Exergetic and engineering analyses of sulphuric acid decomposition process

The purpose of this study was to evaluate the exergetic efficiency of the sulphuric acid decomposition process, which occurs in hydrogen producing thermochemical cycles and chemical energy storage systems. It is a process in which sulphuric acid is decomposed to a gaseous mixture consisting of water, sulphur dioxide and oxygen, using high temperature thermal energy, oxygen as a vector and mostly adiabatic equipment. Parts of the basic process with excessive exergy losses have been identified and a modified flow sheet has been developed and analyzed from thermodynamic and engineering points of view. Thermodynamic analysis of the modified flow sheet indicates that the overall exergetic efficiency of the decomposition process is 79.86%, which represents an improvement of 14.17% over the basic process. Engineering analysis of a plant producing 10⁶ mol of SO₂ per hour shows that the typical levelized cost of chemical exergy production was $ 2.25/GJ exergy from the basic process and $ 1.79/GJ exergy from the modified process.     

Performance Assessment of a Potato Crisp Frying Process

Frying is a common and popular cooking method, which has been widely used in food manufacturing, though it is a very energy-intensive process. Energy analysis has been commonly used to assess the performance of fryers. In this study, we attempted to exergetically assess the performance of a potato crisp frying system, which consists of three main components, a combustor, a heat exchanger, and a fryer. In the analysis, we utilized the actual operational data obtained from the literature. We determined exergy destruction in each system component and the whole system. We calculated universal and functional exergy efficiency values for the system components and compared them with each other. We also undertook a parametric study to investigate how the overall cycle performance was affected by changing the reference environment temperature and some operating conditions. We illustrated the exergy results through the Grassmann (exergy loss and flow) diagram. We calculated the universal exergetic efficiency values of 58, 82, and 77% for the combustor, heat exchanger, and fryer, respectively, with a universal exergetic efficiency value of 4% for the whole frying system. We found that the fryer had the highest functional exergetic efficiency value of 74%, followed by the heat exchanger with 47% and the combustor with 0.08%.     

Exergetic criteria in process optimisation and process synthesis: Opportunities and limitations

The exergy concept is a well established way to express the quality of any kind of process stream, be it energy or matter. Thus detailed exergy analysis of chemical processes can in general be performed and several such applications have been reported in the literature. However, systematic methods for exergetic process optimisation only exist for the comparably less complex area of power systems. For chemical processes a solid optimisation approach based on exergy analysis has yet to be derived. This paper presents a thorough investigation of opportunities and limitations of the exergy concept in both process optimisation and process synthesis. The two tasks are treated separately since their specific requirements and solution strategies are different. As a result of the presented evaluation, concepts for the efficient use of exergetic criteria are proposed.     

环周进汽型喷射器的火用效率分析

对环周进汽型喷射器的工作原理进行了深入分析,从可用能角度出发,建立了分析喷射器运行经济性的火用分析模型,计算得到了不同结构参数和运行参数下喷射器的火用效率,对喷射器火用效率的影响因素进行了分析,并将计算值与实验结果进行了对比。研究结果表明,喷射器的火用效率随入口水温、引射系数和升压比的升高而增大,随蒸汽压力的变化存在最小值,计算和实验得到的各参数对喷射器火用效率的影响规律基本一致,为环周进汽型喷射器的设计和应用提供了理论基础。     

Exergy analysis of desalination by solar-powered membrane distillation units

There has been an increasing interest in using exergy as a potential tool for analysis and performance evaluation of desalination processes where the optimal use of energy is considered an important issue. Unlike energy, exergy is consumed or destroyed due to irreversibilies in any real process and thus provides deeper insight into process analysis. Exergy analysis method was employed to evaluate the exergy efficiency of the “compact” and “large” solardriven MD desalination units. The exergy efficiency of the compact and large units with reference to the exergy collected by the solar collector was about 0.3% and 0.5% but was 0.01% and 0.05%, respectively, when referenced to the exergy of solar irradiance. The exergy efficiency of the flat plate solar collectors in both units varied diurnally and the maxima was 6.5% ad 3% for the compact and large units, respectively. The highest exergy destruction was found to occur within the membrane distillation module.     

Simulation and thermodynamic analysis of an integrated process with H2 membrane CPO reactor for pure H2 production

A one-dimensional steady-state heterogeneous model has been used to simulate the H₂ membrane reactor. The simulation work is the basis for the thermodynamic analysis of the integrated pure H₂ production process. The simulation and analysis also provide a quantitative tool for insight into and understanding the process.The simulation and thermodynamic analysis results indicate that increasing the inlet ratio H₂O/CH₄ cannot enhance the pure H₂ production rate. With increasing the inlet ratio H₂O/CH₄, the overall exergy efficiency of the process decreases, because a large amount of energy is required to obtain the steam.When the geometric parameters of membrane reactor and inlet temperature are given, there is a maximum feeding rate of methane for the integrated process. The pure hydrogen production rate increases with the inlet methane rate increasing, while the overall exergy efficiency decreases as inlet methane rate increases.For the same inlet rate of methane, operating the process at higher inlet temperature increases hydrogen production rate. Whereas, the overall exergy efficiency is lowered.Three suggestions are discussed to improve the overall exergy efficiency. All of them require more equipment investment. There will be an optimal point to balance equipment investment, pure hydrogen production rate and overall exergy efficiency. To find the optimum, thermo-economic analysis will be helpful.     

Unified methodology for thermal energy efficiency improvement: Application to Kraft process

A unified methodology that can be used to identify the interactions between the utilities systems and the process, as well as their impacts on the implementation of energy efficiency measures is presented. It takes into account steam and water systems to analyze the process and formulate energy enhancement measures. It has been applied to an operating Kraft mill in Eastern Canada. The methodology consists of five stages: base-case process definition and characterization, pre-benchmarking, systems interactions analysis, implementation strategy and post-benchmarking. A simulation focused on the energy and water systems is first developed and used as basis of the analysis. The pre-benchmarking characterizes the current energy efficiency of the process by three techniques: energy and exergy content indicators, comparison to the current industrial practice and establishing targets for minimum energy and water requirements determined by the Thermal Pinch and Water Pinch methods. The systems interactions are analyzed to develop complementary energy efficiency measures by applying several energy enhancing techniques. A three-phase strategy is proposed to implement the identified measures. The application of the unified methodology results in an eco-friendly process that does not require fossil fuel for steam production and generates revenues by producing green electricity from biomass. In the case study presented, very significant energy gains have been proposed (26.6% steam requirement reduction and 33.6% fresh water intake reduction).     

Exergetic analysis of atomization process of liquid

The exergy balance equation for liquid atomization is discussed and exergetic efficiency of atomization is defined. On the basis of data taken from the literature some calculations of this efficiency have been carried out for the most widespread cases of atomization, i.e., for pressure and pneumatic atomization. The efficiency is low and does not exceed 1%. Some recommendations about how to carry out the full analysis of all items of the exergetic balance have been made. The results of such an analysis would give some indication of possible improvements in atomizer design and operating parameters.     

Base case process development for energy efficiency improvement, application to a Kraft pulping mill. Part II: Benchmarking analysis

A new procedure for benchmarking analysis has been developed to evaluate the energy efficiency of a chemical process. Benchmarking is performed to identify process inefficiencies before developing energy enhancement measures. The new procedure combines typical techniques, such as the comparison with current practice, with utilization of new performance indicators based on exergy and energy content and the targeting by Pinch Analysis and Water Pinch. All process sections and the steam and water utility systems are evaluated. The procedure consists of five phases. In the first phase the data required is compiled. The second phase consists of comparing the energy and water efficiency of the base case to the current practice of the industry. In the third phase, the new energy and exergy content indicators are used to analyze the efficiency of utilities systems and to quantify the heat rejected by the process. In the fourth phase the minimum energy and water requirements are determined. The last phase is a synthesis by which the inefficiencies are identified and guidelines established for process improvement. Interactions between the utilities systems and the process are developed. The procedure has been applied to an operating Kraft pulping mill in Eastern Canada.     

Comparison of Exergy Losses for Reformers Involved in Hydrogen and Synthesis Gas Production

A user‐coded, customized methodology for exergy balance and analysis of chemical and thermal processes is developed with the Aspen HYSYS process simulator, to assess and explain exergy loss in steam methane reforming. Exergy analysis is presented for four configurations of primary and secondary reformers to produce synthesis gas: (1) an autothermal reformer (ATR) alone, (2) a top‐fired reformer (TFR) alone, (3) ATR‐TFR configured in parallel, and (4) ATR‐TFR configured in series. The same states and feed conditions, i.e., mass flow rate, temperature, pressure, and feed gas composition, are applied to the four reformer configurations considered, with the single ATR showing the lowest exergy loss of about 0.43 W kg^?1 of dry productivity.     

Waste Heat Integration of Coating Paper Machine Drying Process

The paper sheet drying process consumes about 70% of the total energy required in coated papermaking, and almost all the thermal energy used in the process can be found in the exhaust air; thus, it has significant potential to recover the heat. With the aim of saving energy, the recovered energy is usually used to heat different process streams instead of steam.

This article examines the drying process of an operating coating paper machine to demonstrate an optimization method. To study the possibility of improving energy efficiency, thermodynamic analysis was conducted. The reasons why there is so much heat lost during drying were investigated. Based on the results of the energy and exergy analysis, a new waste heat integration scheme is presented. Furthermore, the performance of the proposed scheme has been evaluated. The results of the case study show an energy efficiency improvement of 7.3% and a specific energy consumption reduction of 4.6% with profitable investments.     

Exergy analysis of two-stage steam-water jet injector

Exergy analysis is used as a tool to evaluate exergy losses in the steam-water jet injector so as to improve its overall performance. What this article addresses here is mainly about a parametric study on the injector under various operating conditions, such as different inlet water temperature, inlet steam pressure, pressure ratio, entrainment ratio and flowrate ratio. In addition, the irreversible losses in the component parts of the two-stage injector were analyzed in detail. The results show that the operating parameters have great effects on exergy efficiency of the injector. The average exergy efficiency of the two-stage injector is 21% more than that of the single-stage one. Moreover, calculations based on experimental data indicate that the highest exergy losses due to irreversibility occur in the first-stage mixing chamber. In light of this comparison, the exergy losses occurring in the system are proportional to the exergy efficiency obtained by applying the system.     

Exergy analysis of C2 recovery plants refrigeration cycles

This paper provides an exergy analysis of the multistage refrigeration cycle used for Ethane and heavier hydrocarbons (C₂⁺) recovery plant. The behavior of an industrial refrigeration cycle with propane refrigerant has been investigated by the exergy method. The equations of exergy destruction and exergetic efficiency for the main cycle components such as evaporators, condensers, compressors, and expansion valves are developed. The relations for the total exergy destruction in the cycle and the cycle exergetic efficiency are obtained. An ethane recovery unit with its refrigeration cycle has been simulated to prepare the exergy analysis. Using a typical actual work input value; the exergetic efficiency of the refrigeration cycle is determined to be 43.45% indicating a great potential for improvements. The simulation results reveal that the exergetic efficiencies of the heat exchanger and expansion sections get the lowest rank among the other compartments of refrigeration cycle. Refrigeration calculations have been carried out through the analysis of temperature-entropy (T-S) and pressure-enthalpy (P-H) diagrams where coefficient of performance (COP) was obtained as 1.87. The novelty of this article includes the effect and sensitivity analysis of pressure drop and temperature on the exergy efficiency and coefficient of performance of the cycle.     

气化剂配比对气化炉性能的影响

为了实现能源利用的可持续发展,多联产技术可以对能量进行合理的利用.多联产系统不同于化工或动力分产系统,对气化炉性能有更具体的要求.通过化学平衡和热量平衡方法求解气化炉平衡工作温度以及该温度下的出口煤气成分,研究了气化炉进口气化剂配比对出口煤气成分、冷煤气效率、热效率及火用效率的影响,指出热效率、火用效率最优情况下适应于各煤种的最优氧煤比以及合理的水蒸气耗量,为多联产系统的设计优化提供参考.     

Prorating methods for coupled production of power,fresh water and energy for district heating

The method of allocation of steam generating costs greatly influences the specific costs of heating steam and desalted sea water. This is illustrated by means of a practical example. From the point of view of social economics, the cost allocation method based on the cost of the heating energy generated by a single-purpose heat source yields sound results. The usual criterion for steam cost evaluation, based on business administration viewpoints, is the steam energy content. The advantages of the cost evaluation using the geometric mean of heat and exergy, in comparison with the calorific, the exergetic and the arithmetically averaging methods are shown.     

A dynamic model of the fuel processor for a residential PEM fuel cell energy system

A dynamic numerical model describing an experimental methane fuel processor for a residential PEMFC energy system is presented. In contrast to previous simulation studies of steam reforming of methane, this model includes the various energetic couplings due to spatial proximity and constructive layout of the components. Thus, all significant energy flows inside the system are taken into account, including those caused by unintended conduction and radiation. Steady-state simulations were carried out to investigate the effect of a constructional modification in the reformer, which clearly reveal the significant influence of thermal couplings on the reformer efficiency. Furthermore, dynamic simulations were performed for the start-up procedure of the fuel processor and compared to experimental data. The results demonstrate that the dynamic model is a useful tool for further investigations of unsteady operating conditions and for optimisation with respect to both construction and system operation.     

Comparative analysis of ethanol-steam and -autothermal reforming for hydrogen production using Aspen Plus

A comparative analysis of the ethanol reforming concerning steam and autothermal reformer was conducted to the evaluate parametric conditions for a hydrogen-rich product stream. The present simulation study includes a first attempt to report the optimal parametric conditions for ethanol-steam and -autothermal reformer. Various operating parameters, including temperature, pressure, steam-to-ethanol ratio, and oxygen-to-ethanol ratio, were considered in this analysis. The result illustrated that the hydrogen mole fraction increased with rising temperature in the steam reforming of ethanol, but it remained constant beyond reaction temperature of 750°C. On the other hand, as the pressure and the steam-to-ethanol ratio increased, the H₂ mole fraction decreased. Furthermore, with an enriched oxygen-to-ethanol ratio reactant stream, H₂ and CO contents in the product effluent were found to be reduced in the autothermal ethanol reforming. The results showed that an autothermal reforming strategy under optimized parameters (temperature of 600°C, steam-to-ethanol ratio of 5, and oxygen-to-ethanol ratio of 0.6) produced the maximum H₂ yield (3.78 kmol/h) per mole of ethanol. It was observed that introducing O₂ into the reformer helped reduce the amount of energy required for the steam reforming reaction. This study indicates that, although considerable work has been conducted on reforming catalysts development, simulation-based studies are still useful to help understand the overall process behaviour without undertaking laborious, high-cost involved time-consuming experiments.     

Production of ultrapure hydrogen from biomass gasification with air

An integrated process has been proposed for the production of ultrapure hydrogen from biomass gasification with air. The process consists of an air-blown bubbling fluidized bed gasifier, a steam reformer, and a water-gas-shift membrane reactor. A non-isothermal model has been developed to simulate the fluidized bed gasifier, and a one-dimensional model has also been developed to simulate the steam reformer. The simulation results are compared with the experimental data, and good agreement is obtained. Based on the simulation results, the thermodynamic analysis of the integrated process is carried out. The simulation and analysis provide a quantitative tool for gaining insight into the process.     

Simulation of primary and secondary reformers for improved energy performance of an ammonia plant

The energy consumption of ammonia plants based on steam reforming of naphtha or natural gas can be reduced by shifting the reforming load from the primary to the secondary reformer. It is shown that lowering of the primary reformer operating temperature 20 °C results in 2% increase of unconverted methane content in the outlet stream. The increased methane content can be processed in the secondary reformer if the process air inlet temperature is increased to 400 °C. The lower operating temperature reduces the energy consumption of the primary reformer by about 6% (and the overall consumption by about 2%) and also prolongs the service life of reformer tubes.