This article describes how a fiber-optic turbidity probe may be used as an inferential sensor to aid in the control of commercial-scale batch crystallizations. The discussion focuses on several unseeded crystallization examples involving cooling or cooling plus addition of antisolvent. In a typical control scheme, the fiber-optic probe is used to detect an initial nucleation event, to control a subsequent digestion step for fines dissolution with the potential for modification of nuclei size, number, and purity, and then to monitor a growth period. During the digestion step, temperature is increased and adjusted to achieve a desired reduction in the fiber-optic signal in order to control the extent of digestion. Within Dow, this approach has proven to be robust and cost-effective for numerous commercial-scale batch crystallizations including those with highly fouling or corrosive environments.
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