Three-phase separation

Three-phase separation is used to obtain a gas stream free of liquid and two separate liquid streams: a light liquid phase and a heavy liquid phase.

Process

In general, an AFP ST inlet device is used to calm and distribute the flow at the vessel inlet and to achieve separation of the liquid and gas phases. The streams are therefore forced to pass through several internals simultaneously, but in two different sections of the vessel:

• The upper section treats the gas stream using, for example, a flow rectifier, a roughing device (straightner) and a vane pack.
• The lower section treats the liquid using, for example, a calming baffle, a horizontal parallel plate pack and an overflow weir.

Internals

The configuration of the internals may vary depending on the fluid characteristics and the required performance. An example of a three-phase separator is the so-called “gravity separator”, in which the only internal is usually an ST inlet device, and the light and heavy liquid phases are separated only roughly by the effect of gravity. In other cases, special internals are required to achieve the required performance, such as:

• A double calming baffle instead of a single one.
• A wire mesh coalescer instead of a vane pack.
• A wire mesh demister instead of a vane pack.

Sizing

The sizing of a three-phase separator and its internals is a complex process in which several parameters must be considered:

• Characteristics and properties of the fluids, such as density, viscosity, chemical composition, molecular weight, etc.
• Required residence time in the vessel.
• Required vessel dimensions based on its location and other limitations, such as existing piping to which the vessel must be connected.
• Required process performance, such as the minimum diameter of the liquid droplets to be separated.