Characterizing Petroleum Products with DSC

Fuel is traditionally extracted from crude oil or petroleum. Petroleum contains six different types of substances. The mixture’s composition is specific to the region or area where the oil occurs and includes the following compounds:

• Aromatics
• Cycloalkanes
• Sulfur-containing compounds
• Straight-chain n-alkanes (C_nH_2n+2) with molar masses between 16 and 300 g/mol
• Branched-chain alkanes (iso-alkanes)
• Heterocyclic and polycyclic resins, and bitumens with molar masses of approximately 1000 g/mol

Crude oil distillation produces various fractions, classified as the following:

• Low boiling fractions, such as petrol or gasoline, naphtha, and aviation gasoline
• High boiling fractions such as lubricating oils and heavy oil
• Higher boiling fractions, such as diesel, heating oil or fuel

The residue that follows distillation is known as bitumen or asphalt.

When the distillate is in the liquid state it macroscopically appears as a single-phase mixture, but when it is cooled it leads to the formation of crystals or a multiphase mixture. When the crystalline material is separated, it leads to the following issues:

• A sediment is formed when the crystallized material separates. This can cause a problem, particularly for the storage of heating oils and diesel
• Blockages can occur if the crystallized material is retained in filters
• Asphalt or bitumen products are primarily used to surface roads, however crystallization makes the surface brittle, leading to the formation of cracks

Hydrocarbon distillates primarily contains crystallizable fractions and complex hydrocarbon compounds. Hydrocarbon compounds show a glass transition at low temperatures and remain semi-liquid at room temperature. The petroleum distillate controls the glass transition temperatures of the liquid components. Standard values are -150 °C for gasoline, -130 °C for diesel, and -30 °C for bitumen.

The proportion of the crystallizable fractions is up to 40% for crude oil; 5% to 25% for fuel oil; and 0% to 10% for bitumen. The crystals’ chemical structure is based on the distillate. With regard to bitumen, n-alkanes with 20 to 60 carbon atoms crystallize out; in the case of fuel oils, n-alkanes with 10 to 28 carbon atoms crystallize out; and with crude oil, n-alkanes with 5 to 60 carbon atoms crystallize out. Also present are lightly branched cycloalkanes and iso-alkanes.

Characterization of Petroleum Products with DSC

The melting behavior and glass transition temperature of petroleum products are often used to characterize them. The DSC technique can be used to easily measure these parameters. A standard temperature program used to study petroleum derivatives involves cooling the sample (heavy hydrocarbon compounds) at 10 K/min from room temperature to 100 °C, or cooling the sample (light hydrocarbon compounds such as gasoline and kerosene) to -150 °C.

Following this, the sample is linearly heated at 5 K/min up to end temperatures of 120 °C for bitumen; 100 °C for heavy oil; 80 °C for crude oil, and 50 °C for fuel oils such as diesel or light heating oil. The corresponding heating curves for different samples are depicted in Figure 1, showing different effects.

Figure 1. DSC curves of different petroleum distillates

The glass transition considerably increases the heat capacity at low temperatures, indicated by the step in the heat flow curve. Some of the sample, usually iso-alkanes, crystallizes out, leading to an exothermic peak. Several broad endothermic peaks occur on melting various crystals. The peak’s shape reflects the crystals’ size and weight distribution and is typical of a specific crude oil or specific distillate.

Evaluation of DSC Curves of Petroleum Products

The liquid matrix is integrated with the crystalline components. The step height of the change of the specific heat and the glass transition temperature (T_g) characterize the matrix. It is seen that T_g is in good agreement with the average mole mass of the matrix. One way to measure the amount of the crystallized fractions is to divide the quantified heat of fusion by the melting enthalpy ΔH(T) of a fictive, fully crystallized sample. Figure 2 shows the evaluation of a DSC curve of diesel oil.

Figure 2. Evaluation of a DSC curve of diesel oil

For the compounds used in this analysis, the peak area can be determined by a linear baseline. This typically starts at approximately T_g +30 K (Ti ) and ends at approximately 10 K after the melting process (T_f) is completed. The crystallized material’s melting enthalpy can be measured in the following way:

For medium distillates such as heating oil and gasoline, a third order polynomial can be used to describe ΔH(T). A constant value of 160 J/g is more than adequate. With respect to bitumen, the melting enthalpy is relatively larger and a value of 200 J/g has shown to be ideal. With regard to heavy oils and crude oils, a value of 200 J/g is recommended above 30 °C and a value of 160 J/g below 30 °C.

The problems cited initially with regard to crystals isolating out on cooling are very important for practical reasons. In the case of heavy oils and crude oils, it would be best to determine the crystallization between 80 °C and -20 °C at a cooling rate of 2 K/min, and with regard to medium heavy fuel oils, temperature range can be between 25 °C and -30 °C at a cooling rate of 0.5 K/min.

Marked exothermic peak is seen in these experiments, revealing the course of the crystallization. To assess the corresponding DSC curve, the following typical temperatures are differentiated:

• The cloud or turbidity point, with crude and heavy oils known as the wax appearance temperature (WAT), matches the temperature at which point the crystallization starts (ASTM D2500)
• The Cold Filter Plugging Point (CFPP) matches the temperature below which all crystallizable materials has crystallized (EN 116)
• The flow point (FP) refers to the temperature where the sample’s viscosity is so high that it does not flow (ASTM D97)

A tangential or horizontal baseline is drawn on the left part of the curve to assess the crystallization peak. A 200 J/g value is considered for the crystallization enthalpy.

Example 1: Light Petroleum Distillate

Figure 3. Typical crystallization behavior of diesel oil

The turbidity or cloud point is assumed to be the onset temperature of the crystallization peak (T_onset). If defined in this manner, the turbidity point can be reproducibly determined with an accuracy of ±0.5 K. The values acquired in this way are somewhat lower than those established with the ASTM standard method (T_ASTM). The following correlation was obtained by measuring 50 different light distillates:

    WAT = T_onset = 0.98•T_ASTM -3.6

To measure the CFPP value, the analysis of 40 light petroleum products gave the following correlation between the CFPP determined according to EN 116 and the temperature at which point 0.45% of the crystallizable material has crystallized out (Tc (0.45%)):

    T_c(0.45%) = 1.01•T_{CFPP EN 106} - 0.85

To determine the flow point, the optimum correlation was found to be:

    T_c(1%) = 1.02•T_{flow point ASTM} - 0.28

T_c(1%) represents the temperature at which point 1% of the crystallizable fractions has crystallized out.

Example 2: Heavy and Crude Oils

Figure 4. Typical crystallization behavior of diesel oil

The turbidity point for both crude and heavy oils is determined in the same way as the light petroleum products. If the flow point is less than 0 °C, the crystalline content subsequent to cooling is approximately 2 mass%. The sample’s behavior is established by the noncrystalline matrix for the most part.

Conclusion

DSC measurements provide a fast and dependable characterization of different groups of petroleum products. The melting behavior and the glass transition temperature offer vital data regarding the quality of petroleum derivatives. Additionally, it is also possible to determine the origin of a sample of unknown petroleum as the quantified curves are typical of petroleum derivatives (they are “fingerprints”). Cooling experiments additionally provide practical data about the crystallization behavior of petroleum derivatives.

This information has been sourced, reviewed and adapted from materials provided by Mettler Toledo - Thermal Analysis.

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