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When to Ride on the Wave? Fundamental insights from the wave equation on controlling cryogenic multi-stream heat exchangers during LNG plant startup, normal operation, and shutdown.

Number: 36

Author(s) : BO J., OKASINSKI M.

Summary

Multi-stream heat exchangers have been widely used in many cryogenic processes such as liquefied natural gas (LNG). These heat exchangers are often sizable and costly due to the requirement of large heat transfer area to achieve high thermal efficiency. Due to the dynamic nature of plant operation, these heat exchangers can be subject to thermal shock and stress which, when exceeding the design limit, are the main contributors to tube damage and potential leaks. To minimize the impact of these thermal stresses on the health and longevity of heat exchangers, a good operator/control system moves from one operating point to another smoothly and gradually.
However, such movement cannot be guaranteed without a properly installed and tuned control scheme. In addition, moving too slowly can delay the plant from achieving full rates in a timely manner, thereby negatively impacting revenue. Therefore, it is crucial to have an effective control scheme for the operation of these heat exchangers to maximize plant revenue while minimizing potential damage to the heat exchangers during the entire spectrum of plant operations including plant startup, normal operation, upsets, and shutdown. This paper will focus on effective control schemes for the operation of the main cryogenic heat exchanger (MCHE) in an LNG plant. It can be shown that properly designed and installed control schemes are based on the fundamental wave equation, which captures the dynamic nature of the
MCHE during all phases of plant operation. During normal operation, daily LNG production often varies due to ambient temperature
fluctuations, e.g. higher production during night while lower production during day. Though the variation of production is relatively small around a full rate operating point, the pseudo steady-state wave equation shows that the flow ratio control is effective for normal operation. In contrast, LNG production experiences a large variation from zero to full rate during the plant startup (or from full rate to zero during plant shutdown). The full wave equation shows that the flow ratio control cannot be effective for plant startup/shutdown; rather temperature rate-of-change control should be used. Furthermore, the wave equation can also address when and why to switch between flow ratio control and temperature rate-of-change control depending on the plant production rate. Therefore, based on the wave equation, the optimal control of the MCHE can be automated for maximizing both LNG production and MCHE longevity.

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Details

  • Original title: When to Ride on the Wave? Fundamental insights from the wave equation on controlling cryogenic multi-stream heat exchangers during LNG plant startup, normal operation, and shutdown.
  • Record ID : 30034649
  • Languages: English
  • Subject: Technology
  • Source: 21st International Conference & Exhibition on Liquefied Natural Gas (LNG2026)
  • Publication date: 2026/02/05

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