Choosing a neutralising amine corrosion inhibitor
A review of physical and chemical properties of commonly used amine-based corrosion inhibitors. The authors highlight the relative strengths and weaknesses of amines, particularly with regard to corrosion caused by hydrochloric acid
Philip R Petersen, Petersen Consulting
Samuel A Lordo and Gregg R McAteer, Nalco Company
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The selection of the best neutralising amine, or combination of amines, to help prevent corrosion caused by strong acids in overhead condensing systems has always involved making compromising choices among the various properties of the neutralising amines (and their salts). Each amine has unique properties that govern how rapidly it will evaporate, its potential to form solid or liquid salts, how rapidly it will get into the first drops of condensing water etc.
There are many properties of neutralising amines that must be considered when choosing a product, just as there are many parameters in the design of a tower and overhead system that affect the choice of neutralising amines. Some of the properties of amines that must be considered when selecting a product include:
— Boiling point, Base strength, Equivalent weight
— Hydrochloride salt melting point, Hydrochloride salt formation constant
— Effect of amine, especially excess amine, on water treatment plant
— Vapour-liquid equilibrium (VLE) constant, Interaction of amines.
The effect of these properties on the selection and use of neutralising amines will be discussed further. It is important to remember that all these properties listed are, at a given set of conditions, intrinsic properties. They are immutable (they cannot be changed). For example, ammonia cannot be made to dissolve into the first drops of condensed water more rapidly; ethylenediamine dihydro-chloride cannot be made to be more volatile and less prone to form deposits. The industry has to live with these properties just as the industry has to live with the less than ideal properties of most metals used in refinery and petrochemical plant construction.
While there are potentially a huge number of amines that could be used to prevent strong acid corrosion, the industry is limited to those that are economically viable, those amines that generally cost less than about US$2 /pound (US$4.5/kg). If a neutralising amine product is too expensive, it will not be used regardless of how well it controls system corrosion. While each supplier has one or more amines that they prefer to use for various reasons, they all sell a wide range of amines, including:
NH3 – ammonia
DMEA – dimethylethanolamine
DMIPA – dimethylisopropanolamine
EDA – ethylenediamine
MOPA – methoxypropylamine
MEA – monoethanolamine
Morph – morpholine
Pic – picolines (alkyl pyridines)
TMA – trimethylamine
Some of their physical properties are given in Table 1. Note that, because some of the physical properties of these amines require extensive laboratory time to determine, these properties are considered proprietary and so are given as relative values, rather than the exact value or equation.
In general, discussions will highlight two amines at the extremes of the parameter under consideration, and usually an amine that has an intermediate value.
Why are neutralising amines needed? When crude oil is produced, the crude contains brine that has sodium, calcium, and magnesium chlorides, among other inorganic species, in it. The crude is desalted in the refinery to remove the majority of these salts. These salts need to be removed because two of them, the calcium and magnesium chlorides, will produce hydrogen chloride when heated in the furnaces:
CaCl2 + 2 H2O Æ 2 HCl + Ca(OH)2 (1)
MgCl2 + 2 H2O Æ 2 HCl + Mg(OH)2 (2)
Fortunately, sodium chloride does not undergo this hydrolysis very readily.
As long as there is no water present in the system, the hydrogen chloride gas is not corrosive to carbon steel. When this gas dissolves in water, very corrosive hydrochloric acid is formed. The corrosiveness of the system can be reduced in several ways. If desalting were perfectly done, there would be no salts going into the furnaces and no hydrogen chloride would be formed. Reducing the amount of salt going to the furnaces will reduce the amount of acid produced and thus reduce the amount of corrosion. Diluting the overhead condensed water with wash water will dilute the acids, raising the pH and lowering the corrosion rate. Unfortunately, the amount of wash water needed to do this is normally not practical for many reasons. Diluting the amount of condensed water by a factor of 10 only increases the pH of the overhead water by less than one pH unit because of buffering effects caused by the weak acids in the system.
Neutralising the acid with a base (a source of hydroxide ions) is a very common practice in refineries and petrochemical plants today. By neutralising the acid, the pH is raised, a hydrochlor-ide salt is formed, and corrosion usually decreases.
HCl + M+ + OH– Æ MCl + H2O (3)
Ammonia was the first base used to reduce overhead corrosion and still is extensively used:
HCl + NH3 Æ NH4+ + Cl– (4)
In the 1960s, various organic amines began to be used to neutralise the hydrochloric acid:
HCl + RNH2 Æ RNH3 + + Cl– (5)
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