Implementation of Ethanol Dehydration Using Dividing-Wall Heterogeneous Azeotropic Distillation Column

In this article, the design and optimization procedures of a dividing-wall column for heterogeneous azeotropic distillation (DWC-A) using cyclohexane as an entrainer for ethanol dehydration are investigated. The proposed procedures can detect the optimal values of the design variables and thereby guarantee the minimum energy requirements, which is related to the minimum CO₂ emissions and the lowest total annual cost (TAC). Since ethanol and water form an azeotrope under atmosphere pressure, a conventional heterogeneous azeotropic distillation sequence (CHADS), including an azeotropic column and a recovery column, is usually used to perform the ethanol dehydration process. However, due to high energy requirements and equipment investments of CHADS, the TAC is at a relatively high level. DWC-A can be used to eliminate the condenser of the second column and decrease the degree of back-mixing. Both CHADS and DWC-A are simulated with Aspen Plus®, and the results show that DWC-A has an energy saving of 42.17% and the TAC reduction of 35.18% along with higher thermodynamic efficiency and reduction in greenhouse gas emissions.     

Optimal design of mixed acid esterification and isopropanol dehydration systems via incorporation of dividing-wall columns

This study explores the design of a reactive distillation system and that of a heterogeneous azeotropic system by incorporating dividing-wall column (DWC). The first system involves the esterification of mixed acid (acetic acid and propionic acid) with methanol. Simulation studies are carried out for conventional reactive distillation sequence as well as for reactive dividing-wall distillation system. Both systems are optimized by an iterative optimization procedure. Optimal design results show that the reactive dividing-wall system saves steam consumption by 45.2% and reduces total annual costs (TAC) by 34.5%. The second case investigated is a heterogeneous azeotropic distillation system involving dehydration of crude isopropyl alcohol with cyclohexane as entrainer. Two optimal separation systems are generated, including one with a single-dividing wall column and one with a double-dividing wall column. In comparison with an energy-efficient azeotropic distillation sequence containing two stripping columns by Chang et al. (2012) [1], simulation results show that the former two systems can cut steam usage further by 6.0%. The two systems save about 5.4–6.1% in terms of TAC. DWCs prove to be superior to the convention distillation systems with respect to both cost and energy efficiency.     

Comparison of continuous homogenous azeotropic and pressure-swing distillation for a minimum azeotropic system ethyl acetate/n-hexane separation

Continuous homogenous azeotropic distillation (CHAD) and pressure-swing distillation (PSD) are explored to separate a minimum-boiling azeotropic system of ethyl acetate and n-hexane. The CHAD process with acetone as the entrainer and the PSD process with the pressures of 0.1 MPa and 0.6 MPa in two columns are designed and simulated by Aspen Plus. The operating conditions of the two processes are optimized via a sequential modular approach to obtain the minimum total annual cost (TAC). The computational results show that the partially heat integrated pressure-swing distillation (HIPSD) has reduced in the energy cost and TAC by 40.79% and 35.94%, respectively, than the conventional PSD, and has more greatly reduced the energy cost and TAC by 62.61% and 49.26% respectively compared with the CHAD process. The comparison of CHAD process and partially HIPSD process illustrates that the partially HIPSD has more advantages in averting the product pollution, energy saving, and economy.     

Reducing Energy Consumption and CO2 Emissions in Thermally Coupled Azeotropic Distillation

The design and optimization procedures of a heterogeneous thermally coupled azeotropic distillation sequence with a side stripper (TCADS‐SS) for the purification of isopropanol has been investigated. The proposed procedures can detect the optimal values of the design variables and thereby guarantee the minimum energy consumption, which is related to the minimum CO₂ emissions and the lowest total annual cost (TAC). The procedures are applied to the study of the separation of azeotropic mixtures using the two distillation sequences. In the TCADS‐SS, the top end of the side stripper has both liquid and vapor exchange with the main column, which eliminates a condenser in contrast with the conventional heterogeneous azeotropic distillation sequence (CHADS). The results show that not only reductions in energy consumption and CO₂ emissions but also higher thermodynamic efficiency can be obtained for the TCADS‐SS.     

乙醇脱水塔内两相和三相共沸精馏

研究了乙醇脱水塔内的两相共沸精馏和汽液液三相共沸精馏过程。利用Aspen plus模拟软件对乙醇脱水塔内4种工况的精馏曲线、共沸剂浓度分布、回流量和再沸器能耗进行了分析比较。结果表明,苯做共沸剂时,脱水塔内两相共沸精馏和汽液液三相共沸精馏的精馏曲线、共沸剂浓度分布、回流量和再沸器能耗相近,脱水塔精馏曲线都跨越了精馏边界,并且共沸剂在塔内大多数板上都有较高浓度分布。而环己烷做共沸剂时,两相共沸精馏工况和汽液液三相共沸精馏工况条件下的脱水塔内精馏曲线、共沸剂浓度分布、回流量和再沸器能耗有较大差别。汽液液三相共沸精馏工况条件下,环己烷在塔内大多数板上有较高浓度分布,起到较好的脱水作用,而两相共沸精馏工况条件下脱水塔内共沸剂仅分布在塔顶几块塔板上,塔内多数板上没有起到共沸剂作用。     

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．     

Heterogeneous extractive batch distillation of chloroform-methanol-water: Feasibility and experiments

A novel heterogeneous extractive distillation process is considered for separating the azeotropic mixture chloroform-methanol in a batch rectifying column, including for the first time an experimental validation of the process. Heterogeneous heavy entrainer water is selected inducing an unstable ternary heteroazeotrope and a saddle binary heteroazeotrope with chloroform (ternary diagram class 2.1-2b). Unlike the well-known heterogeneous azeotropic distillation process and thanks to continuous water feeding at the column top, the saddle binary heteroazeotrope chloroform-water is obtained at the column top, condensed and further split into the liquid-liquid decanter where the chloroform-rich phase is drawn as distillate. First, feasibility analysis is carried out by using a simplified differential model in the extractive section for determining the proper range of the entrainer flowrate and the reflux ratio. The operating conditions and reflux policy are validated by rigorous simulation with ProSim Batch Column^® where technical features of a bench scale distillation column have been described. Six reproducible experiments are run in the bench scale column matching the simulated operating conditions with two sequentially increasing reflux ratio values. Simulation and experiments agree well. With an average molar purity higher than 99%, more than 85% of recovery yield was obtained for chloroform and methanol.     

基于双塔精馏的甲醇-碳酸二甲酯分离工艺

基于甲醇和碳酸二甲酯体系的共沸特性,应用双效热集成和分割式热泵两种双塔精馏工艺进行该体系的分离研究。利用文献报道的实验数据对选用的Wilson物性方程中的二元交互作用参数进行回归修正。利用Aspen Plus模拟软件,以年运行总费用最小为目标函数,分别对提出的两种双塔精馏工艺进行模拟与优化,得到合适的工艺参数。模拟结果表明,两种双塔精馏工艺均比单塔加压精馏工艺其能耗更低、年运行总费用更少。就两种双塔精馏工艺而言,分割式热泵精馏工艺较双效热集成精馏工艺,可节约设备投资费用约2.69%,年运行总费用节约17.33%。     

丙酮-甲基异丙基酮-水三元非均相共沸物精馏研究

用剩余曲线图(RCM)方法对丙酮-甲基异丙基酮(MIPK)-水三元非均相共沸物系进行分析,提出通过利用中间液-液分离器,在一个塔中实现丙酮、MIPK、水的分离。结合残余曲线、蒸馏边界线、精馏段和提留段浓度轮廓线、中间液-液分离器相平衡线等曲线对提出的丙酮、MIPK、水三元物系非均相共沸精馏过程进行了详细的分析,对非均相共沸精馏过程的设计和操作都将会起到重要的作用。以MIPK生产中的分离过程为例,建立了节能型精馏流程,结果表明,该流程可以节省设备投资,简化操作,降低生产成本。     

节能型乙醇蒸馏及脱水集成工艺

围绕由生物质发酵醪液生产燃料乙醇过程中精馏与脱水工艺展开研究,提出了精馏塔塔顶部分冷凝技术。通过计算机模拟,着重从能耗方面将该方案与常规工艺以及最近出现的回流段无水乙醇共沸精馏技术进行比较。结果表明,部分冷凝工艺和回流段无水乙醇共沸精馏流程节能效果显著。     

Energy-Efficient Process with a Decanter to Separate Toluene-Methanol-Water Ternary Azeotropic Mixtures

An extractive distillation process with two columns and a decanter was proposed to separate ternary toluene-methanol-water azeotropic mixtures in a previous study. Based on this process, six processes were established to explore further energy-efficient schemes to separate the ternary azeotropic system. The optimal parameters of the process were determined in terms of the minimum total annual cost (TAC). The processes with heat integration perform better than those with a side stream or thermal coupling in terms of energy savings and TAC reduction. A process in which both the feed stream of the extractive distillation column and the entrainer recovery column are preheated by the entrainer recycle stream can reduce energy consumption by up to 27.69 % and TAC by 21.36 %.     

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.     

变压精馏分离甲苯-乙醇体系的经济最优化设计

甲苯-乙醇体系的共沸组成与相对挥发度对压力比较敏感,可用变压精馏分离该体系。利用Aspen化工流程模拟软件,以年度总费用最小为目标函数,以NRTL为热力学计算模型,采用序贯迭代法确定普通、部分热集成和完全热集成的变压精馏最优工艺参数。结果表明完全热集成更具有优势。     

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.     

醋酸正丙酯与醋酸正丁酯的选择与探讨

在 PTA 装置中溶剂脱水正常采用两种方法：常规精馏和共沸精馏。从能耗和物耗以及方便操作的角度,通常选用共沸精馏,而采用共沸精馏时,共沸剂的选择是一个比较关键的因素。笔者从共沸剂的性能、消耗、能耗以及塔的选择方面进行了比较与探讨,为共沸精馏塔的共沸剂选择提供了一个参考。     

Reducing the Energy Demand of Cellulosic Ethanol through Salt Extractive Distillation Enabled by Electrodialysis

One of the main challenges when a biochemical conversion technique is employed to produce cellulosic ethanol is the low concentration of ethanol in the fermentation broth, which increases the energy demand for recovering and purifying ethanol to fuel grade. In this study, two design cases implementing salt extractive distillation—with salt recovery enabled by a novel scheme of electrodialysis and spray drying—along with heat integrated distillation techniques of double-effect distillation and direct vapor recompression are investigated through process simulation with Aspen Plus® 2006.5 for reducing the thermal energy demand. Conventional distillation along with molecular sieve based dehydration is considered as the base case. Salt extractive distillation along with direct vapor recompression is found to be the most economical ethanol recovery approach for cellulosic ethanol with a thermal energy demand of 7.1 MJ/L (natural gas energy equivalents, higher heating value), which corresponds to a thermal energy savings of 23% and cost savings of 12% relative to the base case separation train thermal energy demand and total annual cost.     

Investigating the need of a pre-concentrator column for acetic acid dehydration system via heterogeneous azeotropic distillation

In our previous study [Chien, I.L., Zeng, K.L., Chao, H.Y., Liu, J.H. (2004). Design and control of acetic acid dehydration system via heterogeneous azeotropic distillation. Chemical Engineering Science 59(21), 4547-4567.], an acetic acid dehydration system has been designed. The suitable entrainer selected for that system is iso-butyl acetate. Design and control of the system has been studied in detail to maintain high-purity bottom acetic acid concentration and also keep a small acetic acid loss through top aqueous draw. In that previous study, the feed composition is assumed to contain equal molar of acetic acid and water. However, in a typical waste acid recovery application, the above assumption may be too rich in acetic acid. In this paper, a feed stream containing 80 mol% water and 20 mol% of acetic acid is investigated. Several design alternatives can be deduced including one commonly used in industry by adding a pre-concentrator column in the upstream of a heterogeneous azeotropic distillation column. The necessity of this pre-concentrator column from design and control view points will be thoroughly investigated in this paper. The final recommended process design is a tradeoff between total annual cost (TAC) and operability of the system. The recommended design is a single heterogeneous azeotropic distillation column with aqueous reflux stream. Very wide feed composition and flow rate changes can be handled by this design with proper choice of the overall control strategy.     

节能型甲基异丙基酮分离工艺研究

甲基异丙基酮生产中,水与甲基异丙基酮等形成共沸物,文中运用剩余曲线图(RCM)方法对甲基异丙基酮分离过程进行分析研究,根据非均相共沸精馏原理,提出了在丙酮塔中增加油水分离器的方法脱除待分离物系中的水,从而达到节能的目的。文章在Aspen Plus中建立了该节能型甲基异丙基酮分离过程的模拟模型,进行了计算和分析,并加以实验验证,二者比较吻合。结果表明,该分离工艺即节省了设备投资,又降低了分离能耗,并且提高了脱水效率,改善了产品质量,同时还提高了产品收率,所用方法对简化工艺流程起到了积极作用。     

基于分割式热泵的2-甲氧基乙醇-水精馏工艺模拟

基于2-甲氧基乙醇-水体系的共沸特性,应用分割式热泵精馏用于该体系的分离。采用UNIQUAC方程计算该体系的相平衡数据,并利用实验数据对UNIQUAC方程中的二元交互作用参数进行修正。利用Aspen Plus过程模拟软件中的Radfrac精馏模型和Compr等熵压缩模型,以年总费用最小为目标函数,对提出的分割式热泵精馏工艺进行了模拟与优化,得到了合适的工艺参数,如分割点摩尔分数为x(H2O)=0.17、压缩机进气量为6.16 t/h等关键工艺变量。模拟结果表明,与常规热泵精馏工艺相比,分割式热泵精馏工艺的年总费用可节约34.4%,操作费用可节约36.3%。     

Proposal for the introduction of the concept of multiplicity of solutions in steady-state distillation problems in the syllabus of separation processes

The existence of multiplicity in steady-state separation problems based on the use of equilibrium stage cascades has been known since the second half of the 20^th century. Nevertheless, there are few textbooks -among those usually accessible to Chemical Engineering students- explaining this behavior and even when it is considered it is not deeply treated. In this work, we propose a distillation problem whose resolution allows the instructor the opportunity to introduce some specific characteristics and behavior that may otherwise pass unnoticed. Thus, the simulation of a column for the heterogeneous azeotropic distillation of ethanol using benzene as an entrainer is suggested, and the results obtained do not seem to be compatible with the distillation boundaries of the ternary system. The apparent contradictions can be explained by means of the representation of mass balances on the residue curve map, and allow the students to actually understand the requirements for the feasibility of the separation. The obtainment of two different solutions depending on the initial composition profiles used for starting the calculations allows the instructor to introduce the steady-state multiplicity concept. Errors associated with bad checking of the thermodynamic model are also pointed out.