Design of reactive distillation columns

The design of reactive distillation columns is complicated by the complex interactions between the reaction and separation processes. A comprehensive discussion of the process is given by Sundmacher and Kiene (2003). 

The state-of-the-art can be found in the monograph edited by Sundmacher and Kienle [16], as well as in the books of Doherty and Malone [8], Stichlmair and Fair [14], The case study referring to the manufacturing of fatty esters presents a practical approach to the design of reactive-distillation columns. 

Mahajani S. (1999a). Design of reactive distillation columns for multicomponent kinetically controlled reactive systems. Chemical Engineering Science 54 (10), 1425-1430. 3.2.4, 3.2.4... 

Mahajani S. (19996). Kinetic azeotropy and design of reactive distillation columns. Industrial and Engineering Chemistry Research 38 (1), 177-186. 3.2.4... 

Melles S., Grievink J. and Schrans S.M. (2000). Optimization of the conceptual design of reactive distillation columns. Chemical Engineering Science 55, 2089-2097. 3.2.4, 6.1.3... 

Jackson, J., Grossmann, I.E., A Disjunctive Porgramming Approach for the Optimal Design of Reactive Distillation Columns, Comp. Chem. Eng. 25 (2001), 1661-1673. 

Steady-state designs of reactive distillation columns are developed that are economically optimum in terms of total annual cost, which includes both energy and capital costs. The economics of reactive distillation columns are quantitatively compared with conventional multiunit processes over a range of parameter values (chemical equilibrium constants,... 

These books deal primarily with the steady-state design of reactive distillation columns. Conceptual approximate design approaches are emphasized. There is little treatment of rigorous design approaches using commercial simulators. The issues of dynamics and control... 

In the previous section, the optimum economic steady-state designs of reactive distillation columns were quantitatively compared with conventional multiunit systems for a wide range of chemical equilibrium constants. Relative volatilities (a = 2) were assumed constant. Reactive distillation was shown to be much less expensive than the conventional process. In this section we explore how temperature-dependent relative volatilities affect the designs of these two systems. 

Several hundred papers and patents have appeared in the area of reactive distillation, which are too numerous to discuss. A number of books have dealt with the subject such as (1) Distillation, Principles and Practice by Stichhnair and Fair, (2) Conceptual Design of Distillation Systems by Doherty and Malone," and (3) Reactive Distillation— Status and Future Directions by Sundmacher and Kienle. These books deal primarily with the steady-state design of reactive distillation columns. Conceptual approximate design approaches are emphasized, but there is little treatment of rigorous design approaches using commercial simulators. The issues of dynamics and control stmcture development are not covered. Few quantitative eeonomic comparisons of conventional multiunit processes with reactive distillation are provided. 

We do not discuss the control of reactive distillation columns in this book. Although reactive distillation has been used for many years in specific industrial applications, only recently have systematic studies appeared in the open literature on both steady-state design and dynamic control (Doherty and Buzad, 1992 Sneesby et al., 1997). Because generic understanding of this technology is still in an early stage of development, we feel it would be premature to speculate on specific recommendations. However, we are confident that many of the ideas and techniques discussed in this book will apply directly to this microcosm of the plantwide control problem. 

Reactive distillation, as the name implies, refers to a distillation process that incorporates a reaction and a separation step within a distillation column. The technique offers a key opportunity for improving the structure of a process. - It is a so-called hybrid process, i.e. it merges two different unit operations in a single apparatus, namely reaction and distillation. But the combination of distillation and reactions is possible only if the conditions of both unit operations can be combined. This means that the reactions have to show reasonable data for conversions at pressure and temperature levels that are compatible with distillation conditions. Because of the limited hold-up in distillation column, those reactions having a conversion half-time of 10-30 min are preferred. So, the judicious use of the chemical equilibrium constant is the basis for the design of reactive distillation processes. 

Many patents and publications relating to reactive distillation have been published. A paper on the heuristics of reactive distillation column design contains many references. ... 

Our emphasis is on rigorous simulations, not approximate methods. Rigorous models are used for steady-state design and dynamic analysis of a variety of different types of reactive distillation columns. Several types of ideal systems are studied as well as several real chemical systems. 

In Parts I and II we explored the steady-state designs of several ideal hypothetical systems. The following three chapters examine the control of these systems. Chapter 10 considers the four-component quaternary system with the reaction A + B C + D under conditions of neat operation. Chapter 11 looks at control of two-column flowsheets when an excess of one of the reactants is used. Chapter 12 studies the ternary system A + B C, with and without inerts, and the ternary system A B + C. We will illustrate that the chemistry and resulting process structure have important effects on the control structure needed for effective control of reactive distillation columns. 

From a thermodynamics basis, the transesterification reaction favors the formation of methylphenyl carbonate (Equation 7.4), whilst its further disproportionation in a second-stage continuous reactive distillation column affords DPC with selectivity >99%. Although both reactions occur at a relatively high temperature ( 473 K), optimization of the reaction conditions and engineering design would allow a productivity that fitted with the economics [17, 27]. 

Fixed-bed catalytic reactors and reactive distillation columns are widely used in many industrial processes. Recently, structured packing (e.g., monoliths, katapak, mella-pak etc.) has been suggested for various chemical processes [1-4,14].One of the major challenges in the design and operation of reactors with structured packing is the prevention of liquid flow maldistribution, which could cause portions of the bed to be incompletely wetted. Such maldistribution, when it occurs, causes severe under-performance of reactors or catalytic distillation columns. It also can lead to hot spot formation, reactor runaway in exothermic reactions, decreased selectivity to desired products, in addition to the general underutilization of the catalyst bed. 

Today, there is an increasing interest in the theoretical study and the practical application of integrated reactive separation processes such as reactive distillation columns [1-3] or membrane-assisted reactors [37]. However, to date there is no general method available for designing such processes. For practical applications, it is important to be able to evaluate quickly whether a certain reactive separation process is a suitable candidate to reach certain targets. Therefore, feasibility analysis tools being based on minimal thermodynamic and kinetic information of the considered system are valuable. 

The last step regards the detailed design of the reactive-distillation column and of other operational units. The hydraulic design is consolidated taking into account the optimal traffic of liquid and vapor. Additional internals are provided to ensure uniform distribution of fluids and a sharp residence-time distribution. 

Ind. Eng. Chem. Res., 41, 2735, 2002 Ellenberger, J Krishna, R., Counter-current operation of structured catalytically packed distillation columns, Chem. Eng. Sci., 54,1339-1345, 1999 Krishna, R Hardware selection and design aspects for reactive-distillation columns, Reactive Distillation, eds Sundmacher K. and Kienle, A., Wiley-VCH, Weinheim, Germany, 169-189,... 

Simulation, Modeling, and Design Feasibility Because reaction and separation phenomena are closely coupled in a reactive distillation process, simulation and design are significantly more complex than those of sequential reaction and separation processes. In spite of the complexity, however, most commercial computer process modeling packages offer reliable and flexible routines for simulating steady-state reactive distillation columns, with either equilibrium or kinetically controlled reaction models... 

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