Benzoic acid is an important organic chemical that is used for the production of phenol, sodium

benzoate, benzoyl chloride, benzonitrile and other esters. The usual method of production is by

the liquid phase oxidation of toluene at a temperature between $150$oC and $180$oC and pressure

in the range $5$ to $12$ bar. The stoichiometric reaction is

$2$ C$7$H$8+3$ O$2->2$ C$7$H$6$O$2+2$ H$2$O

The project will consist of the design of a plant to produce $12000$ tonnes$/$year of benzoic acid

$($minimum purity $98\%$ w$/$w C$7$H$6$O$2)$ assuming an operating period of $8000$ hours on stream.

The process will similar to the present industrial practice except that the side reactions, which

produce some benzaldehyde and carbon dioxide will be ignored.

The following parts will be undertaken.

a$)$ A complete material and energy balance on a basis of $100$ kmol toluene entering reactor

b$)$ An outline design of the reactor

c$)$ Design and specifications for various heat exchangers, pumps, and other plant items

d$)$ An economic assessment of the plant.

e$)$ Process improvement ideas and evaluation.

At the end of the project every student will write a short reflection on the design work and their

group$$s process. $80\%$ of the module mark will be assessed from the designs of each of the plant

items to be handed in at the end of each day and $20\%$ from the reflection.Benzoic acid is an important organic chemical that is used for the production of phenol, sodium

benzoate, benzoyl chloride, benzonitrile and other esters. The usual method of production is by

the liquid phase oxidation of toluene at a temperature between $150$oC and $180$oC and pressure

in the range $5$ to $12$ bar. The stoichiometric reaction is

$2$ C$7$H$8+3$ O$2->2$ C$7$H$6$O$2+2$ H$2$O

The project will consist of the design of a plant to produce $12000$ tonnes$/$year of benzoic acid

$($minimum purity $98\%$ w$/$w C$7$H$6$O$2)$ assuming an operating period of $8000$ hours on stream.

The process will similar to the present industrial practice except that the side reactions, which

produce some benzaldehyde and carbon dioxide will be ignored.

The following parts will be undertaken.

a$)$ A complete material and energy balance on a basis of $100$ kmol toluene entering reactor

b$)$ An outline design of the reactor

c$)$ Design and specifications for various heat exchangers, pumps, and other plant items

d$)$ An economic assessment of the plant.

e$)$ Process improvement ideas and evaluation.Feed consisting of fresh and recycled liquid toluene is supplied to the reactor together with air

that has been compressed to a pressure of $8$ bar. The molar flowrate of air employed is

$1\mathrm{.}25$ kmol per kmol toluene and the reaction takes place at $8$ bar and the temperature is

controlled at $165\backslash$deg C by cooling using an external jacket and internal coil to remove the

exothermic heat of reaction.

The contents of the reactor are perfectly mixed and the gross toluene feed rate is adjusted until,

under steady conditions, $15\%$ of the toluene is converted to benzoic acid. Benzoic acid has a

low vapour pressure at the reaction temperature and stays entirely in the liquid phase. Some

toluene and water leave the reactor as vapour along with the nitrogen and unreacted oxygen.

The mol fraction of oxygen in that stream is $0\mathrm{.}0225$ and the partial pressures exerted by the

toluene and water are proportional to their mol fractions in the liquid that leaves the reactor.

The gas$/$vapour mixture from the reactor passes to a cooler condenser where most of the toluene

and water are condensed. The cooler condenser operates at the same temperature as the reactor

and nitrogen and oxygen and residual vapour are vented to the atmosphere at a temperature of

$45\backslash$deg C$.$ In the vented gas, the partial pressures of toluene and water are equal to their saturated

vapour pressures at $45\backslash$deg C$,$ which is also the temperature of the outlet condensate. The

condensate flows to a separator where the toluene and water separate completely into two liquid

phases. The water is discarded and the toluene is returned to the reactor.

The crude liquid product from the reactor passes to a distillation column operated at $1$

atmosphere pressure. The reduction in pressure causes one$-$third of the feed to be vaporised

and the feed enters the column at about $120\backslash$deg C$.$ The distillate contains toluene and water in the

same molar proportions as those in the feed together with a small amount of benzoic acid and

is at a temperature of $110\backslash$deg C$.99\%$ of the toluene is recovered in the distillate, which flows to

a separator to remove the water from the toluene rich stream. The benzoic acid in the distillate

stays dissolved in the toluene as phase separation takes place. The toluene$-$rich mixture is

recycled back to the reactor and the water layer is again discarded.The bottom product from the distillation column contains not less than $97\mathrm{.}5$ mol$\%$ benzoic acid

and is at a temperature of $250\backslash$deg C$.$ It is cooled to $12\backslash$deg C before being passed to a solidification

drum. This device is water$-$cooled and produces a continuous sheet of solid benzoic acid.