Abstract:
A low temperature stable diesel fuel composition includes diesel oil, ethanol, and a solubliser of C 14 -C 18  fatty acids from biodegradable sources e.g. tall oil or depitched tall oil. This fuel remains stable at temperatures as low as −20° C.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of Provisional Application 60/288,109, filed May 3, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to low temperature stable diesel oil/ethanol mixtures and, more particularly, to biodegradable solubliser additives that promote the low temperature stability.  
           [0003]    The hope afforded to nuclear fusion and hydrogen fuel cell technology is likely to be years, or perhaps decades, from commercial viability. In the meantime, demand for oil continues to rise, resulting in increasing pressure to find alternative energy resources. In particular, the world&#39;s finite reserves of oil represent a non-renewable source of energy. For this reason, it is imperative to find ways to reduce the consumption of traditional petroleum-based oil, and increase the use of alternative, renewable fuels.  
           [0004]    The powering of diesel engines accounts for a significant proportion of oil consumption worldwide. A typical diesel oil is a petroleum gas oil (middle distillate), with additives present in very small amounts relative to the diesel oil. These additives improve the combustion efficiency of the diesel oil, resulting in increased engine efficiency and performance, as well as reduced particulate emissions. For example, U.S. Pat. No. 4,451,266 teaches a corresponding additive that can be mixed with diesel oil in a 1:500 to 1:2000 ratio, the additive comprising 3-10% methanol or ethanol, 3-13% halogenated alkene, 3-13% aromatic hydrocarbon, 35-70% of naptha range hydrocarbon, and 20-30% of hydroxy-substituted unsaturated fatty acids.  
           [0005]    To date, research in this field has investigated the production of alternative fuel blends with reduced diesel oil content, capable of powering a diesel engine. In some cases, fuels have been developed which lack diesel oil as a constituent. For example, U.S. Pat. No. 4,929,252 discloses an castor oil/ethanol blend that may be used to power a diesel engine. In another example, U.S. Pat. No. 4,937,655 discloses a vegetable oil/ethanol mixture for use as a diesel engine fuel, wherein the addition of a ketal, acetal or orthoester to the mixture converts any trace of contaminating water to alcohol, thus discouraging phase separation of the oil/ethanol mix. Whilst these “bio-oil” fuels represent interesting alternatives to hydrocarbon-containing fuels, they inevitably contain a high proportion of glycerides. In this regard, it is well understood in the art that use of these fuels can often give rise to glyceride pyrolysis, which unavoidably results in engine gumming. The resulting buildup of deposits within the engine can result in poor engine performance and efficiency. In addition, the high viscosity of “bio-oil” fuels can affect their compatibility for direct use in a diesel engine. In some cases, diesel engines must be significantly modified to accommodate the specific characteristics of these alternative fuels.  
           [0006]    In the light of the problems encountered with “bio-oil” fuels, more recent innovations in diesel engine fuel technology have focussed upon fuel blends that include a proportion of diesel oil. In particular, it is well known in the art that ethanol can be used in diesel oil blends to produce a fuel suitable for use in a diesel engine. Ethanol confers several advantages over alternative constituents. Ethanol can be easily produced on an industrial scale, and thus represents a readily renewable source of energy. As an oxygenate, it could enhance combustion characteristics.  
           [0007]    The key difficulty in providing a diesel oil/ethanol fuel blend, suitable for use in a diesel engine, arises from the limited miscibility of the two components. Although small amounts of ethanol and diesel oil are miscible at room temperature, slight contamination with water separates the mixture into two phases. In addition, the miscibility of diesel oil and ethanol is reduced at lower temperatures. One way of mixing diesel oil and ethanol involves the production of micro-emulsions. For example, U.S. Pat. No. 4,477,258 discloses a diesel fuel emulsion comprising diesel oil and an aqueous solution of ethanol or methanol together with an emulsifying blend of sorbitan monooleate and a water soluble ethoxylated non-ionic surfactant. In another example, U.S. Pat. No. 4,451,265 discloses a hybrid diesel fuel composition in which water and alcohol are held in a stable microemulsion by means of a surfactant system comprising N,N-dimethylethanolamine and a long chain fatty acid.  
           [0008]    It must be mentioned that, to maintain a thermodynamically stable emulsion, the size of the alcohol droplets must be very small. This in turn requires a large quantity of surfactant in the fuel blend. In this regard, surfactant is a relatively expensive constituent, and the cost of the resulting fuel can be unacceptably high. Microemulsions have a further disadvantage with regard to production costs. The production of stable microemulsions requires intensive mixing/stirring high volumes of fuel on an industrial scale that escalates the production cost even higher. Further, when a diesel fuel tank is filled with the emulsion fuel, one must remain with the particular emulsion fuel, and cannot be switched to regular diesel fuel. This is because when emulsion fuel is diluted by regular diesel, the stability of emulsion fuel deteriorates resulting in phase separation.  
           [0009]    As an alternative to microemulsions, it is also possible to mix diesel oil and alcohol together as a solution, which relies upon the miscibility of the two components. Diesel oil and anhydrous ethanol are readily miscible at room temperature. However, it is well known in the art that lower temperatures, or a small amount of contaminating water, result in the separation of the diesel oil and ethanol into distinct phases. A fuel that is easily prone to phase separation is unsuitable for use in a diesel engine. For this reason, it is highly desirable to produce diesel oil/ethanol fuel blends in which the diesel oil and ethanol form a homogeneous and stable solution.  
           [0010]    In one attempt to achieve this objective, U.S. Pat. No. 4,405,337 discloses a diesel oil/alcohol fuel blend comprising a solubliser in the form of castor oil. In this example, the castor oil is shown to induce the miscibility of the diesel oil and an aqueous solution of alcohol, wherein the alcohol component comprises 0.5% water. At 25° C. the castor oil induces solublisation of the diesel oil and alcohol at most relative concentrations of the constituents. However, at 10° C. and at 0° C. the solublisation characteristics are limited. Fuel mixtures comprising less than 20% castor oil, and more than 20% ethanol, will generally separate into two phases at 0° C. U.S. Pat. No. 4,405,337 does not disclose the use of castor oil fuel blends at temperatures below 0° C.  
           [0011]    In another example, U.S. Pat. No. 6,017,369 discloses fuel compositions comprising diesel oil, ethanol, an alkyl ester of a fatty acid, and a stabilizer (of specific ether or ether/amide mixtures). Fuel blends comprising diesel oil/ethanol solutions are described that are stable at temperatures as low as −19° C. Such fuel blends comprise a complex mixture of additives to achieve reasonable levels of low temperature stability.  
           [0012]    In the developed world, vehicles that are powered by diesel engines comprise the majority of smaller vans and trucks, and nearly all large trucks, buses and non-electrically powered trains. In Europe, diesel oil powered cars comprise an increasingly significant market share. This has resulted from a new generation of direct-injection engines, which are more refined and powerful than their predecessors. The modern diesel engine is considerably more efficient than the equivalent gasoline engine, so further improvements to generate “greener” diesel engines, which use even less hydrocarbon-based fuel, could play a key role in the reduction of petroleum consumption and the protection of the environment. However, the development of alternative “hybrid” diesel engine fuels has so far been restricted by the limitation of temperature stability. Much of the developed world exists in regions of cold climates with temperatures frequently falling below 0° C. Other regions with less cold climates nevertheless experience cold conditions during the winter months. Therefore, to be commercially viable, any new fuel for a diesel engine must have the property of low temperature stability.  
           [0013]    It is an object of the present invention to provide improved low temperature stable diesel fuels, in the form of homogeneous liquid mixtures of diesel oil and alcohol, comprising a biodegradable oxygenate as a solubliser. Furthermore, it is an object the present invention to provide a solubliser that may be used in low concentrations to successfully achieve homogeneity of diesel oil/alcohol mixtures at low temperatures, without the risk of glyceride pyrolysis. It is a further object of the present invention to provide fuel blends that exhibit superior levels of stability at temperatures of less than 0° C., preferably less than −5° C.  
         SUMMARY OF INVENTION  
         [0014]    The present invention provides low temperature stable diesel fuel compositions which comprise diesel oil, ethanol, and a solubliser comprising fatty acids having chain lengths of C 14-18 . The inclusion of specific fatty acids in diesel oil/ethanol fuel blends can induce the formation of stable homogeneous solutions at temperatures considerably lower than 0° C., e.g. below −20° C. In the fuel composition of the present invention, it is important to note that the components are dissolved in one another and the fuel is free of emulsions. The ethanol and diesel oil are typically present in the proportion of 5:95 to 85:15 by weight. The solubliser is typically added to the diesel oil/ethanol mixture in an amount up to 20% by weight based on the total fuel composition. The solubliser can be a fatty acid derived from a variety of biodegradable sources. Particularly useful solublisers include tall oil and depitched tall oil (tall oil from which heavy bottoms material has been removed). Depitched tall oil is free of glycerides and thus very suitable as a diesel fuel component. Importantly, tall oil has been unexpectedly found to provide for significantly improved temperature stability of diesel oil/ethanol solutions, thus permitting larger proportions of ethanol to remain dissolved in diesel oil at lower ambient temperatures. The fuels of the present invention may be widely used to power diesel engines under conditions where low ambient temperature is an important consideration. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a graph to show the phase separation characteristics of diesel oil/ethanol mixtures over a range of depitched tall oil concentrations;  
         [0016]    [0016]FIG. 2 is a graph to compare the volume of the separate (ethanol) phase of azeotropic ethanol/diesel oil mixtures in the presence or absence of depitched tall oil (wherein D=diesel oil, ZE=azeotropic ethanol, and DPTO=depitched tall oil);.  
         [0017]    [0017]FIG. 3 is a graph to compare the capacity of depitched tall oil, canola methyl esters and corn methyl esters, to solublise diesel oil and ethanol (wherein D=diesel oil, ZE=azeotropic ethanol, CAME=canola methyl esters, and COME=corn methyl esters); and  
         [0018]    [0018]FIG. 4 is a graph to analyze the capacity of canola methyl esters to solublise diesel oil/ethanol mixtures over a range of temperatures (wherein E=anhydrous ethanol, D=diesel oil, and CAME=canola methyl esters). 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0019]    Examples are provided with particular reference to depitched tall oil as a solubliser. Tall oil is a by-product of the pulping of resinous softwoods such as spruce and pine. Pulp spent liquor is treated with sulfuric acid before being collected as crude tall oil. In this way, the sufuric acid reacts with glycerides present in the liquor. Crude tall oil comprises fatty acids in the C 16  to C 24  range, but the principle components are unsaturated C 18  fatty acids, diterpenic rosin acids and unsaponifiable neutrals such as sterols. Crude tall oil also contains a small amount of sugars. However, when crude tall oil is depitched the rosin acids, neutrals and sugars remain in the heavy ends, an thus depitched tall oil comprises primarily C 16  to C 18  fatty acids. The composition of depitched tall oil therefore provides significant advantages over diesel oil/ethanol solublisers of the prior art, since prior treatment with sulfuric acid renders the oil substantially free of gycerides, thus resulting in reduced glyceride pyrolysis during combustion.  
       EXAMPLE 1  
       [0020]    A series of tests were conducted on different mixtures of anhydrous ethanol, #2 diesel oil, and a solubliser as shown in Table 1 below:  
                                                 TABLE 1                                       Cetane   Flash           Diesel oil ethanol mixtures with   number   point                           a solubliser                Certified #2 diesel oil only   45.7   86           5wt % ethanol + 94wt % #2 diesel +   43.1   16           1wt % depitched tall oil           5wt % ethanol + 94wt % #2 diesel +   43.3           1wt % esterified canola oil           5wt % ethanol + 94wt % #2 diesel +   43.2           1wt % esterified corn oil           5wt % ethanol + 94wt % #2 diesel +   43.6           1wt % esterified soy oil           10wt % ethanol + 88wt % #2 diesel +   39.5   15           2wt % depitched tall oil           20wt % ethanol + 76wt % #2 diesel +   33.2   15           4wt % depitched tall oil                      
 
         [0021]    The cetane numbers and flash points were determined as shown above. In consideration of Table 1, the cetane numbers remain high for fuel mixtures comprising only 5% ethanol. However, at higher ethanol concentrations the cetane numbers are lower. Therefore, the diesel fuel mixtures of the present invention, which comprise higher concentrations of ethanol, may require additional spiking to ensure suitable combustion characteristics. From Table 1, it will be noted that fuels comprising more than 5% ethanol have lower flash points. It is expected that combustion of such fuel mixtures will result in lower NO x  emissions as a result of the lower combustion temperatures. Moreover, the ethanol, a simple oxygenate, in the mixtures will also result in reduced particulate emissions. Therefore, the fuel mixtures may present significant advantages in terms of reduced environmental pollutants.  
       EXAMPLE 2  
       [0022]    The fuel mixtures containing depitched tall oil were tested for temperature stability. This was done by measuring the temperature at which phase separation takes place. As the solubliser, fatty acids in the form of depitched tall oil have been found to be particularly effective. The graph shown in FIG. 1 displays the boundaries of phase separation for anhydrous ethanol and diesel oil at different temperatures and tall oil concentrations. FIG. 1 demonstrates that tall oil considerably enhances the capacity of ethanol and diesel oil to form a solution.  
         [0023]    Two general considerations can be made regarding FIG. 1. Firstly, the graph shows that diesel oil/ethanol mixtures, comprising roughly equal quantities (by weight) of each component, require the highest amount of tall oil additive to ensure that a homogeneous solution is maintained. This point is relevant regardless of the temperature. In mixtures comprising unequal amounts of diesel oil and ethanol, a smaller amount of tall oil may be added to achieve homogeneity. The second consideration relates to temperature. FIG. 1 demonstrates that at lower temperatures, more tall oil is required to achieve homogeneity of a particular diesel oil/ethanol mixture.  
         [0024]    In general, higher amounts of tall oil are needed to ensure complete solublisation of diesel oil and ethanol at lower temperatures. In the prior art, similar observations have been made with alternative solublisers. However, it is important to note that solublisers of the prior art are required in unsatisfactorily high concentrations to achieve solublisation at low temperatures. This is known in the art to potentially give rise to engine gumming and poor engine efficiency. In contrast, the present invention discloses a fuel that is stable at low temperatures, wherein the solubliser can generally be used in concentrations that are considerably lower than those of the prior art. Accordingly, the fuel compositions of the present invention are expected to be less prone to engine gumming and poor engine efficiency.  
         [0025]    According to FIG. 1, at −15° C. the highest amount of tall oil needed is only about 9% of the total volume by weight. With mixtures comprising unequal amounts of diesel oil and ethanol, the amount of tall oil required is even lower. The unexpected superior solublisation properties of tall oil and other similar oils over the solublisers of the prior art represent a significant development in the formulation of “greener” diesel engine fuels, which comprise lower amounts of non-replaceable hydrocarbon-based oil products.  
         [0026]    From FIG. 1 it is also apparent that for a particular diesel oil/ethanol mixture, the amount of tall oil required increases linearly as the temperature decreases. Based on these results, it is expected that diesel oil/ethanol mixtures can be obtained that are stable at temperatures of −40° C. or lower with the addition of less than 20% tall oil. Depitched tall oil is free of sugar and glycerides. Therefore, the fuels of the present invention have the potential to be used successfully in diesel engines at ambient temperatures that are considerably lower than 0° C.  
       EXAMPLE 3  
     Depitched Tall Oil Descreases Phase Separation of Diesel Oil/Azeotropic Ethanol Solutions  
       [0027]    Water contamination of diesel oil/ethanol solutions can result in phase separation of the diesel oil and ethanol. It is known in the art that azeotropic ethanol (ethanol containing 4% water) does not readily form a solution with diesel oil. Experiments were performed to determine the ability of depitched tall oil to increase the solubility of diesel oil and azeotropic ethanol. Diesel oil/azeotropic ethanol mixtures comprising 10%, 20% and 30% azeotropic ethanol were analyzed at temperatures ranging from −15° C. to 20° C. (FIG. 2). The graph demonstrates that separation of the mixtures into 2 distinct phases occurred at all azeotropic ethanol concentrations, regardless of the presence of depitched tall oil. However, for azeotropic ethanol concentrations of 10% or 20%, the presence of depitched tall oil in the mixtures significantly reduced the volume of the separated (ethanol) phase for all temperatures tested. The depitched tall oil therefore induced the ability of the azeotropic ethanol to homogeneously dissolve in the diesel oil.  
       EXAMPLE 4  
     Comparison of Depitched Tall Oil with Methyl Esters of Canola and Corn  
       [0028]    Further experimentation was carried out using a diesel fuel mixture comprising 5% azeotropic ethanol (FIG. 3), to compare the solublisation capacities of depitched tall oil with canola methyl esters and corn methyl esters. In the absence of additive, the mixture remained in two phases at all temperatures tested (−15° C. to 20° C.; FIG. 3). However, when a small amount of depitched tall oil was added to the mixture, the homogeneity was achieved at temperatures ranging from 10° C. to 20° C., although a very small second phase appeared at temperatures lower than 5° C. (FIG. 3). These results suggest that the fuel mixtures of the present invention should preferably comprise less than 0.2% moisture.  
         [0029]    The solublisation characteristics of depitched tall oil were compared with those of canola methyl esters and corn methyl esters. Unexpectedly, the canola and corn methyl esters decreased the ability of the mixture to form a homogeneous solution at all temperatures tested, resulting in an increase in the volume of the separated (ethanol) phase. This was in complete contrast to the depitched tall oil, which increased the homogeneity of the mixture significantly. In conclusion, when moisture is present, the depitched tall oil exhibits properties that induce the formation of diesel oil/ethanol solutions, unlike the selected methyl esters tested.  
       EXAMPLE 5  
     Depitched Tall Oil is a Significantly more Efficient Solubliser than Canola Methyl Esters for Diesel Oil/Anhydrous Ethanol Mixtures  
       [0030]    Experiments were carried out to analyze the ability of canola methyl esters to solublise mixtures of diesel oil and anhydrous ethanol (FIG. 4). The presence of canola methyl esters can maintain homogeneity of mixtures comprising 10% ethanol (anhydrous), only at temperatures higher than 0° C. (0.5% and 2% methyl ester concentration produces near identical results). Therefore, the performance of depitched tall oil as a solubliser is far superior to that of canola methyl esters. In this regard, a 2% concentration of depitched tall oil can maintain a fuel mixture comprising 10% anhydrous ethanol at temperatures as low as −15° C. (see FIG. 1).  
         [0031]    When the anhydrous ethanol concentration is increased to 30%, canola methyl esters (up to 6%) will only maintain a homogeneous solution above 10° C. However, the superior solublisation properties of depitched tall oil (at 7%) maintain the same fuel mixture as a homogeneous solution at temperatures as low as −15° C. (see FIG. 1).  
         [0032]    In conclusion, depitched tall oil exhibits diesel oil/anhydrous ethanol solublisation properties that are significantly superior to those of canola methyl esters.