Why is fractional distillation of petroleum necessary? What type of gases are obtained in the upper part of the fractioning column ?
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Fractional Distillation Definition. Fractional distillation is a process by which components in a chemical mixture are separated into different parts (called fractions) according to their different boiling points. ... The chemical and petroleum industry rely on fractional distillation.The crude petroleum oil obtained from oil field, undergoes fractional distillation and a number of products are generated. These products of fractional distillation include, petrol (or gasoline), naptha, kerosene, diesel oil, lubricating oil, fuel oil, grease and wax, and some residue.The process consists of two, three, or four fractionating columns. The gas stream is first dehydrated and then cooled with refrigeration and/or pressure reduction.
9.8.1.2.1 Three-Column System
The three-column system is used for gas streams containing < 50% CO2. The first column operates at 450-650 psig (3100-4500 kPa) and separates a high-quality methane product in the overhead. Temperatures in the overhead are from 0 to − 140 °F (− 18 to − 95 °C). The second column operates at a slightly lower pressure and produces a CO2 stream overhead, which contains small amounts of H2S and methane. The bottom product contains H2S and the ethane plus components. The third column produces NGL liquids, which are recycled back to the first two columns. It is this recycle that allows the process to be successful. NGL liquids prevent CO2 solid formation in the first column and aid in the breaking of the ethane/CO2 azeotrope in the second column to permit high ethane recoveries.
9.8.1.2.2 Four-Column System
The four-column system is used where CO2 feed concentration exceeds 50%. The initial column in this scheme is a de-ethanizer. The overhead product, a CO2/methane binary, is sent to a bulk CO2 removal column and de-methanizer combination. CO2 is produced as a liquid and is pumped to injection or sales pressure.
9.8.1.2.3 Two-Column System
The two-column system is used when a methane product is not required and is thus produced with the CO2. Very high propane recoveries may be achieved; however, little ethane recovery is achieved. These processes require feed gas preparation in the form of compression and dehydration, which adds to their cost. Such systems are finding applications in enhanced oil recovery (EOR) projects.