(285e) CFD Simulation of a Laboratory-Scale Spray Dryer Fitted with Ultrasonic Nozzle for Production of Skim Milk Powder

Spray drying is a well-established method used in the food, chemical, and pharmaceutical industries for the production of various powder products. The basic principle of spray drying is based on contact between the droplets and the drying air, where the drying air provides the energy for the rapid evaporation of the solvent in the feed and absorbs the solvent vapour. The process of spray drying involves multiphase flow with heat, mass and momentum transfer between the continuous phase of three-dimensional swirling drying air flow and the discrete phase of polydispersed droplets/particles. Therefore, a good understanding of the dynamic flow behaviour in the drying chamber is essential to optimise the process of spray drying and ensure product quality.

Recent studies have shown that Computational Fluid Dynamics (CFD) is a powerful tool for the optimisation of spray dryer operational conditions. CFD is the science of predicting fluid flow, heat and mass transfer, and related phenomena. To predict these phenomena, CFD solves equations for conservation of mass, momentum, energy etc. One important advantage of this approach is the ability to evaluate the effect of the actual air flow pattern, particularly at the air inlet region near the atomiser, on the drying behaviour. CFD simulation can also be used to evaluate regions of high particle deposition flux of food powder which can aid the positioning of air hammers or manipulation of the operating conditions. Advanced modelling approaches based on CFD thereof plays an increasingly important role in developing next-generation drying technology. However, the modelling of the spray drying of dairy products has been relatively neglected. Furthermore, spray drying using the conventional pressure or rotary atomisers have drawbacks of broad droplet size distributions and risk of clogging. These disadvantages can be reduced using ultrasonic nozzles generating uniform droplets and low velocity.

Therefore, the aim of this study is to develop and validate a three-dimensional (3D) CFD model for predicting the air flow fields (i.e., velocity and temperature fields) and particle drying behaviour of skim milk powder produced by a laboratory-scale spray dryer fitted with an ultrasonic nozzle. In the present study, simulations are conducted using commercial CFD solver ANSYS Fluent (version 17). The 3D volume geometry of drying chamber of laboratory-scale spray dryer is sketched with the CAD System (SOLIDWORKS) and inserted into ANSYS Fluent. The Lagrangian-Eulerian approach is implemented to simulate multi-phase flows in sprays and atomisers. Continuous-phase, i.e., air, conservation equations are formulated in the Eulerian model while the droplet or particle equations are set up in the Lagrangian model. Two-way coupling between the continuous and dispersed phases is taken into account in the governing equations. The 3D simulation results give a better understanding of several phenomena that appear during the spray drying process. As such this approach has the potential to considerably reduce the optimisation time for spray drying processes and also assist in finding a solution to the fouling problem.