Understanding process intensification in cyclic distillation systems

The most effective separation possible in distillation columns takes place in the hydrodynamic regime where there is perfect displacement of the liquid and vapor streams. This can be achieved in distillation equipment with separate phase movement (SPM). Such an innovative route for process intensification in distillation is called cyclic distillation. The required process conditions are the lack of liquid outflow from the tray during vapor admission and the lack of liquid mixing in adjacent trays upon outflow of liquid. Remarkably, the throughput of such a column that operates in a controlled cycle mode is two or more times higher than the throughput reachable with conventional operation, at equivalent separation performance.In this study, a theoretical stage model with perfect displacement is proposed and the theory of the process working lines is developed. An adequate mass transfer model is also described along with the mode of calculation of tray columns operating in the cyclic operation mode. Sensitivity analysis was used to determine the effect of the key model parameters. The theoretical developments were implemented at industrial scale and subsequent testing showed an increase in the separation efficiency of 2-3 times as compared to the standard process.     

Experimental validation of a flexible modeling approach for distillation columns with packings

The two main concepts for the modeling of distillation columns are the equilibrium‐stage (EQ) and the nonequilibrium‐stage (NEQ). A model is presented which combines decisive features of both conventional concepts. Based on the idea of a reduced nonequilibrium‐stage (RNEQ), this model can be used for the simulation of distillation columns with packings. In contrast to the conventional NEQ approach, this model neglects the influence of liquid side mass‐transfer coefficients, which ultimately allows to come up with only one empirical equation describing the overall mass transfer. Thus, a considerable reduction in model complexity is reached, which allows for an efficient consideration of new experimental distillation results. Fitted to experimental data, the model is able to predict, how different pressures and chemical systems might affect the separation efficiency. By comparing calculation results with experimentally determined separation efficiencies for three different packing types, these valuable RNEQ qualities are illustrated. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3833–3847, 2014     

规整填料塔多元蒸馏过程混合型模型的模拟

填料塔精馏过程的模拟设计大多采用平衡级模型,但由于单一板效率值的难以确定,很多场合模拟很不成功;而新提出的非平衡级模型认为塔内传质传热均处于非平衡状态,由于方程数和经验性参数多,模型求解非常困难。文章针对规整填料的特点,建立了规整填料塔蒸馏过程的一种混合型模型,模型的主要特征是认为气液二相传质处于不平衡状态,而传热处于平衡状态。模型建立在实际填料基础上,既舍去传统的平衡级模型不确定性,又省略了非平衡级模型中复杂的经验性传热系数和液相传质系数的计算。模型计算值和实验值符合较好,也证实混合型模型既反映实际,又使模型求解变得相对容易。     

多元精馏过程的非平衡级动态模型

提出了-种通用的多元精馏过程非平衡级动态模型。模型中采用传质、传热速率方程表征实际塔板上的传递过程,避免了级效率的计算。通过引入“分离效率函数”提出一种简洁适用的仿真模型和求解算法,分析了速率模型与“平衡级一级效率”模型之间的内在联系。在两个工业精馏塔上进行了仿真计算和实验检验,结果表明,非平衡级动态模型能够准确预测实际塔板上的动态行为。     

分壁式精馏塔精制丁二烯流程模拟

丁二烯是一种重要的石油化工烯烃原料,由于其生产过程能耗高,因此节能降耗成为丁二烯生产工艺的研究热点。利用Aspen Plus模拟软件对丁二烯精制工艺的两套流程进行了模拟研究,考察了分壁式精馏塔(DWC)中内部互连物流连接位置、预分离塔气液相流量和回流比对分离效果和热负荷的影响,对比了相同分离条件下DWC分离流程和传统顺序分离流程的能耗,并根据两套分离流程中塔内液相丁二烯浓度分布情况,分析DWC的节能原因。结果表明,当主塔理论板数105,预分离塔理论板数56,进入预分离塔气相流量1020kmol/h,液相流量890kmol/h,回流比7800时,DWC分离效果最好,丁二烯质量分数可达99.7%,这为DWC精制丁二烯工艺的工业化提供了理论依据。由于DWC有效减少了精馏过程中的返混效应,提高了能量利用率,使其冷凝器可节能29.36%,再沸器可节能29.19%,存在明显的节能优势。     

基于传热/传质的乙烯裂解过程脱甲烷塔进料瓶颈识别及流程重构策略

精馏塔进料的组成与温度会影响塔内质量交换和能量利用,不恰当的进料会导致全塔的分离及用能效果变差。针对多进料精馏塔的组合进料问题提出一种识别不合适进料位置的方法,基于塔板的传热温差和传热量、传质浓度差和传质量计算方法提出应用传热/传质复合曲线识别精馏塔进料瓶颈的方法,并将其应用于裂解装置脱甲烷塔进料瓶颈的识别,采用调整进料位置的流程重构策略实现去瓶颈的操作。流程模拟及瓶颈分析结果表明所提出的方法能识别出脱甲烷塔的进料瓶颈,重构流程的方法能实现去瓶颈的操作,并使全塔的传质传热特性、分离效果变好,能耗降低。     

A Short Note on Steady State Behaviour of a Petlyuk Distillation Column by Using a Non‐Equilibrium Stage Model

A Petlyuk distillation column, considering equilibrium and non‐equilibrium stage models, was studied. Rigorous simulations were conducted using Aspen Plus? RATEFRAC Module for the separation of ternary mixtures. According to the equilibrium model, the energy‐efficient design of the Petlyuk column requires that the intermediate component be extracted from the maximum point in the composition profile in the main column. It was found that, for the intermediate component, mass transfer occurs from the vapour to the liquid phase from the top of the column to the stage where the side stream is extracted, from this point mass transfer occurs in the opposite direction. This point, considering the non‐equilibrium model, corresponds to the stage in which the net mass transfer rate is zero. For the case of two segments per stage, it was found that the heat duties predicted by the equilibrium model are significantly lower than those obtained by using the non‐equilibrium model, which is consistent with previous reported results. However, it is important to say that despite the higher energy duty predicted by the non‐equilibrium model; both models predict significant energy savings.     

Distillation technology – still young and full of breakthrough opportunities

Throughout history, distillation has been the most widespread separation method. However, despite its simplicity and flexibility, distillation still remains very energy inefficient. Novel distillation concepts based on process intensification, can deliver major benefits, not just in terms of significantly lower energy use, but also in reducing capital investment and improving eco‐efficiency. While very likely to remain the separation technology of choice for the next decades, there is no doubt that distillation technology needs to make radical changes in order to meet the demands of the energy‐conscious modern society. This article aims to show that in spite of its long age, distillation technology is still young and full of breakthrough opportunities. Moreover, it provides a broad overview of the recent developments in distillation based on process intensification principles, for example heat pump assisted distillation (e.g. vapor compression or compression–resorption), heat‐integrated distillation column, membrane distillation, HiGee distillation, cyclic distillation, thermally coupled distillation systems (Petlyuk), dividing‐wall column, and reactive distillation. These developments as well as the future perspectives of distillation are discussed in the context of changes towards a more energy efficient and sustainable chemical process industry. Several key examples are also included to illustrate the astonishing potential of these new distillation concepts to significantly reduce the capital and operating cost at industrial scale. © 2013 Society of Chemical Industry     

膜蒸馏跨膜传质过程的新模型--TPKPT

以纯水为介质,用直接接触式膜蒸馏测定了材料或性能参数不同的三种孔疏水膜在不同温度下的渗透通量。根据测量结果计算出了各种膜在不同温度下的渗透系数,发现渗透系数均随着温度的升高而升高;这一结果说明Poiseuille流动在跨膜传质中起着非常重要的作用。据此提出了Knudsen扩散－Poiseuille流动两参数跨膜传质模型,即TPKPT模型。用此模型拟合实验数据,得到了三种实验用膜的模型参数。用这些模型参数计算膜在不同温度下的渗透系数,其值与实验测量值吻合较好,说明TPKPT模型能较好地描述膜蒸馏的跨膜传质过程。     

塔板上流型变化对板效率影响的计算传质学

利用能够描述塔板上气液两相错流过程的流体力学模型,建立了同时模拟气液两相流动与传质过程的数学模型;通过对气液两相传质方程和流体力学模型方程的联立求解,计算得到了塔板上液相速度分布和气液两相浓度分布的数值解,考察了气相完全混合和气相部分混合两种条件下塔板上的气液两相浓度分布,同时考察了气相完全混合时流型变化对塔板效率的影响.     

Extractive distillation: Advances in conceptual design,solvent selection,and separation strategies

Extractive distillation(ED) is one of the most promising approaches for the separation of the azeotropic or closeboiling mixtures in the chemical industry. The purpose of this paper is to provide a broad overview of the recent development of key aspects in the ED process involving conceptual design, solvent selection, and separation strategies. To obtain the minimum entrainer feed flow rate and reflux ratio for the ED process, the conceptual design of azeotropic mixture separation based on a topological analysis via thermodynamic feasibility insights involving residue curve maps, univolatility lines, and unidistribution curves is presented. The method is applicable to arbitrary multicomponent mixtures and allows direct screening of design alternatives. The determination of a suitable solvent is one of the key steps to ensure an effective and economical ED process. Candidate entrainers can be obtained from heuristics or literature studies while computer aided molecular design(CAMD) has superiority in efficiency and reliability. To achieve optimized extractive distillation systems, a brief review of evaluation method for both entrainer design and selection through CAMD is presented. Extractive distillation can be operated either in continuous extractive distillation(CED) or batch extractive distillation(BED), and both modes have been well-studied depending on the advantages in flexibility and low capital costs. To improve the energy efficiency, several configurations and technological alternatives can be used for both CED and BED depending on strategies and main azeotropic feeds. The challenge and chance of the further ED development involving screening the best potential solvents and exploring the energy-intensive separation strategies are discussed aiming at promoting the industrial application of this environmentally friendly separation technique.     

精馏技术研究进展与工业应用

精馏是化学工业中应用最广泛的关键共性技术,广泛应用于石油、化工、化肥、制药、环境保护等行业。精馏具有应用广泛、技术成熟等优点,但存在设备投资大、分离能耗高等问题,因此研究开发新型高效传质元件、开发新型节能精馏技术,具有重要的社会意义和经济价值。本文从精馏塔类型、流体力学性能、传质性能、塔器大型化、过程节能、过程强化等方面,介绍了精馏技术的研究进展与工业应用。对于板式塔,从气液两相流动状态、压降、漏液和雾沫夹带方面研究了塔板的流体力学性能;对于填料塔,从压降、液泛和持液量方面研究了填料塔的流体力学性能,但目前的研究仍以经验关联式为主,缺乏严谨的的理论模型。对于气液两相的传质性能研究,简述了气液两相传质理论,但科学、精准的传质模型尚未提出。对于塔器大型化的应用研究,介绍了塔板、气液分布器和支撑装置等大型化关键技术的工业应用。从精馏过程典型节能技术、耦合节能技术、流程节能技术、低温余热回收和特殊精馏等方面,介绍了精馏过程节能与强化的应用进展。文章最后对精馏过程的传质、强化和集成进行了展望。     

塔内换热精馏的节能原理与数学模型

基于热力学第二定律提出了一种新型精馏节能技术——塔内换热精馏.对塔内换热精馏的概念及其节能原理进行了详细的阐述.分析表明,它尤其适合于塔顶、塔釜温差较大的场合.指出实现塔内换热精馏的关键是寻找既有良好的传热性能、又有良好的传质性能的内构件,并介绍了一种内构件——垂直翅片管,建立了塔内换热精馏的平衡级模型——MESHQ方程组,提出一种利用PRO/Ⅱ计算塔内换热精馏的简化方法.     

分隔壁精馏塔分离醇类混合物的模拟

以分隔壁精馏塔分离乙醇、正丁醇及正己醇为例,建立分隔壁精馏塔稳态模型。用Aspen Plus软件进行模拟,模拟数据与实验数据吻合良好。同时考察了分隔壁精馏塔内液体分配比对产品含量的影响及正丁醇液相组成分布情况。比较了采用分隔壁精馏塔和常规二塔流程分离此物系的节能情况。结果表明,由于分隔壁精馏塔能极大地减少返混现象的产生,故达到相同的分离要求,分隔壁精馏塔比常规精馏的流程更节能,采用分隔壁精馏塔分离此物系时,中间组分的摩尔分数越高,节能效果越好,当进料组成为n(C2H5O)∶n(C4H10O)∶n(C6H14O)=1∶3∶1时,可节能25.9%。分隔壁精馏塔技术是一种节能、经济的新工艺。     

A new model for mass transfer in direct contact membrane distillation

The mass transfer process in direct contact membrane distillation (DCMD) for three kinds of membranes was measured. Water fluxes at different temperatures and the membrane distillation coefficients (MDC) for each membrane were obtained directly from experimental data. The fact that the MDC values of membranes with larger pore size increase with temperature indicates that Poiseuille flow plays an important role in the process of mass transfer through the membrane. Based on this conclusion, a three-parameter model, named the Knudsen diffusion-molecular diffusion-Poiseuille flow transition (KMPT) model, was developed to predict MDC and water flux for membrane distillation. The parameters of the KMPT model for each membrane employed in this study, by which MDC at various temperatures can be determined, were evaluated by a nonlinear regression. The values of MDC and water fluxes for each membrane predicted by KMPT model agree well with that obtained directly from the experiment results. A large contribution of Poiseuille flow to mass transfer was observed and can be attributed to the distribution of large pores in the membranes. The KMPT model also provides a method for estimation of the effect pore size using the ratio of the MDCs; the ratio of the Poiseuille flow to molecular diffusion MDC provides the best estimation.     

反应蒸馏塔中固体催化剂的装填结构

反应蒸馏是集化学反应与蒸馏分离两个过程于一体的新化工单元过程 ,反应蒸馏塔中固体催化剂的装填结构既要满足汽液两相对流流动的需要 ,又要使反应介质与催化剂能够良好接触。简述了对反应蒸馏塔中固体催化剂装填结构的基本要求 ,分类介绍了国内外开发的有特色与应用前景的催化剂装填结构的特点、局限性与应用范围 ,指出了开发新型催化剂装填结构的必要性     

Membrane Distillation of Butanol from Aqueous Solution with Polytetrafluoroethylene Membrane

Nowadays, butanol is considered as biofuel. In this work, separation of butanol from aqueous solution was carried out by vacuum membrane distillation with polytetrafluoroethylene membrane, using mechanical vapor compression on the downstream. Higher flow rates could promote membrane separation with higher total flux and higher separation factor, thus influencing butanol separation. Membrane fouling was observed which exerted a substantial effect on the membrane separation performance with a real fermentation broth. Compared to the traditional distillation process, this procedure is more cost-effective due to ∼ 50 % reduced energy consumption.     

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.     

一个用于精馏塔模拟的新模型

A new computational mass transfer model is proposed for simulating the distillation process by solving the fluctuating mass flux for the closure of turbulent mass transfer equation in order to obtain the concentration profile and the separation efficiency of distillation column. The feather of the proposed model is to abandon the conventional way of introducing the turbulent mass transfer diffusivity (dispersion coefficient) to the turbulent mass transfer equation. To verify the validity of the proposed model, a commercial scale packed column and a sieve tray column were simulated and compared with published experimental data. The simulated results were satisfactorily confirmed in both concentration distribution and separation efficiency.     

A study of mass transfer behaviour in a catalytic distillation column

Catalytic distillation (CD) is a process where reaction and separation take place simultaneously in a distillation column. Recently, a new CD process for the aldol condensation of acetone to diacetone alcohol and mesityl oxide was developed in our laboratory. Owing to the unique structure used to support catalysts in the CD column, mass transfer data are needed to model the CD process. The mass transfer behaviour in the CD column has been studied both for the reactive and non-reactive sections of the column in our laboratory. Based on the experimental results, general correlations for the mass transfer coefficients have been established and the 95% confidence intervals for the associated parameters were obtained. The experimental results indicated that the catalyst bags did not play a significant role for the separation between the vapour and liquid phases. The mass transfer correlations developed in this study have been used successfully to model the CD process for the aldol condensation of acetone carried out in our laboratory.