Top-bin/In-bin-counterflow Drying of Paddy

Paddy or rough, unmilled rice is difficult to dry with high-temperature air because of the susceptibility of rice kernels to fissure. A new in-bin high-temperature drying system has been developed specifically for medium-capacity paddy depots in China. The system consists of a series of three drying bins and two storage bins. Each drying bin contains a top-bin dryer and an in-bin-counterflow dryer. Thus, the paddy is dried in six stages, with tempering after each high-temperature drying stage. The throughput of the system is 12 tonnes of wet rice per hour in reducing the moisture content from 24% to 13%. The system is operating near Shanghai, China. The world production of paddy has been relatively stable - i.e., between 400 and 430 million metric tons (MMT) annually (USDA, 1993); the largest producer is China with 180-185 MMT per year followed by India with 105-110 MMT. Although Thailand and the U.S.A. are relatively small producers with 17-20 MMT and 6-7 MMT of paddy, respectively, they are the largest rice-exporting countries with 1992 exports of 4.7 MMT and 2.4 MMT on a milled rice basis. China's rice export is increasing and is expected to reach 1.0 MMT (milled) soon. Due to the decrease in per capita rice consumption, China at present is storing about 25 MMT of rice, or roughly 20 percent of the annual domestic consumption. The bulk of the rice stored in China is of relatively low quality, and is exported as low-quality rice to Africa at low prices. High-quality rice is sold at a premium, mostly in the domestic market. The main difference between high-quality and low-quality rice is the difference in the percentage of brokens (in general, in world trade high-quality rice is considered to have less than 10% brokens, low-quality rice more than 20%). The price-differential on the world market between low-quality and high-quality rice is about $100 per tonne. Thus, it is essential for the competitive position of the Chinese rice industry that the general quality of the rice crop be upgraded from low-/intermediate-quality to high-quality. Improvement of the rice drying and storage facilities will accomplish this task. Along with the color and cooking qualities, the head yield is the major quality characteristic of milled rice which is defined as the percentage (by weight) of whole kernels in a lot milled from rough rice. In a well-designed rice drying system the decrease in head yield should be limited to 3-4 percentage points (Brooker et al., 1992). Paddy is usually harvested in China at an average moisture content between 20% and 26%, wet basis, depending on the type and variety of paddy and location where the crop is grown. Rapid drying after harvest is essential in preventing the wet rice from discoloring and deteriorating. Proper drying is required to limit the percentage of broken kernels after milling. In Southeast Asia and China much of the paddy produced is sun-dried, a labor-intensive practice of spreading the moist rice kernels in a thin layer 3-5 cm thick on hard soil or concrete floor, and exposing them to the energy of the sun. Intermittent stirring of the rice layer increases the drying rate and the uniformity of the layer-drying process. Frequently the rice-kernel temperatures rise to 60-70°C, resulting in fissuring (cracking) of the kernels (Suhargo, 1993). Milled rice produced from sun-dried paddy, therefore, has a low head yield. Bin-drying of paddy is practiced in many rice-producing areas. The paddy can be dried in steel bins with full perforated floors or in flat storage structures in which appropriate air-distribution systems are placed. Drying can be accomplished with natural air at initial paddy moistures as high as 23-24% if the relative humidity of the ambient conditions is sufficiently low. In the humid tropics the practice is limited to rice in the 17-20% moisture content range (Bowrey and Driscoll, 1986). The use of high temperatures in conventional in-bin rice drying systems is restricted because of the destructive effect of overdrying on rice quality. This paper describes the design of a new concept of in-bin paddy drying which allows the use of moderately-high air temperatures. The system consists of a combination of top-bin and in-bin counterflow dryers arranged in series.