Key points
Introduction
The handling, storage and processing of bulk materials in powder or granular forms have been undertaken for decades – usually with an acceptable degree of process availability.
However, in response to the vagaries of market forces driving raw material procurement strategies, some process steps are now experiencing reliability issues that are often linked to the poorer handle ability of the lower grade raw materials / ingredients.
The handling behaviour of many bulk materials are often recognised as being adversely affected by factors such as increased moisture content, high fines content or supply chain source. All of these factors can interact to create a process that is prone to hanging-up in hoppers or incapable of supporting consistent feed into process steps.
This article will discuss some of the more common causes of processing/flow problems and provide some possible considerations for alleviation of these issues.
The basics
The basis for the design parameters of many handling systems is often a loosely defined description of the bulk solid that is required to pass through the plant.
In the majority of cases, good quality bulk materials will present few problems, however this acceptable performance can often mask a plant that may be operating on a knife edge from an equipment reliability and availability perspective. In such cases even relatively small changes in bulk behaviour can develop into significant causes of down time or reduced throughput.
The discharge problems that are experienced when handling bulk materials that may exhibit less favourable flow characteristics can be categorised into two main groups, these being flow stoppages and flow irregularities (noting that blaming the bulk material for discharge issues is usually only half the story!).
Flow stoppages
Typical discharge stoppages are categories into the following:
- Mechanical arching – the physical interlocking of relatively coarse particles (not a very common issue),
- Cohesive arching – cohesion between fine particles
- “Rat holing” – emptying of the central flow channel developed in a core flow vessel, whereby the bulk of the inventory has sufficient internal strength that it can support itself resulting in the formation of a empty flow channel extending from the top surface down to the outlet.
These flow problems are frequently found in hoppers and silos that have been designed without the use of measured flow properties or equipment types inherited from older plant and pressed into service.
In both instances the need to design equipment to suit the flow characteristics of the powder to be handled will not have been addressed, and recourse to the installation of vibrators or air injection will be the usual approach adopted to initiate or maintain flow.
It should be recognised that unless flow measurements are taken and the information incorporated into a design for a bunker or silo, that the default flow pattern that will develop will be core flow (Fig 1), in which case the bunker/silo will operate on a first in, last out mode of material discharge.
This core flow discharge behaviour will be typified by erratic flow rates which are also sensitive to the head of material in the storage scheme. Outlet sizes for reliable operation also tend to be larger than those required, to deliver the desired flow rate – hence the use of slides or rod gates to restrict outlet sizes is quite common.
The problem with this approach lies in the mismatch between outlet size requirements for reliable discharge and the size require for the process. If a stoppage occurs, the retraction of the throttling gate will often bring no re-initiation of flow until the critical outlet size is exceeded – at which point the flow of material is uncontrollable and the gate must be reintroduced until the next stoppage. A very similar principle applies to the use of butterfly valves.
The two main discharge behaviours that can develop in silos and hoppers are as shown in the figures 1 & 2 below, which illustrate these flow patterns in the context of conical silos, but the principles are equally applicable to rectangular or pyramidal forms of storage vessels.
Fig 1, illustrates a discharge behaviour called core flow, where the bulk particulates discharge preferentially down a vertical channel above the outlet. The bulk particulates are fed into the flow channel from the top free surface and the product around the walls remains static until the level descends to the point where it becomes the top surface and discharges. If this discharge behaviour is present, then it can impact on the process in several ways:
- The vessel will operate on a first-in, last out stock rotation’ which can result in the unpredictable appearance of aged or different bulk particles/ingredient ratios on the process line if the inventory is maintained at a high level for a significant length of time.
- An increased variability in bulk density throughout the discharge cycle of the bin may become apparent due to variations in particle packing due to variability in residence time before discharge (recalling that the vessel will draw down freshly introduced material).
- If the bulk blend is subject to surface effect segregation i.e. tends to separate in such a way that fines enrichment occurs in the central region whilst the periphery of the vessel demonstrates a lack of fines, then initial discharges from the store will be fines rich (correlating to high bulk density or blend imbalance) whilst the final stages of the discharge will tend to feature a higher coarse content (low bulk density or blend imbalance).
By default, this discharge behaviour is most likely to be found in most hoppers.
Fig 2 illustrates a discharge behaviour called mass flow. By contrast this type of discharge is arrived at by designing the vessel/equipment to suit the measured flow characteristics of the worst case bulk material to be handled. If this discharge behaviour is present, then the following benefits can be derived:
- The vessel operates on a first-in, first-out basis and as such maintaining a high inventory level does not affect the residence time of the bulk material being discharged.
- Stagnant regions of material are eliminated.
- The inventory is drawn down evenly through the equipment – thus providing a degree of recombination of radially segregated bulk particulates.
- Discharge can be sustained reliably without resorting to discharge aids.
The above brief descriptions summarise the principle differences in discharge behaviour between the two types of vessel geometries. It should be borne in mind that although the illustrations show conical type vessels, these discharge characteristics can be found in all other shapes and sizes of equipment.
Flow irregularities
During core flow the irregular levels of shear to which materials flowing over static or retarded material are exposed to causes variations in the discharge rate (if unrestricted discharge occurs) or induce variations in particle packing (bulk density) at the outlet.
Such variations in bulk density can result in considerable conservatism being applied to the control set points for operating systems. The problems associated with obtaining good operational efficiency from bulk handling plants has been highlighted many times in the past, with perhaps the most in depth analysis of the problems being the Rand Report (1) produced in 1973 in the USA.
Homogeneity
As mentioned previously, for processes that bulk materials that exhibit a wide variation in size distribution, particle density or particle shape, segregation (de-blending) can be a major source of variable feed rates and inconsistent product quality.
Fig 3 shows the most common type of radial segregation (surface effect), associated with the central gravity loading of materials into silos or hoppers.
Typically, the coarser (or denser) particles will tend to transport more readily down the slope as the heap develops. Thus an analysis of the size distribution of material composing the heap will indicate a lack of coarse under the fill point and an excess of coarse at the periphery.
The importance of appreciating this effect becomes clear if the storage system discharges in core flow, in which case a discharge channel will form through the section of bed immediately above and expand to draw in material around this point.
Fig 3 Radial segregation in a conical silo
A consideration of the effect of this segregation combined with a preferential draw of the inventory is shown in Fig 4.
Fig 4 An example of blend imbalance in a discharge stream for a core flow
vessel discharging segregated material (Wolfson Centre laboratory data)
These types of effects can be apparent for processes fed from silos and bunkers, but a similar effect can occur where stockpiles are extracted from using front end loaders i.e. reclaim is from the periphery of the heap where a higher population of coarse exists – which would equate to a lower bulk density in the initial feedstock entering a conveyor. Such segregation is often an unavoidable consequence of handling bulk materials that exhibit variable particle sizes or densities.
The implications of the reclaim/discharge of segregated material is that not only will the bulk density vary in response to varying particle size distributions, but the handling behaviour will also change (typically adversely impacted upon by increased fines content).
In summary, it is hoped that this brief description of a few common problems and their causes will enable the reader to at the very least be able to diagnose some likely causes of product variability on plant. The main message that should be borne in mind is that there has been over four decades of scientific endeavour in understanding and predicting powder and granular solids behaviour in process plants which can be drawn on to deliver reliable equipment designs.
R.J.Farnish@gre.ac.uk
www.bulksolids.com
The Wolfson Centre for Bulk Solids Handling Technology
University of Greenwich
Central Avenue
Chatham Maritime
Kent ME4 4TB
Tel: +44 (0)208 331 8646
