Chemicals Tank of the Month - #2 Acids, Storage Design Considerations.
Acids, acetic, sulphuric, hydrochloric, hydroflourosilicic HF, highly
corrosive and should be used with EXTREME CARE.
Each acid has different characteristics in process and potentially their
effect on human beings.
your goals with the chemical provider's technical group, a good starting
source of "Things to consider".
One of the first things to do when
planning to use an acid is to study/consult the MSDS (Material Safety Data
Inhalation of Acid Fumes
from a chemical can be as dangerous as contact or ingestion, Extreme care is
to be exercised when designing a storage system for acids and related
When filling a
storage tank, understand that the air inside the tank has become similar to
the chemical itself.
in an acid storage tank will have become acidic and requires some degree of
neutralization or scrubbing to reduce the effect this vent air will have on
the enviornment, humans and equipment. Venting the contents of an acid tank
through a scrubber, (water/air contact scrubber) will remove or lessen the
Ph of the exhaust air.
vessel should be designed to accommodate the aggressive chemical nature of
the particular acid. Plastic tanks, fiberglass and polyethylene do a
tremendous job of providing long terms storage at economical costs. SPECIAL
care is to be given, chemical charts NACE (National Association of Chemical
Engineers) as well as process connections, gaskets an process piping.
Many acid produce heat (exothermic) when diluted with water, this heat can
Note: when combining
chemicals a chemical compound is produced, this compound may have more
aggressive characteristics than the acid in its pure form. Know what
Sodium hypochlorite, bleach in
polyethylene plastic tanks...
AND SODIUM HYDROXIDE
25 years successful experience storing Caustic and Bleach 12-15 % Sodium
Hypochlorite in linear polyethylene 1.9 specific gravity tanks.
Chlorine (bleach) and sodium hydroxide (Caustic) are both in the top twenty
products manufactured by the U.S. chemical industry. In 1999, 10.4 billion
kilograms of sodium hydroxide and 12.1 billion kilograms of chlorine were
produced. These two chemicals are presented together here, because
industrially they are produced simultaneously by the same process, the
electrolysis of brine (aqueous NaCl).
2 Na+(aq) + 2 Cl¯(aq) + 2 H2O(l)
+ H2(g) + 2 Na+(aq) + 2 OH¯(aq)|
electrolysis, chlorine is formed at the anode, and hydrogen and
hydroxide ions are formed at the cathode.
2 Cl¯ Cl2 + 2 e¯
2 H2O + 2 e¯ H2 + 2 OH¯
the Cl2 formed at the anode and the H2 formed
at the cathode can react explosively, they must be kept away from
each other. Furthermore, the hydroxide ions formed at the cathode
can react with any chlorine that remains dissolved in the brine. To
keep the products formed at the two electrodes away from each other,
a porous diaphragm is placed between the two electrodes in the
electrolysis of brine, water is reduced at the cathode. This occurs
because water is more easily reduced than are sodium ions. This is
reflected in their
standard reduction potentials, ï¿½2.71 volts for Na+
versus ï¿½0.83 volt for water. At the anode, where oxidation occurs,
the situation is not as clear. The standard oxidation potential of
water is ï¿½1.23 volts, while that for chloride ions is ï¿½1.36
volts. This means that water is more easily oxidized than chloride
ions. In spite of this, chloride ions are oxidized at the anode, not
water. The reaction that occurs is not what would be predicted by
considering only the standard oxidation potentials because
standard electrode potentials reflect equilibrium conditions,
when no current is flowing. When current begins to flow, the
distribution of ions around the electrodes changes, and the
equilibrium electrode potentials no longer accurately apply. The
potential of a cell depends on the magnitude of the current that is
flowing through it. The difference between the equilibrium
potential, at zero current, and the potential when current flows is
called over voltage. The magnitude of the over voltage depends on
the composition of the electrode and electrolyte, as well as on the
current. Generally, over voltages are small, so predictions of
electrode reactions based on standard electrode potentials are
usually correct. However, in the electrolysis of aqueous
sodium chloride, the over voltage for the oxidation of water,
which is a neutral molecule, is large enough to make it more
difficult to oxidize than chloride ions. (The large over voltage for
the oxidation of water also allows lead storage batteries to be
recharged. If it were not for this large over voltage, the charging
current would oxidize water to oxygen gas instead of PbSO4
50% (by weight)
sodium hydroxide solution is obtained by concentrating the
electrolyte removed from the brine electrolysis apparatus. The
solution is concentrated by heating it to boil off the water. Solid
sodium hydroxide can be obtained from the solution if all of the
water is removed.
chlorine gas and hydrogen gas are collected separately and
piped away from the electrolysis apparatus. The chlorine is dried,
compressed, and liquefied for shipping and storage. Although the
hydrogen can be compressed and stored in cylinders, the commercial
value of hydrogen in not sufficient to warrant this. The hydrogen is
usually burned at the electrolysis plant to provide the
thermal energy used to evaporate water from the sodium
At room temperature, chlorine is a yellow-green gas. It condenses to
a liquid at -34°C and freezes to a solid at -101°C. It has a
piercing, irritating odor, and is caustic to mucous membranes such
as the eyes and lungs. Chlorine is moderately soluble in water, and
aqueous solution contains 0.062 M Cl2, 0.030 M
hypochlorous acid (HOCl), and 0.030 M chloride ions.
Cl2(aq) HOCl(aq) + H+(aq)
Treating this solution with a hydroxide, such as NaOH or Ca(OH)2,
produces a solution containing the hypochlorite ion.
Cl2(aq) + 2 OH¯(aq) OCl¯(aq) +
Cl¯(aq) + H2O(l)
Sodium hypochlorite is the active ingredient of "chlorine" laundry
calcium hypochlorite is used as a disinfectant in swimming pools.
About 20% of the
chlorine produced industrially is used in the manufacture of chlorinated
plastics (mainly polyvinyl
chloride). Another 15% is used in the production of solvents (such as
carbon tetrachloride), 5% in the manufacture of paper, 5% in water
treatment, and the remainder in the production of a variety of other
Gaseous chlorine reacts with hydrocarbons to form chlorinated
hydrocarbons. Chlorine will replace the
hydrogen atoms in methane, CH4, sequentially producing chloromethane,
CH3Cl, methylene chloride, CH2Cl2,
chloroform, CHCl3, and carbon tetrachloride, CCl4. The
last three of these products are important solvents in the chemical
industry. Another important
chlorinated hydrocarbon is
vinyl chloride, CH2=CHCl. About 10 billion pounds of this
substance are manufactured each year, to be turned into polyvinyl chloride,
CH2CHCln. Polyvinyl chloride is used extensively for
piping and plumbing, raincoats, shower curtains, magnetic tape, flooring,
and electrical wire insulation.
Chlorine reacts (explosively, under certain conditions) with
hydrogen to produce hydrogen
Cl2(g) + H2(g) 2 HCl(g)
chloride is extremely soluble in water, forming a solution called
hydrochloric acid. A
saturated solution at 25°C is 12 M in HCl. When it dissolves in
water, HCl ionizes completely, forming H+ and Cl-
Sodium hydroxide is a white solid with a melting point of 3l8°C. It
is very soluble in water. A saturated solution is 18 M in NaOH at 20°C.
Solutions of sodium hydroxide are very corrosive to the skin and other
organic matter. Hence, its name in commerce is
The major industrial uses of sodium hydroxide are in the manufacture of
chemicals (60%), in the paper industry (20%), in the production of aluminum
(5%), and the manufacture of
soaps and detergents (5%).
industry uses the caustic effects of sodium hydroxide on organic materials.
Sodium hydroxide breaks down the lignin in wood.
Lignin is a binder that holds cellulose fibers together in wood. When
the lignin is removed, the freed cellulose fibers can be formed into paper.
The digestive effects of sodium hydroxide on organic materials is the
principle behind such drain
cleaners as Liquid
Plumber, which is a concentrated aqueous solution of sodium
hydrolyzes the ester linkage in
fats and oils to produce glycerol (HOCH2-CHOH-CH2OH)
and sodium salts of
carboxylic acids containing long chains of carbon atoms (e.g.,
stearic acid, CH3(CH2)16COOH). These
salts are soaps. Similar salts of synthetic acids containing long chains of
carbon atoms are called detergents.
TanksystemsCom, LLC has over 25 years successful experience storing 12-15 %
Sodium Hypochlorite in linear polyethylene 1.9 specific gravity tanks.