Key points
Most manufacturing plants employ multiple processing steps to take raw materials or (pre-processed) materials and create a finished particulate product that is required to have consistent bulk properties and functional performance, e.g. taste, dissolution properties, chemical composition etc.
These processing steps might include operations to; create particles (spray drying, centrifuging), modify the particle size (grinding and crushing or comminution and agglomeration), blend or separate different particulates (mixing, grading screening) remove moisture (band dryer), and finishing processes (coating, tableting or packing into big bags, sacks, sachets etc).
In order to achieve these successive processing operations, the bulk particulates must be conveyed through the factory from one step to the next using; mechanical conveyors; screw, chain, vibratory, pneumatic conveyors, belt conveyors or manual handling.
Since these processing operations require a continuous material feed for efficient operation, buffer storage in the form of a hopper, silo, bulk bag is required before each process.
Are your storage and handling equipment weighing you down?
In many industries it is the performance of the storage and handling equipment that can limit the process throughput, due to the potential for material arching over the outlet of a silo or block in a conveyor due to cohesion i.e. particles sticks together when compressed.
Cohesion is primarily a function of particle size, as this reduces inter-particulate forces (van der Waals) dominate over gravity forces. As typical values, most materials start to exhibit cohesion when the particle size reduces below 100microns and some become non-flowing when the particle size approaches 1micron.
However, the forgoing assumes the absence of liquid; the surface tension of the liquid on wet material can generate cohesion when the particle size is of the order of several millimetres.
Processing problems
Free flowing materials can also generate processing problems but presents different challenges. These are the:
- Propensity for the material to segregate, i.e. particles separate by size, shape or density when poured to form a heap, as occurs when they are gravity loaded into a buffer silo or into some types of conveyor.
- Separation of dust or fines (typically below 50microns) occurring when material is charged into an open space at high velocity, e.g. pneumatic loading of a silo or equipment fed by long gravity chutes (e.g. where processing steps feed from one floor down to the next).
Thus process plant design and efficient operation is complicated by the interrelations between the numerous processing operations and the flow characteristic of the storage vessels and conveyors being used, which can also generate unintended changes in the particulate material. For example:
- Process steps that change the particle size, or drive of surface moisture will result in changes in the way the bulk material flows through the conveyors, most operating less efficiently as the particle size reduces, or surface moisture increases (below saturation).
- Furthermore some conveyor types lead to particle attrition in operation. For example in a pneumatic conveyor the particles suffer an impact against each bend in the pipeline leading to particle fracture, fines generation and an unwanted change in the size distribution of the product.
- Another common challenge is that poorly designed silos can drive segregation of particles by size, which can undo preceding blending operations and result in a finished product that is out of specification.
When troubleshooting a processing line or defining modifications to a plant it is critical that a holistic view of the interaction of the complete assembly of equipment. Failure to do so considerably increases the risk associated with such projects.
Most of the above problems can be predicted and equipment can be designed to either eliminate or minimise the effects. However all these design methods require characterisation of the relevant flow properties of the material to generate relationships between particle and bulk scale, which requires testing on representative samples at process appropriate conditions of temperature, relative humidity and storage pressure.
Independent expertise: Providing cost-effective solutions to industrial problems
The Wolfson Centre for Bulk Solids Handling Technology is internationally recognised for its expertise in fields associated with bulk particulate handling and in the science and practical application of technologies to deliver improved process efficiencies. Our mission is to help industry to get powders and bulk materials to behave predictably through processes.
Industry sectors benefiting from our services range widely and include power generation, chemicals, pharmaceuticals, foods, plastics, paper, minerals and cosmetics. Our clients are predominantly based in the UK, although many European and overseas companies take advantage of the independent and bespoke nature of our work.
We have developed our portfolio of services over nearly four decades of interaction with industry, trade associations and professional bodies. We offer consultancy services to industry, conduct industrially focused research related to bulk solids handling issues and offer specialist short courses targeted at industry professionals. If you have a problem with your plant or process, we have the knowledge and experience to provide cost effective answers that you can rely on.
The centre is situated at the University of Greenwich”s Medway Campus, where we are part of the School of Engineering. Our location offers excellent transport links within the UK and EU enabling us to provide a prompt and flexible response to site visit requests and client meetings.
Our experts have between them over one hundred man-years of experience of solving problems in the flow and processing of powders, granules and lump materials, in all industries from pharmaceuticals and food to mining and chemicals; from biomass waste and recycling to household goods and pet foods.
They concentrate on the needs of the industry and share their knowledge through the provision of technical research papers, articles and editorials through the media and at conference and exhibitions where they often present papers or hold workshops.
Our industrial-scale pilot plant is available to test a processing step, or mock up a whole handling process or logistics chain, at full scale or near full scale. This can be used to find and solve the problems off-line, then when you transfer the same process or material to your own or your customers’ plant, you can be confident it will work!
Contact us now on Tel: 020 8331 8648 E-mail: wolfson-enquiries@gre.ac.uk to find out how we can help you.
