Abstract:
Disclosed herein is a process and a plant for recovering ammonia from a mixture including ammonia, acid gas containing H 2 S and/or CO 2  and low-boiling water-soluble organic components. To avoid an enrichment of volatile organic compounds in the acid gas absorber, a partial stream of the liquid phase is withdrawn from an acid gas absorber and processed such that gaseous ammonia with a reduced content of volatile organic components is obtained, which is recirculated into the acid gas absorber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage application under 35 U.S.C. 371 of International Patent Application Serial No. PCT/EP2009/008113, entitled “Process and Plant for Recovering NH 3  from a Mixture Containing NH 3  and Acid Gasses,” filed Nov. 13, 2009, which claims priority from German Patent Application No. 10 2008 058 143.7, filed Nov. 20, 2008. 
     FIELD OF THE INVENTION 
     The present invention relates to a process and a plant for recovering NH 3  from a mixture including NH 3 , acid gas containing H 25  and/or CO 2 , and low-boiling water-soluble organic components, in particular waste water. 
     BACKGROUND OF THE INVENTION 
     The separation of NH 3  from mixtures including NH 3 , acid gas containing H 25  and/or CO 2 , and low-boiling water-soluble organic components, in particular waste water is known and based on the fact that NH 3  forms azeotropes with the acid gases CO 2  and H 25  in aqueous solutions—The higher the temperature and the lower the NH 3  concentration in the liquid, the poorer the azeotropes in acid gases. In a deacidifier, which operates at a high sump temperature, an acid gas free from NH 3  thereby can be obtained as top product, whereas in the sump an aqueous solution is obtained, which contains the entire NH 3  and only a small amount of the acid gases from the inlet. In an NH 3  stripper, which operates at a low sump temperature and generally with a higher NH 3  concentration, a bottom product is obtained, which contains the entire acid gases originating from the deacidifier sump. This requires a relatively small excess of NH 3 . The top product consists of NH 3 , which still contains a small concentration of acid gases. In an acid gas absorber, the acid gases are absorbed from the NH 3  containing acid gas in aqueous NH 3 . In a total stripper, all volatile components are stripped from an aqueous solution of NH 3 , CO 2 , H 25  and a number of further volatile and non-volatile components (depending on the origin of the feedstock), so that a waste water free from volatile components is obtained as bottom product. 
     Usually, the mixture containing NH 3  is added to the sump of the acid gas absorber. To the top of the column, aqueous NH 3  or pure water is charged, which absorbs NH 3  in a cooled absorption circuit, whereby aqueous NH 3  is formed, in which the acid gases are absorbed. The bottom product is supplied to the NH 3  stripper, whose head vapors, which still contain some CO 2 , are introduced into the acid gas absorber. The bottom product of the NH 3  stripper is supplied to the deacidifier. To the top of the deacidifier, pure water is added, which also can originate from the sump of the total stripper, in order to completely absorb NH 3  from the acid gas. The bottom product of the deacidifier is supplied to the total stripper, whose top product is introduced into the sump of the acid gas absorber. The bottom product is NH 3  and waste water free from CO 2 . 
     From DE 25 27 985 a variant of this process is known, in which the liquid feedstock is added to the deacidifier. One part of it is added preheated and at the lower part, whereas another part is added further above, in order to condense steam and absorb most of the NH 3 . 
     In the process disclosed in EP 0 212 690 B1, the main part of the liquid feedstock is added to the total stripper preheated, whereas the rest is supplied cold to the deacidifier. The top product of the total stripper is largely condensed and then added to the NH 3  stripper further down than the sump of the acid gas absorber. This process is particularly favorable with a low acid gas concentration in the feedstock. 
     In the process according to EP 0 654 443 A1 deacidifier and total stripper are combined in one column. In the middle of the column, deacidified vapors are withdrawn and largely condensed at equal pressure. Further above, the non-condensed vapors are recirculated into the deacidifier. The liquid is introduced into the NH 3  absorber, which is combined with the acid gas absorber to one column. 
     The process according to EP 1 135 331 B1 differs from the process described above in that the head vapors of the total stripper are condensed, wherein the liquid is introduced into the NH 3  stripper. 
     In the process mentioned above, volatile water-soluble organic components in particular in the acid gas absorber can be enriched to such an extent that the processes do not work anymore. 
     SUMMARY OF THE INVENTION 
     Therefore, it is the object of the present invention to provide a process and a plant for recovering NH 3 , in which a strong enrichment of organic components is avoided. 
     In accordance with the invention, this object is solved in that a partial stream of the liquid phase is withdrawn from the acid gas absorber and processed such that gaseous NH 3  with a reduced content of volatile organic components is obtained, which is recirculated into the acid gas absorber. 
     Surprisingly, it was found that the content of volatile water-soluble organic components in the liquid phase of the acid gas absorber can be limited by means of the process of the invention such that the same do not disturb the smooth operation. 
     The fractions obtained by processing the withdrawn partial stream of the acid gas absorber, which contain the removed volatile organic components of the liquid phase withdrawn from the acid gas absorber, can be discharged from the process and be supplied to the disposal. 
     In particular, the process can be employed when the low-boiling water-soluble organic components contain alcohols, ketones, nitriles and/or pyridine bases. 
     In accordance with a preferred embodiment of the invention, the partial stream withdrawn from the acid gas absorber is supplied to a first stripping column, in which all volatile components largely are stripped. The top product of the stripping column is partially condensed, and the remaining vapors are recirculated into the acid gas absorber. This involves the advantage that a large part of the originally present water-soluble volatile organic compounds remains in the condensed fraction, whereas the recirculated vapors have a smaller content of volatile organic compounds. 
     To improve the total yield of the process, the partial condensate obtained preferably is supplied to a further stripping column, in which NH 3  and acid gases are stripped to such an extent that the organic components are concentrated in the sump. The top product of the second stripping column, whose content of volatile organic compounds correspondingly is further reduced, is recirculated into the acid gas absorber. 
     It was found that the separation of volatile organic compounds via one or more stripping columns is particularly effective when the stripping column(s) is(are) operated each at a pressure of 2 to 20 bar. Preferably, the pressure is adjusted to 4 to 16 bar, particularly preferably to about 6.5 bar. 
     In a further embodiment, the bottom product of the second stripping column can be expanded to the pressure of the acid gas absorber and be mixed with the bottom product of a column in which the NH 3  is purified, for instance by distillation. In this case, flash vapors are supplied to the acid gas absorber. 
     It may occur that higher-boiling water-soluble organic components are enriched more in the upper part of the NH 3  stripper than in the acid gas absorber. In this case, a liquid or vaporous partial stream can also be withdrawn from the upper part of the NH 3  stripper, which is processed as described above. 
     The NH 3  which ultimately is largely liberated from acid gas at the top of the acid gas absorber preferably is supplied to further cleaning stages and/or liquefied. 
     In accordance with a development of the invention, the aqueous NH 3  directed to the acid gas absorber, which serves as absorbent, is split up. One part of the aqueous NH 3  is charged to the top of the acid gas absorber and one part of the aqueous NH 3  is supplied to the bottommost absorption circuit of the acid gas absorber. Surprisingly, it was found that the total concentration of the volatile organic components in the liquid phase of the acid gas absorber thereby can additionally be reduced. 
     Furthermore, the present invention relates to a plant for recovering NH 3 , which in particular is suitable for performing the process described above. 
     In accordance with the invention, the plant includes an acid gas absorber operated with aqueous NH 3 , wherein downstream of the acid gas absorber a first stripping column is provided, to which a partial stream of the liquid phase from the acid gas absorber is supplied, and wherein the top of the first stripping column is connected with the acid gas absorber. 
     To increase the total yield, the plant of the invention includes a further stripping column in accordance with a preferred embodiment, to which the partial condensate of the first stripping column is supplied and in which NH 3  and acid gases continue to be stripped, wherein the top of the further stripping column is connected with the acid gas absorber. 
     The NH 3  withdrawn from the acid gas absorber is liquefied in a liquefier, wherein in accordance with a further feature of the invention the liquefier is connected with the top of the acid gas absorber via return conduits. 
     In addition, a connection of the liquefier with the bottommost absorption circuit of the acid gas absorber is provided via return conduits. 
     An inventive configuration of the plant consists in that the NH 3  stripping column is connected with the first stripping column via a conduit, so as to be able to direct a liquid or vaporous partial stream from the NH 3  stripping column onto the first stripping column. 
     Further features, advantages and possible applications of the invention can also be taken from the following description of embodiments and the drawing. All features described and/or illustrated form the subject-matter of the invention per se or in any combination, independent of their inclusion in the claims or their back-reference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a process flow diagram of a plant for recovering NH 3  from a mixture containing NH 3  and acid gases in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A waste water to be treated, which usually is a condensate from the degasification or gasification of coal, is supplied in conduit  1 . In column  2 , the waste water is treated with stripping gas from conduit  3 , further stripping gas comes from conduits  58  and  4 . The loaded stripping gas leaves the column  2  via conduit  5  and is supplied to a non-illustrated processing. 
     The water coming from the column  2  via conduit  6  now chiefly contains NH 3 , CO 2  and H 2 S and a low concentration of further gases. Most of the waste water is charged through conduit  7  to a total stripping column  8  in which all strippable gases are removed from the waste water. In the sump of the total stripping column  8 , whose trays are not shown for simplification, a reboiler  9  known per se is provided, which ensures the required buoyancy of the gases. Treated waste water flows off via conduit  10 . A small part of the treated waste water is directed to the top of the deacidifier column  52 , while the rest leaves the plant via conduit  74 . The gases stripped in the total stripping column  8  flow upwards through the gas-permeable tray  11  into the partial condenser  12 , in which they are sprinkled with circulating cooled condensate, so as to condense steam above all. Via conduit  13 , under the influence of a non-illustrated pump, the condensate flows through an external cooler  14  and via conduit  15  back into the partial condenser  12 . Excess condensate is supplied to the total stripping column  8  through the overflow  16 . 
     The top product of the total stripping column  8  leaves the partial condenser  12  in conduit  17  and enters a further condenser  18 , in which intensive cooling is effected, so that a liquid phase is formed. The condenser  18  has a liquid circulation through the conduits  19 ,  20  and  21 , wherein temperature is kept low by an external cooler  22 . The liquid phase of the condenser  18  absorbs no inert gases such as N 2 , H 2 , CO and CH 4 . The same flow off via conduit  4  and serve as additional stripping gases in column  2 . 
     A partial stream of the liquid phase from conduit  19  is withdrawn via conduit  23  and charged to an NH 3  stripping column  24 , in whose sump a reboiler  25  is arranged. The NH 3  stripping column has e.g. ten to thirty trays, wherein the liquid phase from conduit  23  for instance is supplied to a middle tray. Via conduit  26 , the top product of the NH 3  stripping column  24 , a gas rich in NH 3 , which contains rests of acid gases, initially is supplied to the acid gas absorber  27  in the first washing stage  28 . The same is equipped with an absorption circuit with the conduits  29 ,  30  and  31  with an external cooler  32 . From the sump of the acid gas absorber  27 , a partial stream of the absorbate flows as reflux through conduit  33  to the top of the NH 3  stripping column  24 . Above the washing stage  28  of the acid gas absorber  27 , a second washing stage  34  and a third washing stage  35  are located, between which a tray  36  is arranged, which transmits gas to the top and liquid to the bottom. The third washing stage  35  is equipped with an absorption circuit with the conduits  37 ,  38  and  39  and the external cooler  40 . From the third washing stage  35 , virtually pure NH 3  flows through conduit  41  to the NH 3  liquefier  42 . Via conduit  43  liquid NH 3  and via conduit  44  organic components are discharged from the NH 3  liquefier  42  as absorbent, clean water is supplied to the NH 3  liquefier  42  via conduit  45 . As absorption liquid of the acid gas absorber  27 , aqueous NH 3  from the NH 3  liquefier  42  is used, which is discharged via conduit  46  and for one part fed into conduit  37  and supplied to the washing stage  35  via conduit  47  and for the other part is fed into conduit  30  of the absorption circuit of the washing stage  28  via conduit  48 . 
     Under the influence of a non-illustrated pump, the water withdrawn from the sump of the NH 3  stripping column  24  initially flows through conduit  49  to a heat exchanger  50  and through conduit  51  into the deacidifier column  52 , in which a gas mixture rich in NH 3 , CO 2  and H 2 S is stripped by means of the reboiler  53 . After passing through the heat exchanger  55 , the water withdrawn from the sump of column  2  via conduit  6  flows through the conduits  54  and  56  to the deacidifier column  52 . Further above, cooled water discharged from the total stripping column  8  is fed into the deacidifier column via conduit  57 . The gas mixture is washed with the cooled water, before it is supplied via conduit  58  to column  2  for further processing as stripping gas. In the deacidifier column  52 , deacidified water is obtained as bottom product, which contains NH 3  and minor amounts of CO 2  and H 2 S. This bottom product is withdrawn via conduit  59  and initially releases part of its heat in the heat exchanger  50  and then partly flows into the total stripping column  8  via the conduits  60  and  61 . A partial stream adjustable by the valve  62  is discharged via conduit  63  and admixed to the liquid phase, which flows off from the condenser column  18  via the conduits  19 ,  20 . 
     Via conduit  64 , part of the liquid phase of the acid gas absorber  27  is supplied to a first stripping column  65 , in which NH 3 , acid gases and volatile organic components largely are stripped. Via conduit  66 , the bottom product of the first stripping column  65  is conveyed onto a lower tray of the total stripping column  8 . Via conduit  67 , the top product of the first stripping column  65  is supplied to a condenser  68  and cooled to a temperature achievable with cooling water. Most of the water and the organic components is condensed, and part of the NH 3  and the acid gases is absorbed. 
     Via conduit  69  the vapors are recirculated into the acid gas absorber  27 , and via conduit  70  the liquid phase is charged to a second stripping column  71 , in which NH 3  and acid gases are stripped to such an extent that the organic components are concentrated in the sump and only for a small part leave the stripping column  71  with the top product. The top product is recirculated to the acid gas absorber  27  via conduit  72 . 
     In a particular embodiment, part of the recirculated aqueous NH 3  withdrawn via conduit  46  is branched off and not supplied to the top of the acid gas absorber  27  via the conduits  47  and  39 , but via the conduits  48 ,  30  and  31  to the lower absorption circuit of the acid gas absorber  27  via the external cooler  32 . While, as described in the preferred embodiment, total concentrations of the organic components in the liquid phase of the acid gas absorber  27  of 17.5 wt-% are achieved by supplying the aqueous NH 3  as absorbent exclusively to the top of the acid gas absorber  27 , the concentration can be reduced to 9.5 wt-% by introducing a partial stream into the lower absorption circuit of the acid gas absorber  27 . 
     The bottom product of the second stripping column  71  can be supplied via conduit  73  to a further disposal or, upon mixing with the cooled bottom product of a column in which the NH 3  is purified for instance by distillation, be expanded to the pressure of the acid gas absorber  27 . Then, the flash vapor also is supplied to the acid gas absorber  27 . 
     EXAMPLE 
     In a procedure corresponding to the drawing, the contents of NH 3 , CO 2 , H 2 S, H 2 O and organic compounds as indicated in Table 1 were determined for the various conduits: 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 H 2 O 
                 NH 3   
                 H 2 S 
                 CO 2   
                 Org. comp. 
                 Temp. 
                 Pressure 
               
               
                 Conduit 
                 [kmol/h] 
                 [kmol/h] 
                 [kmol/h] 
                 [kmol/h] 
                 [kmol/h] 
                 [° C.] 
                 [bar] 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 193448.9 
                 932.7 
                 26.1 
                 856.6 
                 50.2 
                 43 
                 6 
               
               
                 3 
                 23.8 
                 0.0 
                 8.1 
                 304.2 
                 0.0 
                 47 
                 2 
               
               
                 5 
                 90.7 
                 0.1 
                 30.9 
                 1159.9 
                 0.8 
                 46 
                 2 
               
               
                 6 
                 193373.7 
                 953.8 
                 231.2 
                 184.8 
                 51.2 
                 110 
                 2 
               
               
                 23 
                 11092.4 
                 4245.4 
                 696.2 
                 203.0 
                 201.0 
                 60 
                 3 
               
               
                 26 
                 61.1 
                 1336.8 
                 24.5 
                 0.1 
                 106.1 
                 62 
                 3 
               
               
                 33 
                 832.6 
                 469.9 
                 23.1 
                 0.1 
                 59.3 
                 50 
                 3 
               
               
                 74 
                 194450.8 
                 2.1 
                 0.0 
                 0.0 
                 0.4 
                 63 
                 19 
               
               
                 43 
                 0.0 
                 809.2 
                 0.0 
                 0.0 
                 0.0 
                 43 
                 18 
               
               
                 49 
                 11864.0 
                 3378.5 
                 694.8 
                 203.0 
                 154.2 
                 84 
                 3 
               
               
                 57 
                 9088.6 
                 44.8 
                 10.9 
                 8.7 
                 2.4 
                 40 
                 16 
               
               
                 58 
                 3.4 
                 0.0 
                 227.3 
                 184.6 
                 0.0 
                 45 
                 14 
               
               
                 59 
                 21970.5 
                 3423.3 
                 478.6 
                 27.0 
                 166.6 
                 154 
                 14 
               
               
                 63 
                 8832.1 
                 1376.2 
                 192.4 
                 10.8 
                 63 
                 93 
                 11 
               
               
                 64 
                 337.6 
                 171.2 
                 1.6 
                 0.0 
                 20.9 
                 51 
                 7 
               
               
                 66 
                 313.4 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 163 
                 7 
               
               
                 67 
                 24.2 
                 171.2 
                 1.6 
                 0.0 
                 20.9 
                 100 
                 6 
               
               
                 69 
                 0.2 
                 118.8 
                 0.0 
                 0.0 
                 7.0 
                 36 
                 3 
               
               
                 70 
                 24.0 
                 52.4 
                 1.5 
                 0.0 
                 13.9 
                 50 
                 6 
               
               
                 72 
                 0.2 
                 39.8 
                 0.1 
                 0.0 
                 0.5 
                 40 
                 3 
               
               
                 73 
                 23.8 
                 12.7 
                 1.4 
                 0.0 
                 13.4 
                 90 
                 6 
               
               
                 44 
                 23.3 
                 48.4 
                 0.0 
                 0.0 
                 34.2 
                 95 
                 17 
               
               
                 47 
                 576.1 
                 210.3 
                 0.0 
                 0.0 
                 8.7 
                 70 
                 17 
               
               
                 48 
                 562.9 
                 205.5 
                 0.0 
                 0.0 
                 8.5 
                 70 
                 17 
               
               
                 45 
                 1132.4 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 40 
                 5 
               
               
                   
               
             
          
         
       
     
                                           List of Reference Numerals                                     2    stripping gas column            8    total stripping column            9    reboiler           11   gas-permeable tray           12   partial condenser           14   cooler           16   overflow           18   condenser column           22   cooler           24   NH 3  stripping column           28   washing stage           32   cooler           27   acid gas absorber           35   washing stage           36   overflow           40   cooler           42   NH 3  liquefier           50   heat exchanger           52   deacidifier column           53   reboiler           65   first stripping column           68   condenser           71   second stripping column                        
The remaining reference numerals refer to conduits.