Apparatus For Mixing And Homogenising Bulk Material And Method Of Operating The Apparatus

Abstract

A container with a conical base for the pneumatic mixing of material in powder or granulated form, and with a gas supply means at the bottom has at least one riser pipe arranged therein, said pipe ends in the upper region of the container and is formed with openings or gaps. At least one opening or gap is formed as an annular passage, which divides the riser pipe into a lower section and an upper section. The upper end of the lower section surrounds and is spaced from the lower end of the upper section, whereby said gap is defined.

Parent Case Text

This application is a continuation-in-part of application Ser. No. 49,883, filed June 25, 1970, now abandoned.

Description

BACKGROUND

The invention relates to a container having a conical base for pneumatically mixing material in powder or granulated form, said container having at the bottom outlet a gas supply means and having at least one riser pipe arranged therein, said pipe terminating in the upper region of the container and having openings or gaps.

The invention is further concerned with two different methods of operating the mixing container, by means of which the material is propelled into the riser pipe by gas introduced at the base.

As is know bulk materials which are in a container can be mixed and homogenised by being carried upwardly by means of a gas stream through riser pipes arranged inside or outside the container and being caused to overflow at the upper rim thereof.

It is a disadvantage with these simple arrangements that only material from a high position in the container is supplied to the riser pipes at their lower ends. It is true this arrangement ensures a good transverse mixing of the bed of product disposed at this height, but it does not provide any sufficient longitudinal mixing of the layers or beds of product deposited at different heights in the container.

Apparatus is known in which several riser pipes are arranged inside (German Patent No. 615771) or outside (German Patent No. 669201) of the mixing container in order to produce a better longitudinal mixing, the lower openings forming the supply for the material being arranged at different heights in the container below the level of the product.

In another arrangement (U.S. Pat. No. 3,258,252), the riser pipe arranged in the mixing container is concentrically enclosed by a second perforated pipe of larger diameter, the lower end of this pipe being closed by an annular cover. The powder or granulated material in this case penetrates through the openings in the casing of the perforated pipe into the annular chamber between the two pipes, falls due to gravitational force and is carried upwardly with other material from the bottom of the mixing container by means of a compressed gas stream through the riser pipe.

A particular disadvantage is that, with the said arrangement, an improvement in the longitudinal mixing is only to be achieved with considerable expense in construction. This expense often bears no relationship to the slight improvement in the quality of the mixing which is produced thereby and to the insignificantly shortened mixing time.

In the chemical industry, it is often necessary to carefully clean a mixer before charging the material to be mixed, because many mixers have to be used for different types of products. The standards as regards cleanliness are so thorough that often a man has to climb the mixer and clean the interior thereof by hand. It is to be seen from this that all known mixers having complicated and narrow fittings, small openings, frit-type bottoms, nozzles and positions which cannot be reached or are only accessible with difficulty, are unsuitable for the purposes set forth.

Furthermore, with mixers having aeration nozzles, very high gas velocities are set up locally. The product which is to be mixed and which frequently consists only of loose agglomerates therefore often experiences undesirably heavy abrasion.

THE INVENTION

The invention has for an object to provide a simple mixer which can be easily cleaned and which does not have any complicated fittings, with which a good longitudinal mixing and a good transverse mixing and therefore a short mixing time is ensured. A simple riser pipe without complicated air or product supply means is provided. Furthermore, low gas velocities are required in the mixing pipe and at the feeding position, so that the abrasion of the product remains small.

According to the invention, at least one opening or gap formed as an annular passage is provided on the riser pipe in the mixing container, the said gap dividing the riser pipe into the lower section and an upper section, the upper end of the lower section being widened and surrounding the lower end of the upper section while leaving an annular gap.

The mixing gas is supplied to the container at the bottom end of the mixer housing. Air is advantageously used as mixing gas. The conical bottom of the mixer housing, which terminates in a cylindrical end member, is for this purpose provided with an air supply chamber enclosing the cylindrical end section in annular form, said chamber also having a conical bottom. The conical bottom of the air supply chamber in this case serves simultaneously as the outlet for the product.

A good mixing action of the apparatus as described is only assured if the gas stream provided for the mixing actually only flows through the riser pipe provided for it.

A particularly good longitudinal mixing is obtained when the riser pipe is arranged eccentrically in the container.

It is to be initially assumed that the gas, with the commencement of the mixing operation, selects its path through the riser pipe and through the mixing container in an upward vertical direction. By suitable introduction of the product to be mixed into the mixing container, for example, through a pneumatic feed pipe with a tangential inlet into the container, and by fitting a shielding cone above the upper end of the riser pipe, the container is filled to a selected level between the bottom and top of the riser. After the mixing container has been so filled, the level of product in the annular chamber between the inside wall of the container and the riser pipe is higher than in the latter. The mixing gas now flows for the major part through the riser pipe, because there it has to overcome a smaller flow resistance because of the lower product level. After the loosening of the product in the riser pipe, the filling level therein rises, but the bulk weight inside the riser pipe is then substantially smaller than that inside the mixing container, and the mixing gas also continues to flow for the major part through the riser pipe provided for the mixing. The product to be mixed is raised in the riser pipe by the ascending gas stream, and when the level to which the container is filled is above the top of the annular chamber, is caused to overflow at the upper end thereof. On the conical bottom of the mixer, fresh product slips down into the gas stream ascending to the riser pipe.

Because of the construction of the openings in the riser pipe, when the container is filled as just stated, a part of the product which is in the container at the level of the annular passage falls from this position into the riser pipe. In this way, layers of product from different height positions of the mixing container are homogenised in the riser pipe.

Consequently, a good longitudinal mixing is assured, in addition to the transverse mixing. With a mixing test in a mixing container with a capacity of 3 cubic metres, the mixing time of two hours with a straight-through riser pipe could be reduced to 20 minutes with a riser pipe having an additional annular passage. The same mixing quality was obtained in both tests.

It has proved to be especially advantageous that the annular passages are to be easily cleaned after the mixing process and that no deposits of product occur therein.

When the mixer is filled to above the top of the annular passage, some of the product can naturally drop through the annular passage into the riser pipe. However, the product level in the riserpipe, due to the resistance of the material to flow like a liquid, can at a maximum only reach the height of the lower end of the annular passage or only a short distance above said height. Since the upper end of the annular passage in the riser pipe is situated above the product level in the riser pipe, the difference in the filling heights and thus also the flow resistance in the container and in the riser pipe are sufficiently large to produce required gas flow.

On commencing the mixing operation, the product to be mixed is disposed in the mixing container and in the riser pipe up to at least the height of the upper end of the annular chamber in densely packed form. The mixing gas supplied at the bottom of the mixer has to force the plug of product beneath the bottom end of the riser pipe into the said pipe. In this case, very large shearing stresses have to be overcome at the periphery of the initially still consolidated plug of product. In addition, the plug is also loaded by the weight of the product column in the riser pipe. The pressure necessary for starting up the mixer is usually a multiple of the pressure required for the continued mixing. By way of example, for a mixing container having a capacity of 40 cubic metres (height 10 metres, diameter 2-1/2 metres) and filled with synthetic plastic granulated material (bulk weight: 500 kp/m.sup.3, grain diameter 3 mm.), a starting pressure of more than 20 m. water column is necessary with a gas quantity of 7,000 m.sup.3 /h (at n.t.p.). For the actual mixing operation, on the contrary, only a pressure of 2 m water column is required for the same gas throughput.

There are now to be proposed suitable devices by means of which the pressure necessary on starting the mixer can be lowered.

According to a further development of the invention, the annular passages can for example change in cross-section or be completely closed by inflatable, elastic closure elements arranged therein or by sections of the riser pipe being arranged to be relatively adjusted in height.

Thus, before starting the mixing operation, the product to be mixed can be introduced into the mixing container with the annular passages of the riser pipe closed. The result hereby obtained is that the introduced product can only penetrate from the bottom of the mixing container into the riser pipe, so that the level of product in said pipe only reaches a very small height. The plug of product between the bottom end of the riser pipe and the bottom of the container now has only to be moved upwardly against the shearing stresses at its periphery with supply of the mixing gas, since the weight of the column of product in the riser pipe by which it is additionally loaded is avoided.

In this way, the pressure on starting the last-mentioned mixer can be reduced from 20 m water column to about 2.0 m water column. When the mixing operation has been initiated, the annular passages in the riser pipe are also opened again.

According to a further development of the invention, in the conical lower portion of the mixing container having gas supply chambers at the bottom end, at least one additional gas supply pipe is provided, which opens into the conical mixer bottom at or near the height of the inlet end of the riser pipe.

At the commencement of the mixing operation, the mixing gas can now initially be introduced into the mixing container from the gas supply pipe at the higher position. By this means, the product in the riser pipe is loosened. Thereafter, the gas supply at the bottom of the conical mixing container is gradually opened and the upper gas supply is continuously closed to the same extent. It is still only necessary to loosen the plug of product beneath the riser pipe by the mixing gas which is now flowing in from the bottom of the container. Due to the initially relatively long plug of product inside and outside the riser pipe being loosened in stages, a reduction of the starting pressure is to be obtained, in a similar manner as when closing the annular passages in the riser pipe at the start of the mixing operation.

During the operation of the mixing container, in one embodiment, a filling level is maintained in the container according to the invention above the level of the upper end of the annular passage such that the pressure of the material in the bed in the container exceeds the mixing gas pressure in the region of the annular passage and material enters by way of the annular passage into the riser pipe is mixed with the material introduced at the bottom end of the riser pipe.

This method can always be carried out when, after the introduction of the product to the mixing container, the level of product outside the riser pipe is higher than the upper opening of the annular passage.

In the case where the filling level in the mixing container is kept below the annular passage, another method is proposed for the operation of the mixing apparatus.

The gas introduced at the bottom outlet initially lifts, as before, the material into the riser pipe. A part of the gas transported into the riser pipe is caused to overflow through the annular passage, while periodically another part of the material is lifted into the upper section of the riser pipe, and after reaching a certain height, this collapses to form a plug of material and is mixed together with the material transported from the bottom of the container through the lower riser pipe section and is carried away through the annular passage.

Thus, the invention provides apparatus suitable for pneumatically mixing materials in finely divided form. The apparatus includes a vertically extending container having an inlet opening adjacent its upper end for introduction of finely divided materials, and an outlet means in the lower end outfitted with means for selectively closing the outlet, for withdrawal of mixed materials. The vertically extending riser is disposed in the container above and spaced from the outlet opening, and the riser comprises an upper section and a lower section. The two sections are in overlapping, radially spaced relation, defining an annular passage therebetween. The upper section terminates above the lower section and the lower section has its lower end at least as low as the lower end of the upper section. Means are provided for supplying gas for the mixing, to the container below the lower end of the riser for passage of the gas into the container and pneumatic lifting and mixing of the gas and materials in the riser with the materials filling the container to above the lower end of the riser.

Desirably, the means for supplying gas for the mixing comprises two vertically spaced gas inlet conduits outfitted with valve means for selectively apportioning the flow of gas between the two conduits.

A baffle can be disposed above and spaced from the upper end of the riser for intercepting the mixture of gas and particles issuing from the riser, and separating the particles from the gas. A gas outlet can be disposed above the baffle for discharge of the gas from the container after the separation by the baffle.

In a preferred construction of the riser, the lower section diverges upwardly. Further, in a preferred embodiment, the lower end of the lower section of the riser, terminates below the lower end of the upper section thereof. Also, as mentioned previously, means can be provided for adjusting the flow area of the annular passage, and, desirably, for selectively closing the annular passage.

One embodiment is hereinafter described by way of example and by reference to drawings, wherein:

FIG. 1 is a section through the complete mising apparatus, and

FIG. 2 is a section through one embodiment of the riser pipe.

The mixer which is illustrated in FIG. 1 consists of a cylindrical container 1 having a conical bottom 2 and a removable cover 3. A riser pipe, which consists of a conical lower section 4, which diverges upwardly, and a cylindrical upper section 5, is fixed by stay members 7 a and 7b in the container 1. The two sections of the riser pipe are fitted one within the other, leaving an annular passage 6. In order to center the upper section 5 of the riser pipe in conically widened lower section 6, the upper section 5 is provided with spacer plates 9 welded radially thereon.

Flanged on the conical mixer bottom 2, which terminates in a cylindrical end section 10, is a gas supply chamber 11, said chamber having a conical bottom 12 which serves simultaneously as the outlet for the product and on which is arranged a bucket wheel lock chamber or star valve 13 serving as closure member. The mixing gas delivered by a compressor (not shown) may at will be supplied to the mixing container through pipe conduit 14 by way of the gas supply chamber 11 or through the pipe conduit 15 by way of an opening in the conical mixer bottom 2 and covered by a screen 16. The pipe conduits 14 and 15 can both be closed by means of shut-off valves 17.

Fixed on the radial guide plates 18 in the exhaust gas pipe 19 of the cover 3 of the mixer is the baffle or shielding cone 20 with the rod 21.

The mixing container is filled through a feed pipe 22 opening tangentially into the container, said pipe 22 being subsequently closed by the flap valve 23. The mixing gas is initially supplied through the pipe 15. At this time, the pipe 14 is closed by the flap valve 17. Since the level of the product in the mixing container 1 after the container has been filled is higher than the level in the riser pipe, the gas chooses the path of lower resistance through the riser pipe and loosens the product which is disposed therein. The pipe 14 can now be opened and the pipe 15 can be closed to the same degree. The mixing gas now enters through the gas supply chamber 11 and through the cylindrical end member 10 of the conical mixer bottom 2 into the riser pipe 4 and 5 and thus carries in a vertically upward direction through the riser pipe the product which is disposed beneath the lower riser pipe section 4. The product is then centrifuged radially outwards from the riser pipe. The mixing gas is discharged through the pipe 19. To the same extent as product is withdrawn from the bottom of the mixer into riser pipe, other product slips downwardly from the annular chamber between the riser pipe and the mixer wall 1. Some of the bulk material descends through the annular passage 6 into the riser pipe and is homogenised therein with the product coming from the bottom of the mixer. The mixing process as described here can however only be carried into effect when the level of the product in the mixing container is higher than the upper end of the lower section 4 of the riser pipe.

If such a small quantity of product is introduced into the mixing container that the upper end of the lower riser pipe section 4 projects above the level of the product, then as the mixer is operating, some of the product is discharged with the mixing gas through the annular passage 6 into the mixing container. Furthermore, product is also initially raised in the upper section of the riser pipe 5. As a result, a point is reached at which the pressure necessary for discharging the product from the upper section of the riser pipe is higher than that which is necessary for carrying the product through the annular chamber 6. All the mixing gas now flows through the annular chamber 6, while the product still loosened in the upper section 5 of the riser pipe collapses, with formation of a plug of material. This plug then slowly slips downwardly and becomes admixed with the product transported from the bottom of the mixer through the lower riser pipe section 4. If the plug of material is carried away in this manner, once again the same pressure conditions prevail as those which existed on starting up the mixer. The formation of the plug of product in the upper riser pipe section 5 and the discharge thereof is thus a process which is periodically repeated throughout the entire mixing period.

In the constructional form according to FIG. 2, the upper section 5 of the riser pipe is provided with a rubber or synthetic plastic tube 24 concentrically surrounding the said section. This rubber tube can be inflated by means of compressed air supplied through the pipe 25. As a consequence, it is applied to the wall of the lower riser pipe section 4 and seals off the annular passage 6. In the Figure, the tube is shown in the inflated state.

If the annular gap 6 of the riser pipe is sealed off before the product is introduced into the mixing container, then also no product can fall into the riser pipe during the filling operation. In this way it is possible considerably to reduce the gas pressure necessary for starting the mixer. After having been started, the annular gap is once again opened by reducing the pressure of the gas which is in the tube 24.

EXAMPLE

A mixer as is shown in the drawing having an overall height 10 meters, and a straight cylindrical section of 7.8 meters in length and 2.5 meters in inside diameter, is outfitted with a riser as is shown in FIG. 1, disposed coaxially in the container with respect to the cylindrical section. The lower end of the riser lower section is located about 0.4 meters above the outlet 10, and about 0.2 meters above the inlet 16 of conduit 15. The riser lower section is 440 cm in length, 50 cm in diameter at its lower end and 80 cm in diameter at its upper end. The riser upper section is 550 cm in length and 50 cm in diameter. The over lap of the riser section is 300 cm. The mixer can be filled to about the level indicated in the drawing with polyester granulate of particle size of 3 mm and glass fibers of 0.3 mm diameter and a length of about 5 mm, in the weight proportion of 80 : 20. Using only conduit 15 for start up, the pressure necessary for starting is 1000 mm of water; after start up, using conduit 14, the running pressure is also about 2000 mm of water. Thorough mixing of the batch is completed in about 60 minutes. Star valve 13 can be operated to empty the container. In batch operation, the first runnings discharged by the star valve will not be well mixed and can be saved for use in a later mixing operation. The equipment can be operated continuously.

Claims

What is claimed is:

1. Apparatus suitable for pneumatically mixing materials in finely divided form comprising:

a. a vertically extending container having an inlet opening adjacent its upper end for introduction of finely divided materials, and an outlet means in the lower end outfitted with means for selectively closing the outlet, for withdrawal of mixed materials,

b. a vertically extending riser disposed in the container above and spaced from the outlet opening and comprising an upper section and a lower section, said sections being in overlapping, radially spaced relation defining an annular passage therebetween, with the upper section terminating above the lower section and the lower section having its lower end at least as low as the lower end of the upper section,

c. means for supplying gas for said mixing to the container below the lower end of the riser for passage of gas into the container and pneumatic lifting and mixing of the gas and materials in the riser with the materials filling the container to above the lower end of the riser.

2. Apparatus according to claim 1, the lower end of the riser lower section diverging upwardly.

3. Apparatus according to claim 1, and means for adjusting the flow area of the annular passageway of the riser.

4. Apparatus according to claim 1, and means for selectively closing the annular passageway of the riser.

5. Apparatus according to claim 1, said means for supplying gas for said mixing comprising two vertically spaced gas inlet conduits and valve means for selectively apportioning the flow of gas between the two conduits.

6. Apparatus according to claim 1, the lower end of the lower section of the riser terminating below the lower end of the uppersection of the riser.

7. Apparatus according to claim 1, and a baffle disposed above and spaced from the upper end of the riser for intercepting the mixture of gas and particles and separating the particles from the gas.

8. Apparatus according to claim 7, and a gas outlet above the baffle for discharge of the gas from the container after said separation by the baffle.

9. Process of pneumatically mixing materials in finely divided form with apparatus comprising:

a. a vertically extending container having an inlet opening adjacent its upper end for introduction of finely divided materials, and an outlet means in the lower end outfitted with means for selec-tively closing the outlet, for withdrawal of mixed materials,

b. a vertically extending riser disposed in the container above and spaced from the outlet opening and comprising an upper section and a lower section, said sections being in overlapping, radially spaced relation defining an annular passage therebetween, with the upper section terminating above the lower section and the lower section having its lower end at least as low as the lower end of the upper section,

c. means for supplying gas for said mixing to the container below the lower end of the riser for passage of gas into the container and pneumatic lifting and mixing of the gas and materials in the riser with the materials filling the container to above the lower end of the riser,

said process comprising:

d. filling the container with finely divided material to a level between the upper end of the riser lower section and the upper end of the riser upper section,

e. supplying gas via said gas supplying means for passage therof into the container and pneumatic lifting and mixing of the gas and finely divided materials in the riser,

f. said level of the finely divided material being such that finely divided material during the process flows downwardly through said annular passageway and into the mixture of gas and finely divided material rising through the riser, and

g. maintaining the finely divided material at said level during the process.

10. Process of mixing materials in finely divided form with apparatus comprising:

a. a vertically extending container having an inlet opening adjacent its upper end for introduction of finely divided materials, and an outlet means in the lower end outfitted with means for selectively closing the outlet, for withdrawal of mixed materials,

b. a vertically extending riser disposed in the container above and spaced from the outlet opening and comprising an upper section and a lower section, said sections being in overlapping, radially spaced relation defining an annular passage therebetween, with the upper section terminating above the lower section and the lower section having its lower end at least as low as the lower end of the upper section,

c. means for supplying gas for said mixing to the container below the lower end of the riser for passage of gas into the container and pneumatic lifting and mixing of the gas and materials in the riser with the materials filling the container to above the lower end of the riser,

said process comprising:

d. filling the container with finely divided material to a level between the upper and lower ends of the riser lower section,

e. supplying gas via said gas supplying means for passage thereof into the container and pneumatic lifting and mixing of the gas and finely divided materials in the riser,

f. part of the mixture of gas and finely divided material passing through the riser leaving the riser pipe via said annular passage, while periodically another portion of the finely divided materials rises into the upper section of the riser which portion, after reaching a certain height, collapses to form a plug of material and is mixed together with the material transported from the bottom of the container through the lower section of the riser and discharges through the annular passage.