THE PMR PROCESS, AN INNOVATIVE TECHNOLOGY FOR LARGE LNG TRAINS

Abstract

The forecasts in market growth of LNG and the development of large gas fields are a great stimulus for further enhancing the capacity of LNG trains. The increasing cost of upstream development, including the managing of CO2, deepwater, ice, complex reservoirs and so on, makes downstream economies of scale an imperative. This paper addresses Shell’s approach to large LNG trains in the range of 7–10 Mtpa covering both mechanical and electrical drive options to develop innovative and cost effective designs.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. Due to maximum size constraints of key equipment, an additional liquefaction cycle is required to realise higher LNG capacities. The following solutions are presently applied to extend the capacity above the 5 Mtpa range:adding an additional liquefaction cycle in a three-cycle in series line-up; and,a single pre-cool cycle followed by two parallel liquefaction cycles.Both liquefaction configurations, although of different concepts, have a similar number of equipment items. Shell Global Solutions has developed the latter option as the Parallel Mixed Refrigerant (PMR) process. For precooling either propane or a mixed refrigerant, as used in the Double Mixed Refrigerant (DMR) process, are used. With three well-proven GE Frame 7 gas turbines, 8 Mtpa of LNG production is achieved. With larger drivers such as GE Frame 9 or Siemens V84.2 gas turbines, the LNG capacity increases to above 10 Mtpa.The PMR process for large LNG trains has a number of attractive features:robustness through the use of well-proven equipment;high availability by parallel line-up of the liquefaction cycle. For example, the LNG production is designed to continue at 60% of train capacity if one of the liquefaction cycles trips; and,the optimal power balance between the pre-cool and the two parallel liquefaction cycles results in a high efficiency. Shell’s electrically driven DMR process is also very attractive, particularly for Greenfield applications. This concept is based on a parallel line-up of the refrigerant compressors around a common set of cryogenic spoolwound exchangers and achieves an LNG capacity of more than 8 Mtpa. The power station is driven by gas turbines. The following considerations play a key role in the selection process of electrically driven plants.The gas turbine maintenance is decoupled from LNG production, resulting in a lower downtime. A net increase up to 4% in stream days is possible.High efficiency gas turbines can be selected for the power station and efficiency can be further improved by a combined cycle power plant.The step change in efficiency achieved in a combined cycle power plant is very beneficial in lowering the CO2 and NOx emissions, as well as the feed gas intake.