Stationary reactor with pulsating bed The present invention relates to a method and an apparatus (hereinafter referred to as of the kind described) for the continuous treatment of particulate material wherein the particulate material is treated in a suspended bed in a stationary reactor, gas is injected into the reactor, in order to maintain the suspended bed, through at least one central gas inlet at the bottom of the reactor, during operation the bed pulsates up and down in the reactor, the cross sectional area of the reactor in at least the lower part of the reactor is reduced downwardly towards the gas inlet, assuming, for example, the form of a truncated cone, and the treated material is discharged from the reactor via the gas inlet at the bottom of the reactor A method and an apparatus of this kind is known from our WO 97/28408. In that publication is described a method and an apparatus for the continuous treatment of particulate material, where a suspended bed pulsates in controlled manner upwardly and downwardly in a reactor at such an amplitude that a quantity of the bed material will be precipitated when the bed is situated in its lowermost position, causing the material to drop through the gas stream and to be discharged from the reactor through the gas inlet at the bottom. Generally, the material discharge rate will increase with the weight of the suspended bed, hence stabilizing the operating conditions since an increased feed rate of material to the bed will cause the weight of the bed to be increased until the material discharge rate has grown to such an extent that it corresponds to the feed rate.

However, it has turned out that when the content of the bed exceeds a certain critical weight, which is a characteristic feature of the single reactor, the achievable discharge rate will be at a lower level than the feed rate of fresh material, and this will either result in that a desired rate of production can not be achieved or that the operation becomes unstable.

It is the object of the present invention to provide a method and an apparatus for treating particulate material in a stationary reactor, whereby it will be possible to maintain at all times a desired volume of material in the reactor, while, simultaneously, the production rate of the single reactor is increased.

This object is achieved by a method of the kind described which is characterized in that gas is injected through the walls of the reactor in the lower part of the reactor in the area where the cross sectional area of the reactor is reduced.

The gas being injected may advantageously be a process gas, which is taken to mean a gas which takes part of the actual treatment of the particulate material in the reactor. The injected gas may also be a neutral gas which does not take part of the actual process which is taking place in the reactor.

Depending upon the process parameters and requirements, a maximum of 15%, or 10%, or 5% by volume of the total amount of gas which is fed to the reactor is fed through the walls of the reactor at the lower part of the reactor where the cross sectional area of the reactor is reduced.

The invention also includes an apparatus for carrying out the method according to the invention, the apparatus comprising a stationary reactor which consists of a vertical, substantially cylindrical vessel having at least in the lower part a downwardly decreasing cross sectional area, and the bottom of which is open for simultaneous passage of both gas to the reactor and discharged material from the reactor, the reactor being also provided with one or several inlets for particulate material, characterised in that means are fitted for injecting gas through the walls of the lower part of the reactor where the reactor has a decreasing cross sectional area.

The invention will now be described in further detail with reference to the accompanying drawing, which is

diagrammatical, and shows a plant for nodulization and/or heat treatment of a particulate material.

The plant comprises a reactor with an upper cylindrical part 1 and a lower frustoconical part 2. The reactor is fed with process gas through the bottom via a gas inlet 3 and exhaust gases are discharged through the upper part 1 of the reactor and vented to a separation cyclone 5. The gas inlet 3 also serves as a material outlet. In the separation cyclone 5 larger particles are discharged through the outlet 7 at the bottom of the cyclone whereas smaller particles and exhaust gases are integrated into the process stream or passed on for subsequent treatment via the gas outlet 6. The larger particles from the separation cyclone may advantageously be returned to the reactor via a material inlet 4. Through connections 8, which are located in the lower part 2 of the reactor, a small amount (relative to the total amount of gas which is fed to the process in the reactor) of process gas or neutral gas is added to the process.

During operation of the reactor, fresh feed is supplied via the material inlet 4. The material is directed into the stationary reactor 1,2 and encounters the process gas which subjects the material to swirling action upwardly in the reactor. Subject to the regulation of a number of different operating parameters, such as the freeboard volume, the gas velocity through the reactor, the material feed rate or the temperature in the reactor or the gas inlet, it will be possible for a trained operator to cause the suspended bed of material to pulsate in a controlled manner. It is not within the scope of this invention to indicate the general guidelines for controlling the operation in a stationary reactor with a pulsating bed.

Such procedures are assumed to be known.

The treated material is discharged through the gas inlet 3, while the gas subjecting the particulate material to swirling action in the reactor 1,2 and carrying the finest material portion out of the reactor 1,2 to the

separation cyclone 5. In the separation cyclone 5, the entrained material portion is separated into a coarse fraction and a fine fraction. The coarse fraction may be returned via the duct 7 to the reactor 1,2 whereas the fine fraction is passed on for subsequent treatment, for example in a dust collector, or is passed on to another process step, for example for heating or drying of material in case the reactor has been used for heat treatment.

If the reactor 1,2 is used for heat treatment, the ducts 8 may be burners, in which case the gas being added will be process gas, typically atmospheric air.

If the reactor 1,2 is used as a nodulizer, which involves pelletization of the treated material, the ducts 8 constitute merely gas nozzles. Again, the gas may advantageously be atmospheric air, which, in this case, will be operating as a neutral gas.

In certain instances, it would be desirable to be able to return a material fraction to the reactor. This may be due to the fact that this material fraction has not had the intended time of retention in the reactor. Therefore, by carrying out the method according to the invention it will be possible to separate the material discharged from the reactor via the gas inlet 3, and to return the desired fraction of the separated material to the reactor.