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Drying temperature, air velocity, material thickness during hot air drying usually have a significant influence on drying kinetics and quality attributes of fruits and vegetables. However, under co...

As a measure to enhance drying kinetics, efficiency, and quality of the whole American ginseng root, step-down relative humidity (RH) convective hot air drying strategy was experimentally investiga...

A mathematical model was presented to characterise heat transfer in high-humidity hot air impingement blanching (HHAIB) of cuboid carrots. The model accounts for condensation heat transfer and predicts the sample’s core temperature evolution with time. The simulation was performed at different relative humidity of 20%, 40%, 60%, 80% with constant temperature of 383 K. Results showed that the heat transfer process of HHAIB is divided into two stages based on vapor–liquid phase transition, namely a condensing heat transfer segment and a non-condensing heat transfer segment. In the initial stage of HHAIB, a comprehensive and intense condensation occurred on the sample surface leading to high heat transfer coefficient and greatly enhancing heat transfer, but at the same time creating a huge temperature gradient in the sample. The relative humidity of 40%–60% at 383 K could not only enhance the heat transfer rate but also improve the homogeneity of temperature distribution in the sample. Simulated core temperatures were compared with experimental measurements, showing good agreement with a coefficient of determination R2 of 0.991. The findings of current work provide theoretical basis to better design and control of the process conditions of HHAIB as it elucidates the heat transfer characteristic taking account condensation phenomenon and illustrates the temperature distribution and evolution profiles.

The current work presents energy efficient hot air drying by controlling the relative humidity (RH) including step-down and continuous dehumidification of RH as to specify material based on Weibull and Bi-Di models. The drying characteristics of yam slices of different thickness (6, 12, 18 mm) and longan were investigated using two RH control strategies. Results showed that continuous decrease of RH was energy efficient for longan, while the step-down strategy was more energy efficient for 12 or 18 mm thick yam slices drying. In terms of drying kinetics, the step-down control strategy of RH is more energy efficient for those materials whose drying rate as well as sample’s temperature curves display three distinct stages and sample thickness is relative large. In terms of β and Bi values in Weibull and Bi-Di models respectively, it is observed that the step-down control strategy of RH drying method is more efficient when the β value exceeds 1 and the Bi value was more than 0.1. The current investigation provided an assessment method for the convective hot air drying method by controlling RH to improve drying efficient and save energy consumption.

The drying kinetics and mathematical modeling of hot-air drying of yam slices were investigated under two-stage relative humidity (RH) control strategy with 60 °C and 1.5 m/s as its constant drying temperature and air velocity, respectively. Results indicate high RH in the initial stage results in high sample’s temperature that enhances water diffusion in the falling rate drying period. Within the scope of current work, change in RH in the later drying period has insignificant influence on sample’s temperature rise while low RH can accelerate the drying rate. Compared to drying at constant 20 % RH at the same drying air temperature, the drying strategy of using 40 % RH over the first 15 min and then lowing to 20 % RH for the remainder time yields a shorter drying time. Weibull model adequately described the moisture content variation with time for all experiments with the scale parameter ranging from 105.02 to 122.38 min and the values of shape parameters from 0.988 to 1.183. The effective moisture diffusivity determined from the Weibull model varied from 2.032 to 2.610×10−8 m2/s. The rehydration ratio increased as the overall drying time was reduced. Microstructure examination shows that higher RH in the initial drying stage can lead to a more porous microstructure which enhances drying, while fast drying rate in the initial drying period generates a crust layer which hinders drying.

Industrial drying is one of the most energy intensive unit operations encountered in most industrial sectors. Therefore, much effort has been devoted to energy savings in drying. Most methods applied to convective drying are based on heat recovery from dryer exhaust. However, there are major limitations on how much of the energy can be recovered cost-effectively. Here a new concept based on the relative humidity (RH) control of convective drying of carrot cubes was evaluated with the purpose of enhancing the energy and exergy efficiency of drying. Numerical simulations of heat and mass transfer were performed to investigate hot air drying behavior of carrot cubes. The model fitted well the moisture ratio and the material temperature data with average relative errors of 6.5% and 0.8%, respectively. Results show that both the energy loss caused by exhaust air and the irreversibility loss of thermodynamic process decreased with increase in RH of the drying medium from 4% to 40%. Multistage relative humidity control strategy improved both the drying rate and exergy efficiency. Among the tested schemes, the three-stage RH control strategy, i.e. (1) RH of 40% for 2 h, (2) RH of 20% for 2 h and (3) RH of 4% for 3.5 h was found to be the strategy with the highest exergy efficiency (53.3%). It was almost three times higher than that for constant 4% RH values.

In the present work, the drying kinetics and evolution of sample's core temperature and moisture distribution of yam slices during convective hot-air-drying were investigated. In terms of drying kinetics, the effect of drying temperature (50, 55, 60, 65, 70°C), relative humidity (20, 30, 40, 50%), and sample thickness (5, 7, 9 mm) on drying characteristics of yam slices were studied. Results indicated that all the three factors had significant influence on the drying kinetics, whereas drying temperature gave the most significant effect, followed by relative humidity and sample thickness. Moisture-effective diffusivity and activation energy were calculated, and it was found that the diffusivity was in the range of 5.5454 × 10−10–1.0804 × 10−9 m2/s and the activation energy was 29.528 kJ/mol. Heat and mass transfer models were developed based on the finite element method to calculate the core temperature and moisture distribution of yam slices during drying. Model validation exhibited good agreement...

Thin-layer air impingement drying kinetics of American ginseng slices were investigated under different drying temperatures (35, 40, 45, 50, 55, 60, and 65°C), air velocities (3, 6, 9, and 12 m/s), and sample thicknesses (1, 2, 3, and 4 mm). Results indicated that the drying time is more significantly affected by drying temperature followed by sample thickness and air velocity. Drying rate curves illustrated that the whole drying process happened in the falling rate period. Additionally, from initial drying to the moisture content of about 0.5 kg/kg, the drying rate increased with the increase of drying temperature and air velocity or decreased with the increase of sample thickness. The moisture effective diffusivity (D eff) was determined both neglecting shrinkage and considering shrinkage. The drying activation energy of American ginseng slices was 46.64 kJ/mol determined by Arrhenius relationship. The moisture effective diffusivity and drying activation energy of American ginseng slices were compared to other biomaterials and related issues were also discussed. The current work is useful for designing air impingement drying system, evaluating and saving energy consumption of American ginseng slices.

The drying characteristics of yam slices under different constant relative humidity (RH) and step-down RH levels were studied. A mass transfer model was developed based on Bi-Di correlations containing a drying coefficient and a lag factor to describe the drying process. It was validated using experimental data. Results showed that the drying air with constant RH levels of 20, 30, and 40%, temperature of 60°C, and air velocity of 1.5 m/s had an insignificant effect on drying time. This phenomenon was likely attributed to the fact that higher RH led to a rapid increase in sample’s temperature. The higher sample temperature could provide an additional driving force to water diffusion and thereby promote the moisture movement, which could minimize the negative effect of lower the drying rate in the initial drying stage. Applying air with 40% RH for 15 min in the initial stage achieved the desired color and reduced the drying time by 25% compared to the drying time under continuous dehumidification fr...