Inorganic scaling

Clean-in-place (CIP) units for periodic membrane washing and removal of organic deposits, inorganic scales, and biofouling... 

Removal of iron and inorganic scales is achieved by the use of formulations containing 5 to 10% of acids such as phosphoric or citric acid. 

An operator determines the need for reactor cleansing by draining and visually inspecting the surfaces. Open reactor systems are easily inspected. Systems with sealed vessels are inspected through portholes or manways in the reactor shell. Surfaces of submerged quartz systems become coated with an inorganic scale, very much like boiler scale. This is a particular problem in areas with hard water. Additionally, the inside surface of the quartz and the outer surfaces of the Teflon tubes eventually develop a grimy dust layer, primarily from airborne dirt and water vapor. 

When an alkaline solution is injected into a formation, the concentrations of OH, COs, and SiOs are increased. The increase in OH is from the injected alkaline solution. The increase in is from HCOs because high OH makes the reservoir an alkaline enviromnent and converts HCOs into COs. SiOs is a result of the reaction between the injected alkali and formation minerals. Strong alkalis dissolve the clay minerals and damage the rock surface electric charge and microstructures, which finally results in dispersion and migration of the clay minerals. If seawater is injected, S04 is increased. Ca " and Mg " are from the formation water, cation exchange, and reactions between the injected solution and rock minerals. Sometimes, there is Al. Several inorganic scales can be formed as follows ... 

A typical wash load consists of several types of soils which must be solubilized or degraded by a combination of the detergent and machine. Detergents must control both food soils introduced by the items washed and inorganic scale produced by hardness ions in the wash water. Food soils consisting of either proteinaceous, starchy, or fatty materials must not only be effectively removed from the dishware, but they must also be prevented from redepositing on items being washed. 

Other solids can form in wells. These include asphaltenes and paraffins (waxes), coke, hydrates, inorganic scales, and corrosion products. Consideration of these is beyond the scope of this chapter. 

If the protective current induces precipitation of an inorganic scale on the cathode surface, such as in hard waters or in seawater, the total required current, as described earlier, falls off as the scale builds up. However, at exposed areas of metal, the current density remains the same as before scale formation only the total current density per apparent unit area is less. 

Conditions of the environment that favor dezincification are (1) high temperatures, (2) stagnant solutions, especially if acid, and (3) porous inorganic scale formation. Looking at the situation metallurgically, brasses that contain 15% or less zinc are usually immune. Also, dezincification of a-brasses (up to 40% Zn) can be reduced, as mentioned earlier, by small alloying additions of tin plus a few hundredths percent arsenic, antimony, or phosphorus. 

Conditions of the environment that favor dezincification are high temperatures, stagnant solutions, especially of acid, and porous inorganic scale formation. Other factors that stimulate the process are increasing zinc concentrations and the presence of both cuprous and chloride ions. As the dealloying proceeds a porous layer of pure or almost pure copper is left behind. This reaction layer is of poor mechanical strength. The dezincification process on copper-zinc alloys is therefore very detrimental. 

Bond, L., Mills, C., Whiting, P., et al. (1997). Apparatus to remove inorganic scale from a supercritical water oxidation reactor, J. Cleaner Prod., 5, p. 158. 

Summary of Common Inorganic Scale Forming Compounds "... 

Deposition of inorganic scale may occur during well production. Depending on well conditions and produced water characteristics, different scale types may form. Often, scale formation is associated with a breakthrough of water production. Common scales include calcium carbonate, iron carbonate, calcium sulfate, barium sulfate, strontium sulfate, andiron sulfide. Combinations may also form. Figures 3-2 and 3-3 show well samples of calcium carbonate and iron scale, respectively. 

Production Inorganic scale plugging Calcium carbonate Iron scales (varying acid solubilities) Fines migration... 

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. 

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