PARTICLE-SIZE DETERMINATION BY LASER REFLECTION - METHODOLOGY AND PROBLEMS

The particle size distribution of crystalline solids has progressively become a key parameter in manufacturing processes, as important as ch emical purity. Among the particle size determination and counting syst ems available on the market, very few offer the possibility of continu ous in situ monitoring of the particle size evolution during crystalli zation. For this reason, much interest has been aroused by the appeara nce of the Par Tec 100, patented by Laser Sensor Technology [1, 2]. A study has been carried out in a stirred vessel to verify the precision and reproducibility of particle size measurement and elucidate the in fluence of experimental parameters on data accessible with this instru ment. Optimum reproducibility has logically been achieved by fixing th e highest possible cycle time and taking the mean of several cycles. D eterminations with the Par Tec 100 are influenced variously, according to whether they relate to the total number of particles counted or to the mean size. Thus, the number of counts measured by a particle size probe largely depends on the operating conditions and more particular ly on the hydrodynamic conditions, solvent, temperature and focal poin t position. Its dependence relative to the concentration of the solid in suspension is normal and linear for a solid and for a given monodis perse sample. To establish the relationship between the number of coun ts and the population density would therefore necessitate delicate cal ibration on a case-by-case basis. The mean size determined does not de pend on suspension homogeneity, provided that the stirring speed is su fficient for a statistically significant total count. On the other han d, for a given sample, a displacement of the focal point can lead to c onsiderable variations in the size determined. The optimal focal point position for small sizes is in fact highly sensitive. Lastly, the opt imal position of the focal point is considerably dependent on the true size of the particles, which means that this counter is unsuitable fo r the precise analysis of a dispersed sample since each particle size class would require a different setting of the focal point. In additio n, the sizes determined, irrespective of the products studied, appear to be underestimated for large particles and overestimated for small p articles.