# Effect of Parameters on Vapor Generation in Ship-to-Ship Liquefied Natural Gas Bunkering
## 4 Results and Discussions  
### Effect of Initial Temperature of Receiving Tank  
The effect of the initial temperature of the LNG in the receiving tank on vapor generation was studied by comparing the difference between the vapor mass at the initial condition and that at the completion of bunkering, as illustrated in Figure 3.  
A case study was conducted by changing only the temperature of the receiving tank from the base case condition, as shown in Tables 2-4. The results indicated that with an increase in the initial temperature of the LNG in the receiving tank, both vapor mass difference in the bunker tanks (\(\Delta m_{v,b}\)) and the vapor mass difference in the receiving tank (\(\Delta m_{v,r}\)) were increased. When the temperature of the LNG in the receiving tank was warmer than that of LNG in bunker tanks (\(-147\ ^{\circ}\)C), the net-generated vapor in the overall bunkering system (\(m_{net,v,g,overall}\)) was negative, which could be interpreted as a  
Figure 3: Comparison of vapor mass differences with varying receiving tank initial temperature. partial amount of vapor was condensed. In contrast, in cases where the temperature of the LNG in the receiving tank was colder than that of LNG in the bunker tank, the net-generated vapor mass in the overall bunkering system was positive and decreased as the initial temperature of the LNG in the receiving tank increased. For a more detailed investigation, the variations in the mass flow rate of the vapor return, receiving tank pressure, and amount of vapor return were studied at the initial LNG temperatures of \(-145\), \(-147\), and \(-149\)\({}^{\circ}\)C (in the receiving tank). When the initial temperature of the receiving tank was \(-145\)\({}^{\circ}\)C, i.e., higher than that of the bunker tanks, the rate of vapor return increased sharply once bunkering commenced, and then decreased until the pressures of the bunker tanks and receiving tank were equalized, as shown in Figure 4. Note that the sample data collected at 60 s intervals were used for all subsequent graphs. When the initial temperature of LNG in the receiving tank was \(-149\)\({}^{\circ}\)C, i.e., lower than that of the bunker tanks, the vapor return to the bunker tanks remained zero until the pressure of the receiving tank approximated the bunker tank pressure. From these results, it can be interpreted that a positive value of the vapor mass difference in the bunker tanks (\(\Delta m_{v,b}\)) is mostly due to vapor return in cases of a warmer initial temperature in the receiving tank. In cases where the initial temperature of the receiving tank is colder, the positive value of the vapor mass difference in the bunker tanks (\(\Delta m_{v,b}\)) is mostly due to vapor generation in the bunker tanks, in order to maintain the pressure inside the bunker tanks. Similarly, a negative value of the vapor mass difference (\(\Delta m_{v,r}\)) in the receiving tank in cases of a warmer initial temperature in the receiving tank is mostly due to vapor return to the bunker tanks, and a negative value of the vapor mass difference in the receiving tank (\(\Delta m_{v,r}\)) in cases where the initial temperature of the receiving tank is colder is mostly due to the condensation of LNG vapor. By comparing the total amount of vapor return with vapor mass difference in Figure 3, it can be seen that when the initial temperature of the receiving tank was \(-145\)\({}^{\circ}\)C, the total amount of vapor return was greater than the vapor mass difference in the receiving tank, i.e., \(\Delta m_{v,r}\). This can be interpreted as follows: As the bunkering operation progressed, the pressure in the receiving tank was decreased. Accordingly, the bubble point temperature of the LNG was lowered, resulting in a larger amount of vapor return (\(\int_{0}^{t}\dot{m}_{vapor\ return}=7393\) kg) than the reduced amount of vapor in the receiving tank (\(\Delta m_{v,r}=-4040\) kg).