tailieunhanh - Ebook Chemical and process design handbook: Part 2

(BQ) The book covers state-of-the-art processing operations in the chemical industry with precursors and intermediates combined according to their application within each chapter, presents each of the major chemical processes in logically arranged alphabetical chapters | Speight_Part II_F 11/8/01 3:01 PM Page FLUOROCARBONS Fluorocarbons are compounds of carbon, fluorine, and chlorine with little or no hydrogen. Fluorocarbons containing two or more fluorine atoms on a carbon atom are characterized by extreme chemical inertness and stability. Their volatility and density are greater than those of the corresponding hydrocarbons. However, environmental regulations have restricted the use of many of these compounds. Fluorocarbons are made from chlorinated hydrocarbons by reacting them with anhydrous hydrogen fluoride, using an antimony pentachloride (SbCl5) catalyst. The fluorocarbons trichlorofluoromethane, dichlorodifluoromethane, and chlorodifluoromethane are major fluorocarbon compounds. CCl4 + HF → CCl3F + HCl CCl4 + 2HF → CCl2F2 + 2HCl Difluoromonochloromethane is made by substituting chloroform for the carbon tetrachloride. CHCl3 + 2HF → CHClF2 + 2HCl In the process (Fig. 1), anhydrous hydrogen fluoride and carbon tetrachloride (or chloroform) are bubbled through molten antimony pentachloride catalyst in a steam-jacketed atmospheric pressure reactor at 65 to 95oC. The gaseous mixture of fluorocarbon and unreacted chlorocarbon is distilled to separate and recycle the chlorocarbon to the reaction. Waste hydrogen chloride is recycled by use of water absorption and the last traces of hydrogen chloride and chlorine are removed in a caustic scrubbing tower. 11/8/01 3:01 PM Page FLUOROCARBONS Water Hydrogen chloride absorber Distillation Carbon tetrachloride Sodium hydroxide sulfuric acid Caustic scrubber Hydrogen fluoride Hydrochloric acid Dichlorodifluoromethane Trichlorofluoromethane FIGURE 1 Fluorocarbon manufacture. Spent wash Distillation Distillation Recycle to reactor Recycle chlorocarbons Acid scrubber Speight_Part II_F Spent wash Speight_Part II_F 11/8/01 3:01 PM Page FORMALDEHYDE Formaldehyde (methanal, melting point: –92oC, boiling point: –21oC) is produced .