Mixed-refrigerant

Fig. 3. Propane precooled mixed refrigerant process for natural gas Hquefaction.

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

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%. 

The total plant or train main process bottleneck will probably be identified by the licensor, such as the gasifier for a coal gasification train, the main exchanger for a mixed refrigerant LNG plant train, or the cracked gas compressors for an olefin plant. First and foremost, be sure that the licensor has not made the utility area a bottleneck. This can never be allowed since overloaded utilities could repeatedly shut the entire complex down on a crash basis, adversely impacting economics. 

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

The design of a mixed refrigerant system requires the degrees of freedom to be manipulated simultaneously. These are as follows10. 

Pressure level. The choice of pressure level for the mixed refrigerant evaporation affects the temperature difference between the process cooling curve and refrigerant evaporation curve. Increasing the overall temperature difference will increase the refrigeration power requirements. 

Rather than use the simple cycle shown in Figure 24.44 for the liquefaction of natural gas, much more complex arrangements using multiple cycles (with both pure and mixed refrigerants) and cascade systems can be used. 

Lee G-C, Smith R and Zhu XX (2003) Optimal Synthesis of Mixed-refrigerant Systems for Low-temperature Processes, Ind Eng Chem Res, 41 5016. 

Kinard GE and Gaumer LS (1973) Mixed Refrigerant Cascade Cycles for LNG, Chem Eng Prog, 69 56. 

Ross, H, R. Radermacher, M. DiMarzo, and D. Dirdion, 1987, Horizontal Flow Boiling of Pure and Mixed Refrigerants, Int. J. Heat Mass Transfer 30(5) 979. (3)... 

Fig. 4. Single-pressure, mixed refrigerant cascade system for producing LNG...

Tig. 5. Propane-mixed-refrigerant liquefaction system for producing LNG. L.P, = low pressure H.P. = high pressure MR = mixed refrigerant... 

C3/MR Propane pre-cooled mixed refrigerant DMR Dual mixed refrigerant SMR Single mixed refrigerant MFCP Mixed refluid cascade process... 

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.

---