Design Of Reactive Distillation Columns Research Articles

Reactive distillation for methyl acetate production

We describe a hierarchy of methods, models, and calculation techniques that support the design of reactive distillation columns. The models require increasingly sophisticated data needs as the hierarchy is implemented. The approach is illustrated for the production of methyl acetate because of its commercial importance, and because of the availability of adequate published data for comparison. In the limit of reaction and phase equilibrium, we show (1) the existence of both a minimum and a maximum reflux, (2) there is a narrow range of reflux ratios that will produce high conversions and high purity methyl acetate, and (3) the existence of multiple steady states throughout the entire range of feasible reflux ratios. For finite rates of reaction, we find (4) that the desired product compositions are feasible over a wide range of reaction rates, up to and including reaction equilibrium, and (5) that multiple steady states do not occur over the range of realistic reflux ratios, but they are found at high reflux ratios outside the range of normal operation. Our calculations are in good agreement with experimental results reported by Bessling et al., [Chemical Engineering Technology 21 (1998) 393].

A disjunctive programming approach for the optimal design of reactive distillation columns

A generalized disjunctive programming formulation is presented for the optimal design of reactive distillation columns using tray-by-tray, phase equilibrium and kinetic based models. The proposed formulation uses disjunctions for conditional trays to apply the MESH and reaction kinetics equations for only the selected trays in order to reduce the size of the nonlinear programming subproblems. Solution of the model yields the optimal feed tray locations, number of trays, reaction zones, and operating and design parameters. The disjunctive program is solved using a logic-based outer-approximation algorithm where the MILP master problem is based on the big-M formulation of disjunctions, and where a special initialization scheme is used to reduce the number of initial NLP subproblems that need to be solved. Two examples are presented that include reactive distillation for the metathesis reaction of 2-pentene and for the production of ethylene glycol. The results show that the proposed method can effectively handle these difficult nonlinear optimization problems.

Effect of number of fractionating trays on reactive distillation performance

AbstractSneesby et al. recently suggested that adding trays in the stripping and rectifying sections of a reactive distillation column can degrade performance. This effect, if true, is not only counterintuitive, but very disturbing because it suggests that the design of reactive distillation columns cannot use conservative estimates of tray numbers, that is, we cannot simply add excess trays, as in conventional distillation. The problem is compounded by the uncertainty in vapor–liquid equilibrium data and tray efficiencies. This implies that developing reactive distillation columns would require extensive experimental work at the pilot‐plant and plant stages to find the numbers of stages offering the best performance. Such a scenario would mean long and expensive development programs. This article explores the effect of the number of trays in the rectifying and/or stripping sections of reactive (catalytic) distillation columns. Three reactive distillation systems are used: an ideal hypothetical system, the ETBE system, and the methyl acetate system. Contrary to the published results, it is demonstrated that additional trays do not degrade performance. Two degrees of freedom available in all cases must be carefully chosen for fair comparisons.

Optimisation of the conceptual design of reactive distillation columns

It is shown that the introduction of a different tray holdup in the stripper and rectifyer section of a continuous kinetically controlled reactive distillation column facilitates the design procedure. It also creates a range of design alternatives, generating the possibility to optimise for both the total number of trays and the total liquid holdup in the column. This allows for minimising investment related costs such as the column height and the amount of catalyst. When investigating the influence of product specifications and process parameters such as the heat of reaction and the stoichiometric sum on column design, the introduced range of design alternatives should be compared rather than single designs.

Comparison of equilibrium stage and nonequilibrium stage models for reactive distillation

For modelling reactive distillation columns, two distinctly different approaches are available in the literature: (1) the equilibrium (EQ) stage model, in which the vapour and liquid phases are assumed to be in thermodynamic equilibrium, and (2) the nonequilibrium (NEQ) stage model in which the finite mass transfer rates across the vapour–liquid interface are accounted for. In this paper, these two approaches are compared using two case studies: (a) synthesis of MTBE and (b) hydration of ethylene oxide to ethylene glycol. It is shown that while the phenomena of multiple steady states is exhibited by both modelling approaches, the “window” in which these multiplicities occur is significantly reduced in the NEQ model. It is also shown that in actual column design, some of the steady states calculated by the EQ model cannot be realised due to e.g. flooding or weeping limitations on distillation trays. Another important conclusion that can be drawn from this work is that the hardware design can have a significant influence on the conversion and selectivity. It is concluded that for design of reactive distillation columns we must adopt the NEQ modelling approach.

A new apparatus for phase equilibria in reacting mixtures

The design of reactive distillation columns requires the data for the phase and the reaction equilibrium. The phase equilibrium data for the binary reacting sub-system are difficult to get. A new apparatus has been constructed for reaction rates of half-life of 30 min. Data are presented for methanol with acetic acid and propionic acid at 60°–90°C. These temperatures represent the operation points of technical columns.

Design of reactive distillation columns for multicomponent kinetically controlled reactive systems

A design method for a reactive distillation column for the multi-component reactive system is developed and illustrated. The method is based on the theory of equilibrium and kinetic fixed points in the distillation map. The proposed graphical presentation helps in extrapolating the design aspects that have been developed in the earlier studies on this subject. An example of a four component system is presented to illustrate the proposed design method.

Kinetic Azeotropy and Design of Reactive Distillation Columns

The reactive fixed points in the distillation maps of a reactive distillation column (RDC) with kinetically controlled reactions are identified and their role in deciding the design feasibility has been elucidated. The fixed points at which both reaction and distillation vectors have zero magnitudes correspond to the equilibrium fixed point. It is known that the relative positions of these points for the rectifying and stripping sections determine the value of the minimum reflux ratio. However, apart from these fixed points, there are certain fixed points in the distillation map at which, though the reaction and distillation vectors have nonzero magnitudes, they nullify the effects of each other. These points correspond to the kinetic fixed points and have a special significance. Their positions have direct influence on the feasible product composition. A simple example of an ideal ternary system undergoing a reaction 2B ⇔ A + C has been illustrated to show the importance of kinetic azeotropy in the desig...

98/04274 Effects of unfavorable thermodynamics on reactive distillation column design

98/04274 Effects of unfavorable thermodynamics on reactive distillation column design

Detrimental influence of excessive fractionation on reactive distillation

AbstractReactive distillation has been demonstrated to reduce the capital cost and increase reactant conversion in MTBE production. These advantages are potentially transferable to the production of ETBE, TAME, and other tertiary ethers. However, the design of reactive distillation columns is complicated by interaction between phase and reaction equilibrium. Whereas in conventional distillation, an increase in fractionation is always associated with an improvement in process performance (separation of key components), the same does not necessarily apply to reactive distillation. Using the production of ETBE as an example, several reactive distillation columns were designed for various feed compositions and design philosophies. It was found that the best designs incorporated high reflux ratios with a restricted number of theoretical stages, and that increasing the number of theoretical stages could actually be detrimental to process performance.

Reactive distillation column design with vapor/liquid/liquid equilibria

This paper explores the design of reactive distillation columns with vapor/liquid/liquid equilibria. The number of phases and the phase equilibria on each tray are determined by minimizing the Gibbs free energy, allowing for a variable number of phases on each tray. The minimization problems are embedded within a larger problem that minimizes the annualized cost of the column, creating a bilevel optimization problem. A novel sequential approximation approach that can identify global solutions of the inner optimization problems is also introduced. The sequential approximation approach is employed to design reactive distillation and reactive flash processes.

Reactive Distillation Column Design with Vapor/Liquid/Liquid Equilibria

Reactive Distillation Column Design with Vapor/Liquid/Liquid Equilibria

Some Aspects in the Design of Multicomponent Reactive Distillation Columns with a Reacting Core: Mixtures Containing Inerts

Some aspects related to the design of reactive distillation columns with a reacting core are addressed in this paper. A set of transformed composition variables proposed in a previous paper is used to develop both the design equations and the reactive residue curve maps for quaternary reacting mixtures. Also, conventional residue curve maps are employed to achieve a better physical and conceptual insight into the relationships between the different constitutive parts of a column with a reacting core. Limitations in product specifications of entirely reactive distillation columns are explained by means of reactive residue curve maps, and a new feasibility criterion of a given separation for reactive columns with a reacting core is also developed on the basis of the two extreme operation conditions of a column. Finally, the behavior of column profiles for both high-conversion and low-conversion cases is analyzed and optimal product specifications are suggested to obtain high purity products. As we concluded in previous works, the inerts have a central role in both the design and synthesis of a reactive distillation column. All the concepts are applied to azeotrope-forming mixtures.

Some aspects in the design of multicomponent reactive distillation columns including nonreactive species

Some aspects related to the design of reactive distillation columns are addressed in this paper. A new set of transformed composition variables is proposed for mixtures including one or more components that are inert under the process conditions. This set corresponds to an extension of that suggested by Barbosa and Doherty (1988a, Chem. Engng Sci. 43, 1523–1537) and allows to compute by means of any traditional procedure, the concentration profiles along the column and therefore, the minimum reflux ratio. The compositions of product streams of a reactive distillation column are subject to constraints of thermodynamic nature that can be determined before any attempt to design the column. These thermodynamic constraints do not have a counterpart in conventional distillation and become an essential piece of information in order to select the design variables and specify their values. A parametric analysis of the simultaneous chemical reaction and liquid-vapor equilibrium is suggested as a very useful instrument to select thermodynamically feasible design specifications. In this work, we also present an initial discussion about possible columns sequences to obtain the reaction product free of inert species. The selection and calculation of the relevant variables that are common in the first steps of the design and synthesis of reactive distillation processes are discussed through examples. Finally, throughout the entire paper several interesting physical and operational conclusions regarding inert components in reactive distillation are given. The main conclusion is that the inerts play a key role in the design of a reactive column.

Design and minimum-reflux calculations for double-feed multicomponent reactive distillation columns

A new set of transformed composition variables is introduced to simplify the design equations for double-feed, multicomponent reactive distillation columns. These new equations are used to develop a general method of calculating minimum reflux ratios for reactive distillation columns. The application of these equations to the design of reactive distillation columns for esterification processes is discussed.