Investigation of an alternative operating procedure for fouling management in refinery crude preheat trains

Crude oil atmospheric distillation in petroleum refineries involves a heat exchanger network to heat the crude using hot side streams and pumparounds. This energy integration reduces the furnace load as well as the cold utility consumption, diminishing fuel costs and carbon emissions. During the operation, the effectiveness of the heat exchangers decreases due to fouling. This paper deals with preheat trains composed by multiple parallel branches, where it is investigated an alternative operating policy based on the optimization of stream splits, aiming to manipulate the flow rates according to the fouling status of the existent heat exchangers. The performance of the proposed approach is illustrated by three examples: two networks from the literature and one real network from a Brazilian refinery.     

Fouling Management of Thermal Cracking Units

ABSTRACT

Visbreakers and other thermal cracking units are thermal process units in crude oil refineries that upgrade heavy petroleum, usually residual oils produced from atmospheric or vacuum distillation of crude oil. The associated process streams of these units consist of heavy hydrocarbons with very high viscosities and impurities, resulting in fouling of the heat exchangers used to cool or heat these streams. This paper presents a practical fouling analysis for thermal cracking units in a refinery in Germany. Fouling management at this refinery was initiated as part of the refinery energy-saving program. Following similar analysis of the refinery's crude preheat trains, heat exchanger networks associated in the thermal cracking units were modeled by entering the plant monitoring data, network topology, and heat exchanger geometries into a commercial heat exchanger network simulator, SmartPM. Fouling behaviors of vacuum residue streams and thermal cracker residue streams were identified and quantified. Both chemical reaction fouling and particulate fouling mechanisms were identified to be responsible for the fouling in these streams. Dynamic fouling models were fitted and used to predict fouling of these heavy petroleum streams, which fouled on both the shell and tube sides of the shell-and-tube heat exchangers.     

Heat exchanger fouling model and preventive maintenance scheduling tool

The crude preheat train (CPT) in a petroleum refinery consists of a set of large heat exchangers which recovers the waste heat from product streams to preheat the crude oil. In these exchangers the overall heat transfer coefficient reduces significantly during operation due to fouling. The rate of fouling is highly dependent on the properties of the crude blends being processed as well as the operating temperature and flow conditions. The objective of this paper is to develop a predictive model using statistical methods which can a priori predict the rate of the fouling and the decrease in heat transfer efficiency in a heat exchanger. A neural network based fouling model has been developed using historical plant operating data. Root mean square error (RMSE) of the predictions in tube- and shell-side outlet temperatures of 1.83% and 0.93%, respectively, with a correlation coefficient, R², of 0.98 and correct directional change (CDC) values of more than 92% show that the model is adequately accurate. A case study illustrates the methodology by which the predictive model can be used to develop a preventive maintenance scheduling tool.     

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.     

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.     

Effect of Flow Distribution in Parallel Heat Exchanger Networks: Use of Thermo-Hydraulic Channeling Model in Refinery Operation

Abstract

Parallel branches are commonly observed in industrial heat exchanger networks (HENs). Despite the important relationship between flow distribution and network efficiency, not all parallel branches comprise of flow controllers or not least, flow measurements. When the network is subject to fouling, uncontrolled flow branches can introduce undesired phenomenon such as thermo-hydraulic channeling (THC) [presented at the 2007 HEFC conference; Ishiyama et al., Effect of fouling on heat transfer, pressure drop and throughput in refinery preheat trains]. Recent analysis of crude preheat train heat exchangers has shown the need to use THC models, in particular, for situations where there is insufficient flow measurement data, especially in nonsymmetric branches. This paper revisits the THC model and highlight practical importance of the THC phenomenon through analysis of plant data. The hydraulic aspect of the analysis is strongly linked to the knowledge of deposit thermal conductivity. A case study of a section of a crude refinery HEN is used to illustrate the use of thermo-hydraulic models in data reconciliation to understand flow imbalances caused due to differences in operating conditions and fouling of heat exchangers in each branch of a parallel network.     

Scheduling cleaning in a crude oil preheat train subject to fouling: Incorporating desalter control

Fouling is a serious operating problem in oil refinery distillation preheat trains (PHTs) as the reduction in heat transfer effectiveness not only reduces the overall rate of heat transfer but also causes difficulty in maintaining key temperatures in the network within their defined operating envelopes. This work considers the problem of controlling the desalter inlet temperature by using hot stream bypassing, within a PHT fouling mitigation strategy based on heat exchanger cleaning. The formulation of the problem is incorporated in the PHT simulator described by Ishiyama et al. (2009) [1]. The methodology is illustrated using a case study based on an industrial network subject to fouling, where the fouling rates of heat exchangers were extracted through a data reconciliation exercise. The case study scenarios suggest that our simulation-based tool should be effective in controlling desalter inlet temperature within a fouling management strategy.     

Use of crude oil fouling threshold data in heat exchanger design

The existence of a `threshold' below which chemical reaction fouling of heat transfer surfaces by crude oil does not occur has been identified by Ebert and Panchal [Fouling Mitigation of Industrial Heat-Exchange Equipment, Begell House, 1997, 451–460] and clearly demonstrated by Knudsen et al. [Understanding Heat Exchanger Fouling and its Mitigation, Begell House, 1999, 265–272]. This phenomenon has important implications for the design and operation of heat exchangers in refinery pre-heat trains used for the processing of crudes. In this paper we show how a consideration of the fouling threshold condition can be incorporated into the design procedures for shell-and-tube heat exchangers. We then proceed to show how fouling can be mitigated through attention to heat exchanger design, particularly the choice of configuration. The cost of improperly designed units, based on the conventional use of `fouling factors', is demonstrated.     

The Effect of Fouling on Heat Transfer,Pressure Drop,and Throughput in Refinery Preheat Trains: Optimization of Cleaning Schedules

Optimizing cleaning schedules for refinery preheat trains requires a robust and reliable simulator, reliable fouling models, and the ability to handle the thermal and hydraulic impacts of fouling. The interaction between thermal and hydraulic effects is explored using engineering analyses and fouling rate laws based on the “threshold fouling” concept; the potential occurrence of a new phenomenon, “thermo-hydraulic channeling” in parallel heat exchangers, is identified. The importance of the foulant thermal conductivity is highlighted. We also report the development of a highly flexible preheat train simulator constructed in MATLAB/Excel. It is able to accommodate variable throughput, control valve operation, and different cost scenarios. The simulator is demonstrated on a network of 14 heat exchangers, where the importance of optimizing the flow split between parallel streams is illustrated.     

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.     

Neuro-based formulation to predict fouling threshold in crude preheaters

For preheat exchangers of a crude distillation unit (CDU), operating in conditions such that fouling minimized is crucial. A number of semi-empirical models called “threshold fouling models” were developed by various researchers to predict crude oil fouling behavior in crude preheaters of CDUs. In this study, an artificial neural network (ANN) model has been employed to develop a set of mathematical formulations to identify regions where there is less/no fouling. The comparisons between results of the developed neuro-based formulation and three threshold fouling models showed the use of neuro-based model resulted in significant improvements in terms of predicting crude oil fouling behavior of various laboratory and plant data. The approach of developing neuro-based models to predict fouling behavior can be readily applied in CDUs to identify more accurate fouling/no-fouling operating zones leading to an enhancement in the operation of crude preheaters.     

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.     

Impact of Nonuniform Fouling on Operating Temperatures in Heat Exchanger Networks

ABSTRACT

Investigations of fouling in heat exchangers are mainly focused on two factors: commercial impact due to energy losses, and environmental impact manifested through higher CO₂ emissions. The purpose of this paper is to introduce a third factor relating to safety in operations. This paper presents two case studies, one for a hydroprocessing unit with feed/effluent heat exchangers and another for preheat train exchangers installed upstream of the atmospheric furnace in a refinery crude unit. Due to a wide range of process temperatures examined in both case studies, the heat exchangers in the network are subject to various fouling mechanisms. As illustrated in the pictures of actual tube bundles, some of the exchangers within the network are heavily fouled, while the other exchangers operate in nearly clean conditions. Detailed simulations indicate that nonuniform fouling results in heat exchanger operating temperatures that are significantly higher than those predicted by conventional analyses using uniform fouling. Higher than anticipated process fluid temperatures may result in exceeding the threshold limits for certain corrosion mechanisms and/or significantly higher than expected rates of corrosion.     

Supervision of the thermal performance of heat exchanger trains

In oil refining, heat exchanger networks are employed to recover heat and therefore save energy of the plant. However, many heat exchangers in crude oil pre-heat trains are under high risk of fouling. Under fouling conditions, the thermal performance of heat exchangers is continuously reduced and its supervision becomes an important task. The large number of heat exchangers in pre-heat trains and the change of operation conditions and feedstock charges make the daily supervision a difficult task. This work applies an approach to follow the performance of heat exchangers [M.A.S. Jerónimo, L.F. Melo, A.S. Braga, P.J.B.F. Ferreira, C. Martins, Monitoring the thermal efficiency of fouled heat exchangers – A simplified method, Experimental Thermal and Fluid Science 14 (1997) 455–463] and extends it to monitor the whole train. The approach is based on the comparison of measured and predicted heat exchanger effectiveness. The measured value is computed from the four inlet and outlet temperatures of a heat exchanger unit. The predicted clean and dirty values of effectiveness are calculated from classical literature relations as a function of NTU and of heat capacity ratio (R). NTU and R are continuously adjusted according to mass flow rate changes. An index of fouling is defined for the whole network and the results show the performance degradation of the network with time. The work also suggests that Jerónimo’s index of fouling can be used to estimate the fouling thermal resistance of heat exchangers.     

Analysis of the influence of operating conditions on fouling rates in fired heaters

Fouling due to chemical reaction in preheat trains for the processing of crude oil plays a key role in the operation and maintenance costs and on greenhouse emissions to atmosphere in crude processing plants. A preheat train consists of a set of heat transfer units that provide the crude oil stream the required amount of thermal energy to reach its target temperature either by heat recovery or by direct firing. Fired heaters supply external high temperature heating through the burning of fuel which result in complex heat transfer processes due to the large temperature and pressure changes and vaporization that takes place inside the unit. In this work, a thermo-hydraulic analysis of the performance of fired heaters is carried out through the application of commercial software to solve the mathematical models using finite difference methods; the analysis is applied to the crude side of a vertical fired heater in order to evaluate the impact of process conditions such as throughput and crude inlet temperature (CIT) on the fouling that take place at the early stages of operation. Using a fouling rate model based on thermo-hydraulic parameters, fouling rates are predicted assuming steady state operation and clean conditions. Although variations in process conditions are known to influence fouling rates, little work has been done on the subject. In this work excess air and steam injection are studied as a means to mitigate fouling. Results show that throughput reduction brings about a marked increase in the fouling rates. A decrease in CIT affects only the convection zone and it is found that this effect is negligible. In terms of excess air, it is found that although it affects negatively the heater efficiency it can be used to balance heat transfer between the convection and radiation zone in a way that fouling rates are reduced; however this strategy should be considered right from the design stage. Finally it is observed that steam injection is an effective method to reduce fouling rates since it results in lower film temperatures and larger shear stress.     

Real Time Fouling Diagnosis and Heat Exchanger Performance

The issue of fouling in preheat trains of crude oil distillation units in Petrobras's refineries is a major concern—especially now, as heavier Brazilian crudes with higher asphaltene content are being refined. As the efficiency of the preheat train plays an important role in the energy consumption of a distillation unit, its performance must be tracked as precisely as possible in order to identify operational problems. This work describes an online heat exchanger performance evaluation system based on rigorous simulation of the equipment in order to predict both the operational and clean overall heat transfer coefficient. A real-time comparison between these two values indicates the actual performance of the heat exchanger and of the preheat train. The use of a rigorous process simulator (Petrox from Petrobras) together with a rigorous calculation of the global heat transfer coefficient (using the program Xist from HTRI) allows one to consider aspects that are not usually taken into account in this kind of evaluation. These aspects include crude vaporization after the desalters and variations of crude and products composition with the distillation unit run. The system is being implemented at the biggest Petrobras refinery (360,000 bpd) in a 25 heat exchanger preheat train.     

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.     

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.     

Evaluation of laboratory crude oil threshold fouling data for application to refinery pre-heat trains

`Threshold' models for crude oil fouling present a logical framework for analysing chronic fouling problems in refinery pre-heat trains. Few complete data sets of fouling thresholds in crude oil exist: this paper presents a modelling (correlation) study of the data set obtained by Knudsen et al. [Understanding Heat Exchanger Fouling and its Mitigation, Begell House, New York, 1999, p. 265] for threshold fouling experiments performed in a pilot plant. The threshold fouling equation proposed previously by Ebert and Panchal, and extended by Panchal et al. [Understanding Heat Exchanger Fouling and its Mitigation, Begell House, New York, 1999, p. 273] has been modified following a critique of some of its underlying assumptions. These previous efforts in developing threshold fouling equations were based on observed fouling rates, extrapolated to yield operating conditions that presumably exhibit zero fouling. This study represents an attempt to model measured physical parameters that actually resulted in no observable fouling. The validity of such a model to describe threshold fouling conditions in refinery pre-heat exchangers is discussed.