Quantitative comparison of dynamic controllability between a reactive distillation column and a conventional multi-unit process

A comparison of the steady-state economic optimum designs of two alternative chemical processes was presented in a previous paper [Kaymak, D. B., & Luyben, W. L. (2004). A quantitative comparison of reactive distillation with conventional multi-unit reactor/column/recycle systems for different chemical equilibrium constants. Industrial & Engineering Chemistry Research, 43, 2493–2507]. A generic exothermic reversible reaction A + B ↔ C + D occurs in both flowsheets, which consist of a conventional multi-unit reactor/separator/recycle structure and a reactive distillation column. Results showed that the reactive distillation process is significantly less expensive than the conventional process for a wide range of the chemical equilibrium constant when there is no mismatch between the temperature favorable for reaction and the temperature favorable for vapor–liquid separation.

A reactive distillation column has fewer control degrees of freedom than a conventional multi-unit system. Therefore a reactive distillation column may have worse dynamic response than a conventional process. The purpose of this paper is to compare the dynamic controllability of these two alternative processes.

Three different chemical equilibrium constants are considered. Several control structures are developed for each flowsheet, and their effectiveness is evaluated. Disturbances in production rate and fresh feed compositions are considered.

The conventional multi-unit process provides significantly better control. The operability region is much larger, there is less variability in product quality and the dynamic responses are faster than those of the reactive column. Thus, these results demonstrate that there is a significant trade-off in this system between optimum economic steady-state design and dynamic controllability.     

Design and control of an olefin metathesis reactive distillation column

This paper considers the design and control of a reactive distillation column in which one reactant is consumed and two products are formed (A?B+C). The volatilities are α_B>α_A>α_C, i.e. the reactant is intermediate boiling between the two products. The metathesis of 2-pentene is considered as the demonstrative example. The column has a single feed of the intermediate boiling reactant. The distillate contains mostly light component and the bottoms mostly heavy.Three designs are considered: the base case (low-conversion/low-pressure), a low-conversion/high-pressure case and a high-conversion/high-pressure case. The base design is obtained from the literature, and the other two steady-state designs are optimized with respect to the total annual cost. All the designs are found to be openloop stable. Five control structures are studied for the base design. Then the best two structures are applied to the remaining two designs. This category of reactive distillation exhibits less challenging problems than other categories since it uses a single feed, which eliminates the need for the control structure to perfectly balance two fresh feeds.Simulation results demonstrate that effective dynamic control is provided by a control structure that uses two temperatures to maintain the purities of both product streams. No internal composition measurement is required. This structure is found to be robust and stable and rejects loads and tracks setpoints very well.     

Process alternatives for methyl acetate conversion using reactive distillation. 1. Hydrolysis

In a polyvinyl alcohol (PVA) plant, reaction stoichiometry indicates that equal molar of methyl acetate is generated for every mole of PVA produced. This work explores an alternative to convert methyl acetate back to acetic acid (raw materials of PVA plant), methyl acetate (MeAc) hydrolysis. The design and control of methyl acetate hydrolysis using reactive distillation is studied. Because of the small chemical equilibrium constant (∼0.013) and unfavorable boiling point ranking (MeAc being the lightest boiler), the reactive distillation exhibits the following characteristics: (1) total reflux operation and (2) excess reactant (water) design. The proposed flowsheet consists of one reactive distillation column with a reactive reflux drum, two separation columns, and one water-rich recycle stream. A systematic design procedure is used to generate the flowsheet based on the total annual cost (TAC). Two dominate design variables are: recycle flow rate (for the degree of excess in water) and the overhead impurity level of acetic acid in the product column (to avoid tangent pinch). Finally, the operability of the hydrolysis plant is evaluated. A plantwide control structure is developed followed by process identification and controller tuning. The results show that reasonable control performance can be achieved using simple temperature control for feed flow and feed composition disturbances.     

Effects of feed tray locations to the design of reactive distillation and its implication to control

The effects of feed locations to the design of reactive distillation are explored. In this work, ideal reactive distillation systems are used to illustrate the advantage of feed trays optimization in design and control. Process parameters such as relative volatilities between reactants, relative volatilities between products, column pressure, activation energies, and pre-exponential factors are varied to seek possible generalization. For all systems studied, the percentage of energy saving ranges from 6% to 47%, and this is obtained by simply rearranging the feed locations. Finally, the idea of optimal feed trays is extended to the operation/control of reactive distillation systems. First, steady-state analysis is carried out to find the optimal feed trays as measurable load variable varies. Then, a control structure is proposed to rearrange the feeds as the disturbance comes into the system. The results indicate that, again, substantial energy can be saved by feed rearrangement via the coordinated control structure.     

The second law optimal state of a diabatic binary tray distillation column

A new numerical procedure to minimize the entropy production in diabatic tray distillation columns has been developed. The method was based on a least square regression of the entropy production at each tray. A diabatic column is a column with heat exchangers on all trays. The method was demonstrated on a distillation column separating propylene from propane. The entropy production included contributions from the heat transfer in the heat exchangers and the mass and heat transfer between liquid and vapor inside the distillation column. It was minimized for a number of binary tray distillation columns with fixed heat transfer area, number of trays, and feed stream temperature and composition. For the first time, the areas of heat exchange were used as variables in the optimization. An analytical result is that the entropy production due to heat transfer is proportional to the area of each heat exchanger in the optimal state. For many distillation columns, this is equivalent to a constant driving force for heat transfer. The entropy production was reduced with up to 30% in the cases with large heat transfer area and many trays. In large process facilities, this reduction would ideally lead to 1-2 GWh of saved exergy per year. The most important variable in obtaining these reductions is the total heat transfer area. The investigation was done with a perspective to later include the column as a part in an optimization of a larger process. We found that the entropy production of the column behaved almost as a quadratic function when the composition of the feed stream changed. This means that the feed composition is a natural, easy variable for a second law optimization when the distillation column is a part of a process. The entropy production was insensitive to variations in the feed temperature.     

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.     

Comparative control study of ideal and methyl acetate reactive distillation

The control of an ideal reactive distillation column is compared with that of a similar, but somewhat different, real chemical system, the production of methyl acetate. Similarities and differences are observed. Three control structures are evaluated for both systems. A control structure with one internal composition controller and one temperature controller provides effective control of both systems for both high and moderate conversion designs. A two-temperature control structure is effective when the system is overdesigned in terms of number of reactive trays, holdup and/or catalyst load. Direct control of product purity for the high-conversion/high-purity design is difficult because of system nonlinearity and interaction. Tray temperature control avoids the nonlinearity problem.     

Control System Design for a Single Feed ETBE Reactive Distillation Column

Reactive distillation (RD) is advantageous for the Ethyl Tert‐Butyl Ether (ETBE) synthesis. The steady state model of an ETBE reactive distillation column created using the simulator HYSYS is analyzed to synthesize effective control structures. Since the column exhibits input multiplicity with the dual process objectives of ETBE RD (isobutene conversion and ETBE purity), inferential variables are selected. A control structure that organizes a sensitive tray temperature in the stripping section using the reboiler duty and maintains the temperature difference of reactive trays using the reflux flow, is found to be most suitable. A decentralized PI controller and constrained Model Predictive Controller (MPC) are implemented, and performances are compared for set point tracking and disturbance rejection. MPC control algorithms are implemented in MATLAB and interfaced with HYSYS. Constrained MPC (CMPC) is found to be effective for load disturbance rejection, which frequently occurs in the single feed configuration.     

Design and control of reactive distillation for hydrolysis of methyl lactate

Esterification of raw lactic acid from fermentation broth and then hydrolysis of lactate ester in reactive distillation column is an effective process for purification of lactic acid. Reactive distillation for hydrolysis of methyl lactate is studied. First, the thermodynamic properties and reaction kinetics are analyzed; then the reactive distillation column is designed based on the objective function total annual cost. The effects of tray number of rectifying section, tray number of reactive section, and feed location on the total annual cost are investigated. And a dual-temperature control structure is proposed for the optimal reactive distillation column, and the results show that it works quite well for this hydrolysis system.     

反应精馏合成草酸二乙酯的工艺研究

研究了反应精馏合成草酸二乙酯的工艺条件,在自行设计的反应精馏塔内考察了挟带剂种类及用量、塔釜温度、进料速度、回流进料比对酯化反应的影响。环己烷为较适宜的挟带剂,确定了最佳工艺条件：塔釜温度为110℃,进料速度0.8 mL/min,回流比3。在最佳工艺条件下草酯二乙酯的酯化率达到92.16%。     

Study of heterogeneously catalysed reactive distillation using the D+R tray—A novel type of laboratory equipment

Recently, novel equipment for heterogeneously catalysed reactive distillation, the D+R tray, has been introduced by Schmitt et al. (2009). The present work reports results of measurements of the separation capacity of D+R trays. Furthermore, results of reactive distillation experiments on two test systems are presented that show that the D+R tray can be used routinely for laboratory studies of heterogeneously catalysed reactive distillation. As test systems, the esterifications yielding butyl acetate and hexyl acetate were chosen. Reactive distillation using catalytic packings was studied previously in detail ( [Parada, 2008], [Schmitt et al., 2004] and [Schmitt et al., 2005]) with the same test system. The experiments of the present work are carried out so that a direct comparison is possible. For both test systems, the most important process parameters such as feed rate and mass of catalyst were systematically varied. High conversions of the reactants and high purities of the products were achieved. The reproducibility of the results is excellent. The study shows that the D+R tray is a novel laboratory equipment that facilitates fast and flexible investigations of heterogeneously catalysed reactive distillation processes and that it can be used for studying process designs of columns equipped with either catalytic packings or trays.     

Reactive column profile map topology: Continuous distillation column with non-reversible kinetics

In this paper we present a topologically based approach to the analysis and synthesis of reactive distillation columns. We extend the definition of Tapp et al. [Tapp, M., Holland, S., Glasser, D., & Hildebrandt, D. (2004). Column profile maps part A: Derivation and interpretation. Industrial and Engineering Chemistry Research, 43, 364–374] of a column section in non-reactive distillation column to a reactive column section (RCS) in a reactive distillation column. A RCS is defined as a section of a reactive distillation column in which there is no addition or removal of material or energy. We introduce the concept of a reactive column profile map (RCPM) in which the profiles in the RCPM correspond to the liquid composition profiles in the RCS. By looking at the singular points in the RCPM, it is demonstrated that for a single chemical reaction with no net change in the total number of moles, the bifurcation of the singular points depends on both the difference point as introduced by Hauan et al. [Hauan, S., Ciric, A. R., Westerberg, A. W., & Lien, K. M. (2000). Difference points in extractive and reactive cascades I-Basic properties and analysis. Chemical Engineering Science, 55, 3145–3159] as well as the direction of the stoichiometric vector. These two vectors combine to define what we call the reactive difference point composition. We show that there only certain feasible topologies of the RCPM and these depend only on the position of the reactive difference point composition. We look at a simple example where the vapour liquid equilibrium (VLE) is ideal and show that we can classify regions of reactive difference point compositions that result in similar topology of the RCPM. Thus, by understanding the feasible topologies of the RCPM, one is able to identify profiles in the RCPM that are desirable and hence one is able to synthesize a reactive distillation column by combining RCS that correspond to the desired profile in the RCPM.We believe that this tool will help understand how and when reaction could introduce unexpected behaviors and this can be used as a complementary tool to existing methods used for synthesis of reactive distillation columns.     

Falling film distillation column with heat transfer by means of a vapor chamber – part I: isothermal operation

We describe a new configuration for falling film distillation columns: a heating system provided by a vapor chamber along the entire column with the operation at atmospheric pressure. Experimental tests were carried out with an ethanol–water mixture by providing isothermal while varying the feed flow rate and the temperatures of the feed and vapor chamber. This heating proposal allowed the formation of a temperature gradient inside the distillation column, which was attributed to the mass and heat axial transfer that occurred along the entire length of the unit. Different from the usual recommendation for film units, higher flow rates are more suitable for distillation of ethanol and water, which was attributed to the heat transfer method and the operation being held at atmospheric pressure. Also, it was observed that there is a strong relationship between the feed temperature and vapor chamber temperature. The results are promising and encourage further studies with this new heat transfer proposal.     

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.     

Design and control of reactive distillation–recovery distillation flowsheet with a decanter for synthesis of N-propyl propionate

Reactive distillation (RD) for the synthesis of n-propyl propionate (ProPro) is operated with an excess of the reactant n-propyl alcohol (ProOH). It is simulated as a two-column system which can be easily controlled compared with a single reactive column which is operated neat, because the recovery column acts conceptually as a composition analyzer. The optimal steady-state design of the two-column system with the minimal total annual cost (TAC) is screened first. Then, an effective control scheme is established to handle feed disturbances. Only tray temperatures are required in the dynamic control two-column process. The product purity is held close to the set value 99.5 mol% and large deviation is prevented.     

反应精馏隔壁塔生产乙酸正丁酯的优化与控制

对反应精馏隔壁塔生产乙酸正丁酯过程进行了模拟、优化与控制的系统研究。利用Aspen Plus软件模拟乙酸甲酯与正丁醇的酯交换反应过程,以年总费用（TAC）为目标函数进行过程优化,通过稳态敏感性分析及相对增益矩阵（RGA）判据得到不同的操纵变量与控制变量匹配关系,以此为基础,在Aspen Dynamics平台建立了若干控制结构并进行分析对比。结果表明,利用两股反应物呈比例进料可较为有效地抵抗进料扰动,最后提出的无再沸器热负荷与混合物进料量比值（Q_r/F）控制的改进控制结构CS3,在降低反应精馏隔壁塔控制过程超调量方面有较大的优越性。     

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.     

Distillation columns with vertical partitions

Distillation columns with vertical partitions can separate a feed mixture into 3 or 4 pure fractions. Compared to other column arrangements, their investment costs and energy consumption are lower. Production columns show good results and are easily controlled. Conventional distillation columns produce only 2 pure product streams at the top and at the bottom of the column. Side products are contaminated by light or heavy components, depending on the location of the side stream in the rectifying or stripping section. This disadvantage is set aside by using a distillation column with a vertical partition. The internal separation wall prevents lateral mixing of liquid and vapour in the central part of the column, forming there separate feed and outlet sections. In the presence of a vertical partition, 3 or 4 pure fractions can be obtained in a single distillation step. This is particularly advantageous when heat sensitive components are to be separated. On account of good thermodynamic properties, the energy consumption is 20 to 35% lower than that of other distillation arrangements. The control behaviour is similar or better than that of conventional distillation columns.     

Note: Two-Enthalpy Feed for Distillation with Vapor Feed and Refrigerated Condenser

Abstract

The use of two-enthalpy feed, which involves splitting the feed, condensing part of it, and sending both the liquid and vapor streams to the distillation column, is explored for columns that have a vapor feed and require refrigeration in the column condenser. The use of two-enthalpy feed can reduce both capital and operating costs compared to condensing all the feed. Compared to use of partial cooling and use of a two-phase feed, two-enthalpy feed reduces the condenser heat duty, but the feed cooling occurs at a lower temperature.     

Distributed Heat Supply for Distillation Control to Reduce Feed Composition Disturbance Effects

The introduction of heat in stages combined with a reboiler as a distillation control strategy is proposed aiming to reduce the transition time when a feed composition disturbance occurs. This is the most severe type of perturbation and results in long transition times which are economically undesirable. Experiments were carried out in a pilot distillation column with ethanol and water for testing two different control strategies: conventional, i.e., using both last stage and reboiler temperature controllers, and distributed, i.e., conventional strategy plus stage temperature controllers. The results with this operational approach indicated a significant reduction in the time required for the column to stabilize when compared with the conventional approach. The proposed strategy proved to be a viable alternative, enabling faster dynamics and smaller volumes to be processed outside the predefined quality parameters.