Mass transfer in the absorption of nitrogen oxides in alkaline solutions

The absorption of mixtures of nitrogen oxides into aqueous solutions of NaOH in the range of partial pressures of 0.004‐ to 0.05‐atm NO and 0.002‐ to 0.015‐atm NO 2 was investigated. Absorption experiments were conducted in a gas‐liquid contactor that permitted independent variation of the gas and liquid mass transfer coefficients. The results were analyzed in terms of a model which accounted for diffusion, reaction, and formation of higher oxides and oxyacids in the gas and in the liquid phases. Absorption of both HNO 2 and N 2 O 3 , formed in the bulk gas and within the gas diffusion film, were found to be significant. Using a rate coefficient of 8.8 × 10 −3 mol/s · cm 3 · atm 3 for the formation of HNO 2 in the gas phase (England and Corcoran, 1975), a value of the absorption factor, H √ Dk , for N 2 O 3 of 2.5 × 10 −3 mol/s cm 2 atm was determined at 25°C. This value was found to decrease with temperature and was 1.2 × 10 −3 at 40°C. The use of concentrated base as an absorbent solution prevented the formation of nitric acid mist, a problem encountered in many previous studies of NO x absorption.