Figure 1: overview of internals in 3-phase separators and scrubbers

An inlet device should perform the following functions:

1) Reduce the feed stream momentum and ensure good gas and liquid distribution

This is essential for good functioning of the separator vessel. The distribution of the phases entering the gas and liquid gravity separation sections has to be as good as possible, in order to optimise the separation efficiency in these sections. Maldistribution can lead to a large spread in residence times, decreasing the separation efficiency. Also a gas maldistribution at the entrance of the demister section can locally overload the demister and cause severe carryover.

CFD (Computational Fluid Dynamic) can be a useful tool in determining the fluid distributions inside a vessel. It can therefore be used for vessel internals optimisation and for troubleshooting on existing vessels. Figure 2 shows an example of a vessel with a half pipe inlet device and the resulting gas distribution as calculated with the aid of CFD software. Figure 2 shows that the gas shoots towards the back wall of the vessel and flows upward from there. The resulting gas maldistribution causes local overloading of the cyclone deck.
 

Figure 2: Half pipe inlet device and gas distribution

2) Separate bulk liquids

Any bulk liquids separated at the inlet device, will decrease the separation load on the rest of the separator and thus improve the efficiency. Good bulk separation will also make the separator operation less sensitive to changes in the feed stream. Inlet devices with good bulk separation characteristics are vane type inlet devices and inlet cyclones. Half pipe inlets perform less well because they send both the gas and the liquid heading downward into the vessel.

3) De-foam

If the feed stream has a tendency to foam, an inlet device that prevents or even breaks down foam can significantly improve the separation efficiency of the vessel, or strongly reduce the use of chemicals. Inlet cyclones (Figure 3) perform especially well in this kind of applications, because of the centrifugal forces and shear that are applied to break the foam. The design of inlet cyclones however is a much more complex task than for any ordinary inlet device. Unlike most inlet devices that are positioned in the gas phase, the inlet cyclone is partly submerged in the liquid phase. Care has to be taken with regard to the pressure balancing on the gas and liquid outlets. If these are not correctly designed gas blowby or liquid carry over will occur, which will increase foam formation instead of stopping it.

4) Prevent re-entrainment of already separated liquid and liquid shattering

Inlet devices can actually make things worse inside a separator if they are not correctly designed. Re-entrainment can be caused by blowing gas down, or across the liquid surface at too high velocities. An example of this can be seen in Figure 2. This phenomenon often occurs when vessels with half pipes are operated at higher gas flow rates than what they were designed for. In such cases retrofitting with for example a vane type inlet device can solve the problem. Another problem that can occur is liquid shattering inside the inlet device. This can actually form smaller droplets than were present in the feed stream, making the separation in the rest of the separator even harder. An example where liquid shattering plays an important role is shown in Figure 4. This is a problem that frequently occurs with diverter plate type inlet devices. The liquid smashes into the plate and is broken up in extremely small droplet, leading to carry over because the gravity and demister sections are not able to separate them from the gas stream.

 

Figure 4: Droplet shattering with a diverter plate