Abstract: In recent years, the graphite anticorrosion equipment has been applied to the production of hydrofluoric acid in chlor-alkali, phosphate, phosphate fertilizer and fluorine chemical industries. At present, main graphite equipments include: graphite heat exchanger, graphite condenser, steel lining graphite reaction kettle, steel lining graphite reaction tank, concentrator, reaction tower, packed tower and phosphoric acid extraction tank, prilling tower and other high temperature reaction equipments, heat exchange equipments, cooling equipments, anti-corrosion lining anticorrosive equipments.
In the national chemical, pharmaceutical, metallurgical, textile, defense, electronics, environmental protection and other industries, due to strong corrosive media, equipment will corrode and will exhibit good heat transfer performance. According to relevant data, the direct and indirect losses caused by the corrosion of chemical equipment in China are about 130 billion yuan. How to avoid equipment corrosion, reduce equipment investment and maintenance costs, improve production efficiency and profit is a problem that every chemical enterprise needs to solve. Especially in the chlor-alkali industry, most of the traditional materials cannot be used in high temperature reaction equipment and heat transfer equipment in the phosphoric acid and fluorine chemical industries. If the material selection is not suitable and the equipment is damaged, it will cause huge losses to the enterprise. How to choose good anti-corrosion materials is the key to ensuring normal production. High temperature phosphoric acid, hydrofluoric acid and fluorinated media are extremely corrosive. Stainless steel, aluminum, magnesium, copper and most other alloys are severely corroded by phosphoric acid and are not suitable for use. Ceramic, glass, enamel and other silicate materials have good corrosion resistance and are suitable for all concentrations and temperatures. However, fluorinated media such as fluoride, chlorine, sulfuric acid, and the like, especially in the wet phosphoric acid industry, are not so useful. Various plastics, such as polyethylene, polyvinyl chloride, polystyrene, rubber, etc., such as natural rubber, butyl rubber, fluororubber, etc., are resistant to corrosion by phosphoric acid and hydrofluoric acid. However, the most durable temperature is low, at 65 ° C, can not meet the needs of high temperature reaction and storage equipment; at high temperatures, fluororubber can resist corrosion, but the price is too high. It is widely used in the aerospace and military industries.
Graphite is a typical brittle material with good corrosion resistance and thermal conductivity. Small linear coefficient, high temperature resistance; good processing performance; wear resistance, light weight, high temperature but low strength. The surface of the pressed graphite element is not easy to expand and contract, which can ensure the purity of the product. The disadvantages are low mechanical strength, small elastic deformation and poor impact resistance. Graphite equipment is basically a thick-walled equipment, and generally does not cause damage to the pipe body due to the ultimate in compressive strength. Graphite equipment is mainly used in low temperature and low pressure environments. The design pressure of the graphite equipment is not more than 2.4 MPa, and the design temperature is 60~400 °C. Graphite equipment usually use impregnated graphite. Graphite materials are mainly used in heat exchangers, towers, filters, reactors, absorbers and other equipment. In the design of graphite equipment, the compressive strength characteristics of the graphite material should be maximized so that the graphite element can withstand compressive stress and avoid or reduce tensile stress and bending stress. The direction of the heat exchanger channels must be consistent with the direction of graphite anisotropy. As a heating device, when the resin is impregnated with graphite, pressed graphite, and the binder is combined with a metal material or other materials, the bonding structure should be minimized. The main graphite parts of the equipment should also avoid sticking structures. Equipment corrosion is a very important issue in alkali production. Corrosion not only affects the quality of the product, but also affects the life of the equipment and the consumption of metal materials. Therefore, research and reduction of corrosion in alkali production is one of the main technical problems of the alkali industry. In recent years, due to the rising price of titanium alloys in China, the price of titanium tube heat exchangers is unacceptable. Due to the severe corrosion of the metal material itself and the clogging of the pipeline, the high price and the thermal efficiency, many experts and scholars have begun to pay attention to non-metallic materials. As the only non-metallic material with good corrosion resistance, graphite material has become a key choice. After years of practical experience, graphite equipment has become a mature product, widely used in the production of soda ash industry.
Currently, the traditional method of preparing high quality graphite films is chemical vapor deposition (CVD). However, CVD methods also have some disadvantages such as low throughput, expensive equipment, and adverse reaction conditions. These high quality graphites are typically grown on a catalyst metal substrate such as nickel or copper foil. In practice, they must be transferred to the target substrate by dissolving the catalyst metal substrate. The graphite film produced from the solution can be considered as a graphite composite material, which maintains the good performance of the graphite film to some extent and has been widely reported.
With the development of China’s industry, the use of graphite equipment will continue to extend to other new fields.
Author: YING Yang
