Polyvinyl chloride

The PVC production is considered as a complex industrial process since multiple reactions are involved in the production chain such as naphtha to ethylene, ethylene to

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ethylene dichloride (EDC), EDC to vinyl chloride monomer (VCM), and PVC from VCM polymerization. This study assumes that hydrochloric acid (HC1) generated by the thermal decomposition of EDC is used as the raw material for the oxy-chlorination process. Therefore, the exergy efficiency estimation includes HCl as a useful by-product.

Polyvinyl chloride is commercially produced by bulk, suspension emulsion, and solution polymerization. The suspension process is the major manufacturing technique, nearly 80% of the total PVC is produced by suspension polymerization. Exergy flows of the PVC production process to produce 1 kg PVC resin has been is illustrated in table 3.10, from which the exergy efficiency of PVC production is accounted for 23.7% in present industrial practice.

Table 3.10: Exergy inputs and outputs of PVC resin production.

Process step Input exergy

Naphtha production 37.15 28.09 75.6

Ethylene production 32.81 22.90 69.8

Ammonia (NH3) production 0.24 0.16 67.0

Chlorine (Cl2) production 5.33 2.30 43.1

VCM production 36.74 21.99 59.8

Vinyl chloride polymerization 24.38 19.66 80.6

Note: Calculation based on the process flow information available in Ayres and Ayres 1999;

Narita et al. 2002; Ren et al. 2006.

To calculate the theoretical minimum exergy required for PVC production, the following reactions (equations 3.18-3.21) have been considered where ethylene is converted to PVC through formation of EDC and VCM (Weissermel and Arpe 19997). The theoretical exergy efficiency is estimated to be 52.3% analysing the thermochemical data of the chemical reactions involved in the PVC production process.

CH2=CH2 + Cl2 → CH2Cl-CH2Cl + 209.1 kJ (3.18)

CH2Cl-CH2Cl + 192.7 kJ → CH2=CHCl + HCl (3.19)

2CH2=CH2 + Cl2 + HCl + ¹/2O2 → 2 CH2Cl-CH2Cl + H2O + 510.3 kJ (3.20) In the suspension polymerization process VCM is fed to a reactor in doses, together with a suspension stabilizer, a pH buffer, an anti-foam agent and an initiator (e.g. organic peroxides). VCM reacts to form PVC as follows with up to 80-85% yield:

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nCH2=CHCl → CH2 – CHCl – CH2 – CHCl – (3.21)

Due to the presence of high chlorine content, some of the recycling techniques are not favourable for PVC. In particular, landfilling and composting are not suitable because of known and unknown hazards associated with the oxidative degradation of PVC in the environment (Sadat-Shojai and Bakhshandeh 2011). Incineration and pyrolysis may also be disfavoured because of the large amounts of hydrogen chloride and other toxic products are produced.

Thus, mechanical and chemical recycling are the two main processes of PVC recycling.

The former is preferred when the source of PVC waste is known. This analysis estimates the exergy efficiency of mechanical recycling of PVC assuming an average yield of 90%

PVC from the secondary material. Based on this estimation, theoretical and industrial exergy efficiency of PVC recycling has been quantified 90.5% and 54.2%, respectively.

3.4 Discussion

This section is not intended to make specific proposals for improving the manufacturing system but to determine how efficiently today's systems perform and to pinpoint the improvement potentials of the analysed processes. Table 1 presents the exergy efficiency of the case study materials in present industrial practices both for primary and secondary production processes, demonstrating the corresponding theoretical exergy efficiency.

This table clearly represents that exergy resources are being used very inefficiently, and there is a wide margin to improve the current technologies. Ideally, the exergy efficiency of the case study materials can be improved from 1.1% (ceramic tile recycling) up to 49.1% (primary copper production). Among the primary processes analysed in this study, PP production has been found to be the most efficient process with an exergy efficiency of 53.4%, followed by PVC (23.7%) and steel (16.3%).

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Table 3.11: Exergy efficiency of construction materials: primary and secondary manufacturing processes.

Material Primary production process Secondary production process Exergy efficiency, e₂ potential to achieve more efficient industrial sector since exergy loss in secondary process is far lower compared to primary process (Table 2). It is widely acknowledged that the energy intensity of most of the industrial processes is at least 50% higher than the theoretical minimum determined by the basic laws of thermodynamics (IEA, 2006).

However, results of this analysis show that the actual energy intensity is much higher when exergy based analysis is performed. Table 2 illustrates the practical exergy inputs and theoretical exergy demand in present technologies of the case study materials. These figures demonstrate that primary metal production contributes greater exergy losses.

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Table 3.12: Exergy inputs in primary and secondary production process for 1 kg product output based on present industrial practice and theoretical minimum requirements.

Material Exergy input in the primary production process (MJ)

Exergy input in the secondary production process (MJ)

The results of this study suggest that radical changes are needed in overall processes of the case study materials in order to achieve major breakthroughs in the exergy efficiency of the industry. Changes in process design and development of achievable technology is required in order to improve the resource sustainability of current industrial sector. An example of the achievable technologies that could produce such changes is described briefly in the following section for aluminum production.

The Boehmite process uses similar process flow like the Bayer process but replaces gibbsite seeding with boehmite seeding, and an optimization can be achieved through precipitation of monohydrate (boehmite) rather than trihydrate (gibbsite) alumina under atmospheric pressure in temperatures lower than 90°C (Dash et al., 2009; Panias et al., 2003). Thus, the Boehmite process can be expressed as equation (3.21)

Al2O3·3H2O + 2NaOH → 2NaAlO2 + 4H2O → Al2O3·H2O + 2NaOH + 2H2O (3.21) The process is no longer a chemical cycle like the Bayer process, the final product boehmite differs from the initial gibbsite, producing a product with higher exergy value

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(chemical exergy of boehmite and gibbsite is 1.60 and 0.18 MJ/kg respectively).

Calcination of boehmite to alumina requires 126.46 kJ less energy compared to gibbsite calcinations, equations (3.7) and (3.22).

Al2O3·H2O + 58.7 kJ → Al2O3 + 3H2O (3.22)

The entire alumina production process utilizing the Boehmite process, when compared to the conventional Bayer process, can achieve 19% energy savings and 2% increase in the exergy efficiency (Balomenos et al., 2011). Process efficiency can also be improved by utilizing the high exergy embodied red mud waste instead of sending to landfill. The relatively high exergy embodied in the red mud is mainly due to its high content of Fe2O3, which could characterize red mud as an industrial feedstock rather than an industrial waste.

High temperature Carbothermic reduction is a direct chemical reduction of alumina to aluminium, an alternative to the electrolytic reduction. This process can be theoretically described by the following chemical reaction occurring at temperatures higher than 1900°C:

Al2O3 + 3C + 672.1kJ → 2Al + 3CO (3.22)

The main exergetic benefit from replacing the electrolytic reduction of alumina with a Carbothermic process is that carbon will be used as a direct reducing agent supplying the 47% of the total energy for the process instead of the 8% that is currently being utilized as consumable carbon anodes (Balomenos et al., 2011). The high exergetic cost of transforming carbon into electricity to drive the redox reaction will be avoided, while the alumina reduction will take place in a three-dimensional space thus avoiding heat losses due the volumetric inefficiency of the Hall-Héroult process.