Surface gas-liquid

D. E. Maitire, Unified theoiy of adsoiption cliromatography with heterogeneous surfaces gas, liquid and super-critical fluid mobile phases , ]. Liq. Chromatogr. 11 1779-1807(1988). 

In a bath-type sonochemical reactor, a damped standing wave is formed as shown in Fig. 1.13 [1]. Without absorption of ultrasound, a pure standing wave is formed because the intensity of the reflected wave from the liquid surface is equivalent to that of the incident wave at any distance from the transducer. Thus the minimum acoustic-pressure amplitude is completely zero at each pressure node where the incident and reflected waves are exactly cancelled each other. In actual experiments, however, there is absorption of ultrasound especially due to cavitation bubbles. As a result, there appears a traveling wave component because the intensity of the incident wave is higher than that of the reflected wave. Thus, the local minimum value of acoustic pressure amplitude is non-zero as seen in Fig. 1.13. It should be noted that the acoustic-pressure amplitude at the liquid surface (gas-liquid interface) is always zero. In Fig. 1.13, there is the liquid surface... 

Common to these devices is the intensification of the surface gas-liquid contacting. They will be delt with individually below. 

The most common configuration for a fluid layer that is heated from below is probably to have a solid boundary at z = 0 and a free surface (gas-liquid interface) at z = 1. However, there is a major simplification of the analysis if both surfaces are free, and the results are qualitatively similar regardless of which of the boundary conditions, (12-202) or (12-203), is applied at which boundary. 

Surface tension of SE (OAV) emulsions and its continuous phases, of relevance for the spray drop formation, were measured versus time using the pendant drop method, demonstrated in Fig. 23.7. It was found that the surface tension values of the emulsions were significantly higher than those of their continuous phases including the same surfactant and thickener composition like the related emulsions. It is assumed that the used surfactants were equally active at oil/water interfaces within the emulsions and at the spray drop surface (gas/liquid interface). Consequently, surfactant availability at the gas/liquid interface seemed being restricted in the presence of surfactant-covered emulsion drops. 

The subscripts i and j indicate the various surfaces gas, liquid, solids - and in principle aU combinations are possible, i.e. all of these theories can be used for all interfaces, but there are exceptions. 

Because of the generality of the symmetry principle that underlies the nonlinear optical spectroscopy of surfaces and interfaces, the approach has found application to a remarkably wide range of material systems. These include not only the conventional case of solid surfaces in ultrahigh vacuum, but also gas/solid, liquid/solid, gas/liquid and liquid/liquid interfaces. The infonnation attainable from the measurements ranges from adsorbate coverage and orientation to interface vibrational and electronic spectroscopy to surface dynamics on the femtosecond time scale. 

Figure Bl.22.8. Sum-frequency generation (SFG) spectra in the C N stretching region from the air/aqueous acetonitrile interfaces of two solutions with different concentrations. The solid curve is the IR transmission spectrum of neat bulk CH CN, provided here for reference. The polar acetonitrile molecules adopt a specific orientation in the air/water interface with a tilt angle that changes with changing concentration, from 40° from the surface nonnal in dilute solutions (molar fractions less than 0.07) to 70° at higher concentrations. This change is manifested here by the shift in the C N stretching frequency seen by SFG [ ]. SFG is one of the very few teclnhques capable of probing liquid/gas, liquid/liquid, and even liquid/solid interfaces.

Analytical separations may be classified in three ways by the physical state of the mobile phase and stationary phase by the method of contact between the mobile phase and stationary phase or by the chemical or physical mechanism responsible for separating the sample s constituents. The mobile phase is usually a liquid or a gas, and the stationary phase, when present, is a solid or a liquid film coated on a solid surface. Chromatographic techniques are often named by listing the type of mobile phase, followed by the type of stationary phase. Thus, in gas-liquid chromatography the mobile phase is a gas and the stationary phase is a liquid. If only one phase is indicated, as in gas chromatography, it is assumed to be the mobile phase. 

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. 

These equations assume that there is no drag force at the gas/liquid interface, such as would be produced by gas flow. For a flat surface inclined at an angle 0 with the horizontal, the preceding equations may be modified by replacing g by g sin 0. For films falhng inside vertical tubes with film thickness up to and including the full pipe radius, see Jackson AlChE1, 231-240 [1955]). 

When reactants are distributed between several phases, migration between phases ordinarily will occur with gas/liquid, from the gas to the liquid] with fluid/sohd, from the fluid to the solid between hquids, possibly both ways because reactions can occur in either or both phases. The case of interest is at steady state, where the rate of mass transfer equals the rate of reaction in the destined phase. Take a hyperbohc rate equation for the reaction on a surface. Then,... 

FIG. 15-23 Power for agitation impellers immersed in single-phase liquids, baffled vessels with a gas-liquid surface [except curves (c) and (g)]. Curves correspond to (a) marine impellers, (h) flat-blade turbines, w = dj/5, (c) disk flat-blade turbines witb and without a gas-liquid surface, (d) curved-blade turbines, (e) pitcbed-blade turbines, (g) flat-blade turbines, no baffles, no gas-liquid interface, no vortex. 

Three criteria for scale-up are that the laboratory and industrial units have the same mass-transfer coefficients /cg and E/cl and the same ratio of the specific interfacial surface and liquid holdup Tables 23-9 and 23-10 give order-of-magnitude values of some parameters that may be expected in common types of liquid/gas contactors. 

With normal excess ammonia the gas/liquid ratio is about 3,500 mVm. At this high ratio there is danger of fouling the surface with tariy reaction products. The ratio is brought down to a more satisfactory value of 1,000 to 1,500 by recycle of some of the effluent. 

Liquid holdup is made up of a dynamic fraction, 0.03 to 0.25, and a stagnant fraction, 0.01 to 0.05. The high end of the stagnant fraction includes the hquid that partially fills the pores of the catalyst. The effective gas/liquid interface is 20 to 50 percent of the geometric surface of the particles, but it can approach 100 percent at high hquid loads with a consequent increase of reaction rate as the amount of wetted surface changes. 

Adsorption The gathering of a gas, liquid, or dissolved substance on the surface or interface zone of another substance. 

At any instant, pressure is uniform throughout a bubble, while in the surrounding emulsion pressure increases with depth below the surfaee. Thus, there is a pressure gradient external to the bubble which causes gas to flow from the emulsion into the bottom of the bubble, and from the top of the bubble back into the emulsion. This flow is about three times the minimum fluidization velocity across the maximum horizontal cross section of the bubble. It provides a major mass transport mechanism between bubble and emulsion and henee contributes greatly to any reactions which take place in a fluid bed. The flow out through the top of the bubble is also sufficient to maintain a stable arch and prevent solids from dumping into the bubble from above. It is thus responsible for the fact that bubbles can exist in fluid beds, even though there is no surface tension as there is in gas-liquid systems. 

The power consumed by an agitator depends on its dimensions and the physical properties of the fluids being mixed (i.e., density and viscosity). Since there is a possibility of a gas-liquid surface being... 

Carbon dioxide gas diluted with nitrogen is passed continuously across the surface of an agitated aqueous lime solution. Clouds of crystals first appear just beneath the gas-liquid interface, although soon disperse into the bulk liquid phase. This indicates that crystallization occurs predominantly at the gas-liquid interface due to the localized high supersaturation produced by the mass transfer limited chemical reaction. The transient mean size of crystals obtained as a function of agitation rate is shown in Figure 8.16. 

Gases, liquids, and solids have different physical properties. A gas fills its container, so that if a certain amonnt of gas is transferred from a small container into a large one, the gas will expand to fill the new container. If there is a hole m the top of a container filled with gas, the gas will escape. A liquid keeps the same volume when transferred from one container to another, but takes the shape of the new container. On Earth, a liquid has a flat, horizontal surface. If there is a hole in its container below that surface, the liquid will spill out. A solid keeps both its shape and its volume when transferred from one container to another. 

The definitions above are an abbreviated version of those used in a veiy complex and financially significant exercise with the ultimate goal of estimating resei ves and generating production forecasts in the petroleum industry. Deterministic estimates are derived largely from pore volume calculations to determine volumes of either oil nr gas in-place (OIP, GIP). This volume when multiplied by a recovery factor gives a recoverable quantity of oil or natural gas liquids—commonly oil in standard barrels or natural gas in standard cubic feet at surface conditions. Many prefer to use barrels of oil equivalency (BOE) or total hydrocarbons tor the sum of natural gas, natural gas liquids (NGL), and oil. For comparison purposes 6,000 cubic feet of gas is considered to be equivalent to one standard barrel on a British thermal unit (Btu) basis (42 U.S. gallons). 

Figure 9-6T. (Top) Cascade Mini-Ring, (metal and plastic). Originally used by permission of Mass Transfer, Inc., now, Glitsch, Inc. (middle and bottom) Elevation and plan views of Ballast rings (right) and Cascade Mini-Rings (left). Note how high aspect ratio of former permits occlusion of interior surfaces. Low aspect ratio of Cascade Mini-Rings, on the other hand, favors orientation that exposes internal surfaces for excellent film formation, intimate mixing, and gas-liquid contact. Used by permission of Glitsch, Inc. Bull. 345.

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