Refrigeration plants fractionation

In a refrigeration plant the inlet gas is cooled to a low enough temperature to condense the desired fraction of LPG and NGL. Either freon or propane is used as the refrigerant. Figure 9-2 shows a typical refrigeration plant. 

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. 

Subsequently the gas is precooled in exchanger E-6 and charged to a low temperature fractionator. This tower has a reboiler and a top refluxing system. At the top the conditions are 280psig and —75°F. Freon refrigerant at — 90°F is used in the condenser. The bottoms is recycled to the pyrolysis coil. The uncondensed vapor leaving the reflux accumulator constitutes the product of this plant. It is used to precool the feed to the fractionator in E-6 and then leaves this part of the plant for further purification. 

The case study applies to the refrigeration section of a Liquefied Petroleum Gas Treatment and Storage plant, consisting in six parallel trains of LPG separation and transformation. Inside the complex, the stored LPG is firstly sent to the Dehydration System, where the residual contained water is condensed and removed, and then to the Separation System, where Propane, Butane, and Pentane are separated and sent to the storage units. A part of the separated products is stored at ambient temperature and pressure, while a fraction of commercial propane and butane is refrigerated to be stored at low temperature ready to be loaded on ships. 

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