The term “chlor-alkali" refers to the manufacturing of chlorine (chlor) and a strong base, typically sodium hydroxide (alkali), two chemicals that are simultaneously produced by the electrolysis of brine (a solution of salt in water). These two chemicals are the main products of the chlor-alkali industry. Both are produced in a fixed stoichiometric ratio: for each unit of chlorine produced, 1.13 units of sodium hydroxide are produced.

In terms of chlor-alkali production, energy consumption has always been a strong driver to cost. It is among the highest energy consuming processes due to its dependency on energy intensive electrochemical technology. The cost of electricity makes up around 40-60% of the operating cash costs, and therefore has an influence in the production cycle.

IHS Quarterly recently reported that, “one plant can consume as much electricity as a small country" and energy can account for up to 70% of chlor-alkali variable costs. As a result, engineers have been trying to lower that figure while maintaining production requirements in order to increase overall profit.

Membrane cell technology was found to lower operating costs by an average of around 6%. However, additional caustic soda conccentration steps must be provided, leading to additional energy (steam) requirements.

During the last half of the 19th century, chlorine was almost solely used in the textile and paper industry. Today, that industry amounts to roughly 3% of chlorine demand. The main end-use is for chlorinated compounds such as PVC that account for roughly 34% of the market. Further uses are in water treatment, chlorinated intermediates, inorganic and organic chemicals, among others.

"The ratio of chlorine demand between end users varies greatly between regions," says George Eisenhauer, Director at IHS Chemicals. "Unlike chlorine, sodium hydroxide (also known as caustic soda) has multiple end uses, with none dominant like PVC."

The majority of chlorine is traded in the form of derivatives such as PVC and other vinyls that are primarily shipped to South America and Asia. Unlike chlorine, caustic soda is shipped in its elemental form—typically this means transporting it dissolved in water at a 50% concentration.

At present, caustic soda is a global product with primary trade patterns between northeast to southeast Asia and from North America to South America. “We anticipate that with the continued energy advantage, North America will for the foreseeable future be able to competitively export caustic soda to other regions of the world," says Eisenhauer.

In 2014, global caustic soda demand was 73 million metric tons (MMT) according to IHS Chemicals. The two largest single uses are in the alumina and the pulp and paper industries. Each consumes roughly 14% of caustic soda production.

“Chlor-alkali production is highly fragmented," says Ron Smith, senior consultant at IHS. The top 50 producers make up less than 60% of the total capacity, with Dow Chemical ranking as the largest chlor-alkali producer. Dow currently accounts for about one-fourth of U.S. production and one-sixth of world capacity.

The Process Technologies

Chemical process of a diaphragm cell. Image source: essentialchemicalindustry.orgChemical process of a diaphragm cell. Image source: essentialchemicalindustry.orgThe diaphragm cell process was created in 1851 when the combined processing of caustic soda and chlorine first emerged. Mercury and membrane cell technologies are the other two technologies currently available.

Cell technology varies by region, says Eisenhauer. In the U.S. Gulf Coast, diaphragm technology is dominant with slightly more than 50% of installed capacity. In the past, mercury cell technology was dominant in Europe. However, regulatory changes in Europe mean that mercury cell technology as well as asbestos-based diaphragm technology will be phased out by December 2017. Asia primarily uses membrane cell technology, unlike the rest of the world.

“Many of the producers in Japan converted their plants to membrane cell technology in the early 1980's and since then essentially all new capacity in Asia has been based on membrane cell technology," Eisenhauer says.

Chlor-alkili plants can be energy intensive, with electric power accounting for as much as 70% of variable costs. Energy use is mainly the result of electricity consumption and process steam production.

In principle, producing chlorine and caustic soda deals with passing an electric current through brine, which then detaches and rejoins by the exchange of electrons (delivered by the current) into gaseous chlorine, dissolved caustic soda and hydrogen. Approximately 90% of the electric current is utilized as raw material that cannot be replaced. The remaining 10% is used for lighting and operating pumps, compressors and other equipment. Steam, on the other hand, is used for salt preparation and concentration of caustic soda.

At present, the most cost-competitive technology is the membrane cell technology. Smith says the primary advantage of a membrane cell-based process is that the overall plant process requires less energy than mercury or diaphragm cell-based processes. This can help a plant lower its overall operating costs by around 6%.Chemical process of a membrane cell. Image source: essentialchemicalindustry.orgChemical process of a membrane cell. Image source: essentialchemicalindustry.org

“The membrane cell process shows some capital expense and operating cost advantages. Our analysis indicates that the membrane process has about an 8% capex advantage over the diaphragm process," he says. Total electrical requirements are about 17% lower for a membrane than for a diaphragm.

Moreover, membrane cells can produce a highly pure grade of caustic soda. However, additional caustic soda concentration steps must be provided during the process, leading to additional energy steam requirements. Process steam usage in a diaphragm plant is almost three times what it is in the membrant plant, Smith says.

Diaphragm technology can either be asbestos-based or fluoropolymer-based says Eisenhauer. Asbestos-based diaphragms have environmental concerns related to asbestos itself with associated costs of safe handling and disposal. Fluoropolymer-based diaphragms, meanwhile, are considered equivalent to membrane cells from an environmental standpoint and have a similar level of environmental concerns.

Chemical process of a mercury cell. Image source: essentialchemicalindustry.orgChemical process of a mercury cell. Image source: essentialchemicalindustry.orgPerhaps the most robust of the three technologies is the mercury cell. Mercury cells can handle a wider range of salt quality than the other technologies. However, due to environmental pressures, mercury cells are being phased out in Europe and likely in the United States by 2020, according to Smith. He says producers using this technology will be faced with moving to membrane technology or shutting down their facility.

Design Concepts and Energy Efficiency

To tackle the energy cost component, several technology licensors are following innovative methods on how to optimize the design and operation of chlor-alkali plants.

Some innovations focus on reducing energy consumption within the membrane cell, says Eisenhauer. The Oxygen Depolarized Cathode (ODC) technology is one example. Experts at ThyssenKrupp Uhde Chlorine Engineers started working on developing this technology in the late 1990s. The process suppresses the generation of hydrogen at the cathode. The cathode is replaced by an ODC, in which added oxygen reacts with water in a three-phase process, forming hydroxyl ions. This results in reduced voltage with the same current density, leading to less energy consumption.

Bayer has commercially demonstrated a process for producing chlorine using an ODC. A 215,000 metric tons/yr plant has been built in China, says Smith.Bayer's oxygen depolarized cathode (ODC). Image source: BayerBayer's oxygen depolarized cathode (ODC). Image source: Bayer

"The claimed primary economic benefit is a 30% reduction in electricity consumption when compared to a conventional membrane cell," he says.

Dow uses its own cell design of the filter press bipolar type. Smith says due to Dow's production capacity being concentrated in a few sites, its cell development has followed a different path than for other chlor-alkali technology developers.

“New bipolar membrane cell designs incorporate the zero-gap concept," he says. This eliminates the electrolyte gap between the electrodes and the membrane.

Historically, Dow pioneered the development of bipolar filter-press cells due to advantages that included minimization of the bus between the cells and floor space, and low capital cost. The original patents were issued in 1911 and 1913.

Another design concept is the single-element design of the Uhde bipolar electrolyzer technology. In this approach, each element consists primarily of an anode and a cathode separated by an ion-exchange membrane. Smith says that unlike the filter press design concept used in other electrolyzers, each element is individually sealed by means of a separately bolted flange with gasket. This enables long-term storage of fully assembled elements in working order.

The Next Step

Multiple studies have been undertaken to explore how best to economically and optimally produce chlorine and caustic soda in an environmentally safe way. One technique that may hold promise incorporates fuel cell concepts and electrocatalyst development.

The principle behind a fuel cell is simple: It works as a static device for turning the chemical energy of hydrogen and oxygen in the air into current electricity, water and heat. A fuel cell reverses the electrolysis process in such a way that the electric current breaks down water into its elemental oxygen and hydrogen gases. Fuel cells system integration into a chlor alkali plant can save up to 20% electricity.

Another option could involve using renewable energy conversion systems. "A somewhat new concept in the industry is investing in wind projects," says Eisenhauer. Recently, Dow announced a power purchasing agreement with a wind farm to supply electricity to its Freeport, Texas, plant.