The study of phase heat transfer characteristics and influential factors of wax removal by hot water in pipelines

Most crude oil produced in China is waxy crude oil. When the crude oil is transported along the pipeline, its temperature drops due to the outward heat loss of the pipeline. When the temperature drops below the wax precipitation point of the crude oil, the wax molecules in the crude oil begin to precipitate on the inner surface of the pipelines, which is called waxed. If the pipe is waxed, the flow area will be reduced, and the frictional resistance will be increased, resulting in the low pipeline conveying capacity and even the blocking of the pipelines. It is of great significance to solve this problem for maintaining the normal production of oilfields. Washing with hot water is the most commonly used method to solve this problem. Based on the composition of wax in Daqing crude oil, the wax layer was stratified, the enthalpy method and VOF model were used to simulate the melting process of wax in the pipeline. The phase transition melting process of wax in the pipeline was obtained, the effects of water temperature and wax thickness on the melting process and the time required for complete melting were analyzed. The natural convection was proven to be the main reason for the solid-liquid interface and the irregular temperature field distribution. And the wax in the upper part of the pipeline melted at the fastest rate, the melting rate in the left and right parts of the pipe was slower. The wax in the lower part of the pipeline melted very slowly, which was the weakest link in the entire melting process. Increasing the water temperature from 343.15 to 353.15 K can shorten 44.97% of the melting time, while increasing the water temperature from 353.15 to 363.15 K can only shorten 22.86% of the melting time. The results showed that the wax melting time did not vary linearly with the water temperature. With the increase of the water temperature, its effect on the melting time became weak. When the thickness of wax layer increased from 5 to 10 mm, the time required for complete melting increased from 1159 to 4328 s. With the increase of the wax thickness, the melting time increased sharply. Therefore, in the actual process of wax removal, it is recommended that the wax should be removed by hot water when the wax deposit is not too thick. Otherwise the hot water and wax heat transfer area will be too small, resulting in a longer time for wax removal and more energy consumption.