Removal of Hydrogen Chloride from Inert Gases

3 Removal of Hydrogen Chloride from Inert Gases 

Hydrogen chloride is a toxic and corrosive compound that must not pollute the environment. A process for air purification and hydrogen ehloiide recovery is shown in Fig. 11.1-5. The boiling point of HCl is too low (-85°C) for simple distillation and even for partial eondensation. The process depleted in Fig. 11.1-5 uses the fact that HCl is extremely well soluble in water. Henee, absorption of HCl by water is a promising process. 

Dissolution of the HCl in water sets free a large amount of heat of absorption (approximately as high as the latent heat of evaporation of water). Hence, the liquid in the absorber reaches boiling temperature and, in turn, some water is evaporated by the absorption of HCl. Hence, the conditions in the absorber are quite similar to those in a distillation column. The equilibrium curve of a boiling HCI/H2O liquid is shown in the McCabe-Thiele diagram of Fig. 11.1-6. The presence of the inert gases only reduces the effective pressine of the HCI/H2O system. 

At the top of the absorption column the water vapor is condensed and recycled into the column. To increase the liquid reflux in the column some external water is fed into the column serving as washing agent. The operating line of column C-1 hes above the equilibrium line which is characteristic of absoibers. The azeotrope (maximum azeotrope) does not hinder the absorption process. 

The concentration of the bottom fraction B is as high as 31 wt%. Most of 51 is fed via a heat exchange into distillation column C-2 to be separated into pure HCl (overhead fraction) and an azeotropic mixture (bottom fraction B2). Fraction B2 is recycled into the absorption column C-1. Hence, the operating line of the absorber has a sharp bend at a concentration of 22 wt%. 

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