Life Cycle Assessment model for the chlor-alkali process: A comprehensive review of resources and available technologies

Abstract

Currently, the chlor-alkali sector is shared by three main electrolysis technologies: mercury, membrane and diaphragm cell. As the energy demand of the process is one of its main drawbacks, new technological improvements are emerging such as the replacement of the standard hydrogen-evolving cathode in membrane technology by an oxygen-depolarised cathode (ODC). In this sense, the environmental impacts of novel techniques must be analysed over their entire life cycle to assess properly their integration opportunities. This work develops a life cycle assessment (LCA) model to describe the chlor-alkali European industry. The multi-functional production of chlorine, sodium hydroxide and hydrogen is studied from cradle to gate, including salt production, products treatment and waste management within the system boundaries. While the worst scenario results mercury technique, ODC technology emerges as the most environmentally sustainable *Manuscript Click here to view linked References 2 process. The results suggest the importance of considering every process included, especially salt production and brine preparation, which can involve up to 20% of the total environmental impacts. In fact, taken as reference membrane scenario, results demonstrated that the environmental profile can be reduced by up to 18% when lower energy demanding processes for salt production and NaOH concentration were selected. This improvement percentage overcomes the competitive advantage shown by ODC versus membrane technology (7%). This model is a useful tool not only for the comparative assessment of the environmental sustainability of the different chlor-alkali installations, but also to guide and support the decision-making process in the introduction of emergent technologies in the sector. HIGHLIGHTS  Exhaustive Life Cycle Assessment model to describe the European chlor-alkali sector  Environmental sustainability of current and emerging electrolysis technologies  Importance of every life cycle stage is remarked, especially salt production  Success of emergent technology is challenged by the lack of hydrogen production  Tool to support decision-making process in the introduction of emergent techniques

Hypochlorite Generation from a Water Softener Spent Brine

“…Brine conditioning is a fundamental step for the chlor-alkali process. It requires sodium chloride as an aqueous solution with certain quality requirements, which can vary as a function of the electrolytic technology evaluated [26]. In general, both ions cause physical disruption.…”

“…In the chlor-alkali process, free available chlorine is defined as the sum of Cl 2 (aq), HClO, and OCl − concentrations and it is a measure of the amount of chlorine species that can oxidize any chemical compound. The acidic conditions in the anolyte are also required (pH < 2) to minimize the formation of chlorate [26,29]. Also, the efficiency on the reaction rate of water oxidation is achieved at a low pH value.…”

Decarbonisation of calcium carbonate at atmospheric temperatures and pressures, with simultaneous CO2 capture, through production of sodium carbonate

“…NaOH can have a high carbon footprint, which originates from electricity use during its manufacture via electrolysis of seawater; 42,43 however, with the use of green electricity (e.g., wind turbines), this method of decarbonisation may offer negligible overall CO 2 emissions. The availability of NaOH is not intrinsically limited, since NaOH can be manufactured through electrolysis of sea water, and the seas and oceans of Earth contain B2 Â 10 16 tonnes of Na; the limitation here is the energy demand of the chlor-alkali process (which can be met by renewables) and the utilisation of the very large quantity of Cl 2 that would be produced as a co-product (but diminishing the CaCl 2 co-product from soda ash manufacture).…”

Utilization of hydrogen as clean energy resource in chlor-alkali process

“…This process is also among the highest energy consuming processes due to the high electricity utilization that becomes the key issue to the process feasibility (Jung et al, 2014). However, there are multiple attempts to reduce the energy consumption of the chlor-alkali process using alternative sources of energy (Rabbani et al, 2014a(Rabbani et al, , 2014b(Rabbani et al, , 2016Wang et al, 2014) or by replacing the standard hydrogenevolving cathode in membrane technology by an oxygen-depolarized cathode (Garcia Herrero et al, 2017a, 2017b. The chlor-alkali process applies 90% of the electric current in the process to a brine (water and salt) solution to produce chlorine, hydrogen gas, and sodium hydroxide, or caustic soda solution.…”