Prediction Of Natural Gas Hydrate Formation Conditions In The Presence Of Methanol Using The Trebble-Bishnoi Equation Of State

Abstract

Abstract A methodology for calculating the inhibiting effect of methanol on the formation of gas hydrates in natural gas-water systems is presented. The vapor and liquid phases are modeled using the Trebble-Bishnoi equation of state. The van de Waals-Platteeuw model is used for the solid hydrate phase. In addition to the prediction of the incipient hydrate forming conditions, the compositions of the equilibrium phases are calculated. The predictions agree very well with the available experimental data. The methodology is useful in calculating the amount of methanol required to achieve a desired inhibiting effect, and the amount of methanol carried in the vapor and liquid phases. Introduction Water molecules through hydrogen bonding form a lattice-like structure with cavities. This structure known as empty hydrate lattice is unstable. However, in the presence of natural gas components with molecular diameters smaller than the diameter of the cavities, the structure can become stable. The resulting ice-like crystalline material is known as the gas hydrate. Gas hydrates are non-stoichiometric and are formed in two cubic structures, labeled I and II. Low temperatures and relatively high pressures are in general favorable for hydrate formation. Detailed information on the physical and structural properties of gas hydrates can be found in the literature(1–3), Hammerschmidt(4) showed that the blocking of natural gas transportation lines could result due to hydrate formation. The hydrates could also plug process-piping, valves, orffice plates and other gas processing facilities. It is a common industrial practice to use inhibiting agents to prevent the hydrate formation(5). Electrolytes and alcohols such as methanol and glycols are known to suppress the formation of hydrates. Experimental studies on the inhibition effects of electrolytes have been carried out by Knox et al.(6), Menten et al.(7), Roo et al.(8) and Kubola et al.(9). Experimental data on the depression effect of methanol on cyclopropane hydrate formation have been reported by Menten et al.(7). A systematic experimental study on the effects of methanol and glycols on the hydrate formation from natural gas components and their mixtures has been carried out by Robinson and his co-workers(10–13). For designing gas processing facilities, however, it is very important to have available reliable methods for calculating the depression effects and the amount of inhibitors required. Englezos and Bishnoi(15) recently formulated a computer implementable methodology for calculating the effects of single or mixed electrolytes on gas hydrate formation. The methodology uses thermodynamic models for the calculations. For calculating the depression effects of alcohols and the amounts of alcohols required, the method of Hammerschmidt(4) is widely used in the industry. The method is empirical and the reliability of the calculations is variable(14). Hence, it is desirable to develop reliable methodologies which are based on thermodynamic models and have true predictive capabilities. Anderson and Prausnitz(16) presented a thermodynamics-based method for calculating the inhibiting effects of methanol. They used van der Waals-Plarteeuw model(17) for the solid hydrate phase, Redlich-Kwong equation of state(18) for the vapor phase and the UNIQUAC model for the liquid phase.