03-05-2012, 05:13 PM
Solar Crop Drying System Using Liquid Desiccant
Full Paper-1.pdf (Size: 459.75 KB / Downloads: 32)
Introduction
Drying of food and agricultural products is energy-intensive and uses large amounts of fossil fuels [1]. Liquid desiccant drying enables solar energy to be used as an energy source. This technology allows lower drying temperatures as well as lower time to be used, and is particularly suited to seeds and other temperature-sensitive crops [2, 3]. Liquid desiccant drying allows grain to be dried 24 hours/day, including days with high humidity, rain, mist, frost or fog. As well as using large amounts of fossil fuels, conventional crop dryers are subject to significant shortcomings. High temperature drying can cause breakdown of enzymes, which render the produce unsuitable for consumption. Fumes from burning diesel and other fuels can impart adverse odor or taste, and hot cinders can set fire to the easily-ignited dried product causing loss of the product and destruction of the dryer. Only a narrow range of drying temperatures is suitable for seed products [4]. Most seeds will not germinate if heated above about 42 °C, and peanuts need to be dried at below 37 °C. In tropical climates, high air temperatures and humidity provide a very narrow “temperature window” in which fuel-fired seed dryers can operate. The alternative of field drying, which is widely used in developing countries, is subject to spoilage or insect attack if weather conditions are not favorable. An estimated 20% of the world’s grain production is lost after harvest because of poor handling techniques and ineffective technology [5, 6]. These limitations led Sadaf Rice Processing (SRP) corporation to undertake a collaborative venture with the Babol Nowshirvani University of Technology (NIT) and Materials & Energy Research Centre (MERC) to develop an innovative crop dryer entirely different from fossil fuel dryers used in Iran and many other countries.
Test Procedure and Instrumentation
The experimental testing system consists of two main subsystems, namely an air heating subsystem as shown in Figures 2 and 3 and an air dehumidification and solution regeneration subsystem as demonstrated in Figure 4. In the air heating device which is connected to the batch column, 100 kg of rice of a rather lower humidity is stored and the air fan is turned on while a temperature - humidity sensor is placed within the column at 5 cm above the bottom of the batch. The heater is adjusted at a fixed temperature of 35 °C using a thermostat so that it can turn off whenever the rice temperature is high enough in order to dry the rice column. At the same time the temperature and humidity at the top of the column are measured using a portable humidity-temperature measurement device.
Experimental Results
The experimental results in the tests conducted with no desiccant solution, consists the variation of the product temperature and relative humidity with respect to time, as depicted in Figures 5 and ambient air temperature and relative humidity as shown in Figure 6. As can be seen from Figure 5, the lower crop portion has a sharp decrease in humidity in the first two hours of the experiments, after which the humidity remains almost constant for the rest of the experiment. This is due to the fact that in the first few hours of the experiment the dry air picks up moisture from the product so that the relative humidity of the air is almost the same as that of the product. As a consequence, no additional moisture transfer occurs within the air and product combination, which results in the relative humidity to remain constant for the rest of the test.
Conclusions
An experimental equipment has been developed to investigate the behaviour of crop drying using liquid desiccant under various initial parameters of temperature, air mass flow rate and ambient conditions. Hot water from flat plate solar collectors are used to regenerate the weak desiccant solution. The experiments were conducted in a hot and humid location such as the one prevailing at the city of Babol in the Mazandaran province on the Caspian Sea and initially without using liquid desiccant. It has been shown that low temperature drying will result in less energy consumption and better quality of the product.