Patent Publication Number: US-2009235719-A1

Title: Co2 absorption device for elemental analysis instruments

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a CO 2  absorption device suitable for operation in an elemental analysis instrument, especially for nitrogen determination, particularly an instrument based on the Dumas method. Such an instrument consists of a high temperature sample combustion reactor with a current of oxygen, where the combustion gasses pass into a reduction reactor with the elimination of water, carbon dioxide and any SO 2  present, prior to the gas being sent to a detector, particularly a nitrogen detector. 
     2. Description of the Prior Art 
     The use of chemical filters for CO 2  elimination, which are simply replaced following a certain number of analytical cycles, are known in the art. However, such filters have a number of drawbacks, especially for high weight samples (for example 1-2 g of cereals) since the quantity of CO 2  to be absorbed demands large filters, which have a negative impact on analytical performance. Furthermore, the reaction with large quantities of CO 2  can be highly exothermic and lead to the curing of the absorbent material with increased load loss. The resulting increase in combustion reactor operating pressure reduces the conversion efficiency of the sample into elemental gas. Sending only a percentage of the combustion gas to the filter has been proposed as a solution for obviating such drawbacks, but this influences the accuracy and reproducibility of the analyses, and leads to further complications in the instrument pneumatics. 
     CO 2  filters, acting at the physical level, which can be regenerated by means of heating and passing regenerative gas through, have also been proposed. In cases involving large quantities of CO 2 , such filters must necessarily also have large dimensions, and require long periods of time (of the order of 15 minutes) for their regeneration and subsequent cooling. Furthermore, it is practically essential to provide an upstream water filter, since the CO 2  filter would absorb water more or less irreversibly, with consequential degradation of efficiency. 
     Patent application EP 1586895 illustrates an elemental analysis instrument envisaging a carousel with a number of regenerable CO 2  filters, which are brought in succession into the operating position and then into the regeneration position. This solution allows reduced regeneration times, and the ability to move from one analysis to the next without pausing. However, there are problems with the pneumatic seals and, in the case of heavy samples, the device requires individual, large sized filters and therefore has a tendency to be excessively bulky. 
     SUMMARY OF THE INVENTION 
     The scope of the present invention is therefore that of providing a device for absorbing CO 2 , intended for use in an elemental analysis instrument, that is both regenerable, capable of operating without any moving parts, with high efficiency, and does not require any time for regeneration between one analysis and the next, even for high weight samples. 
     These scopes, and others, which will become evident from the following description, are achieved by a CO 2  absorption device according to claims  1  to  16  operating in an elemental analysis instrument according to claims  17  to  20 . 
    
    
     
       DRAWINGS 
       The device and the instrument according to the invention will be described with reference to a preferred embodiment, illustrated schematically, purely by way of non-limiting illustration, in the attached figures, in which: 
         FIG. 1  is a diagram of an elemental analyser fitted with a CO 2  absorption device according to the invention. 
         FIGS. 2 and 3  are schematic illustrations of the absorption and regeneration supply methods for the filters making up the device according to the invention. 
         FIGS. 4 and 5  schematically depict a control valve for the filters operating in accordance with  FIGS. 2 and 3 . 
         FIG. 6  depicts an example of a CO 2  absorption filter according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The diagram in figure  FIG. 1  refers to an elemental analysis instrument, in the configuration shown, with an automatic sampler  20  capable of sending samples one at a time to an oxidation reactor  21  maintained at a high temperature (approx. 1000° C. or higher). At the same time, the supply of carrier gas, generally consisting of helium, is switched to supplying oxygen in order to achieve the so-called very high temperature “flash” combustion in the reactor  21 . The combustion gasses are then sent, by means of a pneumatic line  22  borne by the carrier, to a reduction reactor  23 , downstream of which the carrier transports the “elemental” gasses, N 2 , CO 2 , H 2 O and possibly SO 2 , by means of said line  22 . 
     A water condenser  24  is fitted to the line  22  in order to remove condensed water, and discharge it externally by means of line  25 . Line  22  then feeds gas to the device  26  which handles the absorption of the CO 2 , any remaining water and any SO 2 , if present 
     The device  26  has two filters, one involved in the absorption stage and one undergoing regeneration by means of heating and passing through a regenerating gas, which may be the same helium carrier, supplied and exhausted by means of line  27 . A gas chromatography column  28  and a detector  29 , to which a reference gas is also supplied by means of line  30 , are arranged downstream in the known manner. 
     The device  26  is shown schematically in  FIGS. 2 and 3 . It consists of two regenerable filters  31  and  32  and pneumatic connections for supplying the same with the combustion gasses and with the regenerating gas. More precisely, with reference to  FIG. 2 , the filter  31 , in absorption mode, is fed using line  22  coming from the water condenser  24  by means of a three-way, two position electrovalve  33 . On emerging from the first electrovalve, the gas passes through a second three-way, two position electrovalve  34  in order to be fed to the gas chromatography column  28 . At the same time, the regenerating gas (helium) is fed into the second filter  32  by means of line  27  through a third, three-way, two position electrovalve  35 , while the exhaust from the filter  32  is discharged to  37  under the control of a fourth, three-way, two position valve. 
       FIG. 3  shows the set-up for absorption by filter  32  and regeneration of filter  31 , obtained by switching over the four electrovalves  33 - 36 . In this case, the combustion gas is fed into filter  32  by means of electrovalve  36  and sent to the gas chromatography column by means of electrovalve  35 . The regenerating carrier is fed into filter  31  by means of electrovalve  34  and exhausted by means of electrovalve  33 . 
     It should be observed that the pneumatic connections shown operate in such a way that filter regeneration always occurs with a flow of carrier in the opposite direction with respect to the flow of gas during the absorption stage for the same filter. This is very important since, as will be appreciated below, it allows improved regeneration conditions, and hence improved device operating conditions. 
     In  FIGS. 4 and 5 , valves  33 - 36  are replaced by a single 10-ways, two position valve  40 . 
     In the first position shown in  FIG. 4 , the regeneration gas, coming in through port  1 , is directed, by means of ports  2 ,  7  and  8 —through the filter  32  and then from the latter, by means of ports  5  and  6 , to exhaust. At the same time, the gasses emerging from the water condenser  24  are sent, by means of ports  4  and  3  to the filter  31  during the analytical stage, and then from the latter, by means of ports  10  and  9 , to the gas chromatography column  28 . In the position shown in  FIG. 5 , the valve  40  sets the filter  31  in the conditions for regeneration by supplying the regenerating gas, by means of ports  1 ,  10 ,  3 ,  2 ,  7  and  6 , while the filter  32  is in analytical mode, and the gasses coming out of the water condenser  24  by means of ports  4  and  5  pass through it and are then conveyed to the gas chromatography column  28  by means of ports  8  and  9 . 
     With reference to  FIG. 6 , each filter  50  consists of an elongated tubular element  51  with an internal diameter preferably comprised of between 4 mm and 10 mm, and length between 50 and 200 cm, optionally folded over into a U-shape for reasons of bulk. The tubular element is made from thermoconductive material, for example a metal, preferably steel, wound around the outer surface of which is at least one heating element  52 , preferably a single wire playing the simultaneous roles of heating element and temperature measuring element during regeneration. 
     The interior volume of the tube is filled with a packing composed of one or more CO 2 -absorbent materials arranged and/or selected so as to provide a CO 2  absorbent power that increases from the filter inlet to the filter outlet in the direction, marked X, taken by the gas during the analysis stage. In particular, said material may be comprised of molecular sieves with granulometry that decreases from the inlet to the outlet in the aforementioned direction, in particular, for example, two different granulometries, as shown the larger in  53  and the finer in  54 , respectively. 
     Still in the direction undertaken by the gas undergoing analysis, upstream of the CO 2 -absorbent material is preferably positioned an absorbent material  55  for any H 2 O not retained by the condenser  24 , and upstream of this latter item at least one SO 2 -absorbent material  56  may be optionally positioned. This layout of the materials making up the filter considerably aids the regeneration stage, which, as already mentioned, occurs with the flow in the opposite direction, so that, during regeneration, any SO 2  and water do not pass through, and therefore have no effect on the CO 2 -absorbent materials. The latter are then treated by the flow of regenerating gas in such a way that the fresh gas first comes into contact with the areas most loaded with CO 2  then little by little moving onto the least loaded areas, towards the end of its path. This improves the regeneration conditions and effects which, thanks also to the other construction details of the filter and its reduced thermal mass, may be completed and the filter cooled within a very short period of time, typically between 3 to 8 minutes. 
     A fan assists with speeding up the filter cooling process, in order to complete the regeneration process in times that are essentially equal to those required for analysis.