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Feeding small quantities of particulate solids
Powder Technology 142 (2004) 1 – 6 www.elsevier.com/locate/powtec
Feeding small quantities of particulate solids Michael Pohorˇely´, Karel Svoboda, Miloslav Hartman * Institute of Chemical Process Fundamentals AS CR, Rozvojova´ 135, 165 02 Prague 6-Suchdol, Czech Republic Received 30 April 2003; received in revised form 18 February 2004 Available online 23 April 2004
Abstract Results of extensive experiments with a laboratory slide feeder are presented. Different particulate materials such as sand, g-alumina, ceramsite, dried sewage sludge, and shredded wood have been employed in testing the equipment for reproducibility of single doses and general reliability. Aside from physical characteristics of the respective particulate materials, the ratio of pocket diameter to particle size has considerable influence on the uniformity of feeding. Minimum flow rates of air have also been determined by experiment as needed for pneumatic transport of the metered solids into a reactive environment. D 2004 Elsevier B.V. All rights reserved. Keywords: Feeding of different particulate materials; Slide feeder; Variations in single doses of different materials
1. Introduction Continual, uniform feeding of the particulate reactant into a chemical reactor often poses a mechanical problem. The situation is further aggravated when the needed amounts of solids are small. However, such circumstances are increasingly common place in conducting experimental research on the fluidized bed reactors, where noncatalytic gas – solid reactions occur. Earlier attempts to develop a reliable, truly continuous, revolving plate feeder equipped with a transport screw were plagued with setbacks. Detailed experimenting disclosed some difficulties, such as short- and long-term variations in the feed rate, stability problems with the plate and screw, particularly at low speed of revolving. Furthermore, the equipment walls tended to foul with powder, and long periods of operation were required to stabilize the process of feeding. Instead of continuous feeding devices, three more or less different types of discontinuous feeders were constructed and tried out in our earlier work [1]. These were a slide feeder, rotary feeder, and a disk feeder. The slide device proved to be the most practical of the three feeders with respect to its operational reliability, reproducibility of doses, range of operation, and minimum particle attrition. This type
* Corresponding author. Tel.: +42-220390254; fax: +42-220920661. E-mail address: [email protected] (M. Hartman). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.03.005
also makes it easier to effectively separate the space of the reactor, often filled with harmful gases, from the feeder. We believe that such a needed and helpful device as the slide feeder deserves to be explored in some practical details. Its use is envisaged, for example, in our ongoing research into the combustion of sewage sludge. It is the aim of this article to provide unbiased experimental data on the metered feeding of different particulate materials by means of a slide feeder.
2. Experimental 2.1. Apparatus Fig. 1 shows the principles of the double-acting, airdriven slide feeder with two containers and an exchangeable sliding plate provided with a cylindrical pocket (hole). The air from the central distribution system was filtered before entering the device and its pressure was maintained at constant level. The construction of the feeder enabled to change the sliding plates of different designs quite easily. The sliding plate was 8 or 10 mm thick (h), constructed of polytetrafluoroethylene (PTFE), and provided with a 8-, 10-, or 12-mm-diameter hole (dh). Other materials for the sliding plates such as bronze or duralumin were also tested. However, they did not compare well with PTFE. The volume of the feeding pockets (holes) was varied from
M. Pohorˇely´ et al. / Powder Technology 142 (2004) 1–6
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Fig. 1. Double-acting, air-driven slide feeder with an interchangeable sliding plate provided with two cylindrical pockets (holes).
0.4021 to 1.1310 cm3. The inside diameter of the tube for pneumatic transport of the metered doses of solids was as large as 9 mm, which corresponds to a cross-sectional area of 0.6362 cm2. Geometrical characteristics of the feeding pockets are given in Table 1. Two pockets in the slide plate make it possible that one pocket can fill whilst the other empties. This allows the slide to work at half the cycle frequency of a single pocket and thereby gives more time for filling and emptying. The alignment of the pockets with the connecting pipes is sensitive to the positional accuracy. Any abutment or ledge can have a profound affect on the transfer of material from one section to the other. The equipment makes it possible to alter the frequency of the doses in the range 0.02 – 0.40 s 1. Our experience indicates, for example, that the intermittent feeding of the solid sorbent results in a practically smooth curve of exit SO2 concentration if the frequency of the doses is higher Table 1 Dimensions and volumes of the cylindrical feeding pocketsa Sliding plate
Diameter, dh (mm)
Height, h (mm)
Volume (cm3)
A B C
8 10 12
8 8 10
0.4021 0.6283 1.1310
a The frequency of doses amounted to f = 0.3667 s 1; the inside diameter of the transport tube was 9 mm and its cross-sectional area, 0.6362 cm2.
than or equal to 0.03 s 1. To avoid development of electrostatic charges, the feeder was thoroughly grounded. 2.2. Materials The present work was conducted with five different sorts of materials: silica sand, g-alumina, ceramsite (fired claystone), digested and dried municipal sewage sludge, and shredded hard wood. Larger batches, which contained no visible inclusions, were crushed (except for sand), sieved carefully, and stored in air-tight containers. The fractions investigated in this study comprised two narrow size ranges: 0.315 – 0.500 mm (d¯p = 0.408 mm) and 1.40 – 1.60 mm (d¯p = 1.500 mm). Particle density was determined by mercury pycnometry, true (skeletal) solids density was determined by helium intrusion analysis. Physical properties of the materials are summarized in Table 2. Using a microscope, the shape and surface characteristics of the materials under investigation were observed in some details. The particles of sand and alumina are isometric. While the sand particles are quite round and smooth, the alumina particles are rather sharp-edged. The ceramsite particles fired at 850 jC are of elongated, irregular shape and exhibit a surface quite rough to the touch. The particles of digested municipal sewage were dried at 105 jC to constant mass. Apart from the isometric particles, different, more or less irregular shapes were also found in the batch of
M. Pohorˇely´ et al. / Powder Technology 142 (2004) 1–6 Table 2 Physical properties of the particulate materials used in this work No. Material
Particle sizea, dp (mm)
Mean particle size, d¯p (mm)
True Particle Loose particle densityc poured densityb (kg m 3) bulk density (kg m 3) (kg m 3)
1 2 3 4 5 6
0.315 – 0.500 0.315 – 0.500 0.315 – 0.500 1.40 – 1.60 1.40 – 1.60 1.40 – 1.60
0.4075 0.4075 0.4075 1.50 1.50 1.50
2530 2202 2248 2248 2171
Sand g-Alumina Ceramsite Ceramsite Dried sludge Wood shreds
2530 1572 1510 1470 1149 550
1426 996 632 670 558 290
a
Determined by sieving. Determined by helium pycnometry. c Determined by mercury pycnometry. b
the sludge and wood particles. One of the major features of these two solids—at least from the standpoint of flow properties—is their fibrous nature. With the aid of a microscope, straight or twisted fibres were clearly visible at the particles’ surface. Irregular shapes, combined with the fibrous character of the sludge and wood particles, contrived to form arches in the container filled with either material. It is of interest to note that the arch only occurred at a certain position in the conical (not cylindrical) section of the container. As an effective remedy for unwanted arching proved to be a simple device knocking at times on the container wall in the vicinity of the troubled spot. In addition to the arching phenomena, the shape and surface characteristics of the particles also exert an important influence on rate and uniformity of the filling and the emptying of the feeding pockets. Our experimental experience indicates that it is the pocket emptying rather than its filling which is more susceptible to flow irregularities. Nevertheless, it was not observed that any material was retained in the pocket between fillings.
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means of calibrated rotameters. Repeated measurements of this important material parameter showed good reproducibility in the range from 5% to 7%. The measurements were carried out at a temperature of 20F0.5 jC. The density at ambient pressure and the viscosity of the filtered and dried air amounted to 1.202 kg m 3 and 1.81. 10 5 Pa s, respectively.
3. Results and discussion Using the ceramsite particles as a reference material in preliminary experiments, the influence of the frequency of doses was tested with the largest pocket (1.131 cm3). It was found out that there was no significant effect on the accuracy of feeding when the frequency of doses was slower than 0.417 s 1, i.e., less than 25 doses per minute. This suggests that the filling and emptying of the largest of the employed pockets took place quite rapidly. Mass of single doses was measured in further work at a frequency of sliding of 0.3667 s 1, i.e. one dose in 2.727 s. Respective deviations in mass of single doses from the mean value are plotted in Fig. 2 for the alumina, ceramsite, and dry sludge particles. As can be seen, there are considerable differences in the reproducibility of single doses of the respective materials. While the alumina particles behaved almost ideally, the mass of the sludge doses fluctuated within the F10% range. The maximum as well as the average deviations from the mean are presented in Table 3. As evident, the doses of nonfibrous materials (sand,
2.3. Procedure In the majority of the experimental runs, 20 single doses in a row were collected and weighed one at a time on an automatic scale with an accuracy of F0.1 mg. Attention was also given to determine a minimum, functional velocity of air (Utr) at which the metered doses of solids are reliably transported from the feeder as outlined in Fig. 1. Although such a quantity is not entirely unequivocal, it is much needed information in experimental work on the continuous fluid-bed units. The delivery pipe was equipped with a transparent glass tube of i.d. = 9 mm that made it possible to observe visually the behaviour of the metered particles in a stream of transporting gas. The value of the gas flow rate used was just sufficient to sustain transport of the metered solids, hence, is the minimum transport velocity at the feed rate. The flow rate of the transporting air, was measured by
Fig. 2. Variations in the mass of single doses of different materials: frequency of doses, f = 0.3667 s 1; ratio of hole diameter to particle size, dh/ d¯p = 8; volume of pocket, 1.131 cm3; mean particle size, d¯p = 1.5 mm; (o) dried sewage sludge particles, mean mass of a single dose w¯i = 0.2125 g; (.) alumina particles, w¯i = 0.9527 g; (x) ceramsite particles, w¯i = 0.5701 g.
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Table 3 Variation in the mass of single doses of different solidsa Materialb
dh/dp
Mean mass of a single dose (g)
Rate of feeding (g h 1)
Maximum deviation (%)
Mean deviation (%)
1 2 3 4 5 6
29.4 29.4 29.4 8.00 8.00 8.00
1.3440 0.9461 0.5702 0.5751 0.2793 0.0781
1785.5 1257.5 743.6 752.6 280.5 119.7
0.441 0.226 1.370 4.36 13.32 31.59
0.246 0.135 0.784 1.900 7.057 15.055
a Based on 20 successive doses. The volume of the cylindrical pocket C (hole) amounted to 1.131 cm3 (dh = 12 mm, h = 10 mm); frequency of doses f = 0.3667 s 1. b Physical properties of the materials are given in Table 2: (1) sand, ¯dp = 0,408 mm; (2) g-alumina, d¯p = 0.408 mm; (3) ceramsite, d¯p = 0.408 mm; (4) ceramsite, d¯p = 1.5 mm; (5) dried sludge, d¯p = 1.5 mm; (6) wood shreds, d¯p = 1.5 mm.
alumina, and ceramsite) can be reproduced with very good accuracy. Understandably, flow intricacies of the fibrous particles (dry sludge solids and wood shreds) manifest themselves in somewhat more pronounced scatter of the doses. From potential sources of the variation in single doses one should consider, aside from general flow properties of the solids, also possible changes in the effective particle size in the course of the experiments due to abrasion (particularly at sludge particles) and/or agglomeration (especially with wood particles). The diameter of the holes or pockets in the slide plate varied from 8 to 12 mm, their depth (thickness of plate) ranged from 8 to 10 mm. Some products included particles as
Fig. 3. Maximum deviation of the mass of single doses as a function of ratio of hole (pocket) diameter to mean particle size: (x) ceramsite; (o) dried sewage sludge; d¯p = 1.5 mm.
Fig. 4. Rate of feeding as a function of pocket (chamber) volume for different materials: frequency of doses, f = 0.3667 s 1; (1) sand, d¯p = 0.408 mm; (2) g-alumina, d¯p = 0.408 mm; (3, 4) ceramsite, d¯p = 0.408 and 1.50 mm, respectively; (5) dried sludge, d¯p = 1.50 mm; (6) wood shreds, d¯p = 1.50 mm. Dimensions of the pockets are given in Table 1.
large as 0.407 and 1.50 mm. As illustrated in Fig. 3, the reproduction of the dose is significantly effected by the ratio of the hole diameter (dh) to mean particle size, d¯p. It appears that the ratio dh/d¯p should not be less than approximately 10.
Fig. 5. Minimum transport velocity of air for the metered particles as a function of particle density. The labelling of the data points is the same as in Fig. 4 and Table 2: (1) sand, d¯p = 0,408 mm; (2) g-alumina, d¯p = 0.408 mm; (3) ceramsite, d¯p = 0.408 mm; (4) ceramsite, d¯p = 1.50 mm; (5) dried sludge, d¯p = 1.50 mm; (6) wood shreds, d¯p = 1.50 mm.
M. Pohorˇely´ et al. / Powder Technology 142 (2004) 1–6
This conclusion aligns with a common ‘rule-of-thumb’ that an orifice should be at least eight times the particle size to avoid the formation of ‘structural arches’ over a circular opening. The volume of the feeding pocket varied from 0.402 to 1.131 cm3 and the particle density ranged from 550 (wood) to 2530 kg m 3 (sand). Accordingly, the rate of feeding in the experiments ranged from approximately 0.050 to about 1.80 kg h 1 as illustrated in Fig. 4. It is necessary to transport the metered doses of particles to a point of interest (e.g., into a burning fluidized bed) without any undesirable effects such as possible particle disintegration, abrasion, or distortion of the bed. Therefore, we also tried to determine for the respective materials, the minimum flow rates of air which ensure reliable pneumatic transport of the particles from the feeder. The minimum transport velocities required (Utr) ranged from approximately 1 to about 3 m s 1. As shown in Fig. 5, Utr is a strong function of the particle density. Using our straightforward procedure [2,3] for calculation of the terminal free-fall velocity (Ut) of a given nonspherical particle, we also assessed the steady-state, free-fall conditions of the tested particles in air (elutriation velocity). The relationship between the experimental Utr a the predicted Ut is not entirely clear. As the data plotted in Fig. 6 indicate, Utr is quite near Ut in the case of the smaller particles (d¯p = 0.407 mm). However, Utr/Ut = 0.3– 0.4 for the particles as large 1.5 mm. It cannot be ruled out that these results are also affected by differences in the velocity profiles developed in the transport tube.
Fig. 6. Ratio of minimum transport velocity to terminal free-fall velocity of the particles in air versus the Archimedes number. The labelling of the data points is the same as in Figs. 4 and 5 and in Table 2: (1) sand, d¯p = 0.408 mm; (2) g-alumina, d¯p = 0.408 mm; (3) ceramsite, d¯p = 0.408 mm; (4) ceramsite, d¯p = 1.50 mm; (5) dried sludge, d¯p = 1.50 mm; (6) wood shreds, d¯p = 1.50 mm.
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A feature of major importance to the reliable and repeatable performance of such a feeding device as ours is also the design of the flow channel that delivers the bulk material into the receiving chamber. To ensure smooth and reliable flow, the system should be designed with radius corners, over-pressure relief and minimum wall friction in the crucial converging section. Products that have intrinsic poor flow properties, such as fibrous and interlocking particulates or cohesive fine materials, usually need particular care and special equipment. 4. Conclusions Different particulate materials such as sand, alumina, ceramsite, dried sewage sludge, and wood shreds were employed to explore an operational capability and accuracy of a laboratory pneumatic—oscillating slide feeder. In the course of the whole work, the developed feeding device proved to be a reliable metering and transporting apparatus. The mass of a single dose of particles was on the order of 10 1 –100 g, depending upon the particle density and the volume of the feeding chamber (pocket). With a frequency of doses as large as 0.367 s 1, the rate of feeding of the above solids varied widely from 0.1 (wood shreds) to 1.8 kg h 1 (sand). Considerable differences were found in the reproducibility of single doses of the respective particulate materials, which reflects their different shape and surface characteristics. The smallest average deviations from the mean 0.14% and 0.25% were found with the isometric and smooth particles of alumina and sand, respectively. Fibrous particles of dried sewage sludge and wood shreds exhibited the most pronounced variations in the mass of single doses. The average deviations for these two solids amounted to 7% and 15%, respectively. To remove or minimize irregularities in the filling and (mainly) emptying of the cylindrical pockets, the ratio of the pocket diameter to mean particle size should be greater than 10. The minimum air velocities needed for pneumatic transport of the metered solids vary from 1 to 3 m s 1, this value being mainly influenced by the particle density. The particles of dry sludge and the wood shreds showed a pronounced tendency to arch in the conical section of the container. List of symbols Ar Archimedes number = (d¯p3gqf(qp qf))lf 2 dh diameter of cylindrical pocket/hole (mm) dp particle size determined by sieving (mm) d¯p mean particle size determined by sieving (mm) f frequency of doses (s 1) g acceleration due to gravity = 9.807 (m s 2) h height of cylindrical pocket/hole Ut terminal, steady-state velocity of an isolated particle in an infinite fluid
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Utr
w¯I
minimum velocity of air at which the metered particles are readily transported in horizontal tube (m s 1) mean mass of single dose (g)
Greek symbols lf fluid viscosity (kg (m s) 1, Pa s) qf fluid density (kg m 3) qp particle density (kg m 3)
Acknowledgements The authors wish to acknowledge the financial support provided for this work by GA AS CR (Grant A 407 2201) and
GA CR (Grant 203/02/0002). We are also very grateful to O. Trnka from The Institute of Chemical Process Fundamentals for contributing to this work with technical assistance.
References [1] M. Hartman, K. Svoboda, O. Trnka, Unsteady-state retention of sulfur dioxide in a fluidized bed with continual feeding of lime and limestone, Ind. Eng. Chem. Res. 30 (1991) 1855 – 1864. [2] M. Hartman, V. Vesely´, K. Svoboda, V. Havlı´n, Explicit relationships for the terminal velocity of spherical particles, Collect. Czechoslov. Chem. Commun. 55 (1990) 403 – 408. [3] M. Hartman, O. Trnka, K. Svoboda, Free settling of nonspherical particles, Ind. Eng. Chem. Res. 33 (1994) 1979 – 1983.
Feeding small quantities of particulate solids Michael Pohorˇely´, Karel Svoboda, Miloslav Hartman * Institute of Chemical Process Fundamentals AS CR, Rozvojova´ 135, 165 02 Prague 6-Suchdol, Czech Republic Received 30 April 2003; received in revised form 18 February 2004 Available online 23 April 2004
Abstract Results of extensive experiments with a laboratory slide feeder are presented. Different particulate materials such as sand, g-alumina, ceramsite, dried sewage sludge, and shredded wood have been employed in testing the equipment for reproducibility of single doses and general reliability. Aside from physical characteristics of the respective particulate materials, the ratio of pocket diameter to particle size has considerable influence on the uniformity of feeding. Minimum flow rates of air have also been determined by experiment as needed for pneumatic transport of the metered solids into a reactive environment. D 2004 Elsevier B.V. All rights reserved. Keywords: Feeding of different particulate materials; Slide feeder; Variations in single doses of different materials
1. Introduction Continual, uniform feeding of the particulate reactant into a chemical reactor often poses a mechanical problem. The situation is further aggravated when the needed amounts of solids are small. However, such circumstances are increasingly common place in conducting experimental research on the fluidized bed reactors, where noncatalytic gas – solid reactions occur. Earlier attempts to develop a reliable, truly continuous, revolving plate feeder equipped with a transport screw were plagued with setbacks. Detailed experimenting disclosed some difficulties, such as short- and long-term variations in the feed rate, stability problems with the plate and screw, particularly at low speed of revolving. Furthermore, the equipment walls tended to foul with powder, and long periods of operation were required to stabilize the process of feeding. Instead of continuous feeding devices, three more or less different types of discontinuous feeders were constructed and tried out in our earlier work [1]. These were a slide feeder, rotary feeder, and a disk feeder. The slide device proved to be the most practical of the three feeders with respect to its operational reliability, reproducibility of doses, range of operation, and minimum particle attrition. This type
* Corresponding author. Tel.: +42-220390254; fax: +42-220920661. E-mail address: [email protected] (M. Hartman). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.03.005
also makes it easier to effectively separate the space of the reactor, often filled with harmful gases, from the feeder. We believe that such a needed and helpful device as the slide feeder deserves to be explored in some practical details. Its use is envisaged, for example, in our ongoing research into the combustion of sewage sludge. It is the aim of this article to provide unbiased experimental data on the metered feeding of different particulate materials by means of a slide feeder.
2. Experimental 2.1. Apparatus Fig. 1 shows the principles of the double-acting, airdriven slide feeder with two containers and an exchangeable sliding plate provided with a cylindrical pocket (hole). The air from the central distribution system was filtered before entering the device and its pressure was maintained at constant level. The construction of the feeder enabled to change the sliding plates of different designs quite easily. The sliding plate was 8 or 10 mm thick (h), constructed of polytetrafluoroethylene (PTFE), and provided with a 8-, 10-, or 12-mm-diameter hole (dh). Other materials for the sliding plates such as bronze or duralumin were also tested. However, they did not compare well with PTFE. The volume of the feeding pockets (holes) was varied from
M. Pohorˇely´ et al. / Powder Technology 142 (2004) 1–6
2
Fig. 1. Double-acting, air-driven slide feeder with an interchangeable sliding plate provided with two cylindrical pockets (holes).
0.4021 to 1.1310 cm3. The inside diameter of the tube for pneumatic transport of the metered doses of solids was as large as 9 mm, which corresponds to a cross-sectional area of 0.6362 cm2. Geometrical characteristics of the feeding pockets are given in Table 1. Two pockets in the slide plate make it possible that one pocket can fill whilst the other empties. This allows the slide to work at half the cycle frequency of a single pocket and thereby gives more time for filling and emptying. The alignment of the pockets with the connecting pipes is sensitive to the positional accuracy. Any abutment or ledge can have a profound affect on the transfer of material from one section to the other. The equipment makes it possible to alter the frequency of the doses in the range 0.02 – 0.40 s 1. Our experience indicates, for example, that the intermittent feeding of the solid sorbent results in a practically smooth curve of exit SO2 concentration if the frequency of the doses is higher Table 1 Dimensions and volumes of the cylindrical feeding pocketsa Sliding plate
Diameter, dh (mm)
Height, h (mm)
Volume (cm3)
A B C
8 10 12
8 8 10
0.4021 0.6283 1.1310
a The frequency of doses amounted to f = 0.3667 s 1; the inside diameter of the transport tube was 9 mm and its cross-sectional area, 0.6362 cm2.
than or equal to 0.03 s 1. To avoid development of electrostatic charges, the feeder was thoroughly grounded. 2.2. Materials The present work was conducted with five different sorts of materials: silica sand, g-alumina, ceramsite (fired claystone), digested and dried municipal sewage sludge, and shredded hard wood. Larger batches, which contained no visible inclusions, were crushed (except for sand), sieved carefully, and stored in air-tight containers. The fractions investigated in this study comprised two narrow size ranges: 0.315 – 0.500 mm (d¯p = 0.408 mm) and 1.40 – 1.60 mm (d¯p = 1.500 mm). Particle density was determined by mercury pycnometry, true (skeletal) solids density was determined by helium intrusion analysis. Physical properties of the materials are summarized in Table 2. Using a microscope, the shape and surface characteristics of the materials under investigation were observed in some details. The particles of sand and alumina are isometric. While the sand particles are quite round and smooth, the alumina particles are rather sharp-edged. The ceramsite particles fired at 850 jC are of elongated, irregular shape and exhibit a surface quite rough to the touch. The particles of digested municipal sewage were dried at 105 jC to constant mass. Apart from the isometric particles, different, more or less irregular shapes were also found in the batch of
M. Pohorˇely´ et al. / Powder Technology 142 (2004) 1–6 Table 2 Physical properties of the particulate materials used in this work No. Material
Particle sizea, dp (mm)
Mean particle size, d¯p (mm)
True Particle Loose particle densityc poured densityb (kg m 3) bulk density (kg m 3) (kg m 3)
1 2 3 4 5 6
0.315 – 0.500 0.315 – 0.500 0.315 – 0.500 1.40 – 1.60 1.40 – 1.60 1.40 – 1.60
0.4075 0.4075 0.4075 1.50 1.50 1.50
2530 2202 2248 2248 2171
Sand g-Alumina Ceramsite Ceramsite Dried sludge Wood shreds
2530 1572 1510 1470 1149 550
1426 996 632 670 558 290
a
Determined by sieving. Determined by helium pycnometry. c Determined by mercury pycnometry. b
the sludge and wood particles. One of the major features of these two solids—at least from the standpoint of flow properties—is their fibrous nature. With the aid of a microscope, straight or twisted fibres were clearly visible at the particles’ surface. Irregular shapes, combined with the fibrous character of the sludge and wood particles, contrived to form arches in the container filled with either material. It is of interest to note that the arch only occurred at a certain position in the conical (not cylindrical) section of the container. As an effective remedy for unwanted arching proved to be a simple device knocking at times on the container wall in the vicinity of the troubled spot. In addition to the arching phenomena, the shape and surface characteristics of the particles also exert an important influence on rate and uniformity of the filling and the emptying of the feeding pockets. Our experimental experience indicates that it is the pocket emptying rather than its filling which is more susceptible to flow irregularities. Nevertheless, it was not observed that any material was retained in the pocket between fillings.
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means of calibrated rotameters. Repeated measurements of this important material parameter showed good reproducibility in the range from 5% to 7%. The measurements were carried out at a temperature of 20F0.5 jC. The density at ambient pressure and the viscosity of the filtered and dried air amounted to 1.202 kg m 3 and 1.81. 10 5 Pa s, respectively.
3. Results and discussion Using the ceramsite particles as a reference material in preliminary experiments, the influence of the frequency of doses was tested with the largest pocket (1.131 cm3). It was found out that there was no significant effect on the accuracy of feeding when the frequency of doses was slower than 0.417 s 1, i.e., less than 25 doses per minute. This suggests that the filling and emptying of the largest of the employed pockets took place quite rapidly. Mass of single doses was measured in further work at a frequency of sliding of 0.3667 s 1, i.e. one dose in 2.727 s. Respective deviations in mass of single doses from the mean value are plotted in Fig. 2 for the alumina, ceramsite, and dry sludge particles. As can be seen, there are considerable differences in the reproducibility of single doses of the respective materials. While the alumina particles behaved almost ideally, the mass of the sludge doses fluctuated within the F10% range. The maximum as well as the average deviations from the mean are presented in Table 3. As evident, the doses of nonfibrous materials (sand,
2.3. Procedure In the majority of the experimental runs, 20 single doses in a row were collected and weighed one at a time on an automatic scale with an accuracy of F0.1 mg. Attention was also given to determine a minimum, functional velocity of air (Utr) at which the metered doses of solids are reliably transported from the feeder as outlined in Fig. 1. Although such a quantity is not entirely unequivocal, it is much needed information in experimental work on the continuous fluid-bed units. The delivery pipe was equipped with a transparent glass tube of i.d. = 9 mm that made it possible to observe visually the behaviour of the metered particles in a stream of transporting gas. The value of the gas flow rate used was just sufficient to sustain transport of the metered solids, hence, is the minimum transport velocity at the feed rate. The flow rate of the transporting air, was measured by
Fig. 2. Variations in the mass of single doses of different materials: frequency of doses, f = 0.3667 s 1; ratio of hole diameter to particle size, dh/ d¯p = 8; volume of pocket, 1.131 cm3; mean particle size, d¯p = 1.5 mm; (o) dried sewage sludge particles, mean mass of a single dose w¯i = 0.2125 g; (.) alumina particles, w¯i = 0.9527 g; (x) ceramsite particles, w¯i = 0.5701 g.
M. Pohorˇely´ et al. / Powder Technology 142 (2004) 1–6
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Table 3 Variation in the mass of single doses of different solidsa Materialb
dh/dp
Mean mass of a single dose (g)
Rate of feeding (g h 1)
Maximum deviation (%)
Mean deviation (%)
1 2 3 4 5 6
29.4 29.4 29.4 8.00 8.00 8.00
1.3440 0.9461 0.5702 0.5751 0.2793 0.0781
1785.5 1257.5 743.6 752.6 280.5 119.7
0.441 0.226 1.370 4.36 13.32 31.59
0.246 0.135 0.784 1.900 7.057 15.055
a Based on 20 successive doses. The volume of the cylindrical pocket C (hole) amounted to 1.131 cm3 (dh = 12 mm, h = 10 mm); frequency of doses f = 0.3667 s 1. b Physical properties of the materials are given in Table 2: (1) sand, ¯dp = 0,408 mm; (2) g-alumina, d¯p = 0.408 mm; (3) ceramsite, d¯p = 0.408 mm; (4) ceramsite, d¯p = 1.5 mm; (5) dried sludge, d¯p = 1.5 mm; (6) wood shreds, d¯p = 1.5 mm.
alumina, and ceramsite) can be reproduced with very good accuracy. Understandably, flow intricacies of the fibrous particles (dry sludge solids and wood shreds) manifest themselves in somewhat more pronounced scatter of the doses. From potential sources of the variation in single doses one should consider, aside from general flow properties of the solids, also possible changes in the effective particle size in the course of the experiments due to abrasion (particularly at sludge particles) and/or agglomeration (especially with wood particles). The diameter of the holes or pockets in the slide plate varied from 8 to 12 mm, their depth (thickness of plate) ranged from 8 to 10 mm. Some products included particles as
Fig. 3. Maximum deviation of the mass of single doses as a function of ratio of hole (pocket) diameter to mean particle size: (x) ceramsite; (o) dried sewage sludge; d¯p = 1.5 mm.
Fig. 4. Rate of feeding as a function of pocket (chamber) volume for different materials: frequency of doses, f = 0.3667 s 1; (1) sand, d¯p = 0.408 mm; (2) g-alumina, d¯p = 0.408 mm; (3, 4) ceramsite, d¯p = 0.408 and 1.50 mm, respectively; (5) dried sludge, d¯p = 1.50 mm; (6) wood shreds, d¯p = 1.50 mm. Dimensions of the pockets are given in Table 1.
large as 0.407 and 1.50 mm. As illustrated in Fig. 3, the reproduction of the dose is significantly effected by the ratio of the hole diameter (dh) to mean particle size, d¯p. It appears that the ratio dh/d¯p should not be less than approximately 10.
Fig. 5. Minimum transport velocity of air for the metered particles as a function of particle density. The labelling of the data points is the same as in Fig. 4 and Table 2: (1) sand, d¯p = 0,408 mm; (2) g-alumina, d¯p = 0.408 mm; (3) ceramsite, d¯p = 0.408 mm; (4) ceramsite, d¯p = 1.50 mm; (5) dried sludge, d¯p = 1.50 mm; (6) wood shreds, d¯p = 1.50 mm.
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This conclusion aligns with a common ‘rule-of-thumb’ that an orifice should be at least eight times the particle size to avoid the formation of ‘structural arches’ over a circular opening. The volume of the feeding pocket varied from 0.402 to 1.131 cm3 and the particle density ranged from 550 (wood) to 2530 kg m 3 (sand). Accordingly, the rate of feeding in the experiments ranged from approximately 0.050 to about 1.80 kg h 1 as illustrated in Fig. 4. It is necessary to transport the metered doses of particles to a point of interest (e.g., into a burning fluidized bed) without any undesirable effects such as possible particle disintegration, abrasion, or distortion of the bed. Therefore, we also tried to determine for the respective materials, the minimum flow rates of air which ensure reliable pneumatic transport of the particles from the feeder. The minimum transport velocities required (Utr) ranged from approximately 1 to about 3 m s 1. As shown in Fig. 5, Utr is a strong function of the particle density. Using our straightforward procedure [2,3] for calculation of the terminal free-fall velocity (Ut) of a given nonspherical particle, we also assessed the steady-state, free-fall conditions of the tested particles in air (elutriation velocity). The relationship between the experimental Utr a the predicted Ut is not entirely clear. As the data plotted in Fig. 6 indicate, Utr is quite near Ut in the case of the smaller particles (d¯p = 0.407 mm). However, Utr/Ut = 0.3– 0.4 for the particles as large 1.5 mm. It cannot be ruled out that these results are also affected by differences in the velocity profiles developed in the transport tube.
Fig. 6. Ratio of minimum transport velocity to terminal free-fall velocity of the particles in air versus the Archimedes number. The labelling of the data points is the same as in Figs. 4 and 5 and in Table 2: (1) sand, d¯p = 0.408 mm; (2) g-alumina, d¯p = 0.408 mm; (3) ceramsite, d¯p = 0.408 mm; (4) ceramsite, d¯p = 1.50 mm; (5) dried sludge, d¯p = 1.50 mm; (6) wood shreds, d¯p = 1.50 mm.
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A feature of major importance to the reliable and repeatable performance of such a feeding device as ours is also the design of the flow channel that delivers the bulk material into the receiving chamber. To ensure smooth and reliable flow, the system should be designed with radius corners, over-pressure relief and minimum wall friction in the crucial converging section. Products that have intrinsic poor flow properties, such as fibrous and interlocking particulates or cohesive fine materials, usually need particular care and special equipment. 4. Conclusions Different particulate materials such as sand, alumina, ceramsite, dried sewage sludge, and wood shreds were employed to explore an operational capability and accuracy of a laboratory pneumatic—oscillating slide feeder. In the course of the whole work, the developed feeding device proved to be a reliable metering and transporting apparatus. The mass of a single dose of particles was on the order of 10 1 –100 g, depending upon the particle density and the volume of the feeding chamber (pocket). With a frequency of doses as large as 0.367 s 1, the rate of feeding of the above solids varied widely from 0.1 (wood shreds) to 1.8 kg h 1 (sand). Considerable differences were found in the reproducibility of single doses of the respective particulate materials, which reflects their different shape and surface characteristics. The smallest average deviations from the mean 0.14% and 0.25% were found with the isometric and smooth particles of alumina and sand, respectively. Fibrous particles of dried sewage sludge and wood shreds exhibited the most pronounced variations in the mass of single doses. The average deviations for these two solids amounted to 7% and 15%, respectively. To remove or minimize irregularities in the filling and (mainly) emptying of the cylindrical pockets, the ratio of the pocket diameter to mean particle size should be greater than 10. The minimum air velocities needed for pneumatic transport of the metered solids vary from 1 to 3 m s 1, this value being mainly influenced by the particle density. The particles of dry sludge and the wood shreds showed a pronounced tendency to arch in the conical section of the container. List of symbols Ar Archimedes number = (d¯p3gqf(qp qf))lf 2 dh diameter of cylindrical pocket/hole (mm) dp particle size determined by sieving (mm) d¯p mean particle size determined by sieving (mm) f frequency of doses (s 1) g acceleration due to gravity = 9.807 (m s 2) h height of cylindrical pocket/hole Ut terminal, steady-state velocity of an isolated particle in an infinite fluid
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Utr
w¯I
minimum velocity of air at which the metered particles are readily transported in horizontal tube (m s 1) mean mass of single dose (g)
Greek symbols lf fluid viscosity (kg (m s) 1, Pa s) qf fluid density (kg m 3) qp particle density (kg m 3)
Acknowledgements The authors wish to acknowledge the financial support provided for this work by GA AS CR (Grant A 407 2201) and
GA CR (Grant 203/02/0002). We are also very grateful to O. Trnka from The Institute of Chemical Process Fundamentals for contributing to this work with technical assistance.
References [1] M. Hartman, K. Svoboda, O. Trnka, Unsteady-state retention of sulfur dioxide in a fluidized bed with continual feeding of lime and limestone, Ind. Eng. Chem. Res. 30 (1991) 1855 – 1864. [2] M. Hartman, V. Vesely´, K. Svoboda, V. Havlı´n, Explicit relationships for the terminal velocity of spherical particles, Collect. Czechoslov. Chem. Commun. 55 (1990) 403 – 408. [3] M. Hartman, O. Trnka, K. Svoboda, Free settling of nonspherical particles, Ind. Eng. Chem. Res. 33 (1994) 1979 – 1983.