Intermediate Fluid Vaporizer (IFV) of LNG system - Working Principle

An Intermediate Fluid Vaporizer (IFV) in an LNG system uses a secondary heat transfer fluid (usually propane) to indirectly vaporize LNG, preventing seawater freezing. It operates via three stages—evaporation of propane by seawater, condensation of propane by LNG, and gas heating—to efficiently regasify LNG into gaseous natural gas at ambient temperatures.

Working Principle & Components

The IFV is typically a shell-and-tube heat exchanger developed by Osaka Gas. It utilizes an intermediate fluid (e.g., propane, freon) to transfer heat from a warm source (seawater) to the cold LNG ( -163°C), which prevents the seawater from freezing on the heat exchanger surfaces.

Stage 1: Evaporator (E-1) - Intermediate Fluid Vaporization

• Warm seawater flows through tubes and releases heat, causing the liquid propane (intermediate fluid) in the shell to boil and vaporize.

Stage 2: Condenser (E-2) - LNG Vaporization

• The vaporized intermediate fluid travels up to the condenser section.
• Cold LNG flows through tubes inside this section, causing the propane vapor to condense back into liquid while the LNG turns into gaseous natural gas.

Stage 3: Heater (E-3) - Natural Gas Heating

• The cold natural gas from E-2 flows through a final heater section, using seawater to heat the gas to the required final output temperature.

Key Features and Advantages

• No Freezing Risk: The intermediate fluid prevents direct contact between seawater and extremely cold LNG, eliminating the risk of ice blockage.
• High Efficiency: Uses titanium tubes to withstand corrosion from seawater, often used in low-temperature seawater conditions.
• Cold Energy Utilization: Because the heat transfer is indirect, the "cold energy" of the LNG can be efficiently recovered for other uses (e.g., power generation, cooling) before the propane is re-used.
• Compact Design: The three-part process (evaporator, condenser, heater) can be integrated into a compact unit.

Intermediate Fluids Used

Commonly used intermediates include propane or dimethylether, chosen for their low boiling points, allowing them to boil at low temperatures while condensing at slightly higher temperatures.