PROCEDURE AND DEVICE FOR DRYING A GRANULATE
The invention relates to a process for drying a granulate in at least two drying vessels by means of heated dryer air, which circulates through the drying vessels mounted in parallel, as well as through a dryer of the drying air itself, and which is heated when it passes through a heating device, the flow of drying air fed to each drying container being strangled to a greater or lesser degree, depending on a temperature that reflects the momentary conditions prevailing in the drying container in question. Such a procedure is known from the
German Report DE 31 31 471 C2. In this case indicated in said report three drying vessels are provided, mounted in parallel, in whose entrance constituted in the form of branch, is incorporated in each case a valve is tranguladota, which is located in front of the drying container, and the which is driven by an adjustment motor depending on the signal received from a temperature sensor, which is located inside the outlet, also in the form of a branch, in front of the drying container. The drying air dryer has adsorption cells, through which circulated drying air as well as some regulating agent (such as hot air) are conducted, and an alternating system. At the outlet of the drying air dryer, the drying air is passed through the single heating device before it enters the pre-entry line for the drying vessels. This known equipment generally works with satisfactory results, but on the other hand it has the disadvantage that all the drying vessels must be fed with a drying air of the same temperature. This system is not very effective when different materials must be dried in the different drying vessels, which due to their different sensitivity to temperature must be dried at different temperatures. Furthermore, it can be said that in the known equipment, comparatively long lines must be isolated between the heating device and the drying vessels in order to avoid excesses in the heat losses. Now, if it were decided to overcome the aforementioned drawbacks by means of a system of placing a heating device of its own, instead of a common heating device for each drying vessel, then it must be taken into account that the distribution of the different amounts of air between the different drying vessels is regulated by the degree of throttling of the drying air streams, fed to the different drying vessels. This can cause in any specific case that one of the heating devices is over-stressed or stressed, that is to say that it suffers a high flow rate of drying air so that there would no longer be the expected heating temperature. of drying air. In order to be able to maintain the temperature theoretically required in all operational states, two or more heating devices with an exaggerated power must therefore be placed, which would constitute a disadvantage. Accordingly, it is the task of the present invention to improve the known method in such a way that, for the different drying vessels, a drying air can be made available at a temperature of a different level, as long as the prescribed or theoretical temperatures of the different streams of drying air can be reached under all operating conditions without the use of heating devices with an excess of power for this purpose. This problem is solved in a method of the kind mentioned at the beginning, and in accordance with the present invention, by the system in which each drying air stream fed to a drying vessel is heated with an individual heating apparatus and that the currents of drying air can not only be controlled depending on the temperature but also depending on each heating performance of the corresponding individual heating deviceIn other words, when a pre-established heating power of a specific heating device is reached, the drying air flow is also throttled in question, so that its heating to a prescribed temperature previously chosen is guaranteed independently of the distribution of air. the amounts of drying air between the different drying air streams. The assembly of these individual heating devices create the possibility of heating the drying air streams that are fed to the different drying vessels at different temperatures, according to the requirements in each case. Should the capacity of an individual heating device be exceeded, since the drying air current in question due to a strong strangulation of the other drying air streams grew strongly and / or for the drying air flow in question previously a comparatively high prescribed temperature existed, then the drying air stream in question undergoes a throttling until the ratio of the flow rate of the drying air to the available heating power again is able to reach the prescribed temperature. This will therefore make it possible to dispense with individual, oversized heating devices, without the danger that under extreme operating conditions in one of the drying vessels the prescribed temperature of the drying air will no longer be reached. The process according to the present invention is carried out efficiently, if used, individual heating devices that can be connected and disconnected automatically according to the needs, in such a way that the additional strangulation of a drying air stream, depending on the the heating power starts at the moment when the individual heating device "in question" is not disconnected within a pre-established period of time The permanent operation of the individual heating device, established during said period, is evaluated here In this case, the heating power is not sufficient to reach the prescribed temperature in the case of a certain momentary flow rate of the drying air The invention also relates to a device used to feed the method according to the invention with a minimum of two containers. of drying, a drying air dryer and a device itivo heater, devices that are incorporated in a drying circuit for the drying air, which has a bellows, and has a previous entry line and a return line as well as branches that connect to the previous lines and that serve for the Parallel arrangement of the drying vessels, a choke valve being incorporated in each branch inlet for the drying container, with each drying container being assigned a temperature sensor, which is connected to a signal transducer line via a signal transmission line. control device used to adjust the throttle valve. Such a device is also known from the German Report DE 31 31 471 C2. This equipment also suffers correspondingly from the disadvantages indicated previously. In order to solve these drawbacks, the known device according to the present invention is improved because on each input branch to a drying vessel an individual heating device is mounted and because each individual heating device is connected through a signal line referring to the heating power to the controlling device for the throttle valve associated with the same drying vessel. With the device configured in this way it is possible to operate the method according to the present invention, as described above, thus achieving the corresponding advantages at the same time. In an advantageous configuration of the device, the individual heating device is mounted inside each input branch, behind the throttle valve. Thus, it is possible to place the individual heating device close to the corresponding drying vessel, which reduces heat losses or, where appropriate, decreases the expense for the taking of insulation measures. At the same time the throttle valve is less loaded, as far as thermal work is concerned.
In another efficient configuration, the temperature sensors are incorporated in each case in the corresponding drying vessels. In this way you can achieve perfect temperature values to control the equipment. An exemplary embodiment of a device according to the invention is illustrated in greater detail in the following, with the help of a schematic drawing. The single figure basically shows, in a top view, a device having two drying containers, illustrated in a side view and with a drying air dryer. In the drawing, a regeneration circuit 8 as well as a drying circuit 9 are illustrated, which are combined in a drying air dryer 10. In the regeneration circuit 8, as seen in the current direction, they are assembled successively a suction filter 1, a bellows or regeneration fan 2, a regeneration heating 3, as well as the two direction change valves 4a, 4b, between which are located two absorption cells 5a, 5b. The two direction change valves 4a, 4b as well as the two adsorption cells 5a, 5b are also assigned in the same way to the drying circuit 9 so that by a change of direction in the direction change valves 4a, 4b, the two adsorption cells 5a, 5b are activated alternately within the regeneration circuit 8 or respectively within the drying circuit 9. The drying circuit 9 is provided with a bellows 6 for the drying air, and of a filter 7 for the return air. The drying circuit 9 belongs to an inlet line 11 and a return line 12 for the recirculated drying air as well as a first branch with a branch inlet 21 and a branch outlet 28 with a second branch having a branch entry 31 and a branch outlet 38, which are connected in each case to the entry line 11 or, if applicable, to the return line 12, and then establish an interconnection passing through a first drying container 20 or in its case a second drying container 30. The two drying containers 20 and 30 serve for the drying of the granular material, that is the granulate, which is fed and discharged in a known manner, so that further explanations the particular is not they judge necessary. The two drying containers 20, 30 are configured in the same way, as is clear from the drawing, and also the corresponding parts of the devices are configured in the same way. Thus each branch entry 21, 31 carries a throttle valve 22a, 32a and an individual heater device 23a, 33a. The heating power is regulated in such a way that the drying air fed to the drying containers 20, 30 has a prescribed temperature, if any, in the granulate subject to drying. For this purpose, a temperature sensor 23e, 33e is mounted on the branch inlet 21, 31 between the individual heating device 23a, 33a and the drying container 20, 30; this sensor is connected through a signal transmitting line 23d, 33d to a control device 23c, 33c, which handles the supply of heat through the heating line 23b, 33b, to the specific heating device 23a, 33a. The throttle valve 22a, 32a makes it possible to adapt the air flow to the granulate flow passing through the drying containers 20, 30, to keep it under optimum drying conditions (drying temperatures). For this purpose, a temperature sensor in each case is incorporated in the containers 20, 30: 22e, 32e, which through a signal transmitting line 22d, 32d informs a control device 22c, 32c about the temperature in the container in question. This control apparatus generates an adjustment signal for the throttle valve 22a, 32a, which signal is transmitted to the valve through a control line 22b, 32b. The control device 23c, 33c for the heating power is connected via a signaling line 24, 34 to the control device 22c, 32c for the throttle valve 22a, 33a and supplies the data on the heating power thereto. The illustrated and described equipment works in the following way: The granulate that has been filled in the drying containers 20, 30, being able to treat different materials with different properties here, it is dried, the drying air recirculated through the drying circuit 9 through the bellows or fan 6 and which is dehumidified, for example, by the adsorption cell 5b, which occurs continuously. During this operative phase, the adsorbent cell 5a is placed inside the regeneration circuit 8 and regenerates accordingly. After a predetermined operating time and in particular in a timely manner before reaching a complete depletion of the adsorption cell 5b, the direction change valves 4a, 4b are changed, so that the newly generated adsorption cell 5a is placed in the circuit of drying 9 and those more or less exhausted adsorption cells 5b are within the regeneration circuit 8, to now undergo a regeneration. This change of placement or alternation between the adsorption cells 5a and 5b continues correspondingly. During the drying system, the drying air fed to the drying containers 20, 30 is heated individually to the individual heating devices 23b, 33b, until reaching the prescribed temperature, established for each case. Depending on the value of the temperature set by the temperature sensors 22e, 32e, it is adjusted through the control apparatus 22c, 32c, and the throttle valve 22a, 32a, until an optimum drying air flow rate is achieved inside the container. corresponding drying 20 or 30. However, in view of the fact that closing a throttle valve 22a or 32a not only decreases the flow rate of the drying air passing through the associated drying vessel 20 or 30, but also leads to an increase of the flow rate of the drying air passing through the other branch with the throttle valve 22a or 32a and through the corresponding drying vessel 30 or 20, an excess in the drying air flow rate can occur in this case, in which case the corresponding individual heating device 32a or 23a, due to its limited heating capacity is no longer able to reach a heating up to the prescribed temperature. Such a condition characterized by the permanent operation (full load) of the individual heating device 23a, 33a, is reported to the control apparatus 22c, 32c through the signal transmitting line on the heating power 24, 34, which causes additional strangulation of the drying air within the overloaded branch so that the temperature of the drying air is again increased to the prescribed level. In the foregoing, it has been pointed out that recirculation of air takes place through the new drying circuit. This should not be interpreted as limiting, since other gases, such as nitrogen, can also be used instead of air inside the drying circuit 9 to be recirculated through the drying vessels.