CN102180778A - Method for preparing low-polymerization-degree polyformaldehyde dialkyl ether from mixture of lower alcohols and application thereof - Google Patents

Abstract

The invention belongs to the technical field of clean utilization of coal and clean diesel fuel, and in particular relates to a preparation method of polyoxymethylene dialkyl ether with a low degree of polymerization that can be used as an additive for fossil diesel fuel. The general structural formula of the polyoxymethylene dialkyl ether with a low degree of polymerization is C _n H _2n+1 O(CH ₂ O) _m C _n H _2n+1 , wherein m=1-5 integers, n=1-5 an integer of . The dialkyl ether with a low degree of polymerization prepared by the present invention has a relatively high CN value, and can be widely and effectively used in diesel fuel without a high degree of polymerization, and the preparation process is relatively simple; The fusel alcohols of the product or the low-carbon mixed alcohols produced in the Fischer-Tropsch synthesis reaction are used as raw materials. On the one hand, they can be put into use without purification treatment, which not only greatly saves production costs, but also provides low-carbon mixed alcohols produced in a large number of industries. The comprehensive utilization provides a better technological route and has obvious economic significance.

Description

Method and application of low-carbon mixed alcohols for preparing polyoxymethylene dialkyl ethers with low polymerization degree

technical field

The invention belongs to the technical field of clean utilization of coal and clean diesel fuel, and in particular relates to a preparation method and application of low-polymerization polyoxymethylene dialkyl ether which can be used as an additive for fossil diesel fuel.

Background technique

Since the two oil crises in the 1970s, energy development and environmental protection have become two strategic themes for the survival and development of human society. With the continuous consumption of natural resources, it is very urgent to actively seek and develop new energy systems. From the perspective of the world's energy structure, coal accounts for 78.9%, and oil only accounts for 6.6%. As the world's largest coal-producing country, my country's energy structure with rich coal and little oil is more obvious. From the perspective of resource utilization and environmental protection, it is of great strategic significance and application prospect to develop "green fuel" by using high-efficiency and clean coal resource technology. Catalytic hydrogenation of CO to synthesize low-carbon mixed alcohols (C1-C5 mixed alcohols) is one of the important ways for the clean utilization of coal resources. In recent years, the application value of low-carbon mixed alcohols in the fields of fuel and chemical industry has gradually emerged, and related research has become increasingly active. With the depletion of oil resources, the future energy structure will shift to coal and natural gas. From the perspective of effective utilization of resources, the research on the production of low-carbon mixed alcohols from synthesis gas through natural gas or coal gasification has broad prospects. The production of low-carbon mixed alcohols from syngas is a breakthrough after the industrialization of methanol from syngas. It shows that people can realize the industrial production of fuel alcohol through the gasification of non-petroleum resources, such as coal and natural gas, to generate syngas, which is more realistic for countries or regions lacking petroleum resources. The synthesis of low-carbon alcohols from synthetic gas as clean fuels should become one of my country's energy development strategies. The research, development and production of synthetic low-carbon mixed alcohols will have a very important impact on my country's future vehicle fuels, fuel additives and environmental pollution prevention. The industrial background and new features of low-carbon alcohol research can be summarized in the following aspects.

1. Alcohol-gasoline blended fuel

With the rapid development of the economy, the environmental pollution caused by various vehicle emissions is becoming more and more serious. At present, MTBE (methyl tert-butyl ether) is widely used as a gasoline additive in the world. Since it was put on the market in 1973, it has become one of the fastest-growing industries in recent years. In 2000, the global MTBE demand reached 20 million tons. However, recent studies have found that MTBE has serious problems as a gasoline additive, such as easy leakage and pollution of drinking water during storage, transportation and use, which will threaten human health. Recently, California, USA has confirmed that MTBE is carcinogenic to humans through animal experiments, and it has been decided to ban it before 2002. In addition, the US Environmental Protection Agency has also decided to gradually ban the use of MTBE in the United States. This provides a huge development opportunity for the development and application of low-carbon alcohols, leading to further heating up of low-carbon alcohol research worldwide. Low-carbon mixed alcohols have been positioned as gasoline additives for their good performance indicators. Low-carbon mixed alcohols have been proven to be high-octane, low-pollutant emission automotive fuel additives. They can be mixed with gasoline to form alcohol-gasoline hybrid fuels, and low-carbon mixed alcohols can also be used directly alone. In view of the cost of substitutes, the maturity of technology and the market, etc., the use of MTBE will continue for a period of time, but it is only a matter of time before it is finally banned. The choice of its substitutes and technical preparations are low for CO hydrogenation synthesis. The carbon mixed alcohol route is one of the most promising technology options for this fuel revolution.

2. Alcohol-diesel mixed components

Low-carbon mixed alcohols are oxygen-containing fuels, but because the physical and chemical properties of low-carbon mixed alcohols are quite different from diesel, it has the following difficulties in the application of diesel engines: (1) When mixed with diesel, if There is a small amount of water, and the hydroxyl groups in the low-carbon mixed alcohol molecules will form hydrogen bonds with water molecules, causing phase separation; (2) The cetane number of the low-carbon mixed alcohols is very low, which also makes diesel alcohol mixed fuel The cetane number drops. According to research, adding 10% low-carbon mixed alcohol to diesel will reduce the cetane number of the mixed fuel by 7.1 units, resulting in increased ignition delay time and worsened cold start. This has a corresponding adverse effect on the combustion characteristics and emission characteristics of the diesel engine. Because the cetane number of diesel oil is the same as the octane number of gasoline, it is the main technical index. The cetane number of diesel oil is required to be 45-49, while the cetane number of low-carbon mixed alcohol is only 3-5. When alcohol with low cetane number is blended with diesel oil, the cetane number of alcohol/diesel blended fuel will be reduced, and the explosive power will be weakened, which will affect the working condition of the engine.

Therefore, it is unreasonable to directly blend low-carbon mixed alcohols with diesel oil. It is necessary to convert low-carbon mixed alcohols into clean components with high cetane number and high flash point through chemical conversion, and then mix them with diesel components for use Fuel for compression ignition engines. Due to the wide use and large consumption of diesel oil, the demand gap of my country's petroleum fuel is mainly reflected in diesel oil, making diesel oil a scarce product in the energy shortage. The country imports a large amount of crude oil and a large amount of finished diesel oil every year. Therefore, the research and development of diesel fuel has important strategic significance for the sustainable development of economy and society.

Cetane Number (Cetane Number, hereinafter referred to as CN value) is an important index used to measure the antiknock performance of diesel. In a diesel engine, the air is first compressed, and then the diesel is injected into the combustion chamber. At this time, the diesel contacts the hot air and is vaporized. When the temperature reaches the self-ignition point, it starts to burn. Usually, the period from fuel injection to natural combustion is called "delay period". The long ignition delay period of diesel oil makes the fuel injected into the cylinder accumulate. Once the fuel is spontaneously ignited, the injected fuel will burn at the same time, resulting in a "knock" phenomenon. The ignition delay period of high CN diesel is shorter than that of low CN diesel. Increasing the CN value of diesel can reduce exhaust pollutant emissions, reduce white smoke, and make cold start faster. my country stipulates that the CN of diesel oil should not be lower than 45. The American Engine Manufacturers Association recommends that diesel engines designed to meet modern exhaust emission standards should use fuels with CN greater than 50, but should not exceed 60.

Due to the limited production of straight-run diesel components with high cetane number in my country (about 30%), and the proportion of FCC diesel components with low cetane number is very large (about 70%), the blended commercially available diesel oil CN generally does not meet the requirements of more than 45. Therefore, increasing the CN value of diesel oil is an urgent problem to be solved.

Polyoxymethylene dimethyl ether (general formula CH ₃ O(CH ₂ O) _m CH ₃ ) is a new type of clean oil additive, which can improve diesel combustion performance, increase CN value, reduce carbon dioxide and NOx emissions, reduce fuel consumption and An effective additive for reducing smoke emission. Because of its vapor pressure, boiling point and solubility in oil, polyoxymethylene dimethyl ether (DMM _3-8 ) with 3≤m≤8 is suitable for oil addition.

At present, the production and preparation of polyoxymethylene dimethyl ether has caused extensive research, such as US patents US 5746785, US 6392102, US 2008/0207954 A1, and Chinese patent CN101182367A, all of which disclose methods for preparing polyoxymethylene dimethyl ether . However, it can also be seen from the above that the dimer with m=2 is the main product in the mixed product obtained by this reaction, and the content of the polymer with 3≤m≤8 suitable for addition is not high, and due to the formation of dimer as the main product The dimer has a lower boiling point, so it also reduces its flash point, so the effect of the reaction mixture as an additive added to diesel fuel is still open to question; It is relatively low, which affects its vapor pressure and boiling point accordingly, so that the degree of polymerization of the added polyoxymethylene dimethyl ether must be kept in the range of 3≤m≤8, and it is known that the production conditions and production process of components with higher polymerization degrees are more difficult. Complex, which also affects the application of polyoxymethylene dimethyl ether as a diesel additive.

The CN value of diesel fuel can be increased by mixing linear ethers. The Chinese patent CN101213274A of BASF discloses a biodiesel fuel mixture comprising polyoxymethylene dialkyl ether, wherein 0.1-20% by weight of polyoxymethylene dialkyl ether of formula RO(CH ₂ O) _n R is added, wherein R is an alkyl group having 1-10 carbon atoms, and n=2-10, and preferably n=3, 4, 5 polyoxymethylene dialkyl ethers and mixtures thereof. Since the molecular weight of R as an end group is slightly larger, the CN value of polyoxymethylene dialkyl ether with a lower degree of polymerization of n=3, 4, and 5 has also reached more than 50. Moreover, since the synthesis process steps of the product with a low degree of polymerization are relatively simple, in terms of production technology, polyoxymethylene dialkyl ether has more advantages than polyoxymethylene dimethyl ether as a diesel additive.

The Chinese patent CN101198576A of BASF discloses a method for preparing polyoxymethylene dialkyl ether from trioxane and dialkyl ether. In this method, trioxane and dialkyl ether selected from dimethyl ether, methyl ethyl ether and diethyl ether are added to the reactor and reacted in the presence of an acidic catalyst to obtain a diesel fuel suitable for use as Additives for polyoxymethylene dialkyl ethers. However, the reaction requires that the reactants and/or catalysts be introduced into the reaction mixture in an amount of water less than 1% based on the reaction mixture, mainly because in the presence of water or alcohol, a large amount of polyoxymethylene glycol and hemicondensation will occur. The chemical reaction of aldehydes will form reactive azeotropes, and the process of distillation and separation is more complicated. This has also led to relatively harsh requirements for the reaction conditions and relatively strict requirements for the reaction raw materials in the reaction process; Special preparation or purchase makes the corresponding cost increase; in addition, this patent only discusses the situation that the end groups of the prepared polyoxymethylene dialkyl ether are methyl and ethyl, and the corresponding polymerization of the polyoxymethylene dialkyl ether Degree is 2-10, and this product also still exists the problem that aforementioned polyoxymethylene dimethyl ether exists.

Contents of the invention

For this reason, the technical problem to be solved by the present invention is to overcome the problem in the prior art that the polymerization degree of polyoxymethylene dimethyl ether as a diesel additive is relatively high and the effective content is low, and then provide a method for preparing polyoxymethylene dimethyl ether with a low degree of polymerization. Alkyl ether method.

Further, the present invention provides a low-polymerization degree polysaccharide prepared by utilizing low-cost alcohol brewing industrial by-products (C2-C5 low-carbon mixed alcohols) or low-carbon mixed alcohols (C1-C5 mixed alcohols) synthesized by CO catalytic hydrogenation. Formaldehyde dialkyl ether method.

Furthermore, the present invention also provides the application of the polyoxymethylene dialkyl ether with a low degree of polymerization as a diesel additive.

In order to solve the above-mentioned technical problems, the preparation method of polyoxymethylene dialkyl ether with low polymerization degree of the present invention is characterized in that, comprising the steps:

(1) Put low-carbon alcohols, polyoxymethylene and acidic catalysts into the reactor, pass through the protective gas to replace the air in the reactor, control the initial pressure of the reactor to 0.1-0.8MPa, and control the reaction temperature to 50 Reaction at -200°C for 10-12 hours;

The polyoxymethylene substance is anhydrous paraformaldehyde and/or paraformaldehyde with a low degree of polymerization;

The molar ratio of the low-carbon alcohols and polyoxymethylene is 1-4:1-4, and the amount of the catalyst is 0.5-5.0% of the total weight of the low-carbon alcohols and polyoxymethylene;

(2) After the reaction is over, deacidify the mixture and adjust the pH value to neutral or slightly alkaline, separate and collect fractions at 150-340°C, and refine to obtain the general structure of C _n H _2n+1 O(CH ₂ O) _m C _n H _2n+1 polyoxymethylene dialkyl ether with a low degree of polymerization, wherein m and n are the same or different values, m is an integer of 1-5, and n is an integer of 1-5.

The low-carbon alcohols are one or a mixture of several alcohols with the general formula C _n H _2n+1 OH, wherein n is an integer of 1-5.

The low-carbon alcohols come from alcohol brewing industrial by-products or Fischer-Tropsch synthesis products.

The molar ratio of the low-carbon alcohols to the polyoxymethylene is 1-2:2-4.

The catalyst is solid superacid or protonic acid.

The protonic acid is one or a mixture of p-toluenesulfonic acid, trifluoromethanesulfonic acid, formic acid, and benzoic acid.

The reaction time of the step (1) is 10-12 hours.

The reaction temperature of the step (1) is 70-180°C.

The deacidification step in the step (2) is to pass the mixture through a solid base loaded with activated carbon. The reactor is a batch type high-pressure reactor.

The protective gas is nitrogen and/or inert gas.

A kind of diesel fuel is characterized in that comprising following component:

The polyoxymethylene dialkyl ether with low degree of polymerization prepared by the above-mentioned method, 1-20wt%

Diesel 80-99wt%.

The polyoxymethylene dialkyl ether with a low degree of polymerization has a cetane number greater than 50, a boiling point of 150-340°C, a density of 0.90-0.97g/mL (20°C), and a flash point of 45-70°C.

In the alcohol brewing industry, some "non-alcoholic" higher alcohols, which are regarded as by-products, are called fusel alcohols (mainly C2-C5 low-carbon mixed alcohols), and their representative alcohols are isoamyl alcohol and isobutanol. Isoamyl alcohol accounts for about 40-50% in the composition of fusel alcohols. Fusel alcohols have poisoning and anesthesia effects on the human body. Long-term drinking of liquor with more fusel alcohols will lead to chronic poisoning. Therefore, the fusel alcohols in the finished alcohol should be reduced as much as possible, and the fusel alcohols are usually extracted in the middle of the rectification tower. At present, the separated and extracted fusel alcohols are generally discarded and treated without research and reuse, which increases the environmental burden on the one hand, and increases the industrial cost in a disguised form on the other hand.

Fischer-Tropsch synthesis is one of the indirect coal liquefaction technologies. It uses syngas (CO and H ₂ ) as raw materials to synthesize paraffin-based liquid fuels under catalyst and appropriate reaction conditions. It is an important process of conversion into light raw materials suitable for engine use, and its products are mostly alkanes and some oxygen-containing organic compounds, such as low-carbon mixed alcohols (mainly C1-C5) and ketones. In recent years, the application value of low-carbon mixed alcohols in the fields of fuel and chemical industry has gradually emerged, and related research has become increasingly active and great progress has been made. However, due to the fact that low-carbon mixed alcohols cannot be directly blended with diesel oil, it is very promising to further develop additive technology suitable for diesel combustion by using low-carbon mixed alcohols with mature technology.

Compared with the prior art, the above-mentioned technical scheme of the present invention has the following advantages. 1. The dialkyl ether with a low degree of polymerization prepared by the present invention has a relatively high CN value, and can be widely and effectively used without a high degree of polymerization. In diesel fuel, the preparation process is relatively simple; 2. The present invention selects low-carbon alcohols and formaldehyde (or paraformaldehyde with a low degree of polymerization, or paraformaldehyde) to react, and the whole process can be completed by feeding only one time; 3. Fusel alcohols as by-products in the alcohol brewing industry or low-carbon mixed alcohols produced in Fischer-Tropsch synthesis reactions are used as raw materials. The comprehensive utilization of low-carbon mixed alcohols produced in the process provides a better technological route and has obvious economic significance.

Description of drawings

In order to make the content of the present invention more easily understood, the present invention will be described in further detail below according to specific embodiments of the present invention in conjunction with the accompanying drawings, wherein

Fig. 1 is the process flow diagram of preparation method of the present invention.

Reference numerals in the figure represent: 1-raw material flow, 2-raw material flow, 3 high-pressure reactor, 4-primary product, 5-filter, 6-secondary product, 7-deacidification separator, 8-three Grade product, 9-atmospheric pressure separation tower, 10-low boiling point fraction, 11-fourth grade product, 12-vacuum separation tower, 13-high polymer component, 14-target product.

Detailed ways

The chemical reaction formula of the method for preparing polyoxymethylene dialkyl ether with low polymerization degree by low-carbon mixed alcohol of the present invention is:

C _n H _2n+1 OH+mCH ₂ O→C _n H _2n+1 O(CH ₂ O) _m C _n H _2n+1 +H ₂ O

As shown in Figure 1, the production process flow for preparing low-polymerization polyoxymethylene dialkyl ethers from low-carbon mixed alcohols according to the present invention is as follows: raw material flow 1 and raw material flow respectively comprising low-carbon alcohols and polyoxymethylene substances 2. Transfer to the intermittent high-pressure reactor 3. At the same time, put in the acidic catalyst, and replace the air in the reactor with protective gas, control the initial pressure of the reactor to 0.1-0.8MPa, and keep the constant temperature at 50-200°C for 10-12 hours; at the end of the reaction, a primary product 4 comprising polyalkoxymethylal with a low degree of polymerization was formed. The resulting primary product 4 is passed through the filter 5 and the solid impurities are filtered to obtain a secondary product 6 substantially free of solid impurities, and then the secondary product 6 is transported into a deacidification separator equipped with activated carbon-loaded solid alkali 7. Carry out deacidification treatment and adjust its pH to neutral or slightly alkaline to ensure that reverse depolymerization will not occur in the subsequent rectification, separation and refining process. The obtained tertiary product 8 enters the atmospheric separation column 9 for separation, wherein the low-boiling fraction 10 with a boiling point (B.P) less than 150 ° C is refluxed into the high-pressure reactor 3 to continue to react again; while the quaternary product 11 composed of other light components Then continue to enter the decompression separation tower 12 for further separation. After separation, the high-polymerization component 13 with m>5 is refluxed into the high-pressure reactor 3 to participate in the reaction again, and the final component 14 with m=2-5 is qualified. The target product is the desired polyoxymethylene dialkyl ether with a low degree of polymerization.

The components and percentages (%) of the selected low-carbon mixed alcohols in the following examples of the present invention are shown in the table below.

serial number _CH3OH C ₂ H ₅ OH C ₃ H ₇ OH C ₄ H ₉ OH C ₅ H ₁₁ OH Example 1 0 5 17 35 43 Example 2 0 15 25 35 25 Example 3 15.7 18.1 23.4 21.5 21.2 Example 4 17 18.5 21.6 22.6 20.2 Example 5 14.5 19.5 27.8 21.5 16.6 Example 6 15.7 22.6 27.6 17.8 16.1 Example 7 16.2 21.8 25.5 20.5 16.1

Example 1

The method for preparing polyoxymethylene dialkyl ether C _n H _2n+1 O(CH ₂ O) _m C _n H _2n+1 with low polymerization degree from low carbon mixed alcohol is:

Add low-carbon mixed alcohol and anhydrous low-degree-of-polymerization paraformaldehyde in the batch-type high-pressure reactor in a molar ratio of 1:2, and choose to account for about 2% of the total mass of low-carbon mixed alcohol and anhydrous low-polymerization-degree paraformaldehyde P-toluenesulfonic acid is used as a catalyst to catalyze and put into the reactor together, fill the air in the reactor with nitrogen, control the initial pressure of the reactor to 0.2Mpa, and keep the reaction at a constant temperature of 90-100°C for 12 hours under stirring at 100rpm. After the reaction is completed, go through the above-mentioned extraction and separation process steps, separate and finally collect the fraction at 150-340° C., and refine the obtained product. According to the detection by chromatography-mass spectrometry (GC-MS), the product distribution of each degree of polymerization is: m=1, 32.1%; m=2, 27.8%; m=3, 13.6%; m=4, 8.7; m=5 , 0.42; m>5, trace; ∑PODAE _2-5 =50.52%. The mixed liquid had a cetane number of 53 and a density of 0.93 g/cm ³ at 20°C.

Add about 10% polyoxymethylene dialkyl ether with a low degree of polymerization to diesel fuel, and the obtained diesel fuel has a cetane number of 46 and a flash point of 58, which can not only meet the ultra-low sulfur diesel index, but also greatly reduce NO _X and other harmful gases are discharged, and the pollution discharge is reduced by more than 30%.

Example 2:

The method for preparing polyoxymethylene dialkyl ether C _n H _2n+1 O(CH ₂ O) _m C _n H _2n+1 with low polymerization degree from low carbon mixed alcohol is:

The mixture of low-carbon mixed alcohol, anhydrous trioxane and anhydrous formaldehyde obtained in the alcohol production process is sequentially added in a molar ratio of 1:1 to the intermittent high-pressure reactor, and the low-carbon mixed alcohol, anhydrous trimer 5% of the total mass of formaldehyde and anhydrous formaldehyde is put into the reaction kettle together as a catalyst, and the air in the reaction kettle is replaced by filling inert gas. -200°C constant temperature reaction for 10 hours. After the reaction is completed, go through the above-mentioned extraction and separation process steps, separate and finally collect the fraction at 150-340° C., and refine the obtained product. The GC-MS method shows that the product distribution of each degree of polymerization is: m=1, 27.6%; m=2, 20.4%; m=3, 16.7%; m=4, 9.3%; m=5, 0.96%; m>5, trace; ∑PODAE _2-5 =47.36%. The mixed liquid had a cetane number of 51 and a density of 0.95 g/cm ³ at 20°C.

Add about 15% polyoxymethylene dialkyl ether with a low degree of polymerization to diesel fuel, and the obtained diesel fuel has a cetane number of 45 and a flash point of 60, which can not only meet the ultra-low sulfur diesel index, but also greatly reduce NO _X and other harmful gases are discharged, and the pollution discharge is reduced by more than 40%.

Example 3:

The method for preparing polyoxymethylene dialkyl ether C _n H _2n+1 O(CH ₂ O) _m C _n H _2n+1 with low polymerization degree from low carbon mixed alcohol is:

Add low-carbon mixed alcohol and anhydrous paraformaldehyde successively in the proportion of molar ratio 2:3 in the intermittent high-pressure reactor, and select trifluoromethanesulfonic acid and The mixture of p-toluenesulfonic acid is put into the reaction kettle together as a catalyst, and the air in the reaction kettle is replaced by nitrogen gas, the initial pressure of the reaction kettle is controlled to 0.6Mpa, and the reaction temperature is maintained at 70-90°C for 10 hours under stirring at 100rpm. After the reaction is completed, go through the above-mentioned extraction and separation process steps, separate and finally collect the fraction at 150-340° C., and refine the obtained product. According to the detection by GC-MS method, the product distribution of each degree of polymerization is: m=1, 30.6%; m=2, 23.8%; m=3, 16.3%; m=4, 7.2%; m=5, 0.87%; m>5, trace; ∑PODAE _2-5 = 48.17%. The mixed liquid had a cetane number of 53 and a density of 0.936 g/cm ³ at 20°C.

Add about 20% polyoxymethylene dialkyl ether with a low degree of polymerization to diesel fuel, and the obtained diesel fuel has a cetane number of 47 and a flash point of 58, which can not only meet the ultra-low sulfur diesel index, but also greatly reduce NO _X and other harmful gases are discharged, and the pollution discharge is reduced by more than 42%.

Example 4:

The method for preparing polyoxymethylene dialkyl ether C _n H _2n+1 O(CH ₂ O) _m C _n H _2n+1 with low polymerization degree from low carbon mixed alcohol is:

Add the mixture of low-carbon mixed alcohol, anhydrous low-polymerization degree paraformaldehyde and anhydrous formaldehyde obtained in the Fischer-Tropsch synthesis reaction in a molar ratio of 3:4 in the intermittent high-pressure reactor, and select the low-carbon mixed alcohol, A mixture of anhydrous low-polymerization paraformaldehyde and anhydrous formaldehyde with a total mass of 3% benzoic acid is put into the reactor as a catalyst, and the air in the reactor is replaced by an inert gas, and the initial pressure of the reactor is controlled to be 0.8 Mpa, keep 100-120° C. constant temperature for 10 hours under stirring at 100 rpm. After the reaction is completed, go through the above-mentioned extraction and separation process steps, separate and finally collect the fraction at 150-340° C., and refine the obtained product. According to the detection by GC-MS method, the product distribution of each degree of polymerization is: m=1, 32.6%; m=2, 26.4%; m=3, 18.3%; m=4, 10.2%; m=5, 0.56%; m>5, trace; ∑PODAE _2-5 = 55.46%. The mixed liquid had a cetane number of 55 and a density of 0.952 g/cm ³ at 20°C.

Add about 5% polyoxymethylene dialkyl ether with a low degree of polymerization to diesel fuel, and the obtained diesel fuel has a cetane number of 45 and a flash point of 61, which can not only meet the ultra-low sulfur diesel index, but also greatly reduce NO _X and other harmful gases are discharged, and the pollution discharge is reduced by more than 20%.

Example 5:

The method for preparing polyoxymethylene dialkyl ether C _n H _2n+1 O(CH ₂ O) _m C _n H _2n+1 with low polymerization degree from low carbon mixed alcohol is:

Add low-carbon mixed alcohol and anhydrous paraformaldehyde in the batch high-pressure reactor in a molar ratio of 1:4, and select formic acid and p-toluenesulfonic acid that account for 2.5% of the total mass of low-carbon mixed alcohol and anhydrous paraformaldehyde The mixture was put into the reactor together as a catalyst, filled with nitrogen to replace the air in the reactor, the initial pressure of the reactor was controlled to be 0.1Mpa, and the temperature was maintained at 120-140°C for 11 hours under stirring at 100rpm. After the reaction is completed, go through the above-mentioned extraction and separation process steps, separate and finally collect the fraction at 150-340° C., and refine the obtained product. According to the detection by GC-MS method, the product distribution of each degree of polymerization is: m=1, 27.8%; m=2, 19.8%; m=3, 14.7%; m=4, 9.3%; m=5, 0.77%; m>5, trace; ∑PODAE _2-5 =44.57%. The mixed liquid had a cetane number of 51 and a density of 0.94 g/cm ³ at 20°C.

Add about 15% polyoxymethylene dialkyl ether with a low degree of polymerization to diesel, and the obtained diesel fuel has a cetane number of 46 and a flash point of 59, which can not only meet the ultra-low sulfur diesel index, but also greatly reduce NO _X and other harmful gases are discharged, and the pollution discharge is reduced by more than 38%.

Embodiment 6:

The method for preparing polyoxymethylene dialkyl ether C _n H _2n+1 O(CH ₂ O) _m C _n H _2n+1 with low polymerization degree from low carbon mixed alcohol is:

Add low-carbon mixed alcohol and anhydrous paraformaldehyde in the batch type high-pressure reactor in a molar ratio of 1:3, and select a solid superacid that accounts for 1% of the total mass of low-carbon mixed alcohol and anhydrous paraformaldehyde as a catalyst. And put it into the reaction kettle, fill inert gas to replace the air in the reaction kettle, control the initial pressure of the reaction kettle to 0.4Mpa, keep 140-170°C constant temperature reaction for 12 hours under stirring at 100rpm speed. After the reaction is completed, go through the above-mentioned extraction and separation process steps, separate and finally collect the fraction at 150-340° C., and refine the obtained product. According to the detection by GC-MS method, the product distribution of each degree of polymerization is: m=1, 26.7%; m=2, 19.5%; m=3, 17.4%; m=4, 8.9%; m=5, 0.54%; m>5, trace; ∑PODAE _2-5 = 46.34%. The mixed liquid had a cetane number of 51 and a density of 0.927 g/cm ³ at 20°C.

Add about 10% polyoxymethylene dialkyl ether with a low degree of polymerization to diesel fuel, and the obtained diesel fuel has a cetane number of 47 and a flash point of 58, which can not only meet the ultra-low sulfur diesel index, but also greatly reduce NO _X and other harmful gases are discharged, and the pollution discharge is reduced by more than 35%.

Embodiment 7:

The method for preparing polyoxymethylene dialkyl ether C _n H _2n+1 O(CH ₂ O) _m C _n H _2n+1 with low polymerization degree from low carbon mixed alcohol is:

In the intermittent high-pressure reactor, the low-carbon mixed alcohol and anhydrous low-polymerization degree paraformaldehyde obtained in the Fischer-Tropsch synthesis reaction are sequentially added in a molar ratio of 4:3, and the low-carbon mixed alcohol and anhydrous low-polymerization degree polyformaldehyde are selected. The mixture of p-toluenesulfonic acid and formic acid with a total mass of formaldehyde of 4% is put into the reactor together as a catalyst, and nitrogen and inert gas are charged to replace the air in the reactor, and the initial pressure of the reactor is controlled to be 0.5Mpa. Maintain a constant temperature of 50-70° C. for 11 hours under stirring. After the reaction is completed, go through the above-mentioned extraction and separation process steps, separate and finally collect the fraction at 150-340° C., and refine the obtained product. According to the detection by GC-MS method, the product distribution of each degree of polymerization is: m=1, 30.8%; m=2, 24.6%; m=3, 17.2%; m=4, 8.3%; m=5, 0.86%; m>5, trace; ∑PODAE _2-5 = 50.96%. The mixed liquid had a cetane number of 52 and a density of 0.917 g/cm ³ at 20°C.

Add about 1% polyoxymethylene dialkyl ether with a low degree of polymerization to diesel fuel, and the obtained diesel fuel has a cetane number of 46 and a flash point of 58, which can not only meet the ultra-low sulfur diesel index, but also greatly reduce NO _X and other harmful gases are discharged, and the pollution discharge is reduced by more than 30%.

Apparently, the above-mentioned embodiments are only examples for clear description, rather than limiting the implementation. For those of ordinary skill in the art, other changes or changes in different forms can be made on the basis of the above description. It is not necessary and impossible to exhaustively list all the implementation manners here. And the obvious changes or changes derived therefrom are still within the scope of protection of the present invention.

Claims (10)

1. the method that low-carbon mixed alcohol prepares polyoxymethylene dialkyl ether with low degree of polymerization, is characterized in that, comprises the steps: (1) Put low-carbon alcohols, polyoxymethylene and acidic catalysts into the reactor, pass through the protective gas to replace the air in the reactor, control the initial pressure of the reactor to 0.1-0.8MPa, and control the reaction temperature to 50 React at -200°C until the components are balanced; The polyoxymethylene substance is anhydrous paraformaldehyde and/or paraformaldehyde with a low degree of polymerization; The molar ratio of the low-carbon alcohols and polyoxymethylene is 1-4:1-4, and the amount of the catalyst is 0.5-5.0% of the total weight of the low-carbon alcohols and the polyoxymethylene; (2) After the reaction is over, deacidify the mixture and adjust the pH value to neutral or slightly alkaline, separate and collect fractions at 150-340°C, and refine to obtain the general structure of C _n H _2n+1 O(CH ₂ O) _m C _n H _2n+1 polyoxymethylene dialkyl ether with a low degree of polymerization, wherein m and n are the same or different values, m is an integer of 1-5, and n is an integer of 1-5. 2. the method for preparing low-polymerization polyoxymethylene dialkyl ethers from low-carbon mixed alcohols according to claim 1, is characterized in that: the low-carbon alcohols are alcohols whose general formula is C _n H _2n+1 OH One or a mixture of several of them, where n is an integer of 1-5. 3. The method for preparing low-polymerization polyoxymethylene dialkyl ethers from low-carbon mixed alcohols according to claim 2, characterized in that: the low-carbon alcohols are from alcohol brewing industrial by-products or Fischer-Tropsch synthesis products. 4. according to the method for preparing the polyoxymethylene dialkyl ether with low polymerization degree according to the arbitrary described low-carbon mixed alcohol of claim 1-3, it is characterized in that: the mole of described low-carbon alcohol substance and described polyoxymethylene substance The ratio is 1-2:2-4. 5. The method for preparing polyoxymethylene dialkyl ethers with a low degree of polymerization according to any one of claims 1-4, wherein the catalyst is a solid superacid or a protonic acid. 6. the method that low-carbon mixed alcohol according to claim 5 prepares polyoxymethylene dialkyl ether with low polymerization degree is characterized in that: described protonic acid is p-toluenesulfonic acid, trifluoromethanesulfonic acid, formic acid, benzoic acid one or a mixture of several of them. 7. The method for preparing polyoxymethylene dialkyl ether with a low degree of polymerization from the low-carbon mixed alcohol according to any one of claims 1-6, characterized in that: the reaction time of the step (1) is 10-12 hours. 8. The method for preparing polyoxymethylene dialkyl ether with a low degree of polymerization from the low-carbon mixed alcohol according to any one of claims 1-7, characterized in that: the reaction temperature of the step (1) is 70-180°C. 9. The method for preparing polyoxymethylene dialkyl ether with a low degree of polymerization according to any one of claims 1-8, characterized in that: the deacidification step of the step (2) is to pass the mixture through a Activated carbon supported solid base. 10. A diesel fuel, characterized in that it comprises the following components: The low degree of polymerization polyoxymethylene dialkyl ether prepared by the method described in any one of claims 1-9, 1-20wt% Diesel 80-99wt%.

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