Limiting Reactive Feeds By Inherently Safe Design Methodology

Semi-batch chemical processing often involves feeding reactive materials into jacketed, agitated vessels producing exothermic reactions. Reactant feed schemes are either automatically controlled via a Basic Process Control System or by manual adjustment. Both methodologies are prone to erroneous control action failure, mechanical component failure, or human error. Feed system failures can lead to sudden undesired excessive feed rates leading to undesired excessive temperature increase by heat of reaction without a means to terminate the feed based on safety constraints that could exceed safe operating limits. What may not be recognized is that temperature excursions due to excessive reactant feed flow can further initiate exothermic, often violent, secondary decomposition reactions with energy release rates much larger than the desired synthesis reaction leading to catastrophic vessel overpressure consequence. This paper will outline a simple, inherently safe design methodology to ensure that potential heat generation does not outrun heat removal capacity. The design steps consist of (1) using a rigorous Reactive Hazards Screening evaluation process to determine safe operating temperature limits, (2) determining vessel heat removal and feed system hydraulic flow capacity, and (3) utilizing a dynamic energy balance to verify that maximum reactant feed flow under worst case failure upset conditions will not exceed an established safe Maximum Temperature Limit for a resulting exothermic reaction.