The compounds with lower boiling points rise faster and higher on the column while those with higher boiling points evaporate slower and do not reach the top of the tower (the vapours condense on the lower trays). The distillation process separates the constituents of the crude oil on the basis of their difference in their boiling point. Refinery gas, (methane, ethane, propane and butane) is removed as a composite gas mixture on the top of the column (Figure 1 and Table 1). The second fraction, naphtha/gasoline, boils in the range of 40-180oC. As the other fractions have higher boiling points, they collect on the lower trays.
Cracking converts heavy fractions from the primary distillation into more useful compounds. For example, when C11H24 is cracked, C9H20 and C2H4 are obtained (Hill, G.C and Holman, J.C, 400-401). C9H20 can be used as gasoline, but more importantly C2H4 can be used as a starting material for the manufacture of products such as plastics, alcohol, diols etc. The cracking process also provides more useful branched-chain alkanes.
Catalytic cracking is a process in which long chain alkanes are converted into hydrocarbons of lower molecular weights by the mediation of catalysts at fairly high temperatures. The catalysts normally consist of Al2O3-SiO2 mixtures (Brown, T.L, LeMay, H.E and Bursten, B.E., 947).