Measurement of Voidage of Sinter Ores in the Tubes - American

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Measurement of Voidage of Sinter Ores in the Tubes Penggang Zhang,* Jiuju Cai, Hui Dong, Zheng Cao, and Yiwei Yang State Environmental Protection Key Laboratory of Eco-Industry, Institute of Thermal and Environmental Engineering, School of Materials and Metallurgy, Northeastern University, Shenyang, 110819, PR China ABSTRACT: An industrial vertical tank (cost $200,000) was produced to recover the heat of sinter ores in our laboratory, the voidage along the height direction needs to be studied in order to determine the best height of sintering layer in it. In this paper, the bulk bed voidage of sinter ores in cylindrical polyvinylchloride tube (PVC-U) was measured by the cross-section image analysis method. The large-diameter tube was used to minimize the wall effects so that we could measure the bulk bed voidage in the center of the tube. The results show that the new method is useful, the voidage increases at lower height and fluctuates slightly between 0.36 and 0.43 in a stable stage, the voidage increases as particle size increases, decreases as the shape factor increases, and increases as the tube to particle diameter ratios decreases. These provide significant voidage data for the study of a vertical tank.

1. INTRODUCTION Energy consumption in the sintering process accounts for about 10% to 15% of the total energy consumption in iron and steel enterprises according to the statistical data, and it is the second consumer in the whole production line in China.1 Annular cooler and belt cooler are the common devices in cooling sinter ores from sintering machines, but there are many defects in the devices, such as serious air leakage and low efficiency of heat transfer in recovering the sensible heat of sinter ores. As a result, a new vertical tank has been produced, which adopts a new technology similar to the coke dry quenching (CDQ) technology, to recover the heat of the hot sinter ores.2 Two important problems in this new device are the gas flow pattern and the pressure drop of the air flowing through sintering layer, which determine the best height of the sintering layer. In all of the factors, voidage is the most important one affecting the airflow distribution and pressure drop in the vertical tank. However, it is difficult for us to measure the voidage in the vertical tank because it is so big and it needs more than 5 tons of sinter ores to pack in. Moreover, random packing, large particle size, nonuniform particles, shape diversity and large tube to particle diameter ratios make the bulk bed voidage in the vertical tank confusing. Therefore, we need to study the bulk bed voidage in an experimental way. Voidage and its distribution have been researched by many scholars. J. Hinkley et al.3 used a new technology to measure the voidage of a bed of sinter feed, in which kerosene was poured into the void of the bed, but the aim of the study they had done was to obtain the voidage distribution as the moisture content of sinter ore changed. From the study of J. Hinkley, we have the idea of using liquid for measuring the voidage of sinter ores. D. Toye et al.4 used an X-ray tomographic scanner as a tool for the analysis of the distribution solid phases in packed columns to obtain the value of the void fraction. J. Kubie5 researched the distribution of voidage in the wall regions of randomly packed beds of uniform spheres, the results of the analysis are in good agreement with available experimental data, a general equation relating the local voidage to the distribution of the spheres is derived and is combined with some simple © 2012 American Chemical Society

observations about the bed to develop a model of particle packing near containing walls. A. De Klerk6 studied the voidage variation in the near-wall region in a randomly packed bed of equal spheres. M. Suzuki et al.7 used X-ray microcomputed tomography to observe the powder packing structure and X-ray micro-CT scan to measure the void fraction profiles of a powder layer formed by piston compression and centrifugal packing. S. Sharma et al.8 used a water substitution method to determine bed voidage and typical bed voidage was observed within 0.1 and 0.19 m inner diameter columns, We decided to measure the bulk bed voidage which was useful in determining the best height of the sintering layer from their typical bed voidage. Roblee et al., 9 Benenati et al.,10 Govindarao, et al.,11 Latham et al.,12 Caulkin et al.,13 du Toit,14 True et al.,15 and Mueller16−18 have studied the void fraction in packed beds. Gotz et al.,19 Poux et al.,20 Zhiyao et al.,21 Farber et al.,22 Forsmo and Vuori,23 Nguyen et al.,24 and Pahk and Klinzing25 measured the voidage in packed beds. In this work, the bulk bed voidage of sinter ores in a cylindrical polyvinylchloride tube (PVC-U) was measured by the cross-section image analysis method. Sinter ores with natural size packing in the tube were selected as the main objects in the study, for they were mixed particle sizes and close to the particle distribution in the industrial vertical tank. The impact factors such as particle size, shape factors, shape diversity of sinter ores, and tube-to-particle diameter ratios of the bulk bed voidage along the height direction were analyzed .

2. EXPERIMENTS 2.1. Materials and Methods. Sinter ores in our experiments were obtained from Anshan Iron and Steel Group Corporation; the bulk density of sinter ores is 1700 kg/m3. Five types of sieves were used to filter sinter ores, and particle sizes ranging from 10 to 20 mm, 20 to 30 mm, and 30 to 40 mm, Received: Revised: Accepted: Published: 10165

March 26, 2012 June 19, 2012 July 10, 2012 July 10, 2012 dx.doi.org/10.1021/ie300801e | Ind. Eng. Chem. Res. 2012, 51, 10165−10171

Industrial & Engineering Chemistry Research

Article

while equivalent diameters (dp) of 15, 25, and 35 mm, respectively, were obtained. In addition, sinter ores with natural size were selected as the main objects in the study, because particles with natural size are identical to the industrial production and suitable for using in a blast furnace. An electronic digital vernier caliper was used to measure the wall thickness of tubes. Four identical stainless steel basins were prepared for soaking and washing the sinter ores. Two different diameters of tubes were used to simulate the vertical tank in which the sinter ores were packed, their sizes were 350 mm (D350) and 450 mm (D450). A tape measure was used to measure the height of tubes. A 14 million pixel camera was used to take the cross-section pictures of sinter ores in the tubes. Soluble starch was mixed in a stainless steel basin with water to a concentration of 30% to fill in the tube. We used starch solution as the background color in the study for these reasons: (1) starch solution has small viscosity and good fluidity; (2) there is no reaction between starch solution and sinter ores; (3) starch solution is cheap to get; (4) starch solution could not cover the surface of the sinter ores; (5) starch solution is opaque and the sinter ores under the layer of the cross-section can not be seen in the experiments. A hacksaw was used to cut off the tube to an appropriate length. Figure 1 is the schematic diagram of the experiment.

Table 2. Number of Experiments in the D450 height of tube (mm)

2.2. Experimental Procedure. Two groups of experiments were done in the study. The number of experiments in D350 is listed in Table 1, and those in D450 are listed in Table 2. In Table 1, A1−A21 experiments were done first, each experiment was done three times (sinter ores were packed in the tube randomly each time) to minimize the influence of the packing method and ensure the reliability of the experiments, the bulk bed voidage is the average value of voidage data from the three times. For example, the voidage of the A1 experiment

height of tube (mm) 250

300

350

400

450

500

15 25 35 natural size

A1 A8 A15 A22

A2 A9 A16 A23

A3 A10 A17 A24

A4 A11 A18 A25

A5 A12 A19 A26

A6 A13 A20 A27

A7 A14 A21 A28

250

300

350

400

450

500

15 25 35 natural size

B1 B8 B15 B22

B2 B9 B16 B23

B3 B10 B17 B24

B4 B11 B18 B25

B5 B12 B19 B26

B6 B13 B20 B27

B7 B14 B21 B28

3. RESULTS AND DISCUSSION The height to particle diameter ratios is large (H/dp > 5), and the tube to particle diameter ratios is also large (D/dp > 9) in the study, thus the wall effects were minimized and the bulk bed voidage of the cross-section could be measured more accurately. Figure 2 shows the grayscale images of the crosssection of sinter ores (dp = 25 mm) in 200 mm and 500 mm height in D350. In the lower height, for example, in 200 mm height, the bulk bed voidage seems to be smaller; in the higher location, the bulk bed voidage is larger. The voidage below all refers to the bulk bed voidage. Figure 3 shows the black and white images of the same crosssection mentioned in Figure 2. We could see the void between the particles more easily and clearly than in Figure 2. Each

Table 1. Number of Experiments in the D350

200

200

is the average value of the voidage data from three random packings. As sinter ores with natural size are mixed particle sizes and the main objects in the study, A22−A28 experiments were done five times for each experiment to exclude the impact of causal factors on packing. The experimental procedure of A1 is expressed as an example. Sinter ores (dp = 15 mm) were soaking for 48 h in the stainless steel basin for washing the particles in order to prevent the impurities from polluting the white background of experiments, and were packed in D350 randomly to 200 mm (H200) height (the value of the height has been marked on the outside of the tube). Starch solution was then poured into D350 to a height in which the gap between particles were filled in and without covering the top surface of the sinter ores. In our study, starch solution existed as a white background for about 6 s, and then starch would precipitate to the bottom of the tube gradually, therefore, it was necessary to shake the tube slightly from side to side so that the background color would recover and reamin for a while. Pictures of cross-section were taken as soon as possible. After that, the sinter ores were poured out, washed by water, and packed randomly in D350 again. New starch solution was poured into D350, and the A1 experiment was repeated another two times. The experiments in Table 2 were done in order to observe the influence of tube diameter on the bulk bed voidage, and they had the same experimental procedure as the A1 experiment did. All the pictures from the camera were imported into the image analysis software. The excess parts of the pictures were removed, only the circular cross-section remaining, and then the pictures were converted into grayscale of 8-bit for the next operation. After adjusting the threshold, all the grayscale images turned to black and white in color; black are the areas occupied by sinter ores, and the white are the areas occupied by starch solution. At last, the voidage of particles in images was analyzed by the software. Experiments were done in the study 406 times, and all the voidage data were numbered and saved for calculation and discussion.

Figure 1. Schematic diagram of the experiment in the study.

particle size dp (mm)

particle size dp (mm)

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dx.doi.org/10.1021/ie300801e | Ind. Eng. Chem. Res. 2012, 51, 10165−10171


Article

3.1. Voidage Distribution along the Height Direction in D350. 3.1.1. Voidage Distribution along the Height Direction as Particle Size Changes. Voidage distribution along the height direction in D350 is plotted with Origin 8.5 in Figure 4 and Figure 5. The tube to particle diameter ratios and the order of experiments with natural size are marked on the two diagrams. There are three curves in Figure 4, the curve (dp = 15) is the voidage for A1 to A7, the curve (dp = 25) is for A8 to A14 and the curve (dp=35) is for A15 to A21. Each point is the average value of the voidage data from three random packing, it can be seen from the three curves that voidage increases between H200 and H325, and reaches to constant in H350. In the curve (dp = 15), the voidage in H200 is 0.205, a small value for the nonuniform particles, as the height increases to H325, the voidage becomes 0.257 and remains between 0.255 and 0.270, which can be seen as a constant as the height continues to grow. The reasons for the changes are (1) in the lower place, the mass of sinter ores packing in the tube is lighter and the force direction of sinter ores under the cross-section is straight down, small particles in the upper layer would fall down and occupy the void of the lower layer, so, for the cross-section in H200, dense packing is the main packing way, thus the voidage is smaller; (2) when the height increases to H300, the force direction of sinter ores starts to change direction because the mass is heavier, including the vertical force and the side force, making the voidage change significantly in Figure 4. When the height increases to H325, most of the force direction is dispersed to the wall, so particles in the center of the tube cannot fall down to the lower layer, and the voidage remains as a constant in the curve. The other two curves in Figure 4 have the same voidage distribution. To investigate the influence of particle size on the voidage, the three curves are compared with each other. It can be seen that voidage increases as the dp of particle size increases. The reason is when large sinter ores are packed in the tube, the mass

Figure 2. Grayscale images of cross-section of sinter ores (dp = 25 mm) in 200 mm and 500 mm height in D350.

Figure 3. Black and white images of cross-section of sinter ores (dp = 25 mm) in 200 mm and 500 mm height in D350.

experiment in the study had the two processing stages (from Figure 2 to Figure 3) before its voidage was obtained. The voidage distribution could be understood literally through the images in Figure 3, but that was not enough for the bulk bed voidage distribution.

Figure 4. Voidage distribution along the height direction in different tube to particle diameter ratios in D350. 10167



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Figure 5. Voidage distribution of sinter ores with natural size along the height direction in D350.

Figure 6. Voidage distribution of sinter ores along the height direction in D450.

that in the curve dp = 25 is between 0.305 and 0.314, the and that in the curve dp = 35 is between 0.365 and 0.387. In Figure 5, the voidage (natural size) is between 0.378 and 0.405 (natural size-1), 0.378 and 0.401 (natural size-2), 0.372 and 0.411 (natural size-3), 0.379 and 0.406 (natural size-4), and 0.370 and 0.402 (natural size-5). The stable voidage in each curve is calculated through taking their arithmetic mean value. The value of voidage is 0.2586 in the curve dp = 15, 0.3117 in the curve dp = 25, 0.3799 in the curve dp = 35 and 0.3939 in the curves (natural size), then the equivalent diameter of natural size in the study can be calculated to be 42 mm by interpolation according to the prediction from Figure 4. This means that sinter ores with natural size packing in the tube can be recognized as about 42 mm in average particle size. Also, it can be seen from Figure 4 and Figure 5 that voidage increases as the tube to particle diameter ratios decreases, this is

in the same height is heavier than the small ones, and the force direction has a component on wall so that few sinter ores could fall down to the lower layer, and the voidage becomes larger. Then it can be predicted that when the equivalent diameter of particle size is more than 35 mm, its curve will have the same distribution and be over the curve (dp = 35) in Figure 4. 3.1.2. Influencing Factors in Voidage Distribution of Natural Size along the Height Direction. Figure 5 shows the voidage distribution of sinter ores with natural size, five curves for the voidage of A21 to A28 experiments represent the experimental results of five times. The voidage has the same distribution in each time despite of the random packing, and the distribution is similar to the one in Figure 4. The difference is that the average value of the voidage in Figure 5 is larger than the voidage of the curve dp = 35 mm in Figure 4. In Figure 4, the voidage in the curve dp = 15 is between 0.255 and 0.270, 10168



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Figure 7. Comparison between the voidage in D350 and the voidage by the mass method.

quite different from the results of J. Theuerkauf et al.,26 F. A. Schneider,27 and N. Zobel et al.28 in the chemical area, in which the voidage decreases as D/dp decreases. In the chemical area, spherical particles form a point contact at the wall in which the voidage is the largest, and the voidage near the wall has influence on the average voidage, making it become larger in whole. In our study, sinter ores are nonspherical, so particles may form a point, a face, or a volume contact at the wall. When face or volume contact occurs, the voidage near the wall is very small which makes the voidage decrease on the whole. The extent of reduction depends on the shape diversity of sinter ores in the experiments, little shape diversity results in little reduction in the value of voidage. Sinter ores are nonspherical and nonuniform particles, yet so many sinter ores are used in the study that it is impossible to measure and calculate the shape factor of sinter ores one by one. However, we used glass spheres with 15 mm diameter as the comparative experiments, and found that the larger the shape factor is (similarity between spheres and sinter ores), the smaller the voidage of sinter ores is. It can be concluded from the Figure 5 that when the diameter of tube is certain, the voidage of sinter ores keeps unchanged between H200 and H240, increases between H250 to H325, and fluctuates quite slightly after H350, which can be recognized as a constant in the study. The equivalent diameter of natural size can be calculated to be about 42 mm. These help us to evaluate the particles distribution along the height direction in the industrial situation and understand the main particle size packing in the vertical tank. As the sinter ores with natural size are the main objects in the study, the result from Figure 5 is one major conclusion in this work. 3.2. Voidage Distribution along the Height Direction in D450. 3.2.1. The Distribution of Voidage along the Height Direction as D and D/dp Change. Figure 6 shows the voidage distribution of sinter ores along the height direction in D450. The eight curves are for B1 to B28 experiments. The curves (natural size) are over the curve dp = 35. The curve dp = 15 is the lowest of all, which is similar with that in Figure 4 and

Figure 5. That is to say, when the diameter of the tube is large enough to ignore the wall effects, then the voidage along the height direction has the similar distribution despite of the shape and the particle size, and the experimental voidage distribution is applicable to the industrial vertical tank. A comparison of Figure 4, Figure 5, and Figure 6 shows that the difference is that the curves in Figure 6 are more stable than the ones in Figure 4 and Figure 5 between H350 to H500. The explanation is that when tube-to-particle diameter ratios are large (D/dp = 13, D/dp = 18 and D/dp = 30, much larger than D/dp = 8, if the D/dp < 8, then wall effects need to be discussed) enough, the voidage along the height fluctuates in a small range, the larger the tube to particle diameter ratios is, the smaller is the range the voidage fluctuates in. For example, the curve (D/dp = 30) fluctuates less than the curves (D/dp = 18 and D/dp = 13). This conclusion further validates the key role of our experiments on the industrial vertical tank. The diameter of cooling section in the vertical tank is 1100 mm and its height is 2100 mm, the tube to particle diameter ratios in it are D/dp = 32, D/dp = 44, and D/dp = 74. Wall effects are ignored in the vertical tank so that every cross-section in a certain height has only one voidage (bulk bed voidage), and the best height of the sintering layer can be determined to be at least 350 mm. 3.3. Validation for the Cross-Section Image Analysis Method. Voidage of sinter ores with natural size was measured in the study, however, errors still exist in the experiments because of the random packing. To cross-check the data of voidage from the images analysis, a mass method was used to measure the voidage of sinter ores with natural size. Figure 7 shows the comparison of the two different ways. One curve is for voidage by mass method, the other is the average value of voidage from Figure 5. It can be seen in Figure 7 that voidage by cross-section image analysis has a better agreement with the results from the mass method, although voidage by the mass method fluctuates relatively large between H350 and H500. The curve (mass method) is below the curve (natural sizeD350), for voidage by mass is the volume voidage. Take the volume of H300 as an example, the voidage of the volume of 10169



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tank. The diameter of cooling section in the vertical tank is 1100 mm, the tube to particle diameter ratios in it are D/dp = 32, D/dp = 44 and D/dp = 74. Wall effects are ignored in the vertical tank so that every cross-section in a certain height has only one voidage (bulk bed voidage), and the best height of the sintering layer can be determined to be at least 350 mm. In fact, the best height of sintering layer can be determined finally only when the pressure drop in the vertical tank is mastered, and this is our next work to be done in the program.

H300 can be regarded approximately as the average voidage of all the cross sections below H300, the voidage of the crosssection of H300 is larger than the voidage of the cross-section of H200 in the above discussions, so the voidage of the crosssection of H300 is larger than the voidage of the volume of H300. The voidage of the cross-section is a constant between H350 and H500, so the voidage by mass also keeps to a constant. From the discussion on Figure 7, it can be concluded that the cross-section image analysis is a useful and accurate method to measure the voidage in the tube, providing the significant voidage data for the industrial vertical tank.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +8613555781039. +86-024-83672218. Fax: +86-024-83672218.

4. CONCLUSIONS This work describes a new method for measuring the bulk bed voidage of sinter ores packing in the tube, and the voidage distribution along the height direction is given with its influencing factors. Large tube-to-particle diameter ratios (D/ dp > 8) and large height-to-particle size ratios (H/dp > 4) are used in all the experiments to minimize the wall effects in the study; all the results in the study are based on this condition. The key findings from this work can be summarized as follows: (1) Cross-section image analysis is a new and useful method for measuring the bulk bed voidage of sinter ores packing randomly in the tube. Mass method is used to crosscheck the voidage data from the cross-section image analysis method, and the two methods have a great agreement in voidage distribution. (2) The bulk bed voidage increases in the lower height (H200 to H325) and gets close to a constant when the height reaches to H350. The force direction of sinter ores changes from straight down to the wall side as height increases, so dense packing is the main packing way in the lower place (H200 to H325) and loose packing is the main packing way after H350. (3) The bulk bed voidage increases as particle size increases if the height keeps unchanged, and the voidage along the height direction has the same distribution if the packed tube is certain. (4) The larger the shape factor of sinter ores is (similarity between spheres and sinter ores), the smaller the voidage in the same height is; the voidage increases as the tube to particle diameter ratios (D/dp) decreases. Sinter ores packing in the tube are nonspherical, so particles may form a point, a face, or a volume contact at the wall. When face or volume contact occurs, the voidage near wall is very small which makes the voidage decrease on the whole. The extent of reduction depends on the shape diversity of sinter ores in the experiments; little shape diversity results in little reduction in the value of voidage. (5) When the tube-to-particle diameter ratio is large enough, the voidage along the height fluctuates in a small range between H350 and H500. The larger the tube-to-particle diameter ratio is, the smaller is range the voidage fluctuates in. (6) The equivalent diameter (dp) of natural size can be calculated to be about 42 mm, and the voidage (natural size) fluctuates between 0.36 and 0.43 in the stable stage (H350 to H500). All the findings from this work help us to evaluate the particle distribution along the height direction in the industrial situation and understand the main particle size in the vertical

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National High Technology Research and Development Program (863 Program for short) of China (Grant No. 2009AA05Z215) and the Natural Science Foundation of Liaoning Province (Grant No. 20102069). Thanks to Cheng Mingming from Tsinghua University for giving some suggestions on dealing with the images.

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REFERENCES

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Measurement of Voidage of Sinter Ores in the Tubes - American

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