I would like to ask why, in the liquefaction of LNG, the pressure of the feed gas is often increased to over 4.5 MPa

LNG liquefaction requires the least energy consumption at around 5 MPa. However, when building a plant, we cannot consider only the energy needed for liquefaction; if a pressure of 5 MPa is required in the cryogenic tank, the pressure in the preceding purification units must be even higher, which means that the pipeline specifications and the design pressure of the equipment also need to be increased. This is especially true for the dehydration unit – due to the high temperatures, the pressure tolerance of flanges decreases significantly, often requiring pipeline specifications to be raised to PN100. This is manageable for small installations where valves and equipment are of smaller size; but for medium or large plants, it leads to a significant increase in the cost of equipment and materials. From the perspective of construction projects, the option that yields the best benefits when taking into account initial investment, operating costs, expected selling price, and payback period is the most suitable one for the owner.

Natural gas will only liquefy when the pressure is above the critical value and then reduced to the critical temperature

Is there a group chat? I’m also new to liquefaction; I just became a technician and want to learn

The written content mainly focuses on facilitating the absorption of carbon dioxide, reducing energy consumption in the liquefaction system, controlling the size of the equipment, as well as the partial pressure for water removal

It should be the latter interpretation: beyond the critical pressure, the energy required for liquefaction is at its lowest

Could you elaborate a bit on the partial pressure of water vapor?