Vapor-liquid, hydrate formation

Reducing pressure at normal surface conditions, such as across a choke, causes a reduction in the temperature of the gas. This temperature reduction could cause the condensation of water vapor from the gas. It also could bring the mixture of gas and liquid water to the conditions necessary for hydrate formation. 

Each quadruple point occurs at the intersection of four three-phase lines (Figure 1.2). The lower quadruple point is marked by the transition of Lw to I, so that with decreasing temperature, Qi denotes where hydrate formation ceases from vapor and liquid water, and where hydrate formation occurs from vapor and ice. Early researchers took Q2 (approximately the point of intersection of line Lw-H-V with the vapor pressure of the hydrate guest) to represent an upper temperature limit for hydrate formation from that component. Since the vapor pressure at the critical temperature can be too low to allow such an intersection, some natural gas components such as methane and nitrogen have no upper quadruple point, Q2, and... 

A hydrate nucleating agent (precipitated amorphous silica) and a quiescent surface inhibitor (sodium dodecyl sulfate) were used in an attempt to initiate hydrates in the bulk phase. While the induction time (for detectable hydrate formation) was not predictable, in every case hydrate was initiated at a surface—usually at the vapor-water interface, but infrequently along the sides of the sapphire tube in the gas phase, and at the metal end-plate below the liquid phase. 

With such low concentrations of components available to form critical nuclei, hydrate formation seems unlikely in the bulk phases. However, at an interface where higher concentrations exist through adsorption (particularly at the vapor-liquid interface where both phases appear in abundance) cluster growth to a supercritical size is a more likely event. High mixing rates may cause interfacial gas + liquid + crystal structures to be dispersed within the liquid, giving the appearance of bulk nucleation from a surface effect. 

FIGURE 3.20 Successive cooling curves for hydrate formation with successive runs listed as Sj < S2 < S3. Gas and liquid water were isochorically cooled into the metastable region until hydrates formed in the portion of the curve labeled Sj. The container was then heated and hydrates dissociated along the vapor-liquid water-hydrate (V-Lyy-H) line until point H was reached, where dissociation of the last hydrate crystal was visually observed. (Reproduced from Schroeter, J.R, Kobayashi, R., Hildebrand, M.A., Ind. Eng. Chem. Fundam. 22, 361 (1983). With permission from the American Chemical Society.)... 

Englezos et al. (1987a,b) generated a kinetic model for methane, ethane, and their mixtures to match hydrate growth data at times less than 200 min in a high pressure stirred reactor. Englezos assumed that hydrate formation is composed of three steps (1) transport of gas from the vapor phase to the liquid bulk, (2) diffusion of gas from the liquid bulk through the boundary layer (laminar diffusion layer) around hydrate particles, and (3) an adsorption reaction whereby gas molecules are incorporated into the structured water framework at the hydrate interface. 

Table 4.4a presents the parameters of Equation 4.2, with an indication of the correlation coefficient. The Kvsi-value charts or equations are used to determine the temperature or pressure of three-phase (Lw-H-V) hydrate formation. The condition for initial hydrate formation from free water and gas is calculated from an equation analogous to the dew point in vapor-liquid equilibrium, at the following condition ... 

Katz (1972) first noted that hydrates could form from heavy liquids such as crude oils that have dissolved gases suitable for hydrate formation. He suggested that the point of hydrate formation from water and a liquid hydrocarbon phase (no gas present) could be predicted using the vapor-hydrate distribution coefficient Kvsi of Equation 4.1 together with the more common vapor-liquid distribution coefficient Kyu (=yi/xa). In this case Equation 4.3 becomes ... 

Since the value of AHu remains constant over a large range of pressures, the maximum in T is determined by the point at which the molar volume change is zero. The volume comparison must be made between the pure liquid hydrocarbon, liquid water, and hydrate, since the hydrocarbon must exist as liquid at pressures between the vapor pressure and the critical pressure. Maxima in hydrate formation temperatures above Q2 have been calculated, but they have yet to be measured. 

Hydrates may also exist in equilibrium with only a fluid hydrocarbon phase (either vapor or liquid) when there is no aqueous phase present. Two-phase (H-V or H-Lhc) regions are shown in the T-x diagram of Figure 4.3. Similarly, Figure 4.3 shows the Lw-H region for hydrates in equilibrium with water containing a small amount of dissolved methane, as in the case for hydrate formation in oceans, as exemplified in Chapter 7. 

Two common misconceptions exist concerning the presence of water to form hydrates in pipelines, both of which are illustrated via the T-x phase equilibrium diagrams in Figure 4.3. The first and most common misconception is that a free water phase is absolutely necessary for the formation of hydrates. The upper three-phase (Lw-H-V) line temperature marks the condition of hydrate formation from free water and gas. Below that temperature and to the right of the hydrate line, however, are two-phase regions in which hydrates are in equilibrium only with hydrocarbon vapor or liquid containing a small (<1000 ppm) amount of water. 

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

Figure 5.13 is the equivalent ethane + water pressure versus temperature phase diagram. Note that the Aq-sI-V line intersects the Aq-V-Lhc line at 287.8 K and 35 bar. Due to differences in the volume and enthalpy of the vapor and liquid hydrocarbon, the three-phase hydrate formation line changes slope at high temperature and pressure from Aq-sI-V to Aq-sI-Lhc, due to the intersectiion of Aq-sI-V line with the Aq-V-Lhc line (slightly higher than the ethane vapor pressure). Note that the hydrate formation pressure for ethane hydrates at 277.6 K is predicted to be 8.2 bar. 

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... 

The optimum temperature of the cold tower is as low as possible without risking formation of a third phase in addition to vapor and aqueous solution. Table 13.24 gives the temperatures at which solid hydrogen sulfide hydrate or liquid hydrogen sulfide form in the system HjS-HjO. At 300 psi, the minimum safe cold tower temperature is around 30°C. The rapid increase in condensation temperature above 300 psi is another reason for this being the optimum pressure. Before the first pilot plant for the GS process was operated, the possibility of hydrate formation was not recognized, and freeze-ups occuned until the cold tower temperature was raised above 30°C. 

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 ... 

This method operates at isobaric condition (constant pressure). The crystallizer with a gas-liquid system is set to pressures at which the hydrate equilibrium temperature needs to be determined. Initially the pressure of the system drops because of dissolution of the gas into the liquid caused by mixing. The pressure of the system is maintained constant by a supply of gas from the reservoir or fluid withdrawal. The system temperature is decreased to induce hydrate formation. The pressure of the system drops because of cooling and hydrate formation. After hydrate formation, the temperature of the system is increased slowly until an infinitesimal amount of hydrate crystals exists in equilibrium with the vapor and liquid phases. During heating, the system pressure is maintained constant by fluid withdrawal. This point is recorded as equilibrium condition. The equUibiium point is determined by visual observation. 

Blockages of valves and pipes can also occur by gas hydrates. Such adducts can be formed by a number of gases with water. In Fig. 7.1-5 the pressure-temperature diagram of the system propane/water with an excess of propane is presented. The line, (g), shows the vapour-pressure curve of propane. Propane hydrate can be formed at temperatures below 5.3°C. At pressures below the vapor pressure of propane a phase of propane hydrate exists in equilibrium with propane gas (Fig. 7.1-5, area b). At higher pressures above the vapor pressure of propane and low temperatures a propane hydrate- and a liquid propane phase were found (area d). In order to exclude formation of gas hydrates these areas should be avoided handling wet propane and other compounds like ethylene, carbon dioxide [14], etc. 

Formation of hydrate nuclei (from aqueous liquid) occurs as heterogeneous nudeation, usually at an interface (either fluid + solid, gas + liquid, or liquid + liquid). When both a nonaqueous liquid and vapor are present with water, hydrates form at the liquid-liquid interface. 

Various surface analysis techniques show that silicate glasses rapidly develop surface compositional profiles when exposed to water. When water is present as a vapor an alkali-rich layer (presumably a hydrated alkali carbonate) forms over the SiOj-rich layer. Water as a liquid dissolves the alkali and leaves the silica-rich film. As long as this SiC -rich film is stable the rate of corrosion due to diffusion is reduced with exposure time. Addition of multi-valent species to the glass or reactant results in formation of a complex protective surface layer in the glass which may be stable over a wide range of environmental conditions. 

Adsorption is the adhesion of molecules of a gas, liquid, or dissolved substance or of particles to the surface of a solid substance. Absorption is the assimilation of molecule into a solid or liquid subsunce, with the formation of a solution or a compound. Sometimes the word sorption is used to indude both of these phenomena. We say that a heated glass vessel adsorbs water vapor from the air on cooling, and becomes coated with a very thin layer pf water a dehydrating agent such as concentrated sulfuric add absorbs water, forming hydrates. Activated alumina sorbs water vapor, probably both by adsorption (the adhesion of a layer of water vapor to the surface of rhe particles.) and by absorptimi (the formation ipf aluimnunt hydroxMe). 

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