WO2012046169A1

WO2012046169A1 - Fuel mixtures composed of light cycle oil and polyoxymethylene dialkyl ethers

Info

Publication number: WO2012046169A1
Application number: PCT/IB2011/054289
Authority: WO
Inventors: Rudolf Sinnen; Eckhard Stroefer; Heinrich Laib; Klaus-Peter Metzner; Markus Siegert; Ferdinand Lippert; Deon Carter; Joe Mclean
Original assignee: Basf Se; Basf (China) Company Limited
Priority date: 2010-10-04
Filing date: 2011-09-29
Publication date: 2012-04-12

Abstract

Fuel mixtures with a cetane number of at least 35 are provided, comprising a) a typical light cycle oil (LCO) fraction with a boiling point in the range from 140^oC to 460^oC and a content of aromatic hydrocarbons of at least 50%, by weight, as component A, b) polyoxymethylene dialkyl ethers of the formula RO(CH₂O)_nR, where n=2-10, and R=C₁-C₄-alkyl, as component B.

Description

Fuel mixtures composed of light cycle oil and polyoxymethylene dialkyl ethers Description The invention relates to fuel mixtures with a cetane number of at least 35, based on a typical light cycle oil (LCO) fraction with a boiling point in the range from 140°C to 460°C and a content of aromatic hydrocarbons of at least 20% by volume.

In a refinery, a series of mineral oil products is obtained from crude oil. The crude oil is first sent to a distillation which is operated at or close to atmospheric pressure (atmospheric distillation). At side draws, various fractions such as gasoline, aviation fuel (kerosene) and gas oil are withdrawn from the distillation column. In the case of gas oil, a distinction is drawn between the heating oil and diesel fractions. The term "middle distillate" encompasses heating oil, diesel fuel and Jet A1 fuel products.

The division of the original crude oil into the different fractions cannot be adjusted arbitrarily, but is variable as a function of the composition of the crude oil used only within certain narrow limits. The result of this fact is that refineries in Europe can meet the great demand for diesel fuel only by increasing the throughput and hence also the amount of gasoline produced. The excess gasoline then has to be exported by Europe to other importer countries for gasoline, for example to America.

There is therefore a great interest in increasing the amount of diesel fuel which can be obtained from a given amount of crude oil. One option is to do this using the portion of the crude oil which is obtained as distillation bottoms in the atmospheric distillation. These distillation bottoms or "residue" are consequently generally supplied to a vacuum distillation wherein vacuum gas oil (VGO) is obtained by side draw removal from the vacuum distillation column. Vacuum distillation has developed to become an important process step for maximizing the yield of materials of value from crude oil.

The vacuum gas oil is generally supplied to an FCC plant (FCC = fluid catalytic cracking) or to a hydrocracker, and processed therein to give low-boiling hydrocarbons. Important low-boiling products are ethylene, propylene and C4 cuts, which are important feedstocks for the chemical industry. Further important products of value are light cracked naphtha (LCN) and high cracked naphtha (HCN). The highest-boiling constituent obtained, directly above the bottom of the column, is what is called light cycle oil (LCO). At the bottom of the column there remains a bitumen-like residue.

Light cycle oil has a high aromatics content, especially a high content of alkyl- substituted naphthalenes. The processes for catalytic treatment with hydrogen (hydrotreating) and for catalytic cracking in the presence of hydrogen (hydrocracking) have often been used in the past to convert LCO to naphtha and low-boiling products. However, both processes require very high capital costs and consume large amounts of hydrogen. The product obtained has to be subjected to a further reforming treatment in order to be usable as gasoline. The company UOP describes, in a conference contribution by V.P. Thakkar et al., "LCO Unicracking Technology - A Novel approach for greater added value and improved returns", ERTC 2004, means of better and less expensive upgrading of LCO to gasoline. However, such a process leads to an increase in the yield of gasolines and is therefore not of interest when the aim is to increase the yield of diesel fuels.

LCO is generally only a moderate blend component for diesel fuels due to its poor ignitability in the engine and its high sulfur content. In the past, it has often been added to the middle distillate of the atmospheric distillation in order to stretch diesel fuel. However, fuels thus produced no longer meet the modern standards. In some cases, it has also been added to residues from the distillation in order to adjust the viscosity. The demand for such "dirty" residue oils which have been rendered free-flowing is, however, decreasing for reasons of environmental protection and engine efficiency. For some time there have been attempts to increase the proportion of LCO from the FCC cracking process. This increases the yield of LCO while reducing the amount of bitumen-like residue in the cracking process, without simultaneously increasing the proportion of the gasoline-like LCN and HCN fractions. In addition, attempts are being made to improve the quality of the LCO obtained, measured by the "cetane number" parameter, in order to be able to use LCO in diesel fuels. BASF Catalysts describes, in a contribution by J. McLean in "eptq.com - the refining, gas & petrochemicals processing website" under the heading "Catalysis" in 2010, page 21 ff., a means by which this can be achieved by optimizing the reaction technology of the FCC cracking process. More particularly, this is achieved by a specific configuration of the catalyst. This involves using a specific zeolite catalyst on a specific matrix.

The cetane number describes the ignitability of diesel fuel. The more unbranched hydrocarbon molecules are present, the more readily the fuel self-ignites. The cetane number of a fuel indicates that it behaves exactly like a mixture of n-hexadecane (cetane) and 1 -methylnaphthalene with the specified proportion by volume of cetane. A mixture with 30 percent cetane has, for example, a cetane number of 30. Shokubai Kasei Giho Vol. 22 (2005), pages 33 - 42 describes a method for upgrading LCO from the FCC process for uses as diesel fuel. LCO comprises large proportions of aromatics and may comprise large amounts of sulfur and nitrogen compounds which lead to a reduction in the cetane number. A process for hydrogenating the LCO over suitable catalysts, preference being given to specific noble metal-zeolite catalysts, can produce, from LCO, oils which have a lower sulfur and aromatics content and are suitable as a blend component for diesel fuels.

The thesis by Ulf Nylen "Ring opening catalysts for cetane improvement of diesel fuels"; University of Stockholm 2005 describes technical means of breaking up the aromatic rings of aromatic hydrocarbons present in LCO and thus increasing the cetane number.

US patent US 5171916 describes improving the quality of LCO by a catalytic alkylation of the aromatics present in the LCO. In addition, the nitrogen and sulfur content is reduced and the boiling point of the mixture is increased.

The methods mentioned for improving the LCO include new complex process steps, require high capital costs and are energy-intensive.

There is therefore a search for further economic means of providing additional diesel fuel from LCO.

It is an object of the invention to provide fuel mixtures which have a cetane number of at least 35 and are suitable for use as diesel fuel on the basis of LCO.

The object is achieved by a fuel mixture having a cetane number of at least 35, comprising

a) a typical light cycle oil (LCO) fraction with a boiling point in the range from 140°C to 460°C and a content of aromatic hydrocarbons of at least 50%, by weight, as component A, and

b) polyoxymethylene dialkyl ethers of the formula

RO(CH₂0)_nR

where

n = 2-10, and

R = C C₄-alkyl, as component B.

It has been found that usable LCO/POMDME mixtures for use as a diesel fuel substitute can be achieved with only comparatively low additions of POMDME. In general, the LCO used as component A comprises at least 60% by weight, preferably at least 70% by weight, especially at least 80% by weight, of aromatic hydrocarbons. In addition, it comprises up to 50% by weight, generally up to 40% by weight, preferably up to 30% by weight and especially not more than 20% by weight of alkanes, alkenes and cycloalkanes having generally at least 9 carbon atoms, typically 9 to 25 carbon atoms.

In a suitable LCO, generally at least 50% by weight, preferably at least 70% by weight, of the aromatic hydrocarbons, based on all aromatic hydrocarbons, are accounted for by aromatic hydrocarbons having 2 or more ring systems. Such aromatic hydrocarbons having 2 or more rings, especially 2 or more fused rings, are especially naphthalene, anthracene, phenanthrene, indane, indene, indole, carbazole, benzothiophene, dibenzothiophene and diphenyl. These may be unsubstituted or mono- or poly-alkyl- substituted, and they are especially substituted by methyl or ethyl. They are generally unsubstituted or mono- or di-alkyl-substituted. The remaining amount of the aromatics present in the LCO is accounted for by unsubstituted or mono- or poly-alkyl-substituted monocyclic aromatics, predominantly by benzene and mono- or poly-alkyl-substituted benzenes (toluene, xylenes, ethylbenzene), and by further monocyclic aromatics such as pyridines.

More particularly, at least 70% by weight of the aromatic hydrocarbons is accounted for by those having two rings, especially by unsubstituted and alkyl-substituted naphthalenes and diphenyls. Suitable LCOs generally have a density in the range of 0.84 - 1 .08 g/cm³. The viscosity is in the range from 3 to 75 centistokes at 40°C (to ASTM D-445).

In general, the flashpoint of LCO used as component A in accordance with the invention is > 55°C (closed cup; determined to ASTM D-92), and the ignition temperature thereof is at least 210°C (determined to ASTM E 659).

LCOs suitable as component A additionally generally have a cetane number of 10 to 30, preferably of 10 to 25. As component B, the inventive fuel mixture comprises polyoxymethylene dialkyi ethers with 2 to 10 oxymethylene units. Further polyoxymethylene dialkyi ethers with up to 20 oxymethylene units (n = 1 1 to 20) and/or only one oxymethylene unit (dialkylformal or methylal; n = 1 ) may be present in small amounts. In general, 90% by weight of the polyoxymethylene dialkyi ethers is accounted for by those with n = 2 - 10. Among these, the dimethyl ethers and diethyl ethers are preferred, particular preference being given to the dimethyl ethers. Preference is further given to polyoxymethylene dialkyi ethers with n = 2, 3, 4, 5 or 6 oxymethylene units, and mixtures thereof. Preferred mixtures comprise at least 80% by weight, especially at least 90% by weight, based on all polyoxymethylene dialkyi ethers, of those with n = 2, 3, 4, 5 or 6 oxymethylene units. Particular preference is given to the dimethyl ethers.

Particular preference is given to polyoxymethylene dialkyi ethers with n = 3, 4 or 5 oxymethylene units, and mixtures thereof. Preferred mixtures comprise at least 80% by weight, especially at least 90% by weight, based on all polyoxymethylene dialkyi ethers, of those with n = 3, 4 or 5 oxymethylene units. Particular preference is given in turn to the dimethyl ethers.

Especially preferred are polyoxymethylene dimethyl ethers with n = 3 or 4 oxymethylene units. Particularly preferred mixtures comprise at least 80% by weight, especially at least 90% by weight, based on all polyoxymethylene dialkyi ethers, of those with n = 3 or 4 oxymethylene units. Particular preference is given in turn to the dimethyl ethers. In a specific embodiment, the mixture consists of polyoxymethylene dimethyl ethers with n = 3 or 4 oxymethylene units.

The polyoxymethylene dialkyi ethers B can, as described in EP-A 1 070 755, be prepared by reacting the appropriate alcohol, preferably methanol, with formaldehyde in the presence of acidic catalysts.

A particularly advantageous process for preparing polyoxymethylene dimethyl ethers which are particularly suitable as component B, especially the polyoxymethylene dimethyl ethers with n = 3 and 4 (trimer, tetramer), proceeds from methylal (n = 1 ) and trioxane. These are fed into a reactor and converted in the presence of an acidic catalyst, the amount of water introduced into the reaction mixture through methylal, trioxane and/or the catalyst being < 1 % by weight, based on the reaction mixture. In the reaction of methylal with trioxane to give the polyoxymethylene dimethyl ethers, no water is formed as a by-product. The reaction is performed generally at a temperature of 50 to 200°C, preferably 90 to 150°C, and a pressure of 1 to 20 bar, preferably 2 to 10 bar. The molar methylaktrioxane ratio is generally 0.1 to 10, preferably 0.5 to 5. Reaction with ethylal affords the corresponding diethyl ethers. The acidic catalyst may be a homogeneous or heterogeneous acidic catalyst. Suitable acidic catalysts are mineral acids such as substantially anhydrous sulfuric acid, sulfonic acids such as trifluoromethanesulfonic acid and para-toluenesulfonic acid, heteropolyacids, acidic ion exchange resins, zeolites, aluminosilicates, silicon dioxide, aluminum oxide, titanium dioxide and zirconium dioxide. Oxidic catalysts may, in order to increase their acid strength, be doped with sulfate or phosphate groups, generally in amounts of 0.05 to 10% by weight. The reaction can be performed in a stirred tank reactor (CSTR) or a tubular reactor. When a heterogeneous catalyst is used, a fixed bed reactor or a fluidized bed is preferred. When a fixed catalyst bed or a fluidized bed is used, the product mixture can subsequently be contacted with an anion exchange resin in order to obtain an essentially acid-free product mixture.

The total amount of water introduced through methylal and trioxane and through the catalyst is < 1 % by weight, preferably < 0.5% by weight, more preferably < 0.2% by weight and especially < 0.1 % by weight, based on the reaction mixture composed of methylal, trioxane and the catalyst. For this purpose, virtually anhydrous trioxane and methylal are used, and any amount of water introduced through the catalyst is correspondingly limited. The hemiacetals (monoethers) or polyoxymethylene glycols formed by hydrolysis in the presence of water from already formed polyoxymethylene dimethyl ether have a comparable boiling point to the polyoxymethylene dimethyl ethers, which complicates removal of the polyoxymethylene dimethyl ethers from these by-products.

In order to obtain polyoxymethylene dimethyl ethers with n = 3 and n = 4 (trimer, tetramer) in a controlled manner, a fraction comprising the trimer and tetramer is removed from the product mixture of the reaction of methylal with trioxane, and unconverted methylal, trioxane and polyoxymethylene dimethyl ethers with n < 3 are cycled into the acid-catalyzed reaction. In a further embodiment of the process according to the invention, the polyoxymethylene dimethyl ethers with n > 4 and especially n > 5 are additionally also recycled into the reaction. Recycling affords a particularly large amount of trimer and tetramer.

In a particularly preferred embodiment, a first fraction comprising methylal, a second fraction comprising the dimer (n = 2) and trioxane, a third fraction comprising the trimer and tetramer (n = 3, 4) and a fourth fraction comprising the pentamer and higher homologs (n > 4) are obtained from the product mixture of the acid-catalyzed reaction of methylal with trioxane. It is especially preferred in this case to perform the separation of the product mixture of the acid-catalyzed reaction of methylal with trioxane in three distillation columns connected in series, the first fraction being removed from the product mixture of the reaction in a distillation column, the second fraction being removed from the remaining mixture in a second distillation column and the remaining mixture being separated into the third and fourth fractions in a third distillation column. In this case, the first distillation column can be operated, for example, at a pressure of 0.5 to 1.5 bar, the second distillation column, for example, at a pressure of 0.05 to 1 bar and the third distillation column, for example, at a pressure of 0.001 to 0.5 bar. Preferably, the first and second fractions, and more preferably additionally the fourth fraction too, are recycled into the reaction.

Where a homogeneous catalyst is used, for example a mineral acid or a sulfonic acid, it remains in the fourth fraction and is recycled into the acid-catalyzed reaction therewith.

Further processes for preparing polyoxymethylene dialkyi ethers proceed from formaldehyde and methanol or from trioxane and dialkyi ethers, and are described, for example, in WO 06/045506, WO 06/134081 , WO 06/134088, WO 07/000428, WO 08/074704 and WO 08/1 19743.

In addition, the inventive fuel mixtures may comprise 0 to 5% by weight, preferably 0 to 1 % by weight, of additives. Customary additives are cetane number improvers, which may be present in amounts of typically up to 1 % by weight. Further additives may include corrosion inhibitors, flow improvers, biocomponents and system detergents.

In general, the inventive fuel mixture comprises 5 to 75% by volume, preferably 10 to 60% by volume and more preferably 10 to 40% by volume of component B.

The unit % by volume is based on the proportion by volume of the component in question, based on all components, before the mixing of the components.

The inventive fuel mixture preferably has a cetane number of at least 35, especially a cetane number of at least 40. Inventive fuel mixtures with a cetane number of at least 35 comprise generally at least 10% by volume, preferably 10 to 30% by volume, of component B. Inventive fuel mixtures with a cetane number of at least 40 comprise generally at least 20% by volume, preferably 20 to 50% by volume, of component B. The present invention also provides a process for preparing a fuel mixture with a cetane number of at least 35, in which

a) a typical light cycle oil (LCO) fraction with a boiling point in the range from 140°C to 460°C and a content of aromatic hydrocarbons of at least 50% by volume, as component A, and

b) polyoxymethylene dialkyi ethers of the formula RO(CH₂0)_nR

where

n = 2-10, and

R = C C₄-alkyl, as component B are mixed. In general, for this purpose, 5 to 75 parts by volume, preferably 10 to 60 and more preferably 10 to 40 parts by volume of component B are mixed with 25 to 95 parts by volume, preferably 40 to 90 parts by volume and more 60 to 90 parts by volume of the further components. In general, for this purpose, components A and B are withdrawn continuously from their storage tanks and combined continuously in the desired mixing ratio in a mixing apparatus. The mixture obtained is discharged continuously into transport vessels, such as tanker wagons, containers or vats, shipped or stored intermediately in product storage tanks.

The mixing apparatus used may be static mixers. A suitable static mixer is described, for example, in EP 0 097 458.

Static mixers are typically tubular apparatuses with fixed internals which serve for mixing of the individual streams over the tube cross section. The homogenization of the feedstocks is brought about by a pressure gradient generated by means of a pump. According to the kind of flow in the static mixer, two basic mixing principles can be distinguished. In mixers with laminar flow, homogenization is effected by division and redistribution of the flow of the individual components. Continual doubling of the number of the layers reduces the layer thicknesses to such an extent that complete macromixing is achieved. Micromixing by diffusion processes depends on the residence time. For mixing tasks with laminar flow, helical mixers or cross-channel mixers are used. Laminar flow resembles normal pipe flow with low shear forces and a narrow residence time distribution.

In mixers with turbulent flow, vortices are generated in a controlled manner in order to homogenize the individual streams in this way. For this purpose, cross-channel mixers and specific turbulence mixers are suitable. Both types of mixers can be used for the process according to the invention. The internals used consist generally of flow-dividing and -deflecting, three-dimensional geometric bodies which lead to redistribution, mixing and recombination of the individual components.

Static mixers are commercial mixing apparatuses and are supplied, for example, by Fluitec Georg AG, Neftenbach, Switzerland for various fields of application.

The invention is illustrated in detail by the examples which follow.

Example

Determination of cetane number to DIN 51773 or ASTM D 6890 - 09 / analogously to DIN EN 15195 The cetane number describes the ignitability of diesel fuel. The more hydrocarbon molecules of unbranched structure are present, the more readily the fuel self-ignites. The cetane number of a fuel indicates that it behaves exactly like a mixture of n-hexadecane (cetane) and 1 -methylnaphthalene with the corresponding proportion by volume of cetane. A mixture containing 30 percent cetane has, for example, the cetane number of 30. Instead of 1 -methylnaphthalene, the synthetically available 2,2,4,4,6,8,8- heptamethylnonane with a cetane number of 15 is also used as a low-ignitability fuel.

The cetane number is determined in Germany to DIN 51773. For this purpose, a specific engine, the BASF engine, is used. This determines the ignitability by varying the amount of intake air with constant ignition delay. The ignition delay is the time between the injection and the self-ignition of the fuel. The control fuels used are diesel fuels of known cetane number. The control fuel is used only to check the engine state of the test engine. The test engine for determination of the cetane number has the following special features: an adjustment device for commencement of fuel injection and amount; an indicator device for determination of commencement of injection; a device for determination of the amount of air sucked in at constant speed. By throttling the amount of air, the final compression is varied. In addition, it has a measuring device for the amount of fuel and a measuring device for the ignition delay.

Cetane number measurements

In a BASF-MWM test engine to DIN 51773, cetane number measurements are conducted on a light cycle oil (LCO). The boiling point of the LCO is between 150 and 420°C, the flash point is 70°C, the ignition temperature is 250°C.

In the case of use of the pure LCOs, there is no longer any self-ignition in the engine, the engine is "silent" and the mixture injected remains partly uncombusted. It can be concluded from the air throttling on examination in a matrix that the cetane number is 16 +/- 6.

For fuel mixtures composed of LCO and a polyoxymethylene dimethyl ether (POMDME) mixture which comprised POMDME with n = 3 and n = 4 in the volume ratios specified, cetane numbers are determined. 80/20 means that 80 parts by volume of the LCO are admixed with 20 parts by volume of the POMDME mixture. The basic mixture comprises POMDME with n = 3 and n = 4 in a ratio of 60 to 40 percent by weight.

100/0 80/20 60/40 40/60 0/100

Cetane number 16+-6 39 59 67 83 For comparison, conventional Aral diesel from a filling station is analyzed; the cetane number is 55. In addition, a mixture of conventional Aral diesel with POMDME in a ratio of 80/20 is analyzed. The cetane number is 61.

What is surprising is the strongly superadditive effect of the addition of POMDME on the cetane number of the LCO/POMDME mixture. Thus, the weighted mean of the cetane numbers of the pure components for an 80/20 mixture is 0.8 ^■ 16 + 0.2 ^■ 83 = 26.1. However, a cetane number of 39 is actually found. For a 60/40 mixture the weighted mean of the cetane numbers of the pure components is 42.8, but a cetane number of 59 is found. For conventional diesel fuel, the cetane number of an 80/20 mixture is 61 and thus corresponds exactly to the weighted mean of the pure components (0.8 ^■ 55 + 0.2 ^■ 83 = 60.6).

Claims

1 . A fuel mixture with a cetane number of at least 35, comprising a typical light cycle oil (LCO) fraction with a boiling point in the range from 140°C to 460°C and a content of aromatic hydrocarbons of at least 50%, by weight, as component A, b) polyoxymethylene dialkyl ethers of the formula

RO(CH₂0)_nR

where

n = 2-10, and

R = C C₄-alkyl, as component B.

The fuel mixture according to claim 1 , wherein component A comprises at least 70% by weight of aromatic hydrocarbons.

The fuel mixture according to claim 1 or 2, wherein at least 70% by weight of the aromatic hydrocarbons of component A are selected from unsubstituted or alkyl-substituted aromatics having at least 2 rings.

The fuel mixture according to any of claims 1 to 3, wherein the polyoxymethylene dialkyl ethers of component A are polyoxymethylene dimethyl ethers.

The fuel mixture according to any of claims 1 to 4, wherein component B comprises at least 80% by weight of polyoxymethylene dialkyl ethers having 2 to 6 oxymethylene units.

The fuel mixture according to any of claims 1 to 5, wherein component B comprises at least 80% by weight of polyoxymethylene dialkyl ethers having 3 to 5 oxymethylene units.

The fuel mixture according to any of claims 1 to 6, which comprises 10 to 60 parts by volume of component B. 8. The fuel mixture according to any of claims 1 to 7, which has a cetane number of at least 40.

The fuel mixture according to any of claims 1 to 8, wherein component A alone has a cetane number of 10 to 25.

A process for producing a fuel mixture with a cetane number of at least 35, in which a) a typical light cycle oil (LCO) fraction with a boiling point in the range from 140°C to 460°C and a content of aromatic hydrocarbons of at least 50% by weight, as component A, and b) polyoxymethylene dialkyl ethers of the formula

RO(CH₂0)_nR

where

n = 2-10, and

R = C C₄-alkyl, as component B are mixed. The process according to claim 10, wherein component A alone has a cetane number of 10 to 25.