Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D45/00—Separating dispersed particles from gases or vapours by gravity, inertia, or centrifugal forces
  • B01D45/04—Separating dispersed particles from gases or vapours by gravity, inertia, or centrifugal forces by utilising inertia
  • B01D45/06—Separating dispersed particles from gases or vapours by gravity, inertia, or centrifugal forces by utilising inertia by reversal of direction of flow

Definitions

• Droplet removers for separating liquid droplets from a gas stream, of a kind generally described by the preamble of claim 1.
• 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.
• 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.
• 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 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.
• 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.
• demisters 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 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.
• 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 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.
• 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.
• baffle plate 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. 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).
• 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.
• the flow path 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 drawing) the flow path is wider and the wall plates thinner.
• 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.
• 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.
• the wall thickness is varied to obtained the flow characteristic that is desired.
• 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.
• 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.
• the minimum width of the flow path 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.
• 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.
• 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.
• the change from a calm to an intense zone will normally be accomplished by the change 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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 thickness variation of the plates only is used to obtain a substantially constant crossection (flow path width) from inlet to outlet.
• 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 Reynold's numbers (turbulent conditions) it is particularly beneficial for an even acceleration throughout the entire bend, as provided by the circle segments. At low Reynold's numbers (laminar flow) this is not so important.
• Droplets in the gas stream are influenced by the acceleration in a way so that they receive a terminal migration velocity, U t , 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:
• Acceleration force (p - p )a( ⁇ c/6)d , where
• the ratio C 2 /C, between concentration of droplets leaving the demister and droplets entering the demister was calculated, providing a direct measure of the effectiveness of separation.
• 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.
• 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.
• 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.
• 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 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.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Separating Particles In Gases By Inertia (AREA)
• Addition Polymer Or Copolymer, Post-Treatments, Or Chemical Modifications (AREA)
• Transition And Organic Metals Composition Catalysts For Addition Polymerization (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)

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Citations (9)

• 1999
  • 1999-12-01 NO NO19995867A patent/NO313867B1/no not_active IP Right Cessation
• 2000
  • 2000-11-06 CA CA002392745A patent/CA2392745A1/en not_active Abandoned
  • 2000-11-06 AU AU15605/01A patent/AU777868B2/en not_active Ceased
  • 2000-11-06 EP EP00978113A patent/EP1246683A1/en not_active Withdrawn
  • 2000-11-06 WO PCT/NO2000/000369 patent/WO2001039865A1/en not_active Application Discontinuation

Patent Citations (9)

Non-Patent Citations (1)

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