Nonequilibrium distillation column

Case-Study for Solver Validation Nonequilibrium Distillation Column Model... 

Two different approaches have evolved for the simulation and design of multicomponent distillation columns. The conventional approach is through the use of an equilibrium stage model together with methods for estimating the tray efficiency. This approach is discussed in Chapter 13. An alternative approach based on direct use of matrix models of multicomponent mass transfer is developed in Chapter 14. This nonequilibrium stage model is also applicable, with only minor modification, to gas absorption and liquid-liquid extraction and to operations in trayed or packed columns. 

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. 

Discuss how the fundamental models of mass transfer in Sections 12.1.7 (binary systems) and 12.2.4 (multicomponent systems) may be used to estimate mass transfer rates for use in a nonequilibrium simulation of an existing distillation column. Your essay should address the important question of how the model parameters are to be estimated. 

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 ... 

Example 6.16 Nonequilibrium Distillation Calculations Using ChemSep Sieve-Tray Column Design... 

Kooijman, H.A. and R. Taylor, Nonequilibrium model for dynamic simulation of tray distillation columns. AIChE Journal, 1995, 41(8) 1852 1863. 

Fig. 9.12 a) Configuration of reactive distillation column for hydration of ethylene oxide to ethylene glycol used by Ciric and Miao (1994). b) Equilibrium model calculations for the ethylene glycol process showing column profiles for liquid phase mole fraction, temperature and vapor phase molar flow, c) Nonequilibrium model calculations for the ethylene glycol process for a column of diameter 1.7 m showing the corresponding column profiles. Details of the simulations are available in Baur et al. 

It is standard practice in the modeling of distillation columns to assume that the liquid is incompressible and perfectly mixed on the trays, and that the vapor and liquid have the same temperature on the tray (thermal equilibrium), whereas the phases could be considered nonequilibrium and some vapor-phase efficiency is sometimes adopted in such cases (e.g., Murphree efficiency). Beyond these possible variants, the basic distillation column model can be reduced to total... 

Baur R., Taylor R. and Krishna R. (2001a). Dynamic behaviour of reactive distillation columns described by a nonequilibrium stage model. Chemical Engineering Science 56, 2085-2102. 2.9.1.2, 2.9.1.2, 2.9.3, 4.3... 

Higler A., Krishna R. and Taylor R. (1999a). Nonequilibrium cell model for packed distillation columns - the influence of maldistribution. Industrial and Engineering Chemistry Research 38, 3988—3999. 

So far, discussing distillation trajectories and their bundles, we proceeded from the fact, that separation stages are equilibrium ( theoretical plates). In real separation process at plates of distillation columns equilibrium is not achieved and the degree of nonequilibrium is different for different components. That leads to decrease of difference between compositions at neighboring plates and to change of curvature of distillation trajectories (Castillo Towler, 1998), but does not influence the location of stationary points of distillation trajectory bundles because in the vicinity of stationary points equilibrium and nonequilibrium trajectories behave equally. Therefore, implemented above analysis of the structure and of evolution of section trajectory bundles is also valid for nonequilibrium trajectory bundles. 

All of the discussions so far regarding distillation lines, residue curves and distillation boundaries have assumed equilibrium behavior. Real columns do not work at equilibrium, and stage efficiency must be accounted for. Each component will have its own stage efficiency, which means that each composition will deviate from equilibrium behavior differently. This means that if nonequilibrium behavior is taken into account, the shape of the distillation lines, residue curves and distillation... 

The rate-based models suggested up to now do not take liquid back-mixing into consideration. The only exception is the nonequilibrium-cell model for multicomponent reactive distillation in tray columns presented in Ref. 169. In this work a single distillation tray is treated by a series of cells along the vapor and liquid flow paths, whereas each cell is described by the two-film model (see Section 2.3). Using different numbers of cells in both flow paths allows one to describe various flow patterns. However, a consistent experimental determination of necessary model parameters (e.g., cell film thickness) appears difficult, whereas the complex iterative character of the calculation procedure in the dynamic case limits the applicability of the nonequilibrium cell model. 

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... 

Nonequilibrium molecular dynamics simulations show that the assumption of local equilibrium in a column with heat and mass transfer is acceptable. The dissipation function in a binary distillation is (Ratkje et al., 1995 Sauar etal., 1997)... 

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. 

Because of nonequilibrium boiling conditions in a simple distillation, the vapors may not contain the true azeotrope, and the heat cost may be too high. Therefore a column is still used to rectify the exact azeotrope at the head of the column however, only a few trays are required. 

The plate theory assumes that the solute is in equilibrium with the mobile and stationary phases. Due to the continuous exchange of solute between the two phases as it progresses down the column, equilibrium between the phases can never actually be achieved. To accommodate this nonequilibrium condition, a technique originally introduced in distillation theory is adopted, where the column is considered to be divided into a number of cells or plates. Each cell is allotted a finite length and, thus, the solute spends a finite time in each cell. The size of the cell is such that the solute is considered to have sufficient residence time to achieve equilibrium with the two phases. Thus, the smaller the plate, the more efficient the solute exchange between the two phases and, consequently, the more plates there are in the column. As a result, the number of theoretical plates contained by a column has been termed the column efficiency. The plate theory shows that the peak width (the dispersion or peak spreading) is inversely proportional to the square root of the efficiency and, thus, the higher the efficiency, the narrower the peak. Consider the equilibrium that is assumed to exist in each plate then... 

Fat lot of good that does you when you have to hand in a sample. So, you turn up the heat, let some of the vapor condense, and take off this top fraction. This raises hell in the column. Nonequilibrium conditions abound— mixing. Arrrgh No more completely pure compound. And the faster you distill, the faster you let material come over, the higher your through-... 

In Section 13.5.1 we presented the data of Vogelpohl for the distillation of two ternary systems acetone-methanol-water and methanol-2-propanol-water in a bubble cap column. Krishnamurthy and Taylor (1985b) simulated these experiments using a nonequilibrium stage model similar to the one described above. The AIChE correlations were used to calculate the mass transfer coefficients. Thermodynamic properties were calculated with the models described by Prausnitz et al. (1980). 

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. 

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. 

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. 

Chapter 12 presents models of mass transfer on distillation trays. This material is used to develop procedures for the estimation of point and tray efficiencies in multicomponent distillation in Chapter 13. Chapter 14 uses the material of Chapter 12 in quite a different way in an alternative approach to the simulation and design of distillation and absorption columns that has been termed the nonequilibrium stage model. This model is applicable to liquid-liquid extraction with very little modification. Chapter 15 considers the design of mixed vapor condensers. 

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