IMPROVED GAS SCRUBBING PROCESS

Abstract

This invention relates to equipment for scrubbing a first substance comprising of a gas using a second substance comprising of a scrubbing fluid. More specifically, the invention relates to the inclusion of an additional performance enhancing feature.

Description

Technical eld In the following document, the term scrubbing should be understood as including the transfer of components (including energy) from the scrubbing uid into the gas as well as the transfer of components (including energy) from the gas into the scrubbing uid. This invention relates to equipment for scrubbing a rst substance comprising of a gas using a second substance comprising of a scrubbing uid. More specifically, the invention relates to the inclusion ofan additional performance enhancing feature. Background Art The removal of gaseous components and / or particulates and / or droplets from a gas stream using a scrubbing uid, is well known, and is often carried out by means of a wet scrubbing process. Similar equipment can be used for transferring components from the scrubbing uid into the gas stream. It is also well known that the size of the scrubbing uid droplets and the relative velocity between those droplets and the gas stream have a strong inuence on the scrubbing efciency during the scrubbing process. In summary, the scrubbing efciency associated with each droplet of scrubbing uid increases as the relative velocity between that droplet and the gas ow is increased. In addition, the scrubbing efciency associated with each droplet of scrubbing uid increases as the size ofthat droplet is decreased. Within South African Patent PCT / ZA 2003 / 000160 (WO 2004 / 039492 A2) technology is described which enables a wet scrubbing process to achieve dust removal efciencies from a gas stream of above 90% for 0.5 micron sized dust particles. This patent describes equipment comprising of a static, co-current contacting device having a plurality of stages dening a ow path, with a ow prole and a ow direction for the gas and the scrubbing uid from an inlet end of the ow path towards an outlet end oftheow path; at least some ofthe stages being shaped to dene a substantially curved ow path having an effective centre of curvature located to one side of the ow path, and wherein each adjacent stage has an effective centre of curvature on an opposite side of the ow path, the ow path characterised in being provided with an edge formation between at least two adjacent stages so as to enhance a launch of the scrubbing uid on an outside of the curved ow path of the adjacent stage that immediately precedes the respective edge formation from that edge formation with a trajectory towards an opposite wall of the ow path, wherein the opposite wall of the ow path is positioned, orientated and shaped to create a landing area that would intersect the ight path of the scrubbing uid at an angle of incidence of less than 35° and to then smoothly continue towards the end of that stage in conformity with the ongoing curvature of the ow path as it continues towards a next stage or towards the outlet end ofthe ow path. In order to achieve the shallow angle of incidence for the scrubbing uid, one design option is for the opposite wall of the ow path that is immediately downstream ofan edge formation to be orientated at an angle (typically <60°) with respect to the direction with which the scrubbing uid is launched (the launch direction). In effect, this means that the whole ofthe ow path that is immediately downstream ofan edge formation has to be orientated at a similar angle in relation to the launch direction of the scrubbing uid. An alternative design option is to provide additional space within the ow path within which the ongoing trajectory (ight path) of the scrubbing uid can become more closely aligned with the orientation ofthe gas ow within the ow path. Note, during the launch process or immediately following the launch, the scrubbing uid typically breaks up into a sheet of droplet, the initial ight path ofwhich is in line with the trajectory with which the scrubbing uid is launched. The net result from these design options is to extend the ight path ofthe droplets of scrubbing uid before they are able to arrive at the opposite wall of the ow path. During this extended ight path, frictional effects between the gas and the droplets cause the droplets to absorb velocity energy from the ow of gas. This energy transfer to the droplets creates an increase in the gas phase pressure drop within the scrubbing process. In addition, because of the increased length of the droplet ight path, the ow path between each edge formation has to be extended, resulting in a substantial increase in the size ofthe necessary equipment. Further, and most importantly as regards the resultant scrubbing performance per unit ofenergy expended, the above referred design options cause the average relative velocity between the gas and the droplets of scrubbing uid to be signicantly smaller than would be the case if the droplets were launched perpendicularly across the ow path. Object ofthe invention It is accordingly a rst object of the present invention to reduce the additional pressure drop and equipment size that is associated with achieving a sufciently shallow angle of incidence for the droplets of scrubbing uid that have been created by an edge formation and subsequently received at an associated landing area both of which have been shaped, positioned and orientated with respect to each other in a manner that is similar to that which is described within South African Patent PCT / ZA 2003 / 000160 (WO 2004 / 039492 A2). In addition, as regards the droplets that are createdby an edge formation, it is a further objective ofthe present invention to maximise the proportion which reach the wall of the ow path that is opposite to that edge formation in a way that enables them to create and be incorporated into a smoothly owing lm of scrubbing uid on that opposite wall. Further, for a given input of scrubbing uid and for a given gas velocity within the ow path, it is an additional objective of the present invention to shape and orientate the walls ofthe ow path that are adjacent to and opposite to an edge formation so as to maximise the relative velocity between the gas ow and the droplets of scrubbing uid throughout their ight path following their creation when the scrubbing uid is launched from that edge formation, whilst at the same time minimising the size ofthe droplets of scrubbing uid that are created. Disclosure ofthe invention According to a rst aspect ofthe invention there is provided equipment for scrubbing a rst substance comprising of a gas, using a second substance comprising of a scrubbing uid, the equipment comprising of a static, cocurrent contacting device having a plurality of stages dening a ow path, with a ow prole and a ow direction for the rst and the second substances from an inlet end of the ow path towards an outlet end of the ow path; at least some of the stages being shaped to dene a substantially curved ow path having an effective centre of curvature located to one side ofthe ow path, and wherein each adjacent stage has an effective centre of curvature on an opposite side of the ow path, the ow path characterised in being provided with an edge formation between at least two adjacent stages so as to enhance a launch of the second substance on an outside ofthe curved ow path of the adjacent stage that immediately precedes the respective edge formation from that edge formation with a trajectory and an ongoing ight path towards an opposite wall ofthe ow path, wherein the opposite wall ofthe ow path immediately downstream of at least one edge formation has a protrusion which protrudes towards the side of the ow path that is associated with the edge formation which is immediately upstream of the protrusion, wherein a rst portion of a surface of the opposite wall immediately upstream of the protrusion is shaped both to create the protrusion and to direct the rst substance in a direction towards and through the trajectory and ight path of the second substance, wherein a second portion of the surface of the opposite wall forms a downstream surface ofthe protrusion and is shaped to intersect the ight path ofthe second substance at an angle ofincidence of less than 35° and to then smoothly continue towards the end of that stage in conformity with the ongoing curvature of the ow path as it continues towards a next stage or towards the outlet end ofthe ow path. The edge formation may be stepped, and is preferably provided with a substantially perpendicular face relative to a surface of the outside of the curved ow path that is immediately upstream of the edge formation to enhance the launch of the second substance. The edge formation may be provided with a ledge subsequent to the edge to provide a rst and a second surface, the rst and the second surfaces preferably being arranged so as to encourage a small back eddy of the rst substance immediately beneath the edge that deects any downwards dribble around the edge of the second substance back up into the underside of the main ow of the second substance as it leaves the edge formation. The rst and second surfaces at each edge formation may be joined with a llet radius to ensure maximum effect from the swirling and scouring action from the back eddy. The rst and second surfaces may have a similar depth and width, preferably, but not exclusively, ofbetween 2 and 10mm. The ow path is preferably provided with an edge formation between successive stages within the ow path. The equipment may be characterized in being capable of achieving removal efciencies ofabove 90% for 0.5 micron sized dust particles. According to a second aspect of the invention there is provided a method for scrubbing a rst substance comprising of a gas, using a second substance comprising of a scrubbing uid, the method including the steps of transporting the gas and the scrubbing uid through a plurality of stages, at least some ofthe stages being shaped to dene a substantially curved ow path having an effective centre of curvature located to one side ofthe ow path, and wherein each adjacent stage has an effective centre of curvature on an opposite side of the ow path, wherein as the rst substance and the second substance progress past an edge formation that is located between adjacent stages within the equipment, the second substance on an outside of the curved ow path of the adjacent stage that immediately precedes an edge formation is launched from that edge formation with a trajectory and an ongoing ight path towards an opposite wall of the ow path; wherein the shape and position of a rst portion of a surface of the opposite wall that is immediately upstream of a protrusion that is located within the opposite wall of the ow path immediately downstream of at least one edge formation directs the rst substance in a direction towards and through the trajectory and ight path ofthe second substance, where the shape and position of a second portion of the surface of the opposite wall that is immediately downstream of the protrusion enables the second substance to be received within a landing area with an angle of incidence of less than 35 ° and then enables the resultant lm ofthe second substance towards a next stage or towards the outlet end ofthe ow path. The methodmay be characterized in being capable of achieving removal efciencies ofabove 90% for 0.5 micron sized dust particles. Specic Embodiment ofthe invention A preferred embodiment of the invention will now be described by means of a non limiting example only and with reference to the various aspects of the invention and the accompanying Figure 1. Figure l shows the shape of a typical ow path for a three contacts gas scrubbing arrangement together with the likely range of ight paths for the droplets of scrubbing uid. Arrow 1 points to the inlet end of the ow path. Arrows 2 and 4 point to the side walls of the ow path. These walls may be the sides of a rectangular or approximately rectangular ow path or they may be the inner and outer walls of an annular ow path. For optimum scrubbing performance within an annular ow path, the ow prole needs to remain the same all of the way around the annulus. For optimum scrubbing performance for a rectangular or approximately rectangular ow path, the ow prole needs to remain the same throughout the width of the ow path. The preferred location for the addition ofthe scrubbing uid would be to uniformly distribute it along the whole of the length ofthe side wall 4, preferably at the entry 3. The necessary means for accelerating the gas that is to be scrubbed to a velocity that would be suitable for the quality of scrubbing that is required and for delivering it to the inlet end would be obvious to a skilled practitioner and are therefore not shown here. For similar reasons, the means for delivering the scrubbing uid to the inlet end ofthe ow path have not been shown. Arrow 5 points to the rst edge formation within this example arrangement and arrow 6 points to the likely range of ight paths for the majority of the droplets that would be launched from this edge formation. Arrow 16 points to the likely range of ight paths for the majority of the droplets that would be launched from the next edge formation, 12. Arrows 8, 18 and 23 point to the three protrusions that are present within this example arrangement. Arrows 7, 17 and 22 point to the respective rst portions of the opposite wall that are associated with these protrusions and arrows 9, 19 and 24 point to the respective second portions of the opposite wall that are associated with these protrusions. In this example arrangement, the approach angles to the landing areas 9, 19 and 24 for the droplets of scrubbing uid are all within the range of20 to 25°. Arrow 10 points to the ongoing smooth surface prole that transfers the lm of scrubbing uid from the landing area 9 towards the second edge formation 12. Arrow 20 points to the similar smooth surface that is upstream ofthe edge formation 21. It is very benecial, but not essential for these portions of the walls of the ow path to be curved as the centrifugal forces associated with a fast moving lm owing around the curve help to maintain the uniformity of the lms thickness and the uniformity of the lms velocity as the lm approaches the subsequent edge formation. Curvature within the section ofthe wall at arrow 4 is similarly helpful. Arrow 11 points towards a short section of straight at the end of the curved surface 10 which is preferably present at all of the edge formations in order to minimise the effect of abrasive wear within these surfaces on the uniformity of the launch angle that is achieved from each edge formation. Arrows 13 and 15 point to the faces of the rst and second steps, respectively, that, preferably, are associated with each edge formation. Arrow 14 points to the llet radius that, preferably, is present within the corner between these steps. Arrow 25 points to the outlet end of the ow path. The many means for separating the used scrubbing uid from the scrubbed gas are well established and should be well known to an experienced practitioner. Therefore, a suitable example is not shown here. Similarly, the means that can be used to convert some or most of the velocity energy that will be present within the scrubbed gases back into pressure energy and for ducting the scrubbed gas to wherever it needs to be sent are also well established and should be well known to an experienced practitioner. Therefore, a suitable example is not shown here. It will be appreciated that many variations in detail are possible without departing from the scope or spirit of the invention as claimed in the claims hereinafter, such as the application ofthe method and equipment to other off-gases and dust emissions.

Claims

In the conclusions below, the following should be understood under the term "scrubbing" be included: the transfer of components (including energy) from the scrubbing fluid to the gas, as well as or in addition to the transfer of components of the gas to the scrubbing liquid. l. Equipment for scrubbing a first substance, consisting of a gas, with the use of a second substance, comprising a scrubbing liquid, whereby the equipment comprises a device bringing a static contact device with a multitude of steps that define a flow trajectory, with a flow profile and a flow direction for the first and second substances from a inlet end of the flow path to an outlet end of the flow trajectory; where at least some of the steps are formed to a to have a predominantly curved flow path with an effective center of curvature positioned on one side of the flow path and where each adjacent staircase has an effective center of curvature at a opposite side of the flow path, where the flow path is characterized by having an edge formation between at least two adjacent stairs to facilitate a launch of the second substance to the outside of the curved flow path of the adjacent staircase that immediately precedes the respective edge formation from that edge formation with a trajectory and a continuous escape route to an opposite wall of the flow path, where the opposite wall of the flow path immediately downstream of the rim formation has a bulge that protrudes towards the side of the flow path that is associated with the edge formation that is located immediately upstream of the bulge, where a first part of a surface of the opposite wall immediately upstream of the bulge is formed to both form the bulge and to guide the first substance in a direction towards and through the trajectory and the escape route of the second substance, where a second part of the surface of the opposite wall a downstream surface of the bulge forms and is formed in such a way that it intersects the flight path of the second substance at an angle of incidence of less than 35° and then continues smoothly to the end of that staircase in accordance with the continuous curvature of the flow path to the next stage or to the end of the flow path.

2. Equipment according to claim 1, where the edge formation is stepped, with a almost perpendicular side with respect to a surface of the outside of the curved flow path located directly upstream of the edge formation is located 3. Equipment in accordance with claim 2, where stepped edge formation is provided of a ridge adjoining the edge to a first and a second surface define.

4. Equipment within the meaning of claim 3, where the first and second surfaces are connected to a rounding radius.

5. Equipment pursuant to claim 4, where the first and second surfaces are have a similar width.

6. Equipment within the meaning of claim 1, where the contact device is provided with edge formation between each step.

7. Equipment in accordance with claim 1, where the flow path is executed and dimensioned to both the angle and the position of each launch relative to the subsequent shape of the flow channel and the controlled change to orient in the direction of the flow gully such that a maximum of scrub fluid is collected, which is launched onto a landing area at the opposite side of the flow pool before a subsequent launch takes place.

8. Method for scrubbing a first substance, consisting of a gas, with a second substance, consisting of a scrubbing liquid, whereby the gas and the scrubbing fluid is transported through a multitude of stages, whereby at at least some steps are shaped in such a way that they have a substantially curved define flow path with an effective center of curvature on one side of the flow path and where each adjacent staircase an effective has center of curvature on an opposite side of the flow path, where the first substance and the second substance, when they pass a edge formation occurring that is located between adjacent stairs in the equipment, the second substance on an outer side of the curved flow path of the adjacent staircase that immediately precedes an edge formation, from that edge formation is launched with a trajectory and a continuous runway to a opposite wall of the flow path; where the shape and position of a first part of a surface of the opposite wall that is directly is located upstream of a bulge situated in the opposite wall of the flow path immediately downstream of at least one ridge formation is located, the first substance in a direction towards and through the trajectory and the flight path leads to the droplets of scrubbing liquid, whereby the shape and position of a second part of the surface of the opposite wall that is directly downstream of the bulge is located, ensures that the second substance is collected within a landing area with an angle of attack of less than 35° and subsequently makes it possible for the resulting film from the second substance to a subsequent one allows flow to the outlet end of the flow path.

9. Method according to claim 8, characterized by being able to a achieve a removal efficiency of more than 90% for dust particles of 0.5 micron.

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