Petroleum fuels pour point

The pour point (ASTM D-97, IP 15) is the lowest temperature at which the fuel oil will flow under specified conditions. The maximum and minimum pour point temperatures provide a temperature window where a petroleum product, depending on its thermal history, might appear in the liquid as well as the solid state. Pour point data can be used to supplement other measurements of cold flow behavior, and the data are particularly useful for the screening of the effect of wax interaction modifiers on the flow behavior of petroleum. The pour point should not be confused with the freezing point, which is an index of the lowest temperature at which the crude oil will flow under specified conditions. Test methods (ASTM D-2386, ASTM D-5901, ASTM D-5972, IP 434, IP 435) for the freezing point are not usually applicable to fuel oil but are more applicable to diesel fuel and aviation fuel. 

Tests of the fuel oil fraction normally include determination of density or specific gravity, total sulfur, aniline point, total acid number, naphthalenes content, smoke point, total nitrogen, viscosity, cloud point (ASTM Test Method D2500, Cloud Point of Petroleum Oils), pour point (ASTM Test Method D97, Pour Point of Petroleum Oils), and calculation of cetane index. Corrosiveness, ash (ASTM Test Method D482, Ash from Petroleum Products), and carbon residue might also be determined in more thorough evaluations. 

Pour point ranges from 213 K (—80°F) for some kerosene-type jet fuels to 319 K (115°F) for waxy No. 6 fuel oils. Cloud point (which is not measured on opaque fuels) is typically 3 to 8 K higher than pour point unless the pour has been depressed by additives. Typical petroleum fuels are practically newtonian liqmds between the cloua point and the boiling point and at pressures below 6.9 MPa (1000 psia). 

The pour point of a crude oil or product is the lowest temperature at which an oil is observed to flow under the conditions of the test. Pour point data indicates the amount of long-chain paraffins (petroleum wax) found in a crude oil. Paraffinic crudes usually have higher wax content than other crude types. Handling and transporting crude oils and heavy fuels is difficult at temperatures helow their pour points Often, chemical additives known as pour point depressants are used to improve the flow properties of the fuel. Long-chain n-paraffins ranging from 16-60 carhon atoms in particular, are responsible for near-ambient temperature precipitation. In middle distillates, less than 1% wax can be sufficient to cause solidification of the fuel. ... 

Some petroleum products, especially those containing higher-molecular-weight compounds such as waxes, do not crystallize rapidly when cooled. Instead, they form a gel-like network throughout the fuel matrix. This network can begin forming at temperatures well above the pour point of a fuel and may render the product unpumpable. 

Crude oil and high-boiling-point, high-viscosity petroleum fractions such as 6 fuel oil, atmospheric tower bottoms, and vacuum gas oil can contain wax which crystallizes at temperatures often above room temperature. It is not unusual for these oils to have base pour points of 100°F (37.8°C) or greater. In order to utilize these heavy oils, the pour point and viscosity of these oils must be reduced. One method which is used to accomplish this is to dilute the heavy oil with lower-viscosity components such as diesel fuel or kerosene. The oil then becomes pumpable at lower temperatures. 

Because the paraffin and mixed-base crudes yield lubricating oil fractions of high quality, means had to be devised in the early days of the petroleum industry to separate the wax from the oil. The removal of wax from petroleum fractions is one of the most important phases in the production of lubricating oils and fuel oils of low pour point, and has received the attention of many investigators. 

However, one of the limitations of using biodiesel fuel for diesel engines is higher cold flow properties compared with petroleum diesel fuel (4). Cold properties consist of cloud point, pour point, and cold filter plugging point. The cloud point is a temperature at which the fuel starts to thicken and cloud, the pour point is a temperature at which the fuel thickens and no longer pours, and the cold filter plugging point is the lowest temperature at which fuel still flows through a specific filter. These... 

Table IV gives the properties of the SRC-II fuel oil compared to a low-sulfur residual oil utilized in a recent combustion test. The SRC-II fuel oil is a distillate product with a nominal boiling range of 350-900°F, a viscosity of 40 Saybolt seconds at 100°F and a pour point below -20°F. Thus, it is readily pumpable at all temperatures normally encountered in transportation of the fuel oil. The fuel oil has a very low content of ash and sediment as well as a low Conradson carbon residue. These characteristics are favorable from the standpoint of particulate emissions during combustion. Tests of compatibility with typical petroleum fuel oils and on stability of the coal distillates over time have not revealed any unusual characteristics that would preclude utilization of these coal-derived fuels in conventional boiler applications.

Petroleum with a high wax content presents difficulties in handling and pumping as well as producing distillate and residual fuels of high pour point and lubricating oils that are costly to dewax. 

In any determination of the pour point, petroleum that contains wax produces an irregular flow behavior when the wax begins to separate. Such petroleum possesses viscosity relationships that are difficult to predict in fuel line operations. In addition, some waxy petroleum is sensitive to heat treatment that can also affect the viscosity characteristics. This complex behavior limits the value of viscosity and pour point tests on waxy petroleum. However, laboratory pumpability tests (ASTM D-3245, IP 230) are available that give an estimate of minimum handling temperature and minimum line or storage temperature. 

Specific gravity, freezing point and pour point data are shown in Table I. For each fuel four kinds of data are given (where available) the range of the data (minimum and maximum values obtained), the average value for all the sanples examined, the military specification requirement for that property, and an average value for a representative petroleum derived fuel. Also shown in the table are the number of fuel samples which were examined in each case. This format is also used for Table II which will follow. 

Pour Point. The pour points of each of two samples of DFM were 20.6°C. This value is well below the specification maximum of -6.7°C (4) and that of an average petroleum derived DFM (11,12). As in the case of the jet fuel freezing points discussed above, the lower pour point of the Shale-II DFM is consistent with its lower distillation end point compared to that of the petroleum DFM as seen in Figure 2. 

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