The separation of a ternary mixture with a decomposing reaction was investigated in a batch distillation column with a side withdrawal (BDS). The process was simulated by a constant molar holdup model solved by the GEAR integration method and optimized by a simulated annealing algorithm. Comparing to the batch rectifier and middle vessel batch distillation column, BDS is favorable for the purification of an intermediate component from light and heavy impurities with a decomposing reaction. The effects of top reflux ratio and side reflux ratio were also investigated. Different combinations of top and side reflux ratio result in different types of product withdrawal, and the optimal operation tends to be in a large top reflux ratio and a small side reflux ratio.