Key points
My academic training as a chemical engineer, back in the mid 80’s, consisted of the usual focus on oil and gas with minimal mention of solids. This is despite the fact that well over 50% of the output of the global chemical industry is in the form of solids and it is well known that the start-up of solids processing plants is more likely to cause problems and fail at the most basic level – for example, falling well short of design capacity. It is even not unheard of for some new solid processing plants to be stopped due to unviable operation.
The understanding of fluids and related unit operations has been the subject of intense study for 100’s of years if we think of Archimedes and more recently William Froude, Osborne Reynolds etc.. Conversely, a numerical approach to the flow of solids was not properly studied until the work of Andrew Jenike (1950’s onwards).
This sets the context for the lack of focus in undergraduate engineering courses, however the serious consequence is that new engineers enter industry with a significant lack of knowledge in how to deal with solids handling.
During my undergraduate training it never occurred to me that this was a problem until I started my first role in industry which was 100% focussed on the processing of solids.
Opening Thoughts – an Industrial Career Playing with Powders
I would like to share my first in plant experience as a new chemical engineer working on a development project. I was the junior member of a team of 5 people who had travelled from the UK based central process development department to one of the company’s plants in France.
The objective was to run granulation trials on a continuous process based only on kilo scale batch experiments where product was transferred by hand from one unit op to the next. The product was a respiratory hazard and therefore necessary to wear air fed helmets and disposable paper suits – not a pleasant experience!
As the most junior member of the team, I was assigned to the bulk bag station to await the flow of product and take samples. I will never forget the experience since there was no means of communication and I stood alone at my station for about 3 hours waiting for the good product that never arrived.
Why ? The sticky agglomerates produced in the granulation step were simply too cohesive to be handled in the continuous fluidised bed cooler and every attempt failed resulting in a lengthy manual dig out of the fluid bed.
Throughout my career I have encountered many examples of problems related to solids handling which, I’m sure if analogous problems occurred in an oil refinery the responsible engineer would be fired whereas in solids process it is still accepted as “one of those things” !
Pictured right a powder doser to a packing line which is meant to have a smooth conical outlet and right a common sight of an operator using a hammer to try and unblock a silo.
As a recently graduated engineer, I was very keen to use engineering calculations to predict what would happen at the large scale. However, I encountered a serious lack in the belief by senior technical management that there was any point in taking measurements at small scale to predict in plant behaviour.
The accepted philosophy was that small scale/pilot plant was only for producing product samples, manufacturing feasibility had to be proven at the large scale. Whilst extremely inefficient, this approach at least worked if you wanted to change the product processed in an existing plant.
However, if you needed to build a new plant, the fall back approach was trials at equipment suppliers. This was hopelessly inadequate since testing equipment in isolation is a very poor substitute for testing the full integrated process.
Typical Industrial Bulk Powder Handling Challenges
– A journey of 30 years illustrating the Evolution of industrial powder flow testing
As my career progressed, I was in the fortunate position to use and contribute to the development of better methods for predictability in the development and scale up of processes involving solids.
The Jenike shear cell for the assessment of powder flow properties was the starting point for many industrial engineers attempting to put some “science” into what was previously an “art”. In the early ‘90’s I was involved in deploying a Jenike shear cell in an industrial environment.
Unfortunately, whilst the merit of the method was not in question, the equipment was very difficult to operate and required a dedicated technician to build the necessary skill to get reproducible results. In an industrial context, this was not practical, and the equipment was eventually donated to a University.
The next development was an automated shear cell tester produced by Dr Ivan Peschl and sold as a user friendly computerised test equipment (pic left). At the time I was involved in the development of a high shear granulation/drying process and a critical design question was “how much do we need to dry to ensure good flow from storage silos”. The Peschl test was very successful in answering this question and the test continued in use for many years.
The Peschl equipment was a great step forward when launched but it’s application was relatively narrow. Working in the development and scale up of solids handling processes, there is a need to put “science” into many more questions which are critical to the successful design and operation of new plants. For example –
- Will my powder flow through a rotary valve without smearing ?
- What size chute do I need for a certain flow rate of my powder ?
- How does speed impact die filling efficiency on my tablet press ?
- etc ……..
In 2010 I was introduced to the FT4 powder tester produced by Freeman Technology. In many areas this equipment has replaced the Peschl test (and others) but with much broader capabilities.
The FT4 was the first universal powder flow tester for measuring powder flow properties and powder behaviour.
In other words, the equivalent of a rheometer for powders. In my experience the FT4 was used successfully on many projects to guide design decisions and remains as the most prominent test for quantifying powder properties and linking measurements to process design.
New Challenges in Complex Formulated Solids Products
A “formulated product” is a product made of a mixture of components having a higher value than the sum of its parts. Typical examples include –
- Detergents
- Foods
- Pharma
- Crop protection/herbicides
Formulated dry (powdered) products bring a particular set of problems for process design and ongoing plant operation –
- Consistency (“right every dose”)
- Demixing / segregation
- Capability to mix low level actives e.g., enzymes
- Filling accuracy – minimise “give away” whilst complying with pack weight regulations
These problems have always been present but the new direction in many formulated products raises the need for these issues to be addressed in process design. For example, automatic dishwashing detergent has moved away from the bottle of powder to tablets and capsules (powder or combination of powder/liquid).
Typically, producers have focussed on ensuring that the composition of a bottle (kg’s) is correct. However, a dishwashing tablet is only about 20g and it is critical for the cleaning performance that every tablet has the right level of every ingredient.
In liquids this requirement is not difficult, in powders, segregation of bulk mixes can lead to significant variation in the composition of individual “doses” used by the consumer.
In this arena, the process engineer responsible for the design of plants to make and pack these products can no longer rely on “gut feel” and experience. It is critical that measurements derived from tests such as the FT4 can be related to full scale plant behaviour.
Industries deploying solids processing are generally accepting that science must replace art to ensure ongoing commercial success. This is also recognised by government supporting initiatives in this field such as the National Formulation Centre based within The Centre for Process Innovation (CPI) in the North East of England.
Closing Thoughts – The Future
How can we achieve the “Lighthouse” vision of linking easily measured powder properties (analogous to a liquid viscosity) to process plant design and operation ? This is critical for the successful start up and operation of processes involving solids –
- Avoiding excess/no flow situations
- Achieving predictable plant performance for solids
- Moving solids processing to better and more advanced levels of process control
Industrially relevant measurement systems for solids are available and becoming much more prevalent than when I started my career. In my experience, senior management at the majority of companies handling powders will no longer accept lengthy starts-ups and ongoing problems for solids plants.
Coupled with vastly superior capabilities in the collection and manipulation of process data from plants (Industry 4.0) we are making significant progress to realise the depth of understanding required to achieve this vision. Compared to fluids, the chemical engineer of solids is still in it’s infancy.
However, economic drivers coupled with the realisation that the old approach is not acceptable, are driving change. Here at DJS Process Consulting, we are proud to have an experienced team who are guiding many of our clients along this journey to realise on time/budget launches of new products and increased consumer satisfaction with always on spec. production.
