Management Systems
3 GENERIC APPLIED PROCESSES AND TECHNIQUES
3.2 Lower olefins
Europe’s main olefin products are ranked on the basis of tonnage in Table 3.1. The Table also indicates what type of process description is provided in the BREF (if any). The most important olefin process is the production of ethylene (and associated butadiene and propylene) from the steam cracking of naphtha or ethane. This process is considered in detail in Chapter 7 as an illustrative process of the sub-sector.
Product Production capacity (kt per year) Process description?
Ethylene 18700 Illustrative Process
Propylene 12100 Illustrative Process (1)
1,3-Butadiene 2282 Illustrative Process (1)
n-Paraffin 833
Acetylene 409
Isobutene 374 √
1-Butene 170 √
Nonylene 150
Note 1: Considered as a co-product of the cracking process.
Table 3.1: Lower Olefin products 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
Some of the other major olefin processes are:
ACETYLENE: The use of acetylene as a chemical intermediate has declined in favour of ethylene, propylene and butadiene. Its use now is mainly restricted to the production of butanediol and as a welding gas. Production is via two distinct routes, either based on calcium carbide (by dry hydrolysis, wet hydrolysis), or from hydrocarbons (by pyrolysis, natural gas oxidation, electric arc) [Austria UBA, 2000 #94] [Environment Agency (E&W), 1999 #7].
Environmental issues of the calcium carbide, and subsequent acetylene, production are:
Environmental issues
Air: Acetylene, ammonia, hydrogen sulphide and phosphine from purging of the generator feed hopper. Acetylene from the purification bed vent during regeneration. Ammonia and hydrogen sulphide from lime pits.
Water: Glycol from raw gas holding tank condensates (contributing to BOD and COD). Water condensate from the cooling of acetylene and combination with the gas holder glycol water seal.
Calcium chloride from dryer blow-down. Ammonia and hydrogen sulphide from the ammonia scrubber used to purify raw acetylene.
Wastes: Carbon and ferro-silicates from the generator (the result of unreacted impurities in the carbide). Chromium and mercury from spent purifier bed solids. Lime hydrate can be re-used (e.g. in cement production, neutralisation).
BUTENE: is produced by the fractional distillation (‘tailing topping’) of mixed butylenes and butanes arising from crackers. It does not involve a chemical reaction. Residual distillation streams are used in other processes and there are no significant point emissions from the process [InfoMil, 2000 #83].
HIGHER OLEFINS: are linear olefins (alpha and internal) in the carbon range C6 to C20. The product from the higher olefin process depends on both the process technology and the feedstock (e.g. ethylene, propylene/butene). The process consists of two complementary techniques [Environment Agency (E&W), 1999 #7]:
Chapter 3
30 Production of Large Volume Organic Chemicals
• oligomerisation synthesis of alpha olefins from ethylene catalysed by a metal ligand catalyst dissolved in a solvent and
• isomerisation / disproportionation in which light C4 olefins and C20+ olefins (plus unwanted C6-C18 olefins) are converted to mid-range C6-C14 internal olefins by molecular rearrangement.
ISOBUTENE (2-METHYLPROPENE): is a raw material for the production of butyl rubber.
Tertiary butyl alcohol (TBA) is catalytically converted to isobutene and water. Crude product is purified by distillation [InfoMil, 2000 #83].
Environmental issues
Air: Carbon oxides, nitrogen oxides, PM10, VOCs.
Water: Bottom stream from distillation column is stripped and biologically treated.
Wastes: None specific to process.
Energy: Endothermic process.
LOWER OLEFIN WASTE WATER ISSUES. A recent survey of German olefin processes quantifies the volume of waste water arisings and the COD/AOX loads after any pre-treatment but prior to biological treatment (Table 3.2). The survey also records the pre-treatment techniques used to make waste waters amenable to biological treatment (Table 3.3).
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 Ethylene/
Propylene/
Acetylene
X (3)
X
1,3-Butadiene X X
Acetylene(1) X X
(1) By thermal route
(2) Figures include all emissions except rainwater and cooling water blowdown.
(3) The CEFIC survey gave a broader range, probably as it included the cracking of heavy feedstock e.g. gas-oil.
Table 3.2: Quantification of waste water arisings from olefin processes [UBA (Germany), 2000 #88]
Treatment technique Product
Incineration Stripping Distillation Extraction Sedimentation
& Flocculation
Hydrolysis Adsorption Ethylene/
Propylene/
Acetylene
X
1,3-Butadiene X X
Acetylene(1) X
(1) By thermal route
Table 3.3: Treatment techniques for olefin process waste waters (excluding biological treatment) [UBA (Germany), 2000 #88]
Chapter 3
Production of Large Volume Organic Chemicals 31
3.3 Aromatics
Table 3.4 gives Europe’s most important aromatic products (in tonnage terms) and also indicates what type of process description is provided in the BREF (if any). The production of benzene, toluene and xylene (BTX) is considered in detail in Chapter 8 as an illustrative process of this sub-sector. It also includes some detail of cyclohexane production because of its close links to the BTX process. The table is followed by brief descriptions of other aromatic processes that have major commercial importance as hydrocarbon intermediates.
Product Production capacity (kt per year) Process description?
Benzene 8056 Illustrative Process
Ethylbenzene 4881 √
Styrene 4155 √
Xylenes (mixed) 2872 Illustrative Process
Toluene 2635 Illustrative Process
Iso-propyl benzene (cumene) 2315 √
Xylene (para) 1342 Illustrative Process
Cyclohexane 1099
Xylene (ortho) 727 Illustrative Process
Alkylbenzene 490
Naphthalene 289 √
Table 3.4: Aromatic products 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
CUMENE: is produced from a reaction between propylene and benzene. The reaction is carried out under pressure at 250 ºC and catalysed by phosphoric acid on kieselguhr. Zeolites can also be used as catalysts. Excess benzene is used to ensure complete conversion of the propylene.
Products are separated by distillation, where propane (present in the propylene feedstock) is removed. Higher alkylated benzene by-products may be converted to cumene by trans-alkylation with additional benzene. Unreacted benzene is recycled to the reactor [Environment Agency (E&W), 1999 #7] [InfoMil, 2000 #83].
Environmental issues
Air: Storage tank blanket gases, purge and let-down gases are generally routed to flare, thereby releasing oxides of carbon.
Water: Phosphoric acid, hydrocarbons and amines from acid pot drainings and decommissioning washes.
Wastes: Spent catalyst and process residues.
ETHYLBENZENE: is a raw material for the production of styrene and propylene oxide. It is produced by the liquid or vapour-phase alkylation of benzene with ethylene over an aluminium chloride or zeolite catalyst. The current technology of choice for ethylbenzene is a liquid phase variant. The product is isolated by successive distillation stages to remove benzene (that is recycled to the feed) and di/tri-ethylbenzene (that is returned to the reactor). Impurities such as methane, hydrogen and ethane are separated from the reactor products and combusted (e.g. in a flare) fuel gas system. In the vapour-phase variant benzene and ethylene are pre-dried with molecular sieves, and these are regenerated using process gas at 220 ºC. The zeolite catalyst is regenerated by burn-off using re-circulated nitrogen containing oxygen. A bleed of gas is vented to atmosphere to remove the resultant carbon dioxide. Special control techniques include double mechanical seals on pumps; the containment of benzene vapours from tanks/loading; stripping of organics from waste water and their combustion in a furnace [Environment Agency (E&W), 1999 #7] [InfoMil, 2000 #83].
Chapter 3
32 Production of Large Volume Organic Chemicals
Environmental issues
Air: Oxides of carbon and oxides of nitrogen from catalyst regeneration and ethylbenzene furnace;
losses of benzene and other compounds from tank vents and loading operations; fugitive losses of ethylene, benzene and ethylbenzene from equipment and fittings; stack emissions of benzene.
Water: Benzene in the dehydration water and hydrocarbons in steam condensate. Treatment by wet air oxidation or VOC stripping (prior to biological treatment). Neutralisation effluents. Cooling water.
Wastes: Spent molecular sieve material. Tars and heavy fractions re-used as raw material or incinerated. Spent zeolite catalysts are regenerated (typically every 4 years) by off-site specialists.
Energy: The reaction is exothermic. Waste organic gases or liquids are recycled or used as fuel.
Discontinuous gases at start-up and shutdown are combusted in a flare without energy recovery.
NAPHTHALENE: is a raw material for the production of phthalic anhydride and is widely used in pharmaceutical processes. The distillation of coal tar produces mostly naphthalene, but also a variety of other by-products (e.g. pyridine bases) [InfoMil, 2000 #83].
Environmental issues
Air: All waste gases are incinerated. Main pollutants are carbon and nitrogen oxides from the incineration.
Water: There are no process related waste water streams. Cleaning water is treated by biological methods.
Wastes: Solid waste is recycled or transported to a processor.
Energy: Endothermic process.
STYRENE: is mainly manufactured in a two-stage process comprising the catalytic alkylation of benzene with ethylene to produce ethylbenzene (EB), followed by the catalytic dehydrogenation of EB to produce styrene. The second commercial process consists of oxidation of EB to ethylbenzene hydro-peroxide, followed by reaction with propylene to give alpha phenyl ethanol and propylene oxide; the alcohol being then dehydrated to styrene. In the catalytic dehydrogenation route, purified EB is vaporised, mixed with superheated steam, and fed to the dehydrogenation reactor. The catalysts are generally formulated on an iron oxide base, sometimes including chromium and potassium. Reaction products are condensed and separated into water and crude styrene phases. Hydrogen-rich process gas is recovered and used as fuel in the steam super-heater and process water is normally purified in a stripper and recycled to the boiler. Crude liquid styrene, consisting primarily of styrene and EB with traces of toluene, benzene and tars, is transferred to storage. Crude styrene is purified using low-temperature vacuum distillation in conjunction with sulphur or nitrogen-based inhibitors to minimise polymerisation of vinyl-aromatic compounds. This process recovers benzene, EB and toluene. Toluene is normally sold, benzene returned to the EB alkylation reactor and EB recycled to the reactor feed. Tars are removed as distillation column residues. Purified styrene is mixed with inhibitor and transferred to storage tanks. In some facilities, an EB/benzene/toluene stream is separated from the crude styrene initially and processed separately [Environment Agency (E&W), 1999 #7] [InfoMil, 2000 #83].
Environmental issues
Air: Hydrogen from catalyst preparation; Benzene and EB from distillation processes; EB, benzene, toluene and styrene releases from the purification process and from storage tanks.
Water: Steam condensate containing EB, benzene, toluene and styrene is stripped prior to central biological treatment.
Wastes: Residue from distillation columns; Sulphur or nitrogen based residues from styrene purification; Spent catalyst.
Chapter 3
Production of Large Volume Organic Chemicals 33
AROMATICS WASTE WATER ISSUES. A recent survey of German aromatic processes quantifies the volume of waste water arisings and the COD/AOX loads after any pre-treatment but prior to biological treatment (Table 3.5). The survey also records the pre-treatment techniques used to make waste waters amenable to biological treatment (Table 3.6).
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 Benzene/
Toluene X X
Ethyl-benzene/
Cumene
X X X
Styrene X X
Note: Figures include all emissions except rainwater and cooling water blowdown.
Table 3.5: Quantification of waste water arisings from aromatic processes [UBA (Germany), 2000 #88]
Treatment technique Product
Incineration Stripping Distillation Extraction Sedimentation
& Flocculation
Hydrolysis Adsorption Benzene/
Toluene X X X
Ethyl-benzene/
Cumene
X X
Styrene X X X
Table 3.6: Non-biological treatment techniques for aromatic process waste waters [UBA (Germany), 2000 #88]
Chapter 3
34 Production of Large Volume Organic Chemicals