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.
A disadvantage of the kinetic modeling methods is the availability of suitable detailed chemical reaction models, stoichiometry, thermodynamic, physical, and transport properties to couple with fluid dynamics models. Fortunately, the development of detailed chemical reaction models has become much more practical over the years as discussed below. Detailed chemical reaction models can be developed quickly and cost effectively using a combination of adiabatic calorimetry testing and advanced computational tools such as Process Safety Office® SuperChems™ software.
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