# Effect of Parameters on Vapor Generation in Ship-to-Ship Liquefied Natural Gas Bunkering
## 4 Results and Discussions
### Effects of LNG Composition  
The LNG composition varies with the natural gas source, processing plant, and customer requirements [12; 31]. In this section, the effects of the LNG composition on the vapor generation and vapor return were analyzed. Four LNG compositions were compared: lean, medium, rich-1, and rich-2, as shown in Table 8. The rich cases were divided into rich-1 and rich-2 to determine the effect of the nitrogen content. To make the comparisons equal, all other design parameters (i.e., except for the LNG compositions in bulker tanks) remained constant, as shown in Tables 2-4.  
The total amount of vapor return in the LNG rich-1 case was smaller than that in the LNG lean case, as the LNG rich-1 case was at a higher pressure than the LNG lean case, as shown in Table 8. In addition, the receiving tank net vapor generation values (\(m_{nt,pg,r}\)) for the lean, medium, and rich-1 cases were 626.14, 434.83, and \(-314.20\) kg, respectively. The negative amount of the receiving tank net vapor generation in the case of LNG rich-1 can be explained by the fact that the receiving tank pressure was lower than the bulker tank pressure for a certain period of time after the start of bunkering,  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Conditions** & **LNG Lean** & \begin{tabular}{c} **LNG Medium** \\ **(Base Case)** \\ \end{tabular} & \
\begin{tabular}{c} **LNG Rich-1** \\ **(Base Case)** \\ \end{tabular} &
\begin{tabular}{c} **LNG Rich-2** \\ **(\%)** \\ \end{tabular} \\ \hline Composition (mole \%) & & & & \\ Methane & 0.985 & 0.923 & 0.860 & 0.860 \\ Ethane & 0.013 & 0.050 & 0.094 & 0.099 \\ Propane & 0.001 & 0.015 & 0.025 & 0.025 \\ Nitrogen & 0.001 & 0.005 & 0.010 & 0.005 \\ n-Pentane & 0.000 & 0.001 & 0.001 & 0.001 \\ n-Butane & 0.001 & 0.006 & 0.010 & 0.010 \\ Temperature (\({}^{\circ}\)C) & \(-147\) & \(-147\) & \(-147\) & \(-147\) \\ Pressure (bar) & 2.916 & 2.941 & 3.077 & 2.803 \\ Total amount of vapor return (kg) & 4224.51 & 4016.52 & 3230.68 & 5296.50 \\ Receiving tank net vapor generation, & 626.14 & 434.83 & \(-314.20\) & 1696.62 \\ \(m_{nt,pg,overall}\) (kg) & \(-167.18\) & 17.01 & 386.24 & \(-134.93\) \\ \hline \hline \end{tabular}
\end{table}
Table 8: Composition of LNG in bulker tanks and results.  
Figure 8: Variation of liquified natural gas (LNG) temperature at the receiving tank inlet for different insulation materials (solid line: temperature in receiving tank inlet, dotted line: heat flow through pipes).  
as shown in Table 8 and Figure 9. Thus, the vapor return did not occur during this period. Thus, in this period, the fraction of the vapor was condensed with the gradual pressure increase. In contrast, in the LNG lean case, the receiving tank pressure gradually decreased until it was equalized with the bunker tank pressure, resulting in a relatively large amount of generated vapor. The values of the net vapor generation in the overall system (\(m_{net,pg,overall}\)) for the lean, medium, and rich-1 cases were \(-\)167.18, 17.01, and 386.24 kg, respectively. The LNG rich-1 case generated a larger amount of vapor due to the volume displacement in the bunker tank, in which the discharged volume was replaced by vapor. To identify the effect of nitrogen in the LNG mixture, the LNG rich-1 and rich-2 cases were compared. The compositional difference between LNG rich-1 and rich-2 was that the rich-2 case had a mole fraction of nitrogen of 0.05 less, and a mole fraction of ethane of 0.05 more than those of rich-1. As shown in Table 8, the receiving tank net BOG generation (\(m_{net,BOG,r}\)) values for the rich-1 and rich-2 cases were \(-\)314.20 and 1696.62 kg, respectively. This was because by reducing the nitrogen mole fraction, the equilibrium pressure of the rich-2 case became lower than that of the rich-1 case; thus, more vapor returned to the bunker tank, generating more vapor in the receiving tank. Overall, it can be determined that the initial pressure of the bunker tank according to the LNG composition at a given temperature significantly affects the vapor generation.