US2831587A

US2831587A - Fluidized solids conveyance

Info

Publication number: US2831587A
Application number: US241842A
Authority: US
Inventors: John S Rearick
Original assignee: MW Kellogg Co
Current assignee: MW Kellogg Co
Priority date: 1951-08-14
Filing date: 1951-08-14
Publication date: 1958-04-22
Anticipated expiration: 1975-04-22

Description

April 22, 1958 J. s. REARICK 2,831,537

FLUIDIZED sou s CONVEYANCE Filed Aug. 14. 1951 INVENTOR. JOHN -s. REARICK BYZ.F.' W ,{1 PM ATTORNEY.

United States Patent G i than! FLUIDIZED SDLIDS CGNVEYAN CE John S. Reariclr, Ironia, N. J., assignor to M. W Kellogg (30., Jersey (Jity, N. 3., a corporation or Deiaware Application August 14, 1951, Serial No. 241,842

7 Claims. (Cl. 21417) This invention relates to improved method and means of transporting finely divided solid material and more particularly pertains to the conveyance of a fluid mass of finely divided solids to a zone of greater pressure than that under which the solids originally exist.

In conveying finely divided solids to a zone of greater pressure, there is available the lock hopper system whereby the solid material is passed into a hopper and the pressure in the hopper is increased to the desired level in order to transport or convey the solids to the zone of greater pressure. This type of an operation has the disadvantage of being discontinuous. As a result, in order to simulate a continuous operation at least two or more of such lock hoppers are employed intermittently to maintain a continuous flow of solids to the zone of greater pressure. The amount of equipment and cost of operating such systems is expensive. Furthermore, such systems do not lend themselves to the maintenance of steady rates of flow of solids as would be desired.

The use of screw pumps and similar devices to transport such materials to a zone of higher pressure is also known, but only relatively low compression ratios can be realized with such equipment without excessive power consumption or other difliculties caused by loss of fluidity. This results from the fact that as the pressure is increased the volume of the gas surrounding the particles is correspondingly reduced until a point is reached at which the mass no longer possesses fluidity and becomes compact and rigid.

For the purposes of the present invention, the compression devices employed can be of any design wherein solids may be transported to regions of higher pressure by means of a screw pump or similar mechanical means. It is essential that the device possess a compression ratio greater than one, so that the solid material will be dis charged under a pressure which is greater than the pressure under which it originally existed. Screw pumps can be made to develop pressure by decreasing gradually the width of the screw flights or by decreasing the steepness of the flight pitch from the inlet to the discharge end. Other techniques are available for accomplishing this purpose and these are known to those skilled in the art.

Generally, for my purpose, the compression ratio of a single state of screw conveyance is in the order of about 1.1:1 to about 4:1. The optimum compression ratio will depend on the characteristics of the material to be conveyed; however, for most practical purposes, the compression ratio will fall in the above range.

The finely divided solid material to be fed to the first stage or" compression may be a fluid or non-fluid mass. The non-fluid mass may constitute a mass of finely divided particles in a quiescent state or settled condition. Under such circumstances, the particles are substantially in contact with one another, and there is absent any mobility which is characteristic of a fluid mass. In order to transport particles which are in a quiescent state, it is necessary to inject gas into the mass, so that at least some fluidization of the particles occurs. This may not be required in the case of a fluid mass. In general, therefore, the present invention is applicable to transporting a mass of finely divided particles which have a density ranging from the bulk or settled density of the particles to as low as about 2 to 5 pounds per cubic foot. Fluid densities depend on the linear velocities of gases passing therethrough or entrapped therein, as well as the density, size and shape of the particles. The size of the particles ranges from about 1 to 400 microns, or they may be of any size which lends to fluidization. The particle density will depend on the type of material which is used and, therefore, for purposes of operation the factors of particle size and gas entrainment may be regulated for control purposes.

As thefluid mass is compressed, initially the volume will be decreased with consequent increase in density without substantial contact of the particles with each other; eventually, however, the gas volume is reduced to a point where the particles are in substantial contact with each other and further compression can be accomplished only with great difliculty; the density of the mass under this condition will correspond with the settled density of the material. Under this condition the material otters considerable resistance to movement and behaves essen tially as a non-fluid solid mass.

From the foregoing, it is apparent that the practical limit of a single stage of compression is determined by the relationship between the feed density and the settled or bulk density of the material being handled and that attempts to employ a greater compression ratio in a single stage will result in excessive power consumption and other operating difliculties. Likewise, any attempt to subject the material as discharged from the first stage to further compression in an additional stage will produce the same result.

By this invention, it is possible to overcome this difliculty and a means is provided to transport fluidized solid materials to regions of higher pressure Without limitation as to the over-all compression ratio. This is accomplished by employing multiple stage compression and by introducing prior to each stage of compression a volume of gas suflicient to fluidize the mass and to insure that the density of the material after compression is less than the bulk density under corresponding conditions of pressure. It will be found that for any given material there is a minimum feed density below which a screw pump will not operate effectively. This sets a practical limit on the maximum amount of gas that can be injected prior to each stage, and as a consequence defines the maximum practical compression ratio for each stage. Ordinarily, it is desired to control the density of the moving mass, so that at the end of each stage of conveyance or compression the mass will have a density which approaches but does not exceed the bulk or settled'density'of the solids.

The number of stages of compression must beat least two in order to derive the benefits of the present invention. The number of stages to be employed will depend on the characteristics of the material to be conveyed, its initial pressure and the final pressure desired.

The term gas as used herein intended to include any material or mixture of materials existing in the gaseous state under the conditions of operation. The selection of the gas will depend on the process requirements of the particular situation; for example, steam, nitrogen, carbon dioxide, or other inert gas might be used in conveying inflammable material; hydrogen or carbon monoxide to provide a reducing atmosphere; etc. The amount of gas employed will ordinarily be controlled to produce a fluid mass having a density in the range indicated above. The quantity required for this purpose'will' Patented Apr. 22, 1958 be governed by the density, size and shape of the particles, as well as the density of the mass prior to undergoing compression. In general, 2.0 to 400 cubic feet of gas measured at operating conditions are required for each stage per 1000 pounds of solids transported. The preferred method of operation is to employ quantities of gas somewhat greater than the minimum required and to provide means for venting the excess. The excess gas may alternatively be returned to the system ahead of the preceding stage of compression to provide all or a portion of the aeration required at that point. The aeration gas can also serve as a means of controlling the temperature of the solids. The gas may have a temperature higher or lower than the solids and be used in quantities which are suflicient to heat or cool the latter to the level desired. The pressure of the gas must be greater than the pressure under which the compressed fluid mass exists at the point of injection. Obviously, this is necessary in order to be able to introduce the gas into the compressed mass.

In order to better understand the nature of my invention, reference will be had to the drawing which illustrates a specific embodiment thereof, and forms a part of this specification.

In the drawing, there are shown two screw pumps in series. It should be understood that my invention includes using as many stages of screw pumps as is desired for a given purpose. Therefore, in the drawing casings or barrels are cylindrical in shape and contain tapered portions 7. The first screw pump is equipped with a hopper 8 which is disposed on'the casing and serves to feed solid material into the conveyor. Each pump contains a shaft 10 which is disposed horizontally along the entire length of the barrels 7. These shafts are driven by motors (not shown) and are made to rotate within the supporting barrel ends 12. Solid and gaseous materials are prevented from leaking through the barrel ends by means of suitable packing glands 14 which fit therein. When the cross-sectional area of the hopper is greater than the projected area of the barrel situated therebelow, the in coming particles undergo a compacting effect as a result thereof. This compacting may cause a defluidization of the mass and may result in bridging or caking of particles at the bottom of the hopper. In order to avoid such an occurrence, the lower portion of the hopper 8 is extended below the barrel 5, forming aeration chamber 16. The earation chamber provides for the introduction of aeration gases, as well as an adequate cross-sectional area for entrance of the fluidized mass to the barrels. Although not essential, the aeration chamber preferably has a crosssectional area at least equal to that of the lower portion of the hopper. Aeration gas is supplied from a header 18 and is fed into the bottom of the aeration chamber by means of valved lines 20 and 21. A portion of the gas passes through the mass of solids entering through hopper 8, while the balance is entrained therein, thereby reduring the density. The fluidized mass is then conveyed laterally by means of flights 2.3 which spiral along the length of shafts 10. The flights and shafts reduce in width and diameter in an amount corresponding to the reduction of the diameter of barrels 7. After the solids pass through the first screw pump, they are discharged into the hopper 25 of the second screw pump. Aeration gas which is introduced into areation chamber 16 below hopper 25 passes upwardly through and is entrained in the mass of solids in hopper 25 which has a level 27. The excess gas leaves the mass of solids in an upward direction and passes through a disengaging zone 29 before being vented through an overhead line 31. The pressure in hopper 25 is maintained by means of a control valve 33 which is installed in the overhead line 31. Furthermore, the aeration gas is fed into the aeration chamber at a rate suflicient to maintain a continuous flow of gas through the mass of solids situated within hopper 25. It is preferred to continuously vent gas from the disengaging zone or the like,

because in such a system there is less of a tendency for uneven distribution of aeration gas through the mass of solids. However, my invention includes within its scope introducing a fixed amount of aeration gas without continuous withdrawal of a portion thereof. The gas thus discharged from the disengaging zone 29 flows through a line 35 which is connected to the areation chamber 16 of the first screw pump. In this way, the aeration gas is used economically for the purpose of aerating the solids of two or more stages of compression. When the amount of gas flowing from line 35 into the aeration chamber of the first screw pump is not sufiicient for maintaining the required density of solids, additional gas may be introduced by means of line 20.

The aerated mass of solids in hopper 25 is conveyed laterally through the second conveyor. The compressed fluid mass from the second conveyor passes into a third hopper 37 which constitutes part of a third screw pump (not shown). As in the case of the second screw pump, gases pass upwardly through hopper 37 and become disengaged from the solids to form a lever 39. Hopper 37 is superimposed by a disengaging zone 41 which contains an overhead line 43 for the discharge of gases therefrom. The pressure in hopper 37 is controlled by means of a control valve 45 which is located in the overhead line 43. The discharged gases are introduced into the bottom of the aeration chamber 16 of the second screw pump by means of a line 47.

The pressure in hopper 37 is greater than the pressure in hopper 25 and likewise the pressure in hopper 25 is greater than the pressure in hopper 8. In each case, the pressure in the respective hoppers represents the pressure to which the solids are compressed as a result of the particular stage of compression. The pressure of the aeration gases in supply line 18 is selected on the basis of the pressure sought in the last stage of compression. Hence, it is necessary to employ only a single aeration gas supply for all stages of compression. However, where expedient, separate gas supplies of the same or different composition may be provided for any or all stages without departing from the spirit of the invention.

The following is an illustration of the practice of the present invention: A finely divided siliceous material having a bulk density of 47 pounds per cubic foot, and having 8 weight percent of the particles coarser than mesh and 47 percent finer than 325 mesh, was fluidized to a density of 28 pounds per cubic foot and transported from a zone at a pressure of 15 pounds per square inch gage to a zone at a pressure of pounds per square inch gage-by the use of three stages of screw pumps in accordance with this invention. The following figures represent the interstage pressures, densities, and aeration gas volumes:

Gas Pressure Density Injected (p. s. i. g.) (p. e. f.) (s. e. 1.]

1,000 solid) Feed-1st Stage 15 28 Discharge-1st Stage 38 Feed-2nd Stage 38 32 35 Discharge-2nd Stage 74 Feed-3rd Stage 74 30 70 Discharge-3rd Stage 140 moving mass of solids before passing through each stage of screw conveyance. This arrangement can be modified by eliminating the initial aeration of the solids feed to the first screw pump and aerating the solid stream only after it has passed through one stage of conveyance. Such a system is adapted to transporting a fluid mass of solid material which does not require aeration for the first stage of conveyance. Another modification is to aerate the moving mass of solid material in the barrel of the screw pump. Under some conditions, the aeration of the solid stream in the barrel of the screw pump is preferred, par ticularly with materials which tend to defluidize readily. Still further, it is contemplated aerating the moving mass of solids at several places in each stage of conveyance. For example, the solid stream can be aerated before and/or after each stage of screw conveyance and/or along the length of the screw pump barrel. The aeration of the moving solid stream in the screw pump barrel can be accomplished by injecting the gas in the middle of the barrel and/or any other place in the barrel where it is desired to prevent the solid stream from defluidizing. This may involve injecting the aeration gas at two or more points along the length of the screw pump barrel.

By suitable design of the screw pump and injection of aeration gas at intermediate points along the barrel, two or more stages of compression may be provided in a single apparatus in accordance with this invention.

Having thus described my invention by furnishing specific illustrations, it should be understood that no undue limitations or restrictions should be imposed by reason thereof.

I claim:

l. A method of conveying finely divided solid material which comprises passing a mass of finely divided solid material through at least two stages of screw pumps, passing the solid material to aerating zones preceding each stage, continuously injecting a gasiform material into said moving solid material in each aeration zone in an amount in excess of that required for a desired fluid density, withdrawing the excess gasiform material from each aeration zone, and passing said excess gasiform material to the preceding aeration zone for fluidization of the moving solid material.

2. An apparatus comprising in combination at least two screw pumps; each screw pump being equipped with an elongated circular casing, a rotatable shaft extending along the entire length of the casing, supporting means whereby the shaft is supported and rotated, screw flights spiraling along the length of the shaft, a hopper superimposed on one end of the casing and communicating therewith for the passage of materials therebetween, an outlet means situated on the end of the casing opposite to the hopper whereby materials are discharged from the casing, an aeration chamber laterally disposed on said casing and located directly below the position of the hopper, said aeration chamber having a cross-sectional area which is at least as great as the cross-sectional area of the hopper at the place where it is connected to the casing, and means for introducing a gas into the aeration chamber; and said screw pumps being connected such that the outlet means of one communicates directly with the hopper of the subsequent screw pump.

3. An apparatus comprising in combination at least two screw pumps; each screw pump being equipped with an elongated circular casing, a rotatable shaft extending along the entire length of the casing, supporting means whereby the shaft is supported and rotated, screw flights spiraling along the length of the shaft, a hopper superimposed on one end of the casing and, communicating therewith for the passage of materials therebetween, an outlet means situated on the end of the casing opposite to the hopper whereby materials are discharged from the casing, an aeration chamber laterally disposed on said casing and located directly below the position of the hopper, said aeration chamber having a cross-sectional area which is at least as great as the cross-sectional area of the hopper at the place where it is connected to the casing, and means for introducing a gas into the aeration chamher; said screw pumps being connected such that the outlet means of one communicates directly with the hopper of the subsequent screw pump; a disengaging zone superposed above said outlet means wherein any gaseous materials may collect; and means for connecting the disengaging zone with the aeration chamber of the preceding screw pump whereby the gaseous material employed between stages is recycled to the preceding stage for utilization.

4. A method of conveying finely divided solids which comprises passing a fluid mass of finely divided solids through at least two stages of compression, and said solids passing into an aeration zone between stages wherein a gaseous material is injected for the purpose of maintaining the solids in a fluid condition.

5. The method of claim 4 wherein the stages of com pression comprise screw pumps.

6. A method of conveying finely divided solid material which comprises passing a non-fluid mass of finely divided material to a screw pump wherein the solid material is first contacted with a gasiformmaterial to form a fluid mass prior to being compressed, passing the fluid mass through at least one more screw pump, and said solids passing into an aeration zone between said screw pumps wherein a gaseous material is injected for the purpose of maintaining the solid material in a fluid condition.

7. A method of conveying a mass of finely divided solid material which comprises passing a mass of finely divided solid material through a series of at least two mechanical stages of compression, each stage of compression being preceded by an aeration zone wherein a gaseous material is injected in an amount in excess of that required for the purpose of maintaining the solid material in a fluid condition, withdrawing the excess gasiform material from each succeeding aeration zone, and passing the same to the previous one.

References Cited in the file of this patent UNITED STATES PATENTS 1,553,539 Kinyon Sept. 15, 1925 2,448,745 Struckman Sept. 7, 1948 FOREIGN PATENTS 589,075 France Feb. 16, 1925

Claims

2. AN APPARATUS COMPRISING IN COMBINATION AT LEAST TWO SCREW PUMPS, EACH SCREW PUMP BEING EQUIPPED WITH AN ELONGATED CIRCULAR CASING, A ROTATABLE SHAFT EXTENDING ALONG THE ENTIRE LENGTH OF THE CASING, SUPPORTING MEANS WHEREBY THE SHAFT IS SUPPORTED AND ROTATED, SCREW FLIGHTS SPRIRALING ALONG THE LENGTH OF THE SHAFT, A HOPPER SUPERIMPOSED ON ONE END OF THE CASING AND COMMUNICATING THEREWITH FOR THE PASSAGE OF MATERIALS THEREBETWEEN, AN OUTLET MEANS SITUATED ON THE END OF THE CASING OPPOSITE TO THE HOPPER WHEREBY MATERIALS ARE DISCHARGED FROM THE CASHING AN AERATION CHAMBER LATERALLY DISPOSED ON SAID CASING AND LOCATED DIRECTLY BELOW THE POSITION OF THE HOPPER, SAID AERATION CHAMBER HAVING A CROSS-SECTIONAL AREA WHICH IS AT LEAST AS GREAT AS THE CROSS-SECTIONAL AREA OF THE HOPPER AT THE PLACE WHERE IT IS CONNECTED TO THE