Evaluating the shape of input pertubation for forced periodic operation

Abstract

Continuous chemical processes are typically designed to operate under steady state conditions. However, there is strong evidence that an optimized forced periodic operation possesses the potential to improve process performance [1]. More demonstration examples are needed to promote such advanced concepts. In this contribution we will present results for two case studies. The first reaction investigated is the liquid phase hydrolysis of acetic anhydride performed in an adiabatic continuous stirred tank reactor (CSTR). The second examples is the heterogeneously catalysed gas phase synthesis of methanol performed in an isothermal and isobaric CSTR. Our theoretical analysis exploits independently determined kinetic models of these reactions [2, 3]. Besides performing numerical process simulations the Nonlinear Frequency Response (NFR) method [4] is used. The magnitude of possible process improvements depends on the applied strategy of forced periodic operation. Besides the input to be perturbed (concentration, flowrate, temperature, …), the forcing frequency and the forcing amplitude as well as the shape of the input modulation are of relevance. In this contribution we will compare the input modulated as harmonic (Fig 1a) and as a square wave function (“bang-bang”, Fig 1b). In order to use the NFR method for the latter input function an approximation via Fourier series is applied [5, 6]. The results reveal improvements of easier to practically implement square wave inputs for both examples considered, compared to harmonic modulation of inputs [5, 6].