Demisting cyclones

Fig. 3.1.1. Sketch of a tangential-inlet cyclone with the flow pattern indicated. The coordinate directions are shown, normally the 2-axis coincides with the axis of the cyclone or swirl tube. To the right, the radial distributions of the axial and tangential gas velocity components are sketched. It is understood that the dust outlet may be the Uquid outlet for the case of a demisting cyclone...

This phenomenon of wall friction even has a bearing on the performance of otherwise smooth-walled, liquid-irrigated or demisting cyclones wherein the water phase on the walls actually exhibits a surprising large hydraulic roughness. See, for example. Fig. 4.2.3. The extent of the effect of liquid on the friction factor depends, for now at least, on experimental laboratory testing, as little has been reported in the cyclone literature. It would appear, however, that data currently available on the effects of liquid films on the friction factor of common gas flow in ordinary pipes could be applicable to any model development work in this area. This, then is one area that is in need of further research. 

It has furthermore been claimed that the position of the vortex end is related to the sharpness of the cyclone cut (Abrahamsen and Allen, 1986). In support of this, the writers have observed considerable mixing of the solids originating in the plane where the precessing vortex tail attaches to the lower walls of model cyclones. This has been observed in both dedusting and demisting cyclones. 

Unlike a solids collecting cyclone, the performance of a demisting cyclone is much more dependent upon the flow conditions that exist in the upstream piping. This, of course, is because the particle or drop size distribution feeding the cyclone is strongly dependent upon such factors as shear rate and surface tension. The shear rate is, itself, a function of the upstream pipe diameter, the superficial gas velocity and the physical properties (namely densities and viscosities) of the gas and liquid phases. 

The method we wish to present here for modeling the performance of vapor-liquid ( demisting ) cyclones follows closely the method presented by Muschelk-nautz and Dahl (1994) and that presented for gas-solids cyclones previously reported in Chap. 6. All the same, to avoid repetition of the formulism, here we shall focus on pointing out differences in the two methods. The reader is encouraged to refer back to Chap. 6 while reading the discussion below. 

Different mechanisms of entrainment from a liquid film on a wall dominate in different film flow regimes. In a classic paper, Ishii and Grolmes (1975) summarize four basic mechanisms for entrainment from a liquid film into a gas flowing co-currently above it. The two that are likely to be relevant in demisting cyclones are illustrated in Fig. 13.7.1. 

It thus seems that these parameters are promising for the formulation of a model for re-entrainment in demisting cyclones. We are of the opinion that the formulation of such a comprehensive model should be an important research priority for the near future, particularly in light of the intensifying interest in off-shore, subsea natural gas processing. 

Figure 13.B.1 depicts a parallel ( multicyclone ) arrangement of six demisting cyclones, with each cyclone discharging its overhead vapors into a common attic chamber and its underflow liquid into a common pool of liquid. The two front cyclones take their feed from the near-wall regions of the inlet duct. The two back cyclones take their feed from the centermost section of the inlet ducting. The middle cyclones take their feed from that section of the inlet...

Fig. 13.B.1. Nonuniform entrance velocity profiie (ieft frame) and associated static pressure difference (right frame) within a set of demisting cyclones...

A worked example for estimation of the static pressure difference between demisting cyclones arising from inlet maldistribution is presented next. 

B Flow Distribution in Parallel Demisting Cyclones 317 13.B.1 Calculation of Flow Distribution... 

Given An industrial demisting cyclone system consists of three pairs of identical cyclones which share a common hopper, as shown in Fig. 13.B.1. It is estimated that the wall friction factor in the ducting leading up to the cyclone is 0.019 or about 30% greater than that for gas-only. 

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