Page 392 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN009K-419 July 19, 2001 20:57

Membranes, Synthetic, Applications 327

3. Double Decomposition
Double decomposition is similar in concept to the sub-
stitution reaction, except that both anion-exchange and
cation-exchange membranes are employed. Simultaneous
interchange of anion–cation pairing takes place to form
products that would otherwise require multistep proce-
dures to prepare and purify. Pure materials can be pro-
duced from crude raw materials by means of double
decomposition, and reactions otherwise impractical by
conventional reaction methods can be performed. An ex-
ample application is the reaction between potassium chlo-
ride and sodium nitrate to produce potassium nitrate and
sodium chloride.
FIGURE 42 Construction of a chlor-alkali membrane unit for elec-
trolysis of brine. (Du Pont Company.)
4. Bipolar Membrane Syntheses
A bipolar membrane consists of a cation-exchange layer
across a cation-exchange membrane to form caustic soda, and an anion-exchange layer, separated by a thin water-
and chloride ions react at the anode to form chlorine gas. filled space. Placing this membrane between cation-
The construction of a commercial chlor-alkali membrane- exchange membranes and electrodes in the orientation
cell assembly is shown in Fig. 42. shown in Fig. 41(d) forms a special electrochemical
The use of polyperfluorosulfonic acid membranes as cell. When direct current is passed through the cell,
the cell separator was first demonstrated about three water between the two layers of the bipolar membrane
decades ago. Yet it was not until the mid-1980s when electrolyzes to release protons and hydroxyl ions into
the economic advantages of membrane cells over the adjoining compartments, where they participate in subs-
traditional mercury- and diaphragm-cell technology were titution reactions. Bipolar membrane technology may be
fully demonstrated—consequent to better membrane considered a second-generation electrochemical synthesis
performance, higher caustic product concentrations, because of its versatility: different arrangements of bipo-
and lower power consumption. Retrofitting chlor-alkali lar membranes together with cation- or anion-selective
facilities with membrane cells accounted for much of the membranes can separate a salt into its constituent acid and
growth and sustenance of this industry over the past two base, or produce purified acid or base streams. Several of
decades. these schemes are shown in Table IX.
By forming an electrolytic cell with both an anion- The schematic shown in Fig. 43(a) is a commercial ex-
exchange membrane and a cation-exchange membrane, ample of this technology. Stack gas is scrubbed with an
acid and alkali can be generated simultaneously. The alkaline solution of sodium hydroxide, sodium sulfite, and
method applies to inorganic salts (as illustrated) and or- sodium sulfate. The sodium sulfite reacts with SO 2 in the
ganic salts (e.g., sodium citrate converted to citric acid and stack gas to form sodium bisulfite. This salt solution is
sodium hydroxide). processed in a bipolar membrane unit [Type (I) shown in
Table IX] to generate an alkaline solution and an acidic so-
lution. The alkaline solution contains regenerated caustic
soda and sodium sulfite, and can be recycled to the scrub-
2. Substitution Reactions
ber, while the sulfurous acid can be further processed to
In substitution reactions, solutions of a salt and an acid sulfur or sulfuric acid for sale.
with the same anion are fed through alternate compart- Bipolar membrane synthesis also holds promise for re-
ments of an array of cation-exchange membranes. The generating spent pickling liquors in stainless steel man-
dissociated metal ions from the salt are removed and re- ufacture. As shown in Fig. 43(b), waste acid laden with
placed by protons to generate the free acid. For exam- metal ions can be continuously neutralized, filtered to re-
ple, amino acids are produced from their sodium salts in move the precipitated metal oxide, and the clarified salt
this way. Compared with conventional neutralization and solution split into its acid and base components in a bipo-
recovery techniques, the membrane-mediated process is lar membrane unit [Type (IV) shown in Table IX]. As
considerably simpler and gives a higher yield of the puri- much as 95% of the hydrofluoric and nitric acid used
fied product. are returned to the pickling bath, thereby solving a waste