Phenol can be manufactured through five different processes. The processes are mentioned below.
- Cumene Peroxidation process
- Ranching process
- Toluene two-stage oxidation process
- Sulphonation process
- Cumene Peroxidation process:
Benzene and purified propylene obtained from the petroleum industry are mixed in liquid or vapour phase in presence of phosphoric acid on kieselguhr. As a result, cumene or isopropyl benzene is formed. The cumene thus formed is made into the form of an emulsion with dilute aqueous sodium carbonate solution, using sodium stearate as an emulsifier. The emulsion is then oxidized in an oxidizer with air under atmospheric pressure for 3 – 4 hours in presence of a catalyst, such as copper, cobalt or manganese salt.
The temperature and pH of the reaction are maintained between 160-260°c and 8.5-10.5 respectively. As a result of oxidation, cumene hydroperoxide is formed. The peroxide thus formed is then decomposed by 5-50%sulphuric acid in an acidifier at 45-65°c under pressure. As a result of decomposition, phenol (15%), acetone (9%), and cumene (73%) are formed along with some α-methyl styrene and acetophenone. These are separated by a separator. The cumene is recycled to be used again and phenol is either extracted or recovered by distillation. The yield is about 92%. Acetone is formed as a byproduct (0.6 lb. per lb of phenol).
2. Rasching process:
This process was developed in Germany in 1940. Benzene is first converted into Chlorobenzene by passing a mixture of benzene vapour, hydrochloric acid vapour and air under normal pressure at about 23°c in presence of a copper iron catalyst, supported on alumina. The reaction is exothermic in nature and so the temperature is maintained constant by external cooling. The conversion .the per pass is 10%
The Chlorobenzene after separation from unchanged reactants is hydrolyzed into phenol by heating with steam at about 400-500°c in presence of a silica catalyst. The conversion is again about 10% per pass in this second step. Hydrogen chloride set free in the reaction is recovered and recycled. Crude phenol (97%) obtained according to the above reaction is purified by distillation under a vacuum. The yield is about 75-85% of benzene. A small amount of HCl is sufficient to convert large amounts of benzene into phenol.
3. Toluene two-stage oxidation process:
Toluene in the liquid phase is oxidized with air in a reactor under 40-70 p.s.i in presence of a soluble cobalt catalyst maintained at 150°c. benzoic acid and water are thus formed. The reaction is exothermic and the temperature is maintained by external cooling. The crude molten benzoic acid at about 150-200°c is transferred from the reactor to the distillation column, where separation of benzoic acid from unreacted toluene and produced water takes place. The toluene is separated and recycled to the first oxidizing reactor.
The pure benzoic acid is fed to a second reactor, where it is oxidized to phenol by air and steam under 20-25 p.s.I at 230°c in presence of cupric benzoate catalyst promoted with manganese. The reaction mass is periodically withdrawn from the second reactor into an extractor, where it is washed with water to remove unwanted tars and benzoic acid and steam are returned to the second reactor. The phenol, water and unreacted benzoic acid are conducted overhead to two distillation columns in series. In the first column, crude phenol is separated from overhead and unreacted benzoic acid is recycled to the second oxidizing reactor. Pure-form phenol is obtained at the second distillation column as an overhead product and supplies aromatics compounds and benzoic acid as a feed for the crude phenol rectification column. The yield of phenol on benzoic acid is about 75- 80%.
4. Sulphonation process:
It is one of the oldest methods of manufacturing of phenol. Benzene sulphonic acid is first prepared by passing vapour of benzene into concentrated sulphuric acid is about 150-170°c. the water formed during the sulphonation process is distilled out because sulfuric acid gets diluted and conditions accelerate the backward reaction to the process. Benzene sulphonic acid should be neutralized by reacting it with aqueous sodium sulphite to form the salt of benzene sulphonic acid. The sodium salt is filtered off and then fused with caustic in a cast iron vessel at about 340-380°c in the ratio (1:3) for about 5-6 hours. As a result, sodium phenate is formed. The melt is cooled, extracted with water and then acidified with sulphur dioxide. The latter is obtained as a result of the neutralization of benzene sulphonic acid with sodium sulphite. the upper oily layer of crude phenol is distilled under a vacuum to get pure phenol. The yield is about 80-90% of benzene. The lower layer contains sodium sulphite which is separated and used for the neutralization of benzene sulphonic acid.
| S.No | Existing Methods | Raw Materials/ Catalyst | Yield | Products | Comments |
| 1 | Cumene peroxidation | Cumene, Air, H2SO4 and Emulsifying agents | 92% | Phenol and acetone | Produces valuable co-product acetone |
| 2 | Toluene two-stage oxidation | Toluene, Air, Cobalt napthalate, Cupric benzoate catalyst | 80% | Phenol and CO2 | Low-cost operation by direct toluene application |
| 3 | Rasching phenol process | Benzene, Air, HCl | 75% | Phenol and HCl as recycling | Feasible under large units |
| 4 | Chlorobenzene caustic hydrolysis | Benzene, Chlorine, NaOH, HCl | 95% | Phenol, NaCl | Economically not feasible |
| 5 | Benzene sulfonate process | Benzene, H2SO4, NaOH | 87% | Phenol, Na2SO3, Na2SO4 | Operates on large batch cycles |
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