Graham T. Polley

Mitigation of Crude Oil Refinery Heat Exchanger Fouling Through Retrofits Based on Thermo-Hydraulic Fouling Models

Crude oil fouling in refinery preheat exchangers is a chronic operating problem that compromises energy recovery in these systems. Progress is hindered by the lack of quantitative knowledge of the dynamic effects of fouling on exchanger heat transfer and pressure drop. The concept of a thermal ‘fouling threshold’, first introduced by Ebert and Panchal, is revisited here alongside models of hydraulic effects of fouling to provide a graphical tool, the modified temperature field plot, for assessing chronic chemical reaction fouling effects in refinery heat exchangers. Fouling data of varying quality, collected from pilot plant and refinery operation, were compared with two previously published threshold fouling models and one based on the Epstein deposition model. The model by Epstein showed the best agreement, primarily because it can accommodate fouling that is mass transfer as well as reaction controlled. The hydraulic analysis indicated that the simple slab approximation for fouling layers gave a reasonably good mapping between heat transfer and pressure drop effects as long as roughness contributions are not significant. Where roughness effects (or tube blockage) are important, the relationship between thermal and hydraulic performance is not straightforward. A case study, based on the network described by Panchal and Huang-Fu, is used to illustrate the thermo-hydraulic effects of fouling and the application of the modified temperature field plot.

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.

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.

Extraction of Crude Oil Fouling Model Parameters from Plant Exchanger Monitoring

Most of the semi-empirical “threshold fouling” models for crude oil fouling in shell-and-tube exchangers have been developed and validated using data collected at what may be considered to be “point” or localized conditions. In practice, both velocity and wall temperature can vary significantly within a heat exchanger, leading to difficulty in applying the models in exchanger design and extracting fouling information from exchanger performance monitoring. A partial simulation model is presented here, incorporating a linear temperature distribution. This short-cut model is compared with a more detailed simulation in order to establish its reliability. Pressure drop using a smooth layer model is also considered. The short-cut approach is employed in a data reconciliation study of an operating crude preheat train, which indicates that the original threshold fouling model of Ebert and Panchal gives a better description of the observed fouling behavior.

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.

Alternative Design Approach for Plate and Frame Heat Exchangers Using Parameter Plots

The simultaneous design and specification of heat exchangers of the plate-and-frame type is analyzed. A pictorial representation of the design space is used to guide the designer toward the selection of the geometry that best meets the heat duty within the limitations of pressure drop. The design space is represented by a bar plot where the number of thermal plates is plotted for three conditions: (1) for fully meeting the required heat load, (2) for fully absorbing the allowable pressure drop in the cold stream, and (3) for fully absorbing the allowable pressure drop in the hot stream. This type of plot is suitable for representing the design space, given the discrete nature of the plate geometrical characteristics, such as effective plate length and plate width. Applications of the use of bypasses as a design strategy are also presented.

Compensating for End Effects in Plate-and-Frame Heat Exchangers

Because fluid flowing through the end channels of plate exchangers only passes heat through one of the channel walls (as opposed to core channels where heat is transferred through both walls), the heat transfer for this element of fluid is less than that for the bulk. The result is that the temperature profile along the end channel can be substantially different from that in the center of the exchanger. This distortion affects each channel in turn. Thus, the temperature profile in the exchanger is not uniform. What may come as a surprise to many is that the end effects can still be significant when the exchanger has many plates. Correction factors accounting for this effect have been proposed by Shah & Focke [1]. The correction factors are dependent upon the ratio of the heat capacity flow rates of two streams, the required effectiveness, and the number of plates used. The results are given in the form of tables relating the correction factor to discrete values of each of these variables. These correction factors are applied to the log mean temperature difference in order to obtain an effective value. Consequently, the procedure treats the exchanger area as a continuous variable. Because exchanger plates come in a range of fixed sizes and plate count is an integer, the exchanger area is a discrete variable. A simple bypass model is presented here that allows the designer to determine and adjust for the end effects while treating the area as a discrete variable. The predictions of the model compare favorably with the tabulated corrections. Using the model, it is possible to determine the end effects for any combination of duty and exchanger size. The implications of the end effects upon the design are briefly considered.

Interpreting and Applying Experimental Data for Plate-Fin Surfaces: Problems with Power Law Correlation

The absence of reliable and theoretically consistent correlations for the prediction of the thermohydraulic performance of plate-fin surfaces from the definition of surface structure is placing a constraint upon plate-fin exchanger design. Many workers have made direct use of the experimental data without fully appreciating the limitations imposed by the way in which the experiments have been conducted. The dangers of such a practice are exposed. The data of Kays and London are then re-analyzed using established heat transfer theory. The comparison between the predicted and reported friction factor and j factor data is good. The predictions compare favorably with experimental measurements over laminar, transitional, and turbulent flow regimes. Predictions for heat transfer in the transitional flow regime can be improved if the transition occurs at a lower Reynolds number. The new equations can be used in design with greater confidence than the use of experimental data alone.

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.

Improving multi-stream heat exchanger design by reducing the number of sections

The total annual cost (TAC) of a Heat Exchanger Network (HEN) is decreased by using Multi-Stream Heat Exchangers (MSHE), enabling a simultaneous heat exchange between more than two streams in a single unit. Several methods have been developed based on pinch concepts to design an MSHE; however, they lead to designs which are both larger and more complex than necessary. The major drawback is they usually result in designs having more individual sections than minimum. In this paper a new procedure for design of MSHE is proposed, which minimizes the number of sections required for a given duty. Having applied the new design procedure in an industrial case study, the results showed a 10% cost reduction compared to current methods.