Characterization of petroleum products with DSC

Conventional fuel is obtained exclusively from petroleum or crude oil. Petroleum is primarily a mixture of 6 different classes of substances. The composition of the mixture is specific to the region where the oil occurs and consists of

• straight-chain n-alkanes (C_nH_2n+2) with molecular masses between 16 and 300 g/mol
• branched-chain alkanes (iso-alkanes)
• cyclo-alkanes
• aromatics
• sulfur-containing compounds
• polycyclic and heterocyclic resins as well as bitumen with molecular masses of typically about 1000 g/mol.

Distillation of the crude oil yields various fractions, which are classified as follows: low boiling fractions, e.g. gasoline (petrol), aviation gasoline, naphtha; higher boiling fractions, e.g. fuel or heating oil and diesel; and high boiling fractions (heavy oil and lubricating oils). The residue after distillation is known as bitumen (or asphalt).

In the liquid state, the distillate appears macroscopically as a single-phase mixture. On cooling, crystals are formed, i.e. a multiphase mixture is obtained. The separation of crystalline material is undesirable and leads to a number of problems:

1. Crystallized material separates out forming a sediment. This is often a problem, especially for the storage of diesel and heating oils.
2. Crystallized material is retained in filters, which can lead to blockages.
3. Bitumen (asphalt) products are mainly used for surfacing roads. Crystallization causes the surface to become brittle and results in the formation of cracks.

Hydrocarbon distillates consist primarily of complex hydrocarbon compounds and crystallizable fractions. The former are partially liquid at room temperature and exhibit a glass transition at low temperatures. The glass transition temperatures of the liquid constituents depend on the petroleum distillate. Typical values are –30 °C for bitumen, -130 °C for diesel and –150 °C for gasoline. The proportion of the crystallizable fractions is between 0% and 10% for bitumen, between 5% and 25% for fuel oil and up to 40% for crude oil. The chemical structure of the crystals depends on the distillate. With fuel oils, n-alkanes with 10 to 28 carbon atoms crystallize out, with bitumen n-alkanes with 20 to 60 carbon atoms; and with crude oil n-alkanes with 5 to 60 carbon atoms. Lightly branched iso-alkanes and cycloalkanes are also present.

Petroleum products are usually characterized by their glass transition temperature and their melting behavior. The measurement of these parameters is very easily performed by DSC. A typical temperature program for the analysis of petroleum derivatives begins by cooling the sample at 10 K/min from room temperature to –100 °C (for heavy hydrocarbon compounds) or to -150 °C (for light hydrocarbon compounds, e.g. kerosene, gasoline). Afterward the sample is heated linearly at 5 K/min up to final temperatures of typically 50 °C (for fuel oils such as light heating oil or diesel), 80 °C (crude oil), 100 °C (heavy oil) and 120 °C (bitumen). Figure 1 shows the corresponding heating curves for different samples. Various effects can be observed. The marked increase of the heat capacity at low temperatures (step in the heat flow curve) is caused by the glass transition. Very often, part of the sample then crystallizes out (usually iso-alkanes), causing an exothermic peak. The melting of the various crystals can lead to several relatively broad, endothermic peaks. The shape of the peak mirrors the size and weight distribution of the crystals and is characteristic of a particular distillate or particular crude oil.

The crystalline components are embedded in the liquid matrix. The matrix is characterized by the glass transition temperature T_g and the step height of the change of the specific heat. The glass transition temperature correlates well with the average mole mass of the matrix. The percentage amount of the crystallized fractions can be calculated by dividing the measured heat of fusion by the (in principle temperature-dependent) melting enthalpy ΔH(T) of a fictive, fully crystallized sample.
For the compounds considered here, a linear baseline can be used for the determination of the peak area. This begins at about T_g +30 K (T_i) and ends at about 10 K after the end of the melting process (T_f). The melting enthalpy of the crystallized material can be estimated in the following way. For medium distillates (gasoline, heating oil), ΔH(T) can be described by a third order polynomial [1]. It is sufficient to use a constant value of 160 J/g. With bitumen, the melting enthalpy is larger. In practice, a value of 200 J/g is generally accepted. With crude oils and heavy oils, a value of 160 J/g is recommended below 30 °C and a value of 200 J/g above 30 °C.

• the turbidity (cloud) point, with crude and heavy oils often also called the wax appearance temperature (WAT), corresponds to the temperature at which crystallization begins (ASTM D2500).
• the CFPP, Cold Filter Plugging Point, corresponds to the temperature below which all crystallizable material has crystallized (EN 116).
• the flow point (FP) is the temperature at which the viscosity of the sample is so high that it no longer flows (ASTM D97).

To evaluate the crystallization peak, a horizontal or tangential baseline is drawn on the left side of the curve. A value of 200 J/g is assumed for the crystallization enthalpy.

The turbidity point is taken to be the onset temperature of the crystallization peak (T_onset). The turbidity point defined in this way can be reproducibly measured with an accuracy of ±0.5 K. The values obtained with this method are slightly lower than those determined using the ASTM standard method (T_ASTM). The measurement of 50 different light distillates led to the following correlation:

WAT = T_onset = 0.98×T_ASTM -3.6

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

T_c(0.45%) = 1.01×_{TCFPP EN 106} - 0.85.

For the determination of the flow point we found the optimum correlation to be:

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

[2]. Here, T_c(1%) is the temperature at which 1% of the crystallizable fractions has crystallized out.

The determination of the turbidity point for heavy and crude oils is performed in a similar way to the light petroleum products. If the flow point is below 0 °C, the crystalline content after cooling is only about 2 mass%. The behavior of the sample is therefore for the most part determined by the noncrystalline matrix [3].