Reinforced temperature control of a reactive double dividing-wall distillation column

The mass and thermal coupling makes the control of the reactive double dividing-wall distillation column(R-DDWDC) an especially challenging issue with a highly interactive nature. With reference to the separation of an ideal endothermic quaternary reversible reaction with the most unfavorable ranking of relative volatilities(A + B ■ C + D with α_(A)>α_(C)>α_(D)>α_(B)), the operation rationality of the R-DDWDC is studied in this contribution. The four-point single temperature control system leads to great steady-state discrepancies in the compositions of products C and D and the reason stems essentially from the failure in keeping strictly the stoichiometric ratio between reactants A and B. A temperature plus temperature cascade control scheme is then employed to reinforce the stoichiometric ratio control and helps to secure a substantial abatement in the steady-state discrepancies. A temperature difference plus temperature cascade control scheme is finally synthesized and leads even to better performance than the most effective double temperature difference control scheme. These outcomes reveal not only the operation feasibility of the R-DDWDC but also the general significance of the proposed temperature difference plus temperature cascade control scheme to the inferential control of any other complicated distillation columns.

Configuring topologically optimum vapor recompressed dividing-wall distillation columns to maximize operating efficiency

For dividing-wall distillation columns(DWDCs) separating a heavy-component dominated and wide boiling-point ternary(HCDWBT) mixture, a significant amount of excessive heat exists inevitably in stripping the heavy-component from the intermediate-component and it can be employed to initiate the development of vapor recompression heat pump(VRHP) assisted DWDC(VRHP-DWDC). Despite dividing wall may locate in the top, middle, and bottom, the optimum VRHP-DWDC is found to involve uniformlytwo VRHP circles. While the first one serves to compress and transform the excessive heat resulted from the separation of the heavy-component from the intermediate-component, the second one to compress and transform the overhead vapor stream of the light-component pre-heated sequentially with the condensate from the first one and the bottom product stream of the heavy-component, both releasing the temperature-elevated latent heat to the pre-fractionator's or common stripping section. The processing of two HCDWBT mixtures of benzene/toluene/o-xylene and n-pentane/n-hexane/n-heptane are selected to assess the derived optimum topological configurations of the VRHP-DWDC and their optimality is confirmed through detailed comparisons with the DWDC and two VRHP-DWDCs involving only one VRHP circle. The proposed strategy helps to tap the full potential of the VRHP-DWDC with considerably alleviated complication in process development.

Temperature inferential control of a reactive distillation column with double reactive sections

Temperature inferential control (TIC) is studied for a reactive distillation column with double reactive sections (RDC-DRSs) processing a hypothetical two-stage consecutive reversible reaction (A + B■C + D, C + B■E + D with αD > αB > αC > αA > αE). Because of the complicated dynamic behaviors, the controlled stages by sensitivity analysis lead to great steady-state deviations (SSDs) in top and bottom product purities. Since TIC involves considerably reduced settling times in comparison with direct composition control, small SSDs in product qualities correspond generally to small transient deviations (TDs) in product qualities. An objective function that measures SSDs in product qualities is formulated to represent the performance of a TIC system and an iterative procedure is devised to search for the best control configuration. The application of the procedure to the RDC-DRS gives considerably suppressed TDs and SSDs in top and bottom product qualities as compared with the one by sensitivity analysis. The method is simpler in principle and less computationally intensive than the current practice. These striking outcomes show the effectiveness of the proposed principle for the development of TIC systems for complicated reactive distillation columns.

Reactive dividing-wall column for the co-production of ethyl acetate and n-butyl acetate

Reactive dividing-wall column(RDWC) technology plays a critical role in the energy saving and high efficiency of chemical process.In this article, the process of co-producing ethyl acetate(EA) and n-butyl acetate(BA) with RDWC was studied.BA was not only the product, but also acted as entrainer to remove the water generated by the two esterification reactions.Experiments and simulations of the co-production process were carried out.It was found that the experimental results were in good agreement with the simulation results.Two kinds of RDWC structures(RDWC-FC and RDWC-RS) were proposed, and the co-production process operating parameters of the two types of RDWC were optimized by Aspen Plus respectively.The optimal operating parameters of the RDWC-FC were determined as follows: 0.6 of the reflux ratio of aqueous phase(RR), 0.66 of the vapor split(R_V) and 0.51 of the liquid split(R_L).And the optimal operating parameters of the RDWC-RS were shown as follows: RR was 0.295 and R_V was 0.61.Furthermore, the energy saving analysis of the co-production process was based on the annual output of 10000 tons of EA, compared with the traditional reaction distillation(RD) to prepare EA and BA, the reboiler duty of the RDWC-FC column could save 20.4%, TAC saving 23.6%; RDWC-RS reboiler energy consumption could save 17.0%, TAC 22.2%.

Vapor recompressed dividing-wall distillation columns:Structure and performance

Due to the topological structure of double columns and multiple separating sections in dividing-wall distillation columns(DWDCs),the development of vapor recompressed dividing-wall distillation columns(DWDC-VRHPs)represents a challenging issue with great complexities and tediousness.For the separations of light-component dominated and wide boiling-point ternary mixtures,because the purification of the light-component from the intermediate-and heavy-components incurs the primary energy dissipation,the application of vapor recompressed heat pumps(VRHP)should be aimed to reduce the irreversibility and this leads to the generation of the optimum topological structures of the DWDC-VRHPs,i.e.,a DWDC plus a two-stage VRHP.The first-stage VRHP is to preheat feed,not only taking the advantages of the small temperature elevation available but also favoring the mass transfer between the vapor and liquid phases through feed splitting.The second-stage VRHP is to reduce further separation irreversibility.The philosophy can be applied to any DWDCs no matter where the dividing wall locates.Two case studies on the separations of ternary mixtures of benzene,toluene,and o-xylene and n-pentane,n-hexane,and n-heptane demonstrate the economic optimality of the proposed DWDC-VRHPs and reveal the inherent interplay between internal and external process integration.

Methodology for Design of Reactive Distillation Column and Kinetics for Isoamylene Etherification Catalysed by Amberlyst 35

Isoamylene from the Fischer-Tropsch syncrude can be transformed to valuable fuel oxygenate additives through an equilibrium limited etherification reaction with methanol. A reactive distillation process is established to increase isoamylene conversion. Facing the challenge of improving product purity at the same time, an equilibrium stage model based design methodology is proposed and illustrated step-by-step for converting the Fischer-Tropsch C_5 olefins to tert-amyl methyl ether(TAME) process by using Aspen Plus. Under the guide of the proposed methodology, the design leads to a TAME product purity of higher than 95% and an isoamylene conversion of higher than 90%. The etherification kinetics over Amberlyst 35 is also studied within a temperature range of 60 ℃ to 75 ℃ to shed more light on the feasibility of process development. The methodology provides an effective reactive distillation column design to achieve the target reactant conversion and product purity simultaneously.

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 + BC + 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.

Unique influences of reboiler inventory control on the operation of totally reboiled reactive distillation columns

In this work,the dynamics and operation of the totally reboiled reactive distillation columns are visualized in terms of transfer function based process models.This kind of processes is found to be characterized by underdamped step responses due to the special topological configuration and the intricate interplay between the reaction operation and the separation operation involved.The under-dampness can be substantially alleviated through the tight inventory control of bottom reboiler and this presents beneficial effects to process dynamics and operation.Two totally reboiled reactive distillation columns,separating,respectively,a hypothetical synthesis reaction from reactants A and B to product C,and a real decomposition reaction from 1,4-butanediol to tetrahydrofuran and water,are employed to demonstrate these uncommon behaviors.The results obtained give full support to the above qualitative interpretation.Despite the strong influences of reaction kinetics and thermodynamic properties of the reacting mixtures,the totally reboiled reactive distillation columns are generally considered to present such unique behaviors and require tight inventory control of bottom reboiler to facilitate their control system development.

Novel design of lubricant-type vacuum distillation process for lube base oils production from hydrocracking tail oil

Dividing-wall columns(DWCs)are widely used in the separation of ternary mixtures,but rarely seen in the separation of petroleum fractions.This work develops two novel and energy-efficient designs of lubricant-type vacuum distillation process(LVDP)for the separation of hydroisomerization fractions(HIF)of a hydrocracking tail oil(HTO).First,the HTO hydroisomerization reaction is investigated in an experimental fixed-bed reactor to achieve the optimum liquid HIF by analyzing the impact of the operating conditions.A LVDP used for HIF separation is proposed and optimized.Subsequently,two thermal coupling intensified technologies,including side-stream(SC)and dividing-wall column(DWC),are combined with the LVDP to develop side-stream vacuum distillation process(SC-LVDP)and dividing-wall column vacuum distillation process(DWC-LVDP).The performance of LVDP,SC-LVDP,and DWC-LVDP are evaluated in terms of energy consumption,capital cost,total annual cost,product yields,and stripping steam consumption.The results demonstrates that the intensified processes,SC-LVDP and DWC-LVDP significantly decreases the energy consumption and capital cost compared with LVDP.DWC-LVDP further decreases in capital cost due to the removal of the side stripper and narrows the overlap between the third lube oils and fourth lube oils.This study attempts to combine DWC structure into the separation of petroleum fractions,and the proposed approach and the results presented provide an incentive for the industrial implementation of high-quality utilization of HTO through intensified LVDP.

HeOH与BuAc酯交换反应共强化系统的评价

鉴于己醇(HeOH)与乙酸正丁酯(BuAc)酯交换反应的不利动力学和热力学特性,采用基于过程强化的反应蒸馏装置是一条有利的途径。常规反应蒸馏塔(CRDC)虽然考虑了反应-分离操作之间的相互耦合,但却完全忽略了分离操作之间的相互耦合,必然导致过大的不可逆性同时可进一步降低系统的稳态性能。单隔离壁反应蒸馏塔(R-SDWDC)的确在一定程度上考虑了分离操作之间的相互耦合,但其外部限定耦合方式致使仍然无法有效降低分离操作的不可逆性。本文基于本课题组最近提出的共强化原理给出了两种双隔离壁反应蒸馏塔(R-DDWDC)的拓扑结构,并将其与CRDC和R-SDWDC进行了深入系统的比较与分析。结果表明,R-DDWDC明显优于CRDC和R-SDWDC,是实施HeOH与BuAc酯交换反应的最佳拓扑结构。得出这一结论的根本原因在于R-DDWDC既能充分考虑分离操作之间的相互耦合,又能有效协调R-DDWDC与反应-分离操作耦合子系统的相互作用。虽然上述结论是通过具有最有利相对挥发度排序的HeOH与BuAc酯交换反应所得到的,但对于任何具有不利动力学与热力学特性的该类反应物系均具有普遍性的指导意义。

反应精馏隔壁塔提纯乳酸工艺研究

以反应精馏隔壁塔替换传统的反应精馏塔,形成了新型双向反应精馏隔壁塔提纯乳酸工艺,并通过灵敏度分析和遗传算法对工艺流程进行了优化,优化后的反应精馏隔壁塔工艺与传统反应精馏工艺相比年度总费用减少10.15%,CO_(2)排放速率降低31.75%,乳酸收率提升4.68%。

反应精馏隔壁塔制二甲醚过程模拟与分析

针对生产二甲醚工艺过程提出了3种反应精馏隔壁塔的设计,利用Aspen Plus模拟软件对常规反应精馏序列和反应精馏隔壁塔进行了流程模拟,并根据隔板的上下侧右侧是否封闭设计3种反应精馏隔壁塔。结果表明:隔板下侧封底以及两侧的结构在经济和环境指标均优于常规反应精馏塔,而隔板上部右侧封顶的结构的经济环境指标均低于常规反应精馏塔。采用隔板下侧封底的结构相比于常规反应精馏塔,年总费用降低了53.54%,CO_(2)排放量降低了61.81%,热力学效率提高了124.86%。

一种有效的双隔壁反应蒸馏塔的综合与设计策略

提出了一种反应蒸馏过程强化原理,在反应和分离操作有效耦合的基础上进一步加强分离操作之间的耦合,得到一种有效的双隔壁反应蒸馏塔。该结构可显著提高过程中的能量利用效率。以理想四元反应和碳酸二甲酯与乙醇之间的酯交换反应为例对所提出的结构进行评价分析,结果表明,与常规和单隔壁结构相比,这种双隔壁反应蒸馏结构能有效降低能耗。此外,基于反应系统的性质对节能效果的影响进行了研究,结果表明,该双隔壁反应蒸馏塔适用于难反应与难分离系统,且具有优越稳态性能。

甲苯氯化连串反应的RD和SRC过程分析与比较

侧反应器与精馏塔集成(SRC)是塔内耦合反应精馏(RD)的新拓展,同时适用于反应与精馏处于相同工况和不同工况两种情形。以甲苯氯化连串反应为研究对象,分别建立了RD和SRC过程的数学模型,在相同总塔板数和塔釜上升蒸气量条件下,计算了相同工况下(反应与精馏压力均为101.3kPa),RD与SRC过程的气/液流率、温度、组成分布,发现两者性能具有一致性。在此基础上,以产品氯化苄的生产成本为目标函数、标志系统产能的氯气进料流率和分配比例为优化变量,对甲苯氯化连串反应的相同工况RD与SRC过程、不同工况SRC过程(反应压力为101.3kPa、精馏压力为10kPa)进行优化,结果表明,相同压力工况下,优化得到的RD与SRC的装置产能与产品质量一致;但在不同压力工况下,SRC的产能明显提升、产品质量也有所提高,反映出不同工况SRC明显的优势。

反应精馏隔壁塔水解醋酸甲酯的控制研究

在稳态模拟的基础上,研究了醋酸甲酯水解的反应精馏隔壁塔的控制策略。首先利用Aspen Plus软件模拟并优化了该工艺,得到最优操作条件;通过稳态敏感性分析得到各塔板温度的相对增益,由非方相对增益矩阵法选择控制变量及相对增益矩阵法确定操纵变量与被控变量的控制关系,以降低系统的耦合程度;采用继电反馈法整定比例积分控制器参数后,最终在AspenDynamics平台上对动态控制进行了模拟。模拟结果表明,通过3个温度控制回路(水进料量控制主塔第28块塔板温度、主塔塔底再沸器热负荷控制主塔第21块塔板温度、侧线塔塔顶回流量控制侧线塔第11块塔板温度),可较好地控制醋酸甲酯水解的化学计量平衡,且产品质量要求与设计值之间的偏差不大于0.01%。

精馏塔设备的设计与节能研究进展

精馏过程是化工过程中最耗能的过程之一。本文总结了常规精馏塔,反应精馏塔以及分隔壁精馏塔这几种类型精馏塔的研究进展。常规精馏塔的研究仍在进行,并主要体现在算法的改进与新算法的提出,以及常规精馏塔在化工高等教学的重要作用上,指出了反应精馏塔和分隔壁精馏塔是近几年精馏塔设计方面的研究热点,如反应精馏塔的模型、求解方法等,分隔壁精馏塔是一种全新的概念因而并未系统化,但却能够明显节省设备投资,并具有良好的节能前景,尤其是在与反应精馏结合后效果特别明显。文章最后对精馏塔的未来研究方向进行了展望。

反应精馏隔壁塔内合成乙酸甲酯的模拟

提出了一种应用反应精馏隔壁塔合成乙酸甲酯的新工艺流程,采用反应精馏隔壁塔替代常规反应精馏流程中的反应精馏塔及甲醇回收塔。利用Aspen Plus模拟软件,对反应精馏隔壁塔及常规流程进行了模拟,比较分析了两种流程塔内液相组成分布,并分析了塔顶回流比与气相分配比对反应精馏隔壁塔的影响。结果显示新流程可以节能11.9%,并能降低设备投资费用和操作费用。

反应精馏应用于差压热耦合精馏技术的模拟

将差压热耦合反应精馏塔应用于乙酸甲酯的合成,利用Aspen Plus软件建立了模型并进行了流程模拟。对提出的工艺进行了单变量分析,进一步确定了各个变量的最佳值。在此基础上,对差压热耦合反应精馏流程的能耗、CO_2排放量方面与常规反应精馏流程进行了全面的分析与比较。结果表明,与传统流程相比,差压热耦合流程可节能达58.79%,并有很好的减排效果。

催化蒸馏合成丙二醇乙醚

提出了在ＮＫＣ－１号催化剂上用催化蒸馏由环氧丙烷和乙醇合成丙二醇乙醚的方法和流程，考察了塔的操作压力、进料量和进料中乙醇／环氧丙烷摩尔比对环氧丙烷转化率、丙二醇乙醚收率和选择性的影响，给出了过程的最适宜操作条件。

共沸-反应精馏隔壁塔制备乙酸乙酯的实验与模拟研究

针对反应精馏中部分物系存在反应物与生成物相对挥发度较小、难以分离的问题,提出共沸-反应精馏隔壁塔结构。以酯化法制备乙酸乙酯为研究对象,对共沸-反应精馏隔壁塔的操作性能进行了分析。在搭建的1套反应精馏隔壁塔实验装置内进行了相应的实验研究,并用Aspen Plus进行了模拟,模拟结果与实验结果一致性良好;对共沸剂用量、液相分配比、塔顶回流比以及原料乙酸质量分数等因素的影响进行了模拟研究;对共沸-反应精馏隔壁塔进行了节能分析。与传统反应精馏相比,共沸-反应精馏隔壁塔可实现重组分乙酸和中间组分水的清晰分离,有效避免中间组分的返混,提高热力学效率。