De-aeration

The most effective mediod of de-aeration is to atomise water into a vessel held under a vacuum. In this way air is stripped out as the atomised water is 

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Before use, the marble chips are washed repeatedly with hot water, and then de-aerated by first etching them with concentrated hydrochloric acid and then boiling them with air-free water under reduced pressure. The chips are then rapidly transferred to the generator small chips should be used and the bulb... 

The relatively concentrated hydrochloric acid is employed so that with ordinary use of the apparatus, spent liquor does not accumulate very rapidly the concentrated acid also ensures a brisk and delicately controlled flow of gas. When the generator is replenished with acid, marble or both, the de-aeration procedure detailed above is repeated until a sufficiently air-free gas supply is obtained. 

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Large power stations use complex feed heating systems before the boiler feed pumps (LP) and after the boiler feed pumps (HP), which can give high overall thermal efficiencies of 39 per cent. However, for the smaller machine, it becomes uneconomic to consider multiple bleeds from the turbine, and the final choice is dictated by the extra cost for the additional complexity against lower running costs due to increased efficiency. As a minimum, a contact type de-aerator is often employed which would extract a small bleed of around 2-3bar from the turbine. 

For watertube boilers it is necessary to maintain low O2 levels, and for this purpose a de-aerator in the feed line is required, which will also provide a degree of feed heating. The steam supply can be taken down from the low-pressure process steam main. 

The other section of the vessel is connected directly to the water system. A correctly positioned air separator will assist in de-aerating the water in the system. 

Oxygen can also be removed from feedwater by thermal de-aeration, or partially removed by skilful design of the feed heating system and blowdown recovery. These processes run without cost to the operator, but save chemicals, and, by reducing the required dose of sulfite into the system, decrease the amount of non-volatile solids added into the boiler. 

High-purity High-pressure boilers De-aeration may be required BS 2486 1978... 

Zembura has made specific use of the rotating disc for investigation of the effect of flow on corrosion reactions. This work has shown that it is possible to determine the type of control (activation or concentration polarisation) of zinc dissolving in 0.1 N Na2S04 (de-aerated), which followed closely the predicted increase in hydrogen ion reduction as the flow rate increased, and proved that in this example... 

For some applications, notably feed-water treatment for high-pressure boilers, removal of oxygen is essential. For most industrial purposes, however, de-aeration is not applicable, since the water used is in continuous contact with air, from which it would rapidly take up more oxygen. Attention must therefore be given to creating conditions under which oxygen will stifle rather than stimulate corrosion. 

This alloy is suitable for high concentrations of de-aerated phosphoric acid providing the concentration is above a certain maximum, which varies with temperature in accordance with the equation. The calculated activation... 

With stainless steels optimum resistance is dependent on aeration, and due account must be taken of this in considering operating conditions where previous experience has been with non-ferrous alloys which require an absence of oxygen for full resistance. There is therefore no need to run off the early distillate from a batch to avoid impurity during initial de-aeration. On the other hand, complete exclusion of oxygen for the benefit of non-ferrous metals in a mixed plant have very serious consequences for the stainless steel, and this must be kept in mind at the design stage. 

In de-aerated 10sulphuric acid (Fig. 3.45) the active dissolution of the austenitic irons occurs at more noble potentials than that of the ferritic irons due to the ennobling effect of nickel in the matrix. This indicates that the austenitic irons should show lower rates of attack when corroding in the active state such as in dilute mineral acids. The current density maximum in the active region, i.e. the critical current density (/ ii) for the austenitic irons tends to decrease with increasing chromium and silicon content. Also the current densities in the passive region are lower for the austenitic irons... 

Fig. 3.50 Corrosion rate versus temperature in de-aerated sea-water continuous test exposure...

In de-aerated conditions, for instance in most central heating systems, little if any attack on copper occurs . As far as drinking waters are concerned, copper is not classified as a toxic substance or hazardous to health. To avoid any difficulties due to unpalatability, the maximum continuous copper content should not exceed 10 p.p.m., with a limit of 3 p.p.m. in water after standing overnight in copper pipes. A review of the subject by Grunau makes reference to 394 published papers. 

Fig. 4.21 Effect of temperature on the anodic behaviour of nickel in 0 025 m H2SO4 -I-0-025 M K2SO4 (pH 1 -3) de-aerated with H2. The curves were determined potentiokinetically at a scan rate of 2 V/h and proceeding from negative to positive (after Cowan and Staehle )...

Fig. 4.22 Effect of chromium content on the anodic behaviour of Ni-Cr alloys in 0-5 M H2SO4 (de-aerated with H2) at 25°C the potential was increased incrementally by 0-025 V every 3 min (after Hodge and Wilde...

Fig. 4.24 Anodic behaviour of Alloy 600 in 0-5 M H2SO4 (de-aerated willi Nj) ai 24°C...

Fig. 4.26 Anodic behaviour of Ni-Cu alloys in 0-5 m H2SO4 (de-aerated with N2) at 25°C the curve was determined potentiokinetically at 0-4 V/h for the 78-3 and 49-9% Ni alloys and at 3 V/h for the 30-4% Ni alloy proceeding from more positive to more negative (after...

Fig. 4.28 Anodic behaviour of Alloys B, C and N in boiling 10% H2SO4 de-aerated with H2 the potential was increased incrementally (after Leonard )...

Fig. 4.29 Anodic behaviour of Ni-Si alloys in 25% FI2SO4 (de-aerated with Nj) at ambient temperature (after Barker and Evans )...

Fig. 4.31 Anodic behaviour of Ni-Ti alloys in HCl + 3 -5% NaCKpFl 1), de-aerated with argon, at 22-2°C the potential was increased by 0-02 V every minute (after Sedriks, el al )...

Fig. 4.32 Anodic behaviour of Ni-Al alloys in 0-5 m H2SO4, de-aerated with H2, at 22°C the potential was increased by 0-01 or 0-02 V every 3 min in the active range and by 0-04 V in the passive range (after Crow, era/. )...

Fig. 4.36 Influence of pH and Cl ion on (he breakdown potential of commercial nickel in alkaline solutions (0-001-5 m NaOH) de-aerated with N2 (after Postlethwaite )...

Fig. 4.37 Influence of the chromium content of Ni-Cr alloys on the breakdown potential in 0-1 M NaCI at 25°C de-aerated with N2 (after Horvath and Uhlig )...

Most of the controlled corrosion studies on beryllium have been carried out in the USA in simulated reactor coolants. The latter have usually been water, aerated and de-aerated, containing small amounts of hydrogen peroxide and at temperatures up to 300-350°C. Many variables have been examined, covering surface condition, chemical composition, temperature, pH, galvanic effects and mechanical stress . 

Variation of pH Between 4 and 8, variation of pH does not appear to have a very significant effect on corrosion rate in de-aerated water. 

Oxygen pitting of boiler tubes by boiler feed water due to inadequate de-aeration is also a problem, but controllable by proper maintenance of de-aerators, coupled with regular boiler feed water analysis, or, preferably, continuous dissolved-oxygen monitoring. 

In practice an excess over the stoichiometric requirement, e.g. of sulphite for de-aeration, is used. 

The following procedure is recommended. The sample solution is de-aerated, then, with the tip of the capillary in the air, the mercury pressure is raised at least 10 cm above the previously found equilibrium height, the capillary is inserted into the cell, and the mercury level is finally adjusted to the desired value. After completion of the measurements the capillary is withdrawn from the cell and washed thoroughly with a stream of water from a wash bottle while the mercury is still issuing from the tip and is being collected in a microbeaker. 

The H-type cell devised by Lingane and Laitinen and shown in Fig. 16.9 will be found satisfactory for many purposes a particular feature is the built-in reference electrode. Usually a saturated calomel electrode is employed, but if the presence of chloride ion is harmful a mercury(I) sulphate electrode (Hg/Hg2 S04 in potassium sulphate solution potential ca + 0.40 volts vs S.C.E.) may be used. It is usually designed to contain 10-50 mL of the sample solution in the left-hand compartment, but it can be constructed to accommodate a smaller volume down to 1 -2 mL. To avoid polarisation of the reference electrode the latter should be made of tubing at least 20 mm in diameter, but the dimensions of the solution compartment can be varied over wide limits. The compartments are separated by a cross-member filled with a 4 per cent agar-saturated potassium chloride gel, which is held in position by a medium-porosity sintered Pyrex glass disc (diameter at least 10 mm) placed as near the solution compartment as possible in order to facilitate de-aeration of the test solution. By clamping the cell so that the cross-member is vertical, the molten... 

In 1976, Hodes et al. [85] reported the stabilization of polycrystalline CdSe photoelectrodes (prepared by electrodeposition and subsequent heat treatment) in de-aerated alkaline aqueous or organic (not specified) solution of S , in which some elemental sulfur was dissolved. It was claimed that besides CdSe, other semiconductors such as CdS, CdTe (n- and p-types), ZnSe, or Bi2S3 are also stable as photoelectrodes in a polycrystalline form in the sulfide solution, and that such cells under AMI sunlight are stable over periods of months, a rather exaggerated claim. It was described in addition how a part or all of the converted energy could be stored in a controlled way in the system by the introduction of an electrode of porous silver. 

To extract and evalnate the color pigments from cochineals Dactylopius coccus Costa), a simple method was developed. The procednre is based on the solvent extraction of insect samples nsing methanol and water (65 35, v/v) and a two-level factorial design to optimize the solvent extraction parameters temperature, time, methanol concentration in mixtnre, and yield. For hydrophilic colorants that are more sensitive to temperatnre, water is the solvent of choice. For example, de-aerated water extraction at low temperatnre was applied to separate yellow saffrole and carthamine from saffron (Carthamus tinctorius) florets that contain about 1% yellow saffrole and 0.3% red carthamine. ... 

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