Modelling coagulation in industrial spray drying : an efficient one-dimensional population balance approach

  • Jonathan Wells

Student thesis: Doctoral Thesis

Abstract

This thesis examines the effects of coagulation on the droplet size distribution within the hollow cone sprays used by industrial spray dryers. A simple model is presented that incorporates the spray's conical shape, droplet transport and coagulation. Both a suitable droplet velocity profile and a coagulation success rate are selected - with the coagulation rate being dependent upon both the size and velocity of colliding droplets.A population balance equation is used to describe how the droplet size distribution is affected by coagulation and this is combined with a conservation equation to capture the droplet transport within the spray. The resulting nonlinear partial integro-differential equation is solved numerically using the cell-average sectional method for the non-local coagulation terms and an essentially non-oscillatory scheme for droplet transport terms.To our knowledge, this is the first time the cell average technique has been applied to the specific conical sprays found within spray drying. A key achievement of our approach is the significant reduction in computational time that it requires to simulate the droplet size distribution throughout the spray. In comparison, alternative computational fluid mechanics techniques take considerably longer to converge on a steady state solution, as they rely on tracking the trajectory of every individual droplet within the system.Numerical results obtained from our model show that coagulation does play an important role within these sprays. Moreover, investigations into the inuence of the atomiser nozzle on the extent of coagulation reveal that an increased amount of coagulation occurs in thin, flattish sprays, whilst very little coagulation is observed in widely dispersed sprays.The model is later extended to include the effects of droplet evaporation. In this case, an increase in the number of smaller droplets near the bottom of the spray is clearly observed in the numerical simulations. Finally, the model's predictions are validated against experimental data obtained from a series of trials conducted within an industrial-scale dryer tower. Once calibrated, the model's predictions are shown to be in very good agreement with the experimental data.
Date of Award1 Apr 2015
LanguageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsEPSRC (Engineering and Physical Sciences Research Council)
SupervisorJohn MacKenzie (Supervisor) & (Supervisor)

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