Removal of deposits

The cleaning or depassivation eflect is of great importance in sonoelectrochemistry, as it can be employed to wash off surface-adsorbed species and reduce blocking of the electrode by adsorption of reaction products. This eflect has been reported, for example, for the depassivation of iron electrodes and for the removal of deposits and in the presence of polymer films on the electrode surface. However, damage of the electrode surface, especially for materials of low hardness such as lead or copper, can also occur under harsh experimental conditions and applied intensities [70, Tf, 80]. 

Figure 4.26 Internal surface of steel heat exchanger tube after removal of deposits. Note the mutually intersecting areas of metal loss.

Removal of deposits and corrosion products from internal surfaces revealed irregular metal loss. Additionally, surfaces in wasted areas showed patches of elemental copper (later confirmed by energy-dispersive spectroscopy) (Fig. 13.12). These denickelified areas were confined to regions showing metal loss. Microscopic analysis confirmed that dealloying, not just redeposition of copper onto the cupronickel from the acid bath used during deposit removal, had occurred. 

Chemical removal of deposits and corrosion products revealed the appearance of the groove (Fig. 14.5). The crevice formed by the incompletely fused weld seam fostered the establishment of differential concentration cells (see Chap. 2). This resulted in localized corrosion and eventual perforation through the greatly thinned tube wall at the bottom of the crevice. The tubercle, which is composed of corrosion products, is a simple result of the corrosion process occurring locally within the crevice. 

B. Removal of Deposited Particles from the Respiratory System... 

From the corrosion point of view, it is very important to control the deposition of scale. Removal of deposited scale by mechanical means is the first step. Standard, industrial water-treating techniques can be used to control scale deposition in general. In deep, hot wells or geothermal wells it is best to avoid untreated makeup water (i.e., geothermal brines). 

Detergent In relationship to fuel technology, a detergent is an oil-soluble surfactant added to fuel aiding in the prevention and removal of deposits. Examples include anionic alkyl aryl sulfonates, cationic fatty acid amides, or nonionic polyol condensates. 

Fluidizer Oil Compounds such as polyisobutylene or refined naphthenic oils which aid in the removal of deposits from the underside of intake valves. 

We conclude, therefore, that the mechanisms of catalytic cracking reactions on nickel metal and nickel carbide are closely comparable, but that the latter process is subject to an additional constraint, since a mechanism is required for the removal of deposited carbon from the active surfaces of the catalyst. Two phases are present during reactions on the carbide, the relative proportions of which may be influenced by the composition of the gaseous reactant present, but it is not known whether the contribution from reactions on the carbide phase is appreciable. Since reactions involving nickel carbide yielded products other than methane, surface processes involved intermediates other than those mentioned in Scheme I, although there is also the possibility that if cracking reactions were confined to the metal present, entirely different chemical changes may proceed on the surface of nickel carbide. 

Stage 2 The physical removal of deposits from heat exchangers and subsequent elimination from the system. 

Figure 5.6 Schematic drawing illustrating the removal of deposited negatively charged colloidal components from the surface of an anion-exchange membrane by reversing the electric field.

Non-stationary operations have found large scale industrial application. An important classical example is catalytic cracking, where oil is exposed with a short residence time to a rapidly deactivating zeolitic catalyst, which is regenerated in a second step by removal of deposited coke. A novel non-stationary process is selective butane oxidation over a regenerable oxidation catalyst (see Chapter 2). Undoubtedly we will see more examples of this type of process, in which the proper catalytic step and the regeneration of the catalytic sites occur in different compartments under different conditions. A nice application involves... 

To improve the removal of deposits on the membrane surface, Bohner and Bradley [179] modified the cleaning procedure described previously. The modified procedure, which was carried out at 54°C, consisted of a 2 min initial rinsing of the... 

The heat exchanger is of shell-and-tube type, where the combustion gases flow in tubes and the heated air in the shell. This construction was selected because of the easy removal of deposits. The principle scheme is shown in Fig 4. 

Boiler Feed Waters.— Impurities in boiler waters not only reduce efficiency and capacity, but also impair quickness of response to demands for steam, increase the rate of deterioration of the boiler, and may produce dangerous conditions. The removal of deposits from a boiler nearly always involves considerable hard labor. Their prevention should be the aim in view. The unintelligent use of proprietary compounds is to be avoided. Some of them contain, besides reagents based on the water analysis, organic compounds which may loosen large masses of scale. 

Region B represents the steady growth of the deposit on the surface. Under these circumstances there is competition between deposition and removal. The rate of deposition gradually falls while the rate of removal of deposit gradually... 

Assuming that there is no removal of deposit taking place the mass flux due to scaling is ... 

Water jets powered by pumps up to 6.89476 x 10 Pa (10,000 psi) have become very useful cleaning tools for lines, heat exchangers, and towers when chemical removal of deposits is not the best method. 

A fire hazard may be safely avoided if the temperature of the material is lower than the relevant minimum ignition or exotherm onset temperature. The greatest risk exists in the part of the drying system where the product moisture content is lowest, such as the separation units (e.g., cyclones and bags). Where dust deposits can accumulate, the requirements are that the maximum permissible temperature of the plant surfaces should not exceed two thirds of the layer MIT. Also, for the dust layer, the time the material is exposed to heat should be taken into account as a controUing factor that will determine cleaning and removal of deposits. 

While surface areas preoccupied by nanobubbles can be prevented from adsorption, electrochemically produced nanobubbles can cause ultimate removal of deposits. 

Associated with electrode damage, the cleaning or depassivation of electrode surfaces is an important aspect of sono-electrochemistry. This effect has heen documented for the depassivation of iron electrodes [35), the removal of deposits [45), and observed when polymer films are present at the electrode surface [46). It is also vital to sonoelectroanalytical methods (see Sect. 2.8.1.2). The oxidation of the carbonyl complex Cr(CO)6 is an example of an electrochemical system that is prone to electrode passivahon [42). In the presence of ultrasound, sustained voltammetric currents can be obtained, which suggested both depassivation and/or a suppression of precipitation this is in contrast with corresponding silent voltammetry in which these phenomena preclude clean voltammetry. 

The degree of build-up of atherosclerotic plaques in patients with lipoprotein di.sorders is determined by the rate of deposition of lipoproteins and by the rate of the removal of deposited lipids from the vascular wall. It is therefore not surprising that ascorbate is also closely connected with this reverse pathway. 

Cleanliness of the gas is important. Lubricant oil must be kept out of the process stream for a variety of reasons. The air used on the compressor seals (see below) must be free of oil and particulate matter. The presence of fine particles in the gas is not as damaging in the short term as it would be in a reciprocating machine. Some users have in fact removed suction screens, preferring to deal with the occasional fine particle than with the large debris associated with a screen failure [35]. Even in the absence of solids contamination, there is a buildup on the impellers. The source is organic materials present in the cell gas. These may come from the brine or from reaction of cell components with the gas. The amount of material formed was greatly reduced by the adoption of metal anode technology. Accumulation of solids on the impellers is not a frequent cause of compressor shutdown, but removal of deposits still should be part of the routine overhaul procedure. 

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