Incipient Hydrate Formation Conditions

The objective next is to determine the hydrate formation pressure at a given 

Open Incipient Hydrate Formation form and select Hydrate Formation P at given T 

CSNGen Uydcate Predictlan Process (CJ Colorado Sshool at KtJies 200I 

FIGURE A.1 Layout of feed screen for fluid composition input. 

TcnperatuEC 35.000 Fahrenheit Pressure 166.97 psia fluueec of Phases Presenc 3 Stable Convergence 

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As indicated in Example 4.1, note the dramatic decrease in hydrate pressure caused by a small amount of propane added to methane, due to the structure change (si to sll). At pressures above incipient hydrate formation conditions, sll hydrates are predicted to be present throughout the entire composition range. 

The methane+ethane+propane+water system is the simplest approximation of a natural gas mixture. As shown in Figure 5.20, the phase equilibria of such a simple mixture is quite complicated at pressures above incipient hydrate formation conditions. One of the most interesting phenomenon is the coexistence of si and sll hydrates which occurs in the interior of some pseudo-ternary phase diagrams. 

The equilibrium behavior of structure H hydrates has only recently been studied. Fig. 4 shows the incipient hydrate formation conditions of a mixture of methane, carbon dioxide, and neo-hexane, as measured by Servio et al. ... 

Activity models have also been used to compute incipient hydrate conditions in the presence of polymers. Englezos and Hall presented a predictive model for the calculation of the incipient hydrate formation conditions in a hydrocarbon-water-polymer system. The activity of water in the polymer-water solution is computed using the UNIFAC model. 

Knowledge of CO2 hydrate formation conditions is very important for rational and design of processes in CO2 capture and sequestration. Several studies have focused on determination of incipient CO2 hydrate formation conditions. The condition at which an infinitesimal amount of hydrate phase is present in equilibrium with liquid phases is referred to as incipient hydrate formation condition [28]. Using different experimental methods, macroscopic hydrate phase equilibrium can be determined... 

The thermodynamics of gas hydrates is a mature subject (Sloan, 2004b). However, new data continue to appear, mainly for mixtures required in new applications, or for more severe conditions of temperature and pressure. A list with experimental data up until the late 1990s is available (Sloan, 1998). From a computational standpoint the main interest is in calculating the incipient hydrate formation pressure for a given hydrate-forming mixture at a particular... 

It should be noted that until the last decade, hydrate predictions using Equation (1) were restricted to incipient hydrate formation - i.e., specifying the temperature and pressure conditions at which the hydrate phase was initiated. The vapor-liquid equilibria analog of this is the determination of the dew point. However, Gupta et... 

Dholabhai, P.D., Incipient Equilibrium Conditions for Methane Hydrate Formation in Aqueous Mixed Electrolyte Solutions, M.Sc. Thesis, University of Calgary, Calgary (1989). 

Kobayashi and co-workers began to use the van der Waals and Platteeuw theory in the 1960s to predict hydrate formation in ternary systems. Parrish and Prausnitz extended the method to prediction of hydrate incipient formation in natural gas systems causing the widespread industrial adoption of the van der Waals and the Platteeuw statistical method. Many academic (e.g.. Holder and co-workers ) and commercial programs (e.g., D.B. Robinson, and Associates ) enabled the gas and oil industry to predict thermodynamic conditions at the incipient formation point, and thereby to prevent hydrate formation in industrial processes. All of the errors in the van der Waals and Platteeuw theory were placed in the solid phase. It may be argued that the theory s unusual success in prediction inhibited motivations for advances in hydrate phase measurements. 

The phase behavior of NGHs has been extensively researched. The temperature, pressure, the composition of the gas, and the state of the aqueous solution determine the incipient conditions of gas hydrate formation. The most common method for representing phase behavior for gas hydrates is by a temperature versus pressure... 

In addition to salts, alcohols, and polymers, the presence of a porous medium will also have an inhibiting effect on the formation of gas hydrates. Clarke, Pooladi-Darvish, Bishnoi derived the following expression for the activity of water in a pore of radius r, and used it along with the model of van der Waals and Platteeuw to model the incipient conditions for... 

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