A mathematical model has been developed for the optimum design of plate columns for the manufacture of nitric acid. The effects of outlet NOx composition, internal geometrical parameters, temperature and pressure have been included in the model. The present model has been simulated using the commercial plant data and has been compared with models published in the literature. Sensitivity of the fixed and operating costs on the optimum values of plate spacing, liquid submergence, excess oxygen, pressure, nitric acid concentration, etc. have been presented.