Comparison of EQ and NEQ Models

Applications of the EQ and NEQ models to RD were reviewed by Taylor and Krishna [1]. Most papers do not compare the two models directly (or even indirectly). Here, we wish to highlight a few studies that reveal the differences between these fundamentally different types of model. 

Kreul et al. [16] used an NEQ model of homogeneous RD and, via a series of case studies, studied the importance of various model simplifications. They found little difference between the full MS description of multi-component mass transfer and 

Baur et al. [17] have compared the EQ and the NEQ models for the MTBE process. They underlined some counter-intuitive features of RD processes. For example, for a methanol feed location yielding a low-conversion steady state, the introduction of mass-transfer resistance (i. e., use of the NEQ model), leads to a conversion higher than that predicted by the EQ model). The introduction of a mass-transfer resistance alleviates a bad situation and has the effect of improving conversion. 

Lee and Dudukovic [18] described an NEQ model for homogeneous RD in tray columns. The Maxwell-Stefan equations are used to describe interphase transport, with the AIChE correlations used for the binary (Maxwell-Stefan) mass-transfer coefficients. Newton s method and homotopy continuation are used to solve the model equations. Close agreement between the predictions of EQ and NEQ models were found only when the tray efficiency could correctly be predicted for the EQ model. In a subsequent paper Lee and Dudukovic [19] presented a dynamic NEQ model of RD in tray columns. The DAE equations were solved by use of an implicit Euler method combined with homotopy continuation. Murphree efficiencies calculated from the results of an NEQ simulation of the production of ethyl acetate were not constant with time. 

Sundmacher et al. [20] used both EQ stage (with Murphree efficiency) and NEQ models to simulate the MTBE and TAME processes. The reactions were handled using both quasi-homogeneous and heterogeneous methods. Simulation results were compared to experimental data obtained in two laboratory-scale columns. A detailed NEQ model was needed to describe the TAME process, but both NEQ and the EQ stage (with an efficiency of 0.8) model could adequately represent the MTBE process. 

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