Ethanol production by direct hydration of ethylene:
Synthetic ethanol which is produced by the hydration of ethylene has a purity of 95-96%, this method is followed by all modern industries having a yield of about 95-97% of ethanol, this is a catalytic gaseous reaction where approximately about 5-6 kg of phosphoric acid is used. Commonly kieselguhr or diatomaceous earth is selected as support for phosphoric acid deposition on their surface.
Ethylene and water are pumped from the spheres and storage tanks respectively to the stream of the recycling pipeline which is fed to the effluent pre-heater shell side and then heated to 300oC in the furnace which uses the gas as fuel along with natural gas, the pressure about 6.8 MPa is maintained by the feed pumps which is controlled by the reactor outlet control valve, a reactor is packed bed and converts reactant to a product with a conversion of 4-25% and a side reaction will produce byproduct acetaldehyde which is sold or hydrogenated to produced ethanol.
CH2=CH2 + H2O →H3PO4→ C2H5OH at 300oC and 6.8MPa
The outlet vapour mixture of the reactor is separated into products and unreacted reactants in the high-pressure separator, before passing through the effluent preheater tube side which is used to heat the feed mixture as the reaction is exothermic one mole of ethylene and water will produce 45.6KJ of energy after complete conversion into ethanol. Unreacted reactants present in the separator top outlet are scrubbed with water to dissolve the ethanol carried over into the recycle stream. The vapours are recycled having 80-85% concentration of ethylene in such a way that the molar ratio of water to ethylene is 0.6:1 and is maintained. The bottom of the separator and scrubber is fed to Hydrogenator to convert acetaldehyde to ethanol on the nickel-packed catalyst by adding hydrogen to acetaldehyde.
CH3CHO + H2O→ C2H5OH
Reacted vapour is fed to the acetaldehyde reduction column to separate Unreacted acetaldehyde and recycle to the reactor are some parts can be sold as by-products based on market demand, the bottom stream is fed to the light and heavy (purifier) column to distil out ethanol and increase it concentration.
Other methods used for the production of ethanol are:
Ethanol production from molasses fermentation