HYDROGEN CHLORIDE ABSORPTION SYSTEM

HCl Gas and Hydrochloric Acid Absorption of HCl Gas

The absorption of hydrogen chloride in water or dilute hydrochloric acid is a central operation in the production of hydrochloric acid.

This technique is often used for gases from

* HCl synthesis units
* Chemical production plants(e.g. MDI/TDI or metallurgical processes)
* Waste incineration units

The absorption system assures clean flue gases and high HCl concentrations in the product acid. The absorption is a highly exothermic process which heats up gas and absorption liquid. Therefore, the absorption is often performed in graphite equipment – graphite absorber and graphite packing columns. Depending on the HCl concentration in the feed gas, the absorption is either isothermal or adiabatic.

Isothermal absorption
Isothermal absorption is more economic for higher concentration HCl gases. Therefore, a watercooled graphite falling-film absorber is used for the main absorption with simultaneous cooling of the produced hydrochloric acid. The falling-film absorber can be operated in a co-current or counter-current mode. In co-current absorption,
both liquid and gas are fed at the top of the absorber. The acid solution and the gas with unabsorbed HCl leave the absorber at the bottom outlet.

This gas is scrubbed in a downstream scrubber column. The weak acid leaving at the bottom is used as absorption liquid for the falling-film absorber.

Adiabatic absorption
Adiabatic absorption is normally applied for low concentration HCl gases with constituents such as solvents that need to be vented.
The HCl-containing gas is fed to the bottom of a graphite scrubbing column, while the absorption liquid is fed at the top. Gas and liquid flow are counter-current to ensure optimum absorption.
The heat of absorption generates an elevated temperature of overheads and bottoms product, thus evaporating additional amounts of water. This water can be removed in a graphite condenser. The hot acid stream is cooled in an integral or separate graphite heat exchanger.