Abstract:
For separating CO 2  from CO 2  -containing gases, especially stack gases and/or blast furnace gases, dimethylformamide is employed as a physical scrubbing medium to ensure high CO 2  purity. After absorption of CO 2 , the DMF is regenerated and returned into the scrubbing stage. Dimethylformamide is utilized as the scrubbing medium.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a process for the separation of CO 2  from CO 2  -containing gases, especially stack or flue gases and/or blast furnace gases, by scrubbing with a physical scrubbing medium which is thereafter regenerated to remove CO 2 , and is then recycled into the scrubbing stage. 
     Several methods are known for obtaining CO 2  from gaseous mixtures. Chemical and physical scrubbing operations, as well as adsorptive separating methods have distinguished themselves as the important processes. Important chemical scrubbing operations include, for example, the NH 3  or monoethanolamine (MEA) scrubbing processes, whereas the methanol scrubbing process conducted at low temperatures is important as a physical scrubbing operation. Adsorptive removal of CO 2  from gaseous mixtures can be effected with the aid of activated carbon or a molecular sieve. Another possibility for separating CO 2  from gaseous mixtures is the regenerator method employing the periodic application of a vacuum to purge the regenerator of CO 2 . 
     Though the technology of removing CO 2  from industrial gases is indeed very old, all the conventional processes exhibit one or more disadvantages. For example, the chemical scrubbing methods require for regeneration a very high thermal input, in the form of low-pressure steam (≧2.5 bar), for example, and such methods initially yield a water-saturated gas which, due to danger of corrosion, must be dried before being further compressed (generally to 140 bar). Physical scrubbing with methanol, as the absorbent, due to the high vapor pressure of methanol may result in very high losses of scrubbing medium. In this process, the stack gas, at ambient pressure, would either have to be compressed to high pressures, or the process would have to be conducted under extremely low temperatures to maintain losses of methanol within economical limits. However, both measures entail excessive costs in energy. Finally, the regenerator process yields a gas having too high a proportion of N 2  which would have to be subsequently separated at high expense in order to obtain sufficiently pure CO 2 . 
     SUMMARY 
     It is therefore an object of the present invention to provide an improved process of the type discussed hereinabove, especially a process wherein deficiencies of the prior art are diminished or eliminated. A further object is to provide a CO 2  product of high purity. 
     Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art. 
     These objects are attained according to the invention by utilizing dimethylformamide as the scrubbing medium. 
     Dimethylformamide (DMF) has proven to be suitable for the separation of CO 2  from gases containing at least about 8, preferably at least about 8 to 30 CO 2  in percent by volume, especially from stack gases and/or blast furnace gases. The vapor pressure of DMF is lower by two powers of ten than that of methanol, but it has practically the same high solubility for CO 2  as methanol. In this connection, the use of DMF as the scrubbing medium offers the substantial advantage of lowering the losses and costs for the operating medium while simultaneously maintaining a level of initial investment costs which, at least, are no higher than when using methanol. Moreover, since the CO 2  is obtained as an anhydrous gas, there is no need for drying the thus-separated CO 2  as is the case, for example, in chemical scrubbing operations. Another advantage in using DMF resides in that gases having any level of sulfur content (SO 2 ) can be processed without the occurrence of any problems in the scrubbing process or any contamination of the CO 2  product. The reason for this is that DMF possesses an extremely high solubility for SO 2  with simultaneously a very high selectivity. Any SO 2  contained in the gas can be transferred out by regeneration of a minimum partial stream of DMF. This partial stream, containing about 1% SO 2  by weight will handled in a small additional column, heated with steam under low pressure. The soluted SO 2  will be taken out on the top of this column as a SO 2  -rich gasstream. This SO 2  stream can be used for chemicals (H 2  SO 4 , elemental sulfur) or can be liquified for transportation. 
     The scrubbing process with the use of DMF is carried out according to a preferred embodiment under a pressure of 1.5-3 bar and at a temperature of -30° to -60° C. Thus, the stack gas which, in most cases, is not present under pressure, need only be compressed to a moderately high pressure. 
     According to a preferred embodiment of the invention, regeneration of the scrubbing medium is conducted by pressure reducing and heating the scrubbing medium. In this process, the regeneration is advantageously conducted under vacuum, specifically under a pressure of 0.05-0.3 bar, and at a temperature of between -60° and -10° C. By operating under such conditions, it is possible to obtain a product purity of CO 2  of up to 99% by volume. 
     The CO 2  liberated during regeneration is then compressed in a plurality of compression steps, e.g., 2, preferably 4 steps, suitably to, in total, about 80 to 200, especially about 140 bar. According to another preferred embodiment of the process of this invention, the loaded scrubbing medium is heated in heat exchange with a partial stream of the product CO 2 , the latter being at a medium pressure, e.g., about 6 to 20, especially about 6 to 15 bar, so that the medium-pressure CO 2  is liquefied during this step. After expansion by pressure reduction, the liquefied CO 2  is vaporized and discharged. In a preferred aspect of the invention, the CO 2  can be vaporized in a cooling coil arranged in the scrubbing column, whereby the heat of solution released during absorption of CO 2  in the scrubbing medium is removed. The removal of the CO 2  product by the use of a vacuum, and the medium-pressure auxiliary CO 2  cycle is preferably integrated into the necessary CO 2  compression system. 
     According to another preferred embodiment of this invention, the CO 2  -containing gas is cooled by means of reversible regenerators prior to the scrubbing step. The regenerators are filled with acid-resistant heat-storage regenerator packing material of a conventional kind. 
     The pure CO 2  obtained by the process of this invention can be utilized especially for injection into petroleum reservoirs. In general, the process of this invention provides CO 2  gas of at least 99.5, and preferably at least 98% by volume. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     The attached drawing is a FIGURE depicting a preferred embodiment of the invention in schematic form. 
    
    
     DETAILED DESCRIPTION 
     Stack gas under ambient pressure containing about 13.5 vol-% of CO 2  is introduced via conduit 1 and compressed to 2.4 bar in a compressor 2. The stack gas is available at a temperature of 130°-150° C. In a regenerator system 3, the stack gas is cooled conventionally to -50° C. The regenerator system 3 consists, in the present example, of two mutually separated, reversible and interchangeably connected regenerators. While, for example, in regenerator (A) stack gas is cooled off on the regenerator packing in a flow direction from the top toward the bottom, the stack gas freed of CO 2  is reheated via a conduit 4 while flowing through regenerator (B) in a direction from the bottom (the temperature level of the scrubbing process) toward the top to a temperature of about 120° C. and is discharged via a conduit 5. The flow through the regenerators is cyclically reversed after a switching time of, for example, three minutes. 
     The stack gas, cooled to -50° C., is fed via a conduit 6 to the lower section of a scrubbing column 7 and freed of CO 2  by physical absorption. The scrubbing medium employed is dimethylformamide in accordance with this invention. This scrubbing medium is introduced via conduit 8 into the upper section of the scrubbing column 7. The stack gas is thus freed of CO 2  countercurrently to downwardly flowing DMF and can be withdrawn from the head of the scrubbing column 7, expanded in turbine 9, heated in heat exchanger 10, with regenerated scrubbing medium to be cooled, and fed via conduit 4 to the regenerator system 3. The ratio of solvent to CO 2  is about 30 to 50 tons solvent to one ton of CO 2 . The number of theoretical plates in the scrubber is between 8 to 20. 
     The loaded scrubbing medium is withdrawn from the sump of scrubbing column 7 via conduit 11 and expanded in valve 12 to about atmospheric pressure. During this step, any concomitantly dissolved N 2  is removed in the gaseous phase; this N 2  is withdrawn from a separator 13 via conduit 14 and fed, together with the regenerated stack gas, in conduit 4 to the regenerator system 3. 
     The remaining scrubbing medium is withdrawn via conduit 15 from separator 13 and expanded via valve 16. During expansion, CO 2  is liberated, present under a pressure of 0.1 bar and removed by means of a fan 17 in conduit 18 from a separator 19. The partially regenerated scrubbing medium is conducted to a further separator 20 and heated therein in heat exchange with a partial stream of the product CO 2  from conduit 21 to -20° C. During this step, additional CO 2  is driven off in the gaseous phase and is admixed via conduit 22 to the CO 2  product from the first separator. The heated and completely regenerated DMF is withdrawn via conduit 22a and cooled in separator 19 in heat exchange with DMF to be regenerated, and again introduced into the scrubbing column 7 by way of conduit 8. 
     The product CO 2  in conduit 18 is compressed in compressor 23 to about 5.7 bar, in compressor 24 to 20 bar and in compressor 25 to 140 bar and discharged. 
     The partial stream of product CO 2  branched off via conduit 21 is thus under a medium pressure of 20 bar at a temperature of about 30° C. This medium-pressure CO 2  is liquefied in heat exchange with regenerated DMF, withdrawn via conduit 26 and, after expansion, via valve 27, to 6 bar, vaporized in a cooling coil 28 arranged in the scrubbing column 7. In this way, the heat of solution liberated during CO 2  absorption in the DMF is removed. Finally, the resultant preheated partial stream of CO 2  is conducted through the regenerator system 3 for further heating, and fed, at a temperature of 20° C. and under a pressure of 5.7 bar via conduit 29 to the CO 2  product upstream of the compressor 24. 
     If the stack gas contains also SO 2  besides CO 2 , this SO 2  is likewise dissolved in the DMF. Therefore, the provision is made to withdraw a partial stream of the partially regenerated DMF via conduit 30 and transfer SO 2  out of this stream. 
     A comparison of the costs for operating media when using a chemical scrubbing operation--an NH 3  scrubbing process has been selected as the example--and when using the DMF scrubbing operation of this invention is set forth below. In both cases, a raw gas containing 13.5 vol-% of CO 2  is the starting material. The product obtained is 10,000 Nm 3  /h of pure CO 2  (99 vol-%) under a pressure of 140 bar. 
     
         ______________________________________        NH.sub.3 Scrubbing                   DMF Scrubbing______________________________________Electrical EnergyMW @ DM 80     4.0 MW =     7.7 MW =          DM 320 per hour                       DM 616 per hourCooling Waterm.sup.3 @ DM 0.05          2,415 m.sup.3 h =                       435 m.sup.3 /h =          DM 121 per hour                       DM 22 per hourLow-Pressure Steam2.5 bartons/h @ DM 12 40 tons/h =  --          DM 480 per hour                       --NH.sub.3 or DMFkg/h NH.sub.3 DM 0.30 per kg          35 kg/h =          DM 10 per hourDMF DM 3.00 per kg          15 kg/h =                       DM 45 per hourCosts for OperatingMediumDM/h           931          683Pfennigs per Nm.sup.3 of CO.sub.2          9.3          6.8______________________________________ 
    
     As can be derived from the comparison, considerable savings in costs can be achieved by employing the process of the present invention. 
     Typical blast furnace and stack gases treatable by this invention have the following approximate ranges of analyses: 
     
         ______________________________________Vol %        blast furnace                   stackgas______________________________________N.sub.2      56         80CO           20         --O.sub.2      --          6CO.sub.2     22         14______________________________________ 
    
     The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples. 
     From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.