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CO2 removal in raw natural gas is required to meet the sale gas specifications. The chemical solvent absorption method is one of the most commonly used technologies. The absorption-based processes, however, require an enormous amount of energy to regenerate the solvent, and are designed to discharge the captured CO2 into the atmosphere directly. The energy efficiency and greenhouse gas emissions should be considered in the gas treatment process. In this study, a new process, combining CO2 removal and compression, is proposed for raw natural gas treatment complexes. Compared with the conventional absorption-based processes, in the new process CO2 is not stripped from rich solvent in one distillation column but through multi-stage flash separators operating under different pressures and temperatures. The CO2 removed from each stage is then compressed separately to the proper pressure for carbon transportation and storage. A mathematical model is then developed to determine the optimum process configuration and operating conditions. A surrogate-based optimization framework is presented in order to strike a good balance between computational efficiency and model accuracy. The complex units are replaced by Kriging surrogate models. Aspen HYSYS is used to generated sampling points for the construction of surrogates. A method of dimension reduction is introduced to compact the surrogate models by selecting essential variables. Finally, a hybrid optimization model comprising of the surrogate models for “complex” units and explicit models for the rest of the process is formulated. The cases show the total annual cost of the CO2 removal and compression can be reduced by 20.6%–28.8% for different sweet gas purity specifications.

Large amounts of low-grade energy in natural gas power plants are not being efficiently recovered or used. They are often emitted directly to the environment. It is quite challenging to retrofit na...

Extractive distillation (ED) processes are widely used for the separation of aromatic and non-aromatic hydrocarbons. Approximate boiling points of components and azeotropes in the mixture need solvents to aid the separation processes. A considerable mass flowrate and recovery of solvents in separation processes leads to significant energy requirement. This study provides a new extractive distillation process for the efficient separation of aromatic and non-aromatic hydrocarbons aided by sulfolane solution. A flash tank and a semi-lean solution stream are introduced to modify existing extractive distillation processes to reduce the reboiler heat duty of the entrainer recovery column and improve the separation performance of the extractive distillation column. The NRTL-RK property method and rigorous unit models in Aspen Plus are used to simulate new and existing processes. No-databank model parameters are regressed to improve the accuracy of simulations. A coordinative strategy is proposed to optimize the significant operating parameters for new and existing processes by combining Aspen Plus with MATLAB. Compared with the optimal existing extractive distillation process, the new extractive distillation process reduces the operating costs by 8.9% when heat integration is not considered, and 13.04% of the total annual cost can be reduced when a heat exchanger network is considered.

Natural gas industrial parks are composed of raw natural gas refining plants, urea plants and power plants. The processes of natural gas refining and utilization in industrial parks are energy-intensive and involve complicated mass and energy networks. This study concentrates on heat and material stream integration for sustainable energy utilization in natural gas industrial parks. Energy integration of industrial parks is reviewed and a mixed integer non-linear programming framework is presented to minimize the total annual cost for the retrofitting of energy integration across plants in natural gas industrial parks. The mathematical framework combines the optimization models of energy-intensive processes, power plant operation and a new heat exchanger network design using some key variables. Total site analysis is proposed for a natural gas industrial park to obtain energy-intensive processes with energy saving potential and determine the process streams for energy integration across plants. The mathematical framework is then applied to the natural gas industrial park and can be solved using the deterministic global optimization solver in a reasonable solution time. The results demonstrate that the static investment payback period is less than two months and the total hot utility decreases by 30.5% through the energy retrofit of the entire natural gas industrial park.

In this work, a novel graphical method is presented to quantitatively identify the optimal feed stage locations of double-feed columns on the basis of the column grand composite curve (CGCC). A double-feed column can be composed of two simple single-feed columns under minimum thermodynamic conditions according to previous research. Then, two material and enthalpy balance envelopes are regulated according to the feed locations of the simple column from the top down. Subsequently the whole simple column is regarded as entire-rectifying and entire-stripping sections to generate the theoretical temperature-enthalpy curves, which are termed as entire-rectifying and entire-stripping curves. Finally, the entire-rectifying and stripping curves of the two simple columns can be combined into three theoretical curves, namely overall entire rectifying, intermediate, and stripping curves. Then, a quantitative approach is proposed to determine the appropriate feed locations of the double-feed columns. By investigating the relationship between the feed points of the CGCC and the intersections of the three theoretical curves, the optimal feed locations can be identified when the enthalpy differences obtained from the CGCC and the theoretical curves reach minimum at each stage. It is found that the appropriate feed stages of a double-feed column are positions which are closest to the intersection points. As a case study, a binary double-feed column is simulated to demonstrate the accuracy and rationality of the graphical method. By optimizing feed locations, the cold and hot utility requirements of the column were reduced by 43.9% and 46.9%, respectively.

Sulfolane Extractive Distillation (ED) processes are widely used for benzene, toluene and xylenes (BTX) separation. Based on the phase equilibrium data and NRTL property method, this paper develops an ED process aided by a co-solvent mixture of sulfolane, N-methyl-2-pyrrolidinone (NMP) and water to improve the recovery of aromatics and reduce the minimum hot utility requirement (Ht,min). Firstly, the relative volatility of aromatics and non-aromatics are calculated and analyzed with the help of MATLAB and the results show that co-solvents have higher selectivity than NMP and higher dissolvability than sulfolane. In addition, a compressive simulation model and optimizing strategy for the new process are presented sequentially. In order to improve the accuracy of modelling, all of the missing parameters in the NRTL database are regressed from literature data or estimated using the Aspen Property Estimator. In particular, considering multi-variables and their interaction, a co-ordinative optimization strategy is proposed based on iterative optimization of local coupling parameters by taking Ht,min as the objective function. Meanwhile the energy consumption and separation efficiency are investigated through adjusting the key operating parameters of the ED process. The results show that the energy consumption of the ED process is significantly reduced and the recovery of aromatics increased from 99.69% to 99.92%.

We report a multi-objective stochastic mixed-integer non-linear programming (MINLP) framework for sustainable retrofit and capability expansion of traditional energy systems in petrochemical complexes. Multiple uncertainties including energy demands, solar radiation and wind speeds are considered in the optimization framework, these are characterized by historical data or normal distributions which are pre-defined with assumed mean values and standard variations. A stochastic reduced order model approach is introduced to describe the uncertainties by a small number of scenarios and their individual probabilities. The optimization framework further accounts for system configuration selection and sizing of the candidate energy conversion equipment, such as thermal storage units, gas turbines, boilers, steam turbines, as well as their operating capacities in each time period. A case study is investigated to demonstrate the performance of the proposed strategy and then the optimization results under three modes (deterministic, stochastic and semi-stochastic programs) are compared.

In a refining complex, an absorption-stabilization process used in the production of end-use petro-products (i.e. stable gasoline and liquefied petroleum gas) is energy-intensive and costly. A new absorption-stabilization process with a two-stage condensation section is introduced in this work to further improve energy-use performance. In the new process, a condenser, a condensed oil tank, and a side-reboiler are integrated into the original process and then a heat integration scheme is performed. Compared with the existing process, the proposed process can reduce the cold utility and hot utility by 17.98% and 25.65%, respectively, as well as decrease the total annual operating costs of the heat exchanger network by 17.48%. Additionally, the process retrofit reduces the annual operating costs of cooling water and steam by about $346,617 at the expense of capital costs around $487,006, and the corresponding payback period is approximately 17 months.

A chemical or petrochemical site is generally made up of several plants that are linked together through process streams. The linking process streams are often cooled down in their source plants, then transferred into storage tanks, and reheated in destination plants. This repeatedly cooling and heating results in low energy-use efficiency and more area installed in heat exchanger network. In this study, we introduce a heat exchanger network superstructure based on stage-wise model for heat integration using hot direct discharges/feeds between plants, and develop a new mixed-integer nonlinear optimization model to simultaneously design heat exchanger network. Unlike conventional HEN design, the model can simultaneously synthesize heat exchanger networks for multiple plants, and be able to address variable supply or target temperatures of process streams. The objective is to minimize total annual cost of heat exchanger networks in source and destination plants. Three examples are used to demonstrate the performance of the proposed model and solution approach. The computational results indicate that the simultaneous design of heat exchanger network for heat integration using hot direct discharges/feeds between plants achieves a significant decrease in total annual cost when compared to the separate design of heat exchanger networks for source and destination plants.

Optimal operational strategy and planning of a raw natural gas refining complex (RNGRC) is very challenging since it involves highly nonlinear processes, complex thermodynamics, blending, and utility systems. In this article, we first propose a superstructure integrating a utility system for the RNGRC, involving multiple gas feedstocks, and different product specifications. Then, we develop a large-scale nonconvex mixed-integer nonlinear programming (MINLP) optimization model. The model incorporates rigorous process models for input and output relations based on fundamentals of thermodynamics and unit operations and accurate models for utility systems. To reduce the noncovex items in the proposed MINLP model, equivalent reformulation techniques are introduced. Finally, the reformulated nonconvex MINLP model is solved to global optimality using state of the art deterministic global optimization approaches. The computational results demonstrate that a significant profit increase is achieved using the proposed approach compared to that from the real operation. © 2016 American Institute of Chemical Engineers AIChE J, 63: 652–668, 2017

A multi-stage activated carbon impregnation system, with a special emphasis on solvent reuse, is introduced and optimized in this study. Experiments and modeling optimization are combined to simultaneously minimize the solute consumption and operating time for the system. The adsorption isotherm and kinetic models are well regressed based on a series of experiments on activated carbon impregnated in triethylene tetramine solution. A multi-objective nonlinear programming optimization framework is then formulated based on the regressed isotherm and kinetic models and material balance, including activated carbon, solute, water and impurities at each stage in the system. The AUGMECON method and global solver ANTIGONE are used to solve the challenging optimization problem, and the pareto optimal solutions are obtained. Additionally, a new dimensionless number is presented to further coordinate the operating parameters for the system. The result shows that the specific solute consumption and operating time are decreased by 65.11% and 85.88%, respectively.

Industrial parks are competitive because of the concentrated utilization of resource and energy. In industrial parks, power plants are energy hubs to supply power and various levels of steam to other enterprises. However, it is still challenging to centralized produce and supply cold energy in industrial parks. We present a process superstructure for the retrofit of power plants to centralized produce and supply cold energy for industrial parks. The energy sources for refrigeration include cold energy from liquefied natural gas, pressure energy of natural gas and low-pressure steam in power plants. A mixed integer nonlinear programming model is formulated to optimize the process superstructure to minimize the total annual cost. The model is solved using the global solver BARON in GAMS environment. The solution results, total annual cost and different price of electricity are analyzed and compared. The best alternative is a hybrid central refrigeration system that includes different energy sources for chilling, and the power consumption of cold energy is reduced by 36%.

BTXs (benzene, toluene and xylenes), used widely as basic chemical intermediates, are separated generally from petroleum naphtha, coking naphtha, and pyrolysis gasoline. These oil fractions contain aromatic and non-aromatic hydrocarbons between C5 to C8 in which certain components have approximate boiling points and form azeotropes. Extractive distillation (ED) is one of the most attractive approaches for separating azeotropic mixtures with approximate boiling points. The aromatic ED technique aided by sulfolane used widely in recent years because of its simple process flowsheet, small equipment investment and high energy efficiency. The solvent and operating parameters are the cores of ED process in the economical design. Some results were published to rapidly screen the solvents as well as process parameter optimization for ED separating process, but few contributions have been reported to optimize an extractive distillation from the view of simultaneous optimization of the solvent composition and operation parameters for process economics. In this work, we propose a systematic optimization approach for energy saving of sulfolane aromatic ED processes based on process simulation and genetic algorithm (GA) optimization. The NRTL property method and rigorous unit models in Aspen Plus are used to simulate the ED processes. In particular, considering multi-variable and their interaction, a coordinating strategy is proposed to optimize the solvent composition and significant operating parameters for the ED processes by combining Aspen Plus with MATLAB. Under given separation specifications, the energy consumption and separation efficiency are investigated and optimized by GA optimization. The results show that the through the above optimization, the minimum heating utility of the ED process is significantly reduced by 29.2%.

Natural gas has emerged as a promising energy source due to its high quality and massive reserves. Operational optimization of natural gas networks including gas treating, blending, compressing, long-distance transmission, supplying and power generation are quite challenging since it contains highly nonlinear processes, complex thermodynamics, and a set of partial differential equations (PDE). In this article, we first introduce a multi-period optimization framework for the large scale natural gas network. Then, we develop a nonconvex mixed-integer nonlinear programming (MINLP) model aiming for better economic performance for the network. Various levels of market price and demand for electricity are considered. The MINLP framework integrates the models of CO2 removing processes, compressors, pipeline transmission and combined-cycle gas and steam turbines. The optimization framework and mathematical model are applied to a case study extracted from an industrial park at South China. Economy and energy performances are investigated and an optimum operating strategy is drawn out. The practicability of the optimization framework and mathematical model makes it more possible to apply to real world problems.

The refining and petrochemical industries generally own process plants and utility systems. Process plants are configured to finish the transformation and separation of materials, and utility systems supply the energy requirements for the process plants. Therefore, integrating two of them is more favorable than optimizing them individually. A coupling mixed integer nonlinear programming model is presented in this work to integrate process plants and utility systems; the objective is to minimize the energy costs to meet the requirements of the process operations and to maintain a steam balance in the total site. The mathematical model includes three parts: the heat integration of the process plants, the optimization of the utility system, and the coupling equations for the site-scale steam integration. The heat integration of the process plant is formulated on the basis of pinch analysis involving heat loads of the process heaters and steam generation and requirements. An optimization of the utility system...

This paper designs an entrainer combined with a sidedraw to enhance the reactive distillation (RD) process of isopropyl acetate (IPAc). Acetic acid (HAc) reacts with isopropanol (IPOH) to generate IPAc and water (H2O). The ratio of IPAc to H2O in the products of esterification is smaller than that in the minimum boiling IPAc–IPOH–H2O azeotrope, resulting in a mass of organic phase reflux to remove the surplus H2O from the top of the RD column. This process consumes a high amount of energy. For better energy efficiency, a feasible design flowsheet includes an RD column, a stripper, a top decanter, a middle decanter, and a sidedraw stream to intensify the azeotropic separation where an entrainer is introduced to carry out the surplus water from the middle of the RD column in the form of a liquid phase. The key design variables in the proposed flowsheet are determined to obtain a minimal total annual cost (TAC). As a result, an optimal process design is drawn out while satisfying the stringent specifications for product purity. These results show that the energy requirements of the IPAc system can be decreased by 27.55%.

North Xinjiang belongs to an alpine region with high altitude. Almost no Chinese jujube was grown 10 years ago in this severely-cold area. However, scientists have developed cultivars of Chinese jujube suitable for this region. As a consequence, the Chinese jujube cultivation increased very fast in recent years and high quality fresh jujube was successfully produced there. The present article reported the growing history and current situation of Chinese jujube industry in Xinjiang, and the practical cultivation techniques for Chinese jujube in North Xinjiang.

A sharp increase in worldwide energy requirements has forced people to exploit novel energy conservation technologies and new alternative energies. Heat integration, as a method of saving energy, i...

A new indicator to represent a maximum capacity expansion rate of distillation processes is presented in this article. A hydraulics-based heuristic strategy consisting of three levels is proposed to address the capacity expansion retrofit redesign of distillation systems. The first level is to evaluate an existing process to get the indicator which reflects quantitatively the capacity expansion rate, where a fractional utilization of area (FUA) method is also employed to screen the process bottlenecks. In the second level, the indicator is used to find the available debottlenecking measures from a measure library which is based on heuristics. In the last level, a new economy indicator called specific annual cost (SAC) is used to sort the available debottlenecking measures, and the corresponding process flowsheets and operation parameters are determined. An industrial capacity expansion retrofit for a light-ends separation plant is conducted so as to evaluate the performance of the proposed indicators and strategy in screening the available retrofit alternatives. Modifications on feeding location, operating pressure, number of theoretical stages, and feeding thermal condition, etc. are proposed to remove the bottlenecks. The maximum capacity expansion rate of the plant is about 40.0%, and the near-optimum retrofit options for different capacity expansion rates such as 20%, 25%, 30%, 35% and 40% are also presented. The results show that the proposed capacity expansion strategy is capable to resolve the issues associated with the retrofit redesign in a systematic and relatively simple way.

Some independent plants making up a chemical or petrochemical site are linked by process streams. Linking process streams are, in general, cooled in up-plants and reheated in down-plants to satisfy process requirements. These streams, even more, travel from up-plants to storage tanks and, then, to down-plants, which results in low energy-use efficiency. Up-plants with multiple hot discharges and down-plants with multiple hot feeds are proposed in this paper, and a T–Q graphic method is presented to target the temperature of multiple hot discharges/feeds between plants. The T–Q diagram is composed of a grand composite curve (GCC) and a composite curve that only involves process streams employed for hot discharge/feed. A mixed integer linear programming (MILP) model is formulated to minimize the total hot and cold utilities of up- and down-plants and to solve the temperature of hot discharge/feed and the heat transfer between plants. Some examples are utilized to demonstrate the performance of the presented...

A multi-period nonlinear programming (NLP) formulation is presented in this paper. This is intended to optimize energy and materials simultaneously for diesel hydrotreating units in order to minimize the total costs which include product income, material cost, energy cost, and other fixed operating costs. The heat exchanger network (HEN) is integrated into the model to address the energy consumption changes with the stream flow and temperature of different production modes. Some process models are regressed from production data or are supplied by refinery engineers, similar to the models for steam turbines and compressors, and distillation towers. An iterative algorithm is developed to solve the complex mathematical model based on HEN simulation. An industrial diesel hydrotreating unit is provided by the Chinese Refinery Complex and is used to demonstrate the performance of the formulated model and algorithm. It is found that the optimized results from the simultaneous consideration of the energy and materials are more profitable than those obtained from other methods that individually optimized the energy or materials.

The electronic structure and effect of vanadium doping of BN nanowires are studied by first principles calculations. For the pure nanowires, it can be found that B atoms move inwards whereas N atoms move outwards, and BN nanowires have a constant band gap about 4.08 eV with larger diameter. The above-mentioned features are in agreement with those of BN nanotubes. We also find that the pure nanowires become more and more stable with increasing diameter. For V-doped BN nanowires, the V atoms move outwards, and the total energies of pair V-doped BN nanowires indicate that the ferromagnetic ground state, and the electronic structures show half-metallicity. The majority of total spin magnetic moments originate from V atoms, and B atoms which near dopant have a little contribution, while N atoms provide a little reverse magnetic moment. This study may be useful in spintronics and nanomagnets.

An absorption and stabilization system is an important process, which is used to produce the industrial raw materials such as dry gas, liquefied petroleum gas, and stable gasoline. However, the existing absorption and stabilization process is heavily energy intensive with low economic benefits. In the existing process, double feeding with different temperatures and identical composition of the desorption tower results in an axial back-mixing behaviour, which weakens efficiencies of the mass and heat transfer of the desorption tower. To solve the problem, a new process with two-stage condensation section is presented in this work. The new process can eliminate the axial back-mixing and improve efficiency of energy-use. Compared with the existing process, the new process improves the separation efficiency of the absorption tower, as well as decreases its cold utility and hot utility by 19.67 % and 24.47 %, respectively.

Crude oil distillation unit (CDU) is significant in refinery, and retrofitting a crude oil distillation unit is more common than building a new one as consideration of the enormous investment cost. The process simulation system PRO/II is used to build a rigorous model for the crude oil distillation unit based on the conventional two-furnace and three-column process flow in refinery that addresses both the distillation columns and the heat exchanger network (HEN) simultaneously. The effect of the differences of crude oil properties on the product yield, pumparounds for heat removals, the design of heat exchanger networks and CO2 emissions in crude distillation units is focused. Furthermore, energy use improvement for crude oil distillation units with consideration of light and heavy crude oil is proposed. Based on the investigation carried out on the existing CDU, the optimum operating conditions and retrofitting solutions have been obtained by two actual crude oil distillation units with three columns and corresponding HEN, effectiveness of the retrofits is finally evaluated by means of thermodynamics and economic analysis. The results show that the retrofits required a fixed capital cost of $102925 and $177215 and saving operation cost is about 355226$/year and 656181$/year. The payback time is approximately0.29 and 0.27 year for the retrofits and optimization of light crude oil distillation process and heavy crude oil distillation process.

A phenomenological equation of exergy transfer, which indicates the relation between exergy flux, exergy resistance and exergy driving forces, is derived by applying non-equilibrium thermodynamics to second law analysis, and simultaneously the expressions of the coefficients of exergy transfer are obtained. The results show that exergy transfers in different forms interact on each other. This study also proposes a simplified expression of exergy transfer coefficients by neglecting some minor engineering couplings in exergy transfers. Furthermore, a mixture consisting of two components is discussed. As an application of this result, thermal exergy transfer within a flat plate is studied.

This paper presents the energy and exergy analyses for a delayed coking unit in a Chinese refinery using the three-link energy structural model. According to the results of energy-use analysis and evaluation, the potentials of energy-use and improvements for energy-savings as well as detail procedures are suggested. The proposed improvements are expected to remarkably lower the energy consumption and to improve the economic profits of the delayed coking unit. Compared to the original unit, the energy consumption of the improved flowsheet decreased by 37.2%, which demonstrates that the applied strategies are suitable for the energy-use analysis and improvement of process systems.

The control volume method has been widely used to describe the rate of exergy destruction due to process irreversibility in a laminar fluid flow stream. In this work, we demonstrate that when the approach is applied to turbulent flows, and the fluid properties are based on averaged values, some information may be lost. We have applied the Reynolds time average method to the set of equations of exergy transfer for turbulent flows and found that the exergy destruction is mainly due to two factors, i.e. the mean flow dissipations and the fluctuating motion dissipations. As a case study, this result is used to model the incompressible turbulent flows through a duct with constant heat flux. The distributions of exergy destruction due to mean flow and fluctuating motion viscous dissipations as well as to mean flow and fluctuating motion heat transfer irreversibility in radial and in axis directions are obtained. The numerical solution of an example with water passing through the duct is obtained and the mechanisms of irreversibility analyzed. It is shown that for a given fluid, the total exergy destruction per unit length is a function of the geometrical parameters and the boundary conditions as well as the Reynolds number. It implies that the design of a thermal system can be optimized through minimizing the exergy destruction in the system.

This report reviews the development of the theoretical framework for exergoeconomics, and proposes a time and space-dependent exergy cost transfer equation. The exergy cost transfer equation, together with the exergy transfer equation, constitutes the theoretical fundament for exergy analysis, exergoeconomic analysis and optimization. With these transfer equations, three theorems regarding to exergoeconomics are therefore derived, expounded and proved, which could be utilized to solve problems in estimating exergy, exergy fluxes and exergy destruction costing.

A heuristic approach is proposed for the 3-staged two-dimensional cutting stock problem with usable leftover, where the unused rectangular segments can be returned to stock for future use. The approach is based on the recursive technique combined with beam search. A forward recursion is used to generate normal segments that consist of homogenous strips, and a beam search heuristic is used to obtain the 3-staged cutting patterns considering usable leftover. The computational results show that the algorithm is effective in reducing material cost.

As is known to all of us, mechanical drawing is a professional course for students majoring in mechanical. In the teaching process, we find it effectively improve students' ability of space imagination if we adopt special teaching methods such as using a variety of teaching methods, developing teaching strategy according to teaching goal, a combination of traditional teaching mode and modern multimedia method. In that case, the teaching process is optimized and the teaching quality is improved. KEYWORD: Teaching strategy; Traditional; Multimedia; Optimization

Based on a three-link model, this paper presents a multi-objective retrofit optimization algorithm that simultaneously takes into account capacity expansions and uses of energy. This approach was successfully applied to retrofit an aromatic fractionation unit. Compared to the original system, the capacity of the retrofitted system increased by 28% and the energy consumption decreased by 81.4%. The proposed retrofit method is suitable for the simultaneous optimization of expansion and energy integration in process systems.

Aiming at the difficulty of maintaining the contour precision of forging die with deep pocket, an optimization algorithm of tool-path generation for High Speed Machining (abbr. HSM) forging die with deep cavity is proposed in this paper. In terms of measuring errors of pocketing die, a mathematical model correlation to the length of a tool-path, the available length of a cutting tool and the profile errors are characterized. After the calculation of the profile errors that vary with the length of the tool-path, the helix shaped tool-path can be created. Moreover, the optimized tool-path based on the algorithm for machining die with deep cavity is also generated by using the smooth curve as transition tool-path. Depending on the Mikron Duro800 HSM Machine Tool, a precision cavity of forging die for shaping internal-star-wheel parts can be machined. The experimental results indicate that the profile precision based on the optimum algorithm has been improved by IT 1?2.

Field emission characteristics of carbon nanotubes (CNTs) were investigated. CNTs were produced by the catalytic decomposition of ethylene on nickel films, which were electroplated onto Mo tips. The SEM images showed that CNTs were quite uniform and disordered; the average diameter is about 100 nm. The measurements were carried out in a diode configuration. As a result, the CNTs showed typical voltage–current characteristics, which qualitatively followed the conventional Fowler–Nordheim equation. In conclusion, CNTs are excellent field emission cathode materials with stable and reproducible field emission characteristics and a lower onset field, which is up to 0.9 V/ μ m.

The optimization of energy-intensive process systems usually depends upon both economics and environment, which often give the proofs for the decision-making for process improvement. Based on the evolutions of exergy flows and the waste flows within an energy-conversion system, a waste treatment subsystem is proposed in this paper, which could be added to treat the wastes rejected from the system. Accordingly, the total system is divided into the main energy-conversion subsystem and the affiliated waste treatment subsystem. Moreover, the environmental-exergoeconomic objectives for optimizing two subsystems and the corresponding constraints were presented by taking account of the thermodynamics, economics and environment simultaneously. A decomposing-coordinating strategy for environmental-exergoeconomic optimization of energy-conversion systems is developed. Coordination between the optimal results from two subsystems makes the total system optimized.