The chemical absorption process

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

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 the physical absorption process, the CO2 is absorbed in a solvent according to Henry s Law and then regenerated using heat, pressure reduction or both heat and pressure reduction. Typical solvents are Selexol (dimethylether of polyethylene glycol) and Rectisol (cold methanol) which are applied at high pressure. At lower pressures, the chemical absorption processes are more economical. The Selexol physical solvent process is frequently specified for coal gasification applications199. 

Based on our previous studies of the dissolution and crystallization kinetics of potassium inorganic compounds based on linear nonequilibrium thermodynamics (Ji et al, 2010 Liu et al, 2009 Lu et al, 2011), we proposed to assume that the kinetic process of CO2 absorption by ILs comprised two steps surface reaction and diffusion, as shown in Fig. 17. Figure 17 demonstrates that when CO2 in the vapor phase and the ILs were in contact, the chemical reaction of CO2 with ILs occurred for the chemical absorption process of CO2 by ILs in the first step, which was named as the surface reaction layer, while for the physical mass transport process of CO2 by ILs in the first step, CO2 in the vapor phase would be transported into the IL phase, which was also named as the assumed surface reaction layer. As for the surface reaction layer, the driving force of the surface reaction was the chemical potential gradient of CO2 between CO2 at the vapor—Hquid interface and gas CO2. After that, in the second step, CO2 in the IL phase would... 

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

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. 8.13 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. 

The small intestine continues the chemical digestion process started in the stomach with a focus on enzymatic cleavage of fats and sugars. Proteins, already partially broken down in the stomach, are cleaved into individual amino acids. Once different food types have been digested completely, they are ready for absorption. The small intestine consists of three regions duodenum, jejunum, and ileum (Table 3.5).5... 

Tamir [5] analyzed the effects of impinging streams enhancing physical and chemical absorption processes. To describe the enhancement of absorption, the following two enhancements were defined to account for the two factors oscillation movement and re-atomization-coalescence of droplets, respectively... 

By assuming (i) that only the above reactions take place, plus the chemical absorption of C02, (ii) that natural gas consists of only CH4, (iii) that air consists of 0.80 mole fraction N2 and 0.20 mole fraction 02, and (iv) that the ratio of conversion of CH4 by processes (7) and (2) is controlled through admitting oxygen for reaction (2) by adding just enough air so that the mole ratio of N2 to H2 is exactly 1 3, consider the overall efficiency of a process in which 1200 m3 (S.T.R) of natural gas yields 1.00 metric tons of NH3. (a) How many moles of NH3 would be formed from... 

Reactive absorption represents a process in which a selective solution of gaseous species by a liquid solvent phase is combined with chemical reactions. As compared to purely physical absorption, reactive absorption does not necessarily require elevated pressure and high solubility of absorbed components because of the chemical reaction, the equilibrium state can be shifted favorably, resulting in enhanced solution capacity [2]. Most of the reactive absorption processes involve reactions in the liquid phase only, but in some of them both liquid and gas reactions occur [3, 4]. 

Usually, the effect of chemical reactions in reactive absorption processes is advantageous only in the region of low gas-phase concentrations due to limitations by the reaction stoichiometry or equilibrium [5]. Further difficulties of reactive absorption applications may be caused by the reaction heat through exothermic reactions and by relatively difficult solvent regeneration [6, 7]. Most of the reactive absorption processes are steady-state operations, either homogeneously catalyzed or auto-catalyzed. Recently, an application of a reactive absorption process based on using secondary amine groups on solid supports as immobilized activators has been reported [8]. 

The C02 is removed in either a chemical, a hybrid, or a physical absorption process. Residual C02 contents are usually in the range of 50 to 1000 ppmv, depending on the type and design of the removal unit. The physical absorption processes may be designed for zero heat consumption. But for comparison with the chemical processes, the mechanical energy requirements have to be considered.53... 

Table II shows the calcium content in paper from the chemical feeder process. With the addition of 9.2 ppm Ca in DI water, the treated newsprint papers calcium content doubled in comparison with the control. The treated Foldur Kraft paper contained three times more calcium than did the unwashed paper. The more calcium that was added in the DI water, the higher the amounts of calcium absorbed in the papers during the washing and deacidification process. However, the absorption of calcium in the paper reached a saturation point. This is the reason why newsprint and Foldur Kraft papers that are treated with 36.4 ppm Ca in the Dl-Ca water imbibed the same amount of calcium as the papers washed with 112.8 ppm Ca in the Dl-Ca water.

At the heart of these processes is the absorber or the reactor of a particular configuration best suited to the chemical absorption or reaction being carried out. Its selection, design, sizing, and performance depend on the hydrodynamics and axial dispersion, mass and heat transfer, and reaction kinetics. 

The main objective of absorption processes is the removal of one or more components from a gas stream using selective solvents. Figure 3 shows a typical absorption process in which, with the help of a selective solvent (absorbent), the undesired compounds (in this case, HjS, COj) are removed from the raw gas (in this case, natural gas) in a multistage countercurrent process. While the purified gas leaves the absorber (saturated with the selective solvent), the absorbent is regenerated in a second column (desorber) and is recycled to the absorber. In the case of absorption, one can distinguish between physical and chemical absorption processes. In the case of physical absorption, the absorber is operated at high pressures and low temperatures while for the desorber the opposite conditions are used. 

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