Numerical simulation of two-phase flows in spray scrubbers

Spray absorbers are widely used to remove SO2 from flue gases of large coal-fired power-plants or garbage incinerators. In such scrubbers the SO2 is absorbed by a suspension of water and limestone. The suspension is dispersed by nozzles to obtain a large interfacial area. There exists a great diversity of designs for spray scrubbers. Towers with cylindrical or square cross section are used. The inlet gas stream is directed radially or tangentially inward and is transported in countercurrent or cocurrent motion with respect to the spray droplets. The nozzles are arranged at different spray bank levels. Full cone or hollow cone sprays with various opening angles can be applied. The main operation parameters are mean axial gas velocity, nozzle pressure and liquid to gas volume flux ratio. A given installation produces a specific pressure drop for the gas stream and a certain efficiency of SO2 removal. Many industrial sized scrubber units suffer from problems associated with uneven distributed flux of gas and slurry droplets within the scrubber cross section. Regions with axial gas velocity much higher than the mean value exist. They cause low local dispersed phase volume fraction and small contact times between the liquid and the gas phase. It is demonstrated that the CFD simulation in a design optimisation procedure effectively can be used to correct gas and spray maldistribution effects.