Heat Capacity Flow Rates Research Articles

Thermodynamic and Heat Transfer Performance of the Organic Triangle Cycle

Compared to the organic Rankine cycle (ORC), the organic triangle cycle (TC) is simpler in structure and is not limited by pinch point temperature differences. TC has been studied to some extent by previous researchers, such as the selection of working fluid, application, and the design of the expander. However, system optimization and parameter analysis of TC are still rare. The thermodynamic performance of TC internal circulation and TC heat recovery systems are investigated by theoretical analysis and numerical simulation, respectively. The results indicate that the expander inlet temperature T3 and heater inlet temperature T2 are key elements impacting the thermodynamic performance of the TC internal circulation. For the TC heat recovery system, an optimal value of the average heat-capacity flow rate of working fluid Cwf is discovered to output the maximum net power output Wnet. Moreover, the total heat transfer coefficients for the heater (kA)h and condenser (kA)c are discussed in relation to Cwf variations. The findings will provide critical guidance for system investment and optimization.

Utility targeting in heat exchanger network with parametric uncertainties

For almost a half-decade, the methodology of Pinch Analysis (PA) and heat integration (HI) has been used to optimize the requirements of external utilities. Though much has been done, optimizations under uncertain conditions still need more work, discussion, and practical implementation. In most of the HI problems, it has been observed that variation in the minimum driving force affects the shape of the source and sink profile eventually the heat exchanger network (HEN) design. Due to changing operating conditions, human errors, and sometimes poor maintenance, it has been experienced that the process plants along with their equipment and vessels do not operate at the desired value upon which they are expected to run. The parameters with variations and uncertainties due to disturbances in the process plants need to be addressed for the desired product output. In this work, the concept of PA has been unified with robust optimization (RO) to target the utility requirements in HEN optimization where the uncertainties in the heat capacity flow rate F and supply temperatures Tin of the source streams are considered. The developed nominal forms of mathematical programming are converted into the deterministic robust counterpart equivalents to incorporate the uncertainties whose set of variations is known. RO is an optimization technique where the information of accurate probability distribution needs not be known; rather, it works in a defined set of uncertainty ensuring the feasibility of the solution. Using two examples from the literature, the developed models have been solved and results for the solutions against the range of uncertainties in parameters have been established. The produced data demonstrates, in the case of uncertain inlet temperature the rise in total utilities is 115% and 225% in examples 1 and 2 for worst case is calculated. Plots of budget parameter Γ vs the utility requirement have also been compared with results. Application of this model will further assist the plant engineers/managers in deciding the requirements of hot utilities and cold utilities under the parametric uncertainties as needed.

Process design and comprehensive comparison of coal- and biomass-fired oxy-combustion power plant

Oxy-combustion is a promising option to achieve large-scale CO2 emission reduction associated with coal-fired power plants. However, its commercial application is greatly restricted due to the large efficiency penalty and high cost of electricity. In this paper, a novel 1000 MW single reheat oxy-combustion power plant is proposed by adopting the heat capacity flowrate matching method to reduce irreversible loss and increase heat recovery of the overall power plant. A quantitative comparison of novel coal- and biomass-fired cases is conducted. The results show that the net electric efficiency of the novel oxy-combustion power plant improves by 0.85% points with a 1.0% points enhancement in the exergy efficiency and a 1.1 $/(MW·h) reduction in the cost of electricity compared with the reference case. When biomass is selected as the fuel, the net electric efficiency of the novel system increases by 1.1% points, and 4.0 MtCO2 is removed from the atmosphere annually. Exergy analysis reveals the energy-saving mechanism of the novel system. Economic analysis indicates that total investment can be recovered within 1.8 years. This work provides an efficient and economic scheme for the design of oxy-combustion power plants.

Synthesis of heat exchanger network with complex phase transition based on pinch technology and carbon tax

The aim of this paper is to integrate carbon emissions into the synthesis of heat exchanger network for an industrial process with a four-stage pyrolysis gas compression cycle and depropanizers. Problem table is used to determine the pinch point for the design of heat exchanger network. Due to the complex phase transition of the mixture in the system, the phase transition is considered as a pseudo-stream with constant heat capacity flow rate according to its thermal load and material characteristics within several individual temperature intervals. The contribution of carbon tax is also introduced to the total annual cost to determine the suitable minimum approach temperature(△Tmin). Compared with traditional pinch point method, △Tmin is determined as 8°C instead of 9°C due to the consideration of carbon emission. The pinch point is obtained at 80.15°C for hot streams and 72.15°C for cold streams, under which the minimum hot and cold utilities required by the heat exchanger network are 28046.89 kW and 69324.05kW respectively. The total annual cost is saved by 4,446,559 USD.a−1 and the carbon emission is reduced by 96689.31t/a. Results indicate that the consideration of carbon emissions based on carbon tax is helpful to obtain a better heat exchanger network in terms of total annual cost and energy efficiency on the way to carbon neutrality.

Adiabatic compressed air energy storage technology

Adiabatic compressed air energy storage technology

A systematic pinch approach to integrate stratified thermal energy storage in buildings

In this study, dynamic streams including space heating, domestic hot water, renewable solar thermal energy and waste heat such as gray water are extracted from a typical multi-family building under extreme winter conditions for one day or 24 h. The maximum heat recovery (MHR) is targeted through an adapted time slice model (TSM) of pinch analysis. Time slices are selected such that all changes in dynamic heat capacity flow rates can be included in the pinch analysis. Moreover, charging and discharging streams of the thermal energy storage (TES) is calculated based on the adapted TSM graphical approach. The integration of a heat pumping system, gray water and solar thermal collectors through mixed direct/indirect heat recovery (i.e. via TES) can reduce hot utility usage in the studied case by as much as 72 percent. An appropriate heat exchanger (HE) and TES network is proposed for the test building to benefit from this underutilized resource. Additionally, the dynamic thermal behavior of the proposed stratified TES is numerically investigated. The results reveals that combined heat loss and thermocline thickness can reduce the heat recovery from TES tank by 10 percent, which is 2 percent of MHR in the test building.

Thermodynamic analysis and improvement of cascaded latent heat storage system using temperature-enthalpy diagram

In order to analyze and improve the cascaded latent heat storage (CLHS) system under the steady heat source, a novel method based on the temperature-enthalpy (T-H) diagram was developed. The heat storage and release processes can be intuitively described by the T-H diagram when the minimum temperature difference ΔTmin related to heat transfer capacity is defined, which is helpful to evaluate the CLHS performance and find heat transfer bottlenecks. The implementation steps of the proposed method were specified in detail, and three typical cases were presented to verify its feasibility. Moreover, an optimization model for the CLHS system was proposed. The optimal melting temperatures of PCMs can be obtained by solving the multivariate linear equations, and meanwhile, a general expression of the optimal melting temperature was derived. Furthermore, the main and interaction effects of the three model parameters were investigated. The results show that a larger stage number Ns, a smaller minimum temperature difference ΔTmin, and a heat capacity flow rate ratio R with a value slightly greater than 1 are recommended. In short, the novel method is primarily graphical and heuristic-based, which provides the benefits of being easy to understand and ready to be used in practice.

Energy recovery enhancement of heat exchanger network by mixing and azeotrope formation

Mixing two streams with different components and/or compositions could enhance heat recovery if they contain azeotrope-forming components. This study investigates the integration of Heat Exchanger Network (HEN) considering mixing and azeotrope formation. Two cold streams are mixed to form a minimum azeotropic mixture and vaporised. Variations in heat capacity flowrate, net heat flow, and Grand Composite Curve are analysed, and their quantitative relations and utility consumption are deduced. Principles behind mixing and azeotrope formation are clarified to enhance energy recovery. A styrene production process is studied using the proposed method. With steam condensate mixed with ethylbenzene and vaporised at the azeotropic composition, the minimum heating and cooling utility consumption can be reduced by 22.87% and 13.88%.

Modeling and optimization of inter-plant indirect heat exchanger networks by a difference evolutionary algorithm

Inter-plant indirect heat integration via intermediate fluid circuit is an efficient energy-saving and heat recovery method, but traditional optimization methods, such as sequential synthesis, may not provide the best solutions. This paper addresses a multi-plant indirect heat exchanger network problem using a two-layer simultaneous synthesis method. To reduce the nonlinear constraints in the simultaneous heat exchanger network model, the outer layer uses a differential evolution algorithm to determine the temperatures of the intermediate fluid, while the inner layer uses a deterministic method to obtain the heat capacity flow rate of the intermediate fluid and the heat exchanger network configuration. Optimization was performed to minimize the total annualized cost, as the sum of utility cost, heat exchanger cost, pump cost, and pipe cost. The differential evolution algorithm in the outer layer improved the simultaneous synthesis efficiency in the inner MINLP model and also provided better results in the case studies.

A computational tool for guiding retrofit projects of industrial heat recovery systems subject to variation in operating conditions

Heat exchanger networks (HEN) in industrial heat recovery systems often consist of large and complex subsystems. Usually, such HENs are subject to variation in operating conditions, such as varying inlet conditions or changing heat capacity flow rates. Additionally, complexities such as stream splits and recycle loops are commonly present in industrial HENs. Therefore, extensive modelling and/or analytical calculations may be necessary when analyzing different retrofit proposals. Furthermore, retrofit opportunities in industrial heat recovery systems are often constrained by operability considerations, i.e. retrofit actions are supposed to have as little impact as possible on the production process to maintain the quality of the core product. In this work, a computational analysis tool is proposed for effective screening of HEN retrofit design proposals at an early stage in the design process. The proposed tool enables fast evaluation of the network’s response, i.e. temperatures and heat loads, when operating conditions change and/or operational settings are manipulated, and it is applicable for a wide range of HEN structures. The practical use of the analysis tool is demonstrated in a case study on the HENs of a large modern Kraft pulp mill.

A Framework for Flexible and Cost-Efficient Retrofit Measures of Heat Exchanger Networks

Retrofitting of industrial heat recovery systems can contribute significantly to meeting energy efficiency targets for industrial plants. One issue to consider when screening retrofit design proposals is that industrial heat recovery systems must be able to handle variations, e.g., in inlet temperatures or heat capacity flow rates, in such a way that operational targets are reached. Consequently, there is a need for systematic retrofitting methodologies that are applicable to multi-period heat exchanger networks (HENs). In this study, a framework was developed to achieve flexible and cost-efficient retrofit measures of (industrial) HENs. The main idea is to split the retrofitting processes into several sub-steps. This splitting allows well-proven (single period) retrofit methodologies to be used to generate different design proposals, which are collected in a superstructure. By means of structural feasibility assessment, structurally infeasible design proposals can be discarded from further analysis, yielding a reduced superstructure. Additionally, critical point analysis is applied to identify those operating points within the uncertainty span that determine necessary overdesign of heat exchangers. In the final step, the most cost-efficient design proposal within the reduced superstructure is identified. The proposed framework was applied to a HEN retrofit case study to illustrate the proposed framework.

Analysis of heat transfer and irreversibility of organic rankine cycle evaporator for selecting working fluid and operating conditions

Organic Rankine cycle (ORC) is suitable to converting the normally hard to utilize low temperature thermal energies, such as geothermal energy, solar energy, and industrial waste heat, to electricity through utilizing low boiling organic working fluids. The performance of ORC system is dramatically affected by the selections of working fluid and working conditions. As a key component of waste heat recovery, the irreversible loss of evaporator also has great influence on the performance of ORC system. In this paper, we study the heat transfer performance in evaporator under the condition that the heat source parameters and pinch point temperature difference are identified. It is found that the heat transfer performance is affected by Cr, the ratio of heat capacity flow rates between the working fluid and the heat source fluid. The equivalent thermal resistance, deducing from the concept of entransy, to measure the irreversability during the heat transfer process is used. Then, the parameter ?r, the ratio between latent heat and sensible heat of working fluid is defined. With the parameters Cr and ?r, we investigate the relationship between the heat transfer and irreversible loss, and deduce the condition that maximum heat transfer and minimum equivalent thermal resistance occurs. Finally, a calculation method is established to choose the optimum working fluid and the evaporation condition.

Performance analysis of printed circuit heat exchanger for supercritical carbon dioxide and water

Printed circuit heat exchanger could be used as pre-cooler in the supercritical carbon dioxide (S-CO2) Brayton power cycle. Due to the drastic variations of S-CO2 properties in pre-cooler, segmental design method is used to investigate the thermal-hydraulic performance of PCHE. When the heat duty is given, the local heat transfer entropy generation number has extreme value, which appears near the intersection point of the heat capacity flow rates of both sides. In addition, the local and total entropy generation mainly depend on the local and total heat transfer entropy generation, and the flow imbalance increases entropy generation, especially under conditions of the higher local heat capacity flow rate of water.

An improved design method for retrofitting industrial heat exchanger networks based on Pinch Analysis

Energy integration relies on matching process hot streams with process cold streams, and utilities are only introduced to balance the deficit in energy consumption. A systematic but simple Pinch Analysis (PA)-based retrofit approach is proposed to lessen the utility consumption of any industrial heat exchanger network (HEN) under new minimum temperature approach (ΔTms) at the cost of minor capital investment. This work is an extended and improved version of our previous work (Li and Chang, 2010). Elaborate visualized tool of T–H diagram for identification of cross-pinch heat load is proposed to fully address the case that phases change in the cross-pinch heat exchanger. Main characters of industrial HENs which are large scale, varied heat capacity flow rate and multiple pinches are fully addressed. Specifically, each cross-pinch exchanger or match is identified and those matches with large cross-pinch loads are disconnected first, and then their heat loads on the hot and cold stream side are split according to pinch temperatures. Next, at either side of or between the pinches, the divided heat loads on each stream are combined and matched according to a systematic Pinch Analysis (PA) based procedure. Retrofit modifications are focused on those cross-pinch matches and utility exchangers, so the retrofit procedure of an industrial HEN is obviously simplified. An industrial case of Crude oil preheat train (COPT) is retrofitted to demonstrate the effectiveness of the proposed approach.

Simultaneous Water and Energy Saving in Cooling Water Networks Using Pinch Approach

Abstract Recirculating cooling water system is the most common practice used for rejection of waste heat to environment. The fresh cooling water is distributed directly without reuse via a heat exchanger networks in parallel configuration to provide cooling duties for hot process stream. Under this configuration, thewater returns to cooling tower with high heat capacity flow rate and low temperature. These conditions will lead to poor performance of cooling tower.In this paper, process integration approach based on Pinch analysis technique used to address energy efficiency issues in cooling water networks for retrofit design problem. The Demand-Composite Analysisis an analytical approach requiring no graphical procedures developed in this work to target simultaneously, prior to the detailed network design, the minimum heat capacity flow rate of fresh cooling water and the maximumtemperature of waste cooling water to maximize the efficiency of cooling tower.The key basis for this methodology is the conversion of the cooling duties to its equivalent demand and source pairs.Agraphical representation called Demand-Composite Curveis then generated to understand the targeting philosophy. To synthesize a network that achieves the target values, the construction ofTemperature Diagram is also proposed as a design methodology for maximum reuse of cooling water.A case study is treated to illustrate the practicability of this approach.

Multi-plant indirect heat integration based on the Alopex-based evolutionary algorithm

Multi-plant indirect heat integration via an intermediate fluid loop is an effective and energy-saving method of heat recovery. It is most suitable in practical applications because it requires fewer inter-plant pipelines and has the advantages of a simple heat exchanger network. A well-designed heat exchanger network will significantly increase economic efficiency and reduce energy consumption in plants. In this paper, a multi-plant indirect heat exchanger network model is developed for recycling heat using intermediate fluid. This model aims to minimize the total annual cost, including utility cost, number of units and heat transfer area cost. An Alopex-based evolutionary algorithm is used to optimize the model and obtain the heat capacity flow rate of intermediate fluids, the temperature of the heat transfer medium and the configuration of the superstructure simultaneously. Results from three examples demonstrate that the proposed model can perform well in multi-plant heat exchanger network synthesis.

Area Targeting of Heat Exchanger Network (Hen) Using a Modified Pinch Technique

Aspen Hysys version 8.6 which is a modified pinch tool was used to minimize the area of two heat exchanger networks problem and the results obtained were compared with those of other methods used by researchers that have solved the same problems. Stream data adopted from previous research work comprising the inlet and outlet temperatures, heat capacity flow rate, and specific heat capacity were imputed and solved for minimum area using the principle of modified pinch technology embedded in the Aspen Hysys software.The minimum approach temperature, composite curve, grand composite curve and the grid diagram representation are all series of steps employed in the methodology to achieve minimal area targeting using the Aspen Hysys. The problems solved in this research compared well with those of other researchers and even obtained a smaller exchanger area when compared with the non-linear programming technique (NLP). For instance, an area of 22.17m2 was obtained in the first example as against 29.84m2 obtained by the non-linear programming techniques, resulting in a percentage difference of 25.7%. Also in the second example solved in this research, an area of 106.4m2 was obtained against 188.9m2obtained with the NLP technique which gives a percentage difference of 43.67%.The overall assessment of the results on area targeting on these problems using Aspen Hysys points to the fact that the traditional pinch technique can still obtain results that are as good as those obtained using mathematical programming techniques in heat exchanger networks (HENs).

Thermodynamic analysis of a modified system for a 1000 MW single reheat ultra-supercritical thermal power plant

Thermodynamics analysis is performed on the modified steam system for a 1000 MW single reheat ultra-supercritical power plant and compared to the reference one. In the modified system, all the flue gas from the economizer is used to heat the partial condensate, and total air for combustion together with the rest of condensate is preheated by extraction steam from turbine. The flowrate of the partial condensate is adjustable to ensure its heat-capacity flowrate equivalent to that of the flue gas in the modified scheme, which avoids the pinch point of temperature difference of the air preheater in reference system. Results show that the exit temperature of flue gas from boiler can be reduced to a lower value without cold end corrosion and clog. The power generation efficiency of the power plant of 48.35% is achieved, which is 1.27% points higher than that of reference unit at the same capacity. The improvement of thermal efficiency is mainly from both the low-grade heat recovery of flue gas by the flue gas-condensate preheater and the small temperature difference between the flue gas and the condensate, therefore a higher inlet water temperature of the economizer can be reached.

Synthesis of flexible heat exchanger networks considering fouling resistance

The conventional synthesis of heat exchanger network (HEN) is performed under nominal conditions with fixed operating parameters. However, it is inevitable for the network configuration to be disturbed by uncertain operating conditions (for example, feed temperatures, heat capacity flow rates, etc.). At the same time, fouling is a non-negligible factor, which will cause sustained decrease to the overall heat transfer coefficient due to the growth of deposit over the heat transfer surface of the heat exchange units. In practice, the fouling resistance is often fixed to its maximum value instead of considering its dynamic growth. In order to make the synthesis more controllable and operable, this paper presents a flexible synthesis method, in which uncertain operating conditions and the growth of the fouling resistances are considered simultaneously. The methodology is sequentially implemented in two steps: the flexibility analysis and the flexible synthesis. By searching the critical directions, the improved flexibility analysis method for evaluating the flexibility index is proposed, which can reflect the simultaneous influence of uncertain operating conditions and fouling resistances. Then, the flexible HEN synthesis method based on the critical operating points identified by the aforementioned flexibility analysis method is presented, with the target of decreasing the complexity of the HEN to avoid over-synthesis and minimizing the total annual cost (TAC) simultaneously. The effectiveness of the methodology is demonstrated by a case study.

Optimal heat exchanger network synthesis with operability and safety considerations

Research on the optimal design of heat exchanger networks (HENs) has primarily revolved around trading off technical design requirements for aspects of economy, such as capital cost of heat exchangers and utilities. As a result, considerations for safety, operability, and flexibility have received much less attention. This study presents a Pinch Analysis-based methodology that considers the inherent safety and operability aspects of an optimal HEN design. The procedure begins with data extraction, followed by utility targeting that gives due consideration to how each process stream impacts the inherent safety of the HEN. This is made possible via the use of a hot and cold Stream Temperature versus Enthalpy Plot (STEP) that prioritises the inherent safety index (ISI) on top of the heat capacity flow rate (CPs) during simultaneous targeting and design of the HEN. The Pinch temperatures and minimum utilities were determined using STEP. At the same time, the hot and cold stream pairs with higher ISIs and those with lower ISIs were matched together so that safety considerations could be emphasised and precautions taken with a particular heat exchanger. The disturbance propagation path through the HEN and the affected streams were also analysed. Network modification was performed using the downstream path concept in order to reduce disturbance propagation downstream of the HEN. The ∆Tmin violations and energy penalties from network changes were assessed. Flexibility and structural controllability of each network option were compared. The highest percentage of change in every stream of the network indicates that network is the most flexible, while the index of structural controllability closest to 1 demonstrates that the network is most controllable. Application of this method within an illustrative case study showed that network 3 was the most flexible as it yielded the highest percentage of change at 22 %. It was also the most controllable as it had a controllability index closest to 1.0, i.e. at 0.917.