Chemical absorption systems

Unfortunately, the analysis of chemical absorption is far more complex than physical absorption. The vapor-liquid equilibrium behavior cannot be approximated by Henry s Law or any of the methods described in Chapter 4. Also, different chemical compounds in the gas mixture can become involved in competing reactions. This means that simple methods like the Kremser equation no longer apply and complex simulation software is required to model chemical absorption systems such as the absorption of H2S and C02 in monoethanolamine. This is outside the scope of this text. 

There is practically nothing about the high-pressure liquid-side-mass transfer coefficient, ku in TBR in the open literature. The only paper published was that of Lara-Marquez et al. [57], The values of kia are determined by using the following chemical absorption systems in the slow reaction regime ... 

It can be concluded from the simple analysis above that impinging streams can only be used for gas-liquid reaction or chemical absorption systems involving fast reaction(s) in liquid for success. 

Absorption can be either physical or chemical. In physical absorption the gas is removed because it has greater solubility in the solvent than other gases. An example is the removal of butane and pentane (C4 -C5) from a refinery gas mixture with a heavy oil. In chemical absorption the gas to be removed reacts with the solvent and remains in solution. An example is the removal of CO2 or H2S by reaction with NaOH or with monoethanolamine (MEA). The reaction can be either irreversible (as with NaOH) or reversible (as with MEA). For irreversible reactions the resulting liquid must be disposed of, whereas in reversible reactions the solvent can be regenerated (in stripper or distillation columns). Thus, reversible reactions are often preferred. Chemical absorption systems are discussed in more detail by Astarita et al. (1983), Kister et al. ( 20081 Kohl (198Z), Kohl and Nielsen (1995), and Zarycki and Chacuk (1993). 

The post-combustion technologies generally consist of chemical absorption systems that are operated at almost atmospheric pressure and with amine-based solvent. The advantage of this technique is the possibility to retrofit existing power plants, but high efficiency... 

A numerical model accounts for the variation in liquid film thickness and back mixing effects. It can be used for a wide range of process conditions in micro-structured falling-film contactors. It enables also investigation of physical and chemical absorption systems with slower reaction kinetics, in which the mass-transfer resistance is concentrated in the liquid phase [143]. 

Discussion of the concepts and procedures involved in designing packed gas absorption systems shall first be confined to simple gas absorption processes without compHcations isothermal absorption of a solute from a mixture containing an inert gas into a nonvolatile solvent without chemical reaction. Gas and Hquid are assumed to move through the packing in a plug-flow fashion. Deviations such as nonisotherma1 operation, multicomponent mass transfer effects, and departure from plug flow are treated in later sections. 

Recovery of the solvent, sometimes by chemical means but more often by distillation, is almost always required, and the recoveiy system ordinarily is considered an integral part of the absorption-system process design. A more efficient solvent-stripping operation normally will result in a less costly absorber because of a smaller concentration of residual dissolved solute in the regenerated solvent however, this may increase the overall cost of solvent recoveiy. A more detailed discussion of these and other economic considerations is presented later in this section. 

Introduction Many present-day commercial gas absorption processes involve systems in which chemical reactions take place in the liquid phase. These reactions generally enhance the rate of absorption and increase the capacity of the liquid solution to dissolve the solute, when compared with physical absorption systems. 

Recommended Overall Design Strategy When considering the design of a gas-absorption system involving chemical reactions, the following procedure is recommended ... 

Traditional Design Method The traditionally employed conventional procedure for designing packed-tower gas-absorption systems involving chemical reactions makes use of overall volumetric mass-transfer coefficients as defined by the equation... 

Piston, screw and centrifugal compressors are used. As many chemical processes, such as oil refining, may have cheap waste heat, large absorption systems will also be found. 

The rate of photolytic transformations in aquatic systems also depends on the intensity and spectral distribution of light in the medium (24). Light intensity decreases exponentially with depth. This fact, known as the Beer-Lambert law, can be stated mathematically as d(Eo)/dZ = -K(Eo), where Eo = photon scalar irradiance (photons/cm2/sec), Z = depth (m), and K = diffuse attenuation coefficient for irradiance (/m). The product of light intensity, chemical absorptivity, and reaction quantum yield, when integrated across the solar spectrum, yields a pseudo-first-order photochemical transformation rate constant. 

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. 

Light absorption by rhodopsin leads to closure of Na+-conductance channels via a chemical messenger system 812... 

Again the radiative association kinetics described above allow a direct comparison for some realistic values of k and k. For most chemically activated systems at the threshold for unimolecular dissociation, the observed radiative rate constants are of the order of 10-100 s and hence are much below the values expected for k of about 10 s . Therefore, the first limit is most likely to be valid, with the interesting conclusion that the observed unimolecular dissociation rate constant will depend only on the photon density and the absorption cross section (rate constant) at a given wavelength. 

Table 23.1 Absorption Systems with Chemical Reaction" ...

Selective Absorption in the Sweetening Process. In recent years removal of the acid gas (H2S, CO2) components from a gas stream has increasingly been by absorption in a solvent system containing amines. While non-reactive solvent sweetening processes are in use, the ability of the basic amine to react chemically with the acid gas to yield water soluble salts has favored the chemical sweetening system. Thus... 

Chemical scrubbing systems for SO/ absorption fall into two broad categories (a) Disposable systems and (b) regenerative systems. Typical of systems in use for a number of years are those that use an aqueous slurry of an insoluble caldum compound, which can be discarded after use. Disposable 02-removal systems use aqueous slurries of finely ground materials, such as lime, limestone or dolomite, to produce a mixture of insoluble sulfites and sulfates. On passing through the scrubber, S02 from the waste gas dissolves to form sulfurous acid S02 ... 

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