Frictional Force Measurement for Multiple Plugs in Dense Phase

Article pubs.acs.org/IECR

Frictional Force Measurement for Multiple Plugs in Dense Phase Pneumatic Conveying of Polymer Particles: An Industry Application Jae Bum Pahk,

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Nestor A. Vasquez,‡ Karl Jacob,‡ and George E. Klinzing*,

†

†

Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States DOW Chemical Company, Midland, Michigan, United States

‡

ABSTRACT: Frictional forces between particles and the pipe wall in dense phase pneumatic conveying were determined using strain gauges attached to the outside of the pipe wall. To perform the frictional force measurement, a special strain measurement section was designed and inserted in the industrial scale pneumatic conveying system. Three different polymer particles, polyolefin elastomer, polyethylene, and SARAN were transported under the different flow conditions of the superficial air velocity and the mass flow rate of solids. The frictional forces were determined from three different locations of the pipe wall, top, bottom, and sides of the pipe. The plug velocity and plug length were also determined by analyzing pressure signals measured at two different locations. The magnitude of frictional force was found to be proportional to the plug length. In general, the largest frictional forces were measured at the top of the pipe. In addition, the frictional forces were related to the particle’s interaction with the pipe (i.e., the area of contact of individual particle and number of particles that interact with the pipe wall) as well as the material properties. It was also found that the minimum fluidization of the particles could be related to the strain signal fluctuations. Materials with a smaller minimum fluidization velocity showed more fluctuations and thus have less stable operation. The results obtained were compared to the smaller scale laboratory pneumatic conveying system previously performed.1

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INTRODUCTION Understanding the basics of the frictional force in pneumatic conveying still requires further studies for both dilute and dense phase conveying. The frictional force exemplifies itself as an overall energy loss or pressure drop. The purpose of this study was to help understand and predict the energy loss by measuring and understanding the frictional forces at various locations around the circumference of the conveying line for dense phase industrial pneumatic conveying. Earlier studies of friction in pneumatic conveying were focused on determining the solid friction factor to estimate the pressure drop. Dozens of researchers explored the friction factor such as Konno and Saito,2 Stemerding,3 Reddy and Pei,4 Van Swaaij, Buurman, and Van Breugel,5 Capes and Nakamura,6 and Stegmaier.7 Other researchers such as Yang determined friction factor for both horizontal pipe8 and vertical pipe.9 More recently, different experimental measuring techniques to determine the frictional force have been developed. Yi10 designed a sliding friction rig to determine the frictional force. He inserted polymer particles into a half pipe shaped test device and measured the frictional force by pulling the test unit with a known force. This work did not measure the frictional force for a moving plug under a dense phase pneumatic conveying condition. Vasquez et al.11 probed the frictional force in dense phase pneumatic conveying using three load cells and a folding rubber connector that united two sections of the pipe to measure frictional forces imparted to one pipe section. When the moving plug is located in the downstream position, the frictional force pulls on the load cells and measures the frictional force. Krull12 measured the frictional force by inserting a shear vane into the pipe. When the plug is moving through the shear vane, the friction between shear vane and © 2012 American Chemical Society

particles interacts with the load cell which is connected to the shear vane measuring the frictional force. In another study Lecrep and Sommer13 determined the wall shear stress and normal stress within a single plug using a piezoelectric probe attached to the pipe wall. For a single plug study, Pahk and Klinzing1 measured the frictional force by using strain gauges attached to the outer surface of the pipe. This device was able to measure the frictional forces generated at four different locations around the pipe’s circumference; the top, bottom, and two sides of the pipe wall. In this present work, the similar frictional force measuring device employed for a single plug was applied to an industrial scale dense phase pneumatic conveying system for multiple plug flow. A scaled-up test section with the strain gauges was designed and inserted in the dense phase pneumatic conveying system at DOW Chemical Co. in Midland, MI, USA. This measurement device allows the friction force measurement without disturbing plug flow when material is being conveyed.

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EXPERIMENT 1. System Description. Experimental tests were performed at DOW Chemical Company, Midland, MI. The schematic of the system is shown in Figure 1. This system was a closed loop system that is 52.4 m in total length. The pipe used in this system was a Schedule 10 carbon steel pipe with inner diameter of 82.8 mm. The system consisted of five horizontal sections (3, 7, 10, 8, and 7.4 m long) and one vertical section of 7 m. A Special Issue: L. T. Fan Festschrift Received: Revised: Accepted: Published: 199

February 28, 2012 June 5, 2012 June 6, 2012 June 6, 2012 dx.doi.org/10.1021/ie3005385 | Ind. Eng. Chem. Res. 2013, 52, 199−206

Industrial & Engineering Chemistry Research

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system. This test section was 1.1 m long, having the strain gauges attached at the center of the test section machined to a wall thickness of 0.7 mm to maximize the signal magnitude from strain gauges. Using milling machine, the outer pipe wall was peeled off, while maintaining the inner diameter of the pipe the same as the overall system. After the strain measurement test section, the pipe was supported by the roller supports, allowing the pipe free to elongate toward the flow direction when tensile frictional force is generated. Since the magnitude of strain signal was small, the strain gauges were connected to the commercial Wheatstone bridge (OMEGA BCM-1) and the signal from this bridge was amplified 100 times by an AD-621 amplifier. The signal was collected by data acquisition card (USB-6210, National Instrument). In addition, four dummy (compensating)14 gauges were used and connected to each strain gauge and Wheatstone bridge to eliminate the effect of temperature changes. The pressure transducers employed were connected directly to the data acquisition card. A LABVIEW software from National Instrument was used to convert the electronic signals collected from the data acquisition card to pressure or strain value. MATLAB software from Mathworks was used to filter the signal noise and analyze the signals to determine plug velocity, plug length, and frictional force. 2. Calibration. Calibrations of the pressure transducers and strain gauges were performed before the actual experimentation began. The strain gauges were calibrated by placing different known weights (4.5, 9.1, and 13.6 kg) hanging on a cord with a pulley to generate tensile force transmitted to the test section. Figure 4 is a schematic of the axial calibration. The strain data

Figure 1. Schematic of system.

rotary air lock feeder was installed below the feed vessel to feed solids into the pipe and plant air is supplied to transport solids. There is a 6.3 m long vertical pipe 3 m after feeder, causes plugs to be generated naturally in this system. Each horizontal and vertical section was connected with a five 2 m long radius bend. Two pressure transducers (Omega PX142-030D5 V) were installed 5.28 m (first transducer) and 4.52 m (second pressure transducer) away from the end of the transport system. These pressure transducers were utilized to calculate the plug velocity and length. Figure 2 indicates how to measure the frictional forces using the strain gauges. When a plug moves through the pipe after the

Figure 4. Axial calibration schematic.

was collected when different known weights were applied to the pipe. The relationship between strain data and tensile force was used to determine the actual frictional force. The strain signals generated during the experiment might contain not only strain due to the frictional force but also contains due to the pressure inside of the pipe and bending of test section due to the weight of a plug. The effect of bending will be explained in a later section. To determine the pressure effect, each end of test section was closed and sealed and pressurized up to 40 kPa. After test section was pressurized, strains were measured continuously while the test section was depressurizing. It was found that the effect of the pressure to strain measurement was small and this effect will not be considered in this study. 3. Materials. Polyolefin elastomer, polyethylene, and SARAN were transported in this study. The material properties of these materials are shown in Table 1. 4. Experimental Conditions. For each material, experiments were performed by changing the volumetric flow rate of the air and mass flow rates of the solids. The volumetric air flow rates were 0.042, 0.05, 0.057 m3/s respectively, and the corresponding superficial air velocities were 9.21, 10.96, and 12.50 m/s respectively. The superficial air velocity was calculated by dividing volumetric air flow rate by pipe cross-

Figure 2. Schematic of the strain measurement principle.

test section, the test section is elongated toward the flow direction due to the frictional force applied to the pipe by the moving plug. The strain gauges attached to the test section also elongated and generated the voltage signal. In this study, the strain signal differences were measured when frictional force was applied to the test section and when no frictional force was applied. Using this strain difference, the frictional force was determined. A specially designed strain measurement test section (Figure 3) was inserted 2.6 m away from the end of

Figure 3. Strain measurement test section (2D view). 200

dx.doi.org/10.1021/ie3005385 | Ind. Eng. Chem. Res. 2013, 52, 199−206


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next plug approaches the pressure transducer while the previous plug still remains in the system at that time. Sometimes, the pressure signal has its maximum value when the plug is about to leave the system. See Figure 7. This phenomenon can be explained by the total number of plugs existing in the system. For example, in most cases, if the total number of the plug in the conveying line is n, but sometimes, before one plug leaves the conveying system, a new plug was formed and started to transport at the inlet of the pipe. In this case, the total number of plugs in the conveying system is n + 1, increasing the overall system pressure. For this case, the pressure of a plug has its maximum immediately before it leaves the system. 1.2. Plug Velocity and Plug Length. From the pressure signal seen in Figure 6b, one can readily determine the velocity and length of the plug. As mentioned before, the pressure increases when it moves past the pressure transducer. The velocity of the plug is determined by dividing the length between two pressure transducers by time differences between a plug arrives at the first transducer and then the second pressure transducer. The length of the plug is determined by velocity of the plug and the time to pass a single pressure transducer. The magnitude of the plug length and velocity are summarized in Table 2. The plug velocity is tied to the solid loading ratio; the ratio of mass flow rate of solid over the mass flow rate of the air. In general, the plug velocity decreases when the solid loading ratio increases. For polyolefin elastomer, with the same superficial air velocity, the plug velocity decreased when the solid mass flow rate was increased. For polyethylene and SARAN, this was not always true because there were many broken plugs observed for these materials, causing a large plug velocity variation. This variation is seen in the histogram of plug velocity shown in Figure 8. In addition, the standard deviations of plug length for these materials were larger than polyolefin elastomer. Broken plugs were observed through the transparent section of the pipe and small dune flow usually trailed the main plug for SARAN and for polyethylene. In this situation, many particles were conveyed in dilute phase following the main plug. 2. Frictional Force. 2.1. Strain Signal Behavior. The frictional force is determined by analyzing the strain signals. A typical strain signal for a multiple plug system is shown in Figure 9. As seen in Figure 9, the strain signals appear to be periodic. A single plug generates a frictional force and this force diminishes when plug exits the conveyer line, thus for the

Table 1. Material Properties

polyolefin elastomer polyethylene SARAN

particle density (kg/m3)

particle size (mm)

902

4

ellipsoidal

885

4.86

cylindrical (rounded) powder

1700

up to 0.022

particle shape

sectional area. At fixed air flow rates (fixed superficial air velocity), tests were performed with different mass flow rates of solids which influenced the plug length. For each test condition, the pressures and strains were measured with the sampling frequency of 5 kHz for 2 min period. The total number of data points for each system configuration was 600 000.

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RESULTS AND DISCUSSION 1. Experimental Challenges. Originally, significant electrostatics were generated and observed in the transport system. This electrostatics generates sharp peak for both pressure and strain signal and made it difficult to measure pressure and strain accurately. In addition, this effect could damage the measurement devices. To minimize the electrostatics observed, additional groundings of the pipe to the metal beams of the building structure were inserted. Figure 5 shows the effect of the electrostatic on both pressure and strain signals. As seen in Figure 5, both pressure and strain signals have a very sharp peaks at the testing time of 85.9 s due to this effect. The electrostatics were reduced after attaching additional grounding wires. Some electrostatics were still observed as the mass flow rate of solids increased for SARAN. Thus some difficulty existed for measuring the plug length and velocity for SARAN. 1.1. General Behavior. Figure 6a and b shows the pressure signal obtained from both pressure transducers versus measurement time. In Figure 6b, pressure in the pipe starts to increase (at point P1s and P2s) when a plug is moving past the pressure transducers. When a plug is leaving the pressure transducer (at point P1e and P2e), the pressure in the pipe decreases slightly and increases again right before it leaves the system. When the plug is leaving from the system (from point Phexit to Ptexit), the pressure drops sharply. When the solid mass flow rate increases, the pressure does not drop completely to zero and then increases again since the

Figure 5. Effect of static electricity for pressure (left) and strain (right) signals. 201



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Figure 6. (a) Pressure signal vs time, polyolefin elastomer (vair = 10.87 m/s, solid mass flow rate =1.34 kg/s). (b) Pressure signal for one of the plugs from part a.

Figure 7. Pressure signal variation for larger number of plugs moving in the pipe (time = 87.5−89.5 s) and normal plug (time = 90−92 s).

Figure 9. Strain signal analysis example.

Table 2. Summary of Average Ranges of Plug Velocity and Plug Length for Each Material

force due to friction and the strain increases. When the plug is leaving the conveying system, the strain decreases because the frictional force is decreased. In Figure 9, when the plug is moving in the strain test section (time = 77.2 or 80.7 s), pipe bending occurs due to the weight of the plug, thus the strain signal from the top and bottom of the pipe responds. Note that the voltage signal from strain gauges at the top and bottom vary in opposite directions. If strain at the top increases, the strain at the bottom decreases due to the effect of bending. The strain at the middle of pipe will remain the same since it is not affected by bending. (Note that the voltage and strain have negative linear relationships.) Even though the bending occurs when the plug is moving through the strain gauge test section, the bending effect did not influenced the strain measurement in this study, since the strain measurement was performed when the plug is moving after the test section. Strains due to the frictional force were measured by noting the difference between maximum and minimum strains. The magnitude of strain differences are converted into the frictional force using axial force calibration result. 2.2. Frictional Force Result. The strain signal for the polyolefin elastomer can be easily understood, but the signal for the polyethylene was more challenging to analyze. This was because there were considerable signal fluctuations with polyethylene and numerous broken plugs were observed. For SARAN, the strain signal fluctuations and signal noise were higher than the other polymers due to more individual particle movement in the plug.


plug velocity (m/s)

plug length (m)

1.81 ± 0.35−5.36 ± 0.77 3.79 ± 1.93−5.48 ± 1.21 4.36 ± 2.30−7.21 ± 2.99

0.63 ± 0.08−1.64 ± 0.52 0.89 ± 0.28−1.70 ± 0.98 2.82 ± 1.50−5.22 ± 2.73

system with multiple plugs, frictional force variation should be periodic. In Figure 9, when a plug is in the position immediately after the test section (75.4 or 78 s), the pipe sees a minimum strain, and as time goes on, the device begins to receive a tensile

Figure 8. Histogram of plug length and plug velocity, Polyethylene, air velocity =10.87 m/s, and mass flow rate of solid = 2.15 kg/s. 202



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maximum P-value from the result of paired t test for each case was 0.0286) . For SARAN, since number of data collected was smaller than other materials, this was not always true (P-value >0.05). Figure 13shows the frictional force measured from

The frictional force for polyolefin elastomer with respect to plug length is shown in Figure 10. The magnitude of frictional

Figure 10. Frictional force with respect to plug length, polyolefin elastomer, vair = 12.5 m/s.

force increases for longer plugs, and the higher mass flow rate generates more frictional forces. A student t test result with very small P value (about 1.8 × 10−7) supports this result. For SARAN and polyethylene (Figures 11 and 12, respectively), the

Figure 11. Frictional force with respect to plug length, SARAN, vair = 9.21 m/s.

Figure 13. Frictional force vs solid mass flow rate measured from different circumferential locations, vair = 9.36 m/s: (a) polyolefin elastomer, (b) polyethylene, (c) SARAN.

different locations for each material. At the top of the pipe, particles were well-fluidized and they interacted with the pipe wall more than with the other sides of the pipe wall. The evidence of fluidizing in a plug has been observed by Klinzing and Pahk15 by measuring the voidage of a single moving plug as well as voidage of packed particles. The voidage of the moving plug was found to be higher than packed particle state, indicating that fluidizing occurs when the plug is moving. In addition, the video result for a moving plug taken from Vasquez16 showed that the particles at the top of the plug moves faster than the bottom of the plug. Thus, one can easily understand that more fluidization occurred at the top, thus frictional force at the top was higher than other sides of the pipe wall. The largest frictional force was observed from SARAN. SARAN has a very small particle size (0.022 mm) compared to the other materials studied. Again, this material has more individual particle contact with the pipe wall and thus generates more frictional force. In addition, SARAN forms longer plugs (up to 12 m), and naturally has more overall contact area. Note that since the distance between the test section and the pipe exit was 7 m, frictional force could not be measured for the longer plug that has a length of more than 7 m.

Figure 12. Frictional force with respect to plug length, polyethylene, vair = 10.96 m/s.

frictional forces tend to increase for longer plugs, but compared to the polyolefin elastomer, this trend was not clearly seen from the results. There are three possibilities to explain this phenomenon. First, the shape of the polyethylene particles is disk-like which may have cause more tumbling movement when they interact with the pipe wall. For SARAN, since particle size is very small, more particle−pipe wall interactions occur and more signal fluctuations were observed (Figure 11). In addition, as mentioned before for these materials, it is difficult to form a plug, and the signal itself fluctuates considerably. One finds that plug length and velocity have considerable variability. In terms of magnitude of frictional force at the measuring location, the largest frictional force was generally observed at the top of the pipe, the sides and bottom of the pipe showed smaller friction for the polyolefin elastomer and the polyethylene with smaller mass flow rate of solid (∼2.28 kg/s) (The 203



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unit area) generated compare to the overall driven pressure when particle plugs are moving. Figure 14 shows the stress ratio

To support further substantiate the contact area behavior, the ratio of relative contact area between the different particles was determined. The area of contact is defined as follows17 ⎡ 3F R ⎤2/3 Acontact = π ⎢ n ⎥ ⎣ 4E* ⎦

(1)

where 1 − ν12 1 − ν2 2 1 = + E1 E2 E*

(2)

where E* is the modified Young’s modulus, R is particle mean diameter, Fn is force normal to the particle, E1 and E2 are Young’s modulus for particle and pipe materials, and ν1 and ν2 are Poison’s ratio of particle and pipe material. Table 3 shows Table 3. Young’s Modulus and Relative Contact Area of a Single Particle material

E

relative area of contact


0.09 0.096 0.6

1 0.92 0.09

the Young’s modulus of transported material and the relative ratio of area of contact to polyolefin elastomer, assuming that the normal force, Fn, was the same for all particles. Figure 11 shows that the frictional force with respect to the plug length for SARAN. The data shows the variability of friction force with plug length at constant solid mass flow when the air velocity was increased. Some of the data indicates that the frictional force increases up to 250 N. The polyolefin eastomer has a smallest frictional force. For this material, the plugs formed easily and rarely break while being transported. Thus both pressure and strain signals were comparatively stable. Polyethylene plugs produce similar magnitude of frictional force as polyolefin elastomer, but due to its instability (many broken plugs), the frictional force plot had more variability than polyolefin elastomer. The signal fluctuation could be explained by exploring the minimum fluidization velocity. The minimum fluidized velocity has been measured directly using fluidized bed, and the result is shown in Table 4.

Figure 14. Stress ratio vs plug length for vair = 9.36 m/s: (a) polyolefin elastomer, (b) polyethylene, (c) SARAN.

with respect to the plug length for each material. As a result, the stress ratio is decreased when the plug length is increased. Similar results were achieved earlier by Pahk and Klinzing13 showing that this ratio decreased rapidly for smaller plugs and decreased slightly for longer plugs. The result for this study indicates that for polyolefin elastomer and polyethylene, since they have relatively smaller plug lengths compare to SARAN, the stress ratio drops rapidly; however for SARAN, which has longer plug lengths, the stress ratio appears to approach an asymptote. In addition, the results showed that the shear stress becomes less important as the plug length is increased. This result can help one to find the flow condition with minimized shear stress and pressure drop. Additional data and further study for longer plugs are needed to support the conjecture described above. 2.4. Compare Result to Previous Single Plug Study. The result of this study for polyolefin elastomer was compared to the previous study of a single plug experiment18 and are shown in Table 5. The materials used for previous study were also polyolefin elastomer with similar size, but different Young’s modulus. The Young’s modulus of polyolefin elastormer for previous study was smaller (0.02 GPa) than this study (0.09 GPa). Intuitively, smaller Young’s modulus materials have larger area of contact for individual particles when they interact with each other and the pipe wall, thus generating larger a frictional force. Different pipe diameters and plug velocity may

Table 4. Minimum Fluidization Velocity for Test Material material

minimum fluidization velocity (m/s)


0.68 ± 0.02 0.52 ± 0.02

Frictional Force Measurement for Multiple Plugs in Dense Phase

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