GAS-SOLIDE CONTACTING METHOD

Abstract: A gas scrubbing process is disclosed whereby particulate material is contacted with a high velocity exhaust gas stream to reduce the resistance to mass transfer attributable to the diffusion boundary layer and provide higher mass transfer rates. The process is particularly suited to the scrubbing of gaseous fluorides from aluminium smelter pot exhaust gas with alumina. Additionally, it has been found that the fines fraction with a particle size of preferably less than 45 mu m adsorbs or chemisorbs a disproportionate amount of contaminant trace metals from the exhaust gas stream. To avoid recycling these trace metals contained in the exhaust gas back to the smelter pot the fines fraction may be elutriated from the coarse fraction.

Description: TITLE: GAS-SOLID CONTACTING METHOD This invention relates to the treatment of exhaust gas streams and in particular the removal of a gaseous component of such streams by solid contact. An Example of a gaseous component in a gas stream which may be removed by solid contact is found in the electrolytic production of aluminium. Gaseous fluorides and particularly hydrogen fluoride are present in a potline exhaust gas as an inevitable consequence of the smelting process. If vented to the atmosphere these gases represent a net loss of fluorides from the process which is important from an economical point of view. Minimizing the fluoride release is also important from an environmental point of view. Gaseous fluorides are presently removed from exhaust gases by a dry scrubbing technique. This involves contacting the raw potline exhaust gas with alumina particles and the gaseous fluorides are chemisorbed and/or physisorbed onto the surface of the alumina. The alumina is then collected in a baghouse, together with any entrained particulates originally in the exhaust gas. In such dry scrubbing techniques, primary contact between the alumina and the exhaust gas can take place in two types of reactors - one type being known as a transport reactor. In a transport reactor, the primary alumina is injected into a low to moderate velocity gas stream before being separated. The particulates are injected into the gas stream for the purpose of entraining the particles and so the velocity of the particulates relative to the gas stream is small. Although such reactors are very simple, they are relatively inefficient gas-solid contactors and a recycle (up to 20 times) is generally employed to achieve the desired scrubbing. The high recycle, results in a high attrition of the alumina particles due to excessive handling and as particle size is an important parameter in aluminium pot feeds, control problems in that pot may arise because of excessive fines. In this type of reactor, the high recycle rate also results in a high proportion of the alumina used in fluoride sorption being carried to the baghouse resulting in a high solids loading in the baghouse. Furthermore, a high recycle rate makes predicting and controlling the rate of fluoride sorption onto the alumina difficult. The other type of reactor uses a conventional fluidized bed system. Such a system involves large initial capital costs and because of the large pressure drop across the bed, a high operating cost can be expected. In both types of reactor, all the alumina is ultimately fed to the cell and so the impurities (carbon, trace metals and other particulates in the exhaust gases) are also recycled to the electrolytic cell. With both of these systems, in practice, it has been found that a large proportion (usually 100%) of the feed inventory for the electrolytic cell must be contacted with the exhaust gas stream to effectively reduce the fluoride concentration to a respectable level. As a result of the impurities in the gas stream the overall quality of the feed is reduced. It is an object of this invention to improve the efficiency of the gas-solid contact, so that a smaller proportion of the alumina inventory can be used to reduce the fluoride levels in the potline exhaust gas. The objectives of any dry scrubbing operation is to adsorb or chemisorb the gaseous contaminants in an exhaust gas stream onto particulate material at the highest possible rate and attain the highest possible loading of contaminant on that particulate material. From a commercial point of view, this should be done by minimizing the operating costs associated with the contacting of gas and solid. The gas phase mass transfer co-efficient and hence mass transfer rate is very much dependent on the resistance to mass transfer of the diffusion boundary layer around the particle. If the boundary layer is reduced considerably by a high slip velocity (relative velocity between particle and gas) the mass transfer is then dependent on the adsorption or chemisorption rate at the particle surface. Generally, this results in the mass transfer proceeding at a much faster rate. Therefore, for a relatively short residence time, the adsorption process can substantially reach equilibria with the particulate material approaching its saturation limit for the gaseous component, before being removed. Consequently, less particulate material is required to handle the same loadings in the gas phase. It has been found that if the particulate material is contacted with a high velocity exhaust gas stream, the effect of diffusion to the particle surface is minimized and its contribution to the overall rate of sorption is negligible. In accordance with the objectives the invention thus provides a process of scrubbing a gaseous component from a gas stream comprising providing said gas stream at a predetermined velocity, entraining in said gas stream particulate material capable of sorbing the gaseous component is significantly reduced and recovering said treated particulate material, characterized in that said predetermined velocity being of such a magnitude and direction relative to the particulate material such that the resistance attributable to the boundary layer diffusion of the gaseous component to the particulate material in the sorption process and recovering said treated particulate material is significantly reduced. Preferably, prior to the scrubbing operation substantially all of the solid material is removed from the gas stream. Preferably the particulate material is alumina and the gaseous component is gaseous fluorides. It has further been found that proportionally, treated fine particulate material (preferably less than 45 μm) contains a larger amount of trace metals than treated coarser particles. Thus the compounding effect of adding impurities in the treated alumina used as feed, can be reduced if the finer particles are elutriated from the treated coarser alumina particles. This has additional benefits as the fine material causes occupational health problems for people working in the vicinity of the smelter pots. Thus in accordance with another aspect, the invention provides a process for treating a gaseous component containing exhaust gas stream comprising contacting a high velocity exhaust gas stream with a particulate material capable of adsorbing or chemisorbing the gaseous component recovering a coarse fraction of particulate material; the sorption of the gaseous component onto said material having substantially reached equilibria allowing a fine fraction of the particulate material to be carried over with the gas stream, and separating and recovering the fine fraction. This process has the advantage that only the fine fraction is carried over with the gas stream to the baghouse thus reducing the solids loading on the bag filter and reducing the cost of operating such equipment. Moreover, particles with a high concentration of such impurities as trace metals, are removed from the system or can be treated separately. If the impurity level in the treated alumina is to be reduced further, it is preferable that substantially all the solids in the exhaust gas stream be removed prior to contact with the sorbing particulate material. Preferably, the contacting process is carried out in an apparatus with a toroidal reaction zone with the gas stream entering from beneath the reaction. The entering exhaust gas stream preferably enters the bottom of the reaction zone at an angle inclined to the axis of the torold such that the gas stream is imparted with a circumferential motion about said axis as it progresses through the reaction zone. In this way a high velocity gas stream may be accommodated in a relatively compact zone. As the gas stream is of a high velocity relative to the particulate material, preferably greater than 1 mis the boundary layer is minimized and the reaction takes place rapidly on the surface of the material. Thus very little contact time is required for the particulate material to reach equilibria. The treated course fraction of particulate material may be taken off after a predetermined residence time and the fine material allowed to be carried over with the gas stream. The foregoing and other features objects and advantages of the present invention will become more apparent from the following description of the preferred embodiments and accompanying drawings in which: FIGURE 1 is a process flow diagram of a dry scrubbing system incorporating an embodiment of the process of the invention, FIGURE 2 is a schematic view of an apparatus for carrying out an embodiment of the process of the invention, FIGURE 3 is a schematic diagram showing the relative velocities of the gas stream and a particle in the reaction zone, FIGURE 4 is a sectional perspective view of an apparatus for carrying out the process in accordance with an embodiment of the invention, FIGURES 5 to 7 are schematic diagrams illustrating the diminishing of the diffusion boundary layer around the particle as the slip velocity increases, FIGURE 8 is a graph showing the effect slip velocity has on mass transfer rate and the relative improvement in the reaction rate, and FIGURES 9, 10, 11 and 12 are graphs showing the amounts of iron, vanadium, nickel and gallium, respectively, adsorbed or chemisorbed onto alumina as a function of particle size. The scrubbing process of the present invention will now be described with reference to the removal of fluorides and trace metals such as vanadium, nickel, gallium and iron from the exhaust gases of an aluminium refining cell. In the overall process flow diagram shown in Figure 1, the raw cell exhaust gas 1 is first passed through a solids removal stage 2 to remove, as stream 3, any particular material such as silica, alumina or carbon which may be entrained in the gas stream. This stage can be series of cyclones or multi-clones or other dust separation devices. The gas stream is then passed to the reactor 4 where fluorides and trace metals are adsorbed or chemisorbed onto the surface of the primary alumina 5. A coarse fraction 6 of treated alumina is then removed and mixed with the feed to an aluminium refining or smelting operation. A finer fraction may then be carried with the gas stream through a baffle 7 to separate the larger of the entrained particles before passed to the baghouse 8. The larger particles 9 in the fine fraction may be returned to the reactor and the remainder 10 are passed to the baghouse for separation from the gas stream as particle stream 11. The particles removed at stream 11 generally have a particle size less than about 45 microns, preferably less than about 20 microns and most preferably less than about 10 microns. If sufficient control can be maintained over the size the particles removed from the reactor as a fine fraction, then a baffle system may not be required. In the apparatus 20 for carrying out the process of the invention as shown in Figures 2 and 4, a fluoride containing exhaust gas stream 21 enters the reactor in the direction of the arrows shown. The bottom of a toroidal reaction zone 22 comprises a number of intake guides 23 which direct the incoming high velocity exhaust gases in a direction at least 80° to the access of the reaction zone so that the gases move in a direction which is substantially circumferential to the reaction zone 22. The gases thus create a swirling motion in the processing zone of the reactor 20 about the axis of the reaction zone 22. Alumina is fed into the reactor via axial conduit 24 which has a diverging conical feeder to provide a uniform distribution of particulate material in the gas stream before entering the toroidal reaction zone at 25. As the particles are contacted with the gas flow, the swirling motion of the gas stream and the velocity of the gas stream relative to the particles causes a shearing or disruption of the diffusion boundary layer as discussed later. The particles initially are moved to the outside of the reaction zone 22 by centrifical force but as more particles are fed into the reactor, the reaction zone fills and the treated particles are forced to the inside of the reaction zone. The treated particles which have completed the sorption process are taken from the reactor via conduit 28 as stream 27 to be mixed with the refining cell or smelter pot feed. The walls of the reactor 29 can extend upwardly from those shown in Figure 4 and taper outwardly (not shown) to keep the finer particles entrained until processed as in Figure 1. As discussed earlier, when the alumina particles enter the reaction zone, the difference between the velocity of the particles and the velocity of the gas is large enough to shear or disrupt the surrounding diffusion boundary layer around the particle. This is illustrated in Figures 5, 6 and 7. In Figure 5, the velocity of the particle is roughly the same as the gas velocity. This results in a slip velocity approaching zero and corresponds to a scrubbing operation performed in a transport reactor. As can be seen in the drawing, the diffusion boundary layer is large and as a consequence high gas phase resistance results. In Figure 6 the slip velocity has increased resulting in a lower diffusion boundary layer resistance and faster gas phase mass transfer. This situation is analogous to a fluidized bed reactor and although improving the scrubbing operation, the higher capital and operating costs detract considerably from the appeal of this type of reactor. When the gas velocity is much greater than the particle velocity as shown in Figure 7, the boundary layer diffusion resistance is greatly reduced. The particles which have a substantially zero velocity in the direction of gas flow are slip velocity above about 1 metre per second provide sufficiently low resistance to gas phase mass transfer to achieve effective gas scrubbing. Although the velocity of the gas may not be large enough to remove all the boundary layer, a sufficient amount of the boundary layer is removed or disrupted so that the contribution of the rate of diffusion of fluorides to the particle in the overall reaction rate is greatly reduced. Thus the rate determining step in the overall reaction is the adsorption or cheraisorption of the fluorides on to the surface of the alumina. As the reaction between fluorides and alumina is both rapid and complete, the contact with alumina can be conducted in a single pass. To handle the large volumes of exhaust gas produced in an aluminium smelter, it is likely that a bank of reactors can be set up in parallel. A typical dry scrubbing operation of gaseous fluorides from smelter pot exhaust gas must be capable of handling 2 - 3 x 10 6 Nm 3 /hr with a concentration of 100-400 ppmHF. Figure 8 is a graph of slip velocity in m/s on the X axis, mass transfer co-efficient in centimeters per second on the Yl axis and percentage relative improvement in reaction rate on the Y2 axis. A fluoridized bed gas scrubber has a slip velocity of typically 0.6 m/s whereas a transport reactor has slip velocity of less than 0.1 m/s. Therefore the benefits in operating at a slip velocity of greater than 1 m/s are clearly evident from Figure 8. For gas scrubbing operations carried out in accordance with the invention, and preferably in a toroidal reaction zone, slip velocities between preferably 4 to 5 m/s can be used with the consequent increase in scrubbing efficiency. As discussed earlier, by using a toroidal reaction zone, the apparatus can be operated to allow a fine fraction of the alumina to be carried over with the gas stream or later recovery. Furthermore, the pressure drop across a toroidal reaction zone of 1 m diameter is 10 - 30 mm H 2 0 which is an order of magnitude less than a corresponding fluidized bed gas scrubber. It has been found by the present applicants that a fine fraction of less than 45 microns adsorbs or chemisorbs a disproportionate amount of, in particular, trace metals. Figures 9, 10, 11 and 12 illustrates the sorption of iron vanadium, nickel and gallium, respectively, in parts per million (Y axis) as a function of particle size (X axis) . In the adsorption or chemisorption of gaseous fluorides from the exhaust gas of an aluminium refining cell onto alumina, both the fluorides and treated alumina are used as feeds for the refining cell. Trace metals such as gallium, nickel, iron and vanadium are considered contaminants and due to their presence in the exhaust gas are adsorbed or chemisorbed onto the alumina simultaneously with the gaseous fluorides. To prevent these trace metals being returned to the refining cell with the treated alumina, the fine fraction which has a disproportionately higher fraction of trace metals, is recovered and not used as feed for the refining cell. From Figures 9 to 12 it is evident that the less than 10 micron fraction has the largest proportion of trace metals but it can be seen that benefits can be gained by separating a fine fraction of up to 45 microns. Table 1 shows the reduction in impurity levels which were obtained by removal of alumina with a particle size of 20 microns and alumina with a particle size of less than 45 microns. It can be seen that the benefits of separating the fine fraction are most noticeable with respect to vanadium and nickel. While the invention has been described mainly in terms of the removal of gaseous fluorides from aluminium cell exhaust gases by contact with alumina, it should be readily understood by those skilled in the art that the process of the invention is equally applicable to other gas scrubbing or gas solid contacting applications which are dependent on the rate of diffusion to the adsorbing or chemisorbing particle. For example, a further application may be in the desulphurization of flue or process gases. Such gases containing sulphur in the form of sulphur dioxide or other oxide species must first be treated prior to venting to minimize harmful effects on the environment prior to release. In common practise treatment may be performed in wet or dry scrubbers. In these cases the solid contact medium may be lime or hydrated lime or zone oxide. Reduction of the diffusion layer around the particles would increase the level of reaction rate and efficiencies.

Claims: CLAIMS: 1. A process of scrubbing a gaseous component from a gas stream comprising providing said gas stream at a predetermined velocity, contacting said gas stream with a particulate material capable of adsorbing or chemisorbing the gaseous component, and recovering said treated particulate material characterized in that said predetermined velocity is of such a magnitude and direction relative to the particulate material such that the resistance attributable to the boundary layer diffusion of the gaseous component to the particulate material in the sorption process is significantly reduced. 2. The process in accordance with claim 1 wherein the predetermined velocity of the gas stream relative to the particulate is greater than 1 m/s. 3. The process in accordance with claim 1 wherein the predetermined velocity of the gas stream relative to the particle is greater than 4 m/s. 4. The process in accordance with claim 3, wherein the predetermined velocity of the gas stream relative to the particle is less than 5 m/s. 5. The process in accordance with claim 1 wherein contact between said gas stream and said particulate material is made in a toroidal reaction zone. 6. The process in accordance with claim 5 wherein said gas stream entering from beneath said toroidal reaction zone is imparted with a substantially circumferential motion and said particulate material enters said reaction zone in a substantially axial direction. 7. The process in accordance with claim 1 or claim 6 wherein a fine fraction of said particulate material is elutriated from a coarse fraction by said gas stream, said fine fraction being entrained in said gas stream and exiting said reaction zone with said gas stream. 8. The process in accordance with claim 7 wherein the adsorption or chemisorption of the gaseous component from the gas stream onto said particulate material has substantially reached equilibria prior to the particulate material exiting the reaction zone. 9. The process in accordance with claim 7 wherein the particulate material is alumina and the gaseous component is a gaseous fluoride. 10. The process in accordance with claim 9 wherein the particle size of the fine fraction is less than about 45um. 11. The process in accordance with claim 10 wherein the particle size of the fine fraction is less than about 20μm. 12. The process in accordance with claim 11 wherein the fine fraction has a particle size of less than about lOum. 13. A process for treating a gaseous component containing exhaust gas stream comprising contacting a high velocity exhaust gas stream with a particulate material capable of adsorbing or chemisorbing the gaseous component, recovering a coarse fraction of particulate material; the sorption of the gaseous component onto said material having substantially reached equilibria, allowing a fine fraction of the particulate material to be carried over with the gas stream and separating and recovering the fine fraction. 14. The process in accordance with claim 13 wherein said high velocity gas stream contacts said particulate material in a toroidal reaction zone, said exhaust stream moving through said reaction zone in a substantially circumferential motion and said particulate material being fed into the reaction zone in a substantially opposing axial direction. 15. The process in accordance with claim 14 wherein the exhaust gas stream velocity relative to the particle velocity is greater than about 1 m/s. 16. The process in accordance with claim 14, wherein the exhaust gas stream velocity relative to the particle velocity is greater than 4 m/s. 17. The process in accordance with claim 16, wherein the exhaust gas stream velocity relative to the particle velocity is less than 5 m/s. 18. The process in accordance with claim 13 or claim 14 wherein the fine fraction has a particle size of less than about 45um. 19. The process in accordance with claim 13 or claim 14 wherein the fine fraction has a particle size of less than about 20μm. 20. The process in accordance with claim 13 or claim 14 wherein the fine fraction has a particle size of less than about lOum. 21. The process in accordance with claim 14 or 14 wherein the particulate material is alumina and the gaseous component removed is a gaseous fluoride. 22. The process in accordance with claim 21, wherein the sorption of fluorides onto the alumina has substantially reached equilibria prior to the treated alumina exiting the reaction zone.

Inventors: GOODES CHRISTOPHER GEOFFREY, KJAR ANTHONY RUDLAND, STILL ROBERT FRANCIS, WELLWOOD GRANT ASHLEY, GOODES, CHRISTOPHER, GEOFFREY, KJAR, ANTHONY, RUDLAND, STILL, ROBERT, FRANCIS, WELLWOOD, GRANT, ASHLEY
Applicants: COMALCO ALU, COMALCO ALUMINIUM LIMITED
Application year: 1991
Main classification: B01D53
IPC codes: B01D53, B01J8