Supercritical Storage

The basic premise of supercritical storage is that the contained fluid exists in a single phase. Thus, during operation, the stored fluid is not allowed to go subcritical, in which case it would exist as both vapor and liquid, in the fixed volume. Therefore, the operative process to maintain the fluid in a single phase is as follows The vessel is initially filled with liquid at atmospheric pressure, state point 1. After the vessel is filled and closed, heat is introduced to the contents, increasing vessel pressure. Since very little vapor initially exists in the vessel, the pressure increase can be considered essentially as a constant-entropy process. At the saturated liquid line, state point 2, the slope of the curve will increase slightly to reflect hydrostatic pressure build-up. A relatively small increase in heat input under this... 

Fig. 2. Pressure-enthalpy diagram for thermally pressurized supercritical storage.

Adsorption of supercritical gases takes place predominantly in pores which are less than four or five molecular diameters in width. As the pore width increases, the forces responsible for the adsorption process decrease rapidly such that the equilibrium adsorption diminishes to that of a plane surface. Thus, any pores with widths greater than 2 nm (meso- and macropores) are not useful for enhancement of methane storage, but may be necessary for transport into and out of the adsorbent micropores. To maximize adsorption storage of methane, it is necessary to maximize the fractional volume of the micropores (<2 nm pore wall separation) per unit volume of adsorbent. Macropore volume and void volume in a storage system (adsorbent packed storage vessel) should be minimized [18, 19]. 

Thus, while models may suggest optimal pore spuctures to maximize methane storage, they give no indication or suggestion as to how such a material might be produced. On the other hand, simple measurement of methane uptake from variously prepared adsorbents is not sufficient to elucidate the difference in the pore structure of adsorbents. Sosin and Quinn s method of determining a PSD directly from the supercritical methane isotherm provides an important and valuable link between theoretical models and the practical production of carbon adsorbents... 

The C02 can be stored in supercritical conditions, rising by buoyancy and can be physically held in a structural or stratigraphic trap, the same way as the natural accumulation of hydrocarbons occurs. The advantage of the capacity of containment system has been demonstrated by the retention of oil for millions of years. If the site is in production, it is used to increase the recovery of oil or gas (EOR recovery - enhanced oil, gas-enhanced recovery - EGR). These operations, EOR/EGR, provide an economic benefit that can offset the costs of the capture, transport and storage of C02. 

Another processing procediue that could involve supercritical fluid extraction with CO2 is the preparation of flavor concentrates from meat lipids for use in mixtures of other natural precursors for the preparation of tynthetic meat flavor additives that serve bofii as antioxidants that prevent warmed-over flavor (WOF) in cooked meat diuing storage and enhance the flavor of the natural products. 

Figure 9 Kawagoe 3 4 Ultra Supercritical, Double Reheat Power Generation System fired with LNG from the Middle East and Indonesia. The largest gas power plant in the world with a rated power of 4.7 GW electric - having a LNG storage tank capacity of 120 000 m3 (Courtesy Kawagoe plant, Japan 2002)...

The Cu-CI thermochemical cycle has been under development for several years. The goal is to achieve a commercially viable method for producing hydrogen at a moderate temperature ( 550°C). This chemical process, if successfully developed, could be coupled with several types of heat sources, e.g. the supercritical water reactor, the Na-cooled fast reactor or a solar heat source such as the solar power tower with molten salt heat storage. The use of lower temperature processes is expected to place less demand on materials of constmction compared to higher ( 850°C) temperature processes. 

Nanoporous carbonaceous materials are appropriate containers for storing molecules under strong confinement [196-207], In the case of the physical adsorption of methane with carbonaceous nanoporous materials, the attractive adsorption forces are only dispersive, therefore very weak hence, efficient carbonaceous materials for methane storage should have the highest possible specific surface areas and pore volume. However, the factor area is just not enough to get an efficient material, that is, the sizes of the pores are significant, especially when adsorption of a supercritical gas is concerned... 

Three different MgO samples were used in the experiments. AP-MgO with a surface area of 385 m2/g was prepared by a sol-gel technique involving high-temperature supercritical drying described in detail earlier.4,13 CP-MgO (281 m2/g) was obtained by decomposition of Mg(OH)2 prepared by hydration of commercial MgO. In both cases the final preparation step was overnight evacuation at 500°C followed by storage under ambient conditions. Their performance was compared to that of a commercial low surface area sample CM-MgO (10 m2/g). 

In geological storage, the C02 is injected as a supercritical fluid, with roughly half the density of water.17 To achieve the necessary supercritical pressures, C02 is injected at high pressure at least 800 m below the surface. 

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