Energy and Exergy Analysis in Spray Drying Systems

Abstract

Drying processes contribute a significant proportion of energy use within industry today, from food and pharmaceuticals to construction products. The need to find and improve methods to increase the energy effectiveness of drying processes without reducing product quality can have significant effects on both financial costs and the cost to the environment through direct or indirect greenhouse gas emissions. These methods, beyond what is already used in industry already (such as improved insulation), are required to be able to be used and understood to make a difference. There are various tools and methodologies which can be used to analyse and estimate the effect of process changes on systems – from ideal rate comparison models (Inversion Temperature) to heat exchanger optimisation tools (Pinch Analysis), to a full energy utilisation integration (Exergy Analysis). It is important to understand when each method is suitable, and the benefits of each. In order to compare the Inversion Temperature (rate-based estimation method) with Pinch Analysis (thermal flow model), a Pinch Analysis was applied to both systems, yielding reasonable results (over open systems in both cases) of 18% energy recovery for the air system and 8.4% (with 86.6% potential) energy recovery for the steam system. The use of exergy analysis over a single unit operation has been explored, showing that while a spray dryer is effective at rapidly drying particulate solids, the energy use is inherently inefficient. An exergy analysis has been extended to two different systems which support the dryer –an electrically supplied system (vapour recompression) and a typical natural gas-driven steam boiler system. The key results are that as the systems get larger and more complex, exergy losses tend to get higher, and the total exergy lost within both systems is significantly higher than just the dryer on its own.