Measurements

The on-line measurement of the relevant process variables forms an essential and often difficult part of the control strategy. For the CSD control the most relevant process variables are the supersaturation and one of the characteristics of the CSD. The main measurement techniques are summarized next.

• Supersaturation. Except for some crystallization systems (sugars) the direct measurement of supersaturation is either impossible or not accurate enough for control purposes. Recently however several studies have shown that it is possible to measure the supersaturation on-line in a crystal-lizer using attenuated total reflection (ATR) probes in combination with Fourier transform infrared (FTIR) devices. Successful applications of density measurements have also been reported.

• On-line CSD measurement. A number of commercial instruments are available nowadays to determine CSD of crystal slurries. Of these the laser diffraction instruments have been shown to give a reliable measurement in diluted suspensions (at particle concentrations below l vol%). A major drawback to the use of these instruments in a process is the need for an on-line dilution system to dilute the crystal suspension. A more recently developed instrument, measuring the attenuation of planar ultrasonic waves, forms an interesting alternative, because suspensions up to a concentration of 30% (by volume) can be measured without dilution.

• In-line CSD measurement. In-line sensors measuring the reflection of laser light and analysing the back-scatter peaks or images enable the analysis of some properties of the CSD inside the crystallizer, which can be of value for control of crystallizers. The main difficulty with the use of these probes is in identification of the relation between the information from the sensor and a process variable which is relevant for the control of the process.

• Obscuration measurement. The obscuration is defined as the fraction of light that is obscured by the crystals present in a flow cell and is a measure of the second moment of the distribution. As with laser diffraction this relation is only valid when multiple scattering can be avoided, i.e. at low particle concentrations. Obscuration measurements have been used to measure the number of fines crystals present in the fines removal loop.

• Particle counter. An optical particle counter measures the number of particles in a predefined size window in the crystallizer simply by counting the number of pulses from a light detector that are caused by the passage of the particles through a laser beam. The size window is defined as a result of the classification function of the funnel used to withdraw the process liquid from the crystallizer and the detection threshold of the detector, which can be adapted. This counter, which has been successfully used to control a DTB crystallizer on a pilot-plant scale, forms an attractive alternative to the expensive CSD measurement devices.

The choice of the signal used by the controller, as well as the choice of the instrument, can be decisive for the performance of the controller. Using a CSD measurement, different characteristics of the CSD can be chosen. Analysis of different possibilities suggests that a reduced signal based on a principle component analysis of the raw diffraction data of a Malvern laser diffraction instrument, yr, gave the best controller behaviour. The median crystal size, however, does not appear to be a suitable signal for control of the CSD, because of large delays in the response and its low sensivity for changes in the small crystal area.

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