What is the significance of lean oil molecular weight in absorption based NGL recovery processes?

The significance of lean oil molecular weight in absorption-based NGL (Natural Gas Liquids) recovery processes is that it directly affects the oil's absorption capacity, circulation rate, and stripping efficiency, ultimately impacting NGL recovery and operating costs. Absorption-based NGL recovery involves contacting a natural gas stream with a liquid absorbent, known as lean oil, to absorb the heavier hydrocarbon components (ethane, propane, butane, etc.). Lean oil is typically a mixture of hydrocarbons with a boiling range higher than the NGLs being recovered. The molecular weight of the lean oil is a critical property that influences its performance. A lean oil with a lower molecular weight generally has a higher vapor pressure, leading to greater lean oil losses through vaporization in the absorber and stripper towers. These losses increase operating costs and can contribute to air emissions. A lean oil with a higher molecular weight has a greater capacity to absorb NGLs because it provides more surface area for interaction with the gas components. This means that a lower circulation rate is required to achieve the same level of NGL recovery, which reduces energy consumption for pumping and regeneration. However, a very high molecular weight lean oil may be more viscous, leading to increased pressure drop in the absorber tower and reduced mass transfer efficiency. The stripping efficiency, which is the ability to remove the absorbed NGLs from the rich oil in the stripper tower, is also influenced by the lean oil molecular weight. A lower molecular weight lean oil is typically easier to strip, requiring less heat input. The optimum lean oil molecular weight is a balance between maximizing absorption capacity, minimizing lean oil losses, and achieving efficient stripping. Selecting the correct lean oil composition is crucial for optimizing NGL recovery and minimizing operating costs.