Safety Aspects during Transient and Steady-State Operation of a Pilot-Scale Distillation Column

Document Type


Publication Date

Fall 2000


Department of Chemical Engineering


In distillation involving flammable liquids, inert gases are used to prevent the formation of flammable or combustible gas mixtures by precluding oxygen from the equipment vapor space. In this study, a pilot-scale distillation column for the separation of ethanol and water was used to directly measure pressure changes within the isolated column and within the inerting system during start-up, steady-state, and transient operation. Our results show the following: 1. Distillation column vacuum relief rates due to reboiler shutdown, as predicted by current literature sources, are conservative by a factor of 50 compared to measurements. 2. During steady-state operation, the difference in solubility of the inert gas (N2) between the feed and distillate streams accounts for pressure changes in an isolated column. For ethanol and water, the nitrogen is more soluble in the distillate, resulting in a column vacuum. For an open system, this effect could result in air being drawn into the system, resulting in potential formation of combustible vapor mixtures. 3. A nitrogen balance model is presented to calculate the changes in column pressure during steady-state column operation. The predictions are conservative by a factor of 2 to 5. 4. For transient operation, a simple model based on fractional utilization of the condenser over-predicts pressure changes resulting from reboiler steam duty changes to within about 30% of the measurements.

Publication Title

Process Safety Progress