The Stress Engineering Process Technology Group has extensive experience in analysis and simulation of drying processes. This example shows a CFD analysis of a powdered detergent spray dryer. The dryer is the counter-current type, with hot air entering through vanes in the bustle of the dryer and wet particles injected through pressure swirl atomizers at the top. The air inlet vanes are angled to generate a swirling air flow in the dryer. This provides longer residence times, allowing the required level of drying to be accomplished with a shorter dryer.
The figures below summarize the results of the CFD simulation. The velocity vector plot is colored by magnitude. This plot shows overall flow pattern within the dryer. Of particular interest is the region near the wall immediately above the inlet jets; here the velocity is very low owing to the separation of the flow from the wall.
The particle track plot shows the mean paths taken by the spray of droplets leaving the atomizer nozzles. Results show that the spray is forced strongly to the wall. This indicates a region of potential problems where product will build up on the dryer wall.
The figures below show the thermal and mass-transfer solution from the CFD model. On the left are contours of water vapor mass fraction ranging from 0% (blue) to 5.8% (red). The right plot shows contours of temperature throughout the dryer. These indicate the presence of a region of cool, wet air adjacent to the wall of the dryer and a core of hot, dry air in the center. This is consistent with the velocity vector plot above which shows relatively low flow at the wall, and the particle tracks which show that cold, wet particles move along the wall.
With the temperature and water mass fraction of the air known, temperature and level of dryness of the particles as they traverse the dryer can be calculated. Using this information we can assess whether the dryer will perform as required. In particular, we can determine whether the residence time is too long, leading to overdrying of the particles and damage to the product. Alternatively, the residence time may be too short or the temperature too low, leading to underdrying of the particles and a sticky product that is difficult to handle. We use these results to guide design changes to either the dimensions of the tower or the process settings. We can then test the modified design using new CFD models. In this way we make CFD analysis part of a "virtual prototyping" process, significantly reducing the time and expense of the eventual physical prototyping process.
