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modelling drying and particle formation in spray towers. Christopher Handscomb Wednesday 9 th May 2007. outline. Introduction to spray drying Modelling approach Continuous phase gas flow Single particle drying Conclusions and further work. what is spray drying?.

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modelling drying and particle formation in spray towers

modelling drying and particle formation in spray towers

Christopher Handscomb

Wednesday 9th May 2007

outline
outline
  • Introduction to spray drying
  • Modelling approach
  • Continuous phase gas flow
  • Single particle drying
  • Conclusions and further work
what is spray drying
what is spray drying?
  • An important technology in industry
  • Used to produce, for example:
    • Pharmaceuticals
    • Food stuffs (e.g. milk powder and coffee)
    • Detergents
  • Unique drying technology combining moisture removal and particle formation
motivation
motivation

A computational model would…

  • predict the effect of process conditions on final product properties
  • guide the operator towards safe and efficient operating conditions
  • facilitate the design of new plant based on physics, rather than correlations
modelling approach
modelling approach
  • Adopt an Eulerian-Lagrangian framework
continuous phase
continuous phase
  • Commercial CFD package – STAR CD – used to model the continuous phase
    • Well known in industry
    • Easy to test different geometries
    • Relatively simple to incorporate sophisticated user defined sub models
  • Test geometry developed representing a generic spray dryer
  • Counter current dryer
  • Single spray nozzle
  • Height: 22m
  • Diameter: 4m
  • 118,807 cells in CFD mesh
continuous phase8
continuous phase

z= 4m

z=0.5m

  • Can fairly easily produce plots of the flow field
  • Consider a single droplet
single particle drying
single particle drying
  • Consider the drying sub-model
  • Modelling assumptions:
    • Three component system:
      • A – solvent;
      • B – solute;
      • D – solid
    • Spherical particles, 1D model
    • Small Biot number  uniform particle temperature
    • Allow for a single centrally located bubble

Assumed ideal binary solution

single particle drying10
single particle drying

wet bulb temperature

boiling temperature

Cheyne, A., Wilson, I., and Bridgewater, J. (2002).

single particle drying11
single particle drying

external coordinates

internal coordinates

advection terms

diffusion terms

source term

  • Population balance for solids

Cheyne, A., Wilson, I., and Bridgewater, J. (2002).

  • Spherical symmetry  reduce to 1-D
  • Solve for the moments of this equation
single particle drying12
single particle drying

assumed independent of internal coordinate (particle size)

  • Variable of interest is solids volume fraction
  • Related to the moments of the population balance equation by:
  • Obtained by solving the moment system:
single particle drying13
single particle drying

Volume Averages

Superficial

Intrinsic

diffusion

evolution

advection

crystallization

Total

  • Volume averaged transport equations for the continuous phase
  • Advection velocity calculated from volume conservation considerations
  • Diffusion coefficient from measurements
single particle drying14
single particle drying
  • Population balance boundary conditions
  • Solute boundary conditions
moving boundary
moving boundary

virtual flux

  • Moving boundary handled through a standard coordinate transformation r  z:
  • This adds a ‘virtual flux’ to all equations
solution method
solution method
  • Problem is a system of PDEs

and coupled ODEs

  • Solved using Numerical Algorithms Group (NAG) library routines for convection-diffusion type equations
  • Finite Volume approach with user-defined flux function
new drying model example
new drying model – example
  • Model described so far can simulate

up to the point of shell formation

  • e.g. Consider a system:
    • Initial 14wt% sodium sulphate solution – no solids
    • Crystallisation model from Rosenblatt et al. (1984):

‘Kinetics of Phase Transitions in the System Sodium Sulphate-Water’

    • Droplet diameter = 1.78mm
    • Drying air T = 373K
    • Droplets initially well mixed
new drying model example18
new drying model – example
  • Compare with experimental data from Nesic and Vodnik (1990) Kinetics of Droplet Evaporation Chem. Eng. Sci.

Experimental data from Nesic and Vodnik (1990) Kinetics of Droplet Evaporation Chem. Eng. Sci.

new drying model example19
new drying model – example

But the new model can give us much more…

Experimental data from Nesic and Vodnik (1990) Kinetics of Droplet Evaporation Chem. Eng. Sci.

conclusions
conclusions…
  • Introduction to spray drying and the associated modelling challenges
  • Results of continuous phase simulation
  • Overview of a new drying model
  • Comparison with experiments for a ‘simple’ case…
work not shown
…work not shown…
  • Drying after shell formation
  • Simulation of detergent droplets drying with experimental comparison
  • Simplified drying models implemented in CFD code
and further work
…and further work
  • Obtain data and validate model for high temperature drying
  • Couple (simplified) model to CFD simulation
  • Compare with existing drying models when used in CFD