Page 102 - Pressure Swing Adsorption
P. 102

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           PSA CYCLES:  BASIC  PRINCIPLES                               77
 76   PRESSURE SWING ADSORPTION
                                    RAFFINATE  PRODUCT
                                          t













 Figure  3.6  Vanatmn  of the  blowdown  stream  comribut1on  to  the  ioss  of raffinate
 product with feed  pressure. A s1mole  mass balance (assummg negligible adsorpnon of
                                         VENT
 the  raffinate  oroduct)  yields  for  the  fraction  of  the  raffinate  product  lost  m  the
 blowdown  stream

 11  + !',,11P1.GXr,.i1ir,u/LP11(Pu -i.),\\dl ..  1
 '!.he  prolilcs  are  shown  for  G = 2.0,  /\ = 1.0  atm,  ipu = 60  s,  .X\, 111  = 0.95,  and
 Xi,J = 0.5.  The numbers on  the curves mdicate  L  /  0  rat10s.
 011
                                          FEED
                                          /a)
 suggested that the purge backwash volume (measured at the purge pressure)
 should exceed the feed  volume (measured at the high ooeratmg pressure) at
 4
 all pomts m the beds dunng each cycle  m order to obtain a pure product.  1n
 practice the purge-to-feed volume should generally be between one and two.
 The relative contributions from  blowdown and purge streams to the total loss
                  COL.  i
 of raffinate  product  deoend  on  the  level  of the  higher  operatmg  pressure.
 Since  the product emerges at  the  high  pressure while  purgmg takes olace  at
 atmosohenc pressure, the actual fraction  of the product stream lost  as  purge
 1s  Quite small and becomes negligible when the pressure ratio is large. On the
 other hand, the  contributmn from  the blowdown  loss  mcreases with  mcreas-
 mg  pressure,  and  becomes  completely  dominant  at  high  ooeratmg pressure,   ,~, u
 as  mav  be  seen  from  Figure  3.6. The  improved  performance of most  of the
 more complex cycles comes from  reduction of the  blowdown  losses.

 3.2.2  Pressure Equalizallon   !  Feed
                    Pressurization  Adsorption  Pressure   Slowdown  Desorption  Pressure
 The first  improvement over Skarstrom·s angina! cycle was the mtroductmn of   Equalization   Equatizolion
 8
 a pressure eaualizataon step proposed by  Berlin. A schematic diagram of the
                                           (b)
 improved. process  and  the  modified  sequence  of ooerat1ons  are  shown  in
 Figure  3.7.  After  the  first  bed  has  been  purged  and  the  second  beli  has   Figure 3.7  (a) Schematic diagram and (b) the sequence M operations in  the  modified
            Skarslrom cycle  including pressure cqualizallon.
 compklt::li  the  high-pressure  adsorption  step,  instead  of hlowmg  down  the
 second  hcd  directly,  the  two  beds  are  connected  through their  product  enc.is
 to equalize  the  pressure.  The first  bed  1s  thus partially  pressurized  with  gas
 from  the outlet region of the second bed. Followmg pressure equalization the
 bccts arc disconnected and the first bed  is oressunzcd with feed  gas while the
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