19.3 Acrylonitrile

19.3.1 Reactions

• • C₃ H₆ + NH₃ + O₂ → C₃ H₃ N + H₂O

• The reaction is exothermic.

• Stoichiometric ratio: C₃ H₆ : NH₃: O₂= 1:1:1.5.

• Operating conditions: 1.5 – 3 atms pressure and 400 – 500°C.

• By products: Acetonitrile and Hydrogen cyanide from side reactions.

• Catalyst: Mo-Bi catalyst.

19.3.2 Process Technology (Figure 19.2)

Figure 19.2 Flow sheet of Acrylonitrile production

• • Propylene + Propane, Air and Ammonia, Steam are compressed to required pressure and are sent to the fluidized catalytic reactor consisting of the Mo-Bi spherical catalyst. The reactor is maintained at 400 – 500°C.

• Cyclone separator is also kept in the fluidized bed reactor in which catalyst and product gases are separated after fluidization. The contact time for fluidization is in the order of seconds.

• The product vapors then enter a water scrubber that does not absorb propane and nitrogen from the products. The products absorbed in the water include acrylonitrile, acetonitrile and other heavy ends.

• The very dilute acryolonitrile (about 3%) solution in water is sent to a fractionator. The fractionators separates acrylonitrile + heavy ends + HCN + light ends as a top product stream and acetonitrile + water + heavy ends as a bottom product.

• The top product then enters an extractive distillation column with water as extractant. The azeotropic distillation column vapour is partially condensed to obtain a vapour, aqueous and organic layer. The vapour consists of Light ends and HCN and is let out. The organic layer consists of acrylonitrile and heavy ends is sent for further purification. The aqueous layer is sent as a reflux to the azeotropic column. In other words, addition of water enabled the formation of a heterogenous azeotropic mixture at the top.

• The bottom product from the azeotropic distillation column enters a product purification unit along with oxalic acid where acrylonitrile is further purified from heavy ends (+ oxalic acid) and is obtained as a 99.5% pure product.

• In similarity to this, the bottom product from the product splitter enters an azeotropic column which produces water as a bottom product. The total condenser in this column generates both aqueous and organic layers. The organic layer is rich in acetonitrile and heavy ends where as the aqueous layer is sent back as a reflux to the azeotropic column.

• The bottom product from the acetonitrile azeotropic column enters a purification unit where distillation principle enables the separation of acetonitrile from the heavy ends.