Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TMPROVED METHOD A~D CûMrûSI T Iûi~ FOIR
REMûVIN~ SULFIDE-CONTAINING SCALE FRûM METAL SURFAC~S
This invention pertains to an improved me~hod
of chemically cleaning acid-soluble, sulfide-containing
scale from metal surfaces. The improved process utilizes
a new cleaning solution which contains an aqueous acid and
glyoxyl~ic acid. The glyoxylic acid is present in amounts
sufficient to prevent or substantially prevent the
evolution of hydrogen sulfide gas.
Many sources of crude oil and natural gas contain
high amounts of hydrogen sulfide. Refineries processing
such crude oil or natural gas commonly end up with sub-
stantial amounts of sulfide-containing scale on the metal
surfaces in contact with the crude oil or gas. This scale
is detrimental to the efficient operation of heat exchangers,
cooling towers, reaction vessels, transmission pipelines, ~
furnaces, etc. Removal of this sulfide-containing scale
has been a substantial problem because conventional acid-
cleaning solutions react with the scale and produce gaseous
20 hydrogen sulfide.
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Hydrogen sulfide gas produced during the
cleaning operation 1eads to several problems.
First, hydrogen sulfide is an extremely toxic gas
and previous techniques have required the entire
system to be vented to an appropriate flare system
(in which the gas is burned) or to a sodium hydroxide
scrubbing system. Neither of these alternatives is
very attractive because the sulfur dioxide and
sulfur trioxide formed during the burning of hydrogen
n sul~ide are substantlal pollutants in and o~ themsel~,es.
The sodium sulfide produced during the scrubbing
system is a solid that presents disposal problems.
It can be land-filled or put into disposal ponds
but only under conditions such that the sodium
sulfide does not contact acid. Sodium sulfide
reacts rapidly with acids to regenerate hydrogen
sulfide. Second, aside from the toxic nature of
hydrogen sulfide, the material causes operational
problems as well because it is a gas. The volume
of gas produced can be substantial. The gas takes
up space within the unit being cleaned and can
prevent the liquid cleaning solution from coming in
contact with all of the metal surfaces. This can
occur, for example, in cleaning a horizontal pipeline
where the gas can form a "pad" over the top of the
flowing liquid and prevent the liquid from filling `
the pipeline and cleaning the entire surface. The
gas produced can also cause the pumps used in the
system to cavitate, lose prime, and/or cease to
function efficiently. And, of course, if enough
gas is generated in a confined vessel the vessel
can rupture.
These problems have been encountered in
the industry and are severe.
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Hydrogen sulfide and acid cleaning solutions
containing hydrogen sulfide can cause severe corrosion
problems on ferrous metals. The corrosion can be
due to attack by acid and/or ferric ion corrosion.
These corrosion problems have been met in the past
by including minor amounts of corrosion inhibitors
in the system. Aldehyde and aldehyde condensation
products (normally with an amine) have been used as
corrosion inhibitors in various systems. For
example, they have been used alone or in combination
with other corrosion inhibitors in aqueous acidic
cleaning solutions and pickling baths or as an
additive to crude oil. Under these systems, however
the aldehyde was included in very minor amounts.
The following patents are representative of how
these aldehydes have been previously used in this
regard: USP 2,426,315; USP 2,606,873; USP 3,077,454;
USP 3,514,410; and USP 3,669,613.
The reaction of hydrogen sulfide with an
aldehyde is a known reaction which has been the
subject of some academic interest. See, for example,
the journal articles abstracted by Chemical Abstracts
in C.A.54:17014h; C.A.63:14690a; C.A.65:9026d. The
references indicate that the product formed by
hydrogen sulfide with formaldehyde is trithiane or
low polymers. This product was also referred to in
USP 3,669,613 cited above. In these references,
the product was produced by bubbling hydrogen
sulfide through the aqueous acid/formaldehyde
systems and the patent indicates that the reaction
should not be attempted at temperatures greater
than about 45 C. The patent also indicates that
the reaction usually reaches completion in from
about 5.5 hours to about 9.5 hours at ambient
temperatures.
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None of the references taught or suggested
the unique phenomenon observed by Frenier et al. and
described in their U.S. Patent 4,220,550.
Frenier et al. discovered that acid-
soluble, sulfide-containing scale could be effectively
removed from metal surfaces without the release of
gaseous hydrogen sulfide by use of an aqueous acid
cleaning composition comprising an aqueous non-
oxidizing acid having at least one aldehyde dissolved
or dispersed therein, with the aldehyde being present
in such compositions in an amount at least sufficient
to prevent or substantially prevent the evolution of
hydrogen sulfide gas. This required at least a stoi-
chiometric amount of aldehyde in the cleaning solu-
tion (i.e., at least one mole of aldehyde per mole
of hydrogen sulfide produced during the cleaning)
and an excess of aldehyde was preferred. By excess
was meant, amounts beyond stoichiometric required
and up to one equivalent weight of aldehyde or more
per equivalent weight of acid. Their best system
was an aqueous sulfuric acid cleaning solution con-
taining excess formaldehyde.
Frenier has described an improvement over
U.S. Patent 4,220,550. The improvement there
described comprises the method of generating the
aldehyde in s:itu during the cleaning process. By
conducting the cleaning process in this manner,
procedural problems associated with the handling of
certain aldehydes (e.g. formaldehydes) was alleviated.
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A novel chemical clea~ing solution has now
heen discovered which comprises an aqueous non-
-oxidizing acid having dissolved therein, glyoxylic
acid.
More particularly, the invention resides in
an aqueous non-oxidizing acid having glyoxylic acid
dissolved therein; said composition having, as one of
its chemical properties, the capability of dissolving
acid-soluble, sulfide-containing scale from a metal sur-
face without the evolution of hydrogen sulfide gas.
The invention further resides in the method
of chemically cleaning acid-soluble, sulfide-containing
scale from a metal surface comprising contacting said
scale with an aqueous acid cleaning composition compris-
ing an aqueous non-oxidizing acid having at least one
aldehyde dissolved or dispersed therein, which aldehyde
is present in an amount at least sufficient to prevent
or substantially prevent the evolution of hydrogen sul-
fide gas, the improvements comprising using glyoxylic
acid as said aldehyde.
The chemical cleaning solution of the invention
is a unique species within the generic disclosure of
Frenier et al. tUSP 4,220,550) in that the reaction pro-
duct(s) of the chemical cleaning solution and the sulfide-
-containing scale (e.g., iron sulfide) are soluble in the
acidic cleaning medium and do not form solid precipitates.
Under treatment conditions, the novel cleaning composi-
tion removes the acid-soluble sulfide-containing scale
from metal surfaces at a rate substantially equal to or
greater than the preferred cleaning composition described
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by Frenier et al. (i.e., solutions comprising aqueous
sulfuric acid with excess formaldehyde). Because of
these factors, the use of the new cleaning composition
in the method described by Frenier et al. results in
an improved cleaning process.
The Frenier et al patent (USP 4,220,550)
describes generically the chemical cleaning composi-
tions, the relative ratio of ingredients, and methods
of use. The primary distinction between the present
invention and the Frenier et al invention resides in
the use of the glyoxylic acid as the aldehyde in the
cleaning solutions. Glyoxylic acid appears to be a
unique species within the genus of aldehydes described
in Frenier et al. Glyoxylic acid corresponds to the
chemical formula
O O
H-C~C-OH.
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As stated in Frenier et al. the acidic
cleaning solutions can utilize a variety of acids
usually at concentrations ranging from about 5 to
about l5 percent. Sulfuric acid and hydrochloric acids
are preferred, and hydrochloric acid is most preferred
in this instance. Frenier et al. also teach the addi-
tion of a compatible acid corrosion inhibitor to the
acid cleaning solution (preferably an amine-based corrosion
inhibitor~ and emphasize the advantage of using the
10 aldehyde in excess in the cleaning solution. The same
teaching similarly applies here. Glyoxylic acid is added
in an amount sufficient to prevent or substantially prevent
the evolution of gaseous hydrogen sulfide when the
cleaning solution is brought in contact with ~n acid-
15 soluble, sulfide-containing scale.
The following example further illustrates the
invention: Example 1: A solution of glyoxylic acid
(7.5 grams ~g) and water ~102 milliliters ~mL) was charged
to a reaction vessel equipped with a gas scrubber con-
20 taining 25 percent aqueous sodium hydroxide. The temper-
ature of the glyoxylic acid/water solution was raised to
150 F in a water bath and iron sulfide ~FeS, 7O5g) was then
added. After the temperature of this mixture reached
150 F, 35 mL of concentrated ~36 percent) hydrochloric acid
25 was introduced and the vessel was quickly sealed. When
the acid was first added, there was a brief initial ;mell
of hydrogen sulfide but no detectable amounts of hydrogen
sulfide after that. Analysis of the sodium hydroxide
scrubbing system using an ORION S electrode gave a
30 zero reading for sulfide. The cleaning solution
dissolved all of the iron sulfide and the spent cleaning
solution was a clear liquid without any noticeable
amounts of solid precipitate. No evolution of hydrogen
sulfide gas was observed during the three hour test.
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