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A number of tests are available to measure low temperature properties of fuels for diesel engines and to estimate their effect on vehicle low temperature operability. The most conservative measure of a fuel’s low temperature operability is Cloud Point. It is also a reasonable estimate of the low temperature operability limit of fuels that do not contain operability additives. The cloud point is defined as the temperature at which a cloud or a haze of wax crystals starts to appear in the fuel under the test conditions. These crystals can collect in filters and eventually lead to blockage of the fuel system. Fuel additives that improve low temperature operability usually have little impact on cloud point. These additives are used in diesel fuel to prevent agglomeration or modify these crystals in other ways to limit filter blockage.
A number of different manual and automatic options for performing this test have been developed.
The most commonly referred to cloud point test is the manual method (ASTM D2500). In this test, a sample is brought to a temperature at least 14°C above the expected cloud point and free moisture is removed. The sample is then poured into a test jar which is closed with a cork fitted with a thermometer measuring the sample temperature on the bottom of the jar.
The sample is cooled in a cooling bath maintained at a constant temperature (Figure 1) by placing the sample jar into a jacket that had previously been placed into the cooling bath to cool. The sample jar is not placed directly into the cooling bath. An air gap of about 5.5 mm is maintained between the outside of sample jar and the inside of the jacket through the use of a felt or cork disk on the bottom of the jacket and a gasket around the outside of the sample jar.
Figure 1. Apparatus for ASTM D2500
Depending on the starting sample temperature and the expected cloud point, a series of cooling baths may be required. Each bath is maintained at a successively lower temperature. This provides a measure of control over the cooling rate of the sample. Note that the cooling rate of this test is not specified and is not constant throughout the test. Typical cooling rates for ASTM D2500 are on the order of 1°C/min . Bath temperature requirements are outlined in Table 1 and Figure 2 illustrates how a typical sample temperature varies over the course of the test.
|Bath||Bath Temperature, °C||Sample Temperature Range, °C|
|1||0±1.5||Start to 10|
|2||-18±1.5||9 to -6|
|3||-33±1.5||-6 to -24|
|4||-51±1.5||-24 to -42|
|5||-69±1.5||-42 to -60|
Figure 2. Typical Bath and Sample Temperature Profile Over ASTM D2500 Manual Cloud Point Test
The sample is placed first in the 0°C bath. At each sample temperature reading that is a multiple of 1°C, the sample is quickly removed without disturbing the sample, examined for cloud and replaced into the jacket. As the sample cools, it may need to be switched to successive baths as outlined in Table 1.
In most cases, wax crystals form at the bottom circumference of the sample jar where the sample temperature is lowest. The first wax crystals appear as a patch of white or milky cloud. The size and position of cloud depends on the nature of the sample. Some samples produce large easily observed clusters while others form barely perceptible clusters. The temperature at which these crystals first appear is the temperature of interest and is reported as the cloud point.
As the sample cools, dissolved water in the sample can come out of solution and form a general haze throughout the entire sample. Generally this should not interfere with the ability to detect the formation of wax crystals. In cases where it does, additional measures are required to remove water from the sample before the analysis is carried out.
Samples containing significant naphthenic or hydrocracked components or those whose cold flow behavior has been altered with additives can be difficult to analyze. Crystal growth can be weak, the contrast poor and the boundary between the crystals and fuel more diffuse. The cloud is these samples can also take the form of a haze that appears throughout the entire sample - much like water haze. To avoid interference from water haze, additional sample drying may be required in these cases as well.
In addition to the manual method, several automatic cloud point test methods have been developed. These tests continuously monitor the sample with an optical system for the formation of wax crystals. The optical system consists of a light transmitter and receiver. The cloud point is detected when there is a drop in incident light at the receiver. The resolution and precision of these automatic methods is generally better than the manual method. Additional details of these automatic methods and a comparison to the manual method are provided in Table 2.
Optical detection stepped cooling rate
Linear cooling rate method
Constant cooling rate method
Miniturized Optical Method
|Applicability||Fuels transparent in layers of 40 mm; cloud point between +49 and -60°C||Fuels transparent in layers of 40 mm; cloud point between +49 and -60°C||Fuels transparent in layers of 40 mm; cloud point between +49 and -60°C||Fuels transparent in layers of 40 mm; cloud point between +49 and -60°C||Fuels transparent in layers of 40 mm; cloud point between +20 and -60°C|
|Manual/Automatic Detection||Manual detection||Automatic optical detection||Automatic optical detection||Automatic optical detection||Automatic optical detection|
|Sample Volume, ml||40||40||≥20||0.15||20|
|Cooling Rate||Not specified, see Figure 2~1°C/min||Not specified, ~1°C/min. Similar to D2500.||1 ± 0.2°C/min||1.5 ± 0.1°C/min||6 ± 3°C/min|
|Report||Nearest 1°C||Nearest 0.1 or 1°C||Nearest 0.1 or 1°C||Nearest 0.1 or 1°C||Nearest 0.1°C|
|Diesel||2°C (-1 to -37°C)||2.2°C (+34 to -56°C)||1.3°C (+34 to -56°C)||1.3°C (+34 to -56°C)||0.026(31.0-X)°C|
X = measured cloud point
|Biodiesel Blends||2°C (+10 to -2°C)||1.2°C (+10 to -2°C)||0.7°C (+10 to -2°C)||0.7°C (+10 to -2°C)|
|Diesel||4°C (-1 to -37°C)||3.9°C (-34 to -56°C)||3.3°C (-34 to -56°C)||2.5°C (-34 to -56°C)||0.034(31.0-X)°C|
X = measured cloud point
|Biodiesel Blends||3°C (+10 to -2°C)||2.7°C (+10 to -2°C)||2.2°C (+10 to -2°C)||0.9°C (+10 to -2°C)|
|Bias Relative to D2500, °C|
|Biodiesel Blends||-||Not determined||Not determined||Not determined|