Efficient structure synthesis of reactive distillation processes using the IDEAS approach

A new synthesis method for reactive distillation processes is proposed. At each stage of a column, vapor–liquid equilibrium (VLE) is assumed and kinetically controlled reaction in liquid phase is considered. First, the liquid composition space is divided into small subspaces. Then, for each subspace a representative liquid composition is decided and assigned to a module corresponding to a stage of a distillation column. Then, after the calculation of the VLE and the reaction rate, the distribution network (superstructure) connecting all modules by vapor and liquid flow paths is constructed. The feature of the proposed model is that all constraints are linear to the optimization variables: the liquid and vapor flow rate and the liquid hold-up. The developed method was applied to the metathesis reaction of 2-pentene, and a completely new process structure was obtained. The effectiveness of the implied structure was confirmed through a comparison with conventional structures.     

Dynamic process intensification: Fundamentals and implementation to ternary distillation

Distillation remains a key technology for separating liquid mixtures. Its versatility comes with the disadvantage of high energy consumption. We previously used empirical arguments to introduce dynamic process intensification (DPI) as a strategy for improving the energy efficiency of binary distillation. In this article, we focus on ternary distillation; we begin by providing a rigorous basis for DPI, then formulate the problem of identifying the operating states for DPI as a nonlinear optimization problem. Via an extensive case study considering a hydrocarbon mixture, we demonstrate that reboiler energy use can be reduced by more than 2.3% relative to an equivalent column operating at steady state. We prove that this result is due to the fact that DPI aims to satisfy product flow and quality constraints on average in time, affording broader opportunities for optimizing column economics than steady-state operation, where constraints are met strictly at all times.     

IDEAS approach to process network synthesis: minimum utility cost for complex distillation networks

This work aims at quantifying the minimum utility cost necessary to complete a given separation task using vapor-liquid equilibrium (distillation) operations. The Infinite DimEnsionAl State-space (IDEAS) process is introduced as a new paradigm for this process synthesis activity. The IDEAS conceptual framework includes all possible process configurations, and yields mathematical programs that are convex (linear), thereby guaranteeing the global optimality of the obtained solution.A case study, on nitrogen/oxygen mixture separation, is employed to illustrate the power of the proposed approach. The obtained IDEAS designs are compared with the corresponding McCabe-Thiele (conventional) minimum utility designs, and are shown to be as much as 50% more thermodynamically and economically efficient.     

Globally optimal power cycle synthesis via the Infinite-DimEnsionAl State-space (IDEAS) approach featuring minimum area with fixed utility

This paper demonstrates the use of the Infinite DimEnsionAl State space (IDEAS) approach in synthesizing optimal power cycles featuring minimum heat exchange area. IDEAS is used to synthesize power cycle networks which include splitters, mixers, pumps, turbines, and heat exchangers and feature a single or multiple working fluid(s). The overall synthesis goal is to minimize heat exchange area requirements, while delivering a fixed percentage of the maximum net power obtainable from a given set of hot and cold utilities. The global optimality of the obtained power cycle network configuration is guaranteed, since IDEAS gives rise to convex (linear) programs. The power of the proposed approach is demonstrated on a case study involving the generation of electricity by a bottoming cycle with a pure ammonia working fluid. Real thermodynamic data for pure ammonia and rigorous equipment models are employed in carrying out the proposed optimization.     

A framework for synthesizing the optimal separation process of azeotropic mixtures

In this work, a systematic framework is introduced to synthesize the optimal separation process of azeotropic mixtures. The proposed framework, which can handle an arbitrary number of components, consists of two main steps: a system analysis and a state‐space superstructure algorithm. The system analysis is composed of some equation‐oriented algorithms to supply basic information for the superstructure, including structure of the composition space, existence of unchangeable points and candidate operations. It is shown that the proposed superstructure featuring multistream mixing is superior to previous ones because it significantly expands the feasible area. Moreover, detailed design parameters such as number of stages and reflux ratio are derived. Additionally, flowsheet feasibility test rules are constructed to facilitate the analysis of the process, and are able to be used as heuristic methods to guide the design of ternary or quaternary systems. Three industrial cases are presented to illustrate the proposed framework. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

二甲苯精馏装置先进控制与节能优化应用

以预测控制和实时优化为代表的先进控制技术可以保证产品质量、提高产率、降低能耗,已经越来越多地应用到石油化工装置上。二甲苯精馏是芳烃生产中的重要单元,具有很大的控制难度,且塔底重沸炉耗能巨大,具有较高的节能空间。针对二甲苯精馏单元的精馏塔和塔底重沸炉在操作和控制中存在的一些问题,根据机理与实际装置数据,建立了该单元的状态空间模型,基于多变量状态反馈预测控制技术开发了该单元的先进控制系统,使装置运行平稳性得到显著改善,提高了精馏塔的分离效果,通过优化促进了装置节能减排。该先进控制系统在两个实际装置应用中都取得了很好的控制和优化效果。     

模拟退火方法用于带有中间换热器精馏塔的优化综合

This article presents a simulated annealing-based approach to the optimal synthesis of distillation column considering intermediate heat exchangers arrangements. T-he number of intermediate condensers and/or intermediate reboilers, the placement locations, the.operating pressure of column, and the heat duties of intermediate heat exchangers are treated as optimization variables. A novel coding procedure making use of an integer number series is proposed to represent and manipulate the structure of system and a stage-to-stage method is used for column design and cost calculation. With the representation procedure, the synthesis problem is formulated as a mixed integer nonlinear programming （MINLP） problem, which can then be solved with an improved simulated annealing algorithm. Two examples are illustrated to show the effectiveness of the suggested approach.     

Integrated design and control of reactive distillation processes using the driving force approach

Superior controllability of reactive distillation (RD) systems, designed at the maximum driving force (design-control solution) is demonstrated in this article. Binary or multielement single or double feed RD systems are considered. Reactive phase equilibrium data, needed for driving force analysis and design of the RD system, is generated through an in-house property prediction tool. Rigorous steady-state simulation is carried out in ASPEN plus in order to verify that the predefined design targets and dynamics are met. A multiobjective performance function is employed to evaluate the performance of the RD system in terms of energy consumption, sustainability metrics (total CO₂ footprint), and control performance. Controllability of the designed system is evaluated using indices like the relative gain array (RGA) and Niederlinski index (N_I ), to evaluate the degree of loop interaction, as well as through dynamic simulations using proportional-integral (PI) controllers and model predictive controllers (MPC). The design-control of the RD systems corresponding to other alternative designs that do not take advantage of the maximum driving force is also investigated. The analysis shows that the RD designs at the maximum driving force exhibit enhanced controllability and lower carbon footprint than the alternative RD designs.     

Study on the nonisothermal crystallization process of mLLDPE/EVA blends using FTIR micro‐spectroscopy

The nonisothermal crystallization process has been investigated by Fourier transform infrared (FTIR) micro‐spectroscopy for the 40/60 wt % blends of metallocene linear low density polyethylene (m‐LLDPE) and ethylene/vinyl acetate copolymer (EVA) at the molecular level. In the cooling process, thermal spectra of mLLDPE/EVA blends were collected between 150°C and 67°C at 1°C interval. According to the van't Hoff equation at constant pressure, the changes of absorbance ratio corresponding to high and low vibrational states were calculated; hereby, apparent enthalpy differences of vibration energy states transformation (?H_v) of characteristic groups could be obtained. Combining with DSC analysis, two exothermal peaks were examined in the crystallization process, corresponding to mLLDPE‐rich and EVA‐rich domains, respectively; while in comparison of the ?H_v values of various characteristic groups corresponding to the two exothermal peaks, the bending vibrational mode of methylene groups has been found to make a prominent contribution to the movement and regular arrangement of mLLDPE and EVA chain segments towards each rich domain in the crystallizing process. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 261–267, 2005     

Translation of variables and implementation of efficient logic‐based techniques in the MINLP process synthesizer MIPSYN

This article describes alternative GDP formulation and convex hull representations for process synthesis problems and their implementation in a unique MINLP process synthesizer MIPSYN. A special translation of variables in mixed‐integer, relaxed, and logic‐based variations has been proposed, which enables modeling and solving process alternatives in a narrowed lifted space of variables, defined by nonzero lower and upper bounds. Based on these translation variations, alternative formulations have been developed for convex hulls, multiple‐term generalized disjunctive programming problems, and logic‐based outer‐approximation algorithm, all of them being specialized for the synthesis of process flowsheets. Several studies were performed and three different large‐scale synthesis problems were solved to test the performance and efficiency of different formulations. This initial research indicates that the proposed alternative convex hull representation usually outperforms the conventional one when solving both MILP and NLP steps in highly combinatorial MINLP process networks problems. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

A hierarchical method to integrated solvent and process design of physical CO2 absorption using the SAFT‐γ Mie approach

Molecular‐level decisions are increasingly recognized as an integral part of process design. Finding the optimal process performance requires the integrated optimization of process and solvent chemical structure, leading to a challenging mixed‐integer nonlinear programming (MINLP) problem. The formulation of such problems when using a group contribution version of the statistical associating fluid theory, SAFT‐γ Mie, to predict the physical properties of the relevant mixtures reliably over process conditions is presented. To solve the challenging MINLP, a novel hierarchical methodology for integrated process and solvent design (hierarchical optimization) is presented. Reduced models of the process units are developed and used to generate a set of initial guesses for the MINLP solution. The methodology is applied to the design of a physical absorption process to separate carbon dioxide from methane, using a broad selection of ethers as the molecular design space. The solvents with best process performance are found to be poly(oxymethylene)dimethylethers. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3249–3269, 2015