Vertical separators, shown in Fig. 2-9, are usually selected when the gas-liquid ratio is high or total gas volumes are low. In a vertical separator, the fluids enter the vessel through an inlet device whose primary objectives are to achieve efficient bulk separation of liquid from the gas and to improve flow distribution of both phases through the separator. Liquid removed by the inlet device is directed to the bottom of the vessel.
The gas moves upward in the gravity settling section, where the liquid droplets fall vertically downward counter-current to the upward gas flow. The settling velocity of a liquid droplet is directly proportional to its diameter. If the size of a liquid droplet is too small, it will be carried up and out with the vapor. Thus, a mist extractor section is added to capture small liquid droplets.
Liquid removed by the mist extractor is coalesced into larger droplets that then fall through the gas to the liquid reservoir in the bottom. Liquid continues to flow downward through liquid collection section to the liquid outlet. As the liquid reaches equilibrium, gas bubbles flow counter to the direction of the liquid flow and eventually migrate to the vapor space.
The ability to handle liquid slugs is typically obtained by increasing vessel height to accommodate additional surge volume. Level control is normally not highly critical and liquid level can fluctuate several inches without affecting the separation performance or capacity of the vessel.
Typical vertical separator L/D ratios are normally in the 2–4 range.
The pressure in the separator is maintained by a pressure controller mounted on the gas outlet. The pressure controller senses changes in the pressure in the separator and sends a signal to either open or close the pressure control valve accordingly. By controlling the rate at which gas leaves the vapor space of the vessel, the pressure in the vessel is maintained.
The liquid dump valve is regulated by a level controller. The level controller senses changes in liquid level and controls the dump valve accordingly. There are seldom any internals in the liquid collection section except possibly a still well for the level control float or displacer. The still well usually consists of walled box or tube, open on the top and bottom.
Its function is to stop wave action in the separator from interfering with the level controller’s operation. Vertical separators are well suited for production containing sand and other sediment and thus are often fitted with a false cone bottom to handle sand production.
As an example of a vertical separator, consider a compressor suction scrubber. In this service the vertical separator:
• Does not need significant liquid retention volume
• A properly designed liquid level control loop responds quickly to any liquid that enters, thus avoiding tripping an alarm or shutdown
• The separator occupies a small amount of plot space
