Towards further internal heat integration in design of reactive distillation columns—Part IV: Application to a high-purity ethylene glycol reactive distillation column

In the first three papers of this series, it has been shown that strengthening internal heat integration within a reactive distillation column involving reactions with high thermal effect is really effective for the reduction of utility consumption and capital investment besides the improvement in process dynamics and operation. One important issue that remains unstudied so far is the influences of reaction selectivity upon the reinforcement of internal heat integration, since the reaction operation is often accompanied by side-reactions and the maintenance of a high selectivity is extremely necessary in process synthesis and design. A reactive distillation column synthesizing high-purity ethylene glycol through the hydration of ethylene oxide is chosen and studied in this work. Because of the unfavorable physicochemical properties of the reacting mixture separated (e.g., the fairly large volatility between the reactants and the existence of a consecutive side-reaction), the process represents a challenging problem for the reinforcement of internal heat integration. Intensive comparison is conducted between the process designs with and without the consideration of further internal heat integration between the reaction operation and the separation operation involved, and it has been found that seeking further internal heat integration still leads to a substantial reduction of energy requirement, in addition to a further abatement in capital investment. Moreover, a favorable effect is again observed upon the process dynamics and operability. These striking outcomes manifest evidently that seeking further internal heat integration should be considered in process synthesis and design irrespective of what a reaction selectivity has been assigned.     

Towards further internal heat integration in design of reactive distillation columns—part I: The design principle

The thermodynamic efficiency of a reactive distillation column involving reactions with a highly thermal effect could sometimes be improved substantially through seeking further internal heat integration between the reaction operation and separation operation. Prudent arrangement of the reactive section and deliberate determination of feed location are the two effective methods that can complement internal heat integration within a reactive distillation column. The reactive section is suggested to properly superimpose onto both the stripping section for exothermic reactions and the rectifying section for endothermic reactions. Feed location should be determined so that the effect of internal heat integration can be maximized between the reaction and separation operations. Two reactive distillation systems, involving a highly exothermic reaction and a highly endothermic one, are employed to evaluate the proposed design philosophy and the results obtained confirm its feasibility and effectiveness. In addition, sensitivity analysis is conducted with respect to the amount of catalyst employed, relative volatilities of reacting mixtures, and thermal condition of feeds. The applicability and potentials of the design principle proposed are highlighted.     

Towards further internal heat integration in design of reactive distillation columns—Part II. The process dynamics and operation

In the first paper of this series, it has been demonstrated that the capital investment and operating cost can frequently be reduced substantially through seeking further internal heat integration between the reaction operation and separation operation for a reactive distillation column involving reactions with highly thermal effect. In this paper, the dynamics and operation of the resultant reactive distillation system is to be examined, with special emphasis focused on the dynamic effect of the supplementary internal heat integration. It has been found that seeking further internal heat integration can sometimes improve process dynamics and lessen difficulties in process operation. This outcome stems from the refined relationship between the reaction operation and separation operation involved and is of great significance in tightening process design for a reactive distillation column containing reactions with highly thermal effect.It should, however, be pointed out that seeking further internal heat integration might also confine severely the flexibility of the resultant reactive distillation column due to the reduction of mass transfer driving forces. When encountering a sharp increase in the product specification that is more relevant to the supplementary internal heat integration, the process might show deteriorated dynamic performance and can even converge to an undesirable steady state where the economical advantages of the supplementary internal heat integration are lost totally. Therefore, some effective measures to increase the redundancy of the resultant process design have to be taken to deal with the side-effect during process development.     

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.     

Balancing design and control of an olefin metathesis reactive distillation column through reactive section distribution

In this contribution, balance of design and control is investigated through reactive section distribution in an olefin metathesis reactive distillation column. Four designs with different strategies for reactive section distribution are studied and these include one with all stages as reactive ones (design-I), one with the distribution of reactive section according to the principle of internal mass integration (design-II), and ones with the extension of the reactive section of the design-II by five stages to either the stripping section (design-III) or the rectifying and stripping sections simultaneously (design-IV). The design-II appears to be more thermodynamically efficient than the design-I but with considerable degradation in process controllability. The design-III and design-IV retain most of the economical advantages of the design-II with only slight deterioration in controllability as compared with the design-I. This outcome demonstrates the fact that the strategy for reactive section distribution could be an effective decision variable for the balance of design and control in the development of reactive distillation columns.     

考虑反应热的乙氧基化反应精馏塔能量内部集成与优化

以正丁醇和环氧乙烷为原料合成乙二醇正丁醚(EGMBE)为例,利用数学模型和模拟分析方法,研究了包含大量反应热效应的乙氧基化反应精馏(ERD)塔中反应热的利用及系统能量优化问题。研究表明,当采用常规的、将反应段直接叠加于提馏段之上的塔设计时,反应热并没有贡献于分离操作或减小再沸器的热负荷,而只是被冷凝器中的冷却介质移走。分析了反应热的可利用途径,提出一种将反应段和提馏段分割、从反应段移出反应热供提馏段加热的内部热集成乙氧基化反应精馏结构--IHIERD。模拟结果表明,IHIERD 将使再沸器的温度从462.5 K降低到420.0 K,冷凝器负荷降低11%,外部能量输入降低14%,节能效果显著。     

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.     

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.     

Temperature control of an ideal heat-integrated distillation column (HIDiC)

In this work, a novel temperature control scheme is derived for an ideal heat-integrated distillation column (ideal HIDiC), in which a temperature difference between two stages is designated as the controlled variable to circumvent the side-effect of continuous pressure variations in the rectifying section. For making an appropriate compensation to the changes in operating conditions, an inferential signal extracted from the feed stage, n/2+1, is used to adjust the set-point of the temperature difference controller. Control of two ideal HIDiCs, separating, respectively, a binary mixture of benzene and toluene and an ideal ternary mixture of hypothetical components A-C is studied. It is demonstrated that the proposed temperature control scheme can retain a stable operation around the vicinity of the nominal steady state with improved dynamic performance and tolerable steady state discrepancies in comparison with the direct composition control scheme. Moreover, the proposed temperature control scheme is also found to be quite robust to the selection of temperature measurements.     

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.     

Challenges and opportunities in integration of design and control

Process synthesis and design of plant operation are related topics but current industrial practice solves these problems sequentially. The implication of this sequential strategy may result in design of processing systems which are very hard to control. This paper presents a discussion on drivers for an integrated approach and outlines the challenges in formulation of such a multi-objective synthesis problem. This discussion is viewed in relation to some of the changing trends in the industry. Significant results have been published which in different ways seek to handle the integrated problem. Further, advancements in control algorithms and software have widened the range of feasible operation and control for strongly interconnected production systems. In light of these advances in different areas of the field, recommendations for further research and initiatives for development of an integrated approach are given with focus on how new results on the short term can improve industrial practice.     

Application of a thermally coupled distillation column with separated main columns to gas concentration process

A modified fully thermally coupled distillation column (FTCDC) for operability improvement is utilized in a gas concentration process. The column consists of a prefractionator and two separated main columns having high distillation efficiency and flexible control structure. The operability of the proposed column is evaluated by examining the open-loop dynamic responses of step input variations with the HYSYS simulation. The simulation result indicates that the modified system can give better control than the original FTCDC. The energy saving and reduction of construction cost are discussed, and the ease of vapor flow manipulation and the elimination of a compressor in the vapor transfer are also evaluated as possible improvements.     

新型复合式内部能量集成的精馏塔的机械设计与水力学模拟

基于HIDiC的理论和Aspen模拟的结果,建立了新型内部能量集成的精馏塔的塔节结构,对筛孔式结构的复合HIDiC塔节做了CFD模拟,探索了不同参数下的水力学性能,优化了新型复合塔节的结构。结果表明:对于筛孔式复合塔节,减小塔板孔径,降低出口堰高有利于提高传质和传热效果。利用SolidWorks软件针对温度与压力下的双重效应做了静应力分析,通过HIDiC理论,模型构建,水力学模拟相互验证,将HIDiC逐板换热的理论与实际模型完美结合。     

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.     

Industrial application of an extended fully thermally coupled distillation column to BTX separation in a naphtha reforming plant

Aromatic compounds are yielded from naphtha reforming in a petrochemical plant, and the products are separated with binary distillation columns for benzene, toluene, xylene and heavy components in sequence. In this study, the first three columns of the fractionation process in the naphtha reforming unit are replaced with an extended fully thermally coupled distillation column (EFTCDC) also known as the extended Petlyuk column. An industrial-sized application of the EFTCDC is examined to compare the performance of the column with a conventional system. From a structural design giving the optimum structure of the column, a practical column structure is derived and used in the HYSYS simulation to find the optimal operation condition for a given set of product specifications. The EFTCDC gives an energy saving of 9.7% over a conventional three-column process. In addition, it is proved that the design procedure is good for an industrial process of 18 components.     

Evaluation of a two-temperature control structure for a two-reactant/two-product type of reactive distillation column

Several different control structures have been proposed for reactive distillation columns. The appropriate control structure depends on the flowsheet and on the type of reactions occurring in the column. If two reactants are involved and if it is desirable to operate the process without any excess of reactant, it is necessary to manage the fresh feed streams so that the stoichiometry is exactly balanced. A composition analyzer that measures an internal composition in the column is often required. However, if two products are produced, it is possible to avoid the use of an analyzer by using two temperatures in the column to adjust the two feed streams. This type of structure was proposed by Roat et al. [Roat, S., Downs. J., Vogel, E., Doss, J., 1986. Integration of rigorous dynamic modeling and control system synthesis for distillation columns. In: Chemical Process Control—CPC III. Elsevier, Amsterdam.] for the ideal reaction A+B↔C+D in one of the earliest papers dealing with reactive distillation control.The purpose of this paper is to explore the effectiveness of this two-temperature control structure for various column designs (number of reactive stages) to quantify the impact of design on controllability. We also discuss the issues of the selection of the trays whose temperatures are to be controlled and the tuning of the two interacting temperature controllers. Disturbances in production rate and fresh feed compositions are made to examine the rangeability of this control structure. Both an ideal reaction system and the methyl acetate system are studied. One of the main conclusions is that the locations of the temperature control trays should be made such that the two temperature controllers both have direct action (an increase in temperature increases feed), which requires negative openloop process gains for both loops.     

Higher energy saving with new heat integration arrangement in heat‐integrated distillation column

In conventional heat‐integrated distillation columns (HIDiCs), the internal heat exchange is executed between the pressurized rectifying section and the stripping section, which are located at the same elevation. In such a structure, the amount of heat exchanged between two sections depends on the temperature profile of both sections. The resulting enthalpy profile inside the column departs from that in reversible distillation, which is the ideal distillation operation in view of energy conservation. More energy saving may be achieved by providing appropriate arrangement of heat exchanges between sections. The interactive graphical design method to determine the appropriate heat exchange arrangement in a previous paper was developed for a binary system. The design method was extended and applied to a multicomponent system by adopting the idea of a quasi‐binary system. Also, a new HIDiC structure that can realize the outcomes of the proposed design method was developed. The economics of the proposed structure was precisely evaluated through a case study of a commercial scale column. It demonstrated that the proposed structure has attractive economics. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3479–3488, 2015     

Systematic retrofit design with Response Surface Method and process integration techniques: A case study for the retrofit of a hydrocarbon fractionation plant

This paper demonstrates the Retrofit Design Approach (RDA) and Response Surface Methodology (RSM) for the retrofit of industrial plants in which assessment of design options for improving existing processes in a site-wide and integrated manner is not straightforward, due to complex design interactions in the process. The design methodology applied in this study is based on the systematic use of a process simulator which is used to identify promising variables through sensitivity analysis. Hence, the most important factors are determined and a reduced model is constructed based on RSM. An optimization framework is then built using the reduced model based on key selected variables, which is optimized to find optimal conditions and performance of the process. This design methodology provides strategic guidelines for determining the most cost-effective design options. The retrofit of a hydrocarbon fractionation plant is presented as an industrial case study. This includes a large number of design options with different process configurations and operating conditions due to the interconnection of distillation columns in sequence and the integrated heat recovery within the plant. The case study results demonstrate the applicability of the proposed approach which is able to effectively deal with a large retrofit problems. This is possible with the aid of process simulation and RSM producing a reduced model which requires considerably less computational effort to solve.     

Falling film distillation column with heat transfer by means of a vapor chamber. Part II: operation with a temperature profile

Based on previous results and a new proposal for the configuration of falling film distillation columns heated by a vapor chamber operating at atmospheric pressure, the present study analyzed a different application for heat transfer by means of an the axial temperature profile in the same vapor chamber, generated by the presence of non-condensable gases. Experimental tests were performed with an ethanol–water mixture by varying the feed flow rate and vapor chamber temperature. This new heat transfer proposal allowed the formation of a temperature gradient inside the distillation tube (similarly to the isothermal proposal previously reported), thus indicating that this heat transfer method is feasible for a distillation unit. The results demonstrated that higher flow rates are suitable for the distillation of ethanol and water in this new proposal; the temperature profile operation shows the feasibility of this new heat transfer proposal in falling film distillation columns and encourages further studies on the optimization of the process and the evaluation of the energy consumption.     

Application of the dividing wall column to olefin separation in fluidization methanol to propylene (FMTP) process

Dividing wall column (DWC) is shown to be energy efficient compared to conventional column sequence for multi components separation, which is used for olefin separation in fluidization methanol to propylene process in the present work. Detailed design for pilot DWC was performed and five control structures, i.e. composition control (CC), temperature control (TC), composition-temperature control (CC-TC), temperature difference control (TDC), double temperature difference control (DTDC) were proposed to circumvent feed disturbance. Sensitivity analysis and singular value decomposition (SVD) were used as criterion to select the controlled temperature locations in TC, CC-TC, TDC and DTDC control loops. The steady simulation result demonstrates that 25.7％and 30.2％duty can be saved for condenser and reboiler by substituting conventional column sequence with DWC, respectively. As for control structure selection, TC and TDC perform better than other three control schemes with smal er maximum deviation and shorter settling time.