Mixed-phase flow

When dealing with a vapor-liquid mixture, 1 assume the flowing velocity and density to be an average of both phases. As long as their combined velocity is above 15 ft/s, we can safely assume both phases are moving in what is called emulsified flow through the piping. 

But what happens if a saturated liquid suddenly starts to boil inside a pipe This is called camtation. For example, hot water saturated with air suddenly enters a section of smaller diameter piping. The resulting increase in velocity causes a reduction in flowing pressure. The reduced pressure causes that water to boil violently. The turbulence created by the generation of bubbles of steam combined with the oxygen cause rapid corrosion due to a combination of localized oxidation and localized erosion. 

Be careful when hot water that has not been deaerated is used in a process plant. Cavitation failures in hot water piping systems are a common problem. 

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Glaser and Litt (G4) have proposed, in an extension of the above study, a model for gas-liquid flow through a b d of porous particles. The bed is assumed to consist of two basic structures which influence the fluid flow patterns (1) Void channels external to the packing, with which are associated dead-ended pockets that can hold stagnant pools of liquid and (2) pore channels and pockets, i.e., continuous and dead-ended pockets in the interior of the particles. On this basis, a theoretical model of liquid-phase dispersion in mixed-phase flow is developed. The model uses three bed parameters for the description of axial dispersion (1) Dispersion due to the mixing of streams from various channels of different residence times (2) dispersion from axial diffusion in the void channels and (3) dispersion from diffusion into the pores. The model is not applicable to turbulent flow nor to such low flow rates that molecular diffusion is comparable to Taylor diffusion. The latter region is unlikely to be of practical interest. The model predicts that the reciprocal Peclet number should be directly proportional to nominal liquid velocity, a prediction that has been confirmed by a few determinations of residence-time distribution for a wax desulfurization pilot reactor of 1-in. diameter packed with 10-14 mesh particles. 

Glaser, M. B. and Litt, M. A.I.Ch.E.Jl. 9 (1963) 103. A physical model for mixed phase flow through beds of porous particles. 

In one mixer-settler design, the mixed phases flow down a shallow trough placed over the settler, which gives them an opportunity to coalesce and separate before entering the settler. In this way, the capacity of the settler is markedly increased, with a concomitant reduction in the inventory of solvent required for a given duty. 

Glaser, M. B., and Lichtenstein, I. Interrelation of packing and mixed phase flow parameters with liquid residence time distribution. AJ.Ch.E. Journal 9, 30 (1963). (II,E)... 

Even today, how to calculate SRVs for mixed-phase flow conditions is the subject of serious debate. The difficulty exists in the fact that the gas and liquid ratios during relief are not necessarily constant. 

Unless more restrictive velocities are specif ted. maximum velocity should not exceed 20ft/s (6 m/s) in mixed phase flow. 

Acoustic/ultrasonic techniques that have been developed into flow-monitoring instruments are Doppler, cross-correlation, and transit-time methods. An ultrasonic Doppler flowmeter has been applied to single-phase turbulent flows and mixed-phase (solid/liquid or gas/liquid) flows. The crosscorrelation technique is mainly for mixed-phase flows, whereas the transit-time method has been applied to single-phase flows, either liquid or gas, in large pipes. 

The Doppler technique measures the frequency shift of scattered waves with respect to incident sound waves. The technique, therefore, requires the presence of scatterers in the flow that is being monitored. The scatterers could be turbulent eddies or vortex shedding for liquid single-phase flows, and solid particles for solid/fluid mixed-phase flows. The basic geometry of a Doppler... 

A coal slurry flow is a practical mixed-phase flow that requires close monitoring to ensure safe and efficient transport of the coal slurry. In addition to the flow rate measurement, detection of gas bubbles and settling of solids is equally important in coal slurry lines. In this section, we will describe a cross-correlation flowmeter that can reliably measure coal slurry flow rates over wide ranges of coal concentration and flow velocity. We will also illustrate how the flowmeter can detect settling of solids and recognize the presence of gas bubbles. Both laboratory and pilot plant flow tests are included. 

In today s industrial applications, Coriolis mass flowmeters are widely used by process engineers to monitor mass flow rate. This meter measures the Coriolis force that depends on the mass momentum of the flow, and, in principle, it can be applied to both single- and mixed-phase flows. Magnetic or optical detectors are generally used to detect mass-flow-related Coriolis acceleration. A brief description of the Coriolis flowmeter will be presented because it is widely used in industrial processes. 

To conclude this chapter, we will present a brief introduction to some emerging technologies for monitoring mixed-phase flow. 

In industry, it is often more important to measure mass flow rate than to monitor volumetric flow rate. To determine the mass flow rate of a mixed-phase flow, one must measure the velocity and concentration of each phase. This becomes a very difficult task and none of the current techniques can accomplish it. In practice, most mass flowmeters measure the relative change of a physical parameter like capacitance. To relate this measurement to solids concentration, the flowmeters must be calibrated against other, direct, methods... 

The capacitive method for measuring solids concentration is based on the change in capacitance in the presence of solids within the sensing volume. For example, in a solid/gas flow, because solids normally have a higher dielectric constant than the gas, the measured capacitance increases with solids concentration. As long as the solids concentration is low, a linear expression for the capacitance of a mixed-phase flow is generally assumed. For mixed-phase flows, Eq. 6.5 can be rewritten as... 

Thus far, we have discussed capacitive techniques for sensing solids concentration in a mixed-phase flow. We must now discuss how to measure flow velocity by the capacitive method, because the ultimate goal is that of measuring mass-flow-rate. Two methods are commonly suggested for... 

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