Modeling of multiphase flows in a spray dryer

Examensarbete för masterexamen
Vasudevan, Sindhuja
Spray drying is a robust and popular unit operation in the pharmaceutical industry. One of the many applications of this unit operation is the manufacture of dry dosage formulations for nasal or pulmonary delivery. It is important that the generated particles from the spray drying process have certain attributes such as flowability, dispersibility and suitable aerodynamic properties, which depend on the particle design. In order to obtain a suitable particle design, a sound understanding of the particle formation process, which includes the physical and chemical mechanisms that control the drying process, is required. In this thesis, the drying process is studied by modeling with the commercial computational fluid dynamics (CFD) code ANSYS Fluent. This multiphase system which includes a gaseous continuous phase and droplet/particle as the dispersed phase is modeled using an Euler-Lagrangian approach. In Euler-Lagrangian modeling, the fluid phase is modeled as a continuum while for the dispersed phase a large number of individual particles is modeled. The different phases are said to be coupled when there is exchange of momentum, mass and energy between them. The significant presence of exchange of mass and heat between the continuous and dispersed phase indicates that the phases are two-way coupled, i.e., the continuous phase transfers mass and heat to dispersed phase and vice-versa. The effect of turbulence on the dispersed phase is accounted through a turbulent dispersion model. The choice of characteristic parameter of the turbulent dispersion model, i.e., the ’number of tries’ is decided based on a sensitivity analysis. This is done by analysing its effect on the residence time distribution of the dispersed phase. Analysis of the drying data obtained from the simulation reveals that the rate of drying of droplets of same initial diameter is different since they follow different trajectories. That is, for droplets of the same initial diameter, the drying rate varies in such a way that the maximum value is around 50% higher than the minimum value. Hence, this implies corresponding variation in solute concentration profile in the droplet and hence, particle structure for particles obtained from droplets of same initial diameter. Additionally, the impact of process operating conditions, especially the mass flow rate of the solution, on the drying rate and hence, the corresponding particle structure is indicated.
Spray Drying, Euler-Lagrangian, Discrete Phase Model, Drying Rate
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