(657b) Incorporating Process Safety in Heat Exchanger Network Synthesis

The development of heat exchanger networks (HEN) has had a profound impact on the economics of operating plants. By efficiently matching hot and cold streams, HEN have resulted in utility savings and improved process economics. Until recently however, little attention has been paid to the safety of HEN. Chan et al. has examined the safety of HEN by applying an inherent safety index [1]. However, to our best knowledge, no prior work has examined the potential for overpressure within HEN synthesis. According to current ASME vessel code, every equipment (including heat exchangers) in a process plant must be analyzed for the potential for overpressure [2]. Overpressure analysis examines the potential of a system to experience a pressure beyond its maximum allowable working pressure, a pressure rating based on the construction of an equipment [3]. A common overpressure scenario often overlooked in heat exchangers is the potential for a tube rupture. Whether due to corrosion, vibration, or other factors that result in a tube failing, this phenomenon allows high pressure tube side fluid to enter the low pressure shell side [4]. Heat exchangers with high differences in pressure between the shell and tube side are most at risk when analyzing this scenario. For an improperly sized relief system, the severity of the aforementioned scenario can be detrimental to safe operation of HEN.

The traditional HEN synthesis literature primarily focuses on the economics of a network [5]. This may result in configurations that significantly increase the likelihood of a tube rupture. To counter this, there exists methods that can predict the pressure profile, and hence the severity, of a tube rupture [6]. This work proposes a metric to examine the safety of a network. A minimum threshold value can be established, which ensures a network is protected from the potential for overpressure. The possibility of a tube rupture is performed on every exchanger for every possible network. The consequence of the tube rupture is examined with different pressure relief valve sizes. Due to a tube rupture scenario being a dynamic event, accurate modeling requires simultaneously solving a series of nonlinear algebraic and partial differential equations (NAPDE). These equations are solved offline, and then imported to the HEN superstructure. The end result is a cost-effective HEN that achieves adequate protection from the possibility of a tube rupture.

References:

[1] Chan, I., Alwi, S. R. W., Hassim, M. H., Manan, Z. A., & Klemeš, J. J. (2014). Heat exchanger network design considering inherent safety. Energy Procedia, 61, 2469-2473.

[2] 2013 ASME boiler code & pressure vessel code. Section VIII: Rules for construction of pressure vessels. Division 1. (2013). New York, NY: ASME.

[3] API Standard 521. (2014). Pressure‐Relieving and Depressuring Systems.

[4] Simpson (1971). “High Pressure Gas-Filled Tubing Rupture in Liquid-Filled Heat Exchangers”. Safety in Design and Operation of Venting Systems Symposium, American Institute of Engineers, California, USA.

[5] Yee, T. F., & Grossmann, I. E. (1990). Simultaneous optimization models for heat integration—II. Heat exchanger network synthesis. Computers & Chemical Engineering, 14(10), 1165-1184.

[6] Harhara, A., Mannan, M. S., & Hasan, M. M. F. (2019). Modeling Shell Side Pressure Profiles for Heat Exchanger Tube Rupture Scenarios. AIChE 2019 Spring Meeting.