Crystallization or gel formation of normal paraffins in diesel fuel under cold weather conditions leading to fuel filter clogging is a common problem. Cold weather operability of biodiesel (B100) and blends with diesel fuel presents additional complexity because of the presence of saturated monoglycerides (SMGs) and other relatively polar species. Currently, the cloud point measurement (a measure of when the first component crystallizes out of solution) is used to define the lowest temperature at which the fuel can be used without causing cold weather issues. While filter plugging issues have declined, there still remain intermittent unexpected problems above the cloud point for biodiesel blends. Development of a fundamental understanding of how minor components in biodiesel crystallize, gel, and transform is needed in order to prevent these unexpected issues. We have found that SMGs, a low level impurity present in B100 from the production process, can crystallize out of solution and undergo a solvent-mediated polymorphic phase transformation to a more stable, less soluble form. This causes them to persist at temperatures above the cloud point once they have some out of solution. Additionally, we have found that SMGs can cause other more soluble, lower melting point minor components in the B100 to co-crystallize and come out of solution. Monoolein, another minor component from the production process is an unsaturated monoglyceride with a much lower melting point and higher solubility than SMGs. It is able to form a co-crystal with the SMGs and is found together with the SMGs on plugged filters we have analyzed in our laboratory. An observation of isolated crystals in the lab led us to believe that the SMGs may also be forming a gel-like network with components of the B100 and diesel fuel. During filtration experiments, we have noted that in some cases a solid layer of crystals forms and blocks the filter completely, while in other cases this does not occur. Because SMGs are polar and can form layered networks once a sufficient amount of crystals have come out of solution, we recently began investigating the ability of SMGs to form a gel network with fuel components as well as with other minor polar components in the fuel in order to obtain a fundamental understanding of the mechanism of formation. It has been well established that this type of phenomena occurs in sub-sea pipelines where a chief crystallizing component begins to crystallize out of solution. Once a sufficient amount of crystals exists, a volume spanning network of solid crystals can trap liquid crude oil and form a solid-like gel network. We are investigating whether this type of phenomena can occur with SMGs and both fatty acid methyl esters from the B100 and normal paraffins from diesel fuel. Additionally, SMGs are well known to incorporate water into their layered crystal structure. Water is often used to stabilize less stable polymorphic forms of SMGs, therefore water was another minor component of interest. Also of interest is glycerin which has been found on clogged filters in our laboratory.
This presentation will describe an investigation of simple model systems containing only a few components to assess their ability to form a gel network with SMGs. We started with surrogate biodiesel blends at 5% fatty acid methyl ester content and continued to add components to investigate their compounding effects. Biodiesel surrogate blends were prepared by blending methyl oleate and methyl linoleate as fatty acid methyl ester components into dodecane and a custom hydrocarbon mixture of decane, undecane, dodecane, tridecane, and tetradecane as diesel fuel components. Once the surrogate blends were prepared, monopalmitin and monostearin were added as the SMG components by themselves. To investigate the compounding effects, additional components were added one at a time to the blends containing SMGs. The additional components were monoolein, water, and glycerin. The cloud point and final melting temperature (the temperature at which the crystals go back into solution) were measured. The samples were then held overnight at 5°C above the cold point of the base diesel blend to precipitate components and were filtered. The time to filter as well as the mass of precipitate was measured. The composition of the crystals was determined by gas chromatography and the crystal form was determined by differential scanning calorimetry. The crystal size and shape was observed and photographed under a microscope. Rheological measurements of the gel-point were also taken for a sub-set of samples to measure the mechanical strength of the potential gel.