Effect of Pressure on the Crystallization of Crude Oil Waxes. I. Selection of Test Conditions by Microcalorimetry

Abstract

Evaluations of the potential for formation of wax deposits in laboratories generally are performed on stabilized oil samples, that is, at atmospheric pressure and without the presence of light components. Therefore, the effects to two important factors that influence the solubility of waxes, namely light fractions and pressure, are not considered. As a consequence, the results may not reflect what really happens in production lines and equipment. The high-pressure microcalorimetry technique (HPμDSC) is indicated for determining the wax appearance temperature (WAT) and crystallization enthalpy of waxes in oils, but due to the complexity of petroleum systems, the reliability of the results depends on the experimental conditions chosen. This paper reports experiments to evaluate the factors that influence the accuracy of the experimental results for determining the WAT and crystallization enthalpy of samples of Brazilian crude oil and condensate, saturated with different types of gases, by using high pressure microcalorimetry (HPμDSC). The results show that heat flow values of more than twice the baseline oscillation, that is, 48 μW, could be considered as a thermal event. It is fundamental to investigate the gas phase saturation envelope to evaluate the viability of pressurizing the reference cell and to determine the most adequate conditions of temperature for the pressurization of the oil samples. Still better results were obtained without pressurizing the reference cell and with exchange of test aliquots after analysis at 50 bar. The minimum saturation time necessary for the oil and condensate samples evaluated was 180 min, and the ideal mass for analysis was around 100 mg. The most suitable procedure was to pressurize the samples rather than depressurize them, with the deviation between the two procedures being <1°C. The standard deviation of the values found in the conditions specified was <0.5°C.

Biodegradable polyester‐based heat management materials of interest in refrigeration and smart packaging coatings

“…The fact that the crystallization temperatures detected with the spectroscopic technique were slightly higher than those obtained through DSC could be attributed, as previously mentioned, to a potentially greater nucleation effect promoted by ice crystals and/or by the ATR crystal itself. Moreover, these samples were pressed over the ATR crystal which could also affect the phase transition temperature, as it can be observed from a typical phase diagram …”

“…From phase diagrams, it can be observed that for a fixed temperature, the material state changes if pressure is modified, i.e., when increasing the pressure, the paraffins can be in solid state, even though the temperature is higher than their crystallization temperature. Although phase diagrams only describe ideal systems, the effect of pressure on the crystallization temperature of some alkanes in real systems has been already reported by some authors …”

A study of asphaltene-resin interactions

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Influence of the chemical structure of additives poly(ethylene‐co‐vinyl acetate)‐based on the pour point of crude oils

“…The evaluation of the crystallization and deposition of waxes from crude oil is carried out in the laboratory by different testing methods, such as measurement of the wax appearance temperature (WAX) and enthalpy of crystallization; determination of the pour point; the cold finger test; and monitoring the viscosity in function of temperature drop …”