Droplet collisions and interaction with turbulent flows for powder manufacturing

Lead Research Organisation: Imperial College London
Department Name: Dept of Mechanical Engineering

Abstract

The following may be a key to the faster growth of powders in spray dryers:
- control of droplet and particle collisions and appropriate outcomes {e.g. droplet coalescence as opposed to separation or bounce-off},
- generation of appropriate droplet or particle concentration 'patterns' - increased 'clustering' in space and time - to increase probability of collisions,
- maintaining these qualities over a wider range of operating conditions.
Although spray drying is an old technology - 'drying' including the process of collision and agglomeration of droplets, semidried and dried particles amongst each other - remarkably little fundamental knowledge exists which would reliably answer the simple question: given this geometry of spray drying tower, what kind of spray(s) and air flow(s) should I produce in order to manufacture powder of the following quality (e.g. humidity, mean size, size distribution, morphology)? The answer must include an explanation of how and why small modifications to the location of atomisers and flow conditions radically changes powder quality. To formulate the answer, we need better and more extensive measurements of the 'fundamental' processes than hitherto: and we must generate new understanding and ideas that will advance our ability to calculate the location, number and outcome (bouncing, coalescence/agglomeration, sticking) of collisions in turbulent flows for realistic liquids (initially feedstock frequently has the consistency of toothpaste on a cold day) and geometries - and check the advance against fundamental, simple yet representative flow geometries.
The overall aim of the proposal is to innovate powder manufacturing in spray dryers by improving the understanding of the probability of droplet or particle collisions in turbulent flows and of the outcome of droplet-droplet, particle-particle or droplet-particle collisions with emphasis on liquid properties and geometries used in powder production through spray drying or other similar processes. The research hypothesis is that the growth of powders in spray drying processes can occur much faster than currently believed through simultaneous multiple droplet collisions, initiated by binary droplet collisions. This is because collisions have a relatively long duration and result in deformed transient shapes (ligaments, discs, etc.) with relatively large 'target' surface area. The ligaments formed during droplet pair collisions can interact with the flow turbulence during a collision event and break up, instead of coalescing into larger droplets. Both these events are not currently taken into account in the design of industrial spray dryers nor in computational models of droplet collisions used for predictions. Our approach will be to make novel time- and spatially- resolved measurements of the liquid, spray and gas motions using optical instrumentation in flow configurations that allow the study of the microscale physics of droplet collisions and in model spray dryers that allow the study of macroscale processes. We will use these results to establish new 'collision kernels' and unique semi-empirical correlations of droplet collision outcomes in turbulent flows and breakup of non-spherical ligaments and propose novel methods for control of powder manufacturing in, and new computational models for predictions of, spray dryers.

Planned Impact

Using the RCUK Typology, this project has impact in five fields:
- Environmental sustainability, protection & impact
- Commercialisation & Exploitation
- Improving social welfare
- Evidence based policy making & influencing public policies
- Increasing public engagement with research & related societal issues
The social, environmental and economic importance of improved manufacturing, in general, and powder production, more specifically, is significant. This project will take an innovative and coordinated approach, pushing forward towards novel, energy efficient powder manufacturing. It will do so in a way which is both general, by working on novel ideas which are applicable across a broad range of applications of whatever scale, and specific, by looking droplet and particle collisions at a local scale. The beneficiaries from this work will therefore be:

Society because of:
- The influence on improved evidence-based policy making
- Energy efficiency and sustainability
- The ideas underlying this project have the potential to improve current technology for powder production

The Industry and the UK Economy generally:
- The broad powder manufacturing industrial sector (e.g. food, pharmaceutical, chemical) because of our detailed focus on the physical processes of powder technology. During the project we will generate detailed information about a wide range of processes of direct relevance to the powder manufacturing industry. These will be of significant and immediate benefit as they fit directly into the Industry's own development plans. This is part of a strong national trend towards efficient and innovative manufacturing.
- Spray drying manufacturing industry through improved control of powder properties and reduced energy cost. This will generate impact over the longer term because the UK will lead the way in developing new and validated methodologies for assessing process efficiency and product control in the context of applying leading edge technology.
 
Description The manufacturing of powders (like detergents or dried coffee) is achieved through agglomeration between droplets and dried solids. The research has quantified the probability of collisions between droplets and particles in a turbulent flow, which can allow development of improved computational models to design more efficient manufacturing processes for powders. It also quantified the rate of liquid deposition on solid particles during the droplet collisions on particles.
These findings have opened new research questions on how powders grow in the real industrial process. This has been pursued by an industrially supported PhD student, who confirmed the identified concepts of spray drying process.
The research has developed specialist skills at the Postdoctoral researcher on the project and the associated PhD student.
Exploitation Route The findings from the grant and the industrially funded PhD student have been used for the assessment of computational efforts in industry. The potential exists to influence the real manufacturing process of powders in spray dryers, based on the new knowledge that has been disseminated to industry.
Sectors Agriculture, Food and Drink,Chemicals,Education,Energy,Environment,Healthcare,Manufacturing, including Industrial Biotechology

 
Description The improved understanding of droplet-particle collisions has been communicated to the supporting industry and is influencing the industrial powder manufacturing design. The industry is also supporting a PhD student, who continued beyond the end of the EPSRC grant and maintained the links with the industry. Therefore, the impact to industry will continue through transfer of knowledge that will lead to more efficient powder manufacturing.
First Year Of Impact 2018
Sector Agriculture, Food and Drink,Chemicals,Education,Energy,Environment,Manufacturing, including Industrial Biotechology
Impact Types Societal,Economic

 
Description Droplet dispersion in spray Dryers 
Organisation Procter & Gamble
Country United States 
Sector Private 
PI Contribution Collaborative research towards the understanding of the dispersion of droplets and collisions between droplets and particles in spray dryers. Designed a model scaled down version of a spray dryer, which has been used to study the dispersion of droplets and particles with laser based optical techniques.
Collaborator Contribution There was assistance in the scaling process of the spray dryer using computational modelling methods available at the industrial laboratory. In addition. advise was provided on the spray drying geometry and flow arrangements in order to design the facility that has been built.
Impact The outputs are related to the industrial process of spray drying for powder. New understanding has been developed which can improve the operation of the powder manufacturing processes and ensure that it becomes more energy efficient.
Start Year 2014