Conceptual design of single-feed hybrid reactive distillation columns

It is well known that reactive distillation offers benefits by integrating distillation and reaction within a single unit. While there are procedures available for the synthesis of non-reactive distillation processes and of reaction-separation systems, the design of reactive distillation columns is still a challenge. This work presents a new synthesis and design methodology for hybrid reactive distillation columns, featuring both reactive and non-reactive sections; reactive equilibrium is assumed. The approach is based on graphical techniques; therefore it is restricted to systems with two degrees of freedom according to Gibbs phase rule. The design method allows rapid and relatively simple screening of different reactive distillation column configurations. The results are useful for initialising more rigorous calculations. The methodology is illustrated for MTBE and ethyl formate production.     

Time optimal start-up strategies for reactive distillation columns

In this contribution, a rigorous process model to simulate the start-up of a cold and empty reactive distillation (RD) column is developed and experimentally validated with a transesterification process. Strategies for time optimal start-up of an RD column are presented. The mostly used strategy for conventional distillation of total reflux for RD is only recommendable with limitations. New, alternative strategies, like the recycling of the off-spec bottom and top product or the initial charging with product, to minimize the necessary start-up time are presented. Suitable strategies can save up to 82% of the needed time for the column start-up.     

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.     

Synthesis of cylohexanol by three-phase reactive distillation: influence of kinetics on phase equilibria

A reactive distillation process is being suggested for the production of the commercially interesting intermediate cyclohexanol from cyclohexene and water, which avoids some of the drawbacks of the conventional liquid-phase cyclohexane oxidation process, especially with respect to operational safety. This reactive distillation process has its own intricate challenges due to the fact that reaction, distillative separation and liquid-phase splitting occur simultaneously. The interaction of these three phenomena is studied using residue curve maps for both, model simulations and experimental data. Based on the fixed point analysis of the residue curves, a novel process is proposed for the reactive separation of cyclohexene/cyclohexane mixtures which is difficult to be carried out by conventional distillation due to very close boiling temperatures.     

背包式反应器与精馏塔耦合合成醋酸甲酯的模拟

针对背包式反应器与精馏塔耦合过程循环流股多,模拟计算收敛难度大的缺点,在Aspen P lus的RadFrac模型中引入Murphree板效率,仅用一个精馏塔模型就描述了这个复杂耦合过程的模拟模型。在固定精馏塔塔板数的情况下,讨论了背包反应器个数和间隔位置、进料位置、回流比和催化剂量等因素对醋酸甲酯合成的影响。初步探索了反应能力和分离能力的匹配问题。模拟结果表明,当采用5个背包反应器,反应器之间间隔4块分离塔板的配置时,在适宜条件下醋酸总转化率可达到96.3%。     

反应精馏及其研究进展

反应精馏是化学反应和精馏分离耦合在一个设备中进行的操作。简述了反应精馏技术,介绍了稳态均相反应精馏过程模拟的平衡级模型、非平衡级模型、非平衡池模型、统计模型、微分模型以及相应的计算方法,并指出了今后的研究方向。     

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.     

Determination of catalytic packing characteristics for reactive distillation

Catalytic distillation still expands its field of applications. New structured catalytic column internals have been developed in recent years and new studies have been reported. A modern structured catalytic packing MULTIPAK^® is a subject of the investigations presented in this paper.

Important parameters of MULTIPAK^® have been examined experimentally in a 250 mm ID laboratory column: pressure drop for dry, prewetted and irrigated packings, flooding line and mass transfer coefficients for the gas and liquid phases.

The correlations obtained have been incorporated into a software tool for the simulation of catalytic distillation processes and simulations have been performed assuming methyl acetate synthesis as a model process. The calculations have been verified using the experiments performed for the same synthesis in a 50 mm ID catalytic distillation column operating continuously and thus reflecting industrial applications. It is concluded that the model represents the real process with satisfactory accuracy, although some deviations can be observed, especially within the reactive zone.     

Conceptual design of single-feed kinetically controlled reactive distillation columns

Reactive distillation, simultaneous reaction and separation within a single unit, represents an exciting alternative to conventional processes, leading to significantly reduced in capital and operating costs. Process design for reactive distillation is facilitated by fast and effective methods for synthesis and conceptual design that take into account reactions that do not instantaneously reach equilibrium. This work presents a new methodology for synthesis and design of single-feed kinetically controlled reactive distillation columns. The design method allows rapid and relatively simple screening of different reactive distillation column configurations. Feasibility is assessed and operating conditions are determined using an extension of boundary-value methods. The approach is limited to systems with four or fewer chemical species. Both fully reactive and hybrid columns are considered. The methodology is illustrated for a metathesis reaction and for MTBE production.     

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 operation of simulated-moving-bed reactors for nonlinear adsorption isotherms and equilibrium reactions

An explicit design procedure for simulated-moving-bed reactors (SMBRs) is presented, incorporating a nonlinear, competitive isotherm, and an equilibrium reaction involving three species, e.g., A⇔B+C. This design procedure is based on an equilibrium theory model of a true-moving-bed (TMB) reactor and it is in close analogy to the well-known design procedure for purely separative SMB-units, the so-called ‘triangle theory’ (Journal of Chromatography A, 769, 3 (1997)). It allows an easy determination of the optimal point of operation and can also evaluate the robustness of an operation point. The operating region of full conversion and complete separation for the TMBR is compared to the one of a purely separative TMB unit, to which only the products are fed. The underlying explicit relations for the new design procedure are presented and their reliability is validated by comparison with experimental data of a SMBR for the synthesis of methyl acetate.     

Influence of column hardware on the performance of reactive distillation columns

We compare the performance of an MTBE synthesis column using two different hardware configurations: (1) a sieve tray column in which the catalyst particles, encased inside wire gauze envelopes, placed along the liquid flow path; (2) a column filled with catalytically active packing of Raschig ring shape. The columns simulations are performed using a rigorous nonequilibrium model. Using the bottoms flow rate of MTBE as continuation parameter it is shown that the two different hardware configurations exhibit significantly different bifurcation diagrams. The sensitivity of this bifurcation diagram has been studied with varying (a) methanol feed, (b) iso-butene feed, (c) inert feed and (d) reflux ratio. We show that the cross-flow contacting on the sieve tray configuration is beneficial to conversion.     

Dynamic rate-based and equilibrium models for a packed reactive distillation column

Dynamic rate-based and equilibrium models were developed for a packed reactive distillation column for the production of tert-amyl methyl ether (TAME). The two types of models, consisting of differential algebraic equations, were implemented in gPROMS and dynamic simulations were carried out to study the dynamic behaviour of reactive distillation of the TAME system. The dynamic responses predicted by the two types of models are similar in general, with some differences in steady-state values. The influence of manipulated variables, such as distillate flow rate, reflux ratio, and reboiler duty, on the dynamic responses of the controlled variables (reactant conversion and product purity) was studied. The dynamic response of reactant conversion is very nonlinear and unconventional, but the response of product purity is well approximated by a linear first-order differential equation. The CPU time required to complete a dynamic simulation of the rate-based model is at least an order of magnitude higher than that for the equilibrium model. Therefore, the dynamic rate-based model is much more complicated than the equilibrium model, and simplification of the rate-based model is necessary for the implementation of model-based control. A new approach was proposed to simplify the dynamic rate-based model by assuming that the mass transfer coefficients are time invariant. This approach was demonstrated to be superior to the conventional simplification methods. It can reduce the number of equations by up to two-thirds and still accurately predicts the dynamic behaviour.     

A fundamental principle and systematic procedures for process intensification in reactive distillation columns

A fundamental principle is developed for process intensification through internal mass and energy integration in reactive distillation columns and three systematic procedures are devised for process synthesis and design. For reactive distillation columns involving reactions with highly thermal effect, process intensification can be achieved with an exclusive consideration of internal energy integration between the reaction operation and separation operation involved. However, in the case of a highly endothermic reaction with an extremely low reaction rate and/or small chemical equilibrium constant, internal mass integration has also to be considered between the reactive section and stripping section. For reactive distillation columns involving reactions with negligibly or no thermal effect, process intensification can be performed with an exclusive consideration of internal mass integration. For reactive distillation columns involving reactions with moderately thermal effect, process intensification must be conducted with a careful trade-off between internal mass and energy integration. Five hypothetical and two real reactive distillation systems are employed to evaluate the principle and procedures proposed. It is demonstrated that intensifying internal mass and energy integration is really effective for process intensification. Not only can the thermodynamic efficiency be improved substantially, but also the capital investment can be further reduced.     

Comparison between a conventional process and reactive distillation for naphtha hydrodesulfurization

Reactive distillation is a useful operation in which reactions and separation take place in the same unit. In this paper, the feasibility of using reactive distillation for hydrodesulfurization of naphtha is explored by simulation and experimentation. It was found that the process can be attractive since initial and operation costs are reduced substantially while the specification of the products can be met.     

Attainable regions of reactive distillation. Part II: Single reactant azeotropic systems

In reactive distillation (RD) one can conveniently manipulate the concentration profiles on the reactive stages by exploiting the difference in volatility of the various components. This property of RD can be advantageously used to improve the selectivity toward the desired product in case of series or series parallel reactions, and obtain a performance superior to the network of conventional reactors. In the previous work [Agarwal, V., et al., 2008. Attainable regions of reactive distillation—Part I. Single reactant non-azeotropic systems. Chemical Engineering Science, submitted for publication], we introduced representative unit models of RD to obtain the attainable regions of RD for non-azeotropic systems. In this work, we extend the approach to a system involving single binary azeotrope. Design guidelines have been formulated based on the residue curve maps, to obtain the improved attainable region with the help of these representative RD models either alone or in the form of their network.     

Interpreting the dynamic effect of internal heat integration on reactive distillation columns

In this work,the impact of internal heat integration upon process dynamics and controllability by superposing reactive section onto stripping section,relocating feed locations,and redistributing catalyst within the reactive section is explored based on a hypothetical ideal reactive distillation system containing an exothermic reaction:A + B ←→ C + D.Steady state operation analysis and closed-loop controllability evaluation are carried out by comparing the process designs with and without the consideration of internal heat integration,For superposing reactive section onto stripping section,favorable effect is aroused due to its low sensitivities to the changes in operating condition,For ascending the lower feed stage,somewhat detrimental effect occurs because of the accompanied adverse internal heat integration and strong sensitivity to the changes in operating condition.For descending the upper feed stage,serious detrimental effect happens because of the introduced adverse internal heat integration and strong sensitivity to the changes in operating condition.For redistributing catalyst in the reactive section,fairly small negative influence is aroused by the sensitivity to the changes in operating condition.When reinforcing internal heat integration with a combinatorial use of these three strategies,the decent of the upper feed stage should be avoided in process development.Although the conclusions are derived based on the hypothetical ideal reactive distillation column studied,they are considered to be of general significance to the design and operation of other reactive distillation columns.     

An integrated reactive distillation process for biodiesel production

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.     

Design of entrainer-enhanced reactive distillation for the synthesis of butyl cellosolve acetate

An innovative entrainer-enhanced reactive distillation (RD) process is presented, which aims to the production of high-purity butyl cellosolve acetate from butyl cellosolve and acetic acid via an esterification reaction. This entrainer-enhanced RD process can procure technical advantages from both heterogeneous azeotropic distillation and RD. Solvents such as cyclohexane, ethylene dichloride, toluene, and octane are considered as candidates in this esterification RD process. The function of entrainers is to simplify the separation between water and acetic acid. For this purpose, the proper entrainer to use is thus evaluated based on its mutual solubility with water in two liquid phases. Simulation results reveal that total annual cost can be substantially reduced when cyclohexane, toluene, and octane are used as entrainers in the RD column. The octane-enhanced RD provides the most economical design in this studied case.