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06321 ChemE Thermodynamics Flash Calcula:on Bruno A. Calfa Last Update: August 27 th , 2014 1

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Page 1: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

06-­‐321  ChemE  Thermodynamics  

Flash  Calcula:on  

Bruno  A.  Calfa    

Last  Update:  August  27th,  2014  

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Page 2: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Basics  of  Flash  Calcula:ons  •  Important  applica:on  of  VLE.  •  Liquid  at  a  pressure  equal  to  or  greater  than  its  bubble  point  

pressure  “flashes”  or  par:ally  evaporates  when  the  pressure  is  reduced,  producing  a  two-­‐phase  system  of  vapor  and  liquid  in  equilibrium.  

•  Consider  a  P-­‐T  flash,  i.e.,  both  pressure  and  temperature  are  specified  as  well  as  the  inlet  (overall)  composi:on.  

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Page 3: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Modeling  (I/II)  Total  mole  balance        Component  mole  balance  Phase-­‐equilibrium  ra:o      •  In  general,  the  K-­‐values  (Ki)  depend  on  composi:on,  T,  and  P.  

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Page 4: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Modeling  (II/II)  •  Combining  equa:ons  and  given  that                                                                          yields:  

 •  This  is  a  nonlinear  algebraic  equa:on.  •  Note  that  only  zi  is  specified.  If  the  K-­‐values  are  independent  of  

composi:ons  (e.g.,  Raoult’s  Law  assump:on),  then  a  simple  itera:ve  solu:on  approach  can  be  used  to  solve  for  the  ra:o  V/F.  Root-­‐finding  methods,  such  as  Newton-­‐Raphson  and  bisec:on,  can  be  employed.  

•  A  more  convoluted  itera:on  procedure  is  needed  if  the  K-­‐values  also  depend  on  composi:ons  (rigorous  models).  

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Page 5: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Numerical  Example  •  Mixture:  (1)methanol  +  (2)1-­‐propanol  +  (3)acetone.  •  P  =  101.325  kPa,  T  =  343.15  K.  •  z1  =  0.4,  z2  =  0.2,  z3  =  0.4.  •  Approximate  Method  

–  Assume  Raoult’s  Law  applies  (use  Antoine  equa:on  to  calculate  satura:on  pressures).  

–  Use  MATLAB’s  func:on  fsolve.  

•  Rigorous  Method  –  Aspen’s  flash  unit  opera:on  with  an  appropriate  rigorous  thermodynamic  

model.  

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Page 6: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Approximate  Method  

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•  See  MATLAB  script  M-­‐file  FlashExampleApproximate.m.  •  Solu:on  

–  V/F  =  0.656  –  f(V/F)  =  2.583  ×  10−8  

Component   yi   xi  methanol   0.427   0.348  

1-­‐propanol   0.116   0.360  

acetone   0.457   0.292  

Page 7: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Rigorous  Method  •  Add  components.  

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Page 8: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Rigorous  Method  •  Select  thermodynamic  model.  Use  Method  Assistant.  NRTL  seems  to  be  

appropriate.  Go  to  the  subfolder  Parameters  under  Methods  to  confirm  that  you  will  be  using  Aspen’s  built-­‐in  binary  interac:on  parameters.  

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Page 9: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Rigorous  Method  •  Switch  to  Simula:on  environment.  Add  flash  unit  opera:on  (Flash2).  •  You  may  rename  the  block  (right  click  -­‐>  Rename  Block).    

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Page 10: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Rigorous  Method  •  Click  on  Material.  Red  arrows  are  mandatory.  Blue  arrows  are  op:onal.  

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Page 11: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Rigorous  Method  •  Draw  streams.  You  can  rename  streams  (right  click  -­‐>  Rename  Stream).  

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Page 12: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Rigorous  Method  •  Configure  feed  stream.  Go  to  folder  Streams,  then  1  (feed  stream).  Use  same  

condi:on  of  feed  for  this  example.  

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Page 13: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Rigorous  Method  •  Configure  flash  unit.  Go  to  folder  Blocks,  then  B1.  

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Page 14: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Rigorous  Method  •  Run  simula:on.  Click  on  blue  play  bukon.  It  converged!  We  are  not  

performing  economic  evalua:ons  at  this  point,  so  click  Close.  

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Page 15: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Rigorous  Method  •  Check  results  (Stream  Summary).  

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Page 16: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Rigorous  Method  •  To  see  molar  frac:ons,  go  to  folder  Setup  -­‐>  Report  Op:ons  -­‐>  Streams.  

Check  Mole  for  Frac:on  basis.  

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Page 17: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Rigorous  Method  •  Rerun  simula:on.  View  Streams  (Custom).  Remember  that  stream  2  is  the  

vapor  stream,  and  stream  3  is  the  liquid  stream.  

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Page 18: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

Summary:  Approximate  vs.  Rigorous  

Approximate   Rigorous  

0.656   0.756  

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Component   yi   xi  Approx.   Rigorous   Approx.   Rigorous  

methanol   0.427   0.427   0.348   0.318  

1-­‐propanol   0.116   0.128   0.360   0.422  

acetone   0.457   0.445   0.292   0.260  

V/F                      

•  More  considerable  devia:ons  in  the  liquid  stream  composi:on.  Ideal  solu:on  (Raoult’s  Law)  may  not  be  a  good  assump:on  for  this  system.  Need  rigorous  ac:vity  coefficients  models.  

•  See  file  FlashExampleRigorous.apwz.  

Page 19: Aspen v8 Tutorial - Flash Calculation - bacalfa.combacalfa.com/TA/ExtraMaterial/Aspen v8 Tutorial - Flash Calculation.pdf · – Aspen’s’flash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’

References  [1]  Green,  D.  W.  (Editor).  (2008).  Perry’s  Chemical  Engineers’  Handbook.  McGraw-­‐Hill  Professional.  New  York,  NY.  USA.  [2]  Seader,  J.  D.;  Henley,  E.  J.;  &  Roper,  D.  K.  (2011).  Separa:on  Process  Principles:  Chemical  and  Biochemical  Opera:ons.  John  Wiley  &  Sons,  Inc.  Hoboken,  NJ.  USA.  

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