Francesco Coletti

Fouling in Crude Oil Preheat Trains: A Systematic Solution to an Old Problem

A major cause of refinery energy inefficiency is fouling in preheat trains. This has been a most challenging problem for decades, due to limited fundamental understanding of its causes, deposition mechanisms, deposit composition, and impacts on design/operations. Current heat exchanger design methodologies mostly just allow for fouling, rather than fundamentally preventing it. To address this problem in a systematic way, a large-scale interdisciplinary research project, CROF (crude oil fouling), brought together leading experts from the University of Bath, University of Cambridge, and Imperial College London and, through IHS ESDU, industry. The research, coordinated in eight subprojects blending theory, experiments, and modeling work, tackles fouling issues across all scales, from molecular to the process unit to the overall heat exchanger network, in an integrated way. To make the outcomes of the project relevant and transferable to industry, the research team is working closely with experts from many world leading oil companies. The systematic approach of the CROF project is presented. Individual subprojects are outlined, together with how they work together. Initial results are presented, indicating that a quantum progress can be achieved from such a fundamental, integrated approach. Some preliminary indications with respect to impact on industrial practice are discussed.

Effects of fouling on performance of retrofitted heat exchanger networks: A thermo-hydraulic based analysis

In refineries, fouling in crude pre-heat trains (PHTs) causes several thermal-hydraulic inefficiencies which lead to increased operating costs (from reduction in throughput and extra fuel burnt at the furnace), carbon emissions, and maintenance issues. The energy recovery performance of PHT can be severely affected over time. Such time varying effects are normally not considered in the design or retrofit of heat exchangers networks. In this paper, an existing PHT network is simulated including its fouling behaviour of over ca. two years. For this purpose, a dynamic, distributed mathematical model for shell-and-tube heat exchangers undergoing crude oil fouling (developed and validated against refinery data in previous work) is used. Three retrofit options aimed at maximizing overall heat recovery are proposed. Simulation results show that networks designs that maximize energy recovery in clean conditions (following traditional pinch rules) may not be best when fouling occurs and that a proper retrofit design must include consideration of time varying fouling effects.

A heat exchanger model to increase energy efficiency in refinery pre heat trains

Abstract Crude oil fouling in pre-heat train heat exchangers has been a major problem in oil refining for decades. The operating problems, increased energy requirements and greenhouse gases emissions which arise from the inefficiencies caused by fouling are discussed. A mathematical model capable of predicting the dynamic behavior of a shell and tube heat exchanger undergoing fouling is used to assess the costs and the environmental impact of the crude distillation unit. Using the model, retrofit options were proposed for an existing industrial unit leading to improved energy efficiency.

Refinery Pre-Heat Train Network Simulation Undergoing Fouling: Assessment of Energy Efficiency and Carbon Emissions

Fouling in crude preheat trains in oil refineries causes additional fuel and production costs, operating difficulties, CO2 emissions, and safety issues. Crude oil fouling deposition mechanisms are still not well understood. Current exchanger design methodologies (based on empirical fouling factors), operating practices, and mitigation solutions (ranging from the use of chemical additives to tube inserts) do not prevent efficiency losses or disruption of operations. Moreover, current analysis and design methodologies neglect local effects and dynamics of fouling, in favor of lumped, steady-state, “averaged” heuristic models. In this paper, a dynamic and distributed model recently developed that accounts for localized fouling growth as a function of process conditions is used to simulate the dynamic behavior of the hot end of a refinery preheat train. The network is simulated by a simultaneous solution of all exchangers, combined according to a desired configuration, within gPROMS, a commercial dynamic simulation environment. The overall network model allows capturing some complex interactions within the network over time and enables the rigorous computation of several key indicators which are highly dependent on fouling. These include throughput reduction, additional energy requirements, and overall economic and CO2 emission impacts.

Detection of changes in fouling behavior by simultaneous monitoring of thermal and hydraulic performance of refinery heat exchangers

Abstract Monitoring of pre-heat trains in oil refineries is a crucial activity to assess the gradual decay in performance of heat transfer equipment due to fouling. It is also important to assist in operational decisions with respect to fouling mitigation options and cleaning strategies. In this paper, a novel graphical representation of time varying operational data is proposed to monitor crude oil fouling. This dynamic thermo-hydraulic plot, named the TH-λ plot, simultaneously captures both thermal and hydraulic performance and presents it in a way easily interpreted by field engineers. Its features and applications are demonstrated here for typical situations using data generated using an advanced dynamic simulation model for shell-and-tube heat exchanger undergoing fouling. The results show that consideration of thermal and hydraulic effects, together with process dynamics, is essential to adequately monitor performance, detect changes in fouling behaviour and properly interpret available data. They also show this is achievable using only measurements of inlet and outlet streams to heat exchangers.

Predicting Refinery Energy Losses Due to Fouling in Heat Exchangers

Abstract Energy efficiency is paramount in an oil refinery and heat integration is critical, especially in the energy-intensive atmospheric distillation unit. Thermal and hydraulic performance of each exchanger in the network used to pre-heat the crude is greatly reduced by the progressive deposition of unwanted material on the thermal exchange surfaces. Here, a detailed mathematical model for a shell and tube heat exchanger undergoing crude oil fouling is used to predict local and average fouling rates and identify critical performance areas in the exchanger. The ease of configuring the model for industrial applications is detailed. With reference to an industrial unit, it is shown that unit averages mask very different local behavior, and that it is possible to achieve precise calculation of heat duty loss caused by fouling over time.

Organic and inorganic fouling in heat exchangers – Industrial case study: Analysis of fouling state

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.

Effects of fouling on performance of retrofitted heat exchanger networks; a thermo- hydraulic based analysis

In refineries, fouling in crude pre-heat trains (PHTs) causes several thermal-hydraulic inefficiencies which lead to increased operating costs (from reduction in throughput and extra fuel burnt at the furnace), carbon emissions, and maintenance issues. The energy recovery performance of PHT can be severely affected over time. Such time varying effects are normally not considered in the design or retrofit of heat exchangers networks. In this paper, an existing PHT network is simulated including its fouling behaviour of over ca. two years. For this purpose, a dynamic, distributed mathematical model for shell-and-tube heat exchangers undergoing crude oil fouling (developed and validated against refinery data in previous work) is used. Three retrofit options aimed at maximizing overall heat recovery are proposed. Simulation results show that networks designs that maximize energy recovery in clean conditions (following traditional pinch rules) may not be best when fouling occurs and that a proper retrofit design must include consideration of time varying fouling effects.

Crude Oil Fouling Deposition, Suppression, Removal, and Consolidation—and How to Tell the Difference

ABSTRACT Crude oil fouling on heat transfer surfaces is often described as the result of two competing mechanisms: a deposition and a deposition-offsetting mechanism. There is uncertainty about whether the offsetting mechanism is suppression (due inhibition of attachment or back-diffusion of foulant from near the wall into the bulk) or removal of foulant already deposited, due to (i) difficulties in experimentally identifying and isolating the key phenomena and (ii) the cumulative measurement of deposition rates by monitoring thermal exchange rates (or resistance) alone. Here, the question is addressed of whether it is conceptually possible to distinguish such phenomena, and if so, in which conditions. A recently developed two-dimensional (2D) deposit model and a thermohydraulic model of a heat exchanger tube are used to assess the system response to removal, suppression, aging, and consolidation (for which a new model is proposed). It is shown that while suppression or removal lead to undistinguishable behavior during overall deposit growth, thermal and hydraulic responses will differ in certain conditions, for which an experimental procedure is suggested. Simultaneous consideration of thermal and hydraulic effects and accurate characterization of the deposit aging and consolidation processes are suggested as a way to allow the unambiguous identification of the dominant deposition-offsetting mechanism.

Model-based monitoring of thermal-hydraulic performance of refinery heat exchangers undergoing fouling

Abstract Monitoring of fouling is key to assess the performance of refinery heat exchangers, estimate resulting economic costs, and assist in planning of mitigation and cleaning. Such monitoring typically addresses just thermal performance, while hydraulic limitations are often also important. A dynamic thermo-hydraulic plot (TH-λ) for simultaneous monitoring the thermal and hydraulic performance of heat exchangers was recently presented. The features and benefits of the plot were illustrated for fixed inlet conditions. In this work, a model-based monitoring solution is presented which enables the application of the TH-λ plot with time-varying inputs. The TH-λ concept is integrated with a software framework that describes a shell-and-tube heat exchanger using a high-fidelity dynamic, distributed system model undergoing fouling. A method is presented to decouple the variations in exchanger performance due to changes in inlet conditions from those due to changes in fouling rates and/or deposit properties. The method and its benefits are illustrated with a case study using typical refinery data.