The present invention relates to a method for drying a fluid or wet product, comprising the steps of providing a drying chamber, an air inlet, a feed tank, and a product recovery system, and feeding a fluid or wet product from the feed tank into the drying chamber at a predetermined flow rate. It furthermore relates to a drying apparatus suitable for carrying out the method.

Processing of a variety of products includes drying a feed of a more or less fluid consistence. Such feed includes pastes, filter cakes and highly viscous liquids, such as agrochemicals, ceramics, dye- stuffs/pigment, inorganic and organic chemicals, and waste products, e.g. sludge, sediments etc. The drying may be carried out in many ways, including the use of contact dryers, band dryers, drum dryers and tray dryers. All of the above drying process have the disadvantage, however, that they entail a long processing time and the need for post-treatment such as milling in order to obtain an acceptable end product. An alternative process includes the use of a flash dryer or pneumatic dryer, in which the product to be dried is fed to a drying chamber, through which the product is passed once and some of its liquid contents being evaporated and discharged. A further development of the flash dryer is a so- called vortex disintegrator dryer, one example of such a dryer being the SWIRL FLUIDIZER™ (GEA Niro). In a vortex disintegrator dryer, such as the SWIRL FLUIDIZER™, the feed is introduced into a drying chamber from a feed tank provided with a dosing screw, which makes it possible to transfer the feed to the drying chamber at a continuous and controllable rate. The drying chamber is provided with a vertical rotary disintegrator at the lower part of the drying chamber, and the drying chamber above the bottom has an air inlet for introducing hot air in a tangential air flow, thus creating the controllable swirling air flow together with the rotary disintegrator. The impeller thus disintegrates the feed, and the liquid contents of the feed are evaporated while being transported upwards in the drying chamber. The spent drying air containing the dried particles flows to a combined exhaust air cleaning and product recovery system through a product/air outlet placed in the top of the drying chamber. Thus, the vortex disintegrator dryer has the ability to handle even the most viscous fluids, which are not even pumpable without dilu- tion.

However, when drying products having a more fluid consistence, such as fluids having a low content of solid matter (dry matter content), or having a low viscosity for other reasons, problems may be experienced in controlling the process in an optimal manner. Both in re- spect of flash dryers and vortex disintegrator dryers, it has been suggested to mix dry product with the fluid or wet product, i.e. the feed in the feed tank, and agitate the dry product and the feed in the tank in order to back-mix the ingredients into a mixture which is subsequently fed into the drying chamber. This process has shown difficult to control and may possibly require a pre-mixer in order to function properly, both of these factors having a disadvantageous effect on the overall process efficiency. Among other things, the relatively high energy consumption required by the back-mixing may be mentioned as a factor that deteriorates the process efficiency, and the risk of damage to the parts of the drying chamber in case the control of the feed mixture does not work properly.

With this background it is an object of the present invention to provide a method for drying a product, by which it is possible to improve the processing conditions with respect to the requirements to the feed and process efficiency.

In a first aspect of the invention, this and further objects are achieved by a process of the kind mentioned in the introduction, which in addition to the steps of providing a drying chamber, an air inlet, a feed tank, and a product recovery system, and feeding a fluid or wet product from the feed tank into the drying chamber at a predetermined flow rate, includes the additional steps of feeding finished dried product to the drying chamber at a flow rate corresponding to controllable settings, mixing the fluid or wet product with the finished dried product in the drying chamber, drying the mixture of fluid or wet product and fin- ished dried product in a gas containing finished dried product, and recovering the product from the gas by means of the product recovery system.

By allowing part of the finished dried product to be fed back into the drying process, a very accurate control of the processing conditions may be obtained. Furthermore, the method provides for more stable operation as regards for instance temperature variation and fluctuations in the power consumption, both of which are substantially reduced in comparison with the prior art approaches. Without being bound by theory, it is believed that these effects may be due to the fact that the finished dried product, which has well-defined and controllable properties with measurable product parameters, has a stabilising function in the process. It is thus possible to operate the drying apparatus even though the feed is more liquid than would under normal circumstances be possible.

It should be understood that a predetermined flow rate of the feed may mean a flow rate controlled on basis of the drying process parameters, e.g. a flow rate based on the actual outlet temperature of the drying gas. In order to ensure that there is a stable supply of finished dry product to be fed into the drying chamber, at least some of the finished dried product may be passed into an intermediate storage container, and the finished dried product fed from the intermediate storage container.

In principle, the settings may be varied in accordance with any program or algorithm for an individual process. Advantageously, the controllable settings are dependent on the predetermined flow rate of the fluid or wet product.

Preferably, the controllable settings are dependent on parameters of the fluid or wet product, such as the dry matter contents, the viscosity, the consistence etc. This makes it possible to control the drying process very precisely.

Although the feed of finished dry product into the drying chamber may continue throughout the entire drying process, it is also possible to feed a larger amount of finished dried product into the drying chamber during the start-up period. In this case, the controllable settings would be dependent on the time elapsed from the start-up of the opera- tion.

In one embodiment, which provides for particularly good mixing in the drying chamber, the drying chamber is provided with an impeller in the lower section of the drying chamber, whereby the finished dried product is fed into the drying chamber above the impeller.

Alternatively, the finished dried product may be fed into the drying chamber at the level of the impeller.

In the method according to the invention, any suitable transportation means may be utilized for feeding the finished dried product into the drying chamber. In one embodiment, the finished dried product is fed into the drying chamber by means of mechanical transportation means, such as a screw conveyor.

In an alternative embodiment, the finished dried product is fed into the drying chamber by means of pneumatic conveying means.

In order to secure proper operation, in particular during the start-up phase, the intermediate storage container may be filled with finished dried product before start-up.

In a second aspect of the invention, an apparatus for drying is provided.

Further advantages and details will emerge from the following description of examples and embodiments.

In the following, the invention will be described in further detail by means of the drawings, in which

Fig. 1 shows a schematic side view of an apparatus for drying in an embodiment of the invention, which is suitable for carrying out the method for drying according to the present invention;

Fig. 2 shows a schematic side view corresponding to Fig. 1 of another embodiment of the drying apparatus.

Referring now to Fig. 1 of the drawings, a drying apparatus gen- erally designated 1 is shown in a first embodiment of the present invention. The structure of the drying apparatus will be described in conjunction with the inventive method for drying a product. Elements of the drying apparatus may thus be modified, while still being able to carry out the method. The drying apparatus 1 comprises a drying chamber 2 of an elongate shape having an upper end 2a and a lower end 2b. A feed tank 3 containing the feed to be dried is connected to the drying chamber 2 via a conduit 4, and the feed is transferred to the drying chamber 2 by means of a pump 5. The term "pump" is to be construed in its broadest sense, i.e. as a means for transporting a fluid of any consistence. For instance, in case of a paste, it may be a screw conveyor. The conduit 4 is passed into the drying chamber 2 via an inlet 6. At or near the lower end 2b of the drying chamber 2, there is a vertical rotary disintegrator provided with blades or bars, in the following denoted impeller 7, mounted on a shaft 8. Drying air is introduced into the drying chamber 2 by means of an air inlet 9 surrounding a section of the drying chamber at the lower end 2b thereof, the section covering for instance 1/3 of the height of the drying chamber 2. The air inlet 9 is supplied with hot drying air by means of fan 10 and heater 11. At the upper end 2a of the drying chamber, a product/air outlet 12 allows for the discharge of spent drying air containing dried particles. The spent drying air is passed, via a conduit 13, to a combined exhaust air cleaning and product recovery system, which in the embodiment shown comprises a bag filter 14; this could alternatively be a cyclone. In the bag filter 14, dried powdery product is discharged at the bottom end thereof by means of valve 15, whereas exhaust gas is removed at the top and sucked by fan 16 on to further cleaning or directly to the ambience.

In the embodiment shown, finished dried product is thus removed from the bag filter 14 and filled into a bulk container 17 for pack- ing into suitable means, such as bags, and to an intermediate storage container, such as hopper container 18 serving as storage to a mechanical transportation means, such as a screw conveyor 19. The screw conveyor 19 ends in the wall of the drying chamber 2 at an inlet 20 for finished dried product. The flow rate of finished dried product through the screw conveyor 19 and into the finished dried product inlet 20 may be controlled in a suitable manner and varied over time during operation of the drying apparatus 1. The flow rate should be regulated in accordance with controllable settings, and is carried out by means of any suitable control means. The flow rate of the finished dried product relative to the flow rate of the fluid or wet product may be controlled in any suitable manner, for instance in dependence of the dry matter content of the feed, the consistence, the viscosity, or even the parameters of the finished dried product etc.

During operation, liquid and pumpable feed is thus filled into the feed tank 3. The feed tank 3 may in a manner known per se be equipped with a rotating agitator, driven from either the top or the bottom and ensuring a steady feed supply to the pump 5 and thus to the drying chamber 6 at a continuous and controllable rate. Simultantaneously, fin- ished dried product is fed from the hopper container 18 via the screw conveyor 19 and into the drying chamber 2 via inlet 20. In the embodiment shown, the inlet 20 is positioned above the impeller 7, for instance slightly above, but could alternatively be positioned at the level of the impeller 7 upper part or lower part. In the lower portion of the drying chamber 2, the fluid or wet product is thus mixed with the finished dried product into a mixture having a suitable consistence for processing.

The intermediate storage container may be filled with finished dried product before start-up in order to secure proper operation. The finished dried product may either be taken from a previous batch, from another storage container and/or be supplemented or replaced partly or in full by another finished dried product.

In the description of the alternative embodiment of the drying apparatus shown in Fig. 2, elements corresponding to elements of the embodiment of Fig. 1 are denoted by the same reference numerals. Only differences from the embodiment of Fig. 1 will be described in detail.

In Fig. 2 it is shown how the finished dried product is passed from the product recovery system, which may also in this embodiment comprise a bag filter 14, directly to the inlet 20 without the use of an intermediate storage container. Instead, pneumatic conveyor 119 draws the desired flow from the bag filter 14 to the drying chamber 2 in accordance with the controllable settings. The pneumatic conveyor is easily controlled and the flow rate may thus be regulated accurately.

Also in this embodiment, it is possible to provide a supplemental supply of finished dried product, either from the process line itself, or from a different process line, in which e.g. another dry product is produced.

Example

A liquid feed in form of a fluid product containing TiO2 having a dry matter content of 50% was fed from the feed tank into the drying chamber at the feed inlet by pumping the feed at a rate of 5000 kg/h. Finished dried product having a dry matter content of 99% was fed into the drying chamber through the inlet for dried product at a rate of 1000 kg/h. The theoretical mixture present only in the drying chamber thus had a dry matter content of 58.2% and was processed at rate of 6000 kg/h. Whereas the product at 50 % when fed directly into the dryer gave unstable operation and unacceptable deposits inside the drying chamber, the addition of dry powder facilitated the operation, and a smooth and stable drying process was possible.