PRINCIPLES OF SEPARATION
Gas- Liquid Separation Fundamentals
Liquid separation from the gas phase can be
accomplished by any combination of the separation
mechanisms previously described.Souders-Brown Equation
for Gravity Settling .This equation can be simplified to
describe the liquid spherical droplet terminal velocity as a
function of the droplet diameter, and the drag coefficient.
The simplified form of the terminal velocity equation is
called the Souders-Brown Equation.The equation is valid for
vertical gas flow, where the drag due to upward gas flow
and the downward gravity force are in balance. The
equation is also frequently used to determine the downward
vertical terminal velocity of droplets in horizontal fluid flow,
even though this relationship is not as rigorous, especially at
higher fluid velocities.The Souders-Brown equation is used in
a number of ways to design equipment for gravity settling
in the oil and gas industry.
A target droplet capture diameter can be specified for a
gravity settling application, and then using the settling laws,
and fluid properties, a drag coefficient, K, and terminal
droplet velocity can be calculated, or determined by
empirical testing. The K-factor is also a function of separator
geometry, including settling space both upstream and
downstream of the mist eliminator.
Gas- Liquid Separation Fundamentals
Mist Eliminators for Gas Liquid Separations
Mechanism of Mist Carryover for Gas-Liquid Mist
Eliminator Devices — Mist eliminators are commonly used
in gas-liquid separation to aid gravity separation in the
removal of liquid so that more efficient, smaller separators
may be used.
To be effective, a mist eliminator must accomplish two
basic functions. First, it must have a means to capture
liquid. Second, it must be able to drain the captured liquid
without allowing re-entrainment into the gas stream.
There are two mechanisms of liquid carryover from a
mist eliminator. In the first mechanism, carryover is due to
droplets of mist which are simply not captured by the
device. The droplets might be too small to be captured or
velocities are too low, causing low efficiency for impactiontype
mist extractors. The second is reentrainment of liquid
after it has already been captured in the mist eliminator.
The majority of separator failures are caused by reentrainment.
This is the mechanism that occurs as the gas
throughput is increased beyond the tolerable limit. Gas
moving through the mist extractor exerts a drag force on
the liquid film of the mist eliminator, causing it to be pulled
toward the trailing edge of the device. If the drag is
excessive, the liquid will be torn off the element and
carried away by the gas stream. As flow rate increases,
the contact efficiency of most mist eliminators improves.
Therefore, increasing gas flow yields improved droplet
capture, but also increases re-entrainment which results in
liquid carryover and limits separation capacity.