Abstract:
The invention relates to a process for the absorptive separation of NH 3  and CH 4  from a gas under high pressure, which at least contains NH 3 , H 2 , N 2  and CH 4 , using a high-boiling, physically acting and regenerable solvent which contains homologues of alkylene glycol-alkyl-ether and which also may contain water, the absorbed components NH 3 , H 2 , N 2  and CH 4  being separated from the laden solvent in at least two further process steps at different pressure rates, thereby withdrawing at least one NH 3 -rich and at least one CH 4 -rich gas fraction from the solvent. This process is particularly suitable to be incorporated as unit in an ammonia production plant.

Description:
This application is a 371 of International Application No. PCT/EP02/03812, filed on Apr. 5, 2002. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a process for the absorptive separation of NH 3  and CH 4  from a gas under high pressure (&gt;50 bar abs.), which at least contains NH 3 , H 2 , N 2  and CH 4 , hereinafter referred to as synthesis gas. NH 3 -rich synthesis gas is chiefly available in processes for generating NH 3  from such synthesis gas, the conversion rate of said processes being really low because of the temperatures, pressures and catalysts that are applied, and the NH 3  produced from synthesis gas having to be removed from a non-reacted gas stream. It is pointed out, however, that the invention is by no means restricted to this specific application. 
     Conventional plants for generating NH 3  from synthesis gas are designed as loop systems operating at high pressure. Said configuration provides for the compression of the synthesis gas that contains H 2 , N 2  and inert gas fractions, inter alia CH 4 , to a high pressure in the first step and then for the feed of the compressed gas to a reactor system in which part of the synthesis gas, i.e. 10 to 20%, is converted to NH 3 . The gas mixture obtained downstream of the reactor system is cooled with the aid of water such that as large a portion as possible of the NH 3  formed condenses and can be withdrawn as liquid. In order to provide for the condensation of a further NH 3  portion of the non-reacted gas mixture, additional cooling to much lower temperatures is required, hence an expensive refrigeration cycle. As it is indispensable to reduce the processing costs to an economic level, NH 3  is separated only up to a residual content of approx. 4 molar % in the non-reacted gas stream. 
     In the case of a product concentration of 20 molar % at a synthesis pressure of 200 bar, for example, the dew point of NH 3  is approx. 57° C. When providing a cooling by means of water, for instance, to 35° C., it is possible to reduce the NH 3  content in the gas to 11.2 molar %, which permits a yield of 59% of the condensable product quantity. As the recycle gas fed to the reactor should have as low an NH 3  concentration as possible, in this particular case 3.8 molar-%, it is common practice to install a low-temperature cycle downstream of the water cooling system so that further product amounts can be condensed at even lower temperatures (e.g. cooling to −10° C. to 0° C.). 
     Upon NH 3  separation a purge stream is permanently withdrawn from the synthesis loop unit which prevents that the loop is enriched with gas fractions that are inert vis-à-vis the NH 3 -producing reaction as, for example, CH 4  entrained by fresh synthesis gas. It is also necessary to recover residual NH 3  and valuable fractions of the synthesis gas from the purge stream withdrawn, said fractions being re-compressed and then recycled to the synthesis loop. The loop is closed downstream of the purge stream withdrawal section by providing a circulator that compensates the pressure drop and by balancing the synthesis gas loss in the reaction through the admixture of fresh synthesis gas to the recycle synthesis gas. 
     But this system has the disadvantage that, for example, the NH 3  separation at 180 bar synthesis pressure can be efficiently carried out down to a residual content of about 4 molar % only. In the case of a reaction system arranged downstream within a synthesis loop or fresh-gas reaction system of the NH 3  separation unit, the said residual content will essentially equal the NH 3  inlet concentration; the dilution caused by the intermediate admixture of fresh synthesis gas would change the NH 3  inlet concentration to a minor extent only. Compared to synthesis gas that has no NH 3  content, the NH 3  inlet concentration of about 4 molar % would only permit a yield of about ⅘ of the NH 3  amount recoverable per loop cycle. 
     Another disadvantage is that an expensive method is required to separate further NH 3  from the purge stream withdrawn if its exploitation is not abandoned. The higher the purge stream rate, the larger the NH 3  amount to be separated. But when the said rate is kept low, the inerts such as CH 4  are enriched in the loop synthesis gas and their partial pressure reduces the yield obtained in the reaction system and the portion of NH 3  that can be recovered with the aid of cooling water. 
     The criteria described in the previous paragraphs also apply to NH 3  production plants that are not designed as loop systems because the NH 3  portion not converted to synthesis gas will be exploited, for example, by downstream synthesis units. This also involves the need to separate as large an NH 3  portion as possible from the synthesis gas downstream of the reaction system and to keep the inerts concentration low. 
     Hence, there has been a keen interest for years on the part of the chemicals industries to exploit by economic methods even small residual amounts of NH 3  contained in the synthesis gas. A series of tests were carried out to remove NH 3  by scrubbing; in most cases the solvent was an aqueous solution. This, however, involved on the one hand the problem to remove the dissolved NH 3  from said solution, on the other hand the need to avoid volatilisation of fractions of the aqueous solution during scrubbing, said fractions entering the synthesis gas and thus causing technical problems in the downstream equipment, for example, poisoning of the catalyst. The said problems aroused the technological prejudice that there is not a safe and economic method to separate the NH 3  from the synthesis gas by scrubbing. Moreover, there had been some interest in a selective removal of inerts, such as CH 4 , from the synthesis loop in order to reduce the necessary purge stream to a minimum. 
     It has also been described, for example, in German patent DE OS 1 924 892, that NH 3  is absorbed from the gas mixture leaving the conversion zone, with the aid of a slowly evaporating organic solvent and that the absorbed NH 3  is recovered upon solvent regeneration. Various alkylene glycol solvents have been suggested but in view of operational problems and related efficiency setbacks, said process has never achieved a breakthrough on the market for over 30 years. Patent WO 90/08736 A1 describes a further process of this type but on account of poor efficiency of this system in NH 3  synthesis plants operated at a loop pressure of &gt;100 bar, this process also failed on the market. A further process is outlined in DD 135 372 which provides for scrubbing to remove NH 3  from off-gas or desorption gas with the aid of organic liquids such as ethylene glycol, di- or triethylene glycol or their mono- or dimethyl ether or mixtures thereof which may also contain up to 20% of water. 
     Hence, the aim of the invention is to overcome the said disadvantage and to provide a very efficient process suited to separate NH 3  and CH 4  from the synthesis gas irrespective of the operating pressure level. 
     SUMMARY OFTHE INVENTION 
     The aim of the invention is achieved as follows: a high-boiling, physically acting and regenerable solvent which contains homologues of alkylene glycol-alkyl-ether and which also may contain water and that is suited to absorb the components NH 3 , H 2 , N 2  and CH 4  from the synthesis gas and to remove said components from the laden solvent in at least two further process steps at different pressure rates, thereby withdrawing at least one NH 3 -rich and at least one CH 4 -rich gas fraction from the solvent. This method is applied to regenerate the solvent. In this context the term “high-boiling” is understood to mean a solvent the vapour pressure of which is sufficiently low to preclude any contamination of the pressure gas under the selected process parameters. The solvent is regarded to be physically acting if it does not form any chemical compound with the NH 3 . And it is regarded as regenerable if the solvent and NH 3  constitute a wide-boiling binary system. 
     As the solvent is regenerable, it is feasible and efficient to design the absorption process as a closed cycle. Compared to other solvents, such as glycols, the homologues of alkylene glycol-alkyl-ether have the advantage that they are very inert to reaction and, hence, they react neither with the substances to be separated nor with other components of the synthesis gas. Moreover, they have a very low viscosity which on the one hand improves the mass transfer in the absorption and on the other hand helps to save pump energy. 
     The process described is particularly suited for the separation of NH 3  and CH 4  in ammonia production plants because it permits in a single absorption step to withdraw from the synthesis gas almost completely the product obtained and the CH 4  normally enriched in the synthesis loop. In comparison to the conventional state of the art, the process has a major advantage, i.e. the operating pressure remains unchanged whereas the partial pressure of the feedstocks are raised, which improves the yield and allows a lower purge stream rate, thus saving plant and operating costs. 
     The absorption process takes place in the temperature range from −30° C. to +50° C., each temperature selected necessitating a suitable solvent from the group of homologues of alkylene glycol-alkyl-ether and water being admixed to said solvent. The temperature range referred for this absorption is 0° C. up to +40° C., which especially applies to cases in which the process described in this invention is used in plants for NH 3  production. 
     The absorption step may either be part of a conventional scrubbing in which the liquid solvent comes directly into contact with the synthesis gas, but it may also take place in devices in which said solvent does not directly come into contact with the synthesis gas. In a further embodiment of the invention the absorption step takes place in a contactor equipped with a diaphragm suitable to partition the gas side from the liquid side and permeable to the gas components but impermeable to the solvent, so that the solvent does not come into direct contact with the synthesis gas. This method has a special advantage because it definitely prevents the penetration of solvent into the synthesis gas so that the steam pressure required for the solvent decreases accordingly vis-à-vis that needed for scrubbing, thereby improving the viscosity and the solubility in NH 3  and CH 4  of the solvent. An additional advantage of this method is that the diaphragm has a substantially larger contact surface with regard to volume than that provided for processes with direct contact of solvent and synthesis gas. It is recommended that the diaphragm be arranged in one or several contactors of modular type and be designed as capillary components conveying the solvent. In comparison to the solvents known to be used for diaphragm contactors according to, for example, EP 0 751 815 B1 the homologues of alkylene glycol-alkyl-ether exhibit a major advantage of lower viscosity, a fact that really permits cost-effective conveyance through capillary components and this constitutes an advantage of the invention. 
     A further embodiment of the invention provides for solvent regeneration in at least three process steps. When implementing this configuration in a plant for NH 3  production from synthesis gas it is recommended that the solvent first passes through the arrangement of at least three process steps designed to reduce the operating pressure and, optionally, increase the operating temperature of the solvent so that the dissolved gases are removed, said steps being called flashing steps. The solvent then flows through a downstream rectification step and a regeneration step operated at atmospheric or negative pressure. The first flashing step is used to reduce the pressure of the laden solvent to a value that permits evaporation of H 2 -rich gas from the solvent. The second step provides for flashing to a pressure that is suited for the development of CH 4 -rich gas and the third step for a further pressure reduction permitting the development of NH 3  vapour. This configuration enables the generation of three gas streams which represent and advantage of the invention. The H 2 -rich gas stream, for example, can be recycled to the NH 3  synthesis system or exploited as heating agent and the CH 4 -rich stream, for example, is suitable for recycling to the plant for generation of NH 3  synthesis gas or exploitable for heating. 
     Further generation of the solvent or of a part-stream thereof is effected by thermal regeneration implemented as rectification, preferably in two steps: first at a pressure above the atmospheric pressure so that the vapours from the column are condensable by an economic method and subsequently below the atmospheric pressure or partial vacuum. This can be turned to an advantage by compressing the vapours to such an extent that it becomes condensable together with the vapours from the upstream regeneration steps. The last regeneration step carried out under partial vacuum alternatively can be implemented as flashing step. A further embodiment of the invention provides for the feed of the desorbed NH 3  vapour to the intake side of a coolant compressor. The liquid NH 3  obtained in the coolant compressor is exploited as reflux for the upstream rectification step. 
     When supplying larger amounts of heat to the flashing steps it is possible to increase the quantity of NH 3  evaporated from the solvent. In this case it is an advantage to re-use low-temperature heat, particularly waste heat from other process steps. It is also possible to implement the flashing steps in a split mode, i.e. decreasing the pressure in a first individual step and raising the temperature in a second. 
     In a further embodiment of the invention the compressed NH 3  vapour is scrubbed with the aid of liquid NH 3  from a refrigeration system and is subsequently recycled to a cold flashing step, so that solvent losses are avoided. Said refrigeration unit can be beneficially integrated into the regeneration process. 
     A further embodiment of the invention provides for a regeneration of the solvent using inert gas. The stripping agent required can be flash gas withdrawn from the process itself or heating gas taken from synthesis gas loop or steam generation unit upstream of the NH 3  synthesis process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a flow diagram for the absorptive separation of NH 3  and CH 4  from a gas under pressure which contains at least NH 3 , H 2 , N 2  and CH 4 ; 
         FIG. 2  is a flow diagram as depicted in  FIG. 1  with further NH 3  degassing stations, solvent recovery from the gaseous NH 3  and a decoupling facility of solvent regeneration; and 
         FIG. 3  is a flow diagram of a process in accordance with the invention as depicted in  FIG. 2  wherein liquid NH 3  vapour is produced in a condensation unit tied into the system. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention is illustrated in the three PFDs which show a typical configuration.  FIG. 1  depicts the invented process which includes an absorption step, several pressure reducing units and a regeneration system for the solvent in a multi-stage desorption. It is possible to provide various locations for NH 3  absorption in the NH 3  production process and, optionally, several absorption devices may be arranged in a single plant section. The representation of just one absorption step is shown in  FIG. 1  and  FIG. 2 , hence, is to be understood that several absorption steps may exist and that the regeneration of solvent and the NH 3  recovery described in this document may also be combined for all absorption steps. 
     NH 3 -rich synthesis gas  1  is fed at a pressure of approx. 180 bar (abs.) to absorption step  2  in which NH 3  is absorbed by a solvent. NH 3 -lean synthesis gas  3  is withdrawn from absorption step  2  and piped to a downstream unit not represented in the diagram. Laden solvent  4  is reduced to a pressure of 60 bar (abs.) in pressure reducing station  5  and then sent to H 2  degassing step  6  in which H 2 -rich non-reacted gas  7  separates from the solvent. Said off-gas  7  may either be admixed to the synthesis gas or be used for heating. Laden solvent  8  undergoes a further pressure reduction to 12 bar (abs.) in pressure reducing station  9  and conveyed to CH 4  degassing station  10  in which CH 4 -rich off-gas  11  separates from the solvent. Said off-gas  11  may either be admixed to the feed gas used for a reforming process to produce synthesis gas or be exploited for heating. Laden solvent  12  is heated in heat transfer station  13  with the aid of regenerated solvent  14  and undergoes pressure reduction to 10 bar (abs.) in pressure reducing station  15  in order to be fed to NH 3  degassing station  16 , thus obtaining NH 3 -rich off-gas  17  from the solvent and exploiting this gas for NH 3  recovery. 
     Solvent  18  that is still laden with NH 3  is fed to pressure desorption step  19  which, for example, may be designed as rectification column and supplies desorbed NH 3  condensate  20  as overhead product. Partly regenerated solvent  21  obtained as bottom product undergoes pressure reduction to 1 bar (abs.) in pressure reducing station  22  and is piped to low-pressure desorption step  23  which may also be designed as rectification column. The NH 3  obtained by this method is recycled to said step  23  via NH 3  vapour recycle line  24 , compression unit  25  and NH 3  recycle line  26 . An extremely beneficial equipment design for this application is to send the vapours from the rectification column, which serves as low-pressure desorption step, directly to the intake side of a coolant compressor and to use the liquid NH 3  thus obtained for reflux so that the functions of compression unit  25  and of cooling the overhead product from low-pressure desorption step  23  are implemented simultaneously. The heat contained in regenerated solvent  27  is exploited in heat transfer station  13  and said solvent  14  is then re-used in absorption step  2 . 
       FIG. 2  illustrates further embodiments of the invention, compared to  FIG. 1 , in particular with further NH 3  degassing stations, solvent recovery from the gaseous NH 3  and decoupling facility of solvent regeneration. The nomenclature of reference numbers  1  to  17  are applicable to  FIG. 2  by analogy to  FIG. 1 , the only difference being pressure reducing station  15  arranged upstream of heat transfer station  13 . This configuration facilitates an incorporation of both steps into one equipment unit; the specialist skilled in the art will select the most beneficial version in each case. 
     Laden solvent  18  is heated in heating device  28  which preferably uses waste heat from other process steps. This entails a shift of the solution equilibrium and, hence, further NH 3 -rich gas is liberated in NH 3  degassing station  29 . This gas should be mixed with NH 3 -rich gas  17 . Laden solvent  31  can be further heated in heat transfer station  32  using regenerated solvent  27  so that the solution equilibrium is further shifted which causes liberation of further NH 3  in downstream NH 3  degassing station  34 . NH 3 -rich gas  35  thus obtained may also be mixed with NH 3 -rich gas  17  and/or  30 . When considering the overall configuration this is a multi-stage heat shifting system from the regenerated solvent to the laden solvent, including additional heat supply, the adequate arrangement of such heat supply station being selectable depending in each case on the local conditions and in particular on the available sources of heat. 
     The NH 3 -rich gas obtained from degassing stations  16 ,  29  and  34  and from admixing stations  17 ,  30  and  35  is cooled to the NH 3  dew point in cooling station  36  prior to the gas compression. Apart from NH 3 , NH 3  vapour  37  also contains small portions of CH 4 , H 2  and evaporated solvent. 
     Laden solvent  38  is piped from NH 3  degassing station  34  and, if necessary, via a further pressure reducing station  39  to pressure desorption step  19  which is shown as a mere stripping column in the flow diagram ( FIG. 2 ). Vaporous NH 3  is obtained but it contains impurities, chiefly solvent portions. Partly regenerated solvent  21  is withdrawn from the bottom, sent to pressure reducing station  22  to be further degassed and then fed to low-pressure desorption step  23  in which the residual NH 3  is removed from the solvent as NH 3 vapour  41 . Regenerated solvent  27  is now recycled via the two heat transfer stations  32  and  13  to absorption step  2  so that this completes the solvent cycle. 
     Contrary to the example shown in  FIG. 1 , the NH 3  vapours separated in the two desorption steps  19  and  23  as shown in the example in  FIG. 2  as well as small portions of the NH 3 -rich gas separated in degassing stations  16 ,  29  and  34  yet contain some solvent which cannot be tolerated in the final product and hence must be removed. According to the invention this requirement is met when NH 3  vapours  43 ,  41  and  37  are combined downstream of cooling stations  42 ,  40  and  36  as well as the subsequent compression units  25  and  45 , thus forming the respective admixtures  44  and  46 . In this context it is recommendable to provide a further compression station  47  to adjust the pressure such that it is easy to liquefy NH 3  vapour  48  at temperatures that need not be very low. The impurities contained in the solvent are removed in post-scrubber  49  which uses liquid NH 3    50  as scrubbing liquid. Purified NH 3  vapour  51  leaves the scrubber at the top, which may be designed as column with few trays, and said vapour can directly be used for further processing or sent to a refrigeration unit for the production of liquid NH 3 . The liquid NH 3 /solvent mixture  52  obtained in post-scrubber  49  is either sent to CH 4  degassing station  10  or mixed with laden solvent  12  so that it is recycled to the solvent loop, thereby reducing the solvent losses. A further advantage is that the pressure constancy of the regeneration system is improved. 
       FIG. 3  shows an embodiment of the process in accordance with the invention and constitutes a supplement to  FIG. 2 . In this case, liquid NH 3  which is used to remove the solvent from the laden NH 3  vapour is produced in a condensation unit tied into the system. 
     Prior to being compressed in station  47 , the NH 3  vapour obtained downstream of admixing station  46  is cooled in station  53  to an extent that bears no risk for the compressor and the compressed NH 3  vapour is further cooled in cooling station  54  so as to reach the NH 3  dew point which, however, must not be exceeded. Subsequent scrubbing takes place in post-scrubber  49  in accordance with the example shown in  FIG. 2  but NH 3  vapour  51  is piped to NH 3  condenser  55  in which NH 3  almost completely condenses. Minor portions of uncondensable gases, chiefly CH 4 , are withdrawn as CH 4 -rich off-gas  56 . The condensed liquid NH 3    57  is mainly drawn off as liquid NH 3  product  58 , but the remaining portion of liquid NH 3    50  is exploited as scrubbing liquid in post-scrubber  49 . 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 Key to diagrams: 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 NH 3 -rich synthesis gas 
               
               
                 2 
                 Absorption step 
               
               
                 3 
                 NH 3  lean synthesis gas 
               
               
                 4 
                 Laden solvent 
               
               
                 5 
                 Pressure reducing station 
               
               
                 6 
                 H 2  degassing step 
               
               
                 7 
                 H 2 -rich off-gas 
               
               
                 8 
                 Laden solvent 
               
               
                 9 
                 Pressure reducing station 
               
               
                 10 
                 CH 4  degassing station 
               
               
                 11 
                 CH 4 -rich off-gas 
               
               
                 12 
                 Laden solvent 
               
               
                 13 
                 Heat transfer station 
               
               
                 14 
                 Regenerated solvent 
               
               
                 15 
                 Pressure reducing station 
               
               
                 16 
                 NH 3  degassing station 
               
               
                 17 
                 NH 3 -rich off-gas 
               
               
                 18 
                 Laden solvent 
               
               
                 19 
                 Pressure desorption step 
               
               
                 20 
                 NH 3  condensate 
               
               
                 21 
                 Partly regenerated solvent 
               
               
                 22 
                 Pressure reducing station 
               
               
                 23 
                 Low-pressure desorption step 
               
               
                 24 
                 NH 3  vapour recycle line 
               
               
                 25 
                 Compression unit 
               
               
                 26 
                 NH 3  recycle line 
               
               
                 27 
                 Regenerated solvent 
               
               
                 28 
                 Heating device 
               
               
                 29 
                 NH 3  degassing station 
               
               
                 30 
                 NH 3 -rich gas 
               
               
                 31 
                 Laden solvent 
               
               
                 32 
                 Heat transfer station 
               
               
                 33 
                 Laden solvent 
               
               
                 34 
                 NH 3  degassing station 
               
               
                 35 
                 NH 3 -rich gas 
               
               
                 36 
                 Cooling station 
               
               
                 37 
                 NH 3  vapour 
               
               
                 38 
                 Laden solvent 
               
               
                 39 
                 Pressure reducing station 
               
               
                 40 
                 Cooling station 
               
               
                 41 
                 NH 3  vapour 
               
               
                 42 
                 Cooling station 
               
               
                 43 
                 NH 3  vapour 
               
               
                 44 
                 Admixing station 
               
               
                 45 
                 Compression unit 
               
               
                 46 
                 Admixing station 
               
               
                 47 
                 Compression station 
               
               
                 48 
                 NH 3  vapour 
               
               
                 49 
                 Post-scrubber 
               
               
                 50 
                 Liquid NH 3   
               
               
                 51 
                 NH 3  vapour 
               
               
                 52 
                 NH 3 /solvent mixture 
               
               
                 53 
                 Cooling station 
               
               
                 54 
                 Cooling station 
               
               
                 55 
                 NH 3  condenser 
               
               
                 56 
                 CH 4 -rich off-gas 
               
               
                 57 
                 Liquid NH 3   
               
               
                 58 
                 Liquid NH 3  product