WO1997027135A1

WO1997027135A1 - Dense phase transport

Info

Publication number: WO1997027135A1
Application number: PCT/AU1997/000037
Authority: WO
Inventors: Leon Fabrikanov; Jeremy Robin Locatelli
Original assignee: Comalco Aluminium Limited
Priority date: 1996-01-22
Filing date: 1997-01-22
Publication date: 1997-07-31
Also published as: AUPN766596A0

Abstract

A method for dense phase pneumatic conveying of particulate material comprises pressurising a feed vessel (10) and commencing flow of gas to a transport line (15). Valve (14) in the solids discharge (13) of feed vessel (10) is opened and the solids flow through solids discharge (13), through feed line (15) and into receiving vessel (16). Receving vessel (16) has all outlet ports therefrom closed during conveying such that the pneumatic conveying system comprises a closed end system. The step of conveying the solids occurs in the transient phase, rather than in a continuous phase common to prior art systems. In a preferred method, the solids are received by a primary receiving vessel and overflow gas and solids from the primary receiving vessel flow into a closed and secondary receiving vessel.

Description

TITLE: DENSE PHASE TRANSPORT

The present invention relates to a method and apparatus for the pneumatic conveying of solid particulate material. The invention is especially suitable for the pneumatic conveying of aluminium powder and aluminium flake. Pneumatic conveying is widely used to transport solid particulate materials, such as powder, pulverized coal, flake material and pellets. In pneumatic conveying, the solid particulate material is moved along a pipe by a flow of gas. Pneumatic conveying techniques include dilute phase pneumatic conveying, in which a relatively large amount of gas is used to transport a relatively small amount of solids, and dense phase pneumatic conveying, in which a relatively small amount of gas is used to transport a relatively large amount of solids.

Pneumatic conveying systems may operate by supplying conveying gas at a positive pressure to the conveying line, or by applying a vacuum to a receiving vessel. In either case, a pressure differential is established to maintain gas flow in the system. In positive pressure systems, the conveying gas is vented from the receiving vessel through a filter or cyclone to minimize carry-over of entrained solids in the vented gas. In vacuum systems, the vacuum equipment continuously removes gas from the system and again the removed gas must be cleaned (usually by passing through a filter or a cyclone) in order to remove entrained solids. Aluminium powder and aluminium flake are used in several industries, for example, as pigments in paints and as blowing agents in lightweight aerated concrete. Aluminium powder and flake causes difficult solids handling problems because they present an explosion hazard. Due to the risk of explosion, aluminium powder and aluminium flake have not been thought of as suitable candidates for transport by dense phase pneumatic conveying due to the unacceptable risk of explosion with these materials inherent in known dense phase pneumatic conveying systems. Therefore, previous systems for pneumatic conveying of aluminium powder have used dilute phase conveying, which results in the powder/air mixture having a powder content below the lower explosion limit. The present invention provides a method and an apparatus for pneumatic conveying that may be suitable for the pneumatic conveying of aluminium powder and aluminium flake. Other forms of hazardous or explosive dusts or powders or particulates may also be handled by the present invention.

In a first aspect, the present invention provides a method for pneumatic conveying of solid particulate material comprising pressurizing a feed vessel containing the solid particulate material, commencing a flow of conveying gas to a transport line, opening a discharge from the feed vessel to thereby cause the solid particulate material to be conveyed from the feed vessel via the transport line to a receiving vessel, wherein discharge of gas and solid particulate material from the receiving vessel during conveying is prevented. The steps of commencing the flow of conveying gas to the transport line and opening the discharge from the feed vessel may occur simultaneously.

In another aspect, the present invention provides a method for pneumatic conveying of solid particulate material characterized in that a closed-end system is used during conveying to thereby prevent discharge of gas and solid particulate material from the system during conveying.

Preferably, discharge of gas and solid particulate material from the receiving vessel during conveying is prevented by closing all outlets from the receiving vessel during conveying. The receiving vessel will typically have a gas outlet and a solids outlet and the method therefore preferably includes the step of closing the gas outlet and the solids outlet of the receiving vessel prior to opening the discharge of the feed vessel.

The method of the invention preferably further comprises allowing the solid particulate material to settle in the receiving vessel after conveying and subsequently bringing the receiving vessel to substantially atmospheric pressure by controllably opening the gas outlet of the receiving vessel. The method may be operated such that at the end of the conveying cycle, the receiving vessel is at above-atmospheric pressure. In this case, opening the gas outlet of the receiving vessel will result in gas exiting the receiving vessel. The release of gas should be controlled such that the velocity of gas within the vessel does not exceed the re- entrainment velocity of the solid particulate material. This will ensure that an undesirable dust cloud will not form in the receiving vessel when venting and will also ensure that solid particulate material is not vented from the receiving vessel. Alternatively, the method may be operated such that the receiving vessel is at sub- atmospheric pressure following completion of the conveying cycle. In this case, opening the gas outlet of the receiving vessel results in gas flowing into the receiving vessel. The rate at which gas is allowed to flow into the receiving vessel is preferably controlled to avoid settling or compaction of the particulate material in the receiving vessel.

The receiving vessel may be held at substantially atmospheric pressure prior to commencement of conveying or it may have a vacuum applied thereto prior to commencement of conveying. In the latter case, the vacuum is preferably applied by means of a vacuum eductor, especially a water eductor.

In order to assist in commencing the flow of solid particulate material from the feed vessel to the transport line, it is possible to fluidise the lower part of the feed vessel. This minimizes the likelihood of the solid particulate material binding or otherwise blocking the discharge from the feed vessel.

In order to pressurize the feed tank, it is necessary to close all outlets in the feed tank through which the pressurizing gas could escape. The feed tank is typically supplied with solids from a bulk store and this solids inlet should be closed. Further, the solids outlet from the feed tank should also be closed during the pressurization step. The step of closing the solids inlet and the solids outlet of the feed tank suitably comprises closing a solids inlet valve and closing a solids outlet valve.

The step of opening a discharge from the feed vessel preferably comprises the step of opening the solids outlet valve. When the solids outlet valve in the feed tank is opened, the pressurized gas in the feed tank causes the solid particulate material in the feed tank to be rapidly forced into the transport lines and along the transport line to the receiving vessel. The conveying gas flowing along the transport line assists in conveying the solid particulate material along the transport line to the receiving tank. During the conveying cycle, it is preferred that the supply of pressurized gas to the feed tank is maintained. In the method of the present invention, the feed vessel is pressurized to a pre- determined pressure. Solids flow is then initiated by opening a discharge from the feed vessel. This causes a rapid flow of material (both gas and solid particulate material) out of the feed vessel and to the receiving vessel. As the receiving vessel is a closed end system during conveying, the pressure within the receiving vessel rapidly increases. At the same time, the pressure within the feed vessel rapidly decreases to a minimum value and then begins to again increase (it being recalled that in the preferred embodiment of the invention the pressurizing gas is continuously being supplied to the feed vessel). Once all the solid particulate material has been removed from the feed vessel, the pressurizing gas is shut-off, the conveying gas is stopped and the inlet to the receiving vessel is closed off. After allowing sufficient time for settling of the solid particulate material in the receiving vessel, which usually takes from 5 to 10 minutes, the receiving vessel is returned to substantially atmospheric pressure by controlled opening of one or more valves in the outlet gas line.

Some solids materials are very difficult to pneumatically convey because they are difficult to fluidise and/or have a tendency to rapidly settle in the transport line during conveying. With such materials, it is preferred that the method of the present invention further includes a by-pass mode in which auxiliary transport gas is injected at one or more locations into the transport line. The auxiliary gas may be injected into the transport line through filters or glass frits.

The method of the present invention differs from known gas conveying methods in that: i) the method is operated with a closed-end receiving vessel. All prior art systems known to the applicant operate with an open-end receiving vessel; and ii) the conveying of solids in the present method involves solid transfer in a transient flow regime. In contrast, known prior art methods use gas flow at a steady state. The method of the present invention transfers solid particulate material in a dust free manner and does not require the use of filters to purge the conveying gas. Therefore, the method is especially suitable for transporting aluminium powder and aluminium flake.

As an extension of the method of the invention, the closed end receiving vessel may comprise a secondary receiving vessel for receiving conveying gas from a primary receiving vessel. The primary receiving vessel may be operated to receive a substantial proportion of the solid particulate material being conveyed, with carry-over conveying gas and any entrained solid particulate material being conveyed to the secondary receiving vessel. As the proportion of solid particulate material carried over to the secondary receiving vessel is small, a number of conveying cycles may be carried out before it becomes necessary to remove the solid particulate material from the secondary receiving vessel. Preferably, the secondary receiving vessel is the end vessel for a number of primary receiving vessels, which allows the possibility of conveying solid particulate material to one primary receiving vessel by the conveying cycle whilst at the same time either taking another primary receiving vessel off-line or removing the solid particulate material from another primary receiving vessel.

Accordingly, in a second aspect, the present invention provides a process for pneumatic conveying of solid particulate material including the steps of: (i) pressurizing a feed vessel containing the solid particulate material: (ii) commencing flow of a conveying gas to a transport line;

(iii) opening a discharge from the feed vessel to thereby convey the solid particulate material through the transport line to a primary receiving vessel; (iv) separating a substantial proportion of the solid particulate material from the gas in the primary receiving vessel; (v) passing the separated gas and any entrained solid particulate material from the primary secondary vessel to a secondary receiving vessel. said secondary receiving vessel being a closed-end system during conveying to prevent discharge of gas and solid particulate material therefrom during conveying. Steps (ii) and (iii) above may occur simultaneously.

The method of the second aspect of the invention may also include the further steps of allowing the solid particulate material in the secondary receiving vessel to settle and subsequently returning the secondary receiving vessel to substantially atmospheric pressure. In this case, the secondary receiving vessel is advantageously arranged such that substantially all of the solid particulate material in the secondary receiving vessel returns to the primary receiving vessel when it settles.

As mentioned earlier, the secondary receiving vessel may be connected by appropriate piping and valving to a plurality of primary receiving vessels.

The method of the second aspect of the present invention may also be operated in a by-pass mode in which auxiliary transport gas is injected at one or more locations into the transport line.

It is preferred that the receiving vessel has a volume of approximately three (3) to six (6), more preferably four (4) to five (5) times the volume of the feed vessel. This allows a sufficient pressure differential to be maintained between the feed vessel and the receiving vessel to maintain good conditions for conveying. In cases where a primary receiving vessel and a secondary receiving vessel are used, the volume of the receiving vessel is the total volume of the primary and secondary receiving vessels (and any piping or conduits intermediate therebetween).

Another preferred feature of the method of the present invention involves passing a purge gas steam through the transport line to remove any settled solid particulate material from the line. The purge stream should precede any period allowed for settling in the receiving vessel.

The method of the present invention is especially suitable for dense phase pneumatic conveying. The present invention also relates to apparatus for pneumatic conveying.

According to a third aspect, the present invention provides apparatus for pneumatic conveying of solid particulate material comprising a feed vessel for containing the solid particulate material to be conveyed, a receiving vessel for receiving conveyed solid particulate material, a transport line for connecting the feed vessel and the receiving vessel, gas supply means for supplying pressurized gas to the feed vessel and the transport line, a feed vessel discharge valve for opening when the feed vessel has been pressurized to a predetermined pressure to thereby allow the solid particulate material to pass therethrough and a receiving vessel valve system for dosing all outlets of the receiving vessel during conveying to prevent discharge of gas and solid particulate material from the receiving vessel during conveying. It will be appreciated that the term "outlets", when used in the context of the receiving vessel, includes all openings in the receiving vessel except the opening connected to the transport line.

Preferably, the receiving vessel valve system includes a gas flow valve means for selectively sealing a gas vent in the receiving vessel and for controllably allowing gas flow into or out of the receiving vessel to bring the receiving vessel to substantially atmospheric pressure. The receiving vessel valve means may also include a solids discharge valve for permitting discharge of solids therethrough when open, the solids discharge valve sealing a solids discharge in the receiving vessel during conveying. The gas supply means is used to (a) pressurize the feed vessel at the start of the conveying cycle and (b) to provide conveying gas to the transport line. The gas supply means may comprise a source of pressurized gas having a gas line to the feed vessel and another gas line to the transport line. Alternatively, the gas supply means may comprise a first source of pressurized gas connected via a gas line to the feed vessel and a second source of pressurized gas connected to the transport line. In either case, the gas lines connecting the source(s) of pressurized gas to the feed vessel and the transport line should include gas valve means to selectively shut-off supply of pressurized gas and provide supply of pressurized gas to the feed vessel and transport line. The feed vessel discharge valve performs several functions, including:

(a) sealing the outlet or discharge from the feed vessel in order to enable the feed vessel to be pressurized at the start of the conveying cycle

(b) to be opened when the feed vessel has been pressurized to a predetermined pressure to allow the solid particulate material and gas to flow therethrough during conveying; and

(c) at the end of the conveying cycle, to close and isolate the feed vessel from the transport line whilst the feed vessel is again filled with solid particulate material.

The receiving vessel can be completely sealed to provide a closed end during the conveying cycle. Furthermore, the receiving vessel can be vented in such a way that formation of an undesirable dust cloud can be avoided. Accordingly, the apparatus of the present invention provides a pneumatic conveying apparatus which does not require a filter or ether gas cleaning means for the gas discharged from the apparatus. This makes the apparatus especially suitable for the pneumatic conveying of aluminium powder and aluminium flake. However, a filter may be provided in the vent line from the receiving vessel to act as a safeguard in case of failure of the vent valve.

In a fourth embodiment, the present invention provides apparatus for pneumatic conveying of solid particulate material including a feed vessel for containing the solid particulate material to be conveyed, a primary receiving vessel for receiving a substantial portion of the solid particulate material being conveyed, a transport lien connecting the feed vessel and the primary receiving vessel, gas supply means for supplying pressurized gas to the feed vessel and the transport line, a feed vessel discharge valve for opening when the feed vessel has been pressurized to a predetermined pressure to thereby allow solid particulate material to pass therethrough, a primary receiving vessel for receiving a substantial portion of the solid particulate material being conveyed, the primary receiving vessel including a gas discharge means for discharging gas and any entrained solid particulate material from the primary receiving vessel, the gas discharge means connected to a secondary receiving vessel whereby gas discharged from the primary receiving vessel passes to the secondary receiving vessel, the secondary receiving vessel having a secondary receiving vessel valve system for closing all outlets of the secondary receiving vessel during conveying to prevent discharge of gas and solid particulate material from the secondary receiving vessel during conveying.

It will be appreciated that the term "outlets", when used in the context of the secondary receiving vessel, includes all openings in the secondary receiving vessel except the opening connected to the primary receiving vessel from which gas and any entrained solids are being received.

Preferably, the secondary receiving vessel is arranged such that settling of any solid particulate material in the secondary receiving vessel returns the solid particulate material to the primary receiving vessel. The primary receiving vessel may be a cyclonic receiving vessel in which the incoming stream of gas and solids enters tangentially. The solids are thereby substantially separated from the gas in the primary receiving vessel.

The secondary receiving vessel valve system is able to close all outlets from the secondary receiving vessel and this enables the secondary receiving vessel to operate as a closed-end for the conveying apparatus. The secondary receiving vessel will include a gas outlet and the secondary receiving vessel valve system includes valve means on the gas outlet. The secondary receiving vessel may also include a solids discharge and this discharge should also be equipped with valve means. In an especially preferred embodiment of the fourth aspect of the invention the apparatus includes a plurality of primary receiving vessels connected to the feed tank by respective transport lines, each of the primary receiving vessels being connected to the secondary receiving vessel. In this embodiment, the secondary receiving vessel is a common end for each of the primary receiving vessel. In a variation of this embodiment, more than one feed vessel is used to feed solids to the primary receiving vessels.

It has been found that good results are achieved when the ratio of the volume of the feed vessel to the volume of the receiving vessel is in the range of 1 :3 to 1 :6, more preferably 1 :4 to 1 :5. If a primary receiving vessel and a secondary receiving vessel are used, the volume of the receiving vessel is calculated as the sum of the volumes of the primary and secondary receiving vessels and any intermediate pipework.

Any valve means known to be suitable in solids handling may be used in the present invention. The valves may be manually controlled or automatically controlled. Preferably, the valves are automatically controlled, with the sequence of opening and closing of the various valves being controlled by a computer controller.

The apparatus of the present invention may also include one or more auxiliary gas inlets in the transport line for injecting auxiliary transport gas inlets in the transport line for injecting auxiliary transport gas into the transport line. This feature is especially suitable if the solids material is difficult to fluidise and/or has a tendency to settle.

The source of pressurized gas may comprise one or more tanks of compressed gas or one or more compressors. If the solid particulate material is aluminium powder or aluminium flake, the gas supplied preferably contains 95-97% nitrogen and 3-5% oxygen.

Preferred embodiments of the present invention will now be described with reference to the accompanying Figures. It will be appreciated that the following description is illustrative of the invention and is not intended to limit the generality of the invention. In the Figures: Figure 1 shows a schematic layout of an apparatus for pneumatic conveying in accordance with the present invention;

Figure 2 shows a schematic layout of another embodiment in accordance with the present invention;

Figure 3 shows a variation of the layout shown in Figure 2 which includes a plurality of primary receiving vessels;

Figure 4 shows a variation of the layout of two primary receiving vessels and a secondary receiving vessel;

Figure 5 is a plot of air pressure vs cycle time for a conveying cycle conducted in a test rig; Figure 6 is a plot of mass of solids in the feed vessel and the receiving vessel vs cycle time for a conveying cycle conducted in a test rig;

Figure 7 is a plot of air mass flowrate in the pressurized gas supply line vs cycle time for a conveying cycle conducted in a test rig; and

Figure 8 is a plot of pressure vs time for a typical prior art steady state dense phase pneumatic conveying apparatus.

In Figure 1 , the pneumatic conveying apparatus includes a feed vessel 10 having an inlet 1 1 through which solid particulate material is feed to feed tank 10. Inlet ( 1 1 ) is controlled by inlet valve 12. Feed vessel 10 has a solids discharge 13. Valve 14 is used to open and close solids discharge 13. Transport line 15 connects feed vessel 10 with receiving vessel 16. Receiving vessel 16 has a volume that is approximately 5 times larger than feed vessel 10. Feed vessel 16 includes a gas vent 17 having a flow control valve 18 therein. A solids discharge 19 having a flow control valve 20 is positioned in the bottom of receiving vessel 16. The receiving vessel shown in Figure 1 is also equipped with a vacuum eductor 21 , or any other devise for creating a vacuum, which can be used to apply an initial vacuum to the receiving vessel at the commencement of the conveying cycle.

The gas supply means in Figure 1 includes a tank of pressurized gas 22 connected via line 23 and 24 to feed vessel 10 and via lines 23 and 25 to the transport line 15. Valves 26 and 27 control the gas flows. For conveying of aluminium powder, gas tank 22 is preferably filled with a pressurized mixture containing 97% nitrogen and 3% oxygen.

A typical conveying cycle using the apparatus shown in Figure 1 includes a step of initially filling feed vessel 10 with solid particulate material through inlet 1 1. It will be appreciated that discharge valve 14 is closed at this time. Once the feed vessel 10 has been filled to a desired level, inlet valve 12 is closed, which seals all of the possible routes for the escape of gas from the feed vessel 10.

In order to commence conveying, valve 26 is opened which results in pressurized gas flowing through lines 23 and 24 and into the feed vessel 10. The gas entering feed vessel 10 through line 24 acts to fluidise the solid particulate material in the lower part of the feed vessel 10. Moreover, the gas entering feed vessel 10 through line 24 also pressurizes feed vessel 10. When the pressure inside feed vessel 10 has reached a predetermined level (as determined by a pressure sensor or a pressure transducer - not shown), valve 27 is opened to commence a flow of conveying gas through transport line 15. Almost simultaneously, discharge valve 14 from feed vessel 10 is opened and the combination of the pressure inside feed vessel 10 and the conveying gas travelling through transport line 15 causes the solid particulate material in feed vessel 10 to be rapidly pneumatically conveyed to receiving vessel 16.

During the conveying cycle, receiving vessel 16 acts as a closed end and this is achieved by closing valves 18 and 20 in receiving vessel 16. This closes all possible outlets for the gas and solids from receiving vessel 16. Prior to commencing conveying, a vacuum may optionally be applied to feed vessel 16 by use of vacuum eductor 21.

Once all of the solid particulate material from feed vessel 10 has been conveyed to receiving vessel 16, a purge of gas is passed through transport line 15 to convey any settled solid particulate material to the receiving tank 16. Following that, valve 26 and valve 14 are closed. A period of between 5 to 15 minutes is then allowed to pass to permit settling of the solid particulate material in receiving vessel 16. Once the solid particulate material has settled, valve 18 is carefully opened to allow the receiving vessel 16 to be returned to atmospheric pressure. If feed vessel 16 is at above atmospheric pressure after the conveying cycle, opening gas flow valve 18 will result in gas being vented out through gas vent 17. In this case, the flow rate from vessel 16 should be controlled such that the gas velocity in the vessel during venting does not exceed the re-entrainment velocity of the gas. This will ensure that a dust cloud is not formed during venting.

Alternatively, receiving vessel 16 may be at sub-atmospheric pressure following the conveying cycle, ln this case, opening gas flow valve 18 will result in gas flowing into receiving vessel 16 via gas vent 17. As gas flows into the vessel, the danger of releasing a dust cloud is negligible. However, the flow rate of gas into the vessel should still be controlled in order to avoid any problems that may be associated with compaction of the solid particulate material if the gas flow into vessel 16 is too high.

Referring now to Figure 2, this shows an arrangement that includes a feed vessel 30, a primary receiving vessel 32 and a secondary receiving vessel 34. Feed vessel 30 is essentially identical to feed vessel 10 of Figure 1 and it has the same arrangement of valving and piping as feed vessel 10 of Figure 1. Similarly, secondary receiving vessel 34 is of an essentially identical layout to secondary receiving vessel 16 shown in Figure 1 and it has the same arrangement of valving and piping as receiving vessel 16 of Figure 1. In particular, secondary receiving vessel 34 includes solids discharge valve 35, gas vent 36 having a gas flow valve 37 and a vacuum eductor 38.

The layout shown in Figure 2 does not show the gas supply means but it will be appreciated that the gas supply means of Figure 2 is essentially identical to that shown in Figure 1.

In Figure 2, feed vessel 30 is connected to primary receiving vessel 32 by transport line 39. Primary receiving vessel 32 includes a discharge valve 40 for discharging solids therefrom. Primary receiving vessel 32 also includes a top outlet 41 through which the conveying gas and any entrained solid particulate material can exit and travel via transport line 42 to receiving vessel 34. Transport line 42 is provided with valve 43 to enable line 42 to be opened and closed to gas flow. Primary receiving vessel 32 acts in a fashion similar to a cyclone in that the incoming stream through transport line 39 enters tangentially. This causes a substantial portion of the solids conveyed through transport line 39 to be separated from the gas in primary receiving vessel 32. However, there will inevitably be some carry over of particulate material in the gas stream leaving primary receiving vessel 32 via outlet 41.

The conveying cycle of the apparatus shown in Figure 2 includes the steps of initially closing valves 35 and 37 in secondary receiving vessel 34. Secondary receiving vessel 34 will then act as a closed end to the apparatus. Feed vessel 30 is pressurized to a predetermined level, after which conveying gas starts to flow through transport line 39 and the discharge valve in feed vessel 30 is opened. This causes rapid conveying of the solid particulate material in feed vessel 30 through transport line 39 into primary receiving vessel 32. The gas and any entrained solids leaves the vessel 32 via outlet 41 and travel through transport line 42 secondary receiving vessel 34.

At the end of the conveying cycle, the gas supply is isolated from the apparatus by closing the appropriate valves and the solids in secondary receiving vessel 34 are allowed to settle. After settling, secondary receiving vessel 34 is returned to substantially atmospheric pressure by opening gas flow valve 37 in gas vent 36.

Figure 3 shows a variation of the apparatus of Figure 2. In Figure 3, the feed vessel 30 and secondary receiving vessel 34 are identical to the feed vessel and secondary receiving vessel shown in Figure 2. Accordingly, they have been given the same reference numerals as Figure 2, as have other features that are common to Figures 2 and 3. The apparatus shown in Figure 3 differs from that shown in Figure 2 in that the apparatus of Figure 3 includes three primary receiving vessels 32A, 32B and 32C. Each of the receiving vessels are connected by respective transport lines 39A, 39B, 39C to feed vessel 30. Each of the transport lines include valves 44A, 44B, 44C to enable the receiving vessels 32A, 32B, 32C to be selectively taken on-line and off-line. Each of the primary receiving vessels is connected to the secondary receiving vessel 34 by transport lines 42A, 42B and 42C, and each of those transport lines are fitted with valves 45A, 45B, 45C.

The arrangement shown in Figure 3, enables one of the primary receiving vessels to be filled with solid particulate material whilst the others are either being emptied or are shut down for maintenance. If, for example, it is desired to fill primary receiving vessel 32A with solid particulate material, valves 44A and 45A are opened, whilst valves 44B, 45B and 44C, 45C are closed. Closing those valves isolates primary receiving vessels 32B, 32C from both the feed vessel and the secondary receiving vessel. Pneumatic conveying then takes place in accordance with the description previously provided for Figure 2.

Figure 4 shows a design variation that may advantageously be used in situations where two (or more) primary receiving vessels are used. Figure 4 shows only the receiving vessels end of the apparatus. In Figure 4, primary receiving vessels 50, 51 are connected via transport lines 52, 53 to a feed vessel (not shown). The primary receiving vessels have respective solids discharged outlets 54, 55 having valves 56, 57 located therein. Similarly, the primary receiving vessels 50, 51 include gas discharge outlets 58, 59 having respective valves 60, 61. The gas discharges 58, 59 are in communication via duct 62 with a secondary receiving vessel 63. Although not shown in Figure 4, secondary receiving vessel 63 is similar to secondary receiving vessel 16 as shown in Figure 1. In operation of the apparatus shown in Figure 4, in order to fill primary receiving tank 50 with solid particulate material, a valve (not shown) in transport line 53 is closed whilst a valve (not shown) in transport line 52 is opened. Valve 56 is closed, valve 60 is opened and valve 61 is closed. During conveying, gas is discharged through gas outlet 58 and it travels through valve 60 and duct 62 into secondary receiving tank 63. When the entrained solids in secondary receiving tank 63 are allowed to settle, they fall downwardly and travel back down gas discharged 58 and settle in primary receiving tank 50.

After the product is conveyed to the receiver vessel it may be gravity discharged into a process vessel or into containers for transportation and sale.

The receiving vessels also may be used as process vessels in their own right i.e. it may be possible to put an oil heated jacket and an agitator shaft into one of the primary receivers.

The apparatus of Figure 4 may also include pipe 64 to connect the secondary receiving vessel 63 to the feed vessel. This allows the possibility of closing valves

60 and 61 and directly filling vessel 63 from the feed vessel. After filling vessel

63 in such manner, the solids may be transported to one or both of vessels 50, 51 by opening valves 60 and/or 61.

In order to test the present invention, a pilot plant scale apparatus having the general layout of the apparatus shown in Figure I was constructed and instrumented as appropriate. The typical parameters measured include the mass of solids in the feed vessel, the mass of solids in the receiving vessel, the air pressure in the feed vessel, the air pressure in the receiving vessel and the air mass flow rate in the gas supply line from the source of pressurized gas. Each of these parameters were logged as a function of time during the conveying cycle, and the results are shown in Figures 5, 6 and 7.

With reference to Figure 5, the air pressure in the feed vessel is shown by the solid line. As can be seen, the air pressure starts at zero (gauge pressure) at time zero. At time ten seconds, pressurization of the feed vessel commences and the pressure inside the feed vessel increases. At this stage the receiving vessel is isolated from the feed vessel because the discharge valve of the feed vessel is closed. Accordingly, the air pressure in the receiving vessel remains at zero.

At time t=70 seconds, the discharge valve on the feed vessel is opened and this causes a "swoosh" of solid particulate material to be conveyed out of the feed vessel, into the transport line and thereafter into the receiving vessel. When the discharge valve of the feed vessel is opened, the air pressure in the feed vessel rapidly drops to a minima at approximately t=85 seconds and thereafter slowly increases again. It should be remembered that the supply of pressurized gas to the feed vessel is maintained during the conveying step. It will also be noted that the air pressure in the receiving vessel rapidly increases when the discharge valve of the feed vessel is opened and it quickly approaches the same value as the air pressure in the feed vessel.

Referring now to Figure 6, the mass of solid in the feed vessel is shown in the dashed line. The mass of solids in the feed vessel remains constant at approximately 80 kg until the bottom discharge valve of the feed vessel is opened. This causes a rapid reduction in the mass of solids in the feed vessel as the solids are rapidly conveyed to the receiving vessel. The mass of solids in the receiving vessel is shown in the solid line in Figure 6 and it can be seen that the mass of solids in the receiving solids rapidly increases upon opening of the discharge valve in the feed vessel. Figure 7 shows a plot of the air mass flow rate from the source of pressurized gas vs cycle time. As can be seen, at time t=10 seconds, the supply of pressurized gas is turned on to the feed vessel and this results in the air mass flow rate increasing to a substantially constant value. At time t=70, the bottom discharge valve of the feed vessel is opened and almost simultaneously conveying gas is feed to the transport line. This results in a large increase in the air mass flow rate and this flowrate is maintained until time t=l 25 seconds, at which stage, the air flow to the conveying test rig is shut-off.

As can be seen from Figures 5, 6 and 7, the solids material is rapidly conveyed from the feed vessel to the receiving vessel in the method of the present invention. This leads to efficiency gains because smaller amounts of conveying gas are required to transport the solids. This allows smaller storage vessels for the conveying gas and lower usage of the conveying gas. This is an important consideration when explosive materials, such as aluminium powder, are being conveyed because the conveying gas must be an inert gas. The amount of gas used is provided by the area under the air mass flow rate vs cycle time curve of Figure 7.

As mentioned earlier and as shown in Figures 5 and 6, conveying of the solid material in the present invention takes place in a transient phase. This is in sharp contrast with known pneumatic conveying systems in which conveying takes place in a continuous manner. Figure 8 shows a typical pressure vs time curve for a conveying cycle used in a prior art pneumatic conveying methods. As can be seen, a continuous pressure difference is maintained between the feed tank and the receiving tank and this pressure differential provides the driving force for the pneumatic conveying. The graph of Figure 8 is clearly very different to the graph shown in Figure 5. As used throughout this specification, the term "pneumatic conveying" should be understood to include conveying by air and also to include conveying using any other conveying gas.

The apparatus shown in Figures 1 to 4 may further include one or more auxiliary gas inlets located in the transport line for injecting auxiliary transport gas into the transport line. This allows operation in the so-called by-pass mode in which auxiliary transport gas is injected into the transport line at one or more locations along the transport line to reduce or prevent settling of the solids and to assist in conveying of the solids. Operation of the method of the present invention in the by-pass mode still entails use of a closed end system and conveying of the solids still involves solids transfer in a transient flow regime. By-pass operation is commonly used on prior art conveying system and accordingly the operation of the by-pass mode of the present invention need not be described further.

Those skilled in the art will appreciate that the invention described herein may be subject to many variations and modifications. It will be appreciated that the invention encompasses all such variations and modifications that fall within its spirit and scope.

Claims

CLAIMS:

1. A method for pneumatic conveying of solid particulate material comprising pressurizing a feed vessel containing the solid particulate material, commencing a flow of conveying gas to a transport line, opening a discharge from the feed vessel to thereby cause the solid particulate material to be conveyed from the feed vessel via the transport line to a receiving vessel, wherein discharge of gas and solid particulate material from the receiving vessel during conveying is prevented.

2. A method as claimed in claim I wherein the steps of commencing the flow of conveying gas to the transport line and opening the discharge from the feed vessel occur simultaneously.

3. A method as claimed in claim 1 or claim 2 wherein discharge of gas and solid particulate material from the receiving vessel during conveying is prevented by closing all outlets from the receiving vessel during conveying.

4. A method as claimed in claim 3 wherein the receiving vessel includes a gas outlet and a solids outlet and the method includes the step of closing the gas outlet and the solids outlet of the receiving vessel prior to opening the discharge of the feed vessel.

5. A method as claimed in any one of the preceding claims wherein the receiving vessel includes a gas outlet and the method further includes allowing the solid particulate material to settle in the receiving vessel after conveying and subsequently bringing the receiving vessel to substantially atmospheric pressure by controllably opening the gas outlet of the receiving vessel.

6. A method as claimed in claim 5 wherein the receiving vessel is at above atmospheric pressure at the end of a conveying cycle and opening the gas outlet of the receiving vessel results in gas exiting the receiving vessel, the opening of the gas outlet of the receiving vessel being controlled such that the velocity of gas within the receiving vessel does not exceed the re-entrainment velocity of the solid particulate material.

7. A method as claimed in claim 5 wherein the receiving vessel is at sub-atmospheric pressure at the end of a conveying cycle and opening the gas outlet of the receiving vessel causes gas to flow into the receiving vessel.

8. A method as claimed in claim 7 further including the step of controlling rate of flow of gas into the receiving vessel to avoid compaction of the solid particulate material in the receiving vessel.

9. A method as claimed in any one of the preceding claims further including the step of applying a vacuum to the receiving vessel prior to commencement of conveying of the solid particulate material.

10. A method as claimed in any one of the preceding claims including the step of fluidising a lower part of the feed vessel to assist flow of solid particulate material from the feed vessel.

11. A method as claimed in any one of the preceding claims wherein the feed vessel includes a solids inlet and a solids outlet and the method includes the step of closing the solids inlet and solids outlet of the feed vessel prior to pressurizing the feed vessel.

12. A method as claimed in any one of the preceding claims further including the step of maintaining supply of pressurized gas to the feed vessel during conveying of the solid particulate material.

13. A method as claimed in any one of the preceding claims further including injecting auxiliary transport gas at one or more locations into the transport line during conveying of solid particulate material therethrough.

14. A process for pneumatic conveying of solid particulate material including the steps of: i) pressurizing a feed vessel containing the solid particulate material; ii) commencing flow of a conveying gas to a transport line; iii) opening a discharge from the feed vessel to thereby convey the solid particulate material through the transport line to a primary receiving vessel; iv) separating a substantial proportion of the solid particulate material from the gas in the primary receiving vessel; v) passing the separated gas and any entrained solid particulate material from the primary secondary vessel to a secondary receiving vessel, said secondary receiving vessel being a closed-end system during conveying to prevent discharge of gas and solid particulate material therefrom during conveying.

15. A method as claimed in claim 14 wherein steps (ii) and (iii) are conducted simultaneously.

16. A method as claimed in claim 14 or claim 15 further including the steps of allowing the solid particulate material in the secondary receiving vessel to settle and subsequently returning the secondary receiving vessel to substantially atmospheric pressure.

17. A method as claimed in claim 16 wherein the secondary receiving vessel is advantageously arranged such that substantially all of the solid particulate material in the secondary receiving vessel returns to the primary receiving vessel when it settles.

18. A method as claimed in any one of claims 14 to 17 further including the step of injecting auxiliary transport gas at one or more locations into the transport line during conveying of the solid particulate material therethrough.

19. A method as claimed in any one of claims 14 to 18 wherein the secondary receiving vessel is connected to a plurality of primary receiving vessels and during conveying of solid particulate material from the feed vessel, the secondary receiving vessel is connected to a primary receiving vessel receiving solid particulate material from the feed vessel and isolated from primary receiving vessel(s) not receiving solid particulate material.

20. A method for pneumatic conveying of solid particulate material characterized in that a closed-end system is used during conveying to thereby prevent discharge of gas and solid particulate material from the system during conveying.

21. An apparatus for pneumatic conveying of solid particulate material comprising a feed vessel for containing the solid particulate material to be conveyed, a receiving vessel for receiving conveyed solid particulate material, a transport line for connecting the feed vessel and the receiving vessel, gas supply means for supplying pressurized gas to the feed vessel and the transport line, a feed vessel discharge valve for opening when the feed vessel has been pressurized to a predetermined pressure to thereby allow the solid particulate material to pass therethrough and a receiving vessel valve system for closing all outlets of the receiving vessel during conveying to prevent discharge of gas and solid particulate material from the receiving vessel during conveying.

22. Apparatus as claimed in claim 21 wherein the receiving vessel valve system includes a gas flow valve means for selectively sealing a gas port in the receiving vessel and for controllably allowing gas flow into or out of the receiving vessel to being the receiving vessel to substantially atmospheric pressure.

23. Apparatus as claimed in claim 21 or claim 22 wherein the receiving vessel valve means includes a solids discharge valve for permitting discharge of solids therethrough when open, the solids discharge valve sealing a solids discharge in the receiving vessel during conveying.

24. Apparatus as claimed in any one of claims 21 to 23 wherein the gas supply means comprises a source of pressurized gas having a gas line to the feed vessel and another gas line to the transport line.

25. Apparatus as claimed in any of claims 21 to 23 wherein the gas supply means comprises a first source of pressurized gas connected via a gas line to the feed vessel and a second source of pressurized gas connected to the transport line.

26. Apparatus as claimed in claim 24 or claim 25 wherein the gas lines connecting the source(s) of pressurized gas to the feed tank and the transport line include gas valve means to selectively shut-off supply of pressurized gas and provide supply of pressurized gas to the feed vessel and transport line.

27. An apparatus for pneumatic conveying of solid particulate material including a feed vessel for containing the solid particulate material to be conveyed, a primary receiving vessel for receiving a substantial portion of the solid particulate material being conveyed, a transport line connecting the feed vessel and the primary receiving vessel, gas supply means for supplying pressurized gas to the feed vessel and the transport line, a feed vessel discharge valve for opening when the feed vessel has been pressurized to a predetermined pressure to thereby allow solid particulate material to pass therethrough, a primary receiving vessel for receiving a substantial portion of the solid particulate material being conveyed, the primary receiving vessel including a gas discharge means for discharging gas and any entrained solid particulate material from the primary receiving vessel, the gas discharge means connected to a secondary receiving vessel whereby gas discharged from the primary receiving vessel passes to the secondary receiving vessel, the secondary receiving vessel having a secondary receiving vessel valve system for closing all outlets of the secondary receiving vessel during conveying to prevent discharge of gas and solid particulate material from the secondary receiving vessel during conveying.

28. Apparatus as claimed in claim 27 wherein the secondary receiving vessel is arranged such that settling of any solid particulate material in the secondary receiving vessel returns the solid particulate material to the primary receiving vessel.

29. Apparatus as claimed in claim 27 or claim 28 wherein the primary receiving vessel is a cyclonic receiving vessel in which the incoming stream of gas and solids enters tangentially.

30. Apparatus as claimed in any one of claims 27 to 29 wherein the secondary receiving vessel includes a gas outlet and the secondary receiving vessel valve system includes valve means on the gas outlet.

31. Apparatus as claimed in any one of claims 27 to 30 wherein the apparatus includes a plurality of primary receiving vessels connected to the feed vessel by respective transport lines, each of the primary receiving vessels being connected to the secondary receiving vessel.

32. Apparatus as claimed in claim 31 wherein the respective transport lines connecting each primary receiving vessel to the feed vessel include valve means for selectively connecting and isolating each primary receiving vessel to the feed vessel.

33. Apparatus as claimed in claim 31 or claim 32 wherein valve means are located in connecting lines connecting each of the plurality of primary receiving vessels to the secondary receiving vessel for selectively connecting each of said primary receiving vessels to the secondary receiving vessel and for selectively isolating each of the primary receiving vessels from the secondary receiving vessels.

34. Apparatus as claimed in any one of claims 21 to 33 wherein the ratio of the volume of the feed vessel to the volume of the receiving vessel (as hereinbefore defined) is in the range of 1 :3 to 1 :6.

35. Apparatus as claimed in any one of claims 21 to 34 wherein the source of pressurized gas comprises one or more tanks of compressed gas or one or more compressors.

36. Apparatus as claimed in any one of claims 21 to 35 wherein the transport line includes one or more injection means for injecting auxiliary transport gas along the transport line.