Dispersion Modeling of a Cloud Generated By Depressurization of a Flashing Multi-Component Liquid System

Depressurization is usually a process operation during which the gas contained in a pressurized system is sent either to a flare or to a cold vent.  However, depressurization can also be the consequence of an unexpected event as, for example, the accidental opening of a vent or an unintended leak of a pressurized system. One of the immediate outcomes is the release and dispersion of a cloud into the atmosphere that creates a hazard when the cloud is flammable and/or toxic.  A depressurization through a flare would also generate a hazardous cloud in the event of a flame out.  The prediction of the harmful extent of such dispersing clouds is a common task when assessing the risk associated with such events.

Depressurization events are of several types which differ in the number and complexity of the mechanisms involved.  In general, there is either a system containing a single compressed vapor phase (referred to in this paper as a “gas system”), or a system containing a flashing liquid phase that is in thermodynamic equilibrium with a vapor phase (referred to in this paper as a “flashing liquid system”.  The flashing liquid is either a pure component or a multi-component mixture. Other variations of these include systems containing a liquid and a vapor phase that are not in equilibrium, systems with multiple liquid phases, and others. However, this paper will focus on the “flashing liquid system” with an emphasis on the multi-component variation, and will make references to the “gas system” for comparison purposes.

With a multi-component flashing liquid system, the characteristics of the vapor released will change over time.  Specifically, the temperature and the composition of the released vapor will change over time.  The resulting dispersing cloud could transition from a buoyant vapor mixture early in the depressurization event to a dense vapor mixture later in the depressurization event.  One immediate consequence is that in stable atmospheric conditions, a dispersing cloud could ascend initially, but after a few minutes it could descend and disperse at ground/sea level.  Not accounting for the changing characteristics of the system could result in a misleading assessment.

With a good consequence modeling program, modeling the time-varying discharge and dispersion of a depressurizing gas system is relatively easy to handle.  However, the same problem for a system containing a flashing liquid under pressure (LNG, LPG, etc.) is more complex even with a good commercial consequence modeling program.  Adding an appropriate accounting of the changing characteristics of a multi-component system further complicates predicting the harmful extent of a dispersing cloud.

The objective of this paper is to present a few methods to appropriately model such a complex dispersion problem. After comparing the merits and demerits of each method, ERM proposes a practical and simple method.