Optimized design of pressure-circulating preparation device of lime sulfur

Abstract: Lime sulfur is a familiar cleaning agent for the prevention and management of diseases and pests in orchards and gardens. Since it is widely applicable for many plant diseases and is not susceptible to resistance, it is a kind of pollution-free pesticide and is suitable for green agricultural production. Lime sulfur is easy to be degraded when it is exposed to the air, which causes it to be not feasible for storage. In the actual production process, the fruit farmers often use iron pot to prepare lime sulfur by boiling calcium hydroxide and sulfur together due to the raw materials being easy to find, and the price being cheap, too. There are many problems using the traditional method to prepare lime sulfur, such as the difficulty of quality control, too long stewing time, the release of hydrogen sulfide and other toxic gases in the environment. To solve these problems, a new stewing device with a reflux condenser installing on the sterilization pot was designed, which consists of pot shell, cover, condensing unit, stirring unit, pressure valve, safety valve and the temperature gauge. The overall structure and the working principle of the reflux condenser were analyzed through the thermodynamics analysis of the condensation/reflowing process of the condensing unit to confirm the parameters of key components. The orthogonal experiment was applied to systematically optimize the influencing factors including mass ratio of raw materials, the pressure, the stewing time and the stirring speed. The results showed that when the length and the diameter of reflux condenser were 0.87 m and 14 mm, respectively, the reflux condensation could make the reaction gas condensed and refluxed and could be applied in the new stewing pot. While the orthogonal experiment was applied to acquire the lime sulfur agent on the new circulative stewing pot, the amount of the generated lime sulfur increased at first and then decreased gradually with the increase of the sulfur content in raw materials. It shows that the stewing pot has higher requirements for selecting raw materials. The content of limes sulfur became high as the heating pressure reached 0.12 MPa, due to that sulfur was converted to liquid state to accelerate the reaction. The stirring velocity would accelerate the reaction to obtain high content of lime sulfur, but the long stewing time would decrease Baume degree in a low level, so overreaction was not beneficial to the lime sulfur. The concentration of lime sulfur could be up to 28.8 °Bé, under the mass ratio of calcium oxide, sulfur powder and water reaching 1:1.8:9, the working pressure of 0.12 MPa, the mixing speed of 240 r/min and the stewing time of 30 min. Compared with the traditional pot preparing lime sulfur mixture, the new device could shorten stewing time from 50 to 30 min with reducing the water supply operation and exhaust emissions. This study can provide a theoretical basis and technical guidance for quickly, safely and efficiently stewing the lime sulfur.