Absorption refrigeration

Fig. 2. Refrigerated absorption process for the production of LPG and natural gasoHne Hquids.

Absorption refrigeration is much less common than compression refrigeration. Absorption refrigeration powers by compressing the refrigerant fluid dissolved in a solvent using a pump. 

Figure 3.4 Refrigerated absorption plants - main flows...

Natural gas contains mostly methane but also small amounts of other chemicals that are valuable in themselves. In a refrigerated absorption process, the natural gas is cooled to — 40 F at 865 psia and sent to an absorber, which uses dodecane as the absorbing media, with a flow rate of 10,000 lb mol/h. Determine the fraction of each of the chemicals absorbed in a tower with 30 stages. 

Figure 10.7 Refrigerated absorption system for removing krypton and xenon from reprocessing off-gases.

Several studies have been devoted to determine the COP and limitations of absorption system operating conditions [21-23]. In order to evaluate the refrigeration absorption system performance, relative to different previously presented configuration, we have developed a numerical program. The calculating procedures of the fluid thermodynamic properties and the performance coefficient were obtained using MAPLE computer tools. 

Condensable hydrocarbon components are usually removed from gas to avoid liquid drop out in pipelines, or to recover valuable natural gas liquids where there is no facility for gas export. Cooling to ambient conditions can be achieved by air or water heat exchange, or to sub zero temperatures by gas expansion or refrigeration. Many other processes such as compression and absorption also work more efficiently at low temperatures. 

Natural gas Hquids are recovered from natural gas using condensation processes, absorption (qv) processes employing hydrocarbon Hquids similar to gasoline or kerosene as the absorber oil, or soHd-bed adsorption (qv) processes using adsorbants such as siHca, molecular sieves, or activated charcoal. Eor condensation processes, cooling can be provided by refrigeration units which frequently use vapor-compression cycles with propane as the refrigerant or by... 

Absorption Systems. Absorption refrigeration cycles employ a secondary fluid, the absorbent, to absorb the primary fluid, refrigerant vapor, which has been vaporized in the evaporator. The two materials that serve as the refrigerant—absorbent pair must meet a number of requirements however, only two have found extensive commercial use ammonia—water and water—Hthium bromide. 

The high solubility of the salt and resultant low water vapor pressure (58) of its aqueous solutions ate usehil ia absorption air conditioning (qv) systems. Lithium bromide absorption air conditioning technology efficiencies can surpass that of reciprocal technology usiag fluorochlorocarbon refrigerants. 

Fig. 2. Dual-pressure process using extended absorption for NO abatement. RC = refrigerated cooling see Figure 1 for other definitions.

Physica.1 Absorption. Whereas chemical absorption rehes on solvent reactions to hold acid gas components in solution, physical absorption exploits gas—hquid solubiUties. The amount of absorption for these solvents is direcdy proportional to the partial pressure of the acid gas components. Thus these processes are most appHcable in situations involving high pressure feed streams containing significant concentrations of acid gas components. To favor absorption, lower temperatures are often employed. Some processes require refrigeration. 

The processes using physical absorption require a solvent circulation proportional to the quantity of process gas, inversely proportional to the pressure, and nearly independent of the carbon dioxide concentration. Therefore, high pressures could favor the use of these processes. The Recitsol process requires a refrigeration system and more equipment than the other processes. This process is primarily used in coal gasification for simultaneous removal of H2S, COS, and CO2. 

Tanks with contents at lower than atmospheric temperature may require insulation to minimize heat absorption. The insulation must have a vapor barrier at the outside to prevent condensation of atmospheric moisture from reducing its effectiveness. An insulation not damaged by moisture is preferable. The insulation techniques presently used for refrigerated systems can be apphed (see subsection Fow-Temperature and Cryogenic Storage ). 

There are many means to obtain refrigerating effect, but here three will be discussed mechanical vapor refrigeratiou cycles, absorption and steam jet cycles due to their significance for industiy. 

Absorption Refrigeration Systems Two main absorption systems are used in industrial application lithium bromide-water and ammonia-water. Lithium bromide-water systems are hmited to evaporation temperatures above freezing because water is used as the refrigerant, while the refrigerant in an ammonia-water system is ammonia and consequently it can be applied for the lower-temperature requirements. 

Coefficient of performance of the absorption cycle is defined on the same principle as for the mechanical refrigeration ... 

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