The design criterion for separator liquid handling capacity is typically based on the following two main considerations:
• Liquid degassing requirements.
• Process control/stability requirements.
Generally, one or the other of these factors will dictate. Liquid capacity is typically specified in terms of residence time, which must be translated into vessel layout requirements for dimensioning purposes. Residence time establishes the separator volume required for the liquid as shown in Eq 7-12:
Typical residence times are shown in Figures 7-20 and 7-21.
Note that except for the Natural gas – condensate application, the residence times specified in Fig. 7-20 are primarily based on process control stability/operability.
These values are primarily intended to reflect liquid degassing requirements. In practice, process control stability and operability requirements will often override the degassing requirements.
Vessel layout recommendations, including liquid handling requirements, are given in Fig. 7-8 and 7-9 for vertical and horizontal separators, respectively.
The retention time requirements given in Figures 7-20 and 7-21 are not specific to vessel orientation. However, the liquid degassing process actually involves the separation of gas bubbles from the liquid phase, which under ideal conditions can be described by the gravity settling equation, Eq 7-1. Similar to liquid droplet settling out of the gas phase, it is easier for a gas bubble to rise perpendicularly through the moving liquid in a horizontal separator than directly against the downflowing liquid in a vertical vessel. Theoretically, for equal liquid residence times, the horizontal separator should be slightly more efficient at degassing. However, this has not typically been an issue in practice. If it is deemed necessary to calculate vessel liquid handling requirements for a degassing constraint according to gravity settling theory, a gas bubble size of 150–200 microns has been suggested by several sources.