n-Propanol aqueous solution shows nonlinearity in liquid level depending on water proportion

n-propanol (boiling point: 97°C) and water (boiling point: 100°C) mixtures cannot distilled by simple distillation. Mixture has a minimum-boiling azeotrope (87.7°C, 71.7wt%). n-Propanol aqueous solution is fed to a tank temperature remained constant at 90°C. A volume of vaporized solution in tank can be changed by a water proportion of feed flow. A volume of vaporized solution can be measured by liquid level. Liquid level of solution shows nonlinearity and input multiplicities.