Use of polyoxymethylene di(alkyl polyglycol) ethers as an additive to diesel fuels to reduce soot emissions in compression-ignition engines

Abstract

Use of polyoxymethylene di(alkyl polyglycol) ethers of the general formula RO(CH2CH2O) n (CH2O) m (CH2CH2O) n R, as an additive to diesel fuel to reduce soot emissions in compression-ignition engines, where R is an alkyl group, n = 1 or 2 and m = 1, and wherein the polyoxymethylene di(alkyl polyglycol) ethers are used in an amount of less than 15% by volume, based on the diesel fuel, and where the diesel fuel contains 1 to 10% by volume of FAME.

Description

[0001] The invention relates to the use of polyoxymethylene di(alkyl polyglycol) ethers as an additive to diesel fuels.

[0002] It is known that soot emissions in compression-ignition engines can be reduced by additives to diesel fuels.

[0003] Such additives include, for example, polyoxaalkanes of the general formula R 1 (-O-CH2-CHR 2 ) n -OR 3 , where R 1 is a straight-chain or branched alkyl radical, R 2 and R 3are identical or different straight-chain or branched alkyl radicals or H and n is ≥ 1. Such polyoxaalkanes are free of carbon atoms with oxidation numbers > + 1. Only those polyoxaalkanes that are essentially free of toxic components are used. Examples of representatives of this class are polyethylene glycol dialkyl ethers. Additions of these polyethylene glycol dialkyl ethers reduce soot formation during diesel engine combustion. For example, the addition of 5 vol.% tetraethylene glycol dimethyl ether to a diesel fuel according to EN 590 reduces the soot emissions of a single-cylinder diesel engine by up to 70% at constant NO x Emissions by 27.4–54.1% depending on the operating point. The reduction in the fuel's calorific value due to the addition of tetraethylene glycol dimethyl ether is slight, at 1.6%. The oxygen content of the mixture is approximately 1.8%.

[0004] The reduction in soot emissions is attributed to the formation of oxygen-containing species during the pyrolysis of these substances in the engine. These inhibit the growth of soot particles. However, polyethylene glycol dialkyl ethers have good solvent properties. Therefore, materials such as elastomers and plastics, as well as coatings that come into contact with polyethylene glycol dialkyl ethers, must be selected with particular care. Backward compatibility with older engines is generally not possible.

[0005] The production of polyethylene glycol dialkyl ethers is usually carried out using the following multi-stage process: a) Alkyl glycols are produced from alkanols and ethylene oxide. Biogenic alkanols, such as methanol, ethanol, and 1-butanol, can be advantageously used for this purpose. Ethylene oxide is produced by catalytic oxidation of ethylene. Ethylene is produced in Europe and the USA by cracking petroleum fractions and, at high oil prices, could also be obtained by dehydrating bioethanol, as is already the case in India. b) The alkyl glycols are reacted with sodium hydroxide to form sodium alkyl glycolates. c) The polyethylene glycol dialkyl ethers are obtained by reacting the sodium alkyl glycolates with alkyl chlorides. (Arpe “Industrial Organic Chemistry”, page 176 / 1777, Wiley-VCH 2007).

[0006] Step c) is particularly critical for environmental reasons due to the use of alkyl chlorides. In addition, production costs are comparatively high.

[0007] Polyoxymethylene dialkyl ethers are known from WO 2007 / 000428. They are less effective in reducing soot emissions. For example, the addition of 10 vol.% oxymethylene dimethyl ether (methylal), the first member of this class of polyoxaalkanes, to a diesel fuel according to EN 590, despite the significantly higher oxygen content of the mixture of 4.1%, only results in a soot reduction of 22.7-41.2%, depending on the engine operating point, compared to the soot emissions of a non-additive diesel fuel according to EN 590. The reduction in calorific value is approximately 4%.

[0008] The production of polyoxyalkylene dialkyl ethers, such as polyoxymethylene dimethyl ether, is known from WO 2008 / 074704. First, methanol is obtained from synthesis gas, which can also be obtained from biomethane or biomass waste through pyrolysis. Methanol is then converted to formaldehyde, and finally, the formaldehyde reacts with methanol via the intermediates methylal and trioxane by acid catalysis, yielding polyoxymethylene dimethyl ether. Polyoxymethylene diethyl ether and polyoxymethylene dibutyl ether are obtained analogously using ethanol or 1-butanol instead of methanol. This process is comparatively cost-effective and can therefore compete with the production of biomass-to-liquids, BtL, from pyrolysis gas.The costs of providing plants for the production of polyoxymethylene dimethyl ethers are significantly lower than those of the Fischer-Tropsch and hydrocracking plants required for the production of BtL.

[0009] The production of polyoxymethylene dialkyl ethers, especially polyoxymethylene dimethyl ether, polyoxymethylene diethyl ether, and polyoxymethylene dibutyl ether, is therefore fundamentally possible from renewable raw materials. The use of biomass syngas as a feedstock thus enables the production of second- and third-generation biofuels.

[0010] Polyoxymethylene dimethyl ethers have good compatibility with frequently used materials such as elastomers and plastics.

[0011] From EP-A-0 014 992 polyethers of the general formula R 1 -O-(AO-) n -R 2 known, in which A represents an ethylene or 1,2-propylene group, R 1represents a C1-C8 alkyl radical and R 2 represents hydrogen or a C1-C4 alkyl radical and n has a value of 1 to 5. Such polyethers are added to diesel fuels in quantities of 15 to 90 vol.% in order to replace conventional diesel fuels entirely or partially with fuels based on methanol and, above all, ethanol.

[0012] WO 86 / 03511 discloses oxygen-containing methane derivatives of the general formula R1-O-CH2-O-R2 with R 1 and R 2 known as alkyl groups. These can be used as fuel additives.

[0013] Furthermore, EP 0 014 992 A1 discloses the use of polyethers R 1 -O-(-AO) n -R 2 (I; A = ethylene; 1,2-propylene; R 1 = C1-C8 alkyl; R 2 =H; C1-C4-alkyl; n = 1-5) and / or acetals R 3 -CH[-O-(-AO-) m -R 4 ]2 (II; R 3 = H; C1-C 12 -alkyl; R 4= Methyl; Ethyl; m = 0-5) alone or in a mixture with up to 45 vol.% ethanol (III) and / or methanol (IV) and / or up to 30 vol.% water (V) and / or up to 85 vol.% diesel fuels.

[0014] US 5520710 A recommends symmetrical or asymmetrical dialkyl and dicycloalkyl ethers or alkylcycloalkyl (polycycloalkyl) ethers with a total of 2 to 24 carbon atoms, in which the oxygen atom is bonded to primary or secondary, but not to tertiary carbon atoms, as clean-burning and cetane number-increasing supplements for diesel fuels.

[0015] US 2763537 A is directed to an improved fuel for compression ignition engines, which consists essentially of a major portion of a mixture of a petroleum-based fuel oil in a specific boiling point range and an oil-soluble aliphatic polyether blending agent having a molecular weight below about 700 and the general formula R-(OX)n -OR', where R and R' are alkyl groups, X is an alkylene-type hydrocarbon group having at least two carbon atoms and n is an integer, both components being present in amounts of more than 5% of the total volume of the fuel and a minor amount of amyl nitrate not exceeding 10% of the total volume of the fuel.

[0016] The standard DIN EN 590, September 2008, Fuels for motor vehicles - Diesel fuel, specifies that a quantity of up to 7 vol.% FAME may be added to commercially available diesel fuel.

[0017] The invention is based on the object of providing additives for diesel fuels to reduce soot emissions in compression-ignition engines that largely eliminate the disadvantages of known additives. The aim is to reduce soot emissions in compression-ignition engines and achieve the highest possible calorific value. Furthermore, the additives should be easily and inexpensively available, preferably from biogenic raw materials. The additives should be non-toxic, preferably also non-harmful to health, and therefore not subject to labeling requirements. Furthermore, the additives should exhibit good material compatibility.

[0018] According to the invention, this object is achieved by the use according to claim 1, namely the use of polyoxymethylene di(alkylpolyglycol) ethers of the general formula RO(CH2CH2O) n (CH2O) m (CH2CH2O) n R, as an additive to diesel fuels for reducing soot emissions in compression-ignition engines, wherein R is an alkyl radical, n = 1 or 2 and m = 1, and wherein the polyoxymethylene di(alkyl polyglycol) ethers are used in an amount of less than 15% by volume, based on the diesel fuel, and wherein the diesel fuel contains 1 to 10% by volume of FAME.

[0019] The production of polyoxymethylene di(alkyl polyglycol) ethers is known per se. This occurs from formaldehyde, paraformaldehyde, or trioxane, and alkyl glycols, RCH2CH2OH, or alkyl polyglycols, R(CH2CH2O). n H, by acid-catalyzed condensation reactions as follows: (CH2O) m + 2 ROCH2CH2OH → ROCH2CH2O(CH2O) m CH2CH2OR + H2O (1) and (CH2O) m + 2 RO(CH2CH2O) n H → RO(CH2CH2O) n (CH2O) m (CH2CH2O) n R + H2O (2).

[0020] For R = CH3 and n = 1, this preparation is described in US-A-4 093 666, for R = n-C4H9 and n = 1, it is known from US-A-2 397 514.

[0021] The synthesis of these substances leads to the incorporation of the polyethylene glycol group (-CH2-CH2-O-), which is particularly active for soot reduction. With the help of the above-mentioned acid-catalyzed condensation reactions (1) and (2), the costly synthesis step of etherifying the alkyl glycols via their sodium salts by reaction with alkyl chlorides is avoided, thus avoiding the environmentally harmful use of alkyl chlorides. At the same time, oxymethylene groups (-CH2-O-) are incorporated, which reduce raw material costs and aggressiveness towards materials the more the higher m is. With higher m, the proportion of biogenic carbon also increases if biomethane is chosen as the starting material for their synthesis.

[0022] Furthermore, the toxicity of polyethylene glycol dimethyl ethers of the formula CH3O(CH2CH2O) n CH3, with n = 1 or 2, which precludes its use as a fuel additive, is eliminated by incorporating the oxymethylene group, CH2O. For example, the oxymethylene di(methylglycol) ether according to the invention, also known as bis(2-methoxyethoxy)methane with the formula CH3OCH2CH2OCH2OCH2CH2OCH3, is not subject to labeling requirements, while the similarly structured diethylene glycol dimethyl ether, CH3OCH2CH2OCH2CH2OCH3, whose soot-reducing properties are well known (SAE paper 2000-01-2886), cannot be used as a fuel additive due to its high toxicity.

[0023] The polyoxymethylene di(alkyl polyglycol) ethers used according to the invention lead to a significant reduction in soot emissions when used as an additive to diesel fuels. Furthermore, a higher calorific value is achieved. As shown above, the polyoxymethylene di(alkyl polyglycol) ethers used according to the invention are easily and inexpensively available, even from biogenic raw materials. Furthermore, they display sufficiently good material compatibility. For example, elastomers do not swell. They are also easily miscible with diesel fuels, as no miscibility gaps occur. Finally, it is advantageous that the use according to the invention advantageously eliminates the need for toxic nitrate-containing compounds, e.g., EHN, 2-ethylhexyl nitrate, as ignition accelerators, since polyoxymethylene di(alkyl polyglycol) ethers also have an ignition accelerator effect and thus also reduce NO x-Emission is reduced.

[0024] For the purposes of the present invention, the term "polyoxymethylene di(alkyl polyglycol) ether" refers to oxymethylene di(alkyl polyglycol) ethers. The term "polyoxymethylene di(alkyl polyglycol) ether" exclusively includes mixtures of such ethers.

[0025] Compounds where n = 1 or 2 and m = 1 can be produced cost-effectively and exhibit good material compatibility. They also have the advantage of being comparatively high in volatility while offering excellent soot reduction, increasing the proportion of premixed and thus low-emission, homogeneous combustion in the engine.

[0026] In a preferred embodiment, n = 1 and m = 1.

[0027] The polyoxymethylene di(alkyl glycol) ether listed below of the general formula RO(CH2CH2O) n (CH2O) m (CH2CH2O) nR, where R is an alkyl radical, n = 1 and m = 1, is particularly preferred: Oxymethylene di(methyl glycol) ether (C7H 16 O4; chain length 11; molecular weight 164.2; oxygen content 39%), Oxymethylene di(methyl diglycol) ether, C 11 H 24 O6, i.e., a compound where n = 2 and m = 1. The chain length is 17 and the molecular weight is 252.3; oxygen content is 38%.

[0028] In a preferred embodiment, R is a straight-chain alkyl radical. This has the advantage that soot reduction is generally higher than with branched alkyl radicals.

[0029] In a further preferred embodiment, R is a methyl, ethyl, or n-butyl radical. Such compounds are particularly well suited for use as second- and third-generation biofuel additives due to the biogenic availability of methanol, ethanol, and n-butanol.

[0030] R is particularly preferably an ethyl or n-butyl radical, especially an n-butyl radical, since miscibility with diesel fuel increases in this order. This has the further advantage that solvents such as FAME can be essentially dispensed with.

[0031] The sum of the chain links (C and O atoms) is preferably < 35, in particular < 30, and particularly preferably about 11 to about 21. This has the advantage that the additives do not exceed the 95% boiling limit of 360°C applicable to the fuel standard EN 590.

[0032] The polyoxymethylene di(alkyl polyglycol) ether is used in an amount of less than 15 vol.% based on the diesel fuel. Preferably, the polyoxymethylene di(alkyl polyglycol) ether is used in an amount of less than 10 vol.%, and more preferably in an amount of less than 5 vol.%, based on the diesel fuel. The lower the amount of polyoxymethylene di(alkyl polyglycol) ether, the lower the additional costs.

[0033] The diesel fuel contains 1 to 10 vol.% FAME. Preferably, the diesel fuel contains 1 to 5 vol.% FAME (EN 590) or 1 to 7 vol.% FAME (DIN 51628). The use of FAME has the advantage of acting as a solubilizer to improve the miscibility of the higher molecular weight polyoxymethylene di(alkyl polyglycol) ethers with diesel fuel.

[0034] The inventive use is also possible in older engines. This means that backward compatibility is also ensured. This is a further advantage of using the additives according to the invention compared to the use of polyethylene glycol dialkyl ethers.

[0035] The invention is further illustrated below with examples. However, these examples are not intended to be limiting or restricting the present invention in any way. Example 1

[0036] A MAN single-cylinder research engine with a displacement of 1.75 l, an engine power of 55 kW, a common rail injection system (rail pressure 1800 bar), a compression ratio of 20.5, an injection start before top dead center of -8° crank angle and an EGR rate of 20% was tested with a fuel mixture of 95 vol.% diesel fuel according to EN 590 and 5 vol.% oxymethylene di(methylglycol) ether (other name: bis(2-methoxyethoxy)methane; C7H 16 O4, boiling point 197 / 205°C, Alfa Aesar, D-76057 Karlsruhe). Diesel fuel without additives according to EN 590 served as the reference fuel.

[0037] The soot reduction was determined using a Micro Soot Sensor from AVL.

[0038] The results are shown in Table 1 below. Table 1 Operating point BP 1 BP2 BP3 BP4 Soot reduction (NO x -Emission level constant) % - 42,7 - 42,7 - 42,7 - 28,2 Speed ​​rpm 914 1542 1542 1800 Torque Nm 75 200 200 200 Medium pressure bar 6,3 15,7 20,7 11,7 Air ratio 3,1 1,35 1,35 1,9

[0039] This experiment shows that the addition of small amounts of oxymethylene di(methyl glycol) ether to diesel fuel leads to a significant reduction in soot, which is greater the lower the excess air during combustion is (see air ratio in Table 1). Example 2

[0040] A fuel mixture of 95 vol.% diesel fuel according to EN 590 and 5 vol.% oxymethylene di(methyl diglycol) ether (bp. 315 / 325°C, DWS Synthesetechnik, D-86356 Neusäß) was tested on the test engine described in Example 1.

[0041] At the operating points described in Example 1, the following soot reduction was found in the fuel mixture using the Micro Soot Sensor compared to a diesel fuel according to EN 590, whereby the NO x -Emission level was kept constant. The results are shown in Table 2 below. Table 2 Operating point BP 1 BP2 BP3 BP3 Soot reduction (NO x -Emission level constant) % - 43,6 - 43,6 - 43,6 - 43,6

[0042] This experiment also shows that the addition of oxymethylene di(methyl diglycol) ether leads to a significant soot reduction, which is even greater the lower the excess air during combustion (see air ratio in Table 1). Oxymethylene di(methyl diglycol) ether contains four ethylene glycol groups per oxymethylene group per molecule and therefore has a slightly higher soot reduction activity than the oxymethylene di(methyl glycol) ether listed in Example 1, which contains only two ethylene glycol groups per oxymethylene group per molecule. Comparison example 1:

[0043] A fuel mixture consisting of 95 vol.% diesel fuel according to EN 590 and 5 vol.% tetraethylene glycol dimethyl ether was tested under the test conditions described in Example 1.

[0044] The results are shown in Table 3 below. Non-additive diesel fuel according to EN 590 was used for comparison. Table 3 Operating point BP 1 BP2 BP2 BP2 Soot reduction (NO x -Emission level constant) % - 41,2 - 41,2 - 52,9 - 52,9 Comparison example 2:

[0045] A fuel mixture consisting of 95 vol.% diesel fuel according to EN 590 and 10 vol.% methylal (oxymethylene dimethyl ether) was tested under the test conditions described in Example 1.

[0046] The results are shown in Table 4 below. Non-additive diesel fuel according to EN 590 was used as the reference fuel. Table 4 Operating point BP 1 BP2 BP2 BP3 Soot reduction (NO x -Emission level constant) % - 22,7 - 22,7 - 41,2 - 22,7

Claims

[1] Use of polyoxymethylene di(alkyl polyglycol) ethers of the general formula RO(CH2CH2O) n (CH2O) m (CH2CH2O) n R, as an additive to diesel fuels to reduce soot emissions in compression-ignition engines, where R is an alkyl radical, n = 1 or 2 and m = 1, and wherein the polyoxymethylene di(alkyl polyglycol) ethers are used in an amount of less than 15% by volume, based on the diesel fuel, and where the diesel fuel contains 1 to 10 vol% FAME. [2] Use according to one of the preceding claims, characterized by that n = 1 and m = 1. [3] Use according to one of the preceding claims, characterized by that R is a straight-chain alkyl radical. [4] Use according to one of the preceding claims, characterized by that R is a methyl, ethyl or n-butyl radical. [5] Use according to one of the preceding claims, characterized by that the sum of the chain links (C and O atoms) of the polyoxymethylene di(alkyl polyglycol) ethers is ≤ 35. [6] Use according to one of the preceding claims, characterized by that the polyoxymethylene di(alkyl polyglycol) ethers are used in an amount of less than 5 vol.%, based on the diesel fuel. [7] Use according to one of the preceding claims, characterized by that the diesel fuel contains 1 to 5 vol.% FAME or 1 to 7 vol.% FAME.