Surface particles, cleaning

The inside of the convection tubes rarely foul, but occasionally the Hquid unsaturates in feedstocks tend to polymerize and stick to the walls and thus reduce the heat transfer. This soft coke is normally removed by mechanical means. In limited cases, the coke can also be burnt off with air and steam. Normally, the outside surface of the convection section fouls due to dust and particles in the flue gas. Periodically (6 to 36 months), the outside surface is cleaned by steam lancing. With Hquid fuel firing, the surface may require more frequent cleaning. 

The SEM-AIA results contain very detailed information for the composite coal/mineral particles and their component parts (i.e., information on size, phase identification, and associations) which can be presented in a number of ways. Tables can be prepared to show the distribution of the sample as a function of particle size and to show the coal-mineral association in terms of bulk properties or in terms of surface properties. For bulk properties, the distribution of coal and minerals is prepared as a function of the total mineral content of the individual particles which can be related to particle density. For surface properties, coal and mineral data are tabulated as a function of the fraction of particle surface covered by mineral matter which can be used to predict the surface properties of the particles and their behavior during surface-based cleaning. Examples of these distributions are given below. 

Note that the preceding equation is for ideal cases, in which the particles are monodis-persed, spherical, and totally elastic, and the contact surface is clean. In practice, the particles are usually nonspherical and polydispersed the collision could have involved some heat loss, plastic deformation, or even breakup and the contact surface may have impurities or contaminants. In these cases, a correction factor tj is introduced to account for the effects of these nonideal factors. The applicable form of the electric current through the ball probe is, thus, given by... 

Because like dissolves like, the nonpolar end (hydrophobic or water-hating part) of the soap molecule can dissolve the greasy dirt, and the polar or ionic end (hydrophilic or water-loving part) of the molecule is attracted to water molecules. Therefore, the dirt from the surface being cleaned will be pulled away and suspended in water. Thus soap acts as an emulsifying agent, a substance used to disperse one liquid (oil molecules) in the form of finely suspended particles or droplets in another liquid (water molecules). 

The most important difference between particles inside the bulk and in the interfacial layer comes from the surrounding environment of the particles the particles inside the bulk are in an isotropic environment, while those in the interface are in an anisotropic environment thus, in the interlayer, the forces between the particles are unbalanced. To reduce the resulting surface pressure, some additional processes occur that must be taken into account. On clean surfaces (for example, on a solid surface in vacuum), these processes are the bond-length contraction or relaxation and reconstruction of the surface particles (Somorjai 1994). It results in significantly reduced spacing between the first and second layers compared to the bulk. The perturbation caused by this movement propagates a few layers into the bulk. The other effect is that the equilibrium position of the particles changes that is the outermost layers can have different crystal structure than the bulk. This phenomenon is the reconstruction. 

Wet cleaning process was the first to be adopted and is still the most commonly used platform for post-CMP cleaning processes for removing leftover surface particles, organic residues, and metallic contaminations. In this section, various wet cleaning methods used in semiconductor process will be reviewed. 

Hydrodynamic jets both high pressure (1,000 to 2,000 psi or higher) and low pressure (< 100 psi) jets are used in industry to clean surfaces. Particle removal from wafers is typically by low pressure sprays during rinsing. While high pressure sprays are more effective in particle removal, especially small particles, they can easily damage wafers. 

A close look at the surface structure of the particles, like that shown in Figure 4.17(a), indicates that the particles are well-faceted. Likewise the surfaces are clean. No evidence of decoration by support overlayers has been obtained for reduction treatments at temperatures lower than 973 K. The same is observed in all the experimental studies at present available for NM/Ce(M)02, catalysts (97,155,235). 

Early work (53) in in-reactor fuel test loops showed that radiolytic oxygen can be suppressed by maintaining 5 to 10 cm Dg/kg DgO dissolved in the coolant and that operation at pH 10 with lithium hydroxide minimizes deposition of magnetite (Fe304) particles on the fuel sheath surfaces. These conditions minimize corrosion and correspond to a minimum solubility of magnetite. With these coolant conditions the fuel surface remains clean and heat transfer is unimpeded—they are the key to the successful use of carbon steel piping, components, and fittings for the CANDU coolant circuit. A simple and effective chemistry control and coolant purification circuit was developed (54). 

Modified surfaces of sample plates can be used for the affinity capture of analytes from crude mixtures directly on target. Such surfaces can significantly enhance detection sensitivity and can be used for simple sample clean-ups. Titania (Ti02)-coated surfaces, particles or sol-gel systems, for example, have been shown to concentrate phosphopeptides very selectively from peptide fingerprint samples [154, 155]. 

Thus, by the use of an inhomogeneous electric field and the consequent ponderomotive force directed tangential to the surface being cleaned, dust particles can be removed more thoroughly this has also been confirmed experimentally, in that the detachment of dust particles by the use of rod-type electrodes in more efficient than detachment by the use of continuous (plate-type) electrodes. 

---