Distillation Control Structure Selection for Energy‐Efficient Operations

Few studies have been reported concerning the energy efficiency of various distillation column control structures. The choice of an energy‐efficient control configuration by incorporating thermodynamics second law in the selection criteria is described. In addition to a relative gain array for assessing control loop interactions, a relative exergy array is used in evaluating the energy efficiency of various control structures. The preferred control structure should have both good operability and good energy efficiency as distillation columns are the major energy consumer in the chemical industry. The performance of the control structures in the dynamic mode is analyzed in detail. The proposed method is demonstrated on two binary distillation columns: methanol‐water separation and benzene‐toluene separation. Dynamic simulation results prove the efficiency of the proposed distillation control structure selection method.     

Solvent selection for extractive distillation using molecular simulation

This study examines a search for a new solvent for extractive distillation using molecular simulation, and the performance of the proposed extractive distillation processes using the solvents is compared to conventional processes. Unlike other solvent search procedures, the proposed solvent finding technique relies on a chemical structure similarity between the solvent and the extracted material without depending on a solvent database. Two new processes of benzene, toluene, and xylenes separation from naphtha reformate and propene/propane separation are compiled with the newly developed solvents for the performance examination. The simulation of the proposed extractive distillation processes demonstrates a significant performance improvement in energy saving and investment cost over conventional processes.     

基于能量目标的芳烃萃取精馏溶剂评价模型

以NRTL活度系数模型为基础，利用Aspen Plus对不同单组分萃取剂回收芳烃的萃取精馏装置进行了全流程模拟和工艺操作参数优化。综合考虑各操作变量及其关联，提出了基于局部耦合参数迭代优化的整体协同优化策略，在保证分离要求的条件下，以能耗为目标，对萃取精馏塔（EDC）、溶剂回收塔（ERC）的进料位置、ERC回流比等关键操作参数进行优化，建立过程能耗的物性关联模型。通过分析不同溶剂对芳烃萃取精馏过程能耗和分离效果的影响，提出了基于能耗目标的芳烃萃取精馏溶剂评价模型，结果表明影响芳烃萃取精馏过程能耗的关键物性为溶剂的分子量及常压沸点，所建立的过程能耗关联模型具有较高的关联性，其R ²均大于0.9，可有效指导萃取精馏溶剂选择，为计算机辅助分子设计提供简化的目标函数。     

分子模拟评价萃取精馏萃取剂的新方法

萃取精馏是强化分离的有效方法,选用不同的萃取剂对萃取精馏过程的可行性和经济性有很大影响。探索性地使用分子模拟软件Material Studio 计算二元物系的混合能来作为萃取剂优劣的判据。以苯和环己烷为待分离物系,选择多种溶剂作为萃取剂,计算萃取剂分子与苯分子之间的混合能以及萃取剂分子与环己烷分子之间的混合能,并计算2个混合能的差值。以文献报导的选择性和以热力学NRTL方程计算无限稀释活度系数得到的选择性为标准来验证使用混合能差值评价萃取剂优劣的合理性。验证结果表明混合能差值基本随选择性的增加而增大,这一结论为进一步深入理论研究和新的萃取剂选择方法的开发打下了基础。     

Mechanism analysis of solvent selectivity and energy-saving optimization in vapor recompression-assisted extractive distillation for separation of binary azeotrope

Octane and p-xylene are common components in crude gasoline, so their separation process is very important in petroleum industry. The azeotrope and near azeotrope are often separated by extractive distillation in industry, which can realize the recovery and utilization of resources. In this work, the vapor–liquid equilibrium experiment was used to obtain the vapor–liquid equilibrium properties of the difficult separation system, and on this basis, the solvent extraction mechanism was studied. The mechanism of solvent separation plays a guiding role in selecting suitable solvents for industrial separation. The interaction energy, bond length and charge density distribution of p-xylene with solvent are calculated by quantum chemistry method. The quantum chemistry calculation results and experiment results showed that N-formylmorpholine is the best solvent among the alternative solvents in the work. This work provides an effective and complete solvent screening process from phase equilibrium experiments to quantum chemical calculation. An extractive distillation simulation process with N-formylmorpholine as solvent is designed to separate octane and p-xylene. In addition, the feasibility and effectiveness of the intensified vapor recompression assisted extraction distillation are also discussed. In the extractive distillation process, the vapor recompression-assisted extraction distillation process is globally optimal. Compared with basic process, the total annual cost can be reduced by 43.2%. This study provides theoretical guidance for extractive distillation separation technology and solvent selection.     

Design of extractive distillation for the separation of close-boiling mixtures: Solvent selection and column optimization

A practical methodology for the design and optimization of extractive distillation is proposed in this work. The extractive distillation is generally applied to the separation of close-boiling mixtures, which by conventional distillation is difficult to separate. The design and optimization of extractive distillation is more complex than that of the conventional distillation when considering the selection of suitable solvent to enhance the separation. Currently, the solvent selection can be effectively handled by the assistance of the computer-aided molecular design (CAMD) approach. The selection result may however be inconclusive due to the lack of accurate or missing parameters in the property model. In this work, the experimental verification and the property parameter determination were proved to be necessary as an additional step to achieve a successful and reliable design. The overall design methodology was illustrated through an industrial separation of C8-Aromatics mixture.     

Separation of tert‐Butyl Alcohol‐Water Mixtures by a Heterogeneous Azeotropic Batch Distillation Process

Tert‐butyl alcohol and water form an azeotrope at normal pressure. Simple distillation cannot be used to separate these two components. In this article, a systematic study of the separation of tert‐butyl alcohol–water mixtures with an entrainer by heterogeneous azeotropic batch distillation was performed. Based upon the thermodynamic behavior of the ternary mixtures, cyclohexane was chosen as the entrainer. It formed ternary and binary heterogeneous azeotropes with the original components. The process feasibility analysis was validated by using rigorous simulation with chemical process simulation software – HYSYS Plant 2.2 and DISTIL 4.1. Simulation results were then corroborated in a batch experimental column for the selected entrainer.     

Membrane‐assisted vapor stripping: energy efficient hybrid distillation–vapor permeation process for alcohol–water separation

BACKGROUND: Energy efficient alternatives to distillation for alcohol recovery from dilute solution are needed to improve biofuel sustainability. A process integrating steam stripping with a vapor compression step and a vapor permeation membrane separation step is proposed. The objective of this work is to estimate the energy and process costs required to make a fuel grade ethanol (0.5 wt% water) from 1 and 5 wt% ethanol aqueous streams using the proposed process. RESULTS: Using process simulation and spreadsheeting software, the proposed membrane‐assisted vapor stripping process was estimated to require as little as 8.9 MJ of fuel‐equivalent energy per kg of fuel grade ethanol recovered from a 1 wt% ethanol feed stream, 2.5 MJ kg^?1 for a 5 wt% ethanol solution. This represents an energy saving of at least 43% relative to standard distillation producing azeotropic ethanol (6 wt% water). Process costs were also found to be lower than for distillation at the 3.0 × 10⁶ kg‐ethanol year^?1 scale modeled. CONCLUSION: In this hybrid system, the stripping column provides high ethanol recoveries and low effluent concentrations while the vapor compression‐membrane component enables the efficient recovery of latent and sensible heat from both the retentate and permeate streams from the membrane system. Published in 2008 by John Wiley & Sons, Ltd.     

Experimental validation of hybrid distillation‐vapor permeation process for energy efficient ethanol–water separation

BACKGROUND: The energy demand of distillation‐based systems for ethanol recovery and dehydration can be significant, particularly for dilute solutions. An alternative separation process integrating vapor stripping with a vapor compression step and a vapor permeation membrane separation step, termed membrane assisted vapor stripping (MAVS), has been proposed. The hydrophilic membrane separates the ethanol–water vapor into water‐rich permeate and ethanol‐enriched retentate vapor streams from which latent and sensible heat can be recovered. The objective of this work was to demonstrate experimentally the performance of a MAVS system and to compare the observed performance with chemical process simulation results using a 5 wt% ethanol aqueous feed stream as the benchmark. RESULTS: Performance of the steam stripping column alone was consistent with chemical process simulations of a stripping tower with six stages of vapor liquid equilibria (VLE). The overhead vapor from the stripper contained about 40 wt% ethanol and required 6.0 MJ of fuel‐equivalent energy per kg of ethanol recovered in the concentrate. Introduction of the vapor compressor and membrane separation unit and recovery of heat from both membrane permeate and retentate streams resulted in a retentate ethanol concentrate containing ca 80 wt% ethanol, but requiring only 2.2 MJ fuel kg^?1 ethanol, significantly less than steam stripping alone. CONCLUSION: Performance of the experimental unit with a 5 wt% ethanol feed liquid corroborated chemical process simulation predictions for the energy requirement of the MAVS system, demonstrating a 63% reduction in the fuel‐equivalent energy requirement for MAVS compared with conventional steam stripping or distillation. Published 2009 by John Wiley & Sons, Ltd.     

进口重芳烃宽馏分生产偏三甲苯与富集均三甲苯

重芳烃分离装置是为分离重整装置二甲苯塔塔底产出的碳九芳烃而设计的。为弥补原料不足,加工组分和馏程变化较大的、以碳九、碳十芳烃为主要成分的重芳烃宽馏分进口原料,经方案选择、工艺模拟计算及流程改进,成功地生产了偏三甲苯与富集均三甲苯工业产品。     

Performance and Energy Consumption of Membrane‐Distillation Hybrid Systems for Olefin‐Paraffin Separation

Light olefin and paraffin are commonly separated by energy‐intensive cryogenic distillation. Membrane/distillation hybrid systems constitute an economical alternative separation process. Different configurations of this hybrid system are studied for olefin‐paraffin separation with emphasis on C₃ separation. An approach based on the McCabe‐Thiele method is applied to analyze different process configurations. A facilitated transport membrane is considered as membrane type. Both new column design and augmentation of an existing distillation column by a membrane module are considered. Numerical examples are considered for the separation of propane from propylene through different hybridization shapes with facilitated transport membranes. The energy requirement can be halved using hybrid systems.     

Simulation of Membrane Distillation for Purifying Water Containing 1,1,1‐Trichloroethane

Vacuum membrane distillation is modeled for the purification of water containing organic matter. The separation medium is a hollow‐fiber membrane contactor that is simplified to a two‐dimensional structure with a single porous membrane wall. The model considers the transport phenomena of a vacuum membrane distillation system in porous media, in which the aqueous volatile organic solution was considered as an incompressible and steady fluid. The numerical simulation of the two‐dimensional model of vacuum membrane distillation for an aqueous solution of 1,1,1‐trichloroethane was established under steady state. The effects of the bulk feed temperature and the feed flow rate on the percentage of 1,1,1‐trichloroethane removal from an aqueous solution are discussed.     

间歇萃取精馏技术的研究进展

对间歇萃取精馏分离技术和进展进行了评述,主要从萃取精馏的溶剂选择与设计、模拟及优化、操作参数对分离过程影响及萃取精馏塔设备等几个方面,介绍了国内外关于间歇萃取精馏新兴分离技术的最新研究动态,最后指出了萃取精馏技术目前存在的问题和今后发展的方向。     

Dynamic modeling and collocation‐based model reduction of cryogenic air separation units

High purity distillation columns and multi‐stream heat exchangers (MSHXs) are critical units in cryogenic air separation plants. This article focuses on modeling approaches for the primary section of a super‐staged argon plant. A full‐order stage‐wise model for distillation columns in air separation units (ASUs) that considers key process phenomena is presented, followed by a reduced‐order model using a collocation approach. The extent of model reduction that can be achieved without losing significant prediction accuracy is demonstrated. A novel moving boundary model is proposed to handle MSHXs with phase change. Simulation results demonstrate the capability of the proposed model for tracking the phase change occurrence along the length of the heat exchanger. Dynamic simulation studies of the integrated plant show that the thermal integration between the feed and product streams captured in the primary heat exchanger is critical to accurately capture the behavior of ASUs. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1602–1615, 2016     

Separation of Ethyl Acetate‐Dichloromethane‐Ethanol by Extractive Distillation: Simulation and Optimization

Chemical process simulation software is adopted to simulate the separation of ethyl acetate‐dichloromethane‐ethanol and water with the aim to use water as the extraction solvent and to meet the requirement of reuse. The influences of solvent ratio, reflux ratio, theoretical plate number, feed position, and solvent position on separation efficiency are discussed. Optimization of orthogonal design and response surface are employed to further optimize the parameters. Relative experimental results prove the reliability of the simulation.     

A simple synthesis method for studying thermally integrated distillation sequences

In this paper an improved method for studying processes for separation of multicomponent mixtures is presented. A thermodynamically oriented procedure was developed for the analysis and synthesis of the best heat‐integrated distillation system. Systematic ranking of possible schemes was used in order to verify the best structure of a heat‐integrated distillation system. An integrability criterion was introduced for the selection of the most promising heat‐integrated sequences. Distillation columns were studied according to their ability for heat integration. The proposed selection of possible distillation columns sequences using rising integrability criteria gave a good approximation of the best heat‐integrated distillation column sequences according to their total annual cost. Computational simulation of the process and pinch analysis were used for thermodynamic optimization of energy consumption. The method is illustrated with five‐ and six‐component example problems.     

Extractive distillation with ionic liquids: A review

Extractive distillation is commonly used for the separation of azeotropic or close‐boiling mixtures in the chemical industry. During the past decade, the use of ionic liquids (ILs) as entrainers has received considerable attention due to their unique advantages when applied in extractive distillation. This work is devoted to providing an easy‐to‐read and comprehensive review on the recent progress made by chemical engineers, focusing on the issues of predictive thermodynamic models, structure‐property relations, separation mechanisms, and process simulation and optimization. This review spans from the molecular level to the industrial scale, to provide a theoretical insight into the molecular interactions between ILs and the components to be separated. Moreover, a comprehensive database on the vapor–liquid equilibria and activity coefficients at infinite dilution concerning ILs is provided as Supporting Information. Concluding remarks are made on the unsolved scientific issues with respect to this promising special distillation technology. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3312–3329, 2014     

Revamping Dimethyl Ether Separation to a Single‐Step Process

Process intensification techniques were recently proposed to improve the eco‐efficiency of the conventional dimethyl ether (DME) purification and methanol recovery distillation sequence, but they all require new specific equipment and hence rather high investment costs leading to several years of payback time. However, the alternative of reusing the existing equipment to revamp the two distillation columns of the downstream processing section into a single‐step separation was so far overlooked in the open literature. To solve the problem of costly DME separation, a novel single‐step DME separation taking place in a dividing‐wall column (DWC) is proposed that effectively integrates in one shell the tasks of DME purification and methanol recovery. The new process is optimized in terms of minimal energy requirements, taking into account the restrictions caused by reusing one distillation column like, such as limited diameter or reboiler/condenser heat duty. The results demonstrate that the DWC alternative is feasible and has better performances as compared to the classic sequence, i.e., 28 % lower operating costs and 20 % less capital investment.     

丙酮-四氢呋喃-水混合物的清洁分离工艺的概念设计

引言 四氢呋喃（THF）既是一种性能优良的贵重有机溶剂,又是一种重要的有机合成中间体,因此在制药、涂料、皮革等领域应用广泛．当四氢呋喃用作溶剂时,由于其不被消耗,往往需要进行回收．但四氢呋喃易与水、丙酮等其他极性溶剂形成共沸物,而生产中又常需要四氢呋喃的纯度足够高,进而增加了分离提纯难度．     

萃取精馏生产高纯度环氧丙烷的工艺研究

丙烯环氧化反应获得的粗环氧丙烷(PO)中含有乙醛、甲醇、甲酸甲酯和水等杂质，由于这些杂质与PO相对挥发度接近于1，普通精馏难以提纯PO；同时，分离过程中PO易发生水解生成1,2-丙二醇(PG)，而PG又导致萃取剂萃取性能下降。据此，结合萃取精馏和液液萃取技术，同时考虑PO水解反应，开发了水洗回收PO和侧线采出共沸物脱除PG流程，在有效脱除杂质的同时，提高了PO的回收率和萃取剂的萃取效率，获得了高纯度PO产品。采用流程模拟软件Aspen Plus对上述流程进行了全流程模拟计算，借助NRTL热力学方法，分析了萃取精馏塔的溶剂比、理论塔板数、原料进料位置、溶剂进料温度等主要工艺参数对分离过程的影响。结果表明该工艺流程合理、可靠，经济性优于现有工艺，可指导工业过程设计和操作优化。