Note: Descriptions are shown in the official language in which they were submitted.
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Odor-Fading Prevention From Organosulfur-
Odori2ed Li uefied Petroleum Gas
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(IR 2855)
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Background of the Invention
S This invention relates to a process for the prevention
of odor-fading from organosulfur chemical-odorized liquefied
petroleum gas (LPG) stored in carbon steel containers. More
particularly, it relates to the process of deactivating the
steel surface of a new or recently cleaned storage container
for organosulfur chemical-odorized LPG by treating said
surface with a deactivating agent prior to the exposure of
said walls to said LPG.
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Commercial LPG is customarily odorized with an organo-
sulfur odorant, very co..~only ethyl mercaptan. Containers
for storing LPG are usually steel or steel-lined cylinders
or tanks which, when new or recently cleaned, have inner
surfaces which may react with the sulfur odorant to form a
complex chemical build-up on the walls of the container.
As a consequence of this complex formation, there is a
depletion of the odorant level in the LPG. This condition,
sometimes referred to as "odor-fading", can be hazardous
to those unknowingly exposed to LPG in the atmosphere.
PRIOR ART
It i9 known that the integrity of trace amounts of
n-butyl mercaptan is well-preserved in a gas chromatographic
transfer system where the stainless steel chromatographic
column is first treated with a solution of benzotriazole in
an inert solvent such as isopropanoll.
In addition, benzotriazole, tolyl triazole, mercapto-
benzothiazole and similar chemical compounds are well known
corrosion inhibitors often employed in lubricant compositions,
1 _ Gramshaw, J.W. and Hussain, A., "Deactivation of a
Metal Transfer Line between a Gas Chromatographic Column and
a Flame Photometric Detector" - Progress in Flavor Research,
Applied Science Publishers, 1979.
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engine coolers, specialty cleaners, hydraulic fluids and the
like, especially on copper or copper-bearing substrates.
Statement of the Invention
This invention is a process for the prevention of odor-
fading from organosulfur-odorized liquefied petroleum gas
stored in containers having active interior steel surfaces
which comprises treating said surfaces with a deactivating
amount of benzotriazole, tolyl triazole, mercaptobenzothiazole,
benzothiazyl disulfide or mixtures thereof, preferably drying
the container, and charging the container with said liquefied
petroleum gas odorized with ethyl mercaptan, thiophane,
dimethyl ~ulfide, or other reduced organosulfur compounds
containing one to five carbon atoms.
Detailed Description of the Invention
This process reduces or eliminates odor-fading from
sulfur-odorized LPG stored in containers having active (new
or recently cleaned) interior steel surfaces. By "active....
interior steel surfaces" is meant inner steel surfaces of
containers which surfaces have not been deactivated to
reacting with sulfur chemicals to form a chemical complex
build-up thereon. Such surfaces may occur in containers
which have either never been used to store organosulfur-
odorized LPG, used only several times for such storage,
or have recently been cleaned during extended service.
Additionally, containers which have been treated either
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chemically or mechanically to remove the deactivating
chemical complex are included within the term.
The organosulfur-odorants used herein include ethyl
mercaptan, thiophane, and dimethyl sulfide. Ethyl
mercaptan, because of its greater use as a commercial
odorant, and because of the inherent reactivity of
mercaptans as a class, can benefit the most by the
metal-deactivating process of this invention. Ethyl
mercaptan, thiophane and dimethyl sulfide are generally
employed as odorants for LPG, either individually or in
blends with each other. The organosulfur-odorant is usually
used to obtain an initial odorant loading of the LPG of
between about 25 and 150 ppm (wt., liquid phase) depending
upon the odorant employed.
As taught in U.S. Patent No. 3,826,631, ethyl mercaptan
and dimethyl sulfide may be combined with an inert chemical
capable of forming a minimum boiling point azeotrope. For
example, these inert chemicals include methyl formate,
n-pentane, isopentane, amylene, isoamylene, chloropropane
or mixtures of these. The azeotrope-former is used in an
amount which is at least sufficient to form an azeotrope
with the amount of organosulfur-odorant to be mixed wlth
the LPG and preferably in a slight excess as described in
the above-mentioned U.S. Patent No. 3,826,631.
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- LPG or liquefied petroleum gas is a well known substance consisting of propane, butane, isobutane and mixtures of
these and other lower hydrocarbons in various proportions.
The deactivating chemicals that are used in the process
of this invention are benzotriazole, tolyl triazole,
mercaptobenzothiazole, and benzothiazyl disulfide. These
materials are solids which must be converted to liquid form, -
preferably by dissolving in an inert solvent, for practical
application to the steel inner surface of the container for
LPG. Solvents for this purpose include, for example,
alcohols, ethers, ketones, esters, and similar liquids.
More particularly, they include, for example, isopropanol,
propylene glycol, acetone, and the like.
The concentration in the solvent solution will depend
on the particular deactivator employed and the length of
time the inner steel surfaces of the container for LPG are
exposed to the solution. In general, a concentration of
deactivator in the solvent will range from about 5% up to
the weight limit of solubility of the deactivating chemical
in the selected solvent. Preferably, from about 20 to about
30% by weight of the deactivator in the solvent may be
employed.
The deactivator solution is used in an amount which
will at least coat the walls of the container and duration
of the deactivator treatment can vary from as little as
about 15 minutes up to many hours, for example, a full day
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or more. However, a range of from several hours up to about
24 hours, depending on the deactivator concentration and
temperature of the solution or steel walls during treatment,
is advantageous. The temperature during treatment can range
from room temperature up to that above which the solvent
boils. A temperature within the range of from about 20 to
about 50 C is generally satisfactory.
After treatment of the container walls with the deacti-
vator solution, the solution is drained and the container
may be purged with an inert gas, for example nitrogen or
odorized propane, or simply allowed to dry.
The following examples demonstrate the process of this
invention.
Example 1
A 20% by weight solution of benzotriazole in isopropanol
was prepared and 50 ml. of this solution was charged into a
new 360 ml. carbon steel container designed for LPG storage.
The co~tainer was rolled in place for 24 hours, drained,
capped and allowed to stand for 48 hours. After 48 hours,
the cap was removed and the container first gently purged
with nitrogen for about S minutes, flushed twice with
liquefied petroleum gas (~5-30 sec. exposure) and then
filled under pressure to 75% of the container volume.
Example 2
100 ml of a 20% by weight solution of benzotriazole
in isopropanol was charged to a new 360 ml. carbon steel
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- container designed for LPG storage. The container was
rolled in place overnight (21 hours) drained and purged
for 15 minutes, twice, immediately following drainage.
The container was then filled under pressure with LPG
to 77% of the container volume.
Tolyl triazole, mercaptobenzothiazole, benzothiazyl
disulfide, and mixtures of these deactivators with each
other and with benzotriazole can be advantageously employed
to replace the benzotriazole in Examples l and 2.
The use of ethyl mercaptan and similar organosulfur-
odorants to precondition or deactivate the inner steel
surfaces of containers for LPG has been contemplated J
however, such method is not practical because of the highly
objectionable odor of this material. Avoiding escape of
the odor in the treatment process would require expensive
handling equipment and, should the odorant accidentally
escape, a severe environmental problem could ensue.
