This document describes a passive observer model for estimating compositions in a multicomponent distillation column using temperature measurements. The goal is to estimate the concentration of NC4 in the distillate. A ternary model using temperature measurements from stage 49 and modeling the dynamics of IC4, NC4 and NC4- provided good performance over different operating conditions with robustness to sensor location changes. The passive observer structure was designed using per-component temperature gradient analysis to identify sensitive components and locations.