20.2.5 Process technology (Figure 20.1)

Figure 20.1 Flow sheet of isoprene production

The process technology consists of three major blocks namely

• • Dimerization-separator-recycle network (DSR)

• Isomerization-separator-recycle network (ISR)

• Pyrolysis-separator-recycle network (PSR)

20.2.6 DSR network

• • Propylene feed is mixed along with unreacted propylene (recovered in a distillation column in the DSR) to enter a mixer.

• The mixer is fed with tripropyl aluminium catalyst (this catalyst is in liquid phase).

• The mixture is compressed to about 200 atms. At these conditions, the mixture is in the liquid phase.

• The liquid phase dimerization feed enters the dimerizatoin reactor.

• The dimerization reactor is a CSTR with dowtherm fluid circulated in the cooling jacket to remove the heat generated from the exothermic reaction.

• After the reaction, the liquid stream is subjected to flash operation. From the flash tower a vapour and liquid stream are produced. The liquid stream consists of the catalyst + solvent where as the vapour stream consists of the unreacted propylene, light ends and the product formed.

• The liquid stream from the flash tower is recycled back to the dimerization CSTR.

• The vapour stream enters a fractionation tower that produces light ends such as C₁-C₂ as the top product. The bottom product consists of propylene and dimerized product i.e., 2-methyl-1-pentene.

• This product is subjected to again fractionator in a separate tower to obtain 2-methyl-1-pentene as the bottom product and propylene as the top product. The top propylene product is then recycled back to mix with the fresh propylene feed stock and enter the mixer unit.

20.2.7 ISR network

• • 2-methyl-1-pentene enters an isomerisation reactor which consists of acid catalyst (on porous carrier).

• The isomerisation reactor is a packed bed reactor operated at desired operating temperature.

• After the reaction, the products are sent to a fractionators that separates unreacted 2-methyl-1-pentene from the isomerized product i.e., 2-methyl-2-pentene.

20.2.8 PSR network

• • 2-methyl-2-pentene enters a pyrolysis furnace.

• Steam is also mixed with the organic vapors so as to quickly facilitate cracking in the pyrolysis furnace.

• The furnace is a shell and tube arrangement with the reactants entering the tubes and the hot flue gases enter the shell.

• HBr catalyst also enters the tubes of the pyrolysis furnace.

• After reaction, the product gases (HBr+product+reactant) are taken out and are quenched with a solvent in a quench tower. The quenching then produces a gas liquid mixture which is sent to a phase separator unit. The liquid stream from the phase separator stream consists of HBr+solvent and this is sent for quenching.

• Fresh HBr can be generated from the HBr + solvent obtained from the phase separator.

• The gas fraction from the phase separator enters a C₁ tower that separates fuel gas and other light ends as a top product.

• The bottom product from this tower consists of polymeric compounds and isoprene is fed to a purification tower to fractionate isoprene from the polymeric compounds.