Direct Hydration Process of Ethylene to Produce Synthetic Ethanol

Ethanol Production

Ethanol production by direct hydration of ethylene:

Synthetic ethanol which is produced by hydration of ethylene has the purity of 95-96%, this method is followed by all modern industries having a yield 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. 

Process description:

Ethylene and water are pumped from the spheres and storage tanks respectively to the stream of the recycling pipe line which is fed to the effluent pre heater shell side and then it heated to 300oC in furnace which is uses off 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 out let control valve, reactor is packed bed and converts reactant to product with a conversion of 4-25% and a side reaction will produce byproduct acetaldehyde which is sold or hydrogenated to produced ethanol.

Main reaction:

CH₂=CH₂ + H₂O →H₃PO₄→ C₂H₅OH at 300^oC 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 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 carry over into the recycle stream. The vapours are recycled having 80-85% concentration of ethylene in such way so that the molar ratio of water to ethylene is 0.6:1 is maintained. The bottom of separator and scrubber is fed to Hydrogenator to convert acetaldehyde to ethanol on the nickel packed catalyst by adding hydrogen to acetaldehyde.

Side reaction:

CH₃CHO + H₂O→ C₂H₅OH

Reacted vapour is fed to acetaldehyde reduction column to separate Unreacted acetaldehyde and recycle to the reactor are some part can be sold as by-product based on market demand, the bottom stream is fed to 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