Retrofitting Crude Oil Refinery Heat Exchanger Networks to Minimize Fouling While Maximizing Heat Recovery

Abstract

The use of fouling factors in heat exchanger design and the lack of appreciation of fouling in traditional pinch approaches have often resulted in crude preheat networks that are subject to extensive fouling. The development of thermal and pressure drop models for crude oil fouling has allowed its effects to be quantified so that techno-economic analyses can be performed and design options compared. The application of these fouling models is described here on two levels: the assessment of increasing heat recovery in stream matches (e.g., by adding extra area to exchangers) and the design of a complete network using the Modified Temperature Field Plot. Application to a refinery case study showed that, at both the exchanger and network levels, designing for maximum heat recovery (e.g., using traditional pinch approaches) results in a less efficient system over time due to fouling effects.     

Evaluation of Crude Oil Heat Exchanger Network Fouling Behavior Under Aging Conditions for Scheduled Cleaning

Heat exchanger network (HEN) fouling is an endemic operational challenge prevalent in many process industries. Its impact on both plant operating cost and productivity is significant and can be compounded by aging effects of the foulant. In this paper, we model and simulate the effect of aging on tube-side fouling and cleaning dynamics in a crude oil refinery preheat train (PHT) comprising a 14-unit HEN. A prescient, HEN modeling and dynamic simulation were performed wherein the transients of fouling and aging as well as the interactions between individual units were captured. To assess the temporal effects, different crude oil deposit (gel) aging scenarios (no aging vs. slow, medial, and fast aging) in the downstream units were considered for the PHTs’ overall heat recovery, cleaning options, and operability. The results show that the deleterious impact of fouling and concomitant aging, quantified in terms of thermal resistances, was significantly reduced by fast aging as opposed to medial, slow, or no aging of the gel deposit. Faster aging rate reflected improved heat recovery and a lesser demand for and lower cost of PHT cleaning. The concomitant higher growth of coke deposit due to aging, however, resulted in greater hydraulic resistance, which is inimical to operability.     

炼油厂脱前原油换热网络的设备优化

利用夹点技术对某炼油厂脱前原油换热网络进行了改造。与原换热网络相比，换热网络的换热设备数目减少了4个，使设备数达到最优化，从而节省了设备的投资。     

Crude Oil Fouling Research: HTRI's Perspective

For 20 years, Heat Transfer Research, Inc. (HTRI), has conducted dedicated, ongoing research into crude oil fouling behavior, specifically on developing test methods to measure fouling resistance over time and comparing fouling tendencies of different crude oils. More than 250 test runs with nineteen crude oils have been conducted. While current methods are sufficient for comparative fouling studies, general methods to predict fouling tendency remain elusive. Recent initiatives have focused our efforts on chemical characterization to screen crude oils and blends for fouling tendency, as well as on identifying thresholds for low fouling. Increasing the shear stress on the heat transfer surface as much as possible can mitigate heat exchanger fouling. Long-term success in controlling fouling depends upon a deeper understanding of the chemical characteristics of an individual crude oil and sound heat exchanger design practices. Current predictive fouling models are limited in their application, but improvements based on chemical characterization look promising.     

一种监测换热器污垢的新方法

在考虑污垢对换热器传热性能影响的基础上,提出换热器当量总污垢热阻和污垢函数的概念,并给出换热器当量总污垢热阻的监测方法,讨论了换热流型、传热有效度ε和冷热流体热容量率比R对换热器当量总污垢热阻的影响。     

Mitigation of Crude Oil Preheat Train Fouling by Design

Fouling in crude oil preheat trains is a significant industrial problem which has restricted the application of process integration techniques such as "pinch technology" in this sector. A semiempirical fouling model for crude oil fouling developed by Panchal and co-workers allows the effects of fouling to be considered at the design stage for such networks. Application of this model at three levels--(1) design of new networks; (2) retrofitting of existing systems; and (3) identification of robust specifications for individual heat exchanger units--is discussed. The design issues are discussed using case studies illustrated by a graphical construction, the temperature field plot. Rigorous optimization of the final designs is not reported. The specification discussion describes how the crude fouling model can be incorporated into existing heat exchanger design software to identify exchanger configurations which are less likely to experience significant fouling over a range of operating conditions. This article concentrates on shell-and-tube designs, but the concepts will be applicable to other exchanger types once a suitable fouling model becomes available.     

Correlating Field and Laboratory Data for Crude Oil Fouling

Crude oil fouling in a laboratory fouling unit was investigated. The study focused on the preheat-train heat exchangers located just before the crude unit furnace and operating at temperatures in excess of 200C. A fouling rate model developed using laboratory data from crude blends was used to predict the threshold conditions where negligible fouling was expected under refinery conditions. The results from the model were compared to actual data from a fouling unit located at a refinery. The article discusses factors that may explain the performance of the model and the observed discrepancies between fouling data obtained in the laboratory and the field.     

Crude Oil Fouling in a Pilot-Scale Parallel Tube Apparatus

Maya crude oil fouling reveals a seemingly straightforward dependency of initial fouling rate on surface temperature, but a maximum is found in the initial fouling rate–velocity relationship, which mirrors that found in a model chemical system of styrene polymerization. The linear dependency of the logarithm of the pre-exponential factor on apparent activation energy for the crude oil is also found in the styrene system. The apparent activation energy for the crude oil ranged from 26.4 kJ/mol at 1.0 m/s to 245 kJ/mol at 4.0 m/s. Such strong dependencies of apparent activation energy on velocity, even at high velocity, are consistent with Epstein's mass transfer reaction attachment model. Surface temperatures at which the fouling rate becomes velocity independent are 274°C and 77°C for Maya crude oil and styrene, respectively. For surface temperatures in excess of this isokinetic temperature, an increase in velocity would lead to an increase in the rate of fouling.     

Industry-Recommended Procedures for Experimental Crude Oil Preheat Fouling Research

Of all heat transfer research arenas, few have the investment return potential of crude oil fouling mitigation. However, crude oil fouling is a very complex phenomenon that occurs via the simultaneous activity of multiple mechanisms. Advances in this field of research are complicated further by the lack of standardized procedures, which would permit unequivocal comparisons of non-proprietary data. As a result, Heat Transfer Research, Inc. formed the Crude Oil Fouling Task Force (COFTF), which is composed of heat transfer experts from many of the world's leading energy companies. The principle endeavor of the COFTF is to ensure that crude oil fouling research is both standardized and industrially relevant. The COFTF recommendations are detailed in this paper.     

Investigating the Impact of Boiling Conditions on the Fouling of a Crude Oil

ABSTRACT

The impacts of nucleate boiling and pressure on crude oil fouling are factors that have not been heavily investigated in previous research. Variables such as wall temperature and fluid velocity/shear are often a main focus, as they are key variables for predictive fouling models, which provide insight to fouling thresholds. Research detailed in this report shows that nucleate boiling and pressure greatly impact the measured fouling rate of a crude oil tested using the Heat Transfer Research, Inc., rotating fouling unit. When nucleate boiling is occurring, the use of fouling resistance plots to measure fouling rates is not a reliable method due to the impact boiling has on the heat transfer coefficient. Visual inspection of fouling deposits to validate fouling resistance data has also been found to be critical. Images of fouling deposits are included. Fouling under nonboiling conditions was shown to increase with increasing pressure.     

Observation of an Isokinetic Temperature and Compensation Effect for High-Temperature Crude Oil Fouling

The initial fouling rates of four crude oils were determined at a nominal bulk temperature of 315°C, an initial heated wall shear stress of 13 Pa, and initial surface temperatures between 375 and 445°C. These initial fouling rates ranged from 1.3(10^{? 6}) to 7.8(10^{? 5}) m² K/kJ. Corresponding Arrhenius plots were linear with the initial fouling rates passing through an isokinetic temperature of 407.5°C. A plot of the natural logarithm of the pre-exponential factors [7.6(10⁴)–5.2(10¹⁵) m² K/kJ] versus the apparent activation energies (128–269 kJ/mol) was also linear, confirming the validity of the isokinetic temperature and the presence of the compensation effect. Below the isokinetic temperature, the relative fouling rates were Crude Oil C > Crude Oil A > Crude Oil D > Crude Oil B; above the isokinetic temperature, the relative fouling rates were reversed (Crude Oil B > Crude Oil D > Crude Oil A > Crude Oil C). Chemical characterization of a fouling deposit suggested that the dominant fouling mechanism at these conditions was coking, with significant contributions from sedimentation (iron sulfide) and corrosion (～ 340 μ m/yr) of the 304 stainless steel test material.     

Simultaneous Consideration of Flow and Thermal Effects of Fouling in Crude Oil Preheat Trains

Given models linking flow resistance and fouling resistance, it becomes possible to simulate the effects of fouling on the hydraulic performance of a refinery preheat train. Such a simulation has been used here to identify when plant throughput will be limited by pressure drop; how throughput can be improved through the cleaning of individual exchangers and groups of exchangers; and how much production can be maintained when individual exchangers are taken off-line. Determination of better operating strategy requires a simulation of both hydraulic and thermal performance. In this article we implement a pragmatic linked model and consider the results from a set of simulations.     

Fouling of Some Canadian Crude Oils

A thermal fouling study was undertaken using three sour Canadian crude oils. Experiments were carried out in a re-circulation fouling loop equipped with an annular (HTRI) electrically heated probe. Fluids at pressures of about 1000–1340 kPa under a nitrogen atmosphere were re-circulated at a velocity of 0.75 m/s for periods of 48 hours, and the decline in heat transfer coefficient followed under conditions of constant heat flux. Bulk temperatures were varied over a range of 200–285^?C, and initial surface temperatures ranged from 300–380^?C. Heat fluxes were in a range of 265–485 kW/m².

Surface temperature effects on fouling of the three oils were compared, and fouling activation energies were estimated. For the lightest oil, a more detailed study of velocity and bulk and surface temperature effects was carried out. The fouling rate decreased slightly with increasing velocity but increased with both surface and bulk temperatures; a rough correlation was developed using a modified film temperature weighted more heavily on the surface temperature. Deposits showed high concentrations of sulfur and minerals, indicating the importance of iron sulfide deposition.     

The Experiment Investigation of Cooling-Water Fouling Mechanism-Associated Water Quality Parameters in Two Kinds of Heat Exchanger

This paper mainly discusses the relevance of some cooling-water quality parameters and the cooling-water fouling mechanism. By using plate and shell-and-tube heat exchanger experimental systems that are built to simulate the practical operation of heat exchangers, we acquired groups of data of water quality parameters such as Fe³⁺ and Cl^? in the cycle process. Then some interaction analyses were made to study the connection between water quality parameters and fouling deposition, and the results indicate that water quality parameters influence each other in a cycle process and the interconnections of water quality parameters have a direct and obvious impact on cooling-water fouling growth in the early operation stage. Later, fouling growth becomes stable for the reason that each parameter achieves a dynamic balance. Cooling-water fouling growth is a comprehensive result of water quality parameter dynamic changes.     

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.     

Use of CFD to Determine Effect of Wire Matrix Inserts on Crude Oil Fouling Conditions

Crude oil fouling rates are strongly affected by both local surface temperature and local surface shear stress. The use of in-tube inserts (such as hiTRAN) in heat exchangers has been shown to be effective in mitigating crude oil fouling while at the same time enhancing heat transfer. However, the introduction of inserts means that there will be axial and radial distributions of both local shear stress and local heat transfer coefficient between the repeating insert–wall contact points, which could mean that there will be local variations in fouling rate. While estimation of local shear stresses and film heat transfer coefficients is facile for bare round tubes, this is no longer the case for tubes fitted with inserts. Accordingly, this article describes a possible solution to the design challenge using computational fluid dynamics (CFD) simulation, the output of which is the temperature and velocity distributions in a three-dimensional geometry of the fluid flow in a tube fitted, for example, with a hiTRAN insert. A simple algorithm is then described for calculating the overall heat transfer coefficient based on the resulting temperature distribution along the wall of the tube. Simulated values of the overall heat transfer coefficient are then compared with those obtained by experiment, showing that there is good agreement, thereby indicating that predicted local values are accurate. Use of CFD in fouling applications now allows the prediction of local conditions when inserts are used and hence can be used to predict whether, and where, fouling might occur.     

原油热化学脱水工艺参数优化方法研究

热化学脱水是原油处理的重要工艺之一.应用数学模型优化脱水参数对于降低脱水运行成本具有重要意义.通过现场或者室内试验获得温度和加药浓度对原油脱水的作用,本文建立了原油脱水工艺约束条件的数学表达式,完善了优化模型.利用建立的数学模型,对脱水工艺参数进行了分析.研究表明,满足脱水指标的运行参数位于脱水温度-加药浓度平面含水等值线上部区域;经济运行的最优点位于满足脱水指标区域的边界上;含水指标影响运行参数的优化结果,指标不同,最优运行温度、加药浓度也不同;在含水指标(含水量降低)提高时,升高脱水温度是较为经济的选择;燃料、药剂价格也影响最优的工艺参数,需要根据价格变化情况进行脱水工艺参数调整.     

Management of Crude Preheat Trains Subject to Fouling

Crude oil refinery preheat trains are designed to reduce energy consumption, but their operation can be hampered by fouling. Fouling behaviors vary from one refinery to the next. Effective management of preheat train operation requires inspection of historical plant performance data to determine fouling behaviors, and the exploitation of that knowledge in turn to predict future performance. Scenarios of interest can include performance based on current operating conditions, modifications such as heat exchanger retrofits, flow split control, and scheduling of cleaning actions. Historical plant monitoring data are frequently inconsistent and usually need to be subject to data reconciliation. Inadequate data reconciliation results in misleading information on fouling behavior. This article describes an approach to crude preheat train management from data reconciliation to analysis and scenario planning based around a preheat train simulator, smartPM, developed at Cambridge and IHS. The proposed methodology is illustrated through a case study that could be used as a management guideline for preheat train operations.     

Fouling of Heat Exchanger Surface: Measurement and Diagnosis

Fouling in heat exchange equipment results in a significant energy toss by increasing heat transfer resistance and fluid frictional resistance. This paper deals with an effective way to monitor fouling and describes a potentially useful diagnostic approach for discriminating different types of deposit in situ. Information regarding deposit type would be useful in selecting an appropriate treatment procedure. Some cautions regarding the indiscriminate use of fouling factor or fouling resistance are also discussed.     

烟气换热器堵灰原因及解决办法

介绍了数家电厂GGH堵灰情况，总结出GGH堵灰的主要原因，并且提出了对策。