Wax Deposition Study in a Cold-finger System with Model Oil

Abstract

Abstract Wax deposition problem occurred during crude oil production and transportation process is a hot one and has brought huge economy losses around the world. This paper focused on the mass and heat transfer process during wax deposition with cold-finger experiments by method of differential scanning calorimetry (DSC) and gas chromatography (GC). Two sets of experiments with 15 °C and 20 °C temperature difference were conducted using the model oil with wax content of 10 wt. % for 120 h. During each experiment, the wax-solvent mixtures at two radial positions of two horizontal planes and deposits are sampled for DSC and GC analysis. The dissolved wax content gradient of the wax-solvent mixture at the same height is from the tank wall to the cold-finger, which is just opposite of its temperature gradient. As the diffusion proceeds, the total proportion of wax with carbon numbers of nC21~nC39 gradually decreases near the tank wall, but increases near the cold-finger. Furthermore, compared with the initial wax-solvent sample, the absolute mass ratios of wax molecules heavier than nC31 become greater in the deposits under 15 °C and 20 °C temperature difference, while the mass ratios of those lighter than nC32 decrease. This variation trend indicates that under this cold-finger temperature, heavy wax molecules continually diffuse into the deposition layer and meanwhile light wax molecules are displaced out of the deposition layer. The nC32 is the critical carbon number (here we defined the carbon number from which can deposite on the cold-finger as the critical carbon number) heavier than which the wax molecules can deposit in this condition. This paper intensely analyzed the wax molecule distribution conditions from the view of heat transfer and mass transfer theories. It will undoubtedly promote the investigation of wax deposition mechanisms of waxy crude oil.