Cyanates / isocyanates

Cyanates and isocyanates contain the radical –NCO. Mono-isocyanates are used commercially, but the term usually refers to diisocyanates.

DIPHENYL METHANE DIISOCYANATE (MDI) is a raw material for the production of polyurethane resins [InfoMil, 2000 #83]. MDI is produced by the phosgenation of diamino diphenyl methane (DADPM). The production of the phosgene and DAPM raw materials is highly integrated into the process. Phosgene is prepared continuously from chlorine gas and carbon monoxide over a carbon catalyst, and then condensed. DADPM is prepared from formaldehyde and aniline with a hydrochloric acid catalyst. After the reaction, HCl is neutralised with caustic soda, and the resulting sodium chloride brine is gravity separated from the DADPM for effluent treatment. Methanol inhibitor in the formaldehyde leaves with the process water. The DADPM is water-washed to remove salt traces and stripped with steam / nitrogen to remove aniline residues. Aniline is condensed and stored for re-use in the DADPM production. The non-condensables from the aniline recovery and the reactor vent gases are sent to the waste gas treatment unit. The DADPM product is stored prior to phosgenation.

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56 Production of Large Volume Organic Chemicals

In the phosgenation section, condensed phosgene is absorbed in monochlorobenzene (MCB) and passed to the phosgenation reactor for reaction with DADPM. The reaction gas consists of mainly HCl and phosgene and is recycled to the absorption column. The off-gases from the absorption column (mainly HCl from the phosgenation reactor and some carbon monoxide) are diverted to the HCl recovery section where HCl is compressed and exported.

The crude MDI mixture is separated from the MCB solvent in three steps. Firstly, the MDI mixture is thermally degassed. The recovered phosgene is returned to the absorption column of the phosgenation section and the recovered MCB is stored for re-use. Secondly, the MDI mixture is purified in a vacuum system and de-chlorinated (to remove HCl) by nitrogen stripping. Here the generated gases are sent to the waste gas treatment unit. The recovered MCB is stored for re-use. In the MCB recovery some phenyl isocyanate is also recovered. The phenyl isocyanate is converted to a MDI isomer and ends up as part of the polymeric MDI product, which contains several MDI isomers. In the splitting section the MDI mixture is split into pure 4,4’ MDI, mixed isomers and polymeric MDI (all of them useful products).

The waste gas treatment section deals with the process vents and vapours from the MCB, HCl and aniline storage. The vents from the DADPM section and the HCl and aniline storage are cooled to condense and recycle DADPM vapours. The uncondensed gases are treated in a caustic scrubber prior to emission to the atmosphere. The other vents from MCB storage, the MDI / MCB separation section and the MDI splitting section are refrigerated and subsequently led to a water and serial caustic scrubber prior to release to the atmosphere. Scrubber liquids are treated in the process water treatment unit.

The process water treatment consists of two parts. The first part, the amine-brine section, treats the DADPM, methanol, aniline and phenol-containing brine from the DADPM section. Phenol is a contaminant in the raw material aniline. Methanol is recovered through fractionation and exported. DADPM and aniline are recovered for re-use through extraction (DADPM in aniline), gravity separation and steam stripping (last stage removal of aniline and methanol prior to discharge of process water). The waste water from this unit is discharged to the central biological waste water treatment plant. The second part of the process water treatment deals with scrubber drains and rainwater and removes MCB through gravity separation and steam stripping. The recovered MCB is returned to the MCB storage. The treated water is discharged to the central biological waste water treatment plant.

Environmental issues

Air: Waste gas emissions from the waste gas treatment units. Fugitive emissions. All raw materials, intermediates and auxiliary products such as MCB, aniline, DADPM, carbon monoxide and HCl are recovered for re-use. Process vents and vents from HCl, aniline, DADPM and MCB tank storage are treated in water and / or caustic scrubber prior to discharge to atmosphere.

Water: Liquid extraction is applied to remove DADPM. Steam strippers are installed to remove aniline and MCB from process and scrubber water discharges to the biological waste water treatment plant. The most important contaminant is phenol.

Wastes: Methanol and halogenated waste from the recovery of MCB from certain off-spec materials are incinerated.

Energy: Phosgene, DADPM and MDI production are exothermic processes but not to the extent that heat recovery options such as steam generation can be applied.

3.5.6 Other

CAPROLACTAM (hexamethyleneimine) is the main raw material for the production of polyamide-6 (nylon). Caprolactam is produced via the intermediate cylohexanone (ketohexamethylene) some of which is used as a solvent in the production of paint. A caprolactam production unit typically consists of four stages [InfoMil, 2000 #83]:

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Production of Large Volume Organic Chemicals 57

1) Cyclohexanone (ANON) plant: Cyclohexanone is produced catalytically from phenol and hydrogen. By-products are cyclohexanol and residues (tar). Cyclohexanol is converted into cyclohexanone whilst generation of hydrogen takes place. Tar is incinerated for heat generation purposes. Waste gas from the reactors contains hydrogen and methane. Methane is an impurity of the hydrogen gas. The waste gas is used as site fuel gas system or flared.

2) Hydro xylamine phosphate oxime (HPO) plant: Oxime, the basic intermediate for caprolactam production, is produced via the phosphate route. This utilises two circular counter current liquid streams of an inorganic liquid (ammonium nitrate, phosphoric acid and water) and an organic stream (mainly consisting of toluene).

3) Hydro xylamine sulphate oxime (HSO) and caprolactam purification plant: Oxime from the HSO route plus the oxime from the phosphate route are converted to caprolactam via the sulphate route.

4) Caprolactam finishing plant: Caprolactam is extracted with benzene. A water wash then removes ammonium sulphate and organic impurities. The remaining benzene is evaporated in a stripper. Further purification is achieved by ion exchange, by catalytic hydrogen treatment, by evaporation and by distillation

Environmental issues

Air: The cyclohexanone plant has emissions of cyclohexanone, cyclohexanol, benzene,

cyclohexane from tank vents and vacuum systems. The HPO plant has emissions of cyclohexanone from tank vents and vacuum systems; toluene tank vent emissions; NOx and NO2 from the catalytic NOx treatment unit. The HSO plant has emissions of cyclohexanone and benzene from tank vents and vacuum systems; SO2 emissions; NOx and NO2 from the catalytic NOx treatment unit. Waste gases from the HPO and HSO plants are used as fuel or flared. Waste gases with nitric oxide and ammonia are converted to nitrogen and water over a catalyst. Benzene tanks are connected to a vapour destruction unit. Vents on oleum, phenol and ammonia storage tanks are equipped with water scrubbers. Balancing lines are used to reduce losses from loading and unloading.

Water: After effluent stripping with steam, the main residual contaminants are caprolactam, cyclohexanone and oxime, and effluent can be treated biologically. The main TOC loads are from the cyclohexanone production. For the manufacture of caprolactam from cyclohexanone, specific water volume is in the range of 0.1 - 1 m³/t and the COD load is 1 - 10 kg/t. Although Ammonia can be separated as a saleable product, effluents may still contain considerable ammonia loads that can be reduced by biological nitrification / denitrification.

Wastes: Tar from cyclohexanone production is incinerated. Catalysts are recovered.

Energy: Waste heat recovery is widely applied.

PYRIDINE (N(CH)5) is manufactured world-wide by the catalysed ammonolysis of acetaldehyde and formaldehyde. Methylpyridine is a by-product of the 2,2-bipyridyl manufacturing process, which involves the use of pyridine. Dimethylpyridine a batch fraction can be produced as a by-product from a wet pyridine recovery plant.

WASTE WATER ISSUES NITROGENATED COMPOUNDS. A survey of German nitrogenated processes quantifies the volume of waste water arisings and COD/AOX loads after any treatment but prior to biological treatment (Table 3.13). The survey also gives the pre-treatment techniques used to make waste waters amenable to biological pre-treatment (Table 3.14).

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58 Production of Large Volume Organic Chemicals

Waste water volume (m³/t product)

COD (kg/t product)

AOX (g/t product) Product

<0.1 0.1 - 1 1 - 10 >10 <0.1 0.1 - 1 1 - 10 >10 <0.1 0.1 - 1 1 - 10 10 - 100 >100

Nitrobenzene X X X

Acrylonitrile X X

Caprolactam X X X

Aniline (hydration of nitrobenzene)

X X X

Aniline (reduction with iron)

X X

TDA X X

TDI (+phosgene)

X X X

Ethanol-amine X X

MDA X X

MDI

(+phosgene) X X X

Note: Figures include all emissions except rainwater and cooling water blowdown.

Table 3.13: Quantification of waste water arisings from nitrogenated processes [UBA (Germany), 2000 #88]

Treatment technique Product

Incineration Stripping Distillation Extraction Sedimentation

& Flocculation Hydrolysis Adsorption

Nitrobenzene X X

Acrylonitrile X X

Caprolactam Aniline (hydration of nitrobenzene)

X X

Aniline (reduction with iron)

X X

TDA X X X

TDI (+phosgene) X

MDA X X

Table 3.14: Non-biological treatment techniques for nitrogenated process waste waters [UBA (Germany), 2000 #88]

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Production of Large Volume Organic Chemicals 59