particle size determination by laser reflection: methodology and problems

10 Part. Part. Syst. Charact. 13 (1996) 10-17

Particle Size Determination by Laser Reflection: Methodology and Problems

Olivier Monnier*, Jean-Paul Klein*, Christian Hoff**, Berthe Ratsimba* (Received: 20 March 1995)

Abstract The particle size distribution of crystalline solids has progressively become a key parameter in manufacturing processes, as important as chemical purity. Among the particle size determination and counting systems available on the market, very few offer the possibility of continuous in situ monitoring of the particle size evolution during crystallization. For this reason, much interest has been aroused by the appearance 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 influence of experimental parameters on data accessible with this instrument. Optimum reproducibility has logically been achieved by fixing the highest possible cycle time and taking the mean of several cycles. Determinations 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 particularly on the hydrodynamic conditions, solvent,

temperature and focal point position. Its dependence relative to the concentration of the solid in suspension is normal and linear for a solid and for a given monodisperse sample. To establish the relationship between the number of counts and the population density would therefore necessitate delicate calibration on a case-by-case basis. The mean size determined does not depend on suspension homogeneity, provided that the stirring speed is sufficient for a statistically significant total count. On the other hand, for a given sample, a displacement of the focal point can lead to considerable variations in the size determined. The optimal focal point position for small sizes is in fact highly sensitive. Lastly, the optimal position of the focal point is considerably dependent on the true size of the particles, which means that this counter is unsuitable for the precise analysis of a dispersed sample since each particle size class would require a different setting of the focal point. In addition, the sizes determined, irrespective of the products studied, appear to be underestimated for large particles and overestimated for small particles.

1 Introduction

The physical properties of crystalline solids, particularly their particle size distribution, have progressively become parameters with the same importance in the manufacturing process as chemical purity. These properties in fact determine the suitability of a solid for processing after crystallization, namely filtration, drying and packaging, and its subsequent use (processing, flowability, dusting and packaging in very small quantities). Measurements designed to characterize physical properties are of two types: measurements for utilization properties (filterability, Jenike index for flowability, bulk density, dissolution kinetics, etc.); and measurements of particle size distribution. The number of particle counters available on the market, operating on different principles, is relatively high. The proposed methods of particle size analysis can be classified into two large series, according to whether they are required to determine the distribution in numbers of particles or their distribution by mass. Furthermore, since the sizes obtained are determined indirectly from physical properties which vary

* 0. Monnier, Prof. J.-P. Klein (to whom correspondence should be addressed), B. Ratsimba, LAGEP, URA CNRS D 1328, Universite Lyon I et CPE Lyon, 43 Boulevard du 1 1 Novembre 191 8, F-69622 Villeurbanne (France). Christian H o f , Sanofi Chimie, Route d’Avignon, F-30390 Aramon (France).

**

from one instrument to another, the particle size distributions established depend on which method is chosen. Table 1 summarizes the principal methods currently in use [5 ] . Mass is usually the parameter of interest to the manufacturer, whereas the researcher is more interested in particle size counts, with a view to determining the population balance in the crystallizer [3, 41. As a general rule, the results obtained with different types of counters are identical for spherical particles. However, they vary considerably from one method to another for asymmetric particles, needles, plates, multifaceted particles as they are currently observed in organic chemistry. Most counters require samples to be taken, with all that this entails by way of ensuring that the sample is representative and stable, the time required for analysis (sampling and measurement) and precautions associated with the toxicity of reactive mixtures [5] .

Table 1: Principal methods of particle size analysis

Results expressed as a function of particle mass

Results expressed as a function of particle counts

Sieving Filtration Sedimentation Elutriation Centrifugation

Optical detection: Absorption Diffraction Reflection

Electrical detection: Coulter counter

Image analysis

05’34-0866/96/0 102-001 0 $l0.00+.25/0 C? VCH Verlagsgesellschaft mbH, D-69469 Weinheirn, 15’96

Part. Part. Syst. Charact. 13 (1996) 10-17 11

A new type of sensor is now available that is able to measure the crystal size distribution directly in situ at full process concentration, from the point of view of its manufacturer. The design of this instrument is such that laser probe can be placed in the process without a need for a sampling system. This is of particular interest in crystallization, for crystal size distribution determination under process conditions and in measuring, for example, the particle size distribution of emulsions [6]. The Par Tec 100 counter, patented by Laser Sensor Technology, under study here enters directly into a suspension and should enable continuous particle size data to be obtained in situ.

2 Particle Size Measurement

2.1 Principle of Particle Size Measurement

The light beam emitted by a laser diode is focused on a focal point in the suspension to be analysed, close to the probe, as shown in Figure 1. The focusing lens is subjected to an eccentric rotation generated electromagnetically, which causes the focal point to be displaced in a circular pattern, the amplitude depending on the distance from the focal point to the probe. In theory, any particle on the trajectory of the focused beam reflects light throughout the time it is scanned by the light ray. The optical pulses which originate from reflections of the probe light from the particles in the suspension are collected by the probe optics and passed via a fibre-optic cable to the detector. They are converted by the detector into electronic signals, which are analysed for their duration in the time domain. A discrimination loop sorts impulse for short rise times. Only impulses originated from particles that pass directly through the focal spot will possess a short rise time and will be accepted. All particles which pass elsewhere than in the focal point will give pulses of much longer rise time and will be eliminated. Furthermore, this focal spot is scanned at a constant rotational speed (4410rpm) so that the time the laser beam takes to scan the particle, and thus the reflection time, is a direct measure of the size of the surface on which the reflection is produced. A counter assigned to each of the distribution segments of these surface sizes registers an increment each time a determination takes place between the boundaries of the corresponding segment. At the end of the cycle, the instrument reconstructs the histogram of sizes as a function of the number of determinations

0 Suspension Fig. 1: Diagram showing the principle of the Par Tec 100 analyser.

conducted, the mean size and the total number of particles counted per cycle. The validity of the determination assumes that the equipment meets certain conditions, in particular: - The light intensity at the focal point must be sufficiently

strong for the retrodiffusing light to be detected by the probe.

~ To prevent the particle size determinations being distorted by the flow of particles, the speed of displacement of the focal point during vibration must be considerably greater than their displacement speed in the solution.

- The focusing of the light must be such that the beam section on the focal plane is at least as small as the smallest of the particles to be detected ( ~ 1 pm). The amplitude of displacement of the focal point, however, has to be larger than the size of the largest particle to be detected.

~ Since the particle size determination is related to the response time of the detectors, the scanning of the focal plane must be conducted at a constant speed.

- Lastly, the discrimination circuit controlling the fact that the impulse received comes from the passage of a particle in the focal plane, and not in front or behind this plane, must be efficient. The impulse is based on the slope of the signal diffused back. If this slope is not adequately “steep”, the signal received does not correspond to a particle at the focal point and the corresponding impulse is eliminated.

Furthermore, the operating conditions must fulfil certain criteria, as defined in Table 2. It should be noted that the intensity of the reflected signal does not enter into the determination of particle size, except to establish whether there is an impulse and if the response time is compatible with a position of the particle at the focal point. The intensity of the response obtained will depend on several factors, such as the diffusion coefficient of the solution, the refractive index of the medium, the absorption of the wavelength used by the particle or the solvent (colour) or the surface characteristics of the particles under study. It will also depend on the distance between the focal point and the probe.

2.2 Equipment Used

There are several types of Lasentec on the market, from the laboratory analyser (Lab-Tec 100) to the industrial analyser (Par Tec 200/250/300), including the version used here (Par Tec 100) which corresponds to the laboratory version of the industrial equipment. This reduced type of the industrial probe developed by Laser Sensor Technology would seem to be a priori the ideal tool to demonstrate the possibilities of the industrial analyser and confidently to undertake the transition from laboratory control to production line control.

Table 2: Basic criteria for correct measurement with a reflection laser counter.

Preliminary operating conditions

Good homogeneity in the suspension

Representative measurement location

Cleanness of the saphire window. There must be no crystallization on this window, particularly at the

strong initial supersaturations.

Concentration area of the suspension: 0.01-30%


The Par Tec system is composed of three parts: the measurement probe; the electronic measurement unit; and the processing and calculation unit. The measurement probe consists of two parts, namely the probe head represented in Figure 1 and containing the laser diode, photodetectors and associated electronics, and the titanium probe body which enters the suspension under analysis, with a sapphire window at the tip enabling the beam to enter the suspension. The electronic unit contains the various sources of supply, the preamplification module, the count card and the motor control card. The processing and calculation unit consists of a micro- computer for data acquisition, communication between the electronic unit and the calculator, data display and processing.

2.3 Data Supplied by the Counter

The instrument used covered 38 size ranges increasing in intervals of (3/2)1/2 from 1.9 to 1000 pm. The measurements are taken during an adjustable time cycle between 0.2 and 3.2 s adapted according to the concentration of the suspension and the response of the product under study. This then gives access to a number of impulses expressed by particle size class or as a total on all classes and to a mean size. The size distributions obtained can be displayed as a fraction, counts by channel or a fraction relative to the cumulative count (larger or smaller than). It is also possible to monitor the development of the measurements over time on a display board, during experimentation, but unfortunately this display is not accessible when the data are being reprocessed. The mean size expressed in pm is calculated conventionally taking the mean size for each class as being the arithmetic mean of the lower and upper limits. The standard deviation around the mean size, and the coefficients of lateral and vertical distortion in relation to one gaussian, make it possible to characterize the shape of the particle size distribution obtained. Since the raw data obtained are expressed in particle numbers as a function of a characteristic size, there are still ways in which the sizes determined can be corrected. Thus, for example, it is possible to correct the statistical dispersion caused by the scanning of spherical particles cutting the focal line by any one of their chords. Our study had essentially two aims, namely, first, to evaluate the counter's characteristics in terms of the reproducibility of the measurements and the determination of parameters that might influence its response, and second, to investigate the correspondence between the particle size determined by Par Tec 100 and the real physical size of the particles in suspension.

3 Reproducibility of the Measurements

Particle size determination, particularly with grains of variable shape, as happens with crystallization when characteristic size is assessed, is by its very nature a delicate procedure. In any discussion of the physical significance of the results, it is essential first to evaluate their precision and reproducibility. Studies in this direction were therefore undertaken by using a 500-ml reactor stirred using a TBI-type radial-flow turbine impeller (with four blades inclined at 45"). The stirring speed was fixed arbitrarily at 750rpm, a value for which the homogeneity of the suspension has been found to be correct.

Figure 2: Diagram of apparatus. Characteristics of the equipment: 500-ml reactor: Tank diameter: 0.100m. Stirring: Impeller diameter: 0.050 m Impeller paddle height: 0.01 Sm. Data acquisition: Computer: PC 486 Printer Data recorder. Heating and cooling: Thermostat-controlled bath.

The particle size probe was placed in the central position (cf. Figure 2). In order to avoid any changes with time, the suspension used consisted of an aqueous suspension of 200 g of glass beads, with a bulk density of 1.45g/cm3, per litre of water. The particle size analysis of this suspension was repeated several times using different measurement cycle times (0.2 and 3.2s). The results obtained for the two variables studied, namely the total counts and the mean size, are given in Figures 3 and 4. On the x-axis we plotted the test reference and on the y-axis the relative deviation of the measured size L or number of counts N, relative to the mean L, or N,, for all the tests carried out for a given cycle time. These results give rise to several observations: - For a cycle time of 0.2 s, the total count is too small to allow

correct statistical processing of the data. This is indicated by poor reproducibility of the determination of mean size.

- For a cycle time of 3.2s, on the other hand, the reproducibility of the determinations is better. The standard deviation of the size determination passes from 2.6% for a cycle time of 0.2s to 0.99% for a cycle time of

8 5 1 ._ r 4

0

0 E

E -I . 2

'O Te&Jo 20 25 30

Fig. 3: Reproducibility of particle size determination. Influence of cycle time on mean size obtained.

Cycle time 3.2s Cycle time 0.2 s

0

0 00 0

0 0 1 .

0 l " " r " " ~ ' " ' l ' " ' " " ' l " ' ' l 0 5 10 15 20 25 30

Test No

Fig. 4: Reproducibility of particle size determination. Influence of cycle time on the total counts.


3.2 s. This can be further improved by taking the mean of the measurements for several cycles.

- Optimum reproducibility can then be obtained by fixing the cycle time as high as possible (3.2 s) and taking the mean of several cycles. It may be very difficult however, to maintain this optimum condition when monitoring on-line such rapid phenomena as nucleation during crystallization.

4 Influence of Experimental Parameters on Determination

The various parameters studied were the concentration of the suspensions, the type of liquid phase, stirring speed, temperature and focal point position. In all cases, the studies were carried out using the experimental assembly shown in Figure 2.

4.1 Influence of the Solid Concentration

The suspension used for the tests consisted of an aqueous suspension of glass beads with particle size determined by sieving between 125 and 250 pm. The measurements were made using a cycle time of 3.2s and readjusting the focal point for each concentration to its optimum according to the method described in Section 4.5. The results obtained for the two variables under study, namely the total counts and the mean size, are given in Figure 5, which appears to show proportionality between the total counts and the concentration in suspension. The mean size determined may be considered as virtually independent of the concentration in solution. However, it is much smaller than that obtained for the glass beads at sieving and which can therefore be considered as a correct value for their real diameter. The linear regression used to correlate the total counts with the concentration of the solid is evidently no longer applicable for concentrations close to zero.

- Mean size E 100; 250001 .. . * . . .'

.c

p 20000 v) $ 15000

! 10000 m ? 5000 .

0 100 200 300 400 500

Concentration C in g of beads per liter of water

Fig. 5: Variation of the total counts and of the obtained mean size as a function of the suspension Concentration.

4.2 Influence of the Type of Liquid

To examine the influence of the type of liquid on the counter response, in addition to water we also tested three organic products with very different refractive indices. The total count is very largely dependent on the type of liquid used, as Figure 6 shows. As regards an organic liquid alone, it appears that the number of counts rises as the refractive index decreases. This results in an increase in the slope of the regression line for each liquid, with a reduction in the refractive index. In Table 3, the results obtained with water were recorded

3000O-j . .

u) 2000

0 1500

I-" 1000

500

T

IbO 260 3b0 4b0

- U - 1

500

WATER TOLUENE ISOOCTANE THF

Concentration in g of beads per liter of liquid

Fig. 6: Total counts observed for calibrated glass beads in different solvents for a counting cycle of 3.2 s.

Table 3: Evolution of the slope of the regression line giving the number of particles counted as a function of their concentration in suspension with variation of the refractive index of the liquid.

Type of liquid Refractive Density Slope index, & (kg/m3) (number m3/kg)

Isooctane 1.391 692 52.43 THF 1.405 889 40.00 Toluene 1.496 867 17.15 Water 1.332 1000 50.49

separately since they do not confirm the results obtained with organic products. Although the refractive index of water (do = 1.332) is less that of isooctane (q? = 1.391), the number of counts obtained in water is always less than that obtained in isooctane. It therefore seems that the refractive index is not the only parameter influencing the number of counts. It will only be possible to determine the predominant factors if a large number of particle-liquid systems are studied. The distributions by size, however, seem to be only negligibly affected by the type of solvent, as is evident from the results compared in Figure 7 . In view of preliminary sieving of the glass beads, the presence of fines in the distribution can be explained in different ways: air microbubbles in suspension, presence of fine particles coming from collisions, reflection of part of the glass beads having flaked off, or fine particles resulting from the cleaning of the glass beads by the solvent.

+ ISOOCTANE

I 10 100 1000 Particle size classes in micrometers

Fig. 7: Particle size distribution for calibrated glass beads in various liquids.


$ 8 0 -

E - g 6 0 - E

m 4 0 - c - - Ln c g 20-

4.3 Influence of Stirring Speed

This parameter was studied using two different suspension solvents, water and toluene. The product in suspension always consisted of glass beads calibrated by sieving to sizes between 125 and 250 pm. The studies were carried out at stirring speeds between 500 and 2000 rpm and concentrations varying from 150 to 500 g of beads per litre of solvent. The results obtained with water are given in Figure 8.

4000 3500

1500

1000 500

-y- 750 rpm .....,.... 1000 rpm - 1250 rpm - 1500 rpm + 1750 rpm - -0.-2000 rpm

i ! j o 2 b 0 2 ~ 0 3 b 0 3 ~ 0 4 b o 4 Concentration in gll

Fig. 8: Influence of stirring speed on the number of counts measured at different concentrations in water.

Non represenlalive 30000 points, air bubbles

20000 (I)

C 3 0

- - 0 + ; 10000

0 100 200 300 400 500

Concentration in gil

Fig. 9: Influence of stirring speed on the number of counts at different concentrations in toluene.

For a given stirring speed, there is a linear relationship between the number of counts and the concentration of beads in the medium. However, if the results obtained at different stirring speeds are compared, it should be noted that the total counts decrease as the stirring speed increases. This phenomenon does not occur when toluene is used as the suspension solvent (Figure 9). Taking the number of counts for different concentrations of solid, it is clear that the normal linear relationship between these parameters only occurs around 750 rpm throughout the range of concentrations studied.

loo 1

04- 100

. I , , -

200 300

Concentration in gil

Stirring speed

+ 500rprn A 750rprn I 1000 rprn

1250 rpm . 1500rpm

1 400

Fig. 10: Influence of stirring speed on the mean size at different concentrations.

8

7 ':h = 3

2

1

0

.I

It11

250 rpm - 2796 Counts

750 rprn - 27345

I I Counts

Fig. 11: Distributions obtained for various stirring speeds (suspension medium: water).

(Inadequate stirring : 750 rpm) Total counts = 3599

8 1 Mean size = 44.1 micrometers

I / L 0 , . . . . . . . . I . . . I...., I . . .v

1 10 100 1000 Particle size classes in micrometers

Fig. 12: Size distribution obtained in a non-homogeneous medium

Adequate stirring: 1500 rpm Total counts: 11366 Mean size: 75,7 micrometers

6 7 n 0

1 10 100 1000 Particle size classes

Fig. 13: Size distribution obtained in a homogeneous medium.

At lower stirring speeds, the suspension is not homogeneous enough, and at higher stirring speeds, the incorporation of air bubbles may account for the increase in the number of counts at high concentrations. With toluene, on the other hand, the tendency observed in tests with water is reversed. The higher the stirring speed, the more significant is the number of counts. Figure 10 shows that the mean size changes only slightly with concentration but increases with stirring speed, which is an artefact. In none of the cases studied does it represent the actual particle size. The size distributions obtained for different stirring speeds are given in Figures 11-13, from which the significance of the total counts on the statistical processing of the data can be demonstrated.


4.4 Influence of Temperature

The influence of temperature was studied by monitoring the evolution of the total counts and the mean size during the heating and cooling of a suspension. The suspension consisted of calibrated glass beads (125- 250 pm) in water at a fixed concentration of 250 g of beads per litre of water. A heating cap was fitted to the equipment used in other tests described above, giving rise to rapid heating slopes of about 64"C/h. Cooling by natural convection allowed the temperature to be reduced at rates of about 30"C/h. The stirring speed was fixed at 750 rpm throughout the tests. Under these conditions, temperature seems to have a notable influence on the total counts detected by the counter, as can be seen in Figure 14. The more the temperature increases, the more the number of counts detected by the probe decreases.

Total counts 15000 -

-k HEATING 6 4 W h $? COOLING 30"Clh

5000

40 60 80 20

Temperature in "C

Fig. 14: Influence of temperature on the total counts for a suspension of calibrated glass beads.

Since the temperature varies in a relatively narrow range (60"C), it is hardly likely that this variation in the equipment response is related to mechanical problems (expansion of the probe, etc.). On the other hand, temperature modifies the physicochemical properties of the suspension. According to the results given above, since the refractive index of a solvent decreases with increase in temperature, there should be an increase in the number of counts, but this does not occur. This confirms the fact that there is a parameter other than the refractive index of the liquid influencing the number of counts and which has not yet been identified. The mean size determined is always relatively independent of the parameter under study, but it always remains considerably below real values.

4.5 Influence of Focal Point Position

Lastly, among the parameters that may influence measurement with the Par Tec 100, and in view of its proposed use for continuous particle size analyses, it seems advantageous to evaluate the influence of focal point position on the data given by the probe.

Table 4: Real mean size of solids used.

Product Mean actual size (pm)

Orgasol: Batch 1601 Orgasol: Batch 123 Orgasol: Batch 121 Latex: Coulter

4.1 10.1 40.5

260

Orgasol Batch 123 - Peak ..____ Total counts

250007 r 3500

0 0.5 1 1.5 2 2.5 3 . 3.5 4 Displacement of the focal point in mm

3000

2500

2000 2

1500

1000

5 00

0

Fig. 15: Observation of the influence of focal point displacement on the total counts. Observation of the sapphire radiation.

Orgasol 40,5 urn - ~ - - - - Mean size 1 20007 __ Peak r 500

J

0 I , I I I , I ~ I l . , ~ . l l . l . ~ I I I 0 0 0.3333 0.6667 1 1.333 1.667 2

Distance from the window in mm

Fig. 16: Display of the optimum position for the focal point. Influence of this position on particle size determination for 40.5 pm particles.

- 800 - 700 -600 5 -500 2 -400 $. -300 3'

Mean size __ Peak

-200 5 -. -100

# O 1.7

Fig. 17: Display of the optimum position for the focal point. Influence of this position on particle size determination for 10.1 pm particles.

12000

10000

8000 Y m 2 6000

4000

2000

0

Orgasol

. . - - . 4.1 urn - IO.lurn - 40.5 u m

3

0 0.5 1 1.5 2 2.5 Distance from the window in mm

Fig. 18: Display of the optimum position of the focal point for different orgasol samples.


100 -

80 -

9 60- B 2 40- @

m v1

Latex 260 urn - Peak r 3500 Size 250 1 __~...

._ 5 cn 200

50; 500

Distance from the window in rnrn

Fig. 19: Display of the optimum position of the focal point for large particles. Influence of this position on particle size determination.

The equipment used for the study was reduced to the bare minimum - a beaker and a magnetic stirrer. The particles studied in suspension in heptane consisted of different batches of orgasol and latex, the mean sizes of which are given in Table 4. Stirring remained constant throughout the test, the cycle time being 1 s, and the mean of 10 cycles being taken. The focal point was displaced with a micrometer screw, a complete turn corresponding to a displacement of 0.9 mm. It is initially placed inside the particle size probe and is progressively moved towards the suspension. An increase in the peak, measurement of the intensity of reflected light corresponding to the sapphire radiation, can be observed (Figure 15). The maximum value of the peak gives the optimum position of the focal point. The different batches studied show that the mean size of the samples determined depends largely on the position of the focal point (Figures 16-19). For small orgasol-type particles, a comparison of Figures 15, 17 and 18 shows that the optimum focal point position is near the sapphire window. Figure 19, on the other hand, shows that for large particles, the optimum focal point is situated further into the solution at a distance between 1.5 and 2 mm from the window. The mean size measured by positioning the focal point at the two limits of this interval varies between 142 and 134 pm respectively for the spherical Latex-type particles used for calibrating the Coulter counter, the real mean size of which is 260 pm It will therefore be difficult, if not impossible, to keep to the optimum measurement conditions throughout crystallization. This type of measurement-size dependence is also found with other particle size determination systems. A counter such as the laser diffraction counter requires the selection of a range of measurements as a function of the size of the particles being analysed. A ZM Coulter counter obliges the user to select the detection aperture as a function of the particle size to be analysed.

Table 5: Characteristics of the solids studied

Solids

5 Comparison of Particle Size Results with Data Obtained by Other Methods on Monodispersed Samples

To test the response of the counter, we used different types of spherical particles of predetermined particle size and a micronized sample of pharmaceutical product (Table 5). The measurements were made in a 400-ml beaker using a magnetic stirrer. The cycle time for determination of the mean sizes was taken as 3.2 s and the data were expressed in counts determined by particle size class. For orgasol and the micronized pharmaceutical product, a dispersing agent was used to facilitate measurement. Its influence is shown in Figure 20, in the case of the micronized pharmaceutical product. It also appears very clearly in the course of particle size determination. For data acquisition, the probe is dipped into the mixture to be analysed and the optimum position of the focal point is determined.

- Micronized pharmaceutical product with additive (Nonarox) A Microntied pharmaceutical product without additive

1 10 100 1000 Particle size classes in pm

Fig. 20: Display of the influence of the dispersing agent on particle size determination with the Par Tec 100.

Measurement is conducted under these conditions and a number of counts and a mean size may be determined. Instead of producing a logical increase in the number of counts, the addition of the dispersing agent causes the measurement to fall abruptly. The focal point adapted to the aggregates no longer allows the particle size of the fine particles to be determined and has to be readjusted. This directly shows the action of the dispersing agent on the particles in suspension, namely, the destruction of the aggregates. The results obtained by products under these conditions are given in Figures 21-23. It appears that the Par Tec 100 tends to underestimate the size of large particles and overestimate the size of small particles. With its sensitivity, which is much lower than that of other

Mean sample size (pm) Determination of mean size (principle)

Coulter microspheres

Bayer Resin

Micronized pharmaceutical product

Orgasol

9.6- 19.1-41-66-87-260

396

4.7-4.8 with 85% of particles <lOmm

4.1-4.77-10.1- 20.13-40.5-49.32

Coulter (resistivity) Cilas (diffraction) Microscope

Microscope

Cilas (diffraction)

Cilas (diffraction) Coulter (resistivity)


jooo 1 Lasentec measurement

1 I a " " " ' I ' " " " ' I '""7 1 10 100 1000

Coulter in um

Fig. 21: Lasentec particle size determination: Latex Etalon Coulter.

t r

. .* . . Lasentec measurement

. ..' ;. _.

:: 0 I

a

4 1 1 10 100

Malvern in urn

Fig. 22: Lasentec particle size determination: orgasol beads.

E ;I 000

6 100 a,

in '; E 10 1 * Latex - Orgasol 6 Pharmaceutical product 1 Bayer Resin

.. . :- 0 -

oe

0

$ ? - I 1 3 1 10 100 1000

Particle size determination (Cilas)

Fig. 23: Comparison of different products tested

particle size determination methods (Cilas; Malvern; Coulter), and the very impressive number of parameters influencing its measurement, this is an instrument to be used with extreme care.

6 Conclusion

This preliminary study shows that particle size determinations by Par Tec 100 are influenced by the different variables studied.

Thus, the number of counts measured by the particle size probe depends considerably on the operating conditions and, more particularly, on the suspension conditions, solvent, temperature, and focal point position. Its dependence in relation to the concentration of a solid in suspension is normal and proves to be linear for a solid and a given size. The linearity relationship, on the other hand, depends on all the parameters mentioned above. To establish the relationship between the number of counts and the population density would, therefore, require delicate calibration on a case-by-case basis. The mean size determined does not depend on the suspension conditions, provided that the stirring speed is sufficient to give a total count that is statistically significant. On the other hand, for a given sample, a displacement of the focal point leads to considerable variations in the size determined. The optimum focal point generally corresponds to the minimum mean size. Furthermore, the position of the optimum focal point for small sizes is very sensitive. Lastly, this optimum focal point position depends largely on the real size of the particles. This means that the system is not suitable for the precise analysis of a dispersed sample since the focal point needs to be adjusted for each particle size range. Monitoring particle size quantitatively during a crystallization test would also require progressive adjustment of the focal point, which is hardly realistic in practical terms. Finally, sizes determined appear to be underestimated for large particles and overestimated for small particles.

7 References

[l] S. K. N. Heflels, E. J . De Jong: Improved operation and control of batch crystallizers. AIChE Symp. Ser. 87 (1991)

[2] B. Singh; Using a Crystal Size Distribution Sensor to Improve the Operation and Control of Batch Crystallizers. Anal. Proc.

[3] P. Marchal, R. David, J. P. Klein, J . Villerrnaux: Crystal- lization and precipitation engineering; An efficient method for solving population balance in crystallization with agglomera- tion. Chem. Eng. Sci. 43 (1988) 59-67.

[4] A. D. Randolph, M . A. Larson: Theory o f Particulate Processes. Academic Press, New York, London 1971.

[5] T. Allen: Particle Size Measurement, Fourth edition. Chapman and Hall, New York 1990.

[6] R. G. Sparks, C. L. Dobbs: The Use of Laser Backscatter Instrumentation for the On-line Measurement of the Particle Size Distribution of Emulsions. Part. Part. Syst. Charact. 10

170-181.

30 (1993) 495-496.

(1993) 279-289.

particle size determination by laser reflection: methodology and problems

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