Page 267 - Pressure Swing Adsorption
P. 267

244                                   PRESSURE SWING ADSORPTION                PSA PROCESSES                                                245
           Table 6.4.  Performance of Bergbau VSA Process for Recovery of.CO 2  from Flue Gas
                                                                                                                                           t
                                          Run  I       Run 2         Run 3
                                                                                                                                                ""'"
                                                                                                                                                OCT-"'E
                            1
                Compressor Pl (m· /h)      65           65            65                                                                    I   eROOUCT
                Compressor P2 (m' /h)     350           160           100                                                                 n
                CO puntv (%)               98.8          98.6         98.7                                                   '
                  2                                                                                                         DESOfiPTION   ,OS()RPTI()N
                Product rate (kg/h)         5.9          4.8           4.1
                Recoverv (%)               72            62           53                                                     L,J
                Total energy of Pl + P2     3.0          1.0           0.8
                kW/hkg CO 2
              Feed:  flue  gas contammg  ~ I I vol% CO at 36 m·' /h (STP);  adsorber:  four beds, each 96 liters packed
                                       2
                                                                                    I
            with wide-pore  CMS.
            So!lrc.-;  Ref.  30.
             molecular sieves, a vacuum desorption system 1s  necessary. A high setectlv1ty                      SEP»IATOFI
             for  CO  is  achieved  by  the  use  of  a  narrow-pore  carbon  molecular  sieve
                                                                                                                                           I
                                                                                                                                               =-
                    2                                                                                                                           LOW
             . adsorbent, similar to that used for mtrogen oroduction. However, the process             @----------.J,                         .........
              differs  from  the  nitrogen  orocess  m  that  the  product  (CO 2 )  is  the  more      mo
              strongly adsorbed component. A  s1mpie  two-bed vacuum swing system oper-                                      {a)
              ating  between  about  1.2  and  0.05  atm  has  been  developed  and  built  at  a
              Japanese steel works.  31   The process schematic 1s  shown m Figure 6.17.  99%
                                                                       3
              product  punty and a recovery of 80%  at  a product rate of 3000  N m /h are
              claimed.  A  more  complex  four:bed  version  of  this  process  has  also  been
              developed to the pilot plant stage. 32   The schematic of the process 1s shown m
              Figure  6.18  and  perfonnance  data  are  given  in  Table  6.4.  As  a  result  of
              environmental  pressures  the  possibility  of  extracting  CO 2  from  the  stack
              gases  of  coal-fired  power  stations  is  under  active  study  at  the  pilot  plant
              stage,31  although, with  current  technoiogy,  the  power  costs  are  too  high  to                SEO               SED
              make such a process econom1cally attractive.



              6. 7  Recovery of Methane from Landfill Gases
              A  somewhat  similar  process  has  been  developed  for  recovery  of methane               FED>
                                                                                                                                            \'ACUL'!'i  OESOIIPTI0:--1
              from·  Jandfill  gases.  12 13 · These gases contam mamly methane (50-65%) and
                               •
              carbon  dioxide  (35-50%)  as  well  as  small  amounts  of nitrogen,  with  many
                                                                                                                     CYCLE  SEQUENCE  CHART
              diffcirent  hydrocarbons  and  sulfur  compounds  m  trace  concentratmns.  A
              two-stage mitial purification process ts  employed.  In the first stage hydrogen    MlSOJl8£1l  l   ADSORP1'[0N     "~"""" ·I  :VACUUM   J
                                                                                                                                  DEPUSSUAI-
                                                                                                                                                  IIEPRl!:SSUlll-
                                                                                                                                  UTION   ·DtSOflPTIOl'il  tATibN
               sulfide  is  removed  at  a  temperature  of 40-50° C  usmg  a  bed  of iodine-im-         COCURRE!IT  lACI.JUH  ·I REPRUSURI-
                                                                                                  MISORlllCR  2   0EPR£SSURI-  DESQ,i.PTION  1A"l'ION   AOSOIIITICHf
               pregnated activated carbon. This acts as an efficient catalyst for conversion to           ZA1'10N
               elemental sulfur, and residual H S levels as low as 1 ppm can be achieved. In                                (b)
                                         2
               the  second  stage  hatocarbons  and  heavier  hydrocarbons  are  removed  in  a   Figure 6.19  (a) Simolified flow  diagram and (b) cycle sequence for the Isos 1v paraffin
               conventional activated carbon adsorber. The 'final stage of the process utilizes   separation process. (From Cassidy and  Holmes, 27   with  perm1ss10n.)
               a four-bed vacuum swmg system to recover methane from the purified landfill
               gas  using  a narrow-pore  carbon  molecular  sieve.  The  remainmg  impurities
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