Method and System for Regenerating Dehumidifier for Use in Charcoal Adsorber

This invention relates to a charcoal adsorber system for selectively holding up such fission gases as xenon and krypton included in off-gas discharged from a nuclear reactor plant. The present invention especially relates to a method and system for regenerating dehumidifiers, which dehumidifiers are provided in the charcoal adsorber system in order to dehumidify the off-gas to be treated.

During operation of a nuclear reactor plant, many kinds of materials are formed through nuclear fission processes. Among those materials are xenon and krypton, which are non-condensable radioactive noble gases, so that before discharging the off-gas to the atmosphere, it is necessary to recover and store the off-gas, such as in a charcoal adsorber system, until the radioactivity of the off-gas decreases to a certain degree so that it will not cause impermissible air contamination. By the charcoal adsorber system, radioactive noble gases are selectively kept for a certain period of time in a charcoal bed until radioactivity is effectively decayed. Meanwhile, between the relative humidity of the off-gas to be treated and the moisture content of the charcoal, there exists a certain relationship and to attain a higher adsorption efficiency of the charcoal, it is required to maintain the off-gas to be passed through the charcoal in a dry state. Usually, moisture in the off-gas to be treated is removed beforehand by passing the off-gas through a dehumidifier which includes a molecular sieve or some other desiccant. However, because of the fact that the dehumidifying ability of the dehumidifier also decreases along with the amount of moisture absorbed in the desiccant, periodic regeneration of the dehumidifier is important. In order to accomplish the regeneration of the dehumidifier, it is known to use a part of the off-gas, which yet includes radioactive noble gases, as regeneration gas. That is, according to the examined Japanese patent application, Publication No. 48-30120, published Sept. 17, 1973, a part of the off-gas which is untreated and includes radioactive noble gas, is introduced to a dehumidifier in a heated state. The heated off-gas removes moisture from the desiccant, which moisture is then separated from this off-gas by some cooling method. The off-gas employed for regeneration is recovered and returned to the charcoal adsorber system for further treatment. Normally, several hundred Nm.sup.3 /hour (normal cubic meters/hour) of regeneration gas is required to complete the regeneration within a desired period of time. However, when the amount of off-gas, which is usually small (about 1 to 10 Nm.sup.3 /h) and variable because the amount is determined by the amount of air captured by leakage in a main condenser of the nuclear plant, becomes smaller than usual, the amount of off-gas for the regeneration becomes insufficient to complete the regeneration of the dehumidifier within a desired period of time. Also, there is another drawback in the above-described regeneration system in that further contamination in the regeneration system can be expected because the radioactive off-gas is employed for the regeneration.

An object of the invention is to provide a method and system for regenerating a dehumidifier for use in a charcoal adsorber system in which the regeneration can be performed in a shorter period of time and independently of the amount of off-gas produced.

Another object of the invention is to provide a method and system for regenerating a dehumidifier for use in a charcoal adsorber system in which the regeneration can be performed without causing any radioactive contamination in the regenerating system.

According to preferred embodiments of the present invention, a method and system for regenerating a dehumidifier for use in a charcoal adsorber system is provided in which the dehumidifier which absorbs moisture from off-gas to be treated in a charcoal adsorber is purged of radioactive materials by fresh purge air and thereafter regenerated by fresh regeneration air. The fresh purge air which may be contaminated by radioactive materials within the dehumidifier is treated in the charcoal adsorber so that contamination of the dehumidifier by radioactive material during regeneration is advantageously prevented. Further, regeneration by fresh regeneration air advantageously reduces the time required for the regeneration as compared with regeneration performed by off-gas. The regeneration of the dehumidifier by fresh regeneration air is preferably performed by circulating the fresh regeneration air within the dehumidifier while heating the fresh regeneration air and separating moisture from the fresh regeneration air. The circulation of the fresh regeneration air advantageously eliminates the unnecessary increase of the amount of off-gas which should be treated in the charcoal adsorber system.

According to other preferred embodiments of the present invention, the pressure of the fresh regeneration air is controlled to be a desirable amount so that pressure changes will not cause leakage of the removed water or the fresh regeneration air during the regeneration process.

These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.

FIG. 1 is a schematic flow diagram showing a charcoal adsorber system according to a first preferred embodiment of the present invention;

FIG. 2 is a schematic flow diagram showing a charcoal adsorber system according to a second preferred embodiment of the present invention; and

FIG. 3 is a schematic flow diagram showing a charcoal adsorber system according to another preferred embodiment of the present invention.

Hereafter, a charcoal adsorber system will be described with reference to FIG. 1, in which main components of the system are a dehumidifier 20, a regeneration system 30 of the dehumidifier 20, and a charcoal adsorber 40.

Off-gas produced as exhaust gas from a nuclear reactor (not shown) is introduced into a cooler condenser 11 through a valve 101 and line 102. Cooling medium is circulated in the cooler condenser 11 and the off-gas introduced into the cooler condenser 11 is cooled down by the cooler condenser 11 to about 6.degree. C. and air dried prior to its passage to the following dehumidifier 20. The temperature in the cooler condenser 11 is maintained higher than 0.degree. C. so that moisture in the off-gas will not freeze in the condenser. Moisture removed partly in the cooler condenser 11 is exhausted as waste water through line 103 for further anti-pollution treatment. The air-dried off-gas is introduced into a dehumidifying stage 20 through a line 104. As explained above, periodic regeneration of the dehumidifier is necessary to obtain effective dehumidifying ability. In this embodiment, a pair of dehumidifiers 21 and 22 are employed to enhance effective use of the dehumidifiers while under regeneration for one of the dehumidifiers. As apparent from the drawing, each of the dehumidifiers is so arranged with switch valves 105 and 106 that they may be used alternatively as one actively dehumidifying and another regenerating for the next use. In this FIG. 1 illustration, the switch valves 105 and 106 are so arranged that the off-gas from the cooler condenser 11 is supplied only to the dehumidifier 21 through the line 107 and exhausted to the next step of the charcoal adsorber 40 through lines 108 and 109. In the dehumidifier 21 (and 22 when in a dehumidifying phase), the air-dried off-gas from the cooler condenser 11 is further dehumidified to a dew point of -30.degree. C. The dehumidifiers 21 and 22 are packed with solid moisture adsorber such as molecular sieve, active alumina or some other desiccant, which is adopted as a means of removing humidity, with a view to enhancing the hold up performance of the following charcoal adsorbing stage. The dehumidifier 22 is shown in FIG. 1 as connected to the regenerating system 30, later explained.

After being dehumidified, the off-gas is introduced into the charcoal absorbers 40, which includes a plurality of charcoal beds, such as beds 41, 42, and 43 connected in series by lines 110 and 111. Adsorption of specific gases by charcoal beds is a physical adsorption phenomenon dependent on their boiling points. The higher the boiling point, the more readily the gas is adsorbed. When the off-gas is passed through a charcoal bed, low-boiling air components such as nitrogen (boiling point -195.8.degree. C.) and oxygen (-182.9.degree. C.), do not undergo adsorption, but high-boiling xenon (-108.1.degree. C.) and krypton (-153.2.degree. C.) are adsorbed reversely on the surface of charcoal, moving slowly along the off-gas stream while repeating adsorption and desorption in the charcoal beds. By holding up or storing radioactive rare gases in the charcoal beds, their radioactivity is effectively decayed until they are discharged to the atmosphere. Before the off-gas is discharged, in order to remove the radioactive aerosol which has passed through the charcoal adsorber 40, the off-gas exhausted through line 112 is passed through a filter 50 further to reduce the radioactivity of the off-gas to a safety degree.

To avoid the outleakage of radioactive off-gas from the charcoal system, a negative pressure is maintained throughout the system. Therefore, the off-gas completing the treatment is discharged out to the atmosphere by the ejector 60. The ejector 60 is provided between lines 113 and 114. Compressed air is introduced through a line 115 to the ejector 60 and the off-gas is discharged through the line 114 and a stack 70 to the atmosphere with a stream of the compressed air without causing any positive pressure in the entire system.

Referring now to the regeneration system 30 for the dehumidifier 22 (or 21), the system is schematically depicted by dotted lines in FIG. 1 and includes a cooler condenser 31, a blower 32 and a heater 33. Before starting operation of the regeneration system, the dehumidifier 22 to be regenerated is purged by employing fresh air to blow away radioactive gases and particles which may cause contamination in the regeneration system during the regeneration process. The purge air is introduced by opening valves 116 and 117 through lines 118, 119, and 120, because the pressure in the dehumidifier 22 is kept negative pressure. The purge air blows away radioactive material in the dehumidifier 22 and then is discharged into the charcoal adsorber 40 through the lines 121, 122, 123 and 109. In this instance, valves 124 and 125 are closed. The air purge is effective to reduce the possibility of contamination in the regeneration system by radioactive materials. Without such air purge, radioactive materials would be spread over the regeneration system and the possibility of same leaking out to the atmosphere during the regeneration process, hereafter disclosed in detail, would increase.

In alternative preferred embodiments, the purge air which has been blown through the dehumidifier 22 is sent to either the cooler condenser 10 (depicted by dash-dot line 123A in FIG. 1) or the dehumidifier 21 (depicted by dash-dot line 123B in FIG. 1) in order to desiccate the purge air before it is treated in the charcoal adsorber 40.

Although the amount of the fresh purge air spent for the air purge, which is about 10 Nm.sup.3 /hour (normal cubic meters/hour).times.3 hours, may cause a temporary increase in the amount of off-gas to be treated in the charcoal adsorber, it is negligible as compared with the capacity of the charcoal adsorber (capacity of practical embodiment of the charcoal adsorber is 1 to 10 Nm.sup.3 /hour.times.30 days). After finishing the air purge, the valve positions are reversed, with valves 116 and 117 being closed and valves 124 and 125 being opened, so that fresh air may be maintained in a closed loop comprising the dehumidifier 22, lines 120, 119, cooler condenser 31, line 126, blower 32, line 127, heater 33, lines 122 and 121. The fresh air in the closed loop is then circulated within the closed loop as fresh regeneration air by means of the blower 32. The regeneration air supplied to the dehumidifier 22 is heated by the heater 33, for example, to about 320.degree. C., so that the temperature at the entrance of the dehumidifier 22 will be about 250.degree. C. in view of heat loss in the conduit lines 122 and 121. Circulation of the regeneration air is continued until the temperature at the exit of the dehumidifier becomes, for example, 200.degree. C. Water absorbed in the desiccant of the dehumidifier is carried away by the heated regeneration air and condensed at the cooler condenser 31, where the regeneration air is cooled about 40.degree. C. The water separated by the cooler condenser 31 is then recovered through a line 128 for further anti-pollution treatment. By continuing the above-described heating-regeneration process, the regeneration is completed in about eight hours in a practical embodiment where the capacity of the dehumidifier is 30 Nm.sup.3 /hour, which is short enough as compared with five days or more, which is a normal design time between successive regenerations of the dehumidifiers.

After finishing the regeneration, the regeneration air is cooled while circulating within the closed loop in certain preferred embodiments. In this case, it is desirable to cool the dehumidifier simply by ceasing the heating in the heater 33 and circulating the regeneration gas through the loop including the cooler 31. Also, it is desirable to reverse the circulating direction of the regeneration air in case of cooling. That is, regeneration air flows from the bottom to the top of the dehumidifier. If there is sufficient time, spontaneous cooling of the dehumidifier may be taken advantage of without circulating the regeneration air within the closed loop. After cooling, the regeneration air is then discharged by opening a valve 129 through a line 128 for further charcoal adsorption treatment. Because the regeneration air discharged from the line 128 normally includes moisture even if it is dehumidified in the cooler condenser 31, it is desirable to return the regeneration air to the cooler condenser 11 for further dehumidifying. In this embodiment, the regeneration air is returned to the inlet line 102 to the cooler condenser 11 through lines 128 and 130.

By the above-described procedures, the regeneration for the dehumidifier 22 is completed well before the time that the dehumidifier 21 requires regeneration. When the dehumidifier 21 absorbs moisture and regeneration is required, exchange of dehumidifiers 21 and 22 is simply made by turning switch valves 105 and 106 from the illustrated position in FIG. 1.

In other preferred embodiments, arrangements are similar to the FIG. 1 construction with the cooler condenser 31 within the closed loop replaced by some other moisture separating apparatus.

Also, in the above-described embodiment of FIG. 1, regeneration air used for the regeneration is returned to the cooler condenser 10 to accommodate use of a cooler condenser 31 having insufficient cooling capacity to remove the moisture from the regeneration air. However, other embodiments are contemplated wherein the cooler condenser 31 has sufficient cooling capacity to remove the moisture; in this case, the regeneration air used for the regeneration is directly sent to the dehumidifier 21 or the charcoal adsorber 40 through lines 130A or 130B for further treatment (lines 130A and 130B being shown as dot-dash lines to depict alternate embodiments).

In the above-described embodiments of FIG. 1, the regeneration air is circulated within the closed loop to maximize the efficiency of regeneration. This is also because the circulation of the regeneration air is effective to eliminate the increase of off-gas which should be treated in the charcoal adsorber system. However, if the capacity of the charcoal adsorber allows, the regeneration air may go through the dehumidifier 21 instead of circulating in the closed loop. Referring to FIG. 2, after completing an air purge by fresh air, fresh air from 118A is continuously supplied while heating by the heater 33 into the dehumidifier 22. The fresh air which passed through the dehumidifier 22 and the cooler condenser 31 can be directly exhausted to the charcoal adsorber system through the lines 130, 130A, or 130B, without circulating in the closed loop.

In these FIG. 2 embodiments, the amount of off-gas to be treated in the charcoal beds 40 increases as compared with the embodiment shown in FIG. 1, so that it is preferable to increase the capacity of the charcoal beds 40.

During the regeneration procedures, using preferred embodiments of the present invention, the pressure in the closed loop may vary substantially due to temperature variation resulting from heating and cooling of the regeneration air. For example, such temperature may vary from room temperature to 350.degree. C. It is desirable to keep the pressure of the regeneration air at a desirable value from the viewpoint of leakage of removed water or regeneration air. In the embodiment shown in FIG. 3, pressure control means are additionally provided to the above-described regeneration system shown in FIG. 1. The pressure in the closed loop is detected by the pressure controller 131. When the pressure in the closed loop increases due to thermal expansion of the regeneration air, signals are generated by the pressure controller 131 which in turn control the valve 129 such that part of the regeneration gas is exhausted outside of the closed loop through the line 130 to maintain the pressure in the closed loop at a desirable value. Also, when the pressure in the closed loop decreases due to thermal contraction of the regeneration air, signals are generated by the controller 131, which in turn control a valve 133 so that fresh air may be supplied through a line 132 into the closed loop.

While we have shown and described several embodiments in accordance with the present invention, it is understood that the same is not limited thereto, but is susceptible of numerous changes and modifications as known to those skilled in the art, and we therefore do not wish to be limited to the details shown and described herein, but intend to cover all such modifications as are encompassed by the scope of the appended claims.