# Investigation of the effect of particle size distribution on the flow properties of bulk solids

Abstract

This paper reports from a Defra sponsored project which aims to develop a toolkit that can be used by formulators to assist in controlling the flow properties of blended or engineered powders. This paper presents the results of an investigation into the effect of the breadth of the particle size distribution on the flow function, bulk density and internal friction of four food grade fillers namely; lactose, dextrose, maltodextrin and sodium chloride. Physical property measurements, including particle size and particle shape, are presented along with bulk flow properties which were measured using the Brookfield Powder Flow Tester (PFT). The flow properties of the four food powders are compared and discussed with reference to the effect of the breadth of the particle size distribution.

1 Introduction

The objective of this work is to investigate the effect of particle size distribution on the flow properties of four filler powders commonly used in the food manufacturing industry. Due to the variety of lactose, we used six different lactose grades to compare different particle size distributions – from wide to very narrow.

Dextrose was used because of its wide size

distribution and by sieving analysis, sections from particle size distribution were removed to be tested in the shear tester. Sodium chloride and maltodextrin have a narrower particle size distributions and the presence of very fine particles gave us the possibility to study the effect of particles below 45 m. Three samples were tested for each flow property measurement and the mean values of the flow functions, bulk densities and friction functions were plotted. These values showed a low standard deviation indicating that the data points tend to be very close to the mean, therefore, they are unappreciable in the graphs. 2 Methods

2.1 Physical properties

Particle size distribution for lactose was measured by laser diffraction using the Malvern Mastersizer with dry powder feeder unit.

Particle size distribution for dextrose, maltodextrin and sodium chloride was measured by sieving analysis using GRADEX 2000 Particle Size Analyzer.

Particle shape has been inferred through images of the four food powders which were obtained using scanning electron microscopy (SEM) and a digital metallurgical microscope L2003 AHTG.

a)

b)

c)

d)

Fig. 1 Comparison of the particle size distributions of the three materials measured by sieving analysis, a) mass percentage (%) for dextrose, maltodextrin and sodium chloride and cumulative percentage by % weight for b)dextrose, c)maltodextrin and d) sodium chloride

D10 (µm)

3

3

5

10

25

55

D50 (µm)

5

9

32

74

90

128

D90 (µm)

9

23

87

140

156

207

Table 1 Comparison of the particle size distributions of 6 different lactose grades, measured by laser diffraction using Malvern Mastersizer

a)

b)

c)

d)

Fig. 2 Comparison of the particle images of the four food powders, a) lactose (2000x), d50=2µm, b) dextrose (12x) full size distribution, c) maltodextrin (25x) full size distribution, d) sodium chloride (12x) full size distribution

2.2 Bulk flow properties measurement

Powder Flow Tester (PFT) from Brookfield Engineering [1] was used for measuring the flow properties of the four food powders – flow function, bulk density and internal friction.

The aim of a shear test is to measure the yield limit of a consolidated bulk solid. The bulk solid specimen is loaded vertically by a normal stress and then a shear deformation is applied on the specimen.

Fig. 3 Brookfield PFT

While the majority of available shear testers [2, 3] undertake flow function measurements at a single consolidation stress level per powder sample, the standard test algorithm for the Brookfield PFT undertakes failure property measurements over multiple increasing consolidation stress levels,...

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