Mixed-refrigerant cycle

The mixed refrigerant cycle is a variation of the cascade cycle and involves the circulation of a single multicoiTmonent refrigerant stream. The simplification of the compression and heat exchange services in such a cycle can offer potential for reduced capital expenditure over the conventional cascade cycle. 

FIG. 11-116 Propane precooled mixed-refrigerant cycle cooling curve for natural gas. 

Ward reported power consumption for a plant using the mixed-refrigerant cycle. Compressor power was 9,460 kw for an LNG production rate of 28,550 m /hr. This converts to 3.37 kw-hr/lb-mole of liquid. The minimum work for a feed-gas pressure of 37 atm, an assumed feed-gas temperature of 80°F, and an assumed heat-sink temperature of 80°F = 1.09 kw-hr/lb-mole of liquid. This gives an efficiency of 32.4%. 

Kaiser, V., O. Salhi, and C. Pocini, Analyze Mixed Refrigerant Cycles, Hydrocarbon Processing,. 57, p. 163, July (1978). 

Natural gas expander cycle 2 Refrigerant expander cycle 3 Cascade cycle 4 Mixed refrigerant cycle... 

The mixed refrigerant cycle (MRC) uses a single mixed refrigerant instead of multiple pure refrigerants as the cascade cycle. The mixed refrigerant consists normally of nitrogen, ethane, propane, butane and... 

Figure 15 Conventional three stage mixed refrigerant cycle.

Figure 16 Single mixed refrigerant cycle. (Prico) [19]...

A comparison of power requirements of different cycles can only be approximate because of the difficulty in defining equivalent operating conditions for the various cycles. Table 4 and Table 5 present values for different cycles, and for different versions of the mixed refrigerant cycle. 

Table 5 Comparison of efficiencies for mixed refrigerant cycles. [221...

FIGURE 8 Mixed refrigerant cycle for liquefying natural gas. 

Another cycle that has been used exclusively for large natural gas liquefaction plants is the mixed refrigerant cycle. Since this cycle resembles the classic cascade cycle in principle, it can best be understood by reference to a simplified flow sheet of that cycle presented in Fig. 6. 

In the past the liquefaction of natural gas used a classic cascade cycle. The process required 120,000 hp for liquefaction of over 150 million standard cubic feet (mmscf) per day. Provisions are made for some of these cycles to use seawater for cooling. Later, baseload LNG plants utilized mixed refrigerant cycles, such as Air Products and Chemicals, Inc. s propane precooled mixed refrigerant system. Baseload plant capacities range from about 70 mmscf/day to about 350 mmscf/day of LNG. Baseload plants move LNG from remote sites by ship to populated areas. For... 

The mixed refrigerant cycle was developed to meet the need for hq-uefying large quantities of natnral 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 natnral gas coohng efficiency, but at a cost of considerable increased process complexity. 

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