AU2005202850B2 - Conveying particulate material - Google Patents

Description

AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): Birrus International Pty Ltd Invention Title: CONVEYING PARTICULATE MATERIAL The following statement is a full description of this invention, including the best method of performing it known to me/us: - 2 CONVEYING PARTICULATE MATERIAL This invention relates to the conveying of particulate material suspended or entrained in a 5 pressurised gas and conveyed with the gas along a duct. It is particularly concerned with the conveying of particulate material in this manner to multiple discharge outlets. 10 BACKGROUND OF THE INVENTION The invention has particular application to pneumatic conveying of powdery particulate material although there will be applications in which the particulate material to be conveyed is not strictly in the form of a powder and/or 15 in which the conveying gas may be a gas other than air. The invention is applicable in the field of "dense phase" conveying in which compressed air at relatively high pressure of the order of 2 bar or more is applied to a pressure vessel charged with a batch of material to be 20 conveyed and the material is conveyed along a duct extending from the pressure vessel. Conveying rates in such systems vary widely, commonly falling in the range 5 to 50 tonne/hr but can be anything from under 1 tonne/hr to over 1000 tonne/hr. Conveying distances can vary 25 between ten meters to several kilometers. The applicant's earlier International Patent Application PCT/AU99/01138 (Publication No. WO 00/39009) discloses a dense phase conveying system in which particulate material entrained in the gas is conveyed 30 along a duct to a single discharge outlet from the duct, generally into a discharge vessel. In that system, a flow constrictor is positioned at the duct discharge outlet so as to produce a discrete pressure drop at the outlet for the purpose of maintaining a higher and more constant 35 pressure in the conveyor duct upstream from the outlet whereby to inhibit increased velocity of the conveyed - 3 material within the duct so reducing duct abrasion and material attrition. The present invention is particularly concerned with conveying systems in which the conveyed material is 5 delivered to multiple discharge outlets, for which the device of WO 00/39009 is unsuitable without additional control devices. One particular application for such systems is in the delivery of aluminium ore (alumina) to electrolytic reduction pots. In a typical aluminium 10 reduction plant, there may be a very large number of reduction pots (a few hundred to a few thousand). Each of these pots requires delivery of alumina ore into a number of outlets inside each pot. To provide even distribution of ore within the pot superstructure hoppers some kind of 15 valve is generally used. Most successful systems provide self-regulating valves that do not require any external control. These devices are installed in the hot and dusty environment of the pot. Frequent maintenance of these devices is generally 20 necessary to guarantee faultless operation. As there are large numbers of these devices (thousands even in a small smelter) there is a considerable cost associated with their maintenance. Additionally when such a device breaks downs the cost in lost production and repairs can be high. 25 The present invention enables controlled distribution through multiple discharge outlets without the need for such complex discharge devices. SUMMARY OF THE INVENTION 30 According to the invention there is provided a method of conveying and distributing particulate material from multiple discharge outlets comprising: locating a quantity of particulate material to be distributed within a pressure vessel; 35 pressurizing the pressure vessel with pressurized gas; - 4 directing the pressurised gas entrained with particulate material from the pressure vessel through a conveyor duct and to multiple discharge outlets; and constricting the gas entrained with particulate 5 material through a constrictor at each discharge outlet; and discharging the particulate material from each discharge outlet whereby the flow area of each constrictor is mutually selected with respect to the flow areas of the 10 other constrictors to produce a controlled particulate material flow rate at each discharge outlet to create a desired overall distribution of the particulate material from the discharge outlets. The particulate material may be discharged at each 15 discharge outlet into a receiving vessel. There may be a separate receiving vessel for each discharge outlet or a common receiving vessel for two or more of the discharge outlets. Preferably, there is a pressure drop across each 20 discharge outlet of at least 30kPa and more preferably between 100 to 400kPa. The pressurised gas is preferably pressurised air that is pressurised in the pressure vessel to at least 300kPa. The interior of the or each receiving vessel may 25 be at atmospheric, reduced or elevated pressure as required by the process. The constrictions may be set to produce an equal particulate material flow rate across all discharge outlets. In this case, a combination of different 30 constrictor cross-sectional areas may be required. In one particular case, it has been found that the first and last discharge outlets in a series of three or more outlets, where the first outlet is upstream of the other outlets, tend to draw more particulate flow than the intermediate 35 outlet or outlets. In order to produce a uniform discharge through all outlets, a greater constriction (smaller cross-sectional area) may be set at the first and - 5 last discharge outlet than the remaining discharge outlets. This decreases the effective cross sectional area of the outlet and decreases the flow rate. Alternatively, the constrictions may be set to 5 produce a desired variation in flow rates across the discharge outlets. This is preferably achieved by setting the constrictions in a combination of effective sizes that will produce the desired uneven distribution. This may include setting the constrictions at the same cross 10 sectional area. According to the invention there is further provided a distribution apparatus for distributing particulate material comprising: a pressure vessel for receiving particulate material; 15 a supply of pressurised gas for pressurising the vessel; and a conveyor for directing the pressurised gas entrained with particulate material to multiple discharge outlets for discharging the particulate material from the 20 duct; wherein a constrictor at each discharge outlet has a flow area that is mutually selected with respect to the flow areas of the other constrictors to provide a controlled particulate material flow rate at the discharge 25 outlet and create a desired distribution of particulate material from the discharge outlets. The apparatus may further comprise one or more receiving vessels to receive particulate material discharged from the discharge outlets. There may be a 30 separate receiving vessel for each discharge outlet or a common receiving vessel for two or more of the discharge outlets. Preferably, there is a pressure drop across each discharge outlet is at least 50kPa and more preferably 35 between 100 to 200kPa.

-6 The distribution apparatus typically controls distribution when the pressure drop across the discharge outlets exceeds approximately 100kPa. Typically, the combined cross-sectional flow areas of 5 the constrictors are less than 50% of the cross-sectional flow area of the duct. The conveyor is preferably a duct having a diameter of 40-1000mm and typically 50-100mm. The size of the constrictors depends on the 10 application. The combined cross sectional area of the constrictors may be sized to produce a pressure drop of 100kPa or more. The combined constrictor cross sectional area may be set to 15% of the cross sectional area of the conveyor which is preferably a duct or a pipe. The 15 individual constrictor cross sectional areas do not vary more than 30% to achieve uniform distribution. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more fully 20 explained one particular embodiment will be described with reference to the accompanying drawings in which: Figure 1 is a schematic drawing of a pneumatic conveyor system in accordance with the present invention; and 25 Figure 2 is a diagram detailing pressure readings taken at various points along the conveyor system over a time frame. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 30 Figure 1 diagrammatically illustrates a "dense phase" pneumatic conveyor system 10 for conveying particulate material to multiple discharge outlets. A quantity of particulate material is fed into pressure vessel 12. Vessel 12 is then sealed and 35 pressurised by a source of compressed air (not shown) connected to the vessel 12 by air supply line 13.

- 7 When the pressure in the vessel 12 reaches somewhere in the order of 400 to 800kPa, control valve 14 at the bottom of the vessel 12 opens to allow particulate material to flow under pressure into conveyor duct 15. 5 The pressure reached inside the vessel 12 is dependent on the particular application of the system and on the type of particulate material conveyed. Particulate material flows through conveyor duct 15, which may be anywhere in length from 10 metres to several 10 kilometres. From conveyor duct 15 particulate material is discharged into receiving vessels 16 through multiple discharge outlets 20. Conveyor duct 15 branches off into a series of discharge ducts 21 that direct the gas entrained with particulate material to the discharge outlets 20. 15 Although four 200 litre receiving vessels 16 are shown in Figure 1, the particulate material need not be received by separate vessels. In many cases multiple discharge outlets are located over a single, wide vessel. Locating the discharge outlets above the vessels 20 allows particulate material to discharge downwardly into the receiving vessels. Constrictors, schematically illustrated as 22 in Figure 1, fitted at the discharge outlets are adapted to significantly reduce the cross sectional area of the 25 outlets 20, that is the flow area, to constrict the passage of gas and particulate material and create a pressure drop of at least 30kPa across each discharge outlet. Under normal operating conditions the pressure drop introduced by the constrictors 22 is approximately 30 100 - 400kPa. However, the pressure drop may reach as high as 600kPa. Each constrictor is in the form of an annular orifice plate having a central circular aperture. However, the aperture need not be circular but may be of other shapes 35 provided the desired distribution is achieved. The constrictor plate is clamped between two flanges 40 at the - 8 end of each discharge duct. Flanges 40 are illustrated schematically in Figure 1. According to the present invention the cross sectional area of each constrictor can be set to provide a 5 controlled material flow rate at each discharge outlet to create a desired overall distribution of particulate material from the discharge outlets. The pressure in conveyor duct 15 drops as the pressurised gas entrained with particulate material 10 progresses down the conveyor duct. The pressure drop at the discharge outlets collectively influence the distribution of particulate material flowing from the discharge outlets. A desired distribution flow can be achieved by selecting the combination of constrictor cross 15 sectional areas that will produce the desired particulate flow distribution. For example, it may be desired that the particulate material should discharge at an even flow rate from every discharge outlet. However, if the same constriction was 20 applied to each discharge outlet an unequal flow distribution would result. This is because the first and last discharge outlets 31,34, that is the outlets closest and furthest from the pressure vessel, would draw a larger flow than the intermediate outlets 32,33. 25 It has also been found that in a four-group discharge outlet system, such as that shown in Figure 1, the last discharge outlet 34, that is the one furthest downstream, also draws more than its equal amount of flow than the second and third discharge outlets 32, 33 situated in 30 between the first 31 and last 34. Accordingly, to produce an equal discharge rate across all discharge outlets it is necessary to vary the cross sectional area of the constrictors 22. Taking the system of Figure 1 as an example, the 35 production of uniform discharge rate involves the selection of different constrictor cross sectional areas to be fitted into individual outlets. In the example - 9 shown the constrictors at outlets 31 and 34 have a 30% smaller diameter than the constrictors at outlets.32 and 33 in order to produce uniform distribution across all outlets. Because there are many variables that affect 5 flow rate and pressure drop in a system, testing will be required for each installation to determine appropriate constrictor diameters and cross sectional areas to provide desired particulate distribution. In practice the variation in constrictor diameters can vary widely, 10 depending on the number of discharge outlets (there will be a larger variation if there are only a small number of outlets), and on the desired throughput of particulate material through each outlet. In the example shown the combined constrictor cross 15 sectional area of the system of Figure 1 was set to 15% of the conveyor duct's cross sectional area and individual constrictor diameters did not vary more than 30% to achieve uniform distribution. In terms of the cross sectional flow area of the conveyor duct of this example, 20 the combined cross-sectional flow area of the constrictors is generally less than 50% of that of the conveyor duct. Figure 2 illustrates pressure readings taken at various points along the conveyor system 10 having the constrictor cross sections of the above example which set 25 the pressure drops across the discharge outlets to produce an equal distribution flow rate. The pressure readings were taken over several minutes. The reading at line A represents the conveying air supply pressure and was taken at point A in Figure 1 on 30 the compressed air supply line 13. Line B represents the pressure in the pressure vessel taken at point B on Figure 1. Line C represents the discharge pressure of the gas entrained with particulate material as it exits the 35 pressure vessel and was taken at point C. Line D represents the end pressure of the conveyed particulate flow before it discharges from the discharge - 10 outlets. This reading was taken at point D on the conveyor duct 15. Line E represents the middle pressure of the conveyed particulate flow after a portion has discharged from first 5 and second discharge outlets 31, 32. This reading was taken at point E on conveyor duct 15. Figure 2 shows that the compressed air supplied to the system (supply pressure A) is at approximately 1700 1900kPa. 10 Before distribution, the pressure in the vessel 12 reaches approximately 500-600kPa. During distribution this pressure slowly declines, as shown by line B, to approximately 300-400kPa. Line C indicates that distribution of particulate 15 material occurs over about 3 minutes. At its peak, the discharge pressure reaches approximately 400kPa. Lines D and E are almost superimposed on one another as the pressure readings and variations experienced at points D & E are almost identical and simultaneous. This 20 is due to the constrictors effectively isolating conditions inside the conveyor duct from any external influences. A relatively large pressure drop across each constrictor (preferably more than 30kPa and typically more than 100kPa) is required to provide effective discharge 25 control. In the example shown the pressure drop introduced by the constrictors exceeds the conveying pressure drop. This may not necessarily be the case. The conveying pressure in other systems will depend on conveying 30 distance, pipeline material and other influences and could be conceivably much larger than the drop caused by the constrictors. The discharge rate during the experiment was controllable within ±4.5% from the average desired distribution. 35 It can be seen from Figure 2 that the pressure drop in the conveyor duct between points C and D is about 200kPa. The pressure drop introduced by the constrictors - 11 is at normal operation a little more than 200kPa and around 300kPa. This is the pressure difference between the discharge outlets and the receiving vessel, which is in the present case at atmospheric pressure. However the 5 receiving vessel could be conceivably at any pressure as long as the pressure upstream of the constrictors can be maintained sufficiently above the vessel pressure to provide effective discharge control. The pressure difference in the conveyor duct between 10 point D and E is small. The above example illustrates a conveyor system where the constrictions at the discharge outlets are set to produce an uniform particulate distribution from the discharge outlets. In that specific example the 15 constrictors at the first and last (fourth) outlets were set at a 14mm diameter and the constrictors at the second and third outlets were set at 20mm. The constrictors could alternatively be set to produce any desired combination of uneven distribution 20 that could be called for in a particular application. This could include the combination where all the constrictors have the same aperture size. Constriction cross section at the discharge outlets can be set by manually changing the constrictor plate with 25 another constrictor plate having an aperture with a different cross sectional area. The absence of any valves, other moving components or controls makes this option the most economic and easiest to maintain. For the convenience of being able to remotely change 30 constrictor cross sectional area the plates may alternatively be equipped with means to automatically change the area of the aperture. For example, the constrictor plate may include a moveable plate that slides to cover and uncover the central aperture thereby changing 35 the constrictor's effective cross-sectional area. Movement of the plate can be controlled from a central controller.

- 12 While the present system has particular application with feeding powdery alumina ore into smelting pots in an aluminium smelter, it is understood that the concept is not limited to this application. Additionally, the system 5 is intended to cover the distribution of all kinds of fine particulate material including flakes, granules or fibres. The present conveyor system is used effectively to distribute particulate material in a controlled manner, whether by way of uniform or non-uniform discharge, to 10 feed process or storage vessels. Elimination of complex mechanical devices from the process environment, which in some applications such as in aluminium smelting can be quite harsh, reduces maintenance costs of the devices themselves as well as maintenance of the pneumatic 15 conveying system. System reliability is increased and downtime decreased. Furthermore, the system can be retro fitted onto existing pneumatic conveying and feeding systems. The conveyor system allows predictable and 20 simultaneous distribution of pneumatically conveyed particulate material from multiple discharge outlets. In the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or 25 variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 30 It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

Claims (29)

1. A method of conveying and distributing particulate material from multiple discharge outlets including: 5 locating a quantity of particulate material to be distributed within a pressure vessel; pressurizing the pressure vessel with pressurized gas; directing the pressurised gas entrained with 10 particulate material from the pressure vessel through a conveyor duct and to multiple discharge outlets; constricting the gas entrained with particulate material through a constrictor at each discharge outlet; and 15 discharging the particulate material from each discharge outlet whereby the flow area of each constrictor is mutually selected with respect to the flow areas of the other constrictors to produce a controlled particulate material flow rate at each discharge outlet to create a 20 desired overall distribution of the particulate material from the discharge outlets.

2. The method claimed in claim 1, including varying the flow area of the constrictor at each outlet to change the 25 overall distribution of particulate material.

3. The method claimed in claim 2, including varying the flow area of each constrictor by replacing the constrictor with a constrictor having a different flow area. 30

4. The method claimed in claim 2, whereby the flow area is varied automatically by way of a mechanism provided in each constrictor that moves to enlarge or reduce the flow area, wherein the mechanism is controlled by a central 35 controller. - 14 5. The method claimed in claim 1, including selecting the flow area of the constrictors to be equal in order to create an overall uneven distribution of particulate material from the discharge outlet.

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6. The method claimed in claim 1, including selecting the flow area of the constrictors to be different to create an overall uneven distribution of particulate material from discharge outlet. 10

7. The method claimed in claim 1, including selecting the flow area of the constrictors to be different to create an overall even distribution of particulate material from the discharge outlet. 15

8. The method claimed in claim 7, including selecting the flow area of the constrictors at the first and last discharge outlets to be smaller than the flow areas of the constrictors at the other outlets, wherein the first 20 outlet is upstream of the other outlets.

9. The method claimed in any one of the preceding claims, including creating a pressure drop across each discharge outlet of more than 30kPa. 25

10. The method claimed in claim 9, wherein the pressure drop is between 100kPa to 400kPa.

11. The method claimed in any one of the preceding 30 claims, wherein the combined cross-sectional area across all constrictors is approximately less than 50% of the cross-sectional area of the conveyor duct.

12. The method claimed in any one of the preceding 35 claims, including discharging particulate material into one or more receiving vessels. - 15

13. The method claimed in claim 12, whereby the receiving vessel is at atmospheric pressure, reduced pressure or at elevated pressure. 5

14. The method according to any one of the preceding claims whereby the pressurized gas is pressurized air that is pressurized to at least 300kPa.

15. A distribution apparatus for distributing particulate 10 material comprising: a pressure vessel for receiving particulate material; a supply of pressurised gas for pressurising the vessel; and a conveyor for directing the pressurised gas 15 entrained with particulate material to multiple discharge outlets for discharging the particulate material from the duct; wherein a constrictor at each discharge outlet has a flow area that is mutually selected with respect to the 20 flow areas of the other constrictors to provide a controlled particulate material flow rate at the discharge outlet and create a desired distribution of particulate material from the discharge outlets. 25

16. The distribution apparatus claimed in claim 15, wherein the flow area at each discharge outlet is variable.

17. The distribution apparatus claimed in claim 16, 30 wherein the flow area is variable by replacing the constrictor with a constrictor having a different flow area.

18. The distribution apparatus claimed in claim 16, 35 wherein the flow area is automatically variable by way of a mechanism that moves to enlarge or reduce the flow area of the constrictor. - 16

19. The distribution apparatus claimed in claim 15, wherein the flow area of the constrictors at all of the discharge outlets are equal. 5

20. The distribution apparatus claimed in claim 15, wherein the flow areas of the constrictors between at least two discharge outlets are different. 10

21. The distribution apparatus claimed in claim 20, wherein the flow areas of the constrictors at the first and last discharge outlets are smaller than the flow area of the constrictors of the other discharge outlets, wherein the first discharge outlet is upstream of the 15 other outlets.

22. The distribution apparatus claimed in any one of claims 15 to 21, wherein the pressure drop across each discharge outlet is more than 30kPa. 20

23. The distribution apparatus claimed in claim 22, wherein the pressure drop across each discharge outlet is between 100 and 400kPa. 25

24. The distribution apparatus claimed in any one of claims 15 to 23, wherein the combined flow areas of all of the constrictors are approximately less than 50% of the cross sectional flow area of the conveyor duct. 30

25. The distribution apparatus claimed in any one of claims 15 to 24, wherein the apparatus includes one or more receiving vessels to receive particulate material from the discharge outlet. 35

26. The distribution apparatus claimed in claim 25, wherein the pressure vessel(s) is kept at atmospheric pressure, reduced pressure or elevated pressure. - 17

27. The distribution apparatus claimed in any one of claims 15 to 26 wherein the pressurized gas is pressurized air. 5

28. A method of conveying and distributing particular material from multiple discharge outlets substantially as herein described with reference to the accompanying drawings. 10

29. A distribution apparatus for distributing particular material substantially as herein described with reference to the accompanying drawings. 15 Dated this 29th day of June 2005 BIRRUS INTERNATIONAL PTY LTD By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and 20 Trade Mark Attorneys of Australia