Models for Decanting Gaseous Fuel Tanks: Simulations with GFSSP Thermal Model (2016)

Transport of fuel from distillation/storage plant to different parts of the world has always been challenging task for Engineers. Different methods have been proposed over the time for transporting fuel efficiently and at low cost which include Marine vessels, Pipelines, Rail Cars and Trucks. In order to transport useful amount of fuel in a reasonably sized tank, we have to liquefy it. While few fuels are easy to liquefy there are great number of fuels which liquefy only under extreme pressure/temperature conditions. Methane has a boiling point of -161.7°C at atmospheric pressure which means it has to be cooled to a much lower temperature in order to turn it into to liquid that can be stored in a tank. In short, Methane is not stored in household tanks because it is hard to liquefy. So large carbon epoxy fiber tanks were developed to transport Methane around the world in gaseous state at high pressures. But when the tank decanting was done in places where ambient temperatures were well below 0°C, it was found that ideal conditions for Methane liquefaction were formed. Gaseous Methane has to lose a lot of energy to liquefy which means that liquid Methane would be much colder than its gaseous counterpart. Now the liquid Methane would cool the tank walls more rapidly than its gaseous counterpart present in the tank and thus this difference in temperature would impart additional thermal stresses on the tank walls. These thermal stresses are result of uneven contraction/expansion of the tank walls and may lead to crack formation in the wall surface which we intend to avoid in all circumstances. The present study is concentrated in identifying different tank decanting conditions where there may arise favorable conditions for liquid Methane formation. Tank decanting simulations were performed for different system temperatures ranging from -50°C to 20°C and flow rates of 0.064kg/s, 0.11975 kg/s and 0.3kg/s respectively were selected for each system temperature. A similar study on Biogas tank decanting was performed and different system conditions were identified where possibility of liquefaction arouse within the tank. Through the tank decanting study carried out on Methane and biogas fuels, few decanting conditions were identified where there was a possibility of liquefaction within the tank. If the decanting was continued from these points of liquefaction, the tank walls would experience immense thermal stresses and there may arise a point where cracking in the tank wall takes place. The tank decanting, either has to be stopped or the decanting flow rate should be reduced further at these points. As the decanting flow rate is reduced the tank wall would have enough time to pump heat into the system and thus avoiding liquefaction within the tank. This process can continue only until the tank wall temperature and the fuel temperature within the tank reaches equilibrium or the fuel temperature reaches its critical point, whichever comes earlier. After this, decanting has to be stopped because any further temperature drop would result in liquefaction which has to be avoided under any circumstances as this would in turn result in tank liner failure. Advisor: Kevin D. Cole