Advances in Water Treatment and Enviromental Management

208 WATER TREATMENTFig. 7b Concentration of VOC in Gas and LiquidIn Fig 7 a the tower is illustrated with liquid entering at the top and leaving at the bottom. Theflow is counter-current with air entering at the bottom and leaving at the top. Subscripts 1and 2 refer to bottom and top conditions.The Operating LineThis represents a material balance in the tower(1)The Equilibrium LineThe equilibrium line is a representation of Henry’s law which at atmospheric pressure is:Where x . is the concentration (mole fraction) of the VOC in the liquid phase which would bein equilibrium when the concentration of the VOC in the vapour is y 1(mole fraction) and theoperating pressure is 1 atmosphere.Values of H for VOCs are quoted below:(2)TCEPCECTC307 ats/mole fraction626 ats/mole fraction761 ats/mole fractionThe relationship used to define the tower dimensions can be presented as follows, where R isthe removal efficiency:(3)Equation (3) shows that the removal efficiency of a packed tower is independent of the inletconcentration x 2 and therefore removal efficiency is the most appropriate means of monitoringthe tower performance.It also shows that the removal efficiency is dependent upon the chemical properties, m and Hand the design variables Z, L m and G m and the mass transfer characteristics K L a.