- Email: [email protected]
Attrition of granules during repeated pneumatic transport
Powder Technology 125 (2002) 82 – 88 www.elsevier.com/locate/powtec
Attrition of granules during repeated pneumatic transport Moriyoshi Konami a,*, Shigeho Tanaka a, Kanji Matsumoto b a
Department of Research and Development, Nisso Engineering Co., Ltd., 1-6-1, Kanda Jinbo-cho, Chiyoda, Tokyo 101-0051, Japan Department of Materials Science and Chemical Engineering, Yokohama National University, 79-5, Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
b
Received 16 July 2001; received in revised form 3 October 2001; accepted 20 December 2001
Abstract We investigated the changes of granules in the attrition process during repeated pneumatic transport, along with the factors that affected these changes. Experiments were conducted with a shot-feed type dense phase pneumatic transporter, a device that allows a low rate of attrition by transporting granule lumps one by one. Easily attrited granulated materials were used. The analysis revealed an equation describing the relationship between the fine particle increment rate (named F) and the mass median diameter of coarse granules. The following results were obtained: (1) the degree of attrition was reduced with the increase in the fine particle ratio (mass fraction) before transport; and (2) the F value increased proportionally to the granule diameter. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Pneumatic transport; Shot feed; Fine particle increment rate; Granule attrition; Repetition; Shape factor; Particle size
1. Introduction The attrition of particles or granulated materials during pneumatic transport is an issue of considerable industrial concern. A good number of dense phase pneumatic transporters have been designed specifically to reduce the particle attrition [1– 5]. There has also been increasing research into the quantification of the attrition phenomenon [6– 8]. For example, factors affecting attrition at bends have been well investigated. It has been shown that the vibration of bends affects the attrition of granules [9], and that long bends are not always useful to reduce such attrition [10]. Moreover, the transport velocity and loading ratio have also been shown to influence the attrition [11]. The strength, size and shape of granules are generally thought to affect the attrition of granules during pneumatic transport. More specifically, the transport air or solid velocity, transport length, fine particle ratio (mass fraction), and loading ratio must all be taken into account as influencing factors. In the case of actual transporters, we must also consider attrition by feeders, a phenomenon that occurs in blow tanks [12] or due to friction in screw feeders [9]. In pneumatic transport with shot-type feeding, single lumps of a solid are formed at a controlled rate and fed into pipelines at precise intervals by the release of com-
*
Corresponding author. Tel.: +81-3-3296-9310; fax: +81-3-3296-9250. E-mail address: [email protected] (M. Konami).
pressed air [13]. In conventional blow tank operation, the pressure drops a great deal between the starting point and ending point of the pipeline. This means that the velocity of the solids varies along the length of the pipeline in this method. Even if a solid starts out at a velocity at the lower limit, it accelerates to a high velocity by the time it reaches the end. In the shot feed method, the drop in pressure is low and constant, hence plugs or slugs of bulk solid travel stably through the entire pipeline. As a consequence, attrition during transportation is lower with the shot feed method than with the conventional blow tank method. While the air velocity, pressure drop, and loading ratio are important factors for pneumatic transport, it is difficult to express the loading ratio for the shot feed method, as the lumps are transported one by one. Repetitive transport tests, tests in which the same powder samples are used repeatedly, are common in industrial experimentation as the samples are usually available in limited amounts. In some cases, these tests are adopted to simulate long pipeline tests with the use of short pipelines in laboratories. Moreover, in actual factories, the granules produced are transported repeatedly through processes for receiving, storage, re-processing, and packaging. In all of these cases, the granule attrition and other changes that result from the repetitive transport should be taken into account. This is essential for the rational evaluation of experiments, and for the design of processes and factories as well. This paper presents findings on the attrition of granules during repeated transport by the shot feed method.
0032-5910/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 ( 0 2 ) 0 0 0 0 2 - 5
M. Konami et al. / Powder Technology 125 (2002) 82–88
83
2. Experimentation Fig. 1 illustrates the system configuration of the transport device used here. The principal components of this equipment were (1) the feed tank; (2) the pipeline and valve V-2, used to feed compressed air into the tank; (3) valve V-6, used to fill the granules into the tank; (4) valves V-1, V-3, and V-4, used to feed the granule lumps into the pipeline; and (5) an orifice ring to prevent the excessive feeding of granules. The feed tank had a volume of 0.0804 m3, and the inner diameter of the orifice ring was 13 mm. A bag filter with a filtration area of 3 m2 was fitted on the receiver to reduce the attrition in the receiver to a negligible level. As shown in Fig. 2, the design of the transport line was modeled after a typical industrial pipeline. The pipeline had a total length of 30 m, including a vertical section spanning a length of 6.3 m, eight bends with a radius of curvature of 200 mm and sections of straight horizontal pipe between the bends. The pipeline was made of stainless steel and had an inner diameter of 17.5 mm. Tests were carried out by repetition of the following operations. After raising the pressure in the feed tank to a specified level, valve V-4 was opened for a specified period to feed the sample to the pipeline via the pressure difference between the feed tank and pipeline. Valve V-4 was then closed, and valve V-5 was opened for a specified period to allow transportation of the granule lumps through the pipeline. The feed time per granule was defined as period during which valve V-4 was left open, and the lump feed interval was defined as
Fig. 2. Dimension of main pipeline for transport.
the period during which valve V-5 was left open. Through the repetition of the above operations, single lumps were continuously fed and transported one by one. The basic setting conditions for the testing were determined based on previous results for shot-feed pneumatic transport [13]. Both the feed tank pressure and pressure for pneumatic transport were set at 50 kPa and regulated by control valve 11) shown in Fig. 1. This pressure level corresponds to between two and three times the transport pressure drop for a single lump of granules, allowing stable transportation at the minimum possible pressure. The superficial velocity of transport air was set at 7.4 ms 1. The lump feed time was set at 0.3 s, a level at which choking is unlikely to occur even when other conditions vary. The lump feed interval was set at 17 s by adding an additional margin to the time required for complete transportation of a single lump. The granules used in this testing were prepared from a fine tungsten – carbide powder at a tablet-forming pressure of 200 MPa by a roller compactor with a clearance of 1 mm between rollers. Once formed, the tablets were disintegrated into a double roll crusher, and fine particles were removed by a 150-Am sieve. The properties of this material are presented in Table 1. Here, the value for granule hardness indicates the average of the maximum stress levels when one granule is ruptured by a 1-mm-diameter rod descending at a rate of 0.4 mm per minute.
Table 1 Properties of granules Granule diameter Original particle diameter Particle density Bulk density
Fig. 1. Schematic diagram of dense phase pneumatic transporter with shot feed.
Angle of repose Carr’s Flowability index Granule hardness
150 to 850 Am 0.8 Am (average) 10,700 kg m 3 4050 kg m 3 (average) (loose 3800, packed 4300) 43j 71.5 points 0.52 N
84
M. Konami et al. / Powder Technology 125 (2002) 82–88
3. Experimental results and discussion 3.1. Definition of fine particle increment rate, F The fine particle increment rate F is defined as the rate of increase of the fine particle ratio (mass fraction) between before and after transport, according to the following equation: F ¼ ðXa Xb Þ=ð1 Xb Þ,
ð1Þ
where, Xa is the fine particle ratio after transport and Xb is the fine particle ratio before transport. The fine particle ratio is defined as the ratio of the mass passed through the 150Am sieve to the overall mass. A circular vibrating sieve was used to measure Xa and Xb. The machine had a 440-mm-diameter sieve, a 0.4-kW power rating, and a sieving time of 10 s. Some degree of granule attrition did occur during the sieving [14], but this was minimized to a negligible level by the short sieving time. Fig. 4. Effect of fine particle ratio on F value.
3.2. Effect of repetitive transport on F Fig. 3 shows the variation of F in repeated transport of identical samples. The total mass of granules for the first run was 5 kg, and this decreased gradually due to the small loss incurred during each transport. The original sample of granules was used in the first run and reused in each subsequent run without removing the fine particles. The results shows that, in general, F decreased as the transportation was repeated. Choking occurred in the sixth repetition, when the fine particle ratio before the transport was 0.436. Furthermore, the F value during the fifth repetition, the repetition before chocking, was increased. This increase was mainly attributed to the high stress between granules in the bends reported in the previous paper [12].
3.3. Effect of the fine particle ratio on F The fine particle ratio, which increased during repeated transport, may have been one of the factors leading to reduction of the F value during the repetition. For this reason, test samples were prepared by adding a fixed mass of fine particles to the original granules after sieving to confirm the effect of only the fine particle ratio. Afterwards, each sample was transported through the test pipeline, and the corresponding F value was measured. Fig. 4 shows the results of this experiment, with the data from the repetitive transport in Section 3.2 superimposed over the figure. The transport test was performed under the basic setting conditions described in Section 2. The results show that F was reduced as the fine particle ratio increased in both cases. Presumably, this was because the particles enclosed the coarse granules and thereby acted as a cushioning medium that reduced the granule stress and levels of attrition. This phenomenon is similar to the cushioning effect observed in crushing processes. The result in Fig. 4 illustrates the difference between the first transport of the original samples and repeated transports. Choking also occurred when the fine particle ratio was set at 0.4 for the first transport, just as observed in the repeated transport, and F rose around the time of the choking. This experiment confirmed that the fine particle ratio has an important influence on the attrition. 3.4. F for repetitive transport without fine particles
Fig. 3. Effect of transport repetition on F value.
To find the effect of fine particles, we removed the fine particles before each individual transport to make Xb = 0, and then examined the relationship between the F value and
M. Konami et al. / Powder Technology 125 (2002) 82–88
85
transport, and (c) corresponds to samples after transport with the fine particles removed. The granules before transportation were irregular and angular in shape as a result of the compression granulation and disintegration. Due to the attrition, we can see that the specific surface areas of these granules became gradually smaller—that is to say, the granules in question became more spherical. To investigate these effects, the shape factors were calculated. The Wadell circularity /c as defined in Eq. (2) was adopted as a shape factor. In order to determine this circularity index, photomicrographs of approximately 50 granules over 150 Am were examined from each sample by commercially available software, thereby allowing calculation of /c as /c ¼ pDH =C,
Fig. 5. Change in F value and circularity with transport repetition.
number of repetitive transports. Fig. 5 presents the result. The initial test samples were prepared by adjusting Xb for original granules to 0. The total mass of granules for the first run was 10.0 kg and then was gradually reduced to 2.6 kg as particles were removed. All other conditions were in accordance with the basic setting conditions. Even under these conditions, F was gradually reduced as the number of transports increased. This gradual reduction was believed to result from the differences in the strength, shape, and size of the granules as the fine particles were removed. 3.5. Change of the shape factor during repetitive transport Fig. 6(a), (b), and (c) presents variations in the shape of granules that resulted from the transportation. Specifically, (a) corresponds to the adjusted original granules with the fine particles removed, (b) corresponds to samples after
ð2Þ
where DH is the projected area diameter (i.e. Heywood diameter, {4A/p}0.5), C is the perimeter of a projected particle, and A is the projected area of a particle. Fig. 5 shows the variation of /c with the number transports. The increment in /c was the most significant after the first transport. After the second and third repetitions the increment became smaller, and by the fourth repetition there was no change at all. At the first transport, when /c was small, F was particularly high. One reason for this may have been the higher susceptibility of granules with irregular surfaces to attrition. In contrast to spheres, granules with large numbers of protuberances or other irregularities on their surfaces more readily succumb to attrition. Another reason for the change in F could have been the process used to manufacture the granules, which seemed to have left a fragile layer in the vicinity of the granule surface. The influence of this layer could not have been substantial, as it would have been removed after the fourth repetition, when /c remained roughly constant, leaving the granule with an almost completely homogenous makeup. As a consequence, we
Fig. 6. Outlook of the granules ( F = 0.393). (a) Adjusted original granules for which fine particles have been removed (Xb = 0); (b) after-transport condition; (c) samples of (b) for which fine particles have been removed.
86
M. Konami et al. / Powder Technology 125 (2002) 82–88
assume that the strength of the granule was stabilized from the fourth repetition onward. 3.6. Effect of granule size on F Upon examination of the effect of granule size on the granule attrition, the relative granule diameter S was introduced as the ratio of the mass median diameter of coarse granules before transport to that of coarse original granules. The term ‘‘coarse granules’’ refers to granules of over 150 Am in diameter. The relationship between S and F was investigated. The variation in S by transport repetition i can be expressed by Eq. (3). See Appendix A for the derivation of this equation. Si =Si1 ¼ ke ð1 Fi Þ1=3
ð3Þ
Although ke is dependent on the difference between the numbers of coarse granules before and after transport, these numbers can be assumed to be constant. Attrition is manifested principally as the chipping of granules, and thus is assumed to leave the initial number of coarse granules unchanged. Only two forces work against this principle. Firstly, a very coarse granule can be crushed into two or more coarse granules. Secondly, a coarse granule can reduced to the size of a fine granule ( < 150-Am diameter) through the process of attrition. In the first case the number of coarse granules increases, and in the second case it decreases. Yet, these phenomena cancel each other out, and under the conditions of the present experiment, the low level of attrition renders them scarcely operative. It can therefore be assumed that the change in the number of coarse granules is negligible. In this case, ke becomes 1 from Eq. (a-9). Fig. 7 plots the values of S obtained in the repeated transport described in Section 3.4. These values were
Fig. 8. Effect of granule diameter on F value.
calculated from the mass median diameters of coarse granules. The values of S, which were calculated from the experimentally obtained F values by Eq. (3) under the assumption that ke is 1, are also given in this figure. This figure shows that ke can be taken as approximately 1. After the fourth repetition, when /c became constant, the only factor that appeared to affect F was the diameter of granules, and thus this relationship was analyzed further. Fig. 8 shows the relationship between F and S, calculated from the data after the fourth repetition under the assumption that ke = 1. Based on this figure, the effect of S on F can be taken as F ¼ 0:1S:
ð4Þ
In the range of test conditions under which shapes were stable, this equation confirms that the amount of attrition is proportional to the granule diameter, as previously reported [6].
4. Conclusions We investigated the attrition of granulated material during repeated pneumatic transport by measuring the increment rate of fine particles. In our analysis, we presented an equation demonstrating the relationship between the fine particle increment rate and the diameter of coarse granules. The following results were obtained: 1.
Fig. 7. Change in granule diameter with transport repetition.
The granule attrition during repeated pneumatic transport was high at first, but the rate of attrition gradually declined and the shape factor of the granules, became constant.
M. Konami et al. / Powder Technology 125 (2002) 82–88
2.
The rate of granule attrition declined as the ratio of fine particles within the granules increased up to a certain value. However, an excess of fine particles led to choking. 3. The fine particle increment rate increased in proportion to the granule diameter.
Acknowledgements The authors thank Mr. A. Iwai of Kimura Chemical Plant and Mr. N. Uchida, a registered consultant engineer, for their useful advice.
and D3i ¼ ð1=ni Þ
ni X
dj3,i :
Next, we introduce the relative granule diameter Si defined by Di/D0, a value based on the mass mean diameter. Thus, we obtain ðSi =Si1 Þ3 ¼ ðni1 =ni Þð1 FÞ:
ðp=6Þqdj3,i1 ¼ W0 ð1 Xb Þ,
ða-1Þ
j¼1
where W0 is the total mass of all granules. In the same way, after i-th transport, we have ni X
ðp=6Þqdj3,i ¼ W0 ð1 Xa Þ:
ða-2Þ
j¼1
The following equation is derived by dividing Eq. (a-2) by Eq. (a-1). ni X
dj3,i ð j ¼ 1 to ni Þ
j¼1 ni1 X
dj3,i1
¼ ð j ¼ 1 to ni1 Þ
ð1 Xa Þ ð1 Xb Þ
ða-3Þ
j¼1
By the definition of F, the right-hand side of Eq. (a-3) becomes ð1 Xa Þ=ð1 Xb Þ ¼ 1 ðXa Xb Þ=ð1 Xb Þ ¼ 1 F: ða-4Þ If we introduced the mass mean diameter for coarse granules before and after transport (Di 1 and Di), we obtain D3i1 ¼ ð1=ni1 Þ
ða-7Þ
And then, ða-8Þ
where
For a group of granules containing coarse granules over a specified diameter (150 Am in this paper), let ni 1 be the number of coarse granules and let dj,i 1 ( j = 1 to ni 1) be the volume equivalent diameter of coarse granules before i transport repetitions. Assuming the density of all granules is q, the total mass of coarse granules before i transport repetition is ni1 X
ða-6Þ
j¼1
ðSi =Si1 Þ ¼ ke ð1 FÞ1=3 , Appendix A
87
ni1 X j¼1
dj3,i1
ða-5Þ
ke ¼ ðni1 =ni Þ1=3 :
ða-9Þ
This concludes the derivation of Eq. (3). Although Xa, Xb, F and ke should be affixed with the suffix i, this suffix was omitted for the sake of simplicity. Nomenclature A projected area of a particle (m2) C perimeter of projected particle figure (m) DH projected area diameter (Heywood diameter) (m) Di mass mean diameter from samples after a repeated number (i) of transports (m) dj,i volume equivalent diameter of coarse granule j of over 150 Am from samples after transport repetition i (m) F, Fi fine particle increment rate under 150 Am from samples after transport repetition i, with attrition as defined by Eq. (1) ( – ) i number of transport repetitions (– ) j number representing each of the coarse granules ( –) ke coefficient defined by Eq. (a-9) (– ) ni total number of coarse granules from samples after transport repetition i ( – ) S, Si relative granule diameter based on mass median diameters; ratio of diameters of coarse granules over 150 Am in samples before transport repetition i vs. original (– ) W0 total mass of granules (kg) Xa fine particle (150 Am under) ratio (mass fraction) after transport (– ) Xb fine particle (150 Am under) ratio (mass fraction) before transport ( –)
Greek letters /c circularity defined in Eq. (4) by Wadell ( –) q density of granules (kg m 3)
88
M. Konami et al. / Powder Technology 125 (2002) 82–88
References [1] M. Konami, Kagaku Souchi 36 (1994) 92 – 101. [2] M. Moriyama, K. Morimoto, A. Iwamoto, S. Onoe, K. Korekata, Kagaku Kogaku 52 (1988) 421 – 425. [3] K. Murata, Y. Joka, Kagaku Souchi 33 (1991) 49 – 55. [4] R. Aoki, H. Murata, H. Homma, H. Wada, Kagaku Kogaku Ronbunshu 6 (1980) 628 – 634. [5] H. Ueda, M. Takei, Y. Zhao, H. Li, M. Ochi, Y. Tomita, K. Horii, Aeronaut. Space Sci. Jpn. 46 (1998) 393 – 397. [6] J.A.S. Cleaver, M. Ghadiri, Powder Technol. 76 (1993) 15 – 22. [7] T. Taylor, Powder Technol. 95 (1998) 1 – 6.
[8] H. Kalman, D. Goder, Adv. Powder Technol. 9 (1998) 153 – 167. [9] H. Kalman, Powder Technol. 112 (3) (2000) 244 – 250. [10] C. Aked, D. Goder, H. Kalman, A. Zvieli, Power Handl. Process. 9 (4) (1997) 345 – 348. [11] S.L. Mckee, T. Dyakowski, R.A. Williams, T.A. Bell, T. Allen, Powder Technol. 82 (1995) 105 – 113. [12] M. Konami, S. Tanaka and K. Matsumoto. Submitted to J. Soc. Powder Technol. Jpn. 39 (2002) 4 – 11. [13] M. Konami, S. Tanaka, A. Iwai, K. Matsumoto, J. Soc. Powder Technol. Jpn. 38 (2001) 236 – 243. [14] M. Satoh, T. Iwasaki, N. Ikemoto, M. Deguchi, T. Oshimi, M. Kondo, J. Soc. Powder Technol. Jpn. 37 (2000) 734 – 741.
Attrition of granules during repeated pneumatic transport Moriyoshi Konami a,*, Shigeho Tanaka a, Kanji Matsumoto b a
Department of Research and Development, Nisso Engineering Co., Ltd., 1-6-1, Kanda Jinbo-cho, Chiyoda, Tokyo 101-0051, Japan Department of Materials Science and Chemical Engineering, Yokohama National University, 79-5, Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
b
Received 16 July 2001; received in revised form 3 October 2001; accepted 20 December 2001
Abstract We investigated the changes of granules in the attrition process during repeated pneumatic transport, along with the factors that affected these changes. Experiments were conducted with a shot-feed type dense phase pneumatic transporter, a device that allows a low rate of attrition by transporting granule lumps one by one. Easily attrited granulated materials were used. The analysis revealed an equation describing the relationship between the fine particle increment rate (named F) and the mass median diameter of coarse granules. The following results were obtained: (1) the degree of attrition was reduced with the increase in the fine particle ratio (mass fraction) before transport; and (2) the F value increased proportionally to the granule diameter. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Pneumatic transport; Shot feed; Fine particle increment rate; Granule attrition; Repetition; Shape factor; Particle size
1. Introduction The attrition of particles or granulated materials during pneumatic transport is an issue of considerable industrial concern. A good number of dense phase pneumatic transporters have been designed specifically to reduce the particle attrition [1– 5]. There has also been increasing research into the quantification of the attrition phenomenon [6– 8]. For example, factors affecting attrition at bends have been well investigated. It has been shown that the vibration of bends affects the attrition of granules [9], and that long bends are not always useful to reduce such attrition [10]. Moreover, the transport velocity and loading ratio have also been shown to influence the attrition [11]. The strength, size and shape of granules are generally thought to affect the attrition of granules during pneumatic transport. More specifically, the transport air or solid velocity, transport length, fine particle ratio (mass fraction), and loading ratio must all be taken into account as influencing factors. In the case of actual transporters, we must also consider attrition by feeders, a phenomenon that occurs in blow tanks [12] or due to friction in screw feeders [9]. In pneumatic transport with shot-type feeding, single lumps of a solid are formed at a controlled rate and fed into pipelines at precise intervals by the release of com-
*
Corresponding author. Tel.: +81-3-3296-9310; fax: +81-3-3296-9250. E-mail address: [email protected] (M. Konami).
pressed air [13]. In conventional blow tank operation, the pressure drops a great deal between the starting point and ending point of the pipeline. This means that the velocity of the solids varies along the length of the pipeline in this method. Even if a solid starts out at a velocity at the lower limit, it accelerates to a high velocity by the time it reaches the end. In the shot feed method, the drop in pressure is low and constant, hence plugs or slugs of bulk solid travel stably through the entire pipeline. As a consequence, attrition during transportation is lower with the shot feed method than with the conventional blow tank method. While the air velocity, pressure drop, and loading ratio are important factors for pneumatic transport, it is difficult to express the loading ratio for the shot feed method, as the lumps are transported one by one. Repetitive transport tests, tests in which the same powder samples are used repeatedly, are common in industrial experimentation as the samples are usually available in limited amounts. In some cases, these tests are adopted to simulate long pipeline tests with the use of short pipelines in laboratories. Moreover, in actual factories, the granules produced are transported repeatedly through processes for receiving, storage, re-processing, and packaging. In all of these cases, the granule attrition and other changes that result from the repetitive transport should be taken into account. This is essential for the rational evaluation of experiments, and for the design of processes and factories as well. This paper presents findings on the attrition of granules during repeated transport by the shot feed method.
0032-5910/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 ( 0 2 ) 0 0 0 0 2 - 5
M. Konami et al. / Powder Technology 125 (2002) 82–88
83
2. Experimentation Fig. 1 illustrates the system configuration of the transport device used here. The principal components of this equipment were (1) the feed tank; (2) the pipeline and valve V-2, used to feed compressed air into the tank; (3) valve V-6, used to fill the granules into the tank; (4) valves V-1, V-3, and V-4, used to feed the granule lumps into the pipeline; and (5) an orifice ring to prevent the excessive feeding of granules. The feed tank had a volume of 0.0804 m3, and the inner diameter of the orifice ring was 13 mm. A bag filter with a filtration area of 3 m2 was fitted on the receiver to reduce the attrition in the receiver to a negligible level. As shown in Fig. 2, the design of the transport line was modeled after a typical industrial pipeline. The pipeline had a total length of 30 m, including a vertical section spanning a length of 6.3 m, eight bends with a radius of curvature of 200 mm and sections of straight horizontal pipe between the bends. The pipeline was made of stainless steel and had an inner diameter of 17.5 mm. Tests were carried out by repetition of the following operations. After raising the pressure in the feed tank to a specified level, valve V-4 was opened for a specified period to feed the sample to the pipeline via the pressure difference between the feed tank and pipeline. Valve V-4 was then closed, and valve V-5 was opened for a specified period to allow transportation of the granule lumps through the pipeline. The feed time per granule was defined as period during which valve V-4 was left open, and the lump feed interval was defined as
Fig. 2. Dimension of main pipeline for transport.
the period during which valve V-5 was left open. Through the repetition of the above operations, single lumps were continuously fed and transported one by one. The basic setting conditions for the testing were determined based on previous results for shot-feed pneumatic transport [13]. Both the feed tank pressure and pressure for pneumatic transport were set at 50 kPa and regulated by control valve 11) shown in Fig. 1. This pressure level corresponds to between two and three times the transport pressure drop for a single lump of granules, allowing stable transportation at the minimum possible pressure. The superficial velocity of transport air was set at 7.4 ms 1. The lump feed time was set at 0.3 s, a level at which choking is unlikely to occur even when other conditions vary. The lump feed interval was set at 17 s by adding an additional margin to the time required for complete transportation of a single lump. The granules used in this testing were prepared from a fine tungsten – carbide powder at a tablet-forming pressure of 200 MPa by a roller compactor with a clearance of 1 mm between rollers. Once formed, the tablets were disintegrated into a double roll crusher, and fine particles were removed by a 150-Am sieve. The properties of this material are presented in Table 1. Here, the value for granule hardness indicates the average of the maximum stress levels when one granule is ruptured by a 1-mm-diameter rod descending at a rate of 0.4 mm per minute.
Table 1 Properties of granules Granule diameter Original particle diameter Particle density Bulk density
Fig. 1. Schematic diagram of dense phase pneumatic transporter with shot feed.
Angle of repose Carr’s Flowability index Granule hardness
150 to 850 Am 0.8 Am (average) 10,700 kg m 3 4050 kg m 3 (average) (loose 3800, packed 4300) 43j 71.5 points 0.52 N
84
M. Konami et al. / Powder Technology 125 (2002) 82–88
3. Experimental results and discussion 3.1. Definition of fine particle increment rate, F The fine particle increment rate F is defined as the rate of increase of the fine particle ratio (mass fraction) between before and after transport, according to the following equation: F ¼ ðXa Xb Þ=ð1 Xb Þ,
ð1Þ
where, Xa is the fine particle ratio after transport and Xb is the fine particle ratio before transport. The fine particle ratio is defined as the ratio of the mass passed through the 150Am sieve to the overall mass. A circular vibrating sieve was used to measure Xa and Xb. The machine had a 440-mm-diameter sieve, a 0.4-kW power rating, and a sieving time of 10 s. Some degree of granule attrition did occur during the sieving [14], but this was minimized to a negligible level by the short sieving time. Fig. 4. Effect of fine particle ratio on F value.
3.2. Effect of repetitive transport on F Fig. 3 shows the variation of F in repeated transport of identical samples. The total mass of granules for the first run was 5 kg, and this decreased gradually due to the small loss incurred during each transport. The original sample of granules was used in the first run and reused in each subsequent run without removing the fine particles. The results shows that, in general, F decreased as the transportation was repeated. Choking occurred in the sixth repetition, when the fine particle ratio before the transport was 0.436. Furthermore, the F value during the fifth repetition, the repetition before chocking, was increased. This increase was mainly attributed to the high stress between granules in the bends reported in the previous paper [12].
3.3. Effect of the fine particle ratio on F The fine particle ratio, which increased during repeated transport, may have been one of the factors leading to reduction of the F value during the repetition. For this reason, test samples were prepared by adding a fixed mass of fine particles to the original granules after sieving to confirm the effect of only the fine particle ratio. Afterwards, each sample was transported through the test pipeline, and the corresponding F value was measured. Fig. 4 shows the results of this experiment, with the data from the repetitive transport in Section 3.2 superimposed over the figure. The transport test was performed under the basic setting conditions described in Section 2. The results show that F was reduced as the fine particle ratio increased in both cases. Presumably, this was because the particles enclosed the coarse granules and thereby acted as a cushioning medium that reduced the granule stress and levels of attrition. This phenomenon is similar to the cushioning effect observed in crushing processes. The result in Fig. 4 illustrates the difference between the first transport of the original samples and repeated transports. Choking also occurred when the fine particle ratio was set at 0.4 for the first transport, just as observed in the repeated transport, and F rose around the time of the choking. This experiment confirmed that the fine particle ratio has an important influence on the attrition. 3.4. F for repetitive transport without fine particles
Fig. 3. Effect of transport repetition on F value.
To find the effect of fine particles, we removed the fine particles before each individual transport to make Xb = 0, and then examined the relationship between the F value and
M. Konami et al. / Powder Technology 125 (2002) 82–88
85
transport, and (c) corresponds to samples after transport with the fine particles removed. The granules before transportation were irregular and angular in shape as a result of the compression granulation and disintegration. Due to the attrition, we can see that the specific surface areas of these granules became gradually smaller—that is to say, the granules in question became more spherical. To investigate these effects, the shape factors were calculated. The Wadell circularity /c as defined in Eq. (2) was adopted as a shape factor. In order to determine this circularity index, photomicrographs of approximately 50 granules over 150 Am were examined from each sample by commercially available software, thereby allowing calculation of /c as /c ¼ pDH =C,
Fig. 5. Change in F value and circularity with transport repetition.
number of repetitive transports. Fig. 5 presents the result. The initial test samples were prepared by adjusting Xb for original granules to 0. The total mass of granules for the first run was 10.0 kg and then was gradually reduced to 2.6 kg as particles were removed. All other conditions were in accordance with the basic setting conditions. Even under these conditions, F was gradually reduced as the number of transports increased. This gradual reduction was believed to result from the differences in the strength, shape, and size of the granules as the fine particles were removed. 3.5. Change of the shape factor during repetitive transport Fig. 6(a), (b), and (c) presents variations in the shape of granules that resulted from the transportation. Specifically, (a) corresponds to the adjusted original granules with the fine particles removed, (b) corresponds to samples after
ð2Þ
where DH is the projected area diameter (i.e. Heywood diameter, {4A/p}0.5), C is the perimeter of a projected particle, and A is the projected area of a particle. Fig. 5 shows the variation of /c with the number transports. The increment in /c was the most significant after the first transport. After the second and third repetitions the increment became smaller, and by the fourth repetition there was no change at all. At the first transport, when /c was small, F was particularly high. One reason for this may have been the higher susceptibility of granules with irregular surfaces to attrition. In contrast to spheres, granules with large numbers of protuberances or other irregularities on their surfaces more readily succumb to attrition. Another reason for the change in F could have been the process used to manufacture the granules, which seemed to have left a fragile layer in the vicinity of the granule surface. The influence of this layer could not have been substantial, as it would have been removed after the fourth repetition, when /c remained roughly constant, leaving the granule with an almost completely homogenous makeup. As a consequence, we
Fig. 6. Outlook of the granules ( F = 0.393). (a) Adjusted original granules for which fine particles have been removed (Xb = 0); (b) after-transport condition; (c) samples of (b) for which fine particles have been removed.
86
M. Konami et al. / Powder Technology 125 (2002) 82–88
assume that the strength of the granule was stabilized from the fourth repetition onward. 3.6. Effect of granule size on F Upon examination of the effect of granule size on the granule attrition, the relative granule diameter S was introduced as the ratio of the mass median diameter of coarse granules before transport to that of coarse original granules. The term ‘‘coarse granules’’ refers to granules of over 150 Am in diameter. The relationship between S and F was investigated. The variation in S by transport repetition i can be expressed by Eq. (3). See Appendix A for the derivation of this equation. Si =Si1 ¼ ke ð1 Fi Þ1=3
ð3Þ
Although ke is dependent on the difference between the numbers of coarse granules before and after transport, these numbers can be assumed to be constant. Attrition is manifested principally as the chipping of granules, and thus is assumed to leave the initial number of coarse granules unchanged. Only two forces work against this principle. Firstly, a very coarse granule can be crushed into two or more coarse granules. Secondly, a coarse granule can reduced to the size of a fine granule ( < 150-Am diameter) through the process of attrition. In the first case the number of coarse granules increases, and in the second case it decreases. Yet, these phenomena cancel each other out, and under the conditions of the present experiment, the low level of attrition renders them scarcely operative. It can therefore be assumed that the change in the number of coarse granules is negligible. In this case, ke becomes 1 from Eq. (a-9). Fig. 7 plots the values of S obtained in the repeated transport described in Section 3.4. These values were
Fig. 8. Effect of granule diameter on F value.
calculated from the mass median diameters of coarse granules. The values of S, which were calculated from the experimentally obtained F values by Eq. (3) under the assumption that ke is 1, are also given in this figure. This figure shows that ke can be taken as approximately 1. After the fourth repetition, when /c became constant, the only factor that appeared to affect F was the diameter of granules, and thus this relationship was analyzed further. Fig. 8 shows the relationship between F and S, calculated from the data after the fourth repetition under the assumption that ke = 1. Based on this figure, the effect of S on F can be taken as F ¼ 0:1S:
ð4Þ
In the range of test conditions under which shapes were stable, this equation confirms that the amount of attrition is proportional to the granule diameter, as previously reported [6].
4. Conclusions We investigated the attrition of granulated material during repeated pneumatic transport by measuring the increment rate of fine particles. In our analysis, we presented an equation demonstrating the relationship between the fine particle increment rate and the diameter of coarse granules. The following results were obtained: 1.
Fig. 7. Change in granule diameter with transport repetition.
The granule attrition during repeated pneumatic transport was high at first, but the rate of attrition gradually declined and the shape factor of the granules, became constant.
M. Konami et al. / Powder Technology 125 (2002) 82–88
2.
The rate of granule attrition declined as the ratio of fine particles within the granules increased up to a certain value. However, an excess of fine particles led to choking. 3. The fine particle increment rate increased in proportion to the granule diameter.
Acknowledgements The authors thank Mr. A. Iwai of Kimura Chemical Plant and Mr. N. Uchida, a registered consultant engineer, for their useful advice.
and D3i ¼ ð1=ni Þ
ni X
dj3,i :
Next, we introduce the relative granule diameter Si defined by Di/D0, a value based on the mass mean diameter. Thus, we obtain ðSi =Si1 Þ3 ¼ ðni1 =ni Þð1 FÞ:
ðp=6Þqdj3,i1 ¼ W0 ð1 Xb Þ,
ða-1Þ
j¼1
where W0 is the total mass of all granules. In the same way, after i-th transport, we have ni X
ðp=6Þqdj3,i ¼ W0 ð1 Xa Þ:
ða-2Þ
j¼1
The following equation is derived by dividing Eq. (a-2) by Eq. (a-1). ni X
dj3,i ð j ¼ 1 to ni Þ
j¼1 ni1 X
dj3,i1
¼ ð j ¼ 1 to ni1 Þ
ð1 Xa Þ ð1 Xb Þ
ða-3Þ
j¼1
By the definition of F, the right-hand side of Eq. (a-3) becomes ð1 Xa Þ=ð1 Xb Þ ¼ 1 ðXa Xb Þ=ð1 Xb Þ ¼ 1 F: ða-4Þ If we introduced the mass mean diameter for coarse granules before and after transport (Di 1 and Di), we obtain D3i1 ¼ ð1=ni1 Þ
ða-7Þ
And then, ða-8Þ
where
For a group of granules containing coarse granules over a specified diameter (150 Am in this paper), let ni 1 be the number of coarse granules and let dj,i 1 ( j = 1 to ni 1) be the volume equivalent diameter of coarse granules before i transport repetitions. Assuming the density of all granules is q, the total mass of coarse granules before i transport repetition is ni1 X
ða-6Þ
j¼1
ðSi =Si1 Þ ¼ ke ð1 FÞ1=3 , Appendix A
87
ni1 X j¼1
dj3,i1
ða-5Þ
ke ¼ ðni1 =ni Þ1=3 :
ða-9Þ
This concludes the derivation of Eq. (3). Although Xa, Xb, F and ke should be affixed with the suffix i, this suffix was omitted for the sake of simplicity. Nomenclature A projected area of a particle (m2) C perimeter of projected particle figure (m) DH projected area diameter (Heywood diameter) (m) Di mass mean diameter from samples after a repeated number (i) of transports (m) dj,i volume equivalent diameter of coarse granule j of over 150 Am from samples after transport repetition i (m) F, Fi fine particle increment rate under 150 Am from samples after transport repetition i, with attrition as defined by Eq. (1) ( – ) i number of transport repetitions (– ) j number representing each of the coarse granules ( –) ke coefficient defined by Eq. (a-9) (– ) ni total number of coarse granules from samples after transport repetition i ( – ) S, Si relative granule diameter based on mass median diameters; ratio of diameters of coarse granules over 150 Am in samples before transport repetition i vs. original (– ) W0 total mass of granules (kg) Xa fine particle (150 Am under) ratio (mass fraction) after transport (– ) Xb fine particle (150 Am under) ratio (mass fraction) before transport ( –)
Greek letters /c circularity defined in Eq. (4) by Wadell ( –) q density of granules (kg m 3)
88
M. Konami et al. / Powder Technology 125 (2002) 82–88
References [1] M. Konami, Kagaku Souchi 36 (1994) 92 – 101. [2] M. Moriyama, K. Morimoto, A. Iwamoto, S. Onoe, K. Korekata, Kagaku Kogaku 52 (1988) 421 – 425. [3] K. Murata, Y. Joka, Kagaku Souchi 33 (1991) 49 – 55. [4] R. Aoki, H. Murata, H. Homma, H. Wada, Kagaku Kogaku Ronbunshu 6 (1980) 628 – 634. [5] H. Ueda, M. Takei, Y. Zhao, H. Li, M. Ochi, Y. Tomita, K. Horii, Aeronaut. Space Sci. Jpn. 46 (1998) 393 – 397. [6] J.A.S. Cleaver, M. Ghadiri, Powder Technol. 76 (1993) 15 – 22. [7] T. Taylor, Powder Technol. 95 (1998) 1 – 6.
[8] H. Kalman, D. Goder, Adv. Powder Technol. 9 (1998) 153 – 167. [9] H. Kalman, Powder Technol. 112 (3) (2000) 244 – 250. [10] C. Aked, D. Goder, H. Kalman, A. Zvieli, Power Handl. Process. 9 (4) (1997) 345 – 348. [11] S.L. Mckee, T. Dyakowski, R.A. Williams, T.A. Bell, T. Allen, Powder Technol. 82 (1995) 105 – 113. [12] M. Konami, S. Tanaka and K. Matsumoto. Submitted to J. Soc. Powder Technol. Jpn. 39 (2002) 4 – 11. [13] M. Konami, S. Tanaka, A. Iwai, K. Matsumoto, J. Soc. Powder Technol. Jpn. 38 (2001) 236 – 243. [14] M. Satoh, T. Iwasaki, N. Ikemoto, M. Deguchi, T. Oshimi, M. Kondo, J. Soc. Powder Technol. Jpn. 37 (2000) 734 – 741.