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. N P T E L 1
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. N P T E L 2
Methane Methane and HCl (to separation) Caustic soda Hot water Chlorine Reactor Stripper HCl scrubber Neutralizer Absorber HCl Methylene dichloride Methyl chloride HCl solution Waste Sulfuric acid Reactor Recycle CH 2Cl 2 Chloroform CH 3Cl Waste Drying Carbon tetrachloride HCl Intermediate CHCl3 Reactor CCl 4 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 350 370 0 C 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 N P T E L 3
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. 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. The bottoms from the stripping 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 where methylene dichloride was distilled and recycled to reactor and desired quantities of chloroform was purified in the chloroforms. 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. Heavy ends were separated from the bottom of the. 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 450 0 C. 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 N P T E L 4
chloride of refrigerant grade. Approximately 85 90% of the condensate distills between -23 to -25 0 C 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 CHCl3 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 N P T E L 5
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 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 : -23.8 0 C : -97.4 0 C : -20.0 0 C : 625 0 C : 1.003gm/cm 3 (-23.8 0 C, liquid) 2.306gm/cm 3 (0 0 C, 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 : 39.6 0 C : -96.7 0 C : None : 556 0 C : 1.33gm/cm 3 (liquid) : Miscible with water USES METHYL CHLORIDE Methyl chloride was widely used as refrigerant N P T E L 6
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 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 N P T E L 7