Adiabatic Absorption
The amount of water for absorption is defined upon specification of the requested outflow concentration level of hydrochloric acid for a known feed mix. For this reason, one achieves the boiling state within the column for high inflowing concentration levels of HCl. The removal of absorption energy is performed at the column head by condensation of water, which is almost free of hydrogen chloride; such condenses in the condensor and is re-directed to the column.

One prefers packed columns(Illustr. 1) for high levels of HCl in the inflow. The minimum scubbing density in packed columns fallsbeyond a certain limiting concentration while a high acid concentration is requested. The use of tray columns is recommended in such cases (Illustr. 2). For small diameters, corrosion-free tray-columns from QVF are equipped with sieve-trays. For bigger diameters, bubble cap trays are deployed, with or without chimney, contingent to the application case.

Illustr. 1: Packed Column	Illustr. 2:Tray Column

If the feed gas contains condensable organic components that are not fully soluble for water, we recommend performing the condensation of the vapour outside of the column in a falling fashion, see illustration 3. The separation of the heterogeneous condensate in a separator allows the split of organic components. The maximum achievable acid concentration is contingent to the water content of the raw gas flow for adiabatic absorption. This relationship is presented in illustration 4. Thus, hydrochloric acid of 30wt% may only be produced if the concentration for hydrogen chloride is greater than 82wt% with reference to inert-free gas state at the column inlet. At this point, it may be repeated that the achievable concentration of hydrochloric acid is impacted in a significantly less scope by the amount of inert gas.

Illustr. 3	Illustr. 4

Design Data for Adiabatic Absorber
A diagram is displayed in illustration 5 that serves to determine an estimateable derivation of the necessary diameter of packed columns for the adiabatic absorption of HCl. Raschig rings made of glass are deployed as column packings. The bundle of curves displayed apply for hydrochloric acid of 30wt%, whereby the volumetric proportion of inert gas serves as a parameter for the displayed bundle of design curves.

Illustr. 5: Packed column

The table compiles the performance data of QVF Standard Absorbers. Our engineering team delivers an individual solution to you, which, of course, fits to your specific application requirements, whereby other corrosion resistant materials may be deployed next to borosilicate glass 3.3.

Next to Raschig rings,the structured packing, called DURAPACK®, which is made of borosilicate glass, may also be used for adiabatic absorption. For small column diameters, the capacity may be significantly increased due to the deployment of the structured packing DURAPACK®, compared to Raschig glass rings (Illustr. 6). For greater column diameters and smallerscrubbing density, which are experienced with high inert gas proportions of the feed gas, the advantage of DURAPACK® is found in the reduced packing heights, in comparison to random packings.

Illustr. 6: DURAPACK®

The use of a tray column is effective if the HCl proportion within the feed gas is so low that the column remains below minimumscrubbing density. (Illustr. 7).

Illustr. 7: Tray Column

This column type operates very effectively even at very low liquid loads, as a sufficient fluid level is retained on the plate due to the baffle plate, thus enabling an intensive contact between gas and fluid.

All three diagrams displayed apply for hydrochloric acid of 30wt%, whereby the volumetric proportion of inert gas within the raw gas flow serves as a parameter.

Control of Adiabatic Absorber
Non-permissibly high HCl emissions must be securely prevented during the operation of an adiabatic absorber, and it must be secured that the concentration of the produced hydrochloric acid is controlled at an almost constant level at the same time. The inflow of absorption water must be controlled continuously for exhaust gas flows, whose composition experiences greater deviations.
It has been found to be very beneficial that the hydrochloric acid exists almost in a boiling state throughout the whole column height for the solution of this control-engineering task. In accordance with illustration 8, a contingency exists between the acid concentration level and the boiling temperature within the concentration level domain of technically manufactured hydrochloric acid. The temperature profile within the column along the temperature axis is deferred in a parallel fashion if inert gas is present, whereby the profile shape remains unchanged.

<<<<<Illustr. 8: Boiling Temperature of Hydrochloric Acid

This can be utilized in order to derive the following control opportunity, which is presented in illustration 9 in a schematic fashion. The temperature difference TD1 between a location near the column head and the bottom of the column, serves as a control variable for the inflowing amount of absorption water. This temperature difference is almost independent from the proportion of inert gas being presented. It must be made secure that themaximum temperature profile does not shift within the column. A shift of temperature profile could occur if a sudden rise of inflowing HClI gas became incidental, for example. In order to prevent this case of temperature shift upwards within the column, control measurement is performed above the temperature difference TD2. As long as this difference exceeds a certain minimum value, the maximum temperature is located at the desired place. In the opposite case, if the temperature difference TD2 moves below the minimum limit, the controller shall impact the inflow of absorption water until the maximum temperature is again relocated to the desired place within the column. An immaculate operation and a steady quality of the outflowing hydrochloric acid are secured by this control arrangement.

Illustr. 9: Temperature Control of an Adiabatic HCl Absorption

If the temperature of the outflowing acid serves as control variable for the inflowing absorption water, one must consider that the outflowing acid does not always attain boiling state in practical operations. A more reliable control is achieved by deploying the QVF Acid Measurement Cell, as displayed in illustration 10.
The boiling temperature of the acid is measured by a resistance thermometer, thus controlling the water amount to the absorber within this cell that may optionally be operated by steam or by electric heating. This technology is preferable for deployment in order to control gas flows that consist of HCl near to 100%. If other components of the feed gas become soluble, e.g. such as supplementary organic materials, the boiling temperature loses its unique function to control hydrochloric acid concentration. In this case, the above described cascade control should preferably be deployed, likewise for pur gas with varying inert gas.

The density of the outflowing acid is a further possible control variable for the amount of absorption water. Deploying a spindle-design chamber may possibly perform a relatively easy measurement of concentration level. The relationship between density and hydrochloric acid concentration is presented in illustration 10.

<<< Illustr. 10: Measurement Cell for Boiling Point

<<< Illustr. 11: Density of Hydrochloric Acid

>>> Isothermal Absorption of Hydrogen Chloride

>>> Concentration of Hydrochloric Acid via the Azeotrope Point

>>> Cleaning of Hydrochloric Acid

>>> Absorption of Hydrogen Chloride