An alternate computational architecture for advanced process engineering

An algorithm for equation-oriented (EO) flowsheeting to which the interfacing of external modular procedures is readily allowed is presented. In the algorithm, all the process units in a flowsheet are solved simultaneously in an EO environment while taking advantage of the external software as it is. The algorithm is the basis for the process simulator MIKAN. The simulator consists of two parts, namely EO main, which is the simulator's executive, and add-on blocks to incorporate external procedures. EO main also serves to solve separation processes. The PBM (Pseudo-Binary-Mixture)-based algorithm for separation processes (Ishii & Otto, 2008) is fully exploited in EO main. The equations of the entire system are composed of a set of equations for separation processes and sets of equations representing the input-output relation of each add-on block. The input-output relation is obtained by numerical perturbation where all the component flow rates are perturbed collectively. Although the proposed perturbation is simple and significantly less expensive, it is very reliable since the interactions among the input variables affecting the output are fully accounted for.Robustness, flexibility and efficiency of the new algorithm have been confirmed by its implementation. A serious drawback with the EO approach is the difficult accessibility to the user when incorporating new or external process models. Easy accessibility is an important attribute of the new algorithm, even though the simulator's executive is highly integrated and complex. It is significant that the derivative information at various levels of a flowsheet is easily obtained in the algorithm. The novel algorithmic capabilities of the algorithm provide a robust platform for the next stage of advanced process engineering.     

大范围工况变化下联塔的严格机理模拟研究

建立基于严格机理的乙烯生产过程联塔系统开放式模型,采用大规模系统优化方法和自动微分方法,对此联塔系统进行大范围工况变化下的模拟.求解结果和收敛效率与Aspen Plus进行了对比.在初值相同、标度相同、建模和求解方式一致的情况下,本文联立求解方法的结果与Aspen Plus的EO模式的求解结果一致;对于进料工况大范围变化的情形,本文的联立求解方法收敛性明显优于Aspen Plus.采用大规模优化计算方法和稀疏自动微分方法对于提高求解效率、降低初值依赖性具有明显的效果.     

New developments and capabilities in prosyn—An automated topology and parameter process synthesizer

This paper describes an improved, equation oriented user friendly version of the modular computer package PROSYN—a mixed-integer nonlinear programming (MINLP) process synthesizer. A number of strategies are described and implemented for the outer approximation and equality relaxation algorithm (OA/ER) in order to reduce the undesirable effects of nonstructured nonconvexities in the master problem. These strategies include use of valid outer-approximations for splitters and mixers and various ways of handling the linearizations. An NLP initializer, model generator and a comprehensive library of models for basic process units and interconnection nodes, and a comprehensive library of basic physical properties have been developed and built in the PROSYN package. In this way PROSYN can be used to solve problems at different levels of complexity: from the simple NLP flowsheeting up to the MINLP synthesis through the modeling and decomposition (M/D) strategy and simultaneous heat integration including HEN costs. Applications with PROSYN are demonstrated with several example problems.     

甲苯歧化装置热高压分离工艺研究及工业应用

针对目前国内甲苯歧化装置反应产物气液分离过程普遍采用冷高压分离工艺导致装置用能不合理的现状,分析了热高压分离工艺的技术先进行以及装置改造成热高压分离工艺需要注意的问题;确定了能够准确分析甲苯歧化反应产物的分析方法及产物组成,利用ASPEN PLUS流程模拟软件分别对热高压分离工艺过程和冷高压分离工艺过程进行了模拟计算与对比,模拟计算结果表明,由冷高压分离工艺改成热高压分离工艺后对循环氢纯度几乎没有影响。通过热量核算,提出了基于热高压分离工艺的甲苯歧化装置新型换热流程。装置按照新流程改造后,装置运行平稳,工业应用结果表明采用该技术后可使1450kt/a的甲苯歧化装置单位能耗下降5.6kg EO/t,证明热高压分离工艺是可行的及新型换热工艺流程的节能优越性。     

A New Numerical Approach for a Detailed Multicomponent Gas Separation Membrane Model and AspenPlus Simulation

A new numerical solution approach for a widely accepted model developed earlier by Pan [1] for multicomponent gas separation by high‐flux asymmetric membranes is presented. The advantage of the new technique is that it can easily be incorporated into commercial process simulators such as AspenPlus^TM [2] as a user‐model for an overall membrane process study and for the design and simulation of hybrid processes (i.e., membrane plus chemical absorption or membrane plus physical absorption). The proposed technique does not require initial estimates of the pressure, flow and concentration profiles inside the fiber as does in Pan's original approach, thus allowing faster execution of the model equations. The numerical solution was formulated as an initial value problem (IVP). Either Adams‐Moulton's or Gear's backward differentiation formulas (BDF) method was used for solving the non‐linear differential equations, and a modified Powell hybrid algorithm with a finite‐difference approximation of the Jacobian was used to solve the non‐linear algebraic equations. The model predictions were validated with experimental data reported in the literature for different types of membrane gas separation systems with or without purge streams. The robustness of the new numerical technique was also tested by simulating the stiff type of problems such as air dehydration. This demonstrates the potential of the new solution technique to handle different membrane systems conveniently. As an illustration, a multi‐stage membrane plant with recycle and purge streams has been designed and simulated for CO₂ capture from a 500 MW power plant flue gas as a first step to build hybrid processes and also to make an economic comparison among different existing separation technologies available for CO₂ separation from flue gas.     

A new algorithm for process flowsheeting

A new algorithm is described for organizing the computations in a process flowsheeting system designed to handle mixed systems of equations and procedures. It is essentially a depth-first implicit enumeration algorithm based on partitioning and tearing techniques, but also uses automatic algebraic manipulation to take further advantage of the structure.     

Reactive distillation for toluene disproportionation: a technical and economic evaluation

The potential of reactive distillation for xylenes manufacture via the toluene disproportionation reaction is considered in detail. The conventional process is carried out in the vapour phase over a fixed bed catalyst at elevated temperature and pressure. Process design and flowsheeting were carried out for both the conventional and reactive distillation processes using the Aspen Plus simulation package, specifically using the facility for evaluation of simultaneous reaction and physical equilibrium within a distillation column calculation block. Using the flowsheeting based process and equipment designs, a detailed comparative cost assessment and economic analysis of the two processes was performed. The economic evaluation is equivocal, and the reactive distillation process does not appear to offer significant benefits. While the reactive distillation approach does allow significant flowsheet simplification, the design compromises required to enable simultaneous reaction and distillation, particularly relating to design pressure, negate the inherent process benefits. Further, changes in reaction selectivity between the two reaction environments influenced not only the process product slate but also forced process design features to suppress an unwanted (heavy) by-product.     

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.     

Problem-oriented modules for process simulation. resolution strategies for simulation and optimization

A new resolution strategy is presented to overcome short-comings of the sequential modular process flowsheeting. Following a logic flow of information, different from the material flow of streams in the flowsheet, changes the mathematical structure of the system of equations that models the units. New programs have been generated, very different from traditional modules. Deciding an appropriate resolution strategy usually leads to a dramatic reduction in required CPU times and to an increased flexibility to handle different problems. The simulation convergence and process optimization can be approached from a different perspective.     

Simulation and heat exchanger network designs for a novel single-column cryogenic air separation process

To realize the industrialization of the novel single-column air separation process proposed in previous work, steady-state simulation for four different configurations of the single-column process with ternary(nitrogen, oxygen and argon) is developed. Then, exergy analysis of the single-column processes is also carried out and compared with the conventional double-column air separation process at the same capacity. Furthermore, based on the steady-state simulation of single-column processes, the different heat exchanger networks(HENs) for the main heat exchanger and subcooler in each process are designed.To obtain better performance for this novel process, optimization of process configuration and operation is investigated. The optimal condition and configuration for this process is consisted as: feedstock is divided into two streams and the reflux nitrogen is compressed at the approximate temperature of 301 K. In addition, HEN is optimized to minimize the utilities. HENs without utilities are obtained for the four different configurations of single-column process. Furthermore, capital costs of the HEN for different cases are estimated and compared.     

分离系统与换热网络同步优化方法的改进：神经元网络—遗传算法

采用神经元网络法和遗传算法，在过程系统用能一致性的基础上对分离系统与换热网络同步优化问题提出了改进的优化模型及优化策略。该方法不仅能够自动、迅速地同步得到分离序列与换热网络联合系统的流程结构与操作参数，而且具有获得全局最优解的能力。最后通过实例说明本方法的有效性。     

Homotopy continuation methods for computer-aided process design

Homotopy continuation methods have been used by the authors and others to solve difficult chemical engineering flowsheeting and design problems involving the solution of simultaneous nonlinear equations. Such methods can fail when: (1) the homotopy path, which one follows from the solution of a simple problem to the solution of the original (difficult) problem, returns to a second solution of the simple problem; (2) the homotopy path strikes an interior boundary of the domain of definition of the original (vector-valued) function; and (3) the homotopy path goes off to infinity. For the first two modes of failure, the use of an affine homotopy is discussed here as a possible remedy. Failure due to an unbounded homotopy path is the subject of current research.     

Study of a continuous separation process for 2-furyl oxirane

The present work is a study on a continuous separation process for 2-furyl oxirane from its reaction medium which is a quaternary mixture. Continuous process flowsheeting has been used in order to compare two different separation processes which are based upon solvent extraction and heteroazeotropic distillation. The improved production process totals five unit operations: synthesis of 2-furyl oxirane; filtration of the reaction medium; heteroazeotropic distillation; synthesis of trimethylsulphonium bromide; and filtration of trimethylsulphonium bromide.     

Computational experiments in equation-based chemical process flowsheeting

The central computational problem in equation-based flowsheeting is the simultaneous solution of a large sparse system of nonlinear algebraic equations. SEQUEL-II, our implementation of the equation-based approach, provides a means for comparing nonlinear equation-solving techniques, in particular the techniques used for computing correction steps, for ge nerating initial guesses, and for evaluating sparse Jacobian matrices. Comparisons are made on the basis of both efficiency and reliability. On a set of fourteen process flowsheeting problems, Powell's dogleg correction step is not only very efficient, but also somewhat more reliable than the Newton step when good initial guesses are not available. Schubert's sparse Jacobian update performs very well, but Bogle and Perkins' new least-relative-change update gives improved performance, when used in connection with either the dogleg or Newton correction step. The use of a hybrid update provides a better approximation of the Jacobian than a sparse update alone, one, and so significantly improves, in terms of number of iterations, the efficiency of the dogleg correction step.     

Conceptual design of distillation processes for mixtures with distillation boundaries: I. Computational assessment of split feasibility

Geometric design methods for the conceptual design of azeotropic distillation processes are fast and efficient tools for the economic screening of different process alternatives. This two‐part series presents a fully automated conceptual design method for finding an optimal recycle policy for the separation of mixtures with distillation boundaries. It does not require visualization and graphical inspection of residue curve or pinch maps and is, hence, not limited to ternary mixtures. The first part introduces a fully computational geometric split feasibility test based on bifurcation analysis. This bifurcation‐based feasibility test can be used as a valuable stand‐alone tool for the assessment of different separation options. It is also one of the core elements of the recycle optimization discussed in the second part of this series. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Simultaneous modular algorithm for steady-state flowsheet simulation and design

The sequential modular algorithm has been used widely for steady-state simulation and design. Consequently, a considerable body of computer programs tailored for such applications exists today. Most of the current research in steady-state calculations, however, is oriented towards simultaneous solution of the entire set of system equations. An unfortunate consequence of this approach is that the software developed for sequential modular calculations can not be employed.

The simultaneous modular algorithm offers many of the advantages of an equation-solving procedure while using the existing sequential modular software. This work presents theoretical analysis and experimental results of various implementations of the simultaneous modular algorithm.

The split fraction model of a unit behavior (y = ax) is shown to have worse convergence properties than Wegstein's algorithm in a sequential modular procedure. Indeed. it is quite a fortuitous set of circumstances when such a model exhibits stable convergence behavior.

The difference split fraction model (dy = adx) is shown to be equivalent to Wegstein's algorithm for a flowsheet with a single recycle loop. On systems with multiple recycle loops, the model exhibits better convergence than Wegstein's algorithm.

The simultaneous modular algorithm can also be implemented as a sparse quasi-Newton procedure to solve systems of non-linear equations. Such implementation allows simultaneous solution of design specifications and flowsheet balances. An initial estimate of the stream variables is obtained by one or more sequential passes through a flowsheet. The system Jacobian matrix is approximated by the difference split fraction unit models. Subsequent Jocobian matrices are obtained by a sparse update procedure.

Covergergence properties of various implementations are demonstrated by examples.     

Process‐based graphical approach for simultaneous targeting and design of water network

A new process‐based graphical approach (PGA) is presented for the simultaneous targeting and design of water network. The PGA is extended from the limiting water profile which was developed for flow rate targeting for a water network. Via PGA procedure, apart from locating the minimum freshwater and wastewater flow rate targets, the water network that corresponds to the minimum flow rate targets is also synthesized simultaneously. The proposed approach handles both fixed load (including operations with water loss and/or gain) and fixed flow rate problems equally well. In addition, the approach can be used to synthesize direct reuse/recycle, regeneration reuse/recycling, and total water network. Furthermore, the proposed approach is applicable for water network with multiple freshwater sources. Three literature examples are presented to illustrate the proposed approach. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Global modular Newton—Raphson technique for simulation of an interconnected plant applied to complex rectification columns

The module-oriented approach to steady state process simulation making use of the global Newton—Raphson method is described. The Newton—Raphson technique gives rise to a flexible and reliable computer program which can be used to solve highly nonlinear heat and material balances. The simulation system consists of a library of subroutines which are capable of automatic generating the residual vector and Jacobian matrix as well. The resulting set of linear equations governing the Newton correction exhibits a sparse structure (sometimes almost band) if the equations are grouped by modules. A modified Gaussian elimination was developed to solve a band matrix having some off-band submatrices. The concept of the generalized Newton—Raphson approach was used to construct a general computer code to solve rectification problems. The important point is the possibility of a straightforward solution of the design problems or problems of “controlled simulation” which via the flowsheeting method cannot be handled easily.     

Equation‐oriented flowsheet simulation and optimization using pseudo‐transient models

Tight integration through material and energy recycling is essential to the energy efficiency and economic viability of process and energy systems. Equation‐oriented (EO) steady‐state process simulation and optimization are key enablers in the optimal design of integrated processes. A new process modeling and simulation concept based on pseudo‐transient continuation is introduced. An algorithm for reformulating the steady‐state models of process unit operations as differential‐algebraic equation systems that are statically equivalent with the original model is presented. These pseudo‐transient models improve the convergence of EO process flowsheet simulations by expanding the convergence basin. This concept is used to build a library of pseudo‐transient models for common process unit operations, and this modeling concept seamlessly integrates with a previously developed time‐relaxation optimization algorithm. Two design case studies are presented to validate the proposed framework. © 2014 American Institute of Chemical Engineers AIChE J 60: 4104–4123, 2014     

Auslegung und Optimierung von hybriden Trennverfahren

Design and Optimization of Hybrid Separation Processes Hybrid separation processes are defined as the combination of at least two different unit operations in different apparatus which contribute to the separation task. Hybrid processes are used for difficult separations, e.g., close‐boiling mixtures and azeotropes, if a single unit operation, e.g., distillation, membranes, extraction, crystallization or chromatography, is not efficient or even not feasible. Because of the structure of a hybrid process which implies two or more unit operations and recycle streams, the design is not straightforward and therefore subject of today's research. In this work general criteria for such a consistent design method are described and a design approach for hybrid separation processes is presented. It bases on rigorous modeling of the unit operations and simultaneous multivariable optimization. The approach feasibility is demonstrated by the separation of an isomer mixture.