Estimating Inlet Drop Size for Two-Phase Mist-Annular Flow

Unlike simple spray towers that operate at gas velocities on the order of 0.6 to 1.5 m/s (2 to 5 ft/s), cyclonic scrubbers are designed to operate at gas velocities of 60 to 180 m/s (200 to 600 ft/s). This makes cyclonic scrubbers much more efficient than spray towers in removing particulates due to the much greater particle-to-liquid relative velocity and turbulence. However, cyclonic type scrubbers are still not as efficient as ventmi scrubbers. 

Spray nozzle plugging is another concern. Nozzles have a tendency to plug either due to particles in the water recycle stream or in the gas stream. Nozzles can also erode internally due to particles in the recycle water. They can also corrode. Strainers are usually required to protect the nozzles from such internal erosion. In any case, the design of the spray header should be such that the nozzles are accessible for routine cleaning or replacement. 

The design of cyclonic scrubbers is usually based on pilot tests, experience with units in similar service and empirical correlations rather than any fundamental model of absorption or particle collection performance. A skid mounted test unit, similar to that shown in Fig. 13.3.6, can give plant personnel a very good indication of the expected performance of a commercial scale unit. 

4 Estimating Inlet Drop Size for Two-Phase Mist-Annular Flow 

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. 

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