11.4 Reaction mechanism & Operating conditions

Reaction mechanism comprises of four basic steps

• •  Carbonium ion formation (Step 1): Here, olefin reacts with acid catalyst to yield carbonium ion.

• Additon reaction (Step 2): Carbonium ion reacts with olefin to generate intermediate carbonium ion.

• Regeneration (Step 3): The intermediate carbonium ion converts to the dimer and generates back the proton on the catalyst surface.

• Isomerization (Step 4): Straight chain proton substituted olefins convert to isomeric carbonium ions.

• Catalysts used: Acid catalysts (H₂SO₄ ) are used.

• Temperature: 150 – 220^oC are used. Too high temperatures give tar deposits.

• Pressure: 25 – 100 atms.

11.5 Olefin polymerization process technology (Figure 11.2)

Figure 11.2 Flow sheet of Olefin Polymerization Technology

 

• •  Caustic wash: C₃-C₄ olefin feed subjected to caustic wash to remove H₂S and other sulphur compounds (such as mercaptans). These tend to poison the catalyst.

•  Water scrubbing: Eventually water scrubbing is carried out to remove dissolved impurities and generate waste water.

•  Polymerization reactor: The reaction mixture is heated, compressed and fed to a polymerization reactor. The reactor design is a shell and tube type design where catalyst is placed in the tube for the reaction to take place and cooling water is circulated in the shell side to control the temperature increase due to the exothermic reaction.

•  Fractionation: Subsequently, the reactor product is fed to a depropanizer and debutanizer to produce propanes, butanes and polymer gasoline. The polymeric product is further stabilization using hydrogenation stabilizer which converts any freely available double bonds to single bonds. The end product is polymer gasoline.

•  The propane produced is partially recycled to the reactor and the other part taken out as a product.