Experimental Investigation and Thermodynamic Modeling of Wax Precipitation in Crude Oil Using the Multi-Solid Model and PC-SAFT EOS

Abstract

Wax precipitation is a significant problem in the oil and gas industry as it may cause plugging of the process equipment and transportation pipelines during oil transportation, production, and refining. Therefore, an accurate thermodynamic model for prediction of the wax appearance temperature (WAT) at especially high pressures can be helpful to deal with this problem. In this work, first, the WAT values for nine types of Iranian live crude oils were measured using a PVT cell equipped with a solids detection system (SDS). Then, an improved thermodynamic model based on the multi-solid (MS) model of wax precipitation, which was coupled with the chain form of the statistical associating fluid theory (PC-SAFT) equation of state (EOS), was developed to correlate the measured experimental data. Also, the MS model of wax precipitation was coupled with the Peng–Robinson (PR) EOS to be compared with the proposed model. Solid–liquid equilibria (SLE) were employed for the pressures above the bubble curve, and vapor–liquid–solid equilibria (VLSE) were utilized below the bubble curve in the P-T diagram. In the proposed model, the crude oil was considered as a mixture of the paraffinic, naphthenic, and aromatic fractions, and the plus fraction has been characterized using the three-parameter gamma probability function theory, to improve the characterization of the previous models based on two-parameter gamma probability function theory. The results of the experiments indicated that the WAT values decreased by increasing the pressure at the lower range of pressures. Also, the correlated values show that the proposed model based on PC-SAFT EOS with the correlated parameters can predict the WAT values better than the MS model with PR EOS with the %AAD below %1 at a wide range of pressures.