Selective removal of nitrogen oxides from catalytic cracking regenerator effluent using wet scrubbing with aqueous sodium chlorite solutions

In most refineries, the flue gas from the regenerator in the fluid catalytic cracking (FCC) unit represents a key source of NOx, SOx, particulate, and CO emissions. Wet scrubbing systems have been used with great success to control particulate and SOx emissions from typical FCC regenerator flue gas. Development of relatively inexpensive and safe additives that can be used in existing scrubbing systems to oxidize and absorb NOx is important in order to increase the industrial options to meet the provisions of the Clean Air Act and recently proposed EPA rules. This paper explores the feasibility of removing NOx from a typical catalytic cracking regenerator effluent gas using a wet scrubbing system with the addition of an oxidizing agent (sodium chlorite). A series of experiments were conducted in a bench-scale packed bed, a bench-scale ejector Venturi scrubber, and a pilot-scale packed bed. Various operating parameters such as gas flow rate, liquid flow rate, pH, operating temperature, gas phase compositions (NO, NO2, SO2, CO2 and O 2), and oxidizing agent concentration, were studied for their effects on NOx oxidation and absorption in the scrubbing systems. Experimental results showed that up to 90% of the NOx and SO2 could be simultaneously removed by the scrubbing systems. Experiments were also conducted using a feed with composition similar to that of a typical effluent from a Thermal DeNOx pretreatment. The low concentration of NOx after the pretreatment can still be removed using this approach.