sugars source in a wide range of food and fermentation applications, as mentioned by Tate & Lyle. [9]. 3.2 Pneumatic conveying tests. Pneumatic conveying ...
CHANGES IN PARTICLE SIZE AND SHAPE IN PNEUMATIC CONVEYING OF DEXTROSE C. Arakaki1, C. Ratnayake1 and G.G. Enstad1,2 1. Telemark Technological R & D Centre Porsgrunn, NORWAY 2. Telemark University College Porsgrunn, NORWAY Abstract – During pneumatic conveying of particulate materials, changes in the size and shape of the individual particles can be observed. These changes can have a great impact in the bulk powder and affect its flow properties in the pipelines and can be wanted or unwanted according to the whole process chain and final use of the powder. In this investigation, dextrose monohydrate particles were pneumatically conveyed repetitively and the size and shape of the particles were characterized by a semi-automatic image analysis method; qualitatively by observing micrographs and quantitatively by using statistical diameters and two shape factors. The mass flow rate of the pneumatic conveying tests was not affected by changes in size and shape of the dextrose particles. 1. INTRODUCTION The behavior and properties of particulate materials are, to a large extent, dependent on particle morphology (shape, texture, etc.) and size distribution, according to Allen [1]. In pneumatic conveying systems, particulate materials of different sizes and shapes are used everyday in industrial settings. Particle attrition is common during pneumatic conveying and usually considered a problem. Attrition can be affected by several parameters; Kalman [2] groups them in three categories, particle strength (material, size and shape), operation parameters (velocity, loading ratio), and bend structure (radius of curvature, material, type of bend, number of bends). Other researchers who have studied the possible parameters influencing attrition are Klinzing [3] and British Materials Handling Board [4]. Size and shape measurements can give an indication of the degree of degradation of the particles. However, particulate materials are rarely perfectly spherical; most of them are irregular in shape, which poses a challenge to determine size and its distribution since many different size measurements can be obtained for the same powder. According to Svarovsky [5], an irregular particle can be described by a number of sizes and there are three groups of definitions: the equivalent sphere diameters, the equivalent circle diameters and the statistical diameters. In the first group are the diameters of a sphere which would have the same property as the particle itself (e.g. the same volume, the same settling velocity, etc.); in the second group are the diameters of a circle that would have the same property as the projected outline of the particles (e.g. projected area or perimeter). The third group of sizes are obtained when a linear dimension is measured (usually by microscopy) parallel to a fixed direction. Particle shape is a fundamental powder property affecting powder packing and thus bulk density, porosity, permeability, cohesion, flowability, caking behavior, attrition and so on. This property can be defined by using qualitative terms or quantitatively mainly by using shape factors (numerical relationships between the various ‘sizes’ of a particle) or shape coefficients (relations between measured sizes and particle volume or surface area), according to Allen [1]. The reader is referred to Rhodes [6] and Allen [1] for different methods of particle size and shape analysis. It is the objective of this paper to show how dextrose monohydrate particles change in size and shape during repetitive pneumatic conveying. Characterization of size and shape of the particles is done qualitatively and quantitatively by image analysis, which is a widely used technique by which each individual particle can be observed and measured.
2. SIZE AND SHAPE MEASUREMENTS Dextrose particles are commonly described in their shape as elongated platelets. The crystals of dextrose are anisotropic, meaning that these particles do not have the same size in all directions, since they are not spherical. In order to obtain a better size characterization of the particle, especially when particles have an elongated shape, more than one number is used. As Pabst et al. [7] suggested, a useful choice is the minimum and maximum Feret diameters. The software program used for this study, Axio Vision 3.0, describes the measurements of Feret diameters as follows: the diameters are determined on the basis of distance measurements. Two parallel straight lines are positioned on opposite sides of the object, like a sliding caliper, at 32 angle positions. The distances are then measured for each angle position. The maximum and minimum value determined are the Feret maximum and Feret minimum diameters (in µm), respectively, as described by Zeiss [8]. For the characterization of shape, sphericity is a commonly used shape factor, however, dextrose crystals are platelets therefore, a two dimensional shape factor is appropriate. The common definition of circularity can be found in literature, Allen [1], Rhodes [6], Svarovsky [5]. Zeiss [8] names this factor Form circle, shown in Eq. (1), with the following description: the measurement parameter describes the form of a region on the basis of its circularity. A perfect circle is given the value 1. The more elongated the region is, the smaller the form factor. Form circle = 4Π
AreaF ……………………………………………………….(1) PerimCroft 2
where: AreaF is the area of a region including any holes it contains PerimCroft is the perimeter of a circular region In addition, a relationship between the Feret diameters was obtained from the software measurements and it is defined as shown in Eq. (2). If the Feret ratio has a low value, the particle is elongated. Values closer to 1 indicate that particles are approaching a circular shape. Feret ratio =
Feret min ………………………………………………………………...(2) Feret max
3. EXPERIMENTAL WORK 3.1 Particulate material Dextrose monohydrate provided by Syral, Belgium was used for the experiments. Crystalline dextrose monohydrate contains 91% dry substance, 99% dextrose content and 0.5% other sugars in the dry substance. It can be used for many applications, such as a sweetening agent or fermentable sugars source in a wide range of food and fermentation applications, as mentioned by Tate & Lyle [9]. 3.2 Pneumatic conveying tests Pneumatic conveying tests were carried out in a rig built at the Dept. of Powder Science and Technology (‘POSTEC’) of the Telemark Technological Research & Development Centre (‘Teltek’). The system comprises of an air compressor, an air dryer, a blow tank, a conveying pipeline, a receiving tank and a filter. Pressure transducers are mounted on the conveying line, on the blow tank and on the air supply lines. There are two compressed air supply lines in the rig. Two air flow meters are installed on these lines. One is supplying air to the blow tank and the other is connected to the by-pass airline. The receiving tank is mounted on load cells in order to measure the mass
accumulation on-line. There is a sampler on the conveying line to take powder samples during the test runs. A sketch of the pneumatic conveying line is shown in Fig. 1; where the pressure transducers are shown as PT. The total length of the pipeline is approximately 26 m including a total length of vertical section of approximately 13 m. There are three 90 degree bends in the rig. The pipeline has an internal diameter of approximately 2 inches (58 mm). The blow tank of the rig has a capacity of 0.4 m3 and can withstand a maximum pressure of 8 bar.
Figure 1: Scheme of the pneumatic conveying rig
Repeated pneumatic conveying tests were done at constant blow tank pressure and air flow values (1.5 bar and 560 Nm3/h). Samples of powder were taken during the tests for the size and shape characterization.
3.3 Particle characteristics study The samples taken on-line from the pneumatic conveying tests were characterized by particle size/shape analysis. A semi-automatic image analysis system comprising of the Carl Zeiss Axioskop (serie nr. 002-10102) microscope coupled with the Axio Cam MRC, were used to take micrographs of the dextrose crystals. Afterwards, the crystals were characterized by using the interactive software, Axio Vision 3.0. The software produced the measurements of particle size and shape required for each individual dextrose crystal. 4. RESULTS AND DISCUSSION Analog to particle size when characterizing powders, when shape factors were measured, the particles also showed a distribution. Therefore, number-weighted distributions were constructed for the Feret ratio and the Form Circle factors using measurements from 2015 particles (Fig. 2 & 3). Fig. 2 & 3 show the shape factors measured after pneumatic conveying the powder once, 5 times, and so on. From Fig. 2, it is not possible to observe any clear difference among the distributions for 1 to 15 runs, meaning that the circularity of the particles is not greatly affected after pneumatically conveying the powder 15 times. At run number 20 there is a clear shift of the curve to the right, indicating the presence of more crystals with higher Form circle values, which would mean that the particles have become more rounded. It is expected that this happens after several test runs, since the particles are colliding with the walls of the pipeline and the edges of the crystals can be broken.
600 1 run 5 runs 10 runs 15 runs 20 runs 25 runs 30 runs 35 runs
500
Number of crystals
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0 0
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1
Form circle
Figure 2: Form circle distribution 700
600
1 run 5 runs 10 runs 15 runs 20 runs 25 runs 30 runs 35 runs
Number of crystals
500
400
300
200
100
0 0
0.1
0.2
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0.7
Feret ratio
Figure 3: Feret ratio distribution
1
As the British Materials Handling Board [4] states, irregular and angular particles are prone to having their corners knocked off in collision and thus become rounder and smaller with time. In Fig. 3, we can observe no distinct difference in the Feret ratio distributions until the test run number 25 when it is possible to see some differences, and shifting of the curves to the right side, meaning that the Feret ratio values are increasing, which indicates that the particles are more compact, less elongated (the Feret maximum diameter of the particles is then decreasing as the number of test runs increase in relation to the Feret minimum diameter). There is no clear difference between the Feret ratio values of the particles after pneumatic conveying for 30 times and 35 times; however, there is a clear difference in Form circle, so the particles are becoming more rounded but the Feret maximum diameter is not decreasing highly in relation with the Feret minimum
diameter after 35 test runs. To give an indication about the shape factor distribution we chose the median of the distributions and compared them to the number of test runs. In Fig. 4 it is clear that only after 15 test runs the circularity mean increases. In the case of the Feret ratio, it increases after run 15 and only slightly. There is a considerable decrease in Feret maximum diameter median values, from approximately 100 µm after the first test run to approximately 80 µm after pneumatic conveying the powder for 30 times in the test rig (Fig. 5). Feret minimum diameter median values decreased much less than the Feret maximum mean values, from approximately 66 µm to 56 µm when comparing the results after the same number of tests. Exact values can be read in Table 1. Therefore, there is a decrease in size of the particles with the test runs, and it is more pronounced in their ‘height’ than their ‘width’, so the particles tend to break first in their longest diameter axis.
120
0.7 0.65
M e d ia n X 5 0 [ m ic r o n s ]
100
M ed ian X 5 0
0.6 0.55 0.5 0.45 0.4 0.35
80 60 40 20 0
0.3 1
5
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Form circle
Figure 4: Feret ratio and Form circle median values vs. number of test runs.
15
25 Feret min
30
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Number of test runs
Figure 5: Feret diameters median values vs. number of test runs.
As other researchers have mentioned, British Materials Handling Board [4] & Konami et al. [10], it is expected that attrition rate decreases with time, tending to a constant value, however, this was not observed in the case of dextrose monohydrate, neither in the size nor in the shape of the particles. There is a clear linear relationship between shape and size of the particles. The Feret maximum diameter values were chosen as an indication of the size and compared with the Form Circle as an indication of particle circularity in Fig. 6. Particles which are more circular, have a lower Feret maximum diameter, so they are more compact, less elongated. There is a dependence of size and shape, therefore. 0.6 0.55 Form Circle Median X 50
0.5
y = -0.0051x + 0.9003 R2 = 0.769
0.45 0.4 0.35 0.3 0.25 0.2 70
75
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105
Feret max Median X50 [microns]
Figure 6: Feret max vs. Form circle
The micrographs shown from Fig. 7 to 10 corroborate the quantitative results presented. There is no visible difference between the particles after 1 test run and after 10. However, after 20 test runs it is possible to observe what seems like fragments broken from bigger particles. Furthermore, it is possible to see still many elongated particles, showing that the ratio between the minimum and maximum Feret diameters has not changed significantly. Most of the particles do not have sharp edges anymore and have rounded borders, therefore the increase in the Form circle values. After 30 tests (see Fig. 10), the particles have broken considerably, and they are much smaller in size. Most particles are circular after 30 test runs; therefore, the Feret ratio has increased as a consequence of a more pronounced decrease of the Feret maximum diameter in comparison with the decrease in Feret minimum diameter.
Figure 7: Dextrose crystals after 1 test run.
Figure 8: Dextrose crystals after 10 test runs.
From Fig. 9 & 10 it is possible to appreciate the relationship between size and shape. It is clear that the smaller particles are more rounded, and the bigger particles preserve some of their sharp edges. It is possible to see from the micrographs (Fig. 7 to 10) that particles can agglomerate, overlap and stick together, as a consequence, since the method used for analysis is a semi-automatic image analysis technique, there can be some level of operational error when interacting with the software to identify individual crystals.
Figure 9: Dextrose crystals after 20 test runs.
Figure 10: Dextrose crystals after 30 test runs.
It is worth mentioning that it seems possible to run tests up to approximately 15 times, as a rough estimation, without changing considerably the powder size and shape parameters studied. In Table 1, the different particle size and shape median values are presented together with the average velocity of air of the pneumatic conveying tests (at the point where the first pressure transmitter of the line is placed), and the mass flow. From the values, it is clear that the velocity of air was approximately the same for every test (blow tank pressure and air flow volumes were held constant). At these conditions, the mass flow of dextrose particles was not affected by the changes in the parameters of shape and size studied, in a considerable way. The variation of mass flow values is not significant considering the total mass in the pneumatic conveying system (approximately 200 kg) and the size of the rig.
Table 1: Pneumatic conveying tests parameters and particle characteristics #Runs
Mass flow
Velocity
Form Circle
Feret Ratio
Feret Max
Feret Min
1 5 10 15 20 25 30 35
kg/s 0.583994 0.685046 0.673235 0.706667 0.673999 0.676473 0.655564 0.645733
m/s 31.62885 30.2232 30.42999 30.46587 30.03873 29.72328 29.71155 29.02371
X50 0.4 0.41 0.39 0.41 0.43 0.46 0.48 0.51
X50 0.64 0.65 0.65 0.65 0.66 0.67 0.68 0.67
X50 101.2 98.6 94.9 90.6 92.5 88 80.3 82.3
X50 65.5 64.2 62.5 60.2 61.1 59.2 55.5 56.5
5. CONCLUSION The size and shape changes of dextrose monohydrate particles in a pneumatic conveying system were quantitatively (by image analysis) and qualitatively (by micrographs) described. The shape factors used, Feret ratio and Form circle exhibited a distribution, analog to particle size. Form circle did not change dramatically after the initial tests, however, after a number of test runs the values of this factor increased and the differences in the distributions were indicating very clearly more rounded particles as a consequence of broken edges. Feret ratio values were less susceptible to the increase in the number of runs, showing less pronounced differences than the Form circle values. However, they showed a tendency of increasing, meaning that the particles became less elongated with the number of pneumatic conveying runs. Both the Feret diameters decreased with the number of test runs, indicating a decrease in particle size, however, the decrease in the Feret maximum diameter was much more pronounced than in the Feret minimum diameter; this means that the particles break on the extremes of their longer axis diameter. A linear relationship was observed between the Feret maximum diameter and the Form circle, indicating that as the particles which were smaller in size, were more rounded. This can be observed in the micrographs too. It is possible to pneumatically convey the particles of dextrose for 15 times in the experimental rig used, without major changes in particle size and/or shape. The mass flow measured in this system was not affected considerably by changes in size and shape.
6. ACKNOWLEDGEMENT The authors wish to acknowledge the support of the POSTEC research group and the EU “Biopowders” Marie Curie Research Training Network. Syral, Belgium is also gratefully acknowledged for providing the powder and analytical instruments used in the experimental work.
7. REFERENCES 1. 2. 3.
T. Allen, Powder sampling and particle size determination. First edition ed. 2003, Amsterdam: Elsevier B.V. 660. H. Kalman, Attrition control by pneumatic conveying. Powder Technology, 1999. 104(3): p. 214-220. G.E. Klinzing, Marcus, R.D., Rizk, F., Leung, L.S., Pneumatic conveying of solids. Second edition ed. Powder Technology Series. 1997, London: Chapman & Hall. 599.
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British Materials Handling Board, Particle Attrition, state-of-the-art-review. Vol. 5. 1987, Clausthal-Zellerfeld: Trans Tech. Publications. L. Svarovsky, Powder testing guide. Methods of measuring the physical properties of bulk powders, ed. B.M.H. Board. 1987, Essex: Elsevier applied science. 146. M. Rhodes, Principles of Powder Technology. 1990, Chichester: John Wiley & Sons Ltd. 439. W. Pabst, C. Berthold, and E. Gregorova, Size and shape characterization of oblate and prolate particles. Journal of the European Ceramic Society, 2007. 27(2-3): p. 1759-1762. C. Zeiss, Axio Vision 3.0 software manual. Tate & Lyle. 2007 [cited; Available from: http://www.tateandlyle.com/. M. Konami, S. Tanaka, and K. Matsumoto, Attrition of granules during repeated pneumatic transport. Powder Technology, 2002. 125(1): p. 82-88.
