Design Of Reactive Distillation Columns Research Articles

Understanding the impact of reactive holdup on process intensification in the design of reactive distillation column

The commercial viability of reactive distillation as a front-runner industrial process intensification technique is limited by slow reactions. It is because the diameter established by the vapor-loading method restricts large holdup volumes (or catalyst amount) to accomplish the essential conversion. However, placing a large amount of catalyst on column trays necessitates either a high tray weir height (limited to excessive column pressure drop) or a large column diameter. This study aims to investigate an alternative design approach by increasing the column diameter beyond that required for vapor loading to retain a large holdup volume. Several combinations of tray weir heights and column diameters were studied and demonstrated through case studies for three industrial processes, and their optimal designs have been reported. High catalyst holdup volume enhanced energy efficiency and overcame hydraulic limitations despite requiring large diameter vessels. These design configurations with optimized catalyst holdup also resulted in improved process economics. Fabricating a wide column diameter is a good conservative engineering procedure considering safety aspects and better design control.

Unraveling the matching of reaction and separation for reactive distillation process based on reaction equilibrium degree (RED)

The multiplicity of matching between reaction and separation for reactive distillation (RD) process leads to a more complex design of RD column, however, the exploration of coupling problem between reaction and separation for the design of reaction section in the RD column is still a blank of RD research. In this article, a design framework for the reaction section in the RD column based on the reaction equilibrium degree (RED) is demonstrated. The interaction relationship between residence degree and separation efficiency of reaction section is established by means of spline interpolation method. Herein, the synthesis of malonic acid (MA) from dimethyl malonate (DM) hydrolysis, as a case study, was investigated to obtain coupling regularities of reaction and separation for specified design targets and further design the reaction section of RD column from principle. The results validated the effectiveness of the proposed framework and provided theoretical guidance for challenging RD process.

Energy Integration and Reactive Distillation for Dimethyl Ether Synthesis via Catalytic Dehydration of Methanol

Distillation is the most widely used separation technique in the process industry. Its main disadvantage is the high consumption of thermal energy: the heat requirements of the distillation columns often represent most part of the total energy cost of the plant, appropriate integration of the distillation columns with the overall process often results in a significant reduction of the energy consumption in many cases.In this work the concept of the integration has been applied by two integrated design approaches:Heat-integrated design; where the distillation system of the process will be heat-integrated with the background process (i.e. integration between the distillation columns and the heat exchangers).Reactive distillation design; where the reaction and distillation will be accomplished in the same unit.The process of dimethyl ether (DME) synthesis by the catalytic dehydration of methanol has been considered as a case study. Heat-integrated design of DME process showed a significant total annual cost saving of 41 % compared to the conventional process, and the reactive DME distillation column design showed highest saving in total annual cost (TAC) by 81 %. In addition to the cost saving of the two proposed designs, also the flue gas emissions have been reduced using process integration methodology, where 51 % of the total emissions has been reduced by the heat-integrated design and 86 % by the reactive distillation column design.

Modelling of heat integrated reactive distillation column (r-HIDiC): Simulation studies of MTBE synthesis

In the present study, heat integrated distillation column is modelled and simulated for the production of Methyl tert-Butyl Ether (MTBE) from methanol and isobutylene using ASPEN Plus®. Also, the objective of this study is to have comparative analysis between the reactive distillation process and an alternative intensified process (r-HIDiC) to produce MTBE. Rigorous simulation study has been conducted to compare the optimal flow sheet of the above two designs. Temperature and composition profiles are studied and compared for both the models and found that the temperature at the last stage of the stripping section in r-HIDiC is 406 K while that of the stripping section in RDC is 427.24 K. This reduced temperature decreases the composition of methanol in bottom product in stripping column in case of r-HIDiC as methanol forms a homogenous azeotrope with MTBE at 408.53 K. Also, it is found that, although with nominal increase in TAC i.e. 16.56%, in r-HIDiC design as compared to the reactive distillation column design which is caused due to the high cost of the additional compressor needed in the vapour compression, however, the r-HIDiC design can save reboiler and condenser heat duties to a considerable amount i.e. by 35.06% and 36.65% respectively.

Reactive Distillation Column Design for Tetraethoxysilane (TEOS) Production: Economic and Environmental Aspects

Alkoxysilanes are silicon compounds containing a Si-OR bond. The applications of tetraalkoxysilanes cover a wide range depending if the Si-OR bond remains intact or is hydrolyzed. Currently, tetraethoxysilane (TEOS) is the most prominent derivative of such silicon compound family. The best known and used method of synthesis of TEOS is the esterification reaction of SiCl4, following the old Von Ebelman’s route (1846). Tetramethoxy- and tetraethoxysilane are obtained by a technology developed in the 1990’s involving the catalyzed direct synthesis of silicon metal and either methanol or ethanol. However, there is few information about novel technologies to produce TEOS, either improved current technology or proposed new approaches. In this work, the reactive distillation (RD) is suggested to produce TEOS as an intensified technology. In general, the concept of process intensification may overcome the traditional process, since it allows design technology with reduction on equipment size, improvement in energ...

Multivariable model predictive control design of reactive distillation column for Dimethyl Ether production

Dimethyl ether (DME) as an alternative clean energy has attracted a growing attention in the recent years. DME production via reactive distillation has potential for capital cost and energy requirement savings. However, combination of reaction and distillation on a single column makes reactive distillation process a very complex multivariable system with high non-linearity of process and strong interaction between process variables. This study investigates a multivariable model predictive control (MPC) based on two-point temperature control strategy for the DME reactive distillation column to maintain the purities of both product streams. The process model is estimated by a first order plus dead time model. The DME and water purity is maintained by controlling a stage temperature in rectifying and stripping section, respectively. The result shows that the model predictive controller performed faster responses compared to conventional PI controller that are showed by the smaller ISE values. In addition, the MPC controller is able to handle the loop interactions well.

Reaction‐separation interaction on the control of ideal two‐feed, two‐product exothermic reactive distillation columns

AbstractIn this paper, the interaction of reaction and separation in an optimal design of an ideal, hypothetical, exothermic, quaternary (A + B↔C + D) reactive distillation (RD) system is explored. The relative volatilities of the components are αC:αA:αB:αD = 8:4:2:1. The closed loop dynamics of the column are evaluated for five different decentralized control structures. The prevention of the accumulation of light reactant A is crucial for successful control. A drastic improvement in the controllability is observed for a modified design of the RD system having a greater number of reactive trays as the accumulation of A is effectively prevented. The controllability improvement is observed to be control structure specific. A comprehensive understanding of the interaction of reaction and separation is very helpful for the design of RD columns of improved controllability. Application of the plant‐wide control principles in the control of RD columns is highlighted.

Reactive distillation columns with two reactive sections: Feed splitting plus external recycle

For the separations of the reacting mixtures with the most unexpected relative volatility order (i.e., the reactants feature the highest and lowest relative volatilities and the products the intermediate ones), feed splitting (FS) should be combined with top-bottom external recycle (TBER) in the design of reactive distillation columns with two reactive sections (RDC-TRS). Despite FS lessens the reactants with the highest and lowest relative volatilities flowing through the intermediate product extraction stage, the TBER is still effective to improve the contact of the reactants in the upper/lower reactive section and intensifies consequently internal mass and energy integration between the reaction operation and the separation operation involved. The design strategy is evaluated in terms of the separations of esterifications of lactic acid with methanol and palmitic acid with isopropanol. The simultaneous employment of FS and TBER is found to favor greatly system performance and the resultant process design is superior to those with one reactive section reinforced by a TBER and/or FS. These outcomes imply that the RDC-TRS with FS plus TBER should be the most competitive alternative for the separations of the reacting mixtures with the most unexpected relative volatility order.

A fixed point methodology for the design of reactive distillation columns

Reactive distillation (RD), one of the best-known examples of process intensification, has been received significant attention. Although the advantages of RD are well documented in the literature, the commercial applications of RD are still limited because of the rather complicated design methodology in multi-component reaction system. In this paper, a modified fixed point method for the design of reactive distillation column with the multi-component reactive system is developed and illustrated. The method is based on the theory of phase equilibrium, kinetic fixed points and element concept in the reactive distillation map. The proposed graphical presentation takes the effect of temperature on the separation and reaction into account, which helps in extrapolating the design details. Three example systems are employed to evaluate the modified fixed point methodology proposed. The results show that the methodology is feasible and computationally efficient even for the complex configurations and multi-component reaction systems.

An integrated reactive distillation process for biodiesel production

Abstract An integrated reactive distillation process for biodiesel production is proposed. The reactive separation process consists of two coupled reactive distillation columns (RDCs) considering the kinetically controlled reactions of esterification of the fatty acids (FFA) and the transesterification of glycerides with methanol, respectively. The conceptual design of the reactive distillation columns was performed through the construction of reactive residue curve maps in terms of elements. The design of the esterification reactive distillation column consisted of one reactive zone loaded with Amberlyst 15 catalyst and for the transesterification reactive column two reactive zones loaded with MgO were used. Intensive simulation of the integrated reactive process considering the complex kinetic expressions and the PC-SAFT EOS was performed using the computational environment of Aspen Plus. The final integrated RD process was able to handle more than 1% wt of fatty acid contents in the vegetable oil. However, results showed that the amount of fatty acids in the vegetable oil feed plays a key role on the performance (energy cost, catalyst load, methanol flow rate) of the integrated esterification–transesterification reactive distillation process.

Synthesis and design of reactive distillation columns with two reactive sections

The synthesis and design of reactive distillation columns with two reactive sections (RDC–TRS) are attempted for the separations of reacting mixtures with somewhat unfavorable ranking of relative volatilities (RM–SURRV, i.e., two reactants are the light and heaviest components and two products the lightest and heavy ones). With the deliberate arrangement of the two reactive sections, the disadvantages by the unfavorable relative volatility can be substantially alleviated, facilitating consequently the reaction operation and separation operation involved. The separations of a hypothetical quaternary reaction, the esterification of acetic acid with methanol, and the transesterification of butyl acetate with ethanol are employed to evaluate the RDC–TRS. The process is found considerably superior to the reactive distillation column with a single reactive section and this demonstrates its feasibility and effectiveness in the separations of RM–SURRV. The RDC–TRS is found to present comparable or even improved performance in comparison with the reactive distillation column with an external recycle from bottom to reactive section and this corroborates it a potentially competitive alternative for the separations of the RM–SURRV. The RDC–TRS is also highlighted for the other kind of RM–SURRV (i.e., two reactants are the lightest and heavy components and two products the light and heaviest ones).

Adopting feed splitting in design of reactive distillation columns with two reactive sections

For the separations of reacting mixtures featuring the heaviest and lightest reactants and intermediate products, although feeding the heaviest/lightest reactant to condenser/reboiler of reactive distillation columns with two reactive sections (RDCs-TRS) at top/bottom favors the reaction operation involved, it poses a detrimental effect to the separation operation involved because the reactants tend to move through the stage for withdrawing the intermediate product and increase consequently separation difficulties. A performance index is derived for the RDC-TRS and feed splitting is adopted to suppress the drawback. The separations of an ideal quaternary reaction, the esterification of lactic acid with methanol, and the esterification of palmitic acid with isopropanol, are employed to evaluate the proposed design philosophy. The resultant design is found considerably superior to the RDC-TRS and exhibits better or at least comparable performance with the reactive distillation column with a top–bottom external recycle. These outcomes indicate that feed splitting is especially effective for enhancing internal mass and energy integration between the reaction operation and separation operation involved. Despite the existence of adverse internal energy integration in case of reactions with thermal effect, the reinforced internal mass integration could still facilitate the resultant design a potentially competitive alternative.

Kinetics Studies on Esterification Reaction of Acetic acid with Iso-amyl Alcohol over Ion Exchange Resin as Catalysts

In this work, the esterification of acetic acid with iso-amyl alcohol catalyzed by acidic ion-exchange resins (TULSION R T-62, T-63 and T-66 MP) has been investigated in a batch reactor to determine the intrinsic reaction kinetics. Iso-amyl acetate is synthesized in this reaction, has the fragrance of strong odor (similar to both banana and pear). The effects of various parameters on the rate of reaction (such as temperature, mole ratio, catalyst loading, and stirrer speed) are studied. The solid- liquid external mass transfer resistance as observed using various stirring speeds is absent and the rate of reaction depends on temperature. Further pseudo-homogenous model is developed for an intrinsically kinetically controlled reaction, which is free from either intra-particle diffusion or external mass transfer resistance. The rate expression developed in this work would be useful in the simulation and design of reactive distillation column for the production of iso-amyl acetate. The resultant kinetic model fitted the experimental data well. The activation energy is found to be 41.88 kJ/kmol for the formation of iso-amyl acetate.

A reactive distillation column with double reactive sections for the separations of two-stage consecutive reversible reactions

In this paper, a novel reactive distillation column with double reactive sections (RDC-DRS) is proposed for the separations of two-stage consecutive reversible reactions. The arrangement of two reactive sections not only allows the careful coordination of the two reaction operations involved, but also provides additional degrees of freedom for the reinforcement of internal mass integration and/or internal energy integration between the reaction operations and the separation operation involved, which could facilitate the RDC-DRS to be more advantageous than the conventional reactive distillation column with a single reactive section (RDC-SRS) in operating cost and capital investment. A representative hypothetical and real two-stage consecutive reversible reactions are chosen to evaluate the steady-state performance of the RDC-DRS. With the constraints of the same total number of stages and the same total number of reactive stages, the RDC-DRS is demonstrated to require less utility consumption than the RDC-SRS and this outcome indicates that the former could be a competitive alternative to the latter in the separations of the two-stage consecutive reversible reactions. The number of reactive sections should therefore be viewed as an important decision variable for the synthesis and design of reactive distillation columns especially in the separations of complicated reacting mixtures involving multiple reversible reactions.

Optimal design of reactive distillation systems: Application to the production of ethyl tert-butyl ether (ETBE)

This work addresses the design of reactive distillation columns to produce ETBE, based on a detailed first-principles model that considers equilibrium and kinetic information, rigorous physical property data, and catalyst deactivation. An evolutionary algorithm is used to generate a sequence of feasible designs with improved characteristics in a sequential solution/optimisation strategy, by specifying the design variables (both integer and continuous) that characterise a particular column configuration. Two classes of optimisation algorithms are compared: genetic algorithms and particle swarm optimisation. The objective function considered is the gross annual profit.The results demonstrate that both algorithms are adequate to solve this design problem. The effect of catalyst deactivation included in the design stage played a determinant role in the optimal column specification. A post-design sensitivity analysis is developed to assess the quality of the solutions obtained, together with the individual effects of each design variable in the optimal configuration identified.

Enhancing mass and energy integration by external recycle in reactive distillation columns

The synthesis and design of reactive distillation columns separating reacting mixtures with the most unfavorable relative volatilities (i.e., the reactants are the heaviest and lightest components with the products being the intermediate ones) are described. The unfavorable thermodynamics poses great difficulties in combining the reaction operation and the separation operation involved and limits severely the potential of reactive distillation columns in the reduction of capital investment (CI) and operating cost. To remove the limitation, we propose two strategies for facilitating the synthesis and design of this kind of reactive distillation columns in this article. One is to arrange prudentially the reactive section so as to strengthen internal energy integration between the reaction operation and the separation operation involved; that is, while the reactive section should be placed at the bottom of the reactive distillation columns separating exothermic reactions, it should be at the top of the reactive distillation columns separating endothermic reactions. The other is to introduce an external recycle flow between the two ends of the reactive distillation columns to reinforce internal mass integration and internal energy integration between the reaction operation and the separation operation involved; that is, whereas the external recycle flow should be directed from the top to bottom of the reactive distillation columns separating exothermic reactions, it should be from the bottom to top of the reactive distillation columns separating endothermic reactions. Separation of four hypothetical ideal (i.e., two quaternary and two ternary systems, respectively) and two real nonideal (i.e., two quaternary systems) reacting mixtures is chosen to evaluate the proposed strategies. The results show that they can considerably lower energy requirement besides a further reduction in CI. © 2012 American Institute of Chemical Engineers AIChE J, 59: 2015–2032, 2013

Short-Cut Method for the Design of Reactive Distillation Columns

The design of reactive distillation (RD) has received significant attention because of the technological and economic advantages obtained from the simultaneous occurrence of reaction and separation. Although the advantages of RD are well-documented in the literature, the commercial applications of RD are still limited because of the control and operation complexity of these separation systems and the need for improved software tools to reliably model and design RD columns. In this study, a robust short-cut method for the design of multicomponent reactive distillation is presented. This method is based on distillation lines and tray-by-tray calculations defined in terms of reaction-invariant composition variables. Our method provides the number of theoretical stages, the operating reflux ratio, the feed tray location, and the top or bottom flow. We use three case studies to demonstrate the effectiveness of the proposed strategy, namely, the reactive systems for the syntheses of ETBE and MTBE in the presenc...

A generalized method for the synthesis and design of reactive distillation columns

Owing to the combination between the reaction operation and the separation operation involved, it is extremely difficult to determine in advance the optimum configuration of a reactive distillation column and this makes process synthesis and design a great challenging task. Currently, no easy-to-use and yet effective methods are available to guide process synthesis and design, restricting considerably the applications and therefore the impacts of reactive distillation columns to the chemical process industry. In this paper, a generalized method is proposed for the synthesis and design of reactive distillation columns in terms of the insights from process intensification. The method is initiated from a simple process design with all feeds of reactants at the middle of the process and all stages as reactive ones. In terms of an economical objective function, it can be evolved into the optimum process design via sequential structure adjustments, including reactive section arrangement, feed stage relocation, feed splitting, and catalyst redistribution. The generalized method proposed is characterized by great simplicity in principle, the capability to tap the full potentials of process intensification, and the high robustness to the initial guess of process configuration as well as the thermodynamic properties of the reacting mixtures separated. Four example systems are employed to evaluate the generalized method proposed and the obtained outcomes demonstrate its effectiveness and applicability to the synthesis and design of various reactive distillation columns.

Optimal design of a reactive distillation column

In this work we develop a MINLP model that can be used to optimize the design of reactive distillation column. MINLP model is formulated in GAMS in such a way that it can be solved locally and globally. In the RD column a component A is converted into product B while vapour and liquid are assumed to be in equilibrium. The objective is to find a design for this process that minimizes the total costs (consisting of capital and operational costs). The design variables of interest are the total number of stages, the number of reactive stages, the location of the reactive stages, the feed tray location and the reflux ratio.

Optimization-based design of reactive distillation columns using a memetic algorithm

The design optimization of reactive distillation columns (RDC) is characterized by complex nonlinear constraints, nonlinear cost functions, and the presence of many local optima. The standard approach is to use MINLP solvers that work on a superstructure formulation where structural decisions are represented by discrete variables and lead to an exponential increase in the computational effort. The mathematical programming (MP) methods which solve the continuous sub-problems provide only one local optimum which depends strongly on the initialization. In this contribution a memetic algorithm (MA) is introduced and applied to the global optimization of four different formulations of a computational demanding real-world design problem. An evolution strategy addresses the global optimization of the design decisions, while continuous sub-problems are efficiently solved by a robust MP solver. The MA is compared to MINLP techniques. It is the only algorithm that finds the global solution in reasonable times for all model formulations.