Entrained gas

Phenomena at Liquid Interfaces. The area of contact between two phases is called the interface three phases can have only aline of contact, and only a point of mutual contact is possible between four or more phases. Combinations of phases encountered in surfactant systems are L—G, L—L—G, L—S—G, L—S—S—G, L—L, L—L—L, L—S—S, L—L—S—S—G, L—S, L—L—S, and L—L—S—G, where G = gas, L = liquid, and S = solid. An example of an L—L—S—G system is an aqueous surfactant solution containing an emulsified oil, suspended soHd, and entrained air (see Emulsions Foams). This embodies several conditions common to practical surfactant systems. First, because the surface area of a phase iacreases as particle size decreases, the emulsion, suspension, and entrained gas each have large areas of contact with the surfactant solution. Next, because iaterfaces can only exist between two phases, analysis of phenomena ia the L—L—S—G system breaks down iato a series of analyses, ie, surfactant solution to the emulsion, soHd, and gas. It is also apparent that the surfactant must be stabilizing the system by preventing contact between the emulsified oil and dispersed soHd. FiaaHy, the dispersed phases are ia equiUbrium with each other through their common equiUbrium with the surfactant solution. 

Segmental and eccentric orifices are frequently used for gas metering when there is a possibility that entrained liqiiids or solids would otherwise accumulate in front of a concentric circular orifice. This can be avoided if the opening is placed on the lower side of the pipe. For hquid flow with entrained gas, the opening is placed on the upper side. The pressure taps should be located on the opposite side of the pipe from the opening. 

Entrained gas from the process lowering NPSH available. 

Reduce entrained gas in liquid by process changes as needed. 

Entrainment of fine particulate matter such as sand and silt in cooling water can contribute significantly to erosion-corrosion. In these cases it is important to eliminate or reduce the amount of particulate by settling or filtration. It may also be necessary to reduce or eliminate entrained gas bubbles. 

Entrained gas and air expands under the reduced pressure of the suction stroke, lowering the suction efficiency. Gas in water-base mud may also deteriorate the natural rubber parts used. Gases are usually separated with baffles or by changing mud composition. 

Figure 32.27 shows how a centrifugal pump is affected, particularly at low flow rates, and the behavior is typical of conventional centrifugal pumps. Figures 32.28 and 32.29 present well-known information on the effects of dissolved and entrained gas on the volumetric efficiency of a positive displacement pump. 

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... 

More pumps fail as a direct result of improper installation than any other single factor. The predominant reasons for these failures include starvation, caused by inadequate or inconsistent suction conditions distortion, caused by pipe strain or improper foundation and turbulent that results from piping or entrained gas problems. Centrifugal pump installation must follow Hydraulic Institute Standards, which provide specific guidelines to prevent these installation and performance problems. This chapter will address the fundamental requirements for proper installation. 

Hydrocyclones are used for removing entrained gas bubbles from liquids, and the extracted gas collects in the gas-core of the secondary vortex before leaving through the vortex finder. Nebrensky(43) points out that because of the low pressure in the region close to the axis, they will also remove dissolved gases(43). 

In entrainer sublimation, an entrainer gas is blown into the vaporisation chamber of a sublimer in order to increase the vapour flowrate to the condensing equipment, thereby increasing the yield. Air is the most commonly used entrainer, though superheated steam can be employed for substances such as anthracene that are relatively insoluble in water. If steam is used, the vapour may be cooled and condensed by direct contact with a spray of cold water. Although the recovery of the sublimate is efficient, the product is wet. The use of an entrainer gas in a sublimation process also provides the heat needed for sublimation and an efficient means of temperature control. If necessary, it may also provide dilution for the fractional condensation at the desublimation stage. Entrainer sublimation, whether by gas flow over a static bed of solid particles or through a fluidised bed, is ideally suited to continuous operation. 

A general-purpose, continuous entrainer-sublimation plant is shown in Figure 15.34. The impure feedstock is pulverised in a mill and, if necessary, a suitable entrainer gas,... 

As discussed in Section 15.5.2, the separation of two or more sublimable substances by fractional sublimation is theoretically possible if the substances form true solid solutions. Gillot and Goldberger(10°) have reported the development of a laboratory-scale process known as thin-hlm fractional sublimation which has been applied successfully to the separation of volatile solid mixtures such as hafnium and zirconium tetrachlorides, 1,4-dibromobenzene and l-bromo-4-chlorobenzene, and anthracene and carbazole. A stream of inert, non-volatile solids fed to the top of a vertical fractionation column falls counter-currently to the rising supersaturated vapour which is mixed with an entrainer gas. The temperature of the incoming solids is maintained well below the snow-point temperature of the vapour, and thus the solids become coated with a thin film (10. im) of sublimate which acts as a reflux for the enriching section of the column above the feed entry point. 

Compression rates typically vary between 0.0025 and 0.0055 for pellet feedstocks. If the compression rate is too low, then the compaction rate of the solid feedstock may not be high enough to force the entrained gas out through the hopper. If the compression rate is too high, poor melting performance and solid bed breakup can occur. For example, LDPE resins can be melted very easily using a screw with a compression rate of 0.0055, while LLDPE resins perform best with a compression rate near 0.0030 [3, 4]. If an LLDPE is extruded using a screw with a compression rate of 0.0055, then solid bed breakup and solid polymer particles in the extrudate are likely to occur. 

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