DEVELOPMENT A MECHANICAL STIRRING UNIT FOR DRYING RICE GRAINS

Abstract

The present study was carried out to develop and evaluate the performance of a mechanical stirring unit for rice crop after harvesting for drying to increase stirring efficiency and decrease both power and cost requirements. The aim of this study is to develop a mechanical stirring-unit for drying rough rice-grains after harvesting. The performance of the developed stirring unit was studied as a function of change in screw paddles speed of (120, 180 and 300 rpm ), paddle inclination angle of( zero , 15 and 30deg) with two paddle shapes (trapezoid and rectangle) and a unit wheel width of (4 and 10 cm). The developed unit performance was evaluated in terms of unit capacity, stirring efficiency and required power. The results showed that the screw trapezoid paddle with inclination angle of 30 0 decreased specific power by 15.79 % and 20.83 % by increasing paddle speed from 120 to 180 rpm and from 180 to 300 rpm, respectively. While, under the same conditions, stirring efficiency increased by 4.39 %, and volumetric capacity decreased by 8.8 % and 17.6 % under conditions of increasing inclination angle from zero to 15 0 and to 30 0 , respectively at the same screw pitch of 20 cm with layer height of 7.0 cm. Total costs with the use of the developed stirring unit decreased by 48.2% comparing to the traditional method (by feet) according to hiring laborers with recent prices. INTRODUCTION n Egypt, rice has been sun dried for years. Sun drying (exposing the product being dried to direct sunrays) is still the most common method used to preserve the agricultural products in most small Egyptian farms and producers. Rice is either spread in the field to dry; gathered into panicles or sheaves and placed on pavement or hung from frames; gathered and dried on nets, mats or canvas; or gathered and placed in a drying barn or grain mill floor. *Agricultural Engineering Research Institute, Agric. Res. Center, Dokki-Giza I Misr J. Ag. Eng., 32 (2): 433 450 FARM MACHINERY AND POWER Misr J. Ag. Eng., April 2015 434 Each method works but has particular problems that must be watched. The traditional method is used by many farmers as it is easy, low cost, widely available, and gives comparable results to more costly mechanized drying methods and also more friendly to the environment. The rice quantity dried by dryers occupied one third of total production because most of the farmers sell fresh rice after harvest. At harvest time rice grain contains a lot of moisture, typically between 20−25%. At such high grain moisture contents there is increased natural respiration in the grain that causes deterioration of the rice. Wet seed, is more vulnerable to pest and disease attack and to deterioration in physical quality. Wet seed also leads to natural respiration during storage creating "hot spots" within the seed lot. These hot spots promote the growth of insects and the development of fungi and other pathogens. Seed life is doubled by each 1% reduction in moisture content below 14%. In general, the longer the intended period for storage, the lower the moisture content should be, i.e. the more you have to dry the seed. Rice constitutes the staple food for a large proportion of the world’s population (Evanson et al., 1996), and its consumers represent one of the most demanding cereal markets with regards to product quality. Kernel quality is thus of utmost importance to the rice processing industry. Soetoyo and Soemardi (1979) demonstrated that paddy can be dried from 24-26% moisture to 14% moisture at depths of 50-100 mm at a rate of 3.3 kg/m2.h for stirred paddy and 1.9 kg/m2.h for unstirred paddy. The grain can reach temperatures as high as 60°C under clear skies and the rate of drying can be extremely high. Under these circumstances kernel cracking and loss of head rice can be appreciable, particularly if paddy is dried to below 14% moisture. Covering the paddy around midday may be beneficial under particularly hot and sunny conditions. Teter (1987) noted that seed paddy can be sun dried at depths of up to 30 mm but that the final stages of drying to 12% moisture should be conducted in the shade to avoid overheating and kernel cracking. Flat-bed dryers can be used with bed depths of up to 0.3 m, air temperatures not exceeding 40°C, and airflows of 1.3-1.7 m3/s per ton of grain. Jeon et al., 1989 stated that all equations were used in many studies concerning thin-layer drying modelling. Rice kernels crack if rewetted FARM MACHINERY AND POWER Misr J. Ag. Eng., April 2015 435 when below a critical moisture content of 13% to 16%. Since the distribution of individual kernel moisture in the panicle is non uniform, cracking or fissuring of some kernels on the standing crop will begin when the average grain moisture drops below 25% for medium grain varieties. Proctor, 1994 stated that the traditional practice of grain drying is to spread crop on the ground, thus exposing it to the effects of sun, wind and rain. The logic of this is inescapable; the sun supplies an appreciable and inexhaustible source of heat to evaporate moisture from the grain, and the velocity of the wind to remove the evaporated moisture is, in many locations, at least the equivalent of the airflow produced in a mechanical dryer. In tropical countries, for at least several months of the year, the mean level of insolation upon the ground is more than 0.5 kW/m2 (measured as a mean over the hours of daylight). The heat available therefore, assuming a 12 hour day, is 21.6 MJ/m2, a quantity theoretically sufficient to evaporate 9 kg of water. Bonazzi et al. 1997 mentioned that to prevent rice damage following harvest of a high moisture content product, the paddy should be dried to such a level of moisture content that will enable safe storage by reducing respiration, and by a prevention of mycotoxins production. This corresponds to a moisture content of about 13-14% (w.b.), which is considered as admissible for safe storage, safe milling and subsequent safe storage as milled rice, with low fungus and insect attack, leading to a minimum deterioration of chemical components, and a minimum of subsequent loss in nutritive values. Two of the main indices used to determine rice quality are head-rice yield and head-rice color. Head-rice yield is accepted as the current measure of commercial physical quality and is defined as the mass percentage of rough rice kernels that remains as head-rice (kernels that are at least 3/4 of the original kernel length) after complete milling. Other frequently reported rice quality parameters include pasting properties, chemical properties, and sensory quality (Daniels et al., 1998; Meullenet et al., 1999; Pearce et al., 2001; Ranalli et al., 2003; Zhou et al., 2003). Post-harvest management of rice plays an essential role in maintaining rice quality. Rough rice is normally harvested at moisture content ranging from 14% to 26%. High-MC rice kernels are subject to FARM MACHINERY AND POWER Misr J. Ag. Eng., April 2015 436 elevated respiration rates due to enzyme activity and mold growth (Dillahunty et al., 2000), which reduce the dry matter content of rice and may produce sufficient energy to be detrimental to product quality (Bradburn et al., 1993). It is generally considered that under typical storage environments, the MC of rough rice must be reduced to less than 13% for safe long-term storage. The development of rice drying techniques was the subject of many studies during these last decades. These studies led to the development of industrial and semi-industrial dryers, unfortunately limited by their costs. In addition to these industrial or artificial methods, other drying methods, known as traditional or natural ones, use sun as energy source. The purpose of this study are to develop and evaluate the performance of a mechanical stirring-unit for drying rough rice-grains after harvesting in order to reduce rice moisture content to the safe level for storage. MATERIALS AND METHODS A study was conducted at El-Serw Agricultural Station to develop and evaluate a hand-propelled unit for stirring rough rice grains after harvesting for sun-drying. It is crucial in sun-drying to stir the seed regularly. This can be done using feet (Fig. 1), by dragging feet through the layer of seed and at the same time mixing and stirring but it is an exhausting task. Rakes (Fig. 2) are also a very effective tool to do this notwithstanding its difficulty. Drying on flat exposed surfaces is the most common way of drying grain after harvesting and threshing. For drying small amounts on the farm grain may be spread on any convenient area of land. Cleaner dried grain can be obtained by drying the grain on plastic sheets, preferably black. Purpose-constructed drying floors are commonly used where there is a need to dry large quantities of grain during the season, e.g. at most rice mills. The floors are usually made of concrete or brick, these materials presenting a relatively smooth and hardwearing surface. Floors should be constructed to withstand the movement of vehicles and sloped or channeled to hasten the runoff of rainwater in rainy regions. The paddy is spread in a thin layer on the floors and raked at intervals, preferably 7-8 times daily, to facilitate even drying. At night the paddy is heaped into FARM MACHINERY AND POWER Misr J. Ag. Eng., April 2015 437 rows and covered with sheeting. In all cases rough rice is covered at night and when it rains to prevent cracking. (Fig. 3) Fig. 1: Traditional stirring rice (by feet). Fig. 2: Rakes used for rice stirring. Fig. 3: Covering rice after sun-drying. In addition, there are machines now available for mixing the grains. Therefore, a field trial has been conducted to evaluate a developed mechanical stirring unit for sun-drying rice crop directly after harvesting. The developed stirring unit The developed stirring unit consists of the following main parts (Fig 4): The frame: The frame is made of a cylindrical steel tube 1.0 inc