Inorganic Chemicals - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 13

dominant in the twentieth century, but has been declining. For example, in the
U.S., Olin has announced that they will convert their mercury cell capacity by
the end of 2012. This will leave Ashta Chemicals in Ashtabula, Ohio and PPG
Industries in Natrium, West Virginia as the only remaining U.S. mercury cell
producers [25]. One major reason is that environmental considerations make
the use of large quantities of mercury less desirable. The mercury process
also consumes more electricity than the other two processes. In a diaphragm
process, the cathode and anode are separated by a permeable diaphragm. The
membrane process is the newest of the three processes and is considered the
most environmentally friendly and energy efficient. As the mercury process
is phased out, the membrane process is becoming the dominant process.
A membrane is a layer of material that selectively allows one component
to permeate through. In the case of the chloralkali process, there are
many requirements that need to be met in the selection of the membrane
material. The membrane needs to be chemically stable to chlorine and
hypochlorite anion even at the high process temperatures of about 100 ∘ C.
There needs to be a low resistance which gives a low voltage drop across
the membrane. Selectivity is important. The membrane needs to selectively
allow sodium ions to pass through. Because of the demanding requirements,
chloralkali membranes commonly employ perfluorinated polymers having
carboxylate or sulfonate groups. These can be prepared by copolymerizing
tetrafluoroethylene, a perfluorovinyl ether, and a perfluorinated comonomer
containing, for example, a sulfonyl fluoride group which is subsequently
hydrolyzed to a sodium sulfonate group. The perfluorosulfonic acid polymers
were invented by Dupont scientists and commercialized under the trade
name Nafion®. ® .
The anode and cathode are separated by an ion exchange membrane which
allows sodium ions to pass through. Sodium ions pass through the membrane
to the cathode where water is reduced to hydrogen and hydroxide. Therefore
high purity sodium hydroxide is prepared in the cathode compartment.
Chloride does not pass through the membrane and is oxidized to chlorine in
the anode compartment.
The major use for chlorine is the chlorination of ethylene to make ethylene
dichloride (this is always referred to as ethylene dichloride, but the more
proper name is 1,2-dichlorethane) by addition of chlorine to the double bond.
Ethylene dichloride, or EDC, is converted into vinyl chloride, which is then
converted to polyvinyl chloride (PVC) polymer. There are many uses for
PVC, but much of the demand is in the building and construction industry
for items such as plumbing, vinyl siding, and window frames. Therefore
the robustness of the construction business has a major effect on PVC
demand and, in turn, chlorine demand. Other uses for chlorine include water
purification, titanium dioxide production, phosgene production and organic