Management Systems
2. Phase separation and purification: The reaction mixture is separated into an upper organic phase (the intermediate product 2-ethylhexenal) and a lower aqueous phase (containing the
3.6 Halogenated compounds
Annex 1 to the IPPC Directive does not elaborate on what products might be considered halogenated but Table 3.15 gives Europe’s most important halogenated organic products (in tonnage terms). The table also indicates what type of process description is provided in the BREF (if any).
Product Production capacity (kt per year) Process description?
1,2-Dichlorethane (EDC) 10817 Illustrative Process
Vinyl chloride (VCM) 6025 Illustrative Process
Methyl chloride 466
Perchloroethylene 434
Methylene chloride 321
Chloroform 318
Epichlorohydrin 290 √
Allyl chloride 244 √
Chloro benzene 233
Tetrachloromethane 205
Trichlorethylene 202
Ethyl chloride 107
Table 3.15: Halogenated organics with European production capacities in excess of 100 kt/yr
[UBA (Germany), 2000 #89] based on Standard Research Institute (SRI) data, Directory of Chemical Products Europe, Vol. II, 1996
Chlorinated products have most commercial importance, and there are few large volume brominated products. The most commercially important halogenation reaction is the production of ethylene dichloride/vinyl chloride (EDC/VCM) and this is considered in detail as an illustrative process in Chapter 12. Other commercially or environmentally important halogenation reactions are:
• the further chlorination of EDC to trichloroethylene and perchloroethylene
• hydrochlorination of methanol to methyl chloride (and further chlorination to methylene chloride)
• hydro-fluorination of chlorocarbons (e.g. chloroform) to hydrochlorofluorocarbons (HFCs).
Halogenation processes are typically large-scale plants where an organic feedstock is reacted with halogen or halide in a pressurised continuous reactor at elevated temperature in the presence of a catalyst. A range of halogenated organic products is formed, which are separated by condensation and distillation in a train of columns depending on the complexity of the mixture. Unwanted by-products are recycled to the process where possible. Unreacted halogen and halide are recovered and returned to the process or other productive use wherever practicable. Where it is necessary to vent a gas stream, the release of VOCs is abated by an appropriate technique (e.g. incineration, adsorption). Emergency vents are directed to a collection system with suitable abatement facilities. Residues such as heavy ends from distillations are incinerated and not released to land.
Although halogenation is characterised by a very wide variety of reaction options, a number of environmental issues are associated with virtually all options, namely [Environment Agency (E&W), 1999 #6]:
- the potential for release of organo-halogens to the air, water and land environments - the potential for formation of dioxins
- sophisticated storage and handling techniques may be required halide and halogen gases are formed and require abatement by water and/or caustic scrubbing.
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60 Production of Large Volume Organic Chemicals
The choice of halogenating agent is obviously dependent on the reaction chemistry, but a consideration of the strengths and weaknesses of the alternative agents gives an insight into some generic factors in their use (Table 3.16).
Halogenating
agent Advantage(s) Disadvantage(s)
Chlorine Many reactions will generate only gaseous by-product streams, which are easily removed from the off-gas.
On completion of the reaction, only small amounts of chlorine usually remain.
Sophisticated storage and handling facilities are normally required.
Bromine As for chlorine.
Can be contained in small, easily handled containers.
As for chlorine.
Iodine Readily available.
Does not require sophisticated storage facilities.
Charging of the solids to the reaction vessel may require special solids handling equipment.
Difficulties with certain waste disposal routes.
Thionyl chloride Sulphuryl chloride
Many reactions will generate gaseous by-product streams, which are most easily removed from the off-gas.
Readily available in containers up to 200 litres.
Does not require sophisticated storage facilities.
Reacts violently with water, alcohols, etc.
On completion of the reaction, considerable excess quantities can remain and these have to be removed by distillation.
Removing the reaction product may be difficult.
Phosphorus tri- and penta-chloride
Readily available in containers up to 150 litres and in bulk.
On completion of the reaction, may be removed by filtration.
Charging of solids to the reaction vessel may require special solids handling equipment.
Effluent containing phosphorus compounds may require specialist treatment.
Reacts violently with water and fumes on contact with moist air.
Aluminium
chloride Readily available.
Does not require sophisticated storage facilities.
Can be used to carry out chemical reactions to produce substances that would be difficult to make using other approaches
Evolves fumes readily in contact with moist air, generating hydrogen chloride and aluminium hydroxide.
Generates a considerable volume of aqueous effluent containing aluminium salts. This effluent is usually highly acidic.
Charging of solids to the reaction vessel may require special solids handling equipment.
Hydrogen
halides Readily available in anhydrous form or aqueous solution.
May generate no gaseous by-product.
Sophisticated storage and handling facilities are often required.
Table 3.16: Comparison of halogenating agents [Environment Agency (E&W), 1999 #6]
Halogenation processes will nearly always involve a reaction vessel (to combine an organic feed with the chosen halogenating agent) and a separation technique (to segregate waste from product). The main releases from reactors will be VOCs (potentially organo-chlorines), halides / halogens, and an aqueous solution of reaction medium (HCI or inorganic salts). Separation processes may create wash waters (from filtration) and VOCs (from evaporation) [Environment Agency (E&W), 1999 #6].
ALLYL CHLORIDE is produced by the chlorination of propylene. The substantial quantities of chlorinated by-products (HCl, dichloropropane, 1,3-dichloropropylene) are separated by distillation and incinerated. HCl is recovered from the incinerator for sale. Waste gases are scrubbed with an alkali liquor and this produces calcium and sodium hypochlorite [InfoMil, 2000 #83]. Some plants may incinerate waste gas and this avoids waste water generation.
CHLORO FLUORO HYDROCARBONS (CFCs) are used as cooling fluids and raw material for production of TFE (tetrafluoro ethylene)-monomer. CFCs are produced from chloroform and
Chapter 3
Production of Large Volume Organic Chemicals 61
hydrogen fluoride, with help of a catalyst. Hydrogen chloride is formed as a by-product and is purified for sale as a 30 % HCl solution in water [InfoMil, 2000 #83].
Environmental issues
Air: Waste gases are thermally incinerated. A 30 % solution of HF in water is recovered for sale.
Chlorine vapours are sent to chlorine destruction. Pollutants are VOCs, aromatic halogenated hydrocarbons, freons, and trichlormethane – mostly from fugitive sources.
Water: Air strippers remove organic compounds (e.g. chloroform) from waste water and pass vapour to incinerator. Main pollutants are inorganic chlorine and fluorine compounds.
Wastes: Used catalyst is regenerated externally.
Energy: Endothermic process.
EPICHLOROHYDRIN is produced by a two-step aqueous phase reaction. In the first stage of epichlorohydrin (chloropropylene oxide) production, allyl chloride and hypochlorite are reacted to produce dichlorohydrin and HCl. In a combined hydrolysis / rectification unit the dichlorohydrin is further reacted with dichloro isopropanol to form epichlorohydrin which is instantly separated from the aqueous solution. The by-products include trichloro propane, tetrachloro propylethers and chloroether [InfoMil, 2000 #83]. The waste water organic load can be reduced by extension of the product rectification column. Lime and other inorganic solids are separated by filtration. The organic load (TOC) mainly consists of glycerine that is easy to biodegrade. As an alternative to biological treatment, the treatment with hypochlorite is applied to remove COD and AOX (reduction 90 % and residual AOX of 3 mg/l). Emissions after treatment are about 3.5 kg COD/ t product, 150 g AOX/t product and 3 g EOX/t product.
Application of sodium hydroxide instead of calcium hydroxide in the aqueous process steps may reduce the release of heavy metals and the related toxicity of the waste water effluent.
WASTE WATER ISSUES HALOGENATED COMPOUNDS. A survey of German processes quantifies the volume of waste water arisings and the COD/AOX loads after any treatment but prior to biological treatment (Table 3.17). The survey also records the pre-treatment techniques used to make waste waters amenable to biological pre-treatment (Table 3.18).
Waste water volume
Note: Figures include all emissions except rainwater and cooling water blowdown.
Table 3.17: Quantification of waste water arisings from halogenated processes [UBA (Germany), 2000 #88]
Treatment technique Product
Incineration Stripping Distillation Extraction Sedimentation
& Flocculation
Table 3.18: Non-biological treatment techniques for halogenated process waste waters [UBA (Germany), 2000 #88]
Chapter 3
62 Production of Large Volume Organic Chemicals