The possibility arises to achieve higher acid concentrations by deploying isothermal absorption, compared to adiabatic absorption. The requirement is to remove the produced absorption heat from the process. Both technical solutions are presented in the following. Heat removal is performed within the apparatus for option 1 (Illustr. 1), whereas heat removal is performed by an external heat exchanger via a pump circulation for option 2 (Illustr. 2). The technical implementation of option 1 is displayed in illustration 1 in form of the so-called falling-film absorber; gas and fluid flow in the same direction through the inner pipe of the shell and tube heat exchanger. The absorption heat energy is transferred to the cooling water, which flows in the shell around the pipes. Hydrochloric acid exits at the bottom; the residual gas flow moves then to the second-stage adiabatic absorber. This second-stage absorber is usually also of need if the absorption process is performed by the facility in accordance with option 2, as presented in illustration 2. It may be deployed as a separate unit or as a supplement topping unit onto the displayed column.
The falling-film-design absorber of QVF 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.

Technical Data for Isothermal Absorber
The table compiles the performance data of QVF Standard Absorbers. The described table data is based on pure HCI gas without inert gas proportions, and applies to 35 wt% hydrochloric acid as target product acid. The size of the second-stage adiabatic absorber is designed to accommodate a volumetric inert gas proportion of 25% for standard facility types. The QVF falling-film absorber may be deployed for the absorption of gases at any location that displays high absorption heat energy, e.g. for the absorption of HBr, NH3, and SO2.

Control of Isothermal Absorber
The inflow of absorption water must be controlled even for the operation of an isothermal absorber. It thus makes sense to chose a measurement variable that substitutes the direct measurement of the concentration level. From all physical properties of hydrochloric acid considered, the density of such lends itself especially appropriate as control variable. The density of hydrochloric acid versus its concentration level is presented in illustration 3, for a temperature of 20°C. This demonstrates a definite contingency between density and concentration level. This definite contingency applies yet only if no other products impact the density.

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<<< Illustr. 3: Density of Hydrochloric Acid

>>> Adiabatic Absorption of Hydrogen Chloride

>>> Concentration of Hydrochloric Acid above the Azeotrope Point

>>> Cleaning of Hydrochloric Acid

>>> Absorption of Hydrogen Chloride