GB2114912A

GB2114912A - Apparatus for scrubbing dust particles from the offgases of a nuclear facility

Info

Publication number: GB2114912A
Application number: GB8304399A
Authority: GB
Inventors: Willy Verachtert; Wilfried Heimerl
Original assignee: Deutsche Gesellschaft fuer Wiederaufarbeitung von Kernbrennstoffen mbH
Current assignee: Deutsche Gesellschaft fuer Wiederaufarbeitung von Kernbrennstoffen mbH
Priority date: 1982-02-17
Filing date: 1983-02-17
Publication date: 1983-09-01
Also published as: FR2521447B1; DE3205536A1; FR2521447A1; GB8304399D0; DE3205536C2; BE895793A; JPS58149000A; BR8300711A

Abstract

To scrub dust particles from the offgas of a nuclear facility, especially from the offgas of a vitrification furnace for highly radioactive waste, a vertically standing counterflow scrubber 6 is used under which a collecting vessel 1 for the scrubbing liquid is arranged. The scrubbing liquid is conducted through a circulatory system to minimize secondary waste. In order to substantially increase the period of time in which the scrubbing liquid can be utilized, the collecting vessel 1 has two partition walls 2, 3 which are arranged so as to be laterally spaced from each other and which extend transversely through the collecting vessel and have a height less than that of the collecting vessel. In this connection, the partition wall 3 adjacent the inlet 10 for the scrubbing liquid is fixedly mounted to the cover of the vessel and the partition wall 2 disposed beyond the first partition wall in the direction of flow is fixedly mounted to the floor of the vessel. The ends of the two partition walls mutually overlap. The drain 9 of the scrubbing liquid from the collecting vessel is arranged behind the lower partition wall. In this manner, the contaminated scrubbing liquid is subjected to a cleansing by means of sedimentation and decantation before recycle to the scrubber at 8 via pipe 11 by means of airlift 12. Contaminated gas enters at 14 and exits at 7; settled sediment is removed via pipe 13. <IMAGE>

Description

SPECIFICATION Apparatus for scrubbing dust particles from the offgases of a nuclear facility This invention relates to apparatus for scrubbing dust particles from the offgas of a nuclear facility, especially from the offgas of a vitrification furnace for highly radioactive wastes.

Offgases develop during the vitrification of highly radioactive liquid wastes in a melting furnace. These offgases have to be freed from entrained dust particles because these particles are radioactive. For this purpose, it is possible to utilize a gas scrubber which can include a column scrubber or a tower scrubber. A collecting vessel is placed beneath the gas scrubber for the scrubbing liquid. The scrubbing liquid is delivered to the head of the gas scrubber and flows from above downwardly through the gas scrubber and drips into the collecting vessel. The furnace offgas is admitted into the gas scrubber at the lower end thereof and flows therethrough from below upwardly in a direction opposite to that of the scrubbing liquid. During the flow through the gas scrubber, most of the dust particles are taken up or transferred to the scrubbing liquid.

The contaminated scrubbing liquid flows into the collecting vessel. The scrubbing liquid is returned to the head of the gas scrubber from the collecting vessel. This circulatory movement of the scrubbing liquid was provided to minimize the occurrence of radioactive secondary wastes.

Air-lift pumping systems have been shown to be very useful for bringing the scrubbing liquid up to the head of the gas scrubber. In air-lift pumping systems, the pumping movement is achieved by introducing bubbles of air into a column of the liquid. The movement of the scrubbing liquid by means of the air-lift pumping system is safer than with a pump since the air-lift requires no moving parts. Furthermore, the possible necessity of exchanging a pump is avoided. No secondary waste occurs because of a contaminated pump and possible exposure of personnel to radiation during an exchange of pumps is eliminated.

A blockage of the air-lift or of the gas scrubber can occur if the scrubbing liquid flowing through the circulation system becomes heavily contaminated after a period of operation and is pumped by the air-lift into the gas scrubber. Even if no blockage occurs, the decontamination efficiency of the gas scrubber will be reduced.

Because of this situation, it was necessary up to now to change the scrubbing liquid at short intervals.

The invention is based on the problem of providing an apparatus for scrubbing dust particles out of the offgas of a nuclear facility wherein the useful life of the scrubbing liquid flowing in the circulation system is substantially extended.

According to the invention there is provided apparatus for scrubbing dust particles from the offgas of a nuclear facility, which comprises a vertical counterflow scrubber, a collecting vessel disposed therebelow for the scrubbing liquid and means for conducting the scrubbing liquid in a circulatory path, wherein the collecting vessel has partition walls which are laterally spaced from each other and which extend transversely through the collecting vessel and have a height less than that of the collecting vessel, a first partition wall which is closest to the inlet for the scrubbing liquid extending downwardly from the top of the vessel and a second partition wall which is disposed downstream thereof in spaced relation thereto extending upwardly from the base of the vessel such that the ends of the two partition walls overlap, a drain for the scrubbing liquid being arranged downstream of the second partition wall.

The contaminated scrubbing liquid flowing from the gas scrubber into the collecting vessel is subjected to a cleansing by means of sedimentation and decantation. The lower partition wall defines an overflow weir for the scrubbing liquid and the upper partition wall on the one hand serves to separate the two compartments defined by the lower partition wall so that offgas cannot move from one compartment to the other and, on the other hand, serves to contain dust particles in the first component by its submergence beneath the level of the liquid in the first compartment.

The contaminated scrubbing liquid is thus subjected to a coarse cleansing which extends the time for which the scrubbing liquid can be utilized.

This intermediate cleansing of the scrubbing liquid in the sedimentation and decantation stages according to the invention achieves a reduced contamination of the gas scrubber and thereby achieves a higher decontamination efficiency of the wet scrubber. A blockage of the gas scrubber or the air-lift pumping system by dust particles entrained in the scrubbing liquid is effectively prevented.

An advantageous embodiment of the invention is provided when more than two partition walls are provided, the partition walls alternately extending downwardly from the top of the vessel and upwardly from the base of the vessel. By means of this embodiment of the invention, the receiving vessel beneath the gas scrubber is divided into more than two compartments, so that several sedimentation stages alternate with several decantation stages.

In a further advantageous embodiment of the invention a conduit to which suction may be applied ends at or adjacent the base of the vessel in a compartment in the vessel which is defined by the second partition wall and which receives liquid from the inlet. The sediment collected on the floor of the compartment receiving the flow of liquid from the gas scrubber can be simply vacuumed away and returned to a ready storage vessel for the highly radioactive waste.

An embodiment of the invention will now be described by way of example with reference to the drawing.

The apparatus for scrubbing dust particles from the offgas of a melting furnace includes a collection vessel 1 having to partition walls 2 and 3 extending transversly therethrough. The lower wall 2 is securely fixed to the base of the vessel and partitions the same into two liquid compartments 4 and 5. The upper wall 3 is mounted on the cover of the housing and projects perpendicularly into compartment 4 of the collection vessel 1. The upper partition wall 3 is disposed in spaced relationship to the partition wall 2 and extends beyond the end of the partition wall 2.

A scrubber column 6 is arranged vertically above the collection vessel 1 and is connected at its lower end with the latter by a pipe 10 which is connected to the collecting vessel in the region of the compartment 4. An outlet conduit 7 for the cleaned offgas is arranged at the head end of the scrubber 6 as is an input conduit 8 for the scrubbing liquid. This input conduit 8 is connected with a drain 9 of the collecting vessel 1 at the base of the vessel in the region of the compartment 5.

A connecting conduit 12 of an air-lift container is connected into the connecting conduit 1 1 between the input conduit 8 and the drain 9.

The transport conduit 13 of a further air-lift device extends downwardly to the floor of the liquid compartment 4.

The collecting vessel 1 includes an offgas input conduit 14 in the region of the compartment 4.

The conduit 14 is arranged at an elevation above the end of the partition wall 2.

The operation of the above-described apparatus is as follows.

The contaminated scrubbing liquid flowing out from the scrubber column 6 flows initially into the first compartment 4. When the level of the liquid reaches the upper edge of the partition wall 2, the scrubbing liquid flows over this upper edge of the partition wall 2 into the second compartment 5.

The partition wall 2 defines an overflow weir for the scrubbing liquid. Dust particles are deposited in the first compartment 4.

The partition wall 3 separates the two compartments 4 and 5 from one another with respect to the offgas. Furthermore, the extension of the separating wall 3 beneath the level of the liquid holds back the lighter particles floating on surface of the solution in compartment 4 and particles which sink in the liquid cannot flow directly over the upper edge of partition wall 2.

A level of liquid is maintained in compartment 5 which is lower than in compartment 4 by means of the air-lift pumping system 12 via conduit 1 1.

The linear flow velocity of the scrubbing liquid as it flows over from compartment 4 into compartment 5 is relatively low because of the large flow cross-section. In this way, the sedimentation of the dust particles in compartment 4 is hardly disturbed. Accordingly, the scrubbing liquid which flows into compartment 5 is substantially freed from dust particles. The scrubbing solution is returned to the head of the scrubber 6 by means of the air-lift pumping system 12.

The sediment collected on the base of the compartment 4 is periodically vacuumed away by means of a second air-lift pumping system 13 and is returned to a storage vessel for the highly radioactive waste.

To increase the cleansing effect of the scrubbing liquid, it is also possible to provide for a subdivision of the collecting vessel 1 into more than two compartments. For this purpose, the subdivision is achieved by partition walls that are mounted alternately from above and below.

Claims

1. Apparatus for scrubbing dust particles from the offgas of a nuclear facility, which comprises a vertical counterflow scrubber, a collecting vessel disposed therebelow for the scrubbing liquid and means for conducting the scrubbing liquid in a circulatory path, wherein the collecting vessel has partition walls which are laterally spaced from each other and which extend transversely through the collecting vessel and have a height less than that of the collecting vessel, a first partition wall which is closest to the inlet for the scrubbing liquid extending downwardly from the top of the vessel and a second.parti#ion wall which is disposed downstream thereof in spaced relation thereto extending upwardly from the base of the vessel such that the ends of the two partition walls overlap, a drain for the scrubbing liquid being arranged downstream of the second partition wall.

2. Apparatus according to claim 1, wherein more than two partition walls are provided, the partition walls alternately extending downwardly from the top of the vessel and upwardly from the base of the vessel.

3. Apparatus according to claim 1 wherein a conduit to which suction may be applied ends at or adjacent the base of the vessel in a compartment in the vessel which is defined by the second partition wall and which receives liquid from the inlet.

4. Apparatus for scrubbing dust particles from the offgas of a nuclear facility substantially as hereinbefore described with . aference to the drawing.