Cascade liquefier

Fig, 3. Conventional or standard cascade system for producing liquefied natural... 

The mixed refrigerant cycle was developed to meet the need for liquefying large quantities of natural gas to minimize transportation costs of this fuel. This cycle resembles the classic cascade cycle in principle and may best be understood by referring to that cycle. In the latter, the natural gas stream after purification is cooled successively by vaporization of propane, ethylene, and methane. Each refrigerant may be vaporized at two or three pressure levels to increase the natural gas cooling efficiency, but at a cost of considerable increased process complexity. 

Liquefied Natural Gas (LNG) is a clean burning fossil fuel. It offers an energy density comparable to petrol and diesel fuels, and produces less pollution. LNG is about 0.2% of the volume of natural gas at standard temperature and pressure, thus making it more economical to transport over long distances. The refrigeration and liquefaction are the key sections of the LNG plant, which typically account for 30-40% of the capital cost of the overall plant (Shukri, 2004). There are several licensed processes available for LNG, but most fall into either a cascade refrigeration or mixed refrigerant scheme. 

With low concentration of CO2 in the raw gas, the CO2 recovery rate at common liquefaction temperatures is small, as is apparent from Fig. 6.6. An improvement in the recovery rate can, for example, be achieved by means of lower hquefaction temperatures. In order to avoid the required installation ofa cascade refrigeration unit with different refrigerants and additional compressors and equipment, liquefied CO2 can be used as refrigerant for the cold generation in a so-called open refrigeration cycle at a sufficiently low temperature level. 

A large part of the CO2 in the residual gases of the stripping column and the liquefier can be liquefied against evaporating CO2, at —50 °C, for example. The liquefied CO2 is separated and evaporated at about 5.5 bara. The evaporation enthalpy is exactly adequate to compensate the required liquefaction enthalpy. The evaporated CO2 is mixed with the inlet flow of the second stage of the COj compressor. Thus the open CO2 cycle with only small amounts of additional equipment (see process flows depicted in bold print in Fig. 6.9) represents an economical alternative to a cascade refrigeration unit. 

A comparison of the cascade and expander cycle units when liquefying stripped natural gas received at 700 psig reveals the following ... 

Use of the cascade cycle makes for a complex plant. However, the utilization of a series of refrigerants in the manner described improves the reversibility of the process, and makes it thermodynamically more efficient. To liquefy 1,000,000 scf/day would take about 460 bhp in a well-designed cascade unit. About 90 of the sweet gas charged to the unit is liquefied, with 10 going to fuel for steam boilers, compressors, electric generators, etc. 

Make a second law analysis of the two-fluid cascade cycle refrigerator liquefier discussed in Problem 4.43. 

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