Absorption of SO2 by solids

As an example, consider the dry absorption of SO2 by magnesium dioxide. The solid magnesium dioxide is brought into contact with SO2 and magnesium sulfate is... 

Liquid Dispersion Spray columns are used with slurries or when the reaction product is a solid. The absorption of SO2 by a lime slurry is an example. In the treatment of phosphate rock with sulfuric acid, offgases contain HF and SiF<. In a spray column with water, solid particles of fluorosilic acid are formed but do not harm the spray operation. The coefficient ki in spray columns is about the same as in packed columns, but the spray interfacial area is much lower. Considerable backmixing of the gas also takes place, which helps to make the spray volumetri-cally inefficient. Deentrainment at the ouUet usually is needed. 

An important gas-solid reaction for wHich porosity decreases with time of exposure to the reactant gas is the absorption of SO2 by calcined limestone (CaO) or dolomite (CaO/MgO) to produce calcium sulfate (CaSO or CaSO./MgO), which has a larger molar volume than the reactant solid. If diffusional limitations in the pores are important, the porosity decrease will be greater near the surface of the solid particle and pores will plug, limiting access to part of the reactant solid. 

Two versions of the ACP have been developed (1) an open-loop configuration in which the dry spent absorbent is simply removed from the system for disposal and fresh alkali is continuously fed to the spray dryer, and (2) a closed-loop process in which the spent absorbent is regenerated and reused. The open-loop system was installed on the 410 MW, Coyote Station at Beulah, North Dakota (Botts et al., 1978). The plant used a spray dryer followed by a fabric filter for simultaneous SO2 and dust removal. As indicated by the flow diagram of Figure 7-32, the process is extremely simple. Because the particulate collected on the fabric filter bags remains on the fobric for a period of time, the gas-solids contact time is extended beyond the particulate residence time in the spray diyo so additional absorption of SO2 by the alkaline material occurs. After this system went conunercial in 1981. no other large soda ash spray dryers were built. About 1990, this scrubber was converted finm soda ash to lime to eliminate solids build-up in the dryer vessel and to use lower cost lime. The... 

In summary, the absorption of SO2 by lime is a rather complex gas-solid reaction. Under conditions likely to be encountered in electric power generation the reaction would appear to be controlled by pore diffusion. Considerable complication is introduced by the fact that the pore structure of the lime will depend on the time-temperature history of the lime during (and perhaps subsequent to) calcination. Furthermore, reaction is accompanied by considerable changes in pore structure, which may cut short the reaction before all the lime has reacted [79, 119]. 

By this model,they(18) were able to explain the effect of solid particles for the absorption of SO2 into aqueous slurries of CaS03,... 

The secondary source of fine particles in the atmosphere is gas-to-particle conversion processes, considered to be the more important source of particles contributing to atmospheric haze. In gas-to-particle conversion, gaseous molecules become transformed to liquid or solid particles. This phase transformation can occur by three processes absortion, nucleation, and condensation. Absorption is the process by which a gas goes into solution in a liquid phase. Absorption of a specific gas is dependent on the solubility of the gas in a particular liquid, e.g., SO2 in liquid H2O droplets. Nucleation and condensation are terms associated with aerosol dynamics. 

Von Getler et al. [23] were the first to consider these processes during spray absorption drying when they examined the material system of SO2 and Ca(OH)2. In the opinion of these authors, the absorption of sulfur dioxide is limited either by the dissolution of solid or by the gas-phase mass transfer in the first drying period of the drop. The product of this reaction causes substantial diffusion resistance for the absorbed sulfur dioxide and thus obstructs further reactions as the calcium hydroxide remains in the core. 

Lime/limestone slurry scrubbing is the dominant commercial technology for flue gas desulfurization 0.). SO2 is absorbed at 50-55°C and pH 5.5-6.0 in an aqueous slurry of excess CaC03 and product solids. The CaS03/CaS04 product is disposed of as solid waste. With greater than 500-1000 ppm SO2 in the flue gas, SO2 absorption is controlled by liquid-film mass transfer resistance because of the limited solubility of SO2 gas and alkaline solids. Additives that buffer between pH 3 and pH 5.5 enhance S02 absorption by providing dissolved alkaline species for reaction with SO2 (8). 

IR absorption bands in amorphous solids are rather broad, with extensive overlap. Direct identification and quantification of the numerous oxidation products, most often of similar chemical structures, in a degraded polymer are difficult. More precise conclusions about the nature of absorbing species can be obtained by selective chemical derivatization. Selective modification of functional groups with reactive gases, such as SF4, NH3, SO2, or NO, results in a shift in absorption band positions, which can then be compared with model compounds to allow for a better chemical assignment of the absorbing species (Table 15.3) [13]. 

Absorption is a process that relies on a solvent s chemical affinity with a solute to dissolve preferably one species into another. It is widely proposed for CO2 separation where a solvent, generally, monoethanolamine (MEA) or a solid absorbent like lithium zirconate is used to dissolve CO2, but not the other components of a flue gas stream. C02-rich solution is typically pumped to a regeneration column, where CO2 is stripped out from the solution and the solvent recycled for a new batch of flue gas. The absorption equipment should be placed after the flue gas desulfurization-step and before the stack. Optimal conditions for absorption are low temperature and high pressure, making this the best location for absorption to occur. In addition, most solvents are easily degraded by compounds such as fly ash, other particulates, 80, (SO2, SO3) and (NO, NO2), so the absorption step must take place after electrostatic precipitation and desulfurization. In a typical absorption process, the C02-lean flue gas is either emitted to the atmosphere or possibly used in other applications e.g. chemical production). 

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