Viscosity Bubble

With their lower viscosity, bubbles will deform more readily than emulsion droplets and therefore be relatively more prone to depart from Stokes law behaviour. 

The scope of possible foam applications in the field warrants extensive theoretical and experimental research on foam flow in porous media. A lot of good work has been done to explain various aspects of the microscopic foam behavior, such as apparent foam viscosity, bubble generation by capillary snap-off, etc.. However, none of this work has provided a general framework for modeling of foam flow in porous media. This paper attempts to describe such a flow with a balance on the foam bubbles. 

In the case of gas emerging from a simple submerged orifice at very low flow rates, bubbles periodically form at the orifice, grow to a certain size, and break away. For media with a reasonably low viscosity, bubble diameter is a function of the interfacial tension and density as indicated below ... 

Oldshue (1966), by using the above equation, showed the various bubble sizes required to have various terminal velocities for fluids with a specific gravity of 1.0 and varying viscosities. If air bubbles are to rise from viscous fluids with any appreciable velocity, then they must have a reasonable size. For example in fluid of 10 poise viscosity, bubbles that are less than 0.76 mm in diameter will remain in the system, and only those that are 2.03 mm or larger will enter and pass out with any appreciable velocity. This, of course, causes much more difficulty in a small-scale fermenter than it would in a large-scale one. 

With their lower viscosity, bubbles will deform more readily than emulsion droplets and, therefore, be relatively more prone to depart from Stokes law behaviour. Hadamard and Rybczynski developed a terminal velocity equation for the creaming of bubbles with a mobile surface ... 

Extrusion. The effects of excess moisture during extrusion may include a foamy melt, lower melt viscosity, bubbles in the extrudate, surging that results in arrow heads and wave forms in the extrudate, surface roughness, and reduced mechanical properties. 

Methods for foam consistency measurements are also reported. In Mitkevitch s procedure [21], foam is generated in a vertical cylinder and a rod is allowed to fall into the lather. The time of fall is used as an assessment of foam consistency. Scott and Thompson [22] described a method for the relative evaluation of detergents according to their foam consistency they showed that foam consistency is influenced by bulk liquid and surface viscosity, bubble size, and geometry. Three instruments for measuring foam consistency are the mobilometer, the consistometer, and a modified viscosimeter. 

Buckland, B.C., Gwewonyo, K., DiMasi, D., Hunt, G., Westerfield, G., and Nienow, A.W. (1988) Improved performance in viscous mycelial fermentations by agitator retrofitting. BiotechnoL Bioertg., 31, TS7—H2. Heijnen, J.J. and Van t Riet, K. (1984) Mass transfer, mixing and heat transfer phenomena in low viscosity bubble column reactors. Chetn. Eng. /, 28, B21-B42. 

In a bubble viscometer, a liquid streams downward in the ring-shaped zone between the glass wall of a sealed tube and a rising air bubble. The rate at which the bubble rises is a direct measure of the kinematic viscosity. The rate of bubble rise is compared with a set of calibrated bubble tubes containing liquids of known viscosities. Bubble viscometers are shown in Figure 7-30. 

HeijnenJJ Van t Riet K Mass transfer, mixing and heat transfer phenomena in low viscosity bubble column reactors, Chem EngJ 28 B21-B42, 1984. 

Unlike gases, liquid viscosity decreases as temperature increases, as the molecules move further apart and decrease their internal friction. Like gases, oil viscosity increases as the pressure increases, at least above the bubble point. Below the bubble point, when the solution gas is liberated, oil viscosity increases because the lighter oil components of the oil (which lower the viscosity of oil) are the ones which transfer to the gas phase. 

The collection of representative reservoir fluid samples is important in order to establish the PVT properties - phase envelope, bubble point, Rg, B, and the physical properties - composition, density, viscosity. These values are used to determine the initial volumes of fluid in place in stock tank volumes, the flow properties of the fluid both in the reservoir and through the surface facilities, and to identify any components which may require special treatment, such as sulphur compounds. 

The ease with which small gas bubbles can escape from the liquid phase is determined by the liquid viscosity higher viscosities imply longer residence times. Typical residence times vary from, some 3 minutes for a light crude to up to 20 minutes for very heavy crudes. 

The specific surface, a, is also relatively insensitive to the duid dynamics, especially in low viscosity broths. On the other hand, it is quite sensitive to the composition of the duid, especially to the presence of substances which inhibit coalescence. In the presence of coalescence inhibitors, the Sauter mean bubble size, is significantly smaller (24), and, especially in stirred bioreactors, bubbles very easily circulate with the broth. This leads to a large hold-up, ie, increased volume fraction of gas phase, 8. Sp, and a are all related... 

Increases in broth viscosity significantly reduce k a and cause bubble size distributions to become bimodal (30). Overall, k a decreases approximately as the square root of the apparent broth viscosity (31). k a can also be related to temperature by the relationship (32)... 

Viscose Aging, Filtration, and Deaeration. After the dissolution step, the viscose cannot be spun into fibers because it contains many small air bubbles and particles. Furthermore, the degree of xanthation is too high, with too many of the xanthate groups in positions dictated by their accessibihty and not in the ideal positions for uniform dissolution. 

Continuous deaeration occurs when the viscose is warmed and pumped into thin films over cones in a large vacuum tank. The combination of the thinness of the Hquid film and the dismption caused by the boiling of volatile components allows the air to get out quickly. Loss of water and CS2 lower the gamma value and raise the cellulose concentration of the viscose slightly. Older systems use batch deaeration where the air bubbles have to rise through several feet of viscose before they are Hberated. 

Flow Past Deformable Bodies. The flow of fluids past deformable surfaces is often important, eg, contact of Hquids with gas bubbles or with drops of another Hquid. Proper description of the flow must allow for both the deformation of these bodies from their shapes in the absence of flow and for the internal circulations that may be set up within the drops or bubbles in response to the external flow. DeformabiUty is related to the interfacial tension and density difference between the phases internal circulation is related to the drop viscosity. A proper description of the flow involves not only the Reynolds number, dFp/p., but also other dimensionless groups, eg, the viscosity ratio, 1 /p En tvos number (En ), Api5 /o and the Morton number (Mo),giJ.iAp/plG (6). 

Because the reaction takes place in the Hquid, the amount of Hquid held in the contacting vessel is important, as are the Hquid physical properties such as viscosity, density, and surface tension. These properties affect gas bubble size and therefore phase boundary area and diffusion properties for rate considerations. Chemically, the oxidation rate is also dependent on the concentration of the anthrahydroquinone, the actual oxygen concentration in the Hquid, and the system temperature (64). The oxidation reaction is also exothermic, releasing the remaining 45% of the heat of formation from the elements. Temperature can be controUed by the various options described under hydrogenation. Added heat release can result from decomposition of hydrogen peroxide or direct reaction of H2O2 and hydroquinone (HQ) at a catalytic site (eq. 19). 

Film. By far the largest appHcation for LLDPE resins (over 60% in the United States) is film. Because LLDPE film has high tensile strength and puncture resistance, it is able to compete with HDPE film for many uses. The toughness and low temperature properties of LLDPE film also exceed those of conventional LDPE. Furthermore, because LLDPE resins exhibit relatively low strain hardening in the molten state and lower extensional viscosity, it can be produced at high rates with Httle risk of bubble breaks. 

Viscosity can also be determined from the rising rate of an air bubble through a Hquid. This simple technique is widely used for routine viscosity measurements of Newtonian fluids. A bubble tube viscometer consists of a glass tube of a certain size to which Hquid is added until a small air space remains at the top. The tube is then capped. When it is inverted, the air bubble rises through the Hquid. The rise time in seconds may be taken as a measure of viscosity, or an approximate viscosity in mm /s may be calculated from it. In an older method that is commonly used, the rate of rise is matched to that of a member of a series of standards, eg, with that of the Gardner-Holdt bubble tubes. Unfortunately, this technique employs a nonlinear scale of letter designations and may be difficult to interpret. 

Density. The density of transparent vitreous sihca is approximately 2.20 g/cm. Translucent and opaque glasses have lower densities owing to the entrapped bubbles. The density of translucent Vitreosil, for example, is 2.07—2.15 g/cm (87,119). The density of transparent vitreous sihca decreases with increasing hydroxyl content and with lower fictive (glass stmcture equihbrium) temperatures. The fictive temperature depends on the thermal history and on glass viscosity (120). 

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