dissolution of the crystal phase followed by crystal regrowth: this enables symmetry breaking in the solid phase. The technique is an effective, simple, and cheap operation, and can promote understanding of the effects of dissolution and recrystallization on chiral symmetry breaking in the solid phase. The heating period leads to the decrease of the size of crystals and the destruction of small crystals; the surviving crystals can then grow during the cooling period. A succession of such cycles allows the autocatalytic transformation from a racemic suspension into pure enantiomer, with an enantiomeric excess (ee) > 99% within a few days. The results demonstrate a possible mechanism for the emergence of homochirality of molecules of biological significance on Earth.
Cocoa butter equivalent (CBE) was produced from a blend of mango kernel fat (MKF) and palm oil mid-fraction (PMF). Five fat blends with different ratios of MKF/PMF (90/10, 80/20, 70/30, 60/40 and 50/50 (%wt)) and pure MKF, PMF and cocoa butter (CB) were characterized. Similar to CB, all fat blends contained palmitic (P), stearic (S) and oleic (O) acids as the main fatty acid components. The triglyceride compositions of all blends were significantly different from CB. However, blend 80/20, which contained higher content of SOS, similar content of POP and lower content of POS compared to CB, exhibited a slip melting point, crystallization and melting behavior most similar to CB and hence it was recommended as CBE. The chosen CBE was then mixed with CB in a ratio of 1:5.64 (wt), mimicking that of typical dark chocolate where 5 % of CBE is added to the finished product. The crystallization behavior, the crystal morphology and bloom behavior of the mixture was investigated and was found to be not significantly different from CB.
An improved process for the deracemization of a racemic conglomerate suspension of enantiomorphs has been created based on principles developed in an earlier method using temperature fluctuations. The method consists of circulating the suspension between two vessels, each controlled at a specific temperature in order to make the process more effective and faster to achieve a homochiral solid state. Crystals in the suspension were partially dissolved in the hot vessel, and the remaining crystals were regrown in the cold vessel. The crystals in the cold vessel have a longer residence time than those in the hot vessel to allow more time for the crystal growth process. The results show that complete deracemization can be achieved via this process far more rapidly than by using the previous temperature cycling (one-vessel) process. Moreover, the new process could easily be scaled up to an industrial scale. The current process can be an effective alternative to currently used enantiopurification methods, with simple processing implementation and low cost.
The effect of a crystal's growth rate history on the current growth behavior was investigated as a cause of crystal growth rate dispersion (GRD), which is a significant problem in crystallizer design and modeling. The solute used in the experiments was sucrose, a high production commodity still crystallized mainly in batch crystallizers, where GRD is most significant. The results show that the growth history of a crystal has a significant effect on the crystal growth rate of that crystal. In particular, a history of rapid crystal growth under high supersaturation causes a roughening of the crystal surface, apparently due to poor surface integration, and subsequent growth occurs at lower rates than would be expected without this growth history. The process of crystal surface healing occurs at low levels of supersaturation over periods of several hours of growth and results in the crystals again reaching their normal growth rate levels. The growth layer formed to heal the surface of the crystal is greater than 100 µm thickness, far in excess of the scale of the apparent surface roughness. This study shows that the crystal perfection near the growing surface of the crystal may be a more significant indicator of crystal growth rate than the overall average perfection in the entire crystal.
The model-based estimation of primary
nucleation rates from the
probability distribution of induction time measurements has been used
extensively in the literature; however, there has been no experimental
validation of this approach via direct measurements of nucleation
rates. In this work, we compared the nucleation rates obtained from
the probability distribution model against that measured from in situ
optical reflectance measurement (ORM) with p-aminobenzoic
acid and l-glutamic acid as model compounds. Results reveal
that the primary nucleation rates obtained from stochastic models
are several orders of magnitude lower than those measured from a direct
particle-counting technique. Although differences in fluid dynamics
due to agitation and crystallizer geometry may have an effect, this
tremendous discrepancy provides strong evidence that primary nucleation
rates obtained from induction time measurements are not scalable and
should not be used for interpretation of nucleation rates in industrial
applications.
The current research has developed
a potential route toward process
optimization for the deracemization of a racemizable conglomerate
forming system. The use of damped temperature cycleswhere
the magnitude of the temperature cycles is reduced as the enantiomeric
excess (ee) in the solid phase increasesis a promising methodology
for optimizing the temperature cycle induced deracemization process.
This process requires significantly less time and energy to reach
an enantiopure state compared with the use of constant amplitude temperature
cycles. There was evidence of some crystal breakage occurring in the
system; however, the limited amount of breakage did not produce any
change in the enantiomeric excess in the solid phase in the absence
of temperature cycles in the system. Hence, the primary mechanism
in this process is neither Viedma ripening nor Ostwald ripening. Scanning
electron micrographs of the crystals taken during the series of temperature
cycles show that the amplification of the ee in the solid phase is
caused by dissolution and growth phenomena induced by the temperature
cycles. The promotion of the larger or faster growing crystals at
the expense of the smaller or slower growing crystals during the cycles
and entrainment from a mother liquor having ee = 0 are both responsible
for this deracemization.
Investigation of the secondary nucleation threshold (SNT) of R-glucose monohydrate was conducted in aqueous solutions in agitated batch systems for the temperature range 10 to 40 °C. The width of the SNT decreased as the induction time increased and was found to be temperature independent when supersaturation was based on the absolute concentration driving force. Nonnucleating seeded batch bulk crystallizations of this sugar were performed isothermally in the same temperature range as the SNT experiments, and within the SNT region to avoid nucleation. The growth kinetics were found to be linearly dependent on the supersaturation of total glucose in the system when the mutarotation reaction is not rate limiting. The growth rate constant increases with increasing temperature and follows an Arrhenius relationship with an activation energy of 50 ( 2 kJ/mol. R-Glucose monohydrate shows significant crystal growth rate dispersion (GRD). For the seeds used, the 95% range of growth rates was within a factor of 6 for seeds with a narrow particle size distribution, and 8 for seeds with a wider distribution that was used at 25 °C. The results will be used to model the significance of the mutarotation reaction on the overall crystallization rate of D-glucose in industrial crystallization.
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