Abstract:
Infrared drying presents several typical advantages, such as the high drying rate without loss of the quality of products, but there is a problem of uneven heat distribution in the drying process. In spouted bed drying, the strong points are efficient mass and heat transfer, particularly on the uniform heating. To combine infrared drying and spouted bed drying can be expected to maintain efficient heat and mass transfer, and concurrently ensure optimal quality of products. However, there is no research on the combination application of the two techniques for drying process until now. In order to improve the drying efficiency and quality of rose, an attempt has been made to investigate the effects of outlet temperature and wind speed on the drying characteristics using a new developed method of infrared drying assisted by spouted bed dryer, and then to establish an ideal model for the drying kinetics during drying. The rose color, rehydration ratio, total content of flavonoids, phenol content and microstructure were characterized under the different drying conditions. The results showed that the increase in the outlet temperature and wind speed can significantly improve the drying rate, while dramatically shorten the drying time. Specifically, the required drying time at 55 ℃ was 69% shorter than that at 45 ℃, and the wind speed of 23.1 m/s was 69% shorter than that of 16.4 m/s. The drying process can be divided into two steps: ascending and descending drying stage, without obvious stage of constant speed. In the early stage of drying, the phenomenon of “bubble bed” appeared, and then a good spouting state formed in the spouted bed as the drying process, where the loss of the material increased rapidly and the drying rate reached the maximum, finally in the descending stage. The effective moisture diffusivity coefficient ranged from 6.703 85×10-10 to 1.382 35×10-9 m2/s, and increased with the increase of the outlet temperature and wind speed, indicating that the increase of outlet temperature and wind speed can lead to the enhancement of thermal effect, which was beneficial to the moisture diffusion. The Midilli model was determined to better describe the drying curves, after the comparison of drying models for 12 thin layers of rose petals. The maximum and minimum rehydration ratio of rose increased by 72.5% and 86.7% under different outlet temperatures and wind speeds, respectively. The high outlet temperature, high wind speed and long drying time can benefit to the reduction of total contents of flavonoids and phenols. It is also found that the proposed method of infrared assisted by spouted bed drying can be more beneficial to the retention of effective components than that in hot air drying, which is commonly-used media in the convention drying technology. The effect of wind speeds on rose color and microstructure was more significant than that of outlet temperatures. With the increase of outlet temperatures and wind speeds, the rose was subject to large deformation, more serious shrinkage during drying. This finding can provide a sound theoretical reference for the research and application of infrared assisted by spouted bed drying technique in plant drying.