Extractive distillation column simulation

Modeling, Simulation and Control of an Extractive Distillation Column... 

The digital simulation of an extractive distillation column was performed in order to understand the dynamic behaviour of the system. Based on this results a considerably simplified dynamic model of sufficient accuracy could be developed. This model was employed in the design of a state observer and of an optimal control. After implementation in the large scale plant this new control system has proved to be highly efficient and reliable. 

Finally, the bottom product of the extractive distillation column, ED-4, can be separated in a simple column since it contains the nonazeotropic species methanol and water only. This column is shown is Fig. 39, again based on a rigorous simulation. 

In this section we use an extractive distillation column as an example to demonstrate how to build a steady-state simulation. This is one column of an overall two-column system for separating isopropanol and water. The detailed design and control of the overall distUlation system will be given in Chapter 10. 

Case 1 Operating the entrainer feed temperature 5 - 15°C below the top temperature of the extractive distillation column (as suggested in Knight and Doherty ). We use 72°C in the following simulation. 

Liano-Restrepo M. and J. Aguilar-Arias, Modeling and simulation of saline extractive distillation columns for the production of absolute ethanol, Comput. Chem. Engng., 27, 527-549 (2003). 

The principle of the perfectly-mixed stirred tank has been discussed previously in Section 1.2.2, and this provides an essential building block for modelling applications. In this section, the concept is applied to tank type reactor systems and stagewise mass transfer applications, such that the resulting model equations often appear in the form of linked sets of first-order difference differential equations. Solution by digital simulation works well for small problems, in which the number of equations are relatively small and where the problem is not compounded by stiffness or by the need for iterative procedures. For these reasons, the dynamic modelling of the continuous distillation columns in this section is intended only as a demonstration of method, rather than as a realistic attempt at solution. For the solution of complex distillation and extraction problems, the reader is referred to commercial dynamic simulation packages. 

The next column C-2 handles the separation of acrylonitrile/ace to nitrile binary by extractive distillation. A large amount of water is necessary to modify the volatility of components. The simulation indicates a ratio solvent/mixture of 10 1, which corresponds roughly to the complete dissolution of acrylonitrile in water. The column has 40 theoretical stages, being simulated as reboiled stripping. Water is introduced on the top stage, the organic feed in the middle. Purified acrylonitrile leaves in top, while acetonitrile is drawn off as a liquid side stream. 

Solvent-based separation through extractive distillation consists of two distillations. The first is an extraction column with two feed (Aspen Distill was used designing this column), while the second is a simple distillation column (the driving force concept was used for designing this column). The design was then verified by rigorous simulation using Aspen Plus . The residue curve map (see Fig. 3) was used... 

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. 

Sometimes reaction rates can be enhanced by using multifunctional reactors, i.e., reactors in which more than one function (or operation) can be performed. Examples of reactors with such multifunctional capability, or combo reactors, are distillation column reactors in which one of the products of a reversible reaction is continuously removed by distillation thus driving the reaction forward extractive reaction biphasing membrane reactors in which separation is accomplished by using a reactor with membrane walls and simulated moving-bed (SMB) reactors in which reaction is combined with adsorption. Typical industrial applications of multifunctional reactors are esterification of acetic acid to methyl acetate in a distillation column reactor, synthesis of methyl-fer-butyl ether (MTBE) in a similar reactor, vitamin K synthesis in a membrane reactor, oxidative coupling of methane to produce ethane and ethylene in a similar reactor, and esterification of acetic acid to ethyl acetate in an SMB reactor. These specialized reactors are increasingly used in industry, mainly because of the obvious reduction in the number of equipment. These reactors are considered by Eair in Chapter 12. 

The block of inter-linked columns offers robust simulation of a combination of complex distillation columns, as heat-integrated columns, air separation system, absorber/stripper devices, extractive distillation with solvent recycle, fractionator/quench tower, etc. Because sequential solution of inter-linked columns could arise convergence problems, a more robust solution is obtained by the simultaneous solution of the assembly of modelling equations of different columns. 

This chapter examines quantitatively, using rigorous simulations, how this design parameter affects the energy and capital investment of the entire system. The focus is the distillate composition trade-off. The example used is the heterogeneous azeotropic distillation process, but the same issue applies in any of the other methods (e.g., extractive distillation) in which a preconcentrator column is used. 

The matrix approach is easily adapted to partial condensers and to columns with side streams (see Problems 6.C2 and 6.C1). The approach will converge for normal distillation problems. Extension to more complex problems such as azeotropic and extractive distillation or very wide boiling feeds is beyond the scope of this book however, these problems will be solved with a process simulator. 

Different physical modes are sometimes available for the same unit operation. A distillation column can, for example, be modeled on the basis of theoretical stages or using a rate-based model, taking into account the mass transfer on the column internals. A simulation of this kind can be used to extract the data for the design of the process equipment or to optimize the process itself During recent years, dynamic simulation has become more and more important. In this context, dynamic means that the particular input data can be varied with time so that the time-dependent behavior of the plant can be modeled and the efficiency of the process control can be evaluated. 

A commercial simulation program, HYSYS, was used for simulation of the fractional distillation column. The flow diagram for acetic acid extraction process is shown in Figure 18.7. The acetic acid concentration used in our design are 20 and 80 wt%. By removing water from the product flow, the acetic acid concentration the top of the distillation column is 38.6 wt%. The distillation column was optimized at 8 plates with feed entering at plate 4. 

CHP production The solid residue from the beer column and me concentrated syrup from the evaporation plant can be used to generate heat to the process and electricity in a CHP plant. If necessary, bark from the debarking unit can also be used. The CHP plant (back-pressure steam turbine unit with steam extraction at appropriate pressme levels to supply utility and process steam) was simulated in Aspen Plus (for input dam, see Figure 4.5). Utility steam is used for process heating in, for example, distillation column reboilers and can be merefore replaced by another hot utility at me same temperature. Process steam is used in the process directly as, for example, in the pretreatment step where steam is used for steam explosion to separate to wood components. As steam in this case is an essential part of the process, it cannot be replaced by another utility. 

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