Chlor-Alkali Plant Expansion, Frankfurt Hoechst, Germany

In 1997, LaRoche, the American industrial ammonia distribution company which manufactures chlor-alkali in Europe, bought Celanese's chlorine plant at the Frankfurt Hoechst Industrial Park.

The plant was run by the subsidiary LII Europe, which was acquired by Akzo Nobel Industrial Chemicals in January 2009.

The plant produces chlorine, caustic soda and chlorinated methane for the manufacturing of specialty chemicals and pharmaceuticals.

In early 2002, LII Europe (now Akzo Nobel) announced a major expansion and replacement of the plant to process 200,000t of chlor-alkali through membrane electrolysis technology. The conversion of the plant was completed in 2004.

In June 2011, AkzoNobel announced an €140m conversion of the plant to increase the production capacity by 50%.

The upgraded facility will employ the latest membrane technology and produce around 250kt of chlorine annually. The renovated plant is expected to be operational in the fourth quarter of 2013.

The increasing requirements in Europe for chlorine and derivatives over the past five years means the current plant is not meeting demand. This expansion should help solve this problem. In Japan and Taiwan the membrane process is almost universal, but in the US it only penetrates around 20% of chlorine manufacturing.

Membrane electrolysis technology

"The plant was run by the subsidiary LII Europe, which was acquired by Akzo Nobel Industrial Chemicals in January 2009."

Membrane electrolysis technology was introduced to process caustic soda, chlorine and hydrogen. Currently the plant uses mercury-processing technology, although this is being phased out all over Western Europe.

The membrane process uses high purity brine to produce a high quality product without environmental repercussions. Developed by polymer chemists, the process currently produces around one-tenth of Europe's chlorine.

The membrane itself is very expensive to produce and Asahi Kasei, the Japanese company that developed the commercial production facility, supplies the technology to globally produce more than five million tons of caustic soda a year. Asahi Kasei began the first commercial chlor-alkali production in 1975 using the membrane electrolysis process.

The Frankfurt Hoechst plant will use the Aciplex F perfluorosulfonic acid membrane. The membrane process for chlor-alkali electrolysis enables greater energy efficiency.

Lurgi Life Sciences, a strategic business unit of Lurgi AG, has been chosen as general contractor and engineering partner and was responsible for construction and environmental controls. The contract for the lump-sum turnkey project was signed in mid-2002. The sub-contract that had been agreed in the second quarter of 2002 was with Infra-Serv Hoechst who provided on-site services.

The electrolysis cells used in modern chlor-alkali plants are large, the cross-section of a single membrane cell can be up to 5m². Each cell can produce 0.65mg/day of chlorine. Ion-exchange membrane cells, using inert diametrically stable electrodes, are replacing those cells using liquid-film mercury cathodes.

The older process leaves traces of mercury in the products and the caustic soda solutions. Traces of mercury released into the environment become highly toxic organomercury compounds, which accumulate in fish and other aquatic organisms.

Mercury processing

Chlorine and caustic soda are the main products made from the chlor-alkali electrolysis technology. Mercury is used as a negative electrode or cathode that works with a titanium anode to keep the highly reactive products involved apart when electricity is passed through brine.

As mercury is extremely toxic, 100% needs to be recycled within the plant to ensure there are no dangers to the environment. The Frankfurt Hoechst plant was one of the cleanest chlorine plants and their decision to upgrade the plant was influenced by the need to expand and the costs of running the plant.

Although chlor-alkali plants are only responsible for 5% of mercury emissions in the atmosphere, chlorine producers in Western Europe have agreed not to build any new mercury processing plants, with many looking at economically viable alternatives in order to comply with a total phase out of the mercury process by 2020.

Therefore one-third of the current chlorine producers using mercury-based process will soon be looking at incorporating this kind of technology to their sites in the near future.

Hydrogen, caustic soda and chlorine products

Chlorine and caustic soda are the base elements of 60% of Europe's chemical industry production turnover, making them the key ingredients in most of the products made by the industry.

"In June 2011, AkzoNobel announced an €140m conversion of the plant to increase the production capacity by 50%."

Chlorine (Cl) cannot exist naturally by itself because it is highly reactive and one of the most plentiful elements in the world. Most chemical processes require it for synthesis, which is why it is being produced on the chemicals and pharmaceuticals industrial site at Frankfurt Hoechst, where it can easily be distributed by pipeline for long-term provision to the other processing companies.

It is used in all industries and in the home in solvents, pharmaceuticals, water treatment, plastics, acids and many more.

Caustic soda (sodium hydroxide - NaOH) is a very versatile alkali and is used as a reactant in the manufacturing of various sodium compounds. For example, it is used to produce sodium hypochlorite, which is used to produce various bleaches and disinfectants.

Caustic soda is used in many industries where it is used to control pH, to breakdown cellulose, in cleaning operations. The main industries where it is used include rayon cellophane, soap and pulp and paper.

Hydrogen is the other major product formed in the chlor-alkali electrolysis process. Because of its high energy non-toxic nature, hydrogen can be used in many processes without any negative environmental impacts.

Lurgi was the general contractor for the LII Europe conversion project, using the well-established AKC membrane technology from Asahi Kasei. The plant was operational by the end of 2004.