Optimized NGL Control System with Actual Flow and Set Point Tracking Feature

Abstract

Consistent ethane recovery in the Natural Gas Liquefication (NGL) process is critical to achieve financial objectives of the NGL processing facility. Joule-Thompson (JT) effect in combination with various processes such as cascade-refrigeration or Residue-Split-Vapor (RSV) are being used in the industry to maximize the ethane recovery from the feed gas varying in the degree of 75% to more than 95%. Identifying the transient conditions and ensuring precise and accurate control throughout is of utmost importance. The transient conditions are categorized as start-up or a scheduled shutdown of the plant, and an upset of the plant. Any of these transient conditions may drive the plant in unstable state which would impact the ethane recovery drastically. This paper discusses a control algorithm that was developed to identify the transient states and to provide an accurate and stabilized control to keep the recovery above the target threshold. During the startup, a typical NGL plant will start its operation in JT mode and will slowly transition into a cooling mode by introducing turboexpander for example. During the shutdown mode, the plant slowly returns to JT mode by shutting down the turboexpanders. During the upset, the turboexpanders can accidently trip to force the plant in an unstable state. In transient states, an accurate control is required to precisely transfer the feed gas volume from turboexpanders to JT equipment or vice versa in a timely manner to minimize the impact such as loss of production or total plant trip. The proposed control algorithm predicts an upset in advance, captures the actual flow of the feed gas passing through the equipment prior to an upset and transforms the captured flow into an equivalent percentage opening of the backup equipment (in case of JT mode, the percentage opening of the JT valve and in case of turboexpander mode, the percentage opening of the Inlet Guided Vanes (IGVs)) to ensure the plant mass balance is maintained. The set point tracking feature of the algorithm ensures that when the normal Proportional Integral Derivative (PID) control is resumed the transfer of control is bump less to avoid any overshooting or undershooting of the overall plant pressure.