CHAPTER THREE

PROPOSAL

3.1 PROCESS DESCRIPTION OVERVIEW

The production of chlorobenzenes from benzene and chlorine involves a multi-step process.

Here's an overview of the process:

CHLORINATION: Benzene is first mixed with chlorine gas in the presence of a catalyst,

typically iron or aluminum chloride. This leads to the formation of various chlorobenzenes,

including monochlorobenzene, dichlorobenzenes, trichlorobenzenes, and tetrachlorobenzenes

depending on the desired product.

SEPARATION: The mixture of chlorobenzenes is then separated by fractional distillation. This

separates the different chlorobenzenes based on their boiling points.

PURIFICATION: The separated chlorobenzenes are then further purified to remove any

remaining impurities, such as unreacted benzene or chlorine. This is typically done through a

series of chemical treatments and additional distillation steps.

BYPRODUCT RECOVERY: The byproducts of the reaction, such as hydrogen chloride gas

and unreacted benzene, are captured and reused in the process to minimize waste.

The resulting chlorobenzenes can be used as intermediates in the production of a variety of

chemicals, including pesticides, dyes, and pharmaceuticals.

3.2 DETAILED PROCESS DESCRIPTION

system consisting of two continuous stirred tanks operating in series at 2.4 bar. Gaseous chlorine

is fed in parallel to both tanks. Ferric chloride acts as a catalyst, and is produced in situ by the
action of hydrogen chloride on mild steel. Cooling is required to maintain the operating

temperature at 328 K. The hydrogen chloride gas leaving the reactors is first cooled to condense

most of the organic impurities. It then passes to an activated carbon adsorber where the final

traces of impurity are removed before it leaves the plant for use elsewhere.

The crude liquid chlorobenzenes stream leaving the second reactor is washed with water and

caustic soda solution to remove all dissolved hydrogen chloride. The product recovery system

consists of two distillation columns in series. In the first column (the "benzene column")

unreacted benzene is recovered as top product and recycled. In the second column (the

"chlorobenzene column") the mono- and dichlorobenzenes are separated. The recovered benzene

from the first column is mixed with the raw benzene feed and this combined stream is fed to a

distillation column (the "drying column") where water is removed as overhead. The benzene

stream from the bottom of the drying column is fed to the reaction system.

3.2.1 DRYING COLUMN

In the production of chlorobenzene from benzene and chlorine, a drying column is commonly

used to remove any moisture that may be present in the reactants or the catalysts. This is

important because water can react with chlorine to form hydrochloric acid, which can then react

with benzene to form undesired byproducts.

One study by Ying et al. (2019) investigated the use of a drying column in the production of

chlorobenzene using iron chloride as a catalyst. The researchers found that the drying column

was effective in removing moisture from the reactants, and that this led to higher yields of

chlorobenzene and lower amounts of unwanted byproducts.

Another study by Liu et al. (2017) also utilized a drying column in the production of

chlorobenzene using aluminum chloride as a catalyst. The researchers found that the drying
column was critical for obtaining high yields of chlorobenzene, as even trace amounts of

moisture could significantly reduce the reaction rate and selectivity.

The use of a drying column in the production of chlorobenzene from benzene and chlorine is an

important step to ensure the purity and efficiency of the reaction. By removing moisture from the

reactants and catalysts, the drying column can help prevent unwanted side reactions and improve

the yield and selectivity of the desired product.

3.2.2 BENZENE FEED PUMP

In the production of chlorobenzene from chlorine and benzene, a benzene feed pump is typically

used to transfer the benzene from a storage tank to the reactor vessel where the chlorination

reaction takes place. The benzene feed pump is a type of positive displacement pump that is

designed to handle liquid benzene, which is a volatile and flammable organic compound. The

pump typically consists of a motor, pump head, and various valves and fittings to control the

flow of benzene.

The pump head may be a piston or diaphragm type, and the motor may be electric or air-powered

depending on the specific requirements of the process. The pump is typically made of materials

that are compatible with benzene, such as stainless steel or special plastics.

The benzene feed pump is an important component of the chlorobenzene production process, as

it ensures a continuous supply of benzene to the reactor vessel, which is critical for maintaining

consistent reaction conditions and product quality. The pump must also be carefully designed and

maintained to prevent leaks or spills of benzene, which can pose a safety hazard to workers and

the environment.
3.2.3 REACTOR SYSTEM

Continuous stirred-tank reactors (CSTRs) are commonly used in the production of

chlorobenzene, typically in a two-stage process. Here's how two CSTRs are used in the

production of chlorobenzene:

1. Chlorination stage: In the first CSTR, benzene and chlorine gas are continuously fed into

the reactor, along with a catalyst such as aluminum chloride or ferric chloride. The

reactor is operated under carefully controlled temperature, pressure, and residence time

conditions to ensure efficient conversion of benzene to chlorobenzene. The reaction is

exothermic, and the heat generated is removed by cooling the reactor jacket or by

external heat exchangers. The effluent from the chlorination reactor is a mixture of

mono-, di-, and trichlorobenzenes, along with unreacted benzene and byproducts such as

hydrogen chloride. The reaction takes place according to the following chemical

equation:

C6H6 + Cl2 --> C6H5Cl + HCl

2. Hydrochlorination stage: The effluent from the first CSTR is then fed into the second

CSTR, where it is mixed with hydrogen chloride gas at a constant flow rate. The reaction

takes place according to the following chemical equation:

C6H5Cl + HCl --> C6H5Cl2 + H2

The reaction is catalyzed by ferric chloride, which promotes the hydrochlorination reaction and

minimizes the formation of unwanted byproducts. The reaction is also exothermic, so the reactor

is cooled to maintain a temperature of around 100-120°C. The residence time is carefully

controlled to achieve maximum conversion of the chlorinated byproducts and unreacted benzene
to chlorobenzene. The effluent from the second CSTR contains chlorobenzene, unreacted

benzene, and byproducts such as hydrogen gas and water.

3. Distillation and purification: The effluent from the second CSTR is then distilled to

purify the chlorobenzene product and separate it from any remaining impurities or

byproducts. The distillation process involves heating the effluent to a temperature of

around 130-140°C to vaporize the chlorobenzene, which is then condensed and collected

in a separate container. The remaining liquid, which contains unreacted benzene and

some impurities, is either recycled back to the first CSTR or sent for further processing

3.2.4 COOLER

After the chlorination reaction is completed, the HCL leaving the reactors is first cooled to

condense most of the organic impurities. It then passes to an activated carbon which acts as an

adsorbent to remove any remaining organic impurities. The activated charcoal has a large surface

area and a high degree of micro porosity, which enables it to adsorb organic impurities and other

contaminants. Specifically, the activated charcoal adsorbs the impurities and contaminants

through a process called van der Waals forces, which are weak attractive forces between the

adsorbent (activated charcoal) and the adsorbate (organic impurities). The impurities become

trapped in the micropores of the activated charcoal, effectively removing them from the

chlorobenzene.

3.2.5 WASHER

In the production of chlorobenzene, a washer is used to remove any residual hydrogen chloride

(HCl) that may be present in the product stream. The washer typically consists of a series of
water and caustic soda (sodium hydroxide) washes, which work together to neutralize the HCl

and remove it from the chlorobenzene. Here's how the process works:

1. Water wash: The chlorobenzene stream is first passed through a water wash, where it is

mixed with water and agitated to facilitate the transfer of HCl from the chlorobenzene

phase to the water phase. HCl is highly soluble in water, so the water wash is effective in

removing most of the dissolved HCl from the chlorobenzene.

2. Caustic soda wash: The chlorobenzene stream is then passed through a caustic soda wash,

where it is mixed with a solution of sodium hydroxide (NaOH). The NaOH reacts with

any remaining HCl in the chlorobenzene to form sodium chloride (NaCl) and water,

according to the following chemical equation:

HCl + NaOH → NaCl + H2O

The NaCl formed in the reaction is insoluble in chlorobenzene and forms a separate layer, which

is removed from the bottom of the washer. The remaining chlorobenzene is then passed through

additional water and caustic soda washes until the desired level of purity is achieved.

The water and caustic soda washes in the washer serve to neutralize any residual HCl in

the chlorobenzene and remove it from the product stream. The use of caustic soda also helps to

minimize the formation of unwanted byproducts, such as di- and tri-chlorobenzenes, which can

result from the reaction of HCl with the chlorobenzene. The resulting purified chlorobenzene can

then be sent for further processing or used in various industrial applications.

3.2.6 BENZENE COLUMN

Benzene column is an important part of the process of producing chlorobenzene, which is an

important industrial chemical used as a solvent, intermediate, and raw material in the

manufacture of a variety of chemicals.

various components in the mixture. The mixture is heated to a specific temperature, causing the

more volatile components to vaporize and rise up the column. The vapors are then condensed

and collected at different points along the column, resulting in the separation of the different

components.

In the production of chlorobenzene, benzene is first reacted with chlorine to produce

monochlorobenzene and hydrogen chloride. The reaction mixture is then fed into a benzene

column, where the unreacted benzene is separated from the monochlorobenzene and hydrogen

chloride.

The benzene column works by using distillation to separate the components of the reaction

mixture based on their boiling points. Benzene has a lower boiling point than

monochlorobenzene and hydrogen chloride, so it vaporizes first and is condensed and collected

at the top of the column. The monochlorobenzene and hydrogen chloride are then collected

separately at the bottom of the column.

The usefulness of the benzene column in chlorobenzene production lies in its ability to separate

the desired product (monochlorobenzene) from the unreacted starting material (benzene) and the

unwanted by-product (hydrogen chloride) in a highly efficient manner. This allows for a higher

yield of monochlorobenzene, which is the desired product, and also helps to minimize waste and

reduce production costs.

3.2.7 CHLOROBENZENE COLUMN

The chlorobenzene column is a distillation column that is used to separate the chlorobenzene

product from the reaction mixture. The column operates based on the principle of fractional
distillation, which involves the separation of a mixture into its component parts based on their

boiling points.

In the case of chlorobenzene, the column operates under vacuum to reduce the boiling point of

the mixture and allow for the separation of the product. The column is typically made of stainless

steel and can be several meters in height.

The chlorobenzene column is typically packed with a material such as Raschig rings or

structured packing to provide a large surface area for the separation process. The reaction

mixture is fed into the column at the bottom and is heated, causing the components with lower

boiling points to vaporize and rise up the column.

As the vapors rise up the column, they come into contact with the packing material, which allows

for the separation of the chlorobenzene product from the other components in the mixture. The

product is then condensed and collected at the top of the column, while the unreacted starting

materials and other byproducts are removed as a liquid from the bottom of the column.

The efficiency of the separation process in the chlorobenzene column depends on a number of

factors, including the temperature and pressure in the column, the type and amount of packing

material used, and the composition of the reaction mixture. Optimization of these factors can

lead to higher yields of chlorobenzene and reduced waste.

3.3 PROCESS ROUTE SELECTION JUSTIFICATION

The process route selection for this production must be carefully justified to ensure the efficient use of

resources and the production of high-quality products.

The selection of the process route for the production of chlorobenzene involves several factors, including

the cost of raw materials, energy consumption, waste generation, environmental impact, and the quality of
the final product. The choice of process route has been based on a thorough evaluation of these factors,

and the selection was made with the objective of optimizing the overall efficiency of the production

process.

One of the most commonly used process routes for the production of chlorobenzene involves the direct

chlorination of benzene with chlorine. This process route offers several advantages, including high yield,

simple operation, and low energy consumption. However, it also produces significant amounts of waste

products, such as hydrogen chloride, and requires the use of expensive catalysts.

Alternatively, an indirect process route can be used, where benzene is first converted to nitrobenzene,

which is then reduced to chlorobenzene. This route offers advantages such as high yield, minimal waste

generation, and the ability to produce high-quality products.

However, it also requires the use of expensive reagents and generates significant amounts of

waste. Therefore, in the justification for process route selection, we have considered all relevant factors

and evaluated the overall efficiency of the production process. In the selection, we have also taken into

account the availability of resources and the environmental impact of the chosen process route.

The selection of the process route for the production of chlorobenzene from benzene and chlorine

has been carefully justified based on a comprehensive evaluation of all relevant factors. The chosen route

has optimized the efficiency of the production process, minimized waste generation, and ensured the

production of high-quality products. And finally, the production process is classified as sustainable and

contributes positively to the industry and the environment.

3.4 PROCESS FLOW SHEETING

a detailed diagram that outlines the steps and equipment involved in a chemical process. The

goal of process flow sheeting is to optimize the design and operation of a process, which can be
achieved by modeling process behavior, performing sensitivity analysis, optimizing the process,

and implementing process control (Dosta et al, 2020).

To begin the design and simulation of a chlorobenzene production plant using ASPEN PLUS, the

plant's flowsheet is first developed to determine the necessary components. The process involves

the chlorination of liquid benzene, which must contain less than 30 ppm by weight of water. The

reactor system consists of two continuous stirred tanks operating in series at 2.4 bar, and gaseous

chlorine is fed in parallel to both tanks. Ferric chloride acts as a catalyst, which is produced in

situ by the action of hydrogen chloride on mild steel. Cooling is required to maintain the

operating temperature at 328 K. The hydrogen chloride gas leaving the reactors is first cooled to

condense most of the organic impurities, and then it passes to an activated carbon adsorber where

the final traces of impurity are removed before leaving the plant for use elsewhere.

The crude liquid chlorobenzene stream leaving the second reactor is washed with water and

caustic soda solution to remove all dissolved hydrogen chloride. The product recovery system

comprises two distillation columns in series. In the first column, unreacted benzene is recovered

as the top product and recycled. In the second column, the mono- and dichlorobenzenes are

separated. The recovered benzene from the first column is mixed with the raw benzene feed, and

the combined stream is fed to a distillation column (the “drying column”) where water is

removed as overhead. The benzene stream from the bottom of the drying column is fed back into

the reaction system. Coulson et al. (2005) provided a detailed explanation of the process.
Fig 3.X: The process flowsheet of the chlorobenzene production