Refrigerating process

In addition to the distinction between open- and closed-cycle systems, refrigeration processes are also described as simple cycles, compound cycles. 

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

Even the best modern low-temperature air separation plant has an efficiency only a small fraction of the theoretical optimum, that is, about 15 to 20 percent. The principal sources of inefficiency are threefold (1) the nonideality of the refrigerating process, (2) the imperfection of the heat exchangers, and (3) losses of refrigeration through heat leak. 

Sidestream compressors are used in refrigeration processes where, for economy, the refrigerant is flashed off at different pressure levels. Ideally, separate compressors could be used to successively compress the gas back up to the condensing pressure level. The pressure ratio for each stage is low enough to enable this to be done with only one or two impellers in each section. Because of this, compressors can be made with all sections in one casing so that mixing of the streams takes place intern.ll I v. 

For any method of heat transfer to take place, a temperature difference is necessary between two faces of a solid body, or at the boundaries of a gas or vapor. Flear transfer will take place only from a high-temperature source to a lower-temperature sink and is an irreversible process unless acted upon by another agency, as is the case with the refrigeration process. 

Figure 12-88. Simplified multilevel refrigeration process an example of three different composition streams entering a single multiwheel case. Also see Figure 12-40C. (Used by permission Peters, K. L. Hydrocarbon Processing, V. 60, No. 5, p. 171, 1981. Gulf Publishing Company. All rights reserved.)...

Hot water or steam can also be used in absorption-type chillers to provide chilled water with the reduction in electrical power needed for the refrigeration process. 

The widest application of the refrigeration process is to provide cooling for air-conditioning. The majority of this is for personal comfort in hot climates or where heat is given off in enclosed spaces. There is an additional demand for industrial manufacturing processes where precise conditions of temperature, humidity and cleanliness are necessary. 

Much attention has been given in recent years to the power consumed in the refrigeration process and the development of more efficient compressors. A few points to consider are ... 

With Midgley pacing up and down behind him, Henne prepared their first CFC compound, dichlorodifluoromethane, later called R-12 for Refrigerant-12. The compound looked promising. Under pressure, it liquefied readily, an important property since the cyclical refrigeration process depended on the compound s continually condensing back into a liquid to... 

Nevertheless, the simplest way to produce low temperature is still the use of cryoliquids (e.g. nitrogen, helium). It must be considered that most low-temperature equipments existing in a laboratory are designed for the use with cryoliquids, and the change to the new technologies is definitely expensive. Also for this reason, we shall briefly describe the properties and the use of cryoliquids used in low-temperature experiments and in particular helium (liquid or gas as used in pulse tubes) which practically intervenes in all refrigeration processes below 10 K. 

In Table 2.1, some properties of cryoliquids are compared with the thermodynamic properties of water. The boiling temperature and the latent heat L are of particular importance for the refrigeration processes. 

Helium vapour pressure and latent heat of evaporation The latent heat of evaporation L and the vapour pressure />vap are fundamental parameters when using these two cryoliquids in the refrigeration process. Figure 2.5 shows L of 3He and 4He as a function of temperature. Note that L ( 20.9 J/g for 4He) is very small in comparison, for example, with that of hydrogen (445 J/g) or of nitrogen (200 J/g). Note also the minimum at 2.2 K in the graph for 4He, in correspondence with the superfluid transition. 

PetroFlux A refrigeration process for removing liquid hydrocarbons from natural gas. Developed by Costain Engineering. Twenty three plants had been operating by 1992. Hydrocarbon Process., 1996, 75(4), 132. 

Because of its large reactor volume, the auto-refrigerated process can operate at very low alkene space velocities of about 0.1 h-1 LHSV (WHSV ca. 0.03 h 1). This design helps in increasing the octane number of the product and lowering acid consumption. The reaction temperature is maintained at about 278 K to minimize side reactions. Spent acid is withdrawn as 90-92 wt% acid. The isobutane concentration in the hydrocarbon phase is kept between 50 and 70 vol%. 

Nonlinear programming Staged-Distillation column (12.1) < Liquid extraction column (12.2) Gas transmission network (13.4) Ammonia reactor (14.2) Alkylation reactor (14.3) CVD reactor (14.5) Refrigeration process (15.2) Extractive distillation (15.3) Operating margin (15.4) Reactor control (16.3)... 

Flow diagram of light hydrocarbon refrigeration process. 

The data in the two previous tables do permit some complex calculations of energy for changes of both state and temperature. Take a mole of water vapor at 100° C and cool it to ice at 0°. The energy released, which must be removed by the refrigeration process, comes from three distinct changes listed in Table 7-4. 

The gauge glass will normally be somewhat colder than the process vessel as a result of ambient-heat losses (an exception to this would be a refrigerated process). For every 100°F decrease in the gauge-glass temperature or level-trol temperature, the specific gravity of the liquid in the glass increases by 5%. This rule of thumb is typical for hydrocarbons only. Aqueous (water-based) fluids are totally different. 

Figure 8.27. An ammonia absorption refrigeration process for a load of 50 tons at 30°F. The conditions were established by Hougen, Watson, and Ragatz (Thermodynamics, Wiley, New York, 1959, pp. 835-842).

---