Biological & Agricultural Engine

Operators of diesel equipment are well aware of the tendency of diesel fuel to gel or solidify at low temperatures. Some of the long chain hydrocarbons in No.2 diesel fuel, known as waxes, will typically start to gel at about -9 to -10^oC. Biodiesel will generally start to gel at higher temperature than #2 diesel fuel. Soybean oil based biodiesel will form crystals at about 0 ^oC and biodiesel from greases and animal fats can form crystals at 20 ^oC or even higher.

Detailed data for the effect of biodiesel on the cold flow properties of diesel fuel can be accessed by clicking here .

At low temperatures, diesel fuel can gel or crystallize and cause the engine to stop. A large amount of research has been done to estimate the lowest temperature at which a given fuel can be used. At least four different techniques are in common use: the Cloud Point, the Pour Point, the Low Temperature Flow Test (LTFT), and the Cold Filter Plugging Point (CFPP). These techniques are described below.

Cloud Point - The cloud point is the temperature at which a cloud of wax crystals first appears in a fuel sample that is cooled under conditions described by ASTM D2500. The cloud point is determined by visually inspecting for a haze in the normally clear fuel. The apparatus used for this test (and the pour point) is shown in Figure 1.

Figure 1. Cloud Point and Pour Point Apparatus

Pour Point - The pour point is the lowest temperature at which movement of the fuel sample can be determined when the sample container is tilted. The apparatus used is the same as for the Cloud Point and is shown in Figure 1. The sample must be cooled following the procedure described in ASTM D97. At every 3^oC of cooling, the sample is inspected and when no movement is detected after 5 seconds, the test is stopped. 3^oC is added to the temperature where no movement was observed and this is the pour point. Pour points are always expressed in multiples of 3^oC.

Low Temperature Flow Test (LTFT) - The LTFT is designed to evaluate whether a fuel can be expected to pass through an engine fuel filtration system. The test determines the lowest temperature at which 180 ml of fuel can be drawn through a 17micron screen in 60 seconds or less with 20 kPa of vacuum. The procedure is defined in ASTM D4539. The test apparatus is shown in Figure 2.

Figure 2. Apparatus for Low Temperature Flow Test

Cold Filter Plugging Point (CFPP) - The cold filter plugging point, as defined by International Petroleum Standard IP-309 and ASTM D 6371-99, is similar to the LTFT test. It determines the lowest temperature where 20 ml of fuel can be drawn through a 45 micron screen in 60 seconds with 200 mm of water (1.96 kPa) of vacuum. The apparatus is shown in Figure 3.

Figure 3. Apparatus for Cold Filter Plugging Point

The cloud point is the highest temperature used for characterizing cold flow and the pour point is the lowest. The LTFT and CFPP temperatures will usually be somewhere between the cloud and pour points.
When diesel fuels from petroleum or biodiesel start to solidify, they initially form microscopic crystals. If allowed to agglomerate, these crystals will grow large enough where they can plug fuel filters and fuel lines. Anti-gelling additives can be used to disrupt the agglomeration process.

Pour point depressants - Most pour point depressants, also known as cold flow improvers, work on similar principles. As the fuel sample is cooled, small wax crystals form. The temperature at which this occurs is the cloud point. As the sample is cooled further, the crystals agglomerate and grow in size until the entire sample solidifies. Most pour point depressants do not alter the initial formation of the crystals and thus they do not generally affect the cloud point. Rather, they inhibit the crystals from combining and growing to a size large enough to plug filters. The additives are generally waxes that are used in small amounts. They surround the small crystals and provide a barrier to agglomeration.

While pour point depressants can lower the allowable operating temperature of the fuel, the most common remedy is to blend #1 and #2 diesel fuel together. Number 1 diesel fuel can generally operate below –40 C (-40° F) without crystallization. So-called "winter blends" of #1 and #2 are used in the northern United States to provide low temperature operability.

ASTM D 975 does not specify a specific value for the cold flow requirements of diesel fuel. Instead, it suggests that one measure of cold flow, the cloud point, which is described below, be no more than 6°C higher than the 10^th percentile minimum ambient temperature for the month the fuel will be used. The 10^th percentile temperature corresponds to the minimum temperature that would be reached no more than 3 days out of 30 for the month. Figure 4 shows the values of 10^th percentile temperatures during the month of January. ASTM D 975 contains similar maps for other low temperature months in the United States.

.

Figure 4. 10th Percentile Minimum Air Temperatures for January (ASTM D975-97)

Biodiesel will generally start to gel at higher temperatures than #2 diesel fuel. Soybean oil-based biodiesel will form crystals at about 0°C and biodiesel from greases and animal fats can form crystals at 20°C or even higher. Low quality biodiesel produced with an incomplete reaction can behave in a similar manner as the mono- and diglycerides crystallize even at high temperatures. Partially reacted monoglycerides containing saturated fatty acids have high melting points and very low solubility in methyl esters. These compounds can fill fuel filters with a creamy deposit. The user may think the fuel was made from a feedstock that provides a high cloud point when in fact the problem is with the completeness of the reaction.

Diesel Fuel Cold Flow Properties