Page 104 - Pressure Swing Adsorption
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78   PRESSURE SWING ADSORPTION   PSA CYCLES:  BASIC  PRINCIPLES          79

 100        second  bed  1s  vented  to complete  the  blowdown.  The  pressure  equaiizat1on
            step conserves energy smce the compressed gas from  the  high-pressure bed  is
            used  to  partially  pressurize  the  low~pressure  bed  and,  smce  this  gas  1s
 80
            partially  depleted  of the  strongly  a<lsoibcd  spcc1Cs,  separative  work  1s  also
            conserved.  Blowdown  losses  are  reduced  to  about  half,  with  consequent
 u          improvement  m  the  recovery  of the  ratfinate  oroduct.  as  mav  be  seen  from
 '
 ~   60
 ~          Figure 3.8.
 ~
 s             Pnor to Beriin•s modification of the Skarstrom cycle,  another natcnt hasecl
 ,,         on a different idea for  rcducmg blowdown  loss was assigned  to, Marsh et al. rn
 0  4-0
            The process scheme, shown m Figure 3.9, reauires an empty tank m addition
            to  the  two adsorbent  beds.  At  the  end of the  high-pressure  adsorot1on  step
 20         but well  before  breakthrough,  the  feed  flow  is  stopped and  the  product end
            of the high-pressure  bed  1s  connected  to  the  empty  tank where  a portion of
            the compressed gas, nch m the raffinate  product. iii  stored. Toe blowctown of
 00  ----=o'=.s,_.---,~.o,_.---1~.s,.....---2~.0----2~.-s- 0   the  high-pressure  bed  is  comoieted  by  venting  to  the  atmosohere  m  the
            reverse-flow  direction.  The  stored  gas  1s  then  used  to  purge  the  bed  after
 Prod. ltow rate (SCFMl
            which the bed 1s  finally  purged with  product gas. The product purge reamre-
 Figure 3,8  Punty and fractmnal  recovcrv of 0  2   ma two-bed PSA a,r separation unit   ment  ts  reduced,  thereby  mcreasmg  the  recovery,  l)ut  the  savings  m  the
 showing improvement  111  recovery ohtamcd by mcluston of pressure equaljzatmn step.
 (From Rd, 4;  rcpnnlcd with  pcrm1ssum.)   soecific energy of separation  Is  less with  this arrangement  than with  a  direct
            pressure cqualizal.lon  stcn.

               3.2.3  Multiple-Bed Systems
 RAFFINATE  PRODUCT
            Further improvements  m  eflic1ency  arc  generally achieved  by  using  multioic
            adsorbent  beds with  a  seauence of oressure  eaualizatton steps  incorporated
            mto  the cycle.  11 12   In  fact,  multiole-bed  systems  also  use  the  blowdown  gas
                          •
            for  purging other  beds.  Since  this  is  done  at a  pressure  level  where  further
            pressure equalization  is  not  worthwhile,  the  resulting  gain  m  recoverv  vields
             an  additional  benefit.  One  such  example  Is  shown  schemat1cally  in  Figure
             3.10,  which  shows  a  typical  large-scale  air  separation  process  for  oxygen
             oroduction. In  this system. which  utilizes three or four columns, one column
             1s  m the actsorotion step and the other two for three) columns are  m various
             stages  of pressurization,  depressurization.  or purging.  The  process  operates
             at  two  intermediate  pressures  between  the  feed  pressure  and  the  exhaust
             pressure (usually atmospheric). At the end of the 'adsorption step, column  i,
             which  1s  at high pressure, Is connected at  the discharge end  to coiumn  2,  and
             the pressures are equalized.  Pnor to pressure eaualizat10n  column  2 has Just
 VENT        completed  the  purge  step  and  1s  essentially  at·  atmosphenc  pressure.  A
             fraction  of the  remaining gas  from  bed  i  1s  used  for  reverse-flow  purging of
             bed 3. When the pressure in  bed 1 has fallen to the reauired level, beds  I  and
             3  are  disconnected  and the  residual  gas from  bed, l  ts  vented  to atmosphere
             from  the  bed  mlet.  Bed  1 1s  then  purged  in  reverse  flow  with  gas  from  the
 FEED        fourth  bed  and  repressurized  to  the  first  intermediate  pressure  from  the
             second bed, which  has JUSt  compieted the adsorption step. Final repressuriza-
 Figure 3.9  Schematic diagram of a  PSA cycle  showing the use of a third  empty tank
 for  rcducmg blowdown  ioss.   tlon  1s  accomplished  using  a  part  of  the  product  gas  and  the  feed  1s  then
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