Abstract:
The recovery of solvents from waste gases takes place according to the state of the art by condensation, freezing out or desublimation of the solvent in recuperative cold reservoirs which can be switched over. In order to reduce the amount of equipment and the cold losses, the condensation and freezing-out operations are run continuously in a shaped-object reservoir through which cooled shaped objects move in a countercurrent to the waste gas.

Description:
BACKGROUND OF THE INVENTION 
     In order to recover solvents from waste gases or else in order to purify waste gases, condenser coolers or recuperative cold reservoirs are used, through which the solvent flows, a process during which the solvents condense out and freeze out. As soon as sufficient solvent has been separated, the flow of waste gas is guided through another group of condenser coolers. The first group is warmed up, and the solvent can be removed as a liquid product. A device of this type is disclosed, for instance, in German preliminary published application no. DE-OS 34 14 246. 
     Even though the processes according to the state of the art function satisfactorily, they require elaborate equipment since a double-installation is necessary to freeze out and to thaw the solvents. Furthermore, the mode of alternating operations calls for high energy consumption. 
     SUMMARY OF THE INVENTION 
     Therefore, the invention is based on the task of creating a process to recover solvents from waste gases or else to purify waste gases, which can be carried out continuously, that is to say, without alternating operations. 
     Accordingly, the inventive idea consists of continuously passing cooled shaped objects, for example, steel spheres, through a shaped-object reservoir. The gas to be purified flows in a countercurrent through the shaped-object reservoir. In this process, the waste gas cools off to such an extent on the shaped objects that the solvent or other vapors to be removed are then separated. The solvent separated in the solid state reaches the lower section of the shaped-object reservoir together with the shaped objects and then it melts due the presence of higher temperatures there. The melted ice drips down together with the resulting condensate and is then carried off to the outside. The warmed up shaped objects are removed from the shaped-object reservoir through a transfer lock and conveyed back to the inlet of the shaped-object reservoir. In this process, they are cooled off in a heat exchanger which can be operated, for example, with liquid nitrogen as the coolant. In a preferred embodiment, prior to cooling off in the heat exchanger, the shaped objects pass through an additional packing in which a flow of cold, purified waste gas passes through these objects and pre-cools them. This packing can also be cooled by means of evaporated nitrogen from the heat exchanger. In another preferred embodiment, the shaped objects removed from the shaped-object reservoir are passed through a dryer before they move to the conveying device. 
     Commonly employed recovery processes entail the disadvantage that, when the solvent is frozen out, the pressure drop in the devices increases steadily, thus giving rise to a non-stationary mode of operation. After a short period of time, the pressure loss is so great that it becomes necessary to switch the waste-gas flow over to a second device while the first one is warmed up and thawed. The cold losses associated with this, the heating energy additionally needed, the complicated equipment and the switching-over all are avoided with the process according to the invention, since freezing up cannot occur as a result of the continuous transportation of the shaped objects from the cold end to the warm end of the shaped-object reservoir. All of the process steps take place simultaneously and continuously, so that devices corresponding to the process according to the invention can be operated in a stationary manner. 
    
    
     THE DRAWINGS 
     FIG. 1 shows a device for carrying out the process in schematic form; and 
     FIG. 2 shows a variant of the device according to FIG. 1, having an additional packing. 
    
    
     DETAILED DESCRIPTION 
     The device shown in FIG. 1 consists of a shaped-object reservoir 1 whose lower section has a connection 2 for the feeding of the waste gases and whose upper section has a connection 3 for the removal of the waste gases. The middle section of the shaped-object reservoir 1 is filled with shaped objects 4 which are steel spheres. The shaped objects 4 lie on a funnel-like perforated plate 5 which ends in a transfer lock 6 which serves for the removal of the shaped objects. Connected to this transfer lock 6, there is a removal tube 7 which leads to a conveying device 8 for the shaped objects. In a corresponding manner, a charging tube 9 leads from the conveying device 8 back to the upper section of the shaped-object reservoir 1. The charging tube 9 is interrupted by a heat exchanger 10. The heat exchanger 10 has a connection 11 which serves to supply liquid nitrogen as the coolant and a connection 12 which serves to remove the gaseous nitrogen which has evaporated. Moreover, in the heat exchanger 10, there is a cooling surface 13 which is linked to the connection 3 and from which a connection 14 leads out of the heat exchanger 10. There is a drain 15 on the bottom of the shaped-object reservoir 1. The arrows (which do not have any position numbers) indicate the direction of flow of the materials. 
     Below, the process for recovering solvents from waste gas is described with reference to the device shown in FIG. 1. 
     Shaped objects 4 leave the conveying device 8 through the charging tube 9 and move to the shaped-object reservoir 1, in which they form a packing which lies on the perforated plate 5. The charging tube 9 is interrupted by a heat exchanger 10 in which the shaped objects 4 are cooled off. The cooling off is done by means of liquid nitrogen which is fed into the heat exchanger 10 through the connection 11 and which is then removed in gaseous form through the connection 12. There is an additional cooling of the shaped objects 4 on the cooling surface 13 which is exposed to cold, purified waste gas from the shaped-object reservoir 1. By means of the transfer lock 6, the shaped objects 4 are fed back to the conveying device 8 through the removal tube 7. The amount of shaped objects 4 removed is regulated by the operation of the transfer lock 6. Therefore, the shaped objects 4 continuously move from the top to the bottom through the shaped-object reservoir 1. 
     The waste gas loaded with solvents passes through the connection 2 into the lower section of the shaped-object reservoir 1. Then the waste-gas flows in a countercurrent with respect to the shaped objects 4 through the shaped-object reservoir 1 towards the top and then leaves this unit through connection 3. In this process, the waste gas cools off on the shaped objects 4 to such an extent that the solvents freeze out. The solvent ice reaches the lower section of the shaped-object reservoir 1 together with the shaped objects and melts, since the temperatures are higher there. The melted ice drips down together with the condensate which has formed in the lower section of the device and is then carried off to the outside through the drain 15. The shaped objects 4 warmed up by the waste-gas flow pass through the transfer lock 6 and through the removal tube 7 and move back to the conveying device 8, while the cold, purified waste gas passes through the connection 3, the cooling surface 13 and the connection 14 on its way out of the device. Naturally, the cold, purified waste gas can also be directly removed through the connection 3 without utilizing its coldness in the heat exchanger 10. 
     FIG. 2 shows a variant of FIG. 1 in which the same reference numbers are employed for the same parts of the installation. The main difference lies in the fact that there is an additional packing 16 of shaped objects 4 which likewise lie on a perforated plate 17 and which can be circulated by means of a transfer lock 18. The packing 16 is integrated into the shaped-object reservoir 1, although it can also be arranged separately. From the packing 16, the shaped objects 4 move through the transfer lock 18 and the removal tube 19 to the heat exchanger 10. Then they pass through the charging tube 20- which corresponds to the charging tube 9 of FIG. 1- and return to the shaped-object reservoir 1. The heat exchanger 10 is likewise exposed to liquid nitrogen which, however, still serves to pre-cool the packing 16 by means of the cooling surface 21 after the nitrogen has been removed through the connection 12 in the gaseous state from the heat exchanger 10. Furthermore, the cold, purified waste gas also passes through the packing 16 before it is removed from the installation through the connection 3. A difference from the embodiment according to FIG. 1 is that the shaped objects removed through the transfer lock 6 and the removal tube 7 also pass through a dryer 22. Purified waste gas or nitrogen can be used for purposes of drying. The drying medium is likewise removed from the dryer 22 through line 23 and fed into the shaped-object reservoir 1, thus making it possible to remove the condensate and solvent components absorbed in the dryer 22. These condensate and solvent components can also be removed separately in an additional condenser. 
     The shaped objects can consist, for instance, of ceramic or glass. They can also be hollow and contain a filling which stores coldness or they can be coated.