AU2002365618A1 - Improved gas scrubbing - Google Patents

Description

WO 03/047724 PCT/AU02/01663 -1 IMPROVED GAS SCRUBBING Field of the Invention 5 The present invention relates to a process for improved gas scrubbing and, in particular, to a process for scrubbing solid particulates and/or soluble gases from gaseous process streams and combustion gas streams emanating from incinerators, power generation plants and metallurgical processes. The invention is particularly applicable to the scrubbing of soluble gasses and/or particulates from a gaseous process stream 10 emanating from an iron ore sinter plant or an alumina production plant. Background of the Invention Industrial gas scrubbing is well known, the principal known methods being: scrubbing using cyclone separators and knockout pots; 15 wet scrubbing, wherein fine dust particles carried by a gas are contacted with a liquid such as water to form a slurry containing the dust particles; electrostatic precipitation; and dry scrubbing, wherein a gas is filtered through a suitable woven fabric material. 20 However, with these methods, either the efficiency is low, such as with the method of scrubbing using cyclone separators, or the method is prohibitively expensive, such as with the dry scrubbing method. The dry scrubbing method also suffers from a disadvantage in that the method is not able to remove volatilised components from the gas stream. 25 Summary of the Invention In accordance with an aspect of the present invention, there is provided a process for scrubbing soluble gaseous and/or particulate material from a gas stream, the process including the steps of: 30 passing the gas stream and a scrubbing medium through an in-line scrubber whilst maintaining a controlled pressure differential between an inlet and an outlet of the in-line scrubber that promotes an interphasic interaction between the gas stream and the scrubbing medium resulting in scrubbing of the soluble gaseous and/or capturing of WO 03/047724 PCT/AU02/01663 2 the particulate material from the gas stream. The controlled pressure differential across the in-line scrubber is preferably at least 8.5 kPa. More preferably, the controlled pressure differential is at least 20 kPa. More 5 preferably still, the controlled pressure differential is at least 30 kPa. In accordance with another aspect of the present invention, there is provided a process for scrubbing soluble gaseous and/or particulate material from a gas stream, the process including the step of: 10 controlling the flow rate of the gas stream and a scrubbing medium through an in-line scrubber such that an interphasic interaction is promoted between the gas stream and the scrubbing medium, resulting in scrubbing of the soluble gaseous and/or capturing of the particulate material from the gas stream. 15 The term "in-line" is used in this specification to describe a unit mounted within a process line or at one end of a process line, eg an inlet or outlet of a process line or pipe. The term "in-line" is also used to indicate that the process is a preferably continuous process rather than a batch process. 2 0 The term "interphasic interaction" is employed to describe any interaction between two or more phases, including mechanical interaction due to, for example, collisions, shearing, friction, compression, extrusion and physical or energy transfers and where the phases that interact are characterised by different relative densities. 25 The term "scrubbing" is employed to describe the cleaning of a gas stream by removal of typically unwanted substances from the gas stream whereby the unwanted substances are transferred from the stream to the scrubbing medium. The unwanted substance would typically be a substance that is considered to be detrimental to either the environment or health or both and may be in the form of a soluble gas, liquid or solid or 30 in the form of insoluble solid particulates. Whilst a chemical reaction may occur during scrubbing, any such reaction would be considered secondary to the cleaning action of a scrubbing process.

WO 03/047724 PCT/AU02/01663 -3 It is recognised that in most if not all scrubbing operations, removal efficiency never reaches 100% and thus it is to be clearly understood that further scrubbing or other cleaning stages may be required to increase the efficiency of removal of unwanted 5 substances using one or more in-line scrubbers arranged either in parallel or in series, alone or in combination with other conventional scrubbing units. Alternatively, the gas stream and scrubbing medium may be recirculated through a single in-line scrubber. Preferably, the in-line scrubber includes a plurality of stages, each stage being arranged 10 to promote cross-migration of phases. In one embodiment the in-line scrubber is a static scrubber. The term "cross-migration" is used in this specification to describe the action whereby the gas and the scrubbing medium are forced to migrate first in one direction and then in 15 substantially the reverse direction. The relative densities of the gas and the scrubbing medium will influence the degree of cross-migration occurring with the more dense phase subjected to a greater degree of cross-migration than the less dense phase. Without wishing to be bound by theory, this action of cross-migration of the phases improves the efficiency of the interphasic interactions occurring in the in-line scrubber. 20 While any suitable scrubbing medium may be employed, depending on the type of soluble gaseous and/or particulate material to be scrubbed from the gas stream, the preferred scrubbing medium is water or an organic solvent. A wetting agent may also be added to the scrubbing medium to enhance solids collection. 25 In one embodiment, the gas stream is a gas stream from an iron ore sinter plant. In an alternative embodiment, the gas stream is a gas stream from an alumina processing plant. 30 Preferably, the process further includes the step of recirculating the scrubbing medium to the in-line scrubber.

WO 03/047724 PCT/AU02/01663 -4 In accordance with another aspect of the present invention, there is provided a system for scrubbing soluble gaseous and/or particulate material from a gas stream, the system including: an in-line scrubber; 5 means for introducing the gas stream into the in-line scrubber; means for introducing a scrubbing medium into the in-line scrubber; and means for generating a controlled pressure differential across the in-line scrubber, the pressure differential being of a magnitude sufficient to pass the gas stream and the scrubbing medium through the in-line scrubber to promote interphasic 10 interaction between the gas stream and the scrubbing medium resulting in scrubbing of the soluble gaseous and/or capturing of the particulate material from the gas stream. The means for generating a controlled pressure differential may be a vacuum pump. In one arrangement, the vacuum pump is a liquid ring vacuum pump. 15 A differential pressure sensor may be provided to provide feedback to a process control circuit for the purpose of maintaining optimum process conditions. In accordance with still another aspect of the present invention, there is provided a 20 system for scrubbing soluble gaseous and/or particulate material from a gas stream, the system including: an in-line scrubber; means for introducing the gas stream into the in-line scrubber; means for introducing a scrubbing medium into the in-line scrubber; and 25 means for controlling the flow rate of the gas stream and the scrubbing medium through the in-line scrubber such that interphasic interaction is provided between the gas stream and the scrubbing medium resulting in scrubbing of the soluble gaseous and/or capturing of the particulate material from the gas stream.. 3 0 Preferably, the means for introducing the gas stream is also the means for introducing the scrubbing medium to the in-line scrubber.

WO 03/047724 PCT/AU02/01663 5- Preferably, the system further includes means for adjusting the pH of the scrubbing medium to a desired pH. The system may include means for clarifying the scrubbing medium and recirculating the clarified scrubbing medium to the in-line scrubber. 5 The system may include at least one knockout pot and/or at least one cyclone separator for further separating particulate material and/or soluble gases from the gas stream. The system may include means for analysing the feed gases at the inlet and the scrubber discharge. 10 Brief Description of the Drawings Preferred embodiments of the present invention will now the described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram representing a process for gas scrubbing in 15 accordance with an embodiment of the present invention; and, Figures 2a and 2b are diagrammatic cross-sectional representations of an in-line scrubber for use in the process represented in Figure 1. Detailed Description of a Preferred Embodiment of the Invention 20 In Figure 1, there is illustrated schematically a gas scrubbing process 10 for scrubbing a gas stream emanating from an iron ore sinter plant. The circuit of Figure 1 illustrates a scrubbing process controlled by setting, measuring and adjusting the minimum pressure differential across an in-line scrubber 18. As an alternative, the control variable of the process could equally be the flow rate of the gas stream and scrubbing medium through 25 the in-line scrubber. Figure 1 is also illustrative of a process for scrubbing solid particulates from a gas stream using a liquid scrubbing medium. In Figure 1, the gas stream flows through a duct 12 and is introduced into an inlet 14. A scrubbing medium, in this example water, is also introduced into the inlet 14 by a 30 plurality of nozzles 16. Although in this example the scrubbing medium is water, it will be understood that other types of scrubbing medium are envisaged, the particular scrubbing medium being selected depending on the target materials which it is desired WO 03/047724 PCT/AU02/01663 -6 to scrub from the gas stream. For example, aqueous or organic solvents may be used. A wetting agent may also be added to the scrubbing medium to further enhance collection of material from the gas stream. The gas stream and the scrubbing medium in the inlet 14 are then forced through an in-line scrubber 18 by, in this example, a first 5 vacuum pump 20 located downstream of the scrubber 18. The discharge stream from the in-line scrubber includes both the scrubbed gas stream and the scrubbing medium loaded with solid particulates. The bulk of the loaded scrubbing medium is recovered from the gas stream using the first knockout pot 22. 10 The overflow from the first knockout pot 22 is directed to first and second cyclone separators 24, 26 in series to remove any additional solid particulates still present in the gas stream downstream of the in-line scrubber. The underflow from the first knockout pot 22 is sent to a scrubbing medium recycle 15 circuit which includes a first tank 30 for receiving the underflow from the first knockout pot 22, a discharge pipe 32 for discharging solids from the first tank 30, a filter 34 for clarifying the scrubbing medium drawn from the tank 30 and a second pump 35 for recirculating the clarified scrubbing medium back to the nozzles 16. Fresh scrubbing medium is also added to the recycling circuit to replace any scrubbing medium 20 discharged to the first discharge pipe 32 with the removed solids. Downstream of the first vacuum pump 20 is a second knockout pot 28 which serves as a backup trap. The second knockout pot 28 communicates with an exhaust duct 29 which serves to transport the scrubbed gas stream to atmosphere. 25 In this example, the vacuum pump 20 is a liquid ring vacuum pump with the liquid being used for a final wetted-particle capturing stage. While the present embodiment is described in relation to a vacuum pump located downstream of the scrubber 18, it will be understood that other means for forcing the gas stream through the scrubber 18 are 3 o possible. For example, the vacuum pump may be replaced with fans or eductors. Also, the vacuum pump or equivalent may be located upstream of the scrubber 18 instead of downstream of the scrubber. However, with this arrangement, the actual gas volume WO 03/047724 PCT/AU02/01663 -7 passing through the scrubber 18 is reduced due to compression, which may enable the number of necessary scrubbers 18 required to scrub a particular gas stream to be reduced. 5 The vacuum pump is arranged such that the minimum pressure drop across each scrubber 18 is within the range 8.5 - 30 kPa, since it is believed that good efficiency is achieved with the scrubber when the minimum pressure drop is within this range. A typical liquid ring vacuum pump can generate a minimum pressure drop across the in line scrubber of up to between 45 and 75 kPa. However, it will be understood that this 10 preferred range may vary depending on the type of scrubber used, the number of stages in the scrubber, and so on. It will also be understood that although one scrubber 18 is shown in Figure 1, in practice any number of scrubbers 18 arranged in series or parallel may be used, the important 15 aspect being that the required gas flow rate is achieved and a sufficient pressure differential across the scrubber(s) is maintained. It is envisaged that a gas scrubbing system comprising a plurality of such in-line scrubbers could be arranged to handle the scrubbing of gas streams in the order of 900,000 to 1,000,000 m 3 i/hour, depending on the capacity of each unit and the number of units arranged in series or parallel. The 20 number of units selected is done so on the basis of the amount of process stream to be scrubbed. A suitable in-line scrubber for use in the present invention is the applicant's FilblastTM Low Shear Reactor Gas (LSR-G ) (hereinafter referred to as the Filblast TM scrubber) 2 5 which is the subject of International Patent Application No. WO 94/29017, the contents of which are incorporated herein by reference. However, although the present invention is described with reference to a FilblastTM scrubber, it is to be understood that any suitable in-line scrubber may be used so long as 30 the in-line scrubber promotes interphasic interaction between a gas stream and a scrubbing medium so as to facilitate scrubbing of the gas under the influence of either a controlled pressure differential between the inlet and the outlet of the in-line scrubber or WO 03/047724 PCT/AU02/01663 -8 by controlling the flow rate through the in-line scrubber. The FilblastTM scrubber is a unit that is capable of subjecting the gas stream and scrubbing medium to elevated pressure and shear and is thus considered a particularly 5 efficient gas cleaning device. When used as an in-line scrubber, the unit functions as a form of hydraulic choke. Typically, the unit would be operated at a set feed pressure required to provide the preferred operating conditions of shear, mass transfer and pressure across the in-line scrubber. For the FilblastTM scrubber, once choke flow characteristics have been achieved, further increasing the pressure will not increase the 10 flow rate through the unit. Within the Filblast tm scrubber, shear is the overriding parameter influencing scrubbing processes, which are mass transfer and diffusion rate controlled. Controlling the flow rate or the pressure differential across the unit in turn controls and provides the shear 15 within the scrubber. The pressure drop across the in-line scrubber is an indication of the effort required to provide the optimum flow for a given hydraulic system. Thus, the operating pressure for the scrubber is the preferred control variable due to the ease of measuring pressure as opposed to measuring flow rate. However, it is to be understood that it is the flow rate that is the prime design parameter applied when using a hydraulic 20 choke of a fixed cross-section. Once the optimum flow rate through the in-line scrubber has been attained, further increasing the operating pressure has no benefit. To achieve a greater degree of scrubbing, the effective number of passes through the in-line scrubber may be increased 25 by either adding a higher capacity unit or installing additional units to operate in parallel while at all times ensuring that the flow rate does not fall below a critical minimum flow rate characteristic of the particular in-line scrubber. Alternatively, the discharge stream from the in-line scrubber may be recirculated for one or more additional passes I through a single in-line scrubber. 30 When scrubbing a gas, the compressibility of the gas must be taken into consideration. Thus, the reduction in gas flow per unit of pressure is not as rapid as in the ease of a WO 03/047724 PCT/AU02/01663 -9 liquid and thus for optimum efficiency, the pressure drop across the unit must be matched to the optimum flow rate depending on the compressibility of the gas and the particular scrubbing medium moving through the in-line scrubber. 5 The FilblastTM scrubber is designed in order to operate at its maximum efficiency when at a level of flow that choking occurs. The FilblastTM scrubber 18 is a multi-phase staged passive scrubber arranged to promote interphasic interaction of a first substance in the form of a liquid phase with a second substance in the form of a non-miscible liquid phase, a solid phase or a gaseous phase. 10 An example of a FilblastTM scrubber 18 is shown in Figures 2a and 2b. As illustrated in Figure 2a, the scrubber 18 includes a plurality of stages C, D, E which define a flow path 19 for two or more substances in the form of different phases. Each stage C, D, E is shaped to define a substantially curved flow path 19 having a centre of curvature 15 located to one side of the flow path 19. The arrangement is such that, in use, as the substances flow through the scrubber 18, the more dense of the phases is subjected to a greater degree of cross-migration than the less dense phase. Each phase migrates first in one direction and then in substantially the opposite direction. The differential cross migration of the two phases generates strong and repetitive interactions between the 20 phases. In this way, none of the phases present in the scrubber 18 are at any time permitted to agglomerate and remain in any portion within the general flow volume, nor are the phases permitted to segregate in any particular part of the flow, which segregation may prevent the repetitive process of interphasic interactions. Also, since each stage C, D, E is shaped to define a smoothly curved flow path, turbulent 25 recirculation eddies in the flow can be substantially eliminated. Any such recirculation eddies and/or turbulent regions would tend to have a deleterious effect on the efficient operation of the scrubber 18 by causing segregation of the different phases. When used as an in-line scrubber, the Fillblast' scrubber 18 promotes interphasic 30 interaction of a gas stream with a scrubbing medium and in this way encourages the target soluble gaseous and/or particulate solid material to become carried by or dissolved in the scrubbing medium.

WO 03/047724 PCT/AU02/01663 - 10 The scrubber 18 is constructed using a rectangular steel frame 21 forming the side walls of a housing for the scrubber 18, and a pair of steel plates 23 forming top and bottom walls of the scrubber 18. The flow chamber within the scrubber 18 is formed by two 5 blocks 25 of a suitable rigid material, for example polyurethane cut to the required shape. In order to further illustrate the present invention, the following example is given for illustrative purposes only, and is not to be taken as limiting the invention in any way. 10 Example 1 Table 1 below shows the results of several tests carried out for an iron ore sinter plant off gas which illustrate the relationship between gas flow through the scrubber 18, pressure drop across the scrubber 18, scrubbing efficiency and power draw for a liquid 15 ring vacuum pump. The table also indicates the number of FilblastTM units used. The asterisk indicates that each scrubber includes a single FilblastTM LSR-G unit. No asterisk indicates that each scrubber includes two LSR-G FilblastTM units in series. Efficiency Power Requirement AP Syst. mgm/m 3 % as Flow FilblastTM MW mmHg Press. Nm 3 /h Number abs Kpa abs. - - 5.95 99.0 1,710 526 44.69 *127 84.4 24.16 90.0 1764 510 37.485 190 76.0 1.75 99.2 1260 714 37.357 157 80.4 8.92 97.1 1368 658 29.780 138 82.9 17.20 90.9 1224 735 29.780 *102 87.7 33.0 87.0 1930 466 34.515 *75 91.3 43.0 88.0 1199 751 33.634 *81 90.3 46.0 79.8 1832 491 34.430 *64 92.8 83.0 59.1 1606 561 24.440 20 The number of scrubbers indicated in each row of the table is calculated on the basis WO 03/047724 PCT/AU02/01663 - 11 that a typical iron ore sinter plant produces approximately 900,000 m 3 per hour of gas for scrubbing. The target maximum amount of material remaining in the exhaust gas after scrubbing 5 was 50 milligram/m 3 . The table indicates that a single LSR-G Filblast T M scrubber will give acceptable cleaning efficiency, while operating under moderate conditions of system pressure and power consumption. Test work conducted with an operating gas flow rate of just under 10 2000 m 3 /hour has reported gas cleaning efficiencies of greater than 99%. An advantage of the unit is that there is a high degree of efficiency. A multi-reactor unit using FilblastTM units is a modular plant design that can be rearranged as required and suffers no scale-up errors. Materials of construction of a particular FilblastTM unit can 15 be adjusted depending on the operating duty and the particular gas to be scrubbed, as well as the particular scrubbing medium employed. Numerous variations and modifications will suggest themselves to persons skilled in the relevant art, in addition to those already described, without departing from the basic 2 0 inventive concepts. All such variations and modifications are to be considered within the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims.

Claims (30)

1. A process for scrubbing soluble gaseous and/or particulate material from a gas stream, the process including the steps of: 5 passing the gas stream and a scrubbing medium through an in-line scrubber under whilst maintaining a controlled pressure differential between an inlet and an outlet of the in-line scrubber that promotes an interphasic interaction between the gas stream and the scrubbing medium resulting in scrubbing of the soluble gaseous and/or capture particulate material from the gas stream. 10

2. A process for scrubbing a gas stream according to claim 1 wherein the controlled pressure differential across the in-line scrubber is at least 8.5 kPa.

3. A process for scrubbing a gas stream according to claim 1 wherein the 15 controlled pressure differential is at least 20 kPa.

4. A process for scrubbing a gas stream according to claim 1 wherein the controlled pressure differential is at least 30 kPa. 20

5. A process for scrubbing soluble gaseous and/or particulate material from a gas stream, including the step of: controlling the flow rate of a gas stream and a scrubbing medium through an in line scrubber such that an interphasic interaction is provided between the gas stream and the scrubbing medium resulting in scrubbing of the soluble gaseous and/or capturing of 25 the particulate material from the gas stream.

6. A process for scrubbing a gas stream according to any one of the preceding claims wherein in-line scrubber is a static scrubber. 30

7. A process for scrubbing a gas stream according to any one of the preceding claims wherein the scrubbing medium is water or an organic solvent. WO 03/047724 PCT/AU02/01663 - 13

8. A process for scrubbing a gas stream according to any one of the preceding claims indicating the step of adding a wetting agent to the scrubbing medium.

9. A process for scrubbing a gas stream according to any one of the preceding 5 claims wherein the gas stream is a gas stream from an iron ore sinter plant.

10. A process for scrubbing a gas stream according to any one of Claims 1 to 8 wherein the gas stream is a gas stream from an alumina processing plant. 10

11. A process for scrubbing a gas stream according to any one of the preceding claims wherein the process includes the step of recirculating the scrubbing medium to the inlet of the in-line scrubber.

12. A process for scrubbing a gas stream according to any one of the preceding 15 claims including the step of recirculating the gas stream and scrubbing medium to the inlet of the in-line scrubber.

13. A process for scrubbing a gas stream according to any one of the preceding claims wherein the in-line scrubber is one of a plurality of in-line scrubbers arranged in 20 parallel.

14. A process for scrubbing a gas stream according to any one of the preceding claims wherein the in-line scrubber is one of a plurality of in-line scrubbers arranged in series. 25

15. A system for scrubbing soluble gaseous and/or particulate material from a gas stream, said system including: an in-line scrubber; means for introducing the gas stream to be scrubbed into the in-line scrubber; 3 0 means for introducing a scrubbing medium into the in-line scrubber; and means for generating a controlled pressure differential across the in-line scrubber, the pressure differential being of a magnitude sufficient to pass the gas stream WO 03/047724 PCT/AU02/01663 - 14 and the scrubbing medium through the in-line scrubber to promote interphasic interaction between the gas stream and the scrubbing medium resulting in scrubbing of the soluble gaseous and/or capturing of the particulate material from the gas stream. 5

16. A system for scrubbing a gas stream according to claim 15 wherein the means for generating a controlled pressure differential is a vacuum pump.

17 A system for scrubbing a gas stream according to claim 16 wherein the vacuum pump is a liquid ring vacuum pump. 10

18. A system for scrubbing a gas stream according to any one of claims 15-17 including a differential pressure to provide feedback to a process control circuit.

19. A system for scrubbing soluble gaseous and/or particulate material from a gas 15 stream, said system including: an in-line scrubber; means for introducing the gas stream into the in-line scrubber; means for introducing a scrubbing medium into the in-line scrubber; and means for controlling the flow rate of the gas stream and the scrubbing medium 20 through the in-line scrubber such that interphasic interaction is provided between the gas stream and the scrubbing medium resulting in scrubbing of the soluble gaseous and/or capturing the particulate material from the gas stream.

20. A system for scrubbing a gas stream according to any one of claims 15 to 19 25 wherein the means for introducing the gas stream is also the means for introducing the scrubbing medium to the in-line scrubber.

21. A system for scrubbing a gas stream according to any one of claims 15 to 20 wherein the system further includes means for adjusting the pH of the scrubbing 30 medium to a desired pH.

22. A system for scrubbing a gas stream according to any one of claims 15 to 21 WO 03/047724 PCT/AU02/01663 - 15 wherein the system includes means for clarifying the scrubbing medium.

23. A system for scrubbing a gas stream according to claim 22 wherein the system includes means for recirculating the clarified scrubbing medium to the in-line scrubber. 5

24. A system for scrubbing a gas stream according to any one of claims 15 to 23, including means for separating solid particulate material from the scrubbed gas stream.

25. A system for scrubbing a gas stream according to claim 24, including at least 10 one knockout pot.

26. A system for scrubbing a gas stream according to claim 24 or 25, including at least one cyclone separator. 15

27. A system for scrubbing a gas stream according to any one of Claims 15 to 26 including means for analysing the gas stream.

28. A system for scrubbing a gas stream according to any one of Claims 15 to 27, including means for analysing an exit gas stream emanating from the in-line scrubber. 20

29. A process for scrubbing a gas stream substantially as herein described with reference to and as illustrated in the accompanying drawings.

30. A system for scrubbing a gas stream substantially as herein described with 25 reference to and as illustrated in the accompanying drawings.