For producing low-molecular C2-C4 olefins, in particular propylene, from methanol and/or dimethyl ether, a multitude of processes are known to those skilled in the art, which are usually based on the reaction of an educt mixture containing steam as well as methanol vapor and/or dimethyl ether vapor on a form-selective zeolite catalyst.
DE 100 27 159 A1 discloses a process for producing propylene from methanol, in which first a vapor mixture containing dimethyl ether is produced from methanol vapor on a first catalyst, before the same is mixed with steam and is reacted in at least two sequentially operated reactors with catalyst beds of form-selective zeolite to obtain a product mixture containing propylene. Subsequently, the product mixture is processed in a separating device comprising several distillation columns, so that there is obtained a fraction rich in propylene with a propylene content of at least 95 vol-%, a fraction containing low-molecular hydrocarbons, which is recirculated to the catalyst beds, and a fraction rich in gasoline hydrocarbons, which is removed from the process. What is, however, disadvantageous in this process is the low yield of propylene, based on the total carbon content of the educt mixture, which among other things is due to the fact that the fraction rich in gasoline hydrocarbons is removed from the process.
From EP 0 882 692 B1 a process is known for producing C2-C3 olefins, in which a mixture of steam and methanol vapor and/or dimethyl ether vapor is reacted in a tubular reactor containing a zeolite catalyst at a temperature between 280 and 570° C. and a pressure between 0.1 and 0.9 bar to obtain a product mixture rich in olefins, which subsequently is separated in a separating device to obtain a C2-C4 olefin fraction with a propylene content of at least 40 wt-%, an aqueous fraction, a gaseous fraction, and a fraction containing C5+ gasoline hydrocarbons. While the three first-mentioned fractions are withdrawn from the process, the product stream containing the C5+ gasoline hydrocarbons is mixed with water, heated in a heater to a temperature of 380 to 700° C., and reacted to obtain C2-C4 olefins in a second reactor containing a zeolite catalyst, before the reaction products are recirculated to the separating device. The yields of C2-C3 olefins obtained with this process, although higher than the yields obtained with the process known from DE 100 27 159 A1, in which the fraction rich in gasoline hydrocarbons is removed from the process, likewise are in need of improvement. In addition, this process is known for its high costs, not least because of the isothermal procedure and the necessary vacuum operation in the tubular reactor.
What is needed is to provide a process for producing C2-C4 olefins, in particular propylene, from an educt mixture containing steam as well as methanol vapor and/or dimethyl ether vapor with a rather high yield.