Inerting The Head Space In Ship Cargo Tanks

By:� Kenneth T. McDonald

The Dow Chemical Company

The quality of some products, such as propylene oxide (PO), is affected by the presence of high oxygen content in the vapor space in storage vessel, such as ship tanks and cargo tanks.� The consequences of oxygen exposure become especially pronounced when the exposure occurs for extended periods of time.� Reducing the oxygen concentration in storage tanks and ship tanks during transit to less than about 500 parts per million (ppm) minimizes many of the detrimental effects of oxygen exposure.� The oxygen concentration is typically reduced by purging and padding the tanks with an inert gas, such as nitrogen.� � However, it is especially difficult and time consuming to purge large ship tanks to the desired low oxygen concentration.� Normally, the oxygen concentration is reduced using a continuous purge from bottom to top and cascaded from tank to tank on chemical tankers.� This method is a time consuming and expensive task.

Marine loading systems for volatile products, such as PO, include a liquid load line and a vapor return line.� The vent gases contain a substantial quantity of product and are returned to the storage tank to minimize losses and emissions.� The gases being vented from the storage tank pass through a refrigerated condenser to separate and recover the PO from the inert gas.

The following methods can be used to optimize the inert gas purging process on ship and cargo tanks:

1.      Prior to loading, all PO cargo tanks and associated piping systems are purged with an inert gas, such as nitrogen.� � The oxygen concentration in the ship tanks...