Optimal design of distillation column using three dimensional exergy analysis curves

This paper presents two main contributions. Firstly, a new exergy graphical method is proposed for optimal design of distillation column with minimum exergy lost. The method is applicable to both grass-root and retrofit cases, respectively. The effect of design and operating parameters of a distillation column on the exergy lost is graphically visualized by three dimensional exergy analysis curves. The curve shows the correlations between exergy lost, design and operating parameters of a distillation column. This technique can be used as an effective method to reduce the simulation effort to search for the optimum design and operating parameters of a distillation column at minimum exergy lost. Besides, visualization also enhances the engineers’ understanding of the column performance. The other contribution is a four-level idealization concept, which is based on three dimensional graphical exergy analysis curves. The concept defines the effect of transport rate and configuration on exergy lost of distillation column. The effectiveness of the method has been demonstrated on a xylene column, which suggested that an implementation of feed pre-heater yields a significant reduction in exergy lost by up to 15.5%.     

Exergy analysis of two cryogenic air separation processes

Two process designs of a cryogenic ASU (air separation unit) have been evaluated using exergy analysis. The ASU is part of an IGCC (integrated gasification combined cycle); it is supplying oxygen and nitrogen to the gasifier and nitrogen to the gas turbine. The two process designs separate the same feed into products with the same specifications. They differ in the number of distillation columns that are used; either two or three. Addition of the third column reduced the exergy destruction in the distillation section with 31%. Overall, the three-column design destroyed 12% less exergy than the two-column design. The rational exergy efficiency is defined as the desired exergy change divided by the total exergy change; it is 38% for the three-column design and 35% for the two-column design. Almost half of the exergy destruction is located in compressor after-coolers. Using this heat of compression elsewhere in the IGCC can be an important way to increase the IGCC efficiency. It is proposed to use it for the pre-heating of ASU products or for the production of steam, which can be used as part of the steam turbine cycle.     

Exergy analysis proves viability of process modifications

Stork Comprimo has developed a commercially available exergy analysis program. With this program several exergy analyses were carried out. Most recently an exergy analysis of a reaction and distillation section within a refinery has been performed. In the reaction section endothermic reactions take place after which the product stream is cooled in a heat exchanger network whereafter the reaction products are separated in a distillation section. From the exergy analysis it can easily be notified that the main part of the losses occurs in the furnaces and distillation columns. A closer look at one of the distillation columns with the largest exergy losses shows that the main part of the losses occurs in the reboiler heated with a furnace. To reduce the exergy losses in this distillation column, Stork Comprimo proposed several process modifications. These modifications are: (1) decreasing operating pressure (lower operating temperature), (2) HP steam reboiling instead of a furnace, (3) splitting feed streams, and (4) recompressing overhead. With these process modifications total exergy losses may be reduced by 70% that directly results in a primary fuel reduction of almost 40% for this column! Splitting of the feed streams has been implemented now and results in an energy saving of 10% and a more stable operation of the column.     

The need for exergy analysis and thermodynamic optimization in aircraft development

This paper outlines a newly emerging body of work that relies on exergy analysis and thermodynamic optimization in the design of energy systems for modern aircraft. Exergy analysis establishes the theoretical performance limit. The minimization of exergy destruction brings the design as closely as permissible to the theoretical limit. The system architecture springs out of this constrained optimization principle. A key problem is the extraction of maximum exergy from a hot gaseous stream that is gradually cooled and eventually discharged into the ambient. The optimal configuration consists of a heat transfer surface with a temperature that decays exponentially in the flow direction. This configuration can be achieved in a counterflow heat exchanger with an optimal imbalance of flow capacity rates. The same optimal configuration emerges when the surface is minimized subject to specified exergy extraction rate. Similar opportunities for optimally matching components and streams exist in considerably more complex systems for power and refrigeration. They deserve to be pursued, and can be approached first at the conceptual level, based on exergy analysis and thermodynamic optimization. The application of such principles in aircraft energy system design also sheds light on the “constructal” design principle that generates all the systems that use powered flight, engineered and natural, cf. constructal theory.     

Simulation,exergy analysis and application of diabatic distillation to a tertiary amyl methyl ether production unit of a crude oil refinery

This paper presents the results of a detailed exergy analysis of a tertiary amyl methyl ether (TAME) unit of a crude oil refinery and the application of diabatic distillation to the depentanizer tower of the unit. Diabatic distillation is a separation process in which heat is not only supplied to the reboiler and extracted from the condenser [as in a conventional (adiabatic) distillation column], but is also transferred inside the column. The process enables operation to approach equilibrium conditions, thus reducing exergy losses and increasing exergy effectiveness. In a TAME unit of a refinery, isoamylenes are converted to TAME. Before transforming the isoamylenes in the reactors, it is necessary to recover them from a catalytic gasoline stream by a depentanization process. The exergy losses of this depentanization process represent about 70% of the total exergy losses of the unit. The results of the exergy analysis of the TAME unit are presented and a detailed exergy analysis of the conventional adiabatic depentanizer column is conducted for comparison purposes. Then, the application of diabatic distillation to the system is evaluated by using cooling water circulating in series from tray to tray in the rectification section and by making the steam emanating from the reboiler circulate in series from tray to tray in the stripping section. The results in terms of the reduction of exergy losses, heating and cooling media flow rates, and cost effectiveness of the diabatic option for the depentanizer section of the plant are compared to the original adiabatic system, and the effect of the diabatization on the overall exergy performance parameters of the depentanizer section and on the whole TAME unit, are presented in this paper.     

660 MW燃煤发电机组的碳捕集系统余热利用和集成系统性能评估

碳捕集与封存（CCS）技术能有效捕获燃煤电厂排放的CO₂但再生能耗大且效率低。为提高燃煤电厂能源利用效率,提出集成有机朗肯循环（ORC）与CCS的太阳能-燃煤发电系统,利用热力学、火用和经济性分析模型对集成系统进行参数敏感性分析。基于外部燃料火用矩阵模型,分析再沸器所需热量中CO₂压缩过程和太阳能集热器的热量占比及集成ORC系统对外部燃料火用贡献度的影响。研究表明：当热源比θ=0.4时的集成系统热经济性能最优且具有较合理的不可逆性;集成ORC系统后锅炉燃煤火用、一、二次再热燃煤火用对系统产品的贡献度均有所提高;随着θ增加,锅炉燃煤火用和一、二次再热燃煤火用对碳捕集系统产品的贡献度逐渐降低;压缩余热火用和太阳能火用的贡献度逐渐增加。     

A real column design exergy optimization of a cryogenic air separation unit

Distillation columns are one of the main methods used for separating air components. Their inconvenient is their high energy consumption. The distillation process is simulated in three types of columns and the exergy losses in the different parts calculated. A sensitivity analysis is realized in order to optimize the geometric and the operational parameters of each type of column. A comparative exergy analysis between the distillation columns considered for cryogenic air separation shows that the exergy efficiency of a double diabatic column, with heat transfer all through the length of the column, is 23% higher than that of the conventional adiabatic double columns.     

传热传质过程和设备的有限时间热力学优化

1前言能源的短缺与高效利用问题是当代世界各国面临的重大社会问题之一。随着世界人口和经济的迅速增长,能源的消耗急剧地增加,并导致能源的严重短缺。因此,研究充分、高效利用现有能源的方式成为我们目前的重要任务。蒸馏作为全球范围内耗能量最大的工业,引起了很多工程界和物     

Exergy analysis of cryogenic air separation

An exergy analysis is performed to analyse the possibilities of fuel saving in the cryogenic distillation process, which is the main method of air separation. It is shown that more than half of the exergy loss takes place in the liquefaction unit and almost one-third in the air compression unit. Minor exergy losses are taking place in the distillation unit and the main heat exchanger. The major cause of exergy loss is the use of compressors and to a lesser extent the use of turbines. Especially, the relatively low rational efficiency of the turbines operating in the cryogenic region is striking. Improvements are suggested which save one-fourth of the exergy loss. For more substantial reductions of the exergy losses in air separation alternative processes have to be used or developed.     

Exergy analysis on combustion and energy conversion processes

When we consider exergy analysis on combustion and thermodynamic processes, we introduce another concept against energy analysis, which is supported by an evaluation of its temperature level. When a higher temperature energy than that an ambient level is taken into consideration, it can be put for some domestic or industrial purpose. A medium temperature energy of 30–60 °C is used for domestic heating, and a high temperature of 200 °C and above is suitable for power generation or process heating. Therefore, we study exergy concept supported by temperature level. When we discuss power generation, a high temperature energy of 1500 °C and above in combined cycle has a higher conversion efficiency than that of 500–600 °C in steam cycle. If we try to apply high temperature air combustion, a preheated air temperature of 1000 °C and above can be produced by exhaust heat recovery from stack gas, which has been developed as a new technology of energy conservation. In this study, the authors present an exergy analysis on combustion and energy conversion processes, which is based on the above-mentioned concept of exergy and energy supported by temperature level. When we discuss high temperature air combustion in furnace, this process shows a higher performance than that of the ambient air combustion. Furthermore, when we discuss the power generation and heat pump processes, the minimum ambient temperature would already be known for each season, and the conversion performance can be estimated by the maximum operating temperature in their cycles. So, the authors attempt to calculate the exergy and energy values for combustion, power generation and heat pump processes.     

Fundamentals and applications of cryogen as a thermal energy carrier: A critical assessment

This paper reviews and assesses the current status of the use of cryogen as an energy carrier, with a focus on the thermodynamic aspects and cryogenic energy extraction. Cryogen as an energy carrier is different from normal heat storage media in that the energy storage in a cryogen occurs through decreasing its internal energy while increasing its exergy. It is shown that cryogens have a higher energy density than other commonly used thermal energy storage media, and cryogen can be efficient working media for recovering low grade heat due to their low critical temperatures. If there are high grade heat sources, a combination of the direct expansion with a Brayton cycle is shown to be the most efficient method to extract the cryogenic exergy for most cryogens. This, however, is not true for hydrogen as its latent heat accounts for only a small portion of the released cold and a simple Brayton cycle is more suitable for the exergy recovery. If there is only ambient and/or a low grade heat source, a combination of direct expansion and a Rankine cycle is more attractive due to its low power consumption in the compression process, and this appears to be more promising when carbon dioxide capture is considered.     

热力循环的特性函数

在热力循环理论中引入了"热流体"。热流体由温度、焓和熵等核心热力学参数来描述,证明了热力循环新函数-工质实际作功能力函数的存在性和回路作功能力原理的正确性,在热力学基本定律和不可逆多能级热力循环理论之间建立了新的联系。给出了新的热力循环特性函数的解析式,并对公式中各项的物理意义进行了详细阐述。将跨能级独立蒸汽冷却器当作辅助循环来处理,无需另外推导公式。工质实际作功能力等同于机械功,其价格恒等于功的价格,用工质实际作功能力概念来建立新的总能系统统一性评价体系,可避开方法中的定价难题。     

A study of industrial steam process heating through exergy analysis

This study investigates using exergy analysis the technical factors that influence the feasibility of substituting steam supplied for other energy sources in industrial heating. Some alternative configurations for the steam‐supply system capable of broadening the range of industries able to use the steam for heating are proposed. When examining the feasibility of substituting steam for other energy currencies for providing process heat, exergy analysis quantitatively determines the increase in process efficiency when a lower value energy currency such as steam is used in place of a higher value energy currency such as electricity. Many industries can benefit from using steam for some or all of their heating requirements. An illustrative example for the Bruce Energy Center in Ontario, Canada is presented to demonstrate the importance of using exergy analysis to assess the feasibility of industrial steam process heating. Some alternate reconfigurations of the Center are considered to supply steam at a variety of thermodynamic states, and better match the steam‐state requirements of many industries. The results suggest that exergy analysis should be used as the central tool in process optimization when the use of large quantities of the steam in energy centers is contemplated. Copyright © 2004 John Wiley & Sons, Ltd.     

Thermodynamic and exergo-economic assessments of a new geothermally driven multigeneration plant

In this paper, a new geothermal-based multigeneration system is designed and investigated in both thermodynamic and economic analyses. The reason to select the geothermal source is that geothermal power is a renewable and sustainable power resource, and also it is not weather dependent. The proposed geothermal-based multigeneration plant is able to produce power, heating, cooling, swimming pool heating, and hydrogen. The main idea in this renewable-based multigeneration system is to create valuable products by using waste heat of subsystems. Then, by applying thermodynamic analyses, the energy and exergy performances of proposed multigeneration system are computed. Also, parametric work has been performed in order to see the impacts of the reference temperature, geothermal fluid temperature, and geothermal water mass flow rate. Finally, exergo-economic analysis based on exergy destruction or thermodynamic losses is done to gain more information about the system and to evaluate it better. According to the calculations, the overall plant's energy and exergy performances are 32.28% and 25.39%. Economic analysis indicates that hydrogen production cost can be dropped down to 1.06 $/kg H₂.     

Energy,exergy and economic analysis of industrial boilers

In this paper, the useful concept of energy and exergy utilization is analyzed, and applied to the boiler system. Energy and exergy flows in a boiler have been shown in this paper. The energy and exergy efficiencies have been determined as well. In a boiler, the energy and exergy efficiencies are found to be 72.46% and 24.89%, respectively. A boiler energy and exergy efficiencies are compared with others work as well. It has been found that the combustion chamber is the major contributor for exergy destruction followed by heat exchanger of a boiler system. Furthermore, several energy saving measures such as use of variable speed drive in boiler's fan energy savings and heat recovery from flue gas are applied in reducing a boiler energy use. It has been found that the payback period is about 1 yr for heat recovery from a boiler flue gas. The payback period for using VSD with 19 kW motor found to be economically viable for energy savings in a boiler fan.     

Investigation of potential of improvement of helical coils based on avoidable and unavoidable exergy destruction concepts

An inevitable problem challenges heat exchanger designers is that the heat transfer augmentation in a thermal system is always achieved at the expense of an increase in pressure loss. Thus, the trade-off by choosing the most proper configuration and best flow condition has become the critical problem for design work. The brief survey on literature shows that optimal Reynolds number of laminar forced convection in a helical tube, was specified based on minimum entropy generation. Therefore, the present study analyzes the thermodynamic potential of improvement for steady, laminar, fully developed, forced convection in a helical coiled tube subjected to uniform wall temperature based on the concept of avoidable and unavoidable exergy destruction. The influence of various parameters such as coil curvature ratio, dimensionless inlet temperature difference, dimensionless passage length of the coil, and fluid properties on avoidable exergy destruction have been investigated for water as working fluid. Results show considerable potential of thermodynamic optimization of helical coil tubes. In addition, a relation for determining the amount of optimum Dean Number is proposed for the range considered in the present study.     

Exergoeconomic assessment of a biomass-based hydrogen,electricity and freshwater production cycle combined with an electrolyzer,steam turbine and a thermal desalination process

Since biomass resources can be found with different contents in most regions of the world, biomass/gasification (Biog) coupling processes can be considered as an attractive and useful technology for integrating in polygeneration configurations. In this regard, a new polygeneration energy configuration based on Biog process is proposed and its conceptual analysis is presented. In the new energy process, a Rankine cycle, a water electrolysis cycle (based on solid oxide electrolyzer, SOE), and a multi-effect desalination (MED) unit are embedded to generate electricity, hydrogen fuel, and freshwater, respectively. The considered polygeneration configuration is comprehensively investigated and discussed utilizing a parametric evaluation and from thermodynamic, energetic and exergoeconomic points of view. Relying on the proposed system can provide a new approach to produce carbon-free hydrogen fuel and freshwater in order to achieve an efficient, modern and green polygeneration configuration. The results indicated that the electrical power generated by the considered polygeneration configuration is close to 1735 kW. In addition, the system is capable of producing almost 9880 kg/h of freshwater and 12.3 kg/h of hydrogen. In such a context, the energy efficiency and total products unit exergy cost were 36.4% and 16.6 USD/GJ, respectively. Also, the system could achieve an exergy efficiency of nearly 17.1%. Moreover, about 8.9 MW of process's exergy is destroyed. The performance of the proposed polygeneration configuration using four different biomass fuels is compared. It was determined that the total products unit exergy costs under paddy husk and paper biomass are approximately 14.8% and 8.6% higher than MSW, respectively.     

Second law analysis of coupled conduction–radiation heat transfer with phase change

This work considers an exergy-based analysis of two-dimensional solid-liquid phase change processes in a square cavity enclosure. The phase change material (PCM) concerns a semi-transparent absorbing, emitting and anisotropically scattering medium with constant thermodynamic properties. The enthalpy-based energy equation is solved numerically using computational fluid dynamics. Once the energy equation is solved, local exergy loss due to heat conduction and radiative heat transfer during the phase change process is calculated by post processing procedures. In this work, the radiation exergy loss in the medium and at the enclosure boundary is taken into consideration. It is found that radiation exergy loss is significant in the high-temperature phase change process. Parametric investigation is also carried out to study the effects of Stefan number, Biot number, Planck number, single scattering albedo and wall emissivity on exergy loss. The results show that the total exergy loss increases with Biot number, single scattering albedo and wall emissivity. The second law effects of the conduction–radiation coupling in the energy equation are also shown in this work.     

A combined thermodynamic cycle used for waste heat recovery of internal combustion engine

In this paper, we present a steady-state experiment, energy balance and exergy analysis of exhaust gas in order to improve the recovery of the waste heat of an internal combustion engine (ICE). Considering the different characteristics of the waste heat of exhaust gas, cooling water, and lubricant, a combined thermodynamic cycle for waste heat recovery of ICE is proposed. This combined thermodynamic cycle consists of two cycles: the organic Rankine cycle (ORC), for recovering the waste heat of lubricant and high-temperature exhaust gas, and the Kalina cycle, for recovering the waste heat of low-temperature cooling water. Based on Peng–Robinson (PR) equation of state (EOS), the thermodynamic parameters in the high-temperature ORC were calculated and determined via an in-house computer program. Suitable working fluids used in high-temperature ORC are proposed and the performance of this combined thermodynamic cycle is analyzed. Compared with the traditional cycle configuration, more waste heat can be recovered by the combined cycle introduced in this paper.     

Thermodynamical analysis of human thermal comfort

Traditional methods of human thermal comfort analysis are based on the first law of thermodynamics. These methods use an energy balance of the human body to determine heat transfer between the body and its environment. By contrast, the second law of thermodynamics introduces the useful concept of exergy. It enables the determination of the exergy consumption within the human body dependent on human and environmental factors. Human body exergy consumption varies with the combination of environmental (room) conditions. This process is related to human thermal comfort in connection with temperature, heat, and mass transfer. In this paper a thermodynamic analysis of human heat and mass transfer based on the 2nd law of thermodynamics in presented. It is shown that the human body's exergy consumption in relation to selected human parameters exhibits a minimal value at certain combinations of environmental parameters. The expected thermal sensation also shows that there is a correlation between exergy consumption and thermal sensation. Thus, our analysis represents an improvement in human thermal modelling and gives more information about the environmental impact on expected human thermal sensation.