The factor by which the ratio of two substances in the feed must be multiplied to give their ratio after treatment. \[\frac{Q_{\text{A}}}{Q_{\text{B}}} = S_{\text{A,B}}\ \frac{Q_{\text{A}}^{'}}{Q_{\text{B}}^{'}}\] where \(Q_{\text{A}}\) and \(Q_{\text{A}}^{'}\) are the final and initial amounts of species A and \(Q_{\text{B}}\) and \(Q_{\text{B}}^{'}\) are the final and initial amounts of species B. Hence \(S_{\text{A,B}}=\frac{E_{\text{A}}}{E_{\text{B}}}\) where \(E\) is the @F02503@. In terms of \(D\),\(n\),\(r\) (where \(n\) is the number of stages and \(r\) the phase ratio), \[S_{\text{A,B}} = \frac{1- (1\,+\,r\ D_{\text{A}})^{-n}}{1- (1\,+\,r\ D_{\text{B}})^{-n}}\]