Scale Inhibition

The problem is basically similar to preventing scale inhibition in washing machines therefore similar chemicals are used to prevent scale deposition. Scale inhibition can be achieved either by adding substances that react with potential scale-forming substances so that from a thermodynamics standpoint the stable region is reached or by adding substances that suppress crystal growth. 

Conventional scale inhibitors are hydrophilic, that is, they dissolve in water. In the case of down-hole squeezing, it is desirable that the scale inhibitor is adsorbed on the rock to avoid washing out the chemical before it can act as desired. However, adsorption on the rock may change the surface tension and the wettability of the system. To overcome these disadvantages, oil-soluble scale inhibitors have been developed. Coated inhibitors are also available. Often, scale inhibitors are not applied as such, but rather in combination with corrosion inhibitors. 

Thennodynamic inhibitors are complexing and chelating agents, suitable for specific scales. For example, for scale inhibition of barium sulfate, common chemicals are ethylenediaminetetraacetic acid (EDTA) andnitrilotriacetic acid. The solubility of calcium carbonate can be influenced by varying the pH or the partial pressure of carbon dioxide (CO2). The solubility increases with decreasing pH and increasing partial pressure of CO2, and it decreases with temperature. 

However, usually the solubility increases with higher temperature. The temperature coefficent of solubility is dependent on the enthalpy of dissolution. An exothermic enthalpy of dissolution causes a decrease in solubility with increased temperature, and vice versa. 

Kinetic inhibitors for hydrate formation may also be effective in preventing scale deposition [1627]. This may be understood in terms of stereospecific and nonspecific mechanisms of scale inhibition. 

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Phosphonic acid is an intermediate in the production of alkylphosphonates that are used as herbicides and as water treatment chemicals for sequestration, scale inhibition, deflocculation, and ion-control agents in oil weUs, cooling tower waters, and boiler feed waters. For example, aqueous phosphonic acid reacts with formaldehyde and ammonium chloride in the presence of hydrochloric acid to yield aminotri(methylenephosphonic acid) [6419-19-8]. 

Scale Inhibitors. When scaling conditions exist, scale inhibitors can be used to control the scaling tendencies, and keep metal surfaces free of scale deposits. Scale inhibitors are chemicals that interrupt and deform the normal crystalline growth pattern of carbonate scales. The three most commonly used classes of scale-inhibiting chemicals used in drilling fluid are [191,197] ... 

Scale Inhibitor. About 3 to 5 gal (20 to 30% active) of scale inhibitor per day should be sufficient for scale inhibition. The treatment can be reduced to 1 to 3 gal per day once the scale formation is under control. The chemical can be introduced to the system by injecting it into the pump suction line. 

Another mechanism of scale inhibition is based on adherence inhibitors. Some chemicals simply suppress the adherence of crystals to the metal surfaces. These are surface-active agents. 

I. R. Collins. Scale inhibition at high reservoir temperatures. In Proceedings Volume. IBC Tech Serv Ltd Advances in Solving Oilfield Scaling Int Conf (Aberdeen, Scotland, 11/20-11/21), 1995. 

In all applications, the chelant may have been added for a variety of purposes. As discussed in Section 10.2, chelants can have roles where calcium and magnesium control is key, such as detergency or scale inhibition. Alternatively, transition metal ion control could be the target, for example, in bleach stabilization. Some applications require both properties simultaneously. Chelants can also act as biocide potentiators or as antioxidants, to prevent trace metal ions from causing oxidative spoilage. This may require that metal concentrations be kept below certain... 

Chelants in bleach-activated powder laundry detergents have a number of key roles to play - bleach stabilization, dye and fabric protection, stain removal, and scale inhibition. 

Wash performance is crucial for the consumer. Stain removal, anti-graying, and color stability with no dye transfer are all important, as these are visible parameters that inform the next purchase choice. Less visible is scale inhibition to protect the machine. The laundry market is highly competitive, so that although chelants are only a small part of the formulation, purchasers want to keep the cost of this part of a package to a minimum. As a result, they are generally only willing to pay a small premium for biodegradability of alternative chelants... 

For any mode of disposal, effluent water from the desalting plants In one area will be collected at a central place for treatment before disposal. Ir. the treatment process, the effluent will be filtered, treated for corrosion and scale Inhibition, bacterial fouling and dcoxygenated. Otner gases like H S and COg if ar.y will also be rcaoved to tbe desired... 

As part of the overall scale-inhibiting threshold effect, the mechanism of crystal growth retardation is complemented by crystal distortion, whereby the crystal fails to successfully agglomerate into a larger crystalline scale. 

The activity of [i-galactosidasc (P-Gal) was studied on a quartz chip using a static micromixer to mix the enzyme and substrate on the ms time scale. Inhibition by phenylethyl-P-D-thio-galactoside was also studied [1048]. In another report, the enzyme P-Gal was assayed on a chip in which P-Gal would convert a substrate, resoruhn-P-D-galactopyranoside (RBG), to resoruhn to be detected fluorescently [1049]. By varying the substrate concentrations and monitoring the amount of resoruhn by LIF, Michaelis-Menten constants could be determined. In addition, the inhibition constants of phenylethyl-P-D-thiogalactoside, lactose, and p-hydroxymercuribenzoic acid to the enzyme P-Gal were determined [1049]. 

Use of scale-inhibiting chemicals to reduce carbonate scaling of flashing wells. Development of pH modification to control silica scaling in power plants. 

Calcium and magnesium scale Inhibited hydrochloric acid, EDTA As for iron oxides... 

Standish, M. L. 1996. A New Polymeric Materi alfor Scale Inhibition and Removal, CORROSION 96, Paper No. 163, NACE International, Conferences Div., Houston, Texas. 

The velocity effects on CO2 corrosion have been studied in several projects dealing with multiphase flow. Mechanistic models based on electrochemistry, reaction kinetics, and mass transfer effects have been developed, e.g. by Nesic et al. [6.28]. A semi-empirical model was presented by de Waard et al. [6.29]. This model and the corresponding experimental results are valid for cases without carbonate scales. Inhibition can be accounted for by inserting an inhibitor efficiency factor. A model mainly based on the same data as the one developed by de Waard et al. is included in the standard NORSOK M-506 [6.30], The application of the NORSOK model is based on a computer program. Common to these models is that they are valid for cases with a bulk phase of water. For mist flow and dewing conditions the calculation basis is inferior, but such conditions give low corrosion rates. 

Sodiumhexametaphosphate (SHMP) is a threshold agent derived from the dehydration of orthophosphoric acid or its sodium salt. It is used to inhibit the formation of calcium carbonate and metalHc sulphate scale. It is mosdy widely used because it offers good inhibition at a low cost. Depending on the concentration of calcium and sulphate, and depending on the CF (Equation 2.31), the dosage is in the range of 2-5 ppm. SHMP can prevent calcium sulphate precipitation up to 150% of the saturation limit. Organopho-sphonates are an improvement over SHMP since they are more resistant to hydrolysis, but are more expensive. They offer scale inhibition and dispersion abihty similar to SHMP [14,40,41]. 

Polyacrylic acids (PAA) are good at both scale inhibition and dispersion, and are more effective than SHMP. Polyacrylic acids with high molecular weight distribution show the best dispersion abiHty at the cost of scale inhibition ability. However, precipitation may occur with cationic polyelectrolytes or multivalent cations such as aluminium or iron, resulting in fouling the membranes. Blend inhibitors are a combination of low (2000—... 

Weng, P.F. (1995). Silica scale inhibition and colloidal silica dispersion for reverse osmosis systems. Desalination, 103, 59-67. 

These requirements eliminate many interesting possibilities for scale inhibition chemistry that are very useful in other crystal growth applications. For example, polymers containing hydrophobic character can be quite effective for inhibiting crystal growth in a simple system, but are generally not suitable for oilfield applications. 

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