Hydrate formation conditions

Table 3.1 gives a set of pressure versus temperature data of equilibrium C02 hydrate formation conditions that were obtained from a series of experiments in a 20 wt % aqueous glycerol solution. The objective is to fit a function of the form... 

Natural gas composition varies from field to field. The main component of purified natural gas generally is methane ( 90 %+) along with other light hydrocarbons. Hence, hydrate formation conditions and the hydrates obtained may vary according to gas composition. Obviously, each component interacts differently with water and hence there will... 

Li, X.-S. Englezos, P. (2006). Prediction of Gas Hydrate Formation Conditions in the Presence of Methanol, Glycerol, Ethylene glycol and Triethylene glycol with the SAFT Equation of State. IndEng Chem Res., 45 (6), 2231-2137. 

The water contents obtained from the graph at temperatures below hydrate-formation conditions represent dew-point formation under metastable equilibrium between gas and liquid water rather than between gas and solid hydrates. The water contents of natural gases in equilibrium with hydrates are significantly lower than the water contents given in Figure 16-18, especially at lower temperatures. 

Fig. 17-4. Hydrate-formation conditions of methane-propane mixtures. (Deaton and Frost, U.S. Bureau of Mines, Monograph 8, 1946, 20.. Courtesy, Bureau of Mines, U.S. Department of the Interior.)...

Figure 17-6 was developed from the data of Figure 17-5.4 Figure 17—6 is a correlation of hydrate-forming conditions for natural gases with various specific gravities. This figure can be used to estimate the conditions under which hydrates will form. The resulting hydrate formation conditions must be used with caution because there is a great discrepancy between the limited published data and the correlation shown in Figure 17-6.5 The differences in hydrate-forming pressures...

When Hammerschmidt (1934) identified hydrates in pipelines, he published a correlation summary of over 100 hydrate formation data points. Shortly afterward, Professor D.L. Katz and his students at the University of Michigan began an experimental study. Because it was impractical to measure hydrate formation conditions for every gas composition, Katz determined two correlative methods. 

Katz s two predictive techniques provided industry with acceptable predictions of mixture hydrate formation conditions, without the need for costly measurements. Subsequently, hydrate research centered on the determination of the hydrate crystal structure(s). Further refinements of the Kvsi values were determined by Katz and coworkers (especially Kobayashi) in Chapter 5 of the Handbook of Natural Gas Engineering (1959), by Robinson and coworkers (Jhaveri and Robinson, 1965 Robinson andNg, 1976), and by Poettmann (1984). 

Each line in Figure 4.2d (except for the lower, almost vertical I-Lw-V lines) bounds hydrate formation conditions listed with a methanol concentration in the free water phase. To the left of each line with H in the label, hydrates will form with a water phase of the given methanol composition to the right of the line hydrates will not form. For example, when the free water phase has 10% methanol, hydrates will not form at pressure-temperature conditions to the right of the line marked 10% MeOH. Yet if no methanol were present, the hydrates would form at pressures and temperatures between the two lines marked 10% and 0% MeOH. Similarly, for process pressure and temperature conditions between the lines marked 10% and 20%, at least 20% methanol in the free water phase would be required to prevent hydrate formation. 

Solubility of Gases Near Hydrate Formation Conditions... 

Example 4.5 Short Cut Calculation of Hydrate Formation Conditions with Salt... 

A typical chart for water content from this period is presented in Figure 4.21. In Figure 4.21 the water content chart at temperatures above the hydrate stability conditions is based primarily on the data of Olds et al. (1942) while the data of Skinner (1948) were the basis for extrapolations to temperatures below the hydrate formation point. A summary chart is given by McKetta and Wehe (1958). However, below the initial hydrate formation conditions, Figure 4.21 represents metastable values, as observed in gas field data by Records and Seely (1951). Kobayashi and Katz (1955) indicated that such concentration extrapolations across hydrate phase boundaries yield severe errors. 

Bansal, V., Christiansen, R.L., Sloan, E.D., Influence of Guest Vapor-Liquid Critical Point on Hydrate Formation Conditions, AIChE/., 39(10), 1735 (1993). 

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. 

Hydrate experimental conditions have been defined in large part by the needs of the natural gas transportation industry, which in turn determined that experiments be done above the ice point. Below 273.15 K there is the danger of ice as a second solid phase (in addition to hydrate) to cause fouling of transmission or processing equipment. However, since the development of the statistical theory, there has been a need to fit the hydrate formation conditions of pure components below the ice point with the objective of predicting mixtures, as suggested in Chapter 5. 

Equilibrium measurements of the solid hydrate phase have been previously avoided due to experimental difficulties such as water occlusion, solid phase inhomogeneity, and measurements of solid phase concentrations. Instead, researchers have traditionally measured fluid phase properties (i.e., pressure, temperature, gas phase composition, and aqueous inhibitor concentrations) and predicted hydrate formation conditions of the solid phase using a modified van der Waals and Platteeuw (1959) theory, specified in Chapter 5. 

Hydrate formation is favored by low temperature and high pressure, but the actual hydrate formation condition is a function of the gas under consideration. Different gases form hydrates at different conditions. 

The hydrate formation conditions for pure H2S (Carroll, and Mather, 1991 and Glew, 2000) and pure C02 (Yang et al., 2000) are well established. However, a review of the literature revealed that there are no data for acid gas mixtures. Thus we must proceed with some caution. 

In fact, none of the methods designed for rapid estimation of the hydrate formation conditions should be used for acid gas mixtures. If one must do such calculations, without a computer and a rigorous model, then it is probably wise to assume the hydrate formation conditions for the acid gas mixture are the same as... 

For accurate prediction of the hydrate formation conditions for acid gases, the advanced methods are preferred. The more advanced methods for predicting hydrate formation are based on the work of van der Waals and Platteeuuw (1959). This is a statistical thermodynamic model. 

Carroll, J.J. 2004. An examination of the prediction of hydrate formation conditions in sour natural gas. GPA Europe Spring Meeting, Dublin, Ireland. 

In the 1940s, Katz had the insight that hydrates were ideal solid solutions rather than stoichiometric compounds. With the ideal solid solution concept, Katz and his students generated two experimentally based prediction methods for hydrate formation conditions, without the knowledge of different hydrate structures. 

Equation (1) indicates that when a guest fills the cavities of a hydrate, the chemical potential of water in the cage is lowered, thereby stabilizing the hydrate phase. In principle. Equation (1) solves the hydrate prediction dilemma - that is, the hydrate formation conditions are determined by the pressures and temperatures that cause equality between the hydrate chemical potential of water in Equation (1), and the chemical potential of water in the other phase(s), as determined by separate equations of state. There are two important terms on the right the molar Gibbs free... 

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. 

Three-phase equilibrium conditions Hydrate formation conditions Gas phase... 

Englezos, P. Bishnoi, P.R. Prediction of gas hydrate formation conditions in aqueous electrolyte solutions. AIChE J. 1988, 34, 1718-1721. 

Zhenquan Lu, Sultan N Chunshuang Jin 2008. Semi-quantitative analysis of factors affecting gas hydrate formation conditions and its fractions. Chinese Journal of Geophysics, 51(1) 125—132. 

Under hydrate-formation conditions, the destruction of the colloidal system will bring available water into contact with hydrocarbon hydrate formers, allowing thermodyan-mically favored growth of the solid phase (hydrate crystals) these crystals can plug a pipe in a matter of minutes. 

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