Method for preparing polyoxymethylene dimethyl ether in anhydrous formaldehyde system

Abstract

The invention discloses a method for preparing polyoxymethylene dimethyl ether in an anhydrous formaldehyde system, and belongs to the field of new energy synthetic fuel and environment-friendly solvent preparation. Aiming at the problems that a water-containing system in the prior art is low in reaction efficiency, separation of residual formaldehyde and formic acid is interfered, and environmental protection and economy are insufficient, the method comprises the following steps: P1, reacting anhydrous gas formaldehyde and methylal with the purity of more than or equal to 99.0% serving as raw materials in a radial flow fixed bed reactor under the catalysis of macroporous cation exchange resin at the temperature of 50-80 DEG C, the pressure of 0.2-0.6 MPa and the air speed of 0.4-1.0 h <-1 > to generate a product containing DMM2-6; under the hydrogenation catalysis of P2 non-supported NiMoW, hydrogenation is carried out at the temperature of 40-80 DEG C and the pressure of 1.0-4.0 MPa to remove aldehyde acid; the P3 is subjected to normal-pressure-reduced-pressure rectification to obtain DMM2 with the purity being larger than or equal to 98.0% and DMM3-6 with the purity being larger than or equal to 98.5%, and side line materials of the first rectification tower are recycled and reused. The formaldehyde conversion rate is greater than or equal to 92%, the DMM2-6 selectivity is greater than or equal to 98%, the product is free of sulfur-nitrogen aromatic hydrocarbons and waste lye, the energy consumption is low, and the raw material utilization rate is high.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of new energy synthetic fuel and environmentally friendly solvent preparation, and particularly relates to a method for preparing polyoxymethylene dimethyl ethers (DMM) in an anhydrous formaldehyde system. n BACKGROUND

[0002] Polyoxymethylene dimethyl ethers (chemical formula: CH3O(CH2O) n CH3, n is usually less than 8) are internationally recognized environmentally friendly solvents and clean fuel components. The State Development and Reform Commission and the Energy Bureau have listed them as a key project for technological innovation in clean and efficient use of coal in the “Energy Technology Revolution Innovation Action Plan (2016-2030)”, and they have irreplaceable value in reducing fuel pollutant emissions and replacing toxic organic solvents.

[0003] From the product characteristics, polyoxymethylene dimethyl ethers have the following advantages: first, they are sulfur-free and aromatic-free, do not produce SO X and aromatic pollutants during combustion, and have high cetane numbers (DMM2 is 63, and DMM 3-6 can reach 78-104) and high oxygen content (45.2%-49.5%). As a diesel blending component (10%-20% addition), they can significantly reduce solid particulate matter, CO and PM emissions in exhaust gas, and do not need to modify the vehicle engine. Second, they are low-toxic and biodegradable, have the lowest toxicity in organic solvents, can be naturally degraded without polluting the soil and water, and can replace toxic solvents such as toluene and xylene for use in the fields of coatings, inks and adhesives, thereby reducing VOC emissions and improving product performance.

[0004] However, the existing polyoxymethylene dimethyl ether preparation technology has many key bottlenecks, which seriously restrict its industrial application and product quality. The specific problems are as follows:

[0005] (1) Water-containing system leads to low reaction efficiency: the existing technology mostly uses water-containing formaldehyde (such as formaldehyde aqueous solution) or methanol-containing raw materials. The presence of water can significantly reduce the conversion rate of formaldehyde, the selectivity and yield of DMM n . On the one hand, water can form azeotropes with DMM2 or produce emulsions during separation, greatly increasing the difficulty of product separation. On the other hand, water can accelerate catalyst deactivation, resulting in the need for high-temperature reaction at 80-110℃. This not only increases energy consumption, but also increases the generation of by-products such as dimethyl ether (DME) and methyl formate (the selectivity of by-products is usually more than 5%).

[0006] (2) Residual formaldehyde and formic acid interfere with separation and purification: the residual formaldehyde (content often reaches 1%-3%) in the synthesis product has a low boiling point characteristic, which will be distilled out in advance during rectification and mixed with DMM​n The distillates are mixed, which makes it difficult to meet the product purity standard (usually less than 95%); meanwhile, formaldehyde changes the phase equilibrium of the mixture, and higher temperature and pressure are required to achieve separation, further increasing energy consumption; the presence of trace amounts of formic acid also corrodes the equipment, and additional alkali solution needs to be added for neutralization, which generates a large amount of waste alkali solution and pollutes the environment.

[0007] (3) Insufficient environmental protection and economy: the existing process uses alkali solution for neutralization of formic acid, and uses DMM n extractant, resulting in problems of waste alkali solution, solid waste (waste catalyst), and residual aromatic hydrocarbon; and the raw material utilization rate is low (formaldehyde conversion rate is usually less than 85%), there is no raw material recycling mechanism, the methanol consumption is high, and it does not meet the requirements of green chemical industry and low-cost production.

[0008] In summary, the existing technology cannot simultaneously meet the industrialization requirements of "high reaction efficiency, high product purity, low energy consumption, and no pollutant emission", and it is urgent to develop a water-free, low-temperature, green, and efficient polyoxymethylene dimethyl ethers preparation technology to break through the above bottlenecks. SUMMARY

[0009] In view of the deficiencies of the existing technology, the present application provides a method for preparing polyoxymethylene dimethyl ethers in a water-free formaldehyde system, which achieves a formaldehyde conversion rate of ≥92%, a DMM 2-6 selectivity of ≥98%; completely eliminates residual formaldehyde and formic acid, simplifies the separation process, and the entire process is free of waste water, waste alkali solution, and organic extractant.

[0010] To solve the above technical problems, the present application adopts the following technical solutions:

[0011] A method for preparing polyoxymethylene dimethyl ethers in a water-free formaldehyde system, comprising the following steps:

[0012] P1, synthesis reaction: using water-free gaseous formaldehyde and methylenedimethyl ether with a purity of ≥99.0% as raw materials, under the action of a macroporous cation exchange resin solid acid catalyst, a condensation reaction is carried out in a radial flow fixed bed reactor at 50-80℃, 0.2-0.6MPa, and a space velocity of 0.4-1.0h -1 Under the above conditions, a product containing polyoxymethylene dimethyl ethers DMM 2-6 is generated.

[0013] P2, hydrogenation supplementary refining: introducing the synthesis product of step P1 into a catalytic hydrogenation supplementary refining tower, under the action of a non-supported NiMoW hydrogenation catalyst, a hydrogenation reaction is carried out at 40-80℃, hydrogen pressure 1.0-4.0MPa, and hydrogen / oil volume ratio 100-300, so that the residual formaldehyde and trace amounts of formic acid in the product are hydrogenated and converted into methanol, and a refined product free of formaldehyde and formic acid is obtained;

[0014] P3, separation and purification: the refined product of step P2 is separated by normal pressure rectification and vacuum rectification in sequence to obtain DMM2 with purity ≥98.0% and DMM with purity ≥98.5% 3-6 .

[0015] Preferably, in step P1, the molar ratio of formaldehyde to methylal is 1:(1-5); the conversion rate of formaldehyde is ≥92%, the selectivity of DMM 2-6 is ≥98%, the residual formaldehyde content is ≤0.65%, the selectivity of by-product methanol is ≤2%, the selectivity of dimethyl ether is ≤0.01%, and the selectivity of methyl formate is ≤0.05%.

[0016] Preferably, in step P1, the pore size of the macroporous cation exchange resin solid acid catalyst (such as styrene macroporous resin) is 10-50 nm, the exchange capacity is 3.0-5.0 mmol / g, and the specific surface area is 300-500 m 2 / g.

[0017] Preferably, in step P2, the mass ratio of Ni, Mo, and W in the non-supported NiMoW hydrogenation catalyst is 1:(2-3):(1-2), the specific surface area of the catalyst is 150-250 m 2 / g, and the average particle size is 20-50 μm.

[0018] Preferably, in step P2, the hydrogenation reaction space velocity is 0.5-2.0 h -1 ; and the formaldehyde content in the product after refining is 0%, and the acid value is ≤0.01 mgKOH / 100 mL

[0019] Preferably, in step P3, the normal pressure rectification is carried out in a first rectification column, the reflux ratio of the first rectification column is (1-3):1, the overhead temperature is 40-43.5 ℃, and the column bottom temperature is 86-97 ℃; the vacuum rectification is carried out in a second rectification column, the reflux ratio of the second rectification column is (2-3):1, the overhead temperature is 81-87 ℃, the column bottom temperature is 125-130 ℃, and the theoretical plate number is 30.

[0020] The DMM 2-6 product obtained in step P3 is free of sulfur, nitrogen, and aromatic hydrocarbons.

[0021] Preferably, in step P3, the mixture containing methylal and methanol is taken out from the side line of the first rectification column, which is recycled to step P1 as raw material.

[0022] A system for implementing the above-mentioned method comprises:

[0023] a radial flow fixed bed reactor, the feed end of which is connected to a formaldehyde feed system and a methylal feed system;

[0024] a catalytic hydrogenation make-up finishing tower, a feed end of which is communicated with a discharge end of the radial flow fixed bed reactor to form a series material flow;

[0025] a first rectifying tower, a feed end of which is communicated with a discharge end of the catalytic hydrogenation make-up finishing tower, a side line of which is provided with a circulation pipeline connected to a feed system of the radial flow fixed bed reactor;

[0026] a second rectifying tower, a feed end of which is communicated with a tower kettle of the first rectifying tower, a top of which is provided with a DMM2 output port, and a bottom of which is provided with a DMM 3-6 output port.

[0027] The technical scheme of the present application has the following advantages:

[0028] A. Reaction efficiency and selectivity are greatly improved: The present application uses anhydrous gas formaldehyde and high-purity methylal (≥99.0%) as raw materials, and combines a radial flow fixed bed reactor and a macroporous cation exchange resin catalyst to realize low-temperature reaction at 50-80℃, with formaldehyde conversion rate ≥92% (existing technology <85%), DMM 2-6 selectivity ≥98% (existing technology <90%), and very low selectivity of dimethyl ether and methyl formate by-products, and raw material utilization rate is significantly improved.

[0029] B. Separation difficulty is reduced and product purity is high: The present application completely converts formaldehyde and formic acid into methanol by using a non-supported NiMoW hydrogenation catalyst, and the refined product is "free of aldehyde, free of acid, and almost free of water", without the need to introduce an extractant or lye; combined with atmospheric-vacuum distillation, DMM2 with purity ≥98.0% and DMM 3-6 with purity ≥98.5% (existing technology <95%) can be easily obtained, and the product is free of sulfur, nitrogen and aromatic hydrocarbons, meeting the requirements of environmentally friendly solvents and clean fuel oil.

[0030] C. Green and environmentally friendly with no pollutant emissions: The present application does not use lye neutralization or introduce aromatic extractants throughout the process, and is free of waste lye, solid waste (long catalyst service life, reusable), and aromatic hydrocarbon residues, meeting the requirements of green chemical industry; at the same time, the methylal-methanol mixture collected from the side line of the first rectifying tower is recycled and reused, with atomic utilization rate close to 100%, and methanol consumption is reduced by more than 30%.

[0031] D. Low energy consumption and strong process stability: The reaction and refining in the present application are carried out at low temperature (40-80℃), with energy consumption reduced by 40% compared with existing technology (80-110℃); the radial flow fixed bed reactor has high mass transfer efficiency and the catalyst is not easy to deactivate; the separation process does not require high pressure, with low risk of equipment corrosion, strong process stability and strong industrial feasibility. BRIEF DESCRIPTION OF DRAWINGS

[0032] In order to more clearly illustrate the specific embodiments of the present application, the drawings required to be used in the specific embodiments will be briefly introduced as follows. Obviously, the drawings described in the following specific embodiments are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative effort on the basis of these drawings.

[0033] Figure 1 The process flow diagram for preparing polyoxymethylene dimethyl ethers in the present application is shown in the figure.

[0034] Figure 2 The schematic diagram of the radial flow fixed bed reactor in the present application is shown in the figure.

[0035] The radial flow fixed bed reactor is internally provided with a raw material distributor, a catalyst bed (loaded with macroporous cation exchange resin) and a product collector. The raw material flows along the radial direction, and the mass transfer efficiency is increased by 20% to 30% compared with the traditional fixed bed.

[0036] The figure is marked as follows:

[0037] 1-radial flow fixed bed reactor; 2-catalytic hydrogenation make-up finishing tower; 3-first rectification tower; 4-second rectification tower.

[0038] S1-anhydrous gaseous formaldehyde; S2-99.0% methylal; S3-synthetic crude product; S4-hydrogenation finishing product; S5-tower top incondensable gas (dimethyl ether, methyl formate); S6-side line circulating raw material (methylal + methanol); S7-DMM 2-6 crude product; S8-DMM2 product; S9-DMM 3-6 product. Specific embodiments

[0039] The present application can be implemented in many different forms, and should not be understood as being limited to the embodiments set forth herein. On the contrary, these embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the concept of the present application to those skilled in the art. All other embodiments obtained by those skilled in the art on the basis of the embodiments in the present application without creative effort shall fall within the scope of protection of the present application.

[0040] The overall process flow of the present application is as follows:

[0041] As Figure 1 , Figure 2As shown, anhydrous formaldehyde S1, 99% formal S2 enters a radial flow fixed bed reactor 1 to synthesize polymethoxy dimethyl ether, reactor outlet stream S3; S3 enters a catalytic hydrogenation complementary refining tower 2 to remove a small amount of formaldehyde and trace formic acid therefrom, catalytic hydrogenation complementary refining tower 2 outlet stream S4; S4 passes through a heat exchanger to enter a first rectifying tower 3, tower top stream S5 is a byproduct dimethyl ether and methyl formate, measuring line stream S6 is methanol and formal (as raw material returned for recycling), tower bottom stream S7 is DMM 2-6 ; S7 enters a second rectifying tower 4, tower top stream S8 is product DMM2, and tower bottom stream S9 is DMM 3-6 product.

[0042] Example 1:

[0043] The present embodiment provides a method for preparing polymethoxy dimethyl ether in an anhydrous formaldehyde system, comprising the following steps:

[0044] P1, the molar ratio of formaldehyde to formal in the synthesis unit is 1:1.5, the reaction temperature is 50°C, the reaction pressure is 0.2 MPa, the space velocity is 0.4 h -1 , and the product composition of radial flow fixed bed reactor 1 outlet stream S3 is shown in Table 1, the formaldehyde conversion rate is as high as 93.64%, the DMM 2-6 selectivity is as high as 98.9%, and the acidity of the reaction system is very low, only 3.7 mgKOH / 100ml.

[0045] Table 1 Reaction conditions and product composition of reactor outlet stream S3

[0046]

[0047]

[0048] P2, hydrogenation complementary refining of stream S3 in a catalytic hydrogenation complementary refining tower 2, using a non-supported NiMoW hydrogenation catalyst, under the conditions of 40°C, a reaction space velocity of 0.5 h -1 , a hydrogen pressure of 4.0 MPa, and a hydrogen / oil ratio of 100, the catalytic hydrogenation complementary refining tower 2 outlet stream S4 composition is shown in Table 2. The high activity and high selectivity of the catalyst convert formaldehyde and formic acid into methanol, and the system acidity is undetectable.

[0049] Table 2 Composition distribution table of catalytic hydrogenation complementary refining tower outlet stream S4

[0050]

[0051] P3, the stream S4 enters the first rectifying tower 3 to separate and purify, the operation condition of the first rectifying tower 3 is that the reflux ratio is 1:1, the column bottom temperature is 86 DEG C, the column top temperature is 40-40.5 DEG C, the column section insulation is 50 DEG C, the dimethyl ether methyl formate is taken out from the column top, the side line takes out the methylal DMM with the purity of 93.2% and the methanol with 6.9%. The stream S7 of the first rectifying tower 3 enters the second rectifying tower 4 to further continuously rectify, the more preferable operation condition of the second rectifying tower 4 is that the reflux ratio is 2:1, the column bottom temperature is 127 DEG C, the column top temperature is 81-82 DEG C, the column section insulation is 85 DEG C, the theoretical column plate number is 30, the stream S8 with the purity of 98.1% DMM2 is taken out from the column top, the stream S9 with the purity of 99.0% DMM is obtained from the column bottom 3-6 .

[0052] Example 2

[0053] The embodiment provides a method for preparing polymethoxy dimethyl ether in anhydrous formaldehyde system, comprising the following steps:

[0054] P1, the molar ratio of the synthesized unit formaldehyde and methylal is 1:1, the reaction temperature is 70 DEG C, the reaction pressure is 0.4 Mpa, the space velocity is 0.6 h -1 The product composition of the outlet stream S3 of the radial flow fixed bed reactor 1 is shown in table 3, the formaldehyde conversion rate is as high as 92.1%, the DMM 2-6 The selectivity is as high as 98.8%, and the acidity of the reaction system is 5.8 mgKOH / 100ml.

[0055] Table 3 reaction condition and product composition table of the reactor outlet stream S3

[0056]

[0057] P2, the stream S3 is hydrogenated and supplementary refined in the catalytic hydrogenation supplementary refining tower 2, a non-supported NiMoW hydrogenation catalyst is used, under the conditions of 60 DEG C, the reaction space velocity is 1.0 h -1 , the hydrogen pressure is 1.0 MPa, the hydrogen oil ratio is 200, the composition of the outlet stream S4 of the catalytic hydrogenation supplementary refining tower 2 is shown in table 4. The catalyst has high activity and high selectivity, and converts the formaldehyde and formic acid into methanol, and the system acidity cannot be detected.

[0058] Table 4 composition distribution table of the outlet stream S4 of the catalytic hydrogenation supplementary refining tower

[0059]

[0060] P3, the stream S4 enters the first rectifying tower 3 to separate and purify, the operation condition of the first rectifying tower 3 is that the reflux ratio is 2:1, the column bottom temperature is 97 DEG C, the column top temperature is 40-43.5 DEG C, the column section insulation is 60 DEG C, the dimethyl ether methyl formate is taken out from the column top, the side line takes out the methylal DMM with the purity of 92.7% and the methanol with 7.3%. The stream S7 of the first rectifying tower 3 enters the second rectifying tower 4 to further continuously rectify, the more preferable operation condition of the second rectifying tower 4 is that the reflux ratio is 3:1, the column bottom temperature is 130 DEG C, the column top temperature is 85-87 DEG C, the column section insulation is 85 DEG C, the theoretical column plate number is 30, the stream S8 with the purity of 98.5% DMM2 is taken out from the column top, the stream S9 with the purity of 99.3% DMM is obtained from the column bottom 3-6 .

[0061] Example 3:

[0062] The embodiment provides a method for preparing polymethoxy dimethyl ether in anhydrous formaldehyde system, comprising the following steps:

[0063] P1, the molar ratio of the synthesized unit formaldehyde and methylal is 1:5, the reaction temperature is 80 DEG C, the reaction pressure is 0.6 Mpa, the space velocity is 1.0 h -1 The product composition of the outlet stream S3 of the radial flow fixed bed reactor 1 is shown in Table 5, the formaldehyde conversion rate is as high as 93.64%, the DMM 2-6 The selectivity is as high as 98.9%, and the acidity of the reaction system is very low, only 3.7 mgKOH / 100ml.

[0064] Table 5 Reaction condition and product composition table of the reactor outlet stream S3

[0065]

[0066] P2, the stream S3 is hydrogenated and supplementary refined in the catalytic hydrogenation supplementary refining tower 2, a non-supported NiMoW hydrogenation catalyst is used, under the conditions that the temperature is 80 DEG C, the reaction space velocity is 2.0 h -1 , the hydrogen pressure is 1.0 MPa, the hydrogen / oil ratio is 300, the composition of the outlet stream S4 of the catalytic hydrogenation supplementary refining tower 2 is shown in Table 6. The catalyst converts the formaldehyde and formic acid into methanol with high activity and high selectivity, and the system acidity cannot be detected.

[0067] Table 6 Composition distribution table of the outlet stream S4 of the catalytic hydrogenation supplementary refining tower

[0068]

[0069] P3, the stream S4 enters the first rectifying column 3 for separation and purification, the operation condition of the first rectifying column 3 is that the reflux ratio is 3:1, the column bottom temperature is 89℃, the column top temperature is 40-43.5℃, the column section insulation temperature is 60℃, the dimethyl ether methyl formate is taken out from the column top, the methylenedimethyl ether DMM with the purity of 92.5% and the methanol with the purity of 7.5% are taken out from the side line. The stream S7 of the first rectifying column 3 enters the second rectifying column 4 for further continuous rectification, the preferable operation condition of the second rectifying column 4 is that the reflux ratio is 3:1, the column bottom temperature is 125℃, the column top temperature is 85-87℃, the column section insulation temperature is 85℃, the theoretical column plate number is 30, the DMM2 with the purity of 98.7% is taken out from the column top as the stream S8, and the DMM with the purity of 99.5% is obtained from the column bottom as the stream S9 3-6 .

[0070] The unmentioned part of the present application is applicable to the prior art.

[0071] Obviously, the above-mentioned embodiments are only examples for clearly illustrating the present application, and are not intended to limit the embodiments. Based on the above-mentioned description, other different forms of changes or variations can be made by those skilled in the art. All the embodiments do not need to be exhausted, and the obvious changes or variations derived from the embodiments are still within the protection scope of the present application.

Claims

1. A process for the production of polymethoxy dimethyl ether in an anhydrous formaldehyde system, characterized in that, Comprising the following steps: P1, synthesis reaction: with anhydrous formaldehyde and methylal with purity ≥ 99.0% as raw materials, under the action of macroporous cation exchange resin solid acid catalyst, condensation reaction was carried out in a radial flow fixed bed reactor (1) at 50-80°C, 0.2-0.6 MPa, space velocity 0.4-1.0 h -1 -1, to generate a product containing polyoxymethylene dimethyl ether DMM 2-6 ; P2, hydrogenation complementary refining: introducing the synthetic product of step P1 into a catalytic hydrogenation complementary refining tower (2), under the action of a non-supported NiMoW hydrogenation catalyst, hydrogenation reaction is carried out at 40-80℃, hydrogen pressure 1.0-4.0MPa, hydrogen / oil volume ratio 100-300, so that the residual formaldehyde and trace formic acid in the product are hydrogenated into methanol, obtaining a refined product free of formaldehyde and formic acid; P3, isolation and purification: the refined product of step P2 is separated by normal pressure rectification and vacuum rectification in turn to obtain DMM2 with purity ≥98.0% and DMM with purity ≥98.5% 3-6 .

2. The method of claim 1, wherein, In step P1 the molar ratio of formaldehyde to methylal is 1 : (1-5); the conversion of formaldehyde is > 92%, the DMM 2-6 selectivity > 98%, residual formaldehyde content < 0.65%; by-products methanol selectivity < 2%, dimethyl ether selectivity < 0.01%, methyl formate selectivity < 0.05%.

3. The method of claim 1, wherein, In step P1, the macroporous cation exchange resin solid acid catalyst has a pore size of 10-50 nm, an exchange capacity of 3.0-5.0 mmol / g, and a specific surface area of 300-500 m 2 / g.

4. The method of claim 1, wherein, In step P2, the mass ratio of Ni, Mo and W in the non-supported NiMoW hydrogenation catalyst is 1:(2-3):(1-2), the specific surface area of the catalyst is 150-250 m 2 / g, and the average particle size is 20-50 μm.

5. The method of claim 1, wherein, The hydrogenation reaction space velocity in the step P2 is 0.5-2.0 h -1 .

6. The method of claim 1, wherein, In step P3, the atmospheric distillation is carried out in the first distillation tower (3), the reflux ratio of the first distillation tower (3) is (1-3):1, the overhead temperature is 40-43.5℃, and the column bottom temperature is 86-97℃; the vacuum distillation is carried out in the second distillation tower (4), the reflux ratio of the second distillation tower (4) is (2-3):1, the overhead temperature is 81-87℃, the column bottom temperature is 125-130℃, and the theoretical plate number is 30.

7. The method of claim 1, wherein, DMM from step P3 2-6 The product is free of sulfur, free of nitrogen, free of aromatics.

8. The method of claim 1, wherein, In step P3, the side line of the first distillation tower (3) takes out a mixture containing methylal and methanol, which is recycled to step P1 as raw material.

9. A system for implementing the method of any of claims 1-8, characterized by Comprising: a radial flow fixed bed reactor (1) whose feed end is connected to a formaldehyde feed system and a methylal feed system; a catalytic hydrogenation complementary refining tower (2) whose feed end is in communication with the discharge end of the radial flow fixed bed reactor (1) to form a series material flow; a first distillation tower (3) whose feed end is in communication with the discharge end of the catalytic hydrogenation complementary refining tower (2), and a side line is provided with a circulation pipeline connected to the feed system of the radial flow fixed bed reactor (1); a second rectifying column (4) whose feed end is communicated with the column still of the first rectifying column (3), whose top is provided with a DMM2 output port, and whose bottom is provided with a DMM2 input port. 3-6 output port.

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