Particle attrition due to rotary valve feeder in a pneumatic conveying system: electrostatics and mechanical characteristics.

Zhang, Yan ; Wang, Chi-Hwa

Particle attrition in the rotary valve of a pneumatic conveying system was studied. The relationship between physical properties of polymer granules and attrition behaviour was compared between intact (fresh/unused) and attrited particles. It was observed that attrited particles become more breakable and have lower flowability. Experiments were conducted in a rotary valve and pneumatic conveying system separately; attrition in each case could be described by the Gwyn function. The influence of particle attrition on electrostatic characteristics was examined. Charge density of attrited particles was higher than that of intact ones, but induced current showed a reverse trend.

On a etudie l'attrition des particules dans une vanne rotative d'un convoyeur pneumatique. On a compare pour des particules intactes (fraiches/non utilisees) et des particules usees, la relation entre les proprietes physiques de granules de polymeres et le comportement d'attrition. On a constate que les particules usees devenaient plus cassables et avaient une plus faible flottabilite. Des experiences ont ete menees separement dans une vanne rotative et un convoyeur pneumatique; dans les deux cas l'attrition pourrait etre decrite par la fonction de Gwyn. L'influence de l'attrition des particules sur les caracteristiques electrostatiques a ete examinee. La masse volumique de charge des particules usees est plus elevee que celles des particules intactes, mais un courant induit montre une tendance inverse.

Keywords: attrition, rotary valve, pneumatic conveying, electrostatic

INTRODUCTION

Particle attrition is a common phenomenon in many industries. It can result in loss of expensive materials and environmental pollution; however, it is also helpful towards the removal of impermeable components on the surface of reacting particles. Some attrition is inevitable and has been reported in a wide range of processes and industries (Bemrose and Bridgwater, 1987).

The rotary valve is widely used in particulate conveying systems (Wypych et al., 2001). However, the conditions under which particles are damaged in such a valve are not well understood. In an earlier study on charge generation in pneumatic conveying systems (Yao et al., 2004), it was found that when airflow rate was low, particles were not transported quickly and piled up in the valve feeder. Under the action of the blades of the rotary valve, particles would be extruded and attrited. As such, breakage of particles within a rotary valve is the main focus of the present work. The rate of attrition of particles could be expressed by a simple function of initial diameter and time (Gwyn, 1969). The initial rate is a function of initial diameter, whereas the decrease in attrition rate of a given size depends only on time. This equation has been verified by laboratory and commercial data. Paramanathan and Bridgwater developed an annular cell that enables solid attrition to be studied (Paramanathan and Bridgwater, 1983a) and investigated the material behaviour and kinetics of attrition (Paramanathan and Bridgwater, 1983b). Neil and Bridgwater also tested granular materials in an annular attrition cell (Neil and Bridgwater, 1994, 1999), a fluidized bed and a screw pugmill (Neil and Bridgwater, 1999). For each experiment, the attrition was characterized using the Gwyn function effectively.

Bemrose and Bridgwater have indicated attrition was affected by many variables, such as properties of the particles (e.g. size, shape, surface, porosity, hardness and cracks), and categorized mechanical tests as single-particle or multi-particle type; whereas, physical properties of particles would be changed mostly after attrition (Bemrose and Bridgwater, 1987). Furthermore, flow properties are sensitive to particle size and size distribution. Attrition also affects the flow behaviour of particles, such as solid flow rate, pipe wall abrasion and the electrostatic charge generation characteristics. The abrasion of pipelines in pneumatic conveying systems can be as significant as the damage inflicted on the particles. Soo observed that pipeline erosion is most severe around pipe bends (Soo, 1980). Such phenomenon arises due to collisions or friction between particles and pipe wall. Moreover, due to collisions or friction with a different material surface, solid particles in pneumatic conveying systems have the propensity to acquire electrostatic charges (Masuda et al., 1976). A previous experimental work (Inculet et al., 1997) demonstrated that the elbows in a pipeline are the major source of triboelectrification of particles in pneumatic conveying systems and these are also important sites of pipe abrasion. The electrostatic charge generation characteristics of granular materials in a pneumatic conveying system have been investigated (Zhu et al., 2004; Yao et al., 2004). This paper also reports on the effect of particle attrition on electrostatics behaviour and shows the relationship between particle size and electrostatic charge generation characteristics. Smeltzer and colleagues studied this phenomenon occurring during pneumatic transport and found that smaller particles had greater electrostatic effects (Smeltzer et al., 1982). Electrostatic characteristics have been applied to measure the mass flow rate, but there have been few investigations on the relationship between electrostatics and solid flow rate (Mathur and Klinzing, 1984).

This work aims to find the attrition characteristics of granular materials in a rotary valve. Physical properties of the granular materials before and after attrition are analyzed and flow behaviours of attrited granules in a pneumatic conveying system, including solid flow rate, abrasion of the pipe wall and electrostatic charge generation characteristics, are studied as well.

MATERIAL AND METHOD

Pneumatic Conveying System and Rotary Valve

The experimental set-up used in the present study was modified from our previous pneumatic conveying system (Yao et al., 2004). The schematic diagram of the modified system is given in Figure 1 and the experimental conditions are listed in Table 1. Solid particles were introduced into the rotary valve (General Resource Corp., Hopkins, Minnesota) (Figure 2) and entrained by air flowing from the compressor. The inner diameter (ID) of the pipe was 40 mm, and the length of the vertical pipe section between two smooth 90[degrees] elbows (R/r = 2) was about 2.97 m, while the horizontal section was about 4.12 m in length. The conveying pipe was made of transparent polyvinyl chloride (PVC) material and had a wall thickness of 5 mm. The entire configuration was held in position using metal castings and various pipe segments joined by connectors. Two types of particles, polyvinyl chloride (PVC) and polypropylene (PP) granules, were used throughout the experiments.

[FIGURES 1-2 OMITTED]

Based on the type of feeder, rotary valve can be classified as the following three types: (1) drop-through rotary valve, (2) side-entry rotary valve, and (3) blow-through rotary valve. The blow-through rotary valve used in this study is commonly seen in industrial-scale pneumatic conveying systems. This style of rotary valve has two ports on the sides near the bottom. The conveying line connects directly to the valve, with the air stream flowing through the rotor pockets. Figure 2 shows the schematic diagram of rotary valve. The rotary valve has 8 pockets with a clearance of 1 mm. The speed of rotary valve was fixed at 25 rpm (5[pi]/6 s-1). The time taken for one pocket to pass through the inlet orifice (40 mm ID) could be calculated from the rolling speed and the sum of the total angular displacement of the pocket ([pi]/4), and the orifice size ([pi]/8). This gives a time interval of 0.45 s. The average weight of solid input to each pocket could also be calculated by multiplying the solid flow rate by 0.45 s. The featured capacity of one pocket is 1.05 litre and the pocket filling percentage is between 1.00 and 1.46%.

Physical Properties of Particles

In order to investigate the attrition characteristics of particles, it was deemed necessary to study their physical properties. In the present section, particle size and shape, load-strain characteristics, flow-time and internal friction angle of PVC and PP samples were measured.

Particle size and shape

Particle size and shape are the basic properties for particulate materials used in this investigation. They were measured simultaneously using a particle size/shape analyzer (Analytical Technologies Pte Ltd, Singapore).

Single particle compression test

Single particle compression test was performed to measure the relationship between load and strain of single particle. Such data is important to understand the attrition process in rotary valve. A single particle was put on the lower support of the Compression Tester (AGS-10kNG, Shimadzu, Kyoto, Japan), and a continuous load was applied until permanent deformation of the particle was achieved. Load and deformation data were recorded during the process.

Granulate test

Flowability is another important parameter for characterizing the physical property of particles. It is supposed to be related to attrition process in the whole pneumatic conveying system. It can be measured by two methods: granulate test and shear strength test. The Granulate Tester (GT/GTB, ERWEKA, Heusenstamm, Germany) was used to determine the flowability of powders and granules as defined by EP (European Pharmacopoeia) by measuring the flow time of a sample of such material. In a typical test procedure, a known mass of particles was allowed to flow out of a hopper with a 25 mm diameter nozzle into a collecting vessel in the Granulate Tester and the total time taken was recorded.

Shear strength test

The Consolidation Isotropic Undrained (CIU) test was used to characterize shear strength. The internal friction angle of polymer particles was measured using a Triaxial Tester (Wykeham Farrance, Slough, England). This parameter determines the friction between particles and can be used to explain the flowability of particles as mentioned above. Each test was applied to 3 specimens, consisting of particles and water in the form of right cylinders of nominal diameter, D, 50 mm, and height, H, approximately equal to twice the diameter. The CIU experiment procedure consisted of four major steps: Firstly, the frozen specimen was prepared; secondly, the specimen in the triaxial apparatus was set up; thirdly, the specimen was allowed to thaw, saturate and consolidate; lastly, the specimen was compressed and the experiment data were read from a computer.

Particle Size Variation Due to Attrition

The procedure for determining particle size distribution following granular attrition was as follows. Firstly, a sample of particles was allowed to undergo attrition for a given time either in a rotary valve feeder or pneumatic conveying system. Secondly, a weight-size analysis on representative samples of the attrited particles was then performed by sieving. Thirdly, the procedure was then repeated for other attrition times to track changes in particle size distribution with respect to attrition time. In this experiment, the sieves were made of metal and grounded using electrical wires. This arrangement ensured that charge generated during shifting would be removed and the electrostatic problem would hardly lead to the inaccuracy of particle size measurements.

Electrostatic Characteristics

During the pneumatic conveying process, frictional contacts between the solid particles and pipe wall generated electrostatic charges. Induced current and particle charge density tests were conducted on the horizontal pipe. These were measured during the pneumatic transport of the four types of particles (intact/ attrited PP and PVC samples) separately through the conveying system using a digital Electrometer (ADVANTEST R8252, Tokyo, Japan) and Faraday Cage (ADVANTEST TR8031, Tokyo, Japan), respectively. In the present work, the term "intact particle" refers to fresh/unused material. The current induced was measured as a function of time by wrapping an aluminum foil sheet tightly over the outer wall of the PVC pipe (labelled 7 in Figure 1). A coaxial line was connected between the high input end of an electrometer and the outer surface of the aluminum foil sheet. A polymer film was then wrapped tightly over the aluminum foil sheet to separate this sheet from another aluminum foil sheet whose external surface was connected to the low input end of the coaxial cable. Subsequently, this external layer of aluminum foil sheet was grounded and used as an extra electrical shield. Experiment data in the form of digital readings were stored in a computer at intervals of 0.5 s. Additionally, the particle charge density was measured using a Faraday Cage at the horizontal segment (labelled 8 in Figure 1). Charged particles were placed into a metal enclosure that was isolated electrically and the amount of charges present was then measured. The mass of particles collected in the cage was measured using an electronic balance to an accuracy of [10.sup.-4] g and the charge density of particles was calculated according to the procedure by Yao et al. (2004).

Pipe Wall Abrasion

The friction and collisions between particles and pipe wall would also lead to pipe abrasion in a conveying system. The material of all pipes used in the present pneumatic conveying system was polyvinyl chloride (PVC) except at three test sections (referred to as the bend, vertical and horizontal sections and shown in Figure 1) where acrylate copolymer film (0.12 mm) was rolled into a cylinder and attached to the inner surface of the pipe wall. Four types of particles (intact/attrited PVC and intact/attrited PP samples) were conveyed separately through the system at an airflow rate of 1600 L/min for about 3 h. The extent of abrasion of the polymer film resulting from frictional forces between moving particles and the film was quantified by measurements of the mass of the film before and after each experiment.

Solid Flow Rate in Pneumatic Conveying System

The present study makes use of electrostatic characteristics of particle to measure the solid flow rate. The solid flow rate was obtained by measuring the induced current with valve 14 shut (Figure 1) so as to allow particles to be conveyed through the system once only. If particles did not flow through the pipe, there was no charge generated, and the output signal of the induced current was close to nil; on the other hand, if particles passed through the pipe, induced current was detected with a nonzero value. Therefore, the time interval for one circulation could be obtained easily by observing the signal of induced current; accordingly, solid flow rate was calculated as the ratio of total mass of granules to time in one circulation. The validity of such a method had been proven using a similar approach (Mathur and Klinzing, 1984) to generate solid flow rate readings comparable to those collected from load cells.

RESULTS AND DISCUSSION

Physical Properties of Particles and Their Variations by Attrition

In this section, attrition effects on different materials and particle behaviours in pneumatic conveying systems are described and compared.

Particle size and shape

In the present study, intact and attrited particles are defined according to their size ranges shown in Table 2. Average size and standard deviation of four types of particles are also described, respectively, in Table 2. It is observed that intact particles have a narrow size distribution while attrited particles have a wider size distribution and all kinds of shapes, for both PVC and PP samples. Undoubtedly, the average size of particles is diminishing with the occurrence of attrition.

In order to study the dependence of particle attrition and flowability on particle shape, a "shape factor" given below is introduced to describe the granular shapes (Watano and Miyanami, 1995),

[empty set] = [P.sup.2] / 4[pi] * S (1)

where S is the projected area and P is the perimeter of this projected area. The shape factors for the samples shown in Table 2 were calculated from the particle size/shape analyzer. It is seen that shape factors of attrited particles have a wide spreading. Yao et al. (2006) demonstrated that the shape factors of particles are significantly more scattered for products from the lower-sized range and this implies that the shapes of these smaller granules are made up of complex combinations of different geometries. From the appearance of attrited particles shown in Figure 3, it is apparent that the mechanism of attrition in the rotary valve is breakage and not abrasion. The latter will usually result in only minute losses in surfaces, edges and corners of particles and thus can be excluded. Besides, heat influence on change of particle shape is not obvious because experiments were operated at room temperature and the process temperature everywhere in the pneumatic conveying system is much lower than the heat deflection temperatures (HDT) of PP and PVC (Table 1).

[FIGURE 3 OMITTED]

Single particle compression test

It is known that breakage occurs when a material is stressed or compressed beyond its failure stress and single particle compression breakage strength varies with size and material properties. However, the relationship between compression test and attrition was seldom investigated. Willem and colleagues compared results from constant strain rate tests, controlled force tests and double spring compression tests where the breakage was acquired and studied in details according to force--displacement curves (Willem et al., 2003). Gorham and co-workers studied damage on PMMA spheres caused by impact and compression and observed a brittle--elastic manner on impact and plastic deformation in compression test (Gorham et al., 2003). Shipway and Hutchings presented a theoretical and experimental study of the fracture of single glass sphere between opposed platens by uniaxial compression, which is relevant to the breakage attrition of particles in granule transport, handling, processing and comminution (Shipway and Hutchings, 1993). The source of breakage of particles is mechanical action between particles and another body. In our experiments, it is the shearing actions of the blades in the rotary valve that leads to fragmentation of particles. The direction of contact, relative velocities and contact stresses between the particles and the blades, shapes of particles, and masses of samples are all important to determine the characteristics of breakage. Therefore, it is necessary to measure the relationship between load and strain of single particle.

Figure 4 shows the change in shape of particle before and after the compression experiment carried out according to the procedure described in the Materials and Method section above. Both PVC cylinder and PP bead were compressed to the shape of a cake. The load-strain graph is also shown in Figure 4 where strain, [epsilon], is defined as the change in magnitude of a reference dimension, expressed as a ratio of axial quantitative change ([DELTA]h) to the original axial height of one particle ([h.sub.0]). The result of the compression test for intact particles is compared to that for attrited particles, respectively, for PVC and PP in Figure 4. Apparently, the curve for attrited particles appears gentler than that of intact ones, both for PP and PVC, showing that the maximum load that attrited particles can withstand decreases as the size of particle decreases. This seems to imply that once a particle is broken, it would be attrited much more easily even under a smaller applied stress. Antonyuk et al. (2005) demonstrated that the highest local tensile stress is generated at the crack release zones in the granules, such as pores and structural defects, and the fracture initiates from this zone. Previously Willem et al. (2003) also reported that the breakage could occur at lower forces due to particular fatigue shown in fatigue curves where the percentage of broken particles were plotted as a function of the compression stress and the number of repeated cycles.

[FIGURE 4 OMITTED]

Granulate test and solid flow rate in pneumatic conveying systems

In pneumatic conveying systems, attrition may be related to the flowability of the impacting bodies. In our experiment, attrition is more likely to occur at low flowability because particles with poor flowability usually accumulate in the rotary valve and are then subjected to the shearing actions of the blades in the valve. In all granulate tests performed, the flow time was calculated according to Equation (2):

Flow Time = measured test time in seconds / weighed sample in grams (2)

Flowability usually depends primarily on particle size, shape and bulk density. Table 2 shows that the flow time increased slightly after the occurrence of particle attrition. This may be due to a strong dependence of flow time on the shape of particles, as the observed widespread shape factor corresponds very well with the fact that attrited particles become much more irregular in shape than original particles. Many researchers (Teunou et al., 1999 and Fitzpatrick et al., 2004) have demonstrated that the flowability of particles reduced with decreasing granular size. Fitzpatrick et al. (2004) investigated 13 food powders and showed that flowability tended to reduce with decreasing particle size. They explained that as particle size decreased, particle surface area per unit mass increased, correspondingly, the greater surface cohesive forces led to more cohesive flow. They also found that the flowability was influenced by the combination of particle size and moisture content and depended only weakly with increasing densities. In our experiment, the particle density of PP is lower than that of PVC (Table 1). Upon comparing the two different kinds of particles, the flowability of PP beads is much better than that of PVC cylinders for both intact and attrited samples. However, this may be attributed more to the shape than density of particles. It is noted that PP beads are ellipsoidal but PVC particles are cylindrical.

In the pneumatic conveying system, the flowability of particles would affect the solid flow rate. When airflow rate was fixed at 1600 L/min, three different cases may arise according to the level of feed valve opening: 75% opening, 100% opening and flood-fed condition (valve removed from the inlet of rotary valve and replaced by a 75 mm ID pipe). Particles with poor flowability, such as PVC samples, would not move smoothly in the systems, and would tend to stick on the hopper and be locked in the rotary valve, even with a valve opening of up to 100%. They can be transported smoothly only in a flood-fed condition, for both intact and attrited samples, giving solid flow rates of 37.62 [+ or -] 4.76 g/s and 37.49 [+ or -] 6.79 g/s, respectively. In contrast, PP samples would generate a strong electrostatic discharge, making the experiment dangerous in this case. Therefore, it was deemed more appropriate for experiments involving both intact and attrited PP to be performed at 75% feed valve opening with respective solid flow rates of 40.67 [+ or -] 3.29 g/s and 40.05 [+ or -] 0.63 g/s. These optimal operating conditions for both PP and PVC samples and the corresponding solid flow rates obtained are listed in Table 3. Therefore, it is demonstrated that the better the flowability of particles, the greater the solid flow rate. PP samples have better flowability than PVC samples, and intact particles show better flowability than attrited ones. The above observation is consistent with the flowability of particles measured in the granulate test.

Shear strength test

Internal friction angle is a measure of granular friction between the particle layers. This parameter is important in designing pneumatic conveying systems as particles with higher internal friction angles can form larger granular piles. In this study, it would be used to characterize the flow behaviour of particles and differences between intact and attrited particles. This is because internal friction and cohesion are both surface interactions to resist powder flow (Fitzpatrick et al., 2004).

From the experimental data obtained, a Mohr's circle was plotted as shown in Figure 5. It represents the state of effective stress at failure; the diameter is defined by points, which represent the major and minor effective principal stress at failure, respectively (BS1377, British Standards Institution). By drawing a tangent line to the Mohr's circles, the internal friction angle, [alpha], of the four kinds of particles are obtained as shown in figures. The friction coefficients (tan?) of the granules are listed in Table 2. The smaller values of internal friction angle and friction coefficient of intact PP samples show that the flowability of PP granule is better than that of PVC, which agrees well with the granulate test result. This will also imply that PVC granules can form larger piles than PP granules. As for the attrition effect, it is observed that the internal friction angles increase after particle attrition, both for PP and PVC samples. This shows that friction among particles is enhanced by attrition, leading to the reduced flowability of attrited particles. For illustration purpose, all physical properties of particles, as well as their variation due to attrition, are summarized in Table 2.

[FIGURE 5 OMITTED]

Particle Attrition Due to Rotary Valve in Pneumatic Conveying Systems

As discussed in the previous section, changes in properties of granulate materials resulting from attrition are important. Particle sizes become smaller in the process of attrition due to abrasion of fine powders from larger particles and formation of larger fragments due to breakage. In the experiment of attrition solely in rotary valve, 939 g PP and 745 g PVC were put into the valve cavity (with the labelled height of 70 cm in Figure 2 (a)) and were allowed to undergo attrition in the rotary valve for different time intervals. These particles were piled up in the valve cavity and the height of pile just touched the blades of valve. If the loading of particles was too low or too high, particles would not be in contact with the blades or would block the running of blades, respectively, and make the experiment dangerous. Similarly, in order to keep the experiment results consistent, the weight of particles was fixed in all tests of attrition due to rotary valve in the conveying system. In this part of the study, intact particles will first undergo breakage to form fragments and then abrasion to produce a finely dispersed product in the rotary valve.

The rotary valve was operated either as a stand-alone device or as part of a pneumatic conveying system. Granular attrition normally occurs at the exit of the rotary valve in the former situation depicted as attrition II in Figure 2 (a), while in the latter, from observations made in the present study, attrition may happen mainly at the entrance (shown as attrition I in Figure 2(a)). When particles were flowing down from the hopper to the rotor pocket, they would first experience the extrusion by the rolling vanes at the entrance of valve. As particles were carried all the way to the exit of valve, air with high flow rate rapidly conveyed particles to the pipe system, thus particles accumulation and attrition at the exit of valve could be minimized. In particular, the size of space between the vanes and casing wall at the entrance and exit varies from a maximum to a minimum during one cycle of rotation. When granules with poor flowability, such as PVC samples, are passed into the rotary valve, the accumulation of particles would result in severe attrition of the granular materials at these positions. Attrition is always accompanied with loud noises and violent shaking of the rotary valve, which plays an important role in causing granular attrition in a typical pneumatic conveying system. To some extent, granular attrition may also be brought about by direct impacts between the granules and vanes of the rotary valve or other granules (Konami et al., 2002).

Attrition solely in rotary valve

The PP or PVC samples after attrition can be divided into two or three sieve cuts. The particle size distribution after attrition in the rotary valve is presented in Figure 6. Here, mass fraction (d[empty set]) is defined by the ratio of mass in one size range to the total mass of particulate samples. It is observed that the mass of both PVC and PP samples with the initial average size of intact particles decrease with time. In contrast, those of other sieve cuts increase slowly. Moreover, the speed of such variation for PVC samples is faster than that for PP samples. Thus, it seems that PVC is more attritable than PP due to lower flowability.

[FIGURE 6 OMITTED]

Attrition by rotary valve in pneumatic conveying systems

Figure 7 shows the size distribution resulting from attrition occurring in a pneumatic conveying system. The same trend, with regards to decrease in amount of particles having the initial average size of intact particles and slow increase in that of particle fragments, is observed for the PVC samples. Remarkably, the size distribution of particles after 1080 min of attrition is very similar to that of particles after 360 min. The quantities of particles with initial size decrease minimally from 63.0% to 61.4% during the period from the 360th minute to the 1080th minute, while those of broken particles with smaller size range basically remain the same in these 720 min. It seems that 360 min is the approximate time scale for the attrition process to be completed and a longer duration beyond this time has no obvious effect on the particle size distribution (Figure 7 (a)).

[FIGURE 7 OMITTED]

As for PP samples the particle size distribution at different times exhibit minimal change as shown in Figure 7 (b). At the end of 360 min, the percentage of intact PP particles is 83.7% while that of PVC particles is only 63.0%. Moreover, very little fine powders of PP (only 0.03%) are generated even after a long attrition time. It may thus be concluded that PP particles are more difficult to be attrited than PVC, as indicated previously.

Compared to Figure 6, the resulting size distribution after attrition in a pneumatic conveying system in Figure 7 is quite similar to that in a rotary valve operating alone. In the former, both PVC and PP particles pile up in the rotary valve for only one third of time in each circulation through the system. The effective time for particle attrition by the rotary valve is calculated to be around 60 min in each 180 min operation and the remaining time (120 min) is the duration of particle transport in systems. That is the reason why similar particle size distributions are obtained despite longer attrition times in the latter. The above reasoning demonstrates that the attrition process in pneumatic conveying system is mostly attributed to rotary valve rather than conveying pipe.

When granules enter the pocket between the two vanes of the running rotary valve, a granular pile is built up (Figure 8). Once the granular pile reaches a certain height, granular attrition occurs between the vanes and casing wall at the entrance of the rotary valve. Many researchers have studied such problems involving granular pile (Komatsu et al., 2001) and associated conditions (Mueth, 2000). The angle of repose of such granular piles is determined by several factors, such as particle size (Carstensen and Chan, 1976), sliding friction coefficient and rolling friction coefficient (Zhou et al., 2001) etc. Sliding or rolling friction coefficients are affected by the shape and type of material of particles, so that these would also affect indirectly the attrition process. Granular materials with low flowability usually have a large angle of repose and form tall heaps. When this occurs in a rotary valve, the granular material would undergo severe attrition. On the other hand, granular materials with high flowability, or correspondingly low angle of repose, would be transported along with the moving vanes of the rotary valve and so avoid attrition. In the present study, both granulate and shear strength tests showed that PP samples had better flowabilities than PVC samples. Thus, the flowability of granular materials is likely to be the primary reason for the higher attritability of PVC than PP. This is despite of the fact that the former has higher particle hardness (PVC: R113, PP: R90).

[FIGURE 8 OMITTED]

Gwyn power law approach

It has been found that a convenient way to describe the size distribution of the product is written as a Schuhmann function (Schuhmann, 1940):

W = [W.sub.T] [(d/[d.sub.T]).sup.G] (3)

Here, d is the particle diameter, [d.sub.T] is the initial particle diameter and G is an exponent characterizing the size distribution . Size analysis is performed by sieving with the initial material having a narrow size distribution and held on a sieve of size [d.sub.T]. W is the mass of the attrited sample that has a size less than d and [W.sub.T] is the mass of the attrited sample having a size less than [d.sub.T].

Gwyn proposed a means of describing attrition and stated that the attrition of initially monodispersed particles could be described empirically by (Gwyn, 1969):

x = K[t.sup.m] (4)

where x is fraction of the initial feed that has undergone attrition at time t and m is an empirical constant; K is another constant, which he argued is a function of initial particle size. Based on Equation (4), the fractional degradation of the initial feed size fraction (x) is defined by the ratio of [W.sub.T] to the mass of the material at the start of the experiment. In the rotary valve experiments the relationship between x and time t is found to be characterized by the Gwyn function (Figure 9) and the averaged values of the Gwyn parameters are summarized in Table 4. In this case, a linear relationship is found between ln x and ln t. In general, the Gwyn formulation is more successful in describing attrtion. It may be concluded that K represents the severity of attrition and the initial attritability of a material, while m deals with the change in material with time (Neil and Bridgwater, 1999). The values of m for the same particles are not very uniform between attrition occurring solely in a rotary valve and that in the pneumatic conveying system. This may be attributed to variations in experimental conditions, including airflow rates and solid flow rates. Combined with the analysis of size distribution, it is seen that PVC particles degrade more rapidly than PP particles and PP particles are substantially more resistant to attrition.

[FIGURE 9 OMITTED]

Effect of Particle Attrition on Pneumatic Conveying Systems

Attrition has a number of different effects, the importance of which should be judged using the appropriate technical criteria in the following sections.

Effect of particle attrition on electrostatic characteristics

In the present study, attempts have also been made to analyze the induced current for the four kinds of particles described above. The induced current of PP samples detected at the horizontal pipe is illustrated in Figure 10. It can be seen that the induced current fluctuates with time and is a composite value resulting from a balance between the electrostatic charge on the particle surface and the pipe wall (Yao et al., 2004). Figures 10 (a, b) show that the magnitude of the induced current decreases with decreasing particle size. Furthermore, in order to eliminate the fluctuation caused by negative and positive values, induced currents, I, were integrated with time according to Equation (5) to obtain the charge Q (Yao et al., 2004):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (5)

[FIGURE 10 OMITTED]

Figure 10 (c) shows that the charge Q increases linearly with time up to 10 000 s. It is observed that the rate of charge accumulation on the pipe wall caused by intact PP particles is larger than that by attrited particles.

The induced currents for intact and attrited PVC samples in the horizontal pipe segment were similarly measured using the Electrometer. Figure 11 shows that the amplitude of fluctuations in the induced current decreases with decreasing particle size: the fluctuation observed with intact samples is about [+ or -] 1.8 x [10.sup.-7] A (Figure 11 (a)) while that with attrited particles is about [+ or -] 0.8 x [10.sup.-7] A (Figure 11 (b)). This can also be deduced from variations of the total accumulated charge on the pipe wall obtained by time integration of the data presented in Figures 11 (a, b). Figure 11 (c) shows that the rate of charge accumulation is larger for larger particles and vice versa. It is observed that the curves for both intact and attrited PVC samples exhibit fluctuations between positive and negative values. This may be due to the particles and pipe wall being made of the same material, PVC (polyvinyl chloride), resulting in charges of either positive or negative polarities being induced on the surface of the particles and pipe wall. Furthermore, due to the fact that PVC is prone to attrition, the particles were broken in the rotary valve during the experiment and this might have led to more complex electrostatic charge generation characteristics.

[FIGURE 11 OMITTED]

The variation of particle charge density, obtained from Faraday Cage, with respect to time for PP and PVC samples is illustrated in Figure 12. It can be seen that particle charge densities of attrited PP are larger than that of intact ones. Similarly, the charge density of attrited PVC samples is also greater than that of intact particles. Notably, the charge density of PVC samples is negative and will be further analyzed in the Particle Material Effect on Electrostatic subsection. In the conveying system, electrostatic charge is generated from the collision, rolling and sliding between particles and pipe wall. Consequently, the degree of particle-wall interaction would determine the rate of charge generation. For a given quantity of particles, the numbers of attrited particles were apparently more than those of intact ones; as a result, the enhanced chance of particle-wall interactions lead to the increase of charge on particles. Choi and Fletcher (1998) demonstrated that the contribution of small particles to particle space charge is more important than that of large particles. In their formulation, particle charge density depended on the total surface area of particles which was higher for small particles.

[FIGURE 12 OMITTED]

The above-mentioned phenomena seem inconsistent with the characteristics of charge generation on the pipe wall, which decreases with decreasing particle size. In order to resolve this contradiction, further experiments were carried out, in which valve 14 (Figure 1) was shut down to allow particles to be conveyed only once through the system. Figure 13 shows the resulting integral of induced current in such a single-pass operation, where the curved sections of the graph correspond to times when particles were moving through the system and the straight sections to times when all particles have stopped. It is easily observed from the two curves that a larger quantity of charge is generated by attrited samples than by intact ones. By a linear fitting of the data obtained, the slopes of the curve provide the average values of the induced current, -5 x [10.sup.-9] A and -3 x [10.sup.-8] A for intact and attrited samples, respectively (Figure 13 (a)). Similarly, Figure 13 (b) shows that the average values of induced current are 5 x [10.sup.-9] A and 8 x [10.sup.-9] A for intact and attrited PVC samples, respectively. The charge generation characteristics of the four types of particles are now consistent with the previous observation (Figure 12) that decrease in particle sizes during attrition corresponds with an increase in amount of charge generated.

[FIGURE 13 OMITTED]

This observation may be explained by a comparison of solid flow rates. It is shown in the previous granulate test that intact particles have much better flowability than attrited particles, regardless of the type of particles. Our previous work (Yao et al., 2004) demonstrated that the equivalent current should be consistent with the measured current on the pipe wall. The equivalent current, [I.sub.c], of a granular flow system due to the motion of charge-carrying particles can be calculated by the following equation:

[I.sub.c] = [Q.sub.p] * SF (6)

where [Q.sub.p] is the particle charge density measured using the Faraday Cage and SF is the particle mass transported per unit time or solid flow rate.

Although attrited particles have larger charge densities than intact particles, a smaller solid flow rate of the former would possibly lead to a corresponding smaller equivalent current [I.sub.c] (product of charge density, [Q.sub.p] and solid flow rate, SF) for attrited particles than intact particles. Similarly, the current on the pipe wall generated by attrited particles are weaker than that generated by intact particles. However, since there is little charge accumulation on the granules in a single-pass type of operation, solid flow rate does not show a strong effect on the equivalent current. Because measurements were performed at the very beginning of the experiment (i.e. after good discharge of the whole system), the result would not be influenced by the initial residual charge in the system. Therefore, induced current in a single-pass operation with intact samples is smaller than that with attrited ones as described in Figure 13.

Interrelationship between electrostatics characteristics and particle flowability

From the above-mentioned analysis, electrostatics affects solid flow rate greatly (Figure 14). During the course of each experiment, solid flow rates of all four types of particles decreased gradually. Due to the effect of charge accumulation on particles and pipe wall, particles tend to stick on the pipe wall and especially the hopper, this in turn results in poor flowability of particles. Considering the material of particles, it is observed that the speed of reduction in solid flow rates for PP samples (Figure 14 (a)) is faster than that for PVC samples (Figure 14 (b)), which can be explained by higher charge generation by PP samples (specification to be given in the next section). Comparing intact particles to attrited particles, it is obvious that the former moved more rapidly than the latter during the entire operation, which is in accordance with the flowability measurement in the granulate test. However, despite the higher charge density of attrited particles, the time dependency of the flow rate of attrited particles seems to be the same as intact ones. The loop of single-pass operation for testing solid flow rate is shown as 5-6-......-14 in Figure 1, thus particles did not pass through the hopper (labelled 4 in Figure 1) and the effect of hopper on particle flowability was isolated. However, as the granulate test shows, the flowability of particles was affected more significantly by the hopper than by the conveying pipes. The major driving force in pipe is aerodynamic drag force, although some charged particles adhered to the pipe wall, the air could still force the particles to move ahead. Given the particle velocity was slightly reduced, the effect was not obvious. However, fully charged particles would stick on the wall of hopper and then reduce the solid flow rate drastically. The degree of such drop in solid flow rate is clearer when more charges were generated on the particle. In principle, the decrease in gradients of the curve for attrited particles would be higher than the corresponding intact particles. Nevertheless, such prediction was not clearly observed in Figure 14 due to the accuracy limit of solid flow rate measurements.

[FIGURE 14 OMITTED]

On the other hand, the flowability of particles has a crucial influence on electrostatic charge generation characteristics, especially for PVC samples conveyed through the system with 100% solids feed valve opening. Intact PVC particles with cylindrical shape and larger size were locked in the rotary valve with little motion through the conveying system. This manifested as several 250 s thin gaps in the plot of induced current against time (Figure 15 (a)). With the attrited PVC samples, plenty of fine powders tend to stick on the feed hopper, causing discontinuity in the flow of particles through the system. Due to poor flowability this is more apparent for attrited particles and shows up as 3 obvious breaks in Figure 15 (b). Furthermore, the range of the induced current data becomes narrower gradually, indicating possible interactions between electrostatics and flowability. A larger amount of electrostatic charges accumulated on pipe wall leads to poorer flowability while at the same time, a smaller flowability will also result in smaller induced currents as demonstrated by Equation (6). In contrast to Figure 15, PVC granules move smoothly and continuously in a flood-fed condition, and the resulting induced current with good flowability of the particles was displayed previously in Figures 11 (a, b).

[FIGURE 15 OMITTED]

Particle material effect on electrostatics

From the above analysis, the most obvious distinction in the electrostatic behaviour between PP samples and PVC samples is the charge polarity on particles. The charge of PP particles is positive and that of PVC particles is negative for both intact and attrited samples, as shown in Figure 12. When two different materials are brought into contact (collide, roll or slide) then separate, electron is able to transfer between these two materials. The relative polarity of the charge acquired due to friction between two materials depends on their respective sequence in a triboelectric series (Diaz and Felix-Navarro, 2004; Yao et al., 2004). According to this series, PP particles would charge the PVC pipe wall positively. However, for two identical materials, PVC granules and PVC pipe, particles can acquire either net positive or negative charges because of the similar material properties between the particles and the pipe wall. Based on the experimental data obtained in this study, there is a higher tendency for negative charges to be generated on the PVC particles than on the pipe wall.

One other difference in electrostatic behaviour between PP and PVC samples, in addition to the charge polarity, is the quantity of charge generated. As may be predicted from the triboelectric series, the potential between PP granules and PVC pipe should be higher than that between PVC granules and PVC pipe. In order to have a fair basis for comparison of the electrostatic charge generation characteristics of PP and PVC samples, the experiment was carried out in a flood-fed condition.

Figure 16 shows the comparison between the induced current obtained from PP and PVC samples. With intact PP samples, the amplitude of the induced current is about [+ or -]3.0 x [10.sup.-7] A. It may reach a high value of 7.0 x [10.sup.-7] A, with discharges observed as sparks across the conveying pipe and indicated as peaks in Figure 16 (a). In contrast, for intact PVC particles, the range of induced current is only about [+ or -]1.8 x [10.sup.-7] A, and no sparking was observed during the experiments (Figure 16 (b)). The abovementioned phenomenon is also observed with attrited samples in the system. The average value of induced current of PP samples is almost 3 times that of PVC samples. However, sparking was observed less frequently as shown in Figures 16 (c) and (d).

[FIGURE 16 OMITTED]

Effect of particle attrition on pipe wall abrasion and electrostatic charge generation mechanism

Friction between particle and wall or other particles generates electrostatic charge through a process known as tribocharging. The mechanism of tribo-electrification of granular flow in horizontal pneumatic conveying was studied in a previous work (Yao et al., 2004). Furthermore, friction between the particles and pipe wall would also lead to pipe abrasion. Pipe wall abrasion can be observed even with the naked eyes as roughness on the surface of the film. In order to characterize the extent of mechanical interaction between the particles and the inner surface of a pipe with a polymer film during a typical conveying process, the film was examined using a scanning electron microscope (SEM). Figure 17 shows SEM pictures (magnified 1000 times) of the fresh film surface and the film after being attrited by PP particles for 3 h. It is evident that the quality of the film surface deteriorates quickly during usage indicating strong sliding effects and frictional forces between particles and the pipe wall. Compared with the original (Figure 17 (a)), films placed at different pipe sections were eroded to different extents. The film at the pipe bend was observed to contain a few holes (Figure 17 (b)), so it is deduced that most of the particles impacted the film with a sharp angle (Yao et al., 2000; Fan et al., 2002). In contrast, several gorges were seen on the films at the vertical and horizontal pipe sections (Figures 17 (c) and (d)), thus we may conclude that the most possible interaction between particles and pipe wall at both vertical and horizontal sections was sliding. Therefore, the mechanisms of charge generation were collision between particles at the bend section and friction between particles and wall at the vertical or horizontal pipe sections.

[FIGURE 17 OMITTED]

In order to describe the degree of pipe wall abrasion, an abrasion ratio is defined as the mass difference of the film before and after the experiment ([DELTA]M) divided by the mass of the original film ([M.sub.0]). Further detailed comparison of abrasion ratio is illustrated in Figure 18. In Figure 18, it is observed that the abrasion ratios in the three pipe sections are increased from horizontal to vertical orientation of the pipe and to the bend section for all four types of particles used. In particular, abrasion is especially significant at the bend section. It is expected that particles impact against the pipe bend with high velocities and large forces and so may cause significant breakage at this point in the system.

[FIGURE 18 OMITTED]

Furthermore, the abrasion resulting from attrited PP samples is more severe than that of intact PP samples (Figure 18 (a)). In this case, the shape of particles is the determining factor. Comparing with the smooth surface of intact PP particles, PP fragments have irregular edges and tough corners (Figure 3), which would seriously damage the pipe wall. The contacting pattern between particles and wall is also modified as attrition takes place. Intact PP particles have an ellipsoidal shape and contact the wall in a point-to-face manner while attrited particles with several smaller planes do so in a face-to-face manner. Therefore, the latter seems to have a larger contact area than the former, which may also result in higher charge generation by attrited particles (Figure 12).

In contrast to PP samples, from Figure 18 (b), the effect of wall abrasion by attrited PVC particles is weaker than that by intact ones. The reason is that abrasion due to PVC samples is mainly caused by the firm edges of the cylindrical particles but the fine powders produced from these intact particles during attrition lack such physical features and so bring weaker damage to the pipe wall. Furthermore, the particle number density (Smeltzer et al., 1982) of attrited particles is higher than that of intact ones even under the same experimental conditions. This may also cause attrited particles to bring about higher charge generation (Figure 12) than intact ones.

CONCLUSIONS

The process of particle attrition in a rotary valve feeder was studied; the physical properties of the granular material used and their variations due to the effects of attrition were studied in relation to the behaviours of particles in a pneumatic conveying system. The parameters investigated included particle size distribution, pipe wall abrasion and electrostatic charge generation characteristics. The main findings:

* The ability of particles to endure compressive stresses after attrition is decreased, the flowability becomes poorer and the internal friction among particles is increased. This was seen for both PVC and PP samples.

* In attrition in a pneumatic conveying system, both PVC and PP display the same general time variation of particle size distribution as that occurring in a rotary valve. A longer attrition time in the conveying system leads to almost the same size distribution as that obtained in the rotary valve with a shorter attrition time. The relationship between fractional degradation, x, and time, t, may be adequately described with the Gwyn function.

* For both PVC and PP samples, the induced current decreases with decreasing particle size, but the charge density shows a reverse trend due to the effects of particle flowability. Due to the charge accumulation, solid flow rates decrease gradually with time for four kinds of particles, and in turn the low flowability leads to diminishing induced current on pipe wall. PVC and PP samples show opposite charge polarities and the quantity of charge generated by PP samples is much larger than that by PVC samples under similar experimental conditions.

* It is found that inter-particles or particle-wall friction and impact can induced the occurrence of charge generation, as well as pipe wall abrasion. Pipe wall abrasion by both PP and PVC samples is greater in a vertical pipe and is most significant at a bend. However, the abrasion resulting from attrited PP samples is more severe than that of intact ones but converse behaviour is observed for PVC samples.

The above results and findings were specific to the test rig and products used, for example, blow-through rotary valve, PVC pipe and PP/PVC particles. Therefore, it is suggested that future work should further extend the experimental conditions to generalize the conclusions, for example, to extend the work to other configurations/ valve designs and to use particles having a greater hardness than those investigated in this study.

ACKNOWLEDGEMENTS

This project is supported by the Science and Engineering Research Council (A * STAR) and National University of Singapore under the grant number R-279-000-208-305. We are grateful to Wee Chuan Lim, Jun Yao and Fong Yew Leong for many helpful discussions on the project. We also extend our sincere thanks to Professor Reginald Tan, Dr. Soo Khean Teoh, Mr. Guangjun Han and Dr. Soon Hoe Chew for their kind support of laboratory equipment.

NOMENCLATURE

d particle diameter

[d.sub.T] initial particle diameter

G exponent characterizing the size distribution

[h.sub.0] original axial height of one particle

[[DELTA].sub.h] axial quantitative change

I induced current

[I.sub.c] equivalent current

K empirical constant

M empirical constant

[M.sub.0] mass of the original film

[DELTA]M mass difference of the film before
 and after the experiment

P perimeter

Q charge

[Q.sub.p] particle charge density

r radius of inner arc of bend

R radius of outer arc of bend

S projected area

SF solid flow rate

t time

W mass of the attrited sample that has a size less
 than d

[W.sub.T] mass of the attrited sample having a size less than
 [d.sub.T]

x fraction of the initial feed

Greek Symbols

[alpha] internal friction angle

[epsilon] strain

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Manuscript received April 13, 2006; revised manuscript received Augusts 19, 2006; accepted for publication August 28, 2006

Yan Zhang (1) and Chi-Hwa Wang (1,2) *

* Author to whom correspondence may be addressed. E-mail address: [email protected]

(1.) Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore, 117576

(2.) Singapore-MIT Alliance, E4-04-10, 4 Engineering Drive 3, Singapore 117576

Table 1. Experimental conditions

Temperature ([degrees]C) 28-30

Relative humidity (RH) in systems 5%

Air pressure (kPa) 500

Air flow rate (L/min) 1600

Air superficial velocity (m/s) 21.2

Solids feed valve 75% opening/100%
 opening/ flood-fed

Roll speed of rotary valve (r/min) 25

Pipe material Polyvinyl Chloride (PVC)

Pipe diameter (inner) (mm) 40.0

Pipe thickness (mm) 5.0

Particles conveying style Circulation

Particle material Polyvinyl Chloride (PVC)

Particle initial size range 3.5 mm - 4.2 mm

Particle shape Cylinder

Particle density (g/[cm.sup.3]) 1.4

Hardness R113

Heat deflection temperature at 67
1.8 MPa ([degrees]C)

Sample mass (g) 745

Temperature ([degrees]C)

Relative humidity (RH) in systems

Air pressure (kPa)

Air flow rate (L/min)

Air superficial velocity (m/s)

Solids feed valve

Roll speed of rotary valve (r/min)

Pipe material

Pipe diameter (inner) (mm)

Pipe thickness (mm)

Particles conveying style

Particle material Polypropylene (PP)

Particle initial size range 2.8 mm - 3.35 mm

Particle shape Ellipsoid

Particle density (g/[cm.sup.3]) 1.1

Hardness R90

Heat deflection temperature at 80
1.8 MPa ([degrees]C)

Sample mass (g) 939

Table 2. Physical properties of particle

Particle PVC

 Intact Attrited

Size range (mm) 3.35~4.2 1.18~3.35

Average size (mm) 4.06 [+ or -] 0.11 2.28 [+ or -] 0.52

Shape factor 1.179 [+ or -] 0.101 1.773 [+ or -] 0.664

Standardized flow
time (s) 0.0103 0.0115

Internal friction angle
([alpha]) 31.2[degrees] 35.8[degrees]

Friction coefficient
(tan[alpha]) 0.61 0.72

Particle PP

 Intact Attrited

Size range (mm) 2.8~3.35 1.18~2.8

Average size (mm) 3.01 [+ or -] 0.13 2.45 [+ or -] 0.29

Shape factor 1.099 [+ or-] 0.201 1.368 [+ or -] 0.616

Standardized flow
time (s) 0.007 0.009

Internal friction angle
([alpha]) 29.2[degrees] 33.7[degrees]

Friction coefficient
(tan[alpha]) 0.56 0.67

Table 3. Comparison of solid flow rates

Particle PP

 Intact Attrited

Solids feed valve 75% opening

Solid flow rate (g/s) 40.67 [+ or -] 3.29 40.05 [+ or -] 0.63

Particle PVC

 Intact Attrited

Solids feed valve Flood-fed

Solid flow rate (g/s) 37.62 [+ or -] 4.76 37.49 [+ or -] 6.79

Table 4. Summary of attrition data using Gwyn approach
(a) Parameters from Gwyn function

Attrition Sample Parameters from Gwyn
 function

 Material m (-) K/[10.sup.-3]
 ([s.sup.-m])

Solely in rotary valve PVC 0.6 1.52

 PP 0.79 0.17

In pneumatic conveying systems PVC 0.42 4.63

 PP 0.63 0.32

(b) Original data for Gwyn functions
 Time (s) ln(t)

PVC samples with initial size of 3.35 ~ 600 6.40
4.10 mm attrited in rotary valve
 2400 7.78

 4200 8.34

 6000 8.70

 7800 8.96

PP samples with initial size of 2.80 ~ 1800 7.50
3.35 mm attrited in rotary valve
 3000 8.01

 4800 8.48

 6600 8.79

 8400 9.04

PVC samples with initial size of 3.35 ~ 10800 9.29
4.10 mm attrited by rotary valve in
the pneumatic conveying system 21600 9.98

 25200 10.13

 64800 11.08

PVC samples with initial size of 2.80 ~ 10800 9.29
3.35 mm attrited by rotary valve in
the pneumatic conveying system 21600 9.98

 32400 10.39

 36000 10.49

 43200 10.67

 x=WT/[sumation]Mass ln(x)

PVC samples with initial size of 3.35 ~ 0.08 -2.54
4.10 mm attrited in rotary valve
 0.15 -1.90

 0.18 -1.70

 0.32 -1.15

 0.39 -0.94

PP samples with initial size of 2.80 ~ 0.07 -2.73
3.35 mm attrited in rotary valve
 0.09 -2.46

 0.13 -2.05

 0.17 -1.77

 0.22 -1.53

PVC samples with initial size of 3.35 ~ 0.17 -1.77
4.10 mm attrited by rotary valve in
the pneumatic conveying system 0.37 -0.99

 0.52 -0.66

 0.39 -0.95

PVC samples with initial size of 2.80 ~ 0.11 -2.20
3.35 mm attrited by rotary valve in
the pneumatic conveying system 0.16 -1.81

 0.20 -1.62

 0.32 -1.14

 0.23 -1.47

Figure 18. Comparison of abrasion ratio at three different pipe
sections. Mass of particle: PP-939 g, PVC-745 g; airflow rate: 1600
L/min; solid feed valve: PP-75% opening, PVC-flood-fed; operation time
3 h (a) PP samples; (b) PVC samples.

(a) PP intact PP attrited

Horizontal 0.025% 0.169%

Vertical 0.179% 0.180%

Bend 0.554% 1.053%

(a) PP intact PP attrited

Horizontal 0.117% 0.029%

Vertical 0.242% 0.119%

Bend 0.463% 0.172%