Separation plant

Air separation plants produce about 99% of the gas, while electrolysis plants produce about 1%. 

Chemical Conversion. In both on-site and merchant air separation plants, special provisions must be made to remove certain impurities. The main impurity of this type is carbon monoxide, CO, which is difficult to separate from nitrogen using distiHation alone. The most common approach for CO removal is chemical conversion to CO2 using an oxidation catalyst in the feed air to the air separation unit. The additional CO2 which results, along with the CO2 from the atmosphere, is then removed by a prepuritication unit in the air separation unit. 

A, 5A, and 13X zeoHtes are the predorninant adsorbents for CO2 removal by temperature-swing processes. The air fed to an air separation plant must be H2O- and C02-ftee to prevent fouling of heat exchangers at cryogenic temperatures 13X is typically used here. Another appHcation for 4A-type zeoHte is for CO2 removal from baseload and peak-shaving natural gas Hquefaction faciHties. 

The Eastman Chemicals from Coal faciUty is a series of nine complex interrelated plants. These plants include air separation, slurry preparation, gasification, acid gas removal, sulfur recovery, CO /H2 separation, methanol, methyl acetate, and acetic anhydride. A block flow diagram of the process is shown in Eigure 3. The faciUty covers an area of 2.2 x 10 (55 acres) at Eastman s main plant site in Kingsport, Teimessee. The air separation plant is... 

The latest of three ethylene recovery plants was started in 1991. Sasol sold almost 300,000 t of ethylene in 1992. Sasol also produces polypropylene at Secunda from propylene produced at Sasol Two. In 1992 Sasol started constmction of a linear alpha olefin plant at Secunda to be completed in 1994 (40). Initial production is expected to be 100,000 t/yr pentene and hexene. Sasol also has a project under constmction to extract and purify krypton and xenon from the air separation plants at Sasol Two. Other potential new products under consideration at Sasol are acrylonitrile, acetic acid, acetates, and alkylamines. 

Medium Heat- Value Gas. Medium heat-value (medium Btu) gas (6,7) has a heating value between 9 and 26 MJ/m (250 and 700 Btu/fT). At the lower end of this range, the gas is produced like low heat-value gas, with the notable exception that an air separation plant is added and relatively pure oxygen (qv) is used instead of air to partially oxidize the coal. This eliminates the potential for nitrogen in the product and increases the heating value of the product to 10.6 MJ /m (285 Btu/fT). Medium heat-value gas consists of a mixture of methane, carbon monoxide, hydrogen, and various other gases and is suitable as a fuel for industrial consumers. 

Fig. 5. Production of helium-group gases in a classical air-separation plant.

Commercially pure (< 99.997%) helium is shipped directiy from helium-purification plants located near the natural-gas supply to bulk users and secondary distribution points throughout the world. Commercially pure argon is produced at many large air-separation plants and is transported to bulk users up to several hundred kilometers away by tmck, by railcar, and occasionally by dedicated gas pipeline (see Pipelines). Normally, only cmde grades of neon, krypton, and xenon are produced at air-separation plants. These are shipped to a central purification faciUty from which the pure materials, as well as smaller quantities and special grades of helium and argon, are then distributed. Radon is not distributed commercially. 

Russia, nitrogen (qv) from the adjacent air-separation plant, and reformed gas with the purified fuel gas stream from the plant. 

Nitrogen is used for pressure maintenance in oil and gas reservoirs for enhanced recovery. It is sometimes used as a miscible agent to reduce oil viscosity and increase recovery in deep reservoirs. Other appHcations include recovery of oil in attic formations, gas cap displacement, and a sweep gas for miscible CO2 slugs. Nitrogen competes with CO2, a more miscible gas with hydrocarbons (qv), in most of these appHcations. The production mode is typically by on-site cryogenic separation plants. In 1990, nitrogen production in enhanced recovery operations was 20 x 10 m /d (750 million SCF/d)... 

Uranium oxide [1344-57-6] from mills is converted into uranium hexafluoride [7783-81-5] FJF, for use in gaseous diffusion isotope separation plants (see Diffusion separation methods). The wastes from these operations are only slightly radioactive. Both uranium-235 and uranium-238 have long half-Hves, 7.08 x 10 and 4.46 x 10 yr, respectively. Uranium enriched to around 3 wt % is shipped to a reactor fuel fabrication plant (see Nuclear REACTORS, NUCLEAR FUEL reserves). There conversion to uranium dioxide is foUowed by peUet formation, sintering, and placement in tubes to form fuel rods. The rods are put in bundles to form fuel assembHes. Despite active recycling (qv), some low activity wastes are produced. 

Media Si. Media are suppHed in several size grades and the grade used varies at each plant. The finer grades improve media stabiUty, but finer particles are more difficult to recover and the feed rate of these finer-grade slurries should be reduced by a factor of 0.5—0.75 to maintain magnetic recovery. A typical size analysis as used in various heavy-media separation plants treating coal (qv) is given in Table 4. 

The electromagnetic separation plant built during World War 11 at Oak Ridge, involved two types of calutrons, alpha and beta. The larger alpha calutrons were used for the enrichment of natural uranium, and the beta calutrons were used for the final separation of U from the pre-enriched alpha product. For the electromagnetic separation process, UO was converted into UCl [10026-10-5] with CCl. The UCl was fed into the calutron for separation. The calutron technique has been used to separate pure samples of and stable isotopes of many other elements. The Y-12 calutron... 

Chemical exchange between hydrogen and steam (catalyzed by nickel—chromia, platinum, or supported nickel catalysts) has served as a pre-enrichment step in an electrolytic separation plant (10,70). If the exchange could be operated as a dual-temperature process, it very likely... 

Both air and oxygen processes can be designed to be comparable in the following areas product quaUty, process flexibiUty for operation at reduced rates, and on-stream rehabiUty (97,182). For both processes, an on-stream value of 8000 h/yr is typical (196). The rehabiUty of the oxygen-based system is closely linked to the rehabiUty of the air-separation plant, and in the air process, operation of the multistage air compressor and power recovery from the vent gas is cmcial (97). 

For the same production capacity, the oxygen-based process requires fewer reactors, all of which operate in parallel and are exposed to reaction gas of the same composition. However, the use of purge reactors in series for an air-based process in conjunction with the associated energy recovery system increases the overall complexity of the unit. Given the same degree of automation, the operation of an oxygen-based unit is simpler and easier if the air-separation plant is outside the battery limits of the ethylene oxide process (97). 

Even the best modern low-temperature air separation plant has an efficiency only a small fraction of the theoretical optimum, that is, about 15 to 20 percent. The principal sources of inefficiency are threefold (1) the nonideality of the refrigerating process, (2) the imperfection of the heat exchangers, and (3) losses of refrigeration through heat leak. 

Experience in air separation plant operations and other ciyogenic processing plants has shown that local freeze-out of impurities such as carbon dioxide can occur at concentrations well below the solubihty limit. For this reason, the carbon dioxide content of the feed gas sub-jec t to the minimum operating temperature is usually kept below 50 ppm. The amine process and the molecular sieve adsorption process are the most widely used methods for carbon dioxide removal. The amine process involves adsorption of the impurity by a lean aqueous organic amine solution. With sufficient amine recirculation rate, the carbon dioxide in the treated gas can be reduced to less than 25 ppm. Oxygen is removed by a catalytic reaction with hydrogen to form water. 

While comparison of the absolute capital costs and costs of electricity among different power systems is difficult and uncertain, the structure of these costs is rather typical, and the costs of component units are usually within known ranges. For an oxygen-blown IGCC power system, the breakdown of the capital cost for the four component units is air separation plant (11 to 17 percent), fuel gas plant (33 to 42 percent), combined-cycle unit (32 to 39 percent), and balance of plant (2 to 21 percent). The breakdown of the cost of elec tricity is capital charge (52 to 56 percent), operating and maintenance (14 to 17 percent), and fuel (28 to 32 percent). 

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