(244u) Condition-Based Cleaning of Refinery HEAT Exchangers Undergoing Crude OIL Fouling

Gradual build-up of fouling deposit on heat transfer surfaces in refinery heat exchangers leads to significant energy losses, operating difficulties and economic penalties. The undesired deposition not only reduces the heat exchange performance of these facilities due to its low conductivity, but also leads to increased pressure drops as the flow area decreases. This blockage may lead, eventually, to losses of production. Despite any mitigation actions, cleaning is required in order to restore the performance of heat exchangers. The economic trade-off between the energy losses due to fouling, the cost of cleaning and loss of production, and on the other hand the improvement after a cleaning, is ultimately the main factor determining the scheduling of cleaning actions.

Recent work [1] led to the development of a dynamic, distributed model for the deposit layer that represents it as a multi-component system undergoing chemical reactions, such as ageing. The model, implemented in a single heat exchanger tube framework, was shown to be able to simulate for the first time with a single model fouling, full cleaning, partial cleaning and fouling resumption after a cleaning, in any order. Rate models for distinct types of cleaning methods (mechanical and chemical) were proposed. The cleaning efficacy depends on the cleaning method and the state of the deposit at the time the cleaning is performed. Fresh deposit are relatively easy to remove, but old deposits, which have been exposed to high temperatures over long periods of time, may be difficult to remove or even non-removable (depending on the cleaning method). Finally, two cleaning modes can be considered: fixed time, for which the duration of the cleaning period is fixed a priori; and condition-based, for which the cleaning lasts until a termination condition is reached. In a later work [2], this improved model of fouling deposits was implemented within Hexxcell Studio^TM, a dynamic, distributed, first-principle thermo-hydraulic model for a shell-and-tube heat exchanger [3,4].

In this work, the framework comprising the new deposit model integrated in the advanced heat exchanger model is used to simulate fouling cleaning cycles for an industrial multi-pass shell–and-tube heat exchanger operating in typical conditions. It is shown that various combinations of mechanical and chemical cleaning (of different extent) can be seamlessly simulated. A cost model, extending previous works to account for several costs associated to cleaning, is proposed and used to assess the most economically advantageous cleaning schedules. Finally, dynamic optimization is explored to find the optimal cleaning schedule for a simple case considering a single heat exchanger.

Acknowledgments

This research was partially performed under the UNIHEAT project. EDB, LL and SM wish to acknowledge the Skolkovo Foundation and BP for financial support. The support of Hexxcell Ltd, through provision of Hexxcell Studio™, is also acknowledged.

[1] Diaz-Bejarano, E., Coletti, F. & Macchietto, S., 2015. A new dynamic model of crude oil fouling deposits and its application to the simulation of fouling-cleaning cycles. Submitted for publication.

[2] Diaz-Bejarano, E., Coletti, F. and Macchietto, S., 2015. Beyond fouling factors: a reaction engineering approach to crude oil fouling modelling. In Heat Exchanger Fouling and Cleaning XI. Enfield (Ireland).

[3] Coletti F, Macchietto S. A Dynamic, Distributed Model of Shell-and-Tube Heat Exchangers Undergoing Crude Oil Fouling. Ind Eng Chem Res. 2011;50(8):4515–4533.

[4] Hexxcell Ltd., 2015. Hexxcell Studio^TM. http://www.hexxcell.com.