An Industrial Case Study on Retrofitting Heat Exchangers and Revamping Preheat Trains Subject to Fouling

Crude refinery preheat trains (PHTs) are a major part of a refining process as these units reduce the amount of thermal energy required to heat the crude oil to its distillation temperature. Fouling is a longstanding problem in the operation of PHTs; the ability of a refinery to process different crude blends or to increase its production capacity depends on the thermal and hydraulic performance of the PHT under fouling conditions. A case study based on a UK refinery preheat train is presented. The fouling behaviour is extracted from the plant’s monitoring database, which enabled fouling behaviour to be identified based on operating flows and temperatures. Techno-economic analyses for heat recovery and fouling mitigation, including retrofit of heat exchangers, use of tube inserts, and network revamp, were conducted. The commercial software tool, SmartPM, was successfully utilized to study heat recovery paths, cleaning schedules and furnace firing capacity on this case study. INTRODUCTION Crude refinery preheat trains (PHTs) are networks of heat exchangers that transfer heat from process streams to the crude in order to raise its temperature before it enters an atmospheric distillation column for fractional separation. Up to 70% of the heat required is provided by the PHT: the remaining heat is provided via a fired heater. Fouling in PHTs is a major economic and environmental problem as it reduces thermal efficiency and throughput capacity of the system. Identifying effective methodologies to manage PHT fouling remains a key research area (Crittenden et al., 1992; ESDU, 2000; Panchal and Huangfu, 2000; Ishiyama et al., 2013). A typical PHT includes units such as a desalter and preflash tower which are required to operate within a constrained set of operating parameters. The PHT is followed by a fired heater, which itself has a maximum furnace duty limit. Crude oil fouling is a complex phenomenon. Fouling is caused by different mechanisms at different locations on the PHT, as a consequence of different chemical and physical mechanisms (Lemke, 1999). Chemical reaction fouling is known to be the dominant mechanism downstream of the desalter (Yeap et al., 2004). Chemical reaction fouling is the formation of deposits on heat transfer surfaces where the fouling precursors are generated by chemical reaction. Two types of reactions are common: formation of gums (when the sulphur content of the crude is high) and decomposition of maltenes to produce insoluble asphaltenes. Asphaltenes are complex polynuclear aromatic compounds and often the cause for chemical reaction fouling in this part of the preheat train (Lambourn & Durrieu, 1983). Other reactions such as those catalysed by FeS and other corrosion products could also be present. Water carry over from the desalter can result in rapid fouling of exchangers operating at the temperature at which the water evaporates. These exchangers are easily identified. Careful analysis of monitoring data provides useful diagnostics. Several quantitative models for calculating (or estimating) crude oil fouling rates have appeared, following the introduction of the ‘fouling threshold’ concept by (Ebert & Panchal, 1997). This semi-empirical approach, originally introduced to evaluate the rate of crude oil tube-side fouling at a local condition (point condition), describes the fouling rate as the combination of a deposition term and a fouling suppression term. The rate exhibits two primary dependencies: it (i) increases with increasing surface (and film) temperature and (ii) decreases with increasing flow velocity. The concept has become an accepted basis for the development of many heat exchanger design and control strategies as reviewed by (Wilson et al., 2005). Complete mitigation of fouling in refinery PHTs is rarely achieved and periodic cleaning of fouled exchangers is a widely practised approach. The scheduling problem of when and which units to clean have been widely researched within the numerical optimization community (Georgiadis et al. 2000; Smaïli et al., 2001; Markowski and Urbaniec, 2005; Ishiyama et al. 2009; Ishiyama et al. 2010). In this manuscript, the hydraulic aspect of the crude oil fouling is revisited. The crude is pumped via centrifugal pumps. These pumps are usually operated under constant rotational speed and are designed to maintain a target throughput even as the network pressure drop increased with fouling. Control of the throughput is achieved through partial opening and closing of the control valves and bypass streams. PHTs can experience hydraulic limitations in two ways: (1) Fouling causes an increase in resistance to flow. If Proceedings of International Conference on Heat Exchanger Fouling and Cleaning 2013 (Peer-reviewed) June 09 14, 2013, Budapest, Hungary Editors: M.R. Malayeri, H. Müller-Steinhagen and A.P. Watkinson Published online www.heatexchanger-fouling.com