Absorption, chemical

In industrial absorption processes the specifications of gas piuity are often very high. These requirements can only hardly be met by physical absorption. A very effective measure to enhance interfacial mass transfer is the addition of substances to the solvent that chemically react with the absorptives. By chemical reaction the capacity as well as the selectivity of absorption can be drastically improved. In a first step the gaseous substance a is physically dissolved in the hqitid. In a second step the dissolved substance a reacts with the active compound h in the hqitid to the product z according to 

The symbol v denotes the stoichiometric coefficient of the reactiom For technical absorption processes only very fast reactions which are reversible at higher temperatures are suited. 

During absorption and succeeding reaction the heat of reaction q is set free. It can be determined from hsted values of the heat of formation , see, for instance, Perry s Chemical Engineer s Handbook (Perry et al. 1997). Its use is e q)lained at the absorption of SO2 in aqueous NaOH solutions at a temperature of 25°C. The reaction is 

Compound State Heat of formation at 25°C in kcal/mol State Heat of formation at 25 °C in kcal/mol 

The reaction considered here is strongly exothermal. In nonideal systems the heat of reaction also depends on concentratioa 

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A sharp separation results in two high purity, high recovery product streams. No restrictions ate placed on the mole fractions of the components to be separated. A separation is considered to be sharp if the ratio of flow rates of a key component in the two products is >10. The separation methods that can potentially obtain a sharp separation in a single step ate physical absorption, molecular sieve adsorption, equiHbrium adsorption, and cryogenic distillation. Chemical absorption is often used to achieve sharp separations, but is generally limited to situations in which the components to be removed ate present in low concentrations. 

The special case involving the removal of a low (2—3 mol %) mole fraction impurity at high (>99 mol%) recovery is called purification separation. Purification separation typically results in one product of very high purity. It may or may not be desirable to recover the impurity in the other product. The separation methods appHcable to purification separation include equiUbrium adsorption, molecular sieve adsorption, chemical absorption, and catalytic conversion. Physical absorption is not included in this Hst as this method typically caimot achieve extremely high purities. Table 8 presents a Hst of the gas—vapor separation methods with their corresponding characteristic properties. The considerations for gas—vapor methods are as follows (26—44). 

Fig. 3. Flow diagram for a chemical absorption process where the horizontal lines within the towers represent trays or packing.

Physica.1 Absorption. Whereas chemical absorption rehes on solvent reactions to hold acid gas components in solution, physical absorption exploits gas—hquid solubiUties. The amount of absorption for these solvents is direcdy proportional to the partial pressure of the acid gas components. Thus these processes are most appHcable in situations involving high pressure feed streams containing significant concentrations of acid gas components. To favor absorption, lower temperatures are often employed. Some processes require refrigeration. 

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. ... 

Consider an aqueous caustic soda solution whose molarity mi = 5.0 kmol/m (20 wt.% NaOH). This solution is to be used in >scH(t>ing H2S from a gaseous waste. The operating range of interest is 0.0 < xi kmoUn ) < 5.0. Derive an equilibrium relation for this chemical absorption over the operating range of interest. 

The chemical absorption of HjS into caustic-soda solution (Astarita and Gioia, 1964) involves the following two reactions ... 

As stated earlier, inhalation is the main route of absorption for occupational exposure to chemicals. Absorption of gaseous substances depends on solubility ifi blood and tissues (as presented in Sections 2.3.3-2.3.5), blood flow, and pulmonary ventilation. Particle size has an important influence on the absorption of aerosols (see Sections 2.3.7 and 3.1.1). 

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. 8.1 shows a diagram of a chemical absorption process described by Chiesa and Consonni [1], for removal of CO2 from the exhaust of a natural gas-fired combined cycle plant (in op>en or semi-closed versions). The process is favoured by low temp>erature which increases the CO2 solubility, and ensures that the gas is free of contaminants which would impair the solvent properties. 

Fig. 8.1. The chemical absorption process (after Chiesa and Consonni [1]).

Manfrida [4] argues that the heat demand and the substantial power loss associated with presssure-swing physical absorption makes it less attractive than chemical absorption, even for high pressure sequestration. The expansion work in the fonner is difficult to recover as. several expanders are needed. 

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. 

Fig. 8.13 shows Cycle B2, a development of Lloyd s simple steam/TCR cycle for CO2 removal, as proposed by Lozza and Chiesa [7J. However, this is a CCGT plant in which the syngas produced by the steam reformer is cooled and then fed to a chemical absorption process. This enables both water and CO2 in the syngas to be removed and a hydrogen rich syngas to be fed to the combustion chamber. 

In cycle D4 [15, since the fuel is burnt with pure oxygen, the exhaust gases contain CO2 and H2O almost exclusively (Fig. 8.21). Cooling the exhaust below the dew point enables the water to condense and the resulting CO2 stream is obtained without the need for chemical absorption. The exjjensive auxiliary plant involved in direct removal of the CO2 is not needed, but of course there is now the additional expense of an air separation plant to provide the pure oxygen for combustion. 

Sada, E., Kumazawa, H. and Lee, C.H., 1983. Chemical absorption in a bubble column loading concentrated slurry. Chemical Engineering Science, 38, 2047-2051. 

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. 

Chemical absorption where a solvent (a chemical) capable of reacting reversibly with the acid gases is used. 

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. 

The BiodeNOx process is a novel process concept to reduce NO emissions from flue gases of stationary sources like power plants and other industrial activities [1]. The concept combines a wet chemicd absorption process with a novel biotechnological regeneration method. In the wet chemical absorption step, flue gas components are absorbed into an aqueous solution of Fe"(EDTA) (EDTA= ethylme-diamino-tetraacetic acid). The following reactions take place ... 

In contrast to the method based on Rayleigh scattering, the procedure based on Mie scattering theory (a) is not restricted to small particles, (b) chemical absorption may be considered, (c) and lastly, it can be extended to permit the use of a nonlinear... 

Separation in absorption is sometimes enhanced by adding a component to the liquid that reacts with the solute. The discussion regarding absorption has so far been restricted to physical absorption. In chemical absorption, chemical reactions are used to enhance absorption. Both irreversible and reversible reactions can be used. An example of an irreversible reaction is the removal of S02... 

In this case, the reactions can be reversed at a regeneration stage in a stripping column by the input of heat in a reboiler. If the solvent is to be recovered by stripping the solute from the solvent, the chemical absorption requires more energy than physical absorption. This is because the energy input for chemical absorption must overcome the heat of reaction as well as the heat of solution. However, chemical absorption involves smaller solvent rates than physical absorption. 

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. 

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. 

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. 

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