Gas entrainment

Gas entrained in the fluid and the flexibiflty of the pipe wall both result in lowering of the wave speed. For deaerated water, the wave speed is about 1250 m/s. Detailed methods of analysis and evaluation of hydraulic transients may be found in the flterature (25). 

Figure 32.29 (a) Effect of entrained gas on liquid displacement (b) solubility of air in oil. Example At 5 in Hg with 3% gas entrainment by volume, pump capacity is reduced to 84 per cent of theoretical displacement... 

Liquid vortexing in suction vessel, thus creating gas entrainment into suction piping. Figure 3-43 suggests a common method to eliminate suction vor-... 

Further increase in the gas flow rate in liquid ring flow leads to a liquid lump flow, of which the high-speed core gas entrains the liquid phase and liquid lumps slide... 

In the Fig.4, it can be seen that the gas hold-up in both riser and downcomer decreases with increasing the draft-tube horn-mouth diameter and approaches the maximum when the draft-tube hom-mouth diameter is 1.05m. However, due to the gas hold-up decreases more in the downcomer, the gas hold-up difference between the downcomer and the riser increases. Therefore, the apparent density difference between the riser and the downcomer enhances, causing higher liquid superficial velocity in the downcomer and in the riser With increasing the hom-mouth diameter. Fig.5 also shows that the existence of hom-mouth promotes the ability to separate gas from liquid and decreases the amount of gas entrained into the downcomer. 

Momentum Dissipation of a Gas-Solid Two-Phase Jet. Gas velocity profiles in a gas-solid two-phase jet inside a fluidized bed were determined at five different horizontal planes perpendicular to jet direction using a pitot tube (Yang and Keaims, 1980). The velocity profiles were integrated graphically, and gas entrainment into a jet was found to occur primarily at the base of the jet. 

Figure 31. Calculated gas entrainment into a jet in a fluidized bed based on Tollmien s Similarity.

The following may be used as a rule of thumb for dimensioning vacuum lines The lines should be as short and as wide as possible. They must exhibit at least the same aoss-section as the intake port at the pump. If particular circumstances prevent shortening the suction line, then it is advisable, whenever this is justifiable from the engineering and economic points of view, to include a roots pump in the suction line. This then acts as a gas entrainment pump vi/hich reduces line impedance. 

Figure 17.11. Types of contactors for reacting gases with liquids many of these also are suitable for reacting immiscible liquids. Tanks (a) with a gas entraining impeller (b) with baffled impellers (c) with a draft tube (d) with gas input through a rotating hollow shaft, (e) Venturi mixer for rapid reactions, (f) Self-priming turbine pump as a mixer-reactor, (g) Multispray chamber. Towers (h) parallel flow falling film (i) spray tower with gas as continuous phase (j) parallel flow packed tower (k) counter flow tray tower. (1) A doublepipe heat exchanger used as a tubular reactor.

In standpipe systems, aeration from the side is often implemented to provide a lubricating effect for the solids flow. Aeration is also used in process applications to provide a stripping gas to remove undesirable gas entrained by the solid particles in the standpipe flow. With side aeration, the multiplicity of flows in the standpipe becomes much more complicated. The flow patterns of solids and gas depend not only on Ap and y but also on the aeration locations and aeration rates. For a single aeration point, the possible number of flow regimes increases to 12 [Mountziaris and Jackson, 1991]. 

The principal reactors used are fluidized bed reactors, called Synthol reactors, in which the feed gas entrains an iron catalyst powder in a circulating flow. The suspension enters the bottom of the fluidized bed reaction section, where the Fischer-Tropsch and the gas shift reactions proceed at a temperature of from 315 to 330°C. These reactions are highly exothermic, as described previously, and the large quantity of heat released must be removed. The products in gaseous form together with the catalyst are taken off from the top of the reactor. By decreasing the gas velocity in another section, the catalyst settles out and is returned for reuse. The product gases are then condensed to the liquid products. 

The ICP-MS has several analytical attractions including very low detection limits [parts per billion to parts per trillion (ppb to ppt) levels], a large linear dynamic range, relatively simple spectra, excellent stability, multielement determination capability, and ability to measure isotopic abundances. Disadvantages are mainly due to the formation of polyatomic interferences from the plasma gas, entrained gases, and matrix elements such as Cl [16]. 

For liquid-liquid mixtures, the calculations of mixing time and power (or Newton) number outlined above are valid for unbaffled vessels only as long as the vortex created by the stirrer does not reach the stirrer head. Otherwise, gas entrainment occurs and the physical properties of the system change. The depth of the liquid-gas interface at the vessel axis with respect to static liquid surface level, HL, can be related to the Froude and Galileo numbers. Some of the reported relationships are summarized in Table XIV. The value of H, at which the vortex reaches the upper impeller blades level can be expressed as... 

The relationships for mixing time and power consumption discussed above assume no gas entrainment. For unbaffled vessels, this occurs only below a certain maximum stirrer speed. This maximum stirrer speed, defined in terms of the dimensionless Froude number (Frmax = N djg), depends upon the... 

The extent of gas entrainment depends on the turbulence at the liquid surface and the downward volumetric flow rate. Thus, the impeller design, diameter, and location are very important. Matsumura et al. (1977) proposed the following correlation for a 0.218 in. diameter vessel using a disk turbine ... 

Qe and Qg are the volumetric flow rates of entrained and sparged gases. Matsumura et al. (1978) also proposed the following correlation for power consumption in the presence of gas entrainment ... 

In waste-water treatment, the energy-efficient transfer of oxygen in the aqueous phase is very critical. Since reactions are slow, a long contact time is required. Both of these can be achieved by a conventional surface aerator or novel UNOX surface aeration system. The minimum impeller speed needed for surface aeration in the absence of gas sparging can be obtained from Eq. (6.30). Similar calculations in the presence of gas sparging can be carried out using either Eq. (6.31) for a turbine stirrer or Eq. (6.36) for other stirrers. The gas entrainment and the power consumption in the presence of gas entrainment can be obtained using Eqs. (6.37) and (6.38), respectively. 

The vapor velocity in a finite-stage contactor column can be limited by the liquid handling capacity of the downcomers or by entrainment of liquid droplets in the rising gases. In most cases, however, downcomer limitations do not set the allowable vapor velocity instead, the common design basis for choosing allowable vapor velocities is a function of the amount of gas entrainment which can result in improper operation or flooding of the column. 

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