Gaseous Carbon Dioxide as the Heat and Mass Transfer Medium in Drying

Using available correlations for heat transfer, a comparative analysis of drying rates in CO₂ and in air was performed for several basic types of dryers. Higher heat transfer rates were found for dryers with active hydrodynamics, which translates into shorter drying time for materials dried in the first drying period. These results were validated by experiments on drying wheat kernels fluidized by air and by CO₂. Shorter drying times by about 20% were confirmed for CO₂, which offers energy savings of about 3% of the heat input to the dryer. Additional energy savings of 4% of the heat load can be expected for drying at temperatures below 100°C because of the lower wet-bulb temperature for CO₂ than that for air. The potential for CO₂ abatement was evaluated based on a case study for drying of distillers' spent grain.