PRODUCTION OF ACETALDEHYDE

There are two methods for producing acetaldehyde by ethylene oxidation: the two-stage techniques established by Wacker-Chemie and the one-stage process developed by Farbwerke Hoechst. Acetaldehyde may be produced in a single step by direct oxidation of ethylene in the gaseous phase in the presence of palladium chloride and water. On a large scale, the procedure is typically carried out as follows: Ethylene is oxidised in a bubble column reactor containing an aqueous solution of CuCl2, CuCl, and PdCl2 with oxygen in a cyclic process at 400°C and 3 bar pressure (absolute pressure), The term "single-stage manufacture" refers to the oxidation of the ethylene yielding acetaldehyde and the reoxidation of the palladium chloride reduced in this process (reoxidation being affected by CuCl2 which is converted into CuCl, which in turn is reoxidized by oxygen) being carried out in one reactor. The gas stream exiting the reactor, which contains steam, acetaldehyde, ethylene, and minor quantities of oxygen, carbon dioxide, acetic acid, crotonaldehyde, and chlorinated chemicals (such as methyl chloride, ethyl chloride, and chloro acetaldehyde), is cooled in a condenser to around 80° to 130° C. The condensate produced, which mostly consists of water and trace quantities of acetaldehyde and acetic acid, is often recycled back into the reactor. Small quantities of copper oxalate and high molecular by-products are also generated and stay persistent in the catalyst solution, whilst the volatile by-products, together with the acetaldehyde and unreacted starting chemicals, exit the reactor. To prevent the buildup of these by-products, a tiny portion of the liquid phase is continually drained from the reactor. After that, the pressure is released, and the dissolved low-boiling chemicals such as acetaldehyde, ethylene, and carbon dioxide flash and are evacuated. The degassed solution is transferred to a regeneration vessel and heated to a temperature of approximately 165° to 180° C. The regenerated solution is recycled into the reactor. After being cooled in the condenser, the gas current exiting the reactor is typically cooled further through heat exchangers to around 30° to 80° C. The acetaldehyde is then cleaned out of the gas stream in a scrubber. After removing a portion of the residual gas (to minimise a buildup of carbon dioxide and inert gas) and adding new ethylene, the leftover gas is returned to the reactor. The condensate from the heat exchangers and the aqueous acetaldehyde solution from the scrubbers are mixed in a collecting tank. This "crude aldehyde" combination is routed via a two-stage distillation process. In this process, low-boiling chemicals (methyl chloride, ethyl chloride) and dissolved gases such as ethylene and carbon dioxide are produced as the overhead in a first stage by extractive distillation using water as an extraction agent. The bottom product is transported to the second distillation process, where pure acetaldehyde is produced as the overhead product. A side stream comprising mostly crotonaldehyde is removed. The high-boiling by-products (particularly acetic acid and chloro acetaldehyde) and water are taken from the bottom. The extracted combination is referred to as "waste water."