Chlorine gas is produced by passing an electric current through brines (common salt dissolved in water). Essential co-products are caustic soda (sodium hydroxide, NaOH) and hydrogen (H2). Therefore, this section addresses chloralkali plants as well. There are three major processes in use: the mercury process, the diaphragm process, and the membrane process. PCDF can be formed in the chlorine cells; concentrations of PCDD are very low.
PCDD and PCDF formation can be relevant when graphite anodes are used. In early years, graphite anodes have been used in diaphragm and mercury cells. Since the membrane technology represents the modern technology, it is not likely that there are membrane plants that use graphite electrodes as the anode. Many industrialized countries replaced the graphite anodes at the beginning of the 1970s, however, the old process using graphite anodes can be a significant source of PCDD/PCDF. Due to the low costs and easy operation, graphite electrodes are commonly used in China, the second largest chloralkali producing country in the world (Wu 2000). Historical production by this method can lead to Hot Spots (see 10b - Production Sites of Chlorine). Limited data shows that PCDF may also be present where titanium anodes are used. The source of the organic carbon may be in rubber sealing rings used in the process.
The Draft Guidelines on BAT/BEP note that the use of graphite electrodes does not constitute BAT (SC BAT/BEP 2004). Modern plants would use the coated titanium anode (BREF 2001c).
It has been reported in the literature (Kannan et al. 1997) that the commercial mixture Aroclor 1268 has been used to lubricate the electrodes. Disposal of process wastes has caused severe environmental contamination.
National inventories should include the PCDD/PCDF release in the residues from chlorine plants that utilize graphite anodes. There will be one emission factor assigned for the residues; other release vectors are negligible although contamination originating from the disposal of the electrode sludge may be relevant. For example in Germany, the dumping site of a former chlorine production plant that used graphite electrodes showed PCDD/PCDF concentrations up to 319 μg I-TEQ/kg (She and Hagenmaier 1994). In sediment from China, concentrations up to 420 μg I-TEQ/kg d.m. were found (Xu et al. 2000). It should be noted that there are two emission factors; one is based on the amount of sludge (= residue) generated and the other is based on one ton of chloralkali produced. There is no emission factor that relates to the amount of chlorine (gas) produced (Table 63).
Table X: PCDD/PCDF emission factors for chlorine production with graphite electrodes
Chlorine/chloralkali production using graphite anodes
20 µg TEQ/ kg sludge
1,000 µg TEQ/ t chloralkali
Ethylene Dichloride or 1,2-Dichloroethane (EDC)
Ethylene dichloride (EDC) is an important intermediate in the manufacture of PVC. In the USA, >90% of the total EDC production is used to produce vinyl chloride monomer (VCM). Most PVC production uses dehydrochlorination (cracking) of ethylene dichloride (EDC)
Production of EDC (two different methods) Direct chlorination of ethylene with chlorine in the presence of a catalyst (chlorides of iron, aluminum, copper, antimony). The process has a high conversion rate. Typically, direct chlorination is carried out in a liquid-phase reactor at temperatures between 50°C and 70°C and pressures around 400-500 kPa. The HCl formed in the process can be recycled into the oxychlorination process.
Oxychlorination of ethylene with hydrochloric acid (HCl) and either air or oxygen is carried out in the presence of a catalyst (usually copper) in a fixed-bed reactor or a fluidized-bed reactor. Temperatures should not exceed 325°C, as higher temperatures will increase formation of by-products (mostly chlorinated C1- and C2-compounds). The first step of the EDC purification process is usually a water quench followed by caustic scrubbing. The water is returned to the process or is steam stripped prior to discharge (see emission factor for discharge water).
Production of VCM
VCM is produced by thermal dechlorination from EDC. The so-called cracking furnace typically operates at around 2,000 kPa at temperatures between 450°C and 650°C. Unreacted raw material is recycled back into the process. VCM (boiling point: -13°C) is separated from byproducts by distillation. High boiling materials may contain various condensation products including PCDD/PCDF. These materials are typically thermally decomposed; in some cases, HCl from the process is recovered and recycled.
Production of PVC
There are the following processes to produce PVC resins:
Bulk (mass,) and
Within the EDC/VCM/PVC industry, the most critical step for PCDD/PCDF generation is the manufacture of EDC via oxychlorination of ethylene. Generation of PCDD/PCDF in VCM pyrolysis is unlikely due to the low concentration of oxygen. Chemical conditions for generation of PCDD/PCDF do not exist in PVC polymerization.
Streams that may contain PCDD/PCDF include any combustion streams, including liquid, liquid/gas or vent gas combustors. In addition, some PCDD/PCDF may reside on catalyst support. Releases of that material differ with the production process.
Fluidized bed catalysis will be accompanied by the catalyst’s particle size distribution. Small particles can be carried over in product vapor and washed out with quench water. The catalyst in fixed bed systems is replaced on approximately an annual basis. As a result, particles from fluid bed systems are typically isolated in solids from wastewater purification. Spent fixed bed catalyst, if discarded, represents an explicit waste stream.
Plant-specific data of PCDD/PCDF releases are available from the EPA TRI reporting. Under TRI facilities that manufacture, process, or otherwise use certain toxic materials are required to report emissions to air, water, and land if they exceed established activity thresholds. The TRI also requires facilities to report their pollution-prevention and recycling data. The Environmental Protection Agency (EPA) compiles TRI data each year, publishes an annual report and makes the data available to the public via the Internet (http://trifacts.org/). In October 1999, EPA added PCDD/PCDF to the TRI inventory to begin in the reporting year 2000. The PCDD/PCDF releases are available from http://www.trifacts.org/dioxin_data/index.html (Carroll 2004).
Emission factors for the EDC/VCM and PVC industry are displayed in Table X. There will be three classes of emission factors splitting between old and modern technology. As a separate class, PVC stand-alone plants are included as class 3. As can be seen, for old technologies, no emission factors to air and for residues are available at present.
Table X: PCDD/PCDF emission factors for the EDC/VCM/PVC industry
In 1995, the European Council of Vinyl Manufacturers (ECVM) set voluntary emission targets as a means of promoting environmental performance. The ECVM Charter, which is a form of self-regulation, includes dioxin emission guidelines based on Best Available Techniques. For the emission of vent gases to the atmosphere the ECVM guideline for dioxin-like components is 0.1 ng I-TEQ/Nm³ (according to (European) normal conditions of 11% O2, 273.15 K or 0°C, 101.3 kPa) and 1 μg I-TEQ/t of EDC in water effluents. These numbers can be taken as rough estimates for calculating PCDD/PCDF releases from state-of-the-art EDC/VCM plants.