Gas-liquid scattering

Keywords Gas-liquid scattering Hybrid QM/MM molecular dynamics Interfacial chemistry Room temperature ionic liquids Squalane... 

Spectroscopy is basically an experimental subject and is concerned with the absorption, emission or scattering of electromagnetic radiation by atoms or molecules. As we shall see in Chapter 3, electromagnetic radiation covers a wide wavelength range, from radio waves to y-rays, and the atoms or molecules may be in the gas, liquid or solid phase or, of great importance in surface chemistry, adsorbed on a solid surface. 

It was predicted in 1923 by Smekal and shown experimentally in 1928 by Raman and Krishnan that a small amount of radiation scattered by a gas, liquid or solid is of increased or decreased wavelength (or wavenumber). This is called the Raman effect and the scattered radiation with decreased or increased wavenumber is referred to as Stokes or anti-Stokes Raman scattering, respectively. 

Very little in the way of advances has occurred since 1971 in the applications of ultraviolet or infrared spectroscopy to the analysis of fluonnated organic compounds Therefore, only gas-liquid chromatography, liquid chromatography, mass spectrometry, and electron scattering for chemical analysis (ESCA) are discussed The application of nuclear magnetic resonance (NMR) spectroscopy to the analysis of fluonnated organic compounds is the subject of another section of this chapter... 

The use of even the very simple models for isothermal operation described in Section IV,B requires a substantial amount of information regarding the elementary iate processes occurring in a gas-liquid-particle operation, as discussed in Section IV,A. While a considerable amount of information of this kind is available in the chemical engineering literature, it is widely scattered. It will be attempted in this section to present a comprehensive review of this information in order to facilitate its use. It is hoped that this review will be of value not only to those chemical engineers directly interested in the practical applications of gas-liquid-particle operations, but also, by pointing to the several areas characterized by very limited information, to those interested in research in this field. 

Backscatter Reflected or scattered radiation in the general direction of the source after it strikes matter (gas, liquid, or solid). 

Gas-liquid mass transfer in fermentors is discussed in detail in Section 12.4. In dealing with in gas-sparged stirred tanks, it is more rational to separate and a, because both are affected by different factors. It is possible to measure a by using either a light scattering technique [9] or a chemical method [4]. Ihe average bubble size can be estimated by Equation 7.26 from measured values of a and the gas holdup e. Correlations for have been obtained in this way [10, 11], but in order to use them it is necessary that a and d are known. 

Let us consider now behaviour of the gas-liquid system near the critical point. It reveals rather interesting effect called the critical opalescence, that is strong increase of the light scattering. Its analogs are known also in other physical systems in the vicinity of phase transitions. In the beginning of our century Einstein and Smoluchowski expressed an idea, that the opalescence phenomenon is related to the density (order parameter) fluctuations in the system. More consistent theory was presented later by Omstein and Zemike [23], who for the first time introduced a concept of the intermediate order as the spatial correlation in the density fluctuations. Later Zemike [24] has applied this idea to the lattice systems. 

Ruiz-Sala et al. (129) described a reversed-phase HPLC method with a light-scattering detector for the analysis of TGs in milk fat. The identification of TGs was carried out by a combination of HPLC and gas-liquid chromatography (GLC), and was based on the equivalent carbon numbers and retention times of different standard TGs. Finally, quantitation of peak areas from HPLC chromatograms was carried out after applying a deconvolution program to the parts of chromatograms with poor resolution. 

From the gas phase scattering data we conclude that a plane wave state for the electron in liquid helium lies at positive energy relative to the vacuum level in agreement with Sommers electron injection experiment. We proceed now to a semiquantitative treatment of free electron states in liquids characterized by a positive scattering length, which will be used to estimate the energy of interaction of a free electron with liquid helium (18). 

Drop size distribution can be indirectly measured by using the light-transmission technique. When a beam of light is passed through a gas-liquid dispersion, light is scattered by the gas bubbles. It was... 

In many practical applications, gas-liquid mass transfer plays a significant role in the overall chemical reaction rate. It is, therefore, necessary to know the values of effective interfacial area (aL) and the volumetric or intrinsic gas-liquid mass transfer coefficients such as kLah, kL, ktaL, kg, etc. As shown in Section IX, the effective interfacial area is measured by either physical e.g., photography, light reflection, or light scattering) or chemical methods. The liquid-side or gas-side mass-transfer coefficients are also measured by either physical (e.g., absorption or desorption of gas under unsteady-state conditions) or chemical methods. A summary of some of the experimental details and the correlations for aL and kLaL reported in the literature are given by Joshi et al. (1982). In most practical situations, kgaL does not play an important role. 

Monochromatic light (in the visible or uv range) is directed through the sample (gas, liquid, crystal, or powder), which must be translucent. Light scattered at 90° or 180° is collected, analyzed, and recorded. The incident light commonly used in modern instruments is from a laser source and may be polarized. The Raman effect involves shifts of frequency of scattered light relative to that from the source... 

The application and scope of infra-red spectra was considerably extended by the discovery by Raman and Krishnan and independently of them by Landsberg and Mandelstam, of what is now known as the Raman effect. When a beam of light passes through a gas, liquid or transparent solid, part of the light is scattered in all directions and if the incident light has a line spectrum, it is found that the scattered light contains exactly the same... 

D. W. Oxtoby and W. M. Gelbart. Depolarized light scattering near the gas-liquid critical point. J. Chem. Phys., 60 3359-3367 (1974). 

The interfacial area is known accurately only in some systems used in laboratory studies falling laminar films, laminar cylindrical jets, undisturbed gas-liquid and liquid-liquid interfaces, and solid castings of known dimensions immersed in liquids. In all reactor systems used industrially such as packed towers, spray towers, and bubble trays, the interfacial area is relatively difficult to determine. Photographic, gamma-ray, light scattering and chemical methods have been used to determine a in bubble dispersions (5, 6, 7, 8, iO, 42). For an average bubble diameter dn, a superficial gas velocity Usa and a bubble rise velocity Un,... 

Before using one of the above relationships, it is judicious to check whether the gas-liquid system of interest is included in the tabulation of experimental diffusivities prepared by Johnson and Babb (J8) or in more recent but scattered tables presented by Davies et al. (D8), Dim et al. (D15), Tyn (T12), Akgerman and Gainer (A2), Perry (P5, p. 3-224), and De Kee and Laudie (Dll). 

A better method is to cool the specimen with a stream of cold gas. Liquid nitrogen, for example, boils at 77°K ( = - I96°C) and can easily produce nitrogen gas at the specimen at about — 150°C. Gas produces much less background scattering than a liquid coolant. 

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