Davy Process Technology (DPT) has recently brought two 1,4-butanediol (BDO) plants on stream. These plants use vapor-phase hydrogenation of diethyl maleate (DEM) starting from maleic anhydride (MA). The first plant was the Shinwa Petrochemical plant located in Ulsan, Republic of Korea, and the second was Tonen Chemical plant located in Kawasaki, Japan. While hydrogenation of DEM was, at the time, claimed to be the most economical route for producing BDO, the dimethyl maleate (DMM) route is proposed as even better. This improvement derives from the ease in separating water from methanol (ethanol and water form an azeotrope) and the increased volatility of the methyl ester. Future licensees will use the DMM route, and this review examines the DMM route.
Our version of the process involves a two-stage esterification. The monomethyl maleate (MMM) forms quickly without catalyst. The DMM is formed in a multistage catalyst bed from MMM flowing countercurrent to dry methanol vapor that carries off the water of reaction, thus driving the equilibrium reaction to completion. The DMM is vaporized into a large excess of recirculating hydrogen and passed through two fixed copper chromite catalyst beds where partial hydrogenation takes place. Partially hydrogenated products are separated and recycled while by-product tetrahydrofuran (THF) is separated for sale and n-butanol is burned. The normally difficult separation of THF from water and methanol is accomplished with two azeotropic distillation columns, one at elevated pressure.
