Drying is an energy intensive process, which accounts for up to 15% of all industrial energy utilizations. Along with the energy shortage and environment issues, it is important to find an energy-saving and environmental friendly drying technique. Drying assisted with heat pump is seen as an ideal way. It uses heat pump to dehumidify exhausted drying air to recover the latent heat of water. It can also reuse the dehumidified air. Hence, it has advantages of energy saving, clean and high dried product quality.

In this study, a Chinese medicine herb, Dong quai was used as wet product dried in the experimental heat pump dryer using carbon dioxide. The drying characteristics in heat pump dryer under various conditions have been investigated. Different drying temperature (45 ,...[ Read more ]

Drying is an energy intensive process, which accounts for up to 15% of all industrial energy utilizations. Along with the energy shortage and environment issues, it is important to find an energy-saving and environmental friendly drying technique. Drying assisted with heat pump is seen as an ideal way. It uses heat pump to dehumidify exhausted drying air to recover the latent heat of water. It can also reuse the dehumidified air. Hence, it has advantages of energy saving, clean and high dried product quality.

In this study, a Chinese medicine herb, Dong quai was used as wet product dried in the experimental heat pump dryer using carbon dioxide. The drying characteristics in heat pump dryer under various conditions have been investigated. Different drying temperature (45 , 55 , 65 °C), drying load (1.0, 1.5 , 2.0 , 3.0 kg), and air velocity (1.0 , 1.5 , 2.0 m/s) have been compared to find out better drying conditions. It was found that high temperature significantly increases the drying rate of Dong quai and shortens drying time. The drying time of 45 and 55 °C drying temperature was 70% and 100% longer than that of 65 °C. The large drying load would decrease drying rate until 3.0 kg. Drying time of 1.5 kg and 2.0 kg was 153% and 200% longer than that of 1.0 kg. But drying time for 3.0 kg was very close to that of 2.0 kg. In terms of energy consumption, high temperature and large drying load in this experiment have higher SMER and save energy. However, high drying air velocity did not have significant influence on drying process in this experiment.

Then, the quality of dried Dong quai was evaluated. The rehydration, HPLC for extract of Dong quai constituent content, and microscope photography were used for quality evaluation. Conclusion was drawn by considering all the evaluation results. It found that the rehydration of product under 55 °C heat pump drying was 113% and 132% higher than that of 65 °C and 45 °C. The rehydration of product under 55 °C heat pump drying was 95, 106, and 122% than sun drying, oven drying and vacuum drying. Although The Z-ligustilide in drying product of 55 °C heat pump dryer is 93% than that of sun drying, but it was at least 138% higher than that of the other products. It means that heat pump drying under 55 °C can give the best drying product quality, and heat pump drying product is competitive in term of quality.