What are the implications of nitrogen presence in the feed gas for liquefaction efficiency?

The presence of nitrogen in the feed gas has several implications for liquefaction efficiency. Primarily, nitrogen increases the refrigeration load required for liquefaction. Nitrogen has a significantly lower boiling point (-196°C) than methane (-162°C), the primary component of natural gas. This means that the liquefaction process needs to cool the nitrogen to a much lower temperature, requiring additional refrigeration duty. This increased refrigeration demand translates to higher energy consumption and reduced overall plant efficiency. Secondly, nitrogen can reduce the heating value of the LNG product. LNG is sold based on its energy content, typically measured as BTU per unit volume. Because nitrogen is an inert gas and does not contribute to the heating value, its presence dilutes the energy content of the LNG. This can reduce the market value of the LNG. Thirdly, nitrogen can affect the stability of the LNG during storage and transportation. If the nitrogen concentration in the LNG is too high, it can lead to stratification, where the LNG separates into layers with different densities and compositions. This can cause "rollover," a phenomenon where the rapid mixing of these layers releases a large amount of vapor, potentially exceeding the tank's venting capacity. Therefore, nitrogen is often removed from the feed gas to meet LNG product specifications and prevent operational problems. This is typically achieved through cryogenic distillation or other separation techniques.