Achieving the Three Dimensions of Mixed Refrigerant Compressor Efficiency

Abstract Investors in small-scale LNG (SSLNG) face the grave challenge of achieving cost efficiency, operational efficiency, and energy efficiency in the equipment they use, all with maximum availability. In this context, the mixed refrigerant compressor's initial design plays a particularly substantial role – and challenge – in both the efficiency and the ROI of an SSLNG plant. In addressing the overall efficiency requirement of the mixed refrigerant compressor, the design must consider several key requirements, such as power consumption, seal leakage rates, and performance degradation during the life of the compressor. Regarding ROI, plant CAPEX, which consists of equipment cost and installation cost (i.e., space, complexity installation time), and OPEX (i.e., power, maintenance interval, and cost) play a vital role. This case study – written jointly by representatives from the turbomachinery OEM and from the end user of an SSLNG plant in China, respectively – will examine how different compressor technologies can address these different dimensions of efficiency requirements. Evaluating different compressor technology solutions for such applications, the authors will examine in detail how a mixed refrigerant compressor can be tailored to meet the needs of a specific plant's processes. Furthermore, the authors describe how a 27 MW (36 200 hp) integrally geared mixed refrigerant compressor deployed in a Chinese SSLNG plant in a single mixed refrigerant cycle supports the achieving of different efficiency parameters. In the compressor design, for example, choosing open or closed impellers can translate into substantial efficiency gains. The case study will also examine how inlet guide vanes (IGVs) allow operators to make full use of agile plant deployment while increasing compressor efficiency by up to 9%. By guiding and regulating inlet flow, IGVs provide more accurate process control at a constant discharge pressure. Variable IGVs can also offer a good operating range and superior partial-load performance for the differing conditions encountered in small-scale LNGs. The integrally geared compressor solution deployed in the case study is designed to deliver 98% reliability and 84% compressor efficiency. In addition, by incorporating seal support, lube oil, and the main compressor on the same skid (single-lift design) is up to 25% smaller than that of inline compressors. The integrally geared approach also allows for intercooling between stages, which improves both energy and cost efficiency by about 2%.