Carbon dioxide scrubbing systems

In order to examine the hydrocarbon contaminant problem in an air separation plant, we may refer to Figure 1, Some contaminants in the entering air will be removed in the prepurification system, in the compressor interstage separators, in the caustic scrubber or other carbon dioxide removal system, in the adsorptive driers, and in the heat exchanger circuit, or in the regenerators of a low-pressure cycle. Those contaminants which pass these points will enter the high-pressure column where they will be washed into the enriched air stream and pass to the hydrocarbon adsorbers. The adsorbers will have substantial capacity for all hydrocarbons except methane. Most of the methane and traces of other hydrocarbons will pass on to the low-pressure column. They will be scrubbed down the column to the reboiler-condenser and, in a cycle as pictured, be continuously removed with the product oxygen. 

The scrubbing liquid must be chosen with specific reference to the gas being removed. The gas solubility in the liquid solvent should be high so that reasonable quantities of solvent are required. The solvent should have a low vapor pressure to reduce losses, be noncorrosive, inexpensive, nontoxic, nonflammable, chemically stable, and have a low freezing point. It is no wonder that water is the most popular solvent used in absorption devices. The water may be treated with an acid or a base to enhance removal of a specific gas. If carbon dioxide is present in the gaseous effluent and water is used as the scrubbing liquid, a solution of carbonic acid will gradually replace the water in the system. 

Multiple parallel SCWO reactors are used to process the accumulated hydrolysate held in the SCWO feed tank. Liquid effluent from the SCWO system containing inorganic salts is processed in an evaporator/crystallizer, where salts are concentrated into salt cakes for disposal and clean water is recycled. The gaseous effluent from the SCWO, containing primarily carbon dioxide and oxygen, is scrubbed, monitored, and filtered through activated carbon before being released to the atmosphere. 

Fluidized-bed combustion systems use a heated bed of sandlike material suspended (fluidized) within a rising column of turbulent air to burn many types and classes of waste fuels. The vendor claims that this technique results in improved combustion efficiency of high moisture content fuels and is adaptable to a variety of waste -type fuels. The scrubbing action of the bed material on the fuel particle is said to enhance the combustion process by stripping away the carbon dioxide and char layers that normally form around the fuel particle. This allows oxygen to reach the combustible material much more readily and increases the rate and efficiency of the combustion process. 

The plant incorporating the air cathode electrolyzer must include a high performance air scrubbing system to eliminate carbon dioxide from the air. Failure to remove C02 adequately results in the precipitation of sodium carbonate in the pores of the cathode this, in turn, affects the transport of oxygen and hydroxide witliin the electrode. Left unchecked, the accumulation of sodium carbonate will cause premature failure of the cathodes. 

When organic fuels are burned, carbon dioxide and water vapor are released along with various amounts of sulfur dioxide and nitrogen oxides. The sulfur and nitrogen oxides in the atmosphere are then further oxidized with the assistance of ultraviolet solar radiation when these gases are scrubbed from the air by precipitation, a dilute solution of sulfuric acid and nitric acid forms. Carbon dioxide itself hydrolyzes to carbonic acid and is important in the marine carbonate buffer system however, it is a weak organic acid and atmospheric concentrations typically lower the pH of distilled water only to about 5.7 (5-6). 

The system has two circulating loops, one for the anolyte solution and one for the catholyte solution. In the anolyte loops, Ce(III) is oxidized to Ce (IV) in the T-cell and passed through the reaction chamber where the organic wastes are introduced gradually. Carbon is converted to carbon dioxide chlorine compounds are converted to elemental chlorine, which is scrubbed and converted to hypochlorite sulfur and other elements are converted to salts, such as sulfates. These salts remain in anolyte solution, which must be periodically replaced as the concentration of the salts increases. 

In a conventional combustion process, such as a power station boiler, the carbon dioxide is contained in the exit flue gas. The concentration is generally fairly low and ranges from about 4 vol.% for a combined-cycle system operating on natural gas to 12-14 vol.% for a traditional boiler fired by pulverized coal. The exhaust can be scrubbed with an amine solution, typically monoethanolamine, which is then heated to release the absorbed carbon dioxide. The low concentration of carbon dioxide in the exit gas means that a huge volume would have to be handled and this would entail the installation... 

The problem of sulfur dioxide pollution is made more complicated by the energy crisis. As petroleum supplies dwindle and the price increases, our dependence on coal will probably grow. As supplies of low-sulfur coal are used up, high-sulfur coal will be utilized. One way to use high-sulfur coal without further harming the air quality is to remove the sulfur dioxide from the exhaust gas by means of a system called a scrubber before it is emitted from the power plant stack. A common method of scrubbing is to blow powdered limestone (CaC03) into the combustion chamber, where it is decomposed to lime and carbon dioxide ... 

The formaldehyde + water + methanol system is just one example for many technically important systems, which are intrinsically chemically reactive. Other examples include aqueous solutions of weak electrolytes, such as amine solutions used to scrub carbon dioxide and other sour gases from gaseous streams, or the solutions containing bases used for chemical extraction of acids from aqueous streams. In many of these cases, the key to the development of predictive thermodynamic models is a quantitative model of the often complex chemical reactions in those mixtures. The necessary information often only can be obtained using spectroscopic methods [25]. 

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