(113c) Retrofitting Heat Exchangers with High Shear Stress Design: Evaluating the Impact on Performance of Crude Pre-Heat Trains

Authors: 
Coletti, F. - Presenter, Hexxcell Ltd
Diaz-Bejarano, E., Hexxcell Ltd
Macchietto, S., Imperial College London
A common solution adopted by heat transfer engineers to mitigate fouling in shell-and-tube heat exchangers is to increase fluid tube-side velocity. As a result, the wall shear stress increases and less fouling material is allowed to accumulate on the tubes' surface. Whilst this approach may indeed reduce fouling, the resulting design is typically one with higher pressure drops which, if not properly considered, may lead to unintended and costly consequences.

In refinery pre-heat trains, as an example, higher pressure drops resulting from increased flow velocity may translate directly into a reduced throughput which can greatly outweigh, from an economic point of view, the benefits of fouling mitigation. To properly assess whether it is beneficial to increase fluid velocity, it is now possible to use software tools capable of capturing the thermal and hydraulic trade-offs that exist between operating conditions (e.g. temperature, shear stress), fouling rates, pressure drops and refinery throughput over time.

In this paper it is shown that the simultaneous assessment of the interacting and non-obvious effects on fouling, pressure drops and throughput as a function of the specific thermal and hydraulic limits of the network, has significant benefits in terms of overall economics. It is also shown how historical plant data can be used to assess alternative retrofit options and evaluate the impact that a new design have on the overall performance of the network.

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