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• A simplified two-fluid model is proposed for critical two-phase flow in microchannels. • The model preserves precision while incorporating fewer constitutive relations. • Proposed an enhanced metastable liquid nucleation model, achieving precise predictions of nucleation pressure. • Advance the understanding of critical flow in microchannels. Critical flow phenomena are present in many chemical engineering processes, such as microchannel heat exchangers, modeling of jet pumps, nuclear safety analysis, and various industrial facilities that involve subcooled or two-phase pressurized fluids. This paper presents a simplified, reliable two-fluid model for accurately simulating critical flow in microchannels. This study develops a five-equation two-fluid model by constructing a mixed energy equation based on the assumption of metastable liquid phase and saturated gas phase. The model utilizes an improved metastable liquid nucleation model as the gas–liquid phase transition boundary to obtain a more accurate flashing inception. It also uses fewer interfacial constitutive relation equations to reduce the occurrence of unstable solutions. The improved nucleation model is validated using experimental data from previous researchers, with prediction errors controlled within 10%. Furthermore, the five-equation model demonstrates substantial improvements in numerical convergence compared to its six-equation counterpart. By comparing critical mass flux predictions, along-channel pressure variations, and interfacial transfer terms, the five-equation model effectively mitigates instability issues observed in the six-equation model while maintaining prediction accuracy. As a result, this model boasts a broader range of applicability and excels in accurately predicting critical flow, even under more complex conditions. [ABSTRACT FROM AUTHOR]

Methods of process integration could be classified in to two categories: the mathematical and conceptual method. The former is outstanding to solve the overall simultaneous synthesis problems, but sometimes, complex calculations will be needed. The conceptual method also could be employed to solve the problem about simultaneous optimization of water and energy but few of them could deal with the multiple contaminants problems. In this paper, a new heuristic approach is proposed to optimize simultaneously the water allocation and heat-exchange network (WAHEN) with both single and multiple contaminants in a fixed flowrate (FF) system. In order to build up this approach solidly, a principle of the single-temperature-peak design is proved through pinch analysis which discloses the interactions between water allocation network (WAN) and energy exchange network (HEN). When WAN only has a single-temperature-peak for each sub-stream, the heat recovery problem of this system could be a threshold problem requiring less energy. As a trade-off between water and energy consumption is established in this design principle, the water and energy consumption could be optimized at one step. Based on this single-temperature-peak design, a novel heuristic approach including two main designing steps, the design of WAN and HEN, is established. A graphical method is employed to design the original water allocation network to ensure it to be a single-temperature-peak type. Next, based on this WAN, WAHEN structures are further generated, which employs a method ensuring that the total energy consumption equals to the minimal value calculated by the first step. Graphical visualization is the advantage of this methodology, and it does not need complex mathematical calculations. Four literature examples are employed to check this new method. That the obtained optimization results are better than those of other works proves its effectivity and advantage. Currently, this paper mainly focuses on the water-reusing network, not the total network which will be investigated in our next work. [ABSTRACT FROM AUTHOR]

The synthesis of heat exchanger networks in chemical plants is vital for energy saving. Most papers presented on this subject have focused on processes having single periods of operation. However, in reality, chemical processes may be multi-period in nature due to changes in environmental conditions, requirements for start-ups and shut-downs, etc. In cases like this, there may be variations in operating parameters such as supply and target temperatures, and flow rates. Further, such processes may involve multiple hot/cold utilities. For processes of this nature, it is imperative to use a mathematical based approach so as to adequately handle the multidimensional nature of the problem. However, solving such models may be difficult except a systematic approach is adopted. In this paper, a modified version of the stage-wise superstructure of Yee and Grossman (1990) is adapted to the synthesis of heat exchanger networks having multiple periods of operations. A new set of solution approaches, which involves solving multi-period MINLP models in a two-step approach is presented. The newly developed method is applied to three examples, out of which two were taken from the literature. In the two examples taken from the literature, the solutions obtained from this study performed better than those presented in the literature. [ABSTRACT FROM AUTHOR]

Abstract: Both shale gas and liquefied natural gas (LNG) contain certain amounts of C2+ hydrocarbons (NGL), which might profitably be recovered. To accomplish this, the shale gas stream needs cold energy to chill down and scrub NGL; while the LNG stream needs hot energy to evaporate and separate from NGL. Thus, integrating both processes of shale gas processing and LNG re-gasification, so as to utilize cold energy from LNG re-gasification process to assist the NGL recovery, has potentially significant benefits on both energy savings and high-value product productions. In this paper, a new methodology to guide the conceptual design and operation for such a process synthesis has been developed, where the uncertainty of shale gas feed rate changes has been considered. The methodology includes four steps of work: superstructure development and data preparation, synthesis model development and solution identification, heat exchanger network design, and final process validation via rigorous simulations. Compared with the recent study of Wang et al. (2013), the paper has two major contributions: (1) a new MILP model with more precise process presentations; and (2) process synthesis with consideration of the uncertainty of shale gas feed rate, which enhanced future operational flexibility of the conceptual design. [Copyright &y& Elsevier]

Highlights • Large-scale HEN synthesis is a first step towards Total Site synthesis. • Two-step MILP/MINLP approach was developed for larges-scale HEN synthesis. • Number of potential matches is significantly reduced, solutions are directed to global optima using MILP TransHEN model. • MINLP model HENSyn use reduced superstructure for HEN synthesis. • Problem with 173 process streams and multiple hot utilities solved. Abstract Although different methodologies for the synthesis of heat exchanger network (HEN) problems have been introduced in the last forty years, there are still significant challenges to be addressed, such as solving large-scale problems. This study focuses on synthesizing large-scale HENs using mathematical programming to achieve near globally optimal solutions based on a two-step MILP/MINLP approach. In the first step a mixed-integer linear programming (MILP) model, TransHEN, is used that obtains a globally optimal solution at selected Δ T min. By utilisation of this model, the most promising matches are selected based on feasibility and viability. The second step entails using the matches selected in the TransHEN of Step 1 in a mixed-integer nonlinear programming (MINLP) model, HENsyn, using a reduced superstructure, to generate a feasible HEN. This study presents also a simultaneous Total Site synthesis with direct heat transfer between processes, and is the first step in the wider project of synthesising an entire Total Site with direct and indirect heat transfer; and is the first step in the wider scope of synthesising an entire Total Site with direct and indirect heat transfer; however, in order to attain this goal, a tool capable of an appropriately handling large number of streams is required. The newly developed procedure has been tested on several case studies, two of which are presented in this paper. For Case study 1 the results obtained lie within the range of best solutions obtained by other authors. Case study 2, consisting of 173 process stream and involving multiple hot utilities, shows the applicability of the developed method to handle large-scale HEN problems. [ABSTRACT FROM AUTHOR]

Highlights • H 2 is autothermally generated from ethanol in a parallel-plates reactor. • Adequate thermal coupling between combustion and reforming of ethanol is predicted. • Feasible integration between the reactor and heat exchangers is proved. • Splitting the fuel feed reduces hot spots while enables extra H 2 yields. • Importance of proper heat loss quantification is evidenced. Abstract This paper deals with the study of the integration of ethanol steam reforming and ethanol combustion in a parallel plates reactor aiming a hydrogen production of 1 kW th equivalent. The study was performed by means of a mathematical model of a non-adiabatic reactor and the associated heat exchangers used for preheating purposes. Focus is given here to the influence of the insulation of the reactor and heat exchangers, the fuel concentration, fuel distribution policy to the reactor, and reforming flowrate. Thermal coupling between combustion and reforming of ethanol in terms of energetic integration is feasible and an adequate behavior of the reactor and the heat exchangers is predicted. The importance of heat losses to the environment is evidenced since they represent about 35–50% of the heat released by the combustion. Ranges of ethanol fuel concentration (0.6–2.0% of ethanol fuel) and distribution (1 or 2 feed ports), and ethanol reforming load (0.37–0.63 kg/h) were studied to find windows for these variables where a satisfactory operation is possible. [ABSTRACT FROM AUTHOR]

Highlights • Synthesis of heat exchangers network considering four types of fuels. • Environmental impact assessment using eco-costs methodology. • Fuzzy-AHP programming for assigning weights to each objective function. • Power generation system operating with coal disadvantageous when considering the impacts. • Power generation system operating with natural gas more advantageous than operating with coal. Abstract In the present paper we propose an optimization model based on fuzzy-Analytic Hierarchy Process (AHP) programming in which the external costs associated with the environmental impacts caused by energy generation and equipment construction were explicitly incorporated in the synthesis of Heat Exchanger Networks (HEN) using the eco-costs methodology. The damage categories that compose the environmental and economic objective functions were human health (HH), ecosystem quality (EQ), resources depletion (RD), greenhouse gases (GW) impacts and HEN cost. The inclusion of the impact in the optimal synthesis of HEN was performed considering the most used fuels in industries: hard coal, heavy and light fuel oil and natural gas. Boilers operating with each one of the cited fuels were considered, and three different technologies for boiler operating with natural gas were included (low NO x natural gas, modulating and non-modulating natural gas boilers). The weights were obtained through a questionnaire directed to a multidisciplinary group of specialists. A case study with two hot and one cold streams was used to test the efficiency of the model and results were compared to a single-objective model and to a model not considering impacts in the objective function. Results showed that the fuzzy-AHP programming model was not able to find HEN costs lower than those obtained by the single-objective model for HENs operating with heavy and light fuel oil. For the HENs operating with hard coal and natural gas, HEN costs were lower than those obtained by the single objective model. The same HEN configuration was obtained for all fuels considered. [ABSTRACT FROM AUTHOR]

• The design optimization problem of gasketed plate heat exchanger is addressed. We solve problems with property variation with temperature and we use ODEs. • The problem is solved using Partial Set Trimming and Smart Enumeration. • The proposed design approach is robust (no convergence issues) and always attains the global optimum. • A comparison with metaheuristics shows this approach has a better performance. In this paper, we present two novel advances in the design optimization of gasketed plate exchangers. Our first novelty is the use of an ordinary differential model that represents the heat transfer and pressure drop along the surface of the plate considering the variation of the thermophysical properties (density, heat capacity, viscosity, etc.) with temperature. Previous literature did not address this aspect of the problem or addressed it only partially. We also take into account all pass arrangements. Although there is no novelty in this, it adds to making our model comprehensive. The second novelty is the design optimization method used: Partial Set Trimming and Smart Enumeration. This method guarantees global optimality, it is robust (no convergence issues), and it is fast. A new feature added to the Set Trimming methodology is Proxy Set Trimming. The advantage of Set Trimming followed by Smart Enumeration is that there is no need to discretize the differential equations and build very large models with the resulting large number of variables and algebraic equations, rendering highly non-convex models difficult to solve using MINLP procedures. A comparison of the performance of the proposed procedure with two different metaheuristic methods showed that this approach presents better computational performance. [ABSTRACT FROM AUTHOR]

We present a new method for the synthesis of mass exchanger networks (MENs) involving packed columns. Simultaneous synthesis of MENs is typically done through the use of mixed-integer nonlinear program (MINLP) optimization, with simplifications made in the mathematical representations of the exchangers due to computational difficulty in solving large non-convex mixed-integer problems. The methodology proposed in this study makes use of the stage-wise based superstructure MINLP formulation for the network synthesis. This stage-wise superstructure model incorporates fixed mass transfer coefficients, fixed column diameters, no pressure drops, and unequal compositional mixing for models. In this paper, the simplified MINLP model is further improved by including a detailed individual packed column design in a non-linear programming (NLP) sub-optimization step, where orthogonal collocation is utilized for the partial differential equations, and optimal packing size, column diameter, column height, pressure drops, and fluid velocities. Detailed designs are then used to determine correction factors that update the simplified stage-wise superstructure models to more accurately portray the chosen design. Once the MINLP is updated with these correction factors, the model is re-run, with new correction factors obtained. This iterative procedure is repeated until convergence between the objective function of the MINLP and that of the NLP sub-optimization is achieved, or until a maximum number of iterations is reached. The methodology is applied to two examples and is shown to be robust and effective in generating new topologies, and in finding superior networks that are physically realizable. [ABSTRACT FROM AUTHOR]

Heat exchanger network (HEN) is one of the most important parts in chemical process industry, owing to its significant advantages of energy recovery. However, models with plentiful HEN structures always result in complex mixed-integer nonlinear programming (MINLP) models with non-convexity of nonlinear constraints, which will cause heavy computational burdens. The computational time of MINLP models and even the feasibility of models without good initial points are still challenging researchers. This paper presents a novel transshipment type model for heat exchanger network synthesis (HENS), which is formulated as a MINLP problem with all linear constraints. Stream splitting, stream by-pass, isothermal and non-isothermal mixing, and recycling flows are all included, while multiple utilities are available in the model. Furthermore, new formulations are presented to calculate the temperature difference of heat exchangers, while a more precise heat exchanger area calculation method by piecewise calculation is developed in the new model. Six literature examples are presented to illustrate the effectiveness and applicability of the proposed model. It is shown that several better results are obtained by the proposed model. [ABSTRACT FROM AUTHOR]

• A new separation technology is applied in process integration system. • Two variant of non-structural models are adapted in the mass exchanger network system. • The advantages of two variant models are analyzed. • A two-step optimization approach for mass exchanger network synthesis is proposed. Mass exchanger networks have been considered a promising solution for reducing impurity in streams and maximizing energy recovery in the chemical industry. This paper adjusts two non-structural models with distinct optimization variables and a heuristic algorithm according to the MEN synthesis characteristics. Considering the requirement of the objective function, a two-step optimization method that combines these two models is proposed. In the first step, a feasible solution is determined by the model using the transferred mass as an optimization variable. In the second step, the aforementioned model is employed to maximize tray utilization by adopting the tray number as an optimization variable. The results of the two cases obtained by the proposed method are 406,330 $/yr and 329,326 $/yr, which are lower than the published results, and is verified on two cases regarding quality and efficiency. [ABSTRACT FROM AUTHOR]

The boiling flow or condensation is widely encountered in many industrial applications for both cooling as well as heating processes. Compact heat transfer devices, such as micro-heat exchangers and evaporators, are extensively used for both cooling as well as heating processes over conventional heat exchangers, such as microelectronic circuits, automobile and aerospace industries, due to high surface area to volume ratio and heat transfer rates, compactness and easy thermal control. For better design of micro- or mini-heat exchangers, a detailed specific knowledge of the multiphase flow and its properties such as the flow pattern during flow boiling, critical heat flux (CHF) and stable operation are very important. This paper provides a state of art review on boiling flow in microchannels since year 2000 till date. Flow patterns formed and the parameters influencing flow pattern transitions, during multiphase heat transfer in micro- or mini-channels, have been reviewed in detail. The flow regimes and flow pattern maps, and modeling approaches considered for boiling flow in micro-channels/devices with various challenges have been discussed. A lot of contradiction between the experimental data has been observed for the analysis of flow regimes and flow pattern maps. Further, the effect of hydrodynamics during flow boiling and CHF on heat transfer coefficient has been discussed in detail. Recently, with the advancement in measurement techniques, the heat transfer measurement technologies have been synchronized with the visualization techniques, which helped in understanding the boiling flow physics in micro- and mini-channels. Therefore, an in-depth understanding of flow patterns and regimes under boiling flow conditions in mini- and micro-channels can be used to predict the boiling heat transfer mechanism, which can be further used for developing better heat transfer models for boiling flow. Further, enhancement in heat transfer coefficient for boiling flow in microchannels, either by using complex microchannel configurations or nanocoating on the microchannel surface, have received attention recently, which have been discussed and analyzed in the present review. Both micro- and mini-channels have number of applications in aerospace, refrigeration and computational systems; therefore further attention is needed for more robust and precise design. [ABSTRACT FROM AUTHOR]

In this paper, semi-analytical and numerical models developed in our previous works to study the dynamic behaviour of natural convection are assessed against the experimental data obtained by means of the L2 Natural Circulation Loop (NCL) of DIME-Tec Labs (University of Genoa). As for the experimental campaign, reference is made to a set of nine experiments performed using water as working fluid and providing a thermal power of 2 kW. This set of data is firstly adopted for the validation of a semi-analytical linear analysis tool aimed at studying the asymptotic behaviour of NCLs through the definition of dimensionless stability maps. Then, two different numerical models (adopted in our previous work to confirm the linear analysis) are assessed, namely an Object-Oriented (O-O) one-dimensional model and a three-dimensional Computational Fluid Dynamics (CFD) model. In this regard, the O-O model represents a fast tool for the evaluation of the most important quantities, such as the velocity and the temperature fields in the loop along the axial coordinate. On the other hand, the CFD tool, which is intended as a support to the 1D analysis, is characterised by a high computational burden, but allows highlighting interesting 3D spatial effects. The validation of these tools is not secondary with respect to that of the stability maps. Actually, the numerical approach is fundamental to study the time-dependent behaviour of both stable and unstable natural circulation regimes, for which the stability maps do not provide information. As for the achieved results, the developed models are able to catch the behaviour of the experimental data. In particular, this outcome is possible if an accurate modelling of both the heat-exchanger section and the piping thermal inertia is considered. [ABSTRACT FROM AUTHOR]

Process effluents recovery can be a tremendous source of revenue as well as a smart way to reduce their environmental footprint for industrial processes. Reusing effluents for heating purposes or reducing resource consumption can lead to substantial operating costs savings if the capital investments required to implement recovery networks are depreciated over an adequate period of time. This paper presents a mixed-integer linear programming model to design mass allocation and heat exchanger networks simultaneously. Mass streams are characterized by their composition and properties. A superstructure is presented to define the interactions between mass allocation and heat exchanger networks. Non-isothermal mixing can occur before and after the heat exchanger network. The objective function is the total annualized costs of both networks. It takes into account the annual operating costs for energy and mass requirements and the capital costs of the heat exchanger network. Two literature case studies are presented to demonstrate the model performances. [ABSTRACT FROM AUTHOR]

• A relatively linear hybrid three-level strategy for retrofitting HENs is presented. • A genetic algorithm produces the structures at the first level. • The total annual cost of each structure is optimized at the second and third levels. • These levels consist of LP and ILP formulations with external search loops. • This method can reduce the TAC of retrofitted HENs by about 0.5%-20% compared to others. This paper proposes a three-level straightforward retrofitting approach for heat exchanger networks (HENs) using a genetic algorithm (GA) coupled with linear programming (LP) and integer linear programming (ILP) formulations. At the first level, the GA produces HEN structures. The local total annual cost (TAC) of these HENs is computed at the second level, consisting of LP, ILP, and an external search loop. LP model is formulated based on the minimum utility consumption, whereas ILP determines the minimum investment cost of implementing potential changes to the existing network to turn it into a GA-generated network. The external search loop is also used to handle the stream splitting ratios. Eventually, the final optimal TAC of the retrofitted network is determined by applying a correction procedure at the third level to the second level results. The results demonstrate that this approach could reduce the retrofitted network's TAC by about 0.5%-20% compared to others. [ABSTRACT FROM AUTHOR]

In this paper, the study of the heat exchange effect on the dynamic behaviour of single-phase natural circulation with internal heat generation is presented. In order to predict natural circulation instabilities, two different methods of analysis are developed and compared. The first approach is a linear analysis in which the governing equations are firstly linearized around a steady-state solution of the system and then treated by means of the Fourier transform. This strategy is adopted to compute, in a semi-analytical way, dimensionless stability maps for different system configurations, highlighting the heat exchange effect on the system dynamics. The second approach consists in numerically solving the nonlinear governing equations and allows investigating some transients of interest. For this purpose, an object-oriented one-dimensional model of natural circulation loops has been developed, and the corresponding results have been compared with RELAP5 and Computational Fluid-Dynamics (CFD) time-dependent simulations. The developed models have been applied to investigate the dynamic behaviour of two loop configurations characterized by large instability regions, namely the Horizontal Heater–Horizontal Cooler (HHHC) and the Vertical Heater–Horizontal Cooler (VHHC). [ABSTRACT FROM AUTHOR]

This study makes use of a novel methodology for the synthesis of heat exchanger networks, which is aimed at overcoming the shortcomings associated with the use of shortcut models to represent individual exchangers in the synthesis network. The new approach entails the use of a number of correction factors to get networks which are based on the use of shortcut models, such as the mixed integer non-linear programming (MINLP) stage-wise superstructure (SWS) of Yee and Grossmann (1990) to more closely represent physically achievable heat exchangers and ensure that the MINLP network topology optimisation step of these models converge on a real design, rather than an approximated one. In this paper, the SWS formulation is used for the generation of an initial network after which its objective function is modified to include the correction factors that force its objective function towards the cost of a network whose individual exchangers are designed using methods such as Bell–Delaware and heuristics. The modified objective function includes parameters that modify the areas obtained by the shortcut based MINLP model so as to more closely represent the areas obtained by the detailed models and also includes a novel method for including the number of shells required for each exchanger duty. The correction factors account for pressure drops, F t correction factors, number of shells, TEMA considerations, and changes to the overall heat transfer co-efficient of each stream match. The methodology is applied to two examples and the solutions are comparable with other solutions obtained in literature and were shown to produce good solutions. The reason that the method is effective is because many potential networks are evaluated during the iterative procedure and the best network, based on the detailed exchanger designs, is chosen. In this way it is possible to use the detailed exchanger designs to “guide” the MINLP optimisation towards more realistic networks and also to generate many different potential networks. [ABSTRACT FROM AUTHOR]

Shell-and-tube heat exchanger has been extensively used in industrial and research fronts for more than a century. However, most of its design procedures are based on empirical correlations extracted from experimental data of long length shell and tube heat exchanger. In this paper, an attempt has been made to investigate the complex flow and temperature pattern in such a short shell and tube type heat exchanger, with and without baffles in the shell side. Heat exchangers of length by hydraulic diameter ratio between 7 < L / D h < 21 ( 0.15 m < D S < 0.6 m ) for unbaffled and L / D h < 7 ( D S = 0.09 m ) for baffled heat exchangers are analysed using CFD code OpenFOAM-2.2.0 for different mass flow rates. It was observed that the cross flow near the nozzle region has a significant contribution towards the heat transfer, hence the conventional heat transfer correlations do not apply to these short heat exchangers. Furthermore, a sensitivity study of turbulence models was performed and it was observed that the standard k – ε model gives best results for the velocity profile as well as heat transfer, provided average y + of the first node adjacent to the heat transfer surface is maintained greater than 15. The commonly used boundary conditions at the exit are not realistic, as it tends to give either incorrect flow and temperature fields, or the solution was found to diverge. Through a sensitivity study of the exit length, it was found that exit length to shell side velocity ratio of 2.5 is required for proper convergence. Finally the effect of flow field on shell side heat transfer coefficient and a comparison with analytical methods are presented. [ABSTRACT FROM AUTHOR]

This paper presents a study on the integration of heat and work. A systematic design methodology for the integration of expanders into sub-ambient heat exchanger networks is developed. The objective is to minimize exergy consumption (or maximize exergy production). Four theorems are proposed and used as the basis of the design methodology. Pinch Expansion, i.e. expansion starts at pinch temperatures, is proven to be a preferred scheme under certain well-defined conditions. Stream splitting is used to maximize the use of Pinch Expansion. An important conclusion is that minimum exergy consumption (or maximum exergy production) can be achieved when expansion starts at pinch temperatures or ambient temperature. [ABSTRACT FROM AUTHOR]

Heat can be converted into work and vice versa, and this fact provides opportunities for the integration between them. This paper presents a systematic methodology for the integration of compressors into heat exchanger networks below ambient temperature. The objective is to minimize exergy consumption. Four theorems are proposed and used as the basis for the design methodology. Pinch Compression, i.e., compression starts at pinch temperature, is proven to be preferred under certain conditions. It is concluded that minimum exergy consumption can be achieved when compression starts at pinch temperatures, ambient temperature or the cold utility temperature. [ABSTRACT FROM AUTHOR]

• A two-surface hybrid approach was introduced for multi-period HENs synthesis. • It was combined linear programming (LP) + imperialist competitive algorithm (ICA). • The ICA is used to find the structural variables on the 1st surface. • The continuous variables are handled with a combined LP and ICA on the 2nd surface. • The relatively linear behavior improved its ability to deal with HENs problems. In this paper, an efficient hybrid approach to deal with multi-period heat exchanger networks (HENs) synthesis, which is inherently formulated as a mixed-integer non-linear programming (MINLP) model, is proposed. In this novel two surfaces approach, the creation and determination of optimal HENs structures are accomplished by an imperialist competitive algorithm (ICA) on the first surface. In each HEN structure, there is a sequence of stages containing the addresses of the exchanger(s). The HENs generated by the ICA are sent to the second surface, where their minimum total annual cost (TAC) is calculated as an overall objective function. This surface works on two levels. For all periods of each HEN, the local optimal solutions are determined based on the maximum energy recovery at the outer level, including an external search loop and a linear programming (LP) model. Based on the outer level results, the ICA is re-used, at the inner level, to find the final minimum TAC of the network. As a result, the MINLP model is transformed into a relatively linear LP + ICA hybrid model, which is easily solvable. The results demonstrate that this approach can sometimes reduce the network TAC even by over 7.2% compared to the literature. [ABSTRACT FROM AUTHOR]

Two phase flows exist as a part of many industrial processes, including chemical processes, nuclear reactor systems, and heat exchangers. In all of these applications the interfacial area concentration is an important parameter for evaluating the interactions between the phases, including drag forces, heat transfer or chemical reaction rates. Many models for interfacial area concentration exist for dispersed bubbly flows; however this type of flow only exists at relatively low void fractions. Very few correlations exist for the prediction of cap-turbulent, slug, or churn-turbulent flows. In this paper a new correlation for predicting the interfacial area concentration beyond bubbly flows in large diameter pipes is derived using a two-bubble-group method (spherical and distorted bubbles as Group-1 bubbles and cap and churn-turbulent bubbles as Group-2 bubbles) and the two-group interfacial area transport equation. The derivation assumes steady state and fully developed flow, and is based on interfacial area transport source and sink terms for large diameter pipes developed by Smith et al., 2012a . Int. J. Heat Fluid Flow 33, 156–167. The resulting equations can be used to predict the void fraction for each group of bubbles and the Sauter mean diameter for each group of bubbles in addition to the total interfacial area concentration. The model is then benchmarked based on the data collected by Schlegel et al., 2012. Exp. Therm. Fluid Sci. 41, 12–22; Schlegel et al., 2014. Int. J. Heat Fluid Flow 47, 42–56. It is found that the correlation predicts the data for Sauter mean diameter of Group 1 bubbles with RMS error of 23.3% and bias of +1.83%. For Group 2 bubbles the RMS error is 24.0% and the bias is +5.35%. This indicates that the correlation somewhat over-predicts the bubble sizes. In spite of this the prediction error remains reasonable compared to the accuracy of previous correlations, and given that the experimental uncertainty can be as high as 15% for some flow conditions. The RMS error and bias in the total interfacial area concentration are 22.6% and −4.29%, respectively. This is consistent with the over-prediction of the Sauter mean diameters, but again is reasonable considering the experimental uncertainty and the prediction error of previous correlations. The model is also able to predict the trends found in the experimental data with varied liquid and gas velocities, representing a large improvement over previous modeling efforts. An expanded database of accurate interfacial area concentration measurements at higher pressures would allow further improvement of the model benchmark and expansion of the range of applicability of the model. [ABSTRACT FROM AUTHOR]

Abstract: The optimal integration of work and its interaction with heat can represent large energy savings in industrial plants. This paper introduces a new optimization model for the simultaneous synthesis of work exchange networks (WENs), with heat integration for the optimal pressure recovery of process gaseous streams. The proposed approach for the WEN synthesis is analogous to the well-known problem of synthesis of heat exchanger networks (HENs). Thus, there is work exchange between high-pressure (HP) and low-pressure (LP) streams, achieved by pressure manipulation equipment running on common axes. The model allows the use of several units of single-shaft-turbine-compressor (SSTC), as well as stand-alone compressors, turbines and valves. Helper motors and generators are used to respond to any demand and excess of energy. Moreover, between the WEN stages the streams are sent to the HEN to promote thermal recovery, aiming to enhance the work integration. A multi-stage superstructure is proposed to represent the process. The WEN superstructure is optimized in a mixed-integer nonlinear programming (MINLP) formulation and solved with the GAMS software, with the goal of minimizing the total annualized cost. Three examples are conducted to verify the accuracy of the proposed method. In all case studies, the heat integration between WEN stages is essential to improve the pressure recovery, and to reduce the total costs involved in the process. [Copyright &y& Elsevier]

Abstract: Conventional network structure modifications for retrofitting heat exchanger networks (HENs) usually require a large network superstructure, which might lead to very large scale retrofit models, and thus increases the computational difficulties of optimization, especially for industrial applications. This paper presents a novel optimization approach of retrofitting HENs for both small scale and industrial scale problems. To overcome the drawback of the existing retrofit research, a simple but efficient method is proposed to determine the network retrofit superstructure based on the concept of pinching match. Then two optimization stages (network structure optimization and investment optimization) are solved with the MILP-based iteration method that has been developed. The proposed approach has many benefits for solving HEN retrofit problems. First, smaller size of retrofit superstructure is provided based on the pinching match analysis. Second, the MILP-based iteration method developed converts conventional a mixed integer non-linear programming (MINLP) to a mixed integer linear programming (MILP), and significantly reduce the computational difficulties of nonlinear programming associated with configurational changes or structural modifications for existing heat exchanging arrangements, which is upgraded based on the MILP-based iterative method proposed by Pan et al. (2012a) for retrofitting HENs with heat transfer intensifications, as the earlier work was only applicable to the cases without network topology modification. In case studies, the new proposed approach is compared with the existing methods in two literature examples and one industrial problem, demonstrating that the new proposed approach is able to obtain better and more realistic solutions for HEN retrofitting considering the economic trade-offs between energy savings and investments. [Copyright &y& Elsevier]

In this paper, scalar mixing by several arrays of vortex generators mounted inside a circular pipe is investigated using numerical simulations. Two flow configurations are studied in which the arrays are in-line and rotated periodically by an angle of 90°. Each vortex generator creates a pair of streamwise vortices which enhances the mixing process in the flow cross-section. It is shown that the alternate configuration, in which the vortex generators are rotated periodically by an angle of 90°, enhances the mixing process relative to the in-line one due to the generation of chaotic advection flow, while in the in-line configuration the flow is regular and the mixing process is only caused by the convective motion of the longitudinal vortices. Both Eulerian and Lagrangian analyses are used to investigate the chaotic behavior. From the Poincaré sections, the alternate rotation of the vortex generators is found to better disperse the fluid particles in the flow cross-section, while in the in-line array the particles are trapped into the vortex core. The Lagrangian study shows that initially close fluid particle paths exhibit an exponential remoteness in the alternate configuration, a sign of chaotic advection flow. This chaotic advection enhances the stretching and folding of the fluid particles which are responsible for mixing in laminar flows. The proposed flow configuration can be used as a multifunctional heat exchanger/reactor for industrial applications such as in chemical reaction and food processing. [ABSTRACT FROM AUTHOR]

• Introducing a simple method to solve HENs synthesis using the GA-MQLP model. • Using GA to find the structural variables. • Using the MQLP, which consists of two inner and outer surfaces, to find the optimal values of each HEN's continuous variables. • Finding the local optimal values of variables by combining an LP and a search loop, on the outer surface. • Increasing probability of finding the global optimal of these variables and the OOF on the inner surface by solving a set of linear equations. In this paper, an efficient as well as reliable approach to deal with heat exchanger networks (HENs) synthesis problems, which is inherently known as a mixed-integer non-linear programming model, is presented. The structural variables as the discrete variables are optimized by a genetic algorithm (GA), whereas continuous variables are handled by a modified quasi-linear programming (MQLP) model. Each HEN is considered as a chromosome consisting of a sequence of genes. Each gene also contains the address of the exchanger(s) in the network. The HENs generated by the GA are sent to the MQLP to calculate their overall objective function (OOF) (i.e. minimum total annual cost (TAC)). The MQLP model includes two inner and outer surfaces. On the outer surface, the local optimal values of the continuous variables are found according to the maximum energy recovery of HEN, while on the inner surface, the globally optimal values of them are found to reach the minimum TAC of HEN. Due to the relatively linear behavior of the proposed method, a comparison of results with references showed that this method can reduce TAC of HENs compared to the studied references by about (0.51% to 2.37%). [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the mathematical modeling of the ice crystallization process occurring during the freezing of sorbet in a continuous scraped surface heat exchanger (SSHE). Two different modeling approaches have been used, both of which include the nucleation, growth and breakage phenomena of the ice crystals. For both models, the rate of ice crystal nucleation and growth were determined by the subcooling degree. The first model combines heat transfer and population balance equations (PBE), assuming plug flow. The evolution of the values of product temperature, mean chord length, ice volume fraction and apparent viscosity were determined as a function of the residence time. The second model is a coupled model of heat transfer and PBE combined with an empirical model of residence time distribution (RTD), which makes it possible to take into account the fact that the fluid fractions flowing throughout the SSHE do not have the same time–temperature history. The values of the variables (product temperature and mean chord length) were determined for each fraction of fluid exiting the SSHE, and the bulk values were then calculated using the RTD. Simulation results were compared to a set of experimental data obtained during the ice crystallization process of sorbet in a continuous SSHE at the laboratory pilot scale. With a first estimated set of model parameters, it has been shown that the experimental tendencies are represented very satisfactorily by the two models within a 10% error limit. These modeling approaches can then be considered as a promising tool for the understanding and the prediction of the ice crystallization process in SSHEs so as to identify new ways to improve the performance of the process. [Copyright &y& Elsevier]

Abstract: Accurate predictions of the sulfuric acid condensation behavior on the surfaces of heat exchangers are crucial for understanding the local low-temperature corrosion characteristics of heat exchangers and designing them. In this paper, a new numerical model has been developed to predict the condensation rate of sulfuric acid and condensate acidic solution concentration on heat exchanger surfaces. By correlating the vapor–liquid equilibrium (VLE) data of H2SO4–H2O solutions from experiments and in conjunction with multi-component diffusion theory, the proposed model obtains numerical solutions of condensation heat transfer under the conditions of a coupled wall and fluid boundary condition and a multi-component mixture of flue gas and the sulfuric acid solution (saturated partial pressure of sulfuric acid and water vapor). The numerical model has been validated by a comparison of the simulation results with available experimental data, and applied to an analysis of the H-type finned tube heat exchanger, which has been widely used in the field of waste heat recovery. The distributions of the condensation rate and condensate concentration on the fin surface are also calculated. The results show that the three dimensional distribution of acid solution concentration is consistent with the fin temperature. An increase in water vapor could result in a sharp reduction of the acid solution concentration and an increase in deposition, which may indicate a serious risk for low-temperature corrosion. In contrast, increasing the flue gas temperature will reduce the corrosion risk by reducing the condensation rate and increasing the acid concentration. [Copyright &y& Elsevier]

Abstract: Carbon capture and storage for power generation systems is a major issue for achieving greenhouse gas emission reductions. Studies are usually dedicated to coal-fired power plants, but few efforts have addressed the challenge of CO2 capture for a gas turbine. In this paper, a recently reported hybrid process based on oxygen enriched air combustion, flue gas recycling and CO2 capture by membranes is explored for a 250MW open cycle gas turbine. An improved configuration based on an energy recovery system integrated with a heat exchanger offers superior performance compared to the reference gas turbine cycle. It is shown that an overall energy requirement below 2GJth/ton of CO2 captured (i.e. around 205kWhelectrical/ton CO2) can be achieved using a highly selective membrane (CO2/N2 selectivity of 200) operated under optimal process conditions. [Copyright &y& Elsevier]

Abstract: This paper analyzes an integrated Cu–Cl thermochemical cycle, Kalina cycle and electrolyzer for hydrogen production. The system operating parameters such as mass fraction, pressure and temperature are varied to investigate their effects on the energy and exergy efficiencies of the with/without heat recovery and integrated system, rate of hydrogen production, and rate of oxygen production. A new Cu–Cl design layout is then presented with a heat exchanger network developed to recover heat within the Cu–Cl cycle in the most efficient manner. The exergy destruction rate in each component is determined and discussed. The results show that increasing the electrolyzer temperature is beneficial up to 328.6K, after which the performance of the cycle results in a negative trend. The exergy efficiency varies more significantly with operating parameters than the corresponding energy efficiency. The maximum exergy destruction in the Cu–Cl cycle occurs in the first separator (16.82kW) and in the heat exchanger of the Kalina cycle (48.05kW), so these components should be considered for performance improvement of the system. [Copyright &y& Elsevier]

Abstract: The general problem of describing the flow distribution in manifolds feeding parallel channels has been addressed in a number of earlier publications. Such systems are used for example as heat exchanger plates, as distributors for fuel cells, or as solar collectors. We address here the "inverse problem", i.e. of finding a design, which provides uniform flow distribution among the channels, for which much less literature is available. We approach that problem by considering the steady-state flow-rates in the branches of 2D rectangular lattice networks with a single inlet and a single outlet, of which the classical parallel channels (ladder) network can be viewed as a special case. The purpose of this paper is to show how to organize the flow resistances on the periphery of the lattice so as to achieve flow uniformity in all inside channels. This is done under the assumption of a resistive network, with linear flow/pressure behavior (pure viscous Stokes flow, neglecting the inertial terms, neglecting or linearizing the effect of branching singularities) allowing completely analytical solutions and comparison of different configurations. The compatibility of uniform flow distribution and uniform Residence-Time-Distribution is discussed. Examples of new conceptual designs of heat exchanger plates, of fluid distributors and of mixers are proposed, based on this possibility of uniform distribution. [Copyright &y& Elsevier]

Abstract: In this paper, the regulation problem of convection governed dynamical processes described by linear distributed parameter models with position-independent inputs is addressed by combining the method of characteristics (MC) and the Malek-Zavarei and Jamshidi theorem. In this approach, the MC is used to transform the first order partial differential equations model of such processes into a set of time-delayed ordinary differential equations whose elements are used to generate certain matrices that satisfy linear matrix inequalities that render the proportional integral (PI) control parameters that guarantee the process stability. The performance of the proposed PI controller is tested via numerical simulations in the temperature regulation of a linear heat exchanger and compared to a tightly tuned classical PI controller. The application of the proposed controller is extended to regulate the outlet concentration in a nonlinear non-isothermal plug flow reactor. It is shown that the proposed controller is able to handle the infinite dimensional nature of the process model yielding smooth and unsaturated responses around the predetermined set-point trajectory in the face of load changes, time varying boundary conditions and set-point changes. [Copyright &y& Elsevier]

Abstract: A methodology to target the minimum energy requirement in water allocation networks where impurity concentrations do not play any significant role is proposed in this paper. The proposed methodology is mathematically rigorous and targets the minimum utility requirement satisfying the minimum approach temperature prior to the detailed design of both the water allocation network as well as the heat exchanger network. The proposed algorithm is presented through a tableau-based calculation procedure. A water allocation network design algorithm is proposed that guarantees the minimum energy targets obtained through the proposed energy recovery algorithm. The proposed methodology is also applied to inter-process heat integration between various water allocation networks. [Copyright &y& Elsevier]

Abstract: In addition to a high void volume and specific area, solid foams possess other properties (low density, good thermal, mechanical, electrical, and acoustical behaviour) that make them attractive for applications such as heat exchangers and reformers. Applications using foams as catalysts or structured catalyst supports have demonstrated higher performance than classical catalysts. Several studies have explored the hydrodynamic behaviour of foams in monophasic and countercurrent systems and have reported very low pressure drops. This paper describes the application of ceramic foam to distillation. The β-SiC foam contains 5 pores per inch (PPI) with a 91% void volume and a surface area of 640m2/m3. Performance parameters including pressure drop for the dry and wet packing, flooding behaviour, and dynamic liquid hold-up were measured in a column of 150mm internal diameter. The mass transfer efficiency in terms of the height equivalent to theoretical plate (HETP) was determined by total reflux experiments using a mixture of n-heptane and cyclohexane at atmospheric pressure. The experimental results were used to develop a set of correlations describing pressure drop and liquid hold-up in terms of a dimensionless number. The hydrodynamic performance and mass transfer efficiency were compared with classical packing materials used in distillation. [Copyright &y& Elsevier]

Abstract: Presented in this paper are experimental test and comparison for several shell-and-tube heat exchangers, one with segmental baffles and four with helical baffles at helix angles of , , and , respectively. The results show that, based on the same shell-side flow rate, the heat transfer coefficient of the heat exchanger with helical baffles is lower than that of the heat exchanger with segmental baffles while the shell-side pressured drop of the former is even much lower than that of the later. Further enhancement techniques should be incorporated in order to enhance shell-side heat transfer based on the same flow rate. The comparison of heat transfer coefficient per unit pressure-drop (and pumping power) versus shell-side volume flow rate shows that (1) the heat exchanger with helical baffles have significant performance advantage over the heat exchanger with segmental baffles; (2) for the same shell inner diameter, the performance of heat exchanger with helical baffles with 30° helix angle is better than that of 20°, and the performance of 40° helix angle is better than that of 50° helix angle. The heat exchanger with helical baffles of 40° angle shows the best performance among the five heat exchangers tested. [Copyright &y& Elsevier]

• The proposed method is based on the theory of Computational Geometry. • A novel method is introduced for evolving and quantifying the uniform distribution of bubble group. • Effectiveness of the new method is demonstrated on both numerical and experimental validation. • Establishing comprehensive evaluation indicators to evaluate the uniform distribution of bubbles. • The method can be used in other engineering fields that require evaluation of mix uniformity. To address the problem of large differences in the mixing time calculated by different characterization methods, this paper proposes an average distance (AD) method, a novel method to measure the distribution uniformity of random irregular bubble populations in direct contact heat exchanger (DCHE), and coupled with several evaluation indicators to study the mixing efficiency. With hypercube line picking theory in computational geometry as the theoretical basis, the AD between any two uniformly distributed points in a unit square as the study object, it is calculated that the normative reference value of AD when the uniform distribution is reached is approximately 0.5214. This study validates this method in a strict and effective manner. With the centroids of the bubble population in DCHE regarded as random points, the AD of bubbles was calculated, and the distribution of bubbles was determined by comparison with 0.5214. Experimental results show that the characteristic curve obtained using the AD method can validate the mixing uniformity of the bubbles. An evaluation indicator δ was established, and a high negative correlation was found between δ ¯ and the average volumetric heat transfer coefficient h v ¯. Using different mixing indicators, different mixing times were obtained by comparing the AD method to existing methods. Therefore, an effective comprehensive evaluation indicator of mixing efficiency φ was established to validate the mixing efficiency in each case. The heat exchange efficiency was verified to be higher when φ < 0.3. The novel method proposed in this study offers a new idea for studying various problems of non-uniform mixing subject to uniformity evaluation and facilitates deep study of the synergistic relationship between multiphase mixing efficiency and the heat transfer process. [ABSTRACT FROM AUTHOR]

Abstract: This paper aims to provide experimental heat transfer results for boiling of CaSO4 solutions on the outside of tube bundles, and also to shed some light into the mechanisms which influence heat transfer fouling under pool boiling conditions. Heat transfer coefficients for three heater rods with an identical diameter of 10.67mm on a 35mm vertical pitch have been measured. The independent variables bulk concentration and heat flux have been varied from 0.8 to 1.6g/L and 33 to 300kW/m2, respectively. The experimental results show that the mechanisms of fouling on the middle and top heater substantially differ from those at the bottom heater, due to the dominant effects of bubble impingement on the heat transfer surfaces. In particular, the competing effects on the degree of wall superheat and the supersaturation of the boiling liquid play a significant role. Spalling effects are more pronounced on the upper heaters where thin, friable deposits were found with low density and adherence, particularly at higher heat fluxes and lower concentrations. In terms of tube bundle efficiency, the best results were found for lower concentrations. [Copyright &y& Elsevier]

This paper demonstrates the use of the Infinite DimEnsionAl State space (IDEAS) approach in synthesizing optimal power cycles featuring minimum heat exchange area. IDEAS is used to synthesize power cycle networks which include splitters, mixers, pumps, turbines, and heat exchangers and feature a single or multiple working fluid(s). The overall synthesis goal is to minimize heat exchange area requirements, while delivering a fixed percentage of the maximum net power obtainable from a given set of hot and cold utilities. The global optimality of the obtained power cycle network configuration is guaranteed, since IDEAS gives rise to convex (linear) programs. The power of the proposed approach is demonstrated on a case study involving the generation of electricity by a bottoming cycle with a pure ammonia working fluid. Real thermodynamic data for pure ammonia and rigorous equipment models are employed in carrying out the proposed optimization. [Copyright &y& Elsevier]

• Design optimization of shell and tube condensers. • The design procedure always identify the global optimum. • The solution is obtained through an integer linear programming. • Comparison with textbook examples show reduction of the heat transfer area. The literature about heat exchanger design optimization encompasses a large number of papers, but only a small fraction is dedicated to phase change systems. This article presents for the first time a linear design optimization approach for horizontal shell and tube condensers that guarantees global optimality. Area is minimized using an integer linear model. This model is obtained from a mathematical reformulation of the original nonlinear thermofluid dynamic equations and it is solved using integer linear programming (ILP). Because of the linear nature of the proposed model, the global optimum is always attained, without the need of using specialized global solvers, selection of good initial estimates or drawbacks related to convergence issues. Numerical results indicate that the proposed approach can present solutions with lower heat transfer areas than two design procedures presented in chemical process design textbooks. [ABSTRACT FROM AUTHOR]

• Discussion on metal foam heat exchangers for various energy-intensive industries. • Comprehensive review of particulate fouling through porous metal foams. • Proposition of oscillatory fluids as a heat exchanger fouling mitigation technique. • Numerical and experimental methods to investigate heat exchanger fouling. Curtailing the ever-increasing global energy demand remains an arduous challenge. The U.S. Energy Information Administration emphasized that the deployment of new heat exchanger technologies could revolutionize how industry uses energy. In this review, we highlight the significance of metal foam heat exchangers as an attractive alternative to traditional heat exchanger technologies. Research on metal foam heat exchangers is steadily gaining momentum. However, metal foam heat exchangers are highly susceptible to fouling which results in a myriad of issues such as low heat exchanger performance and high energy consumption. These issues are further compounded by the fact that the fundamental mechanisms governing particulate fouling in metal foam heat exchangers are poorly understood. As such, the overarching goal of reducing energy consumption and greenhouse gas emissions could be met by deploying energy-efficient heat exchanger technologies and also by gaining a solid comprehension of multiphase flows, heat transfer, and heat exchanger fouling mechanisms. The development of advanced numerical methods to unravel heat exchanger fouling mechanisms could pave the way for an optimized heat exchanger design with minimum energy consumption and greenhouse gas emissions. This paper provides researchers a review of the performance of metal foam heat exchangers for various industrial applications and the implications of particulate fouling on the thermal performance of metal foam heat exchangers. [ABSTRACT FROM AUTHOR]

• A simultaneous multi-plant indirect heat exchanger network model was proposed. • Using DE to search for the temperatures of the two ends in the intermediate fluids circuit. • Deterministic method and heuristic algorithm were combined. • Connection cost and pump related cost were involved. 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. [ABSTRACT FROM AUTHOR]

• The proposed DHISR model incorporates safety redundancy to decrease the risk loss in case of unplanned shutdown in interplant heat integration. • The HEN superstructure model is optimized with a bilevel meta-heuristic algorithm that combines AEA and PSO algorithm. • The cost-sharing strategy of a two-plant HEN uses an improved risk-based Shapley value that considers contributions and shutdown risks simultaneously. Heat exchanger network (HEN) synthesis is important for energy savings and cost reduction. Current research on interplant heat integration has mainly focused on the heat recovery aspect, while ignoring the operating risk due to the unplanned shutdown of other plants. In this paper, a new two-plant direct heat integration model that considers safety redundancy (DHISR) is proposed to decrease the risk loss as far as possible. A bilevel meta-heuristic algorithm that consists of an evolutionary algorithm of pattern extraction (ALOPEX) and a particle swarm optimization algorithm is implemented to optimize the mixed-integer non-linear programming superstructure model. The financial benefits allocated to the plants in the heat integration alliance are determined by the improved two-plant risk-based Shapley value. The results of three examples show that the proposed approach achieves a good performance in the two-plant HEN synthesis and provides good cost distribution. [ABSTRACT FROM AUTHOR]