A holistic approach to bulk-solids-feeding installations can improve the effectiveness and efficiency of operations involving solids handling
Many chemical processes involve the extraction of bulk solid materials at a controlled rate from a storage vessel for delivery into the process. The extraction of solid materials usually requires the use of technology that can deliver the solids into the process at the required feedrate and with minimal variation in rate when in operation. For every industrial installation, bulk solid material type and set of process requirements, a different optimal technology exists. This article presents some of the key considerations involved in obtaining reliable and cost-effective extraction from storage vessels.
One of the most important aspects of efficient feeder installation is to understand that adopting a holistic approach is required to optimize any type of feeder and solids-feeding process. Such an approach considers the storage vessel, the interface between the storage vessel and the feeder along with the feeder itself as a single project entity whereby each element of the process must be specified and designed to work together. For “clean sheet of paper” projects, this approach is usually more easily realized. However, in many cases, engineers are tasked with trying to make the most of retrofits and upgrades where budgets are rarely adequate to support such an ideal approach. In these real-world projects, the need to compromise is important in the delivery of effective solutions.
Consistent volume delivery
A fundamental requirement for most types of solids-feeding installations is that the delivery of solids into the process should proceed with consistent volumes or weights of the solids introduced into the process without any interruptions in feeding. These may sound like fairly undemanding requirements, but in too many solids-feeding cases, they remain elusive for many end users. Success in meeting these requirements is driven by the design of the feeder and the flow-channel profile that it develops, but is also driven by the inherent operational characteristics of the storage vessel.
Achieving any degree of consistency in the delivery of solids into a process requires a correctly designed and installed feeder and consistent bulk-material properties at the outlet (interface with the storage vessel). To do this, it is clearly important to consider the conditions of the bulk material that are prevalent at the outlet, and the influence of the storage vessel on those conditions.
Figure 1. The illustration shows the concept of funnel/core-flow discharge
Depending upon the vessel geometry, internal surface finishes and material characteristics, two basic flow patterns can develop in storage vessels: mass flow and funnel (core) flow. Simplified illustrations of these two operational behaviors appear in Figures 1 and 2.
Figure 2. The diagram illustrates the concept of mass flow discharge
Mass flow is characterized by the friction at the walls of the storage vessel (this is a function of the convergent angle of the vessel, the surface finish of the vessel walls and the interaction of these variables with a specific bulk solid) being sufficiently low to allow material to slide downward when the outlet cross-sectional area is fully active. Because the designed geometry is able to mobilize the friction at the wall, the flow of material toward the outlet approximates a cross-sectional plane drawn down. This flow pattern endows the vessel with the (very desirable) characteristic of operating on a “first-in, first-out” stock rotation — without any stagnant zones or risk of solids retention when the material drains down.
A very beneficial effect of establishing mass flow discharge is that the bulk density at the outlet is largely independent of the head of the material present above the outlet. Similarly, where segregation (by particle size, shape or density) occurs in radial or lateral forms, the cross-sectional drawn down serves to effectively recombine distributed elements at the solids outlet. Thus, the most ideal conditions can be generated at the outlet to support feeder consistency and accuracy — these being minimal variation in bulk density and improved homogeneity (also associated with bulk density but often, more critically, associated with good blend composition).
Designing for mass flow
Attaining mass flow through the storage vessel is of critical importance where the accuracy of feeder operation is a main consideration. Reliability of discharge is a function of the outlet size required to prevent rat-holing or bridging (arching) of the solid material. This dimension can also be calculated for a given type of bulk solid, and should always be incorporated into the design and construction of mass-flow vessels.
Storage vessels that have been constructed without consideration of the bulk-solid properties will by default develop funnel (core) flow (Figure 1). Inappropriately high effective friction at the wall (caused by shallow vessel geometry or by high-friction surface finishes) means that when discharge occurs, material will be retained against the walls, and a preferential flow channel will propagate upward from the outlet. When such channels develop, material is drawn down from the top surface of the solids inventory. This type of flow behavior endows the vessel with a range of characteristics that, while not a major issue for some bulk solids, can cause serious issues for others (typically cohesive or time-dependent materials). These issues include retention, poor stock rotation, cross-contamination and exaggerated segregation effects. From the perspective of feeder performance, the greatest drawbacks are that the bulk density at the outlet is directly influenced by the head of material, and that segregation effects also influence not only bulk density, but also critically adversely impact blend quality.
Where many feeder installations fail is in the use of equipment that is incapable of drawing down from the cross-sectional area of the vessel outlet. As a default, most feeder types are what is termed fixed-volume devices (including standard screws, drag link, belts, rotary valves, and so on), which has the implication that if simply purchased and bolted onto the underneath of a vessel or bunker, a preferential draw will develop though the outlet of the vessel. The result of this is that any vessel (even one designed for mass flow) connected onto the equipment will default to funnel flow (which is the default for the majority of vessels anyhow).
One of the challenges for feeders employed in applications where good repeatability is a key objective, is that, if funnel flow is present (that is, for a fixed-volume-type feeder, suboptimal vessel geometry, or both), the bulk density developed in the feeder will be a reflection of the inventory level at any given moment (that is, the bulk density will oscillate as a reflection of emptying or filling cycles). Obviously, if the range of inventory variation can be controlled by feedback control based on load cells or level probes, then the variation in bulk density will be lessened. This approach is one of several reasons why it is not unusual to find subhoppers used for controlled feeding, where they are fed into from larger-capacity vessels. The less variation in bulk density that occurs, the simpler (and less expensive) the feedback control can be on the feeder.
Figure 3. The graph shows the discharge characteristic for a standard screw feeder and agitator system operating in funnel flow
Figure 3 shows the typical fall off in dose weight over the discharge to empty a small vessel where the geometry and feeder design factors have created funnel flow. The trend is very clear. By contrast, Figure 4 shows a modified version (to support mass flow) of the vessel in use with a modified screw. The stability of the bulk density in the feeder during the drain down to empty the vessel contrasts very strongly with the behavior of the funnel-flow system in Figure 3.
Figure 4. The graph shows the discharge characteristic for a modified screw feeder with the agitator system removed operating in mass flow
Many small-scale dosing systems employ agitators (usually as a default) in their design — the purpose of which is to prevent flow stoppage (since the dimensions of the dosing screws and their corresponding flow-channel volumes are invariably significantly below the rat-hole or arching dimension for the powder being handled). Such agitators may support flow reliability, but can also (in some designs) be responsible for cyclic variability in density conditions. Again refer to Figure 3, where the peaks and troughs in the data are a direct function of the agitator blade proximity to the feed screw during rotation. A good-quality feedback-control loop for screw speed should be able to largely iron this out and, of course, it should be borne in mind that the magnitude of variation will be strongly influenced by many design and operational factors, including, of course, the bulk properties of the powder or granulate being dosed.
Pitch spacing and flow
Taking screw and auger feeders as a specific example of a constant-volume-type feeder, the defining feature for a standard unit in most cases will be that of constant pitch spacing and constant-diameter flights along its length. The unit would typically, but not necessarily, also feature a central shaft along its length to which the flights attach. Some types are shaftless and consequently have a helical appearance. These design features parallel those found in screw conveyors, which are used to transport material either horizontally or at an inclined angle over distances of up to 9–12 ft (before transfer to an additional unit is required).
Therein lies a very important difference in the function of the feeders and conveyors, namely that the former type operates with each “pocket” filled to capacity, while the latter would typically run at below 45% volume. Consider now the operating of a standard (constant-volume) screw feeder under a vessel. During each rotation, a void opens up at the back end of the screw, into which material can be drawn. The subsequent rotation creates another void at the back end of the screw and pushes the previously filled volume forward, but since all of the volumes along the screw are identical, it follows that no further capacity can be generated to draw material down (other than that at the very first pitch). Thus, irrespective of the area of the vessel outlet, the only flow channel that will be generated will be defined by the dimensions of the first pitch spacing and the diameter of the screw. The consequence is that funnel flow will be established. Figure 5 shows this effect.
Figure 5. Preferential drawdown of solids material develops from the first pitch of a constant-capacity screw feeder
The presence of multiple screw sets (for large reclaim bunkers) or agitators above the screw (in small-scale applications) will have no effect on the size of the active flow channel that is created. Thus, for unagitated, large-scale installations, the development of stagnant regions of material can be anticipated to develop. The implications of this can range from a fairly benign “loss of live storage volume” to a potentially hazardous “self-heating and ignition” for combustible materials (such as municipal solid waste, biofuels and so on). The development of draw down only over the area of the first pitch is amply demonstrated in Figure 6, where we can see the effect of operating an agitated screw feed in conjunction with carbon black. In common with many types of cohesive powders, carbon black has been consolidated into the non-flowing zone along the screw — leaving only the active flow channel apparent.
Figure 6. Evidence of a preferential flow channel can be seen in this agitated dosing system (the screw visible through a bed of compacted material)
Variable pitch spacing
Substantial improvements in the size of the flow channel can be obtained by using (in this instance) a screw feeder that has been designed and constructed to develop an increase in capacity between the pitches along its length. This can be achieved through progressively increasing the pitch spacing from the start of the screw forward, the use of a very wide-diameter shaft that reduces in the direction of feed, or a combination of both techniques. The principle of allowing the development of increased capacity along the screw means that there is transport capacity available under the whole outlet area (Figure 7). For a vessel that has inherent funnel flow behavior, this modification allows the development of the largest possible flow-channel area above the screw. This may be adequate to improve flow reliability, but it will definitely deliver a degree of improvement in screw-delivery consistency (by virtue of the larger flow-channel volume acting on the screw).
Figure 7. This diagram shows one of several design approaches to generate increasing capacity along a constant diameter screw
It should be noted that the preceding narrative relates to parallel-diameter screws. On paper, a simpler route to attaining increased capacity would seem to be to use a standard constant-pitch screw and progressively increase the outside diameter in the direction of feed. A calculation of pitch volumes would show that an increase in capacity has been achieved, but functionally, this type of screw would not be recommended for bulk solids other than those that are free-flowing, non-time-dependent and those for which the vessel is drained down to empty on a regular basis (a requirement of best practice common to the operation of conventional funnel-flow vessels where risks associated with long-term resident material exist). An issue arising from the use of tapered screws is presented for less than free-flowing bulk solids, whereby they may bridge or arch over the narrow start of the screw. It is possible that, for some materials, active flow cannot occur until further along the development of the screw diameter (which may be as much as one third to one half of the way along the length of the screw). Allied to the flow-channel diameter restriction imposed by the screw diameter is the shape of the trough in which the screw sits. The trough casing will follow a clearance along the screw, which means that in order to interface with a rectangular vessel outlet, the profile will progress from fully developed at the exit from the vessel to a pronounced “V” profile at the narrow diameter.
This V-shaped trough profile imposes another aspect of flow impediment by virtue of material being supported into the vessel from the flanks of the trough. Screws cannot draw material into their sides (hence material retention onto the flanks during discharge). This is a potential problem not just for tapered screw feeders, but also for parallel screw feeders (which are also invariably interfaced to vessels using V-shaped trough profiles).
Holistic view
Much of this discussion has focused on screw feeders, but the same principles for developing increased capacity over the outlet area apply equally to belts, vibratory trays, drag links, rotary valves and so on
Reliability of flow during the operation of storage vessels cannot be decoupled from the influence of vessel geometry and the characteristics of the feeder. In many cases, attempts at addressing flow problems or retention issues in vessels by installing discharge aids of various types have varying degrees of success. For many plants where bulk-solids handling is problematic, a holistic view of the problem is seldom developed. The consequence is that substantial resources are misapplied because the plant fails to consider the interaction of the vessel and feeder as a single entity that should be designed or specified to work together, rather than being brought together as the result of “catalog engineering.”
Edited by Scott Jenkins
Author
Richard Farnish is a senior consulting engineer at The Wolfson Center for Bulk Solids Handling Technology at the University of Greenwich (Chatham, Kent ME4 4TB, U.K.; Phone: +44 0208 331 8646; Email: [email protected]). The majority of his time at The Wolfson Center is spent undertaking consultancy activities for a wide range of industrial sectors, although he is also involved in the delivery of undergraduate lectures and short courses to industry. A large proportion of his work is linked to troubleshooting bulk solids processes that are underperforming as a result of equipment design issues or product quality problems (segregation, agglomeration, attrition and so on). His research interests relate to optimizing dry-filtration systems. Farnish has worked at the Wolfson Center since 1996. He is a chartered mechanical engineer and a member of the Institution of Mechanical Engineers (CEng MIMechE) in the U.K.