Single Droplet Drying at High Temperatures

Publication Reference: 
ARR-38-14
Author Last Name: 
Bayly
Authors: 
Wael Ebrahim, Andrew Bayly
Report Type: 
ARR
Research Area: 
Particle Formation
Publication Year: 
2016
Publication Month: 
12
Country: 
United Kingdom

Project resources: A first PhD student (Wael Ebrahim) started on the project in April 2017, his work and that of supporting Master’s student projects is reported here. Funding was also secured from the University of Leeds for a second student who will start in November 2017 (Tien Nguyen), his primary focus will be the introduction of mechanical models into the single drop drying framework. Funding has also been received from the EPSRC for a collaborative project, with Durham and Bristol Universities, looking at the fundamentals of structure formation when single drops are dried. This project has just started and wil

Objectives: This project seeks to develop experimental and modelling methods that enable dried particle structure, properties and drying rate to be predicted based on droplet drying history. The project will focus on effects driven by boiling and look to develop material independent models which capture behaviors of industrial interest. In particular it will look to address key limitations in current understanding: 1> the impact of a non-isothermal drying history on particle structure and consequently drying rate; 2> improved measurements of material properties under the non-equilibrium conditions experienced during drying; 3> extension of models to include the mechanics of structure formation.

Approach: Initially, material classes and materials will be identified. Novel experimental rigs and methods will be developed to allow unsteady drying behaviour to be investigated and material properties to be measured under relevant conditions. Established models will be extended to include better material models and mechanical deformation. We will bring this together into a regime map(s) which links material properties, particle size, initial moisture content and drying history to morphology.

Recent Technical Progress:

Benchmarking dryer/drop tube operation – the pilot spray dryer from ProCepT was investigated to check the accuracy of the inlet temperature measurement, check the inlet temperature distribution and to investigate the heat loss. A significant offset in inlet temperature was recorded and a distinct profile in the inlet temperature across the system was measured. Large drops in heat loss were mitigated by insulating the dryer. The dryer will be used as a basis for the drop tube.

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Evaluation of mono-dispersed atomization technologies – several mono-dispersed atomisers were assessed, a customised atomiser from Leeds will be tested on the drop tube

Behaviour of HPMC (one of the model systems identified) has been mapped - the ProCept spray dryer was used to dry HPMC droplets across a range of droplet sizes and dryer temperatures. At all temperatures we saw highly deformed structures, there was no clear, discontinuous, change when the droplet exceeded the boiling temperature. However two distinct structures were noted, a highly deformed, deflated, structure and a smoother spherical structure.

Single drop drying rig development – overall design has been defined and support and pipe work structure complete. The heater control is due to be finished in January and initial commissioning will start then.

Models to estimate the composition distributions of atomised slurry droplets have been developed – a stochastic model has been developed that enables the composition distribution of slurry droplets to be estimated. At low concentrations of solids the solid particle number in each droplet shows a Poisson like distribution.

Models to investigate the prediction of boiling have been developed – a CDC model has been used to estimate the bulk moisture concentration at the droplet boiling temperature. This will be extended to diffusion based models to enable the link between material properties and diffusion to be explored.

Initial experiments into microwave droplet drying have been made – a method for material characterisation by puffing droplets using microwaves has been explored. Initial attempts have proved unsuccessful and work is in progress to develop the technique.

Future Focus - 2017

Rig build and commission: The initial version of the single drop rig is due for commissioning Jan 2017. A mono-dispersed nozzle for the drop tube will be tested in the spray dryer Jan 2017. iv

System mapping: The work with the HPMC highlighted the need to do this for a mono-dispersed drop tube system; consequently this will begin once capability in place. Opportunities to work with alternative techniques at collaborators are also being investigated.

Material properties: Measurement and technique development due to start Q2 2017.

Model development: Approach defined for incorporation of mechanical properties by end of Q2.

Associated work: CFD modelling of the ProCept pilot scale spray dryer which will enable better understanding of the droplet trajectories and drying histories. (Masters project completion May 2017).