In the early stages of distillation process analysis, the residue curve maps are used to check its feasibility. The residue curve map represents the composition that follows the residue in the still of a Rayleigh distillation for several initial conditions. It is demonstrated in the literature by several authors that the compositions followed by a residue curve matches a column composition profile of a packed column operated at infinite reflux flow-rate. In the present work, it is demonstrated that the dimensionless time used to calculate the residue curves corresponds to the number of transfer units by applying mass balances in a section of a packed column operated at infinite reflux flow rate. However, when there is a reaction, the number of moles changes due to the reaction stoichiometry, a different expression is obtained by the residue curves than by the infinite reflux flow rate packed column. It is important to notice that when there is no reaction or the number of stages does not change by the reaction stoichiometry. This expression becomes equal to the Rayleigh distillation expression.