Question 39 TK02 - Tank Vessel Assistant - Liquefied Gas

The Correct Answer is B **Explanation for Option B (At its boiling point):** LNG stands for Liquefied Natural Gas. To maintain natural gas in a liquid state for transportation and storage, it must be cooled to extremely low temperatures, typically around $-162^{\circ}\text{C}$ ($-260^{\circ}\text{F}$) at atmospheric pressure. This specific temperature is the boiling point of methane (the primary component of natural gas) at standard atmospheric pressure. Transporting LNG at its boiling point (often referred to as cryogenic temperature) ensures that the liquid remains stable. As heat inevitably leaks into the storage tanks, a small amount of LNG boils off, maintaining the tank pressure and keeping the remaining liquid at the constant boiling point temperature. This constant boiling (known as boil-off gas or BOG) is a critical characteristic of LNG transport. **Why the other options are incorrect:** * **A) At a pressure in excess of 15.4 psia:** While LNG tanks operate slightly above standard atmospheric pressure (14.7 psia), the key characteristic is the extremely low temperature, not the high pressure. Natural gas transported at room temperature and pressures significantly exceeding 15.4 psia (e.g., hundreds or thousands of psia) is called Compressed Natural Gas (CNG) or simply pipeline natural gas, not LNG. * **C) At its critical temperature:** The critical temperature is the highest temperature at which a substance can exist as a liquid, no matter how much pressure is applied. For methane, the critical temperature is about $-82.3^{\circ}\text{C}$ ($-116.1^{\circ}\text{F}$). LNG is transported far below this temperature (at $-162^{\circ}\text{C}$) to keep it in a liquid state at near-atmospheric pressure, which is essential for economical bulk transport. * **D) At its vapor stage:** LNG is *Liquefied* Natural Gas. If it were in its vapor (gaseous) stage, it would occupy about 600 times the volume, making transportation highly inefficient and uneconomical. The entire purpose of liquefaction is to reduce the volume for transport.