Download Module: 5 Lecture: 29

Module:5
Lecture:29 Methyl chloride and Dichloromethane
Dr. N. K. Patel
Module: 5
Lecture: 29
METHYL CHLORIDE and Dichloromethane
INTRODUCTION
METHYL CHLORIDE
Methyl chloride (CH3Cl) which is also known as chloromethane, R-40 or
HCC 40, is a chemical compound of the group of organic compounds
called haloalkane. It was widely used as a refrigerant. It is a colourless and
extremely flammable gas with a mild sweet odour. However, the odour is
detected at possibly toxic levels.
Methyl chloride is a naturally occurring ozone-depleting gas. CH3Cl
measurements in air extracted from the ice core from South Pole, Antarctica
covers the time period from 160 BC to 1860 AD. CH3Cl levels were elevated
from 900 – 1300 AD by about 50 ppt relative to the previous 1000 years,
coincident with the warm Medieval Climate Anomaly. CH3Cl levels
decreased to a minimum during the Little Ice Age cooling (1650 – 1800 AD),
before rising again to the modern atmospheric level of 550 ppt. These
variations most likely reflect changes in tropical and subtropical conditions,
and raise the possibility that a warmer future climate may result in higher
tropospheric CH3Cl levels
Chloromethane was first synthesized by Jean-Baptiste Dumas and
Eugene Peligot in 1835. They prepared it by boiling a mixture of methanol,
sulfuric acid and sodium chloride. This method is similar which is used today.
The higher toxicity resctct its use in the consumer products.
DICHLOROMETHANE
Dichloromethane (DCM) is an organic compound with the formula
CH2Cl2. Dichloromethane is also known as methylene chloride, methane
dichloride, methylene bichloride, ethylene dichloride. It is a colourless and
volatile liquid with a moderately sweet aroma. It is miscible with many
organic solvents but not miscible with water. It is widely used as a solvent.
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Module:5
Lecture:29 Methyl chloride and Dichloromethane
Dr. N. K. Patel
DCM was first isolated from the mixture of chloromethane and chlorine
in presence of sunlight by the Henri Victor Regnault in 1840. He found it
underground in a cave at the southern tip of France. Impure DCM contains
some neurotoxins in it, so when it was discovered, many people were
overexposed to the chemical and suffered lifelong medical issues or even
death.
MANUFACTURE
1. From methane by chlorination (Methyl chloride and Dichloromethane)
Raw materials
Basis: 1000kg methyl chloride, dichloromethane, chloroform and
carbon tetrachloride by from methane by chlorination
Chlorine
1530kg
Methane
305kg
Reaction
Yield (chlorinated methane) - 99-100% (based on chlorine)
85-99% (based on methane)
Manufacture process
The reaction between chlorine and methane in the presence of light
or a catalyst yieldsmethyl chloride with the formation of methylene
dichloride, chloroform and carbon tetrachloride.
99% pure methane was mixed with chlorine in a mole ratio of 1.7:1. The
preheated reactants were fed to a reactor fitted with mercury arc lamps
which promotes the reaction.
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Lecture:29 Methyl chloride and Dichloromethane
Methane
Methane and HCl
(to separation)
Dr. N. K. Patel
Caustic soda
Neutralizer
Reactor
HCl
scrubber
Stripper
Hot
water
Chlorine
Absorber
HCl
HCl solution
Methylene
dichloride
Waste
Methyl
chloride
Sulfuric
acid
Reactor
Recycle
CH2Cl2
column
Chloroform
CH3Cl
column
Drying
column
Waste
Carbon
tetrachloride
HCl
CCl4
column
CHCl3
column
Intermediate
column
Reactor
Chlorine
Heavy
ends
Figure: Manufacture of Methyl chloride, methylene dichloride, chloroform, carbon
tetrachloride from methane by chlorination
Block diagram of manufacturing process
Diagram with process equipment
Animation
About 65% of the methane was reacted at 350 – 3700C and slightly
above atmospheric pressure with desired residence times. A typical range of
products which leaves the reactor is methyl chloride - 58.5%; methylene
dichloride - 29.3%; chloroform - 9.7%; and carbon tetrachloride - 2.3%. The
effluent gases from the reactor also contain unreacted methane and
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Lecture:29 Methyl chloride and Dichloromethane
Dr. N. K. Patel
hydrogen chloride.
Unreacted methane and HCl were separated from the
chloromethanes by scrubbing the reacted gases with a mixture of liquid
chloromethanes usually a refrigerated mixture of chloroform and carbon
tetrachloride. They were separated in water absorber while methane was
recycled. The chloromethane absorbent, enriched with the chlorinated
products removed from the reaction gases, was charged to a stripping
column. Methyl chloride and some methylene dichloride go over head and
then sent for purification where first hot water wash was given to remove
residual hydrogen chloride, an alkali wash for a neutralization and a strong
sulfuric acid wash to dry the solvent mixture. The methyl chloride and
methylene dichloride were separated by series of fractional column.
The bottoms from the stripping column were recycled to the absorber
while a considerable portion of the liquid was fed to a secondary reactor.In
the reactor more chlorine was added and by photochemically. Hydrogen
chloride vented from the reactor. The reaction products were run into
intermediate column where methylene dichloride was distilled and recycled
to reactor and desired quantities of chloroform was purified in the
chloroforms column. The remaining mass was passed to a third reactor
where it was again chlorinated and produce carbon tetrachloride which
was then purified using carbon tetrachloride column. Heavy ends were
separated from the bottom of the column.
2. Other method
Another methane chlorination process was directed specifically to
methyl chloride; it makes use of a high methane chlorine ratio (10:1 by
volume) and a catalyst such as partially reduced cupric chloride deposited
on pumice. The gases were mixed and passed through a chlorination
furnace where reaction takes place at about 4500C. The contact time varies
in the range of 1 to 20sec.
To remove the hydrogen chloride, the reaction gases were passed
through a water scrubber and then led through a series of water-cooled and
refrigerated condensers under pressure. The mixture of chlorinated
hydrocarbons condensed while unreacted methane was recycled to the
mixing chamber. As all the chlorine was reacted with the methane, so that
little or no chlorine remains in the reaction gases.
Crude methyl chloride was distilled under pressure to yield pure methyl
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Lecture:29 Methyl chloride and Dichloromethane
Dr. N. K. Patel
chloride of refrigerant grade. Approximately 85 – 90% of the condensate
distills between -23 to -250C at atmospheric pressure to give a yield of methyl
chloride of about 80% based on the chlorine charged.
Engineering aspects
 Reaction control
The highly exothermic reaction is a chain initiated and propagated
one involving Cl. radical and can be explosive if not controlled properly.
Considerations to control the reaction are:
 Use of a methane cycle where there is a considerable excess of
CH4 – leads to large amount of mono- and dichloromethane as
product
 Use a liquid as a co-solvent
 Use a nitrogen gas or itself product as diluent
 Chlorine has to be added stage wise along the reactor
 Product ratio control
The methane cycle is the easiest to design and operate but gives
largely CH3Cl. However, the predominate demand for CCl4. This requires a
CH4/Cl2 ratio of between 0.5 and 0.8 with N2 diluent and operation with
recycle, requiring one large furnace or a separate set of furnaces. Another
alternative is to strip out the light chlorinated product from the methane
cycle and add excess chlorine to produce CHCl 3 and CCl4. This also requires
additional equipment.
This reaction was carried out at high temperature in gaseous phase.
The reaction was initiated by heating the reactants. Onces reaction was
started, free radical process rapidly goes on completion with the generation
of substantial amount of heat. If this heat was not controlled then explosion
may occur. For this reason feed ratio of reactant was controlled.
 Utilization of HCl to produce more CH3Cl
In plant where methanol is available, it is available to utilize the
replacement reaction.
This can be carried out as a vapour phase reaction using alumina or
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Lecture:29 Methyl chloride and Dichloromethane
Dr. N. K. Patel
other similar catalyst. In liquid phase zinc or aluminium catalyst was
employed.
The process is also used I plants when methanol and by-product HCl,
not necessarily from methane chlorination.
PROPERTIES
METHYL CHLORIDE
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Molecular formula
Molecular weight
Appearance
Odour
Boiling point
Melting point
Flash point
Autoignition temperature
Density
 Solubility
: CH3Cl
: 50.49gm/mole
: Colourless gas
: Faint, ethereal odour
: -23.80C
: -97.40C
: -20.00C
: 6250C
: 1.003gm/cm3 (-23.80C, liquid)
2.306gm/cm3 (00C, gas)
: Soluble in water
DICHLOROMETHANE
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Molecular formula
Molecular weight
Appearance
Odour
Boiling point
Melting point
Flash point
Autoignition temperature
Density
Solubility
: CH2Cl2
: 84.93gm/mole
: Colourless liquid
: Sweet
: 39.60C
: -96.70C
: None
: 5560C
: 1.33gm/cm3 (liquid)
: Miscible with water
USES
METHYL CHLORIDE
 Methyl chloride was widely used as refrigerant
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Lecture:29 Methyl chloride and Dichloromethane
Dr. N. K. Patel
Used for producing lead-based gasoline additives (tetramethyllead)
As a chemical intermediate in the production of silicon polymers
As a solvent in petrol refining and butyl rubber manufacturing
As a chlorinating and methylating agent
As propellant and blowing agent in polystyrene foam production, as
an extractant for grease oils and resins
 As a local anesthetic and also as an intermediate in drug
manufacturing
 As a catalyst in low-temperature polymerization
 As fluid for thermostatic and thermometric equipment and also used
as herbicide
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DICHLOROMETHANE
 Dichloromethane is useful solvent for many chemical processes as it
having good volatility and dissolves a wide range of organic
compounds
 In food industry it has been used to decaffeinate coffee and tea as
well as to prepare extracts of hops and other flavourings
 As an aerosol spray propellant and as an blowing agent for
polyurethane foams
 In garment Industry for removal of heat-sealed garment
 Used to seal the casing of electric meters
 Used by model building hobbyist for joining plastic component
together
 In civil engineering, for testing of bituminous materials as a solvent to
separate the binder from the aggregate of an asphalt or macadam to
allow the testing of the materials
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