(12b) Experimental Investigation of Change in Chord Length Distribution Measured by Online Backscattering Technique

In the design and study of various particle rate processes knowledge of factors governing the average size and size distribution of particles is an important consideration. The measurement of time varying particle size distribution is difficult and poses many problems due to difference in form i.e. size and shape of particles which brings much complication in characterizing the size distribution. The size distribution is measured by various online and offline techniques. The online techniques are in demand due to difficulty in offline sampling and nonrepresentational of population of particles under measurement.

The use of Lasentec's FBRM as online measurement is widely in use in various fields of particulate process industry such as crystallisation, flocculation, biotechnology and online control of crystallisers.

This is because FBRM offers the potential for monitoring, online in situ measurements of the size distribution of particle populations even in systems with high solid concentrations. FBRM probe requires no sample dilution which has important advantage.

FBRM is found to be ideal for industrial usage as a quality and process control device that provide rapid and accurate data accumulation, with the collected information requiring minimal attention from plant operators [1,2]. Unlike other laser light scattering techniques FBRM does not give direct measurement of particle size distribution. The instrument gives a particle chord length distribution, which is a function of the true particle diameter distribution [3].

Extensive study of the effect of various experimental factors on the chord length measured by FBRM has been done. FBRM has been found to be useful for online in-situ measurements of particle size distribution as long as the factors affecting chord length distribution does not change, factors such as focal position, suspension agitation and orientation once fixed does not change during course of measurement, but the factors such as solid volume fraction and particle material may change in some processes. The effect of such change on the FBRM measurements is shown in this work.

In the present work the influence of particle property on the chord length distribution measured by FBRM measurement is discussed.

It is shown that bigger size transparent crystals have affect on measured chord length distribution. Such affect is more evident in processes involving changes in physical property for example, phase transformation.

During phase transformation of sodium carbonate to sodium carbonate monohydrate the optical property of crystal changes from opaque to transparent. The phase transformation process results in sodium carbonate monohydrate crystals bigger in size than sodium carbonate [5]. However the third moment values of measured chord length distribution shows initial rise and than sudden drop in values. The increase in values is linked to the growth of monohydrate crystals, but the decrease in value remains unexplained. It is assumed that the decrease in chord counts is related to bigger transparent monohydrate crystals affecting the chord measurements by FBRM.

Above assumption is confirmed by carrying out experimental investigation of different size monohydrate crystals measured by FBRM probe.

It is concluded that careful consideration must be given to the particle material and physical property particularly optical property during online measurement of particles by techniques working on similar principle.

References

Redman, T.; Rohani, S.; Strathdee, G. Control of the Crystal Mean Size in a Pilot Plant Potash Crystallizer. Trans Chem E Part A, 1997, 75, 183-192.

Farrell, R. J.; Tsai, Y.C. Nonlinear Controller for Batch Crystallization: Development and Experimental Demonstration. AIChE, 1995, 41, 2318-2321.

Ruf, A.; Worlitschek, J.; Mazzotti, M. Modelling and Experimental Analysis of PSD Measurements through FBRM. Part. Part. Syst. Charact., 2000, 17,167-179.

Shaikh, A.A.; Salman, A.D.; Mcnamara, S.; Littlewood, G.; Ramsay, F.; Hounslow, M.J. In situ observation of the conversion of sodium carbonate to sodium carbonate monohydrate in aqueous suspension. Industrial and Engineering Chemistry, 2005, In the press.