Historically, the gas handling capacity of conventional vertical separators that employ mist extractors has normally been calculated from the Souders and Brown equation, Eq 7-8, using “experience-based” K factors. Typical K values for vertical separators from API 12J7 are presented in Fig. 7-18.
In qualitative terms, the ranges of K given above may be taken to reflect difficulty of the separation conditions, i.e. from non-ideal/difficult to ideal/easy. As indicated in Fig. 7-18, K is also a function of vessel height. This reflects the fact that a certain minimum distance is required to establish a relatively uniform velocity profile before the gas reaches the mist extractor. Theoretically, it is not simply the vessel height that is important with respect to velocity profile, but the vertical height between the inlet device and the mist extractor. As gas handling capacity is based on an allowable limit for liquid carryover into the separated gas stream, and the final liquid removal element is the mist extractor, the mist extractor has a significant influence on the K value used for separator sizing.
The vertical height of the vessel is also influenced by the liquid handling requirements and general vessel layout criteria as indicated in Fig. 7-8. Typically, the liquid phase will occupy the lower third of the vessel height.
A design that optimizes the inlet feed flow condition and utilizes an efficient inlet device, may provide enough feed gas pre-conditioning to allow the vessel diameter to be sized equivalent to the mist extractor. However, traditionally the method typically used has been to “oversize” the gas gravity section, i.e. vessel diameter, relative to the mist extractor. This has generally been done in two ways:
• Derate the mist extractor K factor and use this reduced K value in Eq 7-8 to determine the vessel diameter. Guidelines as to determination of the appropriate deration factor are not well defined. A relatively low liquid loading application with steady flow, a low inlet velocity and a good inlet device should require minimal deration of the extractor K factor, i.e. deration factor approximately equal to 1. On the other hand, an application with significant liquid volumes, unsteady flow, high velocity inlet and a simple diverter plate inlet device may require a deration factor of 0.5. Normally, it would be more economic to improve the inlet flow condition/device than to significantly oversize the vessel relative to the mist extractor requirements.
• Select a separable droplet size and size for the vessel diameter using Eq 7-1. A droplet size of 150 microns has been typically specified. This may be overly conservative for vane pack and cyclonic mist extractors, which generally have higher gas and liquid capacities than mesh pads.
Comparison of Eq 7-8 & Eq 7-1, indicates that
This shows the approximate equivalence of the empirical K and the more theoretical droplet separation sizing methods. However, the value of K in Eq 7-8 as used in practice depends on other factors besides droplet size, drag coefficient, and liquid entrainment loading, including: type of internals, unsteady flow, surface tension, liquid viscosity, foaming, gas velocity profile uniformity, degree of separation required, etc. Additional duration may be required to account for these factors.