Chemical Exergy Research Articles

Numerical investigation of performance trade-off characteristics of a packed bed dehumidifier using aqueous blends of lithium chloride and calcium chloride

Current research in dehumidification systems is mainly focused on thermally driven liquid desiccant systems. However, the selection of proper desiccant solution has a greater significance on the performance of a dehumidifier. In this perspective, current work is presented with governing equations of energy, mass and species balance to study the performance of a dehumidifier. The governing equations are solved using finite difference method coupled with Jacobi’s model for an air–desiccant contact system of a cross-flow packed bed adiabatic dehumidifier operating with different desiccant blends of lithium chloride (LiCl) and calcium chloride (CaCl2) solution. The effect of variation in blend proportion on outlet parameters of a packed bed dehumidifier is patterned under various desiccant to air (L/G) mass flow rate ratios. It is observed that the addition of CaCl2 desiccant solution into LiCl changes the nature of thermo-physical properties of blends‚ which are predicted using non-random two liquid (NRTL) equation. Further, a trade-off analysis among the performance parameters is presented. It demonstrates that dehumidifier performance paradox such as enthalpy effectiveness (ɛh), moisture removal rate (MRR) and moisture effectiveness (ɛm) have a tendency to be optimal for 35% LiCl with 5% CaCl2 blend (BL1) among all blends at L/G ratio of 2. The variation of thermodynamic properties (air temperature, humidity, desiccant temperature and desiccant concentration) inside the dehumidifier module is visualized in terms of contour plots. Furthermore, the irreversibility in heat and mass transfer operation for the optimal blend is investigated in terms of physical and chemical exergy destruction paradigm. The result of such investigation elucidated that the maximum chemical exergy destruction is 0.88 kW, whereas the physical exergy destruction is 0.06 kW.

CO2 methanation process synthesis by superstructure optimization

A superstructure optimization approach to power-to-methane process design that includes heat integration is presented. Carbon dioxide from biogas plants is considered as carbon source. The superstructure includes 13 alternative process technologies in seven layers for power-to-methane processes at their current stage of development. For different scenarios, the most efficient process in terms of product chemical exergy and the most cost-effective process in terms of capital and total annual costs are identified. We consider indirect heat integration via utilities, which for all scenarios, is determined to be a main contributor to both energy efficiency and process cost. The product methane must meet the requirements for feed into the gas grid. The requirements for the gas grid have a direct influence on the most efficient process route. The number of necessary process units is reduced, if hydrogen can be fed to the gas grid. Furthermore, extend of the heat exchanger network determines the trade-off between efficiency and costs, rather than choice of unit operations.

Comparison of Optimization of Exergy Efficiency of a Crude Distillation Unit Using Artificial Neural Network (ANN) and Response Surface Methods (RSM)

The study carried out simulation of the Crude Distillation Unit (CDU) of the New Port Harcourt Refinery (NPHR) and performed exergy analysis of the Refinery. The Crude Distillation Unit (CDU) of the New Port Harcourt refinery was simulated using HYSYS (2006.5).
 The Atmospheric Distillation Unit (ADU) which is the most inefficient unit and where major separation of the crude occurs was focused on. The simulation result was exported to Microsoft Excel Spreadsheet for exergy analysis. The ADU was optimized using statistical method and Artificial Neural Network. Box-Behnken model was applied to the sensitive operating variables that were identified. The statistical analysis of the RSM was carried out using Design Expert (6.0). Matlab software was used for the Artificial Neural Network. All the operating variables were combined to give the best optimum operating conditions.
 Exergy efficiency of the ADU was 51.9% and 52.4% when chemical exergy was included and excluded respectively. The optimum operating conditions from statistical optimization (RSM) are 586.1 K for liquid inlet temperature, 595.5 kPa for liquid inlet pressure and condenser pressure of 124 kPa with exergy efficiency of 69.6% which is 33.0% increment as compared to the base case.
 For the ANN optimization, the exergy efficiency of the ADU was estimated to be 70.6%. This gave an increase of 34.9% as compared to the base case.
 This study concluded that enormous improvement can be achieved both in design feasibility and improved efficiency if the feed operating parameters and other sensitive parameters are carefully chosen. Furthermore, ANN optimization gave better exergy efficiency of 70.6% than RSM optimization of 69.6%.

Energy and exergy analyses of hydrogen production process with aluminum and water chemical reaction

Hydrogen is recognized as an energy carrier and has substantial potential to fulfill future energy demands. This paper conducted energy and exergy analyses on a hydrogen production process that utilized the chemical reaction between aluminum and water. Various aspects of the system’s performance, such as energetic efficiency, exergetic efficiency, sustainability index, and other methods to evaluate the system’s performance were examined. The factors such as the lower heating value (LHV) of hydrogen, the reactant molecules’ enthalpy of formation, and their respective chemical exergy were considered. The energy input from the heater was substantial enough to be taken into consideration; however, the energy input from the pump was considered negligible in the overall process efficiency. The standard reaction efficiency equation displayed the most favourable results at the higher water temperatures. Therefore, the variation in the water medium and NaOH concentration experiments used 70 °C as its water temperature. The variation in water medium test identified tap water as an effective water medium due to its higher energy efficiency, and its ability to achieve the maximum theoretical yield. However, seawater was identified as a viable option for future testing, as it was able to achieve standard energetic and exergetic efficiencies up to 32.40% and 54.35% for the 3-g samples. Additional parametric studies were conducted and compared throughout this study to provide a comprehensive analysis of the system’s performance.

Adaptive neuro fuzzy predictive models of agricultural biomass standard entropy and chemical exergy based on principal component analysis

In order to effectively utilize energy of agricultural biomass, there is a need to evaluate energy potential. For such a purpose, chemical exergy and standard entropy of typical agricultural biomass were examined analytically. Element compositions of the exergy and entropy were acquired for further statistical evaluation. Adaptive neuro fuzzy inference system (ANFIS) was used as the statistical methodology for data analyzing. ANFIS is an efficient estimation model among machine learning techniques. The main weakness of the ANFIS is its dimensionality problem with large inputs. Therefore, the main goal in this study was to estimate the parameters’ influence on the chemical exergy and standard entropy prediction in order to reduce the number of inputs. Principal component analysis was used for presentation of the obtained ANFIS predictive models. Obtained results have shown the best predictive performances for standard entropy based on hydrogen as composite element of the agricultural biomass. Exergy prediction was the best for oxygen as composite element of the agricultural biomass. ANFIS coefficient of determination for standard entropy prediction based on hydrogen is 0.9832 and for chemical exergy prediction is 0.919. The results show the high predictive accuracy of ANFIS models.

Exergy assessment of combustion and EGR and load effects in DI diesel engine using comprehensive two-zone modeling

A model extended to include exergy terms is presented in this investigation, providing more useful results. It complements the energy analysis, based on in-house, validated, advanced, 2-zone combustion model, predicting performance and emissions in direct injection (DI) naturally-aspirated (N/A) diesel engine, operating at two loads with or without exhaust gas recirculation (EGR), with implementation on test results from a diesel obtained by the authors. The exergy terms of each of the two zones (unburned and burned) are identified and computed discretely, while charge chemical exergy is considered. The accurate account of temperature and chemical species histories in the burned zone and the mass entrainment from the unburned zone can lead to a more precise evaluation of the exergy terms of the whole cylinder content, against a single-zone combustion modeling, hence revealing the influence of zoning (presenting also entropy diagrams), which is important if irreversibility is computed from exergy balance. The effect of load is scrutinized and the investigation proceeds using various EGR rates, by keeping constant fueling rate. History diagrams of rate and cumulative exergy terms, for the total cylinder charge and each zone discretely, supply detailed information for the chemical exergy, irreversibility and losses.

A novel approach for optimal energy recovery using pressure retarded osmosis technology: Chemical exergy pinch analysis – Case study in a sugar mill plant

In the present study, pinch analysis is extended taking chemical exergy concept into account. The novel chemical exergy pinch analysis is proposed for sustainable power production by an economic application of pressure retarded osmosis membranes in chemical industries. Chemical exergy composite curves and chemical exergy cascade tables are developed as graphical and numerical tools, respectively. The tools are used to obtain maximum waste energy recovery by achieving various targets and determining the pinch point in a salinity gradient network. Thus, maximum energy recovery and minimum waste treatment are targeted, simultaneously. Moreover, a mathematical model follows the chemical exergy pinch analysis for an economic evaluation of pressure retarded osmosis-retrofitted industries under three probable scenarios. A sugar mill plant is simulated as the case study to validate the model-based analysis. The results showed that chemical exergy pinch analysis could efficiently provide the optimal pressure retarded osmosis -retrofitted industrial networks for decision-making. Having analysed the complex chemical exergy streams by chemical exergy pinch analysis, 11.30 MW net power is recovered with 0.038 $/kWh levelized cost of energy in the case study.

Second law thermodynamic analysis of syngas premixed flames

The exergy loss in laminar premixed flames of syngas is investigated numerically by analyzing the local entropy generation. The effects of the change in the H2 molar ratio in H2-CO mixture at different temperature values (from 300 to 600 K), at atmospheric and high pressure (10 and 50 atm) conditions and for different equivalence ratios (0.5, 1 and 2) are studied identifying both the mainly responsible process for exergy loss and the factors that can positively affect the performance of the syngas combustion. It is found that the chemical reactions are not always the main contribution to the total entropy generation (i.e. to exergy loss) and that the roles of the chemical reactions and heat conduction are strongly affected by pressure. Furthermore, it is underlined that the hydrogen content in syngas worsens the exergy loss due to entropy generation by the combustion process, whereas pressure increase has positive effects. However, these beneficial effects are counterbalanced by an increase of the chemical exergy content in the flue gases (mostly due to unreacted species or incomplete combustion). Therefore, a careful selection of the combustion process conditions is required to meet efficiency constraints if exhaust exergy is not meant to be afterward exploited.

Experimental assessment of heat exchangers with nested helical coils for an absorption heat transformer

ABSTRACT The studies of nested helical coil heat exchangers for an absorption heat transformer are very limited, although this arrangement has a greater heat transfer area in a smaller volume. The heat exchangers are vertically placed to develop the desorption, condensation, evaporation and absorption processes by falling film mode. The usage of energy from point of qualitative view in these times is mandatory, due to the fossil fuels reduction and pollution. This experimental study analyses the behavior the nested helical coils heat exchangers through the first and second law of thermodynamics. The physical and chemical exergy for the main components and whole system is calculated by using the conventional method. All the experimental tests are carried out by varying the source temperature from 343.15 to 358.15 K with heating mass flow rate in the generator between 0.07 and 0.13 kg/s. For this study, the condensation and absorption processes reach the highest exergy destruction at high temperatures, while the solution heat exchanger presents the lowest irreversibility. The exergy coefficient of performance always improves when the supplied heat in the generator increases owing to the heating mass flow rate and temperature, however there is an increment considerably of total irreversibility at 349.15 K.

Dimethoxymethane Production via Catalytic Hydrogenation of Carbon Monoxide in Methanol Media

Dimethoxymethane (OME1) is an important chemical and has vast applications. However, the current commercial method to produce this chemical suffers high exergy loss. In this study, we demonstrate a...

Exergy loss analysis on diesel methanol dual fuel engine under different operating parameters

In order to reveal the mechanism of high thermal efficiency of diesel methanol dual fuel (DMDF) engine, the exergy loss of DMDF engine under different operating parameters is investigated by using experimental and calculative methods based on the second law of thermodynamics. It has been found that DMDF has lower heat transfer exergy loss under all engine loads due to the high latent heat of vaporization of methanol which reduces the temperature in-cylinder. The exhaust exergy loss of DMDF under high load is relatively low compared with that of pure diesel fuel, while it is opposite to that under medium-low load. This is due to the influence of different temperatures on the incomplete combustion of methanol, which also indicates the main cause for the high thermal efficiency of DMDF under high load. Then studies on exergy analysis of DMDF under different temperatures were carried out. The studies covers the effect of intake air temperature on exergy efficiency, the heat transfer exergy loss and the exhaust physical exergy loss increase with rising intake air temperature as well. The results shows that the exhaust chemical exergy loss decreases significantly with the increase of intake air temperature and higher intake air temperature helps to improve exergy efficiency. Additionally, it is concluded that higher methanol temperature and cooling water temperature can improve exergy efficiency by decreasing exhaust exergy loss and combustion irreversible exergy loss. Finally, some feasible measures are proposed to further improve DMDF engine’s performance.

Reinventing exergy as indicator for resource depletion impacts in LCA

While resource aspects are gaining increasing importance for the sustainability assessment of new technologies, the question of how to assess the depletion of abiotic resources is still controversially discussed. Different methodologies exist for their quantification within life cycle assessment (LCA). Among them, thermodynamic approaches have the advantage of considering aspects of absolute quantity (reserves or amount of a substance contained in total in earth’s crust) and of quality (concentration of the target element in the mined resource), making them a potentially appealing approach for assessing resource depletion. However, existing approaches are either far from the original thermodynamic idea of exergy or far too complex and not applicable for resource accounting. This work briefly discusses the suitability of exergy-based approaches for resource assessment, and then suggests a simple but comprehensive methodology for quantifying resource depletion related with the concept of chemical concentration exergy (MDPces). It provides a calculation approach for quantifying the MDPces and estimates the corresponding values for some representative key metals.

Exergy-based fault detection on the Tennessee Eastman process

The exergy-based fault detection method has not yet been applied to a complex industrial system that adequately represents a dynamically changing process. One such system, the Tennessee Eastman process, is commonly used as a benchmark for fault detection methods. In this paper, an exergy-based fault detection approach is applied to the Tennessee Eastman process. This is done to investigate the feasibility of using this approach when confronted with noisy sensor data and control loops masking faulty behaviour. An exergy characterisation was performed on stream data obtained from the Tennessee Eastman process. The exergy characterisation included a new approach to calculate the standard chemical exergy of unknown components. For fault detection, threshold limits were determined for the exergy characterisation when normal operating conditions are assumed. The threshold limits were calculated following the upper and lower control limit determination of the Shewhart control chart. The results showed that this method could quantify both the physical state as well as the chemical features of the process and that 17 out of the 20 considered faults could be detected. This shows that the exergy-based method could be adequately applied to the Tennessee Eastman process.

Evaluating the Exergetic Performance of the Amine Treatment Unit in a Latin-American Refinery.

Recently, exergy analysis has attracted great attention of the scientific community as an attractive tool for evaluating energetic efficiency of any process. In this work, the simulation of the amine treatment unit in a Latin-American refinery was performed in order to apply the exergy analysis tool to identify alternatives of improvement. The industrial amine treatment unit was simulated using Aspen plus software, which provided extended energy and mass balances. To calculate irreversibilities of the process and global exergy efficiencies per stages, the general methodological procedure of exergy analysis was used. To this end, physical and chemical exergies were found for compounds involved within the process. The values estimated for total irreversibilities, exergy of utilities, and exergy of wastes in the treatment of the sulfur-rich amine allowed us to analyze the stages that require reductions in waste generation and utility consumption. For a processing capacity of 72.08 t/h of rich amine, results revealed that the overall exergy efficiency was 83.81% and the total irreversibility was 1.69 × 105 MJ/h, where 23.6% corresponds to the total exergy by residues (3.98 × 104 MJ/h). The novel strategy to use exergy analysis for process optimization proved to be useful to detect critical stages and prioritize actions to improve.

A novel atomic contribution model for the standard chemical exergies of organic compounds

The goal of this study was to propose, a general, yet simple and accurate model for the estimation of the standard molar chemical exergies of various organic compounds, often used in the industries. With this goal in mind, for the first time in literature, the atomic contribution approach was considered, and the feasibility of adopting such a simple method to calculate chemical exergies was investigated. A large data bank, consisting of 4129 organic compounds from wide ranges of families, was gathered from the literature, and divided into training and test datasets. Upon investigating various functionalities, and optimizing using the training databank, two atomic contribution models were proposed to calculate the standard enthalpy and entropy of formation, from which, the standard molar chemical exergy of a compound is calculated. The AARD% value for estimating the standard molar chemical exergy of the entire database, consisting of both the training and testing datasets, was 0.64%, which shows not only the feasibility of applying the atomic contribution approach for the calculation of this property, but also high accuracy as compared to more tedious literature models. Since the proposed models require knowledge of only the chemical formula and the normal boiling point of the desired compound, it is indeed in line with our purpose of simplicity and generality, and can easily be incorporated into computer codes. Considering that it is also global and quite accurate, this model even has great potential to be used in energy and chemical-related software.

Exergy analysis of propane dehydrogenation in a fluidized bed reactor: Experiment and MP-PIC simulation

Exergy can be used to evaluate energy utilization in chemical industry. Although the analysis of the energy consumption in the propane dehydrogenation (PDH) process is of great importance, there is hardly any study of energy or exergy analysis on this topic so far. Thus, the present study focusses on exergy analysis of the PDH process in a fluidized bed reactor (FBR) with both experimental and numerical methods. It is found that the operating temperature and weight hourly space velocity (WHSV) have a great influence on the exergy efficiency of each gas component and the total exergy efficiency. As the temperature increases from 830 K to 890 K, the conversion of propane increases and the total exergy efficiency of product gases increases from 99.06% to 99.66%. As the WHSV increases from 0.40 h−1 to 1.00 h−1, the conversion of propane decreases and the total exergy efficiency of products gases decreases from 99.38% to 98.88%. Also, numerical predictions based on Multi-Phase Particle-in-Cell (MP-PIC) methodology for the PDH fluidized bed reactor are found to be in a very good agreement with experimental data. Based on the numerical predictions of the flow field, temperature field and gas species concentration distributions, detailed distributions of chemical and physical exergies and total exergy in the fluidized bed reactor are quantified. This has made possible to examine the effects of operating conditions on the exergy efficiency and to better understand physical and chemical phenomena occurring in the FBR. Moreover, the quantitative exergy analysis and exergy distributions in the FBR makes it possible to confidently decide on the right operating conditions and guide efforts to reduce sources of inefficiency in the existing PDH designs and evaluate their economics.

An exergy analysis methodology for internal combustion engines using a multi-zone simulation of dual fuel low temperature combustion

Increasing fuel efficiency of internal combustion (IC) engines is an imperative economic and environmental need. To this end, in the present work, an exergy analysis methodology was developed and incorporated within a previously validated, closed cycle, multi-zone, thermodynamic simulation of diesel-methane dual fuel low temperature combustion (LTC). To study in-cylinder exergy transformation, the diesel start of injection (SOI) was varied from 300 crank angle degrees (CAD) to 340 CAD for a fixed engine load 6 bar brake mean effective pressure (BMEP) and speed (1700 rev/min). Gross indicated fuel conversion efficiency (iFCE) and exergetic efficiency (including exhaust exergy at exhaust valve opening) were computed for different SOIs. Results show that at all SOIs ~41–42 percent of the total input exergy was destroyed due to combustion irreversibilities. When combustion started, in-cylinder chemical exergy reduced significantly due to exergy transfers associated with work transfer and heat transfer as well as exergy destruction. While at SOI = 340 CAD (representative of conventional dual fuel combustion), ~14 percent of the fuel chemical exergy exited as exhaust physical exergy, this reduced to ~5.5 percent at SOI = 300 CAD (representative of dual fuel LTC). The “maximum efficiency SOI” was SOI = 310 CAD with an iFCE of 43.6 percent and an exergetic efficiency of 52.2 percent.

Effect of varying fuel types on oxy‐combustion performance

In search for clean energy solutions in a global warming era, oxy-fuel combustion systems are promising. In the study, combustion products are calculated, and exergy analysis is done using the proposed multifeature equilibrium combustion model. And the results obtained for oxy-combustion of different fuels at various oxygen fractions are given in comparison with conventional combustion. For validation, the model results are compared with popular combustion calculation tools, GASEQ and CEA. Effect of oxygen content on oxy-combustion exergy analysis is calculated, also considering changes in equivalence ratio and combustion chamber inlet temperature. Moreover, indicating parameters for combustion performance, temperature ratio, chemical exergy, physical exergy, total specific exergy, and exergy destruction are utilized in the calculations elaborately. Changes in combustion product mole fractions are explained for rich and lean combustion regions. And also, specific exergy results are presented. In terms of exergy destruction, oxy-combustion is more advantageous than conventional combustion. It has been shown that exergy destruction in combustion process with conventional air is approximately 1.5 times higher compared with 21% oxy-combustion, both at different equivalence ratios and at different combustion chamber inlet temperatures. Nowadays, environment-friendly, clean energy production systems are growing in numbers. In this concept, exergetic analyses of combustion for different fuels and greener natural gas, compared with diesel, gasoline, and methanol, are given in comparison. Considering four fuel types, advantageous and disadvantageous cases are presented for oxy-combustion at different oxygen fractions and conventional combustion. As a result, diesel fuel is more advantageous than the other three fuel types, in terms of temperature ratio and exergy. Natural gas combustion appears to be disadvantageous in terms of specific exergy and temperature ratio, but it is the most advantageous in terms of exergy destruction. Consequently, distinctive comparison is done for oxy-combustion and conventional combustion, determining positive and negative effects for different fuels.

Optimization of a DI diesel engine to reduce emission and boost power by exergy and NLPQL method

AbstractThe design and manufacture of efficient diesel engines, which can convert the majority of fuel combustion into a mechanical rotation movement is still growing. Simultaneous investigation of optimum engine configuration and the second law of thermodynamics consideration can best achieve this goal. A baseline 1.8 Ford diesel engine is simulated via AVL Fire software and then the multiobjective nonlinear programming by quadratic Lagrangian method have been applied to find the global optimal design. Then for exergy analysis of the baseline and optimized engine characteristics, a FORTRAN developed code is used. Two basic optimization cases are detected for exergy assessment that are RunID41 (feasible design) and RunID35 (infeasible design) among 55 run points. It was determined that the optimum feasible solution should characterize with low inner wall diameter (Di) with wider half spray cone angle to avoid wall collision in order to stay in the constraint zone and feasible area. The results of exergy also indicates that RunID41 is fuel efficient since the fuel burnt exergy of three design of ID41, ID35, and baseline are 719.52, 694.77, and 643.07 J, respectively. It has also been pointed out that the feasible optimum gives the higher thermo‐mechanical exergy, work exergy, and total exergy, while the infeasible optimum yields a higher heat release rate exergy and chemical exergy as there are no restrictions of injection and combustion chamber geometry set by the designer.

Compression–Expansion Processes for Chemical Energy Storage: Thermodynamic Optimization for Methane, Ethane and Hydrogen

Several methods for chemical energy storage have been discussed recently in the context of fluctuating energy sources, such as wind and solar energy conversion. Here a compression–expansion process, as also used in piston engines or compressors, is investigated to evaluate its potential for the conversion of mechanical energy to chemical energy, or more correctly, exergy. A thermodynamically limiting adiabatic compression–chemical equilibration–expansion cycle is modeled and optimized for the amount of stored energy with realistic parameter bounds of initial temperature, pressure, compression ratio and composition. As an example of the method, initial mixture compositions of methane, ethane, hydrogen and argon are optimized and the results discussed. In addition to the stored exergy, the main products (acetylene, benzene, and hydrogen) and exergetic losses of this thermodynamically limiting cycle are also analyzed, and the volumetric and specific work are discussed as objective functions. It was found that the optimal mixtures are binary methane argon mixtures with high argon content. The predicted exergy losses due to chemical equilibration are generally below 10%, and the chemical exergy of the initial mixture can be increased or chemically up-converted due to the work input by approximately 11% in such a thermodynamically limiting process, which appears promising.