Acid Gas Scrubbing — HCl, H2S, HF, HCN, HBr, NH3, etc.
Acid gasses are produced in many industries including battery production, semiconductor manufacturing, the chemical process industry, combustion processes, metallurgical industries, and the glass industry to name a few. Scrubbing with a neutralizing chemical, a base such as NaOH in aqueous solution, produces non-volatile, soluble salts that are removed by the blowdown process.
Hydrochloric Acid (HCl) is a common combustion product of fuels containing the halide chlorine. HCl is scrubbed with NaOH in a Vertical or Horizontal Packed Bed Scrubber according to the reaction:
HCl + NaOH —⟩ NaCl + H2O
The salt, NaCl, is removed by the blowdown process before it precipitates and contaminates the packing. With sufficient packing height or multiple stages, efficiencies as high as 99.9% can be obtained. Since HCl is highly soluble in water, water alone may provide sufficient absorption of HCl gas in some applications.
Hydrogen Cyanide (HCN) is another acid gas example. It is scrubbed with NaOH and sodium hypochlorite (NaOCl). The reactions between scrubbing liquid and acid gas are as follows:
HCN + NaOH —⟩ NaCN + H2O
Hypochlorite solution: 2NaOH + Cl2 —⟩ NaOCl + NaCl + H2O
NaCN + (2NaOH + Cl2) —⟩ NaCNO + 2NaCl + H2O
The salt, sodium cyanate (NaCNO), is less toxic and can be removed by the blowdown process. The pH is monitored by a pH probe. The pH controller runs the scrubbing liquid chemical metering pumps and maintains pH between 9 and 11 to prevent cyanogen chloride (CNCL) or nitrogen trichloride NCl3 formation.
Hydrogen Sulfide (H2S) is a common pollutant that must be removed because of its odorous and highly corrosive properties. It is often found at wastewater treatment plants, agricultural facilities, and landfills. It is common to remove H2S from methane (CH4) so that the purified Biogas can be used as a fuel source. Monroe Packed Bed Scrubbers use sodium hydroxide (NaOH) as a scrubbing liquid to remove H2S according to the reaction:
H2S + 2NaOH —⟩ Na2S + 2H2O