Referring to the SpiraFlow Case Study, it is seen that after the 1998 retrofit, when the vane pack was replaced by cyclones, there was less liquid carry over from the vessel (43% increase in vessel performance improvement). This substantial reduction in carry over with cyclones is explained by two mechanisms:
Droplet removal characteristics
Re-entrainment of liquids
Droplet removal characteristics
For both the cyclones and the vane packs, droplets are removed as a result of a change in direction of the gas flow. Namely, due to this change in direction, the droplets are subjected to acceleration forces, moving them towards a surface onto which they coalesce, thus establishing separation. In a cyclone a highly swirling gas flow is generated through a static swirl element whereas in a vane pack the flow of the gas only changes direction due to the bends in the corrugated parallel plates. As a result of the higher acceleration forces that are established in a cyclone, a cyclone is far more efficient than a vane pack at removing droplets. This becomes very apparent at increased operating pressures where the separation becomes more difficult due to both the decreased density difference between the gas and the liquid and re-entrainment effects which are discussed later. CDS Engineering has performed extensive tests at pressures up to 40 bar, which show that vane packs fail to separate the small droplets, whereas cyclones show efficient separation due to the higher centrifugal forces that continue to dictate the separation mechanisms, despite the higher gas density. It should also be noted that at lower than design gas throughputs, the droplet removal capabilities of vane packs drop substantially faster than with cyclones.
Re-entrainment of liquids
For both vane packs and cyclones the limiting factor in terms of maximum capacity of the unit is the occurrence of liquid film re-entrainment. This therefore sets the allowable gas throughputs and therefore limits the velocity and hence accelerations within the body of the unit. Ultimately this will therefore limit the droplet size that can be removed by the device. After all it is pointless to separate something that is to re-entrain and be carried-over. The re-entrainment mechanism in vane packs essentially occurs at the end or tip of the corrugated plates. Here, separated liquid that runs along the plate gets torn off due to the shear forces exerted by the gas onto the liquid. As the gas density increases then for a given shear force the gas velocity has to decrease (shear force is proportional to rv?). Additionally when low surface tension fluids are being separated the allowable shear stress also drops. This is why separation problems are generally seen with vane packs at higher operating pressures when the gas densities are high and the surface tensions low. Within axial flow cyclones, this effect is suppressed because of the centrifugal stabilisation caused by the swirling flow of the gas, keeping the liquid film in contact with the cyclone wall. In this way cyclones can process far more gas than vane packs before re-entrainment occurs and therefore still separate the smaller droplets.
The above effects are illustrated in the graphs below. Figure 1 shows the maximum velocity for a vane pack (v max) that is dictated by the re-entrainment rv? limit. The other lines show the minimum velocities required within the vane in order to remove 20, 35 and 70 micron droplets. As can be seen 20 micron droplets can not be removed at gas densities greater than around 10 kg/m? since the required gas velocity would exceed the re-entrainment limit.
Figure 2 shows the maximum throughput for a cyclone (v max) that is dictated by the re-entrainment limit. The other lines show the minimum throughputs required within the cyclone in order to remove 12, 15 and 20 micron droplets. As can be seen all droplets can be removed, even at the higher gas densities, without exceeding the re-entrainment limit.
