Development of a thermodynamic framework for modeling the heat of gas hydrate dissociation in the presence of poly n-vinyl caprolactam

One of the most popular and financially feasible low-dosage hydrate inhibitors (LDHIs) for preventing gas hydrates formation in natural gas pipelines is poly n-vinyl caprolactam (PVCap). There is still disagreement over LDHIs’ thermodynamic effects, even though their better inhibition performance on gas hydrate nucleation and crystal growth has been demonstrated. For a long time, it was assumed that LDHIs do not affect natural gas hydrate dissociation conditions. Nevertheless, PVCap’s status as a thermodynamic hydrate promoter was established a few years ago. This work aims to provide a basic model that may be used to calculate the hydrate dissociation temperature when PVCap is present. For this reason, the van der Waals-Platteeuw solid solution theory is utilized to model the hydrate phase, and the Flory-Huggins (FH) model is used to calculate the water activity when PVCap is present in the aqueous phase. A straightforward correlation based on the hydrate dissociation enthalpy is introduced to obtain the hydrate dissociation temperature in the presence of PVCap. Some variables, including the hydrate dissociation pressure, PVCap molecular weight, and concentration, are included in the proposed model. The enthalpy of hydrate dissociation could be readily calculated using the model, which yields excellent results for the hydrate dissociation temperature for structures I and II when PVCap is present in the aqueous phase. The model performs well for both simple and mixed gas hydrates, and its accuracy is demonstrated by the temperature error obtained from the model for all 50 experimental data points, which is approximately 0.26 K.