REVISED EMERGENCY VACUUM RELIEF DEVICE SIZING FOR ATMOSPHERIC DISTILLATION SYSTEMS

Conventional vacuum relief methodologies are usually protective respon ses; that is, they accomplish their purpose by substitution of an iner t gas (usually nitrogen) for the process gases removed by an external vacuum source, or for condensable vapour collapsed by an internal proc ess mechanism (e.g. condensation). While this approach is theoreticall y possible for all potential vacuum scenarios, it becomes practically impossible to implement for installations where a rapid phase change c an impart near-instantaneous system pressure reductions. The procedure outlined in this paper takes a preventive approach: eliminate the sou rce of vacuum generation before the safe lower system pressure limit i s reached. For distillation and other refluxing systems, this vacuum s ource is usually the main overhead condenser, which is designed to col lapse large volumes of condensable vapour. To eliminate the vacuum sou rce requires elimination of the system's ability to rapidly condense v apour. This goal is accomplished by introduction of inert gas directly into the condensing system to 'blanket' the heat transfer surface and stop condensation. The procedure determines the rate, amount and loca tion for introduction of inert gas. The required design data include: (i) system starting pressure, (ii) maximum allowable system vacuum, (i ii) volume of the condensing system, and (iv) normal system condensing rate. By determining the rate at which the condenser removes vapour v olume from the system, and designing an inert gas delivery system to m eet or exceed this rate, the vacuum generation potential of the system is effectively eliminated using a much smaller quantity of inert gas than with the more traditional volume substitution methods.