Phenol from Benzene | Chemical Technology - Chemical Engineering PDF Download

23.1 Introduction

Phenol can be manufactured from Benzene using several ways

• Benzene hydrochlorination to form Benzyl chloride followed by hydrolysis of benzyl chloride to form phenol.
• Benzene chlorination to form benzyl chloride which is transformed to sodium benzoate and eventually to phenol using NaOH and HCl
• Benzene sulfonate process: In this process, benzene is convered to benzene sulfonate using sulphuric acid and eventually through neutralization, fusion and acidification, the benzene sulfonate is gradually transformed to phenol.
• In this lecture, we restrict our discussion to the manufacture of phenol from
  • Benzene hydrochlorination route
  • Benzene from chlorobenzene route

23.2 Phenol using Hydro chlorination route

23.2.1 Reactions

First reaction

• Benzene + HCl + Oxygen→ Benzyl chloride + Water
• Catalyst: FeCl₃ + CuCl₂
• Operating conditions: 240^oC and atmospheric pressure 

Second reaction

• Benzyl chloride + water→ Phenol  + HCl
• Catalyst: SiO₂
• Here, HCl is regenerated and will be recycled.
• Operating conditions: 350^oC and atmospheric pressure

23.2.2 Process Technology (Figure 23.1) 

Figure 23.1 Flow sheet of manufacture of phenol using hydro chlorination route

• In this process, Benzene is used to extract phenol from phenol +water mixture.  This unit is termed as an extraction unit (liquid liquid extraction principle).  Therefore, this unit takes up fresh benzene and phenol + water mixture and produces two streams namely water stream (bottom product) and benzene + phenol stream (top product).  The water stream is fed to a scrubber unit (i.e., Unit B that will be described later).
• Then onwards, the organic mixture is fed to a distillation column that produces purer benzene as the top product. The bottom product is phenol with other impurities.
• The bottom phenol rich product is sent to the phenol fractionator to obtain waste product as top product and pure phenol as bottom product.
• The purer benzene then enters the hydrochlorination reactor in which a mixture of HCl and O₂ is fed at 220 ^oC.  Under these conditions, Benzene will be also in vapour state.
• Therefore, the reactor is a gas solid reactor.
• The conversions are pretty low and not more than 20 % of the benzene is converted to benzyl chloride.
• Eventually, the products are sent to two fractionators that separate unreacted benzene, crude benzyl chloride and poly benzyl chlorides as various products.  The unreacted benzene is sent back to the hydrochlorination reactor as a recycle stream.
• The crude benzyl chloride then enters an absorber unit A where phenol is used to purify the benzyl chloride from other organic compounds (such as benzene and polybenzyl chlorides).
• The purified benzyl chloride stream then enters the hydrolysis reactor in which water is passed along with benzyl chloride over the silica catalyst.  The reactor itself is a furnace with catalyst loaded in the tubes and hot fuel gases are circulated in the shell to obtain the desired higher temperature.
• Under these conditions, both reactants are in vapour state (with the benzyl chloride boiling point of 179^oC) and therefore, the reaction is also a gas solid reaction.
• After hydrolysis reaction, the product vapors are sent to a partial condenser that separates the HCl from the organic phase.
• The HCl is recycled to the hydrochlorination reactor.
• The phenol rich product stream is sent as a solvent for the scrubber (unit A) that purifies crude benzyl chloride to purer benzyl chloride. The bottom product from the scrubber (i.e., unit A) enters another scrubber (unit B) that receives water from the extractor.
• The unit B enables washing of the phenol to remove any water soluble impurities.  The water from the unit B enters the hydrolysis reactor.

23.2.3 Technical questions 

1. Discuss the merits of the process from waste minimization perspective?

In this flow sheet, the raw material itself is used as a solvent to extract the product. Also, one of the reactants (water) is used as another absorbent. The usage of raw materials and intermediates in the process as absorbents itself is very attractive from waste minimization perspective as waste water streams are not produced significantly.  Also, the solvent used is benzene itself which reduces the complexity of using another solvent and subsequent safety related issues.

2. Comment upon the corrosion issues of the processes?

HCl is very corrosive and therefore, enough precaution shall be taken towards the plant and process design.

3. What impurities are removed in the unit B scrubber using water?

Benzyl chloride has limited solubility with water.  Therefore, it is expected that benzyl chloride is dissolved to some extent in the unit B scrubber where benzyl chloride as an impurity in minor amounts can be removed from phenol.

4. Why do you think make up HCl is required?

Some HCl gets reacted to form poly benzyl chlorides. Some HCl gets lost as a vapour in various operations. Therefore, some make up HCl is definitely required in the process though, HCl is largely regenerated.

5. Compared to other Benzene based phenol production processes, what advantage this process has towards phenol production?

In this process, very little quantities of other raw materials are required.  These are HCl. Air is inexpensive and is freely available.  Therefore, the plant can be build easily as many other auxillary processes are not required provided benzene is available in large quantities in the vicinity. However, one basic drawback is that the fixed costs of units will be high in this case as HCl is involved.

23.3 Phenol from Chlorobenzene route

23.4 Reactions

• There are three reactions to convert benzene to phenol using chlorination route

23.4.1Chlorination 

• Benzene + Cl₂→monochloro benzene
• Operating temperature: 85 ^oC
• Catalyst: Fe or FeCl₃ catalyst

23.4.2Causticization

• Benzyl chloride + NaOH→sodium benzoate
• NaOH is in aqueous media
• Operating conditions: 425 ^oC and 350 atms
• Exothermic reaction

23.4.3 Hydrolysis 

• Sodium benzoate + HCl (aq) →Phenol + NaCl (aq)
• Operating conditions: Nothing specific

23.4.4 Process Technology (Figure 23.2)

Figure 23.2 Flow sheet of manufacture of phenol from chlorobenzene route

• Benzene is first dried. Dry benzene and FeCl₃ catalyst enters the chlorination reactor
• After the solid-gas-liquid reaction, the products are further heated up to enter a fractionator
• The fractionator separates benzene as a top product and monochloro benzene as the bottom product.  The top product also consists of HCl off gas that is obtained as the vapour stream from the partial condenser.
• The benzene is further cooled and sent back to the reactor and also as a reflux to the fractionator.
• The bottom product monochloro benzene is mixed with 10 % NaOH solution and diphenyl oxide to enter a high pressure pump followed with heat integrated heat exchanger that pre-heats the feed to higher temperature.
• The feed then enters the causticization reactor which has cooling water tubes to control the temperature.
• The product stream is cooled using heat integrated exchanger and then enters a neutralizer that is fed with the HCl obtained from the fractionator partial condenser.
• After neutralization, the product phenol is separated from the aqueous phase using gravity settling principle.
• The organic layer rich in phenol is sent to a vacuum column to separate the phenol from diphenyl oxide (bottom product). The bottom product is partially recycled to enter the cauticization reactor.

23.4.5 Technical questions 

1. Why is diphenyl oxide added to the causticization reactor?

Ans: To suppress the formation of more diphenyl oxide at the causticization reactor.

2. How can you regenerate Cl₂ for this process?

Ans: Brine when subjected to electrolysis will produce Cl₂ and NaOH. Therefore, electrolytic process will be beneficial to produce Cl₂ and us e it to the requirements as well as produce excess NaOH and sell it too. This way, the process becomes more commercially attractive.

3. Can the chlorinator and the fractionators be integrated into a single unit where the bottom section is a reactor and the top section is a fractionator?

Ans: Yes, this is possible, as after chlorination the products are in vapour state and they can enter the trays above the reactor section of a single column.  The heavier product in this case is the monochlorobenzene from fractionation perspective and this is the product as well.  Therefore, integrating both reactor and separator in a single unit can reduce the costs significantly.