Temperature of hydrates formation

To avoid hydrate formation, it is necessary either to dry the stream, or to inject a substance that, dissolving the water, lowers its partial fugacity and, consequently, the temperature of hydrate formation. 

The average error of this simplified method is about 3°C and can reach 5°C. Table 4.22 shows an application of this method calculating the temperature of hydrate formation of a refinery gas at 14 bar. Table 4.23 gives an example applied to natural gas at 80 bar. Note that the presence of H2S increases the hydrate formation temperature. 

The initial predictive method by Wilcox et al. (1941) was based on distribution coefficients (sometimes called Kvsi values) for hydrates on a water-free basis. With a substantial degree of intuition, Katz determined that hydrates were solid solutions that might be treated similar to an ideal liquid solution. Establishment of the Kvsj value (defined as the component mole fraction ratio in the gas to the hydrate phase) for each of a number of components enabled the user to determine the pressure and temperature of hydrate formation from mixtures. These Kysi value charts were generated in advance of the determination of hydrate crystal structure. The method is discussed in detail in Section 4.2.2. 

Quadrupole points Q3 and Q4 are unusual in that they have two coexisting hydrate phases. Between these two points is a line along which the two hydrate structures coexist with vapor, this line may be a unique example of the way guest size affects the cavity occupied and the pressure and temperature of hydrate formation. 

For systems with two quadruple points, the hydrate region is bounded by line I-H-V at conditions below Qi, line Lw-H-V between Qi and Q2, as well as line Lw-H-Lhc at conditions above Q2. Hydrates can form at lower temperatures and higher pressures to the left of the region enclosed by the three lines in Figure 4.2b to the right, no hydrates are possible. Upper quadruple point Q2 is often approximated as the maximum temperature of hydrate formation, because line Lw-H-Lhc is almost vertical however see data in Chapter 6 for exceptions. 

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 isobaric operation the system pressure is maintained constant, by the exchange of gas or liquid with an external reservoir. The temperature is decreased until the formation of hydrate is indicated by significant addition of gas (or liquid) from a reservoir. After hydrate formation the temperature is slowly increased (maintaining constant pressure by fluid withdrawal) until the last crystal of hydrate disappears. This point, taken as the equilibrium temperature of hydrate formation at constant pressure, may be determined by visual observation of hydrate dissociation or at a constant temperature as simple hydrates dissociate with heat input. 

Removal of water vapors from the gas. This process is called gas dewatering (dehydration). Since dehydration causes a decrease in the threshold temperature of hydrate formation, this procedure often includes additional steps intended to prevent the formation of hydrates. 

Methods used for prevention of hydrate formation are dictated by physical and chemical nature of a process. Since equilibrium parameters of hydrates formation depend on partial pressure of water vapor in hydrating medium, any action lowering such pressure reduces the temperature of hydrate formation. In practice, the two ways are used dehydration of gas from moisture and the input into gas flow of various water-absorbing substances called inhibitors. 

Under certain conditions of temperature and pressure, and in the presence of free water, hydrocarbon gases can form hydrates, which are a solid formed by the combination of water molecules and the methane, ethane, propane or butane. Hydrates look like compacted snow, and can form blockages in pipelines and other vessels. Process engineers use correlation techniques and process simulation to predict the possibility of hydrate formation, and prevent its formation by either drying the gas or adding a chemical (such as tri-ethylene glycol), or a combination of both. This is further discussed in SectionlO.1. 

Below 65°C, sodium iodide is present ia aqueous solutions as hydrates containing varyiag amounts of water. When anhydrous sodium iodide is dissolved ia water, heat is Hberated because of hydrate formation, eg, AH = —174.4 kJ/mol (—41.7 kcal/mol), when the dihydrate is formed. At room temperature, sodium iodide crystallizes from water as the dihydrate [13517-06-1/, NaI-2H2 02H2O, ia the form of colorless prismatic crystals. 

Anhydrous a-dextrose is manufactured by dissolving dextrose monohydrate in water and crysta11i2ing at 60—65°C in a vacuum pan. Evaporative crysta11i2ation is necessary to avoid color formation at high temperatures and hydrate formation at low temperatures. The product is separated by centrifugation, washed, dried to a moisture level of ca 0.1%, and marketed as a very pure grade of sugar for special appHcations. 

The formation of hydrates downstream of the expander does not appear to be a problem in this faeility. The original design studies indieated that hydrates should not form with this pipeline gas above temperatures of 20°F. The downstream temperature was not expeeted to drop this low. In operation, the downstream temperature has been above 25°F most of the time and has not dropped below 20°F. This is with inlet temperatures normally above 65°F. There have been no indieations of hydrate formation downstream of the expander. Indeed, eleetrie power produetion has aeutually exeeeded the quantity estimated for all years of operation. 

The experiments were conducted at four different temperatures for each gas. At each temperature experiments were performed at different pressures. A total of 14 and 11 experiments were performed for methane and ethane respectively. Based on crystallization theory, and the two film theory for gas-liquid mass transfer Englezos et al. (1987) formulated five differential equations to describe the kinetics of hydrate formation in the vessel and the associate mass transfer rates. The governing ODEs are given next. 

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, 17-7. Depression of hydrate-formation temperatures by inhibitors. (From Handbook of Natural Gas Engineering by Katz et al. Copyright 19,59 by McGraw-Hill Book Company. Used with permission of McGraw-Hiil Book Company.)... 

Fig. 17-8. Depression of hydrate-formation temperatures with methanol and diethylene glycol. (Data from USBM Mono. 8, 32 and Scauzillo, Chem. Eng. Prog. 52, 324.)...

The charts are entered at the intersection of the initial pressure and the initial temperature. The lowest pressure to which the gas can be expanded without danger of hydrate formation is obtained from the abscissa directly below this intersection. 

EXAMPLE 17-3 The pressure and temperature on a 0.7 specific gravity natural gas are 3,000 psia and 16 F. To what pressure can this gas be expanded without danger of hydrate formation and to what temperature will the gas be cooled as a result of the expansion ... 

A 0.70 specific gravity gas is expanded through a choke. Upstream conditions are 3000 psia and 150°F. What is the lowest pressure to which the gas can be expanded prior to onset of hydrate formation What is the temperature at this pressure ... 

Figure 17-7, page 481, Depression of Hydrate-formation Temperatures by Inhibitors... 

Gas molecules are transported to the interface. Long (1994) notes that the gas impingement rate is 1022 molecules/(cm2s) at the normal temperatures and pressures of hydrate formation. Kvamme (1996) indicates this step is transport of molecules through a stagnant boundary. 

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. 

For the upper temperature for hydrate formation, Makogon (1981) suggested a better criterion than the location of Q2 is the P-T condition at which the density of the combined hydrocarbon and water is equal to that of the hydrate. He assumed... 

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. 

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