Hybrid Simulated Moving Bed Processes for the Purification of p‐Xylene

Abstract

A parallel two‐zone and four‐zone SMB hybrid process was developed for p‐xylene purification. By placing a high‐productivity parallel two‐zone SMB in front of an existing four‐zone SMB, feed was enriched in the parallel two‐zone SMB before it was fed to the four‐zone SMB, which allows a higher feed rate to the four‐zone SMB. A general design procedure for hybrid SMB‐SMB process was developed. Simulations were done with Aspen Chromatography and Aspen Plus. Overall adsorbent productivity was increased by more than 100%, but with 3–5% lower overall p‐xylene recovery. With overall recovery of the hybrid process matching the base case, the feed rate was doubled and the overall adsorbent productivity was 50% higher than the base case. The heat duties of distillation columns per unit product were similar for these hybrid processes compared to the base case. If a two‐feed raffinate distillation column was used, the heat duty of distillation columns per unit product in the hybrid process was 2–4% lower than the base case. Both parallel two‐zone and four‐zone SMBs were also developed for use in a hybrid SMB‐crystallization process for p‐xylene purification. SMB productivity can be increased from 31% to 75% by adding additional pumps between columns to operate columns at their pressure limits. Although additional pumps are useful for both systems, the productivities of the parallel two‐zone SMBs were 9% and 16% higher than the corresponding four‐zone SMBs with the same number of recycle pumps. For a four‐zone SMB with two columns per zone and eight recycle pumps, a 114% increase of productivity was achieved, which is 22% higher than a parallel two‐zone with one column per zone and four recycle pumps.     

丁醛精馏塔流程模拟与优化

利用Aspen-plus流程模拟软件对丁辛醇装置丁醛精馏塔建立流程模拟模型,比较模拟结果与设计值,通过灵敏度分析,确定最佳操作参数条件.     

Intensified process for aromatics separation powered by Kaibel and dividing-wall columns

Process intensification in distillation led to major developments, such as reactive distillation, heat-integrated distillation, cyclic distillation, as well as Kaibel and dividing-wall column. Still, the separation of aromatics at industrial scale is carried out typically in a series of conventional distillation columns, with severe penalties on the associated plant footprint, investment and operating costs. To solve this problem, this study investigates novel separation alternatives powered by dividing-wall column (DWC) and Kaibel distillation column. The new sequences using process intensification are able to separate five products (lights, benzene, toluene, xylene and heavies) at high purity levels, in only two distillation columns.AspenTech Aspen Plus^® was used as a computer aided process engineering tool to perform the rigorous simulation and optimization of the new separation alternatives, applied to a simplified industrial case study. In order to allow a fair comparison, all design alternatives were optimized using the sequential quadratic programming (SQP) method.Notably, the novel design with two consecutive DWC units reduces the energy demand by 14%, while the alternative combining a conventional stripper with a Kaibel column leads to over 17% energy savings as compared to the conventional direct distillation sequence. Moreover, the new separation schemes require less equipment and a reduced plant footprint.     

Optimal design of catalytic distillation columns: A case study on synthesis of TAEE

The design of catalytic distillation (CD) columns is a challenging task because of the superposition of chemical reaction and distillation in one apparatus. In this work, a method to design a cost-optimal CD column for chemical systems with large number of components and chemical reactions is presented. The method is based on the following steps: (1) estimation of the number of theoretical stages and catalyst volume by the decomposition of the CD column into a sequence of chemical reactors and non-reactive distillation columns, (2) estimation of the column diameter and operating conditions using an equilibrium stage model, and (3) design of the column applying an optimisation algorithm and using a rigorous non-equilibrium stage model to represent the CD process. The method is applied to determine the optimal column configuration and operating conditions for the synthesis of tert-amyl ethyl ether from ethanol and isoamylenes. Eight components and four chemical reactions were selected to represent the chemical system in the simulations.     

Integrating product and process design with optimal control: A case study of solvent recycling

This paper proposes the simultaneous integration of environmentally benign solvent selection (product design), solvent recycling (process design) and optimal control for the separation of azeotropic systems using batch distillation. The previous work performed by Kim et al. (2004. Entrainer selection and solvent recycling in complex batch distillation. Chemical Engineering Communications 191(12), 1606-1633) combines the chemical synthesis and process synthesis under uncertainty. For batch distillation, optimal operation is also important due to the unsteady state nature of the process and high operating costs. Optimal control allows us to optimize the column operating policy by selecting a trajectory for the reflux ratio. However, there are time-dependent uncertainties in thermodynamic models of batch distillation due to the assumption of constant relative volatility. In this paper, the uncertainties in relative volatility were modeled using Ito processes and the stochastic optimal control problem was solved by combined maximum principle and non-linear programming (NLP) techniques. Then the previous work of optimal solvent selection and recycling was coupled with optimal control. As a real world example for this integrated approach, a waste stream containing acetonitrile-water was studied. The optimal design parameters obtained by Kim et al. (2004. Entrainer selection and solvent recycling in complex batch distillation. Chemical Engineering Communications 191(12), 1606-1633), for this separation were used and the optimal control policy is computed first without considering uncertainties by variable transformation technique. The deterministic optimal control policy improves the product yield by 4.0% as compared to the base case, verified using a rigorous simulator for batch distillation. When the stochastic optimal control policy was computed representing the relative volatility as an Ito process, a similar recovery rate was obtained from simulations, but the batch time was reduced significantly, producing the most profitable operation.     

Analysis of a hybrid distillation-pervaporation column in a single unit: Intermediate membrane section in the rectifying and stripping section

Distillation-pervaporation in a single unit (DPSU) column can perform separations that are not possible in conventional distillation by overcoming distillation boundaries. Unlike conventional hybrid distillation-pervaporation columns, in a DPSU system the pervaporation membrane is located inside the column. The separation by distillation and pervaporation is carried out simultaneously inside the same column section. In a previous work, a simplified model was used to design and analyze distillation-pervaporation in a single unit (DPSU) systems with a hybrid rectifying-pervaporation section, where the membrane constitutes the whole section. In this study, this simplified model is applied to DPSU columns where the membrane partially constitutes the rectifying or the stripping sections, including the model derivation of the stripping section and the operation leaves. The simplified model is applied for the separation of two mixtures with different Serafimov's topology classifications: acetone-isopropanol-water (topology type 1.0-2) and ethyl acetate-ethanol-methanol (topology type 2.0-2b). Thermodynamic limitations are identified for the separation of the ethyl acetate-ethanol-methanol mixture. Multiple operation leaves are produced depending on the liquid composition at the beginning of the membrane section, hindering the conditions that help to overcome the distillation boundary through a DPSU column. For some conditions, a section that is partially constituted by a membrane performs better than if the membrane constitutes the whole section.     

Inter-integration reactive distillation with vapor permeation for ethyl levulinate production: Equipment development and experimental validating

Ethyl levulinate, one of the main derivatives of levulinic acid (LA), is of significant potential as platform chemicals for bio-based materials. The esterification of LA was generally carried out in a conventional batch reactor or in a conventional reactive distillation column. However, traditional methods are hard to deal with equilibrium limited reactions and azeotropic issues. Therefore, the inter-integration reactive distillation with vapor permeation (R-VP-D) process, which integrated reaction, vapor permeation, and distillation into one single unit, is proposed in this paper and validated in the pilot-scale experiments. A comparative study is made between a pilot-scale RD column with and without VP module. Owing to the water-selective VP membrane and the ingenious design of related apparatuses, the R-VP-D process reveal a superiority in LA conversion of 21.9% maximum higher than RD without VP process and removing of product water about 53.6% from VP module, which indicates its promising industrial application in process intensification field.     

异丙醇一步法合成MIBK精馏过程优化

用PROII过程模拟软件对异丙醇一步法合成甲基异丁基酮(MIBK)的精馏系统进行了分析和模拟优化,提出了5塔流程的改进工艺,使MIBK的回收率提高了2.8倍,达到91·7%,而且再沸器能耗由12·79t水蒸气/t产品降低到8·27t水蒸气/t产品。还建立了普通精馏塔和萃取精馏塔小试实验装置,对全工艺流程逐塔进行热态研究,验证了其工业可行性。     

Modelling and simulation of a hybrid process (pervaporation–distillation) for the separation of azeotropic mixtures of alcohol–ether

This work reports the modelling and simulation of a hybrid process, based on the combination of distillation and pervaporation, for the separation of azeotropic mixtures of alcohol–ether. After having selected the separation of methanol‐2‐metoxi‐2,2‐dimethyl ethane (ETHER) as a motivating example the mathematical modelling of the distillation column was achieved and used together with a mass transfer model previously reported for the pervaporation operation in order to simulate the behaviour of the hybrid process for different compositions of the feed stream (case 1: 3.2 wt% methanol, 55.4 wt% C₄, 41.4 wt% ETHER, and case 2: 5.2 wt% methanol, 42 wt% C₄, 52.8 wt% ETHER). Simulation tasks were carried out with the process modelling system gPROMS and the results of alternative process configurations that result from the relative location of the separation technologies have been compared on the basis of the required membrane area. Finally, the design of the pervaporation unit including the overall processing costs is reported. Copyright © 2001 Society of Chemical Industry     

原油常减压蒸馏塔的流程模拟

常减压蒸馏装置是炼油加工的第一道工序,它的运行状况对后续装置乃至全厂都有着重要影响。针对初馏塔、常压蒸馏塔和减压蒸馏塔的稳态流程模拟,选取适合的蒸馏塔中段循环设定变量和减压塔模拟方法,最终流程模拟结果与实际工艺参数相似度极高,说明常减压蒸馏塔的流程模拟能够如实体现实际工艺状态,可以作为指导工艺生产、操作优化的有效方法。     

用于生产TAEE的反应精馏和全蒸发的混合过程

In this study, a reactive distillation column in which chemical reaction and separation occur simultaneously is applied for the synthesis of tert-amyl ethyl ether （TAEE） from ethanol （EtOH） and tert-amyl alcohol （TAA）. Pervaporation, an efficient membrane separation technique, is integrated with the reactive distillation for enhancing the efficiency of TAEE production. A user-defined Fortran subroutine of a pervaporation unit is developed, allowing the design and simulation of the hybrid process of reactive distillation and pervaporation in Aspen Plus simulator. The performance of such a hybrid process is analyzed and the results indicate that the integration of the reactive distillation with the pervaporation increases the conversion of TAA and the purity of TAEE product, compared with the conventional reactive distillation.     

亚硝酸酯合成工艺及反应器选型

简述了酯化反应原理及酯化反应动力学研究进展,提出了酯化反应器应设计为填料塔或板式塔较合适;详细介绍了并流酯化反应工艺与反应精馏工艺流程,比较了两种工艺流程的优缺点及能耗:并流流程的操作弹性大,设备制造简单,但需要设计水洗及干燥设备,蒸汽消耗量大,工艺流程较长;反应精馏工艺流程短,投资减少,但反应精馏塔的设计和制造比较复杂,工艺操作条件严格。     

精馏塔连续放空原因剖析与对策

用脱水塔改造的实际案例,陈述不凝气积聚的观点,及进料中的不凝气在冷凝器中发生积聚是治理精馏塔连续放空的理论依据。文中在多个实际例子中开展了对冷凝工艺的讨论,包括对冷凝器、冷凝流程和压力控制方法的讨论。指出石化工业引进的冷凝工艺,由于采用气液相共用一个出口的冷凝器,造成不凝气积聚,导致压力失控而发生连续放空。文中介绍了传统的气相串联式冷凝工艺的特点和原理,以及在老塔改造中逐步淘汰引进的气液共用一个出口冷凝器的工艺,并在新塔设计中积极推广和应用气相串联式冷凝工艺。     

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.     

焦化蒸氨工艺的模拟与优化

简要介绍了化工流程模拟软件,并利用化工流程模拟软件对唐山中润煤化工有限公司的蒸氨工艺进行了模拟,找到最佳操作条件。模拟结果表明:塔底出水氨浓度随蒸汽量的增加而增大,随进水量的增加而降低,当进水量为60t/h时,最经济的蒸汽量为7t/h;当蒸汽量为10t/h时,在保证塔底出水氨质量浓度小于100mg/L的前提下,最大处理能力为85t/h;当进水量>90t/h后,塔底出水氨质量浓度增加到200mg/L以上。     

Process design and control optimization: A simultaneous approach by multi‐parametric programming

We present a framework for the application of design and control optimization via multi‐parametric programming through four case studies. We develop design dependent multi‐parametric model predictive controllers that are able to provide the optimal control actions as functions of the system state and the design of the process at hand, via our recently introduced PAROC framework (Pistikopoulos et al, Chem Eng Sci. 2015;136:115–138). The process and the design dependent explicit controllers undergo a mixed integer dynamic optimization (MIDO) step for the determination of the optimal design. The result of the MIDO is the optimal design of the process under optimal operation. We demonstrate the framework through case studies of a tank, a continuously stirred tank reactor, a binary distillation column and a residential cogeneration unit. © 2017 American Institute of Chemical Engineers AIChE J, 2017     

Carbon dioxide capture for the oxidative coupling of methane process - A case study in mini-plant scale

The oxidative coupling of methane (OCM) to ethylene is a promising alternative for the oil based industry. In this process, beside the valuable product ethylene, unwanted by-products like CO₂ are produced. Hence, the gas stream has to be refined further. The process is not applied in the industry yet, because of high separation costs. This article focuses particular on the CO₂ purification of the OCM product stream. Therefore a case study was done for a design task of 90% CO₂ capture from 25 vol% in the OCM product gas with an operation pressure of 32 × 10⁵ Pa. Within the article is shown, how to resolve the lack of high separation cost for the purification and the development of an integrated, energy efficient CO₂ capture process for the OCM refinery is described. Therefore a state of the art chemical absorption process using monoethanolamine (MEA) was developed and optimized for the base case. Therefore Aspen Plus^® with the build-in rate based model for the mass transfer with an electrolyte NRTL - approach and chemical equilibrium reactions for the water-MEA-CO₂ system as well as kinetic reactions based on the MEA-REA package was applied. In order to improve the energetic process performance, gas permeation with dense membranes was studied as process alternative. For this purpose a membrane unit was developed in Aspen Custom Modeler^® (ACM). The solution-diffusion model with the free-volume-theory for gas permeation including Joule-Thomson effect as well as concentration polarization (Stünkel et al., 2009) was applied successfully. Furthermore several selective materials for a composite membrane with experimentally determined parameters were studied by this model and it was found, that a matrimide membrane provides the best selectivity performance for the OCM CO₂ capture. Based on this material a membrane module was installed to form a hybrid separation process in combination with the amine based absorption process. The comparison of the state of the art process with the novel hybrid separation process shows an energy saving of more than 40% for the OCM CO₂ capture. In the experimental study the stand alone performance of each unit, as well as the performance of the hybrid process were studied and the results are presented in this article.     

分隔壁萃取精馏塔分离醋酸水溶液的模拟

提出了一种新的单塔萃取精馏精制醋酸水溶液的新工艺,该工艺采用分隔壁萃取精馏塔(DWC-E)替代常规萃取精馏流程的萃取精馏塔及溶剂回收塔,不仅节省了设备投资,而且降低了总能耗。利用Aspen Plus模拟软件,对DWC-E塔及常规萃取流程进行了模拟。DWC-E塔的操作条件:塔板数40块,侧线精馏段的板数10块,回流比2,溶剂摩尔比2.5,在此条件下,比较了常规萃取精馏流程与分隔壁精馏塔内温度、液相组成及汽液相流量的变化。结果表明,DWC-E塔比常规的2塔萃取精馏流程节能23.91%。     

Feed distribution in distillation: Assessing benefits and limits with column profile maps and rigorous process simulation

This contribution describes the column profile map (CPM) methodology for designing distributed feed distillation columns. For non‐sharp product distributions, a case study shows that energy savings of approximately 35% can be obtained if the feed stage(s) are designed optimally. Feed distribution allows capital cost savings, expands operating leaves, and can obtain greater separation feasibility. However, this column only has benefits in ternary and higer‐order systems and when product distributions are non‐sharp. To validate these counter‐intuitive claims, a real Benzene, p‐Xylene, Toluene system is modeled using CPMs, and the resulting design parameters are transported to Aspen Plus^®. Using a sum of squared errors objective function to quantify savings, a cost saving trend very similar to the one predicted by the CPM method is obtained. This article therefore describes a complete design methodology for distributed feed systems and provides convincing evidence of benefits of such a system. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1668–1683, 2013     

甲醇装置精馏塔的设计与使用

介绍了小碳铵厂联醇装置中甲醇精馏塔的设计与使用情况。采用双塔流程,轻组分由预塔分离,从主塔分离出的精甲醇产品纯度达９９８％。