Abstract:
The present invention relates to the field of chemical engineering and technology, in particular relates to the sub-field of synthesis of high quality alternative liquid engine fuel from non-petroleum based feedstock, more particularly relates to a method for regulating and optimizing the synthetic process of polyoxymethylene dimethyl ethers utilizing chemical thermodynamic principle. The process of the present invention Is achieved by conditions wherein the initial temperature of reaction is controlled at 100-120° C., then the temperature is reduced to 50-70° C. by successive stepwise cooling or programmed cooling,, the reaction pressure is controlled at 0.1-4.0 MPa, and the molar ratio of paraformaldehyde or trioxane metered in formaldehyde units to methylal in the feedstock is 1.5:1-8:1. In the process, higher overall yield of the target product can be achieved in the same reaction time, and selectivity of products with higher degree of polymerization of methoxy groups can be increased.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of chemical engineering and technology, In particular relates to the sub-field of synthesis of high quality alternative liquid engine fuel from non-petroleum based feedstock, more particularly relates to a method for regulating and optimizing the synthetic process of polyoxymethylene dimethyl ethers utilizing chemical thermodynamic principle. 
       BACKGROUND OF THE INVENTION 
       [0002]    Recent investigation shows that, the apparent consumption of diesel fuel in China has already mounted up to 167 million tons, which leads to frequent occurrence of short supply of diesel fuel (the domestic demand ratio of diesel fuel to petrol is about 2.5:1, but the production ratio is about 2.3:1). Besides the reasons of unreasonable pricing of different types of oil products, and slow price linkage mechanism of domestic petroleum products with international crude oil, the fundamental reason is the restriction of resource shortage. Traditionally, diesel fuel is made from petroleum based feedstock, and the resource endowment of China characterized in relatively “rich in coal, poor in oil, and lack in gas” leads to increasingly prominent contradiction between petroleum supply and relatively fast sustainable development of economic society. Since China became a net importer of petroleum in 1993, the import quantum increases fast and constantly, and the foreign-trade dependence already exceeded 56% after 2011, it has a severe impact on national strategic security of energy. 
         [0003]    Furthermore, the worsening crude oil quality leads to continuous scale expansion of domestic catalytic processing of heavy oil and increasing percentage of diesel fuel produced by catalytic processing, which results in gradual decline of the cetane number (CN value) of diesel fuel products and significant increase of noxious substance discharged after combustion, therefore, the urgent problem to be solved is to improve the CN value of diesel fuel. 
         [0004]    The tail gas discharged by diesel engine contains, besides CO, CO2 and NOx, a large amount of noxious substance such as unburned hydrocarbon compounds (HC) and particulate matter (PM), which is one of the main sources of PM2.5 contamination in urban air. International Agency for Research on Cancer (IARC) affiliated with World Health Organization (WHO) declared in June, 2012 the decision to promote the cancer hazard ranking of diesel engine tail gas, from “possibly carcinogenic” classified in 1988 to “definitely carcinogenic”. As scientific research advances, now there is enough evidence to prove that diesel engine tail gas is one of the reasons that cause people to suffer from lung cancer. Furthermore, there is also limited evidence indicating that, inhaling diesel engine tail gas is relevant to suffering from bladder cancer. IARC hopes that this reclassification can provide reference for national governments and other decision makers, so as to actuate them to establish more strict discharge standards of diesel engine tail gas. This significant decision undoubtedly puts forward more rigorous requirements of diesel fuel quality. 
         [0005]    Reducing the content of noxious substance such as sulfur, nitrogen and aromatic hydrocarbon in fuels by petroleum refining process such as hydrofining is an effective technical route to improve fuel quality, but has very demanding requirements of hydrogenation catalyst and reaction process, with relatively high processing cost. Internationally, many scientific research institutes are carrying out research and development on production technologies of oxygen-containing blending components of petrol and diesel fuel, especially those diesel fuel blending components with high oxygen and high cetane number, and this has recently become a research hotspot in the technical field of new energy. 
         [0006]    Polyoxymethylene dimethyl ethers (also known as polymethoxymethylal, dimethyl-polyformal, with the general formla of CH 3 (OCH 2 ) n OCH 3 , abbreviated as DMM n , n=8), which is a yellow liquid with a high boiling point, an average cetane number reaching above 76, and increasing dramatically with the increase of its degree of polymerization, an average oxygen content of 47%-50%, a flashing point of about 65.5° C., and a boiling point of about 160-280° C., is a clean diesel fuel blending component with a high cetane number. When blending into ordinary diesel by a certain percentage (e.g., 15 v %), it can significantly increase oxygen content of diesel fuel products, so as to promote sufficient combustion of diesel fuel and to sharply reduce the discharge of combustion-generated pollutants such as NO x , CO and PM, without the need to make any modification in the fuel supply system of the engine. Furthermore, as polyoxymethylene dimethyl ethers added into ordinary diesel cause the diesel to be diluted, accordingly, the contents of aromatic compounds and sulfides in the diesel fuel products are also reduced. 
         [0007]    Synthesis of polyoxymethylene dimethyl ethers may be carried out by processing synthesis gas through a series of steps of methanol, formaldehyde, methylal, and polyformaldehyde etc. The verified coal reserves in China are about 714 billion tons, and developing coal-based methanol industry has huge resource advantages. However, the problem of excessive production capacity of methanol is particularly prominent in recent years. For example, the production capacity of methanol broke through 50 million tons in 2012, but the rate of equipment operation is merely about 50%. Thus the industrial chain of coal chemical industry is in an urgent need to be further extended. Therefore, developing a technologically advanced and economically rational industrial process for synthesizing polyoxymethylene dimethyl ethers based on methanol as upstream feedstock can not only provide a new technology to significantly improve diesel fuel product quality, but also improve the feedstock structure of diesel fuel production, so as to make it more suitable for the resource endowment of domestic fossil energy and enhance the strategic security of domestic supply of liquid fuel for engines. 
         [0008]    In the aspect of synthesis of polyoxymethylene dimethyl ethers, a lot of work has been done at home and abroad, regarding research and development of methods for synthesizing polyoxymethylene dimethyl ether products where n=1−10 by using methanol, methylal, lower alcohol, aqueous formaldehyde solution, paraformaldehyde, etc. as feedstock, in the presence of acidic catalysts. 
         [0009]    In various kinds of feedstock route, more research has been done about the synthesis of polyoxymethylene dimethyl ethers from trioxane or paraformaldehyde and methylal, which includes: 
         [0010]    U.S. Patent US2007/0260094 A1 discloses a preparation process of polyoxymethylene dimethyl ether using methylal and trioxane as feedstock in the presence of acidic catalyst. The water contained in the reaction mixture of methylal, trioxane and acidic catalyst should not exceed 1%. Polyoxymethylene dimethyl ether where n=3 and 4 in the reaction product is separated by rectification, and methylal, trioxane and polyoxymethylene dimethyl ethers with degree of polymerization of n&lt;3, and some n&gt;4 can be recycled. 
         [0011]    A process of catalytic synthesis of polyoxymethylene dimethyl ether with degree of polymerization of methoxy groups at 2-10, by using methylal and trioxane as feedstock, in the presence of homogeneous or heterogeneous acidic catalysts such as liquid mineral acids, sulfonic acids, heteropolyacids, acidic ion-exchange resin, zeolite, etc. at the pressure of 1-20 bar and the reaction temperature of 50° C.-200° C. and under strictly limited condition of the water content introduced into the system, is disclosed in Chinese patent literature CN101048357A of BASF Aktiengesellschaft. By optimization, polyoxymethylene dimethyl ether with degree of polymerization of methoxy groups at 3 and 4 can be separated by distillation through three towers. 
         [0012]    Tianjin University discloses a process of synthesis of polyoxymethylene dimethyl ether using methylal and trioxane as feedstock in Chinese patent literature CU102432441A, which uses cation exchange resin as a catalyst in the fixed bed reactor, under the reaction condition of the temperature of 80° -150° C., the pressure of 0.6 MPa-4.0 MPa and nitrogen atmosphere, and in the main products obtained, n is 3 or 4. 
         [0013]    Furthermore, in recent years abroad, Jakob Burger etc, [i.e., Fuel 89 (2010) 3315-3319] synthesized DMM n  using ion-exchange resin as a catalyst and methylal and trioxane as feedstock in a stirred-tank reactor in laboratory by intermittent operation, which focuses on studying the relationship between the equilibrium composition and reaction temperature, feedstock mass ratio. In China, some colleges and universities such as East China University of Technology, Nanjing University, Lanzhou University of Technology etc. are carrying out some basic and applied basic research in the aspect of chemical thermodynamics, catalyst screening and reaction process. 
         [0014]    In conclusion, there has already been lots of research about preparing target product DMM n  using methylal and paraformaldehyde or trioxane as feedstock, the catalysts involved cover almost all the major types of acidic catalysts, but in the implementation process, no matter what kind of catalyst and reactor are used, the rate of chemical reaction is always very low, and the reaction is generally required to last for hours or even longer, it has become a major challenge which limits large-scale industrialization of this technology. 
       SUMMARY OF THE INVENTION 
       [0015]    The technical problem to be solved by the present invention is, to provide a synthetic process of higher chemical reaction rate, higher one-way yield of target product, high selectivity of target products with higher degree of polymerization of methoxy groups. 
         [0016]    A method for synthesizing polyoxymethylene dimethyl ethers is provided in the present invention, the synthesis reaction is carried out by using paraformaldehyde or trioxane and methylal as feedstock in the presence of acidic catalyst, the initial temperature of reaction is controlled at 100-120° C., then the temperature is reduced to 50-70° C. by successive stepwise cooling or programmed cooling, the reaction pressure is controlled at 0.1-4.0 MPa, and the molar ratio of paraformaldehyde or trioxane metered in formaldehyde units to methylal in the feedstock is 1.5:1-8:1. 
         [0017]    The manner of successive stepwise cooling of the reaction mixture is that the temperature is reduced by 10-20° C. for each step, and then isothermal reaction is carried out, preferably the range of temperature reduced for each step is 10-15° C. 
         [0018]    All kinds of acidic catalysts in prior art which can implement the synthesis of polyoxymethylene dimethyl ethers can be used as the catalyst of the present invention, preferably a strong acidic cation exchange resin, and currently strong acidic cation exchange resin commercially available can achieve the objective of the present invention. 
         [0019]    The amount of the catalyst is equal to 0.3-3 wt % of the total amount of the feedstock, and preferably the amount of the catalyst is equal to 2-3 wt % of the total amount of the feedstock. 
         [0020]    The molar ratio of paraformaldehyde or trioxane metered in formaldehyde units to methylal in the feedstock is 1.5:1-6:1, and preferably 1.5;1-2:1. 
         [0021]    Preferably the pressure is controlled at 1.0-4 MPa, and more preferably 2-3 MPa. 
         [0022]    The reaction time of the synthesis reaction is 2-10 hours, and preferably 4-10 hours. 
         [0023]    As an implementable way, the synthesis reaction is carried out in a single-stage tank reactor using batch operation, and successive stepwise cooling according to time in the reaction process is achieved by a programmed temperature control system. 
         [0024]    As an alternative way, the synthesis reaction is carried out in multi-stage tank reactors connected in series using continuous operation, and successive stepwise cooling in the continuous reaction process is achieved by controlling temperatures of each respective reactor to be different. 
         [0025]    Further, the number of the multi-stage tank reactors connected in series is 2-8. 
         [0026]    More preferably, the tank reactor is slurry bed reactor. 
         [0027]    The present invention further discloses polyoxymethylene dimethyl ethers synthesized by the above-mentioned method. 
         [0028]    The reaction equation of the process of the present invention is as follow: 
         [0000]      CH 3 O(CH 2 O) n-1 CH 3 +HCHO 13  &lt;=&gt;_CH 3 O(CH 2 O) n CH 3 +Q n-1    
         [0029]    where n is degree of polymerization of methoxy groups, and n≧2; Q 1  is the quantity of released heat of the i th  main reaction, and i=n−1. 
         [0030]    Because synthesizing DMM n  using trioxane or paraformaldehyde and methylal as feedstock is a highly exothermic reversible reaction. It is found in research that the relationship between the equilibrium constant of the reaction and the temperature is significantly dependent on the type of feedstock and polymerization degree of methoxy groups in the reaction product. And from the perspective of structural characteristics of the reaction network of the study, the degree of polymerization of methoxy groups in product increases sequentially, and the activity of all kinds of catalysts used so far is generally lower, the rate of reaction is slower. Similarly in view of the above understanding based on thermodynamics of the reaction system itself, the reaction network structure and dynamic characteristics, further taking into account that, when using certain feedstock systems, the target product will be synthesized while notable amount of water will be produced, so as to increase the investment and energy consumption of the post-separation, within the practicable temperature range of the reaction, the equilibrium constant of the synthetic reaction is sensitive to temperature variation, and the level of sensitivity will increase with the increase of degree of polymerization of methoxy groups in product. Based on these understanding of thermodynamic property of the reaction system, the present invention puts forward to use a specially designed slurry bed reactor system under the conditions of eliminating diffusion effects and suitable temperature and pressure to achieve the synthetic reaction. 
         [0031]    For the slurry bed reactors of batch operation, for example, a single-stage slurry bed tank reactor of batch operation, successive stepwise cooling or programmed cooling is used; for the slurry bed reactors of continuous operation, including multi-stage slurry bed tank reactors connected in series, tubular slurry bed reactors, tower-type slurry bed tank reactors and static hybrid-type slurry bed tank reactors etc., the way used is that the spatial distribution of the reaction temperature is optimized, for example, for multi-stage slurry bed tank reactors connected in series of continuous operation, the reaction temperature is reduced by progressively stepwise cooling, so as to repeatedly and duly break through the limitation of thermodynamic equilibrium of the chemical reaction, to increase the average rate of chemical reaction, one-way conversion rate of the feedstock, and the overall one-way yield of the target product, at the same time, to improve the selectivity of target product with suitable degree of polymerization of methoxy groups, so as to strengthen the reaction process. 
         [0032]    The aforementioned technical solutions of the present invention have the following advantages, as compared to the prior art: 
         [0033]    1. For the feedstock system of paraformaldehyde or trioxane and methylal and within suitable operating temperature range of the selected catalyst, in consideration of the characteristic that the equilibrium constant of the synthetic reaction is sensitive to temperature variation and the level of sensitivity will increase with the increase of degree of polymerization of methoxy groups in product, the synthetic process by stepwise cooing is designed in the manner of reaction→being close to chemical equilibrium→cooling to make the equilibrium shift in the direction in favor of producing target product→reacting again, so as to repeatedly and duly break through the limitation of chemical reaction equilibrium, to promote continuously forward reaction, to increase the average rate of chemical reaction, one-way conversion rate of the feedstock, and the overall one-way yield of the target product, especially to improve the selectivity of target product with suitable degree of polymerization of methoxy groups, so as to strengthen the reaction process; compared to the method of maintaining a constant reaction temperature all the time, higher overall yield of the target product can be achieved in the same reaction time, and selectivity of products with higher degree of polymerization of methoxy groups can be increased; 
         [0034]    2. The technical solution can be achieved by using the slurry bed tank reactor of single batch operation, through successive stepwise cooling according to time by a programmed temperature control system, and it also can be achieved by using multi-stage slurry bed tank reactors connected in series of continuous operation, through successive stepwise cooling in the continuous reaction process by controlling temperatures of each respective reactor to be different, which is easy to do. 
         [0035]    3. The reaction technical ideas provided by the present invention can easily be extended to other types of continuous-operation reactors in which DMM n  is synthesized by using paraformaldehyde or trioxane and methylal as feedstock, in the presence of acidic catalyst; 
         [0036]    4. The method of the present invention can effectively shorten the reaction time, improve one-way yield of the product, and no water is produced in the whole system, and subsequent extraction and refinement process is relatively simple. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    In order to make the present invention clearly understood more easily, detailed description is further presented below, in accordance with specific embodiments and in conjunction with accompany drawings, wherein, 
           [0038]      FIG. 1  is a kind of process flow diagram showing the synthetic process of the present invention; 
           [0039]      FIG. 2  is another kind of process flow diagram showing the synthetic process of the present invention; 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0040]    All kinds of strong acidic cation exchange resin catalyst known in the prior art can be selected and used as the catalyst in the technical solution of the present invention, in the following embodiments, D001 macroporous strong acidic styrene type cation exchange resin and 001×7 strong acidic styrene type cation exchange resin produced by Shanghai Jin Kai Resin Co., Ltd (Shanghai resin factory) are taken as examples to expound the technical effect. 
       Embodiment 1 
       [0041]    Experimental device of the process flow of this embodiment is shown in  FIG. 1 . The feedstock solution of paraformaldehyde and methylal is prepared according to a 2:1 molar ratio of paraformaldehyde metered in formaldehyde units to methylal, the solution is added into a 0.3 L single-stage stirred tank reactor, and then D001macroporous strong acidic styrene type cation exchange resin catalyst at the amount of 2 wt % of the overall feedstock is added. The initial pressure of the reaction is controlled at 2.0 MPa, and stirring speed is 250 r/min. And the isothermal reaction experiment using stepwise programmed cooling is carried out in accordance with the following procedures; the reaction mixture is rapidly heated to 100° C., after that the isothermal reaction is carried out for 4 hours; the reaction temperature is rapidly cooled to 90° C. in very short time, then the isothermal reaction is carried out for 2 hours again; the reaction temperature is rapidly cooled to 80° C. in a few minutes, then the isothermal reaction is carried out for 2 hours again; the reaction temperature is rapidly cooled to 70° C. in a few minutes, then the isothermal reaction is carried out for 2 hours again, until the reaction is completed. The sampling is started from when the reaction temperature reaches 100° C. and the timing is started, thereafter samples are taken once per hour for analysis of product composition. 
         [0042]    The overall yield of the target product after 10 hours of reaction is 58.74 wt. %. It is also found that after 5 hours the concentration of DMM 8  in the product has reached about 0.3 wt %. 
       Embodiment 2  
       [0043]    The process flow of this embodiment is the same as Embodiment 1. The reaction feedstock and conditions of this embodiment are similar to Embodiment 1, and the difference is that the reaction temperature is controlled at 100° C. all the time, after 10 hours the reaction is completed. The final overall yield of the target product is 51.66 wt %, DMM 8  is not detected throughout the reaction. 
         [0044]    The concentration distribution of final products in Embodiment 1 and Embodiment 2 is shown in the following table. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                   
               
               
                 Serial number 
                 DMM 2   
                 DMM 3   
                 DMM 4   
                 DMM 5-8   
                 DMM n&gt;8   
               
               
                   
               
             
             
               
                 Embodiment 1 
                 25.45 wt. % 
                 15.12 
                 8.73 
                 9.44 
                 ~0 
               
               
                   
                   
                 wt. % 
                 wt. % 
                 wt. % 
               
               
                 Embodiment 2 
                 22.98 wt. % 
                 13.72 
                 7.33 
                 7.63 
                 ~0 
               
               
                   
                   
                 wt. % 
                 wt. % 
                 wt. % 
               
               
                   
               
             
          
         
       
     
         [0045]    As can be seen by analyzing the table, after the same reaction time of 10 hours, the operation scheme of successive stepwise cooling is compared with the isothermal reaction in which the temperature is kept at the initial temperature of the aforementioned successive cooling, and it is found that the concentration of each kind of the target product of the former is higher than the latter, the overall yield of ΣDMM 2-8  is increased by about 7 percentage points, the proportion of DMM 5-8  in the target product is also higher. It is clearly indicated that successive cooling indeed promote the equilibrium of the reaction system to shift in the direction of producing the target product, and it can not only increase the one-way overall yield of the target product, but also improve the selectivity of target products with higher degree of polymerization of methoxy groups, thus strengthen the synthesis reaction. 
       Embodiment 3 
       [0046]    Experimental device of the process flow of this embodiment is shown in  FIG. 2 . The feedstock solution is prepared according to a 2:1 molar ratio of paraformaldehyde metered in formaldehyde units to methylal, the solution is added into a three-stage combination of 5.0 L slurry bed tank reactors connected in series, the temperature of the first reactor, the second reactor and the third reactor is respectively controlled at 100° C., 80° C. and 60° C., with continuous feeding, and the average reaction time of each tank reactor is kept at about 2 hours. The type of catalyst and its amount used and other reaction conditions are the same to Embodiment 1. The operation is carried out continuously until the system is stable and samples are taken for composition analysis. The final overall yield of the target product is ΣDMM 2-8 =57.22 wt. %. DMM 8  is detected in the final product. 
       Embodiment 4 
       [0047]    The process flow is shown in  FIG. 2 . The feedstock solution is prepared according to a 2:1 molar ratio of paraformaldehyde metered in formaldehyde units to methylal, the solution is added into a three-stage combination of 5.0L slurry bed tank reactors connected in series, the temperature of the first reactor, the second reactor and the third reactor is all controlled at 100° C, with continuous feeding, and the average reaction time of each tank reactor is kept at about 2 hours until the constant state is reached. The type of catalyst and its amount used and other reaction conditions are all the same to Embodiment 3. The operation is carried out continuously until the system is stable and samples are taken for composition analysis. The final overall yield of the target product is ΣDMM 2-8 =53.27 wt. %. DMM 8  is not detected in the final product. 
         [0048]    The concentration distribution of final products in Embodiment 3 and Embodiment 4 is shown in the following table. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                   
               
               
                 Serial number 
                 DMM 2   
                 DMM 3   
                 DMM 4   
                 DMM 5-8   
                 DMM n&gt;8   
               
               
                   
               
             
             
               
                 Embodiment 3 
                 25.02 wt. % 
                 14.54 
                 8.43 
                 9.23 
                 ~0 
               
               
                   
                   
                 wt. % 
                 wt. % 
                 wt. % 
               
               
                 Embodiment 4 
                 23.55 wt. % 
                 13.71 
                 7.50 
                 8.51 
                 ~0 
               
               
                   
                   
                 wt. % 
                 wt. % 
                 wt. % 
               
               
                   
               
             
          
         
       
     
         [0049]    As can be seen by analyzing the table, for the three-stage combination of slurry bed tank reactors connected in series using continuous operation, the operation scheme of successive stepwise cooling is compared with the isothermal reaction in which the temperature of the three reactors is equally kept at 100° C., and after the same reaction time of about 6 hours, it is found that the concentration of each kind of the target product of the former is higher than the latter, the overall yield of ΣDMM 2-8  is increased by about 4 percentage points, the proportion of DMM 5-8  in the target product is also higher. It is clearly indicated that for the multi-stage combination of slurry bed tank reactors connected in series using continuous operation, the reaction process of successive cooling provided by the present invention on the basis of thermodynamic equilibrium principle of the reaction system is also effective, it indeed promote the equilibrium of the reaction system to shift in the direction of producing the target product, it can not only increase the one-way overall yield of the target product, but also improve the selectivity of target products with higher degree of polymerization of methoxy groups, thus strengthen the synthesis reaction. 
       Embodiment 5 
       [0050]    Experimental device of the process flow of this embodiment is shown in  FIG. 1 . The feedstock solution is prepared according to a 1.5:1 molar ratio of trioxane metered in formaldehyde units to methylal, the solution is added into a 0.3 L single-stage stirred tank reactor, and then 001×7 strong acidic styrene type cation exchange resin catalyst at the amount of 3 wt % of the overall feedstock is added. The initial pressure of the reaction is controlled at about 2.0 MPa, and stirring speed is 250 r/min. And the isothermal reaction experiment using stepwise cooling is carried out in accordance with the following procedures: the reaction mixture is rapidly heated to 100° C., after that the isothermal reaction is carried out for 1 hour; the reaction temperature is rapidly cooled to 90° C., then the isothermal reaction is carried out for 1 hour again; the reaction temperature is rapidly cooled to 80° C. in a few minutes, then the isothermal reaction is carried out for 1 hour, and the reaction is completed after 3 hours in total of reaction. The sampling is started from when the reaction temperature reaches 100° C. and the timing is started, thereafter samples are taken once per hour for analysis of product composition. 
         [0051]    The final overall yield of the target product is 47.55 wt. % after 3 hours. 
       Embodiment 6 
       [0052]    The process flow of this embodiment is the same as Embodiment 1, as shown in  FIG. 1 . The reaction feedstock and conditions are similar to Embodiment 5, and the difference is that the reaction temperature is controlled at 100° C. all the time, after 3 hours the reaction is completed. The final overall yield of the target product is 43.26 wt. %. 
         [0053]    The concentration distribution of final products in Embodiment 5 and Embodiment 6 is shown in the following table. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                   
               
               
                 Serial number 
                 DMM 2   
                 DMM 3   
                 DMM 4   
                 DMM 5-8   
                 DMM n&gt;8   
               
               
                   
               
             
             
               
                 Embodiment 5 
                 24.88 wt. % 
                 12.25 
                 5.50 
                 4.92 
                 ~0 
               
               
                   
                   
                 wt. % 
                 wt. % 
                 wt. % 
               
               
                 Embodiment 6 
                 24.30 wt. % 
                 11.27 
                 4.68 
                 3.01 
                 ~0 
               
               
                   
                   
                 wt. % 
                 wt. % 
                 wt. % 
               
               
                   
               
             
          
         
       
     
         [0054]    As can be seen by analyzing the table, the operation scheme of successive stepwise cooling is compared with the isothermal reaction in which the temperature is kept at the initial temperature of the aforementioned successive cooling, and after the same reaction time of 3 hours, it is found that the concentration of each kind of the target product of the former is higher than the latter, the overall yield of ΣDMM 2-9  is increased by about 4.3 percentage points, the proportion of DMM 5-8  in the target product is also higher. It is clearly indicated that successive cooling indeed promote the equilibrium of the reaction system to shift in the direction of producing the target product, it can not only increase the one-way overall yield of the target product, but also improve the selectivity of target products with higher degree of polymerization of methoxy groups, thus strengthen the synthesis reaction. 
       Embodiment 7 
       [0055]    Experimental device of the process flow of this embodiment is shown In  FIG. 2 . The feedstock solution is prepared according to a 1:1 molar ratio of trioxane metered in formaldehyde units to methylal, the solution is added into a three-stage combination of 5.0 L slurry bed tank reactors connected in series, the temperature of the first reactor, the second reactor and the third reactor is respectively controlled at 100° C., 85° C. and 70° C., with continuous feeding, and the average reaction time of each tank reactor is kept at about 1 hour. The type of catalyst and its amount used and other reaction conditions are the same to Embodiment 5. The operation is carried out continuously until the system is stable and samples are taken for composition analysis. The final overall yield of the target product is ΣDMM 2-8 =46-19 wt. %. DMM 8  is detected in the final product. 
       Embodiment 8 
       [0056]    The process flow of this embodiment is shown in  FIG. 2 . The feedstock solution is prepared according to a 1:1 molar ratio of trioxane metered in formaldehyde units to methylal, the solution is added into a three-stage combination of 5.0 L slurry bed tank reactors connected in series, the temperature of reactor of each stage is all controlled at 100° C., with continuous feeding, and the average reaction time of each tank reactor is kept at about 1 hour. The type of catalyst and its amount used and other reaction conditions are all the same to Embodiment 5. The operation is carried out continuously until the system is stable and samples are taken for composition analysis. The final overall yield of the target product is ΣDMM 2-8 =43.07 wt. %. DMM 8  is not detected in the final product. 
         [0057]    The concentration distribution of final products in Embodiment 7 and Embodiment 8 is shown in the following table. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                   
               
               
                 Serial number 
                 DMM 2   
                 DMM 3   
                 DMM 4   
                 DMM 5-8   
                 DMM n&gt;8   
               
               
                   
               
             
             
               
                 Embodiment 7 
                 24.45 wt. % 
                 11.77 
                 5.41 
                 4.56 
                 ~0 
               
               
                   
                   
                 wt. % 
                 wt. % 
                 wt. % 
               
               
                 Embodiment 8 
                 24.26 wt. % 
                 11.21 
                 4.60 
                 3.00 
                 ~0 
               
               
                   
                   
                 wt. % 
                 wt. % 
                 wt. % 
               
               
                   
               
             
          
         
       
     
         [0058]    As can be seen by analyzing the table, for the three-stage combination of slurry bed tank reactors connected in series using continuous operation, the operation scheme of successive stepwise cooling is compared with the isothermal reaction in which the temperature of the three reactors is equally kept at 100° C., and after the same reaction time of 3 hours, it is found that the concentration of each kind of the target product of the former is higher than the latter, the overall yield of ΣDMM 2-8  is increased by about 3.1 percentage points, the proportion of DMM 5-8  of the target product is also higher. It is clearly indicated that for the multi-stage combination of slurry bed tank reactors connected in series using continuous operation, the reaction process of successive cooling provided by the present invention on the basis of thermodynamic equilibrium principle of the reaction system is also effective, it indeed promote the equilibrium of the reaction system to shift in the direction of producing the target product, it can not only increase the one-way overall yield of the target product, but also improve the selectivity of target products with higher degree of polymerization of methoxy groups, thus strengthen the synthesis reaction. 
         [0059]    The above data indicates that through the way of continuous stepwise cooling of the present invention using thermodynamic principle in order to break through the chemical equilibrium in the reaction and to promote continuously forward reaction is conductive to increase the content of the target product in the whole system, and meanwhile the distribution of products with higher degree of polymerization is better. 
         [0060]    Obviously, the aforementioned embodiments are merely intended for clearly describing the examples, rather than limiting the implementation scope of the invention. For those skilled in the art, various changes and modifications in other different forms can be made on the basis of the aforementioned description. It is unnecessary and impossible to exhaustively list all the implementation ways herein. However, any obvious changes or modifications derived from the aforementioned description are intended to be embraced within the protection scope of the present invention.