25.3 DDT

25.3.1 Reactions

•  DDT refers to Dichloro diphenyl trichloroethane.

•  DDT is produced by a reaction between monochlorobenzene and chloral in the presence of Oleum as catalyst.

• •  Monochlorobenzene + Chloral → DDT + H₂O

•  Therefore, to prepare DDT, first we need to prepare Chloral and purify it to the highest standard.

•  Chloral is prepared by first forming chloral hydrate using chlorination of benzene route.

•  The reaction for chloral is presented as

• •  4 Cl₂ + C₂H ₅OH + H₂O → Cl₂ CCH(OH)₂+ 5 HCl

•  Then chloral hydrate is allowed to distill in the presence of sulphuric acid which dessicates the chloral hydrate to chloral.

25.3.2 Process Technology

• •  In the process technology, we first discuss the process technology of chloral which is an important ingredient.

• Dry chlorine is first absorbed into ethanol at room temperature conditions. In this process, the alcohol turns to a syrupy fluid. The operation occurs in a chlorination tank which produces chloral hydrate and HCl.

• From the chlorination tank, HCl is separated by using partial condenser. The uncondensible HCl is sent for gas recovery using scrubbing. The liquid stream from the partial condenser is sent back to the chlorinator.

• The condensate from the chlorination tank is sent to a still, where distillation is facilitated in the presence of sulphuric acid which acts as a dessicant. Eventually, the still is operated at high temperature and for this purpose, steam is used for heating purposes. To control the operation, cooling water is also circulated in the jacket, as chloral hydrate conversion to chloral is a sensitive reaction.

• After the reaction, the batch still produces the waste acid as a product at the bottom and the clear chloral liquid at the top along with the vent gases.

• Eventually chloral is treated with lime to remove dissolved acidic impurities.

• The purified crude chloral is further sent to distillation in another still to remove chloral hydrates present in the stream. Therefore the second distillation unit is also operated in the presence of H₂SO₄ . Vapors released during this step (from the partial condenser) are sent to the vent scrubber.

• Eventually, chloral is produced from the distillation unit and chloral hydrate is recycled back to the chloral hydrate converter to chloral.

• To manufacture DDT, purified chloral, monochlorobenzene, oleum are allowed to enter the DDT condensation unit.

• After condensation, the organic layer and spent acid are withdrawn. The organic layer consists of DDT and monochlorobenzene. This is first neutralized with soda ash.

• After reaction, the organic layer is sent to a dryer where the vapors generated from dryer enter a total condenser followed with gravity settling separator. The gravity settling unit separates monochlorobenzene from other organic impurities. The monochlorobenzene is recycled back to the condensation unit that is meant for preparing the DDT.

• The Dryer produces DDT powder which is sent for casting/pelletization process to obtain the DDT in either flakes or in pellets for sale.

 With this flowsheet, we finish Module 2 in which process technology for good number of petrochemicals has been elaborated. We next move to the third module namely polymer technology.

25.3.3. Uses :

DDT is mainly used as a pesticide. DDT was the first of the modern insecticides. However, the use of this chemical has been restricted to some degree due to its harmful ill effects.