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.

AS) reviewed future monitoring techniques for volatile organic compounds (VOCs). The techniques described included gas chromatography, open-path monitoring, realand near real-time monitoring, passive sampling devices, and sample containment and transport.

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Nitroso compounds, such as 2-methyl-2-nitrosopropane, nitrosobenzene, and 4-nitrosophenol, were tested as volatile inhibitors for vapor-phase inhibition of acrylic acid polymerization during its reflux at 113 °C under reduced pressure. The experimental parameters were set to mimic the conditions employed for the industrial distillation of acrylic acid. Nitrosobenzene was found to be the most efficient vapor-phase inhibitor for the distillation of acrylic acid. The inhibition time was also found to be dependent on the pressure used for the experiments. Decreasing the pressure from 110 to 85 mbar resulted in an almost 2-fold increase in the inhibition time. The combination of nitrosobenzene with a liquid-phase inhibitor, namely, 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl, significantly improved the efficiency of vapor-phase inhibition by decreasing the consumption of nitrosobenzene in the liquid phase.