Membrane system design for multicomponent gas mixtures via mixed-integer nonlinear programming

An optimal design strategy for membrane networks separating multicomponent gas mixtures based on an approximate permeator model and mixed-integer nonlinear programming (MINLP) is proposed. A permeator system superstructure is used to embed a very large number of possible network configurations and allows the permeator feed-side pressure to be fixed or a design variable. A MINLP design model is developed to minimize the total annual process cost by simultaneous optimization of the permeator configuration and operating conditions. Case studies for the separation of acid gases (CO₂ and H₂S) from crude natural gas mixtures with spiral-wound permeators are presented. Permeator configurations are derived for different number of separation stages for both continuous and discrete membrane areas. The method is sufficiently robust to handle product compositions that vary five orders of magnitude. The proposed approach provides an efficient methodology for preliminary screening of multi-stage membrane separation systems for multicomponent gas mixtures.     

Effect of Feed Location on the Performance of Single-Stage Membrane Permeators

Abstract

The effect of feed location on the performance of single-stage membrane permeators was determined based on the minimum unit compressor load (recycle ratio). Since certain feed locations correspond to several well-known permeator configurations (e.g., simple recycle permeator, continous membrane column), it is possible to characterize the relative performance of these configurations for separating binary gas mixtures. For separations involving oxygen, nitrogen, and carbon dioxide, it was found that the location of feed introduction was related to the apparent difficulty of separation. For binary seprations of low to moderate difficulty, the optimum feed location was at a dimensionless axial distance of 0.6 to 0.75 from the top of the column. This feed location corresponded to the continuous membrane column configuration. For difficult separations, the optimum feed location was at the top of the column which corresponded to the simple recycle permeator. Based on this study, the simple recycle permeator configuration outperforms the continuous membrane column for the most difficult separations such as in the separation of oxygen from air. However, the continuous membrane column configuration can be used effectively for less difficult gas separations which cannot be accomplished by a membrane permeator without recycle, but do not require high recycle ratios to achieve the desired separation.     

Feasibility of new pressure swing batch distillation methods

The pressure swing distillation in different batch column configurations is investigated by feasibility study and rigorous simulation calculations. Besides studying the well known batch configurations (rectifier, stripper, middle vessel column) we also suggest two novel configurations such as double column batch rectifier (DCBR) and double column batch stripper (DCBS). The alternate application of a batch rectifier and a batch stripper is also studied. The feasibility method is based on the assumption of maximal separation. The results of the feasibility studies are verified by rigorous simulations based on less simplifying assumptions. The calculations are made by a professional dynamic flow-sheet simulator for the separation of a minimum (ethanol–toluene) and a maximum boiling (water–ethylene-diamine) azeotropic mixture. The different column configurations are compared. The DCBS (for the separation of the minimum azeotrope) and the DCBR (for the maximum azeotrope) showed several advantages (e.g. only one production step without pressure change, lower energy consumption) compared with the other configurations.     

基于NSGA-Ⅱ的模拟移动床色谱分离过程多目标操作优化

多目标优化策略被应用于模拟移动床过程的操作优化中，采用一种基于Pareto最优解的多目标优化算法——NSGA-Ⅱ算法，以分离联萘酚对映体的模拟移动床色谱分离过程作为研究对象，利用模拟移动床TMB数学模型，以分离性能指标作为目标函数进行了多目标操作优化设计。优化结果表明，NSGA-Ⅱ算法得到的非劣解在目标空间分布均匀，算法收敛性和鲁棒性好。基于NSGA-Ⅱ算法的面向分离性能多目标优化设计方法为模拟移动床分离过程的工艺设计和操作指导提供了有效的工具。     

低压脱甲烷系统优化分析

在对低压脱甲烷系统进行计算机模拟的基础上,建立了以系统乙烯损失与能耗之和为目标函数的最优化数学模型;采用可行路径序贯模块法为最优化计算策略,结合广义既约梯度法对该系统实施优化计算,找出了对系统目标函数影响较大的可调决策变量及其最优化条件．对优化决策变量的选择及中等规模化工系统的优化策略进行了有益的探索．     

Multi-objective optimization of industrial styrene reactor: Adiabatic and pseudo-isothermal operation

Multi-objective differential evolution (MODE) is used to optimize an industrial styrene reactor considering productivity, selectivity and yield as the main objectives. Two reactor configurations (single bed adiabatic operation and steam injected pseudo-isothermal operation) and four combinations of objectives consisting of 5 and 7 variables respectively are considered. Pareto optimal solutions are obtained for all combinations of objective functions for both the configurations. The results are compared with those reported in the literature and an industrial operating point. For all the cases considered, MODE is able to give a Pareto front better (in terms of wider range and a better spread) than that obtained using NSGA for both the configurations. Steam injected reactor configuration is better than the adiabatic reactor configuration in terms of performance. The Pareto optimal solutions obtained from such studies provide a wide range of optimal operating conditions from which an appropriate operating point can be selected based on the requirements of the decision maker.     

Non‐Newtonian Heat Transfer on a Plate Heat Exchanger with Generalized Configurations

For the configuration optimization of plate heat exchangers (PHEs), the mathematical models for heat transfer and pressure drop must be valid for a wide range of operational conditions of all configurations of the exchanger or the design results may be compromised. In this investigation, the thermal model of a PHE is adjusted to fit experimental data obtained from non‐Newtonian heat transfer for eight different configurations, using carboxymethylcellulose solutions (CMC) as test fluid. Although it is possible to successfully adjust the model parameters, Newtonian and non‐Newtonian heat transfer cannot be represented by a single generalized correlation. In addition, the specific heat, thermal conductivity and power‐law rheological parameters of CMC solutions were correlated with temperature, over a range compatible with a continuous pasteurization process.     

基于多目标教学优化算法在二甲苯吸附分离过程优化中的应用

以C8芳烃混合物的吸附分离过程作为研究对象, 应用多目标教学优化算法(multi-objective teaching-learning-based optimization algorithm, MOTLBO)对模拟移动床多目标优化问题进行求解。采用TMB方法, 建立了模拟移动床模型, 并对两个典型的模拟移动床多目标操作优化问题进行了优化设计。通过与NSGA-Ⅱ算法的比较, 证明了多目标教学优化算法在求解模拟移动床多目标优化问题上的有效性和优势。此外, 还分析了抽出液流量、抽余液流量以及步进时间等对多目标优化非劣解的影响, 优化结果为模拟移动床分离过程的工艺设计和操作提供了依据。     

高背压供热汽轮机低压部分性能优化

汽轮机高背压供热方式可回收低压缸排汽余热,扩大机组的供热能力,减少高品位抽汽造成的可用能损失,能源转换效率高。供热季运行背压高,低压转子采用了双转子互换技术,低压转子结构的变化使低压部分热力特性发生变化。本文建立了300MW等级高背压供热机组热力系统模型,计算并分析抽汽参数变化对低压加热器附加单耗的影响,并通过参数优化降低供热季低压加热器附加单耗。获得五段和六段抽汽压力优化结果,降低了传热端差,使各级低压加热器温升分配合理,优化后机组发电功率增加507kW,?损减小575.5kW,整体附加单耗下降0.3121g/(kW·h)。以此为基础,进行高背压供热机低压通流部分热力计算,重新分配低压缸各压力级焓降,提高低压缸的通流效率。结果表明：通过对低压回热系统和通流部分优化,低压缸内效率提高至0.9250,机组发电功率增加3068.49kW。     

醇氨联产工艺经济优化方案的讨论

醇氨联产工艺技术可以改变合成氨企业产品结构单一的局面,增强企业的市场应变能力。但该工艺的选用,还需与低能耗、低排放技术相结合。依据生产企业实际投资和操作费用,对中、高压醇烷化以及中、低压甲醇合成进行了分析比较,结果表明:在设计使用年限内综合考虑,高压醇烷化经济性更高,新建醇氨联产装置采用低压甲醇工艺技术的经济性较高,低压甲醇+高压醇烷化是目前醇氨联产工业中比较经济的组合方案。     

Multi-objective operation optimization of olefin separation process for MTO plant

In modern coal processing industries, methanol-to-olefins (MTO) is an important equipment. Its olefin separation process is facing with problems such as the change of raw materials, the loss of olefin products and the high consumption of utilities. Operation optimization is required to achieve maximum benefits under the circumstance of quality assurance and requirements. This article takes the pre-depropanized olefin separation process of Lummus as the research object. And the optimization objectives are the total yield of ethylene and propylene as well as the total energy consumption. Modeling, simulation and multi-objective optimization of the process are conducted. Non-dominated sorting genetic algorithm (NSGA-II) is used to solve multi-objective optimization problem. The simultaneous optimization of 15 operational variables is achieved. Under the current yield, the optimal operation point is found by reducing the reflux ratio of low pressure depropanizer, deethanizer and 1^# propylene tower and so on. The results show that the optimal operating point can reduce energy consumption by 20 MW compared with the existing operating point. The optimization interval of each operation variable corresponding to different trade-off points is determined by the comprehensive analysis of decision variables. It is also found that distillation equipment can operate in different optimal operation intervals.     

NHD脱碳系统节能改进

对原有NHD脱碳流程进行改进，将低压闪蒸槽移至气提塔上部，运用柏努利方程计算了NHD脱碳系统中富液从高压闪蒸槽至低压闪蒸槽的压力降。从而确定了将高压闪蒸槽的位移高度。取消原流程中的富液泵，不仅减少了投资，而且节省了操作费用。     

MTO烯烃分离过程的多目标操作优化

现代煤化工中，甲醇制烯烃 （MTO） 是一个非常重要的装置。其烯烃分离过程面临着原料变动大、烯烃产品损失以及较高的公用工程消耗等问题。这就需要在满足产品规格和需求的情况下，优化操作条件以实现最大效益。以Lummus前脱丙烷的烯烃分离工艺为研究对象，以增加乙烯与丙烯的总收率和降低总能耗为优化目标，对该工艺流程进行建模模拟与多目标优化。采用非支配排序遗传算法（NSGA-II）进行多目标优化的求解，实现了15个操作变量的同时优化。在维持产品收率不变的前提下，可通过降低脱丙烷塔、脱乙烷塔和1^#丙烯精馏塔的回流比等优化措施找到了当前最优操作点。结果表明，该最优操作点与现有操作点相比可降低20 MW能耗。通过对决策变量的综合分析，确定了不同目标权衡下对应的各个操作变量的优化区间，发现精馏塔可以在多个最佳操作区间内运行。     

The structure-breaking effect on oxygen diffusion coefficients in electrolyte and polyelectrolyte solutions

By employing a membrane-covered polarographic oxygen electrode, the oxygen diffusion coefficients and solubilities in aqueous solutions of ammonium chloride, potassium iodide, sodium alginate, sodium carboxymethylcellulose (CMC), and xanthan gum have been measured with a newly developed methodology. Experimental results showed that in all systems studied, oxygen solubilities decrease on addition of solutes. Oxygen diffusion coefficients in aqueous solutions of KI and NH4C1 are found to be higher than that in pure water. In sodium alginate solutions, dilute xanthan solutions, and solutions containing more than 0.3 wt% of CMC, oxygen diffusion coefficients decrease with an increase in polymer concentrations. Interestingly, in dilute CMC solutions and concentrated xanthan solutions containing more than 0.5 wt% of xanthan gum, oxygen diffusion coefficients increase with increasing polymer concentrations, and values exceeding that in pure water are generally observed.     

Simulation of autothermal reforming in a staged‐separation membrane reactor for pure hydrogen production

Steam methane reforming with oxygen input is simulated in staged‐separation membrane reactors. The configuration retains the advantage of regular membrane reactors for achieving super‐equilibrium conversion, but reaction and membrane separation are carried out in two separate units. Equilibrium is assumed in the models given the excess of catalyst. The optimal pure hydrogen yield is obtained with 55% of the total membrane area allocated to the first of two modules. The performance of the process with pure oxygen input is only marginally better than with air. Oxygen must be added in split mode to reach autothermal operation for both reformer modules, and the oxygen input to each module depends on the process conditions. The effects of temperature, steam‐to‐carbon ratio and pressure of the reformer and the area of the membrane modules are investigated for various conditions. Compared with a traditional reformer with an ex situ membrane purifier downstream, the staged reactor is capable of much better pure hydrogen yield for the same autothermal reforming operating conditions.     

Modelling of multicomponent gas separation with asymmetric hollow fibre membranes—methane enrichment from biogas

A mathematical model for the dynamic performance of gas separation with high flux, asymmetric hollow fibre membranes was developed considering the permeate pressure build‐up inside the fibre bore and cross flow pattern with respect to the membrane skin. The solution technique provides reliable examination of pressure and concentration profiles along the permeator length (both residue/permeate streams) with minimal effort. The proposed simulation model and scheme were validated with experimental data of gas separation from literature. The model and solution technique were applied to investigate dynamic performance of several membrane module configurations for methane recovery from biogas (landfill gas or digester gas), considering biogas as a mixture of CO₂, N₂ and CH₄. Recycle ratio plays a crucial role, and optimum recycle ratio vital for the retentate recycle to permeate and permeate recycle to feed operation was found. From the concept of two recycle operations, complexities involved in the design and operation of continuous membrane column were simplified. Membrane permselectivity required for a targeted separation to produce pipeline quality natural gas by methane‐selective or nitrogen‐selective membranes was calculated. © 2012 Canadian Society for Chemical Engineering     

基于均匀分布NSGAII算法的污水处理多目标优化控制

针对污水处理过程控制中能耗过大、出水水质超标严重等问题，提出了一种基于均匀分布的NSGAII(non-dominated sorting genetic algorithm II based on uniform distribution, UDNSGAII)多目标优化智能控制系统。首先，该方法以污水处理能耗和出水水质作为优化目标，建立多目标优化模型。其次，为了获得溶解氧和硝态氮的优化设定值，提高Pareto解的性能，该算法将种群映射到目标函数对应的超平面, 并在该平面上进行聚类以增加解的多样性。此外，加入分布性判断模块和分布性加强模块提高解的分布性。最后，采用比例积分微分(proportional integral derivative, PID)控制器对溶解氧和硝态氮的优化设定值进行底层跟踪控制。为了验证该算法的有效性，采用国际基准的污水处理仿真平台(benchmark simulation model No.1, BSM1)来进行实验。结果显示，所提出的UDNSGAII多目标优化控制方法能够在满足出水水质达标的同时，有效地降低污水处理过程能耗。     

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.     

Synthesis and optimization of membrane cascade for gas separation via mixed‐integer nonlinear programming

Currently, membrane gas separation systems enjoy widespread acceptance in industry as multistage systems are needed to achieve high recovery and high product purity simultaneously, many such configurations are possible. These designs rely on the process engineer's experience and therefore suboptimal configurations are often the result. This article proposes a systematic methodology for obtaining the optimal multistage membrane flow sheet and corresponding operating conditions. The new approach is applied to cross‐flow membrane modules that separate CO₂ from CH₄, for which the optimization of the proposed superstructure has been achieved via a mixed‐integer nonlinear programming model, with the gas processing cost as objective function. The novelty of this work resides in the large number of possible interconnections between each membrane module, the energy recovery from the high pressure outlet stream and allowing for nonisothermal conditions. The results presented in this work comprise the optimal flow sheet and operating conditions of two case studies. © 2017 American Institute of Chemical Engineers AIChE J, 63: 1989–2006, 2017     

Removal of Dissolved VOCs from Water with an Air Stripper/Membrane Vapor Separation System

Abstract

Treatment of water contaminated with volatile organic compounds (VOCs) is a major problem for the United States chemical industry. Currently, VOCs are removed from moderately contaminated wastewater streams by processes such as steam stripping and from dilute wastewaters by air stripping combined with a carbon adsorption off-gas treatment system. This paper describes the development and performance of a hybrid process that combines air stripping with membrane organic-vapor separation to recover VOCs from the stripper off-gas. A number of prototype systems have been constructed and evaluated. The optimum system appears to be a tray stripper fitted with a high-pressure compression-condensation membrane separation unit. Such a system can remove 95 to 99% of the VOCs present in contaminated water; the removed VOCs are recovered as a liquid condensate. The economics of the technology are competitive with alternative processes, particularly for streams containing more than 500 ppm VOC and having flow rates less than 10 to 30 gal/min.