Carbon chemical absorption

While the carbon dioxide/caiistic test method has become accepted, one should use the results with caution. The chemical reaction masks the effect of physical absorption, and the relative values in the table may not hold for other cases, especially distillation applications where much of the resistance to mass transfer is in the gas phase. Background on this combination of physical and chemical absorption may Be found earher in the present section, under Absorption with Chemical Reaction. ... 

There are two main schemes proposed for sequestration of carbon dioxide. The first (referred to as a chemical absorption process), suitable for use at low pressures and temperatures, is usually adopted where the CO2 is to be removed from exhaust flue gases. The second (usually referred to as a physical absorption process), for use at higher pressures, is recommended for separation of the CO2 in syngas obtained from conversion of fuel. 

Fig. 8.7 shows a second example (Cycle A2) of carbon dioxide removal by chemical absorption from a CCGT plant, but one in which the semi-closed concept is introduced— exhaust gas leaving the HRSG is partially recirculated. This reduces the flow rate of the gas to be treated in the removal plant, so that less steam is required in the stripper and the extra equipment to be installed is smaller and cheaper. This is also due to the better removal efficiency achievable—for equal reactants flow rate—when the volumetric fraction of CO2 in the exhaust gas is raised from the 4-6% value typical of open cycle gas turbines to about 12% achievable with semi-clo.sed operation. 

Yagi, H., Iwazawa, A., Sonobe, R., Matsubara, T. and Hikita, H., 1984. Crystallization of calcium carbonate accompanying chemical absorption. Industrial and Engineering Chemistry Fundamentals, 23, 153-158. 

Exit gases from the shift conversion are treated to remove carbon dioxide. This may be done by absorbing carbon dioxide in a physical or chemical absorption solvent or by adsorbing it using a special type of molecular sieves. Carbon dioxide, recovered from the treatment agent as a byproduct, is mainly used with ammonia to produce urea. The product is a pure hydrogen gas containing small amounts of carbon monoxide and carbon dioxide, which are further removed by methanation. 

Mass-transfer rates have been determined by measuring the absorption rate of a pure gas or of a component of a gas mixture as a function of the several operating variables involved. The basic requirement of the evaluation method is that the rate step for the physical absorption should be controlling, not the chemical reaction rate. The experimental method that has gained the widest acceptance involves the oxidation of sodium sulfite, although in some of the more recent work, the rate of carbon dioxide absorption in various media has been used to determine mass-transfer rates and interfacial areas. 

Removal of sulfur from gas streams is generally removal of H2S or removal of S02n. Generally, removal of the sulfur in the form of H2S is much more straightforward than in the form of S02. H2S can be removed by absorption, as already discussed. Chemical absorption using amines is the most commonly used method. However, other solvents can be used for chemical absorption, for example, potassium carbonate. Physical absorption is also possible using solvents such as propylene carbonate and methanol. 

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. 

The development of a commercial mass spectrometer and its application to hydrocarbon gas analysis by the method of Washburn et al. (63) made gas analysis rapid, economical, and, what is even more important, inspired a confidence in the results of routine hydrocarbon gas analysis which was badly lacking. A complex gaseous mixture comprising the atmospheric gases, carbon monoxide, and Ci to C6 hydrocarbons required more than 20 hours of applied time by the previous methods of low temperature fractional distillation coupled with chemical absorption methods. With the mass spectrometer such an analysis is completed in 2 hours or less, about 15 minutes of which is consumed in the... 

Absorption of atmospheric gases Chemical oxidation due to exposure to oxygen in air Change in pH and conductivity due to carbon dioxide absorption Prompt and proper preservation Measurements of field parameters during sampling... 

In the chemical absorption process, the C02 reacts with chemical solvents to form a weakly-bonded intermediate compound that is then broken down by the application of heat. The heat regenerates the original solvent and produces a CO2 stream. Typical solvents are amine- or carbonate-based. Examples are MEA, diethanolamine (DEA), ammonia and hot potassium carbonate. These processes can be used at low C02 partial pressures, but the feed gas must be free of S02, 02, hydrocarbons and particulates. Hydrocarbons and particulates cause operating problems in the absorber199. 

In this study, a and kLa are measured in a TBR in the pressure range [0.3-3.2 MPa] using fast and slow chemical absorption of carbon dioxide into diethanolamine (DEA) aqueous and organic solutions. Only the trickling regime and trickling/pulsing transition have been explored. A simple model to explain the increase of interfacial area and mass transfer... 

Interpretation of 2 D NM R data obtained for cortistatin B (11) showed that it had the same skeleton as 10, but contained a hydroxyl group on C-16 with the (3 configuration. The carbon chemical shift value of 8 214.4 and the IR absorption at 1740 cm indicated the existence ofa ketone at C-16 in cortistatin C (12). Cortistatin D (13) was found to be a 17-hydroxy analog of cortistatin C, which was confirmed by "C NMR deuterium shift experiments as well as HMBC data. Analysis of NOESY data for 13 revealed the orientation of hydroxyl group at C-17 as a [13]. 

Carbon Dioxide Absorption with Chemical Absorbents... 

In the next step the carbon dioxide, mostly from the conversion reaction, and the hydrogen sulfide, if present, are removed from the gas mixture. This is accomplished either by physical or chemical absorption in appropriate solvents. 

Combinations of physical and chemical absorption are also used, as in Shell s Sulfinol process in which a mixture of diisopropanolamine and sulfolane in water is utilized. For the hydrogen sulfide-free gases from the steam-reforming process, chemical scrubbing with activated potassium carbonate solutions or alkanolamines is preferred. In the case of hydrogen sulfide-containing gases from the partial oxidation process, physical absorption alone or in combination with chemical absorption is preferred. 

Chemical absorption in scrubbers, which employ any of water, nitric acid, alkaline solutions, or solutions of urea in water can reduce tail gas NOx concentrations to below 200 ppm [55]. Physical adsorption on molecular sieves is also a feasible control method. Activated carbon cannot be used because of oxidation hazards. Chemical and physical adsorption were improved if both nitric oxide and nitrogen dioxide were present in the tail gas rather than just nitric oxide, perhaps from the formation of dinitrogen trioxide (N2O3). An advantage of any absorption or adsorption system is that the NOx collected is recovered in some form. 

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