THE PROCESS OF PRODUCTION
OF MONOCHLOROBENZENE
1. Direct Chlorination
(Continuos Process)
The process begins with a
series of small, externally cooled cast iron or steel vessels containing the
catalyst (which may consist of Rashig ring of iron or iron wire). The catalyst
used is usually Ferric chloride. This can be added as solution in benzene.
Chlorine is supplied into each vessel through suitably positioned inlets to
maintain a large benzene-to-chorine reaction at all points along the reaction
stream. The temperature is maintained about 20 to 40 for this reaction in order
to minimize the production of dichlorobezene which occur at higher temperature.
Besides, this range of temperature is the best temperature for production of large
amount of monochlorobenzene. This process will produce large amount of
monochlorobenzene and small amount of dichlorobenzene. The feed, which are
liquid benzene and gaseous chlorine are at temperature 25 and atmospheric
pressure then fed to the reactor which operates at 2.4 bars.
The reaction is exothermic
process. Cooling process is required to maintain the temperature at 40 90% of
the HCl formed is first cooled to condense impurities (benzene and chlorinated
product) and then it is scrubbed in a scrubber using refrigerated
chlorobenzene. The crude chlorobenzene stream leaving reactor is washed with
NaOH solution (20wt%) in order to maintained slightly alkaline to protect
downstream equipment from corrosion) in a pre-neutralizer. The product stream
is free from HCl. Then, the product is fed to a Benzene Recovery Column
(distillation column). Here, the bottom is almost slightly 100% pure
chlorobenzene. The top contain 98% by weight of benzene and 2%
chlorobenzene. All the benzene is recycled to the benzene storage via a
purifier. From purifier the monochlorobenzene is sent to the refrigeration
system. The bottom contains monochlorobenzene and dichlorobenzene. This bottom
product is fed to the chlorobenzene column that may be contain 12-25 trays
which operated at 3-7 lb/in2 abs. The temperature may be 100 -200 . The
distillate has purity of 99% monochlorobenzene while bottom has purity of 97%
dichlorobenzene.
This reaction will produce
HCI as the side product. All the desired product and undesired product are then
fed to the Benzene Recovery Column (distillation column). The advantages of
continuous process are, it produce higher amount of monochlorobenzene which is
95% conversion and the process also operate at lower temperature.
P&ID and Review of the Process Production
of Monochlorobenzene from Benzene and Chlorine
2. Batch Process
In the batch process, benzene is contained in a deep, iron or mild steel vessel lined with lead cooling coils. The catalyst that usually used for this process is FeCl3, is added in a benzene solution. Chlorine is fed to into bottom of the chlorinator through a lead covered at temperature 45 in order to minimize the formation of dichlorobenzene. Then the crude chlorobenzene stream and HCl stream are collected and treated in the purification and recovery process.
For
another type of batch process is describe by Faith, Keyes, and Clark’s
Industrial Chemicals. The chlorine is bubbled into a cast iron or steel tank
containing dry benzene with on percent of its own weight of iron filings. The
temperature is maintained at 40°C to 60°C (104°F to 140°F) until density
studies indicate that all benzene is chlorinated. Then, the temperature is
raised to between 55°C and 60°C (131°F to 140°F) for six hours until the
density raises to 1.280g/cm3 (79.91 lb/ft3). The same methods of chlorobenzene
purification and HCl recovery in batch form are then employed. At 100%
chlorination, the products are 80% of monochlorobenzene, 15 % of
p-dichlorobenzene, and 5% of o-dichlorobenzene.
3. Hooker/
Raschiq Process
This
process is conducted at elevated temperature which is in the range of 230 to
270 . This process involve the reaction between benzene and mixture of
hydrochloric acid gas and air in the presence of an oxychlorination catalyst.
This catalyst consists of copper and iron chlorides on an inert support.
Once-through conversion for this process is limited (10 – 15 percent ) to
prevent the excessive formation of polychlorobenzene. The catalyst is put in
the beds to prevent damage since this process is exothermic process. In order
to control the overall temperature, the benzene is injected at lower
temperature. This process is then followed by purification of monochlorobenzene
which can be done by fed the product from the reactor into the distillation
column which is known as brick-lined column.
The top
stream of this column contain water/benzene azeotrope while at the bottom are
1/1 mixture of benzene and chlorobenzenes. The top product which is benzene and
water is recycled back into the reactor while the bottom products which are
benzene and chlorobenzene is neutralized with caustic soda, washed with water
and distillate in two columns to separate the dichlorobenzene,
monochlorobenzene and benzene. Then the process is followed by hydrolysis of the
monochlorobenzene by steam in the presence of tricalcium phosphate or silica
gel base catalyst which can be reactivated periodically to reduce carbon
deposited. The formation of dichlorobenzene in the oxychlorination reaction and
the polyphenols in the hydrolysis process reduce the yield.
The
process contains a few disadvantages. The high temperature in the process
favours high combustion rates of benzene which cause the reaction
uncontrollable. Compare to the other process, this process produce high cost of
vapour phase chlorination process which make it become uneconomical process for
the production of monochlorobenzene. This process also can only produce small
amount of chlorobenzene since this once-through conversion is limited.
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PROCESS
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RASCHIQ PROCESS
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CONTINUOUS
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BATCH
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Raw Material
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· Benzene
· Hydrochloric acid
· Oxygen (air)
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· Benzene
· Chlorine
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· Benzene
· Chlorine
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Reaction Conditions
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Temperature at range 220 - 260 and in
gas-phase
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Temperature at range 20 - 40 and in
liquid -phase
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Temperature at range of 40 - 60 and
in liquid-phase
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Reactor
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Fixed-Bed Reactor
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Continuous Stirrer Tank Reactor
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Batch Reactor
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Catalyst
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Copper and iron chloride
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Ferric chloride
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Ferric Chloride
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Advantages
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· Large economic advantages because HCl produce in the
hydrolysis of chlorobenzene can be used for the oxychlorination of benzene.
· Economy in steam and cooling required for evaporating and
condensing the benzene.
· Less purification operations.
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· Lower operating labor
· Simple operation à liquid
phase
· High conversion of benzene (95%)
· High production of monochlorobenzene
· Produce less by products à only small amount of dichlorobenzene.
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· High production of monochlorobenzene compare raschiq
process.
· Low cost of factory equipment because of the simple design
of batch reactor.
· Reaction it easy to control due to low
temperature.
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Disadvantages
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· Produce many by-products dichlorobenzene,
trichlorobenzene, tetrachlorobenzene and others.
· The benzene conversion is limited,10-15%.
· The reaction is uncontrollable because of the high
temperature.
· High cost of vapour phase chlorination
process.
· Has large investment for corrosion-resistants àhydrochloric acid is highly corrosive
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· High cost of equipments
· Required special material of construction for very low
temperature
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· Lower conversion compare to continuous (80%).
· Produce higher amount of by-products à dichlorobenzene
· Only can produce small scale production.
· Require strict scheduling and control.
· Higher operating labor costs due to equipment cleaning and
preparation time.
· Many people need to operate the process.
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PROCESS SELECTION
Based
on the review and screening, the most suitable process for the production of
the monochlorobenzene is by continuous process. The process was selected
because it is more beneficial compare to batch process and Raschig process.
The selection is based on a few important criteria that need to be
considering in this process. One of the criteria is continuous process can
give higher conversion of monochlorobenzene which is 95% conversion. Besides,
the temperature used for this process is only between 20 - 40 . At this low
temperature, the operating cost can be reduced because it does not required
heating process. Furthermore it is easy to handle the reaction at low
temperature and this range of the temperature is the best temperature to
produce high amount of the monochlorobenzene. Furthermore, the continuous
process also produce high amount of monochlorobenzene and small amount of
dichlorobenzene compared to the other two processes that produce
dichlorobenzene, tri-chlorobenzene, penta-chlorobenzene and also
tetra-chlorobenzene. Another criteria is, for this process the benzene that
been used is in liquid phase which is cheaper compared if we used benzene in
vapor phase. Therefore, it indirectly can reduce the operating cost. Other
than that, the continuous process only need a bit of workforce. So, only a
few workers need to be hired and it indirectly also can reduce the labor
cost.
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