EP2342311B1

EP2342311B1 - Method for improving the oxidation stability of biodiesel as measured by the rancimat test

Info

Publication number: EP2342311B1
Application number: EP09814893.5A
Authority: EP
Inventors: Marc-Andre Poirier; Alex S. Lang
Original assignee: ExxonMobil Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 2008-09-17
Filing date: 2009-09-17
Publication date: 2016-03-09
Anticipated expiration: 2029-09-17
Also published as: EP2342311A1; US20110146139A1; CA2734958A1; EP2342311A4; WO2010033201A1; JP2012503040A; AU2009292637A1

Description

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The present invention relates to biodiesel fuels or mixtures of biodiesel fuel and conventional diesel fuel and to the improvement of the oxidation stability of such fuels by the use of additives.

DESCRIPTION OF THE RELATED ART

Biodiesel fuels which constitute the short chain alkyl esters of vegetable or animal oils or fats, while having been identified in the art as useful for combustion in diesel engines or as blending stock for mixtures with conventional diesel fuels are characterized as being less stable than conventional diesel fuels.
Biodiesel fuels are mixtures of short chain alkyl esters of saturated and unsaturated vegetable oils or animal oils or fats and as such contain olefinic double bonds as well as the ester moiety. Such functionalities are susceptible to oxidative degradation leading to the degradation of the biodiesel and its consequential unsuitability for use due to its lack of long-term stability caused by the presence therein of acids, alcohols, aldehydes, ketones, etc.
The improvement in the oxidation stability of biodiesel has been the subject of investigation leading to the use of various additives and mixtures of additives to effect the desired stabilization.
WO 2008/056203 teaches stabilizer compositions for blends of petroleum and renewable fuels. Mixtures of renewable fuels such as biodiesel, ethanol and biomass mixed with conventional petroleum fuel are stabilized by the addition thereto of a multifunctional additive package which is a combination of one or more additives selected from the group consisting of a free radical chain terminating agent, a peroxide decomposition agent, an acid scavenger, a photochemical stabilizer, a gum dispersant and a metal sequestering agent. Peroxide decomposition agents are selected from the group containing sulfur, nitrogen and phosphorus compounds. Suitable nitrogen-containing compounds are of the general formula:
wherein R, R' and R" can be alkyl linear, branched, saturated or unsaturated C₁-C₃₀, aromatic, cyclic, poly alkoxy, polycyclic. Identified as a useful nitrogen containing compound is N,N-dimethylcyclohexylamine. While N,N-dimethylcyclohexylamine is taught as a useful peroxide decomposition agent, in the examples it is never employed by itself but always in combination with a phenolic anti-oxidant. Reference to Figure 2 of WO 2008/056203 reveals that whereas the use of the combination of 75% phenol and 25% N,N-dimethylcyclohexylamine (at a treat level of 200 mg/l) resulted in an improvement in the relative stability of the fuel as compared to using 100% phenol over all time periods tested, an increase in the amount of N,N-dimethylcyclohexylamine in the additive mixture to 50% significantly reduced the beneficial effect of the additive mixture (still at a treat level of 200 mg/l) in terms of relative stability over all time periods tested as compared to the 75% phenol/25% N,N-dimethylcyclohexylamine mixture with the most significant reduction in benefit being observed over the long term; i.e., at the six hour time period.
US2004/0152930 teaches stable blended diesel fuel comprising an olefinic diesel fuel blending stock containing olefins in an amount of 2 to 80 wt%, non-olefins in an amount of 20 to 98 wt% wherein the non-olefins are substantially comprised of paraffins, oxygenates in an amount of at least 0.012 wt% and sulfur in an amount of less than 1 ppm, the blend diesel being stabilized by an effective amount of a sulfur-free antioxidant. An effective amount of sulfur-free antioxidant is identified as 5 to 500 ppm, preferably 8 to 200 ppm of additive.
The sulfur-free antioxidant is selected from the group consisting of phenols, cyclic amines and combinations thereof. Preferably the phenols contain one hydroxyl group and are hindered phenols. The cyclic amine antioxidants are amines of the formula:
wherein A is a six-membered cycloalkyl or aryl ring, R¹, R², R³ and R⁴ are independently H or alkyl and X is 1 or 2. An example of a sulfur-free antioxidant is given as di-methylcyclohexylamine. See also U.S. Patent 7,179,311 .
"Evaluation of the Stability, Lubricity and Cold Flow Properties of Biodiesel Fuel", J. Andrew Waynick, 6th International Conference on Stability and Handling of Liquid Fuels, Vancouver, B.C., Canada, October 13-17, 1997, pages 805-829 addresses various aspects of biodiesel fuel and reports an example where a blend of 80% low sulfur No. 2 diesel fuel/20% methyl soyate ester biodiesel fuel was combined with 20 ppm N,N-dimethylcyclohexylamine. At page 813 the report states that "although additive C (the N,N-dimethylcyclohexylamine) did not control hydroperoxide or insolubles formulations, it did hold the TAN to a level near that of the fuel blend with anti-oxidant additive A (N-N,di-sec-butyl-p-phenylenediamine) and B (2,6-di-t-butyl-4-methyl phenol)".
U.S. 2008/0127550 discloses stabilized biodiesel fuel composition wherein the stabilizing agent is a combination of: i) one or more compounds selected from the group consisting of sterically-hindered phenolic anti-oxidants; and ii) one or more compounds selected from the group consisting of triazole metal deactivators.
U.S. 2007/0289203 discloses a stabilized biodiesel wherein the stabilizing additive is a mixture of at least one aromatic diamine and at least one sterically-hindered phenol.
U.S. 2007/0151143 discloses a stabilized biodiesel wherein the stabilizing additive is selected from one or more of the group consisting of the 3-arylbenzofuranones and the hindered amine light stabilizers and, optionally, one or more hindered phenolic anti-oxidants.
U.S. 2007/0248740 discloses an additive composition comprising 2,5-di-tert-butyl hydroquinone (BHQ), N,N'-disalicylidenepropylenediamine. The additive is used to stabilize fuel containing at least 2% by weight of an oil derived from plant or animal material.
U.S. 2007/0113467 discloses a biodiesel fuel composition having improved oxidation stability. The fuel contains at least one anti-oxidant that increases the oxidative stability of the fuel selected from the group recited in paragraphs [0006] to [0012] of said published application.
U.S. Patent 3,336,124 discloses stabilized distillate fuel oils and additive compositions for such fuel oils. One additive composition comprises a mixture of: (a) an oil soluble dispersant terpolymer of a particular type; (b) from 0.2 to about 3 parts by weight per part of said oil soluble dispersant tripolymer of N,N-dimethylcyclohexylamine; and (c) a normally liquid inert hydrocarbon carrier solvent in an amount to constitute from about 20% to 80% by weight of the additive composition. See also GB 1,036,384 .

DESCRIPTION OF THE INVENTION

The present invention relates to a method for improving the oxidation stability of biodiesel fuel(s) or mixtures or biodiesel fuel(s) and conventional diesel fuel(s) by the addition thereto of stability additive selected from the group consisting of one or a mixture of N,N-di(C₁-C₅-alkyl)cyclohexylamine, preferably one or a mixture of N,N-di(C₁-C₂-alkyl)cyclohexylamine, most preferably N,N-dimethylcyclohexylamine so as to increase the oxidation stability of said additized fuel as measured by the Rancimat Test by at least about 50%, preferably by at least about 60%, more preferably by at least about 65%, most preferably by 100% or more as compared to the oxidation stability of the fuel without the stability additive, said method comprising adding to the biodiesel fuel or mixture of biodiesel fuel and conventional diesel fuel at least 250 mg, preferably at least 500 mg, more preferably at least 1000 mg of one or a mixture of N,N-di(C₁-C₅-alkyl)cyclohexylamine(s), preferably one or a mixture of N,N-di(C₁-C₂-alkyl)cyclohexylamine(s), more preferably N,N-dimethylcyclohexylamine per liter of the biodiesel fuel or per liter of the biodiesel fuel component in the mixture of biodiesel fuel and conventional diesel fuel. The stability additive is employed in the absence of any phenolic and/or aminic anti-oxidant. For biodiesel fuels or mixture of biodiesel fuel and conventional diesel fuels which have oxidation stabilities of less than six hours, the addition to such fuels of the above-described stability additive in the amounts indicated above can increase the oxidation stability time to at least the six hour minimum induction time necessary to satisfy the EN 14214 specification for stability of biodiesel fuels as measured by the Rancimat Test.
Biodiesel fuels are mixtures of lower, short chain esters of fixed saturated and unsaturated straight chain fatty acids derived from vegetable and/or animal fats and oils. The straight chain fatty acids are, typically, C₁₀ to C₂₆ fatty acids, preferably C₁₂ to C₂₂ fatty acids. The fatty acids are made into biodiesel by transesterification using short chain alcohols; e.g., C₁ to C₅ alcohols, in the presence of a catalyst such as a strong base.
Vegetable and/or animal oils and fats are natural triglycerides and are renewable sources of starting material. Typical vegetable oils are soybean oil, rapeseed oil, corn oil, jojoba oil, safflower oil, sunflower seed oil, hemp oil, coconut oil, cottonseed oil, sunflower oil, palm oil, canola oil, peanut oil, mustard seed oil, olive oil, spent cooking oil, etc., without limitation. Animal fats and oils include beef, pork, chicken fat, fish oil and oil recovered by the rendering of animal tissue.
The biodiesel is made by esterifying one or a mixture of such oils and fats using one or a mixture of short chain; e.g., C₁ to C₅, alcohols, preferably methanol.
Transesterification is effected by the base catalyzed reaction of the fat and/or oil with the alcohol, direct acid catalyzed esterification of the oil and/or fat with the alcohol, conversion of the oil and/or fat to fatty acids and then to alkyl esters with as acid catalyst. In base catalyzed transesterification, the oil and/or fat is reacted with a short chain (such as methanol or ethanol) alcohol in the presence of a catalyst such as sodium hydroxide or potassium hydroxide to produce glycerin and short chain alkyl esters. The glycerin is separated from the product mixture and biodiesel is recovered. Any unreacted alcohol is removed by distillation. The recovered biodiesel is washed to remove residual catalyst or soap and dried.
In the present invention the fuel is either 100% biodiesel, typically identified in the art as B100, or it is a mixture of biodiesel with conventional diesel fuel; i.e., X% conventional diesel fuel and Y% biodiesel which would be identified as BY; for example, 80% conventional diesel and 20% biodiesel fuel would be described as B20.
As indicated above, in the present invention the biodiesel fuel has its stability against oxidation increased by at least 50%, preferably by at least 60%, more preferably by at least 65%, most preferably by 100% or more, as measured by the Rancimat Test by the addition thereto of the above-described stability additive in the amount indicated. Biodiesel fuels or mixtures of biodiesel fuel and conventional fuels exhibiting oxidation stabilities of less than six hours can have their oxidation stabilities increased to an extent sufficient to meet the six hour induction time specification established in EN 14214 for biodiesel fuels as measured by the Rancimat Test (ISO 6886).
It has been found that the biodiesel can be combined with N,N-dimethylcyclohexylamine (DMCHA) in an amount of at least 250 mg, preferably at least 500 mg, more preferably at least 1000 mg of DMCHA per liter of biodiesel fuel (B100) or per liter of the biodiesel fuel component in the mixture of biodiesel fuel and conventional diesel fuel. Expressed differently, one liter of B100 would contain at least 250 mg DMCHA while one liter of B20, for example, would contain 50 mg DMCHA, which is equal to at least 250 mg of DMCHA per liter of biodiesel fuel in the mixture of biodiesel fuel and conventional diesel fuel. Thus, because 5 liters of B20 would contain 1 liter of the transesterified biodiesel fuel itself, 5 liters of B20 would be additized to contain at least 250 mg of DMCHA.
The determination whether the fuel when additized has its oxidation stability increased by at least 50%, preferably by at least 60%, more preferably by at least 65%, most preferably by 100% or more as compared to the fuel without the additive, or, alternatively, meets the minimum six hours induction time is made by subjecting the fuel to the Rancimat Test (ISO 6886, per EN 14112). In the Rancimat Test, samples of liquid are aged at a constant temperature (110°C) while air is passed through the liquid at a rate of 10 liters/hour. The exhaust airflow passes through a measuring cell filled with distilled water. The conductivity of the measuring cell is determined continuously and recorded automatically. As the liquid oxidizes, volatile organic acids, aldehydes, ketones, alcohols, e.g., volatile oxygenates, are produced and taken up by the distilled water. This increases the conductivity of the water. The oxidation process is such that there is a gradual increase in measured conductivity followed by a rapid increase. The inflection point (not a specific value) of the conductivity curve is the measured induction time. The length of the period prior to the rapid increase, known as the "induction period", is a measure of the oxidation stability of the liquid under test. The presence of an effective anti-oxidant will lengthen the induction period. The Rancimat Test has been adopted as a specification test in the qualification of biodiesel fuels.
The biodiesel fuels can contain other performance additives commonly used in conventional diesel fuels such as dispersants, detergents, cetane improvers (such as 2-ethyl hexyl nitrate), demulsifiers, biocides, antifoaming agents, lubricity additives, dyes.
It has been found that to achieve the above-described increase in oxidation stability, it is not necessary to use any supplemental anti-oxidant (e.g., phenolic and/or aromatic amine) in addition to the DMCHA and, such supplemental phenol, preferably hindered phenol and/or amine, preferably aromatic amine (e.g., diphenyl amines or phenylnaphthyl amines) anti-oxidant is not employed in the stabilization process of the present invention.

EXAMPLES

Description of the evaluated fuels:

The Rancimat oxidation tests were run according to EN 14112. B100 canola methyl ester (an unsaturated fatty acid methyl ester) was used as the biodiesel fuel. The N,N-dimethylcyclohexylamine was acquired from Innospec. Dicyclocarbodiimide acid scavenger Additin RC 8500 was acquired from Akzo Nobel. The 2-ethylhexyl-3,4-epoxycyclohexane carboxylate acid scavenger was acquired from ATOFINA.

Fuel 1:: Base Biodiesel Fuel, B100 Canola Methyl Ester (unsaturated fatty acid methyl ester)
Fuel 2:: Fuel 1 + 500 mg/L N,N-dimethylyclohexyamine (DMCHA)
Fuel 3:: Fuel 1 + 1000 mg/L N,N-dimethylcyclohexylamine (DMCHA)
Fuel 4:: Fuel 1 + 500 mg/L Additin RC 8500 acid scavenger
Fuel 5:: Fuel 1 + 1000 mg/L Additin RC 8500 acid scavenger
Fuel 6:: Fuel 1 + 500 mg/L 2-ethylhexyl-3,4-epoxycyclohexane carboxylate acid scavenger (EHEC)
Fuel 7:: Fuel 1 + 1000 mg/L -2-ethylhexyl-3,4-epoxycyclohexane carboxylate acid scavenger (EHEC)

Example 1

This Example shows that N,N-dimethylcyclohexylamine is very effective is increasing the Rancimat oxidation stability.

Test Fuel Additive Rancimat Induction Time, hours Increase Rancimat Induction Time, hours % Increase

1 Fuel 1 None 4.55 0 0%

2 Fuel 2 DMCHA 7.46 2.91 +64%

3 Fuel 3 DMCHA 9.99 5.44 +119%

Example 2

It might have been thought that acid scavengers might retard Rancimat oxidation stability as light acid species are produced during oxidation process. This Example shows that acid scavengers were ineffective in improving the Rancimat induction period.

Test	Fuel	Additive	Rancimat Induction Time, hours	Increase Rancimat Induction Time, hours
1	Fuel 1	None	4.55	0
4	Fuel 4	Additin RC 8500	4.57	0.02
5	Fuel 5	Additin RC 8500	4.56	0.01
6	Fuel 6	EHEC	4.52	-0.03
7	Fuel 7	EHEC	4.62	0.07

Claims

A method for improving the oxidation stability of biodiesel fuel(s) or mixtures of biodiesel fuel(s) and conventional diesel fuel(s), said method comprising the addition to such biodiesel fuel(s) or mixtures of biodiesel fuel(s) and conventional diesel fuel(s) of a stability additive selected from the group consisting of one or a mixture of N,N-di(C₁-C₅-alkyl)cyclohexylamine(s) in an amount of at least 250 mg of said one or a mixture of N,N-di(C₁-C₅-alkyl)cyclohexylamine per liter of the biodiesel fuel or per liter of the biodiesel fuel components in the mixture of biodiesel fuel and conventional diesel fuel wherein the oxidation stability of said additized fuel as measured by the Rancimat Test is increased by at least about 50% as compared to the oxidation stability of the fuel without the stability additive, and wherein the stability additive is employed in the absence of any phenolic and/or aromatic amine antioxidant.
The method of claim 1 wherein the stability additive is one or a mixture of N,N-di(C₁-C₂-alkyl)cyclohexylamine(s).
The method of claim 1 wherein the stability additive is N,N-dimethylcyclohexylamine.
The method of claim 1, 2 or 3 wherein the stability additive is added to the biodiesel fuel(s) or mixture of biodiesel fuel(s) and conventional diesel fuel(s) in an amount of at least 500 mg of said stability additive per liter of the biodiesel fuel or per liter of the biodiesel fuel component in the mixture of biodiesel fuel(s) and conventional diesel fuel(s).
The method of claims 1, 2 or 3 wherein the stability additive is added to the biodiesel fuel(s) or mixture of biodiesel fuel(s) and conventional diesel fuel(s) in an amount of at least 1000 mg of said stability additive per liter of the biodiesel fuel(s) or per liter of the biodiesel fuel(s) component in the mixture of biodiesel fuel(s) and conventional diesel fuel(s).
The method of any one of claims 1 to 5, wherein the oxidation stability is increased by at least about 65%.
The method of any one of claims 1 to 5, wherein the oxidation stability is increased by 100% or more.