Study on the aging and critical carbon number of wax deposition with temperature for crude oils

• The crude oil with 18.6% wax content was used to study the aging process of deposit to reflect the changing of n-paraffin distribution of the real crude oils in deposit with time.
• The higher temperature ranges have an active effect on the aging of wax deposition. Under the same bulk oil temperature, the wax content in deposit decreases and the amount of higher carbon number components in deposit increases (i.e., the hardness of deposit increases) as the coolant temperature increases.
• Both the wax content and the amount of higher carbon number components in deposit increases as the bulk oil temperature increases under constant coolant temperature. Moreover, the temperature has only a little effect on critical carbon number for that change in critical carbon number does not exceed one carbon numbers under different temperature conditions.
• Lower bulk oil temperature provide deposit with less high carbon number components diminishing hardness of the deposit. On the contrary, the higher bulk oil temperature leads to the harder deposit. So the inlet oil temperature, which is not better with higher temperature, should be suitable in the waxy crude oil pipelines for preventing wax deposition and dealing with the pigging problems.

The aging of wax deposition with time and temperature was investigated by a series of experiments using cold finger device and carbon number distribution of the deposit samples was analyzed by high temperature gas chromatography. Deposition experiments were performed with the waxy crude oils from oil field in China, containing 18.6 wt% wax content. In the process of wax deposition aging, wax molecules with carbon number higher than critical carbon number (CCN) would diffuse into the deposit, while those with carbon number less than CCN would diffuse out from the deposit during the experiments. In this study, it is found that the wax content and higher carbon number in deposit all increase as time increases from 3 h to 48 h, so the hardness of deposit increases during the aging of wax deposition. In addition, the higher temperature range has an active effect on the aging of the wax deposition. Comparing the effect of the different temperature conditions, it could be concluded that temperature differential mainly determines the wax content and that hardness of the deposit mainly depends on the bulk oil temperature. Moreover, in this study, it is found that the temperature has only a little effect on critical carbon number (CCN) as change in CCN does not exceed one carbon number under different temperature conditions.