With surging orders, production faces many challenges to meet market demands. Corin Holmes, Operations Manager, Dr. David Craig, Vice President, and Phuong Ly, Project Engineer at Jenike & Johanson explain how understanding flow properties can provide the solution.
In many global industrial processes bulk materials are handled in different forms including, but not limited to, particle size, moisture content, storage at rest and chemical composition. In some instances, the very process can chemically transform the bulk material and change any or all the aforementioned properties. Take for example the production of lithium batteries. While initially this may involve the handling of spodumene rock and associated large particles during the process the rocks are ground and undergo a chemical transformation to extract the fine lithium. When the material is handled as large rocks, ensuring reliable flow will require a certain design. Following transformation though, achieving reliable flow will require different design aspects to be considered, including the interaction of the gas surrounding the fine particles and its effect on flowability. The design of a system to reliably handle a bulk material needs to understand the actual material handled at the appropriate position within the process. Understanding the material is important whether you are looking to increase throughput, design a new system or process, or handle a new type of material in your system.
Dr. Andrew Jenike developed the science of modern-day bulk material handling technology in the 1960s, and since then it has been widely used by designers responsible to design bulk material storage, handling, and transportation systems. The specific bulk characteristics and properties of a material that affect flow, which can in principle be measured, are known as flow properties. To predict how a material will flow in a given piece of equipment, such as a bin or hopper, you must determine the material’s flow properties at representative handling conditions. These properties can be measured using standard material flow properties testing methods, according to ASTM D6128 .
Key flow properties of powder
The flow properties that are generally of most interest with fine powders are presented below. These flow properties refer to the behaviour of the bulk material and arise from the collective forces acting on individual particles, such as van derWaals, electrostatic, surface tension, interlocking, friction, etc. .
Cohesive Strength – The consolidation of powder may result in arching and ratholing within transfer equipment. These behaviours are related to the cohesive strength of the powder, which is a function of the applied consolidation pressure. By measuring the required shear force for various vertical loads, a relationship describing the cohesive strength of the powder as a function of the consolidating pressure can be developed . This relationship, known as a flow function, can be analysed to determine the minimum outlet diameters for bins, press hoppers, blender outlets, etc. to prevent arching and ratholing.
Wall Friction – Used in a continuum model, wall friction is expressed as the wall friction angle or coefficient of sliding friction. The lower the coefficient of sliding friction, the less steep hopper or chute walls need to be for powder to flow along them. This friction coefficient can be measured by sliding a sample of powder in a test cell across a stationary wall surface using a shear tester [1, 2]. This flow property is a function of the powder handled and the wall surface in contact with it. Variations in the material, or the wall surface (type or finish) can have a dramatic effect on the resulting friction coefficient. Wall friction can be used to determine the hopper angles required to achieve mass flow.
Bulk Density – The bulk density of a given powder is not a single but varies as a function of the consolidating pressure applied to it. In a more complete approach, the degree to which a powder compacts can be measured as a function of the applied pressure [1, 4]. The results are often expressed as a straight line on a log-log plot. In bulk solids literature, the slope of this line is typically called compressibility. The resulting data is used to inform equipment design.
Permeability – Flow rate limitations may occur when handling fine powders, due to the expansion and contraction of voids during flow creating air pressure gradients within the powder bed. The permeability of a powder, or its ability to allow air to pass through it, will have a controlling effect on the discharge rate that can be achieved. Permeability is measured as a function of bulk density . Permeability values can be used to calculate the time required for fine powders to settle or deaerate in equipment, the discharge rate of a powder from a bin, and to design efficient drying or purging systems.
Understanding the relevant materials’ flow properties will help optimise and improve the treatment processes efficiency by overcoming flow issues. As occurrences of bulk powder flow issues are strongly affected by the flow pattern during discharge from a bin, it is important to understand the two primary flow patterns that can develop: funnel flow and mass flow, both shown in Figure 1.
In funnel flow, a first-in last-out flow sequence, an active flow channel forms above the outlet, with nonflowing powder at the periphery. In funnel flow, material flows towards the outlet in a channel that is surrounded by stagnant material. When the storage vessel is emptied faster than it is filled, the level of solids within the flow channel drops, causing layers of the solid to slough off the top of the stagnant material and fall into the channel. With cohesive materials the resulting impact pressures may increase the possibility of arching, but if it is sufficiently cohesive, the sloughing of material will not occur and the channel will empty completely and form a stable rathole – a near vertical cavity that empties out above the bin outlet. Material along the walls is in stagnant zones that usually remain in place. In general, funnel-flow bins are usually suitable only for coarse, free-flowing, non-degrading, and abrasive solids in applications where segregation is unimportant. Funnel flow is generally the default flow pattern when little or no consideration is given to the design of the storage vessel and/or interface.
On the other hand, mass flow occurs when all powder in the bin is in motion during discharging. Powder from both the centre and the periphery moves toward the outlet, resulting in a first-in first-out flow sequence. This eliminates stagnant zones, provides a steady discharge with a consistent bulk density, and yields a flow that is uniform and well controlled. Mass flow bins are usually recommended for applications in which the downstream process cannot handle a segregated feed and for handling cohesive materials, fine powders, and any material that may degrade with time. A properly designed mass flow hopper must have appropriate geometry that results in a fully active outlet, so a stable arched shaped obstruction cannot form. In short, the materials’ cohesive strength must be considered in determining the correct outlet size to ensure mass flow.
To illustrate the significance of cohesive strength, one could imagine squeezing dry sand in one’s hand. Once opened the sand is likely to simply fall away as the cohesive strength is low. Repeat the process with wet sand, however, and it will gain sufficient strength to retain its shape once the hand is opened. Typically, with most powders, as the moisture content of the material increases so too does cohesive strength; at least until the material is so wet that lubrication or a slurry is achieved.
A dewatering process such as one that includes a filter press takes a slurry material and squeezes out liquid. Following the press cycle however the material may still contain a significant amount of liquid which will affect the materials’ cohesive strength – this is akin to our hand squeeze example. Most processes will see a range of moisture contents. Now consider each moisture content having different outlet size requirements to prevent arching.
During processing, immediate transfer and consumption of a powder may not always be possible or desired. If the bulk powder is allowed to sit for an extended period of time, several phenomena such as moisture migration, aging, recrystallisation, or reactions which absorb or give off heat may lead to dramatic gains in cohesive strength, agglomeration of smaller particles into larger ones, or caking. For example, a common thermal treatment process known as calcination is used to facilitate decrepitation. During the calcination process, there is an optimal temperature range for decomposition, depending on the material. Additionally, it is important to limit the high temperature during calcination to avoid any undesirable formations. However, after decrepitation, the same material often undergoes a process in which the material is handled at a much lower temperature, such as acid roasting . Temperature change can cause expansion and contraction of particles which can also significantly contribute to increased consolidation and thus increased strength.
External forces such as vibration or storage methods may add to these effects. Each of these factors is a problem by itself, but combining them often compounds flow problems further resulting in cohesive strength increases that make discharge from a bin extremely difficult or impossible. The potential for these effects can be investigated providing valuable insight into the storage requirements for a powder and whether environmental controls or special handling are needed to avoid potential flow issues. Note that the flow behaviour of the same powder in different applications may be quite dissimilar (e.g. a powder that flows well through a bin may flow poorly after calcination). Other processes such as intercalation and purification may also affect the flow properties of the material and understanding the powder’s flow properties can help predict flow behaviour in existing equipment.
Particularly with powder, due to the state of aeration or density of the powder, controlling the flow rate may be a challenge. If the equipment is not properly designed, a fine powder could flow like a liquid and essentially flood uncontrollably, much like a snow avalanche (Figure 3), through the system. Flooding may occur when a rathole collapses, where the falling particles could entrain air and become fluidised. Even if the powder is contained, its bulk density can undergo dramatic variations once fluidised, negatively impacting downstream equipment.
Sometimes, the opposite to uncontrolled or flooded flow may occur. For some fine powders, flow rate limitations may occur when handled. The expansion and contraction of voids during flow can create an upward air pressure gradient at the outlet of mass flow discharge equipment. During discharge, the upward gradient acts counter to gravity thus reducing or limiting the discharge rate. To overcome this issue, the system could be redesigned to minimise the upward air pressure gradient. Alternatively, fluidised feed systems can be used.
Fluidisation involves air flow through a bed of powder, which serves as a force counter to gravity. In a fluidised state, particles are readily separated from each other. The ability of air to separate particles is governed by the flow properties of the powder. The bulk density of a bed will decrease as the air flow through the powder is increased. The permeability of the powder describes the ability of air to move through a stationary bed, which in turn is a function of the bulk density. As the bed dilates and reaches a minimum density, particles separate and move relative to one another; at this point, the pressure drop across the bed remains relatively constant as air velocity increases. The incipient fluidisation point shown in the figure below, is the minimum fluidisation velocity. From this information, the air flow requirements to fluidise a process can be determined.
Additionally, the settling or deaeration of powder could also be a concern. Settling times are influenced by the same properties that affect fluidisation. These properties include the permeability of the bulk powder, as well as the mean size and density of the particles. Segregation could potential occur when entrained air escapes upward and carries with it finer particles that are then deposited on the top surface. Usually, quicker settling times are desired to avoid concerns of flooding and segregation. With the flow properties and equipment design .parameters, the potential for flooding and segregation can be determined.
The measurement of bulk powders’ flow properties should be conducted, and the appropriate flow pattern selected, to inform design/selection of handling equipment. Flow properties data should also be used to analyse existing equipment, prior to modification or retrofitting, in order to prevent materials’ handling problems. Any changes made to the handling system should be reviewed along with the associated flow properties to ensure reliable flow remains.
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