GB580485A - Improvements in or relating to the purification and compression of ethylene - Google Patents

Abstract

Ethylene or a gas containing it is contacted with water or an aqueous solution or dispersion at elevated pressure and a temperature preferably between -5 DEG and 60 DEG C. until at least part of the ethylene is converted into the solid hydrate, the temperature then being raised or the pressure lowered to liberate the ethylene. By separating residual gas or liquid or both from the solid hydrate before decomposing it, purified ethylene is obtained. If the temperature of the hydrate is raised, ethylene at increased pressure is produced which may be cooled while allowing it to contract at constant pressure. Dissociation pressures of ethylene hydrate are given as 6, 20, 70, 450 and 1000 atm. at 0, 10, 20, 40 and 60 DEG C. respectively, and to avoid the necessity for high pressures and low temperatures operation at 10-30 DEG C. is preferred. The time required for formation and decomposition of the hydrate depends on other reaction conditions and may be 1/2 -20 hours per batch of ethylene crystallized, the volume of crystallization vessel required being 10-50 1. per kg. of ethylene in each batch. To speed the decomposition, the temperature may be raised 10-20 DEG C. above the decomposition temperature or the pressure lowered to about 3/4 of the dissociation pressure. Cooling may be employed during the absorption and the heat liberated used for the decomposition. The process may be effected in two closed pressure vessels, one within the other, which latter is insulated on the outside. Alternatively, two or more pressure vessels may be arranged in a common heat exchange medium through which heat passes from the absorbers to the decomposers or a single vessel of large heat capacity may be used. The process may be a batch, continuous or semi-continuous one. It can be used for separating ethylene from other hydrocarbons, hydrogen, nitrogen, oxygen, carbon oxides and other gases. Separation is incomplete when large amounts of gases such as acetylene, carbon dioxide and methane, also forming solid hydrates, are present. In examples: (1) a gas containing 90 per cent ethylene and 10 per cent other hydrocarbons is compressed into a vessel nearly filled with water and kept at 20 DEG C. by cooling. Pressure rises to 100 atm. when a release valve operates to blow off impure gas. After stopping the gas supply, the valve is opened further to give a rapid fall of pressure to 70 atm. when nearly all the impurities are blown off. The valve is partly closed and temperature raised to 30-40 DEG C. A gas containing 98-99 per cent ethylene is evolved at 300 atm.; (2) an ethylene, nitrogen mixture is forced into a pressure vessel half full of water until pressure is 500 atm. Water is pumped in to displace residual gas at the top, liquid drained off at the bottom, and the remaining solid heated to generate ethylene; (3) ethylene containing 1 per cent hydrogen is forced into water at 30 DEG C. until pressure is 350 atm., when impure gas is vented while hydrate forms. On heating to 40 DEG C., 99.7 per cent ethylene is liberated at 350 atm. Specifications 568,290 and 568,292 are referred to.