Management of Fouling in Crude Oil Preheat Trains a report by Ian Wilson,1 Edward M Ishiyama,1,2 Bill R Paterson,1 Graham T Polley3 and Simon J Pugh2
1. Department of Chemical Engineering and Biotechnology, New Museums Site, University of Cambridge; 2. IHS Engineering Science Data Unit; 3. Department of Chemical Engineering, University of Guanajuato
Separation of crude oil into its various fractions by distillation lies at the heart of any refinery. The separating agent is thermal energy and the crude distillation unit can consume up to 4% of the energy content of a barrel of crude even when energy is recovered from the products by heat exchangers arranged in the network known as the preheat train (PHT). The performance of many PHT exchangers is reduced by fouling, caused by deposition of species present in the crude or formed by chemical reactions in the oil. The reduction in heat transfer efficiency requires more heat to be supplied in a furnace, or reduction in throughput. The former leads to increased fuel costs and carbon dioxide emissions: IHS ESDU1
estimated the impact of PHT fouling on refineries worldwide to
generate 36 million tonnes of extra CO2 per year. The latter directly impairs refinery operation and profitability. The largest cost associated with fouling relates to lost production. The profit from processing each barrel of oil currently lies between five and 10 US$/oil barrel (bbl). The crude throughput has to be reduced if:
•
fouling reduces the heat recovery achieved in the PHT to the extent that the firing limit of the furnace is reached;
• the decrease in tube bore and increase in surface roughness caused by fouling deposits results in reduced flow; or
•
when exchangers are removed for cleaning, both thermal and hydraulic factors can result in reduced production.
Simon Pugh is Director of Process Engineering Technology at IHS Engineering Science Data Unit (ESDU) and is based in London. His current role includes the management of the interaction and collaboration between fouling researchers and the oil company members of the IHS ESDU Oil Industry Fouling Working Party. He is currently leading a group of engineers working on the development of a range of design guides to oil industry fouling problems and computer programs for better heat exchanger selection, design and operation, with particular emphasis on improved refinery preheat train management. He holds a degree in Mechanical Engineering from Brunel University.
Edward Ishiyama completed his PhD in 2009 at the Department of Chemical Engineering and Biotechnology, University of Cambridge. His research focused on modelling crude refinery preheat trains subject to fouling. He currently works with IHS ESDU on the development of crude refinery preheat train management software.
Graham Polley has 35 years experience in the development of process engineering technology. This started at HTFS, working on heat exchanger technology and later, at the University of Manchester, working on process integration. He is working on mitigation of fouling in pre-heat trains with IHS ESDU and at the University of Guanajuato.
Bill Paterson recently retired from his senior lectureship in Chemical Engineering at Cambridge. His research interests include reaction engineering, process simulation & synthesis and granular flows. He holds a PhD from the University of Edinburgh and is a chartered engineer.
Ian Wilson is a Reader in Chemical Engineering at Cambridge. He has worked in the area of fouling and cleaning of heat transfer systems since 1988. He is a chartered engineer with research interests including heat transfer, rheology, particle technology and food processing.
The 1973 Legacy
The current landscape of high oil prices and concerns over energy supply was seen in the first oil crisis in 1973. This caused many industries to reduce energy losses by better design of heat exchanger networks for energy recovery, the most popular approach being known as pinch technology. However, these techniques assumed constant heat-transfer performance and prompted the construction of complex networks with relatively low flow velocities and high skin temperatures. Both these factors promote the fouling encountered in PHTs, as shown by the measured fouling rate data in Figure 1. The effect of revamping a PHT using pinch techniques without considering fouling effects is shown in Figure 2. The PHT in question was modelled using a simulation tool, which incorporated the effect of temperature and velocity on fouling rate. The furnace coil inlet temperature (CIT) decreases over time due to fouling, while the pressure drop on the crude side increases due to bore narrowing. Although the pinch technology design offers the best performance when clean, it soon performs less effectively than the unmodified (base) case. The design based on fouling management principles offers a substantially more robust response over the operating period. These principles are based on an understanding of how fouling rates are affected by network configuration and individual exchanger design.2
Fouling Management
Diagnosis Figure 1 shows considerable scatter and highlights the difficulty in predicting the rate of fouling. Fouling in PHTs usually builds up over periods of several months, during which the crude slate will have changed several times. The influence of crude composition is then averaged out, unless chronic fouling occurs, such as when the mixing of incompatible crudes causes precipitation of asphaltenes and other species in the PHT. This can be avoided by appropriate blending tests.3
The data
sets in Figure 1 were obtained by monitoring the thermal performance of units on a PHT and extracting the fouling resistance (Rf) from:
U Uclean =
11 +Rf (1)
where U is the overall heat transfer coefficient and clean denotes the case without fouling.
For a given refinery, data reconciliation of historical operating data and
calculation of local data sets of U and Rf over time represents the most valuable and relevant source of data on fouling. The reasons are that refineries usually operate with similar crude slates and operating policies, so that projection of future activity based on past performance
is reasonable and the value of Rf calculated by (1) incorporates any errors caused by local instrument malfunction and inaccuracy of Uclean,
16 © TOUCH BRIEFINGS 2011
Heat Exchangers
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