The products of the chlor-alkali process continue to be essential for many purposes across the global value chain. Modern chlor-alkali technologies can vastly improve upon the safety and environmental performance of legacy systems
The chlor-alkali production process is one of the most common of all industrial chemical operations. The output of a chlor-alkali system includes chlorine (Cl2), sodium hydroxide (NaOH) and hydrogen gas (H2) — all of which have broad application and growing economic and societal importance. For example, the free chlorine produced by chlor-alkali systems is the most affordable and widely used drinking-water disinfectant in the world. It is also highly effective against nearly all waterborne pathogens. Thus, chlorine serves an essential role as global demand for potable water continues to rise, alongside climate challenges related to rising temperatures and extreme weather events, which can lead to excess algae growth and increased sediment in natural water sources (Figure 1). However, despite its ubiquity, all chlorine is not created equally.
FIGURE 1. As environmental stresses are challenging the availability of potable drinking water, chlorine is increasingly essential as an affordable and easily applicable treatment solution
Available technologies
Today, most free Cl2 is produced electrochemically by water electrolysis in equipment known as chlor-alkali cells. In standard chlor-alkali cell processes, a clean saltwater solution (brine) and water are converted into Cl2, H2 and NaOH. Of the three common cell configurations, one type of legacy system uses mercury cells, while the second depends primarily on asbestos diaphragm cells. The third, more modern type uses polymer electrolyte membranes (PEMs).
The Minamata Convention, an international treaty signed by 128 nations in 2013, set a phaseout date of 2025 for using mercury in the chlor-alkali manufacturing process. After that date, chlor-alkali electrolysis cells will be effectively limited to using diaphragm cells and PEMs.
In addition to the elmination of mercury and asbestos in the process, the advanced membrane chlor-alkali process using PEMs can enable substantial energy savings compared to the two older processes. In fact, the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE; www.energy.gov/eere) has stated that, of the three available chlorine-producing technologies, the mercury cell is the most energy-intensive, consuming about 3,700 Kwh of electricity per metric ton (kWh/m.t.) of Cl2 produced, while the diaphragm cell consumes about 2,900 kWh/m.t. PEM cells are the most energy-efficient, at 2,500 kWh/m.t. [1].
How membrane cells work
A chlor-alkali membrane cell is comprised of two half-cells, one containing an anode (positively charged) and the other a cathode (negatively charged), separated by a PEM (Figure 2).
FIGURE 2. A typical chlor-alkali cell employing a polymer electrolyte membrane (PEM) is shown here
A direct current is applied to the cell while a concentrated salt (NaCl) solution, or concentrated brine, is fed into the anode side of the cell, and water is introduced into the cathode side. The PEM that separates the anode from the cathode is specially formulated so that it will conduct only positive sodium (Na+) ions through it, while preventing passage of negative hydroxyl (OH –) ions into the cathode side of the cell.
The negatively charged chloride ions are attracted to the positively charged anode, where they give up an electron to the anode and coalesce to form chlorine, which exits the anode half-cell as a gas:
2Cl– → Cl2 + 2e– (1)
At the same time, the negatively charged cathode attracts the Na + ions from the brine (anode) side of the cell, through the membrane and into the cathode half-cell. On the cathode side, the water is electrolyzed to produce negatively charged OH – ions:
2H2O + 2e– → 2OH– + H2 (2)
The Na+ ions that have been transported from the anode side of the cell combine with the OH – ions produced at the cathode to form NaOH, commonly known as caustic soda or lye. The NaOH solution is discharged from the cathode side of the cell:
Na+ + OH – → NaOH (3)
Thus, the membrane cell produces a depleted brine solution plus three saleable products — Cl2, H2 and caustic soda — from the original brine and water input. The H2 is becoming ever more valuable as an integral component of the growing hydrogen economy. The caustic soda has uses across the chemical and industrial landscape as one of world’s most-used commodity chemicals. And finally, as mentioned previously, Cl2 can be used to help improve water quality and to make a host of chemicals, including sodium hypochlorite (bleach), a widely used, economical disinfectant that is effective against a wide variety of viruses and bacteria.
PEMs improve the process
Each chlor-alkali environment differs depending on the application, with variables including brine source and purity, water source and purity, electrical costs, available capital, and the labor force’s experience level. Yet one thing remains constant across chlor-alkali facilities — a demand for consistent performance and energy savings.
Consistency and energy savings are crucial when scaling the production of commodities. Chlor-alkali membrane cells offer an environmentally sound solution for producing some of the most widely used commodity inorganic chemicals, through the industrialized electrolysis of NaCl.
The most efficient PEMs are ionomer based, meaning they selectively conduct ions over a wide range of operating conditions. This unique property enables membranes to create higher-quality and sustainable electrochemical separations, even in harsh environments. Additionally, modern, high-performance membranes offer consistent voltage performance, current efficiency, mechanical durability and high impurity resistance over the lifetime of the membrane.
This combination of properties leads to a host of advantages, including the following:
- Fewer process interruptions
- Limited maintenance downtime
- Reduced lifetime costs and total cost of ownership (TCO)
- Consistent production quantities
- Reduced lifetime energy use
Recent developments
Approximately 60% of the production costs involved in the chlor-alkali process come from the operation’s power consumption. Any improvement in membrane voltage or current efficiency reduces overall cell energy consumption, thereby reducing operating costs. At the same time, the lower energy demands reduce the carbon footprint of the generating facility by reducing load and, thus, emissions.
The newest low-voltage membranes can reduce the power consumption not only in new chlor-alkali electrolyzers, but also in existing electrolyzers, if they are chosen at the time of membrane replacement. This is especially true if the low-voltage membranes retain the chemical and mechanical durability of their higher-voltage counterparts.
Because electrolyzer types and process conditions can vary among producers and individual plants, operators should consider the versatility of the membranes they choose. Some membranes for low-voltage performance are designed with the mechanical strength to resist damage from handling, installation and in-process pressure differentials and the chemical resistance to endure brine impurity spikes and impurity accumulations on the membrane, even as they resist degradation from power fluctuations. As membrane manufacturers continue to advance the design and robustness of PEMs, the chlor-alkali process can be counted on to further improve energy efficiency and equipment longevity. ■
Edited by Mary Page Bailey
Reference
1. U.S. Dept. of Energy, Office of Energy Efficiency and Renewable Energy, Industrial Technologies Program, Advanced Chlor-Alkali Technology, https://www1.eere.energy.gov/manufacturing/industries_technologies/imf/pdfs/1797_advanced_chlor-alkali.pdf.
Author
Andy Liu (Email: [email protected]) is currently the product sustainability strategy leader at Chemours, responsible for leading the company in its Sustainable Offerings Corporate Responsibility Commitment Goal, which aims to maximize the Chemours product portfolio’s contributions to the United Nations Sustainable Development Goals. He earned his B.S. in chemistry from the California Institute of Technology and his Ph.D. in inorganic chemistry from Massachusetts Institute of Technology. After working at Northwestern University as a National Institutes of Health Postdoctoral Fellow, Liu joined DuPont as a research & development chemist and served nearly 20 years developing new products, processes and applications. In January of 2009, he assumed responsibilities for developing and executing global strategies for gaining regulatory notification and registration approvals for new-substance commercialization.