Minimum Approach Temperature Research Articles

Maximizing the power output and net present value of organic Rankine cycle: Application to aluminium industry

This study presents an integrated design and optimization of an Organic Rankine Cycle (ORC) for the recovery of waste heat from aluminium production. Non-Linear Programming (NLP) models were developed, with the objectives of maximizing electricity production and the Net Present Value (NPV) of the system. The models account for optimizing the operating conditions and changes in thermodynamic features of the system. The developed models are applied to a case study of Slovenian aluminium company where the performance of three different working fluids (R245fa, R1234yf and R1234ze) are compared. The optimization is performed considering different temperatures and prices of produced hot water and electricity, minimum approach temperature (ΔTmin), concentration of CO2 in flue gas and temperature and flowrate of flue gas. Results show that the selected working fluids for the proposed waste heat-based ORC system have the potential to substitute up to about 830 kW of electricity in a sustainable and economic manner. Out of the three working fluids considered, R245fa showed up to 7.9% efficiency of the ORC cycle and was identified as the best performing working fluid considering both economic viability and the amount of electricity produced by the system, however the refrigerant inherently has higher GHG footprint.

Heat Exchanger Network Optimization in A Natural Gas Dehydration Unit

A dehydration unit using triethylene glycol absorption is a common process in natural gas processing. In its regeneration section, the regenerated lean glycol needs to be cooled before entering the glycol absorber, while the rich glycol is to be preheated before entering the regenerator. This is a good candidate for heat exchanger network (HEN) optimization. In this work, the HEN was revisited using pinch analysis (PA) and mathematical programming (MP) using superstructure. The optimized networks were evaluated using simplified total annual cost (TAC). The PA method using small Tmin (minimum temperature approach) led to the configuration of three exchangers, a heater and a cooler, with minimized utilities. At larger Tmin, the configuration became into two exchangers, two heaters, and a cooler. The minimum calculated TAC is $60,351/year at Tmin = 12.5oC. Furthermore, it was revealed that one heater and one exchanger were two small. Therefore, they were omitted, and the heat load were redistributed to the network. The calculated TAC became $59,224/year. The superstructure approach resulted in two exchangers, a heater, and a cooler; with a calculated minimum TAC of $57,597/year. The two approaches have resulted in very similar minimized TAC.

Effects of cooling and heating sources properties and working fluid selection on cryogenic organic Rankine cycle for LNG cold energy utilization

Cryogenic organic Rankine cycle (ORC) is considered as one of the most attractive solutions to utilize LNG cold energy for power generation. However, its thermodynamic performance is affected by cooling and heating sources and working fluid selection significantly. Thus, it is crucial to quantify the effects of these factors on cryogenic ORC performance. In this work, we proposed a single-stage cryogenic ORC system to utilize LNG cold energy for sustainable power generation. The effects of LNG vaporization pressure, seawater temperature, minimum temperature approach (MTA), and working fluid selection on cryogenic ORC performance were explored quantitatively. The proposed system adopting different single working fluids and binary working fluids was optimized by particle swarm optimization algorithm to maximize specific net power output (SNPO) with respective to different cooling and heating source properties. The results indicated that the LNG vaporization pressure had the most significant influence on ORC performance. Moreover, R1270 exhibited the highest SNPO (89.34 kJ/kg) and exergy efficiency (18.96%), while C2H6 showed the highest thermal efficiency (14.51%). The overall performance was improved significantly by using R1270 and C2H6 (30% and 70%) as binary mixture working fluid at 4000 kPa LNG vaporization pressure. However, performance intensification was marginal for a higher LNG vaporization pressure. These results revealed that binary working fluids were not always superior to single working fluids. Hence, this study provided valuable insights on choosing proper working fluids and optimizing design parameters for cryogenic ORC at different operation conditions.

Neural network-inspired performance enhancement of synthetic natural gas liquefaction plant with different minimum approach temperatures

In this study, a state-of-the-art neural network algorithm (NNA) was explored to improve the overall competitiveness of the single mixed refrigerant (SMR) process for synthetic natural gas (SNG) liquefaction. The NNA approach is inspired by the functions of biological and artificial neural networks. This is the first study to implement the NNA approach, especially to find the energy and cost-saving opportunities in the SMR SNG liquefaction process. Optimized SNG liquefaction processes were analyzed and compared to a recently published SNG liquefaction process optimized by a single-solution-based vortex-search approach. The robustness of the NNA was evaluated against different values of the minimum internal temperature approach (MITA). It is observed that the SMR process corresponding to MITA values of 1.0 °C and 3.0 °C consumes approximately 16% and 2.4% less energy, respectively, compared with the base case. The exergy efficiencies of the optimized process with MITA values of 1.0, 1.5, 2.0, 2.5, and 3.0 °C are 18.52, 13.45, 11.98, 9.60, and 2.24 % higher than the base case, respectively. An economic analysis in terms of total capital investment (TCI) and TAC was also performed. The analysis showed high TCI savings of 3.3% for an MITA value of 3.0 °C compared to the base case, whereas savings in TAC were 6.6%, 7.2%, 8.1%, 7.1%, and 2.7% respectively for MITA values of 1.0, 1.5, 2.0, 2.5, and 3.0 °C. This study will help practitioners design cost-effective liquefaction technologies that would provide clean and affordable energy.

Systematic energy and exergy assessment of a hydropurification process: Theoretical and practical insights

Improvement of process efficiency and reduction of operation costs are of significant importance in industrial chemical plants. In this study, an effective approach is proposed to analyze a hydropurification process, focusing on energy utilization, operating costs, environmental impact (e.g., CO2 emissions), and exergy performance. The hydropurification process includes heat exchangers, pumps, crystallizers, reactor, and furnace. This process is operated under high pressures and temperatures; it is prone to high consumption of energy and considerable loss of work quality. The vital operating parameters affecting the system performance are chosen by employing Taguchi method. The process simulation and/or thermodynamic modeling are carried out using Aspen Plus® software, and by writing codes for the exergy calculation in MATLAB software environment. The results are in acceptable agreement with the industrial data. The optimal conditions lead to 15% reduction in the process exergy destruction. The optimal operation can be established by implementing lower minimum approach temperature (ΔTmin) of the reaction system feed preheaters (e.g., 5 °C), setting a higher operating pressure in the heat exchangers of the feed preparation section (e.g., 96 bar), and enhancing the performance of heat exchangers using oil as heat transfer medium (optimizing furnace operating conditions). Furthermore, the performance of the fourth and sixth heat exchangers can be enhanced by 33.3%; the performance of the fourth crystallizer can be improved up to 18.7%. The furnace, which utilizes the expensive Therminol® 66 (as the heat transfer fluid), experiences a significant exergy destruction due to significant fluctuations in temperature and thermodynamic irreversibility in the combustion process. Implementation of the proposed optimal conditions can lower the operation costs and carbon tax by 9.96% ($20.5/h) and 14.75% ($14.54/h), respectively; and the CO2 emission rate can be decreased by 0.582 t/h. A decrease in ΔTmin of the heat exchangers improves the process specific energy consumption and exergy destruction. Moreover, effective control of the plant and avoidance of high fluctuations in the process parameters can lead to exergy destruction reduction.

Natural gas density measurements and the impact of accuracy on process design

The liquefaction of natural gas is an energy intensive process, requiring at least 5% of the energy associated with methane's lower heating value. Key to estimating and optimizing these energy requirements are process simulations which rely upon calculated thermophysical properties of the natural gas. In particular, the prediction of thermophysical properties of natural gas mixtures at pressure-temperature conditions close to the mixture’s critical point or cricondenbar is challenging but important as often natural gas processes operate close to these conditions. In this work, we present a comprehensive study of two natural gas related systems: (CH4 + C3H8 + CO2) and (CH4 + C3H8 + C7H16) with n-heptane fractions up to 15 mol%. High accuracy measurements of densities, at temperatures from 200 K to 423 K and pressures up to 35 MPa are presented. The extensive experimental data collected for these mixtures were compared with the GERG-2008 equation of state, as implemented in the NIST software REFPROP. The relative deviations of the measured densities from those calculated using the GERG-2008 model range between (−2 to 4)% for all mixtures, presenting a systematic dependent on mixture density and n-heptane content. Finally, a case study is presented that probes the impact of the accuracy of density on the pinch point in a simulated LNG heat exchanger. An uncertainty in the density of 1% is shown to cause significant 30% reduction in the minimum approach temperature difference, suggesting that accurate thermophysical property calculations are key to reducing over-design of processing plant.

Design, evaluation, and optimization of an efficient solar-based multi-generation system with an energy storage option for Iran’s summer peak demand

In summer, the coastal areas of southern Iran suffer from the freshwater shortage and electricity instability. Meanwhile, this region benefits from the high intensity of solar radiation and the huge potential of brackish water resources. Hence, this paper designs a novel cooling, power and pure water trigeneration system for application in this area to mitigate its energy and water crisis. The proposed system is composed of a solar-based Goswami power and cooling production cycle and a multistage flash seawater desalination process. Moreover, a molten salt heating energy storage unit is used to prove its steady operation under uncertain sunlight condition. A comprehensive parametric study is provided to evaluate the system performance by changing the key variables. The numerical results demonstrate that increasing the pressure ratio from 6 to18 causes 3.3% leads to an increase in system efficiencies and minimum total products cost experiences at the pressure ratio of 12.6. Additionally, as the superheating degree increases from 0 to 10 K, the energy efficiency decreases but the exergy efficiency increases. When the ammonia concentration at the rectifier exit is increased from 0.965 to 0.995, the energy efficiency increases 5%, the exergy efficiency has a decreasing trend from 28% to 25.9%, and the minimum total products cost is 131.2 $/GJ. Furthermore, the exergy efficiency gains its maximum, the energy efficiency has a minimum value, and the total products cost grows with higher values of the minimum approach temperature. Finally, the system is optimized under four different scenarios to identify its best operating condition in each condition: three single-objective and one multi-objective optimization cases. The results of the multi-objective optimization are trade-offs between the ones for single-objective optimization cases. In this case, the net power of 12 MW, cooling capacity of 15.74 kW, and the freshwater of 4.72 kg/s are attainable.

Modification in energy and productivity of high-temperature variable pressure humidification–compression

Abstract One of the important approaches in thermal desalination processes is consumed energy reduction. To achieve this aim, three arrangements of humidification–compression (HC) processes are designed. Two single-stage processes and one double-stage HC process are designed and their performances are compared based on desalinated water production, gained output ratio (GOR) and power consumption. An attempt is made to reduce the power consumption and improve the system performance. All three processes are simulated to examine the effect of operation parameters on HC performance. To validate these simulations, the theoretical results are compared with an experimental rig with a humidifier column of 1.5 m height. The results indicate that the simulation values conform to the experimental data. The effect of minimum approach temperature (ΔTmin) on system performance is investigated for three processes subject to constant operating conditions (feed temperature, water mass flow, air mass flow, and pressure ratio). For this purpose, four values of ΔTmin are considered (7.5, 10, 12.5 and 15 °C) for heat exchanger operations. The results indicate that an increase in ΔTmin in all three cases increases desalinated water volume and GOR. Also, the double-stage HC system has higher water production rate (66.08 kg/h) and higher GOR (17.19) compared with its counterparts.

Black Hole-Inspired Optimal Design of Biomethane Liquefaction Process for Small-Scale Applications

Biomethane is regarded as a promising renewable energy source, with great potential to satisfy the growth of energy demands and to reduce greenhouse gas emissions. Liquefaction is a suitable approach for long distances and overseas transportation of biomethane; however, it is energy-intensive due to its cryogenic working condition. The major challenge is to design a high-energy efficiency liquefaction process with simple operation and configuration. A single mixed refrigerant biomethane liquefaction process adopting the cryogenic liquid turbine for small-scale production has been proposed in this study to address this issue. The optimal design corresponding to minimal energy consumption was obtained through the black-hole-based optimization algorithm. The effect of the minimum internal temperature approach (MITA) in the main cryogenic heat exchanger on the biomethane liquefaction process performance was investigated. The study results indicated that the specific energy consumption of modified case 2 with MITA of 2°C was 0.3228 kWh/kg with 21.01% reduction compared to the published base case. When the MITA decreased to 1°C, the specific power of modified case 1 reduced to 0.3162 kWh/kg, which was 24.96% lower than the base case. In terms of exergy analysis, the total exergy destruction of the modified cases 1, 2, and 3 was 31.28%, 22.27%, and 17.51% lower than the base case, respectively. This study’s findings suggested that introducing the cryogenic liquid turbine to the single mixed refrigerant-based biomethane liquefaction process could reduce the specific energy consumption and total exergy destruction significantly. Therefore, this study could provide a viable path for designing and improving the small-scale biomethane liquefaction process.

Thermo-Economic Assessment and Uncertainty Quantification of Hydrofluoroolefin-Based Single Mixed Refrigerant Process for Natural Gas Liquefaction

Among Floating liquefied natural gas (FLNG) operations, natural gas liquefaction is the most energy intensive. Hence, to achieve energy-efficient and eco-friendly FLNG operations that can meet offshore environment requirements, a hydrofluoroolefin-based single mixed refrigerant (SMR) natural gas (NG) liquefaction process is proposed. The proposed process is optimized using a sine cosine paradigm to reduce the overall energy consumption. Then, the economic performance of the proposed process is performed and analyzed against that of the conventional LNG process. Finally, uncertainty quantification is performed to analyze the process outputs under the effects of uncertain key decision variables to guarantee operational reliability. The optimization yields an energy consumption of 0.2317 kW, resulting in a 41.73% energy saving compared to the conventional SMR process. Moreover, the total annualized cost is reduced to 26.02% owing to the appropriate mixed refrigerant selection and process design. The results of the uncertainty quantification analysis indicate that the flow rates of methane and butane, the evaporation pressure, and the condensation pressure are the most influential factors when processing energy consumption and utilizing the minimum internal temperature approach. This study highlights the importance of introducing eco-friendly refrigerants to replace conventional refrigerants used in NG liquefaction processes.

Heat Pump Bridge Analysis Using the Modified Energy Transfer Diagram

Heat pumps are the key technology to decarbonise thermal processes by upgrading industrial surplus heat using renewable electricity. Existing insight-based integration methods refer to the idealised Grand Composite Curve requiring the full exploitation of heat recovery potential but leave the question of how to deal with technical or economic limitations unanswered. In this work, a novel Heat Pump Bridge Analysis (HPBA) is introduced for practically targeting technical and economic heat pump potential by applying Coefficient of Performance curves into the Modified Energy Transfer Diagram (METD). Removing cross-Pinch violations and operating heat exchangers at minimum approach temperatures by combined application of Bridge Analysis increases the heat recovery rate and reduce the temperature lift to be pumped at the same time. The insight-based METD allows the individual matching of heat surpluses and deficits of individual streams with the capabilities and performance of different market-available heat pump concepts. For an illustrative example, the presented modifications based on HPBA increase the economically viable share of the technical heat pump potential from 61% to 79%.

Composition optimization method for mixed refrigerant MR JT cryocooler

In this paper a holistic optimization method of MR J-T mixture composition based on steepest descent method is developed and comprehensively presented. Modelling is carried using Peng-Robinson EoS available in RefProp 10. The optimization tool takes into account many different factors, that are usually applied separately, like minimum temperature approach in recuperative heat exchanger (which corresponds to minimum enthalpy difference under constant temperature), weighted mean temperature difference in recuperator, volume- and mass-based specific cooling powers and COP of the system, pressure and temperature limitations of commercial refrigeration compressors, non-ideal isentropic compression and suction flowrate. Carried optimization show significant coincidence between compositions optimized in terms of COP and volume-based SCP. On the other hand, mass-based SCP was found as inefficient optimization factor in case of volumetric compressors. Moreover, achieved results emphasize general rules of composing MR depending on the cooling temperature.

Optimal design of organic Rankine cycle recovering LNG cold energy with finite heat exchanger size

An optimization study, under a size constraint, was carried out for an organic Rankine cycle (ORC) combined with an LNG regasification plant for recovering LNG cold energy. Typically, many researchers approached to an optimization problem by assuming pinch point or minimum approach temperature difference. As a different point of view, the size constraint was considered in that resources such as thermal energy and equipment size are limited in a real problem. Given this situation, adequate allocation of finite resources is an important issue for the system to maximize performance. Thus, the aim of this study is to understand how to properly utilize the resources when LNG cold energy and total conductance of heat exchangers are limited. Accordingly, the influences of heat duty allocation, UA allocation, and superheating a turbine’s intake on net power were mainly taken into account. Results indicate that, when total conductance for system design increases, the ORC should take more heat duty and total conductance should be weighted to an evaporator. In most cases, the size of heat exchangers should be weighted in the order of evaporator, condenser, and trim heater, provided that total conductance for system design is sufficiently available.

Membrane-Assisted Removal of Hydrogen and Nitrogen from Synthetic Natural Gas for Energy-Efficient Liquefaction

Synthetic natural gas (SNG) production from coal is one of the well-matured options to make clean utilization of coal a reality. For the ease of transportation and supply, liquefaction of SNG is highly desirable. In the liquefaction of SNG, efficient removal of low boiling point impurities such as hydrogen (H2) and nitrogen (N2) is highly desirable to lower the power of the liquefaction process. Among several separation processes, membrane-based separation exhibits the potential for the separation of low boiling point impurities at low power consumption as compared to the existing separation processes. In this study, the membrane unit was used to simulate the membrane module by using Aspen HYSYS V10 (Version 10, AspenTech, Bedford, MA, United States). The two-stage and two-step system designs of the N2-selective membrane are utilized for SNG separation. The two-stage membrane process feasibly recovers methane (CH4) at more than 95% (by mol) recovery with a H2 composition of ≤0.05% by mol, but requires a larger membrane area than a two-stage system. While maintaining the minimum internal temperature approach value of 3 °C inside a cryogenic heat exchanger, the optimization of the SNG liquefaction process shows a large reduction in power consumption. Membrane-assisted removal of H2 and N2 for the liquefaction process exhibits the beneficial removal of H2 before liquefaction by achieving low net specific power at 0.4010 kW·h/kg·CH4.

Combined pinch and mathematical programming method for coupling integration of reactor and threshold heat exchanger network

Abstract A combined Pinch and Mathematical Programming Methodology is proposed to systematically integrate and optimize reactor and Threshold Heat Exchanger Network (HEN). With the reaction kinetics, mass balance and energy balance considered, the energy consumption of the Threshold HEN is analyzed based on the Composite Curves. Relations among the utility consumption of HEN, conversion and reactor temperature are deduced. With the Minimum Temperature Approach taken as a variable and both capital and utility cost considered, a Mixed Integer Nonlinear Programming (MINLP) model is developed to integrate HEN and reactor. Based on this model, the maximum net annual revenue and utility consumptions at different conversions can be identified, as well as the corresponding inlet and outlet temperature of reactors and Minimum Temperature Approach. A duty-annual revenue-conversion-temperature ( Q ˙ -YNAR-X-T) diagram is constructed to illustrate their variation, and intuitively target the optimal conversion and corresponding parameters. A steam-reforming process is studied by the proposed method. The optimal conversion of the reactor is identified to be 0.84, and the net annual revenue can be increased by 27.82% after the optimization.

Thermal Performance of Ammonia in Dual Mixed Refrigerant Cycle of Natural Gas Liquefaction Process

Abstract Mixed refrigerant (MR) system is commonly used for a liquefaction process of liquid natural gas (LNG) plants due to its higher efficiency of heat transfer rate compared to pure refrigerants. The performance of MR system is highly dependent on the variable refrigerant composition, which is challenging to obtain in a practical LNG plant setting. To address this challenge, this study investigates a unique approach to improve the exergy efficiency of liquefaction cycle employing ammonia in the mixture while keeping the MR molar composition constant in dual mixed refrigerant (DMR) cycle. A control strategy is proposed to regulate the MR flowrate through flow control sensors and a series of Joule-Thomason (JT) valves to sustain the desired efficiency of the cycle under various plant’s operation conditions. The robustness and adaptability of two proposed MR compositions were examined under eight cases by varying natural gas (NG) feed pressure and methane concentration. Composite curve plots were utilized as a tool to control the minimum temperature approach (MTA) and to improve exergy efficiency of the cycle. Furthermore, findings revealed that mixtures which included ammonia yielded a reduction in the number of compressors, as well as a reduced the overall amount of compressors rate of shaft work required for the liquefaction cycle. The results emphasize that DMR is most efficient when NG methane concentration is at 75%. Furthermore, the compressor rate of shaft work reduced by 13.3%, while exergy efficiency of the cycle increased by 14.3%, when natural gas methane concentration reduced from 90% to 75%.

A shortcut method for simultaneous energy and heat exchange area optimization with variable stream conditions

Estimating the heat transfer area for heat integration problems with variable stream temperatures and flowrates using conventional methods can be computationally demanding. The problem becomes more challenging when the minimum approach temperature for the heat recovery (ΔTmin) is set to be a variable. This paper presents a novel area targeting shortcut method capable of estimating the total heat exchange surface using the area comprised between the two balanced composite curves, the total amount of heat transferred across the system and the ΔTmin. This innovative technique avoids the use of disjunctive formulations, greatly reduces the number of binary variables and therefore significantly shortens the solution time. Four case studies are tackled to show the numerical performance of the new approach. Results show that, in spite of its apparent simplicity, the proposed method is able to yield reliable and accurate results with a much lower CPU time when compared to existing alternative approaches.

Design and Cost Optimization of Heat Exchangers Network System in a Typical Brewery Plant

Heat exchanger design and cost optimization had been carried out for Pabod Brewery, Port Harcourt, Rivers State, Nigeria using Pinch Technology as process application method. The gross energy expenditure by the plant is 10.44MW at production capacity of 400,000 liters of beer per day. On quantitative aggregate 6.157MW goes for heating and 4.267MW for cooling. A temperature pinch or minimum approach temperature (?Tmin) of 100C, minimum heating utility of 5.04MW and cooling utility of 3.09MW were recorded. Energy upturn of 1.08MW and 1.23MW for the hot and cold flows were measured. This finding correlates to energy conservationS of 18% for hot utility and 21% for the cold utility. Overall improvement in capital and annualized costs of 39% was achieved for the hot and cold utilities. The researchers strongly recommend the outcome of this research to process applications in brewery, chemical, petrochemical, oil and gas industries.

Design and Cost Optimization of Heat Exchangers Network System in a Typical Brewery Plant

Heat exchanger design and cost optimization had been carried out for Pabod Brewery, Port Harcourt, Rivers State, Nigeria using Pinch Technology as process application method. The gross energy expenditure by the plant is 10.44MW at production capacity of 400,000 liters of beer per day. On quantitative aggregate 6.157MW goes for heating and 4.267MW for cooling. A temperature pinch or minimum approach temperature (ΔTmin) of 100C, minimum heating utility of 5.04MW and cooling utility of 3.09MW were recorded. Energy upturn of 1.08MW and 1.23MW for the hot and cold flows were measured. This finding correlates to energy conservationS of 18% for hot utility and 21% for the cold utility. Overall improvement in capital and annualized costs of 39% was achieved for the hot and cold utilities. The researchers strongly recommend the outcome of this research to process applications in brewery, chemical, petrochemical, oil and gas industries.

Simulation, equipment performance evaluation and sensitivity analysis as a comprehensive parametric study of sulfur recovery unit

AbstractThe sulfur recovery unit based on modified Claus process with split flow was simulated, so that the validation results showed satisfactory agreement with the refinery data. In this study, comprehensive investigation was carried out on various parameters of the unit. Concentration changes of compounds were investigated as the process stream passes along various equipment. Composite curves were generated, and the minimum approach temperature was calculated as one of the operational parameters of heat exchangers. Fluid condensation probability was investigated in catalytic reactors. The recovery of sulfur and the amount of sulfur production were calculated by process streams passing along catalytic reactors and condensers respectively. The most influential performance parameters including the fuel flow rate, combustion air flow rate, and bypasses fraction were considered as sensitivity analysis parameters, and their variation influences were investigated on the recovery of sulfur, the ratio of hydrogen sulfide to sulfur dioxide, the amount of produced steam, and the furnace outlet stream temperature. The optimal amounts for fuel flow rate and inlet air flow rate were calculated with the values of 0 and 202 mol/s, respectively, and according to the results, 15% of the mainstream flow rate gives the optimum amount of bypass fraction.