EP0576053B1 - Process for drying aqueous solids in a fluidized bed - Google Patents

Description

The invention relates to a method for drying a water-containing solid in a fluidized bed, which is indirectly heated by at least one heat exchanger device, thereby withdrawing vapors containing water vapor, which contain impurities, and passing part of the vapors as a fluidizing medium through the fluidized bed, the The rest or a further part of the vapors is cooled to form vapor condensate containing the impurities. The solid to be dried can e.g. are coal, lignite, peat, a water-based waste or sludge.

A method of this type is described in German Patent 29 01 723 and in the corresponding US Patent 4,295,281, in German Patent 36 44 806 and in German Patent Application 39 43 366. In this process, a vapor condensate is inevitably formed which contains certain impurities, so that the condensate cannot usually be passed directly into the sewer drainage system.

The invention has for its object to sufficiently clean the vapor condensate formed in the process mentioned at the beginning in a simple and effective manner. According to the invention, this is done by bringing at least part of the vapor condensate in a cleaning zone into direct contact with gaseous or vaporous stripping medium and subtracting partially cleaned condensate and the stripping medium containing impurities from the cleaning zone.

The inventive method of cleaning the vapor condensate can be designed in various ways. It is particularly useful to use water vapor as the stripping medium. It can be recommended that Guide the condensate into the cleaning zone at a temperature that is 0 to 10 ° C below the boiling temperature. As a result, the amount of water vapor used as the stripping medium can be kept low, for example in the range from 1 to 10% by weight of the amount of condensate.

In order to keep the vapor condensate near the boiling point when it enters the cleaning zone, it may also be expedient to relax the hot condensate into the cleaning zone. The boiling temperature corresponding to the relaxation pressure is set automatically. This leads to partial evaporation and thus to partial stripping of the condensate.

In a further process variant, the stripping medium, water vapor, is generated by boiling and evaporating vapor condensate in the cleaning zone itself.

The vapors can be condensed under overpressure (e.g. 1.5 to 10 bar) in the drying process itself, as described in German Patent 36 44 806. However, it is also possible to remove the vapor condensate to be cleaned outside the drying process at about atmospheric pressure (i.e. about the pressure at which the vapors leave the fluidized bed) or else at a pressure of 0.01 to 0.5 bar, e.g. after relaxation in a condensation turbine.

• Fig. 1 das Fließschema einer ersten Verfahrensvariante,
• Fig. 2 das Fließschema einer zweiten Verfahrensvariante und
• Fig. 3 eine Abwandlung des Verfahrens der Fig. 2.

Design options of the method are explained with the aid of the drawing. It shows

• 1 shows the flow diagram of a first method variant,
• Fig. 2 shows the flow diagram of a second process variant and
• 3 shows a modification of the method of FIG. 2.

1, the water-containing solids to be dried are fed through line (1) to a reactor (2) in which a fluidized bed (3) is located. A heat exchanger device (4) through which a heating medium flows is arranged in the region of the fluidized bed (3). Eddy medium exits through pipes (5) which form a nozzle grate. The fluidizing medium which is introduced in line (6) is part of the water vapor-rich vapors which are formed in the fluidized bed (3) when the solid dries.

Vapors containing solids leave the fluidized bed (3) through the channel (9) and first reach a dedusting device (10), e.g. an electrostatic filter or a bag filter. The solids separated out are led back through line (11) into the fluidized bed (3) or removed through line (11a). The largely dedusted vapors leave the dedusting device (10) through line (12) and are divided between lines (13) and (14). The vapors of line (13) are led back through the blower (15) and line (6) as fluidizing medium into the reactor (2).

For the remaining vapors in line (14), it makes sense to use their heat content. In the process of FIG. 1, these vapors are passed through line (17) to a compressor (16), which is preferably designed in several stages. By injecting water through line (20), saturated steam conditions are set in the compressed vapors of line (21). These compressed vapors serve as the heating medium and are fed through the line (21) to the heat exchanger device (4), the vapors at least partially condensing when flowing through the heat exchanger device. The heat of condensation released serves as an effective energy source for indirect heating of the fluidized bed (3). Largely dried solid material slides down between the tubes (5) into the collecting chamber (2a) of the reactor (2) and is drawn off by the metering device (23).

The vapor condensate leaving the heat exchanger device (4) in line (25) contains various types of impurities. In order to purify this condensate at least partially, it is placed in a stripping column (26), which e.g. Contains bottoms or packing. A gaseous or vaporous stripping medium is fed into the lower region of the column (26) through the line (27). Steam is recommended as the stripping medium if the condensate fed through the line (25) into the column (26) has a temperature which is 0 to 10 ° C. and preferably at most 5 ° C. below the boiling temperature. As a result, only a little of the water vapor used as stripping medium is consumed in the column (26) in that this water vapor condenses in the column (26).

Instead of using water vapor as a stripping medium from an external source, the water vapor can also be generated by indirectly heating the boiling of the vapor condensate itself. Boiling and evaporating vapor condensate can e.g. take place in the lower region of the stripping column (26) or outside.

Largely cleaned condensate leaves the column (26) in the line (28) and can, for example, be led into a receiving water. The stripping medium containing impurities is drawn off at the top of the column (26) through line (29) and disposed of, for example by thermal treatment, in particular in an incineration plant. Another possibility is to pass the stripping medium drawn off in line (29) over activated carbon or activated coke for cleaning. If steam is used as the stripping medium, the cleaned water vapor can be dried continue to use, e.g. as a vortex medium. Alternatively, the contaminated stripping medium can also be condensed and the condensate can be disposed of by distributing it on the dried solid material.

In Fig. 2 the reactor (2) with the fluidized bed (3) and the heat exchanger device (4) is again shown to dry the water-containing solids introduced in the line (1). Low-water solids are drawn off in line (24). Insofar as the same reference numbers are used, the explanations already given together with FIG. 1 also apply here. The heating medium that is fed to the heat exchanger device through line (30) is now not compressed vapors, but e.g. Foreign steam or thermal oil. Dedusted vapors are led through line (31) to an expansion turbine (32), which is followed by a condenser (33). The turbine (32) is preferably used to generate electrical energy. The vapor condensate coming from the condenser (33) passes through the line (34) into the stripping column (26), to which stripping medium, in particular water vapor, is fed through the line (27a). This stripping medium can be a partial flow of the dedusted vapors. At least partially cleaned condensate leaves the column (26) in the line (28) and the stripping medium containing impurities is drawn off in the line (29) via a vacuum pump (37).

Fig. 3 shows a modification of the method of Fig. 2, wherein the column (26) is operated at approximately atmospheric pressure. The dedusted vapors from lines (12) and (31) are led to a condensation device (36) according to FIG. 3, the heat of condensation being released being used as desired. The vapor condensate formed passes through line (34) for purification into the stripping column (26), to which stripping medium, for example water vapor, is fed through line (27).

example

In a fluidized bed dryer (2) shown in FIG. 2, 100 t / h of lignite with grain sizes approximately below 8 mm and with a raw moisture content of 62.3% by weight are fed through line (1). 73.5 t / h of saturated steam with a temperature of 160 ° C. and a pressure of 6 bar serve as the heating medium, which is passed through line (30) into the heat exchanger device (4), where the water vapor condenses. The heat required for drying is transferred to the fluidized bed (3), which is heated to approximately 105 ° C.

43.8 t / h of dry lignite with a residual moisture content of 14% by weight and 155.6 t / h of vapors are withdrawn from the dryer through line (24) through channel (9). After they have been dedusted in the electrostatic filter (10), 99.4 t per hour are returned as part of the vapors via line (13), blower (15) and line (6) to the fluidized bed dryer as vortex steam. The remaining 56.2 t / h vapors are, as shown in FIG. 3, fed through line (31) to a condensation device (36), where the vapors are condensed at about 100 ° C., giving off their heat of condensation.

The chemical oxygen demand (COD) of the vapor condensate in line (34) is 110 mg O₂ / l. By stripping with 1 t / h low-pressure saturated steam from line (27), the vapor condensate in the stripping column (26), which has 12 trays, is cleaned to a COD of 50 mg O₂ / l, so that it can be placed in a receiving water can. Via line (29), 1 t / h of contaminated stripping steam is withdrawn from the column with a COD of 3.37 g of O₂ / l and burned.

Claims (10)

1. A process for drying a solid which contains water in a fluidised bed which is indirectly heated by at least one heat exchanger device, wherein vapours which are rich in water vapour and which contain impurities are taken off from the fluidised bed and part of the vapours is passed as a fluidising medium through the fluidised bed, and the remainder or a further part of the vapours is cooled with the formation of a vapour condensate containing the impurities, characterised in that at least part of the vapour condensate is brought into direct contact with a stripping medium in gaseous or vapour form in a purification zone, and partially purified condensate and stripping medium which contains the impurities is taken off from the purification zone.
2. A process according to claim 1, characterised in that water vapour is used as the stripping medium in the purification zone.
3. A process according to claim 1 or 2, characterised in that the stripping medium is produced in the purification zone by boiling and evaporating vapour condensate.
4. A process according to any one of claims 1 to 3, characterised in that the vapour condensate is led into the purification zone at a temperature which is 0 to 10°C below the temperature of ebullition.
5. A process according to any one of claims 2 to 4, characterised in that the amount of water vapour which is led into the purification zone corresponds to 1 to 20 % by weight of the amount of condensate.
6. A process according to any one of claims 1 to 5, characterised in that the vapour condensate is flashed into the purification zone.
7. A process according to any one of claims 1 to 6, characterised in that the stripping medium which contains impurities and which is taken off from the purification zone is subjected to a thermal treatment.
8. A process according to any one of claims 1 to 6, characterised in that the stripping medium which contains impurities and which is taken off from the purification zone is purified over activated carbon or activated coke.
9. A process according to any one of claims 1 to 6, characterised in that the stripping medium which contains impurities and which is taken off from the purification zone is condensed, and the condensate which is thus formed is admixed with the dried solid.
10. A process according to claim 8, characterised in that the stripping medium which is purified over activated carbon or activated coke is water vapour which is fed into the drying process.

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