Page 223 - Pressure Swing Adsorption
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              200                                   PRESSURE SWING ADSORPTION                DYNAMIC MODELING  OF A PSA SYSTEM                           201
                              60,------------~--~                                            recovery,  and  changing  the  mtrogen  equilibrium  affects  both  purity  and
                                                                                             recovery.  An  1rnoroved  moiecular  sieve  for  mtrogen  from  air·  by  pressure
                                                      X                                      swmg adsorption would therefore reamre stronger oxygen equilibrium and/or
                                                                                             slower nitrogen diffusion.  It should  be  recalled  here that there are limits  up
                                                                                             to which such improvements may  be  effectively ex:pioJted  in  this  type  of cycle
                                                                                     I       (see Section 3.4).



                                                                                             5_3  Continuous Countercurrent Models
                                                   Parameters
                                                   F\J  12-8 atm)  --                        In order to understand the contmuous countercurrent ;flow  model  for a  PSA
                                           •        l/V 120-35)  _,,_                        separation process, it 1s  necessary to recall once agam that the steos involved
                                                      G 10-11  ---·
                                                        OPE  □
                                                    SE (0.5-2)  o, •
                                                    SK  (31)-68)  b., •
                                                                                                                    Pr-od!.rct
                               0          6          10         16
                                        Mole % oxygen in product
              Figure 5.9  Effects of some important operatmg parameters on purity and recovery of
               nitrogen  m  a  kinetically controlled  PSA  air separation  process.  Adsorptlon/desorp-
               t1on  time= 60  s,  pressunzat1on/blowdown  time= 15  s,  kinetic  and  eauilibrium  pa-
               rameters  are  given  m  Table  5.8.  The  solid  line  shows  the  effect  of increasmg  the
               adsorotmn  pressure  (in  the  direction  of the  arrow)  for  a  Skarstrom  cycle  with  no
               purge. For two different operatmg pressures the effect of introducing a double-ended
               pressure equalization step, as in the modified cycle 1s shown  by dotted lines leading to
               the  pomts  ( □). The  effect  of increasmg  purge/feed  is  shown  by  cham  dotted  iine   z•O
               leading to pomt (0) at purge/feed= 1.0, and the effect of changing the L/ v H  ratm
                                                                        0
               from  20 s at nomt ( X) to 35 s at pomt ( +) is  mdicated by dot-dash line. The effects
                                                                                                       Praeeurizction   Adeorption   Slowdown
               of increasmg and  decreasmg  the  kinetic  and equiJibnum  oarameters for  oxygen  and                                        Pwgo
               mtrogen (by  factors  of 1.5  and  2.0,  respectively,  relative  to  the exoenmental  values)               (a)
               are also shown.  The open symbols show the effect of changing the vaiues for oxygen,
               and  the  closed  symbols  show  the  effect  of changing  the  values  for  mtrogen.  The
               directions  of the  mcreasmg  vanables  ( L /  v H,  PH,  and  G)  and  the  kinetic  and
                                                 0
               equilibrmm seiectivities (SK  and  SE) are indicated by arrows.                                          Product
                                                                                                                           h
                                                                                                                     z•L
               sure  is  low.  The product purity may  be  significantly improved  m  a  low-feed                           ,. -
               pressure  operation  by  regenerating  the  adsorbent  bed  with  product  ourge.
               However,  the  recovery  1s  reduced  by  mtroducing  purge.  Another  way  of                               ~-
               increasing the product purity when the feed pressure ts low is to increase the                               ~-
               L/vm ratio,  but  product  purge  appears  to  be  more  efficient.  Of  course,
                    1
               when  operated  at  high  feed  pressure  a  very  high-purity  product  may  be                      z-0
               achieved without resorting to any external purge and conseQUentiy, at compa-
               rable  purity,  the  recovery is  much  higher than  that from  a  Skarstrom cycle.                    """°'1'ttcn  Pwgo
               The effects of varying the kinetic select1v1ty (SK  - D /DN,) and the equilib-
                                                          0
               rium  selectivity  (S !  = K 01 /KN)  about  their  experimental  values  are  also                          (b)
                               1
               illustrated.  lt  is  evident  that  varying  the  oxygen  diffus1v1ty  or  equilibnum
                                                                                             Figure  5.10  (a)  Skarstrom  PSA  cvdc.  (h)  Continuous  countcrcurrenl  flow  model
               mainiy  affects  the  ounty.  Varying  the  nitrogen  diffusivity  affects  mainly  the   reoresentat1on of a Skarstrom  PSA  c..yclc.
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