The rapid market growth in LNG and the development of large gas reserves such as the North Field of Qatar demand another step-change in the capacity of LNG trains, as lowest unit cost continues to be a key value driver.
With the standard Propane Mixed Refrigerant (C3/MR) technology, capacities up to 5 Mtpa can be achieved with two GE Frame 7 gas turbines as drivers. For higher LNG capacities new configurations are required. Shell has developed designs for both mechanically driven and electrically driven large LNG trains, featuring low cost and low emissions. The choice is determined by project specific considerations.
The Shell Parallel Mixed Refrigerant process consists of a single pre-cool cycle followed by two parallel liquefaction cycles. For pre-cooling either propane or a mixed refrigerant, like in Shell's Double Mixed Refrigerant Process (DMR), is used. With three well proven GE Frame 7 gas turbines, 8 Mtpa of LNG production is achieved. With GE Frame 9 or Siemens V84.2 gas turbines, the LNG capacity increases to 10 Mtpa; these gas turbines are still novel drivers for the LNG industry but are already used as mechanical drivers in other processes.
The Shell Parallel Mixed Refrigerant Process for large LNG trains has a number of appealing advantages:
Robustness through the application of well-proven equipment without scale-up of equipment.
High reliability and availability by parallel line-up of the liquefaction cycle. For example, if one of the liquefaction cycles trips, LNG production is designed to continue at 60% of train capacity.
The optimal power balance between the two liquefaction cycles (1:2) results in a high efficiency.
The application of Shell's electrically driven DMR process is also very attractive. This concept is based on a parallel line-up of the refrigerant compressors around a common set of cryogenic spoolwound exchangers. Electric motors of 65 MW have already been constructed for LNG service. In combination with Shell's DMR technology an LNG production capacity of 8 Mtpa can be achieved. The power station is driven by gas turbines.