Small-scale LNG — what refrigeration technology is the best?
Currently, low natural gas prices are allowing multiple secondary players in the U.S. market to consider investments in small-scale LNG plants.
T Kohler & M Bruentrup, Linde Engineering
R D Key & T Edvardsson, Linde Process Plants
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As one of the leading technology providers and EPC contractors in this business, Linde is frequently questioned about what refrigeration technology is the best for LNG production. At first glance, there are numerous process alternatives on the market. However, when taking a closer look, the choice simplifies to either single mixed refrigerant (SMR) or nitrogen expander technology. These technologies dominate the small-scale plant capacity range between about 50,000 and 500,000 gallons of LNG per day.
Linde is one of the few players in this business that have experience with and also offer both technologies. This makes Linde ideally suited to provide an unbiased comparison. As usual, there is no simple response to a complex technical matter, so this article is meant to cover a broad range of aspects and guide towards what technology is most suited for what type of application. Other technologies may become relevant for LNG plants with capacities below and beyond the range indicated above, meaning that our observations and conclusions apply only to the mentioned capacity range.
1 Refrigeration Process Design
Two processes have been selected as representative for the two competing liquefaction technologies; both are based on brazed aluminum plate-fin heat exchangers (PFHE) as the main heat exchanger in the liquefaction unit:
• For the SMR, Linde’s proprietary single cycle, Multi stage Mixed refrigerant process LIMUM®
• For the Nitrogen Expander, BHP Billiton’s licensed dual nitrogen expander process, abbreviated N2DExp
Though an arbitrary choice, it is believed that the above processes are representative of the marketplace. For the SMR, the LIMUM process is similar to competing alternatives e.g. enhanced PRICO (Black & Veatch).
On the expander process side, the high specific power requirements limit single expander processes as a widely acceptable option. Other dual expander processes either have different detail process topology or use hydrocarbon components mixed with N2 as refrigerant or are combinations of MR and N2-Expander technology. Hence, the above selection is believed to represent the cornerstones of the modern LNG technology range.
The above process sketches (Figures 1 and 2) include the following differences in equipment count:
2 Refrigeration Process Performance
Selection of plant design parameters, such as ambient design temperature, feed gas pressure and composition, storage tank pressure, flash gas rate, etc. have a significant (+/- 20%) impact on the specific power requirement of an LNG plant. So, for a meaningful performance comparison, it is fundamental to use an equal set of design parameters or, since different processes have their optimum at different conditions, an equal range. For this reason a range of such design parameters has been studied rather than a single arbitrarily chosen point. Also, indication of absolute performance numbers has been avoided for being potentially misleading. Instead, relative differences are provided in the following.
Selection of machinery efficiencies also has quite an impact on such a process comparison. Some literature sets these efficiency values at 100%, pretending to thereby establish an equal basis of comparison. This will, however, lead to a false conclusion: Theoretically, the N2DExp would have up to 15% less power than SMR. To provide a comparison that matches reality, we have selected typical actual machinery efficiencies. We therefore accounted for N2 compressors typically showing better efficiencies (82.5%) than MR compressors (80%), whilst both processes make use of an integrally geared turbo-compressor as cycle compressor, providing optimum, state-of-the-art compression efficiency. For the expander turbines, 85% efficiency was selected.
2.1 Sensitivity Analysis
Figures 3 and 4 show how design ambient temperature impacts the process performance. While the left chart displays that power consumption of any refrigeration process increases with rising ambient temperature, the right chart shows how the N2DExp performs relative to the SMR .
On average the N2DExp cycle requires around 30% more power than the SMR cycle. This power consumption difference is reduced as the ambient temperature increases.
Figures 5 and 6 show how design feed gas pressure impacts on the process performance. The left chart demonstrates that power consumption of any refrigeration process is lower with higher feed gas pressure. The right chart shows how the N2DExp cycle performs relative to the SMR1.
On average the N2DExp cycle requires around 30% more power than the SMR cycle. This power consumption difference is reduced as the feed gas pressure increases.
Overall it can be concluded that the power disadvantage of the N2DExp cycle is lowest for a plant with low design feed gas pressure and high design ambient temperature: Nearly 25% power consumption difference can be reached in such a favourable case, whereas up to 35% power consumption difference may result for the other extreme.
Since refrigeration process efficiency is improved by obtaining a close match between the feed gas and refrigerant (Q/T) cooling curves, composition of the feed gas has an impact too. Analysis of this parameter has been performed and, in our conclusion, only has a moderate effect. The N2DExp cycle tends to perform slightly better on lean feed gases. The improvement may be up to 5% with reference to the difference stated earlier.
The background of this observation is that nitrogen works as a highly efficient refrigerant in cryogenic applications (e.g. the sub-cooling section of Air Product’s AP-X process) but shows poor efficiency at higher temperature levels of the liquefaction process.
For this reason, many N2-expander liquefiers include a precooling unit, thereby providing refrigeration duty at higher temperature levels of the liquefaction process. Fundamentally there are three options for precooling:
• Precooling of the feed gas
• Precooling of the refrigerant
• Precooling of both feed gas and refrigerant
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