The estimation of the product composition profiles for a distillation column is addressed by using an observer based on the differential geometry theory. The estimation problem is challenging because of the markedly nonlinear behavior of the column and because of the ill-conditioning of the observability matrix at the operating condition investigated in the paper. The proposed solution is to consider the estimator structure as a degree of freedom in order to improve the estimator robustness without affecting the capability to accurately reconstruct the composition profile. Guidelines are provided to select the observer structure and the location of the measurement sensors. The observer is designed and tested experimentally on a 10-m high pilot-plant distillation column separating an ethanol/water system. The results indicate that the observer is able to accurately reconstruct the composition dynamics over a wide set of operating conditions.