Note: Descriptions are shown in the official language in which they were submitted.
IMPROVED METHOD AND COMPOSITION FOR
REMOVING SULFIDE-CONTAINING SCALE
FROM METAL SURFACES
This invention pertains to an improved method
of chemically cleaning sulfide-containing scale from
metal surfaces. The novel process utilizes aqueous
acid cleaning solutions containing an aldehyde in
amounts sufficient to prevent or substantially prevent
the evolution of hydrogen sulfide gas, and the aldehyde
is generated in situ.
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 substantial 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 exchanyers, cooling towers, reaction vessels,
transmission pipelines,~furnaces, etc. Removal of this
sulfide-containing scale has been a substantial problem
because conventional acid-cleaning solukions react with
the scale and produce gaseous hydrogen sulfide.
Hydrogen sulfide gas produced during the
cleaning operation leads to several problems. First,
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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 ~o a sodium hydxoxide scrubbing system.
Neither of these alternatives is very attractive because
~he sulfur dioxide and sulfur trioxide formed during
the burning of hydrogen sulfide are substantial pollutants
in and of themselves. The sodium sulfide produced
during the scrubbing system is a solid that presents
disposal problems. It can be lan~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 pxoduced can be substantial.
The gas ta~es 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 hori~ontal 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, i~ enough gas is generated in a confined
vessel the vessel can rupture.
These problems have been encountered in the
industry and are severe.
Hydrogen sulfide and acid cleaning solutions
containing hydrogen sulfide can cause severe corrosion
problems on ferrous metals. The corrosion can be due
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to attack by acid and/or ferric ion corrosion. These
corrosion problems have been met in the past by including
minor amounts o 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 2,426,318; USP 2,606,873; USP 3,077,45~; USP 3,514,4~0;
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 refer-
ences 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 system~ and the patent indi-
cates that the reaction should not be attempted at
temperatures greater than about 45C. The patent also
indicates that the reaction usually reaches completion
in from about 5-1/2 hours to about 9 1/2 hours at
ambient temperatures.
None of the reerences taught or suggested
the unique phenomenon observed by Frenier et al. and
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described in their U.S. Patent No. 4,220,550, issued
September 2, 1980 and entitled "Composition and Method
for Removing Sulfide containing Scale From Metal Surfaces".
Frenier et al. discovered that acid-soluble, sulfide-
containing scale could be effectively removed frommetal 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 stoichiometric amount of aldehyde
in the cleaning solution (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 containing
excess ~ormaldehyde.
It has now been discovered that the Frenier
et al. cleaning process is improved by generating the
aldehyde in situ. Thus, the present invention is 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 comprising 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 sulfide gas, the improvement
comprising generating said aldehyde(s) in situ.
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By conducting the cleaning process according
to the present invention, one can avoid procedural
problems of handling aldehydes per se.
The compositions used in the present invention
can be formulated by any convenient procedure which
results in the formation of the aqueous acid cleaning
composition defined above. This result is most easily
achieved by combining an aqueous non-oxidizing acid
with an aldehyde-releasing organic compound which has a
low vapor pressure and is soluble or dispersable in the
acid. Preferràbly, such aldehyde-releasing organic
compounds are formaldehyde-releasing compounds. Examples
of such aldehyde-releasing compounds are Schiff bases,
condensation products of ammonia or primary amines with
aldehydes, etc. Along these, the class of compounds
which are condensation products of ammonia or aliphatic
primary amines with formaldehyde are preferred and the
condensation product of ammonia with formaldehyde is
most prefexxed.
In many instances, acid is consumed during
the process of generating the aldehyde in situ. This
is easily determined by standard analytical techniques.
The point being, it may be necessary in formulating the
aqueous acid cleaning compositions to allow for this
consumption of acid so tha~ the final formulated material
has sufficient acid strength to clean the acid-soluble,
sulfide-containing scale at a commercially acceptable
rate.
Likewise, the amount of aldehyde which has
been generated in situ can be easily predetermined
and/or measured experimentally by conventional methods.
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The result desired is to have sufficient aldehyde
present in the aqueous acid cleaning composition to
prevent the evolution of hydrogen-sulfide gas when the
cleaning solution comes in contact with the scale. The
stoichiometry of the reaction requires one mole of
aldehyde per mole of hydrogen sulfide which will be
generated. Excess aldehyde is again preferred. Frequently
the aqueous acid cleaning solutions containing the
aldehyde are generated external to the item or vessel
to be cleaned.
In cleaning an item or vessel that will
require circulation of the aqueous acid cleaning solution
(e.g. a contacting tower, heat exchanger or furnace),
it is preferred to ascertain the circulation path
within the system using water. The aldehyde generating
chemical can then be added to the water after which
concentrated acid can be added until the desired cleaning
concentration is achieved. This represents a preferred
embodiment for cleaning many, if not most, systems
since the aldehyde is always present in excess and
effective elimination of gaseous hydrogen sulfide is
assured.
The Frenier et al. patent describes the
aqueous acid cleaning compositions in substantial
detail as well as the techniques for their use. The
primary distinction between the instant invention and
the Frenier et al. invention resides in generating the
aldehyde in situ.
The following examples will further illustrate
the invention:
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Example 1: A solution of hexamethylene
tetramine (HMTA; 5.5g) dissolved in 150 milliliters of
14 percent (by weight) aqueous hydrochloric acid was
charged to a reaction vessel having a mechanical stirrer
and a gas collecting means. Powdered iron sulfide
(FeS; 9.6g) was added to the stirred aqueous acid
solution. The mixture was then heated to 150F and
stirred for 3.5 hours. During this time, approximately
65 percent of the iron sulfide dissolved and 32 milliliters
of gas collected. This gas was tested for hydrogen
sulfide using lead acetate paper. The results were
negatlve .
Example 2: A pipe specimen (1" x 4" x 1/4")
from a petroleum refinery furnace was obtained which
was heavily fouled with a deposit containing iron
sulfide. The specimen was placed in a 2 liter jacketed
flask attached to a gas collecting means. The specimen
was submerged in 600 milliliters of a solution containing
14 percent (by weight) a~ueous hydrochloric acid and
20g HMTA and the flask sealed. The contents of the
vessel were then heated to 150F and maintained at this
temperature for 6 hours. No measurable amount of gas
was observed or collected. The surface of the pipe
specimen was essentially free of scale.
The "spent solvent" was analyzed in each of
Examples 1 and 2 and found to contain 2.5 and 1.5
weight percent dissolved iron, respectively.
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