14.4 Chloromethanes

Chloromethanes namely methyl chloride (CH₃Cl), methylene chloride (CH₃Cl₂), Chloroform (CHCl₃) and Carbon Tetrachloride (CCl₄) are produced by direct chlorination of Cl₂ in a gas phase reaction without any catalyst.

14.4.1 Reactions

     CH₄ + Cl₂ → CH₃Cl + HCl

     CH₃Cl + Cl₂ → CH₂Cl₂ + HCl

     CH₂Cl₂ + Cl₂ → CHCl₃ + HCl

     CHCl₃ + H₂ → CCl₄ + HCl

• • The reactions are very exothermic.

• The feed molar ratio affects the product distribution.  When CH₄/Cl₂ is about 1.8, then more CH₃Cl is produced.  On the other hand, when CH₄ is chosen as a limiting reactant, more of CCl₄ is produced. Therefore, depending upon the product demand, the feed ratio is adjusted.

14.4.2 Process Technology

• • Methane and Cl₂ are mixed and sent to a furnace

• The furnace has a jacket or shell and tube system to accommodate feed pre-heating to desired furnace inlet temperature (about 280 – 300°C).

• To control temperature, N₂ is used as a diluent at times.

• Depending on the product distribution desired, the CH₄/Cl₂ ratio is chosen.

• The product gases enter an integrated heat exchanger that receives separated CH₄ (or a mixture of CH₄ + N₂) and gets cooled from the furnace exit temperature (about 400°C).

• Eventually, the mixture enters an absorber where water is used as an absorbent and water absorbs the HCl to produce 32 % HCl.

• The trace amounts of HCl in the vapour phase are removed in a neutralizer fed with NaOH

• The gas eventually is compressed and sent to a partial condenser followed with a phase separator.  The phase separator produces two streams namely a liquid stream consisting of the chlorides and the unreacted CH₄/N₂.

• The gaseous product enters a dryer to remove H₂O from the vapour stream using 98% H₂SO₄ as the absorbent for water from the vapour.

• The chloromethanes enter a distillation sequence.  The distillation sequence consists of columns that sequentially separate CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄.


Figure 14.2 Flowsheet of Chloromethane production