Abstract:
A scrubber for the cleaning of gases containing several scrubber stages ( 1 - 4 ), where the scrubber stages are arranged in a scrubber tower with the different stages at different levels above each other in the scrubber tower. At least one of the scrubber stages ( 2 - 4 ) above the lowest scrubber stage ( 1 ) comprises, according to the invention, a ring-shaped tank ( 10, 15, 20 ) arranged inside the scrubber tower, which ring shaped tank ( 10, 15, 20 ) is arranged surrounding a central channel ( 9, 14, 19 ) through which the gas that is to be cleaned can pass upwards.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a scrubber for cleaning gases, in particular to a scrubber in the form of a tower containing several scrubbing stages. 
     2. Description of the Related Art 
     Scrubbers are common in the context of cleaning gases, and they are used for, among other applications, cleaning exhaust gases, fumes and other process gases in, for example, the chemical industry. The scrubber is used to clean the gases from contaminants that should not be released into the atmosphere. 
     A scrubber is normally designed as a vertical cylinder, in which a process fluid circulates, normally in a counterflow direction to the gas that is to be cleaned. The process fluid, such as water with or without additives, is pumped around and injected into the scrubber through a nozzle system in the upper part of the scrubber. The gas that is to be cleaned is introduced at the bottom of the scrubber, and is subsequently allowed to rise, in order to leave the scrubber at its top. The process fluid and the gas are in this way brought into contact with each other, whereby certain components that are present in the gas, such as, for example, HCl, SO 2 , and HF, are absorbed in the process fluid. The gas is in this way cleaned from contaminants. 
     In order to improve the contact between the process fluid and the gas that is to be cleaned, what is known as column packing may be installed in the scrubber. The column packing may be manufactured from plastic or a metallic or ceramic material, and it may be designed with various geometrical forms in order to offer a greater or lesser contact area between the gas and the fluid. 
     A scrubber can, as has been mentioned above, be built in several individual stages, where each stage has its own pump circulation circuit and its own nozzle system. The stages are in this case separated by special separation layers that collect the process fluid and lead it onwards to a pump tank from which process fluid can, in turn, be led onwards to the pump circulation system. The pump tank must be able to accommodate a volume of fluid that is sufficiently large such that it can offer satisfactory function of the pump system during normal operation, and, most importantly, during start-up and shut-down sequences when all fluid is to be collected in the tank. 
     In a scrubber that has only one stage, the lower part of the scrubber, the bottom part, acts as pump tank. In scrubber systems with two or more stages, a separate tank volume is required for each stage of the scrubber, such that the fluids from the various stages are not mixed with each other. The traditional manner of constructing this has been to have an external tank located at a lower level than the scrubber stage. 
     With scrubber systems having several stages and with the scrubbers placed in a tower, the solutions described above with the tanks lead to extensive pipe-laying, for both the supply and withdrawal lines for each scrubber. Since towers with several scrubber stages can be very high, severe demands are placed on the pumps that are to circulate the process fluid, such that they can pump fluid to the height that is required for large volumes. Furthermore, the fluid content of the pipes becomes very large, and the tanks must be dimensioned such that they can accommodate all the fluid when the flow is stopped for one reason or another. 
     It is therefore a purpose of the present invention to achieve a scrubber, in particular a scrubber tower with several scrubber stages, in which the problems described above with the laying of pipes and tanks can be solved in a more efficient manner. 
     SUMMARY OF THE INVENTION 
     The purpose of the invention described above is achieved with a scrubber containing several scrubber stages, where the scrubber stages are arranged in a scrubber tower with the different stages at different levels over each other in the scrubber tower, and where, according to the invention, at least one of the scrubber stages above the lowest stage comprises a ring-shaped tank arranged inside the scrubber tower, which ring-shaped tank is arranged surrounding a central channel through which the gas that is to be cleaned can rise. 
     It is appropriate that all scrubber stages above the first stage are provided with such ring-shaped tanks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in more detail in the form of a non-limiting embodiment, shown in the attached drawings, where  FIG. 1  shows a sketch of the principle of how a scrubber tower according to the invention may be designed,  FIG. 2  shows a schematic perspective view of a flow-collection trough to be used between the different stages in a scrubber tower,  FIG. 3  shows schematically the location of a circulation pump according to a first design,  FIG. 4  shows schematically the location of a circulation pump according to a second design, and  FIG. 5  shows schematically the location of a circulation pump according to a third design. 
     
    
    
     DETAILED DESCRIPTION 
     Thus,  FIG. 1  shows schematically a scrubber tower, comprising four scrubber stages  1 ,  2 ,  3  and  4 . The scrubber tower is surrounded by a cylindrical outer cover  5 , and demonstrates at the bottom an inlet  6  for the gas that is to be cleaned. In the lowest scrubber stage  1  a collection tank  7  is formed inside the tower for the fluid that is used for cleaning the incoming gas in the lowest scrubber stage  1 . This fluid can, with the aid of pipes and a circulation pump not shown in the drawing, be fed from the collection tank up to nozzle beams  8  in the upper part of the scrubber stage  1 , and there sprayed out to meet the upward flowing gas. 
     A narrowed section is located inside the scrubber tower  1  above the lowest scrubber stage, as is shown in the drawing, forming a channel  9  in the centre of the tower in order to allow the gas to flow onwards and upwards to the next scrubber stage  2 . A ring-shaped space is formed between the channel  9  and the outer surface  5  of the tower in which a ring-shaped tank  10  can be located, for the reception of scrubber fluid from the second scrubber stage. A separation trough  11 , shown in detail in  FIG. 2 , is arranged uppermost in the channel  9 , with which scrubber fluid arriving from above can be collected and led out to the ring-shaped tank  10  that surrounds the separation trough  11 . Gas that arrives from below through the channel  9  from the scrubber stage  1  can, however, pass through the separation trough  11  and continue its upwards flow. A connection  12  from the ring-shaped tank  10  is also arranged, to which a circulation pump and a pipe, not shown in the drawing, can be connected in order to pump the fluid that has been collected in the ring-shaped tank  10  up to nozzle beams  13  arranged in the upper part of the second scrubber stage  2 , there to be sprayed out and to meet the gas flowing upwards in the second scrubber stage  2 . 
     The third and the fourth stages  3  and  4  of the scrubber are, in an equivalent manner, designed with narrower sections inside the scrubber tower, forming channels  14  and  19  in the centre of the tower in order to allow the gas to flow onwards and upwards to the next stages  3  and  4  of the scrubber. Ring-shaped spaces are formed between the channels  14  and  19  and the outer surface  5  of the tower in which further ring-shaped tanks  15  and  20  can be located, for the reception of scrubber fluid from the third and fourth stages of the scrubber. Separation troughs  16  and  21  are arranged uppermost in the channels  14  and  19 , with which scrubber fluid coming from above can be collected and led out to the ring-shaped tanks  15  and  20  that surround the separation troughs  16  and  21 . Gas that arrives from underneath through the channels  14  and  19  from the scrubber stages  2  and  3  can, however, pass through the separation troughs  16  and  21  and continue on its upward flow. Connections  17  and  22  are also arranged from the ring-shaped tanks  15  and  20 , to each of which a circulation pump and a pipe, see  FIGS. 3-5 , can be connected to pump the fluid that has been collected in the ring-shaped tanks  15  and  20  up to spray beams  18  and  23  arranged in the upper part of the third stage  3  of the scrubber and the fourth stage  4  of the scrubber, there to be sprayed out and meet the upwards flowing gas in these third and fourth scrubber stages  3 ,  4 . 
     The scrubber tower is terminated above the fourth scrubber stage  4  with an outlet  24  for the gas that has been cleaned in the scrubber tower. 
     It is clear that the scrubber tower can comprise fewer than or more than the four scrubber stages that have been shown in the present embodiment. It should also be pointed out that only those parts of the scrubber tower that are of significance for the invention have been shown on the drawings and explained in this text. 
       FIG. 2  shows, as has been described above, a separation trough  11 ,  16 ,  21  used in the scrubber according to the invention. Such separation troughs  11 ,  16 ,  21 , are thus used between the different stages of the scrubber to separate scrubber fluid and lead it to the relevant tank  10 ,  15 ,  20  for recirculation. The separation troughs comprise a number of obliquely placed laminae  25 , along which the fluid can run down into trough channels  26 . Each lamina  25  is provided with an associated trough channel  26 . The trough channels  26  are open at both ends, and extend somewhat more than the complete diameter of the channels  9 ,  14 ,  19 , such that fluid that has entered the trough channels  26  can be led out to the relevant tank  10 ,  15 ,  20 . A space is formed between each pair of laminae  25  and each trough channel through which gas can pass upwards from underneath. This space, however, is covered by the neighbouring lamina, such that no fluid, or essentially no fluid, from above can pass downwards through the separation trough  11 ,  16 ,  21 . 
       FIG. 3  shows a first embodiment of a method of arranging a circulation pump  27  at, for example, the tank  10  in order to circulate the scrubber fluid that has been collected in the tank  10  back to the spray beams  13 . A pump tank  28  is connected outside of the outer surface  5  of the scrubber tower in this embodiment, which pump tank is connected directly to the tank  10  inside of the outer cover through the connection  12 . The circulation pump  27  can be any suitable pump that can feed sufficient quantities of scrubber fluid through a feed pipe  29  up to the spray beams  13 . 
     According to one preferred embodiment, the feed pipe  29  is located inside of the outer cover  5  of the scrubber tower. 
     By arranging, as is shown in  FIG. 3 , a separate pump tank  28  outside of the outer cover  5 , and with the circulation pump  27  also arranged outside of the outer cover  5 , these parts are made easy to access for service and maintenance, while the length of the feed pipe  29  can be limited to the height of the relevant scrubber stage, stage  2  in the example illustrated. It is clear that such a location of the circulation pump and the pump tank is possible for any of the scrubber stages in a scrubber tower, and it is obvious that the length of the feed pipe can be limited to the height of the relevant scrubber stage in this manner. The circulation pump may also have a lower feed capacity than that required if the pump is always placed on the ground. 
       FIG. 4  shows a second embodiment of the manner in which a circulation pump  30  can be arranged at, for example, the tank  15  in order to recirculate the scrubber fluid that has been collected in the tank  15  to the spray beams  18 . In this embodiment, a carrier  31 , such as a floor mounted on a frame, for example, is arranged outside of the outer cover  5  of the scrubber tower, which carrier supports the circulation pump  30  at the same height as the tank  15 . The circulation pump  30  is connected through an inlet pipe  32  and the connection  17  to the tank  15  inside the outer cover. The circulation pump  30  can be any suitable pump that can feed sufficient quantities of scrubber fluid through a feed pipe  33  up to the spray beams  18 . 
     By arranging the circulation pump  30  supported outside of the outer cover  5 , as is shown in  FIG. 4 , connected to the tank  15  inside of the outer cover, the pump is made easy to access for service and maintenance, while the length of the feed pipe  33  can be limited to the height of the relevant scrubber stage, stage  3  in the example illustrated. It is clear that such a location of the circulation pump is possible for any of the scrubber stages in a scrubber tower, and it is obvious that the length of the feed pipe can be limited to the height of the relevant scrubber stage in this manner. The circulation pump may also have a lower feed capacity than that required if the pump is always placed on the ground. 
       FIG. 5  shows a third embodiment of the manner in which a circulation pump  34  can be arranged, for example, in connection with the tank  15  in order to recirculate the scrubber fluid that has been collected in the tank  15  to the spray beams  18 . In this design, a carrier  35 , such as a floor mounted on a frame, for example, is arranged outside of the outer cover  5  of the scrubber tower, which carrier supports the circulation pump  34  at ground level. The circulation pump  34  is connected through an inlet pipe  36  and the connection  17  to the tank  15  inside the outer cover. The circulation pump  34  can be any suitable pump that can feed sufficient quantities of scrubber fluid through a feed pipe  37  up to the spray beams  18 . 
     According to one preferred embodiment, the feed pipe  37  is located inside the outer cover  5  of the scrubber tower. This can also be the case for the inlet pipe  36 . 
     The design according to  FIG. 5  provides ready access to the circulation pump  34  for service and maintenance, but it naturally leads to the laying of longer pipes than the first two embodiments. 
     The scrubber designs according to the present invention can be used for all types of scrubber that comprise two or more stages arranged above each other, independently of the field of application of the scrubber. 
     The scrubber according to the invention can comprise in known manner column packing of the type that has been described in the introduction, in order to offer a larger or smaller contact area between the gas and the fluid. 
     The pipes and the feed pipes that have been shown on the drawings as having been laid outside of the outer surface  5  of the scrubber can advantageously be laid inside of this cover, in order to avoid the risk of freezing during a long period of non-operation, and to make possible factory assembly.