Isothermal Absorption of Hydrogen Chloride
Isothermal absorption process
The absorption process of HCl is called isothermal when the absorption heat is withdrawn along the absorption process. This means that supplementary cooling devices are installed along the absorption column or e.g. a falling film absorption unit is used instead of a column. The absorption process is then carried out at lower temperatures enabling a higher concentration of acid in the liquid than with an adiabatic absorption process.
Boiling point as a function of HCl-concentration in the liquid phase
Falling Film Absorber
Hydrogen chloride feed gas enters at the top of the falling film absorber and flows co-currently with weak acid coming from the tails tower. Heat of solution released as a result of the absorption process is removed by the cooling water flowing in the shell side of the falling film unit.
Product acid at up to 38% hydrochloric acid leaves at the bottom of the falling film absorber at a suitable temperature for storage and is ready for pumping.
The remaining unabsorbed hydrogen chloride gas and any inert gases present leave at the bottom of the falling film unit and enter at the bottom of the tails tower. Process feed water entering at the top of the tails tower absorbs the hydrogen chloride to form dilute hydrochloric acid. The dilute acid flows by gravity to the top of the falling film absorber to serve as the liquid feed to the tubes.
The design of the QVF® falling-film-absorber presents itself as a proven and fully corrosion-resistant apparatus. Special design features ensure that a very even fluid distribution is achieved on the inner side of the pipes.
Packed Column with cooling loops
An alternative to the falling film evaporator is the below shown column with a condenser and 2 additional cooling loops.
For the upper cooling loop the liquid reflux is withdrawn and pumped through a heat exchanger.
The feed of the lower cooling loop is the bottom respectively final product itself.
Falling film absorption unit
Packed column with 2 cooling loops