Refrigeration cascades

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

Sulfuric acid alkylation also is used. In addition to the type of acid catalyst used, the processes differ in the way of producing the emulsion, increasing the interfacial surface for the reaction. There also are important differences in the manner in which the heat of reaction is removed. Often, a refrigerated cascade reactor is used. In other designs, a portion of the reactor effluent is vaporized by pressure reduction to provide cooling for the reactor. 

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

S. Vaidyaraman and C. Maranas (2002) Synthesis of Mixed Refrigerant Cascade Cycles, Chemical Engineerng Communications, 189 (8)... 

Fig. 7.16 Cooling of a typical natural gas against a pure component refrigerant cascade.

Heat Pumps. Because of added capital and complexity, heat pumps are rarely economical, although they were formerly commonly used in ethylene/ethane and propylene/propane spHtters. Generally, the former spHtters are integrated into the refrigeration system the latter are driven by low level waste heat, cascading to cooling water. 

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. 

Figure 3-6. Scheme for liquifying methane using the cascade refrigeration system. 

The following information was used in olefin plant case studies to determine if the ethylene/propylene cascaded refrigeration systems had enough horsepower for various plant operations. The propylene was condensed against cooling water at 110°F and the ethylene was condensed against propylene at -20°F. For comparison, the horsepower requirements for each refrigerant alone are also shown. 

Example. With the example cascaded system at an evaporator temperature of -100°F, the horsepower requirement is 6.2hp/ton refrigeration. A ton of refrigeration is equal to 12,000 BTU/hr. 

Figure 11-56. Cascade type low-temperature refinery refrigeration cycle. (Used by permission Charls, J., Jr. Tech Paper RP-468, 1950. Dresser-Rand Company.

In mechanical refrigeration, a multicomponent refrigerant consisting of nitrogen, methane, ethane, and propane is used through a cascade cycle. When these liquids evaporate, the heat required is obtained from... 

The Mollier diagrams for compound and cascade systems (Figures 2.9 and 2.10) indicate the enthalpy change per kilogram of circulated refrigerant, hut it should he home in mind that the mass flows are different for the low and high stages. 

Smaller items are tested in self-contained cabinets with a chamber the size of a large domestic refrigerator. Two-stage and three-stage systems are used, with R.13 in cascade at the lower end and R.22 for the high stage(s). 

---