Modeling the Static Cooling of Wax–Solvent Mixtures in a Cylindrical Vessel

Abstract

Under subsea conditions, the transportation of ‘waxy’ crude oil through pipelines is accompanied by the precipitation and deposition of higher paraffinic compounds as solids (waxes) onto the cooler surfaces of the pipeline. Wax deposition is more pronounced during shut down of a pipeline since the fluid is held at static conditions. In this study, the static cooling of wax–solvent mixtures in a cylindrical vessel was modeled as a moving boundary formulation involving liquid–solid phase transformation. The deposition process during the transient cooling was treated as a partial freezing/solidification process. Also, the effect of the mixture composition and the cooling rate on the Wax Precipitation Temperature (WPT) or the solubility curve of the wax–solvent mixture was taken into consideration when the bulk liquid phase temperature was lowered below the WAT of the initial mixture composition. The predictions for the transient temperature profiles in the liquid and the deposit region, and the location of the liquid–deposit interface were validated with recently reported experimental results [19]. The predictions were also compared with the predictions for the gelling behavior of wax–solvent mixtures under static cooling reported by Bidmus [19]. The predictions for the temperature profile at seven thermocouple locations and the location of the liquid–deposit interface were in agreement with the experimental results and signified the important role of the solubility curve. The mathematical model presented was based on heat transfer considerations and regarded the deposition process to be thermally driven.