224 results found
Gangar N, Macchietto S, Markides C, 2020, Recovery and utilization of low-grade waste heat in the oil-refining industry using heat engines and heat pumps: an international technoeconomic comparison, Energies, Vol: 13, Pages: 2560-2560, ISSN: 1996-1073
We assess the technoeconomic feasibility of onsite electricity and steam generation from recovered low-grade thermal energy in oil refineries using organic Rankine cycle (ORC) engines and mechanical vapour compression (MVC) heat pumps in various countries. The efficiencies of 34 ORC and 20 MVC current commercial systems are regressed against modified theoretical models. The resulting theoretical relations predict the thermal efficiency of commercial ORC engines within 4–5% and the coefficient of performance (COP) of commercial MVC heat pumps within 10–15%, on average. Using these models, the economic viability of ORC engines and MVC heat pumps is then assessed for 19 refinery streams as a function of heat source and sink temperatures, and the available stream thermal energy, for gas and electricity prices in selected countries. Results show that: (i) conversion to electrical power with ORC engines is, in general, economically feasible for heat-source temperatures >70 ◦C, however with high sensitivity to energy prices; and (ii) steam generation in MVC heat pumps, even more sensitive to energy prices, is in some cases not economical under any conditions—it is only viable with high gas/low electricity prices, for large heat sources (>2 MW) and higher temperatures (>140 ◦C). In countries and conditions with positive economics, payback periods down to two years are found for both technologies.
Diaz-Bejarano E, Coletti F, Macchietto S, 2020, A model-based method for visualization, monitoring, and diagnosis of fouling in heat exchangers, Industrial & Engineering Chemistry Research, Vol: 59, Pages: 4602-4619, ISSN: 0888-5885
A critical review of current methods for monitoring the performance of heat exchangers in the presence of fouling highlights a number of pitfalls. An improved analysis method and visualization of operation data (the TH-λ plot) are proposed, which enable to accurately and rapidly estimate the location and extent of fouling, the properties of the deposit, and their impact on exchanger performance. The method uses advanced dynamic thermo-hydraulic models to analyze the data. The visualization presents this information in a way easily interpreted by field engineers. The superior features are demonstrated on various applications, where traditional methods give poor visibility or outright wrong information about underlying events. These include organic fouling deposition and aging, incomplete cleaning, multicomponent deposits, and changes in fouling behavior. First, the basic concepts are illustrated with idealized examples (constant inlet conditions, using simulated data). The approach is then applied to three real refining case studies, with pressure drop either measured or generated via soft sensors. The results show that the advanced dynamic models used enable to properly integrate and interpret highly variable data measurements, explain complex underlying thermal and hydraulic effects, adequately monitor performance, and rapidly detect changes in fouling behavior. The approach provides a new practical tool for monitoring of heat exchanger performance and early fouling diagnosis.
Lozano Santamaria F, Macchietto S, 2020, Online integration of optimal cleaning scheduling and control of heat exchanger networks under fouling, Industrial and Engineering Chemistry Research, Vol: 59, Pages: 2471-2490, ISSN: 0888-5885
Fouling mitigation is paramount to maintaining the reliable and efficient operation of a heat exchanger network (HEN). From the operational perspective, fouling can be mitigated by changing the flow distribution in the network (control actions), or performing periodic cleanings of the units (scheduling actions). Flow control and scheduling have usually been considered independently, ignoring their interaction. This paper presents an online methodology and implementation that integrates control and scheduling decisions for fouling mitigation in HEN, using first principle models of the heat exchangers subject to fouling. A multiloop NMPC/MHE scheme is proposed to estimate the current state of the HEN, and then define the optimal flow distribution and cleaning schedule over a moving horizon. It is shown that this online scheme reacts rapidly to disturbances and copes with model-plant mismatch by updating the model parameters at an appropriate frequency. The methodology is demonstrated on a real industrial case study involving crude oil fouling in the preheat train of a refinery. Application of the methodology shows that (i) significant economic benefits result relative to the actual historical operation, (ii) the online integration achieves a lower operating cost than that of the optimization of control or scheduling individually, (iii) the effect of disturbances is important and the scheme rejects them efficiently, (iv) updating the prediction models deals effectively with plant-model mismatch and process variability, and gives a sufficiently accurate representation of the underlying process, and (v) the computational effort required to solve all optimization problems is low and allows for the practical online implementation of the scheme.
Oyewunmi O, Lozano Santamaria F, Markides C, et al., Modelling two-phase flows in renewable power generation systems, 5th Thermal and Fluids Engineering Conference (TFEC)
Bailey T, Liu P, Macchietto S, Optimisation of energy and water supply systems for the Dubai Waterfront, 9th International Conference on Foundations of Computer-Aided Process Design, Publisher: Elsevier, Pages: 107-112, ISSN: 1570-7946
Cameron IT, Engell S, Georgakis C, et al., 2019, Education in Process Systems Engineering: Why it matters more than ever and how it can be structured, Computers and Chemical Engineering, Vol: 126, Pages: 102-112, ISSN: 0098-1354
© 2019 Elsevier Ltd This position paper is an outcome of discussions that took place at the third FIPSE Symposium in Rhodes, Greece, between June 20–22, 2016 (http://fi-in-pse.org). The FIPSE objective is to discuss open research challenges in topics of Process Systems Engineering (PSE). Here, we discuss the societal and industrial context in which systems thinking and Process Systems Engineering provide indispensable skills and tools for generating innovative solutions to complex problems. We further highlight the present and future challenges that require systems approaches and tools to address not only ‘grand’ challenges but any complex socio-technical challenge. The current state of Process Systems Engineering (PSE) education in the area of chemical and biochemical engineering is considered. We discuss approaches and content at both the unit learning level and at the curriculum level that will enhance the graduates’ capabilities to meet the future challenges they will be facing. PSE principles are important in their own right, but importantly they provide significant opportunities to aid the integration of learning in the basic and engineering sciences across the whole curriculum. This fact is crucial in curriculum design and implementation, such that our graduates benefit to the maximum extent from their learning.
Santamaria FL, Macchietto S, 2019, Integration of optimal cleaning scheduling and control of heat exchanger networks under fouling: MPCC solution, Computers and Chemical Engineering, Vol: 126, Pages: 128-146, ISSN: 0098-1354
Fouling is a major source of energy inefficiencies that decreases the performance of process units. Fouling mitigation alternatives are required to ensure a sustainable, profitable, and safe operation. In heat exchanger networks, two mitigation alternatives are: flow distribution control, and periodical cleanings. This paper addresses the optimal control and optimal cleaning scheduling, individually and simultaneously, for heat exchangers in refining operations. The problem is formulated as a MINLP, which uses an accurate model of the process, logic disjunctions, and a continuous time representation. To solve it efficiently, it is reformulated as a mathematical program with complementarity constraints (MPCC). The modelling and solution strategies are demonstrated in several case studies, from small to moderately large networks for realistic applications. The formulation is versatile, and large network problems are solved in a reasonable computational time. The integration of optimal scheduling and control decreases the operational cost substantially relative to independent mitigation alternatives.
Diaz-Bejarano E, Coletti F, Macchietto S, 2019, Modeling and prediction of shell-side fouling in shell-and-tube heat exchangers, Heat Transfer Engineering, Vol: 40, Pages: 845-861, ISSN: 0145-7632
Fouling is a challenging, longstanding, and costly problem affecting a variety of heat transfer applications in industry. Mathematical models that aim at capturing and predicting fouling trends in shell-and-tube heat exchangers typically focus on fouling inside the tubes, while fouling on the shell side has generally been neglected. However, fouling deposition on the shell side may be significant in practice, impairing heat transfer, increasing pressure drops, and modifying flow paths. In this paper, a new model formulation is presented that enables capturing fouling on the shell side of shell-and-tube heat exchangers including the effect of occlusion of the shell-side clearances. It is demonstrated by means of an industrial case study in a crude oil refinery application. The model, implemented in an advanced simulation environment, is fitted to plant data. It is shown to capture the complex thermal and hydraulic interactions between fouling growth inside and outside of the tubes, the effect of fouling on the occlusion of the shell-side construction clearances, and to unveil the impact on shell-side flow patterns, heat transfer coefficient, pressure drops, and overall exchanger performance. The model is shown to predict the fouling behavior in a seamless dynamic simulation of both deposition and cleaning operations, with excellent results.
Veys A, Acha V, Macchietto S, A financial accounting based model of carbon footprinting: Built environment example, 29th European Symposium on Computer Aided Process Engineering, Publisher: Elsevier, Pages: 1663-1668, ISSN: 1570-7946
Sharma A, Macchietto S, A computational study on fouling and cleaning operations in milk pasteurisation using a moving boundary model, 29th European Symposium on Computer Aided Process Engineering, Publisher: elsevier
Macchietto S, Engineering success: What does it take to get PSE technologies used?, 29th European Symposium on Computer Aided Process Engineering, Publisher: Elsevier, Pages: 85-90, ISSN: 1570-7946
Behranvand E, Mozdianfard MR, Diaz-Bejarano E, et al., 2019, Cross sectional examination of a fouled tube removed from a crude oil preheat exchanger, Industrial && Engineering Chemistry Research, Vol: 58, Pages: 9651-9664, ISSN: 0888-5885
A first attempt to characterize intact foulant of a refinery preheater is presented. A tube was removed from an exchanger located postdesalter and preflash-drum at a 4-year shutdown, and dissected into undisturbed cut-out rings. Following visual inspection, elemental analysis (X-ray maps and line scans) was carried out and radial concentration profiles of the existing elements were established. A stratified colored foulant layer inside the tube appeared in sine waves fluctuating at both axial and angular directions, likely evidencing for the first time a shadow effect and erosion process. Results agreed with our studies on a comparable exchanger of the same refinery. They confirm the proposed deposition mechanism and simulation results, indicating formation of a stratified foulant consisting inorganics ranging 50 wt %, the presence of an acute inorganic deposition period along the chronic organic–inorganic fouling, and the identification of the foulant phases arrangement, among possible conductivity mixing models, as cocontinuous and effective medium theory structures.
Diaz-Bejarano E, Behranvand E, Coletti F, et al., 2019, Organic and inorganic fouling in heat exchangers: Industrial case study analysis of fouling rate, Industrial & Engineering Chemistry Research, Vol: 58, Pages: 228-246, ISSN: 0888-5885
Fouling rates in refinery heat exchangers with mixed organic/inorganic deposits (frequent in practice) are estimated using a comprehensive model-based thermohydraulic methodology combining data-driven measurements analysis with advanced models. An industrial case study for a heat exchanger over 4 years demonstrates the method. Following an analysis of the fouling state, the dynamic analysis here estimates organic and inorganic fouling rates using constant or time-varying proportionality ratios. Base-line organics deposition rate is described by a typical correlation, inorganics deposition as a perturbation with constant or time-varying proportionality ratios. Deposition rate parameters are estimated from measured pressure drops and validated against temperatures. Results show that the deposition rate ratio varied substantially over time, revealing acute inorganic deposition periods; accounting for inorganics explains well both thermal and hydraulic performances; the time-varying ratio provided a good fit of the data. This is a highly promising new method for predictive monitoring, detection, and diagnosis of fouling.
Sharma A, Macchietto S, 2019, Fouling and Cleaning of Plate Heat Exchangers for Milk Pasteurisation: A Moving Boundary Model, Editors: Kiss, Zondervan, Lakerveld, Ozkan, Publisher: ELSEVIER SCIENCE BV, Pages: 1483-1488, ISBN: 978-0-12-819940-4
Santamaria FL, Macchietto S, 2018, Integration of optimal cleaning scheduling and control of heat exchanger networks undergoing fouling: Model and formulation, Industrial & Engineering Chemistry Research, Vol: 57, Pages: 12842-12860, ISSN: 0888-5885
The performance and operability of heat exchanger networks (HENs) is strongly affected by fouling, which involves the deposition of unwanted material, which reduces the heat-transfer rate and increases the pressure drop, the operational costs, and the environmental impact of the process. Periodical cleaning and control of the flow rate distribution in the HEN are used to mitigate the effects of fouling and restore the performance of the units. The optimal cleaning scheduling has been formulated as a mixed-integer linear programming (MILP) or mixed-integer nonlinear programming (MINLP) problem and is solved using various approaches. The optimal control has been formulated as a nonlinear programming (NLP) problem and is used to define the flow rate distribution of the network. Both problems share the same objective: minimization of the total cost of the operation. In principle, the simultaneous solution of the optimal control problem and the optimal cleaning scheduling problem should provide greater savings than the independent or sequential solution of the two problems, since the interactions of the two mitigation alternatives are considered. However, these two problems have been typically considered separately, because of modeling and solution challenges. Also, it is not quite clear what additional benefit a simultaneous solution may bring. The challenges for solving the integrated problem are the large scale of the associated optimization problem and the different time scales involved in each operational layer. Here, a general and efficient formulation is proposed, using a continuous time discretization scheme for the integrated problem of scheduling and control of HENs subject to fouling. A dynamic model of the heat exchangers is proposed that is sufficiently detailed to represent the physics of interest with novel modifications to address simultaneously their control and scheduling in a network. The problem is formulated as a MINLP and solved using deterministic
Diaz-Bejarano E, Behranvand E, Coletti F, et al., 2018, Identification of thermal conductivity mixing models for heterogeneous fouling deposits, 16th International Heat Transfer Conference, IHTC-16, Pages: 4057-4064, ISSN: 2377-424X
© 2018 International Heat Transfer Conference. All rights reserved. Fouling deposition on heat transfer surfaces is a costly problem in pre-heat trains of crude distillation units. Organic and inorganic species found in the deposits are characterized by markedly different thermal conductivity. Fouling heat transfer resistance depends on the extent of inorganic and organic phases, their structural arrangement, and stratification or layering. Understanding and, if possible, detecting the deposit composition would provide additional capabilities in monitoring, troubleshooting, prediction and optimisation of mitigating remedies. This paper aims at elucidating possible arrangements of the inorganic and organic phases by testing various thermal-conductivity mixing models for heterogeneous materials. A novel model describing the deposit layer as a multi-component structure is used with available plant measurements (pressure drop, temperatures, flowrates) to infer the deposit properties (e.g. thickness, conductivity and composition) over time. Various “mixing” models that describe the arrangement of the organic and inorganic phases in fouling deposits are tested. Radial composition profile of the deposit calculated with various mixing models is compared to the experimental composition analysis of deposits collected upon dismantling of the exchanger after four years of operation. The differences between this approach and a traditional analysis based on fouling resistance (that neglects deposit stratification) are discussed. Results show that the four mixing models tested provide an appreciably different level of agreement between compositions calculated from the plant measurements and the experimental characterization. The best agreement with the experimental data was obtained with a Co-Continuous model (all phases continuous), followed closely by an Effective Media Theory model (all phases discrete).
Diaz-Bejarano E, Coletti F, Macchietto S, 2018, Complex crude oil fouling layers: use of model predictions to detect inorganics breakthrough, Applied Thermal Engineering, Vol: 141, Pages: 666-674, ISSN: 1359-4311
Crude oil fouling models have greatly improved in the past two decades. However, most models focus on the deposition of organic species at high temperatures (i.e. greater than 200 °C). In this paper, a deposit model, capable of capturing simultaneously the deposition of both organic and inorganic species, is used to track deposition history in a shell-and-tube heat exchanger at the hot end of a refinery pre-heat train. The model was previously fitted to plant data and the results compared to the experimental characterization of deposits. It is shown that such a model, together with plant data, can be used (i) to describe the development of complex deposit layers; (ii) to detect and diagnose changes in composition of the deposit. From a practical perspective, it is then possible to alert plant operators of unexpected events such as breakthrough of inorganics at an early stage and help in planning corrective actions.
Diaz Bejarano E, Coletti F, Macchietto S, Identifying the causes of acute fouling in refinery preheat train, 2018 AIChE Spring Meeting and 14th Global Congress on Process Safety, Publisher: American Institute of Chemical Engineers
Crude oil fouling is typically treated in the literature as a phenomenon involving the deposition of organic material on the thermal surfaces of heat exchangers. Little attention is paid to the effects that inorganic species have on the thermal and hydraulic performance of the heat exchangers affected. When deposition of inorganic species occurs, it is often the cause of an upset in operations (e.g. issues with the desalter) that results in episodes of rapid (acute) fouling. In this paper, a method used to detect acute fouling episodes and identifying their causes is presented.
Diaz Bejarano E, Coletti F, Macchietto S, Improving crude oil fouling monitoring, prediction and mitigation strategies in refinery preheat trains, 2018 AIChE Spring Meeting and 14th Global Congress on Process Safety, Publisher: American Institute of Chemical Engineering
Traditional heat exchanger monitoring methodologies rely on fouling resistance calculations that neglect the effects of fouling on pressure drops, and are not able to predict future performance as a function of process conditions, heat exchangers geometry and network configurations. In this paper, an improved approach to monitoring of fouling in refinery pre-heat trains that rely on rigorous predictive models is summarised and illustrated with an industrial case study.
Coletti F, Lozano Santamaria F, Diaz Bejarano E, et al., 2018, Optimization of refinery preheat trains: predictive maintenance and operations improvement, 2018 AIChE Spring Meeting and 14th Global Congress on Process Safety, Publisher: American Institution of Chemical Engineering
Deciding which heat exchanger to clean, when to clean and how to clean in refinery pre-heat trains is a challenging activity that typically relies on operator’s experience. In this paper, an algorithm that allow identifying the most economic cleaning schedule for a given refinery configuration and operating conditions is presented. The method relies on an advanced framework that incorporates rigorous heat exchanger models capable of predicting the fouling behaviour of the refinery as a function of configuration of the individual units and the network, process conditions and time. An industrial case study is presented to illustrate the benefits of the approach, showing that significant improvements over current practice can be obtained.
Coletti F, Diaz Bejarano E, Macchietto S, 2018, Dynamic Data Analysis™ of large scale data to monitor fouling in heat exchanger networks, 2018 AIChE Spring Meeting and 14th Global Congress on Process Safety, Publisher: American Institute of Chemical Engineers
Data collected from heat exchangers networks in the field are typically used to monitor the thermal performance of the individual units. However, the information extracted from the data is limited in quantity and quality by the simplified models typically used in practice. Key decisions such as cleaning of heat exchangers rely on the calculation of derived quantities such as the fouling resistance which lumps together a number of factors contributing to fouling. This approach has been severely criticised in the past by various authors but it is still widely used in the industrial practice . In this paper it is shown that a significantly larger amount of information and insights can be extracted from the same measurements by using rigorous models and a flexible framework. It is shown how this approach leads to a much deeper analysis of the status of the network which, in turn, helps with diagnosis and troubleshooting. An industrial case study is presented to illustrate the benefits.
Behranvand E, Mozdianfard MR, Diaz-Bejarano E, et al., 2018, A comprehensive investigation of refinery preheaters foulant samples originated by heavy crude oil fractions as heating fluids, Fuel, Vol: 224, Pages: 529-536, ISSN: 0016-2361
A deep understanding of the mechanisms responsible for fouling from both crude oils and their fractions is paramount to ensure efficient energy recovery in heat exchangers of crude preheat trains. In this work, seven samples of fouling deposits, carefully collected from a number of refinery heat exchangers processing vacuum gas oil (VGO) and vacuum bottom (VB) streams in an atmospheric crude preheat train were investigated using a range of characterization techniques with the aim of identifying the underlying mechanisms that led to deposition. Characterization of the deposits included morphological and physical examination, fractionating solubility test, Scanning Electron Microscopy-Energy Dispersive X-ray, Combustion Analysis and X-ray Diffraction. In all samples examined, more than 75 wt% of the deposits were identified as inorganic, with about 50 wt% being FeS. At 270–300 °C, FeO(OH) was also identified to be deposited on the tube surfaces made in Cr steel alloy, where more fouling and less corrosion were evident compared to carbon steel (CS). These observations were found in agreement with recent laboratory studies aimed at identifying the role of temperature and tube material in petroleum corrosion. Furthermore, sulphur crystals were found in several VGO fouling samples. Based on the experimental results obtained, a mechanism was proposed to explain the corrosion fouling phenomenon, considered to be the underlying mechanism affecting the refinery. The mechanism involves naphthenic acid attack to the tubes’ metal surface, decomposition of iron naphthenate, disproportion of iron oxide and sulphidation reactions. The results highlighted the importance of studying deposits formed under industrial conditions, timescales and variation of the deposition process, evidenced by the deposit characteristics, along extensive heat exchanger networks.
Santamaria FL, Macchietto S, 2018, Integration of Optimal Cleaning Scheduling and Flow Split Control for Crude Oil Fouling Mitigation in the Operation of Refinery Heat Exchanger Networks, Computer Aided Chemical Engineering, Pages: 1087-1092
© 2018 Elsevier B.V. Fouling in heat exchangers, a slow dynamic process, significantly reduces their thermal and hydraulic performance. For heat exchanger networks (HENs) in particular, mitigation alternatives are needed to restore a profitable and safe operation. Controlling the flow rate distribution in parallel branches of the network is one option, periodically cleaning selected heat exchangers another. These two strategies have been traditionally addressed separately or sequentially, but there is a strong interaction between them. We propose i) a general formulation to model heat exchanger networks under fouling, suitable for ii) the simultaneous optimal control and optimal cleaning scheduling. The resulting large scale dynamic optimization problem with binary variables (MINLP) is reformulated as a mathematical program with complementarity constraints (MPCC) which is solved efficiently for networks of industrially relevant size. The benefit of integrating the two decision layers is demonstrated in a case study for a crude oil preheat train, where the simultaneous solution of the two problems leads to a 25 % savings in operational cost.
Santamaria FL, Macchietto S, 2018, Optimal cleaning scheduling and control of heat exchanger networks: An industrial case study, Pages: 50-57
Copyright © American Institute of Chemical Engineers. All rights reserved. Fouling in heat exchangers of refinery applications has a high impact on operations as it reduces the thermal and hydraulic performance of the units. Fouling mitigation is required to keep the operation closer to the optimal point of low energy consumption, high throughput, and high safety standards. Two fouling mitigation alternatives are commonly used in the operation of large networks: periodic cleanings and flow rate control. The first one can remove the deposit but requires units to be taken out of service, while the second one allows a continued operation, but it just reduces the deposition rate. These two strategies have strong interactions that define the performance of the preheat train. Here we analyze the simultaneous solution of the optimal cleaning scheduling and optimal flow distribution for an industrial case study of the hot end of the preheat train. The problem is formulated as a large scale nonlinear mathematical programming problem with binary variables, which is solved efficiently using complementarity constraints. The optimal simultaneous solution of both problems has an economic benefit over the actual operation of the preheat train (based on past data) of $ 3.2 M over an operation horizon of 4 years.
Macchietto S, Coletti F, Bejarano ED, 2018, Energy Recovery in Heat Exchanger Networks in a Dynamic, Big-data World: Design, Monitoring, Diagnosis and Operation, Computer Aided Chemical Engineering, Pages: 1147-1152
© 2018 Elsevier B.V. Heat exchanger networks (HENs) are often analysed with simplified models which are not sufficiently predictive in the presence of fouling, in particular for operations monitoring, diagnosis and support. A comprehensive approach is presented, with application to oil refining, that moves substantially beyond the usual simplifying assumptions and incorporates: i) far more detailed models of the physics involved ii) detailed dynamics for both slow and fast changes; iii) detailed reaction engineering models of the deposition of fouling deposits on thermal surfaces as well as deposits removal in chemical and mechanical cleanings; iv) the ability to easily generate multi-scale, detailed models of individual exchangers and whole networks; v) consideration of the complex thermal and hydraulic interactions between exchangers and effects on performance; vi) exploiting the abundant plant data available (historical and current) in conjunction with the more sophisticated models; vii) solution of all problems within an easy to use engineering software environment.
Guan S, Macchietto S, 2018, A Novel Dynamic Model of Plate Heat Exchangers Subject to Fouling, Editors: Friedl, Klemes, Radl, Varbanov, Wallek, Publisher: ELSEVIER SCIENCE BV, Pages: 1679-1684
Santamaria FL, Macchietto S, 2018, Optimal cleaning scheduling and control of heat exchanger networks: Problem formulation and solution strategy, Pages: 1-8
© Computing and Systems Technology Division 2018 - Core Programming Area at the 2018 AIChE Annual Meeting. All Rights Reserved. In crude oil processing heat exchanger fouling is a major problem because it reduces the thermal and hydraulic performance of the units which has a large effect in the energy efficiency and operational cost. Fouling mitigation alternatives are mandatory and two of the most common and effective are: flow distribution control in heat exchanger networks, and periodic cleanings. These two mitigation alternatives can be defined individually by solving two distinct optimization problems, an optimal control problem (NLP) for the optimal flow distribution, and an optimal scheduling problem (MINLP) for the cleaning scheduling. However, there are important interactions between these two mitigation strategies that are neglected when addressed independently, while solving the integrated scheduling and control problem has economic benefits, but it is highly challenging. Here we present a general, flexible and accurate formulation to model heat exchanger networks under fouling, and an efficient solution strategy to tackle the optimal control and optimal scheduling problems simultaneoulsy. The formulation is able to capture all the important interactions of the operation, and includes all variables for the various decisions. The integrated control and scheduling problem is reformulated as a MPCC (mathematical problem with complementarity constraints) and then solved using a NLP solver based on interior point methods. A case study of industrial significance is used to illustrate the advantages of the formulation, the efficiency of the solution, and the benefits of considering the control and scheduling optimization problems at the same decision level.
Diaz-Bejarano E, Behranvand E, Coletti F, et al., 2017, Organic and inorganic fouling in heat exchangers – Industrial case study: Analysis of fouling state, Applied Energy, Vol: 206, Pages: 1250-1266, ISSN: 0306-2619
A comprehensive model-based thermo-hydraulic methodology is used to investigate fouling behaviour in refinery heat exchangers where high concentration of inorganics in the deposits was reported. The method combines a data-driven analysis of plant measurements (including pressure drop) with a model-based analysis using advanced models of shell-and-tube heat exchangers undergoing fouling. A deposit model capable of tracking composition and deposition history was extended to include thermal-conductivity mixing models appropriate for various deposit structures. Substantial new and useful information can be extracted from the plant measurements in comparison to current practice: the thickness, the effective conductivity, and the radial conductivity and composition profiles of the deposits, reflecting the exchanger operation history. Episodes of rapid and acute fouling, and deposition of inorganic materials could be identified and quantified. A validation of the approach was carried out by (i) a comparison of averaged predicted and experimental inorganic weight fractions in a mixed deposit sample collected at the end of run, and (ii) an initial comparison of predicted radial inorganics profiles and experimental ones (obtained with SEM-EDX) in deposits from similar exchangers. Both steps yielded surprisingly good agreement. The study indicates that the method employed represents a new powerful, model-based analysis tool for monitoring, diagnosis and troubleshooting of fouling in heat exchangers.
Macchietto S, Coletti F, 2017, Engineering innovation by design: The uniheat project, 2017 AIChE Spring Meeting, Pages: 711-730
Diaz-Bejarano E, Coletti F, Macchietto S, 2017, Thermo-hydraulic analysis of refinery heat exchangers undergoing fouling, AIChE Journal, Vol: 63, Pages: 984-1001, ISSN: 0001-1541
A complete, systematic approach is presented for the analysis and characterization of fouling and cleaning in refinery heat exchangers. Bringing together advanced thermo-hydraulic dynamic models, some new formulations, and a method for dynamic analysis of plant data, it allows: extracting significant information from the data; evaluating the fouling state of the units based on thermal measurements and pressure drops, if available; identifying the range of deposit conductivity leading to realistic pressure drops, if pressure measurements are unavailable; estimating key fouling and ageing parameters; estimating the effectiveness of cleaning and surface conditions after a clean; and predicting thermal and hydraulic performance with good accuracy for other periods/exchangers operating in similar conditions. An industrial case study demonstrates the performance prediction in seamless simulations that include partial and total cleanings for over 1000 days operation. The risks of using thermal effects alone and the significant advantages of including pressure drop measurements are highlighted.
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