Abstrip-packed Columns (1)

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Packed absorption and stripping columns Prof. Dr. Marco Mazzotti - Institut für Verfahrenstechnik 1. HETP - approach Packed columns are continuous contacting devices that do not have the physically distinguishable stages found in trayed columns. In practice, packed columns are often analyzed on the basis of equivalent equilibrium stages using a Height Equivalent to a Theoretical Plate (HETP): HETP ! packed height number of equivalent equilibriu m stages Knowing the value of the HETP and t
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  Packed absorption and stripping columns Prof. Dr. Marco Mazzotti -Institut für Verfahrenstechnik  Packed columns are continuous contacting devices that do not have the physicallydistinguishable stages found in trayed columns. 1. HETP -approach stagesmequilibriu equivalent of number  height  packed HETP  ! HETP nH  ! I n practice, packed columns are often analyzed on the basis of equivalentequilibrium stages using a H eight E quivalent to a T heoretical P late( HETP  ):K nowing the value of the HETP  and the theoretical number of stages n of atrayed column, we can easily calculate the height H  of the column :T he HETP  concept, unfortunately, has no theoretical basis. HETP  values can onlybe calculated using experimental data from laboratory or commercial-size columns.  F or packed columns, it is preferable to determine packedheight from a more theoretically based method usingmass transfer coefficients. 2 . Absorption: Mass transfer approach (HTU, NTU) L, x  1 G , y  1 T  , p Process G , y  2  y  2  < y  s  pe c  L, x  2  F urthermore, we introduce the coordinate z  , whichdescribes the height of the column. T he absorption problem is usually presented as follows. T here is a polluted gas stream coming out from a process. T he pollutant must be recovered in order to clean the gas. At the bottom and the top of the column, thecompositions of the entering and leaving streams are :  )y  ,  x (  11 )y  ,  x (  22 z = 0 z = H  x y  T he green, upper envelope is needed for theoperating line of the absorption column.  F irst, we need a material balance around thegreen, upper envelope of the column. I t is theoperating line, going through the point (x  2  , y  2   ) :   22 y  x  x G L y  ! y* = m x  y  1  ) , (  f  G L f  G L m i  n 21 p¹ º ¸©ª¨ !  )(  1 T hen we need the equilibrium condition :  x  m*  y  ! )(  2 G L m i  n G L  x  1 W e can now draw the equilibrium and operatingline into the diagram. F rom the operating linewith the smallest slope (  L m i  n  /G) , we can get (  L  /G) with the known formula :    x y  2   x  2  y  Gy  L  x Gy  L  x  ! 22
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