Carbon dioxide recovery systems

Conventional carbon dioxide recovery systems fall into the following categories ... 

Mimura, T., Simayoshi, H., Suda, T., Iijima, M., and Mituoka, S. Development of energy saving technology for flue gas carbon dioxide recovery in power plants by chemical absorption method and steam system, Energ. Corners, and Manag., 38(Suppl.), S57-S62, 1997. 

Although carbon dioxide must be generated on site for some processes, there is a trend toward carbon dioxide recovery where it is a major reaction by-product and, in the past, has been vented to the atmosphere. An absorption system, such as the use of ethanolamines (q.v.) or hot carbonate or bicarbonate solutions, is used for concentrating the carbon dioxide to over 99% purity. 

The spiral-wound configuration tFigure 17-lQ is more conplicated but has a significandy higher surface area per unit volume. With proper design of the channels there will be significant turbulence at the membrane surface that promotes mass transfer. These systems have been used for carbon dioxide recovery, UF of relatively clean solutions and RO. 

Gas permeation systems typically use hollow-fiber or spiral-wound membranes, although hollow-fiber systems are more common tBaker. 2004k Cellulose acetate membranes are used for carbon dioxide recovery, polysulfone coated with silicone rubber is used for hydrogen purification, and conposite membranes are used for air separation. The feed gas is forced into the membrane module under pressure. Retentate, which does not go through the membrane, will become concentrated in the less permeable gas. Retentate exits at a pressure that will be close to the input pressure. The more permeable species will be concentrated in permeate. Permeate, which has passed through the membrane, exits at low pressure. The operating cost for a gas permeator is the cost of conpression of the feed gas and the irreversible pressure difference that occurs for the gas that permeates the membrane. A typical hollow-fiber unit will contain 5000 m membrane area per m at a cost of approximately 200/m. ... 

The heating is carried out in a rotating drum by internal steam pipes. The inlet and outlet points are sealed to prevent either loss of the recovery gases or air getting in to dilute the carbon dioxide. The system allows for precise temperature control which is economic in fuel and reduces damage to the drum. 

The appHcation that has led to increased interest in carbon dioxide pipeline transport is enhanced oil recovery (see Petroleum). Carbon dioxide flooding is used to Hberate oil remaining in nearly depleted petroleum formations and transfer it to the gathering system. An early carbon dioxide pipeline carried by-product CO2 96 km from a chemical plant in Louisiana to a field in Arkansas, and two other pipelines have shipped CO2 from Colorado to western Texas since the 1980s. EeasibiHty depends on cmde oil prices. 

The choice of a specific CO2 removal system depends on the overall ammonia plant design and process integration. Important considerations include CO2 sHp required, CO2 partial pressure in the synthesis gas, presence or lack of sulfur, process energy demands, investment cost, availabiUty of solvent, and CO2 recovery requirements. Carbon dioxide is normally recovered for use in the manufacture of urea, in the carbonated beverage industry, or for enhanced oil recovery by miscible flooding. 

The steam balance in the plant shown in Figure 2 enables all pumps and blowers to be turbine-driven by high pressure steam from the boiler. The low pressure exhaust system is used in the reboiler of the recovery system and the condensate returns to the boiler. Although there is generally some excess power capacity in the high pressure steam for driving other equipment, eg, compressors in the carbon dioxide Hquefaction plant, all the steam produced by the boiler is condensed in the recovery system. This provides a weU-balanced plant ia which few external utiUties are required and combustion conditions can be controlled to maintain efficient operation. 

Sodium hydrosulfite is produced through the Formate process where sodium formate solution, sodium hydroxide, and liquid sulfur dioxide reacted in the presence of a recycled stream of methanol solvent. Other products are sodium sulfite, sodium bicarbonate, and carbon monoxide. In the reactor, sodium hydrosulfite is precipitated to form a slurry of sodium hydrosulfite in the solution of methanol, methyl formate, and other coproducts. The mixture is sent to a pressurized filter system to recover sodium hydrosulfite crystals that are dried in a steam-heated rotary drier before being packaged. Heat supply in this process is highly monitored in order not to decompose sodium hydrosulfite to sulfite. Purging is periodically carried out on the recycle stream, particularly those involving methanol, to avoid excessive buildup of impurities. Also, vaporized methanol from the drying process and liquors from the filtration process are recycled to the solvent recovery system to improve the efficiency of the plant. 

Carbon dioxide permeates the membranes at least as readily as does chlorine. This fact will produce a significant increase in CO2 concentration in a recycle system. For best results at highest chlorine recovery, it will pay to keep the CO2 concentration in the membrane feed-gas low. This can be accomplished in most plants by acidification of cell-feed brine. Acidification is highly recommended in any case when a very high degree of chlorine recovery is required, whether by... 

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