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The absorption of hydrogen chloride in water behaves strongly exothermic. The necessary removal of absorption energy is performed in different manners, at least for high HCl concentration levels, as further described in the following adiabatic and isothermal absorption process. Important thermodynamic data for the design of plants are described in the following.
Thermodynamic data for Absorption
The following explanations relate only to the case of physical absorption due to solubility behavior in fluid phase. Solubility processes with regard to chemical reactions are not put into scope.
The absorption equilibrium of the system hydrogen chloride/water is presented in illustration 1. The hydrogen chloride partial pressure values within the gas phase related to the concentration values for hydrochloric acid apply only for constant temperatures. The partial pressure of hydrogen chloride is very low for low concentration levels of hydrochloric acid. It only rises significantly in the case of higher fluid concentration levels.
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| Illustr. 1: Absorption Equilibrium State for HCl/Water |
Illustr. 2: Boiling Temperature of Hydrochloric Acid |
The absorption of hydrogen chloride in water is an exothermal process to about 2100 kJ/kg HCl. That is why the boiling state is usually achieved for high concentration levels of hydrogen chloride within the gas flow. The boiling temperature is contingent to the acid concentration, as displayed in illustration 2. A maximum temperature of 108.6°C is achieved for approx. 21wt% of HCl . The boiling temperature reduces in an almost linear fashion for further increasing concentration levels of hydrochloric acid.
Almost parallel-deferred boiling diagrams unfold for lower total pressures, as e.g. displayed in illustration 2. The equilibrium diagram for a 1 bar pressure system is relevant for the design of absorption under boiling conditions, as displayed in illustration 3. Hydrogen chloride is hardly volatile for low concentration levels of hydrochloric acid, meaning that the gas phase is almost free of hydrogen chloride. The same concentration values are experienced at the azeotropic point for the steam and fluid phase. The volatility of hydrogen chloride strongly increases only beyond this point. The gas phase consists practically of only hydrogen chloride for a fluid concentration level of 40wt% for hydrochloric acid.
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| Illustr. 3: Equilibrium Diagram for HCl/Water |
Illustr. 4: Absorption Equilibrium of SO2 in Water |
A maximum concentration of hydrochloric acid may not be exceeded for absorption under boiling conditions due to the S-shaped equilibrium diagram. It amounts to 35wt% at a total pressure of 1 bar and pure inflow of hydrogen chloride, whereas slightly lower values are adjusted usually.
If further components of the inflowing feed gas can be absorbed, their absorption equilibrium behaviors must also be included in the design of the process. Illustration 4 displays the absorption equilibrium of SO2 in water for different temperatures, as an example. It may be recognized that only low concentrations are achieved in the aqueous phase, in comparison to the system HCl/water. Thus, water is not a good agent for the absorption of SO2. The absorption capability is yet great enough so that such may not to be disregarded.
The concentration levels of components displaying absorption behavior are usually low for technical applications that one may assume that no counteracting affects exist. Thus, the equilibrium behaviors of single components may generally be specified for design independently from each other.
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