Process, absorption chemical

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

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. 

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... 

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. 

Chemical absorption processes Physical absorption processes ... 

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. 

A methodology for predicting the performance of an isothermal packed tower used for chemical ab-eoiplion has been developed by Josbi et al.35 The study was based on the absorption of CO in a hot equeons solution of potassium carbonate however, the general approach is applicable to other chemical absorption processes. 

The removal of acid gases from gas streams can be generally classified into two categories (1) chemical absorption processes and (2) physical absorption processes. There are several such processes that fit into these categories (Tables 23.3 and 23.4) the features of the individual process may vary (Table 23.4 van den Berg and de Jong, 1980 Bodle and Heubler, 1981). 

Chemical absorption including dissociation, a horizontal tangent may be drawn on the partial pressure line at x,- = 0. This is a common case for chemical absorption processes... 

Reaction kinetics In most chemical processes, chemical reactions do not reach the chemical equilibrium. Instead, reaction kinetics are decisive for the yield, the chemical equilibrium calculations are mainly used to determine the maximum yield. Also for the design of reactive distillation processes and chemical absorption processes, the reaction kinetics must be known. As well, the Maurer model for formaldehyde (see Section 13.1) must be regarded as a reactive distillation and especially at low temperatures, the equilibrium is by far not reached. 

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... 

Application of Computational Mass Transfer (II) Chemical Absorption Process... 

The Rectisol and Amisol processes, which were developed in Germany by Liirgi, are physical and physical-chemical absorption processes using organic solvents to remove acid gas and sulfur, respectively, from various gas streams. Both processes use methanol as the physical absorption solvent the Amisol process also uses monoethanolamine (MEA) as the chemical absorber to improve the overaU purification efficiency. The MEA used in the Amisol process is not pertinent to our topic. Only methanol used in the Retisol process is discussed here, and the discussion is also applicable to the physical absorption by methanol used in the Amisol process. 

Conventional high temperature and low temperature shift carbon dioxide removal using a chemical absorption process methanation. 

Several revamp options are available for modification of the carbon dioxide removal section depending on the type of carbon dioxide removal process. The processes mostly used in ammonia plants are chemical absorption processes based on either hot potassium carbonate (HPC) such as Benfield, or Vetrocoke, or amine solutions such as MEA. The chemical carbon dioxide removal processes may be improved or replaced with a physical process in which the absorbent is regenerated by simply flashing off carbon dioxide. In this way the need for regeneration heat may be reduced or eliminated. A physical carbon dioxide removal system may result in energy savings of 0.01-0.35 Gcal/MT ammonia. 

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