Acetylene used as raw material for producing acetaldehyde in petrochemical industries. In theory, there are two methods for acetaldehyde production using acetylene .
- Using solid catalyst in vapour phase system for acetylene hydration.
- By using a mercury-ion (liquid) catalyst in liquid phase for hydration of acetylene.
Industrial process of producing acetaldehyde by hydration of acetylene using mercury-iron catalyst in liquid phase is much simpler in construction and handling the operation. A special designed hydrator converts acetylene to acetaldehyde by managing following chemical reactions.
C2H2 + H2O → CH3CHO + 151 KCal
The hydrator is operated at 1.5 to 2.5 atm pressure and 80-100 oC temperature. Acetylene is fed continuously through the liquid catalyst. The temperature is maintained by steam, it is injected at the bottom of the column. The hydrator is made of stainless steel or with ferrosilicon. The top stream of the hydrator is sent to a cooler. All the water vapour is condensed and recycled. The second cooler takes the outlet from the first one. Acetaldehyde is condensed along with trace of water. Unreacted acetylene and non-condensable vapour is feed to a water scrubber. The water scrubber is operated at temperature of 10 oC. Traces of acetaldehyde and water-soluble compounds are scrubbed down. Remaining gases are used as fuel or recycled to the dehydrator.
The liquid catalyst is a solution made of mercury (II) sulphate dispersed in sulphuric acid. As acetaldehyde is good reducing agent it reduces Hg(II) to Hg(I) and finally reducing Hg(I) to Hg.
- 2Hg2SO4 + H2O + CH3CHO → Hg2SO4 + H2SO4 + CH3COOH
- Hg2SO4 + H2O + CH3CHO → 2Hg + H2SO4 + CH3COOH
Fluidized bed type equipment is used as hydrator. Even zinc oxide, magnesium oxide and iron oxide are used in place of mercury.
In general, calculations 680 kg of acetylene, 0.1 kg of mercury are consumed to produce one ton of acetaldehyde. The conversion of this process is about 50-60% per pass. Catalyst is regenerated through the process.
Process flow-sheet of acetaldehyde manufacturing: