An integrated computational fluid dynamics model for predicting wax deposition in throttling valves

Natural gas is a vital component of the global energy landscape. To meet the growing demand for natural gas, complex and highly engineered transmission pipelines are used to efficiently transport large quantities of natural gas from production sites to end users. The cooling induced by throttling natural gas in transmission pipelines may cause the spontaneous formation of wax crystals in throttle valves, impacting pipeline transmission efficiency and posing risks to production. However, explaining the complex process from wax precipitation to deposition requires more than just considering condensation or deposition alone. In this computation fluid dynamics (CFD) study, we have examined the condensation parameters of natural gas mixtures in a traditional orifice plate throttle valve and evaluated the effects of varying inlet pressures, gas models, and wax content on deposition. The results indicate that nucleation predominantly occurs at the throat and throat outlet of the throttle valve. As inlet pressure increases, the nucleation zone at the throat outlet expands toward the pipe wall. A ring-shaped wax deposition zone forms at the throat exit and 0.9 m from the entrance. With rising inlet pressure, this deposition zone becomes more pronounced but narrower. For low wax content, the deposition rate decreases with increasing inlet pressure and varies with different gas models. Conversely, at higher wax content, the deposition rate increases with higher inlet pressure. The primary contribution of this paper is the use of CFD methods to comprehensively describe the processes of wax crystal precipitation, growth, deposition, and stripping in waxy natural gas following throttling.