LNG technology selectionDr Tariq Shukri, Foster Wheeler, UK, discusses availableLNG technologies and the important criteria for selection.Dual Mixed Refrigerant Process

This is a dual mixed refrigerant process, which is being appliedin the Sakhalin Island project with a capacity of4.8 million tpy per train.Process configuration is similar to the propane pre-cooledmixed refrigerant process, with the precooling conducted by amixed refrigerant (made up mainly of ethane and propane)rather than pure propane. Another main difference is that theprecooling is carried out in SWHEs rather than kettles. Theprecooling and liquefaction SWHEs will be supplied by Linde.The refrigerant compressors are driven by two Frame 7gas turbines. An axial compressor is also used as part of thecold refrigerant compression stages.

www.lngplants.com/

TPG-4140LNG, fundamentale prinsippBengt Olav Neeraas18/09/2007

Large Capacity LNG Plant Development

J.J.B. Pek*, A. van Driel, E.C.J.N. de Jong, R. Klein Nagelvoort

http://www.worldoil.com/Magazine/MAGAZINE_DETAIL.asp?ART_ID=2808&MONTH_YEAR=Feb-2006Global LNG Report Process selection is critical to onshore LNG economics Despite evolving and diversifying, no one liquefaction process is substantially better than others, mandating that project-specific variables and economics be studied. Saeid Mokhatab, University of Wyoming, Laramie, Wyoming, and Michael J. Economides, University of Houston, Houston February 2006DOUBLE MIXED REFRIGERANT PROCESS

Shell developed a Dual Mixed Refrigerant (DMR) process for The cooling for liquefaction of the natural gas is provided by a second liquefaction, Fig. 8, with two separate mixed refrigerant cooling cycles. mixed refrigerant cooling cycle (MR cycle). This cycle's refrigerant One is for pre-cooling gas to about - 50°C (PMR cycle), and the consists of a mixture of nitrogen, methane, ethane and propane. other is for final cooling and liquefaction (MR cycle). This concept Mixed refrigerant vapor from the shell side of the main cryogenic allows the designer to choose the load on each cycle. It also uses heat exchanger is compressed in an axial compressor, followed by proven equipment, e.g. spiral-wound heat exchangers (SWHEs), a two-stage centrifugal compressor. Inter-cooling and initial throughout the process. DMR is the basis of the Sakhalin LNG de-superheating is achieved by air cooling. Further de-superheating plant, with a capacity of 4.8 million t/annum, per train. 5 and partial condensation is achieved by the PMR pre-cooling cycle.

The mixed refrigerant vapor and liquid are separated, and further Process configuration is similar to the propane pre-cooled mixed cooled in the main cryogenic heat exchanger, except for a small refrigerant (PMR) process, with the pre-cooling conducted by a slipstream of vapor MR, which is routed to the end flash exchanger. 14 mixed refrigerant (made up mainly of ethane and propane) rather than pure propane. PMR vapor from the pre-cool exchangers is Shell has also developed technology to further push the propane routed via knock-out vessels to a two-stage centrifugal PMR cycle capacity, by employing double casing instead of single casing compressor. De-superheating, condensation and sub-cooling of the equipment. This reliable method brings the propane-MR process PMR is achieved by using induced-draft air coolers. closer to a capacity of 5 MTPA. Another possibility for the propane-MR

process is to transfer power from the propane cycle to the mixed The PMR compressor is driven by a single gas turbine, equipped refrigerant cycle. The closer coupling between the two cycles by with an electric starter motor/ generator. Another main difference mechanical interlinking of compressors is an operational challenge. is that the pre-cooling is carried out in SWHEs, rather than kettles. The pre-cooling and liquefaction SWHEs will be supplied by Linde. The refrigerant compressors are driven by two Frame 7 gas turbines, equipped with a separate variable speed starter/ helper motor. An axial compressor is also used as part of the cold refrigerant compression stages.

Single mixed refrigerant process has appeal for growing offshore marketTariq Shukri and Michael Barclay, LNG Consultants, Foster Wheeler Energy

The enhanced single mixed refrigerant process is suitable for medium-scale (0.5-1.5 million tonnes per annum) and offshore natural gas liquefaction.

http://www.worldoil.com/magazine/MAGAZINE_DETAIL.asp?ART_ID=3165&MONTH_YEAR=Apr-2007Breaking the offshore LNG stalemate Most technical obstacles to offshore liquefaction have already been overcome. Growing LNG demand may push industry and governments to conquer the remaining hurdles. Saeid Mokhatab , Contributing Editor, LNG, World Oil; and David Wood , David Wood & Associates, Lincoln, UK April 2007

Mixed refrigerant . The MR cycle employs a single mixed refrigerant Fig. 3. Onshore, an air-cooled dual mixed-refrigerant process offers 13 Johnsen, R. J. and P. Christiansen, “LNG production on floating composed of nitrogen and hydrocarbons in place of the cascade cost and time savings due to simplified plant requirements. platforms,” presented at the GasTech 98 Conference, Dubai, cycle's series of pure refrigerants. Various forms of MR have been Offshore, where frequent shutdown and start-up are expected, long Nov. 29–Dec 2, 1998; Naklie, M., “Mobil's floating LNG plant,” used in baseload plants and in smaller-scale plants. The machinery start-up times to precisely blend the refrigerant mix can become costly. presented at the 12th International Conference & Exhibition on configuration is simple, and the power requirement is usually only The DMR cycle minimizes hydrocarbon inventories compared with Liquefied Natural Gas, Perth, Australia, May 4–7,1998; Bliault A.,slightly higher than for a cascade cycle. The refrigerant composition either the single mixed-refrigerant cycle or the C3MR cycle most “Shell floating LNG plant, technology ready for project development,” is specified so that it evaporates over a temperature range similar commonly used onshore. It also gives lower flaring rates in the event Offshore , May 2001; Mayer and Sheffield, 2001; Price and Mortko, 1998.to that of the natural gas being liquefied. This provides close matching of compressor trip and refrigerant blowoff. However, all MR cycles 14 Bliault, 2001.of composite cooling and warming curves for the process gas and the require storage of flammable hydrocarbons. This presents the 15 Pekdemir, T., personal interview, Department of Mechanical and refrigerant, which results in a more efficient thermodynamic process significant challenge of maintaining an offshore safety case and Chemical Engineering, Herriot-Watt University, Edinburgh, UK, July 2001. requiring less power and smaller machinery. Even so, a typical MR requires significant deck space. Any cryogenic plant needs good system usually has a lower efficiency than a cascade cycle because distribution of two-phase streams into heat exchangers, particularly refrigerant flow is high, with associated high thermodynamic losses. MR cycles. Failure to do so can reduce production capacity. Vessel Furthermore, MR is more susceptible to changes in feed gas movement can make good distribution difficult, which may limit MR conditions, which may necessitate large design margins, applications to calm seas. 15 further reducing efficiency.

Mixed refrigerant cycle plants are sensitive to changes in feed gas Mixed-refrigerant technology has been assessed for offshore conditions, as they rely on small temperature differences between liquefaction based on both single mixed-refrigerant and dual the composite cooling and warming streams to give reasonable mixed-refrigerant (DMR) cycles. 13 The latter consumes less power process efficiencies. Onshore, long plant run times are normal and but is more complex. Shell concluded that single mixed refrigerant operators have time to optimize plant performance. Offshore, the is suitable for smaller LNG production capacities of about 2 million opposite may be true. Refrigerant compositions need to be adapted mtpy, while the DMR cycle is suitable for capacities up to 5 million for different feed-gas conditions, which can make performance mtpy. 14 A DMR process forms part of Shell's onshore Sakhalin optimization difficult. Dual mixed-refrigerant technology gives improved Island liquefaction project with a capacity of 4.8 million t/yr per train. operability over single mixed-refrigerant cycles but is expensive for Process configuration is similar to the propane pre-cooled stage all but the largest facilities. Mixed refrigerant plants inevitably take in the cascade process, with the pre-cooling conducted by a mixed longer to start up and stabilize than plants using other refrigerant refrigerant (mostly ethane and propane) rather than pure propane, Fig. 3. cycles because of the need to blend the refrigerant mix precisely.

This is a significant consideration in an environment where frequent startup and shutdown are to be expected.

COMMERCIAL AND TECHNICAL CONSIDERATIONS IN THE DEVELOPMENTS OF OFFSHORE LIQUEFACTION PLANT 23rd World Gas Conference, Amsterdam 2006.Chen-Hwa ChiuSenior Technology AdvisorChevron Energy Technology Company1500 Louisiana StreetHouston, TX 77002

LNG process selection, no easy taskPierre-Yves Martin, Jérome Pigourier, Axens, franceBéatrice Fischer, IFP, FranceHydrocarbon engineering May 2004

NATURAL GAS LIQUEFACTION PROCESSES COMPARISONCOMPARAISON ENTRE PROCEDES DE LIQUEFACTIONDE GAZ NATURELPierre-Yves MartinJérôme PigourierAxens (France)www.axens.frBéatrice FischerIFP (France)Poster PO-39PO-3 9.1

This process (see figure 4) is a dual mixed refrigerant process, with different power on the two cycles, and with two spiral-wound exchangers. Having mixed refrigerant on the first cycle allows to have a smaller condenser, and also to remove the propane compressor bottleneck: For propane compressors, the compressor size, thus the capacity of the unit is limited by the mach number at the tip of the blades. Using a mixed refrigerant, with a lower molecular weight, allows to push further this limit as the mach number is lower with this gas. (see figure 7)

REFERENCES CITED2. R. Nibbelke, S. Kauffman, B. Pek - Liquefaction Process Comparison of C3MR and DMR for Tropical Conditions - GPA 81st annual convention, 2002