Direct scale-up methods have been used to develop relief requirements and vent sizing for runaway reactions since the early 1990s. Direct scale-up methods have been popular because one is able to measure in a laboratory test the required relief size in equivalent vent area per unit mass of a reacting mixture, in 2/kg, and then scale it up to plant scale equipment sizes.
The primary advantage of the direct scale-up methods is simplicity. The user does not have to provide thermodynamic, physical, and transport properties or use complex models for relief sizing. However, direct scale-up methods have a lot of disadvantages and are not capable of providing all the information for safe and optimal design that is now required by recognized and generally accepted good engineering practice (RAGAGEP).
Direct scale-up methods are only valid at the conditions of the test. This includes but is not limited to fill level, relief set pressure, chemical composition, heating rate, and vapor/liquid disengagement characteristics of the test cell and associated vent. Additional tests have to be conducted if different conditions need to be considered. This can be costly both in resources and schedules.
Direct scale-up methods can result in overly conservative venting requirements, especially for gassy systems. While this may be considered to be favorable for vessel protection, an oversized vent can cause safety complications for PRV stability, structural supports, effluent handling, and subsequent safe discharge location for flammable or toxic dispersion. A bigger vent is not necessarily better.
Kinetic modeling methods for pressure relief couple detailed chemical reaction models with fluid dynamics to develop the required vent size. These methods have also been in use since the early 1990s when the American Institute of Chemical Engineers (AIChE) first developed the computer program SAFIRE through its Design Institute for Emergency Relief Systems (DIERS). SAFIRE was later replaced with SuperChems™ for DIERS.
Kinetic modeling methods for pressure relief and effluent handling systems are highly recommended because of their inherent advantages over direct scale-up methods. Once a detailed kinetic model is developed, it can be used over and over again in many process design and modeling applications.
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