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antonio saura Cebrian
long time ago I wrote this paper but it was en spanish Im going to traslate, and I hope that you found here some answer for your question
CAUSES AND REMEDIES OF THE TEST OF COPPER STRIP
The test of the copper strip as we know it today is used in the petroleum industry since 1948 to determine the corrosiveness of fuels or finished products. Basically this test is to dip a sheet of polished copper in the fuel sample and maintained at a specific temperature for a period of time after this period ends is removed from the sample and is visually observed and compared with standard laminated reproductions ( ASTM) to determine the degree of corrosivity of it.
It is important to understand the causes and remedies of the corrosiveness of the copper strip so I am try to explain them.
The root cause for the corrosivity of petroleum products are associated with hydrogen sulfide (H2S), elemental sulfur or polysulfides either alone or combinations thereof.
H2S is a highly toxic compound, is colorless and has a characteristic odor of rotten eggs, is found in most of the hydrocarbons produced in the refinery and it is usually removed by stripping or chemical absorption. Such a small amount of 1 ppm by weight can fail the test of copper. strip
Elemental sulfur (S0), can exist in all three phases (liquid, solid, gas), usually distilled in this element in the overhead of the fractionators, so when the same is found in streams light products is usually due to the oxidation of H2S. A small amount of 2 ppm by weight of elemental sulfur can fail the test of copper strip.
Polysulfides can be formed in the presence of excess sulfur or mercaptans, elemental sulfur can also dissolve and combine to form polysulfides of the form RSSXR , where X is usually 1 to 4 atoms of elemental sulfur dissolved in the sulfide or disulfide. The sulfides are almost never found in distilled product streams. They are often formed during the test run DOCTOR, adding excess of elemental sulfur, its effect on the copper strip is the same as the elemental sulfur.
Historically during the determination of the corrosiveness of fuels, it was observed that the copper strip exhibited a number of different colors. These colors, initially met with the numbers of Indiana (Indiana NUMBERS), which correlated the degree of corrosivity of fuel. It has been determined that the variables that mostly affect the degree of corrosion test methods were:
-The concentration of corrosive sulfur compounds (H2S or S) in the sample.
-The temperature at which the test is performed.
-The time at which the copper strip is exposed in the test.
The standard methods used for qualitative and quantitative determination of the corrosion of the copper strip and the components that cause it are: ASTM-D 130, ASTM-D 1838, ASTM-D 2420, potentiometric titration, Mercury number and DOCTOR test
TOLERANCE IN THE PRODUCT
Hydrogen sulfide, elemental sulfur and polysulfides are the only compounds that cause corrosion of the copper stripl. Because the copper strip develops different colors during corrosive test of several samples, it was thought that the concentration of H2S and S may be directly related to the color produced on the strip during the test, however it was proven that the degree of clouding/discoloration of the strip was not only for the concentration of sulfur compounds, exposure time, temperature, but also the presence of certain mercaptans and corrosion inhibitors impinged on it. It is impossible to quantify that amount or concentration of H2S and S is present based on the final color of the strip, and each of the above elements when they are alone can fail the test at concentrations higher than when there are combinations of them, for example:
A concentration of 5 ppm of S0 in the absence of H2S can fail the test and 0.5 ppm H2S concentrations can cause failure of the test in the absence of S0, but when both concentrations of 1 ppm and 0.01 ppm S of H2S can failure of the test copper strip. This phenomenon suggests that H2S and S react to cause the failure of the test, This explains why a mixture of two products that have passed the corrosion test separately, can fail the test after they was mixing (if one contains elemental sulfur and the other traces of H2S).
There are some compounds or elements that may mask the corrosiveness of the product during the test of copper strip, among them we can indicate:
Mercaptans are generally corrosive to copper, however, they prefer to form a layer of oil soluble copper compounds mercaptides, instead of a dark film insoluble copper sulfide on the strip, the mechanism by which they mask the H2S and S0 is unknown.
In general, injection systems that have rust inhibitors can form a protective film on copper foil during the test, masking the corrosivity of the product.
Traces of peroxide-type compounds do not affect the corrosiveness caused by H2S, but they may inhibit the corrosiveness caused by S0, making the copper foil is less sensitive to attack by elemental sulfur. Organic peroxides have inhibitory effects on copper foil.
If you see spots or specks on the copper strip that is indicative of the presence of water, having to filter the sample and repeat the analysis.
Usually try to take samples in dark bottles and clean and be protected from sunlight, since it could lower the reaction rate during the test on the copper strip, this is possibly due to the formation of peroxide .
EFFECTS OF THE PROCESS
The corrosiveness of the process, as mentioned above is due solely to the presence of H2S, S0 and polysulfides, the following are examples of how these compounds can cause and some of their reactions.
Oxidation of H2S
As is well known that H2S is readily oxidized in the presence of oxygen, or ring-oxygenated compounds in acidic or neutral (pH of 7 or less) to form elemental sulfur oil soluble according to the following reactions:
2H2S + O2 + 2H2O 2S0 (1)
2H2S + H2SO3 + 2H2O 2S0 (2)
2H2S + Fe2O3. H2O + 4H2O + 2fen S0 (3)
The first reaction can not occur in the sweetening process or extraction because the medium is alkaline, but the presence of H2S or metal sulfide is oxidized to thiosulfate and water soluble according to the following reaction:
2H2S + +2 OH- O2 S2O3 = + 3H2O (4)
While the failure of the copper strip on gasoline or heavier products is attributable to both the H2S and / or S0, in the products of liquefied gases (LPG) are almost always attributable to the presence of H2S.
Typically, the concentration of H2S is reduced to less than 1 ppm by weight using a system of amine and / or treatment with caustic prewash. However, if the concentration of H2S in the gas increases or if the unit has an uncontrolled amine, large amount of H2S is going with the flow generating the test to fail. An extraction unit may be able to absorb most of H2S at the expense of a high consumption of caustic. The caustic is consumed when it is oxidized irreversibly according to the following reactions:
4NaOH 2Na2S 2H2S + + 4H2O (5)
2Na2S H2O + 2O2 + Na2S2O3 + 2NaOH (6)
Na2S2O3 + 2O2 2Na2SO4 2NaOH + + H2O (7)
2H2S + 4O2 + 2H2O 4NaOH 2Na2SO4 + (8)
Since the presence of sulfides in the caustic mercapturic suppresses oxidation and because the oxidation kinetics are significantly slower for sulfur than for mercaptides, total sulfur and possibly high corrosiveness of LPG product may last a long time until H2S is removed from the feed to the unit Merox or Merichem.
In those processes where oxygen is present there is great probability of the formation of elemental sulfur (S0) from H2S.
As indicated above because elemental sulfur (S0) is non-volatile products are usually header fail the test of copper strip primarily due to the presence of H2S.
Finished products such as propane and butane have failed the test of the copper strip due to H2S, which sometimes is accumulated in the overhead debutanizer , not being completely removed during treatment with amines. Sulfide ions also can be dragged from the caustic prewash drum due to high speeds on the same, which can generate H2S, by thermal decomposition of sodium sulphide (Na2S) or high-temperature reaction of Na2S with crystals water according to the following reaction:
Na2S + 2H2O 2NaOH + H2S (9)
Another source of generation of H2S, is the hydrolysis of carbonyl sulfide (COS). This is catalyzed molecular sieve dryers. COS hydrolyzed according to the following reaction:
COS + H2S + H2O CO2 (10)
High concentrations of COS in a product stored in the presence of water can be hydrolyzed to H2S since it is catalyzed by iron oxide film of the storage tank.
As mentioned above, the failure of the copper foil can be attributed solely to the presence of elemental sulfur (S0), hydrogen sulfide H2S and sometimes polysulfides. The formation of elemental sulfur is almost always the product of the oxidation of H2S in a neutral or acid medium.
Where there is the presence of H2S and O2, it must be avoided as far as possible that a neutral or acidic aqueous medium exist in order to avoid the formation of elemental sulfur, some refineries injected condensate and monitored the pH of the sample, If the NH3 are present but is insufficient to counteract the effect acidic from H2S an additive may be required to avoid corrosion problems on the copper strip.
The failure of the copper strip test in petroleum products are caused by elemental sulfur, H2S, and sometimes by polysulfides.
Thecopper strip test is a qualitative analysis and can be affected by the presence of mercaptans, corrosion inhibitors, peroxide, sunlight and water in suspension.
The H2S is oxidized in the presence of oxygen and / or oxygenated ring compounds in a neutral or acid to form elemental sulfur, which dissolves in the oil phase and in alkaline or basic thiosulphate formed which will dissolve in the aqueous phase .
im sorry for my english