Equation-oriented optimization of methanol distillation systems using pseudo-transient models

Abstract

Methanol distillation is an energy-intensive and complex integrated process. Up to now, methanol distillation schemes involving energy-saving technologies have been developed based on process simulation, or based on optimization under certain simplified conditions because of the problem of convergence and integer optimization. In this work, six distillation schemes, two of which are proposed for the first time, were optimized using a steady-state optimization algorithm assisted by rigorous pseudo-transient continuation (PTC) models in equation-oriented environments. The optimization results indicated that double-effect distillation could significantly reduce the energy consumption, where the implementation of heat integration had an evident impact on both the energy cost and capital cost. The economic analysis showed that, under the same feed conditions, the total annual cost (TAC) of the five-column scheme was the lowest. There are three groups of heat integrations in the scheme. Since the energy consumption of the methanol distillation process accounts for a large proportion in TAC, adopting multi-effect distillation technology is a good way to save energy in this situation. Although at the expense of increasing the capital cost, the five-column scheme is still the most economic owing to its significant energy saving effect. Finally, after overall comparison of the six schemes, the optimal scheme characteristics are summarized, which is of great reference value for the future design of methanol distillation systems.