S. Dhodapkar,The Dow Chemical Co. L. Bates,Ajax Equip. & G. Klinzing,U. of Pitts
Myths and misconceptions become part of accepted practice if repeated often enough. They are propagated by lack of fundamental particle-technology knowledge and by certain commercial interests. The list of presented here is by no means exhaustive. We have, however, made an attempt to cover a broad range of topics to highlight the widespread nature of these misconceptions. They are broken down by operation, which are ordered alpha-betically for easy reference. We intend to engage the reader’s inquisitiveness with this list, and hope that practi-tioners of solids processing technology will learn to challenge established practices and not always accept them as a given facts.
Drying
Misconception: The required drying time, when a contact dryer is used, does not depend on the equipment size.
Reality: The required residence time to reach desired moisture content in a contact dryer (with heated walls) depends on the size of the equipment. The ratio of volume of material to the surface area of heated walls deter-mines the drying time. This ratio increases as the dryer size increases. Therefore, residence time required in full scale dryers is longer than measured during pilot testing. One must pay close attention to such details during scaleup.
Misconception: The performance of fluid bed dryers can be improved by increasing the gas flowrate.
Reality: This is not always the case. For some products that require long drying time, lowering the gas velocity will improve the dryer performance. By lowering the gas flow, the residence time of material in the dryer can be increased. This will improve dryer performance.
Electrostatics and dust explosion
Misconception: It is possible to drain the static charge from polymer pellets stored in a box or a container by inserting a grounding rod into the container or grounding the metal container.
Reality: Static charge on insulating material can not be drained instantly even if the container is grounded or a grounding rod is inserted. The charge decay relies on surface resistivity of the polymer.
Misconception: Wrapping a grounding wire on a PVC hose is an effective method to discharge the static charge in a pneumatic conveying system.
Reality: The grounding wire simply prevents the operator from experiencing the effect of static charge by direct-ing the force field and grounddirect-ing the surface charges. The material bedirect-ing conveydirect-ing will remain charged. This is not a good practice. A grounding wire can also produce spark when an ungrounded human being touches the hose. Using a charge dissipative (conductive) hose is recommended.
Misconception: Dust explosion characteristics (minimum ignition energy, Kst) are material-specific.
Reality: Additionally, they strongly depend on the particle size. The dust explosion potential increases as the par-ticle size gets smaller. The fines fraction (less than 63 µm) is typically tested for dust-explosion potential. In most cases, particles larger than 420 µm have low dust-explosion potential (per NFPA).
Misconception: Dust explosion in process equipment is the primary reason for loss and destruction.
27 Reality: Secondary dust explosions triggered after the primary explosion due to re-entrainment of settled dust in process area will cause greater damage. Good housekeeping and well engineered systems minimize the likeli-hood of such losses.
Feeding and metering
Misconception: The performance of various feeders can be compared by their accuracy specification.
Reality: The accuracy of a feeder is often quoted as percentage of full scale value. It is more appropriate, however, to compare absolute values. It is equally important to understand the time-average basis for accuracy specifica-tions. Feeder accuracy decreases and feed fluctuations increase at smaller time scales. The time basis must be determined by process requirements.
Misconception: Vent piping on a rotary valve is meant for venting leakage air. Therefore, the piping can be routed in a configuration similar to that of an air line.
Reality: The vent stream contains significant amount of entrained material. It must be designed as if it were a pneumatic conveying system; otherwise, the line may plug.
Misconception: A feed screw delivers a volumetric amount that is proportional to the pitch.
Reality: The amount moved forward by a flooded screw depends on the contact friction between the material and the face of the screw flight. An increase in screw pitch does not necessairly result in a proportional increase in feed rate.
Misconception: A circular casing surrounding a short pitch screw will restrain material from “flushing”.
Reality: If a fine product is dilated to a fluidized condition, it can only be restrained by a positive restriction to the flow channel, such as a rotary valve, and even then some leakage may occur through even small clearances.
Fluidization
Misconception: The fluidization characteristics of a bulk material only depend on average particle size and differ-ence in particle density and fluid density.
Reality: Fluidization behavior is also affected by particle-size distribution and nature of particle surface (mois-ture, stickiness, electrostatics).
Gas-solid separation
Misconception: Each cyclone design can be assigned a unique “efficiency” designation regardless of the applica-tion.
Reality: Overall efficiency of a cyclone depends on grade (or fractional) efficiency of the cyclone and the particle size distribution of incoming dust. The grade efficiency is a function of cyclone design and operating parameters (gas flowrate, gass-solid properties, solid loading). It can also be affected by other factors, such as air leakage from the bottom or presence of bends at the inlet and outlet. Typically, the grade efficiency decreases with par-ticle size. Industrial cyclones have low grade efficiency below 3-5 µm.
“High efficiency” cyclone designs rated at “99.9% efficiency” for coarse particles may separate only a fraction of incoming particulate if the incoming dust is extremely fine.
Misconception: Frequent back pulsing of dust collectors keeps the bags clean and improves the “performance” of a dust collector.
Reality: For most filter media, the efficiency is lowest when the media is clean and there is no cake on the sur-face. The best way to run a dust collector is to back-pulse it as needed (based on pressure drop). That will achieve the highest overall efficiency and longest possible bag life.
Mixing and blending
Misconception: More residence time in a mixer results in better mixing.
Reality: The mixture quality reaches an asymptotic limit after certain duration. The quality of mixture depends on the mixer design and compatibility between the mixing mechanism in the mixer and the mixture. Extended mixing action may cause attrition or fines generation, which then segregates and results in poor mixture quality.
Misconception: Solids mixers can be scaled up by applying scaling laws similar to liquid mixers.
Reality: Scaleup of solids mixers is much more complex. Mixer scaleup will depend on the type of mixer and mechanism of mixing. Various approaches (constant tip speed, Froude number, geometric, kinematic and dy-namic similarity) have been proposed throughout the literature.
Particle characterization
Misconception: It is acceptable to overlay the particle-size distributions obtained from various particle size ana-lyzers on the same plot for comparison as long as they are converted to the same type of distribution (number, surface or volume/mass).
Reality: There are many different ways to describe the ¡®size’ of a particle (volume diameter, projected area di-ameter, Stokes diameter etc.). Various particle size analyzers use different physical measurements to extract the particle size information. They are not always comparable or transformable from one to the other. Stick with one instrument for all measurements in a given process to avoid confusion. Different instruments also handle non-spherical particles differently since there is an inherent assumption of sphericity.
Misconception: Mass-median particle diameter (particle size corresponding to 50% on cumulative mass distri-bution) is the best way to represent a particle size distribution.
Reality: The selection of appropriate “representative” particle diameter for a distribution depends on the applica-tion. For instance, mass median diameter is not sensitive to the presence of fines. The behavior of a bulk material in fluidization, hopper flow and pneumatic conveying applications, however, is strongly influenced by presence of fines. Surface-volume mean diameter is more appropriate than mass median diameter for these applications.
It is defined as the diameter of a sphere having the same volume as the particle of average surface area for the mixture. Other mean diameters are surface mean diameter and volume mean diameters.
29 Misconception: Pickup velocity and saltation velocity are fundamental properties of a material.
Reality: First of all, let us address the definitions of pickup velocity and saltation velocity. Pickup velocity is de-fined as the superficial gas velocity required to entrain particles from a stationary layer on the bottom of the pipe.
Saltation velocity is defined as the minimum superficial gas velocity required to keep particles in suspension for a horizontal gas-solid flow. From a design perspective, the saltation velocity determines minimum gas velocity required at the pickup location. Saltation velocity depends on particle size, particle density, gas density, pipe size and solids loading. Higher velocity is required for larger line size and higher loading.
Misconception: Pneumatic conveying lines can be routed much like utility (air or steam) lines in the plant.
Reality: Good design practice for pneumatic conveying systems requires sufficient straight run after pickup, minimization of bends and directional changes, avoidance of back-to-back bends and avoidance of inclined lines.
Misconception: Increasing air flowrate (or providing more “oomph” to the system) will increase conveying ca-pacity.
Reality: Increasing air flowrate in dilute phase systems will decrease conveying capacity (for the same overall pressure drop). For dense phase systems, the effect is opposite. Transition from the dense-phase region to the dilute-phase region can result in unstable flow for some materials. Increased air flow will always result in higher power consumption.
Misconception: Dense-phase conveying is achieved with high conveying pressure.
Reality: Dense-phase conveying implies “non-suspension” flow in the conveying line. The material is not fully entrained or suspended in the gas stream. While dense phase conveying systems typically run at higher pres-sures, a dilute phase that is long and/or highly loaded can also require higher conveying pressure.
Misconception: Injecting air in the conveying line at various intervals (with “boosters”) will result in a better dense-phase system.
Reality: The need to inject air along the conveying line depends on the bulk material’s permeability and air re-tention characteristics. Materials with high permeability (such as pellets) and high air rere-tention (such as cement) do not require secondary air injection. Excess air injection can result in high gas velocity (dilute phase condi-tions) at the end of the line.
Screening and classification
Misconception: Mesh size is sufficient to specify a screen.
Reality: For wire-mesh screens, mesh size determines the number of apertures per lineal inch. One must sub-tract the thickness of wire to determine the opening available for screening. As the wire diameter increases, the actual opening and percent open area available for screening decreases.
Misconception: The cut size is determined by aperture opening alone.
Reality: The cut size is a function of aperture opening (size and shape), particle shape, screener loading and screener motion. The grade efficiency of a screener is not a step function but an S-shaped curve for non-spherical
particles.
Segregation
Misconception: Mixtures with components of different sizes, shape and density will always segregate in a process.
Reality: In addition to material properties, the severity of segregation depends on operating conditions, process configuration and the scale of operation. For example, a mixture may segregate while loading a 100-ft-tall by 15-ft-dia. silo, although it may not segregate while loading a feeder hopper that is 4-ft tall by 2-ft dia.
Misconception: Large particles will always sink to the bottom.
Reality: Upon vibrating a mixture of particles, the large particles (regardless of density) will rise to the top sur-face. This is known as the “Brazil Nut Effect”. The container geometry and relative size differences in the mixture also play a role in this phenomenon.
Misconception: Mass-flow hoppers will always correct segregation problems in the bin.
Reality: If the segregation in the bin is such that layers of different components are created (such as alternate bands of coarse and fines), then mass-flow design (see CE, Apr. 2006, pp. 40-43) will not correct the segregation problem. The coarse and fine fraction will discharge in alternate sequence. On the other hand, mass-flow design is very effective in alleviating radial segregation (variability across the cross-section) problem. During operation, it is essential to maintain the level of material at a minimum of one bin diameter above the cone section. Oth-erwise, the velocity gradient in the converging section will initially deliver a portion of “rich” material from the central region and finally discharge product that was deposited around the periphery of the stored surface.
Misconception: Large particles always go to the outside of a pile.
Reality: When falling into a mass of dilated (fluidized) bulk, large particles can be captured by “impact penetra-tion” and thereby express fine particles to the outer region.
Solid-liquid separation
Misconception: Higher filtration pressure drop will result in higher filtration throughput.
Reality: Higher pressure drop will not increase the throughput if the cake or the filter medium is compressible.
Higher pressure drop causes reduction in porosity and retards the flow.
Misconception: As long as the media can capture the particle, operating flowrate is not important.
Reality: Operating the filter at lower flowrate will result in better capture of particles. Higher flowrate reduces the chances of particles being caught by the media.
Misconception: The longer one can run a filter (between cleaning cycles), the higher the throughput.
Reality: Filtration rates decrease with time. It is important to analyze the entire filtration cycle to determine whether the extended filtration time (at lower filtration rates) is better or worse than higher filtration rates for shorter duration (due to more frequent cleaning).
31 Misconception: Filter aid helps to increase filtration rate - more is better.
Reality: While filter aid increases filtration rate, it also decreases the actual amount of native solid removed in each cycle. An optimal amount must be determined based on filtration rate improvement and additional cycles that are required to achieve the same capacity.
Misconception: All filters with the same “micron rating” will have the same performance.
Reality: There are no universal standards for specification of micron rating of filter media. Each vendor tests and specifies its medium/media differently. One must pay close attention to the details of the test method. Moreover, the grade efficiency (true performance) of the unit depends on type of filter (such as cartridge, bags and metal screen), material of construction, fabrication and application parameters (such as particle characteristics, operat-ing temperature and liquid viscosity). Bench scale testoperat-ing is highly recommended when there is no prior experi-ence.
Storage
Misconception: A suitable cone angle for bin design (for mass flow) is the same as angle of repose or angle of internal friction or angle of slide.
Reality: Cone angles required for mass flow must be calculated from Jenike’s theory using wall friction data and angle of internal friction from shear test. Angle of repose should not be used for bin design except for estimating its capacity.
Misconception: Stress at the outlet of a large silo (say, 150 m3) is much higher than that of a small silo (say 15 m3) with same outlet dimensions.
Reality: For mass-flow design, the stresses in the vicinity of the outlet are largely independent of the overall size of the silo. This does not hold true for funnel flow silos.
Misconception: Mass-flow pattern gives a true FIFO (first in/ first out) or “plug flow” as observed by uniform draw down of the top surface.
Reality: Broadly speaking, this is true. However, there is significant mixing in the cone section due to velocity gradients. The residence-time distribution depends on the hopper geometry and velocity profile in the hopper.
Steeper and smoother hoppers will result in narrower residence-time distribution.
Misconception: Hoppers should be designed first, and then a feeder selected to suit the process.
Reality: Hoppers and feeders are integral units with interacting properties. The first design decision is to select the appropriate flow regime for the product. This will allow the wall angle to be determined for the form of hop-per selected on the basis of many factors. A minimum outlet size is required for reliable flow and varies accord-ing to the maximum flowrate wanted. Site circumstances, fabrication considerations and a host of other factors influence the ultimate determination of hopper shape and feeder selection, which boil down to the designer’s judgment based on experience.
Misconception: Moisture makes flow more difficult.
Reality: Generally, the flow potential of a bulk material deteriorates with moisture up to a critical value, above
which flow behavior eases because the excess fluid reduces surface tension and behaves more like a lubricant.
Typically, a value of 10 to 12% moisture represents the worst flow condition of a fine, granular solid such a ground coal. Similar behavior has been observed for soft elastomeric and highly frictional pellets.
Misconception: Reliable stream flow can only be achieved by means of mass-flow design.
Reality: Expanded flow (funnel-flow silo with mass-flow cone adapter) can be a viable option, especially when the bin can be emptied periodically.
Misconception: Vibrations always help to discharge material from a silo.
Reality: Vibrations can result in compaction and may adversely affect flow.
Misconception: Chutes designed with 45-deg. inclined surfaces will provide adequate service.
Reality: There is a minimum angle required for proper chute flow that depends upon wall friction and surface adhesion. Allowance must be made for impact and changes of direction. The capacity of a chute drops dramati-cally when its inclined surface is near the critical inclination.
Misconception: Full chutes transfer more than partially loaded chutes.
Reality: Chutes filled over the whole cross section have lower capacity than those operating at 50% fill level in the pipe. Choked (full) chutes are also prone to arching or bridging.
Misconception:Smoother walls always result in lower wall friction angle. Some lining materials have universal low friction values with all bulk materials.
Reality: Friction between bulk material and a surface is a complex phenomenon. Wall friction is a relationship between a given bulk material and a specific contact surface. It is necessary to measure wall friction for each pair. Often times, smooth surfaces can result in higher friction and “low-friction” surfaces may exhibit slip-stick behavior. A surface that has low friction with one bulk solid, compared with an alternative contact surface may have higher friction with another material. Ultra-high-molecular-weight-polyethylene surfaces, for example, exhibit low friction with damp materials due to their hydrophobic nature, but are prone to scouring and high friction with fine, hard particles.
Misconception: Lining hopper walls with low-friction material will improve flow.
Reality: Not always. Low-friction liners on vertical walls and at large spans can increase overpressures, and make the bulk material stronger in the crucial outlet region.
Summary
The misconceptions discussed in this paper are merely the tip of the proverbial iceberg. The chances of success in a solids handling/processing project will be improved if the user is aware of misconceptions and avoids the
The misconceptions discussed in this paper are merely the tip of the proverbial iceberg. The chances of success in a solids handling/processing project will be improved if the user is aware of misconceptions and avoids the