Mathematical and Experimental Studies of Evacuated Tube Solar Dryer for Drying High Value Medicinal Herbs

Abstract

Medicinal plants are a potential source of bio-molecules that play a major role in modern medicines in the treatment of diseases like cancer, diabetes, hypertension, etc. Owing to poor harvest losses of the herbs, spoilage and quality degradation occur. In order to preserve herbs for a longer duration and make them more readily available during the off-season, an appropriate drying method is essential for the removal of moisture. The drying sector consumes more energy, and the energy required for drying should be used without depleting fossil fuels. Therefore, it is necessary to apply the right drying technology to reduce losses and maintain product quality. Solar energy is an alternative to drying medicinal herbs. Therefore, drying food products using solar energy is beneficial in terms of energy and environmental concerns. In this regard, there is a need for the development of a novel and economical solar dryer for quality drying of medicinal herbs. This research work aims to examine drying of three medicinal herbs available in India using an evacuated tube solar dryer. The research work comprises five components; (i) fabrication and investigation of solar air heater for drying application (ii) studies on drying characteristics of medicinal herbs (iii) thin-layer modelling of three medicinal herbs (iv) characterization of dried leaves and quality analysis and (v) 4-E (energy, exergy, environmental and economic analyses). In this research work, as a first step, a preliminary study was carried out on design, fabrication, and testing of an evacuated tube U-type solar collector. The fabricated collector was capable of producing a temperature range of 70-80ºC at the ETSC outlet. So, the developed U-type with ETSC can be useful for medium-temperature industrial process heating applications. As a second step, the research study aimed to dry three potentially available medicinal herbs in India using an ETSC dryer. It briefly described the significance of medicinal herbs, namely neem leaves, Krishna tulsi leaves, and Ashwagandha roots. It also explained the importance of drying these herbs, and sample preparation for drying experiments was also explained. When performing solar drying experiments, continuous measurements of various parameters like solar radiation, relative humidity, velocity, temperature of the air at different locations, and weight of the samples were required. Therefore, different measuring instruments required for exploring a product's drying behaviour were presented, along with their technical specifications. The ETSC dryer reduces the initial moisture content of neem leaves from % (db) to 8.34% (db) in 7 hours, whereas it takes 13 hours in the OSD. The average drying rate in ETSC and OSD processes is 0.004 and 0.0023 % db min-1, respectively. After conducting the drying experiments, as a third step, thin-layer mathematical modelling of herbs carried out using the curve-fitting tool in the MATLAB software to predict the moisture ratio from the obtained experimental data was explained. Moisture ratio expression models were used from the literature and applied to find the best model for determining drying data in the ETSC and OSD modes. The Verma et al., model is the most suitable for the drying of neem leaves in ETSC drying mode with a R2 of 0.9994 and RMSE value of 0.01002. In the OSD experiment, the Modified Henderson and Pabis model is found to be the best fit with a R2 of 0.9984 and an RMSE value of 0.0153. In order to understand the product’s microstructure, and the presence of different bonds, dry leaves must be characterized. Therefore, the results pertaining to the characterization of dried samples were done using a scanning electron microscope (SEM), X-ray diffraction (XRD) analysis, and Fourier transform infrared (FTIR) spectroscopy. Further, the quality analysis of the dried samples was done using color analysis and total phenolic content (TPC). Folic-Ciocalteu method was used to find the TPC of the dried samples. For neem leaves, TPC values in ETSC and OSD are found to be 95±2.4 mg in GAE/gr sample and 36±1.68 mg in GAE/gr sample, respectively. For Krishna tulsi, TPC values are found 7.28±1.8 and 4.97±1.1 for ETSC and OSD, respectively. For the dried Ashwagandha roots, TPC values are found as 32.35±1.4 and 21±2.1 for ETSC and solar dried samples. As a last step, energy, and exergy analyses were carried out for the proposed dryer when drying three medicinal herbs along with sustainability indices of the drying system. In addition, environmental and economic analyses of the developed dryer were estimated and presented for the selected dried herbs. The embodied energy of the fabricated ETSC dryer is 349.874 kWh. For the dryer’s anticipated 20-year lifespan, CO2 mitigation and earned carbon credit values are 13.2 tonnes and INR 10894 to 43576, respectively. All the proposed objectives have been achieved and the proposed dryer is suitable for drying medicinal herbs of high quality and will be beneficial for the medium and small scale entrepreneurs.