Gas-LiquidSeparators-QuantifyingSeparationPerformancePart2-SPEMEB (1)

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Gas/Liquids Separators: Quantifying Separation Performance - Part 2
ArticleinOil and Gas Facilities · April 2015
DOI: 10.2118/1013-0035-OGF
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WATER TREATING INSIGHTS
Gas/Liquids Separators—Part 2
Quantifying Separation Performance
Mark Bothamley, John M. Campbell/PetroSkills
In this second article of a three-part series, methods for
improved quantication of operating performances of
the gas gravity separation, the mist extraction, and the
liquid gravity separation sections of gas/liquid separators
are discussed. ese methods can be used for the selection
and design of new separators, as well as the rating of
existingseparators.
Part 1 of the series in August provided a general
discussion of separation equipment classication, as well
as existing limitations to methods used for quantifying
separator performance.
e main parts of a typical gas/liquid separator, vertical
or horizontal, are shown in Fig. 1, including the feed pipe,
inlet device, gas gravity separation section, mist extractor,
and the liquid gravity separation section. Part 1 discussed
the feed pipe and inlet device.
Gas Gravity Separation Section
e primary function of the gas gravity separation
sectionof a separator is to reduce the entrained liquid load
not removed by the inlet device. Available mist extractors
have limitations on the amount of entrained liquid
(droplets) that can be handled. A secondary, but related,
function is the improvement/straightening of the gas
velocity prole.
In low-liquid-loading applications, such as scrubbers,
pre-separation of liquid droplets may not be required if the
mist extractor can handle the entrainment load. However,
even in this scenario, a relatively uniform gas velocity
distribution should be delivered to the mist extractor to
optimize its performance. e gas gravity separation section
then provides preconditioning of the gas—and its entrained
liquid load—ahead of the mist extractor.
Two approaches to sizing this part of the separator
to remove liquid droplets from the gas are the Ks method
(Souders-Brown equation as shown in Eq. 1) and the
droplet settling theory.
e physical process is the separation of liquid
droplets from the gas phase. Traditionally, the Ks method
has been most commonly used because it usually
provides reasonable results and is easy to use (Ksvalues
for vertical and horizontal separators are available from
many literature sources). e method, primarily an
empirical approach, involves the estimation of an allowable
maximum gas velocity to achieve the required degree of
dropletseparation.
Souders-Brown equation:
V
max=Ks
ρ
l
–ρ
g
ρg .............................................................(1)
e Ks methodology does not readily lend itself to
quantication of gas/liquid separation performance, so will
not be discussed further.
Aer simplifying assumptions to make the calculations
manageable, the droplet settling calculations for sizing the
gas gravity separation section (cross-sectional area and
length) aim at removing a target liquid-droplet size (e.g.,
150 m) and all droplets larger than the target size. Even
this approachis only semiquantitative in that selection of
an appropriate target droplet size is an inherently ill-dened
procedure, and provides little indication as to the amount of
entrainment remaining.
If we think of the gas gravity separation section
as a preconditioning step ahead of the mist extractor,
Fig. 1—Parts of a separator (vertical and horizontal).
Mist extractor
Mist extractor
Feed pipe
Inlet device
Inlet device
Liquid gravity separation section
Feed pipe Gas gravity
separation
section
Liquid gravity
separation
section
Gas gravity
separation section
October 2013 • Oil and Gas Facilities 35

WATER TREATING INSIGHTS
Fig. 2—Effect of inlet device on downstream gas and liquid
velocity profiles (without flow-straightening devices).
No inlet device
Simple diverter/baffle plate
Half-pipe
Cyclonic
Vane-type
L/Di
F, Actual Velocity/Average (Plug Flow) Velocity
3.5
3.0
2.0
1.0
2.5
1.5
0.5
0
024681
012
Vertical
Horizontal
Vg
Vg
Fig. 3—Vertical and horizontal separators showing the gas
flow—droplet settling relationships.
36 Oil and Gas Facilities • October 2013
then it is clear that the requirements for separation
performance of this part of the separator are dependent
on the requirementsof the selected mist extractor.
Particularlyimportant are the allowable liquid loading
(gal/min/2) of the face area of the mist extractor and the
droplet removal capability of the mistextractor.
e uniformity of the gas velocity prole through
the mist extractor and limits on velocity to prevent
re-entrainment are also important factors and will be
discussedlater.
e amount of entrainment and the gas velocity
proleexiting the inlet device were determined by the
method in Part 1 of the series in August 2013.
Fig. 2 shows the quality of the ow distribution
immediately upon exit of the inlet device (L/Di=0)
and the development of the ow prole with distance
downstreamof the inlet device. e quality of the
ow distribution is characterized by the factor F,
the actualaverage velocity/ideal plug ow velocity. F
valuesgreater than 1.0 imply unused cross-sectional
ow area. Useof this factor will allow estimation of the
eective actualvelocity, which can then be used in the
droplet settling calculations for the gas (and liquid)
gravitysections.Note also that the calculated eective
actual velocity for the gas gravity sectionwill be the
velocityat theentrance to the mist extractor section.
Release Point of Entrained Liquids at the Entrance
to the Gas Gravity Separation Section
Fig. 3 shows the gas ow–droplet settling relationships for
vertical and horizontal separators. With the droplet size
distribution and eective actual gas velocity through the gas
gravity separation section established, the droplet settling
calculations are relatively straightforward for a vertical
separator. For a horizontal separator, the eective release
point of the droplets must be established before the settling
calculations can be performed.
e geometry associated with the droplet settling
calculations for a horizontal separator is shown in Fig. 4.
e worst case (most conservative) assumption for the
release point is that all droplets are released at the top
(inlet) of the gas gravity separation section, as shown by the
droplet settling trajectory in Fig. 4. A release point closer to
the gas/liquid interface results in better calculated droplet-
removal performance and a lower entrainment load exiting
the gas gravity separation section (mist extractor inlet). e
type of inlet device and its location has a bearing on the
eective entrainment load release point at the inlet to the
gas gravity separation section. But because of the relatively
chaotic gas ow patterns expected to be exiting from the
inlet device, regardless of the type, attempting to specify the

WATER TREATING INSIGHTS
Fig.4—The geometry associated with the droplet settling
calculations for a horizontal separator.
Mist extractor
Le
hgVt
Vg
Gas gravity separation section
Inlet device
Feed pipe
Liquid gravity separation section
Fig. 5—A schematic of a force balance on a droplet in a flowing
gas stream.
Drag force of gas
on droplet
Direction
of gas flow
Gravitational force
on droplet
Liquid droplet,
Dp
October 2013 • Oil and Gas Facilities 37
vertical position/height of the release point as a point source
is probably not warranted. Additional investigation of this
issue via computational uid dynamics (CFD) modeling
isrecommended.
For the purposes of this article, it will be assumed that
the release of entrained droplets occurs uniformly over the
vertical height of the gas gravity separation section (hg) in
Fig. 4. is is a more optimistic assumption than the release
point depicted at the top of the vessel in Fig. 4, but is more
realistic. e actual release of droplets would be expected to
occur over much of the cross-sectional area of the gas space
at the inlet to the gas gravity separation section, which
is less conservative than the assumption used here. e
current release point assumption does not result in a sharp,
separable droplet cuto size, which is also more realistic.
Droplet Settling Calculations
Droplet settling calculations have typically been based
on the separation of a target droplet size. e primary
assumption is that if all droplets larger than the target
droplet size are removed, the amount of entrainment
remaining would be within the capability of the
mistextractor.
e following discussion does not use this approach;
instead, it is based on quantication of the entrainment
loads, droplet sizes, and liquid-loading capabilities of mist
extractors. Droplet/gravity settling theory is addressed
in other references, so an overview of the subject will be
provided here.
Eq. 2 represents a general form of droplet settling. It is
obtained by performing a force balance on the droplet in
a owing gas stream, as shown in Fig. 5. ese principles
also apply to separation of gas bubbles from liquid and
separation of droplets of one liquid phase from another
liquid phase.
V
t=
4gdp (ρl–ρg)
0.
5
3Cdρg .................................................(2)
TABLE 1TERMINAL VELOCITY EQUATIONS FOR SETTLING LAW REGIONS
Settling Law Reynolds Number (Rep) Terminal Velocity Equation
Stokes' Law <2
V
t=
gd 2
p (ρl–ρg)
18µg ...............................(4)
Intermediate Law 2–500
V
t=
0.1529g0.714dp
1.142
(ρl–ρg)0.714
ρ
g
0.286µ
g
0.428
........(5)
Newton's Law 500–200,000
V
max=Ks
ρ
l
–ρ
g
ρg ...............................(6)
Note: For calculations involving separation of gas bubbles from liquid, the gas viscosity (µg) in the equations is replaced with the liquid viscosity.

WATER TREATING INSIGHTS
38 Oil and Gas Facilities • October 2013
e drag coecient (Cd) is a function of the droplet/
bubble Reynolds number, dened as:
Re
p=
d
p
V
t
ρ
g
µg ............................................................. (3)
e Reynolds number of the droplet includes the
terminal velocity, which makes solving for Cd an iterative
process, much like the solution for the friction factor in
uid-ow calculations.
Fig. 6 shows the relationship between Cd and Rep for
spherical droplets. e full range of Reynolds numbers for
droplets can be divided into the regions associated with the
laws of settling—Stokes’, intermediate, and Newton’s.
Table 1 shows the derived equations for terminal
velocity for the three settling regions.
For the gas gravity separation section, the entrained
droplet sizes of most interest will fall in the intermediate-
law settling region. e very smallest and very largest
droplets will fall into the Stokes’ and Newton’s law
regions,respectively.
Vertical Separators
Using the droplet size distribution and eective actual
velocity correlations discussed in Part 1 in August, the
terminal velocity equation(s) mentioned earlier can be
used to calculate the droplet removal eciency of the gas
gravity separation section. e results of this calculation
will be the entrainment load (gal/MMscf) and the
correspondingdroplet size distribution at the inlet to the
mist extractor. A sharp, separable droplet size cuto will
beobtained.
e F factor (Fig. 2) is needed to calculate the
eective average gas velocity used in the terminal velocity
equation(s) given in Table 1. Because of the relatively
complex interrelationships between variables/parameters
used in the quantitative approach discussed in this series of
articles,an optimization algorithm is used to perform the
iterativecalculations.
Horizontal Separators
e calculations for the horizontal separators are more
complicated than those for a vertical separator. Referring
Fig. 6—The relationship of the drag coefficient (Cd) to the Reynolds number (Rep) for spheres.
Stokes’ Law
100,000.00
10,000.00
1,000.00
100.00
10.00
1.00
0.10
0.001
C
d
Rep
0.01 0.1 110100 1,000 10,000 100,000 1,000,00
0
Newton’s LawIntermediate Law
Cd vs Rep