By Richard Farnish, MPhil CEng MIMechE – Consultant Engineer – The Wolfson Centre for Bulk Solids Handling Technology
The need for characterisation
Many industrial processes are formed from assemblies of “standard” equipments types that offer an economical route to obtaining a functional production system. Invariably, the specifications that are drawn up (and against which equipment is supplied) are stringent in the process requirements that must be met.
The definition of feed rates, operating pressures, and spatial constraints are well within the capabilities of plant engineers to distil into a well written set of requirements. However, the properties of the powders or granular materials that will be handled through such plant are seldom given the thorough consideration that they are due, and as a result references to material properties range from sparse to virtually non-existent in many sets of documentation against which (in many cases) considerable expenditure is planned or undertaken.
The usual result is an element of a plant that can only achieve its planned level of efficiency with a certain sub-group of bulk materials (out of the full range handled on site). Bottle necks formed in the process by these situations are known to lead to significant direct losses to the bottom line of the plants profitability.
The financial (or contractual) losses that are often incurred are typically many magnitudes higher than the initial investment or resources required to adequately define the storage and handling behaviour of bulk materials in either raw material or final blend forms.
In recent decades the provision of test techniques available to industries from all sectors has improved considerably through the research work of academic institutes. The behaviour of bulk materials through handling systems can now be ascertained scientifically for a wide range of commonly found conditions such as: size distribution, density/compressibility, segregation, storage / flow behaviour (vessel geometry for reliable operation), “caking”, particle breakage, adhesion and packing performance.
It can be readily appreciated that the incorporation of this level of information into a contractual specification document not only provides a highly defined process requirement – but also equips companies responding to tender documents with a sufficient level of technical information to ensure that their equipment is suitable.
Flow property measurement
One of the most useful measurements for many types of process operation is that of the flow ability of powders or granular materials. If the flow behaviour of a range of materials (or material conditions) can be determined at the design stage of a project, then the construction of vessels utilising this information can deliver equipment that will not only operate without flow irregularities or stoppages, but will also have the capability to reduce segregation effects.
Approaches exist for obtaining data relating to the flow ability of powders, however many of the most commonly applied techniques such as angle of repose are actually irrelevant from a design perspective and have their origins before the major improvements in the understanding of powder flow behaviour that have taken place over the last five decades.
In recent years there has been a change in the provision of test equipment for use in establishing the flow behaviour of powders. Previously shear testers (the approach used for determining the flow behaviour of powders) had mainly been designed by or supplied to academic institutes or specialist formulation development laboratories. These types of tester were typified by the need to use skilled staff for their operation. More recent types of shear tester are automated and designed specifically to allow the gathering of scientifically valid test data – but without the high initial investment costs associated with traditional testers or the need for skilled operators. Through the use of shear testers the variability of bulk materials can be assessed over a wide range of conditions.
It is now a simple procedure for engineers or formulation scientists to be able to obtain accurate information relating to flow behaviour for bulk materials over a wide range of conditions commonly encountered in process plants.
Raw material cost versus plant efficiency
The ease with which the storage and handling behaviour of bulk materials can be handled can measured can be used in a variety of ways to improve plant operation. A common source of plant inefficiency is the time taken to reconfigure equipment settings to accommodate different sources of raw materials.
This is an increasingly frequent issue for many plants and is the result of raw materials being sourced on the basis of the best “deal” that company buyers can negotiate. In many cases the evaluation process associated with this type of procurement strategy is purely economy at the point of purchase – which can leave the plant engineers with a delivery of materials which may or (more often) may not handle well through the existing plant.
Clearly, if the flow properties of the raw material are taken into account before the final purchasing decision is taken then a true correlation can be draw between the attractiveness of price for the raw materials and their potential to generate process inefficiencies far in excess of the perceived “saving” on purchase price.
There is an improving provision of test techniques and equipment types that industry can use to improve its understanding of bulk material characteristics. This brief article has highlighted just one of these procedures – shear testing, although to do full justice to the finer points and detail of this particular technique would require a considerable number of pages!
It is hoped that the reader is now in a better position to appreciate that the seemingly random nature of many types of process stoppage (i.e. arching or rat holing over outlets) can be directly predicted through the use of shear test data. Similarly, other techniques can be applied to predict variation in bulk density or blend homogeneity – also very common problems in industry.
The first step towards reducing down time or reject materials levels must be to understand the bulk materials that are being handled. Failure to undertake this type of preliminary (or trouble-shooting) activity is to perpetually exist in a “fire fighting” mode of operation rather than one of forward planning.