WO1987004780A1

WO1987004780A1 - Method for the drying of a powdery, granular, chip-formed or equivalent material

Info

Publication number: WO1987004780A1
Application number: PCT/FI1987/000017
Authority: WO
Inventors: Antero Jahkola; Markku Huhtinen; Tapani Peltola
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 1986-01-31
Filing date: 1987-01-29
Publication date: 1987-08-13
Also published as: FI860468A; FI79753C; FI79753B; FI860468A0

Abstract

The material (B) to be dried is, at least for the time of the drying, mixed with a magnetic material (A), and the drying of the material (B) to be dried is carried out in a mixture of the said material (B) to be dried and of the said magnetic material (A) by passing heat into the mixture of the said materials (A) and (B). After the drying, the magnetic material (A) and the dried material (B') are separated from each other, and the dried material (B') is passed further to its purpose of use and the magnetic material (A) is recirculated to the mixture of the material (B) to be dried and the magnetic material (A). The drying is performed by means of the fluidized bed principle so that the mixture of the material (B) to be dried and of the magnetic material (A) is fluidized by means of the drying gas (C). Superheated water vapour is used advantageously as the drying gas (C).

Description

Method for the drying of a powdery, granular, chip-formed or equivalent material

The present invention concerns a method for the drying of a powdery, granular, chip-formed or equivalent material.

The drying method in accordance with the invention is suitable for use in the drying of, e.g., various solid fuels, such as peat, wood chips or shingles, and also of grain and of various powdery or granular products of food, fodder and chemical industries.

Several different methods are known for the drying of solid fuels. A method is prior known from the FI Patent 11200, wherein the drying of the material Containing water is performed regeneratively by mixing hot particles, which are heated in a heat source, e.g., in combustion gases, with the material to be dried. The method involves that the particles, which function as heat carriers, are heated as hot as possible in order to keep the material flow of the particles, which function as heat carriers, reasonable. That is why the method is badly applicable to be used far drying organic materials which are sensitive to the action of heat. This disadvantage can be eliminated by accomplishing the drier by means of the fluidized bed technics. The bed of even temperature is typical for the fluidized bed technics. One drying method is described, e.g., in the FI Patent Application 832049, wherein the drying is performed by means of the fluidized bed principle. In the method of the said paper, the drying is carried cut in a particular drying plant, which consists of a container, through which pipes are passed, through which said pipes the flue gases coning from the boiler are passed. In the container of the drying plant, as a "bed", on the grate a sand layer is fitted, which is heated by means of the flue gases passing through the said pipes. The fuel to be dried is made to pass through the said bed material, whereby the moisture contained in the fuel is vaporized as the fuel passes through the heat sand layer. Hereupon the fuel is passed into a cyclone, wherein the water vapour is separated from the fuel, and hereinafter the fuel is fed into the boiler for combustion.

In the method described above, a considerable amount of the sand used as the bed material remains in the dried fuel, which sand is not recovered until after the burning. Thus, this method is suitable for use only in connection with such boilers in which the fuel is allowed to contain sand. Thus, such a method cannot be used in cases in which the dried material must be as pure as possible, i.e. when no sand is allowed to be present in the dried material. The method of the above FI Pat.Appl. 832049 can, of course, be supplemented by means of various cleaning apparatuses, such as screens or equivalent, but by their means, a sufficient degree of purity is, however, not reached in all cases. Moreover, in the prior-art methods, it has been necessary to keep the particle size of the sand used as the bed material quite large. However, when a large particle size is used, the heat transfer factor between the bed material and the indirect heat exchanger remains quite low.

The object of the present invention is to provide a method for the drying of a powdery, granular, chip-formed or equivalent material which said method does not involve the above drawbacks of the prior-art methods. In view of achieving this, the invention is mainly characterized in that the material to be dried is, at least for the time of the drying, mixed with a magnetic material and that the drying of the material to be dried cut in a mixture of the said material to be dried and of the said magnetic material by passing heat into the mixture of the said materials.

Of the advantages of the method of the present invention as compared with the prior art drying methods, e.g., the following should be mentioned. The separation of the material to be dried from the material in which the drying is performed becomes excellent, because a magnetic material is used as such a material. Thereat, the material to be dried can be separated from the other material, so-called bed material, by using, e.g., magnetic separators. This is why the method of the invention is very well suitable for use in cases in which no foreign matter is allowed to be present among the material dried. Since, owing to the above, the dried material can be separated from the bed material well, the consistency of the bed material may be very finely divided. This again results in the circumstance that the heat transfer factor between the bed material and the indirect heat exchanger has became very good. Owing to the good heat transfer, it is again possible to construct the equipments in which the drying is performed as of small size. This again results in the circumstance that the investment costs of the equipments are considerably lower than those of equipments that make use of prior-art methods. Since a very finely divided material can be used as the magnetic material, the said finely divided material improves the fluidity of the material to be dried in the different parts of the dryer. Moreover, the magnetic material helps to keep the faces of the dryer clean.

In the following, the invention will be described in more detail with reference to the figures in the attached drawing. The apparatuses in accordance with the figures axe shown exclusively in order to illustrate the present invention, and the method of the invention is not confined to the embodiments shown in the figures only.

Fig. 1 is a schematical illustration of the principle of a heatable fluidized bed dryer which can be used for accomplishing the method of the invention.

Fig. 2 is a schematical sectional view of a magnetic separator that can be used in the method of the invention.

To begin with, reference is made to Fig. 1, which is a schematical illustration of the principle of operation of a fluidized bed dryer 10 operating by means of the method of the invention. In the system of Fig. 1, the main components of the equipment are a dryer container 11, in which the drying of the material to be dried is performed, a cyclone 20, in which the drying gas, e.g. superheated water vapour, is separated from the material to be dried, as well as a magnetic separator 30, which separates the dried material from the bed material. The dryer container 11 of the fluidized bed dryer may be, e.g., a vertically positioned cylindrical vessel of the sort shown in Fig. 1. At the lower end of the dryer container 11, near the bottom of the container, a perforated grate 12 is mounted, an which a drying bed of a magnetic material A has been formed as a bed. As the magnetic bed material A, a finely divided particle-like or sand-like material is used, e.g. magnetite, even though other magnetic materials are also suitable for use in the method in accordance with the invention.

Within the dryer container 11, a heat exchanger 14 is installed, in which a circulation of, e.g., pressurized water or steam is arranged, which said water os steam heats the magnetic bed material A as well as the material B to be dried. In the dryer container 11, there is preferably such an amount of the magnetic bed material A that the material encloses the heat exchanger 14 in the container completely. In the lower portion of the dryer container 11, above the grate 12, a feed connection or equivalent 15 is formed, through which the material B to be dried is fed to underneath the bed material A, above the grate 12. The feed duct 15 may preferably be provided, e.g., with a screw-type feeder, by means of which the material B to be dried can be fed into the magnetic bed material A. To the lower portion of the dryer container 11, underneath the grate 12, a feed pipe 19 is farmed, through which the drying gas C used in the drying of the material is fed into the dryer container 11, to underneath the material B to be dried and underneath the magnetic bed material A.

At the upper portion of the dryer container 11, a first exhaust pipe 16 is formed, through which the drying gas C passing through the dryer container 11 and, along with it, a slight quantity of the dried material B' and of the magnetic bed material A are removed out of the dryer container 11 into the cyclone 20. At a level lower than the first exhaust pipe 16, in the dryer container 11, a second exhaust pipe 18 is formed further, which is arranged in the dryer container 11 at such a level that the mixture of the bed material A and the material B to be dried covers the mouth of the said second exhaust pipe 18. The second exhaust pipe 18 is provided, e.g., with a screw conveyor or equivalent, by means of which mixture of the magnetic bed material A and of the dried material B' is removed out of the dryer container 11.

As was already stated above, part of the dried material B' , together with a little quantity of the bed material, is carried along with the drying gas C out of the dryer container 11 along the first exhaust pipe 16 into the cyclone 20. In the cyclone 20 the solid materials are separated from the drying gas, whereinafter the said solid materials are passed cut of the cyclone 20 along an exhaust duct 21 for solid materials into a magnetic separator 30. The drying gas C is again passed out of the cyclone 20 along an exhaust duct 22 into a circulation duct 24, from which it is fed, e.g., by means of a blower 25 frαa the feed pipe 19 again into the dryer container 11.

In Fig. 1, only one cyclone 20 is shown, but in practice it may be advantageous to provide several cyclones 20 one after the other, in which case the drying gas and the solid materials can be separated froa each other better. Above, it was stated further that mixture of the magnetic bed material A and of the dried material B' is removed cut of the dryer container 11 from the top of the bed through the second exhaust pipe 18, e.g., by means of a screw conveyor, being passed into the magnetic separator 30. In practice, the said second exhaust pipe 18 may be connected to the outlet duct 21 for solids after the cyclone 20, e.g., as is shown in Fig. 1.

Above, it was stated that, in a dryer in accordance with Fig. 1, superheated water vapour can be used as the drying gas C instead of the commonly used drying gas, air. The superheated water vapour is obtained to the system in connection with the drying out of the dried material B. Superheated water vapour is in many respects preferable to air. Firstly, if air is used as the drying gas in the drying, the system becomes highly sensitive to fire if the dried material is readily inflammable. When superheated water vapour is used as the drying gas, the energy consumption of the drying process can be reduced essentially, e.g., by condensing the liberated heat for heating purposes. Further, by constructing the dryer so that it comprises several stages, whereat a different pressure is used in each stage, it is possible to use the water vapour liberated from a dryer operating at a higher pressure far heating a dryer operating at a lower pressure. Moreover, in a single-stage dryer, the liberated water vapour can be compressed to a higher pressure and be used for heating the dryer itself.

As was already stated above, after the drying, the mixture of the magnetic bed material A and of the dried material B' is passed along the feed duct 31 into the magnetic separator 30. The construction and operation of the magnetic separator 30 are examined in more detail in Fig. 2. In the magnetic separator 30, the magnetic bed material A and the dried material B' are separated from each other. The magnetic separator 30 is provided with separate cutlet ducts 32 and 33 far the dried material B' and far the magnetic bed material A. After the materials have been separated from each other, the magnetic bed material A is passed from the magnetic separator along the outlet duct 33 back into the system into the dryer container 11. The bed material A may be passed into the dryer container 11 through the top portion, as is denoted with reference numeral 33, or the bed material may be passed into the feed duct 15 for the material to be dried, as is shown by reference numeral 33A and by broken lines. Fig. 1 shows schematically that there is only one magnetic separator 30 in the system. In order to improve the degree of separation, the system may, however, be provided with several magnetic separators placed one after the other. In preliminary separation tests, the dried material B' coming cut of the fluidized bed dryer 10 contained 25 % of magnetic bed material A before the magnetic separator 30, and only 2 % after the magnetic separator 30. Fig. 2 is a schematical sectional view of a magnetic separator 30 which can be used in the method of the invention. The magnetic separator 30 comprises a substantially cylindrical mantle 36, into which a feed duct 31 has been formed for the mixture of the magnetic material A and the dried material B'. as well as outlet ducts 32 and 33 for the dried material B' and the magnetic materials A. Inside the cylindrical mantle 36, substantially coaxially with the said mantle 36, a metal cylinder 34 is mounted as revolving, so that a space of a certain magnitude remains between the outer face of the metal cylinder 34 and the inner face of the mantle 36. Inside the rotable metal cylinder 34, at immediate proximity of the inner face of the metal cylinder, a magnet 35 is provided in accordance with Fig. 2, which said magnet 35 extends in accordance with Fig. 2 in the direction of the circumference of the metal cylinder 34, beginning from the feed duct 34, beyond both of the outlet ducts 32 and 33.

When mixture of the magnetic material A and of the dried material B' is fed into the magnetic separator 30, the magnetic material A adheres to the face of the metal cylinder 34 by the effect of the magnet 35, whereas the dried material B runs in the space between the metal cylinder 34 and the mantle 36 of the magnetic separator freely. The outlet ducts 32 and 33 are arranged in the mantle 36 so that the outlet duct 32 far the dried material B' is placed before the outlet duct 32 for the dried material B' is placed before the outlet duct 32 for the magnetic material A, in the direction of rotation of the metal cylinder 34. Thus, the dried material B' can run out of the said outlet duct, whereas the magnetic material A runs, adhering to the metal cylinder 34 by the effect of the magnet 35. beyond the cutlet duct 32 for the dried material and is detached from the metal cylinder 34 only after it has by-passed the area of effect of the magnet 35, being thereupon removed out of the magnetic separator 30 through the outlet duct 33 for the magnetic material.

The construction of the magnetic separator 30 may also be different from that shown in Fig. 2, for the embodiment shown in the figure is just supposed to illustrate the invention. By using a magnetic material and a magnetic separator, for example as compared with the prior-art fluidized bed dryers, the remarkable advantage is obtained that a considerably more finely divided material can be used as the bed material than in prior art. In the prior-art methods and equipments, it has been necessary to keep the particle size at a level of about 1 mm, whereas in the method of the present invention it is possible to use a bed material whose particle size is, e.g., 0.20 to 0.50 mm. Thereby the heat transfer factor between the bed material and the indirect heat exchanger in the dryer becomes considerably better than in the prior-art methods.

Above, the invention has been described with reference to the figures in the attached drawing. The method of the invention is, however, not supposed to be confined to the embodiments shown in the figures alone. Thus, it is an essential feature of the method of the invention that the material B to be dried is mixed with a magnetic material A and that the drying of the material B to be dried is performed in a mixture of the materials A and B. Further, based an the magnetism it is an essential feature of the invention that, after drying, the materials are separated from each other and the magnetic material separated fromthe mixture is returned to the dryer used in the method.

in other respects, reference is made to the accompanying patent claims, whereat the various details of the invention may showvariation within the scope of the inventive idea defined in the said claims.

Claims

1. Method for the drying of a powdery, granular, chip-formed or equivalent material, characterized in that the material (B) to be dried is, at least for the time of the drying, mixed with a magnetic material (A) and that the drying of the material (B) to be dried is carried out in a mixture of the said material (B) to be dried and of the said magnetic material (A) by passing heat into the mixture of the said materials (A) and (B) and that after the drying, the magnetic material (A) and the dried material (B') are separated from each other based an the magnetism by means of one or several magnetic separators (30) or equivalent and that the dried material (B') is passed further to its purpose of use and the magnetic material (A) is recirculated to the mixture of the material (B) to be dried and the magnetic material (A).

2. Method as claimed in claim 1, characterized in that the drying of the material (B) to be dried is performed in a vessel or equivalent so that the heat required for the drying is passed indirectly through a heat exchanger (14) into the mixture of the material (B) to be dried and the magnetic material (A) .

3. Method as claimed in claim 1 or 2, characterized in that the drying is performed by means of the fluidized bed principle so that the mixture of the material (B) to be dried and of the magnetic material (A) is fluidized by the drying gas (C).

4. Method as claimed in claim 3, characterized in that the drying gas (C) used far the fluidization of the said material mixture (A and B) is circulated in the drying process in a closed circuit.

5. Method as claimed in claim 3 or 4, characterized in that superheated water vapour is used as the drying gas (C).

6. Method as claimed in any of the preceding claims, characterized in that the magnetic material (A) used in the drying consists of a finely divided and particle-formed material.

7. Method as claimed in claim 6, characterized in that the particle size of the magnetic material (A) is of an order of 0.20 to 0.50 mm.

8. Method as claimed in any of the preceding claims, characterized in that the magnetic material (A) is magnetite.