Thermodynamics of hydrate formation

We will restrict our discussion to stoichiometric hydrate. The thermodynamics of hydrate formation has been discussed by Lohani and Grant (45). Assuming that a drug D, forms a hydrate with m moles of water of crystallization, the equilibrium can be... 

The Maddox et al. procedure is based on equations describing the thermodynamics of hydrate formation and applies to either methanol or ethylene glycol. A hand calculation version of the technique includes several simplifying assumptions, but appears to give results that are comparable to the computer version and much closer to experimental data than the previously proposed calculation methods. The hand calculation model for hydrate formation is... 

In a review of the thermodynamics of water, Franks and Reid (1973) showed that the optimum molecular size range for maximum solubility was similar to hydrate stability. Franks and Reid noted, this is not intended to imply that long-lived clathrate structures exist in solution—only that the stabilization of the water structure by the apolar solutes resembles the stabilization of water in a clathrate lattice. Glew (1962) noted that, within experimental error, the heat of solution for ten hydrate formers (including methane, ethane, propane, and hydrogen sulfide) was the same as the heat of hydrate formation from gas and ice, thereby suggesting the coordination of the aqueous solute with surrounding water molecules. 

The inhibition of three-phase hydrate formation is discussed in Section 4.4. These predictions enable answers to such questions as, How much methanol (or other inhibitor) is required in the free water phase to prevent hydrates at the pressures and temperatures of operation Classical empirical techniques such as that of Hammerschmidt (1934) are suitable for hand calculation and provide a qualitative understanding of inhibitor effects. It should be noted that only thermodynamic inhibitors are considered here. The new low-dosage hydrate inhibitors [LDHIs, such as kinetic inhibitors (KIs) or antiagglomerants (AAs)] do not significantly affect the thermodynamics but the kinetics of hydrate formation LDHIs are considered in Chapter 8. 

In this section two prediction techniques are discussed, namely, the gas gravity method and the Kvsi method. While both techniques enable the user to determine the pressure and temperature of hydrate formation from a gas, only the KVSI method allows the hydrate composition calculation. Calculations via the statistical thermodynamics method combined with Gibbs energy minimization (Chapter 5) provide access to the hydrate composition and other hydrate properties, such as the fraction of each cavity filled by various molecule types and the phase amounts. 

Makogon (1981, p. 134) and Berecz and Balla-Achs (1983, p. 102) indicated that methanol can increase the temperature of hydrate formation at concentrations less than 5 mass% (presumably due to the clustering effect), but higher concentrations inhibit formation. Nakayama and Hashimoto (1980) also suggested that several of the alcohols could form hydrates yet further study by Nakayama et al. (1997) caused the opposite opinion. Further measurements by Svartas (1988) also indicated that small methanol amounts do not increase hydrate thermodynamic stability. 

In addition to the change in the theoretical methods applied to hydrates, there have been significant advancements and widespread use of meso- and microscopic tools in hydrate research. Conversely, the typical static experimental apparatus used today to measure macroscopic properties, such as phase equilibria properties, is based on the same principles as the apparatus used by Deaton and Frost (1946). In part, this is due to the fact that the simplest apparatus is both the most elegant and reliable simulation of hydrate formation in industrial systems. In Section 6.1.1 apparatuses for the determination of hydrate thermodynamic and transport macroscopic properties are reviewed. 

This chapter deals with macro-, meso-, and molecular-level thermodynamic and transport hydrate properties of natural gas and condensate components, with and without solute. The feasibility of using these tools to measure the kinetics of hydrate formation and decomposition are also briefly discussed, while the results of these measurements have been discussed in Chapter 3. The results for insoluble substances such as porous media are discussed in Chapter 7. 

From Chapters 4, 5 and 6 thermodynamic data and predictions, the maximum methane concentration (solubility) occurs in the aqueous liquid at equilibrium with hydrates. In order for methane to exsolve the liquid, the solubility must change rapidly as the water rises with corresponding decreases in pressure and temperature. Solubility calculations (Handa, 1990) indicate a change in methane concentration too gradual to account for a significant hydrate amount. Solubility data are needed at conditions of hydrate formation, in order to confirm this model. Preliminary solubility data are available from Besnard et al. (1997). 

The above suite of hydrate sensing tools (thermodynamics, geothermal gradients, kinetics, BSRs, lithology and fluid flow, logging and coring tools, and subsea tools) has enabled an assessment of where hydrates may exist worldwide. On the basis of the data provided by these tools, hydrate formation models such as that of Klauda and Sandler (2005) enable our prediction of hydrate formation sites in nature—notably the a priori prediction of 68 of the 71 sites at which hydrates have been indicated. 

The five studies of hydrate formation given in Section 8.1 are of two types. The first three case studies show thermodynamic (time-independent) methods to prevent plug formation. However, the second type provides a closer, mechanistic look at the physical kinetics (time-dependent) hydrate formation and agglomeration. A goal of this section is to show how these two methods provide two different methods of plug prevention. 

Over the last decade or so, many research efforts have been focused on developing what are termed low-dosage hydrate inhibitors , or LDHIs, that potentially can kinetically inhibit hydrate formation/ LDHIs operate via a much different mechanism than thermodynamic inhibitors such as methanol. They are often effective at concentrations as low as 0.5 wt% and act by delaying the onset of hydrate formation, while thermodynamic inhibitors are effective only at much higher concentrations and act by changing the conditions of hydrate thermodynamic stability, thus shifting the phase diagram. 

Existing methods of technological calculations of the inhibition process [65] are based on the assumption that there exists a thermodynamic balance between liquid (inhibitor) and gas (natural gas) phases. Application of this method allows to determine equilibrium values of concentration of water vapor and inhibitor in a gas at given values of pressure, temperature, inhibitor s mass concentration in the solution, composition of gas, and specific flow rate of inhibitor required for given temperature decrease of hydrate formation ... 

The analyses shows that thermodynamic driving impetus influence on rate of hydrate formation, the ability of stabilizing large cavity of hydrate structure also has important effect on hydrate growth rate. Understanding from the angle of hydrate stability, the ratio(S) of object diameter and the cavity diameter can estimate the stability of cavity. In Table 4,... 

Under thermodynamic conditions outside si and inside sll phase boundaries, the results show that the change in speed of sound was detectable if 5bbl/MMscf of water was converted into hydrates. The results using a commercial device demonstrate that equivalent composition changes can be measured and used to detect early signs of hydrate formation with a similar sensitivity of around 5 bbl/MMscf... 

Alternatively, Bratsch and Lagowski (1985a, b, 1986) proposed an ionic model to calculate the thermodynamics of hydration AGj, A/fJ and ASj using standard thermochemical cycles. This model is based on the knowledge of the values of quantities such as the enthalpy of formation of the monoatomic gas [A/f (M )], the ionization potential sum for the oxidation state under consideration and the crystal ionic radius of the metal ion. This approach, however, is difficult to apply for the actinides since the ionization potentials are, for the most part, unavailable. To overcome this problem, the authors back-calculated an internally consistent set of thermochemical ionization potentials from selected thermodynamic data (Bratsch and Lagowski 1986). The general set of equations developed are ... 

The mechanistic problems associated with the transport of cations (especially Na and K+) through membranes have continued to receive attention. The structures of macrocyclic alkali-metal complexes and the thermodynamics of their formation have been reviewed, as has the general question of the selectivity of the carriers towards the metals. The role of hydration energy has been considered and it has been suggested that, at least in the case of the enniatin ionophores, sandwich... 

On the basis of the limited data available, Moskvin [300] has presented some generalizations of the thermodynamics of the formation of actinide ions in aqueous solutions. His analysis includes discussion of the heat capacities of triply charged actinide ions and the changes in their heat capacities on hydration and when transferred from a crystal lattice to solution. Moskvin concludes that further accumulation of thermochemical data for actinide ions, including those of americium, is one of the most urgent contemporary problems in actinide chemistry. 

For sodium palmitate, 5-phase is the thermodynamically preferred, or equiUbrium state, at room temperature and up to - 60° C P-phase contains a higher level of hydration and forms at higher temperatures and CO-phase is an anhydrous crystal that forms at temperatures comparable to P-phase. Most soap in the soHd state exists in one or a combination of these three phases. The phase diagram refers to equiUbrium states. In practice, the drying routes and other mechanical manipulation utilized in the formation of soHd soap can result in the formation of nonequilibrium phase stmcture. This point is important when dealing with the manufacturing of soap bars and their performance. 

The need for auxiliary heating is another factor that must be carefully evaluated. Due to the nature of the thermodynamic process, the gas discharging from an expander is at a much lower temperature than gas discharging from a regulator station operating within the same pressure bounds. If temperatures downstream of the expander are allowed to drop too low, potential problems may arise, such as hydrate formation and material compatibility. 

A thermodynamic inhibitor alters the chemical potential of the hydrate pha.se such that the hydrate formation point is displaced to a lower temperature and/or a higher pressure. 

Thermodynamic inhibitors Antinucleants Growth modifiers Slurry additives Anti-agglomerates Methanol or glycol modify stability range of hydrates. Prevent nucleation of hydrate crystals. Control the growth of hydrate crystals. Limit the droplet size available for hydrate formation. Dispersants that remove hydrates. 

The thermodynamics of the extraction mechanism is extremely complex. In the initial equilibration of the ion pairs (Scheme 1.6) account has to be taken not only of the relative stabilities of the ion-pairs but also of the relative hydration of the anionic species. Assuming the complete non-solvation of the ion-pairs, the formation of the ion-pair [Q+Y] will generally be favoured when the relative hydration of X- is greater than that of Y. However, in many cases, the anion of the ion-pair is hydrated [8-11] (Table 1.1) and this has a significant effect both on equilibrium between the ion-pairs in the aqueous phase and the relative values of the partition coefficients of the two ion-pairs [Q+X ] and [Q+Y ] between the two phases. 

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