Back to Absorption of Hydrogen Chloride

Adiabatic Absorption of Hydrogen Chloride

Preferred process if maximum concentration of HCl in the liquid phase is not the target

Absorption columns


These absorbers, as their name implies, operate without any heat removal from the reaction zone. In the process hydrogen chloride gas is easily and hence almost quantitively absorbed by fresh water flowing down the column. The maximum possible gas flow for a given column diameter depends basically upon the type of packing material in the column. The minimum flow rate of feed water is reached when an unpermissible amount of HCl is leaving the column at the gas outlet.

Adiabatically operated HCl-absorption column

Adiabatically operated absorption column for HCl

Bubble cap tray column for waste air cleaning

Bubble cap tray column for HCl waste gas absorption

Temperature profile along the column


Depending on the amount of absorbed HCl the generated dissolving heat may vapourise water which rises to the head of the column with any other non-soluble components of the gas stream. They are condensed and refluxed together with the absorption water. The major part of the generated dissolving heat is removed by this condensation process hence outside the absorption process. If the condensation process occurs the temperature downwards along the column is defined by the boiling point of the acid and hence increases downwards along the column up to a maximum value where the azeotropic concentration is reached. From there on the temperature decreases again further downwards to the outlet of the column with increasing HCl concentration. This temperature profile along the column can be used to control the liquid feed flow for a maximum absorption efficiency as illustrated on the following scheme. This scheme shows the temperature profiles for differnet amounts of inert gases. The right curve represents the situation for a feed gas without inert gases. 

Temperature profile along an adiabatic HCl-absorption column

Temperature profile along an adiabatic HCl-absorption column

Achievable acid concentration


The concentration of the HCl in the liquid outlet depends on the over all water/HCl ratio but cannot be higher than the solubility of HCl in water at a given temperature. Hence the maximum achievable acid concentration in the liquid outlet flow depends on the temperature of the liquid acid at the bottom of the column. The lower the temperature is at this outlet point the higher is the maximum achievable concentration of HCl in water. This temperature is determined by the boiling point of the aqueous HCl solution at the bottom of the column. This boiling point is a function of the HCl-content at the bottom which is at a given pressure a result of the ratio of the HCl containing feed gas and the absorption water.

Adiabatic absorbers produce, under normal operating conditions, hydrochloric acid of 30-31% strength. This compares with a theoretical maximum of 35% under such operating conditions.

HCl Partial pressure as a fucntion of HCl-content in the liquid phase

HCl-Partial pressure vs HCl-Content

The water content in the feed gas has of course an impact on the overall water to HCl ratio and hence on the max. achievable HCl concentration. The lower the H2O-content in the feed gas the higher is the maximum achievable HCl concentration in the liquid acid due to the resulting mass balance.

We would be pleased to design the most economic HCl-absorption solution for you.

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