Entrainment Due to Foam

A light, frothy foam is naturally more susceptible to entrainment than is a clear, settled liquid. Knockout drums handling foam, therefore, should be designed for a lower K value. Perhaps the K value that can be tolerated might be reduced by 25 to 35 percent, in the anticipation of such foam. 

Once this foam level rises above the feed inlet nozzle, the vapor in the feed blows the foam up the drum. A massive carry-over of liquid (or foam) into the vapor line then results. I mentioned above that a 

Use of Vortex Tubes marketed by EGS systems has been shown to suppress carry-over of foam. 

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As discussed in Section 1.2.2 the bubble shapes in fairly dry foams and froths (4 gas > 0.83, approximately) are not spheres or distorted spheres, but polyhedrons. In practice there will be distributions of both gas-cell sizes and shapes. In addition to the gas bubbles, froth contains the floated particles, pulp liquor, and a fraction of (hydrophilic) particles that did not float due to bubble attachment, but which were mechanically entrained in the froth. The pulp liquor and these latter particles all have to be allowed to drain back out of the froth. The rate of this drainage will be greatest at the froth-pulp interface (i.e., the bottom of the froth layer) and slowest near the top of the froth layer. Froth drainage equations are discussed elsewhere [53]. The froth needs to be a stable enough foam that some time can be allowed for these drainage processes, and also so that the upper layer(s) of the froth can be swept out of the flotation cell. On the other hand, the froth should not be too stable as a foam so that it will break easily after collection. In addition to the role of the frother, froth stability is also promoted by increasing liquid viscosity. 

Dissolved air is not readily drawn out of solution. It becomes a problem when temperatures rise rapidly or pressures drop. Petroleum oils contain as much as 12% dissolved air. When a system starts up or when it overheats, this air changes from a dissolved phase into small bubbles. If the bubbles are very small in diameter, they remain suspended in the liquid phase of the oil, particularly in high viscosity oils. This can cause air entrainment, which is characterized as a small amount of air in the form of extremely small bubbles dispersed throughout the bulk of the oil. Air entrainment is treated differently than foam and is typically a separate problem. Some of the potential effects of air entrainment include pump cavitation, spongy and erratic operation of hydraulics, loss of precision control, vibrations, oil oxidation, component wear due to reduced lubricant viscosity, equipment shutdown when low oil pressure switches trip, microdieseling... 

A variety of blockages and deposits are observed in different parts of pulp mills and are of a granular to viscous, sticky nature. The deposition of small amounts of these substances on equipment can be handled, but the use of certeiin pulping procedures with some species can lead to rapid accumulations that will cover the surfaces of various parts of the equipment of both the pulp and paper mill, as well as becoming entrained in the pulp. Some of these deposits are due to the addition of de-foaming agents, coating components, and so on, but only those caused by wood extractives are considered here. 

Principal gaseous products from fission and radioactive decay are xenon, krypton, and helium. Using the SCALE/ORIGEN program, the whole core inventories of these elements after 10 years of operation were estimated to be 217 g of xenon, 24 g of krypton, and 1 mg of helium. A considerable amount of these elements would be expected to be entrained in the foam. Consequently, a core clad breach due to pressurization is not credible, and consequences of clad mpture would be relatively insignificant as compared to a commercial power reactor. 

Density changes in tank fluids lead to errors in hydrostatic head level measurements Aeration of fluid, foaming, temperature changes Flow meter errors due to entrained air or gases, partly filled pipelines... 

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