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, 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. The odour is however
detected at possibly toxic levels.
Due to its toxicity, it is no longer present in consumer products.
Chloromethane was first synthesized by the French chemists 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 with the
method is used today.
DICHLOROMETHANE
Dichloromethane (DCM) is an organic compound with the formula
CH2Cl2. It is also known as methylene dichloride. It is a colourless and volatile
liquid with a moderately sweet aroma. It is widely used as a solvent. DCM
was first reported in 1840 by the French chemist Henri Victor Regnault
(1810 - 1878), who isolated it from a mixture of chloromethane and chlorine
that had been exposed to sunlight. It is miscible with many organic solvents
but not miscible with water.
NPTEL
1
Module:5
Lecture:29 Methyl chloride and Dichloromethane
Dr. N. K. Patel
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
Block diagram of manufacturing process
Diagram with process equipment
Animation
The reaction between chlorine and methane in the presence of light
or a catalyst yields methyl chloride with the formation of methylene
dichloride, chloroform and carbon tetrachloride.
NPTEL
2
Module:5
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
Methylene
dichloride
HCl solution
Waste
Methyl
chloride
Sulfuric
acid
Reactor
Recycle
CH2Cl2
column
Chloroform
CH3Cl
column
Drying
column
Waste
Carbon
tetrachloride
Intermediate
column
CHCl3
column
HCl
CCl4
column
Reactor
Chlorine
Heavy
ends
Figure: Manufacture of Methyl chloride, methylene dichloride, chloroform, carbon
tetrachloride from methane by chlorination
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. 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 viz. 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 hydrogen chloride.
Unreacted methane and HCl are 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
NPTEL
3
Module:5
Lecture:29 Methyl chloride and Dichloromethane
Dr. N. K. Patel
were separated in water absorber while methane is recycled. The
chloromethane absorbent, enriched with the chlorinated products removed
from the reaction gases, wass 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 are separated by series of fractional column.
The bottoms from the stripping column were recycled to the absorber
while a considerable portion of the liquid is fed to a secondary reactor. In the
reactor more chlorine was added and by photochemically. Hydrogen
chloride vented from the reactor. The reaction products are 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 is again chlorinated and produce carbon tetrachloride which is then
purified using carbon tetrachloride column. Heavy ends are 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 are 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 pass 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 reacts 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
chloride of refrigerant grade. Approximately 85 – 90% of the condensate
distills between - 23 and -230C at atmospheric pressure to give a yield of
methyl chloride of about 80% based on the chlorine charged.
NPTEL
4
Module:5
Lecture:29 Methyl chloride and Dichloromethane
Dr. N. K. Patel
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 is 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.
 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
other similar catalyst. In liquid phase zinc or aluminium catalyst is employed.
The process is also used I plants when methanol and by-product HCl, not
necessarily from methane chlorination.
NPTEL
5
Module:5
Lecture:29 Methyl chloride and Dichloromethane
Dr. N. K. Patel
PROPERTIES
METHYL CHLORIDE









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










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






NPTEL
Methyl chloride was widely used as refrigerant
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
6
Module:5
Lecture:29 Methyl chloride and Dichloromethane
Dr. N. K. Patel
 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
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
NPTEL
7