Many control-relevant systems present the challenges of nonlinearity, directionality and ill-conditioning for the control systems, and exhibit poor controller performance. This study proposes a Nonlinearity Index to quantify the extent of nonlinearity of such systems. A dynamic nonlinear model of a pilot-scale distillation column operating near the azeotropic region was simulated using Aspen Plus Dynamics. A comparison of the results is made with the Nonlinearity Measure proposed by Du et al. Results show that a significant increase is exhibited in the proposed Nonlinearity Index values of the system as the system moves toward the azeotropic region. Prediction errors of the linear models are also shown to be correlated to the proposed index. Therefore, the proposed Nonlinearity Index is consistent with the existing indicators of nonlinearity, and thus its measurements of system nonlinearity are reliable. Controller performance for the system at higher values of the proposed index further presents its efficacy.