Fixed bed reactors are a sort of common reactors.  In the most common design, a fixed bed reactor has one fixed bed layer or a plurality of fixed bed layers; in the case of a plurality of fixed bed layers, these fixed bed layers are in the same height, or the fixed bed layers are in an incremental height design in the material flow direction, and thereby the feed amounts at the stages are equal to each other or increase stage by stage in the material flow direction.  In “Chemical Engineering” (p. 43-48, No.  3, Vol.  21, 1993), Jinfang N I and Kaihong Z H U et al have described a fixed bed reactor for preparing ethyl benzene from benzene and ethylene, which is designed on the basis of an ideal that the fixed bed layers are in the same height and the feed amounts at the stages are equal to each other; in contrast, in a fixed bed reactor for preparing dimethyl benzene from methyl benzene by methylation and the main fixed bed reactor in the apparatus of Lurgi (a German company) for preparing propylene from methanol, the bed layers are in an incremental height design, and thereby the feed amounts increase stage by stage. 
Propylene is a basic organic chemical material in a great demand, and is mainly obtained from the petroleum processing process.  As the petroleum resources are in short increasingly, more and more attention has been paid to the development of techniques for preparing propylene from non-petroleum resources such as coal or natural gas in China and foreign countries.  The Methanol-To-Propylene (MTP) technique is a new technique that is the most hopeful substitute for the petroleum route.  Preparing a synthetic gas from coal or natural gas and then preparing methanol and dimethyl ether from the synthetic gas is a matured technique.  Hence, since preparing propylene from methanol is a key technique in the coal-to-olefin route, researches on the methods for preparing dimethyl ether from methanol have been made vigorously. 
For example, the patent document U.S. Pat.  No.  2,014,408 describes a method for preparing DME from methanol in the existence of a catalyst such as aluminum oxide, titanium oxide, and barium oxide, wherein, the reaction temperature is preferably 350-400° C. The patent document DE3817816 describes a method for preparing dimethyl ether from methanol by catalytic dehydration in a methanol synthesis apparatus, which utilizes a heat-insulated single-stage fixed bed reactor.  The process flow chart of the above-mentioned method is shown in FIG.  1, wherein, a material fed at a flow rate F1 at temperature T0 is heated up to temperature T1 and then is fed into a catalyst bed layer BED1 in a reactor R1 for reaction. 
The above-mentioned method employs a single-stage heat-insulated fixed bed reactor.  Though the reactor structure is simple, the temperature rise in the catalyst bed layer is almost 130° C., and the hottest-spot temperature, which refers to the peak temperature in the catalyst bed layer in the axis, is almost 400° C.; consequently, the internal structure of the catalyst is unstable, the catalyst may be aged easily and the catalyst life may be shortened.  In addition, since a single-stage fixed bed reactor is used, all feed material has to be heated up to an initiation temperature required for initiating the reaction in the heat-insulated catalyst bed layer; consequently, the energy consumption for the feed material is high.  If the hottest-spot temperature is too low, the reaction will not be completed enough, and the conversion rate will not be high; if the hottest-spot temperature is too high, the reaction will be too violent and may be out of control; consequently, a vicious circle of temperature runaway, occurrence of severe subsidiary reactions, and further temperature rise may occur, not only resulting in material loss and equipment damage, but also causing collapsed catalyst skeletons and shortened catalyst life. 
In recent years, the application of the MTP technique introduced from Lurgi (a German Company) in the coal-based olefin production project of Shenhua Ningxia Coal Group has marked a break-through in the industrial application of the methanol-to-propylene technique.  The technique employs fixed bed reactors and a two-stage reaction process that incorporates pre-reaction and main reaction, wherein, the pre-reaction is a reaction in which the methanol is partially converted into dimethyl ether, while the main reaction is a reaction in which the product of the pre-reaction reacts further in a second reactor to generate propylene.  Owing to the fact that the methanol-to-dimethyl ether reaction in the MTP technique of Lurgi is a strong exothermic reaction and the MTP technique employs single-stage fixed bed reactors, like the techniques in the prior art, the temperature difference is great and the hottest-spot temperature is high in the catalyst bed layer in the methanol-to-dimethyl ether reaction apparatus; consequently, the catalyst may be aged easily, the catalyst life may be shorted, and the energy consumption for the feed material is high. 
The patent document CN103813852A discloses a controlled cooling reactor for preparing dimethyl ether from methanol.  First, the material is fed into a heat-insulated catalyst bed layer in a reactor to initiate the reaction; then, the material passes through a moderating zone, in which the material is cooled by direct or indirect heat exchange; finally, the material passes through a heat-insulated catalyst bed that serves as a conditioning zone and flows out of the reactor.  The moderating zone employs a tubular reactor, in which heat exchange can be executed by direct-flow heat exchange or counter-flow heat exchange or liquid methanol can be injected as a liquid medium.  Though the method effectively controls the hottest-spot temperature in the reaction and utilizes the reaction heat, the reactor design and manufacturing is quite complex, the catalyst charging is very time-consuming; especially, in the moderating zone, where the tubular reactor is located between two fixed beds, the catalyst charging is more labor-intensive and time-consuming.  In addition, all feed material has to be heated up to an initiation temperature required for initiating the reaction in the heat-insulated catalyst bed layer; consequently, the energy consumption for the feed material is high.