Propene (C3H6), often also referred to as propylene, is one of the most important starting substances of the chemical industry. The demand for the base material propylene is increasing worldwide, wherein propylene just like ethylene mostly is produced from petroleum in a steam cracker in a ratio dependent on the process and the raw materials.
To obtain additional propylene, a number of processes exist, such as the PDH process which proceeds from propane as educt. What is known, however, above all is the so-called MTP process, in which olefins are produced from methanol (MeOH) or dimethyl ether (DME) by catalytic conversion on a zeolitic catalyst. By varying the catalyst under process conditions, the selectivity of the products obtained can be influenced and the product spectrum thus can be shifted towards short-chain olefins (then often also the process name Methanol-to-Olefin (MTO)), towards longer-chain products (then often also the process name Methanol-to-Gasoline (MTG)) or towards propylene.
The fundamentals of an MTP process are described for example in DE 10 2005 048 931 A1. From an educt mixture containing steam and oxygenates, such as methanol and/or dimethyl ether, C2 to C4  olefins are produced above all. By a heterogeneously catalyzed reaction in at least one reactor, the educt mixture is converted to a reaction mixture comprising low-molecular olefins and gasoline hydrocarbons. By a suitable separation concept, higher olefins, above all the C5+ fraction, can at least partly be recirculated into the reactor as recycling stream and in said reactor for the most part be converted to propylene, whereby the propylene yield is increased.
One problem when carrying out the MTP process consists in that very pure oxygenates must be used as starting substances. When e.g. methanol is used as educt of the MTP reaction, this methanol must have a degree of purity of the specification AA, which means that the impurities must be smaller than 0.2‰. This mostly requires that in the plant for the production of methanol upstream of the MTP plant a very expensive purification method must be integrated. Usually, three distillation columns are used for this purpose (Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, METHANOL—Process Technology (Eckhard Fiedler, Georg Grossmann, Burkhard Kersebohm, Gunther Weiss, Claus Witte, section 5.4 and Fig. 5). In particular, the high energy costs incurred during the distillation distinctly lower the economy of the process.
In this connection, it is known from U.S. Pat. No. 4,709,113 that the feed of the MTP reactor can directly be guided into the reactor without particular purification. However, there is obtained a high amount of higher olefins, so that both a C5-C8  fraction and a C9+ fraction is obtained and thus the yield of propylene in this process is distinctly lower and in addition the downstream purification method is very complex.
From US 2005/0101478 A1 it is known that when using molecular sieves as catalyst, oxygenates with a lower degree of purity can also be used. However, this method con-fines itself to describing the reaction control via the catalyst and does not discuss the purification of the product spectrum obtained or changes in its composition.
From US 2006/0135632 A1 finally an MTP process is known, in which methanol is pre-purified in a single purification stage and subsequently fed into the MTP reactor. After the conversion to olefins in the reactor, the entire product stream obtained is supplied to a quenching column From the same, a stream rich in olefins is withdrawn and the remaining aqueous stream is supplied to a second column. In this second separating column, the oxygenates are separated from the water contained in the stream, wherein the oxygenate-containing stream subsequently can be recirculated into the reactor. This concept has the disadvantage that directly subsequent to the reactor quenching is effected, i.e. the product stream coming from the reactor is cooled by adding water. The water content in the product stream is distinctly increased thereby. The high water contents which are present in both columns due to the proposed separation concept, however, lead to the fact that here as well very large amounts of energy are required for the purification, so that with an energy balance over the entire process the savings in the field of methanol purification hardly produce any effect or not at all.