CA2392745A1

CA2392745A1 - Demister

Info

Publication number: CA2392745A1
Application number: CA002392745A
Authority: CA
Inventors: Herman Kolderup
Original assignee: Individual
Current assignee: Leiv Eiriksson Nyfotek AS
Priority date: 1999-12-01
Filing date: 2000-11-06
Publication date: 2001-06-07
Also published as: AU1560501A; WO2001039865A1; NO995867L; NO995867D0; EP1246683A1; NO313867B1; AU777868B2

Abstract

Demister comprising corrugated plates arranged adjacent to each other substantially vertically, wherein the gas flow containing liquid droplets is forced to pass between the plates in a substantially horizontal direction and with alternate right hand and left hand deflection at each bend of the plates.
The width of the flow path locally in the direction of the flow between any two adjacent plates is varied by a complementary variation of the thickness of the plates from inlet to outlet, so that a sufficiently high linear gas flow velocity for the purpose in question is obtained in each bend. The flow path width and/or the radius of curvature between any two adjacent plates is varied according to a predetermined pattern from inlet to outlet so that alternately at least one intense zone and at least one calm zone appear, so that there is at least one region in which both large and small droplets are separated from the stream and some large droplets are re-dispersed in the stream, and at least one region in which mainly large droplets are separated from the stream, while varying degrees of draining takes place within the same zones.

Description

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 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.

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 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.

5 PCT/1~1000/00369 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 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.

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.

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.

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 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.

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 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 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 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.

Table 1 CZ/C, ImprovementCz/C~ Improvement 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.

Claims

1. Demister comprising corrugated plates arranged adjacent to each other substantially vertically, wherein the gas flow containing liquid droplets is forced to pass between the plates in a substantially horizontal direction and with alternate right hand and left hand deflection at each bend of the plates, characterized in that the bends of the corrugated plates mainly have the form of circle segments.

2. Demister according to claim 1, characterized in that the corrugated plates are combined of elements including straight segments alternating with circle shaped segments.

3. Demister according to claim 1 or 2, characterized in that the flow path width and/or the radius of curvature between any two adjacent plates is varied according to a predetermined pattern from inlet to outlet so that alternately at least one intense zone and at least one calm zone appears, so that there is at least one region in which both large and small droplets are separated from the stream and some large droplets are re-dispersed in the stream, and at least one region in which mainly large droplets are separated from the stream, while varying degrees of draining takes place within the same zones.

4. Demister according to claim 3, characterized in that it comprises at least one calm zone and at least one intense zone.

5. Demister according to claim 3, characterized in that it comprises at least two calm zones and at least one intense zone.

6. Demister according to claims 1 - 5, characterized in that the flow path width within a randomly chosen "wavelength" of a plate is substantially constant through both flank sections and bends.

7. Demister according to claim 6, characterized in that the flow path width within one "wavelength" of the plate at a point at least one wavelength downstream or upstream from the arbitrary chosen wavelength of claim 6, is substantially constant, but different from the flow path width mentioned in claim 6.

8. Demister according to claims 3-7, characterized in that the distance between two adjacent plates is varied so that there is a minimum distance in a region comprising two subsequent bends and the flank section thereinbetween, followed by an extension to a maximum distance for two subsequent bends and the flank section thereinbetween.

9. Demister according to claim 3, characterized in that in a first part of the demister close to the inlet there is arranged a first region with a defined wavelength and a comparatively narrow and substantially constant flow path, so that the linear gas flow velocity is comparatively high and substantially constant through the bends and flanks of this first region, while closer to the outlet from the demister there is arranged a second region with larger wavelength and a wider flow path than in the first region, said wavelength and flow path being substantially constant through the bends and flanks of this second region, so that the linear gas flow velocity in this second region is substantially constant, but lower than in said first region.

10. Demister according to claims 1-2 or 6, characterized in that flow path width is substantially constant through both flank sections and bends throughout the entire demister.

11. Demister according to any one of the preceding claims, characterized in that the width of the flow path locally in the direction of the flow between any two adjacent plates is varied by a corresponding variation of the thickness of the plates.