Note: Descriptions are shown in the official language in which they were submitted.
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Demister
Device for separating liquid droplets from a gas stream, of a kind generally
described by
the preamble of claim 1. Depending on its application and design, such devices
may be
referred to as droplet removers, deflection (inertial) separators, scrubbers
(oil
terminology), droplet separators (English terminology), demisters, mist
eliminators or
entrainment separators.
Background
Deflection separators are based on the principle that liquid droplets
entrained in a gas
stream have a significantly higher density than the gas, and are caused to
collide with
and stick to one or more wall surfaces by deflecting the gas one or more
times, so that
the flowing gas and liquid particles are exposed to centripetal acceleration.
Ideally all
the droplets would hit a wall surface and be drained away due to gravity at a
direction
perpendicular to the gas flow. In practice this is not easily achieved, due to
the
considerable size variation of the droplets.
When studying the process in more detail, it is observed that many of the
droplets
collide with the wall surfaces when moving through a deflection separator,
allowing a
large volume fraction to be transferred to the liquid film existing on the
wall surface.
However, a substantial number of larger and in particular smaller droplets
(satellite
droplets) will also escape from the wall surface during the collisions. It is
therefore very
difficult and in practice unattainable to reach a near 100 % separation of
droplets
without using a separator that is disproportionately large, causing an
unacceptable
pressure loss.
Droplet separators/ demisters are used under quite different conditions, from
small gas
flow rates at moderate temperature and pressure, to large gas flow rates at
high pressure
and at high temperature. Used in connection with production or refining of oil
and gas
the pressure may be in the range between 50 and 200 atmospheres. It is evident
that
under such conditions the requirements for a good demister are quite different
from the
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requirements for demisters used for gas purification under less severe
conditions,
wherein the pressure is usually around 1 atmosphere.
NO patent application No. 173 262 describes a device for separating liquid,
consisting of
several deflecting plates arranged adjacent to each other with a broad and a
narrower
zone and a bend therein between. At the entrance of each bend an extension is
arranged
to avoid turbulence, with the consequence that the linear velocity of the gas
decreases in
an area where, according to the invention, it is desirable that the velocity
is high.
GB patent No. 1,503,756 describes a demister comprising at least two
adjacently
arranged sinusoidally shaped corrugated main plates, wherein baffle plates
attached to at
least one of the main plates are arranged to reduce the cross-section between
the plates.
One of the depicted embodiments also shows a minimum cross-sectional area
close to
the bend. The design, however, does not avoid the obvious disadvantage of
utilizing
baffle plates, namely enhanced marine growth, increased pressure loss,
increased
entrainment of satellite droplets and in some cases an increased risk of
corrosion. A
design of this kind is therefore not well suited for use under high pressures
and for large
gas flow rates, with a high degree of turbulence etc.
US patent No. 1,926,924 describes a filter to separate solid or liquid
droplets from a gas
stream flowing rapidly through a filter arrangement comprising a number of
plates
arranged adjacent to each other. This embodiment does not have a cross-
sectional
minimum in the bend. Therefore, such a demister will not be able to
quantitatively
separate out droplets, except for quite large droplets.
None of the cited publications show profiles of the plates covering more than
one
"wavelength" of the demister. By "wavelength" in this context is meant a
section of the
demister wherein any one of the single plates, seen from its side edge, draws
a complete
wavelength. If it is intended that any one of these demisters shall include
more than one
bend, it is evident that each subsequent "wavelength" will be an exact copy of
the
foregoing.
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Objectives
The background of the present invention is the oil industry, where high
pressures and
temperatures are common. Under these circumstances turbulence is unavoidable.
Equipment for such purposes are often located to very exposed areas, where
repair
works and replacement are very expensive, and therefore should be minimized.
For this
reason and others, use of baffle plates is not all convenient.
It is thus an object of the present invention to obtain a demister that is
more efficient
than previously known demisters with respect to separating droplets from a
stream of
gas.
It is furthermore an object to provide a demister which is well suited for use
at high
pressures, temperatures and for large gas flow rates, and at operational
conditions where
a significant degree of turbulence is unavoidable.
It is still further an object to provide a demister that when assembled can be
operated
without maintenance for a long period of time.
The invention
The above mentioned objects are achieved by a demister according to the
invention,
which is characterized by the features disclosed by the characterizing part of
claim 1.
Preferred embodiments of the invention are disclosed by the dependent claims.
The most significant disadvantage with the previously known demisters is that
they are
not able to separate the high number of very fine droplets in the gas stream.
Furthermore many of the demisters are not designed to take high flow rates,
turbulence
etc. and are therefore not suited for use in oil related industry.
The most significant feature with the present invention and what clearly
distinguishes
the demister according to the invention from demisters previously described,
is that from
the inlet to the outlet the thickness of the plates are varied in such a way
that the distance
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4
between the plates, or the width of the flow paths, is regulated in a
predetermined way.
If the thickness of the plates is not used as a means for regulating the
distance between
the plates, the distance will as a function of simple geometric considerations
vary in
such a way that the distance will be largest in the bends and least in the
flank sections
where the deflection angle is greatest, as further explained below. The very
simplest
embodiment of the invention is one wherein the thickness of the plates are
varied in such
a way that the distance between the plates is the same and constant throughout
the
demister. It is however, good reasons to make the variations of the plate
thickness in
other ways, as further described in the following.
By expressing that the width of the flow path (the distance between two
adjacent plates)
is to be varied in such a way that a sufficiently high linear velocity is
achieved for the
purpose, is meant that the width in each single case may be optimized
according to the
relevant distribution of droplet size and according to the density of the gas
in question,
so that throughout the demister a velocity is obtained that is close to, but
not beyond, the
limit for re-entrainment of droplets, which is to be explained more in detail
below.
A particularly preferred embodiment of the present invention, and what
distinguishes the
demister according to the invention even further from previously known
techniques, is
varying the plate thickness and/ or the wavelength (and thereby the radius of
curvature)
from inlet to outlet systematically in such a way that the resulting width of
the flow path
and/ or the radius of curvature is varied in a way that ensures alternating
intense and
calm zones through the demister. By "intense zone" is meant a zone
characterized by
high sideways acceleration in the bends, so that even very small droplets will
hit the wall
plates and add to the liquid film. Further, a tear-up of the liquid film will
take place in
the intense zones, so that larger droplets are redispersed in the gas to a
significant extent.
By "calm zone" is meant a zone characterized by lower sideways acceleration in
the
bends, so that mainly larger droplets collide with the wall plates and adds to
the draining
liquid film. Tear-up from the liquid film will occur in only a very limited
degree in a
calm zone.
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Furthermore, it is a central issue that the benefits are achieved with means
that are
simple and provide such a simple design that the demister avoids the problems
related to
extending baffle plates or other extra deflecting means, such as fouling or
scaling,
corrosion or fatigue fracture initiating points in the material during
turbulence.
Utilization of calm and intense zones ensures that in the different steps of
separation, an
optimization is achieved with respect to separation of large droplets,
separation of small
droplets and draining of the liquid film with no further entrainment (re-
entrainment).
Drawings
Fig. 1 is a schematic top view of a segment of a first embodiment of the
invention,
Fig. 2 is a schematic top view of a segment of a second embodiment of the
invention,
Fig. 3 is a schematic top view of a segment of a third embodiment of the
invention,
Fig. 4 is a schematic top view of a segment of a fourth embodiment of the
invention,
Fig. 5 is a schematic top view of a segment of a fifth embodiment of the
invention,
Figure 1 shows an embodiment wherein the wave form of the bends are shaped as
circle
segments. In a first section of the demister (the left part of the drawing)
the flow path is
quite narrow and the wall plates quite thick, while in another section of the
demister (the
right hand part of the drav~~ing) the flow path is wider and the wall plates
thinner. There
are also different "wavelengths" in the two sections, with shorter distance
between the
bends and thereby a lesser radius of curvature in the first section. Within
each segment
the flow path is mainly equal in the bends and in the flank sections, which is
obtained by
making the wall thickness somewhat larger in the bends than in the flank
sections. Both
the difference in the width of the flow path and the radius of curvature
contributes to
making the first section behave as an intense zone, while the second section
behaves as a
calm zone. Each of the zones according to this embodiment extends for several
"wavelengths" of the demister.
Figure 2 shows a variant of the demister depicted in figure 1. Most of the
features of
figure 1 can be recognized, but in figure 2 there are two calm zones and one
intense
zone, arranged so that the gas first enters into a calm zone (the very left
part of the
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6
drawing), thereafter into an intense zone and finally again into a calm zone.
The
variations both with regard to the width of the flow path and the radius of
curvature are
principally the same as for figure 1. A solution with a calm first zone may be
preferable
e.g. when there are a lot of relatively large droplets in the gas that is to
be separated with
as little re-entrainment of droplets as possible.
Figure 3 shows a section of a demister wherein the plate waves have the shape
of sine
curves rather than circle segments. The separating characteristics are
different for
sinusoidal bends compared to bends with the shape of circle segments as
further
described in the following. However, also in this embodiment, the wall
thickness is
varied to obtained the flow characteristic that is desired. In figure 3 the
width of the
flow path is approximately the same in bends and in flank sections. The
embodiment of
the demister showed in figure 3, wherein the width of the flow path is held
constant
from inlet to outlet, constitutes the very simplest form of a demister
according to the
invention.
Figure 4 shows an alternative variant of the demister of figure 3. Thus figure
4 also
shows sinusoidal bends, but the variation in wall thickness is more pronounced
so that
the width of the flow path continuously varies from a minimum at one bend to a
maximum at the next bend and then back again to a minimum at the bend
thereafter. By
arranging the minimum width of the flow path within a bend, this becomes the
point of
the highest linear velocity in the demister, and thereby a high number of
collisions
between droplets and the wall will occur in this area. This design therefore
gives a much
more rapid variation between intense and calm zones than the embodiments
according to
figure 1 and 2. Even though figure 4 shows a complete demister from inlet to
outlet, it
is also possible to combine a certain number of wavelengths as depicted with a
number
of wavelengths according to the same principle, but wherein the repeating
pattern of
intense and calm zones varies within other limits with respect to width of the
flow path
and radius of curvature respectively. In more general terms it may be said
that it is not a
requirement that all intense zones in a demister are equally intense or that
all calm zones
are equally calm.
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Figure 5 shows an embodiment wherein the bends again have the shape of circle
segments, but where the distances between the bends are increased by straight
sections
that have been "spliced" in between each bend. Also for the embodiment
according to
figure 5, the width of the flow path is approximately the same throughout the
entire
shown section of the demister, effected by a conveniently increased wall
thickness at
each bend.
A central feature of the design is based on the observation that when
deflecting the gas
stream, the flow velocity should not be lower in the bend, i.e. where the
deflection takes
place, than it is in sections with more or less linear flow paths. The flow
velocity should
at least be correspondingly high in the bends in order to provide an optimized
separation
of droplets on the wall surfaces. This aspect is not well provided for
previously, which
is reflected in the above cited publications representing prior art. The
background for
this lies in the geometrical fact that when two plates are arranged with a
certain distance
A there between, and bent with identical right hand and left hand bends to a
corrugated
shape, the distance between them in each bend will still be A, while in the
flank sections
the distance seen in direction of the local flow direction, will be B, where B
is
determined by the equation:
B=Axcosv (1)
where v represents the deflection angle (right or left ) from the line
straight ahead. If
said angle gets close to 90° , the distance between the plates will
decrease towards zero,
which corresponds to the change of cos v from 1 to 0 when v increases from
0° to 90°. If
no efforts are made to avoid this effect, the lowest linear velocity of the
gas stream will
be found where it is desired that the velocity is high and preferably highest.
Adjusting the width of the flow path through the demister according to the
invention
may be achieved by adjusting the wall thickness correspondingly
(complementary).
Simply stated, the narrower the flow path, the thicker the plate in the same
region. The
walls do not necessarily have to be solid, it is possible to establish the
varying thickness
by arranging two plates with suitable profiles adjacent to each other with
pockets of
"air" in between, in the regions where additional thickness is required.
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A secondary but still important aspect with the present invention is to
arrange alternate
intense and calm zones as previously mentioned. It is not essential whether
the first
zone is intense or calm, and it is not essential to establish a plurality of
zones. Neither is
it required that all intense zones are equally intense nor that all calm zones
are equally
calm. However, the main point is that there are intense zones with a frequent
number of
collisions between droplets and the wall even for the smallest droplets and
there are
calm zones where large droplets collide with such a low velocity that they
cause a
minimum of re-entrainment of additional - large or small droplets. Typically
from two
to ten or even more zones may be arranged. For natural reasons, i.e. with
regard to size
and cost, there will preferably be fewer zones where each zone extends over
several
wavelengths of the demister, and more zones when there is a change from
intense to
calm zone within each single wavelength.
The change from a calm to an intense zone will normally be accomplished by the
change
1 S to a lesser flow path width and thereby a higher linear gas velocity, but
a similar change
may also be effected by reducing the radius of curvature in the bends, or as
shown in fig.
1 and fig. 2, by a combination of these two measures. There is no absolute
definition of
the characterization of a calm zone and the characterization of an intense
zone, in
relation to the current occurring flow parameters, decided primarily by the
properties of
the gas and the droplets, pressure, gas flow velocity etc. The actual
dimensioning must
be performed in relation to the relevant situation. For one type of process, a
scaling up
my take place by arranging additional demisters of a certain size in parallel,
rather than
dimensioning demisters separately for each specific application.
As shown in the figures, the bends may be sinusoidally shaped or shaped as
circle
segments, but they may also have other shapes. Varying separation
characteristic may
be obtained depending upon the shape chosen, but these are all variations
within the
frame of the invention. Furthermore it is possible to combine profile elements
where
e.g. every second element is linear and every second elements is sinusoidal or
has the
shape of a circe segment (cf. figure 5). The present invention is not limited
to any
particular profile.
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The extension of each zone is also not crucial. One single calm zone may
extend for just
one flank section, whereafter the width of the flow path is narrowed to a
subsequent
intense zone in the bend following. On the other hand an intense (or a calm)
zone may
extend for several subsequent bends, and then be followed by a calm (or an
intense)
zone that may have constant flow path width and radius of curvature for
several
subsequent bends. By "constant" in this context, it is understood that the gas
is exposed
to substantially unchanging conditions, in practice it means that the linear
gas velocity in
a flank section is approximately the same as in the subsequent flank section,
and that the
linear gas velocity in a bend is approximately the same as in the subsequent
bend.
Through a convenient combination of calm zones and intense zones from inlet to
outlet
of the demister, a particularly effective reduction of the amount of droplets
in the gas is
obtained. The number of zones to be applied depends on the situation at hand
and by the
marginal costs of adding still another zone measured against the marginal
benefit from
such an additional zone.
By utilizing a demister with calm and intense zones, cf. Figure 2, the
following general
observation occur:
1. Large droplets collide with the walls in the first calm zone and build a
liquid film.
The gas flow velocity is so low that re-entrainment of new droplets from the
liquid film
occurs only in a very limited extent. Small droplets largely remain in the gas
flow. A
relatively large volume is drained down the wall surfaces.
2. In the intense zone the smaller droplets also collide with the wall
surfaces. Higher
gas flow velocity leads to re-entrainment of some large, but also small
droplets (satellite
droplets) from the film. Due to the intense conditions and the significant
extension of
the zone, many of these collide with each other in the gas, forming larger
droplets,
which again collide with the wall surfaces.
3. In the last calm zone droplets, particularly remaining large ones, will
collide with the
wall surfaces under calm conditions where insignificant further re-entrainment
will take
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place. Draining of additional liquid film takes place in this zone, with
significantly less
volume compared to the first zone and with only a little liquid dispersed in
the gas flow.
As mentioned it is not essential whether the first zone is a calm or an
intense zone. If
there are many large droplets in the gas, it may be beneficial with a calm
first zone in
order to separate and drain a comparatively large liquid volume with a very
limited re-
entrainment of droplets. Under different operating conditions it may be more
convenient with a an intense zone as the first zone at the demisters inlet.
For simpler,
less demanding needs a demister according to the invention may be utilized
wherein the
10 thickness variation of the plates only is used to obtain a substantially
constant
crossection (flow path width) from inlet to outlet.
Basis for calculations
It is possible to do complex calculations on how a demister according to the
invention
will operate, compared to conventional demisters in real-life situations. It
is, however,
very demanding to take into account any parameters that may effect the result
during
ordinary operation. In the following the effect is therefore illustrated with
a basis in
somewhat simplified pre-suppositions.
The deflective effect as such, which is active during a change of direction
and occurs at
every bend in the demister, is inversely proportional to the radius of
curvature. If bends
with a shape of circle segments are used, the acceleration is constant
throughout the
entire bend, which is beneficial with respect to several considerations. If,
on the other
hand, sinusoidally shaped bends are used, the acceleration will increase from
zero at the
deflection tangent between two bends to a maximum acceleration in the middle
of the
bend that is significantly higher than that of the circle segment. A sine
curve's benefit is
that a wavelength occupies only a length of 2/3 of the wavelength of a
corresponding
profile based on circle segments.
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It can be shown that at high particle Reynolds numbers (turbulent conditions)
it is
particularly beneficial for an even acceleration throughout the entire bend,
as provided
by the circle segments. At low Reynolds numbers (laminar flow) this is not so
important.
When gas is flowing between two corrugated plates, the gas is given an
acceleration in
each bend defined by:
a = Uz/R where (2)
U = gas flow velocity between the plates, R - radius of curvature.
Droplets in the gas stream are influenced by the acceleration in a way so that
they
receive a terminal migration velocity, U~, against the wall determined by the
frictional
force between the droplets and the gas equals the acceleration force. The
force balance
on a drop thus is:
Frictional force: = CD(n/4)dp2pgUt2/2 =
Acceleration force = (pp - pg)a(~/6)dp3, where
4da(pp-pg) a where (3)
' 3 CD p8
dp = Diameter of a droplet
pp = Density of a droplet
pg = Density of the gas
CD = Drag factor dependent on Reynolds number, Re, for the droplet based on Ut
Re = p~ Ut dp/~c where
(4)
~ = Viscosity of the gas
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The formulas presented above constitute some of the basis for the
understanding of the
present invention, but it should be emphasized that it is not in the
understanding of the
formulas as such that the invention is conceived, but rather in the
recognition of how
these terms may be best utilized in practice.
Other important parameters in order to make theoretical calculations on these
phenomena are the connection between terminal migration velocity and degree of
separation, the influence of laminar and turbulent flow respectively, and at
what
conditions (critical gas flow velocity) re-entrainment of droplets from the
demister walls
begins. Within the complex reality one is actually facing, it is unavoidable
that
conditions related to dimensioning to some extent must be derived empirically.
This fact
does not prevent that the conditions made subject of the present invention as
defined by
the claims, are universally valid and represent something entirely new within
the
technology of demisters. For further information relating to the theoretical
basis for
calculations in this area, reference is made to:
Monat, J.P. et al.: "Accurate evaluation of Chevron mist eliminators."
Chemical
Engineering Progress vol. 82, no. 12, Dec. 1986 p. 32, and
Calverts et al.: "Entrainment Separators for scrubbers: Final report." Oct.
1973 to June
1975, A. P. T., NTIS Publ. PB- 248050/EPA 650/2-74-119-b (1975).
Based on the above cited theory, calculations have been made to determine how
effective a demister according to the invention will work within one and two
wavelengths of the demister. The starting point is a calm zone where the gas
flow
velocity is close to, but not beyond the limit for re-entrainment.
The theoretical basis was as follows:
Droplet diameter : dp = 104 ~ 10-6 m = 104 ~cm
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Dynamic viscosity of the gas : ~c 1,5 ' 10-5
= Pas
Minimum distance between plates : : Lm;n= 0,004 m
= 4 mm
Specific density of the liquid (droplets):pp 600 kg/m3
=
Specific density of the gas : p 100 kg/m3
=
Maximum gas flow velocity : LJmaX
=
0,38
m/s
Amplitude (deflection) of the bends : a 0.005 m =
= 5 mm
Deflection tangent = angle of deft. of a/2 = 45
the plates:
Based on this, the ratio CZ/C, between concentration of droplets leaving the
demister
and droplets entering the demister was calculated, providing a direct measure
of the
effectiveness of separation. For example a ratio Cz/C, = 0.2 implies that 80%
of the
liquid content in the gas has been removed, while a ratio Cz/C, = 0,05 implies
that 95%
av the liquid content in the gas has been removed.
Example 1
This example relates to sinusoidally shaped plates within one wavelength,
calculated for
a demister with constant wall thickness and a demister with varying wall
thickness
according to the invention respectively, in a way that gives a constant width
of the flow
path over the wavelength. The result is shown in table 1 below.
Example 2
This example relates to sinusoidally shaped plates with an extension of two
wavelengths,
calculated for a demister with constant wall thickness and a demister with
varying wall
thickness according to the invention respectively, in a way that gives a
constant width of
the flow path over the two wavelengths. The result is shown in table 1 below.
Example 3
This example relates to plates with circle shaped segments, extending for one
wavelength, calculated for a demister with constant wall thickness and a
demister with
varying wall thickness according to the invention respectively, in a way that
gives a
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14
constant width of the flow path over the wavelength. The result is shown in
table 1
below.
Example 4
This example relates to plates with circle shaped segments, extending for two
wavelengths, calculated for a demister with constant wall thickness and a
demister with
varying wall thickness according to the invention respectively, in a way that
gives a
constant width of the flow path over the two wavelengths. The result is shown
in table 1
below.
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Table 1
CZ/C, ImprovementCz/C~ Improvement
1 2
wavelength% wavelengths
Sinusoidal, constant0.2738 0.07492
wall
thickness 34% 57%
5 Sinusoidal, according0.1799 (Ex. 1) 0.03234 (Ex. 2)
to
the invention
Circle segment, 0.222 0.0494
constant
wall thickness 44% 68%
Circle segment according0.125 (Ex. 3) 0.0156 (Ex. 4)
10 to the invention
In the examples 1 to 4 is obtained an improvement with regard to the general
aspect of
the invention in the range between 34% (ex. 1 ) and 68% (ex. 4). Further
improvements
may be obtained by utilizing more than two wavelengths and by combining calm
and
15 intense zones regarding a second aspect of the invention.
From the calculations above it is easily understood that the demister
according to the
invention exhibits a dramatic improvement over a conventional demister. This
improvement may be used to obtain a far better separation, a significant
reduction in
dimensioning and costs of the equipment, or a combination of these advantages.
The arrangement of the plates vertically is the most convenient orientation
with regard to
obtain an effective draining of the liquid film from the plates. It is,
however, obvious
that a slight deviation of this orientation will not depart from the scope of
the present
invention.
The deflection angle is not crucial, but it is convenient and common that this
angle is in
the range between 30- 50° and preferably approximately 45° ,
which is also the normal
range for deflection separators.
