Process Selection

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.

PROCESS	RASCHIQ PROCESS	CONTINUOUS	BATCH
Raw Material	·       Benzene ·       Hydrochloric acid ·       Oxygen (air)	·       Benzene ·       Chlorine	·       Benzene  ·       Chlorine
Reaction Conditions	Temperature at range 220 - 260 and in gas-phase	Temperature at range 20 - 40 and in liquid -phase	Temperature at range of 40 - 60 and in liquid-phase
Reactor	Fixed-Bed Reactor	Continuous Stirrer Tank Reactor	Batch Reactor
Catalyst	Copper and iron chloride	Ferric chloride	Ferric Chloride
Advantages	·       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.	·       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.	·       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.
Disadvantages	·       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	·       High cost of equipments ·       Required special material of construction for very low temperature	·       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.
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.