A property‐based approach to the synthesis of material conservation networks with economic and environmental objectives

This article presents a multiobjective optimization model for the recycle and reuse networks based on properties while accounting for the environmental implications of the discharged wastes using life‐cycle assessment. The economic objective function considers fresh sources and treatment costs, whereas the environmental objective function is measured through the eco‐indicator 99. The model considers constraints in the process sinks as well as in the environment based on stream properties such as pH, chemical oxygen demand, toxicity, density, and color, in addition to the composition of the waste streams. A global optimization procedure is developed by indirectly tackling properties through property operators and by segregating the process streams before treatment. Three examples are included, and the results show that it is possible to consider simultaneously the trade‐offs between the total annual costs and the overall environmental impact using the proposed methodology. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Interval based MINLP superstructure synthesis of mass exchange networks

A new simultaneous synthesis approach for mass exchange networks (MENs) is presented. The technique is adapted from the interval based mixed integer non-linear program (MINLP) superstructure (IBMS) synthesis of heat exchanger networks (HENs). The superstructure interval boundary compositions are defined by the supply and target compositions of either the rich or lean set of streams. Each rich and lean stream in the superstructure has the potential of exchanging mass with streams of opposite kind in each interval based on mass transfer feasibility. The model harnesses the strengths of the stagewise superstructure and the pinch technology methods for mass exchange network synthesis (MENS). The IBMS approach simultaneously trades-off the capital and operating costs for MENs. The superstructure composition interval defining approach introduced in this paper enforces the mixing of split streams at equal compositions, hence there is no need to include mixing equations in the model. Fixing the interval boundaries helps to eliminate the complexities involved in initialisations, thus the region of search for the optimum solution is reduced. The IBMS model is applied to MENS problems involving continuous contact and staged columns. It is also extended to problems involving multiple mass separating agents (MSAs) and regeneration. The results obtained compare well with those in the literature.     

The use of a simulation model of the torula yeast process for investment evaluation

A simulation model for the production of torula yeast from sugar cane molasses was developed as a tool for investment evaluation. The model consists of about 20 rather specific equipment and economic modules along with the GEMCS executive program. The simulation model was used to analyze different operating and design conditions for the process. Solutions were found which permitted savings in raw material and energy requirements as well as reductions in the volume of effluents and lower production and investment costs, when compared to the scheme originally considered.     

A kinematic model of continuous separation and classification of polydisperse suspensions

Kinematic models for polydisperse suspensions are based on specifying the solid–fluid relative velocity for each solids species as a function of the local solids concentrations. One such model, the Masliyah–Lockett–Bassoon (MLB) model, is employed herein to simulate continuous separation and classification of polydisperse suspensions. To this end, the clarifier-thickener (CT) setup for the continuous separation of suspensions is extended to a generalized clarifier-thickener (GCT). Discharge streams (or products) are described by new singular sink terms. Combining the GCT setup with the MLB model yields a system of nonlinear conservation laws with a discontinuous flux and a new non-conservative transport term describing the sinks. A numerical algorithm for the solution of this equation is presented along with numerical examples. The model describes the GCT unit with all critical design parameters, and predicts the composition of the overflow, underflow and discharge streams and the spatio-temporal evolution of the solids species concentrations inside the unit.     

Optimization of the sulfolane extraction plant based on modeling and simulation

An optimization system, based on modeling and simulation, was developed for a sulfolane extraction plant. The primary objective of the operation of this plant is to increase benzene composition, which is mostly affected by the recycle streams in the plant. In this work the optimal recycle streams were identified and resulting product compositions were evaluated. In the optimization, suitable parametric models for each process unit were obtained first from the steady-state rigorous modeling and simulation of the sulfolane extraction plant. The parametric models were then employed to develop the optimization system based on the SQP scheme. Results of simulations show promise for further economic improvements over present operation states.     

A hierarchical approach for the synthesis of batch water network

This work addresses the problem of synthesising cost-effective batch water networks where a number of process sources along with fresh water are mixed, stored, and assigned to process sinks. In order to address the complexity of the problem, a three-stage hierarchical approach is proposed. In the first stage, global targets are identified by formulating and solving a linear transportation problem for minimum water usage, maximum water recycle, and minimum wastewater discharge. These targets are determined a priori and without commitment to the network configuration. Next, a network with minimum number of tanks is synthesised by solving a mixed-integer non-linear program. The bilinear constraints are relaxed to transform the program into a mixed-integer linear program that is globally solvable. The third stage is aimed at simplifying the network configuration by minimising the number of network inter-connections. Insights gained from the water pinch analysis are also incorporated into the approach to further reduce the water flows via the placement of water regeneration unit. A case study is solved to illustrate the effectiveness of the proposed procedure.     

Plant optimisation by retrofitting using a hierarchical method: Entrainer selection,recycling and heat integration

Systematic procedures for reducing wastes in complex chemical plants are needed to allow efficient optimisation. Here, a hierarchical procedure was applied to the optimisation of a real industrial plant to reduce wastes as well as energy and raw material consumption. In the case studied, the continuous production of methyl-butynol (MBI), acetylene reacts with acetone. The solvent ammonia and acetylene are recycled to the reactor. Unreacted substrates and by-products are separated from the product stream by distillation. Part of the unreacted acetone can be reused for other purposes after distillation. A substantial part of the unreacted substrates and by-products is delivered to a wastewater treatment plant. These waste streams constitute a substantial problem for the operation of this plant. First, waste streams were characterised and tracked back to their origin. Following the hierarchical design procedure, the overall input–output structure was fixed. The entrainer in the present process was critically examined and options were suggested. Then various recycle schemes were considered for later detailed study. The existing plant was simulated using ASPENPLUS. After adjusting the model to all important aspects of the real process scheme, excellent agreement between actual process performance data and simulation was obtained. The various process schemes were simulated and assessed for their economic and ecological performance. The objective functions used included utility, substrate and catalyst costs, as well as costs for wastewater treatment. Additionally, the environmental burden related to energy supply was accounted for by a carbon dioxide tax as suggested by the Nordic countries. The process changes included separation of unreacted acetone from the product stream and recycling to the reactor. By-products were converted back to substrates in an additional reactor separation system and recycled. In various simulated process configurations and operational schemes substantial economic and ecologic improvements were achieved. This study demonstrates the usefulness of hierarchical approaches combined with process simulation for plant optimisation. ©1997 SCI     

Global optimization of mass and property integration networks with in-plant property interceptors

This paper presents a mathematical programming model for the optimal design of mass and property integration networks that include property interceptors within the structure of the network, as opposed to the end-of-pipe use of such interceptors. The model is based on a recycle and reuse scheme that simultaneously satisfies process and environmental constraints. The properties considered in this work are composition, toxicity, theoretical oxygen demand, pH, density and viscosity. The property mixing rules included in the model give rise to bilinear terms for the property operators, and a global optimization algorithm is used for the solution of the model. The model minimizes the total annual cost of the network, which includes the fresh sources cost and the annualized property treatment system and the piping costs. Three examples are included to show the applicability and advantages of the proposed model.     

Graphical approach to minimum flowrate targeting for partitioning water pretreatment units

Pretreatment is often necessary when the fresh water available to industrial plants is impure, and when some processes are particularly sensitive to contaminants. Partitioning processes such as membrane separation units are often used for such applications. However, the use of pretreatment units adds to capital and operating costs for a water system. Hence, it is of interest to develop design procedures to minimize the cost for such system. This work presents a graphical pinch analysis approach for targeting minimum flowrate of partitioning water pretreatment systems in single component problem. The approach determines how product and reject streams from the treatment unit can be allocated, along with bypassed freshwater, to satisfy multiple process sinks with their respective flowrate and purity requirements. Hypothetical case studies are presented to illustrate the approach, and generalized design principles based on pinch analysis heuristics are drawn from the examples.     

Effect of vacuum gas oil hydrotreating reactor on multiple reactors and hydrogen network

The inlet temperature of the Vacuum Gas Oil (VGO) hydrotreating reactor of a refinery is analyzed with the integration of multiple series reactors and hydrogen network considered. The effect of the inlet temperature (T₁) on hydrogen sinks/sources and the product output is analyzed systematically based on the simulation of the series reactors, including VGO hydrotreating reactor, hydrocracking reactor, fluid catalytic cracking reactor and visbreaking reactor. The general relation between the Hydrogen Utility Adjustment (HUA) and multiple pairs of varying sinks and sources is deduced, and correlations between varying streams and T₁ are linearly fitted. Based on this, the quantitative equation between HUA and T₁ is derived, and corresponding diagram is constructed. The T₁ corresponding the minimum hydrogen consumption is identified to be 345℃.     

带修复操作整型编码遗传算法求解大规模机组组合问题

针对发电机组组合调度问题,提出了一种带修复操作的整型编码遗传算法(r-ICGA)。算法采用整数串的编码方式,有效减小了染色体的长度。同时引入一组新的修复操作来处理约束,将进化过程中产生的新个体修复成为可行个体。与罚函数约束处理方法相比,所提算法不引入惩罚项,避免了针对不可行解的经济负载分配子问题求解,节省了大量计算时间。将所提方法应用于六种不同规模的机组组合问题,仿真结果表明算法的搜索效率更高,求得的调度结果更好。随机组规模增大,算法所需执行时间近似线性地平缓增长,表明r-ICGA算法比其他方法更适合于求解大规模机组组合调度问题。     

Analysis of the Relation between Coupled Sink and Purification Based on Hydrogen Network Integration

In a hydrogen network, sinks and sources are generally connected to reactors or purifiers, which affect their compositions and flow rates. The relationship between these streams is studied based on the integration of the hydrogen network to identify the feasible and optimal operating conditions of reactors and purifiers. Equations are deduced to describe the quantitative relationship between hydrogen consumption, hydrogen concentration, flow rates of coupled sink and source, purification feed, and purified product. The purification and hydrogen‐consuming reactor parameters can be optimized in the design and operation stage of a hydrogen network. The case study proves that the proposed method is simple, easy to understand, and can be applied to identify the variation trend line and feasible region accurately without tedious calculation.     

Nonlinear optimization of steel production using traditional and novel blast furnace operation strategies

The high energy requirements in primary steelmaking make this industrial sector a major contributor to the global emissions of carbon dioxide. Ways to suppress the use of fossil reductants and the emissions from the processes should therefore be developed. The present work applies simulation and optimization for studying the economic feasibility of recycling blast furnace top gas to the combustion zones after CO₂ stripping. The study comprises the unit processes in an integrated steel plant, paying special attention to the blast furnace and the preheating of the blast or the recycled top gas. The system is optimized with nonlinear programming with respect to some central variables under different CO₂ sequestration and emission costs, which yields information about the economic feasibility of the concept. It is demonstrated that the optimal states of the plant show complex transitions, where the costs play a decisive role. It is also shown that hot gas recycling with CO₂ capture and storage would dramatically reduce the harmful emissions from the process. The conditions under which top gas recycling is economically feasible are also reported, as well as the effect of omitting oil injection in a blast furnace with top gas recycling.     

Comparison of process concepts for preparative chromatography

Preparative chromatography is a widely used unit operation in downstream processing. A major step for its implementation is the choice of the process concept. The most popular concept is batch chromatography (BC), which is easy to implement. But also the more complex simulated moving bed chromatography (SMBC) has reached a high significance over the last years. Between these two concepts, closed loop recycling (CLRC) and steady state recycling chromatography (SSRC) promise higher performance than BC and less complexity than SMBC. This paper will analyze the advantages and disadvantages of CLRC and SSRC compared to the both BC and SMBC.These four chromatographic concepts for different separation tasks are compared by dynamic process simulation. Therefore, six isotherms for two component systems are generated and three different feed compositions are regarded. For each case, all mentioned process concepts are optimized and compared with regard to performance. The results show that CLRC and SSRC can be a good choice, especially for difficult separations.     

Safety‐Considered Proactive Flare Minimization Strategy under Ethylene Plant Upsets

Ethylene plant upsets usually lead to flaring of off‐spec products, resulting in significant losses of raw material and energy as well as to air emission problems. Under the premise of plant safe operation, establishing process recycles connecting off‐spec streams to their upper‐stream process can help to reduce flaring during plant upsets. Operational strategies for recycling the potential flaring sources, i.e., effluents from the acetylene reactor and ethylene tower overhead, under various process upsets are developed and analyzed based on rigorous plant‐wide dynamic simulations. Safety considerations on the compressor system performance have been addressed by quantitative comparison of the effectiveness of various recycling strategies. Case studies demonstrate that the safety‐considered flare minimization strategies can proactively reduce the flaring emission amount and upset time and thus have great potentials of economical and environmental benefits to ethylene plants.     

Synthesis of optimal heat-induced separation networks

In this work we introduce the novel problem of synthesizing heat-induced separation networks (HISENs). The essence of HISEN synthesis is to reduce the concentration of a certain component in a number of rich (typically waste) streams from a set of supply compositions to a set of target compositions by using a number of heat-induced separators. A heat-induced separator is any indirect contact unit which employs an energy-separating agent (ESA) to affect separation via phase change. Examples of these systems include condensation, crystallization, vaporization and drying. By combining phase-equilibrium data with enthalpy balances, we develop pinch diagrams that can be used to determine the minimum operating cost of the system. There are two distinctive characteristics associated with the pinch diagrams for the HISENs. First, the location of the pinch point is not restricted to the supply temperatures of streams. Second, heat-induced separators may straddle the pinch point even for networks featuring the minimum cost of energy-separating agents. A systematic procedure is presented to address these characteristics and to synthesize a cost effective network of heat-induced separators. The problem is formulated as an optimization program. A solution procedure that is guaranteed to identify the global solution is devised to solve the problem. In addition, a slightly revised formulation is developed to incorporate the effect of stream bypass/mixing. Several case studies are tackled using the proposed procedures.     

Synthesis of refrigeration system based on generalized disjunctive programming model

Refrigeration system holds an important role in process industries. The optimal synthesis cannot only reduce the energy consumption, but also save the production costs. In this study, a general methodology is developed for the optimal design of refrigeration cycle and heat exchanger network (HEN) simultaneously. Taking the heat integration between the external heat sources/sinks and the refrigeration cycle into consideration, a superstructure with sub-coolers is developed. Through defining logical variables that indicate the relative temperature positions of refrigerant streams after sub-coolers, the synthesis is formulated as a Generalized Disjunctive Programming (GDP) problem based on LP transshipment model, with the target of minimizing the total compressor shaft work in the refrigeration system. The GDP model is then reformulated as a Mixed Integer Nonlinear Programming (MINLP) problem with the aid of binary variables and Big-M Constraint Method. The efficacy of the process synthesis model is demonstrated by a case study of ethylene refrigeration system. The result shows that the optimization can significantly reduce the exergy loss as well as the total compression shaft work.     

Study of masterbatch effect on miscibility and morphology in PET/HDPE blends

The present work studies the morphology in poly(ethylene-terephthalate)/polyethylene (PET/HDPE) polymer blends and its impact on blend properties. Mixing process in blend preparation is the important parameter for the type of obtained blend morphology and final blend properties, so two different mixing processes were used. In the first one, all components are mixed together while another one includes two step mixing procedure using two different types of masterbatch as compatibilizers for PET/HDPE system. Such blends can be considered in terms of PET polymer recycling in the presence of HDPE impurities in order to find suitable compatibilizers, which will enhance the interactions between these two polymers and represents the possible solution in recycling of heterogeneous polymer waste. The morphology of the studied PET/HDPE blends was inspected by scanning electron microscopy to examine the influence of the mixing process and various compositions on blends morphology, and interactions between PET and HDPE. The surface properties were characterized by contact angle measurements. The effect of the extrusion on the samples thermal behaviour was followed by DSC measurements. FTIR spectroscopy was used for the determination of interactions between blend constituents. It can be concluded that the type of mixing process and the carefully chosen compatibilizer are the important factors for obtaining the improved compatibility in PET/HDPE blends.     

Optimum design of integrated liquid recovery plants by variable population size genetic algorithm

Increase in the price of energy sources as well as economic problems have caused cryogenic natural gas plants to become more complex and efficient. After selecting the process configuration, the flow rate, pressure, and temperature of the process fluid streams are determining factors which should be tuned in order to find the optimum condition. Products specification and operating costs of the plant are two significant parameters which should be considered in an optimal design. Moreover, process design limitations contribute to the problem being more difficult. This paper shows how the optimal operating point in an integrated NGL recovery plant can be found through solving a complex constrained optimization problem. A Variable Population size Genetic Algorithm (VPGA) was used for optimization. As well, the role of VPGA algorithm parameters in solving the process design problems is investigated in this study. The analysis showed that the VPGA method has better performance compared to the general GA methods. The plant‐wide net profit increases 12493360 $/year only by changing the selected operating conditions to its optimal value.     

Application of response surface methodology for optimization of purge gas recycling to an industrial reactor for conversion of CO2 to methanol

Nowadays, by the increasing attention to environment and high rate of fuel production, recycling of purge gas as reactant to a reactor is highly considered. In this study, it is proposed that the purge gases of methanol production unit, which are approximately15.018 t·h~(-1) in the largest methanol production complexes in the world, can be recycled to the reactor and utilized for increasing the production rate. Purge gas streams contain 63% hydrogen,20% carbon monoxide and carbon dioxide as reactants and 17% nitrogen and methane as inert. The recycling effect of beneficial components on methanol production rate has been investigated in this study. Simulation results show that methanol production enhances by recycling just hydrogen, carbon dioxide and carbon monoxide which is an effective configuration among the others. It is named as Desired Recycle Configuration(DRC) in this study. The optimum fraction of returning purge gas is calculated via one dimensional modeling of process and Response Surface Methodology(RSM) is applied to maximize the methanol flow rate and minimize the carbon dioxide flow rate. Simulation results illustrate that methanol flow rate increases by 0.106% in DRC compared to Conventional Recycle Configuration(CRC) which therefore shows the superiority of applying DRC to CRC.