Perspectives: Plant InSites
Effectively Remove Droplets
Various factors affect the choice and operation of liquid/vapor separators.
Plants often use separators to remove liquid droplets from vapor streams. To evaluate the gas capacity of these vessels, engineers typically rely on the empirical Souders-Brown correlation:
V = K [(ρL – ρV)/ρV]0.5
where V is the superficial vapor velocity, K is a constant, ρL is the density of the liquid phase and ρV is the density of the vapor phase. The same equation frequently is used to evaluate packing in distillation, stripping and absorption towers. As a concept this makes sense. Tower packing comes into contact with rising vapor and falling liquid. If the liquid can’t fall, the tower will flood. For distillation towers, the limiting value for K often is given as 0.45 for systems requiring low entrainment and up to 0.50 for systems that can tolerate high entrainment. These values are for high-capacity packings that can operate close to the system limit. However, a specific packing may have a much lower capacity.
The Souders-Brown correlation doesn’t account for any effect of separator height. For towers, that isn’t a major issue because most installations have spacings between internal elements of three feet or less.
Separators, however, often have much more vertical space. This allows for higher capacities in a given plot area. With V in ft/sec and density in consistent units, experience-based values of K for separators are:
• 0.167 for 5-ft-high vertical vessels with the stream entering in the middle of the vessel (values for specific services run from 0.12 to 0.24);
• 0.210 for 10-ft-high vertical vessels with entrance in the middle of the vessel (a range of 0.18 to 0.35 for specific services); and
• 0.35 for spherical vessels and vessels with wire-mesh mist eliminators (a variation of 0.22 to 0.39 for specific services).
For horizontal vessels, you can estimate K via:
K = 0.45 (L/10)0.56
where L is the horizontal vessel length in ft. For such vessels, V is calculated based on the open area above the liquid level. This effectively relates the horizontal travel required for the vapor to give enough time for a droplet to settle horizontally.
Vertical separators are best in applications where gas doesn’t contain much liquid. Horizontal separators are more effective for dealing with large liquid volumes. They also better suit three-phase systems if the two liquid phases are being separated at the same time.
The Souders-Brown correlation and K-factor selected imply a minimum droplet size. Here, the situation becomes a bit unclear. Stokes’ Law, Newton’s Law or the Intermediate Law govern droplet settling. Smaller droplets have lower settling velocities; larger droplets have higher ones. Without special internals (e.g., baffles, impingement plates, cyclones or mesh pads), the maximum size of a droplet that will settle depends upon its terminal velocity. If this is less than the velocity of the rising vapor, the droplet will fall out of the vapor. If the terminal velocity exceeds the velocity of the rising vapor, the droplet will ascend with the vapor.
The upper limit for Stokes’ Law is a droplet Reynolds number of 2. Newton’s Law pertains when Reynolds numbers lie in the 500–200,000 range. Between 2 and 500 — the situation for most industrial systems — the Intermediate Law applies. Droplet removal efficiency is a complex function of droplet size, liquid density, vapor density and vapor viscosity. The effective removal limit for vertical separators varies from a droplet size of 40 μ to 170 μ depending upon the system, vapor velocities and liquid load.
Wire-mesh mist eliminators, by providing both inertial and impaction droplet-removal mechanisms, can push limits down into the 3–10-μ range. However, quoted removal efficiencies of 99–99.5% for such droplets often reflect best-case operation. Contamination of the mesh pad with scale, rust or other solids may dramatically reduce this. Corrosion damage to the pad is another major concern.
Mesh pads have a relatively sharp upper limit on vapor velocity. Operation may alter dramatically with as little as 10% change in vapor rate as the pad approaches its upper operating limit.
Finally, mesh pads create liquid/vapor surface area. Much like with packing, this may promote liquid/vapor mass transfer. In fact, mesh pads often serve as the packing in small diameter columns. Liquid composition in the mesh pad may not be as expected. This can lead to fouled pads.
Like all equipment, separators without and with mesh pads have a specific operating envelope for best performance. Running outside that envelope will likely lower efficiencies. Respect their limits and both should work well.
ANDREW SLOLEY, Contributing Editor
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