Solvents for Removing Inorganic Deposits

The inorganic deposits described in Tabie 1 of Chapter 1 usuaily are formed on the surfaces of heat transfer equipment such as power and waste-heat boilers and heat exchangers, and in the various parts of cooling water systems. Iron oxides, copper, and calcium-containing deposits make up the largest portion of the material that requires removal from the metallic surfaces. We will review major developments for the chemical removal of each category of scale. 

There are a very large number of solvents for removing iron oxide deposits. They include mineral acids, organic acids, chelating agents, and mixtures of these chemical classes. The choice of a particular solvent system depends on the metal of construction, exact type and density of the oxide, and cost and disposal considerations. 

Mineral acids used in chemical cleaning operations include hydrochloric acid (HCI), hydrofluoric acid (HF), sulfuric acid (H2SO4), phosphoric acid (H3PO4), nitric acid HNO3), and sulfamic acid (H2NSO3H). Most of these acids have very low pKa values (see Table 1) and are completely ionized at the use strength. An exception is HF, which has a pKa value similar to that of formic acid, and phosphoric acid, which is about 10 times as ionized as HF. 

These acids were the first materials used to remove fouling deposits. The development of corrosion inhibitors to protect metal in contact with the various mineral acids allowed the chemical cleaning industry to provide safe, useful services. 

Hydrochloric acid is the simplest, most common, and most versatile mineral acid. This acid is used on virtually all types of industrial process equipment. It is used at strengths from 5% to 28% (5-10% is the most usual range), and it can be inhibited at temperatures up to about 180°F (82°C) (oilfield acids are used up to about 350°F 

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During the past 30 years many new solvents for inorganic deposits were developed. Low hazard corrosion inhibitors have replaced some of the more toxic materials used to protect the structural surfaces during cleaning. Development of nuclear cleaning methods has extended the life of some very valuable nuclear generating plants. New waste disposal methods have enabled the industry to continue to operate, but have required new chemicals to be developed. Modest improvements for cleaning CP/R equipment include ways to remove FeS and we have seen the development of more environmentally friendly solvents. 

The purpose of the preflush is to remove organic or inorganic scale from the wellbore tubulars prior to injection of the acid stage. An aromatic solvent, such as xylene, can be used to remove hydrocarbon deposits. For asphaltene deposits, specifically, terpene-based solvent solutions can be quite effective. Circulation of 5%-7.5% HCl downhole is adequate to remove rust and other inorganic scale for rust removal in particular, non-add (nearneutral) removal solutions exist and may be preferable at temperatures of MOT (60°C) and higher. 

The approach described here represents a remarkably simple procedure for the preparation of nanometer sized dots in a rather regular pattern. Without further effort we have prepared ordered films of 3x3cm. In principle there is no limitation in size. Besides the diblock copolymers which we employed, also other amphiphilic diblock copolymers form reverse micelles in a non-polar solvent and can be used to bind a large variety of metal compounds like H2PtClg, Pd(Ac)2, TiCU, FeCls, etc.. Chemical transformation of the inorganic compound can be performed before deposition of the micellar film or upon removal/oxidation of the film. While noble metals are deposited in the elementary state, less noble compounds can only be deposited in their oxidic form, i.e., Ti02, Fe203, InO. ... 

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