Physical Exergies Research Articles

The impact of varying cold end diameter on the energy separation in Ranque-Hilsch vortex tube for different working gases: A CFD simulation and thermodynamic analysis

The vortex tube is a potential candidate for sustainable cooling due to its unique feature of energy separation without using any synthetic refrigerant. However, the investigation of energy separation and performance optimization always has been challenging due to its complex physics. In previous literature, the performance of vortex tube has been found affected by several parameters. In order to find the impact of varying cold end diameters with different gases, this paper investigates temperature separation in seven vortex tubes of different cold end diameters operating with three different working gases: He, N2, and CO2. A detailed thermodynamic analysis of their thermal separation performances at different cold mass fractions is presented for each working gas. Exergy analysis has also been conducted and discussed separately. As a result of this extensive investigation, it is found that the larger cold end diameter improves the performance of the vortex tube. The cold and hot temperature separation, cooling, and heating power at higher cold mass fractions are found to improve when operated with helium compared to other gas. However, the COP of cooling and COP of heating at higher cold mass fractions is found to be lower with helium. Improvements in the physical and kinetic exergies at the inlet and both outlets are observed with increasing cold end diameter, except for the kinetic exergy at the hot outlet. Helium gas is found to show more exergy at the inlet and both outlets among all three gases. The performance of vortex tube operating at higher cold mass fraction shows an improvement in exergies at outlets, except kinetic exergies at hot outlet. The total exergy efficiency decreases at cold outlet on increasing cold end diameter. However, the results of actual (physical) exergy efficiency shows an improvement at both outlet for larger cold end diameter. The actual exergy efficiency is calculated by considering the physical exergy alone because the kinetic exergy completely lost into the open atmosphere. Physical exergy efficiency at hot outlet is also found to increase on increasing cold mass fraction, while it decreases at cold outlet. Physical exergy efficiency of cooling and heating are found to more when operated with helium compared to nitrogen and carbon dioxide.

Efficient Exergy Analysis of Chemical Processes through Process Engineering Software

In this work, authors present a simple methodology for computing physical, chemical and total exergies, and the exergy destruction and the exergetic efficiency of simple and complex processes by using the object linking and embedded (OLE) automation by connecting Aspen HYSYS to MS Excel VBA. For this purpose, a simple flowsheet was added to the main one with the same number of streams as that of the components. By introducing all these streams in a mixer and then consecutively conducted to a heater to condition the outlet stream, a separator and a mixer, the VBA application was able to compute the chemical exergy of the different streams of the main flowsheet. To demonstrate the procedure here described seven cases were considered: single streams, a CO2 mixer, a cooler, a distillation column, a reactor, a multiple recirculation process, and different configurations of separating components by distillation, which can be extended to very complex processes. The former was selected to validate the methodology here proposed whereas the rest of cases were used to demonstrate the potential of the tool here developed, which in turn could be used for discriminating among different process alternatives.

Plant wide modelling and thermodynamic optimization of a petroleum refinery for improvement potentials

In this work, plant wide modelling of a petroleum refinery was conducted for the optimization. An Aspen HYSYS® model of a petroleum refinery was used for collection of the information to perform the thermodynamic (exergy) analysis. The plant wide model consisted of alkylation unit, reformer, naphtha hydrotreater, catalytic cracker, diesel hydrotreater, isomerization unit, hydrocracker, and kerosene hydrotreater. Seventeen (17) independent input streams and twenty-eight (28) output streams of the plant were considered for the analysis. The physical and chemical exergies of the streams were summed for assessing the overall exergy efficiency, exergy destruction, and improvement potentials of the plant. Exergy efficiency of the plant was 91.38 % with exergy destruction of 704054.64 kW and exergetic improvement potential of 60707.50 kW. The current study gives an insight into the plant-wide analysis ranging from physical exergies of the process streams and utilities to the chemical exergies of the fuels (product) produced in the refineries.

Maximizing efficiency in solar ammonia–water absorption refrigeration cycles: Exergy analysis, concentration impact, and advanced optimization with GBRT machine learning and FHO optimizer

A detailed analysis of energy and exergy is conducted on a single-effect solar ammonia–water (NH3–H2O) absorption refrigeration cycle (ARC) using TRNSYS and EES software. Considering the physical and chemical exergies, the exergy destruction rate (ĖD) in each component of the system is calculated, highlighting its contribution to the overall ĖD. The study explores the effects of varying refrigerant mass flow rate (ṁᵣ) and ammonia concentration in strong and weak solutions (Xs and Xw) on key performance parameters, including coefficient of performance (COP), exergy efficiency (ĖD), and overall ĖD across a range of generator temperatures (Tg). In this study, a gradient boosting regression tree (GBRT) is employed as a supervised machine-learning technique for classification and regression problems, utilizing boosting to enhance conventional decision tree predictions. The Fire Hawk Optimizer (FHO) approach is also utilized to optimize performance parameters, maximizing COP and ηηEwhile minimizing Tg and ĖD. The GBRT models are developed using available experimental and simulation data, revealing relationships between variables (ṁᵣ, Xs, Xw, and Tg) and outcomes (COP, ĖD, and overall ĖD). The results revealed that the generator exhibits considerable ĖD regardless of operating conditions, underscoring its pivotal role in the ARC. It emerges as the primary ĖD contributor (50 %), followed by the evaporator (17 %) and the absorber (15 %). However, ĖD associated with the recooler, pump, and expansion valves is negligible in comparison. Optimization results reveal that, when minimizing Tg and ĖD, the highest COP and ĖD at Tg of 373.15 K reach 0.8081 and 0.46, respectively.

Assessing Exergy Efficiency in Computer-Aided Modeled Large-Scale Production of Chitosan Microbeads Modified with Thiourea and Magnetite Nanoparticles

Chitosan, the deacetylated derivative of chitin, is a biopolymer with many applications in different sectors, such as pharmaceutical, food, and wastewater treatment, amongst others. It can be used as a source for synthesizing bioadsorbents modified with chelators and nanoparticles for the removal of pollutants. In this report, we conducted an exergy analysis to evaluate the large-scale production of chitosan-based bioadsorbents modified with iron nanoparticles and chelators. The objective was to identify energy inefficiencies and propose technological enhancements to improve energy utilization. The process was simulated using Aspen Plus V.10® software, enabling the quantification of chemical and physical exergies for the species and streams involved. We calculated process irreversibilities, exergy losses, waste exergy, and utility exergy flows for each stage and the overall process. These findings provide valuable insights into optimizing energy utilization in the production of chitosan-based bioadsorbents. The overall exergy efficiency was 4.98%, with the washing and drying stages of nanoparticles and adsorbent synthesis accounting for the largest contribution to process irreversibilities and exergy destruction. To increase the global exergy efficiency of the process, it is proposed to implement process improvement strategies, such as mass or energy integration, to obtain better energy performance.

Numerical study on pollutant emissions characteristics and chemical and physical exergy analysis in Mild combustion

This analytical study appraises the emission characteristics of different pollutants such as CO and NOX as well as conducting chemical and physical exergy analyses on conventional and Mild combustion regimes. The RANS turbulence approach and the k-ε RNG model were utilized in this study to simulate turbulence, whereas the eddy dissipation concept (EDC) model and the GRI–Mech 2.11 chemical mechanism were adopted to treat the turbulence-chemistry interaction. This study evaluated local and global pollutant emissions and exergy efficiency along the reacting jet and explored the effects of temperature and chemical species on the physical and chemical exergies in MILD combustion in comparison to conventional combustion. It is found that the overall exergy efficiency of MILD combustion rises by 1, 3, and 4% for oxygen mass fractions of 3, 6, and 9% in hot oxidizer coflow, with the NOx emissions are 81, 72, and 55% lower than conventional combustion, respectively. Chemical exergy is greater than physical exergy near the fuel nozzle due to high CH4 and H2 concentrations. Chemical exergy reduces along the burner due to chemical reactions, while physical exergy increases due to the increased temperature. However, owing to the trade-off between these two exergies along the flame jet, the total exergy approaches a constant value at large distances from the burner. The maximum total exergy occurs at a distance of 0.12 m from the fuel nozzle, with chemical exergy accounting for 83% of the total exergy. In the outlet (0.5 m from the fuel nozzle), chemical exergy accounts only for 30% of the total exergy. It can also be concluded that the exergy is stabilized at a distance of 0.3 m from the nozzle (60% of the domain length). Despite a higher local CO emission in conventional combustion than in MILD combustion, the CO emission substantially decreases at a smaller distance from the nozzle in conventional combustion due to the high O2 concentration.

Exergy Load Distribution Analysis Applied to the Dehydration of Ethanol by Extractive Distillation

This study presents the analysis of the exergy load distribution in a separation process by extractive distillation for ethanol dehydration. The methodology carried out is divided into three parts: the calculation of the flow exergy considering the physical and chemical exergies of the distillation process; the calculation of the primary and transformed exergy contributions considering the consumed exergy; and finally, the overall process efficiency, which shows the real percentage of energy being used in the process. The simulation of an extractive distillation separation system is carried out using Aspen Plus®, from Aspen Tech Version 9. In general, heat transfer processes (heating or cooling) are the ones that generate the greatest exegetic destruction, which is why they must be the operations that must be optimized. As a result of our case study, the local exergy efficiency of the extractive distillation column is 13.80%, which is the operation with the greatest energy loss, and the overall exergy efficiency of the separation system is 30.67%. Then, in order to increase exergy efficiency, a sensitivity analysis is performed with the variation of the azeotrope feed, number of stages, reflux ratio, and solvent feed variation on ethanol purity to reach an overall efficiency of 33.53%. The purity of ethanol is classified as higher than that of the specified, 99.65%.

Thermodynamic Limitations and Exergy Analysis of Brackish Water Reverse Osmosis Desalination Process.

The reverse osmosis (RO) process is one of the most popular membrane technologies for the generation of freshwater from seawater and brackish water resources. An industrial scale RO desalination consumes a considerable amount of energy due to the exergy destruction in several units of the process. To mitigate these limitations, several colleagues focused on delivering feasible options to resolve these issues. Most importantly, the intention was to specify the most units responsible for dissipating energy. However, in the literature, no research has been done on the analysis of exergy losses and thermodynamic limitations of the RO system of the Arab Potash Company (APC). Specifically, the RO system of the APC is designed as a medium-sized, multistage, multi pass spiral wound brackish water RO desalination plant with a capacity of 1200 m3/day. Therefore, this paper intends to fill this gap and critically investigate the distribution of exergy destruction by incorporating both physical and chemical exergies of several units and compartments of the RO system. To carry out this study, a sub-model of exergy analysis was collected from the open literature and embedded into the original RO model developed by the authors of this study. The simulation results explored the most sections that cause the highest energy destruction. Specifically, it is confirmed that the major exergy destruction happens in the product stream with 95.8% of the total exergy input. However, the lowest exergy destruction happens in the mixing location of permeate of the first pass of RO desalination system with 62.28% of the total exergy input.

Modelling and simulation of industrial multistage flash desalination process with exergetic and thermodynamic analysis. A case study of Azzour seawater desalination plant

Abstract Despite the fact of being intensive energy consumption, MSF is a mature technology that characterised by a high production capacity of high-quality water. The multistage flash (MSF) desalination process is one of the prominent thermal desalination used in the industry of seawater desalination to produce high quantity and high quality of freshwater. However, this process consumes large amount of energy and faces thermal limitations due to its high degree of exergy destruction at several units of the process. Therefore, the research of MSF is still existed to elevate the performance indicators and to resolve the concern of high energy consumption. To rectify these limitations, it is important to determine the units responsible in dissipating energy. This study aims to model an industrial MSF process validated against real data and then investigate the exergy destruction and thermodynamic limitations of the process. As a case study, Azzour MSF seawater desalination plant, located in Al Khiran in Kuwait is under the focus. A comprehensive model is developed by analysing several published models. Specifically, the calculation of exergy destruction has embedded both physical and chemical exergies that identified as a strong point of the model developed. As expected, the highest exergy destruction (55.5%) occurs within the heat recovery section followed by the brine heater with exergy destruction of 28.26% of the total exergy destruction. This study identifies the sections of the industrial process that cause the highest energy losses.

Key issues on the exergetic analysis of H2O/LiBr absorption cooling systems

This paper deals with the key aspects of the exergy analysis of H2O/LiBr absorption refrigeration cycles where, instead of agreement, disparity of opinion exists among researchers. As a result, comparisons with the literature are often difficult or meaningless. Based on an in-depth literature review, the key issues highlighted were: a) the identification of the dead state, b) the calculation of the exergy of the currents, and c) the definition of the exergy efficiency of the devices and of the overall system. This study clarifies controversial and divergent assumptions and proposes a coherent approach. In addition, a comparison with the literature is performed. As a case study, a single effect absorption cycle, refrigerated with water, has been considered here. Related to the dead state, consideration of different subsystems results in practical interest. The results highlight the importance of the correct calculation of the chemical exergy for the exergy analysis of the absorption refrigeration system. It is also described here how to define the rational exergetic efficiency, or fuel-product exergy, according to physical and chemical exergies of the streams. The comparison with the literature shows discrepancy specially in the exergy analysis of the desorber and the absorber, where the chemical exergy plays an important role.

Evaluation of dual crude palm and kernel oil production in North Colombia via computer-aided exergy analysis

Palm production chains in Colombia have some unsatisfied demands that affect their competitiveness. Specific demands include efficiency in energy use. Therefore, in the present report, an exergy analysis for the dual crude palm and kernel oil production process was carried out to determine the main energy sinks and suggest technological improvements that allow better use of energy. For this study, the process was initially simulated in the Aspen Plus ® software, where the chemical and physical exergies of the species and streams involved were quantified. The process irreversibilities, the exergy loss, the exergy of waste, and the exergy of utilities were calculated for each stage and the whole process. An overall exergy efficiency of 18% was achieved, while the highest process irreversibilities contribution was due to the destroyed exergy with the waste in the threshing stage. To increasing the global exergy efficiency of dual crude palm and kernel oil production, it is proposed the evaluation of palm rachis use to obtain biofuels and/or high-value products.

Exergy Study of Sour Water Stripper Unit of Delayed Coker Unit in a Refinery plant: A Real Start-Up Plant

Process sour water produced from refinery contains some hazardous pollutants as (H2S and NH3). Sour water stripper units remove both compounds from water. The two stripped pollutants are sent to Sulphur recovery unit to produce Sulphur and prevent any acidic emissions against environmental regulations. The sour water stripper unit serving the delayed Coker unit was simulated with Aspen HYSYS V.11 and an exergy study was conducted on different equipment. While energy is transformed from a form to another, exergy is destructed in an irreversible process. The total exergy is equal to physical and chemical exergies. Physical exergy is calculated through HYSYS and chemical exergy is calculated through a series of equations embedded in excel. The exergy destruction rates, the destruction efficiency and the percentage share of the destruction of each equipment was calculated. The total unit destruction rate was 2996.25 kW. The stripper showed the highest destruction rate 2592.23 kW and a percentage share of 86.52 % of total destruction. The overall efficiency of exergy is 81.58%. A comparison was conducted between the exergy results of this study with two other exergy studies performed in the same refinery plant. The columns in the three studies showed the highest destruction rates exceeding 78% of the total destruction of each unit. The air coolers showed the second- highest destruction rates in their units with a percentage share exceeding 7% of total destruction. The pumps showed the lowest destruction rates with values of less than 1% of the total destruction of each unit.

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.

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.

Evaluation of Large-Scale Production of Chitosan Microbeads Modified with Nanoparticles Based on Exergy Analysis

Novel technologies for bio-adsorbent production are being evaluated on the lab-scale in order to find the most adequate processing alternative under technical parameters. However, the poor energy efficiency of promising technologies can be a drawback for large-scale production of these bio-adsorbents. In this work, exergy analysis was used as a computer-aided tool to evaluate from the energy point of view, the behavior of three bio-adsorbent production topologies at large scale for obtaining chitosan microbeads modified with magnetic and photocatalytic nanoparticles. The routes were modeled using an industrial process simulation software, based on experimental results and information reported in literature. Mass, energy and exergy balances were performed for each alternative, physical and chemical exergies of streams and chemical species were calculated according to the thermodynamic properties of biomass components and operating conditions of stages. Exergy efficiencies, total process irreversibilities, energy consumption, and exergy destruction were calculated for all routes. Route 2 presents the highest process irreversibilities and route 3 has the highest exergy of utilities. Exergy efficiencies were similar for all simulated cases, which did not allow to choose the best alternative under energy viewpoint. Exergy sinks for each topology were detected. As values of exergy efficiency were under 3%, it was shown that there are process improvement opportunities in product drying stages and washing water recovery for the three routes.

Computer-Aided Exergy Sensibility Analysis of Nitrobenzene Production through Benzene Nitration Using an Acid Mixture

Nitrobenzene is widely produced via benzene nitration to be applied in several industries such as pharmaceutical, textile, and agricultural. In this work, an exergy sensibility analysis was performed with the aim of identifying possible opportunities of process improvements. The irreversibilities, exergy of wastes, and efficiency were calculated per stage through exergy balance. The simulation software Aspen plus V10.1 provided the physical exergies of process streams while chemical exergies were found in literature. A sensibility analysis was also carried out in order to study the effect of efficiency of some stages (polyfunctional reaction and cooling 1) on global exergy efficiency. This analysis reveals that nitrobenzene production from benzene is an efficient process from an exergy viewpoint (88%). The total irreversibilities, total exergy of wastes, and exergy of utilities-inlet were calculated in 41,647,341.85 MJ/h, 5,537,487.3 MJ/h, and 18,137,363.71 MJ/h, respectively. The results obtained from sensibility analysis suggested that heat flow of heat exchangers HE-201, HE-303, and HE-301 could provide energy requirements for heating.

Energy and exergy analysis of a modified absorption cycle: A comparative study

Detailed exergy analysis is conducted on a branched generator-absorber heat exchanger (branched GAX) cycle and the system performance is compared to that for the GAX cycle thermodynamically. By considering the physical and chemical exergies, exergy destruction in each component of the system is calculated and its contribution to the overall exergy destruction is revealed. Parametric studies are also performed to investigate the effects on the coefficient of performance (COP) and the exergy efficiency of some design variables. The results indicate better performance for the branched GAX cycle from the viewpoints of energy and exergy efficiencies. The branched GAX system, however, depends strongly on the condenser temperature which can be accounted as a disadvantage. It is observed that the branched GAX gains an advantage of about 14% in the COP and exergy efficiency for the condenser temperature of 30 °C, however, this advantage goes away as the condenser temperature reaches 50 °C. Furthermore, the total exergy destruction in the branched GAX cycle amounts to 5.17 kW, i.e., 10.6% lower than that for the GAX cycle.

Improvement of a Biorefinery from Palm using Exergetic Sensibility Analysis

Background: Vegetable oils have received a great attention to reduce depending on fossil fuels because of its biodegradable, low toxicity and being derived from renewable resources. Palm oil is the most important vegetable oil and it is transformed commercially into numbers of product. Nevertheless, great amount of energy have been wasted in oil production when residues such as palm rachis and palm cake are not used to obtain value-added products. Objectives: In this work, sensibility analysis was performed to palm-based biorefinery in order to evaluate the effect of process variations (stream leaving hydrogen separation stage and considering sludge as products instead of wastes) and exergy efficiency of gasifier on global exergy efficiency. Methods/Analysis: Physical and chemical exergies of process stream were quantified using a robust commercial simulation software. An exergy balance was performed to determine total exergy entering and leaving the system. Findings: The global exergy efficiency was 37.90% for original process, however, when sludge were considered as products, global exergy efficiency was 37.93%, showing a non-significant increase. In addition, exergy efficiency of gasifier did not affect considerably global exergy efficiency and the total exergy of wastes was reduced when stream of selexol leaving separation stage was assumed as product. Novelty/Improvement: These results indicated that exergy efficiency of palm oil; palm kernel oil and hydrogen production could be improved through adding commercial value to selexol wastes. Keywords: Exergy, Efficiency, Hydrogen, Palm, Vegetable oil

Exergy analysis of hydrogen production from palm oil solid wastes using indirect gasification

Background: The extraction of oil from African palm fruits (Elaeis guineensis) generates solid wastes such as Empty Fruit Bunch (EFB), mesocarp fibers, palm frond, palm trunk and palm kernel shell, which exhibit great capability to be employed for producing value-added products, such as hydrogen. Objectives: In this work, exergy analysis of hydrogen production from EFB was carried out using indirect gasification and purification with selexol in order to evaluate overall efficiency of this process. Methods/Analysis: Based on operational conditions (mass flow, temperature and pressure), the process was modeled using a commercial process simulation software, physical and chemical exergies of streams were calculated, stage and overall irreversibilities and exergy efficiencies were found. Stages that require improvements were identified and sensibility analysis was implemented to increase overall exergy efficiency. Findings: It was found that 36 t/h of EFB produce 2.5522 t/h of hydrogen with an overall exergy efficiency of 20%. Novelty/Improvement: Sensibility analysis suggested that process efficiency in terms of exergy could be improved by recycling selexol to hydrogen separation stage, which led to increase the products exergy to 469,747.05 MJ/h.

Thermodynamic studies and parametric effects on exergetic performance of a steam power plant

ABSTRACTA comprehensive thermodynamic analysis is conducted for an existing 82 MW steam power plant. A detailed assessment on energy loss and exergy destruction for individual components of plants (including boiler sub-parts) is reported. A code is built in EES to evaluate the energy loss, energy efficiency, physical and chemical exergies, exergy efficiency and exergy destruction for each part of plant by considering the real values’ range of operating parameters. Analysis pointed out that the main sources of exergy destruction are boilers followed by the turbines, deaerators and then condensers. In boiler, combustor contributes about 50% in total exergy destruction. The maximum energy loss is reported in boilers followed by condensers, in piping, due to friction and in turbines. The exergy efficiency procured about 42% for boilers, 84% for turbines, 38% for condenser, and 60% for deaerators, respectively. The effects of crucial working parameters on plant performance are analysed.