Distillation nonequilibrium modeling

Higler AP, Krishna R, Taylor R. Nonequilibrium modeling of reactive distillation a dusty fluid model for heterogeneously catalyzed processes. Ind Eng Chem Res 2000 39 1596-1607. 

Sundmacher K, Hoffmann U. Development of a new catalytic distillation process for fuel ethers via a detailed nonequilibrium model. Chem Eng Sci 1996 51 2359-2368. 

The methods based on the equilibrium stage model have existed for over 30 years and refinements continue, but serious development of nonequilibrium models has begun only recently. These methods are an alternative means to the stage model for predicting column performance. They are expected to make inroads, especially for systems for which stage efficiency prediction is very difficult, such as reactive distillation, chemical absorption, and three-phase distillation. However, their progress into systems where efficiency prediction is well-established is likely to be slower. Their complexity due to the restriction to... 

The computer simulation of distillation processes, whether done using equilibrium or nonequilibrium models, requires us to address the following topics ... 

There is a rapidly growing body of literature on nonequilibrium modeling of distillation and absorption processes. An extended bibliography is available at www.chemsep.org/publications. A brief review of other applications follows. 

There is now an extensive literature on using nonequilibrium models for reactive distillation see, e.g., Taylor and Krishna [Chem. Eng. 

Even at steady state, efficiencies vary from component to component and with position in a column. Thus, if the column is not at steady state, then efficiencies also must vary with time as a result of changes to flow rates and composition inside the column. Thus, equilibrium-stage models with efficiencies should not be used to model the dynamic behavior of distillation and absorption columns. Nonequilibrium models for studying column dynamics are described hy, e.g., Kooijman and Taylor [AlChE 41, 1852 (1995)], Baur et al. [Chem. 

In this chapter we discuss some work that has been done towards the development of what we shall refer to as nonequilibrium models of multicomponent distillation (and absorption and extraction) operations. The building blocks of any nonequilibrium model will include... 

In Section 14.4.2 we shall show that the nonequilibrium model provides excellent predictions of the product compositions in the extractive distillation of an acetone (l)-methanol... 

The nonequilibrium model is not limited to simulating distillation operations with no fundamental change, it can be applied to absorption operations as well. A design case study is presented here by way of illustrating the model. This problem is adapted from an application discussed by Krishnamurthy and Taylor (1986). 

Arwickar (1981) reported some results for distillation under total reflux conditions of the system acetone-methyl acetate-methanol. The experiments were carried out in a laboratory scale column of 7.62 cm diameter packed with 0.635 cm Raschig rings. The simulation of total reflux operations using the nonequilibrium model is discussed by Krishnamurthy and Taylor (1985a). In simulations of Arwickar s experiments Taylor et al. used the correlations of Onda et al. (1968) to estimate the mass transfer coefficients in each phase and the effective interfacial area. The average absolute discrepancy between predicted and measured mole fractions was less than 2 mol% for acetone and methyl acetate and less than 4 mol% for methanol. 

Gorak et al. (1991) and Wozny et al. (1991) presented a brief description of their use of a nonequilibrium state model to simulate vacuum distillation of fatty alcohols in columns fitted with structured packing. They found that the compositions predicted by the nonequilibrium model were closer to the experimental data than were the results of an equilibrium stage-HETP calculation. 

Krishnamurthy and Andrecovich (1989) used a nonequilibrium model to simulate cyro-genic distillation processes. 

McNulty and Chatterjee (1992) discuss the use of nonequilibrium models to design packed bed pumparound zones of crude distillation towers. 

Columns with profiles that contain maxima or change rapidly over a small section of the column, for example, azeotropic distillation or nonisothermal gas absorption. Nonequilibrium models can locate such features with greater accuracy. 

There are many excellent texts that discuss the design of distillation columns using equilibrium stage calculations. Some of them were cited in Chapters 12-14. These texts provide a wealth of examples that could be used as the basis for a design using the nonequilibrium model described in Chapter 14. We adapt one such example below (Exercise 14.1) in order to indicate how this might be done. 

Most experimental studies of distillation are carried out at total reflux to prevent loss of materials. How would the nonequilibrium model of Chapter 14 be used to simulate a distillation column operating under total reflux conditions ... 

Gorak and Vogelpohl (1985) present an experimental study of ternary distillation in a packed column. The system used was methanol(l)-2-propanol(2)-water(3) and the column was 0.1 m wide and filled with Sulzer CY packing. Use the nonequilibrium model to simulate their experiments. Investigate the sensitivity of the simulation results to the thermodynamic model parameters. Write an article in the format required by Separation Science Technology that summarizes your calculations. 

Burghardt et al. (1983) present data for the distillation of methanol(l), 2-propanol(2), water(3) and acetone(l), ethanol(2), water(3) in a sieve tray column (0.25-m diameter). Use the nonequilibrium model of Chapter 14 to simulate their experiments. Write up your work in an article in the format required by the Chemical Engineering Journal. 

Taylor, R., Kooijman, H. A., and Woodman, M. R., Industrial Applications of a Nonequilibrium Model of Distillation and Absorption Operations, The Institution of Chemical Engineers Symposium Series No. 128, Distillation and Absorption 1992, MI5-M21 (1992). 

Y Zheng, FTT Ng, GL Rempel. Catalytic distillation A three-phase nonequilibrium model for the simulation of the aldol condensation of acetone. Ind Eng Chem Res. 40 5342-5349, 2001. 

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