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.     

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.     

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.     

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.     

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.     

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.     

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.     

Deposition from Crude Oils in Heat Exchangers

Deposition in flow lines and processing and heat transfer equipment arises from fouling species, which may either be present in the fluid or generated in the vicinity of the equipment surface. Recent research on deposition during heat transfer from petroleum feedstocks is reviewed. For low-sulfur light crude oils, deposition is largely due to particulates and gums. For medium-sulfur crude oils, the formation of iron sulfides plays a major role in deposition. In unstable heavy oil systems, suspended asphaltenes are the fouling species. Trace quantities of impurities such as dissolved oxygen or suspended corrosion products add markedly to deposit formation. The influences of flow velocity, bulk and surface temperatures, and particulate concentrations are demonstrated through experimental results and compared to expectations from simple models. Through an understanding of the key steps in the deposition processes, a rational mitigation strategy can be formulated.     

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.     

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.     

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.     

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.     

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.     

四种原油评价与可行性加工方案探讨

对巴士拉原油、龙卡多原油、梵高原油、邦加原油的一般性质、直馏产品收率、产品性质进行比较和数据分析,结合大连石化现状,对原油加工方案进行探讨.结论认为,巴士拉原油各直馏馏分的收率比例适中,硫含量高,需要与其他硫含量低的原油混炼;龙卡多原油各直馏馏分的收率比例适中,但是硫含量高,酸值达到0.5mgKOH/g,需要与其他硫含量低、酸值低的原油混炼;邦加原油除减压塔底油收率稍低外,其他各直馏馏分的收率比例适中,但酸值高达0.5mgKOH/g,需要与其他酸值含量低的原油混炼;梵高原油为偏重原油,重整原料、航煤和柴油馏分收率小,大部分为减压馏分和渣油馏分,酸值超过1.0mgKOH/g,加工时会严重腐蚀常减压装置,应根据需要掺炼其他酸值低的原油,后续加工可通过催化裂化或加氢裂化等深度加工工艺,生产出高价值的轻馏分油.原油评价为生产计划系统、生产调度排产系统和原油优化调合系统提供数据支持,并为选择效益最优的原油采购方案,确保生产装置平稳运行,保证原油加工效益最大化提供技术支持.     

2010年山东口岸主要进口原油品种的质量浅析

2010年中国石油净进口量达到2.39×108t,进口依存度连续两年突破50%。山东口岸检验原油进口量达到3099.5×104t,占全国进口总量的12.9%,其中进口量超过100×104t的原油品种有7种,包括安哥拉原油共计2种,分别是KISSANJE原油(153.9×104t)、HUNGO原油(280.9×104t),沙特原油共计3种,分别是轻质原油(102.7×104t)、中质原油(183.2×104t)、重质原油(184.4×104t),伊朗重质原油(250.9×104t)和巴西RONCADOR重质原油(127.3×104t),这7种进口原油占山东口岸原油进口总量的41.4%。对这7种进口原油品种的密度、硫含量和水分含量进行检测,发现山东口岸主要进口原油为高硫和中、重质原油,但只有巴西RONCADOR重质原油含水,水分平均含量为0.093%(体积分数);近3年来只有安哥拉HUNGO原油和伊朗重质原油进口比例持续增长,而沙特轻质原油的进口比例则逐年小幅减少。2010年以巴西、委内瑞拉等国为主的南美州原油进口比例呈现明显增长趋势,巴西RONCADOR重质原油的进口比例2010年激增到4.19%。山东口岸2010年原油进口呈现出个别油品增长、油源更加多元化的态势。     

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.