Optimization study on the azeotropic distillation process for isopropyl alcohol dehydration

Modeling and optimization work was performed using benzene as an entrainer to obtain a nearly pure anhydrous isopropyl alcohol product from dilute aqueous IPA mixture through an azeotropic distillation process. NRTL liquid activity coefficient model and PRO/II with PROVISION 6.01, a commercial process simulator, were used to simulate the overall azeotropic distillation process. We determined the total reboiler heat duties as an objective function and the concentration of IPA at concentrator top as a manipulated variable. As a result, 38.7 mole percent of IPA at concentrator top gave the optimum value that minimized the total reboiler heat duties of the three distillation columns.     

A simple,reliable and fast algorithm for the simulation of multicomponent distillation columns

This work has developed a simple, reliable and fast algorithm for the simulation of multicomponent distillation columns, where any equilibrium stage can accept a feed-stream and/or a vapor-side-stream and/or liquid-side-stream. The new scheme considers internal molar overflows and constant relative volatilities to avoid the need of heat balances and vapor–liquid equilibrium calculations. The solution scheme is founded on a Newton-based formulation in block algebra, which relies in a simple, reliable and fast algorithm. Although the proposed calculation scheme can be classified as an approximate method, it is very useful when accurate phase equilibrium and enthalpy data are lacking. Numerical experimentations show good agreement with the results obtained with well-known rigorous simulation approaches.     

Optimal design of multiproduct batch plants considering duplication of units in series

A generalized disjunctive programming (GDP) model for the optimal design of multiproduct batch plants is presented. This general model manages the duplication of units in series to perform a given operation in the process, which is an alternative that has not been considered in previous general approaches. Unlike duplication in parallel, duplication in series is only applicable to some operations which present trade-offs between duplication and other cost-impacting elements in the batch process. In order to use a fixed time and size factor model some assumptions had to be made in the operations that allow the duplication in series. To show the effectiveness of this approach, a plant that produces multiple recombinant proteins is presented and solved.     

Optimum design of Petlyuk and divided-wall distillation systems using a shortcut model

An optimization approach for fully thermally coupled distillation systems for the separation of ternary mixtures is presented. The thermally coupled schemes considered here can be implemented in the form of the original Petlyuk arrangement (with an external prefractionator) or the modified Kaibel arrangement (in the form of a divided-wall column). The approach uses a shortcut design method that allows the system to be modeled as a nonlinear programming problem. Given the practical interest in divided-wall column arrangements, special attention is given in this work to this type of structure. Several cases of study are presented to show the applicability of the proposed approach.     

Approximate design of fully thermally coupled distillation columns

An approximate design procedure for fully thermally coupled distillation columns (FTCDCs) is proposed and applied to example ternary systems. The procedure gives a fast solution of structural and operation design for a preliminary study of the FTCDC. The structural information resolves the design difficulty, caused from the interlinking streams of the column, which is encountered when a conventional design procedure is implemented. The design outcome explains that how the thermodynamic efficiency of the FTCDC is higher than that of a conventional two-column system and how the system of a separate prefractionator is different from a dividing wall structure. From the design result of three example systems with three different feed compositions, the useful performance of the proposed scheme is proved. In addition, the structural design of the FTCDC gives better understanding of the system and leads to high efficiency design of the column.     

A novel MINLP-based representation of the original complex model for predicting gasoline emissions

The Environmental Protection Agency (EPA) introduced Reformulated Gasoline (RFG) requirements as a measure to reduce emissions from gasoline-powered vehicles in certain geographic areas. As part of this effort, the EPA developed empirical models for predicting emissions as a function of gasoline properties and established statutory baseline emissions from a representative set of gasolines. All reformulated gasoline requires certification via this model, known as the Complex Model, and all refiners and importers calculate emissions performance reductions from the statutory baseline gasoline. The current representation of the Complex Model is extremely difficult to implement within refinery operations models or to use in combination with models for designer gasoline. RFG and boutique fuels are key driving forces in the North American refining industry.The RFG models introduce increasingly complex constraints with the major limitation that they are implicitly defined through a series of complicated disjunctions assembled by the EPA in the form of spreadsheets. This implicit and cumbersome representation of the emissions predictive models renders rigorous optimization and sensitivity analysis very difficult to address directly. In this paper, we discuss how the federal government requirements for reformulated gasoline can be restated as a set of mixed-integer nonlinear programming (MINLP) constraints with the aid of disjunctive programming techniques. We illustrate the use of this model with two simple example fuel blending problems.     

Towards further internal heat integration in design of reactive distillation columns—part I: The design principle

The thermodynamic efficiency of a reactive distillation column involving reactions with a highly thermal effect could sometimes be improved substantially through seeking further internal heat integration between the reaction operation and separation operation. Prudent arrangement of the reactive section and deliberate determination of feed location are the two effective methods that can complement internal heat integration within a reactive distillation column. The reactive section is suggested to properly superimpose onto both the stripping section for exothermic reactions and the rectifying section for endothermic reactions. Feed location should be determined so that the effect of internal heat integration can be maximized between the reaction and separation operations. Two reactive distillation systems, involving a highly exothermic reaction and a highly endothermic one, are employed to evaluate the proposed design philosophy and the results obtained confirm its feasibility and effectiveness. In addition, sensitivity analysis is conducted with respect to the amount of catalyst employed, relative volatilities of reacting mixtures, and thermal condition of feeds. The applicability and potentials of the design principle proposed are highlighted.     

复杂精馏系统的合成研究

通过建立精馏换热网络线性规划模型,快速确定最小公用工程量(或费用)的精馏流程和操作参数,使精馏系统的合成问题简化,最后通过文献中的算例验证了模型的可行性。     

Generalized modular framework for the synthesis of heat integrated distillation column sequences

The synthesis of simple and Heat Integrated distillation column sequences is a challenging problem of high economic importance due to its structural and physical complexities and to its economic impact on chemical industries. In this paper this synthesis problem is addressed through a novel systematic superstructure representation method extending the principles of the Generalized Modular Framework (Papalexandri and Pistikopoulos, 1996. A.I.Ch.E. Journal 42 (4), 1010-1032). The method proposes systematic structural and physical models addressing the problem's inherent complexities, generating considerably compact optimization problems while avoiding the use of simplifying assumptions. The method employed for ternary and quaternary non-azeotropic separations is demonstrated over illustrating case studies obtaining the most energy efficient distillation column sequences and Heat Integration schemes and generating substantial energy savings.     

Initialising distillation column models

Difficulties associated with the optimisation of distillation column models by non-linear programming are considered. The paper presents a systematic procedure to enable these difficulties to be overcome and proposes a particular formulation of the distillation column model. A certain limiting case of the column model is examined, that of infinite reflux or zero feed. This limiting case considerably simplifies the model and provides a system of non-linear equations that is readily solved. The solution of this problem gives useful information about the purity that can be achieved in the general case and the number of plates needed to attain a given level of purity. The limiting problem provides starting values for the solution of the general column and suggests a homotopy that can be followed if difficulties arise in obtaining convergence. To obtain a stable form of the limiting case requires the general column model to be formulated in a certain way, which to our knowledge has not previously been considered. The ideas have been successfully tested on various multi-column flowsheets involving distillation columns with heat integration.     

A method for the design of distillation systems aided by ionic liquids

Ionic liquids (ILs) have recently been considered for several applications in chemical processes. Particularly, as they promote an evident salting-out effect over vapor–liquid equilibrium properties and integrate the advantages of a liquid entrainer with the benefits of a solid salt, the use of ILs for the ethanol–water separation has gained wider interest. In this work, a design method for distillation systems aided by ILs is presented. The method is based on tray-by-tray calculations from the outside to the inside of the column, and the feed stage is determined by using a minimum distance concept. The method is illustrated with an ethanol dehydration design. An analysis on the effect of ILs on the column composition profiles is also carried out. The method is shown to provide an effective tool for the conceptual design of these types of systems.     

设计演化算法实现精馏分离序列优化综合

采用演化算法求解具有组合爆炸特征的精馏分离序列优化综合问题。由于精馏分离序列与二叉树之问具有同构性．在数据结构上精馏分离序列可以抽象为二又树,直接采用二叉树结构编码方案可以同时表达个体的基因型和表现形。借鉴生物界父本可以经过有性繁殖或无性繁殖得到子代的现象。对遗传算子进行设计。应冒图论方法建立有效的二叉树结构演化重组机制,从而形成基于双亲或单亲父本的交叉算子。实例表明：双亲遗传操作其空间搜索率较低,而单亲遗传操作其最优解命中率较高。因此。演化算法能够成功解算大规模精馏分离序列优化综合问题。     

An enhanced pseudo-binary-mixture (PBM) algorithm for multistage separation processes

A scale-shift approach is introduced to further refine the pseudo-binary-mixture (PBM) simultaneous correction procedure for the solution of multicomponent, multistage separation problems [Ishii, Y., & Otto, F. D. (2001). An efficient simultaneous correction procedure for multicomponent, multistage separation calculations for non-ideal systems. Computers and Chemical Engineering, 25, 1285–1298; & Ishii, Y., & Otto, F. D. (2003a). A method to extend the domain of convergence for difficult multicomponent, multistage separation problems. Computers and Chemical Engineering, 27, 855–868]. The scale-shift approach takes advantage of the fact that a solution to a stage-wise separation problem is more easily obtained the fewer the number of stages. In the approach the column specified in a given problem is initially downsized to a configuration having a small number of stages and then scaled up to the original one in a multiple-step manner. A solution is obtained at each scale-shift step and the results are used as a set of initial assumptions to solve the subsequent scaled-up problem. Combining the scale-shift approach along with the gradual non-ideal enhancing method with the PBM algorithm provides a more robust and efficient algorithm for the simulation and solution of difficult multicomponent, multistage separation problems including those involving non-ideal mixtures that are highly sensitive to the quality of initial values. A geometric consideration for the gradual non-ideal enhancing method and details of the unique procedure for the algebraic inversion of matrices employed in the algorithm are also presented.     

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.     

Simulation studies on reactive distillation for synthesis oftert-amyl ethyl ether

In this work, a reactive distillation column in which chemical reactions and separations occur simultaneously is applied for the synthesis of tert-amyl ethyl ether (TAEE) from ethanol (EtOH) and tert-amyl alcohol (TAA). A rate-based kinetic model for liquid-phase etherification and an equilibrium stage model for separation are employed to study the reactive distillation. The calculation is carried out using the commercial software package, Aspen Plus. Simulations are performed to examine the effects of design variables, i.e., a number of rectifying, reaction and stripping stages on the performance of reactive distillation column. It has been found that an optimal column configuration for the TAEE production under the study is designed with no rectifying, 4 reaction and 8 stripping stages. With such an appropriate specification of the reactive distillation column, the effects of various operating variables on the TAA conversion and TAEE selectivity are further investigated and the results have shown that the reflux ratio and operating pressure are the most important factors to the operation of the reactive distillation.     

Natural gas sweetening using low temperature distillation: simulation and configuration

ABSTRACT

Hypothesis: Separation of CO₂ from a stream of natural gas using low-temperature distillation process is complicated not only because of the existence of ethane (C₂) and CO₂ azeotrope but owing to the freezing of CO₂. A modified process based on an existing Ryan–Holmes process in the form of a low-temperature distillation is presented here for the separation of sour gases. In addition, solvents were employed to prevent CO₂ freezing and to avoid formation of the C₂-CO₂ azeotrope.

Simulation: Simulation calculations were carried out using a natural gas stream containing CO₂ and H₂S. The process was designed and simulated using Aspen HYSYS 9.

Findings: A low-temperature process for the separation of a feed stream containing up to 30 mole% CO₂ and 2.5 mole% H₂S was achieved, which lowered the levels of these gases to within 50 and 4 ppm, respectively. These values conform to liquefied natural gas specifications. The compositions of the product streams, the required solvent circulation flow rates, and the total energy requirements were estimated. The reported process successfully separated H₂S and produced CO₂ at a pressure of 25.0 bar and a temperature of ?13°C, thereby ensuring its suitability for application in enhanced oil recovery. This described process also delivered the desired natural gas product at 35 bar and ?90°C ready for liquefaction and further cooling.     

Propylene glycol and ethylene glycol recovery from aqueous solution via reactive distillation

Propylene glycol (PG) and ethylene glycol (EG) are recovered from aqueous solution via reaction with acetaldehyde to form acetals in a reactive distillation column. The reaction takes place over Amberlyst 15 cationic exchange resin catalyst, held in structured packing in the column reactive zone. Gycol solution is fed to the column at the top of the reactive zone and acetaldehyde is fed at the bottom of the reactive zone. The acetals produced, 2,4-dimethyl-1,3-dioxolane from PG and 2-methyl-1,3-dioxolane from EG, form minimum-boiling azeotropes with water and exit the top of the column along with excess acetaldehyde; residual water exits the column as the bottoms stream. In a pilot-scale column, steady-state PG conversions of over 90% are obtained. Hydrolysis of both acetals is rapid and allows complete recovery of high purity PG and EG and recycle of acetaldehyde. Simulation of PG acetalization using a kinetic rate-based model in AspenPlus process simulation software gives good agreement with experimental data using an HETP of . The proposed recovery scheme has application for PG and EG recovery from pure polyol solutions and from mixed polyol streams such as those generated in carbohydrate hydrogenolysis.     

ϵ-OA for the solution of bi-objective generalized disjunctive programming problems in the synthesis of nonlinear process networks

There has been an increasing interest in multicriteria optimization (MCO) of nonlinear process network problems in recent years. Several mathematical models have been developed and solved using MCO methodologies including ϵ-constraint, weighted sum, and minimum distance. In this paper, we investigate the bi-objective nonlinear network synthesis problem and propose an effective algorithm, ϵ-OA, based on augmented ϵ-constraint and logic-based outer approximation (OA). We provide theoretical characterization of the proposed algorithm and show that the solutions generated are efficient. We illustrate the effectiveness of our novel algorithm on two benchmark problems. The ϵ-OA is compared to the straightforward use of OA with augmented ϵ-constraint algorithm (ϵ-con + OA), the augmented ϵ-constraint without OA (ϵ-MINLP), and the traditional ϵ-constraint (T-ϵ-con). Based on the results, our novel algorithm is very effective in solving the bi-objective generalized disjunctive programming problems in the synthesis of process networks.