Propane-water system

Carson and Katz5 studied another part of the methane-propane-water system. These authors investigated its behavior when an aqueous liquid, a hydrocarbon liquid, a gas, and some solid were present. It was found that the system was univariant so that the solid consisted of a single phase only. This phase is a hydrate which proved to contain methane and propane in various ratios. They then concluded that these hydrates behaved as solid solutions. It is clear that Carson and Katz measured a part of the four-phase line HllL1L2G. 

Propane - Water System. The interaction parameters for the propane - water system were obtained over a temperature range from 42°F to 310°F using exclusively the data of Kobayashi and Katz (24). This is because among the available literature on the phase behavior of this binary system, their data appear to give the most extensive information. A constant interaction parameter was obtained for the propane-rich phases at all temperatures. The magnitude of the temperature - dependent interaction parameter for this binary was less than that for the ethane - water binary at the same temperature. Azarnoosh and McKetta (25) also presented experimental data for the solubility of propane in water over about the same temperature range as that of Kobayashi and Katz but at pressures up to 500 psia only. However, a different set of temperature - dependent parameters... 

Figure 5.14 Pressure vs. temperature diagram for propane + water system.

Figure 5.15 is the pseudo-binary pressure versus excess water composition diagram for the methane+propane+water system at a temperature of 277.6 K. At 277.6 K the hydrate formation pressures are 4.3 and 40.6 bar for pure propane (sll) and pure methane (si) hydrates, respectively, as shown at the excess water composition extremes in Figure 5.15. As methane is added to pure propane, there will be a composition at which the incipient hydrate structure changes from sll to si as seen in the inset of Figure 5.15, this composition is predicted to be 0.9995 mole fraction methane in the vapor—a very small amount of propane added to a methane+water mixture will form sll hydrates.

Of the possible binary combinations of methane, ethane, and propane, the methane + propane + water system (Figure 5.15) is the simplest. 

Figure 5.15 Pseudo-P- diagram for methane + propane + water system at 277.6 K.

Figure 5.16 is the pseudo-binary pressure versus excess water composition diagram for the methane + ethane + water system at a temperature of 277.6 K. In the diagram, pure ethane and pure methane both form si hydrates in the presence of water at pressures of 8.2 and 40.6 bar, respectively. Note that between the compositions of 0.74 and 0.994 mole fraction methane, sll hydrates form at the incipient formation pressure. Similar to the methane + propane + water system, only a small amount of ethane added to pure methane will form sll hydrates. 

Figure 5.17 shows a predicted pressure versus excess water composition plot for the ethane+propane+water system at 274 K. At 0.0 mol fraction ethane (propane+ water) sll form at approximately 2 bar, and at 1.0 mol fraction ethane (ethane + water) si form at approximately 5 bar. At the intermediate composition of 0.78 mole fraction ethane, a quadruple point (Aq-sI-sII-V) exists in which both incipient hydrate structures are in equilibrium with vapor and aqueous phase. This point will be referred to as the structural transition composition the composition at which the incipient hydrate formation structure changes from sll to si at a given temperature. 

Figure 5.17 Pseudo-P-x diagram for ethane + propane + water system at 274 K.

As the temperature is increased to 277.6 K the pressure versus composition diagram for the ethane + propane + water system changes drastically as shown in Figure 5.18 Between 0.0 and 0.6 mole fraction of ethane, the incipient hydrate structure is sll hydrate. However, if the pressure is increased to approximately 11.45 bar, between 0.3 and 0.6 mol fraction ethane, sll is predicted to dissociate to form an Aq-V-Lhc region. 

Figure 5.20 is a pseudo-ternary phase diagram for the methane + ethane + propane + water system at a temperature and pressure of 277.6 K and 10.13 bar,... 

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. 

Verma, V.K., Hand, J.H., Katz, D.L., in Gas Hydrates from Liquid Hydrocarbons Methane-Propane-Water System, AIChE-VTG Joint Meeting, Munich, September, p. 106... 

Typical results for the HLiG equilibrium are shown for the carbon dioxide-propane-water system and the methane-isobutane water system in Figure 11. The predicted and experimental pressures are compared at the experimentally determined hydrate temperature. The mixtures of carbon dioxide and propane included concentrations from 6 to 84 mole percent propane on a water-free basis in the gas phase and the mixtures of methane and isobutane included concentrations from 0.4 to 63.6 mole percent isobutane on the same basis. It will be seen that the predicted and experimental results compare favorably... 

Recent experiments2 on the equilibrium Hu ice gas at — 3°C in the system HaS-propane-water confirm that these two gases also form mixed hydrates of variable composition, as shown in Fig. 10. In this respect the present system is similar to the system me thane-propane-water of Fig. 7, but unlike the latter it exhibits a minimum pressure (azeotrope). It was further shown that the solution theory of clathrates can account for this interesting phenomenon. For details the reader is referred to ref. 29. 

Water, electronic correlation, 324 methane-propane ternary system, 23 superposition of configuration, 295 Wigner s theory, cellular method," 252, 304, 306, 318... 

The interaction parameters for binary systems containing water with methane, ethane, propane, n-butane, n-pentane, n-hexane, n-octane, and benzene have been determined using data from the literature. The phase behavior of the paraffin - water systems can be represented very well using the modified procedure. However, the aromatic - water system can not be correlated satisfactorily. Possibly a differetn type of mixing rule will be required for the aromatic - water systems, although this has not as yet been explored. 

As mentioned, RMP addresses specific chemicals/materials (compounds) it addresses the accidental release of over one hundred chemical substances. Of the RMP chemicals listed, seventy-seven include acutely toxic chemical compounds and sixty-three flammable gases. Threshold quantity levels range from 500 pounds to 20,000 pounds. USEPA estimates that approximately 100,000+ sources are covered by the rule. The universe includes chemical and most other manufacturers, certain wholesalers and retailers, drinking-water systems, wastewater treatment works, ammonia refrigeration systems, chemical wholesalers and end users, utilities, propane retailers, and federal facilities. 

A pilot-scale demonstration remediating harbor sediment was conducted 1 year before the SITE demonstration. Based on the pilot-scale demonstration, the processing costs for a fuU-scale, 110-ton/day unit were projected to be 230/ton (September 1992 U.S. dollars). It is assumed that the unit will be down approximately 30% of the time for maintenance and design improvements in the first year of operation. Based on this system availability, 28,105 tons can be processed in one year. This cost included estimates for variable costs, fixed costs, and deprecia-tion/insurance. Variable costs include diesel fuel for a mobile generator, hydrogen, and caustic. Fixed costs include labor diesel fuel for pumps, heaters, process equipment, and instrumentation propane, water and sewer and parts and supplies. Depreciation/insurance costs include capital cost depreciated over a 3-year period, general insurance costs, and pollution liabihty insurance. This analysis does not include costs for setup and demobilization (D128007, pp. 5.12-5.14). 

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. 

Figure 7.1-5. Pressure-temperature diagram of the system propane/water [13]. a, Propane gas/water b, propane gas/propane hydrate c, propane liquid/water d, propane liquid/propane hydrate e, propane gas/ice f, hydrate curve g, vapor pressure curve of propane.

Figure 4.2b shows the equivalent of Figure 4.2a to be slightly more complex for systems such as ethane + water, propane + water, isobutane + water, or water with the two common noncombustibles, carbon dioxide or hydrogen sulfide. These systems have a three-phase (Lw-V-Lhc) line at the upper right in the diagram. This line is very similar to the vapor pressure ( V-Lhc) line of the pure hydrocarbon, because the presence of the almost pure water phase adds a very low vapor pressure (a few mmHg at ambient conditions) to the system. 

Holder, G.D., Multi-Phase Equilibria in Methane-Ethane-Propane-Water Hydrate Forming Systems, Ph.D. Thesis, University of Michigan, University Microfilms No. 77-7939, Ann Arbor, MI 48106, (1976). 

The most productive two-phase (H-V or H-Lhc) equilibrium apparatus was developed by Kobayashi and coworkers. The same apparatus has been used for two-phase systems such as methane + water (Sloan et al., 1976 Aoyagi and Kobayashi, 1978), methane + propane + water (Song and Kobayashi, 1982), and carbon dioxide + water (Song and Kobayashi, 1987). The basic apparatus described in Section 6.1.1.2 was used in a unique way for two-phase studies. With two-phase measurements, excess gas was used to convert all of the water to hydrate at a three-phase (Lw-H-V) line before the conditions were changed to temperature and pressures in the two-phase region. This requires very careful conditioning of the hydrate phase to prevent metastability and occlusion. Kobayashi and coworkers equilibrated the hydrate phase by using the ball-mill apparatus to convert any excess water to hydrate. 

Corresponding data for the propane/AOT/water system at 25 C are presented in Figure 8 for W - 1, 5, and 20. In a single phase at W 1 (a) hydrodynamic diameter is nearly invarient with pressure (3.8 0.3 nm) with a slight increase suggested at the very lowest pressures. In a single phase system at W 5 (b),... 

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