ON THE MINLP APPROACH FOR OPTIMAL DESIGN OF CHEMICAL ENGINEERING SYSTEMS (I) A GLOBAL OPTIMIZATION METHOD FOR NONCONVEX MINLP PROBLEMS

An algorithm for global optimization of a class of nonconvex MINLP problems is devel-oped and presented in this paper.By partitioning the variables,dual representation of the primal ofsubproblems and outer-approximation strategy are used to develop a representative relaxed iterativeproblem.Then the original MINLP problem is replaced by a series of subproblems and relaxediterative problems.By exploiting the particular form of the nonconvex MINLP problem,the feasibleregion of this problem is explicitly included in the representative problem,thus the inconvenienceencountered with the GBD method can be avoided.The proposed method is illustrated andinterpreted geometrically with an example problem.     

COMPUTER AIDED OPTIMAL HEAT EXCHANGER NETWORK SYNTHESIS BY A TWO-STAGE METHOD

This paper presents a two-stage method for the optimal heat exchanger network synthesis.In the firststage based on the study of the pinch point properties,a network minimizing the utility consumption and thenumber of exchangers is developed.Usually,this network is close to the optimal.In the second stage,anonlinear programming formulation is developed,and its solution with features of minimum capital andannual operating costs provides an improvement on the network obtained in the first stage.An integratedsoftware package for heat exchanger network synthesis has been developed.This package consists of a sp-readsheet,a graphics program,a data base management system,a menu program,an application networksynthesis program,a network simulation program and a data base of standard size exchangers.The appli-cation of this method is illustrated with several examples which include a complex crude oil preheating heatexchanger network.     

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.     

KINETICS ON EPOXIDATION OF PROPYLENE WITH ETHYLBENZENE HYDROPEROXIDE

In the range of 76° to 1200C,Dalin and others studied the kinetics on molybdenum-catalyzed epoxi-dation of propylene with ethylbenzene hydroperoxide,proposed a mechanism model of this reaction andsuggested that effective concentration of catalyst is constant during the process of epoxidation.In thispaper the reaction has been studied experimentally based on Dalin's investigation.After analyzing thedata obtained in our experiment and those of Dalin,it is shown that the effective concentration of thecatalyst is not a constant but a variable,and its rate of consumption behaves in accordance with the firstorder reaction.Thus,the authors have modified the mechanism model in literature [3] and pointed outthat the deactivation mechanism of the catalyst is different from the inhibition mechanism described byFarberof.     

混合分离过程的有序设计框架

The design of optimal separation flow sheets for multi-component mixtures is still not a solved problem This is especially the case when non-ideal or azeotropic mixtures or hybrid separation processes are considered. We review recent developments in this field and present a systematic framework for the design of separation flow sheets. This framework proposes a three-step approach. In the first step different flow sheets are generated. In the second step these alternative flow sheet structures are evaluated with shortcut methods. In the third step a rigorous mixed-integer nonlinear programming （MINLP） optimization of the entire flow sheet is executed to determine the best alternative. Since a number of alternative flow sheets have already been eliminated, only a few optimization runs are necessary in this final step. The whole framework thus allows the systematic generation and evaluation of separation processes and is illustrated with the case study of the separation of ethanol and water.     

质量交换网络集成的状态空间法

State space approach is an effective method to mass-exchange network (MEN) synthesis. By decomposing the network into two interactive parts, a distribution network and a process operator, the synthesis problem can be formulated into a mixed integer nonlinear programming (MINLP) model. In this article, a generalized state space model based on typical MEN is established and verified in two cases. A new asymmetrical operator and cost index are also adopted to speed up the solution process. The results demonstrate the efficiency of the proposed approach.     

A systematic approach for synthesizing a low-temperature distillation system

In this paper, by combining a stochastic optimization method with a refrigeration shaft work targeting method, an approach for the synthesis of a heat integrated complex distillation system in a low-temperature process is presented. The synthesis problem is formulated as a mixed-integer nonlinear programming (MINLP) problem, which is solved by simulated annealing algorithm under a random procedure to explore the optimal operating parameters and the distillation sequence structure. The shaft work targeting method is used to evaluate the minimum energy cost of the corresponding separation system during the optimization without any need for a detailed design for the heat exchanger network (HEN) and the refrigeration system (RS). The method presented in the paper can dramatical y reduce the scale and complexity of the problem. A case study of ethylene cold-end separation is used to il ustrate the application of the approach. Compared with the original industrial scheme, the result is encouraging.     

Thermodynamic design of a cascade refrigeration system of liquefied natural gas by applying mixed integer non-linear programming

Liquefied natural gas (LNG) is the most economical way of transporting natural gas (NG) over long distances. Liq-uefaction of NG using vapor compression refrigeration system requires high operating and capital cost. Due to lack of systematic design methods for multistage refrigeration cycles, conventional approaches to determine op-timal cycle are largely trial-and-error. In this paper a novel mixed integer non-linear programming (MINLP) model is introduced to select optimal synthesis of refrigeration systems to reduce both operating and capital costs of an LNG plant. Better conceptual understanding of design improvement is illustrated on composite curve (CC) and exergetic grand composite curve (EGCC) of pinch analysis diagrams. In this method a superstruc-ture representation of complex refrigeration system is developed to select and optimize key decision variables in refrigeration cycles (i.e. partition temperature, compression configuration, refrigeration features, refrigerant flow rate and economic trade-off). Based on this method a program (LNG-Pro) is developed which integrates VBA, Refprop and Excel MINLP Solver to automate the methodology. Design procedure is applied on a sample LNG plant to illustrate advantages of using this method which shows a 3.3% reduction in total shaft work consumption.     

多产品间歇过程的分层次在线调度--一种数学规划与遗传算法的混合算法

In this contribution we present an online scheduling algorithm for a real world multiproduct batch plant. The overall mixed integer nonlinear programming (MINLP) problem is hierarchically structured into a mixed integer linear programming (MILP) problem first and then a reduced dimensional MINLP problem, which are optimized by mathematical programming (MP) and genetic algorithm (GA) respectively. The basis idea relies on combining MP with GA to exploit their complementary capacity. The key features of the hierarchical model are explained and illustrated with some real world cases from the multiproduct batch plants.     

Combined Water-Oxygen Pinch Analysis with Mathematical Programming for Wastewater Treatment

Water-oxygen pinch analysis is an effective method to decrease the wastewater quantity and improve the wastewater quality. But when multiple-contaminants are present, the method is difficult to be carried out. In this paper, the method that combines water-oxygen pinch analysis with mathematical programming is proposed. It obtains the general optimal solution and leads to the reuse stream that cannot be found only by pinch analysis. The new method is illustrated by an example, and the annual cost is reduced by 8.43% compared with the solution of literature.     

反应过程中的污染削减方法

To implement the strategy of pollution prevention in the development of reaction processes, studying the effects of reaction conditions such as temperature, pressure and concentration as well as various engineering factors, such as back- mixing, heat and mass transfer, on environmental performance of the reaction processes is very important. In this paper, potential environmental impact balance, proposed in the waste reduction algorithm, and PEI rate equation are recommended as a useful tool to be used to carry out this work. The application of the method is illustrated with the reaction of allyl chloride production.     

用伪并行遗传/模拟退火算法进行多流股换热器网络综合的研究

The multi-stream heat exchanger network synthesis (HENS) problem can be formulated as a mixed integer nonlinear programming model according to Yee et al. Its nonconvexity nature leads to existence of more than one optimum and computational difficulty for traditional algorithms to find the global optimum. Compared with deterministic algorithms, evolutionary computation provides a promising approach to tackle this problem. In this paper, a mathematical model of multi-stream heat exchangers network synthesis problem is setup. Different from the assumption of isothermal mixing of stream splits and thus linearity constraints of Yee et al., non-isothermal mixing is supported. As a consequence, nonlinear constraints are resulted and nonconvexity of the objective function is added. To solve the mathematical model, an algorithm named GA/SA (parallel genetic/simulated annealing algorithm) is detailed for application to the multi-stream heat exchanger network synthesis problem. The performance of the proposed approach is demonstrated with three examples and the obtained solutions indicate the presented approach is effective for multi-stream HENS.     

一种对预测控制系统经济性能评估的实用方法及其工业应用

In this article, an approach for economic performance assessment of model predictive control (MPC) system is presented. The method builds on steady-state economic optimization techniques and uses the linear quadratic Gaussian (LQG) benchmark other than conventional minimum variance control (MVC) to estimate the potential of reduction in variance. The LQG control is a more practical performance benchmark compared to MVC for performance assessment since it considers input variance and output variance, and it thus provides a desired basis for determining the theoretical maximum economic benefit potential arising from variability reduction. Combining the LQG benchmark directly with benefit potential of MPC control system, both the economic benefit and the op-timal operation condition can be obtained by solving the economic optimization problem. The proposed algorithm is illustrated by simulated example as well as application to economic performance assessment of an industrial model predictive control system.     

A procedure for design of hydrogen networks with multiple contaminants☆

It is necessary to reduce hydrogen consumption to meet increasingly strict environmental and product-quality regulations for refinery plants. In this paper, the concentration potential concepts proposed for design of water-using networks are extended to synthesis of hydrogen networks with multiple contaminants. In the de-sign procedure, the precedence of processes is determined by the values of concentration potential of demands. The usage of complementary source pair(s) to reduce utility consumption is investigated. Three case studies are presented to il ustrate the effectiveness of the method. It is shown that the design procedure has clear engineer-ing meaning.     

用于超结构换热器网络热计算和网络综合的显式解

For the optimal design of a heat exchanger network, the inlet and outlet stream temperatures of each heat exchanger in the network should be known. An explicit analytical solution of stream temperatures of an arbitrary connected heat exchanger network was introduced, which is suitable for the thermal calculation of heat exchanger networks. For the heat exchanger network synthesis, this solution was further developed and coupled with the stage-wise superstructure heat exchanger networks. The new calculation procedure reduced the computer memory requirement dramatically. On the basis of this solution, a mathematical model for synthesis of heat exchanger networks with genetic algorithm was formulated, which is always feasible and no iteration is needed. Two examples were calculated with the proposed approach and better results were obtained.     

多管程换热器换热网络的合成

Many methods have been proposed for synthesis of heat exchanger networks in recent years, most of which consider single pass exchangers. In this study some evolutionary rules have been proposed for synthesis of multipass exchanger networks. The method is based on the heuristic that optimal networks should feature maximum energy recovery and have the minimum number of shells. The effectiveness of the developed evolutionary rules is demonstrated through some literature examples.     

OPTIMAL OPERATION OF BATCH DISTILLATION

The maximum-profit problem in the operation of a batch distillation column separating a binary mixture is studied. From the objective function made in this paper, such problem can be transformed under special condition, into either maximum-distillate or minimum-time problem. Reflux ratio is chosen as control variable. Digital method is worked out by using optimal control theory to find the condition of optimal operation. In deriving present mathematic model, the liquid hold-up in plates and in condenser as well as the plate efficiency are taken into account. The proposed optimal operation is compared theoretically with the conventional constant reflux ratio and constant overhead composition policies and the results show that the proposed optimal operation can lead to/nuch higher profits. To demonstrate the practical usefulness of the proposed optimal policy, experimental work is carried out in a 4 sieve trays 0.17M diameter batch distillation column with ethanolwater system. Agreement is found between the experimental data and the computed results from present model.     

基于改进的GSO算法和BP神经网络的氨合成塔出口氨含量软测量模型(英文)

The ammonia synthesis reactor is the core unit in the whole ammonia synthesis production. The ammonia concentration at the ammonia converter outlet is a significant process variable, which reflects directly the production efficiency. However, it is hard to be measured reliably online in real applications. In this paper, a soft sensor based on BP neural network (BPNN) is applied to estimate the ammonia concentration. A modified group search optimization with nearest neighborhood (GSO-NH) is proposed to optimize the weights and thresholds of BPNN. GSO-NH is integrated with BPNN to build a soft sensor model. Finally, the soft sensor model based on BPNN and GSO-NH (GSO-NH-NN) is used to infer the outlet ammonia concentration in a real-world application. Three other modeling methods are applied for comparison with GSO-NH-NN. The results show that the soft sensor based on GSO-NH-NN has a good prediction performance with high accuracy. Moreover, the GSO-NH-NN also provides good generalization ability to other modeling problems in ammonia synthesis production.     

基于瞬时目标函数曲线特性的反应器网络综合

It is believed that whether the instantaneous objective function curves of plug-flow-reactor (PFR) and continuous-stirred-tank-reactor (CSTR) overlap or not, they have a consistent changing trend for complex reactions(steady state, isothermal and constant volume). As a result of the relation of the objective functions (selectivity or yield) to the instantaneous objective functions (instantaneous selectivity or instantaneous reaction rate), the optimal reactor network configuration can be determined according to the changing trend of the instantaneous objective function curves. Further, a recent partition strategy for the reactor network synthesis based on the instantaneous objective function characteristic curves is proposed by extending the attainable region partition strategy from the concentration space to the instantaneous objective function-unreacted fraction of key reactant space. In this paper,the instantaneous objective function is closed to be the instantaneous selectivity and several samples axe examined to illustrate the proposed method. The comparison with the previous work indicates it is a very convenient and practical systematic tool of the reactor network synthesis and seems also promising for overcoming the dimension limit of the attainable region partition strategy in the concentration space.     

SIMULATION OF DISTILLATION PROCESSES WITH REACTIONS IN SERIES

This paper introduces a mathematical model and advances an algorithm,that can be used for simu-lation of reactive distillation processes.This model takes into account the reactions in series,that occurwithin a distillation column and this algorithm is based upon the block tridiagonal matrix technique.In order to accomplish a computation of design and inspect whether or not different configuration of flowsbetween stages can increase the yields of intermediate products,a procedure for solving block tridiagonalmatrix equation with some off-tridiagonal submatrixes and/or submatrixes on the borders is modified.The saponification of propylene chlorophydrin is illustrated to verify the algorithm.It can be seen thatthe results are satisfactory.