Bubble dispersed

Gas-liquid contactors may be operated either by way of gas bubble dispersion into liquid or droplet dispersion in gas phase, while thin film reactors, i.e. packed columns and trickle beds are not suitable for solid formation due... 

This dispersion of the gas passes through several stages depending on the gas feed rate to the underside of the impeller and the horsepower to the impeller, varying from inadequate dispersion at low flow to total gas bubble dispersion throughout the vessel. The open, without disk, radial flow type impeller is the preferred dispersing unit because it requires lower horsepower than the axial flow impeller. The impeller determines the bubble size and interfacial area. 

The low density of gases makes it more difficult to keep the bubbles dispersed. The bubbles will move to the low-pressure areas, that is, behind the impellers, in the trailing vortices close to the impeller, behind the baffles, and at the inner side after a bend. The bubbles will coalesce in these areas with high gas holdup. It is very difficult to design reactors without low-pressure regions where the low-density fluid will accumulate. One such reactor is the monolith reactor for multiphase flow [32, 33]. 

The parameter p (= 7(5 ) in gas-liquid sy.stems plays the same role as V/Aex in catalytic reactions. This parameter amounts to 10-40 for a gas and liquid in film contact, and increases to lO -lO" for gas bubbles dispersed in a liquid. If the Hatta number (see section 5.4.3) is low (below I) this indicates a slow reaction, and high values of p (e.g. bubble columns) should be chosen. For instantaneous reactions Ha > 100, enhancement factor E = 10-50) a low p should be selected with a high degree of gas-phase turbulence. The sulphonation of aromatics with gaseous SO3 is an instantaneous reaction and is controlled by gas-phase mass transfer. In commercial thin-film sulphonators, the liquid reactant flows down as a thin film (low p) in contact with a highly turbulent gas stream (high ka). A thin-film reactor was chosen instead of a liquid droplet system due to the desire to remove heat generated in the liquid phase as a result of the exothermic reaction. Similar considerations are valid for liquid-liquid systems. Sometimes, practical considerations prevail over the decisions dictated from a transport-reaction analysis. Corrosive liquids should always be in the dispersed phase to reduce contact with the reactor walls. Hazardous liquids are usually dispensed to reduce their hold-up, i.e. their inventory inside the reactor. 

Comparison of the boundaries of the observed flow patterns with the analytical criteria derived by Quandt showed that the bubble, dispersed, and annular flow patterns are subclasses of a pressure gradient-controlled flow. Similarly, flow patterns identified as slug, wave, stratified, and f ailing film are subclasses of a gravity-controlled situation. 

With the carrier stream unsegmented by air bubbles, dispersion results from two processes, convective transport and diffusional transport. The former leads to the formation of a parabolic velocity profile in the direction of the flow. In the latter, radial diffusion is most significant which provides for mixing in directions perpendicular to the flow. The extent of dispersion is characterized by the dispersion coefficient/). 

We also want these designations of A, B, C, and Z) to be more general than gases and sohds. The ideas developed in this chapter apply to any continuous fluid reacting with any dispersed phase. Thus the fluid and rigid phases could be gas, hquid, or sohd, for example, gas bubbles (dispersed) reacting with a hquid soluhon (continuous) or a sohd fihn. Examples such as these are important in most mulhphase reactors, the subject of Chapter 12. 

The interfacial gas-liquid area a is a function of the size of the gas bubble dispersion ... 

Note that to continue further with the design of the reactor by taking a value of fA from Fig. 4.3, or using equation 4.15 or 4.17, requires a knowledge of and a for the bubble dispersion as well as a more exact value of kL. These will depend on the type and configuration of the reactor chosen, the flowrates at which it is operated and the physical properties of the chemical species involved. 

The aim of this paper is to make measurements with liquids of various physical properties in order to define the effect of the liquid properties and operating conditions on the parameter /q, and the limits of validity of the literature models for the interpretation of mass transfer coefficients in bubble dispersions. The method, which is used for the measurements, was verified in Part I to minimize misinterpretations. 

T. Reith, Physical aspects of bubble dispersions in liquids, Ph.D. Thesis, Delft Techn. Univ., 1968. 

Many other foods are mixed dispersions, like ice cream which is an emulsion, foam, and suspension. Others abound. Sausages and frankfurters may be considered to be solidified O/W emulsions in which the oil droplets are covered by a protein membrane and dispersed in a gel [293]. Similarly, cakes can be considered to be air bubbles dispersed in a gel phase. 

A foam in which the liquid consists of two phases in the form of an emulsion. Also termed foam emulsion . Example whipped cream consists of air bubbles dispersed in cream, which is an emulsion. See also Foam. See Foaming Agent. 

A state of subdivision in which the particles, droplets, or bubbles dispersed in another phase have at least one dimension between approximately 1 and 1000 nm. 

Under non-equilibrium conditions of bubble formation the bulk viscosity affects bubble dispersity with increase in viscosity the dispersity decreases (see Section 1.1). 

The initial removal of air was carried out at a vacuum of 15 in. Hg. Consequently, below this pressure, additional air was being liberated from the liquid this air appeared as a mass of tiny bubbles, dispersed throughout the volume of water. This lowered the specific weight of the pumped mixture. 

Different authors have identified various flow regimes in large channels. In both vertical and horizontal configurations these include bubbly, dispersed bubbly, slug, pseudo-slug, churn, annular, annular mist and dispersed droplet flows. An important difference in minichannels is that the liquid flow is preferentially laminar. Surface tension effects have more and more influence as the hydraulic diameter is reduced. Gravity becomes negligible compared to surface tension so that the orientation is less influential. 

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,... 

The electroresistivity probe, recently proposed by Burgess and Calder-bank (B32, B33) for the measurement of bubble properties in bubble dispersions, is a very promising apparatus. A three-dimensional resistivity probe with five channels was designed in order to sense the bubble approach angle, as well as to measure bubble size and velocity in sieve tray froths. This probe system accepts only bubbles whose location and direction coincide with the vertical probe axis, the discrimination function being achieved with the aid of an on-line computer which receives signals from five channels communicating with the probe array. Gas holdup, gas-flow specific interfacial area, and even gas and liquid-side mass-transfer efficiencies have been calculated directly from the local measured distributions of bubble size and velocity. The derived values of the disper-... 

Foams may be prepared by either one of two fundamental methods. In one method, a gas such as air or nitrogen is dispersed in a continuous liquid phase (e.g. an aqueous latex) to yield a colloidal system with the gas as the dispersed phase. In the second method, the gas is generated within the liquid phase and appears as separate bubbles dispersed in the liquid phase. The gas can be the result of a specific gasgenerating reaction such as the formation of carbon dioxide when isocyanate reacts with water in the formation of water-blown flexible or rigid urethane foams. Gas can also be generated by volatilization of a low-boiling solvent (e.g. trichlorofluoromethane, F-11, or methylene chloride) in the dispersed phase when an exothermic reaction takes places, (e.g. the formation of F-11 or methylene chloride-blown foams). 

Figure 4 Side view of simulated hydrogen bubble dispersion for different...

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