Innovative NRU design at Pioneer Natural Resources' Fain Gas Plant

Pioneer Natural Resources' Pain Gas Plant underwent a major plant upgrade in 1996. A key component of the expansion was the integration of a nitrogen rejection unit (NRU) into the facility. This posed some interesting and challenging design problems for the project team. Nitrogen rejection units are sensitive to fluctuations in feed conditions and operate best with stable inlet rates, compositions, pressures and temperatures. The Fain plant was experiencing operating problems that would have made the NRU feed conditions unstable. Therefore, the first design challenge was improving plant operations. Because the Fain plant consists of two cryogenic plants operating in parallel, another design challenge was efficiently integrating the NRU into the existing operation. The final design utilized four streams, two from each plant, which were combined to produce two feed streams with distinct compositions. Utilizing a split feed, dual column NRU cold box, the design is tolerant to carbon dioxide while maximizing the methane and helium recovery. Flexibility of the dual column cycle and the use of multivariable optimization in the design enabled the cold box to deliver two residue gas streams that were matched to existing compressors in the facility. The result is an innovative and cost effective design that was readily integrated into the existing operation. Design flexibility and ease of operation was tested early in the NRU plant life when the NGL facilities were unable to deliver the design feed composition and flow rates. Several factors were key to obtaining good performance and operational run-time. First was solving control problems at the interfaces between the NGL plants, NRU and residue compressors. Second was solving operating problems that were impacting product sales and improved marketability of residue sales gas.