Efficiency and economics have motivated RasGas Trains 3 through 5 to implement a novel compressor-driver arrangement comprising both propane and mixed-refrigerant (MR) compressors on a single shaft. One Frame 7E gas turbine drives the low-pressure (LP) and medium-pressure (MP) MR compressors, while a second Frame 7E drives the propane and high-pressure (HP) MR machines. With minor control system adjustments after startup, the refrigerant compressors in RasGas Train 3 are operating reliably.
The propane precooled mixed refrigerant (C3MR) process developed by Air Products has been the dominant process technology employed in the liquefied natural gas (LNG) industry for more than 30 years. This process uses a propane refrigerant system to chill the feed gas from ambient temperature to approximately -33°C and an MR to chill the gas to LNG temperatures. Because pure propane is used in the precooling refrigeration cycle, the minimum temperature of this cycle is limited to the boiling temperature of propane near atmospheric pressure. The total duty that can be transferred to the propane refrigerant is thereby naturally limited to heat that can be transferred within this range.
Because of the propane's limited temperature range, one of the characteristics of the C3MR process arrangement is that the MR refrigeration system requires nearly double the compression power of the propane refrigeration system. The actual difference depends on the number of propane stages, the ambient conditions, the environmental cooling medium, and other process arrangements. To drive these refrigeration compressors, early LNG plants used steam turbines that could be sized to fit the required service (e.g., Marsa el Brega). Later projects determined that gas-turbine drivers were a more efficient and less costly option for driving the compressors.
For procurement, maintainability, and sparing purposes, an operating facility would prefer to have each of the compression services driven by the same type and model of gas-turbine driver. For the C3MR process, such an arrangement is typically inconvenient because of the inherent power consumption mismatch between the propane and MR services.