22.3 Phenol from Toluene Oxidation

22.3.1 Reactions

• • Oxidation to Benzoic acid.

• Toluene + Oxygen → Benzoic acid + Water.

• Catalyst: Cobalt Naphthenate.

• Operating conditions: 150°C and 3 atms.

• Exothermic reaction.

Oxidation of Benzoic acid to phenol

• • Benzoic acid + Oxygen → Phenol + CO_2.

• Catalyst: Cupric Benzoate.

• Operating conditions: 220°C and 1.3 – 1.7 atms.

• This reaction is also exothermic.

22.3.2 Process technology (Figure 22.2)

Figure 22.2 Flow sheet of Phenol manufacture from Toluene Oxidation

• • Fresh and recycled toluene first enter the oxidation reactor.

• The oxidation reactor is a gas-liquid reactor in which air is sparged through a column of liquid (toluene).

• To reduce side reactions, toluene is used as a limiting reactant.

• The catalyst is fed to the reactor along with the toluene stream.

• After reaction, several products are formed including undesired impurities such as benzaldehyde, benzyl alcohol, benzyl benzoate, CO and CO_2.

• The reactor is facilitated with vent after cooling and partial condensation of the air mixed hydrocarbon vapour stream.

• The liquid product from the reactor then enters a distillation column which separates toluene from the other organics.

• The organics then enters a hot water wash unit where hot water extracts benzoic acid. All other impurities don't dissolve in water and leave the wash unit as heavy ends.

• The benzoic acid rich hot water stream is sent to a crystallizer where benzoic acid crystals are formed.

• The benzoic acid crystals are once again washed with water to obtain benzoic acid crystals.

• The benzoic acid crystals are then melted and fed to the second oxidation reactor.

• The second oxidation reactor is fed with air and cupric benzoate catalyst. Steam is also added to the reactor so as to enhance the temperature of the reactants quickly.

• Once again gas liquid reaction is facilitated. Cooling is facilitated by using a jacket or cooling tubes.

• From the reactor two phases are obtained namely vapour and liquid.

• The vapour and liquid both consist of phenol and therefore, both are subjected to series of separation steps to extract phenol and enrich the same.

• The vapour enters a fractionating tower which separates the unreacted benzoic acid from other compounds. The top product is obtained after partial condensation followed by phase separation. The phase separator produces three streams namely the vent stream (that consists of N₂, CO and CO₂), the aqueous water + phenol stream and the organic crude phenol stream. The crude phenol stream is partially sent as a reflux to the fractionators and withdrawn as a product as well.

• The bottom product from the second oxidation reactor consists of organic compounds and is sent to water wash. After water wash, heavy compounds such as tar are removed and phenol is dissolved in water. This water + phenol mixture is mixed with the water + phenol mixture obtained from the fractionators separating benzoic acid.

• The crude phenol is fed to a fractionator to obtain purified phenol as bottom product and phenol + water mixture as a top product. The top product here is an azeotrope.

• The phenol + water coming from three different sources is allowed to enter a distillation column that generates water waste as a top product and a crude phenol stream as a bottom product.