Page 221 - Pressure Swing Adsorption
P. 221

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                                                   PRESSURE SWING ADSORPTION  I
             198                                                                             DYNAMIC MODELING  OF A PSA SYSTEM                            199
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                                                                                                                                              .
                             JO~         ¾_,                                                                40 r        ~
                                          ~                                                                                       L/v •25  s
                           .,               ~--=-~                                                                  Experimental    00
                           u                                                                              C     "   Oiff'uslon  modQJ
                           0                                                                              •
                           ·o                                                                             m
                           a             -~                                                               Q         constant  0
                                                                                                          L
                           C                                                                              ...       vor1oble  0
                           (l_                                                      I
                           C                                                                              C         LDF  model
                                                                                                         4-
                           C                       '                                                      Q  ,..   0   '         l
                           •   □                                                                                                        '
                           ,..
                           m
                                                                                                          •
                           X                   Experim1.mtoi                                              L
                           a                                 "                                            >
                                            Diffusion  modei                                              Q
                           "'   10               constant  D ---                                          0  •  40 r     -------
                           ~                                                                              L
                           a                     variable  0                                             "'
                                           ,  __
                           ""            '  ,-:--,   LOF  model                                                        ~
                                            ...._-=--=--=--=                                                                      LfVOH•37  S
                                         "
                                   L/v H•37  s   'v   _                             i                                           "   "
                                      0
                                      ,.
                              o L~-.l'. :_--~ -:::.::::itS_.L ·-_j                                          0                    '
                               0      2     4      6     B      JO                                            0     2      4     6      B     JO
                                     Adsorption  pressure  (otm)                                                   Adsorption  pn:issurQ  (otm)
                                              !al                                                                            (b)
              Figure  5.8  Effect  of  adsorotton  pressure  on  (a)  mtrogen  produci  ounty  and                 Figure 5.8  (Continued).
              (b)  nttrogen  recovery  at  two different  L /  v H  ratios m  a  kinetically  controlled PSA
                                               0
              air separation process (modified cycle) showmg the companson among the expenmen•
              ta!  data,  the  LDF  model,  and  the  diffusion  model  with  constant  and  vanable
              dilfusivtty.  Exoenmental  conditions  are  given  in  Table 5.7  and  other parameters m   mance based  on  independently measured smgle-comoonent equilibrium  and
              Table 5.8.  (From Rel. 26.)                                                    kinetic data.
                                                                                               The  flexibility  of  a  PSA  s1mulat10n  model  for  accommodating  vanous
                                                                                             cyclic  operations  ts  determtnect  mamlv  by  the  versatility  of  the  fluid  flow
    i
                                                                                             model.  (The flexibility of the  fluid  flow  model  used  here  has  been  discussed
              Industrial  PSA  nitrogen  umts  operate  at  pressures  between  7  and  10  atm   in Section 5.1.) There 1s  therefore no reason to orevent the applicat1on of the
              (with b!owdown to atmosphenc pressure) and at a relatively low  L/voH ratlo    pore diffusion  model to other cycle  configurations.
              ( < 25  s).  A fairly  pure nitrogen product(> 99%)  is  produced. This level  of
              punty is not predicted by either the LDF model or by tlle constant-diffusivity
                                                                                               Effects of Process  Variables
              pore  diffusion  model.  In  fact  at  L/voH = 25  s  the  mtrogen  oroduct  purity
               profiles from the LDF model as well as the constant-diffusivity pore diffusion   Having  established  the  validity  of  the  concentrat,on-deoendent  diffusivity
               model  become  asymptotic  at  an  oxygen  concentration  of about  8%.  It  is   model for air separation over carbon molecular sieve, the modei may now be
               therefore evident that the formation of nitrogen product containing Jess than   used to investigate the effects of some important operating parameters on the
               l %  oxygen, which ts  routmeiy observed in  large-scale industrial umts operat-  performance of the system. Some results are summarized in  Figure 5.9.  For a
               ing at  reiatively low. L/v  rat10s,  1s correctly predicted oniy when allowance   Skarstrorn  cycie  ooerat1on  with  no  purge,  increasing  adsorot1on  pressure
                                   011
               is  made  for  the  concentration dependence of the diffusivity.  The concentra-  improves product ourity at  the  expense of diminishing recoverv.  It  is  shown
               tion dependence of m1cropore diffusivity evidently has a strong effect on the   that  duai-endect  pressure  eoualizat1on  improves  both  ounty  and  recovery
               steady-state  oerfonnance  of  a  kinetically  controlled  PSA  separation.  The   over those obtained by Skarstrom cycle ooeration without purge. Desorption
               vanable-diffusivity model provides a reliable a priori estimate of such oerfor-  without  purge does  not  produce  a  high-ourity oroctuct  when  the  feed  pres-
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