Liquefaction systems for LNG

 Liquefaction systems for LNG

Several LNG plants operate on a modification of the cascade system called the mixed refrigerant cascade (MRC) system, as shown in Fig. 3.23. This system has also been designated as the auto-refrigerated cascade (ARC) system (Linnett and Smith 1970). The operation of this system is made possible by the fact that natural gas is made up of several compo­nents that condense at different temperature levels. These various components may be used to cool the feed stream in the MRC system without using separate cooling circuits for the refrigerants by carefully controlling the composition of the refrigeration cycle gas composition. This allows the use of a single compressor for the recirculating gases instead of a sep­arate compressor for each of the different streams, as in the case of the ordinary cascade system

The natural gas feed stream generally enters the MRC system at 3.9 MPa (565 psia) to 5.3 MPa (765 psia). If the natural gas feed stream pres­sure is sufficiently high, the booster compressor would not be needed. The refrigerant cycle gas mixture is compressed and partially condensed in the compressor aftercooler. The stream is passed to a phase separator from which the propane-rich liquid phase is expanded through a valve and mixed with the return gas stream to furnish cooling in the first three-fluid heat exchanger. The vapor from the phase separator is partially liquefied in the three-fluid heat exchanger and passed to a second phase separator, from which the ethane-rich liquid is expanded through a valve and passed into the second three-fluid heat exchanger.

The vapor from the second phase separator and the natural gas stream are partially condensed in the final three-fluid heat exchanger. At this point, the refrigeration cycle stream is primarily methane, so the stream is expanded through an expansion valve and recirculated to provide cool­ing for the feed stream in the three-fluid heat exchanger. The LNG is expanded down to the storage pressure in the liquid receiver.

The liquefaction of natural gas for such applications as peak-shaving involves several factors not encountered in other cryogenic liquefaction systems, such as air liquefaction plants. In the case of air liquefaction, there are only two components (nitrogen and oxygen) that are present in amounts larger than I percent, whereas natural gas involves three or more components in amounts larger than I percent, including methane, ethane, propane, and nitrogen. These components condense over a wide temperature range. This fact makes the MRC system well suited for LNG plants; however, the system is not applicable for liquefaction of pure gases or mixtures such as air.

References

1. Cryogenic Systems               -             Barron R. F