Heat Exchanger Network Research Articles

A novel two-step approach for multi-plant indirect HENs design

Implementing interplant heat integration indirectly via intermediate fluid circuits is a reliable and promising means of enhancing the overall energy efficiency and reducing the carbon emissions of a chemical industrial zone. Although optimization-based sequential and simultaneous synthesis methods have been developed for interplant heat integration, obtaining optimal multi-plant indirect heat exchanger networks (HENs) remains a challenge due to the problem's size and complexity. Under this content, a two-step method is proposed based on the decomposable structural features of the HENs. In the first step, a mathematical model incorporating superstructure representation and pinch location technique is developed to optimize interplant intermediate stream allocation, by which the optimal configuration of intermediate fluids is determined by balancing utility targets and intermediate stream cost with pumps and pipelines. This approach enables multiple HENs synthesis to become independent, and in the second step, a compact MINLP model is proposed for the cost-optimal HEN design in each plant. The effectiveness and advantages of the proposed method are demonstrated by the better results obtained within reasonable computing times for three literature examples, particularly an industrial-scale problem.

Robust Bayesian optimization for flexibility analysis of expensive simulation-based models with rigorous uncertainty bounds

The performance of emerging biochemical systems rests on their potential to adapt to uncertainties quickly and accurately. Flexibility analysis is a quantitative framework for determining if a system can maintain safe and feasible operation despite uncertainty. Most available methods assume access to equation-oriented models, which can be difficult to obtain in practice. In this paper, we propose a sequential black-box flexibility analysis method, BoFlex, that overcomes this challenge by constructing probabilistic surrogate models over the joint space of uncertain and recourse variables. BoFlex is based on a special alternating confidence bound procedure, which we show finitely converges to a correct solution under mild assumptions on the unknown functions. We also establish a rigorous upper bound on the convergence rate in terms of the maximum information gain of the surrogate model. The advantages of BoFlex are demonstrated on several case studies including a heat exchanger network and a bubble column reactor.

Process modelling and assessment of thermochemical sulfur-iodine cycle for hydrogen production considering Bunsen reaction kinetics

Thermochemical sulfur-iodine (SI) cycle is one of the large-scale, clean, and efficient hydrogen production routes. The Bunsen reaction process is always simulated by a stoichiometric reaction, without considering the realistic Bunsen reaction kinetic process. Herein, a user-defined Fortran module concerning Bunsen reaction kinetics was developed and embedded into a continuous stirred tank reactor (RCSTR), and then a SI flowsheet with 100 L/h hydrogen production was modeled. The key parameter optimization, mass and energy balance were analyzed, the internal heat exchange network was created using the pinch point temperature difference analysis and the principle of energy cascade utilization, along with thermal efficiency evaluation. Bunsen reaction process reaches equilibrium after a period, which is consistent with experiments. The increase of H2SO4 decomposition ratio is beneficial to reduce the energy consumption of system. Increase in temperature has less effect on increasing the equilibrium decomposition ratio of HI. The theoretical thermal efficiency of system reaches 14.0–57.4 % considering the recovery of exothermic heat. This work provides a theoretical basis for the optimization of SI cycle.

Maximization of the Heat Transfer Irreversibility

Abstract The function of a heat exchanger is to transfer heat. In this paper, a second law-based hypothesis is advanced that in heat exchangers, the entropy generated as a result of heat transfer alone, termed productive entropy, is the desired irreversibility and should be maximized while the entropy generated (irreversibilities) by other factors like friction and mixing that do not contribute to this function should be minimized or eliminated to reduce the needed heat transfer area. The hypothesis is proven mathematically for heat transfer between two fluids in a single heat exchanger and also between one hot and two cold fluids in a network of up to four heat exchangers. There currently are two approaches for minimizing the total area (minimum initial cost) of a heat exchanger network (HEN). One uses some empirically based best practices that are generally rooted in the second law, and the other uses optimization algorithms. This paper provides a third approach for HEN optimization, outlining a systematic approach to minimize the area, based on the maximization of productive entropy. The approach identifies the global minimum area for networks with any number of hot and cold streams. It constitutes another method for HEN optimization and an improvement over the existing methods that provide approximate solutions. The methodology is applied to two test cases, and it is shown that this approach improves on the results obtained using the traditional approaches. The approach can be applied to networks using any type of heat exchanger or a combination of different types of heat exchangers.

A frequency domain dynamic simulation method for heat exchangers and thermal systems

Dynamic simulation of thermal systems is crucial for their optimal control but facing the challenge of balancing calculation accuracy and efficiency, where numerically high-efficient and accurate heat exchanger (HX) modeling is a key. Herein, we propose a frequency domain model for HXs' dynamic simulation in the to meet this challenge. The governing equations of HXs are first converted to frequency domain through Fourier transform, and the analytical solution of the converted equations yields the frequency domain HX model in the form of transfer matrix that connects the inlet temperature variations and outlet temperature responses. Furthermore, integrating transfer matrices of HXs into the system's transfer matrix enables a high-efficiency thermal system simulation. Numerical cases of a single HX, a HX network, a district heating network, and a thermodynamic cycle are used to validate the developed method and demonstrate its efficacy by comparing it against finite difference/volume method. Results show that the proposed method could reduce the calculation time by 2–3 orders of magnitude, and the maximum deviation of node temperature is around 1 K. The proposed method can be a powerful tool for the analysis of thermal systems and potentially integrated energy systems.

Unlocking the potential of synthetic fuel production: Coupled optimization of heat exchanger network and operating parameters of a 1 MW power-to-liquid plant

The use of synthetic fuels is a promising way to reduce emissions significantly. To accelerate cost-effective large-scale synthetic fuel deployment, we optimize a novel 1 MW power-to-liquid (PtL) plant in terms of PtL-efficiency and fuel production costs. For numerous plants, the available waste heat and temperature level depend on the operating point. To optimize efficiency and costs, the choice of the operating point is included in the heat exchanger network synthesis. All nonlinearities are approximated using piecewise linear models and transferred to MILP. Adapting the epsilon constraint method allows us to solve the multi-criteria problem with uniformly distributed solutions on the Pareto front. We improved the lowest production costs of 1.89 €/kg and the highest efficiency of 58.08% from the conventional design process to 1.83 €/kg and 61.33%. By applying the presented method, climate-neutral synthetic fuels can be promoted and emissions can be reduced in the long term.

Data-Driven Soft Sensor for Crude Oil Fouling Monitoring in Heat Exchanger Networks

Data-Driven Soft Sensor for Crude Oil Fouling Monitoring in Heat Exchanger Networks

Energy integration of hydrodealkylation (HDA) of toluene

The chemical industry is constantly making advancements. Previously, large equipment was used to transfer heat, but now the industry has modernized. Energy Integration is a trending area of study that can be applied in several sectors to redesign processes. Energy Integration refers to any technique in chemical engineering that results in smaller, environmentally friendly, and more energy-efficient appliances. The goal of energy integration is to improve energy transfer and reduce the equipment's capacity for heat transfer. Pinch analysis has been used in this study to explain the energy integration of Hydrodealkylation (HDA) of the toluene process by finding the best alternative for the Heat-Exchange Network (HEN). Pinch technology offers a systematic approach to reducing energy use across all sites and activities. By using pinch technology, it is possible to determine the proper alterations to the fundamental process parameters that can result in energy savings. In this study, the DWSIM, a chemical process simulator, has been used to simulate the flowsheet of the HDA process. Data obtained from the flowsheet have been used in the pinch analysis to find the best alternative for the heat exchanger network. Heat exchange from hot utility to cold utility has been done in this study, and a proper HEN has also been elaborated. The heating and cooling requirements for the HDA case study have been estimated using pinch analysis. It is estimated that the hot utility (HU) requirement is 64.19 kW and the cold utility (CU) is 202.83 kW. The detailed analysis suggests a 20–30 % reduction in energy consumption.

Energy-saving design and multi-objective optimisation of triple-column pressure-swing distillation configuration for separating n-propanol/benzene/water mixture with multiple azeotropes

The separation of ternary azeotropic mixtures with multiple azeotropes is often encountered in industrial wastewater treatment. This study explores separating the n-propanol/benzene/water mixture with three binary azeotropes and one ternary azeotrope via pressure-swing distillation (PSD). Two conventional PSD schemes are designed based on the ternary phase diagram. The multi-objective genetic algorithm facilitates the optimisation task for each sequence, taking the total annual cost, CO2 emissions, and thermodynamic efficiency as the objective functions. Then, the selection of the optimal solution from the Pareto front is performed using the technique for order preference by similarity to the ideal solution. Subsequently, energy-saving methods, including direct heat integration, heat pump, and heat exchanger network, are introduced into each optimal non-heat-integrated PSD process to address the drawback of tremendous total energy consumption. Finally, the proposed 12 energy-efficient PSD designs are compared with the conventional PSD cases in multiple aspects. The optimisation results highlight the importance of operating pressures as variables in the multi-objective optimisation problem. In addition, the W-B-N sequence shows superiority over the W-N-B sequence in decreasing cost and gas emissions and enhancing energy efficiency. The proposed energy-saving PSD design with direct heat integration, heat pump, and heat exchanger network gives the lowest cost amongst the 14 processes, being up to a reduction of 39% compared to the corresponding conventional PSD design.

A comprehensive review of recent advancements and developments in heat exchanger network synthesis techniques

A comprehensive review of recent advancements and developments in heat exchanger network synthesis techniques

A heuristic algorithm with a dynamic generation strategy for optimizing energy systems

The generation of new heat exchangers during optimization is the main driving force for the structural evolution of the heat exchanger networks. However, the new structure formed by randomly generating heat exchanger using a basic algorithm has only a limited reduction in the target cost, and this aspect has rarely been considered in previous studies. Therefore, this study proposes a new heuristic algorithm with a dynamic generation strategy, to improve the structure evolution capability of heat exchanger networks. The split ratio accompanying the generation strategy based on the concept of the dynamic equilibrium point is established, which improves the generation performance of new heat exchanger. To address the problem of heat exchangers with coupling relationship that are easy to be eliminated in the later stage of optimization due to relatively small heat load, an incentive heat load strategy is designed to coordinate with the former method to provide a certain opportunity for the new heat exchanger to be allocated a large heat load. Compared to the optimal results listed in the literatures, the results of three industrial network cases obtained by the proposed method are 1,711,806 $/yr, 6,665,342 $/yr, and 1,385,391 $/yr. This demonstrates that the proposed scheme improves the global search ability of the basic algorithm, promoting individual structure disturbance and competitiveness of new heat exchange units.

Performance monitoring of heat exchanger networks using excess thermal and hydraulic loads

Fouling in heat exchanger networks (HENs) affects thermal and hydraulic efficiencies resulting in economic penalties to the process industries. Conventionally, overall heat transfer rates of the heat exchangers in HENs are monitored to identify fouling. However, apart from fouling, these rates also vary with inlet conditions. Hence, estimating the degree of fouling accurately based only on heat transfer rates can fail. Previously, we had proposed the use of excess thermal and hydraulic loads as the basis for monitoring fouling in standalone exchangers (Patil et al., 2022). These are quantified as thermal and hydraulic performance indicators that can be easily tracked through suitably constructed charts. In this paper, we extend the approach to HENs wherein deviations in temperature drop across a heat exchanger vis-à-vis the clean condition also depends upon upstream heat exchanger(s). The network performance monitoring charts rely on normalized performance indicators and temperatures across heat exchangers and also have explicitly denoted normal, alert, and alarm regions. The proposed charts are applicable to HENs with or without temperature controllers. Two case studies comprising multiple scenarios of flow rate and setpoint changes are studied. The results show that the method can effectively identify the degree of fouling.

A novel mass–heat exchange network analogy, regression, and synthesis method for mass exchanger networks

Synthesis of mass exchanger network (MEN) is significant to save the material and reduce the pollution for process industries. Considering its small magnitude and concentration constraints, the optimized configuration of MEN can easily violate the concentration constraints, leading to a reduction in the structural diversity during the optimization process compared to that of a heat exchanger network (HEN) of the same size. Moreover, the constraint on the mass transfer driving force based on the equilibrium composition may also have major effects on the optimization process. To address these issues, this study presents a process–equipment–system analogy between the MEN and the HEN, proposing a novel mass–heat exchange network analogy, regression, and synthesis method. First, the generalized HEN was established by using an analogy considering the MEN based on either the original composition or the equilibrium composition. In this regard, a coordination coefficient was introduced to correct the deviation of the analogy relationship between the heat transfer and the mass transfer processes. Then, the generalized HEN was optimized by employing the random walk algorithm with compulsive evolution (RWCE). A regression method was proposed to regress the optimized HEN solution to the MEN expression, allowing the continuous optimization by using the RWCE with finer parameters. The final optimal solution obtained was found to be lower than those reported in the literature with enhanced structural diversity, demonstrating the effectiveness of the presented method and providing new ideas for further synchronous optimization of combined heat and mass exchanger networks.

Modified Mixed-Integer Linear Programming Formulation Implemented in Microsoft Excel to Synthesize a Heat Exchanger Network with Multiple Utilities to Compare Process Flowsheets

In the present work, a modified mixed-integer linear programming model was implemented in Microsoft Excel® and minimized using the Solver tool to obtain information to devise a heat exchanger network with multiple utilities from a set of hot and cold streams and selected utilities by hand. Regarding the mixed-integer linear programming problem, the summation of utility energy was added to the model, and this energy was equal to that from the Temperature Interval method and the Grand Composite Curve. Moreover, feasible temperature ranges for heat exchange were considered according to the second law of thermodynamics. Also, the last two temperature intervals from the rank-ordered ones were assigned for water energy balances. An energy balance was introduced into the algorithm for each interval between its temperature and the process pinch temperature in the case of the boiler-feed water. Seven stream sets collected from the literature were used for the mixed-integer linear programming formulation testing, and six of them are presented in this article. Because of boiler-feed water generation and the low cost of utilities, the annualized cost of a heat exchanger network with multiple utilities can be lower than that of a network without multiple utilities.

Full cycle on-line autonomous reconfiguration of MIMO control system based on REGA pairing principle for heat exchanger networks

This study presents a novel autonomous reconfiguration strategy to achieve full cycle optimal control of the heat exchanger networks. The proposed method aims to select the optimal control structure online to solve the problem of slow time-varying process transition caused by the inevitable fouling accumulation. An auxiliary variable is introduced to increase the degree of control freedom, and a flexible control structure is studied to provide appropriate pairings dynamically between variables. Subsequently, the nonparametric Relative Energy Gain Array (REGA) is considered to form the switching mechanism that determines the switching instants and variable pairing modes between control loops. Furthermore, the online optimization of switching regions ensures the optimal control performance. Finally, one typical experiment for the heat exchanger network in crude distillation unit was conducted and the results illustrated the effectiveness of the proposed control method. Compared with the conventional control system, the available service time is extended by nearly 18 months, and there is still room for further adjustment.

Integration and intensification of thermal processes to increase energy efficiency and mitigate environmental pollution for sustainable development of industry – PRES’22

Integration and intensification of thermal processes contribute greatly to energy efficiency improvement and environmental pollution mitigation, thus, leading to the sustainable development of the industry. A wide range of topic areas have been covered by PRES’22 – e.g., Heat Exchanger Networks (HENs) optimisation, Total Site Integration, heat transfer enhancement and heat exchanger design, renewable energy, waste and fuel processing for improved energy supply, and fundamentals and novel concepts in thermal science applications. All these areas are key components of the contemporary science development directed for increase of efficiency in energy generation and usage to strengthen sustainability in industry. Being carefully selected from totally 413 conference presentations in hybrid mode (154 on site), the papers in special issue adequately represent the main topics in thermal science and engineering discussed during this event. After a thorough review, the current Special Issue accepted 15 excellent contributions that are summarised here. The general advancement observed in the selected articles is energy efficiency improvement at the process level of up to 10–15 %. This, however, has to be further combined with Process Integration measures, to achieve more significant reductions in energy demands, which is the key challenge for enabling renewable sources to become sufficient.The review also points out the future research direction that the integration of renewable energy sources (e.g., solar, wind, biomass) and waste utilisation for energy storage aroused much more attention in recent years, while the heat transfer enhancement and optimisation in energy systems are still fundamental research topics that should be intensively studied.

A zero discharge, carbon-neutral bi-fuel production process for algal biodiesel and renewable hydrogen

This work proposes a novel zero discharge, carbon-neutral bioprocess for the simultaneous production of algal biodiesel and renewable hydrogen. This innovative bi-fuel production process (BiFPP) consists of five units connected in series for biodiesel production, biodiesel purification, glycerol separation, glycerol steam reforming and hydrogen purification. To assess the techno-economic feasibility, the genetic algorithm based multi-objective optimization strategy is developed to minimize the total annual cost and CO2 emission level, and maximize the fuel purity. The optimal BiFPP scheme yields 97.3 mol % biodiesel and 98.8 mol % hydrogen, and thereby meets the standard specifications, ASTM D6751 and EN 14214. A heat exchanger network is designed with thermal coupling to secure the energy savings of 79.5% (heating load) and 50% (cooling load), leading to 93% reduction in utility cost. To achieve carbon neutrality, a CO2 sink for algae cultivation is finally proposed to integrate with this industrial-scale BiFPP process that overall yields a novel zero discharge bi-fuel production technology.

A heuristic method to introduce hot feed into chemical process for energy saving with minor structure modification

Hot feed is a relatively inexpensive strategy to improve the energy utilization in chemical plants. Since hot feed is often an afterthought, retrofitting the heat exchanger network (HEN) with hot feed becomes a key step to decide the energy saving. This paper proposes a heuristic method to introduce hot feed into the existing HEN with minor structure modifications. The concept of heat flow path is extended to fit the hot feed problem. The possible changes of heat exchangers in a path are analyzed in details referring to the heat flow direction. Based on this, the bottleneck and the maximum shift capability of each path can be determined quantitatively. Rules for the creation and selection of new paths are presented to achieve more hot utility saving. A wax oil hydrogenation unit is analyzed to illustrate the proposed method, where heat carried by hot feed can be fully used to save the hot utility. The proposed method obtains more hot utility reduction (3349.0 kW versus 783.8 kW) by the hot feed strategy and a simpler retrofitting scheme than a previous method based on the grand composite curve. Besides, this method can show clear insights in the HEN retrofit with hot feed and it is easy for industrial application.

Process design and energy analysis on synthesis of liquid fuels in an integrated CCUS system

Under the high attention of the international community to CO2 emissions, the capture utilization and storage (CCUS) and the CO2 hydrogenation fuel production have become two popular directions, especially for the combination of the two processes. In this paper, the production of liquid fuels via the methanol synthesis pathway in an integrated CCUS system based on Aspen Plus models was studied. The NH3-CO2-H2O absorption was used to capture CO2 and hydrogen was provided by water electrolysis using renewable energy sources. A two-stage reactor with a recirculating stream was specified for the process of CO2 hydrogenation to synthesize methanol, and the liquid fuel dimethyl ether (DME) was reproduced by dehydrating methanol (Power-to-Liquid). The amount of absorbent used for the carbon capture process was determined and the yield differences due to the number of cycles in the intermediate methanol synthesis reaction were analyzed. It is possible to convert 4.15 t of CO2 per tonne of DME produced at the mass material balance of the process. The optimization of the heat exchange network and the energy requirements for the total process were evaluated. It is calculated that the energy involved in producing 1 ton of DME in the process model is 176 GJ. The analysis of the overall process and the evaluation of models could provide options for the possibility of CCUS engineering application.

A Flexible Heat Exchanger Network Synthesis Method Adapted to Multi-Operation Conditions

This paper presents a flexible HEN (heat exchanger network) synthesis methodology for designing a multiperiod HEN with streams involving phase changes. The methodology is based on an MINLP (mixed integer nonlinear programming) model, identification of critical points, and flexibility index analysis considering phase changes. A nominal multiperiod HEN topology is constructed in the first step. Then each process operating condition’s critical points and flexibility index are calculated to verify the feasibility of the designed HEN under multiple operating conditions. To verify the validity of the method, the proposed methodology will be applied to a case study on an ammonia synthesis process heat transfer network design based on renewable energy. The results show that the method can obtain a flexible heat transfer network that is cost-effective and adaptable to multi-condition production for green ammonia synthesis.