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.     

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.     

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.     

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.     

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 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.     

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.     

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.     

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.     

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.     

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.     

Parameter Estimation of Fouling Models in Crude Preheat Trains

Several fouling mitigation techniques depend on the capacity of predicting fouling rates. Therefore, the identification of accurate fouling rate models is an important task. Crude fouling rates are usually evaluated through empirical or semiempirical models. In both alternatives, there are parameters that must be determined through laboratory or process data. In this context, the article presents an analysis of the parameter estimation problem involving fouling rate models. A proposed procedure for addressing this problem is described through the development of a computational routine called HEATMODEL. An important aspect of this study is focused on the obstacles associated to the search for the optimal set of parameters of the Ebert and Panchal models and its variants. This optimization problem may present some particularities that complicate the utilization of traditional algorithms. In the article, the performance of a conventional optimization algorithm (Simplex) is compared with a more modern numerical technique (a hybrid genetic algorithm) using real data from a Brazilian refinery. The results indicated that, due to the complexity of the parameter estimation problem, the Simplex method may be trapped in poor local optima, thus indicating the importance of the utilization of global optimization techniques for this problem.     

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.     

炉管内聚合物结垢规律测试研究

建立了管内聚合物结垢热阻测试试验台。采用对比法对管内聚合物热阻进行了测试研究,分析聚合物结垢的影响因素。测试结果表明:聚合物浓度越高,聚合物结垢层越厚,热阻也越大;随着混合液温度的升高,管内聚合物垢层及热阻都呈现逐步增大的趋势;随着管内聚合物溶液流速的提高,聚合物热阻下降。     

Experimental Study of Fouling Resistance in Twisted Tube Heat Exchanger

This paper discusses fouling of a twisted tube heat exchanger under different conditions of fluid velocity and heat input. The fluid velocity was varied from 0.5 to 2.0 m/s, whereas the heat input to the heat exchanger was varied from 200 to 800 W. The experimental results show that for low fluid velocity of 0.5 m/s, the fouling resistance showed noticeable variation with respect to heat input, whereas for high velocity ranges, that is, 1.0–2 m/s, the variation in fouling resistance is less. The fouling in twisted tube steadily increases with time for different values of heat input from 1000 min onward for fluid velocity in the range from 1.0 to 2.0 m/s. It is also observed that fouling resistance curves overlap for various values of heat input. During the initial 1000 min of the test duration, the maximum fouling in a twisted tube heat exchanger decreases with increase in fluid velocity from 1.0 to 2.0 m/s. This behavior of the fouling rate can be attributed to the fact that at higher fluid velocity, flow becomes turbulent, and this in turn flushes the fouling particles. The time-series correlations for the fouling resistance are found to be logarithmic in nature.     

Characterization of Crude Oils and Their Fouling Deposits Using a Batch Stirred Cell System

A small (1 L) batch stirred cell system has been developed to study crude oil fouling at surface temperatures up to 400°C and pressures up to 30 bar. Fouling resistance–time data are obtained from experiments in which the principal operating variables are surface shear stress, surface temperature, heat flux, and crude oil type. The oils and deposits are characterized and correlated with the experimental heat transfer fouling data to understand better the effects of process conditions such as surface temperature and surface shear stress on the fouling process. Deposits are subjected to a range of qualitative and quantitative analyses in order to gain a better insight into the crude oil fouling phenomenon. Thermal data that can be obtained relatively quickly from the batch cell provide fouling rates, Arrhenius plots, and apparent activation energies as a function of process variables. The experimental system, supported by computational fluid dynamics (CFD) studies, allows fouling threshold conditions of surface temperature and shear stress to be identified relatively quickly in the laboratory. The data also contribute to existing knowledge about the compensation plot.     

渣油高温换热器结垢原因与防垢剂研究

通过对辽河减粘渣油在高温下形成的积垢物进行分析和测试,查明了渣油高温换热器结垢的原因。建立了渣油高温换热实验室模拟装置,研究了渣油化学组成、加工温度及在设备内停留时间等因素对形成积垢的影响。研制的多功能渣油防垢剂,经实验室模拟装置性能评价,防垢效果良好。     

2006年国际原油市场前瞻

2006年国际原油市场在上升动力和阻力的双重作用下,仍将处于高位盘整时期。一方面,由于原油品种地区和结构性矛盾突出、OPEC的剩余产能不足、中东局势动荡不安等多方面原因,以及基金投机力量的强劲推动,2006年国际原油价格上升动力不减;另一方面,在美元汇率上升、原油需求增速减缓以及原油供给仍有能力持续增加的多重作用下,油价上升遇到一定的阻力。2005年油价的持续高涨直接冲击了中国原油的供销链条,对相关行业产生着重要的影响。2006年,我国必须及早动手,采取措施,积极应对国际石油价格风险。