Integration of Reaction and Separation in a Batch Extractive Distillation Column with a Middle Vessel

HECTOR JOSE MARIA ESPINOSA

The Hague

Simposio; 12 TH European Symposium on Computer Aided Process Engineering-12; 2002

Process intensification through combination of many operations into the smallest number of units is a challenge due to its potential for economic savings and its environmental-friendly nature. The analysis of batch reactive distillation as an alternative to the conventional configuration of a reactor followed by a batch rectifier needs the development of conceptual models to assess the performance of the combined operation. These models should allow the designer to analyze feasible products and evaluate the influence on conversion of variables like vapor flow rate, total reflux time, reaction plus distillation operation time and reflux ratio policy. This contribution focuses on the development of a dynamic conceptual model for a reboiler-reactor with an attached column above it. The main model assumption is a column with an infinite number of stages. Hence, pinch analysis can be used to predict the performance of the combined operation. For a given instantaneous still composition, the key feature of the method is the estimation of the instantaneous minimum reflux (operation at constant distillate composition) or the instantaneous distillate composition (operation at constant reflux ratio or constant Damköhler number) through pinch analysis. Between the two different methods developed for conventional batch rectification ([1], [2]), this contribution explores the usefulness of calculating controlling pinch points from linearization of column profiles at instantaneous still composition as explained in detail elsewhere [1]. By integrating the differential equation system that models the still path evolution in presence of both a kinetically controlled reaction and distillation, the trajectory of main variables (i.e., distillate composition and temperature, still composition and temperature, component recoveries at the top and still, limiting reagent conversion, and reflux ratio) can be obtained without resorting to stage-by-stage calculations. Applied to several highly non-ideal mixtures, the results from the conceptual model can be summarized as follows: i) operation at constant distillate composition (variable reflux policy) is quasi-optimal as it allows a “light” product of constant purity whilst maximizing limiting reagent conversion, ii) operation at constant Damköhler gives rise to a variable reflux policy which does not necessarily gives a product of constant purity nor a maximum in conversion, iii) vapor flow rate strongly affects maximum conversion, iv) initial total reflux time is a key variable as both low and high total reflux times produce a detrimental effect in conversion by either allowing losses of reagents at column top or decoupling reaction and distillation, v) proper operation of batch reactive distillation can be used to overcome azeotropes and distillation boundaries. Finally, the relevance of the conceptual model in deciding among different process alternatives in the synthesis step and in providing good initial profiles for the control variables for the dynamic optimization of the combined process is emphasized. References: [1] Espinosa, J. and Salomone, E. Minimum Reflux for Batch Distillations of Ideal and Nonideal Mixtures at Constant Reflux. Ind. Eng. Chem. Res., 38 (7), 2732-2746 (1999). [2] Espinosa, J.; Brüggemann, S. and Marquardt. W. Application of the Rectification Body Method to Batch Rectification. European Symposium on Computer Aided Process Engineering – 15, 20A, L. Puigjaner and A. Espuña (Editors), Elsevier, 757-762 (2005).