Abstract:
The present invention relates to a method for the determination of the CaCO 3 , content of a scrubbing liquid, which has been separated from a scrubbing liquid circuit of a scrubbing column.

Description:
BACKGROUND  
       [0001]     The present invention relates to a method for the determination of the CaCO 3  content of a scrubbing liquid, which has been separated from a scrubbing liquid circuit of a scrubbing column.  
       DISCUSSION  
       [0002]     Scrubbing columns of the above mentioned type serve for flue gas purification, for example for purifying flue gases from a coal power station or the like. Normally they comprise a scrubbing column having scrubbing liquid nozzles, which are often arranged on several levels, a scrubbing liquid sump, in which scrubbing liquid is collected, and an absorption zone, which extends inside a cylindric receptacle of the scrubbing column from the scrubbing liquid sump towards the upper scrubbing liquid nozzle level. Flue gas is introduced into a lower section of the absorption zone in the scrubbing column, flows upwards from there and leaves the scrubbing column through an outlet provided above the scrubbing liquid nozzles. On its way through the scrubbing column, the flue gas gets into contact with scrubbing liquid emerging from the scrubbing liquid nozzles and is purified, which is described in the following.  
         [0003]     The scrubbing liquid preferably contains, apart from water, alkaline earths, which react with the sulphur oxides present in the flue gas and the sulphur oxides generated in the scrubbing column. Lime in form of calcium oxide, calcium hydroxide, calcium carbonate or the like is in particular used.  
         [0004]     The alkaline earths react with the sulphur oxides present in the flue gas essentially to calcium sulphite, which is bound in the scrubbing liquid. In this manner, the flue gas is purified from the undesirable sulphur oxides and flows out of the purification device afterwards. However, the scrubbing liquid containing the calcium sulphite particles, which are kept floating in this one, flows into the scrubbing liquid sump and is collected there.  
         [0005]     Calcium sulphate, which is generated during the flue gas desulfurization, has similar positive properties as natural gypsum. It is thus a desired by-product of the flue gas purification process, which is won from the scrubbing liquid collected in the scrubbing liquid sump. The calcium sulphate particles are removed together with the scrubbing liquid from the scrubbing liquid sump and are then extracted from the scrubbing liquid in a subsequent process. The calcium sulphate can then be further processed to materials, in particular construction materials.  
         [0006]     For winning calcium sulphate of good quality it has to be taken care that as few alkaline earths as possible are contained in the scrubbing liquid collected in the scrubbing liquid sump, when the scrubbing liquid is removed from the scrubbing liquid sump for winning calcium sulphate.  
         [0007]     On the other hand, it has to be assured that enough alkaline earths are present in the scrubbing liquid, in order to provoke a sufficient reaction in the absorption zone of the scrubbing column.  
         [0008]     For determining the exact concentration of alkaline earths in the scrubbing liquid, it is known to determine the CaCO 3  content of the scrubbing liquid.  
         [0009]     DE-A-19733284 describes a method for measuring the CaCO 3  content of a scrubbing suspension, in particular from the absorber of a flue gas purification device, in which a predetermined constant measuring stream in the bypass is continuously supplied to a pH measuring device and the pH value of the suspension is measured. Herein, the measuring stream is inoculated with an acid at an inoculation point in front of the pH measuring device in temporal intervals and the change of the pH value, which results from the inoculation with the acid, is measured. The CaCO 3  content of the suspension is afterwards determined from the difference of the measured pH values by comparison with data from reference measurements, which have been made in consideration of the residence time of the suspension for the flowing distance between the inoculation point and the pH measuring device. Decisive for the precision of the method is the fact that a certain acid volume is added to the predetermined suspension volume at the inoculation point and both volumes are sufficiently mixed with each other on their way to the pH measuring device, such that a correct measurement of the pH value can be carried out at the pH measuring device. In particular the precise dosage of the suspension and acid volume requires a high effort and can easily lead to inaccuracies, which then also have an effect on the accuracy of the measured pH value. A supervision of a correct dosage is also difficult to realize with this method. As a result, the method has some sources of error, which can adversely affect the reliability thereof.  
         [0010]     DE-A-3809379 describes a method for the determination of the carbonate content of a partially used lime stone suspension, which continuously circulates in a flue gas purification device, for regulating the addition of fresh lime stone powder. Herein, a constant subset of the lime stone suspension is respectively extracted from a branch piping, which subset is separated from the circuit for measuring purposes. Acid is then added to this subset. Finally, the volume of the generated CO 2  is measured at constant temperature and under constant pressure and the value is used for the addition of fresh lime stone powder. Strictly speaking, in this method the gas quantity from the increase of pressure in a gas tight measuring cell and of the temperature in the measuring cell is measured in consideration of the change of volume caused by the addition of acid, and based upon this gas quantity, the carbonate content of the lime stone suspension is determined. In comparison to DE-A-19733284, this is thus an alternative method for the determination of the carbonate content.  
       OBJECTS AND SUMMARY OF THE INVENTION  
       [0011]     It is an object of the present invention to provide an improved or alternative method for the determination of the CaCO 3  content of a scrubbing liquid, which has been separated from a scrubbing liquid circuit of a scrubbing column as well as a corresponding device.  
         [0012]     In the method according to the present invention, a predetermined test volume of scrubbing liquid is at first separated from a continuously available scrubbing liquid volume flow. The test volume is then supplied to a measuring cell, in the same way as a previously dosed, predetermined HCl volume. Both volumes are mixed with each other and react with each other afterwards within a predetermined reaction time. Finally, the pH value of the so produced mixture is measured and the CaCO 3  content is determined based upon the change of the pH value.  
         [0013]     In comparison to DE-3809379, the method according to the invention is an alternative method for the determination of the CaCO 3  content, in which not the volume of the generated CO 2  is measured at constant temperature and under constant pressure, but the pH value of the mixture of scrubbing liquid and HCl, which is present in the measuring cell, is detected.  
         [0014]     In contrast to DE-A-19733284, scrubbing liquid and acid are not mixed in a pipe and then the pH value is measured, but a predetermined test volume of scrubbing liquid is separated from a continuously available scrubbing liquid volume flow and supplied, together with a dosed, predetermined HCl volume, to a measuring cell, both volumes are mixed and finally, after a predetermined reaction time, the pH value is measured. In comparison to DE-A-19733284, the method according to the invention has the advantage that it can be assured that both the scrubbing liquid test volume and the HCl volume can be precisely dosed. A correct mixing of both volumes can also be assured. Accordingly, the pH value determined by means of the method according to the invention is more accurate. Furthermore, the scrubbing liquid volume flow, the test volume in the measuring cell and the dosed HCl volume can for example be supervised by simple means and thus the plausibility of the pH measurement, which has been carried out by means of the method, can be verified, if this is desired.  
         [0015]     According to a preferred variant of the method according to the present invention, the separation of the predetermined scrubbing liquid test volume is carried out by means of a sampling trap, which can be realized by relatively simple means, and the determination of the CaCO 3  content is carried out by means of a pH probe.  
         [0016]     The scrubbing liquid volume flow, from which the predetermined scrubbing liquid volume is separated, is preferably a separately produced scrubbing liquid volume flow. This means that a separate scrubbing liquid circuit and thus a separate scrubbing liquid volume flow is generated for example by means of a closed circular pipeline, which is exclusively used for the measurement of the pH value. This has the advantage that the operation of the closed circular pipeline is independent from foreign pre-pressures, such that a self-supporting, stable circuit can be achieved by the design of the closed circular pipeline and the pump provided in this one. Such an extraction from the absorber and the operation of the closed circular pipeline with a separate pump have proved successful in many installations for realizing pH and density/solids measurements due to their operational reliability. However, the disadvantage of these designs is the higher investment costs for the additional arrangement of these closed circular pipelines.  
         [0017]     It is to be understood that the scrubbing liquid test volume can of course also be extracted from a pipe with pressure inside, e.g. from the pipe, which leads from the scrubbing liquid sump to the scrubbing liquid nozzles. However, an independent pipe for carrying out the CaCO 3  measurement is preferred.  
         [0018]     According to another advantageous embodiment of the method according to the invention, the scrubbing liquid test volume and/or the dosed HCl volume can be modified.  
         [0019]     Furthermore, it is advantageous if the measuring equipment, which is used for the measuring cell, is regularly calibrated, in order to be able to always assure the required accuracy of the CaCO 3  measurement. The calibration is advantageously carried out by wet analytical comparison measurements.  
         [0020]     Furthermore, at least the measuring cell and the sampling trap as well as the associated pipes are cleaned in regular time intervals, in order to correspondingly prevent deposits of solid matters.  
         [0021]     Finally it is preferred according to the method of the invention that the plausibility of the pH measurement is verified by means of supervision of the scrubbing liquid volume flow, the test volume in the measuring cell and the dosed HCl volume, in order to assure a correct functioning of the method.  
         [0022]     The device according to the invention for the determination of the CaCO 3  content of a scrubbing liquid comprises a closed circular pipeline, a pump, which is actively connected to the closed circular pipeline, a sampling trap provided in said closed circular pipeline for separating a scrubbing liquid sample and a measuring cell, which is connected to the sampling trap and which has a measuring equipment for determining the pH value of a scrubbing liquid sample.  
         [0023]     Herein, the measuring equipment advantageously comprises an HCl dosage device and a pH probe for the determination of the pH value.  
         [0024]     Furthermore, a rinsing device and/or an aerating are preferably provided.  
         [0025]     The sampling trap advantageously comprises a by-pass, via which the scrubbing liquid is by-passed by means of the sampling trap during the separation of the scrubbing liquid test volume, in order to not interrupt the scrubbing liquid volume flow in the closed circular pipeline, such that this one is continuously available. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0026]     In the following, a preferred embodiment of the present invention is described in detail with reference to the annexed  FIG. 1  which shows a schematic view of an embodiment of the device according to the invention for the determination of the CaCO 3  content of a scrubbing liquid. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     The represented device comprises a closed circular pipeline  10 , through which scrubbing liquid is pumped by means of a pump  12  from a scrubbing liquid sump  14  of a scrubbing column  16 , which is only partially represented in the drawing. Herein, the scrubbing liquid, which has been extracted from said scrubbing liquid sump  14 , is returned to said scrubbing liquid sump  14  after flowing through the closed circular pipeline  10 , as it is indicated by the arrows A and B. The closed circular pipeline  10  further comprises stop valves  18  and  20 , which alternatively allow or prevent a passage of scrubbing liquid.  
         [0028]     Downstream of said pump  12 , a sampling trap  22  is arranged in the closed circular pipeline, which sampling trap permits to separate a predetermined scrubbing liquid test volume from said closed circular pipeline. The sampling trap  22  is connected via a pipe  24  to a measuring cell  26 , which can be supplied with a scrubbing liquid test volume separated in said sampling trap  22 . Furthermore, pipe  28 , which can be opened and closed by means of the stop valve  28   a , allows the introduction of an HCl volume into said measuring cell  26 , wherein the HCl acid is taken from an HCl store tank  30  and the HCl volume, which shall be supplied to said measuring cell, is dosed in a dosage device  32 . Said measuring cell  26  further comprises an agitator  36  driven by a motor  34 , by means of which the scrubbing liquid and HCl volumes supplied to said measuring cell  26  can be mixed. After mixing both volumes, the pH value of the mixture can be determined by means of a pH probe  38 . By opening a stop valve  42  provided in a pipe  40 , the mixture can be evacuated from the measuring cell after determination of the pH value thereof.  
         [0029]     For by-passing the sampling trap  22 , said closed circular pipeline  10  comprises a by-pass  44  having a stop valve  46 .  
         [0030]     For cleaning said closed circular pipeline  10 , a rinse water pipe  48  having a stop valve  50  is further provided. If said valve  50  is opened, the rinse water can enter in the direction of pump  12  into said closed circular pipeline  10 .  
         [0031]     Finally, a connecting branch  52  is placed at the sampling point  22 , which connecting branch comprises three inlets with corresponding stop valves  52   a ,  52   b  and  52   c . By opening the corresponding stop valves  52   a  or  52   b , rinse water can be introduced via pipe  54  or alternatively compressed air can be introduced via pipe  56  into said connecting branch  52  and with opened stop valve  22   b  into said sampling trap  22 . By opening stop valve  52   c , an aerating can be realized via pipe  58 . A corresponding aeration pipe  60  having a stop valve  62  and a rinse water inlet pipe  64  having a stop valve  66  are connected to said measuring cell  26 .  
         [0032]     The operation of the device represented in  FIG. 1  will be described in detail in the following.  
         [0033]     In the initial position, all valves  18 ,  20 ,  22   a  through  22   d ,  42 ,  46 ,  50 ,  52   a  through  52   c ,  62  and  66  are closed. For starting the represented device, the sop valves  18 ,  20 ,  22   a  and  22   c  are opened and pump  12  is switched on. Then, scrubbing liquid is pumped by means of said pump  12  from said scrubbing liquid sump  14  of scrubbing column  16  into said closed circular pipeline  10  through said sampling trap  22  back into said scrubbing liquid sump  14 .  
         [0034]     If a predetermined scrubbing liquid volume shall be evacuated from said closed circular pipeline  10  by means of said sampling trap  22 , stop valve  22   c  of said sampling trap  22  is at first closed.  
         [0035]     In this way, a predetermined scrubbing liquid volume is dammed up in said sampling trap  22  and is enclosed in this one by closing stop valve  22   a  of sampling trap  22 . When stop valve  22   a  of said sampling trap  22  is closed, stop valve  46  of by-pass  44  is simultaneously opened, such that the scrubbing liquid no longer flows through said sampling trap  22 , but through by-pass  44  back into said scrubbing liquid sump  14 . The predetermined scrubbing liquid volume, which is contained in said sampling trap  22 , is guided via pipe  24  into measuring cell  26  by opening said stop valve  22   d  and  52   c  of said sampling trap  22 . An HCl volume, which is adapted to the scrubbing liquid volume, which has been introduced into said measuring cell  26 , is dosed in the dosage device  32  and is guided via pipe  28  into said measuring cell  26  by opening stop valve  28   a . Agitator  36 , which is driven by means of motor  34 , mixes the scrubbing liquid volume with the HCl volume in said measuring cell  26 . After a predetermined reaction time, the pH value of the mixture, which is present in said measuring cell  26 , is determined by means of pH probe  38 . As soon as the pH value has been determined, the mixture can be evacuated via pipe  40  by opening stop valve  42 .  
         [0036]     For taking the next sample by means of said sampling trap  22 , said stop valves  22   d  and  46  have to be closed again and said stop valve  22   a  has to be opened again, such that scrubbing liquid flows again into the sampling trap.  
         [0037]     If said closed circular pipeline  10  and said by-pass  44  shall be cleaned, valves  22   a  and  22   c  of said sampling trap  22  have to be closed and valve  46  has to be opened. Then, rinse water can flow through rinse water pipe  48  through the corresponding components and afterwards through the opened stop valve  20  back into said scrubbing column  16 . It is also possible to rinse said sampling trap  22  and the connecting pipes thereof and said closed circular pipeline  10  with closed valves  46 ,  22   b  and  22   d  as well as opened valves  50 ,  22   a ,  22   c  and  20 .  
         [0038]     For the individual cleaning of said sampling trap  22 , valves  22   a  and  22   c  thereof have to be closed and valves  22   b  and  22   d  thereof have to be open. By opening valve  52   a , rinse water can flow from pipe  54  through connecting branch  52  into said sampling trap  22  and rinse this one. Form there, it flows through pipe  24  into said measuring cell  26 , then it flows out via pipe  40  by opening stop valve  42 .  
         [0039]     The measuring cell itself can be filled with rinse water via pipe  64  by opening valve  66 . After rinsing, the water also flows out via valve  42  and pipe  40 .  
         [0040]     It is to be understood that the represented device according to the invention is only an example and other modifications and changes are possible without leaving the protected scope of the present invention, which is defined by the annexed claims.